Preparation method of crosslinked fluorine-free waterproof and breathable film

Abstract

The application discloses a preparation method of a crosslinked fluorine-free waterproof and breathable film. The method comprises the following steps: dispersing modified polysiloxane containing active hydrogen groups and paraffin emulsion in deionized water to form a composite dispersion; adding a water-based blocked polyisocyanate crosslinking agent to obtain a pre-reaction coating liquid; centrifugally grading the pre-reaction coating liquid, removing a large-particle unstable phase, and taking the upper uniform dispersion; and coating the upper uniform dispersion on the surface of a porous polymer film substrate and performing heat treatment to form the crosslinked fluorine-free waterproof and breathable film. The water-based process system is adopted, no organic solvent is added in the preparation process, the process is simple, and the obtained film has good waterproofness, breathability and mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of membranes, and specifically to a method for preparing a cross-linked, fluorine-free, waterproof, and breathable membrane. Background Technology

[0002] Traditional waterproof and breathable membranes, such as polytetrafluoroethylene (PTFE) microporous membranes, suffer from environmental problems due to their fluorine content and complex manufacturing processes. Most fluorine-free waterproof and breathable membranes that have emerged in recent years are electrospun membranes. While pore size can be controlled, they often struggle to balance excellent mechanical properties, environmental friendliness, and rapid fabrication. Electrospun fiber membranes, due to their high specific surface area and high porosity, are ideal substrates for preparing waterproof and breathable membranes. However, while most fiber membranes possess some hydrophobicity, it is insufficient to withstand high hydrostatic pressure. Furthermore, the fibers are primarily bonded through physical entanglement, resulting in low strength and susceptibility to damage, limiting their practical applications. Currently, a common solution is to use fluorinated compounds for hydrophobic modification, but this introduces environmental and bioaccumulation risks.

[0003] In existing technologies, silicone resins, wax emulsions, and water-based polymers are commonly used to apply fluorine-free waterproofing finishes to membrane materials or fabrics. Silicone systems have low surface energy, but the resulting coatings sometimes suffer from insufficient adhesion and poor durability. Paraffin emulsions can impart good hydrophobicity to materials, but if they form a continuous, dense layer, they can easily clog pores and reduce breathability. While polymer systems have good film-forming properties, it is often difficult to achieve an ideal balance between waterproofing and breathability.

[0004] The paper "Facile fabrication of environmentally friendly and mechanically robust transparent, waterproof, and breathable fibrous membranes" published by Zhao et al. proposes a thermosetting technology combining green solvent electrospinning and fluorine-free emulsion coating to prepare fiber membranes with high transparency, excellent waterproofness, and good breathability. While fluorine-free emulsions are used to modify the electrospun fiber membranes for waterproofing, the bonding between the coating and the fiber matrix mainly relies on physical encapsulation and limited cross-linking. Its interfacial adhesion is limited, and the coating is prone to peeling under repeated bending or friction, leading to a decrease in waterproof performance.

[0005] For composite systems using water as the dispersion medium and containing paraffin emulsions, modified polysiloxanes, and waterborne blocked polyisocyanate crosslinking agents, the differences in dispersion stability and compatibility of different components in the aqueous phase are more likely to lead to the formation of coarse aggregates and unstable phases. If directly coated onto the surface of porous membranes, this can easily cause uneven film formation and localized pore blockage. Existing technologies lack targeted solutions to this problem. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing a cross-linked, fluorine-free, waterproof, and breathable membrane. This invention uses water as the dispersion medium and employs a modified polysiloxane containing active hydrogen groups, an aqueous paraffin emulsion, and an aqueous blocked polyisocyanate crosslinking agent to construct a fully aqueous composite coating system. To address the problems of dispersion instability and coarse particle aggregation that easily occur in this fully aqueous system, this invention obtains a uniform dispersion through a combination of pre-reaction and centrifugal fractionation, which is then used for surface coating and thermosetting of a porous polymer membrane substrate. The specific steps are as follows:

[0007] Step 1: Disperse the modified polysiloxane containing active hydrogen groups and paraffin emulsion in deionized water at a mass ratio of (0.2-5):1, and stir and mix evenly under heating conditions to obtain a composite dispersion; the modified polysiloxane is an amino-modified polysiloxane or a hydroxyl-modified polysiloxane, preferably an amino-modified polysiloxane.

