Ultraviolet (UV) curable shim

Abstract

Ultraviolet (UV) curable shim useful for filling gaps or spaces is disclosed. The UV curable shim is composed of a layer of reinforcement fibers embedded in a curable resin matrix, which is in a solid state at room temperature and forms a crosslinked material upon exposure to UV light. Two types of resin chemistries can be utilized for the UV curable shim: (i) free radical cure resin composition, which is an acrylate-based composition, and (ii) cationic cure resin composition, which is an epoxy resin-based composition.

Description

ULTRAVIOLET (UV) CURABLE SHIM

[0001] In general, the function of a shim is to eliminate gaps or spaces between assembled structural parts. Such shim is often required in aerospace applications for proper assembly of structural parts and for ensuring structural integrity of the aircraft. Aerospace assembly requires joining the structural parts together at their mating surfaces without leaving any gaps between the mating surfaces that are greater than a predetermined allowance. Gaps greater than the predetermined allowance must be filled with a shim to provide a desirable aerodynamic performance and structural integrity.

[0002] Liquid shims and solid shims are commercially available. Liquid shims for aerospace applications are commonly used for filling gaps of 3mm or less. Such liquid shims are not suitable for larger gaps or voids. For assembling composite components, solid shims are usually composed of composite materials that are similar to the composite components. For aerospace composite components, the liquid and solid shims currently used are based on an epoxy-amine curing chemistry or similar thermal curing chemistry, which requires heating at high temperatures and a long curing cycle.

[0003] There remains a need for solid shims that require less processing time and less energy consumption as compared to the current thermally curable shims, thereby, allowing for higher production rate during assembly of structural parts. For aerospace applications, such solid shims should also have the required physical properties such as high dry and wet glass transition temperature (Tg) after curing and moisture resistance.

[0004] Disclosed herein is an ultraviolet (UV) curable shim useful for filling gaps or spaces. This UV curable shim enables quick cure time (in the order of minutes) and exhibits good physical properties after curing. The UV curable shim according to the present disclosure is composed of a layer of reinforcement fibers embedded in a curable resin matrix, which is in a solid state at room temperature (20-25 °C) and forms a crosslinked material upon exposure to UV light. Prior to UV curing, the curable shim is in the form of a flexible sheet of fiber-reinforced resin matrix, which can be machined into the desired size and shape. This UV curable shim is not B-staged (i.e. , not partially cured) and is stable at room temperature. The term “stable” in this context means that the curable shim can be stored at room temperature, and no crosslinking reaction will occur while the shim is left out at room temperature. Such stability at room temperature is an advantage over other curable shim materials that require cold storage. Moreover, the UV curable shim does not contain any raw materials that are susceptible to advancement by heat, and only UV exposure is required to reach full cure, i.e., no heating to elevated temperatures is required. Unlike theUV curable coatings on the market today that are solvent-based, the resin matrix of the UV curable shim is solvent-free. Solvent-free resin is more preferred because the presence of solvents and volatile organic compounds (VOCs) can cause safety and regulatory issues.

[0005] Additionally, the resin matrix of the UV curable shim contains a unique toughening component that allows for a less brittle, curable material with good handling (or draping) property and low tack (i.e. , low stickiness to the touch). Tack is a measure of mechanical resistance that needs to be overcome in order to separate a material from another substrate. Furthermore, the resin matrix of the shim is formulated to provide a final UV cured material with the desired mechanical properties and moisture and fluid resistance. The desired mechanical properties include high dry and wet glass transition temperature (Tg) after curing. The high wet Tgallows the cured material to be used at a higher than normal service temperatures.

[0006] After UV exposure, the cured shim of the present disclosure may have a dry Tgof greater than 200°C, more specifically, 232 - 250 °C, and a wet Tgof greater than 204°C, more specifically, 215 - 232 °C. Tgvalues disclosed herein are determined by a rheological method using a rheometer in torsional mode. This method measures the Tgfrom the storage modulus onset as described in the article by TA Instruments titled “Measurement of Glass Transition Temperatures by Dynamic Mechanical Analysis and Rheology”, which can be found at the web address https: / / www.tainstruments.com / pdf / literature / RH100.pdf. As an example, the Tgcan be obtained by the storage modulus onset measured on an AR 2000EX rheometer in torsional rectangular mode. Using this rheometer, the onset Tgof cured resin samples can be measured at the intersection of the extrapolated tangents drawn from points on the storage modulus curve before and after the onset of the glass transition event.

[0007] There are two types of resin chemistries that can be utilized for the UV curable shim of the present disclosure: (i) cationic cure resin composition, which is an epoxy resinbased composition, and (ii) free radical cure resin composition, which is an acrylate-based composition.

[0008] UV cure chemistries require a UV energy source to produce UV light to initiate the polymerization reaction. The UV portion of the electromagnetic spectrum light spectrum is broken down into three different wavelength sections, UVA (315-400 nm), UVB (280-315 nm) and UVC (100-280 nm). The most commonly used UV light for industrial applications is UVA (315-400 nm). For both cationic and free radical resin compositions, a photoinitiator is required. The photoinitiator is able to absorb UV light and upon exposure to the correct UV wavelength creates an excited state that can induce a chemical reaction, resulting inpolymerization and curing of the resin composition.Cationic Cure Resin

[0009] The cationic cure resin composition for the purposes disclosed herein contains a combination of cycloaliphatic epoxy resins, an oxetane resin, a sulfonium or iodonium photoinitiator, and a toughening component.

[0010] According to one embodiment, the curable resin matrix of the shim is formed from a cationic cure resin composition, which includes the following components:(a) a combination two or more cycloaliphatic epoxy resins, at least one of which is a solid at room temperature;(b) an oxetane resin (as a reactive diluent or epoxy modifier);(c) a cationic photoinitiator selected from sulfonium and iodonium salts; and(d) a toughening component selected from: polyacrylate liquid elastomers, epoxidized polybutadiene, polyarylsulphone polymers, polyetherdiamine, and acrylic block copolymers.

[0011] In some embodiments, the relative weight percentages of the components, based on the total weight of the resin composition, are as follows:(a) 65% to 75%;(b) 5% to 13%;(c) 0.1% to 5%;(d) 10% to 25%.

[0012] As used in the present disclosure, the term “room temperature” refers to a temperature in the range of 20°C to 25°C.

