Preparation method of microporous coating

Abstract

The invention provides a preparation method of a microporous coating, and relates to the technical field of coatings. According to the invention, the fabric is coated with the polyurethane emulsion prepared by adding the emulsifier, and the coating with a microporous structure is obtained by adopting a method of step-type temperature rise and step-type temperature range control, so that the fabric is good in air and moisture permeability and good in waterproof performance.

Description

Technical Field

[0001] This invention belongs to the field of coating technology and relates to a method for preparing a microporous coating. Background Technology

[0002] A microporous polyurethane coating is applied to the surface of the fabric. This coating allows the fabric to retain the flexibility, hydrophobicity, and weather resistance of polyurethane while achieving breathability, moisture wicking, and quick-drying properties, making it widely used in outdoor sportswear. The key is how to prepare this microporous polyurethane coating. Existing technology 1—Chinese Patent CN119409931A—discloses a water-based polyurethane emulsion containing both water and methyl ethyl ketone (MEK). During the drying process to form a polyurethane film, the large difference in polarity between water and MEK leads to significant differences in volatility. When MEK evaporates from the polyurethane phase, it leaves micropores, thus forming a microporous polyurethane coating. Existing technology 2—Chinese Patent CN119824692A—discloses a microporous breathable fabric that utilizes silica sol-modified carboxymethyl cellulose and organosilicon polyurethane for chemical cross-linking. It leverages the moisture absorption capacity of modified carboxymethyl cellulose and the voids at the composite interface between the carboxymethyl cellulose and polyurethane molecular chains to obtain a microporous coated fabric with good waterproof, breathable, and moisture-wicking properties. However, existing technology 1 requires a relatively large amount of MEK, and MEK evaporation can cause environmental pollution. The preparation process of the aforementioned prior art 2 is relatively complex.

[0003] Therefore, the applicant believes that the existing methods for preparing microporous polyurethane coatings need to be improved. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for preparing a microporous coating.

[0005] The technical solution of the present invention is as follows: A method for preparing a microporous coating involves coating a polyurethane emulsion onto the surface of a substrate, and drying it sequentially at a first temperature, a second temperature, and a third temperature according to the direction of travel of the substrate to obtain the microporous coating. The polyurethane emulsion is obtained by emulsifying liquid polyurethane with an external emulsifier; The liquid polyurethane is isocyanate-terminated; The first temperature is 70-85℃, the second temperature is 95-100℃, and the third temperature is not lower than 120℃.

[0006] Preferably, the wind speed at the first temperature is 2-2.5 m / s; The wind speed at the second and third temperatures is 2.5-3 m / s each.

[0007] Preferably, the third temperature includes a 3-1 temperature segment and a 3-2 temperature segment in sequence according to the travel direction of the substrate; The temperature of the 3-1 temperature range is 120-130℃, and the temperature of the 3-2 temperature range is 140-155℃.

[0008] Preferably, the coating density of the polyurethane emulsion is ag / m³. 2 If the traveling speed of the substrate is bm / min, then 125≤a+2b≤150.

[0009] Preferably, the concentration of the polyurethane emulsion is 10-70 wt%.

[0010] Preferably, the liquid polyurethane has a 100% concentration and a viscosity of 1000-100000 mPa·s at 25°C.

[0011] Preferably, the emulsifier is selected from one or a combination of two or more of polyglycerol fatty acid esters, fatty alcohol polyoxyethylene ethers, Tween emulsifiers, Span emulsifiers, sodium dodecylbenzene sulfonate, sodium dodecyl sulfate, fatty acid polyethylene glycol esters, alkylphenol polyoxyethylene ethers, and sodium lauryl polyoxyethylene ether sulfate. The concentration of the emulsifier in the polyurethane emulsion is 0.5-3%.

