US3532466A - Production of carbon fibres - Google Patents

Description

United States Patent 3,532,466 PRODUCTION OF CARBON FIBRES William Johnson, Farnborough, Roger Moreton, Crookham, near Aldershot, and William Watt, Farnborough, England, assignors to National Research Development Corporation, London, England No Drawing. Filed Nov. 14, 1966, Ser. No. 593,673 Claims priority, application Great Britain, Nov. 16, 1965, 48,566/ 65 Int. Cl. (301]) 31/07 U.S. Cl. 23-2094 4 Claims ABSTRACT OF THE DISCLOSURE Carbon fibers having high tensile strength and high tensile strain characteristics are made by a process which comprises first submitting an organic polymer fiber to a temperature below its carbonizing temperature in an oxidizing atmosphere until the fiber is permeated with oxygen, next carbonizing at a carbonizing temperature in a non-oxidizing atmosphere, combined heat and tension being applied to the fiber during at least one of these two steps, and then heating the carbon fiber to a still higher temperature in an inert atmosphere to increase the ultimate tensile strength and Youngs modulus, the final temperature being limited to a value at which the ultimate tensile strain is in the neighborhood of a peak value. The polymer fiber is preferably polyacrylonitrile and the final temperature in an inert atmosphere is preferably in the range of from 1300" C.l800 C.

This invention relates to high-strength carbon fibres. Processes for producing such fibres by the conversion of a polymer fibre are set forth in the co-pending patent application No. 449,320 filed Apr. 19, 1965, now U.S. Pat. No. 3,412,062.

In general, such processes have been concerned with the production of carbon fibre having both a high ultimate tensile strength and a high value of Youngs modulus (E).

In some fields of work it would be desirable to have carbon fibres which exhibit both high ultimate tensile strength and high ultimate tensile strain characteristics. That is to say, carbon fibres which would be capable of stretching and yet possessing a useful tensile strengthcarbon fibres which could stretch considerably before breakingwould be useful. In processes yielding strong carbon fibres it is possible to increase Youngs modulus at given values of ultimate tensile stress, with a corresponding decrease in the breaking strain. It has, however, been found possible, by continued treatment of the fibres up to a higher value of the ultimate tensile strength, to reach a point of peak value of ultimate strain; up to this point the rate of increase of ultimate tensile strength with heat-treatment-temperature is greater than that of Youngs modulus, but beyond this point the rate of increase of Youngs modulus with heat treatment temperature is greater than that of the ultimate tensile strength, which may indeed decrease. The present invention accordingly consists in bringing the process to an end in the neighbourhood of such a peak value of ultimate tensile strain so as to give high values of both this and the ultimate tensile stress.

Post-treatment of high-strength carbon fibre to temperatures higher than the carbonising temperature, in an inert atmosphere such as an argon atmosphere, will progressively increase Youngs modulus but will increase the ultimate tensile strength more rapidly up to a point beyond which the ultimate tensile strength increases less rapidly than Youngs modulus, or may indeed decrease; at this point the ultimate tensile strain is at a peak value.

3,532,466 Patented Oct. 6, 1970 It is a further feature of the invention to apply this posttreatment up to the neighbourhood of said peak point.

The process disclosed in said co-pending patent application No. 449,320, filed Apr. 19, 1965, now U.S. Pat. No. 3,412,062 is characterised by the step of applying heat and tension together during the conversion of a fibre of an organic polymer such as polyacrylonitrile to carbon fibre. The conversion process comprises heating the polymer fibre to a carbonising temperature in a nonoxidising atmosphere such as hydrogen, and may also optionally comprise a preliminary oxidising treatment such as for example heating in air or preferably in an oxygen-rich atmosphere. The preliminary treatment, if used, should be long enough to ensure complete oxygenpermeation of individual fibres so that they are not left with soft cores which would have a deleterious effect on the ultimately resulting carbon fibres. The heat and tension may be applied to give a combined effect during the carbonising or during the preliminary step, if any, or during both.

In a particular embodiment disclosed in said patent application, a process of producing carbon fibres comprises initially heating fibres of polyacrylonitrile whilst held under tension in an oxidising atmosphere at from ZOO-250 C. for sufficient time to permit substantially complete permeation of oxygen throughout the individual fibres and subsequent further heating of the fibres so formed to a carbonising temperature of at least l,000 C. under non-oxidising conditions.

