Preparation method of polylactic acid for high-heat-resistance food packaging

Abstract

The invention provides a preparation method of polylactic acid for high-heat-resistance food packaging, and relates to the technical field of high polymer materials. Comprising the following steps: synthesizing a hydroxyl-terminated prepolymer, carrying out chain extension on the hydroxyl-terminated prepolymer, preparing hydroxylated nano silicon dioxide, dispersing and melting. Synthesizing a hydroxyl-terminated prepolymer: synthesizing the hydroxyl-terminated prepolymer from L-lactic acid, zinc lactate and ethylene glycol; carrying out chain extension on the hydroxyl-terminated prepolymer: carrying out chain extension reaction on the hydroxyl-terminated prepolymer by using 1, 4-butanediol diglycidyl ether and glycidyl stearate; preparing hydroxylated nano silicon dioxide: reacting ethanol, ammonia water and tetraethyl orthosilicate at 25-40 DEG C for 4-6 hours; and centrifuging to take a solid phase, dispersing the solid phase in a hydrochloric acid solution, and carrying out ultrasonic treatment at 40-60 DEG C for 20-40 minutes. The polylactic acid prepared by the invention has excellent heat resistance and mechanical properties.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer materials, and particularly to a preparation method of polylactic acid for high-heat-resistant food packaging. Background Art

[0002] As a bio-based degradable material, polylactic acid (PLA) is widely used in the field of food packaging due to its excellent biocompatibility and environmental friendliness. However, the heat distortion temperature of traditional PLA materials is usually lower than 60°C, and it is prone to deformation in application scenarios such as high-temperature filling and microwave heating, which severely restricts its popularization in the field of heat-resistant packaging.

[0003] The prior art with publication number CN118496647B discloses a modified polylactic acid heat-resistant and degradable composite material. By acidolysis and oxidation treatment of biomass raw materials, combined with silk fibroin reinforcement and composite nucleating agent blending, the thermal stability and mechanical properties of polylactic acid are improved. Utilizing the biocompatibility of biomass raw materials and silk fibroin to avoid introducing non-degradable components and enhancing the interfacial bonding through amide bonds. However, its process steps are cumbersome, and the elongation at break of the composite material is relatively low, restricting its application scenarios in flexible food packaging.

[0004] The prior art with publication number CN118562103B discloses a preparation method of polylactic acid for high-transparency food packaging. Through three-step processes of starch cationization, sulfuration treatment, and graft copolymerization, the transparency, mechanical properties, and heat resistance of polylactic acid are improved. By cationization and sulfuration treatment, the compatibility between starch and polylactic acid is improved, reducing the interfacial tension. However, the improvement effect of its heat resistance is limited, and further optimization is required if it is used for heat-resistant food packaging.

[0005] The polylactic acid for food packaging prepared by the prior art has the following technical defects: low heat resistance, poor barrier property, insufficient mechanical properties, etc. Summary of the Invention

[0006] Combining the deficiencies of the prior art, the present invention aims to provide a preparation method of polylactic acid for high-heat-resistant food packaging to achieve the following invention objectives: producing polylactic acid with high heat resistance and good mechanical properties.

[0007] To achieve the above invention objectives, the present invention provides the following technical solutions: A preparation method of polylactic acid for high-heat-resistant food packaging, comprising the following steps: Step 1, synthesize a hydroxyl-terminated prepolymer Add L-lactic acid and zinc lactate into the first reaction kettle, introduce nitrogen for protection, heat up to 130 - 150 °C, and stir at a rate of 200 - 500 rpm; turn on the vacuum, keep the vacuum degree at -0.08 MPa to -0.095 MPa, and react for 1 - 2 h. Heat up to 160 - 170 °C, dropwise add ethylene glycol over 30 - 60 min. After the dropping is completed, continue to react for 3 - 4 h to obtain a hydroxyl-terminated prepolymer.

[0008] The feeding mass ratio of the L-lactic acid, zinc lactate and ethylene glycol is 100:(0.3 - 0.6):(5 - 7).

