Modified polylactic acid degradable mulching film for red kidney bean planting and preparation method thereof

Abstract

The application belongs to the field of polymer compositions for mulching films, and particularly relates to a modified polylactic acid degradable mulching film for red kidney bean planting and a preparation method thereof. The modified polylactic acid degradable mulching film comprises the following raw materials in parts by mass: 40-60 parts of polylactic acid, 40-60 parts of polybutylene adipate terephthalate, 15-25 parts of drug-loaded nanometer microspheres, 4-6 parts of coupling agent modified degradation promoter, and 1-3 parts of lubricant. The drug-loaded nanometer microspheres are prepared by adding phenolated lignin into water to adjust pH, adding a pore former and a crosslinking agent, and preparing an aqueous phase; the aqueous phase and an oil phase are mixed and reacted to obtain porous nanometer microspheres; the porous nanometer microspheres are immersed in a pesticide solution for adsorption, and a coating liquid is added to obtain the modified polylactic acid degradable mulching film. The mulching film has strong mechanical properties; and the drug-loaded nanometer microspheres and the coupling agent modified degradation promoter are used in combination to match the degradation rate of the mulching film with the growth cycle of red kidney beans.

Description

Technical Field

[0001] This invention belongs to the technical field of polymer compositions for mulch film, specifically relating to a modified polylactic acid biodegradable mulch film for red kidney bean planting and its preparation method. Background Technology

[0002] Red kidney beans (scientific name: *Vigna angularis*) are herbaceous crops belonging to the genus *Vigna* of the legume family. They are characterized by their tolerance to poor soil and drought, and short growing season. They are a specialty grain crop in the high-altitude and arid / semi-arid mountainous areas of northwestern Shanxi Province, including Kelan, Wuzhai, and Shenchi. The soil in this region is mostly loose loess-based flatlands, ravines, and plateaus, and agricultural production is highly dependent on natural rainfall. The red kidney beans produced here are large, dark in color, and of high commercial quality. They are also rich in various amino acids, vitamins, and minerals, and possess health benefits such as nourishment and lowering blood lipids. Therefore, they are one of the core crops for adjusting the local agricultural industrial structure and increasing farmers' economic income.

[0003] However, the actual cultivation and production of red kidney beans currently faces two major challenges affecting their quality and yield: First, continuous cropping over many years leads to the enrichment of soil pathogens, resulting in increasingly severe soil-borne diseases such as root rot, causing yield reductions (up to 30%-80%) and quality degradation, seriously harming farmers' economic interests. Traditionally, farmers have used crop rotation with cereal crops to alleviate the problem of continuous cropping, but this is difficult to implement due to limitations in farmland size and planting habits, and the rotation cycle (usually more than 3 years) is also difficult to guarantee. Second, the main red kidney bean producing areas in Shanxi Province have low average annual rainfall and high evaporation. After sowing from late April to mid-May, they are prone to spring drought, and during the growing season, they are prone to summer drought. Spring drought causes difficulties in sowing, emergence, and seedling survival, while summer drought hinders growth, leading to significant yield reductions or even crop failure. These two factors combined severely restrict the yield and quality of red kidney beans. Although existing studies have shown that mulching can effectively increase temperature and retain moisture, and alleviate seasonal drought, such as the poly(butylene terephthalate-co-butylene adipate) / starch-based fully biodegradable composite material and its preparation method disclosed in CN103937178B, and the fully biodegradable moisturizing mulch film and its preparation method disclosed in patent CN118006089B, which have respectively developed a polyester blend system with degradation characteristics and a biomass-based moisturizing mulch film, they have provided a technical basis for solving agricultural white pollution and some water retention needs. Ordinary biodegradable mulch film still has significant limitations in its suitability for red kidney bean cultivation: its mechanical properties are poor, with low tensile strength and weak puncture resistance, making it extremely prone to damage in the windy spring environment of northwestern Shanxi, where the ground surface is covered with gravel and crop residues. This leads to premature failure of its moisture retention function, making it difficult to withstand prolonged spring and summer droughts. Its degradation behavior is difficult to control and cannot be synchronized with the complete growth period of red kidney beans. Premature degradation will affect drought resistance, while delayed degradation may entangle the root system and interfere with normal growth. Most importantly, existing mulch film has a single function and lacks the ability to inhibit soil-borne diseases caused by continuous cropping of red kidney beans. It may even create a localized high temperature and high humidity microenvironment, exacerbating the spread and occurrence of soil-borne diseases such as root rot.