[0012] Preferably, the steps of the method for preparing the polyurethane emulsion include: S1. The emulsifier is added to water to prepare an emulsifier solution; S2, 40-70% by weight of the emulsifier solution described in step S1 is mixed with the liquid polyurethane and then stirred and dispersed to obtain an O / W crude emulsion; S3. The remaining emulsifier solution described in step S1 is gradually added to the O / W crude emulsion described in step S2, which is stirred and dispersed at a speed of 3000-8000 rpm. After the addition is complete, stirring is continued for 10-30 minutes, and then the stirring and dispersion is increased to at least 12000 rpm. Stirring and dispersion is continued for at least 30 minutes. The mixture is then allowed to stand and cool to remove bubbles and obtain the polyurethane emulsion.

[0013] More preferably, the concentration of the emulsifier solution in step S1 is 1-5 wt%; The stirring and dispersing speed in step S2 is 600-1200 rpm.

[0014] More preferably, the gradual addition in step S3 refers to adding the remaining emulsifier solution from step S1 to the O / W crude emulsion by spraying.

[0015] The beneficial effects of this invention are: (1) The present invention uses polyurethane emulsion prepared by adding an emulsifier, and heats it in a stepwise manner from low temperature (<100℃) to high temperature (>100℃) to obtain a polyurethane coating with obvious microporous structure, which makes the fabric have good air and moisture permeability.

[0016] (2) The polyurethane emulsion of the present invention adopts a phase inversion emulsification method combined with a two-stage particle size refinement method, which reduces the particle size of the polyurethane emulsion and extends the working time of the polyurethane emulsion, making it suitable for production use. Attached Figure Description

[0017] Figure 1 This is a SEM image of the micropores in the microporous coating obtained in Example 1. Detailed Implementation

[0018] The technical solution of the present invention will be further explained and described below through specific embodiments.

[0019] This invention proposes a method for preparing a microporous coating, which involves coating a polyurethane emulsion onto the surface of a substrate and drying it sequentially at a first temperature, a second temperature, and a third temperature according to the direction of travel of the substrate to obtain the microporous coating. Polyurethane emulsions are obtained by emulsifying liquid polyurethane with an external emulsifier; Liquid polyurethane is isocyanate-terminated; The first temperature is 70-85℃, the second temperature is 95-100℃, and the third temperature is not lower than 120℃.

[0020] In existing technologies, waterborne polyurethane dispersions prepared by self-emulsification are generally used to prepare microporous polyurethane structures. This invention utilizes a polyurethane emulsion prepared with an external emulsifier and a stepped heating method to directly obtain a microporous coating with a relatively uniform pore size, where the micropore diameter can be 0.5-5 μm. At the first temperature of 70-85℃ and the second temperature of 95-100℃, water gradually evaporates. During the fusion process, polyurethane molecules gradually react with active groups such as hydroxyl, amino, and carboxyl groups on the fabric surface to form a film, and also encapsulate a small amount of highly dispersed water molecules (similar to nano-sized water particles dispersed in the initially cured coating). At the third temperature (≥120℃), which is significantly higher than the boiling point of water, the water molecules encapsulated by the polyurethane vaporize, expand, and evaporate upon heating, thereby forming a large number of relatively uniform microporous structures in the polyurethane coating, resulting in a microporous coating with moderate moisture permeability and waterproofness. Therefore, the preparation method of the polyurethane microporous coating of this invention does not require organic solvents or complex preparation methods.

[0021] The substrate of this invention is not particularly limited and can be a fabric commonly used in the clothing industry, such as cotton fabric, linen fabric, cotton-polyester blended fabric, cotton-ammonia blended fabric, cotton-linen blended fabric, etc.

[0022] In some embodiments, the wind speed at the first temperature is 2-2.5 m / s; The wind speeds at the second and third temperatures are 2.5-3 m / s each.

