Multi-layer composite air bubble film and preparation method thereof

Abstract

The invention relates to the technical field of layered packaging materials, in particular to a multi-layer composite air bubble film and a preparation method thereof. The air bubble film prepared by the invention overcomes the problems of poor mechanical properties and weak barrier ability. Casein is hydrophobically modified through the cardanol-based polyurethane prepolymer, an isocyanate group is crosslinked with amino and hydroxyl of the casein, and a hydrophobic olefin chain segment is grafted, so that the hydrophobicity and compatibility are improved, the environment is protected, and harmful residues are avoided; a multi-layer composite structure of a bubble layer, a mechanical enhancement layer and a gas barrier layer is adopted, interface bonding of all the layers is achieved through a compatilizer and a catalyst, and the buffer, enhancement and barrier properties are achieved; the casting temperature, speed and hot pressing parameters are accurately regulated and controlled in the preparation process, compatibility and stability of all layers are ensured, and the prepared air bubble film is excellent in comprehensive performance and suitable for the field of environment-friendly packaging.

Description

Technical Field

[0001] The present invention relates to the technical field of laminated packaging materials, and specifically to a multi-layer composite bubble film and a preparation method thereof. Background Art

[0002] In the modern packaging industry, as a commonly used cushioning packaging material, bubble film is widely used in the packaging of fragile and vulnerable items such as electronic products, glass products, and precision instruments. Traditional bubble films usually consist of a single layer or a simple multi-layer structure, and there is room for improvement in mechanical properties. Their puncture resistance is weak, and they are easily punctured by sharp objects during transportation, resulting in the loss of protection for the internal items; their resilience is poor, and it is difficult to return to the original state after being squeezed, affecting the cushioning effect; their wear resistance is not good, and they are easily damaged by long-term friction and are vulnerable to external environmental corrosion.

[0003] In addition, the gas barrier performance of traditional bubble films is limited, and they cannot effectively prevent the penetration of gases such as oxygen and water vapor, which is not conducive to the preservation of items with high requirements for moisture and oxidation resistance. Moreover, with the continuous improvement of environmental awareness, the relatively high metal content and organic compounds that may be contained in traditional bubble films have caused greater pressure on the environment and do not meet the requirements of sustainable development. Therefore, there is an urgent need to develop a multi-layer composite bubble film with excellent mechanical properties, gas barrier performance, and environmental friendliness, as well as a preparation method thereof. For this reason, a multi-layer composite bubble film and a preparation method thereof are proposed. Summary of the Invention

[0004] The purpose of the present invention is to provide a multi-layer composite bubble film and a preparation method thereof. By hydrophobic modification of casein with a cardanol-based polyurethane prepolymer, its isocyanate groups crosslink with the amino and hydroxyl groups of casein, grafting hydrophobic olefin segments, enhancing hydrophobicity and compatibility, and being environmentally friendly without harmful residues; adopting a multi-layer composite structure of a bubble layer - a mechanical reinforcement layer - a gas barrier layer, and each layer realizes interfacial bonding through a compatibilizer and a catalyst, having both cushioning, strengthening, and barrier properties; during the preparation process, the casting temperature, speed, and hot pressing parameters are precisely controlled to ensure the compatibility and stability of each layer, and the prepared bubble film has excellent comprehensive performance.

[0005] To achieve the above purpose, the present invention provides the following technical solutions: The present invention provides a preparation method of a multi-layer composite bubble film, and the preparation method is as follows: After mixing low-density polyethylene (LDPE), ethylene-vinyl acetate copolymer (EVA), polylactic acid (PLA), high-density polyethylene (HDPE), ethylene-acrylic acid copolymer (EAA), maleic anhydride grafted polyethylene (PE-g-MAH), nano clay, modified casein and hexamethylene diisocyanate (HDI), it is put into a casting machine. After casting, it is pressed by a bubble die forming roller to obtain a bubble layer; EVA can be ExxonMobil™ EVA 25028 Series product with a density of 0.952 g / cm³ and a comonomer content of 27.6%; nano clay can be a product of Zhejiang Fenghong New Materials Co., Ltd. with a density of 1.6 - 1.8 g / cm 3 .

[0006] In the bubble layer, EAA contains polar acrylic acid groups and non-polar ethylene segments, binds to the hydroxyl groups of PLA and the ester groups of EVA through hydrogen bonds, and is physically entangled with LDPE / HDPE at the same time, reducing the interfacial tension between the polar PLA / EVA and non-polar LDPE / HDPE components to achieve compatibility. The maleic anhydride groups in maleic anhydride grafted polyethylene react with the hydroxyl groups of modified casein and the surface hydroxyl groups of nano clay to form ester group covalent bonds, fixing the inorganic filler and enhancing the organic-inorganic interfacial adhesion; the -NCO groups of HDI react with the ester groups of EVA and the hydroxyl groups of PLA to form urethane bonds, promoting the crosslinking of polar polymers and enhancing the overall strength of the bubble layer; at the casting temperature, LDPE / HDPE melts to form a continuous phase, and EVA / PLA is uniformly distributed as a dispersed phase therein. Nano clay is uniformly dispersed through high-speed mixing to avoid stress concentration caused by agglomeration. After casting, the mold is pressed to form a bubble layer; modified casein is compatible with non-polar substances such as LDPE and HDPE through physical entanglement and chemical bonding.

