Plastic photostabilization

Additives such as antioxidants and photostabilizers of low-molecular weight face two major problems (1) they may evaporate during high temperature moulding and extrusion process or (2) they may migrate to the surface of the plastic and get extracted. There are, in general, three ways of overcoming these problems. 

These compounds are often used as photostabilizers in plastic materials. 2-methyIbenzophenone (A) appears to undergo intramolecular H-abstraction in 2-propanol in the excited state. The unstable enol reverts to the ketone in the ground state. The possibility of such a six-membered transition state does not exist for 2-t-butyl benzophenone, and it is as reactive towards H-abstraction as benzophenone itself. 

Plastics have to be stabilized to withstand chemical and physical stresses during different phases of their lifetime. Stabilizers protecting plastics against particular degradation processes were developed and commercialized by various companies. According to their principal activity mechanisms, polymer stabilizers are conventionally classified as antioxidants, photoantioxidants, photostabilizers, heat stabilizers and... 

It is easier to dissolve a stabilizer than to evaporate it [27]. The physical loss of stabilizers due to the leaching from polymer surfa< layers into liquids which come into contact is therefore more serious than volatilization. Problems arise mainly in systems where the degradation process has been concentrated at the surface layer and therefore an efficient surface protection of a polymer is mandatory. This phenomenon takes place mainly in photostabilization of plastics or antiozonant protection of rubbers. The surface loss of stabilizers is extremely serious in very thin profiles or products having a very high surface/mass ratio. 

Triazole and>benzotriazoles have been successfully applied as photostabilizers for fibers, plastics or dyestuffs, for example, and for the protection of human skin from harmful UV irradiation. For a brief summary, see (B-71MI41103). 

One of the primary methods used for protecting liquid and solid photolabile pharmaceuticals and preparations from photodegradation is to package them in colored glass or plastic. Colored/pigmented containers are also suitable for the photostabilization of sterile preparations, such as eye drops. 

Most neat plastic materials used for packaging of pharmaceutical products absorb UV-VIS radiation below 280 nm. Therefore, photostabilization by plastic packaging is only possible by the addition of colorants, UV absorbers, or pigments to the plastic mass. 

One additional use for aluminum foil is as mantles (sleeves) to cover the primary packaging of photosensitive products. For example, mesalazine rectal solutions have been photostabilized using this approach. Without this protective measure the product, packaged and stored in transparent plastic containers, would develop a strong brown discoloration, when exposed to normal indoor daylight (41). 

Hydroxy-4-n-octoxybenzophenone is an effective photostabilizer for a variety of plastic systems. It may be used in food packaging materials as an antioxidant and stabilizer and in addition may be used as a stabilizer in petroleum wax. When used in packaging materials, it prevents UV-radiation from reaching the stored product and increases the stability of the container. 

The timescale for these processes to occur is dependent on the type of polymer, the content of added materials, plasticizers and additives such as antioxidants, photostabilizers, thermostabilizers, etc), filling and reinforcing materials such as particles and fibers, etc. 

The photooxidation of polymers on the other hand continues to remain at a low profile. Bio- and photodegradable plastics continue to be useful for agricultural usage, although interest here is again minimal. The same applies to polymer photostabilization, where commercial applications dominate very much, with emphasis on the practical use of stabilizers. For dyes and pigments stability continues to be a major issue in so far as this section is concerned. 

This paper is an interpretation of the primary photo-oxidation process due to singlet oxygen and some problems in photostabilization of polymers and plastics. Electronic structure, generation, properties and role of singlet oxygen in polymer chemistry are discussed. The reader is also referred to a number of review articles on the subject (1-11). 

DPA are excellent AO and HS in various rubbers and plastics, lubricating greases and oils [27], They have some antifatigue effect and provide no antiozonant protection [1], Their photostabilization activity is rather low. DPA may be used in radiation resistant polymer compositions for cable coatings in nuclear power plants [28]. Various 4,4 -disubstituted DPA contain a fert-alkyl (e.g. 1,1,3,3-tetramethylbutyl 9a, 1,1-dimethylethyl or their combinations), other alkyls or aralkylgroups (e.g. a,a-dimethylbenzyl, 9b) [1,3,24]. A tertiary amine 3-[(diphenylamino)methyl]-5-[(diphenylaminomethyl)thio]-l,3,4-thiadiazolidine-2 (14) was proposed as AO for rubbers [29],... 

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... 

Compounds such as 2-(2-hydroxy-5-methylphenyl)benzotriazole 2 (tinuvin P) absorb UV light in the region of 300 - 400 nm and are used as radiation protection agents, e.g. for the prevention of sunburn, and also as photostabilizers for plastics, rubber and chemical fibres [155]. Tests have been carried out as to the suitability of polynitro-l-phenylbenzotriazoles as explosives [156]. 

J. E. Pickett, Photostabilization of Plastics by Additives and Coatings, in Durability of Coatings and Plastics, R. Ryntz, ed., Hanser Publishers, 2001, pp. 73-98. 

Feller, R. L., Curran, M., Bailie, C. (1981). Photochemical studies of methacrylate coatings for the conservation of museum objects. In S. P. Pappas, F. H. Winslow (Eds.), Photodegradation and photostabilization of coatings based on a symposium sponsored by the Division of Organic Coatings and Plastics Chemistry at the 179th meeting of the American Chemical Society, Houston, Texas, March 26-28, 1980 (pp. 183-196). American Chemical Society, 151. 

It has been shown in the preceding sections that raw polymers are highly susceptible to degradative oxidation. The success of plastic materials, which find apphca-tions in practically any aspect of life, relies heavily on the performance of polymer stabilizers, 70% of which are used for polyolefins. According to their principal protection activity, common polymer stabilizers are conventionally classified as antioxidants, photoantioxidants, photostabilizers, metal deactivators, antiozonants, and heat stabilizers for PVC. 

Most polymers have to be processed into plastics for use in the production of commercially available articles. Plastic materials normally include polymer and a few per cent of one or more different additives such as anti-oxidants, photostabilizers, pigments, dyes, lubricants, fillers, etc. Prior to practical application, polymers are usually granulated and further mixed with other polymers (polymer blends), additives, etc. Such mixtures are further melted in an extruder in the processing step and converted to the final products. 

Besides HALS, another class of photostabilizers used commercially in plastics, elastomer-modified plastics and coatings include preventive stabilizers acting by a preferential absorption of the radiation and transforming the harmful radiation into harmless thermal energy. The role of UVAbs is to inhibit the photo-oxidation of polymeric materials, therefore, they are very important additives used to prevent plastics from photo-degradation by UV. The UVAbs include derivatives of ben-zophenone, triazole, triazine, V-(substituted phenyl) itaconimides, etc. [110-113]. Stabilizers embedded into the polymer matrix are the most photo-reactive components of the system [114—120]. 

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