Fundamentals of Foam Formation

The preparation of a polymeric foam involves first the formation of gas bubbles in a liquid system, followed by the growth and stabilization of these bubbles as the viscosity of the liquid polymer increases, resulting ultimately in the solidification of the cellular resin matrix. 

Foams may be prepared by either one of two fundamental methods. In one method, a gas such as air or nitrogen is dispersed in a continuous liquid phase (e.g. an aqueous latex) to yield a colloidal system with the gas as the dispersed phase. In the second method, the gas is generated within the liquid phase and appears as separate bubbles dispersed in the liquid phase. The gas can be the result of a specific gasgenerating reaction such as the formation of carbon dioxide when isocyanate reacts with water in the formation of water-blown flexible or rigid urethane foams. Gas can also be generated by volatilization of a low-boiling solvent (e.g. trichlorofluoromethane, F-11, or methylene chloride) in the dispersed phase when an exothermic reaction takes places, (e.g. the formation of F-11 or methylene chloride-blown foams). 

Another technique to generate a gas in the liquid phase is the thermal decomposition of chemical blowing agents which generate either nitrogen or carbon dioxide, or both. 

The factors affecting the stability and growth of bubbles in aqueous foams have been reviewed in depth by deVries (3). In order to disperse a given volume of gas in a imit volume of liquid, one must increase the free energy of the system by an amount of energy AF as follows  

According to classical theory, the gas pressure in a spherical bubble is larger than the pressure in the surrounding liquid by a difference Ap, as shown in the following equation  

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Introduction to Foams and Foam Formation 5 FUNDAMENTALS OF FOAM FORMATION... 

The fundamentals of foam formation have been reviewed in detail by a number of authors [17-19]. Therefore, only the main concepts will be reviewed in this chapter. Cellular plastics are produced through several methods, irrespective of cell structure (open or closed). However, the most commonly used methods for cellular PLA are the following ... 

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