Polylactic acid graft copolymer, its preparation method and use

Abstract

The present invention belongs to the technical field of polymer synthesis, and specifically relates to a polylactic acid graft copolymer, its preparation method, and use. The polylactic acid graft copolymer contains a repeating structural unit of the structure represented by formula (A), and is characterized in that it has a right-angle tear strength of 120 kN / m or more, preferably 135-150 kN / m, and a breaking elongation of 30% or more, preferably 35-45%. The polylactic acid graft copolymer prepared by the method described in the technical proposal of the present invention significantly improves the grafting efficiency, and improves the toughness, breaking elongation, and glass transition temperature of the polylactic acid material, and the prepared graft-modified polylactic acid copolymer material is environmentally friendly and completely biodegradable. [Formula 1] JPEG2025502485000015.jpg29169 (R 1 is H or a graft group, and the R 1 At least a portion of the groups is a graft group.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of Chinese Patent Application No. 202210110898.2, filed on January 29, 2022, the contents of which are incorporated herein by reference.

[0002] [Technical field] The present invention relates to the technical field of polymer synthesis, and in particular to polylactic acid graft copolymers, their preparation methods and uses. [Background technology]

[0003] In recent years, research into bio-based composites has attracted attention in academia and industrial production, with the aim of reducing the harm to the natural environment caused by fossil fuels. Polylactic acid (PLA) is a thermoplastic resin with superior strength and processability compared to polyolefins and polyethylene terephthalate (PET), which are commonly used in industrial products. PLA is produced by the polymerization of lactic acid monomer, which is produced by the fermentation of plants such as corn, potato, cotton, and hemp. Lactic acid is a renewable source, and PLA obtained by polymerizing renewable lactic acid also possesses characteristics such as high strength, good processability, and excellent mechanical properties. However, PLA also has several drawbacks, including high brittleness, high hydrophobicity, low impact strength, high cost, and poor stability at high temperatures and certain humidity levels. These drawbacks make PLA difficult to use directly in industrial products like common plastics such as polyethylene and polypropylene, and require modification. Currently, many modification methods are being developed to optimize the performance of PLA by blending it with other plastics, but these methods, while simple, do not fundamentally change the properties of the PLA material. Furthermore, attempts have been made to modify the polylactic acid material itself by other methods, such as graft modification. Maleic anhydride is a polar monomer material and a highly effective graft modifier with a five-membered ring structure. When graft-modifying polylactic acid, it can improve the polarity of the polylactic acid material, and because maleic anhydride has a cyclic structure, it can also modify the toughness of the polymer chain of polylactic acid.

[0004] Currently, the most commonly used method for modifying the properties of other materials using maleic anhydride both in China and abroad is as follows: First, polyethylene, polystyrene, starch, lignin, etc. are grafted with maleic anhydride to obtain a pre-grafted material. Then, the pre-grafted material is blended and added to polylactic acid, improving the toughness of the polylactic acid. However, the effectiveness of this modification method is unclear. Some researchers have used reactive extrusion to directly graft maleic anhydride onto the polymer chains of polylactic acid, thereby changing the non-polar structure of the polylactic acid.

[0005] CN108192033A discloses a method for preparing L-lactide / glycolide / ε-caprolactone terpolymers directly grafted with maleic anhydride. This method involves catalyzing L-lactide, glycolide, and α-caprolactone in a fixed ratio to obtain crude PLLGC. The crude PLLGC is then dissolved in chloroform and reprecipitated with methanol. The white solid is filtered to obtain purified PLLGC. Maleic anhydride is then added to the purified PLLGC and reacted in solution to graft maleic anhydride onto the PLLGC copolymer. While this method produces highly pure polylactic acid, it has many drawbacks, including a maleic anhydride grafting rate of less than 1%. The reaction is performed using solution polymerization, which requires highly toxic organic solvents and results in a low molecular weight polymer, making it unsuitable for industrialization.

[0006] CN103570884A discloses a method for preparing maleic anhydride-modified poly(lactide-co-glycolide). This method involves adding L / D-lactide, glycolide, and maleic anhydride in a fixed ratio to an ampoule and polymerizing them in the presence of stannous octoate catalyst at 130-170°C for 12-60 hours to produce crude poly(lactide-co-glycolide) (MPLGA). The MPLGA is then dissolved in chloroform and reprecipitated with methanol. The resulting white solid is filtered to obtain purified MPLGA. While the polymer prepared by this method has a narrow molecular weight distribution, the grafting reaction of maleic anhydride during the polymerization process poses numerous problems, including low grafting yields, long polymerization times, and low molecular weights, making it unsuitable for industrial production.

