Adhesive bond mechanical interlocking

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. 

Because moisture will break the dispersive interfacial bonds, mechanical interlocking on a micro- or nanoscopic scale is needed between the adhesive/primer and adherend for good... 

In recent years there has been a renewed appreciation of potential beneficial effects of roughness on a macroscale. For example Morris and Shanahan worked with sintered steel substrates bonded with a polyurethane adhesive [61]. They observed much higher fracture energy for joints with sintered steel compared with those with fully dense steel, and ascribed this to the mechanical interlocking of polymer within the pores. Extra energy was required to extend and break these polymer fibrils. 

The surface preparation must enable and promote the formation of bonds across the adherend/primer-adhesive interface. These bonds may be chemical (covalent, acid-base, van der Waals, hydrogen, etc.), physical (mechanical interlocking), diffusional (not likely with adhesive bonding to metals), or some combination of these (Chapters 7-9). 

The scale of the microscopic surface roughness is important to assure good mechanical interlocking and good durability. Although all roughness serves to increase the effective surface area of the adherend and therefore to increase the number of primary and secondary bonds with the adhesive/primer, surfaces with features on the order of tens of nanometers exhibit superior performance to those with features on the order of microns [9,14], Several factors contribute to this difference in performance. The larger-scale features are fewer in number... 

Cyclization is generally very effective in improving the adhesion of TR and SBR to polyurethane adhesives. Rubbers treated with concentrated sulfuric acid yield a cyclized layer on the surface. This layer is quite brittle, and when flexed develops microcracks, which are believed to help in subsequent bonding by favoring the mechanical interlocking of the adhesive with the mbber. 

Adhesion between metallic/organic interfaces is facilitated by a combination of mechanical interlocking, chemical and physical bonding. Physical bonding alone cannot provide for durable, temperature resistant bonds, as van-der-Waals forces present between the metal surface and adhesive molecules are relatively weak. 

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. 

---