A high char yielding matrix resin for use in fabricating carbon-carbon composites comprised of a polymerizable mixture of a diethynylbenzene monomer and an ethynylpyrene monomer.

BACKGROUND OF THE INVENTION 
This invention relates to aromaticacetylene compounds and their utilization 
as ablative materials for re-entry vehicles. In a more particular aspect, 
this invention relates to the synthesis of diethynylbenzene-ethynylpyrene 
copolymers and their use as high char yielding matrix resins for 
carbon-carbon composites. 
The increased use and interest in the operation of re-entry vehicles has 
generated a considerable research effort in an attempt to develop 
structural materials that exhibit high strength and resistance to the 
stresses and strains encountered by space vehicles during their re-entry 
regime. Rocket and missile components, such as turbine blades, nozzles, 
vanes, partitions and, especially, nose cones, are very vulnerable to the 
stress and strain encountered during their re-entry environment. These 
components require structural materials capable of surviving those 
stresses and the elevated temperatures occurring during re-entry. 
Graphite carbon, in the form of a carbon-carbon composite, using pitchas a 
resinous matrix, has been found to be useful in fabricating structural 
component. These materials possess many of the characteristics required by 
structural elements subjected to the stress of a high temperature re-entry 
environment. The carbon-carbon materials have proven to be especially 
effective for nose tip applications and show good thermal stress 
performance. Unfortunately, these materials often times do not show 
sufficient mechanical strength, disclose unpredicted anomalies in their 
ablation characteristics, and require expensive, high pressure processing 
techniques. 
With the present invention, however, it has been found that carbon-carbon 
composites having superior re-entry characteristics can be produced simply 
and easily by utilizing a novel aromaticacetylene copolymer as a high char 
yielding matrix resin for the carbon-carbon composite in lieu of the 
previously used pitch. The novel copolymer matrix resin is derived by 
effecting the copolymerization of a mixture of diethynylbenzene and 
ethynylpyrene. 
The reaction mechanism does not require high pressure processing parameters 
and the resulting copolymer chars easily when utilized as a matrix 
impregnant for graphite fibers. It easily wets the graphite fibers and 
penetrates into the pores of the fibers. 
The present invention replaces the ill defined, variable composition 
pitches utilized heretofore as an impregnant and matrix resin for the 
carbon-carbon composites produced heretofore. The pitches are invariably 
contaminated with S, O, N, P, ash, and other materials. In addition, the 
pitch is not homopolymerizable, therefore, it can exude from the 
impregnated woven carbon composites during processing and by virtue of the 
fact that it is not comprised strictly of aromatic hydrocarbon has a lower 
carbon content than the material of the instant invention. As a 
consequence, pitch has a much lower char yield. The instant invention 
provides processing simplification far beyond the current state of the art 
pitch and provides more dependable performance characteristics for the 
carbon-carbon composite products derived therefrom. 
SUMMARY OF THE INVENTION 
The present invention concerns itself with the synthesis of a novel 
aromatic acetylene copolymer derived from a mixture of diethynylbenzene 
and ethynylpyrene and its use as a matrix resin for carbon-carbon 
composites. The copolymers are unique in that they char very efficiently 
in yields as high as 95%. Additionally, the chars are capable of 
graphitizing when heated to temperatures of 2400.degree.-2800.degree. C. 
The prepolymer mixtures are very fluid when melted and, consequently, they 
can readily impregnate a woven carbon fiber fabric. In addition, they 
homopolymerize when heated above 100.degree. C. and, with a sufficient 
proportion of ethynylpyrene, the homopolymerization rate can be 
controlled, and runaway reactions can be prevented. The novelty of this 
invention resides in the fact that it provides a material system which 
yields high char, graphitizable, low viscosity, easy to process matrix 
resins for carbon-carbon composites. The composites are especially 
effective for use as re-entry missile nose cones. The copolymer of the 
present invention has all the properties necessary to easily produce high 
density carbons with minimal pressure requirements for fabrication. 
