Preparation method of polyvinyl biodegradable composite mulching film

Abstract

The invention belongs to the technical field of preparation of polymeric membrane materials, and particularly relates to a preparation method of a polyvinyl biodegradable composite mulching film. The preparation method comprises the following steps: adding a zinc source, an organic ligand, a photosensitizer and a heat sensitizer into an organic system, uniformly mixing, and reacting to obtain a ZIF-8 compound; the method comprises the following steps: compounding a monomer, a cross-linking agent and an initiator to obtain a pre-polymerized solution, then adding a ZIF-8 compound, uniformly mixing, and reacting to obtain a degradation aid; pBAT, PLA, a degradation auxiliary agent and the like are dried, melted and granulated, and degradation layer master batches are obtained; drying, melting and granulating polyethylene, a gas barrier agent and the like to obtain a supporting layer master batch; and carrying out co-extrusion film blowing on the degradation layer master batch and the support layer master batch to obtain the polyvinyl biodegradable composite mulching film. The polyvinyl biodegradable composite mulching film prepared by the invention still has a relatively stable degradation rate in a high-temperature and high-ultraviolet-intensity environment, and also has good degradability and tensile property.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparation of polymer membrane materials, and particularly relates to a method for preparing a polyvinyl biodegradable composite ground film. Background Art

[0002] A ground film is an agricultural film material with functions of heat preservation, wind resistance, moisture retention and weed suppression. In the existing ground film technology, biodegradable ground films are made of biodegradable materials such as polybutylene terephthalate adipate and polylactic acid. Although they are environmentally friendly, they have problems of poor mechanical properties, high cost and insufficient moisture retention effect, and it is difficult to meet the requirements of the strong wind environment in the northwest region; traditional polyethylene ground films have qualified strength but are difficult to degrade. By improving the traditional polyethylene ground film, an oxidation-biodegradable ground film is prepared by adding a photo / thermal oxidation assistant to a polyethylene substrate, so that the ground film is first oxidized and cracked into low molecular fragments and then degraded by microorganisms. Compared with polyethylene ground films, it is more environmentally friendly, and compared with fully biodegradable ground films, it has lower cost, higher tensile strength and stronger moisture retention effect, which better meets the requirements of technical applications in the agricultural field. On the other hand, existing oxidation-biodegradable ground films still have the problem of uncontrollable degradation. This is because the light and temperature factors are different in different natural environments, resulting in premature activation of the degradation assistant in the oxidation-biodegradable ground film in an environment with long light exposure time and high temperature, and local embrittlement occurs, leading to the failure of moisture retention. Technical personnel hope to combine the degradability of biodegradable ground films with the tensile properties of polyethylene ground films; at the same time, the prepared composite film has a relatively stable degradation rate in an environment with high temperature and high ultraviolet intensity.

[0003] Chinese Patent CN112852059A discloses a biodegradant, which includes 2% - 6% of manganese stearate, 0.4% - 1.2% of iron stearate, 5% - 10% of copper stearate, 0.04% - 0.4% of silane coupling agent, 90% - 70% of low-density polyethylene or polypropylene, and 0.06% - 0.5% of a fertilizer.

[0004] The mechanism by which manganese stearate, iron stearate and copper stearate in this patent promote the degradation of polyethylene is that after metal ions absorb ultraviolet energy, they become excited states, and then initiate the photo-oxidation reaction of plastic polymer chains. Such photo-degradation assistants are prone to premature release due to high ultraviolet intensity, resulting in premature disintegration of the ground film. It is difficult to obtain an ideal degradation rate of the ground film by simply using stearate-based degradation assistants.

[0005] Chinese Patent CN108329571A discloses a biodegradable polyethylene mulch film composition. Calculated by weight parts of raw materials, the composition includes 40 - 50 parts of low-density polyethylene, 50 - 60 parts of linear low-density polyethylene, and 10 - 20 parts of functional masterbatch; the functional masterbatch includes 50 - 60 parts of poly(butylene adipate terephthalate), 40 - 50 parts of calcium carbonate, 5 - 10 parts of metallocene polyethylene, 0.05 - 0.01 parts of fluorine elastomer, 4 - 7 parts of coupling agent, 2 - 4 parts of dispersant, and 8 - 10 parts of solvent.

[0006] Since polyethylene is difficult to degrade in the natural environment, generally, photo-degradation aids, thermal degradation aids, or oxidation degradation aids for promoting degradation need to be added. However, this patent does not involve measures for promoting the degradation of polyethylene, and it is difficult to ensure the degradation performance of the mulch film made from the composition. The mulch film is prone to microplastic residues in the soil; further, it also does not involve measures for controlling the degradation rate of the mulch film. Summary of the Invention

[0007] The purpose of the present invention is to provide a preparation method of a polyethylene-based biodegradable composite mulch film, so that the polyethylene-based biodegradable composite mulch film still has a relatively stable degradation rate in an environment with high temperature and high ultraviolet intensity, and at the same time has good degradability and tensile properties.

