Forces for Shrinkage

Because the liquid wets and spreads over the solid surfaces, pores will be formed in the liquid. The reduction of the liquid-vapor interfacial area provides the driving force for shrinkage or densification of the compact. If the pore in the liquid is assumed to be spherical with radius of r, the pressure difference across the curved surface is given by the Young and Laplace equation ... 

Driving Force for Shrinkage Decrease in Interfacial Energy... 

The removal of a solvent above its critical point occurs with no capillary pressure because there are no liquid-vapor interfaces. Thus in the aerogel process there is a greatly reduced driving force for shrinkage. Compared to xerogels, aerogels are expanded structures (Fig. 3d) that are often more closely related to the structure of the gel that existed at the gel point. 

Driving Forces for Shrinkage explains the role of chemical reactions and the pressure exerted by osmotic, disjoining, and capillary phenomena. The concept of moisture stress, widely used in soil science, is a phenomenological measure of all of these factors. 

Polyethylene (T = 135 °C, Tg < T oond may be lightly crosslinked by a chemical reaction with an organic peroxide at 175 °C. Heat-shrink tubing is made from such a crosslinked polyethylene. When heated at room temperature to about 150 °C, its diameter shrinks by a factor of three or four. Explain the thermal history required to make and use this material and the driving force for shrinkage when heat is applied. 

During pressure sintering, interiDarticle compressive stress, approximated by the externally applied stress and nonnalized by the relative density of the compact p, supplements the surface tension driving force for pore shrinkage ... 

So as to fill the mould at the speed required, high pressure is applied to the melt to compensate for shrinkage, it usually is necessary to force in extra material in a second stage and to prevent over-filling at that time a lower second-stage or hold pressure may be used. 

These results indicate that the solvent removal process in poly(amic acid) film formation is dependent on the residual stress level. Solvent evaporation creates volume changes which in turn generate shrinkage stresses ( 8 MPa at room temperature). This will continue until there is a balance between the internal stress and the driving forces for solvent removal. Further solvent may be removed if this balance is altered by a change in stress or temperature. 

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