1. Introduction
　　
　　The coating finishing of fabrics is an important branch of textile finishing technology. It not only imparts unique style, feel, and appearance to the fabric but also endows the fabric with various functions, greatly enhancing the added value of the product. The main applications of coated products can be divided into three categories: first, clothing and footwear; second, industrial fabrics; and third, decorative fabrics. Among these, clothing and footwear account for the largest usage. Currently, the development of coated products both domestically and internationally is rapid, with functional coating products such as waterproof and moisture-permeable coatings, UV-resistant coatings, thermal insulation coatings, flame-retardant coatings, light-blocking coatings, and conductive coatings emerging. Among them, waterproof and moisture-permeable coatings are undoubtedly one of the more challenging and technically advanced varieties of functional coatings, widely applicable to military uniforms, sportswear, special occupational clothing, ordinary autumn and winter clothing, and footwear fabrics.
　　
　　2. Performance Characteristics of Waterproof and Moisture-Permeable Coatings
　　
　　Waterproof and moisture-permeable coating finishing can endow fabrics with functions such as waterproofing, moisture permeability, wind resistance, and thermal insulation. The human skin requires a certain temperature and humidity to feel comfortable. When a person wears clothing, the ways of heat dissipation include radiation, conduction, convection, and evaporation through sweating. An adult sweats about 60-70 milliliters per hour at rest; about 500 milliliters per day during exercise; and about 1000 milliliters per hour during intense exercise. If sweat cannot be dissipated in time, it not only causes a feeling of dampness due to increased moisture on the skin's surface but also leads to significant heat loss because heat conduction in a humid environment is much greater than in a dry environment. Waterproof and moisture-permeable fabrics can resist rain penetration and cold wind invasion, and they also have the effect of automatically regulating moisture permeability, allowing sweat expelled from the body to be quickly dissipated to the outside of the clothing, thus achieving moisture permeability and thermal insulation, making the body feel comfortable.
　　
　　The diameter of gaseous water molecules is about 0.004um, while the diameter of liquid water molecules is about 100um to 6000um. Therefore, the size of the micropores in the coating film of waterproof and moisture-permeable fabrics must be between the two to allow gaseous molecules to pass through while preventing liquid water molecules from doing so, thus achieving the purpose of waterproofing and moisture permeability. The process of gaseous water molecules passing through can be completed in two ways: one is through the migration of polar groups and hydrophilic groups in the coating film via a "chemical ladder" effect; the other is through the adsorption and desorption actions of the coating film, allowing gaseous water molecules to diffuse from a high concentration side to a low concentration side through the amorphous region.
　　
　　What we refer to as "moisture permeability" is to allow water vapor (gaseous water molecules) to pass through the fabric. The main factors determining the waterproof and moisture-permeable performance of coated fabrics are:
　　
　　(1) The micropore structure of the coating film: If the micropores are evenly distributed and close to the midpoint between the diameters of gaseous and liquid water molecules (about 0.2um to 1.0um), gaseous water molecules can easily pass through while liquid water molecules cannot, facilitating the realization of waterproof and moisture permeability.
　　
　　(2) The molecular structure of the coating film: A certain number of polar groups and hydrophilic groups can improve moisture permeability efficiency.
　　
　　(3) The strength and thickness of the coating film: The microporous structure of the coating film has a certain tensile strength, thus possessing resistance to hydrostatic pressure. The greater the strength of the coating film, the stronger the tensile strength, and the greater its resistance to hydrostatic pressure (waterproofing ability). The thinner the coating film, the poorer the tensile strength, and the worse its resistance to hydrostatic pressure. As the thickness of the coating film increases, the tensile strength improves, and its resistance to hydrostatic pressure also increases. On the other hand, since moisture permeability is achieved by transferring gaseous water molecules through the film, the thickness of the coating film also determines the moisture permeability capability. When the thickness of the coating film increases, the distance for transferring gaseous water molecules increases, leading to more obstacles, making moisture permeability difficult and reducing its moisture permeability capability.