[0008] To achieve the above purpose, the technical solution adopted by the present invention is: The preparation method of the polyethylene-based biodegradable composite mulch film of the present invention includes the following steps: (1) Adding a zinc source, an organic ligand, a photosensitizer, and a thermosensitizer to an organic system, mixing evenly and then carrying out a coordination reaction to obtain a ZIF-8 composite; (2) Compounding a monomer, a cross-linking agent, and an initiator to obtain a prepolymer solution, adding the ZIF-8 composite to the prepolymer solution, mixing evenly and then carrying out a free radical polymerization reaction to obtain a degradation aid; (3) Drying and melt granulating poly(butylene adipate terephthalate), polylactic acid, the degradation aid, a toughening agent, a lubricant, an anti-fogging agent, and a heat stabilizer to obtain a degradation layer masterbatch; (4) Drying and melt granulating polyethylene, a gas barrier agent, a heat stabilizer, an antioxidant, and a compatibilizer to obtain a support layer masterbatch; (5) Co-extrusion blowing the degradation layer masterbatch and the support layer masterbatch to obtain a polyethylene-based biodegradable composite mulch film with a sandwich structure having a support layer in the middle and degradation layers on both sides.

[0009] Wherein: In the said step (1), the zinc source is zinc nitrate hexahydrate, the organic ligand is 2-methylimidazole, the photosensitizer is iron stearate and tetraphenylporphyrin, and the thermal sensitizer is lauroyl peroxide; the molar ratio of the zinc source, the organic ligand, the photosensitizer and the thermal sensitizer is 1:(2-2.5):(0.25-0.35):(0.05-0.1).

[0010] In the said step (1), the organic system is prepared by mixing one of triethanolamine or 4-dimethylaminopyridine with methanol at a molar ratio of 1:(450-800), and the molar ratio of one of triethanolamine or 4-dimethylaminopyridine to the zinc source is 1:(20-40).

[0011] In the said step (1), ultrasonically disperse for 15-35 min until evenly mixed, the coordination reaction temperature is 30-35 °C, and the coordination reaction time is 40-60 min.

[0012] In the said step (2), the monomer is N-isopropylacrylamide, the crosslinking agent is N,N'-methylenebisacrylamide, and the initiator is ammonium persulfate; the molar ratio of the monomer, the crosslinking agent and the initiator is 1:(0.03-0.06):(0.002-0.008), and the molar ratio of the monomer to the zinc source is 5:(0.8-1.2).

[0013] In the said step (2), ultrasonically disperse for 25-35 min until evenly mixed, the free radical polymerization reaction temperature is 75-90 °C, and the free radical polymerization reaction time is 1.5-2.5 h.

[0014] In the said step (3), the toughening agent is one of graphene oxide or zinc oxide nanowhiskers, the lubricant is erucamide, the antifogging agent is glycerol monostearate, and the heat stabilizer is montmorillonite; based on the total mass of the degradation masterbatch being 100 wt%, the degradation masterbatch includes 67-70 wt% of poly(butylene adipate terephthalate), 10-15 wt% of polylactic acid, 7.5-16 wt% of degradation aids, 1.8-2.5 wt% of toughening agent, 1.7-2.0 wt% of lubricant, 0.8-1.2 wt% of antifogging agent and 2-2.5 wt% of heat stabilizer; the drying pressure is 0.01-0.03 MPa, the drying temperature is 55-65 °C, and the drying time is 50-60 min; the melt granulation temperature is 155-165 °C.

[0015] In the step (4), the gas barrier agent is nano-silica, the heat stabilizer is montmorillonite, the antioxidant is tris(2,4-di-tert-butylphenyl) phosphite and pentaerythritol tetra[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate], and the compatibilizer is maleic anhydride grafted polyethylene; based on the total mass of the support layer masterbatch being 100 wt%, the support layer masterbatch includes 95-97 wt% of polyethylene, 1-1.5 wt% of the gas barrier agent, 1-1.5 wt% of the heat stabilizer, 0.5-1 wt% of tris(2,4-di-tert-butylphenyl) phosphite, 0.3-0.6 wt% of pentaerythritol tetra[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate], and 0.2-0.4 wt% of the compatibilizer; the drying pressure is 0.01-0.03 MPa, the drying temperature is 55-65 °C, and the drying time is 50-60 min; the melt granulation temperature is 170-180 °C.

