Preparation method of polyethylene-based biodegradable composite mulch film

Abstract

The present invention belongs to the technical field of polymer film material preparation, and specifically relates to a method for preparing a polyethylene-based biodegradable composite mulch film. The preparation method comprises the following steps: adding a zinc source, an organic ligand, a photosensitizer, and a thermosensitizer to an organic system, mixing and reacting to obtain a ZIF-8 complex; compounding a monomer, a cross-linking agent, and an initiator to obtain a prepolymer solution, subsequently adding the ZIF-8 complex, mixing and reacting to obtain a degradation aid; drying and melt-granulating PBAT, PLA, and the like to obtain a degradation layer masterbatch; drying and melt-granulating polyethylene, a gas barrier agent, and the like to obtain a support layer masterbatch; the degradation layer masterbatch and the support layer masterbatch are co-extruded and blown into a film to obtain a polyethylene-based biodegradable composite mulch film. The polyethylene-based biodegradable composite mulch film prepared by the present invention still has a relatively stable degradation rate in a high temperature and high ultraviolet intensity environment, while also having good degradability and tensile properties.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer film material preparation, and particularly relates to a method for preparing a polyethylene-based biodegradable composite ground film. Background Art

[0002] Ground film is an agricultural film material that provides insulation, wind resistance, moisture retention, and weed control. Existing ground film technologies use biodegradable materials such as polybutylene terephthalate (PET) and polylactic acid. While environmentally friendly, these films suffer from poor mechanical properties, high costs, and insufficient moisture retention, making them difficult to meet the requirements of high winds in the northwest region. Traditional polyethylene ground films, while meeting strength standards, are difficult to degrade. Improvements have been made to traditional polyethylene ground films by adding photo- and thermal-oxidation additives to the polyethylene substrate to create an oxidative-biodegradable dual-degradable ground film. This dual-degradable film is first oxidized and broken down into low-molecular-weight fragments before being degraded by microorganisms. This dual-degradable film is more environmentally friendly than polyethylene ground films, and offers lower costs, higher tensile strength, and enhanced moisture retention than fully biodegradable ground films, making it more suitable for agricultural applications. On the other hand, existing oxidative-biodegradable mulches still suffer from uncontrolled degradation. This is because light and temperature factors vary in different natural environments. As a result, the degradation agents in oxidative-biodegradable mulches can prematurely activate in environments with long light exposure and high temperatures, leading to localized cracking and loss of moisture retention. Technicians aim to combine the degradability of biodegradable mulches with the tensile properties of polyethylene mulches; the resulting composite film would exhibit a more stable degradation rate in environments with high temperatures and high UV intensity.

[0003] Chinese patent CN112852059A discloses a biodegradation agent comprising 2% to 6% manganese stearate, 0.4% to 1.2% iron stearate, 5% to 10% copper stearate, 0.04% to 0.4% silane coupling agent, 90% to 70% low-density polyethylene or polypropylene, and 0.06% to 0.5% fattening agent.

[0004] The mechanism by which manganese stearate, iron stearate, and copper stearate promote polyethylene degradation described in this patent is that the metal ions absorb ultraviolet energy and become excited, triggering a photooxidation reaction in the plastic polymer chains. These photodegradation agents are susceptible to premature release due to high UV intensity, leading to premature disintegration of the mulch film. Using stearate-based degradation agents alone is unlikely to achieve the desired degradation rate for the mulch film.

[0005] Chinese patent CN108329571A discloses a biodegradable polyethylene ground film composition, which comprises, by weight of raw materials, 40-50 parts of low-density polyethylene, 50-60 parts of linear low-density polyethylene, and 10-20 parts of a functional masterbatch; the functional masterbatch comprises 50-60 parts of polybutylene terephthalate-adipate, 40-50 parts of calcium carbonate, 5-10 parts of metallocene polyethylene, 0.05-0.01 parts of a fluoroelastomer, 4-7 parts of a coupling agent, 2-4 parts of a dispersant, and 8-10 parts of a solvent.

