Polymeric composition for pre-impregnated fiber- reinforced composite materials and composite materials obtained therewith

Abstract

There are described a polymeric composition comprising inorganic and optionally organic components, useful as a binder for pre-impregnated fiber-reinforced composite materials, the pre-impregnated fiber-reinforced composite materials obtained using the polymeric composition, and the products obtained by forming and completely setting the aforesaid pre-impregnated fiber-reinforced composite materials. The products of the invention are lightweight, fire-resistant, and stable at high temperatures, with high resistance to yellowing by heat and / or ultraviolet radiation, and are suitable for the production of parts such as aircraft interiors, heat and / or fire barriers, or structural parts intended to operate at high temperatures.

Description

[0001] POLYMERIC COMPOSITION FOR PRE-IMPREGNATED FIBER-REINFORCED COMPOSITE MATERIALS AND COMPOSITE MATERIALS OBTAINED THEREWITH

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a polymeric composition comprising inorganic components and optionally organic components, useful as a binder for preimpregnated fiber-reinforced composite materials, to the pre-impregnated fiber- reinforced composite materials obtained using the polymeric composition, and to the products obtained by forming and completely hardening the aforesaid preimpregnated fiber-reinforced composite materials.

[0004] The composite material according to the present invention finds application as a base material for producing components that are lightweight, fire-resistant and stable at high temperatures, and with high resistance to yellowing by heat and / or ultraviolet radiation, such as, for instance, aircraft interiors, heat and / or fire barriers, or structural parts intended to operate at high temperatures.

[0005] BACKGROUND ART

[0006] As is known, there are many commercial products consisting of a polymeric composition, commonly known as the matrix, reinforced with fibers (in the field, the compact definition of “fiber-reinforced” matrix is also used).

[0007] These products are obtained by variously introducing fibers into the polymeric material of the matrix, which can be of the thermosetting or thermoplastic type, when this is still in the liquid or otherwise fluid state, and then causing the solidification (or hardening) of said polymeric material.

[0008] In the case of thermoplastic matrices, these are first processed by heating at temperatures above the typical glass transition temperature of the material. This allows the polymeric chains to move and slide over each other until the temperature is brought again below the glass transition temperature (Tg), in a reversible process.

[0009] In the case of thermosetting matrices, the processing is performed before an amount of chemical bonds is created between adjacent chains such as to prevent the relative motion thereof, leading the polymer to harden in a non-reversible process; this chemical mechanism is defined as cross-linking, and the solidification processes utilising it are known in the field by the term “curing”, which will be used in the description below. Typically, the thermosetting matrices are cross-linked (or “cured”) by thermal activation.

[0010] Following the solidification of the polymeric composition that impregnates the fibers, a continuous binding matrix is formed, which fixes the fibers together, limits the relative movements thereof, and distributes the mechanical stresses between fibers and matrix.

[0011] The reinforcements can consist of aramid fibers, carbon fibers, glass fibers or ceramic fibers; in a single product, combinations of multiple types of fibers can also be used.

[0012] A class of fiber-reinforced composite materials is the so-called aesthetic ones, having the feature of being stable to yellowing, of both thermal type and resulting from exposure to ultraviolet (UV) radiation. Aesthetic fiber-reinforced composite materials require the fully cured polymeric binding composition (resin) to be transparent and colourless. Furthermore, aesthetic fiber-reinforced composite materials must maintain transparency and colourlessness under the operating conditions of the components.

[0013] In the past, the production technique for fiber-reinforced polymeric material products consisted in arranging the fibers (in the form of fabrics, non-woven fabrics, unidirectional fibers) in one or more layers in a mould and manually impregnating them with the polymeric binding composition in the liquid or fluid state. This manual impregnation (mostly assisted by appropriate tools such as brushes, rollers, spatulas, and the like) is known in the field by the term “wet layup”.

[0014] In the second half of the 1980s, a new methodology was introduced for producing fiber-reinforced polymeric material products, in which the so-called preimpregnated or “prepregs” are used, consisting of sheets or rolls comprising fibers or reinforcing fabrics wetted (impregnated) with a polymeric composition using appropriate machinery; thanks to the composition of the polymeric matrix and the additives contained therein, these prepregs are suitable for long-term storage. Prepregs are commonly produced from thermosetting matrices. The thermosetting matrices used for producing prepregs are typically based on organic resins, generally epoxy, phenol, bismaleimide (BMI), or cyanate ester-based, and already contain all the components required to allow the cross-linking thereof; for this reason, these resins are referred to in the technical jargon as “single-component resins”. During the production of the prepregs, the thermosetting polymer matrix is not cross-linked or is only partially cross-linked, so as to make the prepreg handleable: the matrix must be sufficiently “solidified” so as not to drip from the fibers or not to be too fluid and sticky. In order to avoid the premature aging of the matrix, i.e. , partial premature cross-linking before use, the thermosetting matrix prepregs are stored at controlled temperature.

[0015] Operatively, the process of producing a prepreg involves an impregnation step in which the fibers are arranged on flat surfaces and in such a configuration they are impregnated with the polymeric composition. During such a step, the partial crosslinking of the thermosetting polymeric composition can occur at variable temperatures in the prior art, depending on the polymeric composition used. The material is then cooled to room temperature and, if it is to be stored for long periods of time, it is placed in a refrigerated storage. The prepreg is thus available in the form of rolls or flat workable sheets, which can then be processed to obtain components with even very complex shapes, e.g., by layering and subsequent shaping operations, thermal treating, and finishing operations.

