Reinforced glass and/or ceramic matrix composites

A process for preparing a fiber reinforced, glass matrix composite article of manufacture. The process employs an aqueous wet-laid technique by 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 the 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.

BACKGROUND OF THE INVENTION 
The present invention concerns a process for preparing reinforced glass 
matrix composites. 
Generally, fabrication of glass matrix composites is accomplished by 
impregnating a mat of fibrous material with a glass slurry. The 
impregnated fibers are then dried and stored as a prepreg or used 
directly. In any case, they are cut into desired form and molded under 
pressure and heated to fuse the glass matrix. Typical preparations as 
described above are disclosed in U.S. Pat. Nos. 4,511,663 and 4,485,179. 
U. K. Patent Application 52023/68 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 and hot 
pressed. Modifications of this process can be found in U.S. Pat. No. 
3,681,187. 
U.S. Pat. No. 4,263,367 attempts to improve the reinforcement of glass 
matrix composites by employing premanufactured isotropically laid, i.e., 
in-plane randomly oriented fiberst 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) at 663. Their process involves dispersing chopped carbon fibers 
and 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. 
SUMMARY OF THE INVENTION 
The present invention is a process for preparing a fiber-reinforced, glass 
matrix composite article of manufacture comprising: 
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; 
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 provides precise control of 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 is a process for preparing a 
lofted version of the fiber-reinforced, glass matrix composite article of 
manufacture by adding sequential step: 
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.

DETAILED DESCRIPTION OF THE INVENTION 
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 to form a composite mat. 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 
may then be hot pressed to form a fiber-reinforced glass composite 
article, beneficial results are obtained 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 gives 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. 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 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 about 45 to 
about 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 of manufacture. If the 
amount of glass fibers falls short of about 45 volume percent, it is 
believed 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 the group consisting of 
graphite fibers, metal coated graphite fibers, silica fibers, quartz 
fibers, ceramic fibers, metal fibers and 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. Pat. No. 4,511,663, the 
teachings of which are incorporated herein by reference, discloses the use 
of the following metals: 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. 
Pat. No. 4,485,179, can also be used. 
The amount of reinforcing fibers is suitably from about 3 to about 35 
percent by volume of the solids component. Amounts of reinforcing fibers 
of less than about three volume percent provide inadequate reinforcement. 
Amounts in excess of about 35 volume 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 fibers 
employed have an average length of at least 0. 125 inch (3 mm), preferably 
0.18 inch (4 mm) up to 1.00 inch (25 mm), preferably 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 they 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 about 1 
percent to about 20 percent by volume of the solids component. The amount 
is desirably from about 5 to about 15 volume percent. It has been found 
that with less than about one volume percent of binder, formation of an 
integral, composite mat is quite difficult. On the other hand, with 
greater than about twenty volume 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 
predensification 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 non-reactive 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 about 0 to about 15, desirably from about 0.5 to about 10 percent by 
volume, based on total volume of solids materials. 
The glass matrix composite articles of manufacture are formed, in 
accordance with the present invention, by a multi-step process. A dilute 
aqueous slurry is formed by dispersing the solids component comprising 
glass fibers, reinforcing fibers and at least one binder material in 
water. The slurry is then destabilized and the solids component is 
collected on a porous support. Collection can be assisted by vacuum. The 
collected solids component is dewatered and dried to form a dried mat. A 
single dried mat, or section thereof, may be hot pressed into a 
fiber-reinforced, glass matrix composite article of manufacture. Preferred 
results in terms of thickness and strength are obtained, however, when at 
least two dried mats or sections thereof are stacked together and then hot 
pressed. 
If desired, a thickener may 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 
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 article of manufacture 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.degree. 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 about 5 to about 100, desirably from about 10 to about 
50, percent by volume based upon total volume of solids, are readily 
obtained. Increases in thickness are calculated by dividing the difference 
in thickness by the original thickness (thickness before heating and 
lofting). The lofting is believed to enhance both the flexural and 
insulating properties of the article of manufacture. 
The collected and dried mats can be manufactured on conventional 
paper-making apparatus such as a sheet mold, Fourdrinier or cylinder 
machines. 
