Method for producing polylactic acid polymer

Abstract

The present invention relates to a method for producing a polylactic acid polymer, and more specifically, to a method for producing a polylactic acid polymer having a desired high molecular weight by a lactide ring-opening polymerization reaction using a specific catalyst.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a polylactic acid polymer, and more particularly to a method for producing a polylactic acid polymer having a desired high molecular weight by using a specific catalyst in a lactide ring-opening polymerization reaction. [Background technology]

[0002] Polylactic acid (PLA) is a plant-derived resin obtained from plants such as corn, and has attracted attention as an environmentally friendly material with biodegradable properties and excellent tensile strength and elastic modulus.

[0003] Unlike conventional petroleum-based resins such as polystyrene resin, polyvinyl chloride resin, and polyethylene, polylactic acid has the effect of preventing the depletion of petroleum resources and reducing carbon dioxide emissions, thereby reducing the environmental pollution that is a disadvantage of petroleum-based plastic products. Therefore, as the issue of environmental pollution caused by waste plastics has emerged as a social problem, efforts are being made to expand the scope of application to product areas that use general plastics (petroleum-based resins), such as food packaging materials and containers and electronic product cases.

[0004] Meanwhile, polylactic acid is produced by polymerizing lactic acid produced by microbial fermentation, but direct polymerization of lactic acid only produces a low molecular weight polymer. To synthesize high molecular weight polylactic acid, one method involves polymerizing the low molecular weight polylactic acid obtained by direct polymerization of lactic acid into a higher molecular weight polylactic acid using a chain coupling agent. However, this process is complicated and requires both a coupling agent and an organic solvent, which are difficult to remove.

[0005] Currently, the commercial production process for high molecular weight polylactic acid is a chemical synthesis method in which lactic acid is converted into lactide and polylactic acid is synthesized through a lactide ring-opening reaction. However, even in this case, it is difficult to achieve a high molecular weight suitable for commercialization.

[0006] The Sn(Oct)2 catalyst, which is typically used to achieve high molecular weight, has the following problems: it is a liquid catalyst with high viscosity, making it difficult to accurately measure, and it is vulnerable to oxygen and moisture, making the catalyst itself unstable. In particular, the Sn(Oct)2 catalyst decomposes rapidly at high temperatures, which adversely affects the color of the resin produced and reduces its economic viability.

[0007] Therefore, there is a need to develop a method for producing polylactic acid polymers that have excellent physical properties and can be easily made into high molecular weight polymers so that they can be applied to a variety of industries. Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention provides a method for producing a polylactic acid polymer having a desired high molecular weight by using a specific catalyst in a lactide ring-opening polymerization reaction and controlling the rate of change in molecular weight of the polymer due to changes in polymerization temperature within a specific range.

[0009] In addition, the present invention provides a method for producing polylactic acid polymers that uses a catalyst that exhibits excellent stability even under high-temperature polymerization reaction conditions, making it easy to remove and reuse the catalyst after the reaction is completed, and is economical, and does not cause problems such as changes in the physical properties of the polymer due to catalyst decomposition. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention provides a method for producing a polylactic acid polymer, which includes the following steps:

[0011] First, the method includes a step of producing a polylactic acid polymer by ring-opening polymerization of lactide in the presence of a SnHPO catalyst, The rate of change in weight average molecular weight defined by the following mathematical formula 1 has a positive value. [Mathematical formula 1] Change rate of weight average molecular weight = (M (t+30) -M (t) ) / M (t) *100(%) where M (t) is the weight average molecular weight of the polylactic acid polymer produced by ring-opening polymerization at temperature t°C, M (t+30) is the weight average molecular weight of the polylactic acid polymer produced by ring-opening polymerization at a temperature of t+30°C, t is between 150°C and 250°C.

[0012] Throughout this specification, unless otherwise specified, the terms "comprise" or "contain" refer to the inclusion of a certain component (or ingredient) without any particular limitation, and should not be interpreted as excluding the addition of other components (or ingredients).

[0013] (Method of producing polylactic acid polymer) Currently, the commercial production process for polylactic acid uses a chemical synthesis method that synthesizes polylactic acid through a lactide ring-opening reaction. However, achieving a high molecular weight suitable for commercialization is difficult. In particular, the Sn(Oct)2 catalyst used to achieve high molecular weight has problems: it is a liquid catalyst with high viscosity, making it difficult to accurately measure, and it is vulnerable to oxygen and moisture, making the catalyst itself unstable. In particular, the Sn(Oct)2 catalyst decomposes rapidly at high temperatures, which adversely affects the color of the resin produced and reduces its economic viability.

