A biodegradable plastic mulch film for agriculture and its preparation method

Abstract

This invention discloses a biodegradable mulch film plastic for agriculture and its preparation method, belonging to the technical field of agricultural mulch film materials. The preparation method includes the following steps: boron nitride-terminated isocyanate polybutylene succinate, modified polylactic acid, and stannous octoate are mixed in a weight ratio of 30-40:100:1.3-1.4, and then sequentially blended at 140-150℃ for 60-90 seconds, 160-165℃ for 50-670 seconds, and 180℃ for 30-40 seconds, followed by extrusion granulation to obtain the biodegradable mulch film plastic. This invention, through the melt blending and crosslinking of boron nitride-terminated isocyanate polybutylene succinate and modified polylactic acid, not only prepares a biodegradable mulch film plastic with high heat resistance, high mechanical strength, and good water vapor barrier properties, but also improves melt strength by reducing the melt index, which is beneficial for blown film production.

Description

Technical Field

[0001] This invention belongs to the field of agricultural mulch film materials technology, specifically relating to a biodegradable mulch film plastic for agriculture and its preparation method. Background Technology

[0002] Agricultural mulch film is one of the core technologies and equipment that are indispensable in modern agricultural production. After decades of development, it has been widely used in the planting process of various crops such as grains, vegetables, and cash crops, becoming a key means to improve agricultural production efficiency and ensure the yield and quality of agricultural products. The widespread use of mulch film in agricultural production is essentially to cope with the limitations of the natural environment, optimize crop growth conditions, solve the pain points in traditional planting, and ultimately achieve the goals of improving quality, increasing yield, and increasing efficiency. Its core necessity is mainly reflected in the following aspects: (1) Regulating soil temperature and humidity to meet the needs of crop growth: Soil temperature and humidity are key environmental factors that affect crop seed germination, root growth, and nutrient absorption, and one of the core functions of mulch film is to accurately regulate soil temperature and humidity. In terms of temperature regulation, mulch film can block the loss of soil heat, absorb solar radiation energy and conduct it to the soil surface, significantly increasing the soil temperature. In terms of humidity regulation, mulch film can effectively reduce soil moisture evaporation, lock the deep soil moisture in the cultivated layer, and improve the soil moisture retention capacity. (2) Suppressing weed growth and reducing field management costs: Weeds compete with crops for water, nutrients, light and growing space, which is one of the important factors affecting crop yield. Traditional manual weeding is not only time-consuming and labor-intensive, but also increases production costs. Mulching can effectively suppress weed growth and reduce weed damage from the source. Mulching blocks sunlight from reaching the soil surface, preventing weed seeds from photosynthesizing, thereby inhibiting weed germination and growth. For weeds that have already germinated, the physical covering effect of mulching will hinder their growth, causing them to gradually wither and die. (3) Reducing the occurrence of diseases and pests and ensuring crop growth safety: Mulching can effectively reduce the occurrence of diseases and pests and reduce the degree of damage to crops through physical isolation and environmental regulation. On the one hand, mulching can block pathogens and pests in the soil from contacting crops, reducing soil-borne diseases and underground pests. On the other hand, the effect of mulching in increasing soil temperature and reducing soil moisture can inhibit the reproduction and spread of pathogens, creating an environment unfavorable to the survival of diseases and pests. In addition, some functional mulch films can reflect ultraviolet rays, repel piercing-sucking pests such as aphids and whiteflies, reduce the spread of viral diseases, and further ensure the safety of crop growth. (4) Improve crop yield and quality and increase planting income: Combining the above effects, mulch film can create a stable and suitable environment for crop growth, promote crop growth and development, and thus significantly improve crop yield and quality. Although mulch film technology has brought huge economic benefits, it has also brought significant environmental pollution. The main components of agricultural films used in my country's agricultural production are linear low-density polyethylene or low-density polyethylene. The molecular structure is very stable, and it can exist for a long time under natural conditions and is difficult to degrade. Even during the degradation process, toxic substances will still be dissolved. The large accumulation in the soil leads to changes in the soil physical structure, hinders the downward transport of soil moisture and nutrients, reduces soil porosity and permeability, affects the growth and decline of the number of microorganisms in the soil and the formation of normal soil structure, resulting in a significant reduction in the quality of cultivated land.Furthermore, residual plastic film forms a barrier in the soil, affecting mechanical sowing and seedling emergence, thus reducing crop yields and posing a threat to China's agricultural environment. In recent decades, substandard polyethylene plastic film products have long existed in my country's use of plastic film mulching technology. Coupled with the low level of attention paid to plastic film recycling in previous years and the lack of effective recycling measures, plastic film residue has become an environmental problem in farmland in many areas. At the same time, farmers' lack of comprehensive understanding of the long-term and large-scale accumulation of plastic film in the soil, weak awareness of recycling, and the widespread and excessive use of plastic film mulching further exacerbate the risk of residual pollution.

[0003] Therefore, developing biodegradable mulch film materials to replace traditional non-degradable or difficult-to-degrade mulch film materials is of great significance for solving a series of ecological pollution problems caused by residual mulch film.

