FI107615B - A method for collecting lactide - Google Patents

Abstract

Lactide is removed from polylactide by passing the polymer (1) through a nozzle (2) to form thin threads with such surface area that in normal pressure or vacuum evaporators (3) the lactide evaporates rapidly into a hot carrier gas flow (7), and the polymer settles under gravity on a collecting device (5). The carrier gas flow (7), containing the lactide, is cooled rapidly in crystallisation chamber (10) to a temperature below 100 deg C where the lactide crystallises from the gas, and the crystals are separated from the gas. The resulting polylactide is also claimed. Preferably the nozzle is a perforated nozzle, with perforations of 0.2 to 0.3 mm diameter, or a film nozzle. Evaporation gas is hot nitrogen or air travelling at 0.5 to 1 m/s relative the melt threads. The gas mixture is cooled ( in 10) by cold air at 20 - 40 deg C. Crystallised lactide is separated by cyclone (13) and filtration (14), and is recycled to the process. The resulting polylactide has a concentration of lactide less than 2 wt%.

Description

107616

The invention relates to the recovery of lactide from a process for the production of polylactide.

In recent years, interest in biodegradable polymers, or biopolymers, has increased dramatically and many companies have sought to market packaging materials, hygiene products, agricultural bags and films, or waste bags. 10 In particular, various films have become important. The use of lactic acid polymers in these applications has so far been limited by the high price of the polymers and their susceptibility to degradation by technical processing.

Polyhydroxy acids can be prepared by polycondensation reactions which are typical for the preparation of polyesters or ring opening polymerization from cyclic dimers, lactones. Polyhydroxy acids are thermoplastic polyesters that resemble many conventional polymers.

Polylactides, or condensation polymers based on lactic acid, are particularly attractive for a number of 2 0 reasons, since their main degradation product, lactic acid, is: a non-toxic, generic product widely used in the food and pharmaceutical industries. A high molecular weight polymer can best be prepared by ring opening polymerization from lactide. Lactic acid is optically • · · · ... 1 active, so its dimer comes in four different forms: L, L-lactide, D, D-lactide, · 25 L, D-lactide (mesolactide) and racemate L, L and D, D lactides. Polymerization of these, either as pure compounds or in various mixtures, yields polymers having different stereochemical structures which affect their elasticity and crystallinity and, consequently, their mechanical and thermal properties. The resulting · · * '1 polymers are generally hard and optically clear.

3 Cyclic lactones of hydroxy acids such as lactide, glycolide, ε-caprolactone, etc.

107616 ring-opening polymerization is a known technique in itself. A variety of polymerization processes are known, for example, U.S. Patent No. 5,378,801 for extrusion polymerization, EP 0 664 309-A for biphasic polymerization, and EP 0 499 747-A for polymerization in a stirred reactor.

There are significant differences between US 5089632 and the present application and their application is significantly different. In said US publication, a heated lactic acid oligomer is stripped with gaseous hydrofluoric acid and the stripped lactide-containing gas is rapidly quenched by cooling it to a liquid freon. The solution disclosed in the present application does not use fluorinated hydrocarbons and employs a crystallization atomization process substantially different from US 5089632.

The polymer or copolymer of the invention may be prepared from L-, D- or D, L-15 lactide or mixtures thereof by any of the polymerization processes, batchwise, semi-continuously or continuously. The continuous polymerization can advantageously be carried out in the extruder by polymerization. The polymer is obtained by heating the monomer or mixture of monomers to provide a homogeneous melt and adding a catalyst to polymerize the lactide, thereby opening the ring. The polymer has a molecular weight of 20 (M „) of about 20,000 to 500,000, preferably 40,000 to 300,000. Preferably, the polymer is made from · · ·: Jf * 5 L-lactide.