[0013] The cationic cure resin composition may further include a photosensitizer (or sensitizer), inorganic fillers or flow control agents in particulate form (e.g., powder). In addition to the above mentioned cationic photoinitiator, the cationic cure resin composition may further include a Type I photoinitiator.

[0014] The cationic cure resin composition is devoid of any epoxy curative (or curing agent) containing reactive amino group(s) or other epoxy curatives that require thermal activation and that are typically present in conventional, thermally curable epoxy-based compositions. If selected as a toughening component, polyetherdiamine would be the only amine compound in the resin composition. Preferably, the cationic cure resin composition is devoid of any free radical polymerizable acrylate or methacrylate that is not a polyacrylateliquid elastomer or PMMA-b-PBA-b-PMMA tri-block copolymer. That is, if selected as a toughening component, the polyacrylate liquid elastomer or PMMA-b-PBA-b-PMMA tri- block copolymer, or combination thereof, would be the only acrylate / methacrylate component in the resin composition.Cycloaliphatic Epoxy Resins

[0015] Cycloaliphatic epoxy resins are characterized by non-aromatic saturated rings in their molecular structures. The term “epoxy resin” as used herein refers to monomers, oligomers and precursors containing one or more epoxide groups. The cycloaliphatic epoxy resin for use in the cationic cure resin composition is a compound having at least one alicyclic skeleton (or saturated carbon ring) and two or more epoxy groups per molecule. Suitable cycloaliphatic epoxy resins include compounds expressed by the following general Formula (I):In the general Formula (I), X represents a single bond or a linkage group. The linkage group is exemplified by divalent hydrocarbon groups, carbonyl group ( — CO — ), ether bond ( — O — ), ester bond ( — COO — ), amide bond ( — CONH — ), carbonate bond ( — OCOO — ), and groups that contain two or more of these linked to each other. The divalent hydrocarbon groups are preferably exemplified by straight or branched chain alkylene groups having 1 to 18 carbon atoms (of which those having 1 to 6 carbon atoms are more preferred); and divalent alicyclic hydrocarbon groups (of which divalent cycloalkylene groups are more preferred). The straight or branched chain alkylene groups are typified by methylene, methylmethylene, dimethylmethylene, ethylene, propylene, and trimethylene groups. The divalent alicyclic hydrocarbon groups are typified by 1,2-cyclopentylene, 1,3-cyclopentylene, cyclopentylidene, 1 ,2-cyclohexylene, 1,3-cyclohexylene, 1 ,4-cyclohexylene, and cyclohexylidene groups.

[0016] Examples of suitable di-functional cycloaliphatic epoxy compounds include epoxies having the following chemical structures:where n denotes an integer from 1 to 30.

[0017] In preferred embodiments, the cationic cure epoxy resin composition contains a combination of cycloaliphatic epoxy resins, at least one of which is a solid resin at room temperature.

[0018] An example of a suitable solid cycloaliphatic epoxy resin is EHPE3150 fromDaicel Corp., poly[(2-oxiranyl)-1,2cyclohexanediol] 2-ethyl-2-(hydroxymethyl)-1,3propanediol ether), represented by the following chemical structure:where R represents a straight or branched chain saturated aliphatic hydrocarbon group containing 6 or more carbon atoms; n represents an integer of 1 to 50.

[0019] In some embodiments, the combination of cycloaliphatic epoxy resins includes at least one liquid resin, which is a liquid at room temperature.

[0020] Suitable liquid cycloaliphatic epoxy resins are those having a viscosity of less than 1000 mPa s at 25°C, as determined by Brookfield Viscometer.

[0021] Examples of commercially available liquid cycloaliphatic epoxy resins are Celloxide™ 2021 P (3',4'-Epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate) and Celloxide™ 2081 (3,4-Epoxycyclohexylmethyl-3',4'-epoxycyclohexanecarboxylate modified epsilon-caprolactone) from Daicel Corporation. Additional examples of liquid resins include: 3,4-Epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (Uvicure S105 from Lambson), represented by the following chemical structure:bis((3,4-epoxycyclohexyl)methyl) adipate (Uvicure S128 from Lambson), represented by the following chemical structure:.

[0022] The combination of cycloaliphatic epoxy resins constitutes more than 50% by weight of the resin matrix. More specifically, the total amount of all cycloaliphatic epoxy resins combined may be in the range of 50% to 65% by weight based on the total weight of the resin matrix, and in some embodiments, 65% to 75% by weight.Reactive Diluent

[0023] Oxetane resins having a low-viscosity can function as a reactive diluent when used in combination with the cycloaliphatic epoxy resins. Oxetane resin is a resin having at least one four-membered ring (shown below) of three carbon atoms and one oxygen atom:

[0024] Suitable low-viscosity compounds include oxetane resins having a viscosity of 300 mPa s or less at 25°C, preferably, 15-100 mPa s at 25°C, as determined by Brookfield Viscometer.

[0025] Examples of oxetane resins that can be used as reactive diluents are: 3-Ethyloxetane-3-methanol, commercially available as UviCure S130 from Lambson (Sartomer), a liquid alcohol functional oxetane having a viscosity of 15-25 mPa s at 25°C; 1 ,4-Bis[(3-ethyl-3-oxetanylmethoxy)methyl]benzene, commercially available as Uvicure S150 from Lambson, a liquid difunctional oxetane resin having a viscosity 150-220 mPa s at 25°C); and 4,4-Bis(3-ethyl-3-oxetanyl)methoxymethyl]biphenyl, commercially available as Uvicure S160 from Lambson (a liquid difunctional oxetane resin having viscosity 240-300 mPa s at 25°C).

[0026] The oxetane resin may be present in an amount of 0.1% to 20%, including 5% to 13%, by weight based on the total weight of the resin composition.Photoinitiators

[0027] Suitable cationic photoinitiators are sulfonium salts and iodonium salts that can generate Bronsted acid. Iodonium salts may be selected from diaryl iodonium salts, triaryl iodonium salts, aromatic iodonium salts, and any combination thereof. Sulfonium salts may be selected from aromatic sulfonium salts.

