Epoxy compositions and products comprised thereof

Abstract

The present disclosure provides epoxy resin compositions and composite materials derived therefrom. Also provided are methods for producing said epoxy composition by forming a blend of epoxy resins. Further provided are composite precursors comprising fibrous matter impregnated with the epoxy composition. Further aspects relate to methods for producing composite precursors by combining the epoxy composition with fibrous matter. The disclosure further provides a composite material formed by curing the composite precursor, a method for producing the composite material, an article of manufacture comprising the cured composite material, and a method of manufacturing said article.

Description

[0001] EPOXY COMPOSITIONS AND PRODUCTS COMPRISED THEREOF

[0002] TECHNOLOGICAL FIELD

[0003] The present disclosure relates to self-toughening epoxy compositions.

[0004] BACKGROUND

[0005] Prepregs are intermediate products, usually comprising a reinforcing phase and a polymeric matrix, which need high temperature and pressure to become an engineering composite product.

[0006] Prepregs are used due to their high specific modulus and strength. Such prepregs are usually stored at very low temperatures, typically -18°C to delay curing of the thermosetting matrix and to maintain the tackiness of the prepreg over time. At room temperature the storage time (out-life) is merely a month due to a loss of tack.

[0007] GENERAL DESCRIPTION

[0008] In accordance with a first aspect of the presently disclosed subject matter there is provided an epoxy composition, the composition comprising a liquid epoxy resin and a solid epoxy resin, wherein: said liquid epoxy resin is characterized by viscosity of between about 500 mPa s and about 100,000 mPa s, when measured at 25°C according to ASTM D1084; and said solid epoxy resin is characterized by a softening temperature of between about 60°C and about 160°C when measured according to ASTM D3418 and / or ASTM D1525.

[0009] In accordance with a second aspect of the presently disclosed subject matter there is provided a method of producing an epoxy composition, the method comprises forming a blend comprising a liquid epoxy resin and a solid epoxy resin, wherein: the liquid epoxy resin is characterized by viscosity of between about 500 mPa s and about 100,000 mPa s, when measured at 25°C according to ASTM D1084; and the solid epoxy resin is characterized by a softening temperature of between about 60°C and about 160°C when measured according to ASTM D3418 and / or ASTM D 1525.

[0010] Further, in accordance with a third aspect of the presently disclosed subject matter there is provided a composite precursor comprising fibrous matter and an epoxy composition impregnated within fibrous matter, wherein: the liquid epoxy resin is characterized by viscosity of between about 500 mPa s and about 100,000 mPa s, when measured at 25°C according to ASTM D1084; and the solid epoxy resin is characterized by a softening temperature of between about 60°C and about 160°C when measured according to ASTM D3418 and / or ASTM D 1525.

[0011] In accordance with a fourth aspect of the presently disclosed subject matter there is provided a method of producing a composite precursor, the method comprises mixing the epoxy composition according to the first aspect of the present disclosure and a curing additive, with fibrous matter, the mixing being under conditions that provide impregnation of the fibrous matter with the epoxy composition and the curing additive, the epoxy composition and curing additive being preferably in a form of a blend.

[0012] In accordance with a fifth aspect of the presently disclosed subject matter, there is provided a composite material comprising the presently disclosed composite precursor in cured form.

[0013] In accordance with a sixth aspect of the presently disclosed subject matter, there is provided a method of producing a composite material, the method comprises applying onto a composite precursor curing conditions effective to cause curing of the composite precursor.

[0014] Yet further, in accordance with its seventh aspect, there is provided an article of manufacture comprising the composite material according to the presently disclosed fifth aspect, in a predefined shaped form.

[0015] Finally, in accordance with its eight aspect, the present disclosure provides a method for manufacturing an article of manufacture, the method comprises shaping a composite precursor according to the presently disclosed third aspect and applying curing conditions onto said composite precursor effective to cause curing of the shaped composite precursor.

[0016] BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0018] Figures 1A-1B are plots showing viscosity-temperature relationships for epoxy blends with different weight to weight ratios of liquid component and solid component (L:S ratio) (Figure 1A) and different ratios of liquid component, solid component , liquid rubber and catalyst (Figure IB) according to some examples of the present disclosure.

[0019] Figure 2 is a photograph showing a visual appearance of the curing agent within an epoxy mixture at different mixing temperatures according to some examples disclosed herein.

[0020] Figure 3 is a plot showing FTIR spectra of 60L:40S epoxy composition at different mixing temperatures according to some examples of the present disclosure.

[0021] Figure 4 is a plot showing glass transition temperature (Tg)-fraction of liquid component relationships for epoxy blends according to some examples disclosed herein.

[0022] Figure 5 is a plot showing gel time to temperature relationships for epoxy compositions with different ratios of liquid component and solid component according to some examples of present disclosure.

[0023] Figures 6A-6D are micrographs showing Scanning Electron Microscope (SEM) images of fracture surfaces of 60L:40S CTBN-toughened epoxy compositions showing, according to some examples disclosed herein fracture surfaces of epoxy composition with no CTBN (Figure 6A), with 5 phr CTBN (Figure 6B), with 10 phr CTBN (Figure 6C) and with 15 phr CTBN (Figure 6D).

[0024] Figure 7 is a SEM micrograph showing a tensile fracture of a CTBN-toughened epoxy formulation according to some examples disclosed herein. Figure 8 is a microphotograph showing according to some disclosed examples a drop of epoxy formulation on a single carbon fiber.

[0025] Figure 9 is a plot showing surface tension-temperature relationships for GT6071 / DER 331 blends having L:S ratios of 50L:50S, 60L:40S and 70L:30S according to some examples disclosed herein.

[0026] Figures 10A-10C are SEM photographs showing fractured surfaces after mechanical flexural test according to some examples disclosed herein and specifically showing: fiber crack (Figure 10A), fiber matrix bonding (Figure 10B), and voids due to fiber pull out (Figure IOC).

[0027] Figure 11 is a histogram showing inter laminar shear strength (ILSS) values for vacuum bagged composite laminates (0,90)16 according to some examples disclosed herein.

[0028] Figures 12A-12C are microphotographs showing optical microscopy images of cross-section of an edge showing magnified mid-planes interlaminar failure mode of laminate (0,90)16 specimens according to some disclosed examples and specifically for specimens based on 60L:40S matrix toughened with 12.5 phr CTBN (Figure 12A), 7.5 phr CTBN (Figure 12B) and 7.5 phr CTBN+0.25 phr triphenylphosphine (TPP) (Figure 12C).

[0029] Figure 13 is a histogram showing mode II critical strain energy release rate (GIIc), for neat and toughened systems according to some disclosed examples.

[0030] Figures 14A-14C are SEM micrographs showing end notch flexure (ENF) fractured surfaces of unidirectional carbon fabric composite based on 60L:40S matrix cured with DDS according to some examples disclosed herein and specifically for unmodified (i.e. untoughened) resin (Figure 14A), resin modified (i.e. toughened) with CTBN (Figure 14B), resin modified with CTBN and TPP (Figure 14C).

[0031] Figure 15 is a plot showing complex viscosity-temperature relationships of epoxy compositions with various L:S ratios according to some disclosed examples.

[0032] Figure 16 is a plot showing viscosity-temperature relationships for resins modified with high carboxyl content CTBN according to some disclosed examples. Figure 17 is a logarithmic plot showing a curing rate (P) to 1 / T relationship for epoxy compositions according to some disclosed examples.

[0033] Figure 18 is a microphotograph showing a drop of epoxy formulation on a single carbon fiber according to some disclosed examples.

[0034] Figures 19A-19B are low magnification SEM images showing cured specimens according to some examples disclosed herein and specifically longitudinal section (Figure 19A) and cross-section (Figure 19B) of 40L:60S matrix toughened with 16 phr CTBN and reinforced with carbon fiber.

[0035] Figures 20A-20D are high magnification SEM micrographs showing cured specimens of carbon fiber reinforced 40L:60S epoxy compositions cured with dicyandiamide (DICY) according to some examples disclosed herein and specifically unmodified (untoughened) epoxy / DICY (Figure 20A), toughened with 16 phr CTBNX (Figure 20B), toughened with 16 phr CTBN (Figure 20C), toughened with 16 phr CTBN in presence of 0.35 phr TPP (Figure 20D).

[0036] Figures 21A-21B are SEM images showing a cross-section of cured epoxy / DICY blend according to some examples disclosed herein and specifically toughened with 8 phr CTBN (Figure 21A) and toughened with 8 phr CTBNX (Figure 21B).

[0037] Figures 22A-22B are photographs showing prepregs according to some examples disclosed herein and specifically a prepreg without polyether sulfone (PES) (Figure 22A) and prepreg with 15 wt.% PES (Figure 22B)

[0038] Figure 23 is a plot showing a relationship of curing enthalpy to PES percentage for a 60L:40S epoxy blend according to some examples disclosed herein.

[0039] Figures 24A-24F are SEM micrographs of cured epoxy blends with different PES concentrations after cryogenic fracture, according to some examples disclosed herein and specifically with 0wt% PES (Figure 24A), 5wt% PES (Figure 24B), 10wt% PES (Figure 24C), 15wt% PES (Figure 24D), 20wt% PES (Figure 24E), and 25wt% PES (Figure 24F).

[0040] Figure 25 is a SEM micrograph showing a cured epoxy blend having 5 wt.% PES specimen (the PES droplets are highlighted). Figure 26 is a SEM micrograph showing a phase-in-phase morphology showing epoxy domains (pointed with arrows) within a cured epoxy blend having 5 wt.% PES according to some examples disclosed herein.

[0041] Figures 27A-27C are SEM micrographs showing epoxy specimens having 15 wt.% PES which were etched for a week in DMSO and showing a co-continuous morphology according to some examples disclosed herein and specifically showing a magnification of X40 (Figure 27A), X100 (Figure 27B) and X1000 (Figure 27C).

[0042] Figure 28 is a photograph showing the visual appearance of cured epoxy with different PES concentrations according to some examples disclosed herein.

[0043] Figure 29 is a plot showing complex viscosity-temperature relationship for the hot melt bimodal epoxy resins with concentrations of PES within a range of between 0 wt% to 20 wt% according to some examples disclosed herein.

[0044] Figures 30A-30B are photographs showing PES aggregation in a prepregs having 15wt.% PES according to some examples disclosed herein and specifically for prepregs prepared at fiber pulling speed of 1 m / min (Figure 30A) and 3 m / min (Figure 30B).

[0045] Figure 31 is a plot showing a change of curing enthalpy over time for bimodal epoxy resins having different concentrations of PES according to some examples disclosed herein.

[0046] Figure 32 is a histogram showing change of tackiness over time of the prepregs having different PES content according to some examples disclosed herein.

[0047] Figure 33 is a photograph showing a tackiness test on an aluminum plate, for prepregs that were held 15 weeks at room temperature.

[0048] Figure 34 is a histogram showing a change of inter laminar shear strength (ILSS) overtime of CFRP having varying concentrations of PES, at room temperature according to some examples disclosed herein.

[0049] Figure 35 is a histogram showing interlaminar fracture toughness (mode II) of CFRP having varying concentrations of PES according to some examples disclosed herein. Figure 36 is a photograph showing a fracture surface of CFRP specimens having varying concentrations of PES according to some examples disclosed herein after Mode II and Mode I failure.

[0050] Figure 37 is a photograph showing a side view and zoom-in of the crack zone of fracture surfaces of CFRP specimens having varying concentrations of PES according to some examples disclosed herein after Mode II and Mode I failure.

[0051] Figures 38A-38H are SEM micrographs showing fracture surfaces of CFRP specimens having varying concentrations of PES after Mode II and Mode I failure according to some examples disclosed herein and specifically Mode II fracture surface of a specimen having 0 wt.% PES (Figure 38A), Mode I fracture surface of a specimen having 0 wt.% PES (Figure 38B), Mode II fracture surface of a specimen having 5 wt.% PES (Figure 38C), Mode I fracture surface of a specimen having 5 wt.% PES (Figure 38D), Mode II fracture surface of a specimen having 10 wt.% PES (Figure 38E), Mode I fracture surface of a specimen having 10 wt.% PES (Figure 38F), Mode II fracture surface of a specimen having 15 wt.% PES (Figure 38G), and Mode I fracture surface of a specimen having 15 wt.% PES (Figure 38H).

[0052] DETAILED DESCRIPTION

[0053] The presently disclosed subject matter concentrates on epoxy compositions that, among other uses, are suitable for use as part of prepregs (referred to herein by the term "composite precursor"), on prepregs per se and on products obtained therefrom.

[0054] Prepreg is a short term for "pre-impregnated" and refers to fibrous matter that has been pre -impregnated with a resin system before being processed into a final composite material.

[0055] An efficient impregnation of fibrous matter relies upon balancing properties of the epoxy resin composition, including, but not limited to, viscosity, to achieve a uniform distribution of the epoxy resin throughout the fibrous matter. Without being bound by theory, in order to achieve high-quality prepregs and void-free final composites, the resin system should preferably flow through the pores within the fibrous matter and through the gaps between fiber tows, thus effectively impregnating the fibrous matter. The present disclosure is based on the development of a resin system, with suitable viscosity rendering it fluid or semi-fluid, comprising a solid epoxy resin and a liquid epoxy resin. To this resin system additives may be added to allow for the formation of a prepreg composition (referred to herein by the term precursor composite).

[0056] Thus, in accordance with a first of its aspects, there is provided an epoxy composition comprising a liquid epoxy resin and a solid epoxy resin, wherein the said liquid epoxy resin is characterized by viscosity of between about 500 mPa- s and about 100,000 mPa s, when measured at 25°C according to ASTM D1084; and wherein the solid epoxy resin is characterized by a softening temperature of between about 60°C and about 160°C when measured according to any one of ASTM D3418 and ASTM D1525.

[0057] In some examples of the presently disclosed subject matter, the liquid epoxy resin is characterized by a viscosity, when measured at 25°C according to ASTM D1084, within a range of between about 500 mPa s and about 1,000 mPa s, at times between about 1,000 mPa s and about 5,000 mPa s, at times between about 5,000 mPa s and about 10,000 mPa s, at times between about 10,000 mPa s and about 20,000 mPa s, at times between about 20,000 mPa s and about 50,000 mPa s, at times between about 50,000 mPa s and about 75,000 mPa s, and at times between about 75,000 mPa s and about 100,000 mPa s.

[0058] In some examples of the presently disclosed subject matter, the solid epoxy resin is characterized by a softening temperature, when measured according to any one of ASTM D3418 and ASTM D1525, of between about 60°C and about 80°C, at times between about 80°C and about 100°C, at times between about 100°C and about 120°C, at times between about 120°C and about 140°C, at times between about 140°C and about 160°C.

[0059] One beneficial attribute of presently disclosed subject matter is that suitable viscosity of the epoxy composition and of the prepreg as disclosed herein, is achieved without introducing a solvent into the epoxy composition or into the prepreg.

[0060] As used herein the term "solvent" is intended to mean a chemical substance selected for its ability to dissolve or disperse epoxy resin without undergoing any chemical reaction itself or becoming a covalent part of a cured structure. In the context of the presently disclosed subject matter, it is to be understood that when referring to a solvent, it is to be understood to mean a material with the aforesaid functionally and that would volatize from within the resin at the processing and / or curing temperature.

[0061] Hence, as used herein, the term "solvent" excludes any "reactive diluent" and "plasticizer " since, as further explained hereinbelow, these substances do not volatilize from within the resin at the processing and / or curing temperature.

[0062] In some examples of the presently disclosed subject matter, the term "solvent" encompasses hydrocarbons typically used for adjusting viscosity and improving workability of epoxy resins, including aromatic hydrocarbons, such as toluene, xylene; aliphatic solvents, such as hexane and heptane, and alcoholic solvents, such as isopropanol, ethanol, n-butanol.

[0063] Further, in the context of the presently disclosed subject matter, the term "essentially solvent free" is to be understood to mean that there is no amount of a solvent, such as those known to be used with epoxy resins, that can be detected by acceptable techniques, such as gas chromatography, Fourier Transform Infrared Spectroscopy (FTIR), Nuclear Magnetic Resonance (NMR) Spectroscopy, Thermogravimetric Analysis (TGA), Headspace Analysis, Mass Spectrometry (MS), UV-Vis Spectroscopy.

[0064] Therefore, in some examples of the presently disclosed subject matter, the epoxy composition, as well as the prepreg comprising the same, are essentially solvent-free.

[0065] In some examples of the presently disclosed subject matter, the solid epoxy resin and the liquid epoxy resin are selected to provide the epoxy composition with a viscosity of between about 0.5 Pa s and about 15 Pa s, when measured at the compositions processing temperature. The epoxy composition with this range of viscosity can be defined as a semi-solid composition.

[0066] As used herein, the term "processing temperature" refers to the temperature at which the epoxy composition is sufficiently fluid in order to result in the fibrous matter being impregnated with the epoxy composition. At this temperature, the viscosity of the epoxy composition allows it to penetrate the fibrous matter, such as through the pores within the fibrous matter and the gaps between fiber tows, thus effectively filling the fibrous matter with the epoxy composition. The processing temperature of an epoxy composition in accordance with the presently disclosed subject matter, can be easily determined by mere heating of the composition until the latter becomes sufficiently fluid to be able to impregnate fibrous matter.

[0067] In some examples, the processing temperature comprises a temperature of at least 50°C, at times, at least 60°C, at times, at least 70°C, at times, at least 80°C, at times, at least 90°C.

[0068] In some examples of the presently disclosed subject matter, the solid epoxy resin and liquid epoxy resin are selected to provide the epoxy composition with a viscosity, at processing temperature of the epoxy composition, of between about 0.5 Pa s and about 1 Pa s; at times, between about 1 Pa s and about 3 Pa s; at times, between about 3 Pa s and about 5 Pa s; at times, between about 5 Pa s and about 7 Pa s; at times, between about 7 Pa s and about 9 Pa s; at times, between about 9 Pa s and about 12 Pa s; at times, between about 12 Pa s and about 15 Pa s.

[0069] In some examples of the presently disclosed subject matter, the viscosity at processing temperature is between about 1 Pa s and about 3 Pa s.

[0070] In some examples of the presently disclosed subject matter, the viscosity of the epoxy composition, as defined hereinabove, is achieved by selecting a weight percent ratio between solid epoxy resin and liquid epoxy resin, within a range of about 10:90 and about 90: 10.

[0071] In some examples of the presently disclosed subject matter, the viscosity of the epoxy composition, as defined hereinabove, is achieved by selecting a weight percent ratio between solid epoxy resin and liquid epoxy resin, within a range between about 10:90 and about 20: 80; at times, between about 20:80 and about 30:70; at times, between about 30:70 and about 40:60; at times, between about 40:60 and about 50:50; at times, between about 50:50 and about 60:40; at times, between about 60:40 and about 70:30; at times, between about 70:30 and about 80:20; at times, between about 80:20 and about 90: 10.

