Critical Surface Tension Wetting

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. 

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. 

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... 

Wettability—coupling agents improve the wetting between polymer and substrate (critical surface tension factor). 

Table V. Wetting Angle at the Contact with Water ( fa) and Formamide (- jO and Critical Surface Tension (1 ) at 20°C...

Complete wetting of a solid is only possible if a drop of the liquid spreads spontaneously at the surface, i.e. for 9 = 0 or cos 9=1. The limiting value cos 6 = 1 is a constant for a solid and is named critical surface tension of a solid y... Therefore, only liquids with yl < Vc have the ability to spontaneously spread on surfaces and wet them completely. Tab. 4.2 gives an overview of critical surface tension values of different polymer surfaces [40]. From these data it can be concluded that polytetrafluoroethylene surfaces can only be wetted by specific surfactants with a very low surface tension, e.g. fluoro surfactants. 

The surface properties are of particular interest for composites and coatings. The n = 6 monomer will wet Teflon, and PTFE filled composites can be prepared. The critical surface tension of wetting for the fluoromethylene cyanate ester resin series has been determined from contact-angle measurements on cured resin surfaces. As indicated in Table 2.2, it parallels the fluorine composition and begins to approach the PTFE value of 18 dyn/cm. 

The ability of a liquid to "wet" the membrane material is an indication of that liquids ability to establish and maintain such an interfacial layer. Liquids of surface tension values less than the critical surface tension iy ) of the membrane material are capable of completely "wetting" the polymer. It may be possible therefore, to select membrane materials capable of accomplishing specific separations by their ability to be wet by one solution component but not by the other. For this reason Yc membrane materials is important. By employing the standard techniques of Zisman (43), the critical surface tension for PSF and CA were determined to be 43.0 and 36.5 dynes/cm, respectively. This data indicates that PSF is more readily wet by a larger number of liquids than is CA. Similar measurements for the various sulfonated polysulfones are underway. 

The viscosity or resistance to flow increases as the number of repeat units increases, but physical properties, such as surface tension and density, remain about the same after a DP of about 25. The liquid surface tension is lower than the critical surface tension of wetting, resulting in the polymer spreading over its own absorbed films. The forces of attraction between polysiloxane films are low resulting in the formation of porous films that allow oxygen and nitrogen to readily pass though, but not water. Thus, semipermeable membranes, films, have been developed that allow divers to breath air under water for short periods. 

C Critical surface tension of wetting, mixed series of test liquids. 

With values in the range of about 10-18 mN m 1 perfluorinated liquids have the lowest surface tensions among the known organic liquids, and will completely wet any solid surface. Increasing amounts of hydrogen in the molecule increase the surface tension. Fluorinated solid surfaces, e.g. fluoropolymers, possess very low critical surface tensions yc, which relates to their antistick and low frictional properties, whereas hydrocarbon polymers have substantially higher values (PTFE yc = 16.0 mN m-1 PE yc = 31.0 mN m-1).7... 

Zisman discovered that there is a critical surface tension characteristic of low-energy solids, such as plastics and waxes. Liquids ihat have a lower surface tension than the solid will spread on that solid, while liquids with a higher surface tension will not spread. Examples of critical surface tension values for plastic solids in dynes per cm are "Teflon/ 18 polyethylene, 31 polyethylene terephthalate, 43 and nylon, 42-46. As one indication of the way this information can be used in practical applications, one can consider the bonding of nylon to polyethylene. If nylon were applied as a melt to polyethylene, it would not wet the lower-energy polyethylene surface and adhesion would be poor. However, molten polyethylene would spread readily over solid nylon to provide a strong bond. 

The wetting properties of polyacetylene have been studied by Schonhom et al. 380) who measured a critical surface tension of 51 mN m 1, considerably higher than for other hydrocarbon polymers. This value was attributed to oxidation of the surface as no change was observed on further oxidation. Treatment of polyacetylene with aqueous potassium permanganate renders it hydrophilic, reduces the contact angle for water from 72° to 10° and renders the structure more water-permeable 381)... 

The wetting angle can be measured using simple techniques such as a projector, as shown schematically in Fig. 2.54. This technique, originally developed by Zisman [73], can be used in the ASTM D2578 standard test. Here, droplets of known surface tension, at are applied to a film. The measured values of cos are plotted as a function of surface tension, at, as shown in Fig. 2.55, and extrapolated to find the critical surface tension, ac, required for wetting. 

The wetting of a liquid drop placed on a solid surface has already been described (Section 3.4) by the critical surface tension of the surface and by Young s equation. Temperature is a factor in wetting by aqueous solutions since it influences surfactant solubility. For example, the fastest wetting for polyoxyethylenated non-ionic surfactants is produced by those whose cloud points are just above the use temperature [193]. 

These comments should not be interpreted to mean that measures of wettability are useless at predicting adhesion. They do seem clearly to indicate that contact angles and critical surface tensions reported for wood are not necessarily thermodynamic quantities or well-defined material parameters. Because most contact angles are dynamic values, they should be interpreted with caution and considered as relative measures of adhesion, for which the absolute scale is yet unknown. Further, we need to keep in mind that although wetting is necessary for adhesion, it may not be the limiting factor in many real situations. 

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