A cross-linked polyolefin heat shrinkable film and its preparation process

Abstract

The present invention discloses a cross-linked polyolefin heat shrinkable film and a preparation process thereof, and relates to the technical field of film material preparation. The process comprises the following steps: Step 1: The outer layer, inner layer and core layer raw materials are extruded separately in a single-screw extruder, and then they are combined in a three-layer co-extrusion die head, co-extruded and cooled to obtain a multi-layer composite sheet; Step 2: The multi-layer composite sheet is pulled to a stretching oven for bidirectional synchronous stretching, and then cooled through an air ring, split and rolled up, and cut to obtain a finished product. Advantages: The present invention synthesizes specific inorganic nanomaterials and organic heat-resistant agents, and modifies the surface of the inorganic nanomaterials to accurately control the ratio between the inorganic nanomaterials and the organic heat-resistant agents, thereby significantly improving the compatibility between the two. This improvement not only enhances the thermal stability of the cross-linked polyolefin heat shrinkable film, but also improves its mechanical properties.

Description

Technical Field

[0001] The invention relates to the technical field of film material preparation, in particular to a cross-linked polyolefin heat shrinkable film and a preparation process thereof. Background Art

[0002] Heat-shrink film, as an important packaging material, is widely used in food, beverages, medicine, electronics, and other fields. Cross-linked polyolefin heat-shrink film, in particular, has become a hot topic of research and application in recent years due to its excellent mechanical properties, heat resistance, transparency, and environmental friendliness. However, conventional cross-linked polyolefin heat-shrink film still has several drawbacks that limit its further application. Among them, existing heat-shrink films have insufficient shrinkage at high temperatures, making it difficult to tightly wrap complex-shaped items, affecting packaging quality and product aesthetics. Some cross-linked polyolefin heat-shrink films also lack mechanical properties such as tensile strength and tear strength, making them prone to breakage during transportation and storage, affecting product protection.

[0003] Currently, inorganic and organic heat-resistant additives are commonly added to improve the performance of polyolefin heat-shrinkable films. However, these additives present several challenges: Inorganic materials have poor compatibility with the organic polymer matrix, easily agglomerating and forming stress concentration points, which impact the material's uniformity and mechanical properties. Their weak interfacial bonding forces lead to interfacial delamination under stress, reducing the material's strength and toughness. Organic heat-resistant additives can migrate to the material's surface, affecting its appearance and performance. Furthermore, when mixed, inorganic and organic heat-resistant additives exhibit poor compatibility, leading to phase separation and further compromising the material's uniformity and overall performance.

[0004] In summary, in order to solve the above problems, the present invention provides a process for preparing a cross-linked polyolefin heat shrinkable film. Summary of the Invention

[0005] The object of the present invention is to provide a cross-linked polyolefin heat shrinkable film and a preparation process thereof, so as to solve the problems raised in the prior art.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A preparation process of a cross-linked polyolefin heat shrinkable film comprises the following steps:

[0008] Step 1: The outer layer, inner layer and core layer raw materials are extruded separately in a single-screw extruder, then merged in a three-layer co-extrusion die, co-extruded and cooled to obtain a multi-layer composite sheet;

[0009] Step 2: The multi-layer composite sheet is pulled into a stretching oven for bidirectional synchronous stretching, then cooled by an air ring, split and rolled up, and cut to obtain the finished product.

[0010] More optimally, the outer layer and the inner layer have the same raw materials, including the following substances: by weight, 100-110 parts of terpolymer polypropylene, 10-12 parts of anti-sticking agent, 3-4 parts of heat-resistant agent, 0.1-0.2 parts of initiator; the core layer raw materials include the following substances: by weight, 70-80 parts of low-density polyethylene, 5-8 parts of heat-resistant agent, 0.1-0.2 parts of initiator, and 2-3 parts of lubricant.

[0011] More optimally, the heat-resistant agent comprises a modified inorganic nanomaterial and an organic heat-resistant agent in a mass ratio of (3-4):1.

