Critical surface tension surface

Measurement of contact angles on solids yields data that reflect the thermodynamics of a liquid/solid interaction. These data can be used to estimate the surface tension of the solid. For this purpose, drops of a series of liquids are formed on the solid surface and their contact angles are measured. Calculations based on these measurements produce a parameter (critical surface tension, surface tension, surface free energy etc.), which quantifies the characteristic of the solid surface and its wettability. 

Chemical or physical surface treatments are especially required for structural bonding of low-surface-energy plastics. Low-surface-energy plastics include polyethylene, polypropylene, TPO, and fluorinated polymers. These surface treatments are designed to increase the critical surface tension and improve wetting and adhesion. In addition to increasing the critical surface tension, surface treatments are designed to remove contaminants or weak boundary layers, such as a mold release. 

Figure X-9 shows plots of cos 6 versus 7l for various series of liquids on Teflon (polytetrafluoroethylene) [78]. Each line extrapolates to zero at a certain 7l value, which Zisman has called the critical surface tension 7 since various series extrapolated to about the same value, he proposed that 7 was a quantity characteristic of a given solid. For Teflon, the representative 7 was taken to be about 18 and was regarded as characteristic of a surface consisting of —CF2 — groups.

The critical surface tension concept has provided a useful means of summarizing wetting behavior and allowing predictions of an interpolative nature. A schematic summary of 7 values is given in Fig. X-10 [123]. In addition, actual contact angles for various systems can be estimated since )3 in Eq. X-38 usually has a value of about 0.03-0.04. 

What is the critical surface tension for human skin Look up any necessary data and make a Zisman plot of contact angle on skin versus surface tension of water-alcohol mixtures. (Note Ref. 136.)... 

Take the data from Fig. X-12 on the propyl monolayers and make a Zisman plot to determine the critical surface tension for the surface. 

The interesting implication of Eq. XII-24 is that for a given solid, the work of adhesion goes through a maximum as 7b(a) is varied [69]. For the low-energy surfaces Zisman and co-workers studied, )3 is about 0.04, and Wmax is approximately equal to the critical surface tension yc itself the liquid for this optimum adhesion has a fairly high contact angle. 

Critical surface tension of wettiag, alkane test Hquids only, unless otherwise iadicated. 

Silanes can alter the critical surface tension of a substrate in a well-defined manner. Critical surface tension is associated with the wettabiUty or release qualities of a substrate. Liquids having a surface tension below the critical surface tension, y, of a substrate wet the surface. Critical surface tensions of a number of typical surfaces are compared to y of silane-treated surfaces in Table 2 (19). 

Critical surface tensions for silanes refer to treated surfaces. 

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