MATERIALS | WATERPROOF MEMBRANES
stress – lead to cracking and field damage. Sheet thickness is another factor behind potential lifetime. Although more often associated with pipes, geomembranes can undergo stress cracking and rapid crack propagation under certain conditions. Some particular failures include: ductile yielding – in which the geomembrane is overstrained; slow crack growth, caused by material creep; and brittle failure, where stress concentrations lead to failure. One of the main ways to prevent these mecha- nisms is to restrict UV light – and this can be done by adding carbon black to formulations, he said. “It acts as a light screen, absorbing harmful UV
Above: Geomembranes must be protected from UV exposure in order to extend their lifetimes
with impurities – such as other minerals – which must be removed. One way that this is done is with X-ray separators, which helps to identify and separate these impurities. There are several challenges to developing a
flame retardant for TPO roofing membranes, including: dimension stability; a necessary lifetime of around 20 years; good mechanical properties; and good ageing resistance – under conditions of high temperatures, external UV exposure and weathering factors such as oxygen and humidity. The company developed a number of research
grades, with varying granule size and chemical composition. One grade – Ecopiren 3,5 – is aimed at TPO and PVC roofing membranes. It has a particle size of around 5.5 microns, and magne- sium hydroxide accounts for nearly 93% of the formulation. For a typical membrane, it was used at a 40% loading, with a PP roofing grade co-polymer, for instance. The research grades were tested for mechanical
properties, such as elongation variation after 10 weeks, elongation at break and tensile strength. They also performed well in ageing tests – and in fire tests. “Ecopiren 5,5C, with a low content of Fe2
O3 –
and a 1.5% coating by stearic acid – showed the highest OIT and highest stability during the ageing at 140°C in comparison with synthetic FR fillers,” said Kulichenko.
UV protection As with many types of plastic product, longevity is a key issue. Derek Hepburn, technical director at Colloids, told delegates of some of the factors behind extending the life of waterproof mem- branes using carbon black within the formulation. When assessing possible lifetimes for geomem-
branes, he said that degradation due to environ- mental factors – such as UV exposure and thermal
14 FILM & SHEET EXTRUSION | May 2020
radiation before it reaches the photo chromophic species in the polymer molecule,” he said. “As well as being a UV absorber, carbon black acts as a radical scavenger.” However, carbon black can be designed to give maximum effect by ensuring that molecules are of the correct size and morphology. It can also be combined with other additives to lengthen product life.
By combining the carbon black with a hindered amine light stabiliser (Hals) system also offers higher protection. There is also a synergistic effect between the Hals and an acid scavenger.
Formaldehyde challenge Many compounds, across a range of applications, are now being scrutinised for the use of potentially harmful chemicals. In plastic pipes, for instance, lead- and chromium-based heat stabilisers have been phased out in Europe. In waterproof mem- branes, a related issue is the use of formaldehyde in latex binders. “Formaldehyde is a key raw material with significant health and safety implications,” said Sören Butz, head of technical service for SBU functional solutions at Synthomer in Germany. Formaldehyde is a key component in the
crosslinking of binders in applications such as roofing membranes. Within the textile industry, there are moves to restrict exposure to formalde- hyde – though exact limits have not yet been set. Butz explained how Synthomer has developed a polymer binder for PES roofing felt with the aim of reducing formaldehyde emissions from “in the ppm range” down to levels that are “below the detection limits of Law 112 and VDI 3862.”. The aim was to develop self-crosslinking latex without any emissions and use low temperature curing. The 1K system that the company developed now offers these ultra-low emissions. The self-crosslinking XSBR latex has a high performance, and has no extra requirements on
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