[0004] Biodegradable polyester materials are plastic materials with good degradation properties, and polylactic acid (PLA) is one of the polyester materials with relatively good degradation properties. However, pure PLA exhibits a hard and brittle characteristic due to its high strength, resulting in poor toughness and easy breakage. Its heat resistance is poor, with a heat distortion temperature of around 60℃, making it prone to losing its protective coating function in high-temperature summer environments. Furthermore, PLA has poor moisture barrier properties and low melt strength, making blow molding difficult. Therefore, modifying PLA to enable its application in agricultural mulch films has significant ecological value. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention utilizes the melt blending and crosslinking of boron nitride-terminated isocyanate-polybutylene succinate and modified polylactic acid. This process not only yields a biodegradable mulch film plastic with high heat resistance, high mechanical strength, and good water vapor barrier properties, but also improves melt strength by reducing the melt index, which is beneficial for blown film production. Therefore, this invention solves the technical problems mentioned in the background art. Specifically, the technical solution of this invention includes the following: One objective of this invention is to provide a method for preparing a biodegradable plastic mulch film for agriculture, the method comprising the following steps: Boron nitride-terminated isocyanate polybutylene succinate, modified polylactic acid, and stannous octoate are mixed in a weight ratio of 30~40:100:1.3~1.4, and then sequentially blended at 140℃~150℃ for 60s~90s, 160℃~165℃ for 50s~670s, and 180℃ for 30s~40s before extrusion granulation to obtain the biodegradable mulch film plastic.

[0006] Furthermore, the preparation method of the boron nitride-terminated isocyanate polybutylene succinate includes the following steps: Modified boron nitride, 1,4-butanediol, succinic acid, tetrabutyl titanate, and dehydrating agent are dispersed in a weight ratio of 4~7:140~170:115:0.8~1:200~240, and then heated to 160℃~170℃ for 2h~2.5h. After that, the temperature is further increased to 230℃~240℃ for 3h to obtain boron nitride-polybutylene succinate. Boron nitride-polybutylene succinate, diisocyanate and tetramethylethylenediamine are mixed in a weight ratio of 1:0.1~0.2:0.003~0.005 and heated to 75℃~80℃ for 30min~40min to obtain the boron nitride-terminated isocyanate polybutylene succinate.

[0007] Furthermore, the method for preparing the modified boron nitride includes the following steps: Boron nitride and hydrogen peroxide were mixed at a ratio of 1g:20mL~30mL and heated to 70℃~80℃ for 1h~2h to obtain the modified boron nitride.

[0008] Furthermore, the boron nitride is hexagonal boron nitride with a particle size of 5 μm.

[0009] Furthermore, the hydrogen peroxide has a mass percentage concentration of 30%.

[0010] Furthermore, the dehydrating agent includes toluene.

[0011] Furthermore, the diisocyanate includes hexamethylene diisocyanate.

[0012] Furthermore, the preparation method of the modified polylactic acid includes the following steps: DL-lactic acid, polyhydroxy alcohol compounds, and stannous octoate were mixed in a ratio of 1 mol: 4.1 mol~4.2 mol: 0.5 g~0.6 g and heated to 130℃~135℃ for 2 h~3 h to obtain the first reactant; The first reactant, DL-lactic acid, and stannous octoate were mixed in a weight ratio of 1:4~4.5:0.025~0.03 and heated to 140℃~150℃ for 3h~4h. Then the temperature was raised to 170℃ and reacted for 7h~8h to obtain the modified polylactic acid.

[0013] Furthermore, the polyhydroxy alcohol compound includes pentaerythritol.

[0014] A second objective of this invention is to provide a biodegradable plastic mulch film.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention uses DL-lactic acid as the raw material for synthesizing polylactic acid (PLA), and adds a polyhydroxy alcohol compound as a branching structure to polymerize and obtain modified PLA. This not only disrupts the regular molecular chain arrangement of PLA, hindering the tight packing between molecular chains and thus reducing crystallinity, thereby improving the toughness of PLA, but also enhances the degree of entanglement between PLA molecules through the branching structure, thereby increasing melt strength and improving blown film stability. Next, polybutylene succinate, which has high heat resistance, is used as a reinforcing phase and melt-blended with the modified PLA to improve its heat resistance. However, it was found that the improvement in heat resistance was limited, and the mechanical strength and moisture barrier properties decreased significantly after melt blending. This may be because the two are incompatible materials; direct physical blending results in interfacial incompatibility, preventing the formation of a cross-linked structure, which is detrimental to improving density. Therefore, isocyanate end-capping of polybutylene succinate (PBS) yields isocyanate-terminated PBS. The reactivity of isocyanate is then used to crosslink with modified polylactic acid (PLA), thereby improving the heat resistance of the PLA material. However, it was found that the modified PLA exhibited poor water vapor barrier properties and even worse mechanical strength. This may be because PBS itself has high toughness, and the modified PLA also exhibits improved toughness; the crosslinking of these two materials results in excessive toughness, making the material prone to softening and loss of mechanical strength. Therefore, boron nitride with a layered structure was used as a mechanical reinforcing material. The layered structure improves the diffusion path of water vapor within the material, thus reducing the permeation rate. Surface modification of boron nitride was performed, followed by in-situ polymerization of PBS to disperse and encapsulate the boron nitride. Finally, isocyanate end-capping was performed to obtain boron nitride-terminated isocyanate-terminated PBS. After the boron nitride-terminated isocyanate polybutylene succinate and modified polylactic acid are melt-blended and crosslinked, the biodegradable mulch film is prepared by blowing film using a blown film machine. Detailed Implementation

[0016] The technical solution of the present invention will be clearly and completely described below through embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Unless otherwise stated, all raw materials and reagents used in this invention are commercially available or can be prepared by known methods.

