Viscosity Variation During Addition of Ester and Ether Macrodiols to Isocyanates

Polyurethane blends usually thicken up during the polymerization, but unexpected drop in viscosity could be occasionally observed [1]. In this study, an oligomeric ether-ester macro diol (EEMD of 2700 g/mol) [2] was diluted with polyethylene glycols (PEG of 200 g/mol or 400 g/mol). The polyols were mixed with a trifunctional adduct of hexamethylene -1,6- diisocyanate (HDI3, CAS 3779-63-3) at 1.4 mol excess to initiate addition reactions and assure eventual gelation.

     Within 1 min of mixing dynamic viscosity η was measured using Anton Paar MCR 302 rheometer and a concentric cylinder system. The spindle was lowered leaving 1 mm gap between cylinders and assuring the absence of air bubbles. Viscosity was measured at 3 different shear rates: 1 s-1, 10 s-1, 100 s-1, see fig 1. 

     The theoretically predicted viscosities are somewhat higher than measured.  During early stages of polymerization the viscosity is highly dependent on shear, but later the trends become Newtonian-like [3].  Despite ongoing polymerization, viscosity seems to drop initially, or at least stagnate. It remains unclear what causes such unusual tendencies of viscosity dependence on polymerization