Polylactic acid and preparation method and application thereof

Abstract

The application discloses polylactic acid and a preparation method and application thereof, and belongs to the technical field of high polymer materials. The polylactic acid has specific optical purity, a molecular weight distribution index PDI and a shear viscosity at 190 DEG C and a shear rate of 50 s ‑1 -1, and has good heat sealing performance and mechanical properties, and is suitable for preparing film bag products.

Description

Technical Field

[0001] This application belongs to the field of polymer materials technology, specifically relating to polylactic acid, its preparation method and application. Background Technology

[0002] Polylactic acid (PLA), as a biodegradable material, possesses high transparency, gloss, and breathability, as well as high modulus, full foldability, and entanglement retention. These properties allow PLA to be processed into various products, which can be completely biodegraded into water and carbon dioxide through recycling and composting after disposal. These characteristics make PLA an environmentally friendly material that can replace traditional petrochemical plastics such as PS, PP, and ABS. Especially in the food packaging field, PLA can be processed into various shapes of products, such as food containers (bottles, trays, etc.), films, heat-shrink packaging, breathable packaging, aroma-preserving packaging, shopping bags, and garbage bags, through different processing methods such as melt extrusion, injection molding, blow molding, foaming, and vacuum forming.

[0003] Although polylactic acid (PLA) films have many excellent properties and promising applications, they also face some challenges in their development, such as being brittle and having a narrow processing window, which leads to poor mechanical and heat-sealing properties. Summary of the Invention

[0004] Based on the deficiencies of the existing technology, the purpose of this application is to provide polylactic acid, its preparation method and application, which has good mechanical properties and heat-sealing properties after being processed into a film.

[0005] To achieve the above objectives, in a first aspect, this application provides a polylactic acid with an optical purity of 87 wt.% to 92.5 wt.%, a molecular weight distribution index (PDI) of 1.5 to 3.0, and a shear rate of 50 s at 190 °C. -1 The lower shear viscosity is 20–100 Pa·s.

[0006] The molecular weight distribution index (PDI) is an indicator reflecting the molecular weight distribution in polymer materials. Molecular weight distribution is closely related to the physical and mechanical properties of polymers and processing procedures such as molding, film formation, and spinning. Therefore, controlling molecular weight distribution is an important factor in controlling and improving product quality. (The last sentence appears to be unrelated and refers to a temperature and shear rate of 50s.) -1 Lower shear viscosity reflects the deformation and flow properties of a substance under external force. It mainly refers to the relationship between stress, deformation, deformation rate and viscosity during processing. Maintaining high viscosity at medium and high shear rates is very important for the stability of the processing and is beneficial for obtaining membrane materials with good mechanical properties.

[0007] This application achieves this by controlling the optical purity, molecular weight distribution index (PDI), and temperature of polylactic acid at 190°C and a shear rate of 50 s. -1 The lower shear viscosity is within a specific range, ensuring a suitable melting point, processing temperature, and heat-sealing temperature, which is conducive to heat sealing. At the same time, it avoids the large molecular chains in polylactic acid from becoming entangled and straightened under medium- and high-speed shear during processing, thus preventing unstable flow. This ensures good melt stability under medium- and high-speed shear, which helps improve the stability of the processing and lays the foundation for the resin to have good heat-sealing performance and mechanical properties.

[0008] Optical purity, molecular weight distribution index (PDI), and performance at 190°C and a shear rate of 50 s⁻¹ of polylactic acid. -1 Lower shear viscosity significantly affects the heat-sealing performance of resins, including heat-sealing strength, as well as their mechanical properties such as toughness, stiffness, and strength. The optical purity of polylactic acid (PLA) can be adjusted by changing the L-lactide content in the lactide raw material. Furthermore, the molecular weight distribution index (PDI) of PLA can be adjusted by modifying the polymerization process's temperature, pressure, residence time, catalyst dosage, initiator dosage, first additive dosage, second additive dosage, intermediate polymer viscosity, and monomer content. This adjustment can be achieved at 190℃ and a shear rate of 50 s⁻¹. -1 Adjustment of lower shear viscosity.

