Basic concepts and mechanisms of leather waterproofing

Leather goods have been in widespread demand since ancient times. The earliest use of leather dates back to the Paleolithic era, when our ancestors could already sew leather garments from animal skins. With continuous reforms in tanning technology, people’s demands for leather products have also steadily increased. Leathermaking evolved from simple “raw hides” to “cooked leather” to pursuing the production of tools Waterproof, oil-resistant, and flame-retardant and other multifunctional leathers with excellent performance. At the same time, the emergence of artificial leather with exceptional properties has challenged natural leather.

Waterproof It is an indispensable feature of modern leather. It is the most widely used in daily life clothing leather The demand for waterproof performance on leather uppers is increasing day by day. For many leather products, people’s purchasing criteria have also been incorporated. Is it waterproof? “This point. The widespread use of waterproof leather has made waterproofing seem to be one of the characteristics of leather itself. Nowadays, many waterproofing technologies have been developed,

Basic Concept of Leather Waterproofing

The key to leather waterproofing is to prevent moisture from entering the other side through the body from one side. Since leather itself is hydrophilic, ordinary tanning of natural leather cannot prevent this process. The hydrophilicity of leather comes from the fact that leather itself is woven from collagen fibers in three-dimensional space, with countless capillaries of different radii between the fibers. After tanning, the addition of chemical materials introduces more polar groups to the leather, such as hydroxyl, carboxyl, and amino groups. Due to the principle of similarity compatibility, these polar groups can bond with water and easily bind with it.

When leather comes into contact with water, the presence of many polar groups makes the leather hydrophilic, moistening the leather with moisture. At the same time, the presence of capillaries inside the leather allows the leather to absorb moisture and penetrate into the leather. Waterproofing blocks the occurrence of these processes, so waterproofing can be categorized into the following three types:

(1) Non-wetting: The property of preventing the surface of skin collagen fiber proteins from being wetted by water, i.e., water repellency.

(2) Non-absorbent: The ability to prevent leather from absorbing water and allowing it to penetrate inward, i.e., water resistance.

(3) Water proof: The ability to prevent water from seeping from one side of the leather to the other, i.e., water proof.

The above are three aspects of waterproof performance. Waterproof performance should include these three aspects: the leather’s ability to resist water absorption, water permeability, and wetting by water. But there are already waterproof leather Waterproof performance often cannot achieve all three at once For example, some waterproof leathers can be wetted on the surface but prevent moisture from penetrating inside, offering good water impermeability and poor surface water repellency; Some waterproof leathers, although their surface cannot be wetted, have poor dynamic water resistance. The existence of this phenomenon has led to a rather confused understanding of waterproof leather.

Waterproof mechanism of leather body

finished leather From the outside, they can be divided into coated and coated types crust leather Two floors, for finished leather As for this, we are accustomed to generals crust leather This is called ‘Changing the Body.’ As mentioned earlier, waterproofing refers to the leather’s ability to resist water absorption, permeability, and wetting by water. The first step in waterproofing is to prevent the leather surface from being wetted by water, which involves the issue of wetting solid surfaces. Wettling refers to the interaction between liquid and solid, involving contact between the three phases: gas, liquid, and solid. Surface tension exists on the three-phase contact surfaces. Whether wetting occurs can be determined by surface tension: when the surface tension of a liquid is lower than that of a solid, the liquid can spread flat across the solid surface, thereby wetting the solid; When the surface tension of a liquid is higher than that of a solid, the liquid shrinks on the solid surface in the form of water droplets without diffusing to wet it. In other words, substances with high surface tension cannot wet substances with low surface tension. Therefore, to prevent leather from being soaked by water, the surface tension of the leather must be lower than that of water.

The degree to which a solid is wetted by a liquid is usually expressed by the contact angle. British scientist Thomas Young explained this problem in his famous Young equation: when a liquid is in equilibrium on a solid surface, the degree of wetting on the solid surface can be expressed by the contact angle θ (also called the wetting angle): cosθ = Vs-g-Vl-g Vs-l

In formula 1: θ—at the gas-liquid-solid interface interface, the angle between the tension between the gas-liquid and gas-solid interface; Vs-g — surface tension between solid-gas phases; Vl-g—surface tension between the liquid-gas phase; Vs-l—surface tension between solid-liquid phases. See the diagram below:

Figure 1 is a schematic diagram showing the relationship between contact angle and surface tension. A refers to the case where the liquid wets the solid, while B refers to the case where the liquid wets the solid. By measuring the size of the contact angle, the wetting status of the solid surface can be determined, generally using 90° as the boundary. As described in Figure 1A, the interaction between hydrophilic solids and liquids has a contact angle θ< 90°, and the liquid is laid flat on the solid surface, indicating that the liquid easily wets the solid; As shown in Figure 1B, the liquid tends to shrink on the solid surface to form spherical droplets, with a contact angle θ> 90°, indicating that the liquid does not easily wet the solid, i.e., the interaction between the surface of the hydrophobic solid and the liquid is described.

