Polylactic acid, biodegradable composition and preparation method and application thereof

Abstract

The invention discloses polylactic acid, a biodegradable composition and a preparation method and application thereof, and belongs to the technical field of high polymer materials. The polylactic acid provided by the invention has a specific relationship between optical purity, increased specific viscosity, molecular weight distribution index PDI and melt index and test temperature, and is suitable for processing the membrane material, and the obtained membrane material has high transverse and longitudinal tear strength.

Description

Technical Field

[0001] The present application belongs to the technical field of polymer materials, and specifically relates to polylactic acid, a biodegradable composition, and a preparation method and application thereof. Background Art

[0002] Polylactic acid is a general term for poly L-lactic acid, poly D-lactic acid and poly D,L-lactic acid and their copolymers, including a series of products from semi-crystalline to amorphous. It has a wide range of applications. It can be used as a general-purpose plastic in industrial and agricultural fields, such as agricultural films, desert greening water-retaining materials, fibers, food containers, domestic garbage bags, disposable lunch boxes, etc. It can also be used as a fiber in the textile and clothing fields. It is also non-toxic, non-irritating, and has good biocompatibility, bioabsorbability, biodegradability and easy processing. It is used in biomedical materials, drug sustained-release carriers, surgical implant materials, etc.

[0003] Polylactic acid has a relatively low molecular weight, less molecular chain entanglement, low melt strength, and is difficult to blow film, which limits its application in film products. Generally speaking, the melt strength of polymers can be improved by increasing the molecular weight and introducing long chain branches, but increasing the molecular weight will increase the shear viscosity of polylactic acid, resulting in poor processing performance and poor mechanical properties of film bags; it is difficult to obtain a stable improvement effect by introducing long chain branches, which affects the mechanical properties of the material.

[0004] Therefore, it is urgent to develop a technology to make polylactic acid have good mechanical properties and be suitable for processing membrane materials. Summary of the invention

[0005] Based on the defects of the prior art, the purpose of the present application is to provide a polylactic acid, a biodegradable composition and a preparation method and application thereof. The polylactic acid is suitable for processing film materials, and the obtained film materials have high transverse and longitudinal tear strengths.

[0006] In order to achieve the above-mentioned object, in the first aspect, the present application provides a polylactic acid having an optical purity of 87.3wt.% to 95.2wt.%, a specific viscosity of 1.2 to 1.5, and a molecular weight distribution index PDI of 1.3 to 2.7; in accordance with ISO 1133-1:2022, under a test temperature range of 140°C to 190°C under a load of 5kg, the melt index of the polylactic acid and the test temperature satisfy the following relationship: Y=AX 2 +BX+C, Wherein, Y is the melt index, in g / 10min; X is the test temperature in °C; A is 3×10 -3 ~8×10 -3 ; B is -2.1~-1.0; C is 70~140.

[0007] The inventors have found that polylactic acid that satisfies a specific relationship between the melt index and the test temperature has a higher melt strength. The relationship between the melt index of polylactic acid and the test temperature is affected by factors such as the distribution of molecular weight and the structural characteristics of the molecular chain, and can be regulated by adjusting the amount of additives, catalysts and / or initiators that increase the degree of PLA branching, and the temperature and / or time of the polymerization reaction. In addition, the optical purity can be regulated by adjusting the content of L-lactide in the lactide raw material; the specific viscosity reflects the flow characteristics of the melt and the strength of the internal molecular interaction force, which is affected by factors such as temperature, molecular structure and molecular interaction force, and can be regulated by adjusting the amount of additives, catalysts and / or initiators, and / or the temperature and / or time of the polymerization reaction; the molecular weight distribution index PDI reflects the uniformity of its molecular weight, which is closely related to melt strength, mechanical properties, melt stability, etc., and can be regulated by adjusting the amount of additives, catalysts and / or initiators, and / or the temperature and / or time of the polymerization reaction.

