Polylactic acid and its preparation method and application

Abstract

The present application discloses a polylactic acid and its preparation method and application, belonging to the technical field of polymer materials. The polylactic acid of the present application has an intrinsic viscosity of 0.79-1.06 dl / g, a molecular weight distribution index PDI of 1.49-1.52, a lactide monomer mass percentage content of <0.5%, and a shear viscosity of 0.01-1s under the test condition of 190°C. ‑1 The shear viscosity and shear rate satisfy a specific relationship. Compared with conventional polylactic acid, it has a higher melt flow rate, is stable during the injection molding process, does not stick to the mold and the injection port, has high injection molding efficiency, and has better aging resistance of the obtained products. It is suitable for injection molding to obtain plastic parts with a wall thickness of less than 0.4 mm, such as lunch boxes, bowls, plates, water cups and other food packaging products.

Description

Technical Field

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

[0002] PLA (polylactic acid) is a bio-based, degradable polymer with excellent mechanical properties, gloss, transparency, antibacterial properties and biocompatibility. Compared with traditional plastics, its raw materials are biomass materials, which greatly reduces carbon emissions, and its degradation products are carbon dioxide and water, which are pollution-free to the environment. With the increasing awareness of emission reduction and carbon reduction, PLA has become one of the bio-based materials with great development potential.

[0003] As a polymer material molding processing method, thin-wall injection molding effectively reduces the amount of raw materials, meets the requirements of lightweight and precision products, and is widely used in the manufacture of packaging and precision devices. In the injection molding process, the key condition of thin-wall injection molding is to overcome the flow resistance of the melt in the thin-wall mold cavity and reduce the frozen layer caused by the rapid solidification of the plastic melt. The flow properties of conventional PLA products on the market cannot meet the requirements of thin-wall injection molding. The melt is not easy to fill the cavity, and the mold surface and injection port are prone to adhere to precipitates. The aging resistance of the injection molded products is poor, and the injection molding efficiency is low. At present, the viscosity requirements of thin-wall injection molding are mainly met by adjusting the injection pressure, temperature and injection speed in industrial production, but high temperature and high pressure will cause PLA degradation and affect the mechanical properties of PLA products. Therefore, it is necessary to develop a PLA product with high fluidity and good aging resistance of injection molded products to solve the above technical problems. Summary of the invention

[0004] Based on the defects of the prior art, the purpose of this application is to provide a polylactic acid and a preparation method and application thereof. The polylactic acid has a high melt flow rate, a stable injection molding process, does not stick to the mold and the injection port, has a high injection molding efficiency, and the obtained injection molded products have better aging resistance and are suitable for thin-wall injection molding, such as preparing injection molded parts with a wall thickness of less than 0.4 mm.

[0005] In order to achieve the above-mentioned purpose, in the first aspect, the present application provides a polylactic acid having an intrinsic viscosity of 0.79 to 1.06 dl / g, a molecular weight distribution index PDI of 1.49 to 1.52, a lactide monomer mass percentage of <0.5%; and under the test condition of 190°C, a shear rate of 0.01 to 1s -1 The shear viscosity and shear rate satisfy the following linear relationship: Y = a ln (X) + b,

[0006] Where, Y is the shear viscosity, in Pa·s,

[0007] X is the shear rate, unit is s-1 ,

[0008] a is -8.3 to -7.2,

[0009] b is 20 to 25.

[0010] Compared with conventional polylactic acid, the polylactic acid has a higher melt flow rate, is stable during the injection molding process and does not stick to the mold and the injection port. A higher injection speed can be used for injection molding, the injection molding efficiency is higher, the product qualification rate is higher, and the obtained injection molded products have better aging resistance and are suitable for thin-wall injection molding, such as for preparing injection molded parts with a wall thickness of less than 0.4 mm (for example, lunch boxes, bowls, plates, water cups and other food packaging products).

