Unstabilized polyethylene

In principle, the computational approach to the kinetics of the complex photooxidation process can give meaningful insight into the effects of outdoor weathering of hydrocarbon polymers. For clear amorphous linear polyethylene, the model suggests that the optimum stabilizer would be a molecularly dispersed additive in very low concentration which could trap peroxy radicals. An additive which decomposes hydroperoxides would also be effective but would require higher concentrations. The useful lifetime of unstabilized polyethylene is predicted to vary from a few months in hot weather (100°F) to almost two years in cool weather (45°F), which correlates well with experimental results and general experience. 

Trubiroha [43] reported an action spectrum for development of carbonyl groups (as measured by the change in absorbance at 5.8 microns) of unstabilized polyethylene and polypropylene films. A measurable effect after 1 MJ/sqm of radiant exposure was obtained only for X = 380 nm between 280 and 380 nm the absorption changed logarithmically with wavelength of irradiation. 

The method is capable of determining down to 0.05% of diorganosulphide and tertiary phosphite type antioxidants in polyalkenes with an accuracy of 5%. Table 7.8 shows the results obtained by applying the oxidation method to blends of antioxidants in unstabilized polyethylene. The results are in excellent agreement with the theoretical antioxidant content of these polymers. 

Modelling of photo-oxidation of polyethylene shows that the C—H bonds become oxidized, slowly at first and then more rapidly [878], A 5% C—H bond (1% monomer units) oxidation level has been used to assign a point of failure (marked change in mechanical properties). Under typical conditions, the time to failure (tf) of unstabilized polyethylene would be 3-4 months in temperate climates and shorter in regions of high solar intensity. 

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. 

P. Gijsman and A. Dozeman, Comparison of the UV-degradation chemistry of unstabilized and HALS-stabilized polyethylene and polypropylene, Polym. Degrad. Stab. 1996, 53, 45-50. 

The detrimental environmental degradation of unstabilized commercial polymeric products consisting of polyethylene, polypropylene, poly (vinyl chloride), etc., is frequently due to very small amounts of ketonic carbonyl groups. Electronically excited ketone groups can undergo different processes, in particular the so-called Norrish type I and Norrish type II reactions, as illustrated in Scheme 7.5 for the case of a copolymer of ethylene and carbon monoxide ... 

Viebke, J., Bible, E. et al. Degradation of Unstabilized Medium-density Polyethylene Pipes in Hot Water Applications, Polymer Engineering and Science, 34 (1994) 17, p. 1354- 1361... 

Lassiaz, M., Pouyet, J. Effect of photo-chemical ageing on the tensile properties and behaviour law of unstabilized films of low density polyethylene. Journal of Materials Science, 29 (1994), p. 2177 2181... 

To further ensure biocompatibility, in vitro tests with human cells were carried out [34]. Two different human cell lines were tested on UHMWPE with and without a-tocopherol with respect to cell viability, proliferation, and morphology. Similar proliferation rates were found with both polyethylene samples. Intact morphology was found in light and electron microscopy on each substrate, while the morphologic characteristics of skin fibroblasts were not changed by any material. Normal adherence and spreading of the fibroblasts was found on both stabilized and unstabilized UHMWPE. These results led the authors to conclude that both materials, UHMWPE and a-tocopherol stabilized UHMWPE, do not show any toxic effects to human cells. 

---