Kinetic and chemorheological modeling of thermosetting polyurethanes obtained from an epoxidized soybean oil polyol crosslinked with glycerin
Abstract
Thermosetting polyurethanes were obtained using an aromatic isocyanate and a hydrophobic polyol formulation obtained from epoxidized soybean oil (ESO) crosslinked with glycerin. A systematic DSC analysis of the effect of catalyst type, crosslinker concentration, isocyanate index and ESO crystallization on the cure kinetics was conducted. The combination of a stannic catalyst at 0.2 wt.\% and glycerin at 20 wt.\% produced a cure kinetics governed by autocatalytic heat flow where vitrification played a key role in the formation of chemical bonds. The evolution of Tg as a function of conversion, which followed Di-Benedetto\'s predictions, supported the hypothesis that vitrification was a preponderant phenomenon during cure. The ESO crystallization had a melting endotherm centered at 35C, affecting substantially the availability of reactive hydroxide groups. Dynamic Mechanical Analysis (DMA) of a post-cured sample revealed a Tg centered at 220C, whereas quasi-static flexural mechanical tests revealed a flexural modulus of 2.14 GPa and a flexural strength of 99.4 MPa. Rheological experiments using a rheometer at isothermal conditions supported the hypothesis that vitrification played a key role in the evolution of apparent viscosity. A master model using Kim-Macosko equations was obtained for the proposed formulation.
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