Accordingly, the primary object is to provide an easily processable matrix 
resin for carbon-carbon components. 
Another object of this invention is to provide a carbon-carbon matrix resin 
precursor of specific and known composition which can replace currently 
used pitch which has an undefined and never reproducible composition. 
Still another object of this invention is to provide a carbon-carbon matrix 
resin precursor which can be processed at pressures below 500 psi, thus 
eliminating the need for high pressures in the range of 15,000 psi. 
A further object of this invention is to provide a carbon-carbon matrix 
resin precursor which can cure without runaway exotherms. 
Still a further object of this invention is to provide a carbon-carbon 
matrix resin precursor which graphitizes efficiently into high density 
graphite. 
A still further object of this invention is to provide a low melting 
(&lt;120.degree. C.) carbon-carbon matrix resin precursor. 
A still further object of this invention is to provide a carbon-carbon 
matrix resin precursor which has a sufficiently low vapor pressure such as 
to allow it to be processed without excessive loss due to evaporation. 
A still further object of this invention is to provide a low viscosity, 
carbon-carbon matrix resin precursor which can effectively wet carbon and 
graphite fibers and fabrics and which can efficiently penetrate into the 
pores of the fibers. 
The above and still further objects and advantages of the present invention 
will become more readily apparent upon consideration of the following 
detailed description of its preferred embodiments. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with this invention, it has been found that the above-defined 
objects can be accomplished by synthesizing novel polyaromatic acetylene 
copolymers derived by effecting a reaction between diethynylbenzene and 
ethynylpyrene to produce a novel copolymer which can be effectively 
utilized as a high char yielding carbon-carbon matrix resin precursor. The 
matrix resin precursor of this invention replaces the variable composition 
pitches utilized heretofore as a carbon-carbon matrix resin. In the prior 
art, pitch is procured by trade name. However, its composition varies from 
batch to batch since it is a mixture of an innumerable number of naturally 
occuring compounds. It is never pure, and contains undesirable elements of 
N, S, O, P and ash. In the present state of the art, pitch is used to 
impregnate woven carbon/carbon fabrics at pressures up to 15,000 psi. The 
impregnated masses are then carbonized under pressure followed by 
graphitization up to 2700.degree. C. Subsequently, they are reprocessed 
about 10 times until maximum densification is achieved. The necessary 
repetitious processing results in a very high cost product even though the 
pitch component is very inexpensive. Furthermore, contaminants such as S, 
O, N, P and ash cause the composite's erosion rates to be inconsistent. 
The disadvantages of using pitch as a matrix resin, however, are overcome 
by the present invention. Specifically, the invention is a mixture of two 
compounds whose structures are shown below. 
##STR1## 
By itself diethynylbenzene is too volatile, and polymerizes too rapidly to 
permit its use in the fabrication of carbon-carbon composites. In 
addition, its char is very difficult to graphitize even when heated to 
2700.degree. C. However, it has two excellent advantages, in that the meta 
isomer is a fluid, and it produces a 94-95% yield of char when pyrolyzed. 
In contrast, ethynylpyrene is an excellent graphite former, but produces a 
lower char yield (.perspectiveto.45%) at atmospheric pressure. Its char 
yield can, however, be increased significantly if it is charred under 
pressure (up to 75-80% at 300 psi). Another advantage is its much slower 
thermal polymerization rate and another disadvantage is the fact that it 
is a solid melting in the range of 120.degree. C. 
With the present invention, however, it has been found that a copolymer 
synthesized from a polymerizable mixture of diethynylbenzene and 
ethynylpyrene has all of the advantages and none of the disadvantages. The 
mixture polymerizes at a controllable rate at temperatures in the range of 
120.degree. C., they char very efficiently (over 85%) after 
polymerization, and the chars graphitize efficiently in the 
2300.degree.-2800.degree. C. temperature range. In addition, the mixture 
has a low melting temperature e.g., 60.degree.-80.degree. C. It is also 
fluid enough to allow it to penetrate deeply into the pores and 
intersticies of woven carbon-carbon fabrics. The mixture, thus, has a 
major advantage over the use of pitch or even the individual benzene and 
pyrene monomers as matrix resin components. 