[0007] After mixing polypropylene (PP), nano cellulose, HDPE, PLA, lubricant and compatibilizer, it is extruded by a twin-screw extruder to obtain a mechanical reinforcement layer; PLA can be 4032D and 3052D of NatureWorks. According to the test method of ASTM D1238, at the test conditions of 210 °C and 2.16 kg, the melt flow rate is 7 g / 10 min and 14 g / 10 min; PP can be Moplen HP550J of LyondellBasell with a melt flow rate of 3.0 g / 10 min.

[0008] Preferably, the lubricant in the mechanical reinforcement layer is obtained by mixing ethylene bisstearamide, oleic acid amide and erucic acid amide; the compatibilizer is obtained by mixing maleic anhydride grafted polypropylene and epoxy propane grafted polyethylene.

[0009] In the mechanical reinforcement layer, nanocellulose is uniformly dispersed in the PP / HDPE / PLA matrix through a twin-screw extruder to form a nano-network structure, which undertakes the stress conduction function and improves the tensile strength; maleic anhydride grafted polypropylene and epoxy propane grafted polyethylene are used as compatibilizers. The maleic anhydride groups react with the PLA hydroxyl groups, and the epoxy groups are physically entangled with the HDPE segments, while promoting the crystallization synergy of PP and HDPE and reducing phase separation; the lubricant reduces the melt viscosity, improves the processing fluidity, and at the same time forms a lubricating layer on the material surface, reducing the frictional heat during extrusion and avoiding PLA degradation.

[0010] The ethylene-vinyl alcohol copolymer (EVOH), metallocene polyolefin adhesive resin, modified casein, and PE-g-MAH are mixed and then cast to obtain a gas barrier layer; the EVOH can be EVAL E151 from Kuraray, with a density of 1.14 g / cm³ and a melt flow rate of 1.6 g / 10 min.

[0011] In the gas barrier layer, the hydroxyl groups (-OH) in the EVOH molecular chains form a hydrogen bond network, having high barrier properties; the modified casein reacts with the EVOH hydroxyl groups through the -NCO groups of the cardanol-based polyurethane prepolymer, grafting the cardanol long olefin hydrophobic chain segments, reducing the water absorption and swelling of EVOH, and enhancing the barrier stability in a humid environment; the addition of the compatibilizer PE-g-MAH solves the compatibility problem between the polar EVOH and the non-polar metallocene; the modified casein reacts with the hydroxyl groups of EVOH through the -NCO groups of the cardanol-based polyurethane prepolymer, grafting the cardanol long olefin hydrophobic chain segments, reducing the water absorption and swelling of EVOH, and enhancing the barrier stability in a humid environment.

[0012] The waterborne polyurethane resin, vinyl siloxane, nano-titanium dioxide, N-(2-aminoethyl)-γ-aminopropyltrimethoxysilane (KH792), crosslinking agent, and catalyst are mixed to obtain a corrosion-resistant material; the nano-titanium dioxide can be a product with a particle size of 20 - 40 nm from Xianfeng Nano Materials Technology Co., Ltd.; the waterborne polyurethane resin can be a product of Desmodur® Eco 100.

[0013] The corrosion-resistant material is cured later to obtain a protective layer; the aziridine crosslinking agent reacts with the carboxyl and hydroxyl groups of the waterborne polyurethane to form a three-dimensional network structure, increasing the crosslinking density; the amino group of KH792 reacts with the polyurethane isocyanate group to fix the filler and enhance the coating adhesion; part of the vinyl siloxane migrates to the coating surface to form a hydrophobic layer, blocking the penetration of corrosive media; another part copolymerizes with the unsaturated double bonds in the gas barrier layer under the initiation of di-tert-butyl peroxide free radical initiator and thermal curing conditions, enhancing the tightness of the bonding between layers.

[0014] The bubble layer, mechanical reinforcement layer, and gas barrier layer are hot-pressed, and then the corrosion-resistant material is coated on the surface and cured to obtain a multi-layer composite bubble film.

