The demand for PA degradation has been increased, but the PA industry is still not accepted as a biodegradable polymer, but some of the PAs can be degraded in the soil, which can fulfill future development for degradation.
4.3 Polylactic Acid
Definition
PLA is the most commonly used bio-plastic, and it is a kind of thermoplastic aliphatic polyester. Lactic acid, the precursor of PLA, can be obtained very easily from various raw materials (like corn and starch), which is then polymerized to PLA. Several kinds of PLA are available to include PDLA (Poly-D-lactic Acid), regular PLLA (Poly-L-lactic Acid), PDLA (Poly-D-lactic acid), and Racemic PDLLA (Poly-DL-lactic Acid). They have slightly different characteristics properties but are produced from the same renewable resource (lactic acid).
Naturally occurring polymers, bio-derived plastics, and synthetic bio-based plastics from renewable resources are the existing bases for establishing a sustainable society. Replacing bio-sourced materials over the existing fossil-fuel-based plastics are the prime focus of recent research. Cheap raw materials such as maize, potato, starchy materials, and lignocellulose biomass are feasible for economic lactic acid production. Poly-lactide or polylactic acid (PLA) is the front-runner in the emerging bioplastics market with the best availability and the most attractive cost structure. Theophile-Jules Pelouze first synthesized PLA in 1845 by the lactic acid polycondensation method [16–18]. Later on, Wallace Hume Carothers introduced another method in 1932, which was patented by DuPont in 1954. PLA polymers change from amorphous glassy state to highly crystalline with high glass transition temperature and mechanical property. PLA can be processed into several materials like fused filament fabrication in 3D printers, medical implants (like anchors, screw), and packaging materials. Biodegradability of PLA is a natural phenomenon which is even faster as compared with other bioplastics. Mechanical property and biodegradability can be improved by several methods like-annealing, blending, the composite formation, side-chain modification, etc.
4.3.1 Availability and Production
Lactic acid also has a long invention history with the first reported discovery by Scheele [16, 17] on 1780 as a milk component, later on, Lavoisier named this milk component “acid lactic” in 1789 and Pasteur in 1857 confirmed it as a fermented metabolite rather than milk component [18]. Lactic acid produced by microorganism fermentation or via a synthetic chemical pathway. The demand for lactic acid-based products increasing globally and estimated to be raised around 2,000 kilotons by 2020 [19]; the largest consumer markets in the world are the United States, followed by China and Western Europe [17]. Lactic acid consists of two optical isomers: L (+)-lactic acid and D(−)-lactic acid, which can be prepared as optically pure isomers, i.e., L(+)- or D(−)-lactic acid by microbial fermentation (Figure 4.4) of renewable resources with the correct choice of the microorganisms. Each isomer is advantageous over the other depending upon the application. Optically pure lactic acids are the best choice to make high molecular weight commercial grade bio-plastic rather than the plastics derived from the racemic mixture in the chemical synthesis method (Figure 4.4). Apart from those, other numerous bioresources are available for lactic acid production, like glycerol (a by-product of bio-diesel) and microalgae (harvesting can be possible anywhere with a concise harvesting cycle). Microorganisms producing lactic acid are classified into two groups: bacteria and fungi, and their use depend on the substrates to be fermented. However, lactic acid bacteria (LAB) is the most popular method over the fungal production in terms of production rate caused due to mass transfer limitation and by-products formation. LAB can be classified into two categories depending upon the end fermentation product, homo-fermentative. It converts glucose into lactic acid as the sole product, whereas in the case of hetero-fermentative predominating side products like CO2 and ethanol are also formed along with the desired lactic acid. Several efforts have been given to optimizing lactic acid production through microorganism engineering, and in Table 4.1, a few of them are listed.
Figure 4.4 Overview of different manufacturing methods of lactic acid (a) chemical synthesis and (b) microbial fermentation [20].
Table 4.1 Reports in the literature about recent investigations on the biotechnological production of lactic acid from cheap raw materials.
| Substrate | Microorganism | Fermentation method | Lactic acid | References | |
| Process productivity g/(L.h) | Yield (g/g) | ||||
| Sugarcane bagasse hemicellulose hydrolysate | Bacillus sp. 75 strain 17C5 | Batch | 0.8 | 0.93 | Patel et al., 2004 [49] |
| Corn fiber hydrolysate | Bacillus coagulans MXL-9 | Fed-batch | 0.21 | 0.46 | Bischoff et al., 2010 [50] |
| Biomass derived xylose | Bacillus coagulans NL01 | Batch | 1.04 | 0.75 | Ouyang et al., 2012 [51] |
| Various carbohydrates | Enterococcus faecalis RKY1 | Batch | 5.1 | 0.96 | Yun et al., 2003 [52] |
4.3.2 Polymerization Method
PLA can be synthesized using three ways: (a) The first pathway is condensation polymerization of the L(+) and D(−) isomers of lactic acid which produce low molecular weight PLA, (b) the second route involves ring-opening polymerization of the lactide ring (Figure 4.5). Cargill Dow LLC developed an alternative pathway of melt polycondensation and the use of a tin catalyst to obtain commodity PLA applicable for packaging industries. This pathway involves the pre-polymer formation of aqueous lactic acid and converting into lactide stereoisomer through intramolecular cyclization. In the next step, lactide ring-opening produces high Mw PLA [21] using stannous octoate Sn(Oct)2 as a catalyst [22].