Linear easy-tear retort film, its preparation method and application

Abstract

The application discloses a linear easy-tearing high-temperature cooking film and a preparation method and application thereof. The film comprises a three-layer structure of an upper surface layer, a core layer and a lower surface layer, and the core layer contains copolymerized polypropylene, propylene-based elastomer, styrene-based thermoplastic elastomer, a beta crystal nucleating agent and modified nano-aluminum oxide. During preparation, the core layer raw material is blended with the beta crystal nucleating agent master batch, then is extruded through a multi-layer co-extrusion casting die together with the surface layer material, and is cast on a 80-100 DEG C quenching roller to make the core layer fully generate beta crystal polypropylene; after preheating, two-step longitudinal stretching is performed to make the beta crystal of the core layer be converted into alpha crystal microfibers highly oriented along the longitudinal direction in the stretching, so that ordered intra-layer tearing channels are formed in situ in the film. The film is intact in appearance after high-temperature cooking at 131 DEG C for 30 min, the deviation angle is less than or equal to 3.2 DEG when tearing, the film has excellent high-temperature cooking resistance and linear easy-tearing property, and no penetrating physical damage means is used, and can be applied to the fields of high-temperature cooking packaging of food, medicine and the like.

Description

Technical Field

[0001] This invention relates to the field of packaging technology, and more particularly to a straight, easy-tear, high-temperature retort film, its preparation method, and its application. Background Technology

[0002] With social development, especially the arrival of an aging society and the accelerated pace of life, central kitchens are rapidly emerging. Central kitchens, through standardized and large-scale production, consume enormous quantities of packaging materials such as retort pouches, and face stringent requirements. Retort packaging technology must continuously innovate; the core challenge lies in meeting the stringent requirements of high-temperature sterilization, satisfying consumer expectations for flavor, and simultaneously meeting the demands for consumption upgrades and convenience. After high-temperature treatment, the contents of the retort pouch are kept at a high temperature. The ability to easily open the cooking bag even at high temperatures is a core demand of consumers.

[0003] Traditional packaging often employs simple methods such as cutting tear lines, needle punching, laser perforation / cutting, and tear-guiding grooves to achieve easy, straight-line tearing. However, these methods still have certain limitations. First, the controllability of the tear direction is poor; the straightness is not ideal, and the tear may propagate in an unwanted direction upon opening, leading to leakage or damage to the contents. Second, existing straight-line tear designs compromise the mechanical strength of the material, resulting in reduced packaging strength. This could lead to retort pouches being insufficiently strong to withstand the immense internal pressure at high temperatures, posing a significant technical risk.

[0004] Another technical approach is to design functional films that simultaneously meet the requirements of easy tearing and retort packaging. Commonly used high-temperature retort films are generally made of polypropylene, so research focuses on imparting easy tearing properties to polypropylene films. Chinese patent CN116478477A discloses a straight-tearable polypropylene composite film prepared by a blending method to meet the requirements of easy opening and high-temperature retortability. However, its technical approach mainly relies on multi-component blending, and there is still room for improvement in balancing heat resistance and tear straightness. Chinese patent CN116353172A proposes a scheme to prepare a straight-tearable retort-resistant polyolefin film by blending high-density polyethylene (HDPE) with inorganic powder. This scheme utilizes the specific crystalline structure formed by HDPE during processing to achieve straight tearing properties. However, high-density polyethylene has a low melting point, and its temperature resistance has inherent defects, making it unable to meet the stringent technical requirements of high-temperature sterilization at 131℃ and above, posing a fundamental obstacle to its application in high-temperature retort scenarios. Summary of the Invention

[0005] To address the aforementioned problems in the prior art, this invention provides a linear, easy-tear, high-temperature retort film, its preparation method, and its application.

[0006] The objective of this invention can be achieved through the following technical solutions: To achieve the above objectives, the present invention provides the following technical solution: A straight, easy-tear, high-temperature retort film comprises at least three layers: an upper surface layer, a core layer, and a lower surface layer.

[0007] The upper and lower layers comprise copolymer polypropylene, propylene-based elastomer, and anti-blocking masterbatch.

[0008] The core layer comprises the following components: copolymer polypropylene, propylene-based elastomer, styrene-based thermoplastic elastomer, β-crystal nucleating agent, and modified nano-alumina.

