Multicomponent polyester compound fully degradable plastic bag and preparation method thereof

Abstract

The application discloses a kind of multivariate polyester compound fully degradable plastic bag and preparation method thereof, it is related to fully biodegradable plastic bag preparation technical field, raw material includes aliphatic-aromatic copolyester 50 to 65 parts by weight, polylactic acid 12 to 20 parts, polyhydroxyalkanoate 8 to 15 parts, modified nanocrystalline cellulose 3 to 8 parts, composite plasticizer 2 to 6 parts, crosslinking agent 0.3 to 1.2 parts, antioxidant 0.2 to 0.8 parts, slip agent 0.1 to 0.5 parts;Aliphatic-aromatic copolyester is polybutylene adipate terephthalate and polybutylene succinate terephthalate compound according to mass ratio 2 to 1 to 3 to 1, modified nanocrystalline cellulose is the filler prepared by KH550 silane modification.The application effectively improves the interface compatibility of multivariate blending system, realizes the balance of mechanical property and degradation rate, the tensile strength and tear strength of product are significantly higher than existing similar products, the plasticizer migration rate is greatly reduced, and meets the food contact safety requirements.

Description

Technical Field

[0001] This invention relates to the field of fully biodegradable plastic bag preparation technology, and in particular to a multi-component polyester compound fully biodegradable plastic bag and its preparation method. Background Technology

[0002] This invention utilizes a ternary matrix of aliphatic-aromatic copolyester, polylactic acid, and polyhydroxyalkanoate, combined with epoxy or peroxide crosslinking agents and silane-modified nanocrystalline cellulose. During melt blending, stable covalent bonds can be formed between different polyester molecular chains and between polyester and nanofillers, significantly improving the interfacial bonding of the multiphase system. At the same time, polyhydroxyalkanoate can regulate the degradation rate of the system, achieving a balance between mechanical properties and degradation performance. This effectively solves the defects of existing binary blend systems, such as weak interfacial bonding, difficulty in balancing mechanical properties and degradation rate, and easy bag breakage during use.

[0003] This invention employs a composite plasticizer made by combining small-molecule citrate esters and large-molecule polyethylene glycol. The small-molecule component fills the gaps between polyester molecular chains to provide sufficient plasticizing effect, while the large-molecule component is anchored in the polyester molecular network through hydrogen bonds to form an anti-migration barrier, which greatly reduces the risk of surface migration of the plasticizer. This effectively solves the defects of existing technologies, such as easy migration of small-molecule plasticizers, easy cracking of products after long-term use, and insufficient food contact safety.

[0004] This invention employs a two-stage humidity control process of raw material pre-drying and secondary drying of blended granules, combined with a vacuum degassing process in the melt extrusion stage, to strictly control the moisture content in the system and prevent the polyester from undergoing hydrolysis and chain breakage during the high-temperature melting stage. Combined with a dual-nozzle negative pressure cooling process in the blown film stage, the uniformity of film thickness is improved, effectively solving the defects of existing technologies such as unstable batch performance caused by raw material hydrolysis, poor uniformity of film thickness, and low production yield.

[0005] The technical solution of this invention can be directly adapted to existing traditional PE blown film production lines without large-scale production line transformation. It can adjust the formula parameters according to demand to prepare products with different performance orientations, and adapt to the application needs of multiple scenarios such as supermarket shopping bags, food contact packaging bags, and household waste collection bags. All raw materials used are commercially mature compostable materials, the cost is controllable, and it has high industry promotion value. Summary of the Invention

[0006] To address the problems of poor interfacial compatibility, difficulty in balancing mechanical and degradation properties, easy migration of plasticizers, and low batch stability of existing fully biodegradable plastic bags, this invention provides a multi-element polyester compound fully biodegradable plastic bag and its preparation method.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a multi-element polyester composite fully degradable plastic bag, suitable for daily supermarket shopping, food contact packaging, and household waste collection scenarios, wherein the raw material components are as follows, based on the total weight of the raw materials: The product contains 35-55 parts compostable aliphatic-aromatic copolyester, 20-30 parts bio-based polylactic acid, 10-20 parts microbially synthesized polyhydroxy fatty acid ester, 5-12 parts surface-modified bio-based nanocrystalline cellulose, 2-6 parts fully degradable composite plasticizer, 0.5-2 parts food contact grade crosslinking agent, 0.3-1.2 parts environmentally friendly antioxidant, and 0.2-1 parts fatty amide slip agent. All raw materials comply with the safety requirements of GB 4806.6 Plastic materials and products for food contact.

[0008] Preferably, the aliphatic-aromatic copolyester is at least one of polybutylene adipate terephthalate and polybutylene succinate terephthalate, and its biodegradability meets the requirements of GB / T 28206; wherein the end carboxyl group content of polybutylene adipate terephthalate is ≤20mol / t, the melt index of polybutylene succinate terephthalate is 3~8g / 10min, the test conditions are 190℃ and 2.16kg, the number average molecular weight of the two copolyesters is 50000~120000, the molecular weight distribution coefficient is 1.2~1.8, and the mass ratio of the two copolyesters when used in combination is 1~3:1.

