Rheological characterisation of water based paint using associative rheology modifiers

Abstract

Water-based paints often suffer defects such as storage instabilities, sagging, poor flow and levelling. The performance of paint rheological additives in the presence of acrylic binders used in the formulation of water-based paints, is not well understood. This study utilises the principles of Rheology to investigate the performance of Hydrophobically Modified Ethoxylated Urethane (HEUR) and Hydrophobically Modified Alkali Swellable Emulsions (HASE) rheology modifiers, in the presence of styrene and pure binders in a decorative water-based paint formulation. The performance of HASE and HEUR at concentrations ranging from 0 to 1.2 percent was studied in the presence of pure acrylic and styrene acrylic binders. A total of 12 experimental paint samples were prepared and subjected to rheological testing. The rheological tests employed in this study include flow curves, dynamic amplitude sweep, frequency sweep and three interval thixotropy tests (3ITT). The data obtained was then correlated to paint properties such as storage stability, ease of application, levelling and sagging. HASE was found to have higher thickening efficiency and imparted higher elasticity to paint in the presence of pure acrylic rather than styrene binders. The viscosity curves results showed that paint samples containing HASE have higher viscosities than HEUR, at any given shear rate and irrespective of the type of Binder. The increase in HASE concentration resulted in an increase of paint viscosity throughout the shear rate range; whereas higher HEUR concentration resulted in decrease of paint viscosity in the low shear rate, while high shear viscosity was observed to increase. This finding suggests that higher HEUR concentration can negatively affect the storage stability of water-based paint. Results from the Amplitude sweep test revealed that paints containing HASE have higher storage moduli (G’) than HEUR in the linear viscoelasticity region. The fact that HASE produces higher viscosity and higher G’ values in the linear viscoelastic region (LVER) suggests that paints thickened with HASE possess good storage stability and are not likely to sag after application. It was also established that the higher elasticity and higher viscosity values at the high shear rate of HASE thickened paints can negatively affect the application and post application properties of paint. On the other hand, the low viscosity values at the high shear rates and lower critical strains obtained for paints containing HEUR suggest that these paints can easily be applied and will have an adequate levelling after application. Paint samples containing HASE are expected to produce a larger film thickness during application, as they have high viscosities at high shear rates and larger storage modulus (G’). Results from 3ITT, flow curve and frequency sweeps revealed that paints made of HEUR containing HEUR will have better flow and levelling properties after their application.

The Power Law, Bingham, Casson and Herschel-Bulkley models were used to fit the viscosity curve test results. Power law and Herschel-Bulkley were found to be the best models as they generated smaller values of RMSE, as well as larger adjusted R2 and R2, respectively.