International. In a new paper, scientists at the University of Strathclyde in Scotland investigated the influence of the diol chain extender on the morphology and properties of thermally activated self-repairing polyurethane coatings.
Two sets of water-based polyurethane dispersions were synthesized from polycarbonate polyol with molecular mass of 500 Da and hexamethylene diisocyanate or isophorone diisocyanate. The formulations were prepared without a chain extender, with aliphatic diol with two to five carbon atoms or with diethylene glycol.
The coatings were prepared on sheets of cellulose triacetate, damaged with a steel wool scratch instrument and left to cure at room temperature and at 60 ° C. Self-healing efficiency was examined by comparing haze before damage and at intervals after damage. Samples were analyzed using Differential Scanning Calorimetry, Attenuated Reflectance Fourier Transform Infrared Spectroscopy and Scanning Electron Microscopy. The tests were repeated after 12 weeks to investigate the aging of the polymers. The samples were also tested for their weather stability.
Optimally designed coatings obtained up to 100% recovery in 10 minutes at 60 ° C and partial recovery at room temperature. It was discovered that the self-healing properties of coatings are related to the macroorganization of polymer chains caused by interactions between hard segments and soft segments of the remaining polyurethane, leading to phase mixing promoted by non-symmetric bulky isoforondiysocyanate or phase separation, promoted by symmetric linear hexamethylene diisocyanate.
The length of the chain extender was found to have a great influence on formulations prepared with hexamethylene diisocyanate, increasing phase separation and turbidity with increasing chain length. Diethylene glycol was found to enhance the self-compacting and phase mixing properties of hexamethylene diisocyanate-based materials. The influence of chain extenders was found to be minimal for materials based on isophorone diisocyanate.
Source: https://www.sciencedirect.com
