Photodegradation and Stability of Bisphenol A Polycarbonate in Weathering Conditions

Abstract

Polycarbonates, and especially bisphenol A polycarbonate (BPA-PC), are used in many fields of applications due to their excellent physical and mechanical properties, such as high impact resistance, ductility, and transparency. One major drawback of using polycarbonates in outdoor applications is that the polymer degrades under the influence of UV-light, humidity and oxygen. This undesired photodegradation process is initiated by the absorption of the terrestrial sunlight. Although there has been quite a lot of published work on the degradation of BPA-PC, there is still no consensus on what is happening in outdoor weathering conditions, since most of these studies were performed under different (often accelerated) ageing conditions. The aim of this thesis is to get a more detailed knowledge about the degradation chemistry and the stabilization of bisphenol A polycarbonate in outdoor weathering conditions, which could lead to more stable polymers which will broaden their range of applications. In the literature the chemistry underlying the photodegradation has been ascribed to two different mechanisms: i.e. photo-Fries rearrangement and photo-oxidation. The relative importance of the photo-Fries rearrangements and the photo-oxidation depends on the irradiation wavelengths used. It is known that the photolytic photo-Fries rearrangements take place at short wavelengths (300 nm) only photo-oxidation reactions take place. The wavelength distribution of terrestrial sunlight starts around 300 nm, which implies that both degradation reactions can be present in outdoor weathering conditions. In this thesis it is shown that when BPA-PC is exposed to outdoor weathering conditions, the photo-oxidation reaction is dominant. However, this reaction needs to be initiated by an initiating radical. Small amounts of free radicals can already be su±cient to initiate the autocatalytic photo-oxidation reaction. In the literature the source for initiating radicals is under debate. It was assumed that the initiating radicals were formed by photo-Fries rearrangements, however there was no convincing evidence. In the first part of this thesis, possible initiation sources were explored and their influence on the photo-oxidation reaction was studied. It was shown by using different spectral analysis methods, such as UV-spectroscopy, FT-IR spectroscopy, and fluorescence spectroscopy, that photo-Fries rearrangement products were formed in accelerated outdoor exposure conditions, albeit in very low concentrations. Nevertheless, they do not act as initiators for the photo-oxidation reaction. Thermally produced hydroperoxides, which are known as photo-oxidation initiators for polyolefins also do not influence the photodegradation rate of BPA-PC. The influence of oxygen was determined by degrading BPA-PC films at different oxygen pressures. It was found that charge transfer complexes were formed between oxygen and the polymer. The absorption of these complexes tails into the terrestrial sunlight wavelengths and might lead to photo-oxidation initiating radicals. It was also shown that at higher oxygen pressures the photo-Fries rearrangement reactions are quenched. It was shown that small wavelength fluctuations in the onset of the irradiation spectra (starting from 290 to 300 nm) lead to different ratio between the photo-Fries and the photo-oxidation reaction. Though, in the second part of this thesis a linear relationship between the irradiation intensity and the photodegradation rate, according to the reciprocity law, was found. By retaining the wavelength distribution, the intensity has no effect on the degradation mechanisms. In the third part of this work, di®erent stabilizing techniques were studied. The action of conventional stabilizers and block copolymers based on resorcinol polyarylate, which rearrange themselves into a protective top layer, were compared. The best way to stabilize BPA-PC against photodegradation is to keep the light out by using UV-absorbers or protective coatings. The addition of hindered amine light stabilizers did not greatly affect the degradation rate. In addition, the best results for stabilizing BPA-PC were obtained when the (harmful) UV light was absorbed by hydroxybenzophenones. Especially when a high concentration of UV absorbing species is located on top of the polymer film, i.e. by a secondary film or by a resorcinol polyarylate block copolymer. In the last part of this thesis the results of the accelerated and outdoor tests were compared. Due to a different wavelength distribution of the spectra for the light used in the accelerated test and the terrestrial sunlight, a different ratio between the photo-Fries and the photo-oxidation reactions for accelerated and outdoor weathering was found. Due to this wavelength sensitivity of BPA-PC towards the lowest wavelengths of terrestrial sunlight, it is difficult to make a good lifetime prediction for BPA-PC used in outdoor applications with accelerated tests.