[0009] Furthermore, the styrene-based thermoplastic elastomer is preferably SEPS (styrene-ethylene-propylene-styrene), SEBS (styrene-ethylene-butene-styrene block copolymer), or SEEPS (styrene-ethylene-ethylene-propylene-styrene).

[0010] Furthermore, the β-crystal nucleating agent is at least one of amide compounds or rare earth complex β-crystal nucleating agents; the content of the β-crystal nucleating agent in the core layer is 0.05%-0.3%.

[0011] Furthermore, the upper surface layer, core layer, and lower surface layer are in the following mass fraction ratios: 1:(1-10):1.

[0012] Furthermore, the copolymer polypropylene is selected from one or more of ethylene-propylene binary block copolymer polypropylene and propylene-butadiene binary block copolymer polypropylene.

[0013] Furthermore, the modified nano-alumina is α-alumina modified with a titanate coupling agent.

[0014] The present invention also provides a method for preparing the above-mentioned straight, easy-tear, high-temperature retort film, comprising the following steps: (1) Mix the raw materials of the upper and lower surface layers evenly and add them to the surface layer twin-screw extruder; mix the core layer raw materials and β crystal nucleating agent masterbatch evenly and add them to the core layer twin-screw extruder; (2) The above-mentioned molten polymer is combined into a molten layer of upper surface layer / core layer / lower surface layer through a multi-layer co-extrusion casting die, and then cast onto a rapid cooling roller at a temperature of 80-100℃ to quickly cool and cast a sheet, thereby obtaining a thick sheet with β crystals in the core layer. (3) The multilayer thick sheet is preheated at 120-140℃ and then subjected to two longitudinal stretching steps: the first step is to stretch longitudinally by 1-3 times at 100-120℃, and the second step is to stretch longitudinally by 2-4 times at 110-130℃, and the total longitudinal stretching ratio is controlled to be 4-10 times, and then heat-treated at 130-140℃. (4) After stretching, heat setting, cooling, and surface corona treatment are carried out, and then the product is wound up.

[0015] Furthermore, the processing temperature of the surface twin-screw extruder in step (1) is 180-230℃, and the processing temperature of the core twin-screw extruder is 200-250℃.

[0016] Furthermore, the β-crystal nucleating agent masterbatch mentioned in step (1) is a masterbatch with copolymer polypropylene as carrier and a β-crystal nucleating agent mass concentration of 5%-10%.

[0017] Furthermore, the heat setting temperature in step (4) is 130-140°C, and the cooling temperature is 30-70°C.

[0018] The present invention also provides the application of the above-mentioned straight easy-tear high-temperature retort film in food and pharmaceutical packaging.

[0019] In the technical solution of this invention, the straight-line easy-tear high-temperature retort film utilizes the synergistic combination of formulation design and processing technology to form a unique crystalline orientation structure within the film core layer. Specifically, by introducing a β-crystal nucleating agent into the core layer formulation and precisely controlling the temperature of the quenching roller within a suitable range of 80-100℃ during the casting process, the core layer melt selectively generates a high content of β-crystalline polypropylene during cooling. In the subsequent two-step longitudinal stretching process, under the synergistic effect of tensile stress and temperature, the metastable β-crystals in the core layer undergo a stress-induced β-to-α phase transformation, accompanied by lattice reconstruction and volume shrinkage, generating in-situ, longitudinally highly oriented strip-like microfiber structures within the film. This microfiber structure constructs ordered weak-zone channels within the film, forming an internal path that guides tearing along a straight direction, endowing the film with excellent straight-line easy-tear properties. Meanwhile, the upper and lower surface layers, due to rapid cooling during casting in contact with the quenching roller, primarily generate a dense α-crystalline structure, maintaining the structural integrity and mechanical strength of the film surface.

[0020] The beneficial effects of this invention are as follows: (1) This invention abandons the existing technical route of introducing low-melting-point high-density polyethylene (HDPE) to achieve straight tearability. The entire formulation system is based on high heat-resistant polypropylene, ensuring that the film does not soften, deform or peel off during high-temperature cooking at 131°C, and has a high mechanical strength retention rate. At the same time, by introducing a β-crystal nucleating agent into the core layer and combining it with specific stretching and heat setting processes, while maintaining the film's resistance to high-temperature cooking at 121-135°C, it is endowed with excellent straight tearability, thus solving the technical contradiction of balancing straight tearability and high-temperature resistance.

