Surface-tension-driven flows of coatings: Bondline readout formation.

Abstract

Bondline readout (BLRO) is a coating defect that has recently been observed on automotive, adhesively-bonded, sheet molding compound (SMC) assemblies painted with high-flow/high-glamour clearcoats. BLRO results from surface-tension-gradient flow, which in turn results from temperature gradients generated on the SMC surface during convection heating/curing of the paint. The adhesive bond acts as a heat sink and generates these temperature gradients. The end result is an undesirable outline or imprint in the coating surface of the underlying adhesive bond. To eliminate BLRO, automotive divisions that prefer using high-glamour clearcoats are forced to use metallic body panels, while those divisions that prefer using SMC and other polymeric body panels are forced to use low-glamour clearcoats. Unfortunately, in many cases, this results in weight, cost, and corrosion-resistance penalties or inferior finishes, respectively. In this dissertation, temperature- and concentration-induced BLRO is investigated experimentally and numerically to obtain better insight into the BLRO mechanism so that vehicles with polymeric body panels can ultimately be painted with high-glamour clearcoats without generating BLRO. Non-reacting, single-solvent, polybutylene films were applied on to a conduction heater surface and heated nonuniformly to generate BLRO flow. Time-dependent surface profiles of the films were measured using a non-contact laser profilometer. The calibration, measurement, and data analysis techniques required to obtain accurate results from the profilometer were developed. Five BLRO parameters--initial film thickness, viscosity, heating rate, solvent volatility, and solvent-to-resin surface-tension ratio--were studied to assist in the BLRO mechanism investigation. A one-dimensional (1-D) code, based on the lubrication approximation, and a two-dimensional (2-D) code, based on Patankar's SIMPLER algorithm, were written to obtain detailed quantitative information regarding the surface-tension-gradient driving-force and the resulting pressure and velocity distributions within the film. Profile results predicted numerically were in very good agreement with those measured experimentally. The 1-D code was found to be significantly more efficient than the 2-D code, while producing nearly identical results. Potential BLRO solutions that are based on reducing temperature gradients and controlling clearcoat rheology are proposed. However, a criterion for BLRO acceptability must be determined and actual reactive automotive clearcoats must be modeled before the solutions can be evaluated effectively in future work. A possible BLRO criterion involving a critical surface curvature, and code modifications required to handle automotive clearcoats, are presented.