[0006] Since polyethylene is difficult to degrade in the natural environment, it is generally necessary to add photodegradation aids, thermal degradation aids or oxidative degradation aids to promote degradation. However, this patent does not involve measures to promote the degradation of polyethylene, and it is difficult to ensure the degradation performance of the ground film prepared using the composition. The ground film is prone to produce microplastic residues in the soil; further, it does not involve measures to control the degradation rate of the ground film. Summary of the Invention

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

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] The method for preparing the polyethylene-based biodegradable composite mulch film of the present invention comprises the following steps:

[0010] (1) Add zinc source, organic ligand, photosensitizer and thermosensitizer to the organic system, mix well and then carry out coordination reaction to obtain ZIF-8 complex;

[0011] (2) Compounding the monomer, cross-linking agent and initiator to obtain a prepolymer solution, adding the ZIF-8 complex to the prepolymer solution and mixing well, and then conducting a free radical polymerization reaction to obtain a degradation agent;

[0012] (3) drying and melt-granulating polybutylene terephthalate adipate, polylactic acid, degradation aid, toughening agent, lubricant, antifogging agent and thermal stabilizer to obtain degradation layer masterbatch;

[0013] (4) drying and melt-granulating the polyethylene, gas barrier agent, heat stabilizer, antioxidant and compatibilizer to obtain a support layer masterbatch;

[0014] (5) The degradation layer masterbatch and the support layer masterbatch are co-extruded into a blown film to obtain a polyethylene-based biodegradable composite ground film with a sandwich structure having a support layer in the middle and degradation layers on both sides.

[0015] in:

[0016] In the 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).

[0017] In the step (1), the organic system is prepared by mixing one of triethanolamine or 4-dimethylaminopyridine with methanol in a molar ratio of 1:(450-800), and the molar ratio of one of triethanolamine or 4-dimethylaminopyridine with the zinc source is 1:(20-40).

[0018] In the step (1), ultrasonic dispersion is performed for 15 to 35 minutes until the mixture is uniformly mixed, the coordination reaction temperature is 30 to 35° C., and the coordination reaction time is 40 to 60 minutes.

[0019] In the 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).

[0020] In the step (2), ultrasonic dispersion is performed for 25 to 35 minutes until the mixture is uniformly mixed, the free radical polymerization reaction temperature is 75 to 90° C., and the free radical polymerization reaction time is 1.5 to 2.5 hours.

[0021] In the step (3), the toughening agent is one of graphene oxide or nano zinc oxide, the lubricant is erucamide, the antifogging agent is glycerol monostearate, and the thermal stabilizer is montmorillonite; based on the total mass of the degradation layer masterbatch as 100wt%, the degradation layer masterbatch includes 67~70wt% of polybutylene terephthalate-adipate, 10~15wt% of polylactic acid, 7.5~16wt% of degradation aid, 1.8~2.5wt% of toughening agent, 1.7~2.0wt% of lubricant, 0.8~1.2wt% of antifogging agent and 2~2.5wt% of thermal stabilizer; the drying pressure is 0.01~0.03MPa, the drying temperature is 55~65℃, and the drying time is 50~60min; and the melt granulation temperature is 155~165℃.

[0022] 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 tetrakis [β- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate] pentaerythritol ester, and the compatibilizer is maleic anhydride grafted polyethylene; based on the total mass of the support layer masterbatch as 100wt%, the support layer masterbatch includes 95-97wt% of polyethylene, 1-1.5wt% of the gas barrier agent, 1- 1.5wt% of heat stabilizer, 0.5~1wt% of tris(2,4-di-tert-butylphenyl) phosphite, 0.3~0.6wt% of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] and 0.2~0.4wt% of compatibilizer; drying pressure is 0.01~0.03MPa, drying temperature is 55~65℃, drying time is 50~60min; melt granulation temperature is 170~180℃.

