Effect of Film Properties on Bending Formability of Prepainted Steel Sheets

Abstract

Mechanical and viscoelastic properties of 54 polyester resin systems were measured, and the relationship between the properties and bending formability of prepainted steel sheets on which paints based on the polyester resin systems were applied was investigated. We found that formability increased with increasing elongation and tensile strength of the resin films, and the grass transition temperature (Tg) of the resins strongly influenced on the properties of resin films. We observed that elongation plotted versus Tg of the resin films measured at 20 degree Centigrade showed maximum peak in the resins' Tg around 30 degree Centigrade region, because resins which had low Tg below 30 degree Centigrade showed entropy elasticity at 20 degree Centigrade. On the other hand, tensile strength of resin films increased monotonously with increasing Tg of the resins. Tg of the resins also dominated shape of stress-strain curve (S-S curve). Finally, we proposed that it was important to obtain excellent bending formability that resins were in grass transition range in circumstances in which the prepainted steel sheets were actually used because the area surrounded by the S-S curve was larger compared with the other cases so that the fracture energy of the films was larger, and resins had higher elongation in the mentioned condition.