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Stratospheric Ozone and Human Health Project

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Environmental Effects of Ozone Depletion: 1994 Assessment



A.L. Andrady (USA), M.B. Amin (Saudi Arabia), S.H. Hamid
(Saudi Arabia), X. Hu (China,), and A. Torikai (Japan).

Table of Contents

  1. Summary
  2. Introduction
  3. UV-Induced Damage to Polymers
  4. Spectral Sensitivity
  5. Responses to Increased Solar UV Radiation
  6. Conclusions
  7. References


Synthetic polymers, naturally-occurring biopolymers, as well as some other materials of commercial interest are adversely affected by solar UV radiation. Applications of these materials, particularly plastics, in situations which demand routine exposure to sunlight is only possible through the use of light-stabilizers and/or surface treatments to protect them from sunlight. Any increase in solar UV content due to partial ozone depletion will therefore accelerate the photodegradation rates of these materials, limiting their service lifetimes outdoors.

The nature and the extent of such damage due to increased UV radiation in sunlight is quantified in action spectra. In spite of the several action spectra for polymers, reported in the research literature, the information is often inadequate to make reliable estimates of the increased damage. The specific formulation of the polymer material, the damage criterion employed, and even the manner in which data is interpreted, can often influence the results. However, it is clear from the available data that the shorter wavelength UV-B processes are mainly responsible for photodamage ranging from discoloration to loss of mechanical integrity in polymers exposed to solar radiation. The molecular level interpretation of these changes remain unclear in many instances.

The use of higher levels of conventional light stabilizers in polymer formulations will likely be employed to mitigate the effects of increased UV levels in sunlight. However, such an approach assumes that a) these stabilizers continue to be effective under spectrally- altered sunlight conditions; b) they are themselves photostable on exposure to UV-rich sunlight; and c) they can be sufficiently effective at low enough concentrations to economically serve the purpose. Experimental data bearing on these issues is sparse. On-going research, particularly those relating to extreme-environment exposure of polymers, is expected to shed more light on these unresolved questions. Substitution of the affected materials by more photostable varieties of plastics and other materials also remains an attractive possibility. Both these approaches will add to the cost of plastic products in target applications. With plastics rapidly displacing conventional materials in numerous applications, this is an important consideration particularly in the developing world.

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Most synthetic polymers as well as naturally - occurring biopolymers are readily affected by solar ultraviolet (UV) radiation. The deleterious effects of UV-B on polymers (plastics and rubber) are well known, and in applications which demand routine exposure to solar radiation, photo-stabilizers are commonly used in polymer products to ensure adequate lifetimes. Wood and other biopolymeric materials are similarly affected; surface coating of wood is employed to control the light-induced damage. Applications of particular interest are building products which account for nearly a third of the plastics production in the US as well as in Western Europe. The consistent trend towards increased use of plastics in buildings at the expense of more traditional materials of construction such as metal, glass, mortar, and wood, is a global one and is particularly strong in developing countries with a high demand for low-cost housing. In addition to use in building, polymeric materials are used in numerous other applications where they are routinely exposed to solar radiation (Table 7.1).

The outdoor service lifetimes of plastics building materials, even under present exposure conditions, are determined by their susceptibility to UV-B radiation in terrestrial sunlight. Therefore, a partial depletion of the stratospheric ozone layer, and the resulting increase in UV-B content in sunlight reaching the earth's surface, will have a definite impact on the use of materials in outdoor applications. As both synthetic polymers and biomaterials will undergo light-induced chemical changes, consequent deterioration in useful properties might be expected at significantly faster rates under such conditions. These changes might be mitigated at least in part, however, by the use of higher levels of conventional stabilizers in polymer formulations, by the use of new high-efficiency stabilizer systems, and by the substitution of better UV-resistant types of polymers for outdoor applications of interest. The effectiveness of some of these strategies have not been demonstrated for exposure conditions involving spectrally-altered, UV-B rich sunlight. Increased cost associated with each approach, and their effectiveness, may alter the economics of the use of plastics and rubber in building construction.

