Multilayer composite bubble film and preparation method thereof

Abstract

The present invention relates to the technical field of layered packaging materials, specifically a multi-layer composite bubble film and a preparation method thereof. The bubble film prepared by the present invention overcomes the problems of poor mechanical properties and weak barrier capacity. Casein is hydrophobically modified by a cardanol-based polyurethane prepolymer, and its isocyanate group is cross-linked with the amino group and hydroxyl group of casein, and a hydrophobic olefin segment is grafted to improve hydrophobicity and compatibility, and is environmentally friendly and has no harmful residues; a bubble layer-mechanical reinforcement layer-gas barrier layer multi-layer composite structure is adopted, and each layer is interfacially bonded by a compatibilizer and a catalyst, and has buffering, reinforcement and barrier properties; the casting temperature, speed and hot pressing parameters are precisely controlled during the preparation process to ensure the compatibility and stability of each layer. The obtained bubble film has excellent comprehensive performance and is suitable for the field of environmentally friendly packaging.

Description

Technical Field

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

[0002] In the modern packaging industry, bubble wrap is a commonly used cushioning packaging material, widely used to package fragile and easily damaged items such as electronics, glassware, and precision instruments. Traditional bubble wrap typically consists of a single layer or a simple multi-layer structure. Its mechanical properties need improvement. It has weak puncture resistance, making it easily punctured by sharp objects during transportation, leaving the contents unprotected. It also has poor resilience, making it difficult to recover after being squeezed, affecting its cushioning effect. It also has poor wear resistance, making it susceptible to damage from long-term friction and susceptible to corrosion from the external environment.

[0003] Furthermore, traditional bubble films have limited gas barrier properties and cannot effectively prevent the permeation of gases such as oxygen and water vapor, making them unsuitable for the preservation of items requiring high moisture and oxidation resistance. Furthermore, with increasing environmental awareness, the high metal content and organic compounds that may be present in traditional bubble films pose significant environmental risks and are not in line with sustainable development requirements. Therefore, there is an urgent need to develop an environmentally friendly multi-layer composite bubble film and its preparation method, which exhibits excellent mechanical properties, gas barrier properties, and is environmentally friendly. To this end, a multi-layer composite bubble film and its preparation method are proposed. Summary of the Invention

[0004] The present invention aims to provide a multi-layer composite bubble film and a method for preparing the same. Casein is hydrophobically modified with a cardanol-based polyurethane prepolymer, and its isocyanate groups are cross-linked with the amino and hydroxyl groups of the casein, grafting hydrophobic olefin segments to enhance hydrophobicity and compatibility, while also being environmentally friendly and leaving no harmful residues. A multi-layer composite structure of a bubble layer, a mechanical reinforcement layer, and a gas barrier layer is employed, with each layer achieving interfacial bonding through a compatibilizer and a catalyst, resulting in a film with excellent overall performance, including buffering, reinforcement, and barrier properties. During the preparation process, the casting temperature, speed, and hot pressing parameters are precisely controlled to ensure compatibility and stability of each layer.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] The present invention provides a method for preparing a multi-layer composite bubble film, and the preparation method is as follows:

[0007] 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), nanoclay, modified casein, and hexamethylene diisocyanate (HDI) are mixed and put into a casting machine. After casting, the film is pressed with a bubble film forming roller to obtain a bubble layer. The EVA can be ExxonMobil™ EVA 25028 Series product with a density of 0.952 g / cm³ and a comonomer content of 27.6%. The nanoclay can be a product of Zhejiang Fenghong New Materials Co., Ltd. with a density of 1.6-1.8 g / cm 3 .

[0008] The EAA in the bubble layer contains polar acrylic groups and non-polar ethylene segments, which combine with the hydroxyl groups of PLA and the ester groups of EVA through hydrogen bonds, and are physically entangled with LDPE / HDPE, reducing the interfacial tension between the polar PLA / EVA and non-polar LDPE / HDPE components and achieving compatibility. The maleic anhydride groups in the maleic anhydride-grafted polyethylene undergo esterification with the hydroxyl groups of modified casein and the hydroxyl groups on the surface of the nanoclay to form ester 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 carbamate bonds, promoting cross-linking of polar polymers and improving the overall strength of the bubble layer. At the casting temperature, LDPE / HDPE melts to form a continuous phase, in which EVA / PLA is uniformly distributed as a dispersed phase. The nanoclay is evenly dispersed through high-speed mixing to avoid stress concentration caused by agglomeration. After casting, the bubble layer is formed by mold pressing. The modified casein achieves compatibility with non-polar substances such as LDPE and HDPE through physical entanglement and chemical bonding.

