WO1991013838A1

WO1991013838A1 - A reinforced glass and/or ceramic matrix composite and process for preparing such composites

Info

Publication number: WO1991013838A1
Application number: PCT/US1990/001427
Authority: WO
Inventors: Selim Yalvac; Robert L. Mcgee
Original assignee: The Dow Chemical Company
Priority date: 1990-03-16
Filing date: 1990-03-16
Publication date: 1991-09-19
Also published as: EP0531280A4; JPH05504928A; EP0531280A1

Abstract

A process for preparing a fiber-reinforced, glass matrix composite and a composite as made by the process. The process employs forming a dilute aqueous slurry of solids comprising glass fibers, reinforcing fibers and binder material which may be partially or completely fibrous. The dilute aqueous slurry is destabilized and the solids are collected on a porous support means. They are then dewatered and dried to form a composite mat which can be hot-pressed into an article of manufacture. The hot-pressed article of manufacture may then be heated, in the absence of pressure, to a temperature above the softening point of the glass, but below the degradation temperature of the reinforcing fibers, to provide a lofted composite.

Description

A REINFORCED GLASS AND/OR CERAMIC MATRIX COMPOSITE AND PROCESS FOR PREPARING SUCH COMPOSITES

The present invention resides in a fiber reinforced glass and/or ceramic matrix composite and to a process for preparing such composites.

Generally, fabrication of glass matrix composites is accomplished by impregnating a mat of a reinforcing fibrous material with a slurry of a glass powder or particle. The impregnated mat of reinforcing fibers are then dried and stored as a prepreg or used directly. They can then be cut into a desired shape and molded under heat and pressure to fuse the glass matrix. Typical preparations as described above are disclosed in U.S. Patent Nos. 4,511,663 and 4,485,179.

ϋ. S. Patent No. 3,646,908, issued March 7, 1972, discloses a technique where a continuous length of glass fibers is first pulled over and under spreading rollers to form a tape and then into a bath containing a glass powder slurry. Excess slurry is removed from the wet tape before it is wound onto a flat sided drum so that the turns bond together. The turnings are removed -2-

and hot pressed. Modifications of this process can be found in U.S. Patent No. 3,681,187.

U.S. Patent No. 4,263,367 attempts to improve the reinforcement of glass matrix composites by employing premanufactured isotropically laid, i.e., in-plane randomly oriented fibers, graphite paper mats. After removing the binder material from the mats by solvent immersion or burning, the mats are dipped into a glass slurry. The mats are then stacked with alternating layers of powdered glass and hot pressed. The randomly oriented reinforcing fibers provide enhanced mechanical strength to the glass matrix composites.

Still another attempt to prepare glass matrices with reinforcing fibers is described in the article by Sambell, Bowen and Phillips, "Carbon Fiber Composites with Ceramic and Glass Matrices," Journal of Material Science, 7 (1972), p 663. Their process involves dispersing chopped carbon fibers and a powdered matrix material in isopropyl alcohol. The mixture is continuously agitated while the alcohol is removed by infrared radiant heat until the mixture has a stiff consistency. The mixture is then loaded into a die assembly and hot pressed.

While the above methods are satisfactory, they are quite labor intensive. Simple, more efficient methods with good composition control are desired.

The present invention particularly resides in a process for preparing a fiber-reinforced, glass matrix composite article, comprising the steps of: a. forming a dilute, aqueous slurry having a solids component comprising (1) reinforcing fibers, (2) glass fibers, and (3) at least one binder material; b. destabilizing said aqueous slurry; c. collecting said solids component on a porous support; d. dewatering and drying said collected solids to form a dried composite mat wherein the reinforcing fibers and the glass fibers are comingled and randomly oriented in the plane of the mat; and e. stacking a plurality of said mats or sections thereof and hot pressing said stack under conditions sufficient to fuse the glass fibers into a continuous glass matrix while substantially eliminating the binder material and retaining the integrity of the reinforcing fibers.

The process of the invention provides precise control over the volume fraction of the reinforcing material relative to the matrix material because loss of the matrix material during processing is negligible.

In a related embodiment, the present invention also resides in a process for preparing a lofted version of the fiber reinforced glass matrix composite article by heating the hot pressed composite article, in the absence of pressure, to a temperature above that at which softening of the glass matrix occurs but below that at which the reinforcing fibers degrade, and maintaining the composite article within that temperature range for a period of time sufficient to cause the composite article to loft and increase in thickness. The present invention additionally resides in a glass matrix composite comprising reinforcing fibers of a length of from 3 mm to 25 mm, said reinforcing fibers being selected from graphite fibers, metal-coated graphite fibers, silica fibers, quartz fibers, ceramic fibers, metal or metal alloy fibers, silica carbide, or mixtures thereof, fibers and comprising from 3 to 35 percent by volume of solids in the composite, wherein said reinforcing fibers are substantially uniformly distributed and randomly oriented in the composite.

