Polylactic acid, biodegradable composition, and preparation method and application thereof

Abstract

This application discloses polylactic acid, a biodegradable composition, and its preparation and application, belonging to the field of polymer materials technology. The polylactic acid disclosed in this application exhibits specific optical purity, specific viscosity, molecular weight distribution index (PDI), and melt index versus test temperature, making it suitable for processing into film materials. The resulting film materials exhibit high transverse and longitudinal tear strength.

Description

Technical Field

[0001] The present application belongs to the field of polymer material technology, 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), poly (D,L-lactic acid) and their copolymers, encompassing a range of products from semi-crystalline to amorphous. It has a wide range of applications and can be used as a general-purpose plastic in industrial and agricultural fields, such as agricultural films, water-retaining materials for desert greening, fibers, food containers, household 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, making it suitable for use in biomedical materials, sustained-release drug carriers, surgical implant materials, etc.

[0003] Polylactic acid (PLA) has a relatively low molecular weight, few molecular chain entanglements, and low melt strength, making it difficult to blow film, limiting its application in film products. Generally speaking, polymer melt strength can be improved by increasing molecular weight or introducing long chain branches. However, increasing molecular weight increases the shear viscosity of PLA, resulting in poor processing performance and reduced mechanical properties of film bags. Introducing long chain branches also makes it difficult to achieve consistent improvements, which can affect 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 existing technology, the purpose of this application is to provide a polylactic acid, a biodegradable composition and its preparation method and application. The polylactic acid is suitable for processing film materials, and the obtained film materials have high transverse and longitudinal tear strength.

[0006] In order to achieve the above-mentioned object, in a first aspect, the present application provides a polylactic acid having an optical purity of 87.3 wt.% to 95.2 wt.%, 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 5 kg, the melt index of the polylactic acid and the test temperature satisfy the following relationship: Y=AX 2 +BX+C,

[0007] Wherein, Y is the melt index, in g / 10min;

[0008] X is the test temperature, in °C;

[0009] A is 3×10 -3 ~8×10 -3;

[0010] B is -2.1~-1.0;

[0011] C is 70~140.

[0012] The inventors have discovered that polylactic acid (PLA) that meets a specific relationship between melt index and test temperature exhibits higher melt strength. The relationship between the melt index and test temperature of PLA is influenced by factors such as molecular weight distribution and the structural characteristics of the molecular chain. This relationship can be regulated by adjusting the amount of additives, catalysts, and / or initiators that increase the degree of PLA branching, as well as the temperature and / or time of the polymerization reaction. Furthermore, optical purity can be regulated by adjusting the L-lactide content of the lactide raw material. Specific viscosity reflects the flow characteristics of the melt and the strength of internal intermolecular interactions. It is influenced by factors such as temperature, molecular structure, and molecular interactions, 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 and is closely related to melt strength, mechanical properties, and melt stability. It can be regulated by adjusting the amount of additives, catalysts, and / or initiators, and / or the temperature and / or time of the polymerization reaction.

[0013] The relationship between the melt index and test temperature of polylactic acid, as well as its optical purity, specific viscosity, and molecular weight distribution index (PDI), collectively influence the transverse and longitudinal tear strength of films formed from the resin. This application achieves high melt strength for polylactic acid, suitable for film processing, by controlling the optical purity, specific viscosity, and molecular weight distribution index (PDI) of polylactic acid within specific ranges, and by ensuring a specific relationship between the melt index and test temperature. The resulting films exhibit high transverse and longitudinal tear strength.

[0014] The optical purity of the polylactic acid is 87.3 wt.% to 95.2 wt.%, such as 87.3 wt.%, 87.5 wt.%, 88 wt.%, 89 wt.%, 90 wt.%, 91 wt.%, 92 wt.%, 93 wt.%, 94 wt.%, 95 wt.%, 95.2 wt.%, or an interval formed by any two of the above values. The optical purity of the polylactic acid can be measured as follows:

[0015] An Agilent 8860 gas chromatograph with a CP7502 column was used. The following method was used: 100 ± 10 mg of polylactic acid (PLA) sample was weighed and placed in a hydrothermal reactor. 10.0 mL of methanol and one drop of 1 mol / L NaOH solution were added, and the reactor was sealed. The reactor was placed in a forced-air oven at 150°C. After 60 minutes, the reactor was removed and cooled to room temperature under running water (approximately 10 minutes). The filtered sample solution was transferred to a small glass vial suitable for gas chromatography injection. The analysis was performed according to the specified parameters, with each sample solution tested at least three times. The peaks corresponding to D-methyl lactate and L-methyl lactate were identified based on their retention times, and the peak areas for D-methyl lactate and L-methyl lactate were recorded.

