PLA

Introduction

Polylactic acid or PLA is a biodegradable biopolymer which is mainly acquired from cornstarch. Lactic acid is the monomer unit forming this polymer. The lactic acid monomer is derived by bacterial fermentation of natural sources such as corn, sugarcane, potato, cassava and other biomass. Owing to its availability and low cost, PLA is the biopolymer with highest potential. 

Figure 01: Process of formation of PLA from monomer. Source: Xiang et al., 2017.

Properties

• Density - 1210 -1430 kg m-3 

• Melting point - 150 to 160 °C 

• Crystallinity - amorphous glassy polymer to semi crystalline

• Solubility - insoluble in water but soluble in organic solvents such as dioxane, benzene, THF

The properties of PLA is dependent on the ratio of the l and d isomers of the monomer. l-PLA possesses higher crystallinity. Thus, a higher ratio of the l mesoform in the polymer leads to increased melting point and brittleness. 

Mechanism of Biodegradation

Biodegradation of PLA is possible by hydrolase enzyme. Hydrolase cleaves the ester bond in polylactic acid and produces lactic acid. Microorganisms in soil further decompose lactic acid to carbon dioxide and water. The amount of L and D isomers in the polymer determine the rate of biodegradation. When more of the D isomer is present, the biodegradation is faster. The polymer is semipermeable to water and this makes it more prone to biodegradation. 

Figure 02: Degradation of PLA to lactic acid

References

K. Deshmukh, M. Basheer Ahamed, R.R. Deshmukh, S.K. Khadheer Pasha, P.R. Bhagat, K. Chidambaram, 3 - Biopolymer Composites With High Dielectric Performance: Interface Engineering, Biopolymer Composites in Electronics, Elsevier, 2017, Pages 27-128. 

Xiang Qi, Yiwei Ren, Xingzu Wang, New advances in the biodegradation of Poly(lactic) acid, International Biodeterioration & Biodegradation, Volume 117, 2017, Pages 215-223, ISSN 0964-8305.