US3412062A - Production of carbon fibres and compositions containing said fibres - Google Patents

Description

United States Patent 3,412,062 PRODUCTION OF CARBON FIBRES AND COM- POSITIONS CONTAINING SAID FIBRES William Johnson, Leslie Nathan Phillips, and William Watt, Farnborough, England, assignors to National Research Development Corporation, London, England, a British company No Drawing. Filed Apr. 19, 1965, Ser. No. 449,320 Claims priority, application Great Britain, Apr. 24, 1964, 17,128/ 64 17 Claims. (Cl. 260-37) ABSTRACT OF THE DISCLOSURE According to the invention carbon fibres having high tensile strength and high Youngs modulus are made by the conversion of organic polymer fibre, such as polyacrylonitrile, by the combined effect of heating in a nonoxidising atmosphere to a carbonising temperature of up to about at least 1000 C. and the application of longitudinal tension at some stage of the conversion. The process preferably includes a preliminary step of heating the polymer fibre in an oxidising atmosphere at from 200- 250 C. for sufficient time to achieve complete permeation of oxygen throughout the fibre whilst held under tension such that during oxidation there is little or no longitudinal shrinkage of the fibre. The oxidised fibre may subsequently be carbonised and heat treated without tension to give carbon fibre in some cases having an ultimate tensile strength of 260 lb. per square inch of a Youngs modulus of 60 10 lb. per square inch.

This invention relates to the production of carbon fibres.

Generally speaking, carbon fibres are members of a class of nonmetallic fibres the use of which has been proposed as a high strength and/ or a stiffening element of composite materials of which the matrix material may or may not be metal. It may be shown that such fibres, particularly carbon fibres, usually have their highest specific strength when their diameter is smallest. Now the production of non-metallic fibres, particularly the very small diameter very high strength fibres of which carbon is an example, presents numerous problems. One problem is that of devising means and a process for manufacturing and harvesting fibres of sufficient length that in the composite material, they will bear and enable the composite material to withstand, loads comparable with their individual strength.

In connection with carbon fibres, it has previously been proposed that these should be manufactured by the carbonising of organic materials, such as, for example, cotton-wool and fibres and filaments of synthetic resinous materials. Among the latter materials in particular, polyacrylonitrile fibres have been the proposed subject of experiment but it would appear that only limited success has been achieved.

It is to be noted that the term polyacrylonitrile fibres is used by those skilled in this art to include co-polymers or ter-polymers of acrylonitrile with other monomers e.g. methyl methacrylate or vinyl acetate, either alone or to which have been added polymers compatible with them for example phenolic resins or Friedel-Crafts condensates. It is in this sense that the term polyacrylonitrile fibres is used throughout the specification.

According to the present invention a process of producing carbon fibres comprises heating fibres of polyacrylonitrile while held under tension to a relatively high carbonising temperature under non-oxidising conditions.

Also in accordance with the invention the process comprises a preliminary low temperature oxidising step which, for example, is performed by heating in air.

The high temperature carbonising is performed under vacuum or in a non-oxidising atmosphere such as hydrogen.

In cases where the preliminary low temperature oxidising step forms part of the process, if this step is of too short duration the fibres are left with a soft core and upon subsequent high temperature heat treatment holes are formed in the resulting fibres.

In a further embodiment according to the present invention a process of producing carbon fibres comprises initially heating fibres of polyacrylonitrile whilst held under tension in an oxidising atmosphere at from 200- 250 C. for sufiicient 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 1000 C. under non-oxidising conditions.

The duration of the initial heating required Will depend to a large extent on the diameter of the fibres concerned but for a temperature of 220 C. complete oxygen permeation of the fibres takes place after heating for about 24 hours for 2 /2 denier fibres and after about 50 hours for 4 /2 denier fibres.

The fibres are tensioned so that longitudinal shrinkage which normally takes place during this initial heating is reduced, eliminated or is such as to cause the fibres to elongate.

Further improvements in the characteristics of the fibres produced are achieved if, subsequent to carbonising to about 1000 C. the fibres are further heat treated to above 2000 C. in a non-oxidizing atmosphere.

The tensioning of fibres may also be maintained during the subsequent carbonising and/ or heat treatment.

Several examples of the invention will now be described for the purpose of illustration only.

In each example the raw material was in the form of a multi-filament polyacrylonitrile yarn of which the individual filaments were continuous.

