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 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 an autocatalytic heat flow where vitrification played a key role in the formation of chemical bonds. The evolution of T g as a function of conversion, which followed Di-Benedetto's predictions, supported the hypothesis that vitrification was a preponderant phenomenon during cure. Dynamic Mechanical Analysis (DMA) of a post-cured sample revealed a Tg centered at 220 C, whereas quasi-static flexural mechanical tests shown a flexural modulus of 2.14 GPa and a flexural strength of 99.4 MPa. Rheological experiments 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. The results presented in this work will serve to further extend the use of biobased polymers applied in the polymer composite industry.

The role of post‐cure cycle on the thermomechanical properties of soy‐based polyurethane thermosets

“…As we have already reported in a chemorheology study of this formulation, 17 the evolution of the conversion (cure) is very slow due to vitrification effects. Hence, it is important to analysis the post‐cure residual heat flow, which is a measurement performed by DSC analysis so as to corroborate if the material has reached to the final conversion.…”

“…On the other hand, the ESOP contributed to a less crosslinked microstructure, due to its low hydroxyl value and intermediate molecular weight. It is important to highlight that the hydroxyls present on each precursor compete to react with the isocyanate group, changing the overall cure kinetics of the thermoset polyurethane 17,34,35 . A very important aspect that our group has already studied is vitrification.…”

“…Dibutyltin dilaurate (95%, Sigma Aldrich) was used as the catalyst (DBTDL). A soy‐based polyol (ESOP) with an OH# of 137 mgKOH·g −1 , a functionality of 2.90, a molecular weight of 697 Da and with fully secondary hydroxyl groups was synthesized using the procedure already reported in a previous work 17 . Further details of the physicochemical properties of this polyol can also be consulted there.…”

“…One of the most relevant applications of thermosets is within the polymer composites field, where high hydroxyl functionalities, low viscosities as well as a hydrophobic formulation are key aspects for a successful scale‐up 16 . Our group recently published a chemorheological analysis of a polyurethane thermoset which complied such characteristics, finding as well that the cure of this system was dominated by vitrification effects 17 . The rubbery to glass transition ( T g ) is a typical thermal transition found in polymers and it reduces drastically the cure kinetics during cure 18 .…”

“…By contrast, traditional polyether's are much more hydrophilic. The main issue with vegetable polyols is that the combination of a high hydroxyl functionality and low processing apparent viscosity is usually very difficult to attain 17 . In this regard, biobased crosslinkers, such as glycerin obtained from the transesterification of soybean oil, 31,32 can circumvent this issue.…”