Photo-oxidative degradation mechanisms in styrene–ethylene–butadiene–styrene (SEBS) triblock copolymer
Introduction
Thermoplastic elastomers (TPEs), like all unstabilised organic polymers, degrade upon exposure to sunlight in the presence of oxygen [1]. The types of degradation depend on the environmental conditions, the manufacturing history and the structure of the polymer [2]. TPEs are relatively new development materials, and consequently research studies on their degradation are so far limited. In styrenic elastomers the degradation processes may occur in both the polystyrene and the elastomer phases. The elastomer phase is more liable to degradation because its low Tg ensures permeability to oxygen and ozone, and block copolymers are known to be highly permeable to oxygen. Thus, the type of centre block would determine the behaviour of styrenic thermo-elastomers during ageing [3]. The presence of the polydiene phase in the unsaturated type of styrenic elastomers gives a limited resistance to ageing, because the elastomer segments contain one double bond per monomer unit; these bonds are quite reactive and limit the stability of the product when exposed to high temperature, UV light or ozone. The hydrogenated types such as poly[styrene-b-ethylene-co-butylene-b-styrene] triblock copolymer (SEBS) are saturated, and thus, much more stable.
A previous study on the photo-oxidative degradation of SEBS [4] showed chain scission to occur at the boundary of the polystyrene–olefin phases, forming acetophenone end-groups at the styrene units and carboxylic acids at the olefin chain ends. The olefinic phase exhibited severe oxidation associated with the initial formation of primary hydroperoxides. No evidence of crosslinking was found. This work aims to provide additional data through an understanding of the implications of the styrene blocks in SEBS degradation by means of a comparison with a polystyrene material synthesised under similar conditions. This coupled with the study of varied wavelengths and a more detailed analysis provided comprehension on the implication of the light sensitivity of different phases. Thus, the samples were photo-aged at wavelengths cut-off below 290 nm with a spectral distribution similar to that of sunlight, and with short and long-wavelength low-pressure mercury lamps. The ultraviolet part of the solar spectrum has enough energy to break some chemical bonds and cause photodegradation and the shorter wavelengths are more effective in this regard [5]. The short wavelength part of the ultraviolet radiation is absorbed by the earth's atmosphere, so that the shortest wavelength reaching the surface of the earth is about 290 nm. At wavelengths above 350 nm photo-reactions in polymers are usually very slow. Thus, the wavelength region of 290–350 nm will determine the light stability of polymers [6]. However, according to the first law of photochemistry in order for photodegradation to occur the radiation must not only have enough energy to break some of the bonds but must also be absorbed by the polymer. Light at 365 nm in this work will allow selective excitation of the hydroperoxide groups. Previous work on the ozone stability of SEBS has also shown susceptibility to be due to the presence of impurity double bonds [7], [8].
Section snippets
Materials
The SEBS, of general Structure 1, and polystyrene materials were supplied by Repsol-YPF (Madrid, Spain) in the form of pellets and/or sheets and all the samples were of experimental grade. The polystyrene was prepared in the same process (Li catalysed) as used commercially for the SEBS. The SEBS was compression moulded at 205 °C. The sample grades together with their molecular weights are shown in Table 1. Ratios of polystyrene to olefin are proprietary. All the solvents and chemical products
Results and discussion
Commercially, colour formation is the main problem facing the manufacture and processing of polymers. Colorimetry results showed that the polystyrene had a longer induction time for the formation of chromophores that absorb visible light than SEBS (Fig. 1). However, after this initial yellowing, not visible to the eye, the values of yellowness index (YI) for the SEBS sample reached a plateau and remained relatively constant upon increased irradiation. Both samples showed an increased yellow
Conclusions
Studies of the physical and chemical changes of unstabilised SEBS upon exposure to light showed that both styrenic and olefinic phases undergo degradation, resulting in the discolouration and loss of properties. Initial amounts of hydroperoxides and acetophenone groups, present as impurities from manufacture and processing were measured in the unaged material. These photoactive species are responsible for the photo-initiation reactions in the polymer. Rapid hydroperoxide growth was observed in
Acknowledgements
The authors thank Repsol-YPF, Madrid, for partial financial support of this programme of work and supply of the materials and GPC results.
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