[0021] (2) The present invention controls the total longitudinal stretch ratio within a critical window of 4-10 times, and in conjunction with a specific formulation and stretching temperature, causes the β crystals in the core layer to undergo stress-induced solid-phase transformation, forming a highly oriented dense α crystal microfiber structure without forming micropores or cracks. This allows the film to obtain excellent straight tearability while fully maintaining the density and high barrier properties necessary for retort packaging films.

[0022] (3) This invention is based on mature multi-layer co-extrusion casting and unidirectional stretching equipment. It can be achieved by optimizing key process windows such as casting temperature, stretching temperature and stretching ratio. It does not involve complex post-processing equipment modification, has strong process continuity and high yield. Detailed Implementation

[0023] To further illustrate the technical means and effects of the present invention in achieving the intended purpose, the following detailed description of the specific implementation methods, structures, features and effects of the present invention, in conjunction with preferred embodiments, is provided below.

[0024] The main raw materials used in the following examples and comparative examples are described below: Copolymerized polypropylene: Ethylene-propylene binary block copolymerized polypropylene, melt index 2.0 g / 10min; Propylene-based elastomer: 12% ethylene content, density 0.88 g / cm³ 3 ; Styrene-based thermoplastic elastomers: styrene content <40%, melt flow rate (230°C / 2.16 kg) 0.5-25 g / 10min; Anti-blocking masterbatch: using copolymer polypropylene as a carrier, containing 5% synthetic silica microspheres; Modified nano-alumina masterbatch: α-alumina (particle size 30-60nm) is surface-treated with a titanate coupling agent, using copolymer polypropylene as a carrier, with an alumina content of 20%; β-crystal nucleating agent masterbatch A: Amide nucleating agent TMB-5, with copolymer polypropylene as carrier, concentration 10%; β-crystal nucleating agent masterbatch B: rare earth complex nucleating agent WBG-II, with copolymer polypropylene as carrier, concentration 10%.

[0025] Example 1 A straight, easy-tear, high-temperature retort film is composed of an upper surface layer, a core layer, and a lower surface layer, with a layer thickness-to-mass ratio of 1:8:1.

[0026] The formulations for the upper and lower layers are the same, and by weight, they are: 90 parts copolymer polypropylene, 10 parts propylene-based elastomer, and 2 parts anti-blocking masterbatch.

[0027] The core layer formulation, by weight, is as follows: 67 parts copolymer polypropylene, 15 parts propylene-based elastomer, 5 parts SEBS, 3 parts β-crystal nucleating agent masterbatch A, and 10 parts modified nano-alumina masterbatch.

[0028] The preparation method includes the following steps: (1) Mix the raw materials of the upper and lower layers in the above proportion in a high-speed mixer, and then add them to the surface twin-screw extruder at a processing temperature of 200°C. Mix the core layer raw materials with β crystal nucleating agent masterbatch A, and then add them to the core layer twin-screw extruder at a processing temperature of 210°C.

[0029] (2) The above-mentioned molten polymer is combined into a molten layer of upper surface layer / core layer / lower surface layer through a multi-layer co-extrusion casting die. The die temperature is 230°C, and it is cast onto a quenching roller at a temperature of 90°C to quickly cool the casting sheet, thereby obtaining a thick sheet with β crystals in the core layer.

[0030] (3) The thick sheet is taken to a uniaxial stretching device and preheated at 130°C. Then, it is stretched in two steps: first, it is stretched 2.0 times in the longitudinal direction at 110°C, then stretched 2.5 times in the longitudinal direction at 120°C, and then heat-treated at 135°C.

[0031] (4) After stretching, the film is heat-set at 135°C, then cooled and set by a cooling roller at 50°C, and then subjected to surface corona treatment before being wound up to obtain the finished film.

[0032] Example 2 A straight, easy-tear, high-temperature retort film with the same structure and layer thickness ratio as in Example 1.

[0033] The formulations of the upper and lower surface layers are the same as in Example 1. The core layer formulation, by weight, is: 67 parts copolymer polypropylene, 15 parts propylene-based elastomer, 5 parts SEPS, 3 parts β-crystal nucleating agent masterbatch A, and 10 parts modified nano-alumina masterbatch.