[0072] In some examples of the presently disclosed subject matter, the viscosity of the epoxy composition is achieved by selecting a weight percent ratio between solid epoxy resin and liquid epoxy resin, within a range of between about 50:50 and about 70:30. In some examples of the presently disclosed subject matter, the liquid epoxy resin is selected from the group consisting of liquid Diglycidyl ether of bisphenol-A, Diglycidyl ether of bisphenol -F, epoxy cresol novolac, epoxy novolac, brominated epoxy, and any combination of such liquid epoxy resins.

[0073] In some preferred examples of the presently disclosed subject matter, the liquid epoxy resin is Diglycidyl ether of bisphenol-A of formula (I) wherein n=0.

[0074] In some examples, the solid epoxy resin is selected from the group consisting of solid diglycidyl ether of bisphenol-A, epoxy cresol novolac, epoxy novolac, brominated epoxy, and any combination of same.

[0075] In some examples, the solid epoxy resin is Diglycidyl ether of bisphenol-A of formula (I1) wherein n' is an integer between 1 and 25.

[0076] Notably, the difference between formula (I) and formula (I1) is in the values of the integer "n" .

[0077] While being absent of an epoxy solvent per se, the epoxy composition may contain various additives.

[0078] In some examples, the epoxy composition comprises a toughening agent.

[0079] As used herein the term "toughening agent" is intended to refer to a chemical substance added to epoxy resins to improve toughness, that is, their ability to absorb energy and plastically deform without fracturing, enhance impact resistance, enhance tensile strength, and enhance elongation at break of a resulting product obtained from at least partial curing of the presently disclosed epoxy composition. Toughness can be determined by ASTM D7905.

[0080] In some examples of the presently disclosed subject matter, the toughening agent comprises rubber particles.

[0081] Without being limited thereto, the rubber particles can comprise rubber selected from the group consisting of butadiene rubber, carboxyl-terminated acrylonitrilebutadiene Rubber (CTBN), amine -terminated acrylonitrile-butadiene rubber (ATBN), epoxy-terminated acrylonitrile-butadiene rubber (ETBN), vinyl -terminated acrylonitrilebutadiene rubber (VTBN), core-shell rubber and any combinations of same.

[0082] In some examples of the presently disclosed subject matter, the toughening agent is a thermoplastic toughening agent.

[0083] Without being limited thereto, the thermoplastic toughening agent is selected from the group consisting of polyethersulfone, polyetherimide, polyetheretherketone and any combinations of same.

[0084] In some examples of the presently disclosed subject matter, the toughening agent comprises or is polyethersulfone (PES) of formula (II) wherein n is integer between 1 and 4,000.

[0085] In some examples of the presently disclosed subject matter, the toughening agent comprises inorganic particles and / or fibers.

[0086] In some examples of the presently disclosed subject matter, inorganic particles and / or fibers are selected from the group consisting of silica, polyhedral oligomeric silsesquioxane (POSS), nanoclay, carbon nanotubes, tungsten disulfide, and any combinations of same.

[0087] The amount of the toughening agent in the epoxy composition may vary, depending on the type of epoxy resins used and other additives present. Yet, in some examples of the presently disclosed subject matter, the toughening agent is present in the epoxy composition in amount of between about 1 and about 25 parts per hundred resin (phr).

[0088] In some examples of the presently disclosed subject matter, the toughening agent is in amount of between about Iphr and about 5phr; at times, between about 5phr and about lOphr; at times, between about lOphr and about 15phr; at times, between about lOphr and about 20phr; at times, between about 15phr and about 20phr; at times, between about 20phr and about 25phr.

[0089] In some examples of the presently disclosed subject matter, the epoxy composition comprises a reactive diluent and / or a plasticizer.

[0090] It is to be understood that reactive diluents and plasticizers remain in the resin and become part of the cured structure, unlike solvents which volatilize and are removed during processing or curing.

[0091] In some examples of the presently disclosed subject matter, the epoxy composition comprises a reactive diluent.

[0092] As used herein, the term "reactive diluent" refers to a chemical substance having functional groups capable of undergoing chemical reactions with the epoxy groups of the epoxy resins, in the epoxy composition. The reactive diluent thus actively participates in curing of the epoxy resins and upon completion of curing, the reactive diluent is incorporated, by covalent bonding, into the chemical structure of the final composite material.

[0093] In the presence of a reactive diluent, the liquid epoxy resin is characterized by a viscosity of between about 1 mPa s and about 1,500 mPa s when measured at 25°C according to ASTM D1084; at times, between about 1 mPa s and about 100 mPa s; at times, between about 100 mPa- s and about 200 mPa- s; at times, between about 200 mPa- s and about 400 mPa-s; at times, between about 400 mPa s and about 600 mPa s; at times, between about 600 mPa s and about 800 mPa-s; at times, between about 800 mPa s and about 1,000 mPa s; at times, between about 800 mPa s and about 1,000 mPa s; at times, between about 1,000 mPa s and about 1,200 mPa s, or at times, between about 1,200 mPa s and about 1,500 mPa s.

[0094] Different types of reactive diluents may be used with the epoxy composition. Without being limited thereto, and in accordance with some examples of the presently disclosed subject matter, the reactive diluent is selected from the group consisting of glycerol triglycidyl ether, butyl glycidyl ether, cresyl glycidyl ether, 2-ethylhexyl glycidyl ether, neopentyl glycol diglycidyl ether, phenol diglycidyl ether, o-cresol, p-tert- butylphenol-formaldehyde (PTBP), cardanol, aniline, alcohols, diols, glycols, trimethylol propane and combinations thereof.

[0095] In some examples of the presently disclosed subject matter, the epoxy composition comprises a plasticizer.

[0096] As used herein, the term "plasticizer" (which may also be recognized by the term "non-reactive diluent") is intended to mean a chemical substance that is added to the epoxy composition to make it softer, more flexible, to increase its plasticity, and / or to decrease friction during its handling in manufacture. The plasticizing effect can be determined by the Tg or by the elastic modulus of the cured epoxy

[0097] In some examples of the presently disclosed subject matter, the plasticizer is selected from the group consisting of phthalate, phenol, pine oil, castor oil, coal tar, polyacrylate and combinations thereof.

[0098] In some examples of the presently disclosed subject matter, the epoxy composition comprises a curing additive.

[0099] In the context of the presently disclosed subject matter, the term "curing additive" refers to any chemical substance or combination of substances to be introduced into an epoxy composition for the purpose of facilitating or enhancing the transformation of the epoxy composition from a semi-solid state to a solid state through curing.

[0100] In the context of the presently disclosed subject matter, the term "cured" or "curing" a priori relates to the process that involves chemical cross-linking. The cross linking may be partial, namely, at least a portion of the epoxy moieties undergo crosslinking, to provide a composite material that is defined as solid.

[0101] In some examples of the presently disclosed subject matter, the curing additive is selected from the group consisting of curing agents, accelerators, catalysts and any combinations of same.

[0102] It is to be appreciated that the difference between a curing agent, an accelerator and a catalyst may reside in the amount of the curing additive introduced into the epoxy composition. Typically, yet without being bound by theory, an additive can act as a catalyst when introduced at a first amount, and as an accelerator, when introduced in a second amount, the first amount (of a catalyst) being typically lower than the second amount (of an accelerator).

[0103] The term "curing agent" (at times known by the term "hardener") is used herein to denote any chemical substance that reacts with the epoxy resins to form a cross-linked, thermoset polymer. The curing agent initiates and propagates the chemical reaction that converts the epoxy composition from its semi-solid state to a solid state.

[0104] The term "accelerator" is used herein to denote a chemical substance added to the epoxy resins composition to increase the rate of the curing reaction. It reduces the curing time by enhancing the reactivity of the curing agent and epoxy resins without becoming part of the final composite material structure.

[0105] The term "catalyst" is used herein to denote a chemical substance added to the epoxy resins composition to speed up the curing reaction of the epoxy resins composition without being necessarily incorporated into the final composite material structure.

[0106] In some examples of the presently disclosed subject matter, the curing agent is selected from the group consisting of amines, polyamines, polyamine adducts, polyamides, anhydrides, dianhydrides, catalytic curing agents such as amine salts, boron trifluoride complexes, amine borates, triphenyl phosphine, polysulfide.

[0107] In some examples, the curing agent is a latent curing agent, i.e. a type of curing agent that remains inactive or dormant at room temperature but becomes active and initiates the curing process when exposed to specific conditions, such as elevated temperature, light, or moisture.

[0108] In some examples of the presently disclosed subject matter, the latent curing agent is selected from the group consisting of Diaminodiphenyl sulfone (DDS), dicyandiamide (DICY) and combinations thereof.

[0109] In some examples of the presently disclosed subject matter, the curing agent is in an amount of between about 5 and about 50 parts per hundred resin (phr).

[0110] In some examples of the presently disclosed subject matter, the curing agent is in an amount of between about 5phr and about lOphr; at times, between about lOphr and about 20phr; at times, between about 20phr and about 30phr; at times, between about 30phr and about 40phr; at times, between about 40phr and about 50phr.

[0111] In some examples of the presently disclosed subject matter, the epoxy composition comprises as a curing additive said catalyst.

[0112] In some examples of the presently disclosed subject matter, the catalyst is selected from the group consisting of triphenylphosphine (TPP), amine salts, boron trifluoride complexes, amine borates. In some examples of the presently disclosed subject matter, the catalyst is in amount of between about O.Olphr and about 3phr.

[0113] In some examples of the presently disclosed subject matter, the catalyst is in an amount of between about O.Olphr and about 0.05phr; at times, between about 0.05phr and about O.lphr; at times, between about O. lphr and about 0.5phr, at times between about 0.5phr and about Iphr; at times, between about Iphr and about 1 .5phr; at times, between about 1.5phr and about 2phr; at times, between about 2phr and about 3phr.

[0114] In some examples of the presently disclosed subject matter, the catalyst is in amount of between about 0.5 and about 2. 1 phr.

[0115] In some examples of the presently disclosed subject matter, the epoxy composition comprises, as a curing additive, an accelerator.

[0116] In some examples of the presently disclosed subject matter, the accelerator is selected from the group consisting of imidazole accelerators, urone accelerators and combinations thereof.

[0117] In some examples of the presently disclosed subject matter, the accelerator is selected from the group consisting of 2-methylimidazole, aryldimethylurea, p- chlorophenyl-N,N-dimethylurea, N-3,4-dichlorophenyl-N,N-dimethylurea, N,N- dimethyl-N’ -phenylurea, and N,N”-(4-methyl- 1 ,3 -phenylene )bis(N’ ,N’ -dimethylurea) and any combinations of same.

[0118] In some examples of the presently disclosed subject matter, the accelerator comprises or is 2-methylimidazole.

[0119] In some examples of the presently disclosed subject matter, the accelerator is in amount of between about 0.5phr and about lOphr. In some examples of the presently disclosed subject matter, the accelerator is in amount of between about 0.5phr and about 2phr; at times, between about 2phr and about 4phr; at times, between about 4phr and about 6phr; at times, between about 6phr and about 8phr; at times, between about 8phr and about 9phr; at times, between about 9phr and about 1 Ophr.

[0120] As noted above, it may be that a same chemical substance can function as a catalyst, or as an accelerator, and this may depend on the amount of the substance used.

[0121] A unique property of the epoxy composition and of the composite precursor comprising the same relates to its storage stability. Storage stability can be defined by the composition's shelf life, i.e. the duration under storage conditions, while maintaining functionality / curing capability; and out-life, i.e. the duration under ambient storage conditions, while maintaining functionality / curing capability. The functionality can be determined based on the change from functionality at day 0 of storage. Of particular importance is the out-life of these aspects of the present disclosure.

[0122] In some examples of the presently disclosed subject matter, the disclosed epoxy composition has an out-life of at least 1 month; at times, of at least 2 months, at times, of at least 3 months; at times, of at least 5 months; at times, of at least 6 months.

[0123] In some examples of the presently disclosed subject matter, the disclosed composite precursor has an out-life of at least 1 month; at times, of at least 2 months, at times, of at least 3 months; at times, of at least 5 months; at times, of at least 6 months.

[0124] The functionality can be determined by a measured physical property, in comparison to the same physical property before storage (i.e. at day 0 from production).

[0125] The storage conditions can comprise controlled temperature.

[0126] In some examples of the presently disclosed subject matter, the storage conditions comprise a temperature within a range of between about -50°C and about 14°C.

[0127] In some examples of the presently disclosed subject matter, the storage conditions comprise a temperature within a range of between about -50°C and about 4°C; at times between about -40°C and about 0°C; at times, between about -10°C and about I0°C; at times, between about - 10°C and about 4°C; at times, between about - 10°C and about 0°C; at times, between about 0°C and about 4°C. In some examples of the presently disclosed subject matter, the out-life is determined upon storage at a temperature within a range of between about 10°C and about 40°C.

[0128] In some examples of the presently disclosed subject matter, the out-life is determined upon storage at a temperature within a range of between about 10°C and about 35°C; at times, between about I0°C and about 20°C; at times, between about I0°C and about 25°C; at times, between about 20°C and about 30°C; at times, between about I0°C and about 35°C; at times, between about 20°C and about 40°C.

[0129] In some examples of the presently disclosed subject matter, the out-life is established upon storage at room temperature of about 22°C-25°C.

[0130] In the context of the presently disclosed subject matter, when referring to a material that maintains its functionality, it is to be understood to encompass any retention of at least 50% the measured physical property as compared to the same property when measured before storage.

[0131] In some examples of the presently disclosed subject matter, the shelf-life exhibits retention of at least 60% of a measured physical property; at times, at least 70%; at times, at least 80%; at times, at least 90%.

[0132] In some examples of the presently disclosed subject matter, the shelf-life of the presently disclosed epoxy composition comprises at least 15 days; at times, at least 30 days; at times, at least 60 days.

[0133] In some examples of the presently disclosed subject matter, the epoxy composition exhibits, upon storage at a temperature of about 25°C for a period of about 30 days, at least 60%, of its epoxy groups, as compared to quantity of epoxy groups present in the epoxy composition at day 0 of said period. At times, the composition retains 70%, at times 80% and at times 90% of epoxy groups.

[0134] In some examples of the presently disclosed subject matter, the out-life exhibits retention of at least 60% of a measured physical property; at times, at least 70%; at times, at least 80%; at times, at least 90%.

[0135] In some examples of the presently disclosed subject matter, the out-life of the presently disclosed epoxy composition comprises at least 15 days; at times, at least 30 days; at times, at least 60 days; at times, at least 90 days, at times, at least 120 days, at times, at least 150 days, at times, at least 180 days.

[0136] The epoxy composition can, alternatively or in addition, be characterized by physical properties of its cured state, i.e. after undergoing cross linking (hardening). Such physical properties may include gel time, strain at break and Young's modulus as well as other physical properties. These physical properties are defined herein with respect to the presently disclosed cured composite material, and similarly apply to the epoxy composition and to the presently disclosed precursor composite (the prepreg).

[0137] The epoxy composition can be prepared by mere mixing of the aforementioned components. In accordance with the presently disclosed second aspect, the epoxy composition is prepared by forming a blend (mixture) of the liquid epoxy resin and the solid epoxy resin, wherein the liquid epoxy resin and the solid epoxy resin are as herein defined with respect to the epoxy composition according to the presently disclosed first aspect.

[0138] In some examples of the method of preparing the epoxy composition, according to the presently disclosed second aspect, additives can be added to the blend, such as the herein defined curing additives, the plasticizers, the toughening agent and combinations of same.

[0139] It is to be appreciated that all definitions for terms used in the context of the presently disclosed epoxy composition according to first aspect also apply to the method forming part of the presently disclosed second aspect.

[0140] The method according to the presently disclosed second aspect is solvent free. Accordingly, neither the liquid epoxy resin nor the solid epoxy resin requires dissolution in a solvent in order to achieve a homogenous blend of same.

[0141] In some examples of the presently disclosed subject matter, the forming of the blend is until a homogenous mixture is obtained. To this end, various mixing techniques may be employed.

[0142] Without being limited thereto, the mixing can comprise any one of mechanical stirring, homogenization, shaking, high-shear mixing, ultrasonic mixing and combinations of the same. In some examples of the presently disclosed subject matter, the mixing is at a temperature above 80°C.

[0143] In some examples of the presently disclosed subject matter, the mixing of the blend is at a temperature within a range of between about 80°C and about 150°C.

[0144] In some examples of the presently disclosed subject matter, the formation of the blend is under conditions that cause degassing of the blend.

[0145] As used herein, the term "degassing" is intended to denote a process of removing gas bubbles and / or dissolved gases from a material.

[0146] In some examples the degassing comprises subjecting the epoxy resin blend to reduced pressure to promote the release / extraction of any entrapped gases from within the blend, that may damage the quality of any final product.

[0147] The epoxy resins composition disclosed herein as the first aspect of the presently disclosed subject matter, can be utilized for the formation of prepregs, referred to herein by the term "composite precursor", which constitutes a third aspect of the presently disclosed subject matter

[0148] Thus, in accordance with the third aspect of the present disclosure, there is provided a composite precursor comprising the epoxy composition of the presently disclosed first aspect, fibrous material and at least one curing additive.

[0149] For the sake of simplicity, all terms and definitions provided in connection with the epoxy composition also apply to the presently disclosed precursor composite, mutatis mutandis. As such, the precursor composite may comprise, in addition to the curing additive, the aforementioned toughening agent, plasticizer and other components defined with respect to the epoxy composition. Similarly, properties and other terms defined with respect to the epoxy composition also form part of the definition of the hereindisclosed precursor composite.

[0150] In the context of the presently disclosed subject matter, the term "fibrous matter" should be understood to have its meaning as known with respect to prepregs. Generally, the term fibrous matter is understood to refer to a collection of elongated, thread-like structures composed of materials that provide reinforcement when incorporated into a composite material. These fibers would typically be characterized by their high strength- to-weight ratio, flexibility, and ability to be aligned in various orientations to enhance the mechanical properties of the final composite material. The fibrous matter can be continuous or discontinuous and would be of a type that can be pre -impregnated with the presently disclosed epoxy resin composition to form prepregs.

[0151] In some examples of the presently disclosed subject matter, the fibrous matter is selected from the group consisting of glass fibers, carbon fibers, aramid fibers, basalt fibers, natural fibers, polyethylene fibers, quartz fibers, boron fibers and any combinations of same.

[0152] In some examples of the presently disclosed subject matter, the fibrous matter comprises polymeric fibers, such as, and without being limited thereto, polyethylene fibers, polyester fibers, polyamide fibers, aromatic polyamide (aramid) fibers, and any combinations of same.

[0153] In some examples of the presently disclosed subject matter, the fibrous matter comprises ceramic fibers, such as, and without being limited thereto, alumina fibers, silicon carbide fibers and any combinations of same.

[0154] In some preferred examples of the presently disclosed subject matter, the fibrous material comprises or are carbon fibers.

[0155] The amount of fibrous matter within the composite precursor can be defined as the volume fraction of fibrous matter within the composite precursor.

