When polymer is irradiated with visible light or UV light, they eventually undergo physical and chemical changes which referred to photo-degradation or light-induced degradation (Schnabel, 1981, p.14-15). As polycarbonates (PC) are used outdoor applications, they are frequently exposed to various weathering conditions especially UV radiation from the sunlight. The PC unit can effectively absorb the UV range spectrum to initiate photochemical reaction. The common physical deteriorations of PC are lost of the mechanical strength, surface cracking, brittleness, yellowing and discolouration (Durairaj, 2005 p.527-528; Schnabel, 1981, p.14-15). Figure shows the photomicrograph of the surface phenomenon occurs in the PC sample during photo -degradation process. Before the PC sample in Figure (a) was exposed to photo-aging, the surface did not show any deformations. After the sample was irradiated for 3360 hours, physical impacts such as surface cracking, void and sinks can be observed from Figure (b) (Saron et al., 2006).
The light degradation pathways that occur in the outdoor applications are highly depending on the nature of radiation especially the fraction of UV-B and UV-A. Based on the illustration of Figure below, the harmful UV light (UV-C) with wavelength below 280nm is absorbed by ozone layer located at the stratosphere. As a result, only fraction of UV light with medium wavelength reaches the earth surface to produce greatest influence on the plastic and other polymeric materials. Medium wavelength UV light is consists of UV-A and UV-B, wavelength of UV-B range from 290nm-320nm while UV-A is referred to longer wavelength of 320nm to 400nm. (Arca et al., 1990; Durairaj, 2005).
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Figure: Sunrays exposure on earth. (Ciba Plastic Additive)
Absorption of sunlight has provided sufficient energy for the generation of reactive radicals in the PC. The free radical species can be either formed by direct breakage of chemical bonds prior to short wavelength exposure or excitation of chromophores with longer wavelength light. (Saron et al., 2006). Photodegradation process can exist in two mechanisms, Photo-Fries rearrangement and Photo oxidation. The irradiation wavelength is the major influence on the mechanism that takes place. Photo-Fries rearrangement is more likely to occur at wavelength below 300nm. At longer wavelength (≥340nm), in the presence of oxygen, photo oxidation or chain oxidation process has become predominant (Diepens and Gijsman, 2006; Saron et al., 2006). It is crucial to determine which degradation mechanism is predominated during the outdoor exposure (Diepens and Gijsman, 2006).
2.1.1 Photo-Fries rearrangement
At wavelength below 300nm, photo Fries products is responsible for the discoloration of the PC. Exposure to UV-B radiation causes the aromatic carbonate unit to rearrange into phenylsalicylate (PS) and dihydroxybenzophenone (DHBP) derivatives. The reaction either can be concerted or a radical process (Diepens and Gijsman, 2006; Geretovszky et al., 2002). Instead of the structural rearrangement, the reaction might competitively go through a minor pathway to form other yellowing species. From the illustration of Figure below, radicals that produced from CO-O bond scission can undergo decarboxylation and decarbonylation before the radical recombination or hydrogen abstraction step. This has result in the formation of hydroxy- and dihydroxy-biphenyl units, aromatic ether structures and phenolic derivatives. These compounds are further photolysed into yellowing species to cause yellowing of the PC film (Rivaton et al., 2002; Geretovszky et al., 2002). Several end products of the photo- Fries rearrangement are listed in the Table.
Figure: Photo- Fries rearrangement process via the radical process (Diepens and Gijsman, 2006).
Table: Chemical structures of model compounds for PC and photo-Fries rearrangement products (Diepens and Gijsman, 2006).
2.1.2 Photo oxidation mechanism
When PC is exposed to longer wavelength under the presence of oxygen, the photo induced oxidation will predominate. (Rivaton et al., 2002). The products are hydroperoxides and aromatic ketones. Exposure to a longer UV-A wavelength around 310nm-350nm is also results in the yellowing of the sample. (Diepens and Gijsman, 2006).
With aid of Figure, oxidation pathway is proceeding by a few general sequences. Excitation by UV radiation initiates photolytic cleavage of hydrogen from the polymer backbone to yield primary methylene macroradicals (A). The macroradicals will rearrange into more stable tertiary benzylic radicals (B) or the P• radicals in Figure (Rivaton et al., 2002).
(Rivaton et al., 2002).
The P• radicals is readily reacts with the surrounding oxygen to form peroxy radicals (POO•). An additional P• radicals and hydroperoxide (POOH) are formed when the POO• radicals reacts with another polymeric chain via hydrogen abstraction. This reaction leads to the formation of other free radicals. The same oxidative pathway is repeated by the new formed P• radicals while the unstable POOH is easily decomposed to give hydroxy free radicals (•OH) and alkoxy free radicals (PO•). All the generated radicals are capable of attacking other polymeric chain to propagate the autocatalytic oxidation cycle (Ciba Specialty Chemicals, 1999; Saron et al., 2006). Eventually, the polycarbonates lost its original compound unit and bring impacts to its physical properties (Durairaj, 2005 p.527-528).
