Production and Purification of Lactic Acid and Lactide

Abstract

Natural polymers, biopolymers, and synthetic polymers based on annually renewable resources are the basis for the twenty-first-century portfolio of sustainable, eco-efficient plastics [1]. These biosourced materials will gradually replace the currently existing family of oil-based polymers as they become costand performance-wise competitive. Polylactide or poly(lactic acid) (PLA) is the front runner in the emerging bioplasticsmarket with the best availability and the most attractive cost structure. The production of the aliphatic polyester from lactic acid, a naturally occurring acid and bulk produced food additive, is relatively straightforward. PLA is a thermoplastic material with rigidity and clarity similar to polystyrene (PS) or poly(ethylene terephthalate) (PET). End uses of PLA are in rigid packaging, flexible film packaging, cold drink cups, cutlery, apparel and staple fiber, bottles, injection molded products, extrusion coating, and so on [2]. PLA is bio-based, resorbable, and biodegradable under industrial composting conditions [1, 3, 4]. PLA can be produced by condensation polymerization directly from its basic building block lactic acid, which is derived by fermentation of sugars from carbohydrate sources such as corn, sugarcane, or tapioca, as will be discussed later in this chapter. Most commercial routes, however, utilize the more efficient conversion of lactide—the cyclic dimer of lactic acid—to PLAvia ring-opening polymerization (ROP) catalyzed by a Sn(II)-based catalyst rather than polycondensation [2–6]. Both polymerization concepts rely on highly concentrated polymer-grade lactic acid of excellent quality for the production of highmolecular weight polymers in high yield [2–4, 7]. Purification of lactic acid produced by industrial bacterial fermentation is therefore of decisive importance because crude lactic acid contains many impurities such as acids, alcohols, esters,metals, and traces of sugars and nutrients [4]. The lactide monomer for PLA is obtained from catalytic depolymerization of short PLA chains under reduced pressure [4]. This prepolymer is produced by dehydration and polycondensation of lactic acid under vacuum at high temperature. After purification, lactide is used for the production of PLA and lactide copolymers by ROP, which is conducted in bulk at temperatures above themelting point of the lactides and below temperatures that cause degradation of the formed PLA [4]. Processing, crystallization, and degradation behavior of PLA all depend on the structure and composition of the polymer chains, in particular the ratio of the Lto the D-isomer of lactic acid [2, 4, 6, 8, 9]. This stereochemical structure of PLA can be modified by copolymerization of mixtures of L-lactide and meso-, D-, or rac-lactide resulting in high molecular weight amorphous or semicrystalline polymers with a melting point in the range from 130 to 185C [3, 4, 6–10]. Isotactic PLLA homopolymer—comprising L-lactide only—is a semicrystalline material with the highest melting point, while PLA copolymers with higher D-isomer content exhibit lower melting points and dramatically slower crystallization behavior, until they finally become amorphous at D-contents higher than 12–15% [8–10].