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 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 
Thicken 28 liters of water with 1 gram of xanthan gum. Approximately 19 g 
of a styrene/butadiene latex (50 weight percent solids) binder are added, 
with stirring to the thickened water. Next, 233 g of glass fibers of a 
length about 13 mm and diameter of 13 .mu.m and 60.6 g of nickel-coated 
graphite fibers having a diameter of 7 .mu.m and a length of 4 mm are 
added the aqueous media and stirred until a uniform dispersion is 
obtained. The uniform dispersion is destabilized with a cationic 
flocculant available under the trade name Betz 1260. The water is then 
drained and the solids are collected on a screen. The composite mats thus 
formed are dewatered by pressing and then dried. 
The dried composite mats are packed into a mold cavity 63 mm deep which is 
placed in a furnace and purged with argon to remove oxygen. The furnace is 
then evacuated to a pressure of 1.92 mm Hg and the temperature gradually 
increased to 1,245.degree. C. After about 20 minutes the pressure is 
increased to 2,070 psi (14.3 MPa) and the temperature is allowed to 
decrease at a rate of about 130.degree.-140.degree. C. per hour. At about 
890.degree. C., pressure is decreased to 230 psi (1.6 MPa) to prevent 
microcracking during solidification. The furnace is further cooled and the 
press opened. 
Considerable flash of glass existed on and about the mold. The sample 
removed from the mold 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.degree. C. in an 
air atmosphere to burn off the carbon fibers for 15 minutes. A weight loss 
of 27.26 percent occurred and is attributed to carbon fibers. The burned 
off sample was next reportedly 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 
nickel. This value corresponds favorably with the manufacturer's report of 
47-50 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: 
______________________________________ 
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. 
A specimen of the glass matrix composite, as prepared above, is prepared 
for measurement with a strain gauge. The measurements show an average 
flexural strength of 24,100 psi (166 MPa) and an average flexural modulus 
of 6,185,000 psi (42.6 GPa). The tensile strength is 8,900 psi (61 MPa) 
and the tensile modulus is 6,920,000 psi (47.7 GPa). The specimen exhibits 
these 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 bulk conductivity of the glass matrix composite is 132 
(.OMEGA.cm).sup.-1. This value is very close to the conductivity of 
metals, e.g., aluminum alloy 380 or nickel. 
Magnetic properties of the glass matrix composite include a measured 
saturation value which averages 15.85 emu/g. This corresponds to a 24.38 
percent nickel content (the saturation value for pure nickel is 65 emu/g). 
The glass matrix composite, when further examined with a scanning electron 
microscope, exhibits good bonding (or wetting) between the glass and the 
dispersed fibers. Less bonding is seen at the molded surfaces. This can be 
attributable 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 
reveals 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 glass 
matrix composites by an aqueous wet-laid technique. 
Example 2 
Four liters of water are thickened with 0.5 grams of xanthan gum. To the 
thickened water, approximately 1.40 grams of 25% solids ethylene acrylic 
acid copolymer dispersion in water (commercially available from The Dow 
Chemical Company under the trade designation Primacor.RTM. 4983, the 
solids portion is 20 weight percent acrylic acid, having a melt index of 
300) are added with stirring. Next, approximately 2.02 grams of 60% solids 
polyethylene minifiber pulp (commercially available from Lextar, a 
Hercules-Solvay Company, under the trade designation Pulpex E.RTM.), 
predispersed in a blender, are added. This is followed by the addition of 
12.97 grams of 6.35 mm long quartz fibers (commercially available from J. 
P. Stevens Co. under the trade designation Astroquartz.RTM. 556) fibers 
and 4.33 grams of 9 mm long siliconcarbide(SiC) fibers having a diameter 
of 10 to 15 .mu.m (commercially available from Dow Corning Corporation 
under the trade designation Nicalon.RTM.). All of the ingredients are 
stirred until a uniform dispersion is obtained. The pH is then adjusted to 
four with glacial acetic acid to destabilize the slurry. The destabilized 
slurry is drained onto a sheet forming screen and formed into a mat. The 
mat is passed through nip rolls to remove excess water and then dried. 
Each mat formed in this manner is approximately one (1) mm in thickness 
and has excellent wet and dry strength. 
Four of the mats are stacked together and premolded for five minutes at 
200.degree. C. and 350 psi (2.4 MPa) in order to melt the binders and 
partially densify the mats. The resulting partially densified mat is 
consolidated to 1.5 mm thickness, or approximately 25% of the final 
finished theoretical density. 
Nine round discs of the partially densified mat are cut and firmly stacked 
in a graphite die set (mold). 