Generally, it is preferred to use diethynylbenzene: ethynylpyrene weight 
ratios between 1:4 and 1:1, respectively. However, for some applications 
other ratios may prove to be preferable. Thus, ratios of 1:9 and 9:1 
constitute the range of ratios which have been found useful for the 
purposes of this invention. 
In utilizing the materials of this invention, the mixture of monomers is 
prepared and melted together to provide maximum homogenity. It is then 
impregnated into a degassed woven carbon/carbon fabric. While maintaining 
the pressure between 100 and 500 psi and heating at about 
100.degree.-130.degree. C. for 4-72 hours, polymerization is promoted. 
While maintaining the pressure, the composites are heated above 
500.degree. C. to promote carbonization. Graphitization can then be 
promoted using conventional state-of-the-art graphitization conditions.

Different synthesis procedures were used in the preparation of the 
ethynylated aromatic hydrocarbons of this invention. These procedures are 
illustrated in Table I in general form. In many cases, two or three 
methods were used before the preferred method was identified. Method A was 
used only for the synthesis of meta- and para-diethynylbenzene (DEB) from 
a mixture of divinylbenzene isomers, even though DEB isomers were also 
produced successfully by the Vilsmeier process (Method B). 
The polymerization of ethynylated aromatic hydrocarbons tends to proceed 
quite rapidly at or above 160.degree.-180.degree. C., but runaway 
reactions can also occur at even lower temperatures due to the liberated 
heat of reaction. Nevertheless, controlled polymerizations can be achieved 
at temperatures between 100.degree.-150.degree. C. For example, controlled 
polymerization of a 50:50 mixture of 1-ethynylpyrene and diethynylbenzene 
has also been achieved at 125.degree. C. and a high quality 3D 
carbon-carbon composite was produced. At these lower temperatures, where 
polymerization is slow, pressure is required to prevent monomer 
evaporation. 
Synthesis of small quantities of ethynylpyrene (EP) by the Method C was 
successfully repeated several times. Subsequently, 1 kilogram was 
prepared. This latter material melted at a slightly lower temperature 
(102.degree.-108.degree. C. versus 109.degree.-113.degree. C.), but its 
infrared spectrum indicated that it was identical to the original 
material. The difference in melting range is attributed to a slightly 
different concentration of isomers, probably due to a difference in 
solvents (nitrobenzene vs. methylene chloride) used in the synthesis. The 
1 kilogram lot was used for carbon-carbon composite fabrication purposes. 
TABLE I 
__________________________________________________________________________ 
METHOD A 
##STR2## 
METHOD B 
##STR3## 
METHOD C 
##STR4## 
__________________________________________________________________________ 
In a typical diethynylbenzene-ethynylpyrene blend, thermally induced 
copolymerization occurs, initially producing polymers such as the 
following: 
##STR5## 
Subsequently, the polymerization will proceed to yield an infinite network 
of crosslinked resin. The char yield of such a copolymer is dependent upon 
the pressure at which charring was performed. It was found that a 125 psi 
char yield curve is approximately 28 percent higher than an ambient 
pressure char yield curve when the same are compared. Char yields were 
measured by thermogravimetric analysis on the polymerized blends of the 
ethynylated compounds. Rates of heating affected these yields to some 
extent, but not significantly so. 
For comparison purposes, char yields were also measured on several coal tar 
pitches obtained from Ashland Oil Company. Yields at 800.degree. C. are 
shown after pyrolysis in nitrogen. The values are shown in Table II. None 
of the pitches were outstanding char formers at ambient pressure. The one 
yielding 52 percent char was very high melting. 
