Crystal-free low-gloss matt coated base film and method for producing the same

Abstract

The application discloses a kind of low-gloss matt coating base film without crystal point, it is successively composed of upper surface layer, core layer and lower surface layer, wherein upper surface layer is matt layer, and lower surface layer is anti-sticking layer;The upper surface layer is made of the following mass parts of raw materials: modified inorganic matt powder 1-3 parts, antioxidant 0.1-0.3 parts, polypropylene 52.2-53.5 parts, high-density polyethylene 44.5-45.4 parts.The application also discloses the preparation method of the above-mentioned low-gloss matt coating base film without crystal point.The matt coating base film of the application has the characteristics of low gloss, high haze, excellent matt effect, good mechanical properties, and the film surface is free of large particle crystal point impurities, with excellent appearance quality.

Description

Technical Field

[0001] The present invention relates to the technical field of thin films, in particular to a low-gloss matte coating base film without crystal points and a preparation method thereof. Background Art

[0002] Matte coated base film is a matte film used for coating. As a base film, it features low gloss and can directly impart a matte effect to the coated film. It is primarily used in deep processing industries such as coating and lamination. This requires that matte coated base film not only provide mechanical support but also ensure a good matte effect. With increasing demands for the appearance and performance of film products, there is a need to improve the performance of matte coated base film products. This improvement primarily focuses on improving the quality of matte coated base film, including eliminating large grain defects on the base film surface, improving the surface appearance and matte uniformity, or enhancing the flatness of the matte coated base film to make it more suitable for coating. Grain is a common problem in BOPP film processing. It is a product of overpolymerization of high-melting-point polymers, formed during the long-term high-temperature processing of raw materials. The presence of large grains not only significantly affects the film's surface appearance but also, for matte coated base films, reduces matte uniformity. Therefore, for matte coating film products that require high appearance quality, it is crucial to reduce the large particle crystal point defects of the coating base film. Summary of the Invention

[0003] Based on the technical problems existing in the background technology, the present invention proposes a low-gloss matte coating base film without crystal points and a preparation method thereof.

[0004] The present invention provides a low-gloss matte coating base film with no crystal points, which is sequentially composed of an upper surface layer, a core layer and a lower surface layer, wherein the upper surface layer is a matte layer and the lower surface layer is an anti-sticking layer;

[0005] The upper surface layer is made of the following raw materials in parts by mass: 1 to 3 parts of modified inorganic matting powder, 0.1 to 0.3 parts of antioxidant, 52.2 to 53.5 parts of polypropylene, and 44.5 to 45.4 parts of high-density polyethylene;

[0006] The preparation method of the modified inorganic matting powder comprises the following steps:

[0007] (1) surface-modifying an inorganic matting powder with an aminosilane coupling agent to obtain an aminosilane-grafted inorganic matting powder; dispersing the aminosilane-grafted inorganic matting powder in xylene, then adding dimethylolpropionic acid and a catalyst, heating under reflux and stirring in an inert atmosphere to react, thereby obtaining a hyperbranched polyester-grafted inorganic matting powder;

[0008] (2) dispersing the hyperbranched polyester grafted inorganic matting agent in toluene, then adding perfluorooctanoic acid and catalyst, and stirring and reacting under heating reflux in inert atmosphere to obtain the modified inorganic matting agent.

[0009] Preferably, in step (1), the mass ratio of the amino silane coupling agent to the inorganic matting agent is 1:1-2; preferably, the amino silane coupling agent is silane coupling agent KH-540, KH-550 or a combination thereof.

[0010] Preferably, in step (1), the specific method for surface modification of the inorganic matting agent by the amino silane coupling agent comprises: dispersing the inorganic matting agent in toluene, then adding the amino silane coupling agent, and heating and reacting at 60-70℃ for 2-4h, then filtering, and washing and drying the obtained solid substance.

[0011] Preferably, in step (1), the mass ratio of the dimethylol propionic acid to the amino silane grafted inorganic matting agent is 1:0.25-1, and the mass ratio of the dimethylol propionic acid to the catalyst is 100:0.5-1; the catalyst is p-toluene sulfonic acid.

[0012] In the present application, after the heating reflux stirring reaction of step (1) is completed, the subsequent conventional treatment steps including filtering, washing and drying are also included.