The process of the present invention includes a number of steps, the first of which is forming a dilute aqueous slurry or suspension of a solids component. The solids component comprises glass fibers, reinforcing fibers and at least one binder material. The binder material(s) may also be fibrous. The slurry is then destabilized and wet-laid onto a porous support for collection of the solids components of the slurry on the support to form a composite mat. Collection of the solids component can be assisted by application of a vacuum. The composite mat is then dewatered and dried to form a dried mat wherein the glass fibers, the reinforcing fibers and, if present, the fibrous binding material are comingled and randomly oriented in the plane of the mat. Although a single mat or segment thereof can then be hot pressed to form a fiber- reinforced glass matrix composite article, beneficial results in terms of thickness and strength are obtained, however, when two or more mats, or segments thereof, are stacked together before hot pressing.

The present process provides a number of benefits. First, it facilitates incorporation of various ingredients which make up the solids component. Second, it results in a random orientation of the reinforcing fibers within both the dried mat and the resultant glass matrix composite article. The random orientation provides mechanical properties in the plane of the sheet which are quasi-isotropic, or generally the same regardless of direction in the plane of orientation. Third, it allows preparation of a consistent product wherein the volume fraction of reinforcing fibers relative to the glass matrix in the resultant article of manufacture is reproducible and generally identical to the volume fraction of the reinforcing fibers relative to the glass fibers in the solids component of the dilute aqueous slurry.

The process involves dispersing, in an aqueous media, the glass fibers which form the matrix following heat consolidation of the mats, reinforcing fibers and at least one binder material. Beneficial results are obtained when at least a portion of the binder material is in the form of fibers, e.g., polyolefin fibers. The use of binder fibers for most, if not all,_of the solids component aids in mat formation and collection, minimizes loss of solids and maximizes reproducibility of results. The order of addition of the glass fibers, reinforcing fibers and binder material(s) is not critical. However, desirable results are obtained when the glass fibers are added to the aqueous medium after the reinforcing fibers and binder material(s) are well dispersed.

Glass fibers suitable for purposes of the present invention are those which are dispersible in an aqueous medium and which can be deformed under heat and pressure to fuse into a unitary structure. Soda lime glass, borosilicate glass, quartz and lithium aluminum- silicate glass form suitable glass fibers. The glass fibers usually make up from 45 to 97 percent by volume of the solids component. If the amount of glass fibers exceeds about 97 volume percent, one cannot attain sufficient reinforcement of the glass matrix composite article. If the amount of glass fibers falls short of about 45 volume percent, there will be "matrix starvation" or an insufficient amount of matrix material to fill spaces between the reinforcing fibers following hot pressing of the mats into an article of manufacture.

Reinforcing fibers are suitably selected from graphite fibers, metal coated graphite fibers, silica fibers, quartz fibers, ceramic fibers, metal or metal alloy fibers , silicon carbide fibers, or mixtures thereof. The metal of the metal fibers and the metal coated graphite fibers should, under the hot pressing conditions, be substantially inert to materials of construction for molds used in hot pressing. The reinforcing fibers are beneficially stainless steel fibers or nickel coated graphite fibers.

The metal coating need not be nickel. U.S. Patent No. 4,511,663, discloses the use of the following metals which can be advantageously used as a metal coating for the reinforcing fibers of the invention: Y, Zr, Nb, Mo, Ag, Cd, Ta, W, Zn, Cu, Co, Fe, Mn, Ga, V, Ti, Sc, Al, Mg, Au and Pt. Magnetic or electrical properties of the metals are transferred to the resultant article of manufacture, provided sufficient metal is present. Silicon carbide fibers, as disclosed in U.S. Patent No. 4,485,179, can also be used.

The amount of reinforcing fibers is suitably from 3 to 35 percent by volume of the solids component. Amounts of reinforcing fibers of less than about 3 percent provide inadequate reinforcement. Amounts in excess of about 35 percent are believed to result in matrix starvation.

The reinforcing fibers are essentially uniformly dispersed throughout the glass matrix composite articles formed in accordance with the process of the present invention and randomly oriented in the plane defined by said articles, i.e., there is substantially no preferred orientation of the fibers in the x, y direction. The uniform dispersal and random orientation of the reinforcing fibers is also present in the dried mats from which the glass matrix composite articles of manufacture are formed. The reinforcing fibers employed have an average length of at least 0.125 inch (3 mm), preferably from 0.18 inch (4 mm) up to 1.00 inch (25 mm), more preferably an average length of about 0.75 inch (19 mm) .

