Читать книгу Poly(lactic acid) - Группа авторов - Страница 16

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 cost‐ and performance‐wise competitive. Polylactide or poly(lactic acid) (PLA) is the front runner in the emerging bioplastics market 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 cassava, 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 PLA via 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 high‐molecular‐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 the melting 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 L‐ to 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 185°C [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].

For decades, ROP has been the preferred route to PLA for biomedical applications with small production volumes. PLLA and copolymers with rac‐lactide, glycolide, and ɛ‐caprolactone for resorbable biomedical applications have been produced by, for example, PURAC, previously known as CCA, since the 1970s [5]. Since the 1990s, the ROP concept is also used for high‐volume production of PLA grades for other end uses.

Large‐scale production of PLA, copolymers of L‐ and meso‐lactide, was started in 2002 by a joint venture of Cargill and Dow under the name NatureWorks LLC. Nowadays, since July 1, 2009, NatureWorks LLC is again wholly owned by Cargill and has a production capacity of 140 ktpa for its Ingeo PLA grades in Blair, Nebraska [11].

The attractive price and commercial availability of lactic acid were important reasons why PLA became the first mass‐produced bio‐based polyester. The critical success factor for a final breakthrough of all green chemicals and plastics based on annually renewable materials is economic sustainability. Thus, the very basis of cost‐competitive PLA is an industrial fermentative production process for lactic acid with efficient use of carbohydrates followed by excellent purification technology with minimum generation of by‐products.

An important impulse for the expanding bioplastics market is the commercialization of lactide monomers for PLA by PURAC in 2008. Solid D‐ and L‐lactides are now available in bulk quantities and can be polymerized into a whole range of tailor‐made polylactides by continuous melt polymerization processes, like the technology based on static mixing reactors that was jointly developed by Sulzer and PURAC.

PLA offers an unprecedented market potential to lactic acid producers all over the world, but not all potential players can succeed, because PLA production poses stringent demands to lactic acid quality and price. The chemistry and physics of today's fermentative production and industrial‐scale purification of lactic acid and lactide are the subject of this chapter.