Light-Absorbing Fillers

Several approaches to stabilisation can be used. Light stabilisers will be discussed under four broad categories light-absorbing fillers organic UV absorbing compounds excited state quenchers and photo-antioxidants. 

The addition of screening fillers reduces the penetration of the light into the polymer. 

If a black colour is unacceptable, the rutile form of titanium dioxide (not the anatase form) is a possible alternative. It is available with an unusually small particle size ( 100 nm). White pigments absorb well in the key 300-400 nm range, whereas ultramarine blue ones have little protective potential. 

Coloured pigments have also been used, such as lead oxide, chromium oxide and red iron oxide. However, heavy metals are considered inappropriate for many applications. A few inorganic pigments (notably cadmium sulphide and the anatase form of titanium dioxide) can even act as UV sensitisers, aggravating the degradation. Iron pigments cannot be used in PVC compositions either, because of the risk of forming ferric chloride, which is a catalyst for the decomposition of PVC. 

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There is the possibiUty of a chemical reaction between a plastic and a colorant at processing temperatures. Thermal stabiUty of both the polymer and colorant plays an important role. Furthermore, the performance additives that may have been added to the resin such as antioxidants, stabilizers, flame retardants, ultraviolet light absorbers, and fillers must be considered. The suitabiUty of a colorant in a particular resin must be evaluated and tested in the final apphcation after all processing steps to ensure optimum performance. 

Addition of UV absorbers can greatly reduce the amount of light absorbed by the polymer. Pigmented polymers often reflect incident light so well that few free radicals can be formed. Carbon black used as a filler absorbs UV light very well, and is also effective as a free radical sink. Titanium dioxide, however, is a sensitizer and promotes degradation. 

The art of compounding (110) rises to the fore in the development of polymeric materials. Thus, fillers can be either reinforcing or inert and low price, or serve as pigments. Plasticizers and lubricants need to be added on occasion. Antioxidants and ultraviolet light absorbers lengthen the life of many polymers. Curatives such as sulfur for vulcanization or peroxides reduce flow, producing cross-linking. 

Kimura et al. [107] made a light resistant formulation of a block E-P copolymer by extruding it at 220 °C with Mg(OH)2, and 2-hydroxy-4-octoxybenzophenone. The injection molded product (at 800 kg cm pressure) shows fire retardation also due to MgO filler and the benzophenone derivative is an efficient UV absorber. 

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