[0156] In some examples of the presently disclosed subject matter, the volume fraction of fibrous matter is any volume within a range of between about 40% and about 70%.

[0157] The composite precursor is used for the production of shaped articles. To this end, it is required that the composite precursor exhibit drapeability properties and / or tackiness.

[0158] As used herein, the term " drapeability'' is intended to refer to the ability of the composite precursor to conform to complex shapes and contours without wrinkling or developing defects. The drapeability can be determined by draping the composite precursor over a predefined shape, such as a hemisphere or a dome, under controlled conditions and measuring the extent of coverage without defects.

[0159] In some examples of the presently disclosed subject matter, the composite precursors is characterized by drapeability allowing an extent of coverage of at least 1mm, without introducing wrinkles or folds; at times, between 1mm and 10mm; at times, of at least 2mm; or at least 3mm; or at least 4mm; or at least 5mm; or at least, 6mm; or at least 7mm; or at least 8mm; or at least 8mm.

[0160] As used herein, the term "tackiness" is intended to refer to the ability of the composite precursor to adhere to another surface when they are brought into contact to an extent that it can hold its position but not so sticky that it is difficult to reposition or handle.

[0161] In some examples of the presently disclosed subject matter, the other surface is of another layer of composite precursor, to thus form a two or more layered-laminate structure.

[0162] Thus, in accordance with some examples of the presently disclosed subject matter, the composite precursor disclosed herein are characterized by tackiness sufficient to cause adherence of one layer of composite material to another (typically similar in properties) composite precursor.

[0163] The composite precursor is characterized by a shelf-life and / or out-life as defined with respect to the presently disclosed epoxy composition.

[0164] The presently disclosed epoxy composition and the presently disclosed composite precursor can be characterized by various physical properties.

[0165] In some examples of the presently disclosed subject matter, the physical property is viscosity. The viscosity of the epoxy composition can be determined at the processing temperature, and according to ASTM D1084.

[0166] In some examples of the presently disclosed subject matter, the physical property relates to the amount of epoxy groups free to cross link. The degree of free epoxy groups can be determined by FTIR.

[0167] The presently disclosed epoxy composition and the presently disclosed composite precursor can also be characterized by a physical property of their cured form, i.e. after being subject to curing conditions as described herein with respect to the formation of a composite material according to another aspect of the present disclosure. In some examples of the presently disclosed subject matter, the physical property relates to the degree of cross linking, which can be determined by differential scanning calorimetry (DSC).

[0168] In some examples of the presently disclosed subject matter, the physical property comprises strain at break of a cured material. Strain at break can be determined by tensile tests with specific dimensions as per standardized testing protocols (e.g., ASTM D3039 or ISO 527).

[0169] In some examples of the presently disclosed subject matter, the physical property comprises Young's modulus (modulus of elasticity) of a cured material. Young's modulus can be determined by tensile tests with specific dimensions as per standardized testing protocols (e.g., ASTM D3039 or ISO 527).

[0170] In some examples of the presently disclosed subject matter, the physical property comprises flexural modulus of a cured composite material. Flexural modulus can be determined by tensile tests with specific dimensions as per standardized testing protocols (e.g., ASTM D790).

[0171] In some examples of the presently disclosed subject matter, the physical property comprises flexural strength of a cured composite material. Flexural strength can be determined by tensile tests with specific dimensions as per standardized testing protocols (e.g., ASTM D790).

[0172] In some examples of the presently disclosed subject matter, the physical property comprises fracture toughness (Gue) of a cured composite material. Fracture toughness (Gue) can be determined by End-Notched Flexure (ENF) Test with specific dimensions as per standardized testing protocols (e.g., ASTM D7905).

[0173] In some examples of the presently disclosed subject matter, the physical property comprises inter laminar shear strength (ILSS) of a cured composite material. Inter laminar shear strength (ILSS) can be determined using a short beam shear test, with specific dimensions as per standardized testing protocols (e.g., ASTM D2344 / D2344M).

[0174] In some examples of the presently disclosed subject matter, the epoxy composition or the composite precursor are characterized by, following curing, any one or combination of the following physical properties, each constituting a separate example and each combination constituting a separate example, of the presently disclosed subject matter.

[0175] The composite precursor resulting in a cured composite material with a fibrous matter volume percent of at least 55%.

[0176] The composite precursor resulting in a cured composite material having a void volume percent of below 1.5%. The void content can be determined according to ASTM D3171.

[0177] The composite precursor resulting in a cured composite material having an inter laminar shear strength (ILSS) of at least 40 MPa.

[0178] The composite precursor resulting in a cured composite material having strain to break of at least 1%.

[0179] In some examples of the presently disclosed subject matter, any one of the flexural modulus, flexural strength and fracture toughness of a cured composite material are dependent on the fibrous matter comprised within a composite precursor. In some examples of the presently disclosed subject matter, the fibrous matter is a carbon fiber, and the epoxy composition or the composite precursor are characterized by, following curing, any one or combination of the following physical properties, each constituting a separate example and each combination constituting a separate example, of the presently disclosed subject matter.

[0180] The composite precursor comprises carbon fiber as part of the fibrous matter and provides a cured composite material having flexural modulus of at least 50GPa.

[0181] The composite precursor comprises carbon fiber as part of the fibrous matter and provides a cured composite material having a flexural strength of at least 800MPa.

[0182] The composite precursor comprises carbon fiber as part of the fibrous matter and provides a cured composite material having a fracture toughness (Gnc) of at least 500 J / m2.

[0183] A unique feature of the presently disclosed epoxy composition and / or composite precursor is that they do not have to undergo B-staging before storing and / or prior to applying any curing conditions, as described hereinbelow. As used herein the term "B-staging" is intended to refer to any step of increasing the molecular weight or otherwise increasing a viscosity and / or melting temperature of the epoxy composition or composite precursor by causing partial curing of same.

[0184] The presently disclosed subject matter also provides a method of preparing the presently disclosed composite precursor. Specifically, the method of preparing the composite precursor comprises mixing the epoxy composition of the presently disclosed first aspect, with at least a curing additive and with fibrous matter, under conditions that provide impregnation of the fibrous matter, with the epoxy composition and the at least curing additive.

[0185] For the sake of simplicity, it is to be understood that all definitions and examples provided with respect to the epoxy composition including all its optional components, the curing additive and the fibrous matter also apply for defining the method for producing the composite precursor, mutatis mutandis.

[0186] Further, for the sake of simplicity, it is to be understood that all amounts of components described with respect to the epoxy composition or composite precursor also apply for defining the method for producing the composite precursor, mutatis mutandis.

[0187] In the context of the method for preparing the composite precursor, it is to be understood that the term "impregnating" means the penetration of the epoxy composition into the fibrous matter, e.g. through the pores within the fibrous matter and through the gaps between fiber tows, thus effectively filling the fibrous matter with the epoxy compositon.

[0188] Efficient impregnation can be achieved by various techniques.

[0189] Without being limited thereto, impregnation can be achieved under conditions including at least one processing step selected from the group consisting of hot melt process, film stacking process, wet layup, resin transfer molding (RTM), resin infusion (RI), vacuum-assisted resin infusion (VA-RI), filament winding, spray-up, pultrusion, bulk molding compound (BMC), and sheet molding compound (SMC).

[0190] The epoxy composition and the composite precursor are used, independently, for the preparation of a cured composite material. Thus, in accordance with yet another aspect of the presently disclosed subject matter, there is provided a composite material comprising the presently disclosed epoxy composition or the presently disclosed composite precursor, in cured form.

[0191] The cured form can be determined by any one of the above described properties, including, independently, viscosity, amount of remaining epoxy groups, degree of cross linking, strain at break, Young's modulus, flexural modulus, flexural strength, fracture toughness (Gue), inter laminar shear strength (ILSS).

[0192] Further, the cured composite material can be characterized by any one or combination of the following physical properties, each constituting a separate example and each combination constituting a separate example, of the presently disclosed subject matter:

[0193] - a fibrous matter volume percent of at least 55%.

[0194] - a void volume percent of below 1.5%.

[0195] - an inter laminar shear strength (ILSS) of at least 40 MPa.

[0196] - strain to break of at least 1%.

[0197] - flexural modulus of at least 50GPa, preferably when said fibrous material comprises or is carbon fibers.

[0198] - a flexural strength of at least 800MPa, preferably when said fibrous material comprises or is carbon fibers.

[0199] - a fracture toughness (Gue) of at least 500 J / m2, preferably when said fibrous material comprises or is carbon fibers.

[0200] In yet another aspect of the presently disclosed subject matter, there is provided a method of producing the presently disclosed composite material, the method comprising applying onto a precursor composition, as disclosed herein, at least one curing condition effective to cause curing (also referred to as hardening condition) of the precursor composition.

[0201] As used herein, the term "curing condition" or "hardening condition" refers to any condition that triggers or facilitates cross linking of the presently disclosed epoxy composition in the composite precursor, leading to transition from its semi-liquid (semisolid) viscous state to a solid state.

[0202] In some examples the curing condition comprises at least exposure to ultraviolet (UV) light.

[0203] In some examples the curing condition comprises at least exposure to an elevated temperature, or curing inducing temperature.

[0204] In some examples of the presently disclosed subject matter, the curing temperature is the temperature at which the presently disclosed epoxy composition and / or composite precursor solidifies by cross-linking, and / or turns into a solid body.

[0205] In some examples the curing condition comprises at least exposure to moisture.

[0206] The presently disclosed method of producing the composite material can be characterized by the composite precursor's gel time.

[0207] As used herein, the term "gel time" means the time interval between exposing the composite precursor to the curing conditions and the point at which the composite precursor transitions from a flowable viscous semi-liquid state to a gel-like state (which is before complete solidification).

[0208] In some examples of the presently disclosed subject matter, the composite precursor is characterized by a gel time within a range of between about 5min and about 90min, at times, between about 5min and about lOmin, at times, between about lOmin and about 20min. at times, between about 20min and about 30min, at times, between about 30min and about 45min, at times, between about 45min and about 60min, at times, between about 60min and about 90min.

[0209] The presently disclosed composite material forms part of an article of manufacture. Thus, in accordance with the presently disclosed subject matter, there is also provided an article of manufacture obtained from any one of the presently disclosed epoxy composition, composite precursor or comprising the presently disclosed composite material.

[0210] Finally, the presently disclosed subject matter provides a method for producing an article of manufacture, the method comprising: shaping the presently disclosed composite precursor; and applying at least one curing condition effective to cause curing of the shaped composite precursor.

[0211] The presently disclosed method of producing the article of manufacture may employ, inter alia, the presently disclosed epoxy composition, the presently disclosed composite precursor the presently disclosed composite material and / or the curing conditions described in respect to its preparation. Thus, for the sake of simplicity, all terms and definitions provided in connection with any of the aforementioned also apply to the presently disclosed method of producing the article of manufacture, mutatis mutandis.

[0212] In some examples, and as noted above in connection with the preparation of the composite material, the curing condition can comprise any one of or combination of ultraviolet (UV) radiation, ionizing radiation, electron beam radiation, microwave radiation, moisture, and elevated temperature.

[0213] In some examples of the presently disclosed subject matter, the method of producing the article of manufacture comprises at least an autoclave process.

[0214] In some examples of the presently disclosed subject matter, the method of producing the article of manufacture comprises an Out-of-Autoclave (OOA) process.

[0215] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and / or ordinary meanings of the defined terms.

[0216] The term "about" as used herein indicates values that may deviate up to 1%, more specifically 5%, more specifically 10%, more specifically 15%, and in some cases up to 20% higher or lower than the value referred to, the deviation range including integer values, and, if applicable, non-integer values as well, constituting a continuous range. In some embodiments, the term "about" refers to ± 10 %.

[0217] The indefinite articles “a” and ”« / ■?" as used herein in the description and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one”. It must be noted that, as used in this description and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the content clearly dictates otherwise. The clause “and / or” as used herein in the description and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with “and / or” should be construed in the same fashion, i.e., “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and / or” clause, whether related or unrelated to those elements specifically identified.

[0218] As used herein in the description and in the claims, “or” should be understood to have the same meaning as “and / or” as defined above. For example, when separating items in a list, “or” or “and / or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of or “exactly one of or, when used in the claims, “consisting of will refer to the inclusion of exactly one element of a number or list of elements.

[0219] As used herein in the specification and in the claims, the phrase “at least one” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified.

[0220] It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited.

[0221] Throughout this description (including the Examples) and claims which follow, all transitional phrases such as "comprising", “including” , "carrying", “having”, "containing", “involving”, “holding”, “composed of and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Specifically, it should understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. Only the transitional phrases “consisting of and “consisting essentially of shall be closed or semiclosed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures. More specifically, the terms "comprises" , "comprising", "including", "having" and their conjugates mean "including hut not limited to". The term “consisting of' means “including and limited to”. The term "consisting essentially of means that the composition, method or structure may include additional ingredients, steps and / or parts, but only if the additional ingredients, steps and / or parts do not materially alter the basic and novel characteristics of the claimed composition, method or structure.

[0222] It should be noted that various embodiments of this invention may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range. Whenever a numerical range is indicated herein, it is meant to include any cited numeral (fractional or integral) within the indicated range. The phrases " ranging / ranges between" a first indicate number and a second indicate number and "ranging / ranges from" a first indicate number "to" a second indicate number are used herein interchangeably and are meant to include the first and second indicated numbers and all the fractional and integral numerals there between.

[0223] As used herein the term "process" or "method" refers to manners, means, techniques and procedures for accomplishing a given task including, but not limited to, those manners, means, techniques and procedures either known to, or readily developed from known manners, means, techniques and procedures by practitioners of the chemical, pharmacological, biological, biochemical and medical arts.

[0224] It is appreciated that certain features of the presently disclosed subject matter, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements.

[0225] Various embodiments and aspects of the present invention as delineated herein above and as claimed in the claims section below find experimental support in the following examples.

[0226] Disclosed and described, it is to be understood that the presently disclosed subject matter is not limited to the particular examples, process steps, and compositions disclosed herein as such methods steps and compositions may vary somewhat. It is also to be understood that the terminology used herein is used for the purpose of describing particular embodiments only and not intended to be limiting since the scope of the presently disclosed subject matter will be limited only by the appended claims and equivalents thereof.

[0227] The following examples are representative of techniques employed by the inventors in carrying out aspects of the present invention. It should be appreciated that while these techniques are exemplary of preferred embodiments for the practice of the invention, those of skill in the art, in light of the present disclosure, will recognize that numerous modifications can be made without departing from the spirit and intended scope of the presently disclosed subject matter.

[0228] DESCRIPTION OF NON-LIMITING EXAMPLES

[0229] EXAMPLE 1 - Two fraction DGEBA blends

[0230] Controlling thermal, rheological and mechanical properties by changing the DGEBA molecular weight, and resin properties suitable for hot melt impregnation.

[0231] Materials:

[0232] The following commercial epoxy resins used in this part of the study:

[0233] Diglycidyl ether of bisphenol-A (DGEBA), with different molecular weight. DER 331 with an epoxy equivalent range of 182-192 g / eq, a liquid resin with n=0 in the repeating unit was supplied by Dow. GT 6071 solid bisphenol -A an epoxy resin of medium molecular weight, an equivalent range of 450-465 g / eq with n=2. The resins were blended with the aromatic curing agent diaminodiphenyl sulfone (DDS). The toughening agents- Carboxyl-Terminated Acrylonitrile-Butadiene Rubber (CTBN) 1300*8 and Amine-Terminated Acrylonitrile-Butadiene Rubber (ATBN) 1300*35 supplied by Hyper. Triphenylphosphine (TPP) supplied by Sigma.

[0234] Methods:

[0235] Preparation of neat resin

[0236] A range of preparation variables was examined. Liquid / solid epoxy percentage, weight-to-weight ratios: 30L (liquid):70S (solid), 40L: 60S, 50L: 50S, 60L: 40S, 70L: 30S. Mixing temperature was in the range of 100 °C to 150°C. The Mixing speed was 1000 rpm. The obtained hot melt formulation was poured into a preheated (110°C) aluminum crucible and then degassed in a vacuum oven at 110°C for 30 minutes. Each formulation was stored "un-cured" at 25°C and also in - 18°C. The DDS was added in stoichiometric amount according to the epoxy equivalent. First, the solid epoxy added, after its softening the liquid epoxy added and then the blended for 15 minutes, after that the curing agent was added and mix for 10 minutes. Some of the mixtures were cured at 180°C for 2 h, followed by 1 h at 220°C.

[0237] Preparation of modified systems

[0238] Neat resins were mixed ten minutes more with the rubber toughening agents. Rubber toughening agents were added with a different amount, 5, 7.5,10,12.5 and 15 parts per hundred resin (phr). The formulations with TPP were mixed under a nitrogen atmosphere at 150°C for 2 hours and then cooled to 110°C before adding the curing agent. Regardless of the toughening agent, acceleration with boron fluoride (BF3) with a small amount (1 phr and 2 phr) and using extra DDS 5% and 10% inspected. A boron fluoride monoethylamine complex (BF3-MEA) was mixed at lower temperature 90°C to avoid early curing during mixing. DDS was always the final ingredient that was added to the mixture. The mixtures de-gassed for 30 minutes at 110°C or until no bubbles were detected. Next, it was placed into the appropriate mold or substrate according to the testing method. Finally, the mixtures were cured at 180°C for 2 h, followed by 1 h at 220°C. Sample characterization

[0239] Discovery hybrid rheometer used to determine the viscosities of solid epoxy DGEBA - GT6071 and liquid epoxy DGEBA -DER 331 blends without curing agent at different ratios and with the presence of liquid rubbers ATBN and CTBN, in resin mode, by using a 25 mm diameter parallel plate and a 2 mm gap mode. These viscosities measured by applying a 1 Hz fixed frequency, a heating rate of 2°C / min, in a temperature range of20°C to 130°C.

[0240] Thermal properties and curing characterizing were carried out with Differential Scanning Calorimetry (DSC), Fourier Transform Infra-Red (FTIR) Spectroscopy and a Melt Point Apparatus.

[0241] The DSC measurements were done as follows: heating - cooling - heating at 10°C per minute, to determine the glass transition temperature (Tg), the curing enthalpy and other thermal properties based on the ASTM D3418 protocol. To study the differences in reactivity, curing kinetic tests were made to evaluate the activation energy. The tests were performed at various heating rates: 5, 10 and 15, °C per minute according to ASTM E689.

[0242] FTIR was used to view the reaction progress at different temperatures and to examine the "out-life".

[0243] Tensile tests were carried out using an Instron Model 4481 universal testing machine. Test specimens were cast into silicone molds with a dumbbell geometry. The specimens' dimensions, span, and speed of loading were according to ASTM D638 (for rigid plastics).

[0244] Scanning electron microscopy (SEM) analysis was made on the specimens' fracture surface to investigate morphology. Samples were mounted on sample stubs and coated with alloy combined with silver, in an argon environment. Aspex Explorer machine was used to observe the specimens under an accelerating voltage of 15 kV. The pictures were analyzed with the "hnage-J" software.