[0007] As described above, maleic anhydride-grafted polylactic acid produced by conventional techniques generally has a low molecular weight, a low maleic anhydride graft ratio, and poor mechanical properties and heat resistance. Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the present invention is to provide a polylactic acid graft copolymer in which both the molecular weight and the content of graft groups are significantly improved, so that the polylactic acid graft copolymer material has good mechanical properties, heat resistance, and degradability, and is easy to produce industrially. [Means for solving the problem]

[0009] In order to achieve the above object, the first aspect of the present invention is a polymerizable composition comprising a repeating structural unit of a structure represented by formula (A), The present invention provides a polylactic acid graft copolymer characterized by a right-angle tear strength of 120 kN / m or more, preferably 135 to 150 kN / m, and a breaking elongation of 30% or more, preferably 35 to 45%. [ka] (where R 1 is H or a graft group, and the R 1 At least a portion of the groups are graft groups.

[0010] A second aspect of the present invention is (1) contacting lactide with a grafting monomer under nucleophilic addition reaction conditions to obtain a nucleophilic addition product; Step (2) mixing the nucleophilic addition product obtained in step (1) with a catalyst and an initiator under ring-opening polymerization reaction conditions, and performing low-temperature prepolymerization to obtain a ring-opening polymerization product; and step (3) polymerizing the ring-opening polymerization product obtained in step (2) at high temperature in the presence of an antioxidant under chain extension reaction conditions.

[0011] A third aspect of the present invention provides a polylactic acid graft copolymer prepared by the above method.

[0012] A fourth aspect of the present invention provides the use of the polylactic acid graft copolymer according to the first or third aspect in the preparation of agricultural mulch films, food packaging materials, and medical and sanitary products. [Effects of the Invention]

[0013] Compared with the prior art, the present invention provides a new route for synthesizing polylactic acid graft copolymers: first, graft-modifying lactide with a grafting monomer under nucleophilic addition reaction conditions to obtain a nucleophilic addition product rich in graft groups; then, pre-polymerizing the nucleophilic product rich in graft groups under ring-opening polymerization reaction conditions at low temperature to obtain a ring-opening polymerization product; and finally, polymerizing the ring-opening polymerization product at high temperature under chain extension reaction conditions to obtain a high molecular weight maleic anhydride-grafted polylactic acid copolymer. Compared with the prior art, which involves ring-opening copolymerization followed by grafting, the present invention, which involves grafting followed by ring-opening copolymerization, can effectively increase the graft group content (i.e., the grafting rate), and the subsequent high-temperature polymerization can effectively increase the molecular weight of the polymer.

[0014] Theoretically, the polylactic acid graft copolymer can be obtained with a grafting rate of 50% or even 100% by using the method for preparing polylactic acid graft copolymer according to the present invention. By adjusting the ratio of raw materials, the content of graft groups in the polylactic acid graft copolymer can be effectively adjusted, and thus the properties of the polylactic acid graft copolymer can be adjusted.

[0015] The polylactic acid graft copolymer of the present invention has high flexibility despite its relatively high molecular weight. The prepared graft-modified polylactic acid copolymer material has excellent right-angle tear strength and elongation at break, and is environmentally friendly and completely biodegradable. Because the terminal hydroxyl groups of the polylactic acid graft copolymer are retained, the polylactic acid graft copolymer can be further modified via these functional groups to further adjust the properties of the polylactic acid graft copolymer. [Brief explanation of the drawings]

[0016] [Figure 1] 1 shows infrared absorption spectra of each substance in Example 1: (a) L-lactide (LA), (b) maleic anhydride (MAH), and (c) maleic anhydride-grafted lactide. [Figure 2] 1 shows the infrared absorption spectra of each substance in Example 1, including (a) polylactic acid (PLA), (b) maleic anhydride (MAH), (c) maleic anhydride-grafted polylactic acid, and (d) the infrared difference spectrum after subtracting the spectrum of maleic anhydride-grafted polylactic acid from polylactic acid. [Figure 3] 13C NMR spectra of each substance in Example 1: (a) L-lactide, (b) maleic anhydride-grafted lactide. [Figure 4] 1 shows the 13C NMR spectra of each substance in Example 1: (a) polylactic acid (PLA) and (b) maleic anhydride-grafted polylactic acid. DETAILED DESCRIPTION OF THE INVENTION

[0017] The endpoints of ranges and any values ​​disclosed herein are not intended to be limiting to the exact ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. In the case of numerical ranges, values ​​between the individual range endpoints, between the individual range endpoints and the individual point values, and between the individual point values ​​can be combined with each other to form one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed herein.

[0018] In order to achieve the above object, the first aspect of the present invention is a polymerizable composition comprising a repeating structural unit of a structure represented by formula (A),

[0019] The present invention provides a polylactic acid graft copolymer characterized by a right-angle tear strength of 120 kN / m or more, preferably 135-150 kN / m, and a breaking elongation of 30% or more, preferably 35-45%. [ka] (where R 1 is H or a graft group, and the R 1 At least a portion of the groups are graft groups.

