Prepainted Sheet Steel for Outer Automobile Body Panels: Paint Deformation Behavior

The paint deformation behavior in fully prepainted sheet steel intended for outer automobile body panels is examined in three categories: paint sliding behavior during forming, paint surface roughening during straining leading to loss of coating reflectivity, and dry heat cracking (i.e. time and temperature dependent post-forming paint cracking resulting from viscoelastic strain relaxation). The main findings are: frictional behavior is dictated by the outer coating while pigment particles tend to decrease the measured coefficient of friction; the loss of distinctiveness of image with strain is a result of shear band formation, an inherent deformation mechanism within the polymer coatings; and, dry heat cracks evolve in a two step process where crack nuclei develop during forming and grow as a result of viscoelastic strain relaxation in the coating upon subsequent exposure to heat. Furthermore, pigment particles tend to increase the resistance of a paint coating to the effects of viscoelastic relaxation.