[0245] Results:

[0246] Rheological Properties

[0247] Viscosity is the most important resin parameter that affects impregnation and should be chosen carefully. Parallel plate viscosimetry was used to investigate the viscosity change with different epoxy ratios. As shown in Figure 1A, viscosity increases with solid DGEBA (GT6071) fraction, due to its higher molecular weight. The targeted viscosity at the impregnation temperature is around 1 Pa-s, similarly to the values in solution impregnation. The resin viscosity after impregnation and during lay-up at room temperature, is in the order of 1 x 103Pa-s to 1 x 104Pa-s for a drapeable prepreg, balancing between ductility and drooling prevention during storage.

[0248] Brittleness may lead to fracture during lay-up, especially the automated one. The upper viscosity limit is related to gelation time (viscosity of 1 x 105Pa-s), once the resin has reached the gelation time it is unable to flow in a fluid-like manner. Tackiness should be taken into consideration; the material should be tacky only during the lay-up and not within the "head" of the AFP. The formulation of 50L:50S, 60L:40S and 70L:30S were found suitable for impregnation considering the curing agent reactivity and therefore, were subsequently investigated.

[0249] The viscosity data of DER331 / GT6071 with CTBN, CTBN+TPP or ATBN blends as a function of temperature are presented in Figure IB. The addition of ATBN liquid rubber leads to an increase in viscosity, and by 10 phr this effect is quite significant. On the other hand, the addition of CTBN liquid rubber up to 10 phr does not affect the viscosity curve at 60L:40S. The addition of the catalyst (TPP) slightly increased the viscosity within the operating window for this technology.

[0250] Since the molecular weights of ATBN and CTBN are the same as shown in Table 1, and their viscosities at room temperature are similar, the solubility parameter is the primary parameter influencing the formulation viscosity. The solubility parameter of ATBN is higher than that of CTBN and has therefore more affinity to epoxy.

[0251] Table 1 - Properties of ATBN and CTBN

[0252] Molecular weight (g mol'1) 3500 3500

[0253] Acrylonitrile content (wt%) 18 26

[0254] Viscosity. Brookfield 25°C (Pa*s) 570 500

[0255] Specific gravity 0.960 0 978

[0256] Solubility parameter s (cal / cm3)1,29.14 -10

[0257] Tg (°C) -58 -38 The solubility parameters can be changed by changing the butadiene / acrylonitrile ratio; higher ratios give low solubility parameters, and particle size is also affected. In terms of viscosity, CTBN is preferred. Due to the weak interaction between the epoxy and the CTBN, a TPP catalyst was used to enhance their reaction. FTIR was used to identify the absorption peak of the chemical group that was created during the reaction. The absorption peak of the terminal carboxyl group at 1712 cm"1disappears after the reaction, while a small peak appears at 1740cm"1indicating ester formation between CTBN and DGEBA. Another absorption peak at 3207cm"1confirms the opening of the epoxide ring and formation of a hydroxyl group.

[0258] Thermal Properties

[0259] Temperature-related limitations must be considered, among them, the softening point of the solid DGEBA which is 80°C and the curing agent condition after mixing. The DDS melting point is 180 °C, prepregs based on a DDS require cure temperatures of 177-180 °C and at least above 150°C. A clarity change seems to occur when mixing or de-gassing takes place in the range of 100 °C to 150°C. The formulation looks between murky and transparent, and partial sedimentation is observed. Optical microscopy pictures showed beyond any doubt that particles that sank match the size and shape of the neat DDS particles. In order to understand the causes and effect of this phenomenon, five formulations containing 60L:40S were mixed at various temperatures (110° C, 120°C, 130°C, 140°C, 150°C). Transparency increased at higher mixing temperatures (Figure 2), implying that the DDS begins to dissolve, however dissolution is only partial up to 120°C. At a mixing temperature of 130 °C, the material became transparent, and the curing agent was totally dissolved and uniformly distributed. After two weeks storage at 25 °C the samples, which were mixed at 140 and 150°C became brittle, setting an upper limit for the temperature window.

[0260] Further testing was done to determine the reaction progress at different mixing temperatures by FTIR measurements, using the absorbance of the phenylene group at 830 cm"1as reference peak. Depletion of the epoxide group at 913 cm"1is shown in Figure 3. The decrease in the epoxide peak absorbance is shown in Table 2. As a result, the curing agent begins to react after dissolution at temperatures above 130°C, and control of this temperature is important to prevent curing beyond a desired low degree, while controlling the tackiness of the resulting resin. A loss of up to 20-25% in peak area ratio has been shown to render adequate resin tackiness.

[0261] Table 2 FTIR measurements of epoxide group area ratio

[0262] Mixing temp. Epoxide area Phenylene area

[0263] Area ratio

[0264] [°C] (913cm-1) (830cm-1)

[0265] 110 0.357 1.93 100%

[0266] 120 0.354 1.904 101%

[0267] 130 0.262 1.716 83%

[0268] 140 0.288 1.972 79%

[0269] 150 0.232 1.72 73%

[0270] Fully cured 0.009 2.003 2%

[0271] Tg from DSC analysis

[0272] Figure 4 presents the glass transition temperature (Tg) of systems with different liquid / solid ratio, either unmodified resin or modified with 5, 10, 15 phr of CTBN and also 7.5 phr CTBN with the addition of TPP. The law of mixtures curve was drawn using Tg data from the neat resins, 100% liquid (201°C) and 100% solid (119°C) according to the Fox equation, which should fit for this miscible system. The low molecular weight (Mw) of DER 331 renders a shorter molecular weight between crosslinks, Me, which is the main reason for the Tg rise . The Tg results provide a useful index for crosslink density, in general, shows an increase with increasing degree of crosslinking.

[0273] Data in Figure 4 are average values from 3 samples. Blends of less than 50% liquid were inhomogeneous mixtures showing unstable Tg with a large error. Most Tg values were lower or similar to the Fox equation curve, but the resin with 60% liquid showed a positive synergism. It has been suggested that the Tg of a mixture may change due to hydrogen bonding that may considered as a physical crosslinking mechanism; the higher effective crosslink density may increase the Tg. Hydrogen bond formation depends on amine nucleophilicity, segmental mobility (high Me), and the number of OH groups. It seems that the 60L:40S present a beneficial ratio of crosslink density to hydroxyl content.

[0274] The Tg of the epoxy resin matrix was slightly reduced with the addition of CTBN due to a much lower Tg of the rubber. When the temperatures exceed the glass transition zone, the CTBN phase is in the high-elastic state and can easily deform, which could improve the chain mobility of epoxy and thus result in the depression of the Tg of the DGEBA. Even a small amount of rubber with a medium-high nitrile content and low molecular weight dissolves in the epoxy and plasticizes the epoxy, resulting in a decrease in Tg. A significant reduction of Tg occurred with only 5 phr of rubber, while additional rubber caused a moderate reduction. The Tg of the CTBN phases may increase slightly, due to the inclusion of some epoxy into the CTBN domains. The catalyzed formulation with TPP did not show a significant change in the thermal behavior.

[0275] Out-life

[0276] In addition to shelf life at typical storage temperatures of -18°C, an important functional property of prepregs is the "out-life", meaning the cumulative time the prepreg can last un-cured under "room temperature" conditions. Curing begins as the curing agent comes into contact with the DGEBA, but room temperature reactions are slow enough to allow for this characteristic behavior. The out-life of commercial prepregs is usually in the range of 10 to 30 days. A 60L:40S composition with DDS and 5 phr CTBN was mixed at 110°C and stored at 25°C, then tested via DSC and FTIR after 1, 5, 30 and 50 days. The FTIR test was performed using the area ratio between the phenylene and epoxy groups as presented at the thermal properties section.

[0277] Table 3: DSC and FTIR measurements of enthalpy and epoxide group area ratio of a 60L:40S resin system with DDS and 5 phr CTBN.

[0278] Test time (days) Enthalpy by DSC [J / g] Peak area ratio by FTIR

[0279] 1 203 100%

[0280] 5 190 99%

[0281] 30 167 93%

[0282] 50 73 73%

[0283] Fully cured 0 3%

[0284] The FTIR method should be a more precise indicator of the extent of cure than DSC. At 20% to 25 % less peak area ratio, the loss of prepreg tack may hamper lay-up. Table 3 show that over 90% of the epoxide groups are still present after 30 days, thus matching industry standards for such prepregs. Eventually, the enthalpy of curing decreases by over 25% after 50 days. Gel time analysis

[0285] The decrease in gel time observed in Figure 5 is due to the reaction kinetics. Gel time decreases as the liquid percent increases at a fixed cure temperature, apparently due to the higher number of epoxide groups. Long gel times allow for resin flow, heat transfer and air to be liberated before cure. On the other hand, a long gel times is not economical and will therefore impair the possibility of manufacturing in industries such as automobile and railway. The resulting 15-20 minutes at typical curing temperatures are acceptable for vacuum bag only (VBO) techniques.

[0286] Mechanical properties

[0287] Table 4 exhibits tensile test results for three resins which were found suitable for impregnation. The results shown are for the untoughened systems in order to isolate the particular effect of mixing ratios on the mechanical properties. The samples show linear elastic behavior. It seems that the formulations with 60% and 70% liquid have similar strength, but strength decreases with 50% liquid. The elongation increases and the modulus decreases as the percentage of solid increases reflecting the decrease in crosslink density. Toughness as evaluated by the area under the stress-strain curves is expected to increase with solid epoxy content, and indeed the elongation increases with the addition of solid epoxy, however, the strength levels off so that an optimum is achieved for toughness at 60L:40S. This may be related to the synergy observed in the DSC test, where a local maximum was observed for Tg at 60L:40S ratio. Physical interactions may have contributed to this finding as elaborated before. Generally, in unidirectional (UD) composites, the strain at break of the matrix should exceed that of the fiber in order to attain maximum properties of the composite. For instance, a UD composite with T300 carbon fibers or similar, having an ultimate strain of about 1.3%, could be prepared with 60L:40S resin without a toughener agent.

[0288] Table 4: Tensile test non-toughened systems

[0289] Resin Tensile Strength (MPa) Strain at break (%) Toughness (MPa)

[0290] 50L:50L 26 ±5.1 2 ± 0.2 52

[0291] 6()L:40S 37 5 1.6 ; 0.1 58

[0292] 70L:30S 38±4.0 1.3 ± 0.3 Table 1: Tensile test of 60L:40S toughened systems

[0293] Rubber content Tensile Strain at Modulus Toughness strength break (%) (MPa) (MPa)

[0294] (MPa)

[0295] 0 phr CTBN 37.0 ± 5.7 1.6 ± 0.6 2300 ± 130 58

[0296] 5 phr CTBN 41.5 ± 4.7 2.0 ± 0.2 2200 ± 100 81

[0297] 10 phr CTBN 40.8 ± 6.0 2.4 ± 0.5 1960 ± 140 97

[0298] 15 phr CTBN 29.1 ± 6.0 1.7 ± 0.4 1820 ± 184 50 5. 1 2j± 0 1770 ± 120 87

[0299] Parameters such as the content, particle size, volume, and chemical bonds of the rubber toughener can strongly influence the mechanical properties. The particle size and volume may be controlled by curing temperature, and even by mixing temperatures and environment. The hot melt formulation has a narrow preparation window as learned from parallel plate rheometer and DSC. Therefore, the content of CTBN has to be selected properly. The toughened system's Young Modulus is presented in Table 5. As the concentration of CTBN increases from 0 to 15 phr the modulus decreases. The flexible structure of the rubber causes a decrease in the overall modulus. The tensile strength exhibits a 10% increase by adding 5 to 10 phr rubber, whereas for 15 phr rubber a decrease in strength is observed. The elongation at break also increases with CTBN content, exhibiting a maximum value at 10 phr, but decreases with 15 phr. Hence, the area under the stress-strain curves shows an optimum at 10 phr CTBN. As can be seen in Table 5 the addition of TPP caused a significant improvement in strength and especially the strain at break of the 15 phr CTBN formulation, while the modulus was lower than all other formulations. Higher properties are expected with 10 phr CTBN and TPP.

[0300] Morphology

[0301] Scanning electron microscopy (SEM) was done to investigate the morphology of the specimens fracture surfaces. The morphological development of the CTBN DGEBA blends depends mainly on the competition between the curing and the phase separation rate, which determine whether the mechanism of the reaction-induced phase separation is nucleation and growth or de-mixing. From Figure 6, image analysis shows that both the number and area average domain diameters are found to increase with increasing amount of CTBN in the composition. This increase in the domain size of dispersed CTBN phase is associated with the re-agglomeration or coalescence of the dispersed CTBN, the extent of the coalescence depending on the curing progress and the interaction between the components, as the curing process progresses, the viscosity of the epoxy increases and decreases the diffusion of the rubber particles, therefore high epoxy viscosity or rapid curing will reduce the size of the rubber domains. The compatibility and interaction of rubber with the epoxy matrix is determined by computing the volume fraction and interfacial area of the dispersed rubbery phase. The interfacial area is given by 30 / r, where 0 is the volume fraction of the dispersed phase and r is the domain radius, the results shown in Table 6. The fracture mechanism, which is common when bad adhesion occurs between the particles and the matrix, includes rubber domain cavitation and shear bands. The combination of these two blunts the crack tip which enhances the toughening. As the interfacial area increases, the stress transfer from the matrix to the rubber domains is greater and the described toughening mechanism is enhanced. The interfaciai area increases from 5 phr to 10 phr and then decreases at 15 phr CTBN. Without wishing to be bound thereto, this phenomenon is presently believed to be associated with the vast extent of coalescence in this composition.

[0302] Table 2: The interfaciai areas of the CTBN domains

[0303] CTBN Volume fraction Domain radius (gm) Interfaciai area content (jun)1

[0304] 0 phr -

[0305] 5 phr 0.052 0.9+0.3 0.15

[0306] 10 phr 0.104 1.0+0.3 0.25

[0307] 15 phr 0.156 5.2+3. 1 0.08

[0308] Figure 7 - Figure 8 show formation of crazing and shear bands which are energy absorbing mechanisms. The parallel lines that emerge from the rubber particles in Figure 7 refer to the crack pinning mechanism, which is a common mechanism of well- connected rubbery or rigid particles. Crack pinning at the particles leads to bowing out of the crack path between particles to form secondary cracks, thus increasing the fracture surface. Conclusions

[0309] Thermal, rheological and mechanical properties can be controlled by changing the DGEBA molecular weight, and resin properties suitable for hot melt impregnation can be achieved with DGEBA blends containing two fractions, a liquid like fraction and a solid like fraction, without B-staging or autoclave curing. Differences in viscosity and reactivity of the materials affect the processing window before curing of the system. After curing, the resin properties reflect the relationship between the crosslink density and the strength. Elongation at break comes at the expense of strength, thus rendering an optimum for toughness at 60L:40S. This composition exhibits a local synergism by its Tg value, possibly due to enhanced hydrogen bonding acting as physical crosslinks. The narrow working window using DDS as curing agent must prevent early curing, which starts slowly as the curing agent dissolves in the resin.

[0310] ATBN increased the viscosity of the composition because of the high acrylonitrile percentage. CTBN did not have much effect on viscosity and showed limited but sufficient toughening ability. 10 phr CTBN presented the highest interaction between the rubber domains and the epoxy matrix and thus the best properties for the selected curing route and narrow processing window. The weak chemical bond between the CTBN and epoxy was improved by adding TPP catalyst.

[0311] EXAMPLE 2 - Carbon / hot melt epoxy prepregs for the automated fiber placement (AFP)

[0312] Influence of a DEGBA blend on the properties of composite laminates, resulting from hot melt impregnation of carbon fabrics.

[0313] Experimental outline:

[0314] The variables in this system are the ratio between solid (S) DGEB A to liquid (L) DGEBA, the toughening agent (CTBN) content, as well as the addition of triphenylphosphine (TPP) to enhance the CTBN / DGEBA reaction. The matrix itself was investigated in Example 1. Tensile tests demonstrated an optimal ratio of crosslink density at 60L:40S resin, by balancing elongation and strength. Furthermore, 10 phr CTBN rubber provided the most significant improvement in toughness due to the large interfacial area while minimizing their effect on resin viscosity and the decrease in Tg. The usage of TPP as a liaison between CTBN and DGEBA improved the mechanical properties.

[0315] Materials

[0316] Two commercial DGEBA epoxy resins with different molecular weights were used in this Example: DER 331 liquid resin (n=0 repeating units) with an epoxy equivalent of 182-192 g / eq was supplied by Dow, and GT 6071 solid resin of medium molecular weight (n=2) and 450-465 g / eq was supplied by Huntsman. The resins were blended with the aromatic curing agent diaminodiphenyl sulfone (DDS). The toughening agents CTBN 1300*8 and ATBN 1300*35 were supplied by Hyper. TPP was purchased from Sigma. Carbon fabric T300 plain weave (0 / 90°) and unidirectional tape (0°) were purchased from Composite Materials (Italy).

[0317] Preparation

[0318] 20 or 16 plies of (0\90°) and (0°) carbon fabric were impregnated individually by a hot melt laboratory technique that imitates the impregnation machine, then weighed to adjust the required amount of resin, considering losses during vacuum or pressing. Toughened or untoughened resin was used for hot melt impregnation. 50 to 70 % liquid DEGBA with the complementary amount of solid DGEBA were mechanically mixed in a pre-heated oil bath at 110°C. DDS was added in stoichiometric amount according to the epoxy equivalent. The compositions contained 5, 7.5 and 12.5 phr CTBN as rubber toughening agent as well as 0.1 and 0.25 phr TPP. The stirring, storage, and curing conditions for the prepregs were selected according to recommendations from Example 1. Plies of prepregs were stacked and pressed under a pressure of 3 atm (~1 ton for the 18cm * 18cm area) in case of hot pressing; other laminates were prepared using a vacuum bag. Curing was set at 180°C for 2 hours followed by 1 hour at 220°C. The composition of the composite laminates was examined according to ASTM D3137.

[0319] Characterization

[0320] Parallel plate viscometry- A Discovery Hybrid Rheometer was used to determine the viscosities of GT6071 / DER 331 blends without curing agent at different ratios and in the presence of liquid CTBN, by using a 25 mm diameter parallel plate with a 2 mm gap. Dynamic thermal measurements were run from 20°C to 130°C with a heating rate of 2°C / min, at a constant frequency of 1 Hz.

[0321] Three-point bend flexural tests- The flexural tests were carried out with an Instron Model 4481 universal testing machine. Samples were cut with a diamond saw. The test yielded values of flexural stress, flexural modulus, and flexural strain. All measurements were conducted in three-point bending using a crosshead speed of 1 mm / min, with a span equal to 52 mm. The dimensions of the specimen (57 x 12.7 x 3.0 mm), span, and speed ofthe test were according to ASTM D790. Tests were performed at 50% relative humidity and 23°C.

