Dry prepregs for ceramic matrix composites.

Abstract

A dry prepreg for ceramic matrix composites is disclosed. The dry prepreg comprises a ceramic fiber tow or fabric impregnated with a preceramic matrix containing a low level of aqueous solvent. The preceramic matrix contains an inorganic portion and a binder system. The binder system is described, including a binder and a plasticizer for the binder.

Description

[Technical field]

[0001] The present disclosure relates to prepregs for use in ceramic matrix composites. Such prepregs include a tow or fabric of ceramic fibers impregnated with a dry preceramic matrix. Methods for making articles using such dry preceramic matrix composite prepregs are also described. Summary of the Invention

[0002] Briefly, in one aspect, the present disclosure provides a dry prepreg as defined in claims 1-11.

[0003] In another aspect, the present disclosure provides a method for producing a dry prepreg according to claim 12.

[0004] The above summary of the disclosure is not intended to describe every embodiment of the present invention. The details of one or more embodiments of the present invention are also set forth in the following description. Other features, objects, and advantages of the present invention will be apparent from the detailed description and claims. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0005] Ceramic fibers are well known and available from a variety of commercial sources. Composite materials may be formed by embedding ceramic fibers in a variety of matrices, including polymers (i.e., polymer matrix composites (PMCs), metals (i.e., metal matrix composites (MMCs), and ceramics (i.e., ceramic matrix composites (CMCs)). In some embodiments, ceramic matrix composites comprising reinforcing ceramic fibers embedded in a ceramic matrix can provide superior performance, including high temperature resistance and stability, mechanical strength, hardness, and corrosion resistance.

[0006] CMCs based on oxide ceramic fibers in an oxide ceramic matrix, so-called oxide-oxide (or Ox-Ox) CMCs, are of particular interest, for example, in the aerospace industry. Oxide-based ceramic fibers include, for example, alumina fibers and alumina-silica fibers. Oxide-based ceramic fibers may contain additional components such as boria, alkaline earth metal oxides, alkali metal oxides, and metals. Oxide-based ceramic matrices include those derived from alumina, silica, and mullite.

[0007] Ceramic fibers are available as individual fibers (sometimes called filaments) or as tows. A tow (sometimes called a strand or roving) is a bundle of fibers aligned along a common axis. Tows are available in a variety of forms, including spread tows and round or oval tows. Generally, spread tows may have an aspect ratio (A2 / A1) of 0.05 or less, such as 0.03 or less, or even 0.02 or less, where A1 is the width of the tow and A2 is the thickness. For comparison, conventional round or oval tows may have an aspect ratio in the range of 1 to 0.1. Ceramic fiber tows are also available as yarns, which include multiple tows twisted together, "tapes" include parallel tows stitched together, and fabrics include woven tows. Fabrics and tapes can be prepared from any combination of standard (round or oval) and spread tows.

[0008] In a typical CMC process, ceramic fiber tows are impregnated with a ceramic matrix slurry to form a ceramic prepreg. Generally, a slurry is a dispersion of insoluble solids in a liquid. Ceramic matrix slurries (also called preceramic matrix slurries) typically contain ceramic particles dispersed in a solvent, generally water. The slurry may also include one or more additional components, such as binders, plasticizers, drying control aids, dispersants, and inorganic precursors, e.g., preceramic polymers. Some slurries include ceramic sols, which include a colloidal solution of about 15% to 60% of solids in a solvent, e.g., water.

[0009] In one common method, a woven fabric of ceramic fiber tows is injected into a ceramic matrix slurry to form a prepreg. After impregnation, a fraction of the solvent is typically removed in a process called staging. Staging is used to promote the development of adhesion during layup, a characteristic that facilitates adhesion between adjacent layers and allows their application over tool surfaces with complex contours without delamination or sliding. However, even after staging, the prepreg contains 15%, 25%, or even 40% water by weight, based on the total weight of the matrix. In a subsequent step, the prepreg is formed into the desired shape, sometimes called a "green body," using known methods such as hand layup methods and automated pick-and-place operations.