The binder material is one which effectively assists in the collection of the solids component from the dilute aqueous slurry so that the solids can be destabilized and formed into a mat. Generally, the binder can be in the physical form of a fiber, powder, particle or aqueous dispersions thereof. Typical binder materials include starch, latex dispersions, synthetic polymers and natural polymers. The binder material is beneficially a synthetic or natural polymer.

The binder material is generally present in an amount of from 1 to 20 percent by volume of the solids component. The amount is desirably from 5 to 15 percent. It has been found that with less than about 1 percent of binder, formation of an integral, composite mat is quite difficult. On the other hand, with greater than about 20 percent of binder, hot pressing time is uneconomically increased in an effort to burn off or volatilize the binder. In addition, elimination of resulting porosity is difficult, if not impossible, within an economically reasonable period of time.

Latex binders having anionic or cationic bound charges in an amount sufficient to provide stabilization of the colloid can be employed if desired. Where necessary, a polymeric flocculant opposite in charge to the charged binder can be employed to aid in the destabilization of the colloid.

The binder material is desirably an ethylene/ acrylic acid copolymer, a polyolefin fiber, or a mixture of the copolymer and the polyolefin fiber. Illustrative fibrous binder materials include those formed from polyethylene, polypropylene, polyvinylchloride, polyester, polystyrene, and acrylonitrile/butadiene/ styrene copolymers.

The binder material is desirably a combination of an ethylene/acrylic acid copolymer and polyolefin fibers. This combination is advantageous because the ethylene/acrylic acid copolymer, when flocculated, enhances the wet strength of the collected solids component, or "wet mat". The polyolefin fibers add stiffness to the dried composite mat and aid in pre- densification thereof, presumably by melting and then solidifying.

In addition to the above three main components, other additive-type materials can be admixed in the aqueous slurry so long as they do not interfere with preparation of the glass matrix composite articles of manufacture or substantially degrade the properties thereof. For example, other fibrous materials and particulate fillers can be added to form hybrid composites. Also, colorants, processing aids such as thickeners, flocculants and pH adjusters can be included as well.

Filler materials are not an essential component of the glass matrix composite articles of manufacture prepared in accordance with the present invention. If used, filler materials may be in the form of powders or, preferably, fibers. Although particulate fillers are generally satisfactory, some loss thereof during processing is expected. Suitable particulate fillers include carbon blacks, metallic powders and other materials which are inert or nonreactive under process conditions of the present invention. Combinations of fibrous and particulate fillers can also be used. Illustrative filler material levels fall within a range of from 0 to 15, desirably from 0.5 to 10 percent by volume, based on total volume of solids materials.

If desired, a thickener can be added to the water in an amount sufficient to improve dispersion of the solids component. Also, any part of the solids component can be added in predispersed form to assist in forming a generally uniform dispersion of the solids component. The dried mat(s) may be partially densified before hot pressing.

The dried composite mat, whether densified fully, partially or not at all, can be stored as a prepreg or used directly. In any event, the composite mat is ultimately subjected to hot pressing to completely densify the solids component of the composite mat(s), fusing the glass fibers into a continuous matrix while retaining the generally uniform distribution and random orientation of the reinforcing fibers. Hot pressing also serves to bond the glass matrix to the reinforcing fibers and, if present, filler materials and other additives. The binder materials, volatilized during hot pressing, are not present in the finished article of manufacture. The hot-pressed composite is then cooled and removed from the pressurizing device employed.

The hot-pressed composite article can be further modified as desired by placing the article, or a portion thereof, in an air oven and heating it to a temperature above the softening temperature of the matrix material, e.g., 840°C, and maintaining that temperature for up to an hour or more. The oven is then turned off and allowed to cool. The cooled article is "lofted" in that it has a greater thickness and a lower density, without loss of material. Increases in thickness of at least 5 percent, e.g., from 5 to 100, desirably from 10 to 50, percent are readily obtainable. Increases in thickness are calculated by dividing the difference in thickness by the original thickness (thickness before heating and lofting). The lofting enhances both the flexural and insulating properties of the article of manufacture.

The collected and dried mats can be manufactured on a conventional paper making apparatus such as a sheet mold, Fourdrinier or cylinder machine.

