Exothermic blowing agents

Bio-Foam, Chemical blowing agents, exothermic, Rit-Chem Co. Inc. 

A critical factor is the boiling temperature of the blowing agent and its relationship to the temperature of the walls of the mould and of the reacting mixture. There should be sufficient exotherm to vaporise the blowing agent in the centre of the reacting material but the mould walls should be sufficiently cool to... 

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. 

See entries BLOWING AGENTS, PRESSURE INCREASE IN EXOTHERMIC DECOMPOSITION... 

While drying in a tray dryer at 60°C, a batch decomposed fairly exothermically but without fire. As a blowing agent, it is designed to have limited thermal stability [1], One of the recently calculated values of 87° and 78°C for induction periods of 7 and 60 days, respectively, for critical ignition temperature coincides with the previous value of 8777 days. Autocatalytic combustion is exhibited. 

Foams are commercially produced several ways. Some polymerization processes produce their own foam. Polyurethanes, for example, are very exothermic. When they are formed, if a little water is present, CO2 will be a by-product. As the polymer forms, the CO2 will cause closed cell foam. As another example, a blowing agent can be injected into the molten polymer. The agent will later decompose, giving off a gas when the polymer is heated to melting. Epoxy resins are expanded into foams this way. 

The efficiency of azodicarbonamide and sodium bicarbonate blowing agents for PE foams was considered (253). These systems, which generate GO2 gas, are more suitable for compression moulding of foams. Blends of the blowing agents have a reduced exotherm, so are more suitable for polymer systems that are temperature sensitive, such as ethylene copolymers. 

Journal of Cellular Plastics 36, No.2, March/April 2000, p.126-47 COMPOUND MECHANISM OF THE ENDO-EXOTHERMIC + OR - BALANCED BLOWING AGENT... 

This paper describes experimentation carried out on an exothermic blowing agent to understand the chemistry... 

Thermal decomposition of chemical blowing agents generating either nitrogen or carbon dioxide, or both, by application of heat, or as the result of the exothermic heat of reaction during polymerization. 

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). 

These blowing agents are inert solvents and act as blowing agents by the reaction exotherm of the cationic polymerization. 

In manufacturing phenolic foams, a blowing agent such as R-113 (trichlorotrifluoroethane) is evaporated by using an exothermic reaction (Equation 7), and the blowing agent is included in the polymer. The reaction involved in forming the benzylic ether-type foam is shown below. Similarly, this foam will finally form a network polymer as do resol-type foams. 

Blowing Agents. Blowing agents are divided into two types, one volatUe and the other decomposable. The volatUe type is mainly used for compositions of the resol type. This type is evaporated by exothermic reaction and then forms a blowing gas. On the other hand, the decomposable type is used for novolac-type compositions. This type is... 

As for decomposable matter, examples are sodium carbonate, ammonium carbonate, sodium nitrite, sodium sulfite, and sodium bicarbonate. Other well-known examples are di-N-nitrosopentamethyl-enetetramine, N,N-disubstituted 5-amino-l,2,3,4-thiatriazole, diesters of azodiformic acid, diazonium salts, sulfonhydrazides, and N-alkyl N-nitrosodiacetoneamine (7). The water produced during the exothermic resol condensation reaction is used as a blowing agent for producing high-density foamed composites. 

Volatilization of a low-boiling liquid, either by the heat liberated by an exothermic reaction, or by externally applied heat. Commonly used liquids are chlorofluoro-carbons (CFCs). This is the most widely used technique in the production of rigid polyurethane foams. However, due to the ozone depletion problem in the stratosphere, they must be phased out and industry is presently searching for alternative blowing agents. 

The endothermic or exothermic nature of a blowing agent affects processing economics. One of the factors that frequently controls the cycle time of structural-foam parts, and therefore the number of parts that can be produced in an hour, is the post blow. Post blow results when a part is removed from a mold before the core is sufficiently cooled, causing swelling in areas with the greatest thickness. This swelling is caused by the presence of pressurized gas in the core of the part (21). 

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