CN117946506A - Degradable polylactic resin and application thereof in food packaging - Google Patents

Abstract

A degradable polylactic acid resin and application thereof in food packaging, belonging to the technical field of polymer materials. The degradable polylactic acid resin mainly comprises polylactic acid, modified starch, a crystal nucleating agent and a plasticizer, and the mass ratio is 150-330:55-95:5-20:10-35. The degradable polylactic acid resin obtained by the invention has a tensile strength of 60.3-64.9 MPa, an elongation at break of 288-313%, a heat deformation temperature of 80.9-82.2°C and a notched impact strength of 25.6-27.0 kJ/ ^m2 .

Description

A degradable polylactic acid resin and its application in food packaging

Technical Field

The invention relates to a degradable polylactic acid resin and application thereof in food packaging, belonging to the technical field of polymer materials.

Background technique

Polylactic acid (PLA) is a typical biodegradable material, which is an α-hydroxy acid derivative and has good biocompatibility. The lactic acid monomer for preparing PLA comes from abundant and renewable plant resources. It is made by saccharifying plant starch to obtain glucose and then fermenting it with bacteria. The hydroxyl and carboxyl groups in the lactic acid molecule have strong activity. Under certain conditions, they can be directly dehydrated and condensed to form high molecular weight PLA, or lactide can be synthesized first, and then PLA can be obtained by ring-opening polymerization of lactide. PLA has the basic characteristics of general polymer materials, good mechanical processing properties, low shrinkage, and can be used for most synthetic plastics. It is widely used in packaging materials, 3D printing consumables, and medical devices. However, PLA has defects such as high brittleness, poor toughness, low impact strength, and poor heat resistance, which make it difficult to be widely used in the field of food packaging.

Chinese patent CN111978695A discloses a degradable all-bio-based high-toughness polylactic acid composite material and its preparation method, the polylactic acid composite material includes 5-15 parts of epoxidized vegetable oil, 1-5 parts of radiation crosslinking agent, and 80-95 parts of polylactic acid; all raw materials are mixed evenly according to the ratio, added into a twin-screw extruder for melt extrusion, dried and injection molded to obtain a sample strip, and the sample strip is irradiated by a γ radiation source to obtain the degradable all-bio-based high-toughness polylactic acid composite material of the present invention. The polylactic acid composite material obtained by this patent has relatively low impact strength and not very good toughness, and the preparation method requires radiation of radioactive elements, and the industrialization risk is too high, and it is not suitable for large-scale preparation of polylactic acid materials used in the field of food packaging.

Chinese patent CN101948613A discloses a fully biodegradable high-toughness polylactic acid resin and its preparation method. The fully biodegradable high-toughness polylactic acid resin of the present invention is composed of polylactic acid resin and additives; the additives are generated by the reaction of corn starch, polyethylene glycol and a coupling agent, and through the coupling action of the coupling agent, a part of the polyethylene glycol molecules are grafted onto the surface of corn starch, thereby improving the compatibility of corn starch with the polylactic acid matrix, and at the same time, the free polyethylene glycol molecules are enriched around the corn starch particles to form a core-shell structure, so that the fully biodegradable high-toughness polylactic acid resin composed of the polylactic acid resin and the additive has excellent mechanical properties. The fully biodegradable high-toughness polylactic acid resin prepared by this patent has very low notched impact strength and poor toughness, and it is difficult to meet the application requirements in food packaging.

From the above, we can see that the current polylactic acid biodegradable materials still have significant problems such as high brittleness, poor toughness, low impact strength, and poor heat resistance, which hinder their application in the field of food packaging. Therefore, it is of great and practical significance to develop a biodegradable polylactic acid resin with good toughness, high notch impact strength and good heat resistance.

Summary of the invention

In view of the deficiencies in the above-mentioned prior art, the present invention provides a degradable polylactic acid resin and its application in food packaging, to achieve the following invention objectives: to prepare a degradable polylactic acid resin with high toughness, good heat resistance and impact resistance and apply it to the field of food packaging.

