UV-stable antioxidants

UV absorbers of the benzophenone and benzotriazole types are widely used in polyolefins. Carlsson and Wiles (1976) showed that these molecules are much more effective when incorporated into the polymer than when used as a screen in solution in front of the exposed polymer. They derive their effect in part from being UV-absorb-ers and in part from being UV-stable antioxidants. 

The OIT values of composite materials reflect two principal factors (a) how much of the antioxidants is present in the material and (b) how resistant is the material itself to thermal oxidation. The OIT values of the commercial composite materials determine the sensitivity of the deck boards to oxidative degradation, hence, the lifetime of the boards, first of all in the south. If they are heat/UV stable in the south, no need to worry in the north. 

The phenolic UVAs also trap alkoxyl radicals formed from hydroperoxides during photolysis. By removing initiating radical species from the system by reaction (3.8), they are thus also light-stable chainbreaking antioxidants and UVAs. That is they are autosynergistic because they absorb UV light and also act as UV stable CB antioxidants. 

The exterior durabiHty of relatively stable coatings can be enhanced by use of additives. Ultraviolet absorbers reduce the absorption of uv by the resins and hence decrease the rate of photodegradation. Eurther improvements can be gained by also adding free-radical trap antioxidants (qv) such as hindered phenols and especially hindered amine light stabilizers (HALS). A discussion of various types of additives is available (113). 

David et al. [184] have shown that cool on-column injection and the use of deactivated thermally stable columns in CGC-FID and CGC-F1D-MS for quantitative determination of additives (antistatics, antifogging agents, UV and light stabilisers, antioxidants, etc.) in mixtures prevents thermal degradation of high-MW compounds. Perkins et al. [101] have reported development of an analysis method for 100 ppm polymer additives in a 500 p,L SEC fraction in DCM by means of at-column GC (total elution time 27 min repeatability 3-7 %). Requirements for the method were (i) on-line (ii) use of whole fraction (LVI) and (iii) determination of high-MW compounds (1200 Da) at low concentrations. Difficult matrix introduction (DMI) and selective extraction can be used for GC analysis of silicone oil contamination in paints and other complex analytical problems. 

The antioxidative role of anthocyanins and flavones in providing stable blue flower colors in the angiosperms has been studied. The leaf anthocyanins and flavones showed a protective effect against UV-B radiation. These protective UV-B flavonoids in angiosperms were mainly kaempferol (9), quercetin (10), cyanidin (51), luteolin (65), and isoorientin C-glycoside (flavone C-glycoside, 66) (Fig. 14) [35]. 

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. 

Since UV absorbers cannot completely prevent the initiation reactions above, we felt that better antioxidants were necessary to obtain improved long-term photo-oxidative stability of polyolefins. We reasoned that stable radicals which scavenge radicals formed in the oxidative process might compensate for the imperfections of the absorbers. On the basis of the assumption that the diaryl nitroxyl radicals, rather than their parent diarylamines, are the active antifatigue species in rubber protection, we first attempted to obtain new stable nitroxyl radicals. 

Fully hindered amines are excellent UV stabilizers, but poor thermal antioxidants, for polyolefins. We have found that partially hindered bicyclic amines, such as 3,3-dialkyldecahydroquinoxalin-2-ones, are excellent UV stabilizers and also excellent thermal antioxidants as well. The oxidation of these bicyclic amines with m-chloroperbenzoic acid was studied by electron spin resonance spectroscopy. They form stable, partially hindered nitroxyl radicals (6-line spectra). However, these primary radicals are easily oxidized to a new, fully hindered nitroxyl radical (3-line spectra) which are also very stable. Field desorption mass spectroscopic studies of the oxidation of these amines show that the primary nitroxyl radicals, which form first, lose two hydrogen atoms and add an oxygen. Thus, these bicyclic partially hindered amines are unique in being both stable and at the same time hydrogen atom donors. 

In order for a stabilizer to function well it must also be sufficiently stable to UV light to survive the irradiation process. Photolysis studies were performed with a number of representative antioxidants of the hydrogen donor and peroxide decomposer type (Table III). When the stabilizers are photolyzed alone in n-hexane, conversions range from 10-22% with the hindered phenol, HD-1, the least stable and the aryl phosphite, PD-4, the most stable. When solutions containing both the antioxidant and the photoinitiator are photolyzed, the photoinitiator accelerates the decomposition of the antioxidants by a factor of about five. This results in the total decomposition of the HD-1 which can no longer be detected. The other antioxidants are not completely decomposed. 

Elvax 150 softens to a viscous melt above 70°C, and therefore Is not suitable for temperature service above 70°C when employed In a fabricated module. A cure system was developed for Elvax 150 that results In a temperature-stable elastomer ( ). Elvax 150 was also compounded with an antioxidant and UV stabilizers, which Improved Its weather stability and did not affect Its transparency. The formulation of the encapsulation grade ethylene vinyl acetate Is given In Table I. These Ingredients are compounded Into Elvax... 

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