Note: Descriptions are shown in the official language in which they were submitted.
1 15f~9
1 The present invention relates to polyacrylonitrile (PAN)
2 fibers and partic~larly improved oxidi~ed PAN fibers and the
3 carbonized and graphitiæed forms thereof.
4 Polyacrylonitrile (-C~2CM(CN)-) constitutes a major com-
ponent of many industrial texti]e fihers. Oxidized PAN fiber
6 is potentially useful to form heat protective fabrics as a
7 substitute for asbestos. Carbonized and graphitiæed polyacry-
8 lonitrile (PAN) fibers form composites with other materials,
9 particularly where high strength-to-density and high modulus-
to-density ratios are desired. However, such applications are
11 limited by the ultimate strength, elasticity and diameter of
12 the carbonized and graphitized PAN fibers. Thus, it is not
13 surprising that many attempts have been made to increase
14 strength and elasticity and reduce the diameter of PAN fibers.
Particularly, the smaller the diameter of the fiber, the
16 greater is the ratio of surface area of the fiber to either
17 weight or volume. The greater ratio thus provides increased
18 fiber-to-matrix interface area, distributing the loading on
19 the composite over a greater area so as to improve interlami-
nar shear strength markedly for composite materials utilizing
21 such smaller diameter fibers. Additionally, smaller diameter
22 fibers of improved strength and elasticity are considerably
23 more flexible than larger diameter fibers of similar strength
24 and elasticity, permitting formation of desirably thin woven
fabrics or even braided and knitted fabrics, as composite pre-
26 cursors.
27 Present methods for the production of PAN-based carbon
2a ~ fiber~ cal or the spinning of the PAN, followed by oxidation
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1 ¦ and carbonization of the resulting P~N fibeLs. ~rhe acrylo-
2 ¦ nitrile monomer can be made by several known methods including
3 ¦ direct catalytic adc3ition of hydroyen cyanide to acetylene or
4 ¦ the addition of HCN to eth~lene oxide to give ethylene cyano-
¦ hydrin, followed by dehydration. Polymerization is usually
6 ¦ carried out in an aqueous solution with the polymer precipi-
7 ¦ tating from the system as a fine white powderO
8 ¦ Pure polyacrylonitrile is difficult to spin because it is
9 ¦ not sufficlently soluble in many organic solvents and is not
10 ¦ readily dyed. Consequently, polymers other than a pure PAN
11 ¦ homopolymer are often produced. Thus, a "PAN" fiber may
12 ¦ actually be an acrylic polymer formed primarily of recurring
13 ¦ acrylonitrile units copolymer~zed with a minor proportion of
14 ¦ methyl methacrylate, vinyl pyridine, vinyl chloride and the
15 ¦ like. Th'ese copolymers exhlbit properties substantially simi-
16 ¦ lar to an acrylonitrile homopolymer~ By convention if the
17 ¦ fiber does not contain more than about 15 percent foreign
18 ¦ material it is refered to as polyacrylonitr'ile, and if more
19 ¦ than 15% then as modlfied acrylonitrile. Examples of such
20 ¦ copolymers include PAN fibers produced under trade names such
21 ¦ as Orlon* (E.I. DuPont de Nemours~, Courtelle SAF* (Courtaulds
22 ¦ Ltd.) and Acrilan (Chemstrand)~
23 ¦ The conversion o the PAN to fibers may be accomplished by
24 either dry or wet spinning. In the latter a salt solution of
the polymer is extruded through a spinneret into a liquid which
26 can coagulate the PAN. In the dry spinning process a filament
27 is formed by the evaporation of a volatile solvent from a PAN
28 solution. In either case the filaments are subsequently
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1 ¦ stretched to several times their original lenyth a~ a sliyhtl~
2 ¦ elevated temperature, for example 100C, so as to dra-" out and
3 ¦ align the m~in polymer chains and increase ln-~rchair-~dhesion. '~ur" U.S.
