An unsaturated aliphatic ester polylactic acid graft copolymer, and a preparation method and application thereof

Abstract

The application provides an unsaturated aliphatic ester grafted polylactic acid copolymer, which has a structure shown in the following general formula, wherein R1, R2, … and R n are independently selected from H or wherein R is a substituted or unsubstituted alkyl group with C1-C6, and R1, R2, … and R n are groups, and the number of groups accounts for not less than 25% of the total mole percentage of R1, R2, … and R n The application first copolymerizes the unsaturated aliphatic ester and L-lactide, then prepolymers at low temperature to obtain an HEM-LLA oligomer, and finally polymerizes at high temperature to obtain a copolymer with high molecular weight. More unsaturated aliphatic ester polar groups are successfully introduced on the side chain of the polylactic acid through the grafting reaction, the toughness, elongation at break and glass transition temperature of the polylactic acid material are improved, and the product is green and fully biodegradable.

Description

Technical Field

[0001] This invention relates to the technology of ring-opening polymerization of lactide to synthesize polylactic acid, and more particularly to the preparation method of polylactic acid grafted with unsaturated aliphatic esters, belonging to the field of polymer material synthesis. Background Technology

[0002] With the continuous development of the economy and society, people's production and life are becoming increasingly dependent on fossil energy. However, non-renewable energy sources such as oil and coal are currently in short supply, and the consumption of these energy sources causes serious environmental pollution. Petroleum-based materials are non-degradable, therefore, it is necessary to find green resources to replace oil to alleviate the pressure of resource shortages. Consequently, research on bio-based composite materials has become a hot topic in academia and industrial production in recent years, aiming to reduce the harm to the natural environment caused by the consumption of fossil energy. Compared with polyolefins and polyethylene terephthalate (PET), which are commonly used in industrial products, polylactic acid (PLA) has better strength and processing performance. It is a thermoplastic resin produced by polymerizing lactic acid monomers, which can be produced through the fermentation of plants such as corn, potatoes, cotton, and hemp. The raw materials are renewable, and it has advantages such as high strength, good processability, and excellent mechanical properties. However, PLA also has some drawbacks, such as high brittleness, strong hydrophobicity, low impact strength, and high price. Furthermore, PLA has poor stability at high temperatures and certain humidity levels, which limits its application range.

[0003] Currently, there are two main technologies for preparing polylactic acid (PLA) both domestically and internationally: the one-step method and the two-step method. The one-step method uses protic acid or metal oxide catalysts to directly polymerize lactic acid under vacuum and heating conditions. This method produces PLA with a low molecular weight, making it unsuitable for industrial applications. Industrially, the two-step method is more commonly used. In the first step, lactic acid is reacted with a catalyst to form lactide. In the second step, ring-opening polymerization is performed under reduced pressure and with a catalyst to produce PLA. High-optical-purity PLA materials are then obtained through purification techniques. There are many key factors affecting the preparation of polylactic acid (PLA). First, the optical purity of the lactide raw material used to synthesize PLA must be no less than 99%. In addition, residual free acid and water molecules during the lactide preparation process can also affect the polymerization of PLA. Second, the polymerization mechanism of PLA from lactide can be divided into cationic ring-opening polymerization, anionic ring-opening polymerization, and coordination polymerization. Different catalysts will result in different polymerization methods. Under high-temperature reaction conditions, the ring-opening polymerization of lactide to PLA is prone to racemization and oxidation reactions, which have a certain impact on the purity of the product. Finally, residual lactide monomers that have not participated in the ring-opening polymerization in the PLA product need to be removed in a timely manner, otherwise it will affect the purity and molecular weight of the PLA product.

[0004] Polylactic acid (PLA) suffers from drawbacks such as high brittleness, strong hydrophobicity, and low toughness. Therefore, PLA materials are difficult to directly manufacture industrial products like general-purpose plastics such as polyethylene and polypropylene, and must undergo modification. Currently, a common method is to blend PLA with other plastics to optimize its performance. While this method is simple, it doesn't fundamentally change the properties of PLA. Furthermore, some studies have focused on plasticizing and toughening PLA materials by adding toughening agents, plasticizers, or crosslinking agents to PLA materials or blends with other resins to improve its brittleness and low toughness. Although this has improved the thermodynamic properties of PLA materials to some extent, the modification effect is not significant.

