Thermally initiated micro-crosslinked cold-stretch shrink film, and preparation method and application thereof

Abstract

The present application belongs to the technical field of polyethylene film, and relates to a hot-initiated micro-crosslinking cold-stretching shrinkable film and a preparation method thereof. The hot-initiated micro-crosslinking cold-stretching shrinkable film is a three-layer co-extrusion film, comprising inner layer, middle layer and outer layer films; the inner layer, the middle layer and the outer layer films all comprise pre-crosslinking master batches; the preparation raw materials of the pre-crosslinking master batches comprise mLLDPE, a thermal initiator and a crosslinking aid; the thermal initiator is 1-15 wt% of the mLLDPE, and the crosslinking aid is 1-20 wt% of the mLLDPE. The present application introduces the thermal initiator and the crosslinking aid components, micro-crosslinks in each layer of the film, and enables the film to meet the industry standard requirements under the conditions of realizing thickness reduction and reducing the use of imported mLLDPE raw materials, thereby providing a high-quality cold-stretching shrinkable film and a production method thereof for the market.

Description

Technical Field

[0001] The invention belongs to the technical field of polyethylene films and relates to a heat-induced micro-crosslinked cold-stretch shrink film and a preparation method and application thereof. Background Art

[0002] As a key packaging material for secondary containerization of stacked goods, cold-stretch shrink film not only simplifies handling but also effectively prevents rain erosion and dust contamination, ensuring the integrity of goods. It is particularly suitable for environmentally friendly packaging of products such as synthetic resins. To control costs, the industry continues to explore the ultimate thinning of film. However, this thinning process is often accompanied by a decrease in key physical properties such as puncture resistance, tear strength, and transverse shrinkage. Therefore, how to ensure that the film is thin and lightweight while meeting strict industry standards and packaging requirements has become a pressing technical challenge.

[0003] Given China's leading position in global synthetic resin production, demand for cold-stretch shrink film is showing a significant growth trend. To address the conflict between reducing film thickness and maintaining performance, the industry is pursuing new materials or improving existing materials to achieve the dual goals of "thin yet tight" and "thin yet strong."

[0004] Current conventional cold-stretch shrink film is primarily composed of linear low-density polyethylene (LLDPE) and metallocene linear low-density polyethylene (mLLDPE). While the latter offers superior performance, it is expensive due to its reliance on imports. Furthermore, the thinning process of the film is accompanied by a decline in transverse shrinkage and overall mechanical properties, posing significant challenges to storage and transportation. This can lead to frequent damage to goods during transit or unstable stacking, directly impacting sales performance.

[0005] To address this industry pain point, researchers are working to develop new resin substrates. By optimizing formulations and production processes, they aim to improve the film's mechanical strength and shrinkage efficiency, while also reducing costs and reliance on external raw materials. This requires not only a deep understanding of materials science but also interdisciplinary collaboration, combined with advanced manufacturing technologies. The goal is to achieve sustainable film development while ensuring packaging quality, thereby promoting green transformation and industrial upgrading in the packaging industry. Summary of the Invention

[0006] The present invention aims to address the deficiencies of cold-stretch shrink films in the prior art and provides a heat-induced micro-crosslinked cold-stretch shrink film and a preparation method and application thereof.

[0007] One object of the present invention is achieved by the following technical solutions:

[0008] A heat-induced micro-crosslinked cold stretch shrink film, wherein the heat-induced micro-crosslinked cold stretch shrink film is a three-layer co-extruded film, including an inner layer, a middle layer and an outer layer;

[0009] The inner layer, middle layer and outer layer films all include pre-crosslinked masterbatch;

[0010] The raw materials for preparing the pre-crosslinked masterbatch include mLLDPE, a thermal initiator and a co-crosslinking agent.

[0011] Preferably, the thermal initiator in the pre-crosslinked masterbatch is 1 to 15 wt% of the mLLDPE, and the auxiliary crosslinking agent is 1 to 20 wt% of the mLLDPE.

[0012] Preferably, the mLLDPE is in powder form, with an average particle size of 50 to 500 μm, an MFR value (190°C, 2.16 kg) of 1.0 to 4.0 g / 10 min, and a density of 0.915 to 0.930 g / cm 3 .