[0009] Step 2: Chain-extend the hydroxyl-terminated prepolymer Add cyclopentyl methyl ether and the hydroxyl-terminated prepolymer into the second reaction kettle, heat up to 80 - 100 °C, and stir at a rate of 250 - 350 rpm for 20 - 40 min; successively add 1,4-butanediol diglycidyl ether, glycidyl stearate and dibutyltin dilaurate, introduce nitrogen for protection, and heat up to 120 - 140 °C. Turn on the vacuum, keep the vacuum degree at -0.08 MPa to -0.095 MPa, and react for 3 - 5 h; obtain the chain-extended hydroxyl-terminated prepolymer.

[0010] The feeding mass ratio of the cyclopentyl methyl ether, hydroxyl-terminated prepolymer, 1,4-butanediol diglycidyl ether, glycidyl stearate and dibutyltin dilaurate is (100 - 150):100:(20 - 40):(5 - 10):(0.2 - 0.5).

[0011] Step 3: Prepare hydroxylated nano-silica Add ethanol, ammonia water and tetraethyl orthosilicate into the third reaction kettle, stir at a rate of 500 - 800 rpm, control the temperature at 25 - 40 °C, and react for 4 - 6 h; centrifuge to obtain the solid phase and wash it; disperse the washed solid phase in hydrochloric acid solution, heat up to 40 - 60 °C, and perform ultrasonic treatment at a power of 200 - 400 W for 20 - 40 min, centrifuge and retain the solid phase. The solid phase is washed and dried under vacuum to obtain hydroxylated nano-silica.

[0012] The mass fraction of the ethanol is 70 - 75%; The mass fraction of the ammonia water is 25 - 28%; The amount of substance of the hydrochloric acid solution is 0.05 - 0.2 mol / L; The feeding mass ratio of the ethanol, ammonia water, tetraethyl orthosilicate and hydrochloric acid solution is 100:(5 - 10):(2 - 5):(80 - 120).

[0013] Step 4: Dispersion and melting Hydroxylated nano-silica is added to the chain-extended hydroxyl-terminated prepolymer, and the temperature is raised to 75 - 85 °C, and stirring is carried out at a rate of 700 - 900 rpm for 3 - 5 h to obtain a prepolymer-nano-phase mixed material; the prepolymer-nano-phase mixed material is added to a twin-screw extruder, and the front-section temperature is set at 150 - 160 °C; the middle-section temperature is set at 170 - 180 °C; the rear-section temperature is set at 160 - 170 °C; the screw rotation speed is set at 200 - 300 rpm; high heat-resistant polylactic acid for food packaging is obtained.

[0014] The feeding mass ratio of the hydroxylated nano-silica to the chain-extended hydroxyl-terminated prepolymer is (1 - 3):100.

[0015] Compared with the prior art, the beneficial effects of the high heat-resistant polylactic acid prepared by the present invention are as follows: 1. The polylactic acid prepared by the present invention has excellent mechanical properties, with a tensile fracture stress of 57 - 71 MPa and a notched Izod impact strength of 8.0 - 11.2 kJ / m²; 2. The polylactic acid prepared by the present invention has good heat resistance, with a heat distortion temperature of 117 - 125 °C and a glass transition temperature of 66 - 71 °C; 3. The relative biodegradation rate of the polylactic acid prepared by the present invention is not less than 90%. Detailed implementation manners

[0016] Example 1 A preparation method of high heat-resistant polylactic acid for food packaging Step 1. Synthesize a hydroxyl-terminated prepolymer L-lactic acid and zinc lactate are added to the first reaction kettle, protected by introducing nitrogen, the temperature is raised to 130 °C, and stirring is carried out at a rate of 500 rpm; vacuum is turned on, and the vacuum degree is maintained at -0.08 MPa to -0.095 MPa, and the reaction is carried out for 2 h. The temperature is raised to 165 °C, and ethylene glycol is added dropwise for 30 min. After the addition is completed, the reaction is continued for 3 h to obtain a hydroxyl-terminated prepolymer.

[0017] The L-lactic acid is food-grade lactic acid; The feeding mass ratio of the L-lactic acid, zinc lactate and ethylene glycol is 100:0.5:7.