[0011] The intrinsic viscosity of the polylactic acid is 0.79 to 1.06 dl / g, such as 0.80 dl / g, 0.85 dl / g, 0.9 dl / g, 0.95 dl / g, 1.00 dl / g or 1.05 dl / g. The intrinsic viscosity can be measured using an 1835-type Ubbelohde capillary viscometer with an inner capillary diameter of 0.83 mm. The specific test method is as follows:

[0012] Weigh 0.1250±0.0001g of the 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 intrinsic viscosity;

[0013] The formula for intrinsic viscosity is: η sp =(η-η0) / η0, where η is the viscosity of the sample solution, in Pa·s; η0 is the viscosity of the mixed solvent of phenol and o-dichlorobenzene, in Pa·s; η sp is the intrinsic viscosity, expressed in dl / g.

[0014] The molecular weight distribution index PDI of the polylactic acid is 1.49 to 1.52, such as 1.49, 1.50, 1.51 or 1.52. The molecular weight distribution index PDI can be tested by gel permeation chromatography (GPC), and the specific test method is as follows:

[0015] The measurement was performed using a chromatography system at 40°C, using KF803 and KF805 columns connected in series by SHODEX, a refractive index detector, chloroform as the eluent (flow rate 1 mL / min), and polystyrene as the reference standard.

[0016] The mass percentage of lactide monomer in the polylactic acid is less than 0.5%, such as 0.5%, 0.4%, 0.3%, 0.2%, 0.1%, etc. In some embodiments, the mass percentage of lactide monomer in the polylactic acid is ≤ 0.41%, such as 0.1% to 0.41%. The mass percentage of lactide monomer can be measured by the following method:

[0017] Accurately weigh about 10 mg of the 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.

[0018] The polylactic acid is tested at 190°C with a shear rate of 0.01 to 1 s -1 range, and its shear viscosity and shear rate satisfy the above linear relationship, wherein the a value and b value depend on a variety of complex factors, such as the amount of initiator, the pressure of the first heating reaction, the temperature of the first heating reaction, the time of the first heating reaction, the pressure of the second heating reaction, the temperature of the second heating reaction and the time of the second heating reaction, which reflects that the obtained polylactic acid has a specific microstructure, which makes the melt more stable during the injection molding process, the appearance and size fluctuations of the injection molded products are smaller, the qualified rate is higher, and the degree of degradation of the polylactic acid during the injection molding process is lower, the injection molded products have better aging resistance and a longer shelf life. Optionally, a is -8.3, -8.0, -7.8, -7.5, or -7.2, etc. Optionally, b is 20, 21, 22, 23, 24 or 25, etc. Optionally, R 2 is 0.99 or 1. The shear viscosity of the polylactic acid can be measured by the following method:

[0019] The test was carried out in accordance with the standard requirements of GB / T 33061.10-2016 "Determination of dynamic mechanical properties of plastics Part 10: Determination of complex shear viscosity using parallel plate oscillating rheometer" and JY-T 0590-2020 "General rules for measurement methods of rotational rheometers". The sample size is 10cm*10cm square plate with a thickness of 3mm.

[0020] In some embodiments, the linear correlation R of the linear relationship 2 It is 0.99 to 1, such as 0.99 or 1.

[0021] In some embodiments, the acid value of the polylactic acid is less than 15 mol / t. When the acid value of the polylactic acid is less than 15 mol / t, such as 14.9 mol / t, 14 mol / t, 12 mol / t, 10 mol / t, 8 mol / t, 6 mol / t, 4.9 mol / t, 4.5 mol / t, 4 mol / t, 3 mol / t, 2 mol / t, etc., the aging resistance of the injection molded product is better. In one embodiment, the acid value of the polylactic acid is ≤ 10 mol / t, such as 4 to 10 mol / t, to further improve the aging resistance of the injection molded product. The acid value can be measured by the following method:

[0022] The test instrument used was a Swiss Metrohm 905 fully automatic potentiometric titrator. 0.5 g of PLA was dissolved in 60 mL of dichloromethane to obtain a PLA solution. The PLA solution was titrated with a KOH-EtOH solution having a KOH concentration of 0.0025 mol / L, and the endpoint was determined by potentiometric titration.

[0023] In some embodiments, the mass percentage of L-polylactic acid in the polylactic acid is greater than 95% or the mass percentage of D-polylactic acid is greater than 95%. Optionally, the mass percentage of L-polylactic acid in the polylactic acid is 95.1%, 95.5%, 96%, 97%, 98%, 99% or 99.8%, etc. Optionally, the mass percentage of D-polylactic acid in the polylactic acid is 95.1%, 95.5%, 96%, 97%, 98%, 99% or 99.8%, etc.

