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Diffusion theory is the key principle in coatings for explaining adhesion and the film-forming process; it involves the interfacial diffusion of polymer chain segments, thereby enabling bonding and film formation. This principle applies in two different scenarios within coating systems, as follows: 1. The diffusion principle in terms of coating adhesion – When a coating is applied to a polymer substrate surface, the polymer chain segments from both materials undergo interfacial diffusion due to continuous thermal motion. Only short segments of the molecular chains can diffuse across the interface, while intact macromolecules find it difficult to pass through the entire interface. As a result, a diffusion transition layer with a thickness of approximately 10–1000 angstroms forms at the originally distinct coating-substrate interface. The interface disappears entirely, and the polymer chains interlock to form a network structure, allowing the coating and substrate to bond firmly together. Non-similar polymers are usually incompatible due to large differences in molecular properties and low diffusion coefficients, making effective diffusion difficult to occur ； And this principle only applies when both the substrate and the coating are systems of mobile polymers. 2. The principle of diffusion during the film-forming process of water-based latex coatings. In the recognized three-stage film-forming mechanism of water-based latex coatings, diffusion is the key step in achieving the final shape. The principle is as follows: after the water evaporates in the first stage, the latex particles gradually move closer to each other and change shape, resulting in a dense packed structure ； When the ambient temperature is higher than the glass transition temperature (Tg) of the polymer, the polymer chain segments on the surface of the latex particles gain sufficient activity, allowing them to break through the particle boundaries and diffuse toward adjacent latex particles ； Ultimately, the chain segments of all latex particles diffuse and merge with one another to form a continuous, uniform, and intact solid paint film. The degree of uniformity in molecular diffusion during this process directly determines the strength and properties of the final paint film. This principle is rarely used on its own to explain the adhesion and film formation of coatings; it generally works in conjunction with theories such as mechanical anchoring, electrostatic attraction, and chemical bonding to fully explain the coating properties of coatings.