Stabilization of Polymers

Polyolefins In addition to stabilization of polyolefins against thermal oxidation to reduce the sensitivity to light, stabilization against exposure to light is required for articles to be used outdoors as well as those intended for indoor use [93]. Light stabilizers include UV absorbers of the benzotriazole and the benzophenone types (except for thin sections), HALS, and nickel-containing stabilizers. The latter are used for thin sections such as tapes and films and for surface protection. The type of stabilizer is dictated by the type of polyolefin, its thickness, application, and desired lifetime of the article [20].

The performance of low-molecular-weight HALS for stabilization of polypropylene fibers, films, and tapes is superior to that of other types of light stabilizers even at one tenth the concentration. The performance increases significantly in the concentration range 0.05-1.2%. However, when a pigment is present that acts as UV screener, the contribution of HALS to the light stability of PP multifilaments is considerably reduced. Polymeric HALS is superior to low-molecular-weight HALS in heat-treated fiber as well as in fiber treated with acrylic latex. The inferior performance of low-molecular-weight HALS is due to its tendency to migrate with heat treatment in the former case and to its extraction by the acrylic latex in the latter case.

The combination of HALS with a UV absorber is used in films of polypropylene and polyethylene as well as in thick sections. In films of LDPE, nickel quenchers were commonly used with a UV absorber, except in a very thin film, in which a higher concentration of nickel stabilizer is superior to the combination. The low-molecular-weight HALS are not sufficiently compatible with LDPE at the concentrations necessary, possibly as high as 2%, for the required protection. Incompatibility of HALS with LDPE has been overcome with the development of polymeric HALS. It is considerably better than either the UV absorber or nickel quencher or combinations of the two. For thicker films (100-200 ^m), the combination of a benzophenone-type UV absorber with polymeric HALS is significantly superior to an equivalent amount of polymeric HALS. The type of stabilizers used for linear low-density polyethylene (LLDPE) and ethyl vinyl acetate (EVA) copolymer are similar to those for LDPE. Since LLDPE has superior mechanical properties (elongation at break and tensile strength), thinner films can be used for most applications, and the loss of UV stability with reduction in thickness has to be compensated for by improving the stabilization system.

For UV stabilization of thick sections of all polyolefins, the combination of HALS and a UV absorber are used. The poor efficiency of HALS in unpigmented plaques is considerably improved with the addition of a UV absorber to protect the bulk of the material. However, in titanium-dioxide-pigmented polyethylene, the UV absorber is surpassed by HALS at much lower concentration. Both the stability of the polymer and the lightfastness of pigment are improved. The effect of adding a UV absorber is only marginal because the role of the UV absorber is taken over by the pigment. The effect of 0.5% of the pigment is much greater than the effect of 0.05% UV absorber. For LDPE and LLDPE, only polymeric or high-molecular-mass HALS and some UV absorbers can be used, but for PP, low-molecular-weight HALS may be better than the polymeric, especially in the unpigmented polymer.

Elastomers Stabilization with additives other than carbon black is limited to unpigmented or light-colored elastomers. Substituted salicylanilides protect natural rubber (NR) against UV by screening, and various phenols and aromatic amines inhibit hydroperoxide formation on photooxidation of polybutadiene. Combinations of phenolic antioxidants with benzotriazole UV absorbers and low-molecular-weight HALS are used for stabilization of thermoplastic rubbers (styrene block copolymers) intended for hot melt and solvent-based sealants. High loadings of a UV absorber in combinations with an antioxidant and HALS improves the light stability of a polyurethane sealant.

Styrenic Polymers Stabilization of polystyrene and its copolymers is necessary for articles expected to be exposed to solar radiation or indoor fluorescent lighting. Because of the significant role played by thermal oxidation products in the effect of these sources on the polymers, thermal stabilization at the processing stage is required to reduce their sensitivity to light. The use of a phenolic antioxidant was shown to increase the retention of mechanical properties and, when used in combination with a HALS, further improvement was obtained [94]. It was also shown that certain phenolic antioxidants used in combination with a UV absorber afforded greater light stability than the UV absorber alone [95]. However, the combination of a low-molecular-weight HALS with a UV absorber, such as the benzotriazole-type, provided synergistic stabilization and superior performance over the earlier formulations [20], even for ABS and impact-resistant polystyrenes that are very sensitive to oxidation because of the butadiene component. HALS effectively reduces the fast loss of impact and tensile strength due to surface degradation, and the UV absorber protects the deeper layers that contribute more significantly to the yellowing. Protection by the benzophenone-and benzotriazole-type UV absorbers has been attributed to both the screening mechanism and to transfer of energy from the excited carbonyl groups, that is, the quenching mechanism [96].