[0322] Short beam strength (SBS) - The short beam shear test is used for laminated composites, to determine the maximal shear stress of the carbon fabric binding matrix. The test was performed on an Instron, Model 4481 mechanical tester. Specimens were machined by diamond saw to the specimen dimensions according to ASTM D2344. Inter laminar shear strength (ILSS) values were determined at a crosshead speed of 1 mm / min. At least five specimens were tested for each blend, and the average value was reported.

[0323] End notched flexure (ENF) - The mode II interlaminar fracture toughness, Gue, was measured using the end notched flexure test. Eighteen UD plies (0°)i8 having the dimensions of 118.5 mm long by 16.6 mm wide were machined using a diamond saw. Specimens were nominally 3 mm thick. A 0.0127 mm thick, polytetrafluoroethylene (Teflon / PTFE) film was placed in the mid-plane of the specimen thickness at one end, as a crack starter. In addition to the Teflon spacer, specimens were pre-cracked before testing. The mode II test was conducted using a mechanical tester (Instron, Model 4481) at a test rate of 5 mm / min. The specimens were positioned in the three-point bend fixture with a total span of 66.4 mm, with an initial crack length of 16.6 mm. The end of the shear pre-crack was placed 16.6 mm from the external support. The specimens were unloaded when the maximum load was reached. The fracture toughness, Gue, was calculated as given by Equation (2) and further detailed in Prasad, et al (2011), “Experimental Methods of Determining Fracture Toughness of Fiber Reinforced Polymer Composites under Various Loading Conditions,” J. Miner. Mater. Charact. Eng., vol. 10, no. 13, pp. 1263— 1275:

[0324] 9-P-a2-6

[0325] Eq

[0326] Muation 2: GILILcC = - 2-B(4T- L3+3<X3) Scanning electron microscopy (SEM) was conducted to investigate the fracture surface of the composites. Samples were coated with a silver alloy in an argon environment. An Aspex Explorer SEM was used to observe the specimens at an accelerating voltage of 15 kV.

[0327] Results:

[0328] Wettability

[0329] Example 1 examined the proper resin ratio of liquid and solid DGEBA that may fit hot melt impregnation in terms of viscosity. The resin viscosity after impregnation and during lay-up should be in the range l x 103Pa ■ s to lx 104Pa ■ s to attain a ductile, drapeable prepreg and to prevent drooling during storage. The 50L:50S, 60L:40S and 70L:30S formulations were found suitable for impregnation considering the curing agent reactivity and therefore were subsequently investigated?

[0330] The contact angle between the carbon fiber and the resin is a measure of the wettability; low contact angle is associated with good wettability.

[0331] Optical microscopy with MATLAB calculation, allowed the determination of the wetting angle by measuring the drop height, width and fiber radius. The same wetting angle of 14° was obtained for the three uncured neat resins, while the addition of CTBN increases the wetting angle to 17° degrees; Figure 8 demonstrates a 60L:40S epoxy droplet on the carbon fiber. Wetting angles of 14°-17° indicate relativity good wetting is achieved, compared to commercial resins.

[0332] An adaptation of the Wilhelmy Plate Method for surface tension measurement was implemented using a force machine and small glass plates. A hot plate was used to perform the test at temperatures relevant for the impregnation. The surface tension was calculated from the ratio of the force acting on the glass plate at equilibrium - when the plate emerges from the water, and the circumference times the wetting angle between the matrix and the glass. Figure 9 presents the obtained surface tension for the three unmodified resins. Intermolecular interactions are strongly temperature-dependent, a higher kinetic energy of the molecules leads to a decrease of intermolecular interactions. Due to the significant viscosity change of the DEGBA studied systems with temperature, the surface tension decreases significantly. In general, the total pressure during the impregnation is a sum of the gravitational, capillary and mechanical pressures. A decrease in surface tension causes the capillary effect to be less significant, but on the other hand, viscosity also decreases with temperature. This does not mean impregnation is affected with temperature; it explains there are two temperature-dependent opposite factors. Among the three curves, one can see that as the liquid epoxy content rises the surface tension is lower by 5 to 10 mN / m in the whole temperature range.

[0333] Mechanical properties

[0334] The laminates were molded with a hot press or vacuum bagged, then mechanically tested. The results in Table 7 are compared to a commercial product based on T300 carbon fabric. In order to compare data, the values are normalized to 60 vol% fiber.

[0335] Table 7 -Hot pressed and vacuum bagged composite laminates (0,90)20 . Fiber volume, void content and flexural properties are displayed*.

[0336] Resin 60L:40S 60L:40S 60L:40S 50L:50S 70L:30S Toray Semi-

[0337] CTBN rubber - 7.5 12.5 7.5 7.5 Toughened

[0338] (phr) Epoxy Resin

[0339] Curing method Hot- Hot- Hot-press Vacuum Vacuum NA press press

[0340] Fiber volume % 54 55 57 57 57 60

[0341] (Vf)

[0342] Void % (Vv) 0.9 1.2 0.5 1 0.9 NA

[0343] Flex . Modu hi s 65±3 67 1.6 70±2 63 : 4 64 : 2.1 70

[0344] (GPa)

[0345] Flex. Strength 929±59 902±58 895±35 860±40 872±66 905

[0346] (MPa)

[0347] Strain to break 1.4±0.1 1.5±0.1 1.4±0.1 1.4±0.2 1.3±0.3 1.3

[0348] (%)

[0349] * Flexural properties were normalized to 60% fiber volume

[0350] The flexural moduli follow a rule-of-mixtures behavior. Since the carbon fiber modulus is 230 GPa and the fabric is (0,90) with 60% fiber volume, an elastic modulus of roughly (0.5 * 0.6 * 230 ~) 70 GPamodulus is expected. The tested composites showed similar properties, comparable to theoretical values. Most fracture surfaces are caused by tension in the bottom layers, starting from the lowest ply and upward, rendering a zig-zag side view of the fracture surface which correlates with fibers pull out under tensile loading. Few specimens failed at the top layers with kink bands or micro-buckling related to compression failure. SEM micrographs of the fracture surfaces after bending, are presented in Figure 10A - Figure IOC. DSC tests of the laminates show that the Tg values are similar to those of the neat matrix polymer. In order to understand the matrix effect on the composite properties, inter laminar shear strength (ILSS) and fracture toughness mode II tests were performed. The control over fiber volume and void percent when curing in a hot press is poor, therefore a vacuum bag technique was used.

[0351] The ILSS test uses a small span relative to the thickness of the sample (1:4) in a three-point bend configuration to encourage shear stresses in the middle layers of the laminate. Although the shear forces are the most significant, there may be a variety of other failure modes, such as compression and tension; it is very difficult to assign short beam strength to a single material property. It was found that failure usually occurs at the matrix and through interlaminar failure. In this work, the ILSS allows to compare among the laminates with different solid / liquid DGEBA ratio, and CTBN with or without a catalyst (TPP). The relationship between strength and failure mode was investigated. The ILSS results of laminates with 57% fiber volume percent and 0 to 0.5% void content are shown in Figure 11. A decrease in strength can be seen when adding CTBN to a 60L:40S resin. ILSS decreases by 33% by adding 12.5 phr CTBN and 10% by adding 7.5 phr CTBN. TPP addition leads to a 14% increase of ILSS at constant rubber content. Furthermore, an optimum in mechanical properties is obtained for the 60L:40S matrix; higher (70L:30S) or lower (50L:50S) liquid epoxy contents reduced the ILSS by 5% and 8% respectively. The addition of CTBN does not increase the toughness effectively, and the interfacial breakage of composite disturbs the stress transfer. Toughness increased at 10 phr CTBN due to a larger interface area, the latter being relative to the ratio of domain size to rubber content. Optical microscope images of the plain weave carbon / epoxy laminates are shown in Figure 12A-Figure 12C. Longitudinal fibers along the sample and transverse fibers across the sample can be distinguished. The force was applied from above, (long white arrow side). The brightest areas are the longitudinal fibers and the darker one are the fibers in the transverse direction. Some of the cracks (shown with black arrows) propagate cohesively between transverse fibers, and some of the cracks (pointed at with short white arrows) propagate at the interface between the fiber and the matrix, resulting in complete delamination. In general, when the shear force is applied, the material will fail in the weakest area, in many cases, it will be precisely the interphase between the fiber and the matrix, but in some cases, it may be seen that the crack occurs in the matrix itself between the transverse fibers. Figure 12A -Figure 12C show typical sample edge surface of tested samples with 12.5 phr CTBN (Figure 12A), 7.5 phr CTBN (Figure 12B) and with 7.5 phr CTBN modified with TPP (Figure 12C). It can be seen that for 12.5 CTBN, there are almost no failures in the matrix, only delamination cracks, the interfacial breakage precedes a cohesive matrix breakage. Overall, only 20% of the cracks were in the transverse fibers area. In the 7.5 CTBN sample, the percentage of cracks in the matrix increased to 32%, and with TPP and 7.5 PHR CTBN, the percentage of cracks in the transverse fibers increased to 51%. Another study, which examined changes in the fiber sizing through the ILSS test, found that the surface of the fibers could create chemical bonding with the matrix and thus improve ILSS. It is possible that TPP residues (which take a passive part in the reaction between CTBN and epoxy) can activate the fiber sizing groups and allow interaction between the fiber and the epoxy or even form a covalent bond with the carboxyl group of the reactive rubber.

[0352] Fracture toughness, Mode II

[0353] The mode II critical strain energy release rate (Gnc) of unidirectional laminates was measured by the end notch flexural (ENF) test, data are shown in Figure 13. The highest resistance to the crack propagation occurs with a 60L: 40S matrix.

[0354] In this test 50L:50S and 70L:30S resins show similar fracture toughness values. Addition of 7.5 PHR CTBN has no contribution to the above. However, with the addition of TPP, the fracture toughness value increases significantly, with an optimum at 60L:40S.

[0355] ENF fractured surfaces were analyzed to examine the composite fracture morphologies by SEM. SEM micrographs of unidirectional carbon fabric composite based on 60L:40S matrix cured with DDS, unmodified and modified with CTBN, are given in Figure 14A-Figure 14C. A patern of small cusp plates of different thicknesses can be seen that are oriented perpendicular to the fiber, this fracture type called "hackles" is formed by microcracking in the matrix. At low magnification it is possible to observe the direction of composite breakage, the amount and direction of fiber pull out, and the regularity in the fracture surface, and also the amount of debris. Irregular breakage in the "No CTBN" sample is observed, containing; pull out, rich matrix areas and some block-like hackles perpendicular to the breakage along the side wall of the pullout. The "7.5 phr CTBN" sample at low magnification presents pull out and debris. Debris formation is also a toughening mechanism increasing the amount of energy required for fracture. The modified system with the TPP presents a beter coverage of the fibers and less pull-out and less debris than the system with CTBN without TPP. At high magnification the 7.5 phr CTBN system, too presents hackles, though less developed and with large spacing. It is possible that the debris on the surface are broken hackles that extended, until they finally disengaged from the fiber. The CTBN at the interface between matrix and fibers can increase this phenomenon due to weak interfacial bonding. Meanwhile a relatively beter hackles coverage in the “No CTBN” system can indicate failure via matrix and less in the interface. The patern for the TPP system is much thinner and dense, displays more deformation, reflecting higher fracture surface and a more efficient crack arrest mechanism. Also, the change in hackles angle along the fibers, may indicate a good fibermatrix interaction, and higher shear strain before crack opening. The TPP and CTBN reactivity should be in the same level or even greater than the reactivity of the amine terminal group. This supports the claim that TPP residues may react with the fiber sizing to promote good adhesion between the fiber and the matrix, and not only between the epoxy and the CTBN.

[0356] Conclusions

[0357] The contact angle between the carbon fibers and the resin is a measurement of wettability. Weting angles of 14°-17° indicate relativity good weting, which was achieved for the three uncured neat resins and after the addition of CTBN. Due to the significant change in viscosity with temperature in the DEGBA systems, the surface tension decreases significantly. Since viscosity and surface tension have opposite effects on impregnation, temperature needs to be adjusted for optimum impregnation. Hence, the lowest viscosity is not necessarily the best choice for the prepregging process. The tested composites showed similar flexural properties, comparable to literature values. Triphenylphosphine (TPP) enhances the CTBN / DGEBA interaction and thus further improves fracture toughness and ILSS. When testing ILSS, some of the cracks propagate cohesively, between transverse fibers, and some of the cracks propagate at the interface between the fiber and the matrix. With TPP and 7.5 PHR CTBN, the percentage of cracks in the transverse fibers increased to 51%, compared to 20% for the unmodified resin. When testing fracture toughness (Mode II), the “hackles” pattern in the TPP system is much thinner and dense, displays more deformation, reflecting higher fracture surfaces and a more efficient crack arrest mechanism.

[0358] EXAMPLE 3 - epoxy / DICY systems comprising a blend of liquid and solid resin

[0359] Influence of a DGEBA blend on the properties of composite laminates, resulting from hot melt impregnation of carbon fabrics.

[0360] In this Example, various amounts of dicyandiamide (DICY), different mixtures of two grades of epoxy resins based on diglycidyl ether of bisphenol A (DGEBA) liquid and solid resins (D.E.R 331 and Araldite® GT 6071, respectively) and two different commercial accelerators, 2-methyl imidazole (DYHARD® MI-FF) and difunctional latent uron accelerator (DYHARD® UR500), were used in curing of epoxy / DICY systems comprising a blend of liquid and solid resin. Furthermore, carbon fiber (CF) / epoxy composites were prepared with two different rubber tougheners, standard CTBN and CTBNX, based on a carboxyl terminated copolymer of butadiene and acrylonitrile.

[0361] When using a different liquid / solid epoxy ratio, the glass transition temperature (Tg) and the heat reaction peak were hardly affected. Increasing the imidiazole concentration produced a small increase in the Tg at all ratios and reduced the peak by about 50-65 °C. Increasing the diuron concentration produced a small decrease in the Tg at all ratios and reduced the peak by about 35-55°C. Composite specimens were prepared by hand lay-up by manually impregnating 18 layers of plain weave carbon fabric (195GSM / PW / FT300B 3K) on a hot plate, then curing in a vacuum bag. Flexural strength and ILSS range from 530 to 970 MPa and 33 to 53 MPa, respectively. The mechanical properties are similar to typical values for commercial composite materials. Nevertheless, these compositions do not require "B-staging" in order to get the appropriate tack and drape of the prepreg, and they do not require freezing, remaining stable at room temperature for prolonged times.

[0362] Materials:

[0363] Two grades of diglycidyl ether of bisphenol A (DGEBA) epoxy resin used in this study: liquid (L) epoxy resin with epoxy equivalent weight (EEW) 184-190 Dow’s D.E.R. 331™ and solid (S) epoxy resin with EEW 500-525 Araldite® GT 6071 by Huntsman. Two similar grades of the latent hardener, DICY were used: Dyhard® 100S - a micronized grade of DICY with a particle size of 98% max. 10 pm, typically 8.0-9.5 pm, was supplied by AlzChem AG, and Aradur® 1571 - a homogeneous, agglomerate-free dispersion of 28% by weight of DICY in liquid epoxy resin, with a particle size of 98% max. 10 pm, was purchased from Huntsman. Two accelerators were used, 2- methylimidazole (Dyhard® MI-FF) for elevated temperature curing of epoxy resin formulations, and 3,3'-(4-methyl-l,3-phenylene) bis (1,1 -dimethylurea), diuron (Dyhard® UR500) for use in epoxy resin formulations, both by AlzChem AG. Two grades of rubber tougheners, based on a carboxyl terminated copolymer of butadiene and acrylonitrile (standard CTBN and CTBNX with high content of carboxyl group) supplied by Hyper. Triphenyl Phosphine (TPP) was used at 0.35 pbw (parts by weight) as a coagent to facilitate reaction of the carboxyl groups with epoxy groups.

[0364] Methods:

[0365] Sample preparation

[0366] Samples were prepared from both liquid and solid resins. In order to prepare the resin formulations, the DGEBA epoxy mixture was heated to 120°C to decrease viscosity and the hardener and accelerator were mixed into it with a mechanical stirrer for 30 min. The mixture was then degassed in a vacuum oven at 120°C prior to use.

[0367] Wettability of a single fiber

[0368] The wettability of fibers is useful to predict their adhesion potential to the matrices which they strengthen. The contact angle on a single fiber can characterize its wettability by the contacting liquid in a given environment. The contact angle was determined using optical microscopy with MATLAB calculations. Composite specimens preparation

[0369] Composite specimens were prepared by hand lay-up by manually impregnating 18 layers of 195GSM / PW / FT300B 3k on a hot plate, and curing in a vacuum bag. Vacuum was applied at 180°C for 30 minutes for debulking, and then the selected curing cycle was carried out.

[0370] Differential Scanning Calorimetry (DSC)

[0371] DSC was performed with a Q100 9.9 Build 303 DSC (TA Instruments, USA) under a constant N2 flow of 50 mL / min. The Tflof the cured resins was determined by the second scan (first DSC scan to 220°C at 10°C per minute - cooling - second DSC scan to 220°C at 10°C per minute).

[0372] Non-isothermal scans on fresh and aged uncured resin consisted of a heating cycle from -20°C to 180°C at 10°C min-1. Isothermal scans on uncured resins were performed by rapidly equilibrating to the desired temperature, and holding the temperature for a fixed time. After each non-isothermal or isothermal scan the sample was left in the DSC cell where it was rapidly equilibrated to -20°C and a heating cycle was performed up to 220°C at 10°C min-1, to assess the glass transition temperature and the residual reactivity.

[0373] Rheology

[0374] Complex viscosities as a function of temperature at the various ratios of solid / liquid samples were analyzed using a parallel plate rheometer in oscillatory mode. The resin was molded into a 25.4 mm diameter disk approximately 1.5 mm thick. Temperature was kept below 140°C to prevent initiation of crosslinking of the DICY in the viscosity instrument chamber.

[0375] Mechanical Properties

[0376] The mechanical properties of composite specimens were assessed in both shear and bending modes. Three-point bending was done at room temperature according to ASTM D790 with an Instron 5969 Tensile Machine equipped with a 50KN load cell. Interlaminar shear tests were performed on 3-mm-thick samples according to ASTM D2344 (Short Beam test). Scanning Electron Microscope (SEM)

[0377] The surfaces obtained from the cured composite specimens and the cured resins were examined using a scanning electron microscope with 10-25 kV working energy. All samples were gold coated.

[0378] Activation Energy Measuring by ASTM E698

[0379] The kinetics approach in ASTM E698 requires three or more experiments at different heating rates.

[0380] Results

[0381] The effect of L / S ratio, accelerator type and content on the Tg.

[0382] This study compared resin formulations cured with 4 phr DICY, by measuring the glass transition temperature (Tfl) and by determining the effect of DICY / epoxy ratio and accelerators diuron (UR500) and imidazole (MI-FF) on the resin system.

