UV-stabilizers

PVDC copolymers also tend to be subject to thermal degradation, undergoing the same general types of reaction as PVC, so they also require the use of heat stabilizers. 

Combining different stabilizers acting with different mechanisms may trigger a synergistic effect that can provide excellent protection to the polymer. On the other hand, some combinations result in antagonistic interactions that reduce the efficacy of the stabilizers. 

There is a growing evidence that fillers may adversely affect the performance of UV stabilizers, including practical immobilization and loss of their effect on stabi- 

These data coming from different sources indicate a growing concern that UV stabilizers, which are expensive additives, may not per-foim up to expectations. Table 13.2 shows the amounts of various stabilizers absorbed on carbon black. The absorbed quantities are substantial to raise further questions regarding the effect of this absorption on the performance of stabilizers, considering that carbon black is present in many formulations either as filler or pigment. Table 13.3 shows that carbon black is not the only filler which absorbs UV stabilizers. Fillers used in this experiment, conducted in a cyclohexane suspension, were at low concentrations. The concentration of HALS was typical of many formulations. Results show that tertiary HALS is superior to secondary 

HALS but that absorption on fillers occurs in both cases. Calcium carbonate is the most absorbing filler in this experiment. 

It can be postulated that three factors may play a role in stabilization in the presence of fillers. One factor is the discussed absorption which immobilizes stabilizer to the extent that it cannot perform its function. The second is its desorption capability, which, if it exists, may enhance the performance of stabilizer due to its better retention. However, it should be mentioned that photochemical changes occur at very short time scales therefore, there must always be a sufficient concentration of free (not absorbed) stabilizer to react with radicals. The third factor is the effect of filler on structure and related stability of stabilizer. The reduced stability of polypropylene in the presence of some stabilizers in Table 13.4 can be explained only by the formation of degradation products. It 

This harmless dissipation of the UV energy stabihzes the polymer. Another type of UV stabilizer is a class known as hindered amine light stabilizers (HALS). Many of them are based upon tetramethylpiperidines. 

Some specific examples are shown with their original Ciba-Geigy Corp. names, but they are sold under other names by other companies. 

HALS can be very effective. One explanation of their effectiveness is that the hindered amine oxidizes to form a nitroxyl radical, which can react with a polymer radical to form an alkoxy amine. The alkoxy amine can terminate peroxy radicals and in the process regenerate the nitroxyl radical. 

The following measures can be taken to stabilize polymers against light-induced degradation prevention of light absorption, deactivation of excited states, destruction of already formed per compounds, and preven- 

Addition of UV absorbers can greatly reduce the amount of light absorbed by the polymer. Pigmented polymers often reflect incident light so well that few free radicals can be formed. Carbon black used as a filler absorbs UV light very well, and is also effective as a free radical sink. Titanium dioxide, however, is a sensitizer and promotes degradation. 

Each transparent polymer must be stabilized against light-induced degradation by incorporation of UV stabilizers. UV absorbers should absorb UV light without the formation of free radicals. For commercial purposes, they should absorb under 420 nm, since the maximum sensitivity of many plastics lies between 290 and 360 nm. UV absorbers for cosmetic purposes, however, should absorb under 320 nm, since the human skin shows a sharp maximum sensitivity at 297 nm. 

Some UV absorbers, such as, for example, o-hydroxy benzophenones or 2-(2 -hydroxyphenyl)benztriazoles, absorb the incident light in the hydrogen bonds and convert it into IR radiation. Other compounds, such as the phenyl salicylates, first change photochemically into o-hydroxy benzophenones. 

In addition to being capable of absorption and having a protective effect, UV absorbers for plastics also must be compatible with the substrate, possess a high light-fastness, remain stable under the working conditions, not be poisonous, and not impair color-fastness, e.g., in fibers. 

