KR20160060296A - Method for removal of lactide from polylactide - Google Patents

Abstract

According to the present invention, disclosed is a method for producing polylactic acid through ring-opening polymerization by using lactide as a main ingredient. Production process simplicity, efficiency, and mass-production properties are ensured, and polylactic acid having high molecular weight and high purity is produced. In particular, non-reacted residual lactide may be effectively removed in a short time. The method for producing polylactic acid through ring-opening polymerization by using lactide as a main ingredient comprises the steps of: (a) sequentially dissolving, mixing, and polymerizing the lactide; cooling polymerized polylactic acid and making the same into pellets; and (c) removing residual lactide from the pellets of polylactic acid at a temperature which is lower than a polymerization temperature of step (a). Step (c) comprises a step of simultaneously removing the residual lactide while a temperature is increased and pressure is decreased.

Description

METHOD FOR REMOVAL OF LACTIDE FROM POLYLACTIDE BACKGROUND OF THE INVENTION < RTI ID = 0.0 > [0001]

The present invention relates to a method for producing polylactic acid, and more particularly, to a method for efficiently removing residual lactide after polylactic acid polymerization.

Polylactic acid (PLA) can be produced by a polycondensation reaction which is a production method of a polyester or ring-opening polymerization from a cyclic lactone (dimer). Ring-opening polymerization of cyclic lactones of hydroxy acids such as lactide and the like is already known in the art.

In ring-opening polymerization of lactide, there is a polymerization equilibrium between the lactide monomer and the poly (lactic acid) polymer. The equilibrium shifts toward the lactide monomer at high temperature and toward the poly (lactic acid) polymer at low temperature. Therefore, the lactide monomer in the reaction mixture necessarily remains at a high temperature of 150 ° C or higher. In addition, when the polymerization is carried out at a low temperature, the lactide monomer may be eliminated, but if it takes a considerably long period of time, a high molecular weight polylactic acid can be obtained.

On the other hand, as a method for producing high molecular weight poly (lactic acid), a melt polymerization method is generally used. However, this melt polymerization method has several disadvantages. For example, the melt viscosity of the prepolymer sharply increases at a high conversion rate, and it is very difficult to uniformly mix the high viscosity poly (lactic acid) with stirring. It also has the disadvantage of causing hot spots along the walls of the reaction vessel. In addition, the polylactic acid produced in this manner requires additional cooling and molding steps. In such a process, the lactide remaining after the reaction causes loss and contamination of the polylactic acid, thereby lowering the production yield in the manufacturing process .

Japanese Patent Registration No. 3055422 discloses a method in which L-lactide and a catalyst are put into a reaction tank to carry out polymerization, and when a polymerization reaction proceeds over a certain period of time, polylactic acid is supplied to cool the polymerized product, There is a problem in that the polymerization process including solid phase polymerization and unreacted monomer removal step takes a long time of 100 hours or more.

Japanese Patent Registration No. 3589333 discloses a method in which a polylactic acid pellet is introduced into a crystallizer from a raw material feeder to crystallize the polylactic acid pellet first and the crystallized polylactic acid pellet is sent to a reactor in the form of a hopper, But the process time including the crystallization time and the removal of the residual monomer is about 30 hours, which is also a long time.

The present invention relates to a process for producing poly (lactic acid) by ring-opening polymerization using lactide as a main raw material, which is capable of producing poly (lactic acid) having high molecular weight and high purity with simplicity, efficiency and mass productivity of the production process, And a method for efficiently removing lactide in a short period of time.

In order to solve the above problems, the present invention provides a method for producing polylactic acid by ring-opening polymerization using lactide as a main raw material, comprising the steps of: (a) mixing and mixing the lactide; (b) cooling and pelletizing the polymerized poly (lactic acid); And (c) removing residual lactide from the pelletized poly (lactic acid) at a temperature lower than the polymerization temperature of the step (a), wherein the step (c) And removing the lactide at the same time. The present invention also provides a method for producing polylactic acid.

