Rapid Photothermal Synthesis of Polyurethane from Blocked Isocyanates

Abstract

Though it has been shown that the photothermal heating by pulsed lasers can provide localized heat to cure polymers at an enhanced rate without bulk temperature changes, such an approach has been unsuccessful at driving chemical transformations that require large increases in bulk temperature─such as the curing of blocked isocyanates. We show that photothermal heating using 1 W of continuous wave laser power directed toward a mixture of 6 wt % carbon black in a blocked isocyanate is sufficient to reach temperatures near 142 °C. Additionally, using both infrared and nuclear magnetic resonance spectroscopies, we demonstrate that this heat is sufficient to drive the deblocking of a trimer of hexamethylene diisocyanate blocked by methyl ethyl ketoxime. We also show that 1 s of such heating produces the same degree of deblocking as 8 h in an oven held at 160 °C. Finally, we demonstrate that photothermal heating can also drive the formation of a urethane bond that is spectroscopically identical to that produced after oven heating at 160 °C for 1 h. This work shows that photothermal heating with carbon black and a CW laser can provide bulk heat necessary for high temperature reactions while maintaining the photothermally induced kinetic advantage of localized heat.