[0020] The polylactic acid graft copolymer may be represented by the following general formula: [ka] (where R1 ’’ , R2 ’’ , ... and R n ’’ are each independently selected from H, a graft group represented by formula (I), and a graft group represented by formula (II).

[0021] According to the present invention, the content of the graft group is 1 The content is 10 mol % or more of the total amount, preferably 30 mol % or more, and more preferably 40 mol % or more.

[0022] According to the present invention, the graft group may be any of various groups that can be grafted to lactide / polylactic acid and can increase the right-angle tear strength of the polylactic acid material to 120 kN / m or more, preferably 135 to 150 kN / m, and the breaking elongation to 30% or more, preferably 35 to 45%, and preferably the graft group has a structure represented by formula (I) and / or formula (II). [ka] (wherein R1, R2, R3, R4, and R5 are each independently hydrogen or a C1-C4 substituted or unsubstituted alkyl, and R6 is a C1-C6 substituted or unsubstituted alkyl or a C6-C20 aryl.)

[0023] In the present invention, the C1-C6 substituted or unsubstituted alkyl may be any one of methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, tert-pentyl, neopentyl, n-hexyl, and isohexyl, and the alkyl may be substituted with one or more of halogen, nitro, primary amino, secondary amino, and tertiary amino. The halogen may be one or more of fluorine, chlorine, bromine, and iodine.

[0024] The C6 to C20 aryl may be any one of phenyl, methylphenyl, ethylphenyl, dimethylphenyl, diphenyl, biphenyl, and polyphenyl.

[0025] According to the present invention, taking a graft copolymer having maleic anhydride as a graft group as an example, the polylactic acid graft copolymer 13 The presence of a peak for the carbon atom of the anhydride group at a chemical shift position of approximately 169 ppm in C NMR and a peak for the carbon atom remaining after the carbon-carbon double bond of maleic anhydride is cleaved at a chemical shift position of approximately 72 ppm are characteristic of the mid-chain grafting method of the present invention. In the present invention, the term "approximately" refers to an error within the acceptable range in the art caused by possible factors such as test equipment and personal operation, and is generally ±5 ppm. Therefore, in the present invention, the method of ring-opening polymerization followed by grafting is referred to as chain-end grafting.

[0026] According to the present invention, the weight average molecular weight of this polylactic acid graft copolymer is 1×10 5 -4×10 5 The polylactic acid graft copolymer of the present invention has high flexibility despite its relatively high molecular weight. In the prior art, the molecular weight of polylactic acid grafted to the chain end is usually 3×10 5 The right-angle tear strength is less than 100 kN / m, usually 80 to 95 kN / m, and the breaking elongation is 10% or less, usually 5 to 10%.

[0027] According to the present invention, the glass transition temperature of this polylactic acid graft copolymer is 80 to 100° C. The glass transition temperature of the polylactic acid graft copolymer according to the present invention is higher than the glass transition temperature of polylactic acid grafted to the chain end in the prior art, which is advantageous in improving the heat resistance of the material.

[0028] According to the present invention, the polylactic acid graft copolymer contains at least one hydroxyl end group, which allows further modification of the polylactic acid graft copolymer through this functional group. The presence of this hydroxyl end group can be determined by chemical titration.

[0029] In the present invention, the right-angle tear strength and breaking elongation are measured by a universal mechanical testing machine, the weight-average molecular weight is measured by gel chromatography, the glass transition temperature is measured by differential scanning calorimetry, and the graft ratio is measured by chemical titration. These will be explained in detail below.

[0030] In a second aspect of the invention, the method comprises: (1) contacting lactide with a grafting monomer under nucleophilic addition reaction conditions to obtain a nucleophilic addition product; Step (2) mixing the nucleophilic addition product obtained in step (1) with a catalyst and an initiator under ring-opening polymerization reaction conditions, and performing low-temperature prepolymerization to obtain a ring-opening polymerization product; and step (3) of polymerizing the ring-opening polymerization product obtained in step (2) at high temperature in the presence of an antioxidant under chain extension reaction conditions to increase the molecular weight of the polylactic acid graft copolymer.

[0031] The present invention employs a method of grafting followed by ring-opening polymerization, which can effectively ensure the grafting rate, and can ensure that a polylactic acid graft copolymer with a higher molecular weight can be obtained through high-temperature polymerization, thereby providing the resulting polymer with high flexibility and heat resistance.

[0032] Preferably, the grafting monomer is a monomer of formula (i) and / or formula (ii). [ka] (Here, R1', R2', R3', R4', R5', and R6' correspond to and are similar to R1, R2, R3, R4, R5, and R6 defined above, respectively, and will not be described in detail here.)