The process of the present invention is illustrated by the following examples wherein all parts and percentages are by volume unless otherwise specified. Volume percentages for the hot-pressed composite mats are based upon the assumption that loss of matrix material, most likely to occur during the hot pressing step, is, for all practical purposes, zero. The binder materials are, of course, volatilized during processing. Examples of the present invention are identified by Arabic numerals whereas comparative examples are identified by capital alphabetic characters.

Example 1

28 liters of water were thickened with one gram of xanthan gum. Approximately 19 g of a styrene/ butadiene latex (50 weight percent solids) binder were added, with stirring to the thickened water. Next, 233 g of glass fibers (having a diameter of 13 microns and a length of about 1.25 cm) and 60.6 g of nickel- coated graphite fibers (having a diameter of 7 microns and a length of about 6.35 mm) were added to the aqueous media and stirred until a uniform dispersion was obtained. The uniform dispersion was destabilized with a cationic flocculant obtainable under the trade name Betz 1260. The water was then drained and the solids were collected on a screen. The composite mats thus formed were dewatered by pressing and then dried. The drained (white water) analysis indicated the absence of any type of fiber.

Several dried composite mats were placed in a mold, having a mold cavity depth of 63 mm, which was then placed in a furnace and purged with argon to remove oxygen. The furnace was then evacuated to a pressure of 1.92 mm Hg and the temperature gradually increased to 1245°C. After about 20 min. the pressure was increased to 2070 psi (14.3 MPa) and the temperature was allowed to decrease at a rate of about 130°C to 140°C per hour. At about 890°C, the pressure was decreased to 230 psi (1.6 MPa) to prevent microcracking during solidification. The furnace was further cooled, the press opened, and the fiber shell glass matrix composite removed.

Considerable flash of glass existed on and about the mold. The sample was removed from the mold and weighed. The sample weighed 30.69 g and had a density of 2.462 g/cc.

The composition of the sample was determined by grinding a small portion of the sample. The small ground portion was heated to 750°C for 15 min. in an air atmosphere to burn off the carbon fibers. A weight loss of 27.26 percent had occurred and is attributed to carbon fibers. The burned off sample was next repeatedly etched with concentrated nitric acid to dissolve the nickel, a weight loss of 22.32 percent occurred. The nickel coated graphite fibers were thus determined to be 45.02 weight percent (wgt.%) nickel. This value corresponds favorably with the manufacturer's report of 47 to 50 percent nickel content.

Various volumes are reported for the density of graphite fibers, from 1.75 g/cc to 1.82 g/cc. The observed composition of the composite is: -13-

Weight Volume Volume

Percent Percent* Percent**

Glass 50.42 52.33 53.17

Graphite Fiber 27.26 41.06 40.12

Nickel 22.32 6.61 6.72

*Graphite density taken as 1.75 g/cc. **Graphite density taken as 1.82 g/cc.

10

A specimen of the glass matrix composite, having a dimension of 7.62 cm x 11.4 mm x 2.2 mm, as prepared above, was measured with a strain gauge. The measurements showed an average flexural strength of

15 24,100 psi (166 MPa) and an average flexural modulus of 6,185,000 psi (42.6 GPa) . The tensile strength was 8,900 psi (61 MPa) and the tensile modulus was 6,920,000 psi (47.7 GPa). The specimen exhibited these 0 physical properties in a quasi-isotropic fashion, meaning that the strength is the same in any direction within the plane of the specimen tested.

The electrical conductivity of the glass matrix

25 composite was 132 (Ωcm)^"1. This value is very close to the conductivity of metals, e.g., aluminum alloy 380 or nickel.

Magnetic properties of the glass matrix -,_Q composite include a measured saturation value which averaged 15.85 emu/g (electromagnetic units/g). This corresponds to a 24.38 wgt.% nickel content (the saturation value for pure nickel is 65 emu/g).

The glass matrix composite, when further examined with a scanning electron microscope, exhibited good bonding (or wetting) between the glass and the dispersed fibers. Less bonding was seen at the molded surfaces. This was attributed to the high nickel coated graphite fiber content. Bonding can be improved by employing lower amounts of reinforcing fiber.

Further analysis of the composite by energy dispersive x-ray spectroscopy revealed dewetting of the nickel from the surface of the graphite fibers. This phenomenon is believed to be easily correctable simply by varying the mold conditions.

In summary, this example demonstrates the successful preparation of a glass matrix composite by an aqueous wet-laid technique.

