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Modeling the Ultraviolet Photodegradation of Rigid Polyurethane Foams Christopher R. Newman and Daniel Forciniti* Department of Chemical Engineering, University of MissourisRolla, Rolla, Missouri 65409-1230
Before the Montreal Protocol of 1987 and the subsequent phasing-out of chlorofluorocarbons (CFCs) in industrial applications, rigid polymer foams were made using these compounds as secondary blowing agents. The CFCs remain trapped in the gaseous part of the cellular foam structure, and once discarded these foams constitute a significant reservoir for the environmental release of ozone-depleting chemicals. Environmental degradation of the foam accelerates this process. Of particular interest in this work is the degradative effect of ultraviolet (UV) light on rigid polyurethane foams. Foams were subjected to accelerated weathering conditions and then viewed with a scanning electron microscope. The thin cell membranes near the foam surface degrade when exposed to UV light, leaving only a network of polymer struts that offers negligible resistance to the escape of CFCs or any other gases contained within. This effect has been reproduced qualitatively through simulated weathering of a computer-generated foam structure. If enough is known about the optical properties and photosensitivity of the polymer foam, this simulation technique can be used to estimate the rate of weathering penetration in any situation where photodegradation is the primary concern. Introduction It has been estimated that, between 1971 and 1985, approximately 14.6 billion pounds of polyurethane foam was produced and that approximately 24 million pounds is discarded as scrap each year.1 Provided that the foams were created prior to the phasing out of chlorofluorocarbons (CFCs) in industrial use, they can contain significant amounts (20-30% w/w) of CFCs within the void spaces of the foam and dissolved in the polymer matrix. Once discarded, these foams can be exposed to (among other factors) the degrading effects of sunlight, moisture and heat. Kesari1 has discussed measures for assessing the CFC content of polyurethane foams and for extracting these compounds from the foams. In addition, he subjected the foams to variations in heat, UV light, and natural weathering conditions to determine the effects on the rate of CFC-11 (Freon) release. As one might expect, he determined that increasing the intensity of these factors results in increased escape rates. In particular, he found a 3-fold increase in escape rate in the presence of UV radiation. A number of authors have investigated the harmful effects of ultraviolet light on polyurethanes.2-5 The degradation mechanisms and the effects on the mechanical properties of polyurethane foams are wellestablished. For polyurethanes based on aromatic diisocyanates, the photosensitivity is thought to be due to an aromatic diurethane bridge.3 The monoquinone-imide structure in reaction 1 is the chromophore thought to be responsible for the yellowing of polyurethane during exposure, whereas the diquinone-imide is responsible for the brownish or amber color that polyurethanes exhibit after longer periods of exposure. In addition to reaction 1, which results in the discoloration of the polymer, Osawa3 has proposed the fol* Author to whom correspondence should be addressed. Telephone: (573) 341-4427. E-mail:
[email protected]. Fax: (573) 341-4377.
lowing UV-induced chain-scission reactions that liberate CO and CO2:
The alkoxy, amino and alkyl radicals in eqs 2 and 3 initiate further degradation reactions. For example, two amino radicals can combine to yield an intermediate that then reacts with an alkoxy radical to form diazo compounds. These diazo compounds are also brownish in color. Azo products can also be formed through a light-initiated photo-Fries rearrangement.2 The alkyl radicals formed in reaction 3 can also react with oxygen to yield aldehydes, peroxy radicals, and hydroxy radicals.2 For the most part, investigations into the UV degradation of polyurethanes have focused on the resultant discoloration and failure of mechanical properties.3,5
10.1021/ie0009738 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/27/2001
Ind. Eng. Chem. Res., Vol. 40, No. 15, 2001 3347 Table 1. Weathering Chamber Exposure Times for Foam Samples
Figure 1. Foam sample configuration.