TABLE II 
______________________________________ 
Pitch Char 
______________________________________ 
RD-131-R (low sulfur No. 240) 
22% 
RD-130-R (No. 260) 30% 
RD-129-R (No. 240) 30% 
RD-132-R (No. 210C) 52% 
RD-128-R (No. 170) 13% 
______________________________________ 
TABLE III 
______________________________________ 
CHAR YIELDS OF VARIOUS A-15 PITCH BLENDS 
CONTAINING POLMERIZABLE ADDITIVES 
Additive DTBP, Char Yield at 
Additive Concentration, % 
% 80.degree. C., % 
______________________________________ 
Diethynylbenzene 
20 50-84 
33 63 
50 84-88 
Divinylbenzene 
100 12 
20 59 
33 3 61 
Phenylacetylene 
100 2 
100 6 after 215.degree. C. 
post-cure 
Diethynylbiphenyl 
50 81 
______________________________________ 
Other char yield measurements were made on A-15 pitch blends containing 
additives such as diethynylbenzene, divinylbenzene, phenylacetylene and 
diethynylbiphenyl. In one case, ditertiary butyl peroxide was added to 
catalyze polymerization. The blends studied are shown in Table III. 
Carbon-carbon materials utilizing the resin matrix precursor of this 
invention were prepared as unidentified components. They were graphitized, 
and their microstructure studied with a scanning electron microscope 
(SEM). The unidirectional composites were prepared by vacuum-pressure 
impregnating yarn bundles wrapped in a simple graphite fixture. These 
components were prepared in accordance with the following schedule: 
1. Wind dry yarn bundles on graphite fixture. 
2. Vacuum-pressure impregnation with matrix pressure. 
3. Polymerize the matrix at low temperature and pressure (usually ambient 
atmospheric pressure and 120.degree.-200.degree. C.). 
4. Graphitization to 2700.degree. C. 
Table IV lists the unidirectional specimens prepared and examined. In the 
examination procedure, small composite segments were cut from the mandrel 
and prepared using standard metallographic techniques. The polished 
specimens were then oxygen plasma-etched, and examined at magnifications 
from 20.times. to 10,000.times.. Graphitization, or lack of it, was 
apparent in high magnification photomicrographs of the oxygen plasma 
etched specimens; these studies were relied on to define graphitizability. 
In some instances, a polymer would graphitize in the vicinity of fibers 
while not developing graphitic structure a few fiber diameters away from 
the interface. These studies verified the conditions of the graphitic or 
nongraphitic matrix in matrix pockets and in fiber bundles. DEB, EP and 
mixtures of the two were chosen for fabrication of test billets. Table V 
shows the processing steps for the pure compounds and mixture of EP and 
DEB. 
An objective of this invention was predicated on improved carbon being 
obtained under mild processing conditions. The selection of EP and DEB was 
compatible with this objective. DEP and EP have carbon to hydrogen ratios 
which are higher than those of most organic compounds. Experimental values 
have been obtained for comparison (Table VI). Since yield will depend upon 
experimental conditions, temperatures and pressures at which the data were 
obtained are included. Commonly, more than 95% of the weight loss occurs 
below 850.degree. C. Most of the information presented was obtained in 
two-step processing in which case the product yield fractions for each 
step gives the overall yield of that material to the indicated 
temperature. For example, the 20 DEB; 80 EP mixture was first cured at 75 
psi and 300.degree. C. with a 96.1% yield: then the resulting polymer was 
heated to 850.degree. C. at 75 psi with a 79% yield for that process. The 
product of those yield figures gave the tabulated value of 75% 
(850.degree. C., 75 psi). For comparison, Table VII is included to show 
the state of pitch processing technology for Allied Chemical 15 V pitch. 
