Embrittlement and Fragmentation

In semi-crystalline polymers such as polyolefins, initial oxidation occurs in the amorphous tie molecules between crystallites and sometimes even in the inter-lamellar amorphous chains within crystallites. This allows a relatively low levels scission to cause a disproportionately large changes in mechanical properties due to brittle failure in the amorphous regions (Celina, 2013). In PP for instance, beginning stages of bulk embrittlement corresponded to only 0.01% of oxidation (Fayolle et al., 2004). 

Fragmentation is an important precursor to biodegradation as it increases the surface area of plastic available to microbes to interact with. However, the kinetics of fragmentation (as opposed to that of degradation) and the evolution of particle sizes during fragmentation of plastics outdoors are essentially unknown. 

A distinction needs to be made between degradation-induced fragmentation described earlier and that due to entirely physical causes. Marine borers consuming polystyrene (EPS) foam floats (Davidson, 2012) and insects or rodents attacking 

FIGURE 64 Development of surface cracks on PP surfaces on exposure to a filtered xenon light source (600W/m ) at 42°C and at different durations of exposure. Source Reproduced with permission from Yakimets et al. (2004). 

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Fig. 4. Embrittlement and fragmentation of polymer under the influence of wind and weather as a result of carbonyl formation.

The effect of such an additive on the UV degradation of polyethylene is shown in Fig. 3 which shows carbonyl formation in the polymer with time in the presence and absence of additive. It can be shown that carbonyl formation correlates directly with the physical properties of the polymer which rapidly loses strength and embrittles. Embrittlement and fragmentation are particularly rapid in the case of the more crystalline polymers which crumble even under the influence of wind and weather (see Fig. 4). The polymer particles so formed are chemically modified by oxidation to be now subject to attack by normal bacterial action and it is anticipated that these will quickly become part of the soil. 

Table 10.9 lists some common zinc anode alloys. In three cases aluminium is added to improve the uniformity of dissolution and thereby reduce the risk of mechanical detachment of undissolved anode material . Cadmium is added to encourage the formation of a soft corrosion product that readily crumbles and falls away so that it cannot accumulate to hinder dissolution. The Military Specification material was developed to avoid the alloy passivating as a result of the presence of iron . It later became apparent that this material suffered intergranular decohesion at elevated temperatures (>50°C) with the result that the material failed by fragmentation". The material specified by Det Norske Veritas was developed to overcome the problem the aluminium level was reduced under the mistaken impression that it produced the problem. It has since been shown that decohesion is due to a hydrogen embrittlement mechanism and that it can be overcome by the addition of small concentrations of titanium". It is not clear whether... 

Stage B, a rapid catalysed photooxidation to embrittlement (fragmentation) of the polymer which continues to oxidize in Stage C, due to the presence of prooxidant species which catalyse thermal oxidation and lead to rapid incorporation of the plastic into the soil structure with ultimate conversion to carbon dioxide and water by the combined effects of abiotic and biotic oxidation. 

Although there is a shift in fragmentation (embrittlement) time between the hot, sunny elimes and temperate climes, it can normally be predicted with reasonable aeeuraey based on incident energy (mJ/m ) and temperature. 

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