This is not surprising as the practical applications of polyurethanes are jeopardized by such changes and therein lies the interest in minimizing this degradation. One standard technique for measuring the extent of degradation is to measure changes in tensile strength/ elongation as determined by stress-strain curves.3 In addition, changes in polyurethane solution viscosity5 or solubility might be measured. These changes are due to changes in the molecular weight of the polyurethane (which can be measured directly as another analysis technique).3 Finally, the course of degradation can be monitored via changes in the UV, visible, and infrared absorption spectra of polyurethane. For example, Rek and Bravar5 followed the course of UV degradation of polyurethane viscometrically and through changes in the IR, UV/visible, and NMR spectra. All of the above-mentioned investigations have involved polyurethane films or polyurethane in solution. Abu Zeid et al.4,6 employed photoacoustic spectroscopy as a method of monitoring the course of controlled UV degradation and degradation that occurs as a result of natural weathering in polyurethane foams. They postulated that residual catalyst is responsible for the environmental susceptibility of polyurethane foams.6 To the best of the authors’ knowledge, our investigation is the first to focus on the structural changes that occur near the surface of rigid polyurethane foams as a result of environmental exposure. It is also the first that has sought to connect the increased release of gases contained within the void spaces of rigid polyurethane foams with these changes. In this investigation, the small-scale physical changes in the surface of the foam structure that occur as a result of weathering have been monitored by electron microscopy, and the qualitative nature of these changes has been reproduced through the use of simulated UV exposure on a computerconstructed foam model. It is hoped that the insights gained through this simulation can be used to more accurately predict the release rates of gases contained within environmentally exposed rigid foams. Experimental Section The polyurethane foam used in this work came from an old refrigerator backing that was retrieved from a landfill. Small cylindrical plugs of foam were cut from the center of this rectangular slab with a cork-boring device. The surfaces of these cylinders were then leveled with a razor blade. The plugs were approximately 1 cm in diameter and 2 cm in height. The plugs were then mounted (three to a plate) on aluminum plates with a high-strength epoxy (see Figure 1), which were then inserted into a QUV accelerated weather tester (model QUV, Q-Panel Lab Products, with a UVA-340 lamp) to simulate long periods of environmental exposure. The top surface of each foam plug directly faced the UV light bulbs. The accelerated weathering chamber was programmed to provide alternating 12-h cycles of ultraviolet radiation
sample
duration of UV (h)
duration of condensation (h)
total time (h)
1 2 3 4 5
80.0 198.8 284.0 391.7 460.5
115.7 209.6 306.7 448.6 499.7
195.7 408.4 590.7 840.3 960.2
Figure 2. Preparation of samples for SEM.
and condensation. The temperature of the chamber was around 50 °C for the condensation cycle and 80 °C for the ultraviolet cycle. At approximately 200-h intervals, a plate was removed (as a rough estimate, 200 h of QUV exposure corresponds to about 1 year of natural weathering). The actual durations of UV, condensation, and total exposure for each sample are given in Table 1. A few power outages and UV bulb replacements are responsible for the disparity between the UV and condensation times. However, the number of hours for each condition that elapsed after the samples were inserted was accurately maintained on analogue dials on the weathering chamber. Scanning electron microscopy (SEM) was used to analyze the samples at a specimen angle of 0° (model JEOL JSM T330A, 20 kV). Because the foam is nonconducting, all samples were treated with a gold/palladium sputter-coater before they were analyzed with the microscope. After each sample was removed from the weathering chamber, it was stored in a plastic bag in a dark area until the day on which it was analyzed. Before being viewed under the electron microscope, each sample had to be cut near the base of the cylinder to remove it from the aluminum plate. Some samples were left as is so that the exposed surface could be viewed head-on, whereas others were cut with a razor blade down the length of the cylinder so that the depth of the membrane destruction could be measured (see Figure 2). From the micrographs of these “half-cylinders” and the electron microscope’s length scale, one can obtain an estimate of the depth of the region that was affected by this exposure. All foam samples underwent severe surface discoloration while in the weathering chamber. However, this discoloration only penetrated a very short distance into the foam, as revealed when the cross sections were prepared for electron microscopy. Simulated Weathering The first step in simulating the effect of ultraviolet light was to create a computer model of the foam that reproduced as many of the known foam characteristics as possible while maintaining computational feasibility for the structure-building process and the subsequent weathering simulations. The structure should be spacefilling and cellular, and the cell geometry should capture the nature of the foam as viewed under the electron microscope. With these considerations, it was decided to model the foam structure as a stacking of orthic tetrakaideca-
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Figure 3. Orthic tetrakaidecahedron.
