Surface energy critical wetting tension

Coran and Patel [33] selected a series of TPEs based on different rubbers and thermoplastics. Three types of rubbers EPDM, ethylene vinyl acetate (EVA), and nitrile (NBR) were selected and the plastics include PP, PS, styrene acrylonitrile (SAN), and PA. It was shown that the ultimate mechanical properties such as stress at break, elongation, and the elastic recovery of these dynamically cured blends increased with the similarity of the rubber and plastic in respect to the critical surface tension for wetting and with the crystallinity of the plastic phase. Critical chain length of the rubber molecule, crystallinity of the hard phase (plastic), and the surface energy are a few of the parameters used in the analysis. Better results are obtained with a crystalline plastic material when the entanglement molecular length of the... 

Since the time that study was made, a large body of experimental data has accumulated which shows that regular and predictable changes in solid-liquid contact angles result from changes in the outermost layer of atoms in the solid surface. A "wettability spectrum" has been published [14],and the utility of the "critical surface tension of wetting" as an index of solid surface energy has been well established. 

A complete wetting of a solid is only possible for spontaneous spreading of a drop of the liquid at the surface, i.e. for = 0 or cos = 1. For a specific solid surface of low surface energy, a linear correlation is observed between cosO and the surface tension. This is demonstrated for polytetrafluoroethylene in Figure 3.6. The limiting value, i.e. cos = 1, is a constant for a solid and is called the critical surface tension of a solid, Xc- Therefore, only liquids with y < yc are able to spontaneously spread on surfaces and to wet them completely. 

Besides viscosity, the surface-wetting ability of underfills is critical to capillary flow. For capillary flow to occur, the underfill material must wet the surfaces so that the advancing contact angle is less than 90°. " Also, for capillary flow, the intramolecular forces of attraction among adhesive molecules must be weaker than the intermolecular attraction of the adhesive for the die, the substrate, and the solder surfaces.t This occurs when the surface tension of the underfill is lower than the surface energy of... 

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