The natural touch

Paul Boughton

Sustainably sourced fibre reinforcements – ranging from banana fibres to wood flour – can boost the properties of plastics and reduce carbon footprint. Lou Reade reports.

Fibre reinforcement helps engineering plastics to replace metals in a host of applications by boosting mechanical properties like stiffness and strength. Many of the most demanding automotive and electronics applications would still have to be made in metal, if it were not for the use of fibre reinforced plastics. Glass fibre continues to dominate but there is also increasing use of carbon fibre reinforced parts, especially for structural parts in the automotive sector.

The whole field of fillers and reinforcements for plastics is undergoing change, in the face of the green revolution. While there is no end in sight for the use of conventional fibres, a new generation of naturally derived materials is finding use in a variety of industries.

Some of these, like wood plastic composites (WPCs), are fully commercialised. WPCs usually comprise a commodity plastic like PVC or polyethylene (PE), with added wood flour or wood fibres. This creates a new type of material that can be cut and shaped just like wood – but is far more durable and weather resistant.

On deck

The material was popularised in North America – where it was used as a replacement for wood in fencing and decking panels – and has now spread to Europe. In fact, WPC decking panels were used in London’s Olympic Stadium, as one of its many examples of sustainable construction.

The 4000 sq m O2 VIP platform was made of WPC panels from Vannplastic, based near Chester in the UK.

Its Ecodek panels are a blend of recycled high density polyethylene (HDPE), wood fibres and compatibilising additives.

The main ingredient (HDPE, 55 per cent) is mainly derived from recycled milk bottles. This material costs about 75 per cent of the price of virgin material, but is of high quality. Wood fibres – from post-industrial beech hardwood from a sawmill in northern France – account for 40 per cent of the mix. The remaining 5 per cent is the pigments, UV stabilisers and coupling agents needed to hold everything together.

“Lots of companies skimp on the additives, but we invest a lot in them,” says Alex Collins, technical director at the company.

Although the formulation for the boards was quite standard, he says the testing was very rigorous.

“The Olympic Delivery Authority (ODA) take safety very seriously, so we had to do lots of physical testing,” he says. “I was pleasantly surprised that our boards performed so well.”

There was also a need to redesign the panels slightly, for safety reasons.

“The organisers didn’t want debris falling through the gaps – in order to minimise the risk of fire – so the boards had to be redesigned,” says Collins.

Working with stadium designer Populous, the company redesigned the edges so that they were ‘S’ shaped – allowing an overlap. This closed the gap to debris and litter, but still allowed rainwater to drain through.

In a one-month production surge, Vannplastic produced 110 tonnes of decking. The decking area accounts for around 10 per cent of the Olympic stadium’s circumference.

And it’s not just wood fibres that can strengthen plastics in this way. Researchers at the Universiti Teknologi Mara in Malaysia have created a new type of WPC using a polypropylene (PP) matrix with fibres from the kenaf plant. They say that their new composite materials have higher performance and durability than existing alternatives.

Their study assessed the use of powdered kenaf core fraction (which makes up about 65% of the whole stem of the plant) as a filler material.

Kenaf stems contain two fibre types, called bast and core. Dosing with a compatibiliser strengthened the bond between the ground kenaf core (GKC) and plastic in the WPC. This improved stress transfer and increased strength and stiffness, allowing more filler to be used. Reducing the amount of plastic while increasing the amount of GKC – without sacrificing strength, stiffness or durability – would result in ‘greener’ WPC products.

WPCs of PP and GKC fibre, dosed with compatibiliser in the right amount, bridged the interface between GKC and plastic, improved stress transfer and increased strength and stiffness – as well as allowing a higher filler loading of 65 per cent.

Panel game

There are many reasons for choosing natural fibre reinforcements. They may be readily available, and offer clear benefits (as in the case of wood fibres for WPCs); they can also help to reduce the carbon footprint or recyclability of a product, which is becoming more important.

