Influence of wetting agent chemistry on associative interactions in hydrophobically modified cellulose ether systems

Abstract

Hydrophobically modified cellulose ethers are widely used as associative thickeners in water-based paints and their properties depend on the interactions with wetting agents and dispersed particles in the paint composition. These interactions are important for optimization of the composition with respect to viscosity, stability, and application. This project investigated two hydrophobically modified cellulose ether thickeners with different chemistry, hydrophobically modified ethyl hydroxyethyl cellulose (HM-EHEC) and hydrophobically modified methyl ethyl hydroxyethyl cellulose (HM-MEHEC), to explore the effect of methyl substitution on the polymer. The interactions with a number of nonionic surfactants were studied, and their structure and concentration were compared by measuring the rheological behavior, stability, particle size, and surface activity. Rheological measurements of the viscosity demonstrated the dependence of both surfactant structure and methyl substitution on the polymer. Branched surfactants produced slightly higher viscosity than the linear ones, and a lower ethylene oxide (EO) content significantly increased it. HM-EHEC generally exhibited higher viscosity and the range was larger than for systems containing HM-MEHEC, indicating a weakened hydrophobic association with methyl substitution. The presence of propylene oxide (PO) groups shifted the viscosity maximum toward lower surfactant concentrations. Stability was evaluated using backscattering (BS) measurements, with samples containing latex as well as the cellulose ether thickeners and surfactants. Phase separation was proven to increase with increased surfactant concentration, likely due to depletion flocculation. Dynamic light scattering (DLS) measurements supported the formation of larger aggregates at higher surfactant concentration, immediately after sample preparation. Over time, the method suggested adsorption of polymer-surfactant complexes onto the latex particles. Surface tension measurements also indicated complex dynamics of the polymer-surfactant systems since the results deviated from ideal critical association behavior, especially for surfactants with low critical micelle concentration (CMC). Overall, the nonionic surfactants affected the associative behavior of the cellulose ethers differently and their structures affected the viscosity and surface tension of the systems. The choice of cellulose ether thickener, surfactant, and surfactant concentration is important to adjust for the application. The findings contribute to an improved understanding of the interactions between surfactants and polymers in waterborne paints and provide insights for future paint formulations.