Role of Air Bubble Inclusion on Polyurethane Reaction Kinetics

Abstract

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).

Ostwald Ripening Modulation by Organofluorine Additives in Rigid Polyurethane Foams

“…The first foaming stage (stage I) was characterized by a sharper change in N , while the second foaming stage (stage II) was characterized by a slighter decrease, depending on the strength and the type of degeneration mechanisms that affected the morphology evolution. In both foaming stages, we can observe no formation of new bubbles that could be attributed to nucleation, already observed in our previous works, , and it can be explained by the different levels of energy barriers that the gas/liquid system overcomes. In fact, gas molecules present in the reacting medium (CO 2 provided by blowing reaction, PBA and OFA evaporation), once supersaturated, would diffuse toward pre-existing air bubbles (included during the mixing stage) rather than nucleate due to no energy barrier to overcome .…”

“…In the first case (Figure a), a sample characterized by a coarser bubble size distribution than that obtained in the case of a PUF with an added catalyst was observed. The occurrence can be ascribed to an unbalanced competition between polymerization and a blowing reaction in which the polymer structure could not withstand bubble expansion . After 30 min, a still ongoing PU foaming process was observed due to delayed reaction kinetics caused by the absence of catalyst within the PU recipe.…”

“…Figure depicts an example of the bubble morphology evolution during the (a) PU and (b) PIR foaming process in case of neat formulations and their corresponding HFB and NFP additions at three parts. From the initial microphotograms, a finer cellular structure can be noticed for the neat PIR formulation as a result of the different involved chemical mechanisms − as well as air bubble inclusion. ,, After 300 s from the mixing, the effect of the HFB on the morphology details is readily evident, translating in a reduced cellular structure and a more pronounced effect for the PIR formulation. Interestingly, the use of NFP induced a stronger effect than that of HFB.…”

“…A simple and inexpensive optical observation system, already reported in refs and , has been setup and is shown in Figure . A high-speed camera (CMOS, model DMK 33UX178) from the Imaging source (Bremen, Germany) with a resolution of 3072 pixel × 2048 pixel (6.3 MP) and a frame rate of 60 fps has been used to collect microphotograms during the foaming process.…”

“…Use of these types of additives for a specific PU formulation was reported in a patent application. 19 This work, introducing the "controlled inhibition of bubble Ostwald ripening" by use of OFAs in the context of rigid PUFs, is the follow up of previous works aimed at the fundamental understanding of the foaming mechanisms occurring during the PU foaming process and dealt with (1) the development of an inexpensive optical apparatus for processing and characterization of thermoset foaming materials, 20 (2) the investigation on the competing bubble formation mechanisms by a methodical approach that allowed one to separate bubble nucleation contribution from aeration, 20 (3) the measurement and modeling of the effect of air bubble inclusion on the PU reaction kinetics, 21 and (4) the characterization study of the OFA effect on the morphological properties as well as thermal conductivity and mechanical strength of PUFs and PIRFs. 22 The well-grounded studies allowed us to discern the different bubble degeneration mechanisms and surgically intervene on the OR, producing microcellular, low density PUFs (67 μm and 27.66 kg•m −3 ), and PIRFs (40 μm and 32.78 kg•m −3 ).…”