EP0066389A2

EP0066389A2 - Thermal stabilization of acrylonitrile copolymer fibers

Info

Publication number: EP0066389A2
Application number: EP82302439A
Authority: EP
Inventors: Salvador Olivé; Gisela Henrici Olivé
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1981-05-15
Filing date: 1982-05-13
Publication date: 1982-12-08
Also published as: EP0066389A3; CA1182957A; JPS57193523A

Abstract

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

Field of Invention

This invention relates to the thermal stabilization of fibers formed from a copolymer of acrylonitrile and vinyl bromide.

Background of Invention

The first carbon fibers ever appear to have been made by Edison, who used them as electrical resistance in Light bulbs. Prepared by pyrolysis of cellulose threads, these carbon fibers had poor mechanical properties.
In modern times, the interest in carbon fibers is based mainly on their use as reinforcement in epoxy or polyester resins (composites). Early in the development of composites, in the forties, glass fibers were used to provide 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 retatively poor elastic modulus, so their use is confined to applications where high modulus is not required, for example, for silos, tanks and boat bodies. As increasingly more demanding end uses for composites were aspired, for example by the automotive and aeronautic industry, other fibrous materials had to be developed, which would offer not only high tensile strength, but also high 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 of their outstanding strength-to-weight and stiffness- to-weight ratios. Carbon fibers consist essentially (>99.5% by weight) of carbon. They can, in principle, be made from many organic, fiber forming materials, however only three such materials have gained industrial importance: rayon, polyacrylonitrile (PAN) and pitch. Rayon is injured by a relatively high carbon loss during carbonization as the oxygen contained in rayon fibers tends to be released as CO or C0₂. Pitch based carbon fibers have relatively poor tensile properties, unless they are prepared 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 in an oxygen-containing atmosphere at a moderate temperature for prolonged periods of time. Mechanistically the modification involves the oligomerization of the nitrile groups to form the so-called ladder structure comprising dihydropyridine moieties. Intermolecular reaction of nitrile groups also occurs resulting in crosslinking. As all free radical polymerizations of double and triple bonds, this reaction is strongly exothermic.
The resulting structure contains conjugated -C₌N- groups which cause color formation. A light yellow color can be observed at about 150°C, and after heating in a vacuum at temperatures of 180 - 200°C, polyacrylonitrile has a copper color. The cyclization reaction becomes particularly critical between 200 and 300°C in an oxygen containing atmosphere. If uncontrolled the exothermic oligomerization 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 stabilized fibers up to about 1400°C results in high strength carbon fibers while heating up to about 3000⁰C results in high modulus carbon fibers or graphite fibers.
Control of the thermal stabilization step has been achieved heretofore by heating the precursor fiber at moderate temperatures over a long period of time extending up to several hours which becomes a very expensive procedure. The present invention markedly reduces the time required for stabilization of the fiber and the stabilized product results, upon additional heat treatment, in carbon fibers with excellent properties.

Summary of Invention

The present invention comprises a process for the thermal stabilization of a fiber derived from a copolymer containing from up to about 15 percent by weight of vinyl bromide and not less than about 85 percent by weight of acrylonitrile whereby said fiber is heated at a temperature of about 220° to 330°C in an oxygen atmosphere for about 10 to 30 minutes.

