Architectural coatings/paints play a very important role in our world, making our houses and workplaces beautiful, comfortable, and energy-efficient. Polymeric components of coating formulations include latex or binder, dispersants, and thickeners or rheology modifiers [RM]). This review is devoted to the RMs, primarily associative thickeners [AT], specifically highlighting the relationship between the thickener molecular structure, paint formulation, rheology, and application. We briefly describe the chemistries of various associative thickeners (“hydrophobic alkali-swellable emulsion” [HASE], hydrophobically modified hydroxyethylcellulose [HMHEC], and “hydrophobically modified ethoxylated urethane” [HEUR]) polymers. Concentrating on the HEUR thickeners, we then review the structure and rheology of their aqueous solutions and of model paints containing HEURs and latex particles in water. For model paints, we review experimental studies (rheology, NMR, and SANS) and theoretical models from molecular-level classical Molecular Dynamics [MD] to mesoscale coarse-grained MD to large-scale rheology calculations. Lastly, influence of rheological metrics on specific paint application features (flow and leveling, sag resistance, and brush drug) is discussed. We conclude by highlighting opportunities and challenges for future research.

Commercial paints are complex formulations and their application properties are typically explained based on their steady shear viscosity profiles. Hydrophobically modified ethoxylated urethane (HEUR) polymers are widely used as rheology modifiers (RM) in waterborne paints. In this work we formulated a series of HEUR-based model paints with matching high-shear viscosity (0.12 Pa-s at 10,000 s(-1)). Compositionally these paints were similar, differing only in the HEUR end-hydrophobe length (Cn, n = 10-18) and the amount used to obtain the targeted high-shear viscosity. Their linear viscoelastic response was dominated by a transient network of HEUR-bridged latex particles as revealed from oscillatory shear measurements. The low (steady) shear viscosities were related to the HEUR end-hydrophobe length while the high-shear viscosities tend to approach the targeted value. The first normal stress difference (N-1) at high shear changed sign from negative for low Cn (n = 10, 12) to positive for high Cn (n = 16, 18) paints. Further, brush drag was measured in real time where the typical application shear rate was approximate to 10,000 s(-1). Paint brush drag was found to increase monotonically with HEUR Cn and this variation/trend could not be explained through their high-shear viscosity. It is proposed that the observed trend can be explained based on the paint N-1 sign, where a negative N-1 provides a compressive force pulling the brush towards the surface and facilitates paint transfer at relatively lower drag. In contrast, a positive N-1 exerts a tensile force tending to pull the brush out from the surface and extra force is required to overcome this effect. This work emphasizes the importance of normal stress (along with viscosity) to understand paint properties, especially for high-shear applications.

Hydrophobically modified ethoxylated urethane (HEUR) thickeners are widely used as rheology modifiers for waterborne paints. Although the rheology of HEUR solutions in water is fairly well-understood, 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 behaviors 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". We also note that the oscillatory shear behavior exhibits a power-law spectrum of relaxation times, unlike the single-Maxwellian behavior of pure HEUR solutions. On the basis of 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 characteristic lifetimes of the bridge. The model is found to be in good qualitative and semiquantitative agreement with the experiments for both steady shear and oscillatory shear.

A new family of cellulosic ether polymeric excipients has been recently engineered for fabrication of amorphous solid dispersions of active pharmaceutical ingredients via hot-melt extrusion (HME). These hydroxypropyl methyl cellulose excipients enable plasticizer-free melt processing at much lower temperatures (135-160 °C) due to their substantially reduced glass transition temperatures ( Tg = 98-110 °C). The novel amorphous cellulose ethers were found to be rheologically solidlike well above their glass transition ( Tg + 70 °C). We demonstrate that in the pharmaceutically relevant HME processing temperature range these polymers behave similarly to yield-stress fluids and flow only when the applied stress exceeds a critical stress value. This critical stress value (0.50 ± 0.05 MPa, 150 °C) is surprisingly high but is easily achieved under typical HME conditions. The origin of their yield-stress fluidlike behavior is hypothesized to arise from hydrogen bonds of the HPMC materials that act as physical cross-links and do not relax within the measured temperature and time window unless the applied stress exceeds the critical stress value. Support for this hypothesis arises from infrared spectroscopic estimates of the free and bound hydrogen bond levels at end-use temperatures.

