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CN-121975288-A - Lignin modified polylactic acid antibacterial degradable food packaging material

CN121975288ACN 121975288 ACN121975288 ACN 121975288ACN-121975288-A

Abstract

The invention discloses a lignin modified polylactic acid antibacterial degradable food packaging material which comprises, by weight, 60-85% of polylactic acid, 5-20% of modified lignin, 3-10% of a plasticizer, 2-8% of a compatilizer and 0.1-1% of an antioxidant, wherein 0.5-2% of a food-grade antibacterial agent can be optionally added. The modified lignin is prepared by adopting an alkali treatment-maleic anhydride grafting two-step method, the grafting rate is 3.2-5.8%, and the compatibility with polylactic acid is obviously improved. The finished product is prepared by high-speed mixing, sectional temperature-control melt blending and molding processing, the tensile strength is more than or equal to 35MPa, the elongation at break is more than or equal to 45%, the thermal decomposition temperature is more than or equal to 330 ℃, the oxygen permeability is less than or equal to 25cm < 3 >/(m <2 > < 24h </1 > and 0.1 MPa), the water vapor permeability is less than or equal to 10 g/(m <2 > < 24 h), the soil degradation rate is more than or equal to 60% in 60 days, the antibacterial rate of the antibacterial product to pathogenic bacteria is more than or equal to 90%, and the migration quantity accords with the food contact safety standard. The invention solves the defects of large brittleness, poor thermal stability and insufficient barrier property of polylactic acid, reduces the material cost by 10-20% by utilizing the lignin of agricultural and forestry wastes, can be widely applied to the fields of food fresh-keeping films, disposable tableware and the like, and has extremely high industrialization value.

Inventors

  • HU ZHIHAN

Assignees

  • 中国海洋大学

Dates

Publication Date
20260505
Application Date
20260319

Claims (9)

  1. 1. The lignin modified polylactic acid antibacterial degradable food packaging material is characterized by being prepared by synergetic compatibility of the following components in percentage by weight: 60-85% of polylactic acid, 5-20% of modified lignin, 3-10% of plasticizer, 2-8% of compatilizer and 0.1-1% of antioxidant, and optionally 0.5-2% of food-grade antibacterial agent; The modified lignin is lignin grafted with maleic anhydride after alkali treatment, the grafting rate is 3.2-5.8%, the compatibility with polylactic acid is improved through grafting modification, the rigidity and the barrier property of the material are enhanced, the weight average molecular weight of the polylactic acid is 8-12 ten thousand, the tacticity is not lower than 95%, and the basic mechanical property and the degradable property are provided for the material.
  2. 2. The lignin-modified polylactic acid antibacterial degradable food packaging material according to claim 1, wherein the food-grade antibacterial agent is selected from nano zinc oxide or epsilon-polylysine, and the particle size of the nano zinc oxide is 20-50 nanometers.
  3. 3. The lignin modified polylactic acid antibacterial degradable food packaging material according to claim 1 is characterized in that the preparation method of the modified lignin comprises the steps of crushing lignin to 50-100 microns in particle size, stirring and treating ten milliliters per gram of lignin with a sodium hydroxide solution with the mass fraction of 5-10% at 60-80 ℃ for 2-4 hours according to a liquid-solid ratio, filtering and washing to be neutral, drying at 105 ℃ for 8 hours, adding maleic anhydride with the mass of 10-20% of lignin and dicumyl peroxide with the mass of 0.5-1.5% of an initiator, reacting for 1-3 hours at 120-140 ℃ in a double-screw extruder, and rotating the screw of the extruder at 40-60 revolutions per minute.
  4. 4. The lignin modified polylactic acid antibacterial degradable food packaging material according to claim 1, wherein the plasticizer is one or more of tributyl citrate, polyethylene glycol 400 and glycerin, and the mass ratio of tributyl citrate to polyethylene glycol 400 is one to one during compounding.
  5. 5. The lignin modified polylactic acid antibacterial degradable food packaging material according to claim 1, wherein the compatilizer is maleic anhydride grafted polylactic acid or ethylene-vinyl acetate copolymer, the grafting rate of the maleic anhydride grafted polylactic acid is 1.2-2.0%, and the vinyl acetate content of the ethylene-vinyl acetate copolymer is 15-20%.
  6. 6. The lignin modified polylactic acid antibacterial degradable food packaging material according to claim 1, wherein the antioxidant is formed by compounding an antioxidant 1010 and an antioxidant 168 in a ratio of one to one by mass.
  7. 7. The lignin-modified polylactic acid antibacterial degradable food packaging material according to claim 1, wherein the material has a tensile strength of not less than 35 MPa, an elongation at break of not less than 45%, a thermal decomposition temperature of not more than 330 ℃, an oxygen transmission rate of not more than 25 cubic centimeters per square meter per twenty-four hours per 0.1 MPa, a water vapor transmission rate of not more than 10 grams per square meter per twenty-four hours, a 60-day soil degradation rate of not less than 60%, an antibacterial rate of not less than 90% for 24 hours against E.coli and Staphylococcus aureus after addition of a food-grade antibacterial agent, a heavy metal migration rate of not more than 0.01 mg per square meter, and an organic matter migration rate of not more than 5mg per square meter.
  8. 8. A method for preparing the lignin-modified polylactic acid degradable food packaging material according to any one of claims 1 to 7 comprising the steps of: (1) Drying the raw materials, namely vacuum drying polylactic acid, modified lignin and a compatilizer for 4-6 hours at the temperature of 80-100 ℃ and the vacuum degree of minus 0.08-minus 0.1 megapascal, and removing water to avoid hydrolysis and degradation of the polylactic acid; (2) Mixing ingredients, namely weighing the components according to weight percentage, firstly putting polylactic acid, modified lignin and compatilizer into a high-speed mixer, mixing for 5 to 8 minutes at 300 to 500 revolutions per minute, then adding plasticizer, antioxidant and optional antibacterial agent, and continuing mixing for 5 to 7 minutes to ensure that the components are uniformly dispersed; (3) Adding the mixture into a double-screw extruder, wherein the temperature of each section of the extruder is 140-150 ℃ in a feeding section, 160-170 ℃ in a compression section, 170-180 ℃ in a homogenizing section and 165-175 ℃ in a machine head, the rotating speed of the screw is 30-60 revolutions per minute, extruding and granulating, and the temperature is controlled in a sectional manner to ensure that the components are fully melted and not degraded; (4) The molding processing comprises the steps of adopting a film blowing or injection molding process to process, wherein the die head temperature is 160-170 ℃, the blowing ratio is 2.5-3.0, the traction speed is 5-8 m/min, the charging barrel temperature is 165-180 ℃, the die temperature is 30-40 ℃, the injection pressure is 80-100 megapascals, and the pressure maintaining pressure is 60-80 megapascals, and the product performance uniformity is ensured by precisely controlling the molding parameters.
  9. 9. Use of the lignin modified polylactic acid degradable food packaging material according to any one of claims 1-7 in food preservative films, disposable tableware or food packaging boxes, wherein the material adapts to the use requirements of different food packaging scenes by virtue of excellent mechanical properties, barrier properties, degradation properties and safety properties brought by component cooperation.

