3.3.4 Chain Extension with Bis‐Epoxies
Bis‐epoxies have been reported to be useful for chain extending PLA. The epoxy groups can react both with hydroxyl groups and with carboxylic acid groups (Figure 3.6).
However, the latter reaction has been found to be the more rapid one. The opening of the epoxy ring yields a secondary hydroxyl, but this one does not readily react further with remaining epoxy groups [51].
FIGURE 3.6 Chain‐extension reactions of LA‐based prepolymers using bis‐epoxies.
3.4 LACTIC‐ACID‐BASED POLYMERS BY RING‐OPENING POLYMERIZATION
The generally applied ROP process for polylactides involves three separate steps: polycondensation, lactide manufacturing, and ring‐opening polymerization (Figure 3.1). All three chemical processes have basically been known for a long time. Carothers et al. [74] did the first observations on the reversible formation of the ring‐formed dimer of α‐hydroxy acids, and the self‐condensation ability of LA was discovered even longer back in time [75]. The results of these pioneering works have later been utilized in further scientific studies, as well as in making and improving technically and economically feasible processes. The three different processes deal with a number of critical steps. Some of these issues are intrinsically present in all the steps of the manufacturing process due to the nature of the LA molecule, while other issues are generated in the separate process steps as a result of the processing conditions. The most crucial parameters are summarized below along with a summary of the harmful effects that can be seen in the PLA:
1 (a) Racemization. The racemization may have its origin in the optical purity of LA or be generated, which is pronounced in any of the process steps. An increased amount of the antipodal structure of the repeating unit will result in drastic changes in the crystallization behavior and eventually affect many other properties of the end products [76, 77].
2 (b)Lactide Purity. The lactide can contain impurities such as acids or oligomers formed during the depolymerization or purification step. The presence of impurities in the lactide and the amount thereof will affect polymerization rate, molecular weight, or both [78].
3 (c)Residual Monomer Content. The presence of residual lactide in the polymer and the amount thereof will have harmful effects on the performance of the polymer during processing and may also cause undesired property changes in the end products [79].
A few complete process descriptions going from LA to polylactide can be found in the literature, for instance, the Cargill process [80], the Inventa Fisher process [81], and the Boehringer process [82]. However, most references are found on newer scientific results and detailed process improvements, which will be discussed in the following sections.
3.4.1 Polycondensation Processes
Many technical processes involve esterification reactions, and these have traditionally been of main importance in the preparation of polyesters. Previous chapters have dealt with the preparation of mainly high‐molecular‐weight LA polymers involving polycondensation. This chapter focuses on processes where the polycondensation is a process step in the ROP polylactide manufacturing chain and the PLA prepared is generally of low molecular weight.
The lactide manufacturing is done by depolymerization of PLA that preferably is in the M w range of 400–2500 g/mol [83]. Both catalyzed and uncatalyzed polycondensation reactions of LA as such have been known as an industrial process since the 1940s and can be considered to be common knowledge [84]. This explains the fact that not more than a few relevant patents can be found for the polycondensation of LA into low‐molecular‐weight polymer. The patent literature found is mainly related to other inventions in connection with the polycondensation process, for instance, the use of different catalyst systems, such as solid inorganic catalysts containing alumina silicate [85] or alkali metal compounds [26]. The former reference also provides the conditions for the polycondensation processes in terms of temperature increase (from 105 to 150°C) along with pressure reduction (350–30 mmHg). Further references are related to technical solutions for the polycondensation process. One invention describes the production of an LA polycondensate with a degree of polymerization of 1.59–2.63 and immediately separation of the lactide contained in the polycondensate [86]. Another uses an adiabatic reactor at 120–180°C and has a recycling loop, and it eliminates the water vapor [87]. Two additional references for polycondensation processes focus on improving water removal [88, 89].
3.4.2 Lactide Manufacturing
It was earlier mentioned that the reversible lactide formation from polycondensated LA was initially explored by Carothers. He furthermore observed that temperature and pressure could be manipulated for pushing the equilibrium toward the lactide product. This was utilized later for the preparation of lactide, but the presence of other species (e.g., LA, water, lactoyllactic acid, lactoyllactoyllactic acid, and higher oligomers) necessitates further purification of the crude lactide to make it useful for polymerization purposes.
Various technologies for lactide manufacturing are found in the literature. Batch‐wise or continuous manufacturing processes have been described, as well as the use of different catalysts [90, 91]. A typical manufacturing process on an industrial scale involves heating PLA to 130–230°C at reduced pressure in the presence of 0.05–1.0 wt% of tin dust, or an organic tin compound derived from a carboxylic acid having up to 20 carbon atoms, in such a way that the produced lactide is distilled off and the PLA is continually or batch‐wise replenished [92]. In some processes, a fluid is used to make the separation of lactide more efficient. This can, for instance, be done by stripping off and recovering lactide from a gaseous nonreactive feed containing LA polycondensate [93]. It has been shown that keeping the crude lactide at elevated temperatures above the melting point prior to distillation for several hours will result in an increased LA oligomer content which enables a more efficient purification process [94]. The crude lactide will in most cases contain different impurities that will make the monomer mixture unsuitable for direct ring‐opening polymerization as such. The optical purity, acid number, and yield of the lactide will accordingly affect the economy of the manufacturing to a large extent. There are mainly three purification approaches suggested in the literature: solvent‐assisted purification, crystallization from the melt, and purification in the gas phase.
3.4.2.1 Solvent‐Assisted Purification
Solvent‐assisted purification has been described in both the scientific and the patent literature. The most commonly used method for purifying lactide is by crystallization from ethyl acetate or from toluene and subsequent drying of the lactide under vacuum [95, 96]. A few industrial processes for manufacturing and purifying lactide from solvents have been described in the patent literature. In one reference, lactide is purified by washing with C4–12