[0008] The relationship between the melt index and the test temperature of polylactic acid, the optical purity, the specific viscosity and the molecular weight distribution index PDI jointly affect the transverse and longitudinal tear strength of the film formed by the resin. The present application controls the optical purity, specific viscosity and molecular weight distribution index PDI of polylactic acid within a specific range, and the melt index and the test temperature meet a specific relationship, so that the polylactic acid has high melt strength and is suitable for processing film materials, and the formed film has high transverse and longitudinal tear strength.

[0009] The optical purity of the polylactic acid is 87.3wt.% to 95.2wt.%, such as 87.3wt.%, 87.5wt.%, 88wt.%, 89wt.%, 90wt.%, 91wt.%, 92wt.%, 93wt.%, 94wt.%, 95wt.%, 95.2wt.% or an interval formed by any two of the above values. The optical purity of the polylactic acid can be measured as follows: Agilent 8860 gas chromatograph and CP7502 column were used. The test method was as follows: 100±10 mg of polylactic acid sample was weighed into a hydrothermal reactor, 10.0 mL of methanol was added, 1 drop of NaOH aqueous solution with a NaOH concentration of 1 mol / L was added, and the hydrothermal reactor was sealed. The hydrothermal reactor was placed in a 150°C blast oven; after 60 min, the hydrothermal reactor was taken out and cooled to room temperature with running water (about 10 min). The filtered sample solution was transferred to a small glass bottle suitable for gas chromatography injection. The test was performed according to the specified parameters, and each sample solution was tested at least 3 times. The peaks corresponding to D-methyl lactate and L-methyl lactate were determined according to the retention time, and the peak areas of D-methyl lactate and L-methyl lactate were recorded.

[0010] [%] A DML : Peak area of ​​D-methyl lactate A LML : Peak area of ​​L-methyl lactate According to the peak areas of D-methyl lactate and L-methyl lactate, the content of L-lactic acid in polylactic acid is calculated according to the above formula, which is the optical purity of polylactic acid.

[0011] The specific viscosity of the polylactic acid is 1.2 to 1.5. For example, the specific viscosity of the polylactic acid is 1.2, 1.3, 1.4, 1.5 or an interval formed by any two of the above values. The specific viscosity of the polylactic acid is measured using an 1835-type Ubbelohde capillary viscometer with an inner diameter of 0.83 mm. The specific test method is as follows: Weigh 0.1250±0.0001g of a polylactic acid sample and put it into a 250mL conical flask, add 25mL of a mixed solvent of phenol and o-dichlorobenzene (the volume ratio of phenol to o-dichlorobenzene is 3:2), heat to dissolve and then cool to room temperature to obtain a sample solution; then place it in a water bath at 25±0.01℃ for 10min, and then measure the elution time of the sample solution and the above-mentioned mixed solvent of phenol and o-dichlorobenzene at 25℃ in an Ubbelohde capillary viscometer, and calculate the specific viscosity; The formula for increasing specific viscosity is: η sp =(η-η 0 ) / η 0 , Wherein, η is the viscosity of the sample solution, in Pa·s; η 0 is the viscosity of the mixed solvent of phenol and o-dichlorobenzene, in Pa·s; η sp is the specific viscosity of polylactic acid, dimensionless.

[0012] The molecular weight distribution index PDI is 1.3 to 2.7, such as 1.3, 1.5, 1.8, 2.0, 2.2, 2.4, 2.6, 2.7 or the interval formed by any two of the above values. The PDI of the polylactic acid can be measured by gel permeation chromatography (GPC), such as using Waters' ACQUITY APC™ equipment for testing, the test temperature is 40°C, using ACQUITY APC™ XT45, XT200 and XT459 chromatographic columns in series, solvent: tetrahydrofuran, mobile phase flow rate: 0.5 mL / min, using polystyrene standards as standard samples, and taking the average of three results.

[0013] The A is 3×10 -3 ~8×10-3 , such as 3×10 -3 , 4×10 -3 , 5×10 -3 , 6×10 -3 ,7×10 -3 , 8×10 -3 Or the interval formed by any two of the above values.

