Polymer stabilization photooxidation protection

U.V. Stabilizers.—Many of the papers discussed in preceding sections contain discussion of the nature and mechanism of u.v. stabilizers. A brief survey of antioxidants and stabilizers used in the plastics industry has appeared,28 and some new photostabilizers, including a polymeric u.v. absorber and a surface-grafted antioxidant, have been proposed.280 The mechanism of nickel chelate stabilizers has been further discussed,281 and the diffusion of and loss of light stabilizers in poly(olefins) described.282 As part of an attempt to understand the transformations of stabilizers during the ageing of polymers, the photooxidation of 2,6-di-t-butyl-4-methylphenol sensitized by Methylene Blue has been studied.283 U.v. light protection by sunscreens, with mechanisms of interest to the polymer field, has been described.284... 

This chapter reports the results of the literature that concerns the photooxidation of polymer nanocomposites. The published studies concern various polymers (PP, epoxy, ethylene-propylene-diene monomer (EPDM), PS, and so on) and different nanofillers such as organomontmorillonite or layered double hydroxides (LDH) were investigated. It is worthy to note that a specific attention was given to the interactions with various kinds of stabilizers and their efficiency to protect the polymer. One of the main objectives was to understand the influence of the nanofiller on the oxidation mechanism of the polymer and on the ageing of the nanocomposite material. Depending on the types of nanocomposite that were studied, the influence of several parameters such as morphology, processing conditions, and nature of the nanofiller was examined. 

Low-molecular-weight HALS, a hydroperoxide inhibitor, has been shown to considerably reduce photooxidation of PBT [107]. It can be expected that the combination of a UV absorber with HALS will be particularly effective. There is little data on stabihzation of thermoplastic polyester elastomers, but the stabilizers used to protect polymers that degrade by free-radical chain oxidations should also be efficient for these types. Thus, UV absorbers of the benzotriazole and benzophenone types may be used for their screening capabihties, and synergistic effects can be expected with the addition of HALS to reduce the rate of chain oxidations. 

The use of polypropylene (PP) outdoors is limited by its weatherability. The main cause of degradation is photooxidation, promoted by ultraviolet (UV) irradiation, and in hot sunny dimates components made from PP may fail within a few days outdoor service unless they are suitably protected. Failure is usually by brittle fracture resulting from surface embrittlement. Chemical stabilizers added to the polymer can increase the lifetime very considerably and should be used even if a component is to be exposed to sunlight only occasionally. Much of the research into polymer weathering and stabilization has been conducted on PP because of its importance and its sensitivity to outdoor conditions. 

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