The mechanism of decomposition is very important both with pitch and with 
these candidate precursor materials. With DEB, little or no gas evolution 
was observed during polymerization and a high overall carbon yield was 
obtained with low pressure processing. With pure EP this is not the case; 
low pressure char was very porous, the polymerization process apparently 
entrapping bubbles of gases evolved during the polymerization reaction. 
Ambient pressure cure gave a low yield. EP resembles pitch in this 
characteristic. Mixtures of EP and DEB gave yields of about 80% carbon at 
300 psi or lower while 900 psi pressure is required to achieve that 
percentage yield with pitch. It is concluded that most improvement of 
yield in a mixture occurs by adding 40 or 50 weight percent of DEB to EP. 
TABLE IV 
__________________________________________________________________________ 
UNIDIRECTIONAL COMPOSITES 
Ultimate 
Cure State of State of 
Specimen Temp Polymerized Mass 
Graphitized 
Number 
Material .degree.C. 
(as received) 
Mass 
__________________________________________________________________________ 
1 Unidirectional composite 
300 Coherent composite 
Coherent 
with 1-ethynylpyrene 
structure 
matrix 
2 Unidirectional composite 
300 Coherent composite 
Generally 
20% 1,3-diethynylbenzene 
structure coherent, some 
in 1-ethynylpyrene delamination 
(melting or de- 
polymerization 
occurred during 
graphitization 
processing) 
3 Unidirectional composite 
300 Coherent composite 
Coherent 
20% 1,3-diethynylbenzene 
structure 
in 1-ethynylpyrene matrix 
(pressure cured) 
__________________________________________________________________________ 
TABLE V 
______________________________________ 
DEB PROCESSING 
##STR6## 
EP PROCESSING 
##STR7## 
EP & DEB MIXTURE PROCESSING 
##STR8## 
______________________________________ 
TABLE VI 
______________________________________ 
CARBON YIELDS OF 1,3-DIETHYNYLBENZENE (DEB) 
AND 1-ETHYNYLPYRENE (EP): THEORETICAL 
AND EXPERIMENTAL 
Theo- Experi- Processing Conditions 
retical 
mental Temp. .degree.C. 
Pressure (psi) 
______________________________________ 
DEB 95.2 94% 2700 15 
EP 95.6 44% 300 75 
20% DEB: 80% EP 
95.5 76% 850 75 
33% DEB: 67% EP 
95.4 89% 850 125 
______________________________________ 
TABLE VII 
______________________________________ 
CARBON GRAPHITE YIELD VERSUS PYROLYSIS 
PRESSURE FOR 15-V PITCH (T.sub.F = 2700.degree. C.) 
Pressure Yield 
(psi) (%) 
______________________________________ 
15 24 
300 65 
920 82 
5,000 82 
15,000 80-84 
______________________________________ 
The compounds of this invention polymerize into a char precursor 
graphitizable at low temperature. This feature determines the nature of 
the matrix pocket microstructure of the composite, and results in a 
material similar in structure to high pressure processed pitch 
carbon/carbon. Experience shows this microstructure to have better 
performance characteristics in erosion resistance and ablation as well as 
better basic mechanical and thermal properties which relate to that 
performance. 
DEB's major disadvantage, when used alone as a matrix precursor is the 
requirement for critical temperature control during polymerization. The 
larger the mass of material being processed, the more difficult it becomes 
to control the polymerization, especially when using a constant 
temperature process. Inside a preform, this may not be as great a problem 
as with the excess of liquid puddled on top of a preform, as was required 
in these experiments, to assure complete impregnation of the composite. 
High vapor pressure and an associated low boiling point make processing 
difficult, both as the pure compound and in a mixture. To maintain a given 
composition, time becomes critical; time of application of vacuum is held 
to a minimum, and hard vacuum is avoided. 
EP when used alone, has some advantages: it shows only a very small or 
nonexistent exotherm in its polymerization, and it is easily graphitized. 
