EP0623153A1 - Procede continu de fabrication de lactide et de polymeres de lactide - Google Patents

Procede continu de fabrication de lactide et de polymeres de lactide

Info

Publication number
EP0623153A1
EP0623153A1 EP93903632A EP93903632A EP0623153A1 EP 0623153 A1 EP0623153 A1 EP 0623153A1 EP 93903632 A EP93903632 A EP 93903632A EP 93903632 A EP93903632 A EP 93903632A EP 0623153 A1 EP0623153 A1 EP 0623153A1
Authority
EP
European Patent Office
Prior art keywords
lactide
lactic acid
crude
catalyst
ester
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
EP93903632A
Other languages
German (de)
English (en)
Inventor
Patrick R. Gruber
Eric S. Hall
Jeffrey J. Kolstad
Matthew Lee Iwen
Richard D. Benson
Ronald L. Borchardt
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Cargill Inc
Original Assignee
Cargill Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Family has litigation
First worldwide family litigation filed litigation Critical https://patents.darts-ip.com/?family=27542241&utm_source=google_patent&utm_medium=platform_link&utm_campaign=public_patent_search&patent=EP0623153(A1) "Global patent litigation dataset” by Darts-ip is licensed under a Creative Commons Attribution 4.0 International License.
Priority claimed from US07/825,059 external-priority patent/US5142023A/en
Priority claimed from US07/927,149 external-priority patent/US5247058A/en
Priority claimed from US07/926,901 external-priority patent/US5258488A/en
Priority claimed from US07/935,566 external-priority patent/US5247059A/en
Application filed by Cargill Inc filed Critical Cargill Inc
Priority to ES98203427T priority Critical patent/ES2345307T3/es
Priority to EP98203427A priority patent/EP0893462B1/fr
Publication of EP0623153A1 publication Critical patent/EP0623153A1/fr
Ceased legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
    • C08G63/00—Macromolecular compounds obtained by reactions forming a carboxylic ester link in the main chain of the macromolecule
    • C08G63/78—Preparation processes
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
    • C08G63/00—Macromolecular compounds obtained by reactions forming a carboxylic ester link in the main chain of the macromolecule
    • C08G63/02—Polyesters derived from hydroxycarboxylic acids or from polycarboxylic acids and polyhydroxy compounds
    • C08G63/06—Polyesters derived from hydroxycarboxylic acids or from polycarboxylic acids and polyhydroxy compounds derived from hydroxycarboxylic acids
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
    • C08G63/00—Macromolecular compounds obtained by reactions forming a carboxylic ester link in the main chain of the macromolecule
    • C08G63/02—Polyesters derived from hydroxycarboxylic acids or from polycarboxylic acids and polyhydroxy compounds
    • C08G63/06—Polyesters derived from hydroxycarboxylic acids or from polycarboxylic acids and polyhydroxy compounds derived from hydroxycarboxylic acids
    • C08G63/08—Lactones or lactides

Definitions

  • the present invention relates to processes for the continuous production of lactide and lactide polymers from crude lactic acid and esters of lactic acid in the field of biodegradable polymers.
  • step n of said polymerization reaction is represented by:
  • the resulting polylactic acid is limited to a low molecular weight polymer of limited value, based on physical properties, due to a competing depolymerization reaction in which the cyclic dimer of lactic acid, lactide, is generated.
  • the polymerization reaction rate decelerates until it reaches the rate of the depolymerization reaction, which effectively, limits the molecular weight of the resulting polymers.
  • An example of this equilibrium reaction is represented below. 0
  • lactic acid exists in two forms which are optical enantiomers, designated as D- lactic acid and L-lactic acid.
  • D-lactic acid, L- lactic acid or mixtures thereof may be polymerized to form an intermediate molecular weight polylactic acid which, upon further polymerization, generates lactide as earlier disclosed.
  • the lactide, or the cyclic dimer of lactic acid may have one of three types of optical activity depending on whether it consists of two L- lactic acid molecules, two D-lactic acid molecules or an L-lactic acid molecule and a D-lactic acid molecule combined to form the dimer. These three dimers are designated L-lactide, D-lac ide and meso-lactide, respectively.
  • D,L-lactide a 50/50 mixture of L-lactide and D-lactide with a melting point of about 126°C is often referred to in the literature as D,L-lactide.
  • DeVries U.S. Patent No. 4,797,468, recently disclosed a process for the manufacture of lactide polymers utilizing a solvent extraction process to purify lactide prior to polymerization. With DeVries' disclosure, the inventor recognized that existing literature recommends purification of lactide by several recrystallization steps. It is believed that processes prior to DeVries solvent extraction method, have generally utilized a recrystallization step to purify the crude lactide in order to obtain a source of lactide suitable for polymerization.
  • the biogradable polylactide polymers must also possess physical properties suitable for application in non-medical products presently utilizing petrochemical- based polymers such as packaging materials, paper coatings and any other disposable articles.
  • Nieuwenhuis et al. disclose that lactide polymers derived from polymerization of mixtures of the three lactides result in polymers with a variety of useful physical properties, including improved biodegradability.
  • no commercially viable process for the large- scale manufacture of such lactide polymers is believed to have been disclosed to date.
  • Lactic acid is commercially available and manufactured from several known processes. Representative examples of such processes are disclosed by Glassner et al. (European Patent Application, EP
  • optical activity of either lactic acid or lactide is known to alter under certain conditions, with a tendency toward equilibrium at optical inactivity, where equal amounts of the D and L enantiomers are present.
  • Relative concentrations of D and L in the starting materials, the presence of impurities or catalysts and time at varying temperatures and pressures are known to affect the rate of such racemization.
  • Muller U.S. Patent No. 5,053,522 discloses that the preparation of optically pure lactide from an optically pure lactic acid feed is possible when utilizing appropriate conditions and catalysts.
  • a process which controls the optical purity of the resulting lactide to desired degrees or minimizes overall costs and maximizes yield of the lactide product.
  • a commercially-viable lactide purification system which allows production of polymer grade lactide, from crude lactic acid, which may subsequently be polymerized to produce a variety of non-medical- related polylactide polymers suitable for replacing existing petrochemical-based polymers.
  • Bellis U.S. Patent No.
  • 4,727,163 discloses the use of an ester of an alpha-hydroxy acid on a thermally-stable polyether core to manufacture highly pure cyclic esters such as lactide.
  • Bhatia U.S. Patent No. 4,835,293 discloses a process for preparing highly pure cyclic esters such as lactide by heating a polymer of the corresponding alpha-hydroxy acid or its ester or a copolymer of the alpha-hydroxy acid or its ester and a thermally-stable polyether core in the presence of an inert gas with the cyclic ester being carried from this reaction with the inert gas to a solvent system.
  • Bellis et al. PCT Application No.
  • WO 92/00292 published January 9, 1992 disclose a continuous catalyzed vapor phase process for the production of dimeric cyclic esters such as lactide by converting the corresponding alpha-hydroxy carboxylic acid or ester in the vapor phase over a solid catalyst such as silica alumina and preferably silica alumina having a high silica content, in the presence of a carrier gas.
  • a solid catalyst such as silica alumina and preferably silica alumina having a high silica content
  • the present invention provides a continuous process for the production of lactide polymers from a crude lactic acid feed source.
  • the crude lactic acid feed may be any available combination of the optical enantiomers D-lactic acid and L-lactic acid in solution with a hydroxylic medium such as water or other solvent such as methanol, ethanol, propanol, butanol, isopropanol, isobutanol, or the like, or mixtures thereof.
  • the crude lactic acid or source of lactic acid could also be an ester of lactic acid, such as methyl lactate, ethyl lactate, propyl lactate, butyl lactate, isopropyl lactate, isobutyl lactate or the like, or mixtures thereof.
  • the crude lactic acid may also be a mixture of lactic acid and esters of lactic acid.
  • An ester of lactic acid suitable for use in the present invention may generally be defined by the formula:
  • R is a Cj-Cs linear or branched alkyl.
  • ester of lactic acid When an ester of lactic acid is used as the source of a crude lactic acid for the process of the present invention, the polymerization reaction along with the depolymerization reaction and the equilibrium between the two compounds are altered.
  • the ester group, or more particularly the linear or branched alkoxy group leaves during the polymerization reaction from one end of the lactate molecule while a hydrogen is cleaved from the hydroxy group on the opposite end of another lactate molecule.
  • a condensation reaction by-product of the formula ROH is formed.