[0013] Preferably, in step (2), the mass ratio of the perfluorooctanoic acid to the hyperbranched polyester grafted inorganic matting agent is 1:0.5-1, and the mass ratio of the perfluorooctanoic acid to the catalyst is 100:2-4; the catalyst is p-toluene sulfonic acid.

[0014] In the present application, after the heating reflux stirring reaction of step (2) is completed, the subsequent conventional treatment steps including filtering, washing and drying are also included.

[0015] In the present application, the inert atmosphere refers to the atmosphere formed by non-reactive gas, wherein the non-reactive gas can be nitrogen, argon, helium, etc.

[0016] Preferably, the polypropylene is binary or ternary polypropylene copolymer, and the melt index at 230℃ under 2.16kg is 3-5g / 10min; the density of the high density polyethylene is 0.920-0.961g / cm 3 , and the melt index at 230℃ under 2.16kg is 0.03-0.08g / 10min.

[0017] Preferably, the inorganic matting agent is silica with a particle size of 2-5μm; the antioxidant is antioxidant 1010, antioxidant 168 or a combination thereof.

[0018] Preferably, the core layer is made of the following raw materials by mass fraction: polypropylene 99-97 parts, antistatic agent 1-3 parts; the lower surface layer is made of the following raw materials by mass fraction: polypropylene 95-99 parts, anti-adhesion agent 1-5 parts.

[0019] Preferably, the anti-adhesion agent is Constap AB6018PP, and the antistatic agent is antistatic agent PBT.

[0020] Preferably, the matt coated base film has a thickness of 12-25 μm, the upper surface layer has a thickness of 2-5 μm, and the lower surface layer has a thickness of 0.5-2 μm.

[0021] The application further provides a preparation method of the matt coated base film, comprising: adding raw materials of a core layer into a main extruder to heat and melt, to obtain a core layer melt; adding raw materials of an upper surface layer and a lower surface layer into two auxiliary extruders respectively to heat and melt, to obtain an upper surface layer melt and a lower surface layer melt; combining and extruding the core layer melt, the upper surface layer melt and the lower surface layer melt in a die to obtain a cast sheet; and sequentially subjecting the cast sheet to cooling, longitudinal stretching, transverse stretching, and then performing corona treatment on the surface of the upper surface layer to a darcy value of 38-45, to obtain the matt coated base film.

[0022] Preferably, the raw materials of the core layer are heated and melted at a temperature of 247-252 ℃, the raw materials of the upper surface layer are heated and melted at a temperature of 250-258 ℃, the raw materials of the lower surface layer are heated and melted at a temperature of 220-250 ℃, and the die temperature is 248-255 ℃.

[0023] Preferably, the longitudinal stretching comprises: preheating at 120-125 ℃, then stretching at a temperature of 115-120 ℃ and a stretching ratio of 4.2-4.6 times, and then setting at 118-122 ℃.

[0024] Preferably, the transverse stretching comprises: preheating at 158-162 ℃, then stretching at a temperature of 151-154 ℃ and a stretching ratio of 7.5-8.5 times, and then setting at 156-158 ℃ and cooling at room temperature.

[0025] The application has the following beneficial effects:

[0026] The matt coating base film of the present application adopts a blending system of polypropylene and high-density polyethylene to form a rough surface of the matt layer, and combines the addition of inorganic matt powder, so as to realize good matt effect, and can achieve high haze and low gloss, i.e., haze≥80% and gloss≤15%; the surface of the inorganic matt powder is treated by grafting of hyperbranched polyester, and the hyperbranched polyester is further modified by fluorination, on the one hand, the surface modification of the hyperbranched polyester improves the dispersibility of the inorganic matt powder in the matt layer and the flowability of the inorganic matt powder, so as to avoid the formation of large-particle crystal points on the surface of the matt layer due to the agglomeration or poor flowability of the inorganic matt powder during processing; on the other hand, the fluorination treatment of the hyperbranched polyester on the surface of the inorganic matt powder can also improve the lubricity between the melt and the equipment during processing, so as to inhibit the generation of large-particle crystal points caused by the adhesion of the melt on the surface of the processing equipment; in combination with the adjustment of the process parameters such as the stretching temperature, the processing performance and the mechanical properties of the film are ensured, and the generation of large-particle crystal points caused by over-polymerization of the polymer due to the excessively high processing temperature is avoided; in summary, the matt coating base film of the present application has the characteristics of low gloss and high haze, excellent matt effect, good mechanical properties, and no large-particle crystal point impurities on the surface of the film, and has excellent appearance quality. DETAILED DESCRIPTION

[0027] The technical solutions of the present application will be described in detail below through specific examples.

