Greener and stronger - new biopolymer developed

Materials World magazine
,
7 May 2012
Biopolymer bottle "degrading" in grass

A biopolymer blend has been developed that provides improved mechanical and processing properties over PET. The material can be used for applications in the packaging and aerospace industries. Professor Alma Hodzic from the University of Sheffield reports.

A new fully bio-based, naturally biodegradable biopolymer that could replace PET in commercial uses has been invented. Consisting of 90% polylactic acid (PLA), the naturally biodegradable biopolymer has excellent mechanical and physical properties and is used primarily for packaging applications. It is produced from sustainable plant feedstock, and has a lower carbon footprint and non-renewable energy usage than any mineral thermoplastic, including recycled polyethylene terephthalate (RPET).  

In principle, PLA can be recycled either by thermoplastic methods or by hydrolytic cracking back down to a monomer, although this process is still in development. Furthermore, the original commercial strength of PLA is retained in the new product’s biodegradation through a two-stage process consisting of hydrolysis to low molecular weight oligomers, followed by complete digestion by micro-organisms.

At room temperature, PLA has high modulus and strength but very poor toughness. This is largely due to its glass transition point, which lies between 50 and 60°C. In certain applications, this presents further problems due to deformation and loss of strength under storage conditions in warmer climates. Solutions to these problems exist, largely by control of polymer chemistry, producing copolymers and branched chains. With the aim of producing a tougher, commercially viable thermoplastic that is still biodegradable, various approaches have been examined based on thermoplastic compounding or blending. The majority of work on PLA nanocomposites has focused on improving strength and modulus. However, for many thermoplastic applications this may not be required.

Two common naturally biodegradable polymer additives constitute 10% of the blend for the biopolymer – called Floreon – providing the base polymer with enhanced ductility, toughness and thermal resistance. These properties are achieved through careful selection of molecular weights in the additives, allowing the formation of the phase-separated nanocomposite during melt compounding of the blend. The additives are dispersed throughout the base biopolymer with a degree of phase separation, to form energy-absorbing nanoglobules in a modified matrix. This allows improved performance without changes to its chemistry.

Standard PLA may deform above 40°C and lose aesthetic appeal, preventing the biopolymer from being widely adopted for standard packaging. Some attempts have been made to use PLA in bottle applications. However, success has been limited because the bottle must be refrigerated and the high cost of raw materials has meant a high overall price.

Improved production

One of the main advantages of the material is in lower processing temperatures compared to polymers with similar mechanical properties. Floreon is produced by twin-screw melt extrusion of PLA and the additives into standard size pellets, and can be further processed by applying any standard thermoplasticforming technique around the softening region between 70 and 105°C.

Strength and ductility are the main advantages of this biopolymer, which translate into performance characteristics of the nanocomposite being slightly more flexible compared to PLA. It can also be used at higher environmental temperatures, and does not require refrigeration, although it can be stored at low temperatures, and can be handled and packaged like all similar polymer products.

The main scientific aim of this project was to produce a fully biodegradable material that saves on processing energy and retains translucency despite its complex microstructure. The behaviour of bio-based materials during the processing conditions can be quite challenging compared to the synthetic polymers.

Project Manager, Dr Peter Bailey, experimented with numerous combinations of biopolymer blends in the laboratories of the Composite Systems Innovation Centre at Sheffield University, UK. Although many biopolymer additives enhanced toughness and crystallisation of PLA, a particular combination of three polymers with specific molecular weight distributions showed improved performance in processability, as well as finished properties, that led to the formation of the new polymer blend. Although there is much work still to do, Dr Bailey commented, ‘We are now running production trials to bring a wide range of applications to market’.

The next phase of the project will be to increase the gas barrier properties of the biopolymer and expand its range of applications in food packaging.

End of life

As with all naturally biodegradable materials, Floreon can only biodegrade once in contact with soil. In some cases, a slight increase in opacity can be noticed after some time, however, this does not influence the quality of the material. Compared to PET, the new product has 46% lower energy consumption in raw material production. With basic recycling, the material is collected, cleaned and melted down into new bottles. With full recycling, the material is collected, cleaned and broken down into its component parts to make brand new PLA. Depending on the soil temperatures, it can completely disintegrate in fewer than two months and fully biodegrade within six months. The bacteria responsible for biodegradation have the optimum metabolitic activity between 30–55°C, and the material will biodegrade slower at lower temperatures.

Various bioplastics are already being used in thin film applications and the size of the overall market in 2010 was 12% compared to their oil-based counterparts. This market share is expected to grow to 25% by 2020, mainly due to the improvement in biopolymer manufacturing technology, price reduction and the expansion of innovations such as rigid packaging and loaded components. Furthermore, the incentives that promote high calorific-value, bio-based materials into waste for energy have secured another waste management option at the beginning of the new cycle where some composting plants may not yet be geared for biopolymers composting.

The author wishes to thank the University of Sheffield, especially Dr Simon Hayes – Department of Materials Science and Engineering, Dr Patrick Fairclough – Department of Chemistry and Shaun Chatterton – CPD Plc.

Further information

Professor Alma Hodzic: a.hodzic@sheffield.ac.uk