  • the overall polymerization reaction is represented by: 0 0 0 0 0
  • step n of said polymerization reaction is represented by: 0 0 0 0 0
  • polymer chains may contain an ester end group wherein said polymer would be defined as a polylactic ester polymer.
  • polylactic acid or polylactic acid molecules to include the polymer chains disclosed above with an ester end group or end cap as formed from the polymerization of an ester of lactic acid.
  • condensation reaction by-products include both water and chemical compounds of the general formula ROH wherein R is a Ci-C 8 linear or branched alkyl.
  • the composition of the crude lactic acid feed source and the design and operating conditions of the process disclosed herein will affect the optical purity of the final polylactide polymer product.
  • the process disclosed herein provides for the control of racemization to advantageously produce a polymer grade lactide of selected optical purity and composition. Because racemization can be controlled, it is possible to project the optical purity and composition of the resulting product based on that of the starting material and on processing conditions. When polymerized, the resulting polylactide can have desirable physical properties for a wide variety of non- medical related applications.
  • impurities such as color bodies, carbohydrates, proteins, amino acids, salts, metal ions, aldehydes, ketones, carboxylic acids, esters of carboxylic acids, esters of carboxylic acids or other organic acids, may be present in the crude lactic acid feed.
  • Applicant's process disclosed herein overcomes problems associated with producing a polymer grade lactide when such contaminants are present.
  • the crude lactic acid is first fed to an evaporator, continuously.
  • a portion of the water or solvent or any condensation reaction by-product is removed from the crude lactic acid.
  • the water or solvent or any condensation reaction by-product is removed as a vapor from the evaporator and discarded or recycled.
  • the evaporator thus concentrates the lactic acid in the crude feed. It is believed there will be some condensation reaction occurring and the lactic acid may start to form oligomers and low molecular weight polymers during the evaporation step, producing a condensation reaction by-product. A small quantity of lactide may also form.
  • This concentrated lactic acid is next fed to a prepolymer reactor, which in reality is a further evaporator.
  • evaporation and prepolymerization stages may be combined into one step.
  • Applicants have discovered the benefit of utilizing two steps that allow first removing uncontaminated water or solvent in the evaporation step which is readily discarded or reused without treatment.
  • the vapor stream from the prepolymerization reactor is greatly reduced in volume, yet contains some lactic acid. Recycling back through the initial evaporation step allows recovery of any lactic acid carryover, thus preventing loss of any valuable feed material.
  • the prepolymer product from the prepolymer reactor, polylactic acid or PLA, is fed to a lactide reactor.
  • a catalyst is simultaneously and continuously fed to the lactide reactor.
  • catalysts are known, such as metal oxides, metal halides, metal dusts and organic metal compounds derived from carboxylic acids or the like. It is believed, any such catalyst may be utilized in the process disclosed herein. Polymer properties will, however, vary.
  • the prepolymer and catalyst are mixed together in a static mixer to facilitate an even distribution of catalyst within the prepolymer.
  • the solution within the lactide reactor would quickly come to an equilibrium distribution of lactide and polylactic acid with the temperature and catalyst employed.
  • Heat is added to vaporize the crude lactide which is continuously removed from the lactide reactor, thus driving the depolymerization reaction, resulting in the net production of lactide as the contents of the lactide reactor seek equilibrium. It is believed that concentrations of unreactive high-boiling polylactic acid and other non-volatile impurities will concentrate in the solution within the lactide reactor. It is believed this will require a purge stream to remove such impurities. In a preferred embodiment of the present invention, a portion of the purge stream of unreactive high-boiling polylactic acid or other non-volatile impurities in the solution within the lactide reactor may be recycled to a point prior to the lactide reactor system or fed to polymerization.
  • any long chain lactic acid polymers will undergo transesterification to form lower molecular weight polylactic acids which may be utilized as a feed source to the lactide reactor. This allows further maximization of yield due to reduced loss of valuable feed material.
  • the crude lactide vapor is composed of a mixture of all three possible lactides: L-lactide, D- lactide, and meso-lactide, in various combinations.
  • This crude lactide may be fed directly to a distillation system as a vapor for purification.
  • this stream is fed to a partial condenser in which the lactide condenses and the majority of the water and other impurities remain as vapors and are recycled back to the lactide reactor or other upstream process equipment such as the evaporator or prepolymer reactor.
  • the condensed crude lactide is fed directly to a distillation system for purification.
  • the distillation system of the present invention may be any conventional distillation column. However, in a preferred embodiment of the present invention the distillation column is utilized to separate the crude lactide into at least three component streams.
  • the overhead low-boiling stream or distillate stream contains those components which have a lower boiling point than lactide, such as lactic acid, water or other solvent and condensation reaction by-products which may be present within the system.
  • the second stream is a high-boiling bottoms stream which contains those components which are higher boiling than a lactide, such as oligomers of lactic acid having at least two lactic acid units in the polymer chain. Both of these streams may contain lactide in various concentrations depending upon equipment design and operation.
  • the third component stream would be the substantially purified lactide which would be removed from the column as an intermediate-boiling sidedraw stream.
  • Substantially purified lactide is any lactide suitable for polymerization to form useful polylactide for application in non-medical related products.
  • Substantially purified lactide is a mixture of lactide and other constituents with a concentration of lactide of about 75% by weight lactide, more preferably about 85% to 95% by weight lactide and most preferably about 99% by weight or greater lactide.
  • the intermediate boiling sidedraw stream as removed from the distillation column may not be pure enough to be substantially purified lactide, thus in an alternative embodiment, the intermediate-boiling sidedraw stream upon concentration in the distillation system may be fed to a second distillation system within which the lactide is concentrated to form a substantially purified lactide.
  • the overhead low-boiling stream may also contain the linear dimer of lactic acid and the linear dimer of lactate esters collectively referred to herein as DP2, or alternatively, such stream may be extracted from the distillation column as a second intermediate- boiling sidedraw stream at a point in the distillation column wherein said DP2 concentration is maximum at steady state. It is recognized that DP2 has a higher boiling point than lactide and should be concentrated in the high boiling bottoms stream, however, applicants recognized DP2 may be formed by ring-opening decomposition of lactide during distillation.
  • An alternative to a standard distillation column could be incorporated in a preferred embodiment which would include a reactor vessel with a distillation column mounted directly on top of such vessel to create a single enclosed area within which reactions and distillation to purify the lactide take place.
  • catalyst and prepolymer are fed directly to the reactor wherein crude lactide is generated.
  • the crude lactide is vaporized as it is generated and flows directly up into the distillation column mounted on top of the lactide reactor.
  • the lactide is purified.
  • the top-mounted distillation column would be utilized in the same manner as a conventional distillation system, however, the physical structure of said column varies.
  • the crude lactic acid feed to the process disclosed herein may include a source such as lactic acid, but also the crude lactic acid may be the ester of a lactic acid such as methyl lactate or ethyl lactate.
  • the crude lactic acid feed may also be a mixture of an ester of lactic acid and lactic acid.
  • the first such modification includes addition of a catalyst or reacting in the presence of a catalyst the ester of lactic acid to form the condensation product or prepolymer, polylactic acid (as defined herein) .
  • catalysts which may be used include soluble homogeneous catalysts or insoluble solid, supported catalysts.
  • the key to selection of such catalyst is necessity of having acid functionality to provide the necessary protons in the case of an acid or a positive charge density in the case of a Lewis acid, to catalyze the condensation of the ester of lactic acid to form a prepolymer polylactic acid.
  • These catalysts are referred to herein as having acid functionality.
  • Applicants have also discovered that when a catalyst is added to promote the condensation of the ester of lactic acid to form a prepolymer polylactic acid, the same catalyst also promotes depolymerization of the formed polylactic acid to generate crude lactide.
  • the benefits include downsizing of equipment with the need for less surface area and the ability to conduct the condensation reaction to form the prepolymer and the depolymerization reaction to form lactide within the same reactor or reaction vessel.
  • the soluble catalyst remains with the higher boiling material which includes prepolymer and some of the lactide which was generated, this bottoms stream is recycled back to the lactide reactor.
  • applicants utilize an insoluble supported catalyst which is contained within a catalyst bed reactor or catalyst bed reactor system. The ester of lactic acid is fed to this catalyst bed reactor system wherein prepolymer and lactide are both generated in the presence of the solid, supported catalyst. Also generated is condensation reaction by-product. The fact that the supported catalyst is insoluble eliminates the ring-opening reaction problems within the distillation system as mentioned above because it is retained in the catalyst bed reactor.
