The choice of substrate for textile chemical powder-free pastes directly influences their adhesion stability, a process that involves multiple factors, including substrate surface properties, chemical structure compatibility, and environmental adaptability. As a functional coating material, powder-free pastes require stable adhesion through substrate surface energy, polarity matching, and microstructural adaptation. However, differences in the physical and chemical properties of different fiber materials can significantly vary adhesion performance.
Substrate surface energy is a key parameter determining the wettability of powder-free pastes. High-surface-energy substrates, such as glass fibers and metallized fibers, due to their high surface free energy, enable spontaneous spreading with the low-surface-tension components of the powder-free paste, promoting the formation of intermolecular forces. In contrast, low-surface-energy substrates, such as polytetrafluoroethylene fibers and certain modified synthetic fibers, require substrate pretreatment or powder-free paste formulation adjustments to enhance interfacial interactions. For example, plasma treatment of polypropylene fibers can significantly increase their surface polarity, making polar groups (such as hydroxyl and carboxyl groups) in the powder-free paste more susceptible to hydrogen bonding or chemical bonding.
Compatibility of the substrate's chemical structure is crucial for long-term adhesion stability. Natural fibers (cotton and linen), due to their high hydroxyl content, are highly reactive with the isocyanate and epoxy components of the powder-free paste, forming a stable covalent bond network. The ester or amide structures of synthetic fibers (polyester and nylon) require specific functional group matching. For example, treating the substrate surface with an amino-containing silane coupling agent can enhance the adhesion of the polyurethane component of the powder-free paste. Chemical mismatches can lead to interfacial phase separation and coating delamination under conditions of moisture, heat, or mechanical stress.
The substrate's microstructure directly influences the mechanical anchoring of the powder-free paste. Porous substrates (such as nonwovens and sponge fibers) allow the powder-free paste to penetrate through capillary action, forming a three-dimensional network and significantly improving peel strength. Smooth substrates (such as high-density polyethylene film) require adjustment of the powder-free paste's rheological properties. This can be achieved by increasing thixotropy or introducing microsphere fillers to create a mechanical interlocking structure at the coating-substrate interface. Experiments have shown that corona-treated polypropylene film improves the adhesion of powder-free paste, which is directly related to the enhanced mechanical bond resulting from increased surface roughness.
Environmental adaptability is a key dimension of adhesion stability. In high-temperature environments, the matching of the thermal expansion coefficients of the substrate and the powder-free paste is crucial. If the difference in thermal expansion coefficients is too large, repeated thermal cycling can lead to interfacial stress concentration and cracking of the coating. For example, when applying powder-free paste to the surface of glass fiber-reinforced composites, a resin system with a thermal expansion coefficient close to that of the glass should be selected to avoid interfacial debonding during high-temperature operation.
Dynamic stress tolerance requires a gradient transition in elastic modulus between the substrate and the powder-free paste. Flexible substrates (such as spandex and elastic polyester) require a low-modulus powder-free paste to dissipate energy through molecular chain entanglement, while rigid substrates (such as carbon fiber and aramid) require a high-modulus powder-free paste to resist stress concentration through chemical bonding. This modulus matching effectively prevents fatigue fracture of the coating during repeated bending or stretching.
Substrate pretreatment processes can influence adhesion stability. Plasma treatment, chemical etching, or the application of primers can modify the substrate's surface morphology and chemical activity. For example, treating aluminum alloy surfaces with a primer containing siloxane not only forms a chemically adsorbed layer but also creates a nanoscale rough structure through hydrolysis and condensation of the siloxane, significantly improving the adhesion of the powder-free paste.
From a formulation design perspective, substrate characteristics dictate the choice of powder-free paste components. For polar substrates, the proportion of polar resin can be increased; for non-polar substrates, a compatibilizer or surface modification is required. This targeted design allows powder-free pastes to achieve a balance of adhesion stability and functionality on a variety of substrates, meeting the diverse demands of industrial coating materials.
