Can the bio-plastic known as polylactic acid (PLA) be used in engineering?

The corn derived bio-plastic polylactic acid (PLA) has largely stood in the shadows of oil derived plastics since its discovery some eighty years ago. However, with increasing talk of potential uses, what has changed?

The increasing desire to use materials that are naturally derived or environmentally friendly is bringing polylactic acid (PLA) back in to the gaze of the engineer. This coupled with concerns around the volatility of oil prices has led to a lot of interest in finding a more natural source of plastic materials.

"Traditional plastic materials come from oil and as we know there are issues as to how much oil there actually is," says Chi Lam, technical and training manager at thermoplastic, polymers and elastomers distributor, Distrupol. "There is genuine concern about oil reserves. Also with the immerging BRIC countries there is now more demand on oil. Everyone is starting to look for, and develop, plastics from other sources."

PLA has been helped out of the shadows with the recent advance in the fermentation of dextrose, obtained from corn, which has reduced the cost to manufacture the material. But how useful is the material and is there a danger of early adoption based on hype?

"PLA has been around for quite a while and there are some good applications that people want to, and are, using it for," says Lam. "So, I don't think it is hype. But for engineering applications it does have some weaknesses compared to conventional materials."

The PLA market is developing interest as a renewable source material and one of its most significant properties is it biodegrades. Plastic materials can have a bad reputation for their environmental impact and have faced much criticism for the lasting impact with the supermarket plastic bag coming to symbolise the feeling. This is perhaps where PLA offers most potential; packaging.

"The life of packaging is very short so it is fantastic for that kind of application," says Professor Malcolm Fox, head of research and development at Nylacast. "But, if you want to make a little gear out of it, you don't want that hydrolysing and falling apart over a period of time."

PLA's production can be controlled to produce a range of molecular weights and there are two forms – the D and L – which are optical isomers. These have different properties and though they can be mixed together to tailor the PLAs properties, it's difficult to imagine it being used for many engineering applications.

Demand for PLA mostly comes from those that are keen on renewable source materials or are particularly attracted by its biodegradability. For example, it could be used to make an implanted medical device that harmlessly biodegrades inside the body rather than having to be removed at a later date. However, the material is still produced in relatively small volumes and those that are attracted to it usually have to pay the premium.

"At the moment it is still too expensive and it under performs compared to Nylon, polyethylene and propylene," says Professor Fox. "If it could benefit from economies of scale and there was a lot more of it available at a lower cost, then it would probably be used by more people in more applications.

"However, ethylene and propylene are produced on a vast scale and that is what makes polythene and polypropylene, and the harder versions of them, so cheap. So to a certain extent PLA has got to be made in much greater volumes before it can start to compete."

Of course, many plastic developers are already busy modifying formulations of PLA to enable better mechanical performance at lower production costs. However, modification can involve combining it with petrochemicals, which for many, defeats the object in the first place.

Distrupol's Lam adds: "A lot of our suppliers are keen to develop plant or renewable source plastics, but because of the weaknesses of PLA, it is never going to be a drop in solution.

"However, if you look at a material called Hytrel RS – a thermoplastic elastomer developed by DuPont – it is made from 30-70% renewable sources yet has the same performance as the standard Hytrel. These biopolymers will never replace 100% of the oil-derived materials, but there is plenty of room for growth. At the moment it makes up 2% of our sales, but if it grows to 10% that is a few million tons."

Indeed, Nylacast – like most Nylon companies – has looked at developing a bio-based Nylon 6 as the price of its current materials are so closely tied to the price of oil, which again has shown considerable volatility in recent years.

Professor Fox says: "It would come from vegetable beans rather than corn to make a synthetic castor oil. There are already some materials around with some German car companies using bio-based Nylon in the non-structural parts of the engine."

One of the major concerns raised around the development of any bio-derived material is how it impacts the environment and whether its proliferation would compete with food crops. Biofuels experienced considerable backing, and media hype, a few years ago but further reports discovered that Western appetite for the 'green' fuel was resulting in virgin rainforest being chopped down to farm the necessary crops. And in the US it was documented that the reduction in corn produced for human consumption made the price rise to such an extent it caused a country wide shortage of tortillas in Mexico, one of the countries food staples.

Distrupol's Lam says: "It's perhaps through that experience with biofuels that plastic producers have taken so long to be seen to do anything. They are conscious of that kind of unforeseen impact. A lot of suppliers are very sensitive about this issue. When they make a claim they want to be very sure about what will actually be delivered. They use words like 'developed from non-food-competing sources' or 'developed from farming waste'. The last thing they want is to proclaim a green material only to find it's causing more harm than good."

As a result, there is a lot of work trying to develop plastic materials from biomass – essentially waste organic materials. Other research is looking to development plastics derived from fungi or algae to create plastics. However, some might argue these still compete as the water needed to produce the algae on mass will take away drinking water or water for food crop irrigation.

At present many bio-derived materials in engineering are brought solely for green credentials. However, if engineers are going to start specifying bio-derived materials the cost, availability and physical properties need to be better than whatever it is they are to replace.

Author
Justin Cunningham

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