DE3602330A1 - METHOD AND DEVICE FOR PRODUCING GRAPHITE FIBERS - Google Patents

Description

Process and device for the production of graphite fibers

The invention relates to graphitized polyacrylonitrile (PAN) fibers and more particularly to PAN fibers which have a considerably increased modulus of elasticity.

Graphitized PAN fibers are used to create composites with other materials, in particular for applications where a high strength to density ratio and a high modulus of elasticity ratio too density is required. These applications are, however, due to the ultimate strength, due to the modulus of elasticity and limits the diameter of the graphitized PAN fibers. Accordingly, numerous attempts have been made has been used to increase the strength and especially the modulus of elasticity of PAN fibers.

The currently used processes for the production of Graphite fibers based on PAN use a spinning of the PAW, which is an oxidation, a carbonization and finally a graphite solution of the resulting PAN fibers follows. The acrylonitrile monomer can be produced by various known methods including direct ones catalytic addition of hydrogen cyanide to acetylene or addition of HCN to ethylene oxide to form ethylene cyanohydrin to produce, followed by dehydration. The polymerization is usually carried out in an aqueous solution

carried out, the polymer precipitating out of the system as a fine white powder.

Pure polyacrylonitrile is difficult to spin because it is not sufficiently soluble in many organic solvents and because its fibers do not dry quickly. As a result, polymers other than pure PAN homopolymers are usually made commercially. Thus, a PAN fiber can actually be an acrylitic polymer, which was primarily produced from repeating acrylonitrile units which are combined with a smaller proportion of methyl methacrylate, vinyl pyridine, vinyl chloride and the like to form a copolymer. These copolymers have numerous properties that are similar to an acrylonitrile homopolymer. Conventionally, a fiber that contains no more than about 15 % foreign matter is referred to as polyacrylonitrile, and if it has more than 15% it is referred to as modified acrylonitrile fiber. Examples of such copolymers include PAN fibers sold under the following trademarks: Orion (EI DuPont de Nemours), Courtelle SAF (Courtaulds Ltd.) and Acrilan (Chemstrand).

To bring about a conversion of the PAN fibers into carbon fibers, the PAN fibers are first oxidized and ^{heated to approximately 220 °} C., while at the same time being exposed to oxygen or gases containing oxygen, such as air, nitrogen oxides and sulfur dioxide. The heating causes the creation of the conductor structure by a cyclic reaction, whereby some of the CHg groups are oxidized and HCN evolved

The PAN fibers can be further oxidized at higher temperatures up to 300C. Thereafter, the oxidized PAN fibers at temperatures of 3OO be ⁰ ° C to 800 G in a non-oxidizing atmosphere, such as nitrogen, argon, helium or hydrogen, is heated to effect carbonization. During this stage, HCK and other products of the decomposition reaction of PAN and some of the oxygen that was adsorbed during the oxidation are also released as gases. which are arranged in an aromatic ring structure.

The strength and the modulus of these carbonized fibers increases rapidly up to 1400 ^° C. A further heating above 1400 ^° C. causes an increase in the modulus of elasticity, but the tensile strength is reduced, apparently because the structure of the carbonized fibers becomes more representative of pure graphite. As a result, conventional fibers are usually offered in a carbonized form with low elastic modulus and high strength, or in graphitized form with high elastic modulus but low tensile strength.

The presence of oxygen in the oxidized fiber, which was adsorbed during the oxidation, leaves the formation of the graphite-crystalline structure during the carbonization and graphitization progress. During carbonization and graphitization, oxygen and

Carbon-containing compounds are released during the creation of the micro-fiber structure associated with the high modulus graphite fibers. A large proportion of oxygen is released above 1400 ⁰ C carbonization. Direct high temperature graphitization of a 140 ° C carbon fiber results in oxygen elimination during the formation of the high modulus fiber structure. The overall effect is a breakdown of the fiber and the furnace structure. In view of the relatively high temperatures used, high energy inputs are required to obtain a graphitized fiber with a given modulus of elasticity.

Accordingly, the object of the invention is to provide an improved graphite fiber made from a PAN precursor to create and develop a method for making the improved graphite fiber.