Example 2

4 liters of water were thickened with 0.5 g of xanthan gum. To the thickened water, approximately 1.40 g of 25 percent solids ethylene acrylic acid copolymer dispersion in water (commercially available from The Dow Chemical Company under the trade designation PRIMACOR^® 4983, the solids portion was 20 percent by weight acrylic acid having a melt index of 3000) were added with stirring. Next, approximately 2.02 g of 60 percent solids polyethylene minifiber pulp (commercially available from Lextar, a Hercules-Solvay Company, under the trade designation PULPEX E^®), predispersed in a blender, were added. This was followed by the addition of 12.97 g of 6.35 mm long quartz fibers (commercially available from J. P. Stevens Co. under the trade designation ASTROQϋARTZ^®) and 4.33 g of 9 mm long silicon carbide (SiC) fibers (commercially available from Dow Corning Corporation under the trade designation NICALON^®) having a diameter of from 10 to 15 microns. All of the ingredients were stirred until a uniform dispersion was obtained. The pH was then adjusted to 4 with the addition of glacial acetic acid to destabilize the slurry. The destabilized slurry was drained onto a screen and formed into a mat. The mat was passed through nip rolls to remove excess water and then dried. Each mat formed in this manner was about 1 mm thick and had excellent wet and dry strength.

Four of the mats were stacked together and premolded for 5 min. at a temperature of 200°C and at a pressure of 350 psi (2.4 MPa) in order to melt the binders and partially densify the mats. The resulting partially densified mat was consolidated to 1.5 mm thickness, or approximately 25 percent of the final finished theoretical density.

9 round discs of the partially densified mat were cut and firmly stacked in a graphite die set (mold). Graphite foil was used to line the die to prevent sticking. The die set was placed into the vacuum furnace and pressed. The mold was heated in an argon purged vacuum to 600°C in 20 min. The vacuum applied during the cycle was 0.13 atm.

After 20 min. of elapsed time, pressure was applied at 113 psi (779 kPa) . The temperature was then increased to 1645^βC in 60 min. more and the pressure increased to 8500 psi (58.6 MPa). While maintaining the pressure at 58.6 MPa, the temperature was then lowered to 1000°C over a period of 60 min. after which the pressure was lowered to 170 psi (1171 kPa). The temperature was decreased to 100°C, over a period of 90 min., the furnace was opened and the composite pieces removed from the die set.

2 disc specimens according to Example 2 are found to have densities of 2.235 g/cc and 2.241 g/cc. The samples have a theoretical density at a ratio of quartz/SiC fiber of 75/25 of 2.278 g/cc. The bulk resistivity was found to be between 2.0 X 10⁴ and 2.0 X 10⁵ ohm-cm.

Example 3

40 liters of water were thickened with 2 g of xanthan gum thickener. 43.5 g of 92 percent stainless steel fibers (commercially available from 5 Bekaert under the trade designation BECKINOX™) having an average length of 6 mm and a diameter of 8 microns, were stirred for 5 min. into the slurry in order to debundle the fibers. The stainless steel fibers as purchased, were coated with polyvinyl alcohol amounting to 0 8 percent by weight, based on the total weight of the fibers. The polyvinyl alcohol is water soluble. Next, 17.14 g of 35 percent solids ethylene acrylic acid copolymer dispersion (commercially available from _t- The Dow Chemical Company under the trade designation PRIMACOR^® 4990) and 35 g of 40 percent solids poly¬ ethylene pulp (commercially available from Lextar, a Hercules-Solvay Company, under the trade designation PULPEX E^®) were added. Next, 140 g of magnesia-alumina- 0 silicate glass fibers having an average length of 12.7 mm and a diameter of 13 microns, (commercially available from Owens-Corning Fiberglas Corp. under the trade designation S-2 glass) were added. Stirring continued throughout the addition of ingredients and after until a uniform dispersion resulted. Finally the slurry was destabilized by adjusting the pH to acidic with 100 ml of 28 percent acetic acid.

10 batches of the slurry of 4 liters each, were prepared into mats by draining the destabilized slurries onto a screen. The aqueous media was visually examined after passing through the screen. No fibers were observed and the aqueous media was found to be translucent. These mats were passed through nip rolls to remove excess water and dried. 5 of the mats, 0 measuring 12 in. x 12 in. (30.5 cm x 30.5 cm) were pressed at a temperature of 200°C and at a pressure of 350 psi (2.4 MPa) in order to melt the binders and preconsolidate the mats. The resulting preconsolidated _C- composite mat had a thickness of approximately 100 mils (0.25 cm) and a density which was 25 percent of the final theoretical density. 2 of these preconsolidated mats were cut into a total of 8 pieces, each having a dimension of 6 in. x 6 in. (15.2 cm x 15.2 cm). The 8 0 pieces were stacked and placed in a graphite die set which was then placed in a vacuum furnace. After a vacuum was drawn to 200 mm of Hg, the pressure on the die set was set at 200 psi (1.38 MPa) and the press was heated to 770°C. This pressure and temperature 5 combination was maintained for 20 min. The temperature was then increased to 1000°C at which time the pressure was increased to 1000 psi (6.89 MPa). The 1000°C and 1000 psi (6.89 MPa) conditions were maintained for 50 0 min. The temperature was then lowered at a rate of 2.5°C/min. to a temperature of 775°C while maintaining a pressure of 1000 psi (6.89 MPa). At a temperature of 775°C, the pressure was reduced to 200 psi (1.4MPa) and the furnace was turned off. 2 hours later, the die set was removed from the furnace and allowed to cool in air to a temperature of 100°C.