hedrons (see Figure 3) of equal size. Such a configuration has the additional advantage that, if the characteristic dimension f (the analogue of the radius for a sphere) of the tetrakaidecahedron is chosen as some integral multiple of 21/2, the coordinates of all center points and vertices take on integral values. Because it will later be necessary to assign each point in the foam structure to a corresponding element in a threedimensional matrix, this is a very convenient feature. Of course, this type of structure is an approximation. The electron micrographs show that a true foam consists of a distribution of cell sizes and, within each cell, a distribution of side lengths and numbers of sides. In addition, the struts in a tetrakaidecahedral configuration do not meet at the thermodynamically favored tetrahedral angle of 109°28′, but for the purposes of this weathering simulation, a stacking of orthic tetrakaidecahedrons maintains enough of the known characteristics of a rigid foam structure to be reasonable. The sides of this polyhedron form the struts of the foam, and the points at which multiple struts meet form the nodes. Taken together, the struts and nodes constitute the nondegradable backbone of the foam. The faces of the polyhedron correspond to the thin, degradable cellular membranes. The computer programs that build the foam structure were all written in Matlab. The user can adjust a set of variables to determine the dimensions of the cell array and the relative thicknesses of the nodes, struts, and membranes. The structure-building programs calculate the spacing of the cells and the size of the matrix necessary to hold all of the relevant information about the foam and subsequently fill in the various parts of the foam. Once the unit cell (see Figure 4) resembles the target structure, the cell array can be constructed. Figure 5 shows a 4 × 4 × 4 stacking of the unit cells. A more complete discussion of tetrakaidecahedral geometry, the rationale for choosing this geometry, and the individual algorithms used to create the foam are given in the Supporting Information. Initially, the three-dimensional matrix that contains all of the relevant information about the foam has all elements equal to zero. The final value of each element signifies whether the corresponding integer-valued point in space is part of a node, strut, membrane or the void space within cells. Rounding the value of all points to integers means that the struts are not perfectly straight and the membranes are not perfectly planar, but this effect becomes unnoticeable as the characteristic dimension of cell is increased (see Figure 4 with f ) 8 × 21/2). The foam block used in the weathering simulation was a 4 × 4 × 6 stacking of unit cells. The upper plane of this block was chosen to be the “weathered” surface (because of the symmetry of the cell stacking, the choice
Figure 4. Simulated foam unit cell.
Figure 5. 4 × 4 × 4 stacking of cells.
is arbitrary). Because the cell stacking is staggered and the edges therefore uneven, a smaller control volume that contains only points within the foam is defined. Periodic boundary conditions are imposed on the sides of this control volume, creating a continuous foam plane that is six layers deep in the direction of weathering. The artificial foam structure is then weathered by striking the upper surface with a predetermined number of photons. Each photon is initially characterized by a random location on the upper surface and a random angle of entry. These photons can be absorbed, reflected, or transmitted through the various parts of the foam in the course of the simulation, with the probabilities of these outcomes estimated using the physical properties of the foam (absorbance at a given wavelength and refractive index) and some basic laws of optics. A photon can only exit the weathering loop by being reflected from the top surface, transmitted through the bottom of the foam, or absorbed by some part of the foam. Results and Discussion Figures 6 and 7 show a pair of images of the foam surface before and after weathering (∼1000 h), respectively. The unweathered surface still has the complete
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Figure 6. Foam surface before weathering.
Figure 9. Depth at which membranes could be found partially intact vs duration of UV exposure.
