Polyethylene oxide deterioration

The equations described above constitute the schematic oxidation process for polyethylene but observation and detailed study of each reaction are very important in order to elucidate the oxidation mechanism. Many investigations of polyethylene oxidation have been based on analysis of the final products described in Eq. (7.22). In this section, direct ESR observations of the radical species in the reaction equations are introduced. The direct observation is very important to clarify the mechanism of oxidation and to design methods to prevent deterioration. Reaction Eqs. (7.15)-(7.21) can be examined either qualitatively or quantitatively based on ESR results. 

Deterioration occurs in uncolored polyethylene exposed to weather. Ultraviolet light causes photoactivated oxidation. Satisfactory weathering formulations contain 2-2.5% well-dispersed carbon black and stabilizers, The carbon black prevents ultraviolet light penetration. 

Especially above room temperature many polymers degrade in an air atmosphere by oxidation that is not light-induced (heat ageing). A number of polymers already show a deterioration of the mechanical properties after heating for some days at about 100 °C and even at lower temperatures (e.g. polyethylene, polypropylene, poly(oxy methylene) and poly(ethylene sulphide)). 

Post-irradiation oxidative effects lead to a considerable deterioration of the mechanical properties of polypropylene and of its blends with a low content of polyethylene. The elongation at the breaking point of the polymer falls sharply even at doses of lower than 100 kGy which are used for example during the sterilization of medical instruments. In blends consisting of more than 30% w. of low density polyethylene, the influence of post-irradiation oxidative effects is markedly diminished [151]. 

Polyethylene is one of the few polymers which may sometimes be used in the pure form, i.e. uncompounded with other additives, although it will usually contain some anti-oxidant. However, in outdoor applications PE is attacked by UV light resulting in embrittlement and general deterioration in properties. 

Three polymers, listed in Table 26.6, were selected for further work in which the effect on ITV of fillers, antioxidants and UV absorbers was to be examined. The effect of fillers was explored in the fluorine containing copolymer, the silicone copolymer and polyethylene polyethylene was selected for antioxidant and UV absorber study because of the three it is the most susceptible to oxidation. These results showed that there was a general deterioration compared with pure polymers, although hydrated alumina showed some benefit in polyethylene and silicone rubbers. However such results were not general - alumina hydrate for example deteriorates the performance of fluorine copolymers. The following tables summarize these effects. (Tables 26.6, 26.7, 26.8). 

In the absence of light, most polymers are stable for very long periods at ambient temperatures. However, above room temperature many polymers start to degrade in an air atmosphere even without the influence of light. For example, a number of polymers show a deterioration of mechanical properties after heating for some days at about 100 °C and even at lower temperatures (e.g., polyethylene, polypropylene, poly(oxy methylene), and poly(ethylene sulfide)). Measurements have shown that the oxidation at 140 °C of low-density polyethylene increases exponentially after an induction period of 2 h. It was concluded that thermal oxidation, like photooxidation, is caused by autoxidation, the difference merely being that the radical formation from the hydroperoxide is now activated by heat. The primary reaction can be a direct reaction with oxygen (Van Krevelen and Nijenhuis 2009) ... 

Figure 4.8 Low-density polyethylene (LDPE) suspected to have low levels of thermal degradation (by a deterioration of physical properties) may show negligible differences in the infrared. Initial oxidative reaction takes place adjacent to the vinylidene units [151], and treatment with SO2 will lead to a reduction in pendant methylene absorption, which can be further intensified by prior heating. This is illustrated in this figure by comparing the treatment effects on a complaint (oxidised, right) and control (left) samples. A, the samples as received B, after SO2 treatment C, heated at 100°C for 18h D, heated at 100°C for 18h then S02-treated. Reproduced from ref. 151, by permission of Elsevier Applied Science Publishers Ltd, Barking.

In polyethylene, we find increasing promotion of oxidation hy various metals in the following order [61] Cu > Fe > Zn > Pb > Mo > Ti Al. In PE-HD manufactured with Ziegler-Natta catalysts, residues in concentrations as low as 1 ppm can lead to considerable deterioration in oxidative stability [55]. 

Hydrocarbons such as polyethylene and polypropylene are readily available and inexpensive, but they are not sufficiently soluble to cast as films. Unsubstituted hydrocarbons are easily oxidized and their nonstick properties rapidly deteriorate in exterior usage. Halogenated... 

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