[0028] In the following examples and comparative examples, the melt index of the polypropylene at 230℃ under 2.16kg is 3.15g / 10min, the density of the high-density polyethylene is 0.920-0.961g / cm 3 , the melt index at 230℃ under 2.16kg is 0.05g / 10min, the inorganic matt powder is silicon dioxide with a particle size of 2-5μm, the anti-adhesive is Constap AB6018PP, the antioxidant is antioxidant 1010 and antioxidant 168 in a mass ratio of 2:1, and the antistatic agent is antistatic agent PBT.

[0029] Example 1

[0030] A crystal-point-free low-gloss matt coating base film, which is composed of an upper surface layer, a core layer and a lower surface layer in sequence, wherein the upper surface layer is a matt layer, and the lower surface layer is an anti-adhesive layer;

[0031] The upper surface layer is made of the following raw materials in parts by mass: modified inorganic matt powder 2 parts, antioxidant 0.2 parts, polypropylene 52.8 parts, and high-density polyethylene 45 parts.

[0032] The preparation method of the modified inorganic matt powder comprises the following steps:

[0033] (1) dispersing the inorganic matting powder in toluene, then adding silane coupling agent KH-550, heating at 60℃ for 4h, then suction filtration, washing and drying the obtained solid substance to obtain the amino-silane grafted inorganic matting powder, wherein the mass ratio of inorganic matting powder to silane coupling agent KH-550 is 1:2;

[0034] dispersing the amino-silane grafted inorganic matting powder in xylene, then adding dimethylol propionic acid and p-toluene sulfonic acid, heating under reflux with stirring under nitrogen atmosphere, then suction filtration, washing and drying the obtained solid substance to obtain the hyperbranched polyester grafted inorganic matting powder, wherein the mass ratio of dimethylol propionic acid to amino-silane grafted inorganic matting powder is 1:0.25, and the mass ratio of dimethylol propionic acid to p-toluene sulfonic acid is 100:0.5;

[0035] (2) dispersing the hyperbranched polyester grafted inorganic matting powder in toluene, then adding perfluorooctanoic acid and p-toluene sulfonic acid, heating under reflux with stirring under nitrogen atmosphere, then suction filtration, washing and drying the obtained solid substance to obtain the modified inorganic matting powder, wherein the mass ratio of perfluorooctanoic acid to hyperbranched polyester grafted inorganic matting powder is 1:0.5, and the mass ratio of perfluorooctanoic acid to p-toluene sulfonic acid is 100:2;

[0036] The core layer is made of 98 parts of polypropylene and 2 parts of antistatic agent; the lower surface layer is made of 97 parts of polypropylene and 3 parts of anti-adhesive agent;

[0037] The thickness of the matting coated base film is 20 μm, the thickness of the upper surface layer is 3.5 μm, and the thickness of the lower surface layer is 1.5 μm.

[0038] The preparation method of the matting coated base film comprises the following steps: adding the raw materials of the core layer into a main extruder to melt at 247-252℃ to obtain a core layer melt, adding the raw materials of the upper surface layer and the lower surface layer into two auxiliary extruders to melt, respectively, to obtain an upper surface layer melt and a lower surface layer melt, wherein the melting temperature of the raw materials of the upper surface layer is 250-258℃, and the melting temperature of the raw materials of the lower surface layer is 220-250℃; combining and extruding the core layer melt, the upper surface layer melt and the lower surface layer melt in a die to obtain a cast sheet, wherein the temperature of the die is 248-255℃; the cast sheet is sequentially subjected to cooling, longitudinal stretching, transverse stretching, and then the surface of the upper surface layer is subjected to corona treatment until the dynes value reaches 42 to obtain the matting coated base film; the longitudinal stretching comprises the following steps: preheating at 122℃, then stretching at a temperature of 118℃ and a stretching ratio of 4.3 times, and then setting at 120℃; the transverse stretching comprises the following steps: preheating at 160℃, then stretching at a temperature of 153℃ and a stretching ratio of 8 times, and then setting at 157℃, and then cooling at room temperature.

