Treatment Of Carbon Fibers

Abstract

A method of treating high modulus, high strength carbon fiber to improve its bonding characteristics in a resin matrix comprising immersing the fiber in concentrated nitric acid for 4- 8 hours at refluxing temperature.

Description

BACKGROUND OF THE INVENTION

This invention relates to carbon fibers of the type suitable for use as a reinforcement in a composite material and more particularly relates to a method of treating carbon fiber to improve its bond with the supporting matrix in a composite material.

High modulus, high strength carbon and graphite yarns, having an ultimate tensile strength of at least 100 .times. 10.sup.3 psi and a Young's modulus of at least 16 .times. 10.sup.6 psi, because of certain unfavorable surface characteristics, do not bond strongly to resins such as those suitable for use in applications of the aerospace industry, e.g. the epoxy or polyamide resins. The bond of such fibers to the resins has characteristically been poor typically yielding graphite fiber-resin matrix composites with low shear strengths; generally in the range of 3,500-4,500 psi for low fiber content composites (20-45 vol %) and below 3,500 psi for high fiber content composites (45-65 vol %).

While it has been suggested to surface treat carbon fibers in order to improve their shear strength by various methods, such as for example, by oxidation, the resulting degradation of other fiber properties, especially tensile strength, has been a problem.

SUMMARY OF THE INVENTION

The present invention relates to the treatment of carbon fiber in a process wherein the surface of the fiber is activated with no significant deterioration of the desirable filament properties. As used herein, the term carbon fiber relates to flexible carbon or graphite filamentary material available in any elongated textile form such as yarns, braids, felts, etc. or in monofilament form.

In accordance with this invention carbon fiber is exposed to boiling nitric acid by immersion therein for an extended period of 4-8 hours. Carbon-resin composites fabricated with carbon fibers treated according to the present invention exhibited a high resistance to shear failure while maintaining high flexural strengths.

BRIEF DESCRIPTION OF THE DRAWINGS

An understanding of the invention will become more apparent to those skilled in the art by reference to the following detailed description when viewed in light of the accompanying drawings, wherein:

FIG. 1 is a graph illustrating the effect of contact time on composite short beam shear strength;

FIG. 2 is a graph illustrating the relation between fiber volume and composite short beam shear strength;

FIG. 3 is a graph illustrating the relation between fiber volume and composite flexural strength; and

FIG. 4 is a graph showing the correlation between specific surface area and short beam shear and transverse tensile strength.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The technique of treating carbon fibers according to the present invention comprises immersing carbon fibers in boiling, concentrated nitric acid for a period of time sufficient to activate the fiber surface with little or no concomitant degradation in fiber properties.

The process was performed on a batch basis by winding the yarn onto a spool and lowering it into an acid filled resin kettle which was provided with a reflux condenser and which served as the reaction vessel. The spool was comprised of two series of parallel rods concentrically arranged at two selected radial distances from a central rod, all rods being coated with Teflon and supported between two Teflon coated hubs. The carbon yarn was wound about each series of parallel rods so that there existed a space between the yarn layers. After the nitric acid treatment, the yarn was rinse cycled by rinsing in distilled water for three 15 minute periods, rinsing in a dilute solution of ammonium hydroxide for 10 minutes, rinsing in distilled water and then rinsing in acetone for two 15 minute periods. After the rinse-cycling, the yarn was air dried in a hood and finally dried in an air-circulating oven at 100.degree. C for 15 minutes, impregnated with resin by passage therethrough and wound onto a drum in tape form and processed into a composite.

During experimentation, commercially available Thornel 50 yarn, Hitco HMG-50 and Morganite I yarns were utilized with conditions and resulting properties as shown in Tables I and II. ##SPC1## ##SPC2##

Further tests, wherein the tensile strengths of treated and untreated yarns were made are shown in Table III. ##SPC3##

In FIG. 1, the effect of various contact times of carbon fiber with 70% HNO.sub.3 at 120.degree. C on short beam shear strength of a composite having a 2,256-0820 epoxy resin matrix is shown. It can be seen, as a result of testing, that in order to achieve significant shear strength improvement, a minimum of 4 hours contact time is necessary. Further it can be seen that contact times of greater than 8 hours while not detrimental, produce no significant increase in shear strength.

FIGS. 2 and 3 show the comparison between untreated fibers and those treated according to the present invention with respect to the effects of fiber volume on shear and flexural strength. In each case, the treated fibers display a significant increase in strength regardless of volume fraction.

It is believed that the great improvement of bonding characteristics of the carbon yarns treated according to the present nitric acid treatment process are due primarily to the increase in specific surface area and in the increase of surface reactivity caused by the treatment. Increasing the exposure of the fiber causes a steady increase in the surface area with a very gradual increase in shear and transverse tensile strength. The results in Table IV below, for example, indicate that there exists a definite correlation between the surface properties (specific surface area and the concentration of acid sites per unit area of fiber based on sodium hydroxide adsorption) and the observed increases in shear strength. In the treatment, it has been found that the number of acid sites per unit area increases initially and then levels off and diminishes. In the particular experiment illustrated by Table IV, it can be seen that while the concentration of acid sites/unit area falls off as the surface area increases to 24 m.sup.2 /g, it is still greater than the untreated fiber. It thus appears that both the increase in surface area and in surface reactivity contribute to the improvement in shear strength of the composite. ##SPC4##

In FIG. 4, the relation between specific surface area and short beam and transverse tensile strength is shown.

What has been set forth above is intended primarily as exemplary to enable those skilled in the art in the practice of the invention and it should therefore be understood that, within the scope of the appended claims, the invention may be practiced in other ways than as specifically described.

Claims

What is claimed is:

1. A method for improving the bonding characteristics of high strength, high modulus carbon fiber with resin matrix material without significantly degrading the mechanical properties of the fiber which comprises, prior to impregnating the carbon fibers with said resin matrix material, immersing said carbon fibers in concentrated nitric acid at refluxing temperature for at least 4 hours.

2. The method of claim 1 wherein the fiber is immersed from 4 through 8 hours.

3. The method of claim 2 wherein the acid is 70% nitric acid.

4. A method for improving the bonding characteristics of high strength, high modulus carbon fibers with resin matrix material without significantly degrading the mechanical properties of the fibers which comprises, prior to impregnating the carbon fibers with said matrix material, subjecting said fibers to the action of concentrated nitric acid at refluxing temperature for a period of time sufficient to increase the specific surface area of the fibers to 3.4-7.3 m.sup.2 /g.

5. A method for improving the bonding characteristics of high strength, high modulus carbon fibers with resin matrix material without significantly degrading the mechanical properties of the fibers which comprises, prior to impregnating said carbon fibers with said resin matrix material, exposing said fibers to the action of concentrated nitric acid at refluxing temperature to cause an increase in the number of active sites at the fiber surface as measured by NaOH absorbed per unit area and continuing the exposure of the fiber to the acid at least until the number of active sites at the fiber surface ceases to increase.

6. A method for the production of a carbon filament-resin composites having a high resistance to shear failure while maintaining high flexural strength comprising, exposing high modulus, high strength carbon filaments to concentrated nitric acid at refluxing temperature for at least 4 hours, and impregnating said treated carbon filaments in a resin matrix.

7. The method of claim 6 wherein said filaments are exposed for 4-8 hours.

8. The method of claim 6 wherein said resin is epoxy or polyimide resin.