[0023] Appropriate wind speeds can improve the uniformity of coating heating, and ensure a suitable rate of moisture absorption. For example, the wind speed at the first temperature can be 2 m / s, 2.2 m / s, 2.5 m / s, etc.; the wind speeds at the second and third temperatures can be 2.5 m / s, 2.7 m / s, 3 m / s, etc., respectively. There are no particular restrictions on the residence time of the substrate at the first, second, and third temperatures, as long as sufficient heating time is allowed for polyurethane film formation and / or moisture evaporation. For example, the residence time at the first temperature can be 0.5-5 min, the residence time at the second temperature can be 0.5-5 min, and the residence time at the third temperature can be 0.5-8 min. In this invention, residence time refers to the heating time corresponding to each temperature. For example, a residence time of 0.5-5 min at the first temperature means that the heating time at the first temperature is 0.5-5 min, including the substrate passing through the heating device in a continuous conveying manner. For example, if the length of the heating device is 20 m, and the substrate coated with polyurethane emulsion passes through the heating device at a speed of 5 m / min, then the residence time (i.e., the heating time) is 4 min.

[0024] In some embodiments, the third temperature includes a third-1 temperature segment and a third-2 temperature segment in sequence according to the travel direction of the substrate; The temperature range for temperature segment 3-1 is 120-130℃, and the temperature range for temperature segment 3-2 is 140-155℃.

[0025] The third temperature range uses a stepped heating method, which is beneficial for both the evaporation of moisture to form micropores and for the complete evaporation of moisture, resulting in a better performance microporous coating. The residence time for the 3-1 temperature range can be 0.5-5 minutes, and the residence time for the 3-2 temperature range can also be 0.5-5 minutes.

[0026] In some embodiments, the coating density of the polyurethane emulsion is set to ag / m³. 2 If the traveling speed of the substrate is bm / min, then 125≤a+2b≤150.

[0027] Different substrates have different densities (basis weight), resulting in different coating amounts (coating density) for the polyurethane emulsion. Generally, the higher the substrate basis weight, the greater the coating amount and the thicker the coating. This necessitates a slower substrate travel speed to promote the formation of a more uniform microporous structure and facilitate moisture evaporation. This invention has found that when the coating density and travel speed satisfy the relationship 125≤a+2b≤150, the polyurethane coating exhibits better pore-forming and drying effects. For example, a substrate basis weight of 20-40 g / m³... 2 The amount of adhesive applied can be 20-30g / m². 2 The substrate traveling speed can be 60 m / min; the substrate basis weight is 40-60 g / m. 2 The amount of adhesive applied can be 30-45g / m². 2 The substrate traveling speed can be 50m / min; the substrate basis weight is 60-80g / m³. 2 The amount of adhesive applied can be 45-55g / m². 2 The substrate traveling speed can be 45m / min; the substrate basis weight is 80-100g / m³. 2 The amount of adhesive applied can be 55-65g / m². 2 The substrate traveling speed can be 35 m / min; the substrate basis weight is 100-120 g / m³. 2 The amount of adhesive applied can be 65-75g / m². 2 The substrate traveling speed can be 30m / min; the substrate weight is 120-140g / m³. 2 The amount of adhesive applied can be 75-90g / m². 2 The travel speed of the substrate can be 25m / min.

[0028] For example, the value of a+2b can be any value among 125, 130, 135, 140, 145, 150, etc., or any value in between, without any particular restrictions.

[0029] In some embodiments, the concentration of the polyurethane emulsion is 10-70 wt%. For example, the concentration of the polyurethane emulsion can be any value or any value between 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, etc., without particular limitation. Further, the concentration of the polyurethane emulsion can be 30-60 wt%.

[0030] In some embodiments, the liquid polyurethane is 100% concentration and has a viscosity of 1000-100000 mPa·s at 25°C. In this invention, liquid polyurethane refers to pure polyurethane, without solvents and / or diluents. The viscosity of the polyurethane (at 25°C) can be any value or any value between 1000 mPa·s, 5000 mPa·s, 10000 mPa·s, 20000 mPa·s, 30000 mPa·s, 40000 mPa·s, 50000 mPa·s, 60000 mPa·s, 70000 mPa·s, 80000 mPa·s, 90000 mPa·s, and 100000 mPa·s, without particular limitation. Further, the viscosity of the liquid polyurethane at 25°C can be 10000-30000 mPa·s. When the viscosity of liquid polyurethane is high, such as 50,000 mPa·s or higher, it can be appropriately heated to lower the temperature, such as heating to 50-60℃.

