Bright Future - Still marginally present in the global plastic production, bioplastics are nevertheless experiencing a rapid growth. Among them, polylactide (PLA) is predicted to have a bright future as new production units are being built worldwide. This increased availability will however bring the focus on end-of-life options. Chemical recycling is adding to the current possibilities: it is a thermal depolymerization of PLA waste from which a highly purified lactic fraction is recovered and reused in a cradle-to-cradle cycle.

Technical Background

Chemical recycling is a closed loop where the used PLA is recovered and recycled back into its original form, lactic acid. Lactic acid is a chiral molecule and has two optical isomers. One is known as L(+) lactic acid and the other, its mirror image, is D(-) Lactic. L(+) Lactic acid is the biologically important isomer.

During the polymerization and the production of the original product, the treatment generates a racemization of the lactic acid. If PLA is mainly made of L(+) lactic acid, only a small quantity of D(-) lactic will remain in the final product. Generally speaking, the isomeric fraction of the incoming PLA waste will be found in similar proportions in the resulting lactic acid. Galactic has developed a process in order to reach a high L polymer grade of lactic acid, suitable for the manufacturing of virgin PLA grades. It is therefore impossible to make a distinction between an end product made from virgin lactic acid and another one obtained from recycled source, hence the "cradle-to-cradle" claim (no downcycling). Furthermore, the recycled lactic acid may also be used in a wide range of other industrial applications such as solvents and detergents.

Practical Aspects

Concretely, PLA waste comes in all kinds of shapes and sizes: Size reduction is usually the first mechanical conversion needed to guarantee that input streams are rather homogeneous. The recycling process is designed to be as flexible as possible, allowing the broadest range of PLA waste to be recycled. In almost all cases, physical impurities, additives, colors and other possible contaminants are effectively chemically separated from the useful material to be converted.

Currently, the industry generates the largest stream of PLA waste. Because of the sources (out-of-specification material, trial runs, start-up procedures, trimmings, etc.) the material flow is generally very clean and does not need specific sorting. Possible contaminants can also be easily identified. Wastes from domestic streams are by far the most complex scenario. Increasing numbers of PLA-made products will understandably impact mixed waste streams. But technical solutions are already available today: Near-infrared equipment (NIR) can effectively remove a PLA fraction from domestic streams. A study published in June 2008 by the WRAP (Waste & Resources Action Program) in England indicated that such NIR equipment could achieve a purity of 97 % at a throughput of 3mT/h. Incidentally, this purity dropped to 94 % for the PET fraction.

The question is thus not a technical one but a financial one, as low volumes of PLA waste can be presently retrieved from domestic streams. However, these volumes are on the rise in light of the ongoing research that broadens the scope of possible PLA applications. As an intermediary step between industrial and consumer waste streams, partnerships are concluded with cities or event organizers to collect PLA waste that is locally generated during a specific event (e.g. cups during a music festival or sport competition, carpets in exhibition centers, etc.). In this case, some limited contamination can be expected - usually food and drinks. This is also an opportunity to educate end consumers on bioplastics and their related end-of-life options.

Current Situation

Galactic's PLA recycling unit is located in Escanaffles, Belgium, where 2,000 tons of waste material can be treated annually. The process is robust and able to handle a large scope of contaminants. However, PLA formulations are becoming increasingly more complex requiring occasional adaptations of the recycling procedures.

Environmental Impact

From a LCA's perspective, recovering the material (instead of burning or composting it) has a tremendous impact, since further land use is avoided. The percentage of PLA content in the waste mass is however a predominant factor. If the waste does contain a high percentage of organic waste with a relatively low PLA content, other options like composting should be considered. A PLA content of more than 75-80 % is required for chemical recycling.

Compared to the (avoided) production of crops, sugar extraction and conversion into lactic acid, chemical recycling not only requires a fraction of the electricity and steam previously needed but also considerably reduces the use of water. This guarantees the economical viability of such recycling model, as the recycled lactic acid costs may not exceed the manufacturing of a virgin grade.