[0018] Preparation Example 1 The preparation method of modified boron nitride is as follows: 20g of hexagonal boron nitride with a particle size of 5μm and 400mL of 30% hydrogen peroxide were dispersed by ultrasonic power at 400W for 30min. The mixture was then transferred to a reaction vessel, and the rotation speed was controlled at 400r / min. The reaction vessel was then heated to 70℃ and reacted at this temperature for 1h. After the reaction, the mixture was allowed to cool naturally to room temperature and removed. The precipitate was then obtained by centrifugation. The precipitate was washed with deionized water until the pH of the wash water reached neutral. The precipitate was then dried in a vacuum drying oven at 60℃ to remove water, yielding modified boron nitride.

[0019] Preparation Example 2 The preparation method of modified boron nitride is as follows: 20g of hexagonal boron nitride with a particle size of 5μm and 500mL of 30% hydrogen peroxide were dispersed by ultrasonic treatment at 400W for 30min. The mixture was then transferred to a reaction vessel, and the rotation speed was controlled at 400r / min. The reaction vessel was then heated to 70℃ and reacted at this temperature for 1.5h. After the reaction, the mixture was allowed to cool naturally to room temperature and removed. The precipitate was then obtained by centrifugation. The precipitate was washed with deionized water until the pH of the wash water reached neutral. The precipitate was then dried in a vacuum drying oven at 60℃ to remove water, yielding modified boron nitride.

[0020] Preparation Example 3 The preparation method of modified boron nitride is as follows: 20g of hexagonal boron nitride with a particle size of 5μm and 500mL of 30% hydrogen peroxide were dispersed by ultrasonic power at 400W for 30min. The mixture was then transferred to a reaction vessel, and the rotation speed was controlled at 400r / min. The reaction vessel was then heated to 80℃ and reacted at this temperature for 1.5h. After the reaction, the mixture was allowed to cool naturally to room temperature and removed. The precipitate was then obtained by centrifugation. The precipitate was washed with deionized water until the pH of the wash water reached neutral. The precipitate was then dried in a vacuum drying oven at 60℃ to remove water, yielding modified boron nitride.

[0021] Preparation Example 4 The preparation method of modified boron nitride is as follows: 20g of hexagonal boron nitride with a particle size of 5μm and 600mL of 30% (w / w) hydrogen peroxide were dispersed using ultrasonic power at 400W for 30min. The mixture was then transferred to a reaction vessel, and the rotation speed was controlled at 400r / min. The reaction vessel was then heated to 80℃ and reacted at this temperature for 2h. After the reaction, the mixture was allowed to cool naturally to room temperature and removed. The precipitate was then obtained by centrifugation. The precipitate was washed with deionized water until the pH of the wash water reached neutral. The precipitate was then dried in a vacuum drying oven at 60℃ to remove water, yielding modified boron nitride.

[0022] Preparation Example 5 The preparation method of modified boron nitride is as follows: 20g of hexagonal boron nitride with a particle size of 5μm and 600mL of 30% hydrogen peroxide were dispersed by ultrasonic treatment at 400W for 30min. The mixture was then transferred to a reaction vessel, and the rotation speed was controlled at 400r / min. The reaction vessel was then heated to 90℃ and reacted at this temperature for 4h. After the reaction, the mixture was allowed to cool naturally to room temperature and removed. The precipitate was then obtained by centrifugation. The precipitate was washed with deionized water until the pH of the wash water reached neutral. The precipitate was then dried in a vacuum drying oven at 60℃ to remove water, yielding modified boron nitride.

[0023] Preparation Example 6 The preparation method of modified boron nitride is as follows: 20g of hexagonal boron nitride with a particle size of 500nm and 600mL of 30% hydrogen peroxide were dispersed by ultrasonic treatment at 400W for 30min. The mixture was then transferred to a reaction vessel, and the rotation speed was controlled at 400r / min. The reaction vessel was then heated to 80℃ and reacted at this temperature for 2h. After the reaction, the mixture was allowed to cool naturally to room temperature and removed. The precipitate was then obtained by centrifugation. The precipitate was washed with deionized water until the pH of the wash water reached neutral. The precipitate was then dried in a vacuum drying oven at 60℃ to remove water, yielding modified boron nitride.

[0024] Preparation Example 7 The preparation method of modified boron nitride is as follows: 20 g of hexagonal boron nitride with a particle size of 5 μm was added to an ethanol solution prepared by mixing 800 mL of anhydrous ethanol and 200 mL of deionized water, and then dispersed by ultrasonic power of 400 W for 30 min. After dispersion, the mixture was heated to 70 °C and stirred at 300 r / min for 70 min. After the reaction was completed, the precipitate was collected by centrifugation, washed successively with anhydrous ethanol and deionized water, and finally dried in a vacuum drying oven at 60 °C to remove water, thus obtaining modified boron nitride.

[0025] Preparation Example 8 The preparation method of boron nitride-terminated isocyanate polybutylene succinate is as follows: 8g of the modified boron nitride obtained in Preparation Example 1, 280g of 1,4-butanediol, 230g of succinic acid, 1.8g of tetrabutyl titanate, and 400g of toluene were weighed and dispersed evenly using a disperser. Then, under nitrogen protection, the mixture was heated to 160℃ and reacted for 2 hours. During the reaction, water was separated and discharged using a water separator. Toluene was then removed under reduced pressure by maintaining a vacuum of 50 Pa. The vacuum was then maintained at 200 Pa, and the temperature was further increased to 230℃ for 3 hours. After the reaction was complete, nitrogen gas was introduced to restore the pressure to atmospheric pressure, and then the temperature was lowered to 75°C and maintained to obtain boron nitride-polybutylene succinate. Hexamethylene diisocyanate was added to the boron nitride-polybutylene succinate at 0.1 times its weight, and then tetramethylethylenediamine was added at 0.003 times its weight. The mixture was stirred at 200 rpm and reacted at this temperature for 30 minutes. After the reaction was complete, the mixture was allowed to cool naturally to room temperature and discharged to obtain boron nitride-terminated isocyanate-based polybutylene succinate.