In order to achieve the above-mentioned invention object, the present invention adopts the following technical scheme:

A degradable polylactic acid resin and its application in food packaging, wherein the degradable polylactic acid resin mainly comprises polylactic acid, modified starch, a crystal nucleating agent and a plasticizer;

The crystal nucleating agent is a compound of modified palygorskite and hyperbranched polyglycidol;

The preparation method of the degradable polylactic acid resin comprises three steps: preparation of modified starch, preparation of a crystal nucleating agent, and melt blending;

The following are further improvements to the above technical solution:

Step 1, preparation of modified starch

Add amylose to an acetic acid aqueous solution, control the stirring rate to 600-950 rpm, raise the temperature to 55-80°C, then add trifluoroacetic acid, stir and react at a constant temperature for 3-6 hours under condensation reflux, quickly cool to 0-5°C, centrifuge, wash the obtained solid with anhydrous ethanol for 3-5 times, dry it in a vacuum oven at 40-55°C for 4-9 hours, and obtain fluffy starch powder, then put the starch powder into a reactor, add a methanol solution of potassium hydroxide, control the stirring rate to 450-750 rpm, raise the temperature and keep the temperature constant to 45-60°C, then add trimethylsilyl chloride, continue to stir and react for 3-7 hours under condensation reflux, cool to room temperature, filter, wash the filtrate with deionized water until neutral, put it in a vacuum oven, and dry it at 50-65°C for 6-10 hours to obtain modified starch;

The molecular weight of the amylose is 1.2×10 ⁴ ~2.3×10 ⁵ g/mol;

The mass fraction of acetic acid in the acetic acid aqueous solution is 14-35wt%;

The mass ratio of the amylose to the acetic acid aqueous solution is 1-4:20;

The added mass of trifluoroacetic acid is 2-7wt% of the mass of amylose;

The starch powder and the methanol solution of potassium hydroxide have a mass ratio of 5 to 40:80;

The methanol solution of potassium hydroxide, wherein the mass fraction of potassium hydroxide is 4-15wt%;

The added mass of trimethylchlorosilane is 9-20wt% of the mass of starch powder.

Step 2: Preparation of crystal nucleating agent

The palygorskite is placed in an oven, dried at 90-115°C for 5-10 hours, and then placed in a dry reactor. Anhydrous ethanol is then added, the stirring rate is controlled at 350-650 rpm, the temperature is raised and kept constant at 50-80°C, and N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane is added. After constant temperature stirring and reaction for 5-11 hours under condensation reflux, the temperature is cooled to room temperature and filtered. The filtrate is dried at 60-85°C for 10-18 hours to obtain modified palygorskite. The modified palygorskite is placed in a vacuum stirred kettle, and the vacuum pump is turned on to evacuate the kettle to a negative pressure of -0.1 to -0.096 MPa. The vacuum is continuously evacuated to maintain the negative pressure for 15 to 24 hours. Then, the vacuum pump is turned off and the feed valve is opened. The hyperbranched polyglycidol is sucked back into the stirred kettle by the negative pressure in the kettle. After the modified palygorskite is completely immersed in the hyperbranched polyglycidol, the feed valve is closed. After being sealed and left to stand for 25 to 36 hours, the negative pressure is removed, and the material is filtered. The filtered solid is the crystallization nucleating agent.

The particle size of the palygorskite is 100-5000 nm;

The added mass of the anhydrous ethanol is 1.5 to 3.5 times the mass of the palygorskite;

The added mass of the N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane is 1-4wt% of the mass of palygorskite;

The molecular weight of the hyperbranched polyglycidol is 1000-7500 g/mol.

Step 3: Melt blending

The polylactic acid, modified starch, crystal nucleating agent and plasticizer are mixed in a mass ratio of 150-330:55-95:5-20:10-35, and then put into a kneader. The mixture is stirred and hot mixed for 10-25 minutes at a temperature of 80-110° C. and a stirring rate of 55-100 rpm. The mixture is transferred into a twin-screw extruder while hot, and the speed of the twin-screw extruder is controlled at 30-55 rpm. The heating temperatures are sequentially controlled to be 135-146° C. for the first zone, 147-158° C. for the second zone, 159-172° C. for the third zone, 173-183° C. for the fourth zone, and 173-185° C. for the fifth zone. After extrusion, the mixture is rapidly cooled to room temperature for granulation. The obtained particles are degradable polylactic acid resin, which are stored in a dry container.

The plasticizer is a mixture of sorbitan fatty acid ester and triglycerol monopalmitate;

The mass ratio of the sorbitan fatty acid ester to triglycerol monopalmitate is 25-60:100;

The polylactic acid has a melting point of 140-190°C, a melt index of 2.0-20 g/10 min under the test conditions of 190°C and 2.16 kg, and a heat deformation temperature of 55-65°C.

