Читать книгу Poly(lactic acid) - Группа авторов - Страница 35

The synthesis of lactide was first described by Pelouze in 1845 [71]. He investigated the self‐esterification of lactic acid by heating and driving off water and obtained a prepolymer that was no longer fully miscible with water. Upon continued heating of the prepolymer, he noticed that in a certain distillate fraction nice crystals were formed. He was able to deduce the chemical formula and gave the name “lactid” to the substance. An improved procedure was described in a patent by Gruter and Pohl in 1914 [72]. Lactic acid was self‐esterified at 120–135°C, and air was drawn in to remove the water. Next, zinc oxide was added as a catalyst and lactide was distilled off under vacuum at 200°C. In practice, modern industry cannot dispense with this concept of thermal catalytic depolymerization for lactide production. A major step forward was the use of a tin catalyst, a frequently used coordinating catalyst in polymerizations, in the process. The general scheme of lactide manufacture including the purification is shown in Figure 1.7.

TABLE 1.3 Physical Properties of the Lactides

	Unit	D‐Lactide	L‐Lactide [6]	meso‐Lactide	rac‐Lactide
CAS number		13076‐17‐0	4511‐42‐6	13076‐19‐2	116559‐43‐4
Molecular weight	g/mol	144.12	144.12	144.12	—
Melting point	°C	96–97	96	53 [64]	125 [6]
Boiling point	°C	—	—	—	142 (20 mbar) [64]
Heat of fusion	J/g	—	146	128 [64]; 118 [6]	185 [6]
Heat of vaporization	kJ/mol	—	63	—	—
Solid density	g/mL	—	1.32–1.38	1.32–1.38 [6]	—
Liquid viscosity	mPa s	—	2.71 (110°C); 2.23 (120°C); 1.88 (130°C)	—	—

FIGURE 1.7 Schematic illustration of lactide manufacture by thermal catalytic depolymerization of lactic acid oligomers.

In the past two decades, several papers have appeared on lactide manufacture [73, 74]. A main underlying problem in understanding all information is that the reaction from oligomer to lactide is an equilibrium reaction. To pull the reaction toward the right, lactide must be withdrawn from the system. In reaction engineering terms, this means that the chemical kinetics of the reaction cannot be understood without consideration of the method and efficiency of lactide removal. In terms of know‐how described in patents, this means that reported lactide production rates depend to a large extent on the geometry of the equipment in which lactide synthesis is performed and that provides for removal of lactide vapor from the reaction zone.

In modern chemical technology, one of the goals is to fully understand a given system, capture the knowledge in models to describe experimental work, and ultimately use these models to design, optimize, and debottleneck large‐scale equipment. For the present system, this means that one must develop process know‐how on chemical kinetics and thermodynamics of lactide and HL oligomers, and on physical phenomena related to equipment design.

These aspects will be relevant for both the prepolymerization and the synthesis of lactide, as these chemical systems are highly similar. In practice, however, lactide synthesis is more complex as chemistry, recovery and type of equipment are intertwined, and the viscous nature of reaction mixtures requires special attention.

With these aspects in mind, the information on the lactide synthesis that can be found in the literature is summarized below.