[0026] Preparation Example 9 The preparation method of boron nitride-terminated isocyanate polybutylene succinate is as follows: 10g of the modified boron nitride obtained in Preparation Example 2, 300g of 1,4-butanediol, 230g of succinic acid, 1.8g of tetrabutyl titanate, and 440g of toluene were weighed and dispersed evenly using a disperser. Then, under nitrogen protection, the mixture was heated to 165℃ and reacted for 2 hours. During the reaction, water was separated and discharged using a water separator. Toluene was then removed under reduced pressure by maintaining a vacuum of 50 Pa. The vacuum was then maintained at 200 Pa, and the temperature was further increased to 230℃ for 3 hours. After the reaction was complete, nitrogen gas was introduced to restore the pressure to atmospheric pressure, and then the temperature was lowered to 75°C and maintained to obtain boron nitride-polybutylene succinate. Hexamethylene diisocyanate was added to the boron nitride-polybutylene succinate at 0.1 times its weight, and then tetramethylethylenediamine was added at 0.003 times its weight. The mixture was stirred at 200 rpm and reacted at this temperature for 35 minutes. After the reaction was complete, the mixture was allowed to cool naturally to room temperature and discharged to obtain boron nitride-terminated isocyanate-based polybutylene succinate.

[0027] Preparation Example 10 The preparation method of boron nitride-terminated isocyanate polybutylene succinate is as follows: 12g of the modified boron nitride obtained in Preparation Example 3, 320g of 1,4-butanediol, 230g of succinic acid, 1.9g of tetrabutyl titanate, and 460g of toluene were weighed and dispersed evenly using a disperser. Then, under nitrogen protection, the mixture was heated to 165℃ and reacted for 2.5h. During the reaction, water was separated and discharged using a water separator. Toluene was then removed under reduced pressure by maintaining a vacuum of 50Pa. Next, the vacuum was maintained at 100Pa, and the temperature was further increased to 235℃ for 3h. After the reaction was complete, nitrogen gas was introduced to restore the pressure to atmospheric pressure, and then the temperature was lowered to 80°C and maintained to obtain boron nitride-polybutylene succinate. Hexamethylene diisocyanate was added to the boron nitride-polybutylene succinate at 0.15 times its weight, and then tetramethylethylenediamine was added at 0.004 times its weight. The mixture was stirred at 200 rpm and reacted at this temperature for 35 minutes. After the reaction was complete, the mixture was naturally cooled to room temperature and discharged to obtain boron nitride-terminated isocyanate-based polybutylene succinate.

[0028] Preparation Example 11 The preparation method of boron nitride-terminated isocyanate polybutylene succinate is as follows: 14g of the modified boron nitride obtained in Preparation Example 4, 340g of 1,4-butanediol, 230g of succinic acid, 2g of tetrabutyl titanate, and 480g of toluene were weighed and dispersed evenly using a disperser. Then, under nitrogen protection, the mixture was heated to 170℃ and reacted for 2.5h. During the reaction, water was separated and discharged using a water separator. Toluene was then removed under reduced pressure by maintaining a vacuum of 50Pa. Next, the vacuum was maintained at 100Pa, and the temperature was further increased to 240℃ for 3h. After the reaction, nitrogen gas was introduced to restore the pressure to atmospheric pressure, and then the temperature was lowered to 80°C and held to obtain boron nitride-polybutylene succinate. Hexamethylene diisocyanate was added to the boron nitride-polybutylene succinate at 0.2 times its weight, and then tetramethylethylenediamine was added at 0.005 times its weight. The mixture was stirred at 200 rpm and reacted at this temperature for 40 minutes. After the reaction, the mixture was allowed to cool naturally to room temperature and discharged to obtain boron nitride-terminated isocyanate-coated polybutylene succinate.

[0029] Preparation Example 12 The preparation method of boron nitride-terminated isocyanate polybutylene succinate is as follows: The modified boron nitride in Preparation Example 11 was replaced with the modified boron nitride obtained in Preparation Example 5, and the rest of the preparation process was the same as in Preparation Example 11.

[0030] Preparation Example 13 The preparation method of boron nitride-terminated isocyanate polybutylene succinate is as follows: The modified boron nitride in Preparation Example 11 was replaced with the modified boron nitride obtained in Preparation Example 6, and the rest of the preparation process was the same as in Preparation Example 11.

[0031] Preparation Example 14 The preparation method of boron nitride-terminated isocyanate polybutylene succinate is as follows: The modified boron nitride in Preparation Example 11 was replaced with the modified boron nitride obtained in Preparation Example 7, and the rest of the preparation process was the same as in Preparation Example 11.

[0032] Preparation Example 15 The preparation method of boron nitride-terminated isocyanate polybutylene succinate is as follows: The amount of succinic acid in Preparation Example 11 was increased to 340g, while the rest of the preparation process remained the same as in Preparation Example 11.

[0033] Preparation Example 16 The preparation method of boron nitride-terminated isocyanate polybutylene succinate is as follows: In Preparation Example 11, hexamethylene diisocyanate was replaced with terephthalic diisocyanate, and the rest of the preparation process was the same as in Preparation Example 11.