4 ¦ Patent No 3,729,549 to Gump et al i.ssued April 24, 1973 indicates that the
¦ fibers are sometimes oriented by hot drawing over a heated
6 ¦ shoe at a draw ratio of about 3:1 to about 7:1. In one
7 ¦ instance stretching PAN fibers some fourteen times reportedly
8 ¦ produced a fiber with a Young's modulus and strength of 2.7 x
9 ¦ 106 psi and 130 x 103 psi, respectivelyO
10 ¦ The denier of the resulting PAN fibers, such as those
11 usea as precursors for oxidation and carbonization, generally
12 measures from 1.3 to 3. The spinning and production of PAN
13 fibers with a denier of less than 1.3 as such precursors has
14 proven impractical, since such fibers have been heretofore too
fragilP for process;ng.
16 To effect conversion of the PAN fibers, the latter are
17 heated to about 220C while exposed to oxygen or oxygen-
lB containing gases such as air, nitrous oxide and sulphur
19 dioxide. The heating encourages the formation of a ladder
structure~ while some of the CH2 groups are oxidized and HCN
21 is evolved. This may be ideall~ summarized as~
22 ~ O
23
24 \ ~ CH~ ~ CH2\ ~ \ ~ C ~ ~CH2~ ~ C ~ ~CH2
CH CH ~H + 2 ~ - CH CH CH CH
26 I 1 1 200C
27 CN CN CN to ~C ~ ~C ~ ~C ~ CN
28 300C N N NH
_
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11S610~)
1 ¦ Some references in the prior a~k indicate that P~N fibers
2 ¦ should be prevented from shrirlkiny or drawn slightly during
3 ¦ oxidation in order ko restrain the P~N fibers from reverting
4 ¦ to their weak unstretched state. ~lowever, U.S~ Patent ~o.
~ 4,100,004, issued ~o Moss et al., indicates that some fibers
6 ¦ (not identified as to source) may generally be stretched by at
7 ~ least 50%, though no experimental values greater than 50% are
8 ¦ revealed and stretching of the fibers is generally limited to
9 ¦ resulting carbonized fiber diameters of 6 microns or more.
¦ Ihe ma~m amount o~ stretching indicated by ~le prior art is a~proximately
11 ¦ 95%, i.e. slightl~ less than double. H~ever, even a slight amount of stretch
12 ¦ ing during oxidation is generally avoided. In fact, general
13 ¦ industrial practice is to allow the fibers to shrink slightly
14 ¦ during oxidation in order to avoid any damage to the fibers at
15 ¦ this stage of the process.
16 ¦ The PAN fibers may be further oxidized at higher tem-
17 1 peratures up to about 300C~ Thereafter, to effect
18 ¦ carbonization, the oxidized PAN fibers are heated to tem-
19 ¦ peratures of 300 to 1400C in a nonoxidizing atmosphere, such
20 ¦ as-nitrogen, argon, helium or hydrogen. During this stage HCN
21 and other products from the decomposition reaction of PAN are
22 also released as gases. This release is accompanied by the
23 build-up in the fiber of ribbons consisting largely o~ carbon
24 atoms arranged in aromatic ring structures.
The strength and modulus of these carboni~ed fibers
26 increases rapidly up to about 1400C. However, while further
27 heating beyond about 1400C continues to increase the elastic
28 modulus, it reduces tensile strength, apparently because the
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s-tructure of the carhoni.~ed fibers hecomes rflo:re r~p~es~rl~at:i~fA
of true graphite. Conse~luent:Ly, cornrnerc-;al fiber-; are usually
offered in a carbonlzed form with low mo~ulus and high
strength or in graphitized forrn w:i-th high rnodulus and 10W
s-trength. For e~.ample, :in one case, heating P~N fibers from
about 1400C to about 2400C reported:Ly resul-ted in a decrease
in strength from approximately 3.1 GN/m2 (4.~8 x 10 psi) to
2~2 GN/m2 (3.2 x 105 psi), but an increase in the modulus
from approximately 230 GN/m (33~4 x 106 psi~ to 500 GN/m
(72.5 x 106 psi).
From the foreyoing it can be seen that restrictions
on the size of precursor fiber employed and the amount to which
the fibers can be stre-tched during oxidation, place l.imits on
the strength and elasticity as well as the diameter of the
resulting oxidized, carbonized or graphitized fibers produced.
~dditionally, in view of the relatively high temperatures
involved, high inputs of energy are required to obtain an
oxidized PA~, carbon or graphitized fiber of a given modulus
and elasticity.