[0009] Polypropylene (PP), nanocellulose, HDPE, PLA, lubricant and compatibilizer are mixed and extruded through a twin-screw extruder to obtain a mechanical reinforcement layer;

[0010] The PLA can be NatureWorks' 4032D and 3052D. According to the ASTM D1238 test method, the flow rates of 4032D and 3052D are 7 g / 10 min and 14 g / 10 min at the test conditions of 210°C and 2.16 kg. The PP can be LyondellBasell's Moplen HP550J, with a flow rate of 3.0 g / 10 min.

[0011] Preferably, the lubricant in the mechanical reinforcement layer is obtained by mixing ethylene bisstearamide, oleamide and erucamide; and the compatibilizer is obtained by mixing maleic anhydride grafted polypropylene and propylene oxide grafted polyethylene.

[0012] The nanocellulose in the mechanical reinforcement layer is evenly dispersed in the PP / HDPE / PLA matrix through a twin-screw extruder, forming a nano-network structure, which plays a stress conduction role and improves the tensile strength; maleic anhydride grafted polypropylene and propylene oxide grafted polyethylene serve as compatibilizers, the maleic anhydride groups react with the PLA hydroxyl groups, and the epoxy groups are physically entangled with the HDPE chain segments, while promoting the crystallization synergy of PP and HDPE and reducing phase separation; the lubricant reduces the melt viscosity, improves processing fluidity, and forms a lubricating layer on the material surface, reducing frictional heat during extrusion and avoiding PLA degradation.

[0013] Ethylene-vinyl alcohol copolymer (EVOH), metallocene polyolefin adhesive resin, modified casein and PE-g-MAH are mixed and cast to obtain a gas barrier layer; EVOH can be Kuraray's EVAL E151, with a density of 1.14 g / cm³ and a flow rate of 1.6 g / 10 min.

[0014] The hydroxyl groups (-OH) in the EVOH molecular chain in the gas barrier layer form a hydrogen bond network, which has high barrier properties. The modified casein reacts with the hydroxyl groups of EVOH through the -NCO groups of the cardanol-based polyurethane prepolymer to graft the long olefin hydrophobic chain segments of cardanol, reducing the water absorption and expansion of EVOH and improving the barrier stability in humid environments. The addition of the compatibilizer PE-g-MAH solves the compatibility problem between polar EVOH and non-polar metallocenes. The modified casein reacts with the hydroxyl groups of EVOH through the -NCO groups of the cardanol-based polyurethane prepolymer to graft the long olefin hydrophobic chain segments of cardanol, reducing the water absorption and expansion of EVOH and improving the barrier stability in humid environments.

[0015] A corrosion-resistant material is obtained by mixing a waterborne polyurethane resin, vinyl silicone, nano-titanium dioxide, N-aminoethyl-γ-aminopropyltrimethoxysilane (KH792), a crosslinking agent, and a catalyst. The nano-titanium dioxide may have a particle size of 20-40 nm and is a product of Xianfeng Nanomaterial Technology Co., Ltd. The waterborne polyurethane resin may be a product of Desmodur® Eco 100.

[0016] The corrosion-resistant material is cured to obtain a protective layer; the aziridine crosslinker reacts with the carboxyl and hydroxyl groups of the water-based polyurethane to form a three-dimensional network structure, and the crosslinking density is increased; the amino group of KH792 reacts with the isocyanate group of the polyurethane to fix the filler and enhance the adhesion of the coating; the vinyl siloxane partially migrates to the surface of the coating to form a hydrophobic layer, blocking the penetration of corrosive media; the other part copolymerizes with the unsaturated double bonds in the gas barrier layer under the conditions of di-tert-butyl peroxide free radical initiator and thermal curing, thereby improving the tightness of the bonding between the layers.

[0017] After the bubble layer, the mechanical reinforcement layer and the gas barrier layer are hot-pressed, a corrosion-resistant material is coated on the surface and solidified to obtain a multi-layer composite bubble film.