[0010] In another CMC process, individual tows of ceramic fibers are impregnated with a ceramic matrix material to form a ceramic prepreg (also called a towpreg). For example, the ceramic fibers can be dipped into a ceramic matrix slurry to infuse the ceramic fiber bundles with solvent and ceramic particles to form a prepreg. This wet prepreg may be wound onto a core for use in subsequent processes. In other cases, the wet prepreg is immediately transferred to further processing along the impregnation step. In the layup process, the wet prepreg is formed into the desired shape, sometimes called a "green body," using known methods such as drum winding, filament winding, automated tape layup, and advanced fiber placement.

[0011] After layup, the slurry-infused fabric- or tow-based structure is dried and consolidated, often in an autoclave, to remove the solvent, compress the layup, and reduce or eliminate voids. The consolidated layup is then fired to remove any remaining solvent, burn out the organics, and sinter the ceramic particles to form a fired CMC article.

[0012] To produce a green composite, the matrix components must be combined with the fibers to achieve an acceptable volume fraction. Published data provide reliable guidance regarding the optimal volume fractions of components in oxide-oxide CMCs. For example, in a typical ceramic fiber-reinforced composite, the ceramic fibers typically account for about 45% to 55% by volume of the composite, with an additional 20% to 25% by volume contributing to porosity. The remaining 20% ​​to 35% by volume is therefore accounted for by the fired ceramic matrix. However, the specific volume percentages depend on the materials selected. Because shrinkage during firing is very small, the volume fractions of the components in the consolidated pre-fired laminate are close to the volume fractions in the final part.

[0013] Aqueous slurries containing low amounts of organic binder and high amounts of water are frequently used in the manufacture of CMC in fabric-based lay-up procedures. For example, a typical ceramic matrix slurry may contain about 0% to 8% by weight of binder based on the total weight of the ceramic matrix slurry. Plasticized binder systems typically contain about 0% to 5% by weight of binder and about 2% to 10% by weight of plasticizer. Unplasticized systems may contain about 25% to 45% by weight of water, while plasticized systems may contain lesser amounts of water, for example, 15% to 35% by weight of water.

[0014] Due to excessive drying, these fabrics have a limited "out time" during which they can be processed without losing their desirable adhesive and handling properties. Such prepregs are not considered stable and their use requires special storage and handling conditions to ensure the viability of the material. For example, such prepregs may need to be refrigerated during shipping and storage to maintain the desired moisture content. In addition, they may require a rapid lay-up process to avoid drying and may need to be handled in a controlled humidity environment to avoid excessive loss of moisture.

[0015] These issues become more important as the industry moves from hand layup procedures to automated processing involving, for example, "pick and place" or filament winding of prepreg fabrics and advanced fiber placement of towpregs. These processes require slurries that are tacky enough to be self-adhesive. Preferably, they have a longer time during layup and do not require extensive humidity control.

[0016] Automated methods of forming CMC from towpregs place even more demands on the slurry. Ideally, a ceramic matrix slurry would result in a towpreg that can be used with AFP hardware currently used in the polymer matrix composites industry. Such a towpreg could provide stability leading to long out times, easy handling, sufficient tack to guide the tow over a desired radius (e.g., about 1500 mm) during layup, and seamless lamination between plies during the consolidation process at pressure / temperature combinations practical in an autoclave. Such a towpreg should also be fired to produce a composite with the properties and performance desired for a CMC. Generally, the water-based slurry compositions used in typical fabric-based hand layup methods lack the desired balance of properties and have many of the disadvantages associated with slurry-based fabric layup.

[0017] This disclosure relates to "dry" prepregs. Generally, it may not be possible to remove all water from a composition. For example, if the components are hygroscopic, some moisture may remain despite attempts at complete drying. Thus, as used herein, a "dry" prepreg comprises ceramic fibers impregnated with a preceramic matrix that contains 4 wt.% or less water based on the total weight of the ceramic matrix. In some embodiments, a dry prepreg contains 2 wt.% or less, or even 1.5 wt.% or less water based on the total weight of the ceramic matrix. As used herein, "preceramic matrix" refers to a ceramic matrix before the organic materials have been removed and the inorganics have been fired and sintered.