[0023] In the step (5), the co-extrusion blown film refers to the use of a co-extrusion blown film unit to prepare a polyethylene-based biodegradable composite film. 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 areas: a feeding section, a compression section, and a metering section. The screw length-diameter ratio of the extruders A and C is 32:1, and the screw length-diameter ratio of the extruder B is 28:1. The degradation layer masterbatch is put into the extruders A and C, the support layer masterbatch is put into the extruder B, and the extruders A and C are The extrusion temperature of extruder A is: 155~165℃ in the feeding section, 165~170℃ in the compression section, and 170~175℃ in the metering section; the extrusion temperature of extruder B is: 175~180℃ in the feeding section, 185~190℃ in the compression section, and 190~195℃ in the metering section; the screw speed of the three extruders A, B and C is 35~45rpm, the melt pressure is 10~12MPa, the pulling speed is 8~10m / min, and the blow-up ratio is 2.5:1.

[0024] In the step (5), extruders A and C perform extrusion molding to obtain degradation layer A and degradation layer B, respectively, and extruder B performs extrusion molding to obtain a support layer; the degradation layer A, degradation layer B and support layer are gathered in the die head, the die head film blowing temperature is 180~185℃, and the die head extrusion molding is performed to obtain a polyethylene-based biodegradable composite mulch film, the thickness of the polyethylene-based biodegradable composite mulch film is 0.008~0.012mm, 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, and the thickness ratio of degradation layer A, support layer and degradation layer B is 1:1:1; the cooling temperature at the die head outlet is 15~20℃.

[0025] In the present invention, maleic anhydride grafted polyethylene serves as a compatibilizer, and the maleic anhydride groups (-CO-O-CO-) in the molecule form hydrogen bonds or transesterification reactions with ester groups or hydroxyl groups in polybutylene terephthalate-adipate and polylactic acid. At the same time, its non-polar polyethylene chain segments are compatible with the polyethylene substrate, forming a bridge effect, thereby increasing the interfacial compatibility between polybutylene terephthalate-adipate, polylactic acid and polyethylene.

[0026] The beneficial effects of the present invention are as follows:

[0027] (1) The present invention prepares ZIF-8 by reacting zinc nitrate hexahydrate with 2-methylimidazole in an organic system. The organic system includes one of triethanolamine or 4-dimethylaminopyridine and methanol, so that the reaction can be carried out at a lower temperature, reducing the reversible dissociation phenomenon of the photosensitizer / thermosensitizer and reducing the loss of the photosensitizer / thermosensitizer.

[0028] In addition, the photosensitizer and thermosensitizer in the present invention are embedded within the ZIF-8 nanopores through π-π interactions or coordinated adsorption. The confinement effect of the nanopores can suppress the thermal vibration of the photosensitizer and thermosensitizer, thereby preventing their decomposition during subsequent high-temperature processing. Furthermore, ZIF-8 also immobilizes the photosensitizer and thermosensitizer through π-π interactions or coordinated adsorption, providing a buffering effect against high temperatures and high UV intensity in the external environment. This avoids the problem of premature release of degradation agents and premature disintegration of traditional oxidative and biological dual-degradable mulch films caused by high temperatures and high UV intensity. In addition, ZIF-8 will have the 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, and its highly symmetrical and highly rigid molecular structure can form a stable physical barrier, reducing 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 collapse of the ZIF-8 skeleton under acidic conditions. The iron stearate molecule is formed by the combination of iron ions and the carboxylate radicals of long-chain stearic acid through coordination bonds, and its Fe 2+ / Fe 3+ Ion pairs can quench excess oxygen free radicals in acidic soil, preventing reactive oxygen free radicals from attacking the organic ligands of ZIF-8, thereby protecting the integrity of the ZIF-8 skeleton.

[0029] (2) The present invention utilizes monomer N-isopropylacrylamide, cross-linking agent N,N'-methylenebisacrylamide, and initiator ammonium persulfate to react to prepare thermosensitive polymer poly N-isopropylacrylamide (PNIPAM). PNIPAM, as a thermosensitive polymer hydrogel, has a narrow critical release temperature (LCST, about 33-36°C). Near the critical release temperature, the release rate of the photo / thermosensitizer is unstable and fluctuates greatly. The present invention combines PNIPAM with ZIF-8 complex to utilize the synergistic effect of the two.