[0018] During the hot pressing process after the composite layers are formed, the preheating zone softens the surface of the bubble layer and the mechanical layer, activates the polar groups in the compatibilizer, the carboxylic acid groups of EAA and the anhydride groups of PE-g-MAH, and prepares for the interfacial reaction; the LDPE chain segments begin to move, the EVA / PLA phase interface becomes blurred, and the interfacial tension drops to; then the EVOH reaches the melting point and melts and interpenetrates with the metallocene adhesive resin to form a diffusion interface layer; the EAA of the bubble layer and the PP-g-MAH of the mechanical layer react through the carboxylic acid group-anhydride group; the HDI of the bubble layer and the tetrabutyl titanate of the mechanical layer accelerate the cross-linking reaction at high temperature; the shaping cooling zone quickly solidifies the interfacial reaction products, retains the bubble layer structure, and the EVOH recrystallizes to form a composite interface of physical entanglement and chemical bonding between the layers.

[0019] Preferably, the modified casein is prepared by adding casein into an internal mixer, slowly adding a cardanol-based polyurethane prepolymer, mixing, drying, and freezing the mixture after internal mixing; casein.

[0020] Preferably, the cardanol-based polyurethane prepolymer is obtained by reacting cardanol and diisocyanate; the cardanol may be Cardolite NX-9203LP.

[0021] Preferably, the diisocyanate is one of 4,4'-methylenebis(phenyl isocyanate), hexamethylene diisocyanate, isophorone diisocyanate and toluene diisocyanate.

[0022] The reaction principle of the cardanol-based polyurethane prepolymer is as follows, with the diene structure of cardanol and 4,4'-methylenebis(phenyl isocyanate) as examples for diisocyanate.

[0023]

[0024] After the cardanol-based polyurethane prepolymer is prepared, it is reacted with casein to prepare modified casein. The reaction principle is as follows:

[0025]

[0026] It mainly reacts with the amino group in casein, or reacts with the carboxyl group in casein, or reacts with both the amino group and the carboxyl group at the same time, but eventually the isocyanate group is consumed; Casein-NH2 represents casein, indicating that the amino group in casein undergoes a chemical reaction.

[0027] Preferably, the melt index of LDPE is 0.18-4.0 g / 10 min; the melt index of HDPE is 0.06-0.45 g / 10 min; the melt index of EAA is 8.2-38.0 g / 10 min;

[0028] LDPE may be ExxonMobil LD 2119.LN series, ExxonMobil LD 01820 series and ExxonMobil LD 4020.BA; EAA may be ExxonMobil Adhere EAA38015 and ExxonMobil Adhere EAA8206; HDPE may be ExxonMobil HD 5001, ExxonMobil HD 5703 and ExxonMobil HD 5805; the metallocene polyolefin adhesive resin may be ExxonMobil Exact 3020FL, having an ethylene content of 11 wt%, a melt index of 1.20 g / 10 min at 190°C.

[0029] More preferably, the melt index of LDPE is 2.1-4.0 g / 10 min; and the melt index of HDPE is 0.30-0.45 g / 10 min.

[0030] The present invention also provides a multilayer composite bubble film, which is prepared by any of the above preparation methods; the multilayer composite bubble film is composed of four layers of material: 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 to primarily promote transesterification between the hydroxyl groups of EVOH in the bubble layer and the ester groups of PLA in the mechanical layer, forming covalent bonds. It also promotes physical entanglement between the metallocene polyolefin and PP / HDPE in the gas barrier layer, thereby enhancing the bond strength between the layers. Di-tert-butyl peroxide is added to the protective layer to generate free radicals through decomposition under hot pressing conditions, promoting 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.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. Casein is hydrophobically modified using a cardanol-based polyurethane prepolymer. The isocyanate group in the prepolymer undergoes a cross-linking reaction with the amino and hydroxyl groups of casein, and the hydrophobic cardanol long-chain olefin is grafted onto the surface of the casein molecule, forming a steric hindrance effect and effectively shielding the hydrophilic group. This not only significantly improves the hydrophobicity of 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 avoids harmful residues that may be introduced by traditional modifiers. The modified casein acts as an interface bridge, enhancing the interlayer bonding strength of composite structures such as bubble layers and gas barrier layers. At the same time, the raw material cardanol is derived from natural renewable resources. No harmful solvents are released during the entire modification process, giving the product environmentally friendly attributes. Testing has shown that no polluting heavy metals or polybrominated organic pollutants were detected, meeting the requirements of green packaging materials.