[0013] More preferably, the MFR value of the mLLDPE is 2.0 to 3.0 g / 10 min, and the density is 0.916 to 0.918 g / cm 3 .

[0014] Melt Flow Rate (MFR) is a method used to measure the melt fluidity of thermoplastics under specific conditions. The MFR value is the mass of polymer melt flowing through a capillary tube of standard diameter and length in 10 minutes at a specified temperature and pressure (typically 190°C, 2.16 kg). MFR is a critical quality control parameter in the plastics industry because it directly correlates to the plastic's processing performance and some mechanical properties.

[0015] Preferably, the thermal initiator is an organic peroxidation initiator, including one or more of di-tert-butyl peroxide (DTBP), benzoyl peroxide (BPO), tert-butyl hydroperoxide (TBHP), diethylhexyl peroxide (EHP), and tert-butyl perbenzoate (TBPB). More preferably, di-tert-butyl peroxide (DTBP) is used.

[0016] Preferably, the auxiliary cross-linking agent is a composite auxiliary cross-linking agent comprising triallyl isocyanurate (TAIC) and maleic anhydride (MAH).

[0017] Preferably, the mass ratio of tripropylene isocyanurate to maleic anhydride in the auxiliary cross-linking agent is (3-1): (1-3).

[0018] Preferably, the raw materials for preparing the pre-crosslinked masterbatch further include an antioxidant and a lubricating rheological agent.

[0019] Preferably, the antioxidant in the pre-crosslinked masterbatch is 0.01-0.1 wt% of mLLDPE, and the lubricating rheological agent is 0.01-0.5 wt% of mLLDPE.

[0020] Preferably, the antioxidant includes one or more of antioxidant 1076, antioxidant 1010, antioxidant 1310, antioxidant 330, and antioxidant 168. Antioxidant 1076 is further preferred.

[0021] Preferably, the lubricating rheological agent includes one or more of oxidized polyethylene wax, low molecular weight EVA, and low molecular weight polylactic acid, and the weight average molecular weight Mw thereof is 1000-3000.

[0022] It is further preferred that the thermal initiator is 1 to 5 wt% of mLLDPE in the pre-crosslinked masterbatch of the inner and outer layer films, and is 1.5 to 10 wt% of mLLDPE in the pre-crosslinked masterbatch of the middle layer film, wherein the content of the thermal initiator in the pre-crosslinked masterbatch of the inner and outer layer films is less than that in the pre-crosslinked masterbatch of the middle layer film.

[0023] Further preferably, the co-crosslinking agent is 1 to 15 wt% of the mLLDPE in the pre-crosslinked masterbatch of the inner and outer layer films, and is 1.5 to 20 wt% of the mLLDPE in the pre-crosslinked masterbatch of the middle layer film, wherein the content of the co-crosslinking agent in the pre-crosslinked masterbatch of the inner and outer layer films is less than that in the pre-crosslinked masterbatch of the middle layer film.

[0024] Preferably, the method for preparing the pre-crosslinked masterbatch comprises the following steps: mixing the pre-crosslinked masterbatch raw materials at high speed, and then kneading and granulating them in an extruder to obtain the pre-crosslinked masterbatch.

[0025] Preferably, the mixing and granulation is carried out in a twin-screw mixing extruder, the screw length-diameter ratio of the extruder is (30-60):1, and the melt temperature is 130-165°C.

[0026] The second object of the present invention is achieved through the following technical solutions:

[0027] A method for preparing a heat-induced micro-crosslinked cold stretch shrink film comprises the following steps: blowing the raw materials for preparing the inner layer, middle layer and outer layer of the heat-induced micro-crosslinked cold stretch shrink film, and cooling to obtain the heat-induced micro-crosslinked cold stretch shrink film.

[0028] Preferably, the raw materials for preparing the inner layer, middle layer and outer layer of the heat-induced micro-crosslinked cold stretch shrink film include LLDPE, mLLDPE and pre-crosslinked masterbatch.

[0029] Preferably, the mass ratio of LLDPE and mLLDPE in the inner film is (60-80):(20-40); the mass ratio in the middle film is (25-45):(55-75); and the mass ratio in the outer film is (40-60):(40-60).

[0030] Preferably, the pre-crosslinked masterbatch is 1 to 5 wt% of LLDPE and mLLDPE.