The smaller the contact angle, the better the wettability; When θ=0°, it indicates that the solid surface is thoroughly wetted; when θ=180°, it is completely unwetted. Therefore, to prevent the leather surface from getting wet and maintain water repellency, a contact angle θ>90° is visually required, which can be achieved by reducing the leather’s surface energy and thereby changing its surface tension. In addition to changing the contact angle to prevent the leather surface from becoming wet, it should also be noted that leather itself is made of collagen fibers, meaning there are countless capillaries of different radii. Capillary phenomena are extremely easy to occur, which further increases the leather’s water absorption. Therefore, to make leather waterproof, it is also necessary to consider improving the hydrophobicity of the internal fibers of the leather.

Capillary phenomena occur because the adhesion and cohesion forces in the molecular adhesion layer create a crescent shape on the liquid surface, while the presence of surface tension creates additional pressure on the curved liquid surface, creating a pressure difference between the curved liquid surface and the liquid below the horizontal surface. This pressure difference causes the liquid inside the capillary to rise or fall, offsetting the additional pressure and thus balancing the pressure difference. The rise or fall of liquid in the capillary can be determined by the Young-Laplace equation, which describes the relationship between the additional pressure at the bent liquid surface, the surface tension of the liquid, and the radius of curvature, as shown in Equation 2. △p=γ( 1 R1 + 1 R2 )

Formula 2: Where: △p — pressure difference inside and outside the liquid surface; γ—surface tension coefficient; R1, R2—radius of principal curvature of the liquid surface. See the diagram below:

As shown in Figure 2 α, β, σ Three-phase coexistence system distribution diagram in capillaries, if three-phase equilibrium occurs in a capillary tube with radius r, the contact angle between α and β phase is θ. The two are shown in Figure 3 Schematic diagram of the Young-Laplace equation in capillaries. If θ < 90°, then △p < 0. The liquid surface in the capillary is concave, and the force applied to the liquid below is the pulling force, which causes the liquid to rise along the tube wall, allowing the liquid to enter the capillary and wetting the solid. If θ> 90°, the liquid level inside the △p >0 capillary becomes convex. The convex surface exerts pressure on the lower liquid surface, making it difficult for the liquid to climb into the capillary and to wet solids. Therefore, to prevent capillary action, the contact angle must be < 90°, but this is achieved by reducing the surface energy of the capillary inner surface to change its surface tension.

Waterproofing mechanism of leather coatings

The leather surface is the first to come into contact with water. Besides preventing water wetting by changing the surface of the leather, a waterproof coating can be added to replace the leather surface, making it the first line of defense for the leather’s waterproofing. The key to this coating is that… crust leather Wet coating adhesion and primer penetration in three aspects. The coating of the base layer is the foundation of the entire finish, and the adhesion of the base layer is crucial to the finishing process, making the adhesion of the coating crucial. The impact on coating adhesion can be discussed from both physical and chemical perspectives.

Smooth surfaces are less effective for coating adhesion than worn surfaces because worn surfaces have many wrinkles, small bumps, and irregularities, and have a larger surface area than unworn smooth surfaces, which are easier to bond. Using chemical crosslinking agents for coating can improve the coating’s waterproofing and adhesion strength. The penetration of the base slurry is also a key influencing factor. Leather itself is a porous material with a fibrous structure. According to the wetting and viscous fluid laws, the penetration rate of the base paste into the leather body depends on many factors.

The Sandmeyer formula describes the relationship between permeation rate and surface tension, viscosity, and contact angle. Formula 4: Permeation rate = porosity × surface tension ×cosθ viscosity

From Equation 4, it can be seen: (1) The cosine function value (cosθ) of the wetting angle is proportional to the permeation rate of the bottom slurry, indicating that wetting plays a key role in the permeation of the bottom slurry. (2) Increasing porosity, increasing surface tension, increasing the cosine value of the contact angle (COSΘ), and reducing liquid viscosity all help increase the wetting rate of the liquid. At the same time, the penetration depth of the bottom slurry should also be considered. The factors affecting penetration depth are basically the same as those affecting penetration speed, but increasing the penetration depth of the bottom slurry is achieved by reducing viscosity, decreasing the wetting angle, and increasing the surface tension of the bottom slurry. Reducing the wetting angle and increasing surface tension are contradictory, so surface tension should be adjusted appropriately to allow deeper penetration of the bottom slurry. Although the permeation rate is proportional to surface tension—the greater the surface tension, the greater the penetration speed—does not necessarily mean greater surface tension is better. In addition to the factors mentioned above, the penetration of the sole paste into the leather surface is also related to its concentration, crust leather The state (water content, porosity, charge polarity, etc.) and the coating method are related.