[0015] During the hot pressing process after the lamination of each layer, the preheating zone softens the bubble layer and the surface of the mechanical layer, activating the polar groups in the compatibilizer, the carboxylic acid groups of EAA and the anhydride groups of PE-g-MAH, preparing for the interfacial reaction; the LDPE segments start to move, the EVA / PLA phase interface becomes blurred, and the interfacial tension drops to; then EVOH reaches its melting point and melts, interpenetrating with the metallocene adhesive resin to form a diffusion interface layer; the EAA in the bubble layer reacts with the PP-g-MAH in the mechanical layer through the carboxylic acid group-anhydride group reaction; the HDI in the bubble layer and the tetrabutyl titanate in the mechanical layer accelerate the cross-linking reaction at high temperature; the shaping and cooling zone rapidly cures the interfacial reaction products, retains the bubble layer structure, EVOH recrystallizes, and a composite interface with physical entanglement and chemical bonding is formed between the layers.

[0016] Preferably, the modified casein is obtained by adding casein to a mixer and slowly adding the cardanol-based polyurethane prepolymer, followed by drying and freezing after mixing; for casein.

[0017] Preferably, the cardanol-based polyurethane prepolymer is obtained by reacting cardanol with a diisocyanate; the cardanol can be Cardolite NX-9203LP.

[0018] Preferably, the diisocyanate is one of 4,4'-methylenebis(phenyl isocyanate), hexamethylene diisocyanate, isophorone diisocyanate, and toluene diisocyanate.

[0019] Among them, the reaction principle of the cardanol-based polyurethane prepolymer is as follows. The reaction of cardanol with a diisocyanate is demonstrated with the diene structure in cardanol and 4,4'-methylenebis(phenyl isocyanate) as an example. After the preparation of the cardanol-based polyurethane prepolymer, it reacts with casein to prepare the modified casein. The reaction principle is as follows: It mainly reacts with the amino groups in casein, or with the carboxyl groups in casein, or with both amino and carboxyl groups simultaneously, but ultimately the isocyanate groups are consumed; where Casein-NH2 represents casein, indicating that the chemical reaction occurs with the amino groups in casein.

[0020] Preferably, the melt index of LDPE is 0.18 - 4.0 g / 10min; the melt index of HDPE is 0.06 - 0.45 g / 10min; the melt index of EAA is 8.2 - 38.0 g / 10min; The LDPE can be ExxonMobil LD 2119.LN series, ExxonMobil LD 01820 series, and ExxonMobil LD 4020.BA; the EAA can be ExxonMobil Adhere EAA38015 and ExxonMobil Adhere EAA8206; the HDPE can be ExxonMobil HD 5001, ExxonMobil HD 5703, and ExxonMobil HD 5805; the metallocene polyolefin adhesive resin can be ExxonMobil Exact 3020FL, with an ethylene content of 11 wt% and a melt index of 1.20 g / 10 min at 190 °C.

[0021] More preferably, the melt index of LDPE is 2.1 - 4.0 g / 10 min; the melt index of HDPE is 0.30 - 0.45 g / 10 min.

[0022] The present invention also provides a multi-layer composite bubble film, which is prepared by any of the above preparation methods; the multi-layer composite bubble film is composed of 4 layers of materials, which are, from the outside to the inside, a protective layer, a gas barrier layer, a mechanical reinforcement layer, and a bubble layer. Tetrabutyl titanate is added to the mechanical reinforcement layer, which mainly promotes the transesterification reaction between the hydroxyl groups of EVOH in the bubble layer and the ester groups of PLA in the mechanical layer to form covalent bonds, and at the same time promotes the physical entanglement of the metallocene polyolefin in the gas barrier layer with PP / HDPE, improving the bonding strength between layers; di-tert-butyl peroxide is added to the protective layer, which generates free radicals under hot pressing conditions to promote the copolymerization of the unsaturated double bonds of 1,3-divinyl-1,1,3,3-tetramethoxydisiloxane with the unsaturated bonds of the gas barrier layer.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The casein is hydrophobically modified by the cardanol-based polyurethane prepolymer. The isocyanate groups in the prepolymer react with the amino and hydroxyl groups of the casein to graft the hydrophobic cardanol long-chain olefin onto the surface of the casein molecule, forming a steric hindrance effect and effectively shielding the hydrophilic groups; not only significantly improving the hydrophobicity of the casein, enabling it to be compatible with non-polar polymers such as high-density and low-density polyethylene through physical entanglement and chemical bonding, but also avoiding the harmful residues that may be introduced by traditional modifiers; the modified casein serves as an interfacial bridge, enhancing the interfacial bonding force of composite structures such as the bubble layer and the gas barrier layer. At the same time, the raw material cardanol is derived from natural renewable resources, and no harmful solvents are released during the entire modification process, endowing the product with environmental protection properties. After testing, no polluting heavy metals, polybrominated organic pollutants, etc. are detected, meeting the requirements of green packaging materials.