[0033] 2. The multi-layer composite design of "bubble layer-mechanical reinforcement layer-gas barrier layer" is adopted, with each layer achieving interfacial chemical bonding through compatibilizers and catalysts. The bubble layer is based on LDPE and EVA, with PLA and nanoclay introduced. A regular bubble structure is formed through tape casting, utilizing the elastic deformation of the bubbles to absorb impact energy. The nanoclay and modified casein synergistically enhance the matrix strength. The mechanical reinforcement layer uses PP and HDPE as the backbone, with nanocellulose evenly dispersed to form a network structure. Lubricants are used to improve processing fluidity, significantly enhancing tensile strength and impact resistance. The gas barrier layer forms a dense structure through hydrogen bonding between EVOH and modified casein, and physical interlocking of the metallocene adhesive resin, effectively blocking oxygen permeation. The compatibilizer acts during the hot pressing process to achieve a firm bond between the layers, preventing delamination between the layers and ensuring the material combines excellent tensile strength, impact toughness, and gas barrier properties.

[0034] 3. During the preparation process, material performance is optimized through precise control of process parameters at each stage. Controlling the casting temperature ensures full melt compatibility between LDPE and EVA, preventing PLA degradation due to high temperatures. Reasonable temperature regulation ensures thorough blending of the nanocellulose with the polymer matrix, forming a stable reinforcing phase. The casting speed of the gas barrier layer is optimally matched, and three-stage temperature control is employed during hot pressing and lamination to ensure sufficient interfacial reaction between the layers while preventing structural damage to sensitive materials like EVOH caused by high temperatures. Furthermore, the corrosion-resistant protective layer achieves efficient crosslinking of the waterborne polyurethane by adding silane coupling agents and aziridine crosslinkers, forming a dense coating. The result is a multi-layer composite bubble film with excellent overall performance and stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Schematic diagram of the preparation method of the multi-layer composite bubble film of the present invention;

[0036] Figure 2 Graph showing the tensile strength and Izod notched impact strength test results of Examples 1-5 of the present invention;

[0037] Figure 3 FTIR characterization diagram of the cardanol-based polyurethane prepolymer and modified casein of the present invention. DETAILED DESCRIPTION

[0038] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0039] See also Figures 1 to 2The present invention provides a multi-layer composite bubble film and a preparation method thereof, and the technical solution is as follows:

[0040] Preparation Example 1

[0041] Synthesis of cardanol-based polyurethane prepolymer: Weigh 50.0 g of 4,4'-methylenebis(phenyl isocyanate) (MDI) and store it in a sealed container away from light for later use.

[0042] Prepolymerization process: 31.55g of cardanol was placed in a 250mL three-necked flask and placed in an oil bath. The oil bath was turned on at 200rpm and heated to 110°C. Dehydration was carried out at a vacuum degree of -0.09MPa for 30min to ensure that the moisture content of cardanol was <0.05%. After dehydration, dry nitrogen was introduced for protection, the temperature was lowered to 60°C, and MDI was slowly added. The addition was completed within about 10min. At this time, the system temperature rose to 80°C due to the exothermic reaction. The temperature was maintained at 80°C and the reaction was continued for 1.5h. During this period, samples were taken every 15min and the -NCO content was detected by di-n-butylamine titration until it reached the theoretical value. After the reaction, the transparent light yellow cardanol-based polyurethane prepolymer was poured into a dry container while hot, sealed and refrigerated for later use.

[0043] Preparation of modified casein: 35 g of dehydrated casein was added to a high-speed shear mixer at 8000 rpm and stirred for 15 min to form a uniform material. The internal mixer was preheated to 80°C and the rotor speed was set to 60 rpm. The uniform material was transferred to the internal mixer and 8.0 g of cardanol-based polyurethane prepolymer was slowly added within 5 min. After internal mixing for 20 min, a light yellow solid was obtained. After drying, the modified casein was obtained; it was immediately placed in an aluminum foil sealed bag and stored in a freezer at -18°C.