Hydrophobically modified ethylene oxide urethane (HEUR) associative thickeners are widely used to modify the rheology of waterborne paints. Understanding the normal stress behavior of the HEUR-based paints under high shear is critical for many applications such as brush drag and spreading. We observed that the first normal stress difference, N-1, at high shear (large Weissenberg number) can be positive or negative depending on the HEUR hydrophobe strength and concentration. We propose that the algebraic sign of the N-1 is primarily controlled by two factors: (a) adsorption of HEURs on the latex surface and (b) the ability of HEURs to form transient molecular bridges between latex particles. Such transient bridges are favored for dispersions with small interparticle distances and dense surface coverages; in these systems; HEUR-bridged latex microstructures flow-align in high shear and exhibit positive N-1. In the absence of transient bridges (large interparticle distances, low surface coverage), the dispersion rheology is similar to that of weakly interacting spheres, exhibiting negative N-1. The results are summarized in a simplified phase diagram connecting formulation, microstructure, and the N-1 behavior.

The structure of aqueous dispersions of poly(oligo(ethylene oxide) monomethyl methacrylate)-grafted silica nanoparticles was characterized using contrast variation small-angle neutron scattering studies. Modeling the low hybrid concentration dispersion scattering data using a fuzzy sphere and a polydisperse core-shell model, demonstrated that the polymer chains are highly swollen in the dispersions as compared to the dimensions of the free polymer chains in dilute solution. At higher hybrid concentrations, the dispersions were well described using a Percus-Yevick approximation to describe the structure factor. These structural characterization tools are excellent starting points for effective molecular level descriptors of dewetting and macroscopic phase transitions for polymer tethered hybrid nanoparticle systems.

Recent advances in catalyst technology have enabled the synthesis of olefin block copolymers (OBC). One type is a “hard-soft” OBC with a high density polyethylene (HDPE) block and a relatively low density polyethylene (VLDPE) block targeted as thermoplastic elastomers. Presently, one of the major challenges is to fractionate HDPE segments from the other components in an experimental OBC sample (block copolymers and VLDPE segments). Interactive high temperature liquid chromatography (HTLC) is ineffective for OBC separation as the HDPE segments and block copolymer chains experience nearly identical enthalpic interactions with the stationary phase and co-elute. In this work we have overcome this challenge by using liquid chromatography under the limiting conditions of desorption (LC LCD). A solvent plug (discrete barrier) is introduced in front of the sample which specifically promotes the adsorption of HDPE segments on the stationary phase (porous graphitic carbon). Under selected thermodynamic conditions, VLDPE segments and block copolymer chains crossed the barrier while HDPE segments followed the pore-included barrier solvent and thus enabled separation. The barrier solvent composition was optimized and the chemical composition of fractionated polymer chains was investigated as a function of barrier solvent strength using an online Fourier-transform infrared (FTIR) detector. Our study revealed that both the HDPE segments as well as asymmetric block copolymer chains (HDPE block length ≫ VLDPE block length) are retained in the separation and the barrier strength can be tailored to retain a particular composition. At the optimum barrier solvent composition, this method can be applied to separate effective HDPE segments from the other components, which has been demonstrated using an experimental OBC sample.