[0014] The B is -2.1~-1.0, such as -2.1, -2.0, -1.9, -1.8, -1.7, -1.6, -1.5, -1.4, -1.3, -1.2, -1.1, -1.0 or an interval formed by any two of the above values.

[0015] The C is 70-140, such as 70, 80, 90, 100, 110, 120, 130, 140 or an interval formed by any two of the above values.

[0016] Preferably, the mass percentage of lactide monomer in the polylactic acid is less than 0.5%, such as 0.49%, 0.45%, 0.40%, 0.35%, 0.30%, 0.25%, 0.20%, 0.15%, 0.10% or an interval formed by any two of the above values.

[0017] More preferably, the mass percentage of lactide monomer in the polylactic acid is 0.1% to 0.45%.

[0018] The mass percentage of the lactide monomer in the polylactic acid can be measured by the following method: Accurately weigh about 10 mg of polylactic acid product and dissolve it in 4 mL of chloroform solvent containing 1 mg / mL internal standard (pentamethylbenzene) at room temperature. After complete dissolution, use 10 mL of n-hexane to precipitate the high molecular weight product. Take the liquid phase and inject it into a 2 mL injection bottle through a polytetrafluoroethylene needle filter (pore size of 0.45 μm). Use gas chromatography for detection, and calculate the lactide monomer content by the integral area ratio of the specific peak.

[0019] In a second aspect, the present application provides a biodegradable composition, comprising the following components in parts by weight: 58-88 parts of biodegradable polyester, 4-10 parts of the polylactic acid, 10-30 parts of inorganic filler, and 0.1-0.6 parts of additives. The biodegradable composition not only has high melt strength, but also has good processing performance, is not prone to crystal point problems, and is suitable for processing film materials by adding specific polylactic acid and controlling the content of the above components within a specific range.

[0020] Preferably, the biodegradable polyester includes PBAT (polybutylene adipate terephthalate).

[0021] Preferably, the melt flow rate of the PBAT measured at 190°C and 2.16kg load according to ISO 1133-1-2011 is 3-5g / 10min. For example, the melt flow rate of the PBAT measured at 190°C and 2.16kg load according to ISO1133-1-2011 is 3.0g / 10min, 3.2g / 10min, 3.4g / 10min, 3.6g / 10min, 3.8g / 10min, 4.0g / 10min, 4.2g / 10min, 4.4g / 10min, 4.6g / 10min, 4.8g / 10min, 5.0g / 10min or an interval formed by any two of the above values.

[0022] Preferably, the molar ratio of AA (adipic acid) to PTA (terephthalic acid) in the PBAT is (1.0-1.1): 1. For example, the molar ratio of AA to PTA in the PBAT is 1.0: 1, 1.02: 1, 1.04: 1, 1.06: 1, 1.08: 1, 1.1: 1 or an interval formed by any two of the above values.

[0023] Preferably, the inorganic filler includes at least one of calcium carbonate and talc.

[0024] Preferably, the D50 particle size of the inorganic filler is ≤5 μm. For example, the D50 particle size of the inorganic filler is 5 μm, 4 μm, 3 μm, 2 μm, 1 μm, or an interval formed by any two of the above values.

[0025] More preferably, the D50 particle size of the inorganic filler is 2-4 μm.

[0026] The D50 particle size of the inorganic filler is measured according to GB / T 19077.1-2008 "Particle Size Analysis by Laser Diffraction Method", the dispersion used is water, and the sample concentration is 5 wt%.

[0027] Preferably, the auxiliary agent includes at least one of an opening agent and a lubricant.

[0028] Preferably, the opening agent includes at least one of talc, silicon dioxide and PE wax.

[0029] Preferably, the lubricant includes at least one of erucamide, oleamide, monostearate glycerol, pentaerythritol stearate, PE wax, and ethylene bisstearamide EBS.

