Effect of Film Crystallinity on Environmental Stress Cracking in Polyester Film Laminated on Steel Sheets for Interior Building Materials

Polyvinyl chloride coated steel sheets were generally used as interior building materials because of their embossing decorativeness, bending formability and durability. However the polyvinyl chloride generates harmful gases when it's disposed of. Hence substitution of the resin was demanded from the environmental point of view. We have developed polyester film laminated steel sheets as the substitution and already reported that especially polybutylene terephthalate (PBT) film laminated ones were superior to the polyvinyl chloride coated ones as conventional materials. In some tests for durability, however, we found that crack occurred at the bent portion of the film when it was dipped in alkaline solution. A degree of the crack changed by laminating conditions of the film and aging time after bending. This phenomena implies that the transition of crystallinity in the film due to the laminating condition relates to the generation of the cracking. Here, the crack is considered to be a kind of environmental stress cracking. In this paper, we investigated the cracking at the bent portion of the PBT film laminated steel sheets, which was caused by dipping into alkaline solution. The crack size of the bent portion was larger than that occurred at the plane portion. The more the crystallinity of the film had, the larger the crack size was. In low crystallinity, the crack was inclined to decrease with aging after bending. At 40°C aging, the crack rapidly decreased when comparing to that at 25°C aging.