[0005] Chinese patent CN105504729A describes mixing glycidyl methacrylate, methyl methacrylate, butyl methacrylate, γ-methacryloyloxypropyltrimethoxysilane, and the initiator dicumyl peroxide in a mass ratio of 15:20:50:15:0.5 to form a mixture. Then, the mixture is combined with poly(L-lactic acid) (molecular weight 5 × 10⁻⁶). 4The mixture was added to a mixer at a mass ratio of 1:1 and mixed evenly. Then, it was melt-extruded and granulated in a twin-screw extruder at a temperature of 190℃. The screw speed was adjusted to allow the material to remain in the mixer for 8 minutes. The granulation process yielded the modified polylactic acid material. Although the polylactic acid matrix and the filler are chemically bonded together by a modified coupling agent, resulting in an elastomer of a certain thickness between them, thus improving toughness and adhesion, the simple linking method between the macromolecular chains has limited effect on the modification of polylactic acid. Chinese Patent 106519195A reacts methyl methacrylate with hydroxyethyl methacrylate monomers to obtain a macromolecular initiator with terminal hydroxyl groups in the side chain, which then initiates the ring-opening polymerization of cyclic ester monomers (such as glycolide, lactide, or caprolactone) to obtain comb-shaped polyester. The comb-shaped polyester is then blended with polylactic acid (PLA) and electrospun to obtain modified PLA fibers. The main chain of the comb-shaped polyester is a methyl methacrylate-hydroxyethyl methacrylate copolymer structure, which is flexible and elastic, and plays a toughening and strengthening role on the PLA matrix. By adjusting the ratio of polymethyl methacrylate and polyethyl methacrylate segments in the copolymer structure, PLA fibers with different modification effects can be obtained. Although the polyester carrying polyester side chains solves the problem of poor compatibility between the main chain and PLA, the toughness modification effect of PLA is not significant only when the PLA end groups carry polyester side chains. Chinese Patent 108129813A provides a polylactic acid resin composition for injection molding materials. The composition comprises 70%–95% polylactic acid resin and 5.0%–30% modified masterbatch. The modified masterbatch contains a double-crosslinked copolymer A and copolymer B with a core-shell structure. Copolymer A has a shell of poly-D-lactic acid and a core of crosslinked acrylate-glycidyl methacrylate copolymer, forming a secondary crosslink. Copolymer B has a shell of poly-L-lactic acid and a core of crosslinked acrylate-glycidyl methacrylate copolymer, also forming a secondary crosslink. The weight ratio of copolymer A to copolymer B is 2:3–3:2. This technology achieves good toughening and heat resistance for polylactic acid, but does not fundamentally change the thermodynamic properties of polylactic acid.

[0006] In summary, the current processes for preparing and modifying polylactic acid (PLA) mainly have the following problems: (1) High quality requirements for lactide monomer raw materials, and the content of free hydroxyl groups (free acid and water) in lactide monomers seriously affects the degree of PLA polymerization. (2) Pure PLA products have poor heat resistance and low toughness, and require modification processing for industrial application. (3) Simply adding plasticizers and toughening agents to PLA materials cannot fundamentally improve the thermodynamic properties of PLA. (4) Grafting or esterification reactions on the side chains or end groups of PLA macromolecular chains have low reactivity and poor modification effect. Summary of the Invention

[0007] To address the above shortcomings, this invention provides an unsaturated aliphatic ester-grafted polylactic acid copolymer and its preparation method. Employing a reaction route different from existing technologies, it prepares a high molecular weight unsaturated aliphatic ester-grafted polylactic acid material with a high grafting rate of the unsaturated aliphatic ester. The unsaturated aliphatic ester-grafted polylactic acid copolymer exhibits both good toughness and heat resistance, is green and biodegradable, and is easy for industrial production.

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

[0009] The first aspect of this invention aims to provide an unsaturated aliphatic ester-grafted polylactic acid copolymer, which is a compound having the general formula I:

[0010]

[0011] Where R1, R2, ... and R n Selected independently from H or Wherein R is a substituted or unsubstituted alkyl group of C1-C6, preferably a substituted or unsubstituted alkyl group of C1-C4, and R1, R2, ... and R n for The number of groups accounts for the proportions of R1, R2, ... and R n The total molar percentage is not less than 25%, preferably not less than 30%, further preferably not less than 35%, and most preferably not less than 40%.

[0012] Furthermore, the copolymer has a weight-average molecular weight of 1.5 × 10⁻⁶. 5 -4.5×10 5 2.5×10 5 -4.0×10 5 .

[0013] Furthermore, the copolymer has a right-angle tear strength of not less than 130 kN / m, preferably 135-155 kN / m; an elongation at break of not less than 35%, preferably 40-55%; and a glass transition temperature of 85-100℃.

[0014] In this invention, the right-angle tear strength and elongation at break are measured by a universal mechanical testing machine; the weight-average molecular weight is measured by gel chromatography; and the glass transition temperature is measured by differential scanning calorimetry. These are described in detail below.

