Process for the preparation of polylactic acid polymers

Abstract

The present disclosure relates to a method of preparing a polylactic acid polymer, and more particularly, to a method of preparing a polylactic acid polymer having a desired high molecular weight by using a specific catalyst in a lactide ring-opening polymerization reaction.

Description

[Technical Field]

[0001] Cross-reference to related applications

[0002] This application claims priority to Korean Patent Application No. 10-2021-0087959, filed on July 5, 2021, the entire disclosure of which is incorporated herein by reference.

[0003] This disclosure relates to a method for preparing a polylactic acid polymer, and more specifically, to a method for preparing a polylactic acid polymer having a desired high molecular weight by using a specific catalyst in the ring-opening polymerization reaction of lactide. [Background Technology]

[0004] Polylactic acid (PLA) is a plant-derived resin extracted from plants such as corn. As an environmentally friendly material with biodegradable properties and excellent tensile strength and elastic modulus, it has attracted much attention.

[0005] Unlike petroleum-based resins such as polystyrene, polyvinyl chloride, and polyethylene, polylactic acid (PLA) has the effect of preventing the depletion of petroleum resources and inhibiting carbon dioxide emissions, thus reducing the environmental pollution associated with petroleum-based plastic products. Therefore, given that environmental pollution caused by waste plastics has become a social problem, efforts are underway to expand its application to areas that use general plastics (petroleum-based resins), such as food packaging materials, containers, and electronic product casings.

[0006] Meanwhile, polylactic acid (PLA) is manufactured by polymerizing lactic acid produced through microbial fermentation, but direct polymerization of lactic acid only yields low molecular weight polymers. To synthesize high molecular weight PLA, methods are known to use chain coupling agents to polymerize low molecular weight PLA obtained through direct polymerization of lactic acid to obtain PLA with higher molecular weights. However, the disadvantages are the complexity of the process and the fact that the coupling agent is used in conjunction with organic solvents, making it difficult to remove.

[0007] Current commercially available high-molecular-weight polylactic acid (PLA) production processes employ a chemical synthesis method that converts lactic acid into lactide, and then synthesizes PLA through the ring-opening reaction of lactide. However, even under these conditions, achieving commercially viable high molecular weight PLA remains a challenge.

[0008] In the case of conventional Sn(Oct)₂ catalysts used to achieve high molecular weight, the problem is that as a liquid catalyst, it has high viscosity, which is difficult to measure accurately, and it is fragile to oxygen and moisture, thus reducing the stability of the catalyst itself. In particular, there is the problem of rapid decomposition of Sn(Oct)₂ catalysts at high temperatures, which adversely affects the color of the prepared resin and also reduces economic efficiency.

[0009] Therefore, there is a need to develop a method for preparing polylactic acid polymers with excellent physical properties and easy to achieve high molecular weight, so that they can be applied to various industries. [Summary of the Invention]

[0010] [Technical Issues]

[0011] One object of this disclosure is to provide a method for preparing polylactic acid polymers with desired high molecular weights by using a specific catalyst in the ring-opening polymerization reaction of lactide and controlling the rate of change of polymer molecular weight with polymerization temperature within a specific range.

[0012] Another object of this disclosure is to provide a method for preparing polylactic acid polymers that promotes catalyst removal and reuse after the reaction by using a catalyst that exhibits excellent stability even under high-temperature polymerization reaction conditions, and is economical, and does not cause changes in the physical properties of the polymer due to catalyst decomposition.

[0013] [Technical Solution]

[0014] To achieve the above objectives, according to this disclosure, a method for preparing polylactic acid polymer is provided, comprising the following steps:

[0015] First, the method includes the step of ring-opening polymerization of lactide in the presence of a SnHPO3 catalyst to prepare polylactic acid polymer, and

[0016] The rate of change of weight-average molecular weight, as defined by the following mathematical formula 1, has a positive value:

[0017] [Mathematical Expression 1]

[0018] Weight-average molecular weight change rate = (M (t+30) –M (t) ) / M (t) ×100(%)

[0019] M (t) The weight-average molecular weight is the polylactic acid polymer prepared by ring-opening polymerization at temperature t °C.

[0020] M (t+30) The weight-average molecular weight of the polylactic acid polymer prepared by ring-opening polymerization at temperature t+30℃ is given.

[0021] t is between 150℃ and 250℃.

[0022] Unless specifically mentioned herein, the terms “including” or “comprising” mean including certain elements (or components) without any limitation and should not be construed as excluding the inclusion of other elements (or components).

