Antioxidants and ultraviolet absorber

In view of this wavelength dependence on photooxidation further studies were carried out to determine the role of antioxidants and ultraviolet absorber-antioxidant combinations in photooxidation reactions. 

The response of polyurethanes to thermally activated autoxidation depends upon polymer structure. In general, polyurethane degradation by this mechanism is suppressed by the addition of antioxidant to the polymer. Ultraviolet initiated autoxidation is suppressed by a suitable screen (e.g. carbon black, titanium dioxide) or a combination of antioxidant and ultraviolet absorber. Irganox 1010 and Tinuvin P (Ciba-Geigy) are particularly suitable antioxidant and ultraviolet absorbers, respectively, for polyurethanes. Polyurethane structures with enhanced resistance to ultraviolet initiated autoxidation may be possible. 

Denning and Marshall [6] devised a method in which polymer extracts are examined for antioxidants and ultraviolet absorbers at two column temperatures in order to overcome the problem that whereas some componnds have a relatively low retention time, others of higher molecular weight have very long retention times at 250 "C. 

Figures 4.4 and 4.5 are gas chromatograms representing the separation of groups of antioxidants and ultraviolet absorbers identified at 250 C and at 300 C.

Dissolve sample A in a small amount of chloroform and make up to volume in a 5 cm calibrated flask. Transfer sample B quantitatively to a 5 cm calibrated flask and dilute to volume with ethanol. Use sample A solution for the determination of antioxidant and ultraviolet absorber, and sample B solution for the determination of organotin compounds. 

Such structural changes are a consequence of chemical reactions of which the most common are oxidation, ozone attack, dehydrochlorination and ultraviolet attack. (Reactions due to high-energy radiation or to high temperature are not considered here as causing natural aging.) Over the years many materials have been introduced as antioxidants, antiozonants, dehydrochlorination stabilisers and ultraviolet absorbers—originally on an empirical basis but today more and more as the result of fundamental studies. Each of these additive types will be eonsidered in turn. 

The mechanisms of inhibition by peroxide decomposers, metal deactivators, and ultraviolet absorbers are fairly well understood in general terms, although many details of the individual reactions remain to be elucidated. Classifying a preventive antioxidant into one of the three categories above will only rarely describe its entire function. The dual behavior of dialkyl dithiophosphates in the liquid phase has been mentioned. Many other phosphorus- and sulfur-containing antioxidants commonly classified as peroxide decomposers can also act as chain breakers. Similarly, the structure of many metal deactivators and ultraviolet absorbers indicates that they must also have some chain-breaking activity. 

Four main types of antioxidants are commonly used in polypropylene stabilizer systems although many other types of chemical compounds have been suggested. These types include hindered phenolics, thiodi-propionate esters, aryl phosphites, and ultraviolet absorbers such as the hydroxybenzophenones and benzotriazoles. Other chemicals which have been reported include aromatic amines such as p-phenylenediamine, hydrocarbon borates, aminophenols, Zn and other metal dithiocarbamates, thiophosphates, and thiophosphites, mercaptals, chromium salt complexes, tin-sulfur compounds, triazoles, silicone polymers, carbon black, nickel phenolates, thiurams, oxamides, metal stearates, Cu, Zn, Cd, and Pb salts of benzimidazoles, succinic acid anhydride, and others. The polymeric phenolic phosphites described here are another type. 

Good aging Generally requires the addition of antioxidants and ultraviolet (UV) absorbers if exposed for a long term to weathering, but the elastomer has good retention of properties in normal use. 

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. 

Antioxidants and UV absorbers (see Section 11.2) are also used as secondary PVC stabilizers [67]. They become necessary, since double bonds created in degraded PVC can absorb radiation in the ultraviolet range which energy is high enough to produce more dehydrochlorination. Furthermore, these radiations could decompose peroxides or hydroperoxides formed or already present in PVC and induce radical promoted oxidation of the polymer chains [68,69]. The wavelength range that may produce these undesired effects is believed to go from 310 to 400 nm [79-82]. UV absorbers used for PVC are mainly benzo-triazol, benzophenone derivatives, zinc oxide, and titanium oxide. 

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). 

The peroxide decomposer will drastically reduce the number of radicals, which can then be more effectively mopped up by the chain-breaking materials. A widely used combination is 4-methyl-2,6,di-t-butylphenol and dilauryl thiodipropionate. It is possible to envisage most powerful combinations where a chain-breaking antioxidant, a regenerating agent, a peroxide decomposer, a metal deactivator and an ultraviolet absorber are all employed together. 

The plasticizer-range alcohols are largely used as feedstock for production of high molecular weight diesters of phthalic, adipic, azelaic, and sulftiric acids. All these are used primarily in plasticizers for polyvinyl chloride (PVC) and other plastics. The plastics industry also uses them as additives for heat stabilization, to control the viscosity of PVC plastisols, ultraviolet absorbers, flame retardants, and antioxidants. They are also found in synthetic, lubricants, agricultural chemicals, and defoamers. 

Polystyrene light stabilization has been achieved with a variety of ulfravioler absorbers including the benzophenones, benzotriazoles. and salicylates. While yellowing of polystyrene occurs in many applications, it is particularly noticeable in diffusers used with fluorescent lights. This problem has been effectively solved by using ultraviolet light absorbers. In this instance, superior stabilization is achieved when the ultraviolet absorber is used in conjunction wirh specific antioxidants. 

Figures 6, 7, and 8 show the stabilizing efficiency of the combinations consisting of the above absorbers and the antioxidant 2,4,6-tri-fert-butyl-phenol, on Fade-ometer exposure. The total additive concentration in each polystyrene sample was 0.25%. The data show that the rate of discoloration of the polymer was inhibited to a greater extent with the ultraviolet absorber-antioxidant combinations than with the absorber alone. Only a small amount (0.075%) of the antioxidant was required for increased effectiveness of the combination.

Certain phenolic antioxidants in combination with ultraviolet absorbers improve substantially the light stability of polystyrene. Combinations of this type can be used advantageously to prevent polymer discoloration in indoor fluorescent lamp fixture applications. However, studies of this type require further investigation, and an explanation must be sought for the striking differences in behavior of the various antioxidants tested. 

The fact that XXXV displays light-protective activity, decelerating CDA destruction at ultraviolet irradiation not only in vacuum (Figure 2. 16, curve 3), but in the air (Figure 2.16, curve 2), moreover, curves 2 and 3 have the character close to curve 1, that undoubtedly testifies in favour of XXXV action not only as ultraviolet absorber, but also as an antioxidant. Possibility to decelerate the rate of CDA photodestruction by the products of inhibitors transformation is important, too. 

The photodegradation of polymers has been studied mainly from the practical angle, since polymers have many commercial uses. Various kinds of stabilizers, such as ultraviolet absorbers and antioxidants, have been developed but the mechanism of stabilization has not yet been completely clarified from the point of view of the photophysical and photochemical processes involved in polymer systems. 

Ultraviolet Absorbers, Light Stabilizers, and Antioxidants. Ultraviolet absorbers and light stabilizers can be added to urethane coatings in order to increase their resistance to discoloration and to outdoor weathering. UV absorbers are employed to eliminate the radiation in the region of 334 and 411 um that sensitize the polyurethanes and promote an autoxidation process. Light stabilizers on the other hand do not absorb UV radiations but inhibit or control the degradation by other means. 

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