The Prediction of Paint Properties from Rheological Data
Advancing the chemical engineering fundamentals
Rheology (T2-4)
Keywords: Paint, rheology, rheometer, modelling
A paint can be regarded as a complex mixture of mutually interacting components, usually formulated for specific application conditions. Rheology is the science of deformation and flow. All forms of shear behaviour can be viewed as lying between two extremes: the flow of ideal viscous liquids on the one hand and the deformation of ideal elastic solids on the other. The behaviour of all real materials, such as paint, is based on the combination of both the viscous and the elastic portion and therefore, it is called visco-elastic materials.
To a degree matched by few other areas, rheology determines the success for coatings. Though all other properties can be acceptable, a coating will usually not meet with success if the rheology is not acceptable. Experienced formulators say that more than half the cost of new product development is consumed in “getting the rheology right”. Moreover, apparently “minor” changes in a raw material or process can cause significant and unexpected variability in product rheology. For all these reasons, rheological analysis is a vital and cost-effective tool for the coatings industry. The development of relationships between paint properties and the relevant rheological test methods has been identified as a major gap in the coatings industry.
Rheological measurements were performed using a MCR 300 (Paar-Physica, Stuttgart, Germany). Rotational (ROT) and oscillatory (OSC) measurements were performed on all the paint mixtures. Rotational measurements were: Constant high shear (HS), constant low shear (LS), flow curve (FC) and three-interval-thixotropy-test (3-ITT). Rotational measurements were performed with the cone and plate measuring system CP50 (diameter = 50mm, angle = 1˚, cone-plate distance = 0,050 mm). The measurements were performed in a controlled shear rate (CSR) mode and the resulting shear stress was measured and used for modelling purposes. Oscillatory measurements were: Amplitude sweep (AS), frequency sweep (FS), time sweep (TS), three-interval-thixotropy-test (3-ITT), and extra low frequency test (XLF). Oscillatory measurements were performed with the plate and plate measuring system PP50 (diameter = 50mm, plate-plate distance = 0.25 mm). The measurements were performed in a controlled shear deformation (CSD) mode and the resulting shear stress and phase angle was measured and used for modelling purposes. All measurements were performed at a constant temperature (23 ± 0.2˚C).
A total of 43 paint properties were measured. This includes properties where the relationship between rheology and the specific paint property is well established, e.g. sagging, spatter, leveling, in-can stability, etc. Other paint properties were also measured where the relationship between rheology and the specific paint property is not that well established, e.g. gloss, burnish, water permeability, etc. All properties were measured according to international standards such as American Standard Test Methods (ASTM) and Coatings Research Group International (CRGI).
Models were developed using both Multiple Linear Regression as well as Artificial Neural Networks to predict the following paint properties from the rheological data: Dry Burnish 20, Dry Burnish 60, Dry Burnish 85, Wet Burnish 20, Wet Burnish 60, Wet Burnish 85, Water Permeability, Krebs viscosity, Sag, Open time, Gloss 20, Gloss 60, Gloss 85, Dirt Pick-Up, Opacity, Hiding Power and Dry Film Thickness. Accurate predictions in the critical paint properties that are used for quality control (Krebs Viscosity, Opacity, Gloss and Dry Film Thickness) are possible as long as raw materials are varied within plus/minus 20% of the standard formulation.
Presented Thursday 20, 15:00 to 15:20, in session Rheology (T2-4).