Another object of the invention is to provide a graphitized PAN fiber with a high elastic modulus to accomplish.

Another specific object of the invention is to provide an improved method of making To create graphite fibers with a high modulus of elasticity, starting from acrylitic polymers that consist mainly of recurring acrylonitrile units.

Another object of the invention is to provide an improved method by which a high modulus of elasticity can be obtained at process temperatures

that are lower than the current generation a graphitized Paser temperatures used, with essentially the same modulus of elasticity the same starting material is obtained.

According to the invention, to solve the problem the fiber temperature over several graphitization stages repeated cyclically to remove the oxygen and purify the fiber, resulting in a fiber product with a higher nodule and the furnace a Maintains longer service life. The technique according to the invention with multiple graphitization stages leads to a cleaner one Atmosphere during the final high temperature graphitization. There is less erosion of the surface layers of the fibers in this cleaner atmosphere compared to the conventional one-step graphitization and this leads to the graphite fibers having a higher strength and a greater modulus of elasticity obtain.

Further refinements of the invention emerge in connection with the following description. The invention thus comprises methods with several stages and the relation and order of several such treatment stages in Associations with properties and relationships with elements that emerge from the description below.

An exemplary embodiment of the invention is described below with reference to the drawing. The only figure of the Drawing (Fig. 1) shows a schematic representation of the process sequence.

1, PAN fibers or threads in the form of a sheet, tape or rope 20 made up of many threads are drawn off from the fiber supply spool 22 by a "known constant-speed device J4 "and 31, and cooperating with a winder 32, serves to keep the multifilamentary sheet, rope or ribbon under sufficient uniform tension to stretch the PAN fibers to the extent desired for oxidation . The fiber tow 20 is passed under tension through an oxidation chamber, such as a gradient 35 "where the temperature is between 240 ⁰ C and 275 ° C. The dew is then drawn off through the oven 35 into an oxidizing atmosphere at a rate sufficient to obtain sufficient residence time in the oven and to ensure adequate oxidation, ie 10 to 11% by weight oxygen content is deposited in the fibers when they leave the oven.

The furnace 35 may comprise a plurality of heating zones in the typical case, preferably in the temperature range of about 200 ⁰ C to about 260 ⁰ C, but can vary from about ⁰ 180 G at the entrance to 300 ⁰ C at the outlet of the furnace. Of course, several separate ovens with one or more heating zones could also be used to produce a series of temperature levels. Likewise, an oven could be provided with a single heating stage maintained at a particular temperature, the oven arrangement depending on the final properties of the fiber product.

An oxidizing agent is supplied to the furnace 35 via a line 36 supplied, which oxygen and oxygen-containing gases, for example air, nitrogen oxides and sulfur dioxide, contains. According to the drawing, the oxidizing agent is only fed to the inlet of the furnace 35, however, this oxidizing agent can also be used at various points along the path of travel of the fibers be injected when they are oxidized.

By venting the furnace 35 through a line 38 can A pressure reduction and recirculation of the oxidation reaction and the thermal decomposition products of PIIT are achieved will.

When leaving the furnace 35, the oxidized rope 20 is passed through a carbonization furnace 40. During the carbonization, the oxidized PAN threads or fibers are heated in a non-oxidizing atmosphere, for example nitrogen, argon, helium or hydrogen, to temperatures ranging from approximately 300 ^° C. to approximately 900 ^° C., and this heating takes place in the carbonization furnace 40. The strength and modulus of the oxidized fibers increase rapidly during this stage as carbon dioxide, water, carbon monoxide, HCN, NH3 and other products are released and the aromatic ring structure of carbon is produced. Then the fiber is introduced into a second carbonization furnace 41, in order to complete the carbonization at a temperature of about 1400 ⁰ C.