[0024] 2. A multi-layer composite design of "bubble layer - mechanical reinforcement layer - gas barrier layer" is adopted, and chemical bonding at the interface is achieved between each layer through a compatibilizer and a catalyst. The bubble layer uses LDPE and EVA as the matrix, PLA and nano-clay are introduced, and a regular bubble structure is formed through casting molding. The elastic deformation of the bubbles is utilized to absorb impact energy. At the same time, the nano-clay and modified casein synergistically enhance the matrix strength; the mechanical reinforcement layer uses PP and HDPE as the framework, and nano-cellulose is uniformly dispersed to form a network structure, and lubricants are used to improve the processing fluidity, significantly enhancing the tensile strength and impact resistance; the gas barrier layer forms a dense structure through the hydrogen bonding between EVOH and modified casein and the physical occlusion of the metallocene adhesive resin, effectively blocking the oxygen permeation. Each layer is firmly bonded through the action of the compatibilizer during the hot pressing process, avoiding delamination between layers, making the material have excellent tensile strength, impact toughness and gas barrier properties.

[0025] 3. During the preparation process, the material properties are optimized by precisely controlling the process parameters of each link. The casting temperature is controlled to ensure the full melting and compatibility of LDPE and EVA, and avoid the degradation of PLA due to high temperature; the temperature is reasonably regulated to ensure the full blending of nano-cellulose and the polymer matrix, forming a stable reinforcing phase; the casting speed of the gas barrier layer is reasonably matched, and a three-stage temperature control is adopted during hot pressing composite, which not only ensures the full reaction at the interface of each layer, but also avoids the structural damage of sensitive materials such as EVOH caused by high temperature; in addition, the corrosion-resistant protective layer realizes the efficient cross-linking of waterborne polyurethane by adding silane coupling agent and aziridine cross-linking agent, forming a dense coating, and finally a multi-layer composite bubble film with excellent and stable comprehensive performance is prepared. Brief Description of the Drawings

[0026] Figure 1 It is a drawing of the preparation method of the multi-layer composite bubble film of the present invention; Figure 2 It is a drawing of the test results of the tensile strength and cantilever beam notched impact strength of Examples 1 - 5 of the present invention; Figure 3 It is an FTIR characterization drawing of the cardanol-based polyurethane prepolymer and modified casein of the present invention. Detailed Embodiments

[0027] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work shall fall within the protection scope of the present invention.

[0028] Please refer to Figures 1 to 2 , the present invention provides a multi-layer composite bubble film and its preparation method, and the technical solutions are as follows: Preparation Example 1 Synthesis of cardanol-based polyurethane prepolymer: Weigh 50.0 g of 4,4'-methylenebis(phenyl isocyanate) (MDI), and store it in a dark place in a sealed container for later use.

[0029] Prepolymerization reaction process: Place 31.55 g of cardanol in a 250 mL three-necked flask, and place it in an oil bath. Start heating the oil bath to 110 °C at 200 rpm, and dehydrate it under a vacuum of -0.09 MPa for 30 min to ensure that the water content of cardanol is <0.05%. After dehydration is completed, introduce dry nitrogen for protection, cool down to 60 °C, and slowly add MDI, which is added dropwise within about 10 min. At this time, the system temperature rises to 80 °C due to the exothermic reaction. Maintain the temperature at 80 °C and continue the reaction for 1.5 h. During this period, take samples every 15 min and detect the -NCO content by the dibutylamine titration method until the theoretical value is reached; after the reaction is completed, pour the transparent light yellow cardanol-based polyurethane prepolymer into a dry container while it is hot, seal it, and store it in the refrigerator for later use; Preparation of modified casein: Add 35 g of anhydrous casein to a high-speed shear mixer and stir at 8000 rpm for 15 min to form a homogeneous material. Preheat the internal mixer to 80 °C, set the rotor speed to 60 rpm, transfer the homogeneous material to the internal mixer, and slowly add 8.0 g of cardanol-based polyurethane prepolymer within 5 min. After internal mixing for 20 min, discharge to obtain a light yellow solid, and obtain modified casein after drying; then immediately pack it into an aluminum foil sealed bag and store it frozen at -18 °C.

[0030] Preparation Examples 2 - 4 Differing from Preparation Example 1, the following preparation conditions are changed, as shown in Table 1 specifically. In Table 1, MDI is 4,4'-methylenebis(phenyl isocyanate), HDI is hexamethylene diisocyanate, IPDI is isophorone diisocyanate, and TDI is toluene diisocyanate.

[0031] Table 1 Preparation of Modified Casein Preparation of Comparative Examples Except for the following changes in preparation conditions, other methods are the same as those in Preparation Example 3.

[0032] Preparation of Comparative Example 1 The amount of IPDI used is 22.3 g.

[0033] Preparation of Comparative Example 2 After mixing anhydrous casein and cardanol, conduct internal mixing instead of mixing with cardanol-based polyurethane prepolymer.

[0034] Preparation of Comparative Example 3 Anhydrous casein is not subjected to hydrophobic modification treatment.

[0035] Preparation of Comparative Example 4 Anhydrous casein is directly mixed with cardanol-based polyurethane prepolymer, without using the method of adding and slowly adding cardanol-based polyurethane prepolymer later.