[0016] In the step (5), coextrusion blown film means preparing a polyethylene-based biodegradable composite mulch film using a coextrusion blown film unit. The coextrusion blown film unit includes three extruders A, B, and C and a die head. The extruders A, B, and C are divided into three functional zones: a feeding section, a compression section, and a metering section. The screw length-diameter ratio of extruders A and C is 32:1, and the screw length-diameter ratio of extruder B is 28:1; the degradation layer masterbatch is put into extruders A and C, and the support layer masterbatch is put into extruder B. The extrusion temperatures of extruders A and C are: 155-165 °C in the feeding section, 165-170 °C in the compression section, and 170-175 °C in the metering section; the extrusion temperature of extruder B is: 175-180 °C in the feeding section, 185-190 °C in the compression section, and 190-195 °C in the metering section; the screw speeds of the three extruders A, B, and C are 35-45 rpm, the melt pressure is 10-12 MPa, the traction speed is 8-10 m / min, and the blow-up ratio is 2.5:1.

[0017] In the step (5), extruders A and C are extruded and formed to obtain the degradation layer A and the degradation layer B respectively, and extruder B is extruded and formed to obtain the support layer; the degradation layer A, the degradation layer B, and the support layer converge in the die head. The die head blown film temperature is 180-185 °C, and the die head is extruded and formed to obtain a polyethylene-based biodegradable composite mulch film. The thickness of the polyethylene-based biodegradable composite mulch film is 0.008-0.012 mm, and the film structure of the polyethylene-based biodegradable composite mulch film is a sandwich structure of degradation layer A - support layer - degradation layer B. The thickness ratio of the degradation layer A, the support layer, and the degradation layer B is 1:1:1; the cooling temperature at the die head outlet is 15-20 °C.

[0018] In the present invention, maleic anhydride grafted polyethylene is used as a compatibilizer. The maleic anhydride groups (-CO-O-CO-) in its molecules form hydrogen bonds or transesterification reactions with the ester groups or hydroxyl groups in poly(butylene adipate terephthalate) and polylactic acid. At the same time, its non-polar polyethylene chain segments are compatible with the polyethylene substrate, forming a bridging effect and increasing the interfacial compatibility between poly(butylene adipate terephthalate), polylactic acid and polyethylene.

[0019] The beneficial effects of the present invention are as follows: (1) In the present invention, ZIF-8 is prepared by reacting zinc nitrate hexahydrate with 2-methylimidazole in an organic system. The organic system includes one of triethanolamine or 4-dimethylaminopyridine and methanol, enabling the reaction to proceed at a lower temperature, reducing the reversible dissociation phenomenon of the photo / thermosensitive agent and reducing the loss of the photo / thermosensitive agent.

[0020] In addition, in the present invention, the photosensitizer and the thermosensitizer are embedded in the ZIF-8 nanopores through π-π interaction or coordination adsorption. The confinement effect of the nanopores can inhibit the thermal vibration of the photosensitizer and the thermosensitizer, thereby inhibiting the decomposition of the photosensitizer and the thermosensitizer caused by subsequent high-temperature processing. In addition, ZIF-8 also fixes the photosensitizer and the thermosensitizer through π-π interaction or coordination adsorption, providing a buffering effect against high temperature and high ultraviolet intensity in the external environment, avoiding the problem that the degradation additives in traditional oxidation-biodegradable mulch films are prematurely released due to high temperature and high ultraviolet intensity, resulting in premature disintegration of the mulch film. In addition, ZIF-8 has a problem of accelerated decomposition in acidic soil. In the present invention, tetraphenylporphyrin and iron stearate are not only photosensitizers but also have the effect of improving the acid resistance of ZIF-8: tetraphenylporphyrin can also be embedded in the nanopores of ZIF-8 through π-π stacking. Its highly symmetric and rigid molecular structure can form a stable physical barrier to reduce the penetration of acidic protons in the soil; at the same time, its macrocyclic structure can enhance the mechanical strength of ZIF-8 and inhibit the framework collapse of ZIF-8 under acidic conditions. The iron stearate molecule is formed by the coordination bond between iron ions and the carboxylate group of long-chain stearic acid. Its Fe 2+ / Fe 3+ ion pair can quench the excessive oxygen free radicals in acidic soil, avoiding the attack of reactive oxygen free radicals on the organic ligands of ZIF-8, thereby protecting the integrity of the ZIF-8 framework.

[0021] (2) The present invention uses monomer N-isopropylacrylamide, crosslinking agent N,N'-methylenebisacrylamide, and initiator ammonium persulfate to react to prepare the thermosensitive polymer poly(N-isopropylacrylamide) (PNIPAM). As a thermosensitive polymer hydrogel, PNIPAM has a relatively narrow lower critical solution temperature (LCST, around 33 - 36 °C), and near the lower critical solution temperature, it will cause the release rate of the photo / thermosensitizer to be unstable and fluctuate greatly. The present invention combines the PNIPAM with the ZIF-8 composite and utilizes the synergistic effect of the two.