[0039] Example 2

[0040] A low-gloss matt coating base film without crystal points is composed of an upper surface layer, a core layer and a lower surface layer in sequence, wherein the upper surface layer is a matt layer and the lower surface layer is an anti-sticking layer;

[0041] The upper surface layer is made of the following raw materials in mass parts: modified inorganic matt powder 1 part, antioxidant 0.1 part, polypropylene 53.5 parts, high-density polyethylene 45.4 parts;

[0042] The preparation method of the modified inorganic matt powder comprises the following steps:

[0043] (1) The inorganic matt powder is dispersed in toluene, then silane coupling agent KH-550 is added, heated at 65℃ for 3h, then filtered, and the obtained solid substance is washed and dried to obtain amino-silane grafted inorganic matt powder, wherein the mass ratio of the inorganic matt powder to the silane coupling agent KH-550 is 1:1.5;

[0044] The amino-silane grafted inorganic matt powder is dispersed in xylene, then dimethylol propionic acid and p-toluene sulfonic acid are added, heated and stirred under nitrogen atmosphere, then filtered, and the obtained solid substance is washed and dried to obtain hyperbranched polyester grafted inorganic matt powder, wherein the mass ratio of the dimethylol propionic acid to the amino-silane grafted inorganic matt powder is 1:0.5, and the mass ratio of the dimethylol propionic acid to the p-toluene sulfonic acid is 100:0.8;

[0045] (2) The hyperbranched polyester grafted inorganic matt powder is dispersed in toluene, then perfluorooctanoic acid and p-toluene sulfonic acid are added, heated and stirred under nitrogen atmosphere, then filtered, and the obtained solid substance is washed and dried to obtain the modified inorganic matt powder, wherein the mass ratio of the perfluorooctanoic acid to the hyperbranched polyester grafted inorganic matt powder is 1:0.6, and the mass ratio of the perfluorooctanoic acid to the p-toluene sulfonic acid is 100:3;

[0046] The core layer is made of the following raw materials in mass parts: polypropylene 98 parts, antistatic agent 2 parts; and the lower surface layer is made of the following raw materials in mass parts: polypropylene 97 parts, anti-sticking agent 3 parts;

[0047] The thickness of the matt coating base film is 20μm, the thickness of the upper surface layer is 3.5μm, and the thickness of the lower surface layer is 1.5μm.

[0048] The preparation method of the above matt coating base film is the same as that of Example 1.

[0049] Example 3

[0050] A low-gloss matt coating base film without crystal points is composed of an upper surface layer, a core layer and a lower surface layer in sequence, wherein the upper surface layer is a matt layer and the lower surface layer is an anti-sticking layer;

[0051] The upper surface layer is made of the following raw materials in mass parts: modified inorganic matting powder 3 parts, antioxidant 0.3 parts, polypropylene 52.2 parts, high density polyethylene 44.5 parts;

[0052] The preparation method of the modified inorganic matting powder comprises the following steps:

[0053] (1) The inorganic matting powder is dispersed in toluene, then silane coupling agent KH-550 is added, heated at 70°C for 2h, then filtered, the obtained solid substance is washed and dried to obtain amino-silane grafted inorganic matting powder, wherein the mass ratio of inorganic matting powder to silane coupling agent KH-550 is 1:2;

[0054] The amino-silane grafted inorganic matting powder is dispersed in xylene, then dimethylol propionic acid and p-toluene sulfonic acid are added, heated and stirred under nitrogen atmosphere, then filtered, the obtained solid substance is washed and dried to obtain hyperbranched polyester grafted inorganic matting powder, wherein the mass ratio of dimethylol propionic acid to amino-silane grafted inorganic matting powder is 1:1, and the mass ratio of dimethylol propionic acid to p-toluene sulfonic acid is 100:1;

[0055] (2) The hyperbranched polyester grafted inorganic matting powder is dispersed in toluene, then perfluorooctanoic acid and p-toluene sulfonic acid are added, heated and stirred under nitrogen atmosphere, then filtered, the obtained solid substance is washed and dried to obtain the modified inorganic matting powder, wherein the mass ratio of perfluorooctanoic acid to hyperbranched polyester grafted inorganic matting powder is 1:1, and the mass ratio of perfluorooctanoic acid to p-toluene sulfonic acid is 100:4;

[0056] The core layer is made of the following raw materials in mass parts: polypropylene 98 parts, antistatic agent 2 parts; the lower surface layer is made of the following raw materials in mass parts: polypropylene 97 parts, anti-adhesive agent 3 parts;

[0057] The thickness of the matting coated base film is 20μm, the thickness of the upper surface layer is 3.5μm, and the thickness of the lower surface layer is 1.5μm.