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Polylactide is in equilibrium with the lactide of its monomer. This has been shown to be sometimes also advantageous as monomers and oligomers may act as plasticizers of% · * \% · 25 polymers. However, it results in rapid hydrolysis and causes sticking problems in the processing of the polymer. In addition, the presence of the monomer lowers the thermal stability during melt processing. When aiming at conventional products made by melt processing techniques, the residual monomer must be removed from the polymer. An acceptable monomer content of * ·· * is less than 2% by weight, preferably less than 1% by weight.

The lactide can be removed from the polymer by extraction or evaporation from the polymer melt.

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The problem with evaporation is that evaporation times are too long and the polymer begins to degrade. Evaporation methods are listed in EP 0 499 747 -A. These include removal of the lactide by evaporation from the falling polylactide belts, thin film evaporation and the use of a vacuum extruder type evaporator.

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Preferably, the lactide can be removed by a process that effectively removes the residual lactide in the polymer at the final stage of polymerization without degradation of the polymer. The process is based on improved material transfer as the lactide evaporates from the polymer melt and is further described in the co-pending application 10 of this patent application.

The object of the present invention has been to find an efficient method for recovering the separated lactide.

The lactide recovered by the process of the invention is in powder form and is easy to process and can be fed back into the polymerization process. The recovered lactide has a very high purity and an appropriate optical structure.

The recovery method has a good yield, about 100%.

:: 20 * · ·: r? The lactide-containing gas mixture leaving the lactide evaporator is rapidly cooled, thereby causing the lactide to thicken. Typically, the gas mixture is led to an ejector or gas nozzle, to which cold air is supplied. The temperature of the lactide gas when entering the ejector is 120 - 300 ° C, depending on the evaporation method. The lactide-containing gas is cooled to a temperature of less than 100 ° C, preferably to a temperature range of 20 ° C to 40 ° C, whereupon it condenses to form small lactide crystals in the gas. The cooled lactide powder / gas mixture is sprayed into the thawing chamber, where the lactide powder is separated from the gas. The chamber exhaust gas is filtered to completely separate the m m · ♦ ♦ lactide powder.

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The method is described in more detail in the following example.

Example 5

The lactide used in the experiments was recovered by evaporation after the L-lactide polymerization process.

The lactide-containing gas at 200 ° C was mixed with cold air in the 10 gas mixer of Figure 1. The ratio of cold air supply to lactide-containing hot gas was 10 kg feed air / kg lactide-containing gas. The lactide completely sublimated as dust into the cooled gas. The agitated lactide dust was recovered by vibration from the chamber walls and separated from the air stream by filtration, after which it flowed freely into the lactide collection vessel.

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The yield of recovered lactide was over 90%.

The separated lactide was analyzed by DSC analysis and found that the recovered lactide was pure L-lactide and was not racemated by evaporation or; : 20 during recovery. Figure 2 shows a DSC image showing purity of L-lactide.

If a copolymer of D and L lactide is made, then the recovered lactide is also a mixture.

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Claims (7)

107616 A process for recovering a lactide from a lactide-containing gas characterized in that the hot lactide-containing gas is rapidly cooled by crystallization spraying to a temperature below 100 ° C, whereby the lactide is triturated to form lactide crystals which are separated from the gas. Method according to Claim 1, characterized in that the cooling is effected by introducing a gas mixture into a gas mixer to which cold air is simultaneously supplied. Process according to claim 1 or 2, characterized in that the gas mixture is cooled to a temperature of 20 to 40 ° C. A process according to any one of the preceding claims, characterized in that the mixture of the lactated truncated lactide and gas is sprayed into a trapping chamber in which the lactide 15 crystallizes. Process according to one of the preceding claims, characterized in that the crystallized lactide is separated by filtration. Process according to any one of the preceding claims, characterized in that the lactide recovered is so pure that it can be directly recycled to the process. • ·· · • φ + φ φ Φ φ φ The lost lactide ····! In accordance with any one of the preceding claims. 111 use in the lactide polymerization process. • »· · i% · i« m oC * · 25 · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·· · * · ♦ »» ♦ ♦ χ * * * * * * * * # # # # # # # 1 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10.