[0030] The biodegradable composition described in the present application may contain common additives such as antioxidants, light stabilizers, plasticizers, and compatibilizers without impairing the effects of the present application. Among them, the antioxidant may be selected from at least one of hindered amine antioxidants, phenolic antioxidants, phosphite antioxidants, etc.; the light stabilizer may be selected from at least one of hindered amines, o-hydroxybenzophenone, etc.; the plasticizer may be selected from at least one of citrate, glycerol, polyethylene glycol, etc.; the compatibilizer may be selected from maleic anhydride grafted polymers, etc.

[0031] Exemplarily, the preparation method of the biodegradable composition comprises the following steps: feeding the biodegradable polyester, polylactic acid and additives through the main feeding port of a twin-screw extruder, feeding the inorganic filler through the side feeding port, melt extruding granulation, cooling, air drying, pelletizing, drying, homogenizing, and obtaining the biodegradable composition. The aspect ratio of the twin-screw extruder can be selected as (44-56):1, and the melt extrusion temperature can be selected as 180°C-200°C.

[0032] In a third aspect, the present application provides an application of the polylactic acid or the biodegradable composition in film bag products. For example, the polylactic acid or the biodegradable composition can be used to prepare packaging bags and the like.

[0033] In a fourth aspect, the present application provides a method for preparing the polylactic acid, comprising the following steps: Mixing lactide, additives, catalysts and initiators, and preheating to obtain a preheated mixture; The obtained preheated mixture is subjected to a first reaction to obtain a prepolymer having a specific viscosity of 0.6 to 1 and a lactide monomer mass percentage of 15% to 30%; The obtained prepolymer is subjected to a second reaction to obtain an intermediate polymer having a specific viscosity of 1.20 to 1.50 and a lactide monomer content of 1% to 5% by weight; Adding a catalyst deactivator to the obtained intermediate polymer, and then evaporating it for 10 to 60 minutes at a pressure of 0 to 300 Pa and a temperature of 180 to 230° C. to obtain polylactic acid; The additive is a multifunctional epoxy compound, and the functionality of the multifunctional epoxy compound is ≥3; The mass percentage of L-lactide in the lactide is 87.5% to 95%; Calculated based on the mass of the lactide, the mass of the additive is 0.1% to 0.5%.

[0034] Preferably, the functionality of the multifunctional epoxy compound is 3 to 6.

[0035] The multifunctional epoxy compound is used to increase the branching degree of PLA. Preferably, the additive includes at least one of N,N-diglycidyl-4-glycidyloxyaniline, N,N,N',N'-tetraglycidyl-4,4'-diamino-3,3'-diethyldiphenylmethane, epoxyglycerol triester (CAS No. 8013-07-8), and pentaerythritol glycidyl ether (CAS No. 3126-63-4).

[0036] The mass percentage of L-lactide in the lactide can be measured by the following method: weigh 100±10 mg of lactide sample into a hydrothermal reactor, add 10.0 mL of methanol, then add 1 drop of 1 mol / l NaOH aqueous solution, seal the hydrothermal reactor, and then put it into an oven at 150°C. After 60 minutes, take out the hydrothermal reactor, cool it to room temperature (about 10 minutes) with running water, and then perform gas chromatography test. According to the peak area ratio of D-methyl lactate to L-methyl lactate, the mass percentage of L-lactide in the lactide is calculated. The gas chromatography test uses an Agilent 8860 gas chromatograph and a CP7502 chromatographic column.

[0037] Preferably, the acid value of the lactide is ≤10 mol / t. More preferably, the acid value of the lactide is 1-7 mol / t.

[0038] The acid value of the lactide can be measured by the following method: using a Swiss Metrohm 905 fully automatic potentiometric titrator as the testing instrument, dissolving 0.5 g of lactide in 60 mL of dichloromethane to obtain a lactide solution; titrating the lactide solution with a KOH-EtOH (EtOH is ethanol) solution having a KOH concentration of 0.0025 mol / L, and determining the endpoint by potentiometric titration.

[0039] Preferably, based on the mass of the lactide, the mass of the catalyst is 0.01% to 0.1%, the mass of the initiator is 0.17% to 0.25%, and the mass of the catalyst deactivator is 0.01% to 0.1%.