[0016] Thermosetting resin-based prepregs acquire superior mechanical features after the complete cross-linking of the resins with which they are impregnated. Such an operation occurs during a step of hot forming the prepreg. The cross-linking of the resins can, in fact, be completely achieved during the forming step, or be completed in a subsequent post-curing step, if the forming step is conducted at lower temperatures and / or for not sufficiently long times.

[0017] In conventional prepregs, the cross-linked organic resins give the composite material obtained good mechanical resistance features.

[0018] Prepregs having organic resins as their matrix, once cured (and possibly postcured), cannot however operate at temperatures generally above 250 °C for long periods of time due to softening (the glass transition temperature, better known as Tg, is exceeded) and / or thermal degradation of the polymeric matrix. This limits the use thereof and the possibility of replacing metal materials, characterized by good mechanical properties but much heavier when compared to fiber-reinforced composite materials.

[0019] Moreover, a conventional prepreg based on organic resins generally has the following limitations: - low operating temperature of the composites obtained after complete crosslinking of the matrix; such composites, while having excellent mechanical resistance features, can hardly work above 250 °C in an oxidizing atmosphere for a long period;

[0020] - many organic resins (in particular, the epoxy ones) have low fire-resistance and emission of toxic fumes when exposed to flames. Fire-resistance can be improved with the addition of flame-retardant compounds (phosphates, hydrates, brominated compounds) and / or inert fillers serving the purpose of hindering the diffusion of oxygen and decreasing the mass fraction of flammable material, thus lowering the heat emitted during combustion. However, the addition of these additives deteriorates the mechanical properties of the composite material, increases the specific weight thereof, decreases the workability thereof, and in the case of brominated compounds, increases the toxicity of the combustion fumes;

[0021] - phenolic resins have good fire-resistance features without the necessity of adding additives, but they are toxic in the working environment and classified as carcinogenic.

[0022] In order to overcome these problems, the use of resins with a strong inorganic character has been suggested, typically silicon-based compounds (siloxane resins, silazane resins, etc.).

[0023] Matrices based on resins with a strong inorganic character have much higher fire-resistance features than organic ones, but they have the disadvantage of being mechanically less performant and require much longer curing and possible postcuring cycles than those usually in use in the industry, strongly limiting the possible uses thereof.

[0024] An example of prepreg composites with an inorganic matrix is reported in patent US 11 ,577,477 B2, which describes a prepreg obtained from siloxane and silsesquioxane resins with good workability, mechanical, and thermal stability features. In order to obtain products from the prepregs of this document, it is necessary to perform curing treatments at temperatures not less than 200 °C for a time between 60 and 150 minutes using the "compression moulding" technique at a pressure not less than 40 bar, or at temperatures between 150 °C and 300 °C at a pressure between 2 and 14 bar and for a time between 1 and 6 hours using the "vacuum bagging" technique. By summing the post-curing treatment times and those required for the heating ramps (in the patent, a heating rate of 2 °C / min is recommended to obtain good results), cycles with a duration of more than 10 hours are obtained, which thus involve significant energy consumption as compared to the curing cycles of organic resins, which require cycles at temperatures varying between 110 °C and 150 °C with total times between about 2 and 4 hours.

[0025] There is thus a need to produce prepregs that can lead, upon complete crosslinking of the polymeric binder composition, to composite materials having greater thermal stability and lower flammability than currently possible with organic resins, but having better mechanical properties and / or require more cost-effective production processes as compared to inorganic resins.

[0026] It is a first object of the present invention to provide a pre-impregnated (prepreg) fiber-reinforced composite material which overcomes or at least reduces the problems of the prepregs of the prior art, and in particular which allows obtaining finished products that are resistant to fire and high temperatures with curing and post-curing times and temperatures comparable to those of common prepregs based on organic resins, with various curing processes common in the field, such as autoclave curing, oven curing, or hot platen press curing.

[0027] It is another object of the present invention to provide a pre-impregnated fiber- reinforced composite material of aesthetic type, i.e. , endowed with high resistance to yellowing upon exposure to high temperatures or UV radiation.

[0028] It is another object of the present invention to provide a prepreg that remains flexible and workable for long periods of time, allowing a storage period before use between 15 and 22 days at temperatures around room temperature and over 12 months if stored at low temperatures (approximately about -18 °C).

[0029] SUMMARY OF THE INVENTION

[0030] These objects are achieved with the present invention, which in a first aspect thereof relates to a polymeric composition comprising: a) between 90.0 and 99.8% by weight of a mixture of at least one siloxane resin functionalized with hydroxyl groups and at least one siloxane resin functionalized with ether groups, OR, in which R is a C1-C4 alkyl radical, wherein:

[0031] - said at least one siloxane resin functionalized with hydroxyl groups is solid at room temperature, is present in an amount between 80 and 85% of the mixture of siloxane resins, has an average molecular weight between 1200 and 4500 Da determined by Gel Permeation Chromatography (GPC) and the hydroxyl groups are from 1 % to 6% of the weight of the resin;

[0032] - said at least one siloxane resin functionalized with ether groups is liquid at room temperature, is present in an amount between 15 and 20% of the mixture of siloxane resins and has an average molecular weight between 700 and 4000 Da determined by Gel Permeation Chromatography (GPC); b) a catalyst for the cross-linking reaction of siloxane resins selected from an imidazole catalyst and a superacid catalyst, in which when the catalyst is an imidazole catalyst, it is present in an amount between 0.2 and 1 % by weight of the composition, and when the catalyst is a superacid catalyst, it is present in an amount between 0.5 and 5% by weight of the composition; c) optionally, one or more additives in a total amount not exceeding 9.8% by weight.

