# Patent

## Title
Method for producing aliphatic polyester having increased molecular weight

## Abstract

The problem to be solved by the present invention is to provide a method for continuously and effectively producing an aliphatic polyester having an increased molecular weight which prevents evaporation of diisocyanate at the step of mixing the diisocyanate with the aliphatic polyester prepolymer in a molten state at a temperature not lower than the melting point of the prepolymer and enables uniform mixing of the diisocyanate with the aliphatic polyester prepolymer. In order to solve this problem, the present invention provides a method for producing an aliphatic polyester having an increased molecular weight comprising the steps of:

    
    
        (i) quantitatively injecting a diisocyanate into an aliphatic polyester prepolymer which has a number average molecular weight of 5000 or higher and has terminal hydroxyl groups and in which at least one acid component is a succinic acid compound, in a molten state at a temperature not lower than the melting point thereof, wherein
        the amount of diisocyanate is equivalent to between one tenth and two times the amount of the hydroxyl groups in the aliphatic polyester prepolymer,
        (ii) quantitatively and continuously mixing the diisocyanate-containing aliphatic polyester prepolymer obtained at step (i) by a static mixer, discharging same from the static mixer, and then supplying same to a coupling reaction tank, and
        (iii) reacting the aliphatic polyester prepolymer with the diisocyanate in the coupling reaction tank.

## Background
TECHNICAL FIELD
The present invention relates to a method for continuously and effectively producing an aliphatic polyester having an increased molecular weight. The obtained polyester having an increased molecular weight is a high-quality resin.
BACKGROUND ART
Conventionally, the high molecular weight polyesters used in films, sheets, fibers, and other molded articles were mostly aromatic polyesters such as polyethylene terephthalate and polybutylene terephthalate.
Aliphatic polyesters generally regarded as being biodegradable have attracted attention in recent years from the point of environmental protection and the like. As a method for producing the aliphatic polyesters, a method comprising directly esterifying an aliphatic dicarboxylic acid with an aliphatic diol or a method comprising conducting transesterification of an alkyl ester of aliphatic dicarboxylic acid or an anhydride thereof with an aliphatic diol to obtain a glycol ester or a low polymer thereof and then stirring the same by heat under high vacuum for polycondensation has been known.
Polymerization proceeds by removing a released component outside the system in the polycondensation reaction as above. It is generally conducted by distilling the released component from the system conditioned at a high temperature and under high vacuum. For example, Patent publication 1 discloses a method for producing an aliphatic polyester having an increased molecular weight comprising esterifying an aliphatic dicarboxylic acid with a glycol component and conducting a reaction to remove glycol from the formed polyester diol in the presence of a catalyst at a temperature of 180-230° C. under high vacuum at 0.005-0.1 mmHg.
However, it is not sufficient to put the system under high vacuum, but it is necessary to make the surface area of the reactants for polycondensation sufficiently large and effectively renewed. This is because the released component present on the surface of the reactants can be removed more easily. In particular, as the viscosity of the reactants is increased and it is difficult for the released component to diffuse from the reactants in the latter half of the polycondensation reaction, it is necessary to make the surface area of the reactants larger by mechanical stirring so as to effectively renew the surface. As the desired surface area and effective surface renewal cannot be achieved in using a conventional polymerization apparatus equipped with conventional stirring blades, problems where the reaction does not proceed sufficiently and an aliphatic polyester having an increased molecular weight cannot be obtained occur. Formation of a film or sheet using a polybutylene succinate having a low molecular weight is difficult.
In addition, as there is a limit to the direct polymerization described above, Patent publication 2 proposes preparation of an aliphatic polyester having an increased molecular weight by adding a diisocyanate having an isocyanate group in an amount one tenth to two times equivalent to the amount of the hydroxyl groups to an aliphatic polyester prepolymer substantially having terminal hydroxyl groups in a molten state at a temperature not lower than the melting temperature thereof. It has been known that the aliphatic polyester having an increased molecular weight obtained by this method has a weight average molecular weight (Mw) of 200,000 or higher that cannot be achieved by a conventional direct polymerization method and that formability and physical properties of films thereof are excellent. However, by this method, reduction of accuracy in controlling molecular weight and formation of gelation and fish eyes sometimes occurred when adding the isocyanate to a polymerization tank as in the conventional method.
PRIOR ART REFERENCE
Patent Publications


    Patent publication 1: Japanese Patent Laid-Open No. 5-310898
    Patent publication 2: Japanese Patent No. 2825969


SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
The inventors of this application have studied the reduction of accuracy in controlling molecular weight and formation of gelation and fish eyes and as a result, they found that when an isocyanate is added to a polymerization tank at a high temperature not lower than the melting point of the aliphatic polyester prepolymer so as to increase the molecular weight, the isocyanate evaporates due to the high temperature and not all of the amount of isocyanate added is used for the reaction and that the evaporated isocyanate is adhered to the inner wall of the reaction tank vapor-phase portion.
Accordingly, the object of the present invention is to provide a method for continuously and effectively producing an aliphatic polyester having an increased molecular weight which prevents evaporation of diisocyanate at the step of mixing the diisocyanate with the aliphatic polyester prepolymer in a molten state at a temperature not lower than the melting point of the prepolymer and enables uniform mixing of the diisocyanate with the aliphatic polyester prepolymer.
Means for Solving the Problems
The inventors of this application keenly studied to achieve the above object, and as a result, found that a high-quality aliphatic polyester having an increased molecular weight in which gelation or formation of fish eyes are reduced can be obtained by quantitatively injecting a diisocyanate into an aliphatic polyester prepolymer in a molten state at a temperature not lower than the melting point thereof, sufficiently stirring and uniformly mixing same, and then subjecting same to a coupling reaction.
Namely, the present invention is:
(1) a method for producing an aliphatic polyester having an increased molecular weight comprising the steps of:
(i) quantitatively injecting a diisocyanate into an aliphatic polyester prepolymer which has a number average molecular weight of 5000 or higher and has terminal hydroxyl groups and in which at least one acid component is a succinic acid compound, in a molten state at a temperature not lower than the melting point thereof, wherein
the diisocyanate comprises isocyanate groups in an amount equivalent to between one tenth and two times the amount of the hydroxyl groups in the aliphatic polyester prepolymer,
(ii) quantitatively and continuously mixing the diisocyanate-containing aliphatic polyester prepolymer obtained at step (i) by a static mixer, discharging same from the static mixer, and then supplying same to a coupling reaction tank, and
(iii) reacting the aliphatic polyester prepolymer with the diisocyanate in the coupling reaction tank;
(2) a method for producing an aliphatic polyester having an increased molecular weight according to (1),
wherein a mixing tank is further provided between the static mixer and the coupling reaction tank and the aliphatic polyester prepolymer of (ii) discharged from the static mixer is introduced into the mixing tank and is further discharged from the mixing tank under stirring to be supplied into the coupling reaction tank;
(3) a method for producing an aliphatic polyester having an increased molecular weight according to (2),
wherein the mixing tank of (ii) has an inlet of the diisocyanate-containing aliphatic polyester prepolymer in its upper portion, an outlet for discharging a mixture of the aliphatic polyester prepolymer with the diisocyanate in its bottom portion, and at least one partitioning plate and stirring blade;
(4) a method for producing an aliphatic polyester having an increased molecular weight according to (3),
wherein said at least one partitioning plate and stirring blade are spaced one above the other;
(5) a method for producing an aliphatic polyester having an increased molecular weight according to (3) or (4),
wherein the stirring blade is a multistage blade;
(6) a method for producing an aliphatic polyester having an increased molecular weight according to any one of (1)-(5),
wherein the diisocyanate is injected by a non-pulsating pump at step (i) above;
(7) a method for producing an aliphatic polyester having an increased molecular weight according to any one of (1)-(6),
wherein the reaction at step (iii) above is conducted under stirring;
(8) a method for producing an aliphatic polyester having an increased molecular weight according to (7),
wherein the reaction at step (iii) above is conducted under stirring by a helical ribbon blade or a twisted lattice-shape blade;
(9) a method for producing an aliphatic polyester having an increased molecular weight according to any one of (1)-(6),
wherein the reaction at step (iii) above is conducted without stirring;
(10) a method for producing an aliphatic polyester having an increased molecular weight according to any one of (1)-(9),
wherein at least one polyalcohol component in the aliphatic polyester is ethylene glycol; and
(11) a method for producing an aliphatic polyester having an increased molecular weight according to any one of (1)-(9),
wherein at least one polyalcohol component in the aliphatic polyester is 1,4-butanediol.
Effect of the Invention
According to the method of the present invention, a high-quality aliphatic polyester having an increased molecular weight with less occurrence of gelation and less formation of fish eyes can be continuously produced in increasing the molecular weight of an aliphatic polyester. In addition, according to the production method of the present invention, accuracy in controlling the melt-flow rate (MFR) of the obtained aliphatic polyester having an increased molecular weight is high.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic drawing of the production method of the present invention.
FIG. 2 is a schematic drawing of the production method corresponding to a comparative example.
FIG. 3 is an embodiment of mixing tank 7 used in the present invention.



