MATERIALS | WPCs
Right: Heldebrant: “Our product works like WPC decking, but stores about 5% by weight of CO2 inside”
He said the US decking market is about 3.5bn linear board feet, and about 1ft of 2 x 4 of this material will weigh about 1kg. “If you factor in 5wt% of 3.5bn
kilos, it starts adding up substan- tially,” he says. “This isn’t just a small niche product. It could potentially remove the carbon emissions of more than 50,000 cars in the decking market alone.” The research challenge, he said, was to find a
product that was ‘CO2 negative’ and would store it for a long time – while also turning a profit. The fibres begin life as two waste products:
brown coal and lignin – a waste product from papermaking. These are then chemically treated – to add ester group’s to the particles’ surface – so that they can be compounded into a plastic matrix. “Esters are essentially carboxylic acids, which are
a captured form of CO2,” said Heldebrant. The team wanted to put CO2 onto the surface of
the particles in the composite to make the material even more environmentally friendly – while improving its mechanical performance. They used a chemical reaction to form a bond between captured CO2 molecules and a group called a phenol – which is abundant in wood products like lignin. After the reaction, the lignin and coal particles contained 2–5% CO2 by weight. To test the feasibility of this approach, the team turned to a classic chemical reaction to form a new chemical bond between CO2 and a functional group called a phenol, which is abundant in wood products like coal and lignin. After undergoing the reaction, the lignin and coal particles contained 2–5% CO2 by weight. The team mixed varying ratios of these particles with HDPE to form composites and tested their properties. A composite with 80% filler maximised the CO2 content while showing strength and durability that met international building codes for decking materials. It was made using friction extrusion. The researchers used the material to form 10ft composites that look and feel similar to standard WPC boards. Heldebrant claims the new boards are 18%
cheaper than standard versions. They also store more CO2 than is released during their manufac- ture and lifetime, he says. The researchers plan to make more composite
formulations and test their properties. They believe that carbon-negative composites could be devel- oped for many building materials, such as fencing and siding. In the meantime, they are looking to
26 PIPE & PROFILE EXTRUSION | Winter 2024
commercialise the decking boards – and say their ‘carbon-negative’ decking could be available as soon as they summer.
Joining up WPCs illustrate the environmental benefit of combining wood and plastics. In the case of decking, for instance, WPCs are more durable than
timber equivalents, giving an environmental
benefit. Meanwhile, scientists at Graz University of
Technology in Austria have devised other ways of combining plastics with wood – but in this case, by sticking one solid element to the other. Using wood, they argue, has an environmental advantage as it is a renewable resource. In both cases, joining is achieved without using
either traditional adhesives or mechanical fasten- ers. One technique is based on 3D printing, while the other uses ultrasonics. The first method, called AddJoining, uses a 3D printing technique called fused filament fabrication (FFF) to deposit molten polymer directly onto the surface of the wood substrate. The printed material penetrates into the wood pores, where a chemical reaction occurs – similar to the reaction of glue with wood. The technique was used to make overlap joints for mechanical testing – which took less than an hour. It was also used to make demonstrators of possible aerospace applications – such as skin- stringer connections, which connect the plane’s body to the outer ‘skin’ – without adhesives or rivets. A technique called laser texturing creates
precise, micron-sized ‘grooves’ on the wood surface, to act as anchor points and increase the area of the wood-polymer interface. ‘This greater contact area increases the mechan-
ical strength of the hybrid structure,’ says Sergio Amancio, who leads the research. The second technique uses ultrasonic joining,
where the key is to control energy transfer, pressure and temperature to avoid damaging the wood while melting the polymer. Surface preparation and material compatibility are also critical. The technique could be used in applications such as automotive interior panels and furniture where ‘natural aesthet- ics and functional durability is needed’. Amancio believes the techniques could be
commercialised within three to five years, depend- ing on capital investment and future R&D budgets.
Sustainable benefit In a 2024 paper in Polytechnica, Daniel Friedrich of
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