Stabilizers ultraviolet absorbers

Plasticizer Heat stabilizer Ultraviolet absorber Flame retardant Biocide... 

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). 

Further improvement in light stability may be achieved by addition of small quantities of ultraviolet absorbers. Typical examples include phenyl salicylate, 2,4-dihydroxybenzophenone, resorcinol monobenzoate, methyl salicylate and stilbene. 

Since acetal resins are degraded by ultra violet light, additives may be included to improve the resistance of the polymer. Carbon black is effective but as in the case of polyethylene it must be well dispersed in the polymer. The finer the particle size the better the ultra violet stability of the polymer but the poorer the heat stability. About 1.5% is generally recommended. For white compounds and those with pastel colours titanium dioxide is as good in polyacetals as most transparent ultraviolet absorbers, such as the benzophenone derivatives and other materials discussed in Chapter 7. Such ultraviolet absorbers may be used for compounds that are neither black, white nor pastel shade in colour. 

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. 

In terms of weatherabilityf polystyrene does not exhibit ultraviolet stability and is not considered we ather-re sis tan t as a clear material. Continuous, long-term exposure results in discoloration and reduction of strength. Improvement in weatherability can be obtained by the addition of ultraviolet absorbers, or by incorporating pigments. The best pigmenting results are obtained with finely dispersed carbon black. 

Ultraviolet absorbers continue to be the most widely used stabilizers. Such products must have long-term stability to ultraviolet light, be relatively nontoxic, heat stable, have little color, must not sensitize the substrate, and must be priced at levels which the plastics processor can tolerate. The principal classes of chemicals meeting these requirements at present are the 2-hydroxybenzophenones, and 2-(2/-hydroxyphenyl)benzotriazoles, substituted acrylates, and aryl esters. Typical compounds representative of these classes are 2-hydroxy-4-octoxybenzophenone, 2-(2 -hy droxy-5 -me thylphenyl) be nzotn azole, ethyl-2-cyano-3,3-diphenyl acrylate, diinethyl / -methoxybenzylidene maJonate, and / -ter -octylphenyl salicylate. 

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. 

The stabilizers chosen for evaluation include different types of heat and light stabilizers selected to represent different mechanisms of action as well as chemical compositions (ArJi). Types of stabilizers evaluated include benzotriazole and benzophenone light stabilizers [ultraviolet (UV) light absorbers], hindered amine light stabilizers (HALS, catalytic radical scavengers), hindered phenol heat stabilizers (antioxidant radical scavengers), and thioester heat stabilizers (antioxidant hydroperoxide decomposers). 

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. 

Ultraviolet absorbers are added to polypropylene resins which are used where exposure to sunlight is high. These chemicals are essentially ineffective as heat or processing stabilizers. 

Here we discuss a new class of polypropylene stabilizers—the polymeric phenolic phosphites. These compounds exhibit unique, broad-spectrum activity which may allow simplification of polypropylene stabilizer systems. The most active species are synergistic with thiodipro-pionate esters, are effective processing stabilizers when used alone or with other compounds, and contribute to photostability. Compounds of this type appear to function as both free radical scavengers and peroxide decomposers, and through a mechanism not yet completely understood, allow significant reductions in the concentration of ultraviolet absorbers required to achieve high levels of photostability. 

The significance of the interaction between PPP and ultraviolet absorbers, such as the hydroxybenzophenones, becomes evident when the high cost of absorbers is considered. The potential cost for polymeric phenolic phosphites is low by comparison. By using a polymeric phenolic phosphite in place of a normal hindered phenolic in the heat-stabilizer system, enhanced photostability is obtained, and less ultraviolet absorber is required for a given degree of stability. 

Stabilizers. Ideally, the metallic stabilizer should perform four major functions in a vinyl system (1) HC1 acceptor (2) ultraviolet absorber (3) antioxidant (4) reactive dieneophile. 

The absorption spectrum for a typical stabilizer of this type, 2-hydroxy-4-dodecyloxybenzophenone, is shown in Figure 8 (10) along with that of poly (ethylene—CO). Over the near ultraviolet range, the absorption coefficient of the stabilizer is much greater than that of the polymer, and even at low concentrations the stabilizer will absorb most of the light, thereby protecting the polymer. 

Among the types of ultraviolet absorbers that may be used as stabilizers are salicylates, o-hydroxybenzophenones, o-hydroxyarylbenzotri-azoles, and certain acrylonitriles. The stabilization of polystyrene by other additives has also been reported. Matheson and Boyer (10) found that certain aliphatic amines and amino alcohols improved the light stability of the polymer. 

Ultraviolet Absorber-Phenol Combinations. Ultraviolet absorbers are commonly used to stabilize polystyrene against light. Experimentally, it was found that their efficiency in this polymer does not increase linearly with increasing concentration. Figure 4 shows that above 0.2% concentration only a minimal increase in stabilization is obtained with 2-hydroxy-... 

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. 

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