Polyurethane foaming with engineered CO2-releasing nanoparticles: From the thickening effect to the industrial applications of the blowing agents

Abstract

We explored polyurethane (PU) foaming with CO2-releasing nanoparticles from CO2 adducts of poly(propylene glycol) (PPG)-grafted polyethylenimines. The amphiphilic nature of these CO2 adduct macromolecules drives the formation of nanoparticles in the white components (i.e. foaming mixtures without isocyanates), with the hydrophobic PPG side-chains stretching outward. The outermost PPG terminal groups intimately contact the foaming mixture and thus largely determine the particle size, while the side-chain polymerization degree, ranging from 1 to 5, plays a minor role. The blowing agents with methoxyl terminal groups increased the viscosity of the white components very rapidly due to the small particles dispersed therein (e.g. 63–65 nm after 7 d aging) that caused a strong thickening effect. Normal PU foams could only be obtained within 1 h of white-component aging. Longer aging made the wall of the growing bubbles difficult to flow due to the strong thickening effect and the nanoparticles in the wall further weakened the wall while releasing CO2, both contributing to the failure of blowing. In contrast, the blowing agents with butoxyl terminal groups demonstrated a very slow increase in the viscosity and the pot life of the white components could be prolonged up to 90 d, due to the relatively large dispersed-particles (e.g. about 200 nm at aging for 60 d). The CO2 content as high as ∼25% has been achieved in the blowing agents with the shortest side chains whose polymerization degree n = 1. The PU foams obtained had densities below 50 kg/m3 and compressive strengths over 230 kPa, both satisfying the demands of many industrial applications.