In-situ Dynamic Mechanical Analysis during polyurethane foaming: relationship between modulus build-up and reaction kineticsThursday (08.10.2020) 09:50 - 10:10
Water-blown polyurethane foams are produced from the simultaneous reaction of isocyanate with water and hydroxyl-containing compounds, reactions known as blowing and polymerization, respectively. During the foaming process, the delicate equilibrium between these reactions gives rise to a material that changes quickly from a viscous liquid with dispersed gas bubbles, to a low-modulus gel with packed pores and subsequently to a solid foam with a supramolecular architecture and stable cellular structure. The quality of the final product depends to a large extent on the kinetics of reaction and generation of the cellular structure. In addition, to prevent degeneration of the cellular structure or collapse, the expansion undergone by the polymer needs to be coupled with an adequate viscosity increase.
In this work, we present a novel methodology based on the use of Dynamic Mechanical Analysis (DMA) to investigate the modulus development of polyurethane foams during growing. Furthermore, the use of DMA allows to register information about the reaction and curing times, the density changes undergone by the test sample during foaming and curing, and the phase response of the material to sinusoidal stress.
Four formulations with different amount of water and catalyst have been used to test the technique’s potential. Additionally, the moduli increase and characteristic reaction times registered during the in-situ DMA experiments have been contrasted with the rate of isocyanate conversion measured with Fourier Transform Infrared spectroscopy (FTIR). The results show that the moduli measured with DMA are sensitive to changes in the formulation and in good agreement with the speed of the reaction measured by FTIR. This novel technique allows obtaining information about the change in the mechanical properties of the PU matrix during foaming which would be very helpful to understand the degeneration mechanisms taking place (drainage, coalescence and/or coarsening).