Wherein the elevated temperature is from 130 to 180 ° C at room temperature and the reduced pressure is from 0.01 to 2 torr at normal pressure.

And the temperature is raised at a rate of 1 to 10 ° C / min.

The decompression is performed at a pressure of 100 to 300 torr at a pressure of 10 to 30 torr / min and at a pressure of 30 to 50 torr to a pressure of 0.01 to 2 torr.

And the pelletized poly (lactic acid) has an average particle diameter of 1 to 50 mm.

And the polylactide in which the remaining lactide is removed has a weight average molecular weight of 50,000 to 500,000.

And the remaining lactide-free polyactic acid has a residual lactide content of 0.5 wt% or less.

The method for producing poly (lactic acid) according to the present invention is a method for producing poly (lactic acid) by ring-opening polymerization using lactide as a main raw material, comprising the steps of removing residual lactide from the solid phase, The present invention can provide a method for effectively removing residual lactide in an amount of 0.5 wt% or less in a short period of time.

Hereinafter, preferred embodiments of the present invention will be described in detail. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. Throughout the specification, when an element is referred to as "including " an element, it means that it can include other elements, not excluding other elements, unless specifically stated otherwise.

The inventors of the present invention have found that, in the method of producing poly (lactic acid) by ring-opening polymerization using lactide as the main raw material, the problem that takes a long time to remove residual lactide purity is observed, The residual lactide can be efficiently removed in a short period of time when the residual lactide is simultaneously removed.

Accordingly, the present invention provides a process for producing poly (lactic acid) by ring-opening polymerization using lactide as a main raw material, comprising the steps of: (a) (b) cooling and pelletizing the polymerized poly (lactic acid); And (c) removing residual lactide from the pelletized poly (lactic acid) at a temperature lower than the polymerization temperature of the step (a), wherein the step (c) And removing the lactide at the same time.

The step (a) may be a polylactic acid polymerization step, using lactide and a catalyst in polylactic acid polymerization, and optionally using an initiator.

Examples of the lactide include L-lactide composed of L-lactic acid, D-lactide composed of D-lactic acid, meso-lactide composed of one L-form and one D- D, L-lactide (or rac-lactide) in which 50:50 tide is mixed can be used.

If the raw material lactide contains water, it may affect the degree of polymerization during polymerization. Therefore, it is preferable to use lactide from which moisture has been removed by drying. Accordingly, it is preferable that the lactide supplied in step (a) is firstly dried under an inert gas or dry air, then mixed with the catalyst, and melted at a temperature of 100 to 120 ° C before being supplied.

Examples of the catalyst include lead oxide, calcium oxide, aluminum oxide, iron oxide, calcium chloride, zinc acetate, paratoluene sulfonic acid, stannous chloride, stannous sulfate, stannous oxide, stannic oxide, 1-tin, tetraphenyltin, tin powder, titanium tetrachloride and the like can be used, and tin compounds such as tin octylate can be preferably used. The content of the catalyst is preferably 0.0005 to 5 parts by weight, more preferably 0.001 to 1 part by weight based on 100 parts by weight of lactide.

The initiator is a substance that initiates a polymerization reaction. Non-limiting examples of the initiator include alcohols and the like, and the content thereof is not particularly limited and may be in a molar ratio of 1: 0.00005 to 1: 0.1 based on the lactide monomer.

The temperature of the polymerization reaction is not particularly limited, but it is preferably about 170 to 210 ° C, more preferably 180 to 200 ° C because polylactic acid can be formed through a smoother polymerization reaction.

The reaction tank that can be used in the polymerization reaction is not particularly limited, such as stirring type or tubular shape, and a reaction vessel having an agitator such as an impeller, a spider impeller, a paddle impeller or the like may be used. Such a reaction tank may be composed of two or more stages, and may be composed of a stirrer or a reaction tank having a reaction form depending on the viscosity.

The polymerized poly (lactic acid) may have a weight average molecular weight of 20,000 to 200,000, and the residual monomer content may be generally 0.5 to 15% by weight, more usually 1 to 10% by weight, based on the poly (lactic acid).

