Preparation method of breathable film

Abstract

The invention provides a preparation method of a breathable film. The preparation method comprises a longitudinal stretching step and a transverse stretching step, in the longitudinal stretching step, heat treatment is carried out on the composite cast film, the composite cast film is longitudinally stretched to obtain the composite longitudinal stretching film, the composite cast film comprises a plurality of layers of laminated cast films, and in the step, the heat treatment temperature is not higher than the melting point of the cast film; in the transverse stretching step, the composite longitudinal stretching film is subjected to heat treatment through infrared irradiation, and the composite longitudinal stretching film is transversely stretched to obtain the composite two-way stretching film. The preparation method is high in production efficiency, and the breathable film prepared by the preparation method has the characteristics of high breathability, leakage resistance, balanced mechanical properties and light weight.

Description

Technical Field

[0001] This application relates to the field of breathable membrane preparation technology, and in particular to a method for preparing a breathable membrane. Background Technology

[0002] Breathable membranes are typically produced using linear low-density polyethylene (LLDPE) as the base material, which is formed by unidirectional longitudinal stretching after incorporating a specific proportion of calcium carbonate particles. This process relies solely on longitudinal stretching to separate the filler from the matrix interface and create micropores, without involving transverse stretching. This results in uneven micropore distribution and a decline in the overall performance of the breathable membrane.

[0003] The core limitation of traditional biaxial stretching processes for breathable membranes lies in their support for single-layer membrane processing. When using biaxial stretching, the single-layer membrane must be stretched laterally after longitudinal stretching. This single-layer operation stems from the heating method of traditional lateral stretching: hot air in the oven simultaneously blows heat from both the top and bottom surfaces of the film. If multiple composite membranes are processed directly, the middle membranes cannot fully receive the hot air energy due to the obstruction of the surface membranes, leading to an imbalance in interlayer temperature distribution. Insufficient preheating of the middle membranes not only reduces the uniformity of micropore expansion but also causes interlayer differences in pore size and orientation, ultimately weakening the membrane's leak-proof properties and breathability consistency. Simultaneously, the need to separate and stretch multiple membranes layer by layer significantly increases process complexity and negatively impacts production efficiency. Summary of the Invention

[0004] This invention provides a method for preparing a breathable membrane. This method has high production efficiency, and the breathable membrane prepared by this method has the characteristics of high air permeability, leak-proofness, balanced mechanical properties, and lightweight.

[0005] A method for preparing a breathable membrane includes: a longitudinal stretching step and a transverse stretching step; in the longitudinal stretching step, a composite cast film is heat-treated and longitudinally stretched to obtain a composite longitudinally stretched film, wherein the composite cast film comprises multiple layers of cast films, and in this step, the heat treatment temperature is not higher than the melting point of the cast film; in the transverse stretching step, the composite longitudinally stretched film is heat-treated by infrared irradiation and transversely stretched to obtain a composite biaxially stretched film.

[0006] This invention provides a method for preparing a breathable membrane. In the longitudinal stretching step, the composite cast film is heat-treated and longitudinally stretched. The heat treatment temperature is controlled to be no higher than the melting point of the cast film. This not only enables the composite longitudinally stretched film to have high longitudinal tensile strength, but also allows the composite cast film to stretch synchronously in the longitudinal stretching step, maintaining good consistency. As a result, the surface of each cast film forms a pore structure with similar pore size distribution after the longitudinal stretching step, providing a good base film structure foundation for the subsequent transverse stretching step. In the transverse stretching step, the composite longitudinally stretched membrane is heat-treated by infrared irradiation. Multiple composite longitudinally stretched membranes can be uniformly heated and synchronously stretched. Based on the micropores formed in the longitudinal stretching step, the micropore size and distribution are further controlled, resulting in a composite biaxially stretched membrane with a uniformly sized and distributed nanoscale microporous structure on its surface. These nanoscale microporous structures effectively block liquid permeation while allowing water vapor to pass through freely. Furthermore, the biaxially stretched membrane possesses high transverse tensile strength, which matches the longitudinal tensile strength obtained in the transverse stretching step. In addition, the breathable membrane prepared by the method provided by this invention has a low basis weight, requiring less raw materials in the preparation process, achieving a balance of high breathability, leak-proof properties, and mechanical performance. Therefore, the preparation method provided by the present invention can achieve simultaneous transverse stretching of multiple membranes without splitting the composite longitudinal stretching membrane, which improves the production efficiency of breathable membranes. Furthermore, the breathable membranes made by this process have the characteristics of high air permeability, leak-proofness, balanced longitudinal and transverse performance, and lightweight.