Table 7.1. Materials Routinely Exposed to Solar UV-B Radiation. _____________________________________________________________________
1.       Building Materials: Plastics - Pipes, water storage tanks, window/door frames, siding, gutters, roofing, glazing, exterior fascia, cable coverings, and conduits. Wood used in buildings.
2.       Outdoor Furniture and Surfaces: Stadium seats, park benches, beach furniture and artificial turf.
3.       Transportation Applications: Composites, other polymers, and wood, used in Aircrafts, Automobiles, and Marine Vessels. Automotive and aircraft tires.
4.       Agricultural Applications: Greenhouse coverings, mulch films, and irrigation hoses.
5.       Coatings and Paints: Coatings for protection of outdoor surfaces, outdoor artwork, dyes, highway pavement markings, and road signs.
6.       Textile Products: Fabrics used outdoors (e.g. sails), geomembranes, netting and commercial fishing gear.
7.       Biopolymers: Wool, human hair, Chitin/Chitosan*.
8.       Packaging: Heavy-duty sacs.
9.       Miscellaneous: Resins for restoration of outdoor statues, leather products, solar panel materials, paper or paperboard products used outdoors.

      * Shell of crabs and shrimp is composed of Chitin, the second most abundant biopolymer. Chitosans occur in the cell wall of fungi.

Severity of the impact of increased UV-B levels on the outdoor lifetimes of materials depend on both the geographic location of exposure and the susceptibility of the particular material to UV-B radiation. While the higher latitudes will experience the high levels of ozone depletion, the high ambient temperatures in the near-equator regions will tend to severely magnify the effect of even a very marginal increase in solar UV-B in these regions. The effects of a uniquely harsh combination of high levels of solar UV-B from spectrally altered sunlight, and the high temperatures leading to severe heat build-up in materials exposed outdoors, on the lifetime of building products at these latitudes is not well understood.

The cost advantage offered by durable plastic building products have made them popular in developing countries including those in near-equator regions. Wood and natural materials have long been the conventional building materials for dwellings in these regions. With the lifetimes of both plastics and wood affected by UV-B increase, a partial ozone depletion will have very significant socio-economic impacts on these populations.

To make a realistic assessment of the impact of a partial ozone-layer depletion on materials, several key types of data are needed. These are, a) the spectral sensitivity of plastics and biomaterials of interest, b) dose -response data to estimate the increased damage to be anticipated as a result of the UV-B increment in terrestrial sunlight, and c) data on the effectiveness of conventional and new types of light stabilizers in specific polymers, under exposure to spectrally-altered sunlight. Only a fraction of the needed data is currently available, and a reliable quantitative assessment is therefore difficult at the present time. However, spectral sensitivity data on several relevant polymers, data on high-UV desert exposures of polymers, and some preliminary information on stabilizer effectiveness, have been reported in recent years. The availability of such data continues to increase the reliability of the assessment process.

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UV-Induced Damage to Polymers

The chemistry of UV-induced damage to polymers is not completely understood. The basic chemical processes that occur in key polymers exposed to solar UV-B radiation, however, have been broadly identified.

Discoloration of materials due to formation of colored chemical species from photoreactions is a primary consequence of exposure of polymers to UV radiation. While mechanical properties also suffer on continued exposure, the rate of discoloration often determines the service life of the product. In the case of poly(vinyl chloride), (PVC), typical formulations used in the building applications ( for instance in siding, and in window frames), the predominant change caused by UV-B is the discoloration resulting from photo-dehydrochlorination of the polymer [Andrady et. al. ,1989, Andrady et. al. ,1990]. The yellowing obtained is uneven and gradually increases with prolonged exposure. Adequate stabilization with an opacifier (rutile titania) controls the rate of discoloration in white profiles used in siding, window frames and pipes [Titow, 1984]. Polycarbonates used in glazing also undergoes yellowing [Andrady et. al., 1992], but due to a combination of photo-Fries rearrangement (shown below) and oxidative reactions [Factor and Chu, 1980] . In this reaction the bisphenol A units photoisomerize into phenyl salicylate units and possibly to dihydroxybenzophenone units. The use of UV absorbers in the formulation is necessary to control rate of yellowing [Davis and Sims, 1983].