[0033] In a second aspect thereof, the invention relates to the pre-impregnated fiber- reinforced composite materials obtained using the above-described polymeric composition.

[0034] In a further aspect thereof, the invention relates to the products obtained by forming and completely curing the aforesaid pre-impregnated fiber-reinforced composite materials.

[0035] BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The invention will be described in detail below with reference to Figures 1 -3, which show dynamic mechanical analysis (DMA) plots obtained according to the standard method ASTM D7028 for the fiber-reinforced composite materials of the invention produced with different curing cycles.

[0037] DETAILED DESCRIPTION OF THE INVENTION

[0038] In the following description and in the claims, the following abbreviations and definitions are adopted:

[0039] - “room temperature” means a temperature between 20 and 25 °C;

[0040] - “polymeric binder composition” means the polymeric composition of the first aspect of the invention.

[0041] The applicant has observed that with the use of the particular siloxanes of the invention, it is possible to obtain polymeric compositions to be used as prepreg binders which, once subjected to curing and possible post-curing treatment, give rise to polymeric matrices of fiber-reinforced composite materials endowed with the desired features, i.e., better fire and high-temperature resistance than fiber-reinforced composite materials with an organic polymeric matrix, and more cost-effective production processes and better mechanical properties as compared to fiber- reinforced composite materials in which the polymeric matrix is only inorganic.

[0042] In the first aspect thereof, the invention thus relates to a polymeric composition to be used as a binder in prepregs.

[0043] The aforesaid polymeric composition comprises inorganic resins and a crosslinking catalyst, in addition to optional additives.

[0044] The first component of the composition is a mixture of siloxane resins functionalized with hydroxyl groups and siloxane resins functionalized with ether groups.

[0045] The siloxane resins functionalized with hydroxyl groups (OH groups) have an average molecular weight between 1200 and 4500 Da, and the OH groups are present in an amount between 1 % and 6% by weight of the molecule. These siloxane resins appear as solid resins at room temperature. The siloxane resins functionalized with OH groups are between 80% and 85% by weight of the mixture of siloxane resins functionalized with hydroxyl groups and siloxane resins functionalized with ether groups.

[0046] The siloxane resins functionalized with ether groups (OR groups) are liquid resins at room temperature with an average molecular weight between 700 and 4000 Da. The radical R of the ether is a C1-C4 alkyl (methyl, ethyl, n-propyl, isopropyl, n- butyl, isobutyl, sec-butyl, and tert-butyl). Preferably, the radical OR is a methoxy radical (OCH3) and is present in an amount between 13% and 36% by weight of the molecule. The siloxane resins functionalized with OR groups are between 15% and 20% by weight of the mixture of siloxane resins functionalized with hydroxyl groups and siloxane resins functionalized with ether groups.

[0047] The molecular weight of these siloxane resins is determined by Gel Permeation Chromatography (GPC), following the methodology of standard ISO 13885-1 :2020, Part 1 , using tetrahydrofuran (THF) as a solvent.

[0048] The siloxane resins useful for the purposes of the invention are widely available on the market.

[0049] Examples of siloxane resins that are solid at room temperature are some resins functionalized with hydroxyl groups, such as the Silres® series resins from Wacker Chemie AG, in particular the Silres® REN 168, Silres® SY 300, Silres® IC 836, Silres® 603, and Silres® 604 resins; resins from the Dowsil® series (THE DOW CHEMICAL COMPANY), such as Dowsil® RSN 0217, Dowsil® RSN-0220, Dowsil® RSN-0233, Dowsil® RSN-0249, and Dowsil® RSN-0255, for example.

[0050] Examples of liquid siloxane resins at room temperature, having ether functionality (-OR), are some resins from the Silres® series by Wacker Chemie AG, e.g., the resins functionalized with methoxy groups Silres® MSE 100, Silres® SY231 , Silres® IC 232, or the resins Dowsil® 3074 Intermediate, Dowsil® 3037 Intermediate, Dowsil® 2405, Dowsil® US-CF 2403 by The DOW Chemical Company.

[0051] The imidazole catalysts are present in the composition of the invention in amounts between 0.2 and 1 % by weight, preferably from 0.2 and 0.5% by weight; these catalysts are commercially available. Imidazole catalysts useful for the purposes of the present invention are, for example, the product Dyhard® PI-FF by ALZCHEM AG, based on 2-phenylimidazole, the adduct of 2,4-diamino-6-[2’- methylimidazolyl-(1 ’)]-ethyl-s-triazine with isocyanuric acid dihydrate (Curezol 2MA- OK - SHIKOKU), reaction mass of 2-ethyl-4-methyl-1 H-imidazole-1-propiononitrile (Curezol 2E4MZ-CN - SHIKOKU), 2-ethyl-5-methyl-1 H-imidazole-1-propiononitrile, or 1 -[(2-methyl-1 H-imidazol-1 -yl)methyl]-2-naphthalenol (Aradur 3123 by HUNTSMAN).