[0018] Step 2. Chain-extend the hydroxyl-terminated prepolymer Cyclopentyl methyl ether and the hydroxyl-terminated prepolymer are added to the second reaction kettle, the temperature is raised to 100 °C, and stirring is carried out at a rate of 350 rpm for 20 min; 1,4-butanediol diglycidyl ether, glycidyl stearate and dibutyltin dilaurate are added in sequence, protected by introducing nitrogen, and the temperature is raised to 130 °C. Vacuum is turned on, and the vacuum degree is maintained at -0.08 MPa to -0.095 MPa, and the reaction is carried out for 4 h; a chain-extended hydroxyl-terminated prepolymer is obtained.

[0019] The feeding mass ratio of the cyclopentyl methyl ether, the hydroxyl-terminated prepolymer, 1,4-butanediol diglycidyl ether, glycidyl stearate and dibutyltin dilaurate is 100:100:30:7:0.4.

[0020] Step 3: Prepare hydroxylated nano-silica Add ethanol with a mass fraction of 75%, ammonia water with a mass fraction of 25% and tetraethyl orthosilicate into the third reaction kettle, stir at a rate of 800 rpm, control the temperature at 40 °C, and react for 5 h; centrifuge to obtain the solid phase and wash it; disperse the washed solid phase in a hydrochloric acid solution, heat up to 60 °C, and perform ultrasonic treatment at a power of 400 W for 20 min, centrifuge and retain the solid phase, and the solid phase is washed and vacuum dried to obtain hydroxylated nano-silica.

[0021] The amount of substance of the hydrochloric acid solution is 0.05 mol / L; The feeding mass ratio of the ethanol, ammonia water, tetraethyl orthosilicate and hydrochloric acid solution is 100:5:3:110.

[0022] Step 4: Dispersion and melting Add hydroxylated nano-silica to the chain-extended hydroxyl-terminated prepolymer, heat up to 80 °C, stir at a rate of 900 rpm, and react for 3 h to obtain a prepolymer-nano-phase mixed material; add the prepolymer-nano-phase mixed material to a twin-screw extruder, set the front section temperature: 150 °C; middle section temperature: 180 °C; rear section temperature: 170 °C; set the screw speed: 200 rpm; to obtain polylactic acid for high heat-resistant food packaging.

[0023] The feeding mass ratio of the hydroxylated nano-silica and the chain-extended hydroxyl-terminated prepolymer is 1:100.

[0024] Example 2 A preparation method of polylactic acid for high heat-resistant food packaging Step 1: Synthesize a hydroxyl-terminated prepolymer Add L-lactic acid and zinc lactate into the first reaction kettle, introduce nitrogen for protection, heat up to 140 °C, and stir at a rate of 400 rpm; turn on the vacuum, and keep the vacuum degree at -0.08 MPa to -0.095 MPa, and react for 1.5 h. Heat up to 170 °C, dropwise add ethylene glycol, and the dropping time is 60 min. After the dropping is completed, continue to react for 4 h to obtain a hydroxyl-terminated prepolymer.

[0025] The L-lactic acid is food-grade lactic acid; The feeding mass ratio of the L-lactic acid, zinc lactate and ethylene glycol is 100:0.3:5.

[0026] Step 2: Chain-extend the hydroxyl-terminated prepolymer Add cyclopentyl methyl ether and hydroxyl-terminated prepolymer into the second reaction kettle, heat up to 90 °C, stir at a rate of 300 rpm for 30 min; successively add 1,4-butanediol diglycidyl ether, glycidyl stearate and dibutyltin dilaurate, introduce nitrogen for protection, and heat up to 120 °C. Turn on the vacuum, keep the vacuum degree at -0.08 MPa to -0.095 MPa, and react for 5 h; obtain chain-extended hydroxyl-terminated prepolymer.

[0027] The feeding mass ratio of the cyclopentyl methyl ether, hydroxyl-terminated prepolymer, 1,4-butanediol diglycidyl ether, glycidyl stearate and dibutyltin dilaurate is 150:100:20:10:0.5.

[0028] Step 3: Prepare hydroxylated nano-silica Add ethanol with a mass fraction of 75%, ammonia water with a mass fraction of 28% and tetraethyl orthosilicate into the third reaction kettle, stir at a rate of 500 rpm, control the temperature at 32 °C, and react for 6 h; centrifuge to obtain the solid phase and wash it; disperse the washed solid phase in hydrochloric acid solution, heat up to 40 °C, and perform ultrasonic treatment at a power of 250 W for 40 min, centrifuge and retain the solid phase, and the solid phase is washed and vacuum dried to obtain hydroxylated nano-silica.

