Purification of Lactide

A crude lactide stream produced in the lactide synthesis reactors contains lactic acid, lactic acid oligomers, water, meso-lactide, and further impurities. Two main separation methods, distillation and crystallization, are currently employed for lactide purification. Crystallization may be carried out either by solvent crystallization or melt crystallization. The most used method for production of ultra-pure lactide in laboratory is by repeated recrystallization of a saturated lactide solution in mixtures of toluene and ethyl acetate [15, 23, 24]. Lactide purification using C4-12 ethers [25], and an organic solvent that is immiscible with water to extract the solution with water [26] are also reported. Melt crystallization is more practical in industry for lactide purification. Several types of equipment are described in the literature for melt crystallization [17, 27-30]. This method uses the differences in the melting points of L-, D-, and meso-lactide for separating the different lactides from each other. In a distillation process, the crude lactide is first distilled to remove the acids and water, and then meso-lactide is separated from lactide [11, 31]. Different methods are reported in the literature for distillation purification of lactide [32, 33]. In... 

ROP of lactide was already demonstrated by Carothers in 1932, but the products had low molecular weight. New developments in the purification of lactide monomer and improvements in the synthesis techniques led recently to PLA with suitable properties being synthesized by this method. The lactide monomer was obtained from oligomeric PLA in an internal transesterification (back-biting) process performed at high temperatures and reduced pressure. This process, due to racemization, yields a mixture of the three possible dilactide forms D,D-lactide, L,L-lactide and D,L-lactide (meiu-lactide) (Scheme 6.3). The desired isomer can be obtained from this mixture by distillation and crystallization. 

Gruber PR, Hall ES, Kolstad JJ, Iwen ML, Benson RD, Borchardt RL (1994) Continuous process for manufacture of lactide polymers with purification by distillation. US Patent 5357035... 

Groot W, van Krieken J, Sliekersl O, de Vos S (2010) Production and purification of lactic acid and lactide. In Poly(lactic acid) synthesis, stmetures properties, processing, and applications, John Wiley Sons, Inc., Hoboken, New Jersey, pp 1-18... 

PLA offers an unprecedented market potential to lactic acid producers all over the world, but not all potential players can succeed, because PLA production poses stringent demands to lactic acid quality and price. The chemistry and physics of today s fermentative production and industrial-scale purification of lactic acid and lactide are the subject of this chapter. 

Solvent Crystallization. A commonly used laboratory method for lactide purification is recrystallization from mixtures of toluene and ethyl acetate [4]. Lactide of extremely high purity can be obtained by repeated crystallization with different toluene/ethyl acetate ratios. Several patents also mention the use of solvents for the crystallization of lactide, but for large scale, melt crystallization without the use of solvents is preferred. 

The specifications and allowed impurity levels of lactide monomer for PLA are defined by the polymerization mechanism and the applied catalyst. PLA is commercially produced by ROP of lactides in bulk. The tin(II)-catalyzed process offers good control over molecular weight and reaction rate provided that it is performed in the absence of impurities such as water, metal ions, lactic acid, or other organic acids. Purification of crude lactides is therefore indispensable for the industrial manufacture of high molecular weight PLA (M > lOOkg/mol). In fact, lactide is the ultimate form of lactic acid, in its dehydrated and purest form. 

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. 

While direct polycondensation of LA should be the cheapest route to PLA, the ring-opening polymerization (ROP) of lactide is the method used commercially. Though the ROP of lactide was first studied long back (1932), only low molecular weight polymer was produced until lactide purification techniques were devised by DuPont in 1954. Over the past decades, many researchers have studied the... 

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. 

Although PLA is synthesized from renewable resources such as starch, the production up to the polymeric form requires much energy and many steps (Fig. 9.1] i.e., fermentation and purification of lactic acid, condensation for oligomerization, thermal degradation into lactide, and polymerization [9], if a highly selective depolymerization of PLA to the cyclic monomer, lactide, can be achieved with high efficiency, it will then be possible to reproduce PLA via the shortest and most energy efficient route. 

The biodegradable polymer available in the market today in largest amounts is PEA. PEA is a melt-processible thermoplastic polymer based completely on renewable resources. The manufacture of PEA includes one fermentation step followed by several chemical transformations. The typical annually renewable raw material source is com starch, which is broken down to unrefined dextrose. This sugar is then subjected to a fermentative transformation to lactic acid (LA). Direct polycondensation of LA is possible, but usually LA is first chemically converted to lactide, a cyclic dimer of LA, via a PLA prepolymer. Finally, after purification, lactide is subjected to a ring-opening polymerization to yield PLA [13-17]. 

---