[0040] Preferably, the temperature of the preheated mixture is 90-150°C.

[0041] Preferably, the first reaction is carried out at a pressure of 0-50 kPa and a temperature of 170-200° C. for 1-3 h.

[0042] Preferably, the second reaction is carried out at a pressure of 0.5-2 MPa and a temperature of 180-210° C. for 1-3 h.

[0043] Preferably, the catalyst comprises a tin compound. More preferably, the catalyst comprises stannous octoate, SnCl 2 SnCl4 SnBr 2 SnBr 4 , at least one of butyltin tri(2-ethylhexanoate), monobutyltin hydrate, dibutyltin dilaurate, and tetraphenyltin.

[0044] Preferably, the initiator comprises a hydroxyl-containing compound. More preferably, the initiator comprises at least one of butanediol, dodecanol and lactic acid.

[0045] Preferably, the catalyst deactivator includes at least one of phosphorous acid, monostearate phosphate, and distearate phosphate.

[0046] In some embodiments, in the process of obtaining the preheated mixture using lactide, additives, catalysts and initiators, the mixing and preheating treatment is performed in a mixer. For example, the mixer is a static mixer.

[0047] In some embodiments, the first reaction is carried out in a reactor having a stirring device. In one embodiment, the reactor having a stirring device is a continuous reactor, such as a fully mixed flow tank reactor. As an example, the fully mixed flow tank reactor is the stirred vessel 2 disclosed in CN101820996A.

[0048] In some embodiments, the second reaction is carried out in a plug flow reactor. As an example, the plug flow reactor is the tubular reactor 5 disclosed in CN101820996A.

[0049] In addition to being prepared by the above-mentioned preparation method, the polylactic acid can also be prepared by other preparation methods, for example, by a method comprising the following steps: Mixing lactide, a catalyst, an initiator and a first additive, and preheating to 112-125° C. to obtain a preheated mixture; The obtained preheated mixture is polymerized at a temperature of 175-185° C. for 1.5-2.5 hours, and then polymerized at a temperature of 200-210° C. for 1-2 hours to obtain an intermediate polymer having a specific viscosity of 1.45-1.5 and a lactide monomer mass percentage of 3-3.2%; Adding a catalyst deactivator to the obtained intermediate polymer, and then evaporating for 25 to 35 minutes at a pressure of 75 to 85 Pa and a temperature of 200 to 210° C. to remove volatiles, thereby obtaining a polymer after removing volatiles; Add the second additive to the obtained polymer after removing volatiles, then mix, react at 190-200°C for 0.3-0.5h, and then pelletize and dry to obtain polylactic acid; based on the mass of lactide, the mass of the first additive can be selected to be 0.04%-0.06%, the mass of the catalyst can be selected to be 0.004%-0.006%, the mass of the initiator can be selected to be 0.15%-0.25%, the mass of the catalyst passivator can be selected to be 0.004%-0.006%, and the mass of the second additive can be selected to be 0.2%-0.3%. Among them, the catalyst can be selected from tin compounds, such as stannous octoate, SnCl 2 SnCl 4 SnBr 2 SnBr 4 , butyltin tris(2-ethylhexanoate), hydrated monobutyltin oxide, dibutyltin dilaurate, tetraphenyltin; the initiator can be selected from hydroxyl-containing compounds, such as at least one of butanediol, dodecanol, and lactic acid; the first additive is used to reduce thermal degradation side reactions and improve the hue of PLA, and can be selected from at least one of triphenyl phosphite, triphenyl phosphate, and trinonylphenyl phosphite; the catalyst deactivator can be selected from at least one of phosphorous acid, monostearate phosphate, and distearate phosphate; the second additive can be selected from at least one of triphenylmethane triisocyanate, dimethyltriphenylmethane tetraisocyanate and other isocyanates. In the process of mixing and preheating lactide, the catalyst, the initiator and the first additive to prepare the preheated mixture, it can be selected in a device with dispersion and heating functions, such as a static mixer. In the process of preparing the intermediate polymer using the preheated mixture, it can be carried out in a reactor with a stirring device; in one embodiment, the reactor with a stirring device is a continuous reactor, such as a fully mixed flow kettle reactor, and illustratively, the fully mixed flow kettle reactor is the stirring container 2 disclosed in CN101820996A; in addition, in some embodiments, the process is carried out in two fully mixed flow kettle reactors, specifically as follows: the obtained preheated mixture is first polymerized in the first fully mixed flow kettle reactor at a temperature of 175~185°C for 1.5~2.5h, and then transported to the second fully mixed flow kettle reactor at a temperature of 200~210°C for 1~2h to obtain an intermediate polymer with a specific viscosity of 1.45~1.5 and a lactide monomer mass percentage of 3~3.2%. The above evaporation treatment can be carried out in an evaporator, such as a thin film scraper evaporator. The reaction after adding the second additive to the obtained polymer after the volatile matter is removed can be carried out in a device with dispersion and heating functions, such as a static mixer.