[0009] Preferably, the modified nanocrystalline cellulose is cotton-derived nanocrystalline cellulose modified with silane coupling agent KH550. The preparation process involves dispersing cotton-derived nanocrystalline cellulose in a 70% (v / v) aqueous ethanol solution, adjusting the pH of the system to 8-9, adding 3-8% (w / v) of silane coupling agent KH550 by mass of nanocrystalline cellulose, stirring and reacting at 60-70°C for 2-4 hours, filtering, washing with anhydrous ethanol, and vacuum drying to obtain the product. The product has a grafting rate ≥6%, a moisture content ≤2%, a particle size of 20-80 nm, a particle size distribution coefficient ≤1.5, and a crystallinity ≥75%.

[0010] Preferably, the composite plasticizer is obtained by compounding tributyl citrate, acetylated tributyl citrate, and polyethylene glycol 2000 in a mass ratio of 1~2:1:0.3~0.5. All components meet the requirements for compostability and food contact safety. The compounding process involves first adding the three components into a stirred tank with a heating jacket in proportion, and stirring and mixing for 15~20 minutes at 40~50℃ and 100~200 r / min to obtain a uniform and transparent composite plasticizer system with a moisture content ≤0.1% and no mechanical impurities.

[0011] Preferably, the crosslinking agent is at least one of trimethylolpropane triglycidyl ether and dicumyl peroxide, both of which are food contact grade, wherein the epoxy value of trimethylolpropane triglycidyl ether is 0.6~0.7 eq / 100g, and the purity of dicumyl peroxide is ≥99%; the antioxidant is obtained by compounding antioxidant 1010 and antioxidant 168 in a mass ratio of 1:1, and the compounding process is to put the two antioxidants into a high-speed mixer and mix them for 5~10 minutes at room temperature and a speed of 200~300 r / min, and the volatile matter of the product is ≤0.5%; the slip agent is at least one of erucamide and oleamide, with a melting point of 70~85℃ and an acid value ≤1mgKOH / g.

[0012] Preferably, a method for preparing the aforementioned multi-polymer polyester compound fully degradable plastic bag, suitable for industrial continuous production, includes the following steps: S1. Weigh each raw material according to the weight proportions. Place the aliphatic-aromatic copolyester, polylactic acid, and polyhydroxy fatty acid ester in a vacuum drying oven at 60-80℃ with a vacuum degree of -0.08~-0.1MPa and dry for 4-8 hours. During the drying process, replace the gas in the oven with a blower every 1-2 hours to avoid cross-reaction of the raw materials. After drying, use a Karl Fischer moisture analyzer to test the moisture content and ensure that the moisture content of the three polyester raw materials is ≤50ppm. Unqualified raw materials are returned for re-drying. S2. The dried three types of polyester, modified nanocrystalline cellulose, composite plasticizer, crosslinking agent, antioxidant, and slip agent are put into a high-speed mixer with a constant temperature jacket. Nitrogen gas is introduced throughout the process to prevent the raw materials from oxidizing. The mixture is mixed for 10 to 15 minutes at 80~100℃ and 300~500r / min. After the mixture is completed, the material is cooled to below 40℃ and discharged to obtain a loose and non-lumpy premix. S3. The premixed material is fed into a twin-screw extruder with a length-to-diameter ratio of 40~48:1 for melt blending and extrusion granulation. The temperatures of each section of the twin-screw extruder from the feed section to the die head are set as follows: 130~140℃, 145~155℃, 160~170℃, 175~185℃, 185~190℃, and 180~185℃, respectively. The die head temperature is 175~180℃, and the screw speed is 150~300r / min. Granulation is carried out by air-cooled pelleting to obtain blended granules with a particle size of 3~5mm and a melt index of 4~10g / 10min. The test conditions are 190℃ and 2.16kg. S4. The blended granules are fed into a single-layer co-extrusion blown film machine to blow film and make bags. The temperature of each section of the blown film machine is set to 120~180℃, the blow-up ratio is 2.5~4, the traction speed is 10~20m / min, the film bubble is cooled by top blowing, and the bag is sealed by heat sealing at a temperature of 120~150℃, finally obtaining a multi-polyester composite fully degradable plastic bag.

[0013] Preferably, in the melt co-extrusion process of step S3, the twin-screw extruder is equipped with two independent vacuum exhaust ports, located at the third and seventh sections of the extruder respectively. Each exhaust port is independently connected to a vacuum stabilizing device, and the vacuum degree is maintained at -0.06~-0.09MPa with a vacuum degree fluctuation range of ≤±0.005MPa, continuously discharging small molecule impurities in the melt system.