Wood and paper also undergo yellowing discoloration on exposure to solar UV-B [Andrady, 1991; Forsskahl, 1993, Heitner, 1993]. when the lignin component in wood undergoes photodegradation. In a study involving 75 varieties of commercially important wood, 65 percent were found to discolor due to UV light [Sanderman, 1962]. On exposure to UV radiation, the fractions of both holocellulose and lignin reduce and that of extractives increase, but the percent reduction in lignin is relatively higher than that of cellulose [Hon, 1993]. Cellulose in wood has been shown to undergo a free radical mediated degradation on exposure to wavelengths < 340 nm. Electron spin resonance spectroscopic data suggest scission of the C5 - C6 bond in glucose units in the molecule during irradiation [Hon, 1981].

Wool readily undergoes light induced yellowing due to solar UV-B radiation. The amino acid residues, particularly tryptophan, histidine and cystine, degrade extensively on irradiation. Some free radical mediated main-chain scission of wool molecules also accompany photoyellowing [Launer, 1965]. Role of UV-B radiation in generating free-radicals in human hair has also been reported [Jahan 1987]. Presence of free-radicals often leads to degradation in polymer materials.

Both photo-initiated thermooxidative processes and photodegradation reactions may lead to chain-scission in polymers exposed to solar radiation. These are often free-radical processes and may also involve concurrent cross linking. As the desirable mechanical properties of polymers are a consequence of their long chain-like molecular structure, chain scission leads to deterioration of these properties. This in turn impacts their outdoor service lives. Polyethylene films exposed to solar UV-B radiation readily loose their extensibility and strength [Hamid et. al. 1988], as well as their average molecular weight [Andrady et. al., 1993]. While the change in UV-induced loss in elongation was shown to generally correlate with the development of carbonyl functionalities, it is preferable to use both measures when predictions of weatherability of polyethylene is attempted [Tidjani, 1993]. In polyethylene, photoinitiation is thought to originate from polymer-oxygen complexes while in polypropylene the initiation is via photolysis of tertiery hydroperoxide groups [Gugumus, 1994]. Solar UV-B is also known to degrade polystyrene foam [Andrady et. al., 1991], a popular packaging and material. These changes in bulk mechanical properties reflect changes in macromolecular chain scission ( and/or cross linking) resulting from photodegradation. Changes in viscosity or gel permeation characteristics of polymers have been used [Torikai, 1993a ; Andrady et. al., 1993] to establish molecular level changes during photodegradation.

It has been suggested that unless the ozone losses exceed an arbitrary value of 15 percent in summer months, the increased deleterious effects on polymers might be minimal [Prickett, 1994]. These conclusions based on biologically effective UV doses and the geographic variations of terrestrial UV currently observed, do not take into account the synergistic effects of the temperature in polymer photodegradation. In near-equator regions (for instance, in Dhahran, Saudi Arabia) where the ambient air temperatures can be as high as 50 C, even a very small increase in UV-B levels can translate into significant increases in the rate of degradation. A more reliable assessment must be based on activation spectra of relevant polymer formulations.

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Spectral Sensitivity

The spectral sensitivity of polymers is determined from exposure experiments using monochromatic radiation, or from exposures to filtered xenon sources ( whose spectral irradiance distribution is designed to closely approximate terrestrial sunlight at unit air mass) and a series of cut-on filters. Early experimental data is of limited value because of incomplete descriptions of the polymer formulations and processing techniques used in sample preparation, and because mercury vapor lamps were used as sources in these studies. Such lamps emit short wavelength UV radiation not typically found in terrestrial sunlight.

Table 7.2 summarizes the available data on spectral sensitivity of polymeric materials. Data on UV-A sensitive materials are not included because ozone depletion is expected to mainly affect the UV-B region of the solar spectrum. Data generated using a borosilicate-filtered xenon source, with cut-on filters to separate the effect of different spectral regions allow the identification of spectral regions most effective in bringing about the damage of interest. Such activation spectra will be source-specific, and those reported in the Table are specific for filtered - xenon source spectrum and the indicated damage criterion only. Spectral sensitivity can also be studied in experiments where materials are exposed to monochromatic radiation. Using either experimental approach it is possible to estimate the damage obtained per available photon, and plot as a function of wavelength of exposure, to obtain an action spectrum. Information from the two approaches should agree in instances where there is no significant synergism or mutual cancellation of effects obtained at different wavelengths. In the case of PVC, the yellowing discoloration brought about by the UV-B region of the solar spectrum is offset to some extent by the photobleaching of chromophores afforded by the 500nm - 600 nm band of visible light [Andrady et. al., 1989]. In spite of the high quantum efficiency of photobleaching reaction [Decker et. al., 1988] relative to that of yellowing, this apparantly plays only a minor role in the overall photodegradation process, and the monochromatic wavelength sensitivity data was consistent with activation spectra for yellowing under white light [Andrady, 1989a]. Figure 7.1 illustrates the action spectra of common polymer formulations for yellowing.