[0034] Preparation Example 17 The preparation method of polybutylene succinate is as follows: 340g of 1,4-butanediol, 230g of succinic acid, 2g of tetrabutyl titanate, and 480g of toluene were weighed and dispersed evenly using a disperser. Then, under nitrogen protection, the mixture was heated to 170℃ and reacted for 2.5h. During the reaction, water was separated and discharged using a water separator. Toluene was then removed under reduced pressure by maintaining a vacuum of 50Pa. The vacuum was then maintained at 100Pa, and the temperature was further increased to 240℃ for 3h. After the reaction was completed, nitrogen was introduced to restore the pressure to atmospheric level, and the mixture was then allowed to cool naturally to room temperature before being discharged to obtain polybutylene succinate.

[0035] Preparation Example 18 The preparation method of boron nitride-terminated isocyanate polybutylene succinate is as follows: 1,4-Butanediol, 230g of succinic acid, 2g of tetrabutyl titanate, and 480g of toluene were weighed and dispersed evenly using a disperser. Then, under nitrogen protection, the mixture was heated to 170℃ and reacted for 2.5h. During the reaction, water was separated and discharged using a water separator. Toluene was then removed under reduced pressure by controlling the vacuum at 50Pa. The vacuum was then controlled at 100Pa, and the temperature was further increased to 240℃ for 3h. After the reaction, nitrogen was introduced to restore the pressure to atmospheric pressure, and then the temperature was lowered to 80℃ and maintained to obtain polybutylene succinate. Hexamethylene diisocyanate was added to the polybutylene succinate at 0.2 times its weight, and tetramethylethylenediamine was added at 0.005 times its weight. The mixture was stirred at 200r / min and then reacted at this temperature for 40min. After the reaction is complete, the product is naturally cooled to room temperature and discharged to obtain polybutylene succinate with terminal isocyanate.

[0036] Preparation Example 19 The preparation methods of modified polylactic acid include the following: DL-lactic acid was first dehydrated at 80℃ and under vacuum of 100 Pa. 1 mol of DL-lactic acid, 4.1 mol of pentaerythritol, and 0.5 g of stannous octoate were weighed and mixed. Nitrogen gas was introduced as a protective gas, and the mixture was heated to 130℃ and stirred for 2 hours to obtain the first reactant. DL-lactic acid was then added back in at four times its weight and mixed with the first reactant. Stannous octoate was then added back in at 0.025 times its weight and mixed with the first reactant. The mixture was then heated to 140℃ under nitrogen protection and reacted for 3 hours. Water generated during the reaction was discharged through a water separator. The temperature was then raised to 170℃, and a vacuum of 50 Pa was applied. The reaction was continued for 7 hours. After the reaction was completed, the mixture was restored to atmospheric pressure and allowed to cool naturally to room temperature before being discharged to obtain modified polylactic acid.

[0037] Preparation Example 20 The preparation methods of modified polylactic acid include the following: DL-lactic acid was first dehydrated at 80℃ and under a vacuum of 100 Pa. 1 mol of DL-lactic acid, 4.1 mol of pentaerythritol, and 0.5 g of stannous octoate were weighed and mixed. Nitrogen gas was introduced as a protective gas, and the mixture was heated to 130℃ and stirred for 3 hours to obtain the first reactant. DL-lactic acid was then added back in at 4.2 times its weight and mixed with the first reactant. Stannous octoate was then added back in at 0.025 times its weight and mixed with the first reactant. The mixture was then heated to 140℃ under nitrogen protection for 3.5 hours. Water generated during the reaction was discharged through a water separator. The temperature was then raised to 170℃, and a vacuum of 50 Pa was applied. The reaction was continued for 7 hours. After the reaction was completed, the mixture was restored to atmospheric pressure and allowed to cool naturally to room temperature before being discharged to obtain modified polylactic acid.

[0038] Preparation Example 21 The preparation methods of modified polylactic acid include the following: DL-lactic acid was first dehydrated at 80℃ and under vacuum of 100 Pa. 1 mol of DL-lactic acid, 4.2 mol of pentaerythritol, and 0.6 g of stannous octoate were weighed and mixed. Nitrogen gas was introduced as a protective gas, and the mixture was heated to 135℃ and stirred for 3 hours to obtain the first reactant. DL-lactic acid was then added back in at 4.4 times its weight and mixed with the first reactant. Stannous octoate was then added back in at 0.028 times its weight and mixed with the first reactant. The mixture was then heated to 145℃ under nitrogen protection for 3.5 hours. Water generated during the reaction was discharged through a water separator. The temperature was then raised to 170℃, and a vacuum of 30 Pa was applied. The reaction was continued for 8 hours. After the reaction was completed, the mixture was restored to atmospheric pressure and allowed to cool naturally to room temperature before being discharged to obtain modified polylactic acid.

[0039] Preparation Example 22 The preparation methods of modified polylactic acid include the following: DL-lactic acid was first dehydrated at 80℃ and under a vacuum of 100 Pa. 1 mol of DL-lactic acid, 4.2 mol of pentaerythritol, and 0.6 g of stannous octoate were weighed and mixed. Nitrogen gas was introduced as a protective gas, and the mixture was heated to 135℃ and stirred for 3 hours to obtain the first reactant. DL-lactic acid was then added back in at 4.5 times its weight and mixed with the first reactant. Stannous octoate was then added back in at 0.03 times its weight and mixed with the first reactant. The mixture was then heated to 150℃ under nitrogen protection and reacted for 4 hours. Water generated during the reaction was discharged through a water separator. The temperature was then raised to 170℃, and a vacuum of 30 Pa was applied. The reaction was continued for 8 hours. After the reaction was completed, the mixture was restored to atmospheric pressure and allowed to cool naturally to room temperature before being discharged to obtain modified polylactic acid.