[0015] The second aspect of this invention aims to provide a method for preparing unsaturated aliphatic ester-grafted polylactic acid copolymers, comprising the following:

[0016] (1) L-lactide, The mixture was reacted with an initiator to obtain an unsaturated aliphatic ester-grafted L-lactide product (HEM-LLA), which was then purified; among which, R in the C1-C6 substituted or unsubstituted alkyl group;

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

[0018] (3) The HEM-LLA oligomer obtained in (2) is mixed with an antioxidant and polymerized at high temperature to obtain an unsaturated aliphatic ester grafted polylactic acid copolymer (HEM-PLLA).

[0019] Further, the initiator in step (1) is dicumyl peroxide or 2,5-dimethyl-2,5-bis-(tert-butylperoxide)hexane, preferably dicumyl peroxide; the amount of initiator added is 1-10% of L-lactide by weight, preferably 1-5%.

[0020] Further, step (1) involves a constant-temperature reaction under an inert gas flow, preferably nitrogen, and a vacuum condition. The reaction temperature is 50-150℃, preferably 80-120℃, and the reaction pressure is 2-100 kPa, preferably 20-60 kPa. The reaction time is 1-20 h, preferably 2-10 h. The gas flow rate under nitrogen protection is 0.5-6 m / s, preferably 2-4 m / s. It is one or more of methyl methacrylate, ethyl methacrylate, and butyl methacrylate, preferably ethyl methacrylate, based on the weight of L-lactide. The amount added is 150-350% of L-lactide, preferably 200-300%.

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

[0022] Furthermore, the purification in step (1) involves removing unreacted L-lactide from the grafting reaction using vacuum distillation to obtain purified unsaturated aliphatic ester-grafted L-lactide (HEM-LLA). Specifically, the vacuum distillation temperature is 100-200℃, preferably 120-150℃; the pressure is 0-100 kPa, preferably 10-50 kPa; and the time is 1-10 h, preferably 2-6 h.

[0023] The main reaction that occurs in step (1) is shown in the following equation:

[0024]

[0025] Furthermore, 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] Furthermore, in step (2), the amount of catalyst added is 0.1-5% of HEM-LLA by weight, preferably 0.5-3%; the amount of initiator added is 0.05-5.0% of HEM-LLA, preferably 0.5-2%.

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

[0028] Furthermore, the antioxidant mentioned in step (3) is one of phosphite esters, alkyl polyphenols and thiobisphenols, preferably triphenyl phosphite, and the amount of antioxidant added is 0.1%-5% of HEM-LLA oligomer by weight, preferably 1-3%.

[0029] Furthermore, the polymerization temperature in step (3) is 150-250℃, preferably 160-200℃.

[0030] Furthermore, in step (3), a twin-screw extruder is used to carry out the reaction process. HEM-LLA oligomer and antioxidant are added to the twin-screw extruder for polymerization reaction, high-temperature reactive compounding, extrusion and granulation are performed, and finally unsaturated aliphatic ester grafted polylactic acid copolymer material (HEM-PLLA) is obtained.

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

[0032]

[0033] The technical objective of the third aspect of this invention is to provide applications of the above-mentioned polylactic acid graft copolymer, which is used in fields such as agricultural mulch film, food packaging, and medical and hygiene products.

[0034] Compared with existing technologies, the present invention has the following advantages:

[0035] (1) Compared with the prior art, the present invention provides a novel route for synthesizing polylactic acid graft copolymers. First, unsaturated aliphatic ester (HEM) is grafted onto L-lactide (LLA) to obtain unsaturated aliphatic ester grafted L-lactide (HEM-LLA). Then, HEM-LLA is prepolymerized at low temperature to obtain HEM-LLA oligomers. Finally, HEM-LLA oligomers are polymerized at high temperature to obtain high molecular weight unsaturated aliphatic ester grafted polylactic acid copolymers.

[0036] (2) The method of the present invention performs a grafting reaction of unsaturated aliphatic esters before the polymerization of lactide, which greatly improves the grafting efficiency and increases the molecular weight of the polymer. More unsaturated aliphatic ester polar groups are successfully introduced into the side chain of polylactic acid through the grafting reaction, which improves the toughness and elongation at break of polylactic acid materials. In addition, the preparation method provided by the present invention can theoretically obtain polylactic acid graft copolymers with a grafting rate of 50% or even 100%. By controlling the raw material ratio, the content of grafting groups in polylactic acid graft copolymers can be effectively controlled, thereby controlling the performance of polylactic acid graft copolymers.

[0037] (3) Due to the introduction of a large number of unsaturated aliphatic ester structures on the side chain of polylactic acid, the glass transition temperature of polylactic acid material is increased, and the prepared graft-modified polylactic acid copolymer material is green and fully biodegradable.

