CA1182957A - Thermal stabilization of acrylonitrile copolymer fibers - Google Patents

Abstract

Abstract of the Disclosure The use of copolymers of acrylonitrile and vinyl bromide as carbon fiber precursors permit substantial reduction in the stabilization time without concomitant reduction in carbon fiber properties.

Description

5~

UiERMAL STAHILIZATION OF ACRYLONITRILE
COPOIYMER FI~ERS
~L:L~ Qf_ln~sntion 5This invent;on relates to the thermal stabili-zation of f;bers formed from a copolymer of acrylon;-tr;le and v;nyl bromide.
~a~ckaround of Invent;on The first carbon fibers ever appear to have been made by Edison, who used them as electrical resist-ance in light bulbs. Prepared by pyrolysis of cellulose threads, these carbon f;bers had poor mechanical properties.
In modern times, the interest in carbon fibers ;s based ma;nly on their use as reinforcement ;n epoxy or polyester resins ~composites). Early in the develop-ment of composites~ in the forties, glass fibers were used to prov;de tensile strength to the formable matrix, which acts as an agglomerant, and transfers stress to the fiber. Glass fibers have a high tensile strength, but a relatively poor elast;c modulus, so their use is confined to applicat;ons where high modulus is not required, for example, for silos, tanks and boat bodies. As increas-ingly more demanding end uses for composites were aspired, for example by the automotive and aeronautic industry, 2S other fibrous materials had to be developed, which would -1a- 07-1068 offer not only high tensile streng-th, but aLso hiyh elastic modulus. Such fibers consist mainly of light elements such as boron~ carbon or beryllium, but also of carbides, nitrides, silicides and oxides~ Among these~ carbon fibers are potentially the most interesting, in particular for the automotive industry, because 5~

-2- 07-106~
of their outstanding strength-to-weigh~ and stiffness~
to-weiyht ratios. Carbon fibers consist essentially (~99.5~ by weight) of carbon. They can, in principle, be made from many organic~ fiber forming ma~erials, however only three such materials have gained industrial importance: rayon, polyacrylonitrile (PAN) and pitch. Rayon is injured by a relatively high carbon loss during carbon;zation as the oxygen con-tained in rayon fibers tends to be released as C0 or C02. Pitch based carbon fibers have relatively poor tensile properties, unless they are pr~pared from extremely purified (expensive) mesophase pitch.
At present, polyacrylonitrile appears to be the most widely used starting material for carbon fibers.
Fibers formed from an acrylic homopolymer or copolymer can be modified to enhance their thermal stability by heating ;n an oxygen-containing atmos-phere at a moderate temperature for prolonged periods of time~ Mechanistically the modifica~ion involves the oligomerization of the nitrile groups to form the so-called ladder structure comprising dihydro-pyridine moiet;es. Intermolecular reaction of nitr;le groups also occurs result;ng in crosslinking~ As all free radical polymer;zations of double and triple bonds, this reaction is strongly exothermic.
The result;ng structure containsconjugated -C=N- groups wh;ch cause color formation. A light yellow color can be observed at about 150Cq and after heat;ng in a vacuum at temperatures of 180 -200C9 polyacrylon;trile has a copper color. Thecyclization reaction becomes particularly critical bet~een 200 and 300C in an oxygen containing atmosphere. If uncontrolled the exotherm;c oligo-merization of nitrile groups can become explosive and the fibers fuse. However, if a suitable temperature regimen and sufficient time are provided, an acrylic precursor fiber can become black, infusible and resistant to flame. Such a fiber is said to be stabilized and can be further heat treated to form a carbon fiber or a graphite fiber. Heating such stabi-lized fibers up to about 1400~C. results in high strength carbon fibers while heating up to about 3000C. results in high modulus carbon fibers or graphite fibers.

Control of the thermal stabilization step has been achieved heretofore by heating the precursor Eiber at moderate temperatures over a long period of time extending up to several hours which becomes a very expen-sive procedure. The present invention markedly reduces the time required for stabilization oE the fiber and the stabilized product results, upon additional heat treatment, in carbon fibers with excellent properties.

Summar_ of Inventlon In accordance with one embodiment oE the pre-sent invention, there is provided a process Eor stabiliz-ing a Eiber derived from a copolymer containing from about 2 to 15 percent by weight of vinyl bromide and from about 85 to 9~ percent by weight of acrylonitrile characterized in that the fiber is heated continuously at a temperature ranging from about 220 to 330C. in an oxygen atmosphere for about 10 to 30 minutes.