Temperature control would be less of a problem if the pure compound could 
be processed. The graphitizability which carries over into its use in a 
mixture makes it useful in matrix formation. Its normal state as a solid 
at room temperature complicates processing by requiring elevated 
temperature impregnation. The melting point of 115.degree. C. is not 
inordinately high, but does limit its working temperature range. 
Unfortunately, it has a comparatively low carbon yield and yields porous 
char. 
The DEB-EP mixtures of this invention, however, have been selected because 
of an apparent synergism which results in: (1) a reduced exotherm; (2) 
greater ease of graphitization; (3) improved carbon yield relative to EP 
alone, and dense char similar to that of DEB (and associated absence of 
gas evolution); and finally, (4) polymerization to a set microstructural 
framework within matrix pockets. The matrix material fortunately inherits 
the good characteristics of both components while minimizing the 
disadvantage of the pure compounds. The solubility of EP in DEB, 
increasing with increasing temperature, allows a reasonable working range 
for impregnation with their mixture (70.degree.-100.degree. C.). Table V 
shows the processing steps for the pure compounds and a mixture of EP and 
DEB. 
Further studies to determine the effectiveness of the precursor material of 
this invention were carried out using 3-dimensional PAN minibillets (1 cm 
cylinders) of PAN fibers cut from a larger 3-D preform. 
These minibillets are identified as specimens 22, 23 and 24. Reaction 
temperature limits were investigated by processing in a modified DIA 
apparatus in order to program the long, closely controlled temperature 
cycles required for curing and pyrolyzing to 700.degree. or 300.degree. C. 
Samples of these billets were checked for weight pickup in single cycle 
and multiple cycle processing. The required scale-up to larger equipment 
and larger amounts of precursor material was done by adapting a United 
States autoclave which had vacuum to 300 psi pressure, capability as well 
as reasonable temperature control. To maintain closer temperature control, 
samples were processed within containers which are inserted into cavities 
within a large graphite block. This large mass was needed for its thermal 
inertia to eliminate temperature cycling. Chromelalumel thermocouples 
inserted into two positions within the block indicated that constant and 
controllable temperature was maintained within .+-.1.degree. C. over 
periods of 16 or more hours in the most recent processing cycles. 
Two one-inch cubes (specimens 25 and 26) cut from the same PAN 3-D preform 
were partially processed in this fashion. The processing proceeded as 
depicted in the flow diagram of Table VIII. Water immersion measurements, 
conducted by ASTM procedure C20-46, were used to monitor processing and to 
determine the amount of precursor required to fill the billet in the 
following cycle. An excessive amount of matrix precursor is undesirable 
because of the potential problem of a runaway reaction during 
polymerization. In each cycle the billets were processed in a close 
fitting steel container conserving expensive experimental materials and 
minimizing the potential for exotherm. Processing details and billet 
characteristics are shown in Table IX. With the first minibillet, specimen 
25, pure DEB was used as the matrix precursor. The second, specimen 26, 
was processed with a 50:50 weight percent mixture of DEB and EP. ASTM C-20 
data were not obtained for each step of the processing of this billet but 
the bulk density curve for densification appeared to be reasonably good 
considering the exotherm experienced in cycle 1 of the densification. 
Analysis based on data available showed that because of the closed 
porosity developed in the first and second cycles, this point represents a 
90-100 percent efficiency in filling of the available (open) pore space. 
Difficulties with DEB processing, and the consideration discussed above, 
led to the use of the mixture of compounds used with billet 26. This 
billet processed reasonably well, but the density curve and porosity data 
show that if continued on the same basis as the first four cycles, it 
probably would not fully densify. The reason for this has been observed in 
the nature of the char which developed. Gas evolution, which apparently 
occurs during polymerization of the EP component when it is a large 
fraction of the total mixture, causes the char to have low density and 
some closed porosity. This may have resulted from evaporation of the DEB 
component of the mixture because of two factors: the large open volume of 
the autoclave and the vacuum processing of the DEB component with its high 
vapor pressure. 