[0009] The optical purity of the polylactic acid is 87 wt.% to 92.5 wt.%, such as 87.5 wt.%, 88 wt.%, 89 wt.%, 90 wt.%, 91 wt.%, 92 wt.%, or any two of the above values ​​within the range. The optical purity of the polylactic acid can be measured by the following method:

[0010] An Agilent 8860 gas chromatograph with a CP7502 column was 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, and 1 drop of mol / L NaOH aqueous solution was added. The hydrothermal reactor was then sealed. The reactor was placed in a 150℃ forced-air oven for 60 min. After 60 min, the reactor was removed and cooled to room temperature by running water (approximately 10 min). The filtered sample solution was transferred to a small glass vial suitable for gas chromatography injection. Tests were 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 determined based on the retention times, and the peak areas of D-methyl lactate and L-methyl lactate were recorded.

[0011]

[0012] A DML Peak area of ​​D-methyl lactate

[0013] A LML Peak area of ​​L-methyl lactate

[0014] Based on the peak areas of D-lactic acid methyl ester and L-lactic acid methyl ester, the content of L-lactic acid in polylactic acid is calculated according to the above formula, which is the optical purity of polylactic acid.

[0015] The molecular weight distribution index (PDI) of the polylactic acid is 1.5 to 3.0, such as 1.5, 1.8, 2.0, 2.2, 2.4, 2.6, 2.8, 3.0 or any two of the above values ​​forming a range.

[0016] The polylactic acid was subjected to a temperature of 190°C and a shear rate of 50 s. -1 The lower shear viscosity is 20–100 Pa·s, such as 20 Pa·s, 30 Pa·s, 40 Pa·s, 50 Pa·s, 60 Pa·s, 70 Pa·s, 80 Pa·s, 90 Pa·s, 100 Pa·s, or any two of the above values ​​within the range. The polylactic acid exhibits this viscosity at 190°C and a shear rate of 50 s⁻¹. -1 The shear viscosity can be tested according to the standard requirements of GB / T33061.10-2016 "Determination of dynamic mechanical properties of plastics - Part 10: Determination of shear viscosity using a parallel plate oscillating rheometer" and JY-T 0590-2020 "General rules for measurement methods of rotational rheometer". The sample size is a 10cm*10cm square plate with a thickness of 3mm.

[0017] Preferably, the Z-average molecular weight of the polylactic acid is 19,000 to 31,000 Da. For example, the Z-average molecular weight of the polylactic acid is within the range formed by any two of the following values: 19,000 Da, 20,000 Da, 21,000 Da, 22,000 Da, 23,000 Da, 24,000 Da, 25,000 Da, 26,000 Da, 27,000 Da, 28,000 Da, 29,000 Da, 30,000 Da, 31,000 Da.

[0018] More preferably, the polylactic acid has a Z-average molecular weight of 22,500 to 27,500 Da.

[0019] The Z-average molecular weight of polylactic acid (PLA) reflects the viscosity properties of the polymer and is closely related to its structure. It is crucial for the melt strength and mechanical properties of PLA products during processing. Controlling the Z-average molecular weight of PLA within the range of 19,000–31,000 Da, particularly within the range of 22,500–27,500 Da, helps improve the balance of PLA's heat-sealing strength, dart impact strength, puncture strength, and other mechanical properties, resulting in better overall performance of the film and bag products made from it.

[0020] The Z-average molecular weight of polylactic acid can be adjusted by modifying the temperature, pressure, residence time, catalyst dosage, initiator dosage, first additive dosage, second additive dosage, viscosity of intermediate polymer, and monomer content during the polymerization process.

[0021] The Z-average molecular weight and PDI of the polylactic acid can both be determined by gel permeation chromatography (GPC), such as using Waters' ACQUITY APC. TM The equipment was tested at a temperature of 40℃ using ACQUITY APC. TM The chromatographic column was set up in series with three columns: XT45, XT200 and XT459. The solvent was tetrahydrofuran and the mobile phase flow rate was 0.5 mL / min. Polystyrene standard was used as the standard sample. The results were taken as the average of three measurements.

