US3775535A - Process for the production of carbon fibre products - Google Patents

Abstract

CARBON FIBRE PRODUCTS, SUCH AS FILAMENTS, STAPLE FIBRES, YARNS, WOVEN FABRICS, KNITTED FABRICS, AND NON-WOVEN FABRICS, ARE PRODUCED BY CARBONIZING CORRESPONDING FIBRE PRODUCTS OF ACRYLONITRILE POLYMERS IN A PROCESS COMPRISING A TWO STEP HEAT TREATMENT. IN THE PROCESS THE POLYMERIC FIBRE PRODUCT IS FIRST HEATED IN AN ATMOSPHERE CONTAINING OXYGEN AT A TEMPERATURE IN THE RANGE OF 150*C. TO 400*C. AND THEREAFTER CARBONIZED IN A FURNACE AT A TEMPERATURE OF AT LEAST 1000*C., THE FIBRE PRODUCT BEING PASSED BETWEEN SAID STEPS THROUGH A LIQUID INERT TO ACRYLONITRILE POLYMERS AND FORMING A GAS-TIGHT SEAL AT THE INLET OF SAID FURANCE. SUITABLE LIQUIDS INCLUDE THE EUTECTIC MIXTURE OF DIPHENYL AND DIPHENYL ETHER, BENZENE, HEAVY GASOLINE, AND MERCURY.

Description

NOW 1973 R. WEISBECK ETAL 3,775,535

PROCESS FOR THE PRODUCTION OF CARBON FIBRE PRODUCTS Filed March 11, 1971 Inventors: &OLAND WEI SBECK, OTTFRIEL PnMPUS, CARLHANS SULING, LOTT-um PREI'S.

United States Patent 3,775,535 PROCESS FOR THE PRODUCTION OF CARBON FIBRE PRODUCTS Roland Weisbeck, Odenthal VoisWinkeI, Carlhans Siiling, Odenthal-Hahnenberg, Gottfried Pampus, Leverkusen, and Lothar Preis, Cologne, Germany, assignors to Bayer Alrtiengeselisclraft, Leverkusen, Germany Filed Mar. 11, 1971, Ser. No. 123,369 Claims priority, application Germany, Mar. 14, 1970, P 20 12 286.5 Int. Cl. C01b 31/07 US. Cl. 423-447 1 Claim ABSTRACT OF THE DISCLOSURE Carbon fibre products, such as filaments, staple fibres, yarns, Woven fabrics, knitted fabrics, and non-woven fabrics, are produced by carbonizing corresponding fibre products of acrylonitrile polymers in a process comprising a two-step heat treatment. In the process the polymeric fibre product is first heated in an atmosphere containing oxygen at a temperature in the range of 150 C. to 400 C. and thereafter carbonized in a furnace at a temperature of at least 1000 C., the fibre product being passed between said steps through a liquid inert to acrylonitrile polymers and forming a gas-tight seal at the inlet of said furnace. Suitable liquids include the eutectic mixture of diphenyl and diphenyl ether, benzene, heavy gasoline, and mercury.

The invention relates to a process for the production of carbon fibre products, such as filaments, staple fibres, yarns, woven fabrics, knitted fabrics, or non-woven fabrics, with improved fibre properties by carbonization of the corresponding fibre products from acrylonitrile polymers in a two-step heat treatment of the fibre products.

It is known to manufacture carbon fibres-starting from polyacrylonitrile fibresin two steps, the organic fibres being exposed, in the first step, to a heat treatment between 200 and 400 C. in an atmosphere containing oxygen, and being carbonized, in the second step, in a flowing atmosphere containing hydrogen.

In most cases the heat treatment is carried out in air. In general, a tensile stress is applied to the fibres during the heat treatment. The carbonization is preferably carried out in flowing hydrogen, at times also in flowing inert gas or in a flowing gas mixture consisting of hydrogen and inert gases. During the carbonization, the temperature is in general increased by less than 300 C./ hour, up to the region of 1000 C. In some cases, the temperature is raised to between about 1500 and 1600 C. The carbonization can be followed, in a third step, by a graphitization at temperatures of up to 3000 C., in inert gases.

The two steps heat treatment and carbonization are in most cases carried out in two separate furnaces. This is standard in continuous processes. The fibre products must then be introduced continuously at one side of the furnace and discharged again at some other place. When carbonizing under hydrogen and/or inert gas, air should be prevented from entering the furnace through the orifices through which the fibrous material is being transported. To avoid damage to the fibrous material at the edges of the orifices, the orifices should be relatively large. Nevertheless, the gas quantities (hydrogen, inert gas) necessary for maintaining the flowing atmosphere during carbonization should be kept as small as possible.

