Polymers and the Environment 3
During the 1960s percipient environmentalists became aware that the increase in volume of synthetic polymers, particularly in the form of one-trip packaging, presented a potential threat to the environment, what became evident in the appearance of plastics packaging litter in the streets, in the countryside and in the seas [8]. PVC (see Figure 1) is a good example. Although the density of PVC is around 1.4, hollow parts may float in the oceans, which have a density of about 1.03. PVC has a high concentration of chlorine atoms in an organic chemical structure that is new in nature (i.e. a xenobiotic), what renders it very recalcitrant [9]. On the other hand, PVC degrades easily under the action of light or heat, and its decomposition is catalyzed by the HCl released, forming a poly-unsaturated structure which is very degradable. In oceans the HCl might be removed by the moving water and also neutralized by the cations existing in the alkaline medium (pH ~ 8). To increase stability, 1 to 5% of additives based on transition metals, such as salts, derivatives and complexes of Pb, Zn, Cd and Sn are commonly employed [10]. In order to get a more flexible PVC, plasticizers based on phthalate are commonly used, many of them having chronic toxicity to animals, showing body growth problems (ie, teratogenic effects) and reproduction complications in humans. Small fragments of PVC molecules can evaporate. Just as halogenated solvents, these molecules are very inert and can rise to the stratosphere, contributing to the destruction of the ozone layer [9]. In addition to the accumulation in the environment and to the possible toxicity of the additives, it was realized that the incineration of PVC generated many toxic products such as dioxins, due to the high concentration of chlorine atoms present [1].
Figure 1. Basic structures of the main thermoplastic polymers in the present: a) polyethylene (PE), b) polypropylene (PP), c) polystyrene (PS), d) polyvinyl chloride (PVC), and e) polyethylene terephthalate (PET).
Polycarbonate (plastic) and epoxy resins (coating and adhesive) are normally produced with bisphenol A as one of the monomers. This substance may also be used as an additive for plastics. It is an endocrine disruptor (it can mimic hormones) [1]. Some studies have shown toxicity, carcinogenic effects and possible neurotoxicity at low doses in animals [1115]. In the case of decomposition of the resin, this toxic monomer might be released into the