Compared with the prior art, the present invention achieves the following beneficial effects:

1. The present invention uses a mixed solution of trifluoroacetic acid and acetic acid to perform oxidation treatment on the terminal hydroxyl group of amylose, so that the terminal of amylose becomes a carboxyl group with stronger reactivity, and then the terminal carboxyl group of amylose reacts with trimethylchlorosilane in an alkaline environment, so that the terminal of amylose becomes a silane group with strong non-polarity, which promotes the compatibility between amylose and polylactic acid resin in the subsequent melt blending process, so that amylose plays a better role in strengthening and toughening;

2. The crystal nucleating agent with hyperbranched polyglycidol as a functional component prepared by the present invention is firstly hydrophobically modified with N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane to reduce the polarity of the inner and outer surfaces of the palygorskite, and then the hyperbranched polyglycidol is loaded in the micropores inside the palygorskite by using the negative pressure adsorption process and the high adsorption of the palygorskite itself. After the crystal nucleating agent with this structure is uniformly mixed with the polylactic acid matrix resin during the melt blending process, the surface and internal adsorption of the palygorskite are The hyperbranched polyglycidol will partially desorb or overflow after being heated. The hyperbranched polyglycidol contains a large number of hydroxyl groups, which can form strong hydrogen bonds with the terminal carboxyl groups of polylactic acid, so it has very significant heterogeneous nucleation and induced crystallization effects. In addition, the size of palygorskite is small, which can provide many heterogeneous nucleation points, and then can greatly accelerate the crystallization rate of polylactic acid polymers, deepen the degree of microcrystallization and fine crystallization of polylactic acid resins during the molding process, thereby significantly improving the mechanical properties and heat resistance of degradable polylactic acid resins.

3. The present invention adds two plasticizers, sorbitan fatty acid ester and triglycerol monopalmitate. Test data show that the addition of these two plasticizers can significantly improve the toughness of polylactic acid resin, while having little effect on heat resistance;

4. The degradable polylactic acid resin obtained by the present invention has a tensile strength of 60.3-64.9 MPa, an elongation at break of 288-313%, a heat deformation temperature of 80.9-82.2°C, and a notched impact strength of 25.6-27.0 kJ/m ² .

Detailed ways

The preferred embodiments of the present invention are described below. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

Example 1: A method for preparing a degradable polylactic acid resin

Step 1, preparation of modified starch

Add amylose to an acetic acid aqueous solution, control the stirring rate to 850 rpm, raise the temperature to 70°C, then add trifluoroacetic acid, and react under constant temperature stirring for 5 hours under condensation reflux, then quickly cool to 2°C, centrifuge, wash the obtained solid with anhydrous ethanol 4 times, dry it in a vacuum oven at 50°C for 7 hours, and obtain fluffy starch powder, then put the starch powder into a reactor, add a methanol solution of potassium hydroxide, control the stirring rate to 600 rpm, raise the temperature and keep it constant at 50°C, then add trimethylsilyl chloride, continue stirring and react under condensation reflux for 6 hours, cool to room temperature, filter, wash the filtrate with deionized water until neutral, put it into a vacuum oven, and dry it at 60°C for 7 hours to obtain modified starch;

The molecular weight of the amylose is 2.0×10 ⁵ g/mol;

The mass fraction of acetic acid in the acetic acid aqueous solution is 19wt%;

The mass ratio of the amylose to the acetic acid aqueous solution is 3:20;

The added mass of trifluoroacetic acid is 5wt% of the mass of amylose;

The starch powder and the methanol solution of potassium hydroxide have a mass ratio of 27:80;

The methanol solution of potassium hydroxide, wherein the mass fraction of potassium hydroxide is 9wt%;

The added mass of trimethylchlorosilane is 15wt% of the mass of starch powder.