[0018] In some embodiments, the first step used to prepare the dry prepregs of the present disclosure is similar to that currently used in which ceramic fiber tows or fabrics are infused with a ceramic matrix slurry. However, the slurry composition used in the present disclosure allows all or substantially all of the water to be removed prior to layup. As a result, such dry prepregs can be stored and shipped without the need for special conditions such as freezing. They also have a long out time that allows for a slower layup process without the need for humidity control.

[0019] Generally, the ceramic matrix slurry of the present disclosure includes an aqueous solvent, an inorganic component, and an organic component. As used herein, an aqueous solvent is a solvent system that includes at least 80% by weight, such as at least 90% by weight, or even at least 95% by weight water. In some embodiments, the aqueous solvent is at least 99% or even 100% water. Generally, any suitable co-solvent that is soluble or miscible in water may be present in the aqueous solvent, provided that the total amount of co-solvent is 20% by weight or less, such as 10% by weight or less, 5% by weight or less, or even 1% by weight or less. The total amount of any particular co-solvent should not exceed the solubility of water.

[0020] As used herein, to distinguish solvents from other liquid materials that may be present in the system, a solvent is liquid at room temperature and has a boiling point of 150° C. or less at atmospheric pressure. To facilitate subsequent processing steps, in some embodiments, the solvent has a boiling point of 125° C. or less, or even 100° C. or less at atmospheric pressure. Suitable organic co-solvents, used alone or in combination, include, for example, alcohols (e.g., methyl, ethyl, isopropyl, and t-butyl alcohol), acetates, aromatic hydrocarbons (e.g., benzene, toluene, and xylene), and aliphatic hydrocarbons (e.g., heptane, hexane, and octane). In some embodiments, isopropanol, ethanol, butanol, and various acetates may be preferred due to toxicity concerns.

[0021] In general, the inorganic component may include any known ceramic particles. The ceramic particles are selected based on their compatibility with the ceramic fibers and the desired end-use properties. In some embodiments, the ceramic particles are oxide ceramic particles. Common ceramic particles include sol-gel and particulate-based refractory materials. Exemplary refractory particles include alumina (Al2O3), and hydrates of alumina (e.g., boehmite or aluminum trihydrate) and silica (SiO2), alumina-coated silica, and mullite. Sol-gels include colloidal suspensions of particles in the size range of 2 nanometers to 300 nanometers. In some embodiments, the sol contains about 5% to about 50% by weight of refractory particles. Particulate slurries can use much larger refractory particles, such as, for example, 0.2 microns to 5 microns, but in some embodiments, even particles larger than 2 microns may cause filtration problems.

[0022] In this disclosure, the organic component includes a binder system to impart the desired tack, drying stability, and lamination properties to the prepreg without the higher levels of water previously required. The binder system includes one or more binders and, optionally, one or more plasticizers. Generally, a plasticizer is an organic material that is miscible with the binder and reduces the composite modulus or glass transition temperature (Tg) relative to the unplasticized binder.

[0023] The composition of the binder system is selected to impart the desired tack during handling and laying up as a dry prepreg (i.e., without the need for water). In some embodiments, the desired tack may be achievable at room temperature (about 20° C.), which may lead to problems handling the prepreg prior to layup. In some embodiments, the dry prepreg has low tack or no tack at ambient conditions (e.g., 20° C.-25° C.). Such prepregs may then be "heat activated" to impart the desired tack required for layup. In some embodiments, the binder system is selected to impart the desired level of tack at temperatures of 30° C. or higher, 40° C. or higher, or even 50° C. or higher. In some embodiments, the desired tack is achieved at temperatures of 150° C. or lower, such as 100° C. or lower, or even 70° C. or lower. Thus, in some applications, an external heat source (e.g., infrared and laser heaters) may be used to increase the tack at layup.