[0008] Step 2: Mix the composite dispersion obtained in Step 1 with the water-based blocked polyisocyanate crosslinking agent at a mass ratio of (2-10):1, and stir and mix evenly under heating conditions to obtain the pre-reaction coating liquid;

[0009] Step 3: Centrifuge the pre-reaction coating liquid obtained in Step 2 at a speed of 3000-5000 rpm for 3-10 min to remove the deposited large-particle unstable phase, and take the upper or upper middle layer of homogeneous dispersion as the coating liquid.

[0010] Step 4: Apply the coating solution obtained in Step 3 to the surface of the porous polymer membrane substrate with a coating thickness of 1-5 μm, and then heat treat it at 100-180℃ for 5-30 min to form a cross-linked fluorine-free waterproof and breathable membrane.

[0011] The heating and stirring temperature in step one is 40–80°C, and the stirring time is 10–60 min; the heating and stirring temperature in step two is 40–80°C, and the stirring time is 10–60 min.

[0012] The paraffin emulsion mentioned in step three is an aqueous paraffin emulsion with a solid content of 40%.

[0013] The substrate mentioned in step four is one of polyurethane film, polyacrylonitrile film, and polyamide film.

[0014] The beneficial effects of this invention are:

[0015] In this invention, the modified polysiloxane containing active hydrogen groups can undergo a cross-linking reaction with the deblocked isocyanate groups after heat treatment during the heat treatment process, thereby forming a cross-linked structure with high bonding strength. The waterborne blocked polyisocyanate cross-linking agent has good stability and is deblocked after high-temperature treatment. Paraffin emulsion can provide a low surface energy hydrophobic phase, which helps to improve the waterproof ability of the coating. Since there are compatibility differences between paraffin emulsion and modified polysiloxane in aqueous systems, this invention removes large unstable phase particles in the system by combining pre-reaction and centrifugal fractionation, and then coats the uniform dispersion onto the surface of the porous membrane, thereby reducing the risk of pore blockage and improving the uniformity of coating distribution.

[0016] It exhibits high water resistance and mechanical strength. During heat treatment, the water-based blocked polyisocyanate crosslinking agent, after being deblocked, can react with the active hydrogen groups in the modified polysiloxane, enhancing the bonding between the coating and its substrate. This improves the coating's resistance to friction, bending, and washing.

[0017] It boasts excellent air permeability and durability. High-speed centrifugation removes unstable large particles from the coating solution caused by compatibility differences, helping to reduce coating unevenness and localized pore blockage. This results in a uniform, micro-rough structure on the fiber surface rather than dense blockage, ensuring effective passage of air and water vapor. The robust chemical cross-linking network makes the coating water-resistant, abrasion-resistant, and has a long service life.

[0018] The overall performance is well-balanced. This invention does not use a single organosilicon modification system, but instead uses modified polysiloxane, paraffin emulsion, and water-based blocked polyisocyanate crosslinking agent to construct a composite fluorine-free coating. Through the synergistic effect of each component, the waterproofness, mechanical strength, and durability of the membrane material are improved.