[0022] The molecular chain of PNIPAM is composed of hydrophilic amide groups and hydrophobic isopropyl units, forming a reversible conformational change between hydrophilic and hydrophobic. Below the lower critical solution temperature, the amide groups form strong hydrogen bonds with water molecules in the air, and the PNIPAM molecular chains are fully hydrated and extended, showing hydrophilicity. At this time, the ZIF-8 structure is fully exposed, enabling ZIF-8 to maintain a certain release rate of the photo / thermosensitizer even in environments with low temperature, short light exposure time, and large day-night temperature difference, reducing the residue of the plastic film fragments in the soil. While above the lower critical solution temperature, the thermal energy destroys the hydrogen bonds, and PNIPAM as a whole shows hydrophobicity. The PNIPAM molecular chains dehydrate and fold, and then contract into a hydrophobic aggregated state, reducing the exposure degree of ZIF-8, so that the plastic film will not disintegrate prematurely in an environment with long-term high temperature and high ultraviolet intensity. Detailed implementation mode

[0023] The present invention will be specifically described and illustrated below in conjunction with the embodiments.

[0024] The raw materials in the examples and comparative examples are all commercially available products. Sources of some raw materials: tris(2,4-di-tert-butylphenyl) phosphite, also known as antioxidant 168, provided by BASF; pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate], also known as antioxidant 1010, provided by BASF; polyethylene refers to linear low-density polyethylene, provided by Qilu Petrochemical, grade 7042; maleic anhydride grafted polyethylene, provided by Exxon, grade PE 1040.

[0025] Example 1 Preparation of degradation aid Prepare an organic system by mixing triethanolamine and methanol at a molar ratio of 0.1:50 for standby; at room temperature, add 10 mol of zinc nitrate hexahydrate and 22 mol of 2-methylimidazole to 200 mol of the organic system, then add 2 mol of iron stearate, 1 mol of tetraphenylporphyrin, and 0.8 mol of lauroyl peroxide, and ultrasonically disperse for 20 min; start stirring, heat up to 35 °C, and carry out a coordination reaction for 40 min; after the reaction is completed, naturally cool to room temperature, centrifuge to obtain the solid phase, wash with flowing methanol for 20 min, and obtain the ZIF-8 composite after vacuum drying.

[0026] Dissolve 50 mol of N-isopropylacrylamide, 2.5 mol of N,N'-methylenebisacrylamide, and 0.1 mol of ammonium persulfate in deionized water to form a prepolymer solution; add the ZIF-8 composite to the prepolymer solution and disperse it ultrasonically for 25 min. Start stirring, and under a nitrogen atmosphere, heat up to 75 °C for free radical polymerization reaction. After reacting for 2.5 h, cool to room temperature, centrifuge to separate the solid phase, and wash it with flowing deionized water for 20 min to obtain a degradation aid.

[0027] Prepare a polyvinyl biodegradable composite mulch film Based on the total mass of the degradation layer masterbatch being 100 wt%, mix 70 wt% of poly(butylene adipate terephthalate), 15 wt% of polylactic acid, 7.5 wt% of the degradation aid, 2.5 wt% of graphene oxide, 1.7 wt% of erucamide, 0.8 wt% of glycerol monostearate, and 2.5 wt% of montmorillonite at 0.01 MPa and 65 °C for 50 min; after drying, place it in a twin-screw extruder and melt granulate at 165 °C to prepare 55 kg of the degradation layer masterbatch for standby.

[0028] Based on the total mass of the support layer masterbatch being 100 wt%, mix 97 wt% of polyethylene, 1 wt% of nano-silica, 1 wt% of montmorillonite, 0.5 wt% of antioxidant 168, 0.3 wt% of antioxidant 1010, and 0.2 wt% of maleic anhydride grafted polyethylene at 0.02 MPa and 65 °C for 50 min; after drying, place it in a twin-screw extruder and melt granulate at 180 °C to prepare 25 kg of the support layer masterbatch for standby.

[0029] Prepare a coextrusion blown film unit, which includes three extruders A, B, and C and a die head. Extruders A, B, and C can be divided into three functional zones: a feeding section, a compression section, and a metering section. The screw length-diameter ratio of extruders A and C is 32:1, and the screw length-diameter ratio of extruder B is 28:1; add the degradation layer masterbatch to extruders A and C, and add the support layer masterbatch to extruder B; the extrusion temperatures of extruders A and C are: 160 °C for the feeding section, 168 °C for the compression section, and 172 °C for the metering section; the extrusion temperature of extruder B is: 178 °C for the feeding section, 188 °C for the compression section, and 192 °C for the metering section; the screw speeds of the three extruders A, B, and C are 42 rpm, the melt pressure is 11 MPa, the traction speed is 9 m / min, and the blow-up ratio is 2.5:1. Extruders A and C extrude and form to obtain degradation layer A and degradation layer B respectively, and extruder B extrudes and forms to obtain the support layer; degradation layer A, degradation layer B, and the support layer converge in the die head. The die head blown film temperature is 182 °C, and the cooling temperature at the die head outlet is 16 °C. After blowing film forming through the die head, a polyvinyl biodegradable composite mulch film is obtained.

