The Processing of Biobased Plastics – Part 3: PLA

In the previous two parts of this series, we introduced the current market of bioplastics and the generally available methods of processing. This information provides a concrete basis for a deep dive into the processing characteristics of specific bioplastics, such as PLA.

Polylactic acid (PLA) is a thermoplastic polyester that is biodegradable under certain circumstances. It shows proven biodegradability with anaerobic digestion and industrial composting3. The polyester consists of the monomer lactic acid (LA) (Figure 8), which is produced from biobased feedstock, such as sugar cane or sugar beets, by way of fermentation. The subsequent production of PLA from the monomer building blocks can be achieved through biological means, or chemical means in the form of polycondensation or ring-opening polymerization4. As such, the process can be a biobased or chemical based process.

Fig. 1: Chemical structure of Polylactic Acid (PLA)

The melting point of PLA is highly dependent on the crystallinity of the polymer, where a higher crystallinity indicates a higher melting point. PLA usually exhibits a high crystallinity, and thus a high melting point, however, the crystallinity can be modified by integrating meso-lactide or d-lactide monomers into the polymer 5. The melting temperature can range from 130 to 230 ˚C, while the processing temperature ranges from 185 to 250 ˚C 6,7. Thermal degradation due to chain scission occurs at 200 ˚C 6, which illustrates the necessity of reducing the melting point of the previously mentioned method (or other methods) to allow for the possibility of a lower processing temperature. Developments in this field have inevitably led to successful results, because products, such as Total Corbion PLA in Europe, are now commercially available 8.

Processing of PLA during extrusion can be improved with additives, such as plasticizers, strengthening agents or inorganic fillers. The mechanism of strengthening agents, for example Polystyrene, and inorganic fillers, for example calcium carbonate, are the same. They reduce the crystallinity and thus reduce TP. Additional improvement in processing can be achieved by producing polymer blends with for example biobased and degradable PBS. Extruded films produced from these blends show improvement in tear resistance, elongation and flexibility compared to pure PLA9.

Injection moulding of PLA has a troublesome reputation, but the reputation dissuades its use more than reality. To improve processing and reduce the brittleness of the product, the PLA is blended with thermoplastic elastomers. Such elastomers come in the form of biobased and degradable PBS. The processing of PLA requires lower than normal processing temperatures, but they are not outside the realm of specification. Concerns are raised over the necessity of a time consuming drying process to remove water and prevent hydrolysis degradation, but usually surface drying is sufficient10. In fact, NatureWorks produces PLA articles with injection moulding 11.

During blow moulding the PLA parison is only heated to 90 ˚C, well below the degradation temperature. Usually additives are added to improve energy absorption to achieve the 90 ˚C processing temperature. Optimization of the stretching process and thus the thickness of the articles needs to be optimized. This is to prevent strain hardening and the appearance of white areas in the plastic with excessive stretching, or variable thickness due to low stretching. An additional benefit for symmetrically radially shaped PLA articles is a reduced brittleness that can be attributed solely to the form of the article. Such forms have a reduced brittleness to PLA sheets12. NatureWorks’ Ingea 7032D PLA is a commercially available variant for the process of blow moulding13. No commercial articles that are produced via compression moulding seem to be available. However, it is possible to produce articles in the form of composites with compression moulding. Such articles can for example be produced with jute fibres14. Transfer moulding is generally done with thermosets, whereas PLA is a thermoplastic. PLA sheets are thermoformed at temperatures between 80 and 110 ˚C, which is significantly below the degradation temperature. The form of the article also reduces brittleness compared to PLA sheets12. PLA is considered to be the most easily applicable biobased biodegradable polymer suitable for the additive manufacturing. Scaffold systems are suitable for medical application. In products other than medical application is where pure PLA is considered to be insufficient, since the products are of low strength. This problem can be solved by creating PLA composites with (natural)-fibre reinforcement7.

Fig 2: Fused Filament Fabrication (FFF) with PLA monofilament.

Article by Wybren Kalsbeek

 

References

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  8. PLA, T. C. Total Corbion PLA announces the first world-scale PLA plant in Europe. https://www.total-corbion.com/news/total-corbion-pla-announces-the-first-world-scale-pla-plant-in-europe/ (2020).
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  10. Greendot Bioplastics. How has the relationship between injection molders and bioplastics changed ? https://www.greendotbioplastics.com/injection-molders-made-bioplastics-work/?print=pdf.
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