Abstract: According to the functional requirements of microfiber synthetic leather for protective shoes, the waterproof and moisture permeable mechanism of microfiber synthetic leather base fabric and dry veneer polyurethane (PU) film was discussed. The design of microfiber synthetic leather base fabric for protective shoes was designed. Structure, and select hydrophilic polyurethane resin as the surface layer resin of microfiber synthetic leather and control the micropore diameter of PU film to meet the requirements of waterproof and moisture permeability. At the same time, the application fields of microfiber synthetic leather for protective shoes are introduced.
Key words: protective shoes; microfiber; synthetic leather; moisture permeability; waterproof; labor insurance shoes; anti-static shoes; protective toe safety shoes;
Protective shoes are shoes that are worn in the workplace to protect the feet from injury. They are part of the individual labor protection products or individual soldier protection systems in developed countries in Europe and America. Recently, more and more countries require staff to Wearing protective shoes to prevent possible dangers during work, some industries or companies in China also require people to wear protective shoes when working, so the market for protective shoes will become larger and larger. With the introduction of many environmental protection laws and regulations, as well as the high cost and limited source of leather, and in some specific occasions (such as aseptic work), the dermis is prone to decay and the bacteria are difficult to meet the requirements. Therefore, people began to seek alternatives to the dermis. In recent years, protective shoes made of polyurethane microfiber nylon artificial leather (microfiber synthetic leather) have become more and more popular.
Compared with leather, the defect of microfiber synthetic leather is that it does not absorb sweat and breathe, making people uncomfortable and lame. The super-fiber synthetic leather for protective shoes is specially designed based on the moisture absorption and permeability characteristics of the leather. It has both moisture permeability and waterproof properties, that is, the shoes prevent the outside rain from penetrating the shoe leather while wearing, and at the same time The sweat inside the shoes is discharged, making people comfortable.
In this paper, the moisture-permeable and waterproof mechanism of microfiber synthetic leather base fabric was discussed, and the preparation method of moisture-permeable waterproof microfiber synthetic leather base fabric was obtained. The dry laminating process of microfiber synthetic leather was discussed. The moisture permeability and waterproof mechanism of microfiber synthetic leather polyurethane (PU) resin and super fiber synthetic leather PU film were analyzed. The preparation of microfiber synthetic leather for protective shoes was carried out. Providing a theoretical basis, it is possible to replace natural leather with microfiber synthetic leather.
2 Process design of microfiber synthetic leather for protective shoes
Synthetic leather for protective shoes must be in accordance with EN345 or ISO20345 to be circulated in foreign markets (EN345 or ISO20345 is the European and American standard for access to protective footwear). The moisture permeability index of the superfibrous synthetic leather for protective shoes requires water vapor permeation ≥0.8mg/(cm2·h) and water vapor permeability coefficient ≥1.5mg/cm2, while the waterproof index requires water absorption rate ≤30% and water wear. The permeability is ≤0.2mg. In order to meet the requirements of moisture permeability and waterproof technology, the super-fiber synthetic leather for protective shoes is designed from the two parts of the microfiber synthetic leather base fabric and the dry veneer.
2.1 Process design of microfiber synthetic leather base fabric for protective shoes
2.1.1 Waterproof and moisture permeable mechanism of microfiber synthetic leather base cloth for protective shoes
To study the moisture permeability and waterproof function of the super-fiber synthetic leather base cloth for protective shoes, the first is to analyze the mechanism of moisture permeability and waterproof of the microfiber synthetic leather base cloth. Moisture transmission is actually two processes of adsorption and transfer. First, the water vapor (ie, sweat) inside the shoe is adsorbed on the inner side of the shoe, and then transferred from the inside of the shoe to the outside of the shoe. The adsorption process requires a water vapor adsorption point on the microfiber synthetic leather base cloth. The adsorption point can be that water molecules are adsorbed by hydrogen bonds or porously adsorbed. Without adsorption points, water vapor does not adsorb, and the sweat inside the shoes cannot be used. Transfer out, so when designing the microfiber synthetic leather base fabric, the microfiber synthetic leather base fabric should have a certain hydrophilic absorption function. The nylon fiber of water and synthetic leather base fabric easily forms hydrogen bonds and has certain hydrophilicity, so the base fabric has certain hydrophilicity. In addition, the more pores of the microfiber synthetic leather base fabric, the larger the specific surface area. The easier it is to adsorb water vapor. In the water vapor penetration, if the pore diameter of the microfiber synthetic leather base fabric is large and the opening ratio is high, the water vapor transfer speed is fast and the amount of transfer is large. Fig. 1 shows the structure of a superfibrous synthetic leather base fabric for protective shoes. It can be seen that the cross section of the microfiber synthetic leather base fabric for protective shoes has a plurality of micropores which facilitate the transfer of moisture in the shoe.
