Ranking common plastics

FIGURE 8.4 Plastic pyramid originally proposed in 1998 by Van der Naald and Thorpe. 

However, the LCA rankings of these were based on had a limited scope, and were assessed on a volume basis (as opposed to a functional unit such as a number of containers of specified capacity). The footprint of a product must be taken together with its functionality or the contribution the product makes to human lifestyle. This requires the footprint be assessed on a functional unit basis. The Institute for Agriculture and Trade Policy (lATP) (2005) study on food-use plastics also ranked PVC as a hazardous plastic and recommends avoiding its use (plasticized with DEHA or other plasticizer). It advocates avoiding PS (which can leach residual styrene) and PC (which can leach BPA) plastics as well, with any food-contact products. 

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. 

While there is some justification for using such rankings as a general guide to material selection, there are shortcomings in the approach. The most obvious is that 

TABLE 8.8 Hazard Levels of Common Plastics Estimated from Monomer Characteristics 

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Fig. 2.80 is typical of the effects which may be observed with several common plastics materials. C ite apart from the changes in impact strength with temperature an important lesson which should be learned from this diagram is that the ranking of the materials is once again influenced by the test conditions. For example, at 20°C polypropylene is superior to acetal whereas at — 20°C it... 

In tenns of the physicochemical behavior of coal during the liquefaction process, some mention must also be made of the phenomenon of coal plasticity (Chapter 9). Plasticity is particularly evident in coals of the bituminous rank during the plastic condition (which usually occurs in the tanperature range 325°C-350°C [615 -660 ]), the plastic mass has a tendency to adhere quite strongly to a variety of surfaces. Thus, reactor plugging could be a common result of the tendency of various coals to exhibit plastic behavior and there has been a considerable amount of effort directed to resolving this particular problem. 

The same is not true, however, for fires that involve PVC materials [24] or for uncontrolled incineration of PVC waste. While the plastic is not particnlarly flammable in a fire due to its high halogen content (among common polymers, PVC ranks among the best in terms of time to ignition), considerable amounts of the metal compounds can be released [24] into the atmosphere. Research aimed at replacing these metal-based stabilizers with alternatives that are safer in fires is valuable in responding to this real problem associated with plastic fires. 

It is commonly assumed that the size of the plastic zone in the crack plane and in the direction perpendicular to crack length is of the same size 1. The size scale I of the crack tip plastic zone relative to specimen dimensions B, D) and crack length (2a) is used to rank failure modes (82) and can be used to describe the displacement of the state of stress from the reference state of plane strain. 

Different types of stress situations are common in fatigue testing—including constant strain, alternating tension and compression, or flexural stress. These stresses may cycle around some preset limit, or they may alternate from a mean stress or strain of zero. These various methods will produce significantly different results in plastics. Therefore, the test conditions should simulate as closely as possible the actual application. Effects of various variables and the ranking of different materials may easily be reversed by choosing different test conditions. 

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