[0030] The PNIPAM molecular chain is composed of hydrophilic amide groups and hydrophobic isopropyl units. This reversible conformational transition occurs between hydrophilic and hydrophobic states. Below the critical release temperature, the amide groups form strong hydrogen bonds with water molecules in the air, fully hydrating and extending the PNIPAM molecular chain, rendering it hydrophilic. This fully exposes the ZIF-8 structure, allowing it to maintain a consistent rate of photosensitizer release even in low temperatures, short photoperiods, and environments with large day-night temperature swings, thereby reducing the amount of film debris left in the soil. Above the critical release temperature, thermal energy destroys the hydrogen bonds, rendering the PNIPAM structure hydrophobic. The PNIPAM molecular chain dehydrates, folds, and then contracts into a hydrophobic aggregate, reducing ZIF-8 exposure and preventing premature disintegration in environments with prolonged high temperatures and high UV intensity. DETAILED DESCRIPTION

[0031] The present invention is described and illustrated in detail below with reference to the embodiments.

[0032] The raw materials in the examples and comparative examples are all commercially available products, and some of the raw materials are from: 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, brand 7042; maleic anhydride grafted polyethylene, provided by Exxon, brand PE 1040.

[0033] Example 1

[0034] Preparation of prodegradants

[0035] An organic system was prepared with triethanolamine and methanol in a molar ratio of 0.1:50 for standby use; 10 mol of zinc nitrate hexahydrate and 22 mol of 2-methylimidazole were added to 200 mol of the organic system at room temperature, followed by the addition of 2 mol of ferric stearate, 1 mol of tetraphenylporphyrin and 0.8 mol of lauroyl peroxide, and ultrasonic dispersion was performed for 20 minutes; stirring was started, the temperature was raised to 35°C, and the coordination reaction was carried out for 40 minutes; after the reaction, the mixture was naturally cooled to room temperature, centrifuged to obtain a solid phase, washed with flowing methanol for 20 minutes, and vacuum dried to obtain a ZIF-8 complex.

[0036] 50 mol of N-isopropylacrylamide, 2.5 mol of N,N'-methylenebisacrylamide, and 0.1 mol of ammonium persulfate were dissolved in deionized water to form a prepolymer solution. The ZIF-8 complex was added to the prepolymer solution and ultrasonically dispersed for 25 minutes. Stirring was initiated and the temperature was raised to 75°C under a nitrogen atmosphere for free radical polymerization. After 2.5 hours, the mixture was cooled to room temperature and centrifuged to obtain a solid phase, which was then washed with running deionized water for 20 minutes to obtain the degradation agent.

[0037] Preparation of polyethylene-based biodegradable composite mulch film

[0038] Based on the total mass of the degradation layer masterbatch as 100wt%, 70wt% of poly(butylene terephthalate-adipate), 15wt% of polylactic acid, 7.5wt% of a degradation aid, 2.5wt% of graphene oxide, 1.7wt% of erucamide, 0.8wt% of glycerol monostearate and 2.5wt% of montmorillonite were dried at 0.01MPa and 65°C for 50min; after drying, the mixture was placed in a twin-screw extruder and melt-granulated at 165°C to prepare 55kg of degradation layer masterbatch for standby use.

[0039] Based on the total mass of the support layer masterbatch as 100wt%, 97wt% polyethylene, 1wt% nano-silica, 1wt% montmorillonite, 0.5wt% antioxidant 168, 0.3wt% antioxidant 1010 and 0.2wt% maleic anhydride grafted polyethylene were dried at 0.02MPa and 65°C for 50min; after drying, they were placed in a twin-screw extruder and melt-granulated at 180°C to prepare 25kg of support layer masterbatch for use.