[0031] Preferably, the thickness ratio of the inner layer, middle layer and outer layer of the heat-induced micro-crosslinked cold stretch shrink film is (1-3):(3-6):(1-3), and more preferably 1:3:1.

[0032] Preferably, in the method for preparing the heat-induced micro-crosslinked cold-stretch shrink film, the raw materials for preparing the inner layer, middle layer and outer layer films are blown in a three-layer co-extrusion blown film machine.

[0033] A three-layer co-extrusion blown film machine is a device used to produce plastic film. It consists of three extruders, each with an independent heating and control system, and a common die. In this machine, three different plastic granules are squeezed through their respective extruders into three melt streams, which are then mixed together inside the die to form a three-layer co-extruded plastic film.

[0034] Preferably, the melt temperature of the inner film blown film processing is 175-185°C; the melt temperature of the middle film blown film processing is 185-195°C; and the melt temperature of the outer film blown film processing is 180-190°C.

[0035] The third object of the present invention is achieved through the following technical solutions:

[0036] Application of a heat-induced micro-crosslinked cold stretch shrink film in synthetic resin secondary containerization kits.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] 1. This invention provides a heat-induced micro-crosslinked cold-stretch shrink film. By introducing a thermal initiator and a co-crosslinking agent, micro-crosslinking occurs within each layer of the film. This innovation significantly improves the shrinkage rate and overall mechanical properties of the film while maintaining the same thickness as conventional films.

[0039] 2. The present invention provides a heat-induced micro-crosslinked cold-stretch shrink film with high strength and excellent transverse shrinkage. This film can be reduced in thickness and use less imported mLLDPE raw material while still meeting industry standards, providing the market with a high-quality cold-stretch shrink film and its production method.

[0040] 3. This invention provides a thermally initiated micro-crosslinked cold-stretch shrink film. By optimizing the raw materials and designing a low degree of crosslinking in the inner and outer film layers, the polyethylene molecular weight is increased without affecting the film's processing properties, effectively preventing the formation of crystal points in the film. Furthermore, the introduction of a lubricating rheological agent into the raw materials promotes uniform dispersion of the thermal initiator and co-crosslinking agent components in the resin matrix, reducing localized over-crosslinking caused by uneven dispersion, thereby preventing the formation of crystal points and ensuring that the film's mechanical properties are not affected.

[0041] 4. The present invention provides a method for preparing a heat-induced micro-crosslinked cold-stretch shrink film. Pre-crosslinked masterbatches with varying degrees of crosslinking are first prepared for each layer, followed by a three-layer co-extruded film. This method can specifically optimize the performance of each layer, prevent the formation of crystal points, improve the film's transparency and appearance, and simplify the co-extrusion process. It also adapts to diverse market demands, reduces reliance on expensive raw materials, ensures consistency in the crosslinking process, and thus improves the quality and stability of the final product. DETAILED DESCRIPTION

[0042] The technical solution of the present invention will be further described below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the contents of the present invention, those skilled in the art may make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the claims appended hereto.

[0043] In the following examples and comparative examples, the mLLDPE used is in powder form with an average particle size of 50 to 500 μm, an MFR value (190°C, 2.16 kg) of 2.0 g / 10 min, and a density of 0.916 g / cm 3 .

[0044] Example 1

[0045] The raw materials for preparing the pre-crosslinked masterbatch of the heat-induced micro-crosslinked cold-stretch shrink film in this embodiment are: mLLDPE (average particle size of 50 μm), thermal initiator DTBP, co-crosslinking agent (a mixture of TAIC and MAH in a mass ratio of 3:1), antioxidant 1076, and lubricating rheological agent (oxidized polyethylene wax, weight-average molecular weight Mw of 1000).

[0046] The thermal initiator is 1 wt% of mLLDPE in the inner and outer layer pre-crosslinking masterbatch and 1.5 wt% of mLLDPE in the middle layer pre-crosslinking masterbatch;

[0047] The cross-linking agent is 1.5 wt% of mLLDPE in the inner and outer layer pre-cross-linking masterbatch and 4 wt% of mLLDPE in the middle layer pre-cross-linking masterbatch;

[0048] The antioxidant content in the inner, middle, and outer pre-crosslinked masterbatch is 0.08 wt% of mLLDPE;

[0049] The lubricating rheological agent is 0.05 wt% of mLLDPE in the pre-crosslinked masterbatch of the inner layer, the middle layer and the outer layer.