[0012] More optimally, the preparation process of the modified inorganic nanomaterial is:

[0013] S1: adding illite to a sodium hydroxide solution, heating to 40-50°C, stirring and soaking for 1-2 hours, filtering and washing with deionized water until neutral; then transferring to a hydrochloric acid solution, stirring and soaking for 1-2 hours at room temperature, filtering and washing with deionized water until neutral, and drying to obtain pretreated illite;

[0014] S2: Mix the pretreated illite with deionized water, heat to 70-80°C, add hydrochloric acid dropwise to adjust the pH to 2-3, then slowly add anhydrous ethanol solution of titanium tetrachloride dropwise. After the addition is complete, continue the reaction for 2-3 hours, cool, wash, dry, and transfer to a muffle furnace for calcination at 500-550°C for 2-3 hours to obtain a composite inorganic material;

[0015] S3: The composite inorganic material is dissolved in anhydrous ethanol, ultrasonically dispersed for 30-40 minutes, hydrochloric acid is added to adjust the pH to 3-4, a mixed solution of 3-aminopropyltriethoxysilane and ethanol is added, stirred for 30-40 minutes, reacted at 60-70°C for 3-4 hours, cooled to room temperature, washed with anhydrous ethanol, vacuum-dried, ultrasonically dispersed in deionized water, a mixed solution of 3-mercaptopropyltriethoxysilane and methanol and an alkaline solution are added, the temperature is raised to 70-80°C, the reaction is carried out for 5-8 hours, washed, and dried to obtain a modified inorganic nanomaterial.

[0016] More optimally, the mass ratio of the 3-aminopropyltriethoxysilane to the 3-mercaptopropyltriethoxysilane is 1:(2-3).

[0017] More optimally, the pretreated illite comprises the following substances: by weight, 20-30 parts of illite, 244-250 parts of sodium hydroxide solution, and 210-215 parts of hydrochloric acid solution; wherein the mass fraction of the sodium hydroxide solution is 20-25%, and the mass fraction of the hydrochloric acid solution is 10-15%;

[0018] The composite inorganic material comprises the following substances: by weight, 25-30 parts of pretreated illite, 260-280 parts of deionized water, and 42-45 parts of anhydrous ethanol solution of titanium tetrachloride.

[0019] More optimally, the preparation process of the organic heat-resistant agent is as follows: 2-amino-4-pentenoic acid and calcium carbonate are added to tetrahydrofuran, ultrasonically dispersed for 30-40 minutes, then placed in an ice-water bath at 0°C, and a benzene solution of 3,5-bis(tert-butyl)-4-hydroxyphenylpropionyl chloride is slowly added dropwise. After the addition is complete, the temperature is raised to 50-60°C, and the reaction is carried out at a constant temperature for 20-25 hours. After cooling to room temperature, the mixture is filtered, washed, and dried to obtain the organic heat-resistant agent.

[0020] More optimally, the organic heat-resistant agent raw material includes the following components: by weight, 11-12 parts of 2-amino-4-pentenoic acid, 200-220 parts of calcium carbonate, 250-180 parts of tetrahydrofuran, and 18-20 parts of a benzene solution of 3,5-bis(tert-butyl)-4-hydroxyphenylpropionyl chloride.

[0021] More optimally, the extrusion temperature of the outer layer and the inner layer in the single-screw extruder is 180-190°C; the extrusion temperature of the core layer in the single-screw extruder is 170-180°C.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] This invention significantly improves the compatibility of a cross-linked polyolefin heat-shrinkable film by synthesizing specific inorganic nanomaterials and organic heat-resistant agents, modifying the surface of the inorganic nanomaterials, and precisely controlling the ratio of the inorganic nanomaterials to the organic heat-resistant agent. This improvement not only enhances the thermal stability of the cross-linked polyolefin heat-shrinkable film but also improves its mechanical properties. The details are as follows:

[0024] First: In the scheme, by controlling the mass ratio of modified inorganic nanomaterials to organic heat-resistant agents at (3-4):1, the compatibility of the two can be effectively balanced. The thiol group and amino group on the modified inorganic nanomaterial can react with the vinyl and carboxyl groups of the organic heat-resistant agent to reduce the migration of the organic heat-resistant agent. However, if the proportion of inorganic nanomaterials is too high, they may agglomerate in the organic polymer matrix to form stress concentration points, thereby affecting the uniformity and mechanical properties of the film; conversely, if the proportion is too low, the thermal stability of the heat shrinkable film will be significantly reduced. Similarly, too much organic heat-resistant agent may cause phase separation inside the film, affecting the uniformity and overall performance of the film; and when the proportion of organic heat-resistant agent is too low, the antioxidant properties and flexibility of the film will decrease, causing the performance of the film to decline rapidly in high temperature and oxidative environments, affecting its service life;

