In the production of Kraft silicone paper, the pretreatment of the kraft substrate is a crucial step in ensuring the stability and strong adhesion of the final product. This process requires multiple specialized steps to deeply process the kraft substrate, eliminating its natural defects and endowing it with the physicochemical properties suitable for the silicone coating. The following explanation covers seven aspects: substrate cleaning, morphological stabilization, surface activation, structural adjustment, impurity control, dimensional standardization, and pre-coating treatment.
Initial cleaning of the kraft substrate is the first step in pretreatment. As a natural material, kraft may have residual grease, dander, dust, and other impurities on its surface. These substances can hinder direct contact between the silicone layer and the kraft fibers, leading to decreased adhesion strength. Therefore, it is necessary to thoroughly remove surface contaminants using mechanical rollers, high-pressure water washing, or organic solvent wiping. Some processes also employ ultrasonic cleaning technology, utilizing high-frequency vibration to peel away tiny particles embedded in the fiber gaps, ensuring the substrate surface cleanliness meets industrial-grade standards.
The natural morphological stability of kraft is poor, and it is prone to shrinkage, curling, or deformation due to changes in environmental temperature and humidity. This can affect the accuracy of subsequent coating processes. To address this issue, pretreatment requires a heat-setting process to fix the shape of the cowhide. Specifically, the cowhide substrate is placed in a constant-temperature oven and heated for a specific time at a suitable temperature. This allows the fiber molecular chains to rearrange and form a stable structure. Simultaneously, a tension control system stretches the cowhide to eliminate internal stress, ensuring it remains flat and does not warp during coating.
The fibers on the cowhide surface are naturally arranged. When directly coating with silicone, uneven penetration of the adhesive may result in weak areas at the bonding interface. To enhance the adhesion between the silicone layer and the cowhide, the cowhide surface needs to be activated. Common methods include corona discharge treatment, which uses a high-voltage electric field to break down the molecular structure of the cowhide surface, creating micropores and increasing roughness; or using chemical etchants to locally dissolve the fiber surface, exposing more active groups. These treatments significantly increase the dyne value of the cowhide surface, allowing the silicone coating to form a chemical bond rather than simple physical adhesion.
As an animal tissue, cowhide contains natural oils and collagen, which may affect the curing reaction of the silicone or cause the finished product to yellow or become brittle. Pre-treatment requires degreasing to remove excess oil, typically using organic solvent soaking or enzymatic hydrolysis to break down fat molecules. Simultaneously, given the fibrous structure of cowhide, some processes involve partial fiber peeling or softening to adjust its hardness and elasticity, making it more suitable for the flexibility required by silicone paper.
The cowhide substrate may carry microorganisms or chemical contaminants. If not thoroughly removed, these can trigger side reactions during the silicone coating curing process, leading to defects such as bubbles and cracks in the product. Therefore, pre-treatment must include strict disinfection and purification steps, such as using ultraviolet irradiation to kill surface bacteria or removing residual chemicals through acid-base neutralization. Some advanced processes also incorporate ion exchange resin adsorption technology to further purify impurities such as metal ions in the cowhide fibers.
As a natural material, cowhide exhibits individual variations in thickness and width. Directly coating with silicone will result in uneven thickness in the finished product, affecting performance. Pre-treatment requires precision cutting equipment to uniformly trim the cowhide substrate to standard dimensions, while simultaneously using a sander to lightly sand the surface to eliminate localized bumps or depressions. For cowhide exceeding the thickness limit, a layered peeling technique is used to separate it into multiple thin sheets, ensuring that the thickness error of each substrate sheet is controlled within a minimal range.
To further enhance the adhesion strength between the silicone layer and the cowhide, some processes involve pre-coating a primer onto the cowhide surface. This primer is typically a silane coupling agent or an acrylic adhesive; its molecular structure can form covalent bonds with the cowhide fibers and react with the organic groups in the silicone, thus creating a "chemical bridge" between the two. After pre-coating, the cowhide substrate undergoes drying and curing to stabilize the primer layer, providing an ideal bonding interface for subsequent silicone coating.
In the production of Kraft silicone paper, the pretreatment of the cowhide substrate is a complex system engineering process encompassing cleaning, shaping, activation, purification, standardization, and functionalization. Each step requires strict control of process parameters to ensure that the physicochemical properties of the cowhide substrate meet the high-precision requirements of silicone coating, ultimately achieving a perfect bond between the cowhide and silicone, endowing Kraft silicone paper with excellent weather resistance, flexibility, and environmental adaptability.
