Fluorinated solid surfaces

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... 

The effects of fluorination on solid-surface free energies parallel the liquid trends Perfluormated polymers have the lowest cntical surface tensions, which directly relate to their antistick properties [19], but substimtion of fluorine by hydrogen or by the more polarizable ehloiine atom markedly raises their surface free energy. 

Etchant species (for example, fluorine atoms) diffuse to the surface of the material and adsorb onto a surface site. It has been suggested (20) that free radicals have fairly large sticking coefficients compared with relatively inert molecules such as CF4, so adsorption occurs easily. In addition, it is generally assumed (20) that a free radical will chemisorb and react with a solid surface. Further, surface diffusion of the adsorbed species or of the product molecule can occur. 

Product desorption is a crucial step in the etch process. A free radical can react rapidly with a solid surface, but unless the product species has a reasonable vapor pressure so that desorption occurs, no etching takes place. For instance, when an aluminum surface is exposed to fluorine atoms, the atoms adsorb and react to form AIF3. However, the vapor pressure of AIF3 is 1 torr at 1240 C thus etching is precluded at ambient temperatures. 

Fluorination is quite effective for introduction of hydrophobicity to the solid surface. Touhara et al. [33] reported that ACF can be fiuorinated even below 473 K and the fluorine content can be controlled by the reaction temperature. ACF is composed of micrographites and the fluorination changes the unit structure of ACF from the micrographite to polycyclohexane frame. The predominant surface of ACF consists of the conjugated C=C bonds, whereas that of the fiuorinated ACF is covered with the C-F bonds. Accordingly, the fiuorinated ACF can offer the micropore having a perfect hydrophobicity. 

The addition of surfactant to the adhesive changes the surface properties of the polymers, and consequently changes the wetting of the solid surface. As Fig. 5.1 shows, the addition of 1% OP-10 or of the fluorinated alcohol CF3(CF2)CH20H to an adhesive based on a 40% solution of PBMA in MMA decreases the interfadal angle for wetting of the steel siuface by the adhesive from 72° to 30-50°. 

Hence, the high effectiveness of fluorinated coatings is associated with their ability to lower the critical surface tension, Yc of the treated surface well below that of any fluid other than a fluorocarbon (Rao and Baker, 1994). It is worth recalling that the lower the Yc of a solid surface. 

Despite their potential application, the wetting properties of aqueous surfactant solutions on superhydrophobic (SH) solid surfaces have been rarely investigated. In this work single surfactant solutions and binary mixtures of non-ionic and ionic surfactants have been studied in contact with textured and non-textured surface coatings obtained utilizing nanoparticles and a fluorinated polymer. 

It was to meet the need for hydrocarbon, oil-resistant seals, gaskets, O-rings, and so on in the automotive and aerospace industries that PMTFPS was first developed. PDMS is an oleophilic polymer, quite unsuited to this application. All the n-alkane values used in the determination of solid surface tension of PDMS earlier in this chapter are instantaneous values or taken as soon after drop deposition as possible. All n-alkane contact angle values on PDMS decline inexorably to zero as the drop swells the polymer and eventually disappears. The CF3- cluster of PMTFPS was found to combine excellent solvent resistance with adequate thermal stability for the minimum degree of fluorination, critical to furnishing a cost-effective product. The... 

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