TECHNOLOGY | WOOD PLASTIC COMPOSITES
certification and chair of the ASTM subcommittee on WPC decking. Wagner explains that ASTM D 7032 outlines
Above: Wood fibre filled PLA pellets and 3Dprint filaments produced in a Fraunhofer WKI development project
Mixaco’s HM/KM Wood mixing system is specifi- cally designed for WPC and provides a dehumidify- ing effect that reduces moisture. The system comprises an HM Heating Mixer and KM High Efficiency Horizontal Cooler, adapted with other features for mixing of wood or natural fibres with PVC/PP/PLA or other polymers. “Even with a moisture above 10% [in incoming material] we can achieve a residual moisture of less than 1% in the final blend,” said Gehring. If moisture is not effectively removed, the
material will stick to surfaces and flowability of the blend is affected. “Moisture inside a WPC blend affects the
process stability in further processing,” he said. He adds that Mixaco has experimented with WPC mixing and has defined a standard mixing process for most applications using its system.
Testing for building Testing requirements for WPC in the US are governed by ASTM D7032, which was adopted into the US model building codes beginning in 2009, with only editorial updates since the original 2004 version, according to Craig Wagner, chief engineer for Intertek Building & Construction product
testing requirements for composite decking materials that include end-use factors (for example, flexural performance, load, creep recovery), biodeterioration, and surface burning characteris- tics. “IBC [International Building Code] Chapter 26 and IRC [International Residential Code] R507 both apply to deck boards, stair treads, handrails, and guards, and evaluate structural performance (span/ load rating), end-use factors (temperature and moisture effects), durability (weathering), and termite and decay resistance,” he said. Wagner says that one area of regulation and testing that has expanded over recent years is the adoption of special fire resistance testing for applica- tion in areas prone to wildfires, commonly known as Wildland Urban Interface (WUI) requirements. Another aspect of testing that WPC processors should be aware of is the need to investigate the range of colours that might be used in a product. “This is not so much a concern for the colours (pigments) making a significant difference in the various test results, as it is more for making sure that you’re addressing differences in the base material compound. Sometimes one or more capstock colors are a different material from the others [for example, acrylic or PVC]. Another variable in [coloured] compounds could be different additives [such as] the amount of additive for UV stabilisation. UV resistance is one of the primary requirements of the product standard and the complete range of different compounds used must be considered in the evaluation,” he said. Recycled plastics can potentially have more variability than virgin plastics, which is another consideration in testing. “A trend that we’re seeing in the use of recycled plastics is manufacturers getting into the plastics recycling business to gain greater control over the
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IMAGE: FRAUNHOFER WKI/MANUELA LINGNAU
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