MODE FOR CARRYING OUT THE INVENTION
The present invention relates to a method for continuously and stably producing an aliphatic polyester having an increased molecular weight on an industrial scale comprising injecting a diisocyanate into an aliphatic polyester prepolymer which has a number average molecular weight of 5000 or higher and has terminal hydroxyl groups and in which at least one acid component is a succinic acid compound, in a molten state at a temperature not lower than the melting point thereof, wherein
the diisocyanate comprises isocyanate groups in an amount equivalent to between one tenth and two times the amount of the hydroxyl groups in the aliphatic polyester prepolymer.
The succinic acid compound used in the present invention is succinic acid or a derivative thereof (such as diesters, monoesters, and anhydride thereof). Specific examples thereof include succinic acid; succinic acid esters such as dimethyl succinate and diethyl succinate; and succinic anhydride. Among them, succinic acid, dimethyl succinate, and succinic anhydride are preferable. Succinic acid or derivatives thereof may be used alone or in combination of two or more thereof.
Dicarboxylic acid compounds other than the succinic acid compound may be used as part of the acid components above. Specific examples thereof include dicarboxylic acids having a linear or branched alkylene group such as adipic acid, suberic acid, sebacic acid, azelaic acid, decane dicarboxylic acid, dodecane dicarboxylic acid, octadecane dicarboxylic acid, and dimeric acid; esters of the dicarboxylic acids such as dimethyl adipate and dimethyl malonate; acid anhydrides such as maleic anhydride, itaconic anhydride, and adipic anhydride; and oxycarboxylic acids such as malic acid, tartaric acid, and citric acid. Among them, adipic acid or an adipic acid derivative such as dimethyl adipate is preferable.
The amount of the dicarboxylic acid component other than the succinic acid compound above is about 0-35 mol %, preferably about 0-25 mol %, with respect to the total amount of dicarboxylic acids as the acid components.
Glycol is used as a polyalcohol for producing the aliphatic polyester used in the production method of the present invention. Examples thereof include aliphatic glycols having a linear or branched alkylene group such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, 1,2-pentanediol, 2,4-pentanediol, 1,6-hexanediol, 1,2-hexanediol, neopentyl glycol, and 2,2-diethyl-1,3-propanediol; epoxides corresponding to 1,2-glycol; alcohols having a valency of three or higher such as trimethylolpropane; and diepoxides. Among them, ethylene glycol and 1,4-butanediol are preferable. 1,4-Butanediol is more preferable.
The amount of the glycol component used is different, depending on the physical properties of the desired polyester, but in general, it is 1.02-1.5 mole, preferably 1.03-1.2 mole, with respect to one mole of the dicarboxylic acid component. If it is less than 1.02 mole, the content of the aliphatic polyester prepolymer having terminal hydroxyl groups becomes small.
In general, an aliphatic polyester is produced in the presence of a catalyst. Catalysts may be used alone or in combination of two or more thereof. A wide range of catalysts used in transesterification reaction can be used. Examples of the catalysts include protonic acids such as sulfuric acid, p-toluenesulfonic acid, and phosphoric acid and derivatives thereof; metal compounds comprising a metal such as Li, Mg, Ca, Ba, La, Ce, Ti, Zr, Hf, V, Mn, Fe, Co, Ir, Ni, Zn, Ge, and Sn (for example, organic metal compounds comprising the metal such as organic acid salts, metal alkoxides and metal complexes (acetylacetonate and the like) and inorganic metal compounds comprising the metal such as metal oxides, metal hydroxides, carbonates, phosphates, sulfates, nitrates and chlorides.). Among these metal compound catalysts, titanium compounds, especially, organic titanium compounds such as titanium alkoxides including titanium tetraethoxide titanium tetraisopropoxide, and titanium tetrabutoxide are preferable. The amount of these metal compound catalysts is about 0.005-1 mole, preferably about 0.01-0.3 mole, with respect to 100 moles of the total amount of the acid components.
In the method of the present invention, an organic or inorganic phosphorus compound may be used as a catalyst with the metal compound catalyst above (such as an organic titanium compound). A polymer having an increased molecular weight can be obtained in a short polymerization time by using the metal compound above in combination with the organic or inorganic phosphorus compound.
The organic or inorganic phosphorus compound is exemplified as below.
(a) Phosphoric acid and organic esters thereof: commercially available products thereof include phosphoric acid, alkyl or aryl acidic phosphates (in which the alkyl or aryl group is methyl, isopropyl, butyl, octyl, phenyl, and naphthyl group), and the like.
(b) Phosphonic acid and organic esters thereof: commercially available products thereof include methyl phosphonate, ethyl phosphonate, aryl phosphonates such as phenyl phosphonate and naphthyl phosphonate, dibutyl butyl phosphonate, and the like. Substituents such as alkyl groups (C1-4 alkyl groups and the like such as methyl group), halogen atoms (fluorine atom, chlorine atom, and the like), alkoxy groups (C1-4 alkoxy groups and the like such as methoxy group), nitro group, and the like may be bonded to the aromatic rings of the aryl phosphonates above.
(c) Phosphorous acid and organic esters thereof: examples thereof include dibutyl hydrogen phosphite, triphenyl phosphite, diphenyl isodecyl phosphite, and tris isodecyl phosphite.
The amount of the organic or inorganic phosphorus compound in using the metal compound catalyst in combination with the organic or inorganic phosphorus compound as catalysts is 1-100 moles, preferably 5-33 moles, with respect to 100 moles of the metal compound catalyst (such as an organic titanium compound).
The type of diisocyanate used in the present invention is not particularly limited, but a commercially available product per se can be used.
Examples thereof include 2,4-trilene diisocyanate, a mixture of 2,4-trilene diisocyanate and 2,6-trilene diisocyanate, diphenylmethane diisocyanate, P,P′-diphenyl diisocyanate, 1,6-naphthylene diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, isophorone diisocyanate, and hexamethylene diisocyanate.