Step 2: Preparation of crystal nucleating agent

The palygorskite was placed in an oven, dried at 105° C. for 8 hours, and then placed in a dry reactor. Anhydrous ethanol was added, the stirring rate was controlled to 550 rpm, the temperature was raised and kept constant at 70° C., and N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane was added. After constant temperature stirring and reaction for 9 hours under condensation reflux, the mixture was cooled to room temperature for filtration, and the filtrate was dried at 76° C. for 15 hours to obtain modified palygorskite. The modified palygorskite was then placed in a vacuum stirred reactor, and the vacuum pump was turned on to evacuate the reactor to a negative pressure of -0.099 MPa. The negative pressure was maintained for 19 hours, and then the vacuum pump was turned off and the feed valve was opened. The hyperbranched polyglycidol was sucked back into the stirred reactor by the negative pressure in the reactor. After the modified palygorskite was completely immersed in the hyperbranched polyglycidol, the feed valve was closed, and the negative pressure was removed after standing for 29 hours. The discharged material was filtered, and the filtered solid was the crystallization nucleating agent.

The particle size of the palygorskite is 500 nm;

The added mass of the anhydrous ethanol is 1.8 times the mass of the palygorskite;

The added mass of the N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane is 3wt% of the mass of palygorskite;

The molecular weight of the hyperbranched polyglycidol is 3600 g/mol.

Step 3: Melt blending

Polylactic acid, modified starch, crystal nucleating agent and plasticizer are mixed in a mass ratio of 195:80:16:17, and then put into a kneader. The mixture is stirred and hot mixed for 17 minutes at a temperature of 95° C. and a stirring rate of 90 rpm. The mixture is transferred into a twin-screw extruder while hot, and the speed of the twin-screw extruder is controlled to 39 rpm. The heating temperatures are sequentially controlled to 139° C. for zone 1, 148° C. for zone 2, 162° C. for zone 3, 178° C. for zone 4 and 180° C. for zone 5. After extrusion, the mixture is rapidly cooled to room temperature for granulation. The obtained particles are degradable polylactic acid resin, which are stored in a dry container.

The plasticizer is a mixture of sorbitan fatty acid ester and triglycerol monopalmitate;

The mass ratio of the sorbitan fatty acid ester to triglycerol monopalmitate is 29:100;

The polylactic acid has a melting point of 180°C, a melt index of 12 g/10 min under the test conditions of 190°C and 2.16 kg, and a heat deformation temperature of 62°C.

Example 2: A method for preparing a degradable polylactic acid resin

Step 1, preparation of modified starch

Add amylose to an acetic acid aqueous solution, control the stirring rate to 600 rpm, raise the temperature to 55°C, then add trifluoroacetic acid, and react under constant temperature stirring for 3 hours under condensation reflux, then quickly cool to 0°C, centrifuge, wash the obtained solid with anhydrous ethanol 3 times, and dry it in a vacuum oven at 40°C for 4 hours to obtain fluffy starch powder, then put the starch powder into a reactor, add a methanol solution of potassium hydroxide, control the stirring rate to 450 rpm, raise the temperature and keep it constant at 45°C, then add trimethylchlorosilane, continue stirring and react under condensation reflux for 3 hours, cool to room temperature, filter, wash the filtrate with deionized water until neutral, put it into a vacuum oven, and dry it at 50°C for 6 hours to obtain modified starch;

The molecular weight of the amylose is 1.2×10 ⁴ g/mol;

The mass fraction of acetic acid in the acetic acid aqueous solution is 14wt%;

The mass ratio of the amylose to the acetic acid aqueous solution is 1:20;

The added mass of trifluoroacetic acid is 2wt% of the mass of amylose;

The starch powder and the methanol solution of potassium hydroxide have a mass ratio of 5:80;

The methanol solution of potassium hydroxide, wherein the mass fraction of potassium hydroxide is 4wt%;

The added mass of trimethylchlorosilane is 9wt% of the mass of starch powder.

Step 2: Preparation of crystal nucleating agent

The palygorskite is placed in an oven, dried at 90° C. for 5 hours, and then placed in a dry reactor. Anhydrous ethanol is then added, the stirring rate is controlled to 350 rpm, the temperature is raised and kept constant at 50° C., and N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane is added. After constant temperature stirring and reaction for 5 hours under condensation reflux, the mixture is cooled to room temperature and filtered. The filtrate is dried at 60° C. for 10 hours to obtain modified palygorskite. The modified palygorskite is then placed in a vacuum stirred tank, and the vacuum pump is turned on to evacuate the tank to a negative pressure of -0.1 MPa. The vacuum is continuously evacuated to maintain the negative pressure for 15 hours, and then the vacuum pump is turned off and the feed valve is opened. The hyperbranched polyglycidol is sucked back into the stirred tank by the negative pressure in the tank. After the modified palygorskite is completely immersed in the hyperbranched polyglycidol, the feed valve is closed, and the mixture is allowed to stand for 25 hours in a closed state, the negative pressure is removed, and the discharged material is filtered. The filtered solid is the crystallization nucleating agent;