[0025] Second: In this scheme, by using aminosilane coupling agent and mercaptosilane coupling agent to modify the composite inorganic material and controlling the ratio of the two to 1: (2-3), a combination reaction of multiple groups can be achieved. Among them, the mercapto group can react with the vinyl group in the organic heat-resistant agent to form a stable sulfide bond; the amino group can react with the carboxyl group in the organic heat-resistant agent to form a stable amide bond. This multi-group combination reaction can further improve the overall performance of the material; however, if too much aminosilane coupling agent is grafted, due to the hygroscopicity of the amide bond, the resulting heat shrinkable film may absorb moisture in a humid environment, thereby affecting its use scenario; on the contrary, if too much mercaptosilane coupling agent is grafted, the thermal stability of the material may be reduced;

[0026] Third: The multi-layered illite used in the scheme has good interlayer bonding force and mechanical strength, providing a good dispersion platform for inorganic nanomaterials and organic heat-resistant agents. This structure can effectively inhibit the thermal shrinkage of the material at high temperatures and ensure the dimensional stability of the film in high-temperature environments. DETAILED DESCRIPTION

[0027] Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative work shall fall within the scope of protection of the present invention.

[0028] It should be noted that the following parts are by weight, and all raw materials involved in the present invention are purchased from manufacturers without any special restrictions. Examples include: In the following examples, 2-amino-4-pentenoic acid CAS is 16338-48-0; 3-aminopropyltriethoxysilane CAS is 919-30-2; and 3-mercaptopropyltriethoxysilane CAS is 14814-09-6.

[0029] Example 1: A process for preparing a cross-linked polyolefin heat shrinkable film, comprising the following steps:

[0030] Step 1: 100 parts of terpolymer polypropylene, 10 parts of paraffin wax, 3 parts of heat-resistant agent, and 0.1 parts of dicumyl peroxide are mixed as the raw materials for the outer layer and the inner layer; 70 parts of low-density polyethylene, 5 parts of heat-resistant agent, 0.1 parts of dicumyl peroxide, and 2 parts of calcium stearate are mixed as the raw materials for the core layer; wherein the heat-resistant agent includes a modified inorganic nanomaterial and an organic heat-resistant agent in a mass ratio of 3:1;

[0031] Step 2: Extruding the outer layer and the inner layer in a single-screw extruder at 180° C.; extruding the core layer in a single-screw extruder at 170° C., and then merging them in a three-layer co-extrusion die, co-extruding, and cooling to obtain a multi-layer composite sheet;

[0032] Step 3: The multi-layer composite sheet is pulled into a stretching oven for bidirectional synchronous stretching, then cooled by an air ring, split, rolled up, and cut to obtain the finished product;

[0033] Among them, the preparation process of modified inorganic nanomaterials is:

[0034] S1: adding 20 parts of illite to 244 parts of 25 wt% sodium hydroxide solution, heating to 40°C, stirring and soaking for 1 hour, filtering and washing with deionized water until neutral; then transferring to 210 parts of 15 wt% hydrochloric acid solution, stirring and soaking for 1 hour at room temperature, filtering and washing with deionized water until neutral, and drying to obtain pretreated illite;

[0035] S2: 25 parts of pretreated illite were mixed with 260 parts of deionized water, the temperature was raised to 70-80°C, hydrochloric acid was added dropwise to adjust the pH to 2, and then 42 parts of anhydrous ethanol solution of titanium tetrachloride was slowly added dropwise. After the addition was complete, the reaction was continued for 2 hours, and the mixture was cooled, washed, dried, and transferred to a muffle furnace and calcined at 500°C for 2 hours to obtain a composite inorganic material;

[0036] S3: 10 parts of the composite inorganic material were dissolved in anhydrous ethanol, ultrasonically dispersed for 30 minutes, hydrochloric acid was added to adjust the pH to 3, and then a mixed solution of 3-aminopropyltriethoxysilane and ethanol was added, stirred for 30 minutes, reacted at 60°C for 3 hours, cooled to room temperature, washed with anhydrous ethanol, vacuum dried, transferred to deionized water, and a mixed solution of 3-mercaptopropyltriethoxysilane and methanol and an alkaline solution were added, heated to 70°C, reacted for 5 hours, washed, and dried to obtain a modified inorganic nanomaterial; wherein the mass ratio of 3-aminopropyltriethoxysilane to 3-mercaptopropyltriethoxysilane is 1:2;