Fig. 7.1. Spectral sensitivity of selected polymeric materials.

Scission (and sometimes cross linking) of macromolecular chains making up the polymer, is a common consequence of photodegradation. These changes result in a drastic reductions in the mechanical integrity of the polymer and therefore influence their useful lifetimes outdoors. Recently the action spectra for chain scission in several polymers including polystyrene [Torikai, 1993b], polyethylene [Andrady et. al. 1994], and polycarbonate [Torikai et. al., 1993a] have been reported. Activation spectra based on tensile properties of materials are of particular interest as the mechanical integrity of plastics is frequently measured in terms of tensile properties. Figure 7.2 shows relevant data for {ethylene -carbon monoxide (~1 %) } copolymer film exposed to white light from a xenon source [Andrady et. al. 1994]; a sharp transition from almost no effect to drastic deterioration is obtained around 330 nm. The standard error associated with elongation at break measurements is generally large, and the scatter in Figure 7.2 is not unusual. A more fundamental measure of chain-scission is the change in solution viscosity, resulting from a change in average molecular weight. Recently viscosity data on polycarbonate [Torikai et. al., 1993a], and polystyrene [Torikai, 1993b] were reported, with either 300 nm or 280 nm - 300 nm being identified as the wavelengths most effective in causing chain scission. A quantum yield of 0 - 1 x 10-3 scission events per photon was reported for polycarbonate.

Table 7.2. Reported Spectral Sensitivity Data on Materials


Polyehtylene [PE]                                                           
 LLDPE Base Polymer         Optical Density (UV/Vis) 260*      Torikai.(1993c)      
LLDPE + Flame retardant                                
 HDPE + Flame retardant     Optical Density (UV/Vis) 300
 LDPE Base Polymer          Discoloration            310       Hirt (1967) 
Poly(vinyl chloride) [PVC]                                     
 Base Polymer film          Discoloration (Y.I.)     320       Hirt (1967)   
 Rigid PVC + titania        Discoloration (Y.I.)     310-325   Andrady (1989)        
 Base polymer film          Spectroscopy (FTIR)      355-385   Martin (1971)        
Polypropylene [PP]                                                          
 Base Polymer Film          Spectroscopy (FTIR)      260-280   Torikai (1993b)       
 PP + Flame retardant       Chain Scission (GPC)     260-280
 PP molded pieces           Extensibility (Tensile)  315-330   Andrady (1994)        
Polystyrene [PS]
 Base Polymer               Optical Density (UV/Vis) 260-320   Torikai (1993b)       
 PS + Flame retardant (I)   Optical Density (UV/Vis) 280
 PS + Flame retardant (II)  Optical Density (UV/Vis) 310             
 Base Polymer               Chain Scission (GPC)     280
 PS + Flame retardant (I)   Chain Scission (GPC)     300
 PS + Flame retardant (II)  Chain Scission (GPC)     300
 PS (photodegradable)       Discoloration            320-345   Andrady (1994)        
Polycarbonate [PC]
 Base Polymer               Chain Scission (Visc.)   280-320   Torikai (1993a)       
 PC extruded sheet          Discoloration (Y.I.)     280       Andrady (1994)        
 PC + photostabilizer       Discoloration (Y.I.)     310-340   Andrady (1994)        
Poly(methyl metha-crylate)
 Base Polymer               Chain Scission (Visc.)   300       Mitsuoka (1993)        
Copolymers and Blends
 ECO copolymer (1% CO)      Optical Density (UV/Vis) 280-340   Aoki (1992)
                            Extensibility (Tensile)  <320      Andrady (1994)        
 PC/PMMA Blend              Chain Scission (Visc.)   <280      Osawa (1991)  
 Wood Pulp (mechanical)     Discoloration (Y.I.)     334-354   Andrady (1991)        
 Wood Pulp (refiner pulp)   Brightness  (UV/Vis)     450-500   Forsskahl (1993)        
 Wool                       Discoloration (Y.I.)     290-311   Launer (1965)        
                            Discoloration (Y.I.)     280*      Lennox (1971)        