[0007] Preferably, in the transverse stretching step, the difference between the lowest temperature in the heat treatment and the melting point of the cast film is denoted as ΔT1, where ΔT1 satisfies -35℃≤ΔT1≤-5℃. By further limiting the difference between the lowest temperature in the heat treatment and the melting point of the cast film in the transverse stretching step, ΔT1, to -35℃≤ΔT1≤-5℃, the composite longitudinally stretched film is in a suitable thermoplastic state during transverse stretching. This allows the composite longitudinally stretched film to fully soften and uniformly stretch, which not only promotes the transversely ordered expansion of longitudinal micropores and facilitates the control of micropore size and distribution, thereby forming a nanoscale microporous structure with uniform size and distribution, but also enables the breathable membrane to achieve excellent thickness uniformity.

[0008] Preferably, the transverse stretching step includes a preheating section, a stretching section, and a setting section. The temperature difference between the end point of the preheating section and the temperature of the stretching section is denoted as △T2, and △T2 satisfies 15℃≤△T2≤50℃. The temperature difference between the setting section and the stretching section is denoted as △T3, and △T3 satisfies -5℃≤△T3≤50℃.

[0009] In the transverse stretching step, the temperature difference between the preheating and stretching sections, and between the setting and stretching sections, is rationally controlled to form a stable temperature gradient. The preheating section promotes uniform temperature increase across multiple films by raising the temperature, providing a temperature basis for the subsequent stretching section. During the stretching stage, a uniform thermal field enables multiple films to stretch transversely simultaneously, promoting the orderly expansion of micropores formed during longitudinal stretching. The setting stage balances internal stress and stabilizes the microporous structure through temperature adjustment, reducing deformation rebound. By controlling the temperature difference ΔT2 between the preheating stage and the stretching stage to be 15℃≤ΔT2≤50℃, the ductility of the membrane material can be effectively maintained, improving the membrane's air permeability and thickness uniformity. By controlling the temperature difference ΔT3 between the setting and stretching stages to be -5℃≤ΔT3≤50℃, the microporous structure formed in the stretching stage can be shaped and set, further precisely controlling the micropore size and distribution, improving the pore uniformity on the composite biaxially stretched membrane, thereby enhancing the air permeability and thickness uniformity of the breathable membrane.

[0010] Preferably, the heat treatment time in the transverse stretching step is 60 to 100 seconds.

[0011] Preferably, the stretching speed ratio in the longitudinal stretching step is denoted as λ1, where λ1 satisfies 1 < λ1 ≤ 2.5.

[0012] Preferably, the stretching speed ratio in the transverse stretching step is denoted as λ2, where λ2 satisfies 1 < λ2 ≤ 2.5.

[0013] The longitudinal stretching speed ratio in this scheme is calculated as the ratio of the exit linear velocity to the inlet linear velocity of the processed object during the longitudinal stretching step. The transverse stretching speed ratio in this scheme is calculated as the ratio of the exit membrane width to the inlet membrane width of the processed object during the transverse stretching step. During the transverse stretching process, the two edges of the composite longitudinally stretched membrane are clamped and fixed to the chain by chain clamps, and the chain runs along the chain track. In the preheating section, the chain clamp spacing is kept basically consistent with the initial width of the composite longitudinally stretched membrane, so that the composite longitudinally stretched membrane is uniformly heated and softened. In the stretching section, the chain track gradually widens to achieve the stretching of the composite longitudinally stretched membrane in the transverse direction. In the shaping section, the width of the chain track narrows, and the width of the composite longitudinally stretched membrane in the transverse direction shrinks by 8% to 12%, which helps the composite biaxially stretched membrane to shrink and shape after stretching, and improves the uniformity of the pore formation of the composite biaxially stretched membrane. The reasonable setting of the stretching speed ratio in each stage can regulate the stretching deformation and shrinkage, effectively improve the size consistency and distribution uniformity of the microporous structure, thereby further improving the breathability and leak-proof performance of the breathable membrane.

[0014] Preferably, the thickness of the cast film is 25~55μm; the composite cast film includes n cast films stacked together, where n satisfies 2≤n≤5.

[0015] Preferably, before the longitudinal stretching step, a composite step is further included, which includes: stacking and compositing multiple layers of cast film to obtain a composite cast film, and heat-treating the composite cast film; the difference between the melting point of the composite cast film and the highest temperature of the heat treatment in the composite step is ΔT4, and ΔT4 satisfies 5℃≤ΔT4≤15℃.

