Features of Adhesion

One of the most important factors in adhesive bonding is the surface condition of the substrate, i.e. the surfaces of the materials to be joined. Since adhesion takes place only at the interface between the component and the adhesive, it is evident that surface preparation or chemical pretreatment has a crucial bearing on the quality of the adhesive bond. 

Considering this more closely, cohesion is defined as forces between atoms or molecules within the adhesive. Therefore, cohesion differs from adhesion, where adhesion forces occur between the different materials making up the surface of substrate and the adhesive. 

Adhesion is related to surface properties, and hence not only the substrates but also the surface structure influences adhesion. Surface structure is not usually flat, rather it exhibits mountains and valleys as well as pores (Fig. 39). 

Contact angle vertical Bad wettability Low surface tension 

Contact angle pointed Good wettability High surface tension 

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Although the primary purpose of this chapter is to discuss mechanical testing and strength of adhesive joints, the reader should be aware that ASTM covers a wide variety of tests to measure other properties. ASTM, for example, includes standard tests to measure the viscosity of uncured adhesives, density of liquid adhesive components, nonvolatile content of adhesives, filler content, extent of water absorption, stress cracking of plastics by liquid adhesives, odor, heat stability of hot-melt adhesives, ash content, and similar properties or features of adhesives. 

It should be noted that proportional increase in the shear breaking stresses with compression stresses is probably the characteristic feature of adhesive joints. Evidently, adhesive joints resist the combined action of normal compression and shear much better than the actions... 

An adhesive is a material that is applied to the surfaces of articles to join them permanently by an adhesive bonding process. An adhesive is a substance capable of forming bonds to each of the two parts when the final object consists of two sections that are bonded together. A feature of adhesives is the relatively small quantities that are required compared to the weight of the final objects. 

Consideration of all aspects of joint microstructure would require a scope that includes most of materials science. In this article, attention will be directed particularly to micro-structural features of adhesive and of substrate surface that can be altered to affect the performance of an adhesive joint. 

Microstructure of joints D E PACKHAM Microstructural features of adhesive and substrate and their effect on adhesion... 

Fi is considerably smaller than the force F4 i.e., in the first case (curve 1), the majority of the particles are torn off more readily than in the fourth case (curve 4). Here, evidently, adhesion is better evaluated on the basis of the detachment force for an adhesion number of 50% (see Fig. 1.2.a). We must also note that certain cases are likewise possible, although less probable, in which the detachment force F50 will be equal for a family of curves (Fig. I.2.b) but will not reflect the features of adhesive interaction. 

Features of Adhesion on Rough Surfaces. Actual surfaces may have irregularities that change the area of contact between particles and surface, the gap between the contiguous bodies, and the adhesive interaction. 

Specific Features of Adhesion of Irregularly Shaped Particles. Irregular particles are characterized by three measurements length, width, and height. It is not feasible to compare the adhesion of irregular particles on the basis of these three measurements. Hence it is necessary to reduce the dimensions of irregular particles to a single dimension equivalent to these three. 

The concept of the sphericity factor still cannot be used fully in evaluating the specific features of adhesion for irregularly shaped particles since it does not really account for the relationship among the height, length, and width of the particle. This relationship is taken into account more fully in a concept that we have considered previously [162], that of equivalent size of particles, as determined by means of Eqs. (III. 14)-(III. 16). 

Features of Adhesive Interaction in Liquid Media. The adhesion of particles in liquid media has a number of special features in comparison with adhesion in air. These features are determined by the fact that the dust-covered surface is surrounded by liquid rather than air. The liquid medium changes the nature of the adhesive forces. Capillary and electrical forces (see Sections 15-17) do not appear in liquid media. Any charges that are on the particles will leak off in a liquid medium, and donor-acceptor processes do not take place on the wet surface. No capillary forces will be manifested since no liquid bridge (meniscus) tending to draw up the adherent particle can be formed in the contact zone. 

Features of Adhesion of Cylindrical Particles and Other Regularly Shaped Particles. As in air, particle adhesion in liquid media has a number of special features when the particles are not spherical. First, let us consider the adhesion of particles to a glass surface in an aqueous medium when the particles are cylinders with a diameter of 40 fim and a length within the range of 100-600 jLim. The distribution of adherent particles with respect to adhesive force in a liquid medium, the same as in air (see Fig. V.l 1), follows a log-normal law [194]. 

Features of Adhesion of Particles with Different Sizes, in Liquid Media. As noted previously, we find a log-normal distribution of particles with respect to force of adhesion in liquid media. The integral adhesion curves for particles of different sizes may intersect. Such an intersection does occur in the adhesion of spherical glass particles to glass surfaces, as indicated by the point K in Fig. 

Features of Adhesion of Particles to Paint and Varnish Coatings... 

In order to define the specific features of adhesive interaction, we investigated the adhesion of spherical glass particles to a painted surface as affected by treatment of particles and surface with surfactant solutions of different concentrations, the surfactant film being allowed to dry for different periods before rinsing it off with distilled water. The following adhesion numbers were obtained ... 

Features of Adhesion Processes under Industrial and Agricultural Production Conditions... 

Particular attention needs to be paid to establishing an effective system of quality assurance for adhesive connections. Non-destructive test methods based on ultrasound, X-rays or the measurement of electrical or thermal conductivity are available but only of limited value in practice. Specific data on adhesion cannot always be obtained by these methods. This section examines the issue of quality assurance from a practical point of view. Many years of experience have shown that only a quality assurance system that takes into account the specific features of adhesive bonding technology can produce consistently satisfactory results over an extended production period. The information outlined here should be viewed as a general checklist to be adapted to the specific requirements of each manufacturing environment. 

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