Figure 7. Foam surface after ∼1000 h of weathering. Figure 10. High-magnification view of strut weathering.
Figure 8. Cross-sectional view of weathered foam edge.
cellular structure with membranes intact, whereas the weathered surface has only the network of struts remaining in the top layers of cells. In Figure 8, the weathered edge of a foam cross section that had been in the QUV apparatus for ∼1000 h is shown. Below a certain depth, the membranes are still somewhat intact, but above this region, only the network of struts remains. For each set of samples, approximately 30 micrographs were taken at a variety of locations along the surface and near the edge of the cross section and at a number of different magnifications. Using the cross-
section micrographs and the length scale provided by the electron microscope, the depth at which membranes could still be found at least partially intact was estimated. Because of the lack of precision of this method, an average over the micrographs for each set of samples was used. The number of electron micrographs in which a reliable visual estimation of the weathering depth can be made varies with each data set. Figure 9 shows the estimated depth at which cell membranes could be found at least partially intact versus the number of hours of UV exposure in the QUV weathering apparatus. The error bars in Figure 9 represent the 95% confidence intervals for these depth estimations. The depth of the weathering effect increases with prolonged exposure, as one might expect. However, the rate at which this effect penetrates into the foam appears to decrease with time. From Figures 7 and 8, it appears that the thicker parts of the rigid foam (the struts) persist long after the membranes have been degraded away. These parts are not immune to the effects of weathering (see Figure 10), but they are considerably more robust than the comparatively fragile membranes. Although the foam samples in the QUV were subjected to multiple environmental factors and, as mentioned previously, some of these factors might work in concert, it is the authors’ belief that UV exposure is primarily responsible for the phenomena shown in Figure 9. As has been reported,7 UV degradation of rigid
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polyurethane foams is exclusively a surface phenomenon, in that only the several topmost layers appear to be affected, whereas other mechanisms (such as thermal degradation and to a lesser extent thermal oxidation) act on the entire foam volume. Furthermore, the discoloration of the surface of the samples and the short penetration of this discoloration suggest that the UV radiation-induced reactions shown in eq 1 are taking place. Undoubtedly, variations in temperature must play a role in the degradation of the foam and probably make it more susceptible to UV degradation. Still, the effect of UV radiation seems to dominate, for example, the release of CFC from the foams. This has been confirmed by Kesari,1 who found a 3-fold increase in the release of Freon from foam samples at 60 °C and under UV light versus the same samples at 60 °C and in the dark. It has been argued4 that thermal oxidation will occur (to some extent) at temperatures as low as 60 °C. However, no evidence has been found in this work to indicate that the kind of damage observed is due to variations in the temperature. To accurately simulate environmental UV exposure, some knowledge of the optical and physical properties of polyurethane foam is required. These quantities are necessary to determine what fraction of the incident photons will be absorbed or reflected when they encounter parts of the foam in the course of the simulation. First, one needs to know which UV wavelengths are particularly harmful to polyurethane. According to Lappin,8 polyurethanes are especially susceptible to UV light at the 254- and 310-nm lines. For reasons that will be discussed later, only photons of wavelength 310 nm were considered in these simulations. By assuming that the cell membranes behave like thin polyurethane films, the probability of an incident photon being reflected, transmitted, or absorbed can be calculated if one knows the refractive index of the polyurethane film and its absorbance at 310 nm. The details of these calculations are presented in Appendix B of the Supporting Information. Even if one could obtain reliable estimates of the above-mentioned quantities, there are still certain obstacles to approaching quantitative accuracy in a simulation such as this. First, the simulated foam structure has no obvious scale for determining the distance between any two points in the structure. To obtain such a scale, one needs only know the approximate diameter of a typical cell in a rigid polyurethane foam. Figure 11 is an example of an electron micrograph that will allow a reliable estimation of the cell diameter (250 µm was used in these simulations). From Figure 9, the depth to which the UV degradation penetrates after around 460 h of UV exposure can be estimated as about 2-3 cell diameters. Second, one needs an accurate estimate of the terrestrial UV flux at the geographical location of interest. Knowledge of the terrestrial UV flux is necessary to make the connection between the number of photons that have struck the upper surface and time. Unfortunately, estimating this quantity can be very difficult, because it is a function of a host of variables in itself, including altitude, latitude, time of year, and time of day. The terrestrial UV flux can vary well over an order of magnitude for each of these factors. Generally, the abundance of UV photons decreases significantly with decreasing wavelength. This is why the effect of degradation at the 254-nm line has been ignored. Finally,
Figure 11. Unweathered surface for cell-diameter determination.