[0383] Effect of the epoxy L / S ratio

[0384] In Table , the influence of the epoxy L / S ratio on the Tg and Tpeakis described. The L / S epoxy ratio does not significantly affect either Tg or Tpeak.

[0385] Table 8: Effect of epoxy L / S ratio on Tg and Tpeakwith 4 phr DICY

[0386] L / S ratio DICY [phr] Tfl[°C] Tpeak[°C]

[0387] 30 / 70 4 113 196

[0388] 35 / 65 4 113 197

[0389] 40 / 60 4 112 193

[0390] Influence of DICY content

[0391] The influence of the DICY epoxy ratio for a nonaccelerated system is displayed in Table . The same value of Tg, 121 °C, was obtained for 3 and 8 phr DICY while a value of 116°C was obtained for 6 phr, yet Tg seems to be invariant in this range of DICY contents. Table 9. Tg of nonaccelerated system at different DICY weight ratio

[0392] L / S ratio DICY [phr] Tfl[°C]

[0393] 40 / 60 3 121

[0394] 40 / 60 6 116

[0395] 40 / 60 8 121

[0396] Effect of the Accelerator Type and Content

[0397] Table and Table show the influence of the amount of accelerator on the Tg and the temperature at the exotherm peak, Tpeak. Increasing the accelerator content is expected to reduce the Tg and the Tpeakof the reaction. However, while the Tpeakdecreases with diuron increase, the Tg stays constant (104°C) and much lower than that of the non-accelerated resin mixture (Table 8). Imidazole compositions show an even stronger effect on Tpeak, while Tg remains unaffected. These compositions, especially at 1.6 phr imidazole, allow reducing the curing temperature without sacrificing the Tg when compared to non-accelerated compositions.

[0398] Table 10 Tg and Tpeakas function of diuron content

[0399] L / S ratio DICY [phr] diuron [phr] 7^ [°C] Tpeak[°C]

[0400] 40 / 60 4 0 104 155

[0401] 40 / 60 4 1.6 104 146

[0402] 40 / 60 4 2.4 104 139

[0403] Table 11 Tg and Tpeakas function of imidiazole content

[0404] L / S Ratio DICY [phr] imidiazole [phr] Tg[°C] Tpeak[°C]

[0405] 40 / 60 4 0.4 117 146

[0406] 40 / 60 4 1.6 120 136

[0407] 40 / 60 4 2.4 117 129

[0408] Complex viscosity

[0409] Error! Reference source not found, and Error! Reference source not found, show the viscosity as function of temperature for different ratios of liquid / solid mixtures and CTBN. The viscosity diagram shows a typical hot melt prepreg viscosity profile for 30-40% liquid epoxy. L / S epoxy ratio effect on viscosity.

[0410] The viscosity is initially high and decreases with increasing temperature to less than 1 Pa-s at about 120°C and then levels off to a plateau.

[0411] Using the rheology test the appropriate mixture of L / S epoxy resin was chosen by comparing the viscosity profile to a commercial prepreg rheology data. To simplify the research a 40% liquid epoxy, 60% solid epoxy ratio was chosen, with constant amount of 7 phr DICY.

[0412] The viscosity of 30L-40L samples is about 1000 Pa-s at 35-40°C and less than 10 Pa-s at 80-120°C. Mixtures were found appropriate according to their tack, drapeability, and low viscosity at the impregnation temperature ("process window").

[0413] The CTBNX effect on the viscosity.

[0414] The presence of CTBN rubber in the epoxy matrix induces a slight increase of the viscosity of the resin. However the mixtures are still at the "process window" as defined.

[0415] Measurements of the Activation Energy by DSC

[0416] Since the molecular weight (MW) of the resin affects crosslink density, we can speculate on its effect on reaction kinetics, given the possible dependence of the kinetics on the degrees of freedom and the free volume among the molecules. Therefore the activation energy was calculated for different epoxy resin mixture and with CTBN and CTBN+TPP.

[0417] The dynamic DSC scans were performed at scanning rates of 5, 10 and 15 °C / min. The DSC temperature ramps performed on fresh uncured resin formulations revealed different curing kinetics depending on the formulation. All curves fit the kinetic models very well, showing a linear dependence of log P upon 1 / T (R2>0.99) as shown by the slopes in Error! Reference source not found.. The activation energies are shown in Table .

[0418] Table 12: Activation energy results

[0419] Ea(kJZmole) Ea(kJZmole)

[0420] Resin content (Ozawa method) (Kissinger Method)

[0421] 20L / 80S / 7phr DICY 179 181 40L / 60S / 7phr DICY 174 173

[0422] 60L / 40S / 7phr DICY 184 186

[0423] 40L / 60S / 7phr DICY / CTBN 174 173

[0424] 40L / 60S / 7phr DICY / CTBN +

[0425] TPP156 156

[0426] 100L / 7phr DICY 200 202

[0427] The activation energy values are very high (above 150kJ / mol). This can be related to the latent nature of DICY and the fact that no accelerator is used in this formulation. Most probably, the Activation Energy decreases with epoxy solid content, due to the higher EEW of the solid epoxy fraction.

[0428] Gel time by DSC

[0429] Gelation is one of the most important properties of thermoset polymers and has direct influence on storage and shelf life as well as processing parameters. The gel time determined in this work is the time required to start crosslinking in isothermal cure. Results for the gel time obtained by isothermal cure at 180°C in different concentrations of CTBN. Contrary to expectations, there was no significant dependence of gel time on CTBN concentration. The gel time obtained was around 15 minutes for all the samples. In the present work CTBN 1300x9 was used as rubber toughener. CTBNX 1300x9 has number-average molecular weights of about 3400 and his functionality is 2.4. The higher functionality leads to high exothermic energy but does not shorten the gel time.

[0430] Wettability of single fibers

[0431] The contact angle between the solid fiber and the impregnating liquid matrix is the accepted measure of wettability.

[0432] Optical microscopy with MATLAB calculation, allowed the determination of the wetting angle by measuring the drop height, width and fiber radius over a carbon single fiber. A low wetting angle of less than 12° was obtained for all the resins, either neat or containing CTBN or CTBN+TPP.

[0433] Figure 18 depicts an epoxy droplet on the carbon single fiber. Table 13: Results of contact angle for 4 different resin mixtures obtained by measuring 4 different drops of resin on a single fiber of carbon fiber

[0434] Resin content Contact angle Std dev

[0435] 40L / 60S / 7phr DICY 7.2 .11

[0436] 60L / 40S / 7phr DICY .99 2.0

[0437] 40L / 60S / 7phr DICY / CTBN .91 0.5

[0438] 40L / 60S / 7phr DICY / CTBN + TPP .128 1.3

[0439] Properties of vacuum bag cured carbon fiber composites

[0440] The size of the rubber particles increased with increasing rubber content. The increase in the domain size of the dispersed CTBN phase is associated with the reagglomeration or coalescence of the dispersed rubber particles, which is more prominent at higher concentrations of the dispersed CTBN phase. These phenomena lead to decrease in all mechanical properties as shown in Table and Table .

[0441] Interlaminar shear strength properties (ILSS)

[0442] ILSS results of neat epoxy / DICY are similar to typical commercial prepregs (such as RP549). However, a continuous decrease in modulus was observed as the concentration of the CTBN / CTBNX phase increased (Table ). In the worst case, the modulus decreased from 52 MPa (the highest ILSS measured) to about 38 MPa with increasing CTBNX content. Thus, 12 phr CTBNX led to a decrease of 27% in modulus when compared to the neat epoxy resin. This decrease can be due to the lower modulus of the elastomer phase, or the relative amount of dissolved rubber as rubber content increases. The weak chemical bond between CTBN and epoxy was improved by adding triphenylphosphine (TPP) catalyst. Adding TPP increased the ILSS by 34% for CTBN and only by about 4% for CTBNX. Both results are lower than for neat epoxy / DICY system. Table 14: ILSS of vacuum bag cured composites of unmodified as well as CTBN / CTBNX (resin content of40L / 60S + 7phr DICY)

[0443] Toughener

[0444] Content ILSS (MPa) S.D

[0445] [phr]

[0446] 0 phr 52 2

[0447] 8 45 1

[0448] 12 41 1

[0449] CTBNX

[0450] 12 + 0.35 phr TPP 43 1

[0451] 16 37 1

[0452] 8 43 1

[0453] 12 36 1

[0454] CTBN

[0455] 16 33 1

[0456] 16 + 0.35 phr TPP 45 1

[0457] Flexural strength properties

[0458] The highest flexural strength (786 MPa) was obtained for the unmodified DICY / epoxy system (Table ). The flexural properties decreased steadily with CTBN / CTBNX content. The decreases in the flexural strength are attributed to the presence of the low modulus rubber particles in the epoxy matrix; the rubber that is incorporated into the resin matrix is responsible for the reduction in flexural strength of the modified samples. The optimum formulation containing 8 phr of CTBNX and 12 phr of CTBN gave maximum flexural strength of 691 MPa and 631 MPa respectively, but still less than neat epoxy / DICY system. Nevertheless, some compositions exhibited higher strain to break.

[0459] The reduction in flexural strength is due to the presence of some amount of rubber that remains dissolved in the epoxy matrix.

[0460] Table shows the plasticizing effect of the dissolved rubber, which is also reflected in the increasing of flexural strain. The system's average flexural strain increased from 1.57% at unmodified specimens to 1.76% and 1.72% at 8 and 12 phr CTBNX respectively, butthen decrease to 1.64% for 12 phr CTBNX + TPP (mismatch) and 1.4% for 16 phr CTBNX. 8 and 12 phr CTBN didn’t change the strain while adding TPP to 16 phr CTBNX increased the strain from 1.27% to 1.76% as expected.

[0461] Flexural strength specimens failed mostly in flexure mode for 8 and 12 phr CTBN / CTBNX while in higher concentration of rubber the specimens failed in shear or mixed mode. This may be related to the debonding around the rubber in the epoxy modified with high-rubber content.

[0462] Table 15 : The flexural strength properties of vacuum bag cured composites of unmodified as well as CTBNX / CTBN-modified compositions of systems A (40% L + 60% S + 7phr DICY) Morphology by SEM

[0463] SEM images

[0464] The SEM micrographs were quantified to find the size of the dispersed particles. SEM micrographs image obtained from resin reinforced by carbon fiber specimens do not show any CTBN / CTBNX particles. This is apparently due to the fact that the resin percentage is low in the composite material. The representative SEM micrographs obtained from carbon fiber specimens can be seen in Error! Reference source not found.A (longitudinal view) and Figure 19B (cross section) at low magnification, and at higher magnification in Figures 20A-20D.

[0465] SEM images of fractured surfaces of the unreinforced rubber modified cured DICY / epoxy blends reveal a two-phase morphology (the samples were fractured under liquid nitrogen). The soft elastomeric phase is phase separated from the hard epoxy matrix during the early stage of cure. The representative SEM micrographs of cured DICY / epoxy blends are given in Figures 21A-21B.

[0466] Conclusions

[0467] A carbon / epoxy solvent-less prepreg using DICY as a curing agent without B staging was successfully developed, based on mixtures of two grades of liquid / solid (L / S) diglycidyl ether of bisphenol A (DGEBA) epoxy resins.

[0468] A solid epoxy content of about 60% has good impregnation properties, drape and tack when compared to commercial prepregs. The 40L:60S resin with 7phr DICY showed the best results.

[0469] A Tg of ~112°C was measured, indistinctively of the liquid / solid epoxy ratio. Accelerators strongly decrease the reaction temperature. Diuron decreases Tg by 10°C, while imidazole increases Tg to 120°C.

[0470] CTBN / CTBNX have no significant effect on viscosity. An expected reduction in flexural modulus and strength is shown, and although some toughening is demonstrated in some compositions by a higher strain to failure, the 40L:60S / 7phr DICY system performs well without these thougheners. If used, TPP should be added as co-agent to facilitate reaction of the carboxyl groups of CTBN with epoxy, preferently under nitrogen atmosphere. EXAMPLE 4 - PES - toughened hot melt bimodal epoxy prepregs for carbon / epoxy composites

[0471] Influence of a DGEBA blend on the properties of composite laminates, resulting from hot melt impregnation of carbon fabrics.

[0472] This Example examined the properties of a prepreg and the resulting composite made of carbon fiber with an innovative bimodal matrix combining two epoxy grades: a liquid epoxy resin based on DGEBA with an EEW of 184-190 (60%) and a solid epoxy resin based on DGEBA with an EEW of 500-525 (40%). The compositions included a polyether sulfone (PES) thermoplastic additive at 0, 5, 10, 15 and 20 wt%. The curing agent was dicyandiamide (DICY) throughout this Example. The prepreg was tested for its tackiness and reactivity, and the mechanical properties of the cured composite included interlaminar shear strength and mode II interlaminar fracture toughness. DSC and FTIR results revealed that PES particles extend the shelf life of the matrix at room temperature. The tackiness of the prepreg without PES is relatively high and remains stable for over six months at room temperature, however PES causes some decrease in tackiness. An initial increase in interlaminar shear strength is seen with 15 and 20 wt% PES, but with time ILSS values level off, and after four to six months they are indifferent to PES content. Nevertheless, addition of 10 wt% PES leads to a five-fold increase in mode II interlaminar fracture toughness.

[0473] Experimental

[0474] Materials

[0475] The following materials were used for the study: DER 331 - a liquid epoxy resin based on DGEBA with EEW of 184-190 (Dow Chemicals); GT6071 - a solid epoxy resin based on DGEBA with EEW of 500-525 (Huntsman); ARADUR 1571 - dispersion of 28% by weight of DICY in liquid epoxy resin (Huntsman), non-functionalized Polyether Sulfone (PES) - Virantage®, VW - 10300 FP, Lot: 1437000040 (SOLVAY), and HexTow® AS4C carbon fiber - 12K (1.78 g / cm3) by Hexcel. The molecular structures are shown in Error! Reference source not found..

[0476] GW1 -solid Epoxy resin.

[0477] ARADUR 1571 - Dispersion of 28% by weight of DICY in liquid Epoxy resin

[0478] Poiyether Sulfone

[0479] Preparation of Hot melt bimodal resin, prepregs and laminates

[0480] Bimodal epoxy resin

[0481] Solid (40% by weight) and liquid (60% by weight) epoxy mixture compositions were prepared by a suitable calculation with 7pph latent curing agent DICY. The components were put into a beaker, where the GT6071 (solid Epoxy) is in a crushed, powdery state. The beaker was heated to 110 °C (in a silicone oil bath). The mixture was prepared by mixing with a mechanical stirrer: 10 min of initial mixing at low velocity, 210 rpm, and another 20 min at high velocity, 1050 rpm until a homogeneous white mixture was obtained. The temperature was kept at 110°C. In the next step, the PES was added at the required amount to prepare mixtures with 0, 5, 10, 15 and 20 wt.% PES. Mixing was done for 20 min at high velocity, 1050 rpm at 110°C. Prepregs

[0482] In this study a hot melt fiber impregnation machine (CDI CENTURY DESIGN, San Diego, USA) was used to produce prepregs from every series. Figure 22 is showing an example of prepreg without PES (Figure 22A) and with 15 wt.% PES (Figure 22B) Machine parameters are: fiber pulling speed of 3 m / min, 0.15 mm film thickness of the resin (the distance between the rolls of the feed area), 40 psi - impregnation pressure and 20 psi - squeeze pressure and a designated temperature depending on a chosen viscosity of 20 [Pa.sec].

[0483] Laminates

[0484] The prepregs were laid-up manually in the fiber direction (unidirectional, UD). The curing cycle was 30 minutes at 80°C, 1 hour at 150°C, 2 hours at 180°C and 1 hour at 200°C at a heating rate of 5 — m rin-.

[0485] Methods

[0486] Differential scanning calorimetry (DSC)

[0487] A DSC (QI 00, TA Instruments) was used to determine the curing temperature, curing enthalpy and Tg. The test was done under conditions of heating - cooling - heating

[0488] (from -20°C to 300°C). The heating (and cooling) rate was The curing enthalpy of bimodal epoxy resins was monitored every 3 weeks while the resins were stored at room temperature.

[0489] Scanning Electron Microscopy (SEM)

[0490] SEM (JSM-IT200, JEOL) was used to observe the specimens of bimodal epoxy cured matrix and CFRP series at an accelerating voltage of 15 kV with gold and palladium plating made for 45 seconds.

[0491] Parallel plate viscometry

[0492] A Parallel Plates rheology test was performed from 20°C to 200°C at a heating rate °C of 5 The plate diameter was 25 mm, the spacing between plates was 2400 micrometer and the frequency 1Hz. Fourier-transform infrared spectroscopy

[0493] An FTIR. spectrometer (Broker Alpha-P) was operated even' 3 weeks to monitor the curing process of resins stored at room temperature. The areas under the epoxide group peak (884.08-928.49cm-1) and that ofthe phenylene group (785.91-853.70 cm-1) were calculated.

[0494] Prepreg-prepreg tackiness

[0495] Tackiness test was done by preparing 3 specimens 25 mm wide and 170 mm long for every series of prepregs made of bimodal epoxy resin, with and without PES. The specimens were prepared by attaching 2 prepregs 70 mm wide and of the specified length with adhesive tape to the end of each prepreg to facilitate gripping in the force machine, then cut with scissors to a width of 25 mm. The peel test speed was 300 mm / min with a 20N load cell.

[0496] Calculation of vol% carbon fiber / matrix and voids in the laminate

[0497] Volume fractions were measured according to ASTM D3171.

[0498] Dynamic mechanical analysis (DMA)

[0499] One specimen from every series of unidirectional laminate [0] 12 with dimensions 13mmx65mmx 1 ,5mm was cut with a diamond saw under cooling water. The specimens were tested in a model Q 800 DMA (TA Instruments), operated in three -point-bending mode at 1Hz, at a heating rate of 3°C / min, in the temperature range of 25°C to 150°C.

[0500] Inter laminar shear strength (ILSS)

[0501] Five specimens from every series of unidirectional laminate [0] 36 of dimensions 8mmx24mmx4mm were cut with a diamond saw under cooling water. The specimens were tested according to ASTM D2344 at a speed of 1 mm / min with 16mm span. The test was performed in an Instron Model 4481 mechanical tester.

[0502] Mode II interlaminar fracture toughness, GIIC

[0503] Gue was found according to ASTM D7905. Laminates made of 40 unidirectional plies [O]4o, 200 mm long by 22 mm wide, were machined using a diamond saw. Specimens were 4 mm thick. A 0.0127 mm thick, polytetrafluoroethylene (Teflon / PTFE) film 50 mm long was placed in the mid-plane thickness of the laminates at one end, creating an initial crack length of 30 mm. The specimens were positioned in a three-point- bend fixture with a total span of 100 mm, then a Mode II test was conducted using a mechanical tester (Instron, Model 4481) at a test rate of 0.5 mm / min with a 50kN load cell. The specimens were unloaded when the maximum load was reached.

