lobal Perspective

The Prospects for Carbon as a Structural Material Thomas Abraham Business Communications Company, Inc.

Except for the naturally occurring graphites, most manufactured carbon and graphite fibers are made from mixtures of carbonaceous materials (e.g., petroleum coke, carbon black and anthracite coal) with binders such as coal tar pitch. These materials are baked to a rigid, hard structure by heat treatment at temperatures in the range of 900 to 1,800°C. To form "synthetic graphite" or "graphitized products," the baked carbon is subjected to an addi­tional heat treatment at temperatures of 2,200°C or higher, resulting in a change in the crystallographic struc­ture to the "graphitic" form.

In the last twenty years, carbon and graphite fibers have been widely adopted as fibrous reinforcements in composites. All graphite fibers are manufactured by the pyrolysis of an organic precursor in an inert atmo­sphere. The different types of carbon and graphite fibers are rayon-based, polyacrylonitrile (PAN)-based and pitch-based.

In carbon-carbon composites, high­grade carbon fibers are oriented along a certain direction and set within a polycarbon matrix. The mechanical properties of materials thus obtained extend considerably their field of ap­plication, which was once limited by the insufficient performance of poly­crystalline graphites. There are sev­eral ways to fabricate such composites, the most important being chemical vapor infiltration and liquid vapor impregnation. The reinforcements used are carbon fiber and cloth.

Carbon-graphite is frequently speci­fied for applications where extreme environmental conditions exist or where chemical inertness is required. Monolithic graphites have been used for some time in high-temperature and friction-resistant applications. How­ever, several new applications are emerging for monolithic graphites and carbon fiber-reinforced carbon-matrix (C/C) composites.

Monolithic graphites and C/C com­posites have many desirable high­temperature properties, including high strength, high modulus and low creep. In addition, C/C composites have high fracture toughness and thermal-shock resistance. Their low-temperature strength and moduli are comparable to

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resin-matrix composites. Other advan­tages ofC/C composites are light weight and tailorability. Because these com­posites' properties are maintained to 2,000°C, they represent the premier materials for high-temperature, inert­atmosphere applications or short-time, high-temperature applications in non­inert environments.

Monolithic graphites and C/C com­posites are particularly attractive for applications such as refractory linings,

advanced heat-engine components, tooling systems for fabrication of ad­vanced composites, electrical and com­munications components, missile and other aerospace parts, brushes and brush plates, brake linings, corrosion­resistant parts for the chemical indus­try, parts for metal processing and cut­tingtool manufacturing industries, and biomedical implants.

Thus far, the U.S. Navy, Air Force and NASA have supported research and development in C/C composites. Lately, this has been changing as more companies investigate potential uses other than military and space applica­tions. A recent study, Structural Car­bon: High Performance, High Value Added-A Technical/Economic Review by the Business Communications Com­pany, emphasizes that the carbon fi­bers and C/C composites constitute a very high growth segment among advanced materials. Along with mono­lithic carbon and carbon fibers, C/C

composites constituted a market of $419 million in 1988 and will grow at a rate of 12.6% annually. Currently, the market is shared more or less equally in sales by each of these three seg­ments. By 1993, the demand will ap­proach $760 million.

In today's structural carbon market, the report points out that monolithic carbon accounts for 90% of consump­tion. However, while fibers and com­posites represent only 10% of consump­tion, they are high-value-added mate­rials, and they represent approximately 66% of total sales. By 1993, the com­bined share of the fibers and compos­ites will increase to 19% in volume and 78% in value.

In pounds, the demand for carbon fibers and C/C composites will approxi­mately double by 1993. In value, the demand will increase 2.4 times for fi­bers and 1.9 times for C/C composites. Fibers will demonstrate the largest average annual growth rate, 19.5%, because of their increasing use in composites. C/C composites will have an annual growth rate of 13.2%.

Currently, there is an over-capacity of carbon fiber production, with de­mand being about 66% of the total U.S. capacity (8.5 million pounds). Most PAN resin is currently imported from Europe and Japan. However, with the U.S. requiring use of domestically produced PAN precursor for defense applications, several companies are building production facilities.

The structure of the U.S. graphite­fiber industry is fairly complex, with several players and significant overlap of activities. A few companies are fully integrated (i.e., they produce carbon fi­bers, prepregs and finished composite shapes, selling them in the merchant market and using them captively as well). In terms of U.S. industry, Union Carbide is the market leader for mono­lithic carbon/graphite. For carbon fi­bers, the leading suppliers are Hercu­les, Amoco and BASF. These com­panies along with AKZO and Cour­taulds/Grafil account for 90% of the total market.

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