[0029] The amount of substance of the hydrochloric acid solution is 0.2 mol / L; The feeding mass ratio of the ethanol, ammonia water, tetraethyl orthosilicate and hydrochloric acid solution is 100:10:2:80.

[0030] Step 4: Dispersion and melting Add hydroxylated nano-silica into the chain-extended hydroxyl-terminated prepolymer, heat up to 85 °C, stir at a rate of 700 rpm, and react for 3.5 h to obtain a prepolymer-nano-phase mixed material; add the prepolymer-nano-phase mixed material into a twin-screw extruder, set the front section temperature: 160 °C; middle section temperature: 170 °C; rear section temperature: 160 °C; set the screw speed: 250 rpm; obtain polylactic acid for high heat-resistant food packaging.

[0031] The feeding mass ratio of the hydroxylated nano-silica and the chain-extended hydroxyl-terminated prepolymer is 3:100.

[0032] Example 3 A preparation method of polylactic acid for high heat-resistant food packaging Step 1: Synthesize hydroxyl-terminated prepolymer Add L-lactic acid and zinc lactate into the first reactor, introduce nitrogen for protection, heat up to 150 °C, and stir at a rate of 200 rpm; turn on the vacuum, keep the vacuum degree at -0.08 MPa to -0.095 MPa, and react for 1 h. Heat up to 160 °C, dropwise add ethylene glycol over 40 min. After the dropwise addition is completed, continue to react for 4 h to obtain a hydroxyl-terminated prepolymer.

[0033] The L-lactic acid is food-grade lactic acid; The feeding mass ratio of the L-lactic acid, zinc lactate and ethylene glycol is 100:0.6:6.

[0034] Step 2, chain-extend the hydroxyl-terminated prepolymer Add cyclopentyl methyl ether and the hydroxyl-terminated prepolymer into the second reactor, heat up to 80 °C, and stir at a rate of 250 rpm for 40 min; successively add 1,4-butanediol diglycidyl ether, glycidyl stearate and dibutyltin dilaurate, introduce nitrogen for protection, and heat up to 140 °C. Turn on the vacuum, keep the vacuum degree at -0.08 MPa to -0.095 MPa, and react for 3 h; obtain a chain-extended hydroxyl-terminated prepolymer.

[0035] The feeding mass ratio of the cyclopentyl methyl ether, hydroxyl-terminated prepolymer, 1,4-butanediol diglycidyl ether, glycidyl stearate and dibutyltin dilaurate is 130:100:40:5:0.2.

[0036] Step 3, prepare hydroxylated nano-silica Add ethanol with a mass fraction of 70%, ammonia water with a mass fraction of 28% and tetraethyl orthosilicate into the third reactor, stir at a rate of 700 rpm, control the temperature at 25 °C, and react for 4 h; centrifuge to obtain the solid phase and wash it; disperse the washed solid phase in a hydrochloric acid solution, heat up to 50 °C, and perform ultrasonic treatment at a power of 200 W for 35 min, centrifuge and retain the solid phase. The solid phase is washed and dried under vacuum to obtain hydroxylated nano-silica.

[0037] The amount of substance of the hydrochloric acid solution is 0.1 mol / L; The feeding mass ratio of the ethanol, ammonia water, tetraethyl orthosilicate and hydrochloric acid solution is 100:7:5:120.

[0038] Step 4, dispersion and melting Hydroxylated nano-silica is added to the chain-extended hydroxyl-terminated prepolymer, and the temperature is raised to 75 °C, and stirring is carried out at a rate of 800 rpm for 5 h to obtain a prepolymer-nano-phase mixed material; the prepolymer-nano-phase mixed material is added to a twin-screw extruder, and the front-section temperature is set at 155 °C; the middle-section temperature is 175 °C; the rear-section temperature is 165 °C; the screw rotation speed is set at 300 rpm; poly(lactic acid) for high heat-resistant food packaging is obtained.

[0039] The feeding mass ratio of the hydroxylated nano-silica to the chain-extended hydroxyl-terminated prepolymer is 2.5:100.