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Figure: General steps for photo-oxidation pathway (Saron et al., 2006).
Figure: Photo-oxidation process of bispenol A polycarbonate (Rivaton et al., 2002).
Mechanism that predominates during the outdoor exposure:
In order to overcome the undesired photodegradation process, the actual mechanism involved during the outdoor exposure should be investigated. Hence, the fraction of UV-A and UV-B must be first determined from the solar spectrum (Diepens and Gijsman, 2006). According to the Arca et al., (1990), solar spectrum reported in Figure contains higher UV-A fraction rather than the UV-B (Arca et al. 1990).
Figure: Seasonal and annual variations in relative solar UV-A radiation at 357deg;N (Arca et al. 1990).
Then, the study was performed by irradiating the polycarbonate films under suntest lamp with different wavelength range. Diepens and Gijsman, (2006) had used Atlas Suntest (CPS) lamp and Suntest XXL+ (XXL+) lamp to examine the degradation products. Before running the experiment, the solar spectrum and both suntesters were obtained and compared. Based on the observation to Figure, the terrestrial sunlight produced irradiation wavelength above 300nm especially the region above 330nm. Comparing the two suntester, the main difference between the CPS and XXL+ is the wavelength range. The CPS contained wavelength less than 290nm whereas the spectrum of XXL+ consisted of the wavelength above 298nm. The small wavelength difference is expected to produce significant result on the mechanism involved. (Diepens and Gijsman, 2006).
Figure: Comparison of the different irradiation wavelength spectra with the solar spectrum (Diepens and Gijsman, 2006).
UV radiation at wavelength below 300nm is responsible for the Photo Fries rearrangement process. To prove this phenomenon, the BPA-PC films were irradiated under both sunsterter and their Photo-Fries products were measured. The result is shown in Figure, the formation rate of PS can be represented by UV absorbance at 320nm. It can be seen that the UV absorbance for both irradiated samples were increasing with the irradiation hours. Obviously, the formation rate of PS is greater for the sample tested with CPS. This indicated that the lower irradiation wavelength emitted by CPS is mosre likely to enhance the rate Photo Fries degradation (Diepens and Gijsman, 2006).
Figure: UV absorbance of BPA-PC at 320nm when irradiated under suntester CPS and XXL (Diepens and Gijsman, 2006).
The irradiated samples were examined using FT-IR in ATR mode to study the surface degradation chemistry. Figure shows the spectrum obtained and each absorption band indicated the presence of different degradation products. PS and DHBP produce absorption band at 1689 cm-1 and 1629 cm-1 respectively. Absorption band at 1713 cm-1 is ascribed to aliphatic chain acids which indicate the oxidation process. Besides that, a weak shoulder of cyclic anhydrides was formed at region 1840 cm-1. Then, the relative absorbance for each absorption band were obtained and depicted in Figure. After 400 hours irradiation, the all absorptions were rapidly increasing with increased irradiation period. Among all the degradation products, the aliphatic chain acids were presence in the greatest concentration (Diepens and Gijsman, 2006).
Figure: FT-IR spectra of irradiated PC films in the carbonyl region for various irradiation times in CPS (Diepens and Gijsman, 2006).
Figure: Relative absorbance at different wavelengths for increasing irradiation times in CPS (Diepens and Gijsman, 2006).
In addition, the aged PC films were analyzed using MALDI-TOF to detect the side chain oxidation products. The MALDI-TOF spectrum of the irradiated PC films under CPS for 477 hours is shown in the Figure below while peaks identification was tabulated in the Table. Peaks A, B and C were initially present in the undegraded PC whereas peaks D, E and F appeared after sample aging. These additional peaks were caused by formation of side chain oxidation products. This mass spectroscopy technique failed to detect any Photo- Fries products because structural rearrangement will not cause molecular weight change in the PC (Diepens and Gijsman, 2006).
Figure: MALDI-TOF spectrum of bisphenol A polycarbonate film degraded for 477 h in CPS (Diepens and Gijsman, 2006).
Table: Structural assignments of the peaks appearing in the MALDI-TOF spectra (Diepens and Gijsman, 2006).
In order to test the presence of hydroxyperoxides, the irradiated PC films were examined with a nitrogen ramp experiment using chemiluminescence. The total light intensity (TLI) measured was proportional to the concentration of hydroxyperoxides in the PC films. According to Figure, the TLI increases within the first 400 hours implied that oxidation of PC had started from the beginning. After 400 hours, TLI maintained at almost constant level whereby the consumption and production of hydroxyperoxides started to establish equilibrium.
