Of cohesion

Most solid surfaces are marred by small cracks, and it appears clear that it is often because of the presence of such surface imperfections that observed tensile strengths fall below the theoretical ones. For sodium chloride, the theoretical tensile strength is about 200 kg/mm [136], while that calculated from the work of cohesion would be 40 kg/mm [137], and actual breaking stresses are a hundreth or a thousandth of this, depending on the surface condition and crystal size. Coating the salt crystals with a saturated solution, causing surface deposition of small crystals to occur, resulted in a much lower tensile strength but not if the solution contained some urea. 

The statement was made that the work of adhesion between two dissimilar substances should be larger than the work of cohesion of the weaker one. Demonstrate a basis on which this statement is correct and a basis on which it could be argued that the statement is incorrect. 

Because the work of cohesion, of the adhesive is 2 CJ, equation 3 follows. 

The amount of set imparted in steam-setting depends on the pH and moisture content of the wool as well as the time and temperature of setting. In continuous machines the conditions are mild and Htde permanent set is imparted. In batch machines, the wool is often set at temperature up to 136°C for 3—5 min. This imparts large amounts of permanent set to the fibers. Impregnation of the wool with reducing agents increases the rate of permanent setting. The amount of cohesive set imparted in both batch and continuous machines depends on the temperature of the fibers when the fabric is released from the wrapper cloth. The cooler the fabric, the more cohesive the set imparted. 

It has a low solubility parameter (15.3 MPa ) which differs considerably from that of the cellulose dinitrate (21.8 MPa ). This indicates that compatibility is not simply due to similarities of cohesive forces but also to some form of interaction probably involving the carbonyl group. 

Visually, failure was mostly eohesive within the adhesive (see Figs. 34 and 46). However, there was a small area of apparent interfacial failure ( initiation zone ) located at one end of each substrate. Line scans were eondueted aeross the initiation zone, from the edge of the substrate to the area of cohesive failure within the adhesive. From the line scans, it was apparent that there were patehes of polymer present in the initiation zone, even when failure appeared to be interfaeial (see Fig. 46). SIMS images of the initiation zone were constructed for various mass numbers (see Figs. 47-49). The images showed well-defined cation-rieh... 

Surface energies are also associated with failure of an adhesive bond, because failure involves forming new surfaces and the appropriate surface energies have to be provided. The surface energy term may be the work of adhesion, VTa, or the work of cohesion, VTcoh. depending on whether the failure is adhesive or cohesive. For phases 1 and 2, these are defined as follows [lOj ... 

Despite these early successes in the commercialization of acrylic polymers, no acrylic PSAs were manufactured on a larger scale until many years later. One of the primary reasons for the initial commercial failure of the acrylic PSAs was their lack of cohesive strength. Unlike the higher Tg, plastic-like polymers obtained from monomers like methylmethacrylate, polymers synthesized from alkyl acrylates typically formed sticky, cold-flowing materials with little if any utility. 

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. 

The ideal soil is defined as a loose, granular medium that is devoid of cohesion but possesses internal friction. In contrast, an ideal cohesive medium is one that is devoid of internal friction. Real soils generally fall between the foregoing two limiting definitions. 

Unconfined compression tests are used to estimate the shearing strength of cohesive soils. 

Lowdin, P.-O., Advances in Phys. 5, 1, Quantum theory of cohesive properties of solids/ ... 

Morita, A., and Takahashi, K., Progr. Theoret. Phys. [Kyoto) 19, 257, Theory of cohesive energy of LiH crystal—the method of semilocalized crystalline orbitals."... 

Consistency, working time, setting time and hardening of an AB cement can be assessed only imperfectly in the laboratory. These properties are important to the clinician but are very difficult to define in terms of laboratory tests. The consistency or workability of a cement paste relates to internal forces of cohesion, represented by the yield stress, rather than to viscosity, since cements behave as plastic bodies and not as Newtonian liquids. The optimum stiffness or consistency required of a cement paste depends upon its application. 

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