[0024] Because the plasticizer is miscible with the binder or at least a portion of the binder, it functions to increase tack and flexibility by lowering the Tg or complex modulus. Thus, in some embodiments, plasticizers can be added to lower the required layup temperature, increase tack, and improve room temperature handling of the dried prepreg. In some embodiments, the plasticizer is soluble or miscible in an aqueous solvent, such as water.

[0025] In some embodiments, suitable plasticizers may be liquids at room temperature. However, such materials may be distinguished from solvents because the boiling point of plasticizers is high enough that they do not appreciably evaporate at room temperature and do not dry out with the water and co-solvents in the slurry. Thus, the plasticizer should have a boiling point at atmospheric pressure that is at least 50° C., such as at least 75° C., or even at least 100° C. higher than the highest boiling solvent in the system. In some embodiments, the plasticizer has a boiling point at atmospheric pressure of at least 200° C., at least 250° C., or even at least 275° C. In addition, the plasticizer should be selected such that it will evaporate or burn off with other organics during post-layup steps, such as during consolidation or firing processes.

[0026] Generally, the components of the binder system and their relative amounts are selected to impart a balance of properties required throughout the CMC manufacturing process. For example, in some embodiments, the binder is soluble in the aqueous solvent used in the preceramic matrix slurry, while maintaining compatibility with the coating process and retaining or imparting an acceptable viscosity that facilitates impregnation into the tow or fabric.

[0027] Generally, the ceramic matrix slurries of the present disclosure contain a higher amount of binder system (organic portion) than prior art compositions. In some embodiments, the ceramic matrix slurries of the present disclosure may contain about 10% to 20% by weight (e.g., 10% to 15% by weight) of binder. In some embodiments, the binder system may include a plasticizer, such as about 2% to 5% by weight (2.5% to 3.5% by weight). The amount of plasticizer and binder depends on the selection of these materials and their properties. In some embodiments, no plasticizer is required. In some embodiments, the weight ratio of plasticizer to binder is 5:95 to 30:70, such as 10:90 to 30:70, or even 15:85 to 25:75.

[0028] For a given preceramic matrix, a higher amount of organic binder system can improve adhesion and consolidation, but if the volume ratio of organics to ceramic is too high, there will be too much porosity in the fired CMC when the organics are removed. Conversely, if the volume ratio of organics to ceramic is too low, the fired ceramic matrix phase may have acceptable porosity, but at the expense of poor adhesion and poor consolidation during layup. A similar tradeoff exists between the volume percent of fiber and the amount of preceramic matrix impregnated in the tow. Applying a large amount of matrix to the tow can impart improved adhesion, but the resulting fiber volume percent after firing may be too low.

[0029] After layup, the binder system should be soft and deformable / flowable under typical consolidation conditions and below the decomposition temperature of the organic materials. Consolidation typically occurs in an autoclave under vacuum. In some embodiments, under such conditions, some or all of the plasticizer may be evaporated or removed. Removal of this portion of the organics may aid in densification and reduce the amount of porosity that is formed when the remaining organics are burned off during the firing process.

[0030] The inventors have discovered that by proper selection of binder system components, a plasticizer is not required to achieve consolidation. Thus, although a plasticizer may be required to impart the desired mechanical properties (e.g., tackiness) required for the layup, in some embodiments, the binder alone can impart the desired properties required for consolidation. In some embodiments, the binder softens and becomes flowable at temperatures of 250° C. or less (e.g., 200° C. or less, or even 150° C. or less). In some embodiments, pressure may be used to aid in consolidation. In some embodiments, the binder is selected such that a pressure of 1400 kPa (about 200 psi) or less is required, such as 1000 kPa or less, or even 700 kPa or less.

[0031] The combination and relative amounts of binder and plasticizer are selected to function even at very low water content. Generally, in this disclosure, the role of water is primarily as a processing aid to disperse the ceramic precursor to provide a fluid slurry that can be easily impregnated into the tow or fabric. Unlike prior art systems, water is not required for further process steps, such as to impart the necessary tack for layup. Thus, after impregnation, the prepreg of the present disclosure can be dried to remove all or nearly all of the water, while retaining most or substantially all of the plasticizer, the operation being facilitated by the large boiling point difference between the solvent and the plasticizer. Because the water remaining in the dried prepreg is minimal and not necessary for the desired performance, the dried prepreg can impart improved out time and drying stability compared to conventional systems with large amounts of water.

