Carbon and graphite fibers are made from rayon, PAN (polyacrylonitrile), or petroleum based pitch with PAN based fibers producing the best combination of properties. Rayon was developed as a precursor prior to PAN but is rarely used today due to its higher cost and lower yield. Petroleum based pitch based fibers were developed as a lower cost alternative to PAN but are mainly used to produce high and ultra-high modulus graphite fibers. A comparison of the PAN and pitch manufacturing processes is shown in Fig. 7.5. Both carbon and graphite fibers are produced as untwisted bundles called tows. Common tow sizes are 1k, 3k, 6k, 12k, and 24k, where k = 1000 fibers. Immediately after fabrication, carbon and graphite fibers are normally surface treated to improve their adhesion to the polymeric matrix. Sizings, often epoxies without a curing agent, are frequently applied as thin films (1% or less) to improve handleability and protect the fibers during weaving or other handling operations.
Several other fibers are occasionally used for polymeric composites. Before carbon was developed, boron fiber was the original high performance fiber. Boron is a large diameter fiber that is made by pulling a fine tungsten wire through a long slender reactor where it is chemically vapor deposited with boron. Since it is made one fiber at a time, rather than thousands of fibers at a time,
Fig. 7.5. PAN and Pitch Fiber Manufacturing Processes
Fig. 7.5. PAN and Pitch Fiber Manufacturing Processes it is very expensive. However, due to its large diameter and high modulus, it exhibits outstanding stiffness and compression properties. On the negative side, it does not conform well to complicated shapes and is very difficult to machine. High temperature ceramic fibers, such as silicon carbide (Nicalon), aluminum oxide, and alumina-boria-silica (Nextel), are frequently used in ceramic matrix composites, but rarely in polymeric composites.
The matrix holds the fibers in their proper position; protects the fibers from abrasion; transfers loads between fibers; and provides interlaminar shear strength. A properly chosen matrix will also provide resistance to heat, chemicals, and moisture; have a high strain-to-failure; cure at as low a temperature as possible and yet have a long pot or out-time life, and not be toxic. The most prevalent thermoset resins used for composite matrices (Table 7.3) are polyesters, vinyl esters, epoxies, bismaleimides, polyimides, and phenolics.
Matrices for polymeric composites can be either thermosets or thermoplastics. Thermoset resins usually consist of a resin (e.g., epoxy) and a compatible curing agent. When the two are initially mixed, they form a low viscosity liquid that cures as a result of either internally generated (exothermic) or externally applied heat. The curing reaction, as shown schematically in Fig. 7.6, forms a series of crosslinks between the molecular chains so that one large molecular network is formed, resulting in an intractable solid that cannot be reprocessed on reheating. On the other hand, thermoplastics start as fully reacted, high viscosity materials that do not crosslink on heating. On heating to a high enough temperature, they either soften or melt, so they can be reprocessed a number of times. Although a lot of research and development has been conducted on thermoplastic composites, thermoset resins are by far the most widely used resin systems for current high performance composite applications.
Table 7.3 Relative Characteristics of Composite Resin Matrices1
Polyesters Used extensively in commercial applications. Relatively inexpensive with processing flexibility. Used for continuous and discontinuous composites.
Vinyl Esters Similar to polyesters but are tougher and have better moisture resistance.
Epoxies High performance matrix systems for primarily continuous fiber composites.
Can be used at temperatures up to 250-275° F. Better high temperature performance than polyesters and vinyl esters.
Bismaleimides High temperature resin matrices for use in the temperature range of 275-350° F
with epoxy-like processing. Requires elevated temperature post-cure.
Polyimides Very high temperature resin systems for use at 550-600° F. Very difficult to process.
Phenolics High temperature resin systems with good smoke and fire resistance. Used extensively for aircraft interiors. Can be difficult to process.
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