[0014] Preferably, before the blended granules obtained in step S3 are fed into the blown film machine hopper, they are first placed in a vacuum drying oven at 70~90℃ with a vacuum degree of -0.08~-0.1MPa and dried for 3~6 hours. During the drying process, the granules are turned over once every 1 hour by a matching turning device to ensure that the granules are heated and dried evenly. After drying, the moisture content is tested by a Karl Fischer moisture analyzer to ensure that the moisture content of the blended granules is ≤30ppm. The dried granules are transported to the blown film machine in an insulated and sealed hopper to prevent the granules from absorbing moisture from the air.

[0015] Preferably, during the film blowing process in step S4, the cooling air ring is a dual-outlet negative pressure air ring with the upper and lower air outlets having outlet angles of 30°~45° and 60°~75° respectively. The cooling air is sent into the air ring after passing through three stages of coarse, medium and high efficiency filtration. The outlet air temperature is 15~25℃, the outlet air velocity is 5~10m / s, and the outlet air velocity uniformity is ≥95%.

[0016] Preferably, the multi-component polyester compound fully degradable plastic bag prepared in step S4 has a thickness of 0.015~0.05mm, a thickness uniformity of ≥92%, a haze of ≤30%, a tensile strength of ≥18MPa as tested according to GB / T 1040.3, an elongation at break of ≥350% as tested according to GB / T1040.3, a right-angle tear strength of ≥60kN / m, a heat-sealing strength of ≥30N / 15mm, a heavy metal content that meets the requirements of GB4806.6, a biodegradability rate that meets the requirements of GB / T 19277.1, and a degradation rate of ≥90% after 180 days under composting conditions, meeting the requirements of GB / T 20197 compostable plastics.

[0017] The present invention has the following beneficial effects: This invention utilizes a ternary matrix of aliphatic-aromatic copolyester, polylactic acid, and polyhydroxyalkanoate, combined with epoxy or peroxide crosslinking agents and silane-modified nanocrystalline cellulose. During melt blending, stable covalent bonds can be formed between different polyester molecular chains and between polyester and nanofillers, significantly improving the interfacial bonding of the multiphase system. At the same time, polyhydroxyalkanoate can regulate the degradation rate of the system, achieving a balance between mechanical properties and degradation performance. This effectively solves the defects of existing binary blend systems, such as weak interfacial bonding, difficulty in balancing mechanical properties and degradation rate, and easy bag breakage during use.

[0018] This invention employs a composite plasticizer made by combining small-molecule citrate esters and large-molecule polyethylene glycol. The small-molecule component fills the gaps between polyester molecular chains to provide sufficient plasticizing effect, while the large-molecule component is anchored in the polyester molecular network through hydrogen bonds to form an anti-migration barrier, which greatly reduces the risk of surface migration of the plasticizer. This effectively solves the defects of existing technologies, such as easy migration of small-molecule plasticizers, easy cracking of products after long-term use, and insufficient food contact safety.

[0019] This invention employs a two-stage humidity control process of raw material pre-drying and secondary drying of blended granules, combined with a vacuum degassing process in the melt extrusion stage, to strictly control the moisture content in the system and prevent the polyester from undergoing hydrolysis and chain breakage during the high-temperature melting stage. Combined with a dual-nozzle negative pressure cooling process in the blown film stage, the uniformity of film thickness is improved, effectively solving the defects of existing technologies such as unstable batch performance caused by raw material hydrolysis, poor uniformity of film thickness, and low production yield.

[0020] The technical solution of this invention can be directly adapted to existing traditional PE blown film production lines without large-scale production line transformation. It can adjust the formula parameters according to demand to prepare products with different performance orientations, and adapt to the application needs of multiple scenarios such as supermarket shopping bags, food contact packaging bags, and household waste collection bags. All raw materials used are commercially mature compostable materials, the cost is controllable, and it has high industry promotion value. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the process for preparing a multi-component polyester composite fully degradable plastic bag and its preparation method proposed in this invention; Figure 2 This is a grouped bar chart comparing the core mechanical properties of embodiments and comparative examples of the present invention; Figure 3 This is a radar chart comparing the comprehensive performance of the embodiments and comparative examples of the present invention across all dimensions. Figure 4 The figures shown are biline graphs of plasticizer migration rate and compost degradation rate in the embodiments and comparative examples of this invention. Figure 5 This is a scatter plot showing the correlation between the amount of modified nanocrystalline cellulose added and its performance in this invention. Detailed Implementation

[0022] The following will refer to the appendices in the embodiments of the present invention. Figures 1-5 The technical solutions in the embodiments of the present invention are clearly and completely described herein. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0023] This specific embodiment addresses four types of defects in existing fully degradable plastic bags: poor interfacial compatibility between polylactic acid and aliphatic-aromatic copolyester blends, easy migration of plasticizers, performance degradation due to raw material hydrolysis during the preparation process, and difficulty in balancing mechanical properties and degradation rate. The following examples and comparative examples provide a detailed description. All experimental raw materials and operating parameters meet the requirements for repeatability. Those skilled in the art can repeat the experiments based on the following description to obtain the corresponding results.

