Williams, A.T. & Simmons, S.L.
“Plastics can be defined as organic materials con- taining molecules of high molecular weight (i.e. be- tween 104 and 107) which can be moulded to shape by the application of pressure at moderately high tempera- tures. Once moulded they may retain their plasticity in the manner of polyethylene or nylon (thermoplastics) or they may become permanently hard and brittle like bakelite (thermosetting plastics)” (Higgins 1988, p. 243). Until the beginning of the twentieth century emphasis was placed on the destruction of complex organic com- pounds to produce larger numbers of simpler materials. It was only then that chemists learnt to rebuild some of the products of destructive processes to produce sub- stances which did not occur naturally. Of major impor- tance amongst such substances produced were the su- per-polymers (plastics).
be used almost interchangeably. There is frequently a tendency to presume that plastics degrade virtually com- pletely by biodegradation. However, in most instances photodegradation is the major process involved” (Klemchuk 1990, p. 188). Photodegradation is the proc- ess by which ultra-violet light (in sunlight) reduces the molecular weight of polymers, causing the plastic to become brittle and disintegrate. In contrast, biodegrada- tion may be defined as the “breakdown of the physical and chemical properties of a structure by the action of living organisms - typically fungi and bacteria” (Lloyd 1987, p. 20). Many studies have been undertaken to explore the biodegradation of plastics (Nykvist 1974; Klemchuk 1990; Lloyd 1987). Klemchuk (1990, p. 183) made the interesting observation “that all commercial packaging plastics are not biodegradable, because their molecular weights are too high and their structures are too rigid for assimilation by organisms”.
“Two general mechanisms are usually considered for degradable plastics, namely photodegradation and biodegradation. Unfortunately, care is not taken to de- fine which mechanism is involved in a particular proc- ess with a degradable plastic and the two have come to
An important study by Potts et al. (1972) found most commercial thermoplastics to be immune to fungal at- tack. Although polyethylene film initially supported growth, it was apparently due to additives. Longer term studies (two and eight years) of soil burial, showed that
Fig. 1. The’Christmas tree’ effect.