[0022] Preferably, the intrinsic viscosity of the polylactic acid is 1.22–1.57 dL / g. For example, the intrinsic viscosity of the polylactic acid is within the range of 1.22 dL / g, 1.3 dL / g, 1.4 dL / g, 1.5 dL / g, 1.57 dL / g, or any two of the above values. The intrinsic viscosity of the polylactic acid is measured using an Ubbelohde capillary viscometer (Type 1835, 0.83 mm inner diameter). The specific test method is as follows:

[0023] Weigh 0.1250±0.0001g of the sample and place it in a 250mL conical flask. Add 25mL of a mixed solvent of phenol and o-dichlorobenzene (volume ratio of phenol to o-dichlorobenzene is 3:2). Heat to dissolve and then cool to room temperature to obtain the sample solution. Then place it in a water bath at 25±0.01℃ for 10min. Then measure the outflow time of the sample solution and the above-mentioned mixed solvent of phenol and o-dichlorobenzene at 25℃ using an Ubbelohde capillary viscometer and calculate the intrinsic viscosity.

[0024] The formula for intrinsic viscosity is: η sp =(η-η0) / η0,

[0025] Where η is the viscosity of the sample solution, in Pa·s;

[0026] η0 is the viscosity of the mixed solvent of phenol and o-dichlorobenzene, in Pa·s;

[0027] η sp It is the intrinsic viscosity, measured in dL / g.

[0028] Preferably, the polylactic acid contains less than 0.5% by mass of lactide monomer. More preferably, the polylactic acid contains 0.08% to 0.43% by mass of lactide monomer. The polylactic acid contains lactide monomer in a range of 0.49%, 0.45%, 0.40%, 0.35%, 0.30%, 0.25%, 0.20%, 0.15%, 0.10%, 0.08%, or any two of the above values. The polylactic acid contains lactide monomer by mass of mass of lactide monomer can be determined by the following method:

[0029] Accurately weigh approximately 10 mg of the product and dissolve it at room temperature in 4 mL of chloroform containing 1 mg / mL of internal standard (pentamethylbenzene). After complete dissolution, precipitate the high molecular weight product with 10 mL of n-hexane. Take the liquid phase and inject it into a 2 mL sample vial through a PTFE needle filter (0.45 μm pore size). Detect the sample using gas chromatography and calculate the lactide monomer content by the integral area ratio of a specific peak position.

[0030] The acid value of the polylactic acid can be determined by the following method:

[0031] The test instrument used was a Metrohm 905 fully automatic potentiometric titrator. 0.5g PLA was dissolved in 60mL of dichloromethane to obtain a PLA solution. The PLA solution was titrated with a KOH-EtOH solution with a KOH concentration of 0.0025mol / L, and the endpoint was determined by potentiometric titration.

[0032] Secondly, this application provides a biodegradable composition comprising the following components in parts by weight: 52-91 parts of biodegradable polyester, 3-12 parts of polylactic acid, 7-35 parts of inorganic filler, and 0.2-0.6 parts of additives.

[0033] Preferably, the biodegradable polyester includes PBAT (polybutylene terephthalate).

[0034] Preferably, the melt flow rate measured by the PBAT at 190°C and 2.16 kg load is 3.0–5.0 g / 10 min, according to ISO 1133-1-2011. For example, the melt flow rate measured by the PBAT at 190°C and 2.16 kg load is 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 any two of the above values ​​within a range, according to ISO 1133-1-2011.

[0035] Preferably, the molar ratio of AA (adipic acid) to PTA (terephthalic acid) in the PBAT is (1.0 to 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 any two of the above values ​​within a range.

[0036] Preferably, the inorganic filler includes at least one of calcium carbonate and talc.

[0037] Preferably, the D50 particle size of the inorganic filler is ≤6μm. For example, the D50 particle size of the inorganic filler is within the range formed by any two of the following values: 6μm, 5μm, 4μm, 3μm, 2μm, 1μm.

[0038] More preferably, the D50 particle size of the inorganic filler is 1–6 μm.

[0039] The D50 particle size of the inorganic filler was determined according to GB / T 19077.1-2008 "Particle Size Analysis by Laser Diffraction", and the dispersion used was water with a sample concentration of 5 wt%.

[0040] Preferably, the additive includes at least one of an opening agent and a lubricant.

[0041] Preferably, the opening agent includes at least one of talc, silica, and PE wax.

[0042] Preferably, the lubricant comprises at least one of erucamide, oleamide, glyceryl monostearate, pentaerythritol stearate, PE wax, and ethylene bis-stearamide (EBS).