[0031] In some embodiments, the emulsifier is selected from one or a combination of two or more of polyglycerol fatty acid esters, fatty alcohol polyoxyethylene ethers, Tween emulsifiers, Span emulsifiers, sodium dodecylbenzene sulfonate, sodium dodecyl sulfate, fatty acid polyethylene glycol esters, alkylphenol polyoxyethylene ethers, and sodium lauryl polyoxyethylene ether sulfate. The concentration of emulsifier in the polyurethane emulsion is 0.5-3%. The external emulsifiers used in this invention are mainly nonionic and / or anionic emulsifiers, which can be used alone or in combination, such as a combination of nonionic and nonionic emulsifiers, or a combination of nonionic and anionic emulsifiers.

[0032] In some embodiments, the steps of the method for preparing a polyurethane emulsion include: S1. Add the emulsifier to water to prepare an emulsifier solution; S2, 40-70% by weight of the emulsifier solution from step S1 is mixed with liquid polyurethane and stirred to disperse, thus obtaining an O / W crude emulsion; S3. The remaining emulsifier solution from step S1 is gradually added to the O / W crude emulsion from step S2, which is stirred and dispersed at a speed of 3000-8000 rpm. After the addition is complete, stirring is continued for 10-30 minutes. Then, the stirring and dispersion is increased to at least 12000 rpm and stirred and dispersed for at least 30 minutes. The mixture is then allowed to stand and cool to remove bubbles and obtain a polyurethane emulsion.

[0033] To obtain a polyurethane emulsion with good stability, this invention employs a reverse emulsification method combined with a two-stage particle size reduction method. The first particle size reduction after the reverse emulsification involves adding the remaining emulsifier solution and high-speed stirring and dispersion (3000-8000 rpm), followed by ultra-high-speed stirring and dispersion (not less than 12000 rpm, e.g., 12000-20000 rpm). This allows the obtained polyurethane emulsion to achieve a stability time of 72-96 hours at room temperature, meeting the requirements for on-site production. Furthermore, the obtained polyurethane emulsion has a milky white, semi-transparent appearance and a low particle size, which facilitates a more uniform and highly dispersed polyurethane film layer to encapsulate moisture during heating and drying, resulting in a finer microporous structure. Conversely, if the polyurethane emulsion has a large particle size, the dispersion of moisture encapsulation is insufficient (similar to the larger water particles in a partially cured polyurethane coating), resulting in larger micropores. While this provides good air and moisture permeability, it leads to poor waterproofing, failing to meet the requirements of both breathability and waterproofing.

[0034] In some embodiments, the concentration of the emulsifier solution in step S1 is 1-5 wt%; for example, the concentration of the emulsifier solution can be any value or any value between 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, 5 wt%, etc., without any particular limitation. In step S2, the stirring speed is 600-1200 rpm.

[0035] In some embodiments, step S3 involves gradually adding the remaining emulsifier solution from step S1 into the O / W crude emulsion via spraying.

[0036] The remaining emulsifier solution is added to the O / W crude emulsion via spraying. This increases the contact area between the emulsifier solution and the O / W crude emulsion, thereby improving the dispersion effect of the emulsifier solution on the O / W crude emulsion, promoting particle size reduction, and enhancing the stability of the polyurethane emulsion. For spraying, a nozzle can be used for application.

[0037] The technical solution of the present invention will be further described and explained below with reference to various preparation examples and embodiments. Unless otherwise specified, the parts mentioned in the following preparation examples and embodiments are parts by weight.

