The effect of structure-property relationships on the formability of pigmented polyester coatings

Pre-painted metal sheet (PPM) is used in applications from domestic appliances to architectural cladding. The coating provides excellent aesthetics and corrosion protection, but must possess excellent formability to not fail due to the large strains applied during the folding and hemming processes used to produce components. Therefore it is important to understand how the coating formulation affects the free-film properties and formability of a coating system. Polyester coatings crosslinked with hexa(methoxymethyl)melamine (HMMM) with a Tg of ∼40 °C were used, pigmented with TiO2. The glass transition temperature was increased by increasing the crosslinker content (from 5% to 30%), decreasing the adipic acid content (from 24% to 12%) and decreasing the molecular weight (from Mn = 3300 g/mol to Mn = 1500 g/mol). The chemical structure of the resin had little effect on the formability of the coatings when the test temperature was normalised with respect to Tg. The formability measured using the Erichsen cupping and T-bend tests is related to the tensile properties of the free-films. The damage induced by the T-bend is greater than that by the Erichsen cupping test due to the higher applied deformation rates in T-bend tests. Increasing the apparent yield and fracture stress increases the likelihood of damage at lower deformation levels, whilst increasing the strain to failure decreases the likelihood of damage in both the T-bend and Erichsen cupping tests. The strength of these correlations reduces with an increase in T-bend level as the magnitudes of the strains applied are reduced. This work takes a holistic approach to correlate structure and tensile properties with formability in both the Erichsen cupping test and T-bend test for the first time, enabling industry to improve coating performance significantly but cost-effectively.