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Introduction

A wide variety of synthetic and naturally-occurring high polymers absorb solar ultraviolet radiation and undergo photolytic, photooxidative, and thermo-oxidative reactions that result in the degradation of the material (Rabek,1995: Scott, 1990: Wypych, 1990). The degradation suffered by these materials can range from mere surface discoloration affecting the aesthetic appeal of a product to extensive loss of mechanical properties that severely limit its performance. The deleterious effects of solar UV-B radiation in particular, on wood, paper, biopolymers and polymers (plastics and rubber), are well known. The phenomenon is of special interest to the building industry which relies on polymer building products that are routinely exposed to sunlight during use. Most of the common polymers used in such applications contain photostabilizers to control photodamage and to ensure acceptable lifetimes under outdoor exposure conditions. In both the U.S. and Western Europe about 20-30 percent of the annual production of plastics resin is used by the building sector (Mader, 1992). The use of plastics in building applications is popular in the developing world as well because of the low cost and the ease of use of plastics components compared to the conventional metal, glass, mortar, wood and other materials. Plastics are used in other products such as outdoor furniture, fishing gear, and marine crafts, that are also routinely used outdoors. Table 7.1 illustrates the diversity of products that fall into this category. A large body of research literature on polymers deals directly with the issue of increasing the useful lifetimes of such products under routine outdoor exposure conditions (Shlyapnikov et al., 1996: Scott, 1985: Burn, 1994: Wypych, 1990).

Table 7.1 Plastics materials routinely exposed to solar UV radiation
 
Building applications  Plastic window and door frames, siding, mobile home skirting, gutters and downspouts, conduits, cable covering, flooring, outdoor furniture. Exterior fascia and soffit [rigid PVC formulations] 
  Membrane roofing, geomembranes, weather-stripping [plasticized PVC, EPDM rubber, other rubbers]
  Glazing, covers for lighting fixtures [polycarbonate and acrylics]
  Varnishes and coatings used to protect surfaces. Highway marking paints. Resins used in the repair of monuments
Agricultural Applications Irrigation hoses, pipes, netting [Polyethylene and PVC] Tanks for storage of water [Unsaturated polyester, and PE]
  Mulch films and greenhouse films [PE and PVC]
Transportation Automobile tires [rubber]. Plastics used in automobile, aircraft, and marine vessel construction [composite]
Other Fishing nets, sails, outdoor temporary housing, outdoor furniture, fibers and textiles
Biopolymers Wool, human hair, wood, chitinaceous materials

PE: Polyethylene, PP: Polypropylene, PVC: Poly(vinyl chloride), EPDM: Ethylene -propylene- diene monomer

It is mainly the ultraviolet radiation in sunlight that presently determines the useful lifetime of even adequately photostabilized plastic products in outdoor applications. Any increase in the UV-B content in terrestrial solar radiation due to a partial depletion of the stratospheric ozone layer is therefore expected to have an impact on the outdoor lifetimes of this category of materials. The damage to polymers under exposure to UV-B radiation is generally intensity-dependent. While the incremental increase in UV-B in solar radiation due to ozone depletion is expected to be small, the efficiency of polymer degradation processes at these wavelengths is generally high. Marginal increases in solar UV levels can therefore translate into a noticeable decrease in the service life of polymer products. In applications such as organic protective coatings, electrical cable jackets and plastic fishing gear, premature failure of the material can involve significant indirect economic losses that greatly exceed the replacement cost of the polymer material. With these applications, it is not easy to estimate the magnitude of the losses consequent to increased UV in sunlight.

    The severity of the ozone-layer depletion and the consequent enhancement of UV-B in terrestrial solar radiation is latitude dependent. Most depletion of the ozone layer occur at the higher latitudes where the largest increases in the UV levels is expected. While the solar radiation environment at these locations may become harsher due to additional UV-B, most of them enjoy relatively moderate temperatures that slow down the degradation reactions of materials. The change in the ozone column at low latitudes is relatively small, but these regions presently experience high ambient temperatures as well as high solar UV-B insolation. The service life of plastics under such harsh conditions is reported to be dramatically reduced. For instance, the tensile strength of white poly(vinyl chloride), PVC, pipes exposed for 24 months in Dhahran (Saudi Arabia) decreased by 43 percent while an exposure of the same duration in Florida resulted in only a 26 percent decrease in the property (Hussein et al., 1995). The combination of high ambient temperatures as responsible for the reduced lifetime of the product.. Even a small increase in solar UV-B levels can dramatically accelerate the deterioration processes in locations where the ambient temperature is high. In the tropical developing countries housing construction mostly relies on lumber and other plant materials while the use of plastics is also on the increase. Plastics are also extensively used in irrigation, water distribution and run-off applications, water storage tank construction, fishing nets, and agricultural films. A decrease in the service lives of these items can have a serious socio-economic impact on the populations in these countries.

