by Caterina Plenzick, Anselm Wohlfahrt, Rafael Vinz & Lukas Keller
Raw materials – basic building blocks/ starting materials
PLA is a biodegradable, bio-based, thermoplastic polyester.
Through the biotechnological fermentation of sugar, lactic acid is produced in bioreactors with the help of bacteria from the Lactobacillus family, which is then polymerised. The starting materials for PLA are renewable raw materials such as corn starch or sugar cane. They serve as a carbohydrate source for the fermentation process. In addition, PLA can be produced from algae and organic residues such as old bread or olive pits. Up to 900 grams of PLA can be obtained from one kilogram of sugar.
Biotechnological production – PLA varieties
Polylactic acid is produced from lactic acid by polymerisation. In this process, the chemical chains of the monomers of the lactic acid are linked together. Polylactic acid can be produced in two chemical variants, D-lactide and L-lactide. This results in PLA in the variants PDLA and PLLA. Depending on how these building blocks are proportionally and structurally integrated into the molecular chains of PLA, the morphology as well as the thermal and mechanical properties of the plastic differ.
Material Design – Additives
There are about 20 different unadditivated PLA variants available on the market. They are manufactured by two companies in the USA. Since pure PLA is not suitable for industrial applications, an application-specific modification of the material is necessary. Such a modification can be achieved by mixing in additives and/or fillers.
Crystalline PLA can be used for film production, injection moulding or for various plastic extrusion processes. In comparison, amorphous PLA can be used for thermal forming processes such as deep drawing.
Properties – Material development
The diversity of additivated PLA allows it to be processed with almost all common plastic processing methods. PLA can be produced in transparent, crystalline or rigid form. In addition, it has a high mechanical strength and resistance to oils and fats. This makes it comparable to other oil-based plastics in industry. In particular, the need to use a material that is less harmful to the environment but guarantees the same material properties has led to the rapid, successful entry of PLA into the plastics market as a competitive product.
Design potential – applications
PLA can be used to make films, fibres or foams for many product applications. One well-known application of PLA is as a filament for FDM 3D printing processes. Other applications where PLA can be used are short-life products such as disposable tableware, rubbish bags or packaging. In medical technology, PLA is used as a material for temporary, internal body prostheses thanks to its high biocompatibility and biodegradability, as PLA can degrade in the body over time. PLA is also used in the textile sector, as yarn and as non-woven fabric. Basically, it can be stated that PLA has the potential to replace many applications of oil-based plastics.
PLA compared to conventional plastics
Compared to oil-based plastics, PLA has many advantages. The raw material of PLA is renewable plant-based raw material. Furthermore, the production of the raw materials for PLA requires a fraction of the energy that is used for the production of conventional plastics. In addition, PLA offers new material properties and a wide range of disposal options (material recycling, composting, climate-neutral energy recovery).
The disadvantages of PLA are its lower temperature resistance (0-80 °C) and, compared to other plastics, its higher moisture and oxygen permeability. Especially when used as a packaging material, this can severely limit its application. Furthermore, the long-term effects of PLA on humans and the environment have not yet been fully researched.
Recycling – hurdles & potentials
In order to enable efficient recycling, the various PLA compounds should be collected as sorted as possible. Due to the currently still low quantities of PLA in post-consumer waste, and the variety of PLA blends, a systematic sorting of PLA is not yet economical. Currently, only 1% of global plastic production is bio-based. PLA accounts for 10% of this. Presently, PLA packaging is incinerated or used as a substitute fuel in industry. If PLA were separated by type, it could be mechanically recycled up to ten times. For this purpose, it can be shredded and processed into new products with the addition of fresh PLA. However, this process is associated with quality losses because the polymer chains in the PLA are shortened by the shredding. If the polymer chains of the material have become too short through mechanical recycling, it is possible to feed the material into a chemical recycling process. In this process, the material is reprocessed into lactic acid, which can be used as a starting material for new PLA. A systematic, single-variety collection of PLA plays an even greater role here, as the purity of the lactic acid recovered from the PLA determines the quality of the PLA at the end of the polymerisation process. A raw-material, single-variety recycling of PLA would therefore be a sensible and necessary alternative to the current thermal utilisation of the material in order to reduce environmental pollution.
Significance of the material at regional level
In 2015, a total of approximately 5.9 million tonnes of plastic waste was generated in Germany. Of this, 45% was recycled materially and sorted into thermoplastic re-granulates. However, 53% was sent for energy recovery. Since bio-based plastics are chemically new materials, they mostly end up in the established disposal channels for plastic waste. In this context, adapting sorting systems to separate PLA material streams could be a useful step towards more efficient recycling of PLA – especially at regional level. There are already pilot research projects that confirm the energy efficiency of systems based on disposal, sorting of PLA waste and its recycling into recyclate. In terms of manufacturing, local cultivation of sugar beet or other suitable indigenous crops could be significant in the regional context. But at the moment, such a scenario is rather unlikely.
Possible cycles and alternative raw materials
Mechanical and chemical recycling both offer the possibility of establishing material cycles, the efficiency of which depends on the application area and context. Other cycles could be based on the use of alternative raw material sources. Residues from agriculture, food production and animal farming could offer carbohydrate-based resources. The possible use of whey, olive pits or old bread for the production of lactic acid have already been explored in case studies, with the additional aim of forming suitable cycles. The major environmental impacts of PLA production arise in the agricultural sector; the use of genetically modified algae as a raw material or even as a production platform could eliminate the use of agricultural land and thus significantly reduce the overall energy consumption of PLA production; in addition, this would also avoid most of the harmful side effects of any agricultural production.
What does “bio” stand for in PLA plastic?
Generally, a distinction is made between (semi-)synthetic polymers – industrially produced substances that are the main component of plastic production – and biopolymers. In contrast to synthetic polymers, the basic building blocks for biopolymers are not synthesised industrially, but in living organisms. PLA falls under the category of synthetic polymers because the substance is artificially produced in the laboratory. PLA is considered a so-called bio-plastic because it is produced on the basis of renewable raw materials. The manufactured molecular structure of PLA is biodegradable and compostable. PLA has a high biocompatibility, it is hydrolysed into its basic elements when implanted into living beings. In the process, it does not produce any toxic or questionable substances.
PLA meets the criteria of EU standard 13432. The biodegradability of PLA strongly depends on the chemical composition and the use of additives or copolymers. Complete composting can only be achieved in industrial plants under defined conditions. This is already standard in some countries in southern Europe. Additives in the PLA compounds can make composting even more difficult, as the residues of the additives deteriorate the quality of the compost. In Germany, biodegradable rubbish bags are not composted, as biowaste is only composted naturally in Germany. Since the biodegradable bags or packaging in natural composts do not decompose as quickly as the organic waste, the packaging is filtered out and incinerated. Basically, for each product application of PLA, it should be considered whether the material can be better circulated in a technical recycling loop instead of composting it.
Open questions or visions
– Which cycles could be generated, based on a systemic pre-sorting and collection of PLA waste, in the sense of a sensible recycling of the material?
– Which raw materials offer a sensible alternative for the production of PLA in the long term? And in which contexts?
– What would systems look like based on this idea?
– Could long-lasting product applications increase the value of the material and its resources? For which other applications does the performative development of the material make sense?