[0038] (4) The product of the present invention retains the terminal hydroxyl groups of the polylactic acid graft copolymer, thereby enabling the polylactic acid graft copolymer to be further modified through the functional group to further regulate the properties of the polylactic acid graft copolymer.

[0039] (5) The bulk prepolymerization reaction of L-lactide is carried out in a reaction vessel and coupled with a twin-screw extruder for copolymerization. The reaction operation is simple and easy to industrialize.

[0040] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0041] Figure 1 The infrared absorption spectra of HEM-LLA and PLLA (polylactic acid) products from Example 1 are shown. Detailed Implementation

[0042] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way. Unless otherwise specified, the reagents, materials, and instruments involved in the following embodiments can be obtained through legitimate commercial channels. Unless otherwise specified, the testing and inspection methods involved in the following embodiments are existing testing and inspection methods in the prior art. The following embodiments clearly and completely describe the technical solutions of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. All obvious variations or modifications derived from the present invention still fall within the scope of protection of the present invention.

[0043] The following are the test methods involving performance parameters in the embodiments:

[0044] (1) Grafting rate of grafted groups:

[0045] The grafting rate of grafted groups in polylactic acid graft copolymers was determined by chemical titration. Specifically, a certain mass of polylactic acid graft copolymer was weighed and placed in an Erlenmeyer flask, a certain amount of tetrahydrofuran was added to dissolve it, and a small amount of thymol blue / DMF indicator was added. The solution was titrated with an excess of 0.05 mol / L potassium hydroxide in ethanol solution and back-titrated with 0.01 mol / L HCl-isopropanol solution.

[0046] Grafting rate = n2 / n1 × 100%, n1 = (m0 - m2) / M1, n2 = m2 / M2, that is, grafting rate = [(0.05V KOH -0.01V HCl )×2×M1] / [m0-(0.05V KOH -0.01V HCl [m0] × 2 × M2; where m0 is the mass of the polylactic acid graft copolymer weighed, in g; m2 is the total mass of the grafted groups on the polylactic acid molecular chain, calculated based on the consumed KOH; n1 is the degree of polymerization of polylactic acid, i.e., the amount of substance of lactic acid unit; n2 is the amount of substance of the grafted groups on the polylactic acid molecular chain; M1 is the molar mass of lactic acid; M2 is the molar mass of the grafted groups; V KOH V represents the volume of potassium hydroxide ethanol solution added, in mL; HCl The volume of HCl-isopropanol solution consumed in the titration is expressed in mL.

[0047] (2) Right-angle tear strength and elongation at break: measured by a universal testing machine in accordance with GB / T1039-1992 standard.

[0048] (3) Glass transition temperature: Measured using a differential scanning calorimeter (DSC) (DSC 204, Netzsch, Germany). The test conditions were as follows: 5–10 mg of sample was heated from room temperature to 200 °C at a heating rate of 10 °C / min and held at isothermal temperature for 5 min to eliminate the thermal history of the sample; then the sample was rapidly cooled to -20 °C, and then heated from -20 °C to 200 °C at a heating rate of 10 °C / min; nitrogen gas was introduced as a protective gas throughout the test, with a flow rate of 50 mL / min.

[0049] (4) Polymer weight-average molecular weight: Measured by gel permeation chromatography (GPC). The molecular weight of the polylactic acid graft copolymer samples was characterized using gel permeation chromatography (GPC Waters 1515 system). Chromatographically pure THF was used as the mobile phase at a flow rate of 1.0 mL / min and a column temperature of 35 °C. Before testing, the sample was dissolved in THF at a concentration of 5 mg / mL, filtered through a 0.22 μm polytetrafluoroethylene (PTFE) membrane, and injected. Calibration was performed using narrow-distribution polystyrene (PS) standards.

[0050] (5) Detection of terminal hydroxyl groups: The phthalic anhydride method was used to determine the hydroxyl groups according to the standard GB / T 12008.3-2009.

[0051] Example 1

[0052] (1) L-lactide and ethyl methacrylate were added to a reaction vessel, and dicumyl peroxide was added as an initiator. The reaction was carried out under nitrogen flow and vacuum conditions at a constant temperature to obtain ethyl methacrylate-branched L-lactide. The reaction temperature for graft modification was 95℃, the reaction pressure was 35 kPa, and the reaction time was 2 h. The gas flow rate under nitrogen protection was 2 m / s. Based on the weight of L-lactide, ethyl methacrylate accounted for 200% of L-lactide, and dicumyl peroxide accounted for 1% of L-lactide.

