A polylactic acid graft copolymer, a preparation method and application thereof

Abstract

The application provides a polylactic acid graft copolymer and a preparation method and application thereof, the copolymer has a compound or mixture with a general formula I structure, L-lactide, maleic anhydride and an initiator are mixed and reacted, maleic anhydride grafted L-lactide is obtained first, then the maleic anhydride grafted L-lactide is mixed with a catalyst and an initiator, low-temperature prepolymerization is carried out to obtain a MA-LLA oligomer; the MA-LLA oligomer is mixed with an antioxidant, high-temperature polymerization is carried out, and a maleic anhydride grafted polylactic acid copolymer material is obtained. The application provides a brand-new route for synthesizing the polylactic acid graft copolymer, the grafting efficiency is greatly improved, the polymer molecular weight is increased, more maleic anhydride polar groups are successfully introduced on the side chain of the polylactic acid through the grafting reaction, the toughness and elongation at break of the polylactic acid material are improved, the glass transition temperature is improved, and the prepared graft modified polylactic acid copolymer material is green and fully biodegradable.

Description

TECHNICAL FIELD

[0001] The present application relates to the synthesis of polylactic acid by ring-opening polymerization of lactide, in particular to a method for preparing a high molecular weight polylactic acid graft copolymer, and belongs to the field of polymer material synthesis. BACKGROUND

[0002] With the continuous development of the economy and society, people's production and life are becoming more and more dependent on fossil energy. However, currently, non-renewable energy such as oil and coal is in short supply, and the consumption of these energy sources also causes serious environmental pollution. Oil-based materials are non-degradable, so it is necessary to find green resources to replace oil to alleviate the pressure of resource shortage. Therefore, in recent years, the research on bio-based composites has become a popular topic in academia and industrial production, aiming to reduce the harm of consuming fossil energy to the natural environment. Compared with polyolefins and polyethylene terephthalate commonly used in industrialized commodities, polylactic acid (PLA) has better strength and processing performance. It is a thermoplastic resin formed by polymerization of lactic acid monomer, and lactic acid can be fermented from plants such as corn, potatoes, cotton, and hemp. The raw materials can be regenerated, and have the advantages of high strength, good processing performance, excellent mechanical properties, etc. However, PLA also has some defects, such as high brittleness, strong hydrophobicity, low impact strength, high price, and poor stability at high temperature and certain humidity, which limits its application range.

[0003] Currently, there are two main methods for preparing polylactic acid at home and abroad (one-step method and two-step method). The one-step method uses protonic acid or metal oxide catalysts to directly polymerize lactic acid into polylactic acid under vacuum and heating conditions. This method has a low molecular weight of polylactic acid and is not suitable for industrial application. The two-step method is commonly used in industry. In the first step, lactic acid is converted into lactide under the action of a catalyst. In the second step, polylactic acid is synthesized by ring-opening polymerization under the action of a catalyst under reduced pressure. Then, high-optical-purity polylactic acid material is obtained through purification technology. There are many key factors that affect the preparation of polylactic acid. First, the optical purity of the lactide raw material for synthesizing polylactic acid should not be less than 99%. In addition, the residual free acid and water molecules in the lactide preparation process also affect the polymerization of polylactic acid. Second, the polymerization mechanism of lactide to polylactic acid can be divided into cationic ring-opening polymerization, anionic ring-opening polymerization, and coordination polymerization. Different catalysts have different polymerization modes. Lactide ring-opening polymerization to synthesize polylactic acid is prone to racemization and oxidation under high-temperature reaction conditions, which affects the purity of the product. Finally, the lactide monomer that does not participate in the ring-opening polymerization of polylactic acid needs to be removed in time, otherwise it will affect the purity and molecular weight of the polylactic acid product.

[0004] Polylactic acid has the disadvantages of brittleness, strong hydrophobicity and low toughness, so it is difficult to directly produce industrial products like polyethylene and polypropylene, and it must be modified. At present, the most common method is to optimize its performance by blending polylactic acid with other plastics. Although this method is simple, it does not fundamentally change the properties of polylactic acid. In addition, some studies modify polylactic acid itself, such as graft modification. Maleic anhydride is a good grafting modifier. Maleic anhydride is a polar monomer material with a five-membered ring structure. Grafting modification of polylactic acid with maleic anhydride can improve the polarity of polylactic acid on the one hand, and on the other hand, due to the cyclic structure, it can also modify the toughness of the polylactic acid macromolecular chain. At present, the most common method is to graft maleic anhydride onto polyethylene, polystyrene, starch, lignin, etc., and then add it to polylactic acid by blending to improve the toughness of polylactic acid. However, the modification effect of this method is not obvious. Some studies use reactive extrusion to directly graft maleic anhydride onto the polylactic acid macromolecular chain to change the non-polar structure of polylactic acid. Although this method is simple, the grafting rate is low and the modification effect is not significant.

