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
  • C08G63/88—Post-polymerisation treatment
• • 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
• • 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
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J3/00—Processes of treating or compounding macromolecular substances
  • C08J3/12—Powdering or granulating

Definitions

• the present invention relates to lactide . particles, more specifically to lactide particles which are stable enough to be stored and transported at room temperature and which have a quality high enough for use as starting material for polylactic acid.
• D-lactic acid, L-lactic acid, or mixtures thereof may be polymerized to form an intermediate molecular weight polylactic acid which, after the ring-closure reaction, generates lactide as earlier disclosed.
• the lactide (sometimes also referred to as dilactide) , 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-lactide, and meso-lactide, respectively.
• a 50/50 mixture of L-lactide and D-lactide with a melting point of about 126 0 C is often referred to in the literature as D, L-lactide.
• the 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 enantiomers 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.
• the optical purity of the lactic acid or the lactide is decisive for the stereochemistry of the polylactid acid obtained upon ring-opening polymerization of the lactide. With respect to polylactic acid, stereochemistry, and molecular weight are the key parameters for polymer quality.
• the transport and storage of melted lactide is also detrimental to the quality of the lactide because racemization, hydrolysis, and oxidation reactions are accelerated at these temperatures.
• the same problem occurs in the direct conversion process when the residence time of the lactide is not precisely controlled.
• the present invention is directed to stable lactide particles wherein the surface/volume ratio of the particle is lower than 3000 m ⁇ l .
• lactide particles that fulfill this requirement are stable enough for storage and transport at room temperature and can readily be used as starting material for the production of lactic acid for bulk applications.
• lactide present in the particles according to the invention contains more than 95% by weight D- or L-lactide, preferably more than 98.5% by weight D- or L-lactide, most preferably more than 99.5% D-or L-lactide by weight.
• the lactide particles according to the invention can be prepared by subjecting lactide (for instance in the melted or crystalline powdery form) to a shaping process.
• Suitable shaping processes are extrusion, pastillation, prilling, flaking etcetera.
• the particles formed in the shaping process can be considered pellets, pastilles, granules and/or agglomerates. These terms are used throughout the description dependent from the term commonly used in the shaping process concerned.
• melted By melted is meant that at least part of the lactide is at a temperature at or above the melting point of the lactide.
• the apparatus used for the shaping process, or at least those parts that will be in contact with the lactide preferably are prepared from corrosive-resistant material such as stainless steel. Further, to avoid water uptake of the lactide particles, the shaping process is preferably conducted under inert gas or dry atmosphere such as under nitrogen or dry air.
• cylindrical or rod-like particles By means of extrusion through one or more dies cylindrical or rod-like particles can be obtained. When looking at the surface/volume ratio of the lactide particles, these cylindrical or rod-shaped particles are preferred. This shaping process is further preferred because processing equipment for the preparation of polylactide from lactide readily can handle particles of this shape because of the relatively uniform particle size and shape.
• the extruder is optionally cooled to avoid local overheating of the lactide. Any extruder conventionally used in the plastics, metal powder, food and ceramics industry such as screw extruders, such as single- and twin-screw extruders and radial screen extruders etcetera is suitable.
• Suitable pastillation machines are for instance the disc pastillator, ex GMF® or a rotoformer ® ex Sandvik.
• the lactide is melted and droplets are placed on a disk or belt with controlled temperature.
• the surface/volume ratio of the resulting substantially hemi-spherical lactide particles is somewhat higher than for cylindrical or rod-shaped particles, hemispherical lactide particles are preferred because processing equipment for the preparation of polylactide from lactide readily can handle particles of this shape because of the relatively uniform particle size and shape.
• pastilles usually can easier be dosed in polylactic acid reactors especially when reactive extrusion polymerization is used.
• relatively uniform means that at least 90 percent by weight of the pastilles are within plus/minus 30 percent of the mean diameter. Preferably, at least 95 percent by weight of the particles are within plus/minus 10 percent of the mean diameter.
• substantially hemi-spherical means that the form of the particle is basically hemi-spherical, but can be flattened somewhat, i.e. the height of the particle is between 50 and 30% of its diameter.