[0050] The preparation method of the pre-crosslinked masterbatch includes the following steps: the pre-crosslinked masterbatch raw materials of the inner layer, the middle layer and the outer layer are mixed at high speed respectively, and then mixed and granulated in a twin-screw compounding extruder. The screw length-diameter ratio of the extruder is 30:1 and the melt temperature is 155°C to obtain the pre-crosslinked masterbatch of each layer respectively.

[0051] In the heat-induced micro-crosslinked cold stretch shrink film of this embodiment, the mass ratio of LLDPE and mLLDPE in the inner film is 70:30; the mass ratio in the middle film is 35:65; and the mass ratio in the outer film is 50:50. The pre-crosslinked masterbatch of each layer is 2 wt% of LLDPE and mLLDPE.

[0052] The preparation method of the heat-induced micro-crosslinked cold stretch shrink film includes the following steps: feeding the raw materials for preparing the inner layer, middle layer and outer layer films of the heat-induced micro-crosslinked cold stretch shrink film into a three-layer co-extrusion film blowing machine for film blowing processing according to the thickness of 1:3:1, the melt temperature of the inner layer film blowing processing is 180°C, the melt temperature of the middle layer film blowing processing is 190°C, the melt temperature of the outer layer film blowing processing is 185°C, and the film thickness is 122 μm. After cooling, a heat-induced micro-crosslinked cold stretch shrink film is obtained.

[0053] Example 2

[0054] The raw materials for preparing the pre-crosslinked masterbatch of the heat-induced micro-crosslinked cold stretch shrink film in this embodiment are: mLLDPE (average particle size 100 μm), thermal initiator DTBP, co-crosslinking agent (a mixture of TAIC and MAH in a mass ratio of 1:3), antioxidant 1076, and lubricating rheological agent (oxidized polyethylene wax, weight-average molecular weight Mw is 3000).

[0055] The thermal initiator is 5wt% of mLLDPE in the inner and outer layer pre-crosslinking masterbatch and 8wt% of mLLDPE in the middle layer pre-crosslinking masterbatch;

[0056] The cross-linking agent is 15wt% of mLLDPE in the inner and outer layer pre-cross-linking masterbatch and 20wt% of mLLDPE in the middle layer pre-cross-linking masterbatch;

[0057] The antioxidant content in the inner, middle, and outer pre-crosslinked masterbatch is 0.08 wt% of mLLDPE;

[0058] The lubricating rheological agent is 0.2 wt% of mLLDPE in the pre-crosslinked masterbatch of the inner layer, the middle layer and the outer layer.

[0059] The rest is the same as in Example 1.

[0060] Example 3

[0061] The raw materials for preparing the pre-crosslinked masterbatch of the heat-induced micro-crosslinked cold-stretch shrink film in this embodiment are: mLLDPE (average particle size 150 μm), thermal initiator DTBP, co-crosslinking agent (a mixture of TAIC and MAH in a mass ratio of 2:1), antioxidant 1076, and lubricating rheological agent (oxidized polyethylene wax, weight-average molecular weight Mw is 2000).

[0062] The thermal initiator is 3wt% of mLLDPE in the inner and outer layer pre-crosslinking masterbatch and 4wt% of mLLDPE in the middle layer pre-crosslinking masterbatch;

[0063] The cross-linking agent is 7.5wt% of mLLDPE in the inner and outer layer pre-cross-linking masterbatch and 15wt% of mLLDPE in the middle layer pre-cross-linking masterbatch;

[0064] The antioxidant content in the inner, middle, and outer pre-crosslinked masterbatch is 0.08 wt% of mLLDPE;

[0065] The lubricating rheological agent is 0.15 wt% of mLLDPE in the pre-crosslinked masterbatch of the inner layer, the middle layer and the outer layer.

[0066] The rest is the same as in Example 1.

[0067] Example 4

[0068] The difference between the heat-induced micro-crosslinked cold stretch shrink film of this embodiment and that of Example 1 is that the average particle size of mLLDPE is 200 μm, the lubricating rheological agent is low molecular weight EVA (weight average molecular weight Mw1000), and the film blowing process is carried out according to conventional film thickness. The thickness of the obtained cold stretch shrink film is 131 μm, and the rest is the same as that of Example 1.