[0040] Prepare a co-extrusion film blowing unit, which includes three extruders A, B, and C and a die head. Extruders A, B, and C can be divided into three functional areas: feeding section, compression section, and 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 temperature of extruders A and C: the feeding section is 160°C, the compression section is 168°C, and the metering section is 172°C; the extrusion temperature of extruder B: the feeding section is 178°C, the compression section is 188°C, and the metering section is 192°C; the screw speed of the three extruders A, B, and C is 42rpm, the melt pressure is 11MPa, the pulling speed is 9m / min, and the blow-up ratio is 2.5:1. Extruders A and C extrude to obtain degradation layer A and degradation layer B respectively, and extruder B extrude to obtain support layer; degradation layer A, degradation layer B and support layer are gathered in the die head, the die head film blowing temperature is 182°C, the cooling temperature at the die head outlet is 16°C, and the polyethylene-based biodegradable composite mulch film is obtained after die head film blowing.

[0041] The thickness of the polyethylene-based biodegradable composite mulch is 0.01 mm. The membrane structure of the polyethylene-based biodegradable composite mulch is a sandwich structure of degradation layer A-support layer-degradation layer B. The thickness ratio of degradation layer A, support layer and degradation layer B is 1:1:1.

[0042] Example 2

[0043] Preparation of prodegradants

[0044] An organic system was prepared with triethanolamine and methanol in a molar ratio of 0.1:45 for later use; 8 mol of zinc nitrate hexahydrate and 16 mol of 2-methylimidazole were added to 180 mol of the organic system at room temperature, followed by the addition of 2.0 mol of tetraphenylporphyrin, 0.8 mol of ferric stearate, and 0.8 mol of lauroyl peroxide, and ultrasonic dispersion was performed for 35 minutes; stirring was started, the temperature was raised to 30°C, and the coordination reaction was carried out for 55 minutes; after the reaction, the mixture was naturally cooled to room temperature, centrifuged to obtain a solid phase, washed with flowing methanol for 20 minutes, and vacuum dried to obtain a ZIF-8 complex.

[0045] 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 complex was added to the prepolymer solution and ultrasonically dispersed for 35 minutes. Stirring was initiated and the temperature was raised to 85°C under a nitrogen atmosphere for free radical polymerization. After 2 hours of reaction, the mixture was cooled to room temperature and centrifuged to obtain a solid phase, which was then washed with running deionized water for 20 minutes to obtain the degradation agent.

[0046] Preparation of polyethylene-based biodegradable composite mulch film

[0047] Based on the total mass of the degradation layer masterbatch as 100wt%, 67wt% poly(butylene terephthalate-adipate), 10wt% poly(lactic acid), 16wt% degradation aid, 1.8wt% graphene oxide, 2.0wt% erucamide, 1.2wt% glycerol monostearate and 2wt% montmorillonite were dried at 0.02MPa and 60°C for 55min; after drying, the mixture was placed in a twin-screw extruder and melt-granulated at 160°C to prepare 55kg of degradation layer masterbatch for standby use.

[0048] Based on the total mass of the support layer masterbatch as 100wt%, 95wt% polyethylene, 1.5wt% nano-silica, 1.5wt% montmorillonite, 1wt% antioxidant 168, 0.6wt% antioxidant 1010 and 0.4wt% maleic anhydride grafted polyethylene were dried at 0.01MPa and 55°C for 55min; after drying, they were placed in a twin-screw extruder and melt-granulated at 175°C to prepare 25kg of support layer masterbatch for use.

[0049] A co-extrusion film blowing unit is prepared, which includes three extruders A, B, and C and a die head. Extruders A, B, and C can be divided into three functional areas: feeding section, compression section, and 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 temperature of extruders A and C is 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 speed of the three extruders A, B, and C is 35 rpm, the melt pressure is 10 MPa, the pulling speed is 8 m / min, and the blow-up ratio is 2.5:1. Extruders A and C extrude to obtain degradation layer A and degradation layer B respectively, and extruder B extrude to obtain support layer; degradation layer A, degradation layer B and support layer are gathered in the die head, the die head film blowing temperature is 185°C, the cooling temperature at the die head outlet is 20°C, and the polyethylene-based biodegradable composite mulch film is obtained after die head film blowing.