Detailed Description

The acrylic polymer utilized as the starting material is formed primarily of recurring acrylonitrile units. In general the starting material acrylic polymer contains not less than 85 percent by weight of acrylonitrile units and not more than about 15 percent by weight of vinyl bromide units. Preferably the polymer precursor consists of from about 85 to 98 percent by weight of acrylonitrile and from about 2 to 15 percent by weight of vinyl bromide. More preferably the vinyl bromide constitutes 4 to 6 percent by weight of the starting material polymer. A vinyl bromide content of 4.2 percent by weight is the most desirable.
The acrylic precursor typically is provided as a continuous length of a fibrous 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 material generally is comprised of 0.7 to 2.1 denier filaments,preferably 1.5 denier filaments.
The polymeric fibrous precursor is heated in a continuous furnace, featuring a temperature profile ranging from about 220° to 330⁰C, in an oxygen containing atmosphere, until there is formed 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 time 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 oxidizing atmosphere comprised of from 2 to 50 percent oxygen and an inert gas, such as nitrogen, argon or helium may be utilized.
The precursor polymeric fibrous material is highly oriented and this characteristic is maintained or enhanced by stretching the precursor during stabilization which ultimately enhances the tensile strength of the carbon fibers produced therefrom. Orientation of the fibrous material is achieved primarily by stretching (6-13 fold) during the spinning of the filamentary material. The optimum amount of stretching which may be applied during stabilization depends upon not only the amount of stretch applied during spinning of the polymeric material but also upon the particular vinyl bromide content. A lower spinning stretch and a higher vinyl bromide content permit higher stabilization stretch. Up to 15X stretch may be imparted during stabilization to the fibrous material employed in the present invention.
Generally the desirable draw-ratio during stabilization is determined by increasing the stretch level until filament breakage appears, and backing off so that undamaged fiber is produced.
Typically in practicing the present invention 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 furnace. A similar drive system is mounted at the furnace exit, and each drive system is independently controlled. By varying the linear speed at the upper and lower nip rolls the amount of stretch imparted to the fiber 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 established 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 1450°C. In order to maintain the high orientation, shrinkage should be avoided during this step. The properties of the final carbon fibers, all other conditions being essentially the same, depend considerably on the temperature profile in the furnace and on the residence time of the fiber therein as illustrated in the following Table I.
It is apparent that utilization of profiles having a higher entrance and end temperature renders possible shorter stabilization times and also yields carbon fibers having excellent tensile properties. Carbon fibers having the best tensile properties result from precursor fibers which were stabilized in 15 to 20 minutes.
The acrylonitrile/vinyl bromide polymeric precursor offers certain advantages not found in known precursor materials. Vinyl bromide has two important properties which are not found combined in any of the other potential comonomers,for example, vinyl chloride, methyl methacrylate, methyl acrylate or vinyl acetate. The first such property is a small molar volume. Due to the small volume of the bromine side chain, vinyl bromide as a comonomer does not essentially reduce the high molecular order, which can be obtained in stretched polyacrylonitrile fibers. Since good carbon fiber tensile properties are a direct consequence of molecular orientation, the advantage is evident.
Intimately related to the greater molecular order is the relatively high melting point, which helps to prevent fiber fusion, thus permitting stabilization at higher temperatures than with other precursors. Since higher reaction temperature means higher reaction rate, it is possible to achieve shorter stabilization 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 during heat treatment, at a temperature significantly below that required for main chain scission (main chain C-C bond in polyacrylonitrile: 71 kcal/mol). The breaking of the C-Br bond gives radicals, without fragmentation of a macromolecule. The radicals may initiate the oligomerization of CN groups at a lower temperature, compared to the cases where main chain scission is the lowest energy radical source, for example, acrylonitrile/methyl methacrylate, acrylonitrile/ methyl acrylate, or acrylonitrile/vinyl acetate. This not only spreads the heat evolution over a greater temperature range, thus preventing fiber fusion, but also takes care of early crosslinking of macromolecules. As a consequence, the formation of small fragments is reduced in the temperature range of main chain scission.
The very reactive and mobile bromine radical most probably does not take part directly in the oligomerization reaction. Presumably it abstracts the nearest hydrogen it finds, creating a new carbon radical able to initiate the CN oligomerization:
Hydrobromic acid is very likely to initiate, additionally, the CN oligomerization by a different, ionic mechanism, as known for succinotrile:
Since part of the bromine appears again as covalently bonded, there might be even a truly catalytic effect of 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. (C-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-1: 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 acrylonitrile/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/m²; tensile strength about 2.76 GN/m² density 1.7 g/mL. Prior to our invention we have been unable to obtain such high quality carbon fibers with the reduced stabilization times as herein disclosed.
The polymeric precursor material described herein may be prepared by any conventional polymerization 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. However, the preferred practice utilizes suspension polymerization wherein the polymer is prepared in finely divided form for immediate use in the filament forming operations. Suspension polymerization according to batch or semi-continuous methods can be used. The preferred method however is continuous polymerization involving the gradual addition of monomers and the continuous withdrawal of polymer.
The polymerization is catalyzed by means of conventional free radical catalysts well known in the art. Included among these are organic and inorganic peroxides containing the peroxy group:
A wide variation in the quantity of peroxy compound is possible. 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 also 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 it is possible to obtain polymerization to a substantial extent at lower temperatures than otherwise would be required. Suitable redox agents are sulfur dioxide, the alkali metal and ammonium bisulfites, and sodium formaldehyde sulfoxylate. The catalyst may be charged at the outset of the reaction, it may be added continuously or in increments throughout the reaction for the purpose of maintaining a more useful concentration of catalyst in the reaction mass. The latter method is preferred because it tends to make the resultant polymer more uniform in regard to its chemical and physical properties.
Although the uniform distribution of the reactants throughout the reaction mass for the suspension 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 dispersing agents. Suitable reagents for this purpose 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 compounds, 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 conditions of polymerization. In general, however, from 0.1 to 1.0 weight percent based on the weight of the monomers can be employed.
The dispersion polymerizations are preferably conducted in stainless steel or glass-lined vessels provided with means for agitating the contents therein. Generally, rotary stirring 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.
The polymers from which the filaments are produced in accordance with the present invention have specific viscosities within the range of 0.1 to 0.3. The specific viscosity value as employed herein is represented by the formula:
Viscosity determinations of the polymer solutions and solvents are made by allowing said solutions flow by gravity at 25°C. through a capillary viscosity tube. In the determinations herein a polymer solution containing 0.1 gram of the polymer dissolved in 100 ml of N,N'dimethylformamide is employed. The most effective polymers for the preparation of filaments are those of uniform physical 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 properties of heat and light stability which carry over from 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 dissolving the vinyl bromide copolymers in a polar organic solvent such as dimethylacetamide, dimethylformamide or dimethylsulfoxide and adjusting the polymer solids to about 25 percent. The polymer dope can then be extruded into a coagulation bath, washed, stretched and passed through a finish bath before drying. Variations in the process for fiber preparation are well known in the art.
In general wet-spinning techniques can be used for the acrylonitrile/vinyl bromide copolymers, if the somewhat Lower solubility (as compared to copolymers such as acryLonitriLe/vinyL acetate 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 112°C. The dope is deaerated in a vacuum (22 mm Hg) at 85°C for one hour and spun into a sLow-coaguLant medium (e.g., 60X 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 suitable.

