Polyethylene degradation, time-controlled

Degradable Low-Density Polyethylene (LDPE) Films. Typical xenon arc embrittlement times for unstabilized low-density polyethylene are l600-2UOO hrs. The controls chosen for this research are listed in Table II the thin LDPE garment bag embrittled in 1820 hr and the EcoPlastics LDPE control became brittle after 2075 hr xenon irradiation. The thin garment bag was not brittle, as determined by a fold test, after 18 months outdoor exposure. The Dupont "Sclair" LDPE control listed in Table II was unusual since it had short embrittlement times of 1175 hr in the Weather-Ometer and 6.5 months outdoors. This film was used as a yardstick to measure the Degradable LDPE films since a Degradable Plastic should not last more than 6 months, perhaps less, outdoors and 1200 hr in the xenon arc Weather-Ometer. 

Wall thickness fluctuations of up to 5% may occur as a result of the uneven temperature in the molten polymer during rotation. Also, because of rate of cooling which is not always reproducible, deviations in the dimensions of the finished product may amount to 5%. Requirements for this process are that the material can be molten completely, that the melt is of sufficiently low viscosity, and that the molten polymer does not degrade too rapidly. Besides plasticized PVC, high and low density polyethylene (HOPE and LDPE) are often used, as well as copolymers of PE such as EVA (ethylene-vinyl acetate). Cycle times vary between 3 and 40 min, depending on the wall thickness. Cycle time is predominantly controlled by the heating and cooling time. 

It was shown that the kinetics of photo-oxidation of polyethylene is characterized by the superposition of two phenomena. The first corresponds to an exponential increase in the concentration of the carbonyl groups with time and is observed when the kinetics are controlled by the diffusion of oxygen. The second one is not controlled by diffusion, but corresponds to a linear increase of the carbonyl concentration with time and takes place in degraded samples. This is explained in terms of chain rupture in the amorphous regions of the polymer, allowing free access of oxygen [567]. 

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