Toughness Considerations

In addition to the effects on modulus and strength, the use of fillers can, also in most cases, improve polymer toughness, that is, its ability to resist crack propagation, normally expressed as the area under the stress/strain curve. Impact strength, a common measure of toughness at high strain rates or dynamic conditions is the most important property for real application conditions. 

In short-fiber composites, areas around fiber ends, areas of poor adhesion, and regions of fiber-fiber contacts may reduce the resistance to crack initiation by acting as stress concentrators. However, fibers may also reduce crack propagation by diverting cracks around the fibers or by bridging cracks. Materials with high-impact 

The energy dissipated in forming a unit amount of new surface, F, by debonding is 

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These arguments have been applied to the development of tough ceramic laminates. In general, ceramic materials such as silicon carbide, aluminum nitride, or zirconium oxide are brittle. They fail catastrophically when a single crack penetrates the material. Using the arguments above, a new type of ceramic laminate has been invented, to prove that the introduction of correct interfaces raises the toughness considerably. " ... 

Special silane coupling agents that produce a chemical reaction with the polymer may improve stiffness and/or toughness considerably, but they tend to be expensive, and other routes are worth investigating. 

Mechanical and Thermal Properties. The first member of the acrylate series, poly(methyl acrylate), has fltde or no tack at room temperature it is a tough, mbbery, and moderately hard polymer. Poly(ethyl acrylate) is more mbberflke, considerably softer, and more extensible. Poly(butyl acrylate) is softer stiU, and much tackier. This information is quantitatively summarized in Table 2 (41). In the alkyl acrylate series, the softness increases through n-octy acrylate. As the chain length is increased beyond n-octy side-chain crystallization occurs and the materials become brittle (42) poly( -hexadecyl acrylate) is hard and waxlike at room temperature but is soft and tacky above its softening point. 

The cured polymers are hard, clear, and glassy thermoplastic resins with high tensile strengths. The polymers, because of their highly polar stmcture, exhibit excellent adhesion to a wide variety of substrate combinations. They tend to be somewhat britde and have only low to moderate impact and peel strengths. The addition of fillers such as poly (methyl methacrylate) (PMMA) reduces the brittleness somewhat. Newer formulations are now available that contain dissolved elastomeric materials of various types. These mbber-modifted products have been found to offer adhesive bonds of considerably improved toughness (3,4). 

Poly(phenylene sulfide) (PPS) is another semicrystalline polymer used in the composites industry. PPS-based composites are generally processed at 330°C and subsequently cooled rapidly in order to avoid excessive crystallisation and reduced toughness. The superior fire-retardant characteristics of PPS-based composites result in appHcations where fire resistance is an important design consideration. Laminated composites based on this material have shown poor resistance to transverse impact as a result of the poor adhesion of the fibers to the semicrystalline matrix. A PPS material more recently developed by Phillips Petroleum, AVTEL, has improved fiber—matrix interfacial properties, and promises, therefore, an enhanced resistance to transverse impact (see PoLYAffiRS containing sulfur). 

For a part to exhibit stmctural stiffness, flexural moduH should be above 2000 N/mm (290,000 psi). Notched l2od impact values should be deterrnined at different thicknesses. Some plastics exhibit different notch sensitivities. For example, PC, 3.2 mm thick, has a notched l2od impact of 800 J/m (15 fdbf/in.) which drops to 100 J/m (1.9 fflbf/in.) at 6.4-mm thickness. On the other hand, one bisphenol A phthalate-based polyarylate resin maintains a 250-J /m (4.7-fdbf/in.) notched l2od impact at both thicknesses. Toughness depends on the stmcture of the part under consideration as well as the plastic employed to make the part. Mechanical properties, like electrical properties, ate also subject to thermal and water-content changes. 

Since toughness and abrasion resistance are likely to he opposing properties, considerable judgment is required in deciding where, in this series, the best prospect lies, especially if economic considerations are important. The choice is easiest at the extremes. 

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