[0004] Therefore, the main red kidney bean producing areas urgently need a special biodegradable mulch film that can effectively resist spring and summer droughts, ensure soil moisture throughout the entire growth period, adapt to the growth period of red kidney beans to achieve controllable degradation, and also help alleviate or eliminate soil-borne diseases caused by continuous cropping. This is of great significance for breaking through the bottleneck of red kidney bean planting and production and ensuring the agricultural economy. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a modified polylactic acid (PLA) biodegradable mulch film for red kidney bean cultivation and its preparation method. The mulch film raw materials of this invention incorporate drug-loaded nanospheres and a coupling agent-modified degradation promoter. The drug-loaded nanospheres are prepared by first using phenolic lignin, a pore-forming agent, and a cross-linking agent as raw materials via a reverse emulsion method to form porous nanospheres. After adsorbing pesticides, these nanospheres are coated with a composite film containing an organotin catalyst and carboxyl-terminated PLA, achieving stable controlled release of pesticides. Under the high temperature and the action of the organotin catalyst during mulch film preparation, the carboxyl-terminated PLA forms an interpenetrating network with the hydroxyl groups on the porous nanospheres and the substrate polymer, giving the mulch film excellent mechanical properties. Furthermore, by controlling the relative amounts of the drug-loaded nanospheres and the coupling agent-modified degradation promoter, the degradation rate of the mulch film can be precisely controlled to match the complete growth cycle of red kidney beans.

[0006] To achieve the above objectives, the following technical solution is adopted:

[0007] A modified polylactic acid biodegradable mulch film for red kidney bean cultivation comprises the following raw materials in parts by weight: 40-60 parts polylactic acid, 40-60 parts polybutylene adipate terephthalate, 15-25 parts drug-loaded nanospheres, 4-6 parts coupling agent, modifier, and degradation promoter, and 1-3 parts lubricant; the drug-loaded nanospheres are prepared by a method comprising the following steps:

[0008] 1) Phenolic lignin is added to water, the pH is adjusted, and pore-forming agents and cross-linking agents are added to prepare an aqueous phase; organic solvents and emulsifiers are used to prepare an oil phase; the aqueous phase and oil phase are mixed evenly, heated to react, centrifuged, washed with water, and dried to obtain porous nanospheres;

[0009] 2) The porous nanospheres were immersed in a pesticide solution for adsorption, and a coating solution including an organotin catalyst and carboxyl-terminated polylactic acid was added. After centrifugation and drying, the drug-loaded nanospheres were obtained.

[0010] In step 1), the method for preparing phenolic lignin is well known in the art. In a specific embodiment of the present invention, phenolic lignin is prepared by a method including the following steps: adding alkali lignin, phenol, and a catalyst to water, and heating to reflux to react and obtain phenolic lignin. The reaction time in the method for preparing phenolic lignin is 0.5-2 hours. After the reflux reaction is completed, the following post-treatment operations are also included: cooling, acid precipitation, filtration, washing, and drying. The alkali lignin is prepared by a conventional alkali dissolution and acid precipitation method, with a weight-average molecular weight of 8000-10000. The catalyst is selected from one or a combination of two of sodium hydroxide and potassium hydroxide. The mass ratio of alkali lignin, phenol, catalyst, and water is 100:25-55:5-10:300-500.

[0011] In step 1), the mass ratio of phenolic lignin, water, porogen, and crosslinking agent is 100:300-500:30-50:10-15. The pH is adjusted to 10-11 using an alkaline solution with a concentration of 1-2 mol / L. The alkaline solution is selected from one or a combination of two of sodium hydroxide solution and potassium hydroxide solution. The porogen is polyethylene glycol with a number average molecular weight of 1000-2000. The crosslinking agent is selected from one or a combination of two or more of malondialdehyde, succinal, glutaraldehyde, and adipaldehyde.

[0012] In step 1), the organic solvent is selected from one or more combinations of liquid paraffin, cyclohexane, and white oil. The emulsifier is selected from one or more combinations of Span-85, Span-80, Span-60, Tween-80, Tween-60, and OP-10. The mass ratio of the organic solvent to the emulsifier is 100:2-4. When mixing the organic solvent and the emulsifier, the mixture is stirred and emulsified at a speed of 500-1000 r / min for 5-20 min to prepare an oil phase. Preferably, the emulsifier is a mixture of Span-80 and Tween-80 with a mass ratio of 1-3:1.

[0013] In step 1), the volume ratio of the aqueous phase to the oil phase is 1:3-5. The aqueous and oil phases are mixed uniformly by adding the aqueous phase dropwise to the oil phase at a rate of 0.3-0.5 mL / min, while the oil phase is stirred at a speed of 8000-10000 r / min. The reaction conditions are: 6000-9000 r / min, 60-90℃, and 1-3 h. The water washing is performed at 30-60℃ for 30-60 min under ultrasonic power of 30-50 kHz. The drying is performed at 60-80℃ under a vacuum of 0.01-0.09 MPa for 6-12 h.