[0019] This invention employs a fully aqueous process system. Water is the primary dispersion medium, paraffin emulsion is an aqueous emulsion, and the crosslinking agent is an aqueous blocked polyisocyanate crosslinking agent. No organic solvents are required during preparation, resulting in advantages such as good environmental friendliness and high process safety. Furthermore, addressing the issue of large aggregates easily generated in fully aqueous composite systems, centrifugal fractionation improves the uniformity of the coating liquid, helping to maintain the membrane's permeable structure. It is environmentally friendly, fluorine-free, simple to implement, and has a short preparation cycle. Compared to complex processes such as in-situ polymerization, this invention uses a blending-centrifugation-coating-heat treatment process, which is simple to operate and prepare, suitable for continuous industrial production, and resolves the contradiction between the performance and process requirements of traditional fluorine-free coatings. Attached Figure Description

[0020] Figure 1 This is a SEM image of the waterproof and breathable membrane prepared according to Example 1.

[0021] Figure 2 This is a pore size distribution diagram of the waterproof and breathable membrane prepared according to Example 1. Detailed Implementation

[0022] The following detailed description, with reference to specific embodiments, provides a further detailed explanation of the preparation method of a waterproof and breathable membrane provided by the present invention. It should be understood that the specific examples are for illustrative purposes only and do not limit the scope of application of the present invention. Any modifications and variations made to the present invention that do not depart from the purpose and scope of the present invention fall within the protection scope of the present invention.

[0023] Example 1:

[0024] Step 1: Disperse 2g of amino-modified polysiloxane and 10g of paraffin emulsion in 88g of deionized water, and stir at 50℃ for 60 min to obtain a composite dispersion.

[0025] Step 2: Mix the composite dispersion with 1.5g of water-based blocked polyisocyanate crosslinking agent and stir at 60℃ for 30 min to obtain the pre-reaction coating liquid;

[0026] Step 3: Centrifuge the pre-reaction coating liquid at 4000 rpm for 5 min, discard the large unstable particles deposited at the bottom, and take the upper homogeneous dispersion and let it stand for later use.

[0027] Step 4: Coat the obtained dispersion onto the surface of the polyurethane fiber membrane with a coating thickness of 2 μm, and then heat-treat it in an oven at 150℃ for 15 min to form a cross-linked fluorine-free waterproof and breathable membrane.

[0028] The waterproof and breathable membrane fabricated using the process described in Example 1 was evaluated for its hydrostatic resistance using the hydrostatic pressure method (GB / T 4744-2013 - Textiles - Test and Evaluation of Waterproof Performance - Hydrostatic Pressure Method). The hydrostatic pressure resistance was 48 kPa. Simultaneously, the air permeability was measured to be 3200 ml per minute at an air pressure of 7 kPa and an area of ​​1 square centimeter. The mechanical properties of the membrane material were evaluated using the tensile properties method (GB / T 3923.1-2013 - Textiles - Tensile Properties of Fabrics), and the stress of the waterproof and breathable membrane was 10.28 MPa.

[0029] Example 2:

[0030] Step 1: Disperse 5 g of amino-modified polysiloxane and 5 g of paraffin emulsion in 90 g of deionized water and stir at 60°C for 40 min to obtain a composite dispersion.

[0031] Step 2: Mix the composite dispersion with 1 g of aqueous blocked polyisocyanate crosslinking agent and stir at 60°C for 30 min to obtain a pre-reaction coating liquid;

[0032] Step 3: Centrifuge the pre-reaction coating liquid at 5000 rpm for 3 min, discard the large unstable particles deposited at the bottom, and take the upper homogeneous dispersion and let it stand for later use.

[0033] Step 4: Coat the obtained dispersion onto the surface of the polyacrylonitrile fiber membrane with a coating thickness of 3 μm, and then heat-treat it in an oven at 130℃ for 20 min to form a cross-linked fluorine-free waterproof and breathable membrane.

[0034] The waterproof and breathable membrane fabricated using the process described in Example 2 was evaluated for its hydrostatic resistance using the hydrostatic pressure method (GB / T 4744-2013 - Textiles - Test and Evaluation of Waterproof Performance - Hydrostatic Pressure Method). The hydrostatic pressure resistance was 61 kPa. Simultaneously, the air permeability was measured to be 2300 ml per minute at an air pressure of 7 kPa and an area of ​​1 square centimeter. The mechanical properties of the membrane material were evaluated using the tensile properties method (GB / T 3923.1-2013 - Textiles - Tensile Properties of Fabrics). The stress of the waterproof and breathable membrane was 12.71 MPa.