  • the preferred overall process disclosed herein allows for the continuous manufacture of lactide polymers from a crude lactic acid with little or no waste of raw material lactic acid feed or ester of lactic acid feed. This is accomplished by maintaining the crude lactide which was generated in the lactide reactor as a liquid or vapor and avoiding the yield loss associated with the recrystallization step traditionally used to purify the lactide. The purified lactide leaving the distillation system is further maintained as a liquid and fed into a polymerization process.
  • this system allows recovery of any meso-lactide which may be present or formed within the disclosed process and which is normally lost in a recrystallization process. Further, the problems associated with handling solid materials are eliminated. These problems are well-documented by D. D. Deane and E. G. Hammond in "Coagulation of Milk for Cheese-Making by Ester Hydrolysis", J. Dairy Science, v. 43, pp. 1421- 1429 (1960) and Nieuwenhuis et al. (U.S. Patent 5,053,485) which are incorporated herein by reference. The problems of storing such solids for any time period are also disclosed by Deprospero et al. (U.S. Patent 3,597,449) and F. E.
  • the present system also allows use of crude lactic acid streams which contain impurities. As designed, the present system allows for removal of both low-boiling and high-boiling impurities prior to distillation of the crude lactide stream which is subsequently polymerized.
  • the prior art fails to disclose a process with such advantages. Further, Applicants have found that impurities may prevent catalyst activation. In a preferred embodiment, this problem is overcome by first activating the catalyst by heating a mixture of the catalyst and purified lactic acid or lactide, then feeding such activated catalyst with the crude lactic acid feed. It is believed the prior art contains no such teaching.
  • the present system further allows use of a crude lactic acid which is comprised of esters of lactic acid.
  • Applicants have designed an overall process which allows processing of commercially-available esters of lactic acid to polymers of lactide which is commercially feasible as a cost-competitive replacement for petroleum-based polymers.
  • Applicants have found it is necessary to utilize a catalyst with acid functionality to promote the condensation of the ester of lactic acid to form a prepolymer.
  • Applicants have also found that such catalysts promote the depolymerization of the prepolymer to form lactide.
  • the detrimental effects of using a catalyst with acid functionality are also disclosed by applicants along with process modifications to overcome such detriments.
  • the acid- functional catalyst creates instability within the distillation system, in that such catalyst causes the ring-opening reaction of lactide to form linear dimers and other oligomers of higher degree of polymerization while the lactide is being purified in the distillation column of the present system.
  • Applicants have disclosed the use of a flash distillation system with short residence time to separate the lactide from a soluble acid-functional catalyst.
  • applicants have disclosed the use of a solid, insoluble catalyst alone or on a support with said catalyst contained within a catalyst bed reactor system. The insolubility of the catalyst overcomes the stability problems within the distillation system. It is believed the prior art contains no such teachings.
  • Figure 1 is a flow diagram of the preferred overall process steps of the present invention
  • Figure 2 is a detailed schematic representation of a preferred polylactide polymer production system in accordance with the present invention
  • Figure 3 is a graph showing the effects of sodium on lactide production rate and optical purity
  • Figure 4 is a graph which represents the relationship between the optical purity of generated lactide in relation to the molecular weight of the feed to the lactide reactor;
  • Figure 5 is a graph showing the relationship between catalyst concentration and optical purity of the resulting lactide
  • Figure 6 is a graph showing the effect of hydroxyl impurities on polymer molecular weight at different temperatures
  • Figure 7 is a graph showing the effect of hydroxyl impurities on polymer molecular weight at different catalyst concentrations
  • Figure 8 is a graph showing the equilibrium lactide concentration as a function of temperature
  • Figure 9 is a schematic representation of a preferred lactide reactor/distillation column configuration
  • Figure 10 is a schematic diagram of a preferred alternative polylactide polymer production system
  • Figure 11 is a schematic representation of a preferred alternative polylactide polymer production system incorporating catalyst removal
  • Figure 12 is a schematic representation of a preferred alternative polylactide polymer production system incorporating a catalyst bed reactor system.
  • DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As required, detailed embodiments of the present invention are disclosed herein. However, it is to be understood that the disclosed embodiments are merely exemplary of the present invention which may be embodied in various systems. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to variously practice the present invention.
  • FIG. 2 represents an overall schematic flowchart encompassing the preferred process disclosed herein.
  • a crude lactic acid feed (2) is provided.
  • the crude lactic acid feed may be of various grades. This could include USP, food grade, or any other solution in a hydroxylic medium.
  • a hydroxylic medium is a medium which contains molecules having a hydroxyl group, mediums such as water, methanol, ethanol, propanol, butanol, isopropanol, isobutanol and the like, preferably having a number of carbon atoms in a range from 0-4, more preferably in a range from 0-2.
  • the crude lactic acid can include from about 1% to about 99% by weight lactic acid, preferably, from about 1% to about 85%, more preferably from about 5% to about 50%.
  • the crude lactic acid feed is a solution of about 15% lactic acid and about 85% water which is commercially produced.
  • Many manufacturing processes for producing crude lactic acid are known in the art, such as Glassner et al., (European Patent Application, EP 393818, October 24, 1990); G. Machell, "Production and Applications of Lactic Acid", Industrial Chemist and Chemical Manufacturer, v. 35, pp. 283-90 (1959) and Kirk Othmer, Encyclopedia of Chemical Technology, "Lactic Acid", v. 12, pp. 177-78 (2nd ed. 1963), which are incorporated herein by reference.
  • the source of crude lactic acid (2) could be in the form of the ester of lactic acid of the general formula:
  • esters are methyl lactate, ethyl lactate, propyl lactate, butyl lactate, isopropyl lactate or isobutyl lactate or mixtures thereof. These esters are known intermediate products of the lactic acid process disclosed above and incorporated herein by reference.
  • the crude lactic acid may be a mixture of these esters of lactic acid or a mixture of these esters of lactic acid or a mixture of one or more esters of lactic acid with lactic acid.
  • lactic acid includes two optical isomers, the L and D enantiomers. Either optical isomer or any combination thereof may be utilized as a crude lactic acid feed to the present reactor system.
  • the crude lactic acid feed (2) may contain other impurities, such as color bodies, carbohydrates, proteins, amino acids, salts, metal ions, aldehydes, ketones, other carboxylic acids and esters of carboxylic acids.
  • the overall system incorporated in the present invention includes outlets for such impurities so that their impact on final polymer products is limited. These outlets are designated as (10), (28), (99), (71) and (66) on Figure 2. Each will be discussed in greater detail below.
  • the need for a more costly purified crude lactic acid feed is eliminated.
  • a fluid transfer mechanism (4) is provided to transport the crude lactic acid feed (2) through an optional in-line mixer (6) in a pipeline (8) to an evaporator system (22).
  • the evaporator system (22) is utilized to concentrate the crude lactic acid feed (2) by removing water or any other solvent or hydroxylic medium which is used as a carrier for the lactic acid, such as methanol, ethanol or the like and any condensation reaction by-products.
  • the evaporator system (22) may be of any conventional type known in the art, such as a multiple effect evaporator, a wiped film evaporator, a falling film evaporator, or any other conventional system.
  • vacuum evaporation is utilized to reduce racemization.
  • Water vapor or solvent vapor in reference to the hydroxylic medium or condensation reaction by-product, is removed from the evaporator via a transfer line (18), and condensed in a condenser (16).
  • the condensed liquid is transferred in a pipeline (14) to a fluid transfer mechanism (12) , such as a pump or the like.
  • the fluid transfer mechanism (12) transports the condensed water or solvent via pipeline (10) as a waste stream and is discarded.
  • the lactic acid may be concentrated to a weight percent lactic acid of about 50% to about 99%, preferably from about 75% to about 99% and more preferably from about 85% to about 99%.
  • the evaporator system (22) is utilized to concentrate the crude lactic acid feed from about 15% lactic acid up to about 85% lactic acid.
  • the concentrated lactic acid is transferred via a fluid transfer mechanism (24) through a pipeline (26) to a prepolymer reactor (38).
  • the prepolymer reactor (38) is essentially a second evaporator system of any conventional type which is utilized to further remove water or solvent from the crude lactic acid feed.
  • a portion of the water or solvent vapor now also includes such water or solvent produced from the lactic acid polymerization reaction previously disclosed, the condensation reaction by-product.
  • the water or solvent vapor leaves the prepolymer reactor (38) via line (32) and is condensed in a condenser (30).
  • the condensed liquid is transferred via pipeline (36) to a transfer mechanism (34), with the transferred liquid comprising water or solvent with small amounts of lactic acid and other impurities present.
