Teflon Tension

A zero or near-zero contact angle is necessary otherwise results will be low. This was found to be the case with surfactant solutions where adsorption on the ring changed its wetting characteristics, and where liquid-liquid interfacial tensions were measured. In such cases a Teflon or polyethylene ring may be used [47]. When used to study monolayers, it may be necessary to know the increase in area at detachment, and some calculations of this are available [48]. Finally, an alternative method obtains y from the slope of the plot of W versus z, the elevation of the ring above the liquid surface [49]. 

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.

However, on rigid substrates, the growth of dry zones is accompanied by a rim of excess liquid with width X (Fig. 10). As the dewetting proceeds, X increases. For short times and < K, the growth of dry patches is controlled only by surface tension forces and the dewetting speed is constant. A constant dewetting speed of 8 mm-s has been measured when a liquid film of tricresyl phosphate (TCP) dewets on Teflon PFA, a hard fluoropoly-mer of low surface free energy (p. = 250 MPa, 7 = 20 mJ-m ). 

The n-A curves were measured with a trough equipped with a moving blade and a piezoelectric device (Figure 2). Both the trough (286 mm long and 70 mm wide) and blade were coated with Teflon. The subphase temperature was kept within 0.1 °C by use of a water jacket connected with a thermostated circulation system, and the environmental air temperature was kept at 18 °C. The surface tension was measured with a Wilhelmy plate made of filter paper (25 x 25 x 0.25 mm) using a piezoelectric device. The surface pressure(ji) is defined as ... 

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. 

Young s equation at liquid —solid l iq uid2 has been investigated in various systems where it has been found that the liquid-solid-liquidi surface tensions meet at a given contact angle. For example, the contact angle of water drop on Teflon is 50° in octane (Chattoraj and Birdi, 1984 see Figure 5.3). 

In the water-Teflon-octane system, the contact angle, 9, is related to the different surface tensions as follows ... 

It was described exhaustively before that the molecules at the surface of a liquid are under tension due to asymmetrical forces, which gives rise to surface tension. However, in the case of solid surfaces, one may not envision this kind of asymmetry as clearly, although a simple observation might help one to realize that such surface tension analogy exists. For instance, let us analyze the state of a drop of water (10 pL) as placed on two different smooth solid surfaces (e.g., Teflon and glass). One finds that the contact angles are different (Figure 5.4). 

The magnitude of ycr for Teflon of 18 mN/m thus suggests that -CF2- groups exhibit this low surface tension. The value of ycr for -CH2-CH3- alkyl chains gave a higher value of 22 mN/m than for Teflon. Indeed, from experience, one also finds that Teflon is a better water-repellant surface than any other material. The magnitudes of ycr for different surfaces are seen to provide much useful information (Table 5.4). 

Dynamic Mechanical Measurements. Films were prepared by casting the acetone solution of sample No. 2 onto a Teflon sheet after adding curing agents. The sample was allowed to stand at room temperature for one day, and then cured at 130°C for 2 hours. The dynamic mechanical spectroscopic data were measured in tension with a Rheovibron DDV-II (Toyo Baldwin Co. Ltd.) at a frequency of 110 Hz with a heating rate of about l°C/min. 

EXAMPLE 6.4 Surface Tension and the Height of a Meniscus at a Wall. Calculate the height to which an n-octane surface will climb on a Teflon wall (this is the same system used in Example 6.3) if y is 22 mj m 2J = 30°, and p = 0.70 g cm 3. Comment on the ease or difficulty of making this measurement. 

The intercept at 0 = 0 is viewed as a kind of critical state for these systems, and the corresponding surface tension is represented by yc. What is the value of yc for Teflon ... 

Perfluorocarbons. In 1966, it was demonstrated (27) that a laboratory mouse could survive total immersion in a perfluorochemical (PFC) solution. This material, similar to commercial Teflon, is almost completely inert and is insoluble in water. A water-soluble emulsion was prepared that could be mixed with blood (28), and in 1968 (29) the blood volume in rats was completely replaced with an emulsion of perfluorotributylamine [311-89-7], C12F27N. The animals survived in an atmosphere of 90—100% 02 and went on to long-term recovery. However, the 02 content of the perfluorochemicals has a linear dependence on the partial pressure of oxygen, P, as can be seen in Figure 1. The very high 02 tension required to transport physiologic amounts of 02 (12) and the propensity of the perfluorocarbon to be taken up by the reticuloendothelial cells were considered to be severe limitations to the development of clinically useful perfluorocarbon blood substitutes (30). 

Procedure. To form a BLM, a small amount (.— 0.005 ml.) of lipid solution was applied via a Teflon capillary attached to a micrometer syringe. The formation characteristics leading to the black state were observed under reflected light at 20-40 X magnification. Other precautions that should be exercised are essentially those described previously (10). The bifacial tension of BLM was measured as follows. After the membrane had become completely black (except at the Plateau-Gibbs border), the infusion-withdrawal pump was started. The pressure difference across the BLM was continuously monitored and reached a maximum when the membrane was hemispherical. The interfacial tension was calculated from this point using Equation 3. 

Stearic acid (1.16 g) was dissolved in 100 ml of ethanol. A10 pi portion of the resulting solution was pipetted onto a dean surface of a dilute HC1 solution (in a shallow tray) where it spread to form a monolayer of stearic acid. Tire layer was compressed (by moving a Teflon barrier across the tray) until the surface pressure n started to rise sharply and reached 20 dyn / cm. Note that n = y0-y where y is the measured surface tension with the film present and yc is the higher surface tension of water alone. Tire compressed film occupied a 20 x 24 cm area. Calculate the cross-sectional area of an alkyl chain in stearic acid. [See J. B. Davenport, in Biochemistry and Methodology of Lipids (A. R. 

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. 

When a drop of liquid is placed on a solid surface the liquid may form a bead on the surface, or it may spread to form a film. A liquid having a strong affinity for the solid, i.e., if its surface tension is less than the critical surface tension of the surface, yc, will seek to maximize its contact (interfacial area) and spread to form a film. A liquid with much weaker affinity, i.e., if its surface tension is above yc, will form into a bead. The critical surface tensions of solids range from 18 mN/m for Teflon to about 46 mN/m for nylon. 

Examination of the relevant theory indicates that the adjuvant effect of surface-active agents on herbicide action is maximized when the quantity FI = yL cos 0, or the film pressure at the liquid/solid interface, has a maximum value. Measurement of surface tension of 1.0% aqueous solutions and of contact angle on a number of substrates (Teflon, paraffin) and plant-leaf surfaces (soybean, com) as a function of hydrophile-lipophile balance show at least one maximum, and these values are in good agreement with earlier experimental data on herbicidal activity. 

Clean, flat, homogeneous surfaces can be obtained in the laboratory, but otfly with difficulty. You must expect all surfaces around you to be rough and contaminated. So don t expect accurate results from amateur experiments on surface phenomena. The cleanest surfaces around tend to be those of low surface-tension materials such as Teflon and polyethylene films. High-tension surfaces are always contaminated, if only by water adsorbed from air. Clean aluminium foil is perhaps as close as you can get. You often get best results with a little tapping or vibration as this reduces the difference between the two angles. 

---