Controllable biodegradable high-strength polylactic acid composite mulching film and preparation method thereof

Abstract

The application belongs to the technical field of biodegradable polymer materials, and relates to a controllable biodegradable high-strength polylactic acid composite mulching film and a preparation method thereof. The mulching film is a three-layer composite structure, and the layers from top to bottom are a surface layer, a middle layer and an inner layer. The raw materials of the surface layer include PLA, PPC, PCL and a core-shell structure degradation regulator. The core-shell structure degradation regulator takes an imidazole phosphate as a core and takes calcium pyrophosphate as a shell. The raw materials of the middle layer include PLA, PBAT, an epoxy-containing compatibilizer and an epoxy-containing chain extender. The raw materials of the inner layer include PLA, starch and humic acid. The core-shell structure degradation regulator has a slow-release catalytic function and has a regulating effect on the degradation of the mulching film. The surface layer of the composite mulching film provides a degradation regulating function. The middle layer provides mechanical support. The inner layer activates soil bacteria and promotes crop growth. The three layers realize the contradictory requirements of high initial strength, long functional period and rapid degradation in the later period through synergistic action.

Description

Technical Field

[0001] This invention belongs to the field of biodegradable polymer materials technology, specifically relating to a high-strength polylactic acid composite mulch film that can be controlled to biodegrade and its preparation method. Background Technology

[0002] Biodegradable mulch film refers to plastic film that can be degraded by microorganisms in the soil under natural environmental conditions. It is mainly used for ground cover to increase soil temperature, retain soil moisture, and maintain soil structure, thus providing insulation, moisture retention, and protection against pests and diseases caused by certain microorganisms, thereby promoting crop growth. Furthermore, biodegradable mulch film requires no recycling; it is degraded into non-toxic small molecules by microorganisms in the soil and does not harm the ecological environment. Therefore, the use of biodegradable mulch film is an effective way to solve the "white pollution" problem caused by traditional polyethylene (PE) mulch film and achieve sustainable development of agricultural mulch film.

[0003] The raw materials for biodegradable mulch films are divided into natural biodegradable materials and synthetic biodegradable materials. Natural biodegradable materials mainly include cellulose, starch, chitin, and protein. These materials are widely available, can decompose naturally, and their products are completely non-toxic. However, most of them lack thermoplasticity, making them difficult to process and mold, and often cannot be used alone. Currently developed synthetic biodegradable materials mainly include polylactic acid (PLA), polybutylene terephthalate-co-butylene adipate (PBAT), polycaprolactone (PCL), polypropylene carbonate (PPC), and polybutylene succinate (PBS). Agricultural PLA mulch films have insufficient mechanical properties (tensile strength <20MPa), poor flexibility (elongation at break <50%), and an uncontrollable degradation rate. PBAT has good flexibility and extensibility, making it suitable for film production, but its degradation is slow. PPC mulch films have low mechanical strength and unstable thermal properties, making them unsuitable for use alone. PCL mulch films have high elasticity and flexibility, but low tensile strength. Each of the above-mentioned single biodegradable films has some problems. To solve these problems, blending, grafting, or copolymerization are generally used to achieve complementary performance. However, improving mechanical properties often leads to a decrease in degradation performance, and vice versa. Biodegradable mulch films, on the other hand, require maintaining functional integrity during the crop's demand period, possessing good mechanical strength and a basically intact physical structure, and not undergoing significant degradation leading to functional loss. That is, the functional period of the biodegradable mulch film (the time from when it is laid in the field until it begins to degrade initially, i.e., the period when the mulch film fully functions) must be longer than the crop's demand period (generally, for conventional crops, a functional period of more than 90 days is ideal). Simultaneously, after the functional period, the biodegradable mulch film must be able to rapidly break down and completely degrade into water, carbon dioxide, and other biomass in a natural soil environment (ideally, under composting conditions, the degradation rate should reach over 90% after 180 days). In other words, existing agricultural mulch film technologies cannot simultaneously address the issues of high initial strength, long functional period, and rapid degradation in the later stages; and they often have only one function.

[0004] CN117643246A discloses a nitrogen-rich biodegradable mulch film and its preparation method. The mulch film consists of three layers: A, B, and A6. The surface layer (A6) comprises 40-95 wt% PBAT, 0-30 wt% PLA, 0-30 wt% PPC, 0-50 wt% inorganic mineral powder, 0-2 wt% erucamide, 0-2 wt% BIPB crosslinking agent, 0-10 wt% carbon black masterbatch, and 0-5 wt% UV stabilizer. The core layer (B6) comprises 40-95 wt% PBAT, 0-50 wt% starch, 0-10 wt% water, 0-20 wt% urea, 0-20 wt% monoglyceride, and 0-20 wt% glycerol. This mulch film can protect crops during the early stages of growth by conserving moisture, increasing soil temperature, and suppressing weeds. During the later stages of growth, it accelerates degradation and releases nitrogen fertilizer to provide nutrients to the crops. However, the initial strength of the mulch film is low, with a lateral tensile strength of only about 30 MPa.

