Lactide production

Figure 8.30 Energy consumption (a) and carbon dioxide equivalent amount (b) for PLLA production. In Case (1), used PLLA is disposed by reclamation and equivalent amount of PLLA is produced from corn. In Case (2), used PLLA is disposed by incineration, the energy is utilized for power generation, and equivalent amount of PLLA is produced from com. In Case (3), used PLLA is recycled by hydrolytic degradation, lactide production and polymerization, and PLLA with equivalent amount of which is lost during recycling is produced from com [433]. (Reproduced from [433] with permission from Society of Environmental Science, Japan 2009.)...

The dehydration of lactic acid to make the prepolymer should start with an —OH to —COOH ratio of 1 1. All other components with —OH and —COOH functionality disrupt the stoichiometric balance and may be incorporated as comonomers during prepolymerization, which limits the final lactide production yield from lactic acid. Little public information is available on the technical and economic relationship between lactic acid quality and lactide synthesis. Only a few patents mention the effect of metal impurities on racemization [68,69]. Stereochemical purity is one of the key parameters determining lactic acid purity. 

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. In order to pull the reaction toward the right, lactide must be withdrawn from the system.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 engineering terms, this means that mass transfer of lactide from the liquid to the gas phase decreases as viscosity increases. The balance between lactide production and lactide removal plays a role in aU experiments that one might want to investigate on lab scale. For example, catalyst concentrations of 0.05-0.2 wt% tin(II) octoate are mentioned in the literature, but traditional experiments to verify the order of the reaction for the catalyst are difficult because of the influence of mass transfer limitations. 

Cationic impurities such as sodium ions have no direct effect on lactide production rate, but the sodium content has a direct correlation with the m. yo-lactide content in the crude lactide [67, 87]. 

Ingeo PLA is easily processable and suitable as amorphous biopackaging material as a result of its relatively high meso-lactide content. The downside is the poor resistance to elevated temperatures (low heat distortion temperature, HDT) during transportation, storage, and use of articles produced from this bioplastic. mc o-Lactide— which contains an L- and a D-isomer—is an unavoidable side product of lactide production and must be separated from L- and D-lactides of high stereochemical purity. 

It was earlier mentioned that the reversible lactide formation from polycondensated lactic acid was initially explored by Carothers. He furthermore observed that manipulation of the temperature and pressure could be utilized 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., lactic acid, water, lactoyllactic acid, lactoyl-lactoyllactic acid, and higher oligomers) necessitates further purification of the crude lactide to make it useful for polymerization purposes. 

Though Sn(Oct)2 are effective catalysts for L,L-lactide production from PLLA [43], there are concerns regarding their... 

MgO even in the lower temperature range. This characteristic antiracemization effect of MgO is due to the lower basicity of Mg compared to Ca. At temperatures lower than 270°C, the pyrolysis of PLLA/MgO (5 wt%) composite occurred causing unzipping depolymerization, resulting in selective L,L-lactide production. 

Lactide production technologies have been in use since the 1930s, with the related pubhcation by Carothers et al. (1932) about the reversible polymerization of six-membered cyclic esters. After that, lactide technology underwent a period of inactivity because the purity of lactide was insufficient for large-scale production. Lactide technology did well after DuPont developed a purification technique. This ultimately led towards mass-scale production by NatureWorks. This section mainly focuses on the mass-scale lactide production as developed by Cargill—DuPont (currently known as NatureWorks) in the early phases, as well as some related lactide technologies. 

---