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.

Dependence of Photothermal Rate Enhancements of Urethane Formation upon the Nature of Light Exposure and Solution Viscosity

“…For the pure alcohols, the polyol is roughly an order of magnitude more viscous than butanediol, while the butanediol is an order of magnitude more viscous than butanol. The first system (HDI + K-Flex 188) has been used in our prior research 24 and produces a linear polymer that forms a solid at high conversions. Moving to butanediol allows us to reduce the viscosity of the initial reactive mixture while still producing a linear polymer that will form a solid at high conversions.…”

“…We chose to examine the influence of these parameters using the reaction between alcohol and isocyanate groups to form urethanes, driven by the photothermal effect of gold nanoparticles (AuNPs). Our group has shown that this approach is effective for generating cross-linked polymers from viscous prepolymers, − but has not yet sought to extend this to low-viscosity systems or those that do not generate polymers. Additionally, our early work on urethane formation , focused on using high-powered pulsed lasers in order to attain the highest local temperatures we could, while more recent work has shown that CW lasers can also be effective .…”

“…Our group has shown that this approach is effective for generating cross-linked polymers from viscous prepolymers, − but has not yet sought to extend this to low-viscosity systems or those that do not generate polymers. Additionally, our early work on urethane formation , focused on using high-powered pulsed lasers in order to attain the highest local temperatures we could, while more recent work has shown that CW lasers can also be effective . Thus, this particular reaction seemed well-suited for an initial investigation into the effects of viscosity and laser exposure time.…”

“…Additionally, our early work on urethane formation 20,23 focused on using high-powered pulsed lasers in order to attain the highest local temperatures we could, while more recent work has shown that CW lasers can also be effective. 24 Thus, this particular reaction seemed well-suited for an initial investigation into the effects of viscosity and laser exposure time.…”

Photothermal catalysis: Recycling polymer to monomer with carbon quantum dots

“…We hypothesized that photothermal catalysis could merge the advantages of photocatalysis -the temporal and spatial control over radical formation -with the generality of heat. [13][14][15][16][17][18] Fig. 1: Employing photothermal catalysis for chemical recycling to monomer.…”