Influence of prepolymer molecular weight on the rheology and kinetics of HEUR-thickened Latex Suspensions

Hydrophobically Modified Ethoxylated Urethanes (HEURs) were synthesized by a step growth polymerization of polyethylene glycol (PEG, Mn = 4000, 6000, 8000) with isophorone diisocyanate (IPDI). The thickeners were produced with different sizes of the hydrophilic section by changing the molar ratios of reactants. The hydrophobic ends, 1-tetradecanol (C14), was constant for all prepared samples. The structure, molecular weight and polymer dispersity index (PDI) of HEUR were characterized by 1H NMR, IR and GPC respectively. The rheological behavior of the HEUR-thickened latex (Latex/HEUR) was determined by oscillatory and flow experiments. Dramatic increases in viscosity of Latex/HEUR are obtained. Under flow conditions the Latex/HEUR fits the Cross model, showing two regions, a Newtonian and a shear thinning one. Broad thixotropy are observed in hysteresis tests and three interval thixotropy test (3ITT) with decreasing molecular weight of the prepolymer, which was analyzed using a second-order kinetic equation. The results of atomic force microscopy (AFM) and the flow-levelling performance tests for Latex/HEUR films show smoother surface and better levelling with the increase of the hydrophilic component size.