It has been known that the creep strength of polyvinyl chloride (PVC) in conc. nitric acid is remarkably lower than that in air. This behavior has been explained from the viewpoint of the plasticizing effect of nitric acid in PVC. However, the diffusion process of nitric acid in PVC has not yet been made clear.
In this paper the diffusion of conc. nitric acid in rigid polyvinyl chloride was studied experimentally by using the film-adhesion method which is similar to the film-roll method. The tests were carried out under unloaded and loaded conditions at temperatures 25 to 55°C. The results obtained are as follows:
(1) Under the unloaded condition, the diffusion process is Fickian and the surface concentration of nitric acid in PVC does not change with immersion time. The diffusion coefficients of nitric acid are in the range of 10-12-10-11cm2/sec.
(2) In the case of tensile stress levels less than 75kg/cm2, the diffusion process is almost the same as that in the case of the unloaded condition at 40°C. At a stress level of 95kg/cm2, the diffusion process is non-Fickian where the surface concentration increases with immersion time.