[0504] Results

[0505] Thermal characterization

[0506] The presence of PES particles affects resin crosslinking. Therefore, blends of 0, 5, 10, 15 and 20wt% PES with hot melt bimodal epoxy resin, 40s, were tested by DSC to determine the curing enthalpy and the curing (peak) temperature. The curing enthalpy of the blends in Error! Reference source not found, behaves according to the rule of mixtures where 100% PES is referenced as zero curing -enthalpy. This means that if PES affects crosslinking, it shows no cumulative effect with concentration. The curing peak temperature, Table 3, is stable, but may increase by 2°C due to some steric hindrance effected by PES. PES is an aromatic polymer, and the sulfur link is stable because of resonance and steric hindrance of the benzene rings. Hence, PES has high temperature stability.

[0507] Table 3: Characteristics of the 40S resins in the various mixtures of PES by curing enthalpy, curing temp, and Tg.

[0508] Content of PES (wt.%) 0 5 10 15 20

[0509] Curing temp. Tpeak[°C] 194 194 196 196 196

[0510] Two transition temperatures (Tg) were obtained. To understand their source, the following mixtures were also investigated: a two-component PES-free blend (DER331 / Aradurl571); Aradurl571; 40S with 7pph DICY (DYHARD 100SH); and DER331 with 7pph DICY (DYHARD 100SH). In all the compositions two transition temperatures were found and we conclude that the two Tg values are characteristic of DICY crosslinking. 3.1.2. Morphology

[0511] The morphology of the curing compositions of bimodal epoxy with (and without) PES was analyzed by SEM (Error! Reference source not found.A-Figure 24F). Dispersed droplets of about 8 microns in diameter are shown at 5wt.% PES, in a phase- within-phase morphology. The continuous phase in Error! Reference source not found, is epoxy, the droplets are PES, and the dispersed phase within the droplets in Error! Reference source not found, is epoxy. At 10wt.% PES, the phase-within-phase morphology is more evident.

[0512] Between 15 to 25wt.% PES, it is more difficultto identify the type of morphology. Therefore, a 15wt.% PES specimen was etched in DMSO to dissolve the PES and enhance the contrast. According to Error! Reference source not found.A - Figure 27C the morphology is co-continuous.

[0513] Etching leaves voids in the epoxy phase behind the missing PES domains proving the phase-within-phase and bimodal morphology. In this bimodal morphology the minor phase comprises dispersed PES particles in two average diameters: 30pm and 2.5 m.

[0514] Error! Reference source not found, shows the blends after curing in an aluminum pan. Phase separation begins at 5 wt.% PES.

[0515] The hot melt is impregnated at a relatively low temperature at which curing is prevented. The resin is applied to a roll-to roll nip gap, and adheres to siliconized paper as the paper advances towards the impregnation zone where a pressure cylinder impregnates the resin into the carbon fibers. In order to determine the optimal impregnation temperature, a rheological characterization was performed. The dependence of viscosity on temperature is shown in Error! Reference source not found.. Initial tests revealed that the recommended working viscosity was 20 [Pa.sec] so that on the one hand the wetting of the resin on the paper is uniform and on the other hand the matrix can permeate and impregnate the fibers well. Therefore, the selected temperatures for impregnating the prepregs are: 55, 60, 67, 70 and 73°C for resins with 0, 5, 10, 15 and 20wt.% PES respectively. The graph shows the effect of PES addition to the viscosity of the various compositions. The higherthe PES content, the higherthe viscosity. All graphs in Figure 29 follow a typical thermoset behavior: the viscosity is high at room temperature and decreases with heating (similarly to a thermoplastic material) but at about 140°C curing begins, and viscosity increases due to the increasing molecular weight. The local maximum in the 10 wt.% PES composition at 140°C reflects phase separation of the PES while performing the test on a Parallel Plate device. Phase separation leads to an increase in PES particles size and therefore viscosity fluctuations occur.

[0516] 3.2. Prepreg characterization

[0517] Prepregs prepared in the hot melt impregnation machine were characterized for fiber volume fraction, see Table 4. PES content was up to 20 wt.%, beyond this value (e.g. 25 wt.%) the higher viscosity does not allow for a good impregnation of the fibers.

[0518] Table 4: Prepregs characterization by fiber volume.

[0519] Prepreg 0 wt.% 5 wt.% 10 wt.% 15 wt.% 20 wt.% series PES PES PES PES PES fiber 53 60 58 58 58

[0520] The process parameters were tuned so as to achieve a nominal fiber volume fraction of 60%. However, the PES-free composition reached a lower fiber volume fraction, possibly due to the lower resin viscosity. Certain difficulties were dealt with when running the impregnation process with PES-modified resin. Aggregation of PES particles were observed, as exemplified in Error! Reference source not found. 30B for a 15 wt.% PES composition. At low speed (1 m / min), the residence time between rolls is longer and the impregnation pressure causes an increase in the fiber tension which squeezes out some resin and leads to agglomeration of the PES particles (Figure 30A). At medium fiber pulling speed (3 m / min) the lower residence time reduces the pressure and the fiber tension, less resin is squeezed out and PES particles are less driven towards one another, thus contributing to a better distribution of the PES (Figure 30B).

[0521] Composite characterization

[0522] Dynamic Mechanical Analysis (DMA) of laminates with varying PES content was performed to characterize the storage and loss moduli and Tg (Table 17). Fiber and void volume fraction were determined as well. Table 5: 60L:40S carbon / epoxy composites, effect of PES content on Tg (°C), carbon fiber and void volume fractions.

[0523] Table 5 shows the Tg values, fiber and void content by volume. At 10 wt.% PES, Tg is relatively low and relates to the relatively high percentage of voids. At 0 and 15 wt.% PES, a shoulder can be seen in the tan 8 graph. The somewhat higher matrix content helps to reveal a second Tg value around 100 to 110°C, previously found by DSC (119°C and 121°C respectively at 0 and 15 wt.% PES).

[0524] Monitoring the Curing Process

[0525] To test the stability of the resins at room temperature over time, curing enthalpy tests were performed every three weeks by a DSC device. In Error! Reference source not found., the relative change with time of the curing enthalpy for each resin is shown. The curve for the neat, PES-free resin rapidly decreases monotonically with time. With increasing PES content, the curing enthalpy tends to rather keeps its original value. Fluctuations may be related to temperature variations, sample inhomogeneity and other sources of noise. For instance, sedimentation of the insoluble components may happen, driven by the different densities: 1.16 and 1.18 for the liquid and solid epoxy, 1.404 for DICY and 1.37^^ for PES. The increase in enthalpy of the 100% liquid epoxy with 15% PES (0%S_15%PES) is most probably due to PES sedimentation leading to the sampling of richer in resin. Nevertheless, compared to 100% liquid epoxy without PES (0%S_0%PES) the liquid / solid bimodal epoxy composition is much more stable at room temperature.

[0526] Analysis of FTIR peak areas associated with epoxide chemical groups (at 915cm-1) is shown in Error! Reference source not found, to 21. Changes are not monotonous with time, and we can assume that small changes (up to about 5%) reflect some segregation and imperfect distribution of the PES and / or DICY particles, partly because of the aforementioned sedimentation. Beyond this noise, the bimodal epoxy blend without PES shows a decrease already after 3 weeks. In contrast, PES-containing compositions seem to maintain the level of epoxide groups, some of them even after 28 weeks. PES may contribute to this behavior and help stabilize the system. The aromatic PES sulfur link is stable because of resonance and steric hindrance of the benzene rings. The sulfone groups -SO2- are stable, and the two oxygens have each two pairs of unshared electrons which can interact through strong hydrogen bonding with adjacent molecules.

[0527] Table 18: FTIR analysis of the epoxy group area (884.08-928.49 cm”1)

[0528] 6 2.200 2.121 1.902 2.126 2.194

[0529] 10 2.121 2.466 2.376 2.350 2.317

[0530] 13 2.151 2.023 1.835 1.908 1.764

[0531] 21 1.917 2.025 1.800 1.840 1.790

[0532] 25 1.693 1.966 1.778 1.672

[0533] 28 2.466 2.322 2.144 2.218

[0534] Table 19: FTIR analysis of the phenylene group area (785.91-853.70cmx) week Phenylene area [830 cmx| number 0 wt.% 5 wt.% 10 wt.% 15 wt.% 20 wt.% PES

[0535] PES PES PES PES

[0536] 0 11.871 13.768 13.9 13.841 13.635

[0537] 3 12.889 12.889 13.583 13.457 13.444

[0538] 6 12.225 13.193 13.005 13.217 13.309

[0539] 10 12.165 14.334 14.116 14.173 14.135 3 417

[0540] 21 10.032 12.987 13.432 13.451 13.642 25 9.121 11.647 12.894 12.955

[0541] 28 14.605 14.351 14.388 14.298

[0542] Table 6: Epoxide to phenylene area ratios [at 915cm-1 / 830cm-1] for 60L:40S compositions, kept at room temperature. week Area ratio [at 915cm-1 / 830cm-1] number 0 wt.% 5 wt.% 10 wt.% 15 wt.% 20 wt.% PES

[0543] PES PES PES PES

[0544] 0 0.192 0.153 0.144 0.141 0.137

[0545] 3 0.163 0.163 0.143 0.134 0.132

[0546] 10 0.174 0.172 0.168 0.166 0.164

[0547] 13 0.186 0.157 0.141 0.141 0.131

[0548] 21 0.191 0.156 0.134 0.137 0.131

[0549] 25 0.186 0.169 0.138 0.129

[0550] 28 0.169 0.162 0.149 0.155

[0551] Table 21: Epoxide group change [%] of bimodal epoxy resin with 0, 5, 10, 15 and 20 wt.% PES. week Epoxide group change [%] number 0 wt.% 5 wt.% 10 wt.% 15 wt.% 20 wt.% PES

[0552] PES PES PES PES

[0553] 0 0 0 0 0 0

[0554] 3 -15 7 -1 -5 -4

[0555] 6 -6 5 1 14 20

[0556] 10 -9 12 17 18 19

[0557] 25 -3 10 -4 -8

[0558] 28 -12 6 3 10

[0559] In conclusion, DSC and FTIR results support that PES has a stabilizing effect leading to extended shelf life of the matrix at room temperature (“out-life”) at least 28 weeks, e.g. at least six months.

[0560] Prepreg tackiness

[0561] In a series of peeling tests, the tackiness of prepreg - prepreg joints was tested (Error! Reference source not found.). Prior to each test, a prepreg stickiness test was performed on a mold surface. The tackiness of the prepreg without PES is relatively high and remains stable for over six months at room temperature. With 20 wt.% PES the tackiness dropped to zero at week #15 (Error! Reference source not found.), leading to unraveling of the 9-fiber tows and loss of tape continuity. Furthermore, tackiness decreases with increasing PES content. These findings confirm that PES is detrimental to prepreg tackiness. Keeping in mind that the vol.% of the resin in the prepreg is: 57, 40, 42, 42 and 42% for a series of 0, 5, 10, 15 and 20wt.% PES; after 24 weeks the tackiness of the neat prepreg decreased by only 30% (from 0.85 to 0.60 “^) and that of the 10 wt.% PES prepreg decreased by 35% (from 0.31 to 0.2 Inter laminar shear strength (ILSS)

[0562] Laminates of the various prepregs with varying PES content were tested for interlaminar shear strength, at zero time (i.e., in the first week of production), and after being stored at room temperature for 15 and 24 weeks. An initial increase in interlaminar shear strength is seen with 15 and 20 wt% PES, but with time ILSS values level off, and after four to six months they are indifferent to PES content (Error! Reference source not found.). For 20wt.% PES the initial ILSS value is the highest (75MPa) and this points out the toughening effect of PES. After 24 weeks values dropped to 48 MPa. PES seems to sediment / sink in the matrix and adhere to the fibers and thus the ILSS decreases to the typical values for this system, being similar to literature values.

[0563] Fracture Toughness

[0564] Mode II interlaminar fracture toughness shows that GIIC increases significantly with PES (Error! Reference source not found.5). Already at 5 wt.% PES fracture toughness increases by a factor of four, and further improvements result with increasing PES content. Once Mode II testing was completed, a Mode I test was done manually to reveal the failure mode. Specimens after failure are shown in Error! Reference source not found. - Figure 37. A change in hue is visible along the three different zones of the specimens (Teflon, mode II and mode I zones). Specimens with 0 and 5wt.% PES show adhesive failure, specimens with 15 and 20wt.% PES show cohesive failure, and a mixed failure mode appears at I0wt.% PES. SEM micrographs (Error! Reference source not found.) of specimens with 5-20 wt.% PES show the contribution of PES to toughness in the form of stretch marks. Thermoplastic PES can deform and bridge incipient microcracks. This deformation is one of the energy dissipation mechanisms of the composite with PES.

[0565] Conclusions

[0566] The hot melt bimodal epoxy resin, comprising solid (40%) and liquid (60%) components, has proven its effectiveness in withstanding room temperature for more than half a year, and with PES the stability is further increased. DSC and FTIR results revealed that PES particles extend the shelflife of the matrix at room temperature (“out-life”). The tackiness of the prepreg without PES was relatively high and remained stable for over six months at room temperature, however PES causes some decrease in tackiness. An initial increase in interlaminar shear strength was observed with 15 and 20 wt% PES, but with time ILSS values levelled off, and after four to six months they were indifferent to PES content. Nevertheless, the addition of 10 wt% PES led to a five-fold increase in mode II interlaminar fracture toughness. This matrix system may contribute to the relief of storage conditions during transport and storage, and lower the power consumption in cooling storage compartments.

[0567] LIST OF PARAGRAPHS

[0568] The following statements / paragraphs disclose features and / or embodiments of the present disclosure. It is to be appreciated that any combination of these two or more of these paragraphs, or parts of paragraphs, constitute part of the invention and that there should be no limitation to the number of paragraphs that can be combined, as part of the presently disclosed subject matter:

[0569] 1. An epoxy composition comprising a liquid epoxy resin and a solid epoxy resin, wherein: said liquid epoxy resin is characterized by viscosity of between about 500 mPa s and about 100,000 mPa s, when measured at 25 °C according to ASTM DI 084; and said solid epoxy resin is characterized by a softening temperature of between about 60°C and about 160°C when measured according to ASTM D3418 and / or ASTM DI 525.

[0570] 2. The epoxy composition according to paragraph 1, wherein said solid epoxy resin and said liquid epoxy resin are, independently selected to provide the epoxy composition with a viscosity of between 0.5 and 15 Pa s when measured at a temperature between room temperature and up to curing temperature of the epoxy composition.

[0571] 3. The epoxy composition according to any one or combination of the above paragraphs wherein said liquid epoxy resin and said solid epoxy resin are present at a liquid: solid weight percent ratio within a range of about 10:90 and 90: 10.

[0572] 4. The epoxy composition according to any one or any combination of the above paragraphs, being essentially solvent-free.

[0573] 5. The epoxy composition according to any one or any combination of the above paragraphs, wherein said liquid epoxy resin is selected from the group consisting of liquid Diglycidyl ether of bisphenol -A, Diglycidyl ether of bisphenol-F, epoxy cresol novolac, epoxy novolac, brominated bisphenol A epoxy, brominated novolac epoxy, and any combination of same.

[0574] 6. The epoxy composition according to any one or any combination of the above paragraphs, wherein said liquid epoxy resin is Diglycidyl ether of bisphenol-A of formula (I) wherein n=0.

[0575] 7. The epoxy composition according to any one or any combination of the above paragraphs, wherein said solid epoxy resin is selected from the group consisting of solid diglycidyl ether of bisphenol-A, epoxy cresol novolac, epoxy novolac, brominated bisphenol A epoxy, brominated novolac epoxy, and any combination of same.

[0576] 8. The epoxy composition according to any one or any combination of the above paragraphs, wherein said solid epoxy resin comprises or is diglycidyl ether of bisphenol- A of formula (P), wherein n' is an integer between 1 and 25.

[0577] 9. The epoxy composition according to any one or any combination of the above paragraphs, comprising a toughening agent.

[0578] 10. The epoxy composition according to any one or any combination of the above paragraphs, wherein, when relevant, said toughening agent are rubber particles.

[0579] 11. The epoxy composition according to paragraph 10, wherein said rubber particles comprise a rubber selected from the group consisting of Butadiene Rubber, Carboxyl- Terminated Acrylonitrile-Butadiene Rubber (CTBN); Amine-Terminated Acrylonitrile- Butadiene Rubber (ATBN), Epoxy-Terminated Acrylonitrile-Butadiene Rubber (ETBN), Vinyl-Terminated Acrylonitrile-Butadiene Rubber (VTBN), and core-shell rubber.

[0580] 12. The epoxy composition according to paragraph 9, wherein said toughening agent is a thermoplastic toughening agent.

[0581] 13. The epoxy composition according to paragraph 12, wherein said thermoplastic toughening agent is selected from the group consisting of polyethersulfone, polyetherimide, polyetheretherketone and combinations thereof.

[0582] 14. The epoxy composition according to paragraph 9, wherein said toughening agent is selected from the group consisting of silica, polyhedral oligomeric silsesquioxane (POSS), nanoclays, carbon nanotubes, tungsten disulfide, and combinations thereof.

[0583] 15. The epoxy composition according to any one or any combination of paragraphs 9 to 14, comprising said toughening agent in an amount of between about 1 and about 25 parts per hundred resin (phr).

[0584] 16. The epoxy composition according to any one or any combination of the above paragraphs, comprising a reactive diluent.

[0585] 17. The epoxy composition according to paragraph 16, wherein said reactive diluent is selected from the group consisting of diglycidyl ether of phenol, o-cresol, PTBP, cardanol, aniline, alcohols, diols, glycols, trimethylol propane.

[0586] 18. The epoxy composition according to any one or any combination of the above paragraphs, comprising a plasticizer.

[0587] 19. The epoxy composition according to paragraph 18, wherein said plasticizer is selected from the group consisting of phthalate, phenol, pine oil, castor oil, coal tar, polyacrylate and combinations thereof.

[0588] 20. The epoxy composition according to any one or any combination of the above paragraphs, comprising a curing additive.

[0589] 21. The epoxy composition according to paragraph 20, wherein said curing additive is a curing agent.

[0590] 22. The epoxy composition accordind to paragraph 21, wherein said curing agent is Diaminodiphenyl sulfone (DDS) and / or dicyandiamide (DICY). 23. The epoxy composition according to any one of paragraphs 21 or 22, comprising said curing agent in amount of between about 5 and about 50 phr.

[0591] 24. The epoxy composition according to paragraph 20, wherein said curing additive is a catalyst.

[0592] 25. The epoxy composition according to paragraph 24, wherein said catalyst is selected from the group consisting of triphenylphosphine (TPP), amine salts, boron trifluoride complexes, amine borates.

[0593] 26. The epoxy composition according to any one of paragraphs 24 or 25, comprising said catalyst in amount of between 0.01 and 3 phr.

[0594] 27. The epoxy composition according to paragraph 20, wherein said curing additive is an accelerator.