[0032] Other process steps also influence the selection of binder system components and amounts. For example, low carbonization materials may be desirable because organics may be burned off during the firing process. In addition, the total amount of binder required to achieve the desired flow, compaction, and adhesion should be selected to provide the desired amount of porosity in the fired CMC, for example, 20% to 25% by volume in some embodiments.

[0033] Suitable binders include many water-soluble organics, such as polyurethanes, polyvinyl alcohols, polyolefins, and the like. Oxide (e.g., ethylene oxide, propylene oxide, and copolymers thereof), polyvinyl acetate, polyoxazoline, polyacrylamide, and polyvinylpyrrolidone, and copolymers and blends of any of these. In addition, common ceramic binders and additives, including carboxymethylcellulose, cellulose gum, guar gum, and acacia gum, can be used, either by themselves or in combination.

[0034] Suitable plasticizers include, for example, glycols (eg, polyethylene glycol, propylene glycol), polyols (eg, glycerol), sugars, and sugar alcohols. The present invention includes the following aspects. (Item 1) 1. A dry prepreg comprising tows of ceramic fibers impregnated with a preceramic matrix comprising 45 volume % to 60 volume % inorganic oxide and 40 volume % to 55 volume % organic binder system, based on the total volume of the preceramic matrix, wherein the preceramic matrix comprises 4 weight percent or less of an aqueous solvent, based on the total weight of the preceramic matrix. (Item 2) 2. The dry prepreg of claim 1, wherein the organic binder system comprises a polymeric binder and a plasticizer for the polymeric binder. (Item 3) 3. The dry prepreg of claim 2, wherein the binder comprises a polyoxazoline. (Item 4) 4. The dry prepreg according to claim 2 or 3, wherein the plasticizer comprises glycerol. (Item 5) 5. The dry prepreg according to any one of items 2 to 4, wherein the weight ratio of plasticizer to binder in the binder system is from 10:90 to 30:70, for example, from 15:85 to 25:75. (Item 6) 6. The dry prepreg of any one of the preceding claims, wherein the preceramic matrix comprises at least 96 weight percent of the inorganic oxide and organic binder system, based on a total weight of the preceramic matrix. (Item 7) 7. The dry prepreg according to any one of items 1 to 6, wherein the aqueous solvent comprises at least 95% by weight of water. (Item 8) 8. The dry prepreg of any one of the preceding claims, wherein the preceramic matrix comprises 1.5 wt. % or less of an aqueous solvent, based on the total weight of the preceramic matrix. (Item 9) 9. The dry prepreg according to any one of the preceding claims, wherein the prepreg is a fabric comprising a plurality of tows of ceramic fibers impregnated with the preceramic matrix. (Item 10) 10. The dry prepreg according to any one of items 1 to 9, wherein the preceramic matrix comprises 50 vol.% to 60 vol.% of an inorganic oxide and 40 vol.% to 50 vol.% of the organic binder system, based on a total volume of the preceramic matrix. (Item 11) 11. The dry prepreg according to any one of items 1 to 10, wherein the ceramic fiber tows have an aspect ratio (A2 / A1) of 0.05 or less, A1 being the width of the tow and A2 being the thickness of the tow. (Item 12) 1. A method for producing a dry prepreg, comprising the steps of: (i) impregnating a ceramic fiber tow with a ceramic matrix slurry comprising an inorganic oxide, an organic binder, and an aqueous solvent; (ii) drying the slurry to form a preceramic matrix comprising 45 volume percent to 60 volume percent of the inorganic oxide and 40 volume percent to 55 volume percent of the organic binder system, based on a total volume of the preceramic matrix, the preceramic matrix comprising 4 weight percent or less of the solvent, based on a total weight of the preceramic matrix; A method comprising: EXAMPLES

[0035] [Table 1]

