3 INDUSTRIAL PRODUCTION OF HIGH‐MOLECULAR‐WEIGHT POLY(LACTIC ACID)

      Anders Södergård Mikael Stolt and Saara Inkinen

3.1 INTRODUCTION

Polymers based on lactic acid (PLA) are the most promising category of polymers made from renewable resources. They are not only compostable and biocompatible but also processable with standard processing equipment. The properties of lactic‐acid (LA)‐based polymers vary to a large extent depending on the ratio between, and the distribution of, the two stereoisomers or other comonomers [1–3]. The polymers can be manufactured by different polymerization routes, which are schematically described below (Figure 3.1).

      PLA of high molecular weight is most commonly made by ring‐opening polymerization (ROP) of the ring‐formed dimer, dilactide (lactide; 3,6‐dimethyl‐1,4‐dioxane‐2,5‐dione), which is made by depolymerization of the polycondensed LA (2‐hydroxypropanoic acid). This route is a two‐step reaction that usually involves additional purification steps and is therefore costly. It is often stated in the art that the preparation of high‐molecular‐weight PLA by a direct dehydration condensation reaction is not feasible due to the equilibrium not favoring a high‐molecular‐weight polymer. PLA prepared from polycondensation has low molecular weight and poor mechanical properties and therefore is not suitable for many applications. The commercial interest for solving this problem has increased because of the need of cost‐effective approaches in the manufacturing of LA‐based polymers with a high molecular weight. Solvent‐assisted polycondensation is one way to overcome this problem [4] and melt polycondensation followed by solid‐state polycondensation is another one [5]. The third approach to achieve high‐molecular‐weight LA‐based polymers is to utilize the terminal groups of the prepolymer in linking processes where a linking agent is employed [6]. Such prepolymers can be composed of solely one stereoisomer, combinations of D‐ and L‐lactoyl units in various ratios, lactic acid in combination with other hydroxy acids, or di‐/multifunctional comonomers. If the lactic acid is polycondensated in the presence of difunctional monomers (e.g., diols or diacids), the resulting prepolymer will have the same end groups in both chain ends; that is, the prepolymer is a telechelic macromer [6].

      Since the first commercial products of lactic‐acid‐based polymers were introduced in the market by DuPont, Chronopol, and Cargill at the end of the 1980s and the beginning of the 1990s, PLA has been successfully commercialized by several companies and subsequently found use in several different end‐product categories [7]. However, PLA was initially used mainly in medical applications [8], which continue to be an important field for some PLA producers [9]. Today, PLA is one of the most important bio‐based and biodegradable polymers suitable for a wide variety of applications [10].

      The unique position of PLA in the field of bio‐based products is related to its processability, appearance, and properties such as mechanical strength and barrier. In addition to the medical sector, other uses of PLA include food and nonfood packaging, food service ware, agricultural materials, durable goods, electronics, building, construction, and 3D‐printing [11, 12]. Since 2005, the business of bio‐based material production has grown significantly. In 2020, 18.7% (i.e., 395,000 metric tons) of the total annual production capacity of bioplastics was attributed to PLA [13].

The growth of industrial PLA production volumes was initially driven by (i) oil prices, as high crude oil price was reflected in the pricing of traditional polymers and in energy costs; (ii) factors related to legislation, as favorable legislation and government support made the use of bio‐based materials an attractive option to traditional polymers; and (iii) image, as an increased awareness among brand owners and retailers made compostable and renewable products real alternatives with added value in terms of sustainability.