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Oscillatory and steady shear rheology of model HEUR-thickened waterborne paints Valeriy V. Ginzburg, Tirtha Chatterjee, Alan Isamu Nakatani, and Antony Keith Van Dyk Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01711 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018
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Oscillatory and steady shear rheology of model HEUR-thickened waterborne paints Valeriy V. Ginzburg1*, Tirtha Chatterjee2, Alan I. Nakatani3, and Antony K. Van Dyk4
1
The Dow Chemical Company, Materials Science and Engineering, Building 1702, Midland, MI 48674
2
The Dow Chemical Company, Dow Water and Process Solutions, 400 Arcola Road, Collegeville, PA
19426 3
The Dow Chemical Company, Analytical Science, 400 Arcola Road, Collegeville, PA 19426
4
The Dow Chemical Company, Dow Coating Materials, 400 Arcola Road, Collegeville, PA 19426
* Author for correspondence. Email
[email protected] Keywords: paints; latex; rheology; colloidal stability; associative thickeners; oscillatory shear; steady shear; Cox-Merz rule; transient network
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Abstract Hydrophobically modified ethoxylated urethane (HEUR) thickeners are widely used as rheology modifiers for waterborne paints. While the rheology of HEUR solutions in water is fairly wellunderstood, their impact on the rheology of waterborne latex/pigment suspensions (formulated paints) is more complicated. We study the shear rheology of model HEUR/latex/TiO2 suspensions in water, and investigate the dependence of both oscillatory and steady shear behavior on the strength of the HEUR hydrophobes. We observe that in both oscillatory and steady shear experiments, rheological curves could be shifted onto a single master curve, demonstrating a “time-hydrophobe superposition” (THS). We also note that oscillatory shear behavior exhibits a power-law spectrum of relaxation times, unlike the single-Maxwellian behavior of pure HEUR solutions. Based on these results and earlier experimental and theoretical findings, we propose that the rheology of the HEUR-thickened latex/TiO2 suspensions is mainly determined by the transient network of HEUR-bridged latex particles, with a broad distribution of the bridge characteristic lifetimes. The model is found to be in good qualitative and semi-quantitative agreement with experiments for both steady and oscillatory shear.
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Introduction Hydrophobically modified ethoxylated urethanes (HEUR) represent an important class of additives in waterborne paints and the coatings industry.1-9 HEUR-thickened paints exhibit good flow properties, including leveling and sag resistance. In the past 30 years, HEUR thickeners have been the subject of numerous studies, both experimental and theoretical. Most of those studies concentrated on the structure and rheology of pure HEUR aqueous solutions.10-15 It has been conclusively demonstrated that HEUR molecules associate into so-called “flower micelles”, and the micelles then form a transient network.10-11 The structure and viscoelastic behavior of HEUR solutions has been studied, both experimentally and theoretically by numerous authors.14, 16-28 It was shown that the linear viscoelasticity can be accurately described by a simple model with a single Maxwell element where the dashpot viscosity, η, and the spring modulus, G, are related via the Green-Tobolsky29 relationship, η = Gτ = νRTτ; here ν is the density of physical crosslinks, and τ is the lifetime of an average crosslink. Nonlinear elasticity of transient networks was shown to be much more complex, with many systems exhibiting shear thickening (followed by shear thinning) while others showed only shear thinning. The early approach of Tanaka and Edwards18-21 predicted primarily the shear thinning behavior, while the subsequent refinements by Tripathi et al.,14 Indei22-24 and others described the appearance of shear thickening due to shear enhanced formation of network strands and/or finite extensible nonlinear elasticity (FENE) of the strands. Independently, Watanabe and co-workers proposed a different approach in which they related shear thickening to anisotropy in the network strand destruction.15, 26-28 Even though some uncertainty about the origins of shear thickening and its dependence on the HEUR concentration, hydrophobe strength, and backbone molecular weight still remain, overall the rheology of pure HEUR aqueous solutions seems to be understood reasonably well.
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The picture is much less clear, however, when one considers a formulated paint, with HEURs and latex (and, potentially, pigments, surfactants, dispersants, and other additives) mixed in water. The original view was that in a paint formulation, HEUR molecules form a transient network of flower micelles, the same way as in solution.5-6, 30-31 Some of the micelles or individual HEURs can adsorb onto the latex surface, thus connecting the latex particles to the network; the adsorption of HEURs onto latexes has been studied extensively by numerous researchers.32-37 Overall, the rheology of the model paint is determined by a complex interplay between the HEUR adsorption onto the latex particles, formation of the micellar network, bridging of the particles by the polymers, and potentially clustering of the particles and formation of large aggregates. In recent years, experimental studies by Van Dyk and co-workers38-43 and theoretical research by Larson and co-workers39, 44-49 offered a slightly different picture of the HEUR-thickened waterborne paints. Within this framework, for the typical HEUR concentration range (< 3 wt.%) used in commercial paints, all – or practically all – HEUR molecules are adsorbed onto latex surfaces, as was convincingly demonstrated with pulsed field gradient nuclear magnetic resonance (PFGNMR) spectroscopy.42 The thickener molecules primarily form loops and transient bridges between the particles. Further, the stretching, relaxation, and rearrangement of those bridges is responsible for the viscoelasticity of the overall fluid. If the particles are connected by strong, long-lived bridges, they end up forming aggregates or fractal clusters as observed in neutron scattering experiments.38, 40-41 The interplay between the adsorption and the ability of the hydrophobes to re-arrange between bridges and loops turns out to be critical to the rheology, specifically to the first normal stress difference at high shear (positive for the case of strong adsorption combined with effective re-arrangements, and negative otherwise).43 Simulations by Larson and co-workers, likewise, helped demonstrate the preference of the HEUR molecules to adsorb on the latex surfaces instead of forming micelles and micellar networks.44-45, 47-48 Further simulations showed that for HEURs with hydrophobe-latex adsorption energies > 8 kBT (at or
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near room temperature), the fraction of molecules adsorbed onto latexes is close to 100% in compositions typical for waterborne paints.39 All these results are combined to develop a coarse-grained model to study the rheology of latex dispersions with HEUR bridges, where the bridge creation and destruction in shear flow is modeled using a population balance approach49 or directly with HEUR bridges modeled as finitely extensible nonlinear elastic (FENE) dumbbells.50 Further coarse-graining and consideration of flow hydrodynamics is likely required to model the rheology of waterborne paints in realistic detail. Building on these recent advances, here we analyze the shear rheology of model waterborne paints. We use HEURs with various hydrophobe strengths, similar to those studied in ref. 42, and measure their oscillatory shear rheology in solution, and both oscillatory and steady shear rheology in a model paint (waterborne formulation containing latex, pigment, and HEUR). We show that for the model paint both oscillatory and steady shear responses obey the principle of “time-hydrophobe superposition”, thus supporting the hypothesis that the main contribution to viscosity comes from the HEUR bridges between the latex particles. We also demonstrate that the bridged particle hypothesis helps explain the power-law relaxation time spectrum for the oscillatory shear, and suggest simple scaling expressions describing the rheology under steady shear.