[0036] Test Example 1 The modified casein obtained from the preparation examples and the preparation comparative examples was cold-rolled at room temperature and then hot-pressed. It was hot-pressed at 100 °C and 20 Mpa for 3 min, cooled and placed at room temperature for 24 h, and then hot-pressed again with the hot-pressing temperature and time remaining unchanged and the pressure being 40 MPa. After that, it was cooled to room temperature. According to Part of GBT1040-2006 "Determination of Tensile Properties of Plastics", the tablet was prepared into a dumbbell shape, and the water contact angle was measured using a contact meter after equilibration in an incubator. The test results were the average of three measurements, and the final test results are shown in Table 2.

[0037] Table 2 Hydrophobic test results of the modified casein of Preparation Examples 1-4 and Preparation Comparative Examples 1-4 As Figure 3 shown, the cardanol polyurethane prepolymer shows an absorption peak at 2271 cm -1 , which is the characteristic absorption peak of the isocyanate group; an absorption peak appears at 1730 cm -1 , which is the characteristic absorption peak of the urethane bond; a phenolic hydroxyl absorption peak of unreacted cardanol appears at 3450 cm -1 , and a characteristic absorption peak of the benzene ring appears at 720 cm -1 . A characteristic absorption peak of the benzene ring skeletal vibration appears at 15800 cm -1 . The above results indicate that the cardanol polyurethane prepolymer was successfully synthesized; then the casein was modified with the cardanol polyurethane prepolymer. The characteristic absorption peak at 2271 cm -1 in the modified casein disappeared, indicating that the isocyanate groups reacted and were consumed, probably reacting with the carboxyl or amino groups in the casein; and the carbonyl stretching vibration peak shifted towards the high wave direction (1654 cm-1), indicating that the modified casein was successfully prepared.

[0038] The modified casein prepared by the present invention has improved hydrophobic properties, and the hydrophobic angles of Preparation Examples 1-4 are 47.8°-53.9°. Cardanol and diisocyanate are pre-polymerized. Cardanol contains phenolic hydroxyl groups (-OH) and long-chain unsaturated olefins, and MDI contains isocyanate groups (-NCO). The two react through an addition reaction to form a polyurethane prepolymer; the unsaturated olefin segments endow the prepolymer with flexibility, and the benzene ring structure enhances rigidity, forming a molecular chain with both rigidity and flexibility; by controlling the molar ratio of diisocyanate to phenolic hydroxyl groups contained to 2:1, a prepolymer terminated with -NCO is obtained; the prepolymer is used for the modification of casein. Natural casein contains a large number of hydrophilic amino groups (-NH2), carboxyl groups (-COOH) and hydroxyl groups (-OH), resulting in strong hygroscopicity. The -NCO groups of the cardanol-based polyurethane prepolymer react with the amino and hydroxyl groups of casein, grafting the hydrophobic cardanol long-chain olefins and polyurethane segments onto the casein molecules, forming a hydrophobic network structure and reducing the exposure of hydrophilic groups, thereby improving hydrophobicity; during the reaction process, high speed is used to fully disperse casein, increasing the contact area with the prepolymer. The preheating of the internal mixer and the setting of a fixed rotation speed provide a suitable melt mixing environment. The prepolymer is slowly added to avoid local agglomeration and ensure uniform grafting reaction. In Preparation Comparative Example 1, the amount of IPDI used is insufficient, the crosslinking density is low, the hydrophobic chain segments are not completely covered, and the hydrophobicity decreases significantly compared with the preparation examples; in Preparation Comparative Example 2, cardanol is directly mixed with casein, lacking -NCO group crosslinking and only physical adsorption, resulting in a weak hydrophobic modification effect; in Preparation Comparative Example 3, no hydrophobic modification treatment is carried out, containing a large number of hydrophilic groups and having strong natural hydrophilicity; in Preparation Comparative Example 4, the prepolymer is not added slowly through internal mixing, the prepolymer is unevenly dispersed, and some areas are not reacted, so the hydrophobic modification effect decreases.

[0039] Example 1 The overall preparation method is as Figure 1 shown. The raw materials used in the following process are the modified casein prepared by the method of Preparation Example 3; The preparation method of the bubble layer is as follows: 40 parts of environmentally friendly low-density polyethylene (LDPE) resin, 10 parts of ethylene-vinyl acetate copolymer (EVA), 10 parts of polylactic acid (PLA), 13 parts of high-density polyethylene (HDPE), 0.5 part of hindered phenol antioxidant 1010, 1 part of nano-clay, 4 parts of modified casein, and 3 parts of compatibilizer. The compatibilizer consists of 2 parts of ethylene-acrylic acid copolymer (EAA) and 1 part of maleic anhydride grafted polyethylene, and 0.5 part of crosslinking agent HDI; the melt index of EAA is 38 g / 10 min; these raw materials are added to a high-speed mixer and pre-mixed at 90 °C for 12 min to make each component fully and evenly mixed; the mixed bubble layer raw materials are put into a casting machine, the casting temperature is set at 160 °C, the casting speed is 1.5 m / min, the die head temperature is 175 °C, and the die head gap is 0.3 mm; and it is pressed by a bubble die forming roller to make a bubble layer film with uniform thickness.