The particle size of the palygorskite is 100 nm;

The added mass of the anhydrous ethanol is 1.5 times the mass of the palygorskite;

The added mass of the N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane is 1wt% of the mass of palygorskite;

The molecular weight of the hyperbranched polyglycidol is 1000 g/mol.

Step 3: Melt blending

Polylactic acid, modified starch, crystal nucleating agent and plasticizer are mixed in a mass ratio of 150:55:5:10, and then put into a kneader. The mixture is stirred and hot mixed for 10 minutes at a temperature of 80° C. and a stirring rate of 55 rpm. The mixture is transferred into a twin-screw extruder while hot, and the speed of the twin-screw extruder is controlled at 30 rpm. The heating temperatures are sequentially controlled to 135° C. for zone 1, 147° C. for zone 2, 159° C. for zone 3, 173° C. for zone 4 and 173° C. for zone 5. After extrusion, the mixture is rapidly cooled to room temperature for granulation. The obtained granules are degradable polylactic acid resin, which are stored in a dry container.

The plasticizer is a mixture of sorbitan fatty acid ester and triglycerol monopalmitate;

The mass ratio of the sorbitan fatty acid ester to triglycerol monopalmitate is 25:100;

The polylactic acid has a melting point of 140°C, a melt index of 2.0 g/10 min under the test conditions of 190°C and 2.16 kg, and a heat deformation temperature of 55°C.

Example 3: A method for preparing a degradable polylactic acid resin

Step 1, preparation of modified starch

Add amylose to an acetic acid aqueous solution, control the stirring rate to 950 rpm, raise the temperature to 80°C, then add trifluoroacetic acid, and react under constant temperature stirring for 6 hours under condensation reflux, then quickly cool to 5°C, centrifuge, wash the obtained solid with anhydrous ethanol 5 times, dry it in a vacuum oven at 55°C for 9 hours, and obtain fluffy starch powder, then put the starch powder into a reactor, add a methanol solution of potassium hydroxide, control the stirring rate to 750 rpm, raise the temperature and keep the temperature constant to 60°C, then add trimethylchlorosilane, continue stirring and react under condensation reflux for 7 hours, cool to room temperature, filter, wash the filtrate with deionized water until neutral, put it into a vacuum oven, and dry it at 65°C for 10 hours to obtain modified starch;

The molecular weight of the amylose is 2.3×10 ⁵ g/mol;

The mass fraction of acetic acid in the acetic acid aqueous solution is 35wt%;

The mass ratio of the amylose to the acetic acid aqueous solution is 4:20;

The added mass of trifluoroacetic acid is 7wt% of the mass of amylose;

The starch powder and the methanol solution of potassium hydroxide have a mass ratio of 40:80;

The methanol solution of potassium hydroxide, wherein the mass fraction of potassium hydroxide is 15wt%;

The added mass of trimethylchlorosilane is 20wt% of the mass of starch powder.

Step 2: Preparation of crystal nucleating agent

The palygorskite is placed in an oven, dried at 115° C. for 10 hours, and then placed in a dry reactor. Anhydrous ethanol is then added, the stirring rate is controlled to 650 rpm, the temperature is raised and kept constant at 80° C., and N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane is added. After constant temperature stirring and reaction for 11 hours under condensation reflux, the mixture is cooled to room temperature for filtration, and the filtrate is dried at 85° C. for 18 hours to obtain modified palygorskite. The modified palygorskite is then placed in a vacuum stirred tank, and the vacuum pump is turned on to evacuate the tank to a negative pressure of -0.096 MPa. The vacuum is continuously evacuated to maintain the negative pressure for 24 hours, and then the vacuum pump is turned off and the feed valve is opened. The hyperbranched polyglycidol is sucked back into the stirred tank by the negative pressure in the tank. After the modified palygorskite is completely immersed in the hyperbranched polyglycidol, the feed valve is closed, and the negative pressure is removed after standing for 36 hours. The discharged material is filtered, and the filtered solid is the crystallization nucleating agent;

The particle size of the palygorskite is 5000nm;

The added mass of the anhydrous ethanol is 3.5 times the mass of the palygorskite;

The added mass of the N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane is 4wt% of the mass of palygorskite;

The molecular weight of the hyperbranched polyglycidol is 7500 g/mol.

