The development of organic coatings for strip steels with improved resistance to photodegradation

• Main Author: Robinson, A. J.
• Published: Swansea University 2005
• Subjects: 669
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.638692

A flat panel reactor assembly has been designed and built which using a closed loop flow system is accurately able to rank TiO₂ photoactivity into different grades (A, B and C with A being the most stable) by measuring the rates of photogenerated carbon dioxide (CO₂). Using a suitable long path length cell this apparatus is able distinguish between A grade photoactivity in a short time compared to conventional weathering techniques. During prolonged irradiation an increase in degradation rate was observed which is a result of HC1 acid catalysis. Additions of Hydrotalcite removed this catalytic pathway. The next phase of the work added a further ingredient to the PVC films in the form of a phthalate plasticizer. The initial rates of photodegradation were generally increased by plasticizer addition. This is a reflection of both greater oxygen transport in the film and the preferential oxidation of pre-adsorbed plasticizer molecules on the TiO₂ surface. This degradation does not yield hydrochloric acid and so there is no catalytic effect in these samples and the rate of CO₂ formation decreases with time. For a phthalate free plasticizer (mesamoll) which is a sulphonic acid ester, one of the oxidation/hydrolysis products was a sulphonic acid and another sulphuric acid. These catalyse the TiO₂ and so in this instance an acceleration in rate was observed. In the next section of work reported the effects of black, yellow, red, green and blue pigments were evaluated. It was found that the yellow pigments (based on Titania) were ineffective at slowing down photodegradation, and that black was effective only at high concentrations where its light absorbance was enough to overcome the additional charge percolation that it gave as a low concentration additive. Blue and green gave the best stability due to their absorbing the lower energy wavelengths but red dye additions had an accelerating effect due to a form of dye sensitisation of the TiO₂. In the final section of the work a collection of fully formulated panels were prepared and subject to accelerated QUVA weathering. This work showed the importance of TiO₂ grade with low grade TiO₂ giving faster gloss failure and lead to significant delamination which is related to elevated plasticizer loss. Further work on these panels demonstrate that meaningful lifetime extensions were possible through the application of a topical treatment including plasticizer and UV absorber.