What is PLA?
PLA - poly(lactic acid) - is a biodegradable polymer and a thermoplastic. In contrast to many traditional fuel-based plastics, it is produced from renewable resources, which allows classifying PLA in the bioplastic category. Indeed, it is actually known as "the corn plastic", as in the United States and Canada it is obtained from corn starch. It can be also derived from tapioca starch or sugarcane .
Chemically, PLA is a polymer of lactic acid units, or a polyester to be more specific. Lactic acid is, on its turn, a three carbon compound with an acid and an alcohol functions. These two functions can condense themselves, joining two molecules of acid lactic and producing a molecule of water on the ligation reaction. This reaction can be imagined as the first step of PLA polymerization, but actually, for producing PLA chemically or biologically, we need some intermediate steps that will be explained on the following sections .
PLA may be the polymer with the broadest range of applications because of its ability to be stress crystallized, thermally crystallized impact modified, filled, copolymerized and processed in most polymer processing equipment.
At a temperature of 20-30 °C, it remains solid and keeps its shape; but at 70ºC or more, it becomes malleable and thus, it can be formed into films, fibers, or injection molded (to make bottles for instance) .
Figure 1. Structure of PLA. Polymer made of n monomers of lactic acid.
In today’s world of green chemistry and concern for the environment, PLA has additional drivers that make it unique in the marketplace. In spite of this unique combination of characteristics, its commercialization is limited by high production costs.
Production of PLA
Biomass-derived polymers can be synthesized fully by microorganisms, but most of them combine biological and chemical synthesis in an hybrid process. In the case of PLA, the starting material lactic acid is made by a fermentation process using 100% renewable resources.
However, the polymer requires a complex procedure of chemical synthesis that can result into expensive costs and chemical side-products which are not environmental friendly. For these reasons, bioproduction can be a good alternative for PLA industrialization.
PLA can be prepared by both direct condensation of lactic acid and by the ring-opening polymerization (ROP) of the lactide. In ROP, lactic acid is condensed to give a prepolymer, and then it is dehydrated to make lactide. Then, lactide is submitted to a chemical process that uses a metal catalyst to make high-molecular-weight PLAs . Figure 1 shows an schema of the process.
The final molecular weight of PLA is limited by the reversibility of the direct condensation and the difficulties of removing trace amounts of water in the late steps of polymerization. The last step of ROP gives advantage over that, and that is the reason for which the chemical production is focused on ROP.
Figure 1.* Chemical synthesis of PLA (From ). Three steps for production of high-molecular-weight PLA, starting from lactic acid produced by fermentation.
Advantadges of Bioproduction
Bioproduction is gaining popularity nowadays, as it allows materials to be entirely synthesized by biological processes, using microorganisms. It uses knowledge from metabolic engineering at the systems-level, by integrating systems biology, synthetic biology, protein engineering and evolutionary engineering. With bioproduction, materials come from sustainable carbon sources, instead of limited fossil fuels. PLA has its biodegradability as eco-friendly characteristic, but the process of chemical production has a negative environmental impact. When bioproducing PLA, we are advancing on the microbial factories paradigm, which has a great potential to help reducing climate change, decreasing society’s reliance on fossil resources and improving energy security .
Apart from its environmental advantages, bioproduction of PLA is also beneficial for its use on biomedical applications. During chemical synthesis there are used organometallic catalysts that are difficult to remove completely from the final product. This could generate toxicity problems when used in medicine. Thus, bioproduction would reassure a metal-free polymer compound .
*Note: Figure 1 belongs to the original paper cited in . The article has Creative Commons Attribution Licence and the corresponding author gave us permission to use it.
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