Step 3: Melt blending

Polylactic acid, modified starch, crystal nucleating agent and plasticizer are mixed in a mass ratio of 330:95:20:35, and then put into a kneader. The mixture is stirred and hot mixed for 25 minutes at a temperature of 110° C. and a stirring rate of 100 rpm. The mixture is transferred into a twin-screw extruder while hot, and the speed of the twin-screw extruder is controlled to 55 rpm. The heating temperatures are sequentially controlled to 146° C. for zone 1, 158° C. for zone 2, 172° C. for zone 3, 183° C. for zone 4 and 185° C. for zone 5. After extrusion, the mixture is rapidly cooled to room temperature for granulation. The obtained granules are degradable polylactic acid resin, which are stored in a dry container.

The plasticizer is a mixture of sorbitan fatty acid ester and triglycerol monopalmitate;

The mass ratio of the sorbitan fatty acid ester to triglycerol monopalmitate is 60:100;

The melting point of the polylactic acid is 190°C, the melt index is 20g/10min under the test conditions of 190°C and 2.16kg, and the heat deformation temperature is 65°C.

Application of biodegradable polylactic acid resin in food packaging:

The degradable polylactic acid resin obtained in the above embodiment can be made into a film-shaped degradable food packaging by extrusion casting, foaming, phase separation, spinning and other methods;

Comparative Example 1: Based on Example 1, step 1, preparation of modified starch, is not performed. In step 3, in melt blending, 80 parts of modified starch are replaced by 80 parts of amylose. The specific operation is as follows:

The step 1, preparation of modified starch is not performed;

The operation of step 2 is the same as that of embodiment 1;

Step 3: Melt blending

The polylactic acid, amylose, crystal nucleating agent and plasticizer are mixed in a mass ratio of 195:80:16:17, and then put into a kneader. The mixture is stirred and hot mixed for 17 minutes at a temperature of 95°C and a stirring rate of 90 rpm. The mixture is transferred into a twin-screw extruder while hot, and the speed of the twin-screw extruder is controlled to 39 rpm. The heating temperatures are sequentially controlled to 139°C in the first zone, 148°C in the second zone, 162°C in the third zone, 178°C in the fourth zone and 180°C in the fifth zone. After extrusion, the mixture is rapidly cooled to room temperature for granulation. The obtained granules are degradable polylactic acid resin, which are stored in a dry container.

The molecular weight of the amylose is 2.0×10 ⁵ g/mol;

The plasticizer is a mixture of sorbitan fatty acid ester and triglycerol monopalmitate;

The mass ratio of the sorbitan fatty acid ester to triglycerol monopalmitate is 29:100;

The polylactic acid has a melting point of 180°C, a melt index of 12 g/10 min under the test conditions of 190°C and 2.16 kg, and a heat deformation temperature of 62°C.

Comparative Example 2: Based on Example 1, step 2, preparation of a crystallization nucleating agent, is not performed. In step 3, in melt blending, 16 parts of the crystallization nucleating agent are replaced by 16 parts of polylactic acid in equal amounts. The specific operation is as follows:

The operation of step 1 is the same as that of embodiment 1;

Step 2, preparation of a crystal nucleating agent, is not performed;

Step 3: Melt blending

The polylactic acid, modified starch and plasticizer were mixed in a mass ratio of 211:80:17, and then put into a kneader. The mixture was stirred and hot mixed for 17 minutes at a temperature of 95°C and a stirring rate of 90 rpm. The mixture was transferred into a twin-screw extruder while hot, and the speed of the twin-screw extruder was controlled to 39 rpm. The heating temperatures were sequentially controlled to 139°C in the first zone, 148°C in the second zone, 162°C in the third zone, 178°C in the fourth zone, and 180°C in the fifth zone. After extrusion, the mixture was quickly cooled to room temperature for granulation. The obtained granules were degradable polylactic acid resin, which were stored in a dry container.

