A growing biodegradable plastics industry

C&I Issue 14, 2009

Imagine a polymer with a low heat distortion temperature and poor impact resistance. It is also difficult to form into products and nearly twice the cost of other resins. This, in a nutshell, is a biodegradable plastic.

  Despite these shortcomings, biodegradables have come a long way. Ten years ago, Natureworks had only just begun to commercialise polylactic acid (PLA), its corn-derived biodegradable resin. Now, its 140,000 t/year plant in Nebraska is close to capacity – and the company is assessing locations for a second manufacturing facility. But in that time, PLA and other biodegradables have yet to extend their reach beyond their initial target market.

  ‘Most applications of biodegradable plastics are still in packaging, or for products like agricultural film,’ says Aniruddh Menon, a research associate at Frost & Sullivan. ‘But a major concern for end users is price/performance ratio – and this is one of the factors that will decide the fate of bioplastics. In many cases, these materials cannot match the performance of conventional resins.’

  Bioplastics suppliers recognise this. Last year, NatureWorks held a huge congress in Las Vegas – ‘Innovation Takes Root’ – which brought together many players that will help PLA leap into more demanding applications. There is a huge effort to improve the products that already exist – and to open up new avenues. One slightly vexing problem will be to make the additives themselves biodegradable – a growing demand from end-users.

  ‘This will depend very much on additive manufacturers,’ says Menon. ‘They are doing the research to make bio-based additives. I would expect to see joint ventures between them and resin manufacturers.’

Tough challenge
Toughness – or impact resistance – is the amount of physical punishment that a material will take. A moderate improvement, achievable by adding impact modifiers, would allow PLA to withstand more punishing processing conditions. This would mean faster line speeds, and less product damage during production.

  Swiss company Sukano says that its impact modifier can help to prevent cracks and splinters during the production of PLA film and sheet. DuPont makes similar claims – and independent tests confirm that its Biomax Strength impact modifier can have extra benefits.

  John Christiano, of machinery extrusion company Davis-Standard, ran samples of impact modified PLA through a conventional film processing line. He says that the additive improved impact resistance of PLA by a factor of two or three, when added at a loading of 2%. But it also delivered other advantages. ‘It lowered melt temperature by around 3°C, which reduced power consumption by up to 25%,’ he says. ‘You would not see such a large improvement with a conventional polymer.’

  If commercialised, this kind of technique could cut the cost of producing biodegradable film and sheet – thanks to lower energy bills – and help it compete with conventional polymers.

  A combination of additives and process technology is likely to overcome some of PLA’s other potential drawbacks. At Innovation Takes Root, Roger Avakian, chief technology officer at US polymer compounder PolyOne, made a long list of PLA’s shortcomings, plus a list of additives that could solve them. The problems included high cost and low melt viscosity, as well as poor heat resistance. PLA can usually withstand temperatures of around 50°C, but PolyOne has developed ways of raising it.

  ‘Higher HDT [heat distortion temperature] and improved impact strength are key enabling properties,’ says Avakian. He says that new blends, which Polyone expects to commercialise by the end of this year, will deliver higher heat resistance at low load (66psi), as well as an impact strength up to 25 times higher than the 0.5ft-lb/inch of ‘neat’ PLA. ‘We have at least two different platforms in development,’ he says. ‘Improvements in HDT range from 85 to 110°C, and impact strength from 1.5 to over 12 ft-lb/inch, depending on the grade.’

  This would lead to applications in both thermoforming and injection moulding, competing with materials such as high impact polystyrene, ABS and ‘some engineering thermoplastics’. ‘We see applications in consumer, electronics, packaging, and transportation,’ says Avakian.

  Some other PLA developments revealed at Innovation Takes Root included making stronger fibres and textiles from the material; Samsung’s use of PLA/ polycarbonate blends to make a mobile phone handset and Toyota’s use of PLA reinforced with kenaf, a natural fibre, for various components.