The object of the present invention is to provide relatively large orifices for the fibre products in the carbonization furnace and, nevertheless, safely avoiding the penetration of air into the furnace while extremely economizing on hydrogen and/ or inert gas.

Patented Nov. 27, 1973 p CC Therefore the invention relates to a process for producing of carbon fibre products by carbonization in a two-step heat-treating of fibre products from acrylonitrile polymer.

The process comprises heating in the first step the fibre products in an atmosphere containing oxygen at temperatures of between 150 and 400 C. and heating in the second step at carbonization temperatures of 1000 C. and higher, passing the fibre product between the first and the second step through a liquid being inert to acrylonitrile polymers, said liquid effecting a gas-high seal against the inlet of the fibre product into the second step of carbonization.

Surprisingly the liquid, of which always a small part partly caused by the finite vapour pressure of every liquid, but mainly owing to the wettability and viscosity of the liquidenters the carbonization furnace together with the fibre product, has an influence on the properties of the carbonized fibre product. When properly selecting the liquid, this influence may result in an improvement of the fibre properties.

The special advantages of the process according to invention consist in that the fibre product enters the carbonization furnace without being damaged, that the furnace at the material inlet side is provided with a virtually gas-tight seal towards the outside atmosphere (and is only open for gases at the material outlet side; the furnace atmosphere is discharged here together with the pyrolysis waste gases), and that the fibre properties are being influenced and improved by the liquid carried over into the furnace and by its vapour. The heat treatment is carried out at temperatures between 150 and 400 C., while the carbonization takes place at temperatures up to 1000 C. and higher (1600 C.). The process is carried out in such a manner that the liquid, which is inert towards the fibre product, is located in a trough which is closed by a lid in such a manner that a gas-tight seal is provided. The lid has an orifice through which the fibre product is introduced into the liquid. The orifice is adapted to the fibre product. It should not be unnecessarily large, so as to keep the evaporation of the liquid low. A tube attachment, which is mounted gas-tight at the inlet into the carbonization furnace, projects into the liquid from the bath. The fibre product is passed by means of direction-changing rollers or guide rollers through the liquid into the tube attachment and into the carbonization furnace. The arrangement of the trough and of the tube attachment are so chosen that the liquid cannot run out of the trough into the furnace.

When using filaments, staple fibres, fibre strands, or yarns as fibre products, it is preferred to use circularly cylindrical funnels for introducing these fibre products through the lid of the trough into the liquid and circularly cylindrical tube attachments for introducing the material into the carbonization furnace. Rectangular slit orifices are used for plane structures such as woven fabrics, nonwoven fabrics, and the like.

Appropriate liquids are those whoe handling is relatively easy and does not require any appreciable precautionary measures and which do not attack the fibrous material either in liquid or in vapour-like or in dissociated form. Organic substances liquid at room temperature are very suitable. Liquids preferably used are heavy gasoline and aromatics, for example aromatic hydrocarbons and high-boiling ethers such as diphenyl ether or mixtures of diphenyl and diphenyl ether or mixtures of aromatic hy drocarbons with one another and with aromatic ethers, which are liquid below C., especially at room temperature and at normal pressure. Mercury and also other almost non-wetting metal melts are suitable as well. The temperature of the liquid lies below 100 C., preferably between 0 and 50 C.

It is advisable to use a transparent trough-for example of thick-walled glass or plasticand transparent liquids, so that the fibrous material can be observed at any time during its travel through the trough.

The amount of liquid which passes into the carbonization furnace depends on the following parameters:

The amount of vapour passing into the furnace only depends on the vapour pressure of the liquid at the predetermined liquid temperature and on the internal crosssection of the tube attachment which dips into the liquid. The amount of liquid passing into the furnace depends above all on the wetting of the fibers by the liquid. In the case of good wetting, the amount carried away depends on the viscosity of the liquid and on the specific surface area of the fibrous material. The higher the viscosity and the greater the surface of the material, the more liquid is carried away into the furnace. The surface of the fibres depends on the starting material and on the heat treatment in the oxygen-containing atmosphere and is, in BET determination, generally of the order of 100 m. g.

In the carbonization furnace, organic liquids are decomposed at least partly. The decomposition products may form new compounds. Decomposition may proceed down to carbon, which, under favourable conditions, can separate on the fibre products or in pores of the fibres opened to the outside. When organic, especially aromatic liquids are used, the process according to the invention results in fibre products of carbon of especially smooth appearance, good flexibility, and relatively high values for tensile strength and modulus of elasticity. The carbon content of the fibre products produced by the process according to the invention is generally somewhat higher than that of fibre products produced by carbonization without liquid bath.