[0037] Among them, the preparation process of the organic heat-resistant agent is as follows: 11 parts of 2-amino-4-pentenoic acid and 200 parts of calcium carbonate are added to 250 parts of tetrahydrofuran, ultrasonically dispersed for 30 minutes, then placed in an ice water bath at 0°C, and 18 parts of 3,5-bis(tert-butyl)-4-hydroxyphenylpropionyl chloride benzene solution are slowly added dropwise. After the addition is completed, the temperature is raised to 50°C, the reaction is carried out at a constant temperature for 20 hours, and after cooling to room temperature, the mixture is filtered, washed, and dried to obtain the organic heat-resistant agent.

[0038] Example 2: A process for preparing a cross-linked polyolefin heat shrinkable film, comprising the following steps:

[0039] Step 1: 110 parts of terpolymer polypropylene, 12 parts of paraffin wax, 4 parts of heat-resistant agent, and 0.2 parts of dicumyl peroxide are mixed as the raw materials for the outer layer and the inner layer; 80 parts of low-density polyethylene, 8 parts of heat-resistant agent, 0.2 parts of dicumyl peroxide, and 3 parts of calcium stearate are mixed as the raw materials for the core layer; wherein the heat-resistant agent comprises a modified inorganic nanomaterial and an organic heat-resistant agent in a mass ratio of 4:1;

[0040] Step 2: Extruding the outer layer and the inner layer in a single-screw extruder at 190° C.; extruding the core layer in a single-screw extruder at 180° C., and then merging them in a three-layer co-extrusion die, co-extruding, and cooling to obtain a multi-layer composite sheet;

[0041] Step 3: The multi-layer composite sheet is pulled into a stretching oven for bidirectional synchronous stretching, then cooled by an air ring, split, rolled up, and cut to obtain the finished product;

[0042] Among them, the preparation process of modified inorganic nanomaterials is:

[0043] S1: 30 parts of illite were added to 250 parts of 25 wt% sodium hydroxide solution, heated to 50°C, stirred and soaked for 2 hours, filtered and washed with deionized water until neutral; then transferred to 215 parts of 15 wt% hydrochloric acid solution, stirred and soaked for 2 hours at room temperature, filtered and washed with deionized water until neutral, and dried to obtain pretreated illite;

[0044] S2: 30 parts of pretreated illite were mixed with 280 parts of deionized water, the temperature was raised to 80°C, hydrochloric acid was added dropwise to adjust the pH to 3, and then 45 parts of anhydrous ethanol solution of titanium tetrachloride was slowly added dropwise. After the addition was complete, the reaction was continued for 3 hours, and the mixture was cooled, washed, and dried. The mixture was transferred to a muffle furnace and calcined at 550°C for 3 hours to obtain a composite inorganic material;

[0045] S3: 20 parts of the composite inorganic material were dissolved in anhydrous ethanol, ultrasonically dispersed for 40 minutes, hydrochloric acid was added to adjust the pH to 4, and then a mixed solution of 3-aminopropyltriethoxysilane and ethanol was added, stirred for 40 minutes, reacted at 70°C for 4 hours, cooled to room temperature, washed with anhydrous ethanol, vacuum dried, transferred to deionized water, and a mixed solution of 3-mercaptopropyltriethoxysilane and methanol and an alkaline solution were added, heated to 80°C, reacted for 8 hours, washed, and dried to obtain a modified inorganic nanomaterial; wherein the mass ratio of 3-aminopropyltriethoxysilane to 3-mercaptopropyltriethoxysilane is 1:3;

[0046] Among them, the preparation process of the organic heat-resistant agent is: add 12 parts of 2-amino-4-pentenoic acid and 220 parts of calcium carbonate to 280 parts of tetrahydrofuran, ultrasonically disperse for 40 minutes, then place in an ice water bath at 0°C, slowly add 20 parts of 3,5-bis(tert-butyl)-4-hydroxyphenylpropionyl chloride benzene solution, after the addition is completed, heat to 60°C, react at a constant temperature for 25 hours, cool to room temperature, filter, wash, and dry to obtain the organic heat-resistant agent.