Figure 12. Layer number vs number of cycles necessary to achieve 45% membrane destruction.
knowledge of the quantum yield for chain scission or other degradation reactions is also necessary. An absorbed photon can produce a number of effects, including but not limited to heat, the proposed chain-scission reactions in eqs 2 and 3, the discoloration reactions in eq 1, or nondestructive fluorescence. The fraction of photons that produce a given effect is often referred to as the quantum yield for that effect. This quantity can be difficult to find in the literature, and its measurement is outside the scope of this work. All of these issues and the associated calculations are addressed in detail in Appendix B (Supporting Information). Despite these obstacles, the qualitative nature of rigid foam degradation, as shown in Figure 9, has been successfully simulated. Figure 12 shows the results for a 1 million-photon simulation. The simulation provides progress reports for 10 cycles (or every 105 photons in this case). After each cycle has been completed, the program evaluates the loss of membrane by calculating the percentage of initial membrane points remaining for each value of z. Figure 12 shows the number of cycles required to achieve 45% or greater membrane destruction. Because of the symmetry of the foam stacking, the resulting weathering profile shows some periodicity at intervals equal to one-half of the cell diameter. The reason for this is shown in Figure 13. A stacking six layers deep has eight planes that contain the upper or
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Figure 13. Cross-sectional view of foam stacking.
lower surface of a layer of cells (dotted lines in Figure 13). The weathering profile has seven distinct regions corresponding to the seven layers in Figure 13. This periodicity is an artifact of the computer-generated foam geometry. Real foam does not have this symmetric layering. This also results in some aberrations in the weathering profile. Some z values contain significantly fewer initial membrane points than others (especially those just above and below the eight planes, which contain mostly node and strut points). A few hits at these levels will cause a much larger percentage change than at other levels. The data analysis techniques used in generating Figure 12 circumvent both of these effects by considering the foam to be composed of discrete layers of thickness equal to one-half of the cell diameter as in Figure 13 and by not considering a layer to be degraded until the majority of z values within the layer have achieved the predetermined level of membrane destruction. In Figure 12, this level has been set at 45%, which is just slightly less than the maximum level of destruction exhibited by the fifth layer upon completion of the simulation. The degradation of the bottom two layers of cells (6 and 7) was not considered in Figure 12 (neither layer reached 45% membrane destruction). However, the presence of these two layers is necessary to ensure that end effects can be ignored while monitoring the degradation of the upper five layers. Out of 1 million photons simulated, only 78 passed through all seven layers and exited through the bottom of the foam block. This is less than 0.09% of the total number of photons that were absorbed by the membranes in the course of the simulation. The weathering profile shown in Figure 9 suggests that the remaining network of struts provides a shielding effect against the weathering of layers beneath it. This is, of course, a simplistic explanation for a possibly more complex phenomenon that might include oxygen starvation. Atmospheric oxygen will replace the CFC gas in the cells opened by photodegradation. The resulting oxidation products (carbonyl groups and hydroperoxides) are, in turn, destroyed by photolysis and generate new free radicals, thus leading to the well-known autoaccelerated photooxidation. The deceleration observed in Figure 9 could then also be due to the reduced penetration of air in the deep-lying layers. Considering the sizes of the pores, however, this is very unlikely. The membranes nearest to the surface of the sample have little protection from the photons of UV light bombarding the surface and degrade quickly. After these layers of membrane degrade away, the deeper layers receive only a fraction of the incident UV, because the remaining strut network absorbs and reflects some of the subsequent photons. This effect is clearly borne
Figure 14. Number of membrane hits vs cycle number.