In one aspect the invention provides in a method of
oxidizing fibers of an acrylic polymer comprising recurring
acrvlonitrile units, wherein the fibers are oxidized in an
oxidizing atmosphere heated to a temperature range between
about 180C to about 300C preparatory to carbonization of the
fibers, and dur.i.ng oxidation the fibers are drawn under tension,
: the improvement comprising the steps of permeating the polymer
with a carboxylic acid capable of forming its anhydride when
heated to within the temperature range, and drawin~ the permeated
fibers under tension during oxidation of the fibers in the
atmosphere within the temperature range, the carhoxylic acid
being present initial].y in -the polymer in an amount sufficient
to permit improved drawing of the fiber to at least double
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the oriy:inal f:it)er ]encJt:h, and to an avera(3e fiber diatrleter
of less than 10 m:icrons.
The inventiorl also provides novel fiher.c,.
The invention i~ basically accomplishecl by mixing
the acrylic polymer with a carboxyl:ic acid 50 tha-t the po:Ly-
mer fiber is perrneated therewith, the acid being one which,
upon heating the fiber to an oxidiziny ternperature,
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1 will form the anhyc]ride correspondin~ ~o that acid (and it is
2 postulated that the anhydride will equilihrate within the
3 fiber with the acid); oxidizing the treated fibers in an
4 oxidizing atmosphere; and stretching the fibers during the
oxidation process. The acid and/or its anhydride appears to
6 act as a plasticizer and reduces the fiber yield stress and
7 increases fiber plasticity such that during oxidation the
8 treated fibers may be drawn by at least 40~ and as much as
9 300~ or more than similar but untreated fibers. The acid
should be present in the fiber during oxidation, in a quantity
11 sufficient to improve the stretchability the noted amount.
12 The relative amount of acid used depends then upon such fac-
13 tors as the nature of the acid, the choice of the particular
14 fiber as to constituents and diameter, the length of time
allowed to permit the acid to permeate the fiber to some
16 desired extent, etc~ and can easily be determined empirically
17 for each set of parameters.
18 ~ther objects of the invention will in part be obvious and
19 will in part appear hereinafter. The invention accordingly
comprises the processes involving the several steps and the
21 relation and order of one or more of such steps with respect
22 to each of the others, and the products possessing the
23 features, properties and relation of elements which are
24 exemplified in the following detailed disclosure and the scope
of the application all of which will be indicated in the claims.
26 For a fuller understanding of the nature and objects of
27 the present invention, reference should be had to the
28 ~ following ailed desc~ ~tion taken in connection with the
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1 ¦ accompanying drawing wherein:
2 ¦ Fig. 1 is a sch~!atic of the apparatus used to carry out
¦ one embodiment of the present invention; and
4 ¦ Fig. 2 is a representation of a typical temperature gra-
¦ dient of an oxidation furnace employed in one embodiment of
6 ¦ the instant invention.
7 Referring now to Fig. 1, PAN fibers in the form of a
8 multifilament sheet, tow or web, 20, are pulled from fiber
9 supply spool, 22, by constant speed devicel 24, which
comprises a pair of electric drive rollers, 25 and 260
11 Tensioning device 28 of known type, typically comprising three
12 rollers, 29, 30, and 31, in conjunction with take-up device 32
13 is intended to place the mul~ifilament sheet, tow or web in
14 sufficient uniform tension to draw the PAN fibers to the
extent desired during oxidation~ In this regard it is pre-
16 ferable to stretch the PAN fibers during the Qxidation process,
17 more than the prior art and up to as much as quadruple the
18 stretchabili~y of the untreated fibers, since the greater the
]9 stretching accomplished, the more one will achieve the pur-
poses of the invention to produce higher strength and modulus
21 fibers.
Z2 The fiber tow 20, is then transferred under tension
23 through an oxidation chamber such as multizone gradient fur-
24 nace 34, so as to provide a proper residence time, as
discussed below. Upon leaving furnace 34 the oxidized and
26 stretched PAN fiber of tow 20 is taken up on known constant
27 speed take-up device 32, before being passed to a carbonizing
28 zone for further treatment.