[0005] CN117229612B discloses a PBAT / PPCP polymer film, its preparation method, and its application in the field of biodegradable mulch films. The PBAT / PPCP polymer film comprises the following raw materials by weight percentage: 65-75% PBAT, 20-30% PPCP, and 1-5% compatibilizer. The preparation method involves weighing each raw material, granulating it in a twin-screw extruder to obtain mixed granules, and then blow-molding the mixed granules using a blown film extruder to obtain a polymer film. This application leverages the characteristic that both PBAT and PPCP contain ester groups, introducing p-phenylenediamine as a compatibilizer. Through an amine-ester exchange reaction, segmental intercalation between PBAT and PPCP is achieved, thereby improving their compatibility and ultimately forming a stable new polymer with excellent properties in blown film formation, mechanical properties, water vapor barrier properties, and functional life. This polymer film exhibits good degradation performance, completely degrading within 5 months, but its functional life is short, only 60 days, and its initial strength is also relatively low. Summary of the Invention

[0006] Given that existing agricultural biodegradable mulch films cannot simultaneously meet the performance requirements of high initial strength, long functional period, and rapid degradation in the later stages, this invention provides a high-strength polylactic acid composite mulch film with controllable biodegradability and its preparation method. The three-layer composite mulch film is prepared using a three-layer co-extrusion blown film machine. The surface layer regulates the degradation rate; the middle layer provides mechanical support to ensure the overall mechanical strength of the composite mulch film; and the inner layer is close to the crop to promote crop growth. The synergistic effect of the three layers achieves the "contradictory requirements" of high initial strength, long functional period, and rapid degradation in the later stages of the polylactic acid-based composite mulch film, achieving a good balance in mechanical properties, functional period, and degradation rate.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A high-strength polylactic acid composite mulch film with controllable biodegradability has a three-layer composite structure, consisting of a surface layer, a middle layer, and an inner layer from top to bottom. The surface layer is made of PLA, PPC, PCL, and a core-shell degradation regulator, wherein the core-shell degradation regulator has an imidazole phosphate core and a calcium pyrophosphate shell. The middle layer is made of PLA, PBAT, an epoxy-containing compatibilizer, and an epoxy-containing chain extender. The inner layer is made of PLA, starch, and humic acid.

[0009] Furthermore, the thickness ratio of the surface layer, the middle layer, and the inner layer is (1~1.5):(1.5~2):1. In use, the surface layer faces upwards and the inner layer faces downwards, meaning the inner layer is close to the crop.

[0010] Further, the raw materials of the surface layer include: 10-15 wt% PPC, 10-15 wt% PCL, 6-8 wt% core-shell structure degradation regulator, and the remainder is PLA; the raw materials of the intermediate layer include: 20-25 wt% PBAT, 3-5 wt% epoxy-containing compatibilizer, 0.5-0.8 wt% epoxy-containing chain extender, and the remainder is PLA; the raw materials of the inner layer include: 15-20 wt% starch, 3-5 wt% humic acid, and the remainder is PLA.

[0011] Furthermore, the raw material of the intermediate layer also includes 0.5~1.0wt% talc powder with a particle size D50 of 2~4μm. Talc powder can improve the crystallinity of PLA as a nucleating agent, and it can also play a reinforcing role as a rigid particle, thereby further improving the mechanical properties of the intermediate layer. The raw materials of the surface layer, intermediate layer and inner layer also include 0.2~1wt% of a mildew inhibitor, which is a long-chain alkyl quaternary ammonium salt, specifically selected from at least one of dodecyl dimethyl benzyl ammonium chloride and didecyl dimethyl ammonium chloride.

[0012] Further, the imidazole phosphate ester is selected from at least one of 1-butyl-3-methylimidazolium phosphate ([BMIM][DBP]), 1-butyl-3-methylimidazolium phosphate diethyl ester ([BMIM][DEP]), 1-ethyl-3-methylimidazolium phosphate dibutyl ester ([EMIM][DBP]), and 1-ethyl-3-methylimidazolium phosphate diethyl ester ([EMIM][DEP]).

[0013] Furthermore, the preparation method of the core-shell structure degradation regulator includes the following steps:

[0014] (S1) Dissolve the emulsifier in an aqueous pyrophosphate solution to form an aqueous phase solution;

[0015] (S2) Add imidazole phosphate to the aqueous solution and stir to emulsify, to obtain an oil-in-water emulsion;

[0016] (S3) The calcium salt aqueous solution is added dropwise to the emulsion, and an interfacial chemical deposition reaction is carried out under stirring. After the reaction is completed, the core is cooled, washed and dried to obtain a core-shell structure degradation regulator with imidazole phosphate core and calcium pyrophosphate shell. The amount of calcium salt is 15~25wt% of imidazole phosphate.

[0017] Preferably, the amount of emulsifier used in step (S1) is 3-5 wt% of the pyrophosphate aqueous solution, and the emulsifier is a nonionic surfactant with an HLB value of 12-15, such as at least one selected from O-10, NP-10, and OP-10; the concentration of the pyrophosphate aqueous solution is 4-8 wt%, and the pyrophosphate is at least one selected from sodium pyrophosphate and potassium pyrophosphate.

[0018] Preferably, the amount of imidazole phosphate ester used in step (S2) is 15~20wt% of the aqueous solution; the conditions for stirring and emulsification are: temperature 45~55℃, time 20~40min, and stirring speed 500~700r / min.

