Critical surface tension of the substrate

FIGURE 3.4 Contact angle of an uncured epoxy adhesive on four substrates of varying, critical surface tension. Note that as the critical surface tension of the substrates decreases, the contact angle increases, indicating less wetting of the surface by the epoxy adhesive.4... 

Figure 23 Relationship between peel force and critical surface tension of the substrate for acrylic, polyvinyl ether and O natural rubber adhesives [103], The contact time...

Good wetting of the substrate surface is essential for developing reliable bonds. Adhesives that do not wet the surface will not spread out and fill substrate surface irregularities. Wetting occurs when the surface tension of the liquid adhesive is lower than the critical surface tension of the substrates being bonded (see Section 6.1). 

Wetting of plastic surfaces is much more complex than wetting clean metal surfaces. Plastics and adhesives are both polymeric materials and thus have similar physical properties, including wetting tensions. Plastic-bonded joints do not have the large difference between the critical surface tension of the substrate and that of the adhesive. 

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). 

PDMS based siloxane polymers wet and spread easily on most surfaces as their surface tensions are less than the critical surface tensions of most substrates. This thermodynamically driven property ensures that surface irregularities and pores are filled with adhesive, giving an interfacial phase that is continuous and without voids. The gas permeability of the silicone will allow any gases trapped at the interface to be displaced. Thus, maximum van der Waals and London dispersion intermolecular interactions are obtained at the silicone-substrate interface. It must be noted that suitable liquids reaching the adhesive-substrate interface would immediately interfere with these intermolecular interactions and displace the adhesive from the surface. For example, a study that involved curing a one-part alkoxy terminated silicone adhesive against a wafer of alumina, has shown that water will theoretically displace the cured silicone from the surface of the wafer if physisorption was the sole interaction between the surfaces [38]. Moreover, all these low energy bonds would be thermally sensitive and reversible. 

The surface energy (critical surface tension) of solids is measured by a method developed by Zisman.9 In this method a series of contact angle measurements are made with various liquids with known surface tensions on the solid to be tested. The contact angle 9 is plotted as a function of the yLV of the test liquid. The critical surface tension is defined as the intercept of the horizontal line cos 9=1 (i.e., when the contact angle is 0°) with the extrapolated straight-line plot of cos 9 against yLV of the liquids. The yLV at this intersection point (i.e., where a hypothetical test liquid would just spread over the substrate) is defined as the critical surface tension of the solid. 

Ethyltriethoxysilane (ETES). The critical surface tensions of the ETES films obtained on silica by retraction from 1% solutions in a-chloro-naphthalene are plotted in Figure 2A. The adsorption time—i.e., the abscissa—is the time the silica substrate was allowed to remain in contact with the adsorbate solution. Adsorption times longer than 20 hours did not produce any further decrease in yc and solutions containing 0.1% and 5% ETES gave values within 1 dyne/cm. of those in Figure 2A. Attempts to obtain ETES films from solution in isopropylbicyclohexyl were unsuccessful—the solutions did not retract from the test surfaces even after 20 hours adsorption time. Isopropylbicyclohexyl has a surface tension of 34.4 dynes/cm. so if the ETES adsorbed to form films having yc values of 33-35 dynes/cm., as it had from -chloronaphthalene, then the bicyclohexyl solution would not be expected to retract. 

Mohandas and co-workers (18), confirming previous findings of Weiss and Blumenson (19), have also shown that cells in an environment free of adsorbable proteins (which rapidly modify the surface properties of polymeric or inorganic substrates) will exhibit a similar direct relationship between their adhesion and the critical surface tension of the surface they contacted. DiflFerential adhesion of red blood cells was measured by determining the fraction of cells retained on a surface after the application of well-calibrated shear stresses (IS). In protein-free experiments, the red cells (themselves dominated in adhesive interactions by their protein membranes) had greatest adhesion to glass, intermediate adhesion to polyethylene and siliconized glass, and least adhesion to Teflon. 

As a general rule, good adhesion can be achieved when the surface tension of the substrate (natural fibres) is larger than that of the matrix. [64] have shown that the critical surface tension can be modified through AAPP treatment (Fig. 6.3). Different UgnoceUulosic fibres can be seen behaving differently if exposed to the same... 

There are a few points to be kept in mind. The liquid membrane must be nonvolatile since it is subjected to vacuum on the permeate side and can evaporate on the feed side and be lost to the feed solution (Figure 8.1.49(d)). Further, wetting of the porous membrane structure spontaneously by the membrane liquid present outside requires that the surface tension of the membrane liquid should be equal to or lower than the critical surface tension of the polymeric or ceramic substrate being employed. For example, y,. for polypropylene (PP) is -33 dyne/cm. Many organic liquids will spontaneously wet it. However, y,. values for various fluoropolymers (FPs) are usually <25... 

This theory proposes that adhesion results from molecular contact between two materials and the surface forces that develop. The first step in bond formation is to develop interfacial forces between the adhesive and the substrates. The process of estabtishing continuous contact between the adhesive and the adherend is called wetting. For an adhesive to wet a soUd surface, the adhesive should have a lower surface tension than the critical surface tension of the solid. This is precisely the reason for surface treatment of plastics, which increases their surface energy and polarity. 

Inhibitors form a film on the surface that blocks the dissolution of the substrate. Adsorption of an alkyl-thiol to the surface of the CU3AU alloy resulted in an inaeased surface tension of the gold film this is observed as an increase in the value of <., which depresses dissolution of copper. This behavior resembles inhibition of copper corrosion on a pure copper surface where benzotriazole increases the potential to start significant copper dissolution this was demonstrated by Cruickshank et using in situ AFM. However, when the critical potential for benzotriazole film... 

Extrapolation of the straight Hne to cos9 = 1 (9 = 0) gives the critical surface tension of wetting y. Any Hquid with y v Tc vvill give 9 = 0, that is, it wets the surface completely y,. is the surface tension of a liquid that just spreads on the substrate to give complete wetting. 

This concept of a critical surface tension for spreading on low-energy surfaces is one that was developed by Zisman and coworkers (Fox, 1950 Shafrin, 1960 Zisman, 1964). They demonstrated that, at least for low-energy substrates, in order to wet the substrate the surface tension of the wetting liquid must not exceed a certain critical value that is characteristic of the particular substrate. 

Thus, for example, in the presence of some highly fluorinated carboxylic acids and their salts, the value yc for polyethylene is decreased from its usual value of almost 31 mN/m to about 20 mN/m (Bernett, 1959) by adsorption of the fluorinated surfactants onto the polyethylene surface, with the result that solutions of these surfactants having surface tensions less than the normal yc for polyethylene do not spread on it. The requirement that the surface tension of the wetting liquid be reduced by the surfactant to some critical value characteristic of the substrate is thus a necessary, but not sufficient, condition for complete spreading wetting. A surfactant solution whose surface tension is above the critical tension for the substrate does not produce complete wetting, but a solution whose surface tension is below the critical tension for the substrate may or may not produce complete wetting (Schwarz, 1964). 

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