[0595] 28. The epoxy composition according to paragraph 27, wherein said accelerator is selected from the group consisting of 2 -methylimidazole, p-chlorophenyl-N,N- dimethylurea), N-3,4-dichlorophenyl-N,N-dimethylurea, N,N-dimethyl-N’-phenylurea, and N,N”-(4-methyl - 1 ,3 -phenylene)bis (N ’ ,N ’ -dimethylurea) .

[0596] 29. The epoxy composition according to any one of paragraphs 27 or 28, wherein said accelerator in amount of between 0.5 and 10 phr.

[0597] 30. The epoxy composition according to any one or any combination of the above paragraphs, wherein, when relevant, upon subjecting to curing inducing conditions, the composition has a gel time of between 5 and 90 minutes.

[0598] 31. The epoxy composition according to any one or any combination of the above paragraphs, retaining at least 60% epoxy groups when stored at a temperature of about 25°C for a period of about 30 days as compared to percent epoxy groups present in the epoxy composition at day 0 of the storage period.

[0599] 32. The epoxy composition according to any one or any combination of the above paragraphs, which following curing into a cured epoxy, the cured epoxy has at least one of the following physical properties: strain at break of at least 1%;

[0600] Young's modulus of at least 2500MPa. 33. A method of producing an epoxy composition, the method comprises forming a blend comprising a liquid epoxy resin and a solid epoxy resin, wherein said liquid epoxy resin is characterized by viscosity of between about 500 mPa s and about 100,000 mPa s, when measured at 25°C according to ASTM D1084; and said solid epoxy resin is characterized by a softening temperature of between about 60°C and about 160°C when measured according to ASTM D3418 and / or ASTM DI 525.

[0601] 34. The method according to paragraph 33, wherein said solid epoxy resin and said liquid epoxy resin are selected to provide upon said mixing process a viscosity of between 0.5 and 15 Pa s when measured at the temperature of 80 °C.

[0602] 35. The method according to any one of paragraphs 33 or 34 wherein said liquid epoxy resin and said solid epoxy resin are present at a liquid: solid weight percent ratio within a range of between about 10:90 and about 90: 10.

[0603] 36. The method according to any one or any combination of the above method-related paragraphs, wherein said liquid epoxy resin is selected from the group consisting of liquid Diglycidyl ether of bisphenol-A, Diglycidyl ether of bisphenol-F, epoxy cresol novolac, epoxy novolac, brominated bisphenol A epoxy, brominated novolac epoxy, brominated epoxy, and any combination of same.

[0604] 37. The method according to any one or any combination of the above method-related paragraphs, wherein said liquid epoxy resin is Diglycidyl ether of bisphenol-A of formula (I) wherein n=0.

[0605] 38. The method according to any one or any combination of the above method-related paragraphs, wherein said solid epoxy resin is selected from the group consisting of solid Diglycidyl ether of bisphenol-A, epoxy cresol novolac, epoxy novolac, brominated bisphenol A epoxy, brominated novolac epoxy, brominated epoxy and any combination of same.

[0606] 39. The method according to any one or any combination of the above method-related paragraphs, wherein said solid epoxy resin is diglycidyl ether of bisphenol-A of formula wherein n' is an integer between 1 and 25.

[0607] 40. The method according to any one or any combination of the above method-related paragraphs, comprising introducing into the blend a toughening agent as defined in any one of claims 9 to 15.

[0608] 41. The method according to any one or any combination of the above method-related paragraphs, comprising introducing into said blend a curing additive.

[0609] 42. The method according to paragraph 41, wherein said curing additive is as defined in any one or any combination of paragraphs 20 to 29.

[0610] 43. The method according to any one or any combination of the above method-related paragraphs, wherein said blend is essentially solvent-free.

[0611] 44. The method according to any one or any combination of the above method-related paragraphs, comprising subjecting said blend to at least one of mechanical stirring, homogenization, shaking, blending, high-shear mixing, and ultrasonic mixing.

[0612] 45. The method according to any one or any combination of the above method-related paragraphs, wherein said mixing of the blend is at temperature of at least 80°C.

[0613] 46. The method according to any one or any combination of the above method-related paragraphs, comprising subjecting said blend to a reduced pressure to cause release of entrapped gases from within said blend.

[0614] 47. A composite precursor comprising fibrous matter and an epoxy composition impregnated within said fibrous matter, said epoxy composition comprises a liquid epoxy resin, a solid epoxy resin and a curing additive, wherein: said liquid epoxy resin is characterized by viscosity of between about 500 mPa s and about 100,000 mPa s, when measured at 25°C according to ASTM D1084; and said solid epoxy resin is characterized by a softening temperature of between about 60 and about 160oC when measured according to ASTM D3418 or ASTM DI 525.

[0615] 48. The composite precursor according to paragraph 47, wherein said solid epoxy resin and said liquid epoxy resin are selected such to provide a viscosity of said epoxy composition in the absence of said fibrous matter between 0.5 and 15 Pa- s when measured at the temperature of 80 °C.

[0616] 49. The composite precursor according to any one of paragraphs 47 or 48, wherein said liquid epoxy resin and said solid epoxy resin are present at a liquid: solid weight percent ratio within a range of about 10:90 and about 90: 10.

[0617] 50. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, characterized in that said composite precursor is essentially solvent-free.

[0618] 51. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, wherein said liquid epoxy resin is selected from the group consisting of liquid Diglycidyl ether of bisphenol-A, Diglycidyl ether of bisphenol-F, epoxy cresol novolac, epoxy novolac, brominated bisphenol A epoxy, brominated novolac epoxy, and any combination of same.

[0619] 52. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, wherein said liquid epoxy resin is Diglycidyl ether of bisphenol-A of formula (I) wherein n=0.

[0620] 53. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, wherein said solid epoxy resin is selected from the group consisting of solid diglycidyl ether of bisphenol-A, epoxy cresol novolac, epoxy novolac, brominated bisphenol A epoxy, brominated novolac epoxy, and any combination of same.

[0621] 54. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, wherein said solid epoxy resin comprises or is diglycidyl ether of bisphenol-A of formula (P): wherein n' is an integer between 1 and 25.

[0622] 55. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, wherein said epoxy composition is uncured.

[0623] 56. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, comprising a toughening agent.

[0624] 57. The composite precursor according to paragraph 56, wherein said toughening agent comprises rubber particles.

[0625] 58. The composite precursor according to paragraph 56, wherein said toughening agent is a thermoplastic toughening agent.

[0626] 59. The composite precursor according to paragraph 56, wherein said toughening agent is an inorganic toughening agent.

[0627] 60. The composite precursor according to any one or any combination of paragraphs 56 to 59, comprising said toughening agent in amount of between 1 and 25 phr.

[0628] 61. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, wherein said curing additive comprises or is a curing agent.

[0629] 62. The composite precursor according to paragraph 61, wherein said curing agent comprises or is Diaminodiphenyl sulfone (DDS) and / or dicyandiamide (DICY).

[0630] 63. The composite precursor according to any one of paragraphs 61 or 62, comprising said curing agent in amount of between 5 and 50 phr. 64. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, wherein said curing additive comprises or is a catalyst.

[0631] 65. The composite precursor according to paragraph 64, wherein said catalyst is selected from the group consisting of triphenylphosphine (TPP), amine salts, boron trifluoride complexes, amine borates.

[0632] 66. The composite precursor according to any one of paragraphs 64 or 65, wherein said catalyst is in amount of between 0.01 and 3 phr.

[0633] 67. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, wherein said curing additive comprises or is an accelerator.

[0634] 68. The composite precursor according to paragraph 67, wherein said accelerator is selected from the group consisting of 2 -methylimidazole, p-chlorophenyl-N,N- dimethylurea), N-3,4-dichlorophenyl-N,N-dimethylurea, N,N-dimethyl-N’-phenylurea, and N,N”-(4-methyl - 1 ,3 -phenylene)bis (N ’ ,N ’ -dimethylurea) .

[0635] 69. The composite precursor according to any one of paragraphs 67 or 68, wherein said accelerator is in amount of between 0.5 and 10 phr.

[0636] 70. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, wherein said fibrous matter is selected from the group consisting of glass fiber, carbon fiber, aramid fiber, polyethylene fiber, polyester fiber, polyamide fiber, aluminum oxide fiber, silicon carbide fiber and combinations thereof.

[0637] 71. The composite precursor according to paragraph 70, wherein said fibrous matter is carbon fiber.

[0638] 72. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, wherein a volume fraction of said fibrous matter is within a range of between about 40% and about 70% out of the total volume of said composite precursor. 73. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, wherein at curing temperature, said precursor has a gel time of between 5 and 90 minutes.

[0639] 74. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, wherein, when said fibrous matter comprises carbon fibers, said composite precursor has at least one of the following physical property: fiber volume percent of at least 55%; void volume percent of below 1.5%; flexural modulus of at least 50GPa; flexural strength of at least 800MPa; fracture toughness (GIIC) of at least 500 J / m2; inter laminar shear strength (ILSS) of at least 40 MPa; and strain to break of at least 1% said physical property being measured on a sample of said composite precursor that has been subjected to curing.

[0640] 75. The composite precursor according to any one or any combination of the above composite precursor-related paragraphs, having at least one of: i) tackiness and ii) drapeability of at least 1mm.

[0641] 76. A method of producing a composite precursor, the method comprising mixing an epoxy composition according to any one or any combination of the above epoxy composition-related paragraphs and a curing additive with a fibrous matter under conditions that provide impregnation of said fibrous matter with said epoxy composition and said curing additive.

[0642] 77. The method according to paragraph 76, wherein said fibrous matter is selected from the group consisting of glass fiber, carbon fiber, aramid fiber, polyethylene fiber, polyester fiber, polyamide fiber, aluminum oxide fiber, silicon carbide fiber and combinations thereof. 78. The method according to any one of paragraphs 76 or 77, wherein said fibrous matter is carbon fiber.

[0643] 79. The method according to any one or any combination of the above paragraphs relating to method of producing a composite precursor, wherein a volume fraction of said fibrous matter is within a range of between about 40% and about 70% out of the total volume of said composite precursor.

[0644] 80. The method according to any one or any combination of the above paragraphs relating to method of producing a composite precursor, wherein said conditions that provide impregnation comprises at least one processing step selected from the group consisting of hot melt process, film stacking process, wet layup, resin transfer molding (RTM), resin infusion (RI), vacuum-assisted resin infusion (VA-RI), filament winding, spray-up, pultrusion, bulk molding compound (BMC), sheet molding compound (SMC).

[0645] 81. A composite material comprising a composite precursor according to any one or any combination of composite precursor-related paragraphs in cured form.

[0646] 82. A method of producing a composite material, the method comprises applying onto a precursor composition according to any one or any combination of the above composite precursor-related paragraphs curing conditions effective to cause curing of the precursor composition.

[0647] 83. An article of manufacture comprising a composite material according to paragraph 81 in a predefined shaped form.

[0648] 84. A method for manufacturing an article of manufacture, the method comprising: shaping a composite precursor according to any one or any combination of the above composite precursor-related paragraphs; and applying curing conditions effective to cause curing of the shaped composite precursor.

[0649] 85. The epoxy composition according to any one or any combination of the above epoxy composition-related paragraphs, or the composite precursor according to any one or any combination of the above composite precursor-related paragraphs having an out- life of at least 1 month.

[0650] 86. The epoxy composition according to any one or any combination of the above epoxy composition-related paragraphs, or the composite precursor according to any one or any combination of the above composite precursor-related paragraphs having an out- life of at least 6 months.

Claims

CLAIMS:

1. An epoxy composition comprising a liquid epoxy resin and a solid epoxy resin, wherein: the liquid epoxy resin is characterized by viscosity of between about 500 mPa s and about 100,000 mPa s, when measured at 25°C according to ASTM D1084; and the solid epoxy resin is characterized by a softening temperature of between about 60°C and about 160°C when measured according to ASTM D3418 and / or ASTM D 1525.

2. The epoxy composition of claim 1, wherein the solid epoxy resin and said liquid epoxy resin are, independently selected to provide the epoxy composition with a viscosity of between 0.5 and 15 Pa s when measured at a temperature between room temperature and up to curing temperature of the epoxy composition.

3. The epoxy composition of claim 1 or 2 wherein the liquid epoxy resin and the solid epoxy resin are present at a liquid: solid weight percent ratio within a range of about 10:90 and 90: 10.

4. The epoxy composition of any one of claims 1 to 3, being essentially solvent-free.

5. The epoxy composition of any one of claims 1 to 4, wherein the liquid epoxy resin is selected from the group consisting of liquid Diglycidyl ether of bisphenol-A, Diglycidyl ether of bisphenol-F, epoxy cresol novolac, epoxy novolac, brominated bisphenol A epoxy, brominated novolac epoxy, and any combination of same.

6. The epoxy composition of any one of claims 1 to 5, wherein the liquid epoxy resin is Diglycidyl ether of bisphenol-A of formula (I)wherein n=0.

7. The epoxy composition of any one of claims 1 to 6, wherein the solid epoxy resin is selected from the group consisting of solid diglycidyl ether of bisphenol-A, epoxy cresol novolac, epoxy novolac, brominated bisphenol A epoxy, brominated novolac epoxy, and any combination of same.

8. The epoxy composition of any one of claims 1 to 7, wherein the solid epoxy resin comprises or is diglycidyl ether of bisphenol-A of formula (I'),wherein n' is an integer between 1 and 25.

9. The epoxy composition of any one of claims 1 to 8, comprising atoughening agent and / or a reactive diluent and / or plasticizer and / or a curing additive.

10. The epoxy composition of claim 9, wherein the toughening agent is in an amount of between about 1 and about 25 parts per hundred resin (phr).

11. The epoxy composition of claim 9, wherein the curing additive is a curing agent present in amount of between about 5 and about 50 phr.

12. The epoxy composition of claim 9, wherein the curing additive is a catalyst present in amount of between 0.01 and 3 phr.

13. The epoxy composition of claim 12, wherein the curing additive is an accelerator present in amount of between 0.5 and 10 phr.

14. The epoxy composition of any one of claims 1 to 13, characterized by at least one of: upon subjecting to curing inducing conditions, the epoxy composition has a gel time of between 5 and 90 minutes; the epoxy composition retains at least 60% epoxy groups when stored at a temperature of about 25°C for a period of about 30 days as compared to percent epoxy groups present in the epoxy composition at day 0 of the storage period; following curing into a cured epoxy, the cured epoxy has at least one of the following physical properties: strain at break of at least 1%;Young's modulus of at least 2500MPa.

15. A method of producing an epoxy composition, the method comprises forming a blend comprising a liquid epoxy resin and a solid epoxy resin, wherein the liquid epoxy resin is characterized by viscosity of between about 500 mPa s and about 100,000 mPa s, when measured at 25°C according to ASTM D1084; and the solid epoxy resin is characterized by a softening temperature of between about 60°C and about 160°C when measured according to ASTM D3418 and / or ASTM D 1525.

16. The method of claim 15, wherein the solid epoxy resin and the liquid epoxy resin are selected to provide upon said mixing process a viscosity of between 0.5 and 15 Pa s when measured at the temperature of 80°C.

17. The method of claim 15 or 16 wherein the liquid epoxy resin and the solid epoxy resin are present at a liquid: solid weight percent ratio within a range of between about 10:90 and about 90: 10.

18. The method of any one of claims 15 to 17, wherein the liquid epoxy resin and the solid epoxy resin are as defined in any one of claims 1 to 14.

19. The method of any one of claims 15 to 18, comprising introducing into the blend a toughening agent and / or a curing additive20. The method of any one of claims 15 to 19, wherein the blend is essentially solvent- free.

21. The method of any one of claims 15 to 20, comprising subjecting the blend to at least one of mechanical stirring, homogenization, shaking, blending, high-shear mixing, and ultrasonic mixing.

22. The method of any one of claims 15 to 21, comprising subjecting the blend to a reduced pressure to release of entrapped gases from within the blend.

23. A composite precursor comprising fibrous matter and an epoxy composition impregnated within said fibrous matter, the epoxy composition being according to any one of claims 1 to 14.

24. The composite precursor of claim 23, being essentially solvent-free.

25. The composite precursor of claim 23 or 24, wherein the epoxy composition is uncured.

26. The composite precursor of any one of claims 24 to 25, wherein the fibrous matter is selected from the group consisting of glass fiber, carbon fiber, aramid fiber, polyethylene fiber, polyester fiber, polyamide fiber, aluminum oxide fiber, silicon carbide fiber and combinations thereof.

27. The composite precursor according to any one of claims 23 to 26, wherein a volume fraction of the fibrous matter is within a range between about 40% and about 70% out of the total volume of the composite precursor.

28. The composite precursor of any one of claims 23 to 27, wherein at curing temperature, the precursor composition has a gel time of between 5 and 90 minutes.

29. The composite precursor of any one of claims 23 to 28, wherein, when the fibrous matter comprises carbon fibers, the composite precursor has at least one of the following physical property: fiber volume percent of at least 55%; void volume percent of below 1.5%; flexural modulus of at least 50GPa; flexural strength of at least 800MPa; fracture toughness (Gnc) of at least 500 J / m2; inter laminar shear strength (ILSS) of at least 40 MPa; and strain to break of at least 1% the physical property being measured on a sample of the composite precursor that has been subjected to curing.

30. The composite precursor of any one of claims 23 to 29, having at least one of: i) tackiness and ii) drapeability of at least 1mm.

31. A method of producing a composite precursor, the method comprising mixing (i) an epoxy composition according to any one of claims 1 to 14 and a curing additive with (ii) a fibrous matter, under conditions that provide impregnation of the fibrous matter with the epoxy composition and the curing additive.

32. The method of claim 31, wherein the mixing is such to provide a volume fraction of the fibrous matter to be within a range between about 40% and about 70% out of the total volume of the composite precursor.

33. The method of claim 31 or 32, wherein the conditions that provide impregnation comprises at least one processing step selected from the group consisting of hot melt process, film stacking process, wet layup, resin transfer molding (RTM), resin infusion (RI), vacuum-assisted resin infusion (VA-RI), filament winding, spray-up, pultrusion, bulk molding compound (BMC), sheet molding compound (SMC).

34. A composite material comprising a composite precursor of any one of claims 23 to 30, in cured form.

35. A method of producing a composite material, the method comprises applying onto a precursor composition according to any one of claims 23 to 30, curing conditions effective to cause curing of the precursor composition.

36. An article of manufacture comprising a composite material of claim 34 in a predefined shaped form.

37. A method for manufacturing an article of manufacture, the method comprising: shaping a composite precursor according to any one of claims 23 to 30; and applying curing conditions effective to cause curing of the shaped composite precursor.

38. The epoxy composition of any one of claims 1 to 14, or the composite precursor of any one of claims 23 to 30 having an out-life of at least 1 month.

39. The epoxy composition of any one of claims 1 to 14, or the composite precursor of any one of claims 23 to 30 having an out-life of at least 6 months.

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