[0019] Preferably, in step (S3), the amount of calcium salt used is 20-25 wt% of imidazole phosphate, the calcium salt is at least one of calcium hypochlorite, calcium chloride, and calcium bicarbonate, and the concentration of the calcium salt aqueous solution is 3-5 wt%; the reaction conditions are: temperature 40-50℃, stirring speed 150-250 r / min, and the calcium salt aqueous solution is added dropwise within 60-90 min.

[0020] Further, the PLA has a weight-average molecular weight of 80,000 to 150,000 Da and a melt index of 10 to 20 g / 10 min at 190°C / 2.16 kg; the PPC has a weight-average molecular weight of 80,000 to 150,000 Da and a melt index of 10 to 15 g / 10 min at 190°C / 2.16 kg; the PCL has a weight-average molecular weight of 60,000 to 100,000 Da and a melt index of 12 to 18 g / 10 min at 160°C / 2.16 kg; the PBAT has a weight-average molecular weight of 100,000 to 150,000 Da and a melt index of 8 to 15 g / 10 min at 190°C / 2.16 kg; and the starch is at least one of corn starch, sweet potato starch, potato starch, and cassava starch. PLA, PPC, PCL, PBAT, and starch are the main materials used in this invention, all of which are biodegradable polymers, and each has different mechanical properties, flexibility, and processing properties. PLA, while rigid, is brittle and serves as the matrix material for each layer, providing basic strength. PPC, a hydrophobic resin with a tight molecular structure, offers good water and oxygen barrier properties; its application in the surface layer enhances the moisture retention of the composite mulch film. PCL, with its high elasticity and flexibility, provides initial flexibility in the surface layer. PBAT, a semi-crystalline polymer, is flexible, has a low modulus, and good ductility; its blending with PLA in the intermediate layer improves the mechanical properties of the intermediate layer, and its lower degradation rate compared to PLA helps maintain the structural stability of the intermediate layer. Starch, a natural biodegradable polymer, is easily degraded and used in the inner layer, which is close to the crop; the starch activates soil microorganisms, promoting crop growth. Combining these biodegradable materials in different layers of the composite mulch film allows for complementary performance and a synergistic effect.

[0021] Further, the epoxy-containing compatibilizer is a linear copolymer of glycidyl methacrylate and methacrylate, with an epoxy equivalent of 250-350 g / mol and a number-average molecular weight of 2000-6000, for example, selected from CE-SZ01; the epoxy-containing chain extender is selected from at least one of ADR-4468 and ADR-4370S; the humic acid has a carboxyl content of 1.8-2.5 mmol / g, of which the fulvic acid content is 8-15 wt%.

[0022] The total thickness of the polylactic acid composite mulch film described in this invention is 0.01~0.05mm, and different thicknesses are suitable for different crops and natural environments.

[0023] The inventors discovered through research that the core-shell structure degradation regulator, specifically the imidazole phosphate salt, can catalyze the degradation of PLA. This may be because the imidazole phosphate contains cations such as [BMIM]. + and [EMIM] + Carbonyl groups of PLA can be polarized; anions such as [DBP]- and [DEP] - It can nucleophilically attack ester bonds, thereby lowering the hydrolysis energy barrier and thus facilitating biodegradation. Therefore, the core-shell structured degradation regulator prepared by this invention, using imidazole phosphate esters as the core and calcium pyrophosphate as the shell, has a slow-release catalytic function, thus regulating the degradation of composite mulch films. In the early stage of composite film degradation, the core layer is protected by the shell layer, thereby preventing rapid degradation of the composite mulch film in the early stage and the resulting loss of water and nutrients; while in the middle and late stages of composite film degradation, the calcium pyrophosphate in the shell layer gradually dissolves in the soil to form Ca. 2+ and HPO4 2- This releases imidazole phosphate esters from the core layer, which then act as a catalytic agent in the middle and later stages of composite mulch film degradation, accelerating the degradation process. In other words, the core-shell structure degradation regulator prevents the composite mulch film from degrading too rapidly in the initial stages, or too slowly and incompletely in the later stages; simultaneously, the Ca formed by the dissolution of the shell layer... 2+ and HPO4 2- It is also a nutrient required for plant growth.

[0024] The composite mulch film of this invention has a three-layer composite structure. The surface layer, due to the action of a core-shell degradation regulator, prevents the composite mulch film from degrading too rapidly in the early stages, nor too slowly or incompletely in the later stages. Simultaneously, PPC has good water and oxygen barrier properties, improving the moisture retention performance of the composite mulch film. Furthermore, PLA is brittle, and the appropriate addition of PCL can improve its flexibility. The middle layer, through blending with PBAT and the action of epoxy-containing compatibilizers and chain extenders, improves its mechanical strength, thus acting as a skeletal support to ensure the overall mechanical strength of the composite mulch film. This is because although PLA and PBAT blends can complement each other's properties, their compatibility is poor. However, through the ring-opening reaction of the epoxy groups in the compatibilizers and chain extenders, PLA and PBAT are bound at the end groups, thereby improving their compatibility. At the same time, the chain extender can achieve polymer chain growth, which helps to further improve mechanical properties. The inner layer is close to the crop, and through the action of starch and humic acid, it activates the soil microbiome, thereby promoting crop growth. The synergistic effect of the three layers achieves the "contradictory requirements" of high initial strength, long functional period and rapid degradation in the later stage of the polylactic acid base film; in addition, each layer of the composite mulch film in this invention uses PLA as the matrix, so the compatibility between the three layers is good.

