US3476703A - Treatment of carbon fibres and composite materials including such fibres - Google Patents

Description

United States Patent US. Cl. 260-37 6 Claims ABSTRACT OF THE DISCLOSURE The invention comprises a process of treating the surface of carbon fibres by heating the fibre in an oxidising atmosphere at not more than 1000 C. for sufiicient time to produce a weight loss of between 0.05 and preferably not more than 6.0 percent from the fibres, the process enabling a pitted fibre surface to be obtained enabling a good bond to be obtained between such treated fibres and a supporting matrix in a composite material.

This invention relates to carbon fibres of the type'suitable for use as a reinforcement in a composite material.

An example of the type of carbon fibre to which the invention relates are those disclosed in co-pending patent application Ser. No. 449,320, now U.S. Patent Ser. No. 3,412,062, issued Nov. 19, 1968.

As disclosed in this co-pending application it has been proposed to use carbon fibres disclosed therein as a reinforcement in a composite material which comprises a reinforcement of carbon fibres in a matrix of material such as an epoxy, polyester or Friedel-Crafts type resin. By Friedel-Crafts type resin is meant a resin formed from an aromatic compound with an-aromatic linking agent which has two groups, such as methoxymethyl or chlorornethyl, attached to an aromatic nucleus, by means of a polycondensation reaction involving the nuclear hydrogen atoms. The resins are described in more detail in the literature such as the Transactions and Journal of the Plastics Institute (London) Volume 32, N0. 101, pages 298-302 (-1964). Carbon fibers produced according to US. Patent No. 3,412,062, are of the high strength, high modulus type having. an ultimate tensile strength of at least 100x10 pounds per square inch and a Youngs modulus of at least 16x 10 pounds per square inch parallel to the fiber axis.'

The strength of such a composite material, and in particular the shear strength parallel to the fibres in cases where there is selective orientation of the fibres, is dependent to some extent on the nature of the bond achieved between the reinforcing fibres and the matrix.

It has been suggested that the strength of the bond between the fibres and the matrix may be improved by treatment of the fibres which would modify the surface by a process such as oxidation. However, some surface treatments would be likely to reduce the strength of the fibres themselves to such an extent that there was little or no overall gain in strength of the composite material.

We have, however, discovered a controlled surface treatment of the carbon fibres which has little, if any, detrimental effect on their strength but which nevertheless enables composite material of considerably enhanced shear strength to be produced.

Accordingly the present invention is concerned with the provision of a process of treating carbon fibre to obtain fibre surface characteristics which enable an improved bond to be obtained between such treated fibres and a supporting matrix in a composite material, and, with composite material incorporating such treated fibre as a reinforcement.

A process of treating carbon fibres according to the present invention comprises heating carbon fibres in an oxidising atmosphere at. such a temperature and fora limted period of time sufiicient to produce a loss of weight by the fibres of at 1east'0 05 percent and preferably not more than 6.0 percent from the fibres.

The heat treatment may most conveniently take place in air in which case the temperature at which heating takes place with not exceed 1000 C. and will more normally be within the range of from 350-850 C.

The individual fibres must be suificiently separated during the heat treatment process and the temperature, period of heating and gas flow over the fibres must be much that all the fibres are effectively subjected to the surface oxidation treatment.

An oxygen rich or pure oxygen atmosphere, or, an atmosphere containing an oxide of nitrogen may be used.

forcement of carbon fibre which has been treated by -a process disclosed above.

Several examples will now be given:

EXAMPLE 1 Carbon fibres from a batch with a mean strength of 250x10 lb. per square inch of a type produced in accordance with copending application Ser. No. 449,320 and which had been heat treated at from 2500-2600 C. were heated for 1 /2 hours at 550 C. and had a weight loss of 0.9 percent. 20 fibres were tested separately for tensile strength after this oxidation and had a mean strength of 253 X 10 lb. per square inch.

EXAMPLE 2 Using carbon fibres of the same type disclosed in and as treated as in Example 1, but using an epoxy resin in place of the polyester resoin as a matrix, it was found that the shear strength of the composite parallel to the fibre axes using the treated carbon fibre reinforcement was 9500-10500 pounds force per square inch as compared to 2500-3000 pounds force persquare inch using untreated carbon fibres.

EXAMPLE '4 68.8 g. and 89.75 g. of carbon fibres of the type disclosed in Example 1 and which had been heat treated at 25002600 C. of average diameter 7.4 microns and of length 14 inches were placed, respectively, into two openended ceramic tubes 14 inches long by 3 inches diameter.