[0036] Slurry E was prepared by first adding 1266.7 g of a 30 wt% aqueous solution of AQUAZOL 50 (B-1) to a high density polyethylene bottle. Then, 4.7 g of HNO3 (dispersant) and 95 g of glycerol (P-1) were added and mixed until uniformly distributed. Next, about 40 drops of 1-octanol (defoamer) and 20 drops of SOLWET L-77 (surfactant) were added and mixed until uniformly distributed. Finally, 1865.5 g of ALMATIS A1000SG alumina (AL-2) was added to the jar and the mixture was ball milled for at least 12 hours. The resulting slurry was uniform in appearance and had a viscosity of 1500 cP to 2500 cP. Ceramic matrix slurries A to D were prepared in a similar manner by combining the ingredients in the amounts summarized in Table 2.

[0037] Slurry F was made by first adding 6.5 g of HNO3 to 703.4 g of NALCO 1056 colloidal silica (Si-2). Next, 210 g of deionized water was added, followed by 437 g of AQUAZOL 50 solids (B-2). The mixture was stirred overnight. Then, 109.3 g of glycerol (P-1), approximately 40 drops of 1-octanol, and 20 drops of SILWET L-77 were added and mixed until evenly distributed. Finally, 1906.2 g of ALMATIS A1000SG alumina (AL-2) was added, and the mixture was ball milled for at least 12 hours. [Table 2]

[0038] Dry towpregs were prepared using 10,000 denier Nextel 610 ceramic fiber tows from 3M Company. The spread tows had a nominal width of 6 mm and were sized with PEG 20k at a nominal coat weight of 1.2 wt%. The ceramic matrix slurry in Table 2 was placed in a pan. The spread tows of ceramic fiber were unwound and passed through the slurry. As the tows were submerged in the slurry, they were supported by a partially submerged driven roller. After exiting the slurry, a gapped metering roll was driven at approximately the same speed as the driven roll was used to regulate the amount of slurry injected into the tow. The gap between the metering roll and the driven roll controlled the amount of slurry to be between 40% and 80% by weight of the dry weight of the tow.

[0039] After the slurry was weighed, the wet prepreg was transferred to a liner and passed through a convention oven set at 95°C-100°C. The line speed was adjusted to produce a dry towpreg containing less than about 3% water by weight. The dry towpreg, containing ceramic fibers impregnated with the preceramic matrix, was wound on a spool with a liner for subsequent use. The composition of the dry preceramic matrix of the towpreg is summarized in Table 3. Samples were produced at different loadings, expressed as both weight % and volume % of fiber in the dry towpreg. The volume percent of inorganics based on the total volume of the dry ceramic matrix ("vol. % inorganics") and the weight ratio of plasticizer to binder in the binder system ("Wt.P:B") are also reported. [Table 3]

[0040] In a typical AFP layup process, ambient conditions are expected to be about 20° C. and about 45% relative humidity (% RH). A sample of Towpreg F was stored at about 22° C. and about 26% RH and had a baseline moisture content of 0.9 wt% based on the total weight of the preceramic matrix (i.e., excluding the weight of the ceramic fibers). This sample was then held at 22° C. and 54% RH for 66 hours. The conditioned sample had a moisture content of 1.1 wt%. Three additional towpreg samples with other slurry systems were tested in the same manner. The baseline moisture content ranged from 0.6 wt% to 0.9 wt%, and the conditioned moisture content ranged from 0.9 wt% to 1.3 wt%.

[0041] AFP process. Dry towpregs were laid down using an automated fiber placement (AFP) machine commercially available from Electroimpact. Dry towpregs were laid down using a single spool feed and 30kg to 60kg of compaction was applied through a 50mm to 60mm diameter roller. Layup speeds varied from 4 meters to 25 meters per minute depending on the tackiness of the dry towpreg samples. Infrared heating was used to provide composite part surface temperatures of 30°C to 150°C, typically 35°C to 55°C.