[0016] Preferably, in the composite step, the heat treatment includes a first preheating treatment and a first thermal composite treatment. In the composite step, the temperature of the first preheating treatment is not higher than the temperature of the first thermal composite treatment. The multilayer cast film is composited before the longitudinal stretching step, so that the interlayer bonding of the cast film is tight, effectively preventing the interlayer separation phenomenon in the subsequent longitudinal stretching step and transverse stretching step, and providing a basis for uniform pore formation.

[0017] Preferably, after the transverse stretching step, a heat treatment step is also included. The difference between the melting point of the composite biaxially oriented film and the highest temperature of the heat treatment is denoted as △T5, and △T5 satisfies 5℃≤△T5≤15℃.

[0018] Preferably, after the transverse stretching step, the heat treatment step includes a second preheating treatment and a second thermal bonding treatment, and the temperature of the second preheating treatment is not higher than the temperature of the second thermal bonding treatment. After the transverse stretching step, the porous membrane layer is structurally strengthened by heat treatment, which can effectively release the tensile residual stress between the composite biaxially oriented membranes, strengthen the interlayer bonding force, further stabilize the microporous structure morphology, and help improve the air permeability, leak-proofness and mechanical strength of the breathable membrane. Attached Figure Description

[0019] Figure 1 This is a SEM image of the breathable membrane obtained in Example 1. Detailed Implementation

[0020] To better understand and implement this application, the technical solutions of this application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only some of the embodiments of this application, and not all of them.

[0021] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0022] Unless otherwise stated, all numerical values ​​for the amounts of expressed components, reaction conditions, etc., used in the specification and claims are to be understood as being modified by the term "about". Therefore, unless otherwise indicated, the numerical parameters set forth herein are approximate values ​​that can be varied to obtain the desired performance.

[0023] The word “and / or” as used in this article refers to one or all of the elements mentioned.

[0024] The terms "include" and "contain" as used in this article cover both cases where only the mentioned elements exist and cases where other unmentioned elements exist in addition to the mentioned elements.

[0025] All percentages in this application are weight percentages unless otherwise stated.

[0026] Unless otherwise stated, the terms “a,” “an,” “an,” and “the” as used in this specification are intended to include “at least one” or “one or more.” For example, “a component” refers to one or more components, and therefore more than one component may be considered and may be employed or used in the implementation of the described embodiments.

[0027] Example 1 This embodiment provides a method for preparing a breathable membrane, including the following steps: S1. Casting: Homopolymer PP with a melt flow index (MI) of 2.0 is mixed with 0.4% by mass of aromatic amide β-crystal nucleating agent, and then melt-blended and extruded through a twin-screw extruder. The extruder body temperature is set to 200℃ and the die temperature is set to 235℃. The melt is then cast onto the surface of a casting roller at a temperature of 100℃ and cooled and solidified to obtain a cast film with a thickness of 40μm.

[0028] S2. Lamination: The three cast films are stacked and the stacked cast films are heat-treated. The heat treatment includes a first preheating treatment and a first heat lamination treatment. During the first preheating treatment, the stacked cast films pass sequentially through a first preheating roller at a temperature of 80°C, a second preheating roller at a temperature of 100°C, a third preheating roller at a temperature of 120°C, and a fourth preheating roller at a temperature of 150°C. The temperature of the first heat lamination treatment is 155°C.

[0029] S3. Longitudinal stretching: The heat-treated composite cast film is sequentially passed through a first longitudinal stretching preheating roller at a temperature of 70°C, a second longitudinal stretching preheating roller at a temperature of 90°C, a third longitudinal stretching preheating roller at a temperature of 110°C, and a fourth longitudinal stretching preheating roller at a temperature of 140°C. Then, it is subjected to single-point longitudinal stretching at a temperature of 140°C and a stretching speed ratio of 1.5. After that, it is shaped by a setting roller at a temperature of 140°C to obtain a composite longitudinal stretching film.

[0030] S4. Transverse Stretching: The composite longitudinally stretched film is heat-treated by infrared irradiation in a transverse stretching oven for 80 seconds. Along the conveyor belt direction, the heat treatment includes a preheating section (in the preheating section, the composite longitudinally stretched film passes sequentially through a first transverse stretching preheating zone at 150°C, a second transverse stretching preheating zone at 160°C, a third transverse stretching preheating zone at 170°C, and a fourth transverse stretching preheating zone at 180°C), a stretching section (stretching section temperature is 145°C), and a setting section (setting section temperature is 170°C). The composite longitudinally stretched film is then transversely stretched under a stretching speed ratio of 2.0 to obtain a composite biaxially stretched film.