[0050] Compared with the prior art, the beneficial effect of the present application is that the present application ensures that the optical purity, specific viscosity and mass percentage of lactide monomer of polylactic acid are within a specific range, and the melt index and the test temperature satisfy a specific relationship, thereby ensuring that the melt strength is high and suitable for processing film materials, and the obtained film materials have high transverse and longitudinal tear strengths. DETAILED DESCRIPTION

[0051] In order to better illustrate the purpose, technical scheme and advantages of the present application, the present application will be further described below in conjunction with specific embodiments and comparative examples, the purpose of which is to understand the content of the present application in detail, rather than to limit the present application. All other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present application. The experimental reagents and instruments involved in the implementation of this application, unless otherwise specified, are commonly used commercially available ordinary reagents and instruments. In the present application, the technical features described in an open manner include closed technical schemes composed of the listed features, and also include open technical schemes containing the listed features.

[0052] Example 1 This embodiment provides a method for preparing polylactic acid, comprising the following steps: 3000 kg of lactide, catalyst, initiator and additives were mixed in a static mixer and preheated to temperature T 0 , obtaining a preheated mixture; The obtained preheated mixture is transported to a fully mixed flow tank reactor at a pressure of P 1 , Temperature T 1 and time t 1 The first reaction was carried out under the condition of 1 And the lactide monomer content is α 1 Prepolymers; The obtained prepolymer is transferred to a plug flow reactor at a pressure of P 2 , Temperature T 2 and time t 2 The second reaction is carried out under the condition of 2 And the lactide monomer content is α 2 Intermediate polymers; The obtained intermediate polymer is added with a catalyst deactivator and then transported to a thin film scraper evaporator at a pressure of P 3 , Temperature T 3 and time t 3 Evaporating under low temperature to remove unreacted lactide monomer, and pelletizing to obtain polylactic acid; Among them, the acid value Q and the L-lactide content α of the lactide used 0 , type and amount of catalyst 1(calculated based on the mass of lactide), type and amount of initiator 2 (calculated based on the mass of lactide), type and amount of additives m 3 (calculated based on the mass of lactide), type and amount of catalyst deactivator m 4 The results of the above tests are shown in Table 1 (calculated based on the mass of lactide), the temperature of the preheated mixture, the pressure, temperature and time of the first reaction, the second reaction and the evaporation, the specific viscosity of the prepolymer and the intermediate polymer and the lactide monomer content.

[0053] Embodiments 2 to 8 These embodiments all provide a method for preparing polylactic acid. The differences between these preparation methods and Example 1 are shown in Table 1 or Table 2.

[0054] Example 9 This embodiment provides a method for preparing polylactic acid, comprising the following steps: 3000 kg of lactide (same as in Example 1, same batch), stannous octoate catalyst, butanediol initiator and triphenyl phosphite, a first additive, were mixed in a static mixer and preheated to 120° C. to obtain a preheated mixture; The obtained preheated mixture was conveyed to a first fully mixed flow tank reactor, polymerized at a temperature of 180°C with a residence time of 2 hours, and then conveyed to a second fully mixed flow tank reactor, polymerized at a temperature of 205°C with a residence time of 1.5 hours to obtain an intermediate polymer with a specific viscosity of 1.48 and a lactide monomer mass percentage of 3.1%.