[0005] Chinese patent CN10892033A polymerizes L-lactide, glycolide, Ƹ-caprolactone in a certain proportion under the action of a catalyst to prepare crude PLLGC. The crude PLLGC is dissolved in chloroform, and then re-precipitated with methanol to obtain white solid, i.e. fine PLLGC. By adding maleic anhydride and reacting in solution, maleic anhydride is grafted onto the PLLGC copolymer. Although the synthesized polylactic acid has high purity, the grafting rate of maleic anhydride is not more than 1%, and the solution polymerization method is used in the reaction, and toxic organic solvents are used, and the synthesized polymer has low molecular weight, which is not conducive to industrial production. Chinese patent CN103570884A adds L / D-lactide, glycolide, maleic anhydride into an ampoule in a certain proportion, and polymerizes at 130-170°C for 12-60h under the action of stannous octoate catalyst to obtain crude polypropylene glycolide (MPLGA). The MPLGA is dissolved in chloroform, and then re-precipitated with methanol to obtain white solid, i.e. fine MPLGA. Although the prepared polymer has a narrow molecular weight distribution, the maleic anhydride grafting reaction is carried out simultaneously during the polymerization process, the grafting rate is low, the polymerization time is long, and the polymer has low molecular weight, which is not conducive to industrial production. Chinese patent CN102643418B removes water from lactic acid by blowing inert gas under normal pressure; pre-polymerization under gradually reduced pressure: pre-polycondensation under low vacuum degree 1000-10000 Pa and high vacuum degree 20-1000 Pa, temperature 60-160°C, time 1-10h, to obtain lactic acid pre-polymer; melt polycondensation in the presence of titanium-based composite catalyst using lactic acid pre-polymer as raw material to finally obtain polylactic acid; although no organic solvent is used, the production equipment is simple and the operation is convenient, but the polylactic acid synthesized by one-step method has low molecular weight, which cannot meet the standard of industrial plastics. Chinese patent CN104861157A isomerizes lactide under the catalytic action of an alcohol alkali metal salt, the optical purity of L-lactide or D-lactide in the lactide is 90%-100%, to obtain an isomer mixture of meso-lactide, D-lactide and L-lactide; the mixture is subjected to polymerization reaction to obtain polylactic acid with low optical purity and high molecular weight; although the preparation method has a simple process route and is easy to operate, the optical purity of polylactic acid is low.

[0006] In summary, the current polylactic acid preparation existing process mainly has the following problems: (1) high quality requirement for lactide monomer raw material, the free hydroxyl content (free acid and water) in lactide monomer seriously affects the polymerization degree of polylactic acid. (2) racemization and oxidation reaction easily occur at high temperature during lactide ring-opening polymerization. (3) uninvolved lactide monomer in polylactic acid product needs to be removed in time (devolatilization process). (4) pure polylactic acid product has poor heat resistance and low toughness, and needs to be modified for industrial application. SUMMARY

[0007] To address the above shortcomings, this invention provides a polylactic acid graft copolymer and its preparation method. By employing a reaction route different from existing technologies, a high molecular weight maleic anhydride-grafted polylactic acid copolymer material (MA-PLLA) is prepared. The maleic anhydride grafting rate is high, and the MA-PLLA material has both good toughness and heat resistance. It is also green, biodegradable, and easy to industrialize.

[0008] To achieve the above technical objectives, the technical solution adopted by this invention is as follows:

[0009] The first aspect of the present invention aims to provide a polylactic acid graft copolymer, which is a compound or mixture having the structure of general formula I:

[0010]

[0011] Ⅰ

[0012] Where R1, R2, ... and R n Each independently selected from H or And R1, R2, ... and R n In the diagram, R1, R2, ... and R n for The percentage is no less than 30%.

[0013] Furthermore, R1, R2, ... and R n In the diagram, R1, R2, ... and R n for The percentage should be no less than 35%, and further preferred to be no less than 40%.

[0014] Furthermore, the molecular weight of the polylactic acid graft copolymer is 1.0 × 10⁻⁶. 5 ~4.0×10 5 2.0×10 is preferred. 5 ~3.5×10 5 .

[0015] The second aspect of the present invention aims to provide a method for preparing polylactic acid graft copolymers, comprising the following:

[0016] (1) L-lactide, maleic anhydride and initiator were mixed and reacted to obtain maleic anhydride-grafted L-lactide (MA-LLA), and the product was purified;

[0017] (2) The MA-LLA obtained in (1) is mixed with a catalyst and an initiator, and prepolymerized at low temperature to obtain MA-LLA oligomers;

[0018] (3) mixing the MA-LLA oligomer obtained in (2) with an antioxidant, and performing high-temperature polymerization to obtain a maleic anhydride grafted polylactic acid copolymer material (MA-PLLA).

[0019] Further, the initiator in step (1) is dicumyl peroxide or 2,5-dimethyl-2,5-bis-(tert-butylperoxy) hexane, preferably dicumyl peroxide; the amount of the initiator added is 0.1-5% by weight of L-lactide, preferably 0.5-2.0%.

[0020] Further, step (1) is a constant-temperature reaction under the protection of an inert gas stream, preferably nitrogen, and under vacuum conditions, the reaction temperature is 50-150°C, preferably 80-120°C, the reaction pressure is 2-100 kpa, preferably 20-60 kpa; the reaction time is 0.5-10 h, preferably 2-6 h; the gas flow rate of the nitrogen stream protection is 0.5-6 m / s, preferably 2-4 m / s; the amount of maleic anhydride is 100-300% of L-lactide by weight, preferably 150-200% of L-lactide.