Next, embodiments of the present invention will be explained with reference to FIG. 1. FIG. 1 is an example of the scheme of the production method of the present invention.
In the specification of this application, the descriptions, i.e., weighing hopper 1 for supplying the acid component comprising the succinic acid compound to esterification tank 3, glycol storage tank 2 from which the glycol component is supplied to esterification tank 3, esterification tank 3 in which the acid component and the glycol component are mixed and esterified, transesterification reaction tank 4 in which glycol is removed by transesterification reaction from the aliphatic polyester prepolymer formed in esterification tank 3, diisocyanate storage tank 5, static mixer 6 for mixing the aliphatic polyester prepolymer with the diisocyanate, mixing tank 7 optionally provided in which the aliphatic polyester prepolymer is further mixed with the diisocyanate, coupling reaction tank 8 for obtaining the aliphatic polyester having an increased molecular weight in which the mixed aliphatic polyester prepolymer and diisocyanate are reacted, and pelletizer 9 for pelletizing the aliphatic polyester having an increased molecular weight obtained as a final product are used.
Glycol storage tank 2 and esterification tank 3 are connected by pipe 10a. Esterification tank 3 and transesterification reaction tank 4 are connected by pipe 10b. Transesterification reaction tank 4 and static mixer 6 are connected by pipe 10c Diisocyanate storage tank 5 is connected to pipe 10c via pipe 10d. Static mixer 6 and mixing tank 7 are connected by pipe 10e. Mixing tank 7 and coupling reaction tank 8 are connected by pipe 10f. Coupling reaction tank 8 is connected to pelletizer 9 by pipe 10g. 
The acid component comprising the succinic acid compound is supplied from weighing hopper 1 to esterification tank 3. When an acid component other than the succinic acid compound is used, a mixture previously prepared with the succinic acid compound may be supplied from weighing hopper 1 to esterification tank 3 or another weighing hopper may be used to supply the other acid component to esterification tank 3 separately from the succinic acid compound.
A catalyst is added from a tank (not shown) for supplying the catalyst to esterification tank 3. The catalyst used at each step may be added to esterification tank 3 at the same time or a catalyst needed in transesterification reaction tank 4 may be added.
Glycol is supplied through pipe 10a to esterification tank 3. Pipe 10a is provided with pump 11a and a weighing apparatus for weighing the amount of glycol supplied, so that the supply by the pump can be automatically stopped. The pump used herein includes centrifugal pumps, turbine pumps, propeller pumps, and the like.
In addition, the temperature of pipe 10a is set to a temperature not lower than the melting point of glycol so as to prevent crystallization of the glycol. The temperature is different, depending on the kind of glycol, but in general, it is preferably set to 30 to 50° C.
Ingredients are preferably reacted under stirring in esterification tank 3 for promoting the reaction efficiently. Esterification tank 3 is preferably provided with a stirring blade. For example, a vertical stirring tank equipped with a stirring blade having a vertical rotation shaft (such as a paddle blade and a turbine blade) is used.
After the predetermined amounts of the acid component, the glycol component, and the catalyst are supplied to esterification tank 3, esterification reaction is conducted under a nitrogen gas atmosphere. In general, the reaction temperature is from 140 to 250° C., preferably from 145 to 245° C. If the temperature is lower than 140° C., the reaction rate is slow and is not practical. If the temperature is higher than 250° C., the formed polymer may be thermally decomposed. In general, the reaction pressure is normal pressure, but the pressure in the system may be reduced in the latter half of the reaction, for example, to from 5 to 100 mmHg (from 665 to 13300 Pa) so as to shorten the reaction time. The reaction time is not particularly limited, but is generally from 6 to 12 hours.
The aliphatic polyester prepolymer obtained in esterification tank 3 has a number average molecular weight of from about 500 to 5,000.
The aliphatic polyester prepolymer obtained in esterification tank 3 is rapidly delivered to transesterification reaction tank 4 through pipe 10b. A catalyst is added as necessary for conducting the transesterification reaction. At this step, the molecular weight of the formed aliphatic polyester prepolymer is increased mainly by transesterification reaction (condensation reaction by removing glycol) of the aliphatic polyester prepolymers. When the number average molecular weight becomes 5,000 or higher, an aliphatic polyester prepolymer having terminal hydroxyl groups is formed.
The temperature in the transesterification reaction is from 200 to 250° C., preferably from 210 to 240° C. If the polymerization temperature is too low, the reaction time becomes longer and the production efficiency is lowered. If the polymerization temperature is too high, the polymer is colored and decomposed products are easily formed. In order to promote the condensation reaction by removing glycol, the pressure in the system should be reduced. The pressure finally reached at the time of completion of reaction is from 0.1 to 5.0 mmHg (from 13 to 666 Pa) and the reaction time is about 5 to 10 hours.
Although the esterification reaction and the transesterification reaction in the present invention are bulk polymerization without using solvents, solvents may be used as necessary.
Polymerization apparatuses generally used in polyester prepolymer production such as esterification tank 3 above and coupling polymerization apparatuses can be used as transesterification tank 4. Examples of these polymerization apparatuses include vertical stirring tanks equipped with a stirring blade having a vertical rotation shaft (such as a double helical ribbon blade and a twisted lattice-shape blade).
The aliphatic polyester prepolymer having a number average molecular weight of 5,000 or higher and having terminal hydroxyl groups is delivered to static mixer 6 through pipe 10c. The number average molecular weight of the aliphatic polyester prepolymer generally obtained by transesterification reaction is from 5000 to 30,000, preferably from 10,000 to 20,000.