5~
-3a-In accordance wi-th another embodiment of the present invention, there is provided a process for the manufacture of carbon fibers wherein acrylic fibers are stahilized by heating in an oxygen containing atmosphere while stretching and thereafter carbonizing the thus stabilized fiber, characterized by employing an acrylic fiber which contains up to 15 percent by weight of vinyl bromide as a comonomer and which has been heated at a temperature ranging from about 220 to 330C. in the oxygen containing atmosphere for about 10 to 30 minutes.

Detailed Description The acrylic polymer utilized as the starting material is :Eormed primarily of recurring acrylonitrile units. In general the starting material acrylic polymer contains not less than 85 percent by weight of acrylo-nitrile units and not more than about 15 percent by weight of vinyl bromide uni~s. Preferably the polymer precursor consi.sts of from about 85 t.o 98 percent by weight of acrylonitrile and from i~ -g~35~
-4- 07-106~
about 2 to 15 percent by weight of v;nyl bromide.
More preferably the vinyl bromide constitutes 4 to 6 percent by weight of the starting material polymer. A v;nyl bromide content of 4.2 percent S by weight ;s the most desirable.
The acrylic precursor typically is provided as a continuous length of a f;brous material and may be in a variety of physical configurations, such as, for example multifilament tows, yarns, strands or similar fibrous forms. The fibrous polymer mater-ial generally is comprised of 0.7 to 2.1 denier filaments,preferably 1.5 denier filamentsa The polymeric fibrous precursor is heated in a continuous furnace, featuring a temperature profile ranging from about 220 to 330C, in an oxygen containing atmosphere, until there is ~ormed a stabilized fibrous material which retains its original configuration substantially intact and which is non-burning when subjected to an ordinary match flame. Typically the fibrous material requires a residence t;rne in the furnace of about 10 to 30 minutes.
Preferably the oxidizing atmosphere is air however other such atmospheres may be employed.
For example,an oxid;~;ng atmosphere compr;sed of from 2 to 50 percen~ oxygen and an inert gas, such as nitrogen, argon or helium may be utilized.
The precursor polymeric fibrous material is h;ghly oriented and this characteristic is ma;n-ta;ned or enhanced by stretching the precursor duringstabilization which ultimately enhances the tensile strength of the carbon fibers produced therefrom.
Orientation of the fibrous material is achieved primarily by stretching (6-13 folcl) during the spin-ning of the filamentary material. The optimumamount of stretching which may be applied during stabilization depends upon not only the amount of stretch applied durinq spinning of the polymeric material but also upon the particular vinyl bromide content. A lower sp;nning stretch and a higher vinyl bromide content permit higher stabilization stretch.
Up to 15% stretch may be imparted during stab;liza-t;on to the fibrous material employed in the present invention.
Generally the desirable dra~-ratio during stabilization is determined by increasing the stretch level until fi1ament breakage appears, and backing off so that undamaged fiber is produced.
Typically in practicing the present inven-tion the polymeric fibrous precursor is passed through a heated furnace provided with an oxygen-containing atmosphere by conventional means. For example a bobbin of spun fibrous precursor may be mounted on a free wheeling mandrel and redirected through a nip roll.
The nip roll is closed and the motor speed is adjusted to obtain the desired feed rate into the furnaceO
A similar drive system ;s mounted at the furnace exit, and each drive system is independently controlled.
9y varying the linear speed at the upper and lower nip rolls the amount of stretch ;mparted to the f;ber may be controlled. The furnace employed may be,for example, a vertical, tubular furnace of about 20 feet in length. The furnace temperature profile can be establ;shed by wiring and controlling the heaters in five independent~ approximately equally long zones with Zone 1 being the entrance zone, and Zone 5 being the exit zone.
After the fibers have been stabilized they may be completely carbonized in times as short as 2 minutes without detrimental effects on the resultant carbon fibers by heating in an inert atmosphere at a temperature which may vary from about 1200 to about 6- 07-106~

1450C. In order to maintain the high orientation, shrinkage should be avoided dur;ng this step. The properties of the final carbon fibers, all other conditions being essentially the same, depend con-S siderably on the temperature profile in the furnaceand on the residence time of the fiber therein as illustrated in the following Table I.

