Fluoride-free hydrophobic and oleophobic dustproof composite gauze and preparation method thereof

Abstract

The invention discloses a fluorine-free hydrophobic oleophobic dustproof composite gauze and a preparation method thereof, the composite gauze comprises a PET gauze substrate and a nanofiber layer formed through electrostatic spinning, and the nanofiber layer is prepared from a spinning solution composed of 10-15% of a fluorine-free polymer, 3-10% of a fluorine-free waterproof agent and 75-85% of a solvent; the preparation method comprises the following steps: dissolving the fluorine-free polymer and the fluorine-free waterproof agent in a solvent to prepare a spinning solution; forming a nanofiber layer with the diameter of 300-2000nm on a PET (Polyethylene Terephthalate) gauze by adopting an electrostatic spinning process, and controlling the spinning interval to be 16-25cm, so that fibers and a base material form a flat anchor point structure; and applying a fluoride-free hydrophobic and oleophobic agent through a padding process, and drying and curing. The prepared gauze has a micron network structure with the pore diameter of 10-20 microns, has excellent hydrophobicity, high air permeability and durability, does not contain fluorine compounds completely, and is particularly suitable for the fields of electronic product protection and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of functional materials, in particular to a fluorine-free hydrophobic and oleophobic dust-proof composite screen and a preparation method thereof. BACKGROUND

[0002] In the application field of electronic products such as mobile phones and speakers, the composite screen needs to provide certain hydrophobic and oleophobic dust-proof effect, and also needs to achieve good air permeability. Therefore, the microstructure of the composite screen plays a crucial role in product design. Most composite screens use a method of bonding waterproof and air-permeable films with screens to achieve the hydrophobic and oleophobic dust-proof effect, but this method is not only relatively complex, but also greatly reduces the air permeability of the screen. With the increasingly stringent environmental regulations and consumers' attention to health and safety, traditional fluorine-containing waterproof and oil-proof materials (such as PFOS, PFOA, etc.) are gradually being restricted or prohibited due to their persistent organic pollutant (POPs) characteristics. However, most high-performance waterproof and oil-proof textiles on the market still rely on fluorine-containing compounds, and their application in the fields of textiles, filtration, medical treatment, etc. faces major environmental challenges. Therefore, the development of fluorine-free hydrophobic and oleophobic materials has become a current research hotspot.

[0003] In the application of air filtration and dust-proof screen, the material not only needs to have waterproof and oil-proof performance, but also needs to maintain good air permeability. The traditional method usually uses coating finishing technology, that is, a waterproof and oil-proof agent is coated on the surface of the substrate. However, this kind of method has the following problems: the coating is easy to fall off: the conventional coating has weak adhesion to the substrate, and the performance decreases after long-term use or friction; poor air permeability: the dense coating will block the fiber pores, affecting the air permeability; dependent on fluorine-containing chemicals: even some "short-chain fluorinated" products still have potential environmental risks.

[0004] In recent years, electrospinning technology has shown advantages in the field of functional textiles due to its ability to produce micro-nano fiber networks. By adjusting the fiber diameter and packing density, waterproof and oil-proof functions can be achieved while maintaining high air permeability. However, the existing electrospinning waterproof and oil-proof materials still have the following problems: insufficient adhesion between fibers and substrate: nanofibers are only physically accumulated on the surface of the substrate, which is easy to peel off; insufficient strength due to too fine fibers, poor waterproof and oil-proof durability: relying only on fiber fineness to achieve hydrophobicity, lacking chemical modification, performance decay after encountering water / oil; inaccurate process control: spinning parameters (such as liquid supply speed, receiving distance) are not optimized, resulting in uneven fiber distribution or uncontrollable porosity.

[0005] Therefore, in view of the above, the present application provides a fluorine-free hydrophobic and oleophobic dust-proof composite screen and a preparation method thereof, aiming to provide an environmentally friendly, high-adhesion, air-permeable and durable fluorine-free hydrophobic and oleophobic dust-proof composite screen. SUMMARY

[0006] The application aims to provide a fluorine-free hydrophobic and oleophobic dust-proof composite screen and a preparation method thereof, so as to prepare a fluorine-free hydrophobic and oleophobic dust-proof composite screen which is environmentally friendly, has high bonding force and is breathable and durable.

