Polylactic acid
Polylactic acid (PLA) is a polyester based on lactic acid, and has been known since the 18th century. Lactic acid is, after glycolic acid, the most simple hydroxy acid, and capable of self-polymerization (polycondensation), under release of water.
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PLA has been synthesized for the first time 200 years ago, but it has not been until very recently that it has found applications in the industry.
Lactic acid can be synthesized by chemical means, but is generally produced by the fermentation of sugars by micro-organisms. Glucose is most widely used, but Hycail is also developing PLA from lactic acid made by fermentation of lactose out of whey-permeate, a waste product from cheese production.
In general, natural fermentation will yield L(+) lactic acid. Chemical synthesis of lactic acid will yield a racemic mixture of L(+) and D(-).
Pure poly(L)lactic acid has a very regular molecular structure and therefore forms crystalline domains in the matrix. This will yield a more stiff and harder material then the polymer which has some D lactic acid built in. D-lactic acid is normally found as a impurity in fermented lactic acid, but will also be formed out of the L-form in the process to make PLA. A little bit of D is necessary to tune the material, as it will disturb the crystallinity of the material just enough to get the right balance of properties like stiffness and toughness.
PLA’s properties resemble those of polystyrene. PLA is however biologically degradable, when subjected to the right conditions. It is therefore very well suited for waste treatment through composting. It will also improve the compost quality by increasing the C/N ratio in the organic waste. Because lactic acid is made out of natural sugar resources, which are in turn made by photosynthesis out of CO2 in plants, the material can be regarded as “CO2-neutral” and therefore renewable.
PLA can be processed as a standard thermoplastic on existing industrial equipment.
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