The hot-pressed stack of mat pieces had a density of 2.93 g/cc which equals the theoretical density. In other words, there were substantially no voids in the stack. The composite after hot pressing contained 92 volume percent (vol.%) glass matrix (magnesia-alumina-silicate) and 8 vol.% stainless steel reinforcing fibers, corresponding to 22 wgt.% reinforcing fibers and 78 wgt.% glass matrix. Strain gauge measurements of a portion of the stack using the three point bending mode resulted in an average flexural stress of 19810 psi (136.5 MPa) and an average flexural modulus of 14.6 x 10⁶ psi (100.6 x 10⁹ Pa).

Resistivity in ohm-centimeters (Ωcm) and electromagnetic interference (EMI) shielding values, determined by the Aperture Box method, in decibels at various frequencies and as measured in megahertz (Mhz), are shown in the table which follows Example 5.

Example 4

Example 3 was duplicated with two exceptions. The amount of stainless steel fibers was reduced to

32.6 g and the amount of glass fibers was increased to 150 g. This provided a solids component of the mat material of 75 wgt.% glass fibers, 15 wgt.% reinforcing fibers and 10 wgt.% binder materials. After hot pressing to form a matrix, the composite comprised

16.7 wgt.% reinforcing fibers and 83.3 wgt.% glass matrix, corresponding to 90 vol.% glass matrix and

6 vol.% stainless steel reinforcing fibers. A second method of determining EMI Shielding values is known as the Transmission Line method. Use of the second method of this example produced the following results: 30 MHz - 58 dB; 100 MHz - 58 dB; 300 MHz - 61 dB; 1000 MHz - 73 dB. The differences between these results and those in the table are due to the enhanced degree of accuracy of this method over the Aperture Box method.

Example 5

Example 3 was duplicated with two exceptions. The amount of stainless steel fibers was reduced to 21.7 g and the amount of glass fibers increased to 160 g. This provided a solids component of the mat having 80 wgt.% glass fibers, 10 wgt.% reinforcing fibers and 10 wgt.% binder materials. After hot pressing to form the glass matrix, the composite contained 96 vol.% glass matrix and 4 vol.% stainless steel reinforcing fibers, corresponding to 11 wgt.% reinforcing fibers and 89 wgt.% glass matrix.

Resistivity/Shielding Data

Shielding Data (dB) Vol. _% at Various Frequencies

Ex. Reinforcing Resistivity _Fibers (Ωcm) ^ ^ ^^ _(Mh2)

5 4 0.49 55 37 36 57 4 6 0.21 68 48 44 66

3 8 0.08 64 58 45 65 Example 6

Example 3 was duplicated with 2 exceptions. The amount of stainless steel fibers was reduced to 16.3 g and the amount of glass fibers increased to 165 g. This provided a solids component of the mat material having 82.5 wgt.% glass fibers, 7.5 wgt.% reinforcing fibers and 10 wgt.% binder materials. After hot pressing to form a glass matrix, the composite contained 94 vol.% of the glass matrix and 3 vol.% of 0 the SS reinforcing fibers, corresponding to 8.3 wgt.% reinforcing fibers and 91.7 wgt.% glass matrix.

Comparative Example A

5 In a comparative method, a glass matrix composite was prepared in accordance with the procedure of Example 1 using glass microspheres instead of glass fibers, as follows: the pH of 28 1 of water was adjusted to 8 with NH₄OH; 112.9 g of a 24.8 percent 0 solids ethylene acrylic acid was added, with stirring, as a binder; 206.6 g of glass microspheres-and 50.4 g of nickel-coated graphite fibers (5 mm in length) were added, with stirring; the pH was adjusted to 4 by adding _t- acetic acid to destabilize the suspension; the destabilized suspension was drained using a screen to form a wet mat; the wet mat was dewatered by passing it through nip rolls; and the dewatered mat was dried. The dried mat was then hot pressed into a glass matrix 0 composite. The theoretical content of reinforcing fibers in the hot-pressed glass matrix composite was calculated to be 17.9 vol.%. The actual content of the fibers in the hot-pressed glass matrix composite was 46.8 vol.%. The difference between the theoretical and the actual results is due to the loss of glass microspheres during mat formation and subsequent dewatering and due to the loss of glass matrix material during hot pressing of the mats. Although the latter loss can be minimized by tighter control of hot pressing conditions, the former loss is difficult to minimize.