1. Column 1 abbreviations. LLDPE - Linear Low Density Polyethylene, HDPE - High Density Polyethylene, LDPE - Low Density Polyethylene, PMMA - Poly(methyl methacrylate), ECO - copolymer of ethylene and carbon monoxide.
2. Column 2 abbreviations. UV/Vis. : UV - Visible Spectroscopy, Y.I.: Yellowness Index, FTIR: Fourier Transform Infra-red Spectroscopy, GPC: Gel Permeation Chromatography.
3. Column 3. Wavelength interval in the white light (filtered xenon - source ) spectrum at which maximum damage was obtained. Single wavelengths refer to data from monochromatic exposure experiments, and indicate the wavelength at which maximum damage was obtained. A '*' indicates that this was also the shortest wavelength used in the experiment.


Fig. 7.2. Elongation at break for (ethylene -carbon monoxide) copolymer films exposed behind cut-on filters to a white light source ( Borosilicate-filtered Xenon source).

Activation spectra for yellowing of biopolymeric materials such as wood pulp [Andrady 1991] and wool [Lennox 1971] have also been reported. With wool and paper made from mechanical pulps, premature yellowing takes place on exposure to solar UV-B light. Figure 7.3 shows the activation spectrum for yellowing of newsprint paper made from pinewood (Pinus taeda ) exposed to white light from a xenon source.


Fig. 7.3. Activation spectrum for yellowing of newsprint paper exposed to a white light source ( Borosilicate-filtered Xenon source).

Increased rates of UV-B induced degradation of polymers may also affect post-consumer plastic waste management technologies such as recycling and photodegradable polymers. Depending on the geographic location, plastics in the litter stream will undergo higher extents of photodegradation prior to collection for recycling. The significance of this incremental exposure on recyclate quality is not clear at this time.

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Responses to increased Solar UV Radiation

In principal, there are two basic approaches to maintaining current service lives of selected materials in spite of a moderate increase in UV levels in sunlight. One is to substitute materials; more photoresistant, albeit more expensive, polymers or other materials might be used for those applications that demand routine exposure to sunlight. For instance, the PVC formulations used in exterior profile (for instance in window frames) might be replaced with better weather-resistant copolymers such as (acrylonitrile -butadiene - styrene), or with PVC - capped with films or layers of selected polymers with superior weatherability [Moore, 1994]. Alternatively, an effort might be made to use conventional or novel light - stabilizers to address the problem. Polymers such as PVC are inherently photolabile, and their outdoor use is possible only due to the effectiveness of light- and heat-stabilizer technology, with impressive classes of photostabilizers such as hindered amine systems (HALS) being recently developed [Al-Malaika 1983]. It is likely and certainly not unreasonable to expect the polymer industry worldwide to explore the full capability of stabilizers to address the problem of increased solar UV-B component. The cost of UV stabilizer systems is a very significant component of the cost of plastic formulations used in outdoor applications; with polyethylene greenhouse film formulations, as much as 30 percent of the compound cost might be ascribed to the photostabilizer. Increasing levels of stabilizer will therefore have a definite impact on the economics of plastics use in outdoor applications.