  • This liquid may be discarded through line (28) or may be recycled through line (29) back to a static mixer or other mixing mechanism and fed once again through line (8) to the evaporator (22).
  • the remaining liquid in the prepolymer reactor is continuously transferred via transfer mechanism (40) through line (42) to a hold tank (44).
  • the prepolymer reactor (38) is utilized to remove adequate water or solvent and condensation reaction by-product from the lactic acid to cause polymerization up to a molecular weight of about 100 to about 5000, preferably about 200 to about 3000, and more preferably about 400 to about 2500.
  • the molecular weight of the polylactic acid leaving the prepolymer reactor impacts the chemical purity as well as the optical purity of the crude lactide. This in turn will affect the distillation and the properties of the final polymer product
  • the evaporator system (22) and the prepolymer reactor (38) could be combined into a single system which provided removal of water or solvent sufficient to concentrate the lactic acid feed and also to polymerize such lactic acid.
  • the systems are separate to take advantage of recognized differences in the composition of the vapor leaving the evaporator (22) at line (18) and the vapor leaving the prepolymer reactor (38) in line (32).
  • the first step of concentrating the crude lactic acid in the evaporator (22) from 15% lactic acid to 85% lactic acid results in substantially pure water or solvent leaving the evaporator in line (18), which may be readily discarded without treatment.
  • the vapor in line (32) leaving the prepolymer reactor (38) will necessarily contain lactic acid and other impurities which are carried over in the evaporation process. These impurities will preferably need to be recycled or treated before discarded.
  • Applicants take advantage of the economic benefits of removing nearly pure water (or solvent) in the evaporator (22) and reduced recycle or waste treatment of the vapor leaving the prepolymer reactor (38) .
  • the evaporator system (22) and pre-polymer reactor (38) may be replaced by a series of batch evaporators that concentrate the lactic acid and produce pre-polymer. The series of batch systems may be operated to provide a net continuous supply of pre-polymer.
  • the hold tank (44) is maintained at a temperature sufficient to keep the polylactic acid in a flowable liquid state.
  • the hold tank (44) is, however, only a feature of the preferred embodiment, recognizing the control problems which may result from direct feed to the lactide reactor (60).
  • This liquid is transferred via transfer mechanism (46) through a pipeline (48) to a static in-line mixer or other mixing mechanism (50).
  • a catalyst is added to the polylactic acid.
  • any means of adding the catalyst to the polylactic acid would be appropriate; however, the static mixer (50) utilized in the preferred embodiment allows more even distribution of the catalyst within the liquid.
  • the catalyzed polylactic acid is transferred via transfer line (54) to the lactide reactor (60). It is well recognized in the art that polylactic acid maintains a dynamic equilibrium with its depolymerization product, lactide, as represented by the reaction below:
  • this reaction is catalyzed by such compounds as tin dust, tin halide, tin oxide, and organic tin compounds derived from C : -C 20 carboxylic acids, as disclosed by Muller in U.S. Patent No. 5,053,522, which is incorporated herein by reference.
  • Other metals such as zinc, and their halides, oxides and organic compounds thereof, have been recognized by the art as possible catalysts for the lactide reaction. It is believed any metals of Groups IV, V or VIII of the Periodic Table and compounds thereof, are possible catalysts for generating lactide.
  • tin oxide is utilized as catalyst.
  • the catalyst is activated prior to feed.
  • lactide As lactide is generated within the lactide reactor (60), it is removed as a vapor continuously through line (58). The removal of lactide further drives the depolymerization reaction. It is believed that some high-boiling or non-volatile contaminants present in the feed to the entire system will concentrate in the lactide reactor and necessitate inclusion of a purge stream (62).
  • Example 1 below details the effect of cationic impurities and Example 11, the detrimental effects of concentrating metal ions in the lactide reactor (60). It is believed other impurities would have a similar effect and necessitate the purge stream (62).
  • this purge stream (56) may be recycled back to the static mixer (50) and recatalyzed and fed to the lactide reactor (60) .
  • the purge stream may be fed to the polymerization reactor (110) via line (68), if such polymer is desired.
  • a transfer mechanism (64) is provided to transport the purge stream optionally to a point prior to the evaporator (22) such as the static mixer (6) or to the feed line (26) to the prepolymer reactor (38) or to waste through line (66) or to a static mixer (104) for polymerization in a polymerization reactor (110).
  • the lactide vapor leaving the lactide reactor (60) in line (58) is partially condensed in a condenser (72).
  • the uncondensed vapor consists of residual lactic acid and water or solvent, along with some lactide which remains uncondensed.
  • This vapor stream leaves the system through line (90) and is condensed in a condenser (92), the liquid thus being transferred by transfer mechanism (96) through line (94).
  • This liquid may optionally be discarded through line (99) or recycled through line (98) back to the crude lactic acid static mixer (6). In the preferred embodiment, this stream is recycled in order to recover and utilize any lactic acid or lactide which is not condensed in partial condenser (72).
  • the condensed crude lactide leaving condenser (72) via line (74) is transferred via fluid transfer mechanism (76) through line (78) to a distillation system (80) for purification of the lactide.
  • a distillation system (80) for purification of the lactide.
  • partial condensation may not be necessary and the crude lactide vapor may be fed directly to the distillation system (80).
  • the distillate leaving the distillation system (80) in line (82) is composed of water or solvent, some residual lactic acid, and some lactide carryover.
  • This stream is condensed in condenser (84) and transferred via fluid transfer mechanism (88), in line (86), and may be discarded or recycled back to a point prior to the evaporator (22), such as the static mixer (6) or the feed line (26) to the prepolymer reactor (38) through line (71), or more preferably recycled in line (70) back to the static mixer (50) to be recatalyzed and re-fed to the lactide reactor (60).
  • This preferred embodiment allows minimization of waste by preventing loss of lactic acid or converted lactic acid to lactide from the feedstock.
  • the refined lactide is removed from the distillation system (80) via transfer mechanism (100) in line (102) and fed to a polymerization reactor (110).
  • the distillation system (80) may include more than one distillation column or a flash drum.
  • the prepolymer is fed to a static in-line mixer or other mixing mechanism (50) through a pipeline (48).
  • a catalyst is added to the polylactic acid or prepolymer through a pipeline (52).
  • the catalyzed prepolymer is then fed to the combined reactor/top-mounted distillation column (200) through a pipeline (54).
  • the well-recognized dynamic equilibrium between the polylactic acid and its depolymerization product, lactide results in an approximate concentration of 5 weight % lactide within the reactor portion of the reactor with top-mounted distillation column (200).
  • the lactide is vaporized within the lactide reactor with top-mounted distillation column (200) and travels directly as a vapor into the top-mounted distillation column indicated by (202).
  • the crude lactide is refined and purified.
  • a low-boiling overhead stream leaves the distillation column in a pipeline (203) to a condenser (204).
  • a portion of the condensed low-boiling overhead stream is removed from the system via a pipeline (205) while a second portion of said stream is refluxed back to the distillation column through a pipeline (206).
  • the low- boiling overhead stream is composed of water or solvent, some residual lactic acid and some lactide carry over.
  • Components within the distillation column that boil at a temperature higher than lactide create a high- boiling bottoms stream which, in the present configuration, travels down distillation column (202) directly into the reactor with top-mounted distillation column (200) as generally indicated by (208) where it is further reacted to form additional lactide.
  • a re-boil tray may optionally be located in the bottom portion of the top-mounted distillation column (202) to facilitate purification.
  • the substantially purified lactide stream is removed as an intermediate-boiling sidedraw stream from distillation column (202) through a line (201).
  • the substantially purified lactide may be cooled and/or fed to a polymerization process as disclosed above. As previously disclosed, it may be necessary to subject the substantially purified lactide to further purification in a second distillation system prior to polymerization. One of skill in the art would make this decision based on design and operation of the first distillation system in light of desired final polymer properties. Applicants recognize it may be necessary to withdraw a purge stream from the bottom of the reactor with top- mounted distillation column (200). This may be accomplished via a pipeline (209) as was indicated above in the description of the process of Figure 2.
  • the overhead low-boiling stream, leaving the system in ⁇ pipeline (205), may be discarded or recycled back to an earlier point in the process to recover the lactic acid, lactide or any other valuable component present in said stream.
  • a sidedraw stream may be withdrawn from the column in a pipeline (207) at a point where lactic acid and linear dimer are high in concentration. This stream may be recycled back to the reactor with top-mounted distillation column (200) or, alternatively, fed back to the catalyst-mixing mechanism (50) through a pipeline (48).