[0055] Adding phosphorous acid, a catalyst deactivator, to the obtained intermediate polymer, conveying it to a thin film scraper evaporator, and then evaporating it for 30 minutes under the conditions of a pressure of 80 Pa and a temperature of 210° C. to remove volatiles, thereby obtaining a polymer after removing volatiles; A second additive, triphenylmethane triisocyanate, is added to the obtained polymer after removing volatiles, and then the mixture is evenly mixed in a static mixer, the residence time is controlled to be 0.4 h, the reaction temperature is 195° C., and polylactic acid is obtained after pelletizing and drying. Calculated based on the mass of lactide, the mass of the first additive is 0.05%, the mass of the catalyst is 0.005%, the mass of the initiator is 0.2%, the mass of the catalyst passivator is 0.005%, and the mass of the second additive is 0.25%.

[0056] Comparative Example 1 This comparative example provides a polylactic acid, which is a polylactic acid resin with the brand name LX930 produced by Total Energy Cobien.

[0057] Comparative Examples 2 to 10 These comparative examples all provide a method for preparing polylactic acid. The differences between these preparation methods and Example 1 are shown in Table 3 or Table 4.

[0058] Effect Example 1 The polylactic acid obtained in the above embodiments and comparative examples was made into film materials according to the following method: PLA, PBAT and additives are fed from the main feeding port of a twin-screw extruder, and inorganic fillers are fed from the side feeding port, melt-extruded into granules, cooled, air-dried, granulated, dried, and homogenized to obtain a biodegradable composition.

[0059] PLA is obtained according to the methods in Examples 1 to 9 and Comparative Examples 1 to 10, and the dosage is 5 parts by weight; PBAT is selected from the KB100 product of Kingfa Science & Technology Co., Ltd., and the melt flow rate of the PBAT under a load of 2.16 kg at 190°C is 4.1 g / 10 min according to ISO 1133-1-2011, and the dosage is 70 parts by weight; the inorganic filler is HTPUTtra5L talc powder of Liaoning Aihai Company, with a particle size of 2 μm at D50, and the dosage is 25 parts by weight; the processing aid is erucic acid amide, and the dosage is 0.5 parts by weight. The twin-screw extruder selected for melt extrusion has an aspect ratio of 48:1, and the melt extrusion processing temperature is 185°C.

[0060] The obtained biodegradable composition was subjected to film blowing to obtain a film bag, the screw aspect ratio was 32:1, a spiral flow channel die head was used, the air ring was a double-opening air ring, the film blowing processing temperature was set at 150°C, the blow-up ratio was 3.5, and the film thickness was controlled at 20μm. The obtained film material was subjected to the following performance tests: Tear strength: Tear strength test was performed on film bags at 23°C according to standard ISO 6383-2:2004.

[0061] The test results are shown in Table 5.

[0062] Table 1 Table 2 Table 3 Table 4 Table 5 From the above data, it can be seen that the polylactic acid in each embodiment of the present application is suitable for film processing, and the obtained film has high transverse and longitudinal tear strength, such as a transverse tear strength of more than 3600mN and a longitudinal tear strength of more than 2100mN. Comparative Examples 1 to 10 do not meet the requirements of the optical purity, specific viscosity, molecular weight distribution index PDI and / or melt index of polylactic acid and the relationship between the test temperature, resulting in deviations in the transverse and longitudinal tear strength of the film.

[0063] Effect Example 2 The PLA obtained in Example 1 was used to prepare biodegradable compositions A and B according to the formula in Table 6, and then these biodegradable compositions were processed into film bags according to the processing method in Example 1. The other raw materials used were the same as in Example 1, and the test was carried out according to the test method in Example 1.

[0064] Table 6 Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present application rather than to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, ordinary technicians in this field should understand that the technical solution of the present application can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present application.