[0023] (Preparation method of polylactic acid polymer)

[0024] Currently, commercially available polylactic acid (PLA) production processes utilize a chemical synthesis method involving the ring-opening reaction of lactide rings. However, achieving commercially viable high molecular weights remains a challenge. Specifically, when using Sn(Oct)₂ catalysts to achieve high molecular weights, the high viscosity of the liquid catalyst makes precise measurement difficult, and its vulnerability to oxygen and moisture reduces its inherent stability. Furthermore, the rapid decomposition of Sn(Oct)₂ catalysts at high temperatures negatively impacts the color of the prepared resin and reduces economic efficiency.

[0025] Therefore, the inventors discovered that by using a SnHPO3 catalyst with excellent high-temperature stability and controlling the reaction conditions in the ring-opening polymerization reaction of lactic acid oligomers, the catalytic activity is not reduced even under high-temperature conditions, and high molecular weight polylactic acid polymers can be easily obtained, thus completing this invention. Furthermore, the catalyst can be easily removed and reused after the reaction is complete, resulting in excellent economic efficiency.

[0026] According to one embodiment of this disclosure, the method includes the step of performing ring-opening polymerization of lactide in the presence of a SnHPO3 catalyst to prepare a polylactic acid polymer.

[0027] The SnHPO3 catalyst is tin(II) phosphite, which is the phosphite anion (PO3). - SnHPO3 is a tin (Sn) salt. SnHPO3 catalysts do not decompose even under the high temperatures required for the ring-opening polymerization of lactide, making them easy to remove and reuse after the reaction, thus offering excellent economic benefits. Furthermore, SnHPO3 catalysts are conveniently used as solid powder heterogeneous catalysts.

[0028] The SnHPO3 catalyst content relative to lactide can be from 100 ppm mol to 1,000 ppm mol, preferably from 150 ppm mol to 900 ppm mol, 300 ppm mol to 700 ppm mol, or 500 ppm mol to 700 ppm mol. Within these ranges, ring-opening polymerization can be promoted while the formation of byproducts can be suppressed.

[0029] Ring-opening polymerization is preferably carried out at temperatures above 150°C, between 150°C and 250°C, or between 150°C and 240°C. Even when using a SnHPO3 catalyst within these temperature ranges, the polymerization reaction can be readily carried out without altering the catalyst activity. Furthermore, by conducting ring-opening polymerization within these temperature ranges, desired high-molecular-weight polylactic acid polymers can be readily formed, and the formation of byproducts can be minimized. Additionally, desired high-molecular-weight polylactic acid polymers can be readily prepared without decomposing the SnHPO3 catalyst or altering the polymer's color. However, when the reaction temperature is below 150°C, the activity of ring-opening polymerization may decrease.

[0030] More preferably, ring-opening polymerization can be carried out at temperatures above 160°C, above 170°C, above 180°C, or below 240°C. Within these ranges, polylactic acid polymers with desired high molecular weights can be readily prepared without the aforementioned problems.

[0031] The ring-opening polymerization reaction was carried out in the presence of SnHPO3 catalyst. In particular, the rate of change of weight-average molecular weight, as defined by the following mathematical formula 1, has a positive value:

[0032] [Mathematical Expression 1]

[0033] Weight-average molecular weight change rate = (M (t+30) –M (t) ) / M (t) ×100(%)

[0034] M (t) The weight-average molecular weight is the polylactic acid polymer prepared by ring-opening polymerization at temperature t °C.

[0035] M (t+30) The weight-average molecular weight of the polylactic acid polymer prepared by ring-opening polymerization at temperature t+30℃ is given.

[0036] t is between 150℃ and 250℃.

[0037] The rate of change is the rate of change in the weight-average molecular weight of polylactic acid (PLA) prepared by increasing the ring-opening polymerization temperature by 30°C. It can be understood as an indicator of the degree of catalyst decomposition at high temperatures. A positive rate of change means that the weight-average molecular weight of the polymer increases with increasing polymerization temperature. Theoretically, it refers to greater than 0%, but in practice, it can refer to approximately 1% or more, approximately 3% or more, or approximately 5% or more.

[0038] In conventional ring-opening polymerization using Sn(Oct)2, a problem exists where 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. Therefore, the ring-opening polymerization reaction disclosed herein solves this problem, while exhibiting a positive weight-average molecular weight change rate as described above.

[0039] Preferably, the weight-average molecular weight change rate can be 10% to 80%, 20% to 70%, or 40% to 60%. Even when the polymerization reaction is carried out at high temperatures within the above range, it is suitable for achieving excellent high molecular weights without decomposing the catalyst, and is economical because the catalyst can be recycled.