[0058] The preparation method of the matting coated base film is the same as that of Example 1.

[0059] Comparative Example 1

[0060] A low-gloss matting coated base film without crystal points, which is composed of an upper surface layer, a core layer and a lower surface layer in sequence, wherein the upper surface layer is a matting layer, and the lower surface layer is an anti-adhesive layer;

[0061] The upper surface layer is made of the following raw materials in mass parts: inorganic matting powder 2 parts, antioxidant 0.2 parts, polypropylene 52.8 parts, high density polyethylene 45 parts;

[0062] The core layer is made of the following raw materials in parts by mass: polypropylene 98 parts, antistatic agent 2 parts; the lower surface layer is made of the following raw materials in parts by mass: polypropylene 97 parts, anti-adhesion agent 3 parts;

[0063] The matt coated base film has a thickness of 20 μm, the upper surface layer has a thickness of 3.5 μm, and the lower surface layer has a thickness of 1.5 μm.

[0064] The preparation method of the matt coated base film is the same as that of Example 1.

[0065] Comparative Example 2

[0066] A low-gloss matt coated base film without crystal points is composed of an upper surface layer, a core layer and a lower surface layer in sequence, wherein the upper surface layer is a matt layer, and the lower surface layer is an anti-adhesion layer.

[0067] The upper surface layer is made of the following raw materials in parts by mass: hyperbranched polyester grafted inorganic matt powder 2 parts, antioxidant 0.2 parts, polypropylene 52.8 parts, high-density polyethylene 45 parts.

[0068] The preparation method of the hyperbranched polyester grafted inorganic matt powder comprises the following steps:

[0069] The inorganic matt powder is dispersed in toluene, then silane coupling agent KH-550 is added, heated at 60°C for 4 h, then filtered, and the obtained solid substance is washed and dried to obtain amino-silane grafted inorganic matt powder, wherein the mass ratio of the inorganic matt powder to the silane coupling agent KH-550 is 1:2.

[0070] The amino-silane grafted inorganic matt powder is dispersed in xylene, then dimethylol propionic acid and p-toluene sulfonic acid are added, heated and stirred under nitrogen atmosphere, then filtered, and the obtained solid substance is washed and dried to obtain hyperbranched polyester grafted inorganic matt powder, wherein the mass ratio of the dimethylol propionic acid to the amino-silane grafted inorganic matt powder is 1:0.25, and the mass ratio of the dimethylol propionic acid to the p-toluene sulfonic acid is 100:0.5.

[0071] The core layer is made of the following raw materials in parts by mass: polypropylene 98 parts, antistatic agent 2 parts; the lower surface layer is made of the following raw materials in parts by mass: polypropylene 97 parts, anti-adhesion agent 3 parts;

[0072] The matt coated base film has a thickness of 20 μm, the upper surface layer has a thickness of 3.5 μm, and the lower surface layer has a thickness of 1.5 μm.

[0073] The preparation method of the matt coated base film is the same as that of Example 1.

[0074] Test Example

[0075] The performance of the matt coated base film of Example 1 and Comparative Examples 1-2 was tested, wherein the haze was tested according to GB / T 2410-2008, the gloss of the matt layer was tested according to the standard of GBT 8807-1988; the counting method of the crystal points was as follows: 5 areas with a size of 150m x 150m were randomly observed and counted on the surface of the film matt layer, and the total number of the crystal points with a size of more than 0.4mm in the 5 areas was recorded. The test results are shown in Table 1:

[0076] Table 1

[0077] Haze (%) Gloss (45°) (%) Crystalline point number Example 1 82 10 0 Comparative Example 1 80 12 8 Comparative Example 2 Comparative Example 3 81 9 5