[0016] [%]

[0017] A DML : Peak area of D-methyl lactate

[0018] A LML : Peak area of L-methyl lactate

[0019] According to the peak areas of D-methyl lactate and L-methyl lactate, the content of L-lactic acid in the polylactic acid is calculated according to the above formula, which is the optical purity of the polylactic acid.

[0020] 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:

[0021] Weigh 0.1250±0.0001g of a polylactic acid sample 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 the solution in a water bath at 25±0.01°C for 10 minutes, and then measure the elution time of the sample solution and the mixed solvent of phenol and o-dichlorobenzene at 25°C in an Ubbelohde capillary viscometer, and calculate the specific viscosity;

[0022] The formula for increasing specific viscosity is: η sp =(η-η0) / η0,

[0023] Where η is the viscosity of the sample solution, in Pa·s;

[0024] η0 is the viscosity of the mixed solvent of phenol and o-dichlorobenzene, in Pa·s;

[0025] η spis the specific viscosity of polylactic acid, dimensionless.

[0026] 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 any two of the above values. The PDI of the polylactic acid can be measured by gel permeation chromatography (GPC), such as using a Waters ACQUITY APC™ instrument, at a test temperature of 40°C, using three ACQUITY APC™ XT45, XT200, and XT459 columns in series, using tetrahydrofuran as the solvent, a mobile phase flow rate of 0.5 mL / min, and using polystyrene standards as the standard sample. The results are averaged three times.

[0027] 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 range formed by any two of the above values.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] More preferably, the mass percentage of lactide monomer in the polylactic acid is 0.1% to 0.45%.

[0032] The mass percentage of the lactide monomer in the polylactic acid can be measured by the following method:

[0033] Accurately weigh approximately 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, precipitate the high molecular weight product with 10 mL of n-hexane. Take the liquid phase and pass it through a polytetrafluoroethylene needle filter (0.45 μm pore size) into a 2 mL injection vial. Detect by gas chromatography, and calculate the lactide monomer content by the integrated area ratio of the specific peak position.

[0034] In a second aspect, the present application provides a biodegradable composition comprising the following components by weight: 58-88 parts of a biodegradable polyester, 4-10 parts of the polylactic acid, 10-30 parts of an inorganic filler, and 0.1-0.6 parts of an additive. By adding a specific polylactic acid and controlling the contents of the aforementioned components within specific ranges, the biodegradable composition not only has high melt strength but also good processing properties, is less susceptible to crystallization problems, and is suitable for processing film materials.

[0035] Preferably, the biodegradable polyester includes PBAT (polybutylene adipate terephthalate).

[0036] Preferably, the PBAT has a melt flow rate of 3 to 5 g / 10 min at 190° C. and a load of 2.16 kg as measured according to ISO 1133-1-2011. For example, the PBAT has a melt flow rate of 3.0 g / 10 min, 3.2 g / 10 min, 3.4 g / 10 min, 3.6 g / 10 min, 3.8 g / 10 min, 4.0 g / 10 min, 4.2 g / 10 min, 4.4 g / 10 min, 4.6 g / 10 min, 4.8 g / 10 min, 5.0 g / 10 min, or an interval formed by any two of the above values as measured at 190° C. and a load of 2.16 kg as measured according to ISO 1133-1-2011.

[0037] 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 a range formed by any two of the above values.

[0038] Preferably, the inorganic filler includes at least one of calcium carbonate and talc.

[0039] 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.

[0040] More preferably, the D50 particle size of the inorganic filler is 2-4 μm.

[0041] The D50 particle size of the inorganic filler is determined 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%.

[0042] Preferably, the auxiliary agent includes at least one of an opening agent and a lubricant.

[0043] Preferably, the anti-blocking agent includes at least one of talc, silicon dioxide, and PE wax.

[0044] Preferably, the lubricant includes at least one of erucamide, oleamide, monostearate glyceric acid, pentaerythritol stearate, PE wax, and ethylene bisstearamide EBS.

[0045] The biodegradable composition described herein may contain common additives such as antioxidants, light stabilizers, plasticizers, and compatibilizers, provided that the effects of the present invention are not impaired. The antioxidant may be at least one of hindered amine antioxidants, phenolic antioxidants, and phosphite antioxidants; the light stabilizer may be at least one of hindered amines and o-hydroxybenzophenone; the plasticizer may be at least one of citrate, glycerol, and polyethylene glycol; and the compatibilizer may be a maleic anhydride grafted polymer.