  • the presence of dimer at a point above the point at which lactide is withdrawn as a sidedraw stream is due to ring-opening decomposition reactions.
  • the polymerization process may be of any conventional design known to the art, such as that disclosed by J. Leenslag and A. Pennings, "Synthesis of High Molecular Weight Poly (L-lactide) Initiated with Tin 2-Ethylhexanoate", Makromol . Chem. , v. 188, pp. 1809-14 (1987) and F. Kohn et al., "The Ring-Opening Polymerization of D,L-Lactide in the Melt Initiated with Tetraphenyltin, J. Applied Polymer Science , v. 29, pp. 4265-77 (1984), which are incorporated herein by reference.
  • An alternative source of crude lactic acid for the process disclosed above is an ester of lactic acid.
  • Applicants have discovered that processing an ester of lactic acid to form a polylactide polymer requires alterations of the above-disclosed general process. These modifications or alternative processing steps are described below in detail. It is recognized, however, that the general disclosure utilizing lactic acid as the crude feed to the general overall process as disclosed in Figure 2 and Figure 9 and described in detail above, is equally applicable to the processes disclosed below with the modifications incorporated therein.
  • esters of lactic acid include methyl lactate, ethyl lactate, propyl lactate, butyl lactate, isopropyl lactate, isobutyl lactate and mixtures thereof.
  • the condensation of an ester of lactic acid results in formation of a low molecular weight prepolymer.
  • the polymerization reaction is generally indicated by the formula below:
  • step n of said polymerization reaction is represented by:
  • polymer chains may contain an ester end group wherein said polymer may be defined as a polylactic ester polymer.
  • polylactic acid or polylactic acid molecules are defined herein to include polylactic acid molecules formed from the polymerization of lactic acid as disclosed previously and polylactic ester polymers as disclosed above produced from an ester of lactic acid feed.
  • Catalysts such as those used in other polyester condensation and esterification reactions are also useful.
  • Specific catalysts would include tin(II) ethylhexanoate, p-toluene sulfonic acid, Fascat ® 9100 and Fascat ® 9102 (Fascat ® catalysts available commercially from Atochem North America Inc, Philadelphia, PA) .
  • Supported catalysts or solid catalysts with some acid functionality or Lewis acid functionality would also be useful. These would include the solid catalysts disclosed by Bellis et al. in PCT Application No. WO 92/00292, which is incorporated herein by reference.
  • a preferred catalyst is, however, the supported polymeric resin catalysts available from Rohm and Haas commercially under the tradename Amberlyst ® as a series of catalysts. Specific examples utilizing such Amberlyst ® catalysts are outlined below. Also useful are the solid catalysts of the type described by Sir John Meurig Thomas in Scientific American , April 1992, pp. 112-118. Mineral acid catalysts are also known to catalyze the above esterification reactions and would be useful in these applications.
  • an ester of lactic acid is fed to an evaporator (260) in a pipeline (250).
  • a soluble homogeneous catalyst such as those outlined above is fed to the evaporator through a pipeline (251).
  • the catalyst and ester of lactic acid could be fed together into a static mixer prior to being fed to the evaporator (260).
  • the ester of lactic acid is heated, and the hydroxylic medium, within which the ester of lactic acid may be in solution, is evaporated.
  • the ester of lactic acid may be without a hydroxylic medium.
  • the evaporator (260) may be operated at atmospheric pressure or more preferably under reduced pressure to allow for lower temperature of operation and evaporation.
  • the reactions to form lactide are continued, preferably under conditions of elevated temperature and reduced pressure.
  • the mixture of lactide and polylactic acid within the lactide reactor (270) is removed through a pipeline (271) and fed to a distillation system (280).
  • the mixture of lactide and polylactic acid is a crude lactide.
  • the crude lactide will also contain low molecular weight oligomers of lactic acid or the ester of lactic acid, including DP2 and DP3.
  • the crude lactide is purified.
  • the purified lactide is removed as an intermediate- boiling sidedraw stream from the distillation system (280) through a pipeline (282). As disclosed in Figure 2, this purified lactide may be fed to a polymerization process.
  • a low- boiling overhead stream leaves distillation system (280) through a pipeline (281) where it is condensed in a condenser (288) with a portion of the condensed low- boiling overhead stream being removed from the system through a pipeline (289) and the remainder utilized as a reflux stream back to distillation system (280) through a pipeline (290).
  • the low-boiling overhead stream will contain components that are lower-boiling than lactide. They may include water, dimer of lactic acid and other oligomers, and a concentration of lactide. The concentration of lactide in the overhead stream will be varied depending on distillation system (280) operation.
  • a high-boiling bottoms stream is removed from the distillation system (280) through a pipeline (283) .
  • This high-boiling bottoms stream will generally include those materials higher-boiling than lactide, along with a concentration of lactide which will vary depending upon the operation of distillation system (280).
  • the high-boiling bottoms stream may be recycled back to the lactide reactor (270) via a pipeline (286) or, alternatively, to the evaporator (260) through a pipeline (287).
  • other sidedraw streams may be removed from the distillation system (280). These are generally indicated by a pipeline (284) and a pipeline (285).
  • these sidedraw streams may be located at a point in the distillation column where a component such as DP2 or the dimer of lactic acid is in high concentration and may be withdrawn from the distillation system (280) and recycled back to the lactide reactor (270) or evaporator (260) .
  • Figures 11 and 12 are directed to process modifications to remove catalysts from the crude lactide stream prior to purification.
  • FIG 11 a preferred overall system utilizing an ester of lactic acid feed and a soluble homogeneous catalyst with the crude lactide being subjected to a flash distillation to remove said homogeneous catalyst prior to distillation of the lactide.
  • An ester of lactic acid is fed to a combined evaporator/reactor (300) through a pipeline (302).
  • the ester of lactic acid may be any mixture of esters of lactic acid and may also be in a hydroxylic medium as a solution.
  • a catalyst is fed to the combined evaporator reactor (300) through a pipeline (301).
  • This catalyst may be any acid-functional or Lewis acid-functional soluble catalyst as detailed above.
  • the ester of lactic acid and soluble catalyst may be combined in a mixing mechanism or static mixer prior to being fed to the combined evaporator/reactor (300).
  • sufficient residence time is allowed to both condense the ester of lactic acid to form a polylactic acid and to depolymerize the polylactic acid to form lactide.
  • concentrations of the polylactic acid and lactide will depend upon temperature and residence time as the system approaches equilibrium as outlined above.
  • Condensation reaction by-product of the general formula ROH is removed from the combined evaporator/reactor (300) through a pipeline (303) as a vapor and condensed in a condenser (304) with subsequent removal from the system in a pipeline (305).
  • the lactide and polylactide reaction product or crude lactide which was formed in the combined evaporator reactor (300) is removed from said reactor (300) in a pipeline (306) and fed to a flash distillation system (310).
  • the flash distillation system (310) is designed with minimum residence time and preferably low pressure operation to quickly flash a quantity of the lactide from the mixture.
  • the soluble catalyst remains with the liquid stream which is removed from the bottom of the flash distillation system through a pipeline (328) and recycled back to the combined evaporator/reactor (300).
  • the lactide which is flashed as a vapor in the flash distillation system (310) is removed through a pipeline (311) and fed to a distillation system (320). This stream contains lactide and some polylactic acid of low degree of polymerization and low boiling impurities.
  • the distillation system (320) may be any distillation system as disclosed above as, for example, in Figure 10.
  • the mixture of lactide and polylactic acid is a crude lactide.
  • the crude lactide will also contain low molecular weight oligomers of lactic acid or the ester of lactic acid including DP2 and DP3.
  • the crude lactide is purified.
  • the purified lactide is removed as an intermediate- boiling sidedraw stream from the distillation system (320) through a pipeline (326). As disclosed in Figure 2, this purified lactide may be fed to a polymerization process.
  • a low- boiling overhead stream leaves distillation system (320) through a pipeline (321) where it is condensed in a condenser (322) with a portion of the condensed low- boiling overhead stream being removed from the system through a pipeline (323) and the remainder utilized as a reflux stream back to distillation system (320) through a pipeline (324).
  • the low-boiling overhead stream will contain components that are lower-boiling than lactide. They may include water, dimer of lactic acid and other oligomers, and a concentration of lactide. The concentration of lactide in the overhead stream will be varied depending on distillation system (280) operation.
  • a high-boiling bottoms stream is removed from the distillation system (320) through a pipeline (327).
