Critical surface tension concept

The critical surface tension concept has provided a useful means of summarizing wetting behavior and allowing predictions of an interpolative nature. A schematic summary of 7 values is given in Fig. X-10 [123]. In addition, actual contact angles for various systems can be estimated since )3 in Eq. X-38 usually has a value of about 0.03-0.04. 

Explain briefly the critical surface tension concept and how its value for PMMA has been estimated. Which of the following liquids will fully wet PMMA diethylene glycol, formamide and acetone ... 

The theory of repellency is discussed in Chapter 11. The relationship between repellency and the structure of the fluorinated repellent is in agreement with the critical surface tension concept developed by Zisman [319]. Shafrin and Zis-man [320] determined the critical surface tensions of fu-perfluroalkyl-substituted -heptadecanoic acids and the wettabilities of their monolayers. The wettabilities... 

Although the critical surface tension concept has been criticized for its empirical nature [27], critical surface tension data have been very useful in developing water- and oil-repellent finishes. One reason for this practical impact may be the similarity between the systems studied by Zisman and the surfaces of water-and oil-repellent finishes. 

Zisman was one of the first to recognize that the critical surface tension concept is strictly empirical and to suggest that needs to be replaced by parameters having a thermodynamic or statistical mechanical basis [26]. Fox and Zisman [30] have cautioned that jc varies between liquid types and that it is not a measure of the surface energy of the solid yso-... 

Because of the deficiencies and limitations of the critical surface tension concept, the need to determine the surface free energy of solids has remained. Because the surface free energy of solids is difficult to measure, attempts have been made to estimate the surface free energy from interaction with liquids. 

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... 

The critical surface tension value for most inorganic solids is in the hundreds or thousands of dynes per centimeter. For polymers and organic liquids, it is at least an order of magnitude lower. Critical surface tension is an important concept that leads to a better understanding of wetting and adhesion. 

This concept of a critical surface tension for spreading on low-energy surfaces is one that was developed by Zisman and coworkers (Fox, 1950 Shafrin, 1960 Zisman, 1964). They demonstrated that, at least for low-energy substrates, in order to wet the substrate the surface tension of the wetting liquid must not exceed a certain critical value that is characteristic of the particular substrate. 

In addition to overcoming experimental difficulties, the Naval Research Laboratory group has contributed many important generalizations and new concepts-for example, the concepts of low energy solid [33,69], critical surface tension [33,61], and autophobic liquid [32,41]. These developments have also stimulated the search for an interpretative scheme capable of yielding values of solid-vapor and solid-liquid interfacial tensions. 

Fox and Zisman first proposed the concept of critical surface tension in the early 1950s. In Zisman s method, the relationship between the contact angle of various liquids on a solid and the surface tension of the liquids are investigated. Specifically, cos 9 is plotted against yLv (known as Zisman plot) in which a straight line is often obtained when a homologous series of liquids are used to wet the solid s surface (non-linear for non-homologous liquids). 

The availability of a wider range of experimental results and advances in the theory of polymer wettability lead to a refinement of the concept of critical surface tension. Dann noted a pronounced curvature of some of the cos 6 versus klf plots, and the tendency for different liquids to give somewhat different values of Yq. He explained these points as follows. 

Despite these considerations, the critical surface tension is an empirical parameter, often quoted and relatively easy to measure, which helps characterize the surface of a low-energy solid such as a polymer. Values of this are available from review articles and Bandrup and Immergut and Grulke s Polymer Handbookf A much-simplified practical routine based on the concept is used in ASTM D-2578 to test the surface of plastic film see Surface characterization by contact angles - polymers. 

The concept of critical surface tension was proposed by Fox and Zis-man [93-95]. An empirical, linear relation was foimd when plotting between the cosine of the advancing angle and the surface tension of a series of homologous liquids (referred to as a Zisman plot). The critical surface tension (yc) equals the surface tension of the liquid, when it is extrapolated to a zero contact angle on the soUd ... 

Another useful concept for assessing the wettability of surfaces is that introduced by Zisman and collaborators [121], namely the critical surface tension of wetting, that was discussed in detail in Chapter 11. These authors found that, for a given surface and a series of related liquids such as n-alkanes, siloxanes or dialkyl ethers, cos 0 is a reasonably linear function of la- The surface tension at the point where the line cuts the cos 0=1 axis is known as the critical surface tension of wetting, y. It is the surface tension of a liquid that would just spread to give complete wetting. 

The degree of wetting is controlled by thermodynamic and kinetic factors. The driving force for wetting a substrate is dependent on the surface energies of the adhesive and the substrate. The Zisman concept of critical surface tension provides the basis for estimating the surface energy of solids [212]. Typical values of critical surface tension (in mN/m) for polymer substrates follow [216] ... 

Before discussing the conditions dictating optimum thermodynamic adhesion, it is in order to introduce the concept of 7q, the critical surface tension of wetting. It is obvious from equation (2) that for the determination of W, one must know the values of and Tsl (TIv can easily be measured), and these are not easily accessible because of many experimental difficulties associated with measuring solid surface free energies. However, in cases where equation (1) is valid, the difference of Tgy - Tsl can be determined by measuring 6. ... 

The themiod3Tiamic approach to the description of adhesion has many advantages as compared with some other theories. It does not require knowledge of the molecular mechanism of adhesion but considers only the equilibrirun processes at the polymer-solid interface. The approach to the problem developed by Zisman is widely accepted. Zisman introduced the concept of the critical surface tension of wetting as a value which is found by extrapolation of the dependence of cos on y to cos =1, i.e., when liquid fully spreads on the surface. The value Yj found by extrapolation is considered as the critical surface tension of a solid. If the value y is known, the equilibrium contact angle can be predicted for any liquid on any surface. If yj < y, the contact angle equals zero and the liquid spreads on the surface. 

Surface tensions are readily determined for liquids, unlike solids. To ascribe surface energy values to solids, the concept of critical surface tension y, as developed by Zisman, is utilized. As a generalization, a liquid will spread on a surface which has a higher surface tension or free surface energy than that of the liquid. 

There are two important approaches for studying wetting phenomena the Zisman s plot and the associated concept of the critical surface tension and the use of interfacial theories. These two approaches are presented in this section. The approach based on interfacial theories is somewhat more complex but provides more information. When combined with the Young equation for the contact angle (Chapter 4), interfacial theories can be used... 

An alternative method uses a concept called critical surface tension, proposed by Fox and Zissman to characterize the surface energy of solids. A plot cosine of the contact angle (cos 6), and liquid—vapor surface tension (yiv), yields a straight line for a homologous series of liquids (Fig. 3.5). Nonho-mologous liquids yield a curved line that may not be easily extrapolated. The intercept of the line at cos (0) equal to one is defined as the critical surface tension of the polymer (yc). Values of 18 dynes/cm for... 

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