[0014] In step 2), the pesticide solution includes an antibacterial agent and an organic solvent, with the antibacterial agent content in the pesticide solution being 3-5 wt%. The antibacterial agent is selected from one or more combinations of carbendazim, chlorothalonil, thiophanate-methyl, tebuconazole, and oxadixyl. The organic solvent is selected from one or more combinations of chloroform, acetone, and dichloromethane. The ratio of the porous nanospheres, pesticide solution, and coating solution is 100-180 g: 1 L: 0.05-0.1 L. The impregnation is performed under ultrasonic conditions at a power of 50-150 W for 3-5 hours. The coating solution is a homogeneous mixture of an organotin catalyst, carboxyl-terminated polylactic acid, and a solvent in a mass ratio of 0.01-0.03: 30-40: 100-150. The solvent is selected from one or more combinations of chloroform and dichloromethane. The weight-average molecular weight of the carboxyl-terminated polylactic acid is 30,000-90,000. The organotin catalyst is selected from one or more of dibutyltin oxide, stannous octoate, and dibutyltin dilaurate. Terminal carboxyl polylactic acid acts as a coating agent. The coating solution is added dropwise at a uniform rate over 20-40 minutes to a mixture of porous nanospheres and pesticide solution stirred at 300-500 rpm.

[0015] The coupling agent modified degradation agent is prepared by formulating a mixture of degradation agent, alkylsilane coupling agent and water, adding the mixture to a ball mill jar for ball milling, filtering and drying.

[0016] The degradation promoter, alkylsilane coupling agent, and water are mixed in a mass ratio of 100:3-5:100-200, with a ball milling speed of 300-600 rpm and a ball milling time of 1-3 hours. The alkylsilane coupling agent is selected from one or more combinations of ethyltriethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, and n-propyltriethoxysilane. The degradation promoter is selected from one or more combinations of ferric stearate, cerium stearate, manganese stearate, and N,N-dibutyldithiocarbamate.

[0017] The polylactic acid has a weight-average molecular weight of 100,000 to 150,000.

[0018] The polybutylene adipate terephthalate has a number-average molecular weight of 80,000 to 120,000.

[0019] The lubricant is selected from one or more of erucamide, oleamide, and stearamide.

[0020] This invention also provides a method for preparing the above-mentioned modified polylactic acid biodegradable mulch film for planting red kidney beans, comprising the following steps:

[0021] Drug-loaded nanospheres, coupling agents, modified degradation promoters, lubricants, polylactic acid, and polybutylene adipate terephthalate were mixed evenly, extruded and granulated, and blown into film to obtain a modified polylactic acid biodegradable mulch film for red kidney bean cultivation.

[0022] The extrusion granulation temperature is 120-190℃. The blown film temperature is 155-195℃.

[0023] The mulch film is 8-20 μm thick.

[0024] The degradation rate of the plastic film is 8-15% after 45 days, 25-35% after 60 days, 60-70% after 90 days, and ≥95% after 120 days. The degradation rate is the tensile elongation at break loss (longitudinal) / (%) calculated according to standard GB / T20197-2006. During the seedling stage (approximately 45 days after sowing), the plastic film needs to remain intact to fully exert its heat preservation and moisture retention functions. From the seedling stage to the flowering stage (45-60 days after sowing), the plastic film still needs to maintain a certain degree of integrity to continue maintaining soil temperature and humidity. From the pod formation stage to the grain filling stage (60-90 days after sowing), the plastic film can begin to gradually degrade, gradually weakening its heat preservation effect and preventing excessive soil heat and worsening of root rot in the later stages. During the maturity period, the plastic film is basically degraded 90 to 120 days after sowing. After 120 days, the degradation rate reaches more than 95%. The plastic film is rapidly degraded into fragments and further decomposed by soil microorganisms, which not only does not affect the planting of subsequent crops, but also avoids white pollution.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] I. The mulch film of this invention mainly solves the problems of mechanical properties, degradation time adaptability, and prevention and control of soil-borne diseases in red kidney bean planting by using drug-loaded nanospheres, coupling agents, modified degradation agents, and other methods. On the one hand, porous nanosphere carriers are prepared using phenolic lignin, pore-forming agents, and cross-linking agents via a water-in-oil reverse emulsion method. Pesticides are then loaded onto these carriers, and finally, a composite membrane containing organotin catalysts and terminal carboxyl polylactic acid is coated onto them to obtain pesticide-loaded nanospheres. Because the carrier contains numerous benzene ring structures and abundant pores, it can achieve efficient pesticide loading and inhibit premature pesticide leakage. The composite membrane improves the dispersibility of the lignin-loaded nanospheres while further controlling pesticide release, achieving sustained and stable release of disease-controlling components. On the other hand, because alkali lignin molecules also have numerous active hydroxyl groups, under the high temperature and organotin catalyst during mulch film preparation, they react with terminal carboxyl polylactic acid in the composite membrane and the polymers in the substrate to form a chemically bonded interpenetrating network. When under stress, the stress is uniformly transferred to the entire network of microspheres and the substrate through covalent bonds. Combined with the porous structure of the microspheres, the elastic deformation absorbs tensile and puncture energy, effectively resisting damage caused by windy and gravelly environments and ensuring stable soil moisture.

