Dispersion force adhesion

The electrostatic adhesive force is proportional to whereas the dispersion force F does not depend on the charge on the toner, and for a given set... 

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 ... 

There is much evidence that there are many cases in which the interaction between liquids and solids cannot be described in terms of dispersion forces alone. For example, Dann [75] found significant non-dispersion-force contributions to the work of adhesion between ethanol/water mixtures, mixed glycols, and polyglycols and a mixture of formamide and 2-ethoxyethanol against a variety of solids. The nature of these other interactions , however, were at first the subject of some dispute. We may account for them in a general way with a term /sl inserted into Eq. 11 ... 

Fowkes [107] has argued that the van der Waals contribution to the work of adhesion in solids arises mainly from the dispersion forces. Moreover, the work of adhesion can be approximated by the geometric mean of the dispersion contributions to the surface energies yf and according to... 

Among all the low energy interactions, London dispersion forces are considered as the main contributors to the physical adsorption mechanism. They are ubiquitous and their range of interaction is in the order 2 molecular diameters. For this reason, this mechanism is always operative and effective only in the topmost surface layers of a material. It is this low level of adhesion energy combined with the viscoelastic properties of the silicone matrix that has been exploited in silicone release coatings and in silicone molds used to release 3-dimensional objects. However, most adhesive applications require much higher energies of adhesion and other mechanisms need to be involved. 

Fowkes and co-workers also clearly demonstrated that the physical Interaction of polymers with neighboring molecules was determined by only two kinds of interactions London dispersion forces and Lewis acid-base interactions (21) Calculations based on this concept were shown to correct many of the problems inherent in the solubility approach. They were also able to use the concept to study the distribution of molar heats of absorption of various polymers onto ferric oxides, and thereby more accurately described the requirements for adequate adhesion to steel substrates (21) In the symposium on which this book is based, Fowkes summarized work showing that the polar Interactions between polymers and metal surfaces that are... 

Adherent films would not necessarily require formation of covalent bonds at the Interface, since localized lntermolecul u dispersion forces that u e operative In the adsorption of coatings (with good wetting properties) should provide stable interfacial bond conditions. Among candidate materials which could fulfill the requirements of good adhesion and substrate protection from moisture are epoxy-modified polyurethanes and epoxy-siloxane polymers. 

Dispersion forces57 , which result from temporary variations in the distribution of electron density in atoms, can account for up to 90 per cent58 of the adhesion forces between non-polar polymers and metal substrates (bond energy 0.5-5 Kcal/ mole)50 . However, for the adhesion of epoxy resins and other polar polymers to metals, dispersion forces are of secondary importance when compared to the electromagnetic and mechanical interactions discussed above. 

This equation assumes that the tip and sample are of the same material and no deformation occurs. If the tip and the surface are made of different materials, the cohesion energy wc=2y should be replaced by the work of adhesion. For a hydrocarbon polymer, where mainly dispersion forces are responsible for the tip-surface interaction, the work of adhesion can be estimated as wa =2(YtdYs)1/2>... 

The London dispersion forces are present and important in most adsorption processes and in adhesive interactions between dissimilar materials. The free energy of interaction per unit area between materials 1 and 2 in contact is where W 2 -s... 

Whatever the mechanism is, particles adhere spontaneously if, at constant temperature and pressure, the Gibbs energy G of the system decreases. The main contributions to the Gibbs energy of particle adhesion A Gad are from electrostatic, hydrophobic and dispersion forces,1 5 and, furthermore, in case of protein adsorption, from rearrangements in the structure of the protein molecule.6 9 When the sorbent surface is not smooth but hairy , additional, mainly steric, interactions come into play.4,10 12 Hairy surfaces are often encountered in nature as a result of adsorbed or grafted natural polymers, such as polysaccharides, that reach out in the surrounding medium with some flexibility. Interaction of particles with such hairy surfaces will be dealt with in section 3. 