[0069] Example 5

[0070] The difference between the heat-induced micro-crosslinked cold stretch shrink film in this embodiment and that in Example 1 is that the average particle size of mLLDPE is 250 μm, the lubricating rheological agent is low molecular weight EVA (weight average molecular weight Mw3000), the proportion of the thermal initiator in the pre-crosslinked masterbatch of the inner layer, middle layer and outer layer films is the same, and the rest is the same as in Example 1.

[0071] The thermal initiator in the inner layer, the middle layer and the outer layer pre-crosslinked masterbatch is 1 wt% of mLLDPE, and the rest is the same as in Example 1.

[0072] Example 6

[0073] The difference between the heat-induced micro-crosslinked cold stretch shrink film of this embodiment and that of Example 1 is that the average particle size of mLLDPE is 300 μm, the lubricating rheological agent is low molecular weight EVA (weight average molecular weight Mw 2000), the proportion of the thermal initiator in the pre-crosslinked masterbatch of the inner layer, middle layer and outer layer films is the same, and the rest is the same as in Example 1.

[0074] The thermal initiator in the inner layer, middle layer and outer layer pre-crosslinked masterbatch is 1.5 wt% of mLLDPE, and the rest is the same as in Example 1.

[0075] Example 7

[0076] The difference between the heat-induced micro-crosslinked cold stretch shrink film of this embodiment and that of Example 1 is that the average particle size of mLLDPE is 400 μm, the lubricating rheological agent is low molecular weight polylactic acid (weight average molecular weight Mw1000), the proportion of the co-crosslinking agent in the pre-crosslinked masterbatch of the inner layer, middle layer and outer layer films is the same, and the rest is the same as in Example 1.

[0077] The amount of the auxiliary cross-linking agent in the inner layer, the middle layer and the outer layer pre-cross-linking masterbatch is 1.5 wt % of the mLLDPE, and the rest is the same as in Example 1.

[0078] Example 8

[0079] The only difference between the heat-induced micro-crosslinked cold stretch shrink film of this embodiment and that of Example 1 is that the average particle size of mLLDPE is 500 μm, the lubricating rheological agent is low molecular weight polylactic acid (weight average molecular weight Mw3000), the proportion of the co-crosslinking agent in the pre-crosslinked masterbatch of the inner layer, middle layer and outer layer films is the same, and the rest is the same as in Example 1.

[0080] The amount of the co-crosslinking agent in the inner layer, the middle layer and the outer layer pre-crosslinking masterbatch is 4 wt % of the mLLDPE, and the rest is the same as in Example 1.

[0081] Comparative Example 1

[0082] The only difference between the cold stretch shrink film of this comparative example and Example 1 is that no pre-crosslinked masterbatch is added to the inner layer, middle layer and outer layer film, and conventional cold stretch shrink film raw material ratio and film thickness are used, and the rest is the same as Example 1.

[0083] In the cold stretch shrink film of this comparative example, the mass ratio of LLDPE and mLLDPE in the inner film is 40:60; the mass ratio in the middle film is 20:80; and the mass ratio in the outer film is 30:70. The obtained film thickness is 131 μm.

[0084] Comparative Example 2

[0085] The only difference between the cold stretch shrink film of this comparative example and Example 1 is that the thermal initiator is 0.5wt% of mLLDPE in the inner and outer pre-crosslinked masterbatch and 0.8wt% of mLLDPE in the middle pre-crosslinked masterbatch, and the rest is the same as Example 1.

[0086] Comparative Example 3

[0087] The cold stretch shrink film of this comparative example differs from that of Example 1 only in that the crosslinking aid is 20 wt % of mLLDPE in the inner and outer pre-crosslinked masterbatch and 25 wt % of mLLDPE in the middle pre-crosslinked masterbatch. The rest is the same as in Example 1.

[0088] Comparative Example 4

[0089] The cold stretch shrink film of this comparative example is different from that of Example 1 only in that no thermal initiator is added to the pre-crosslinked masterbatch, and the rest is the same as that of Example 1.

[0090] Comparative Example 5

[0091] The cold-stretch shrink film of this comparative example is different from that of Example 1 only in that no auxiliary cross-linking agent is added to the pre-cross-linked masterbatch, and the rest is the same as that of Example 1.