[0024] Unless otherwise specified, all raw materials used in the embodiments and comparative examples meet the following requirements: Aliphatic-aromatic copolyesters: Polybutylene terephthalate (PBAT) with a number-average molecular weight of 80,000, a molecular weight distribution coefficient of 1.5, and a terminal carboxyl group content of 15 mol / t; Polybutylene terephthalate (PBST) with a number-average molecular weight of 70,000, a molecular weight distribution coefficient of 1.4, and a melt index of 5 g / 10 min (190℃, 2.16 kg). Both meet the biodegradability requirements of GB / T 28206.

[0025] Polylactic acid (PLA): Bio-based polylactic acid, L-lactic acid content ≥99%, number average molecular weight 100,000, melt index 6 g / 10 min (190℃, 2.16 kg).

[0026] Polyhydroxy fatty acid ester (PHA): Poly-3-hydroxybutyrate-3-hydroxyvalerate (PHBV), hydroxyvalerate content 8 mol%, number average molecular weight 60,000, melt index 4 g / 10 min (190℃, 2.16 kg).

[0027] Modified nanocrystalline cellulose: Cotton-derived nanocrystalline cellulose modified with KH550 was prepared by dispersing cotton-derived nanocrystalline cellulose with a particle size of 30 nm and a crystallinity of 80% in a 70% (v / v) ethanol aqueous solution, adjusting the pH of the system to 8.5, adding 5% (w / v) of silane coupling agent KH550, stirring and reacting at 65 °C for 3 h, filtering, washing three times with anhydrous ethanol, and vacuum drying at 60 °C for 12 h to obtain the grafting rate of 7.2%, moisture content of 1.2%, and particle size distribution coefficient of 1.3.

[0028] Composite plasticizer: Tributyl citrate (TBC), acetylated tributyl citrate (ATBC), and polyethylene glycol 2000 (PEG2000) are compounded in proportion. The compounding process is to put the three components into a stirred tank with a heating jacket and stir at 45°C and 150 r / min for 18 min to obtain a homogeneous and transparent system with a moisture content of 0.08%.

[0029] Crosslinking agent: Trimethylolpropane triglycidyl ether (TGIC) is food contact grade with an epoxy value of 0.65 eq / 100g; dicumyl peroxide (DCP) has a purity of ≥99.2%.

[0030] Antioxidant: Antioxidant 1010 and Antioxidant 168 are mixed at a mass ratio of 1:1 at room temperature at 250 r / min for 8 min to obtain the product, with a volatile content of 0.3%.

[0031] Slip agents: Erucamide melting point 78℃, acid value 0.8mgKOH / g; Oleamide melting point 72℃, acid value 0.7mgKOH / g.

[0032] Example 1 The multi-component polyester compound fully degradable plastic bag prepared in this embodiment has the following raw material components by weight: 35 parts PBAT, 30 parts PLA, 20 parts PHBV, 5 parts modified nanocrystalline cellulose, 2 parts composite plasticizer (TBC:ATBC:PEG2000=1:1:0.3), 0.5 parts TGIC, 0.3 parts compound antioxidant, and 0.2 parts erucamide.

[0033] The preparation method is as follows: S1. Weigh each raw material according to the specified weight proportions. Place PBAT, PLA, and PHBV in a vacuum drying oven at 60℃ with a vacuum degree of -0.08MPa and dry for 8 hours. Replace the gas in the oven with forced air every hour. After drying, use a Karl Fischer moisture analyzer to test the moisture content of the three raw materials, which are 42ppm, 45ppm, and 40ppm, respectively, all meeting the requirement of ≤50ppm. This step strictly controls the moisture content of the raw materials, solving the defect in existing technologies where excessive moisture in the raw materials leads to polyester hydrolysis and performance degradation during melt blending.

[0034] S2. The dried three types of polyester, modified nanocrystalline cellulose, composite plasticizer, TGIC, compound antioxidant, and erucamide are put into a high-speed mixer with a constant temperature jacket. Nitrogen gas is introduced throughout the process to prevent oxidation of the raw materials. The mixture is mixed for 15 minutes at 80°C and 300 r / min. After mixing, the material is cooled to 38°C and discharged to obtain a loose and non-lumpy premix.

[0035] S3. The premixed material is fed into a twin-screw extruder with a length-to-diameter ratio of 44:1 for melt blending and extrusion granulation. The temperatures of each section of the twin-screw extruder are set sequentially from the feed section to the die head as 130℃, 145℃, 160℃, 175℃, 185℃, and 180℃, with the die head temperature at 175℃ and the screw speed at 150 r / min. During the extrusion process, the vacuum degree of the two independent vacuum exhaust ports of the 3rd and 7th sections of the barrel is maintained at -0.06MPa, with a fluctuation range of ≤±0.005MPa, to continuously remove small molecule impurities from the melt system. Air-cooled pelletizing is used to obtain blended granules with a particle size of 3~5mm and a melt index of 4.2g / 10min (190℃, 2.16kg).

[0036] The blended granules were placed in a vacuum drying oven at 70℃ with a vacuum degree of -0.08MPa and dried for 6 hours. The granules were turned over every hour to ensure uniform drying. After drying, the moisture content was 27ppm, which meets the requirement of ≤30ppm. The granules were transported to the blown film machine in an insulated and sealed hopper to avoid secondary water absorption.