[0040] Preparation Example 23 In Preparation Example 22, pentaerythritol was replaced with glycerol, and the rest of the preparation process was the same as in Preparation Example 22.

[0041] Preparation Example 24 In Preparation Example 22, pentaerythritol was replaced with pentaerythritol glycidyl ether, and the rest of the preparation process was the same as in Preparation Example 22.

[0042] Preparation Example 25 The amount of pentaerythritol used in Preparation Example 22 was increased to 8 mol, while the rest of the preparation process remained the same as in Preparation Example 22.

[0043] Example 1 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 300g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 8, 1kg of modified polylactic acid obtained in Preparation Example 19, and 1.3g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 140℃ for 60s, then blended at 160℃ for 50s, and finally blended at 180℃ for 30s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0044] Example 2 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 340g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 9, 1kg of modified polylactic acid obtained in Preparation Example 20, and 1.3g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 145℃ for 70s, then blended at 160℃ for 60s, and finally blended at 180℃ for 30s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0045] Example 3 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 380g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 10, 1kg of modified polylactic acid obtained in Preparation Example 21, and 1.4g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 145℃ for 80s, then blended at 165℃ for 60s, and finally blended at 180℃ for 40s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0046] Example 4 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 400g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 11, 1kg of modified polylactic acid obtained in Preparation Example 22, and 1.4g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 150℃ for 90s, then blended at 165℃ for 70s, and finally blended at 180℃ for 40s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0047] Comparative Example 1 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 400g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 12, 1kg of modified polylactic acid obtained in Preparation Example 22, and 1.4g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 150℃ for 90s, then blended at 165℃ for 70s, and finally blended at 180℃ for 40s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0048] Comparative Example 2 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 400g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 13, 1kg of modified polylactic acid obtained in Preparation Example 22, and 1.4g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 150℃ for 90s, then blended at 165℃ for 70s, and finally blended at 180℃ for 40s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0049] Comparative Example 3 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 400g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 14, 1kg of modified polylactic acid obtained in Preparation Example 22, and 1.4g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 150℃ for 90s, then blended at 165℃ for 70s, and finally blended at 180℃ for 40s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0050] Comparative Example 4 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 400g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 15, 1kg of modified polylactic acid obtained in Preparation Example 22, and 1.4g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 150℃ for 90s, then blended at 165℃ for 70s, and finally blended at 180℃ for 40s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0051] Comparative Example 5 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 400g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 16, 1kg of modified polylactic acid obtained in Preparation Example 22, and 1.4g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 150℃ for 90s, then blended at 165℃ for 70s, and finally blended at 180℃ for 40s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0052] Comparative Example 6 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 400g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 17, 1kg of modified polylactic acid obtained in Preparation Example 22, and 1.4g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 150℃ for 90s, then blended at 165℃ for 70s, and finally blended at 180℃ for 40s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0053] Comparative Example 7 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 400g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 18, 1kg of modified polylactic acid obtained in Preparation Example 22, and 1.4g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 150℃ for 90s, then blended at 165℃ for 70s, and finally blended at 180℃ for 40s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0054] Comparative Example 8 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 400g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 11, 1kg of modified polylactic acid obtained in Preparation Example 23, and 1.4g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 150℃ for 90s, then blended at 165℃ for 70s, and finally blended at 180℃ for 40s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0055] Comparative Example 9 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 400g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 11, 1kg of modified polylactic acid obtained in Preparation Example 24, and 1.4g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 150℃ for 90s, then blended at 165℃ for 70s, and finally blended at 180℃ for 40s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0056] Comparative Example 10 A method for preparing a biodegradable plastic mulch film for agricultural use, the process is as follows: 400g of boron nitride-terminated isocyanate polybutylene succinate obtained in Preparation Example 11, 1kg of modified polylactic acid obtained in Preparation Example 25, and 1.4g of stannous octoate were added into a single-screw extruder through the feed port. The speed was adjusted to 50r / min. The mixture was first blended at 150℃ for 90s, then blended at 165℃ for 70s, and finally blended at 180℃ for 40s. After blending, the mixture was extruded and granulated to obtain biodegradable mulch film plastic.

[0057] The biodegradable plastic mulch films obtained in Examples 1-4 and Comparative Examples 1-10 were dried in a vacuum drying oven at 70°C for 12 hours and then subjected to the following tests: (a) Melt Flow Index The biodegradable plastic film obtained in Examples 1-4 and Comparative Examples 1-10 was tested for its mass after 10 minutes at 190°C and 2.16 kg pressure using a melt flow rate meter. The results are shown in Table 1 below.

[0058] Table 1 Melt Index Source of materials Melt index (g / 10min, 190℃) Example 1 3.02g Example 2 2.78 Example 3 2.66 Example 4 2.61 Comparative Example 1 7.33 Comparative Example 2 5.81 Comparative Example 3 2.49 Comparative Example 4 8.07 Comparative Example 5 2.37 Comparative Example 6 7.14 Comparative Example 7 6.85 Comparative Example 8 27.35 Comparative Example 9 4.83 Comparative Example 10 3.09 (ii) Heat resistance The softening temperatures of the biodegradable plastic films obtained in Examples 1-4 and Comparative Examples 1-10 were tested using a Vicat softening point temperature meter. The results are shown in Table 2 below.

