Polyethylene predicting stability

Two different approaches for lifetime prediction are presented. The underlying lifetime limiting processes have been identified in two cases. Mathematical expressions of chemical/physical relevance were used for the lifetime predictions for PE hot-water pipes and cables insulated with plasticized PVC. Accelerated testing, extrapolation and validation of the extrapolation by assessment of the remaining lifetime of objects aged during service conditions for 25 years were successfully applied to cables insulated with chlorosulfonated polyethylene. Polyolefin pipes exposed to chlorinated water showed a very complex deterioration scenario and it was only possible to find a method suitable for predicting the time for the depletion of the stabilizer system. 

Energy transfer. To model this mechanism of stabilization, a reaction (number 50, Table I) was included to allow for quenching of the excited ketone by an additive (Ql) with a rate constant comparable to the upper limit for diffusion of a small molecule in a polymer matrix. Figure 8 shows that up to 1M concentration (about 8 wt-%) of quencher had minimal effect on the time to failure (5% oxidation). This assumes completely random distribution of both the excited ketones and the stabilizer as in the calculation of Heller and Blattman (34). Such a bi-molecular process is too slow to compete with the fast unimolecular reactions of the excited ketone, and thus stabilization by such transfer is predicted to be ineffective in polyethylene. Allowance must be made, however, for special cases in which the excitation energy can effectively migrate (e.g., in some aromatic polymers), in which case the bimolecu-lar process may become competitive with the other chemical processes from the excited states. 

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. 

The oxidation of high energy irradiated polymers progresses in relation with their basic component structure and formulation, which is initiated by the scission of backbones. The activation energy (Ea) required for oxidative degradation is the key of stability evaluation. The values of activation energies are placed on the range between 100 and 120 kJ moF for different classes of insulation materials (polyethylene, ethylene-propylene copolymer, poly(vinyl chloride) used in the manufactures of cables for nuclear power plants [176]. The life time allows the prediction... 

Unfortunately, any attempt to predict the long-term stability of polyethylene based on an Arrhenius plot of high-temperature oxidative induction times measured above the melting point fails when projected to lower temperatures where polyethylene is a semicrystalline solid (Bair 1973 Chan et al. 1978). The reasons for this nonlinear behavior appear to be associated with complex chemical and physical interactions that behave differently in the solid state than in the melt. 

Separations of complex FAME mixtures, with chain lengths up to 24 carbons and up to six double bonds, including aU co3 and co6 acids, can be achieved on capillary columns of polyethylene glycol or Carbowax type columns (usually of dimensions 25-30 m long, 0.25 mm i.d., and a film thickness of 0.2 xm) as recommended in the AOCS official method Ce lb-89 (American Oil Chemists Society, 2005). A GC profile of FAME from a fish oil/sunflower oil mixture on a column of this type is shown in Figure 2.3. These columns are preferred because of the stability, and the predictable order in which all compounds elute. An analysis on this type of column would normally be the first step for a sample of unknown composition. Typically in our laboratOTy, the following conditions are used an... 

Since the calculated free energy of interaction is largely dependent on the value chosen for Xi, then evidence of the effect of salts on Xi could lead to significant conclusions on the stability of emulsions in the presence of salts. Several reports of non-ionic surfactants and polyethylene glycols bear out the contention that electrolytes dehydrate the ethylene oxide chains and promote their salting out . This is what the study of the effect of NaCl and Nal referred to earlier (see Fig. 8.3) aimed to display-that salting in and salting out would have an effect on stability as predicted by Equation 8.30. 

Malik, J., Tuan, D., Spirk, E. Lifetime prediction for hindered amine light stabilizer-stabilized low-density polyethylene and pol3qiropylene. Polym. Degrad. Stab. 47, 1-8 (1995)... 

Whilst this prediction of in vivo stability is largely borne out in practice, there is some evidence that other factors are involved and that unexpected degradation mechanisms operate within the body. One of the first observations of degradation of these polymers was made by Oppenheimer" who was working on the carcinogenic properties of plastics. In attempting to elucidate the mechanisms by which plastic films induced tumours after subcutaneous implantation in rodents, certain radiolabelled polymers were employed. " C-labelled polystyrene, polyethylene and poly(methyl methacrylate) were implanted and urine, faeces and respiratory CO2 were monitored for periods over a year. With the polystyrene, nothing radioactive was excreted in the urine until 21 weeks, but some radioactivity was detected after this time. With polyethylene, radioactive species were excreted after 26 weeks and with poly(methyl methacrylate), this occurred after 54 weeks. 

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