[0031] S5. Composite: The composite biaxially oriented film is subjected to heat treatment, which includes a second preheating treatment and a second heat-composite treatment. During the second preheating treatment, the composite biaxially oriented film passes sequentially through a first preheating roller at a temperature of 80°C, a second preheating roller at a temperature of 100°C, a third preheating roller at a temperature of 120°C, and a fourth preheating roller at a temperature of 150°C. The temperature of the second heat-composite treatment is 155°C.

[0032] S6. Winding: The heat-treated composite biaxially oriented membrane roll is slid and peeled apart using a sheet separator until individual sheets are wound up to obtain the breathable membrane. The SEM image of the breathable membrane is shown below. Figure 1 As shown.

[0033] The longitudinal stretching speed ratio in this scheme is calculated as the ratio of the exit linear velocity to the inlet linear velocity of the processed object during the longitudinal stretching step. The transverse stretching speed ratio in this scheme is calculated as the ratio of the exit membrane width to the inlet membrane width of the processed object during the transverse stretching step.

[0034] Example 2 This embodiment provides a method for preparing a breathable membrane, including the following steps: S1. Casting: Homopolymer PP with a melt flow index (MI) of 2.0 is mixed with 0.4% by mass of aromatic amide β-crystal nucleating agent, and then melt-blended and extruded through a twin-screw extruder. The extruder body temperature is set to 200℃ and the die temperature is set to 235℃. The melt is then cast onto the surface of a casting roller at a temperature of 100℃ and cooled and solidified to obtain a cast film with a thickness of 26μm.

[0035] S2. Lamination: Two of the above-mentioned cast films are stacked and the stacked cast films are heat-treated. The heat treatment includes a first preheating treatment and a first heat lamination treatment. During the first preheating treatment, the cast film passes sequentially through a first preheating roller at a temperature of 80°C, a second preheating roller at a temperature of 100°C, a third preheating roller at a temperature of 120°C, and a fourth preheating roller at a temperature of 150°C. The temperature of the first heat lamination treatment is 155°C.

[0036] S3. Longitudinal stretching: The heat-treated composite cast film is sequentially passed through a first longitudinal stretching preheating roller at a temperature of 70°C, a second longitudinal stretching preheating roller at a temperature of 90°C, a third longitudinal stretching preheating roller at a temperature of 110°C, and a fourth longitudinal stretching preheating roller at a temperature of 140°C. Then, it is subjected to single-point longitudinal stretching at a temperature of 140°C and a stretching speed ratio of 1.3. After that, it is shaped by a setting roller at a temperature of 140°C to obtain a composite longitudinal stretching film.

[0037] S4. Transverse Stretching: The composite longitudinally stretched film is heat-treated by infrared irradiation in a transverse stretching oven for 60 seconds. Along the conveyor belt direction, the heat treatment includes a preheating section (in the preheating section, the composite longitudinally stretched film passes sequentially through a first transverse stretching preheating zone at 150°C, a second transverse stretching preheating zone at 160°C, a third transverse stretching preheating zone at 170°C, and a fourth transverse stretching preheating zone at 180°C), a stretching section (stretching section temperature is 145°C), and a setting section (setting section temperature is 170°C). The composite longitudinally stretched film is then transversely stretched under a stretching speed ratio of 1.5 to obtain a composite biaxially stretched film.

[0038] S5. Composite: The composite biaxially oriented film is subjected to heat treatment, which includes a second preheating treatment and a second heat-composite treatment. During the second preheating treatment, the composite biaxially oriented film passes sequentially through a first preheating roller at a temperature of 80°C, a second preheating roller at a temperature of 100°C, a third preheating roller at a temperature of 120°C, and a fourth preheating roller at a temperature of 150°C. The temperature of the second heat-composite treatment is 155°C.

[0039] S6 winding: The composite biaxially oriented membrane roll after the above heat treatment is separated into individual sheets by a sheet separator until the individual sheets are wound up to obtain a breathable membrane.

[0040] The longitudinal stretching speed ratio in this scheme is calculated as the ratio of the exit linear velocity to the inlet linear velocity of the processed object during the longitudinal stretching step. The transverse stretching speed ratio in this scheme is calculated as the ratio of the exit membrane width to the inlet membrane width of the processed object during the transverse stretching step.

[0041] Example 3 This embodiment provides a method for preparing a breathable membrane, including the following steps: S1. Casting: Homopolymer PP with a melt flow index (MI) of 2.0 is mixed with 0.4% by mass of aromatic amide β-crystal nucleating agent, and then melt-blended and extruded through a twin-screw extruder. The extruder body temperature is set to 200℃ and the die temperature is set to 235℃. The melt is then cast onto the surface of a casting roller at a temperature of 100℃ and cooled and solidified to obtain a cast film with a thickness of 53μm.

