Linking formability to the structure – property relationships of coil coatings

Abstract

Polyester coatings crosslinked with hexa(methylmethoxy)melamine (HMMM) are commonly used in coil coating, a highly automated and efficient process used to produce pre-painted metal sheet (PPM). The PPM undergoes large deformations when formed into end-products, ranging from white-goods to architectural cladding, which are used in diverse environments. As the coatings protect the substrate and provide excellent aesthetics, it is critical for them to withstand failure by cracking. The thermo-mechanical properties of polyester coatings with different chemistries were determined using dynamic mechanical analysis, differential scanning calorimetry and tensile testing. The formability of the coatings was examined using industry standard T-bend and Erichsen cupping tests. The effects of varying the crosslinker content from 5 % to 30 % with respect to resin, replacing phthalic anhydride with isophthalic acid in the resin, varying the adipic content from 12 % to 24 %, and varying the resin molecular weight (Mn) from 1500 g/mol to 3300 g/mol were investigated. Increasing the crosslinker content, and decreasing the adipic content and Mn decreases the formability. This is attributed to an increasing Tg and restrictions on polymer chain movement. The addition of TiO2 pigment increases the stiffness and induces a toughening effect at elevated temperatures. The surface strains in the T-bend test (~ 225 % at 0T, reducing to ~ 27 % at 4T) and Erichsen cupping test (~ 38 % at IE = 9 mm) were quantified for the first time using finite element analysis, and experimentally verified. A study on the parameters affecting the strains in the Erichsen cupping is presented, and a novel analytical model to predict the applied strains in T-bend tests has been proposed.