[0007] The application achieves the above-mentioned purpose by the following technical solutions.

[0008] The fluorine-free hydrophobic and oleophobic dust-proof composite screen comprises a PET screen base and a nanofiber layer formed by electrospinning, wherein the nanofiber layer is made of a spinning solution containing the following components by mass percentage: 10-15% of a fluorine-free polymer, 3-10% of a fluorine-free water repellent, and 75-85% of a solvent; the nanofiber has a diameter of 300-2000 nm, the network structure formed by cross-linking of the nanofiber has a pore size of 5-20 μm, and the fiber-to-screen contact surface forms a flat anchor point structure.

[0009] Further, the network structure formed by cross-linking of the nanofiber has a pore size of 5-20 μm, so as to prevent the pore size from being too large and water droplets from easily penetrating, and prevent the pore size from being too small and the air permeability from being low.

[0010] Preferably, the fluorine-free polymer is at least one selected from thermoplastic polyurethane elastomer (TPU), polyacrylonitrile and polystyrene (PS); and the fluorine-free water repellent is an acrylate or polyurethane oil-based fluorine-free water repellent.

[0011] Preferably, the acrylate oil-based fluorine-free water repellent is one or more selected from methyl methacrylate-butyl acrylate copolymer (MMA-BA), ethyl acrylate-butyl acrylate copolymer (EA-BA), stearyl acrylate-methyl methacrylate copolymer (SA-MMA), lauryl acrylate-hydroxyethyl acrylate copolymer (LA-HEA), polydimethylsiloxane-acrylate graft copolymer, epoxy soybean oil modified acrylate, alkylphenol polyoxyethylene ether acrylate, stearyl methacrylate (SMA) and behenyl acrylate (BEA).

[0012] Preferably, the polyurethane oil-based fluorine-free water repellent is one or more selected from hexamethylene diisocyanate type polyurethane (HDI type), isophorone diisocyanate type polyurethane (IPDI type), dicyclohexyl methane diisocyanate type polyurethane (HMDI type), polydimethylsiloxane modified polyurethane, polyether modified polyurethane and castor oil modified polyurethane.

[0013] Preferably, the solvent is a mixed solvent of N'N-dimethylformamide, N'N-dimethylacetamide and ethyl acetate, and the mass percentage of the amide solvent in the mixed solvent is 70-80%.

[0014] The application also claims to protect a preparation method of the above-mentioned fluorine-free hydrophobic and oleophobic dust-proof composite screen, which comprises the following steps:

[0015] S1, Spinning solution preparation: the solvent is loaded into a stirring device, and the fluorine-free polymer and the fluorine-free water-repellent agent are dispersed in the solvent for stirring. After the solution forms a homogeneous system, it is left to stand for 3-6 h, and the bubbles in the spinning solution are discharged;

[0016] S2, Electrospinning: the spinning solution with the bubbles discharged in step S1 is added to an electrospinning device, and a PET mesh is used as a receiving substrate material. The voltage high-voltage system is turned on to control the positive voltage at 16-25 KV and the negative voltage at 1-10 KV. Nanofibers with a diameter of 0.3-2.0 pm are obtained, and the nanofiber composite mesh is collected through a take-up and pay-off assembly;

[0017] S3, Post-processing: using a dip-nip process, the nanofiber composite mesh is placed on the take-up and pay-off assembly of a dip-nip drying all-in-one machine, and an aqueous fluorine-free hydrophobic and oleophobic agent is applied for coating, followed by heating and drying. Finally, the fluorine-free hydrophobic and oleophobic dust-proof composite mesh treated by dip-nip is wound through the winding assembly.

[0018] Preferably, in step S1, the stirring time is 12-24 h, and the stirring speed is 200-800 rpm.

[0019] Preferably, in step S2, the liquid supply flow rate is controlled at 10-30 mL / h to adjust the liquid output of the nozzle at 30-60 mL / h, and the running speed of the conveying belt for collection is controlled at 10-30 mm / s to control the density of the nanofibers. The vehicle speed is controlled at 5-30 mm / s and the liquid supply flow rate is controlled at 10-30 mL / h to control the density of the nanofibers on the mesh and the pore size of the nanofiber mesh.