[0042] S2. Lamination: Five of the above-mentioned cast films are stacked and the stacked cast films are heat-treated. The heat treatment includes a first preheating treatment and a first heat lamination treatment. During the first preheating treatment, the cast film passes sequentially through a first preheating roller at a temperature of 80°C, a second preheating roller at a temperature of 100°C, a third preheating roller at a temperature of 120°C, and a fourth preheating roller at a temperature of 150°C. The temperature of the first heat lamination treatment is 155°C.

[0043] S3. Longitudinal stretching: The heat-treated composite cast film is sequentially passed through a first longitudinal stretching preheating roller at a temperature of 70°C, a second longitudinal stretching preheating roller at a temperature of 90°C, a third longitudinal stretching preheating roller at a temperature of 110°C, and a fourth longitudinal stretching preheating roller at a temperature of 140°C. Then, it is subjected to single-point longitudinal stretching at a temperature of 140°C and a stretching speed ratio of 2.0. After that, it is shaped by a setting roller at a temperature of 140°C to obtain a composite longitudinal stretching film.

[0044] S4. Transverse Stretching: The composite longitudinally stretched film is heat-treated by infrared irradiation in a transverse stretching oven for 100 seconds along the conveyor belt direction. The heat treatment includes a preheating section (in the preheating section, the composite longitudinally stretched film passes through a first transverse stretching preheating zone at 150°C, a second transverse stretching preheating zone at 160°C, a third transverse stretching preheating zone at 170°C, and a fourth transverse stretching preheating zone at 180°C), a stretching section (stretching section temperature is 145°C), and a setting section (setting section temperature is 170°C). The composite longitudinally stretched film is then transversely stretched at a stretching speed ratio of 2.0 to obtain a composite biaxially stretched film.

[0045] S5. Composite: The composite biaxially oriented film is subjected to heat treatment, which includes a second preheating treatment and a second heat-composite treatment. During the second preheating treatment, the composite biaxially oriented film passes sequentially through a first preheating roller at a temperature of 80°C, a second preheating roller at a temperature of 100°C, a third preheating roller at a temperature of 120°C, and a fourth preheating roller at a temperature of 140°C. The temperature of the second heat-composite treatment is 155°C.

[0046] S6. Winding: The composite biaxially oriented membrane roll after the above heat treatment is separated into individual sheets by a sheet separator until the individual sheets are wound up to obtain a breathable membrane.

[0047] The longitudinal stretching speed ratio in this scheme is calculated as the ratio of the exit linear velocity to the inlet linear velocity of the processed object during the longitudinal stretching step. The transverse stretching speed ratio in this scheme is calculated as the ratio of the exit membrane width to the inlet membrane width of the processed object during the transverse stretching step.