By way of contrast, no loss of matrix material or reinforcing fibers was observed in Examples 1 through 6 wherein the matrix material was in the form of glass fibers.

Example 7

5 4 liters of water were thickened with 0.5 g of xanthan gum. Approximately 1.4 g ethylene acrylic acid copolymer dispersion (0.35 g solids) were added, with stirring, to the thickened water. Next, approximately 2.02 g PULPEX E^® polyethylene minifiber pulp (1.21 g 0 solids), predispersed in a blender, were added. This mixture was followed by the addition of 12.97 g of quartz fibers having an average length of 6.35 mm and a diameter of 9 microns, and 4.33 g of silicon carbide c fibers having an average length of 9 mm. All the ingredients were stirred until a uniform dispersion was obtained. The pH was adjusted to the acidic level (approximately pH 4) with the addition of acetic acid to destabilize the suspension. The slurry was drained onto 0 a screen to form a mat which was then dewatered, pressed and dried. Mats formed in this manner were approximately 1 mm in thickness and had excellent wet and dry strength. 4 of the dried mats were stacked onto each other and molded at a temperature of 200°C, and a pressure of 350 psi (2.41 MPa) in order to melt the binders and partially consolidate the mats. The resulting partially consolidated mat had a thickness of approximately 1.5 mm and a density which was approximately 25 percent of theoretical density.

Round discs of the partially densified mat were cut (3.81 cm diameter) and stacked in a graphite die set. A total of 11 of these discs (10.02 g) were placed in the mold. Graphite foil discs (.013 cm) were placed between the rams and the material to prevent sticking after solidification and compaction. The die set was placed into a hot press furnace and pressed. The mold (die set) was heated in an argon-purged vacuum to a temperature of 600°C in approximately 20 min. under a pressure of 70 lbs. (40 psi or 276 Pa) and held at this temperature and pressure for 30 min. to burn off the binders. The temperature was then increased to 1,645°C over a period of 1 hour after which the pressure was increased to 1,770 lbs (1,000 psi or 6.9 MPa). This combination of temperature and pressure was held for 15 min. The temperature was then lowered to 1,000°C after which the pressure was reduced to 300 lbs (170 psi or 1.2 MPa). Finally, the temperature was reduced to 100°C and the furnace was opened. The mold was then opened and the molded article removed. The molded article had quasi-isotropic properties and a density of 2.252 g/cc which corresponds to 1.1 percent residual porosity (theoretical density was 2.278 g/cc). No extrusion of quartz was observed. An analysis of the composite showed that it contained 78 vol.% of the glass matrix and 22 vol.% of the Silicon Carbide reinforcing fibers. Comparative Ex. B - Preparation of a Lofted Composite

160 grams of borosilicate glass microspheres (commercially available from PQ Industries under the trade designation 3000 E) and 40 grams of 9 mm long silicon carbide (SiC) fibers (commercially available from Dow Corning Corporation under the trade designation Nicalon^®) were slurried in 14 1 of water. The pH was adjusted to 8 with NH₄OH. Next, 64.5 g of 25 percent solids ethylene acrylic acid copolymer dispersion (commercially available from The Dow Chemical Company under the trade designation PRIMACOR^® 4983) were added with stirring. All of the ingredients were stirred until a uniform dispersion was obtained. The pH was then adjusted to 4 with acetic acid to destabilize the slurry. The destabilized slurry was drained onto a screen and formed into a mat. The mat was passed through nip rolls to remove excess water and then dried.

The dried composite mat was then cut into 3 in. x 3 in. (7.6 cm x 7.6 cm) squares. Enough of the mats to provide a total weight of 55.5 g were stacked in a graphite mold which was placed in a furnace and purged with argon to remove oxygen. The furnace was then evacuated to a vacuum of 28 in. (71 cm) Hg and the temperature was increased to 600°C over a period of 5 min. After 10 min., the pressure was increased to 228 psi (1.6 MPa) and the temperature increased, over a 20 rain, period, to 800°C. Next, the pressure was increased to 1013 psi (7 MPa) and the temperature increased, over a 45 min. period, to 1,175°C. The pressure was then further increased to 1563 psi (10.8 MPa) after which the temperature was raised to 1275°C over a 10 min. period. The pressure was further increased to 2163 psi (14.9 MPa) and maintained at that level for a period of 150 min. while the temperature was allowed to fall to 800°C.