[0052] The superacid catalyst is present in amounts from 0.5 to 5%, preferably from 1 to 4% by weight of the polymer composition.

[0053] The superacid catalysts are preferably of the blocked superacid type; suitable compounds are quaternary ammonium compounds, in which the counterion is an SbFe- ion. Catalysts of this type are commercially available: suitable catalysts for the purposes of the invention are the products K-PURE® CXC-1612 and K-PURE® CXC- 1614 by KINGS INDUSTRIES.

[0054] The polymeric composition according to the invention can contain additional compounds such as: reactive silanes, additives with a deaerating function or suitable for modifying the rheology of the resin. When present, these additives are in an amount generally less than or equal to 9.8% by weight of the polymeric composition. Reactive silanes useful for the purposes of the invention are, for example, 3- (glycidoxypropyl)trimethoxysilane (EVON IK), 3-(glycidoxypropyl)triethoxysilane (EVONIK), or silicone-based tougheners, produced for example by SILTECH CORPORATION. Useful deaerating additives are, for example, the products BYK A535, BYK A530, and BYK A525 (BYK CHEMIE AG). Rheology modifiers are, for example, layered silicate (Garamite® 7305 - BYK CHEMIE AG), or fumed silica, e.g., the products Aerosil R202 (EVONIK) or Cab-O-Sil TS-720D (CABOT).

[0055] Unlike similar known compositions, the composition of the present invention contains no solvents.

[0056] The polymeric composition is generally prepared by dissolving the solid siloxane resin in the liquid siloxane resin at a temperature in the range between 50 and 70 °C under stirring. Once the complete dissolution of the solid siloxane resin in the liquid siloxane resin is achieved, any additives (silane compounds and / or tougheners and / or rheology modifiers and / or deaerating additives) can be added to the resulting solution, still operating within the same temperature range. The catalyst is added to the resulting mixture, maintaining the temperature between 50 and 70 °C, under stirring. Once the dispersion is completed, the composition is ready to be used for the impregnation process.

[0057] The polymeric binder composition of the invention has viscosity values in the range between 150,000 and 65,000 mPa*s in the temperature range between 50 and 70 °C, which values naturally decrease as the temperature increases. These viscosity values were determined by means of a rotational viscometer of the Brookfield® type at a shear rate equal to 1 s-1, or equivalently using a rheometer in a rotational mode, parallel plates at a shear rate equal to 1 s-1, and are ideal for the impregnation process of technical fabrics (carbon fiber, glass fiber, or a combination thereof) through the Hot-Melt process.

[0058] The polymeric composition according to the invention having the above- mentioned viscosity values is a flexible and elastic solid at room temperature (range 20 - 25 °C), free from droplet formation, allowing for easy impregnation of the fibers without any phenomena of separation of the polymeric composition according to the invention from the fibrous mass. The rheological profile of the polymeric composition according to the invention makes it particularly suitable for the impregnation process of fibrous masses, i.e., for producing prepregs. In a second aspect thereof, the invention relates to the pre-impregnated fiber- reinforced composite materials obtained using the above-described polymeric composition.

[0059] The fibers can have any chemical nature and can be, for example, carbon fibers, glass fibers, aramid fibers, quartz fibers, ceramic fibers, or mixtures thereof. In particular, useful for the purposes of the present invention are the carbon fibers obtained from PAN (polyacrylonitrile) and / or pitch, fiberglass (S-glass or E-glass), or the ceramic fibers obtained from alumina or from silica / alumina. Preferably, the fibrous component of the prepregs of the invention consists of carbon fibers or glass fibers only.

[0060] The content of fibers and polymeric binder composition can be varied as a function of the features that are to be obtained in the final products to be made from the prepregs of the invention. Products made from composite materials with high fiber content are more suitable for applications in which high mechanical performance is required; products made from composite materials with a high percentage of matrix are more suitable for applications in which higher service temperatures, greater fire protection, greater resistance to abrasion and / or scratching, and better aesthetic features are required.

[0061] In the prepregs of the present invention, the polymeric composition is from 35% to 45% by weight, while the fibrous component (fabric or non-woven fabric) is from 55% to 65% by weight.

[0062] In the prepregs of the invention, the fibers can be arranged in unidirectional webs, i.e. , fibers aligned along a single direction, or in fabrics with different weaving styles. Furthermore, fibers arranged in non-woven fabrics (non-wovens), both virgin and recycled, can be used. The fibers are arranged on a flat surface, in a single layer or preferably in multiple overlapping layers, and the layer thus obtained is then impregnated with the polymeric composition described above. The curing of the composition results in the complete polymerization (cross-linking) of the composition, creating a rigid structure in which the fabric fibers have a structural support function (and in some cases an aesthetic one), while the polymeric composition of the invention forms a matrix in which the fabric fibers are embedded, preventing the relative movements thereof.

[0063] Operatively, for producing a prepreg of the invention, the polymeric composition described above is first processed at a temperature between 45 and 75 °C so as to obtain a continuous resin film by casting between calenders preheated between 40 and 65 °C. The polymeric composition between the calenders forms a film that is collected on non-adhesive paper, known as release paper. The film obtained is cooled to room temperature and used to impregnate the fibrous mass by coupling the film with the fibrous mass itself using machinery known in the art. The coupling of the resin film with the fibrous mass is carried out at temperatures varying between 50 and 80 °C; the composite material (prepreg) thus obtained is then left to cool to room temperature.