  • This high-boiling bottoms stream will generally include those materials higher-boiling than lactide, along with a concentration of lactide which will vary depending upon the operation of distillation system (320) .
  • the high-boiling bottoms stream may be recycled back to the combined evaporator/reactor (300) via a pipeline (327).
  • other sidedraw streams may be removed from the distillation system (320). These are generally indicated by a pipeline (329). With proper design these sidedraw streams may be located at a point in the distillation column where a component such as DP2 or the dimer of lactic acid is in high concentration and may be withdrawn from the distillation system (320) and recycled back to the combined evaporator/reactor (320).
  • FIG. 12 is a preferred embodiment for utilizing an ester of lactic acid feed material to form polylactide without the instability created by the presence of a catalyst within the distillation system.
  • a solid supported catalyst or solid catalyst is utilized in a catalyst bed reactor system (350).
  • the catalyst bed reactor system may be filled with any solid supported catalyst or solid, insoluble catalyst as disclosed above which has an acid functionality or Lewis acid functionality.
  • There are many ways in which to operate the catalyst bed reactor system which are generally known in the art and include a fixed bed, an ebbulated bed, or a fluidized bed system. Key to operation is preventing the catalyst from leaving the catalyst bed reactor (350) and entering the distillation system (360) as indicated in Figure 12.
  • the ester of lactic acid or the ester of lactic acid in hydroxylic medium is fed to the catalyst bed reactor system (350) through a pipeline (352). This material may be fed either as a liquid or a vapor.
  • the above-stated reactions are allowed to occur in the presence of a solid supported or solid catalyst.
  • the concentration of lactide and polylactide will vary, depending upon residence time within the catalyst bed system, the removal of condensation reaction by-product, the catalyst selected, along with temperature and pressure.
  • the condensation reaction by-product may be flashed and removed through a pipeline (368) from the catalyst bed reactor system or, alternatively, may remain a liquid and be fed to the distillation system (360) through a pipeline (351).
  • the mixture of lactide and polylactic acid leaving catalyst bed reactor (350) is a crude lactide.
  • the crude lactide will also contain low molecular weight oligomers of lactic acid or the ester of lactic acid including DP2 and DP3, which are collectively referred to as, and included in, the definition of polylactic acid. It may also contain condensation reaction by- product.
  • This material is fed to distillation system (360).
  • the crude lactide is purified.
  • the purified lactide is removed as an intermediate-boiling sidedraw stream from the distillation system (360) through a pipeline (365). As disclosed in Figure 2, this purified lactide may be fed to a polymerization process.
  • a low- boiling overhead stream leaves distillation system (360) through a pipeline (361) where it is condensed in a condenser (362) with a portion of the condensed low- boiling overhead stream being removed from the system through a pipeline (367) and the remainder utilized as a reflux stream back to distillation system (360) through a pipeline (363).
  • the low-boiling overhead stream will contain components that are lower-boiling than lactide. They may include water, dimer of lactic acid and other oligomers, and a concentration of lactide. The concentration of lactide in the overhead stream will be varied depending on distillation system (360) operation.
  • a high-boiling bottoms stream is removed from the distillation system (360) through a pipeline (366).
  • This high-boiling bottoms stream will generally include those materials higher-boiling than lactide, along with a concentration of lactide which will vary depending upon the operation of distillation system (360).
  • the high-boiling bottoms stream may be recycled back to the catalyst bed reactor (350) via a pipeline (366).
  • other sidedraw streams may be removed from the distillation system (360). These are generally indicated by a pipeline (369).
  • these sidedraw streams may be located at a point in the distillation column where a component such as DP2 or the dimer of lactic acid is in high concentration and may be withdrawn from the distillation system (360) and recycled back to the catalyst bed reactor (350).
  • a non-lactide monomer to the purified lactide leaving the distillation system (80).
  • This co-monomer may be added via line (101).
  • the co-monomers are fed to the polymerization reactor (110) and polymerized to form a co-polymer.
  • Many co-polymers of polylactide are known to the art. These include P. Dave, N. Ashar, R. Gross, S. McCarthy, "Survey of Polymer Blends Containing Poly (3-hydroxybutyrate-co-16% hydroxyvalerate) , Polymer Preparation, American Chemical Society, v. 31 (1), pp. 442-3 (1990); B. Riedl and R.
  • Prud'ans Prud'ans, "Thermodynamic Study of Poly(vinyl chloride)-Polyester Blends by Inverse Gas Phase Chromatography", J. Polymer Science , Part B, vol. 24(11), pp. 2565-82 (1986); H. Younes and D. Cohn, "Phase Separation in Poly(ethylene glycol)/Poly(lactic acid) Blends, European Polymer J . , v. 24(8), pp. 765-73 (1988); Smith et al. (European Patent Application, EP 209371, January 21, 1987); Pines et al. (European Patent Application EP 109197, May 23, 1984); J. Zhu, Y. Shao, W. Sui, S. Zhang, H.
  • the preferred overall system described herein is a complex combination of many known chemical engineering unit operations. So that the benefit of the overall combination may be recognized, Applicants herein disclose in further detail the selection, operation, and benefits of selecting such unit operations, along with actual laboratory experimental results exemplifying the disclosed advantages.
  • the crude lactic acids fed to this process (2) may be made up of L-lactic acid or D-lactic acid, or combinations thereof.
  • the composition of the feed does not translate directly through the entire process to define the composition of the polymer product leaving the polymerization reactor (110) through line (108). Applicants recognize that racemization, or conversion of one optical enantiomer to the other, may occur.
  • racemization is driven by such factors as temperature, pressure, time at a given temperature or pressure, the presence of catalysts or impurities, and relative concentrations of the two enantiomers at any given time.
  • the degree of racemization is defined herein by the percent conversion of the optical enantiomer that is present in excess of 50%. As an equation, this calculation would be defined as:
  • each unit operation is controlled to a degree that allows production of a purified lactide mixture with selected chemical and optical composition.
  • the optical composition of the lactide mixture is determined by the relative abundance of D- and L-lactic acid sub-units in the polylactic acid within the lactide reactor.
  • the blend of lactide isomers used to produce the polymer affects the physical properties of the polymer, including the biodegradability.
  • the evaporator (22) is operated to minimize residence time so that there is little or no effect on optical purity.
  • the prepolymer reactor (38) is also operated to minimize racemization. This includes reducing the residence time within the reactor.
  • the crude lactide produced in the lactide reactor (60) will be a mixture of the three possible lactides which may be generated from L- and D-lactic acid. These include L-lactide, D-lactide, and meso- lactide.
  • the concentration of catalyst added to static mixer (50) also affects the degree of racemization and composition of the crude lactide product.
  • the catalyst concentration level is adjusted based on desired properties of the final polymer product.
  • Example 12 the quality of the crude lactide charged to the distillation system has a significant effect on the operation of said system.
  • acidic impurities such as lactic acid and low molecular weight oligomers, which are formed by ring opening reactions of lactic acid or water with lactide, can cause premature polymerization in the distillation system.
  • side reactions may be controlled by partially condensing the crude vapor prior to feeding to distillation to remove impurities.
  • the distillation system (80) may also be operated to control racemization of the lactide and other side reactions.
  • this system is designed to minimize racemization by utilizing a packed column distillation system which minimizes liquid holdup, along with a thermal-siphon reboiler which limits residence time of the bottom liquids, and utilizing a minimum reflux ratio to further reduce holdup time in the column. It is, however, recognized that other distillation systems may be utilized with varying impact on the optical purity of the purified lactide and resultant polymer product.
  • the distillation system (80) is utilized as a purification step for the lactide so that crystallization of the crude lactide is unnecessary in order to produce a lactide product of suitable purity for polymerization.
  • the lactide reactor (60) is also designed in a preferred embodiment, maximizing surface area between liquid and vapor so that liquid lactide can more easily vaporize. This allows for rapid removal of the generated lactide, which in turn further drives the reaction.
  • a system which utilizes purification steps other than crystallization increases yield.
  • the use of distillation as a purification step also prevents the need to handle solids with the problems with equipment and contamination inherent in such operations.
  • a catalyst with acid functionality whether in terms of a true acid which is a source of protons, or a Lewis acid which is a source of positive charge density, is used to facilitate the condensation reaction.
  • the presence of the catalyst with acid functionality renders the crude lactide relatively unstable during purification by distillation.
  • the decomposition of the lactide during purification is avoided by utilizing an insoluble, solid supported catalyst.
  • a soluble homogenous catalyst may be utilized in forming the crude lactide, however, the crude lactide is then subjected to a flash distillation with the catalyst being removed in the bottoms high-boiling stream.