[0092] Comparative Example 6

[0093] The cold-stretch shrink film of this comparative example is different from that of Example 1 only in that no lubricating rheological agent is added to the pre-crosslinked masterbatch, and the rest is the same as that of Example 1.

[0094] Comparative Example 7

[0095] The cold stretch shrink film of this comparative example is different from that of Example 1 only in that TAIC is used as the auxiliary cross-linking agent in the pre-cross-linked masterbatch instead of the TAIC / MAH composite auxiliary cross-linking agent, and the rest is the same as that of Example 1.

[0096] Comparative Example 8

[0097] The cold stretch shrink film of this comparative example is different from that of Example 1 only in that the auxiliary cross-linking agent in the pre-cross-linked masterbatch is MAH instead of the TAIC / MAH composite auxiliary cross-linking agent, and the rest is the same as that of Example 1.

[0098] The cold stretch shrink films obtained in the examples and comparative examples were tested using national standards, and the testing method was as follows:

[0099] The tensile strength is measured in accordance with GB / T 1040.3-2006;

[0100] The elongation at break is measured in accordance with GB / T 1040.3-2006;

[0101] The right-angle tearing strength is measured in accordance with QB / T1130;

[0102] Dart impact strength is measured in accordance with GB / T 9639.1-2008;

[0103] The elastic recovery rate is measured according to the method specified in 5.6.6 of T / ZZB 1008-2019 standard;

[0104] The specific test results are shown in Table 1.

[0105] Table 1 Film performance test data

[0106]

[0107]

[0108] As shown in Table 1, from the Examples and Comparative Example 1, it can be seen that the thermally initiated micro-crosslinked cold stretch shrink film of the present invention incorporates thermal initiators and co-crosslinking agents into each layer of the film, enabling micro-crosslinking. Compared to conventional films, this film can achieve reduced thickness and use less imported mLLDPE raw material while still meeting industry standards. This provides the market with a high-quality cold stretch shrink film and its production method.

[0109] It can be seen from Example 4 and Comparative Example 1 that, under the condition of the same thickness as a conventional film, the technical solution of the present invention can significantly improve the shrinkage rate and comprehensive mechanical properties of the film.

[0110] It can be seen from Examples 1 to 3 and 5 to 8 that when the content of thermal initiator and cross-linking aid in the inner and outer layer films is less than that in the middle layer film, that is, a lower degree of cross-linking is designed in the inner and outer layer films, the molecular weight of polyethylene can be increased without affecting the processing properties of the film, effectively avoiding the generation of crystal points in the film, thereby further improving the overall performance of the film.

[0111] As can be seen from Example 1 and Comparative Examples 2-8, the comprehensive mechanical properties, such as strength and shrinkage, of heat-induced micro-crosslinked cold-stretch shrink films prepared without using the raw material ratios specified in the technical solution of the present invention are significantly inferior to those of the examples. This further demonstrates the importance and effectiveness of the technical solution of the present invention in improving the comprehensive performance of cold-stretch shrink films.

[0112] The various aspects, embodiments, and features of the present invention should be considered in all respects as illustrative and not limiting, the scope of which is defined solely by the claims. Other embodiments, modifications, and uses will be apparent to those skilled in the art without departing from the spirit and scope of the invention as claimed.

[0113] In the preparation method of the present invention, the order of the steps is not limited to the order listed. Persons skilled in the art will appreciate that variations in the order of the steps are within the scope of the present invention without inventive effort. Furthermore, two or more steps or actions may be performed simultaneously.

[0114] Finally, it should be noted that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit its implementation. Persons skilled in the art may make various modifications, additions, or substitute similar methods for the described specific embodiments. It is not necessary and impossible to provide comprehensive examples of all implementations here. However, obvious variations or modifications arising from the essential spirit of the present invention remain within the scope of protection of the present invention, and interpreting them as any additional limitations would be contrary to the spirit of the present invention.

