Polytetrafluoroethylene surface tension

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.

Surface Protection. The surface properties of fluorosihcones have been studied over a number of years. The CF group has the lowest known intermolecular force of polymer substituents. A study (6) of liquid and solid forms of fluorosihcones has included a comparison to fluorocarbon polymers. The low surface tensions for poly(3,3,3-trifluoropropyl)methylsiloxane and poly(3,3,4,4,5,5,6,6,6-nonafluorohexyl)methylsiloxane both resemble some of the lowest tensions for fluorocarbon polymers, eg, polytetrafluoroethylene. 

Fluorocarbon soHds are rare in defoamer compositions, presumably on account of their cost. SoHd fluorine-containing fatty alcohols and amides are known. The most familiar fluorocarbon soHd is polytetrafluoroethylene [9002-84-0]. Because it is more hydrophobic than siHcone-treated siHca, it might be expected to perform impressively as a defoamer component (14). However, in conventional hydrocarbon oil formulations it works poorly because the particles aggregate strongly together. In lower surface tension fluids such as siHcone and fluorocarbon oils, the powdered polytetrafluoroethylene particles are much better dispersed and the formulation performs weU as a defoamer. 

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. 

For the experiment, the dorsal skin of young rats (Wistar or a comparable strain) is shaved and washed with an antibiotic solution (containing, e.g., streptomycin, penicillin, chloramphenicol, and amphotericin in concentrations inhibiting bacterial growth). After skin excision, excess fat is peeled off and the skin is placed over the end of a polytetrafluoroethylene tube with the epidermal side in touch with the hollow cylinder. The skin is fixed with an O-ring and the tube interior is sealed. The side of the dermis is then submersed in a magnesium sulphate solution (154 mM). The samples are applied at 30°C to the epidermal side of the skin in such a way that the skin interface is fully covered. After the incubation time, the substances are removed with prewarmed water the skin surface tension is decreased with ethanol which is subsequently replaced with magnesium sulphate solution (154 mM). 

A fluoroepoxy compound was made from a fluorodiepoxy resin cured with an amine adduct. The use of fluoroepoxies is most advantageous in bonding fluorocarbons such as polytetrafluoroethylene. Good bond strengths are achieved on fluorocarbons without the need to surface-treat the substrate because the surface tension of the fluoroepoxy adhesive is reduced from about 45 to 33 dyn/cm.25... 

Hence a plot of cos B versus / /yi. should yield a straight line with an inclination given by the surface tension of the solid. Good and Girifalco used several liquids to verify this behavior. As an example they determined the surface tension of polytetrafluoroethylene and octadecylamine monolayers to be roughly 28 and 30 mN/m, respectively. 

Silicone polymers exhibit significantly lower surface tension than most solids. Exemptions are only polyolefins and polytetrafluoroethylene. Therefor it is not surprising that much of today s business for silicon based materials depends on their surface activity. 

It has generally been observed that urethane systems, which are cured on the surface of some low energy materials, are also free from adhering to them, i.e. they are self-releasing. Materials which show this characteristic relative to urethanes in a very effective way include at least three different types of plastics polytetrafluoroethylene, polyethylene, and polypropylene. These materials all have a defined critical surface tension sc less than about 30 dynes/cm. Assuming that this value is near, the liquid vapor surface tension lv value of an effective IMR urethane systems, then the work of adhesion as given by Equation 4 is as follows ... 

Two common liquid membrane support materials, polytetrafluoroethylene and polypropylene, have critical surface tensions of 18 mN/m and 35 mN/m, respectively. Manufacturers often supply critical surface tensions for their porous films. Liquids with a surface tension, y, less than the critical surface tension will probably wet the surface. Therefore, hydrocarbons will wet polypropylene, but water (y = 72 mN/m) will not. Shafrin and Zisman (30) have summarized critical surface tension data for many materials and correlated the data such that critical surface tensions may be estimated from knowledge of the functional groups in the chemical structure of the surface. 

For the two surfaces in question, plots of adhesion tension versus the surface tension of the liquid phase are given in Figure 2. Of special note are the nearly constant values of the adhesion tensions for various homologous series of hydrocarbon liquids in contact with polytetrafluoroethylene. 

Complications such as these extend also to the case of polytetra-fluoroethylene. The large difference in estimated solid-vacuum tensions between this polymer and polyethylene is not imexpected, since a proportionately large difference exists for the liquid surface tensions of hydrocarbons and fluorocarbons having five to eight carbon atoms [58]. The underlying cause of this difference is, however, more obscure. The inter molecular forces for fluorocarbons apparently have features wuich lead to anomalous behavior, at least from the point of view of solubility parameter theory [59]. Thus, theoretical calculations of the surface tension for the bare solid in the case of polytetrafluoroethylene would face a number of difficulties not encountered with paraffin crystals. 

Surface tensions for the bare solids are estimated to be polytetrafluoroethylene, 34 paraffin, 45 and polyethylene, 55 dynes per cm. 

CONCERNING THE SURFACE TENSION, CRITICAL SURFACE TENSION, AND TEMPERATURE COEFFICIENT OF SURFACE TENSION OF POLYTETRAFLUOROETHYLENE. 

Figure 3.6. Influence of the surface tensions of various fluids on the wetting of polytetrafluoroethylene according to Fox and Zismann (cf. ref. 4)...

Fluorocarbons. Fluorocarbons, such as polytetrafluoroethylene (TFE), polyfluororethylene propylene (FEP), polychlorotrifluoroethylene (CFE), and polymonochlorotrifluoroethylene (Kel-F), are notoriously difficTilt to bond because of their low surface tension. However, epoxy and polyurethane adhesives offer moderate strength if the fluorocarbon is treated prior to bonding. 

Our studies showed that the inner surface of the cardiovascular System was hydrophobic surface with Yc(Zis.)=29 dyne/cm. We could easily select polytetrafluoroethylene as the material with a smaller critical surface tension than 29 dyne/cm to make test materials. Figure-3 shows the surface properties of the inner surface of the cardiovascular system and polytetrafluoroethylene in relation to wetting properties viewed from Zisman s plots. How should we treat polytetrafluoroethylene to minimize the difference in wetting properties between the surface of polytetrafluoroethylene and the inner surface of the cardiovascular system. We tried to improve the surface properties of polytetrafluoroethylene by using the graft copolymerization method and expansion method. 

Table 12.1 (7) summarizes the surface tensions of selected polymers. While the surface tension of most polymers varies from about 20 to 50 erg/cm these values are low compared to the surface tension of water, 72.94 erg/cm or mercury, 486.8 erg/cml Thus water beads up on polymers with low surface tensions, not wetting them. For example, low surface tension underlies the utility of the Teflon polytetrafluoroethylene frying pan. 

Figure 7.6. A typical Zisman plot (cosine of 6 vs. liquid surface tension) for a polytetrafluoroethylene ( Teflon ) surface in contact with liquid /7-alkanes. The critical surface tension y for this system is I8 mJ/m (from ref. (5)), reproduced with permission from American Chemistry Society...

Table 14.4. Liquid surface tension, /iv, at which maximum (Max) or minimum (Min) in sedimentation volume occurs, for several polymers PTFE, polytetrafluoroethylene PVDF, poly(vinylidene fluoride) PVF, poly(vinyl fluoride) HOPE high-density polyethylene PA 66, poly(hexamethylene adipamide) (nylon 6, 6) PSF, polysulfonate. Experiments were performed at room temperature, and the extrema are to be understood as being the surface tensions of the polymers (from refs (65, 66)) ... 

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