On-line, non-Newtonian capillary rheometry for continuous and in-line coatings production

Moving from traditional batch production into in-line or continuous coatings production requires accurate on-line quality control instruments. The aim of the present work was to investigate the principle of non-Newtonian capillary rheometry for quality control purposes. In the investigation, three series of acrylic-based viscoelastic coating samples with different types and concentrations of pigments and thickening agents were used, and the rheological measurements were compared to results obtained with the so-called Stormer viscometer and an advanced off-line rheometer. A detailed analysis of the potential measurement implications was also conducted. For shear stresses from 15.0 to 350.0 Pa (the upper boundary), the novel capillary rheometer was found to provide results in good quantitative agreement with the advanced rheometer when sample holding time, and thereby shear history, was properly controlled. At a shear stress between 1.0 Pa (lower boundary) and 15.0 Pa, the agreement was not as good, with a difference in results of the non-Newtonian capillary rheometer and the advanced rheometer between 15% and 74%. The resolution of the capillary rheometer was sufficiently high to allow detection of the rheology changes associated with variations in coating formulations of pigment volume and rheology modifier concentrations. In summary, for fast on-line evaluation of coating rheology, the principle of capillarity has been demonstrated to be a varied and robust technique.