[0027] II. By adjusting the relative amounts of drug-loaded nanospheres and coupling agent-modified degradation promoters, this invention can precisely control the degradation rate of the mulch film, matching it with the complete growth cycle of red kidney beans, meeting the early temperature and humidity requirements of red kidney beans, and enabling rapid degradation in the later stages. Detailed Implementation

[0028] The present invention will be further described below with reference to specific embodiments, but is not limited to the contents of the specification. Unless otherwise specified, all "parts" mentioned in the embodiments of the present invention are parts by weight. All reagents used are commercially available in the art.

[0029] Alkali lignin with a weight-average molecular weight of 8000 is sourced from Shanghai Ruidong Chemical Group Co., Ltd.

[0030] Alkali lignin has a weight-average molecular weight of 10,000, catalog number 471003, and is sourced from Sigma-Aldrich.

[0031] The PEG has a number-average molecular weight of 2000, catalog number P103723, and is from Aladdin.

[0032] The PEG has a number-average molecular weight of 1000, catalog number P103719, and is from Aladdin.

[0033] The carboxyl-terminated polylactic acid has a weight-average molecular weight of 50,000, model number DG-LH070, and comes from Jinan Daigang Bioengineering Co., Ltd.

[0034] The carboxyl-terminated polylactic acid has a weight-average molecular weight of 90,000, model number DG-LH100, and comes from Jinan Daigang Bioengineering Co., Ltd.

[0035] Polylactic acid with a weight-average molecular weight of 60,000, catalog number P875095, is from Maclean's.

[0036] Polylactic acid with a weight-average molecular weight of 150,000, catalog number P742431, is from Maclean's.

[0037] Polylactic acid with a weight-average molecular weight of 110,000, catalog number P890263, is from Maclean's.

[0038] Polybutylene adipate terephthalate (P909229) has a number-average molecular weight of 120,000 and is sourced from Maclean's.

[0039] Mesoporous silica microspheres M758933, with a particle size of 200nm and a pore size of 2nm, product number M758933, are from McLean.

[0040] Example 1

[0041] 1) Add 100 kg of alkali lignin with a weight average molecular weight of 8000, 55 kg of phenol, and 10 kg of sodium hydroxide to 300 kg of water. Heat to reflux and react for 1 hour. Cool to room temperature, add 2 mol / L hydrochloric acid to precipitate until no precipitate forms, filter, wash with water until neutral, and dry to obtain phenolic lignin. Add 100 kg of phenolic lignin to 300 kg of water, adjust the pH to 11 with 1 mol / L sodium hydroxide solution, stir until the phenolic lignin is completely dissolved, and add 50 kg of pore-forming agent PEG. P103723 and 15 kg of crosslinking agent glutaraldehyde were mixed to obtain an aqueous phase. 100 kg of liquid paraffin and 4 kg of emulsifier (a mixture of Span-80 and Tween-80 in a 1:1 mass ratio) were stirred at 1000 r / min for 10 min to prepare an oil phase. 1 L of the aqueous phase was added dropwise at 0.5 mL / min to 5 L of the oil phase, which was stirred at 9000 r / min. The mixture was heated to 90 °C and reacted for 1 h. After centrifugation, the resulting precipitate was ultrasonically washed with water at 50 kHz and 60 °C for 30 min. Finally, it was dried under vacuum of 0.03 MPa and 60 °C for 12 h to obtain porous nanospheres.

[0042] 2) Prepare a 5wt% pesticide solution using tebuconazole and chloroform; prepare a coating solution using stannous octoate, carboxyl-terminated polylactic acid DG-LH070, and chloroform at a mass ratio of 0.03:40:100; immerse porous nanospheres in the pesticide solution at a ratio of 100g:1L under ultrasonic conditions at 50W for 5h, stop the ultrasonication, stir at 500rpm, and simultaneously add 0.1L of coating solution dropwise over 40min. Centrifuge the solution, and dry the resulting precipitate under vacuum of 0.03MPa and 60℃ for 12h to obtain drug-loaded nanospheres.

[0043] 3) Iron stearate, ethyltriethoxysilane and water are mixed in a mass ratio of 100:5:200 to prepare a mixture. The mixture is added to a ball mill jar and ball milled at 600 rpm for 1 hour. The mixture is then filtered and dried to obtain the coupling agent modified degradation promoter.