[0047] Example 3: A process for preparing a cross-linked polyolefin heat shrinkable film, comprising the following steps:

[0048] Step 1: 108 parts of terpolymer polypropylene, 11 parts of paraffin wax, 3.5 parts of heat-resistant agent, and 0.15 parts of dicumyl peroxide are mixed as the raw materials for the outer layer and the inner layer; 75 parts of low-density polyethylene, 6 parts of heat-resistant agent, 0.15 parts of dicumyl peroxide, and 2.5 parts of calcium stearate are mixed as the raw materials for the core layer; wherein the heat-resistant agent comprises a modified inorganic nanomaterial and an organic heat-resistant agent in a mass ratio of 3.5:1;

[0049] Step 2: Extruding the outer layer and the inner layer in a single-screw extruder at 185° C.; extruding the core layer in a single-screw extruder at 175° C., and then merging them in a three-layer co-extrusion die, co-extruding, and cooling to obtain a multi-layer composite sheet;

[0050] Step 3: The multi-layer composite sheet is pulled into a stretching oven for bidirectional synchronous stretching, then cooled by an air ring, split, rolled up, and cut to obtain the finished product;

[0051] Among them, the preparation process of modified inorganic nanomaterials is:

[0052] S1: adding 25 parts of illite to 248 parts of 25 wt% sodium hydroxide solution, heating to 45°C, stirring and soaking for 1.5 hours, filtering and washing with deionized water until neutral; then transferring to 212 parts of 15 wt% hydrochloric acid solution, stirring and soaking for 1.5 hours at room temperature, filtering and washing with deionized water until neutral, and drying to obtain pretreated illite;

[0053] S2: 28 parts of pretreated illite were mixed with 270 parts of deionized water, the temperature was raised to 75°C, hydrochloric acid was added dropwise to adjust the pH to 3, and then 45 parts of anhydrous ethanol solution of titanium tetrachloride was slowly added dropwise. After the addition was complete, the reaction was continued for 3 hours, and the mixture was cooled, washed, dried, and transferred to a muffle furnace and calcined at 550°C for 3 hours to obtain a composite inorganic material;

[0054] S3: 15 parts of the composite inorganic material were dissolved in anhydrous ethanol, ultrasonically dispersed for 35 minutes, hydrochloric acid was added to adjust the pH to 3, and then a mixed solution of 3-aminopropyltriethoxysilane and ethanol was added, stirred for 30 minutes, reacted at 60°C for 3 hours, cooled to room temperature, washed with anhydrous ethanol, vacuum dried, transferred to deionized water, and a mixed solution of 3-mercaptopropyltriethoxysilane and methanol and an alkaline solution were added, heated to 75°C, reacted for 5.5 hours, washed, and dried to obtain a modified inorganic nanomaterial; wherein the mass ratio of 3-aminopropyltriethoxysilane to 3-mercaptopropyltriethoxysilane is 1:2.5;

[0055] Among them, the preparation process of the organic heat-resistant agent is: add 11.5 parts of 2-amino-4-pentenoic acid and 210 parts of calcium carbonate to 260 parts of tetrahydrofuran, ultrasonically disperse for 35 minutes, then place in an ice water bath at 0°C, slowly add 20 parts of 3,5-bis(tert-butyl)-4-hydroxyphenylpropionyl chloride benzene solution, after the addition is completed, heat to 55°C, keep constant temperature reaction for 23 hours, cool to room temperature, filter, wash, and dry to obtain the organic heat-resistant agent.

[0056] Comparative Example 1: No organic heat-resistant agent was added, and the rest was the same as Example 3, specifically as follows:

[0057] Step 1: 108 parts of ternary copolymer polypropylene, 11 parts of paraffin wax, and 3.5 parts of modified inorganic nanomaterials are mixed as the raw materials for the outer layer and inner layer; 75 parts of low-density polyethylene, 6 parts of modified inorganic nanomaterials, and 2.5 parts of calcium stearate are mixed as the raw materials for the core layer;

[0058] Step 2: Extruding the outer layer and the inner layer in a single-screw extruder at 185° C.; extruding the core layer in a single-screw extruder at 175° C., and then merging them in a three-layer co-extrusion die, co-extruding, and cooling to obtain a multi-layer composite sheet;

[0059] Step 3: The multi-layer composite sheet is pulled into a stretching oven for bidirectional synchronous stretching, then cooled by an air ring, split, rolled up, and cut to obtain the finished product;

[0060] Among them, the preparation process of modified inorganic nanomaterials is:

[0061] S1: adding 25 parts of illite to 248 parts of 25 wt% sodium hydroxide solution, heating to 45°C, stirring and soaking for 1.5 hours, filtering and washing with deionized water until neutral; then transferring to 212 parts of 15 wt% hydrochloric acid solution, stirring and soaking for 1.5 hours at room temperature, filtering and washing with deionized water until neutral, and drying to obtain pretreated illite;

[0062] S2: 28 parts of pretreated illite were mixed with 270 parts of deionized water, the temperature was raised to 75°C, hydrochloric acid was added dropwise to adjust the pH to 3, and then 45 parts of anhydrous ethanol solution of titanium tetrachloride was slowly added dropwise. After the addition was complete, the reaction was continued for 3 hours, and the mixture was cooled, washed, dried, and transferred to a muffle furnace and calcined at 550°C for 3 hours to obtain a composite inorganic material;

[0063] S3: 15 parts of the composite inorganic material were dissolved in anhydrous ethanol, ultrasonically dispersed for 35 minutes, hydrochloric acid was added to adjust the pH to 3, and then a mixed solution of 3-aminopropyltriethoxysilane and ethanol was added, stirred for 30 minutes, reacted at 60°C for 3 hours, cooled to room temperature, washed with anhydrous ethanol, vacuum dried, transferred to deionized water, and a mixed solution of 3-mercaptopropyltriethoxysilane and methanol and an alkaline solution were added. The temperature was raised to 75°C, reacted for 5.5 hours, washed, and dried to obtain a modified inorganic nanomaterial; wherein the mass ratio of 3-aminopropyltriethoxysilane to 3-mercaptopropyltriethoxysilane is 1:2.5.

[0064] Comparative Example 2: No modified inorganic nanomaterial was added, and the rest was the same as Example 3, specifically as follows:

[0065] Step 1: Mix 108 parts of terpolymer polypropylene, 11 parts of paraffin wax, and 3.5 parts of an organic heat-resistant agent to form the outer and inner layer materials; mix 75 parts of low-density polyethylene, 6 parts of an organic heat-resistant agent, and 2.5 parts of calcium stearate to form the core layer materials;

[0066] Step 2: Extruding the outer layer and the inner layer in a single-screw extruder at 185° C.; extruding the core layer in a single-screw extruder at 175° C., and then merging them in a three-layer co-extrusion die, co-extruding, and cooling to obtain a multi-layer composite sheet;

[0067] Step 3: The multi-layer composite sheet is pulled into a stretching oven for bidirectional synchronous stretching, then cooled by an air ring, split, rolled up, and cut to obtain the finished product;

[0068] Among them, the preparation process of the organic heat-resistant agent is: add 11.5 parts of 2-amino-4-pentenoic acid and 210 parts of calcium carbonate to 260 parts of tetrahydrofuran, ultrasonically disperse for 35 minutes, then place in an ice water bath at 0°C, slowly add 20 parts of 3,5-bis(tert-butyl)-4-hydroxyphenylpropionyl chloride benzene solution, after the addition is completed, heat to 55°C, keep constant temperature reaction for 23 hours, cool to room temperature, filter, wash, and dry to obtain the organic heat-resistant agent.

[0069] Comparative Example 3: An excess amount of modified inorganic nanomaterial was added, and the rest was the same as in Example 3, specifically as follows:

[0070] Step 1: 108 parts of terpolymer polypropylene, 11 parts of paraffin wax, 3.5 parts of heat-resistant agent, and 0.15 parts of dicumyl peroxide are mixed as the raw materials for the outer layer and the inner layer; 75 parts of low-density polyethylene, 6 parts of heat-resistant agent, 0.15 parts of dicumyl peroxide, and 2.5 parts of calcium stearate are mixed as the raw materials for the core layer; wherein the heat-resistant agent comprises a modified inorganic nanomaterial and an organic heat-resistant agent in a mass ratio of 5:1;

[0071] Step 2: Extruding the outer layer and the inner layer in a single-screw extruder at 185° C.; extruding the core layer in a single-screw extruder at 175° C., and then merging them in a three-layer co-extrusion die, co-extruding, and cooling to obtain a multi-layer composite sheet;

[0072] Step 3: The multi-layer composite sheet is pulled into a stretching oven for bidirectional synchronous stretching, then cooled by an air ring, split and rolled up, and cut to obtain the finished product.