[0038] Preparation Example 1 Polyglycerol fatty acid esters are added to water to prepare an emulsifier solution with a concentration of 3 wt%. 50 parts of emulsifier solution were gradually added to 100 parts of polyurethane (viscosity 10000 mPa.s at 25℃, NCO group-terminated) and stirred and dispersed at 800 rpm. After the addition was complete, stirring and dispersion were continued for 15 min to obtain O / W crude emulsion. The stirring speed of the above O / W crude emulsion was adjusted to 5000 rpm, and 30 parts of the above emulsifier solution were added dropwise. After the addition was complete, stirring was continued for 20 minutes, then the stirring speed was increased to 12000 rpm, and stirring was continued for 40 minutes. After standing and cooling to room temperature, vacuum degassing was performed to obtain a milky white and translucent polyurethane emulsion. The time it took for the polyurethane emulsion to remain stable at 20℃ and 40% humidity was measured to be 72 hours.

[0039] Preparation Example 2 The difference between this preparation example and Preparation Example 1 is that in Preparation Example 1, the 30 parts of emulsifier solution were added by spraying through a nozzle instead of dripping. All other steps remained unchanged. A milky white, semi-transparent polyurethane emulsion was obtained and remained stable for 96 hours at 20°C and 40% humidity.

[0040] Preparation Example 3 Polyglycerol fatty acid esters are added to water to prepare an emulsifier solution with a concentration of 1 wt%. 60 parts of emulsifier solution were gradually added to 100 parts of polyurethane (viscosity 20000 mPa.s at 25℃, NCO group-terminated) and stirred and dispersed at 900 rpm. After the addition was completed, stirring and dispersion continued for 15 min to obtain O / W crude emulsion. The stirring speed of the above O / W crude emulsion was adjusted to 6000 rpm. 40 parts of the above emulsifier solution were added to the crude emulsion by spraying. After spraying, stirring continued for 20 minutes, then the stirring speed was increased to 16000 rpm, and stirring continued for 45 minutes. The mixture was allowed to stand and cool to room temperature, and then vacuum degassed to obtain a milky white, semi-transparent polyurethane emulsion. The time it took for the polyurethane emulsion to remain stable at 20℃ and 40% humidity was measured to be 80 hours.

[0041] Comparative Preparation Example 1 The stirring speed of the same weight of O / W crude emulsion from Preparation Example 1 was adjusted to 5000 rpm. 30 parts of the emulsifier solution from Preparation Example 1 were added dropwise, and stirring continued for 40 minutes after the addition was complete. The mixture was then allowed to cool to room temperature and degassed under vacuum to obtain a milky white, opaque polyurethane emulsion. The time it took for the polyurethane emulsion to stabilize at 20°C and 40% humidity was measured to be 24 hours. Judging from the appearance of the polyurethane emulsion (milky white and opaque), the particle size of the polyurethane emulsion in this comparative preparation example 1 was coarser than that in Preparation Example 1.

[0042] Therefore, this comparative preparation example omits the second ultra-high-speed stirring and dispersion compared to preparation example 1, resulting in significantly worse stability.

[0043] Example 1 The polyurethane emulsion from Preparation Example 1 was coated onto the surface of a cotton-polyester blended fabric with a basis weight of 55 g / m². 2The polyurethane emulsion coating amount is 37g / m². 2 The fabric travels at a speed of 50 m / min. The fabric is dried sequentially at three temperatures in the direction of travel: a first temperature (80℃ for 2 min, wind speed 2.0 m / s), a second temperature (100℃ for 3 min, wind speed 2.5 m / s), and a third temperature (120℃ for 5 min, wind speed 2.5 m / s), to obtain a microporous coating.

[0044] SEM images of the micropores in the microporous coating obtained in this embodiment are attached. Figure 1 As shown, the pore size of most of the micropores is between 0.5 and 5 μm.

[0045] Example 2 The difference between this embodiment and Embodiment 1 is that in Embodiment 1, the third temperature is divided into temperature segment 3-1 (120℃ residence time 2min, wind speed 2.5m / s) and temperature segment 3-2 (140℃ residence time 3min, wind speed 2.5m / s). The remaining steps remain unchanged.

[0046] Comparative Example 1 The difference between this comparative example and Example 1 is that in Example 1, the polyurethane emulsion was replaced with anionic aqueous polyurethane dispersion FS-1940A at a concentration of 40 wt%. The remaining steps remained unchanged.