• a sieving step is performed after the shaping to avoid dusting during transport and further processing to form polylactide.
• prilling lactide droplets fall in a liquid bath and thus spherical particles can be obtained. If water is used for the bath, extensive drying of the lactide particles is necessary.
• particles with an average diameter of at least 3 millimeters are preferred, because then an optimum surface/volume ratio is ensured. More preferably the particles have an average diameter between 3 and 10 millimeters .
• the water content of the lactide is an important factor for the stability of the lactide particles.
• the lactide Contamination by water vapor leads to ring-cleavage causing the lactide to convert to lactoyl lactic acid and lactic acid. It was found that if the water content is below 200 ppm the stability of the lactide particles when stored at room temperature in air-tight and vapor-tight packages is ensured for several months. Preferably, the water content is below 100 ppm because it further increases the stability of the lactide.
• the water content of the lactide can be measured by means of a Karl-Fisher titration as will be known by the artisan.
• the acid content of the lactide is important for the stability and quality of the lactide.
• the presence of lactic acid and or lactoyl lactic acid in the feed to the final polymerization step will result in polymers of limited molecular weight.
• the free acid content is below 50 milli-equivalents per Kg lactide (meq.Kg "1 ) the stability of the lactide particles when stored at room temperature in air-tight and vapor-tight packages is ensured for several months.
• the acid content is below 20 meq.Kg x because it further increases the stability of the lactide.
• the acid content is between 0 and 10 meq.Kg "1 .
• the acid content can be measured by means of titration using for instance sodium methanoate or potassium methanoate, as will be clear for the artisan.
• the lactide used as starting material for the shaping process may have been prepared by any conventional lactide process such as water removal from a lactic acid solution or condensation reaction of lactate esters, followed by a ring-closure reaction in a lactide reactor with the help of a catalyst.
• the crude lactide is further purified by for instance distillation and/or crystallization prior to the shaping process.
• the lactide reactor can be of any suitable type that is designed for heat sensitive materials.
• a reactor that can maintain a uniform film thickness, such as a falling film or agitated thin-film evaporator is most preferred, because film formation increases the rate of mass transfer.
• lactide can quickly form and vaporize, and as lactide vaporizes, more lactide is produced as dictated by the polylactic acid/lactide equilibrium reaction.
• these lactide reactors are operated under reduced pressure such as between about 1 mmHg and 100 mmHg.
• the temperature of the lactide formation is kept between 150 0 C and 250 0 C.
• tin ⁇ II oxide or tin(0ct) 2 catalyst is used for lactide formation.
• Stabilizers may also be added to the lactide reactor in order to facilitate lactide formation and discourage degenerative lactic acid and lactide reactions.
• Stabilizers such as antioxidants, can be used to reduce the number of degradation reactions that occur during the process of polylactic acid and lactide production. Stabilizers may also reduce the rate of lactide formation during this process. Therefore, efficient production of lactide requires proper reactor design for minimal thermal severity and a proper balance between the catalyst and any use of process stabilizers .
• the stabilizing agent may include primary antioxidants and/or secondary antioxidants.
• Primary antioxidants are those which inhibit free radical propagation reactions, such as alkylidene bisphenols, alkyl phenols, aromatic amines, aromatic nitro and nitroso compounds, and quinones.
• secondary antioxidants include: phosphites, organic sulfides, thioethers, dithiocarbamates, and dithiophosphates .
• Antioxidants, when added to the lactide reactor can reduce the extent of racemization during lactide production. This reduction indicates that the addition of antioxidants is an additional means to control optical purity.
• Antioxidants include such compounds as trialkyl phosphites, mixed alkyl/aryl phosphites, alkylated aryl phosphites, sterically hindered aryl phosphites, aliphatic spirocyclic phosphites, sterically hindered phenyl spirocyclics, sterically hindered bisphosphonites, hydroxyphenyl propionates, hydroxy benzyls, alkylidene bisphenols, alkyl phenols, aromatic amines, thioethers, hindered amines, hydroquinones, and mixtures thereof.
• phosphite-containing compounds are used as process stabilizing antioxidants.
• phosphite-containing compounds are used.
