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Principles of Adhesion and Delamination
Regarding adhesion, there are primarily two theories: the diffusion theory, which suggests “like bonds strongly to like,” and the adsorption theory, which posits that substances bind due to adsorption.
1. Diffusion Theory
This theory is particularly effective for interlayer adhesion and adhesion to plastics. The compatibility between the paint resin and the substrate’s resin influences the range of Van der Waals forces, subsequently affecting the adhesion strength. The better the compatibility, the better the affinity, assessed by the solubility parameter (δ). Think of it as “like tends to be near like” in terms of δ.
2. Adsorption Theory
On the other hand, the adsorption theory is effective for adhesion to metals. Removing impurities and oxide films from the metal surface, increasing active adhesion sites, and introducing polar groups into the paint resin enhance adhesion.
3. Causes of Delamination
An old proverb from past literature states, “Adhesion is a battle with water, and maintaining adhesion is a struggle with internal stress.” After the formation of a paint film, moisture and water vapor permeate the film, reaching the primer or substrate interface. The water dissolves substances at the interface, deactivating adhesion sites and weakening adhesion strength. If adhesion is weak, water easily penetrates the interface.
Another cause of delamination is internal stress. The product of the paint film’s shrinkage strain and Young’s modulus results in internal stress.
Internal Stress = (Young’s Modulus of the Paint Film) × (Shrinkage Strain)
Shrinkage Strain = (Difference in Linear Expansion Coefficients of the Paint Film and the Substrate) × (Temperature Difference)
Over time, as the paint film shrinks, tensile forces develop internally, adversely affecting adhesion. For example, cardboard painted with oil paint will curve outward due to the contraction force of the paint, similar to how internal tensile forces can lead to delamination. Maintaining adhesion means minimizing initial internal stresses and preventing their increase. Achieving this requires designing paint with stress-relieving features or a lower coefficient of thermal expansion.