out in the simulations. The first cycle of photons is sufficient to cause over 60% membrane destruction in the first layer and nearly 40% destruction in the second layer. Subsequent cycles cause considerably less membrane damage. Figure 14 shows the number of membrane hits per cycle of photons. For the purposes of these simulations, it was assumed that the network of struts and nodes did not undergo significant degradation. Figure 10 is a high-magnification electron micrograph of a small piece of the strut network after ∼460 h of UV exposure. Clearly, the struts are not immune to the effects of weathering, but even after this amount of exposure, they still remain intact in this partially degraded state and can continue to partially shield ever-deeper layers. However, it is not possible to know for certain whether parts of the strut network have fallen off in the course of weathering or whether the partially degraded strut network provides comparable protection to the fully intact strut network used in the simulation. If more were known about the evolution of the physical state of the struts, this information could be incorporated into the simulation. In these simulations, only the absorption of polyurethane at 310 nm was considered. Most polymers are susceptible to UV light at a number of different wavelengths corresponding to the types of bonds present in the starting material. Environmental weathering involves exposure of the foam to the entire spectrum of terrestrial UV radiation. To make the simulation more realistic, photons of various energies could be considered and distributed in correspondence with the local UV spectrum (if available). To this end, the absorbance of the polymer at each possible wavelength would have to be programmed into the simulation (this would mandate that only discrete wavelengths could be used). If some functional relationship between wavelength and absorbance were available, this would allow a continuous range of wavelengths to be simulated. Despite these difficulties, it is the belief of the authors that valuable insight can be gained from such simulations. If reliable estimates of the local terrestrial UV flux, the relevant quantum yields, and the optical properties of the polymer are available, the simulation can provide a reasonable estimate of the rate at which the thickness of the weathered surface layer grows with time. This information can be used in conjunction with theory to predict the rate at which residual blowing agents, such as CFCs, escape from the foam in situations where ultraviolet light is the primary source of
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foam degradation. In addition, the simulations could be used to predict the effect of changes in the foam geometry on the foam’s response to UV exposure. For example, if the thickness of the struts and membranes could be manipulated via the reaction conditions, the simulations could provide an estimate of the associated changes in weathering properties. Similarly, by changing the relevant parameters within the program, the simulations could be used to predict the effect of a change in the optical properties (refractive index and absorbance) of the polymer on its weathering characteristics. Conclusions The structural nature of environmental surface degradation of rigid polyurethane foam has been investigated, and an explanation has been offered to explain the observed results by focusing on the role of ultraviolet light in the degradation process. It has been shown that certain parts of the rigid foam geometry degrade faster than others and that this degradation is sufficient to allow any gases contained within these layers to escape. However, the longer-lived parts of the foam provide a shielding effect that retards the penetration of this weathering effect. This shielding effect has been reproduced in the simulated UV exposure of a computer-generated foam. The weathering pattern generated by the simulations is qualitatively similar to the experimentally observed pattern. If enough is known about the local terrestrial UV flux and the relevant optical properties of the polymer, it is believed that the