One of the core functions of carbon fiber primer is to seal the micropores and crevices on the substrate surface. Its composition design must balance permeability, filling properties, and chemical stability. The resin matrix, as the main body of the primer, is typically made of epoxy resin or modified polyurethane. These materials have low viscosity and high fluidity, allowing them to penetrate deep into the pore structure of the carbon fiber substrate through capillary action. During curing, epoxy resin forms a three-dimensional cross-linked network. The flexibility and rigidity of its molecular chains are balanced, enabling it to both fill micron-sized pores and form a dense sealing layer, preventing the intrusion of external moisture, gases, and corrosive media.
Fillers are key components for improving the sealing performance of the primer. Nanoscale silica is a commonly used choice due to its high specific surface area and excellent dispersibility. Its particle size is typically between 10-100 nanometers, capable of filling even smaller pores on the carbon fiber surface while enhancing the mechanical strength of the coating. In addition, flaky fillers such as talc and mica powder can form physical barriers through layering, further blocking the connectivity of pores. These fillers require surface modification to improve compatibility with the resin matrix and prevent reduced sealing performance due to uneven dispersion.
Leveling agents optimize the coating performance of the primer. Carbon fiber surfaces have numerous irregular uneven structures; if the primer's leveling properties are insufficient, pinholes or craters can easily form after curing. Silicone-based leveling agents reduce surface tension, allowing the primer to spread automatically during coating, forming a uniform and continuous film. The high bond energy of the silicon-oxygen bonds in their molecular structure imparts good temperature resistance and chemical stability to the coating, ensuring long-lasting sealing performance.
Sealants are functional components designed specifically for the unique structure of carbon fiber substrates. Some primers contain low-molecular-weight epoxy resins or reactive diluents. These substances maintain fluidity in the early stages of curing, allowing them to penetrate the tiny gaps between carbon fiber bundles and form chemical bonds with the substrate. As the curing reaction progresses, the sealant molecules gradually cross-link, forming a tightly bonded sealing layer. This chemical sealing effect, combined with physical filling, significantly improves the primer's adaptability to complex porous structures.
Coupling agents enhance sealing performance by improving interfacial compatibility. Carbon fiber surfaces typically contain polar groups, while resin matrices are mostly non-polar or weakly polar; insufficient compatibility can easily lead to coating peeling. Silane coupling agents, such as KH-550 and KH-560, contain amino or epoxy groups at one end of their molecules that can react with the carbon fiber surface, and alkoxy groups at the other end that can bind to the resin matrix, thus improving interfacial adhesion through chemical bridging. This strengthening effect allows the primer to adhere more tightly to the substrate surface, reducing the risk of porosity exposure due to interfacial defects.
The choice of curing agent directly affects the sealing performance of the primer. Amine curing agents, such as polyamides and modified aromatic amines, form a highly cross-linked network structure through ring-opening polymerization with epoxy resins. Highly cross-linked coatings have lower air permeability and water absorption, effectively sealing substrate pores. Furthermore, the addition of latent curing agents allows the primer to remain stable at room temperature, and curing can be triggered by heating or adding a catalyst after application, avoiding insufficient penetration due to premature curing. The sealing properties of carbon fiber primer are the result of the synergistic effect of multiple components. The resin matrix provides the basic sealing framework, fillers enhance the physical filling effect, leveling agents optimize coating uniformity, sealants deeply penetrate micro-gaps, coupling agents strengthen interfacial bonding, and curing agents ensure coating densification. These components, scientifically proportioned, form a comprehensive sealing system for the pores of the carbon fiber substrate, providing a stable and durable adhesion foundation for subsequent coatings.