[0024] The mass percentage of left-handed polylactic acid and right-handed polylactic acid in the polylactic acid can be measured by the following method: weigh 100±10 mg of polylactic acid sample in 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 in an oven at 150°C. After 60 minutes, take out the hydrothermal reactor, cool it to room temperature with running water, and then perform gas chromatography test. According to the peak area ratio of D-polymethyl lactate to L-polymethyl lactate, the mass percentage of left-handed lactide and / or the mass percentage of right-handed lactide in lactide are calculated. The gas chromatography test uses an Agilent 8860 gas chromatograph and a CP7502 chromatographic column.

[0025] In the second aspect, the present application provides the application of the polylactic acid in plastic parts with a wall thickness of 0.1 to 0.4 mm. The polylactic acid can be injection molded to obtain products with a wall thickness of 0.1 to 0.4 mm, such as food packaging products such as lunch boxes, bowls, plates, water cups (including reusable water cups, airline cups, etc.) with a wall thickness of 0.4 mm, 0.3 mm, 0.2 mm or 0.1 mm, which not only has high injection molding efficiency, but also has a high product qualification rate and a long shelf life.

[0026] In a third aspect, the present application provides a method for preparing the polylactic acid, comprising the following steps:

[0027] Mixing lactide, additives, catalysts and initiators, and preheating to obtain a preheated mixture;

[0028] The preheated mixture is subjected to a first heating reaction to obtain a prepolymer having an intrinsic viscosity of 0.6 to 0.7 dl / g and a lactide monomer mass percentage of 25% to 35%;

[0029] The prepolymer is subjected to a second heating reaction to obtain a polymer having an intrinsic viscosity of 0.80 to 1.05 dl / g and a lactide monomer mass percentage of 4% to 5%;

[0030] A catalyst deactivator is added to the polymer, and then evaporated for 20 to 40 minutes under the conditions of a pressure of 0 to 300 Pa and a temperature of 190° C. to 220° C. to obtain the polylactic acid.

[0031] In some embodiments, the acid value of the lactide is ≤10 mol / t, and the mass percentage of L-lactide is >95% or the mass percentage of D-lactide is >95%. Optionally, the acid value of the lactide is 10 mol / t, 8 mol / t, 6 mol / t, 4 mol / t or 1 mol / t, etc. Optionally, the mass percentage of L-lactide in the lactide is 95.1%, 95.5%, 96%, 97%, 98%, 99% or 99.8%, etc. Optionally, the mass percentage of D-lactide in the lactide is 95.1%, 95.5%, 96%, 97%, 98%, 99% or 99.8%, etc.

[0032] 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 solution having a KOH concentration of 0.0025 mol / L, and determining the endpoint by potentiometric titration.

[0033] The mass percentage of left-handed lactide and the mass percentage of right-handed lactide in the lactide can be measured by the following method: weigh 100±10 mg of lactide sample in 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 in an oven at 150° C. After 60 minutes, take out the hydrothermal reactor, cool it to room temperature 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 left-handed lactide and / or the mass percentage of right-handed lactide in the lactide are calculated. The gas chromatography test uses an Agilent 8860 gas chromatograph and a CP7502 chromatographic column.

[0034] Lactide can be fresh lactide or a mixture of fresh lactide and recycled lactide, wherein fresh lactide refers to lactide prepared from lactic acid; recycled lactide refers to lactide recovered by evaporation during the preparation of polylactic acid.

[0035] In some embodiments, the temperature of the preheated mixture is 100-130°C; the first heating reaction is carried out at a pressure of 0-50 kPa and a temperature of 165-185°C for 1.5-2 hours; and the second heating reaction is carried out at a pressure of 1.0-1.2 MPa and a temperature of 185-195°C for 1.5-2 hours.