[0028] Examples of suitable sulfonium salts are: (sulfanediyldibenezene-4,1-diyl) bis(diphenylsulfonium) bis(hexafluoroantimonate) (commercial available as Speedcure 976sfrom Lambson); (sulfanediyldibenezene-4,1-diyl) bis(diphenylsulfonium) bis(hexafluoroantimonate) (commercial available as Speedcure 976D from Lambson); and (4-{[4-(diphenylsulfanylium)phenyl]sulfanyl}phenyl)diphenylsulfonium bishexafluoro- phosphate in propylene carbonate (commercial available as Speedcure 992 from Lambson).

[0029] Examples of suitable iodonium salts are: Bis(4-dodecylphenyl)iodonium hexafluoroantimonate (commercial available as Speedcure 937 from Lambson); Bis-(4-t-butylphenyl)-lodonium hexafluorophosphate (commercial available as Speedcure 938 and Speedcure 938D in a reactive solvent from Lambson); and 4-lsopropyl-4’- methyldiphenyliodonium Tetrakis(pentafluorophenyl)borate (commercial available as Speedcure 939 from Lambson).

[0030] The amount of cationic photoinitiator in the resin matrix may be within the range of 0.1%-5% by weight based on the total weight of the resin composition.

[0031] In addition to the above mentioned cationic photoinitiator, the cationic cure resin composition may further include a Type I photoinitiator, in an amount of 0.01% to 1% in weight, based on the total weight of the resin composition, in order to catalyze the reaction. The Type I photoinitiator will be described in more detailed subsequently in reference to the free-radical cure resin composition.Photosensitizers

[0032] In some embodiments, photosensitizers (or sensitizers) are added to increase the rate of photoinitiated polymerization or shift the wavelength at which polymerization occurs. When a photosensitizer is used, photoinitiators absorbing at shorter wavelengths can be used in combination.

[0033] Suitable photosensitizers include anthracenes, xanthones, thioxanthones, benzophenones, acetophenones, titanocene, methanones, xanthenones, pyrenemethanols, pyrene, perylene, quinones, benzoyl esters, and any combination thereof. Specific examples of anthracenes are 9,10-Dibutoxyanthracene and 9,10-Diethoxyanthracene. Examples of commercially available anthracene photosensitizers are those in the ANTHRACURE UVS series from Kawasaki Kasei Chemicals Ltd.

[0034] The amount of photosensitizer in the resin matrix may be within the range of 0.01% to 5% by weight based on the total weight of the resin composition.Fillers and Flow Control Agents

[0035] Inorganic fillers or flow control agents in particulate form (e.g., powder) may be added to the resin composition of the curable resin matrix to modify rheology or impart additional properties such as heat resistance, fire resistance or conductivity to the resin matrix. Inorganic fillers may include fumed silica, ceramic microspheres, calcium carbonate, alumina, and any combination thereof. The ceramic microspheres are small, spherical, hollow bodies. Each microsphere consists of an outer shell enclosing a hollow core. An example of commercially available ceramic microspheres are sold by Zeelan Industries, Inc. under the trade name Zeeospheres®, for example, G-200, G210 and W-200.

[0036] The amount of inorganic filler or flow control agent in the resin composition may be within the range of 0.1% to 5% by weight based on the total weight of the resin composition.Free Radical Cure Resin

[0037] The free radical cure resin composition for the purpose disclosed herein contains a low-viscosity (meth)acrylate monomer, a solid acrylate polymer or a solid cellulose ester polymer, a dendritic acrylate oligomer, a free radical photoinitiator, and a toughening component.

[0038] According to one embodiment, the free radical cure resin composition includes the following components:(a) a free-radical polymerizable, low-viscosity (meth)acrylate monomer or oligomer having a viscosity of less than 24,000 cP (mPa-s) at 60°C;(b) a methyl methacrylate homopolymer or copolymer that is a solid at room temperature or a cellulose ester polymer that is a solid at room temperature, and preferably, the cellulose ester polymer has a Tgof greater than 100°C, where Tgis determined by a rheological method using a rheometer in torsional mode;(c) thioether dendritic acrylate;(d) one or more free-radical photoinitiator(s); and(e) a toughening component selected from: polyacrylate liquid elastomers, epoxidized polybutadiene, acrylic block copolymers, and combinations thereof.

[0039] The term "(meth)acrylate" as used herein refers to both acrylate and methacrylate, as well as acrylic acid and methacrylic acid. As used herein, the term “room temperature” refers to a temperature in the range of 20°C to 25°C.

[0040] The free radical polymerizable monomer or oligomer may be selected from urethane (meth)acrylates, epoxy (meth)acrylates with no epoxy functionality, and polyester (meth)acrylates. In some embodiments, the free-radical polymerizable acrylate or methacrylate monomer or oligomer is a urethane (meth)acrylate monomer or oligomer having a functionality of 1 to 6.

[0041] The free radical cure resin composition may further include one or more additional components selected from: inorganic fillers or flow control agents in particulate form (e.g., powder), acrylate functional impact modiiers, and wetting agents.

[0042] It is preferred that the free radical cure resin composition is free of epoxy resin. Thus, it is distinguishable from the hybrid resin compositions that contain polymerizable acrylates, epoxy resins, photoinitiators and epoxy curing agents. Such hybrid resin compositions contain both a UV curable constituent and a thermally curable constituent.

[0043] The following Embodiments A and B are representative of the radical cure resin compositions:Embodiment AEmbodiment BThe amounts shown are in “wt%”, which refers to weight percentage based on the total weight of the resin composition.(Meth)acrylate Monomers

[0044] The (meth)acrylate monomer may be chosen from alkyl acrylates and / or alkyl methacrylates. The monomers may be aliphatic, linear and / or branched acrylic and / or methacrylic monomers, and / or cyclic methacrylate monomers, and / or aromatic methacrylate monomers. Preferably, the (meth)acrylic monomer is selected from acrylic acid, methacrylic acid, alkyl acrylic monomers, alkyl methacrylic monomers and mixtures thereof, the alkyl group containing from 1 to 22 linear, branched or cyclic carbons; the alkyl group preferably containing from 1 to 12 linear, branched or cyclic carbons.

[0045] The (meth)acrylate monomers may have functionality of 1 to 6, i.e. , monofunctional up to hexafunctional.

[0046] One or more low-viscosity or liquid (meth)acrylate monomer(s) may be added to the free radical cure resin composition as a diluent to lower the viscosity of the resin composition. Such diluent monomer is in liquid form at room temperature or has a viscosityof less than 100 cP at 25°C.

