Nickel light stabilizers

The early recognition of the role of stable nitroxyl free radicals, e.g., 2,2,6,6-tetramethyl-4-oxopiperidine, and their hindered amine precursors, in polymer stabilization soon led to the development of the hindered amine light stabilizer (HALS) class of photoantioxidants. The first HALS, Tinuvin 770, AO-33, (commercialized in 1974) proved to offer much higher UV-stabil-ity to polymers than any conventional UV-stabilizer available at the time such as UV-absorbers, nickel compounds and benzoates. Table 3). 

As with thermal stabilizers, photostabilizers must satisfy basic chemical and physical requirements (see the section titled ""Antioxidant permanence effects of chemical and physical factors ). In addition, they must be photo-stable, i.e., stable to UV-light, to withstand continuous periods of UV-exposure, without being prematurely destroyed or effectively transformed into sensitizing products. There are essentially three classes of compounds that are categorized as photostabilizers/ photoantioxidants UV-absorbers and pigments, peroxide decomposers including nickel complexes, and sterically hindered amine light stabilizers. 

Other alloys with steel and other metals Electroplating Coins and jewelry Nickel-cadmium batteries Catalysts for hydrogenation Light stabilizer in polyolefins... 

Several nickel chelates have found an industrial application as light stabilizers, mostly in polyolefins. The formula of some of these compounds together with their absorption spectra are shown below (Fig. 29) [129]. 

Several nickel compounds also are efficient light stabilizers, typical representative is Argastab 2002 (Ciba-Geigy). 

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... 

Among the light stabilizer classes available commercially, only a few may be used in a broad range of plastics. Thus, nickel compounds are used almost exclusively in polyolefins, whereas poly( vinyl chloride) is stabilized with UV absorbers only. 

Among the numerous commercial light stabilizers only a few are suitable for low density polyethylene (LDPE). This is mainly due to the fact that most light stabilizers are not sufficiently compatible at levels of concentration necessary for the required protection and so they bloom more or less rapidly. Initially, UV absorbers of the benzophenone and benzotriazole types were used to protect LDPE materials. With the development of nickel-quenchers, significant improvement in the light stability of LDPE films has been achieved. For reasons of economy, however, combinations of nickel-quenchers with UV absorbers are mainly used. The service life of LDPE films may be increased by raising the concentrations of light... 

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]. 

To increase the light stability of styrene-based plastics, stabilizers such as benzophenones, benzotri-azoles and organic nickel compounds are usually added. 

Three different classes of compounds form the major and most important commercial categories of photostabilizers for PP. These are based on nickel complexes (those containing sulfur ligands function primarily as peroxide decomposers, PD), UV-absorbers (UVA), e.g. based on 2-hydroxybenzophenone and 2-hydroxyphenylbenztriazole, and sterically hindered amine light stabilizers (HALS). 

Highly effective UV stabilizing systems can also be achieved by using synergistic mixtures of photoantioxidants/stabilizers with complementary antioxidant mechanisms, e.g. the use of combinations of nickel complexes of dithioic acids, NiDRC, NiDRP, with UV absorbers such as UV531, or hindered piperidine light stabilizers with UV absorbers, e.g. the benzotriazoles. 

Effective quenchers are based on nickel complexes (phenolate, dithiocarbamate, di-thiophosphate) and are used almost exclusively in polyolefins at a concentration of 0.2-0.5%. Nickel complexes impart a green coloration to plastic articles and their importance was significantly reduced with the introduction of hindered amine light stabilizers (HALS). 

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