Note: Descriptions are shown in the official language in which they were submitted.
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55544
TITLE
Improvement of Carbon Fiber Strength
Background of the Invention
The production of carbon fiber from pitch is
well known. See, for example, U.S. Patent N~s.
4,005,183 and 4,504,454. It is believed that the
maximum tenacity which these fibers are capable of
achieving is not attained due in part to defects in
the fibers and/or at the fiber surface. The present
invention seeks to enhance the fiber tenacity of such
fibers.
Summary of the Invention
This invention provides a method for
increasing the tenacity of pitch-based carbon fiber
yarn comprising impregnating a pitch-based carbonized
or partially carbonized fiber yarn with a
carbonizable resin precursor, polymerizing the
precursor in situ to form the resin and subjecting
the impregnated yarn to a temperature in excess of
1000C in an inert atmosphere to carbonize the resin.
; Detailed Description of the Invention
;
The carbon fiber yarn to be strengthened in
accordance with the present invention is a
pitch-based yarn. This yarn may be prepared by the
general procedures described in UOS~ Patent No.
4,005,183. Eitber carbonized or partially carbonized
yarn may be employed.
A ~olution of a carbonizable resin precursor
~ is applied to the yarn. The object is to impregnate
the fibers of the yarn with a material which will
polymerize or polymerize further to a resinous
product which will remain in place and leave a carbon
residue within the fiber upon carbonization. Thus,
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the term "resin precursor" is intended to include
unpolymerized or partially polymerized materials.
Polymerization or resinification is often facilitated
by application of heat. The resin must be capable of
being carbonized, the usual temperature of
carbonization being above 1000Co Among suitable
carbonizable resin precursors are partially
polymerized phenolic condensation products, epoxy
resins, furfural, furfuryl alcohol, partially
polymerized furfuryl alcohol resin, urea condensation
products, acrylic resins, vinyl resins, propylene
glycol, etc. A sufficient amount of carbonizable
resin precursor must be absorbed by the yarn to
provide strengthening. Amounts yielding between
about 0.1 and 10% of resin based on the weight of the
yarn prior to impregnation have been found
satisfactory. Preferably the weight gain is kept
below 5~, particularly with partially carbonized
fiber because such fibers are less able to absorb the
resin precursors.
It is preferred that the resin precursor
have a high coking factor, that is, a high percentage
of carbon yield when subjected to carbonization.
Volatile impregnants which are entirely driven off in
the heatinq step are clearly unsuitable.
The yarn is passed through a bath containin~
the carbonizable resin precursor. If the resin
prerur~or is furfuryl alcohol, a partially
polymerized furfuryl alcohol or combination thereof,
it is desirable to incorporate a latent catalyst
along with the precursor. These are commercially
available and recommended for the purpose of
catalyzing the polymerization of the resin precursor
at elevated temperatures. One such catalyst is a
complex of boron trifluoride and monoethylamine.
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Another catalyst is maleic anhydride. Use of a
latent catalyst permits application of a low
viscosity solution to the fiber with subsequent
polymerization at the elevated temperatures. If the
precursor were to polymerize significantly prior to
application, the treating bath would be so viscous as
to allow only a coating to be formed. Under such
circumstances, the ~ibers of the yarn would stick to
each other and become damaged or adversely affected
for use in composites where penetration of matrix
medium between the yarn fibers is of utmost
importance. Suitable solvents are those which will
readily evaporate without leaving any harmful
residues. Acetone has been found useful for this
purpose.
As mentioned above, the viscosity of the
impregnation bath should be sufficiently low as to
permit permeation of the resin precursor into the
fibers of the yarn, i.e., to fill the voids, cracks
and other defects of the fiber. If the bath is too
viscous, the yarn itself, rather than the fibers
thereof, will entrap such amounts of resin precursor
I as to cause the fibers to ~tick tv each other. Such
I fibers often break when attempts are made to separate
them from each other. It will be understood that the
resin content of the impregnating solution increases
with age because partial polymerization is taking
place. As this occurs, the viscosity of the bath
increases and will influence the degree of fiber
permeation.
