Association mechanisms, HEURs

Hydrophobically modified, ethoxylated urethanes (HEURs) are the thickeners of choice in exterior coatings and in corrosion-resistant, high-gloss industrial latex finishes. This chapter presents an overview of our research efforts and discusses approaches to the synthesis of HEUR polymers with different molecular architectures, geometrical influences on solution properties, and viable mechanisms by which this class of associative thickener effects desirable coatings rheology and applied film properties. 

Once an associative thickener is completely desorbed from the latex, it behaves like a non-adsorbing conventional thickener and can flocculate the latex by the depletion mechanism (see Section 13.3.1.1). Flocculation phase diagrams can be constructed showing the state of flocculation or deflocculation of the latex at a specified solids content, over a range of thickener and surfactant or cosolvent concentrations [97]. Because of their lower molar mass and less volumefilling backbones, the threshold concentration for desorbed HEUR thickeners to cause depletion flocculation is much higher than that for high molar mass cellulose ethers. 

Associative thickeners are low-molecular-mass water-soluble polymers with at least two hydrophobes such as hydrophobically modified ethylene oxide-urethane block copolymers (HEUR) or hydrophobically modified hydroxyethylcellulose (HMHEC). The hydrophobes can associate with themselves or with hydrophobes on surfactant, cosolvent, latex, and pigment. This sets up a loose network that is sensitive to mechanical disturbance but re-forms quickly. The result is that pigment settling and film sag are reduced because the network structure increases the low shear viscosity, but the formulations show easy... 

Associative thickeners increase the viscosity of the paint by interacting with other paint ingredients through bridging of particles (surface to surface) or interaction of adsorbed layers. The interactions are assumed to be through hydrophobic forces. The operative mechanism is illustrated in Figure 5.9. Important groups in this class are associative acrylic thickeners, associative cellulose ethers and hydrophobically modified ethoxylated urethanes (HEUR). 

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