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1 ¦ During subsequent carbonization by well-known techinque~,
2 ¦ the oxidized P~N fibers are heated in a nonoxidizing ato-
3 ¦ mosphere, such as nitrogen, argon, helium or hydrogen to tem~
4 ¦ peratures of about 300 to 1400C. The strength and modulus of
¦ the oxidized fibers increases rapidly during this stage as
6 ¦ carbon dioxide, water, carbon monoxide, HCN, NH3, and other
7 1 products are released and aromatic ring structures of carbon
8 ¦ are formed. The carbonized fibers may then be further heated
9 ¦ and graphitiæed under an inert gas at temperatures up to 3000
10 ¦ C. Both carbonization and graphitization may be carried out
11 in one or more stages, during which the fibers are generally
12 placed under some tension. Further details of such treatment
13 are not believed to be required herein, since such are well
14 known in the prior art as illustrated by Mos.s et al. in U.S.
Patent No. 4,100,004 and Gump et al in U.S. Patent No.
16 3,729,549.
17 Multizone gradient furnace 34 comprises a number of
18 heating zones preferably ranging in temperature from a low of
19 about 200C to a high of about 260C, but varying from as much
as 180C at the entrance to 300C at the exit of the furnace.
21 A typical temperature gradient is depicted in Eigure 2 in
22 terms of temperature of the furnace atmosphere at a given
23 distance from the furnace entrance. Of course, a series of
24 separate furnaces with one or more heating zones may be
employed to establish a series of temperature stages.
26 Likewise, a single heating zone furnace held at a particular
27 temperature may also be appropriate depending upon the ulti-
28 ~ mate prope es desired in the fiber product.
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1 ¦ An oxygenation medium comprising oxygen and oxygen con-
2 ¦ taining yases such as air, nitrous oxide and sulphur dioxide
3 ¦ is supplied to furnace 34 by line 36. Although only
4 ¦ shown as supplied at the inlet of furnace 34, the oxygenation
¦ medium may be injected into the furnace at various points
6 ¦ along the path of the fibers as they are oxidized.
7 ¦ Pressure relief and recirculation of the oxidation reac-
8 ¦ tion and thermal decomposition products of PAN as well as any
9 ¦ unreacted gases can be achieved by venting furnace 34 through
10 ¦ line 38, although it may be desirable to permit the gases in
11 ¦ the furnace to remain relatively stagnant to encourage the
12 ¦ postulated equilibrium between the vaporized acld and its
13 anyhydride in the fiber. A main component of these decom-
14 position products is HCN, particularly during oxidation of the
fibers. However, other components include CO, CO2, H20, NH3,
16 as well as a number of intermediate hydrocarbons and nitriles,
17 including acetonitrile and acrylonitrile.
18 In accordance with the present invention, a carboxylic
19 acid which, at the temperatures used to oxidize the fiber,
will substantially vaporize and convert to the corresponding
21 anhydride with which the acid can be in material equilibrium
22 at least in the fiber in part, is mixed with the PAN fiber.
23 Inasmuch as such temperatures are in the range of about 180 to
24 300C, it is apparent that formic acid is excluded insamuch as
it decomposes at such temperatures and does not form an an-
26 hydride. Other carboxylic acids may not vaporize at such tem-
27 peratures or may not thermally form their anhydride in suffi-
28 cient amounts to maintain a substantially balanced equilibrium
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1 ¦ i.e. the reaction goes virtually to completion in one direc-
2 ¦ tion or the other. Thus, both mono and polycarboxylic acids
3 ¦ are useful. Typically, such diverse carboxylic acids as ace-
I tic acid and itaconic acid are acceptable for purposes of the
invention~ Mixing of the fiber and acid can occur in the ori-
6 ginal manufacturing process for the fiber, or the fiber can
7 be permeated or impregnated with the carboxylic acid by imbi-
8 bition in an appropriate solution of the acid. The imbibition
9 time to impregnate the fiber depends upon the composition of
the fiber, particularly the na~ure of the interstitial voids
11 provided by the introduction of copolymers and other materials
12 into the original fiber. Typically, imbibition times of from
13 one minute to several hours can be used, but the longer imbi-
14 bition times seem to provide the better results.
The plasticizing action of the carboxylic acid and/or its
16 anhydride is believed to facilitate molec~lar motion in the
17 PAN fibers. In any event, some acids may be less readily
18 absorbed in the PAN fiber than others, depending upon the
19 steric aspects of the acid and the molecular structure of the
particular fiber. Thus, care must be taken to insure proper
21 mixture of fiber polymer and concentration of the acid, both
22 in amount and in time as the case may be, to allow absorption
23 of the latter into the acrylic fibers. ~lomopolymer acrylo-
24 nitrile exhibits little if any permeation by carboxylic acids25 from a soaking bath, even over extended periods of time, so
26 the desired acid should be incorporated into the fiber pre-
27 ferably at the time of spinning.