Rubber as the Disperse Phase. In polyblend systems, a rubber is masticated mechanically with a polymer or dissolved in a polymer solution. At the conclusion of blending, a rubber is dispersed in a resin as particles of spherical or irregular shape. We can further subdivide this system into three classes according to the major intermolecular forces governing adhesion (a) by dispersion forces—e.g., the polyblend of two incompatible polymers, (b) by dipole interaction—e.g., the polyblend of polyvinyl chloride and an acrylonitrile rubber (56), and (c) by covalent bond—e.g., an epoxy resin reinforced with an acid-containing elastomer reported by McGarry (43). 

In these models, the method consists of (i) assuming that a certain type of interaction such as van der Waals dispersion forces (Didier and Jupille 1994, Naidich 1981) or image forces (Stoneham et al. 1995) is predominant at metal/oxide interfaces, (ii) calculation of the adhesion energy resulted from these... 

Owens and Wendt proposed an even more general equation for the work of adhesion between two arbitrary media [1121. In analogy to dispersion forces they used the geometric mean also for the other force components ... 

It has been shown that the free energy of adhesion can be positive, negative, or zero, implying that van der Waals interactions can be attractive as well as repulsive [130,133,134]. While Eq. (14) can, strictly speaking, be expected to hold only for systems that interact by means of dispersion forces only, there are no restrictions on Eq. (15). Since this equation describes very well the fundamental patterns of the behavior of particles, including macromolecules, independent of the type of molecular interactions present, it was found to be convenient to define an "effective Hamaker coefficient that reflects the free energy of adhesion [130],... 

When toner particles are transferred from the photoreceptor to the receiver, some particles invariably remain and must be removed before the subsequent image-forming cycle. Toner to photoreceptor adhesion results from electrostatic and dispersion forces which must be overcome to separate the toner particles from the photoreceptor (Nebenzahl et al.. 1980 Mastrangelo, 1982). Many... 

Physical adsorption is a universal phenomena, producing some, if not the major, contribution to almost every adhesive contact. It is dependent for its strength upon the van der Waals attraction between individual molecules of the adhesive and those of the substrate. Van der Waals attraction quantitatively expresses the London dispersion force between molecules that is brought about by the rapidly fluctuating dipole moment within an individual molecule polarizing, and thus attracting, other molecules. Grimley (1973) has treated the current quantum mechanical theories involved in simplified mathematical terms as they apply to adhesive interactions. 

It can be derived from Antonow s rule, 15.7.4], applying it to partial wetting but accounting for the adhesion between solid and liquid, assuming it to be dominated by the Van der Waals, or dispersion, parts of the surface tensions, y and y. Various studies have shown that [5.7.5] is quite effective for materials that mainly interact through dispersion forces and that it remains a reasonable approximation for systems in which other interactions also operate. The root in the r.h.s. of [5.7.5] stems from the assumption that Berthelot s principle may be applied. In sec. 2.11b we argued that this principle may be applied only to the energetic part of the interfacial tensions and that a more correct form is... 

Equation (5.7.5], and variants thereof, have been widely invoked to assess the surface tension of solid surfaces by working with organic liquids (such as hydrocarbons) in which dispersion forces prevail. For those one may set = y " so that the equation can be solved for y , which, in turn, may be equated to y for a Van der Waals solid. However, we see from [5.7.7] that the situation is more complicated. First, neglection of the TAS term is not allowed, it may account for 20-30% of the interaction Helmholtz energy. Second, even if an assumption is made on, or if it is directly measured from the heat of adhesion, only the... 

Of the different types of forces responsible for intermolecular attraction, the foremost are the London or dispersion forces that act between all atoms and account for virtually all of the molecular attraction or cohesion in all molecules except the very polar molecules (described later). Dispersion forces are short-range interactions, effective at about 4 A, and rapidly decrease with the sixth power of the distance between molecules. Therefore, the adhesive polymer molecule must be flexible enough to come within this range of interaction with the rigid adherent surface under the conditions of bond formation. 

---