[0044] 4) Add 25 kg of drug-loaded nanospheres, 6 kg of coupling agent, modified degradation agent, 3 kg of erucamide, 60 kg of polylactic acid P742431, and 40 kg of polybutylene adipate terephthalate P909229 to a high-speed mixer and mix evenly. Extrude and granulate, blow film to obtain a modified polylactic acid biodegradable mulch film for red kidney bean planting with a film thickness of 10 μm and a film width of 70 cm.

[0045] The extrusion granulation temperatures are as follows: Zone 1: 130℃, Zone 2: 160℃, Zone 3: 170℃, Zone 4: 180℃, Zone 5: 180℃, Zone 6: 180℃, Zone 7: 180℃, Zone 8: 180℃, Die head: 180℃, Screw speed: 200rpm.

[0046] The blown film temperature is: Zone 1: 155℃, Zone 2: 165℃, Zone 3: 170℃, Zone 4: 170℃, Zone 5: 170℃; the main screw speed is 150rpm; the traction speed is 30m / min; and the blow-up ratio is 3.

[0047] Example 2

[0048] The rest is the same as in Example 1, except that in step 1), the amount of porogen PEG P103723 is 30 kg.

[0049] Example 3

[0050] The rest is the same as in Example 1, except that in step 1), the amount of crosslinking agent glutaraldehyde is 10 kg.

[0051] Example 4

[0052] The rest is the same as in Example 1, except that in step 1), malondialdehyde is used instead of glutaraldehyde as the crosslinking agent.

[0053] Example 5

[0054] The rest is the same as in Example 1, except that in step 2), the porous nanospheres are immersed in the pesticide solution for 5 hours under ultrasonic conditions at a ratio of 180g:1L.

[0055] Example 6

[0056] The rest is the same as in Example 1, except that in step 2), the amount of coating solution used is 0.05L.

[0057] Example 7

[0058] The rest is the same as in Example 1, except that in step 4), the amount of drug-loaded nanospheres used is 15 kg.

[0059] Example 8

[0060] The rest is the same as in Example 1, except that in step 4), the coupling agent modified degradation agent is 4 kg.

[0061] Example 9

[0062] The rest is the same as in Example 1, except that in step 4), polylactic acid P742431 is replaced with an equal mass of polylactic acid P890263.

[0063] Example 10

[0064] The rest is the same as in Example 1, except that in step 4), the amount of polylactic acid P742431 used is 40 kg.

[0065] Example 11

[0066] 1) Add 100 kg of alkali lignin with a weight average molecular weight of 8000, 25 kg of phenol, and 5 kg of sodium hydroxide to 300 kg of water. Heat to reflux and react for 1 hour. Cool to room temperature, add 2 mol / L hydrochloric acid to precipitate until no precipitate forms, filter, wash with water until neutral, and dry to obtain phenolic lignin. Add 100 kg of phenolic lignin to 500 kg of water, adjust the pH to 11 with 1 mol / L sodium hydroxide solution, stir until the phenolic lignin is completely dissolved, and add 50 kg of porogen PEG. P103719 and 15 kg of crosslinking agent glutaraldehyde were mixed to obtain an aqueous phase. 100 kg of liquid paraffin and 4 kg of emulsifier (a mixture of Span-80 and Tween-80 in a 1:1 mass ratio) were stirred at 1000 r / min for 10 min to prepare an oil phase. 1 L of the aqueous phase was added dropwise at 0.5 mL / min to 3 L of the oil phase, which was stirred at 9000 r / min. The mixture was heated to 90 °C and reacted for 1 h. After centrifugation, the resulting precipitate was ultrasonically washed with water at 50 kHz and 60 °C for 30 min. Finally, it was dried under vacuum of 0.03 MPa and 60 °C for 12 h to obtain porous nanospheres.

[0067] 2) Prepare a 3wt% pesticide solution using carbendazim and chloroform; prepare a coating solution using dibutyltin oxide, carboxyl-terminated polylactic acid DG-LH070, and chloroform at a mass ratio of 0.03:40:100; immerse porous nanospheres in the pesticide solution at a ratio of 180g:1L under ultrasonic conditions at 50W for 5h, stop the ultrasonication, stir at 500rpm, and simultaneously add 0.1L of coating solution dropwise over 40min. Centrifuge the solution, and dry the resulting precipitate under vacuum of 0.03MPa and 60℃ for 12h to obtain drug-loaded nanospheres.

[0068] 3) Iron stearate, ethyltriethoxysilane and water are mixed in a mass ratio of 100:5:200 to prepare a mixture. The mixture is added to a ball mill jar and ball milled at 600 rpm for 1 hour. The mixture is then filtered and dried to obtain the coupling agent modified degradation promoter.