[0033] According to the present invention, in step (1), a grafting reaction between lactide and a grafting monomer such as maleic anhydride is carried out in the presence of an initiator, whereby maleic anhydride is grafted onto lactide via double bond cleavage to obtain a graft product through a nucleophilic addition reaction.

[0034] In the present invention, the lactide is at least one selected from L-lactide, D-lactide, and meso-lactide, and preferably L-lactide and / or D-lactide.

[0035] According to the present invention, the initiator is a peroxide initiator, preferably dicumyl peroxide and / or 2,5-dimethyl-2,5-bis-(t-butylperoxy)hexane.

[0036] According to the present invention, the amount of the initiator added is 0.1 to 5 wt %, preferably 0.5 to 2 wt %, of the amount of lactide used.

[0037] According to the present invention, step (1) is a constant temperature reaction under an inert gas flow, preferably a nitrogen gas flow, and vacuum conditions, and the conditions for the nucleophilic addition reaction include a reaction temperature of 50 to 150°C, preferably 80 to 120°C, a reaction pressure of 2 to 100 kPa, preferably 20 to 60 kPa, a reaction time of 0.5 to 10 hours, preferably 2 to 6 hours, and an air flow velocity of the nitrogen flow protection of 0.5 to 6 m / s, preferably 2 to 4 m / s.

[0038] In the present invention, unless otherwise specified, all pressures referred to above refer to absolute pressures.

[0039] According to the present invention, in step (1), the lactide has an optical purity of 99.0 to 99.6%, preferably 99.4 to 99.6%, and either homemade or commercially available lactide can be used.

[0040] According to the present invention, in order to obtain better toughness, the graft ratio of the graft group (i.e., the content of the graft group in the polylactic acid material) is improved, and preferably, the amount of the graft monomer added is 100 to 300 wt %, preferably 150 to 200 wt %, of the amount of lactide used.

[0041] Taking maleic anhydride and L-lactide as an example, the main reactions occurring in step (1) are shown in the following formulas (a) and (b). [ka]

[0042] Theoretically, the grafting rate of the product obtained by formula (a) is 100%, and the grafting rate of the product obtained by formula (b) is 50%.

[0043] According to the present invention, this method further comprises the step of removing the lactide that has not been involved in the nucleophilic addition reaction by vacuum distillation after the step (1), and subjecting the resulting product to the reaction in step (2).

[0044] According to the present invention, the conditions for the reduced pressure distillation include a temperature of 100 to 200°C, preferably 120 to 150°C, a pressure of 0 to 100 kPa, preferably 10 to 50 kPa, and a time of 1 to 10 hours, preferably 2 to 6 hours.

[0045] According to the present invention, in the step (2), the catalyst catalyzes the ring-opening reaction of lactide to which a graft group has been grafted, and is preferably at least one of stannous octoate, zinc lactate, trialkylaluminum, triisobutylaluminum, and stannous chloride, more preferably stannous octoate.

[0046] According to the present invention, the amount of the catalyst added may be 0.1 to 5 wt %, preferably 0.5 to 3 wt %, of the total weight of the nucleophilic addition product.

[0047] According to the present invention, the initiator in step (2) is an alkylhydroxy or aromatic hydroxy initiator, which initiates the polymerization reaction after the ring-opening reaction of the grafted lactide, and the initiator is preferably at least one selected from glycerol, xylitol, ethylene glycol, and triphenylphosphine, and preferably triphenylphosphine.

[0048] According to the present invention, the amount of the initiator added may be 0.05 to 5 wt %, preferably 0.3 to 2 wt %, of the total weight of the nucleophilic addition product.

[0049] According to the present invention, step (2) is low-temperature polymerization under negative pressure conditions in an inert gas flow, preferably a nitrogen flow, and the conditions for the ring-opening polymerization reaction in step (2) include a reaction temperature of 100 to 150°C, preferably 120 to 140°C; a reaction pressure of 100 to 1000 kPa, preferably 200 to 500 kPa; a reaction time of 2 to 15 hours, preferably 3 to 8 hours; and an air velocity of the nitrogen flow of 0.5 to 6 m / s, preferably 2 to 4 m / s.

[0050] According to the present invention, the antioxidant in step (3) prevents oxidation of the polylactic acid oligomer in step (2) and ensures further polymerization and chain extension reactions at higher temperatures. The antioxidant can be any of various substances that can prevent oxidation of polylactic acid at high temperatures and do not adversely affect the polylactic acid material, such as at least one selected from phosphite-based antioxidants, alkylpolyphenol-based antioxidants, and thiobisphenol-based antioxidants, and is preferably triphenyl phosphite.