[0020] Preferably, in step S2, the spinning distance is controlled at 18-25 cm to make the nanofibers reach the mesh surface before being completely dried, form a fiber flat anchor point structure with the mesh surface, and enhance the adhesion of the nanofibers to the mesh surface. The control of the spinning distance is based on the formation of the flat anchor point to avoid the spinning distance being too close, causing the fibers to form a bundle. At the same time, it also avoids the spinning distance being too far, failing to form a fiber flat anchor point, resulting in insufficient adhesion.

[0021] Preferably, in step S3, the concentration of the aqueous fluorine-free hydrophobic and oleophobic agent is 15%-20%, the vehicle speed in the dip-nip process is 2-5 m / min, the roller shaft distance is adjusted to control the nip percentage at 50%-70%, and the heating and drying temperature is 130-150 °C for 3-5 min.

[0022] Preferably, the flat anchor point is 120%-200% of the diameter of the original fiber after fiber swelling. A too high fiber swelling rate can easily cause the fibers to break during the dip-nip process, damaging the fiber mesh structure.

[0023] The working mechanism of the present application: the present application provides a fluorine-free hydrophobic and oleophobic dust-proof composite screen and a preparation method thereof. The nanofiber structure compounded on the traditional screen improves the dust-proof and water drop direct penetration performance of the traditional screen. In addition, the anchor point structure formed by the nanofiber and the screen avoids the adverse phenomena such as nanofiber falling caused by the padding process. The main method is to mix polyurethane or other fluorine-free polymer materials with a specific solvent system and add an oily fluorine-free water repellent ingredient to form a spinning solution. Then, the large-scale electrospinning equipment is used to directly spin on the screen, and the nanofiber and the screen body structure form a specific anchor point, thereby forming a composite screen structure. The composite screen forms a micron-level network structure with a certain cross-linking density on the mesh of the screen to improve the dust-proof performance of the screen and prevent water drops from directly penetrating the screen. Then, the fluorine-free hydrophobic and oleophobic agent is fixed to the surface of the composite screen by padding, and finally a safe and healthy product with excellent waterproof and breathable properties and without fluorine is obtained.

[0024] Due to the use of the above technical solution, the present application has the following beneficial effects compared with the prior art:

[0025] 1. The present application adopts a completely fluorine-free formula, completely avoiding the environmental pollution risk of traditional fluorine-containing materials (such as PFOS / PFOA). The use of water-based fluorine-free hydrophobic and oleophobic agent ensures that the product meets international environmental protection standards, and there is no harmful substance emission in the production process, realizing green manufacturing.

[0026] 2. The present application has excellent functional performance. The unique nanofiber anchor point structure makes the product have super strong bonding force and excellent durability. The micron network structure (5-20 μm) controlled accurately can realize efficient dust-proof, excellent waterproof and good breathability.

[0027] 3. The present application has the advantages of process innovation. The integrated electrospinning process realizes the simplification of production process and the reduction of energy consumption. The key process parameters are innovated: the optimized spinning distance (18-25 cm) ensures the fiber anchoring effect; the precise spinning control (fiber diameter 300-2000 nm) and the intelligent padding process (padding percentage 50-70%) are realized.

[0028] 4. The product of the present application perfectly balances the protection and breathability, and is especially suitable for: high-end electronic products such as mobile phones and speakers, medical protection equipment, and precision instrument protection field. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical features, objectives and effects of the embodiments of the present application or the technical solutions in the prior art, the drawings required to be used in the description of the embodiments or the prior art will be briefly introduced. Obviously, some of the drawings described below are some embodiments of the present application, and other drawings can also be prepared according to these drawings without creative labor for those skilled in the art.

[0030] Figure 1 is a scanning electron microscope image of the composite screen of the embodiment 1 of the present application, and the anchor point structure is indicated by the white arrow, and the scale is 20 μm;

[0031] Figure 2 is a scanning electron microscope image of the composite screen of the embodiment 2 of the present application, and the scale is 20 μm;

[0032] Figure 3 is a scanning electron microscope image of the composite screen of the embodiment 1 of the present application after being coated with a fluorine-free hydrophobic and oleophobic agent by padding, and the scale is 20 μm;

[0033] Figure 4 is a contact angle diagram of the composite screen of the embodiment 1 of the present application with water and oil (diiodomethane);

[0034] Figure 5 is a contact angle diagram of the composite screen of the comparative example 1 of the present application with oil (diiodomethane) obtained by spraying process;

[0035] Figure 6 is a scanning electron microscope image of the composite screen of the comparative example 2 of the present application without forming anchor point structure, and the scale is 20 μm. DETAILED DESCRIPTION

[0036] In order to have a more clear understanding of the technical features, objectives and effects of the present application, the specific implementation schemes will be described in detail.