[0064] The impregnation process is carried out using conventional machinery in the production industry of pre-impregnated fiber-reinforced composite material, in particular machinery used for producing the aforesaid materials by the hot-melt process (solvent-free).

[0065] Following the impregnation process, the polymeric composition according to the invention forms a non-cross-linked or only partially cross-linked matrix, which fills the gaps of the fibrous mass.

[0066] The pre-impregnated fiber-reinforced composite material according to the invention can be made in the form of tapes supported by a polymeric film or siliconized paper, which are commonly wound to form rolls ready to be used or stored awaiting use. These tapes can then be processed to obtain components with much more complex shapes, e.g., by layering and forming. Once formed, these components can be subjected to complete cross-linking (curing) to obtain final products with the desired properties.

[0067] In order to obtain the final product, the pre-impregnated fiber-reinforced composite material obtained as described above is then subjected to a thermal cycle of thermoforming, which leads to the curing of the material itself, resulting in the desired component.

[0068] In detail, thermoforming leads to the polymerization (curing) of the constituent resins of the polymeric binder composition, resulting in the formation of a solid matrix consisting of mutually cross-linked polysiloxane and / or polysilsesquioxane resins, which fills the fiber gaps of the fiber-reinforced composite material (prepreg).

[0069] The prepregs of the present invention have the feature of having higher thermomechanical resistance than that of prepregs made from organic resins, while simultaneously requiring more cost-effective production processes as compared to inorganic resins.

[0070] The thermoforming of the prepregs of the invention can be carried out in an autoclave through a standard curing cycle used by prepreg users, especially in the automotive field, which includes a ramp from room temperature up to 135 °C, ramp (2 ± 0.5) °C / min, hold for 90 min at 135 °C and pressure 6 bar; or through a standard curing cycle used especially in the aerospace industry, which includes, still operating in an autoclave, a ramp from room temperature up to 180 °C, ramp (2 ± 0.5) °C / min, hold for 60 min at 180 °C and pressure 6 bar.

[0071] Alternatively, the prepregs of the invention can be thermoformed in a vacuum oven with a standard curing cycle used by prepreg users, which includes a ramp from room temperature up to 135 °C, ramp (2 ± 0.5) °C / min, hold for 90 min at 135 °C and pressure 6 bar.

[0072] Finally, the prepregs of the invention can be thermoformed by treating in a hot platen press (compression moulding) through a curing cycle of 15 min at 180 °C, at a pressure between 6 and 10 bar.

[0073] Preferably, the thermoforming is preceded by the lamination of the preimpregnated fiber-reinforced composite material with the polymeric binder composition of the invention, followed by a vacuum bagging process, also a standard practice for prepreg users.

[0074] Advantageously, in order to further increase the thermal performance, after a curing cycle with curing hold at 135 °C, in an autoclave or oven, a component produced from a prepreg of the invention can be subjected to post-curing in an oven with a ramp from room temperature up to 180 °C, ramp (2 ± 0.5) °C / min, and hold for 60 min at 180 °C.

[0075] The treatment times of the prepregs of the invention, including curing and possible post-curing treatment, vary approximately between 2 and 5 hours.

[0076] In a further aspect thereof, the invention is directed to the products obtained by forming and completely curing the above-described prepreg composites.

[0077] With the prepregs of the invention, it is possible to produce all the products that can be produced with known prepregs, but due to the performance advantages of the new polymeric component, they also allow the production of products with improved features. As already mentioned, conventional prepregs with organic matrix do not ensure operating temperatures generally higher than about 250 °C.

[0078] The prepregs of the invention are particularly suitable for applications at high temperatures or when high fire resistance is required, in conditions under which organic-based composites could not operate. These improved thermal properties at high temperatures are confirmed by the DMA-Tg measurements (performed according to ASTM D7028) which show E’ onset temperatures above 350 °C (Figures 1 , 2, and 3). Furthermore, the products produced from the prepregs of the invention show improved features in aesthetic applications, which require stability intended as the resistance to both heat and UV yellowing.

[0079] The features of a product obtained by forming and completely curing the prepreg according to the invention are:

[0080] - high thermal features;

[0081] - service (operating) temperature up to 500 °C for long periods and resistance to short exposure to temperatures above 500 °C;

[0082] - high flame resistance;

[0083] - low emission of toxic fumes in the case of exposure to fire;

[0084] - high resistance to yellowing.

[0085] Applications for which the composites obtained from the prepreg of the invention can be of interest are, just to name a few examples: heat or flame barriers, components with a structural function that must operate at high temperatures, and aesthetic components with the additional function of protecting composite and noncomposite components.

[0086] Finally, in the last aspect thereof, the invention is directed to the process for producing the above-described prepreg.

[0087] Further features and advantages of the invention will become more apparent from the following Examples, intended for illustrative and non-limiting purposes only.

