Waterborne bio-based anti-corrosion coatings, using water as a dispersion medium and combining the properties of bio-based materials, offer significant advantages in environmental friendliness and sustainability. However, their adhesion to metal surfaces is often constrained by material characteristics, processing conditions, and environmental factors. Improving adhesion requires a comprehensive approach encompassing substrate pretreatment, formulation optimization of waterborne bio-based anti-corrosion coatings, improvement of application processes, and environmental control to achieve a stable bond between the coating and the metal substrate.
Substrate pretreatment is a fundamental step in enhancing adhesion. The presence of oil, rust, or oxide layers on the metal surface hinders direct contact between the waterborne bio-based anti-corrosion coatings and the substrate, forming a weak boundary layer and leading to decreased adhesion. Therefore, the substrate surface must be thoroughly cleaned using physical or chemical methods: physical methods include sandblasting, grinding, or high-pressure water jet cleaning, which removes surface impurities and increases roughness, providing mechanical anchoring points for the waterborne bio-based anti-corrosion coatings; chemical methods employ acid or alkali washing to dissolve the oxide layer on the metal surface, exposing the active metal substrate. For example, after phosphate treatment, a dense phosphate conversion film forms on the surface of steel substrates. This film enhances the chemical bond between waterborne bio-based anti-corrosion coatings and the substrate, while also providing an additional corrosion barrier.
Formulation optimization of waterborne bio-based anti-corrosion coatings is a key method for improving adhesion. The resin system of waterborne bio-based anti-corrosion coatings needs to balance the environmental friendliness of the bio-based components with the stability of film-forming properties. By introducing functional monomers or modifiers, the polarity, molecular weight, and crosslinking density of the resin can be adjusted to better suit the metal substrate. For example, introducing phosphate ester groups into the resin molecular chain can form stable chemical bonds on the metal surface, significantly improving adhesion; adding silane coupling agents can build a "chemical bridge" between the resin and metal interface, forming covalent bonds through the reaction of silanol groups with hydroxyl groups on the metal surface, enhancing interlayer adhesion. Furthermore, the selection and ratio of pigments and fillers also affect adhesion: Adding appropriate amounts of flake-like pigments and fillers (such as mica powder) can form a laminated structure, reducing internal stress in the coating and increasing the contact area between the coating and the substrate; while nano-sized pigments and fillers can fill microscopic defects in the coating, improving density and indirectly enhancing adhesion.
Improving the application process is equally crucial for enhancing adhesion. Coating thickness must be strictly controlled within a reasonable range: too thin a coating may lead to discontinuities and failure to form a complete protective film; too thick a coating is prone to cracking or peeling due to stress accumulation. Precise control of the application environment is also necessary: excessively high temperatures accelerate solvent evaporation, causing the coating to dry too quickly and preventing the release of internal stress; excessively low temperatures may prolong drying time, increasing the risk of coating contamination. Humidity control is particularly important: in high humidity environments, water molecules easily accumulate at the coating-metal interface, replacing the adsorption of resin polar groups on the metal surface, leading to decreased adhesion. Therefore, a relatively dry environment should be chosen during application, or air humidity should be reduced using heating and dehumidification equipment.
Adding adhesion promoters is an effective auxiliary means to improve adhesion. For water-based systems, modified silane coupling agents or phosphate ester accelerators can be used: the former reacts with the metal surface at room temperature to form stable chemical bonds; the latter requires activation under baking conditions and is suitable for paint systems. In addition, polymeric accelerators (such as Casheda LA A2013) can enhance the mechanical bonding between the coating and the substrate through molecular chain entanglement, while also improving flexibility and reducing adhesion loss due to substrate deformation.
Post-treatment processes are crucial for maintaining long-term adhesion. During coating curing, solvent evaporation and resin cross-linking reactions can lead to internal stress accumulation, causing adhesion degradation. Therefore, gradient heating or staged curing processes are necessary to allow the solvent to evaporate slowly and the resin to fully cross-link, reducing internal stress. For coatings subjected to immersion or wet-dry cycling, resins with excellent water permeability (such as chlorinated rubber resin) can be introduced into the formulation, or a sealing agent can be applied during post-treatment to prevent moisture from penetrating the interface and maintain stable adhesion.