A fundamental study of the separation of homopolymers from polystyrene-block-polymethylmethacrylate (PS-b-PMMA) by liquid chromatography with preloaded discrete and continuous adsorption promoting barriers was performed. The impact of barrier composition on the separation of block copolymers (BCP) was studied by a dual detection (ultraviolet (UV) and evaporated light scattering (ELSD) detectors) system that enabled monitoring both barrier composition and BCP separation simultaneously. The separation of homopolymers from BCP by preloaded discrete adsorption promoting barriers was validated via a series of control experiments by blending known amounts of homopolymers PS or PMMA with PS-b-PMMA, and the resulting chromatograms were free from co-elution of homopolymers and BCP. Quantitation of homopolymers and BCP by ELSD was also demonstrated. The influence of BCP chemical composition on the separation by preloaded discrete adsorption promoting barriers was investigated. Results showed a PS-b-PMMA having 90 wt% PMMA co-eluted with homopolymer PMMA, whereas PS-b-PMMA samples having lower amounts of PMMA block could be separated from homopolymer PMMA, successfully. Attempts at using a preloaded solvent gradient for separating homopolymers from block copolymers were unsuccessful. UV detection of the solvent gradient revealed significant deviation in solvent composition compared to the nominally loaded gradient. This deviation was due to the interaction of strong desorption solvent with column stationary phase. As such, the barrier composition in the preloaded gradient method was not as expected. Therefore, one can obtain undesired separation results by preloaded solvent gradients.

We have investigated the in situ mesoscale structure of paint formulations under shear using ultra small-angle neutron scattering (rheo-USANS). Contrast match conditions were utilized to independently probe the latex binder particle aggregates and the TiO2 pigment particle aggregates. Two different latex chemistries and two different hydrophobically modified ethylene oxide urethane (HEUR) rheology modifiers were studied. The rheo-USANS data reveal that both the latex particles and the TiO2 particles form transient aggregates which are fractal in nature. The structures depend on the chemistry of the binder particles, the type of rheology modifier present and the shear stress imposed upon the formulation. The aggregate size of both the latex and pigment generally decreases with increasing shear stress. In two of the formulations studied, the latex and TiO2 correlation lengths remain large even at high shear stress and are characteristic of TiO2 crowding. In a third formulation, shear induces string-like aggregate structures of TiO2, and a further increase in shear leads to pigment particles becoming more uniformly dispersed. The changes in the latex and pigment transient aggregate structures correlate with the changes observed in their viscosity flow curve profiles. We have used this correlation to develop an elementary viscosity prediction model based on the structural parameters extracted from the rheo-USANS data. Using a single fitting parameter and only the latex transient fractal aggregate structural parameters, good agreement between the measured and calculated viscosity is obtained. This implies that the structural parameters extracted from the scattering data are representative of the colloidal structure under shear and that energy dissipation from transient fractal aggregates of latex is the predominant mechanism of viscosity creation in HEUR thickened latex paints.

Submitted for the MAR15 Meeting of The American Physical Society Modeling the Adsorption of Hydrophobic Ethoxylated Urethane (HEUR) Thickeners onto Latex Surfaces using Self-Consistent Field Theory1 VALERIY GINZBURG, ANTONY VAN DYK, TIRTHA CHATTERJEE, Dow Chemical Co, SHIHU WANG, RONALD LARSON, University of Michigan — Hydrophobic Ethoxylated Urethane (HEUR) polymers are widely used as rheology modifiers (thickeners) in waterborne latex paints. Recently, it has been shown that the thickening effect of HEURs in paints is largely determined by their adsorption onto latex surfaces, this adsorption being a function of polymer structure, latex surface chemistry, and total available latex surface. Here, we describe the application of Self-Consistent Field Theory (SCFT) to calculate adsorption isotherms of several model HEURs onto ideal hydrophobic latex surfaces. Unlike earlier SCFT studies of adsorption, we explicitly take into account the role of HEUR micelles and competition between adsorption and micellization. The results are compared with experimental data and coarse-grained molecular dynamic (CG-MD) simulations, and good qualitative and semi-quantitative agreement is found. 1This work was supported by The Dow Chemical Company Valeriy Ginzburg Dow Chemical Co Date submitted: 05 Nov 2014 Electronic form version 1.4