Foam Expansion and Its Control

One such process is the TAP process, the basic patent being held by Dow. It was developed in Japan by Asahi in conjuction with Toshiba. Foam expansion after mould filling is made possible by use of retractable mould cores. Because of the difficulty of allowing expansion in more than one direction this process has been largely limited to the production of flat products. Efficient gas sealing systems are also vital and the process needs close control. For this reason it has not been widely used in either Europe or North America. 

An important condition which has to be fulfilled when using this method for foam dispersity determination is the absence of an excess hydrostatic pressure in the foam liquid phase. This pressure is equalized to a considerable extent when an equilibrium distribution the foam expansion ratio and the border pressure along the height of the foam column is established. This can be controlled by measuring the pressure in the Plateau borders at a certain level of the foam column by means of a micromanometer. However, if this condition is overlooked, the hydrostatic pressure can introduce a considerable error in the results of bubble size measurements, especially in low expansion ratio foams. Probably, it is the influence of the unrecorded hydrostatic pressure that can explain the lack of correspondence between the bubble size in the foam and the excess pressure in them, observed by Aleynikov[49]. The... 

The properties of a low expansion ratio foam are controlled mainly by changing the rate of the kinetic processes running in it. To decelerate the hydrodynamic processes in order to preserve its structure for a longer time (for instance, in the formation of polymer or frozen foams), the following measures can be recommended i) to use a foam generation mode that allows producing a foam of a uniform expansion ratio such are the stream type generators and some mechanical devices that mix the solutions ii) to produce a foam of maximum dispersity... 

The formation and preservation of the uniform cell structure of either the solid polymer or frozen foam depend on the properties of the initial foam, i.e. on its expansion ratio and stability. On the one hand, the former two processes are determined mainly by the rates of drainage and internal foam collapse. On the other hand, however, they are affected by the rate of polymerisation and the increase in strength of the foam material. That is why optimisation of the processing properties of polymer foams can be achieved by decreasing both the rate of microsyneresis and drainage, and the rate of internal foam collapse. Along with that precise control of the rate of solidification, reduction of the water content and introduction of plastifiers are needed. 

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