[0047] An example of a suitable acrylate diluent monomer is EBECRYL® I BOA (Isobornyl acrylate) supplied by Allnex, a monofunctional liquid monomer which polymerizes when exposed to sources of free radicals. Another example is Sartomer SR 351 LV (Trimethylolpropane triacrylate or TMPTA) supplied by Sartomer, a tri-functional liquid monomer.(Meth)acrylate Oligomers

[0048] As used herein, an "oligomer" is a polymeric unit containing 2 to approximately 100 monomer units, i.e. the degree of polymerization is 2 to approximately 100.

[0049] The oligomers may be monofunctional or multifunctional (meth)acrylates. The oligomers may contain an aliphatic or aromatic backbone structure.

[0050] Urethane (meth)acrylate oligomers are particularly suitable. Urethane (meth)acrylate is, in general, a reaction product of at least one hydroxyl functional methacrylate and an isocyanate, along with optional alcohols or diols in the presence of a catalyst. In general, urethane (meth) acrylates have excellent weathering characteristics and good abrasion resistance. Additionally they have good adhesion to plastic and metals and impart chemical and solvent resistance.

[0051] Examples of commercially available urethane acrylate oligomers include: EBECRYL® 8606, an aliphatic urethane tetra-acrylate; EBECRYL® 4859, a difunctional aliphatic urethane methacrylate; EBECRYL® 5781 , an aliphatic urethane diacrylate;EBECRYL® 5129, a hexafunctional aliphatic urethane acrylate, all from Allnex; and Bomar® BR-941, a hexafunctional, aliphatic polyether urethane acrylate supplied by DYMAX.

[0052] Epoxy (meth)acrylates have no epoxy functionality, but can have hydroxyl functionality. They are fast curing resins with excellent chemical resistance, adhesion and high hardness. Examples of commercially available epoxy (meth)acrylates include Sartomer® CN120A75 and Allnex EBECRYL 3708, EBECRYL 605 (modified Bisphenol A epoxy diacrylates) and the like.

[0053] Polyester (meth) acrylates are synthesized from organic acids and polyols. They have lower viscosities and excellent scratch and abrasion resistance. They are fast curing and have reasonable exterior durabilities.

[0054] Examples of commercially available polyester acrylates include: Sartomer®CN704, Sartomer® CN2610, Sartomer®203 from Arkema, Ebecryl® 837, Ebecryl® 892, and Ebecryl® 812 from Allnex.Solid Methacrylate and Cellulose Ester Polymer

[0055] Examples of commercially available solid methacrylates include: Elvacite 4026, a solid methylmethacrylate copolymer having Tgof 75°C after curing; and NeoCryl® B-728, a methylmethacrylate homopolymer that is a solid at room temperature and has a Tg of 111°C after curing.

[0056] An example of a suitable cellulose ester polymer is Bomar JL-106E, an acrylamidomethyl-substituted cellulose ester polymer that is a solid at room temperature and has a Tg of 118°C after curing.Dendritic Acrylate

[0057] The thioether dendritic acrylate is a dendritic oligomer with a hyper-branched structure and a large number of reactive functional groups. Here, the term "dendritic" refers to a shape in which a monomer is branched in a radiating form from one nucleus and polymerizes and spreads radially. The preferred thioether dendritic acrylate has a functionality of at least 15, more specifically, 15 to 50. In a preferred embodiment, the thioether dendritic acrylate has a functionality of 30 and a Tgof greater than 350°C (662°F) upon curing, where Tg is determined by Dynamic mechanical analysis (DMA). As an example, such 30-functional thioether dendritic acrylate is commercially available as BDT- 4330 from Bomar (formerly Dymax Oligomers & Coatings). When the thioether dendritic acrylate is incorporated into the curable resin matrix formulation, it is possible to achieve a dry Tg of greater than 200°C, for example, 240°C-250°C for the cured, hardened matrix.Free-Radical Photoinitiators

[0058] Free radical photoinitiators can be classified as Type I and Type II. Type I photoinitiators cleave upon exposure to radiation, producing radical species which are capable of initiating the polymerisation of unsaturated compounds. Type II photoinitiators are compounds which do not fragment upon exposure to radiation and will not typically initiate radical-chain polymerisation unless a co-initiator is present. Upon exposure to radiation, interaction between the Type II photoinitiator and the co-initiator leads to the generation of radical species which can initiate the polymerisation of UV-curable resins.

[0059] Suitable Type I photoinitiators include: acetophenones; hydroxyacetophenones; aminoacetophenones; and phosphine oxides.

[0060] Specific examples are: 2-hydroxy-2-methyl-1 -phenylpropanone (SpeedCure®73); 1 -hydroxycyclohexyl phenyl ketone (SpeedCure® 84); 1-[4-(2-hydroxyethoxy)-phenyl]- 2-hydroxy-2-methyl-1-propane-1-one (SpeedCure® 2959); 2,2-dimethoxy-2- phenylacetophenone (SpeedCure® BKL); 2-methyl-1-[4-(methylthio)phenyl]2- morpholinopropan-1-one (SpeedCure® 97); 2-Benzyl-2-dimethylamino-4- morpholinobutyrophenone (SpeedCure® BDMB); 2,4,6-trimethylbenzoyl- diphenylphosphine oxide (SpeedCure® TPO); and Phenyl bis(2,4,6-trimethylbenzoyl)-phosphine oxide (SpeedCure® BPO).

[0061] Suitable Type II photoinitiators include: benzophenones and substituted benzophenones; benzoylformates; and thioxanthones.

[0062] Specific examples are: benzophenone (SpeedCure® BP); 4-methyl- benzophenone (SpeedCure® MBP); Methyl-2-benzoylbenzoate (SpeedCure® MBB); 4- phenylbenzophenone (SpeedCure® PBZ); Methylbenzoylformate (SpeedCure® MBF); 2- isopropylthioxanthone (SpeedCure® 2-ITX); 1-chloro-4-propoxythioxanthone (SpeedCure® CPTX); and 2,4-diethylthioxanthone (SpeedCure® DETX).

