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The stabilisation of PVC plastisol using hydrotalcite (HT). / Glyn Martin

Swansea University Author: Glyn Martin

Abstract

The main focus of this EngD Thesis is an investigation into the use of Hydrotalcite (HT) as a photodegradation stabiliser for polyvinyl chloride (PVC) used as a coating for pre-finished steel. The work builds on the use of model systems, both unplasticised and plasticised, in comparing HT with other...

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Published: 2007
Institution: Swansea University
Degree level: Doctoral
Degree name: EngD
URI: https://cronfa.swan.ac.uk/Record/cronfa42522
Abstract: The main focus of this EngD Thesis is an investigation into the use of Hydrotalcite (HT) as a photodegradation stabiliser for polyvinyl chloride (PVC) used as a coating for pre-finished steel. The work builds on the use of model systems, both unplasticised and plasticised, in comparing HT with other commercial stabilisers and culminates with the full formulation and testing of paint systems containing all the relevant commercial pigments and stabilisers. In the first section of work, a flat panel reactor was used to accurately measure the rates of photogenerated carbon dioxide (CO2) as an in-situ measurement of degradation. In unplasticised PVC pigmented with 30% photoactive titanium dioxide, there was a transition to a higher rate of CO2 evolution after a set amount of degradation which is attributed to the formation of hydrochloric acid. In this instance, the addition of up to 10% HT in model system removes this catalytic effect as the HT exchanges chloride ions for carbonate. This initial result suggested that HT could be a useful stabiliser and the results on model systems showed it to be far more consistent in its performance over a wide range of concentrations compared to commercial barium/zinc and tin based stabilisers. In model films containing plasticiser molecules based on phthalates the effect of HCl catalysis was not found to occur. An increase in initial rate of CO2 evolution and an absence of HCl production indicates preferential attack on the plasticiser rather than the PVC matrix. When more sulphonic acid ester (phthalate free) systems are used, acidic fragments and HCl are produced and these lead to a similar acceleration in degradation with UV exposure time observed with PVC alone. This indicated that in such systems the addition of a stabiliser that could remove hydrochloric acid would be beneficial. In near commercial PVC plastisol systems prepared on steel substrates, HT has been compared to historic (tin), existing (barium zinc) and future (calcium zinc) stabilisers. In all cases, regardless of titanium dioxide grade, the HT performed very well in terms of colour retention and gloss following QUVA weathering cycles with the HT preventing dehydrochlorination within the coating which leads to darkening on exposure. In the final section of work completely stabilised and fully formulated paint systems were prepared using the HT and commercial stabilisation systems and exposed to extended QUVA and QUVB cycles. Even in these systems the HT is demonstrated to perform consistently and is in almost all respects superior to existing stabilisation chemistries. The positive effects are shown for both white and coloured systems. In a summary, it seems that HT is able to stabilise PVC coatings applied to steel substrates and exposed to arduous weathering cycles. This stabilisation reflects the ability of HT to remove hydrochloric acid within the film which reduces dehydrochlorination, coating darkening and degradation. HT is a cheap mineral material and is extremely easy to blend into paint and as such is a potentially promising pigment addition to make to painted product to reduce acid catalysed degradation.
Keywords: Materials science.
College: Faculty of Science and Engineering

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