The figure shows a device for carrying out the process continuously. The fibre product, consisting of an acrylonitrile polymer, for example a fibre strand, is dravm from the stock roll 1 possessing the brake drum 2 via the rollers 4 with the aid of the first pair of tension rollers 3 and the second pair of tension rollers 5, through the tubular furnace 6 possessing the quartz tube 7 and the funnels 8. A defined amount of air is passed into the tube 7 via the regulator 8a. At the lower roller of the second pair of tension rollers there is a knife 11. The fibre strand is passed via the direction-changing roller 10 and through the funnel 18 into the trough having a lid 16, and is passed via the direction-changing rollers 19, the dip tube 20, and the direction-changing rod 21 through the liquid 17 into the tubular furnace 12 having a quartz tube 13. In the quartz tube 13, in which the carbonization takes place, an oxygen-free gas is introduced via the regulator 12a. The carbon fibre band is wound up on the roll 14.

Following examples illustrate more particularly the invention.

EXAMPLE 1 An endless fibre strip consisting of 6000 individual fibres of acrylonitrile homopolymer, each of 1.8-1.9 dtex., is located on a stock roll 1, the axis of which is borne horizontally so that it can rotate. A brake drum 2 with variable adjustable braking action is seated on the axis.

The fibre strand is continuously drawn from the stock roll between two vertical guide rods by means of a first pair of tension rollers 3 of which one rollers is driven by the synchronous motor. Both rollers are cylindrical; their axes are arranged parallel to one another in a vertical plane. The driven, lower roller consists of mirror-polished high-quality steel. The upper roller is rubber-coated, borne so that it is rotatable about its axis, and is pressed with adjustable firmness against the drive rollers, with the axes of both rollers remaining parallel. Between the stock roll and the first pair of tension rollers there are four rollers 4 alternatingly borne in two different horizontal planes, which on drawing the strip from the braked stock roll convert the almost circular strip cross-section into a rectangular cross-section of large width and small height.

A second pair of tension rollers 5 of exactly the same construction and the same nature, of which the lower drive roller is driven by a synchronous motor running at the same speed as the synchronous motor of the first pair of tension rollers, draws the fibre strand through an electrically heated tubular furnace 6 of 130 cm. length. The heater winding of the furnace is so arranged that a constant temperature of 230 C. prevails over cm. length and a gradual temperature rise of 4-5 C./cm. prevails at the furnace inlet side. A quartz tube 7 of 5 cm. internal diameter is located in the furnace. Glass funnels 8 are cemented into the tube at both ends, and these ensure that the strip is axially guided through the furnace.

At the furnace inlet side, 50 1. (measured at N.T.P.) of air/hour are blown in via the regulator 8a, and these together with the reaction vapours leave the furnace through the funnel orific at the other end. The strand runs through the heat-treatment furnace at a speed of 71 cm./hour. The length of the strand is kept constant, whilst travelling through the heat-treatment furnace, by means of the two pairs of tension rollers at the inlet and outlet of the furnace.

The strand which leaves the heat-treatment furnace again has an almost circular cross-section, and this is converted by two rollers 9, borne in different horizontal planes, into a rectangular cross-section of large width and small height, before the strip passes between the two tension rollers of the second pair. This results in uniform tension of the strip.

Behind the second pair of tension rollers, the strand is drawn vertically downwards via a direction-changing roller 10, in order to pass through the carbonization furnace in a lower horizontal plane and in the converse direction (relative to the direction in the heat-treatment furnace).

To avoid difliculties which are caused by broken fibres, a knife 11-inclined at about 45 to the horizontal plane-is located with the cutting edge tightly against the lower roller above the level of the axis. The strand slides over the inclined knife plane.

The carbonization occurs in a 300 cm. long tubular furnace 12 which possesses ten separate adjacent heater windings, each of about 30 cm. length' (in the direction of the axis of the tube). At the ends, each of the ten windings are wound more tightly to avoid subsidiary minirna in the temperature profile.

Each winding is heated via its own variable transformer in such a way that a strictly monotonously rising temperature curve results along the axis of the tube between the inlet and outlet of the furnace. The furnace inlet is at 150 C. The maximum temperature, which is reached shortly before the furnace outlet, is 1050 C. A quartz tube 13 of 5 cm. internal diameter and smooth glazed internal wall lies in the furnace. The strand is drawn through this tube by means of a winding machine 14 with a cycling drive and slipping clutch.

The carbonization takes place in a stream of hydrogen 1. [measured at N.T.P.] H /hour). The hydrogen is introduced via the regulator 12a at the same end of the furnace as the strip, but through a separate orifice. Excess hydrogen and reaction vapours leave the furnace at the other end, together with the carbonized strip.