EXAMPLE 1

A copolymer consisting of 95.8% acrylonitrile and 4.2% vinyl bromide, with a specific viscosity η _sp=0.173, is wet-spun from dimethylacetamide 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 applied temperature profile is as follows:

Zone 1: heating up to 260°C, zone 2: plateau at 260°C, zone 3: heating to 300°C, zone 4: plateau at 300°C, zone 5: heating to 330°C. The residence time of the fiber in the furnace is 15 minutes. A stabilization stretch of 15% is applied. The atmosphere is air. The stabilized fiber has a tenacity of 1.8 g/denier and an elongation of 4.8%. After carbonization at 1400°C, the carbon fiber has a sonic modulus, _Es, of _{292 GN/}m², a tensile strength σ of 2.95 GN/m and a density ρ of 1.73 g/ml.

EXAMPLE 2

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 330°C. The residence time is 11 minutes; 14X stabilization stretch. Carbon fiber properties:

E_s = 262 GN/m², δ = 2.29 GN/m² ρ = 1.72 g/ml.

EXAMPLE 3

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 310°C, zone 4: plateau at 310°C, zone 5: heating to 330°C. Residence time: 20 minutes, stretch 15%. Carbon fiber properties: E_s = 292 GN/m², δ= 3.07 GN/m², ρ = 1.72 g/ml.

EXAMPLE 4

A copolymer consisting of 93.6% of acrylonitrile and 6.4% of vinyl bromide is spun into a fiber, with spinning stretch 10%. The fiber is stabilized with the following temperature profile:

Zone 1: heating up to 260°C, zone 2: plateau at 260°C, zone 3: heating to 300°C, zones 4 and 5: plateau at 300°C. The residence time is 30 minutes, stabilization stretch is 7.5%. Carbon fiber properties: E_{s =} 234 GN/m², δ= 2.90 GN/m², ρ = 1.70 g/mL.

Claims

₁. A process for stabilizing a fiber, characterized in that the fiber is 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 and said fiber is heated continuously at a temperature ranging from about 260 to 330°C. in an oxygen atmosphere for about 10 to 30 minutes.

2. A 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. A process of either Claim 1 or Claim 2, characterized in that the copolymer contains from 4 to 7 percent by weight of vinyl bromide.

4. A process of either Claim 1 or Claim 2, characterized in that the copolymer contains about 4.2 percent by weight of vinyl bromide.

5. A process of any of the preceding claims, characterized in that the residence time at the said temperature is 15 to 20 minutes.

6. A process for the manufacture of a carbon fiber, characterized in that an acrylic fiber that contains up to 15 percent by weight of vinyl bromide as a comonomer is stabilized by heating in an oxygen containing atmosphere while stretching and thereafter carbonizing the thus stabilized fiber.

7. A process of Claim 6, characterized in that the acrylic fiber is stabilized by a process according to any of Claims 1 to 5.