[0036] In some embodiments, based on the mass of the lactide, the mass of the 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%, and the mass of the catalyst deactivator is 0.001%-0.1%. Optionally, based on the mass of the lactide, the mass of the additive is 0, 0.001%, 0.005%, 0.01%, 0.05%, 0.08% or 0.1%. Optionally, based on the mass of the lactide, the mass of the catalyst is 0.001%, 0.005%, 0.01%, 0.05%, 0.08% or 0.1%. Optionally, based on the mass of the lactide, the mass of the initiator is 0.17%, 0.19%, 0.21%, 0.23% or 0.25%, etc. Optionally, based on the mass of the lactide, the mass of the catalyst deactivator is 0.001%, 0.005%, 0.01%, 0.05%, 0.08% or 0.1%.

[0037] In some embodiments, the additive includes triphenyl phosphate, triethyl phosphate, triphenyl phosphite; the catalyst includes a tin compound; the initiator includes a hydroxyl-containing compound; the catalyst deactivator includes at least one of phosphorous acid, monostearate phosphate, and distearate phosphate. As an example, the catalyst includes at least one of stannous octoate, SnCl2, SnCl4, SnBr2, SnBr4, SnO, tri(2-ethylhexanoate)butyltin, hydrated monobutyltin oxide, dibutyltin dilaurate, and tetraphenyltin; the initiator includes at least one of butanediol, dodecanol, and lactic acid.

[0038] In some embodiments, the first heating reaction is carried out in a reactor having a stirring device. In a specific 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.

[0039] In some embodiments, the second heating reaction is carried out in a plug flow reactor. As an example, the plug flow reactor is the tubular reactor 5 disclosed in CN101820996A.

[0040] Compared with the prior art, the beneficial effects of the present application are: the polylactic acid of the present application has a high melt flow rate, the injection molding process is stable and does not stick to the mold and the injection molding port, the injection molding efficiency is high, and the obtained injection molded products have good aging resistance. It is suitable for injection molding to obtain plastic parts with a wall thickness of less than 0.4 mm, such as lunch boxes, bowls, plates, water cups and other food packaging products. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 The shear viscosity and shear rate variation trend diagram of the polylactic acid obtained in Example 1;

[0042] Figure 2 The shear viscosity and shear rate variation trend diagram of the polylactic acid obtained in Comparative Example 3;

[0043] Figure 3 This is a trend diagram of shear viscosity and shear rate of the polylactic acid obtained in Comparative Example 4. DETAILED DESCRIPTION

[0044] 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 are all commonly used ordinary reagents and instruments unless otherwise specified. 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.

[0045] Example 1

[0046] This embodiment provides a method for preparing polylactic acid, comprising the following steps:

[0047] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.005%, the mass of initiator butanediol is 0.2%, and the mass of additive triphenyl phosphite is 0.05%) are mixed in a static mixer and preheated to 125° C. to obtain a preheated mixture, wherein the acid value of lactide is 3.2 mol / t and the mass percentage of L-lactide is 98.3%;

[0048] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 5 kPa and a temperature of 165° C. for a reaction time of 2 hours to obtain a prepolymer having an intrinsic viscosity of 0.60 dl / g and a lactide monomer mass percentage of 35%;

[0049] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.01 MPa and a temperature of 185° C. for 2 hours to obtain a polymer having an intrinsic viscosity of 0.93 dl / g and a lactide monomer mass percentage of 4.2%;

[0050] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.004% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 20 Pa and a temperature of 200° C. for 40 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0051] Example 2

[0052] This embodiment provides a method for preparing polylactic acid, comprising the following steps:

[0053] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.001%, the mass of initiator butanediol is 0.20%, and the mass of additive triphenyl phosphite is 0.02%) are mixed in a static mixer and preheated to 118° C. to obtain a preheated mixture, wherein the acid value of lactide is 3.5 mol / t and the mass percentage of L-lactide is 99.4%;

[0054] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 25 kPa and a temperature of 178° C. for a reaction time of 2 hours to obtain a prepolymer having an intrinsic viscosity of 0.64 dl / g and a lactide monomer mass percentage of 25%;

[0055] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.2 MPa and a temperature of 192° C. for a reaction time of 1.6 h to obtain a polymer having an intrinsic viscosity of 0.92 dl / g and a lactide monomer mass percentage of 5.0%;

[0056] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.002% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 5 Pa and a temperature of 204° C. for 40 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0057] Example 3

[0058] This embodiment provides a method for preparing polylactic acid, comprising the following steps:

[0059] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.004%, the mass of initiator butanediol is 0.16%, and the mass of additive triphenyl phosphite is 0.09%) are mixed in a static mixer and preheated to 130° C. to obtain a preheated mixture, wherein the acid value of lactide is 8.2 mol / t and the mass percentage of L-lactide is 99.4%;

[0060] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 25 kPa and a temperature of 178° C. for a reaction time of 2 hours to obtain a prepolymer having an intrinsic viscosity of 0.7 dl / g and a lactide monomer mass percentage of 34%;

[0061] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.2 MPa and a temperature of 192° C. for a reaction time of 1.6 h to obtain a polymer having an intrinsic viscosity of 1.04 dl / g and a lactide monomer mass percentage of 4.5%;

[0062] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.002% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 5 Pa and a temperature of 204° C. for 30 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0063] Example 4

[0064] This embodiment provides a method for preparing polylactic acid, comprising the following steps:

[0065] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.008%, the mass of initiator butanediol is 0.15%, and the mass of additive triphenyl phosphite is 0.10%) are mixed in a static mixer and preheated to 126° C. to obtain a preheated mixture, wherein the acid value of lactide is 9.9 mol / t and the mass percentage of L-lactide is 99.4%;

[0066] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 25 kPa and a temperature of 178° C. for a reaction time of 2 hours to obtain a prepolymer having an intrinsic viscosity of 0.6 dl / g and a lactide monomer mass percentage of 32%;

[0067] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.2 MPa and a temperature of 192° C. for a reaction time of 1.6 h to obtain a polymer having an intrinsic viscosity of 1.02 dl / g and a lactide monomer mass percentage of 4.8%;

[0068] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.002% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 5 Pa and a temperature of 204° C. for 30 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0069] Example 5

[0070] This embodiment provides a method for preparing polylactic acid, comprising the following steps:

[0071] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.010%, the mass of initiator butanediol is 0.22%, and the mass of additive triphenyl phosphite is 0.07%) are mixed in a static mixer and preheated to 103° C. to obtain a preheated mixture, wherein the acid value of lactide is 3.5 mol / t and the mass percentage of L-lactide is 99.4%;

[0072] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 50 kPa and a temperature of 180° C. for a reaction time of 2 hours to obtain a prepolymer having an intrinsic viscosity of 0.67 dl / g and a lactide monomer mass percentage of 27%;

[0073] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.08 MPa and a temperature of 190° C. for a reaction time of 1.5 h to obtain a polymer having an intrinsic viscosity of 0.87 dl / g and a lactide monomer mass percentage of 5.0%;

[0074] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.015% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 203 Pa and a temperature of 213° C. for 32 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0075] Example 6

[0076] This embodiment provides a method for preparing polylactic acid, comprising the following steps:

[0077] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.015%, the mass of initiator butanediol is 0.25%, and the mass of additive triphenyl phosphite is 0.03%) are mixed in a static mixer and preheated to 114° C. to obtain a preheated mixture, wherein the acid value of lactide is 3.5 mol / t and the mass percentage of L-lactide is 99.4%;

[0078] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 45 kPa and a temperature of 172° C. The reaction time was 1.6 hours, and a prepolymer having an intrinsic viscosity of 0.66 dl / g and a lactide monomer mass percentage of 27% was obtained;

[0079] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.12 MPa and a temperature of 187° C. for a reaction time of 1.8 h to obtain a polymer having an intrinsic viscosity of 0.79 dl / g and a lactide monomer mass percentage of 4.7%;

[0080] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.013% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 282 Pa and a temperature of 218° C. for 31 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0081] Example 7

[0082] This embodiment provides a method for preparing polylactic acid, comprising the following steps:

[0083] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.012%, the mass of initiator butanediol is 0.23%, and the mass of additive triphenyl phosphite is 0) are mixed in a static mixer and preheated to 122° C. to obtain a preheated mixture, wherein the acid value of lactide is 4.6 mol / t and the mass percentage of L-lactide is 98.4%;

[0084] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 12 kPa and a temperature of 166° C. The reaction time was 1.8 hours, and a prepolymer having an intrinsic viscosity of 0.62 dl / g and a lactide monomer mass percentage of 29% was obtained;

