Bubble Growth and Stabilization

The final cell population density of the foam decreases due to cell coalescence and cell coarsening. These two outcomes of the bubble growth mechanism should be reduced because fine-celled foams have better mechanical, thermal, and acoustical properties. Therefore, stabilization is critical to prevent cell wall rupture caused by excessive thinning. 

It should also be pointed out that cell wall rupture may be important for the formation of open-cell foams while cell wall thinning and rupture must be avoided for the production of closed-cell foams. In either case, there is an optimum processing temperature window and viscoelastic properties of the polymer for foaming. 

Several authors have pointed out that when there are two adjacent bubbles of different sizes, the gas pressure in the small cell is greater than that in the larger one. The difference in pressure between the two bubbles (Apf) having radii of rj and is given by [17, 19] 

One can notice from Equation 17.3 that the gas will tend to diffuse from the smaller bubble into the larger one because of the pressure difference in the cells. Then the two cells will become one larger cell (cell coarsening) [24]. 

Stabilization of bubbles is accomplished by cooling the foamed molten plastic to provide the necessary increase in viscosity. In this stage, the temperature of the melt is one of the most important process parameters because it affects the 

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Typically, physical foaming is a three-step process (1) mixing a blowing gas is dissolved in the polymer to form a homogeneous solution (2) bubble nucleation subsequent pressure release or temperature increase induces phase separation due to the thermodynamic instability, and gas starts to form nuclei and (3) bubble growth and stabilization. 

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