[0037] The present application will be further described below in combination with the embodiments, but the present application is not limited to the following embodiments. The implementation conditions used in the embodiments can be further adjusted according to different requirements of specific use, and the implementation conditions indicated are the conventional conditions in the industry. The technical features involved in each embodiment of the present application can be combined with each other as long as there is no conflict.

[0038] Embodiment 1

[0039] Referring to the accompanying drawings of Figure 1 , and Figure 3 , and Figure 4 , the present embodiment provides a preparation method of a fluorine-free hydrophobic and oleophobic dust-proof screen, which comprises, by mass percentage: thermoplastic polyurethane elastomer (TPU) 12%; fluorine-free waterproof agent 6%; solvent 82%;

[0040] The fluorine-free waterproof agent is an oily fluorine-free waterproof agent, the oily fluorine-free waterproof agent is a polyurethane-based oily fluorine-free waterproof agent, and the polyurethane-based oily fluorine-free waterproof agent is a polydimethylsiloxane modified polyurethane.

[0041] The solvent is a mixed solvent of N'N-dimethylformamide, N'N-dimethylacetamide and ethyl acetate, and the mass percentage of the amide solvent in the mixed solvent is 75%; the specific steps are as follows:

[0042] S1, spinning solution preparation: the solvent is loaded into a stirring device, TPU and fluorine-free waterproof agent are dispersed in the solvent, stirring for 18h, the stirring speed is 350rpm, the spinning solution is allowed to stand for 6h after forming a homogeneous system, and the bubbles in the spinning solution are discharged;

[0043] S2, electrospinning: the spinning solution with bubbles discharged in step S1 is added to an electrospinning device, a 300-mesh PET gauze is used as a receiving substrate material, a voltage high-voltage system is turned on to control the voltage positive pressure to be 22KV and the negative pressure to be 3KV, the liquid supply flow rate is 20ml / h, the spinning distance is 20cm, the speed is 20mm / s, and the diameter of the nanofiber obtained is 1.2μm; the density of the nanofiber on the gauze and the pore size of the nanofiber web are controlled by controlling the speed and the liquid supply flow rate; the binding force between the nanofiber and the gauze (the formation of a flat anchor point structure) is controlled by controlling the spinning distance; and the nanofiber composite gauze is collected by a winding and unwinding assembly;

[0044] S3, post-treatment: using a dip-nip process, the nanofiber composite gauze is placed on the winding and unwinding assembly of a dip-nip drying all-in-one machine, an aqueous fluorine-free hydrophobic and oleophobic agent is applied, the concentration is 18%, the speed is set to 3m / min, the roller spacing is adjusted to control the nip percentage to be 60%, heating and drying are performed, the heating and drying temperature is 140℃, the time is 4min, and finally the fluorine-free hydrophobic and oleophobic dust-proof composite gauze treated by dipping and nipping is collected by a winding assembly.

[0045] Example 2

[0046] Referring to the accompanying drawings, Figure 2 , the embodiment is based on the above-mentioned example 1, and the same parts as the above-mentioned example 1 will not be described in detail;

[0047] In this embodiment, the mass percentage includes: thermoplastic polyurethane elastomer (TPU) 9%; polystyrene (PS) 3%; fluorine-free waterproof agent 6%; solvent 82%.

[0048] Comparative Example 1

[0049] Referring to the accompanying drawings, Figure 5The present comparative example was prepared based on the above-mentioned Example 1, and the same parts as those of the above-mentioned Example 1 are not described herein.

[0050] In the present comparative example, the dust-proof composite screen was obtained by using a spraying process.

[0051] Comparative Example 2

[0052] Referring to the drawings, Figure 6 Comparative Example 2 is a composite screen structure formed by controlling the spinning distance to be 27 cm, and the present comparative example is a screen without an anchor point structure.