[0085] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.01 MPa and a temperature of 191° C. for a reaction time of 1.9 h to obtain a polymer having an intrinsic viscosity of 0.86 dl / g and a lactide monomer mass percentage of 4.5%;

[0086] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.018% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 300 Pa and a temperature of 220° C. for 27 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0087] Example 8

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

[0089] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.012%, the mass of initiator butanediol is 0.22%, and the mass of additive triphenyl phosphite is 0.07%) are mixed in a static mixer and preheated to 130° C. to obtain a preheated mixture, wherein the acid value of lactide is 4.9 mol / t and the mass percentage of right-handed lactide is 95.1%;

[0090] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 15 kPa and a temperature of 170° C. for a reaction time of 1.8 hours to obtain a prepolymer having an intrinsic viscosity of 0.65 dl / g and a lactide monomer mass percentage of 35%;

[0091] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.04 MPa and a temperature of 190° C. for a reaction time of 2 hours to obtain a polymer having an intrinsic viscosity of 0.89 dl / g and a lactide monomer mass percentage of 4.5%;

[0092] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.020% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 120 Pa and a temperature of 210° C. for 32 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0093] Comparative Example 1

[0094] This comparative example provides a method for preparing polylactic acid, comprising the following steps:

[0095] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.005%, the mass of initiator butanediol is 0.2%, and the mass of additive triphenyl phosphite is 0.05%) are mixed in a static mixer and preheated to 125° C. to obtain a preheated mixture, wherein the acid value of lactide is 3.3 mol / t and the mass percentage of L-lactide is 98.3%;

[0096] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 5 kPa and a temperature of 165° C. for a reaction time of 2 hours to obtain a prepolymer having an intrinsic viscosity of 0.6 dl / g and a lactide monomer mass percentage of 35%;

[0097] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.01 MPa and a temperature of 185° C. for a reaction time of 2 hours to obtain a polymer having an intrinsic viscosity of 0.92 dl / g and a lactide monomer mass percentage of 4.2%;

[0098] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.004% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 20 Pa and a temperature of 200° C. for 15 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0099] Comparative Example 2

[0100] This comparative example provides a method for preparing polylactic acid, comprising the following steps:

[0101] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.005%, the mass of initiator butanediol is 0.28%, and the mass of additive triphenyl phosphite is 0.05%) are mixed in a static mixer and preheated to 125° C. to obtain a preheated mixture, wherein the acid value of lactide is 3.2 mol / t and the mass percentage of L-lactide is 98.3%;

[0102] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 5 kPa and a temperature of 160° C. for a reaction time of 1.5 hours to obtain a prepolymer having an intrinsic viscosity of 0.54 dl / g and a lactide monomer mass percentage of 37.6%;

[0103] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.01 MPa and a temperature of 185° C. for a reaction time of 2 hours to obtain a polymer having an intrinsic viscosity of 0.73 dl / g and a lactide monomer mass percentage of 4.6%;

[0104] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.004% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 20 Pa and a temperature of 200° C. for 40 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0105] Comparative Example 3

[0106] This comparative example provides a method for preparing polylactic acid, comprising the following steps:

[0107] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.005%, the mass of initiator butanediol is 0.15%, and the mass of additive triphenyl phosphite is 0.05%) are mixed in a static mixer and preheated to 125° C. to obtain a preheated mixture, wherein the acid value of lactide is 3.5 mol / t and the mass percentage of L-lactide is 98.4%;

[0108] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 5 kPa and a temperature of 165° C. for a reaction time of 2 hours to obtain a prepolymer having an intrinsic viscosity of 0.73 dl / g and a lactide monomer mass percentage of 23.4%;

[0109] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.01 MPa and a temperature of 185° C. for a reaction time of 2 hours to obtain a polymer having an intrinsic viscosity of 1.12 dl / g and a lactide monomer mass percentage of 6.5%;

[0110] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.004% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 20 Pa and a temperature of 200° C. for 40 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0111] Comparative Example 4

[0112] This comparative example provides a method for preparing polylactic acid, comprising the following steps:

[0113] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.005%, the mass of initiator butanediol is 0.22%, and the mass of additive triphenyl phosphite is 0.05%) are mixed in a static mixer and preheated to 125° C. to obtain a preheated mixture, wherein the acid value of lactide is 3.2 mol / t and the mass percentage of L-lactide is 98.6%;

[0114] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 5 kPa and a temperature of 154° C. for a reaction time of 3 hours to obtain a prepolymer having an intrinsic viscosity of 0.58 dl / g and a lactide monomer mass percentage of 25.9%;

[0115] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.01 MPa and a temperature of 192° C. for 3 hours to obtain a polymer having an intrinsic viscosity of 0.87 dl / g and a lactide monomer mass percentage of 5.5%;

[0116] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.004% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 20 Pa and a temperature of 200° C. for 40 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0117] Comparative Example 5

[0118] This comparative example provides a method for preparing polylactic acid, comprising the following steps:

[0119] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.005%, the mass of initiator butanediol is 0.22%, and the mass of additive triphenyl phosphite is 0.05%) are mixed in a static mixer and preheated to 125° C. to obtain a preheated mixture, wherein the acid value of lactide is 3.3 mol / t and the mass percentage of L-lactide is 98.3%;

[0120] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 5 kPa and a temperature of 190° C. The reaction time was 1.3 hours, and a prepolymer having an intrinsic viscosity of 0.78 dl / g and a lactide monomer mass percentage of 22.8% was obtained;

[0121] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.01 MPa and a temperature of 180° C. for a reaction time of 2 hours to obtain a polymer having an intrinsic viscosity of 0.88 dl / g and a lactide monomer mass percentage of 6.4%;

[0122] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.004% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 20 Pa and a temperature of 200° C. for 40 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0123] Comparative Example 6

[0124] This comparative example provides a method for preparing polylactic acid, comprising the following steps:

[0125] 3117 kg of lactide, catalyst stannous octoate, initiator butanediol and additive triphenyl phosphite (calculated based on the mass of lactide, the mass of catalyst stannous octoate is 0.005%, the mass of initiator butanediol is 0.28%, and the mass of additive triphenyl phosphite is 0.05%) are mixed in a static mixer and preheated to 125° C. to obtain a preheated mixture, wherein the acid value of lactide is 3.4 mol / t and the mass percentage of L-lactide is 98.3%;

[0126] The obtained preheated mixture was conveyed to a fully mixed flow autoclave reactor, and a first heating reaction was carried out at a pressure of 5 kPa and a temperature of 165° C. for a reaction time of 2 hours to obtain a prepolymer having an intrinsic viscosity of 0.63 dl / g and a lactide monomer mass percentage of 33.8%;

[0127] The obtained prepolymer was transported to a plug flow reactor, and a second heating reaction was carried out at a pressure of 1.01 MPa and a temperature of 185° C. for a reaction time of 2 hours to obtain a polymer having an intrinsic viscosity of 0.72 dl / g and a lactide monomer mass percentage of 4.3%;

[0128] The obtained polymer was added with phosphorous acid (the mass of phosphorous acid was 0.004% of the mass of the lactide used above), and then transported to a thin film scraper evaporator, evaporated at a pressure of 20 Pa and a temperature of 200° C. for 40 minutes to remove unreacted lactide monomer, and pelletized to obtain polylactic acid.

[0129] The intrinsic viscosity, molecular weight distribution index PDI, lactide monomer mass percentage, and acid value of the polylactic acid obtained in each embodiment and comparative example are shown in Table 1; under the test conditions of 190°C, the shear rate is between 0.01 and 1s -1The shear viscosity and shear rate of the polylactic acid obtained in each embodiment and comparative example satisfy the following linear relationship: Y = a ln (X) + b, wherein Y is the shear viscosity, in Pa·s; X is the shear rate, in s -1 ; a, b and linear correlation R 2 See Table 1 for the values.

[0130] The polylactic acid obtained in each embodiment and comparative example was tested for performance using the following test method:

[0131] (1) Polylactic acid was heated and melted at 190°C and injection molded. The injection speed is shown in Table 2. The injection molded product is a conical water cup with an opening side length of 74 mm, a bottom side length of 45 mm, and a height of 74 mm. Its thickness is shown in Table 2. The qualified rate of the injection molded product was detected by visual inspection and dimensional accuracy measurement. If there were no defects such as missing material, flash, shrinkage, burning, deformation, impurities, etc., and the opening side length deviation was <1.0 mm, the bottom side length deviation was <1.0 mm, and the thickness deviation was <5%, then the product was judged to be qualified. Qualified rate = (number of qualified products / total output) × 100%.