The tubes were placed on refractory bricks in a furnace so that the tubes were in the centre region of the furnace.

The temperature in the furnace was raised to 600 C. and kept at this temperature for one hour and at the same time a flow of oxygen of 4 litres per minute was passed through the furnace.

After cooling down the two bundles of fibres were weighed.

Weight loss of fibres from the first tube was 2.44 percent.

Weight loss of fibres from the second tube was 2.54 percent.

Percent weight Shear strength,

loss of fibres sq. in. X10

EXAMPLE 6 This was in all ways similar to that of Example 5 except that the samples were from a different batch of similar fibres. The results were as follows:

Percent weight Shear strength, loss of fibres sq. in. X

Time of heating in air at 600 C.:

It is to be noted that at the values of shear strength given the composite specimens failed in different manner than interlaminar shear indicating that the ultimate shear strength was greater than the figures quoted.

EXAMPLE 7 In this case separate 7 inch long samples of carbon fibres produced in accordance with the disclosure of copending application Ser. No. 449,320 and which had been heat treated for 1 hour at 1500 C. in an inert atmosphere to produce carbon fibres of 8 microns diameter having high strength and high strain were heated in air at the temperatures shown in the following table and with the resulting percentage weight loss and interlaminar shear strength when incorporated in a matrix:

Sheer Strength, lb./sq. in

Percent Weight loss after oxidation Temperature, C.

Time of heating in air:

It is to be noted with particular reference to Examples 5, 6 and 7 above that the effect of the oxidation treatment even for a relatively short period, in one case only 30 minutes, is to almost double the interlaminar shear strength.

EXAMPLE 8 Carbon fibres of the type disclosed in Example 1 and which had been heat treated at from 2500-2600" C. were heated in air at 500 C. for 400 minutes, The treated fibres were found to have a weight loss of 1.22 percent and when incorporated in a resin matrix were found to have a shear strength greater than 8.3 x 10 lb. per square inch.

Electron microscopic examination of examples of oxidised carbon fibres show a pitted surface and it is believed that the higher interlaminar shear strength obtained results from the pitted surface providing a good key for the matrix.

The pits range in size from 300 to 400 angstroms at low oxidation weight loss up to nearly 1000 angstroms when the pits coalesce into grooves running parallel to the longitudinal fibre axis.

In Examples 2, 3, 5, 6, 7 and 8 above the ratio of carbon fibre to matrix material in all cases was approximately 62 percent by weight.

We claim:

1. A process of treating carbon fiber having an ultimate tensile strength of at least 10 pounds per square inch and a Youngs modulus parallel to the fibre axis of at least 16X 10 pounds per square inch to improve the bonding characteristics of said fibre to a resin matrix comprising heating the fibre in an oxidizing atmosphere at a temperature of not more than 1000 C. for a time sufficient to provide a pitted surface on said fibres and to produce a weight loss of from 0.05 to 6 percent by weight based on the weight of the carbon fibre.

2. A process of treating carbon fibres as claimed in claim 1 wherein carbon fibres are heated in air at a temperature of at least 350 C.

3. A process according to claim 1 wherein the carbon fibres are heated in oxygen for about 1 hour at a temperature within the range of from 450600 C.

4. Carbon fibre having improved bonding characteristics to a resin matrix comprising carbon fibre having an ultimate tensile strength of at least l00 10 pounds per square inch and a Youngs modulus parallel to the fibre axis of at least 16X 10 pounds per square inch, the surface of said fibre being pitted with a plurality of microscopically visible pits produced in said surface representing a weight loss of said fibre of from about 0.05 to 6 percent by weight based on the weight of the carbon fibre.

5.. A composite material comprising a cured resin matrix containing a plurality of carbon fibres as claimed in claim 4 disposed therein.

6; A composite material as claimed in claim 5 wherein the resin matrix comprises a resin selected from the group consisting of a polyester, epoxy and Friedel-Crafts type resin.

.References Cited UNITED STATES PATENTS 2,796,331 6/1957 Kauffman at al. 2,799,915 7/1957 Barnett et al.

3,053,775 9/1962 Abbott 252-421 OTHER REFERENCES Schmidt et al., Chemical Engineering Progress, vol. 58, No. 10, October 1962, pp. 4050.

Schmidt et al., Filamentous Carbon and Graphite, Technical Report AFML-TR-65-l60, August 1965, pp. 11-13.

ALLAN LIEBERMAN, Primary Examiner U.S. Cl. X.R. 23-2091; 106-307; 26040