[0021] Further, the optical purity of the L-lactide in step (1) is 99.0-99.6%, preferably 99.4-99.6%, and a self-made or purchased L-lactide is used.

[0022] The main reaction occurring in step (1) is shown in the following formula:

[0023]

[0024] Further, the purification in step (1) is performed by removing the L-lactide not involved in the grafting reaction by vacuum distillation to obtain purified maleic anhydride grafted L-lactide (MA-LLA). Specifically, the temperature of the vacuum distillation is 100-200°C, preferably 120-150°C; the pressure is 0-100 kpa, preferably 10-50 kpa; the time is 1-10 h, preferably 2-6 h. The grafting rate can be calculated by the mass difference before and after vacuum distillation and the molar ratio of the chemical reaction.

[0025] Further, the catalyst in step (2) is selected from one or more of stannous octoate, zinc lactate, trialkylaluminum, triisobutylaluminum, and stannous chloride, preferably stannous octoate; the initiator is selected from at least one of glycerol, xylitol, ethylene glycol, and triphenylphosphine, preferably triphenylphosphine.

[0026] Further, the amount of the catalyst added in step (2) is 0.1-5% by weight of MA-LLA, preferably 0.5-3%; the amount of the initiator added is 0.05-5.0% of MA-LLA, preferably 0.5-2%.

[0027] Further, the step (2) is low-temperature polymerization under the conditions of inert gas flow, preferably nitrogen flow, and negative pressure, the temperature of the polymerization reaction is 100-150 DEG C, preferably 120-140 DEG C, the reaction time is 2-15 h, preferably 3-8 h, the reaction pressure is 100-1000 kpa, preferably 200-500 kpa, and the gas flow rate of the nitrogen flow is 0.5-6 m / s, preferably 2-4 m / s.

[0028] Further, the antioxidant in the step (3) is one of phosphite, alkyl polyphenol and thio-bisphenol, preferably triphenyl phosphite, and the addition amount of the antioxidant is 0.1%-5% of the MA-LLA oligomer, preferably 1-3% by weight.

[0029] Further, the temperature of the polymerization reaction in the step (3) is 150-250 DEG C, preferably 160-200 DEG C.

[0030] Further, the step (3) adopts a double-screw extruder to realize the reaction process, the MA-LLA oligomer and the antioxidant are added into the double-screw extruder to carry out the polymerization reaction, high-temperature reactive mixing extrusion granulation is carried out, and finally the maleic anhydride grafted poly-lactic acid copolymer material (MA-PLLA) is obtained.

[0031] The main reaction processes of the steps (2) and (3) are shown in the following formula:

[0032]

[0033] The second aspect of the present application is to provide the poly-lactic acid grafted copolymer prepared by the above method. The grafted modified poly-lactic acid copolymer material prepared by the method of the present application has a high molecular weight, also has good toughness and elongation at break, a high glass transition temperature, and is green and fully biodegradable.

[0034] The third aspect of the present application is to provide the application of the poly-lactic acid grafted copolymer, which is applied to the fields of agricultural mulching film, food packaging and medical and health products.

[0035] Compared with the prior art, the present application has the following advantages:

[0036] (1) Compared with the prior art, the present application provides a new route for synthesizing the poly-lactic acid grafted copolymer, first grafts maleic anhydride (MA) to L-lactide (LLA) to obtain maleic anhydride grafted L-lactide (MA-LLA), then prepolymers the MA-LLA at low temperature to obtain MA-LLA oligomer, and finally polymerizes the MA-LLA oligomer at high temperature to obtain the maleic anhydride grafted poly-lactic acid copolymer with a high molecular weight.

[0037] (2) The method of the present application first carries out the grafting reaction of maleic anhydride before the polymerization of lactide, greatly improves the grafting efficiency, increases the molecular weight of the polymer, successfully introduces more maleic anhydride polar groups on the side chain of polylactic acid through the grafting reaction, and improves the toughness and elongation at break of the polylactic acid material.

[0038] (3) Since a large number of maleic anhydride cyclic structures are introduced on the side chain of polylactic acid, the glass transition temperature of the polylactic acid material is improved, and the prepared grafted modified polylactic acid copolymer material is green and fully biodegradable.

[0039] (4) The bulk prepolymerization reaction of L-lactide in the reaction kettle is coupled with the way of double-screw extruder copolymerization, the reaction operation is simple, and the industrial production is easy.

[0040] Other features and advantages of the present application will be described in detail in the following specific embodiments. DETAILED DESCRIPTION

[0041] The present application will be further described below in conjunction with examples. The reagents, materials and instruments involved in the following examples, if not specifically stated, can be obtained through normal commercial channels. The test and inspection methods involved in the following examples, if not specifically stated, are the existing test and inspection methods in the prior art. The following examples clearly and completely describe the technical solutions of the present application. Obviously, the described examples are only a part of the examples of the present application, not all examples. Any obvious changes or changes still within the scope of protection of the present application.

