Lactide purification

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... 

A lactide synthesis reactor invariably produces a crude lactide stream that contains lactic acid, lactic acid oligomers, water, mc50-lactide, and further impurities. The specifications for lactide are stringent mainly for free acid content, water, and stereochemical purity. Basically, two main separation methods, distillation and crystallization, are currently employed for lactide purification ... 

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. 

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... 

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]. 

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... 

ROP (Figures 12.20 and 12.21) is the route which delivers by far the highest proportion of PLA chips available on the market. The other routes produce only minor amounts or did not get past the pilot scale. Figure 12.21 depicts the steps of an ROP process, starting from lactic add. In the first part lactide is formed, which - after fine purification is converted by ROP to PLA. 

PLA is a crystal clear, transparent material when amorphous that becomes hazier the higher the crystallinity. Crystallized material is opaque. When producing lactide, meso-lactide is formed as a by-product. It is difficult to separate the meso-lactide from the L-lactide in the purification step. When polymerizing L-lactide with small contents of meso-lactide a co-polymer is formed. Increasing meso-lactide leads to decreasing crystallinity. With more than 10-15% meso-lactide the polymer is amorphous. 

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... 

As one-pot reactions by the simultaneous initiation of both polymerizations always affect one another, control of the overall process is often very difficult to achieve. Another fairly new strategy, the AROP of lactones [210-213], lactides [214] or benzyl-L-glutamate [215] and the controlled radical polymerization of vinyl monomer, which take place in one-pot but in consecutive fashion, has been introduced by several groups. In this strategy, the AROP of lactones, lactides, or benzyl-L-glutamate can st be initiated by either an enzymatic or a metal catalyst at low temperature. In a second step, ATRP of MMA [210-213], tBMA [212], or 2-hydroxyethyl MA [214] and NMRP of styrene [211, 215] can be activated by increasing the reaction temperature and injecting the ATRP catalyst, respectively (Scheme 11.48). The reaction was conducted in one-pot, without any intermediate work-up and purification. 

The lactide monomer for PLA is obtained from catalytic depolymeiization of short PLA chains under reduced pressure [4]. This prepolymer is produced by dehydration and polycondensation of lactic acid under vacuum at high temperature. After purification, lactide is used for the production of PLA and lactide copolymers by ROP, which is conducted in bulk at temperatures above the melting point of the lactides and below temperatures that cause degradation of the formed PLA [4]. 

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. 

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. 

PLA grades for more demanding applications that require better heat resistance are achievable by stereocomplexation with PDLA [89]. This is only effective with PLA grades of high stereochemical purity. In order to prepare high-quality PLA, it is necessary to start with lactide monomers with the highest possible stereochemical purity, that is, the lowest mejo-lactide content that is technically and economically achievable by purification. 

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. 

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