A Review on the Thermodynamic Aspects of Thermoplastic Foams

Both physical and chemical foaming agents can be used in the two common methods for the production of polymeric foams. The physical foaming agent is dissolved in the polymer at a pressure and temperature beyond its critical condition. The foam structure is formed by a sudden pressure drop in the mixture in three steps; nucleation, growth, and coalescence. In the nucleation step, thermodynamic instability is formed in the mixture. This induces a tendency in the solvent molecules for a phase transition from a supersaturated state to a gas state in the direction of instability reduction. By overcoming the energy barrier, the free energy of the system is reduced and gas stable nuclei are formed in the free volumes of the polymer chains. Growth and coalescence of the nucleic take place with the diffusion of the gas molecules inside them. The growth step is finally stopped and the foam structure is established. With the increase in the nucleation efficiency, the growth rate is reduced. Therefore, prediction of nucleation rate is an important factor for controlling the thermoplastic foam structures. Investigation of the nucleation step has been doable with the aid of nucleation theories. The classical nucleation theory is a prominent method for the investigation of nucleation phenomena in thermoplastic foams, but due to the existing of a divergence between its theoretical values with the experimental data, some modifications have been introduced into this theory. Other competing theories such as the Density Functional Theory and the Self Consistent Field Theory have also been developed. The main aim of this review paper is an investigation of the existing theories for thermoplastic foams.