[0035] Example 3:

[0036] Step 1: Disperse 1 g of amino-modified polysiloxane and 15 g of paraffin emulsion in 84 g of deionized water and stir at 80°C for 30 min to obtain a composite dispersion.

[0037] Step 2: Mix the composite dispersion with 3 g of aqueous blocked polyisocyanate crosslinking agent and stir at 60°C for 30 min to obtain a pre-reaction coating liquid;

[0038] Step 3: Centrifuge the pre-reaction coating liquid at 3000 rpm for 10 min, discard the large unstable particles deposited at the bottom, and take the upper uniform dispersion and let it stand for later use.

[0039] Step 4: Coat the obtained dispersion onto the surface of the polyamide fiber membrane with a coating thickness of 5 μm, and then heat-treat it in an oven at 180℃ for 5 min to form a cross-linked fluorine-free waterproof and breathable membrane.

[0040] The waterproof and breathable membrane fabricated using the process described in Example 3 was evaluated for its hydrostatic resistance using the hydrostatic pressure method (GB / T 4744-2013 - Textiles - Test and Evaluation of Waterproof Performance - Hydrostatic Pressure Method). The hydrostatic pressure resistance was 72 kPa. Simultaneously, the air permeability was measured to be 1500 ml per minute at an air pressure of 7 kPa, covering an area of ​​1 square centimeter. The mechanical properties of the membrane material were evaluated using the tensile properties method (GB / T 3923.1-2013 - Textiles - Tensile Properties of Fabrics). The stress of the waterproof and breathable membrane was 20.6 MPa.

[0041] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and improvements 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 protection scope of the present invention.

Claims

1. A method for preparing a cross-linked, fluorine-free, waterproof, and breathable membrane, characterized by the following steps: S1. The modified polysiloxane containing active hydrogen groups and paraffin emulsion are dispersed in deionized water and heated and stirred to obtain a composite dispersion, wherein the mass ratio of the modified polysiloxane to the paraffin emulsion is (0.2~5):

1. S2. Add an aqueous blocked polyisocyanate crosslinking agent to the composite dispersion obtained in step S1, and heat and stir to obtain a pre-reaction coating liquid, wherein the mass ratio of the composite dispersion to the aqueous blocked polyisocyanate crosslinking agent is (2-10):1; S3. Centrifuge the pre-reaction coating liquid obtained in step S2 at a speed of 3000-5000 rpm for 3-10 min to remove the deposited large-particle unstable phase, and take the upper uniform dispersion as the coating liquid. S4. The coating liquid obtained in step S3 is coated onto the surface of a porous polymer membrane substrate with a coating thickness of 1 to 5 μm. Then, it is heat-treated at 100 to 180°C for 5 to 30 minutes to obtain a cross-linked fluorine-free waterproof and breathable membrane.

2. The method according to claim 1, characterized in that: The modified polysiloxane containing active hydrogen groups in S1 is an amino-modified polysiloxane or a hydroxyl-modified polysiloxane, preferably an amino-modified polysiloxane.

3. The method according to claim 1, characterized in that: In step S1, the heating and stirring temperature is 40–80°C, and the stirring time is 10–60 min; in step S2, the heating and stirring temperature is 40–80°C, and the stirring time is 10–60 min.

4. The preparation method according to claim 1, characterized in that, Both steps S1 and S2 use water as the dispersion medium, and no organic solvents are added during the preparation process.

5. The method according to claim 1, characterized in that: The S3 paraffin emulsion is an aqueous paraffin emulsion with a solid content of 40%.

6. The method according to claim 1, characterized in that: The substrate in S4 is one of polyurethane film, polyacrylonitrile film, and polyamide film.

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