At the same time, the polymer itself may be bio-based – so using a naturally sourced filler or reinforcement makes perfect sense.

A good example of this is in the pan-European Cayley project, which aims to develop panels for the transport sector based on renewable polymers and natural fibres. The panels, which are used as sidewalls, ceilings, fairings and overhead lockers in applications such as aircraft, buses, ships and trains, are usually made from materials like phenolic resin and glass fibre. The project partners – Boeing Research and Technology Europe, Aimplas of Spain, Invent of Germany and Lineo of Belgium – intend to replicate the panels using more sustainable materials, while creating a commercially viable manufacturing method.

At the recent JEC Europe event in Paris, project co-ordinator Maik Wonneberger of Invent said the panels were likely to be made from polymers based on linseed oil (rather than petroleum), and natural fibres like flax. Flax fibres have similar mechanical properties to established materials: their Young’s modulus is comparable with that of glass fibre, while their elongation at break approaches that of carbon fibre. Also, their density (at 1,45g/cm3) is lower than any conventional fibre.

The panels would be produced by pressing together a pre-impregnated fibre material with thermoplastic sheets. The partners aim to develop a resin that cures within 15 minutes.

Wood in the car

Automotive giant Ford is testing a cellulose fibre-based plastic called Thrive – from Canadian wood products supplier Weyerhaeuser – for stiffness, durability and temperature resistance.

As well as cutting carbon footprint, Ford discovered other advantages of the material: it created parts that were 10 per cent lighter – and could be made 20-40 per cent faster – than those using traditional glass fibres.

“Using composites with cellulose fibres makes sense,” said Ellen Lee, plastics research technical expert at Ford. “Their excellent thermal stability allows us to extend the range of potential automotive applications for natural fibre materials. With increased use of these renewably sourced materials, we can reduce the environmental footprint of our products while accruing a variety of benefits across our entire supply chain.”

This material is only one example of natural fibre reinforcement used in Ford cars: the Flex uses wheat straw filler in its plastic bins; door bolsters in the Escape use kenaf fibre; and the Focus Electric uses a wood fibre-based material in its doors.

Weyerhaeuser says that Thrive will initially be used in household goods and automotive parts, but also has potential for office furniture, kitchenware, small and large consumer appliances and other industrial goods. It is available in as a masterbatch for custom compounders, or as ready-to-mould thermoplastic pellets for moulders.

“Thrive composites are low mass, yet demonstrate excellent tensile strength and flexural properties,” said Don Atkinson, vice president of marketing and new products for Weyerhaeuser’s cellulose fibres business. “They can improve moulding cycle times by up to 40 per cent.”

He added that the compounds also caused less wear and tear on processing equipment compared with those containing glass fibres.

The compounds are available as cellulose blended with polypropylene (PP), with both high and low melt flow indices. Because cellulose fibres are compatible with many polymers, Weyerhaeuser plans to expand their use beyond PP.

Packed with fibre

It seems that researchers have tried reinforcing plastics with just about every inedible part of every foodstuff they can find.

Scientists at the University of Kassel in Germany have assessed the effectiveness of cellulose-reinforced, bio-based polyamide. PA is usually reinforced with glass fibre, but the growing number of bio-based grades of PA has inspired the hunt for natural alternatives.

The researchers found that cellulose improved the mechanical properties of two grades of PA. Their heat distortion temperature could almost compete with the glass fibre-reinforced versions, they said.

Similarly, researchers at the University of Waterloo in Canada substituted talc with wheat straw, as a filler for polypropylene. They found some product improvements: while talc boosted flexural modulus at loadings of 20 per cent, wheat straw was effective at higher loadings (30-40 per cent). However, the effect on mechanical effect was limited.

And researchers from Bishop Moore College in Kerala, India found that a 20 per cent loading of banana micro fibrils in low density polyethylene (LDPE) helped to boost mechanical properties (such as tensile strength) by 20 per cent. Higher loadings caused the fibres to agglomerate, which reduced tensile strength.