[0043] For example, the preparation method of the biodegradable composition includes the following steps: feeding biodegradable polyester, polylactic acid, and processing aids through the main feed port of a twin-screw extruder, feeding inorganic filler through the side feed port, melt extruding and granulating, cooling, air drying, pelletizing, drying, and homogenizing to obtain the biodegradable composition. The length-to-diameter ratio of the twin-screw extruder can be selected as (44–56):1, and the melt extrusion temperature can be selected as 180℃–200℃.

[0044] Thirdly, this application provides the use of the polylactic acid or the biodegradable composition in film and bag products. For example, the polylactic acid or the biodegradable composition can be used to prepare packaging bags, etc.

[0045] Fourthly, this application provides a method for preparing the polylactic acid, comprising the following steps:

[0046] The lactide, first additive, catalyst and initiator are mixed and preheated to obtain a preheated mixture;

[0047] The preheated mixture was subjected to a first reaction to obtain a prepolymer with an intrinsic viscosity of 0.7–0.9 dl / g and a lactide monomer mass percentage of 25%–35%.

[0048] A second additive is added to the obtained prepolymer, and then a second reaction is carried out to obtain an intermediate polymer with an intrinsic viscosity of 1.20 to 1.50 dl / g and a lactide monomer mass percentage of 4% to 5%, wherein the second additive includes a bifunctional epoxy compound.

[0049] A catalyst passivator was added to the obtained intermediate polymer, and then the polymer was evaporated for 20 to 40 minutes under a pressure of 0 to 300 Pa and a temperature of 190 to 220 °C to obtain the polylactic acid.

[0050] The lactide in question has a L-lactide content of 88% to 92.5% by mass.

[0051] Based on the mass of the lactide, the mass of the first additive is 0-0.1%, the mass of the catalyst is 0.001%-0.1%, the mass of the initiator is 0.17%-0.25%, the mass of the second additive is 0.05%-0.5%, and the mass of the catalyst passivator is 0.001%-0.1%.

[0052] The first reaction was carried out for 1 to 2.5 hours under pressure of 0 to 60 kPa and temperature of 160 to 190 °C; the second reaction was carried out for 1 to 2.5 hours under pressure of 0.5 to 1.5 MPa and temperature of 180 to 200 °C.

[0053] The mass percentage of L-lactide in the lactide can be determined 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 place it in an oven at 150℃ for 60 min. After 60 min, remove the hydrothermal reactor, cool it to room temperature with running water, and then perform gas chromatography. Calculate the mass percentage of L-lactide in the lactide based on the peak area ratio of D-lactate to L-lactate. The gas chromatography test was performed using an Agilent 8860 gas chromatograph with a CP7502 column.

[0054] Preferably, the bifunctional epoxy compound includes at least one of diglycidyl ether (CAS No.: 2238-07-5), cyclohexane-1,2-dicarboxylic acid diglycidyl ester (CAS No.: 5493-45-8), neopentyl glycol diglycidyl ether (CAS No.: 17557-23-2), and phthalic acid diglycidyl ester (CAS No.: 7195-45-1).

[0055] Preferably, the acid value of the lactide is ≤10 mol / t. The acid value of the lactide can be selected from a range of 10 mol / t, 8 mol / t, 6 mol / t, 4 mol / t, 2 mol / t, 1 mol / t, or any two of the above values.

[0056] In some embodiments, the acid value of the lactide is 1 to 7 mol / t.

[0057] The acid value of the lactide can be determined by the following method: The testing instrument is a Swiss Metrohm 905 fully automatic potentiometric titrator. 0.5g of lactide is dissolved in 60mL of dichloromethane to obtain a lactide solution. The lactide solution is titrated with a KOH-EtOH solution with a KOH concentration of 0.0025mol / L, and the endpoint is determined by potentiometric titration.

[0058] Preferably, the first additive includes at least one of triphenyl phosphate, triethyl phosphate, and triphenyl phosphite; the catalyst includes a tin compound; the initiator includes a hydroxyl-containing compound; and the catalyst passivator includes at least one of phosphorous acid, monostearate phosphate, and distearate phosphate.

[0059] Preferably, the catalyst comprises at least one of stannous octoate, SnCl2, SnCl4, SnBr2, SnBr4, SnO, tris(2-ethylhexanoate)butyltin, hydrated monobutyltin oxide, dibutyltin dilaurate, and tetraphenyltin.