[0088] MATERIALS, INSTRUMENTS, AND METHODS

[0089] Siloxane resins:

[0090] Silres® 604 solid OH functionalized siloxane resin (Wacker Chemie AG)

[0091] Si Ires® MSE 100 liquid OCH3 functionalized siloxane resin (Wacker Chemie AG) Silres® SY231 liquid OCH3 functionalized siloxane resin (Wacker Chemie AG) Dowsil® 3074 Intermediate liquid OCH3 functionalized siloxane resin (The DOW Chemical Company)

[0092] Dowsil® 3037 Intermediate liquid OCH3 functionalized siloxane resin (The DOW

[0093] Chemical Company)

[0094] Dowsil® 2405 liquid OCH3 functionalized siloxane resin (The DOW Chemical

[0095] Company)

[0096] Dowsil® US-CF 2403 liquid OCH3 functionalized siloxane resin (The DOW

[0097] Chemical Company)

[0098] Silres® IC 232 liquid OCH3 functionalized siloxane resin (Wacker Chemie AG)

[0099] Dowsil® RSN 0233 solid OH functionalized siloxane resin (The DOW Chemical

[0100] Company)

[0101] Dowsil® RSN 0255 solid OH functionalized siloxane resin (The DOW Chemical

[0102] Company)

[0103] Catalysts

[0104] Dyhard® PI-FF (ALZCHEM AG)

[0105] Curezol 2MA-OK (SHIKOKU)

[0106] Curezol 2E4MZ-CN (SHIKOKU)

[0107] Aradur 3123 (HUNTSMAN)

[0108] K-PURE® CXC-1612 (KINGS INDUSTRIES)

[0109] K-PURE® CXC-1614 (KINGS INDUSTRIES)

[0110] Additives

[0111] Silanes:

[0112] 3-(glycidoxypropyl)trimethoxysilane (EVONIK),

[0113] 3-(glycidoxypropyl)triethoxysilane (EVONIK),

[0114] Deaerators:

[0115] - BYK® A530 (BYK Chemie AG)

[0116] - BYK® A535 (BYK Chemie AG)

[0117] - BYK® 088 (BYK Chemie AG)

[0118] - BYK® 361 N (BYK Chemie AG)

[0119] Rheology modifier:

[0120] - Garamite® 7305 (BYK Chemie AG)

[0121] - Aerosil R202 (EVONIK)

[0122] - Cab-O-Sil TS-720D (CABOT).

[0123] INSTRUMENTS DMA (dynamic mechanical analyzer) - Instrument DMA 242 E Artemis - Netzsch® equipped with 3-point bending clamp. ASTM D7028 (frequency 1 Hz, temperature ramp 5 °C / min).

[0124] Preparation of laminates for curing and post-curing cycle tests and DMA analysis:

[0125] Autoclave MAROSO. Mould: polished aluminum plate, thickness 6 mm.

[0126] Curing cycles (autoclave):

[0127] - ramp from room temperature up to 135 °C at a heating rate equal to 2 °C / min, pressure 6 bar; isotherm 90 min at 135 °C, pressure 6 bar;

[0128] - ramp from room temperature up to 180 °C at a heating rate equal to 2 °C / min, pressure 6 bar; isotherm 60 min at 180 °C, pressure 6 bar.

[0129] Post-curing cycles: OVEN equipped with PLC control.

[0130] Post-curing cycles: ramp from room temperature up to 180 °C at a heating rate equal to 2 °C / min, isotherm 1 h at 180 °C.

[0131] EXAMPLE 1

[0132] This example relates to the preparation of a first prepreg of the invention.

[0133] In the preparation of this prepreg, the methyl / phenyl siloxane resin Silres® 604, solid at room temperature and having a melting point in the range between 55 and 80 °C, and the methyl methoxy siloxane resin Silres® MSE 100, liquid and having a kinematic viscosity between 20 and 35 mm2 / s at room temperature, determined according to ASTM D445, were used.

[0134] 199.20 g of liquid resin Silres® MSE 100 were placed in a clean and perfectly dry stainless steel container. The liquid resin was heated to a temperature between 70 and 80 °C, under stirring (mechanical stirrer equipped with a Cowles impeller) at a speed of 100 rpm; 796.81 g of solid resin Silres® 604 were then added in four subsequent additions. After each addition of Silres® 604, under stirring at a speed of 100 rpm, the mixture was left under stirring at a temperature between 70 and 80 °C for a time of 10 min until a transparent viscous solution was obtained.

[0135] The stirring speed of the solution was increased to 200 rpm, and 3.98 g of imidazole catalyst Curezol® 2 MA-OK were added thereto under stirring. The mixture was left under stirring for 10 minutes.

[0136] The polymeric binder composition was poured into a metal container and then cast between calenders preheated to a temperature between 50 and 60 °C in order to obtain a film.

[0137] The resin, evenly distributed between the calenders, width 127 cm, was deposited on release paper running at a speed of 2 m / min. The resin film on the release paper was cooled and wound onto a cardboard core.

[0138] The roll of polymeric binder composition film was then used to impregnate a fabric made from a carbon fiber fabric of areal mass 200 g / m2with a twill type weave. The weight ratio between the polymeric binder composition and the fabric was 42 / 58.

[0139] The impregnation of the fabric was carried out by passing between calenders preheated at a temperature between 55 and 65 °C. The speed of the impregnation machinery was equal to 2.5 m / min. The resulting prepreg was cooled and coated with polyethylene film. The assembly, i.e., resin film (polymeric composition) / fibrous mass / polyethylene film, was wound onto a cardboard core. The roll of prepreg was placed in a polyethylene bag, which was immediately sealed and placed in a refrigerated warehouse at a temperature of -18 ± 2 °C.