As the viscosity of the aliphatic polyester prepolymer is high, it is necessary to raise the temperature to turn it into a molten state in mixing the aliphatic polyester prepolymer with the diisocyanate. However, if the aliphatic polyester prepolymer in a molten state is simply mixed with the diisocyanate, the diisocyanate evaporates to move to the head space of the mixing tank and cannot be mixed uniformly at the predetermined ratio. In addition, the diisocyanate adheres to the inner wall surface of the mixing tank vapor-phase portion, and does not contribute to the reaction, so, a prepolymer having a sufficiently increased molecular weight cannot be obtained.
For that reason, evaporation of the diisocyanate can be prevented by using a pump and quantitatively injecting the diisocyanate without generating vapor-phase diisocyanate directly into the aliphatic polyester prepolymer through an inlet provided in pipe 10c which enables injection of the diisocyanate.
As another embodiment, static mixer 6 is provided with an inlet which enables injection of the diisocyanate and the diisocyanate may be injected from the inlet (not shown). The inlet is preferably provided at the upstream portion (on the side of pipe 10c) from the center of the static mixer.
Pipe 10c is provided with pump 11b. This pump is provided for delivering the high-temperature aliphatic polyester prepolymer having a high viscosity and needs to be a quantitative pump which can deliver the prepolymer quantitatively and continuously through pipe 10c. The quantitative pump used herein is preferably a volume pump, a rotating pump that can be used for the prepolymer having a high viscosity. Specific examples of the rotating pump include gear pumps, screw pumps, and vane pumps. Among them, gear pumps which have high quantitativity and are the most suitable for delivering a fluid having a high viscosity are preferable.
In order to increase the quantitativity, the quantitative pump is driven by a servomotor, whereby the flow rate can be controlled.
The delivery rate of the aliphatic polyester prepolymer through pipe 10c is affected by the scale of the apparatus, the diameter of pipe 10c and the like, but generally is maintained to be constant by which the average residence time in the mixing tank at the following step becomes from 2 to 10 minutes. If the delivery rate is faster than the above, the diisocyanate may not be sufficiently mixed with the aliphatic polyester prepolymer. If it is too slow, the reaction proceeds excessively to increase the viscosity of the polymer, which cannot be easily discharged from the mixing tank.
The rate is adjusted mainly by the rotational speed of pump (11b).
The aliphatic polyester prepolymer in a molten state is delivered in such a manner that it is filled in pipe 10c. Namely, pipe 10c is filled with the aliphatic polyester prepolymer such that there are few voids in the pipe. The void ratio in pipe 10c is less than 5%, preferably less than 2%, more preferably less than 1%. It is preferable that the diisocyanate is injected into the aliphatic polyester prepolymer without contacting the void portion (vapor phase) in pipe 10c. If the void ratio is large, the diisocyanate injected from pipe 10d evaporates and remains in the void space, whereby the diisocyanate cannot be efficiently mixed with the aliphatic polyester prepolymer at the following step.
If static mixer 6 is provided with an inlet, through which the diisocyanate is injected, static mixer 6 is filled with the aliphatic polyester prepolymer in a molten state in the same manner as described above. The void ratio in static mixture 6 is less than 5%, preferably less than 2%, more preferably less than 1%. It is preferable that the diisocyanate is injected into the aliphatic polyester prepolymer without contacting the void portion (vapor phase) in static mixer 6.
Diisocyanate storage tank 5 is connected to pipe 10c or the inlet provided in static mixer 6 via pipe 10d. Pipe 10d is provided with quantitative pump 11c, which enables a quantitative and continuous supply of the diisocyanate to the aliphatic polyester prepolymer. The quantitative pump used is preferably a non-pulsating pump. In this case, a reciprocating pump suitable for delivering a small amount of a fluid having a low viscosity is preferably used. Examples thereof include diaphragm pumps, piston pumps, and plunger pumps. It is preferable to use plunger pumps connected in line so as to stabilize the flow rate and increase the accuracy. In order to further increase the quantitativity, the quantitative pump may be driven by a servomotor to control the flow rate.
The non-pulsating pump used herein means a pump comprising a plurality of pumps in line whose total volume is constant, namely which can pump flow volumes without pulsation. One is used in a wide range of applications.
The diisocyanate comprising isocyanate groups in an amount equivalent to one tenth to two times the amount of the hydroxyl groups of the aliphatic polyester prepolymer is quantitatively injected. A pump is used to pressurize the diisocyanate in pipe 10d at from 0.5 to 1 MPa at room temperature.
When the isocyanate is injected through pipe 10c, the injected diisocyanate-containing aliphatic polyester prepolymer is quantitatively and continuously supplied from the inlet of static mixer 6 through pipe 10c. When the diisocyanate is injected into static mixer 6, injecting and delivering it into the static mixer occur at the same time.
The static mixer used in the present invention means a static mixing device which has no driving members, but mixes and disperses a fluid by making the fluid pass through the pipe and blades called mixing elements arranged in line in the tube or flow path.
Static mixers used as static mixer 6 are grouped into two types, i.e., spiral types and the stator type, depending on the shapes of mixing elements. The spiral type static mixer is preferably used because the injected diisocyanate-containing aliphatic polyester prepolymer has a high viscosity. In order to obtain a sufficient mixed state, it is preferable that the temperature be from 160 to 200° C. and the delivery rate from 5.0 to 27 cm/sec when the aliphatic polyester prepolymer is passed through the static mixer.
As the aliphatic polyester prepolymer is a fluid having a medium level of viscosity such as several hundred poises, the number of elements is must be 6 or more, preferably 12 or more, by which an excellent mixing state can be obtained.
In the present invention, after the aliphatic polyester prepolymer is mixed by the static mixer, it is delivered directly to coupling reaction tank 8, where it can be reacted, but mixing tank 7 may be provided for another mixing step.