[0060] Preferably, the initiator includes at least one of butanediol, dodecanol, and lactic acid.

[0061] Preferably, the temperature of the preheated mixture is 100–130°C.

[0062] In some embodiments, the mixing and preheating process for obtaining the preheated mixture using lactide, a first additive, a catalyst, and an initiator is carried out in a mixer. For example, the mixer is a static mixer.

[0063] In some embodiments, the first reaction is carried out in a reactor equipped with a stirring device. In one embodiment, the reactor equipped with the stirring device is a continuous reactor, such as a stirred tank reactor. As an example, the stirred tank reactor is the stirred vessel 2 disclosed in CN101820996A.

[0064] 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.

[0065] The polylactic acid can be prepared by the above-described preparation method, or by other preparation methods, such as a method including the following steps:

[0066] The lactide, first additive, catalyst and initiator are mixed and preheated to obtain a preheated mixture;

[0067] The preheated mixture was subjected to polymerization using the following gradient temperature program to obtain an intermediate polymer with an intrinsic viscosity of 1.20–1.50 dl / g and a lactide monomer content of 4%–5% by mass:

[0068] The temperature is raised to a first temperature in the first instant, and then held at the first temperature for a second time.

[0069] The temperature is raised to the second temperature at the third time point, and then held at the second temperature for a fourth time point.

[0070] The temperature is raised to the third temperature at the fifth time point, and then held at the third temperature for a sixth time point.

[0071] The temperature is increased to the fourth temperature at the seventh time point, and then held at the fourth temperature for the eighth time point.

[0072] The first temperature is 128–132℃, the second temperature is 148–152℃, the third temperature is 168–172℃, and the fourth temperature is 188–192℃. The first, second, third, fourth, fifth, sixth, seventh, and eighth times are each 20–40 minutes. The sum of the first, second, third, fourth, fifth, sixth, seventh, and eighth times is 4–5 hours.

[0073] A catalyst passivating agent was added to the obtained intermediate polymer, and then the polymer was evaporated for 20 to 40 minutes under a pressure of 0 to 300 Pa and a temperature of 190 to 220 °C to remove volatiles, thus obtaining a polymer after the removal of volatiles.

[0074] A second additive is added to the obtained polymer after volatile matter removal, and the reaction is carried out at 180℃~190℃ for 0.5~1h. The resulting polylactic acid is then obtained after extrusion granulation and drying. The preheating temperature of the mixture can be selected as 100~130℃; the extrusion temperature can be selected as 180~200℃; based on the mass of lactide, the mass of the first additive can be selected as 0~0.1%, the mass of the catalyst can be selected as 0.001%~0.1%, the mass of the initiator can be selected as 0.17%~0.25%, the mass of the catalyst passivator can be selected as 0.001%~0.1%, and the mass of the second additive can be selected as 0.05%~0.5%. The acid value of lactide can be selected to be ≤10 mol / t, such as 10 mol / t, 8 mol / t, 6 mol / t, 4 mol / t, 2 mol / t, 1 mol / t, or any two of the above values ​​within a range; the mass percentage of L-lactide in the lactide can be selected to be 88%–92.5%; the first additive can be selected to include at least one of triphenyl phosphate, triethyl phosphate, and triphenyl phosphite; the catalyst can be selected to include tin compounds, and the tin compounds can be selected to include stannous octoate, SnCl2, SnCl4, SnBr2, Sn At least one of Br4, SnO, tri(2-ethylhexanoate)butyltin, hydrated monobutyltin oxide, dibutyltin dilaurate, and tetraphenyltin; the initiator may be a hydroxyl-containing compound; the catalyst deactivator may be at least one of phosphorous acid, monostearate phosphate, and distearate phosphate; the second additive includes a bifunctional epoxy compound, which may be at least one of diglycidyl ether, cyclohexane-1,2-dicarboxylic acid diglycidyl ether, neopentyl glycol diglycidyl ether, and diglycidyl phthalate.

