Abstract
Environmental problems, such as global warming and plastic pollution have forced researchers to investigate alternatives for conventional plastics. Poly(lactic acid) (PLA), one of the well-known eco-friendly biodegradables and biobased polyesters, has been studied extensively and is considered to be a promising substitute to petroleum-based polymers. This review gives an inclusive overview of the current research of lactic acid and lactide dimer techniques along with the production of PLA from its monomers. Melt polycondensation as well as ring opening polymerization techniques are discussed, and the effect of various catalysts and polymerization conditions is thoroughly presented. Reaction mechanisms are also reviewed. However, due to the competitive decomposition reactions, in the most cases low or medium molecular weight (MW) of PLA, not exceeding 20,000–50,000 g/mol, are prepared. For this reason, additional procedures such as solid state polycondensation (SSP) and chain extension (CE) reaching MW ranging from 80,000 up to 250,000 g/mol are extensively investigated here. Lastly, numerous practical applications of PLA in various fields of industry, technical challenges and limitations of PLA use as well as its future perspectives are also reported in this review.
Highlights
It has been estimated that until today, global polymers’ total production has reached about 9 billion tons [1]
This review summarizes monomer synthesis of lactic acid and lactide, new synthesizing methods in Poly(lactic acid) (PLA) production introduced over the past few years and attempts to explain their mechanisms
Nagahata et al [36] reported the synthesis of PLA with a molecular weight of ≈16.000 g/mol by a single-step direct polycondensation of Lactic Acid (LA), which can be achieved by effectively using microwave irradiation
Summary
It has been estimated that until today, global polymers’ total production has reached about 9 billion tons [1]. This review summarizes monomer synthesis of lactic acid and lactide, new synthesizing methods (melt polycondensation and ring opening polymerization) in PLA production introduced over the past few years and attempts to explain their mechanisms. (c) The “one-step liquid-phase process”, where water removal takes place during the ring-closing reaction and lactide is synthesized directly from an aqueous LA solution though condensation rather than through transesterification This method introduces zeolites as heterogenous catalysts to the bioplastic industry. One of the main origins for the recent extended use of PLA is attributed to the costeffective production of high-molecular-weight PLA polymers (higher than ∼100,000 Da) These polymers can be produced applying various techniques, including azeotropic dehydrative condensation, direct condensation polymerization, and/or polymerization through lactide formation [35]. The polycondensation system of LA involves two reaction equilibria: (1) dehydration equilibrium for esterification and ring-chain equilibrium involving the (2) depolymerization of PLA into lactide (Scheme 2) [22]
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