[0132] (2) Product shelf life: The injection molded products obtained above were sealed with a 300 μm thick PE packaging film and stored at 25° C. The shelf life of the products was determined based on whether the intrinsic viscosity was maintained above 60%.

[0133] The test results are shown in Table 2.

[0134] Table 1

[0135]

[0136]

[0137] Table 2

[0138]

[0139]

[0140] It can be seen from Table 2 that the polylactic acid obtained in each embodiment of the present application has good fluidity, the injection speed in the injection molding process is fast, which is above 350 mm / s, the product molding cycle is short, and the efficiency is high; the qualified rate of injection molded products is high, which is above 99%; the aging resistance is good, and the shelf life is more than 6 months, which is suitable for preparing injection molded parts with a wall thickness of less than 0.4 mm.

[0141] Comparative Examples 1 to 6 have a low qualified rate of injection molded products because the lactide monomer content, intrinsic viscosity, PDI and / or shear viscosity in the polylactic acid are not within the scope of this application, and the aging resistance of Comparative Examples 1 to 2 and 4 to 6 is biased and the shelf life is short.

[0142] 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 intrinsic viscosity of the polylactic acid is 0.79-1.06 dl / g, the molecular weight distribution index PDI is 1.49-1.52, the lactide monomer mass percentage is less than 0.5%; and under the test condition of 190°C, the shear rate is 0.01-1s -1 range, the shear viscosity and shear rate satisfy the following linear relationship: Y = aln (X) + b, Where, Y is the shear viscosity, in Pa·s, X is the shear rate, unit is s -1 , a is -8.3 to -7.2, b is 20 to 25.

2. The polylactic acid according to claim 1, characterized in that The mass of lactide monomer of the polylactic acid is 0.1% to 0.41%.

3. The polylactic acid according to claim 1, characterized in that The acid value of the polylactic acid is less than 15 mol / t.

4. The polylactic acid according to claim 3, characterized in that The acid value of the polylactic acid is 4 to 10 mol / t.

5. Use of the polylactic acid as described in any one of claims 1 to 4 in plastic parts with a wall thickness of 0.1 to 0.4 mm.

6. The method for preparing polylactic acid according to any one of claims 1 to 4, characterized in that: The following steps are involved: Mixing lactide, additives, catalysts and initiators, and preheating to obtain a preheated mixture; The preheated mixture is subjected to a first heating reaction to obtain a prepolymer having an intrinsic viscosity of 0.6 to 0.7 dl / g and a lactide monomer mass percentage of 25% to 35%; The prepolymer is subjected to a second heating reaction to obtain a polymer having an intrinsic viscosity of 0.80 to 1.05 dl / g and a lactide monomer mass percentage of 4% to 5%; A catalyst deactivator is added to the polymer, and then evaporated for 20 to 40 minutes under the conditions of a pressure of 0 to 300 Pa and a temperature of 190° C. to 220° C. to obtain the polylactic acid.

7. The method for preparing polylactic acid according to claim 6, characterized in that: The acid value of the lactide is ≤10 mol / t, and the mass percentage of L-lactide is >95% or the mass percentage of D-lactide is >95%.

8. The method for preparing polylactic acid according to claim 6, characterized in that: The temperature of the preheated mixture is 100-130°C; the first heating reaction is carried out at a pressure of 0-50kPa and a temperature of 165-185°C for 1.5-2h; the second heating reaction is carried out at a pressure of 1.0-1.2MPa and a temperature of 185-195°C for 1.5-2h.

9. The method for preparing polylactic acid according to claim 6, characterized in that: Calculated based on the mass of the lactide, the mass of the 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%, and the mass of the catalyst deactivator is 0.001%-0.1%.

10. The method for preparing polylactic acid according to claim 6, characterized in that: The 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 deactivator includes at least one of phosphorous acid, monostearate phosphate, and distearate phosphate.

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