Syneresis and Rheology Mechanisms of a Latex-HEUR Associative Thickener System

• Main Author: Smith, Travis Bruno
• Format: Others
• Published: DigitalCommons@CalPoly 2015
• Subjects: Polymer Chemistry
• Online Access: https://digitalcommons.calpoly.edu/theses/1502; https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=2653&context=theses

Rheology modifiers are used in paints and coatings to ease their application to a surface, prevent sagging once applied, and allow the leveling of brushstrokes, among other benefits. The early rheology modifiers were hydroxyethyl celluloses (HECs), a type of non-associative thickener that is relatively inexpensive and synthesized from cellulose, which is abundant. However, coatings that are modified with HECs tend to suffer from poor leveling and syneresis (phase separation). HECs have since been replaced with associative thickeners (ATs). These thickeners, when properly formulated, produce stable dispersions that have improved rheological properties, yet, unlike HECs, are sensitive to changes to the coating formulation. This drawback has encouraged research that attempts to predict the phase behavior and rheology of systems that are modified with ATs. This work is concerned with the phase behavior and rheology of waterborne latex / hydrophobically-modified ethoxylated urethane (HEUR) AT systems. When latex volume fraction is held constant, the amount of HEUR (and surfactant) in the mixture determines whether the system experiences syneresis. Dispersion phase diagrams (DPDs) of such systems have been previously studied, but the rheology of the mixtures used to prepare the DPDs have not been studied in any detail. A study on the rheology of phase separated latex / HEUR mixtures that were prepared with commercial materials was done at Cal Poly and showed a correlation between syneresis and complex rheology. However, a proper analysis was limited because the compositions and chemical structures of the commercial materials were not well known. To better understand the relationships between phase behavior and rheology, waterborne latex / HEUR mixtures were prepared from latex and HEURs that were made at Cal Poly. Three series of mixtures were studied: commercial latex / commercial HEUR (I), commercial latex / Cal Poly HEUR (II), and Cal Poly latex / Cal Poly HEUR (III). The latex volume fraction was held constant at 0.25 and the concentration of HEUR was varied from 0–2.0 wt%. Mixtures were allowed to equilibrate for 7 days, syneresis was measured on day 7, and steady-state viscosities over a shear rate range of 0.01–1000 s-1 were determined on days 7–9 with a DHR-2 rheometer (TA Instruments) that was outfitted with 40 mm, 2o cone. The mixtures were also studied by microscopy and dynamic oscillatory testing. The chemical structures of the Cal Poly HEURs were determined by proton nuclear magnetic resonance spectroscopy (1H NMR) and the molecular weight by gel permeation chromatography (GPC). From this study, a correlation between syneresis and complex rheology was observed in I. Similar trends were observed in phase-separated II and dispersed (not phase-separated) III, though II with over 0.4 wt% HEUR were ejected from the cone / plate geometry at 1–100 s-1 and III did not demonstrate syneresis. Further investigation of dispersed II and phase-separated III is recommended to confirm the presence of the syneresis–rheology correlation of I in both II and III. In addition to these trends, only 2.1–4.2 wt% II were able to be analyzed by the single-mode Maxwell model. Also the transition from phase-separated to stable dispersion was observed with a polarized microscopy at 5x magnification. In conclusion this study represents progress in the ongoing study at Cal Poly to better understanding the mechanisms behind the syneresis and rheology of these latex / HEUR AT dispersions.