TABLE VIII 
______________________________________ 
##STR9## 
##STR10## 
##STR11## 
______________________________________ 
TABLE IX 
__________________________________________________________________________ 
3-D BILLETS PREED DURING PROCESS DEVELOPMENT 
Specimen Matrix No. of 
Graphitization Temperature 
No. Type 
Size Precursor 
Cycles 
(.degree.C.) 
__________________________________________________________________________ 
22 3-D 
1 cm cylinder 
DEB 2 2600 (fast heat ramp, no 
PAN hold at T.sub.gr) 
23 3-D 
1 cm cylinder 
2 EP: 1 DEB 
1 2600 (slow heat ramp, 1/2 hr 
PAN hold at T.sub.gr) 
24 3-D 
1 cm cylinder 
DEB 1 2600 (slow heat ramp, 1/2 hr 
PAN hold at T.sub.gr) 
25* 3-D 
1 .times. 1 .times. 1 in. 
DEB 2 2600 
PAN 
cube 
26 3-D 
1 .times. 1 .times. 1 in. 
1 EP: 1 DEB 
4 2700, 2300 
PAN 
cube 
27* 3-D 
2 .times. 2 .times. 3 in. 
1 EP: 1 DEB 
2 2700 
PAN 
__________________________________________________________________________ 
*A preliminary pyrolysis was at too high a temperature and resulting in a 
exotherm without any apparent damage to the billet. 
The process development work on chars of pure compounds, unidirectional 
minibillets, and small 3D composites were carried out to prepare for 
fabrication of a modest sized billet which would be cut into test 
specimens (mechanical, erosion, ablation) to obtain data for comparison 
with data on carbon-carbon materials processed by standard methods such as 
HiPIC EISP processes for missile nosetips. The 2 in..times.2 in..times.3 
in. billet Specimen 27 provided sufficient material for characterization. 
The larger size of this billet necessarily involved further scale-up in 
the use of matrix precursor. In the first impregnation-polymerization 
cycle, the temperature program which proved satisfactory for 1 inch cubes 
was not suitable for the larger billet as processed. The reaction ran 
away, as the temperature of the chamber reached 135.degree. C., resulting 
in a low density porous char. The apparent reason for the runaway reaction 
is that with the larger mass of liquid precursor the exothermic heat 
developed on polymerization was not conducted away through the liquid fast 
enough to prevent a continually increasing temperature. To avoid the 
exotherm, either (1) polymerization must be accomplished at a low 
temperature for a longer time, or (2) the amount of excess monomer used 
must be reduced. The former conditions were used for the second 
impregnation of billet 27. The temperature of polymerization, previously 
135.degree.-145.degree. C. for small billets, was reduced to 123.degree. 
C. 
To summarize the present invention, it has been found that a new low 
pressure impregnated and carbonized carbon/carbon composite can be 
produced from a mixture of high char yielding acetylenic precursor 
materials; namely, a copolymerizable mixture of 1,3-diethynylbenzene and 
ethynylpyrene. Processing methods have been developed which allow 
densification to 1.8 g/cm.sup.3. The partial polymerization step is done 
at low pressure and low temperature (300 psi, 123.degree. C.) over a 
period of about 16 hours. The material is comparable to HiPIC densified 
carbon/carbons in microstructure, mechanical properties, (except shear) 
and thermal expansion. The advantages of this material are: HiPIC-like 
microstructure from a low pressure process; thermomechanical properties 
comparable to HiPIC composites; reproducible matrix material composition 
(synthetic precursor of low impurity content, therefore, multisource 
acquisition and improved quality control are possible); reduction in 
processing cycles from other LoPIC processing to achieve comparable 
density (high char yield); and potential for development of much larger 
composites because of low pressure processibility. 
While the invention has been described with particularity in reference to 
specific embodiments thereof, it is to be understood that the disclosure 
of the present invention is for the purpose of illustration only and is 
not intended to limit the invention in any way, the scope of which is 
defined by the appended claims.