Graphite-fiber-reinforced thermoplastics, properties

Table 2.19 Typical properties of Discontinuous Graphite-Fiber-Reinforced Thermoplastic Composites (8)...

Experimental results are presented that show that high doses of electron radiation combined with thermal cycling can significantly change the mechanical and physical properties of graphite fiber-reinforced polymer-matrix composites. Polymeric materials examined have included 121 °C and 177°C cure epoxies, polyimide, amorphous thermoplastic, and semicrystalline thermoplastics. Composite panels fabricated and tested included four-ply unidirectional, four-ply [0,90, 90,0] and eight-ply quasi-isotropic [0/ 45/90]s. Test specimens with fiber orientations of [10] and [45] were cut from the unidirectional panels to determine shear properties. Mechanical and physical property tests were conducted at cold (-157°C), room (24°C) and elevated (121°C) temperatures. 

In formulating reinforced thermoplastics, glass fibers with lengths less than 1/4 in. are generally used. Commonly, milled fibers are selected. The fibers improve the physical properties of the base resin, in particnlar the heat-deflection temperatnre. Some thermoplastics are reinforced with graphite fibers to give electromagnetic interference protection. Aramid fibers with thermoplastics resnlt in excellent wear and abrasion resistance. 

Conductive reinforcements in thermoplastics are a new application of composites [2891. High temperature resistant thermoplastics, such as PPS, PEI, PPO and liquid crystal polyesters, have been combined with conductive fibers, generally chopped graphite and nickel coated graphite. Fiber distribution, orientation and fracture morphology were determined by optical and SEM techniques. Poor bonding in the nickel coated graphite composite, as observed in SEM tensile fractures, resulted in lowered tensile and impact properties. 

Quantitative predictions of the effects of fillers on the properties of the final product are difficult to make, considering that they also depend on the method of manufacture, which controls the dispersion and orientation of the filler and its distribution in the final part. Short-fiber- and flake-filled thermoplastics are usually anisotropic products with variable aspect ratio distribution and orientation varying across the thickness of a molded part. The situation becomes more complex if one considers anisotropy, not only in the macroscopic composite but also in the matrix (as a result of molecular orientation) and in the filler itself (e.g., graphite and aramid fibers and mica fiakes have directional properties). Thus, thermoplastic composites are not always amenable to rigorous analytical treatments, in contrast to continuous thermoset composites, which usually have controlled macrostructures and reinforcement orientation [8, 17]. 

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