[0025] Secondly, the present invention provides a method for preparing a high-strength polylactic acid composite mulch film that is controllably biodegradable, comprising the following steps:

[0026] (1) Mix the raw materials of the surface layer, the raw materials of the middle layer and the raw materials of the inner layer according to the mass ratio, and then melt extrude, pelletize and dry to obtain the corresponding surface masterbatch, middle layer masterbatch and inner layer masterbatch;

[0027] (2) Add the surface masterbatch, intermediate masterbatch and inner masterbatch into a three-layer co-extrusion blown film machine, and obtain a high-strength polylactic acid composite mulch film that can be controlled to degrade by blown film forming.

[0028] Further, in step (1), each raw material is pre-dried and then melt-extruded, wherein the starch is pre-dried to a moisture content of ≤0.5wt%; the conditions for melt extrusion are: melt extrusion and granulation in a twin-screw extruder, the melt temperature is 160~180℃, and the screw speed is 200~300r / min; the drying is carried out at 60~80℃ for 12~24h.

[0029] Further, in step (2), the blowing machine has a rotation speed of 70~120 r / min, a temperature of 160~180℃, a blowing ratio of 2~3:1, and a traction speed of 10~20 m / min. The above parameters are adjusted according to the thickness of the composite film.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. This invention prepares a degradation regulator with imidazole phosphate as the core and calcium pyrophosphate as the shell through interfacial chemical deposition reaction. The imidazole phosphate in the core layer, protected by the calcium pyrophosphate in the shell, has a slow-release catalytic function, thus regulating the degradation of the composite mulch film. This prevents the degradation from being too rapid in the initial stage or too slow and incomplete in the later stages. Simultaneously, the Ca2+ formed by the dissolution of the shell layer... 2+ and HPO4 2- It is also a nutrient required for plant growth.

[0032] 2. The composite mulch film in this invention has a three-layer composite structure. The surface layer provides degradation regulation function; the middle layer provides mechanical support, ensuring the overall mechanical strength of the composite mulch film; and the inner layer activates soil microorganisms and promotes crop growth. The synergistic effect of the three layers realizes the "contradictory requirements" of high initial strength, long functional period and rapid degradation in the later stage of the polylactic acid base film, achieving a good balance in mechanical properties, functional period and degradation rate. Moreover, each layer of the composite mulch film uses PLA as the matrix, so the compatibility between the three layers is good.

[0033] 3. The composite mulch film of the present invention has good mechanical properties, with an initial tensile strength of over 45 MPa, an elongation at break of over 75%, and a puncture resistance of ≥15 N / mm.

[0034] 4. The composite mulch film of the present invention has a functional period of more than 90 days, which makes it less likely to degrade and break in the early stage, thus preventing the rapid loss of water and nutrients in the soil; and the degradation rate (ISO 14855 standard) reaches more than 92% at 180 days, that is, a good balance is achieved between the long functional period, the rapid degradation in the later stage and the high degradation rate in the final stage. Detailed Implementation

[0035] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to the following embodiments.

[0036] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are commercially available unless otherwise specified. Unless otherwise specified, all parts are parts by weight.

[0037] 1-Butyl-3-methylimidazolium dibutyl phosphate ([BMIM][DBP]) and 1-ethyl-3-methylimidazolium diethyl phosphate ([EMIM][DEP]) were both selected from Hubei Keji Biomedical Technology Co., Ltd.

[0038] PLA particles were selected from Shanxi Biomass New Materials Industry Research Institute Co., Ltd., with a weight-average molecular weight of approximately 110,000 Da and a melt index (190℃ / 2.16kg) of 10 g / 10min.

[0039] The PPC granules were selected from Boda Dongfang New Chemical (Jilin) ​​Co., Ltd., with a weight-average molecular weight of approximately 130,000 and a melt index (190℃ / 2.16kg) of 12.7g / 10min.

[0040] PCL particles were selected from Hubei Biaoyue Biotechnology Development Co., Ltd., with a weight-average molecular weight of approximately 80,000 Da and a melt index (160℃ / 2.16kg) of 15.5 g / 10min.

[0041] PBAT particles were selected from Shanxi Biomass New Materials Industry Research Institute Co., Ltd., with a weight-average molecular weight of approximately 110,000 Da and a melt index (190℃ / 2.16kg) of 10 g / 10min.

[0042] The humic acid was selected from Xinjiang Shuanglong Humic Acid Co., Ltd., model SL-HA-5, with a carboxyl content of 2.2 mmol / g, of which fulvic acid content was 11.2 wt%.

[0043] The epoxy-containing compatibilizer CE-SZ01 and the epoxy-containing chain extender ADR-4370S are both selected from BASF.

[0044] The talc powder was selected from Baige Group, with a particle size D50 of 2.5μm.

