Introduction
1.1. Introduction
1.1.1. Poly (lactic acid)
PLA is the first renewable bio-based synthetic polymer that has been exploited for commercialization at a large scale (Goncalves et al., 2013; Rudnik et al., 2011). PLA has great potential to replace conventional non-renewable petroleum-based polymers (Zhang et al., 2014). It is derived from renewable resources such as corn, sweet potato, cane molasses and beet sugar. It is an environment friendly thermoplastic polymer which is a bio-based, biocompatible and biodegradable material (Zhang et al., 2014; Maharana et al., 2009;
Reddy et al., 2013). PLA has proved to be the most promising biodegradable polymer which has wide variety of applications. Due to its desirable properties such as transparency, bio-safety and compostability; it has promising applications in consumer goods, fibers, biomedicines and packaging which may result in exorbitant use of it in near future. Hence, systematic and rigorous study of degradation for PLA is of great concern towards its recyclability and nontoxic incineration process.
Poly lactic acid (PLA) lies in the class of degradable polymers which are primarily synthesized in the laboratory using monomers which are generally derived from bio feedstock. PLA is an example of the class where stereospecific lactic acid is prepared by the fermentation of starch based agro feedstock such as sugarcane, corn, sugar beet pulp, etc. The basic building block for PLA is lactic acid (LA), which exists in two optically active configurations L(+) lactic acid and D(-) lactic acid (figure 1.2). LA (2-hydroxy propanoic acid) is an organic acid having bifunctionality as it has −COOH and −OH groups. LA is produced by fermentation of different
carbohydrates (namely glucose, sucrose, lactose and maltose) obtained from renewable resources such as sugar cane, sweet potato and corn (Maharana et al., 2009). The fermentation can be performed by maintaining few important parameters such as atmospheric conditions, temperature, pH-value and agitation. LA can be obtained by conducting fermentation in batch or in continuous process by using bacteria, fungi or yeast (Sodergard et al., 2002). LA can also be synthesized directly from sugars such as glucose, sucrose and fructose using solid Sn-Beta zeolite catalyst (Kimura, 2013). Another intermediate monomer for PLA synthesis is lactide and obtained by depolymerization of low molecular weight PLA under reduced pressure to give a mixture of L-lactide, D-lactide, or meso-lactide (figure 1.2.). Lactide is also formed by the dimerization of polycondensated lactic acid (Sodergard et al., 2002).
Figure 1.2. Structure of isomeric lactides.
Synthetic environment friendly polymers with precursors from natural resources such as PLA, have shown enormous potential to substitute a wide variety of conventional fossil based packaging plastics and have been demonstrated to be commercially viable. In view of this, a detailed description about various aspects of PLA such as synthesis, properties and processing have been discussed below. PLA belongs to the family of aliphatic polyesters. It is a
biodegradable polymer with a reasonable shelf life for a wide variety of consumer products, such as paper coatings, films, moulded articles, and fiber applications (Datta et al., 1995).
PLA can be synthesized by various synthesis routes such as direct polycondensation, azeotropic dehydrative condensation, ring opening polymerization (ROP), melt polycondensation (MP) and solid state polymerization (SSP) of low molecular weight PLA (Lunt et al., 1998). LA is polymerized by direct polycondensation to yield a viscous to brittle glassy material with molecular weight up to 10,000 Da depending on the polymerization conditions. The statistical presence (low concentration) of reactive end groups causes various unwanted side reactions such as transesterification, ester exchange and backbiting equilibrium reactions, which favours the formation of lactide as byproduct (Garlotta, 2002). Subsequently, in order to avoid this unwanted equilibrium, PLA synthesis from LA is performed in the presence of a catalyst and organic solvents via azeotropic dehydrative condensation process.
In this process, water is removed from the reaction system via azeotropic distillation with the organic solvent (Ajioka et al., 1995). The average molecular weight of the PLA depends on the presence of moisture content, organic solvent and the type of solvent. When the solvent has high moisture content, ~ 400-500 ppm, the average molecular weight of PLA obtained is 15,000-50,000 Da. Although the polymerization time is also high (~50 h), under optimum conditions, this process yields PLA with molecular weight greater than 105 Da, (Enomoto et al., 1994). In order to achieve high molecular weight PLA, polycondensation synthesis of moderate molecular weight PLA is first carried out through the melt polycondensation (MP) of LA in the presence of an appropriate catalyst (Moon et al., 2001). Subsequently, the molecular weight is enhanced by lowering the temperature below the melting point of PLA, and carrying out SSP of the moderate molecular weight PLA prepolymer. In solid state
polymerization, reaction can be favored over depolymerization and other side reactions, because of restricted mobility of PLA chains and their end groups, and due to high activation energy required for unwanted side reactions. Particularly, in the process of crystallization of the resultant pre-polymer, both reactive end groups and catalyst, are concentrated in the amorphous region leading to the preferential condensation between the reactive end groups of pre-polymer, yielding PLA of high molecular weight upto 600,000 Da (Moon et al., 2001).
Scheme 1.1. illustrates the process of polymerization of lactic acid. As one of the simplest polymerization, addition polymerization of lactide, the dimer of lactic acid is capable of yielding high molecular weight PLA in short polymerization times (minutes to a few hours) via ROP.