The plasticizer is a mixture of sorbitan fatty acid ester and triglycerol monopalmitate;

The mass ratio of the sorbitan fatty acid ester to triglycerol monopalmitate is 29:100;

The polylactic acid has a melting point of 180°C, a melt index of 12 g/10 min under the test conditions of 190°C and 2.16 kg, and a heat deformation temperature of 62°C.

Comparative Example 3: Based on Example 1, in step 3, in melt blending, no plasticizer is added, and 17 parts of plasticizer are replaced by 17 parts of polylactic acid. The specific operation is as follows:

The operations of steps 1 and 2 are the same as those of Example 1;

Step 3: Melt blending

The polylactic acid, modified starch and crystal nucleating agent are mixed in a mass ratio of 212:80:16, and then put into a kneader. The mixture is stirred and hot mixed for 17 minutes at a temperature of 95°C and a stirring rate of 90 rpm. The mixture is transferred into a twin-screw extruder while hot, and the speed of the twin-screw extruder is controlled to 39 rpm. The heating temperatures are sequentially controlled to 139°C in the first zone, 148°C in the second zone, 162°C in the third zone, 178°C in the fourth zone and 180°C in the fifth zone. After extrusion, the mixture is quickly cooled to room temperature for granulation. The obtained particles are degradable polylactic acid resin, which are stored in a dry container.

The polylactic acid has a melting point of 180°C, a melt index of 12 g/10 min under the test conditions of 190°C and 2.16 kg, and a heat deformation temperature of 62°C.

Performance Testing:

The degradable polylactic acid resin obtained in Examples 1, 2, 3 and Comparative Examples 1, 2, 3 was hot-pressed into sheets, and then the heat deformation temperature was tested according to the relevant requirements of GB/T1634 national standard, and the notched impact strength was tested according to GB/T 1043.1-2018 Determination of impact properties of simply supported beams of plastics. In addition, a phase separation molding method was used to prepare a degradable food packaging film, firstly, the degradable polylactic acid resin was dissolved in a chloroform solution to obtain a casting solution, the mass ratio of the degradable polylactic acid resin to the chloroform was 1:25, after vacuum degassing, the casting solution was poured onto a horizontal glass plate, and a film was formed by scraping with a scraper, and the film was formed after drying at room temperature in a fume hood for 8 hours to obtain a degradable food packaging film, and then the relevant properties of the degradable food packaging film were tested according to GB/T 1040. 3-2006 Determination of tensile properties of plastics Part 3: Test conditions for films and sheets, and the specific results are shown in Table 1;

Table 1

From the data in Table 1, it can be seen that in Comparative Example 1 in which starch is not modified, the mechanical properties and heat deformation temperature are the lowest among all the embodiments and comparative examples, which indicates that starch is not modified and has very poor compatibility with polylactic acid, resulting in a sharp decline in various properties of the polylactic acid resin; in Comparative Example 2, no crystallization nucleating agent is added, and the mechanical properties of the degradable polylactic acid resin are significantly reduced, and the heat deformation temperature is reduced to the lowest. It can be seen that the crystallization nucleating agent can significantly improve the crystallinity of the polylactic acid resin, and plays a very critical role in improving the heat resistance; in Comparative Example 3, two plasticizers, sorbitan fatty acid ester and triglycerol monopalmitate, are not added, and the notched impact strength of the mechanical properties decreases significantly, which indicates that the plasticizer plays a relatively important role in improving the toughness of the polylactic acid resin.

The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims (5)

1. A degradable polylactic acid resin, which is characterized in that:

The degradable polylactic acid resin mainly comprises polylactic acid, modified starch, a crystallization nucleating agent and a plasticizer;

The mass ratio of the polylactic acid to the modified starch to the crystallization nucleating agent to the plasticizer is 150-330:55-95:5-20:10-35;

The preparation method of the modified starch comprises the following steps: adding amylose into an acetic acid aqueous solution, controlling the stirring speed to be 600-950 r/min, heating to 55-80 ℃, adding trifluoroacetic acid, stirring at constant temperature for reacting for 3-6 hours under a condensation reflux state, rapidly cooling to 0-5 ℃, centrifugally separating, washing the obtained solid with absolute ethyl alcohol for 3-5 times, drying in a vacuum oven at 40-55 ℃ for 4-9 hours to obtain fluffy starch powder, then placing the starch powder into a reaction kettle, adding a methanol solution of potassium hydroxide, controlling the stirring speed to be 450-750 r/min, heating to 45-60 ℃, then adding trimethylchlorosilane, continuously stirring for reacting for 3-7 hours under a condensation reflux state, cooling to room temperature, filtering, washing the filtrate with deionized water to be neutral, and drying at 50-65 ℃ for 6-10 hours to obtain modified starch;