[0037] In this step, the crosslinking reaction between the crosslinking agent TGIC and the end groups of the polyester molecule is as follows: TGIC epoxy group + R-COOH (polyester terminal carboxyl group) → R-COO-CH2-CH(OH)-R'; TGIC epoxy group + R-OH (polyester terminal hydroxyl group) → RO-CH2-CH(OH)-R'; This reaction occurs simultaneously during melt blending, allowing for the grafting of different types of polyester molecular chains at the same time. This enhances the interfacial bonding of the multiphase system and overcomes the poor compatibility of multi-component polyester blends in existing technologies. S4. The blended granules are fed into a single-layer co-extrusion blown film machine for blown film production and bag making. The temperature of each section of the blown film machine is set to 120~170℃, the blow-up ratio is 2.5, and the traction speed is 10m / min. The cooling air ring is a dual-outlet negative pressure air ring with upper and lower outlet angles of 30° and 60° respectively. The cooling air is filtered through three stages of coarse, medium, and high-efficiency filters before entering the air ring. The outlet air temperature is 15℃, the outlet air velocity is 5m / s, and the air velocity uniformity is 96%. The heat sealing temperature is 120℃, ultimately yielding a 0.015mm thick multi-component polyester composite fully biodegradable plastic bag. This step uses a dual-outlet cooling air ring to ensure uniform cooling of the membrane bubble, solving the defects of poor thickness uniformity and large performance fluctuations in existing technologies. Example

[0038] The multi-component polyester compound fully degradable plastic bag prepared in this embodiment has the following raw material components by weight: The ingredients are: 55 parts PBST, 20 parts PLA, 10 parts PHBV, 12 parts modified nanocrystalline cellulose, 6 parts composite plasticizer (TBC:ATBC:PEG2000 = 2:1:0.5), 2 parts DCP, 1.2 parts compound antioxidant, and 1 part oleamide. The preparation method is as follows: S1. Weigh each raw material according to the weight proportions, and place PBST, PLA and PHBV in a vacuum drying oven at 80℃ with a vacuum degree of -0.1MPa for 4 hours. Replace the gas in the oven with a blower every 2 hours. After drying, the moisture content of the three raw materials is measured to be 38ppm, 40ppm and 35ppm respectively, all of which meet the requirement of ≤50ppm.

[0039] S2. The dried three types of polyester, modified nanocrystalline cellulose, composite plasticizer, DCP, compound antioxidant, and oleamide are added to a high-speed mixer with a constant temperature jacket. Nitrogen gas is introduced throughout the process for protection. The mixture is mixed for 10 minutes at 100°C and 500 r / min. After mixing, the material is cooled to 35°C and discharged to obtain a loose, non-lumpy premix. In this embodiment, the small-molecule TBC and ATBC in the composite plasticizer provide plasticizing efficiency, while the large-molecule PEG2000 is anchored in the interstices of the polyester molecular chains, solving the defects of easy migration and poor product stability of single plasticizers in the prior art.

[0040] S3. The premixed material is fed into a twin-screw extruder with a length-to-diameter ratio of 44:1 for melt blending and granulation. The temperatures of each section of the twin-screw extruder from the feed section to the die head are set to 140℃, 155℃, 170℃, 185℃, 190℃, and 185℃ respectively, with the die head temperature at 180℃ and the screw speed at 300 r / min. During extrusion, the vacuum degree of the two vacuum exhaust ports is maintained at -0.09MPa, with a fluctuation range of ≤±0.005MPa. The mixture is air-cooled and pelletized to obtain blended granules with a particle size of 3~5mm and a melt index of 9.7g / 10min (190℃, 2.16kg). The blended granules are placed in a 90℃ vacuum drying oven with a vacuum degree of -0.1MPa and dried for 3 hours, turning the granules every hour. After drying, the moisture content is 22ppm, which meets the requirement of ≤30ppm. The granules are then conveyed to the blown film machine using an insulated and sealed hopper.

[0041] S4. The blended granules are fed into a single-layer co-extrusion blown film machine to blow film and make bags. The temperature of each section of the blown film machine is set to 130~180℃, the blow-up ratio is 4, and the traction speed is 20m / min. The air outlet angles of the upper and lower air outlets of the cooling air ring are 45° and 75° respectively. The cooling air is sent into the air ring after being filtered through three stages. The outlet air temperature is 25℃, the outlet air velocity is 10m / s, and the air velocity uniformity is 97%. The heat sealing temperature is 150℃, and finally a multi-polyester composite fully degradable plastic bag with a thickness of 0.05mm is obtained. Example

[0042] The multi-component polyester compound fully degradable plastic bag prepared in this embodiment has the following raw material components by weight: 25 parts PBAT, 20 parts PBST (the mass ratio of the two is 1.25:1, which meets the 1~3:1 compounding requirement of claim 2), 25 parts PLA, 15 parts PHBV, 8 parts modified nanocrystalline cellulose, 4 parts composite plasticizer (TBC:ATBC:PEG2000=1.5:1:0.4), 1 part TGIC, 0.8 parts compound antioxidant, and 0.6 parts erucamide.

