Comparative effects of two different crosslinkers on the properties of vegetable oil-based polyurethanes

Abstract

Thermoset polyurethanes (PUs) were prepared from a polyol derived from castor oil, 4,4′-methylene diphenyl diisocyanate (MDI) and different trifunctional low-molecular-weight crosslinkers, biobased glycerol (Gly) and petroleum-derived trimethylolpropane (TMP). The synthesis was carried out in bulk and without catalyst via one-step polymerization varying the components equivalent weight ratio, Polyol:MDI:Gly and Polyol:MDI:TMP, respectively. The physicochemical, morphological, thermal, dynamic–mechanical, and mechanical properties of the PUs were determined. The success of the reaction between the polyol and MDI was confirmed by Fourier transform infrared spectroscopy. The dynamic–mechanical and the mechanical properties as well as hardness were determined and related to the concentration of the low-molecular-weight crosslinkers utilized (Gly or TMP). However, important differences were observed between the synthesized two series, due to phase separation produced during the curing reaction, which affected more the materials prepared from TMP. Scanning electron microscopy images and dynamic–mechanical results confirmed this difference, related to the reactivity of primary and secondary hydroxyls present in the crosslinkers. Thermogravimetric analysis also showed to be sensible to the different structure of the crosslinkers with TMP leading to more thermally stable samples. Finally, measurements of water contact angle indicated that the surfaces were mostly hydrophobic with minor differences between the samples.