[0030] The thickness of the polyvinyl biodegradable composite ground film is 0.01 mm. The film structure of the polyvinyl biodegradable composite ground film is a sandwich structure of degradation layer A - support layer - degradation layer B, and the thickness ratio of degradation layer A, support layer and degradation layer B is 1:1:1.

[0031] Example 2 Preparation of degradation aid Triethanolamine and methanol were prepared into an organic system in a molar ratio of 0.1:45 for standby; at room temperature, 8 mol of zinc nitrate hexahydrate and 16 mol of 2-methylimidazole were added to 180 mol of the organic system, and then 2.0 mol of tetraphenylporphyrin, 0.8 mol of iron stearate and 0.8 mol of lauroyl peroxide were added, and ultrasonic dispersion was carried out for 35 min; stirring was started, the temperature was raised to 30 °C, and the coordination reaction was carried out for 55 min; after the reaction was completed, it was naturally cooled to room temperature, and the solid phase was obtained by centrifugal separation, washed with flowing methanol for 20 min, and dried in vacuum to obtain the ZIF-8 composite.

[0032] 50 mol of N-isopropylacrylamide, 1.5 mol of N,N'-methylenebisacrylamide and 0.2 mol of ammonium persulfate were dissolved in deionized water to form a prepolymer solution; the ZIF-8 composite was added to the prepolymer solution and ultrasonic dispersion was carried out for 35 min. Stirring was started, and under a nitrogen atmosphere, the temperature was raised to 85 °C for free radical polymerization reaction. After the reaction for 2 h, it was cooled to room temperature, and the solid phase was obtained by centrifugal separation, washed with flowing deionized water for 20 min to obtain the degradation aid.

[0033] Preparation of polyvinyl biodegradable composite ground film Based on the total mass of the degradation layer masterbatch being 100 wt%, 67 wt% of polybutylene adipate terephthalate, 10 wt% of polylactic acid, 16 wt% of degradation aid, 1.8 wt% of graphene oxide, 2.0 wt% of erucic acid amide, 1.2 wt% of glycerol monostearate and 2 wt% of montmorillonite were dried at 0.02 MPa and 60 °C for 55 min; after drying, they were placed in a twin-screw extruder and melt granulated at 160 °C to prepare 55 kg of degradation layer masterbatch for standby.

[0034] Based on the total mass of the support layer masterbatch being 100 wt%, 95 wt% of polyethylene, 1.5 wt% of nano-silica, 1.5 wt% of montmorillonite, 1 wt% of antioxidant 168, 0.6 wt% of antioxidant 1010 and 0.4 wt% of maleic anhydride grafted polyethylene were dried at 0.01 MPa and 55 °C for 55 min; after drying, they were placed in a twin-screw extruder and melt granulated at 175 °C to prepare 25 kg of support layer masterbatch for standby.

[0035] Prepare a coextrusion blown film unit, which includes three extruders A, B, and C and a die head. The extruders A, B, and C can be divided into three functional zones: a feeding section, a compression section, and a metering section. The screw length-diameter ratio of extruders A and C is 32:1, and the screw length-diameter ratio of extruder B is 28:1. Add the degradable layer masterbatch to extruders A and C, and add the support layer masterbatch to extruder B. The extrusion temperatures of extruders A and C are: 165 °C in the feeding section, 170 °C in the compression section, and 175 °C in the metering section. The extrusion temperature of extruder B is: 180 °C in the feeding section, 190 °C in the compression section, and 195 °C in the metering section. The screw speeds of the three extruders A, B, and C are 35 rpm, the melt pressure is 10 MPa, the traction speed is 8 m / min, and the blow-up ratio is 2.5:1. Extruders A and C are extruded and molded to obtain the degradable layer A and the degradable layer B respectively, and extruder B is extruded and molded to obtain the support layer. The degradable layer A, the degradable layer B, and the support layer converge in the die head. The die head blowing film temperature is 185 °C, and the cooling temperature at the die head outlet is 20 °C. After blowing film forming through the die head, a polyethylene-based biodegradable composite plastic film is obtained.

[0036] The thickness of the polyethylene-based biodegradable composite plastic film is 0.012 mm. The film structure of the polyethylene-based biodegradable composite plastic film is a sandwich structure of degradable layer A - support layer - degradable layer B, and the thickness ratio of degradable layer A, support layer, and degradable layer B is 1:1:1.