[0042] The toughness and elongation at break of the grafted modified polylactic acid copolymer material synthesized in the following examples are tested by a universal mechanical testing machine; the heat resistance of the grafted modified polylactic acid copolymer material is characterized by a differential scanning calorimeter; the molecular weight of the grafted modified polylactic acid copolymer material is characterized by a gel permeation chromatograph; the grafting rate of maleic anhydride of the polymer is determined by a chemical titration method, specifically: a certain mass (m0) of MAH-g-PLA is placed in a conical flask, a certain amount of tetrahydrofuran is added to dissolve it, a little thymol blue / DMF indicator is added dropwise; an excess of (V KOH ) 0.05 mol / L potassium hydroxide ethanol solution is titrated, and 0.01 mol / L HCl-isopropyl alcohol solution (V HCl ) is back-titrated; the grafting rate n2 / n1, n1=(m0-m2) / M1, n2=m2 / M2, that is, the grafting rate = [(0.05V KOH -0.01V HCl ) x 2 x 90.08] / [m0-(0.05V KOH -0.01V HCl) x 2 x 98.06]; wherein m0 is the mass of the weighed MAH-g-PLA, m2 is the total mass of MAH on the molecular chain of polylactic acid, n1 is the polymerization degree of polylactic acid, i.e. the amount of substance of lactic acid units, n2 is the amount of substance of MAH on the molecular chain of polylactic acid, M1 is the molar mass of lactic acid, M2 is the molar mass of maleic anhydride, V KOH is the volume of the added potassium hydroxide ethanol solution, V HCl is the volume of the titration consumed HCl-isopropyl alcohol solution.

[0043] Example 1

[0044] (1) L-lactide and maleic anhydride were added to a reaction kettle, and a initiator, dicumyl peroxide, was added, and constant temperature reaction was carried out under nitrogen flow and vacuum condition, to obtain maleic anhydride grafted L-lactide. The reaction temperature of the graft modification was 95°C, the reaction pressure was 35 kpa, the reaction time was 2h, and the gas flow rate of the nitrogen flow protection was 2m / s; the maleic anhydride was 140% of the L-lactide, and the dicumyl peroxide was 0.5% of the L-lactide, based on the weight of the L-lactide.

[0045] (2) The L-lactide which did not participate in the grafting reaction in step (1) was removed by vacuum distillation to obtain purified maleic anhydride grafted L-lactide (MA-LLA); the temperature of the vacuum distillation was 125°C, the pressure was 15 kpa, and the time was 2h.

[0046] (3) The maleic anhydride grafted L-lactide (MA-LLA) obtained in step (2) was subjected to low-temperature prepolymerization under the action of a catalyst and an initiator, under the condition of simultaneous nitrogen flow and negative pressure, to obtain MA-LLA oligomer; the reaction temperature of the low-temperature prepolymerization was 130°C, the reaction time was 3h, the reaction pressure was 300 kpa, and the gas flow rate of the nitrogen flow protection was 3m / s; the catalyst stannous octoate was 0.5% of the MA-LLA, and the initiator triphenylphosphine was 0.2% of the MA-LLA, based on the weight of the maleic anhydride grafted L-lactide (MA-LLA).

[0047] (4) The MA-LLA oligomer of step (3) was added to a twin-screw extruder for polymerization reaction, and an antioxidant was added, and high-temperature reactive mixing extrusion granulation was carried out, to finally obtain a maleic anhydride grafted polylactic acid copolymer material (MA-PLLA); the antioxidant triphenyl phosphite was 0.5% of the MA-LLA oligomer, based on the weight of the MA-LLA oligomer, and the reaction extrusion temperature of the reactive mixing extrusion granulation of the twin-screw extruder was 180°C.

[0048] Performance characterization: the right angle tear strength of the maleic anhydride grafted polylactic acid copolymer material is 130 KN / m, the elongation at break is 35%, the glass transition temperature is 90℃, and the molecular weight of the polymer is 2.5 x 10 5 The maleic anhydride grafting rate of the polymer is 42.5% as determined by chemical titration method.

[0049] Example 2

[0050] (1) L-lactide and maleic anhydride were added to the reaction kettle, and the initiator dicumyl peroxide was added. The reaction was carried out under constant temperature, nitrogen flow and vacuum conditions to obtain maleic anhydride grafted L-lactide. The reaction temperature of graft modification was 100℃, the reaction pressure was 35kpa, the reaction time was 3h, and the gas flow rate of nitrogen flow protection was 2.5m / s; the maleic anhydride was 145% of L-lactide, and the dicumyl peroxide was 0.6% of L-lactide based on the weight of L-lactide.

[0051] (2) The L-lactide not involved in the grafting reaction in step (1) was removed by vacuum distillation to obtain purified maleic anhydride grafted L-lactide (MA-LLA); the temperature of vacuum distillation was 130℃; the pressure was 15kpa, and the time was 2.5h.

[0052] (3) The maleic anhydride grafted L-lactide (MA-LLA) obtained in step (2) was subjected to low-temperature pre-polymerization under the action of catalyst and initiator, and under the conditions of nitrogen flow and negative pressure to obtain MA-LLA oligomer; the reaction temperature of low-temperature pre-polymerization was 135℃, the reaction time was 4h, the reaction pressure was 350kpa, and the gas flow rate of nitrogen flow protection was 3m / s; the catalyst stannous octoate was 0.6% of MA-LLA, and the initiator triphenylphosphine was 0.25% of MA-LLA based on the weight of maleic anhydride grafted L-lactide (MA-LLA).

