Analysis of Heat Transfer Mechanisms in Polyurethane Rigid Foam

Nanocomposites of Rigid Polyurethane Foam and Graphene Nanoplates Obtained by Exfoliation of Natural Graphite in Polymeric 4,4′-Diphenylmethane Diisocyanate

The influence of graphene nanoplates (GNPs) obtained by the ecofriendly exfoliation of natural graphite has been addressed on the mechanical and thermal insulating properties of rigid polyurethane foams (RPUFs). Few-layer GNPs with few defects were prepared in polymeric 4,4′-diphenylmethane diisocyanate (pMDI) under ultrasonication to obtain a GNP/pMDI dispersion. GNP/pMDI dispersions with different GNP concentrations were used to prepare RPUF nanocomposites via in situ polymerization. An important finding is that the GNP/pMDI dispersion exhibits lyotropic liquid crystalline behavior. It was found that the unique orientation of GNPs above the concentration of 0.1 wt% in the dispersion affected the mechanical and thermal insulation properties of the RPUF nanocomposites. GNP/RPUF nanocomposites with GNP concentrations at 0.2 wt% or more showed better thermal insulating properties than neat RPUF. The lyotropic liquid crystalline ordering of GNPs provides stable nucleation for bubble formation during foaming and prevents bubble coalescence. This decreases the average cell size and increases the closed cell content, producing GNP/RPUF nanocomposites with low thermal conductivity. Furthermore, GNPs incorporated into RPUF act as a barrier to radiant heat transfer through the cells, which effectively reduces the thermal conductivity of the resulting nanocomposites. It is expected that the nanocomposite of RPUF investigated in this study can be applied practically to improve the performance of thermal insulation foams.

Properties of polyurethane foam with fourth-generation blowing agent

Climate change makes it imperative to use materials with minimum global warming potential. The fourth-generation blowing agent HCFO-1233zd-E is one of them. The use of HCFO allows the production of polyurethane foam with low thermal conductivity. Thermal conductivity, like other foam properties, depends not only on the density but also on the cellular structure of the foam. The cellular structure, in turn, depends on the technological parameters of foam production. A comparison of pouring and spray foams of the same low density has shown that the cellular structure of spray foam consists of cells with much less sizes than pouring foam. Due to the small size of cells, spray foam has a lower radiative constituent in the foam conductivity and, as a result, a lower overall thermal conductivity than pouring foam. The water absorption of spray foam, due to the fine cellular structure, also is lower than that of pouring foam. Pouring foam with bigger cells has higher compressive strength and modulus of elasticity in the foam rise direction. On the contrary, spray foam with a fine cellular structure has higher strength and modulus in the perpendicular direction. The effect of foam aging on thermal conductivity was also studied.

Melamine-formaldehyde rigid foams – Manufacturing and their thermal insulation properties

The manufacturing of novel melamine-formaldehyde rigid foam material, by blowing the melamine-formaldehyde (MF) resin emulsion with pentane and further catalytic and thermal curing, is presented in this work. The process of foaming is described in terms of particular process parameters, which are; the proportions of blowing, curing, emulsifying agents. The examination of the foam, by SEM images, shows that the foam pore sizes are in the range from 150 to 250 µm. The thermal characterization of the obtained foams, is described in terms of thermal conductivity contributions of solid, gas and radiation conduction to total thermal conductivity at atmospheric and vacuum condition. The foam with densities from 50 to 80 kg/m3 achieve thermal conductivity at an atmospheric pressure of 33–34 mW/(m × K), while in a vacuum of 6–7 mW/(m × K). Compared to other organic polymer foams, MF foams have superior fire resistance and chemical stability. The innovation of MF rigid foams presented here, compared to other well-known MF flexible foam, is in their rigid structure, combined with low density and thermal conductivity, which makes this particular foam potentially useful in the manufacture of vacuum insulation panels (VIP).

FTIR Analysis of Chemical Gradients in Thermally Processed Molded Polyurethane Foam

Thermally processed PU foams are examined as a function of processing temperatures (25, 45, 65, and 85°C) at the side, middle, and center of a simple cylindrical mold. The PU foams show both chemical and morphological differences as a function of the processing temperature and radial position within the mold. Thermal degradation of uretoneimine structures, the emergence of carbodiimide structures, and extent of reaction of isocyanate groups are measured using photoacoustic FTIR spectroscopy. Chemical gradients and morphology differences between the side, middle, and center of the molded foam are observed for all processing temperatures. The data indicate that thermal activation at the center of the mold is important for samples regardless of processing temperature. Furthermore, in spite of thermal processing at temperatures well above the decomposition of uretoneimine structures (40°C), chemical gradients remain within the simple molded foams.

Physical Properties of Soy-Phosphate Polyol-Based Rigid Polyurethane Foams

Water-blown rigid polyurethane (PU) foams were made from 0–50% soy-phosphate polyol (SPP) and 2–4% water as the blowing agent. The mechanical and thermal properties of these SPP-based PU foams (SPP PU foams) were investigated. SPP PU foams with higher water content had greater volume, lower density, and compressive strength. SPP PU foams with 3% water content and 20% SPP had the lowest thermal conductivity. The thermal conductivity of SPP PU foams decreased and then increased with increasing SPP percentage, resulting from the combined effects of thermal properties of the gas and solid polymer phases. Higher isocyanate density led to higher compressive strength. At the same isocyanate index, the compressive strength of some 20% SPP foams was close or similar to the control foams made from VORANOL 490.

Bi-cellular Foam Structure of Polystyrene from Extrusion Foaming Process

This article investigates the foaming process of bi-cellular polystyrene foams blown with n-butane and water in extrusion. The bi-cellular foam structure has two types of cells: large cells ranging from 0.1 to 1.2 mm and small cells ranging in size from about 5% to about 50% of the average large cell size, which constitute more than 90% of the total cell volume. A bi-cellular structure has outstanding heat insulation property. In order to generate a bi-cellular structure, a water-blowing technology was used. This technique is environmentally benign and economical since butane and water are used as blowing agents. Despite these advantages, the foaming process and mechanism of bi-cellular foams have not been identified in detail. Therefore, in this article, an attempt has been made to enhance our knowledge and understanding of the foaming behavior of bi-cellular polystyrene foam. The effects of n-butane, water, and silica (a nucleating agent) on the foam cell morphology are presented.

Mass Transport of Cell Gases in Carbon Dioxide Blown PET (Polyethylene Terephthalate) Foam Insulation

Polyethylene terephthalate (PET) foam is a possible replacement option for polyurethane (PUR) foam as insulation material in district heating pipes. In this study, the diffusion coefficients and activation energies of cell gases in carbon dioxide blown PET foam (densities 148–157 kg·m−3) were determined at temperatures between 23 °C and 90 °C. The foam thermal ageing due to the mass transport of air into and carbon dioxide out of the foam was about ten times slower in PET foam than in PUR foam. The thermal conductivities of the PET foam boards were determined in a heat flow meter apparatus. The contribution to the foam thermal conductivity due to conduction in the solid polymer and radiation within the cell voids was determined to 17 mW·m−1·K−1 at 20 °C. This is higher than the value estimated for PUR foam in district heating pipes, 12 mW·m−1·K−1. This contribution can probably be reduced by developing low density PET foam and reducing the cell size.