[0053] (2) The L-lactide that did not participate in the grafting reaction in step (1) was removed by vacuum distillation to obtain purified ethyl methacrylate grafted L-lactide (HEM-LLA); the vacuum distillation temperature was 125℃, the pressure was 15kpa, and the time was 2h.

[0054] (3) The ethyl methacrylate-grafted L-lactide (HEM-LLA) obtained in step (2) is prepolymerized at low temperature under the action of a catalyst and initiator, with the addition of nitrogen gas flow and negative pressure to obtain HEM-LLA oligomers; the reaction temperature of the low-temperature prepolymerization is 130℃, the reaction time is 3h, the reaction pressure is 300kpa, the gas flow rate of the nitrogen gas flow protection is 3m / s, and the catalyst stannous octoate is 0.5% of HEM-LLA by weight of ethyl methacrylate-grafted L-lactide (HEM-LLA); the initiator triphenylphosphine is 0.2% of HEM-LLA.

[0055] (4) The HEM-LLA oligomer from step (3) is added to a twin-screw extruder for polymerization reaction, and an antioxidant is added at the same time. The mixture is then subjected to high-temperature reactive compounding, extrusion and granulation to finally obtain ethyl methacrylate-grafted polylactic acid copolymer material (HEM-PLLA). The antioxidant triphenyl phosphite is 0.5% of the weight of the HEM-LLA oligomer, and the reactive extrusion temperature for reactive compounding, extrusion and granulation in the twin-screw extruder is 180°C.

[0056] Performance characterization: The ethyl methacrylate-grafted polylactic acid copolymer material was found to have a right-angle tear strength of 140 kN / m, an elongation at break of 40%, a glass transition temperature of 85 °C, and a molecular weight of 2.1 × 10⁻⁶. 5 The ethyl acrylate grafting rate of the polymer was determined to be 36.2% by chemical titration. The presence of terminal hydroxyl groups was also determined by chemical titration.

[0057] Figure 1 The infrared absorption spectra of HEM-LLA and PLLA (polylactic acid) products from Example 1 are shown below. Figure 1 It can be seen that both are at 2998cm -1 An asymmetric stretching vibration peak of CH exists at 1750 cm⁻¹. -1 The characteristic absorption peak of the C=O group is present at 1190 cm⁻¹. -1 1100cm -1 The peak at 2930 cm⁻¹ represents the stretching vibration of COC in PLLA. Comparing the infrared absorption spectra of the two, it can be seen that 2930 cm⁻¹ is the peak of the stretching vibration of COC in PLLA. -1 The presence of the characteristic absorption peak of -CH2- indicates that ethyl methacrylate was successfully grafted onto the macromolecular chain of PLLA in the form of a free radical under the action of an initiator.

[0058] Example 2

[0059] (1) L-lactide and ethyl methacrylate were added to a reaction vessel, and dicumyl peroxide was added as an initiator. The reaction was carried out under nitrogen flow and vacuum conditions at a constant temperature to obtain ethyl methacrylate-grafted L-lactide. The reaction temperature for graft modification was 100℃, the reaction pressure was 35 kPa, and the reaction time was 3 h. The gas flow rate under nitrogen protection was 2.5 m / s. Based on the weight of L-lactide, ethyl methacrylate accounted for 220% of L-lactide, and dicumyl peroxide accounted for 2% of L-lactide.

[0060] (2) The L-lactide that did not participate in the grafting reaction in step (1) was removed by vacuum distillation to obtain purified ethyl methacrylate grafted L-lactide (HEM-LLA); the vacuum distillation temperature was 130℃, the pressure was 15 kPa, and the time was 2.5 h.

[0061] (3) The ethyl methacrylate-grafted L-lactide (HEM-LLA) obtained in step (2) is prepolymerized at low temperature under the action of a catalyst and initiator, with the addition of nitrogen gas flow and negative pressure to obtain HEM-LLA oligomers. The reaction temperature of the low-temperature prepolymerization is 135℃, the reaction time is 4h, the reaction pressure is 350kpa, the gas flow rate of the nitrogen gas flow protection is 3m / s, and the catalyst stannous octoate is 0.6% of HEM-LLA by weight of ethyl methacrylate-grafted L-lactide (HEM-LLA); the initiator triphenylphosphine is 0.25% of HEM-LLA.

[0062] (4) The HEM-LLA oligomer from step (3) is added to a twin-screw extruder for polymerization reaction, and an antioxidant is added at the same time. The mixture is then subjected to high-temperature reactive compounding, extrusion and granulation to finally obtain ethyl methacrylate-grafted polylactic acid copolymer material (HEM-PLLA). The antioxidant triphenyl phosphite is 0.6% of the weight of the HEM-LLA oligomer, and the reactive extrusion temperature for reactive compounding, extrusion and granulation in the twin-screw extruder is 180°C.

