A Study of Various Parameters Affecting Adhesion of Coatings to Metal Substrates

Abstract: The adhesion of coatings is of high importance in the coating industry and a more thorough understanding of adhesion behavior is required. In this thesis work, seven parameters affecting adhesion of silane-modified poly(urethane urea) (PUSi) coatings on pretreated steel and aluminum substrates were studied. These parameters include substrate type, dry film thickness (20-30 and 60-70 μm), solid content (40, 60, and 70 wt%), resin ratio between two different types of PUSi (PUSi-A: PUSi-B = 70:30, 50:50, and 30:70 wt ratio), crosslinking density, additive, and curing condition. The different pretreatments of substrates include solvent wiping, sandblasting, phosphating, and galvanizing. A commercial paint product (‘yellow topcoat’) was used as a reference for the study of substrates and additives. Several formulations of clearcoat, prepared from the same PUSi resins as the commercial paint product, were mainly used in every experiment. The obtained coatings were tested for their adhesion properties using cross hatch adhesion test, bending test, and humidity resistance test. The film hardness and thermo-mechanical properties were evaluated with König pendulum hardness test and Dynamic Mechanical Analysis (DMA), respectively. Surface energies of all substrates were analyzed with Contact Angle Measurement (CAM). The PUSi-A and PUSi-B resins used in the coating formulations were characterized with Differential Scanning Calorimetry (DSC), Size-Exclusion Chromatography (SEC), and Fourier Transform Infrared Spectroscopy (FT-IR). The results showed a correlation between poor adhesion properties and the relatively low surface energies of some substrates, namely cold-rolled steel (CRS), industrial ACE aluminum, and standard Q aluminum. The use of silane-functional crosslinking agent and silane adhesion promoters in the coatings has greatly enhanced adhesion. The increase in film hardness via increased crosslinking density also did not hinder the adhesion due to the presence of silane groups in the crosslinker. Additionally, increased time and temperature during curing showed positive effects. However, the variation of resin ratio, solid content, and film thickness did not offer significant adhesion improvement in this study.