[0044] Preparation Example 2-4

[0045] The difference from Preparation Example 1 is that the following preparation conditions are changed, as shown in Table 1. In Table 1, MDI is 4,4'-methylenebis(phenyl isocyanate), HDI is hexamethylene diisocyanate, IPDI is isophorone diisocyanate, and TDI is toluene diisocyanate.

[0046] Table 1 Preparation of modified casein

[0047]

[0048] Preparation of the comparative example Except for the following changes in preparation conditions, other methods are consistent with Preparation Example 3.

[0049] Preparation Comparative Example 1 The amount of IPDI used was 22.3 g.

[0050] Preparation Comparative Example 2: Dehydrated casein was mixed with cardanol and then kneaded instead of being mixed with cardanol-based polyurethane prepolymer.

[0051] Preparation Comparative Example 3: Dehydrated casein was not subjected to hydrophobic modification treatment.

[0052] Preparation Comparative Example 4: Dehydrated casein was directly mixed with the cardanol-based polyurethane prepolymer without using the post-addition and slow addition methods of the cardanol-based polyurethane prepolymer.

[0053] Test Example 1

[0054] The modified casein obtained in the preparation example and the comparative example was hot pressed after cold refining at room temperature, hot pressed at 100°C and 20 MPa for 3 minutes, cooled and placed at room temperature for 24 hours, and hot pressed again. The hot pressing temperature and time remained unchanged, and the pressure was 40 MPa. After cooling to room temperature, the pressed tablets were prepared into a dumbbell shape in accordance with GBT1040-2006 "Determination of Tensile Properties of Plastics". After balancing in a constant temperature box, the water contact angle was tested using a contact instrument. The test results were averaged three times. The final test results are shown in Table 2.

[0055] Table 2 Hydrophobicity test results of modified casein of Preparation Examples 1-4 and Comparative Examples 1-4

[0056]

[0057] like Figure 3 As shown, cardanol polyurethane prepolymer is at 2271cm -1 There is an absorption peak at 1730cm, which is the characteristic absorption peak of isocyanate group; -1 There is an absorption peak at 3450cm -1 The phenolic hydroxyl absorption peak of unreacted cardanol appears at 720 cm -1 The characteristic absorption peak of benzene ring appears at 15800 cm -1 The above results show that the cardanol polyurethane prepolymer was successfully synthesized. Then the cardanol polyurethane prepolymer was used to modify casein. The modified casein had a characteristic absorption peak at 2271cm -1 The disappearance of the characteristic absorption peak indicated that the isocyanate group was consumed by reaction, possibly by reacting with the carboxyl or amino group in casein; and the carbonyl stretching vibration peak shifted to the high-wave direction (1654 cm-1), indicating that the modified casein was successfully prepared.

[0058] 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 prepolymerized. Cardanol contains phenolic hydroxyl groups (-OH) and long-chain unsaturated olefins, and MDI contains isocyanate groups (-NCO). The two react to form a polyurethane prepolymer through an addition reaction. The unsaturated olefin chain segments give the prepolymer flexibility, and the benzene ring structure enhances rigidity, forming a rigid and flexible molecular chain. By controlling the molar ratio of diisocyanate to the phenolic hydroxyl groups at 2:1, a prepolymer terminated with -NCO is obtained. The prepolymer is used to modify casein. Natural casein contains a large number of hydrophilic amino groups (-NH2), carboxyl groups (-CO The -NCO group of the cardanol-based polyurethane prepolymer reacts with the amino and hydroxyl groups of casein to graft 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 the hydrophobicity. During the reaction, the high speed allows the casein to be fully dispersed, increasing the contact area with the prepolymer. The preheating of the internal mixer and the setting of a fixed speed provide a suitable melt mixing environment. The prepolymer is added slowly to avoid local agglomeration and ensure that the grafting reaction proceeds evenly. In the preparation of Comparative Example 1, the amount of IPDI was insufficient, the cross-linking density was low, the coverage of the hydrophobic segments was incomplete, and the hydrophobicity was significantly reduced compared with the preparation example; in the preparation of Comparative Example 2, cardanol was directly mixed with casein, lacking -NCO group cross-linking, only physical adsorption, and the hydrophobic modification effect was weak; in the preparation of Comparative Example 3, no hydrophobic modification treatment was performed, and it contained a large number of hydrophilic groups and had strong natural hydrophilicity; in the preparation of Comparative Example 4, the prepolymer was not slowly added through banburying, the prepolymer was unevenly dispersed, and some areas did not react, so the hydrophobic modification effect was reduced.