Graphite foil is used to line the die to prevent sticking. The die set is 
placed into the vacuum furnace and pressed. The mold is heated in an argon 
purged vacuum to 600.degree. C. in 20 minutes. The vacuum during the cycle 
is 0.13 atm. 
At 20 minutes elapsed time pressure is applied at 113 psi (779 KPa). The 
temperature is then increased to 1645.degree. C. in 60 minutes more and 
the pressure is increased to 8500 psi (58.6 MPa). While maintaining the 
pressure of 58.6 MPa the temperature is then lowered to 1000.degree. C. 
over a period of 60 minutes after which the pressure is lowered to 170 psi 
(1172 KPa). The temperature is next decreased to 100.degree. C. over 90 
minutest the furnace is opened and the composite pieces are removed from 
the die set. 
Two 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 is 
found to be between 2.0.times.10.sup.4 and 2.0.times.10.sup.5 ohm-cm. 
Example 3 
Forty liters of water are thickened with 2 grams of xanthan gum thickener. 
Forty-three and one-half grams of 92% solids stainless steel fibers having 
a diameter of 8 .mu.m and a length of 6 mm (commercially available from 
Bekaert under the trade designation Beckinox.TM.) are stirred for five 
minutes into the slurry in order to debundle the fibers. The stainless 
steel fibers as purchased are coated with polyvinylalcohol amounting to 8 
weight percent of the total fiber weight. The polyvinylalcohol is water 
soluble. Next 17.14 grams of 35% solids ethylene acrylic acid copolymer 
dispersion (commercially available from The Dow Chemical Company under the 
trade designation Primacor.RTM. 4990) and 35 grams of 40% solids 
polyethylene pulp (commercially available from Lextar, a Hercules-Solvay 
Company, under the trade designation Pulpex E.RTM.) are added. Next, 140 
grams of magnesia-alumina-silicate glass fibers having a length of 13 mm 
and a diameter of 13 .mu.m (commercially available from Owens-Corning 
Fiberglas Corp. under the trade designation S-2 glass) are added. Stirring 
continues throughout the addition of ingredients and after until a uniform 
dispersion results. Finally the slurry is destabilized by adjusting the pH 
to acidic with 100 ml of 28 percent acetic acid. 
Ten four-liter batches of the slurry are prepared into mats by draining 
destabilized slurries onto a screen. The aqueous media is visibly examined 
after passing through the screen. No fibers are observed. The aqueous 
media is translucent. These mats are passed through nip rolls to remove 
excess water and dried. Five of the mats, measuring 12 inches by 12 inches 
(30.5 centimeters by 30.5 centimeters), are pressed at 200.degree. C. and 
350 psi (2.4 MPa) in order to melt the binders and preconsolidate the 
mats. The resulting preconsolidated composite mat is square, about 12 
inches (30.5 centimeters) on a side, and approximately 100 mils (0.25 cm) 
thick. It has a density which is 25% of the final theoretical density. Two 
of these preconsolidated mats are cut into a total of eight 6 inch by 6 
inch (15.2 cm by 15.2 cm) pieces. The eight pieces are stacked and placed 
in a graphite die set which is then placed in a vacuum furnace. After a 
vacuum was drawn to 200 .mu.m of Hg, the pressure on the dieset is set at 
200 psi (1.4 MPa) and the press is heated to 770.degree. C. This pressure 
and temperature combination is maintained for 20 minutes. The temperature 
is then increased to 1000.degree. C. at which time the pressure is 
increased to 1000 psi (6.9 MPa). The 1000.degree. C. and 1000 psi 
conditions are maintained for 50 minutes. The temperature is then lowered 
at a rate of 2.5.degree. C. per minute to a temperature of 775.degree. C. 
while maintaining 1000 psi pressure. At 775.degree. C., the pressure is 
reduced to 200 psi (1.4MPa) and the furnace is turned off. Two hours 
later, the die set is removed from the furnace and allowed to cool in air 
to 100.degree. C. 
The hot-pressed stack of mat pieces has a density of 2.93 g/cc which equals 
the theoretical density. In other words, there are substantially no voids 
in the stack. The composite after hot pressing comprises 22 weight percent 
reinforcing fibers and 78 weight percent glass matrix. Strain gauge 
measurements of a portion of the stack using the three point bending mode 
provide an average flexural stress of 19810 psi (136.6 MPa) and an average 
flexural modulus of 14,600,000 psi (100.7 GPa). 