The applied tension is such that longitudinal shrinkage which normally would take place during this preliminary oxidising step is reduced or eliminated, or is such as to cause the fibres to elongate. The preliminary oxidising step may be carried out by heating the fibres in an oxygen-rich atmosphere or in commercially pure oxygen, the fibres being spaced from each other to allow a free flow of oxygen between them.

This process can yield a high-strength carbon fibre. There is also disclosed a further post treatment of the carbon fibre in an inert atmosphere of, for example, argon at 2500 C. which increases Youngs modulus without increasing the ultimate tensile strength.

In a particular process according to the present invention for producing carbon fibres of maximum strength and maximum breaking strain the post treatment consists in heating the high-strength carbon fibre in an inert atmosphere such as argon for at least one hour at from 1300* to 1800" C.

Prior to the conversion process disclosed above, the fibres may be stretched to a greater extent than is normal in the production of polyacrylonitrile fibres for textile purposes.

In one example according to the invention polyacrylonitrile fibres of 20 micron diameter were heated in air at 220 C. for 44 hours whilst held under tension. The fibres were then carbonised by heating to a temperature of 1,000 C. in an atmosphere of hydrogen. The resulting fibres were then heated in accordance with the following table and the results were as indicated:

Youngs Breaking modulus (E) strain parallel percent to the equals Temp. of heat treatmen U.T.S., fibre axis, U.T.S./ for 1 hour, C. p.s.i. ps 1 E(mod.)

In another example in accordance with the invention carbon fibres of average dia. 7 microns were made from Young's modulus (E) parallel to the fibre axis, p.s.i.

26. 3X10 30. 8X10 31. 4X10 0. 33. 1X10 1. 1. 1 0

Breaking strain Temp. of heat treatment for 1 hour, 0. Edno'df It will be seen from the tables that within the range of heat treatment temperature of from 13001800 C. the ultimate tensile strength and the breaking strain both reach maximum levels.

Thus carbon fibres can be produced which have the desirable characteristics of having both high tensile strength and high breaking strain.

We claim:

1. In a process of producing carbon fibers having high ultimate tensile strength and high Youngs modulus wherein an organic polymer fiber is heated to a temperature below its carbonizing temperature in an oxidizing atmosphere until the fiber is permeated with oxygen and the fiber is subsequently carbonized at a carbonizing temperature in a non-oxidizing atmosphere, tension being applied to said fiber during at least one of these two steps, and wherein the carbonized fiber is heated further at a temperature higher than the carbonizing temperature in an inert atmosphere to increase the ultimate tensile strength and Youngs modulus of the carbonized Al fiber, the improvement which comprises conducting said further heating of the carbonized fiber for at least one hour at a maximum temperature of from 1300 C. to 1800 C. at which temperature the tensile strain of the carbonized fiber is increased to an approximately maximum value.

2. A process according to claim 1 in Which the organic polymer fiber is polyacrylonitrile.

3. A process of producing carbon fiber according to claim 1 in which the tension is applied to the fiber while it is heated in an oxidizing atmosphere at a temperature of from 200 C.250 C. for a time sufficient to permit substantially complete permeation of oxygen throughout the fiber.

4. A process of producing carbon fiber according to claim 2 in which the tension is applied to the fiber while it is heated in an oxidizing atmosphere at a temperature of from 200 C.-250 C. for a time sufficient to permit substantially complete permeation of oxygen throughout the fiber.

References Cited UNITED STATES PATENTS 2,799,915 7/1957 Barnett et a1. 81l5.5 X 2,913,802 11/1959 Barnett 8115.5 X 3,285,696 11/1966 Tsunoda 23209.1 3,313,597 4/1967 Cranch et al. 23-209.3 3,412,062 11/1968 Johnson et al. 260-37 OTHER REFERENCES Shindo Osaka Kogyo Gijutsu Shikensho Hokoko, No. 317 (1961), Govt. Ind. Res. Inst. Osaka, Japan, pp. 1, 7, 1827, and 4049.

EDWARD J. MEROS, Primary Examiner US. Cl. X.R. 23-209.l, 209.2