Lactide crude

The solution to this problem has been to isolate the lactide and to polymerize this directly using a tin(ii) 2-(ethyl)hexanoate catalyst at temperatures between 140 and 160 °C. By controlling the amounts of water and lactic acid in the polymerization reactor the molecular weight of the polymer can be controlled. Since lactic acid exists as d and L-optical isomers, three lactides are produced, d, l and meso (Scheme 6.11). The properties of the final polymer do not depend simply on the molecular weight but vary significantly with the optical ratios of the lactides used. In order to get specific polymers for medical use the crude lactide mix is extensively recrystallized, to remove the meso isomer leaving the required D, L mix. This recrystallization process results in considerable waste, with only a small fraction of the lactide produced being used in the final polymerization step. Hence PLA has been too costly to use as a commodity polymer. 

Although increasing the catalyst concentration results in the faster overall reaction rate, enhanced rate of racemization, which is unwanted in the production of optically pure lactide may also occur. Higher synthesis temperatures, longer reaction times and presence of some metal cations such as sodium and potassium have the same effect on the stereochemical purity of the crude lactide [14, 15, 19]. Released metals through corrosion and carboxylic acid impurities formed during lactic acid fermentation are other sources of impurities [10,14, 20-22]. 

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... 

K. Kubota and Y. Murakami, Process for producing lactides and process for purifying crude lactides, US Patent 5463086, assigned to Dainippon Ink and Chemicals, Inc. (Tokyo, JP), October 31,1995. 

Because the composition of a mixture comprising lactic acid oligomers and lactide is governed by chemical equilibria, a prepolymerization exhibits relatively high concentrations of lactide (HL2-H2O-L2 equUibrium) around DP 2. Sinclair et al. distilled these fractions to recover lactide, but the crude lactide was quite impure, which may prevent economical processing [73]. In hindsight, the patent describes trials to optimize Pelouze s original lactide synthesis without catalyst [71]. 

Basic Research on Batch Lactide Synthesis and the Catalysts Used Noda and Okuyama reported on the batch synthesis of lactide from DP 15 prepolymer with various catalysts at 4—5 mbar and 190-245°C [74]. In a batch synthesis with 50 g of oligomer in a stirred flask, the evolution rate of crude lactide is rather constant and then starts to dechne and the conversion levels off at 80-90%. The tin catalyst performed best compared to other catalysts and showed the lowest levels of racemization. Tin octoate... 

Continuous Synthesis In 1992, Gruber et al. [68] described a continuous lactide synthesis in which prepolymer is fed continuously to a reactor, crude lactide is evaporated under vacuum, and residue is removed. Typical operating conditions for the reactor were residence time around 1 h, vacuum pressure 4 mbar, temperature 213°C, and catalyst amount 0.05 wt% tin(II) octoate on feed. The conversion per pass was around 70%, and the overall yield was increased by recycling the residue to the lactic acid section of the process, where the oligomers are hydrolyzed again. 

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 ... 

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. 

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]. 

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. 

Remove crude lactide as a vapor from lactide reactor... 

As mentioned earlier, the stereocomplex composition of the lactide produced is dependent on the initial crude lactic acid... 

In the polymerization of lactides to polylactide the crude lactides have to be purified with regard to residual water (<50 ppm) and free acidity (<0.1% meq/kg). Depending on the type of lactides used, for the ring opening polymerization, polymers with different properties can be generated. For this process the necessary step of dimerization of lactic acid has the disadvantage of increasing production costs. 

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