[0040] The preparation method of the mechanical reinforcement layer is as follows: Take 50 parts of polypropylene (PP) particles, 2 parts of nanocellulose (diameter 10 - 100 nm, length 0.5 - 5 μm), 20 parts of high-density polyethylene (HDPE), 10 parts of polylactic acid (PLA), 2 parts of ethylene bisstearamide, 1 part of oleic acid amide, 1 part of erucic acid amide, 0.2 part of catalyst tetrabutyl titanate, and 4 parts of compatibilizer: maleic anhydride grafted polypropylene and propylene oxide grafted polyethylene are mixed in a mass ratio of 3:1, added to a twin-screw extruder, and reserved; melt-blended and extruded at a temperature of 200 °C to obtain a composite material; then the composite material is extruded through an extrusion molding machine at a temperature of 215 °C to form a mechanical reinforcement layer.

[0041] The preparation method of the gas barrier layer is as follows: Prepare 90 parts of ethylene-vinyl alcohol copolymer (EVOH) particles; at the same time, prepare 18 parts of metallocene polyolefin adhesive resin, 6 parts of modified casein, and 1.5 parts of compatibilizer PE-g-MAH. The above substances are mixed evenly and subjected to casting molding through a casting machine at a temperature of 175 °C. During the casting process, control the casting speed at 1.5 m / min to ensure uniform film thickness and obtain a gas barrier layer.

[0042] The raw materials for preparing the corrosion-resistant protective layer are as follows: Take 90 parts of waterborne polyurethane resin, 5 parts of 1,3-divinyl-1,1,3,3-tetramethoxydisiloxane, 10 parts of nano-titanium dioxide, 0.5 part of KH792, 1 part of aziridine crosslinking agent, and 0.5 part of di-tert-butyl peroxide. After mixing, add them to a stirring kettle and stir at 55 °C for 40 min to fully disperse and make a corrosion-resistant material.

[0043] The preparation method of the multi-layer composite bubble film is as follows: Adopt a multi-layer hot pressing and extrusion technology to composite each layer; from the outside to the inside, they are a corrosion-resistant protective layer, a gas barrier layer, a mechanical reinforcement layer, and a bubble layer; by precisely controlling the preheating zone at 140 °C, the melt composite zone at 185 °C, and the shaping and cooling zone at 60 °C, ensure that each layer is closely adhered; after composite, the order and thickness of each layer are as follows: the gas barrier layer is 20 μm, the mechanical reinforcement layer is 40 μm, and the bubble layer is 120 μm. A layer of polyurethane corrosion-resistant protective layer is coated on the surface of the composite film by roll coating, and the coating thickness is controlled at 1 μm; after coating, dry and cure at a temperature of 75 °C to improve the hardness and smoothness of the film surface and obtain a multi-layer composite bubble film.

[0044] Examples 2 - 5 Different from the preparation method of Example 1, the following preparation methods are adjusted as shown in Tables 3-1 to 3-3 specifically.

[0045] Table 3-1 Preparation method of the bubble layer Table 3-2 Preparation Methods of Bubble Layer and Mechanical Reinforcement Layer Table 3-3 Preparation Methods of Gas Barrier Layer, Corrosion-Resistant Protective Layer and Multilayer Composite Bubble Film The four layers in Table 3-3 are, in sequence, gas barrier layer, mechanical reinforcement layer, bubble layer and corrosion-resistant protective layer.

[0046] Comparative Example Except for the following adjusted preparation conditions, other methods are the same as those in Example 3.

[0047] Comparative Example 1 The melt index of HDPE is 0.06 g / 10 min.

[0048] Comparative Example 2 The melt index of LDPE is 0.18 g / 10 min.

[0049] Comparative Example 3 The melt index of EAA is 8.2 g / 10 min.

[0050] Comparative Example 4 No modified casein is added to the bubble layer and gas barrier layer.

[0051] Comparative Example 5 Unmodified casein is added to the bubble layer and gas barrier layer.

[0052] Comparative Example 6 No EAA, maleic anhydride grafted polyethylene and catalyst HDI are added during the preparation of the bubble layer.

[0053] Comparative Example 7 No ethylene bisstearamide, oleic acid amide, erucic acid amide and catalyst tetrabutyl titanate are added during the preparation of the mechanical reinforcement layer.

[0054] Comparative Example 8 No compatibilizer obtained by adding maleic anhydride grafted polypropylene and propylene oxide grafted polyethylene is added during the preparation of the mechanical reinforcement layer.