Claims

1. A heat-induced micro-crosslinked cold stretch shrink film, characterized in that: The heat-induced micro-crosslinked cold-stretch shrink film is a three-layer co-extruded film, including an inner layer, a middle layer and an outer layer; The raw materials for preparing the inner layer, middle layer and outer layer films include LLDPE, mLLDPE and pre-crosslinked masterbatch; The mass ratio of LLDPE and mLLDPE in the inner layer film is (60-80): (20-40), the mass ratio in the middle layer film is (25-45): (55-75), and the mass ratio in the outer layer film is (40-60): (40-60); the pre-crosslinked masterbatch is 1-5wt% of LLDPE and mLLDPE; The raw materials for preparing the pre-crosslinked masterbatch include mLLDPE, a thermal initiator, a co-crosslinking agent, an antioxidant and a lubricating rheological agent; The auxiliary cross-linking agent is a composite auxiliary cross-linking agent comprising tripropylene isocyanurate and maleic anhydride, wherein the mass ratio of tripropylene isocyanurate to maleic anhydride in the auxiliary cross-linking agent is (3-1): (1-3); The thermal initiator is present in the pre-crosslinked masterbatch of the inner and outer films at 1-5 wt% of the mLLDPE, and in the pre-crosslinked masterbatch of the middle film at 1.5-10 wt% of the mLLDPE, wherein the content of the thermal initiator in the pre-crosslinked masterbatch of the inner and outer films is less than that in the pre-crosslinked masterbatch of the middle film; The cross-linking aid is present in the pre-cross-linked masterbatch of the inner and outer films at 1-15 wt% of mLLDPE, and in the pre-cross-linked masterbatch of the middle film at 1.5-20 wt% of mLLDPE, wherein the content of the cross-linking aid in the pre-cross-linked masterbatch of the inner and outer films is less than that in the pre-cross-linked masterbatch of the middle film.

2. The heat-induced micro-crosslinked cold stretch shrink film according to claim 1, characterized in that: The thermal initiator in the pre-crosslinked masterbatch is 1-15 wt% of mLLDPE, and the auxiliary crosslinking agent is 1-20 wt% of mLLDPE.

3. The heat-induced micro-crosslinked cold stretch shrink film according to claim 1, characterized in that: The mLLDPE is in powder form, with an average particle size of 50-500 μm, an MFR value of 1.0-4.0 g / 10 min, and a density of 0.915-0.930 g / cm 3 .

4. The heat-induced micro-crosslinked cold stretch shrink film according to claim 1, characterized in that: The thermal initiator is an organic peroxidation initiator, including one or more of di-tert-butyl peroxide, benzoyl peroxide, tert-butyl hydroperoxide, diethylhexyl peroxide, and tert-butyl perbenzoate.

5. The heat-induced micro-crosslinked cold stretch shrink film according to claim 1, characterized in that: The antioxidant is 0.01-0.1wt% of mLLDPE, and the lubricating rheological agent is 0.01-0.5wt% of mLLDPE; The weight average molecular weight Mw of the lubricating rheological agent is 1000-3000.

6. The heat-induced micro-crosslinked cold stretch shrink film according to claim 1, characterized in that: The preparation method of the pre-crosslinked masterbatch comprises the following steps: mixing the pre-crosslinked masterbatch raw materials at high speed, and then kneading and granulating them in an extruder to obtain the pre-crosslinked masterbatch; the kneading and granulating are carried out in a twin-screw kneading extruder, the screw aspect ratio of the extruder is (30-60):1, and the melt temperature is 130-165°C.

7. A method for preparing a heat-induced micro-crosslinked cold stretch shrink film according to any one of claims 1 to 6, characterized in that: The preparation method comprises the following steps: blowing the raw materials for preparing the inner layer, middle layer and outer layer of the heat-induced micro-crosslinked cold stretch shrink film, and cooling to obtain the heat-induced micro-crosslinked cold stretch shrink film.

8. The preparation method according to claim 7, characterized in that The thickness ratio of the inner layer, middle layer and outer layer of the heat-induced micro-crosslinked cold stretch shrink film is (1-3): (3-6): (1-3); The raw materials for preparing the inner layer, middle layer and outer layer films are blown into films in a three-layer co-extrusion blown film machine.

9. The preparation method according to claim 7, characterized in that The melt temperature of the inner film blown film processing is 175-185°C; the melt temperature of the middle film blown film processing is 185-195°C; and the melt temperature of the outer film blown film processing is 180-190°C.

10. Use of the heat-induced micro-crosslinked cold stretch shrink film according to any one of claims 1 to 6 in a synthetic resin secondary containerization package.

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