Date of Award

9-2020

Degree Name

MS in Polymers and Coatings

Department

Chemistry & Biochemistry

College

College of Science and Mathematics

Advisor

Dr. Raymond Fernando

Advisor Department

Chemistry & Biochemistry

Advisor College

College of Science and Mathematics

Abstract

Rheology modifiers such as hydrophobically-modified ethoxylated urethane (HEUR)thickeners are included in waterborne latex coatings to optimize shear-rate dependent viscosity and other rheological properties. While these HEUR polymers are commonly used in industry, the complex chemical interactions that contribute to rheological properties are still not completely understood. Prior work in this area has focused on understanding latex-HEUR and latex-surfactant-HEUR interactions that affect rheological properties. Additionally, studies have been previously conducted to understand the relaxation mechanisms of complex interactions present in HEUR-thickened waterborne latex coatings under various dynamic conditions. The objective of this work is to extend the experimental work to fully-formulated coatings and determine the effects of additional ingredients in a fully-formulated system.

Coating formulations were prepared with a target 90 KU (Kreb Units) viscosity, having 0.23wt% HEUR. The pigment volume concentration (PVC) and non-volatiles by volume (NVV) were kept constant at 19.87% and 30.47%, respectively. An analysis of phase stability (presence or absence of syneresis), flow sweep (10-2 to 103 s-1), oscillatory strain (10-2 to 102 %), and oscillatory frequency (10-2 to 102 Hz) data was carried out in an attempt to determine connections among these properties. Furthermore, brief comparisons were made with previous results on latex-HEUR and latex-HEUR-surfactant systems that utilized the same HEUR thickener and latex used in this study. In the fully-formulated system, 0.23wt% HEUR was found to be in excess of what is needed to saturate latex surfaces. This HEUR level is less than half of the level needed to saturate latex surfaces in simpler latex-HEUR systems in previous studies. Fully-formulated coatings, in addition to having TiO2 and other ingredients are more crowded than the previous systems. It appeared that a depletion flocculation mechanism dominated at low surfactant concentrations for fully-formulated systems in this study as evident from syneresis; large HEUR aggregates appear to build enough osmotic pressure to drive aggregation of latex and pigment particles resulting in depletion flocculation. At increasing surfactant levels, the depletion flocculation mechanism was negated allowing the associative HEUR bridge networks to dominate and stabilize the system. Phase stability for fully-formulated systems in this study were associated with Newtonian viscosity plateaus on flow sweeps, strain hardening on oscillatory strain sweeps, and formation of high frequency moduli plateaus in frequency sweeps. Further increase of surfactant concentration appeared to disrupt the stable latex-HEUR network due to competitive adsorption of surfactant on latex particles, resulting in syneresis from bridging flocculation.

Possible correlations between phase stability and high relaxation times were seen, although further analysis of relaxation time data and simulations will need to be carried out to better understand the behavior of HEUR in fully-formulated systems.

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