The carbonized fibers are cooled and can be stored for further treatment or directly under one

inert gas at a temperature below 1400 ⁰ C are graphitized. According to the invention, the graphitization is carried out cyclically over not less than two or more than four heating cycles, in which the fiber preferably rapidly (for example between 2 to 5 minutes) from a comparatively lower temperature (for example room temperature or from another temperature which is suitably maintained , typically below the carbonization temperature) is heated to a graphitization temperature (for example to about 1800 ⁰ C to about 2800 ⁰ C). The fibers are then left at this graphitization temperature for a very short time (for example less than half a minute and preferably about 15 to 20 seconds) and then they are quickly cooled (for example within a few seconds) to the original low temperature. Conveniently, this "process can be carried out by moving the fibers at room temperature at about 30 to 60 cm / min through a narrow (for example 8 cm wide) zone and then back into the room temperature environment. The average residence time is thus in the hot zone at about 10 seconds and the temperature gradients from cold to hot and from hot to cold are extremely steep. Even better results are achieved if the maximum graphitization temperatures reached in each successive cycle are greater than the maximum temperature of the previous one Cycle so that the last cycle with the highest temperature is carried out.

The graphitization cycles are each within a graphitization furnace 4-2 in a controlled, not

oxidized atmosphere (for example argon, nitrogen, etc.) carried out, which can optionally also contain a graphitization catalyst such as a mixture of triethylene borane and hydrogen, a mixture of diborane and methane or the like. The catalyst apparently improves the increase in modulus by healing defects or holes that have remained in the original fiber structure during the treatment. It is important that the fiber be prevented from shrinking during graphitization by maintaining tension on the fiber as it is unwound from the supply spool. Shrinking the fiber would result in non-uniform graphite build-up and the maximum modulus possible could not be obtained at a given graphitization temperature.

The furnace 42 typically includes a plurality of spaced apart hot zones 44, 46 and 48, the are separated by zones maintained at room temperature, and each hot zone has control means, generally at 50 »52 and 54- are shown to be the temperature to adjust. Instead, the oven 42 may consist of several separate individual ovens, each with one is equipped with appropriate heat control. Means in the form of a winder 62 exercise a predetermined Pull on the fibers as they pass through the oven 42.

The cyclic graphitization allows the removal of oxygen and other volatile compounds before the final graphitization takes place. This is in

essentially a cleaning process with the aim of increasing the carbon content of the fibers from 95 to 100 %. Best results are obtained with a pure carbon fiber prior to the creation of the high modulus graphite structure, and a high strength, high modulus fiber and improved furnace life result from reduced erosion from oxygen contamination.

The following examples illustrate the invention and the advantages obtained thereby. These examples are presented for the purpose of illustration only, so that the invention is not limited to the specific conditions of the examples. In order to perform baseline measurements, a test with a Paser based on PACT, carried out originally carbonized at 1400 ⁰ C and then graphitized directly at 2800 C in an atmosphere of nitrogen, hydrogen or Triäthylboran was. The resulting properties were measured as follows: ^{391,522 x 10 6} Pa (56.8 10 ⁶ psi) for the modulus of elasticity and 172.25 kp / mm (24-5 χ 10 -4 psi) for the tensile strength.

Example I.

A PAN-based precursor carbon fiber (which had previously been heated to 1400 ° C.), which was identical to the carbon fibers used to obtain the basic measurements, was ^{graphitized for about 15 to 20 seconds at 2300 °} C. and immediately subjected to room temperature cooling . Then one followed

Graphitization to 2800 C and then a quick cool down to room temperature. At the two elevated temperatures, the furnace atmosphere contained nitrogen, Hydrogen and TriäthyIbοran. The measured properties for the treated fibers were as follows at the respective temperature:

1400 ⁰ C

Module 275 790 TO ⁶ Pa (40 χ 10 ⁶ psi)

Tensile strength 351.54 kp / mm ² (500 χ 10 ⁵ psi)

2500 ° C

Module 348 874 10 ⁶ Pa (50.6 χ 10 ⁶ psi)

Tensile strength 274.20 kp / mm ² (390 χ 10 ^ psi)

2800 ⁰ C

Module 479 185 10 ⁶ Pa (69-5 x 10 ⁶ psi)

Tensile strength 258.73 kp / mm ² (368 χ 10 ⁵ psi)

Of course, the elastic modulus obtained by treatment at the final temperature was markedly opposite the basic measurements improved.