The basic question then would be whether increasing levels of conventional photostabilizers in common polymer formulations will result in a concomitant increase in the service life of materials exposed to spectrally - altered, UV-rich sunlight? Early investigations of the issue in CIAP (Climatic Impact Assessment Program) assumed that such an approach will be successful and even calculated (based on Beer - Lambert Law) factor increases in stabilizer needed to offset a given increase in total UV radiation levels [Shultz, 1975]. These calculations, however, were not based on experimental data pertaining to specific stabilizer / polymer combinations. Some data is also available from industry sources and in technical literature on the effectiveness of higher levels of photostabilizer in typical formulations in increasing the service life of polymers. However, this data pertains almost exclusively to photodamage to materials from sunlight with present-day levels of UV-B. The effect of increasing levels of a common type of photostabilizer in polyethylene film is shown in Figure 7.4 to illustrate the efficiency of the additive in maintaining service life, and to show that the protective action can be a non-linear function at high levels of the additive. This data is on Chimasorb 944 LD stabilized LDPE films exposed for a three year period under desert conditions in Dhahran, Saudi Arabia [Hamid 1994]. The high ambient temperatures in desert locations will further exacerbate the problem of maintaining reasonable lifetimes for plastic products exposed to sunlight with increased UV-B component. In an empirical study on LDPE weathering under desert conditions Hamid [Hamid 1991] found the elongation at break to be a sensitive indicator of the extent of weathering. The study found total UV-B as well as total sunlight to correlate particularly well with changes in properties of the polymer on exposure. Another relevant study was carried out by Bauer [Bauer 1990, 1992] on photo-degradation of organic coatings photostabilized by HALS compounds. A "Harsh" exposure involving high-intensity short-wave UV radiation, and an "Ambient" exposure with simulated solar radiation were used to rank a series of HALS. The ranking of the effectiveness of different stabilizers was very different under the two exposure conditions. Some commercial HALS compounds are known to be photolyzed on exposure to short wavelength UV radiation [Al-Malaika, 1983 ; Chen, 1988].


Fig. 7.4. Solar UV radiation needed to reduce the elongation at break by 50 percent, of LDPE films containing different levels of UV stabilizer.

Hitherto, there was little need to address the question of spectrally-altered sunlight and its effect on weathering of materials. The efficiency and effectiveness of conventional stabilizers under spectrally-altered sunlight has not been studied and therefore remains essentially unknown.

Increase in levels of photostabilizers may offset the increased rate of photodegradation caused by higher solar UV levels resulting from a partial ozone depletion. However, the effect of higher stabilizer levels on other useful properties of the material must also be considered; at least in the case of rigid PVC profile formulations increased titania levels can lead to several negative consequences [Mastro, 1983]. An attractive alternative is the use of improved grades, specially coated grades, of rutile to obtain higher levels of protection at low levels of additive [Day, 1990]. The relative importance of different approaches to mitigation, including the use of alternate materials of superior UV resistance, will invariably be determined by the costs associated with each strategy. Insufficiant data is available to estimate these costs and therefore the impact on building materials industry at this time.

In the case of biomaterials, the mitigation of the effects of higher UV levels in sunlight is considerably more difficult. While wood surfaces can be treated either with coatings or other stabilizers, the same is generally not true of paper made of mechanical pulps. Light - induced yellowing and the related loss in brightness is a key factor limiting the use of mechanical pulps [Hemmingson, 1989]. With materials such as wool, chitins, natural fibers used in netting and cordage, the impact of damage due to increased solar UV levels has not been established with any degree of certainty. While UV-induced degradation reactions in these materials are known, the extent to which such changes interfere with the performance of these materials, or affect the economic value of these materials ( specially with wool or paper) has not been comprehensively addressed.

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Both naturally occurring biopolymer materials as well as synthetic polymers undergo degradation reactions on exposure to solar UV-B radiation. With synthetic polymers, it is the effective photostabilization that ensures adequate lifetimes for products used outdoors even under present exposure conditions. Any increase in the UV-B content of terrestrial sunlight must therefore reduce the service life of products based on these materials.

Some relevant action spectra for typical formulations of common polymers are available. In spite of many recent pertinent contributions in the literature, a complete understanding of the wavelength sensitivity of key formulations used in building applications has not been achieved. To be useful in models assessing damage, the action spectra have to address relevant formulations of more common polymers, and pertain to those properties (often mechanical properties) of interest to end-users. In the case of biomaterials routinely exposed to sunlight, even less data is available; action spectra are known only for a few of these.

With synthetic polymers there is a likelihood that either increasing the levels of conventional stabilizers, or the use of novel stabilizers, will alleviate some of the deleterious effects of increased UV-B in sunlight. However, insufficient data on weathering studies based on spectrally altered white light, precludes confirmation of the efficacy of this strategy. Intensity - dependence of the key light stabilizers for polymers, under different and relevant light - temperature domains has not yet been reported. Therefore the economic feasibility of this primary industry response to stratospheric ozone depletion, cannot be realistically assessed at this time.

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