[0063] The photoinitiator(s) may be used in conjunction with an amine synergist. Suitable amine synergists are aminobenzoates. Specific examples are: ethyl-4- (dimethylamino)benzoate (SpeedCure® EDB); 2-ethylhexyl-4-(dimethylamino)benzoate (SpeedCure® EHA); and 2-butoxyethyl-4-(dimethylamino)- benzoate (SpeedCure® BEDB). All SpeedCure products mentioned herein are available from Lambson.Acrylate Functional Impact Modifier

[0064] Acrylate functional impact modifier may be added to the free radical cure resin composition to provide additional toughening without impeding the cure properties due to the acrylate functionality. If present, the amount of impact modifier is up to 10 wt% based on the total weight of the resin composition.

[0065] An example is Paraloid 2691A available from Dow Plastics, a Methacrylate- Butadiene-Styrene (MBS) polymer in the form of particles. Methyl methacrylatestyrene (MBS), also known as impact modified styrene methyl methacrylate (SMMA), has rubber particles dispersed throughout its methyl methacrylate-styrene polymer matrix. It is a clear engineering and commodity thermoplastic that has excellent transparency, high impactstrength, good stiffness and good resistance to many chemicals.Wetting Agents

[0066] A wetting agent may be added to the cationic cure or free-radical cure resin composition to improve surface adhesion to composite or metallic substrates, leading to better adhesion properties. The amount of wetting agent may be up to 5 wt% based on the total weight of the resin composition.

[0067] A suitable wetting agent, in general, consists of a hydrophilic segment and a hydrophobic segment. Wetting agents utilized for this application can be of the following types: siloxanes, polyethermodified siloxanes, sulfosuccinates, Gemini surfactants (dimeric structures, composed of two hydrophobic chains and two hydrophilic heads, linked by a spacer at or near the head groups), and alcohol alkoxylates.

[0068] One commercially available example is SURFYNOL® AS 5080 available from Evonik, an organic wetting agent composed of a blend of Gemini surfactants. Such wetting agent has the ability to reduce both eguilibrium and dynamic surface tension. Another example of a suitable wetting agent is BYK-3763 from BYK, a silicone-containing surface additive, which improves substrate wetting and prevents surface defects. Such wetting agent is a solution of a polyether-modified polydimethylsiloxane. An example of an alcohol alkoxylate is SURFYNOL® AS 5180 from Evonik.Fillers and Flow Control Agents

[0069] The inorganic fillers and flow control agents that may be added to the free radical cure resin composition are the same as those described above for the the cationic cure resin composition. If present, the amount of fillers or flow control agent may be up to 10 wt% based on the total weight of the resin composition.Toughening Component

[0070] The toughening component used in the cationic cure resin composition and the free radical cure resin composition may be selected from: polyacrylate liquid elastomers, epoxidized polybutadiene, polyarylsulphone polymers, polyetherdiamine, and acrylic block copolymers.

[0071] Polyacrylate liquid elastomer is a liquid without solvent at room temperature. Particularly suitable is (meth)acrylate-functionalized poly(acrylate).

[0072] The (meth)acrylate-functionalized poly(acrylate) desirably has at least two(meth)acrylate functional groups, is the combination of two or more mono-functionalized (meth)acrylate-functionalized poly(acrylate)s, or is a mono-functionalized (meth)acrylate- functionalized poly(acrylate) in combination with a (meth)acrylate-functionalized poly(acrylate) having at least two (meth)acrylate functional groups.

[0073] The (meth)acrylate-functionalized poly(acrylate)s may be derived from a blend of at least 80 percent by weight of certain mono-functionalized (meth)acrylate monomers selected from alkyl, arylalkyl, hydroxy alkyl, and phenyl (meth)acrylates and 0-20 percent by weight of mono-functional, non-(meth)acrylate vinyl monomers selected from maleic anhydride, maleate, fumarate and vinyl esters, acrylonitrile, styrene, butadiene, vinylazlactone, N,N-dimethylacrylamide, N-vinylpyrrolidinone, acryloyl morpholine, N- vinylcaprolactam and N-vinylformate.

[0074] The (meth)acrylate-functionalized poly(acrylate) may in other words be defined with respect to the following backbone:-(A)x-(B)y- where A represents one or more co-monomer units selected from non-functional alkyl or arylalkyl mono (meth)acrylates and B represents one or more co-monomer units selected from alkyl or arylalkyl mono-functionalized (meth)acrylates selected from alkoxyalkyl or aryloxyalkyl with or without substitution with groups containing the hetero atoms, O, S, N, P, or halogen, or non-(meth)acrylate vinyl monomers such as maleic anhydride, where x and y represent the fraction of A and B, respectively, and x is at least 0.30 and at most 0.95 and y is at least 0.05 and at most 0.70. While the structure above suggests a block copolymer, random incorporation of the A and B units distributed within the polymer backbone may occur, and may be preferable.

[0075] The (meth)acrylate-functionalized poly(acrylate) may have a number average molecular weight (Mn) in the range of 1,000-100,000, such as in the range of 5,000-80,000, or 5000-50,000 as determined by Gel Permeation Chromatography (GPC) method.

[0076] The GPC measurement can be carried out with a polystyrene gel column using chloroform as the mobile phase, and the number average molecular weight (Mn) can be determined relative to polystyrene standards.

[0077] Examples of suitable (meth)acrylate-functionalized poly(acrylate) liquid elastomers are those supplied by Kaneka Corporation under the trade designation, XMAP, such as XMAP RC100C and XMAP SA100S.

[0078] The epoxidized polybutadiene is formed by by introducing an epoxy group through oxidation of the vinyl group of 1,2-polybutadiene. The preferred epoxidized polybutadiene has following chemical formula:where m = 4-11 ; n = 12-35.

[0079] Examples of commercially available products are JP-100 (viscosity of 220 Poise at 45°C) and JP-200 (viscosity of 1000 Poise at 45°C) from Nippon Soda Co., Ltd.

[0080] Polyarylsulphone polymers include polyether sulfone (PES), polyether ether sulfone (PEES), and a copolymer of PES and PEES,

[0081] The preferred polyetherdiamine has a weight average molecular weight (MW) of 10,000 g / mol or higher as determined by gel permeation chromatography (GPC).Particularly suitable is an amine-functionalized butyl ether elastomer having the following chemical structure:H2N - ■ CH2CH2CH2CH2O- CH2CH2CH2CH2NH2 LJn where n is the degree of polymerization and is an integer from 50 to 150.