[0050] The thickness of the polyethylene-based biodegradable composite mulch is 0.012 mm. The membrane structure of the polyethylene-based biodegradable composite mulch is a sandwich structure of degradation layer A-support layer-degradation layer B. The thickness ratio of degradation layer A, support layer and degradation layer B is 1:1:1.

[0051] Example 3

[0052] Preparation of prodegradants

[0053] An organic system was prepared with 4-dimethylaminopyridine and methanol in a molar ratio of 0.1:80 for later use; 12 mol of zinc nitrate hexahydrate and 30 mol of 2-methylimidazole were added to 240 mol of the organic system at room temperature, followed by the addition of 1.5 mol of ferric stearate, 1.5 mol of tetraphenylporphyrin and 0.6 mol of lauroyl peroxide, and ultrasonic dispersion was performed for 15 min; stirring was started, the temperature was raised to 32°C, and the coordination reaction was carried out for 60 min; after the reaction was completed, the mixture was naturally cooled to room temperature, centrifuged to obtain a solid phase, washed with flowing methanol for 20 min, and vacuum dried to obtain a ZIF-8 complex.

[0054] 50 mol of N-isopropylacrylamide, 3 mol of N,N'-methylenebisacrylamide, and 0.4 mol of ammonium persulfate were dissolved in deionized water to form a prepolymer solution. The ZIF-8 complex was added to the prepolymer solution and ultrasonically dispersed for 30 minutes. Stirring was initiated and the temperature was raised to 90°C under a nitrogen atmosphere for free radical polymerization. After 1.5 hours, the mixture was cooled to room temperature and centrifuged to obtain a solid phase, which was then washed with running deionized water for 20 minutes to obtain the degradation agent.

[0055] Preparation of polyethylene-based biodegradable composite mulch film

[0056] Based on the total mass of the degradation layer masterbatch as 100wt%, 68.2wt% of polybutylene terephthalate-adipate, 14wt% of polylactic acid, 10.2wt% of degradation aid, 2.4wt% of nano zinc oxide, 1.9wt% of erucamide, 1.1wt% of glycerol monostearate and 2.2wt% of montmorillonite were dried at 0.03MPa and 55°C for 60min; after drying, the mixture was placed in a twin-screw extruder and melt-granulated at 155°C to prepare 55kg of degradation layer masterbatch for use.

[0057] Based on the total mass of the support layer masterbatch as 100wt%, 96wt% polyethylene, 1.3wt% nano-silica, 1.3wt% montmorillonite, 0.6wt% antioxidant 168, 0.5wt% antioxidant 1010, and 0.3wt% maleic anhydride grafted polyethylene were dried at 0.03MPa and 60°C for 60min; after drying, they were placed in a twin-screw extruder and melt-granulated at 170°C to prepare 25kg of support layer masterbatch for use.

[0058] Prepare a co-extrusion film blowing unit, which includes three extruders A, B, and C and a die head. Extruders A, B, and C can be divided into three functional areas: feeding section, compression section, and 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 temperature of extruders A and C: the feeding section is 155°C, the compression section is 165°C, and the metering section is 170°C; the extrusion temperature of extruder B: the feeding section is 175°C, the compression section is 185°C, and the metering section is 190°C; the screw speed of the three extruders A, B, and C is 45rpm, the melt pressure is 12MPa, the pulling speed is 10m / min, and the blow-up ratio is 2.5:1. Extruders A and C extrude to obtain degradation layer A and degradation layer B respectively, and extruder B extrude to obtain support layer; degradation layer A, degradation layer B and support layer are gathered in the die head, the die head film blowing temperature is 180°C, the cooling temperature at the die head outlet is 15°C, and the polyethylene-based biodegradable composite mulch film is obtained after die head film blowing.

[0059] The thickness of the polyethylene-based biodegradable composite mulch is 0.008 mm. The membrane structure of the polyethylene-based biodegradable composite mulch is a sandwich structure of degradation layer A-support layer-degradation layer B. The thickness ratio of degradation layer A, support layer and degradation layer B is 1:1:1.

