Chemical Blowing Agents CBA

Chemical blowing agents generate a gas, typically nitrogen, water, carbon monoxide or carbon dioxide, by a chemical reaction that can be either exothermic (i.e., heat is generated chemically and given out by the system, resulting in a rise in temperature) or endothermic (heat is absorbed 

More than half of all commercial CBAs are used with PVC, but they can be employed with polyolefins, polystyrene, PET and rubber. They are sold as powders, with or without a coating, and in the form of pastes, dispersions or masterbatches. Commercial grades often contain several ingredients in addition to the active constituents listed below, in order to satisfy complex requirements. 

Azodicarbonamide (AZC) is by far the most widely used CBA. It can foam PVC, including plasticised PVC, as well as the polyolefins, the styrenics, polyamides, PPO and some thermoplastic elastomers. AZC decomposes exothermically at around 210 °C to give nitrogen gas, along with other degradation products such as mea, cyanuric acid and cyamelide as solid by-products. Another possible residue is semicarbazide, traces of which have been found in foamed gaskets made of PVC blown with azodicarbonamide (see Chapter 7). 

Since loadings range from 0.05% to 20% by weight, the amoimt of these residual substances can be significant. 

Carbon monoxide and isocyanuric acid may also be formed, and if the decomposition temperature is high enough, ammonia may be detectable. 

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Chemical blowing agents (CBAs). These are solids that decompose to give a gas at the processing temperature. In addition to the demands above, the CBA must possess the following ... 

Chemical blowing agents (CBAs) decompose at various processing temperatures to form a gas (Table 8.3). The most important criterion for selection of a chemical blowing agent is that the decomposition temperature matches the processing temperature of the plastic. Little or... 

In either case, the gas, if evenly dispersed, expands to form the cells in the plastic. There are a number of different ways to bring about the formation of cells, depending on the gas being used, the chemical blowing agent (CBA) the type of plastic resin, and/or the particular process being used (7). 

On the other hand, the use of chemical blowing agents (CBAs) has increased. These agents generate the gas necessary for stractural foam by either decomposition or chemical reaction in the melt. Factors to consider when selecting a blowing agent for a particular thermoplastic stmctural foam are (21) ... 

For example, in profile foam operations, where higher density foams are desired, it is general practice to use chemical blowing agents (CBA). These materials, which decompose under polymer melt processing conditions to yield gases to expand the polymer, are usually dry-blended with the resin and then fed to the extruder. They can also be compounded directly into the resin by the polymer supplier and provided as a ready-to-use masterbatch. 

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