[0053] Comparative Example 3

[0054] The present comparative example is a commercially available pure screen without a composite.

[0055] Comparative Example 4

[0056] The present comparative example was prepared based on the above-mentioned Example 1, and the same parts as those of the above-mentioned Example 1 are not described herein.

[0057] The present comparative example is a composite screen without a dip-and-pad process.

[0058] The dust (5 μm) filtration efficiency of the composite screen prepared in Example 1 and the pure screen without a composite of Comparative Example 3 was compared, and the comparison results are shown in Table 1.

[0059] Comparative Example 5

[0060] The present comparative example is different from Example 1 in that the spinning height is controlled to be 15 to 16 cm, and an anchor point structure is obtained, and the fiber swelling rate is > 200%.

[0061] Test Example 1

[0062] The sample was cut into a long strip shape having a width of 25 mm and a length of 300 mm, and the peeling force of the nanofiber on the screen was tested by using a crown 7972G tape and a universal testing machine through a 180-degree peeling force test method.

[0063] The filtration efficiency of the composite screen prepared in Example 1 and the pure screen without a composite of Comparative Example 3 was detected, and the detection results are shown in Table 1.

[0064] Table 1

[0065]

[0066] The hydrophobic and oleophobic properties of the composite screen prepared in Example 1, the dust-proof composite screen obtained by using a spraying process of Comparative Example 1, and the pure screen without a composite of Comparative Example 3 were detected, and the detection results are shown in Table 2.

[0067] Table 2

[0068] Test item Example 1 Comparative Example 1 Comparative Example 3 Hydrophobic angle 133±2.8° 129±2.1° 126.3±3.6° Oleophobic angle 124±23.2° 103±4.3° 95±1.6°

[0069] The composite mesh prepared in Example 1, the composite mesh without padding and rolling process of Comparative Example 4, and the pure mesh without composite of Comparative Example 3 were subjected to air permeability detection, and the detection results are shown in Table 3.

[0070] Table 3

[0071] Test item Example 1 Comparative Example 4 Comparative Example 3 Air permeability (mm / s @ 125 Pa) 1689.7±32.9 1768.2±40.1 1892.8±36.2

[0072] The peeling forces of the nanofiber meshes with different anchor point structures are shown in Table 4.

[0073] Table 4

[0074] Test item Anchor structure name Peeling force (gf) Comparative Example 2 No anchor structure 47.18 Example 1 Anchor structure 1 (fiber swelling 60-70%) 206.72 Comparative Example 5 Anchor structure 2 (fiber swelling 200%) 130.64

[0075] The anchor point structure can be formed by controlling the spinning distance in Example 1 or adding PS in Example 2 (see Figure 1 and Figure 2 ); Figure 4 is the hydrophobic and oleophobic effect of the fluorine-free dust-proof mesh composite mesh of Example 1; Figure 5 is an effect diagram of the dust-proof mesh obtained by using a spraying process in Comparative Example 1, and the oil-repellent effect is slightly poor; Figure 6 is a mesh diagram without anchor points in Comparative Example 2, and the fibers are prone to falling off and peeling off. Table 3 shows that the air permeability of the composite mesh can still maintain ≥ 88% after padding treatment, compared with the pure mesh.

[0076] In summary, the present application adopts a completely fluorine-free formula, completely avoiding the environmental pollution risk of traditional fluorine-containing materials (such as PFOS / PFOA), and the use of water-based fluorine-free hydrophobic and oleophobic agent ensures that the product meets international environmental protection standards, and there is no harmful substance emission in the production process, realizing green manufacturing; the present application has excellent functional performance, and the unique nanofiber anchor point structure makes the product have super strong binding force and excellent durability; the precisely controlled micron network structure (5-20 μm) can realize efficient dust prevention, excellent waterproofness, and good air permeability; the process innovation advantage of the present application realizes the simplification of the production process and the reduction of energy consumption by using an integrated electrospinning process; the key process parameter innovation: the optimized spinning distance (18-25 cm) ensures the fiber anchoring effect; the precise spinning control (fiber diameter 300-2000 nm) and the intelligent padding process (padding percentage 50-70%); the product of the present application perfectly balances the protection and air permeability, and is particularly suitable for: high-end electronic products (mobile phones, speakers, etc.), medical protection equipment, and precision instrument protection fields.