[0051] According to the present invention, the amount of the antioxidant added is 0.1 to 5 wt %, preferably 0.5 to 2 wt %, of the total weight of the ring-opening polymerization product.

[0052] According to the present invention, in the step (3), the temperature of the chain extension reaction is 140 to 250°C, preferably 160 to 200°C.

[0053] According to the present invention, the step (3) is preferably carried out in a twin-screw extruder. The reaction process is carried out in a twin-screw extruder. The ring-opening polymerization product and the antioxidant are added to the twin-screw extruder, followed by high-temperature reactive kneading, extrusion, and granulation, and finally the polylactic acid graft copolymer is obtained.

[0054] The high-temperature polymerization conditions include an inlet temperature of the twin-screw extruder of 140 to 160° C. and an outlet temperature of 170 to 190° C. The residence time of the material in the twin-screw extruder is preferably 13 to 23 minutes.

[0055] The method for preparing polylactic acid graft copolymers according to the present invention involves carrying out a bulk prepolymerization reaction of lactide in a reactor and copolymerizing the mixture in combination with a twin-screw extruder, which simplifies the reaction procedure and facilitates industrial production.

[0056] Taking maleic anhydride and L-lactide as an example, the main reaction processes in steps (2) and (3) are shown in the following formulas (c) and (d): [ka]

[0057] In the above formulas (c) and (d), x and y represent the degree of polymerization of MA-LLA, where y is greater than x, and z represents the number of moles.

[0058] A third aspect of the present invention provides a polylactic acid graft copolymer prepared by the above method.

[0059] The polylactic acid graft copolymer prepared by the above method has a right-angle tear strength of 120 to 150 KN / m, a breaking elongation of 30 to 45%, a glass transition temperature of 80 to 100°C, and a molecular weight of 1 × 10 5 ~4×10 5 The graft rate is 40 to 50%.

[0060] A fourth aspect of the present invention provides the use of the polylactic acid graft copolymer according to the first or third aspect in the preparation of agricultural mulch films, food packaging materials, and medical and sanitary products.

[0061] The polylactic acid graft copolymer has a high grafting rate, which allows for improved flexibility and heat resistance while maintaining a higher molecular weight, making it suitable for use in the preparation of agricultural mulch films, food packaging materials, and medical and sanitary products.

[0062] The present invention will be described in more detail below with reference to examples. In the following examples, all reagents used are commercially available unless otherwise specified.

[0063] The following is a characteristic parameter testing method according to the present invention.

[0064] (1) Graft ratio of graft groups The graft ratio of the polylactic acid graft copolymer was measured using chemical titration. Specifically, a certain mass of polylactic acid graft copolymer was weighed into an Erlenmeyer flask, dissolved in a certain amount of tetrahydrofuran, and a small amount of thymol blue / DMF indicator was added dropwise. The solution was titrated with an excess of 0.05 mol / L potassium hydroxide in ethanol, and then back-titrated with 0.01 mol / L HCl-isopropyl alcohol solution. Grafting rate = n2 / n1 × 100%, n1 = (m0 - m2) / M1, n2 = m2 / M2, i.e., grafting rate = [(0.05V KOH -0.01V HCl )×2×M1] / [m0-(0.05V KOH -0.01V HCl ) × 2 × M2], where m0 is the weighted mass of the polylactic acid graft copolymer (unit: g), m2 is the total mass of grafted groups on the polylactic acid molecular chain calculated based on the consumed KOH, n1 is the degree of polymerization of polylactic acid, i.e., the amount of lactic acid units, n2 is the amount of grafted groups on the polylactic acid molecular chain, M1 is the molar mass of lactic acid, M2 is the molar mass of the grafted groups, V KOH is the volume of potassium hydroxide ethanol solution added (unit: mL), V HCl is the volume (in mL) of the HCl-isopropyl alcohol solution consumed by the titration.

[0065] (2) Right-angle tear strength and breaking elongation: measured using a universal mechanical testing machine according to GB / T 1039-1992 standard.

[0066] (3) Detection of terminal hydroxyl: Use phthalic anhydride method and measure according to GB / T 12008.3-2009 standard.

[0067] (4) Glass transition temperature: Measured using a differential scanning calorimeter (DSC) (DSC 204, Netzsch-Feinmahltechnik GmbH). The test conditions were as follows: Sample weight was 5-10 mg. First, the sample was heated from room temperature to 200°C at a heating rate of 10°C / min, and held at that temperature for 5 minutes to remove the sample's thermal history. After that, the sample was rapidly cooled to -20°C, and the temperature was then increased from -20°C to 200°C at a heating rate of 10°C / min. Nitrogen was introduced as a protective gas throughout the test process. The nitrogen flow rate was set to 50 mL / min.