[0048] Example 4 This embodiment prepares a breathable membrane according to Example 1. The difference between this embodiment and Example 1 is that in the heat treatment of the transverse stretching step (S4), the final temperature of the preheating section is 180℃, the stretching section temperature is 130℃, and the setting section temperature is 170℃. The difference between the lowest temperature in the transverse stretching heat treatment and the melting point of the cast film is denoted as ΔT1, where ΔT1 = 130 - 165 = -35℃. Apart from the above differences, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0049] Example 5 This embodiment prepares a breathable membrane according to Example 1. The difference between this embodiment and Example 1 is that in the heat treatment of the transverse stretching step (S4), the final temperature of the preheating section is 180℃, the stretching section temperature is 165℃, and the setting section temperature is 160℃. The difference between the lowest temperature in the transverse stretching heat treatment and the melting point of the cast film is denoted as ΔT1, where ΔT1 = 160 - 165 = -5℃. Apart from the above differences, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0050] Example 6 This embodiment prepares a breathable membrane according to Example 1. The difference between this embodiment and Example 1 is that in the heat treatment of the transverse stretching step (S4), the final temperature of the preheating section is 180℃, the stretching section temperature is 125℃, and the setting section temperature is 170℃. The difference between the lowest temperature in the transverse stretching heat treatment and the melting point of the cast film is denoted as ΔT1, where ΔT1 = 125 - 165 = -40℃. Apart from the above differences, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0051] Example 7 This embodiment prepares a breathable membrane according to Example 1. The difference between this embodiment and Example 1 is that in the heat treatment of the transverse stretching step (S4), the final temperature of the preheating section is 180℃, the stretching section temperature is 170℃, and the setting section temperature is 165℃. The difference between the lowest temperature in the transverse stretching heat treatment and the melting point of the cast film is denoted as ΔT1, where ΔT1 = 165 - 165 = 0℃. Apart from the above differences, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0052] Example 8 This embodiment prepares a breathable membrane according to Example 1. The difference between this embodiment and Example 1 is that in the heat treatment of the transverse stretching step (S4), the final temperature of the preheating section is 170℃, the stretching section temperature is 160℃, and the setting section temperature is 170℃. The difference between the final temperature of the preheating section and the temperature of the stretching section is denoted as ΔT2, where ΔT2 = 170 - 160 = 10℃. Apart from the above differences, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0053] Example 9 This embodiment prepares a breathable membrane according to Example 1. The difference between this embodiment and Example 1 is that in the heat treatment of the transverse stretching step (S4), the final temperature of the preheating section is 190℃, the stretching section temperature is 135℃, and the setting section temperature is 170℃. The difference between the final temperature of the preheating section and the temperature of the stretching section is denoted as ΔT2, where ΔT2 = 190 - 135 = 55℃. Apart from the above differences, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0054] Example 10 This embodiment prepares a breathable membrane according to Example 1. The difference between this embodiment and Example 1 is that in the heat treatment of the transverse stretching step (S4), the final temperature of the preheating section is 180℃, the stretching section temperature is 160℃, and the setting section temperature is 150℃. The difference between the setting section temperature and the stretching section temperature is denoted as ΔT3, where ΔT3 = 150 - 160 = -10℃. Apart from the above differences, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0055] Example 11 This embodiment prepares a breathable membrane according to Example 1. The difference between this embodiment and Example 1 is that in the heat treatment of the transverse stretching step (S4), the final temperature of the preheating section is 180℃, the stretching section temperature is 135℃, and the setting section temperature is 190℃. The difference between the setting section temperature and the stretching section temperature is denoted as ΔT3, where ΔT3 = 190 - 135 = 55℃. Apart from the above differences, the materials, formulation ratios, and preparation operations used in this embodiment are strictly consistent with those in Example 1.

[0056] Comparative Example 1 This comparative example prepares a breathable membrane according to Example 1. The difference between this comparative example and Example 1 is that in the preparation of the breathable membrane, the S4 transverse stretching step is replaced by oven heating instead of infrared irradiation, and the heat treatment time is 32 seconds. Apart from the above differences, the materials, formulation ratios, and preparation operations used in this comparative example are strictly consistent with those in Example 1.

[0057] Comparative Example 2 This comparative example prepared a breathable membrane according to Example 1. The difference between this comparative example and Example 1 is that in the preparation of the breathable membrane, the S4 transverse stretching step was replaced by oven heating instead of infrared irradiation, and the heat treatment time was 64 seconds. Apart from the above differences, the materials, formulation ratios, and preparation operations used in this comparative example were strictly consistent with those in Example 1.

[0058] Comparative Example 3 This comparative example prepares a breathable membrane according to Example 1. The difference between this comparative example and Example 1 is that, in the process of preparing the breathable membrane, the temperature for unidirectional longitudinal stretching in step S3 is 170°C, and the temperature of the setting roller is 170°C. Apart from the above differences, the materials, formulation ratios, and preparation operations used in this comparative example are strictly consistent with those in Example 1.

[0059] Test case 1. Test subjects: The breathable membranes prepared in Examples 1-11 and Comparative Examples 1-3 were used as test subjects in this test example.

[0060] 2. Test items: (1) Water Vapor Transmission Rate (WVTR): Referring to ASTM D6701, under the condition of 38°C, a humidity gradient of 90%RH / 0%RH is formed on both sides of the test object. After water vapor permeates from the high humidity side to the low humidity side, it is carried to the infrared sensor by the dry carrier gas. The infrared sensor calculates the water vapor transmission rate by detecting the change in water vapor concentration.

[0061] (2) Leakage prevention performance: Lay three layers of filter paper flat on the work platform, add 10 mL of test solution to the middle of the filter paper; after standing for 30 seconds, place the test object on the filter paper (use side of the test object facing up), then cover the test object with two layers of toilet paper (or one layer of filter paper), and press it down with a 10 kg weight. After standing for 5 minutes, remove the weight and observe whether the toilet paper (or filter paper) leaks test solution. The test result judgment criteria are: if no more than 3 penetration points with a diameter <0.3 mm appear during the test, the rating is "qualified"; if any penetration point with a diameter ≥0.3 mm appears during the test, the rating is "unqualified".