Materials and methods Sample preparation Hydrophobically modified ethylene oxide urethane (HEUR) rheology modifiers (RMs) used in this study were telechelic in nature with a backbone PEO molecular weight ~ 30,000 g/mol end-capped by alkyl hydrophobes. The strength of the hydrophobe is expressed in terms of the equivalent number of methylene groups.51 The end-hydrophobe strength was systematically varied in between 10 to 18. In our
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nomenclature HEURX represents a HEUR molecule with end-hydrophobe strength of X equivalent methylene groups where a higher X value denotes stronger hydrophobicity. Further details on the HEUR chemistry were reported in a previous publication.42 Aqueous HEUR solutions with 10 wt% concentration were prepared in 40 g batches by directly adding a measured amount of HEUR to water and mixing on a Cole-Parmer Roto-Torque rotary mixer overnight. All the model paint formulations studied here were made with commercial acrylic binders with a nominal particle diameter of 150 nm and a size polydispersity of ~ 10%. For all the paints, initially a stock solution was prepared with latex emulsion (in aqueous solvent), along with H2O, pigment (TiO2, Ti-Pure R-706, The Chemours Company), surfactant (TRITON™ X-405 surfactant, The Dow Chemical Company), dispersant (TAMOL™ 1124 dispersant, The Dow Chemical Company) and defoamer (BYK024). Pigment dispersion was performed in a Cowles high speed disperser. Calculated amounts of the rheology modifier (premixed with water) were added to the stock solutions and mixed using a mechanical stirrer. pH adjustment was done on the final sample by titrating with AMP-95 to maintain a pH value of 9.0 (to ensure long-term shelf-stability). Relevant composition details are reported in Table 1. The typical density of the wet paints was ~ 1.25 g/cc. Table 1: Description of model paint formulation used in this study
Paint
Name
Latex volume solid (φlatex), (v/v)
Model
0.28
Total volume solid b (VS) , (v/v) 0.35
Pigment Volume c concentration (PVC), (%) 20
d
RM concentration (wt.%) 0.44
a
a
All paints contained a fixed (0.2%, w/w) amount of surfactant and 1 wt% (based on pigment) dispersant VS = φlatex + φpigment, where φpigment = pigment volume solids c PVC = 100*φpigment/VS d HEUR type: HEURX where X=10/12/14/16/18 b
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Rheology Dynamic frequency sweeps under oscillatory shear were performed on a Rheometrics Fluid Spectrometer (RFS-2) using 50 mm diameter parallel plate fixtures at 25 °C with a 0.5 mm gap. The frequency (ω) sweeps were conducted from 0.1 rad/s to 100 rad/s in six equally spaced logarithmic increments per decade of frequency. Depending on the viscosity of the samples, the applied sinusoidal strain amplitude (γ0) was varied between 5% and 15%. A strain amplitude sweep measurement (at fixed frequency) was performed on each sample (using a separate aliquot) to determine the linear viscoelastic regime. Lower viscosity samples necessitated the use of higher strains to obtain sufficient torque. The storage and loss moduli, G’(ω) and G”(ω), respectively, and complex viscosity (η*) were measured as a function of the frequency. The magnitude of the complex viscosity can be expressed as:
η = *
(G ' )2 + (G")2 ω
=
G*
ω
.
Strain rate sweeps under steady shear were performed on a TA Instruments AR-G2 stress rheometer using a 40 mm diameter stainless steel, upper parallel plate and Peltier plate temperature controlled lower plate at 25 °C and a 0.5 mm gap. The rate sweeps were conducted from 0.01 s-1 to 10,000 s-1 in five equally spaced logarithmic increments per decade of shear rate. All the samples were hand-mixed prior to loading using a spatula and no additional pre-shear or flow-conditioning was performed before the rate sweeps. Measurements were stopped if samples were expelled from the gap between the fixture plates due to inertia. The steady-shear viscosity and normal force were recorded as a function of the applied shear rate. The typical relative precision (twice the standard deviation divided by the mean value, in %) of the suspension viscosity measured by the rotational rheometer is