[0059] Example 1

[0060] The overall preparation method is as follows Figure 1 As shown, the raw materials used in the following process are the modified casein prepared by the method of Preparation Example 3;

[0061] 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 parts of hindered phenol antioxidant 1010, 1 part of nanoclay, 4 parts of modified casein and 3 parts of a compatibilizer, the compatibilizer consisting of 2 parts of ethylene-acrylic acid copolymer (EAA) and 1 part of maleic anhydride grafted polyethylene, and 0.5 parts of a cross-linking 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 minutes to ensure that the components are fully and evenly mixed; the mixed bubble layer raw materials are put into a casting machine, and the casting temperature is set to 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 the bubble layer film is pressed with a bubble film forming roller to form a bubble layer film with uniform thickness.

[0062] The preparation method of the mechanical reinforcement layer is as follows: take 50 parts of polypropylene (PP) particles, 2 parts of nanocellulose (diameter 10-100nm, length 0.5-5μm), 20 parts of high-density polyethylene (HDPE), 10 parts of polylactic acid (PLA), 2 parts of ethylene bisstearamide, 1 part of oleamide, 1 part of erucamide, 0.2 parts 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 set aside; melt blending and extrusion are carried out 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.

[0063] The gas barrier layer was prepared by preparing 90 parts of ethylene-vinyl alcohol copolymer (EVOH) particles; 18 parts of a metallocene polyolefin adhesive resin, 6 parts of a modified casein, and 1.5 parts of a compatibilizer, PE-g-MAH. The above substances were mixed evenly and then cast using a casting machine at a temperature of 175°C. During the casting process, the casting speed was controlled at 1.5 m / min to ensure uniform film thickness, thereby obtaining a gas barrier layer.

[0064] The raw materials for preparing the corrosion-resistant protective layer are as follows: 90 parts of water-based polyurethane resin, 5 parts of 1,3-divinyl-1,1,3,3-tetramethoxydisiloxane, 10 parts of nano-titanium dioxide, 0.5 parts of KH792, 1 part of aziridine crosslinker and 0.5 parts of di-tert-butyl peroxide are mixed and added into a stirring tank, stirred at 55°C for 40 minutes, and fully dispersed to make a corrosion-resistant material.

[0065] The preparation method of the multi-layer composite bubble film is as follows: the layers are compounded by multi-layer hot pressing extrusion technology; from the outside to the inside, they are corrosion-resistant protective layer, gas barrier layer, mechanical reinforcement layer and bubble layer; by precisely controlling the preheating zone at 140°C, the melt compounding zone at 185°C and the shaping cooling zone at 60°C, the layers are ensured to fit closely together; after compounding, the order and thickness of the layers are: gas barrier layer 20μm, mechanical reinforcement layer 40μm and bubble layer 120μm, and a layer of polyurethane corrosion-resistant protective layer is applied on the surface of the composite film by roller coating, and the coating thickness is controlled at 1μm; after coating, it is dried and cured at a temperature of 75°C to improve the hardness and smoothness of the film surface to obtain a multi-layer composite bubble film.

[0066] Examples 2-5

[0067] Different from the preparation method in Example 1, the following preparation method has been adjusted, as shown in Tables 3-1 to 3-3.

[0068] Table 3-1 Preparation method of bubble layer

[0069]

[0070] Table 3-2 Preparation methods of bubble layer and mechanical reinforcement layer

[0071]

[0072] Table 3-3 Preparation methods of gas barrier layer, corrosion resistant protective layer and multi-layer composite bubble film

[0073]

[0074] The four layers in Table 3-3 are gas barrier layer, mechanical reinforcement layer, bubble layer and corrosion resistant protective layer.

[0075] Except for the following preparation conditions being adjusted, other methods were the same as those in Example 3.

[0076] Comparative Example 1 The HDPE melt index is 0.06 g / 10 min.

[0077] Comparative Example 2 The LDPE melt index was 0.18 g / 10 min.

[0078] Comparative Example 3 The melt index of EAA is 8.2 g / 10 min.