[0053] (4) The MA-LLA oligomer of step (3) was added to a twin-screw extruder for polymerization reaction, and an antioxidant was added, and then high-temperature reactive mixing extrusion granulation was carried out to finally obtain a maleic anhydride grafted polylactic acid copolymer material (MA-PLLA); the antioxidant triphenyl phosphite was 0.6% of MA-LLA oligomer based on the weight of MA-LLA oligomer, and the reaction extrusion temperature of reactive mixing extrusion granulation by twin-screw extruder was 180℃.

[0054] Performance characterization: the right angle tear strength of the maleic anhydride grafted polylactic acid copolymer material is 135 KN / m, the elongation at break is 37%, the glass transition temperature is 92℃, and the molecular weight of the polymer is 2.7 x 10 5 The maleic anhydride grafting rate of the polymer is 44.6% as determined by chemical titration method.

[0055] Example 3

[0056] (1) L-lactide and maleic anhydride were added to a reaction kettle, and a initiator, dicumyl peroxide, was added. The reaction was carried out under constant temperature and nitrogen flow and vacuum conditions to obtain maleic anhydride grafted L-lactide. The reaction temperature of the graft modification was 100°C, the reaction pressure was 40 kPa, the reaction time was 3 h, and the gas flow rate of the nitrogen flow protection was 3 m / s. The maleic anhydride was 150% of the L-lactide, and the dicumyl peroxide was 0.65% of the L-lactide, based on the weight of the L-lactide.

[0057] (2) The L-lactide that did not participate in the grafting reaction in step (1) was removed by vacuum distillation to obtain purified maleic anhydride grafted L-lactide (MA-LLA). The temperature of the vacuum distillation was 130°C, the pressure was 20 kPa, and the time was 3 h.

[0058] (3) The maleic anhydride grafted L-lactide (MA-LLA) obtained in step (2) was subjected to low-temperature prepolymerization under the action of a catalyst and an initiator, with the addition of nitrogen flow and negative pressure conditions. The reaction temperature of the low-temperature prepolymerization was 135°C, the reaction time was 4.5 h, the reaction pressure was 350 kPa, the gas flow rate of the nitrogen flow protection was 3.5 m / s, the catalyst stannous octoate was 0.65% of the MA-LLA, and the initiator triphenylphosphine was 0.3% of the MA-LLA, based on the weight of the maleic anhydride grafted L-lactide (MA-LLA).

[0059] (4) The MA-LLA oligomer of step (3) was added to a twin-screw extruder for polymerization reaction, and an antioxidant was added. The reaction was carried out by high-temperature reactive mixing extrusion and granulation to finally obtain a maleic anhydride grafted polylactic acid copolymer material (MA-PLLA). The antioxidant triphenyl phosphite was 0.6% of the MA-LLA oligomer, and the reaction extrusion temperature of the reactive mixing extrusion and granulation by the twin-screw extruder was 185°C, based on the weight of the MA-LLA oligomer.

[0060] Performance characterization: The right-angle tear strength of the maleic anhydride grafted polylactic acid copolymer material was measured to be 142 KN / m, the elongation at break was 39%, the glass transition temperature was 94°C, and the molecular weight of the polymer was 2.9 x 10 5 The maleic anhydride grafting rate of the polymer was 48.2% as determined by chemical titration method.

[0061] Example 4

[0062] (1) L-lactide and maleic anhydride are added to a reaction kettle, and a initiator, dicumyl peroxide, is added, and the reaction is carried out under the conditions of constant temperature, nitrogen flow and vacuum, to obtain maleic anhydride grafted L-lactide. The reaction temperature of the graft modification is 105°C, the reaction pressure is 45 kPa, the reaction time is 3.5 h, the gas flow rate of the nitrogen flow protection is 3 m / s; the maleic anhydride is 155% of the L-lactide, and the dicumyl peroxide is 0.7% of the L-lactide, based on the weight of the L-lactide.

[0063] (2) The L-lactide that does not participate in the grafting reaction in step (1) is removed by vacuum distillation to obtain purified maleic anhydride grafted L-lactide (MA-LLA); the temperature of the vacuum distillation is 130°C; the pressure is 25 kPa, and the time is 3.5 h.

[0064] (3) The maleic anhydride grafted L-lactide (MA-LLA) obtained in step (2) is pre-polymerized at low temperature under the action of a catalyst and an initiator, and under the conditions of simultaneous nitrogen flow and negative pressure, to obtain MA-LLA oligomer; the reaction temperature of the low-temperature pre-polymerization is 140°C, the reaction time is 4.5 h, the reaction pressure is 350 kPa, and the gas flow rate of the nitrogen flow protection is 3.5 m / s; the catalyst, stannous octoate, is 0.75% of the MA-LLA, and the initiator, triphenylphosphine, is 0.35% of the MA-LLA, based on the weight of the maleic anhydride grafted L-lactide (MA-LLA).

[0065] (4) The MA-LLA oligomer of step (3) is added to a twin-screw extruder for polymerization reaction, and an antioxidant is added, and the reaction is carried out by high-temperature reactive mixing extrusion and granulation, to finally obtain a maleic anhydride grafted polylactic acid copolymer material (MA-PLLA); the antioxidant, triphenyl phosphite, is 0.65% of the MA-LLA oligomer, based on the weight of the MA-LLA oligomer, and the reaction extrusion temperature of the reactive mixing extrusion and granulation by the twin-screw extruder is 185°C.

