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Why is adhesion considered the most fundamental property of a coating?

When discussing the value of water-based coatings, their two fundamental and core functions are to protect the substrate from environmental damage (such as corrosion, wear, UV radiation, chemicals) and to provide aesthetic decorative effects (color, gloss, texture). However, these two functions are built on excellent and long-lasting adhesion.  

Imagine, no matter how excellent the protective performance of the coating is (such as weather resistance, corrosion resistance), or how exquisite its appearance is, once the coating falls off or peels off from the surface of the substrate, all protective barriers will instantly collapse, exposing the substrate to external erosion; All the aesthetic effects have also vanished.  

Therefore, the first step in the formulation design and technical application of water-based coatings must focus on solving the fundamental problem of how to make the coating firmly adhere to the target substrate. This is not an easy task, as there are many types of substrates in practical applications - metals (iron, aluminum, stainless steel), plastics (PP, ABS, PC), wood, glass, etc. - their surface energy, chemical composition, microstructure (roughness, porosity), and surface tension characteristics vary greatly. The matching degree between the surface tension of the substrate and the surface tension of the water-based coating is a key factor determining the initial adhesion. A formula may adhere perfectly to steel, but may exhibit shrinkage, fish eyes, or even inability to adhere on plastic (especially low surface energy plastics such as PP, PE) or certain treated metal surfaces.  

It can be said that adhesion testing is the first step in evaluating the performance of water-based coatings. If the initial adhesion of the coating on the substrate (whether tested by scratch, pull off, or other standard methods) cannot meet the required standards, then testing its color accuracy, weather resistance, hardness, flexibility, chemical resistance, and other properties in the future will be completely meaningless - because any advanced performance built on an unstable adhesion foundation is like a castle in the air, with no practical application value.  

However, solving the initial adhesion problem is only the first step in a long journey. During the protection process, water-based coatings will face various harsh environmental tests: immersion in water (water resistance), high-temperature water boiling (water boiling resistance), salt spray environment (salt spray resistance), intense temperature cycling (high and low temperature resistance), acid and alkali corrosion (acid and alkali resistance), and contact with various chemicals (chemical resistance). These environmental stresses not only directly challenge the intrinsic performance of the coating, but also continuously attack the interface between the coating and the substrate. Moisture may penetrate the interface, leading to hydrolysis, swelling, or electrochemical corrosion; Temperature changes may cause stress in the coating and substrate due to differences in thermal expansion coefficients; Chemical media may break chemical bonds at interfaces or dissolve interface substances.  

Therefore, the adhesion of a truly reliable water-based coating is by no means a one-time pass. After completing each key environmental performance test (such as 96 hours of water boiling resistance, 500 hours of salt spray resistance, 10 cold and hot cycles, etc.), its adhesion must be rigorously evaluated immediately or after a specified recovery time (such as 4 hours, 24 hours to simulate stress release or water evaporation process).  

So it can be said that adhesion is the most fundamental and critical performance testing indicator for coatings.