[0069] 4) Add 25 kg of drug-loaded nanospheres, 6 kg of coupling agent, modified degradation agent, 3 kg of erucamide, 40 kg of polylactic acid P742431, and 60 kg of polybutylene adipate terephthalate P909229 to a high-speed mixer and mix evenly. Extrude and granulate, blow film to obtain a modified polylactic acid biodegradable mulch film for red kidney bean planting with a film thickness of 10.2 μm and a film width of 70 cm.

[0070] The extrusion granulation temperatures are as follows: Zone 1: 130℃, Zone 2: 160℃, Zone 3: 170℃, Zone 4: 180℃, Zone 5: 180℃, Zone 6: 180℃, Zone 7: 180℃, Zone 8: 180℃, Die head: 180℃, Screw speed: 200rpm.

[0071] The blown film temperature is: Zone 1: 155℃, Zone 2: 165℃, Zone 3: 170℃, Zone 4: 170℃, Zone 5: 170℃; the main screw speed is 150rpm; the traction speed is 30m / min; and the blow-up ratio is 3.

[0072] Comparative Example 1

[0073] The rest is the same as in Example 1, except that in step 2), polylactic acid P875095 of equal mass is used to replace carboxyl-terminated polylactic acid DG-LH070.

[0074] Comparative Example 2

[0075] The rest is the same as in Example 1, except that in step 2), stannous octoate is not added to the coating solution.

[0076] Comparative Example 3

[0077] The rest is the same as in Example 1, except that step 4) does not contain a coupling agent modified degradation agent.

[0078] Comparative Example 4

[0079] 1) Prepare a 5wt% pesticide solution using tebuconazole and chloroform; prepare a coating solution using stannous octoate, carboxyl-terminated polylactic acid DG-LH070, and chloroform at a mass ratio of 0.03:40:100; immerse mesoporous silica microspheres M758933 in the pesticide solution at a ratio of 180g:1L under ultrasonic conditions at 50W for 5h, stop the ultrasonication, stir at 500rpm, and simultaneously add 0.1L of coating solution dropwise over 40min. After the addition is complete, centrifuge the solution, and dry the resulting precipitate under vacuum of 0.03MPa and 60℃ for 12h to obtain drug-loaded nanospheres.

[0080] 3) Iron stearate, ethyltriethoxysilane and water are mixed in a mass ratio of 100:5:200 to prepare a mixture. The mixture is added to a ball mill jar and ball milled at 600 rpm for 1 hour. The mixture is then filtered and dried to obtain the coupling agent modified degradation promoter.

[0081] 4) Add 25 kg of drug-loaded nanospheres, 6 kg of coupling agent, modified degradation agent, 3 kg of erucamide, 60 kg of polylactic acid P742431, and 40 kg of polybutylene adipate terephthalate P909229 to a high-speed mixer and mix evenly. Extrude and granulate, blow film to obtain a modified polylactic acid biodegradable mulch film for red kidney bean planting with a film thickness of 10 μm and a film width of 70 cm.

[0082] The extrusion granulation temperatures are as follows: Zone 1: 130℃, Zone 2: 160℃, Zone 3: 170℃, Zone 4: 180℃, Zone 5: 180℃, Zone 6: 180℃, Zone 7: 180℃, Zone 8: 180℃, Die head: 180℃, Screw speed: 200rpm.

[0083] The blown film temperatures are: Zone 1: 155℃, Zone 2: 165℃, Zone 3: 170℃, Zone 4: 170℃, Zone 5: 170℃; main screw speed: 150rpm; traction speed: 30m / min; blow-up ratio: 3. The difference is that step 1) is omitted, and mesoporous silica microspheres are used instead of self-made porous nanospheres.

[0084] The mulch films prepared in the above embodiments and comparative examples were subjected to the following performance tests:

[0085] 1. Tensile strength: Tested according to standard GB / T 1040-2006.

[0086] 2. Right-angle tear strength: Tested according to standard QB / T 1130-1991, Test Method for Right-Angle Tear Performance of Plastics.

[0087] 3. Photodegradation: Natural exposure test was conducted according to 6.3 of standard GB / T20197-2006. The tensile elongation at break was tested at 0 days, 45 days, 60 days, 90 days and 120 days according to standard GB / T35795-2017. The tensile elongation at break loss rate (longitudinal) / (%) relative to day 0 was calculated. The value of the tensile elongation at break loss rate (longitudinal) / (%) was recorded as the degradation rate.

[0088] Tensile elongation at break loss L = (L0 - L') / L0 × 100%

[0089] L - Elongation at break loss, %; L0 - Elongation at break before natural exposure, %; L´ - Elongation at break after natural exposure, %.