[0045] Preparation of core-shell structure degradation regulators

[0046] Preparation Example 1

[0047] (S1) Dissolve 4 parts of emulsifier O-10 in 100 parts of 4 wt% sodium pyrophosphate aqueous solution to form an aqueous phase solution;

[0048] (S2) Add 20 parts of 1-butyl-3-methylimidazolium dibutyl phosphate ([BMIM][DBP]) to the aqueous solution prepared in step (S1) and stir to emulsify. Control the emulsification temperature at 50℃ and stir at 600 r / min for 30 min to obtain an oil-in-water emulsion.

[0049] (S3) 60 parts of a 5 wt% calcium chloride aqueous solution were added dropwise to the emulsion prepared in step (S2). The interfacial chemical deposition reaction was carried out under stirring at 200 r / min, and the calcium chloride aqueous solution was added dropwise within 60 min. After the addition was completed, stirring was continued for 20 min. Then, after centrifugation and washing twice with ether, the emulsion was dried at 80 °C for 24 h in a drying oven to obtain a core-shell structure degradation regulator a with an imidazole phosphate core and a calcium pyrophosphate shell. The particle size D50 of the regulator was 0.9 μm as measured by a laser particle size analyzer.

[0050] Preparation Example 2

[0051] The rest is the same as in Preparation Example 1, except that: in step (S1), the concentration of sodium pyrophosphate aqueous solution is 5.5 wt%; in step (S2), 1-ethyl-3-methylimidazolium-diethyl phosphate ([EMIM][DEP]) is used to replace [BMIM][DBP] by mass; and in step (S3), the amount of 5 wt% calcium chloride aqueous solution is 80 parts. Finally, a core-shell structure degradation regulator b with an imidazolium phosphate core and a calcium pyrophosphate shell is obtained. The particle size D50 of the regulator is 1.0 μm as measured by a laser particle size analyzer.

[0052] Preparation Example 3

[0053] The rest is the same as in Preparation Example 1, except that: in step (S1), the concentration of sodium pyrophosphate aqueous solution is 6.5 wt%; in step (S3), the amount of 5 wt% calcium chloride aqueous solution is 100 parts, and finally a core-shell structure degradation regulator c with imidazole phosphate core and calcium pyrophosphate shell is obtained. The particle size D50 of the substance is 1.1 μm as measured by a laser particle size analyzer.

[0054] Example 1

[0055] (1) 67.5 wt% PLA particles, 12 wt% PPC particles, 12 wt% PCL particles, 7 wt% core-shell structure degradation regulator a (prepared in Preparation Example 1), 0.8 wt% talc powder and 0.7 wt% dodecyl dimethyl benzyl ammonium chloride were added to a mixer and stirred evenly at 500 r / min. Then, the mixture was added to a twin-screw extruder for melt extrusion, water cooling and pelleting. The melt temperature was set to 175 °C, the extrusion temperature was set to 165 °C, the screw extrusion speed was 250 rpm, and the pelleted material was dried at 60 °C for 24 h to obtain surface masterbatch.

[0056] 72.7 wt% PLA particles, 22 wt% PBAT particles, 4 wt% CE-SZ01, 0.6 wt% ADR-4370S and 0.7 wt% dodecyl dimethyl benzyl ammonium chloride were added to a mixer and stirred evenly at 500 r / min. Then, the mixture was added to a twin-screw extruder for melt extrusion, water cooling and pelleting. The melt temperature was set to 180℃, the extrusion temperature was set to 170℃, the screw extrusion speed was 250 rpm, and the pelleted material was dried at 60℃ for 24 h to obtain intermediate layer masterbatch.

[0057] 77.5 wt% PLA granules, 18 wt% pre-dried starch (pre-dried to a moisture content ≤0.5 wt%), 4 wt% humic acid and 0.5 wt% dodecyl dimethyl benzyl ammonium chloride were added to a mixer and stirred evenly at 500 r / min. Then, the mixture was added to a twin-screw extruder for melt extrusion, water cooling, and pelleting. The melt temperature was set to 165℃, the extrusion temperature was set to 160℃, the screw extrusion speed was 250 rpm, and the pelleted material was dried at 60℃ for 24 h to obtain the inner layer masterbatch.

[0058] (2) The surface masterbatch, intermediate masterbatch and inner masterbatch are added to the three-layer co-extrusion blown film machine. The temperature of the feeding section is set to 160℃, the temperature of the melting section is set to 175℃, the temperature of the die head is set to 170℃, the blowing ratio is 2.5:1, and the thickness ratio of the surface layer, intermediate layer and inner layer is controlled to 1:2:1 by controlling the flow rate ratio of each masterbatch. After molding, the air ring is cooled, the height of the condensation line is controlled at 3cm, the traction speed is 15m / min, and then it is cut and rolled up to obtain a three-layer structure controllable degradable high-strength polylactic acid composite mulch film with a total thickness of 0.03mm.

[0059] Example 2

[0060] The rest is the same as in Example 1, except that in step (1), the core-shell structure degradation regulator b prepared in Preparation Example 2 is used instead of the core-shell structure degradation regulator a.