Green additives
Researchers at the University of Kassel in Germany have assessed the strengthening effects of different natural fibres on PLA. They say they have managed to improve physical properties using wheat and rye husks. ‘Natural bio-fibre composites are emerging as a viable alternative to synthetic fibre-reinforced composites, especially in the automotive and building industries,’ says Abdullah Al Mamun, a researcher on the project.

  These fillers are of particular interest because they are waste products: the husks account for around 20% of the crop’s weight, but are inedible. The researchers also looked at the effect of soft wood fibre, as a comparison.

  Each type of fibre was mixed (at 40% weight) with PLA, then injection moulded and subjected to physical testing. The two additives showed quite different effects. Compared with the wood fibre composite, wheat husks had a 20% increased effect on tensile strength, while rye husks were 40% lower. In the Charpy strength test – which measures the energy absorbed by a material during fracture – wheat and rye husks were 35% and 20% better than soft wood fibres, respectively.

 ‘This type of lignocellulosic waste is ecofriendly, available and cheap,’ says Al Mamun. ‘Wheat and rye husk reinforced composites could be used together with wood fibre composites, or as a substitute.’

Film awards
PLA’s main application – as a film material – is far from forgotten. A widespread technique used to make barrier films is being increasingly applied to PLA. Biaxial orientation involves stretching a film in two directions to improve strength and barrier properties. It is routinely used to make films out of polypropylene (‘BOPP’) and PET (‘BOPET’). Now it is being applied to PLA to create ‘BOPLA’, which currently accounts for less than 1% of ‘bi-oriented’ production.

  There are even some natural advantages: PP is stretched around five times in the machine to create BOPP – this is just three-and-a-half times for PLA. In the transverse direction, the elongation is a factor of 10 for PP, but just five for PLA.

  ‘Biodegradable film is quite a young and booming market,’ says Christian Aigner, head of marketing at film machinery company Brueckner. ‘Growth rates in this area are expected to be well above 10 per cent per year.’ He points to a number of existing applications that could be replaced with BOPLA, such as twist-wrap cellophane, for confectionery; multi-layer food packaging film; and shrink sleeve films.

  Direct material substitutions are often impractical, but Aigner says that existing production lines – used to make BOPP, for example – can be adapted to produce BOPLA. They would need to take account of differences between PLA and other plastics, such as PLA’s sensitivity to humidity. Other changes are needed for resin handling, extruder layout and the electrical system.

  ‘Each line needs an individual check and recommendation for a retrofit,’ says Aigner.

  Once film has been stretched, it is commonly metallised – and this has begun to happen with PLA. Celplast Metallized Products (CMP), a US film specialist, has developed a metallised version of PLA, called Enviromet. A clear roll of PLA film is placed in a high vacuum metallising chamber, air is pumped out, then high purity aluminium wire is evaporated at the bottom of the chamber and film is run over this vapour at high speeds. The aluminium condenses on the film surface, creating a high barrier layer.

  Such a technique will be used on a groundbreaking snack food package. Frito-Lay’s SunChips, crisps which are sold widely in the US, are packed in three-layer bags. The outer layer recently shifted to PLA – and the manufacturer, a subsidiary of PepsiCo, expects to make all three layers out of PLA by next year. Dante Ferrari, chief operating officer of CMP, says: ‘This is a large volume application that is likely to switch to metallised PLA by next year.’

  Further in the future – perhaps within five years – Aniruddh Menon says bioplastics could develop into a specialist resin for medical products. ‘One key trend in medical devices is towards home care treatment,’ he says. ‘These devices are commonly designed to be disposable – but they are made from conventional plastics.’

 If the performance of biodegradables could be raised to compete with engineering resins such as polyamide or polycarbonate – and they can gain the necessary medical approvals – he thinks they could have a significant impact on the market. ‘The medical devices market is growing fast, and could be a key area for bioplastics,’ he says.

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