Photostabilizers

Hydroxyphenylbenzotriazoles have long been used as photostabilizers of polymers. For further details, see the 1990 review by Rabek B-90M1 401-01). The photostabilizing mechanism of o-hydroxyphenylbenzotriazoles has been well investigated 87JPC1408). The absorption and fluorescence spectra of these photostabilizers have been the subjects of many publications (see Section 4.01.2.9). There are a great number of commercially available 2-(o-hydroxyphenyl)benzotriazoles. They have been widely used in the photostabilization of polypropylene, polystyrene, polyacrylates. 

polyamides, unsaturated crosslinked polyesters, ABS, and wood B-90MI 401-01 . Di- and tri-benzotriazole photostabilizers, such as (874) and (875) are synthesized from 2-nitro-benzenediazonium salts and an excess of 1,3-dihydroxybenzene or 1,3,5-trihydroxybenzene 85Mi 40i-0l . The dibenzotriazole derivatives (874b and 875b) can be used as polymerizable acrylic UV absorbers 84PB237 . A few or/Ao-urethane and -trimethylsilane substituted 2-phenylbenzotriazoles (876 and 877) show similar photostabilization activity b-90MI 401-01 . Weather resistance of low-density polyethylene is improved by the addition of a benzotriazole-type photostabilizer 90M140i-04 . 

The basic free radical chain mechanism involved in light-initiated oxidation of PP is essentially the same as that involved in its thermally-initiated oxidation, with the main difference lying in the rate of chain initiation which is much higher in photooxidation resulting in a shorter 

Photostabilizers for pol)rpropylene and factors affecting their performance 

To be effective photostabilizers, these compounds must satisfy not only the basic chemical (i.e. intrinsically active) and physical (i.e. solu-blility, low diffusion and volatility, nonextractability) requirements, but must also be stable to UV light and must withstand continuous periods of exposure to UV light without being destroyed or effectively transformed into sensitizing products. Other factors that can affect the ultimate photostability of the polymer are sample thickness, polymer crystallinity and presence of other additives, e.g. pigments, fillers [1, 2]. 

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). 

Catalyst for Ionic daeompoaHlon of paroidda paroxidolytic antioxidant 

Quenchers Quenchers are used to capture the excess energy of excited chromo-phores [Eqs. (94) and (95)], which is then dissipated as vibrational energy, before they can initiate harmful reactions. 

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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Copper-based thermal stabilizers are also effective photostabilizers for nylon. They can be added before polymerization, or the soluble salts (eg, CuSO can be appHed to fibers as part of the finish or to fabrics as post-treatments. The effectiveness of the copper salt—alkah haUde system added to prepolymer in retarding phototendering and photoyeUowing of the resulting spun yam is illustrated in Figure 5. 

H-l,2,3-Benzotriazole, 2-phenyl-oxidation, 5, 721 as photostabilizers, 5, 731 synthesis, 5, 722 Benzotriazoles acylation, 5, 54 antifogging agents... 

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. 

The photostabilizing efficiency of polymer-bound HALS in i-PP and E-P copolymers were studied and compared with commercial HALS (i.e., Tinuvin 770 and Chimassorb 944) by measuring the carbonyl index at 1720 cm. Plots of carbonyl index versus irradiation... 

J. F. Rabek and B. Ranby, Photodegradation, Photo-oxidation and Photostabilization of Polymers, Interscience, New York (1975). 

Photodegradation of polyethylene waste can be markedly accelerated via its grafting with acrylamide [98], In contrast, photostabilization of polyethylene and polypropylene can be achieved as a result of the grafting of 2-hydroxy-4-(3-methacryloxy-2-hydroxy-propoxy) benzophenone using gamma radiation [ 147], In this case, the grafted compound acting as a U V stabilizer is chemically bound to the backbone chain of the polymer and its evaporation from the surface can be avoided. 

When the photostabilization of a polymer material is to be obtained through such a surface treatment process, it is all important to make sure that the protective effect will last throughout the service life and therefore to ensure a long-term adhesion of the coating onto the substrate. This can be best achieved by promoting a grafting reaction between the two elements (20). For that purpose, the photoinitiator was partly incorporated in the top layer of the PVC plate by a surface treatment with an acetone solution. Upon UV-irradiation of the resin-coated sample, the following reactions are expected to occur ... 

Ultraviolet radiation, effect on commercial polymers, 201 Ultraviolet-cured coatings hydroxy-benzotriazolc, 213 photostabilization of PVC, 213 Ultraviolet-visible spectrum,... 

In Photodegradation and Photostabilization of Coatings Pappas, S., et al. ACS Symposium Series American Chemical Society Washington, DC, 1981. 

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