Upon removal of the yarn from the bath,
excess surface fluid is gently removed as by padding
with an absorbent material to avoid damaging the
yarn. Exposure of the yarn to air at room
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temperature for extended periods allows solvents from
the impregnating solution to evaporate and the
resin precursor to partially cure. Alternatively,
this may be achieved by slowly heating the yar~ from
room temperature to final polymerization
temperatures. In any event, the yarn must be heated
to completely polymerize the impregnant. Conditions
for such polymerization arle well known in the art.
In general, the yarn will !be heated to temperatures
in the range of about 100 to 200C for periods of at
least five minutes. Following this polymerization,
the yarn is subjected to carbonization conditions,
namely, treatment in an inert atmosphere, e.g.,
nitrogen, argon, etc., at temperatures of above
1000C until the resin has carbonized.
Testing Procedures
Tenacity, elongation and modulus (T/E/M~ are
measured according to ASTM D-3379-75.
The following examplcs are submitted to
illustrate the invention and are not intended as
l limiting. In each case, the pitch-based yarn was
! impregnated by passage through a bath containing the
resin precursor.
1 The yarn conta;ned 496 filaments each having
`~ 25 a diameter of about 10 microns. The carbonized yarn
(at 1600C~ had an initial modulus of about 900 to
~000 grem~ per denier ~gpd).
Example 1
Carbon fiber yarn was impregnated with
furfuryl alcohol containing 1~ latent catalyst
(commercially available complex of boron trifluoride
and monoethylamine ~BF3.MEA). The fibers were heated
at 4C/min. to 150~C and held at 150C for 32 minutes
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: to polymerize the furfuryl alcohol. After
polymerization the fibers were found to have gained
1.45% of their original weight. The fibers were then
carbonized at 15B4C. An 11% improve~ent in tensile
strength over the control fiber resulted tT/E/rl
impregnated fiber: 14.6gpd/1.22%/1055gpd, T/~/M
control fiber: 13.2gpd/1.17~/988 gpd).
Example 2
Carbon fiber yarn was impregnated with the
solution used in Example 1. The solution had been
aged for 22 days. The furfuryl alcohol was
polymerized under the same conditions as in Example
1. After polymerization, the fibers were found to
have gained 8.7~ of their original weight. The
fibers were then recarbonized at 1584C. A 22%
improvement in tensile strength over the control
fiber resulted (T/E/M impregnated fiber:
16.1gpd/1.25~/997gpd, T/E/M control fiber:
13.2gpd~1.17~/988gpd).
Example 3
Carbon fiber yarn was impregnated using a
90/10% by wt. mixture of furfuryl alcohol and
furfural ~odified furfuryl alcohol resin with 2~ of
the latent catalyst of Example 1. The fibers were
heated at 4C/min~ to 120~C and held at 120C for 32
minutes to polymerize the furfuryl alcohol/resin.
.fter polymerization, the fibers were found to have
gained 3.2~ of their original weight. The fiber~
were then recarbonized at 1600C. A 29% improvement
in tensile strength over the control fiber resulted
(T/E/M impregnated fiber: 14.2gpd/1.23%/10259pd,
T/E/M c~ntrol fiber: ll.Ogpd/.95~/1081gpd).
Example 4
Carbon fiber yarn was impregnated using a
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90/10% by wt. mixture of furfuryl alcohol and
furfural modified furfuryl alcohol resin with 2% of-
the latent catalyst of Example 1. The fibers were
heated at 4C/min to 120 and held at 120C for 32
minutes to polymerize the iEurfural modified resin.