When mixing tank 7 is used, the aliphatic polyester prepolymer mixed by the static mixer is quantitatively and continuously provided from the inlet of mixing tank 7 through pipe 10e. 
A vertical mixer may be used as mixing tank 7, and has a stirring shaft equipped with stirring blades. Stirring blades are categorized into propeller types, paddle types, turbine types, ribbon types, and special paddle types. These stirring members may be used in combinations of two or more thereof.
Further, one embodiment of mixing tank 7 will be explained with reference to FIG. 3. Mixing tank 7 in FIG. 3 is provided with inlet 7a for ingredients at its upper portion, outlet 7b for the mixture at its bottom portion, stirring shaft 7c, partitioning plate 7d, stirring blades 7e and 7f, and nitrogen inlet 7g. 
Inlet 7a is connected to pipe 10e. The injected diisocyanate-containing aliphatic polyester prepolymer is introduced from inlet 7a as shown by a dashed arrow. The introduced ingredient is a fluid having a high viscosity. It is delivered to the bottom portion under stirring. It is important to uniformly mix the ingredients so as to obtain a high-quality aliphatic polyester having an increased molecular weight. Therefore, partitioning plate 7d is provided to maintain an appropriate residence time so as to achieve sufficient stirring by the stirring blades. (Partitioning plate) 7d is fixed to (stirring shaft) 7c. 
Stirring blades are provided above and below partitioning plate 7d, 7e and 7f represent stirring blades in FIG. 3. Stirring blade 7e is a stirring blade arranged at an angle of 90° to the horizontal surface. 7f represents a stirring blade arranged at an angle of 45° or 135° to the horizontal surface. One or more stirring blades can be provided. It is preferable to use multistage blades as shown in FIG. 3 because efficient stirring can be realized.
It is important that the diisocyanate and the aliphatic polyester prepolymer are delivered at a constant rate in static mixer 6 and mixing tank 7.
Although retention times of the introduced ingredients are different, depending on the scale and the like of static mixer 6 and mixing tank 7, in general, the introduced ingredients are retained in static mixer 6 for from 5 to 30 seconds and mixing tank 7 for from 2 to 10 minutes and then discharged.
Nitrogen introduced from nitrogen inlet 7g is used for promoting discharge of the polymerization product in the mixing tank. The pressure in the mixing tank is adjusted by nitrogen to from 0.15 to 0.5 MPa, preferably from 0.2 to 0.4 MPa. The stirring blade rotational speed is adjusted to from 50 to 200 rpm.
The mixture uniformly blended in static mixer 6 and mixing tank 7 optionally provided is delivered to coupling reaction tank 8 through the pipe. A polymerization apparatus that may be used in transesterification reaction tank 4 or a high viscosity polymerization apparatus may be used as the coupling reaction tank. Examples of these polymerization apparatuses include vertical stirring tanks equipped with a stirring blade having a vertical rotation shaft (such as a double helical ribbon blade and a twisted lattice-shape blade). Ingredients can be reacted without stirring in coupling reaction tank 8. In that case, the stirring blades in the stirring tank are not rotated or a reaction tank without stirring blades may be used.
The reaction temperature in coupling reaction tank 8 is from 130 to 210° C., preferably from 160 to 200° C. If the reaction temperature is too low, as the prepolymer crystallizes and is not fluid, stirring cannot be achieved. If the temperature is too high, as the reaction rate becomes too fast, sufficient stirring cannot be achieved. At this step, in general, reaction is carried out for from 2 to 10 hours, preferably from 3 to 8 hours under conditions at normal pressure and at a temperature within the above range.
Nucleators, pigments, dyes, heat stabilizers, antioxidants, weather resistant agents, lubricants, antistatic agents, fillers, reinforcing agents, flame retardants, plasticizers, other polymers, and the like may be added after completion of the reaction, as necessary.
The aliphatic polyester having an increased molecular weight produced by the above method is delivered to pelletizer 9 through pipe 10g, where it is cut into a desired shape (for example, pellets).
As the aliphatic polyester having an increased molecular weight has a high viscosity, pipe 10g is equipped with pump 11d. A pump similar to pump 11b may be used as pump 11d. 
The aliphatic polyesters having an increased molecular weight obtained by the production method of the present invention can be formed into films, sheets, fibers, foam bodies, and other molded articles by conventional molding methods such as mold injection, hollow molding, extrusion, and the like. In addition, as the aliphatic polyesters having an increased molecular weight obtained by the production method of the present invention are biodegradable, they are suitably used in garbage bags, agricultural films, cosmetic containers, detergents, and the like, fishing lines, fishnets, ropes, suture threads, food wrapping materials, medical containers, and the like.
EXAMPLES
The present invention will be explained in more detail below with reference to examples, but it is not limited thereto.
Molecular weight determination is carried out by the GPC analysis below.
Shodex GPC SYSTEM-11 (manufactured by SHOWA DENKO K.K.)
Eluent: CF3COONa 5 mM/HFIP (hexafluoroisopropanol)
Sample column: HFIP-800P and HFIP-80M×two sets
Reference column: HFIP-800R×two sets
Polymer solution: 0.1 wt % HFIP sol., 200 μl
Column temperature: 40° C., flow rate: 1.0 ml/min, pressure: 30 kg/cm2 
Detector: Shodex RI
Molecular weight standard: PMMA (Shodex STANFARD M-75)
MFR (melt flow rate) determination was carried out, in accordance with JIS-K-7210, at a temperature of 190° C. under the load of 2.16 kg.
Gel and/or FE (gelation and/or fish eyes) were determined by the following steps. A film having a thickness of 30 μm was formed by inflation molding at a molding temperature of 180° C. The film was cut into 50 cm×50 cm squares. Any Gel and/or FE (with a size of 0.2 mm or larger) in a 30 cm×30 cm center space of the cut film were counted visually. Ten Gel and/or FE portion with a size of from 0.2 to 0.5 mm were evaluated as one Gel and/or FE portion with a size of 0.5 mm or larger. Samples were evaluated to be assigned with any of five grades determined based on the inequalities below.