[0014] The present inventors have therefore confirmed that by using a SnHPO catalyst with excellent high-temperature stability in the ring-opening polymerization of lactic acid oligomer and controlling the reaction conditions, the catalytic activity does not decrease even under high-temperature conditions, and high-molecular-weight polylactic acid polymers can be easily produced, thereby completing the present invention. Furthermore, the catalyst is easy to remove and reuse after the reaction is complete, making it economical.

[0015] According to one embodiment of the present invention, the method includes the step of preparing polylactic acid polymer by ring-opening polymerization of lactide in the presence of SnHPO3 catalyst.

[0016] The SnHPO3 catalyst is Tin(II) Phosphite, which is a phosphite anion (PO3 - The SnHPO3 catalyst is a tin (Sn) salt of SnHPO3. The SnHPO3 catalyst does not decompose even under the high temperature conditions required for the ring-opening polymerization of lactide, making it easy to remove and reuse after the reaction is complete, resulting in excellent economic efficiency. In addition, the SnHPO3 catalyst is a solid powder-type heterogeneous catalyst, making it easy to use.

[0017] The SnHPO catalyst may be contained in an amount of 100 ppmmol to 1,000 ppmmol relative to lactide, preferably 150 ppmmol to 900 ppmmol, 300 ppmmol to 700 ppmmol, or 500 ppmmol to 700 ppmmol. Within the above ranges, ring-opening polymerization can be promoted while suppressing the generation of by-products.

[0018] The ring-opening polymerization may be carried out at a temperature of 150°C or higher, preferably 150°C to 250°C or 150°C to 240°C. Even when using a SnHPO3 catalyst within this temperature range, the polymerization reaction can be easily carried out without a change in catalyst activity. Furthermore, by carrying out the ring-opening polymerization within this temperature range, polylactic acid polymers having the desired high molecular weight can be easily formed and the generation of by-products can be minimized. Furthermore, polylactic acid polymers having the desired high molecular weight can be easily produced without decomposition of the SnHPO3 catalyst or a change in polymer color. On the other hand, if the reaction temperature is lower than 150°C, the activity of the ring-opening polymerization may be reduced.

[0019] More preferably, the ring-opening polymerization may be carried out at a temperature of 160° C. or higher, 170° C. or higher, 180° C. or higher, or 240° C. Within the above range, a polylactic acid polymer having the desired high molecular weight can be easily produced without the above-mentioned problems.

[0020] The ring-opening polymerization reaction is carried out in the presence of a SnHPO3 catalyst, and in particular, the rate of change in weight average molecular weight defined by the following mathematical formula 1 has a positive value. [Mathematical formula 1] Change rate of weight average molecular weight = (M (t+30) -M (t) ) / M (t) *100(%) M (t) is the weight average molecular weight of the polylactic acid polymer produced by ring-opening polymerization at temperature t°C, M (t+30) is the weight average molecular weight of the polylactic acid polymer produced by ring-opening polymerization at a temperature of t+30°C, t is between 150°C and 250°C.

[0021] The rate of change is the rate of change in the weight-average molecular weight of polylactic acid produced by increasing the ring-opening polymerization reaction temperature by 30°C, and can be interpreted as an index that explains the degree of catalyst decomposition at high temperatures. A positive value for the rate of change means that the weight-average molecular weight of the polymer increases with an increase in polymerization reaction temperature. Theoretically, this means a rate of change greater than 0%, but in practice, it can mean about 1% or more, about 3% or more, or about 5% or more.

[0022] In the case of Sn(Oct)2, which has been used in conventional ring-opening polymerization, the catalyst decomposes at the high polymerization temperature of the ring-opening reaction, resulting in a decrease in the weight average molecular weight of the polymer, making it difficult to achieve the desired high molecular weight. However, the ring-opening polymerization reaction of the present invention solves this problem by providing a positive rate of change in weight average molecular weight as described above.

[0023] Preferably, the weight average molecular weight change rate may be 10% to 80%, 20% to 70%, or 40% to 60%. Within this range, even when the polymerization reaction is carried out at a high temperature, it is suitable for achieving excellent molecular weight without decomposition of the catalyst, and is economical because the catalyst can be reused.

[0024] The measurement of the weight average molecular weight of the polymer will be explained in more detail in the experimental examples to be described later.