    The crucial role of temperature on the weathering of polyethylenes was illustrated in a recent study on desert exposure of polyethylene films. Two sets of polyethylene film samples one maintained at 25 C at all times in an air-cooled, UV-transparent enclosure, and the other left under the much higher ambient temperature, were exposed to sunlight outdoors. The air temperature varied in the range of 26 C to 36 C during the period of exposure. However, the surface temperature of plastics exposed to sunlight can be much higher (by as much as 60 C for common plastics depending on color and thickness) than that of the surrounding air due to heat build-up (Rabonavitch et al., 1983). Figure 7.1 shows the change in extensibility of the films obtained for each set of samples. Samples kept at the lower temperature deteriorated much slower relative to those at ambient temperature although both were exposed to the same dose of solar UV radiation. It is the synergistic effect of high temperature and solar UV radiaion that is responsible for the rapid degradation of the polyethylene films under these conditions. The findings are consistent with the observation that weathering rates of common plastics are very much slower when exposed floating in sea water in marine environment, compared to those exposed on land (Andrady, 1990). Water acting as a heat sink is able to maintain low sample temperatures, retarding deterioration.

    For a given material, the decrease in service life due to increased UV levels in will invariably be determined by a) the spectral irradiance distribution of sunlight and environmental factors such as the ambient temperature ; b) the spectral sensitivity and the dose-response characteristics of the material; and c) the efficacy of the available light-stabilizers under spectrally-altered light conditions (Andrady, 1997). Any serious assessment of the extent of anticipated photodamage requires quantitative information pertaining to each of these. The solar radiation data and the temperatures for most locations of interest are reliably known. A growing body of data on the spectral sensitivity of common polymers as well as some compounded systems used in specific applications (such as in PVC plastic siding) are becoming available (Andrady, 1997). However, the information on the dose-response characteristics of the degradation in plastics and natural materials, as well as on the effect of temperature on the UV-induced degradation, remain scant. Perhaps the least amount of information is available on the effectiveness of the conventional light-stabilizers when used under UV-enhanced sunlight conditions. Figure 7.2 shows the basic steps involved in photodamage and illustrates the various strategies commonly used to mitigate light-induced degradation of polymers.
 
Fig. 7.1. Change in the extensibility of polyethylene film samples exposed in Dhahran, Saudi Arabia. The open symbols are for samples maintained at 45 C during the exposure, and the filled symbols are samples exposed under ambient conditions [Hamid 1998].

Effective light absorbers such as benzotriazoles, benzophenones and phenyl esters, as well as hindered amine light stabilizers (HALS) are presently used in plastics formulations intended for outdoor use (usually at a 0.05 -2.0 wt percent level). Improved stabilizers are introduced into the market periodically. Reliance on increased concentrations of these conventional light-stabilizers to maintain present service lifetimes of plastics products is the most likely response of the plastics industry to counter the effects of increased solar UV B levels. Reported data, such as those for harsh desert weathering experiments suggest that HALS and titania opacifier (Summers et al., 1983) used at higher levels can increase the service life of common plastics, considerably. However, the potential of the conventional photostabilizers to breakdown under exposure to enhanced UV solar radiation, possibly decreasing their effectiveness, is a concern. Even some commercial HALS compounds are reported to be photolyzed by UV radiation (Pan et al, 1993) but no action spectra for the breakdown of even the common photostabilizers are available. Novel and more effective light-stabilizers might be developed to supplement the existing compounds.

Fig. 7.2. A schematic diagram of the various stages of light-induced damage in polymers and its mitigation.

With organic light stabilizers such as hindered amines, increasing the stabilizer level in the composition will have little or no impact on processibility of the resin. The cost, however, will be significantly affected as the contribution of the stabilizer cost to the total cost of a product such as greenhouse films can be as much as 30 percent. With inorganic opacifiers such as titania or carbon used with resins such as PVC for instance, higher levels will affect processibility, power consumption, and even the lifetime of processing equipment, due to increased melt viscosity. Capstock technology (Moore, 1994) where a photolabile polymer is covered by a surface cap layer of the same material rich in light stabilizer, or a photoresistant polymer is also a promising approach As the incremental cost of these efforts rise, it is possible that other weather-resistant polymers will become more cost-effective for particular high-value applications.

    In the case of high-value wood, protective surface coating will remain the primary means of controlling light-induced damage. With low-quality material (that is generally left unprotected) used in dwelling construction in developing countries a decrease in useful lifetime is expected. Use of wood in building might involve additional costs for more frequent painting or other maintenance.


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