[0068] (5) Weight-average molecular weight of polymer: Measured by gel permeation chromatography (GPC). The molecular weight of polylactic acid graft copolymer samples is characterized using gel permeation chromatography (GPC Waters 1515 system). Chromatographically pure THF is the mobile phase, the flow rate is 1.0 ml / min, and the column temperature is 35°C. Before testing, the sample is dissolved in THF at a concentration of 5 mg / mL and filtered through a polytetrafluoroethylene (PTFE) filter membrane with a pore size of 0.22 μm, followed by calibration with narrow-dispersion polystyrene (PS) standards. Example 1

[0069] (1) L-lactide and maleic anhydride were added to a reactor, and dicumyl peroxide was added as an initiator. The nucleophilic addition reaction was carried out by maintaining the temperature under nitrogen flow and vacuum conditions at 105°C, 45 kPa, and 3.5 hours, with a nitrogen flow rate of 2 m / s. The amount of maleic anhydride added was 155 wt% of the amount of L-lactide used, and the amount of dicumyl peroxide added was 0.7 wt% of the amount of L-lactide used. (2) The L-lactide not involved in the nucleophilic addition reaction in step (1) was removed by vacuum distillation to obtain the purified nucleophilic addition product. The vacuum distillation was carried out at a temperature of 130°C, a pressure of 25 kPa, and a time of 3.5 hours. Chromatographic detection revealed that the purified product was maleic anhydride-grafted L-lactide (MA-LLA). (3) The maleic anhydride-grafted L-lactide (MA-LLA) obtained in step (2) was subjected to ring-opening polymerization in a nitrogen flow atmosphere with the aid of a catalyst and initiator. Low-temperature prepolymerization was performed to obtain MA-LLA oligomer as the ring-opening polymerization product. The ring-opening polymerization was carried out at a reaction temperature of 140°C, a reaction time of 4.5 h, a reaction pressure of 350 kPa, and a nitrogen flow rate of 3.5 m / s. The amount of stannous octoate added as a catalyst was 0.75 wt% of the amount of MA-LLA used, and the amount of triphenylphosphine added as an initiator was 0.35 wt% of the amount of MA-LLA used. The weight-average molecular weight of the MA-LLA oligomer was 5,500. (4) The MA-LLA oligomer obtained in step (3) was added to a twin-screw extruder together with an antioxidant, and high-temperature reactive mixing was performed to induce chain extension of the MA-LLA oligomer. The resulting material was then extruded and granulated, yielding a polylactic acid graft copolymer (MA-PLLA). Based on the weight of the MA-LLA oligomer, the amount of triphenyl phosphite added as an antioxidant was 0.65 wt% of the amount of MA-LLA oligomer used. The reactive extrusion temperature during reactive mixing and extrusion granulation in the twin-screw extruder was 185°C, and the residence time of the material in the twin-screw extruder was 15 minutes. Here, the infrared spectra of L-lactide, maleic anhydride, and maleic anhydride-grafted lactide are shown in Figure 1a, b, and c, respectively. The infrared difference spectra of PLA, maleic anhydride, maleic anhydride-grafted polylactic acid (MAH-g-PLA), and maleic anhydride-grafted polylactic acid (MAH-g-PLA) and polylactic acid after subtraction are shown in Figure 2a, b, c, and d, respectively. 13 The C NMR spectra of polylactic acid and maleic anhydride-grafted polylactic acid are shown in Figure 3a and b, respectively. 13 The C NMR spectra are shown in Figure 4a and b, respectively. As can be seen from FIG. 1, in the infrared spectrum of maleic anhydride-grafted lactide, -1 The out-of-plane bending vibration peak of the olefinic hydrogen of =CH appears at 1600 cm-1 The -C=C- stretching vibration peak appeared, which indicated that maleic anhydride was successfully grafted onto lactide. As can be seen from Figure 2, in the infrared difference spectra of MAH-g-PLA and pure PLA, -1 The absorption peak of the C=O group appears at 1190 cm -1 , 1130cm -1 and 1093 cm -1 The absorption peaks of COC appear in the infrared spectrum. Compared with PLA, the absorption peaks of carbonyl and ether bonds are enhanced in the infrared spectrum of MAH-g-PLA, which indicated that MAH was successfully grafted onto the PLA polymer chains. Comparing Figure 3 a and b, the peaks of two carbon atoms in the anhydride group increased at chemical shifts of 70 ppm and 18 ppm, indicating that maleic anhydride was successfully grafted onto L-lactide. Comparing Figure 4a and b, there was a peak of the carbon atom of the anhydride group at a chemical shift of 169 ppm, and a peak of the carbon atom after the carbon-carbon double bond of maleic anhydride was cleaved at a chemical shift of 72 ppm, which indicated that MAH was successfully grafted onto the PLA polymer chain. The presence of terminal hydroxy groups was confirmed by chemical titration. Examples 2 to 11