[0025] Meanwhile, if necessary, before the ring-opening polymerization, the polymerization reactor may be purged to an inert condition. Specifically, after the reactor is connected, vacuum-argon purging may be performed about three times to purify the inside of the reactor to an inert condition.

[0026] If necessary, before the ring-opening polymerization, lactide and the SnHPO catalyst may be independently pretreated at 50°C to 70°C and 7 mbar to 10 mbar for 1 hour to 3 hours. This pretreatment step can remove oxygen, moisture, impurities, etc. from the lactide and the SnHPO catalyst.

[0027] (Polylactic acid polymer) According to one embodiment of the present invention, there is provided a polylactic acid copolymer produced by the above-mentioned production method.

[0028] The weight average molecular weight (Mw) of the polylactic acid polymer may be 20,000 to 50,000, and preferably 30,000 to 50,000.

[0029] The number average molecular weight (Mn) of the polylactic acid polymer may be 18,000 to 33,000, and preferably 25,000 to 33,000.

[0030] The polydispersity index (PDI) of the polylactic acid polymer may be 1.18 to 1.5, preferably 1.2 to 1.5.

[0031] The YI (yellow index) of the polylactic acid polymer is 13 to 33, preferably 13 to 31, or 13.3 to 30.3.

[0032] The methods for measuring the weight average molecular weight, number average molecular weight, polydispersity index and YI will be described in detail in the experimental examples below.

[0033] (Goods) According to another embodiment of the present invention, there is provided an article comprising the polylactic acid polymer. [Effects of the Invention]

[0034] As described above, the method for producing a polylactic acid polymer according to the present invention provides a method for producing a polylactic acid polymer having a desired high molecular weight by using a specific catalyst in the ring-opening polymerization reaction of lactide and controlling the rate of change in molecular weight of the polymer due to changes in polymerization temperature within a specific range.

[0035] Furthermore, by using a catalyst that exhibits excellent stability even under high-temperature polymerization reaction conditions, it is possible to produce a polylactic acid polymer that is economical, easy to remove and reuse the catalyst after the reaction is completed, and does not cause problems with changes in the physical properties of the polymer due to catalytic decomposition.

[0036] Furthermore, the polylactic acid polymer produced by the above method has a high molecular weight and is excellent in color change stability. [Brief explanation of the drawings]

[0037] [Figure 1] FIG. 1 shows the results of XRD analysis of the powder-type SnHPO3 catalyst obtained in Production Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0038] Hereinafter, the present invention will be described in more detail with reference to the following examples, but the following examples are merely illustrative of the present invention and are not intended to limit the scope of the present invention. [Example]

[0039] <Production example> Example 1 - SnHPO3, Tin(II) phosphite catalyst synthesis A three-neck round flask was placed in an oil bath, and 10 g of phosphorous acid (H3PO3) was added to the flask and dissolved in the solution by refluxing under nitrogen purging conditions and heating to 74°C. Then, 9.5 g of tin(II) oxide was added in 5 to 10 portions while heating the flask to 180°C and dissolving.

[0040] When the viscous mixture in the flask becomes transparent, turn off the oil bath and slowly cool to room temperature (24±1°C). Adding water to the flask precipitates a solid. The remaining H3PO3 is removed by washing with 5±2°C methanol and 5±2°C water, and the resulting solid powder catalyst is vacuum dried. The resulting powder catalyst is then subjected to XRD analysis, the results of which are shown in Figure 1.

[0041] The absence of a tin oxide peak in the XRD pattern confirmed the synthesis of the catalyst. A single phase of SnHPO3 was observed in the PLA catalyst, and neither the tin oxide nor H3PO3 crystalline phases used in the synthesis were observed. The crystal grain size of the SnHPO3 catalyst was approximately 150-165 nm.

[0042] <Examples and Comparative Examples> Example 1 After setting up a 0.5L lab-scale glass reactor, the inside of the reactor was replaced with Ar while maintaining Ar conditions overnight. A Viton O-ring was attached to a 500mL beaker, and lactide was added at approximately 30-40% (approximately 170g) of the beaker level. After fastening the reactor, a leak test was performed using a diaphragm pump and vacuum controller, maintaining 10mbar for 1 minute. After this, a vacuum-argon purge was performed approximately three times using a Schlenk line to replace the inside of the reactor with inert conditions. Finally, the reactor was replaced with Ar conditions.