3~7 ~7~ 07-1 068 J r~l O~--~ 00 ~ O ~ ~r~ l ~ r~l N
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E U_ ~J
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L ~ 11:) C
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v)ro 2 ~ O O O O OJ O O ' Q O O r~l Of~l 0 ~r~1 C' ~ 8 O` O~ O C~ ~ `O O` `O O` `O
ou~ x ~~ ~ ~ ~r~l ~ N O OJ ~ r~l N i~l '~O` U~
~-- 1~1 ., Va, C _ O
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Q~ L ~ H U1 t~ O L _ ~V) ~ ~ ~ ~ ~ J L
a~ L (/~ O ~ 11~ m m m m c~ c, c~ ~ UJ ~ ~ ~ ~ ~ ~ ~
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r~ ~-- E ~ ~O ~ ~ ~ ~O oo ~ r~l r~l ~ ~ ~ ~ ~- _ _ _ ,~

3~57 It is apparent that utilization of profiles having a higher entrance and end temperature renders pos~ble shorter stabilization times and also yieldscar ~on fibers haviny excellent tensile properties. Carbon fibers having the best tensile properties result from precursor fibers which were stabilized in lS
to 20 m;nutes.
The acrylon;tr;le/vinyl bromide polymer;c pre-cursor offers certa;n advantages not found in known precursor materials. V;nyl bromide has two important properties which are not found combined in any of the other pQtential comonomers,for example, vinyl chloride, methyl methacrylate, methyl acrylate or v;nyl acetate.
The f;rst such property ;s a small molar volume.
Due to the small volume of the brom;ne s;de chain, vinyl bromide as a comonomer does not essentially reduce the high molecular order, which can be obtained in stretched polyacrylonitr;le fibers. Since good carbon f;ber tens;le propert;es are a d;rect conse-quence of molecular or;entation, the advantage isevident.
Intimately related to the greater molecular order is the relat;vely high melt;ng point, which helps to prevent f;ber fus;on, thus perm;tt;ng stab;l;zation at h;gher temperatures than w;th other precursors~ Since h;gher react;on temperature means higher react;on rate~ ;t ;s possible to achieve shorter stabil;zation times.
The second such property is the very weak C-Br bond~ The weak C-Br bond (-65 kcal/mol) is the first bond to break dur;ng heat treatment, at a temperature s;gnificantly below that required for ma;n cha;n sc;ssion (main chain C-C bond in poly-acrylonitrile: 71 kcal/mol). The breaking of the C-Br bond g;ves radicals, without fragmentation of a macromolecule. The radicals may initiate the oligo-merization of CN groups at a lower temperature, ~L~8~2~5~

compared to the cases where main chain scission is the lowest energy radical source, for example, acrylonitrile/methyl methacrylate, acrylonitr;le/
methyl acrylate, or acrylonitrile/vinyl acetate.
This not only spreads the heat evolution over a greater temperature range, thus prevent;ng f ;ber fusion, but also takes care of early crossl;nk;ng of macromolecules~ As a consequence, the formation of small fragments is reduced in the temperature range of main chain scission.
The very reactive and mobilè bromine radical most probably does not take part d;rectly in the oligomer;zat;on react;on. Presumably ;t a~stracts the nearest hydrogen it finds, creating a new carbon rad;cal able to ;n;t;ate the CN ol;gomer;zat;on:
R - CH2 - CH - R' + ~r*
CN
- CH2 - C* - R~ + HBr CN
Hydrobromic acid is very l;kely to ;n;t;ate, add;t;on-ally, the CN ol;gomer;zat;on by a d;fferent, ;on;c mechan;sm~ as known far succ;notr;le:

