The internal stress of coating films

Abstract

In organic coatings, serious defects or film failure (cracking, bending and peeling) may be caused by internal stresses which adversely affect adhesive and cohesive properties (elasticity) of the coating film. Internal stress may be composed of the sum of: internal stress arising during drying (shrinkage); thermal stress due to differences in the coefficient of thermal expansion of coating and substrate during changes in ambient temperature; and stress associated with polymer degradation. Topics covered in the article include: origin of residual internal stresses; factors affecting stress (glass transition temperature T g , Young's modulus, crosslinking, plasticization by solvent or plasticizer, pigment, coating thickness); change in stress (absorption and desorption of water, thermal aging, outdoor exposure to ultraviolet radiation); and practical aspects of the relation of internal stress to adhesion and cracking.