[0034] The preparation method is the same as in Example 1, namely, the temperature of the quenching roller is 90°C, the preheating temperature is 130°C, the first step is to stretch 2.0 times at 110°C, the intermediate heat treatment is 135°C, the second step is to stretch 2.5 times at 120°C, the heat treatment is 135°C, and then cooling.

[0035] Example 3 The difference from Example 1 is that: a straight, easy-tear high-temperature retort film is composed of three layers: upper, core, and lower, with a layer thickness-to-mass ratio of 1:5:1.

[0036] The formulations for the upper and lower surface layers are: 85 parts copolymer polypropylene, 15 parts propylene-based elastomer, and 2 parts anti-blocking masterbatch.

[0037] The core layer formulation is as follows: 67 parts copolymer polypropylene, 15 parts propylene-based elastomer, 5 parts SEEPS, 3 parts β-crystal nucleating agent masterbatch A, and 10 parts modified nano-alumina masterbatch.

[0038] In the preparation process, the surface extrusion temperature is 210℃, the core extrusion temperature is 220℃, and the die temperature is 235℃. The quench roll temperature is 85℃. The preheating temperature is 120℃. The first stretching step is performed at 105℃ for a stretch of 2.5 times, and the second stretching step is performed at 115℃ for a stretch of 2.2 times. The heat treatment temperature is 130℃, followed by cooling.

[0039] Example 4 The difference from Example 1 is that: a straight, easy-tear high-temperature retort film with a layer thickness-to-mass ratio of 1:10:1.

[0040] The formulations for the upper and lower surface layers are the same as in Example 1. The core layer formulation is: 67 parts copolymer polypropylene, 15 parts propylene-based elastomer, 5 parts SEBS, 3 parts β-crystal nucleating agent masterbatch B, and 10 parts modified nano-alumina masterbatch.

[0041] In the preparation process, the quenching roller temperature is 95℃. Preheating is 135℃, the first step involves stretching at 115℃ to 1.8 times the original length, followed by heat treatment at 140℃, and the second step involves stretching at 125℃ to 3.3 times the original length. Heat setting is performed at 140℃.

[0042] Example 5 The difference from Example 1 is that: a straight, easy-tear high-temperature retort film with a layer thickness-to-mass ratio of 1:7:1.

[0043] The formulations for the upper and lower surface layers are: 80 parts copolymer polypropylene, 20 parts propylene-based elastomer, and 3 parts anti-blocking masterbatch.

[0044] The core layer formulation is as follows: 67 parts copolymer polypropylene, 15 parts propylene-based elastomer, 5 parts SEBS, 3 parts β-crystal nucleating agent masterbatch B, and 10 parts modified nano-alumina masterbatch.

[0045] In the preparation process, the core layer extrusion temperature is 220℃, the die temperature is 235℃, the quench roll temperature is 80℃, the preheating temperature is 140℃, the first step is a 1.5-fold stretch at 120℃, the second step is a 3.0-fold stretch at 125℃, and the heat treatment is 135℃.

[0046] Comparative Example 1 Based on Example 1, β-crystal nucleating agent masterbatch A is not added to the core layer; its amount is supplemented by copolymer polypropylene, and the rest is the same as in Example 1.

[0047] Comparative Example 2 Based on Example 1, the temperature of the quenching roller in step (2) is set to 120°C (exceeding the 80-100°C range of this invention), and the rest is the same as in Example 1.

[0048] Comparative Example 3 A three-layer film with a layer thickness-to-mass ratio of 1:8:1. The upper and lower surface layers are the same as in Example 1. The core layer formulation is: high-density polyethylene (HDPE, melt index 0.5 g / 10min, density 0.956 g / cm³). 3 65 parts of propylene-based elastomer, 15 parts of modified talc masterbatch (30% talc content, with homopolymer PP as carrier) and 20 parts of β-crystal nucleating agent; does not contain β-crystal nucleating agent.

[0049] Comparative Example 4 Based on Example 1, SEBS is not added, and everything else remains the same as in Example 1.

[0050] Preparation process: Surface extrusion temperature 200℃, core extrusion temperature 210℃, die head temperature 230℃; quench roll temperature 110℃; after preheating, the thick sheet undergoes one-step longitudinal stretching with a total stretching ratio of 4.0 times, without intermediate heat treatment; heat setting temperature 115℃, followed by cooling and winding.

