DENSIFYING CARBON-CARBON COMPOSITES WITH SUGAR
R. J. Price and G. H. Reynolds
MSNV~, Inc., P. O. Box 865, San Marcos, CA 92079
K. M. Kearns
Wright Laboratory, Wright-Patterson Air Force Base, Dayton, OH 45433-7750
Because sugars release so much water vapor as they are
converted from liquid to solid, foaming occurs and tends
to compromise the effectiveness of impregnation.
The objective of the present program was to
identify sugar-based liquid phase densification processes
for carbon-car~n that would minimize or totally
eliminate the need for non-oxidizing high temperature
processing.
Introduction
Over 50% of the cost of carbon-carbon composites
is associated with conventional densification that
requires processing at high temperatures in nonoxidizing environments. Simplifying and increasing the
efficiency of the densification could result in as much as
a 33% reduction in the final cost of simple highpr<xtuction carbon-carlxm composites.
An effective approach for lowering the
densiiication cost of carbon-carbon composites
fabricated by the liquid impregnation and pyrolysis
method would be to minimize or totally eliminate the
need for non-oxidizing high-temperature processing.
Sugars and other carbohydrates are low-cost materials
that have long been used as carbon precursors for
various types of sorptive carbon and as cements for
monolithic graphite components. Sucrose solutions were
used in Great Britain in the 1950s for densifying low
permeability graphite components for gas cooled nuclear
reactors (ref. 1).
Experimental
Preliminary work on melt infiltration of carbon
fiber preforms with sugars showed that the disaccharides
sucrose and maltose formed melts that were too viscous
for effective impregnation. On the other hand, the
monosaccharides fructose, glucose and ribose formed
fluid, wetting melts. For the bulk of the work I3-D
fructose (melting poin t 103°C) and a-D glucose
(melting point 146°C) were used. Infiltration
experiments were conducted on coupons cut from a 1.6
mm thick braided carbon fiber preform fabricated from
HM fiber (Techniweave, Inc.) and on a 14 ply 2-D layup of plain weave T-300 carbon fabric clamped between
perforated plates in a fixture. The fiber volume percent
in both dry preforms was about 55%. The preforms were
vacuum melt-infiltrated at about 20°C above the melting
point of the sugar. The preforms were pyrolyzed in air to
a peak temperature ranging from 250°C to 325°C. Reinfiltration and pyrolysis was repeated up to six times.
Final carbonization was at 950°C for 2 hr. in an
atmosphere of argon.
Comparison of Sugars with Traditional
Carbon Precursors
Furfuryl alcohol resin, phenolic resin, and pitches
derived from both coal and petroleum are most often
used to produce carbon-carbon composites. The
pyrolysis carbon yield is typically 50 wt. % or less unless
expensive high-pressure autoclave pyrolysis is
employed. Pyrolysis of both pitches and resins produces
a variety of volatile compounds that are noxious,
poisonous, and, in some cases, are known carcinogens.
Sugars have a carbon to water ratio of unity, so
their carbon content is 40 wt %. Sugar melting points
fall in the range 90-200°C. Because of their low carbon
content compared with traditional impregnants,
pyrolysis char yields are lower (for example, 25 wt % for
sucrose pyrolyzed to 850°C). A key advantage of the
sugars over the traditional matrix precursor impregnants
is their decomposition to form carbon at low
temperatures. Decomposition is almost complete by
350°C, which is too low for appreciable oxidation.
Results and Discussion
Pyrolysis to 300-325°C in air was mfficient to
produce a hard cation char from both fructose and
glucose. The char yield from the sugar introduced in the
first infiltration-pyrolysis cycle was 35-45%, but
decreased to 20% or less in subsequent cycles. The
thickness of the preforms increaseA by 20-50% in the
first infiltration cycle but changed only slightly in
subsequent cycles. Table 1 shows representative values
for the amount of sugar infiltrated and the amount of
520
matrix carbon after pyrolysis. The amount of sugar
infiltrated per cycle decreased after the first cycle and
this decrease, combined with the lower char yield in
higher cycles, limited the reduction, in porosit3~ and
increase in bulk density that were achieved. Somewhat
better densities were obtained with the braided preforms
than with the 2-D lay-up, probably because the braided
structure allows more open channels from the surface to
the interior. There was little difference in densification
efficiency between specimens infiltrated with fructose or
with glucose.
Figures 1 and 2 show the microstructure of a
carbonized glucose-infiltrated specimen. Carbon char
can be seen throughout the thickness, but unfilled pores
are also evident. Wetting of the fiber bundles by molten
sugar and penetration into the fiber bundles (figure 2)
a ~ r e d to be more effective in a glucose-infiltrated
specimen than in a fructose-infiltrated specimen. The
matrix carbon had a featureless appearance resembling
glassy carbon. X-ray diffraction on carbon residue from
both fructose and glucose heated to 950°C in argon
showed no indication of crystalline graphite.
It is likely that pressure infiltration and autoclave
pyrolysis, or hybrid processing using both sugars and
traditional densification processes, will be needed to
obtain substantial improvements in densification.
Nevertheless, infiltration with sugars and atmospheric
pyrolysis is a simple and effective approach to reducing
overall processing costs of carbon-carbon composites.
Acknowledgments
This work was supported by the U.S. Air Force
SBIR program, contract number F33615-96-C-5064.
References
Boyland, D. A. and BrightweU, J. W., "process for
Production of Low Permeability Carbon and
Resultant Article", U.S. Patent No. 3,026,214,
March 20, 1962.
Table 1: Properties of Sugar-Infiltrated Preforms
Preform
Braided
Braided
2-D
,
,
' Infiltrant
'
'Fructose
Glucose
Glucose
Voi. Fr. Sugar
(1 st ~cle)
0.31-0.55
0.31-0.45
0.25
Vol. Fr. Sugar ' Vol. Fr. Matrix
After P~olysis
(5th cycle)
0.09-0.18
O.19--0.20
0.08-0.10
0.12--0.31
0.16
0.07
,
,
.
.
.
.
Bulk Density 'After
Pyrolysis (~,/cm3)
1.24
1.18
1.02
.
.
.
.
.
.
.
Vol. Fr. Pores
After Pyrolysis
0.35
0.33
0.42
,
, ,
Figure 2: Glucose-infiltrated braided preform, 500x
Figure 1" Glucose-infiltrated braided preform, 30x
521