Bulk resistivity in ohm-centimeters (.OMEGA.cm) and electromagnetic 
interference (EMI) shielding values, determined the Aperture Box method, 
in decibels at various frequencies, as measured in megahertz (Mhz) are 
shown in the table which follows Example 5. 
Example 4 
Example 3 is duplicated with two exceptions. The amount of stainless steel 
fibers is reduced to 32.6 grams and the amount of glass fibers is 
increased to 150 grams. This provides a solids component of the mat 
material of having 75 weight percent glass fibers, 15 weight percent 
reinforcing fibers and ten weight percent binder materials. After hot 
pressing to form a glass matrix, the composite comprises 16.7 weight 
percent reinforcing fibers and 83.3 weight percent glass. 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 is duplicated with two exceptions. The amount of stainless steel 
fibers is reduced to 21.7 grams and the amount of glass fibers is 
increased to 160 grams. This provides a solids component of the mat having 
80 weight percent glass fibers, ten weight percent reinforcing fibers and 
ten weight percent binder materials. After hot pressing to form a glass 
matrix, the composite comprises 11 weight percent reinforcing fibers and 
89 weight percent glass. 
Example 6 
Example 3 is duplicated with two exceptions. The amount of stainless steel 
fibers is reduced to 16.3 grams 
TABLE 
__________________________________________________________________________ 
Resistivity/Shielding Data 
Weight Volume 
Percent Percent Shielding Data (dB) 
Reinfor- Reinfor- 
Resis- 
at Various Frequencies 
Example 
cing cing tivity 
30 100 300 1000 
Number 
Fibers 
Fibers 
(.OMEGA.cm) 
(Mhz) 
(Mhz) 
(Mhz) 
(Mhz) 
__________________________________________________________________________ 
5 11.0 4 0.49 55 37 36 57 
4 16.7 6 0.21 68 48 44 66 
3 22.2 8 0.08 64 58 45 65 
__________________________________________________________________________ 
and the amount of glass fibers is increased to 165 grams. This provides a 
solids component of the mat material having 82.5 weight percent glass 
fibers, 7.5 weight percent reinforcing fibers and ten weight percent 
binder materials. After hot pressing to form a glass matrix, the composite 
comprises 8.3 weight percent reinforcing fibers and 91.7 weight percent 
glass. 
Comparative Example A 
An alternative method for preparing a glass matrix composite like that of 
Example 1 is as follows: adjust the pH of 28 liters of water to 8 with 
NH.sub.4 OH; add, with stirring, 112.9 grams of a 24.8% solids ethylene 
acrylic acid as a binder; add, with stirring, 206.6 g of glass 
microspheres and 50.4 g of nickel-coated graphite fibers (5 mm in length); 
adjust the pH to 4 by adding acetic acid to destabilize the suspension, 
drain the destabilized suspension using a screen to form a wet mat; 
dewater the wet mat by passing it through nip rolls; and dry the dewatered 
mat. The dried mat is then hot pressed into a glass matrix composite. The 
theoretical volume percent of reinforcing fibers in the hot-pressed glass 
matrix composite is 17.9. The actual volume percent of nickel-coated 
graphite fibers in the hot-pressed glass matrix composite is 46.8. 
The difference between the theoretical and and actual results is explained 
by loss of glass microspheres during mat formation and subsequent 
dewatering and by 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 is 
observed in Examples 1-6 wherein the matrix material is in the form of 
glass fibers. 
Example 7 
Four liters of water are thickened with 0.5 g of xanthan gum. Approximately 
1.4 g ethylene acrylic acid copolymer dispersion (0.35 g solids) are 
added, with stirring, to the thickened water. Next, approximately 2.02 g 
Pulpex E polyethylene minifiber pulp (1.21 g solids), predispersed in a 
blender, is added. This is followed by the addition of 12.97 g of 6.35 mm 
quartz fibers and 4.33 g of 9 mm silicon carbide fibers. All the 
ingredients are stirred until a uniform dispersion is obtained. The pH is 
adjusted to the acidic level (approximately pH 4) with acetic acid to 
destabilize the suspension. The slurry is drained onto a screen to form a 
mat which is then dewatered, pressed and dried. Mats formed in this manner 
are approximately 1 mm in thickness and have excellent wet and dry 
strength. 