Claims

1. A polylactic acid, characterized in that: The optical purity of the polylactic acid is 87.3wt.%~95.2wt.%, the specific viscosity is 1.2~1.5, and the molecular weight distribution index PDI is 1.3~2.7; according to ISO 1133-1:2022, under a load of 5kg and a test temperature range of 140°C~190°C, the melt index of the polylactic acid and the test temperature satisfy the following relationship: Y=AX 2 +BX+C, Wherein, Y is the melt index, in g / 10min; X is the test temperature in °C; A is 3×10 -3 ~8×10 -3 ; B is -2.1~-1.0; C is 70~140.

2. The polylactic acid according to claim 1, characterized in that The mass percentage of lactide monomer in the polylactic acid is less than 0.5%.

3. The polylactic acid according to claim 2, characterized in that The mass percentage of lactide monomer in the polylactic acid is 0.1% to 0.45%.

4. A biodegradable composition, characterized in that The invention comprises the following components in parts by weight: 58 to 88 parts of biodegradable polyester, 4 to 10 parts of the polylactic acid according to any one of claims 1 to 3, 10 to 30 parts of inorganic filler, and 0.1 to 0.6 parts of auxiliary agent.

5. The biodegradable composition according to claim 4, characterized in that At least one of the following conditions is met: S1. The biodegradable polyester comprises PBAT; S2. The inorganic filler comprises at least one of calcium carbonate and talc; S3. The auxiliary agent includes at least one of an opening agent and a lubricant.

6. Use of the polylactic acid according to any one of claims 1 to 3 or the biodegradable composition according to claim 4 or 5 in film bag products.

7. A method for preparing polylactic acid according to any one of claims 1 to 3, characterized in that: The following steps are involved: Mixing lactide, additives, catalysts and initiators, and preheating to obtain a preheated mixture; The obtained preheated mixture is subjected to a first reaction to obtain a prepolymer having a specific viscosity of 0.6 to 1 and a lactide monomer mass percentage of 15% to 30%; The obtained prepolymer is subjected to a second reaction to obtain an intermediate polymer having a specific viscosity of 1.20 to 1.50 and a lactide monomer content of 1% to 5% by weight; Adding a catalyst deactivator to the obtained intermediate polymer, and then evaporating it for 10 to 60 minutes at a pressure of 0 to 300 Pa and a temperature of 180 to 230° C. to obtain polylactic acid; The additive is a multifunctional epoxy compound, and the functionality of the multifunctional epoxy compound is ≥3; The mass percentage of L-lactide in the lactide is 87.5% to 95%; Calculated based on the mass of the lactide, the mass of the additive is 0.1% to 0.5%.

8. The method for preparing polylactic acid according to claim 7, characterized in that: The functionality of the multifunctional epoxy compound is 3 to 6.

9. The method for preparing polylactic acid according to claim 7, characterized in that: The additive includes at least one of N,N-diglycidyl-4-glycidyloxyaniline, N,N,N',N'-tetraglycidyl-4,4'-diamino-3,3'-diethyldiphenylmethane, epoxyglycerol triester, and pentaerythritol glycidyl ether.

10. The method for preparing polylactic acid according to claim 7, characterized in that: At least one of the following conditions is met: S1. The acid value of the lactide is ≤10 mol / t; S2. Based on the mass of the lactide, the mass of the catalyst is 0.01% to 0.1%, the mass of the initiator is 0.17% to 0.25%, and the mass of the catalyst deactivator is 0.01% to 0.1%; S3. The temperature of the preheated mixture is 90 to 150 ° C; S4. The first reaction is carried out at a pressure of 0 to 50 kPa and a temperature of 170 to 200 ° C for 1 to 3 hours; S5. The second reaction is carried out at a pressure of 0.5~2MPa and a temperature of 180~210°C for 1~3h; S6. The catalyst comprises a tin compound; S7. The initiator comprises a hydroxyl-containing compound; S8. The catalyst deactivator includes at least one of phosphorous acid, monostearate phosphate, and distearate phosphate.

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