[0140] EXAMPLE 2

[0141] This example relates to the preparation of a second prepreg of the invention.

[0142] 193.50 g of Silres® MSE 100 liquid resin were introduced into a clean and perfectly dry stainless steel container. The liquid resin was heated to a temperature between 70 and 80 °C, under stirring (mechanical stirrer equipped with a Cowles impeller) at a speed of 100 rpm; 773.99 g of Silres® 604 solid resin were then added in four subsequent additions. After each addition of Silres® 604, under stirring at a speed of 100 rpm, the mixture was left under stirring at a temperature between 70 and 80 °C for a time of 10 min until a transparent viscous solution was obtained.

[0143] The stirring speed of the solution was increased to 200 rpm, and 32.51 g of superacid catalyst K-PURE® CXC-1614 were added thereto under stirring. The mixture was left under stirring for 12 minutes.

[0144] The polymeric binder composition was poured into a metal container and then cast between preheated calenders at a temperature between 50 and 60 °C in order to obtain a resin film.

[0145] The resin, evenly distributed between the calenders, width 127 cm, was deposited on release paper running at a speed of 2 m / min. The resin film on the release paper was cooled and wound onto a cardboard core. With the roll of polymeric binder composition film thus obtained, a prepreg was then produced under the same conditions described in Example 1 (200 g / m2carbon fiber fabric, twill weave, polymeric binder composition / fabric ratio equal to 42 / 58 by weight, passage between preheated calenders at a temperature between 55 and 65 °C). The speed of the impregnation machinery was equal to 2.5 m / min. The resulting prepreg was cooled and coated with polyethylene film. The assembly, i.e. , resin film (polymeric composition) / fibrous mass / polyethylene film, was wound onto a cardboard core. The roll of prepreg was placed in a polyethylene bag, which was immediately sealed and placed in a refrigerated warehouse at a temperature of -18 ± 2 °C.

[0146] EXAMPLE 3

[0147] This example relates to the assessment of the maintenance of the workability of the prepregs of the invention stored for 15 days at room temperature.

[0148] An L-shaped metal mould was prepared, treated with release agent, and conditioned for 24 h at a temperature of 21 ± 2 °C.

[0149] On this mould, a sample of prepreg prepared as described in Example 1 was laminated, conditioned for 15 days at a temperature of 21 ± 2 °C immediately after being produced.

[0150] The adhesion and flexibility of the sample were verified. The adhesion is an indicator of the preservation of the features of the material as just produced, and it was determined through application and measurement of the residence time of the sample on the vertical face of the metal mould. The sample remained in contact with the vertical metal face for 15 min, the same residence time measured on a similar sample just produced, which indicates that the prepreg of the invention maintained the features of the “fresh” sample during the 15 days of conditioning.

[0151] The flexibility of the sample was then evaluated after 15 days of conditioning at 21 ± 2 °C, comparing it also in this case with that of a similar prepreg sample just produced. The test was carried out by application of the prepreg through lamination on the internal 90° angle of the mould. Also in this case, the flexibility properties of the prepreg conditioned for 15 days turned out to be similar to those of the sample just produced, confirming that the prepregs of the invention maintain their workability features unchanged after storage of at least 15 days at room temperature.

[0152] EXAMPLE 4

[0153] The prepreg obtained in Example 1 was subjected to thermoforming, after lamination on a flat aluminium plate and vacuum bagging. The curing procedure was carried out in an autoclave at a pressure of 6 bar, with a cycle comprising a heating ramp from room temperature to 135 °C with a heating rate of 2 °C / min and hold at 135 °C for 90 minutes, which leads to almost complete cross-linking and allows removing the component from the mould. The cured matrix is transparent and colourless.

[0154] The thermoformed product thus obtained was subjected to dynamic mechanical analysis (DMA) according to standard ASTM D7028. The result of the test is shown in Figure 1 , and indicates a transition at a temperature above 430 °C.

[0155] EXAMPLE 5

[0156] The prepreg obtained in Example 1 was subjected to thermoforming in an autoclave at 6 bar with vacuum bagging, after lamination on an aluminium sheet. In this case, the thermal cycle included a heating ramp from room temperature to 135 °C with a heating rate of 2 °C / min and hold at 135 °C for 90 minutes, and a subsequent post-curing treatment comprising heating from 135 °C to 180 °C with a rate of 2 °C / min and hold of 60 min at 180 °C. The cured matrix is transparent and colourless.

[0157] The thermoformed product thus obtained was subjected to dynamic mechanical analysis (DMA) according to standard ASTM D7028. The result of the test is shown in Figure 2, and indicates a transition at a temperature above 440 °C.

[0158] EXAMPLE 6

[0159] The prepreg obtained in Example 1 was subjected to thermoforming in an autoclave at 6 bar with vacuum bagging, after lamination on an aluminium sheet. In this case, the thermal cycle included a heating ramp from room temperature to 180 °C with a heating rate of 2 °C / min and hold at 180 °C for 60 minutes. The cured matrix is transparent and colourless.

[0160] The thermoformed product thus obtained was subjected to dynamic mechanical analysis (DMA) according to standard ASTM D7028. The result of the test is shown in Figure 2, and indicates a transition at a temperature above 600 °C.

