Method of carbonizing polyacrylonitrile fibers

This invention relates to a method of carbonizing polyacrylonitrile fibers (PAN fibers) by exposing the fibers at an elevated temperature to an oxidizing atmosphere, then exposing the oxidized fibers to an atmosphere of an inert gas such as nitrogen containing a carbonaceous material such as acetylene. The fibers are preferably treated with an organic compound, for example benzoic acid, before the exposure to an oxidizing atmosphere. The invention also relates to the resulting fibers. The treated fibers have enhanced tensile strength.

This invention relates to the carbonization of polyacrylonitrile (PAN) 
fibers. 
BACKGROUND ART 
Carbonized PAN fibers find use as reinforcement in various materials of 
construction, principally plastic materials. It is desirable that the 
fibers have a high tensile strength and for certain purposes that they 
also have a high electrical resistance. 
Such fibers intended as reinforcement are oxidized by air at a relatively 
low temperature, typically about 260.degree. C., and are then subjected to 
carbonization in an inert atmosphere at an elevated temperature. The 
carbonization is typically carried out in an atmosphere of nitrogen and it 
may be carried out at a relatively low temperature, for example 
1200.degree. to 1500.degree. C. or at a higher temperature, for example 
about 2500.degree. to 3000.degree. C. The carbonized fibers resulting from 
the lower temperature carbonization have higher tensile strength than 
those prepared at the high temperature. 
OBJECT OF THE INVENTION 
It is an object of the present invention to improve upon the methods of 
producing carbonized PAN fibers and to produce PAN fibers of higher 
tensile strength, and higher modulus than heretofore and/or of higher 
electrical resistance. 
SUMMARY OF THE INVENTION 
In accordance with the invention PAN fibers, after the oxidation step 
described above, are carbonized, preferably at an even lower temperature 
than indicated above, in the presence of a gaseous active carbon 
substance, such as, for example, acetylene and/or others mentioned 
hereinbelow. As will be apparent from the examples and data presented 
hereinbelow, tensile strength is greatly improved. Preferably, prior to 
the oxidation step, the PAN fibers are subjected to contact at a 
relatively low temperature, typically at about 140.degree. to 200.degree. 
C., with molten benzoic acid or other similar liquid or gaseous 
conditioning material.

