The morphology of rigid polyurethane foam matrix and its evolution with time during foaming – New insight by cryogenic scanning electron microscopy

Abstract

This paper presents an investigation of the morphology of growing polyurethane (PU) rigid foams during the very first seconds of the process by cryogenic scanning electron microscopy (cryo-SEM) performed at -150°C. The heterogeneous nature of the initial mixture has been revealed with the presence of sub-micron size physical blowing agent droplets (isopentane), µm size dispersed phase nodules, and large air bubbles dispersed in a continuous matrix. Following the evolution of the microstructure during foaming by cryo-SEM suggested that the isopentane liquid droplets (undissolved part of the physical blowing agent) did not vaporize to create their own bubbles. These observations were confirmed by showing that the number of air bubbles per unit volume (∼10 6 bubble/cm 3 ) was similar to the cell population density of the final foam (∼10 6 cell/cm 3 ), while the number of isopentane droplets initially present was found to be six orders of magnitude higher (∼10 12 droplet/cm 3 ). This all means that isopentane molecules initially dissolved in the continuous phase diffuse into the preexisting air bubbles with no energy barrier to overcome (non-classical nucleation) whereas the isopentane droplets simply act like reservoirs. Finally despite our best efforts, there is still some doubt whether polymeric 4,4'-diphenylmethane diisocyanate (PMDI) is dispersed in the polyol phase, or polyol dispersed in PMDI.

Biobased Polyurethane Foams Based on New Polyol Architectures from Microalgae Oil

“…48 This theory is partially discussed and contested for the benefit of the pockets of air entrapped during the mixing stage. 49 The blowing agent droplets migrate into the air bubbles. In the case of a slow network formation, air bubbles and thus cells grow bigger through a coalescence phenomenon combined with Ostwald ripening.…”

Self-foaming polymers: Opportunities for the next generation of personal protective equipment

“…In particular, the use of low-boiling point chlorofluorocarbons (CFCs) has been preferred to water initiated self-blowing for a long time due to the lower thermal conductivity of CFCs as compared to CO2. [107] Moreover, CO2 is known for its high diffusivity through the cells walls that results in pressure drop and ultimately in dimensional instability of the foams. [108,109] With the hazardous environmental effects of CFCs (ozone depletion), and of the substitutes that have been proposed after the Montreal protocol (hydrofluorocarbon, HFC), there have been a regain of interest for waterblown PUFs over the past decades.…”

Statistical Characteristics of Polymer Grouting Material Microstructure

“…e polymer grouting material has a kind of cell structure, whose internal feature parameters mainly include cell shape [34], cell size [39], cell size distribution [25], and cell content [10]. e parameters of roundness, cell diameter, and porosity are often used to measure the above geometric characteristics.…”

“…Two-dimensional image analysis methods include microimage acquisition and computer image analysis methods. e 2D image of the cell structure is usually obtained by optical microscope (OM) [21][22][23] or scanning electron microscope (SEM) [24,25]. e objective magnification of SEM can be changed from 10 times to more than 300,000 times.…”