Ostwald Ripening Modulation by Organofluorine Additives in Rigid Polyurethane Foams

Abstract

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.

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

“…An important question is why the addition of the FC measurably increases the number of bubbles present in the premix. In fact, we made this observation 13 for different blending techniques (double-syringe blending, ultra turrax blending) and it has also been recently reported by Brondi et al 19 for blending with a mechanical impeller. One mechanism may be that even though the same number of bubbles are generated with and without FC, the bubbles without FC undergo a more rapid ageing process.…”

“…However, the two graphs in the SI show that, the FC has a negligible in uence on the ripening process. This is in contrast to the argument that the FC acts as Ostwald ripening inhibitor in PUR foam formulations which is often evoked in the literature and in patents 17,19 . This surprising lack of in uence of the FC on the Ostwald ripening can be easily explained by the fact that the liquid and the gas matrix are both saturated with FC: even though the FC is not soluble in the A-component, it is present in the form of tiny droplets (as shown in the Supporting Information Figure S3) which diffuse and serve as e cient FC reservoirs.…”

“…Advancing to solve this puzzle, Brondi et al 19 very recently started analyzing the FC-driven pore size reduction from a mechanistical point of view. They also argue that FCs retard diffusional coarsening of the expanding gas bubbles during PUR foam blowing and solidi cation, which therefore gives rise to foams with a smaller mean pore size.…”

Introducing the I&ECR Virtual Collection of Industrial and Industrially Relevant Research (2022–2023)

“…Engineering chemistry related to Materials and Interfaces is another strong topic area for I&ECR . Here, studies in the collection include a novel methodology of improving polyphenyl sulfide (PPS)-based membranes selected as ACS Editors’ Choice in June 2023, an improvement of lubricant oil properties using carbon nanomaterials, synthetic strategies to prepare γ-alumina by coprecipitation (a cover article in January 2023) and FeNi particles by spray pyrolysis, , the impact of pressure and temperature on sorption of gases by fluoroelastomers, methodologies to improve polymer/clay composite design, novel carbonaceous materials for Li batteries, and a study of rigid polyurethane foam Ostwald ripening from a third member of the 2022 Class of Influential Researchers …”

“…Here, studies in the collection include a novel methodology of improving polyphenyl sulfide (PPS)-based membranes selected as ACS Editors' Choice in June 2023, 24 an improvement of lubricant oil properties using carbon nanomaterials, 25 synthetic strategies to prepare γ-alumina by coprecipitation (a cover article in January 2023) and FeNi particles by spray pyrolysis, 26,27 the impact of pressure and temperature on sorption of gases by fluoroelastomers, 28 methodologies to improve polymer/clay composite design, 29 novel carbonaceous materials for Li batteries, 30 ripening from a third member of the 2022 Class of Influential Researchers. 31 Although most studies in Process Systems Engineering come from academic groups, the topic area is very important to industrial researchers, with papers in the collection an August 2023 cover article on the impact of H 2 /N 2 ratio for NH 3 synthesis, 32 a methodology for multipurpose batch plant scheduling, 33 an image-based control method for an agrochemical crystallization process, 34 and a process for preparing propanol from methane. 35 The Separations topic area is one where industrial authors demonstrate consistent innovation.…”