Aging and Stabilization of Polyolefins

The data available in the literature, mostly published in the last decade, pertain chiefly to the aging of high-pressure polyethylene, and to a lesser degree to polyolefins produced on complex catalysts. There are practically no data on the destruction of copolymers. 

Even the first works [1-6] showed that during the aging process the polyolefin is mainly subject to oxidative destruction, which is accelerated 

TABLE 3. Variation of the Characteristic Viscosity of Pols ropylene During Reprocessing and Use at Increased Temperatures 

This chapter cites data characterizing the processes of oxidation and photodestruction of polyolefins of the series polyethylene—copolymers of ethylene and propylene—polypropylene, and methods of their stabilization against the destructive action of oxygen and light. 

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Determination of the residual antioxidant content in polymers by HPLC and MAE is one way to determine the amoimt needed for reasonable stabilization of a material, and also to compare different antioxidants and their individual efficiencies. During ageing and oxidation of PE, carboxyhc acids, dicarboxylic acids, alcohols, ketones, aldehydes, n-alkanes and 1-alkenes are formed [86-89]. The carboxyhc acids are formed as a result of various reactions of alkoxy or peroxy radicals [90]. The oxidation of polyolefins is generally monitored by various analytical techniques. GC-MS analysis in combination with a selective extraction method is used to determine degradation products in plastics. ETIR enables the increase in carbonyls on a polymer chain, from carboxylic acids, dicarboxyhc acids, aldehydes, and ketones, to be monitored. It is regarded as one of the most definite spectroscopic methods for the quantification and identification of oxidation in materials, and it is used to quantify the oxidation of polymers [91-95]. Mechanical testing is a way to determine properties such as strength, stiffness and strain at break of polymeric materials. 

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. 

In actual use, the catalytic oxidative d radatlon of polyolefins Is controlled, to a large extent, by the additives and contaminants present In the polymer. Although the stability of polyethylene Is related Initially to the antioxidant concentration and type, upon aging the critical factor Is the rate of loss of the antioxidant. Figure 4 shows typical examples of effective stabilizer loss, due to migration on aging. The samples, shown... 

The loss of effective stahlllzatlon Is predominantly physical, not chemical, as Identification of stabilizer on the surface of aged polyolefins has shown.(6, ) The concentration of antioxidant dissolved in the polymer and the resulting oxidative stability decrease with time and approach equilibrium for the temperature of concern. The entire process Is complex, with stabilizer solubility only a few parts per million at room tem-peratureC ) and diffusion of stabilizer from the polymer rapid. 

The polyolefin life trajectory is also called ageing, the term that is preferred if the long-term changes of polyolefin properties due to weathering come into the play. It may involve the participation of physical processes such as recrystallization, loss of stabilizers by bleaching and mechanically initiated embrittlement. In the older literature, the reader may encounter also the term corrosion that was implemented from metals. 

Polymethylpentene has a high crystalline melting point of 240°C, coupled with useful mechanical properties at 204°C and retention of form stability to near the melt point. However, the polymer is brittle (fiber or rubber additives are usually advised for improved toughness), ages poorly (the use of antioxidants is recommended), has high gas permeability, and is relatively expensive. Polymethylpentene s chemical resistance is very good and typical of the polyolefins. Its transparency is close to the theoretical optimum for thermoplastics. Polymethylpentene also has excellent electrical properties with power factor, dielectic constant, and volume resistivity on the same order as PTFE fluorocarbon. 

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