Fluorocarbon-driven pore size reduction in polyurethane foams: an effect of improved bubble entrainment

Polyurethane (PU) foams are created via the chemical reactions arising after the blending of two initially liquid components (polyols and isocyanates). They are widely used for thermal insulation, for which a small pore size is required. Some of the most efficient pore size–reducing agents have proven to be per- and polyfluorinated carbons (FCs) which are simply added in small quantities to the initially liquid mixture. However, despite their long-standing use, their modes of action have only recently begun to be studied in detail. One widely accepted explanation of their action is that they supposedly suppress diffusional gas exchange between bubbles in the liquid-foam state of the nascent PU foam (foam coarsening). However, using a new double-syringe mixing technique, we show that FCs actually act at a much earlier state of the process: they facilitate the entrainment of tiny air bubbles into PU foam systems during the initial blending process. These bubbles serve as sites for heterogeneous nucleation during the foaming process, and their large number leads to a significant reduction of the characteristic pore size. More importantly, we also demonstrate that the same overall relation is found between the air bubble density and the final pore size for systems with and without FC. Combined with a detailed analysis of the pore size distribution, we argue that the main pore size–reducing effect of FCs is to facilitate air entrainment and that foam aging–related effects only play a minor role.