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This study investigates the effect of liquid-type organofluorine additives (OFAs) on the morphology, thermal conductivity and mechanical properties of rigid polyurethane (PU) and polyisocyanurate (PIR) foams. Foams were characterized in terms of their morphology (density, average cell size, anisotropy ratio, open cell content), thermal conductivity and compressive as well as flexural properties. Based on the results, we observed that OFAs efficiently reduced the average cell size of both PU and PIR foams, leading to improved thermal insulating and mechanical properties.

In this study, we investigated the influence of mixing conditions on the foaming process of water blown polyurethane (PU) foams obtained at different mixing speeds (50, 500, 1000 and 2000 rpm). In particular, the morphological evolution during the foaming process, in terms of the bubble size and bubble density, was studied via optical observations, while the effects on the reaction kinetics were monitored using in situ FTIR spectroscopy. At the slow mixing speed (50 rpm), no air bubbles were included and the early foaming process was characterized by the formation of new bubbles (CO2 nucleation), provided by the blowing reaction. Later on, it was observed that the coalescence affected the overall foaming process, caused by the gelling reaction, which was inhibited by the indigent mixing conditions and could not withstand the bubbles expansion. As a result, a PU foam with a coarse cellular structure and an average bubble size of 173 µm was obtained. In this case, the bubbles degeneration rate, dN/dt, was −3095 bubble·cm−3·s−1. On the contrary, at 500 rpm, air bubbles were included into the PU reaction system (aeration) and no formation of new bubbles was observed during the foaming process. After this, the air bubbles underwent growth caused by diffusion of the CO2 provided by the blowing reaction. As the gelling reaction was not strongly depleted as in the case at 50 rpm, the coalescence less affected the bubble growth (dN/dt = −2654 bubble·cm−3·s−1), leading to a PU foam with an average bubble size of 94 µm. For the foams obtained at 1000 and 2000 rpm, the bubble degeneration was first affected by coalescence and then by Ostwald ripening, and a finer cellular structure was observed (with average bubble sizes of 62 µm and 63 µm for 1000 rpm and 2000 rpm, respectively). During the first foaming stage, the coalescence was less predominant in the bubble growth (with dN/dt values of −1838 bubble·cm−3·s−1 and −1601 bubble·cm−3·s−1, respectively) compared to 50 rpm and 500 rpm. This occurrence was ascribed to the more balanced process between the bubble expansion and the PU polymerization caused by the more suitable mixing conditions. During the late foaming stage, the Ostwald ripening was only responsible for the further bubble degeneration (with dN/dt values of −89 bubble·cm−3·s−1 and −69 bubble·cm−3·s−1, respectively).

In this study, we investigated the preservation mechanisms induced by the use of organofluorine additives (OFAs), at different contents on the foaming process of water–pentane blown rigid polyurethane and polyisocyanurate foams (PUFs and PIRFs). The morphological evolution was observed via optical observation and characterized in terms of average bubble size and bubble density. For both PUFs and PIRFs, the foaming process was formed by a two-stage mechanism, consistent with the bubble growth and degeneration. The first foaming stage was affected by combined bubble coalescence and Ostwald ripening (OR) in the case of PUFs, while coalescence only in the case of PIRFs. The second foaming stage was affected by only OR for all the PU and PIR formulations. Interestingly, it was revealed that OFAs can act with two possible mechanisms: (i) enhanced air bubble inclusion during the mixing stage and (ii) OR inhibition effect on the bubble degeneration. The dual function was related to the unique chemical structures of the OFAs, conferring a high dissolution capacity for the gas phase such as air bubbles included in the polymeric phase, in which they are insoluble. In particular, the OR inhibition was regulated by the addition of OFA that possibly induced two effects: (i) a reduction in system surface tension that mitigated the Laplace pressure and (ii) the incompatibility of OFA with the polymeric phase that remains confined within the air bubbles, leading the partial pressure of OFA to contribute together with the partial pressure exerted by the other solutes so that the pressure of the internal bubble counterbalances the pressures of Laplace and the reacting mixture. In addition, morphology features such as foam density, average bubble size, anisotropy ratio, and open cell content were also measured. Based on the results, we observed that OFAs efficiently reduced the average bubble sizes of both PUFs and PIRFs, with negligible effects on the foam density, weakening of the bubble walls, and orientation of the bubble distribution.

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