[0075] Compared with the prior art, the beneficial effects of this application are as follows: This application selects the optical purity, molecular weight distribution index (PDI), and temperature of polylactic acid (PLA) at 190°C and a shear rate of 50 s⁻¹. -1 With a lower shear viscosity within a specific range, polylactic acid possesses both excellent heat-sealing properties (such as high heat-sealing strength) and mechanical properties (such as good dart impact resistance and high puncture strength), making it suitable for the preparation of film and bag products, especially as a component in the preparation of film and bag products based on PBAT material. Detailed Implementation

[0076] To better illustrate the purpose, technical solutions, and advantages of this application, the following description, in conjunction with specific embodiments and comparative examples, aims to provide a detailed understanding of the content of this application, rather than limiting it. All other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of this application. Unless otherwise specified, the experimental reagents and instruments involved in the implementation of this application are commonly used commercially available reagents and instruments. In this application, the technical features described in an open-ended manner include both closed-ended technical solutions composed of the listed features and open-ended technical solutions that include the listed features.

[0077] Example 1

[0078] This embodiment provides a method for preparing polylactic acid, including the following steps:

[0079] 3000 kg of lactide, stannous octoate catalyst, butanediol initiator and triphenyl phosphite first additive were mixed in a static mixer and preheated to temperature T1 to obtain a preheated mixture.

[0080] The resulting preheated mixture was fed into a fully mixed flow reactor and subjected to a first reaction at pressure P1, temperature T2 and time t1 to obtain a prepolymer with intrinsic viscosity η1 and lactide monomer content α2.

[0081] The obtained prepolymer is fed into a plug flow reactor, a second additive is added, and a second reaction is carried out under pressure P2, temperature T3 and time t2 to obtain an intermediate polymer with intrinsic viscosity η2 and lactide monomer content α3.

[0082] After adding a catalyst passivator to the obtained intermediate polymer, it is fed into a thin-film scraped evaporator and evaporated at pressure P3, temperature T4 and time t3 to remove unreacted lactide monomers. The polymer is then pelletized to obtain polylactic acid.

[0083] The acid value A and L-lactide content α1 of the lactide used, the type and amount of catalyst m1, the type and amount of initiator m2, the type and amount of the first additive m3, the type and amount of the second additive m4, the type and amount of catalyst passivator m5, the preheating temperature of the mixture, the pressure, temperature and time of the first reaction, the second reaction and evaporation, the intrinsic viscosity of the prepolymer and intermediate polymer and the lactide monomer content are all shown in Table 1.

[0084] Examples 2-8 and Comparative Examples 1-7

[0085] These examples and comparative examples all provide a method for preparing polylactic acid, and the differences between these methods and Example 1 are shown in Tables 1 and 2.

[0086] Example 9

[0087] This embodiment provides a method for preparing polylactic acid, including the following steps:

[0088] 3000 kg of lactide (same as in Example 1, from the same batch), stannous octoate catalyst, butanediol initiator, and triphenyl phosphite first additive were mixed in a static mixer and preheated to 120°C to obtain a preheated mixture.

[0089] The resulting preheated mixture was fed into a tubular reactor and continuously polymerized at a temperature gradient of 130℃-150℃-170℃-190℃ with a residence time of 4 hours, yielding an intermediate polymer with an intrinsic viscosity of 1.43 dl / g and a lactide monomer mass percentage of 4.1%. The specific temperature program for continuous polymerization is as follows:

[0090] The temperature was increased to 130℃ over 30 minutes, and then held at 130℃ for 30 minutes.

[0091] The temperature was increased to 150℃ over 30 minutes, and then held at 150℃ for 30 minutes.

[0092] The temperature was raised to 170℃ over 30 minutes, and then held at 170℃ for 30 minutes.

[0093] The temperature was raised to 190℃ over 30 minutes, and then held at 190℃ for 30 minutes.

[0094] Phosphorous acid, a catalyst passivator, was added to the obtained intermediate polymer and then fed into a thin-film scraped evaporator. The evaporator was then evaporated for 40 minutes at a pressure of 20 Pa and a temperature of 200 °C to remove volatiles, thus obtaining a polymer after the removal of volatiles.

[0095] The second additive, cyclohexane-1,2-dicarboxylic acid diglycidyl ester, was added to the obtained polymer after the volatiles were removed. The mixture was then thoroughly mixed using a static mixer, with a residence time controlled at 0.7 h and a reaction temperature of 185 °C. After extrusion granulation and drying, polylactic acid was obtained, with the extrusion temperature at 180 °C. Based on the mass of lactide, the mass of the first additive could be selected as 0.05%, the catalyst as 0.005%, the initiator as 0.2%, the catalyst passivator as 0.005%, and the second additive as 0.19%.