At the same time, the pore diameter of the microfiber synthetic leather base fabric is generally large and uneven, and the rainwater is easily passed through. In order to meet the use of the superfibrous synthetic leather base fabric for protective shoes, it is also necessary to carry out the microfiber synthetic leather base fabric. It is processed to have a certain hydrophobic property to reduce the speed and the amount of rainwater passing through the super-fiber base fabric, so that the rainwater does not easily reach the inside of the shoe, thereby achieving the waterproof function.
In summary, in the design of the production process of the superfibrous synthetic leather base fabric for protective shoes, the superfibrous base fabric should have a certain water vapor adsorption point (ie, hydrophilic function), that is, the microfiber base fabric has a high opening ratio. And with a proper aperture, at the same time, the microfiber base fabric has a certain waterproof function, achieving both moisture permeability and waterproof.
2.1.2 Preparation of microfiber synthetic leather base cloth for protective shoes
According to the analysis of the above-mentioned moisture-permeable and waterproof mechanism, we designed a super-fiber synthetic leather base cloth for protective shoes. The specific process is as follows: polyethylene and nylon 6 are blended and stirred according to a certain weight ratio, at a temperature of 220-300 ° C, By melt-plasticizing by a single-screw extruder, the polyethylene and nylon island fibers having a fineness of 4 to 7 denier are spun, and the fibers are cut into short fibers having a length of 40 to 60 mm, which are subjected to non-woven needle punching and non-woven fabric. A superfibrous synthetic leather base fabric is obtained by flattening, impregnating a wet polyurethane slurry, reducing toluene, expanding oil and the like. The resulting three-dimensional network of microfiber synthetic leather fabrics is similar to leather, and its tear strength, shape retention, uniformity, chemical resistance, and water resistance and mildew resistance exceed natural leather.
At the same time, the superfibrous synthetic leather base fabric combined with the polyurethane slurry impregnation and reduction processing technology with porous structure forms a large number of micropores, and has a large specific surface area and strong water absorption. Therefore, from the internal microstructure and material, as well as the appearance of texture and physical properties, as well as the feel of people, the microfiber synthetic leather fabric can compete with high-grade natural leather. The process is as follows: spinning → needle-punched non-woven fabric → ironing → impregnation → reduction → expansion drying → microfiber synthetic leather base fabric using soft hydrophobic treatment agent such as silicone or fluorine-silicon → impregnation → pressing and drying A microfiber synthetic leather base fabric for protective shoes having a certain hydrophobic function is obtained.
2.2 Design of superfibre synthetic leather veneer for protective shoes
2.2.1 Waterproof and moisture permeable mechanism and design of superfibre synthetic leather veneer PU microporous film for protective shoes
In order to solve the contradiction between the moisture permeability and the waterproof of the superfibrous synthetic leather for protective shoes, it is also necessary to design the PU film structure of the dry veneer, and it is required that the microfiber synthetic leather surface also has micropores with appropriate pore diameters.
Waterproofing mainly prevents rainwater from penetrating the leather surface into the shoe. When the moisture is transmitted, the water vapor inside the shoe is transferred to the outside of the shoe. The water vapor penetrates in the micropores of the microfiber synthetic leather and passes through the gas. The diameter of the water vapor molecules is 4×10-4μm. As long as the micropore diameter of the microfiber leather surface is larger than 4×10-4μm, the water vapor can be used. It is transferred from the inside of the shoe to the outside of the shoe and has a moisture permeability function. When the rainwater passes through the micropores of the microfiber synthetic leather, the surface tension is caused when the rainwater contacts the micropores. If the pore diameter is small, the rainwater is difficult to enter the shoe through the micropores, thereby achieving the purpose of waterproofing. Studies have shown that the minimum diameter of various rain mists that can pass through the micropores is: fog is 20μm, light rain is 400-900μm, and moderate rain and heavy rain are above 2000μm. Therefore, the micropore diameter of the dry veneer PU film can be designed by utilizing the difference in the pore diameter of the water vapor and the rainwater passing through the micropores.
Therefore, in the dry veneering process, the PU film after the veneer is produced with a certain number of micropores with a certain pore diameter, and the pore diameter of the micropores is controlled to be 4×10 −4～20 μm, which allows the water vapor to penetrate freely. At the same time, it is difficult to pass rainwater to achieve the purpose of moisture permeability and waterproofing. At present, the methods for producing micropores in the synthetic leather surface include laser mechanical punching method, foaming agent micropore method, solvent volatilization and pore-forming method. As can be seen from Fig. 2, the microfiber synthetic leather mask for protective shoes has a pore size of 5 to 8 μm, which is advantageous for the penetration of water vapor molecules from the inside of the shoe to the outside of the shoe while suppressing the entry of water into the shoe.