[0046] Exemplarily, the method for preparing the biodegradable composition includes the following steps: feeding a biodegradable polyester, polylactic acid, and an additive through the main feed port of a twin-screw extruder, feeding an inorganic filler through a side feed port, melt-extruding and pelletizing, cooling, air-drying, pelletizing, drying, and homogenizing to produce the biodegradable composition. The twin-screw extruder may have an aspect ratio of (44-56):1, and the melt extrusion temperature may be 180°C to 200°C.

[0047] 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, etc.

[0048] In a fourth aspect, the present application provides a method for preparing the polylactic acid, comprising the following steps:

[0049] mixing lactide, additives, a catalyst and an initiator, and preheating the mixture to obtain a preheated mixture;

[0050] 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 content of 15% to 30% by weight;

[0051] performing a second reaction on the obtained prepolymer 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;

[0052] Adding a catalyst deactivator to the obtained intermediate polymer, and then evaporating it under the conditions of pressure of 0-300 Pa and temperature of 180-230° C. for 10-60 minutes to obtain polylactic acid;

[0053] The additive is a multifunctional epoxy compound, and the functionality of the multifunctional epoxy compound is ≥3;

[0054] The mass percentage of L-lactide in the lactide is 87.5% to 95%;

[0055] Calculated based on the mass of the lactide, the mass of the additive is 0.1% to 0.5%.

[0056] Preferably, the functionality of the multifunctional epoxy compound is 3 to 6.

[0057] 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, epoxy triglyceride (CAS No. 8013-07-8), and pentaerythritol glycidyl ether (CAS No. 3126-63-4).

[0058] The mass percentage of L-lactide in 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, and then add one drop of 1 mol / l aqueous NaOH solution. The hydrothermal reactor is sealed and placed in a 150°C oven. After 60 minutes, the reactor is removed and cooled to room temperature with running water (approximately 10 minutes). Gas chromatography is then performed. The mass percentage of L-lactide in the lactide is calculated based on the peak area ratio of D-methyl lactate to L-methyl lactate. Gas chromatography is performed using an Agilent 8860 gas chromatograph using a CP7502 column.

[0059] Preferably, the acid value of the lactide is ≤10 mol / t. More preferably, the acid value of the lactide is 1-7 mol / t.

[0060] The acid value of the lactide can be measured by the following method: using a Swiss Metrohm 905 fully automatic potentiometric titrator, 0.5 g of lactide is dissolved in 60 mL of dichloromethane to obtain a lactide solution; the lactide solution is titrated with a KOH-EtOH (EtOH is ethanol) solution having a KOH concentration of 0.0025 mol / L, and the endpoint is determined by potentiometric titration.

[0061] 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%.

[0062] Preferably, the temperature of the preheated mixture is 90-150°C.

[0063] Preferably, the first reaction is carried out at a pressure of 0-50 kPa and a temperature of 170-200° C. for 1-3 hours.

[0064] 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 hours.

[0065] Preferably, the catalyst comprises a tin compound. More preferably, the catalyst comprises at least one of stannous octoate, SnCl2, SnCl4, SnBr2, SnBr4, butyltin tris(2-ethylhexanoate), hydrated monobutyltin oxide, dibutyltin dilaurate, and tetraphenyltin.

[0066] Preferably, the initiator comprises a hydroxyl-containing compound. More preferably, the initiator comprises at least one of butanediol, dodecanol, and lactic acid.

[0067] Preferably, the catalyst deactivator includes at least one of phosphorous acid, monostearic acid phosphate, and distearic acid phosphate.

[0068] In some embodiments, during the process of obtaining the preheated mixture using lactide, additives, catalyst, and initiator, the mixing and preheating are performed in a mixer, for example, a static mixer.

[0069] 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.

[0070] 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.

[0071] 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:

[0072] Mixing lactide, a catalyst, an initiator, and a first additive, and preheating to 112-125° C. to obtain a preheated mixture;

[0073] The 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 content of 3-3.2% by weight;

[0074] adding a catalyst deactivator to the obtained intermediate polymer, and then evaporating the mixture 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;