[0060] Comparative Example 1

[0061] 7.5 wt % of the prodegradant was replaced with an equal mass of a mixture consisting of 2 mol of ferric stearate, 1 mol of tetraphenylporphyrin, 0.5 mol of lauroyl peroxide, and the balance of polyethylene. That is, the coordination reaction and free radical polymerization reaction were not performed. The remaining steps and raw materials used were the same as in Example 1.

[0062] Comparative Example 2

[0063] 7.5 wt % of the prodegradant was replaced by an equal mass of a mixture consisting of a ZIF-8 complex and the remainder polyethylene, i.e., no free radical polymerization reaction was performed. The remaining steps and raw materials used were the same as in Example 1.

[0064] Comparative Example 3

[0065] The ZIF-8 complex was replaced with an equal mass mixture consisting of 2 mol of iron stearate, 1 mol of tetraphenylporphyrin, 0.5 mol of lauroyl peroxide and the balance of polyethylene, i.e., no coordination reaction was performed. The remaining steps and raw materials used were the same as in Example 1.

[0066] Comparative Example 4

[0067] Iron stearate was replaced by nano zinc oxide, and the remaining steps and raw materials were the same as those in Example 1.

[0068] Comparative Example 5

[0069] Tetraphenylporphyrin was replaced by nano zinc oxide, and the remaining steps and raw materials were the same as those in Example 1.

[0070] Comparative Example 6

[0071] Iron stearate and tetraphenylporphyrin were replaced by nano zinc oxide, and the remaining steps and raw materials were the same as those in Example 1.

[0072] Implementation Effect

[0073] Take the mulch films prepared in Examples 1 to 3 and Comparative Examples 1 to 3 and cut them into 50 cm × 50 cm samples for later use; set the ultraviolet intensity in the artificial climate box to 800-1000 μW / cm 2 , temperature 35~55℃ and pH=4.5~4.5 to simulate different light intensity, surface temperature and soil physical and chemical properties in one day, and evaluate the activation efficiency of photosensitivity and thermosensitive oxidants and the degradation behavior of samples in soils with different physical and chemical properties. 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 the embodiments and comparative examples are the same.

[0074] Physical performance test:

[0075] Tensile strength and elongation at break: Samples were taken regularly (5 days, 30 days, 60 days and 120 days) to measure the changes in tensile strength (MPa) and elongation at break (%) to evaluate the loss of mechanical properties of the samples.

[0076] Weight loss rate: The weight loss rate (%) was calculated by weighing method. The calculation formula of weight loss rate is weight loss rate = (original mass - final mass) / original mass × 100%.

[0077] Specific physical performance test data are shown in Table 1.

[0078]

[0079] As can be seen from Table 1, compared with Comparative Examples 1 to 3, the weight loss rate of the polyethylene-based biodegradable composite mulch prepared by the present invention changes more smoothly over time in high temperature, strong ultraviolet and acidic soil environments, which is consistent with the rhythm of agricultural production, without premature disintegration, and has good stability in the degradation process; the polyethylene-based biodegradable composite mulch prepared by the present invention has both the degradability of a fully biodegradable mulch and the tensile properties of a polyethylene mulch, the initial tensile strength reaches the performance parameters of a polyethylene mulch, the tensile strength changes more smoothly over time, and the elongation at break changes less over time, indicating that the support layer and the degradation layer are degraded synchronously, without local brittle cracks or delamination, and the interface compatibility is good; the changes in tensile strength and elongation at break during a 120-day usage cycle meet the requirements of the mulch for moisture conservation, wind resistance and weed control.

[0080] In addition, experiments have shown that Comparative Examples 4 to 6 exhibit a premature decrease in tensile strength and a premature increase in weight loss rate, wherein the rate of decrease in tensile strength and the rate of increase in weight loss rate of Comparative Example 6 are faster than those of Comparative Examples 4 and 5. Therefore, further experiments are unnecessary.