[0063] Performance characterization: The ethyl methacrylate-grafted polylactic acid copolymer material was found to have a right-angle tear strength of 144 kN / m, an elongation at break of 43%, a glass transition temperature of 87 °C, and a molecular weight of 2.3 × 10⁻⁶. 5 The ethyl methacrylate grafting rate of the polymer was determined to be 38.1% by chemical titration. The presence of terminal hydroxyl groups was also determined by chemical titration.

[0064] Example 3

[0065] (1) L-lactide and ethyl methacrylate were added to a reaction vessel, and dicumyl peroxide was added as an initiator. The reaction was carried out under nitrogen flow and vacuum conditions at a constant temperature to obtain L-lactide grafted with ethyl methacrylate. The reaction temperature for graft modification was 100℃, the reaction pressure was 40 kPa, the reaction time was 3 h, and the gas flow rate under nitrogen protection was 3 m / s. Based on the weight of L-lactide, ethyl methacrylate accounted for 240% of L-lactide, and dicumyl peroxide accounted for 3% of L-lactide.

[0066] (2) The L-lactide that did not participate in the grafting reaction in step (1) was removed by vacuum distillation to obtain purified ethyl methacrylate grafted L-lactide (HEM-LLA); the vacuum distillation temperature was 130℃, the pressure was 20 kPa, and the time was 3 h.

[0067] (3) The ethyl methacrylate-grafted L-lactide (HEM-LLA) obtained in step (2) was prepolymerized at low temperature under the action of a catalyst and initiator, with the addition of nitrogen gas flow and negative pressure to obtain HEM-LLA oligomers. The reaction temperature of the low-temperature prepolymerization was 135℃, the reaction time was 4.5h, the reaction pressure was 350kpa, the gas flow rate of the nitrogen gas flow protection was 3.5m / s, and the catalyst stannous octoate was 0.65% of HEM-LLA by weight of ethyl methacrylate-grafted L-lactide (HEM-LLA), and the initiator triphenylphosphine was 0.3% of HEM-LLA.

[0068] (4) The HEM-LLA oligomer from step (3) is added to a twin-screw extruder for polymerization reaction, and an antioxidant is added at the same time. The mixture is then subjected to high-temperature reactive compounding, extrusion and granulation to finally obtain ethyl methacrylate-grafted polylactic acid copolymer material (HEM-PLLA). The antioxidant triphenyl phosphite is 0.6% of the weight of the HEM-LLA oligomer, and the reactive extrusion temperature for reactive compounding, extrusion and granulation in the twin-screw extruder is 185°C.

[0069] Performance characterization: The ethyl methacrylate-grafted polylactic acid copolymer material was found to have a right-angle tear strength of 150 kN / m, an elongation at break of 48%, a glass transition temperature of 90 °C, and a molecular weight of 2.46 × 10⁻⁶. 5 The ethyl methacrylate grafting rate of the polymer was determined to be 39.5% by chemical titration. The presence of terminal hydroxyl groups was also determined by chemical titration.

[0070] Example 4

[0071] (1) L-lactide and ethyl methacrylate were added to a reaction vessel, and dicumyl peroxide was added as an initiator. The reaction was carried out under nitrogen flow and vacuum conditions at a constant temperature to obtain ethyl methacrylate-grafted L-lactide. The reaction temperature for graft modification was 105℃, the reaction pressure was 45 kPa, the reaction time was 3.5 h, and the gas flow rate under nitrogen protection was 3 m / s. Based on the weight of L-lactide, ethyl methacrylate accounted for 260% of L-lactide, and dicumyl peroxide accounted for 4% of L-lactide.

[0072] (2) The L-lactide that did not participate in the grafting reaction in step (1) was removed by vacuum distillation to obtain purified ethyl methacrylate grafted L-lactide (HEM-LLA); the vacuum distillation temperature was 130℃, the pressure was 25 kPa, and the time was 3.5 h.

[0073] (3) The ethyl methacrylate-grafted L-lactide (HEM-LLA) obtained in step (2) is prepolymerized at low temperature under the action of a catalyst and initiator, with the addition of nitrogen gas flow and negative pressure to obtain HEM-LLA oligomers. The reaction temperature of the low-temperature prepolymerization is 140℃, the reaction time is 4.5h, the reaction pressure is 350kpa, the gas flow rate of the nitrogen gas flow protection is 3.5m / s, and the catalyst stannous octoate is 0.75% of HEM-LLA by weight of ethyl methacrylate-grafted L-lactide (HEM-LLA), and the initiator triphenylphosphine is 0.35% of HEM-LLA.