[0078] The above merely describes the preferred specific embodiments of the present application, but the protection scope of the present application is not limited to this. Any person skilled in the art, according to the technical solution and the inventive concept of the present application, makes equivalent replacement or change within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A haze-free low-gloss matte coated base film, characterized by, The upper surface layer is a matt layer, and the lower surface layer is an anti-sticking layer; The upper surface layer is made of the following raw materials by mass: modified inorganic matt powder 1-3 parts, antioxidant 0.1-0.3 parts, polypropylene 52.2-53.5 parts, and high-density polyethylene 44.5-45.4 parts; The preparation method of the modified inorganic matt powder comprises the following steps: (1) the inorganic matt powder is surface-modified by using an amino silane coupling agent to obtain amino silane grafted inorganic matt powder; the amino silane grafted inorganic matt powder is dispersed in xylene, then dimethylol propionic acid and a catalyst are added, and the mixture is heated to reflux and stirred under an inert atmosphere to obtain hyperbranched polyester grafted inorganic matt powder; (2) the hyperbranched polyester grafted inorganic matt powder is dispersed in toluene, then perfluorooctanoic acid and a catalyst are added, and the mixture is heated to reflux and stirred under an inert atmosphere to obtain the modified inorganic matt powder.

2. The non-glossy low-gloss matte coated base film according to claim 1, characterized by, In step (1), the mass ratio of dimethylol propionic acid to amino silane grafted inorganic matt powder is 1:0.25-1, and the mass ratio of dimethylol propionic acid to catalyst is 100:0.5-1; the catalyst is p-toluene sulfonic acid.

3. The haze-free, low-gloss, matte coated base film according to claim 1, wherein, In step (2), the mass ratio of perfluorooctanoic acid to hyperbranched polyester grafted inorganic matt powder is 1:0.5-1, and the mass ratio of perfluorooctanoic acid to catalyst is 100:2-4; the catalyst is p-toluene sulfonic acid.

4. The haze-free, low-gloss, matte coated base film according to claim 1, wherein, The polypropylene is a binary or ternary polypropylene copolymer, and the melt index at 230°C, 2.16 kg is 3-5 g / 10 min; the high-density polyethylene has a density of 0.920-0.961 g / cm 3 The melt index at 230°C, 2.16 kg is 0.03-0.08 g / 10 min.

5. The haze-free, low-gloss, matte coated base film according to claim 1, wherein, The inorganic matt powder is silicon dioxide with a particle size of 2-5 μm; and the antioxidant is antioxidant 1010, antioxidant 168 or a combination thereof.

6. The haze-free, low-gloss, matte coated base film according to claim 1, wherein, The core layer is made of the following raw materials by mass: polypropylene 99-97 parts and antistatic agent 1-3 parts; and the lower surface layer is made of the following raw materials by mass: polypropylene 95-99 parts and anti-sticking agent 1-5 parts.

7. The non-glossy, low-gloss, matte coated base film according to claim 6, characterized by, The anti-sticking agent is Constap AB6018PP, and the antistatic agent is antistatic agent PBT.

8. The matt coated base film according to claim 1, characterized in that The matt coating base film has a thickness of 12-25 μm, the upper surface layer has a thickness of 2-5 μm, and the lower surface layer has a thickness of 0.5-2 μm.

9. A method for producing a matt coated base film according to any one of claims 1 to 8, characterized by, The method comprises the following steps: The raw materials of the core layer are added into a main extruder to be heated and melted to obtain a core layer melt, the raw materials of the upper surface layer and the lower surface layer are added into two auxiliary extruders to be heated and melted to obtain an upper surface layer melt and a lower surface layer melt, the core layer melt, the upper surface layer melt and the lower surface layer melt are combined and extruded in a die to obtain a cast sheet, and the cast sheet is sequentially subjected to cooling, longitudinal stretching, transverse stretching, and then the surface of the upper surface layer is subjected to corona treatment to have a dyne value of 38-45 to obtain the matt coating base film.

10. The method for preparing a matte coating base film according to claim 9, wherein: The heating and melting temperature of the raw materials of the core layer is 247-252 ℃, the heating and melting temperature of the raw materials of the upper surface layer is 250-258 ℃, the heating and melting temperature of the raw materials of the lower surface layer is 220-250 ℃, and the die temperature is 248-255 ℃; The longitudinal stretching comprises the following steps: preheating at 120-125 ℃, stretching at a temperature of 115-120 ℃ and a stretching ratio of 4.2-4.6 times, and then setting at 118-122 ℃; The transverse stretching includes preheating at 158 to 162°C, stretching at a temperature of 151 to 154°C and a stretching ratio of 7.5 to 8.5, and setting at 156 to 158°C, followed by cooling at room temperature.