CHz - fH2 fH2 ~ CIH2 CH2 - C~2 CN CN C C~ C r \~N ~NH ¦~N/ ~NH
Br~ 8r i7 ~10- 07-1068 Since part of the bromine appears again as covalently bonded, ~here might be even a truly cataLytic effect o-f the bromine.
The advantage of the small molar volume of vinyl bromide is shared by monomers like ethylene, vinyl chloride or vinyl iodide. But, the first two do not have a labile bond which could give radicals before main chain scission takes place~ tC-C in polyethylene: 82 kcal/mol; C-C1 in polyvinylchloride:
78 kcal/mol). Vinyl iodide, on the other hand, has too labile a bond (C-I: 53 kcal/mol) which probably would not even survive dope preparation temperatures.
Vinyl chloride, in a polymer or copolymer, is known to decompose thermally by dehydrochlorination rather than by a radical break. However, HCl is considerably less prone than HBr to attack CN groups.
Hence, for several reasons vinyl bromide may be expected to be superior to vinyl chloride.
The acrylon;trile/vinyl bromide precursor fiber permits rapid stabilization without fiber damage, and consequently with resulting carbon fibers (1400 C) of high quality: sonic modulus about 276 GN/m ; tensile strength about 2.76 GN/m density 1.7 g/ml~ Prior to our invention we have been unable to obtain such high qual;ty carbon fibers with the reduced stabilization times as herein disclosed.
The polymeric precursor mater;al described herein may be prepared by any conventional polymeriza-tion procedure, such as mass polymerization methods, solution polymerization methods, or procedures wherein the monomers are dispersed in the reaction medium, either by suspension or emulsion. The polymerization is normally catalyzed by known catalysts and is carried out in equipment generally used in the art.
Howeverr the preferred practice utilizes suspension polymerization wherein the polymer is prepared in finely divided form for immediate use in the filament ~ 07-1068 forming operations. Suspension polymerization accord-ing to batch or semi-continuous methods can be used~
The preferred method however is continuous polymeriza-tion involving the gradual addition of monorners and the continuous withdrawal of polymer.
The polymerization ;s catalyzed by means of conventional free radical catalysts well known in the art. Included among these are organic and inor-ganic peroxides containing the peroxy group:
~ O - O ~
A wide variation in the quantity of peroxy compound ;s poss;ble. For example, from 0~1 to 3.0 percent based on the weight of the polymerizable monomer may be used.
The well known redox catalyst system al~so may be used. Redox agents are generally compounds in a lower valence state which are readily oxidized to the higher valence state under the conditions of reaction. Through the use of this reduction-oxidation system ;t is poss;ble to obta;n polymerization -to a substantial extent at lower temperatures than other-wise would be required. Su;table redox agents are sulfur dioxide, the alkali metal and ammonium bisul~
fitesl and sodium formaldehyde sulfoxylate. The catalyst may be charged at the outset of the reaction, it may be added continuously or in increments through-out the reaction for the purpose of maintaining a more useful concentration of catalyst in -the reaction mass~ The latter method ;s preferred because it tends to make the resultant polymer more uniform in regard to its chem;cal and physical properties.

Although the uniform distribution of the reactants throughout -the reaction mass for the sus-pension polymerization technique can be achieved by vigorous agitation, it is generally desirable to promote the uniform distribution oF reagents by using inert wetting agents, emulsion stabilizers, or dispers;ng agents~ Suitable reagents for this pllrpose are the water soluble salts or fatty acids, such as sodium oleate and potassium stearate, mixtures of water soluble fatty acid salts, such as common soaps prepared by the saponification of animal and vegetable oils, the amino soaps such as salts of triethanolamine and dodecylmethylamine, salts of resin acids and mixtures thereof. The water soluble salts of half esters of sulfonic acids and long chain aliphatic alcohols, sulfonated hydrocarbons, such as alkyl aryl sulfonates, and others of a wide variety of wetting agents, which are in general organic com-pounds, containing both hydrophobic and hydrophilic radicals. The quantity of emulsifying or dispersing agent will depend upon the particular agents selected, the ratio of monomer to be used and the condit;ons of polymerizat;on. In general, however, from 0.1 to 1.0 weight percent based on the weight of the monomers can be employed~
The dispers;on polymerizations are prefer-ably conducted in stainless steel or glass-lined vessels provided with means for agitating the contents there;n. Generally, rotary stirr;ng devices are the most effective means of incurring the intimate contact of the reagents, but other methods may be successfully employed, for example, by rocking or rotating the reactors. The polymerization equipment generally used is conventional in the art.

¢~S7 The polymers from which the filaments are produced in accordance with the present inven-tion have specific viscosities within the range of 0.1 to 0.3. The specific viscosity value as employed herein is represented by the formula:
Time of flow of polymer solutions in seconds_ sp Time of flow of the solvent in seconds Viscosity determinations of the polymer solutions and solvents are made by allowing said sol-utions flow by gravity at 25C. through a capillaryviscosity tube. In the determinations herein a poly-mer solution containing 0.1 gram of the polymer dis-solved in 100 ml of N,N'dimethylformamide is employed The most effective polymers for the preparation of filaments are those of uniform phys;cal and chemical properties and of relatively high molecular weight.
Filaments prepared from the copolymers of the present invention possess excellent properties of strength and dimensional stability as well as proper-ties of heat and light stability which carry overfrom the polymer. The polymer dopes comprising 10-25 percent solids of copolymers may be spun according to conventional wet~ dry or dry jet-wet methods. In general useful filaments have been manufactured by dissolv;ng the v;nyl brom;de copolymers in a polar organ;c solvent such as dimethylacetamide, dimethyl-formamide or dimethylsulfoxide and adjusting the polymer solids to about 25 percent. The polymer dope can then be extruded ;nto a coagulat;on bath, washed, stretched and passed through a finish bath before drying. Variations in the process for fiber prepar-ation are well known in the art.

