Stabilization of Polymeric Materials Against Environmental Effects

STABILIZATION OF POLYMERIC MATERIALS AGAINST ENVIRONMENTAL EFFECTS 

Ultraviolet absorbers were among the first organic stabilizers used. They are colorless compounds that strongly, but selectively absorb ultraviolet radiation and harmlessly dissipate it as heat so that it does not lead to photosensitization. They are also characterized by their very good stability to the absorbed radiation. However, based on the UV absorption mechanism alone, they can only provide limited protection to surface layers and thin samples, for example fibers and films. In accordance with the Beer-Lambert absorption law, the amount of radiation reaching any particular layer diminishes exponentially with the distance from the exposed surface. Thus, the effectiveness of protection via screening of the actinic radiation from the polymer by the UV absorber increases with sample thickness. Protection by UV absorbers is most effective when the additive is concentrated on the surface, such as when it is incorporated in a thin film coextruded over the polymer [75]. 

Specific combinations of low- and high-molecular-mass HALS as well as the combination of two kinds of polymeric HALS have given pronounced synergistic protection to polyolefins [91], However, the combination of two low-molecular-mass HALS usually does not have more than an additive effect and some specific compounds may have a strong antagonistic effect. Some combinations of low-and high-molecular-mass HALS may also be antagonistic to one another. The performance of the combinations of two HALS stabiUzers cannot be predicted but must be determined experimentally. 

Polyolefins In addition to stabilization of polyolefins against thermal oxidation to reduce the sensitivity to light, stabilization against exposure to light is required for articles to be used outdoors as well as those intended for indoor use [93]. Light stabilizers include UV absorbers of the benzotriazole and the benzophenone types (except for thin sections), HALS, and nickel-containing stabilizers. The latter are used for thin sections such as tapes and films and for surface protection. The type of stabilizer is dictated by the type of polyolefin, its thickness, application, and desired lifetime of the article [20]. 

The combination of HALS with a UV absorber is used in fihns of polypropylene and polyethylene as well as in thick sections. In films of LDPE, nickel quenchers were commonly used with a UV absorber, except in a very thin film, in which a higher concentration of nickel stabilizer is superior to the combination. The low-molecular-weight HALS are not sufficiently compatible with LDPE at the concentrations necessary, possibly as high as 2%, for the required protection. Incompatibility of HALS with LDPE has been overcome with the development of polymeric HALS. It is considerably better than either the UV absorber or nickel quencher or combinations of the two. For thicker films (100-200 ftm), the combination of a benzophenone-type UV absorber with polymeric HALS is significantly superior to an equivalent amount of polymeric HALS. The type of stabilizers used for linear low-density polyethylene (LLDPE) and ethyl vinyl acetate (EVA) copolymer are similar to those for LDPE. Since LLDPE has superior mechanical properties (elongation at break and tensile strength), thinner films can be used for most applications, and the loss of UV stabihty with reduction in thickness has to be compensated for by improving the stabilization system. 

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Areas requiring special attention in the future include improved stabilization processes for existing polymers and the synthesis of new polymeric materials which are more resistant against environmental corrosion than those presently used. The implications of the stability of natural polymers for the prediction of service life of synthetic polymers,the general effects of hydrolysis, the light-induced hydrolytic degradation are relevant research topics. 

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