[0062] (3) Tensile strength: The longitudinal tensile strength and transverse tensile strength of the test object are tested in accordance with GB / T 1040.1.

[0063] (4) Longitudinal tensile strength / transverse tensile strength: The longitudinal tensile strength / transverse tensile strength is calculated as the ratio of longitudinal tensile strength to transverse tensile strength, which is used to characterize the degree of balance of the mechanical properties of the test object.

[0064] (5) Gram weight: Referring to GB / T 31729-2015, under the conditions of a temperature of 23℃±2℃ and a relative humidity of 50%±10%, the area and mass of the test object were tested. The mass per unit area of ​​the test object was calculated according to the following formula:

[0065] u — Mass per unit area of ​​the test subject, in grams per square meter; m — Mass of the test subject, in grams; s — Area of ​​the test object, in square millimeters.

[0066] 3. Test Results: Table 1. Relevant performance tests of breathable membranes

[0067] The test data of the breathable membranes provided in Examples 1-11 and Comparative Examples 1-3 were compared, and the results are shown in Table 1. It can be found that the breathable membranes provided in Examples 1-11 have high breathability, leak-proof properties, balanced mechanical properties, and lightweight characteristics. Compared with the breathable membranes provided in Examples 1-11, in the process of preparing the breathable membranes in Comparative Examples 1 and 2, the membrane of the middle layer was not heated enough, which led to membrane breakage during the transverse stretching process, resulting in the inability to produce breathable membrane products. In the process of preparing the breathable membrane in Comparative Example 3, the membrane adhered to the stretching roller, resulting in the inability to produce breathable membrane products.

[0068] Compared to Example 1, the breathable membrane in Comparative Example 1 was heated in an oven during the transverse stretching step for 32 seconds; the breathable membrane in Comparative Example 2 was heated in an oven during the transverse stretching step for 64 seconds. The test results show that Comparative Example 1 could not produce a breathable membrane product; in Comparative Example 2, even extending the heat treatment time in the transverse stretching step did not result in a breathable membrane product. This is because, during the transverse stretching heat treatment, hot air in the oven simultaneously heats both the upper and lower surfaces of the composite longitudinally stretched membrane. At this time, the membrane material in the middle layer of the composite longitudinally stretched membrane is blocked by the surface membrane material, preventing the hot air energy from being fully transferred to the interior of the composite longitudinally stretched membrane. During the transverse stretching process, the membrane material breaks due to insufficient toughness of the middle layer, making it impossible to produce a breathable membrane product. Even extending the heat treatment time in the transverse stretching step does not prevent the production of a breathable membrane product.

[0069] In Comparative Example 3, the heat treatment temperature during the longitudinal stretching step of the breathable membrane preparation process was higher than the melting point of the cast film, making it impossible to produce a breathable membrane product. The reason is that the heat treatment temperature during the longitudinal stretching step was higher than the melting point of the cast film, causing the cast film to transform from a solid to a molten state and adhere to the stretching roller. This prevented the formation of a porous structure during longitudinal stretching and caused it to rupture, thus preventing the production of a breathable membrane product.

[0070] The difference between the breathable membranes provided in Examples 1 and 4-7 lies in the difference between the minimum temperature during heat treatment and the melting point of the cast film in the transverse stretching step. Test results show that as the difference between the minimum temperature during heat treatment and the melting point of the cast film increases, the water vapor transmission rate of the breathable membrane also increases, as do its longitudinal and transverse tensile strengths. In the examples provided above, considering the thickness uniformity of the breathable membrane, the breathable membranes provided in Examples 1, 4, and 5 exhibit better thickness uniformity than those in Examples 6 and 7. In Examples 1, 4, and 5, during the preparation of the breathable membrane, the difference between the minimum temperature during heat treatment in the transverse stretching step and the melting point of the cast film, ΔT1, is further limited to -35℃ ≤ ΔT1 ≤ -5℃. This ensures that the composite longitudinally stretched membrane is in a suitable thermoplastic state during transverse stretching, allowing it to soften fully and stretch uniformly. This not only promotes the orderly transverse expansion of longitudinal micropores and facilitates the control of micropore size and distribution, resulting in a nanoscale microporous structure with uniform size and distribution, but also enables the breathable membrane to achieve excellent thickness uniformity. Among the examples provided above, the breathable membrane provided in Example 1 exhibits the best overall performance, combining high breathability, leak-proof properties, balanced mechanical properties, lightweight characteristics, and excellent thickness uniformity.

