Wetting and work of adhesion

A review of the fundamentals of snrface science will establish both the technical basis for adhesion and provide methods to qnantitatively determine a surface s ability to be bonded. From a physics point of view, objects and materials are either attracted or repulsed from one another. Since adhesives are typically used in the liquid state, the terms -phillic and -phobic are often used to describe the attraction or repulsion, respectively. For example, materials that attract water are called hydrophilic, while those that repulse water are hydrophobic. Water is a good example, since historically many adhesives have been solvent borne, bnt now with increasing environmental consciousness there is a large effort to replace the solvent-borne adhesives with aqueous or water-borne adhesives. 

It should be apparent that it is of interest to either know or rank the surface attraction of adherend materials to prospective adhesives. The surface attraction of a material is referred to as the surface energy, surface free energy, or surface tension. Therefore, measurement of the surface energy of a material will rank the attraction of that material to the prospective adhesive. If possible it is desired to measure the attraction of the adhesive to the snrface materials that are to be used as adherends. Often, this is not possible and a more fundamental measurement is made to determine the surface energy. 

FIGURE 11.2 Contact angle measurement, (a) High surface energy substrate and (b) low surface energy substracte. 

The preceding section has discussed the philosophy behind the surface measurements. Now the theory will be presented, which will allow for quantification of the attraction between the solid and liquid. Young s equation for the balance of forces when a liquid drop is placed on a surface is 

Ysv = surface tension, or surface free energy of the solid-vapor interface Ys, = surface tension, or surface free energy of the solid-liquid interface Yiv = surface tension, or surface free energy of the liquid-vapor interface 0 = the angle of the liquid drop on the surface. 

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The work of adhesion (see Wetting and work of adhesion. Contact angles and inter-facial tension) is a simple example of a concept that could be regarded as a fundamental measure of adhesion between two surfaces. 

Note that the work of adhesion is defined here as the free energy change required to separate phases 1 and 2 cleanly along the interface and to place each in a separate vacuum enclosure (Fig. la). It is sometimes defined in a subtly different way, see Wetting and work of adhesion, Eqns. 1 and 4. 

These ideas are farther developed in other articles, in particnlar. Surface characterization by contact angles - polymers. Wetting and spreading, Wetting and work of adhesion, Contact angles and interfacial tension. Pre-treatments of polymers and Pretreatment of metals prior to bonding. 

Wetting and work of adhesion J F PADDAY Thermodynamic works of adhesion, wetting, spreading and cohesion Young s equation... 

Wetting. Advancing work of adhesion for methylene iodide, formamide, and ethylene glycol on the three carbon fiber specimens is presented in Table IV. 

Wetting Criteria, Surface and Interface Free Energies, and Work of Adhesion In a solid-liquid system, wetting equilibrium may be defined from the profile of a sessile drop on a planar solid surface. Young s equation [36], relating the surface tension Y of materials at the three-phase contact point to the equilibrium contact angle 6, is written as... 

Figure 17.4 Schematic diagram of the interactions for the contact between a solid and a liquid with the SFE value being the same as the ST value, 50mN/m. The top panel Ulustrates the case of complete wetting as a result of liquid and solid having the same degree of polarity. The bottom panel shows the case of incomplete wetting. The values for IFT, contact angle and work of adhesion were calculated according to the OWRK approach.

Combining theories of interfacial tensions with Young equation and work of adhesion for studying wetting and adhesion... 

It can be shown, in the context of the assumption that only dispersion forces operate across the interface (i.e. j/l = vt nd ys = vi), that optimal wetting and maximal work of adhesion may not be criteria consistent with one another. Either Eq. 9 or Eq. 11 gives for these conditions ... 

W. quantifies the specific, discrete interactions that exist between a wetting liquid and a substrate. These interactions may be Van der Waals, acid-base, or covalent. The reversible work of adhesion is the product of the areal density of these interaction sites (or attachment points) and the energy per attachment point ... 

Adhesion is created by primary and secondary forces according to the theory of adsorption interaction. This theory is applied the most widely for the description of interaction in particulate filled or reinforced polymers [30]. The approach is based on the theory of contact wetting and focuses its attention mainly on the influence of secondary forces. Accordingly, the strength of the adhesive bond is assumed to be proportional to the reversible work of adhesion (W ), which is necessary to separate two phases with the creation of two new surfaces. 

Equation 24 is derived from the Young (Eq. 21), the Dupre (Eq. 8) and the Eowkes (Eq. 10) equations by assuming complete wetting (cos 0 =0). Measurements with polar solvents give the polar component of the surface tension, but acid/base constants and the corresponding work of adhesion can also be calculated from them ... 

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