~8 In acc ance with the present invention, fiber residence
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1 1~56~1~'3
1 ¦time in the furnace should generally not be less than 2 minu-
2 ¦tes, and is preferab~y in the range from 2 minutes to 120
3 ¦minutes, since the plasticizing effect is not lmmediate.
4 ¦ Consequently, oxidizing the fibers slowly is favored~
¦ In this regard the acrylic polymer which is utilized in
6 ¦ the present process is formed either entirely of recurring
7 ¦ acrylonitrile units, or of recurring acrylonitrile units copo-
~ ¦ lymerized with a minor proportion of one or more vinyl
9 ¦ units to produce a copolymer exhibiting properties substan-
10 ¦ tially similar to an acrylonitrile homopolymer, particularly
11 ¦ with regard to the time needed to undergo oxidation. As to
12 the temperature used in the oxidation process, while acrylo-
13 nitrile homopolymers can be used in the present process, other
14 PAN copolymer fibers which oxidize over a wide temperature
range are preferred.
16 In the prior art, the acrylic fibers were stretched, if at
17 all, during oxidation by approximately not more than 95%
18 their original length. In contrast the treatment of the
19 fibers with carboxylic acid in accordance with the present
inventiont allows the treated acrylic fibers to be stretched
21 during oxidation, as much as 300~ or more compared to the
22 untreated fiber, thus increasing the ultimate strength and
23 modulus of the resulting carbon fibers by as much as 40% and
24 50% or more, respectively. The increase in the strength and
e]asticity of the oxidized acrylic fibers is believed to occur
26 because improved extension of the fibers causes greater chain
27 orientation than has been heretofore possible.
28 A9 rea ly appreciated by those skilled i- the art, these
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1 improved properties are obtained at llttle or no increase in
2 cost, since the plasticizing effect may be obtained by simple
3 addition of inexpenslve carboxylic acids to the fiber. Addition-
4 ally, overall energy requirements to produce a given strength
or modulus of oxidized or carbonized PAN fiber are reduced,
6 since the fibers obtain greater strength and elasticity at an
7 earlier stage of the process, thus reducing the number of sta-
8 ges and the extent of heating required. Concomitant equipment
9 savings are likewise obvious to those skilled in the art.
Use of the carboxylic acid and/or its anhydride at the
11 oxidizing temperatures as a plasticizing rnedium also allows
12 smaller size fiber precursors to be processed and smaller
13 diameter oxldized PAN and carbon fibers to be produced than
14 previously possible. Prior art processes required precursor
fibers of at least approximately 1.3 denier, or greater, and
16 produced oxidized PAN fibers of 12 microns or larger in
17 diameter and carbon fibers of 6 microns or more in diameter.
18 In contrast, the present invention allows the processing of
19 fiber precursors of 1.2 denier or less and the production of
oxidized PAN fibers as small as 3 microns in diameter as well
21 as carbon fibers of as small as 2 microns in diameter with
22 little or no increase in process requirements.
23 The following examples ~urther illustrate the invention
24 and the advantages resulting therefrom. These examples are
presented solely for illustration, such that the invention
26 sho~ld not be construed as being limited to the particular
27 conditions set forth in the examples.
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1 11S6~09
1 ¦ EXAMPLE I
2 ~
3 ¦ E. I. DuPon~ Or~on* brand ~iber is a cornrnercially available
4 ¦PAN fiber with copolymer units interspersed throughout the
¦ fiber structure. On information and belief, the composition
6 ¦of the fiber is 94~ polyacrylonitrile and 6% methyl acrylate.
7 ¦ A thermal gradient from 220 to 240C was established in fur-
8 ¦ nace 34 in several steps. Pure oxygen served as the oxygena-
9 I tion medium supplied through line 36. The drawn and oxidized
10 ¦ PAN fiber, untreated with any acid, exhibited a maximum draw
11 ¦ ratio of 1.27, corresponding to a 27~ elongationO
12 1
13 ¦ EXAMPLE II
14 1
Example I was repeated, but prior to oxidizing the PAN
16 iber, the latter was soaked for one minute in a 8.3% itaconic
17 acid solution in a dip tank, then put throuyh squeeze rolls to
18 express excess fluid, and placed into the oxidiæing chamber.