[0043] The preparation method is as follows: S1. Weigh each raw material according to the weight proportions, and place PBAT, PBST, PLA, and PHBV in a vacuum drying oven at 70℃ with a vacuum degree of -0.09MPa for 6 hours. Replace the gas in the oven with a blower every 1.5 hours. After drying, the moisture content of the four raw materials is tested and found to be ≤45ppm, which meets the requirements.

[0044] S2. The dried four types of polyester, modified nanocrystalline cellulose, composite plasticizer, TGIC, compound antioxidant, and erucamide are put into a high-speed mixer with a constant temperature jacket. Nitrogen gas is introduced throughout the process for protection. The mixture is mixed for 12 minutes at 90°C and 400 r / min. After the mixture is completed, the material is cooled to 36°C and discharged to obtain a loose and non-lumpy premix.

[0045] S3. The premixed material is fed into a twin-screw extruder with a length-to-diameter ratio of 44:1 for melt blending and granulation. The temperatures of each section of the twin-screw extruder from the feed section to the die head are set sequentially as follows: 135℃, 150℃, 165℃, 180℃, 188℃, and 182℃, with a die head temperature of 178℃ and a screw speed of 220 r / min. During extrusion, the vacuum degree at the two vacuum exhaust ports is maintained at -0.08 MPa, with a fluctuation range of ≤±0.005 MPa. Air-cooled pelleting yields blended granules with a particle size of 3~5 mm and a melt index of 6.8 g / 10 min (190℃, 2.16 kg). The blended granules are placed in an 80℃ vacuum drying oven with a vacuum degree of -0.09 MPa and dried for 4.5 h, turning the granules every 1 h. After drying, the moisture content is 25 ppm, which meets the requirements. The granules are then conveyed to the blown film machine using an insulated and sealed hopper.

[0046] S4. The blended granules are fed into a single-layer co-extrusion blown film machine to blow film and make bags. The temperature of each section of the blown film machine is set to 125~175℃, the blow-up ratio is 3.2, and the traction speed is 15m / min. The air outlet angles of the upper and lower air outlets of the cooling air ring are 38° and 68° respectively. The cooling air is sent into the air ring after three-stage filtration. The outlet air temperature is 20℃, the outlet air velocity is 7m / s, and the air velocity uniformity is 96.5%. The heat sealing temperature is 135℃, and finally a multi-polyester composite fully degradable plastic bag with a thickness of 0.025mm is obtained.

[0047] This embodiment uses a blend of PBAT and PBST as the matrix, combined with the heterogeneous nucleation effect of modified nanocrystalline cellulose, to balance the mechanical toughness, stiffness and degradation rate of the product, thus solving the defect of the difficulty in balancing mechanical properties and degradation rate in the prior art.

[0048] Comparative Example 1 This comparative example uses existing conventional fully degradable plastic bag preparation technology. The raw materials, by weight, are: 60 parts PBAT, 40 parts PLA, 8 parts unmodified nanocrystalline cellulose, 4 parts single plasticizer TBC, 1 part TGIC, 0.8 parts compound antioxidant, and 0.6 parts erucamide. The preparation method is as follows: S1, PBAT and PLA are dried in a 70℃ forced-air drying oven for 6 hours, and the moisture content after drying is 120 ppm. S2, The dried PBAT and PLA are added to a high-speed mixer and mixed at 300 r / min at room temperature for 10 minutes to obtain a premix without nitrogen protection.

[0049] S3. The premixed material is fed into a twin-screw extruder for melt blending and granulation. The extrusion temperature is 130~180℃. There is no vacuum exhaust device. The resulting blended granules are used directly for blown film production without secondary drying.

[0050] S4. Blow-blown film to make bags, with a blow-blowing ratio of 3.2 and a cooling air of room temperature natural air, to obtain a fully biodegradable plastic bag with a thickness of 0.025mm.

[0051] This comparative example corresponds to a typical solution in the prior art, which has the following defects: insufficient drying of raw materials, lack of vacuum exhaust, single plasticizer, and unmodified nanofiller.

[0052] Table 1. Raw material ratios for each embodiment and comparative example.

[0053] This table clarifies the differences in raw material composition between each embodiment and the comparative example. Embodiments 1 to 3 all use a ternary polyester matrix, modified nanocrystalline cellulose, and a composite plasticizer, which completely covers all the raw material characteristics of claim 1. The comparative example uses a binary polyester matrix, unmodified nanofillers, and a single plasticizer, which corresponds to the typical formulation of the prior art. This table can intuitively demonstrate the formulation differences between the present invention and the prior art.

[0054] Table 2. Performance test results of each embodiment and comparative example.