Ns=(N×25)/T 

T: film thickness (μm), N: number of Gel and/or FE portion with a size of 0.5 mm or larger

Evaluation 1: Ns<1

    
    
        2: 1≦Ns<3
        3: 3≦Ns<10
        4: 10≦Ns<20
        5: 20≦Ns
    
    


Regarding the determination of foreign substances, substances (such as black spots) in 200 g of resin pellets were confirmed visually and were evaluated to be assigned with any of grades A to E, depending on the size and number thereof.
A: 10 or less foreign substances with a size of 0.1 mm or less, 5 or less foreign substances with a size of more than 0.1 and less than 0.5 mm, 1 or less foreign substances with a size of 0.5 mm or more
B: 20 or less foreign substances with a size of 0.1 mm or less, 10 or less foreign substances with a size of more than 0.1 and less than 0.5 mm, 2 or less substances with a size of 0.5 mm or more
C: 30 or less foreign substances with a size of 0.1 mm or less, 20 or less foreign substances with a size of more than 0.1 and less than 0.5 mm, 3 or less foreign substances with a size of 0.5 mm or more
D: 31 or more foreign substances with a size of 0.1 mm or less, 50 or less foreign substances with a size of more than 0.1 and less than 0.5 mm, 3 or less foreign substances with a size of 0.5 mm or more
E: 31 or more foreign substances with a size of 0.1 mm or less, 51 or more foreign substances with a size of more than 0.1 and less than 0.5 mm, 4 or more foreign substances with a size of 0.5 mm or more
Example 1
An aliphatic polyester was produced, in accordance with the production flow chart shown in FIG. 1 (where mixing tank 7 is not included).
(Esterification Reaction)
All the ingredients, i.e., 3589 kg of 1,4-butanediol (39.8×103 mol: glycol excess ratio of 104.4%) supplied from glycol storage tank 2 using a centrifugal pump, 4500 kg of succinic acid (38.1×103 mol) from weighing hopper 1, and 810 g of titanium tetraisopropoxide from a catalyst supply tank were charged into a vertical stirring tank with a jacket equipped with paddle blades (esterification tank 3) under a nitrogen atmosphere. The ingredients were stirred at a temperature of 145 to 225° C. at normal pressure for carrying out the esterification reaction. Water formed in the esterification reaction was distilled. When the amount of distilled water was greater than 1390 kg, the pressure in the tank was reduced to 60 mmHg (8000 Pa) and then was maintained for one hour to complete the esterification reaction. The reactant solution was delivered to a vertical stirring tank with a jacket equipped with ribbon blades (transesterification reaction tank 4). The delivered reactant solution was stirred at a temperature of 225 to 240° C. The pressure in the tank was finally reduced to 2 mmHg (267 Pa). After 10 hours, cooling was started. Reduction in pressure was stopped when the temperature reached 190° C. and the inside of the tank was put under a nitrogen atmosphere. Then, 3.28 kg of IRGANOX 1010 (manufactured by BASF: hindered phenol-based antioxidant) was added, the ingredients were cooled to 180° C., and 1.05 kg of phosphorous acid was added to conduct glycol removal reaction (transesterification reaction). The average molecular weight of the product determined by GPC analysis was 10,600.
(Coupling Reaction Step)
The aliphatic polyester prepolymer obtained at the transesterification reaction step above was delivered into static mixer 6 (manufactured by NORITAKE CO., LIMITED: 2 1/2(4)-N10ES-532-2 (WN)) at a constant flow rate (3250 kg/h) (total amount of 6500 kg) by a gear pump (pump 11b) at a temperature of 180° C. Pipe 10c was filled with the aliphatic polyester prepolymer and there were no voids in the pipe. Hexamethylene diisocyanate was injected into the pipe at a flow rate of 35.15 kg/h (OH/NCO=1/0.68: total amount of 70.3 kg) by a non-pulsating pump (pump 11c) to be delivered to static mixer 6. The ingredients were agitated and stirred in static mixer 6 and were delivered into coupling reaction tank 8. The coupling reaction by mixing was further conducted for 7 hours at 180° C. under stirring with helical ribbon blades as long as the blades could rotate.
After completion of the reaction, the resultant product was delivered into an extruder by a gear pump and pelletized by a pelletizer 9 to obtain pellets of an aliphatic polyester having an increased molecular weight.
MFR, Gel, and FE values obtained after a series of polymerization steps was repeated ten times without disassembling and cleaning of the reaction apparatus are shown in Table 1.
Example 2
Pellets of an aliphatic polyester having an increased molecular weight were obtained in the same manner as in Example 1 except that coupling reaction tank 8 without stirring blades was used and the reaction in the reaction tank was conducted without stirring.
Example 3
Pellets of an aliphatic polyester having an increased molecular weight were obtained in the same manner as in Example 1 except that the aliphatic polyester prepolymer delivered from static mixer 6 was further delivered to mixing tank 7 for agitation and stirring. Mixing tank 7 was provided with three partitioning plates and four stirring blades. The rotational speed of the stirring blades was 100 rpm. The time required for the solution to be passed through mixing tank 7 was three minutes.
Example 4
Pellets of an aliphatic polyester having an increased molecular weight were obtained in the same manner as in Example 3 except that coupling reaction tank 8 without stirring blades was used and the reaction in the reaction tank was conducted without stirring.
Comparative Example 1
A summary of the production steps of Comparative example 1 is shown in FIG. 2. Steps before the transesterification reaction were the same as in Example 1. In Comparative example 1, there was no static mixer 6 or mixing tank 7 and diisocyanate storage tank 5 was directly connected to coupling reaction tank 8 via pipe 10d. 
In Comparative example 1, the esterification reaction step and the transesterification reaction were completed as in Example 1. The obtained aliphatic polyester prepolymer was delivered into coupling reaction tank 8 (delivery rate: 3,000 kg/hr, total amount: 6500 kg) in accordance with the flow chart shown in FIG. 2. After completion of delivery, the predetermined amount (70.3 kg) of hexamethylene diisocyanate was injected from the upper portion (vapor phase) of the coupling tank. The ingredients were agitated and stirred by helical ribbon blades, were maintained at 180° C. for 8 hours, and then were pelletized by pelletizer 9 to obtain an aliphatic polyester having an increased molecular weight.
The diisocyanate measured was charged into diisocyanate storage tank 5 and was delivered while being pressurized with nitrogen. (As it could be delivered by adjusting the pressure of nitrogen, the diisocyanate could be quantitatively delivered without using a pump.)
MFR, Gelation, and FE values obtained after a series of the polymerization steps was repeated ten times are shown in Table 1.