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Figure: TLI of PC films irradiated in CPS for different irradiation times (Diepens and Gijsman, 2006).
The irradiated samples were further analyzed with fluorescence spectroscopy to detect the photo-Fries products. PS gave the fluorescence signal around 480nm and its fluorescence intensity is depicted in Figure. After that, the amount of the PS was calculated and the calculation shown that 1g of PC contains 4 x 10-6 mol PS unit. Only about 1 PS unit can be found in 1000 PC units. This has indicated the photo-Fries products of the irradiated PC are only presence in limited concentration (Diepens and Gijsman, 2006).
Figure: Fluorescence of PC dissolved in DCM irradiated for different ageing times in the CPS. Inset shows the intensity at 480 nm for increasing ageing times (Diepens and Gijsman, 2006).
Different analysis studies have shown that both photo-Fries rearrangement and photo-oxidation can occur in the outdoor photo-degradation of the BPA-PC. Nevertheless, the results proved that oxidation process are more predominate under the outdoor exposure. Photo-Fries reaction only takes place in negligible level (Diepens and Gijsman, 2006).
Besides the influence of the solar spectrum, many researchers believe that the atmospheric oxygen is another critical factor on influencing the mechanism involved during the photo-sdegradation. To further confirm the photochemical reaction is mainly due to oxidation process, the influence of the oxygen is also being studied (Diepens and Gijsman, 2008).
Figure: Solar spectrum measured at Sanary France on 1-06-2001 (solid line) and the UV absorption spectrum of an undegraded BPA-PC film (- -) (Diepens and Gijsman, 2008).
From the solar spectrum shown in Figure, the UV-B radiation that will induce photo-Fries activity is present in a limited intensity. Hence, the occurrence of the photo-Fries under the terrestrial sunlight is not significance and does not play an important role. The absorption spectrum of the undegraded PC also indicated that this polymer does not absorb the UV-A radiation (Diepens and Gijsman, 2008).
Hence, other compounds such as chromophoric impurities and atmospheric oxygen are associated with the absorption at wavelength region above 300nm. Some previous researchers postulated that absorption towards longer wavelength is related to the formation charge transfer complexes (CTCs). Under the presence of oxygen, chromophoric impurities can absorb the solar radiation and leads to pre-oxidation process. Consequently, some CTCs will formed and cause the red shift in the polymeric absorption spectrum (Diepens and Gijsman, 2008).
Figure and show the relationship between the oxygen and the formation rate of the CTCs in the BPA-PC. When the BPA-PC films were placed in a reactor under different air pressure, the absorption spectrum for each pressure was recorded. A notable observation from both figures is the absorption is increased with increasing air pressure. In Figure, the absorption at 5 bar argon pressure is greatly reduced compared to 5 bar of air pressure. This phenomenon suggested that concentration of oxygen is responsible for the increased absorption above 280nm. On the other hand, the oxygen can form transient complexes between the BPA-PC, these labile complexes will enhance the photo oxidation by shifting the absorption into solar spectrum. Even though there is only small amount of CTCs absorb at sunlight spectrum, it is sufficient enough to trigger the photo-oxidation process. Another conclusion can be made is the formation rate of the CTCs is proportional to the oxygen concentration (Diepens and Gijsman, 2008).
Figure: Difference spectra of UV absorption spectra of undegraded BPA-PC at different air and 5 bar argon pressures with the UV absorption spectrum of BPA-PC at atmospheric pressure (Diepens and Gijsman, 2008).
Figure: UV absorption at 284 nm for different air pressures for undegraded BPA-PC (Diepens and Gijsman, 2008).
Besides the formation of CTCs, high concentration of oxygen is able to quench the photo Fries rearrangement. According to Figure, the absorption band at 320nm increase faster at 5 bar of argon pressure and 1 bar of air pressure. A negative slope is obtained in Figure, this provides evidence that the formation rate of the phenylsalicyclate is decreased with increasing CTCs. Thus, atmospheric oxygen can act as photo- Fries quencher and promote the autocatalytic oxidation process in BPA-PC.
Figure: UV absorption at 320 nm for different air pressures against irradiation time with an irradiation wavelength of 250 nm (Diepens and Gijsman, 2008).
Figure: Influence of air and argon pressures on the slope of the absorbance at 320 nm against irradiation time with an irradiation wavelength of 250 nm (Diepens and Gijsman, 2008).
Various studies have indicated the oxygen play an important role in the predomination of the light degradation mechanism. Small amount of initiating radicals triggered by oxygen are capable of propagating the autocatalytic oxidation process. High oxygen level enhances the oxidation rate and quenches the photo-Fries reaction at the same time. As a result, photo-Fries reaction becomes less significant in the outdoor weathering of BPA-PC. These observations strongly confirmed the previous discussion whereby the photo-oxidation process is predominated in the BPA-PC (Diepens and Gijsman, 2008).