The step (b) is a step of pelletizing the poly (lactic acid) polymerized in the step (a), and the poly (lactic acid) obtained by the polymerization is cooled in a water tank or the like, or a water cooler such as a belt cooler or a drum cooler It is preferable to pelletize it through a cooling step which does not come into contact. At this time, the poly (lactic acid) is cooled to a glass transition temperature (Tg) or less and then cut to a certain size, but it is preferable to cut the poly (lactic acid) to a size of 1 to 50 mm.

The step (c) is a step of removing lactide remaining in the polymerization process. In the present invention, crystallization, drying, and residual lactide removal of polylactic acid can be performed simultaneously.

That is, the residual lactide can be removed by rotating and vacuuming equipment such as a rotary vacuum dryer, so that crystallization and drying of the poly (lactic acid) can proceed while the lactide remaining on the pellets is removed. Such residual monomer removal equipment may be used in any form such as a stirred tank, a mixer, and a tumble dryer. On the other hand, it is possible to prevent fusion problems which may occur in the poly (lactic acid) crystallization by installing a blocking plate in the equipment.

In the present invention, the removal of the residual lactide is preferably carried out at a temperature equal to or higher than the melting temperature of the residual monomer but lower than the melting temperature of the poly (lactic acid) in order to reduce the chromaticity and molecular weight due to the residual monomer in the polymerized poly (lactic acid). Specifically, the removal of the residual lactide may be carried out at a temperature of 100 to 180 ° C, preferably 120 to 160 ° C. In this case, in order to remove residual lactide at a low level in a short time, Tide removal is performed.

That is, in the present invention, at the same time, the residual lactide removal is performed at the same time in a state where at least the temperature increase and the pressure decrease are performed.

The temperature can be raised from 130 to 180 ° C at normal temperature, preferably from 150 to 170 ° C at 40 to 60 ° C, and the temperature raising rate is set to 1 to 10 ° C / min, preferably 2 to 6 ° C / min . The decompression may be performed at a pressure of 0.01 to 2 Torr, preferably 0.01 to 1 Torr, at a pressure of 100 to 300 Torr at a pressure of 10 to 30 Torr / min and at a pressure of 0.1 to 2 Torr at a pressure of 30 to 50 Torr , It is desirable to decompress the faster speed in the latter half.

The residual lactide removal process may be continued at the final temperature and the depressurized state after the temperature elevation and decompression are simultaneously performed, and the residual lactide may be removed simultaneously with the temperature increase and the depressurization, It is possible to produce a polylactic acid having a high purity with a residual lactide content of 0.5 wt% or less even in a short time of 10 hours, preferably 1 to 5 hours, more preferably 1 to 3 hours, and most preferably 1 to 2 hours .

In the present invention, the weight average molecular weight of the polylactic acid from which the residual lactide is removed may be 50,000 to 500,000, and preferably 100,000 to 300,000.

Example  One

4 kg of vacuum-dried L-lactide (Mushashino chemical lab.) Was charged into the reactor and nitrogen and water and oxygen remaining in the monomer and the reactor were removed. The reactor was then heated with stirring at 80 rpm to melt the lactide. Alcohol (lactide weight: initiator weight ratio = 400: 1) was added as a catalyst and 30 ppm of tin octylate as a catalyst. Thereafter, the reactor was heated at 185 DEG C with stirring at 100 rpm to effect polymerization. Upon reaching the constant torque value, the reaction was terminated and the polylactic acid was pelletized and the molecular weight was analyzed. As a result, polylactic acid having a weight average molecular weight of 129,665 was obtained, and the residual monomer was 4.2% by weight. The polymerized poly (lactic acid) was then introduced into a reactor capable of temperature control and stirring. The reactor was stirred at 10 rpm and the temperature was raised from 50 ° C. to 150 ° C. at a rate of 4 ° C./min. The pressure was reduced from 760 torr to 200 torr at a rate of 20 torr / min and the pressure was reduced to 40 torr / Respectively. Thereafter, the removal process was performed for a total of 1 hour while maintaining the final temperature and the reduced pressure. The final residual monomer was 0.48 wt%, and the weight average molecular weight was 124,325.

