PVC STABILISERS | MATERIALS
New formulations of stabilisers and pigments can help to protect dark- coloured PVC products – such as rain gutters – against the effects of the weather, writes Jörg Fröhlich
Optimising weathering for dark-coloured PVC
PVC has become one of the most important com- mercial polymers due to the wide variety of its applications. This also applies to architecture and the construction industry, where PVC is used, including window profiles, roller shutters, sidings, hollow soffits and rain gutters. What these outdoor PVC applica- tions have in common is that they are exposed to extreme weather conditions – a combination of summer heat, high UV radiation and moisture. White PVC products are very resistant to heat and light because of the excellent reflective properties of the commonly used titanium dioxide (TiO2
) pigment. Dark colours, such as brown or
anthracite grey, however, run the risk of bleaching and discoloration over time as a consequence of changes in their chemical structure. In a joint research study, Baerlocher and Chemours have looked into overcoming this problem by developing new formulations for grey and brown PVC profiles, using calcium-based heat stabilisers and TiO2
pigments.
Outdoor challenge During the summer, outdoor PVC products – espe- cially dark-coloured ones – can become very hot due to solar irradiation. This can result in tempera- tures of 80°C and more. In addition, the products are exposed to high UV radiation – while moisture
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accumulates in the PVC and acts as a reaction medium. Under these conditions, the absorbed UV energy can trigger reactions with oxygen or water molecules. The radicals formed are reactive enough to destroy organic bonds in the polymeric matrix, degrading the PVC. The elimination of hydrochloric acid (HCI) from the polymer – the so-called dehydrochlorination – happens as a parallel process. The free HCI can act as a catalyst for further elimination reactions, resulting in a self-accelerated mechanism of PVC degradation. The pigments – inorganic colour pigments or – become more accessible at the
white TiO2 surface. Furthermore, calcium carbonate (CaCO3 ), a
commonly used filler, is washed out from the PVC leading to the formation of microscopic holes. Since these degradation processes typically lead to a porous surface with a whitish appearance, this phenomenon is often referred to as ‘chalking’ which, besides discoloration, results in a loss of mechanical properties and a loss of gloss.
Stable performance Neat PVC has a low thermal stability during processing in the melt. Hence, stabilising additives are added to protect it during processing and to improve the final product. The stabilisers are
July/August 2018 | PIPE & PROFILE EXTRUSION 13
Main image: The formula- tion of the stabiliser is key to significant improvement of the weather- ing resistance of the final product
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