Cohesion, adhesion and

Adhesion and cohesion are other manifestations of molecular interactions. The work of adhesion refers to the free energy difference between two states of the system in the first one, phase 1 is in contact and in equilibrium with phase 2 (surface free energy per unit area yxf) (Fig. 5(a)) and in the second one, phase 1 and phase 2 are in contact and in equilibrium with their own vapors (surface free energy per unit area Y + Vi) (Fig. 5(b)). Therefore, Wa is 

Work of cohesion refers to the same diffetence when phase 1 is equal to phase 2. In that case, the fitst system has 2eto surface energy while for the second one it is 

Adhesion between a pure liquid and a solid maybe described in terms of the interfacial and surface tensions [18]. Consider the reversible process of splitting a unit area of the Solid-Liquid interface in such a way as to create a unit area of the Solid-Vacuum interface and a unit area of the Liquid-Gas interface (as shown in Fig. 6.3a). 

Adhesion between the two phases is defined as the reversed process and the Gibbs energy of adhesion is given, at fixed P and T, by the Dupr6 equation [4, 19-21] 

Inert materials, capable of neither acid nor base interactions, are called apolar. It should be noted here that this nomenclature [22, 23] has nothing in common with polarity of molecules per se, as measured by their respective dipole moments. 

Dispersion forces are universal because they attract all molecules together, regardless of their specific chemical nature. The potential energy of dispersion attraction between two isolated molecules decays with the sixth power of the separation distance. Based on the so-called Hamaker theory (i.e., the method of pair-wise summation of intermolecular forces) or the more modern Lifshitz macroscopic treatment of strictly additive London forces, it is possible to develop the so-called Lifshitz-Van der Waals expression for the macroscopic interactions between macroscopic-in-size objects (i.e., macrobodies) [19, 21], Such an expression strongly depends on the shapes of the interacting macrobodies as well as on the separation distance (non-retarded or retarded interaction). For two portions of the same phase of infinite extent bounded by parallel flat surfaces, at a distance h apart, the potential energy of macroscopic attraction is  

The values of An for the various substances interacting across vacuum or across a medium can be found in Refs. [24, 25], It should be noted that Hamaker constants for interaction across a medium are usually much lower in comparison with the related values under vacuum (e.g., in the case of two macroscopic bodies of quartz at short distances apart, Ancquals 41.3 x 10 J for interaction across vacuum and 1.3 X 10 ° J for interaction across water [25]). 

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Derive the expression (in terms of the appropriate works of adhesion and cohesion) for the spreading coefficient for a substance C at the interface between two liquids A and B. 

The intermolecular forces of adhesion and cohesion can be loosely classified into three categories (7) quantum mechanical forces, pure electrostatic... 

Fillers. Fillers are not commonly added to CR adhesives. Calcium carbonate or clay can be primarily added to reduce cost in high-solids CR mastics. Maximum bond strength is obtained using fillers with low particle size (lower than 5 [jim) and intermediate oil absorption (30 g/100 g filler). In general, fillers reduce the specific adhesion and cohesion strength of adhesive films. Although polychloroprene is inherently flame retardant, aluminium trihydrate, zinc borate, antimony trioxide or... 

This chapter first reviews the general structures and properties of silicone polymers. It goes on to describe the crosslinking chemistry and the properties of the crosslinked networks. The promotion of both adhesive and cohesive strength is then discussed. The build up of adhesion and the loss of adhesive strength are explained in the light of the fundamental theories of adhesion. The final section of the chapter illustrates the use of silicones in various adhesion applications and leads to the design of specific adhesive and sealant products. 

Pure PDMS networks are mechanically weak and do not satisfy the adhesive and cohesive requirements needed for most applications in which the silicone adhesive joint is subjected to various stresses. For crosslinked silicones to become high performing adhesives, they need to be strengthened. 

The performance of a product where adhesion plays a role is determined both by its adhesive and cohesive properties. In the case of silicones, the promotion of adhesion and cohesion follows different mechanisms [37]. In this context, adhesion promotion deals with the bonding of a silicone phase to the substrate and reinforcement of the interphase region formed at the silicone-substrate interphase. The thickness and clear definition of this interphase is not well known, and in fact depends on many parameters including the surface physico-chemistry of... 

In the search for an adhesive system, the researcher designs a polymeric network and formulation and builds in adhesive and cohesive strength. A combined... 