[0079] Comparative Example 4 No modified casein was added to the bubble layer and the gas barrier layer.

[0080] Comparative Example 5 Unmodified casein was added to the bubble layer and the gas barrier layer.

[0081] Comparative Example 6 During the preparation of the bubble layer, EAA, maleic anhydride grafted polyethylene and catalyst HDI were not added.

[0082] Comparative Example 7 During the preparation of the mechanical reinforcement layer, ethylene bisstearamide, oleamide, erucamide and the catalyst tetrabutyl titanate were not added.

[0083] Comparative Example 8 During the preparation of the mechanical reinforcement layer, no compatibilizer obtained by grafting maleic anhydride onto polypropylene and propylene oxide onto polyethylene was added.

[0084] Comparative Example 9 No compatibilizer PE-g-MAH was added during the preparation of the gas barrier layer.

[0085] Comparative Example 10 1,3-divinyl-1,1,3,3-tetramethoxydisiloxane and nano-titanium dioxide are not added to the corrosion-resistant protective layer.

[0086] Comparative Example 11 KH792, aziridine crosslinking agent and di-tert-butyl peroxide were not added to the corrosion-resistant protective layer.

[0087] Comparative Example 12 The casting speed was 2.0 m / min.

[0088] Comparative Example 13 The temperature of the melt lamination zone during the hot pressing lamination process was 210°C.

[0089] Test Example 2

[0090] The multilayer composite bubble films obtained in Examples 1-5 and Comparative Examples 1-13 were subjected to tensile strength and Izod notched impact strength tests. The tensile strength test was conducted in accordance with GB / T 1040.3-2006 "Determination of Tensile Properties of Plastics - Part 3: Test Conditions for Film and Sheeting"; the Izod notched impact strength test was conducted in accordance with the method in GB / T 1843-2008 "Determination of Izod Impact Strength of Plastics." A sample with a length and width of 80 mm × 10 mm was cut from the bubble film. A V-shaped notch was machined in the middle of the sample using a special notch cutter. 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 sample was fixed on a cantilever beam fixture with the notch located opposite the impact surface to ensure that the sample was clamped tightly without loosening. An appropriate pendulum energy was selected based on the estimated strength of the sample to prevent the sample from not breaking or being excessively broken. The pendulum was released to impact the back of the notch of the sample. The impact energy consumed when the sample broke was recorded. The final test results are shown in Tables 4 and Figure 2 shown.

[0091] Table 4 Test results of tensile strength and Izod notched impact strength

[0092]

[0093] By optimizing the preparation method of the bubble layer, mechanical reinforcement layer, gas barrier layer and corrosion resistant protective layer, the final multi-layer composite bubble film has good mechanical properties. Figure 2 The results show that the tensile strength under the conditions of Examples 1-5 is 159-180 MPa, and the Izod notched impact strength is 68-80 J / m 2 . The melt index of HDPE, LDPE and EAA used in Comparative Examples 1-3 is relatively low, resulting in poor melt fluidity, which in turn leads to brittle fracture in local nano-agglomeration areas, increased extrusion internal stress, and rapid expansion of the generated microcracks under impact loads, thereby reducing material performance; Comparative Example 4 does not add modified casein and Comparative Example 5 adds unmodified casein, the hydrophilicity of the barrier layer increases, water molecules penetrate and weaken the EVOH crystallization interface, and the interlayer bonding force decreases; and without modification, the compatibility of casein with non-polar components is poor; the bubble layer of Comparative Example 6 lacks compatibilizer and catalyst, the interface bonding force decreases, LDPE / PLA phase separation, and the impact energy absorption capacity decreases; the bonding performance between layers deteriorates; Comparative Example 7 does not add lubricant in the mechanical reinforcement layer, the processing performance decreases, and the tensile strength decreases compared with Example 3 The decrease is not obvious, but the cantilever notched impact strength is significantly reduced; in comparative example 8, no compatibilizer is added, PP / HDPE / PLA phases are separated, and the performance of the bubble film is reduced; in comparative example 9, no compatibilizer is added during the preparation of the body barrier layer, the interface bonding between EVOH and metallocene polyolefin is weak, the interlayer peeling, and the interface defects become the source of impact cracks; in comparative example 10, the corrosion-resistant protective layer does not add the above substances, and the mechanical properties are reduced; in comparative example 11, no cross-linking agent and catalyst are added to the corrosion-resistant protective layer, the water-based polyurethane is insufficiently cured, the bonding performance with the gas barrier layer is reduced, the tensile strength is reduced, and the cantilever notched impact strength is reduced; in comparative example 12, the casting speed is too fast, the solvent is not fully evaporated, the porosity of the bubble layer increases, and the performance is reduced; in comparative example 13, the hot pressing temperature is too high, EVOH decomposes, holes are generated in the barrier layer, and stress concentration at the edge of the hole induces brittle fracture.