[0040] Comparative Example 1 A preparation method of poly(lactic acid) for food packaging Step 1, synthesize prepolymer L-lactic acid and zinc lactate are added to the first reaction kettle, protected by introducing nitrogen, the temperature is raised to 150 °C, and stirring is carried out at a rate of 200 rpm; vacuum is turned on, and the vacuum degree is maintained at -0.08 MPa to -0.095 MPa, and the reaction is carried out for 1 h. The temperature is raised to 160 °C and the reaction is continued for 4 h to obtain a prepolymer.

[0041] The feeding mass ratio of the L-lactic acid to the zinc lactate is 100:0.6.

[0042] Step 2, chain-extended hydroxyl-terminated prepolymer Cyclopentyl methyl ether and hydroxyl-terminated prepolymer are added to the second reaction kettle, the temperature is raised to 80 °C, and stirring is carried out at a rate of 250 rpm for 40 min; 1,4-butanediol diglycidyl ether and dibutyltin dilaurate are added in sequence, protected by introducing nitrogen, the temperature is raised to 140 °C, vacuum is turned on, and the vacuum degree is maintained at -0.08 MPa to -0.095 MPa, and the reaction is carried out for 3 h; a chain-extended hydroxyl-terminated prepolymer is obtained.

[0043] The feeding mass ratio of the cyclopentyl methyl ether, the hydroxyl-terminated prepolymer, 1,4-butanediol diglycidyl ether and dibutyltin dilaurate is 130:100:40:0.2.

[0044] Step 3, melt extrusion The chain-extended hydroxyl-terminated prepolymer is added to a twin-screw extruder, the front-section temperature is set at 150 °C; the middle-section temperature is 180 °C; the rear-section temperature is 170 °C; the screw rotation speed is set at 200 rpm; poly(lactic acid) for food packaging is obtained.

[0045] Example 4 A forming and post-curing method of poly(lactic acid) Step 1, forming Poly(lactic acid) is placed in a mold, the temperature is set at 160 °C, the pressure is 8 MPa, hot pressing is carried out for 10 min, and it is slowly cooled to room temperature to obtain a sample.

[0046] Step 2: Post-curing Place the sample in an oven, set the temperature to 80 °C, anneal for 6 h, increase the temperature at a rate of 1 °C / min, and maintain at 100 °C for 2 h after reaching 100 °C.

[0047] Performance testing: The polylactic acid prepared in Examples 1-3 and Comparative Example 1 was molded and post-cured according to the method of Example 4. The heat distortion temperature was tested according to "GB / T 1634.2-2019 Plastics - Determination of heat distortion temperature - Part 2: Plastics and hard rubbers", the glass transition temperature was tested according to "GB / T 19466.2-2004 Plastics - Differential scanning calorimetry (DSC) - Part 2: Determination of the glass transition temperature", the tensile fracture stress was tested according to "GB / T 1040.2-2022 Plastics - Determination of tensile properties - Part 2: Test conditions for moulded and extruded plastics", the notched Izod impact strength was tested according to "GB / T 1043.1-2008 Plastics - Determination of the notched Izod impact strength - Part 1: Non-instrumented impact test", and the relative biodegradation rate was tested according to "GB / T 19277.1-2011 Determination of the ultimate aerobic biodegradability of materials under controlled composting conditions - Method by measuring the evolved carbon dioxide - Part 1: General method"; the test results are shown in Table 1.

[0048] Table 1 Performance test results of polylactic acid

[0049] As can be seen from Table 1, the heat distortion temperature of the high heat-resistant polylactic acid prepared in Examples 1-3 of the present invention reaches 117-125 °C, the glass transition temperature reaches 66-71 °C, the tensile fracture stress reaches 57-71 MPa, the notched Izod impact strength reaches 8.0-11.2 kJ / m², and the relative biodegradation rate is not less than 90%. Compared with Comparative Example 1, while the heat resistance is significantly optimized, the mechanical properties are also greatly improved.

[0050] It should be noted that the above examples are only relatively representative implementation schemes of the present invention, and do not limit the scope of patent protection. Any technical personnel in the relevant field who make technical scheme adjustments or equivalent replacements within the framework of the claims of the present invention shall be deemed to fall within the scope of patent protection of the present invention.