Example 1 Raw material in the form of a 4% denier bundle of about 500 polyacrylonitrile fibres each about 27 microns diameter and about 5 inches long was suspended in a vertical furnace tube with Weights totalling 10 grams attached so as to apply tension to the fibres. The furnace was evacuated and evacuation pumping was continuously maintained thereafter during heat treatment. The heat treatment consisted of raising the temperature of the furnace to l,000 C. at a rate of 15 C. per hour. After this treatment the carbonised fibres had the following properties:

Carbonised density gram/cc 1.6 Final carbon filament diameter microns 11-13 Tensile strength of indivdual fibres p.s.i-- x10 Youngs modulus do 16.5 x 10 Failing strain percent 0.7

As a further step the carbon fibres were heated up to a temperature of 2500 C. for 1 hour under argon gas at one atmosphere pressure. After this treatment, during which the fibres shrank in the diametric direction but not in the direction of the fibre axis, the density had changed from 1.70 gm./-cm. to 1.90 g./cm. The Youngs modulus of the fibres as a result of this treatment changed from 16.5 10 p.s.i. to an average value of 35 10 p.s.i. The average tensile strength after the high temperature treatment was 200x10 p.s.i.

3 Example 2 In this example material as in Example 1 was used and the heat treatment procedure was similar, but instead of carbonising the filaments under vacuum an atmosphere of hydrogen was maintained in the furnace at a pressure of cm./Hg. The properties of the carbonised filaments after treatment were as follows:

Density of the carbonised fibres grams/cc 1.6 Final carbon filament diameter microns 11-14 Tensile strength p.s.i- 135 Youngs modulus p.s.i l2.9 10 Failing strain percent 1 In the examples above described it has been found that The room temperature curing step led to the production of a product similar in properties to that resulting from Example 4.

. Example 6 A bundle of carbon fibres produced as in Example 1 were impregnated with a solution of a Friedel-Craft type resin, in particular a diphenyl oxide based resin made by reacting the constituents in the following proportions:

1 mol diphenyl oxide 1.2 mols para-dichloroxylylene in a dichloroethane solution suificient to give 40% solid content in the mixed resin. A Friedel-Crafts type resin is formed from an aromatic compound with an aromatic the individual filaments shrink whilst their original circulinking agent which has two chloromethyl or methoxylar cross-section is maintained. The data given indicates m thyl groups attached to an aromatic nucleus by means that the diameter is halved approximately. of a polycondensation reaction involving the nuclear hy- Examples 3, 4, 5 and 6 which follow describe the use drogen atoms and y be aided y the Presence of a of carbon fibres produced according to Examples 1 and 2 Small amount of Fl'ledel'craft YP catalyst Such 38 Stanas Strengthening elements i composite t i l nic chloride. After evaporation of the dichloroethane, the impregnated fibrous mass was shaped and heat cured at EXtimPle 3 180 C. for 2 hours under a pressure of 500 p.s.i. to form The following constituents were compounded in a high Strength fibre relnfofced hard Shit comPQSlte rubber mill:

Grams 20 Among the advantages oifered by the process of the Fluoroelastomer c0 po1ymer of vinylidine fl id invention are the superior strength of the carbonlsed and hexafluoro propylene (Viton B) 100 filaments compared w1th those produced from regener- Magnesium Oxide 5 ated cellulose fibres, the latter in general having a tensile Dicinnamylidene hexamethylene diamine (curing strength of about 50x10 p.s.i., also that the fibres have agent) 3 a particularly smooth surface making them more suitable Carbon fibres Produced as in Example 1 20 than those made from regenerated cellulose as reinforcement in composite materials. The fibres were fed into the charge at the nip of the rolls where they broke into short lengths about A; of an inch Example 7 10118 and thereafter became uniformly distrlbuted g 2 /2 denier polyacrylonitrile fibres heated to achieve 01111 the charge y the pb g- The Pompouhded complete permeation of oxygen throughout the fibres for Charge was Shaped y mouldlng and Cured 1n the mould 24 hours at 220 C. in air, then carbonised in a nony healing at for 1 ur a further heat treated oxidising atmosphere to 1000 C. and heat treated to a fOr 24 hOUTS- The end Product e a dense 2500 C. resulted in fibres having a tensile strength of composite rubber of great toughness and having pood 250x10 p.s.i., whereas when the initial heating at 220 strength retaining properties at elevated temperatures. was only applied for 2 hours the resulting fibre strength E 1 4 was only 100x10 p.s.i.