[0161] Comment on the results

[0162] The results obtained from the experimentation confirm that a prepreg of the invention can lead, upon complete cross-linking of the polymeric binder composition, to composite materials having high thermal stability, being capable of operating up to 500 °C for a time not less than 1000 hours, superior to what can be obtained with organic matrix resins. Furthermore, it was possible to verify that the components obtained from the prepreg of the invention can be obtained in times in the range from 15 min (by a hot plate press process) to 283 min (about 5 h). The products made from composite materials with cured polymeric matrix according to the invention further have high fire-resistance and heat barrier properties, and high resistance to yellowing upon exposure to UV rays or high temperatures.

[0163] As previously mentioned, the pre-impregnated fiber-reinforced composite material according to the present invention finds particular application as a base material for producing, by thermoforming and completely curing the polymeric binder composition, components such as: heat or fire barriers, components with structural function that must operate at high temperatures, components that in the case of fire do not participate in the combustion and the degradation products of which (mainly fumes) are in low amounts and have low toxicity, and also components with an aesthetic function given the resistance to yellowing.

Claims

CLAIMS1 . Polymeric composition comprising: a) between 90.0 and 99.8% by weight of a mixture of at least one siloxane resin functionalized with hydroxyl groups and at least one siloxane resin functionalized with ether groups, OR, wherein R is a C1 -C4 alkyl radical, wherein:- said at least one siloxane resin functionalized with hydroxyl groups is solid at room temperature, is present in an amount between 80 and 85% of the mixture of siloxane resins, has an average molecular weight between 1200 and 4500 Da determined by Gel Permeation Chromatography (GPC) and the hydroxyl groups constitute from 1% to 6% of the weight of the resin;- said at least one siloxane resin functionalized with ether groups is liquid at room temperature, is present in an amount between 15 and 20% of the mixture of siloxane resins and has an average molecular weight between 700 and 4000 Da determined by Gel Permeation Chromatography (GPC); b) a catalyst for the cross-linking reaction of siloxane resins selected from an imidazole catalyst and a superacid catalyst, wherein when the catalyst is an imidazole catalyst it is present in an amount between 0.2 and 1 % by weight of the composition, and when the catalyst is a superacid catalyst it is present in an amount between 0.5 and 5% by weight of the composition; c) optionally, one or more additives in a total amount not exceeding 9.8% by weight2. Polymeric composition according to claim 1 , wherein in said at least one siloxane resin functionalized with ether groups, the OR radical is a methoxy radical (-OCH3) and is present in an amount between 13% and 36% by weight of the molecule.

3. Polymeric composition according to any one of claims 1 or 2, wherein said imidazole catalyst comprises a compound selected among 2-phenylimidazole,the adduct of 2,4-diamino-6-[2'-methylimidazolyl-(T)]-ethyl-s-triazine with isocyanuric acid dihydrate, reaction mass of 2-ethyl-4-methyl-1 H-imidazole-1- propiononitrile, 2-ethyl-5-methyl-1 H-imidazole-1-propiononitrile and 1 -[(2- methyl-1 H-imidazol-1 -yl)methyl]-2-naphthalenol.

4. Polymeric composition according to any one of the preceding claims, wherein said imidazole catalyst is present in an amount between 0.2 and 0.5% by weight.

5. Polymeric composition according to any one of claims 1 or 2, wherein said superacid catalyst is a quaternary ammonium compound with a SbFe- counterion.

6. Polymeric composition according to any one of claims 1 , 2 or 5, wherein said superacid catalyst is present in an amount between 1 and 4% by weight.

7. Polymeric composition according to any one of the preceding claims, wherein said one or more additives are selected from reactive silanes, additives with deaerating function and rheology modifiers.

8. Method for preparing a polymeric composition of any one of the preceding claims, comprising the steps of dissolving said at least one siloxane resin functionalized with hydroxyl groups in said at least one siloxane resin functionalized with ether groups at a temperature in the range between 50 and 70 °C under stirring, possible subsequent addition to the liquid mixture thus obtained of said additives operating in the range between 50 and 70 °C under stirring, and final addition of said catalyst, maintaining the temperature between 50 and 70 °C under stirring.

9. Pre-impregnated fiber-reinforced composite material comprising between 35% and 45% by weight of a polymeric composition of any one of claims 1 to 7 and between 55% and 65% by weight of fibers.

10. Pre-impregnated fiber-reinforced composite material according to claim 9, wherein said fibers are selected among carbon fibers, glass fibers, aramid fibers, quartz fibers, ceramic fibers or mixtures thereof, and are present in said material in the form of unidirectional tape, fabric or non-woven fabric.

11. Method for producing a pre-impregnated fiber-reinforced composite material of any one of claims 9 or 10, comprising treating a polymeric composition of any one of claims 1 to 7 at a temperature of between 45 and 75 °C and casting it between calenders preheated between 40 and 65 °C to obtain a film, and subsequently coupling the polymeric composition film thus obtained and said fibers at a temperature of between 50 and 80 °C.

12. Product obtained by thermoforming a pre-impregnated fiber-reinforced composite material of any one of claims 9 or 10, wherein the thermoforming process is carried out by heating to a temperature between 135 and 180 °C, possibly followed by a subsequent treatment at 180 °C, for a total time between 2 and 5 hours, operating in an autoclave, in an oven or in a hot plate press with a pressure between 6 and 10 bar.

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