[0075] A second additive is added to the polymer obtained after removing volatiles, followed by mixing, reacting at 190-200° C. for 0.3-0.5 h, and then pelletizing and drying 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%. The catalyst may be a tin compound, such as at least one of stannous octoate, SnCl2, SnCl4, SnBr2, SnBr4, butyltin tris(2-ethylhexanoate), hydrated monobutyltin oxide, dibutyltin dilaurate, and tetraphenyltin; the initiator may be a hydroxyl-containing compound, such as at least one of butanediol, dodecanol, and lactic acid; the first additive, used to reduce thermal degradation side reactions and improve the hue of PLA, may be at least one of triphenyl phosphite, triphenyl phosphate, and trisnonylphenyl phosphite; the catalyst deactivator may be at least one of phosphorous acid, monostearic acid phosphate, and distearic acid phosphate; and the second additive may be at least one of isocyanates such as triphenylmethane triisocyanate and dimethyltriphenylmethane tetraisocyanate. The preheating of the lactide, catalyst, initiator, and first additive to prepare the preheated mixture may be performed in a device with dispersing and heating functions, such as a static mixer. The process of preparing the intermediate polymer from the preheated mixture can be carried out in a reactor equipped with a stirring device. In one embodiment, the reactor equipped with a stirring device is a continuous reactor, such as a fully mixed-flow tank reactor. Exemplarily, the fully mixed-flow tank reactor is the stirred vessel 2 disclosed in CN101820996A. In some embodiments, the process is carried out in two fully mixed-flow tank reactors, specifically as follows: the preheated mixture is first polymerized in a first fully mixed-flow tank reactor at a temperature of 175-185°C for 1.5-2.5 hours, then transferred to a second fully mixed-flow tank reactor for polymerization at a temperature of 200-210°C for 1-2 hours, yielding an intermediate polymer having a specific viscosity of 1.45-1.5 and a lactide monomer content of 3-3.2% by weight. The evaporation treatment can be carried out in an evaporator, such as a scraped-film evaporator. The reaction after adding the second additive to the resulting, devolatileized polymer can be carried out in a device equipped with dispersion and heating functions, such as a static mixer.

[0076] Compared with the existing technology, the beneficial effect of the present application is that: by selecting the optical purity, specific viscosity and mass percentage of lactide monomer of polylactic acid within a specific range, and the melt index and test temperature satisfying a specific relationship, the present application ensures that the melt strength is high, which is suitable for processing film materials, and the resulting film materials have high transverse and longitudinal tear strengths. DETAILED DESCRIPTION

[0077] In order to better illustrate the purpose, technical solutions and advantages of the present application, the present application will be further described below in conjunction with specific embodiments and comparative examples. Its purpose is to understand the content of the present application in detail, rather than to limit the present application. All other embodiments obtained by those of ordinary skill in the art 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 are all commonly used commercially available reagents and instruments unless otherwise specified. In this application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions comprising the listed features.

[0078] Example 1

[0079] This embodiment provides a method for preparing polylactic acid, comprising the following steps:

[0080] 3000 kg of lactide, a catalyst, an initiator, and additives were mixed in a static mixer and preheated to a temperature T0 to obtain a preheated mixture;

[0081] The obtained preheated mixture is conveyed to a fully mixed flow tank reactor, and a first reaction is carried out at a pressure P1, a temperature T1 and a time t1 to obtain a prepolymer having a specific viscosity of η1 and a lactide monomer content of α1;

[0082] The obtained prepolymer is transferred to a plug flow reactor and subjected to a second reaction at a pressure P2, a temperature T2 and a time t2 to obtain an intermediate polymer having a specific viscosity of η2 and a lactide monomer content of α2;

[0083] After adding a catalyst deactivator, the obtained intermediate polymer is transported to a thin film scraper evaporator, evaporated at pressure P3, temperature T3 and time t3 to remove unreacted lactide monomer, and pelletized to obtain polylactic acid;

[0084] Table 1 contains the acid value Q and L-lactide content α0 of the lactide used, the type and amount m1 of the catalyst (calculated based on the mass of lactide), the type and amount m2 of the initiator (calculated based on the mass of lactide), the type and amount m3 of the additive (calculated based on the mass of lactide), the type and amount m4 of the catalyst deactivator (calculated based on the mass of lactide), the temperature of the preheated mixture, the pressure, temperature, and time of the first reaction, second reaction, and evaporation, the specific viscosity of the prepolymer and intermediate polymer, and the lactide monomer content.

[0085] Examples 2 to 8

[0086] 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.

[0087] Example 9

[0088] This embodiment provides a method for preparing polylactic acid, comprising the following steps:

[0089] 3000 kg of lactide (the same batch as in Example 1), stannous octoate catalyst, butanediol initiator, and triphenyl phosphite as the first additive were mixed in a static mixer and preheated to 120° C. to obtain a preheated mixture;

[0090] The obtained preheated mixture was conveyed to a first fully mixed flow tank reactor and polymerized at a temperature of 180°C with a residence time of 2 hours. The mixture was then conveyed to a second fully mixed flow tank reactor and polymerized at a temperature of 205°C with a residence time of 1.5 hours to obtain an intermediate polymer having a specific viscosity of 1.48 and a lactide monomer mass percentage of 3.1%.