Four of the dried mats are stacked onto each other and molded at 
200.degree. C., 350 psi (2.41 MPa) in order to melt the binders and 
partially consolidate the mats. The resulting partially consolidated mat 
has a thickness of approximately 1.5 mm and a density which is 
approximately 25 percent of theoretical density. 
Round discs of the partially densified mat are cut (3.81 cm diameter) and 
stacked in a graphite die set. A total of eleven of these discs (10.02 g) 
are placed in the mold. Graphite foil discs (0.013 cm) are placed between 
the rams and the material to prevent sticking after solidification and 
compaction. The die set is placed into a hot press furnace and pressed. 
The mold (die set) is heated in an argon-purged vacuum to 600.degree. C. 
in approximately 20 minutes under a pressure of 70 lbs (40 psi or 276 Pa) 
and held at this temperature and pressure for 30 minutes to burn off the 
binders. The temperature is then increased to 1,645.degree. C. over a 
period of 1 hour after which the pressure is increased to 1,770 lbs (1,000 
psi or 6.9 MPa). This combination of temperature and pressure is held for 
15 minutes. The temperature is then lowered to 1,000.degree. C. after 
which the pressure is reduced to 300 lbs (170 psi or 1.2 MPa). Finally, 
the temperature is reduced to 100.degree. C. and the furnace is opened. 
The mold is then opened and the molded article is removed. The molded 
article has 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. 
Comparative Example B 
One hundred sixty 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.RTM.) are 
slurried in 14 liters of water. The pH is adjusted to 8 with NH.sub.4 OH. 
Next, 64.5 grams of 25% solids ethylene acrylic acid copolymer dispersion 
(commercially available from The Dow Chemical Company under the trade 
designation Primacor.RTM. 4983) are added with stirring. All of the 
ingredients are stirred until a uniform dispersion is obtained. The pH is 
then adjusted to four with acetic acid to destabilize the slurry. The 
destabilized slurry is drained onto a screen and formed into a mat. The 
mat is passed through nip rolls to remove excess water and then dried. 
The dried composite mat is then cut into three inch by three inch (7.6 cm 
by 7.6 cm) squares. Enough of the mats to provide a total weight of 55.5 
grams are stacked in a graphite mold which is placed in a furnace and 
purged with argon to remove oxygen. The furnace is then evacuated to a 
vacuum of 28 (71 cm Hg) inches Hg (i.e. a pressure of 2 inches of Hg, 5 cm 
Hg) and the temperature is increased to 600.degree. C. over a period of 5 
minutes. After 10 minutes, the pressure is increased to 228 psi (1.6 MPa) 
and the temperature is increased, over a twenty minute period, to 
800.degree. C. Next, the pressure is increased to 1013 psi (7 MPa) and the 
temperature is increased, over a 45 minute period, to 1,175.degree. C. The 
pressure is then further increased to 1563 psi (10.8 MPa) after which the 
temperature is raised to 1275 .degree. C. over a ten minute period. The 
pressure is further increased to 2163 psi (14.9 MPa) and maintained at 
that level for a period of 150 minutes while the temperature is allowed to 
fall to 800.degree. C. The pressure is then decreased to 1660 psi (11.4 
MPa) and the temperature is allowed to fall to 740.degree. C. at which 
time the pressure is further decreased to 1010 psi (7 MPa). Further 
cooling takes place over 95 minutes as the temperature falls to 
200.degree. C., after which the pressure is decreased to 228 psi (1.6 
MPa). After further cooling to 80.degree. C., pressure is relieved, the 
press is opened and a hot-pressed composite is removed from the mold. 
The hot-pressed composite has 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, measuring 1.0 by 1.0 by 0.055 inch (2.54 by 
2.54 by 0.13 cm) and weighing 2.26 grams, is placed into a furnace and 
heated in air to a temperature of 840.degree. C. and maintained at that 
temperature for 60 minutes. The furnace is then cooled and the piece is 
removed from the furnace. The composites new dimensions are 1.003 by 1.003 
by 0.0723 inch. No loss of material is observed. The new density is 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 
is calculated. 
Although the densified composite is not made by the process of the present 
invention because glass particles rather than glass fibers are used to 
prepare the composite mats which are subsequently hot pressed to fully 
consolidate the composite similar results are obtained with densified 
composites prepared in accordance with the present invention, e.g., the 
composite formed in Example 7.