[0082] An example of a suitable polyetherdiamine is Poly(Oxy-1,4-Butanediyl), Alpha-(4- Amino-Butyl)-Omega-(4-Aminobutoxy), commercially available as Dynamar™ HC 1101 from Dyneon, a division of 3M, with MW of about 10,000 g / mol.

[0083] Suitable acrylic block copolymers are those sold under the commercial name Nanostrength® supplied by ARKEMA. A preferred block copolymer is Nanostrength® M52N, a poly(methyl methacrylate-butyl acrylate-methyl methacrylate) (PMMA-b-PBA-b- PMMA) block copolymer, which is a tri-block copolymer constituting of a center block of poly (butyl acrylate) and two side blocks of poly (methyl methacrylate).Reinforcement Fibers

[0084] The reinforcement fibers for the shim may be in the form of a woven or nonwoven fabric. The woven fabric is composed of continuous fibers in a weaving pattern. Theweaving pattern is not limited and may be plain weave, twill weave, basket weave, satin weave and the like. The nonwoven fabric is composed of randomly oriented fibers. The fibers may be selected from glass fibers, carbon fibers, aramid fibers, quartz fibers, polyester fibers, and combinations thereof.

[0085] The areal weight of the reinforcement fabric may be in the range of 500 to 1500 g / m2(or gsm). In some embodiments, the fiber reinforcement is a heavy, woven glass fabric such as a twill woven E-glass fabric having an areal weight of 876 gsm.

[0086] The resin content relative to fiber reinforcement may be between 30% and 70% by weight, preferably in the 50% resin content range. The thickness of the UV cure coated material may be between 0.01 in and 0.06 inches.Methods of Making

[0087] The components of the cationic cure or free radical cure resin composition may be mixed, for example, by adding them to a high speed mixing vessel with a shear blade. Liquid resins may be added and stirred at room temperature. For the addition of solid acrylate resin, cellulose ester polymer or tougheners, the temperature of the mixture would need to be raised to the melting temperature of the components, e.g., 150°F-250°F (or 65°C- 121 °C). Once a homogenous mixture is obtained, the resin composition can be coated onto a reinforcement fabric using a conventional film coating technique, creating a supported, UV curable resin film. The coating temperature can be in the range of 150°F-250°F (or 65°C- 121°C).

[0088] A fabric may be impregnated with the resin film to form a UV curable shim, also called “prepreg”. Impregnation may be done by continuously moving a fabric web and pressing a continuous resin film against on side of the fabric web or pressing two resin films on opposite sides of the fabric web. Impregnation may be facilitated by the application of heat and pressure, for example, 150°F-180°F (or 65.5°C-82.2°C) and 40-60 psi of pressure. The conditions during impregnation does not result in a cured resin.

[0089] Following impregnation of the fabric with resin film(s), the UV curable prepreg can be cut, shaped and prepared for UV curing or alternatively stored at room temperature conditions until ready for use. The UV curable prepreg is tacky to the touch and can be easily placed onto a substrate such as a composite or metallic substrate. The tacky prepreg can still be removed and modified if needed prior to curing.

[0090] Any conventional UV light source may be used for UV curing, for example,mercury vapor lamps or UV LED lamps. The type, intensity and wavelength of the UV lamp can vary. The wavelength of the UV lamp can be between 100 and 400 nm. Preferably, the UV light will be a light source in the UVA range (315-400 nm). The lamp power may be 50 watts to 500 watts. The distance of the UV lamp from the substrate can vary dependent on the intensity of the light source.

[0091] For shims containing cationic cure resin compositions, the cure time may be from 1 minute to 1 hour, plus a possible additional post cure or dark cure period. For shims containing free radical cure resins, the cure time may be from 30 seconds to 10 minutes. The shims in sheet form (also referred herein as “plies”) can be stacked and cured so that multiple plies can be cured simultaneously. The number of plies is dependent on light intensity of the UV lamp. For example, the number of plies may be 2 to 5.EXAMPLESExample 1Free radical curing formulations

[0092] Table 1 shows resin compositions that were formulated for free radical curing. Amounts are reported in weight percentage (wt%) based on the total weight of the resin composition.TABLE 1

[0093] The components of each resin composition was mixed using a high-speed mixer with a high shear blade. In order to dissolve the solid acrylate resins, the mixture was heated to 230°F (110°C). The components except for the photoinitiators were blended until homogeneous or about 30 minutes. Then the photoinitiator was added as the last step and blended for about 5 minutes.

[0094] To form shims from the resin compositions disclosed in Table 1, each resin composition was de-aired following mixing and then coated at 200°F (93.3°C) to form two UV curable resin films. Following coating, a fabric carrier was placed between the two UV curable films. The resin films were pressed into the fabric carrier using a 30-minute hold at 180°F (82.2°C) and 60 psi.

[0095] Each of the resin compositions disclosed in Table 1 was cured and assessed to determine the glass transition temperature (Tg). Tgwas measured using an AR 2000EX rheometer in torsional rectangular mode and the Tgresults are reported in Table 2. For this test, each resin composition placed in a 50mm x 12mm x 3mm mold and cured by placing the mold under UV light for 2 minutes to ensure a full cure. The cured sample was measured in torsional mode from 23°C to 275°C with a 0.05% strain. To determine wet Tg, the cured sample was conditioned at 160°F (70°C) and 95% relative humidity for 30 days.TABLE 2

[0096] The results in Table 2 show that Resins A, B, C , D and E were able to be fully cured in 30 seconds to 5 minutes and exhibited very high glass transition temperature for UV cured materials, Tgvalues of up to 450°F (232°C). Such values are higher than those typical for acrylate based materials, which usually have Tg values of up to 350°F (176°C).

[0097] Cured resin B was further tested to determine its resistance to fluid exposure and the results are reported in Table 3. The solvents for fluid exposure in Table 3 were chosen based on common fluids that can be encountered in aerospace and industrial applications. JP-4 is a jet fuel / jet propellant containing a mixture of aliphatic and aromatic hydrocarbons, corrosion inhibitor and anti-icing additives. Skydrol, from Eastman, is a fire-resistance hydraulic fluid that is made of fire-resistance phosphate ester base stock, with a number of oil additives to inhibit corrosion.TABLE 3Example 2Cationic curing formulations

[0098] Five resin compositions (Resins F-J) suitable for cationic curing were prepared according to the formulations shown in Table 4. Amounts are reported in weight percentage (wt%) based on the total weight of the resin composition.