2.2.2 Waterproof and moisture permeable mechanism and design of hydrophilic resin for superfibre synthetic leather veneer for protective shoes
The penetration of water vapor molecules into the PU mask is also the first process of adsorption and re-infiltration, so the choice of PU mask material is also very important. Xie Fuchun et al. used polyethylene glycol as a soft segment to synthesize polyurethane with excellent hydrophilic properties. PU film made of PU resin with hydrophilic groups (such as ethoxylate) in the soft segment structure can be used. The water vapor molecules are adsorbed by the hydrogen bond. Since the water vapor pressure in the shoe is larger than the outer side of the shoe, the water vapor molecules are continuously transmitted to the outside of the shoe along the gap between the polyurethane molecular chains, thereby exerting a moisture permeability effect.
In summary, in the dry veneer process design, if the mask has no micropores, the water vapor can be transferred to the other side of the microfiber synthetic leather surface even by the hydrophilic PU resin, but it is easy to be hydrophilic PU mask material. When the water film is formed, it is difficult to rapidly and continuously transfer the water vapor to the other side of the microfiber synthetic leather surface, and the moisture permeability is not good. Therefore, when designing a superfibre synthetic leather veneering process for protective shoes, a hydrophilic resin is used, and a micropore having a pore diameter of 4×10 −4 to 20 μm is designed to allow water vapor molecules to pass freely. Water, but it is difficult to enter the shoes to achieve the purpose of moisture permeability and waterproof.
3 Application fields of microfiber synthetic leather for protective shoes
According to the statistics of the Hong Kong Labour Department, about one-third of industrial accident safety accidents are caused by treading objects, touching objects or being injured by objects. The result is often foot injuries and severe disability. In foreign countries, especially in the western developed countries, great importance is attached to foot protection, requiring employees to wear professional shoes. China has also issued a series of product testing standards for foot protection. Therefore, protective shoes have broad market prospects. At present, China has the largest professional shoe production base in Asia.
With the continuous advancement of society, there are more and more varieties of individual labor protection products. In Europe and the United States, the previous single safety shoes are now divided into protective shoes, anti-static shoes, protective toe safety shoes, military shoes, nurses according to professional characteristics. Shoes, administrative shoes, Latin dance shoes and other types, so the application of superfibrous synthetic leather for protective shoes is more extensive, as follows:
For safety shoes: uppers for shoes such as protective shoes, anti-static shoes, toe safety shoes and safety anti-smash shoes. The uppers of these shoes are mainly black or brown, and are mainly used in mining, construction, metallurgy, transportation, harvesting and other industries to protect the toes. It requires the shoe leather to be moisture-permeable and waterproof, and the physico-mechanical strength of the super-fiber synthetic leather for protective shoes has exceeded that of the leather. Therefore, the super-fiber synthetic leather for protective shoes is currently very popular at home and abroad.
For professional shoes: uppers for shoes such as nurse shoes, executive shoes and women's work shoes. Among them, nurse shoes are very common. At present, many hospitals at home and abroad use special nurse shoes to prevent the needle from falling and stabbing the foot when the needle is shot. The nurse's shoes are mainly white, and the upper is required to be moisture-permeable, waterproof, resistant to bending, soft and comfortable. Many countries in Europe and the United States require executives to wear executive shoes to work, women wear women's work shoes, these shoes also require the upper leather to have moisture permeability.
For military shoes: In the past, most of them were made of cowhide. At present, the leather has been gradually replaced with microfiber synthetic leather, which requires the leather surface to have moisture permeability and waterproof function.
For women's Latin dance shoes: early use of cowhide or sheepskin production, the current use of breathable, moisture permeability of super-fiber synthetic leather production.
With the increasingly specialized division of labor, the growing awareness of individual protection, and the growing scarcity of dermis, the protective footwear leather with breathable, moisture-permeable and waterproof functions will have a broad market and will be gradually applied in sports shoes and running shoes. In the future, gradually replace the leather.
In order to achieve the function of waterproofing and satisfying the air permeability, the process design of the superfibrous synthetic leather for protective shoes should be started from the microfiber base fabric and the dry veneering process. The microfiber synthetic leather base fabric needs more micropores in the structure. The structure is designed to facilitate the diffusion of water vapor from the inside of the shoe; at the same time, the superfibrous synthetic leather base fabric must have a certain hydrophobic property to prevent rainwater and the like from entering the shoe. In the dry veneer, it is not only necessary to use hydrophilic PU resin to improve the breathability of the protective shoes, but also to have micropores with suitable pores on the leather surface to facilitate the transfer of moisture in the shoe while preventing water from entering the shoe and causing wetness. foot.
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