[0074] (4) The HEM-LLA oligomer from step (3) is added to a twin-screw extruder for polymerization reaction, and an antioxidant is added at the same time. The mixture is then subjected to high-temperature reactive compounding, extrusion and granulation to finally obtain ethyl methacrylate-grafted polylactic acid copolymer material (HEM-PLLA). The antioxidant triphenyl phosphite is 0.65% of the HEM-LLA oligomer by weight, and the reactive extrusion temperature for reactive compounding, extrusion and granulation in the twin-screw extruder is 185°C.

[0075] Performance characterization: The ethyl methacrylate-grafted polylactic acid copolymer material was found to have a right-angle tear strength of 153 kN / m, an elongation at break of 54%, a glass transition temperature of 92 °C, and a molecular weight of 2.56 × 10⁻⁶. 5 The ethyl methacrylate grafting rate of the polymer was determined to be 40.2% by chemical titration. The presence of terminal hydroxyl groups was also determined by chemical titration.

[0076] Example 5

[0077] (1) L-lactide and ethyl methacrylate were added to a reaction vessel, and dicumyl peroxide was added as an initiator. The reaction was carried out under nitrogen flow and vacuum conditions at a constant temperature to obtain ethyl methacrylate-grafted L-lactide. The reaction temperature for graft modification was 110℃, the reaction pressure was 55 kPa, the reaction time was 4 h, and the gas flow rate under nitrogen protection was 4 m / s. Based on the weight of L-lactide, ethyl methacrylate accounted for 280% of L-lactide, and dicumyl peroxide accounted for 5% of L-lactide.

[0078] (2) The L-lactide that did not participate in the grafting reaction in step (1) was removed by vacuum distillation to obtain purified ethyl methacrylate grafted L-lactide (HEM-LLA); the vacuum distillation temperature was 135℃, the pressure was 30 kPa, and the time was 3.5 h.

[0079] (3) The ethyl methacrylate-grafted L-lactide (HEM-LLA) obtained in step (2) is prepolymerized at low temperature under the action of a catalyst and initiator, with the addition of nitrogen gas flow and negative pressure to obtain HEM-LLA oligomers. The reaction temperature of the low-temperature prepolymerization is 140℃, the reaction time is 5h, the reaction pressure is 400kpa, the gas flow rate of the nitrogen gas flow protection is 4m / s, and the catalyst stannous octoate is 0.8% of HEM-LLA by weight of ethyl methacrylate-grafted L-lactide (HEM-LLA); the initiator triphenylphosphine is 0.4% of HEM-LLA.

[0080] (4) The HEM-LLA oligomer from step (3) is added to a twin-screw extruder for polymerization reaction, and an antioxidant is added at the same time. The mixture is then subjected to high-temperature reactive compounding, extrusion and granulation to finally obtain ethyl methacrylate-grafted polylactic acid copolymer material (HEM-PLLA). The antioxidant triphenyl phosphite is 0.7% of the weight of the HEM-LLA oligomer, and the reactive extrusion temperature for reactive compounding, extrusion and granulation in the twin-screw extruder is 190°C.

[0081] Performance characterization: The ethyl methacrylate-grafted polylactic acid copolymer material was found to have a right-angle tear strength of 148 kN / m, an elongation at break of 46%, a glass transition temperature of 89 °C, and a molecular weight of 2.37 × 10⁻⁶. 5 The ethyl methacrylate grafting rate of the polymer was determined to be 39.1% by chemical titration. The presence of terminal hydroxyl groups was also determined by chemical titration.

[0082] Comparative Example 1

[0083] Polylactic acid graft copolymers are prepared by first polymerizing L-lactide and then grafting it.

[0084] (1) L-lactide (LLA) was prepolymerized at low temperature under the action of a catalyst and initiator, and under nitrogen flow and negative pressure conditions to obtain LLA oligomers. The reaction temperature for low-temperature prepolymerization was 130℃, the reaction time was 3h, the reaction pressure was 300kPa, the flow rate of the nitrogen flow was 3m / s, and the amount of stannous octoate catalyst added was 0.5wt% of the amount of LLA, and the amount of triphenylphosphine initiator added was 0.2wt% of the amount of LLA.

[0085] (2) The LLA oligomer obtained in step (1) is added to a twin-screw extruder for polymerization reaction, and ethyl methacrylate and antioxidant are added at the same time. The mixture is then subjected to high-temperature reactive compounding, extrusion and granulation to finally obtain ethyl methacrylate-grafted polylactic acid material (HEM-PLLA). The amount of ethyl methacrylate added is 25 wt% of the amount of LLA oligomer (and the inventor's experiments have verified that even if the amount of ethyl methacrylate added is increased, the grafting rate is not improved). The amount of antioxidant triphenyl phosphite added is 0.5 wt% of the amount of LLA oligomer. The reactive extrusion temperature of the twin-screw extruder for reactive compounding, extrusion and granulation is 180°C.