[0061] Example 3

[0062] The rest is the same as in Example 1, except that in step (1), the core-shell structure degradation regulator c prepared in Preparation Example 3 is used instead of the core-shell structure degradation regulator a.

[0063] Example 4

[0064] The rest is the same as in Example 1, except that the amount of raw materials used in each layer in step (1) is different, specifically:

[0065] (1) 65.5 wt% PLA particles, 10 wt% PPC particles, 15 wt% PCL particles, 8 wt% core-shell structure degradation regulator a (prepared in Preparation Example 1), 0.8 wt% talc powder and 0.7 wt% dodecyl dimethyl benzyl ammonium chloride were added to a mixer and stirred evenly at 500 r / min. Then, the mixture was added to a twin-screw extruder for melt extrusion, water cooling and pelleting. The melt temperature was set to 175 °C, the extrusion temperature was set to 165 °C, the screw extrusion speed was 250 rpm, and the pelleted material was dried at 60 °C for 24 h to obtain surface masterbatch.

[0066] 73.5 wt% PLA particles, 20 wt% PBAT particles, 5 wt% CE-SZ01, 0.8 wt% ADR-4370S and 0.7 wt% dodecyl dimethyl benzyl ammonium chloride were added to a mixer and stirred evenly at 500 r / min. Then, the mixture was added to a twin-screw extruder for melt extrusion, water cooling and pelleting. The melt temperature was set to 180℃, the extrusion temperature was set to 170℃, the screw extrusion speed was 250 rpm, and the pelleted material was dried at 60℃ for 24 h to obtain intermediate layer masterbatch.

[0067] 79.5 wt% PLA granules, 15 wt% pre-dried starch (pre-dried to a moisture content ≤0.5 wt%), 5 wt% humic acid and 0.5 wt% dodecyl dimethyl benzyl ammonium chloride were added to a mixer and stirred evenly at 500 r / min. Then, the mixture was added to a twin-screw extruder for melt extrusion, water cooling, and pelleting. The melt temperature was set at 165℃, the extrusion temperature was set at 160℃, the screw extrusion speed was 250 rpm, and the pelleted material was dried at 60℃ for 24 h to obtain the inner layer masterbatch.

[0068] (2) Same as Example 1.

[0069] Example 5

[0070] The rest is the same as in Example 1, except that the amount of raw materials used in each layer in step (1) is different, specifically:

[0071] (1) 67.5 wt% PLA particles, 15 wt% PPC particles, 10 wt% PCL particles, 6 wt% core-shell structure degradation regulator a (prepared in Preparation Example 1), 0.8 wt% talc powder and 0.7 wt% dodecyl dimethyl benzyl ammonium chloride were added to a mixer and stirred evenly at 500 r / min. Then, the mixture was added to a twin-screw extruder for melt extrusion, water cooling and pelleting. The melt temperature was set to 175 °C, the extrusion temperature was set to 165 °C, the screw extrusion speed was 250 rpm, and the pelleted material was dried at 60 °C for 24 h to obtain surface masterbatch.

[0072] 70.5 wt% PLA particles, 25 wt% PBAT particles, 3 wt% CE-SZ01, 0.8 wt% ADR-4370S and 0.7 wt% dodecyl dimethyl benzyl ammonium chloride were added to a mixer and stirred evenly at 500 r / min. Then, the mixture was added to a twin-screw extruder for melt extrusion, water cooling and pelleting. The melt temperature was set to 180℃, the extrusion temperature was set to 170℃, the screw extrusion speed was 250 rpm, and the pelleted material was dried at 60℃ for 24 h to obtain intermediate layer masterbatch.

[0073] 76.5 wt% PLA granules, 20 wt% pre-dried starch (pre-dried to a moisture content ≤0.5 wt%), 3 wt% humic acid and 0.5 wt% dodecyl dimethyl benzyl ammonium chloride were added to a mixer and stirred evenly at 500 r / min. Then, the mixture was added to a twin-screw extruder for melt extrusion, water cooling, and pelleting. The melt temperature was set to 165℃, the extrusion temperature was set to 160℃, and the screw extrusion speed was 250 rpm. The pelleted material was dried at 60℃ for 24 h to obtain the inner layer masterbatch.

[0074] (2) Same as Example 1.

[0075] Example 6

[0076] The rest is the same as in Example 1, except that the thickness ratio of the surface layer, intermediate layer and inner layer in step (2) is 1.5:2:1.

[0077] Comparative Example 1

[0078] The rest is the same as in Example 1, except that: in step (1), no core-shell structure degradation regulator is used in the raw materials of the surface layer, and the amount of PLA is 74.5 wt%.

[0079] Comparative Example 2

[0080] The rest is the same as in Example 1, except that: in step (1), [BMIM][DBP] is used to replace the core-shell structure degradation regulator a in the raw material of the surface layer, that is, the core-shell structure is not made, and [BMIM][DBP] is directly added to the raw material of the surface masterbatch preparation.

[0081] Comparative Example 3

[0082] The rest is the same as in Example 1, except that: in step (1), PBAT and epoxy-containing compatibilizers are not used in the raw materials of the intermediate layer, and the amount of chain extender is increased accordingly. That is, the raw material ratio of the intermediate layer is: 98.1 wt% PLA particles, 1.2 wt% ADR-4370S and 0.7 wt% dodecyl dimethyl benzyl ammonium chloride.