[0055] Comparative Example 9 No compatibilizer PE-g-MAH is added during the preparation of the gas barrier layer.

[0056] Comparative Example 10 No 1,3-divinyl-1,1,3,3-tetramethoxydisiloxane and nano-titanium dioxide are added to the corrosion-resistant protective layer.

[0057] Comparative Example 11 No KH792, aziridine crosslinking agent and di-tert-butyl peroxide are added to the corrosion-resistant protective layer.

[0058] Comparative Example 12 The casting speed is 2.0 m / min for all.

[0059] Comparative Example 13 The temperature of the melt composite zone during the hot pressing and composite process is 210 °C.

[0060] Test Example 2 The multi-layer composite bubble films obtained in Examples 1-5 and Comparative Examples 1-13 were tested for tensile strength and Izod notched impact strength. The tensile strength was tested according to GB / T 1040.3-2006 "Plastics - Determination of tensile properties - Part 3: Test conditions for films and sheets"; the Izod notched impact strength was tested according to the method of GB / T 1843-2008 "Plastics - Determination of Izod impact strength". Specimens with length and width dimensions of 80 mm × 10 mm were cut from the bubble film, and a V-notch was machined in the middle of the specimen using a special notching tool. The notch angle was 45°, and the remaining thickness was 1 / 2 of the original thickness (i.e., the notch depth was 50% of the thickness). The specimen was fixed on the Izod fixture, and the notch was located in the opposite direction of the impact surface to ensure that the specimen was clamped without looseness; the appropriate pendulum energy was selected according to the estimated strength of the specimen to avoid the specimen not breaking or being overly broken, and the pendulum was released to impact the back of the specimen notch; the impact energy consumed when the specimen broke was recorded; the final test results are shown in Table 4 and Figure 2 as follows.

[0061] Table 4 Test Results of Tensile Strength and Izod Notched Impact Strength By optimizing the preparation method of the bubble layer, mechanical reinforcement layer, gas barrier layer and corrosion-resistant protective layer for the final multi-layer composite bubble film, the final multi-layer composite bubble film has good mechanical properties. Table 4 and Figure 2 the results show that under the conditions of Examples 1-5, the tensile strength is 159-180 MPa, and the Izod notched impact strength is 68-80 J / m 2。In Comparative Examples 1-3, the melt indices of HDPE, LDPE, and EAA used were low, resulting in poor melt fluidity, which in turn led to brittle fracture caused by local nano-aggregation regions, increased extrusion internal stress, and rapid propagation of microcracks under impact loads, thereby reducing the material properties; in Comparative Example 4, modified casein was not added, and in Comparative Example 5, unmodified casein was added, increasing the hydrophilicity of the barrier layer, weakening the water molecule penetration to the EVOH crystal interface, and decreasing the interfacial bonding force; and without modification treatment, the compatibility between casein and non-polar components was poor; in Comparative Example 6, the bubble layer lacked a compatibilizer and a catalyst, resulting in a decrease in interfacial bonding force, phase separation of LDPE / PLA, and a decrease in impact energy absorption capacity; the bonding performance between layers deteriorated; in Comparative Example 7, no lubricant was added to the mechanical reinforcement layer, resulting in a decrease in processing performance, and the tensile strength decreased slightly compared to Example 3, but the Izod notched impact strength decreased significantly; in Comparative Example 8, no compatibilizer was added, resulting in phase separation of PP / HDPE / PLA and a decrease in the performance of the bubble film; in Comparative Example 9, no compatibilizer was added during the preparation of the body barrier layer, resulting in weak interfacial bonding between EVOH and metallocene polyolefin, delamination between layers, and interfacial defects becoming the source of impact cracks; in Comparative Example 10, the above substances were not added to the corrosion-resistant protective layer, resulting in a decrease in mechanical properties; in Comparative Example 11, no crosslinking agent and catalyst were added to the corrosion-resistant protective layer, resulting in insufficient curing of the waterborne polyurethane, a decrease in the bonding performance with the gas barrier layer, a decrease in tensile strength, and a decrease in the Izod notched impact strength; in Comparative Example 12, the casting speed was too fast, the solvent volatilization was insufficient, the porosity of the bubble layer increased, and the performance decreased; in Comparative Example 13, the hot pressing temperature was too high, EVOH decomposed, holes were generated in the barrier layer, and stress concentration at the hole edges induced brittle fracture.

[0062] Test Example 3 The barrier properties of the multilayer composite bubble films obtained from the above examples and comparative examples were tested, mainly referring to the differential pressure method in GB / T 1038-2000. A pressure difference was formed on both sides of the specimen, and the change in the gas volume on the low-pressure side was measured (unit: cm³ / (m²·24h·0.1MPa)). The final test results are shown in Table 5, where OTR is the oxygen transmission rate.