DETAILED DESCRIPTION OF THE INVENTION 
As active carbon sources any one or a mixture of the following may be used: 
Methane, acetylene, ethylene, fuel gas and aliphatic hydrocarbons 
generally, provided they can be used in gaseous form; benzene and other 
aromatic hydrocarbons; also nonhydrocarbon organic materials such as 
carbon disulfide. In general any carbonaceous material that is gaseous at 
the temperature involved and which decomposes at temperatures below 
1000.degree. C. within a reasonable period of time, of the order of 
minutes, may be used for this purpose provided it does not react 
destructively with the PAN fibers. 
In place of benzoic acid other aromatic carboxylic acids may be used such 
as phthalic acid, terephthalic acid and naphthoic acid. Hydroquinone may 
also be used. They may be used in liquid, e.g. molten state or in the 
gaseous state. 
The examples below will serve further to illustrate the practice and 
advantages of the invention. In carrying out the experimental work, a 
standard procedure and a modified procedure were employed as follows: 
STANDARD PROCEDURE 
A tow of 96 filaments of 2.3 denier PAN was oxidized at 260.degree. C. over 
a contact time of about 3 hours under a load of 15 g in a tubular reactor 
of about 2.45 cm diameter to produce a fiber with less than 5% shrinkage 
or extension. The sample was then heated at a rate of 20.degree. C./min to 
temperatures within a range of 450.degree.-1100.degree. C. in a flow of 20 
cc/min nitrogen atmosphere under a load of about 50 g in a tubular reactor 
of about 1.9 cm diameter. 
MODIFIED PROCEDURE 
In Examples 2, 4, 5 and 6 the same PAN tow was first passed through a 
molten benzoic acid bath at 175.degree. with 1 g loading over a contact 
time of about 3 hours to produce fibers with less than 10% shrinkage or 
extension. In each of Examples 2 to 6 (after treatment with benzoic acid 
in Examples 2, 4, 5 and 6) the samples were oxidized at the control 
conditions under a load of 50 g to produce fibers with less than 5 percent 
shrinkage or extension. The oxidized fibers were then carbonized at the 
control conditions, except for the presence, as indicated, of a mixture of 
acetylene-nitrogen as the atmosphere (5.34% acetylene, 94.66% nitrogen). 
The results are set forth in Table 1 below. It will be seen that 
substantial improvement in tensile strength resulted from carbonizing 
(Example 3) in an atmosphere of nitrogen and acetylene and that the best 
results were obtained by pre-treatment with benzoic acid followed by 
oxidation and carbonization in an atmosphere of nitrogen and acetylene 
(Example 4). Example 5 shows that treatment with benzoic acid, carbonizing 
in an atmosphere of nitrogen, cooling and then carbonizing in an 
atmosphere of nitrogen and acetylene substantially improves tensile 
strength but not as much as in Example 4 where the initial (and only) 
carbonization is carried out in an atmosphere of nitrogen and acetylene. 
TABLE 1 
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Effect of Processes on Tensile Strength of Final Carbon Fiber 
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Product 
Benzoic Acid, 
Air Oxidation at 
100% N.sub.2, at 
5.34% C.sub.2 H.sub.2 /N.sub.2 
Sample Treatment 
175.degree. C., hr 
260.degree. C., hr 
.degree.C. 
at .degree.C. 
% Elongation 
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Oxidized and carbonized in 
-- 3 700 -- 0.76 
N.sub.2 (standard process) 
Treated with benzoic acid, 
3 3 700 -- 0.77 
oxidized and carbonized 
In N.sub.2 
Oxidized and carbonized in 
-- 3 -- 700 0.90 
C.sub.2 H.sub.2 
Treated with benzoic acid, 
3 3 -- 700 1.22 
oxidized and carbonized 
In C.sub.2 H.sub.2 
Treated with benzoic acid, 
3 3 700 700 -- 
oxidized and carbonized 
in N.sub.2, cooled and 
carbonized in C.sub.2 H.sub.2 
Treated in benzoic acid 
3 -- -- 700 -- 
and carbonized in C.sub.2 H.sub.2 
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Tensile 
% Change in 
Modulus 
Resistance 
Strength 
Tensile Strength 
Sample Treatment 
(psi .times. 10.sup.-6) 
(.OMEGA. cm) 
(psi) 
From (A) 
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1. 
Oxidized and carbonized in 
11.9 4.18 90596 
0.00 
N.sub.2 (standard process) 
2. 
Treated with benzoic acid, 
12.4 9.10 95160 
+5.04 
oxidized and carbonized 
In N.sub.2 
3. 
Oxidized and carbonized in 
13.8 1.32 124400 
+37.30 
C.sub.2 H.sub.2 
4. 
Treated with benzoic acid, 
12.6 3.18 154170 
+70.20 
oxidized and carbonized 
In C.sub.2 H.sub.2 
5. 
Treated with benzoic acid, 
-- -- 129980 
+43.50 
oxidized and carbonized 
in N.sub.2, cooled and 
carbonized in C.sub.2 H.sub.2 
6. 
Treated in benzoic acid 
-- -- 103100 
+13.70 
and carbonized in C.sub.2 H.sub.2 
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Note: Figures in Columns 1 and 2 are approximate. 
In Table II the effects of acetylene concentration and temperature on 
tensile strength are set forth. The modified procedure was used in all 
cases. 
TABLE II 
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Effect of Acetylene Concentration and Temperature on Tensile Strength 
during Carbonization 
of a PAN Sample Treated in Benzoic Acid at 175.degree. C. for 3 hr and 
Air-Oxidized at 260.degree. C. for 3 hr. 
Acetylene Tensile Strength, 
Resistance, Modulus 
Sample 
Concentration, % 
Temperature, .degree.C. 
(psi) ohm-cm 
% Elongation 
(psi .times. 10.sup.-6) 
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1 5.34 647 122,730 44.1 n.a. n.a. 
2 5.34 705 154,170 3.18 1.22 12.6 
3 5.34 800 191,180 .058 0.74 25.8 
4 5.34 845 169,550 .019 0.70 24.2 
1 11.3 610 73,590 254.7 -- -- 
2 11.3 705 124,450 1.54 -- -- 
3 11.3 750 136,320 .212 -- -- 
4 11.3 805 116,860 .054 -- -- 
1 14.02 500 68,370 18300 -- -- 
2 14.02 590 116,930 657.6 -- -- 
3 14.02 655 144,900 9.52 -- -- 
4 14.02 703 133,280 n.a. -- -- 
Standard 
process 
0.00 705 90,590 4.18 0.76 11.9 
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Table III illustrates the effect of varying the procedure. In the first 
case there was a straight heating in an atmosphere of acetylene and 
nitrogen. In the other cases the fiber was heated to 500.degree. C. in 20 
cc/min flow of pure N.sub.2, held for 10 minutes while the acetylene 
mixture replaced the N.sub.2, heated to the set temperature, the gas flow 
decreased to 0.5-1.0 cc/min during the 5-minute hold period, and the 
system purged with 90 cc/min pure N.sub.2 on cooldown. 
TABLE III 
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The Effect of Variation of Carbonization Procedure 
on Tensile Strength 
Acetylene 
Tensile Strength, 
Resistance Modulus 
Procedure 
Temperature, .degree.C. 
Concentration, % 
psi ohm-cm 
% Elongation 
(psi .times. 10.sup.-6) 
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Straight 
845 5.34 171,500 0.0191 
0.71 24.2 
heat-up at 
20.degree. C./min 
in C.sub.2 H.sub.2 
Modified 
840 5.34 235,200 .0199 
0.93 25.3 
" 2.0 256,433 -- 1.01 25.4 
" 1.0 225,400 -- 1.00 22.5 
" 0.0 104,920 -- 0.61 17.2 
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