[0083] Comparative Example 4

[0084] The mulch film has a double-layer structure, containing only the surface layer and inner layer of Example 1, specifically:

[0085] The surface masterbatch and inner masterbatch from Example 1 were added to a two-layer co-extrusion blown film machine. In the blown film machine, the temperature of the feeding section was set to 160°C, the temperature of the melting section was set to 175°C, the temperature of the die head was set to 170°C, the blow-up ratio was 2.5:1, and the film was cooled by the air ring after molding. The height of the condensation line was controlled at 3cm, and the traction speed was 15m / min. Then, the film was cut and wound up to obtain a double-layer composite mulch film with a total thickness of 0.03mm.

[0086] Testing and Analysis

[0087] The composite mulch films prepared in the above embodiments and comparative examples were subjected to the following performance tests:

[0088] a) Mechanical properties

[0089] According to the relevant provisions of GB / T 1040.3-2006 standard, the prepared composite mulch film was subjected to tensile testing using a universal testing machine to test the transverse tensile strength and transverse elongation at break. Five parallel tests were conducted, and the average value of the test results was taken. According to the relevant provisions of ASTM D1709 standard, the puncture resistance was tested using a Φ38mm hemispherical dart through the free-fall dart method.

[0090] b) Degradation performance

[0091] Degradation performance was tested from two dimensions: functional period and degradation rate.

[0092] Functional period: According to the relevant provisions of EN 17033-2018, the time from when the plastic film is laid in the field until the breaking elongation of the plastic film drops to 50% of the initial value is the functional period, which is also the period when the plastic film fully plays its role.

[0093] Degradation rate: Tested under composting conditions according to the relevant provisions of ISO 14855-1-2012 standard. The degradation time and mass change of the sample were recorded. The degradation rate was calculated based on the mass change, using the following formula:

[0094] Degradation rate (%) = (1 - M / M0) × 100%

[0095] Where M0 is the initial mass of the mulch film, and M is the mass of the mulch film after a certain degradation time. The test results of mechanical properties and degradation performance are shown in Table 1.

[0096] Table 1 Performance Tests

[0097]

[0098] As shown in Table 1, the polylactic acid composite mulch film prepared according to the embodiments of the present invention has good initial mechanical properties, with an initial transverse tensile strength of 45 MPa, an initial transverse elongation at break of over 75%, and an initial puncture resistance of over 15 N / mm. Furthermore, it has a long functional period of over 90 days, which prevents the composite mulch film from rapidly degrading and breaking in the early stages, thus preventing the rapid loss of water and nutrients from the soil. At 180 days, the degradation rate of the composite mulch film (ISO 14855 standard) reaches over 92%. In other words, the composite mulch film prepared according to the present invention balances the performance requirements of high initial strength, long functional period, and rapid degradation in the later stages, resolving the contradiction between "high initial strength and long functional period" and "rapid degradation in the later stages" in the prior art.

[0099] In Comparative Example 1, no core-shell degradation regulator was used in the surface layer. Although the resulting composite mulch film had good initial mechanical properties, its degradation rate was low in the later stages. In Comparative Example 2, imidazole phosphate esters were used directly in the surface layer without shell protection. The degradation rate of the resulting composite mulch film was uncontrollable, and it degraded rapidly in the early stages, resulting in a short functional period. In Comparative Example 3, no PBAT or epoxy-containing compatibilizers were used for reinforcement and modification in the intermediate layer. Although the degradation performance of the resulting composite mulch film was acceptable, its initial mechanical properties were poor. In Comparative Example 4, the composite film was only a two-layer structure and did not contain an intermediate layer that provided strength support. The resulting composite mulch film had poor initial mechanical properties and a slightly shorter functional period.

Claims

1. A high-strength polylactic acid composite mulch film with controllable biodegradability, characterized in that, The material has a three-layer composite structure, consisting of a surface layer, a middle layer, and an inner layer from top to bottom. The surface layer comprises PLA, PPC, PCL, and a core-shell degradation regulator, with the core-shell degradation regulator using imidazole phosphate esters as the core and calcium pyrophosphate as the shell. The middle layer comprises PLA, PBAT, an epoxy-containing compatibilizer, and an epoxy-containing chain extender. The inner layer comprises PLA, starch, and humic acid. The surface layer comprises 10-15 wt% PPC, 10-15 wt% PCL, 6-8 wt% core-shell degradation regulator, and the remainder PLA. The middle layer comprises 20-25 wt% PBAT, 3-5 wt% epoxy-containing compatibilizer, 0.5-0.8 wt% epoxy-containing chain extender, and the remainder PLA. The inner layer comprises 15-20 wt% starch, 3-5 wt% humic acid, and the remainder PLA.

2. The high-strength polylactic acid composite mulch film with controllable biodegradability according to claim 1, characterized in that, The thickness ratio of the surface layer, intermediate layer, and inner layer is 1~1.5: 1.5~2:

1.