[0066] Performance characterization: the right-angle tear strength of the maleic anhydride grafted polylactic acid copolymer material is 145 KN / m, the elongation at break is 40%, the glass transition temperature is 96°C, and the molecular weight of the polymer is 3.05 x 10 5 The grafting rate of maleic anhydride of the polymer is 49.7% as determined by chemical titration method.

[0067] Example 5

[0068] (1) L-lactide and maleic anhydride are added to a reaction kettle, and a initiator, dicumyl peroxide, is added, and the reaction is carried out under the conditions of constant temperature, nitrogen flow and vacuum, to obtain maleic anhydride grafted L-lactide. The reaction temperature of the graft modification is 110°C, the reaction pressure is 55 kPa, the reaction time is 4 h, the gas flow rate of the nitrogen flow protection is 4 m / s, the maleic anhydride is 160% of the L-lactide, and the dicumyl peroxide is 0.8% of the L-lactide.

[0069] (2) The L-lactide that does not participate in the graft reaction in step (1) is removed by vacuum distillation to obtain purified maleic anhydride grafted L-lactide (MA-LLA). The temperature of the vacuum distillation is 135°C, the pressure is 30 kPa, and the time is 3.5 h.

[0070] (3) The maleic anhydride grafted L-lactide (MA-LLA) obtained in step (2) is pre-polymerized at low temperature under the action of a catalyst and an initiator, and under the conditions of nitrogen flow and negative pressure, to obtain MA-LLA oligomer. The reaction temperature of the low-temperature pre-polymerization is 140°C, the reaction time is 5 h, the reaction pressure is 400 kPa, the gas flow rate of the nitrogen flow protection is 4 m / s, the catalyst stannous octoate is 0.8% of the MA-LLA, and the initiator triphenylphosphine is 0.4% of the MA-LLA.

[0071] (4) The MA-LLA oligomer of step (3) is added to a twin-screw extruder for polymerization reaction, and an antioxidant is added, and then high-temperature reactive mixing extrusion granulation is carried out, to finally obtain a maleic anhydride grafted polylactic acid copolymer material (MA-PLLA). The antioxidant triphenyl phosphite is 0.7% of the MA-LLA oligomer, and the reaction extrusion temperature of the reactive mixing extrusion granulation of the twin-screw extruder is 190°C.

[0072] Performance characterization: The right-angle tear strength of the maleic anhydride grafted polylactic acid copolymer material is 140 KN / m, the elongation at break is 38%, the glass transition temperature is 93°C, the molecular weight of the polymer is 2.82 x 10 5 , and the maleic anhydride grafting rate of the polymer is 46.3% as determined by chemical titration method.

[0073] Example 6

[0074] (1) L-lactide and maleic anhydride are added to a reaction kettle, initiator 2,5-dimethyl-2,5-bis-(tert-butyl peroxy) hexane is added, and constant temperature reaction is carried out under nitrogen flow and vacuum condition, to obtain maleic anhydride grafted L-lactide. The reaction temperature of graft modification is 95℃, the reaction pressure is 35kpa, the reaction time is 2h, and the gas flow rate of nitrogen flow protection is 2m / s; the maleic anhydride is 140% of L-lactide, and the 2,5-dimethyl-2,5-bis-(tert-butyl peroxy) hexane is 0.5% of L-lactide, based on the weight of L-lactide.

[0075] (2) The L-lactide not involved in the grafting reaction in step (1) is removed by vacuum distillation to obtain purified maleic anhydride grafted L-lactide (MA-LLA); the temperature of vacuum distillation is 125℃, the pressure is 15kpa, and the time is 2h.

[0076] (3) The maleic anhydride grafted L-lactide (MA-LLA) obtained in step (2) is pre-polymerized at low temperature under the action of a catalyst and an initiator, and under the conditions of simultaneous nitrogen flow and negative pressure; the reaction temperature of low-temperature pre-polymerization is 130℃, the reaction time is 3h, the reaction pressure is 300kpa, and the gas flow rate of nitrogen flow protection is 3m / s; the catalyst zinc lactate is 0.5% of MA-LLA, and the initiator glycerol is 0.2% of MA-LLA, based on the weight of maleic anhydride grafted L-lactide (MA-LLA).

[0077] (4) The MA-LLA oligomer of step (3) is added to a twin-screw extruder for polymerization reaction, and an antioxidant is added, and then high-temperature reactive mixing extrusion granulation is carried out, to finally obtain a maleic anhydride grafted polylactic acid copolymer material (MA-PLLA); the antioxidant thio-bisphenol is 0.5% of the MA-LLA oligomer, and the reaction extrusion temperature of reactive mixing extrusion granulation of the twin-screw extruder is 180℃, based on the weight of the MA-LLA oligomer.

[0078] Performance characterization: the right-angle tear strength of the maleic anhydride grafted polylactic acid copolymer material is 125KN / m, the elongation at break is 30%, the glass transition temperature is 88℃, and the molecular weight of the polymer is 2.13×10 5 The grafting rate of maleic anhydride of the polymer is 40.2% as determined by chemical titration method.