[0042] Flat layups of the towpregs in Table 3 were prepared using the AFP process with layup speeds of 6 m / min to 20 m / min. 15 x 15 cm square specimens were prepared with 15 to 23 plies, each containing parallel straight towpregs. The first ply was deposited onto a release liner substrate and subsequent plies were applied onto the previously deposited dry towpreg layer using a 0 / 90 layup pattern. Towpreg B with 51 wt% fiber and Towpreg F with 51 wt% fiber were 、 Indicates poor adhesion to itself. , was unsuccessful.All other samples of dried towpreg showed excellent interlayer adhesion and good adhesion to the release liner. The samples ranged from 27% to 49% fiber by weight. Towpreg A had the lowest adhesion to the liner substrate, but this was compensated for by applying more heat, more pressure, and using a slower layup speed.

[0043] Towpregs C (at 44 wt% fiber) and D (at 49 wt% fiber) were deposited onto substrates with different tack levels and adhesion was evaluated by applying the dry towpregs in arcs of decreasing radius. A carbon substrate (HEXCEL HEXPLY M21E) was used as the "high" tack substrate and PET was used as the low tack substrate. Adhesion was judged qualitatively based on how the tows laid down. "Excellent" adhesion indicates the tows are free of wrinkles, bubbles, or other defects. "Good" adhesion indicates minimal defects, while "poor" adhesion indicates multiple defects.

[0044] Using the AFP procedure at 6 m / min, a series of dry towpregs were laid down in arcs of decreasing radius of curvature from 5000 mm to 250 mm. Both dry towpregs showed excellent adhesion to the carbon substrate out to a radius of 1250 mm and good adhesion at 1000 mm. Towpreg C showed excellent adhesion to the PET substrate out to a radius of 1500 mm and good adhesion at 1250 mm. Towpreg D showed excellent adhesion to the PET substrate out to a radius of 2500 mm and good adhesion at 2000 mm.

[0045] NEXTEL fabric DF-11 (manufactured by 3M Company) vinegar Prepreg in Rally B, Fill with 52% by weight of wet slurry based on the total weight of the prepreg. The fabric was passed twice using a benchtop laminator with a gap of 0.56 mm (22 mils). The prepreg was dried at ambient conditions to less than 3 wt. % water based on the weight of the preceramic matrix, resulting in 44 wt. % dry preceramic matrix based on the total weight of the dry prepreg. It became .

[0046] In one experiment, two 1.3 cm x 5.1 cm (0.5'' x 2'') pieces of dry prepreg were cut from the sample and oriented 90 degrees to each other. The cross construct was placed between two sheets of polyimide, which were then topped with two 10 cm x 10 cm aluminum plates. The stack was placed into a WABASH hydraulic press that was preheated to 90°C. The platens were then compressed to approximately 320 kg (700 lbs) and held for 5 minutes. The sample was then removed and allowed to cool to ambient temperature. The resulting cross ply exhibited sufficient structural integrity to be picked up by either piece of dry towpreg.

[0047] In a second experiment, 10 x 10 cm panels of dry prepreg fabric were placed next to each other with a 5 cm overlap and hot pressed. The contact area of ​​the above example intersection (1.3 cm x 1.3 cm = 1.7 cm 2 ) to the larger contact area of ​​this overlapping sample (5 × 10 cm = 50 cm 2 Considering the above, the press load was increased to 3200 kg. Press conditions were 90°C and 5 minutes. The resulting part showed sufficient structural integrity to be picked up by either panel of dry towpreg and maintained adhesion to adjacent panels.

[0048] Slurry E was impregnated into a fiber preform and dried to remove water. The resulting dried towpreg was 73 wt% preceramic matrix, 26 wt% ceramic fiber, and 1 wt% residual moisture. The dried prepreg material was used to fabricate a 19-ply layup using an automated fiber placement machine. The layup was vacuum bagged and consolidated in an autoclave with the following temperature and pressure profile: Upon initialization, the pressure was immediately increased to 1.3 MPa (195 psi) over 15-30 minutes, and the temperature was increased at 1.4 °C / min (2.5 °F / min) to 121 °C (350 °F) and held for 4 hours. Upon removal, the laminate was not tacky, was stiffer, and had a more uniform texture and appearance compared to the input layup, with adjacent bleeder layers being uniformly covered with matrix. The consolidated laminate was sintered in a box furnace by heating at 1° C. / min to 650° C., then 10° C. / min to 1175° C., followed by a 2 hour hold. The sintered laminate had a short beam shear strength of 15 MPa, a fiber fraction of about 26% by volume, and a total porosity of 32% by volume.