　　
　　1. Introduction
　　
　　The coating finishing of fabrics is an important branch of textile finishing technology. It not only imparts unique style, feel, and appearance to the fabric but also endows the fabric with various functions, greatly enhancing the added value of the product. The main applications of coated products can be divided into three categories: first, clothing and footwear; second, industrial fabrics; and third, decorative fabrics. Among these, clothing and footwear account for the largest usage. Currently, the development of coated products both domestically and internationally is rapid, with functional coating products such as waterproof and moisture-permeable coatings, UV-resistant coatings, thermal insulation coatings, flame-retardant coatings, light-blocking coatings, and conductive coatings emerging. Among them, waterproof and moisture-permeable coatings are undoubtedly one of the more challenging and technically advanced varieties of functional coatings, widely applicable to military uniforms, sportswear, special occupational clothing, ordinary autumn and winter clothing, and footwear fabrics.
　　
　　2. Performance Characteristics of Waterproof and Moisture-Permeable Coatings
　　
　　Waterproof and moisture-permeable coating finishing can endow fabrics with functions such as waterproofing, moisture permeability, wind resistance, and thermal insulation. The human skin requires a certain temperature and humidity to feel comfortable. When a person wears clothing, the ways of heat dissipation include radiation, conduction, convection, and evaporation through sweating. An adult sweats about 60-70 milliliters per hour at rest; about 500 milliliters per day during exercise; and about 1000 milliliters per hour during intense exercise. If sweat cannot be dissipated in time, it not only causes a feeling of dampness due to increased moisture on the skin's surface but also leads to significant heat loss because heat conduction in a humid environment is much greater than in a dry environment. Waterproof and moisture-permeable fabrics can resist rain penetration and cold wind invasion, and they also have the effect of automatically regulating moisture permeability, allowing sweat expelled from the body to be quickly dissipated to the outside of the clothing, thus achieving moisture permeability and thermal insulation, making the body feel comfortable.
　　
　　The diameter of gaseous water molecules is about 0.004um, while the diameter of liquid water molecules is about 100um to 6000um. Therefore, the size of the micropores in the coating film of waterproof and moisture-permeable fabrics must be between the two to allow gaseous molecules to pass through while preventing liquid water molecules from doing so, thus achieving the purpose of waterproofing and moisture permeability. The process of gaseous water molecules passing through can be completed in two ways: one is through the migration of polar groups and hydrophilic groups in the coating film via a "chemical ladder" effect; the other is through the adsorption and desorption actions of the coating film, allowing gaseous water molecules to diffuse from a high concentration side to a low concentration side through the amorphous region.
　　
　　What we refer to as "moisture permeability" is to allow water vapor (gaseous water molecules) to pass through the fabric. The main factors determining the waterproof and moisture-permeable performance of coated fabrics are:
　　
　　(1) The micropore structure of the coating film: If the micropores are evenly distributed and close to the midpoint between the diameters of gaseous and liquid water molecules (about 0.2um to 1.0um), gaseous water molecules can easily pass through while liquid water molecules cannot, facilitating the realization of waterproof and moisture permeability.
　　
　　(2) The molecular structure of the coating film: A certain number of polar groups and hydrophilic groups can improve moisture permeability efficiency.
　　
　　(3) The strength and thickness of the coating film: The microporous structure of the coating film has a certain tensile strength, thus possessing resistance to hydrostatic pressure. The greater the strength of the coating film, the stronger the tensile strength, and the greater its resistance to hydrostatic pressure (waterproofing ability). The thinner the coating film, the poorer the tensile strength, and the worse its resistance to hydrostatic pressure. As the thickness of the coating film increases, the tensile strength improves, and its resistance to hydrostatic pressure also increases. On the other hand, since moisture permeability is achieved by transferring gaseous water molecules through the film, the thickness of the coating film also determines the moisture permeability capability. When the thickness of the coating film increases, the distance for transferring gaseous water molecules increases, leading to more obstacles, making moisture permeability difficult and reducing its moisture permeability capability.
