MATERIALS | PVC STABILISERS
Right: Heat and UV radiation from sunlight have a
detrimental effect on all types of outdoor PVC products
usually applied as so-called ‘one-pack’ systems. These tailor-made blends of ingredients include acid scavengers and co-stabilisers. The former are usually metal salts which can neutralise free HCl. The latter are well-defined organic substances, which should prevent the continuous release of HCl and protect the chemical structure of the PVC. Additionally, organic anti-oxidants are typically added to a one-pack to suppress oxidative degra- dation of the PVC. One-packs also contain various lubricants – ei- ther crude-oil-based hydrocarbons or waxes derived from oleochemical feedstock. These lubricants regulate the melting behaviour of the PVC in the processing machines and enhance its ability to withstand the shear stresses involved. In recent years, most PVC processors have changed their stabilisers from lead-based to calcium-based systems. In terms of the weather- ability of the final products, these traditional systems were sometimes superior to calcium- based systems.
The role of TiO2 As a white pigment, TiO2
is widely known as part of
the formulation of white PVC products, imparting whiteness, brightness and high opacity. Its ability to absorb UV light can significantly improve the weatherability and durability of white-coloured polymer products. Nonetheless, it can also play an important role in
dark-coloured PVC applications. In grey or anthra- cite products, it is usually blended with carbon black to achieve the optimum colour. Dark brown PVC products are typically colour-matched by red, yellow and black pigments. In most brown applica- tions, no TiO2
using small amounts of TiO2
Right: Tailored formulations can help to extend the life of dark PVC rain gutters
is added for colouring reasons. By in all dark-coloured
PVC articles, though, the PVC matrix gains extra
weathering protection. Pure TiO2
is known as a photo catalyst generat-
ing reactive photo radicals at its untreated surface – a property which is not desired in PVC products.
To prevent photo-catalytic degradation, TiO2 pigments for outdoor PVC applications are typically coated with inorganic oxides such as silicon dioxide, aluminium oxide and others. This surface treatment creates an extra energy barrier and prevents the formation of photo radicals.
Study details For the study, three different colours were selected for the formulation of PVC-U profiles: white, dark brown and anthracite grey. For the pigmentation, the focus lay on two different TiO2
pigment grades: one
was a highly durable rutile grade; the second was a super-durable rutile grade with a heavier coating. Three different stabiliser/lubricant one-packs
from Baerlocher – for use in PVC profiles such as gutters, sidings or window profiles – were employed, including one lead-based and two calcium-based systems – standard and premium. These systems are meant for use in weathered PVC-U applications but are not specifically optimised for dark colours. The calcium-based premium grade stabiliser contains higher levels of stabilising components than the standard grade. This results in better values in the so-called Congo Red thermal stability test of 28 min in the white window profile formulation compared to 24 min with the standard stabiliser grade. Lubrication of the three one-packs was adjusted in such a way that similar process conditions could be used for the preparation of all samples. The dry- blends with the various stabiliser one-packs (includ- ing processing aids) and the different pigments were prepared in a standardised mixing process. Extruded strips were prepared on a conical
twin-screw extruder with a flat die and calibrator. The 14 PIPE & PROFILE EXTRUSION | July/August 2018
www.pipeandprofile.com
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