[0051] Comparative Example 5 Using the finished film obtained in Example 1, a straight semi-tangent line (not penetrating) with a depth of 20% of the film thickness was processed along the transverse surface of the film using an ultraviolet laser, with a line spacing of 5 mm.

[0052] Comparative Example 6 Based on Example 1, step (3) of two-step longitudinal stretching is changed to one-step stretching to 5 times, and the rest remains the same as in Example 1.

[0053] Comparative Example 7 The same formulation and preparation steps as in Example 1 were followed, except that the two longitudinal stretching conditions in step (3) were different: the longitudinal stretching ratio in the first step was adjusted to 1.2 times, and the longitudinal stretching ratio in the second step was adjusted to 1.5 times, with the total longitudinal stretching ratio controlled at 1.8 times. The other conditions were the same as in Example 1.

[0054] Comparative Example 8 The same formulation and preparation steps as in Example 1 were followed, except that the two longitudinal stretching conditions in step (3) were different: the longitudinal stretching ratio in the first step was adjusted to 3 times, and the longitudinal stretching ratio in the second step was adjusted to 4 times, with the total longitudinal stretching ratio controlled at 12 times. The other conditions were the same as in Example 1.

[0055] Comparative Example 9 Based on Example 1, the temperature of the first step of the two-step longitudinal stretching was changed to 130°C, while the rest remained the same as in Example 1.

[0056] Comparative Example 10 Based on Example 1, the temperature of the second step of the two-step longitudinal stretching was changed to 140°C, while the rest remained the same as in Example 1.

[0057] Performance testing: Straight-line tear performance: A 5mm long slit is cut in the transverse direction of the film as the tear starting point, and the film is manually torn along the longitudinal direction at a uniform speed of 250mm / min. The maximum deviation angle of the tear path from the longitudinal axis is measured. High-temperature retortability: Referring to the test requirements for heat resistance of retort pouches in GB / T 10004-2008 "Dry lamination and extrusion lamination of plastic composite films and bags for packaging", the film was made into 200mm×150mm retort pouches, filled with 120mL of deionized water, and heat-sealed; placed in an autoclave, the temperature was set at 131℃, the pressure at about 0.22MPa, and the retort time was 30min; after depressurization and cooling to room temperature, the pouches were removed and their appearance was inspected, observing for any abnormalities such as bag breakage, obvious deformation, interlayer peeling, and peeling at the heat-sealed area; and the tensile strength before and after retort was tested according to GB / T 1040.3-2006, and the strength retention rate was calculated.

[0058]

[0059] The test results show that Examples 1-5 all exhibit excellent straight tear resistance (deviation angle 1.3°-3.2°) and high-temperature cooking resistance at 131°C (intact appearance, strength retention rate 89%-93%), meeting the dual requirements of cooking resistance and easy tearing. In contrast, Comparative Example 1, lacking a β-crystal nucleating agent, has a tear deviation angle as high as 13.0°, completely lacking straight tear resistance; Comparative Example 2, with a quenching roller temperature of 120°C exceeding the preferred range, has insufficient β-crystal formation, resulting in a deviation angle of 8.5°; Comparative Example 3, using HDPE instead of copolymer polypropylene, suffers severe deformation and melting after cooking at 131°C, failing to meet the high-temperature sterilization requirements, highlighting the heat resistance advantage of the all-PP system of this invention; Comparative Example 4, without adding SEBS and with the other formulations and processes identical to Example 1, has a film tear deviation angle of 3.0°, although it still maintains a certain degree of straight tear resistance. While the film exhibited good tear-guiding properties, after high-temperature cooking at 131℃ for 30 minutes, localized microcracks appeared inside the core layer, reducing the tensile strength retention rate to 82%. The lack of SEBS resulted in the microfibrillation weak zones formed by the β-to-α phase transition in the core layer lacking effective elastic toughening protection under the high internal pressure and thermal stress of high-temperature cooking. Microcracks easily initiated and propagated, damaging the integrity of the film structure. Comparative Example 5, although achieving some tear-guiding effect through laser semi-tangential physical method, resulted in cracking and leakage along the cut after cooking, with a strength retention rate of only 61%. Comparative Example 6, by changing the two-step stretching to a one-step stretching, significantly increased the deviation angle. Comparative Example 7 had a total longitudinal stretching ratio as low as 1.8 times, insufficient to induce sufficient stress-induced phase transformation and microfibrillation in the β crystals. The film did not form an effective tear-guiding channel, and the tear deviation angle reached as high as 12.5°, proving that linear tearability is lost when the stretching ratio is below the lower limit. Comparative Example 8, with a total longitudinal stretch ratio as high as 12 times, although forming a highly oriented microfiber structure that resulted in a tear deviation angle of only 1.8°, excessive stretching led to the debonding of the microfiber interfaces and the formation of interconnected micropores. After cooking, microleakage occurred, and the strength retention rate plummeted to 58%, proving that the sealing performance failed when the stretch ratio exceeded the upper limit. Comparative Example 9 had a higher first-step stretching temperature (130°C), and Comparative Example 10 had a higher second-step stretching temperature (140°C), with deviation angles increasing to 6.8° and 7.1° respectively. This indicates that suitable stretching temperature ranges in both steps are conducive to the formation of ordered microfiber tear channels.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A straight, easy-tear, high-temperature retort film, characterized in that, It contains at least three layers: an upper surface layer, a core layer, and a lower surface layer; The upper and lower surface layers comprise copolymer polypropylene, propylene-based elastomer, and anti-blocking masterbatch. The core layer comprises the following components: Copolymer polypropylene, propylene-based elastomers, styrene-based thermoplastic elastomers, β-crystal nucleating agents, and modified nano-alumina.