TABLE 1








Example 1
Example 2













Batch
MFR
Gel.
Foreign
MFR
Gel.
Foreign


No.
(g/10 min)
FE
matter
(g/10 min)
FE
matter
















1
1.4
1
A
1.5
1
A


2
1
1
A
1.8
1
A


3
1.6
1
A
1.2
1
A


4
1.2
1
A
0.8
1
A


5
1.8
1
A
1
1
A


6
0.9
1
B
0.9
1
A


7
1.3
2
B
1.4
2
A


8
0.8
2
A
1.6
1
A


9
1
2
B
1.2
1
B


10
1.4
1
A
1.1
2
B


Average
1.24


1.25




S.D.
0.32


0.32





















TABLE 2










Comparative



Example 3
Example 4
example 1

















MFR

For-
MFR

For-
MFR

For-


Batch
(g/10
Gel.
eign
(g/10
Gel.
eign
(g/10
Gel.
eign


No.
min)
FE
matter
min)
FE
matter
min)
FE
matter





 1
1.3 
1
A
0.8 
1
A
3  
1
A


 2
0.9 
1
A
1.3 
1
A
2.2 
1
A


 3
1.2 
1
A
1.5 
1
A
2.8 
1
A


 4
0.8 
1
B
1.2 
1
A
0.5 
2
B


 5
0.8 
2
B
1  
1
A
1.8 
2
A


 6
1.2 
1
A
1.2 
1
A
2.5 
3
C


 7
1.1 
1
A
0.8 
1
A
2.8 
5
C


 8
1.4 
2
A
1.3 
2
B
1.6 
5
D


 9
0.8 
1
B
0.9 
1
A
3.1 
5
C


10
0.9 
2
B
1.4 
2
B
1.9 
5
D


Average
1.04


1.14


2.22




S.D.
0.23


0.25


0.80









The desired value of MFR is 1 g/10 min. The level of distribution of values was less and the average value of 10 batches was relatively close to the desired value in the examples, while the level of distribution was significant and the average value was remarkably different from the desired value in the Comparative example. In particular, it is indicated that the standard deviations were small and sufficiently controlled MFRs were obtained in Examples 3 and 4 using mixing tank 7.
Further, there is a tendency for the evaluation grades on Gel, FE, and foreign substance to become poorer as the batch number increased. This indicates that it was difficult to maintain the quality of the polymer when produced continuously.
EXPLANATION OF SYMBOLS


    1: Weighing hopper; 2: glycol storage tank, 3: esterification tank, 4: transesterification reaction tank, 5: diisocyanate storage tank, 6: static mixer, 7: mixing tank, 7a: inlet, 7b: outlet, 7c: stirring shaft, 7d: partitioning plate, 7e and 7f: stirring blades, 7g: nitrogen inlet, 8: coupling reaction tank, 9: pelletizer, 10a-g: pipes, 11a-d: pumps

## Claims

1. A method for producing an aliphatic polyester having an increased molecular weight comprising the steps of:
(i) quantitatively injecting a diisocyanate into an aliphatic polyester prepolymer which has a number average molecular weight of 5000 or higher and has terminal hydroxyl groups and in which at least one acid component is a succinic acid compound, in a molten state at a temperature not lower than the melting point thereof, wherein
the diisocyanate comprises isocyanate groups in an amount equivalent to between one tenth and two times the amount of the hydroxyl groups in the aliphatic polyester prepolymer;
(ii) quantitatively and continuously mixing the diisocyanate-containing aliphatic polyester prepolymer obtained at step (i) by a static mixer, discharging same from the static mixer, and then supplying same to a coupling reaction tank, and
(iii) reacting the aliphatic polyester prepolymer with the diisocyanate in the coupling reaction tank.

2. The method for producing an aliphatic polyester having an increased molecular weight according to claim 1,
wherein a mixing tank is further provided between the static mixer and the coupling reaction tank and the aliphatic polyester prepolymer of (ii) discharged from the static mixer is introduced into the mixing tank and is further discharged from the mixing tank under stirring to be supplied into the coupling reaction tank.

3. The method for producing an aliphatic polyester having an increased molecular weight according to claim 2,
wherein the mixing tank in (ii) has an inlet of the diisocyanate-containing aliphatic polyester prepolymer in its upper portion, an outlet for discharging a mixture of the aliphatic polyester prepolymer with the diisocyanate in its bottom portion, and at least one partitioning plate and stirring blade.

4. The method for producing an aliphatic polyester having an increased molecular weight according to claim 3,
wherein said at least one partitioning plate and stirring blade are spaced one above the other.

5. The method for producing an aliphatic polyester having an increased molecular weight according to claim 3,
wherein the stirring blade is a multistage blade.

6. The method for producing an aliphatic polyester having an increased molecular weight according to claim 1,
wherein the diisocyanate is injected by a non-pulsating pump at step (i) above.

7. The method for producing an aliphatic polyester having an increased molecular weight according to claim 1,
wherein the reaction at step (iii) above is conducted under stirring.

8. The method for producing an aliphatic polyester having an increased molecular weight according to claim 7,
wherein the reaction at step (iii) above is conducted under stirring by a helical ribbon blade or a twisted lattice-shape blade.

9. The method for producing an aliphatic polyester having an increased molecular weight according to claim 1,
wherein the reaction at step (iii) above is conducted without stirring.

10. The method for producing an aliphatic polyester having an increased molecular weight according to claim 1,
wherein at least one polyalcohol component in the aliphatic polyester is ethylene glycol.

11. The method for producing an aliphatic polyester having an increased molecular weight according to claim 1,
wherein at least one polyalcohol component in the aliphatic polyester is 1,4-butanediol.