[0079] Example 7

[0080] (1) L-lactide and maleic anhydride are added to a reaction kettle, and a initiator, dicumyl peroxide, is added, and the reaction is carried out under the conditions of constant temperature, nitrogen flow and vacuum, to obtain maleic anhydride grafted L-lactide. The reaction temperature of the graft modification is 100°C, the reaction pressure is 40 kPa, the reaction time is 3 h, the gas flow rate of the nitrogen flow protection is 3 m / s, the maleic anhydride is 150% of the L-lactide by weight, and the dicumyl peroxide is 0.65% of the L-lactide by weight.

[0081] (2) The L-lactide that does not participate in the graft reaction in step (1) is removed by vacuum distillation to obtain purified maleic anhydride grafted L-lactide (MA-LLA). The temperature of the vacuum distillation is 130°C, the pressure is 20 kPa, and the time is 3 h.

[0082] (3) The maleic anhydride grafted L-lactide (MA-LLA) obtained in step (2) is pre-polymerized at low temperature under the action of a catalyst and an initiator, and under the conditions of nitrogen flow and negative pressure, to obtain MA-LLA oligomer. The reaction temperature of the low-temperature pre-polymerization is 135°C, the reaction time is 4.5 h, the reaction pressure is 350 kPa, the gas flow rate of the nitrogen flow protection is 3.5 m / s, the catalyst, triisobutyl aluminum, is 0.65% of the MA-LLA by weight, and the initiator, ethylene glycol, is 0.3% of the MA-LLA by weight.

[0083] (4) The MA-LLA oligomer of step (3) is added to a twin-screw extruder for polymerization reaction, and an antioxidant is added, and then high-temperature reactive mixing extrusion granulation is carried out, to finally obtain a maleic anhydride grafted polylactic acid copolymer material (MA-PLLA). The antioxidant, triphenyl phosphite, is 0.6% of the MA-LLA oligomer by weight, and the reaction extrusion temperature of the reactive mixing extrusion granulation of the twin-screw extruder is 185°C.

[0084] Performance characterization: The right-angle tear strength of the maleic anhydride grafted polylactic acid copolymer material is 138 KN / m, the elongation at break is 37%, the glass transition temperature is 92.5°C, the molecular weight of the polymer is 2.74 x 10 5 , and the maleic anhydride grafting rate of the polymer is 45.6% as determined by chemical titration method.

[0085] Example 8

[0086] (1) L-lactide and maleic anhydride are added to a reaction kettle, initiator 2,5-dimethyl-2,5-bis-(tert-butyl peroxy) hexane is added, and constant temperature reaction is carried out under the conditions of nitrogen flow and vacuum, to obtain maleic anhydride grafted L-lactide. The reaction temperature of the graft modification is 110°C, the reaction pressure is 55kpa, the reaction time is 4h, and the gas flow rate of the nitrogen flow protection is 4m / s; the maleic anhydride is 160% of the L-lactide, and the initiator 2,5-dimethyl-2,5-bis-(tert-butyl peroxy) hexane is 0.8% of the L-lactide, based on the weight of the L-lactide.

[0087] (2) The L-lactide not involved in the grafting reaction in step (1) is removed by vacuum distillation to obtain purified maleic anhydride grafted L-lactide (MA-LLA); the temperature of the vacuum distillation is 135°C; the pressure is 30kpa, and the time is 3.5h.

[0088] (3) The maleic anhydride grafted L-lactide (MA-LLA) obtained in step (2) is subjected to low-temperature prepolymerization under the action of a catalyst and an initiator, under the conditions of simultaneous nitrogen flow and negative pressure, to obtain MA-LLA oligomer; the reaction temperature of the low-temperature prepolymerization is 140°C, the reaction time is 5h, the reaction pressure is 400kpa, and the gas flow rate of the nitrogen flow protection is 4m / s; the catalyst stannous octoate is 0.8% of the MA-LLA, and the initiator glycerol is 0.4% of the MA-LLA, based on the weight of the maleic anhydride grafted L-lactide (MA-LLA).

[0089] (4) The MA-LLA oligomer of step (3) is added to a twin-screw extruder for polymerization reaction, and an antioxidant is added, and high-temperature reactive mixing extrusion granulation is carried out, to finally obtain a maleic anhydride grafted polylactic acid copolymer material (MA-PLLA); the antioxidant thiodiphenol is 0.7% of the MA-LLA oligomer, and the reaction extrusion temperature of the reactive mixing extrusion granulation of the twin-screw extruder is 190°C, based on the weight of the MA-LLA oligomer.

[0090] Performance characterization: the right-angle tear strength of the maleic anhydride grafted polylactic acid copolymer material is 142KN / m, the elongation at break is 38.7%, the glass transition temperature is 93.5°C, and the molecular weight of the polymer is 2.89×10 5 The grafting rate of maleic anhydride of the polymer is 40.2% as determined by chemical titration method.

[0091] Comparative Example 1

[0092] In CN108192033A, L-lactide, glycolide and ε-caprolactone are first reacted to generate a ternary copolymer, and then maleic anhydride is grafted, and from the example results, it can be known that the molecular weight of the grafted copolymer is 8.5×10 4-1.38×10 5 The grafting rate of maleic anhydride is less than 1%, which is far lower than the grafting rate of the present application.