[0055] Table 2 shows that the performance of Examples 1 to 3 is significantly better than that of Comparative Example 1, which uses the prior art. The tensile strength and elongation at break of the Examples are increased by 45.6% to 94.4% and 100% to 133.3% respectively compared to the Comparative Example, verifying the effect of the multi-component polyester crosslinking system on improving interfacial compatibility; the plasticizer migration rate is only 3.7% to 11.1% of that of the Comparative Example, verifying the anti-migration effect of the composite plasticizer; the 180-day compost degradation rate is not less than 90%, while meeting the mechanical performance requirements, verifying the balancing effect of the formulation system on mechanical properties and degradation rate; the thickness uniformity is increased by 13.9% to 16.7% compared to the Comparative Example, verifying the effect of drying process, vacuum exhaust, and dual-vent cooling on the stability of the product. The heavy metal content of all Examples meets the food contact safety requirements and can be directly applied to food packaging scenarios.

[0056] Verification of poor compatibility defects in multi-phase polyester blends: The crosslinking agent used in the example undergoes a grafting reaction with the terminal hydroxyl and terminal carboxyl groups of the three polyesters during melt blending. At the same time, the KH550 coupling agent on the surface of the modified nanocrystalline cellulose can form chemical bonds with the hydrophobic polyester chain and the hydrophilic cellulose, significantly improving the interfacial bonding force of the multiphase. Compared with the formulation without modified nanofiller and without ternary polyester synergy, the mechanical properties are significantly improved.

[0057] Verification of plasticizer migration defects: In the composite plasticizer used in the example, small molecule TBC and ATBC fill the gaps between polyester molecular chains to provide plasticizing effect, while large molecule PEG2000 is anchored to polyester molecular chains through hydrogen bonds to prevent small molecule plasticizers from migrating to the surface of the product. In the comparative example, a single TBC plasticizing scheme is used, and the plasticizer migration rate is reduced by about 90%. This technical feature corresponds to the composite plasticizer feature of claim 4.

[0058] Verification of performance degradation caused by raw material hydrolysis: The example uses a two-stage vacuum drying process to control the moisture content of raw materials to ≤50ppm and the moisture content of blended granules to ≤30ppm. At the same time, the melt co-extrusion process uses a vacuum exhaust device to remove small molecule by-products in the system, avoiding hydrolysis and chain breakage of polyester molecular chains at high temperatures. In contrast, the process without strict moisture control and vacuum exhaust significantly improves the mechanical property stability of the product.

[0059] The example uses a ternary polyester compound system of PBAT / PBST / PLA / PHA, in which PBAT and PBST provide toughness, PLA provides stiffness, PHA regulates the degradation rate, and modified nanocrystalline cellulose acts as a heterogeneous nucleating agent to improve crystallization uniformity. While ensuring that the mechanical properties meet the requirements for use in plastic bags, the composting degradation rate after 180 days is ≥90%, which meets the standards for compostable plastics. Compared with the comparative binary polyester system, which has a degradation rate of only 78%, this example achieves a balance between performance and degradability.

[0060] The reaction between the crosslinking agent TGIC and the polyester end groups can proceed spontaneously within the temperature range of 130~190℃ during melt blending, without the need for additional catalysts. No small molecule toxic byproducts are generated during the reaction process, which meets food contact safety requirements. This reaction can form covalent bonds between different types of polyester molecular chains, avoiding phase separation in multiphase systems and improving the compatibility of the blend system at the molecular level. This directly addresses the defect of weak interfacial bonding in multi-component polyester blends in existing technologies.

[0061] Reference Figure 2 This figure visually highlights the effectiveness of this invention in addressing the core mechanical defects of biodegradable plastic bags. Existing conventional processes use simple blending of binary polyesters, resulting in poor interfacial compatibility, a tensile strength of only 12.5 MPa, and a heat-sealing strength of less than 20 N / 15 mm, making them prone to breakage and leakage. This invention constructs a covalent bond interface of multiple polyesters using a TGIC crosslinking agent, combined with modified nanocrystalline cellulose reinforcement. The three embodiments show an increase in tensile strength of 45.6%-94.4% and an increase in heat-sealing strength of 77.8%-133.3%. Embodiment 3, a general-purpose type, exhibits the best performance, fully meeting the usage requirements of different load-bearing scenarios.

[0062] Reference Figure 3 This radar chart comprehensively demonstrates the multi-dimensional performance advantages of this invention. Existing technologies have significant shortcomings: poor mechanical properties, severe plasticizer migration, and low thickness uniformity, failing to simultaneously meet performance and environmental requirements. This invention, through a combination of processes including two-stage vacuum drying, in-situ crosslinking, composite plasticizing, and dual-vent cooling, achieves simultaneous improvements in mechanical strength, heat-sealing performance, migration resistance, degradation efficiency, and production stability. Example 3 demonstrates optimal performance across all indicators, offering the highest overall cost-effectiveness and making it suitable for high-end applications such as food packaging.