  • EXAMPLE 1 Effect of Cationic Impurities on Optical Purity Na 2 S was added at levels of 20, 200, and 1000 ppm to purified lactic acid (Purac heat stable grade) of composition 85% L-lactic acid and 15% water. The lactic acid was then polymerized to form PLA with a number average molecular weight of about 650 g/mol. H 2 S was removed under vacuum while the PLA was being formed, leaving Na + ions in solution. The PLA was then used to generate lactide at 10 mm Hg with 0.05 wt.% SnO catalyst (Aldrich cat. no. 24,464-3, Tin (II) oxide, 99 + %) and a constant heat input, allowing reactor temperature to float. The results are shown in the following table and in Figure 3:
  • the Effect of PLA Molecular Weight on Optical Purity Lactide was generated from several samples of PLA (also known as polylactic acid or prepolymer) each sample having a different average molecular weight.
  • the conditions under which the lactide was generated in each experiment include: 10 mm Hg pressure, 0.05 wt.% SnO as catalyst (same catalyst as Example 1), run to approximately 73% conversion of PLA to lactide, constant heat input and stirring speed.
  • the reaction temperature was allowed to float, reaching the value necessary to dissipate the heat input. This method of running tends to give a constant lactide production rate, within limits.
  • EXAMPLE 3 The Effect of Catalyst Concentration on Optical Purity Lactide was produced using SnO catalyst (catalyst of Example 1) from PLA with a molecular weight of 650 g/mol at several catalyst levels and two pressures. The power input was held constant (Variac setting of 75) and the reaction temperature was allowed to seek an equilibrium level. The percentage meso in the crude lactide for each experiment is shown in the table below and graphically in Figure 5.
  • catalyst levels above 0.1 wt.% lead to an increase in the content of meso-lactide, for both pressures studied. The increase occurs even though the overall lactide generation rate increases and the reaction temperature decreases. The content of meso-lactide also increases at very low concentrations of catalyst, resulting in a minimum for 0.02-0.10 wt.% SnO. In a preferred embodiment, catalyst concentrations may be varied depending on desired final polymer physical properties.
  • Example 1 demonstrated that some form of purge of the liquid in the reactor (reactor bottoms) would probably be necessary for a continuous operation. This example demonstrates a surprising benefit if such purge stream is recycled back to the catalyst addition stage.
  • PLA was produced from the same lactic acid used in Example 1, utilizing the same method. This was used to generate lactide at 1 mm Hg with 0.05 wt.% SnO as catalyst (catalyst of Example 1). The reaction was run to 72% conversion, at which point the lactide production rate had begun to decline significantly. The heat was turned off and the flask was cooled to 150°C under a nitrogen atmosphere. 390 gms of 88% L-lactic acid was added to the 80 gms of residue.
  • the flask was adapted to generate a new batch of PLA, simulating the recycle of reactor bottoms to the evaporator section.
  • the new PLA was heated under vacuum until the molecular weight was about 650 g/mol (by end group titration) .
  • a GPC (gel permeation chromatography) analysis showed that the reactor bottoms had been completely broken down and been reabsorbed into the PLA, with no sign of high molecular weight reactor bottoms.
  • the lactide generation was rerun using the same conditions as before, and it was surprisingly found that the lactide production rate increased: from 0.86 hr -1 for the first run (prior to recycle) to 1.03 hr -1 for the second run (recycle, no additional catalyst) .
  • the reaction temperature was 213°C for the first run and 215°C for the second run.
  • the composition of the crude lactide and of the reactor bottoms were similar for the two cases.
  • the reactor bottoms is recycled to a point prior to the lactide reactor to increase overall yield from the lactic acid feed.
  • the lactide is a mixture of 80% L- and 20% D,L- lactide, recrystallized to high purity. 40 gm of lactide is charged to a flask with magnetic stirring. A THF (Tetrahydrofuran, Burdick and Jackson, high purity, non-spectro) solution containing L-lactic acid, water, or both is added to the lactide. The flask is lowered into an oil bath at 140-160 C C to melt and mix the monomer. This is held for five minutes after complete melting (about 15 minutes total) . A starting sample is pulled for GC and/or water analysis. A catalyst solution of 10 wt.
  • % ti (II) 2-ethylhexanoate (Johnson Mathey Electronics, Tech. Grade) in toluene is added and allowed to react for 1 minute. Five gram samples are then pipetted into silanized and nitrogen flushed 20 ml vials. These are quickly placed into a temperature controlled oil bath. Vials are pulled and frozen at various time intervals up to 4 hours.
  • the samples are prepared for analysis by breaking the polymer out of the vials and dissolving in THF at room temperature on an orbital shaker (about 1-6 hours for 5 grams of polymer in 125 mis THF) .
  • the mixture is then diluted to 1% in THF and analyzed utilizing GPC analysis to determine the molecular weight and percent conversion.
  • EXAMPLE 6 EXAMPLE 6 :
  • hydroxyl impurities are controlled to desired levels to control the physical properties of the resulting polymer product.
  • EXAMPLE 7 Polymer Molecular Weight is Controlled by Impurity Level and is Nearly Independent of Catalyst Concentration
  • EXAMPLE 8 Eguilibrium Concentration of Lactide in Polylactic-Acid PLA of 650 MW was heated at atmospheric pressure with either 0.00, 0.05, or 0.15 wt% SnO as a catalyst (Catalyst of Example 1) . The mixtures were held at three different desired temperature for 20 minutes, at which time 10 wt% of purified L-lactide was added to the mixture with stirring. The vessel was fitted with a condenser to prevent the loss of water or other volatile components.
  • EXAMPLE 9 Relative Rates of Racemization Samples of PLA (with and without SnO (catalyst of Example 1) as catalyst) and lactide were heated and stirred for four hours at 200°C at atmospheric pressure in a round bottom flask fitted with a condenser to prevent loss of volatile components. The samples were then allowed to cool and the optical purity of the PLA was determined by saponification followed by a measurement of the optical rotation. The lactide sample was analyzed by GC to determine the meso-lactide content, which was then converted to a measurement of optical purity.
  • racemization occurs fastest in PLA which is exposed to catalyst.
  • racemization is controlled in the lactide generating reactor.
  • another area of racemization control will be the evaporators which are used to prepare PLA, because of the long residence times and the possible inclusion of catalyst and catalyzing impurities.
  • the residence time of the lactide in the distillation column will be kept low, minimizing the potential for racemization.
  • Lactide was produced from PLA at several catalyst levels and at two pressures to determine the effect of mass transfer.
  • the catalyst was SnO (catalyst of Example 1) with constant heat input at a power setting on the Variac of 75%.
  • Hg vs 10 mm Hg results in significantly higher net lactide production rates and a lower concentration of meso-lactide.
  • the lactide reactor is operated under vacuum to facilitate mass transfer.
  • Lactic acid was concentrated and polymerized to form low molecular weight polylactic acid (MW range of about 600-2200) and fed to a continuous pilot scale reactor for the production of lactide. At the end of 1- week of operation a sample of the reactor liquid was taken and analyzed for metals. The results are shown below.
  • the metals profile clearly shows corrosion of the stainless steel reaction system, either in the formation of the prepolymer or in the lactide generating reactor.
  • the high metals content which represents the build-up over a week with no purge on the reactor bottoms, is detrimental to the lactide formation process. The data below demonstrate this effect.
  • the Effect of Acidic Impurities on Distillation Lactide was produced in a continuous pilot plant at rates of 2-5 kg/hr.
  • the starting materials were Purac lactic acid of about 85% concentration.
  • a PLA prepolymer having a range of molecular weights from about 400-2000 MW was made batchwise by heating first at atmospheric pressure and then under vacuum. The prepolymer was used to supply the continuous feed to the lactide reactor. The reactor was run at a temperature of 220-240 °C and pressure of about 35 mm Hg.
  • EXAMPLE 13 The Beneficial Effects of Catalyst Activation on Lactide Generation Rates
  • the sources of the PLA were: A) Purac heat stable grade lactic acid, B) a test sample of lactic acid from
  • Lactech, Inc. which was produced by fermentation, and A/B) a 50/50 mix of the PLA produced from each of the previous two sources.
  • Lactide was generated in a laboratory apparatus (three-neck flask with mechanical stirring, primary condenser operated at 85-100°C, and receiving flasks and traps) at constant temperature of 230°C and constant pressure of 10 mm Hg.
  • Initial PLA charge was 360 gms of material having an average molecular weight of about 650.