[0094] Test Example 3

[0095] The multilayer composite bubble films obtained in the above examples and comparative examples were subjected to barrier performance testing, primarily referring to the pressure differential method in GB / T1038-2000. A pressure differential was established across the sample, and the change in 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 oxygen transmission rate.

[0096] Table 5 OTR test results of Examples 1-5 and Comparative Examples 1-13

[0097]

[0098] 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 resulting microcracks become oxygen diffusion paths; Comparative Example 4 does not add modified casein, resulting in the exposure of the EVOH hydrophilic groups, which destroys the crystallization after moisture absorption and expands the oxygen channel; Comparative Example 5 adds unmodified casein, which increases the water absorption performance, causes the hydrophilic groups to expand by moisture absorption, and expands the distance between the EVOH molecular chains; 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, a decrease in the cross-linking density, which deteriorates the oxygen barrier performance; 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 increases; in comparative example 13, the temperature is too high, which has a destructive effect on each layer and reduces the oxygen barrier performance.

[0099] Test Example 4

[0100] 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.

[0101] Table 6 Metal and organic matter test results

[0102]

[0103] 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.

[0104] While 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 these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a multi-layer composite bubble film, characterized in that: The preparation method is as follows: Low-density polyethylene, ethylene-vinyl acetate copolymer, polylactic acid, high-density polyethylene, nanoclay and modified casein are mixed and put into a casting machine, and then pressed by a bubble film forming roller to obtain a bubble layer. The modified casein is prepared by adding casein into an internal mixer, slowly adding a cardanol-based polyurethane prepolymer, mixing, drying, and freezing the mixture. The polypropylene, nanocellulose, the high-density polyethylene, polylactic acid, a lubricant and a compatibilizer are mixed and then extruded through a twin-screw extruder to obtain a mechanical reinforcement layer; The ethylene-vinyl alcohol copolymer, the metallocene polyolefin adhesive resin and the modified casein are mixed and then cast to obtain a gas barrier layer; The corrosion-resistant material is obtained by mixing waterborne polyurethane resin, vinyl silicone, nano titanium dioxide, a cross-linking agent and a catalyst; After the bubble layer, the mechanical reinforcement layer and the gas barrier layer are hot-pressed, the corrosion-resistant material is coated on the surface and solidified to obtain the multi-layer composite bubble film.

2. The method for preparing a multi-layer composite bubble film according to claim 1, wherein: The lubricant in the mechanical reinforcement layer is obtained by mixing ethylene bisstearamide, oleamide and erucamide; and the compatibilizer is obtained by mixing maleic anhydride grafted polypropylene and propylene oxide grafted polyethylene.

3. The method for preparing a multi-layer composite bubble film according to claim 1, wherein: The cardanol-based polyurethane prepolymer is obtained by reacting cardanol and diisocyanate.

4. The method for preparing a multi-layer composite bubble film according to claim 3, wherein: The diisocyanate is one of 4,4'-methylenebis(phenyl isocyanate), hexamethylene diisocyanate, isophorone diisocyanate and toluene diisocyanate.

5. A multi-layer composite bubble film, characterized in that: The multi-layer composite bubble film is prepared by the preparation method according to any one of claims 1 to 4; the multi-layer composite bubble film is composed of four 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.

6. The multi-layer composite bubble film according to claim 5, characterized in that: The raw materials for preparing the bubble layer include low-density polyethylene, ethylene-vinyl acetate copolymer, high-density polyethylene and ethylene-acrylic acid copolymer; the raw materials for preparing the mechanical reinforcement layer include polypropylene and nanocellulose; and the raw materials for preparing the gas barrier layer include modified casein.

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