In situ characterization of dynamic structures of coatings

Abstract

We have developed new methods and apparatus to characterize the structure changes of coatings in situ. The techniques enabled the study of critical factors to control during drying or curing process to avoid excess materials use and coating defects. The instruments have been used to investigate the effect of process conditions on the structure development and final coating properties. A magnetic microrheometer for in situ measurement of local viscosity of coatings was designed since conventional bulk rheometry cannot be used to follow the temporal and spatial gradients of viscosity in drying or curing coatings. Micron-sized magnetic probe particles under a magnetic field gradient act as probes for local rheological responses in coatings. Viscosity-time profiles were measured in drying aqueous PVA coatings, and the results revealed the correlation between sagging defects and viscosity build-up. The development of viscosity gradient through the thickness in UV curing epoxy coatings was also characterized to study wrinkling defects, skin formation, and structure or composition gradients through the coating thickness. The microstructure of drug-polymer coatings was also characterized using confocal Raman microscopy. A drying apparatus was built to control the coating method, drying temperature, and air flow during the characterization since the size and distribution of drug phase and polymer structure in the coating strongly depend on the process conditions. The dependence of environmental inputs to the drug and polymer coating morphology during drying was investigated in order to elucidate and optimize either the processing conditions or coating formulation.