Example II

Pre-treated carbon fibers based on PAN, the were identical to those used in Example I were graphitized in three cycles by using the

Fibers were brought from room temperature to the treatment temperatures and quickly back to room temperature, these cycles comprising heating to 1800 ^° C., 2300 ^° C. and 2800 ^° C., the heating lasting 15 to 20 seconds in each case. The furnace atmosphere at all three temperatures contained nitrogen, hydrogen and triethylborane. The properties at 1800 ⁰ G were not measured, but those at 2300 ⁰ C and 2800 ⁰ C, which resulted as follows:

2300 ° C

Module 407 480 - 10 ⁶ Pa (59-1 χ 10 ⁶ psi)

Tensile strength 284.74 kp / mra ² (405 x 10 ^ psi)

2800

Module 496 422 10 ⁶ Pa (72.0 χ 10 ⁶ psi)

Tensile strength 259.43 kp / mm ² (369 χ 10 ⁵ psi)

It is noteworthy that the additional first heating cycle to 1800 ^° C. made it possible to achieve a noticeable improvement in the modulus of elasticity compared with the products according to Example I.

Example III

The same type of PAN carbon fiber was graphitized cyclically, in each case at 1800 ⁰ C, 2300 ° C and 2800 ⁰ C, each for 15 to 20 seconds. The furnace atosphere

at 1800 ⁰ C consisted only of nitrogen, and at 2300 ⁰ C and 2800 0 the atmosphere contained nitrogen, hydrogen and triethylborane. The measured results for all three process temperatures were as follows:

1800 ⁰ C

Module 350 611 10 ⁵ Pa (4-3.6- χ 10 ⁶ psi)

Tensile strength 232.01 kp / mm ² (330 χ 10 ⁵ psi)

2300 ° C

Module 381 969 10 ⁶ Pa (.55.4- χ 10 ⁶ psi)

Tensile strength 209.51 kp / mm ² (298 χ 10 ^ psi)

2800 ⁰ C

Module 530 896 * 10 ⁶ Pa (77-0 χ 10 ⁶ psi)

Tensile strength 213.73 kp / mm ² (304- χ 10 ^ psi)

Since certain modifications can be made in the above method without departing from the scope of the invention, the embodiments discussed above should not be viewed as limiting.

Claims (1)

Patent claims: 1. Process for the production of graphite fibers from
prepared carbonized fibers based on polyacrylonitrile (PAN "), thereby ge indicates that the fibers in a non-oxidizing atmosphere between
Graphitization temperatures and relatively cooler temperatures are heated and cooled for at least two cycles. Method according to claim 1, characterized in that the cyclical heating and the cyclical cooling over not
more than four cycles are performed. Process according to Claim 1, characterized in that the relatively cooler temperature is below the carbonization temperature at which the carbonized fibers are produced
became. BATH ORIGINAL 4-. Process according to Claim 3 »characterized in that the carbonization temperature is ^{approximately 1400 ° C.} 5. Method according to claim 1, characterized that the fibers stay on for less than half a minute during each cycle the graphitization temperature can be left. 6. The method according to claim 5 »characterized in that the fibers on the Graphitization temperature can be held for about 15 to 20 seconds during each cycle. 7- method according to claim 5? known thereby show that the temperature of the Fibers between the relatively cooler temperatures and the graphitization temperature in just a few Seconds is changed. 8. The method according to claim 1, characterized that the graphitization temperature in each subsequent cycle is higher than the graphitization temperature of the previous one Cycle. 9. The method according to claim 8, characterized in that the graphitization temperature of at least the last cycle is approximately 2800 ^° C. 10. The method according to claim 1, characterized in that the fiber during the graphitization is placed under tensile stress to such an extent that shrinkage of the fibers is prevented will. 11. Apparatus for carrying out the method according to any one of claims 1 to 10, characterized in that a heater is provided which heats up the fiber under tension at temperatures of about 1400 ⁰ C, in order to achieve carbonization of the fiber, and in that means are provided are to cyclically heat and rapidly cool the carbonized fibers between the graphitization temperature and a temperature below about 1400 ⁰ C for at least two cycles under tension.