[0073] Comparative Example 4: The amount of mercaptosilane coupling agent used was increased, and the rest was the same as in Example 3, as follows:

[0074] Step 1: 108 parts of terpolymer polypropylene, 11 parts of paraffin wax, 3.5 parts of heat-resistant agent, and 0.15 parts of dicumyl peroxide are mixed as the raw materials for the outer layer and the inner layer; 75 parts of low-density polyethylene, 6 parts of heat-resistant agent, 0.15 parts of dicumyl peroxide, and 2.5 parts of calcium stearate are mixed as the raw materials for the core layer; wherein the heat-resistant agent comprises a modified inorganic nanomaterial and an organic heat-resistant agent in a mass ratio of 3.5:1;

[0075] Step 2: Extruding the outer layer and the inner layer in a single-screw extruder at 185° C.; extruding the core layer in a single-screw extruder at 175° C., and then merging them in a three-layer co-extrusion die, co-extruding, and cooling to obtain a multi-layer composite sheet;

[0076] Step 3: The multi-layer composite sheet is pulled into a stretching oven for bidirectional synchronous stretching, then cooled by an air ring, split, rolled up, and cut to obtain the finished product;

[0077] Among them, the preparation process of modified inorganic nanomaterials is:

[0078] S1: adding 25 parts of illite to 248 parts of 25 wt% sodium hydroxide solution, heating to 45°C, stirring and soaking for 1.5 hours, filtering and washing with deionized water until neutral; then transferring to 212 parts of 15 wt% hydrochloric acid solution, stirring and soaking for 1.5 hours at room temperature, filtering and washing with deionized water until neutral, and drying to obtain pretreated illite;

[0079] S2: 28 parts of pretreated illite were mixed with 270 parts of deionized water, the temperature was raised to 75°C, hydrochloric acid was added dropwise to adjust the pH to 3, and then 45 parts of anhydrous ethanol solution of titanium tetrachloride was slowly added dropwise. After the addition was complete, the reaction was continued for 3 hours, and the mixture was cooled, washed, dried, and transferred to a muffle furnace and calcined at 550°C for 3 hours to obtain a composite inorganic material;

[0080] S3: 15 parts of the composite inorganic material were dissolved in anhydrous ethanol, ultrasonically dispersed for 35 minutes, hydrochloric acid was added to adjust the pH to 3, and then a mixed solution of 3-aminopropyltriethoxysilane and ethanol was added, stirred for 30 minutes, reacted at 60°C for 3 hours, cooled to room temperature, washed with anhydrous ethanol, vacuum dried, transferred to deionized water, and a mixed solution of 3-mercaptopropyltriethoxysilane and methanol and an alkaline solution were added. The temperature was raised to 75°C, reacted for 5.5 hours, washed, and dried to obtain a modified inorganic nanomaterial; wherein the mass ratio of 3-aminopropyltriethoxysilane to 3-mercaptopropyltriethoxysilane is 1:5.

[0081] Performance test: (1) The finished products obtained in the examples and comparative examples were tested for tensile strength according to the provisions of GB / T1040.3; (2) The finished products obtained in the examples and comparative examples were tested for shrinkage at different temperatures; the results are shown in the following table:

[0082]

[0083]

[0084] Table 1

[0085] Conclusion: Example 3, the best example of the present invention, significantly improves the tensile strength and shrinkage of a cross-linked polyolefin heat-shrinkable film by precisely controlling the ratio of the modified inorganic nanomaterial and the organic heat-resistant agent. Furthermore, the synergistic effect of the two is crucial for improving material performance. The absence of either component (as in Comparative Examples 1 and 2) significantly degrades material performance. Furthermore, the ratios of the various components must be strictly controlled; excessive amounts of both the modified inorganic nanomaterial (as in Comparative Example 3) and the mercaptosilane coupling agent (as in Comparative Example 4) can lead to decreased material performance.