Claims

1. A dry prepreg comprising tows of ceramic fibers impregnated with a preceramic matrix, the preceramic matrix comprising from about 45 volume percent to about 60 volume percent of an inorganic oxide and from about 40 volume percent to about 55 volume percent of an organic binder system, based on the total volume of the preceramic matrix; the preceramic matrix is ​​free of aqueous solvent or the preceramic matrix contains an amount of aqueous solvent that is 4 weight percent or less based on the total weight of the preceramic matrix; The dry prepreg, wherein the organic binder system comprises a polymeric binder selected from the group consisting of polyurethane, polyvinyl alcohol, polyolefin oxide, polyvinyl acetate, polyoxazoline, polyacrylamide, polyvinylpyrrolidone, and copolymers and blends thereof.

2. 2. The dry prepreg of claim 1, wherein the organic binder system comprises the polymeric binder and a plasticizer for the polymeric binder.

3. 3. The dry prepreg of claim 2, wherein the polymer binder is water soluble.

4. 4. The dry prepreg of claim 2 or 3, wherein the plasticizer comprises glycerol.

5. A dry prepreg according to any one of claims 2 to 4, wherein the weight ratio of the plasticizer to the polymer binder in the organic binder system is from 10:90 to 30:

70.

6. The dry prepreg of any one of claims 1 to 5, wherein the preceramic matrix comprises at least 96 weight percent of the inorganic oxide and organic binder system, based on the total weight of the preceramic matrix.

7. The dry prepreg according to any one of claims 1 to 6, wherein the aqueous solvent comprises at least 95% by weight of water.

8. The dry prepreg of any one of claims 1 to 7, wherein the preceramic matrix contains an amount of aqueous solvent equal to or less than 1.5 wt%, based on the total weight of the preceramic matrix.

9. The dry prepreg of any one of claims 1 to 8, wherein the dry prepreg is a fabric comprising a plurality of tows of ceramic fibers impregnated with the preceramic matrix.

10. 10. The dry prepreg of claim 1, wherein the preceramic matrix comprises from 50 to 60 volume percent of the inorganic oxide and from 40 to 50 volume percent of the organic binder system, based on a total volume of the preceramic matrix.

11. The dry prepreg according to any one of claims 1 to 10, wherein the ceramic fiber tows have an aspect ratio (A2 / A1) of 0.05 or less, A1 being the width of the tow and A2 being the thickness of the tow.

12. 1. A method for producing a dry prepreg, comprising the steps of: (i) impregnating a tow of ceramic fibers with a ceramic matrix slurry comprising an inorganic oxide, an organic binder system comprising a polymeric binder, and an aqueous solvent; (ii) drying the slurry to form a preceramic matrix. Including, the preceramic matrix comprises from 45 volume percent to 60 volume percent of the inorganic oxide and from 40 volume percent to 55 volume percent of the organic binder system, based on a total volume of the preceramic matrix; The method, wherein the preceramic matrix is ​​free of aqueous solvent or the preceramic matrix contains an amount of aqueous solvent that is 4 weight percent or less based on the total weight of the preceramic matrix.

13. 13. The method of claim 12, wherein the organic binder system further comprises a plasticizer for the polymeric binder, and the weight ratio of the plasticizer to the polymeric binder in the organic binder system is from 10:90 to 30:

70.

14. The method of claim 12 or 13, wherein the polymeric binder is water-soluble.

15. 15. The method of any one of claims 12 to 14, wherein the polymeric binder is selected from the group consisting of polyurethanes, polyvinyl alcohols, polyolefin oxides, polyvinyl acetates, polyoxazolines, polyacrylamides, polyvinylpyrrolidones, and copolymers and blends thereof.

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