[0063] Table 5 OTR test results of Examples 1-5 and Comparative Examples 1-13 The multi-layer composite bubble film obtained by the present invention has good barrier properties, and the OTR results under the conditions of Examples 1-5 are 2.9-4.6cm³ / (m²·24h·0.1MPa). The melt index of the materials in Comparative Examples 1-3 is too low, resulting in a decrease in processing performance, and the generated microcracks become oxygen diffusion paths; Comparative Example 4 does not add modified casein, resulting in the exposure of the EVOH hydrophilic group, the crystallization is destroyed after moisture absorption, and the oxygen channel is expanded; Comparative Example 5 adds unmodified casein, the water absorption performance is increased, the hydrophilic group absorbs moisture and expands, and the distance between the EVOH molecular chains is stretched; Comparative Example 6 lacks the above substances in the bubble layer, resulting in a decrease in the bonding force between the layers, and at the same time, the cross-linking density is reduced, and the oxygen barrier performance becomes poor; Comparative Examples 7 and 8 lack key substances in the mechanical reinforcement layer, and the mechanical reinforcement The bonding performance between the layer and the bubble layer and the gas barrier layer is reduced, the compatibility of the materials between the individual layers is reduced, the raw materials between the layers are unevenly bonded, and the OTR value is increased; in comparative example 9, there is no PE-g-MAH compatibilizer, the EVOH / metallocene interface is peeled off, and oxygen penetrates along the interface defects; in comparative examples 10 and 11, the corrosion-resistant layer lacks siloxane / TiO2, the degree of crosslinking and reaction is reduced, and the barrier performance is reduced; in comparative example 12, the casting speed is too fast and the solvent is not fully evaporated, and the porosity of the bubble layer is increased; in comparative example 13, the temperature is too high, which has a destructive effect on each layer and the oxygen barrier performance is reduced.

[0064] Test Example 4 The multilayer composite bubble films prepared in Examples 1-5 were subjected to metal and organic tests, and the final test results are shown in Table 6.

[0065] Table 6 Metal and organic test results The test results show that the multi-layer composite bubble film prepared by the present invention has good environmental protection, and no polluting heavy metals, polybrominated organic pollutants, plasticizers, halogen elements and restricted polluting perfluorinated compounds are detected.

[0066] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A preparation method of a multi-layer composite bubble film, characterized in that, The preparation method is as follows: After uniformly mixing low-density polyethylene, ethylene-vinyl acetate copolymer, polylactic acid, high-density polyethylene, nano-clay and modified casein, put them into a casting machine, and after casting, press them through a bubble mold forming roller to obtain a bubble layer; After mixing polypropylene, nano-cellulose, the high-density polyethylene, polylactic acid, lubricant and compatibilizer, extrude them with a twin-screw extruder to obtain a mechanical reinforcement layer; After mixing ethylene-vinyl alcohol copolymer, metallocene polyolefin adhesive resin and the modified casein, carry out the casting to obtain a gas barrier layer; After mixing waterborne polyurethane resin, vinyl siloxane, nano-titanium dioxide, crosslinking agent and catalyst, obtain a corrosion-resistant material; After hot-pressing the bubble layer, the mechanical reinforcement layer and the gas barrier layer, coat the corrosion-resistant material on the surface and cure to obtain the multi-layer composite bubble film.

2. The preparation method of a multi-layer composite bubble film according to claim 1, characterized in that: In the mechanical reinforcement layer, the lubricant is obtained by mixing ethylene bis-stearamide, oleic acid amide and erucic acid amide; the compatibilizer is obtained by mixing maleic anhydride grafted polypropylene and epoxy propane grafted polyethylene.

3. The preparation method of a multi-layer composite bubble film according to claim 1, characterized in that: The modified casein is obtained by adding casein into a mixer, slowly adding cardanol-based polyurethane prepolymer, and then carrying out mixing and drying, and freezing.

4. The preparation method of a multi-layer composite bubble film according to claim 3, characterized in that: The cardanol-based polyurethane prepolymer is obtained by reacting cardanol with diisocyanate.

5. The preparation method of a multi-layer composite bubble film according to claim 4, characterized in that: The diisocyanate is one of 4,4'-methylenebis(phenyl isocyanate), hexamethylene diisocyanate, isophorone diisocyanate and toluene diisocyanate.

6. A multi-layer composite bubble film, characterized in that: The multi-layer composite bubble film is prepared by the preparation method described in any one of claims 1-5; the multi-layer composite bubble film is composed of 4 layers of materials, and from the outside to the inside are a protective layer, a gas barrier layer, a mechanical reinforcement layer and a bubble layer in turn.

7. A multi-layer composite bubble film according to claim 6, characterized in that: The preparation raw materials of the bubble layer include low-density polyethylene, ethylene-vinyl acetate copolymer, high-density polyethylene and ethylene-acrylic acid copolymer; the preparation raw materials of the mechanical reinforcement layer include polypropylene and nano-cellulose; the preparation raw materials of the gas barrier layer include modified casein.

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