[0047] Comparative Example 2 The difference between this comparative example and Example 1 is that in Example 1, the heating at the first and second temperatures was omitted, while the heating at the third temperature was retained, and the dwell time at the third temperature was adjusted from 5 minutes to 10 minutes. The remaining steps remained unchanged.

[0048] Comparative Example 3 The difference between this comparative example and Example 1 is that in Example 1, the heating at the second and third temperatures was omitted, while the heating at the first temperature was retained, and the dwell time at the first temperature was adjusted from 2 minutes to 10 minutes. The remaining steps remained unchanged.

[0049] Comparative Example 4 The difference between this comparative example and Example 1 is that in Example 1, the polyurethane emulsion was replaced with the polyurethane emulsion of Comparative Preparation Example 1. All other steps remained unchanged.

[0050] Example 3 The polyurethane emulsion from Preparation Example 2 was coated onto the surface of a cotton-polyester blended fabric with a basis weight of 30 g / m². 2 The polyurethane emulsion coating amount is 25g / m². 2The fabric travels at a speed of 60 m / min. The fabric is dried sequentially in the direction of travel at the first temperature (80℃ for 3 min, wind speed 2.5 m / s), the second temperature (100℃ for 4 min, wind speed 2.5 m / s), and the third temperature (120℃ for 3 min and 150℃ for 3 min, wind speed 2.5 m / s) to obtain a microporous coating.

[0051] Example 4 The polyurethane emulsion from Preparation Example 2 was coated onto the surface of a cotton-ammonia blended fabric with a basis weight of 95 g / m². 2 The polyurethane emulsion coating amount is 60g / m². 2 The fabric travels at a speed of 35 m / min. The fabric is dried sequentially in the direction of travel at the first temperature (80℃ for 4 min, wind speed 2.0 m / s), the second temperature (100℃ for 2 min, wind speed 3.0 m / s), and the third temperature (120℃ for 2 min and 150℃ for 5 min, wind speed 3.0 m / s) to obtain a microporous coating.

[0052] Comparative Example 5 The difference between this comparative example and Example 4 is that in Example 4, the fabric travel speed was adjusted from 35 m / min to 25 m / min. The remaining steps remained unchanged.

[0053] Comparative Example 6 The difference between this comparative example and Example 4 is that in Example 4, the fabric travel speed was adjusted from 35 m / min to 55 m / min. The remaining steps remained unchanged.

[0054] Example 5 The polyurethane emulsion from Preparation Example 3 was coated onto the surface of a cotton-ammonia blended fabric with a basis weight of 125 g / m². 2 The polyurethane emulsion coating amount is 85g / m². 2 The fabric travels at a speed of 25 m / min. Following the direction of travel, the fabric is dried sequentially at three temperatures: a first temperature (75℃ for 5 min, wind speed 2.0 m / s), a second temperature (98℃ for 2 min, wind speed 3.0 m / s), and a third temperature (120℃ for 4 min, 140℃ for 1 min, and 150℃ for 2 min, wind speed 2.5 m / s), to obtain a microporous coating.

[0055] Example 6 The polyurethane emulsion from Preparation Example 3 was coated onto the surface of a cotton-linen blended fabric with a basis weight of 60 g / m². 2 The polyurethane emulsion coating amount is 45g / m². 2The fabric travels at a speed of 45 m / min. The fabric is dried sequentially in the direction of travel at three temperatures: a first temperature (80℃ for 4 min, wind speed 2.0 m / s), a second temperature (95℃ for 1 min, wind speed 2.5 m / s), and a third temperature (130℃ for 6 min, wind speed 2.5 m / s), to obtain a microporous coating.

[0056] Performance testing Moisture permeability: Tested according to JIS L1099 A1. The higher the moisture permeability, the better the air and moisture permeability.

[0057] Static water pressure resistance: Tested according to GB / T4744-2013. Better static water pressure resistance indicates better waterproofing. Because the amount of polyurethane emulsion applied in this invention is relatively low and the coating is thin, the overall static water pressure resistance is not very high.