Description

Lignin modified polylactic acid antibacterial degradable food packaging material Technical Field The invention belongs to the technical field of high polymer materials, in particular relates to a degradable food packaging material, and especially relates to a lignin modified polylactic acid antibacterial degradable food packaging material which is suitable for food contact products such as food fresh-keeping, disposable tableware, packaging films and the like and covers packaging scenes of various foods such as fresh foods, cooked foods, snacks and the like. Background Polylactic acid (PLA) is a biodegradable polymer material derived from renewable resources such as corn starch, has good biocompatibility, no toxicity and smell, and complete degradation, and is an ideal material for replacing traditional petroleum-based plastics (such as polyethylene and polypropylene). But pure PLA has obvious defects of insufficient tensile strength (about 28-32 MPa), low elongation at break (only 8-12%), high brittleness, easy breakage, easy cracking in disposable tableware, easy tearing of packaging films, poor thermal stability, about 290-305 ℃ of thermal decomposition temperature, narrow processing window, easy degradation due to temperature fluctuation in the processing processes of extrusion, film blowing and the like, uneven product performance, high Oxygen Transmission Rate (OTR) of 45-55cm 3/(m2 -24 h 0.1MPa, about 15-20 g/(m 2 h) of Water Vapor Transmission Rate (WVTR), insufficient barrier performance, difficulty in meeting the fresh-keeping requirements of foods such as fresh foods, cooked foods and the like, and limitation of the application of the pure PLA in high-requirement food packaging scenes. Lignin is the main component of agricultural and forestry waste (straw, wood chip and bagasse), has annual output of over 5000 ten thousand tons, is widely available and low in cost (about 800-1200 yuan/ton), has good thermal stability and rigidity, and the benzene ring structure in the molecular structure can enhance the barrier property of the material. However, the surface hydroxyl groups of unmodified lignin are dense, the polarity is stronger, PLA is a weak polar polymer, the compatibility of the two is poor, the direct blending can lead to the agglomeration phenomenon inside the material, the mechanical property is reduced, and the tensile strength is even lower than that of pure PLA. In the prior art, lignin is added in a physical blending mode in part of the schemes, the compatibility problem is not solved, the tensile strength of the material is only 30-33MPa, the elongation at break is less than 20%, the thermal stability is not obviously improved, and few chemical modification schemes (such as silane modification and epoxy modification) improve the compatibility, but have the problems of complex process, harsh reaction conditions (high temperature and high pressure), high cost of a modifier (the price of the silane coupling agent is about 8000-12000 yuan/ton) and the like, so that industrialization is difficult to realize. Meanwhile, the existing modification scheme is characterized in that the single performance is improved, or only the mechanical performance is improved, or only the degradability is optimized, the mechanical property, the thermal stability, the blocking property and the food contact safety are not simultaneously considered, and the migration risk of the modifier in part of the scheme possibly exists, so that the modifier does not meet the standards of GB 4806.7-2016 plastic materials and products for food contact. In addition, the cost of the pure PLA material is high (about 18000-22000 yuan/ton), the popularization of the pure PLA material in the middle-low end food packaging market is limited, the cost can be effectively reduced by high-efficiency utilization of lignin, but a mature low-cost and high-performance modification system is not formed in the prior art, so that the application proportion of lignin in the PLA packaging material is generally lower than 10%, and the recycling utilization rate is insufficient. Disclosure of Invention Aiming at the problems of large brittleness, poor heat stability, insufficient barrier property, poor lignin modification compatibility, high industrialization cost, single performance and insufficient food contact safety of the existing PLA food packaging material, the lignin modified polylactic acid degradable food packaging material which has reasonable component design, simple preparation process and excellent comprehensive performance and meets the food contact safety standard is provided, and the synergistic promotion of mechanical, heat stability, barrier and degradation performance is realized by optimizing the modification process and the component proportion, meanwhile, the lignin addition proportion is improved, the material cost is reduced, the recycling utilization of agricultural and forestry wastes is realized, and the industrialized