[0040] The measurement of the weight-average molecular weight of the polymer will be explained in more detail in the experimental examples described later.

[0041] On the other hand, if necessary, the polymerization reactor can be replaced with inert conditions before ring-opening polymerization. Specifically, after the reactor is secured, it is purged with vacuum and argon approximately three times to replace the internal conditions of the reactor with inert conditions.

[0042] In addition, if necessary, before ring-opening polymerization, lactide and SnHPO3 catalyst can be pretreated independently at 50°C to 70°C and 7 mbar to 10 mbar for 1 to 3 hours. The pretreatment step can remove oxygen, moisture, impurities, etc. from the lactide and SnHPO3 catalyst.

[0043] (polylactic acid polymer)

[0044] According to one embodiment of the present disclosure, a polylactic acid polymer prepared according to the above preparation method is provided.

[0045] The weight-average molecular weight (Mw) of the polylactic acid polymer can be from 20,000 to 50,000, preferably from 30,000 to 50,000.

[0046] The number-average molecular weight (Mn) of the polylactic acid polymer can be from 18,000 to 33,000, preferably from 25,000 to 33,000.

[0047] The polydispersity index (PDI) of polylactic acid polymers can be from 1.18 to 1.5, preferably from 1.2 to 1.5.

[0048] The YI (yellowing index) of the polylactic acid polymer can be from 13 to 33, preferably from 13 to 31, or from 13.3 to 30.3.

[0049] The methods for measuring weight-average molecular weight, number-average molecular weight, polydispersity index, and YI will be described in detail in the experimental examples described later.

[0050] (Products)

[0051] According to yet another embodiment of this disclosure, an article comprising a polylactic acid polymer is provided.

[0052] [Beneficial Effects]

[0053] As described above, the method for preparing polylactic acid polymer disclosed herein provides a method for preparing polylactic acid polymers with a desired degree of high molecular weight by using a specific catalyst in the ring-opening polymerization reaction of lactide and controlling the rate of change of polymer molecular weight with polymerization temperature within a specific range.

[0054] Moreover, by using a catalyst that exhibits excellent stability even under high-temperature polymerization conditions, the following polylactic acid polymers can be prepared, which facilitate the removal and reuse of the catalyst after the reaction and are economical, and do not cause changes in the physical properties of the polymer due to catalyst decomposition.

[0055] Furthermore, the polylactic acid polymer prepared according to the above method has a high molecular weight and excellent color change stability. [Attached Image Description]

[0056] Figure 1 The XRD analysis results of the powdered SnHPO3 catalyst obtained in Preparation Example 1 are shown.

Detailed Implementation Methods

[0057] Hereinafter, embodiments of the present disclosure will be described in detail with reference to examples. However, the following examples are for illustrative purposes only, and the detailed description of the present disclosure is not limited to these examples.

[0058] <Preparation Example>

[0059] Preparation Example 1 – Synthesis of SnHPO3, i.e., tin(II) phosphite catalyst

[0060] Prepare a three-necked round-bottom flask in an oil bath. Add 10g of phosphorous acid (H3PO3) to the flask and then heat to 74°C under nitrogen purging to dissolve it by reflux. Then, while heating the flask to 180°C, add 9.5g of tin(II) oxide in 5 to 10 portions and dissolve it.

[0061] When the viscous mixture in the flask became transparent, the oil bath was turned off, and the mixture was slowly cooled to room temperature (24±1℃). When water was added to the flask, a solid precipitated. This precipitate was washed with methanol at 5±2℃ and water at 5±2℃ to remove residual H3PO3, and then vacuum dried to obtain a solid powdered catalyst. The obtained powdered catalyst was then analyzed by XRD, and the results are shown below. Figure 1 .

[0062] The absence of tin oxide peaks in the XRD pattern confirms the successful synthesis of the catalyst. A single-phase SnHPO3 was observed in the PLA catalyst; the tin oxide and H3PO3 crystalline phases used in the synthesis were not observed. The grain size of the SnHPO3 catalyst is approximately 150 to 165 nm.

[0063] <Examples and Comparative Examples>

[0064] Example 1

[0065] After setting up a 0.5L laboratory-scale glass reactor, Ar conditions were maintained overnight, and the reactor interior was purged with Ar. After installing the Viton O-ring into a 500mL beaker, approximately 30 to 40% (about 170g) of lactide was added based on the beaker level. After securing the reactor, it was maintained at 10 mbar for 1 minute using a diaphragm pump and vacuum controller for a leak test. Then, approximately three vacuum-argon purgings were performed using a Schlenk line to purge the reactor interior to inert conditions, ultimately purging it to Ar.