[0091] Adding phosphorous acid, a catalyst deactivator, to the obtained intermediate polymer, conveying the polymer to a thin film scraper evaporator, and then evaporating the polymer at a pressure of 80 Pa and a temperature of 210° C. for 30 minutes to remove volatiles, thereby obtaining a polymer after devolatileization;

[0092] A second additive, triphenylmethane triisocyanate, is added to the obtained polymer after removing volatiles, and then the mixture is uniformly mixed in a static mixer, with a residence time of 0.4 h and a reaction temperature of 195° C., followed by pelletization and drying to obtain polylactic acid. 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 deactivator is 0.005%, and the mass of the second additive is 0.25%.

[0093] Comparative Example 1

[0094] This comparative example provides a polylactic acid, which is a polylactic acid resin with the brand name LX930 produced by Total Energy Cobien.

[0095] Comparative Examples 2-10

[0096] 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.

[0097] Effect Example 1

[0098] The polylactic acid obtained in the above examples and comparative examples was made into a film material according to the following method:

[0099] 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. The products are melt-extruded into granules, cooled, air-dried, granulated, dried, and homogenized to prepare a biodegradable composition.

[0100] PLA was obtained according to the methods of Examples 1-9 and Comparative Examples 1-10, with an amount of 5 parts by weight. PBAT was selected from Kingfa Science & Technology Co., Ltd., brand KB100, with a melt flow rate of 4.1 g / 10 min at 190°C and a load of 2.16 kg, as measured according to ISO 1133-1-2011. The amount used was 70 parts by weight. The inorganic filler was HTPUTtra5L talc powder from Liaoning Aihai Company, with a D50 particle size of 2 μm, with an amount of 25 parts by weight. The processing aid was erucamide, with an amount of 0.5 parts by weight. The melt extrusion process was performed on a twin-screw extruder with an aspect ratio of 48:1, and the processing temperature was 185°C.

[0101] The resulting biodegradable composition was blown into film bags using a screw length-to-diameter ratio of 32:1, a spiral flow channel die head, a double-opening air ring, a set temperature of 150°C, a blow-up ratio of 3.5, and a film thickness of 20 μm. The resulting film was subjected to the following performance tests:

[0102] Tear strength: Tear strength test was performed on film bags at 23°C according to standard ISO 6383-2:2004.

[0103] The test results are shown in Table 5.

[0104] Table 1

[0105]

[0106] Table 2

[0107]

[0108] Table 3

[0109]

[0110] Table 4

[0111]

[0112] Table 5

[0113]

[0114] The above data demonstrate that the polylactic acid used in each of the examples of this application is suitable for film processing, and the resulting films exhibit high transverse and longitudinal tear strengths, such as transverse tear strengths exceeding 3600 mN and longitudinal tear strengths exceeding 2100 mN. Comparative Examples 1-10 exhibited discrepancies in transverse and longitudinal tear strength due to the relationship between the optical purity, specific viscosity, molecular weight distribution index (PDI), and / or melt index of the polylactic acid and the test temperature not meeting the requirements.

[0115] Effect Example 2

[0116] The PLA obtained in Example 1 was used to prepare biodegradable compositions A and B according to the formulations in Table 6. These biodegradable compositions were then 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 tests were carried out according to the testing method in Example 1.

[0117] Table 6

[0118]

[0119] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not intended 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, those skilled in the art should understand that the technical solutions of the present application may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A polylactic acid, characterized in that The optical purity of the polylactic acid is 87.5 wt.% to 95 wt.%, the specific viscosity is 1.2 to 1.5, and the molecular weight distribution index (PDI) is 1.3 to 2.

7. According to ISO 1133-1:2022, under a test temperature range of 140° C. to 190° C. under a load of 5 kg, 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 to -1.0; C is 70~140.

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

3. The polylactic acid according to claim 2, wherein The mass percentage of the 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, wherein At least one of the following conditions is met: S1. The biodegradable polyester includes 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, a catalyst and an initiator, and preheating the mixture 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 content of 15% to 30% by weight; 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 under the conditions of 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, wherein: The functionality of the multifunctional epoxy compound is 3 to 6.

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

10. The method for preparing polylactic acid according to claim 7, wherein: 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 to 2 MPa and a temperature of 180 to 210 ° C for 1 to 3 hours; 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, monostearic acid phosphate, and distearic acid phosphate.

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