Testing methods contact angle

The surface energy (critical surface tension) of solids is measured by a method developed by Zisman.9 In this method a series of contact angle measurements are made with various liquids with known surface tensions on the solid to be tested. The contact angle 9 is plotted as a function of the yLV of the test liquid. The critical surface tension is defined as the intercept of the horizontal line cos 9=1 (i.e., when the contact angle is 0°) with the extrapolated straight-line plot of cos 9 against yLV of the liquids. The yLV at this intersection point (i.e., where a hypothetical test liquid would just spread over the substrate) is defined as the critical surface tension of the solid. 

An alternate method frequently mentioned in the literature is the so-called captive bubble technique [42]. Experimentally, an air bubble of known volume is produced at the tip of a microsyringe. This bubble is then injected into a tank containing the test liquid. The test surface is positioned on the top of the test liquid and 2-3 mm above the injected air bubble. A captive bubble is formed when the air bubble floats upward and be captured by test surface. Contact angle 6, formed at the three phase contact line, follows the same Young s equation in Eq. (2.1) and can be calculated from the drop profile of the bubble in a manner similar to the sessile drop method. A schematic of the captive bubble method is shown in Fig. 2.11. 

An astronaut team has, as one of its assigned experiments, the measurement of contact angles for several systems (to test the possibility that these may be different in gravity-free space). Discuss some methods that would be appropriate and some that would not be appropriate to use. 

Measurement of Surface Activity. Each surface-active property can be measured in a variety of ways and the method of choice depends on the characteristics of the substance to be tested. The most frequendy determined properties are surface tension (Y5q, Ylg) i t if cial tension (Yll> Tlg) contact angle (9), and CMC. 

Many of the most widely used methods are based on measuring the contact angles of a series of test liquids on the solid surface, and evaluating the surface energies via Young s equation, Eq. 4 above. 

Test Methods. Surface tension (y) measurements were taken by Wil-helmy method (25+0.1°C). Critical micelle concentrations (cmc) were obtained from Y logC curves. Contact angle. Type GI, Japan. Wetting test. Canvas disk method, CIS,HG-2-380-66. Foam test, Ross-Miles lather method. Emulslbillty was determined by mixing 20 ml of 2.5%... 

The antistatic properties of the step 2 product were evaluated by preparing tapes of blends with polystyrene by extruding in a twin-screw extruder using a flat die at 200°C. The contact angle of the tapes was measured using the sessile drop method and water as the measuring liquid. Testing data not supplied by author. 

An average of eight measurements was taken as the contact angle. Water and methylene iodide were used as test liquids. Polarity of the dried latex films were estimated according to the method of Kaelble (10). 

Test methods used to determine the uniformity of substrates are numerous and vary with the type of material. They are generally the same tests used to characterize the material or to determine its fundamental physical properties. Tests that are commonly employed are hardness, tensile strength, modulus, and surface characteristics such as roughness or contact angle with a standard liquid. Often a test similar to the nonvolatile test mentioned above is used to determine if there are any compounds in the substrate that are capable of out-gassing on exposure to elevated temperatures. Moisture content of certain hydroscopic polymers, such as nylon and polycarbonate, is also known to affect adhesion. 

Owing to the last problem the contact angle method has mainly been applied to low-energy surfaces like polymers 1112,122— 126J or silicas grafted with alkyl chains [ 127, 28. Some results are summarized in Table 9. In addition, it is often used to determine the surface tension of materials with a high specific surface area like powders or porous substances 1129-133). In that case the rise of the test liquids in a capillary filled with the substance is measured. 

For the adsorption tests, a sample of silica (Merk) with a specific surface area of 388 m g measured by the BET method was used. The solid specimens used to measure the contact angles were microscope glass slides and pieces of quartz polished using a rotating plate covered with a polishing cloth impregnated with 10 pm diamond polishing particles. 

These properties are listed in order of usefulness for comparative review purposes. Liquid surface tension is the most fundamental property, because it pertains only to the material in question (provided the material is adequately pure) and the technique used for measurement. All the other properties listed are dependent also on solvents, contact-angle test liquids, and liquid or solid substrates selected. For solids, approaches such as the Owens-Wendt analysis (7) have supplanted the Zisman method (18) in recent years, but data from the Zisman method for organosilicon polymers are more available compared with data from the Owens-Wendt approach. Some useful data on aqueous surface tensions and Langmuir troughs are also available. Data for other listed properties are of less fundamental use and rather scanty. 

The technique consists in measuring the B (e.g. water) flow rate (/) through a membrane impregnated with A (e.g. isobutanol or mixtures of alcohols and water) as a function of the pressure difference AP. We have to note that it is possible to modify the method from "pressure controlled" to "flow controlled" in order to reduce the test time and increase its flexibility [126]. At a certain minimum pressure the largest pores become permeable, while the smaller pores still remain impermeable. This minimum pressure depends mainly on the type of membrane material (contact angle), type of permeate (surface tension) and pore size. When all pores are filled with B, the liquid flux / through the membrane becomes directly proportional to the pressure. 

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