19 On drawing the fiber during oxidation to a maximum temperature
of 255C, a maximum draw ratio of 1.44 was obtained, an im-
21 provement of .20 in the draw ratio.
22
23 EXAMPLE III
24 _
Example II was repeated but the fiber was after soaking,
26 additionally pretreated in air saturated with 8.5% itaconic
27 acid solution at 190C for four hours beore oxidation. In
28 this instance, a maximum draw ratio of 1.56 was obtained.
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1 S 6'1 0 s~
I ~1 EXAMPLE IV
2 I
3 ¦ Courtauld's S~F Courtelle*~ib~r (hereinafter referrred to
4 ¦ as SPF ) is a commerciall~ availab~e PAN fiher with copolymer
5 ¦ units interspersed throughout the ~iber structure. This fiber
¦ is believed to differ from the DuPont Orlon* brand of acrylic
7 ¦ fiber used in Example I in that, on information and belief, the
8 ¦ composition of the Courtelle* fiber is 93% polyacrylonitrile,
9 ¦ 6% methyl acry]ate and 1~ itaconic acid. An SAF* 3000 fiber
10¦ tow o~ 1.2 d'tex of this Courtelle* fiber was drawn in three
11¦ stages to 300% during oxidation with a residence time of about
12¦ 1 hour. A thermal graaient from 220C to 260C was establish-
13¦ ed in furnace 34 in three steps of 10 to 20 degrees each ~i.e.
14~ 220-230, 230-240, 240-260). Pure oxygen served as the oxyge-
15¦ nativn medium supplied through line 36. The maximum draw ratio
16¦ was 3.08 resulting in a 208% elongation at the end of the
17¦ first oxidation stage (max. temp. 243C)~ a considerable
18¦ improvement over the similar DuPont fiber in which no car-
19 boxylic acid was present prior to treatment~
Four tows of the stretched fiber were collimated to make
21 one 12,000 fiber tow during the last step of the oxidation
22 process. The drawn and oxidized PAN fiber was then carbonized
23 continuously between 300 and 900C under nitrogen and there-
24 after graphitized under nitrogen at 1400C and at 2300C in
two steps. The graphite fiber diameter was found to be
26 approximately 3 microns and a tensile strength of 46 x 104 psi
27 and tensilé modulus of 64 x 106 psi were observed. This
28 represents a 31~ increase in tensile strength and a 30%
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11S64U9
1 increase in the modulus, since a yraphitized fiber prepared
2 under identical conditions, but without drawing and use o a
3 plasticizing medium during oxidation, had a diameter o
4 approximately 7 microns with a tensile strength of 35 x
104 psi and a tensile modulus of 50 x lU5 psi.
7 EXAMPLE V
9 Example IV was repeated, but prior to oxidizing the PAN
fiber, the latter was soaked for one minute in a 6% itaconic
11 acid solution in a dip tank, then put through squee~e rolls to
12 express excess fluid, and placed into the oxidizing chamber.
13 On drawing the iber during oxidation, a draw ratio o 3.61
14 was obtained.
1 EXAM LE VI
1 Example V was repeated three times but in each case the
1 PAN fiber was soaked in a only water. Upon drawing the fiber
2 during oxidat;onl respective draw ratios of 3.03, 3.03 and
2 3.17 were observed, in excellent agreement with the results of
2 Example IV.
2 EXAMPLE VII
2 Example V was repeated using 10% acetic acid solution as
2 the dip. On fiber drawing the observed maximum draw ratio was
2 3.60.
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1 ¦ EXAMPLE VIII
2 l
3 ¦ Example V was repeated using a 1~ acetic acid solution as
4 ¦ the dip. A draw ratio of 3.37 was obtained upon drawing the
5 ¦ fiber.
6 1
7 Although the invention has been described with preferred
8 embodiments, it is to be understood that variations and modi-
9 fications may be resorted to as will be apparent to those
skilled in the art. Such variations and modifications are to
11 be considered within the purview and scope of the following
12 claims.
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