3. The high-strength polylactic acid composite mulch film with controllable biodegradability according to claim 1, characterized in that, The raw materials of the intermediate layer also include 0.5~1.0wt% talc powder with a particle size D50 of 2~4μm; the raw materials of the surface layer, intermediate layer and inner layer also include 0.2~1wt% mildew inhibitor, which is a long-chain alkyl quaternary ammonium salt, specifically selected from at least one of dodecyl dimethyl benzyl ammonium chloride and didecyl dimethyl ammonium chloride.

4. The high-strength polylactic acid composite mulch film with controllable biodegradability according to claim 1, characterized in that, The imidazole phosphate ester is selected from at least one of 1-butyl-3-methylimidazolium phosphate, 1-butyl-3-methylimidazolium phosphate, 1-ethyl-3-methylimidazolium-dibutyl phosphate, and 1-ethyl-3-methylimidazolium-diethyl phosphate.

5. The high-strength polylactic acid composite mulch film with controllable biodegradability according to claim 1, characterized in that, The preparation method of the core-shell structure degradation regulator includes the following steps: S1. Dissolve the emulsifier in an aqueous pyrophosphate solution to form an aqueous phase solution; S2. Add imidazole phosphate to the aqueous solution and stir to emulsify, to obtain an oil-in-water emulsion; S3. Add the calcium salt aqueous solution dropwise to the emulsion and carry out the interfacial chemical deposition reaction under stirring. After the reaction is completed, cool, wash and dry to obtain a core-shell structure degradation regulator with an imidazole phosphate core and a calcium pyrophosphate shell. The amount of calcium salt used is 15~25wt% of the imidazole phosphate.

6. The high-strength polylactic acid composite mulch film with controllable biodegradability according to claim 5, characterized in that, In step S1, the amount of emulsifier used is 3-5 wt% of the pyrophosphate aqueous solution. The emulsifier is a nonionic surfactant with an HLB value of 12-15, selected from at least one of O-10, NP-10, and OP-10. The concentration of the pyrophosphate aqueous solution is 4-8 wt%, and the pyrophosphate is at least one of sodium pyrophosphate and potassium pyrophosphate. The amount of imidazole phosphate ester used in step S2 is 15~20wt% of the aqueous solution, and the stirring emulsification conditions are: temperature 45~55℃, time 20~40min, stirring speed 500~700r / min; In step S3, the amount of calcium salt used is 20-25 wt% of imidazole phosphate ester, and the calcium salt is at least one of calcium hypochlorite, calcium chloride, and calcium bicarbonate. The concentration of the calcium salt aqueous solution is 3-5 wt%. The conditions for the interfacial chemical deposition reaction are: temperature 40-50℃, stirring speed 150-250 r / min, and the calcium salt aqueous solution is added dropwise over 60-90 min.

7. The high-strength polylactic acid composite mulch film with controllable biodegradability according to claim 1, characterized in that, The PLA has a weight-average molecular weight of 80,000 to 150,000 Da and a melt index of 10 to 20 g / 10 min at 190°C / 2.16 kg; the PPC has a weight-average molecular weight of 80,000 to 150,000 Da and a melt index of 10 to 15 g / 10 min at 190°C / 2.16 kg; the PCL has a weight-average molecular weight of 60,000 to 100,000 Da and a melt index of 12 to 18 g / 10 min at 160°C / 2.16 kg; the PBAT has a weight-average molecular weight of 100,000 to 150,000 Da and a melt index of 10 to 20 g / 10 min at 190°C / 2.16 kg. The concentration is 8-15 g / 10 min under the following conditions; the starch is at least one of corn starch, sweet potato starch, potato starch, and cassava starch; the epoxy-containing compatibilizer is a linear copolymer of glycidyl methacrylate and methacrylate, with an epoxy equivalent of 250-350 g / mol; the epoxy-containing chain extender is selected from at least one of ADR-4468 and ADR-4370S; the carboxyl content of the humic acid is 1.8-2.5 mmol / g, of which the fulvic acid content is 8-15 wt%.

8. The method for preparing the controllable biodegradable high-strength polylactic acid composite mulch film according to any one of claims 1-7, characterized in that, Includes the following steps:

1. Mix the raw materials of the surface layer, the intermediate layer, and the inner layer according to the mass ratio, and then perform melt extrusion, pelletizing, and drying to obtain the corresponding surface layer masterbatch, intermediate layer masterbatch, and inner layer masterbatch.

2. Add the surface masterbatch, intermediate masterbatch and inner masterbatch to a three-layer co-extrusion blown film machine, and obtain a high-strength polylactic acid composite mulch film that can be controlled to degrade through blown film forming.

9. The preparation method according to claim 8, characterized in that, In step 1, the raw materials are pre-dried and then melt-extruded, wherein the starch is pre-dried to a moisture content ≤0.5wt%; the melt extrusion conditions are: melt extrusion and granulation in a twin-screw extruder, with a melt temperature of 160~180℃ and a screw speed of 200~300r / min; the drying is carried out at 60~80℃ for 12~24h. In step 2, the blowing machine operates at a speed of 70-120 r / min, a temperature of 160-180℃, a blowing ratio of 2-3:1, and a traction speed of 10-20 m / min. These parameters are adjusted according to the thickness of the composite mulch film.

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