[0086] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A process for preparing a cross-linked polyolefin heat shrinkable film, characterized in that: The following steps are involved: Step 1: The outer layer, inner layer and core layer raw materials are extruded separately in a single-screw extruder, then merged in a three-layer co-extrusion die, co-extruded and cooled to obtain a multi-layer composite sheet; Step 2: The multi-layer composite sheet is pulled into a stretching oven for bidirectional synchronous stretching, then cooled by an air ring, split, rolled up, and cut to obtain the finished product; The outer layer and the inner layer are made of the same raw materials, including the following substances: 100-110 parts by weight of terpolymer polypropylene, 10-12 parts by weight of anti-sticking agent, 3-4 parts by weight of heat-resistant agent, and 0.1-0.2 parts by weight of initiator; the core layer is made of the following substances: 70-80 parts by weight of low-density polyethylene, 5-8 parts by weight of heat-resistant agent, 0.1-0.2 parts by weight of initiator, and 2-3 parts by weight of slip agent; The heat-resistant agent comprises a modified inorganic nanomaterial and an organic heat-resistant agent in a mass ratio of (3-4):1; The preparation process of the modified inorganic nanomaterial is as follows: S1: adding illite to a sodium hydroxide solution, heating to 40-50°C, stirring and soaking for 1-2 hours, filtering and washing with deionized water until neutral; then transferring to a hydrochloric acid solution, stirring and soaking for 1-2 hours at room temperature, filtering and washing with deionized water until neutral, and drying to obtain pretreated illite; S2: Mix the pretreated illite with deionized water, heat to 70-80°C, add hydrochloric acid dropwise to adjust the pH to 2-3, then slowly add anhydrous ethanol solution of titanium tetrachloride dropwise. After the addition is complete, continue the reaction for 2-3 hours, cool, wash, dry, and transfer to a muffle furnace for calcination at 500-550°C for 2-3 hours to obtain a composite inorganic material; S3: The composite inorganic material is dissolved in anhydrous ethanol, ultrasonically dispersed for 30-40 minutes, hydrochloric acid is added to adjust the pH to 3-4, a mixed solution of 3-aminopropyltriethoxysilane and ethanol is added, stirred for 30-40 minutes, reacted at 60-70°C for 3-4 hours, cooled to room temperature, washed with anhydrous ethanol, vacuum-dried, ultrasonically dispersed in deionized water, a mixed solution of 3-mercaptopropyltriethoxysilane and methanol and an alkaline solution are added, the temperature is raised to 70-80°C, the reaction is carried out for 5-8 hours, washed, and dried to obtain a modified inorganic nanomaterial.

2. The process for preparing a cross-linked polyolefin heat shrinkable film according to claim 1, wherein: The mass ratio of the 3-aminopropyltriethoxysilane to the 3-mercaptopropyltriethoxysilane is 1:(2-3).

3. The process for preparing a cross-linked polyolefin heat shrinkable film according to claim 1, wherein: The pretreated illite comprises the following materials: 20-30 parts by weight of illite, 244-250 parts of sodium hydroxide solution, and 210-215 parts of hydrochloric acid solution; The mass fraction of the sodium hydroxide solution is 20-25%, and the mass fraction of the hydrochloric acid solution is 10-15%; The composite inorganic material comprises the following substances: by weight, 25-30 parts of pretreated illite, 260-280 parts of deionized water, and 42-45 parts of anhydrous ethanol solution of titanium tetrachloride.

4. The process for preparing a cross-linked polyolefin heat shrinkable film according to claim 1, wherein: The preparation process of the organic heat-resistant agent comprises the following steps: adding 2-amino-4-pentenoic acid and calcium carbonate to tetrahydrofuran, ultrasonically dispersing for 30-40 minutes, then placing the mixture in an ice-water bath at 0°C, slowly dripping a benzene solution of 3,5-bis(tert-butyl)-4-hydroxyphenylpropionyl chloride, heating the mixture to 50-60°C after the dripping is complete, reacting at a constant temperature for 20-25 hours, cooling the mixture to room temperature, filtering, washing, and drying the mixture to obtain the organic heat-resistant agent.

5. The process for preparing a cross-linked polyolefin heat shrinkable film according to claim 4, wherein: The organic heat-resistant agent raw material comprises the following components: by weight, 11-12 parts of 2-amino-4-pentenoic acid, 200-220 parts of calcium carbonate, 250-180 parts of tetrahydrofuran, and 18-20 parts of a benzene solution of 3,5-bis(tert-butyl)-4-hydroxyphenylpropionyl chloride.

6. The process for preparing a cross-linked polyolefin heat shrinkable film according to claim 1, wherein: The extrusion temperature of the outer layer and the inner layer in the single-screw extruder is 180-190°C; the extrusion temperature of the core layer in the single-screw extruder is 170-180°C.

7. The finished product obtained by the preparation process of a cross-linked polyolefin heat shrinkable film according to any one of claims 1 to 6.

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