Bio composites become a genuine alternative

Tom Shelley cuts through some of the green hype and highlights where biologically produced materials and sustainability really can bring benefits

Despite the many promises, 'green materials' have frequently failed to live up to expectations, often falling short of their quoted performance and, in many cases, not possessing the green credentials first thought. But, there are signs that things are dramatically improving as they increasingly offer significant engineering benefit over conventional oil derived materials.

The term composite has commonly come to represent a carbon fibre weave infused with an oil based resin to form a reinforced plastic. However, examples of composites are numerous and include fibre glass – or fibre reinforced polymer - and Kevlar reinforced plastics.

But, the use of natural fibres as a reinforcement is showing the potential to offer process benefits relative to those materials reinforced with glass and, in some cases, these have properties that are superior to conventional plastics.

Additionally, the resins can be made from biological sources that can be designed to biodegrade. These have potential applications in the medical sector for implants used on broken bones, which instead of needing to be removed would gradually biodegrade as the bones heal. The big benefits though would come from much wider use in terms of sustainably and the ability to recycle.

And, although many materials claim to be green, they perhaps do not tell the whole story. Toyota says it has developed the world's first injection moulded material to be derived from plant matter saying, 'the new Ecological Plastic is used throughout the new Prius, and will be featured widely in Toyota's new model ranges in the future. In the short term, Toyota's aim is for Ecological Plastic to be used for up to 60% of a car's interior components.'

Apart from the fact that plant derived plastics based on polylactate, thermoplastic starch (TPS) and polyhydroxybutyrate (PHB) have been being injection moulded for years, there has to be doubt as to whether something that is derived from food crops, or grown on land that would otherwise be used for food crops is truly ecologically friendly. But, to be fair, Toyota says it is pursuing research and development of ecological plastics that are made from waste plant products or weeds for avoiding a conflict with food.

PHB has the strongest claim to being considered a genuine 'green' alternative because it can be derived from bacteria fed on organic waste, has good mechanical properties, and has the great advantage that it can be used for temporary surgical implants and sutures that don't have to be physically taken out afterwards.

In addition to issues relating to sustainability, there can be good engineering reasons for using plant derived polymers. Non-biodegradable oil derived plastic packaging clog up watercourses and pollute the countryside the world over, whereas biologically derived materials decay and rot when they come in to contact with wet ground.

The breakdown of plant derived polymers can be speed up by adding various different additives the material and surrounding environment. The phenomena has been studied and developed by Bibi Nelson as part of her Masters in Innovation Design Engineering at the Royal College of Art.

She discovered that adding some plant food to TPS makes the material breakdown in just 10 to 20 days. "When plant roots detect high concentrations of nutrients, they release an exudate which stimulates microbial growth," she says. "The microbes breakdown the plastic and the roots can then absorb the nutrients."

She added that the melting temperature of her plastic is 120°C, and yield strength 10MPa, which is similar to that of low density polyethylene (LDPE).

Purely mechanical advantages are claimed for SABIC Innovative Plastics' newest bio-based composites. Its has developed two LNP Thermocomp materials, the first is reinforced with 20% curauá, a fibre derived from a plant cultivated in South America, as an alternative to glass fibres. It claims that the natural fibres are not as abrasive as glass or mineral reinforcement. Brazil based Permatec, the largest Latin American manufacturer of automotive sun visors, is currently using the material to mould the frame and bracket of a new product.

The second is a grade of polypropylene (PP) reinforced with 30% wood flour that is intended to replace wood. Despite looking similar to wood, it is more resistant to fungi and has better dimensional stability. Compared with unfilled PP, or 30% mineral reinforced PP, it also offers a lower specific gravity, higher flexural strength and retained impact strength, giving it a strength to weight advantage. It can be extruded or injection moulded, and provides a colourable surface with the natural grain of wood.

A similar philosophy lies behind a 'liquid wood' being studied by Ford's European Research Centre in Aachen. It is assessing a wood plastic compound (WPC) produced by a rubber compounding process. The material is already used for making household terrace building panels but Ford want to use it for vehicle interior parts and battery trays. However, because it is 60 to 80% wood it presently has too high a viscosity to injection mould.

Already in use on cars is another liquid wood plastic made from waste generated by paper makers. The process was developed by researchers at the Fraunhofer Institute for Chemical Technology (ICT) in Pfinztal, Germany and is being sold by spin off company Tecnaro.

Emilia Regina-Kauffmann, tem leader of the project at ICT says: "We mix lignin with fine natural fibres made of wood, hemp or flax and natural additives such as wax. From this, they produce plastic granulate that can be melted and injection moulded."

The material is already being used for making car interior parts, loudspeaker enclosures and ball point pens. One of its advantages over conventional plastics is that it shrinks very little on moulding.

Like aircraft OEMs, automotive manufacturers are increasingly looking to reduce the weight of vehicle structures, in a bid to improve fuel efficiency. But, legislation regarding the disposal of vehicles is getting ever more stringent meaning the OEMs are increasingly assessing the use of bio-based composites, which is a key driver for much of the developments.

However, it is not just the automotive companies that set to gain from the new material breakthroughs. The increasing number of genuine 'green materials' on the market open up opportunities for engineering designers from all areas of industry to reassess the environmental impact and sustainability of their products.

Tom Shelley

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