Balance between adhesion and cohesion

The equilibrium situation which exists after a liquid drop is brought into contact with a smooth homogeneous substrate depends on the balance between adhesion and cohesion free energies involved. Three cases can be identified ... 

The creep of a PSA can be minimized by increasing the molecular mass of the adhesive and by cross-linking it. But there are limitations, because some creep is necessary to wet the surface and to relax stress concentrations when the adhesive is stressed. This often is referred to as the balance between adhesion and cohesion. The relevance of creep to PSAs is discussed in depth in [211, p. 97 ffj. 

Pressure-sensitive adhesives seem to possess a critical balance between adhesion and cohesion. The phenomenon of pressure-sensitive tack has been described as a complex function of flow and diffusion. The role of resins in pressure-sensitive adhesives has been interpreted recently as improving tack by a two-step process (1) reducing the modulus and viscosity of the adhesive to give faster, more complete wetting of the substrate and (2) raising the Tg of the adhesive. Plasticizers and solvents accomplish the first objective but not the second This interpretation is consistent with the... 

The field of solvent-free pressure-sensitive adhesives is an upcoming industrial technology. Such products find application as self-adhesive technical and medical products. The main aim is to manufacture solvent-free acrylic PSAs (A-PSAs), which have excellent performance and provide a good balance between adhesive and cohesive strengths. PSAs can act as extruder adhesives or UV-polymerised adhesives. Such adhesives, through their viscoelastic fluid state, can build joints without any requirement to change this flow state during or after application. 

Wetting is the ability of a liquid drop to keep contact with a solid surface. The degree of wetting is qualified by a contact angle (9 ), at which the liquid-vapor interface meets the solid-liquid interface, as shown in Figure 1. The contact angle is governed by the balance between adhesive and cohesive forces and it provides an inverse measure of wettability. 

Tackifiers. Resins are generally added to adjust the desired tack. In general, resins must be used with plasticizers to obtain a good balance between tack and cohesive strength. Typical tackifiers are polyterpenes, although hydrocarbon resins and modified rosins and rosin esters can also be used. In some cases, terpene-phenolics or phenol-formaldehyde resins are added to increase adhesion. 

When a small-diameter glass tube, or capillary, is placed in water, water rises in the tube. The rise of liquids up very narrow tubes is called capillary action. The adhesive forces between the liquid and the walls of the tube tend to increase the surface area of the liquid. The surface tension of the liquid tends to reduce the area, thereby pulling the liquid up the tube. The liquid climbs until the force of gravity on the liquid balances the adhesive and cohesive forces. Capillary action helps water and dissolved nutrients move upward through plants. 

The balance between tack and cohesive strength may be varied by the balance between unvulcanized and prevulcanized latex. The prevulcanized latex should be clarified after prevulcanization, i.e., centrifuged to remove excess vulcanizing components. This is to prevent post-cure in the adhesive, which may reduce tack too much. 

The intermolecular forces of adhesion and cohesion can be loosely classified into three categories quantum mechanical forces, pure electrostatic forces, and polarization forces. Quantum mechanical forces give rise both to covalent bonding and to the exchange interactions that balance tile attractive forces when matter is compressed to the point where outer electron orbits interpenetrate. Pure electrostatic interactions include Coulomb forces between charged ions, permanent dipoles, and quadrupoles. Polarization forces arise from the dipole moments induced in atoms and molecules by the electric fields of nearby charges and other permanent and induced dipoles. 

From the above discussion, it is clear that the stabilization or failure of graphite electrodes depends on a delicate balance between passivation phenomena (due to the formation of highly cohesive and adhesive surface films) and a buildup of internal pressure due to the reduction of solution species inside crevices in the graphite particles. This delicate balance can be attenuated by both solution composition (EC-DMC vs. EC-PC or PC, etc.) and the morphology of the graphite particles (i.e. the structure of the edge planes and the presence of crevices). 

The contact angle 0 depends on the balance between the sohd/Vapour (ygy) and solid/liquid (yg ) interfacial tensions. The angle which a drop assumes on a solid surface is the result of the balance between the adhesion force between solid and liquid and the cohesive force in the liquid,... 

The work of adhesion (see Chapter 1,1) reflects the degree to which unsaturated molecular interactions between solids and liquids in contact are balanced. The value of cos 9, which is symbatic to the work of adhesion, is also a measure of the degree of similarity between the solid surface and a liquid (liophilicity). Polar surfaces that are wetted by water well are hydrophilic, while those poorly wetted (solid hydrocarbons, and particularly fluororinated polymers) are hydrophobic. Since the value of 0 is determined by both the work of adhesion and the work of cohesion, a comparison of the contact angles formed by different liquids at the same solid surface does not allow one to compare the works of adhesion (the degree of similarity in the nature of the liquid and solid) directly. For example, polar surfaces are equally wetted well by both water and hydrocarbons. 

---