Critical surface tension fluorocarbon surfaces

The critical surface tension has been evaluated by means of advancing contact angle measurements for water, various hydrocarbon liquids and for plasma polymerized fluorocarbon films " " . The most comprehensive, however, is that of Yasuda... 

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. 

We should emphasize that the critical surface tension is a (semi-)empirical parameter and is not the surface tension of the solid, although it is close to this value, as will be seen in Section 6.2.2. Extensive critical surface tension data are available for many polymers and other solids, e.g. as shown in Table 6.1. As expected, the fluorocarbon surfaces have the lowest critical surface tensions, ranging from only 6 mN m [-CF3] to 28 mN m [-CFH-CH2-]. The hydrocarbon polymeric surfaces have critical surface tensions around 30 mN m, while chloro-and nitrated hydrocarbon surfaces have higher critical surface tensions, around 40-45 mN m . The exact value depends on the precise polymeric surface considered. 

The repellency of fluorocarbon finishes depends on the structures of the fluorocarbon segment, the nonfluorinated segment of the molecule, the orientation of the fluorocarbon tail, the distribution and the amount of the fluorocarbon moiety on fibers, and the composition and geometry of the fabric [101]. The relationship between repellency and the structure of the fluorocarbon segment is in accord with the critical surface tension concept developed by Zisman and co-workers (see Chapter 11). Shafrin and Zisman [102] determined the wettabilities and critical surface tensions of -perfluoroalkyl substituted 77-heptadecanoic acids synthesized by Brace [103]. Once the seven outmost carbon atoms are fully fluorinated x = 7), the wettability of monolayers of the acids F(CF2)a(CH2)i6COOH approaches that of the perfluorocarboxylic acid F(CF2).vCOOH (Fig. 12.2). This suggests that a terminal perfluoroalkyl chain of seven carbon atoms is sufficiently... 

Surface tension studies of the most common fluorosilicone, poly(3,3,3-trifluoropropylmethylsiloxane) (PTFPMS), give unexpected results. Compared with (PDMS), PTFPMS has a higher liquid surface tension, a similar critical surface tension of wetting, and a considerably lower solid surface tension, as determined by water and methylene iodide contact angles and the method of Owens and Wendt (67). These results are summarized in Table X (7, 67, 72-74, 76, 77), in which PTFPMS is compared with two other fluorocarbon polymers, poly(tetrafluoroethylene) (PTFE) and poly(chlorotrifluoroethylene) (PCTFE). PTFE behaves like PTFPMS, whereas PCTFE behaves like PDMS. 

Four polymers with different surface compositions were used in this study—polystyrene (PS), poly(methyl methacrylate) (PMMA), polyacrylamide (PAM), and a poly(vinylidene chloride) (PVeC) copolymer (containing 20% polyacrylonitrile). Polystyrene has essentially a hydrocarbon surface, whereas the surfaces of poly (methyl methacrylate) and polyacrylamide contain ester and amide groups, respectively. The surface of the poly(vinylidene chloride) copolymer on the other hand will contain a relatively large number of chlorine atoms. The presence of acrylonitrile in the poly(vinylidene chloride) copolymer improved the solubility characteristics of the polymer for the purposes of this study, but did not appreciably alter, its critical surface tension of wetting. Values of y of these polymers ranged from 30 to 33 dynes per cm. for polystyrene to approximately 40 dynes per cm. for the poly(vinylidene chloride) copolymer. No attempt was made to determine e crystallinity of the polymer samples, or to correlate crystallinity with adsorption of the fluorocarbon additives. 

Branching of the fluorocarbon chain decreases the efficiency of a fluorinated surfactant in surface tension reduction [57,58]. In analogy, a condensed (spread) monolayer of a perfluorinated n-alkanoic acid has a lower critical surface tension than its terminally branched isomer. Bemett and Zisman [59] attributed the effect of branching to different molecular packing and carbon chain adlineation. 

The critical micelle concentrations of fluorinated surfactants are compared with those of hydrocarbon surfactants in Table 6.11 [59,157]. The cmc values for C7 and Cg fluorinated surfactants are approximately equal to those for Ci 1 and C12 hydrocarbon surfactants, respectively. The greater hydrophobicity of the fluorocarbon chain enhances the amphiphilic character of the surfactant and increases surface activity, reflected in surface tensions below 20 dyn/cm and lower cmc values [1,159]. Perfluorination reduces cmc values about four times per —CF2— group. The A value [or K value in Eq. (35)] does not change considerably with the type of the surfactant. Because the relation expressed by Eq. (34) is exponential, the cmc value of a fluorinated surfactant is approximately equal to that of a hydrocarbon surfactant with a hydrocarbon chain 1.5-1.7 times longer than the fluorocarbon chain [59,157,160]. 

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