[0081] Comparative Examples 4 to 6 lack one or both of iron stearate and tetraphenylporphyrin, and thus cannot achieve the synergistic effect of iron stearate and tetraphenylporphyrin in improving the acid resistance of ZIF-8, 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 thermosensitizers leading to mulch film disintegration.

Claims

1. A method for preparing a polyethylene-based biodegradable composite mulch film, characterized in that: The following steps are involved: (1) Add zinc source, organic ligand, photosensitizer and thermosensitizer to the organic system, mix well and then carry out coordination reaction to obtain ZIF-8 complex; (2) Compounding the monomer, cross-linking agent and initiator to obtain a prepolymer solution, adding the ZIF-8 complex to the prepolymer solution and mixing well, and then conducting a free radical polymerization reaction to obtain a degradation agent; (3) drying and melt-granulating polybutylene terephthalate adipate, polylactic acid, degradation aid, toughening agent, lubricant, antifogging agent and thermal stabilizer to obtain degradation layer masterbatch; (4) drying and melt-granulating the polyethylene, gas barrier agent, heat stabilizer, antioxidant and compatibilizer to obtain a support layer masterbatch; (5) The degradation layer masterbatch and the support layer masterbatch are co-extruded and blown into a film to obtain a polyethylene-based biodegradable composite ground film with a sandwich structure having a support layer in the middle and degradation layers on both sides; The zinc source is zinc nitrate hexahydrate, the organic ligand is 2-methylimidazole, the photosensitizers are iron stearate and tetraphenylporphyrin, and the thermosensitizer is lauroyl peroxide.

2. The method for preparing the polyethylene-based biodegradable composite mulch film according to claim 1, characterized in that: In step (1), 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 method for preparing the polyethylene-based biodegradable composite mulch 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 in a molar ratio of 1:(450-800), and the molar ratio of one of triethanolamine or 4-dimethylaminopyridine with the zinc source is 1:(20-40).

4. The method for preparing the polyethylene-based biodegradable composite mulch 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 method for preparing the polyethylene-based biodegradable composite mulch film according to claim 1, characterized in that: In step (2), the monomer is N-isopropylacrylamide, the cross-linking agent is N,N'-methylenebisacrylamide, and the initiator is ammonium persulfate; the molar ratio of the monomer, the cross-linking 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 method for preparing the polyethylene-based biodegradable composite mulch 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 method for preparing the polyethylene-based biodegradable composite mulch film according to claim 1, characterized in that: In step (3), the toughening agent is one of graphene oxide or nano zinc oxide, the lubricant is erucamide, the antifogging agent is glycerol monostearate, and the thermal stabilizer is montmorillonite; based on the total mass of the degradation layer masterbatch as 100wt%, the degradation layer masterbatch includes 67~70wt% of polybutylene terephthalate-adipate, 10~15wt% of polylactic acid, 7.5~16wt% of degradation aid, 1.8~2.5wt% of toughening agent, 1.7~2.0wt% of lubricant, 0.8~1.2wt% of antifogging agent and 2~2.5wt% of thermal stabilizer; and the melt granulation temperature is 155~165℃.

8. The method for preparing the polyethylene-based biodegradable composite mulch film according to claim 1, characterized in that: 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 as 100wt%, the support layer masterbatch includes 95~97wt% of polyethylene, 1~1.5wt% of the gas barrier agent, 1~1.5wt% of the heat stabilizer, 0.5~1wt% of tris(2,4-di-tert-butylphenyl) phosphite, 0.3~0.6wt% of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] and 0.2~0.4wt% of the compatibilizer; and the melt granulation temperature is 170~180℃.

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

10. The method for preparing the polyethylene-based biodegradable composite mulch film according to claim 9, characterized in that: In step (5), extruders A and C perform extrusion molding to obtain degradation layer A and degradation layer B, respectively, and extruder B performs extrusion molding to obtain a support layer; the degradation layer A, degradation layer B and support layer are gathered in a die head, the die head film blowing temperature is 180-185°C, and the die head film blowing molding is performed 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 polyethylene-based biodegradable composite mulch 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.

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