[0096] Comparative Example 8

[0097] This comparative example provides a polylactic acid, specifically Total Energy Corian's LX930 polylactic acid resin.

[0098] Example 1

[0099] The polylactic acid obtained from the above embodiments and comparative examples was used to prepare film materials according to the following method:

[0100] PLA, PBAT, and processing aids are fed into the main feed port of a twin-screw extruder, while inorganic fillers are fed into the side feed port. The mixture is melt-extruded and granulated, cooled, air-dried, pelletized, dried, and homogenized to obtain a biodegradable composition.

[0101] The PLA was prepared according to Examples 1-4 and Comparative Examples 1-9, with a dosage of 10 parts by weight. PBAT was selected from Kingfa Science & Technology Co., Ltd.'s product A400. According to ISO 1133-1-2011, the melt flow rate of PBAT at 190℃ and 2.16kg load was 3.7g / 10min, with a dosage of 55 parts by weight. The inorganic filler was HTPUTtra5L talc powder from Liaoning Aihai Co., Ltd., with a D50 particle size of 2μm, and a dosage of 35 parts by weight. The processing aid was erucamide, with a dosage of 0.5 parts by weight. The twin-screw extruder used for melt extrusion had an L / D ratio of 48:1, and the melt extrusion processing temperature was 180℃.

[0102] The obtained biodegradable composition was subjected to blown film processing to obtain film bags. The screw length-to-diameter ratio was 32:1, a spiral flow channel die head was used, the air ring was a double-outlet air ring, the blown film processing temperature was set at 150℃, the blow-up ratio was 3.5, and the blown film thickness was controlled at 20μm. The resulting film material was subjected to the following performance tests:

[0103] Heat seal strength: The heat seal strength of the film bag is tested at 23℃ according to standard QB / T2358-1998.

[0104] Dart impact: The film bag was subjected to a dart impact test at 23°C in accordance with standard ISO 7765-1-1988.

[0105] Puncture strength: The puncture strength of the film bag was tested using a PARAM XLW(PC) intelligent electronic tensile testing machine with a puncture needle diameter of 6mm at 23℃.

[0106] The test results are shown in Table 2.

[0107] Table 1

[0108]

[0109]

[0110]

[0111]

[0112] Table 2

[0113]

[0114] As can be seen from the above data, the polylactic acid films prepared in the various embodiments of this application have good heat-sealing and mechanical properties, such as heat-sealing strength of 18 N / 15 mm or higher, dart impact strength of 200 g or higher, and puncture strength of 9 N or higher. Comparative Examples 1-8, due to the optical purity, molecular weight distribution index (PDI), and / or at 190°C and a shear rate of 50 s, show that the polylactic acid exhibits good heat-sealing and mechanical properties. -1 High or low lower shear viscosity results in low heat-sealing strength, dart impact strength, and / or puncture strength of the membrane material.

[0115] A comparison of Examples 1 and Examples 5-8 shows that when the Z-average molecular weight of polylactic acid is in the range of 22,500-27,500 Da, its heat-sealing strength, dart impact strength, and puncture strength are better balanced.

[0116] Example 2

[0117] The PLA obtained in Example 1 was used to prepare biodegradable compositions A and B according to the formulations in Table 3. These biodegradable compositions were then processed into film bags according to the processing method in Effect Example 1 and tested according to the test method in Effect Example 1.

[0118] In the biodegradable composition A, PBAT is the A400 product of Kingfa Science & Technology Co., Ltd. (different from the PBAT batch in Effect Example 1). According to ISO 1133-1-2011, the melt flow rate of this PBAT at 190℃ and 2.16kg load is 3.0g / 10min. The inorganic filler is HTPUTtra5L talc powder from Liaoning Aihai Co., Ltd., with a D50 particle size of 1μm. The additive is silica.

[0119] In biodegradable composition B, PBAT is Kingfa Science & Technology Co., Ltd.'s A400 product (different from the PBAT batch in Effect Example 1). According to ISO 1133-1-2011, the melt flow rate of this PBAT at 190°C and 2.16 kg load is 5.0 g / 10 min. The inorganic filler is Omia's 5T-JI calcium carbonate with a D50 particle size of 5 μm. The additive is pentaerythritol stearate.

