Influence of Polyester Powder Coating Chemistry on Adhesion Durability of Silicone Sealants

Abstract

Powder coated aluminum is one of the major substrates that is enjoying continuous market growth in window and door glazing as well as in structural glazing applications. In 1998, a European Community directive classified triglycidyl isocyanurate (TGIC) as toxic. TGIC was used as the curing agent in the fabrication of polyester powder coatings on aluminum. This environmental decision has led polyester powder suppliers to adopt new chemical compositions and to develop alternatives to TGIC-based powder coatings. These modifications have brought changes in the bulk properties and in the surface properties of the coatings, which are of particular interest with regard to the adhesion properties of silicone sealants. In this study, various commercial and developmental polyester powder coated aluminum substrates from different manufacturers were evaluated. An attempt was made to relate the surface energy, chemical composition and morphology of polyester powder coatings to the adherence of various silicone sealants. Overall, it was difficult to find a straight correlation between the adhesive bonding performances of the sealant studied and the contact angle data acquired from the various powder coating surfaces. This indicates that changes in the surface free energy due to substitution of the resins components used to prepare the coating are not solely responsible for the changes in adhesive performance of sealants. However, XPS and SEM analysis have clearly shown that in some coatings a wax is present at the surface. The presence of these waxes has a detrimental effect on subsequent adhesive bonding capabilities of silicone sealants, independent of whether a TGIC-containing or TGIC-free powder system was used. On a particular series of coatings, tensile-adhesive joint and peel-adhesive testing has shown a reasonably good correlation with conclusions from surface analysis. In particular, the higher the Polar component of the total surface energy, the better the adherence of silicone sealants was on polyester powder coated aluminum. Cleaning the coating with white spirit was effective in increasing the polar component of the total surface energy of the coating, which enhanced adhesive bonding of the silicone sealant. The effect of the silicone sealant chemistry (tin versus titanium ester catalyst, for instance) was also analyzed.