Environmental Stability and Degradation Mechanisms of Polymeric Materials

ENVIRONMENTAL STABILITY AND DEGRADATION MECHANISMS OF POLYMERIC MATERIALS 

The rubber-modified aromatic polymeric materials, for example, high-impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS) are the least stable because of the rapid photooxidation of the rubber component, which sensitizes the polymer to further oxidation. Diene-based rubbers are also susceptible to degradation by ozone, which causes chain scission of the main-chain 

The photostabiUty of different types of polyolefins varies depending on their chemical structures, crystallinity, and morphology. Polypropylene (PP) is especially sensitive to UV radiation and must be stabilized against the effects of solar 

Polyethylene (PE) is inherently less sensitive to oxidative attack than PP, but stabilization of PE is also mandatory for outdoor use. The stability varies with the type of polyethylene and manufacturing process. Linear low-density polyethylene (LLDPE) (1-octene comonomer) is significantly less sensitive to photooxidation than low-density polyethylene (LDPE) with comparable density and molecular weight [20, 21]. Generally, LDPE is less susceptible to photooxidation than high-density polyethylene (HDPE). The most fundamental difference between polyethylene homopolymers and polypropylene is the behavior of hydroperoxides toward photolysis. On photooxidation, hydroperoxides accumulate in PP, but decrease rapidly on UV exposure of PE. In copolymers of polyethylene with vinyl acetate, the stabihty depends on the content of vinyl acetate. The higher the content, the more the copolymers act like polyvinyl acetate, which is more susceptible to photooxidative degradation than polyethylene. 

Aromatic polyamides are more susceptible to the effects of sunlight than the aliphatic types. Significant yellowing and rapid decrease in molecular weight and 

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Acetal homopolymer is a highly crystalline thermoplastic manufactured by polymerization of formaldehyde and capping the two ends of the polymer chain with acetate groups (Table 6.2). It is called polyoxymethylene (POM) and has a backbone consisted of repeating —CH2O—units. Acetal copolymers are produced by copolymerization of trioxane and small amounts of a comonomer. The comonomer randomly distributes carbon—carbon bonds in the polymer chain, which stabilizes the resin against environmental degradation. The low cost of acetals compared to other polymers with similar performance and their mechanical, chemical, and electrical properties, allows them to replace metal and other structural materials in many applications. 

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