[0086] Product performance: The measured right-angle tear strength of the ethyl methacrylate-grafted polylactic acid copolymer material is 108 kN / m, the elongation at break is 15%, the glass transition temperature is 65℃, and the molecular weight of the polymer is 1.8 × 10⁻⁶. 5 The ethyl methacrylate grafting rate of the polymer was determined to be 1.6% by chemical titration.

[0087] The presence of terminal hydroxyl groups could not be detected by chemical titration.

Claims

1. An unsaturated aliphatic ester-grafted polylactic acid copolymer, characterized in that, It is a compound with the general formula I: ， Ⅰ Where R1, R2, ... and R n Each independently selected from H or Where R is a C1-C6 substituted or unsubstituted alkyl group, and R1, R2, ... and R n for The number of groups accounts for the proportions of R1, R2, ... and R n The total molar percentage shall not be less than 30%.

2. The unsaturated aliphatic ester-grafted polylactic acid copolymer according to claim 1, characterized in that, R1, R2, ... and R n for The number of groups accounts for the proportions of R1, R2, ... and R n The total molar percentage shall not be less than 35%.

3. The unsaturated aliphatic ester-grafted polylactic acid copolymer according to claim 2, characterized in that, R1, R2, ... and R n for The number of groups accounts for the proportions of R1, R2, ... and R n The total molar percentage shall not be less than 40%.

4. The unsaturated aliphatic ester-grafted polylactic acid copolymer according to claim 1, characterized in that, R is a C1-C4 substituted or unsubstituted alkyl group.

5. The unsaturated aliphatic ester-grafted polylactic acid copolymer according to claim 1, characterized in that, Its right-angle tear strength is not less than 130kN / m, and its elongation at break is not less than 35%.

6. The unsaturated aliphatic ester-grafted polylactic acid copolymer according to claim 1, characterized in that, Its weight-average molecular weight is 1.5 × 10⁻⁶. 5 -4.5×10 5 .

7. The unsaturated aliphatic ester-grafted polylactic acid copolymer according to claim 1, characterized in that, Its glass transition temperature is 85-100℃.

8. A method for preparing the unsaturated aliphatic ester-grafted polylactic acid copolymer material according to claim 1, comprising the following: (1) L-lactide, The product was reacted with an initiator to obtain the unsaturated aliphatic ester-grafted L-lactide product HEM-LLA, which was then purified; among which, R in the C1-C6 substituted or unsubstituted alkyl group; (2) The HEM-LLA obtained in (1) is mixed with a catalyst and an initiator, and prepolymerized at low temperature to obtain HEM-LLA oligomers; (3) The HEM-LLA oligomer obtained in (2) is mixed with an antioxidant and polymerized at high temperature to obtain an unsaturated aliphatic ester-grafted polylactic acid copolymer material.

9. The preparation method according to claim 8, characterized in that, The initiator mentioned in step (1) is dicumyl peroxide or 2,5-dimethyl-2,5-bis-(tert-butylperoxy)hexane, and the amount of initiator added is 1-10% of L-lactide by weight.

10. The preparation method according to claim 8, 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 1-20h.

11. The preparation method according to claim 8, characterized in that, The aforementioned It is one or more of methyl methacrylate, ethyl methacrylate, and butyl methacrylate.

12. The preparation method according to claim 8, characterized in that, Based on the weight of L-lactide, The amount added is 150-350% of L-lactide.

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

14. The preparation method according to claim 8, characterized in that, The catalyst 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 0.1-5% of HEM-LLA by weight.

15. The preparation method according to claim 8, 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 0.05-5.0% of HEM-LLA by weight.

16. The preparation method according to claim 8, characterized in that, Step (2) is low-temperature polymerization under inert gas flow and negative pressure conditions. The polymerization temperature is 100-150℃, the reaction time is 2-15h, and the reaction pressure is 100-1000kPa.

17. The preparation method according to claim 8, characterized in that, The antioxidant mentioned in step (3) is one of phosphite, alkyl polyphenol and thiobisphenol, and the amount of antioxidant added is 0.1%-5% of HEM-LLA oligomer by weight.

18. The preparation method according to claim 8, characterized in that, The polymerization temperature in step (3) is 150-250℃.

19. The preparation method according to claim 8, characterized in that, Step (3) uses a twin-screw extruder to carry out the reaction process. HEM-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 polylactic acid graft copolymer.

20. The application of the unsaturated aliphatic ester-grafted polylactic acid copolymer of claim 1, wherein the copolymer is used in the fields of agricultural mulch film, food packaging and medical and hygiene products.

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