The rust preventive effect of the phosphate coating process has its origin in the strong bonding that exists between the OH groups of phosphoric acid and the iron atoms, and in the poor electric conductivity of the phosphate film. This study was undertaken in the hope that rust preventive effect could be improved by the use of linear polyphosphate or reticular ultraphosphate. It was found that the ultraphosphate film had a rust preventive effect superior to that of the orthophosphate film, especially when the film was formed on a rusty steel surface. A theoretical explanation of the results is presented.
The diffusion phenomena of solvents through the films of alkyd and epoxy-ester resins were investigated from a kinetic point of view in order to elucidate the relations between the film structure and the swelling of the film. It is well known that the swelling is related to such film properties as chemical, stain, corrosion, blister and overcoating-resistances, as well as the adhesion. At the early stage of swelling the Fick's law Qt/Q∞=4/√π·√Dt/h2 holds, whereQt and Q∞ are the degrees of swelling after time t and after the equilibrium is reached, respectively, h is the thickness of the film, and D is the diffusion coefficient. The D value was determined from the slope of Qt-√t plot. The activation energy of diffusion, U, was calculated from the temperature dependency of D. The values of D and U for methanol, acetone and benzene were 10-7-10-9 cm2/sec and 10-15 kcal/mole, respectively. As the Mc, the molecular weight between two cross-linking sites, became smaller, D became smaller and U larger. The cross-linking points prolong the diffusion pass, thus showing the detour effect. The difference in the solubility parameters (δ) of polymer and solvent is a measure of the mutual affinity. When Mc was kept constant, D became large as the affinity was increased. As the degree of unsaturation of modified oil of the alkyd resin was increased, δ decreased and D for benzene increased. The dependency of the kind of solvent on D was in the order, methanol<benzene<acetone. Phenyl group, having a limited molecular motion, disturbed the solvent diffusion.