　　
　　(4) The variety and specification of the base fabric: Fabrics with a smooth surface structure, greater thickness, and density tend to have better waterproofing ability after coating finishing. Moreover, the moisture permeability of the fabric before coating has a significant impact on the moisture permeability testing after coating, so evaluating the moisture permeability of a coating agent should not only look at the moisture permeability after coating but should also compare the moisture permeability before and after coating.
　　
　　3. Waterproof and Moisture-Permeable Coating Agents
　　
　　Currently, the coating agents used in textiles are mainly two types: polyacrylate (PA) and polyurethane (PU). PA has a relatively abundant supply of raw materials and intermediates, is low in price, and its synthesis and polymerization technology are relatively easy to master. PA coating agents have advantages such as strong adhesion, transparency without yellowing, wash resistance, and aging resistance, but they also have disadvantages such as stickiness after multiple coatings, lower water resistance, uncomfortable hand feel, poor elasticity, and poor cold resistance. Their performance can only meet general requirements and is not suitable as waterproof and moisture-permeable coating agents. PU coating agents have unique properties; they possess high elasticity, high modulus, low-temperature resistance (below -30°C), wear resistance, a soft and pleasant hand feel, and a certain degree of hydrophilicity, making them suitable for developing waterproof and moisture-permeable coating agents.
　　
　　PU coating agents are block polymers composed of alternating hard and soft segments. The hard segment gives PU strength and elastic modulus, composed of various diisocyanates and chain extenders, among which the selectable diisocyanates mainly include 2,4 and 2,6-toluene diisocyanate (TDI), 4,4-diphenylmethane diisocyanate (MDI), 1,6-hexamethylene diisocyanate (HDI), and 3,5,5-trimethyl-2-cyclohexenone diisocyanate (PDI), etc. The chain extenders mainly include diethylene triamine (DETA) and 3,3'-dichloro-4,4'-diaminodiphenylmethane (MOCA), etc. The soft segment makes PU soft and elastic, composed of polyols, and the size of the molecular weight can also affect the softness and hardness of PU. There is a wide selection of polyols, which can be small molecular weight polyols or polyether or polyester polyol prepolymers with a certain molecular weight.
　　
　　PU is generally made from diisocyanates, polyether polyols or polyester polyols, chain extenders, and catalysts, through solution polymerization or bulk polymerization. The specific synthesis method depends on different varieties and requirements. Many properties of PU, such as glass transition temperature, melting point, modulus, elasticity, tensile strength, water absorption, and moisture permeability, can be achieved by changing the types and molecular weights of polyether or polyester diol prepolymers, the types of diisocyanates, the ratio of hard to soft segments, and the types and amounts of chain extenders.
　　
　　The PU coating agents for textiles can be divided into two main categories: solvent-based and water-dispersible. The main solvents for solvent-based agents include dimethylformamide (DMF), butanone, toluene, xylene, isopropanol, ethylene glycol methyl ether, ethylene glycol ethyl ether, butanol, ethyl acetate, and their mixtures. The outstanding advantages of solvent-based agents are good film-forming properties, water resistance, adhesion, and better film smoothness compared to water-dispersible agents. Additionally, due to the high latent heat of vaporization of solvents being less than that of water, the production speed is fast, and the production efficiency is high. However, their disadvantages are also evident, as most solvents have certain toxicity, volatilize and pollute the environment, and the working conditions in the workshop are poor.
　　
　　Compared to solvent-based coating agents, water-dispersible coating agents have advantages such as clean and simple operation, no environmental pollution, and a wide range of performance styles, making them the development direction for PU coating agents.
　　
　　In terms of waterproof and moisture-permeable coatings, the performance of solvent-based waterproof and moisture-permeable coating agents, such as film strength, smoothness, waterproof performance, and wash resistance, is superior to that of water-dispersible waterproof and moisture-permeable coating agents.
