PLA properties improved through stretching and crystallisation

By making clever use of the specific crystallisation behaviour of polylactic acid (PLA), it has proved possible to produce a PLA bottle that is both virtually transparent and tough, but which hardly, if at all, loses its shape at temperatures up to 90 °C.

PLA is a versatile, 100% bio-based and bio-compostable plastic that is already used in a wide range of products. PLA could be used for an even greater number of applications if properties such as toughness, maximum service temperature, and barrier characteristics are improved. As part of the PLA-StIC project, researchers from Eindhoven University of Technology and Wageningen UR Food & Biobased Research joined forces with seven industrial partners from the entire production chain (from raw material supplier to end-user application) to try and improve these properties by taking advantage of PLA’s specific crystallisation behaviour during processing. A decision was made to demonstrate the results of the project through the production of bottles; the principles found can also be applied to the production of fibres, films, thermoformed products and foams.

Crystal formation during stretching

‘PLA crystallises relatively slowly and most PLA products currently on the market are hardly crystalline, if at all,’ explains Gerald Schennink, researcher at Wageningen UR Food & Biobased Research. A well-known example is the PLA cups currently used at major festivals which are amorphous rather than crystalline and therefore only suitable for cold drinks. ‘And yet it is not difficult to make crystalline and semi-crystalline PLA cups which are also suitable for holding hot drinks,’ Schennink says. This involves, among other things, taking advantage of the orientation and stretching processes that occur during the manufacturing of these products.

Purity of PLA determines crystalline behaviour

‘It is important to ensure close compatibility between the choice of raw material and the production parameters,’ Schennink explains. When making a PLA product, it is important to know whether the polymer is synthesized from only left-rotating or right-rotating lactic acid, or both monomers are used to form what is known as a copolymer. If a polymer consisting purely of left-rotating lactic acid (PLLA) is added as an additive to a PLA polymer or copolymer with little or no left-rotating lactic acid, a type of crystal (stereocomplex) develops during processing which not only has a higher melting point but also accelerates the entire crystallisation. This mechanism was applied during the research.

Thermostable and transparent PLA bottles

The study shows that it is possible to produce a PLA bottle that is transparent and tough, and which hardly, if at all, loses its shape at temperatures up to 90 °C. However, the bottle is not yet suitable for carbonated drinks. ‘Carbon dioxide leaks through the bottle wall,’ explains Pim Lohmeijer, researcher at the Laboratory of Polymer Materials of the Eindhoven University of Technology. ‘Although the formed crystals may improve the bottle’s barrier properties, the blow moulding process stretches the material to such an extent that minuscule voids develop.’ It may be possible to prevent the formation of these voids by studying the conditions of the stretching process more closely.

Model provides greater insight

To better understand the relationship between stretching conditions, crystallisation behaviour and the properties of the final product, Lohmeijer and Schennink tried to create a model which captures the various processes that occur during stretching. Researchers at Eindhoven University of Technology concentrated on the in-line study of the crystallisation behaviour during 1D stretching of PLA tapes. Researchers at Food & Biobased Research focused on performing 2D stretch tests of films and sheets, and ultimately the production of bottles. The combined research yielded information relating to raw material choice, crystal formation and the final properties of stretched, crystalline PLA products. Finally, the research team discovered that not only crystallinity, but also orientation of the non-crystallised material is important for the final properties of a PLA product. This knowledge can be used, for example, in the production of shrink films.