[0043] The catalyst inlet (septum) was opened, and 500 ppmmol (equivalent to the amount of lactide added) (approximately 120 mg) of the SnHPO3 powder catalyst from Preparation Example 1 was weighed and added. Using a Schlenk line, the reactor was heated to 60°C (lower mantle 60°C) for approximately 2 hours under vacuum conditions to remove impurities. After replacing the reactor with Ar, the temperature was increased to 240°C (lower mantle 240°C, upper mantle 240°C). After the reaction temperature reached 240°C, the reaction was continued for 3 hours to produce polylactic acid polymer.

[0044] Example 2 A polylactic acid polymer was produced in the same manner as in Example 1, except that the polymerization reaction temperature was 210°C.

[0045] Example 3 A polylactic acid polymer was produced in the same manner as in Example 1, except that the polymerization reaction temperature was 180°C.

[0046] Comparative Examples 1 to 3 Polylactic acid polymer was prepared in the same manner as in Examples 1 to 3, except that Sn(Oct)2 catalyst was used at a content of 50 ppmol instead of SnHPO3 catalyst.

[0047] <Experimental Example> The properties of the copolymers prepared in the above examples and comparative examples were evaluated as follows.

[0048] 1) Molecular weight characterization The weight average molecular weight, number average molecular weight, and polydispersity index of the polymers prepared in the examples and comparative examples were measured by gel permeation chromatography (GPC, Tosoh ECO SEC Elite), and the results are shown in Table 1. Solvent: chloroform (eluent) Flow rate: 1.0ml / min Column temperature: 40℃ Standard: Polystyrene (corrected by a cubic function)

[0049] 2) Color change characteristics evaluation The polymers prepared in the examples and comparative examples were each prepared into pellet specimens using a twin-screw extruder. The yellow index (YI) of the prepared specimens was measured according to ASTM E 313 [D65 / 10], and the results are shown in Table 1.

[0050] [Table 1]

[0051] As can be seen from Table 1, in Examples 1 to 3, it was confirmed that the weight average molecular weight of the synthesized polymer increased with increasing ring-opening polymerization reaction temperature. This confirmed that the SnHPO catalyst exhibited excellent activity without being decomposed even when the polymerization reaction was carried out at high temperatures.

[0052] On the other hand, in the case of Comparative Examples 1 to 3, it was confirmed that the weight average molecular weight of the polymer synthesized decreased as the ring-opening polymerization reaction temperature increased, which resulted in decomposition of the catalyst and a significant decrease in activity at high temperatures.

[0053] In addition, Comparative Example 3, which was polymerized at 180°C, had a YI value similar to that of Example 3, which was polymerized at the same temperature. However, in Comparative Examples 2 and 1, it was confirmed that as the polymerization temperature increased to 210°C and 240°C, the YI value of the polymer increased significantly compared to the Examples.

Claims

1. SnHPO ３ The method includes a step of ring-opening polymerizing lactide in the presence of a catalyst to produce a polylactic acid polymer, The rate of change in weight average molecular weight defined by the following mathematical formula 1 has a positive value: Method for producing polylactic acid polymer. [Mathematical formula 1] Rate of change of weight average molecular weight = (M （ｔ+３０） -M （ｔ） ) / M （ｔ） *100 (%) M （ｔ） is the weight average molecular weight of the polylactic acid polymer produced by ring-opening polymerization at temperature t°C, M （ｔ+３０） is the weight average molecular weight of the polylactic acid polymer produced by ring-opening polymerization at a temperature of t + 30°C, t is 150° C. to 250° C., and the weight average molecular weights (M(t+30) and M(t)) of the polylactic acid polymer are each independently 20,000 to 50,000.

2. 2. The method for producing a polylactic acid polymer according to claim 1, wherein the change rate of the weight average molecular weight defined by the mathematical formula 1 is 10% to 80%.

3. The method for producing a polylactic acid polymer according to claim 1, wherein the ring-opening polymerization is carried out at a temperature of 150°C to 240°C.

4. SnHPO ３ 2. The method for producing a polylactic acid polymer according to claim 1, wherein the catalyst is contained in an amount of 100 ppmmol to 1,000 ppmmol relative to the lactide.

5. The lactide and SnHPO ３ The catalysts are each independently pretreated at 50° C. to 70° C. and 7 mbar to 10 mbar (700 Pa to 1,000 Pa) for 1 hour to 3 hours; The method for producing the polylactic acid polymer according to claim 1.

6. 2. The method for producing a polylactic acid polymer according to claim 1, wherein the polylactic acid polymer has a number average molecular weight of 18,000 to 33,000.

7. 2. The method for producing a polylactic acid polymer according to claim 1, wherein the polylactic acid polymer has a PDI of 1.18 to 1.5.

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