2. The straight, easy-tear, high-temperature retort film according to claim 1, characterized in that, The β-crystal nucleating agent is at least one of amide compounds or rare earth complex β-crystal nucleating agents; the content of the β-crystal nucleating agent in the core layer is 0.05%-0.3%.

3. The straight, easy-tear, high-temperature retort film according to claim 2, characterized in that, The upper surface layer, core layer and lower surface layer are in the following mass fraction ratios: 1:(1-10):

1.

4. The straight, easy-tear, high-temperature retort film according to claim 1, characterized in that, The copolymer polypropylene is selected from one or more of ethylene-propylene binary block copolymer polypropylene and propylene-butadiene binary block copolymer polypropylene.

5. The straight, easy-tear, high-temperature retort film according to claim 1, characterized in that, The modified nano-alumina is α-alumina modified with titanate coupling agent.

6. A method for preparing a straight, easy-tear, high-temperature retort film as described in any one of claims 1-5, characterized in that, Includes the following steps: (1) Mix the raw materials of the upper and lower surface layers evenly and add them to the surface layer twin-screw extruder; mix the core layer raw materials and β crystal nucleating agent masterbatch evenly and add them to the core layer twin-screw extruder; (2) The above-mentioned molten polymer is combined into a molten layer of upper surface layer / core layer / lower surface layer through a multi-layer co-extrusion casting die, and then cast onto a rapid cooling roller at a temperature of 80-100℃ to quickly cool and cast a sheet, thereby obtaining a thick sheet with β crystals in the core layer. (3) The multilayer thick sheet is preheated at 120-140℃ and then subjected to two longitudinal stretching steps: the first step is to stretch longitudinally by 1-3 times at 100-120℃, and the second step is to stretch longitudinally by 2-4 times at 110-130℃, and the total longitudinal stretching ratio is controlled to be 4-10 times, and then heat-treated at 130-140℃. (4) After stretching, heat setting, cooling, and surface corona treatment are carried out, and then the product is wound up.

7. The preparation method according to claim 6, characterized in that, The processing temperature of the surface twin-screw extruder in step (1) is 180-230℃, and the processing temperature of the core twin-screw extruder is 200-250℃.

8. The preparation method according to claim 6, characterized in that, The β-crystal nucleating agent masterbatch mentioned in step (1) is a masterbatch with copolymer polypropylene as carrier and a β-crystal nucleating agent mass concentration of 5%-10%.

9. The preparation method according to claim 6, characterized in that, The heat setting temperature in step (4) is 130-140℃, and the cooling temperature is 30-70℃.

10. An application of the straight, easy-tear, high-temperature retort film as described in any one of claims 1-5, characterized in that: The straight, easy-tear, high-temperature retort film is used in the packaging of food and pharmaceuticals.

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