Wetting liquid, adhesion

Fig. XII-13. Dlustration of adhesion between two plates due to a meniscus of a wetting liquid.

Gent, A.N. and Schultz, J., Effect of wetting liquids on the strength of adhesion of viscoelastic materials. J. Adhes., 3, 281-294 (1972). 

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 ... 

This interaction energy is reversible because removal of the wetting liquid from the surface only requires the disruption of these interaction sites. Solidification of the liquid into an adhesive changes the requirements for dewetting, however. 

As explained under the adsorption theory of adhesion,3 an adhesive must first wet the substrate and come into intimate contact with it. (A brief description of the adsorption theory of adhesion is presented in the section below.) The result of good wetting is simply that there is greater contact area between adherend and adhesive over which the forces of adhesion (e.g., van der Waals type of forces) may act. For good wetting, the surface free energy (surface tension yLV) of the liquid adhesive must be less than that (critical surface tension yc) of the solid adherend, or... 

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 equation is basic for determination of interfacial tension and for explaining phenomena of wetting and adhesion [iii-v] including electrochemical experiments [vi]. For the work of adhesion between two immiscible liquids see -> Dupre equation. 

The study of forces between deformable interfaces can be broken into two categories, the interactions between two sets of deformable interfaces (e.g., two oil drops in water), or a rigid particle and a single deformable interface. Study of the forces in these systems is motivated by the prevalence of both types of systems (drop-drop or drop-rigid particle) in industrial problems. For example, wetting and adhesion of oil emulsions in porous media are concerns in the petroleum industry for both liquid/liquid separations and oil recovery [1]. An understanding of the interaction forces between... 

Phenomenologically speaking, wetting and adhesion are related. Adhesion is the attachment of particles, or drops, onto surfaces, or of two macrobodies of whatever geometry to each other. Consider a solid or liquid sphere, approaching a surface S, through medium 2, until attachment and spreading, if possible, occurs (see flg. 5.7). This process can be divided into two steps. 

Adhesives wet, flow, and set to a solid during bond formation. The transformation from liquid adhesive to solid bond can be achieved in a number of ways. Where the adhesive is a polymer, the initial starting material is a liquid monomer or prepolymer that, under the conditions of bonding with heat, pressure, and/or catalyst, polymerizes to the solid polymer in the glue line. It is also usual to apply solutions of preformed polymers in suitable solvents to the faces of adherends, and allow bond formation to take place with evaporation of solvent. Alternatively, polymers that can be melted or softened to flow at elevated temperatures can be applied as hot melt adhesives to form the bond on cooling. With porous adherends like wood, penetration of the pores by liquid or molten adhesives is an important factor in bond formation. 

The second widely used method for preparing polyurethane foam laminates is the liquid adhesive, or wet process. Here special adhesives, either in the form of water solutions, or as solutions in organic solvents, are applied to either the fabric or the foam. The equipment is conventional. The water or solvent is evaporated, and the bond may be set by drying or curing at elevated temperature (20). One of the adhesives used for this application is based on acrylic interpolymer latices. This adhesive is used in the manufacture of thermal garments and insulated bags. Carpet underlay can also be made by adhesive laminating (7). 

Major new insights into the surface science of silicone materials were catalyzed by the work of deGennes, whose theories of polymer wetting and adhesion have inspired many to explore his provocative themes [7]. These explorations focused attention on the need for a polymeric material that is well characterized, liquid over a wide molecular weight range, with controlled molecular weight distribution and crosslinkable in a controlled fashion. Dimethylpolysiloxane is the best available candidate and has become central to a revolution in polymer surface physics. 

The equation is from Dupre (1869). In this process any reduction of the interfacial tension between substrate and the wetting liquid results in an increased tendency for adhesion to occur, but reduction of either the surface tension of the liquid or the surface tension of the substrate decreases the tendency of adhesion to occur. This accounts for the poor adhesion of substances to low-energy surfaces, especially when the natures of substance and substrate are very different (i.e., ySL is large). 

Fig. 5.4 Random cut through part of an agglomerate or a particulate bulk solid mass and explanations of how strength may be caused, (a) Pore volume filled with a matrix binder, (b) Pore volume filled with a wetting liquid, (c) Liquid bridges at the coordination points, (d) Adhesion forces at the coordination points.

The same influence of porosity on strength is obtained for agglomerates that are completely saturated with a wetting liquid (for example water). In this case, the sum of all adhesion forces, A , is replaced by the liquid s surface tension, a (Eq. 5.2). 

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