simulations can provide a reliable estimate of the rate at which the weathering effect penetrates into the foam. This technique could be adapted to estimate the rate at which any rigid polymer foam is photodegraded, and this information could be used to predict the escape rates of gases contained within the foam. Supporting Information Available: We have included in the supplementary information (available in the electronic version of this manuscript) details of the building of the foam model and the rationale behind the selection of some of the model parameters, as well as details about the weathering simulation. The material in Appendix A is a step-by-step description of the procedure followed to build the foam. A discussion of the tetrakaidecahedral geometry, the rationale for choosing this geometry, and the individual algorithms used to create the foam are given in this appendix. The first portion of Appendix B contains information about the energy and intensity of terrestrial UV irradiation, the procedure followed to determine the size of typical rigid polyurethane foam cell and its equivalence in the foam model, and estimates of the absorbance of radiation by
the foam, as well as a discussion about the problems faced to estimate quantum yields. In the second part of Appendix B, we present details of the weathering simulation. The fate of the photons entering the foam is described and snap shots of the simulation are presented. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Kesari, S. Assessment of Entrapped Freon in Polyurethane Foams and Its Release into the Atmosphere. M. S. Thesis, University of MissourisRolla, Rolla, MO, 1996. (2) Gajewski, V. Chemical Degradation of Polyurethane. Rubber World 1990, 202 (Sept 6), 15-18. (3) Osawa, Z. Photodegradation and Stabilization of Polyurethanes. Dev. Polym. Photochem. 1980, 3, 209-236. (4) Abu-Zeid, M. E.; Nofal, E. E.; Tahseen, L. A. Photoacoustic Study of UV, UV-Thermal, and Weathering Degradation of Rigid Foam Polyurethane. J. Appl. Polym. Sci. 1984, 29, 2443-2451. (5) Rek, V.; Bravar, M. Ultraviolet Degradation of PolyesterBased Polyurethane. J. Elastomers Plast. 1983, 15, 33-41. (6) Abu-Zeid, M. E.; Nofal, E. E. Effect of Catalyst Residues on the Degradation of Rigid Foam Polyurethane. J. Appl. Polym. Sci. 1986, 31, 2407-2415. (7) Abu-Zeid, M. E., Marafi, M. A., Nofal, E. E., Anani, A. A. Study of natural weathering of polyurethane rigid foam by photoacoustic spectroscopy. J. Photochem. 1982, 18, 347-353. (8) Lappin, G. R. Ultraviolet-Radiation Absorbers. In Encyclopedia of Polymer Science and Technology; Bikales, N. M., Mark, H. F., Gaylord, N. G., Eds.; Interscience Publishers: New York, 1969; Vol. 14, pp 125-148. (9) Webb, J. D.; Czanderna, A. W. Dependence of the Predicted Outdoor Lifetimes of Bisphenol A Polycarbonates on the Terrestrial UV Irradiance Spectrum. Sol. Energy Mater. 1987, 15, 1-8. (10) Barker, R. E. The Availability of Solar Radiation Below 290 nm and Its Importance in Photomodification of Polymers. Photochem. Photobiol. 1968, 7, 275-295. (11) Kueper, S.; Brannon, J. KrF Laser Ablation of Polyurethane. In Lasers in Microelectronics Manufacturing; Proceedings of the Meeting, San Jose, CA, Sept. 10, 11, 1991; Braren, B. Ed.; SPIE Press: Bellingham, WA, 1991; Vol. 1598, pp 27-35. (12) Torikai, A.; Ohno, Masakuni; Fueki, Kenji. Photodegradation of Poly(methyl methacrylate) by Monochromatic Light: Quantum Yield, Effect of Wavelengths, and Light Intensity. J. Appl. Polym. Sci. 1990, 41, 1023-1032. (13) Q-Panel Lab Products Home Page, http://www.q-panel.com/ html/quv.html (accessed Nov 2000). (14) Chandra, R.; Thapliyal, B. P.; Soni, R. K. Stabilization of Polyurethane Films against Thermal and Photo-Oxidative Degradation. Polym. Degrad. Stab. 1993, 39, 93-101. (15) Fowles, G. R. Introduction to Modern Optics, 2nd ed.; Holt, Rinehart and Winston, Inc.: New York, 1975; pp 38-46. (16) Kapoor, S. K.; Pandey, C. D.; Joshi, J. C.; Dawar, A. L. Polyurethane Coatings for Integrated Optics Applications. Thin Solid Films 1988, 161, L79-L81.
Received for review November 16, 2000 Revised manuscript received March 19, 2001 Accepted May 8, 2001 IE0009738
