Adhesion mechanical interlocking theory

Mechanical Interlocking Theory. A practical adhesion can be enhanced if the adhesive is applied to a surface which is microscopically rough. 

Four main theories of adhesion, namely 1. adsorption, 2. electrostatic, 3. diffusion, and 4. mechanical interlocking theories, are very well known. 

The mechanical interlocking theory assumes that adhesion is due to irregularities on the surface into which the liquid material can penetrate. Upon solidification, the solid adhesive material is held in place by the geometry on the adsorbed layer. Mechanical interlocking is thus enhanced by increasing the surface roughness or porosity of the solid material. 

If a substrate has an irregular surface, then the adhesive may enter the irregularities prior to hardening. This simple idea gives the mechanical interlocking theory, which contributes to adhesive bonds with porous materials such as wood and textiles. An example is the use of iron-on patches for clothing. The patches contain a hot melt adhesive which, when molten, invades the textile material. 

However, the attainment of good adhesion between smooth surfaces exposes the mechanical interlocking theory as not being of wide applicability. For example, the elegant work of Tabor et al. [1,2] who studied the adhesion between two perfectly smooth mica surfaces and Johnson et aL [3] who examined the adhesion to optically smooth rubber surfaces, clearly demonstrates that adhesion may be attained with smooth surfaces. Also, detailed examination of surfaces roughened by typical industrial pretreatment methods, for example, grit-blasted metallic substrates (Fig. 3.1), usually reveals little... 

The theory of mechanical interlocking explains adhesive bonding with the physical coupling of surface irregularities, roughness. It can be applied for the solution of problems emerging in the textile and paper industry, but cannot describe adhesive interaction of smooth surfaces like in the case of glass. 

As the science of adhesion has developed, various theories of adhesion have been advocated for one material or another. With wood as a substrate, mechanical interlocking, interdiffusion of polymers, intermolecular attractive forces, and covalent chemical bonding all have been proposed, either individually or collectively, to explain adhesion. In reality, no experiments reported to date have been able to disprove the existence of any one of these mechanisms, or to quantify their relative importance. A most exasperating feature of research on adhesion to wood is that factors presumed to be independent in experiments are never totally independent. 

Most of the surface treatments aim to bring roughness on the adherend, in order to satisfy the mechanical adhesion theory proposed by McBain [1], which is mainly aimed to enhance the mechanical interlocking of the adhesive with the adherend. To this end, solid abrasion remains one of the most widespread techniques solid particles are directed onto the composite surface using different particle sizes at controlled impact speeds. This kind of treatment makes it possible... 

While the adsorption theory is the most accepted one, mechanical interlocking comes into play in case of substrates with a special kind of roughness such as galvannealed steel where the liquid can spread into cavities and thereby interlock with the substrate. The diffusion theory does not play an important role for polymer-metal interfaces. The contribution of the electrostatic theory is not easy to estimate. However, the electrical component of the adhesive force between the planar surfaces of solids becomes important if the charge exchange density corresponds to 10 electronic charges, meaning about 1% of the surface atoms [71]. 

A number of adhesion theories have been proposed to identify the formation of adhesive forces. The contributed adhesion mechanisms are (1) chemical bonding such as chemisorption theory (2) physical interaction such as polarization, electrostatic, and diffusion theory (3) thermodynamical interpretation such as adsorption theory and (4) mechanical interlocking. No single theory exists to explain the entire property of adhesion oti various substrates and adhesives. However, those theories may provide a guideline to understand the principle of the adhesion as the following details (Fig. 2). 

Historically, mechanical interlocking, electrostatic, diffusion, and adsorp-tion/surface reaction theories have been postulated to describe mechanisms of adhesion. More recently, other theories have been put forward for adhesive bonding mechanism (Table 1.1). It is often difficult to fully ascribe adhesive bonding to an individual mechanism. A combination of different mechanisms is most probably responsible for bonding within a given adhesive system. The extent of the role of each mechanism could vary for different adhesive bonding systems. An understanding of these theories will be helpful to those who plan to work with adhesives. 

For FRCs, the stress transfer at the interface between the matrix and fibre phases is determined by the degree of adhesion. Effective transfer of stress and load distribution throughout the interface is possible when strong adhesion exists at the interfaces. The adhesion phenomenon is described in several theories including mechanical interlocking, absorption, primary bonding, interdiffusion, electronic theories, etc. 

Venables to argue for the importance of mechanical interlocking (see Mechanical theory of adhesion) in adhesion to porous anodized surfaces. 

The mechanical theory of adhesion predicts mechanical interlocking as the driving force for all adhesion, and is especially useful for materials with rough surfaces, and/or porous... 

Mechanical theory. A simple explanation is mechanical interlocking. The liquid adhesive penehates the pores or between fibres then sets. In order to separate the adherends, the adhesive needs to be stretched and maybe broken. 

Understanding adhesion implies identifying the dominant mechanism (thermodynamic adsorption, specific adsorption, diffusion, mechanical interlocking), but combinations of these theories or mechanisms are also possible. The four major mechanisms of adhesion are illustrated in Figure 6.6. 

Mechanical interlocking is the dominant theory for coatings and adhesives on wood and porous materials. In these cases, it is worth trying to increase the surface roughness (or porosity) by chemical or mechanical surface treatment. However, it is under debate whether the roughness helps the adhesion due to the existoice of this interlocking adhesion mechanism (Bendey and... 

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