s~

In general wet-spinning techniques can be used for -the acrylonitrile/vinyl bromide copolymers, i-f the somewhat lower solubility (as compared to copolymers such as acrylonitrile/vinyl ace-tate or acrylonitrile/
methyl acrylate) is taken care of. Solution, for example, in dimethylacetamide, storage and pumping are made at elevated temperature. Typically, a 25 percent solution of acrylonitrile/vinyl bromide copolymer is prepared at 112C. The dope is deaerated in a vacuum (22 mm H9) 1û at 85C for one hour and spun into a slow-coagulant medium (e.g., 60% dimethylacetamide, 40% water, at 40 C) with a jet stretch of 0.7 to 1.25 X. Orientation is accomplished using a boiling water cascade, steam tube, and~or a surface heated godet~ Stretching factors from 6 to 13 X are su;table.

A copolymer consisting of 95.8% acrylo nitr;le and 4.2% vinyl bromide, with a specific viscosity i~ 5p=0.173, is wet-spun from dimethyl-acetamide into a fiber, with a spin stretch of 13 X.
The fiber is subsequently stabilized by passing it through a tubular, 20 ft. long furnace, featuring 5 approximately equally long temperature zones. The appl;ed temperature profile is as follows:
Zone 1: heating up to 260 C, zone 2: plateau at 260C, zone 3: heat;ng to 300C, zone 4: pla~eau at 300C, zone 5: heating ~o 330C. The residence time of the fiber in the furnace is 15 minutes. A
stabilization stretch of 15% is applied. The atmos-phere is a;r. The stab;l;zed fiber has a tenacity of 1.~ g/denier and an elongat;on of 4.8%. Af~er carbon;zation at 1400 C, the carbon fiber has a sonic modulus, E , of 292 GN/m , a tensile strength~of 2.95 GN/m and a density~ of 1.73 g/ml.

5'7 The precursor fiber of Example 1 is stabilized with the following temperature profile:
Zone 1. heating up to 260 C~ zone 2 plateau at 260 C~ zone 3: heating to 330 C, zones 4 and 5:
plateau at 330C. The residence time is 11 minutes;
14% stabilization stretch. Carbon fiber properties:
Es = 262 GN/m2,6~= 2~ 29 GN/m2~ 72 g/ml.

The precursor fiber of Example 1 is stabil1zed with the following temperature profile:
Zone 1: heating up to 260C~ zone 2: plateau at 260C~ zone 3: heating to 310C, zone 4: plateau at 310C~ zone 5: heating to 330C~ Residence time:
15 20 minutes, stretch 15~. Carbon fiber properties:
Es = 292 GN/m2, C~= 3~07 GN/m2,~ 72 g/ml.
EXA~PLE 4 A copolymer consisting of 93~6~o of acrylonitrile and 6.4% of vinyl bromide is spun into 20 a fiber, with spinning stretch 10%. The fiber is stabilized with the following temperature profile:
Zone 1: heating up to 260C, zone 2: plateau at 260 C~ zone 3: heating to 300C~ zones 4 and S:
plateau at 300 C. The residence time is 30 minutes, 2S stabilization stretch is 7~5%~ Carbon fiber properties:
Es = 2~4 GN/m , ~= 2~90 GN/m ,jO = 1.70 g/ml.

Claims (7)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A process for stabilizing a fiber derived from a copolymer containing from about 2 to 15 percent by weight of vinyl bromide and from about 85 to 98 percent by weight of acrylonitrile characterized in that said fiber is heated con-tinuously at a temperature ranging from about 220 to 330°C. in an oxygen atmosphere for about 10 to 30 minutes.

2. The process of claim 1 characterized in that said fiber is heated in a continuous furnace featuring a temperature profile ranging from about 260 to 330°C. with a residence time of about 10 to 30 minutes.

3. The process of claim 1 characterized in that the copolymer contains from 4 to 7 percent by weight of vinyl bromide.

4. The process of claim 1 characterized in that the copolymer contains 4.2 percent by weight of vinyl bromide.

5. The process of claim 1 characterized in that the residence time is 15 to 20 minutes.

6. A stabilized fiber derived from a copolymeric precur-sor containing from about 2 to 15 percent by weight of vinyl bromide and from about 85 to 98 percent by weight of acrylo-nitrile when prepared by the process of claim 1.

7. A process for the manufacture of carbon fibers wherein acrylic fibers are stabilized by heating in an oxygen containing atmosphere while stretching and thereafter carboniz-ing the thus stabilized fiber, characterized by employing an acrylic fiber which contains up to 15 percent by weight of vinyl bromide as a comonomer and which has been heated at a tem-perature ranging from about 220° to 330°C. in said oxygen containing atmosphere for about 10 to 30 minutes.