Since the locus of failure can clearly distinguish between adhesive and cohesive failures, the following discussion separates loss of adherence into loss of adhesion and loss of cohesion. In the loss of cohesion it is the polysiloxane network that degrades, which can be dealt with independently of the substrate. The loss of adhesion, however, is dependent on the cure chemistry of the silicone, the chemical and physical properties of the substrates, and the specific mechanisms of adhesion involved. 

We have attempted to relate the basics of silicone chemistry to applications where adhesion is an important property. These applications cover a vast industrial arena that does not make a review of this sort easy. Instead, we focused on the fundamental aspects of silicone physics and chemistry and related them to adhesion and adherence properties. We have attempted to use a logical structure to help the reader understand silicone adhesion. Adhesion and cohesion have been considered as they both determine the ultimate performance of an adhesive joint. 

The design of a silicone adhesive naturally considers both the creation of adhesive and cohesive strengths to provide the performance needed. 

Standard-grade PSAs are usually made from styrene-butadiene rubber (SBR), natural rubber, or blends thereof in solution. In addition to rubbers, polyacrylates, polymethylacrylates, polyfvinyl ethers), polychloroprene, and polyisobutenes are often components of the system ([198], pp. 25-39). These are often modified with phenolic resins, or resins based on rosin esters, coumarones, or hydrocarbons. Phenolic resins improve temperature resistance, solvent resistance, and cohesive strength of PSA ([196], pp. 276-278). Antioxidants and tackifiers are also essential components. Sometimes the tackifier will be a lower molecular weight component of the high polymer system. The phenolic resins may be standard resoles, alkyl phenolics, or terpene-phenolic systems ([198], pp. 25-39 and 80-81). Pressure-sensitive dispersions are normally comprised of special acrylic ester copolymers with resin modifiers. The high polymer base used determines adhesive and cohesive properties of the PSA. 

An EB-curable struetural adhesive formulation usually eonsists of one or more crosslinkable oligomeric resins or prepolymers, along with such additives as reactive diluents, plasticizers, and wetting agents. The oligomer is an important component in terms of the development of mechanical properties. The adhesive and cohesive properties depend on the crosslink density, chemical group substitution, and molecular organization within the polymer matrix. Adhesion is achieved... 

The compressive forces may be contained by adhesion and cohesion in the oxide-metal system experiments designed to reveal the presence and... 

The greater the viscosity of a liquid, the more slowly it flows. Viscosity usually decreases with increasing temperature. Surface tension arises from the imbalance of intermolecular forces at the surface of a liquid. Capillary action arises from the imbalance of adhesive and cohesive forces. 

Because of the unavoidable tendency of granular solids to become triboelectrically charged when handled, it is no surprise that electrostatic phenomena are often quite pronounced in fluidized and spouted beds. The vigorous motion of fluidized particles—with constant particle-particle and particle-wall contacts—guarantees that electrical charging will take place. Electrostatic adhesion and cohesion, observed and recorded in the very earliest experimental investigations of fluidization, were immediately identified as experimental nuisances to be overcome. Somewhat later, the hazardous nature of electrostatics came to be appreciated. 

Adhesion. The modulation of adhesion with light is used in some important commercial applications of photopolymer technology. The adhesion and cohesion of a photopolymer to other polymeric materials can either be increased or decreased upon the application of light, depending upon the chemistry of the photopolymer formulation and surface properties of the films in the product structure. The... 

Compare the data given in Table A.4.2 and interprete them in terms of interfacial energy, adhesion, and cohesion. 

As you know, intermolecular forces act between both like and unlike molecules. Adhesive forces are intermolecular forces between two different kinds of molecules. Cohesive forces are intermolecular forces between two molecules of the same kind. Adhesive and cohesive forces affect the physical properties of many common phenomena in the world around you. 

The addition of silica fume to dry-mix shotcrete in proportions of 10-15% by mass of Portland cement substantially improves the adhesive and cohesive properties of the freshly applied shotcrete. Silica fume creates a very dense and sticky mix with an almost complete lack of bleeding. These characteristics of the material are attributed to the extreme densification achieved by packing of the ultrafine silica fume particles between the cement particles in the plastic (fresh) shotcrete. Practical ramifications of these characteristics include the observations which follow [107, 117-119]. 

Rebound Because of marked improvements of adhesion and cohesion, the amount of rebound is significantly reduced. In controlled tests, rebound of a conventional dry-mix shotcrete, applied to a thickness of 50 mm (2 in) to the smooth overhead surface of concrete deck slabs, was measured at 40% by mass (the rebound was caught in tarpaulins and weighed). Rebound under the same circumstances was reduced to 25% by mass in dry-mix silica fume shotcrete. 

Adhesive and cohesive forces at binder bridges which are not freely movable... 

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