2-Hydroxybenzophenones 2-hydroxyphenylbenzotriazoles Sterically hindered amines Salicylates 

Cinnamate derivatives Resorcinol monobenzoates Oxanilides p-hydroxybenzoates 

6-terf-Butyl-2-(5-chloro-2H-benzotriazol-2-yl)-4-methylphenol (U-vinul 3026) 

Ethyl-2-cyano-3,3-diphenylacrylate (Uvinul 3035) (2-Ethylhexyl)-2-cyano-3,3-diphenylacrylate (Uvinul 3038) 

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Uv methods Uv-quenchers Uv radiation uv Radiation Uv Radiation Uv stability Uv stabilizers... 

Decabrom has poor uv stabiUty ia styrenic resias and causes significant discoloration. The use of uv stabilizers can minimize, but not eliminate, this effect. For styrenic apphcations that require uv stabiUty, several other brominated flame retardants are more suitable. In polyolefins, the uv stabiUty of decabrom is more easily improved by the use of stabilizers. 

TetrabromophthalicAnhydride. Tetrabromophthalic anhydride [632-79-1] (TBPA) is widely used as a reactive flame retardant in unsaturated polyesters as weU as the precursor to a number of other fine retardants. Polyesters prepared from this compound have relatively poor photochemical stabiUty and tend to discolor upon exposure to light. This tendency to discolor can be reduced, but not eliminated, by the use of uv stabilizers. 

The chemical industry manufactures a large number of antioxidants (qv) as well as uv stabilizers and their mixtures with other additives used to facilitate resin processing. These companies include American Cyanamid, BASE, Ciba—Geigy, Eastman Chemical, Elf Atochem, Enichem, General Electric, Hoechst—Celanese, Sandoz, and Uniroyal, among others. The combined market for these products in the United States exceeded 900 million in 1994 and will reach 1 billion in the year 2000. 

Methyl methacrylate is often used in combination with styrene to improve light transmission and uv stabiUty in fiame-retardant glazing appHcations. Phosphate ester (triethyl phosphate) additives are also included to supplement fiame-retardant efficiency benzophenone uv stabilizers are required to prevent yellowing of these uv-sensitive resins. 

Other uses of 4-methylphenol include its conversion to a benzotriazole uv stabilizer, 2-(2 -hydroxy-5 -methylphenyl)benzotriazole [2440-22-4] (45). The benzotriazole-based uv stabilizer makes possible the extended use of thermoplastics in outdoor appHcations. Other minor appHcations for 4-methylphenol include its use in the production of novolak or resole phenoHc resins. It is also used in the production of certain dyes and fragrances (Table 3). 

Another use of 4-/ f2 octylphenol is ia the productioa of uv stabilizers. 4-/ f2 -Octylpheaol reacts with sulfur dichloride to yield the thio-hisphenol derivative, which thea reacts with nickel acetate to form 2,2 -thiobis(4-/ f2 octylphenolate)-A/-butylamiQe nickel [14516-71 -3]. This type of stabilizer is widely used in the production of outdoor carpeting based on polypropylene fibers. Nickel compounds give a green discoloration which limits their apphcatioas. A second class of uv stabilizers based on the benzotriazole stmcture. 2-(2 -hydroxy-5 -/ f2 octylphenyl)benzotriazole [3147-75-9] is produced from 4-/ i -octylphenol (55). 

A major use for 2,4-di-/ f2 -amylphenol is in the production of uv stabilizers the principal one is a benzotriazole-based uv absorber, 2-(2 -hydroxy-3, 5 -di-/ f2 -amylphenyl)-5-chlorobenzotriazole [25973-55-17, which is widely used in polyolefin films, outdoor furniture, and clear coat automotive finishes (56). Another significant use for 2,4-di-/ f2 -amylphenol is in the photographic iadustry. A number of phenoxyacetic acid derivatives of... 

The growth rate for 2,4-di-/ f2 -amylphenol is predicted to be above GNP growth rate, mainly driven by the use of the benzotriazole derivative as a uv stabilizer for clear-coat appHcations in auto finishes (see Table 3). 

The largest use for 2,4-dicumylphenol is in a production of a uv stabilizer of the benzotriazole class, 2-(2 -hydroxy-3, 5 -dicumylphenyl)benzotriazole [70321-86-7] which is used in engineering thermoplastics where high molding temperatures are encountered (67). The high molecular weight of... 

