Improving the adaptability of carbon fiber primers on flexible substrates requires a multi-dimensional collaborative approach, encompassing surface treatment, formulation optimization, process adjustment, and performance verification. This aims to address issues such as insufficient adhesion and coating cracking caused by differences in substrate flexibility, surface energy, and deformation characteristics.
The surface energy of flexible substrates is typically lower than that of rigid materials, making it difficult for carbon fiber primers to spread sufficiently and form a uniform coating. For example, some polymer films or fiber fabrics have smooth surfaces lacking microscopic roughness, preventing coating molecules from binding to the substrate through mechanical anchoring. To address this issue, laser cleaning or plasma treatment technologies can be employed. Laser cleaning removes impurities from the substrate surface using a high-energy beam, while simultaneously creating micro/nano-scale rough structures to increase the contact area. Plasma treatment bombards the surface with active particles, introducing polar groups containing oxygen and nitrogen, thereby increasing surface energy. For instance, plasma treatment of polyimide film surfaces can increase their surface energy from 32 mN/m to 58 mN/m, significantly enhancing the wettability of the primer.
Traditional carbon fiber primer formulations are primarily designed for rigid substrates. After curing, they exhibit high crosslinking density but insufficient flexibility, leading to stress concentration and coating cracking when flexible substrates are bent or stretched. Improvements include using low-modulus resin matrices, such as polyurethane or acrylate elastomers, which have lower glass transition temperatures (Tg) and maintain flexibility over a wide temperature range; introducing flexible segments, such as polycaprolactone (PCL) or polybutadiene (PB), to reduce coating hardness through copolymerization or blending; and adding nanofillers, such as fumed silica or carbon nanotubes, to enhance coating mechanical properties while utilizing their nano-effects to disperse stress. For example, a primer with 2% fumed silica can increase elongation at break by 40% while maintaining adhesion.
The deformation characteristics of flexible substrates require primer application processes with greater adaptability. Traditional spraying or brushing methods are prone to uneven coating thickness due to substrate vibration, while dip coating or roller coating, although achieving uniform coverage, may introduce pinhole defects due to rapid solvent evaporation. Improved processes include employing electrostatic spraying technology, which uses charge adsorption to ensure uniform coating deposition on the substrate surface, reducing overspraying; and optimizing drying and curing conditions, such as using a staged heating method. First, the solvent evaporates slowly at a low temperature to prevent the coating surface from hardening too quickly and forming a hard shell, then the cross-linking reaction is completed at a high temperature to ensure thorough curing of the coating interior. For example, a carbon fiber primer for flexible electronic devices, using a process of pre-drying at 60℃ for 10 minutes and curing at 120℃ for 30 minutes, exhibits significantly better coating flexibility than traditional processes.
Flexible substrates are often used in applications involving dynamic deformation or complex environments, requiring the primer to possess excellent bending resistance, impact resistance, and chemical corrosion resistance. Coating performance can be comprehensively evaluated through testing methods simulating actual working conditions, such as repeated bending tests (bending radius ≤5mm, cycles ≥100,000), thermal cycling tests (-40℃ to 80℃, 50 cycles), and salt spray tests (5% NaCl solution, 72 hours). For example, after the above tests, a carbon fiber reinforced composite primer showed no cracking or peeling, and its adhesion remained at Grade 1, demonstrating its potential for engineering applications.
The diversity of flexible substrates places higher demands on the universality of primers. For example, the surface properties of carbon fiber fabrics and polymer films differ significantly, necessitating the development of general-purpose primers or targeted formulations. By adjusting the ratio of resin matrix to curing agent, or by adding different functional additives, "one primer for multiple uses" can be achieved. For instance, a certain carbon fiber primer, by adding 0.5% silane coupling agent, can be applied to both polyester films and carbon fiber reinforced plastics, achieving Grade 0 adhesion in both cases.
The lightweight nature of flexible substrates requires primers to improve performance while minimizing coating thickness and weight. By optimizing formulations and processes, coating thickness ≤30μm and weight increase ≤5g/m² can be achieved, meeting the weight reduction requirements of aerospace, new energy vehicles, and other fields. For example, a certain ultra-thin carbon fiber primer uses a high-solids-content resin system, with a coating thickness of only 25μm, yet possesses excellent weather resistance and corrosion resistance.
Improving carbon fiber primers for flexible substrates requires consideration of materials science, surface engineering, and process optimization. This involves enhancing substrate compatibility through surface treatment, improving coating flexibility through formulation design, adapting to dynamic deformation through process adjustments, and ensuring reliability through performance verification. Ultimately, this will enable the coating and flexible substrate to work synergistically, expanding the application boundaries of carbon fiber materials in high-end manufacturing.
