Ostwald Ripening Modulation by Organofluorine Additives in Rigid Polyurethane Foams

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.