[0063] Reference Figure 4 The conventional biaom process, as shown in the broken line graph, uses a single small-molecule plasticizer with a migration rate as high as 2.7%, posing a food safety hazard. Furthermore, the degradation of the binary polyester system is uncoordinated, with a degradation rate of only 78% after 180 days. This invention employs a TBC / ATBC / PEG2000 composite plasticizer system. The large-molecule PEG anchors the molecular chain, reducing the plasticizer migration rate to 0.1%-0.3%. Simultaneously, the ternary polyester compound regulates the degradation rate, achieving a compost degradation rate exceeding 90% after 180 days, thus balancing food safety and environmental friendliness.

[0064] Reference Figure 5This scatter plot reveals the intrinsic relationship between the core reinforcing component of this invention and product performance. When the amount of modified nanocrystalline cellulose added is increased from 5 parts to 8 parts, the surface KH550 coupling agent forms chemical bonds with the polyester, and heterogeneous nucleation improves crystallinity uniformity, resulting in a continuous increase in tensile strength and tear strength, while thickness uniformity is simultaneously improved. When the addition amount exceeds 8 parts, the nanofiller is prone to agglomeration, and the mechanical properties begin to decline. This invention controls the addition amount within the range of 5-12 parts, with 8 parts being the optimal value, achieving the best balance between reinforcing effect and processing stability.

[0065] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A multi-component polyester composite fully biodegradable plastic bag, characterized in that, The raw materials for preparation, by weight, include: 35-55 parts aliphatic-aromatic copolyester, 20-30 parts polylactic acid, 10-20 parts polyhydroxy fatty acid ester, 5-12 parts modified nanocrystalline cellulose, 2-6 parts composite plasticizer, 0.5-2 parts crosslinking agent, 0.3-1.2 parts antioxidant, and 0.2-1 parts slip agent.

2. The multi-element polyester composite fully degradable plastic bag according to claim 1, characterized in that, The aliphatic-aromatic copolyester is at least one of polybutylene adipate terephthalate and polybutylene succinate terephthalate, with a number average molecular weight of 50,000 to 120,000.

3. The multi-element polyester composite fully degradable plastic bag according to claim 1, characterized in that, The modified nanocrystalline cellulose is cotton-derived nanocrystalline cellulose modified with silane coupling agent KH550, with a particle size of 20~80nm and a crystallinity ≥75%.

4. The multi-element polyester composite fully degradable plastic bag according to claim 1, characterized in that, The composite plasticizer is obtained by compounding tributyl citrate, acetylated tributyl citrate, and polyethylene glycol 2000 in a mass ratio of 1~2:1:0.3~0.

5.

5. The multi-element polyester composite fully degradable plastic bag according to claim 1, characterized in that, The crosslinking agent is at least one of trimethylolpropane triglycidyl ether and dicumyl peroxide; the antioxidant is obtained by compounding antioxidant 1010 and antioxidant 168 in a mass ratio of 1:1; and the slip agent is at least one of erucamide and oleamide.

6. A method for preparing a multi-component polyester composite fully degradable plastic bag, used to prepare the multi-component polyester composite fully degradable plastic bag according to any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Weigh each raw material according to the weight parts, and place the aliphatic-aromatic copolyester, polylactic acid, and polyhydroxy fatty acid ester in a vacuum drying oven at 60~80℃ for 4~8h, and control the moisture content to ≤50ppm. S2. The dried three types of polyester, modified nanocrystalline cellulose, composite plasticizer, crosslinking agent, antioxidant, and slip agent are put into a high-speed mixer and mixed for 10 to 15 minutes at 80~100℃ and 300~500r / min to obtain a premix. S3. The premixed material is fed into a twin-screw extruder for melt blending and extrusion granulation to obtain blended granules. The temperature of each section of the twin-screw extruder is set to 130~190℃ and the screw speed is 150~300r / min. S4. The blended granules are fed into a blown film machine to blow film and make bags, resulting in a multi-polyester composite fully degradable plastic bag. The temperature of each section of the blown film machine is set to 120~180℃, the blow-up ratio is 2.5~4, and the traction speed is 10~20m / min.

7. The method for preparing a multi-component polyester composite fully degradable plastic bag according to claim 6, characterized in that, During the melt co-extrusion process in step S3, the vacuum level of the twin-screw extruder is maintained at -0.06 to -0.09 MPa.

8. The method for preparing a multi-component polyester compound fully degradable plastic bag according to claim 6, characterized in that, The blended granules obtained in step S3 are dried in a vacuum drying oven at 70~90℃ for 3~6h before blown film, and the moisture content is controlled to be ≤30ppm.

9. The method for preparing a multi-component polyester compound fully degradable plastic bag according to claim 6, characterized in that, During the blowing process in step S4, the outlet air temperature of the cooling air ring is 15~25℃, and the cooling air velocity is 5~10m / s.

10. The method for preparing a multi-component polyester compound fully degradable plastic bag according to claim 6, characterized in that, The plastic bag prepared in step S4 has a thickness of 0.015~0.05mm, a tensile strength ≥18MPa, an elongation at break ≥350%, and a degradation rate ≥90% after 180 days under composting conditions.

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