[0090] 4. Planting Experiment: Liyuanping Village, Kelan County. The experimental sites were red kidney bean replanting land for three consecutive years and corn & wheat rotation land for two consecutive years. Soil fertility in the 0-20cm depth of both plots was as follows: organic matter 12-13g / kg, total nitrogen 0.9-1.0g / kg, total phosphorus 0.75-0.90g / kg, total potassium 18.5-19.5g / kg, available phosphorus 14.0-15.5mg / kg, and available potassium 127-129mg / kg.

[0091] Land preparation: 1-3 days before sowing, spread 1000 kg / mu of well-rotted cow and sheep manure, deep plow and harrow to a depth of 15-18 cm, and level the land.

[0092] Sowing under film: Sow British red kidney beans under film on May 15th, with a sowing depth of 4-5cm, row spacing of 40cm, plant spacing of 25cm, and 2-3 plants per hole, with a film spacing of 45cm.

[0093] Management: The experiment was conducted under natural rainfall conditions, without supplemental irrigation, topdressing, or pesticide application. After emergence, conventional field cultivation and weeding management were carried out.

[0094] Harvest: September 15th.

[0095] After harvesting, randomly select 100 fruits from the harvested fruits in the same type of mulched plots as a group, weigh them, and randomly select the average value of 5 groups as the weight of 100 fruits.

[0096] Among them, the corn and wheat fields were not covered with plastic film for two consecutive years after crop rotation, and the weight of 100 grains after harvest was 48g.

[0097] Table 1 Performance Test Results

[0098]

[0099] Table 2 Degradation rate performance test results

[0100]

[0101] As shown in Table 1, the drug-loaded nanospheres endow the mulch film with excellent mechanical properties. The longitudinal tensile strength is in the range of 45.1-50.8 MPa, and the longitudinal right-angle tear strength reaches 162.9-179.3 kN / m. The longitudinal right-angle tear strength of Example 1 reaches a peak of 179.3 kN / m, which is significantly better than that of Comparative Examples 1-4. This indicates that under the action of organotin catalyst, the carboxyl-terminated polylactic acid forms an interpenetrating network with the hydroxyl groups on the surface of the porous nanospheres and the polyester molecules of the mulch film substrate. Under stress, the stress can be uniformly transferred through covalent bonds. Combined with the porous structure of the lignin microspheres, energy absorption is achieved, which can greatly improve the tensile and tear resistance of the mulch film.

[0102] The 100-seed weight test results show that the 100-seed weight of red kidney beans in continuously cropped fields corresponding to this invention is 51.0-63.9g, which is higher than the 48g of 100-seed weight in fields with continuous corn and wheat rotation without mulching, and significantly higher than the 44.2-57.7g of comparative examples 1-4. This indicates that the porous structure of the drug-loaded nanospheres and the dual controlled-release effect of the polylactic acid coating enable the sustained and stable release of pesticides, preventing soil-borne diseases in continuously cropped fields and ensuring the normal growth and development of red kidney beans. Analysis of the 100-seed weight test results and the degradation rate test results shows that there is no positive or negative correlation between 100-seed weight and degradation rate, indicating that 100-seed weight is the result of the combined effect of mulch degradation rate and sustained and stable pesticide release.

[0103] As shown in Table 2, the degradation performance of the mulch film prepared in this invention exhibits a degradation rate of 8.2-14.5% at 45 days, 25.1-33.5% at 60 days, 60.2-69.5% at 90 days, and ≥95.5% at 120 days, perfectly matching the 90-120 day growth cycle requirements of red kidney beans. The low degradation rate of the mulch film during the early growth stage (first 60 days) allows it to maintain structural integrity and fully utilize its heat preservation and moisture retention functions. During the later growth stage (60-120 days), the degradation rate accelerates, allowing for rapid degradation after the red kidney beans mature and are harvested. The 120-day degradation rate meets the standard, preventing soil residue from affecting subsequent crop planting. In contrast, Comparative Examples 1, 2, and 4 show excessively rapid degradation rates in the early stages, easily leading to premature damage and loss of moisture retention. Comparative Example 3, lacking a coupling agent to modify and promote degradation, has a degradation rate of only 32.7% at 120 days, resulting in significant mulch film residue and failing to meet farmland environmental protection requirements.

[0104] The above detailed description is a specific description of one of the feasible embodiments of the present invention. This embodiment is not intended to limit the patent scope of the present invention. All equivalent implementations or modifications that do not depart from the present invention should be included within the scope of the technical solution of the present invention.