[0093] Comparative Example 2

[0094] The prior art method is to first polymerize L-lactide and then graft:

[0095] (1) L-lactide (LLA) is pre-polymerized at low temperature under the action of a catalyst and an initiator, with nitrogen flow and negative pressure, to obtain LLA oligomers; the reaction temperature of the low-temperature pre-polymerization is 130°C, the reaction time is 3h, the reaction pressure is 300kpa, the gas flow rate of the nitrogen flow protection is 3m / s, the catalyst stannous octoate is 0.5% of L-lactide (LLA) by weight, and the initiator triphenylphosphine is 0.2% of L-lactide (LLA) by weight.

[0096] (2) The LLA oligomers of step (1) are added to a twin-screw extruder for polymerization reaction, maleic anhydride (MA) and an antioxidant are added at the same time, and high-temperature reactive mixing extrusion granulation is carried out, finally obtaining a maleic anhydride grafted polylactic acid material (MA-PLLA); the maleic anhydride is 0.5-5% of the LLA oligomers by weight, the antioxidant triphenyl phosphite is 0.5% of the MA-LLA oligomers, and the reactive extrusion temperature of the twin-screw extruder for reactive mixing extrusion granulation is 180°C.

[0097] Performance characterization: the right-angle tear resistance of the maleic anhydride grafted polylactic acid copolymer material is 103KN / m, the elongation at break is 14%, the glass transition temperature is 70°C, and the molecular weight of the polymer is 2.1×10 5 The grafting rate of maleic anhydride is 0.71%, which is far lower than the grafting rate of the present application.

[0098] Generally, the copolymer material obtained by the above method has a right-angle tear resistance of 90-112KN / m, an elongation at break of 12-16%, a glass transition temperature of 65-75°C, and a polymer molecular weight of 1.8×10 5 -2.2×10 5 The grafting rate of maleic anhydride is 0.4-0.9%.

Claims

1. A method for preparing a polylactic acid graft copolymer, characterized in that, It includes the following: (1) L-lactide, maleic anhydride and initiator were mixed and reacted to obtain maleic anhydride-grafted L-lactide, and the product was purified. (2) The maleic anhydride-grafted L-lactide obtained in (1) was mixed with a catalyst and an initiator and prepolymerized at low temperature to obtain MA-LLA oligomers; (3) The MA-LLA oligomer obtained in (2) is mixed with an antioxidant and polymerized at high temperature to obtain maleic anhydride-grafted polylactic acid copolymer material.

2. The preparation method according to claim 1, characterized in that, The initiator mentioned in step (1) is dicumyl peroxide or 2,5-dimethyl-2,5-bis-(tert-butylperoxy)hexane.

3. The preparation method according to claim 2, characterized in that, The amount of initiator added is 0.1-5% of L-lactide by weight.

4. The preparation method according to claim 3, characterized in that, The amount of initiator added is 0.5 to 2.0% of L-lactide by weight.

5. The preparation method according to claim 1, characterized in that, Step (1) is a constant temperature reaction under inert gas flow and vacuum conditions, with a reaction temperature of 50~150℃, a reaction pressure of 2~100kpa, and a reaction time of 0.5~10h.

6. The preparation method according to claim 1, characterized in that, In step (1), the maleic anhydride is 100 to 300% of L-lactide by weight.

7. The preparation method according to claim 6, characterized in that, In step (1), the maleic anhydride is 150-200% of L-lactide by weight.

8. The preparation method according to claim 1, characterized in that, The purification described in step (1) involves removing L-lactide that did not participate in the grafting reaction by vacuum distillation.

9. The preparation method according to claim 8, characterized in that, The vacuum distillation is carried out at a temperature of 100~200℃ and a pressure of 0~100kPa.

10. The preparation method according to claim 1, characterized in that, The catalyst mentioned in step (2) is selected from one or more of stannous octoate, zinc lactate, trialkylaluminum and stannous chloride, and the amount of catalyst added is 0.1 to 5% of maleic anhydride grafted L-lactide by weight.

11. The preparation method according to claim 1, characterized in that, The initiator mentioned in step (2) is selected from at least one of glycerol, xylitol, ethylene glycol and triphenylphosphine, and the amount of initiator added is 0.05 to 5.0% of maleic anhydride grafted L-lactide by weight.

12. The preparation method according to claim 1, characterized in that, Step (2) is low-temperature polymerization under inert gas flow and negative pressure conditions, with the polymerization reaction temperature being 100~150℃.

13. The preparation method according to claim 1, characterized in that, The low-temperature prepolymerization reaction time in step (2) is 2~15h, and the reaction pressure is 100~1000kpa.

14. The preparation method according to claim 1, characterized in that, The antioxidants mentioned in step (3) are phosphites, alkyl polyphenols and thiobisphenol antioxidants. The amount of antioxidants added is 0.1% to 5% of the MA-LLA oligomer by weight.

15. The preparation method according to claim 1, characterized in that, The polymerization temperature in step (3) is 150~250℃.

16. The preparation method according to claim 15, characterized in that, The polymerization temperature in step (3) is 160~200℃.

17. The preparation method according to claim 1, characterized in that, Step (3) The reaction process is carried out using a twin-screw extruder. MA-LLA oligomer and antioxidant are added to the twin-screw extruder for polymerization reaction. High-temperature reactive compounding, extrusion and granulation are performed to obtain maleic anhydride-grafted polylactic acid copolymer material.

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