The preparation method of the crystallization nucleating agent comprises the following steps: placing palygorskite into an oven, drying at 90-115 ℃ for 5-10 hours, placing the dried palygorskite into a dried reaction kettle, adding absolute ethyl alcohol, controlling the stirring rate to be 350-650 r/min, heating and keeping the temperature to be 50-80 ℃, adding N- (beta-aminoethyl) -gamma-aminopropyl methyl dimethoxy silane, performing constant-temperature stirring reaction for 5-11 hours in a condensation reflux state, cooling to room temperature, filtering, drying filtrate at 60-85 ℃ for 10-18 hours to obtain modified palygorskite, placing the modified palygorskite into a vacuum stirring kettle, starting a vacuum pump to pump the kettle to negative pressure of-0.1 to-0.096 MPa, continuously vacuumizing to maintain the negative pressure for 15-24 hours, closing a feed valve of the vacuum pump, reversely sucking hyperbranched polyglycidyl into the stirring kettle by virtue of the negative pressure in the kettle, closing the feed valve after the modified palygorskite is completely immersed in the hyperbranched polyglycidyl, sealing for 25-36 hours, removing the negative pressure, discharging, filtering, and obtaining the solid which is the crystallization nucleating agent;

The plasticizer is a mixture of sorbitan fatty acid ester and triglycerol monopalmitate;

The mass ratio of the sorbitan fatty acid ester to the triglycerol monopalmitate is 25-60:100;

the melting point of the polylactic acid is 140-190 ℃, the melt index is 2.0-20 g/10min under the test condition of 190 ℃ and 2.16 kg, and the heat distortion temperature is 55-65 ℃.

2. The degradable polylactic acid resin according to claim 1, wherein:

the molecular weight of the amylose is 1.2X10 ⁴~2.3×10⁵ g/mol;

the mass fraction of acetic acid in the acetic acid aqueous solution is 14-35wt%;

The mass ratio of the amylose to the acetic acid aqueous solution is 1-4:20;

The adding mass of the trifluoroacetic acid is 2-7wt% of the mass of the amylose;

the mass ratio of the starch powder to the methanol solution of potassium hydroxide is 5-40:80;

The methanol solution of potassium hydroxide comprises, by mass, 4-15% of potassium hydroxide;

the adding mass of the trimethylchlorosilane is 9-20wt% of the mass of the starch powder.

3. The degradable polylactic acid resin according to claim 1, wherein:

The particle size of the palygorskite is 100-5000 nm;

the added mass of the absolute ethyl alcohol is 1.5-3.5 times of the mass of the palygorskite;

the addition mass of the N- (beta-aminoethyl) -gamma-aminopropyl methyl dimethoxy silane is 1-4wt% of the mass of palygorskite;

The molecular weight of the hyperbranched polyglycidyl is 1000-7500 g/mol.

4. The degradable polylactic acid resin according to claim 1, wherein:

The degradable polylactic acid resin is obtained by melt blending, and the specific method comprises the following steps: mixing polylactic acid, modified starch, crystallization nucleating agent and plasticizer according to the mass ratio of the degradable polylactic acid resin, putting the mixture into a kneader, controlling the temperature to be 80-110 ℃, the stirring speed to be 55-100 r/min, stirring and hot mixing for 10-25 min, transferring the mixture into a double-screw extruder while the mixture is hot, controlling the rotating speed of the double-screw extruder to be 30-55 r/min, sequentially controlling the heating temperature to be one region to 135-146 ℃, two regions to 147-158 ℃, three regions to 159-172 ℃, four regions to 173-183 ℃ and five regions to 173-185 ℃, extruding, rapidly cooling to room temperature, granulating, and putting the obtained granules into a dry container for storage, wherein the degradable polylactic acid resin is obtained.

5. Use of the degradable polylactic acid resin according to claim 1 in food packaging, characterized in that:

The degradable polylactic acid resin is prepared into the film-shaped degradable food package by extrusion casting molding, foaming molding, phase separation molding and spinning molding.