Additives exothermic blowing agents

Polyether-based foams account for more than 90% of all flexible polyurethane foams. The properties of foams are controlled by the molecular structure of the precursors and the reaction conditions. In general, density decreases as the amount of water increases, which increases the evolution of carbon dioxide. However, the level of water that can be used is limited by the highly exothermic nature of its reaction with isocyanate, which carries with it the risk of self-ignition of the foamed product. If very low density foams are desired, additional blowing agents, such as butane, are incorporated within the mixing head. 

The proper balance between viscosity and gas evolution can be controlled by a number of factors such as a suitable type and concentration of catalyst and surfactant, the presence of a nucleating agent (not always necessary) (17,18) and control of reaction temperature (or exotherm). Additional factors that must be considered are the use of a suitable chemical blowing agent, which is especially important for the production of thermoplastic foams, and the formation of oligomers (prepolymers) which exhibit higher viscosities than monomers in the preparation of thermoset foams (e.g. polyurethane foams). 

The breadth of the exothermic curve (195 °C to 225 °C) indicates the temperature range over which the decomposition of azo occurs. The shape of the peak indicates the uniformity of the decomposition. In addition, the size of the peak, i.e., the area under the exothermic curve, is a quantitative measure of the amount of blowing agent that has decomposed in the sample. This integrated area under the exothermic peak is referred to as heat of decomposition, AH. ... 

However, an important consideration with the chemical types is that in addition to an exotherm resulting from the chemical breakdown of the compoimd, unless steps are taken to seal the system there will also bean endotherm due to the hberation of the gas. This loss of heat from the system will cancel out some of the exotherm and interfere with the data. The use of high-pressure, sealed pans is therefore very important in studies involving chemical blowing agents as these ensure that no volatiles can escape during the analysis. 

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