Plastics classification criteria

The chemical resistance of plastics is frequently illustrated in so-called resistance tables (e.g., with the classifications resistant, moderately resistant, nonresis-tant ) [3, 17, 21, 23]. The evaluation criterion is the change in mass caused by swelling (increase) or solution (reduction). 

A 2011 study by Lithner et al. (2011) assessed the environmental and health hazards posed by plastics, based on the toxicity of their monomers. The classification is not inherent to the polymer as it is based primarily on residual monomer (with selected additives, plasticizer, and flame retardants). The ranking (see Table 8.8) can have relevance only for occupational exposures and in some food-contact uses of plastics. Also, future advances in residual monomer reduction technology and green substitution of additives can change the status of a polymer in this assessment. Where recyclabihty" is used as a ranking criterion, it generally refers to technical recyclabihty that has little to do with if the resin will in fact be recycled in practice. 

A very simple explanation of the effect of notching has been given by Orowan [95], For a deep, symmetrical tensile notch, the distribution of stress is identical to that for a flat frictionless punch indenting a plate under conditions of plane strain [102] (Figure 12.31). The compressive stress on the punch required to produce plastic deformation can be shown to be (2 + 7t)K, where K is the shear yield stress. For the Tresca yield criterion the value is l.Sloy and for the von Mises yield criterion the value is 2.82oy, where 0 is the tensile yield stress. Hence for an ideally deep and sharp notch in an infinite solid the plastic constraint raises the yield stress to a value of approximately 2>Oy which leads to the following classification for brittle-ductile behaviour first proposed by Orowan [95] ... 

Rigid Piastic n For the purposes of general classification, a plastic that has a modulus of elasticity ether in flexure or in tension greater than 700 MPa (100 kpsi) at 23°C and 50% relative humidity when tested in accordance with ASTM methods D 747, D 790, D 638, or D 882 (ASTM D 883). This simple ASTM criterion has not always been adequate, especially with respect to vinyls whose impact strengths and other properties can vary widely while elastic modulus remains fairly constant. Vinyls are classified as rigid if their moduli are 1.4 GPa or higher, semirigid firom 0.4 to 1.4 GPa, and flexible below 0.4 GPa. 

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