Example  2

The polylactic acid was prepared in the same manner as in Example 1, except that the removal step was carried out for a total of 2 hours while maintaining the temperature elevation and decompression state in Example 1. The final residual monomer was 0.24 wt%, and the weight average molecular weight was 122,970.

Example  3

The polylactic acid was prepared in the same manner as in Example 1, except that the removal step was carried out for 3 hours in total while maintaining the temperature elevation and decompression state in Example 1. The final residual monomer was 0.07% by weight and the weight average molecular weight was 120,576.

Comparative Example  One

The residual monomer removal process was carried out at a reduced pressure of 40 torr / min from room temperature and from 760 torr to 0.5 torr without heating in Example 1, and then the removal process was carried out for a total of 3 hours while maintaining the final reduced pressure state. Polylactic acid was prepared in the same manner as in Example 1. The final residual monomer was 4.0% by weight, and the weight average molecular weight was 128,710.

Comparative Example  2

A polylactic acid was prepared in the same manner as in Example 3, except that the residual monomer removal step was carried out in the state where the temperature was raised to 150 ° C. in Example 3. The final residual monomer was 1.64% by weight, and the weight average molecular weight was 117,534.

Comparative Example  3

A polylactic acid was prepared in the same manner as in Example 3, except that the residual monomer removal step was carried out at a temperature elevated to 110 ° C in Example 3. The final residual monomer was 2.21% by weight, and the weight average molecular weight was 113,467

From the results of the above Examples and Comparative Examples, it can be seen that when the present invention includes a step of removing residual lactide in a solid phase while simultaneously removing residual lactide in a state where the temperature is elevated and the pressure is decreased, a residual lactide It can be confirmed that polylactic acid having a content reduced to 0.5 wt% or less, preferably 0.1 wt% or less can be produced. At this time, even if the removal time is increased at the final temperature elevation and depressurized state (Example 3), the residual monomer removal efficiency is not increased, so that simultaneously removing the remaining lactide in the state where the temperature rise and the depressurization are performed More important than anything else.

On the other hand, when the residual monomer removal process is performed only in the reduced pressure (Comparative Example 1) or the residual monomer removal process is performed while the reactor temperature is raised under the reduced pressure (Comparative Examples 2 and 3), it has a high molecular weight, Level is not remarkable in the case of simultaneous execution in the state of elevated temperature and reduced pressure (in the embodiment).

The preferred embodiments of the present invention have been described in detail above. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing detailed description, and all changes or modifications derived from the meaning, range, and equivalence of the claims are included in the scope of the present invention Should be interpreted.

Claims (7)

A method for producing poly (lactic acid) by ring-opening polymerization using lactide as an essential material,
(a) mixing and polymerizing the lactide;
(b) cooling and pelletizing the polymerized poly (lactic acid); And
(c) removing residual lactide from the pelletized poly (lactic acid) at a temperature lower than the polymerization temperature of step (a)
Wherein the step (c) comprises simultaneously performing the removal of the remaining lactide in a state where the temperature is elevated and the pressure is reduced. The method according to claim 1,
Wherein the elevated temperature is from 130 to 180 DEG C at room temperature and the reduced pressure is from 0.01 to 2 torr at normal pressure. 3. The method of claim 2,
Wherein the elevated temperature is performed at a rate of 1 to 10 DEG C / min. 3. The method of claim 2,
Wherein the decompression is carried out at a pressure of 100 to 300 torr at a pressure of 10 to 30 torr / min and at a pressure of 30 to 50 torr to a pressure of 0.01 to 2 torr. The method according to claim 1,
Wherein the pelletized poly (lactic acid) has an average particle diameter of 1 to 50 mm. The method according to claim 1,
Wherein the residual lactide-free poly (lactic acid) has a weight average molecular weight of 50,000 to 500,000. The method according to claim 1,
Wherein the remaining lactide-free polylactic acid has a residual lactide content of 0.5 wt% or less.