[0037] Example 3 Prepare a degradation aid Prepare an organic system by mixing 4-dimethylaminopyridine and methanol at a molar ratio of 0.1:80 for standby. At room temperature, add 12 mol of zinc nitrate hexahydrate and 30 mol of 2-methylimidazole to 240 mol of the organic system, then continue to add 1.5 mol of iron stearate, 1.5 mol of tetraphenylporphyrin, and 0.6 mol of lauroyl peroxide, and ultrasonically disperse for 15 min. Start stirring, heat up to 32 °C, and carry out a coordination reaction for 60 min. After the reaction is completed, cool naturally to room temperature, centrifuge to obtain a solid phase, wash with flowing methanol for 20 min, and obtain the ZIF-8 composite through vacuum drying.

[0038] Dissolve 50 mol of N-isopropylacrylamide, 3 mol of N,N'-methylenebisacrylamide, and 0.4 mol of ammonium persulfate in deionized water to form a prepolymer solution. Add the ZIF-8 composite to the prepolymer solution and ultrasonically disperse for 30 min. Start stirring, and under a nitrogen atmosphere, heat up to 90 °C for a free radical polymerization reaction. After reacting for 1.5 h, cool to room temperature, centrifuge to obtain a solid phase, and wash with flowing deionized water for 20 min to obtain the degradation aid.

[0039] Prepare a polyethylene-based biodegradable composite plastic film Based on the total mass of the degradation layer masterbatch being 100 wt%, 68.2 wt% of poly(butylene terephthalate-co-adipate), 14 wt% of polylactic acid, 10.2 wt% of a degradation aid, 2.4 wt% of nano-zinc oxide, 1.9 wt% of erucamide, 1.1 wt% of glycerol monostearate, and 2.2 wt% of montmorillonite are dried at 0.03 MPa and 55 °C for 60 min; after drying, they are placed in a twin-screw extruder and melt granulated at 155 °C to prepare 55 kg of the degradation layer masterbatch for later use.

[0040] Based on the total mass of the support layer masterbatch being 100 wt%, 96 wt% of polyethylene, 1.3 wt% of nano-silica, 1.3 wt% of montmorillonite, 0.6 wt% of antioxidant 168, 0.5 wt% of antioxidant 1010, and 0.3 wt% of maleic anhydride grafted polyethylene are dried at 0.03 MPa and 60 °C for 60 min; after drying, they are placed in a twin-screw extruder and melt granulated at 170 °C to prepare 25 kg of the support layer masterbatch for later use.

[0041] Prepare a co-extrusion blown film unit, which includes three extruders A, B, and C and a die head. Extruders A, B, and C can be divided into three functional zones: a feeding section, a compression section, and a metering section. The screw length-diameter ratio of extruders A and C is 32:1, and the screw length-diameter ratio of extruder B is 28:1; the degradation layer masterbatch is added to extruders A and C, and the support layer masterbatch is added to extruder B; the extrusion temperatures of extruders A and C are: 155 °C in the feeding section, 165 °C in the compression section, and 170 °C in the metering section; the extrusion temperatures of extruder B are: 175 °C in the feeding section, 185 °C in the compression section, and 190 °C in the metering section; the screw speeds of the three extruders A, B, and C are 45 rpm, the melt pressure is 12 MPa, the traction speed is 10 m / min, and the blow-up ratio is 2.5:1. Extruders A and C are extruded and formed to obtain the degradation layer A and the degradation layer B respectively, and extruder B is extruded and formed to obtain the support layer; the degradation layer A, the degradation layer B, and the support layer converge in the die head. The die head blown film temperature is 180 °C, and the cooling temperature at the die head outlet is 15 °C. After blown film forming through the die head, a polyethylene-based biodegradable composite plastic film is obtained.

[0042] The thickness of the polyethylene-based biodegradable composite plastic film is 0.008 mm. The film structure of the polyethylene-based biodegradable composite plastic film is a sandwich structure of degradation layer A - support layer - degradation layer B, and the thickness ratio of degradation layer A, support layer, and degradation layer B is 1:1:1.

[0043] Comparative Example 1 Replace 7.5 wt% of the degradation aid with an equal mass of a mixture composed of 2 mol of iron stearate, 1 mol of tetraphenylporphyrin, 0.5 mol of lauroyl peroxide, and the balance of polyethylene, that is, no coordination reaction and free radical polymerization reaction are carried out. The remaining steps and raw materials used are the same as in Example 1.

[0044] Comparative Example 2 Replace 7.5 wt% of the degradation aid with an equal mass of a mixture composed of a ZIF-8 composite and the balance of polyethylene, that is, no free radical polymerization reaction is carried out, and the remaining steps and raw materials used are the same as those in Example 1.

[0045] Comparative Example 3 Replace the ZIF-8 composite with an equal mass of a mixture composed of 2 mol of iron stearate, 1 mol of tetraphenylporphyrin, 0.5 mol of lauroyl peroxide, and the balance of polyethylene, that is, no coordination reaction is carried out, and the remaining steps and raw materials used are the same as those in Example 1.

[0046] Comparative Example 4 Replace iron stearate with zinc oxide nanoparticles, and the remaining steps and raw materials used are the same as those in Example 1.