Polyamides: Aliphatic and Aromatic Polyamides require stabilization to reduce the appearance and mechanical changes caused by exposure to the environment. Various types of stabilizers are used, often in combination, in both aliphatic and aromatic types to effectively protect them. Carbon black is a very effective stabilizer. It has been reported that nylons containing carbon black retained a reasonable proportion of their tensile properties after 12 years in the tropics [38]. Reduction in yellowing, an important practical problem, is achieved by the addition of ultraviolet stabilizers or suitable pigments or dyes [20, 97]. Good light stability was also achieved with HALS, both as free molecules and bound to the chain ends [98]. However, the best performance of nylons is obtained when it is stabilized with the combination of a UV absorber, a hindered amine stabilizer, and a phenolic antioxidant [20], analogous with results found with other thermoplastics such as polyolefins and styrenic polymers. In aromatic polyamides, benzotriazole-type UV absorbers act by quenching the excited states as well as by absorption of UV radiation [20]. The addition of a metal deactivator was shown [98] to effectively inhibit the photosensitized oxidation of nylon 6,6 by iron oxide, a known trace-metal salt introduced during the manufacturing process. The addition of cupric ion to aromatic polyamide fibers at concentrations of about 2.5% was shown [99] to significantly retard photodegradation, although it was insufficient to stabilize the polymer for certain long-term end-use applications.

Polyvinyl Chloride (PVC) The light stability of PVC without added light stabilizers is a function of the thermal stabilizer system. Because of the high temperatures required for its compounding and fabrication, good thermal stabilizers must be used during processing in order to provide a product with adequate weatherability [100, 101]. Scavenging of HCl and inhibition of formation of hydroperoxides or promotion of their decomposition to nonradical species are the most important stabilizing mechanisms. Ba/Cd salts and organic tin carboxy-lates confer some UV stability to rigid PVC on outdoor exposure by inhibiting the formation of UV-absorbing groups during processing. However, for adequate performance, the use of stabilizers that protect against the damaging effects of sunlight is mandatory, especially for transparent and translucent articles, even with the use of a good thermal stabilizer system.

Outdoor performance can be greatly improved by the addition of an ultraviolet absorber, such as the orthohydroxybenzotriazole or orthohydroxybenzophenone type, particularly when used in combination with an HCl scavenger [101]. In pigmented PVC, appropriate thermal stabilizers in combination with adequate pigments can confer sufficient UV stability for numerous outdoor applications. For example, rutile TiO2 at high loadings, up to 12% and more, imparts superior light stability. In some cases, addition of a UV absorber is required for best performance. Some pigmented formulations, such as those containing photosensitive forms of TiO2 and pigments having extractable iron as a contaminant, catalyze degradation of PVC. The metal can be introduced as a component of the darker color pigments or as an impurity in one of the additives [102-104].

Low-molecular-weight HALS has almost no effect, except in impact-modified PVC in which HALS in combination with a benzotriazole-type UV absorber outperforms UV absorbers used alone in the same concentration. In TiO2-pigmented impact-modified PVC, HALS is much more effective than UV absorbers. In transparent plasticized PVC films used in greenhouses, stabilization by HALS is comparable to that of the benzophenone- and benzotriazole-type UV absorbers. However, the best performance is obtained with the combination of HALS and a UV absorber.

Polycarbonate (PC) Protection of polycarbonate materials against the effects of solar radiation is mandatory to reduce yellowing and loss of tensile strength. Incorporating a UV-absorbing stabilizer, typically a benzotriazole type, has been shown to effectively protect a 1-mil film of bisphenol A polycarbonate (BPA-PC) against both forms of degradation caused by exposure to UV radiation [105]. A non-UV-absorbing stabilizer, such as one of the nonreactive types of hindered amines, used either alone or in combination with the UV absorber, offers some improvement by reducing surface degradation. However, for adequate protection against the environmental effect on impact strength, the surface requires either a coating containing a high concentration of an ultraviolet absorber or impregnation with a solution containing the UV absorber. Thermal stabilization during processing to reduce the initial color of a PC article is usually accomplished with a phosphite additive. The yellow color formed during processing is reduced on initial exposure, particularly if the light source emits long-wavelength UV and short-wavelength visible radiation.

Polyesters UV absorbers have been used as efficient stabilizers of the linear polyesters, polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) as well as for improving the lightfastness of dyes in polyester fibers during outdoor exposure. Effective stabilization of high-molecular-weight PET fibers by a UV absorber was shown [106] to depend on the level of uptake and its uniform distribution throughout the fiber cross section. A benzophenone-type stabilizer provided considerably better protection than the benzotriazole- and benzotriazine-type stabilizers because it diffused throughout the fiber whereas the other two stabilizers only penetrated peripherally. Since the UV absorbers act as carriers for the dye, the diffusion pattern of the UV stabilizer also influences the penetration of the dye and its protection by the UV absorber.

Low-molecular-weight HALS, a hydroperoxide inhibitor, has been shown to considerably reduce photooxidation of PBT [107]. It can be expected that the combination of a UV absorber with HALS will be particularly effective. There is little data on stabilization of thermoplastic polyester elastomers, but the stabilizers used to protect polymers that degrade by free-radical chain oxidations should also be efficient for these types. Thus, UV absorbers of the benzotriazole and benzophenone types may be used for their screening capabilities, and syn-ergistic effects can be expected with the addition of HALS to reduce the rate of chain oxidations.

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