• the amount of process stabilizer used can vary depending upon the optical purity desired of the resulting lactide, the amount and type of catalyst used, and the conditions inside of the lactide reactor. Normally amounts varying form 0.01 to 0.3 wt . % process stabilizer can be used.
• dehydration or anti-hydrolysis agents may be used. These dehydration agents favor the formation of lactide. Further, they may be used in a later stage of the manufacturing process for polylactic acid as well as for preventing chain scission by water.
• the carbodiimide compound is a compound having one or more carbodiimide groups in a molecule and also includes a polycarbodiimide compound.
• a monocarbodiimide compound included in the carbodiimide compounds dicyclohexyl carbodiimide, diisopropyl carbodiimide, dimethyl carbodiimide, diisobutyl carbodiimide, dioctyl carbodiimide, diphenyl carbodiimide, naphthyl carbodiimide, etc. may be exemplified.
• industrially easily available compounds such as dicyclohexyl carbodiimide, diisopropyl carbodiimide or products like Stabaxol® by Rheinchemie are used.
• stabilizers and dehydration agents may be added to the lactide at a later stage, such as for instance prior to the shaping and/or after the shaping step. If the stabilizers are added to the lactide after shaping, the stabilizers may be sprayed or coated onto the lactide particles.
• the lactide particle usually comprises more than 95% by weight lactide, preferably more than 98.5% by weight lactide, most preferably more than 99.5% by weight.
• the shaping process can either be combined with the preparation and or purification, or not. For instance, if the lactide is obtained form distillation, it makes sense to directly couple a pastillation machine to the distillation column because the lactide is already in its melted form. If the final purification step of the lactide comprises crystallization, the use of an extruder is more opportune. Said extrusion can also take place at a later point in time.
• the droplet falling rate and the disc rotation speed were matched in order to get circular arrays of pastilles on the disc.
• the position of the nozzle over the disc was adapted to start a new array, thus producing a cooling disc ultimately covered with concentric arrays of pastilles.
• Pastilles did not stick to the metal disc and could be collected easily. Solidified lactide pastilles of uniform dimensions could thus be produced (Average particle diameter 5.5- 6 mm with a thickness of between 1.6-1.8 mm).
• the extruder was operated with a screw speed of 150 rpm and L-lactide powder was metered in water-cooled zone 1 at a solids rate of 1.8-2.4 Kg/h by means of a volumetric feeder.
• the temperature of the white paste discharged from the die was 88-92 0 C.
• the resulting strands broke spontaneously when they fell down some 20-40 cm upon discharge from the extruder onto an RVS tray.
• cylindrical pellets with a randomly distributed length of several millimeters are obtained (the particle diameter was about 3 mm while the length varied from 5 to 15 mm) .
• the white lactide pellets initially exhibited a free lactic acid content of 4 meq/Kg.
• the stability of powdery material having a diameter of about 1 mm was measured after storage for 1 year in air-tight and vapor-tight bags (comprising a polyethylene inner bag and an aluminum outer bag) with a hole in it.
• the initial free acid content was 0.080 meq/Kg. After 1 year at 4 0 C the free acid content was increased to 0.09 meq/Kg and after 1 year at 25 0 C the free acid content was increased to 1131 meq/Kg. This means shows that powdery material is not stable enough for storage at room temperature for several months.
• the stability of powdery material having a diameter of about 1 mm was measured after storage for 1 year in a single polyethylene bag (vapor-tight but not air-tight) .
• the initial free acid content was 0.09 meq/Kg. After 6 months at 25 0 C the free acid content was increased to 405 meq/Kg, and thus not suitable anymore as a starting material for the preparation of polylactid acid.

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• Chemical Kinetics & Catalysis (AREA)
• Medicinal Chemistry (AREA)
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• Organic Chemistry (AREA)
• Polyesters Or Polycarbonates (AREA)
• Processes Of Treating Macromolecular Substances (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
• Medicinal Preparation (AREA)
• Biological Depolymerization Polymers (AREA)
• Heterocyclic Compounds That Contain Two Or More Ring Oxygen Atoms (AREA)
• Processing And Handling Of Plastics And Other Materials For Molding In General (AREA)

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