[0070] The polylactic acid graft copolymer was prepared according to the same steps and methods as in Example 1, except that the reaction conditions, material types, and amounts of materials added were as shown in Table 1. Example 12

[0071] A polylactic acid graft copolymer was prepared by the method of Example 1, except that the nucleophilic addition reaction conditions were a temperature of 170° C. and a reaction pressure of 5 kPa. Example 13

[0072] A polylactic acid graft copolymer was prepared by the method of Example 1, except that the amount of maleic anhydride added was 310 wt % of the amount of L-lactide used, and the amount of dicumyl peroxide added was 6 wt % of the amount of L-lactide used. Example 14

[0073] A polylactic acid graft copolymer was prepared by the method of Example 1, except that the amount of stannous octoate added as a catalyst in the ring-opening polymerization reaction was 5 wt % of the amount of the nucleophilic addition product used, and the amount of triphenylphosphine added as an initiator was 5 wt % of the amount of the nucleophilic addition product used. Example 15

[0074] The polylactic acid graft copolymer was prepared by the method of Example 1, except that the amount of antioxidant added in the chain extension reaction was 5 wt% of the amount of ring-opening polymerization product used, and the temperature of the ring-opening polymerization reaction was 250°C. Example 16

[0075] A polylactic acid graft copolymer was prepared by the method of Example 1, except that maleic anhydride was replaced with methyl methacrylate. Example 17

[0076] A polylactic acid graft copolymer was prepared by the method of Example 1, except that L-lactide in the nucleophilic addition reaction was changed to D-lactide. Comparative Example 1

[0077] As in the prior art, polylactic acid graft copolymers were prepared by a method in which L-lactide was polymerized and then grafted. (1) L-lactide (LLA) was subjected to low-temperature prepolymerization under conditions of nitrogen flow and negative pressure in the presence of a catalyst and an initiator to obtain an LLA oligomer. The low-temperature prepolymerization was carried out at a reaction temperature of 130°C, a reaction time of 3 hours, a reaction pressure of 300 kPa, and a nitrogen flow rate of 3 m / s. Based on the weight of L-lactide (LLA), the amount of stannous octoate added as a catalyst was 0.5 wt% of the amount of LLA used, and the amount of triphenylphosphine added as an initiator was 0.2 wt% of the amount of LLA used. (2) The LLA oligomer prepared in step (1) was added to a twin-screw extruder for polymerization, followed by the addition of maleic anhydride (MA) and an antioxidant. High-temperature reactive kneading was performed, followed by extrusion and granulation, resulting in the production of a maleic anhydride-grafted polylactic acid material (MA-PLLA). Based on the weight of the LLA oligomer, the amount of maleic anhydride added was 2.5 wt% of the amount of LLA oligomer used, and the amount of triphenyl phosphite added as an antioxidant was 0.5 wt% of the amount of LLA oligomer used. The extrusion temperature for the reactive kneading and extrusion and granulation reaction in the twin-screw extruder was 180°C. Chemical titration does not confirm the presence of terminal hydroxy groups. Comparative Example 2

[0078] The polymer was prepared according to the method disclosed in Example 1 of CN 104371082 A. Into a high-pressure reactor, add 30g of lactide, 3g of maleic anhydride, 9mg of stannous octanoate, 3mg of benzoyl peroxide, and 50ml of toluene. Introduce nitrogen for 5 minutes to remove oxygen gas. After reacting under sealed conditions at 200°C for 20 hours, cool with water. Open the reactor, add the solution to a portion of anhydrous methanol, stir to remove unreacted substances, then suction filter, and vacuum dry the powder obtained by suction filter to obtain a discolored polylactic acid modified with maleic anhydride. The presence of terminal hydroxy groups was confirmed by chemical titration. Comparative Example 3

[0079] The polymer was prepared according to the method disclosed in Example 1 of CN 104371082 A, except that the amount of maleic anhydride added was 140 wt% of the amount of L-lactide used. The presence of terminal hydroxy groups was confirmed by chemical titration.

[0080] [Table 1] JPEG0007792527000009.jpg228169

[0081] [Table 2]

[0082] As can be seen from Table 2, by using the method disclosed in the present invention and carrying out the grafting reaction of the grafting group before polymerizing lactide, the grafting efficiency is significantly improved, and the toughness, breaking elongation, and glass transition temperature of the polylactic acid material are improved. The prepared graft-modified polylactic acid copolymer material is environmentally friendly and completely biodegradable.

[0083] Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, multiple simple modifications can be made to the technical solution of the present invention, including combining each technical feature in other appropriate ways, and these simple modifications and combinations should also be considered as the content disclosed in the present invention, and all fall within the protection scope of the present invention.