After polymerization, the Eibers were found to have
gained 6.1% of their original weight. The fibers
- were then recarbonized at 1600C. A 24~ improvement
in tensile strength over the control fiber resulted
(T/E/M impregnated iber: 15.1gpd/1.18~/1148gpd,
T/E/M control fiber: 12.2gpd/1.023%/lOg5gpd).
Example 5
Carbon fiber yarn was impregnated using a
10~ by wt. solution of furfural modified furfuryl
alcohol resin dissolved in acetone. The fibers were
then heated at 4C/min to 120C and held at 120C for
32 minutes. The resulting weight gain was 7.7~ of
j their original weight. The fibers were recarbonized
at 1600C. A 12~ improvement in tensile strength
over the control fiber resulted (T/E/M impregnated
fiber: 15.0gpd/1.41%/945, T/E/M control fiber:
13.4gpd/1.34~/868gpd).
Example 6 - Comparative Example
i Carbon fiber yarn was impregnated using
acetone containing 5% of the latent catalyst of
Example 1. The fibers were then heated to 4C/min to
j 120C. and held at 120C for 32 minutes. The
i resultins weight gain was less than 1~. The fibers
were then recarbonized at 1600C. No increase in
strength resulted (T/E/M impregnated fiber:
13.7gpd/1.13%/1131gpd, T/E/M control fiber:
14.~gpd/1.07%/1272gpd). This example shows treatment
with a non-resin forming medium did not result in
strength improvement.
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Example 7
Carbon fiber yarn was impregnated using
propylene glycol containing 5% of the latent catalyst
of Example 1. The fibers were then heated at 4C/min
to 200C and held at 200C for 32 minutes. A resin
was formed from propylene glycol under these
conditions. The resulting weight gain was less than
1%. The fibers were then recarbonized at 1600C. A
10% improvement in tensile strength over the control
fiber resulted (T/E/M impregnated fiber: 13.2gpd/
1.05%/1133gpd, T/E/M control fiber:
12.Ogpd/.947%/1141gpd).
Example 8
Partially carbonized yarn (heated to 1000C~
was impregnated with furfuryl alcohol containing 2%
of the latent catalyst of Example 1 which had been
aged 72 hours. The fibers were heated at 4C/min to
120C and held at 120C for 32 minutes to polymerize
the furfuryl alcohol. The resulting weight gain was
less than 1%. The fibers were then oarbonized at
1599C. A 24.5~ improvement in tensile strength over
the control fiber resulted. (T/E/M impregnated
fiber: 14.5gpd/1.18~/1083gpd, T/E/~ control fiber:
l 11.6gpd/97%/1080gpd~.
! 25 Example 9
Carbon fiber was impresnated with furfural
containing 2% of the catalyst of Example 1. The
fibers were heated at 4C/min to 120CC and held at
I20C for 32 minutes to polymerize the furfural.
~ 30 After polymerization, the fibers were found to have
gained 0.1% of their original weiqht (weight gain
from dipping was 3~ he fibers were then
recarbonized at 1600C. A 14% improvement in tensile
~trength over the control fiber resulted (T/~/M
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impregnated fiber: 13.8gpd/1.06%/1194gpd, T/E/M
control fiber: 12.2gpd/1.023%/1095gpd).
Exampl~ 10
Carbon fiber was impregnated using a 95/5%
by wt. mixture of furfuryl alcohol and furfural
modified furfuryl alcohol resin with 5~ maleic
anhydride added as a latent catalyst. The fibers
were heated at 4C/min to 120C and held at 120C for
32 minutes to polymerize the furfuryl alcohol/resin.
After polymerization, the fibers were found to have
gained 6.1~ of their ori~inal weight ~weight gain
aftçr dipping was 160%). The fibers were then
recarbonized at 1594C. A 20% improvement in tensile
strength over the control fiber resulted ~T/E/M
impregnated fiber: 14.6gpd/1.17%/1155gpd, T/E/M
control fiber: 12.2gpd/1.02%/1087gpd).
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