[0077] The above embodiments only express several implementation manners of the present application, and the description is relatively specific and detailed, but should not be understood as a limitation on the patent scope of the present application. It should be noted that, for ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A fluorine-free, hydrophobic, oleophobic, and dust-proof composite mesh, characterized in that: The invention comprises a PET mesh base and a nanofiber layer formed by electrostatic spinning. The nanofiber layer is made of a spinning solution containing the following components in mass percentage: 10-15% fluorine-free polymer, 3-10% fluorine-free waterproofing agent, and 75-85% solvent. The diameter of the nanofibers is 300nm-2000nm, the pore size of the network structure formed by cross-linking the nanofibers is 5μm-20μm, and a flat anchor point structure is formed on the contact surface between the fibers and the mesh.

2. The fluorine-free hydrophobic, oleophobic and dust-proof composite mesh according to claim 1, characterized in that: The fluorine-free polymer is selected from at least one of thermoplastic polyurethane elastomer (TPU), polyacrylonitrile or polystyrene; the fluorine-free waterproofing agent is an acrylate or polyurethane oily fluorine-free waterproofing agent.

3. The fluorine-free hydrophobic, oleophobic and dust-proof composite mesh according to claim 1, characterized in that: The solvent is a mixed solvent of N'N-dimethylformamide, N'N-dimethylacetamide and ethyl acetate, and the weight proportion of the amide solvent in the mixed solvent is 70-80%.

4. A method for preparing the fluorine-free hydrophobic, oleophobic and dust-proof composite mesh according to any one of claims 1 to 3, characterized in that: The steps include: S1. Preparation of spinning solution: Add the solvent into a stirring device, disperse the fluorine-free polymer and the fluorine-free water repellent in the solvent and stir until the solution forms a homogeneous system, then let it stand for 3 to 6 hours to remove the bubbles in the spinning solution; S2, electrospinning: adding the spinning solution from step S1 with bubbles removed into an electrospinning device, using PET mesh as a receiving substrate, turning on the high-voltage system to control the positive voltage to 16-25 kV and the negative voltage to 1-10 kV; obtaining nanofibers with a diameter of 0.3-2.0 μm, and then collecting the nanofiber composite mesh through a reel assembly; S3. Post-processing: Using a dipping and rolling process, the nanofiber composite mesh is placed on the reeling and unreeling assembly of the dipping, rolling and drying machine, coated with a water-based fluorine-free hydrophobic and oleophobic agent, and then heated and dried. Finally, the dipping and rolling-treated fluorine-free hydrophobic and oleophobic dust-proof composite mesh is reeled up through the reeling assembly.

5. The method for preparing the fluorine-free hydrophobic, oleophobic and dust-proof composite mesh according to claim 4, characterized in that: In step S1, the stirring time is 12 to 24 hours, and the stirring speed is 200 to 800 rpm.

6. The method for preparing the fluorine-free hydrophobic, oleophobic and dust-proof composite mesh according to claim 4, characterized in that: In step S2, the liquid output of the nozzle is adjusted to 30mL / h~60mL / h by controlling the liquid supply flow rate to 10~30mL / h, and the running speed of the collection conveyor belt is 10mm / s~30mm / s to control the density of the nanofibers; the density of the nanofibers on the mesh and the pore size of the nanofiber mesh are controlled by controlling the vehicle speed to 5~30mm / s and the liquid supply flow rate to 10~30mL / h.

7. The method for preparing the fluorine-free hydrophobic, oleophobic and dust-proof composite mesh according to claim 4, characterized in that: In step S2, the spinning distance is controlled to be 18 to 25 cm so that the nanofibers reach the mesh surface before being completely dried, and adhere to the mesh surface to form a structure of flat fiber anchor points.

8. The method for preparing the fluorine-free hydrophobic, oleophobic and dust-proof composite mesh according to claim 4, characterized in that: In step S3, the concentration of the aqueous fluorine-free hydrophobic and oleophobic agent is 15% to 20%; the vehicle speed in the dipping and rolling process is 2 to 5 m / min; the roller spacing is adjusted in the dipping and rolling process so that the rolling allowance is controlled at 50% to 70%; the temperature of the heating and drying is 130° C. to 150° C., and the time is 3 to 5 minutes.

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