Claims

1. A modified polylactic acid biodegradable mulch film for planting red kidney beans, characterized in that, The raw materials comprise the following parts by weight: 40-60 parts polylactic acid, 40-60 parts polybutylene adipate terephthalate, 15-25 parts drug-loaded nanospheres, 4-6 parts coupling agent, modifier, and degradation promoter, and 1-3 parts lubricant; the drug-loaded nanospheres are prepared by a method comprising the following steps: 1) Phenolic lignin is added to water, the pH is adjusted, and pore-forming agents and cross-linking agents are added to prepare an aqueous phase; organic solvents and emulsifiers are used to prepare an oil phase; the aqueous phase and oil phase are mixed evenly, heated to react, centrifuged, washed with water, and dried to obtain porous nanospheres; 2) The porous nanospheres were immersed in a pesticide solution for adsorption, and then a coating solution containing organotin catalyst and carboxyl-terminated polylactic acid was added. After centrifugation and drying, the drug-loaded nanospheres were obtained.

2. The modified polylactic acid biodegradable mulch film for red kidney bean cultivation according to claim 1, characterized in that, In step 1), the method for preparing phenolic lignin is as follows: alkali lignin, phenol, and catalyst are added to water, and the mixture is heated to reflux to react and obtain phenolic lignin; the mass ratio of alkali lignin, phenol, catalyst, and water is 100:25-55:5-10:300-500; the alkali lignin is prepared by conventional alkali dissolution and acid precipitation method, and the weight average molecular weight is 8000-10000; the catalyst is selected from one or a combination of two of sodium hydroxide and potassium hydroxide.

3. The modified polylactic acid biodegradable mulch film for red kidney bean cultivation according to claim 1, characterized in that, In step 1), the mass ratio of phenolic lignin, water, porogen, and crosslinking agent is 100:300-500:30-50:10-15; the pH adjustment is performed by adjusting the pH to 10-11 with an alkaline solution of concentration of 1-2 mol / L; the porogen is polyethylene glycol with a number average molecular weight of 1000-2000; and the crosslinking agent is selected from one or more combinations of malondialdehyde, succinate, glutaraldehyde, and adipaldehyde.

4. The modified polylactic acid biodegradable mulch film for planting red kidney beans according to claim 1, characterized in that, In step 1), the organic solvent is selected from one or more of liquid paraffin, cyclohexane, and white oil; the emulsifier is selected from one or more of Span-85, Span-80, Span-60, Tween-80, Tween-60, and OP-10.

5. The modified polylactic acid biodegradable mulch film for planting red kidney beans according to claim 1, characterized in that, In step 1), the mass ratio of the organic solvent to the emulsifier is 100:2-4; the volume ratio of the aqueous phase to the oil phase is 1:3-5.

6. The modified polylactic acid biodegradable mulch film for planting red kidney beans according to claim 1, characterized in that, In step 2), the ratio of the porous nanospheres, pesticide solution, and coating liquid is 100-180g:1L:0.05-0.1L; the pesticide solution includes an antibacterial agent and an organic solvent, and the antibacterial agent content in the pesticide solution is 3-5wt%; the antibacterial agent is selected from one or more combinations of carbendazim, chlorothalonil, thiophanate-methyl, tebuconazole, and hymexazol.

7. The modified polylactic acid biodegradable mulch film for planting red kidney beans according to claim 1, characterized in that, In step 2), the coating solution is a mixture of organotin catalyst, carboxyl-terminated polylactic acid, and solvent in a mass ratio of 0.01-0.03:30-40:100-150; the carboxyl-terminated polylactic acid has a weight-average molecular weight of 30,000-90,000; the organotin catalyst is selected from one or more of dibutyltin oxide, stannous octoate, and dibutyltin dilaurate.

8. The modified polylactic acid biodegradable mulch film for planting red kidney beans according to claim 1, characterized in that, The coupling agent modified degradation accelerator is prepared by mixing a degradation accelerator, an alkylsilane coupling agent, and water, adding the mixture to a ball mill jar for ball milling, followed by filtration and drying. The mass ratio of the degradation accelerator, alkylsilane coupling agent, and water is 100:3-5:100-200, the ball milling speed is 300-600 rpm, and the ball milling time is 1-3 hours. The alkylsilane coupling agent is selected from one or more combinations of ethyltriethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, and n-propyltriethoxysilane. The degradation accelerator is selected from one or more combinations of ferric stearate, cerium stearate, manganese stearate, and N,N-dibutyldithiocarbamate.

9. The modified polylactic acid biodegradable mulch film for planting red kidney beans according to claim 1, characterized in that, The polylactic acid has a weight-average molecular weight of 100,000-150,000; the polybutylene adipate terephthalate has a number-average molecular weight of 80,000-120,000.

10. The method for preparing the modified polylactic acid biodegradable mulch film for red kidney bean cultivation according to any one of claims 1-9, characterized in that, Includes the following steps: Drug-loaded nanospheres, coupling agents, modified degradation promoters, lubricants, polylactic acid, and polybutylene adipate terephthalate were mixed evenly, extruded and granulated, and blown into film to obtain a modified polylactic acid biodegradable mulch film for red kidney bean cultivation.

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