Читать книгу Poly(lactic acid) - Группа авторов - Страница 74

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.