[0047] Comparative Example 5 Replace tetraphenylporphyrin with zinc oxide nanoparticles, and the remaining steps and raw materials used are the same as those in Example 1.

[0048] Comparative Example 6 Replace iron stearate and tetraphenylporphyrin with zinc oxide nanoparticles, and the remaining steps and raw materials used are the same as those in Example 1.

[0049] Implementation Effect Take the plastic films prepared in Examples 1 to 3 and Comparative Examples 1 to 3, cut them into samples of 50 cm × 50 cm size for standby; set the ultraviolet intensity in the artificial climate chamber to 800 - 1000 μW / cm 2 、temperature to 35 - 55 °C and pH value of acidity and alkalinity to 4.5 - 4.5 to simulate different light, surface temperature and soil physical and chemical properties within a day, and evaluate the activation efficiency of photosensitive and thermosensitive oxidants and the degradation behavior of the samples in soils with different physical and chemical properties, where 800 - 1000 μW / cm 2 represents a strong ultraviolet environment, 4.5 - 5.5 represents an acidic soil environment, and 35 - 55 °C represents a high temperature environment. The experimental conditions of each example and comparative example are the same.

[0050] Physical Property Test: Tensile Strength and Elongation at Break: Sampling and measurement are carried out regularly (5 days, 30 days, 60 days, and 120 days) to test the changes in tensile strength (MPa) and elongation at break (%), so as to evaluate the loss of mechanical properties of the samples.

[0051] Weight Loss Rate: Calculate the weight loss rate (%) by the weighing method. The calculation formula for the weight loss rate is weight loss rate = (original mass - final mass) / original mass × 100%.

[0052] Specific physical property test data are shown in Table 1.

[0053]

[0054] As can be seen from Table 1, compared with Comparative Examples 1-3, the weight loss rate of the polyvinyl biodegradable composite mulch film prepared by the present invention changes smoothly with time in high temperature, strong ultraviolet and acidic soil environments, conforms to the agricultural production rhythm, and there is no phenomenon of premature disintegration, having good stability during the degradation process; the polyvinyl biodegradable composite mulch film prepared by the present invention has both the degradability of the fully biodegradable mulch film and the tensile properties of the polyethylene mulch film. The initial tensile strength reaches the performance parameters of the polyethylene mulch film, the tensile strength changes smoothly with time, and the elongation at break changes little with time, indicating that the support layer and the degradation layer degrade synchronously, and there is no local brittle fracture or delamination phenomenon, and the interfacial compatibility is good; the changes in the tensile strength and elongation at break during the 120-day service cycle meet the service requirements of the mulch film for moisture preservation, wind resistance and weed suppression.

[0055] In addition, it has been experimentally proven that Comparative Examples 4-6 show premature reduction of tensile strength and premature increase of weight loss rate. Among them, the reduction rate of tensile strength and the increase rate of weight loss rate of Comparative Example 6 are faster than those of Comparative Examples 4 and 5. Therefore, there is no need to conduct further experiments.

[0056] Due to the lack of one or both of iron stearate and tetraphenylporphyrin in Comparative Examples 4-6, the synergistic effect of iron stearate and tetraphenylporphyrin in improving the acid resistance of ZIF-8 cannot be achieved, resulting in premature disintegration of the mulch film; therefore, iron stearate and tetraphenylporphyrin jointly improve the acid resistance of ZIF-8 through different mechanisms, thereby alleviating the problem of premature release of photosensitizers and thermal sensitizers leading to the disintegration of the mulch film.

Claims

1. A preparation method of a polyvinyl biodegradable composite mulch film, characterized in that, It includes the following steps: (1) Add a zinc source, an organic ligand, a photosensitizer, and a thermosensitizer to an organic system. After mixing evenly, carry out a coordination reaction to obtain a ZIF-8 composite; (2) Compound a monomer, a crosslinking agent, and an initiator to obtain a prepolymer solution. Add the ZIF-8 composite to the prepolymer solution, mix evenly, and then carry out a free radical polymerization reaction to obtain a degradation aid; (3) Dry and melt granulate poly(butylene adipate-co-terephthalate), polylactic acid, the degradation aid, a toughening agent, a lubricant, an anti-fogging agent, and a heat stabilizer to obtain a degradation layer masterbatch; (4) Dry and melt granulate polyethylene, a gas barrier agent, a heat stabilizer, an antioxidant, and a compatibilizer to obtain a support layer masterbatch; (5) Coextrusion blow mold the degradation layer masterbatch and the support layer masterbatch to obtain a polyethylene-based biodegradable composite mulch film with a sandwich structure having a support layer in the middle and degradation layers on both sides.