Claims

1. A preparation method of polylactic acid for high heat-resistant food packaging, characterized in that: It includes steps of synthesizing a hydroxyl-terminated prepolymer, chain-extending the hydroxyl-terminated prepolymer, preparing hydroxylated nano-silica, dispersing and melting; The synthesizing of the hydroxyl-terminated prepolymer: Synthesize the hydroxyl-terminated prepolymer from L-lactic acid, zinc lactate and ethylene glycol; The chain-extending of the hydroxyl-terminated prepolymer: Carry out a chain-extending reaction on the hydroxyl-terminated prepolymer with 1,4-butanediol diglycidyl ether and stearic acid glycidyl ester; The preparing of the hydroxylated nano-silica: React ethanol, ammonia water and tetraethyl orthosilicate at 25 - 40 °C for 4 - 6 h; Centrifuge to obtain the solid phase, and then disperse it in hydrochloric acid solution, and carry out ultrasonic treatment at 40 - 60 °C for 20 - 40 min.

2. The preparation method of polylactic acid for high heat-resistant food packaging according to claim 1, characterized in that: The synthesizing of the hydroxyl-terminated prepolymer: Mix L-lactic acid and zinc lactate, introduce nitrogen for protection, and heat up to 130 - 150 °C; Turn on the vacuum and react for 1 - 2 h; Heat up to 160 - 170 °C, dropwise add ethylene glycol, and the dropping time is 30 - 60 min. After the dropping is completed, continue to react for 3 - 4 h to obtain the hydroxyl-terminated prepolymer.

3. A preparation method of polylactic acid for high heat-resistant food packaging according to claim 1, characterized in that: The chain-extending of the hydroxyl-terminated prepolymer: Mix cyclopentyl methyl ether and the hydroxyl-terminated prepolymer, heat up to 80 - 100 °C, stir at a rate of 250 - 350 rpm for 20 - 40 min; Sequentially add 1,4-butanediol diglycidyl ether, stearic acid glycidyl ester and dibutyltin dilaurate, introduce nitrogen for protection, and heat up to 120 - 140 °C; Turn on the vacuum and react for 3 - 5 h.

4. A preparation method of polylactic acid for high heat-resistant food packaging according to claim 1, characterized in that: The mass ratio of the L-lactic acid, zinc lactate and ethylene glycol is 100:(0.3 - 0.6):(5 - 7).

5. A preparation method of polylactic acid for high heat-resistant food packaging according to claim 1, characterized in that: The chain-extending reaction: The solvent is cyclopentyl methyl ether, and the catalyst is dibutyltin dilaurate; The mass ratio of the cyclopentyl methyl ether, the hydroxyl-terminated prepolymer, 1,4-butanediol diglycidyl ether, stearic acid glycidyl ester and dibutyltin dilaurate is (100 - 150):100:(20 - 40):(5 - 10):(0.2 - 0.5).

6. A preparation method of polylactic acid for high heat-resistant food packaging according to claim 1, characterized in that: The feeding mass ratio of the ethanol, ammonia water, tetraethyl orthosilicate and hydrochloric acid solution is 100:(5 - 10):(2 - 5):(80 - 120).

7. A preparation method of polylactic acid for high heat-resistant food packaging according to claim 1, characterized in that: The dispersing and melting: Add the hydroxylated nano-silica into the chain-extended hydroxyl-terminated prepolymer, heat up to 75 - 85 °C, and react for 3 - 5 h to obtain a prepolymer-nano-phase mixed material; Add the prepolymer-nano-phase mixed material into a twin-screw extruder for melt extrusion.

8. A preparation method of polylactic acid for high heat-resistant food packaging according to claim 7, characterized in that: The dispersing and melting: The feeding mass ratio of the hydroxylated nano-silica and the chain-extended hydroxyl-terminated prepolymer is (1 - 3):

100.

9. A preparation method of polylactic acid for high heat-resistant food packaging according to claim 7, characterized in that: The melt extrusion: Set the front section temperature of the twin-screw extruder: 150 - 160 °C; The middle section temperature: 170 - 180 °C; The rear section temperature: 160 - 170 °C; Set the screw speed: 200 - 300 rpm.

10. A preparation method of polylactic acid for high heat-resistant food packaging according to claim 1, characterized in that: The mass fraction of the ethanol is 70 - 75%; The mass fraction of the ammonia water is 25 - 28%; The amount of substance of the hydrochloric acid solution is 0.05 - 0.2 mol / L.

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