Water-blown foam mechanism

The continuous evolution of carbon dioxide from water-blown foam from the earliest times of reaction is a mechanism for significant heat loss from free-rise foam, the magnitude of which will depend on the scale of foaming. This observation offers an explanation for the departure from adiabaticity, lower than expected maximum reaction temperature, in smaller scale foaming. 

The dimensional stability of low density, water blown rigid PU foams for pour-in-place thermal insulation applications was improved by the use of a phthalic anhydride based polyester polyol containing a dispersed cell opening agent. The foam systems obtained allowed some of the carbon dioxide to be released through the cell windows immediately after filling of the cavity, and to be rapidly replaced by air. Studies were made of the flowability, density, open cell content, dimensional stability, mechanical properties, thermal conductivity and adhesion (particularly to flame treated PE) of these foams. These properties were examined in comparison with those of HCFC-141b blown foams. 21 refs. 

The same PU foam was also treated with a mixture of glycerine and water (up to 40%) to investigate the effective decomposition through the combined mechanisms of hydrolysis and glycolysis. The presence of water slowed down the process to 10 min at 160 °C, while working at higher temperatures resulted in undesired trans-esterification and pyrolysis products. However, this hydroglycolysis approach did dramatically reduce the amount of solvent required. In addition, due to the presence of water, the resultant polyols formed could be easily used in new formulations of water-blown PU foams. 

Lin Y, HsiehF, Huff HE, lannottiE. 1996. Physical, mechanical, and thermal properties of water-blown rigid polyurethane foam containing soy protein isolate. Cereal Chem 73 189-196. 

Polyolefin foams are easier to model than polyurethane (PU) foams, since the polymer mechanical properties does not change with foam density. An increase in water content decreases the density of PU foams, but increases the hard block content of the PU, hence increasing its Young s modulus. However, the microstructure of semi-crystalline PE and PP in foams is not spherulitic, as in bulk mouldings. Rodriguez-Perez and co-workers (20) showed that the cell faces in PE foams contain oriented crystals. Consequently, their properties are anisotropic. Mechanical data for PE or PP injection mouldings should not be used for modelling foam properties. Ideally the mechanical properties of the PE/PP in the cell faces should be measured. However, as such data is not available, it is possible to use data for blown PE film, since this is also biaxially stretched, and the texture of the crystalline orientation is known to be similar to that in foam faces. 

The crystal structure and three-dimensional network prevent cellulose from behaving as a polyol for PU syntheses. To overcome this problem a liquefaction process in the presence of organic solvents was developed and resulted in products suitable for PU synthesis. Yan and co-workers (2008) liquefied corn stalk, an agricultural by-product, and tested it for the synthesis of PU foams blown by water. The report revealed that such polyurethane foams had excellent mechanical and thermal properties and could be used as heat insulating materials. 

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