[0099] The components of each resin composition were mixed using a high-speed mixer with a high shear blade. In order to dissolve the solid components, the mixture was heated to 230°F (110°C). The components except for the photoinitiators were blended until homogeneous or about 30 minutes. The photoinitiator was added as the last step and blended with the other components for about 5 minutes.[000100] Each of the resin compositions disclosed in Table 4 was cured by placing each resin composition in a 50mm x 12mm x 3mm mold and placing the mold under UV light for 2 minutes to ensure a full cure (i.e. , 100% degree of cure) and the curing time is also reported in Table 4.TABLE 4[000101] The cationic cure compositions in Table 4 differ from commercially available, thermally curable epoxy-based resins because of their reduced curing time. Resins F-J all cure in less than 60 minutes when exposed to UV light. Commercially available epoxy based composite materials usually require curing time of greater than 60 minutes, more typically from 90 minutes to 360 minutes, for example, Solvay MTM® 46 prepreg, Toray 2511 prepreg, and Hexcel HexPly® M21 prepreg. Consequently, the faster curing times provided by UV curable resins of Table 4 lead to increased production and processing rates. The higher throughput and continuous processing in turn leads to lower costs and lower energy use for a more sustainable process.[000102] To form shims from the resin compositions disclosed in Table 4, each resin composition was de-aired following mixing and then coated at 200°F (93.3°C) to form twoUV curable resin films. Following coating, a fabric carrier was placed between the two UV curable films. The resin films were pressed into the fabric carrier using a 30 min hold at 180°F (82.2°C) at 60 psi.Example 3Comparative free radical curing formulations[000103] For comparison, resin compositions (Mixes 1-4) suitable for free radical curing were prepared according to the formulations disclosed in Tables 5 and 6. Amounts are reported in weight percentage (wt%) based on the total weight of the resin composition.[000104] The resulting resins after mixing were cured via UV radiation and the Tgof the cured materials was measured and reported in Tables 5 and 6. The Tg was measured using the AR 2000EX rheometer in torsional rectangular mode. To determine wet Tg, the cured sample was conditioned at 160°F (70°C) and 95% relative humidity for 30 days.TABLE 5TABLE 6[000105] The cured resins based on the resin compositions disclosed in Tables 5 and 6 (Mixes 1-4) display much lower dry and wet Tgas compared to the cured resins based on the resin compositions shown in Table 1. The lower Tg values limit the applications of such resins in many aerospace and industrial applications, which require robust and high strength materials. Furthermore, the lower Tg values (which are typical for most free radical curing acrylate formulations) have widely limited the use of UV curing for certain applications, such as aerospace applications.

Claims

CLAIMSWhat is claimed is:

1. An ultraviolet (UV) curable shim comprising a layer of reinforcement fibers embedded in or impregnated with a curable resin matrix, wherein said curable resin matrix is in a solid state at room temperature (20-25 °C) and comprises:(a) at least one free-radical polymerizable acrylate or methacrylate monomer or oligomer having a viscosity of less than 24,000 cP at 60°C;(b) a methyl methacrylate homopolymer or copolymer that is a solid at room temperature or a cellulose ester polymer that is a solid at room temperature;(c) thioether dendritic acrylate;(d) one or more free-radical photoinitiator(s); and(e) a toughening component selected from: polyacrylate liquid elastomers, epoxidized polybutadiene, and acrylic block copolymers, and combinations thereof.

2. The UV curable shim according to claim 2, wherein the at least one free-radical polymerizable acrylate or methacrylate monomer or oligomer (a) is a urethane (meth)acrylate having a functionality of 1 to 6.

3. The UV curable shim according to claim 1 or 2, wherein cellulose ester polymer has a Tgof greater than 100°C, where Tgis determined by a rheological method using a rheometer in torsional mode4. The UV curable shim according to any one of the preceding claims, wherein the at least one free-radical polymerizable acrylate or methacrylate monomer is in liquid form at room temperature or has a viscosity of less than 100 cP at 25°C.

5. The UV curable shim according to any one of the preceding claims, wherein the thioether dendritic acrylate has a functionality of at least 15, preferably, a functionality of 30.

6. The UV curable shim according to any one of the preceding claims, wherein the toughening component is (meth)acrylate-functionalized poly(acrylate).

7. The UV curable shim according to any one of claims 1 to 5, wherein the toughening component is epoxidized polybutadiene having following chemical formula:where m = 4-11 ; n = 12-35.

8. The UV curable shim according to any one of claims 1 to 5, wherein the toughening component is a poly(methyl methacrylate-butyl acrylate-methyl methacrylate) (or PMMA-b- PBA-b-PMMA) tri-block copolymer.

9. The UV curable shim according to any one of the preceding claims, wherein the free radical photoinitiator(s) is / are selected from Type I and Type II photoinitiators, preferably, Type I photoinitiators are selected from: acetophenones, hydroxyacetophenones, aminoacetophenones, and phosphine oxides, and Type II photoinitiators are selected from: benzophenones and substituted benzophenones, benzoylformates, and thioxanthones.

10. The UV curable shim according to any one of the preceding claims, wherein relative amounts of components (a) to (e), in weight percentages (wt%) based on the total weight of the resin matrix, are as follows:(a) 1-10 wt% free-radical polymerizable acrylate or methacrylate having a viscosity of less than 24,000 cP at 60°C;(b) 15-30 wt% methyl methacrylate homopolymer or copolymer that is a solid at room temperature or a cellulose ester polymer that is a solid at room temperature;(c) 15-50 wt% thioether dendritic acrylate;(d) 0.1-10 wt% free-radical photoinitiator(s); and(e) 2-10 wt% toughening component.

11. The UV curable shim according to any one of the preceding claims, wherein the curable resin matrix further comprises at least one polymerizable oligomer containing 2 to 100 monomer units and selected from urethane (meth)acrylates, epoxy (meth)acrylates with no epoxy functionality, and polyester (meth)acrylates, preferably, in an amount of 25-50 wt% based on the total weight of the resin matrix.