[0059] Table 2 Vicat softening temperature Source of materials Vicat softening temperature (°C) Example 1 81.3 Example 2 84.6 Example 3 86.7 Example 4 88.1 Comparative Example 1 74.7 Comparative Example 2 76.5 Comparative Example 3 90.3 Comparative Example 4 57.9 Comparative Example 5 91.1 Comparative Example 6 58.3 Comparative Example 7 60.2 Comparative Example 8 69.6 Comparative Example 9 60.8 Comparative Example 10 61.3 (III) Mechanical Properties The biodegradable plastic film obtained in Examples 1-4 and Comparative Examples 1-10 were prepared into strip-type 2 specimens of 150mm×20mm×0.5mm according to GB / T1040.3-2006 Determination of tensile properties Part 3. The tensile strength and elongation at break of the insulating materials obtained in Examples 1-4 and Comparative Examples 1-6 were then measured by a universal testing machine, and the results are shown in Table 3 below.

[0060] Table 3 Mechanical Properties Source of materials Tensile strength (MPa) Elongation at break (%) Example 1 25.1 180.09 Example 2 26.3 189.45 Example 3 27.2 196.77 Example 4 27.6 201.38 Comparative Example 1 15.3 108.65 Comparative Example 2 16.8 113.06 Comparative Example 3 30.1 267.13 Comparative Example 4 14.8 176.03 Comparative Example 5 30.2 270.08 Comparative Example 6 15.9 72.74 Comparative Example 7 16.1 182.31 Comparative Example 8 26.4 54.72 Comparative Example 9 17.2 181.03 Comparative Example 10 17.5 184.23 (iv) Blow molding The biodegradable plastic film obtained in Examples 1-4 and Comparative Examples 1-10 was blown into plastic film with a thickness of 0.01 mm using a blown film blowing machine at 180°C. The success of the blown film blowing was observed, and the results are shown in Table 4 below.

[0061] Table 4 Blow Molding Properties Source of materials Was the blow molding successful? Example 1 yes Example 2 yes Example 3 yes Example 4 yes Comparative Example 1 yes Comparative Example 2 yes Comparative Example 3 yes Comparative Example 4 yes Comparative Example 5 yes Comparative Example 6 yes Comparative Example 7 yes Comparative Example 8 Bubble bursting leads to blow molding failure. Comparative Example 9 yes Comparative Example 10 yes (v) Water vapor transmission The film prepared by the above blow molding process is cut into pieces with an area of ​​40 cm². 2 The water vapor transmission rate of the circular thin film sample was tested in accordance with GB / T1037-2021 in a test environment with a temperature of 38℃ and a relative humidity of 90%. The results are shown in Table 5 below.

[0062] Table 5 Water vapor transmission rate Source of materials <![CDATA[Water vapor transmission rate (g / m 2 ·24 h)]]> Example 1 92.67 Example 2 86.93 Example 3 82.41 Example 4 80.22 Comparative Example 1 124.62 Comparative Example 2 126.52 Comparative Example 3 75.09 Comparative Example 4 123.71 Comparative Example 5 71.46 Comparative Example 6 125.31 Comparative Example 7 126.09 Comparative Example 9 127.14 Comparative Example 10 127.68 (vi) Biodegradation After the above water vapor permeation test, the film was dried in a vacuum chamber at 70°C for 12 hours and then buried at a depth of 5 cm from the soil surface. It was then taken out after 110 days. The biodegradation was represented by the weight loss rate (%) = (weight before degradation - weight after degradation) / weight before degradation. The higher the value, the better the degradation effect. The results are shown in Table 6 below.

[0063] Table 6 Degradation status Source of materials Weight loss rate (%) Example 1 73.62 Example 2 74.87 Example 3 75.22 Example 4 75.61 Comparative Example 1 73.91 Comparative Example 2 74.66 Comparative Example 3 26.49 Comparative Example 4 73.77 Comparative Example 5 30.12 Comparative Example 6 74.19 Comparative Example 7 74.43 Comparative Example 9 74.83 Comparative Example 10 74.92 The following conclusions can be drawn from Tables 1-6 above: (1) Through Examples 1 to 4, it can be found that the biodegradable plastic film prepared by the present invention has a low melt index, high melt strength, which is conducive to blow molding. The elongation at break, mechanical strength, water vapor barrier and heat distortion temperature are all good, and the biodegradability is also good.

[0064] (2) Comparative Example 1 shows that although the melt index of the prepared biodegradable plastic film was reduced and the elongation at break and heat distortion temperature were increased, the effect was not obvious. Moreover, its mechanical strength and water vapor barrier performance were poor. This may be because in this system, the purpose of using hydrogen peroxide to oxidize boron nitride is to modify its surface with oxygen-containing functional groups such as hydroxyl groups, so that the stability of the mixture and dispersion can be increased through hydrogen bonding during mixing. This is conducive to the encapsulation and dispersion of polybutylene succinate through in-situ polymerization, thus solving the agglomeration of boron nitride. However, excessive oxidation may cause excessive peeling between layers and structural collapse, thus losing the reinforcing effect.

[0065] (3) Comparative Example 2 shows that although the melt index of the prepared biodegradable plastic film was reduced and the elongation at break and heat distortion temperature were increased, the effect was not obvious. Moreover, its mechanical strength and water vapor barrier performance were poor. This may be because the boron nitride with a fine particle size has a higher surface energy, stronger intermolecular forces, and higher agglomeration. Under the modification method of this system, the modification effect is poor, and the in-situ polymerization, encapsulation and dispersion uniformity may be poor, which may prevent the advantages of boron nitride from being brought into play.

[0066] (4) Comparative Example 3 shows that although the mechanical strength and water vapor barrier properties of the prepared biodegradable plastic film have been further improved, its biodegradability is poor. This may be because although silane coupling agent can improve the interfacial bonding between inorganic and organic materials, silane coupling agent has poor degradability and when combined with polylactic acid, it affects the degradation performance of polylactic acid.