Claims

1. Contains a repeating structural unit of the structure represented by formula (A), The right-angle tear strength is 120 kN / m or more and the breaking elongation is 30% or more, 【Chemistry 1】 (where R １ is H or a graft group, and the R １ At least a portion of the groups are graft groups.) the content of the graft group is 40 mol % or more of the total amount of R 1 ; A polylactic acid graft copolymer, characterized in that the graft group has a structure represented by formula (I). 【Chemistry 2】 (wherein R 1 and R 2 are each independently hydrogen or a substituted or unsubstituted C1-C4 alkyl.)

2. A polylactic acid graft copolymer as described in claim 1, having a right-angle tear strength of 135 to 150 kN / m and a breaking elongation of 35 to 45%.

3. １３ 2. The polylactic acid graft copolymer according to claim 1, wherein a peak is present at a chemical shift of about 169 ppm and a peak is present at a chemical shift of about 72 ppm in CNMR.

4. The weight average molecular weight is 1 x 10 ５ ~4 x 10 ５ The polylactic acid graft copolymer according to claim 1, wherein

5. The polylactic acid graft copolymer according to claim 1, which has a glass transition temperature of 80 to 100°C.

6. The polylactic acid graft copolymer according to claim 1, which contains a terminal hydroxyl group as determined by a phthalic anhydride method.

7. Step (1) of contacting lactide with a monomer of formula (i) and / or formula (ii) under nucleophilic addition reaction conditions to obtain a nucleophilic addition product; Step (2) mixing the nucleophilic addition product obtained in step (1) with a catalyst and an initiator under ring-opening polymerization reaction conditions, and performing low-temperature prepolymerization to obtain a ring-opening polymerization product; and step (3) polymerizing the ring-opening polymerization product obtained in step (2) at high temperature in the presence of an antioxidant under chain extension reaction conditions. 【Transformation 3】 (where R １ ', R ２ ', R ３ ', R ４ ', and R ５ Each ' is independently hydrogen or a substituted or unsubstituted C1-C4 alkyl; R ６ ' is a substituted or unsubstituted C1-C6 alkyl, or a C6-C20 aryl.

8. The preparation method described in Claim 7, wherein in step (1), the nucleophilic addition reaction is carried out in the presence of an initiator.

9. The method of claim 8, wherein the initiator is dicumyl peroxide and / or 2,5-dimethyl-2,5-bis-(t-butylperoxy)hexane.

10. The preparation method according to claim 8, wherein the amount of the initiator added is 0.1 to 5 wt % of the amount of lactide used.

11. The preparation method according to any one of claims 7 to 10, wherein the conditions of the nucleophilic addition reaction include a reaction temperature of 50 to 150°C, a reaction pressure of 2 to 100 kPa, and a reaction time of 0.5 to 10 h.

12. The preparation method according to any one of claims 7 to 10, wherein the total amount of the monomers represented by the formula (i) and the formula (ii) added is 100 to 300 wt% of the amount of lactide used.

13. The preparation method according to any one of claims 7 to 10, further comprising the step of removing the lactide not involved in the nucleophilic addition reaction by distillation under reduced pressure after the step (1), and subjecting the obtained product to the reaction in step (2).

14. In the step (2), the catalyst is at least one selected from stannous octoate, zinc lactate, trialkylaluminum, triisobutylaluminum, and stannous chloride; the amount of the catalyst added is 0.1 to 5 wt % of the total weight of the nucleophilic addition product; and / or the initiator is at least one selected from glycerol, xylitol, ethylene glycol, and triphenylphosphine; The preparation method according to any one of claims 7 to 10, wherein the amount of the initiator added is 0.05 to 5 wt% of the total weight of the nucleophilic addition product.

15. The preparation method according to any one of claims 7 to 10, wherein the conditions of the ring-opening polymerization reaction in step (2) include a reaction temperature of 100 to 150°C, a reaction pressure of 100 to 1000 kPa, and a reaction time of 2 to 15 h.

16. In the step (3), the antioxidant is at least one selected from the group consisting of phosphites, alkyl polyphenols, and thiobisphenol antioxidants; The preparation method according to any one of claims 7 to 10, wherein the amount of the antioxidant added is 0.1 to 5 wt% of the total weight of the ring-opening polymerization product.

17. The preparation method according to any one of claims 7 to 10, wherein in step (3), the temperature of the chain extension reaction is 140 to 250°C.

18. The method according to any one of claims 7 to 10, wherein step (3) is carried out in a twin-screw extruder.

19. Use of the polylactic acid graft copolymer according to any one of claims 1 to 6 in the preparation of agricultural mulch films, food packaging materials, and medical and sanitary products.

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