[0066] The catalyst diaphragm was opened, and 500 ppm mol of SnHPO3 powder catalyst from Preparation Example 1 (approximately 120 mg relative to the lactide feed amount) was weighed and added. The reaction mixture was heated to 60°C (60°C for the lower jacket) under vacuum using a Schlenk line for approximately 2 hours to remove impurities. After replacing the reactor with Ar, the temperature was increased to 240°C (240°C for both the lower and upper jackets). The reaction was carried out for 3 hours after reaching 240°C to prepare the polylactic acid polymer.

[0067] Example 2

[0068] Except that the polymerization reaction was carried out at a temperature of 210°C as in Example 1, the polylactic acid polymer was prepared in the same manner as in Example 1.

[0069] Example 3

[0070] Except that the polymerization reaction was carried out at a temperature of 180°C as in Example 1, the polylactic acid polymer was prepared in the same manner as in Example 1.

[0071] Comparative Examples 1 to 3

[0072] Polylactic acid polymers were prepared in the same manner as in Examples 1 to 3, except that a Sn(Oct)2 catalyst with a content of 50 ppm mol was used instead of a SnHPO3 catalyst in Examples 1 to 3.

[0073] <Experimental Example>

[0074] The properties of the polymers prepared in the Examples and Comparative Examples are evaluated below.

[0075] 1) Evaluation of molecular weight characteristics

[0076] 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 below.

[0077] Solvent: Chloroform (eluent)

[0078] Flow rate: 1.0 ml / min

[0079] Column temperature: 40℃

[0080] Standard: Polystyrene (corrected using a cubic function)

[0081] 2) Evaluation of color-changing characteristics

[0082] For the polymers prepared in the Examples and Comparative Examples, pellet samples were prepared using a twin-screw extruder, and the YI (yellowing index) of the prepared samples was measured according to ASTM E 313 [D65 / 10], and the results are shown in Table 1 below.

[0083] [Table 1]

[0084]

[0085] As shown in Table 1, it can be confirmed that in Examples 1 to 3, the weight-average molecular weight of the synthesized polymers increases with increasing ring-opening polymerization temperature. Therefore, it can be confirmed that even when the polymerization reaction is carried out at high temperatures, the SnHPO3 catalyst does not decompose, exhibiting excellent activity.

[0086] On the other hand, it can be confirmed that in the cases of Comparative Examples 1 to 3, as the ring-opening polymerization temperature increases, the weight-average molecular weight of the synthesized polymer decreases, and the catalyst decomposes at high temperatures, significantly reducing its activity.

[0087] Furthermore, it can be confirmed that Comparative Example 3, which polymerizes at 180°C, has a similar YI value to Example 3, which polymerizes at the same temperature. However, when comparing Comparative Examples 2 and 1, the YI value of the polymers increases significantly compared to the Examples as the polymerization temperature increases to 210°C and 240°C, respectively.

Claims

1. A method for preparing a polylactic acid polymer, the method comprising: Ring-opening polymerization of lactide was carried out in the presence of SnHPO3 catalyst to prepare polylactic acid polymer. The rate of change of weight-average molecular weight, as defined by the following mathematical formula 1, has a positive value: [Mathematical Expression 1] Weight-average molecular weight change rate = (M (t+30) –M (t) ) / M (t) ×100(%) M (t) The weight-average molecular weight is the polylactic acid polymer prepared by ring-opening polymerization at temperature t °C. M (t+30) The weight-average molecular weight of the polylactic acid polymer prepared by ring-opening polymerization at temperature t+30℃ is given. t is between 150℃ and 250℃.

2. The method for preparing polylactic acid polymer according to claim 1, wherein: The weight-average molecular weight variation rate, as defined by Equation 1, is 10% to 80%.

3. The method for preparing polylactic acid polymer according to claim 1, wherein: The ring-opening polymerization was carried out at 150°C to 240°C.

4. The method for preparing polylactic acid polymer according to claim 1, wherein: The SnHPO3 catalyst content relative to the lactide is from 100 ppm mol to 1000 ppm mol.

5. The method for preparing polylactic acid polymer according to claim 1, wherein: The lactide and the SnHPO3 catalyst were each independently pretreated at 50°C to 70°C and 7 mbar to 10 mbar for 1 to 3 hours.

6. The method for preparing polylactic acid polymer according to claim 1, wherein: The polylactic acid polymer has a weight-average molecular weight of 20,000 to 50,000.

7. The method for preparing polylactic acid polymer according to claim 1, wherein: The polylactic acid polymer has a number-average molecular weight of 18,000 to 33,000.

8. The method for preparing 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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