[0067] (5) Comparative Example 4 shows that although the melt index of the prepared biodegradable plastic film was reduced and the elongation at break was increased, the material itself was soft and its mechanical strength, water vapor barrier and heat distortion temperature were poor. This may be due to the low alkyd ratio, which inhibited the growth of the molecular chain of polybutylene succinate during polymerization, and affected the synthesis of polybutylene succinate, thus making its reinforcing effect on polylactic acid worse.

[0068] (6) Comparative Example 5 shows that although the mechanical strength and water vapor barrier properties of the prepared biodegradable plastic film have been further improved, its biodegradability is poor. This may be because the rigid structure of the aromatic ring inhibits its degradability.

[0069] (7) Comparative Example 6 shows that although the melt index of the prepared biodegradable plastic film was reduced, the elongation at break, mechanical strength, water vapor barrier and heat distortion temperature were all poor. This may be because polybutylene succinate and modified polylactic acid are two materials that are incompatible at the interface. Melt blending is only a physical blending and cannot achieve further chemical cross-linking, which is not conducive to performance improvement.

[0070] (8) Comparative Example 7 shows that although the melt index of the prepared biodegradable plastic film was reduced and the elongation at break was increased, the material itself was soft and its mechanical strength, water vapor barrier and heat distortion temperature were poor. This may be because polybutylene succinate itself has high toughness and the toughness of modified polylactic acid was also improved. The cross-linking of the two leads to excessive toughness of the material, which makes it easy to soften and reduce its performance.

[0071] (9) Comparative Example 8 shows that although the prepared biodegradable plastic film has certain mechanical strength and heat distortion temperature, it has a high melt index, low elongation at break, and poor material toughness, which is not conducive to blow molding of plastic film. This may be because glycerol has a poorer branching modification effect and poorer toughness modification in the polylactic acid synthesis process compared with pentaerythritol.

[0072] (10) Comparative Example 9 shows that although the prepared biodegradable plastic film has a certain elongation at break and a low melt index, the mechanical strength, water vapor barrier and heat distortion temperature difference are poor. This may be due to the excessively long branched molecular chain structure of pentaerythritol glycidyl ether, which may cause the molecular chains to be too loosely packed, excessively reducing the crystallinity, and thus affecting the performance of the material.

[0073] (11) Comparative Example 10 shows that although the prepared biodegradable plastic film has a certain elongation at break and a low melt index, the mechanical strength, water vapor barrier and heat distortion temperature difference are poor. This may be because if pentaerythritol is used in too much of this system, the star-shaped branched structure will be too large, which may also make the molecular chains too loosely packed, excessively reduce the crystallinity, and thus affect the performance of the material.

[0074] The embodiments described above provide a detailed explanation of the technical solutions and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Various changes and modifications can be made to the present invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed.

Claims

1. A method for preparing a biodegradable plastic mulch film for agriculture, characterized in that, The preparation method includes the following steps: Boron nitride-terminated isocyanate polybutylene succinate, modified polylactic acid, and stannous octoate are mixed in a weight ratio of 30~40:100:1.3~1.4, and then sequentially blended at 140℃~150℃ for 60s~90s, 160℃~165℃ for 50s~670s, and 180℃ for 30s~40s before extrusion granulation to obtain the biodegradable mulch film plastic.

2. The method for preparing a biodegradable agricultural mulch film according to claim 1, characterized in that, The preparation method of the boron nitride-terminated isocyanate polybutylene succinate includes the following steps: Modified boron nitride, 1,4-butanediol, succinic acid, tetrabutyl titanate, and dehydrating agent are dispersed in a weight ratio of 4~7:140~170:115:0.8~1:200~240, and then heated to 160℃~170℃ for 2h~2.5h. After that, the temperature is further increased to 230℃~240℃ for 3h to obtain boron nitride-polybutylene succinate. Boron nitride-polybutylene succinate, diisocyanate and tetramethylethylenediamine are mixed in a weight ratio of 1:0.1~0.2:0.003~0.005 and heated to 75℃~80℃ for 30min~40min to obtain the boron nitride-terminated isocyanate polybutylene succinate.

3. The method for preparing a biodegradable agricultural mulch film according to claim 2, characterized in that, The method for preparing the modified boron nitride includes the following steps: Boron nitride and hydrogen peroxide were mixed at a ratio of 1g:20mL~30mL and heated to 70℃~80℃ for 1h~2h to obtain the modified boron nitride.

4. The method for preparing a biodegradable agricultural mulch film according to claim 2, characterized in that, The diisocyanate includes hexamethylene diisocyanate.

5. The method for preparing a biodegradable agricultural mulch film according to claim 1, characterized in that, The preparation method of the modified polylactic acid includes the following steps: DL-lactic acid, polyhydroxy alcohol compounds, and stannous octoate were mixed in a ratio of 1 mol: 4.1 mol~4.2 mol: 0.5 g~0.6 g and heated to 130℃~135℃ for 2 h~3 h to obtain the first reactant; The first reactant, DL-lactic acid, and stannous octoate were mixed in a weight ratio of 1:4~4.5:0.025~0.03 and heated to 140℃~150℃ for 3h~4h. Then the temperature was raised to 170℃ and reacted for 7h~8h to obtain the modified polylactic acid.

6. The method for preparing a biodegradable agricultural mulch film according to claim 5, characterized in that, The polyhydroxy alcohol compounds include pentaerythritol.

7. A biodegradable mulch film prepared by the method for preparing a biodegradable mulch film for agriculture as described in any one of claims 1 to 6.