[0120] Table 3

[0121] Components / parts by weight Biodegradable Composition A Biodegradable Composition B PLA 3 12 PBAT 90 53 Inorganic packing 7 35 Processing aids 0.6 0.2 Heat seal strength / N / 15mm 19.5 18.8 Dart impact strength / g 204 228 Puncture intensity / N 10.1 9.4

[0122] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A polylactic acid, characterized in that, The polylactic acid has an optical purity of 87 wt.%–92.5 wt.% and a molecular weight distribution index (PDI) of 1.5–3.0, and is suitable for use at 190°C and a shear rate of 50 s. -1 The lower shear viscosity is 20–100 Pa·s.

2. The polylactic acid as described in claim 1, characterized in that, The polylactic acid has a Z-average molecular weight of 19,000 to 31,000 Da, preferably 22,500 to 27,500 Da.

3. The polylactic acid as described in claim 1, characterized in that, The intrinsic viscosity of the polylactic acid is 1.22–1.57 dL / g.

4. The polylactic acid as described in claim 1, characterized in that, The polylactic acid contains less than 0.5% by mass of lactide monomer, preferably 0.08% to 0.43%.

5. A biodegradable composition, characterized in that, It comprises the following components in parts by weight: 52-91 parts of biodegradable polyester, 3-12 parts of polylactic acid as described in any one of claims 1 to 4, 7-35 parts of inorganic filler, and 0.2-0.6 parts of additives.

6. The biodegradable composition according to claim 4, characterized in that, At least one of the following conditions must be met: S1. The biodegradable polyester includes PBAT; S2. The inorganic filler includes at least one of calcium carbonate and talc; S3. The additives include at least one of the following: opening agent and lubricant.

7. The use of polylactic acid as described in any one of claims 1 to 4 or the biodegradable composition as described in claim 5 or 6 in film and bag products.

8. A method for preparing polylactic acid as described in any one of claims 1 to 4, characterized in that, Includes the following steps: The lactide, first additive, catalyst and initiator are mixed and preheated to obtain a preheated mixture; The preheated mixture was subjected to a first reaction to obtain a prepolymer with an intrinsic viscosity of 0.7–0.9 dl / g and a lactide monomer mass percentage of 25%–35%. A second additive is added to the obtained prepolymer, and then a second reaction is carried out to obtain an intermediate polymer with an intrinsic viscosity of 1.20 to 1.50 dl / g and a lactide monomer mass percentage of 4% to 5%, wherein the second additive includes a bifunctional epoxy compound. A catalyst passivator was added to the obtained intermediate polymer, and then the polymer was evaporated for 20 to 40 minutes under a pressure of 0 to 300 Pa and a temperature of 190 to 220 °C to obtain the polylactic acid. The lactide in question has a L-lactide content of 88% to 92.5% by mass. Based on the mass of the lactide, the mass of the first additive is 0-0.1%, the mass of the catalyst is 0.001%-0.1%, the mass of the initiator is 0.17%-0.25%, the mass of the second additive is 0.05%-0.5%, and the mass of the catalyst passivator is 0.001%-0.1%. The first reaction was carried out for 1 to 2.5 hours under pressure of 0 to 60 kPa and temperature of 160 to 190 °C; the second reaction was carried out for 1 to 2.5 hours under pressure of 0.5 to 1.5 MPa and temperature of 180 to 200 °C.

9. The method for preparing polylactic acid as described in claim 8, characterized in that, The bifunctional epoxy compound includes at least one of diglycidyl ether, cyclohexane-1,2-dicarboxylic acid diglycidyl ester, neopentyl glycol diglycidyl ether, and diglycidyl phthalate.

10. The method for preparing polylactic acid as described in claim 8, characterized in that, The acid value of the lactide is ≤10 mol / t, preferably the acid value of the lactide is 1 to 7 mol / t; And / or, the first additive includes at least one of triphenyl phosphate, triethyl phosphate, and triphenyl phosphite; And / or, the catalyst comprises a tin compound; And / or, the initiator includes a hydroxyl-containing compound; And / or, the catalyst passivating agent includes at least one of phosphorous acid, phosphate monostearate, and phosphate distearate; And / or, the temperature of the preheated mixture is 100–130°C.

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