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

Abstract

A systematic study of the post-cure cycle of soy-based polyurethane thermosets was conducted as a function of crosslinker content, post-cure temperature and time. An improvement of 207% of the flexural strength, up to 175 MPa, was measured by increasing the crosslinking content to 60 wt% and by using a post-cure cycle temperature of 110°C. Similarly, the flexural modulus also increased by 199%, up to 3.2 GPa, while maintaining a flexural strain to failure of 6.60%. In situ studies of the post-cure cycle using dynamical mechanical analysis revealed the interplay between the two interpenetrating polymer networks as a function of crosslinker content. At a crosslinker content below 40 wt%, two rubbery to glass transition temperatures (Tg) were found at 60 and 200°C, respectively. At 60 wt%, only one Tg at temperatures close to the degradation temperature was identified (250°C). Scanning electron microscope micrographs of the fracture surface revealed the formation of stress induced cracks which were mitigated through the use of a temperature stepped post-cure cycle. The formation of soft segments with high degradation temperatures (485°C) were revealed by thermogravimetric analysis, which can be attributed to the presence of additional covalent bonds such as allophanate and carbodiimide that were also identified through Fourier transform infrared analysis. The results revealed in this work are key for the development of biobased polyurethanes applied for the polymer composite industry. In this regard, this study represented the first analysis of the post-cure cycle of a soy-based polyurethane thermoset.