Polystyrene photooxidation

In contrast to the above, Figure 21 shows that the antioxidant did not inhibit the rate of either discoloration or carbonyl formation under mercury-arc exposure. Grassie and Weir (5) also reported that under 2537-A. irradiation the antioxidant, 2,6-di-tert-butyl-p-cresol, had no effect on the rate of polystyrene photooxidation. It may be concluded,... 

Further work will be necessary to determine the products formed on light exposure and thus elucidate more fully the mechanism of polystyrene photooxidation. Such investigations are quite useful for stabilization purposes, provided, however, that careful consideration is given to the irradiation conditions used. 

The kinetics of polystyrene photooxidation have been monitored and the ultraviolet absorbing products achieve a photothermal... 

The first move in this direction was to improve the weatherability of impact-resistant polystyrene. Because polybutadiene, the most widely used rubber in impact-resistant polystyrene, is unsaturated, it is sensitive to photooxidation, and impact-resistant polystyrene is therefore not suitable for outdoor applications. A saturated rubber might be able to help here. In the ABS sector this has been successfully tried out with acrylate rubber (77) and EPDM (78, 79), and the latter has also been used in impact-resistant polystyrene (80, 81) This development has elicited satisfactory responses only in certain areas and more work still has to be done. For instance, attempts have been made to improve resistance to weathering by using silicone rubber (82 ). This approach is effective, but economic factors still stand in its way. Further impetus may also be expected from stabilizer research. Hindered secondary amines (83), to which considerable attention has recently been paid, are a first step in this direction. 

Despite the numerous papers devoted to photooxidation of hydrocarbon polymers [21], the initiation step has not been clearly established yet even for polyethylene or polystyrene which were the most studied [22,23]. Difficulties which follow from solution of this problem consist in the necessity of analysis of small amounts of decomposing unstable structures and products which are thereby formed. Moreover, photoinitiation does not include one reaction only but the overall complex of many chemical and physical processes, which importance depends on experimental conditions. 

Hindered Phenols. Because of the relationship between photooxidation and discoloration, we became interested in the effect of hindered phenols which are well-known oxidation inhibitors. A preliminary study was carried out to determine the relative effectiveness of a variety of such compounds, many of which are commercially available. The additives were incorporated into polystyrene at 0.1% concentration, and 50-mil molded plaques were exposed to a carbon-arc Fade-ometer. 

Role of Antioxidants. The inhibition of color development by phenolic antioxidants is rather interesting since it points out the relationship between discoloration and oxidation. It is generally assumed that the photooxidation of polystyrene is a free radical reaction involving oxygen attack on a polymer radical, produced by the action of ultraviolet light. 

In addition to the above two ketones, additional carbonyl compounds have been suggested to explain the discoloration and absorption spectra of irradiated polystyrene. Reiney (12) and more recently Zapol skii (13) postulated that further photooxidation results in the formation of cr-diketones. 

While there appears to be some discrepancy in the conclusions derived from these studies, the difference can be explained as being caused by the exposure conditions. To illustrate the effect of irradiation wavelength on the photooxidation, unstabilized polystyrene films were exposed to the following three ultraviolet light sources having different emission characteristics (Figure 16) ... 

Wettability and Constitution of Photooxidized Polystyrene and other Amorphous Polymers... 

The effect of ultraviolet irradiation in air on the wettability of thin films of amorphous polymers has been studied. With poly(vinyl chloride), poly(methyl methacrylate), poly(n-butyl methacrylate), poly (ethylene terephthalate), and polystyrene the changes in contact angles for various liquids with irradiation time are a function of the nature of the polymer. A detailed study of polystyrene by this technique and attenuated total reflectance spectra, both of which are sensitive to changes in the surface layers, indicates that the contact angle method is one of the most sensitive tools for the study of polymer photooxidation in its early stages. The method is useful in following specific processes and in indicating solvents to be used in the separation and isolation of photooxidation products. 

That photooxidation is indeed responsible for the observed changes is indicated by a comparison with the results obtained with films irradiated in vacuum. Both polystyrene and poly(n-butyl methacrylate) irradiated in vacuum showed no changes in contact angles after exposures up to 120 min. with poly (ethylene terephthalate), contact angles for all of the liquids tended to increase slightly. 

To relate the wettability changes more firmly to the photooxidation processes and products, a detailed study was carried out with polystyrene. This polymer was selected because the formation of oxidation products in the hydrocarbon surface gave rise to large changes in wettability and because these products would be readily accessible to optical methods of analysis. The ultraviolet absorption spectrum of polystyrene shows a sharp cut-off, and the extinction coefficients for the radiation absorbed are sufficiently high that almost all of the photochemical reaction should be confined to the surface layers. 

Figure 7. Effect of photooxidation product removal on water contact angles with polystyrene...

This can be demonstrated with polystyrene. During photooxidation, three types of products will form (a) material which volatilizes during the irradiation and therefore does not affect the contact angle (b) extractable products and (c) residual non-extractable products, mostly polymeric. In Figures 7 and 8 are shown plots of the contact angles for water and for ethylene glycol, respectively, on irradiated polystyrene... 

Attempts to reconstitute a photooxidized surface by reevaporation of various extracts on both unirradiated and irradiated polystyrene surfaces were only partially successful. Deposition of the material from a methanol extract on unirradiated polystyrene, a film irradiated 2.5 hours and extracted with water, and on a film irradiated 2.5 hours and extracted with methanol reduced the water contact angle by 55°, 50°, and 51°, respectively. The same experiments with a water extract, either as such or evaporated to dryness and redissolved in methanol, produced erratic results both from film to film and on the same film. The latter result may be due to the formation of islands of material. Such inconsistencies may be the result of differences in the way various photooxidation products reorient on redeposition or to changes in the orientation... 

Under irradiation with polychromatic light at X > 300 nm and 60 °C, representative of outdoor exposures, polystyrene (PS) homopolymer, copolymers and blends do not directly absorb the incident radiation. It is well known that the photooxidation of these polymers results from light absorption by chromopho-ric impurities [1,2]. Photooxidation generates modifications of the chemical structure of the material, which results in the formation of oxidized groups, the development of discoloration and the loss of the initial mechanical properties. 

PHOTOOXIDATION OF BLENDS OF POLYSTYRENE AND POLY(VINYL METHYL ETHER) (PVME-PS)... 

The materials analyzed were blends of polystyrene (PS) and poly(vinyl methyl ether) (PVME) in various ratios. The two components are miscible in all proportions at ambient temperature. The photooxidation mechanisms of the homo-polymers PS and PVME have been studied previously [4,7,8]. PVME has been shown to be much more sensitive to oxidation than PS and the rate of photooxidation of PVME was found to be approximately 10 times higher than that of PS. The photoproducts formed were identified by spectroscopy combined with chemical and physical treatments. The rate of oxidation of each component in the blend has been compared with the oxidation rate of the homopolymers studied separately. Because photooxidative aging induces modifications of the surface aspect of the material, the spectroscopic analysis of the photochemical behavior of the blend has been completed by an analysis of the surface of the samples by atomic force microscopy (AFM). A tentative correlation between the evolution of the roughness measured by AFM and the chemical changes occurring in the PVME-PS samples throughout irradiation is presented. 

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