Crosslink density and the development of internal stress in organic coatings

The corrosion test performance of organic coatings on certain substrates no ted for relatively weak adhesion (unphosphatized steel and hot dipped galva nized steel) is dominated by internal stress in the coating. Internal stres s develops due to a mismatch of the physical dimensions between a coating a nd a substrate to which it is bound(1)). This mismatch can be the result of temperature and humidity changes, chemical reactions and gain or loss of m aterial. The magnitude of the stress is related to many factors, including: chemical and structural nature of the polymer, extent of crosslinking, tem perature of the curing reaction, and the presence of plasticizers. Of particular interest is the change in internal stress on cyclic exposure to corrosive aqueous media such as found in corrosion testing or the natura l exposure environment. Unexpectedly large increases in internal stress are observed for some commercially important coating types with these increase s correlated to corrosion failures. In some instances, water sensitivity of the coating as evidenced by high water permeability increase on exposure o r loss of water extractable materials on exposure can explain the observed internal stress behavior, but anomalies exist. In these cases structural ch anges occurring as the polymer coils, uncoils, and coils again during cycli c water exposure may also play an important role.