Study on the Method of Predicting Service Life of Coated Steel Sheets (IX)

Preparing the Map on Predicting the Service Life of Polyvinyl Chloride Resin Coated Galvanized Steel Sheet

Abstract

The deterioration of a polyvinyl chloride resin coated steel sheet subjected to an 8-year outdoor -weathering test in five areas was examined in respect of the appearance, chemical and physical changes of the film and the change in adhesive strength. At the same time, the correlations between the change of the material properties and the effects of the deterioration factors were studied. The results are :
(1) The deterioration of the polyvinyl chloride resin coated steel sheet considerably depends on the coating color and the place of the outdoor weathering test. A coating color with poor weatherability varies more in deterioration in different places of the outdoor weathering test.
(2) There is a correlation between the decrease in the amount of extender pigment (CaCO₃) in the film and such appearance changes as the film cracking, reduction in gloss or discoloring. The change of the amount of extender pigment is effectively reduced by increasing the mixing amount of the titanium dioxide pigment in the film.
(3) The reaction in which polyvinyl chloride resin is decomposed to generate crosslinking materials or low molecular weight compounds varies with the mixing amount of titanium dioxide pigment. This is probably due to the photochemistry of titanium dioxide pigment against the polyvinyl chloride resin.
From the results of analysis thus obtained, it is possible to prepare a map for service life evaluation of polyvinyl chloride resin coated steel sheets. This map shows that the deterioration is promoted most conspicuously at the Pacific Ocean coasts in western Japan. As far as the blue-colored film is concerned, it has been found that deterioration is accelerated about 1. 65 times more intensely in Amagasaki where the deterioration factors are severe than in Obihiro where the deterioration factors are gentle.