The pressure was then decreased to 1660 psi (11.4 MPa) and the temperature allowed to fall to 740°C, at which time the pressure was further decreased to 1010 psi

(7 MPa). Further cooling took place over 95 min. as the temperature was reduced to 200°C, after which the pressure was decreased to 228 psi (1.6 MPa). After further cooling to 80°C, the pressure was relieved, the press was opened, and the hot-pressed composite removed from the mold.

The hot-pressed composite had a density of 2.517 g/cc which corresponds to 1.1 percent residual porosity (theoretical density of 2.545 g/cc).

Example 8

A small piece of the composite of comparative example B, measuring 1.0 in. x 1.0 in. x 0.055 in. (2.54 cm x 2.54 cm x 0.13 cm) and weighing 2.26 g, was placed into a furnace and heated in air to a temperature of 840°C and maintained at that temperature for 60 min. The furnace was then cooled and the piece removed from the furnace. The composites new dimensions were 1.003 in. x 1.003 in. x 0.0723 in. (2.548 cm x 2.548 cm x 0.184 cm). No loss of material was observed. The new density was 1.89 g/cc. By dividing the difference between the thickness after heating and before heating by the thickness before heating a lofting percentage of 32 was calculated.

Although this densified composite was not made by the process of the present invention because glass particles rather than glass fibers were used to prepare the composite mats which were subsequently hot pressed to fully consolidate the composite, similar results were obtained with densified composites prepared in accordance with the present invention, e.g., the composite formed in Example 7.

Claims

Claims:

1. A process for preparing a fiber-reinforced, glass matrix composite, comprising the steps of: a. forming a dilute, aqueous slurry having a solids component comprising: (1) reinforcing fibers, (2) glass fibers, and (3) at least one binder material; b. destabilizing said aqueous slurry; c. collecting said solids component on a porous support; d. dewatering and drying said collected solids to form a dried composite mat wherein the reinforcing fibers and the glass fibers are comingled and randomly oriented in the plane of the mat; and e. stacking a plurality of said mats, or sections thereof, and hot pressing said stack under conditions sufficient to fuse the glass fibers into a continuous glass matrix while substantially eliminating the binder material and retaining the integrity of the reinforcing fibers.

2. The process of Claim 1, wherein the solids component comprises from 3 to 35 volume percent reinforcing fiber, from 45 to 97 volume percent glass fiber and from 1 to 20 volume percent binder material.

3. The process of Claim 1 or 2, wherein the reinforcing fibers have a length of from 3 mm to 25 mm and are selected from graphite fibers, metal coated graphite fibers, silica fibers, silicon carbide fibers, quartz fibers, ceramic fibers, metal or metal alloy fibers, or mixtures thereof.

4. The process of Claim 3, wherein the reinforcing fibers are selected from stainless steel fibers, nickel coated graphite fibers, or mixtures thereof.

5. The process of any one of the preceding claims, wherein the glass fibers are selected from soda lime glass, borosilicate glass, quartz, lithium-aluminum glass, or mixtures thereof.

6. The process of any one of the preceding claims, wherein the binder material is selected from starch, latex dispersions, a synthetic or natural polymer in an amount of from 1 to 20 percent by volume.

7. The process of Claim 6, wherein the binder is selected from ethylene/acrylic acid copolymers, polyolefin fibers, or mixtures thereof.

8. The process of Claim 1, including the additional step of: f. heating the hot-pressed composite article, in the absence of pressure, to a temperature above that at which softening of the glass matrix occurs but below that at which the reinforcing fibers degrade, and maintaining the composite article within that temperature range for a period of time sufficient to cause the composite article to increase in thickness by at least 5 percent based upon the quotient of the difference in thickness divided by the original thickness.

9. A glass matrix composite comprising reinforcing fibers of a length from 3 mm to 25 mm selected from graphite fibers, metal-coated graphite fibers, silica fibers, quartz fibers, ceramic fibers, metal fibers, silicon carbide fibers from 3 to 35 percent volume of solids wherein the fibers are generally uniformly distributed and randomly oriented in the glass matrix composite.

10. The composite of Claim 9, wherein the glass fibers are present in an amount of from 45 to 97 percent volume and are selected from soda lime glass, borosilicate glass, quartz, lithium aluminum-silicate glass, or mixtures thereof.

11. The composite of Claim 9 or 10, wherein the composite has a porosity of from 5 to 50 percent.