Rubber. The mbber industry consumes finely ground metallic selenium and Selenac (selenium diethyl dithiocarbamate, R. T. Vanderbilt). Both are used with natural mbber and styrene—butadiene mbber (SBR) to increase the rate of vulcanization and improve the aging and mechanical properties of sulfudess and low sulfur stocks. Selenac is also used as an accelerator in butyl mbber and as an activator for other types of accelerators, eg, thiazoles (see Rubber chemicals). Selenium compounds are useflil as antioxidants (qv), uv stabilizers, (qv), bonding agents, carbon black activators, and polymerization additives. Selenac improves the adhesion of polyester fibers to mbber. 

Additives are used extensively in compounding this resin. Antioxidants (qv), uv stabilizers, especially carbon black (qv), and fillers (qv) such as glass fibers, sihca, or clay provide properties desirable for various purposes. 

Many materials are subject to deterioration by ultraviolet light (UV). particularly many of the plastics and fiberglass materials. Fiberglass materials for outside use should be specified as UV-stabilized, and most plastics installed outdoors should be carbon-impregnated (black in color). It is particularly recommended that plastic cable ties, which secure cables in cable trays, be carbon-impregnated if installed outdoors. 

The ESIPT mechanism of phototautomerization allows the employment of types 346 and 347 compounds and their analogs as UV stabilizers useful for synthetic polymers. Such a stabilizer -rapidly converts the energy ab-... 

A multidimensional system using capillary SEC-GC-MS was used for the rapid identification of various polymer additives, including antioxidants, plasticizers, lubricants, flame retardants, waxes and UV stabilizers (12). This technique could be used for additives having broad functionalities and wide volatility ranges. The determination of the additives in polymers was carried out without performing any extensive manual sample pretreatment. In the first step, microcolumn SEC excludes the polymer matrix from the smaller-molecular-size additives. There is a minimal introduction of the polymer into the capillary GC column. Optimization of the pore sizes of the SEC packings was used to enhance the resolution between the polymer and its additives, and smaller pore sizes could be used to exclude more of the polymer... 

The UV stabilizing action of nickel and iron complexes (e.g., NiDRC and FeDRC) is strongly concentra-... 

The early recognition of the role of stable nitroxyl free radicals, e.g., 2,2,6,6-tetramethyl-4-oxopiperidine, and their hindered amine precursors, in polymer stabilization soon led to the development of the hindered amine light stabilizer (HALS) class of photoantioxidants. The first HALS, Tinuvin 770, AO-33, (commercialized in 1974) proved to offer much higher UV-stabil-ity to polymers than any conventional UV-stabilizer available at the time such as UV-absorbers, nickel compounds and benzoates. Table 3). 

Irg 1076, AO-3 (CB), are used in combination with metal dithiolates, e.g., NiDEC, AO-30 (PD), due to the sensitized photoxidation of dithiolates by the oxidation products of phenols, particularly stilbenequinones (SQ, see reaction 9C) (Table 3). Hindered piperidines exhibit a complex behavior when present in combination with other antioxidants and stabilizers they have to be oxidized initially to the corresponding nitroxyl radical before becoming effective. Consequently, both CB-D and PD antioxidants, which remove alkyl peroxyl radicals and hydroperoxides, respectively, antagonise the UV stabilizing action of this class of compounds (e.g.. Table 3, NiDEC 4- Tin 770). However, since the hindered piperidines themselves are neither melt- nor heat-stabilizers for polymers, they have to be used with conventional antioxidants and stabilizers. 

Since 2-hydroxy-4-alkoxybenzophenones are widely used to stabilize polystyrene, flexible and rigid PVC, celluloses, acrylics, and polyolefins such as PE and PP, the polymeric UV stabilizers shown in Table 1 are used with polystyrene, polymethylmethacrylate, and cellulose triacetate (CTA). The polymeric-HALS are used in polyolefins. 

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