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The degradation of plastic litter in rivers: implications for beaches - page 8 / 10





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Williams, A.T. & Simmons, S.L.

Fig. 6. LDPE Degradation trials (four months): Sample % tensile strength re- tention at three exposure areas.

Wypych (1990) discussed plastic degradation in re- lation to percentage retention of tensile properties such as strength and elongation. Fig. 6 shows results in this format for the three stranding positions. Buried samples showed the greatest tensile strength retention, dropping no lower than 90%, whilst bank-side and suspended samples showed similar retention rates at approximately 80%. Based on the known relationship between de- creasing molecular weight and decreasing tensile strength (Wypych 1990), results appeared to demonstrate that some decrease in molecular weight occurred during the first month of exposure for bank-side and suspended samples, but with little further loss. Reduction in mo- lecular weight of these samples as opposed to buried samples is likely to be due to photodegradation of ex- posed specimens. Slight decreases in tensile strength (and molecular weight) of buried samples not exposed to photodegradation may be explained by losses of low molecular weight contaminants in the polymer which are easily biodegraded (Seal 1988).

process. Overall, samples did not exhibit rapid degrada- tion upon exposure to environmental conditions. Initial changes in physical properties were rapid, but were followed by little subsequent activity.

On rinsing samples after exposure, buried samples were shown to maintain their original appearance, whilst bank-side and suspended samples exhibited obvious colour bleaching and dirt impregnation. This is an area which could be investigated in future work. Plastics are frequently buried in coastal dune systems and the au- thors have subjected biodegradable plastics to similar studies. Degradation of plastics was supposed to be complete over a 30 day time period, but because of periodic burial, the plastic strips were still basically intact after 4 months (unfortunately, a large scale storm took away these boards). The conclusion was clear, plastics degrade very slowly in the beach environment.


Although decreasing tensile strength may be related to reduced molecular weight, losses may be accounted for by photo or biodegradation, as seen above. Elonga- tion retention is known to exhibit greater sensitivity to changes occurring during photodegradation and corre- lates well to chemical changes during photo-oxidation. Elongation retention results show retention as low as 35 % for bank-side and suspended samples, as opposed to 75% for buried samples, possibly indicating the over- all importance of photodegradation in the degradation

Light and mobile litter i.e. plastic sheeting, is trans- ported to beaches by rivers in estuarine areas prior to fragmentation either in the riverine environment or beach. Degradation observations over a 4 month period indi- cated that initial plastic (LDPE) breakdown in rivers was rapid, especially during the first week of exposure. Subsequent breakdown was slow and, during the survey period, samples did not reach a stage where fragmenta- tion was likely. Sample burial altered the degradation

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