Enhancing UV protection of clear coated exterior wood by reactive UV absorber and epoxy functional vegetable oil

• Since ancient times wood has played a large role in human activities, both in terms of fuel and for construction purposes. A drawback with wood in exterior applications is, however, its susceptibility to photo-initiated degradation, caused by radiation from the sun. Hence, wood needs to be protected against ageing by means of different surface treatments. The work presented in this thesis describes a possible method for protecting wood against photo-initiated degradation. The system comprises the UV absorber, 2-hydroxy-4(2,3-epoxypropoxy)-benzo-phenone (HEPBP), which has a primary epoxy group with the ability to react covalently with hydroxyl groups, enabling reaction with hydroxyl groups of the wood (preferably phenolic hydroxyls in lignin). The epoxy functional vegetable oils (soybean and linseed oil) contain secondary epoxy groups which theoretically also enables reaction with the hydroxyl groups of the wood. This ability and a possible reaction with wood could result in a more long term protection since it prevents leaching of the protective substances. The study includes evaluation of several reaction parameters of the pretreatment, such as temperature, time, the influence of the oil, and also the photo protective ability of the sole pretreatment. The results show that a reaction temperature of 102 °C or higher results in presence of the reactants on the surface even after extraction, indicating grafting. Two different reaction procedures imply that reaction for 16 h results in slightly better results compared to a 1 h dipping procedure with 2 h subsequent drying in an oven. However, in terms of time and energy consumption the difference is considered too small to defend using the longer reaction procedure. Colour measurements of samples before, during and after ageing indicate a better performance of the pretreated samples compared to the untreated, and an effect of the oil is also noticed, giving the colour change a more stable increase that levels off instead of continuing to increase. The study also takes into account the use of a clear coating together with the pretreatment to study the performance of a complete coating system, in terms of photo protection. Coating of the pretreated samples is shown to work, with uncompromised adhesion as a result. Evaluations of natural and accelerated weathering of the full systems indicate only minor degradation after 1400 h of accelerated ageing or 14 months of natural exposure. After 4000 h of accelerated ageing, visible signs of degradation are detected, but the pretreated samples perform slightly better than the untreated. After 26 months of natural exposure the samples had too much mould in order to perform a reliable evaluation of the photo induced degradation. Overall the pretreatment is concluded to have a photostabilising effect of the wood.

Ämnesord

NATURVETENSKAP  -- Kemi -- Polymerkemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Polymer Chemistry (hsv//eng)

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