  • the catalyst charge was 0.045 wt% as Sn for each type of catalyst.
  • PLA B is not as reactive toward lactide production as is PLA A.
  • the effect is particularly pronounced with
  • EXAMPLE 14 Distillation of Crude Lactide
  • the overhead fraction from example 12 was collected in three receivers, containing 14%, 36%, and 28% of the crude lactide charge, respectively.
  • the first fraction contained over 5 wt% lactic acid and was discarded.
  • Fractions 2 and 3 were combined and redistilled, yielding a lactide fraction with total acidic impurities of 4.4 milliequivalents/mol of lactide.
  • This fraction was polymerized using the polymerization technique of example 5 (temperature 180°C, catalyst/monomer ratio 1:5000), yielding a polymer with number average molecular weight of 42,100 at 100% conversion and weight average molecular weight of 76,300. Actual conversion was 84.6% at 2 hours.
  • Examples 22 and 23 give additional data on the course of a reaction and indicate that the distributions observed probably correspond to equilibrium.
  • Example 24 demonstrates probable control of polymer molecular weight when utilizing a lactate ester by controlling impurities as was previously shown above when utilizing lactic acid and accompanying impurities.
  • Examples 25-27 demonstrate the use of heterogenous or solid, supported catalysts for the production of lactide from an ester of lactic acid. Unless otherwise indicated, the following experimental materials and test procedures were utilized in Examples 15-27:
  • Methyl lactate, ethyl lactate, and butyl lactate were supplied by PURAC America, Inc. under the tradename PURASOLV and used without further purification.
  • the material initially boiled at 140°C.
  • the overhead consisted primarily of ethyl lactate and the expected condensation byproduct ethanol.
  • the pot material was colorless and contained 45% ethyl lactate, undetermined oligomers, and 2.0% D- and/or L- lactide.
  • DP2 12% DP3, 3% DP4, 0.4% meso-lactide and 2.6% D- and/or L-lactide, based on response factors for pure lactide and ester.
  • Response factors for the oligomers were assumed to be equal to the response factor for the ester. This assumption was used in all subsequent examples wherein corrections for response factors are used.
  • Fascat* 9102 (Atochem North America, Inc.) catalyst at atmospheric pressure under a nitrogen atmosphere to a temperature of 190°C, for 2.5 hours.
  • the pot material was deep orange color and contained 45% butyl lactate, undetermined oligomers, and 2.0% D- and/or L-lactide, based on response factors for pure lactide and ester. A white precipitate was observed.
  • Fascat ® 9100 (Atochem North America, Inc.,
  • EXAMPLE 20 Lactide Generated from Methyl Lactate in the Presence of Fascat* 1 9102 Catalyst
  • Fascat' 1 ' 9102 (Atochem North America, Inc.) catalyst at atmospheric pressure under a nitrogen atmosphere to a temperature of 190°C, for 3.6 hours.
  • the ethyl lactate content of the second overhead is estimated to be about 52%, with the remaining material being ethanol.
  • Control Agent Ethyl Lactate Lactic Acid Weight % Control Agent Polymer Molecular Weight
  • the polymers formed with ethyl lactate as the control agent had a molecular weight which averaged 37% higher than the polymers formed with lactic acid as the control agent. This implies that purification requirements to meet a specified molecular weight may be slightly lower for a process based on lactate esters rather than on lactic acid feed, if autogenous impurities are used as the molecular weight control agent.
  • Heating was resumed with the flask at a pressure of 45 mm Hg. A slight boil was maintained as the material was heated to 120°C, removing another 63.5 ml of material. The methyl lactate content of the second overhead is estimated to be about 62%, with the remaining material being methanol. Total heating time for this phase was 5.5 hours. The material was a dark orange color.
  • EXAMPLE 26 2.5 kilograms of methyl lactate were heated with 3.5 wt.% Amberlyst ® 36 resin (Rohm and Haas) catalyst at 80 mm Hg, under a nitrogen atmosphere, to a temperature of 104 C C, for 10.75 hours. The material initially boiled at 70°C. The overhead consisted primarily of methanol with some methyl lactate, total volume of distillate - 505 ml at 81% methanol. The material was allowed to cool, and the catalyst exchanged with fresh Amberlyst ® 36 resin at 4.5 wt.%. Heating was resumed with the flask at a pressure of 40 mm Hg.
  • Amberlyst ® 36 resin Amberlyst ® 36 resin
  • composition of the pot was monitored as a function of time, with the results shown below. These concentrations are calculated upon correction for response factors as detailed above. The results are very similar to those obtained using homogeneous catalysts.
  • a ring-opening reaction was performed using the heterogenous catalyst, Amberlyst ® 36.
  • the conditions are similar to those of Example 23, which used a homogenous catalyst.

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  • Chemical & Material Sciences (AREA)
  • Health & Medical Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Medicinal Chemistry (AREA)
  • Polymers & Plastics (AREA)
  • Organic Chemistry (AREA)
  • Polyesters Or Polycarbonates (AREA)
  • Heterocyclic Compounds That Contain Two Or More Ring Oxygen Atoms (AREA)

Abstract

L'invention concerne un procédé de production continue de lactide et de polymères de lactide purifiés à partir d'acide et de polymères de lactide purifiéslactique ou d'un ester d'acide lactique consistant à produire de l'acide polylactique brut, de préférence en présence d'un élément catalyseur dans le cas de l'ester d'acide lactique afin d'obtenir un sous-produit de réaction de condensation et un acide polylactique; et à dépolymériser l'acide polylactique pour obtenir du lactide brut, ceci suivi d'une purification ultérieure de lactide brut dans un système de distillation. Un lactide purifié est ensuite polymérisé pour obtenir des polymères de lactide.
EP93903632A 1992-01-24 1993-01-22 Procede continu de fabrication de lactide et de polymeres de lactide Ceased EP0623153A1 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
ES98203427T ES2345307T3 (es) 1992-01-24 1993-01-22 Proceso continuo para la fabricacion de lactida y polimeros de lactida.
EP98203427A EP0893462B1 (fr) 1992-01-24 1993-01-22 Procédé continu pour la préparation de lactide et polylactides

Applications Claiming Priority (11)

Application Number Priority Date Filing Date Title
US07/825,059 US5142023A (en) 1992-01-24 1992-01-24 Continuous process for manufacture of lactide polymers with controlled optical purity
US825059 1992-01-24
US926901 1992-08-07
US07/927,149 US5247058A (en) 1992-01-24 1992-08-07 Continuous process for manufacture of lactide polymers with controlled optical purity
US07/926,901 US5258488A (en) 1992-01-24 1992-08-07 Continuous process for manufacture of lactide polymers with controlled optical purity
US927149 1992-08-07
US07/935,059 US5274073A (en) 1992-01-24 1992-08-24 Continuous process for manufacture of a purified lactide
US935059 1992-08-24
US935566 1992-08-24
US07/935,566 US5247059A (en) 1992-01-24 1992-08-24 Continuous process for the manufacture of a purified lactide from esters of lactic acid
PCT/US1993/000568 WO1993015127A1 (fr) 1992-01-24 1993-01-22 Procede continu de fabrication de lactide et de polymeres de lactide

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EP93903632A Ceased EP0623153A1 (fr) 1992-01-24 1993-01-22 Procede continu de fabrication de lactide et de polymeres de lactide

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KR (1) KR100261850B1 (fr)
AT (1) ATE466897T1 (fr)
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CA (1) CA2128509C (fr)
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AU3482193A (en) 1993-09-01
FI113661B (fi) 2004-05-31
KR100261850B1 (ko) 2000-08-01
JPH07503490A (ja) 1995-04-13
WO1993015127A1 (fr) 1993-08-05
EP0893462A2 (fr) 1999-01-27
BR9305788A (pt) 1997-02-18
NO309530B1 (no) 2001-02-12
ES2345307T3 (es) 2010-09-20
FI943487A (fi) 1994-09-21
NO942728L (no) 1994-09-23
CA2128509C (fr) 2006-09-19
EP0893462B1 (fr) 2010-05-05
FI943487A0 (fi) 1994-07-22
NO942728D0 (no) 1994-07-21
ATE466897T1 (de) 2010-05-15
EP0893462A3 (fr) 1999-10-20
DE69334330D1 (de) 2010-06-17
MX9300333A (es) 1993-07-01
CA2128509A1 (fr) 1993-08-05
JP3258324B2 (ja) 2002-02-18
AU677174B2 (en) 1997-04-17
NZ249033A (en) 1996-05-28

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