A stable thick-walled trough 15 with lid 16 contains 2.5 l. of benzene of room temperature. The strand is guided through a glass funnel 18 inserted in the lid into the benzene and is drawn via two direction-changing rollers 19 of polished high-quality steel through a glass tube 20 of 8 mm. internal diameter and having rounded edges, which dips into the benzene, and over a glass rod 21 arranged horizontally at the level of the tubular furnace axis and at right angles to the axis, axially through the carbonization furnace. The glass funnel 18 and the glass tube 20 are inserted in the lid 16 in a gas-tight manner; the lid is connected to the trough 15 in a gas-tight manner. The consumuption of liquid benzene is about 3 ml./hour.

The speed of the strand is 66 cm./hour; it is lower than the speed during the heat treatment in air, because the carbonization causes the strip to shrink. carbonization up to 1050 C. takes 4.6 hours. The slipping clutch of the winding machine ensures that the strip is always slightly tensioned.

The carbon fibre strand manufactured in this way has a completely smooth, glossy appearance, and is very flexible and suitable for textile applications. It has a C content of 98% and a specific electric resistance of 16 10- x cm. The mean cross-section of the carbonized individual fibre is 60x10" cm The tensile strength was measured to be 1.3 kgf./crn. and the modulus of elasticity 2.3 10 kgil/cmF.

If the same fibre strand, after heat treatment and before carbonization, is drawn through a heavy naphtha of a boiling point of 123 C. (instead of through benzene) and if the heat treatment and the carbonization remain absolutely unchanged, the result is again a carbon fibre strand of smooth, glossy appearance and good flexibility. The C content is 97%, the specific electric resistance 18 10 S2 x cm., the mean cross-section of the carbonized individual fibre 60X 10- cmf The determination of the tensile strength and the modulus of elasticity resulted in 1.2 10- kgfJcm. and l.9 l0 kgf./cm.

EXAMPLE 2 A fibre strand consisting of 3000 individual fibres of an acrylonitrile homopolymer, each of 2.4 dtex., is heattreated and carbonized precisely as described in Example 1, the only difference being that this strip is drawn through a trough which is filled with the eutectic mixture of diphenyl and diphenyl ether: 27% by weight of diphenyl, 73% by weight of diphenyl ether of room temperature (2.5 1.). The consumption of liquid mixture is about 1 mL/hour.

The carbon fibre strand produced under these conditions has a perfectly smooth, glossy appearance, but a black shade slightly dilferent from that of the strip produced according to Example 1. The C content is 98%, the specific electric resistance 14 10' Q x cm., the mean cross-section of the carbonized individual fibre 80x10" cm. The tensile strength is 1.7)(10 kgf./cm. and the modulus of elasticity 2.6)(10 kgf./cm.

If the same fibre strand, which was continuously carbonized here in Example 2 by the process according to the invention, is carbonized discontinuously with equal heat treatment (the fibre strip is wound up on a highquality steel frame, so that on heat treatment it cannot change its length) and equal carbonization, but without the strip making contact with a liquid before the carbonization and no liquid (also not its vapour) being present during the carbonization, a carbon fibre strip is obtained which is less smooth and glossy, has a C content of 96%, a mean fibre cross-section of X l0 cm. and a specific electric resistance of 19 10 Q x cm. The tensile strength is 1.2 l0- kgf./cm. and the modulus of elasticity 1.9 10 kgf./cm.

What We claim is:

1. In a process for the productiop of a carbon fiber product by carbonization in a two-step heat treatment of a fibre product of an acrylonitrile polymer which comprises passing the fibre product continuously through a first heating zone wherein the fibre product is heated in an atmosphere containing oxygen at ISO-400 C. and thereafter through a second heating zone in which the fibre product is heated to at least 1000 C. and carbonized in a furnace having an orifice inlet opening for the fibre product, the improvement which comprises, between said first and second heating zones, passing the fibre product through an organic liquid forming a gastight seal at said orifice inlet opening preventing the penetration of air into the carbonizing furnace at said opening, said organic liquid being the eutectic mixture of diphenyl and diphenyl ether.

References Cited UNITED STATES PATENTS 3,552,923 1/1971 Carpenter 23-2091 3,533,743 10/1970 Prescott et a1. 23209.1 3,539,295 10/1970 Ram 23-209.1 3,615,212 10/ 1971 Whitney et al. 23209.4

EDWARD J. MEROS, Primary Examiner US. Cl. X.R. 264-29 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,775,535 I Dated November 21, 197 3 lnveflofls) Roland Weisbeck et a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 6, line 19, (claim'l, line-4), change "the" to --said polymeric".

Signe dend sea led this 3rd d ay of December. 1974.

(SE L) Attest: f MCCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner Of Patents FORM po-wsd (10-69) I USCOMM-DC QOS'IO-PUO O 10.1. IOVIIIICIY "um-a ornu nu 0-0-"4

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