[0071] The difference between the breathable membranes provided in Examples 1, 8, and 9 lies in the temperature difference between the end point of the preheating section and the stretching section in the transverse stretching step. Comparing the test results of the breathable membranes provided in Examples 1, 8, and 9, it can be found that as the temperature difference between the end point of the preheating section and the stretching section in the transverse stretching step increases, the water vapor permeability of the breathable membrane also increases. In the examples provided above, the breathable membrane provided in Example 1 has better thickness uniformity than that in Examples 8 and 9. In the process of preparing the breathable membrane in Example 1, by controlling the temperature difference ΔT2 between the end point of the preheating section and the stretching section to satisfy 15℃≤ΔT2≤50℃, the extensibility of the membrane material can be effectively maintained, and the breathability and thickness uniformity of the membrane can be improved. The difference between the breathable membranes provided in Examples 1, 10, and 11 lies in the temperature difference between the shaping section and the stretching section in the transverse stretching step. In the breathable membranes provided in Examples 1, 10, and 11, the breathable membrane provided in Example 1 exhibits better thickness uniformity than those in Examples 10 and 11. In the preparation of the breathable membrane in Example 1, by controlling the temperature difference ΔT3 between the shaping section and the stretching section to satisfy -5℃≤ΔT3≤50℃, the microporous structure formed in the stretching section can be shaped and fixed, further precisely controlling the micropore size and distribution, improving the pore uniformity on the composite biaxially oriented membrane, thereby enhancing the breathability and thickness uniformity of the breathable membrane. Among the examples provided above, the breathable membrane provided in Example 1 has the best overall performance, possessing high breathability, leak-proof properties, balanced mechanical properties, lightweight characteristics, and excellent thickness uniformity.

[0072] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention, but such modifications or substitutions are all within the scope of protection of the present invention.

Claims

1. A method for preparing a breathable membrane, characterized in that, include: Longitudinal stretching step; Transverse stretching step; In the longitudinal stretching step, the composite cast film is heat-treated and longitudinally stretched to obtain a composite longitudinally stretched film. The composite cast film includes multiple cast films stacked together. In this step, the heat treatment temperature is not higher than the melting point of the cast film. In the transverse stretching step, the composite longitudinally stretched film is heat-treated by infrared irradiation and then transversely stretched to obtain a composite biaxially stretched film.

2. The preparation method according to claim 1, characterized in that, In the transverse stretching step, the difference between the lowest temperature in the heat treatment and the melting point of the cast film is denoted as ΔT1, where ΔT1 satisfies -35℃≤ΔT1≤-5℃.

3. The preparation method according to claim 1, characterized in that, The transverse stretching step includes a preheating section, a stretching section, and a shaping section. The difference between the end temperature of the preheating section and the temperature of the stretching section is denoted as △T2, and △T2 satisfies 15℃≤△T2≤50℃; The temperature difference between the shaping section and the stretching section is denoted as △T3, and △T3 satisfies -5℃≤△T3≤50℃.

4. The preparation method according to claim 1, characterized in that, In the transverse stretching step, the heat treatment time is 60 to 100 seconds.

5. The preparation method according to claim 1, characterized in that, The stretching speed ratio in the longitudinal stretching step is denoted as λ1, where λ1 satisfies 1 < λ1 ≤ 2.

5.

6. The preparation method according to claim 1, characterized in that, The stretching speed ratio in the transverse stretching step is denoted as λ2, where λ2 satisfies 1 < λ2 ≤ 2.

5.

7. The preparation method according to claim 1, characterized in that, Before the longitudinal stretching step, a composite step is also included, which includes: stacking multiple layers of the cast film to obtain a composite cast film, and heat treating the composite cast film; the difference between the melting point of the composite cast film and the highest temperature of the heat treatment in the composite step is ΔT4, and ΔT4 satisfies 5℃≤ΔT4≤15℃.

8. The preparation method according to claim 7, characterized in that, In the composite step, the heat treatment includes a first preheating treatment and a first thermal composite treatment, wherein the temperature of the first preheating treatment is not higher than the temperature of the first thermal composite treatment.

9. The preparation method according to claim 1, characterized in that, After the transverse stretching step, a heat treatment step is also included. The difference between the melting point of the composite biaxially oriented film and the highest temperature of the heat treatment is denoted as △T5, and △T5 satisfies 5℃≤△T5≤15℃.

10. The preparation method according to claim 9, characterized in that, The heat treatment steps include a second preheating treatment and a second heat-combining treatment, wherein the temperature of the second preheating treatment is not higher than the temperature of the second heat-combining treatment.

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