Effect of formulation variables on coating properties and performance in the deep draw process

This study demonstrated correspondences between certain coating properties and observed failure modes in the deep draw process. Mode I failure, coating delamination at the cup edge, correlated with residual compressive stresses in the drawn coating. Neither crosslinker concentration nor prepolymer molecular weight in the ranges studied significantly affected residual stresses. However, residual stresses were strongly affected by the amount of wax in the coating formulation to the extent that coatings with less than 1.5% wax exhibited mode I failure, whereas coatings with 1.5% or more did not fail. Some of the coatings that exhibited mode I failure also exhibited mode II failure. Loss of adhesion on the cup wall, mode II failure, correlated with adhesive strength of the coating to the metal substrate, which in the present study was most strongly affected by crosslinker concentration. Coatings with 15 and 20% crosslinker exhibited extended areas of mode II failure, whereas coatings with 10% crosslinker showed no mode II failure. Increasing the molecular weight of the prepolymer improved adhesion somewhat, but not enough to affect the failure mode in the coatings tested, and wax had no measurable effect on the adhesion.