A Model Study for Prediction of Performance of Automotive Interior Coatings: Effect of Cross-Link Density and Film Thickness on Resistance to Solvents and Chemicals 05-12-02-0007

Automotive interior coatings for flexible and rigid substrates represent an important segment within automotive coating space. These coatings are used to protect plastic substrates from mechanical and chemical damage, in addition to providing colour and design aesthetics. These coatings are expected to resist aggressive chemicals, fluids, and stains while maintaining their long-term physical appearance and mechanical integrity. Designing such coatings, therefore, poses significant challenges to the formulators in effectively balancing these properties.

Among many factors affecting coating properties, the cross-link density (XLD) and solubility parameter (δ) of coatings are the most predominant factors. In general, the higher the XLD (i.e., more number of cross-links between the polymeric chain per unit volume of coating network), the lower the free volume between polymer chains and the lower the permeability to the diffusion of solvents and chemicals at a given film thickness. Coatings with optimum XLD are desirable as XLD also affects various mechanical properties like flexibility, hardness, and toughness. Coating formulators often use time-consuming trial-and-error studies to determine formulation with optimum XLD.

In this study, a range of 2K-polyurethane automotive clear coats with varying XLDs were formulated and applied on widely used automotive plastic substrates at varying dry-film thicknesses (DFT). The XLD of these coatings were determined by equilibrium swelling technique using various solvents and also by dynamic mechanical analysis (DMA). Additionally, these coatings were tested for chemical, physical, and mechanical properties as per standard test protocols. Analysis of the results provided useful insight into the effect of XLD, DFT, and δ on chemical resistance of the coatings. The outcome of this research can be useful for coating formulators, specifiers, and end-users in optimization of formulations, application processes, and trouble shooting.