　　
　　Waterproof and moisture-permeable coating products can generally be divided into the following three categories: first, high moisture permeability and high water pressure products, which generally require a moisture permeability of 30009/(m2d) and water resistance of 2000mmH2O; this type of product is more challenging to produce; second, products with higher moisture permeability requirements, generally reaching 30009/(m2d), but with lower water pressure requirements, around 500mmH2O; third, products with higher water pressure requirements but lower moisture permeability requirements.
　　
　　4. Waterproof and moisture-permeable coating finishing process
　　
　　4.1 Coating base fabric and pretreatment
　　
　　Theoretically, all fabrics can be used as base fabrics for coating finishing, but in waterproof and moisture-permeable coating finishing, cotton, linen, silk, polyester, nylon, viscose, and their blended products are still the most commonly used. Most woven fabric varieties can be suitable for waterproof and moisture-permeable coating finishing, such as plain fabric, twill fabric, elastic fabric, Oxford fabric, taffeta, satin, and straight grain structures. With the continuous innovation and development of coating agent varieties and coating finishing processes, low-grade fabrics can be processed into high-grade coated products, greatly increasing the added value of the products.
　　
　　The pretreatment of the coating base fabric is not significantly different from that of other dyeing and finishing processes, but the requirements for the fabric surface are slightly higher, such as complete singeing, a smooth surface, no cotton knots, no weft skew, and no loose edges. For dyed varieties, attention should be paid to the performance of the dye varieties to prevent excessive discoloration of the coated products due to the interaction between the solvents in the coating agents and the dyes.
　　
　　Prevent excessive discoloration of the coated products due to the interaction between the solvents in the coating agents and the dyes.
　　
　　4.2 Waterproof and moisture-permeable coating finishing process
　　
　　Waterproof and moisture-permeable coating finishing can be done using either wet coating or dry direct coating methods.
　　
　　4.2.1 Wet coating
　　
　　Wet coating, also known as coagulation coating, generally uses DMF solvent-based coating agents applied to the base fabric, which is then immersed in a water coagulation bath to extract the solvent (DMF), allowing the coating film to form micropores and achieve moisture permeability.
　　
　　Process flow:
　　
　　Base fabric - pretreatment (dyeing) - shaping - PU coating - coagulation (solvent extraction) - washing - drying - finishing - finished product. The water resistance of wet coating products can generally reach over 100mmH20, with moisture permeability reaching 3000g/(m2d). This method has a mature process and supporting additives, but the steps are complicated, solvent recovery is troublesome, and the adhesion strength and film strength are not high, while the processing cost is relatively high.
　　
　　4.2.2 Dry direct coating
　　
　　Dry direct coating equipment and processes are simple, easy to operate, and have lower processing costs. The key to this type of coating is the coating agent, as its performance will determine the waterproof and moisture-permeable properties, hand feel, elasticity, film strength, and other characteristics of the coated products.
　　
　　Process flow:
　　
　　(1) Single-agent coating agent: Base fabric - pretreatment (dyeing) - shaping - direct coating - drying - baking - finishing - finished product.
　　
　　(2) Double-agent coating agent: Base fabric - pretreatment (dyeing) - shaping - applying bottom glue - drying - applying surface glue - drying - baking - finishing - finished product.
　　
　　Conclusion
　　
　　PU waterproof and moisture-permeable coating products have good elasticity, high strength, low-temperature resistance (below -30 degrees), wear resistance, a soft and comfortable hand feel, and good waterproof and moisture-permeable effects, with promising application prospects. The technology content of dry direct coating using waterproof and moisture-permeable coating agents is high, marking the advanced level of coating processing. The required equipment is simple, and the process is convenient, making it the development direction for future waterproof and moisture-permeable products. Currently, there is a significant gap between domestic research and production of waterproof and moisture-permeable coating agents and advanced foreign levels, with high moisture permeability and high water pressure coating agents still mainly imported. Therefore, accelerating the research and development of high moisture permeability and high water pressure coating agents and their application processes is of great significance.