[0058] The results are shown in Table 1 below.

[0059] Table 1

[0060] Therefore, as can be seen from the results of the above embodiments and comparative examples, the present invention uses an external emulsification method for polyurethane emulsion combined with a gradient heating method to obtain a microporous coating with good moisture permeability and waterproofness on the surface of the fabric, resulting in good overall performance.

[0061] As described above, the basic principles, main features, and advantages of the present invention have been shown and described. Those skilled in the art should understand that the present invention is not limited to the above embodiments, which are merely preferred embodiments and should not be construed as limiting the scope of the invention. All equivalent changes and modifications made in accordance with the scope of the patent and the description should still fall within the scope of the present invention. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a microporous coating, characterized in that, A polyurethane emulsion is coated onto the surface of a substrate, and then dried sequentially at a first temperature, a second temperature, and a third temperature according to the direction of travel of the substrate to obtain the microporous coating. The polyurethane emulsion is obtained by emulsifying liquid polyurethane with an external emulsifier; The liquid polyurethane is isocyanate-terminated; The first temperature is 70-85℃, the second temperature is 95-100℃, and the third temperature is not lower than 120℃.

2. The method for preparing a microporous coating according to claim 1, characterized in that, The wind speed at the first temperature is 2-2.5 m / s; The wind speed at the second and third temperatures is 2.5-3 m / s each.

3. The method for preparing a microporous coating according to claim 1, characterized in that, The third temperature, according to the direction of travel of the substrate, sequentially includes a 3-1 temperature segment and a 3-2 temperature segment; The temperature of the 3-1 temperature range is 120-130℃, and the temperature of the 3-2 temperature range is 140-155℃.

4. The method for preparing a microporous coating according to claim 1, characterized in that, a The coating density of the polyurethane emulsion is ag / m³. 2 If the traveling speed of the substrate is bm / min, then 125≤a+2b≤150.

5. The method for preparing a microporous coating according to claim 1, characterized in that, The concentration of the polyurethane emulsion is 10-70 wt%.

6. The method for preparing a microporous coating according to claim 1, characterized in that, The liquid polyurethane is 100% concentration and has a viscosity of 1000-100000 mPa·s at 25°C.

7. The method for preparing a microporous coating according to claim 1, characterized in that, The emulsifier is selected from one or a combination of two or more of the following: polyglycerol fatty acid esters, fatty alcohol polyoxyethylene ethers, Tween emulsifiers, Span emulsifiers, sodium dodecylbenzene sulfonate, sodium dodecyl sulfate, fatty acid polyethylene glycol esters, alkylphenol polyoxyethylene ethers, and sodium lauryl polyoxyethylene ether sulfate. The concentration of the emulsifier in the polyurethane emulsion is 0.5-3%.

8. The method for preparing a microporous coating according to claim 1, characterized in that, The steps of the method for preparing the polyurethane emulsion include: S1. The emulsifier is added to water to prepare an emulsifier solution; S2, 40-70% by weight of the emulsifier solution described in step S1 is mixed with the liquid polyurethane and then stirred and dispersed to obtain an O / W crude emulsion; S3. The remaining emulsifier solution described in step S1 is gradually added to the O / W crude emulsion described in step S2, which is stirred and dispersed at a speed of 3000-8000 rpm. After the addition is complete, stirring is continued for 10-30 minutes, and then the stirring and dispersion is increased to at least 12000 rpm. Stirring and dispersion is continued for at least 30 minutes. The mixture is then allowed to stand and cool to remove bubbles and obtain the polyurethane emulsion.

9. The method for preparing a microporous coating according to claim 8, characterized in that, The concentration of the emulsifier solution in step S1 is 1-5 wt%; The stirring and dispersing speed in step S2 is 600-1200 rpm.

10. The method for preparing a microporous coating according to claim 8, characterized in that, The gradual addition in step S3 refers to adding the remaining emulsifier solution from step S1 to the O / W crude emulsion by spraying.

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