Chemical Blowing Agents CBAs

Examples of chemical blowing agents are azo compounds, N-nitroso-compounds, and sulphonyl hydrazides, which yield 10()-3()() cm of nitrogen per gram of compound at temperatures of 90-2 7 5°C. Azodicarbonamide is widely used, having a decomposition temperature of 230-235°C, which can be reduced to 155-200°C by means of metal compounds such as lead and zinc stabilizers. It can thus match the temperatures at which the melt viscosity of many polymers is suitable for foaming, and is used (typically) in calendered PVC and PVC plastisols and in structural foam forms of polyethylene, polypropylene, PVC, polystyrene, and ABS. 

Endothermal blowing agents require energy for decomposition and gas release therefore stops quickly after the supply of heat is terminated. Shorter cooling periods are needed and moulding cycles are therefore shorter. The base materials are bicarbonate and citric acid - which are also used as food additives, and present no handling problems. 

Other blowing agents are under development for foaming engineering polymers and/or to overcome problems occurring, such as corrosion of screws, barrels, and moulds, and uneven cell structure. 

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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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