Xampe When treated in the above manner the fibres shrink A bundle of unbroken carbon fibres about 4 inches during the initial heating to 220 C. in air by as much as long produced according to Example 2 was tightly packed 40% and whilst the carbon fibre obtained by subsequent in a test tube of /8" internal diameter and a cold setting carbonising and heat treating to 2,500 C. has a tensile catalysed polyester resin poured over them. The resin strength which may be satisfactory for certain circumconsisted of polyester resin mixed with methyl ethyl stance the Youngs modulus is relatively low. If tension ketone peroxide and a 6% solution of cobalt naphthenate is applied to the fibres during the initial heat treatment in white spirit in proportions thus: 100 parts of the resin to 220 C. both the tensile strength and Youngs modulus to 3 parts by weight of each of the other two constituents. can be increased as is shown by the following table:

Properties of the fibre Properties of the fibre Length after carbonising to after heat treating to Load applied toayarn 01100 change 1,000 C. in an inert; 2,500 C. in an inert filaments of 2% denier durin atmosphere atmosphere polyacrylonitrile fibres, 220 C. for 24 hours at 220 C., treat- Young's Young's grains ment, Tensile modulus Tensile modulus percent strength, axially strength, axially p.s.i. of fibre, p.s.i. of fibre,

p.s.i. p.s.1.

-40 100x10 13x10 230x10 30x10 -12 100 10 16x10 100x10 38X10G +2 120x10 20 10 120x10 47 10 +15 200 10 21 1o 200x10 53x10 +36 200x10 21x10 200x10 x10 The fibres were evenly wetted by the resin mixture and Example 8 constituted a longitudinal reinforcement in the high strength rigid composite resin/ fibre formed by the curing and setting of the resin at room temperature.

Example 5 Parts by weight 60 Epoxy resin Polyamide hardener (Versamid 12S) 40 drogen at 15 cms. pressure.

The properties of some of the fibres were then measured and the remainder heated to 2500 C. in a carbon tube furnace under one atmosphere pressure of argon.

A further selection of the resulting fibres were tested and the remainder heated to 2900 C. under the same 1 atmosphere of argon conditions. The properties of the fibres resulting from this final treatment were then meas ured.

The results were as follows:

We claim:

1. A method of making carbon fibers having a Youngs modulus parallel to the fiber axis of not less than 16 10 pounds per square inch comprising the steps of oxidizing an organic polymer fiber by simultaneously heating the fiber in an oxidizing atmosphere at a temperature of from about 200 C. to 250 C. for a time sufficient to permit substantially complete permeation of oxygen throughout the core of the fiber while the fiber is held under longitudinal tension, said tension being sufficient at least to limit shrinkage of the fibers during heating to not more than about 12% of the length of the fiber, and carbonizing the fiber by heating the oxidized fiber in a non-oxidizing atmosphere to a temperature of up to about at least 1000 C.

2. A method according to claim 1 wherein said organic polymer is polyacrylonitrile.

3. A method according to claim 2 wherein longitudinal tension is applied to said fiber during said oxidizing step such that the change in length of the fiber during oxidizing is within the range of 12% shrinkage to 36% extension based on the length of the fiber before oxidizing.

4. A method according to claim 2 wherein said fiber is stretched during said oxidizing step up to about 36% based on the length of the fiber before oxidizing.

5. A method according to claim 2 including a further heat treatment comprising heating the carbonized fiber in a non-oxidizing atmosphere at a temperature above said carbonizing temperature and up to about 3000" C.

6. A method according to claim 2 wherein said fiber is heated during said oxidizing step for at least 24 hours.

7. A method according to claim 1 wherein said oxidized fiber is held under tension during said carbonizing step.

8. A method according to claim 5 wherein said carbonized fiber is held under tension during said further heat treatment.

9. Carbon fiber made by the process of claim 1 and having a Youngs modulus parallel to the fiber axis of not less than 16 10 pounds per square inch.

10. Carbon fiber made by the process of claim 5 having a Youngs modulus parallel to the fiber axis of not less than 38x10 pounds per square inch.

11. A composite material comprising a matrix containing a plurality of carbon fibers as claimed in claim 9 distributed therein.

12. A composite material according to claim 11 wherein said matrix comprises rubber.

13. A composite material according to claim 12 wherein said rubber comprises fluoroelastomer.

14. A composite material according to claim 11 wherein said matrix comprises synthetic resin.

15. A composite material according to claim 14 wherein said resin comprises polyester.

16. A composite material according to claim 14 wherein said resin comprises an epoxy resin.

17. A composite material comprising a matrix containing a plurality of carbon fibers as claimed in claim 10 distributed therein.

References Cited UNITED STATES PATENTS 2,796,331 1/1957 Kaufiman et al. 23209.1 2,799,915 7/1957 Barnett et al. 23-2091 3,285,696 11/1966 Tsunoda 23--209.1

ALLAN LIEBERMAN, Primary Examiner.