Peter Kamarchik

Crosslink density and the development of internal stress in organic coatings

The corrosion test performance of organic coatings on certain substrates noted for relatively weak adhesion (unphosphatized steel and hot dipped galvanized steel) is dominated by internal stress in the coating. Internal stress develops due to a mismatch of the physical dimensions between a coating and 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 material. The magnitude of the stress is related to many factors, including: chemical and structural nature of the polymer, extent of crosslinking, temperature 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 natural exposure environment. Unexpectedly large increases in internal stress are observed for some commercially important coating types with these increases correlated to corrosion failures. In some instances, water sensitivity of the coating as evidenced by high water permeability increase on exposure or loss of water extractable materials on exposure can explain the observed internal stress behavior, but anomalies exist. In these cases structural changes occurring as the polymer coils, uncoils, and coils again during cyclic water exposure may also play an important role.

Applications of X-ray techniques to coatings analysis

3 Diffraction in coating systems 83 3.1 Interaction of X-rays with matter 83 3.2 Experimental procedures. 84 3.3 Qualitative analysis. 88 3.4 Quantitative analysis. 89 3.5 Particle dimension measurement. 91 3.6 Applications of diffraction techniques 91 3.6.1 Pigments 91 3.6.2 Pretreatment quality 93 3.6.3 Crystallinity of polymers and resins 93 3.7 General coating problems 95