Characterization of clearcoats containing phosphoric acid-functionalized acrylic polyols for automotive precoated metal sheet coatings

Abstract

In this study, the various physical and mechan- ical properties of clearcoats prepared through a new crosslinking method were investigated. The method was aimed at developing clearcoat systems to improve the deep-draw processing and formability performance in precoated metal (PCM) sheets for automotive applica- tions. From phosphoric acid-functionalized acrylic poly- ols (PAFAPs) first synthesized in this study and glycidyl methacrylate-modified acrylic copolymer (GMAMAC), phosphoric acid-GMA modified acrylic polyols (PAGMAPs) were newly prepared as new binders in automotive clearcoats. Several clearcoats were formu- lated with different molecular weights and hydroxyl contents from PAFAPs and GMAMAC. Using clear- coats themselves, the crosslinking reactions for these clearcoats were compared by evaluating the curing behaviors with a rigid-body pendulum test (RPT) and the changes of chemical structures via attenuated total reflectance FTIR spectroscopy. The mechanical proper- ties of the clearcoats were systematically characterized, using dynamic mechanical analysis (DMA) and universal testing machine analysis (UTM). Also, various tests were carried out using PCM sheets by depositing clearcoats above the same PCM-based primers and basecoat layers on galvanized steel. The fracture and deformation patterns related to surface damages on the clearcoat surface were visualized using a nano-scratch test, in association with atomic force microscopy. In particular, deep-draw processing tests, based on forming process simulations, were employed to scrutinize the effect of clearcoats developed in this study on the forming feature in PCM sheets. From the results of RPT, DMA, and UTM tests, the primary crosslinking networks of PAGMAPs from the synthesized PAFAPs and GMAMAC, and also succeeding secondary crosslinking networks between PAGMAPs and blocked isocyanates, were closely corre- lated with the degree of crosslinking (Xc), in accordance with the molecular weight between crosslinks (Mc), and glass transition temperature (Tg). As a result, the presented clearcoats with a long pendulum period, a low rubbery modulus, and a large tensile strain value, which are the significant factors for developing automo- tive PCM sheet technology, have truly demonstrated more superior formability during the deep-draw process. It is confirmed that properties of clearcoats with tough- ness and flexibility could be optimally controlled by PAGMAPs for automotive coatings.