12. The UV curable shim according to any one of the preceding claims, wherein the curable resin matrix further comprises Methacrylate-Butadiene-Styrene (MBS) polymer, preferably, in an amount of 0.1-10 wt% based on the total weight of the resin matrix.

13. The UV curable shim according to any one of the preceding claims, wherein the curable resin matrix further comprises a wetting agent, preferably, in an amount of 0.1-5 wt% based on the total weight of the resin matrix.

14. The UV curable shim according to claim 13, wherein the wetting agent is selected from: siloxanes, polyethermodified siloxanes, sulfosuccinates, Gemini surfactants (dimericstructures, composed of two hydrophobic chains and two hydrophilic heads, linked by a spacer at or near the head groups), and alcohol alkoxylates.

15. The UV curable shim according to any one of the preceding claims, wherein the curable resin matrix further comprises aminobenzoate, preferably, in an amount of 0.1-5 wt% based on the total weight of the resin matrix.

16. The UV curable shim according to any one of the preceding claims, wherein the curable resin matrix further comprises an inorganic filler in particulate form, preferably, in an amount of 0.1-10 wt% based on the total weight of the resin matrix.

17. The UV curable shim according to any one of the preceding claims, wherein the inorganic filler is selected from: fumed silica, ceramic microspheres, calcium carbonate, alumina, and any combination thereof.

18. The UV curable shim according to any one of the preceding claims, wherein the curable resin matrix is free of epoxy resin.

19. An ultraviolet (UV) curable shim comprising a layer of reinforcement fibers embedded in or impregnated with a curable resin matrix, wherein said curable resin matrix is in a solid state at room temperature (20-25 °C) and comprises:(a) a combination two or more cycloaliphatic epoxy resins, at least one of which is a solid at room temperature (20-25 °C);(b) an oxetane resin;(c) a cationic photoinitiator selected from sulfonium and iodonium salts; and(d) a toughening component selected from: polyacrylate liquid elastomers, epoxidized polybutadiene, polysulfone polymers, acrylic block copolymers, and combinations thereof; and wherein the curable resin matrix is devoid of any epoxy curative (or curing agent) containing reactive amino group(s) that can crosslink with the cycloaliphatic epoxy resins.

20. The UV curable shim of claim 19, wherein relative amounts of the components, in weight percentage based on the total weight of the resin matrix, are as follows:(a) 65% to 75% cycloaliphatic epoxy resins combined;(b) 5% to 13% oxetane resin;(c) 0.1 % to 5% cationic photoinitiator;(d) 10% to 25% toughening component.

21. The UV curable shim according to claim 19 or 20, wherein the tougheningT1component comprises a polyacrylate liquid elastomer and the resin matrix is devoid of any other polymerizable acrylate or methacrylate.

22. The UV curable shim according to any one of claims 19 to 21, wherein the toughening component comprises an epoxidized polybutadiene having following chemical formula:where m = 4-11; n = 12-35.

23. The UV curable shim according to any one of claims 19 to 21, wherein the toughening component comprises a copolymer of polyethersulfone and polyetherethersulfone (PES-PEES).

24. The UV curable shim according to any one of claims 19 to 21, wherein the toughening component comprises a poly(methyl methacrylate-butyl acrylate-methyl methacrylate) (PMMA-b-PBA-b-PMMA) tri-block copolymer and is devoid of any other polymerizable acrylate or methacrylate.

25. An ultraviolet (UV) curable shim comprising a layer of reinforcement fibers embedded in or impregnated with a curable resin matrix, wherein said curable resin matrix is in a solid state at room temperature (20-25 °C) and comprises:(a) a combination two or more cycloaliphatic epoxy resins, at least one of which is a solid at room temperature (20-25 °C);(b) an oxetane resin;(c) a cationic photoinitiator selected from sulfonium and iodonium salts; and(d) a polyetherdiamine having the following chemical structure:where n is an integer from 50 to 150, and wherein the polyetherdiamine is the only amine compound with reactive amine group in the curable resin matrix.

26. The UV curable shim according to claim 25, wherein relative weight percentage ofthe components, based on the total weight of the resin matrix, are as follows:(a) 65% to 75% cycloaliphatic epoxy resins combined;(b) 5% to 13% oxetane resin;(c) 0.1 % to 5% cationic photoinitiator;(d) 10% to 25% toughening component.

27. The UV curable shim according to claim 25 or 26, wherein the curable resin matrix is devoid of any polymerizable acrylate or methacrylate.

28. The UV curable shim according to any one of claims 19 to 27, wherein the solid cycloaliphatic epoxy resins is poly[(2-oxiranyl)-1 ,2cyclohexanediol] 2-ethyl-2- (hydroxymethyl)-l ,3-propanediol ether).

29. The UV curable shim according to any one of claims 19 to 28, wherein the combination of cycloaliphatic epoxy resins includes a cycloaliphatic epoxy resin that is a liquid at room temperature (20-25 °C).

30. The UV curable shim according to claim 29, wherein the liquid cycloaliphatic epoxy resin is3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylatebis(3,4-epoxycyclohexylmethyl) adipate, or a combination thereof.

31. The UV curable shim according to any one of claims 19 to 30, wherein the curable resin matrix further comprises a photosensitizer, preferably, in an amount of 0.01% to 5% by weight based on the total weight of the resin matrix.

32. The UV curable shim according to claim 31 , wherein the photosensitizer is selected from: anthracenes, xanthones, thioxanthones, benzophenones, acetophenones, titanocene, methanones, xanthenones, pyrenemethanols, pyrene, perylene, quinones, benzoyl esters, and any combination thereof.

33. The UV curable shim according to any one of claims 19 to 32, wherein the curable resin matrix further comprises an inorganic filler in particulate form, preferably, in an amount of 0.1% to 5% by weight based on the total weight of the resin matrix.

34. The UV curable shim according to claim 33, wherein the inorganic filler is selected from: fumed silica, ceramic microspheres, calcium carbonate, alumina, and any combination thereof.

35. The UV curable shim according to any one of the preceding claims, wherein the layer of reinforcement fibers is in the form of a woven or nonwoven fabric, preferably, woven fabric.

36. The UV curable shim according to any one of the preceding claims, wherein the reinforcement fibers are selected from glass fibers, carbon fibers, aramid fibers, quartz fibers, polyester fibers, and combinations thereof.