2. The preparation method of the polyvinyl biodegradable composite ground film according to claim 1, wherein In step (1), the zinc source is zinc nitrate hexahydrate, the organic ligand is 2-methylimidazole, the photosensitizer is iron stearate and tetraphenylporphyrin, and the thermosensitizer is lauroyl peroxide; the molar ratio of the zinc source, the organic ligand, the photosensitizer, and the thermosensitizer is 1:(2-2.5):(0.25-0.35):(0.05-0.1).

3. The preparation method of the polyethylene-based biodegradable composite ground film according to claim 1, characterized in that, In step (1), the organic system is prepared by mixing one of triethanolamine or 4-dimethylaminopyridine with methanol at a molar ratio of 1:(450-800), and the molar ratio of one of triethanolamine or 4-dimethylaminopyridine to the zinc source is 1:(20-40).

4. The preparation method of the polyvinyl biodegradable composite ground film according to claim 1, characterized in that In step (1), the coordination reaction temperature is 30-35 °C, and the coordination reaction time is 40-60 min.

5. The preparation method of the polyvinyl biodegradable composite ground film according to claim 1, characterized in that, In step (2), the monomer is N-isopropylacrylamide, the crosslinking agent is N,N'-methylenebisacrylamide, and the initiator is ammonium persulfate; the molar ratio of the monomer, the crosslinking agent, and the initiator is 1:(0.03-0.06):(0.002-0.008), and the molar ratio of the monomer to the zinc source is 5:(0.8-1.2).

6. The preparation method of the polyvinyl biodegradable composite plastic film according to claim 1, characterized in that In step (2), the free radical polymerization reaction temperature is 75-90 °C, and the free radical polymerization reaction time is 1.5-2.5 h.

7. The preparation method of the polyvinyl biodegradable composite ground film according to claim 1, characterized in that, In step (3), the toughening agent is one of graphene oxide or zinc oxide nanoparticles, the lubricant is erucamide, the anti-fogging agent is glycerol monostearate, and the heat stabilizer is montmorillonite; based on the total mass of the degradation layer masterbatch being 100 wt%, the degradation layer masterbatch includes 67-70 wt% of poly(butylene adipate-co-terephthalate), 10-15 wt% of polylactic acid, 7.5-16 wt% of the degradation aid, 1.8-2.5 wt% of the toughening agent, 1.7-2.0 wt% of the lubricant, 0.8-1.2 wt% of the anti-fogging agent, and 2-2.5 wt% of the heat stabilizer; the melt granulation temperature is 155-165 °C.

8. The preparation method of the polyvinyl biodegradable composite plastic film according to claim 1, wherein In step (4), the gas barrier agent is nano-silica, the heat stabilizer is montmorillonite, the antioxidant is tris(2,4-di-tert-butylphenyl) phosphite and pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate], and the compatibilizer is maleic anhydride grafted polyethylene; based on the total mass of the support layer masterbatch being 100 wt%, the support layer masterbatch includes 95 - 97 wt% of polyethylene, 1 - 1.5 wt% of the gas barrier agent, 1 - 1.5 wt% of the heat stabilizer, 0.5 - 1 wt% of tris(2,4-di-tert-butylphenyl) phosphite, 0.3 - 0.6 wt% of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate], and 0.2 - 0.4 wt% of the compatibilizer; the melt granulation temperature is 170 - 180 °C.

9. The preparation method of the polyvinyl biodegradable composite plastic film according to claim 1, wherein In step (5), co-extrusion blown film means preparing a polyethylene-based biodegradable composite mulch film using a co-extrusion blown film unit. The co-extrusion blown film unit includes three extruders A, B, and C and a die head. The extruders A, B, and C are divided into three functional zones: a feeding section, a compression section, and a metering section; the biodegradable layer masterbatch is put into extruders A and C, and the support layer masterbatch is put into extruder B. The extrusion temperatures of extruders A and C are: 155 - 165 °C in the feeding section, 165 - 170 °C in the compression section, and 170 - 175 °C in the metering section; the extrusion temperature of extruder B is: 175 - 180 °C in the feeding section, 185 - 190 °C in the compression section, and 190 - 195 °C in the metering section.

10. The preparation method of the polyethylene-based biodegradable composite plastic film according to claim 9, characterized in that, In step (5), extruders A and C are extruded and formed to obtain the biodegradable layer A and the biodegradable layer B respectively, and extruder B is extruded and formed to obtain the support layer; the biodegradable layer A, the biodegradable layer B, and the support layer converge in the die head. The die head blown film temperature is 180 - 185 °C, and the die head blown film is formed to obtain a polyethylene-based biodegradable composite mulch film. The thickness of the polyethylene-based biodegradable composite mulch film is 0.008 - 0.012 mm. The polyethylene-based biodegradable composite mulch film has a sandwich structure of biodegradable layer A - support layer - biodegradable layer B, and the thickness ratio of the biodegradable layer A, the support layer, and the biodegradable layer B is 1:1:1.

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