Diffusion adhesion

Other aspects of interfacial science and chemistry are examined by Owen and Wool. The former chapter deals with a widely used chemistry to join disparate surfaces, that of silane coupling agents. The latter chapter describes the phenomenon of diffusion at interfaces, which, when it occurs, can yield strong and durable adhesive bonds. Brown s chapter describes the micromechanics at the interface when certain types of diffusive adhesive bonds are broken. The section on surfaces ends with Dillingham s discussion of what can be done to prime surfaces for adhesive bonding. 

The polymer particle size in latexes is far too small to he effective for adhesive applications. Therefore, many of these very tiny budding Uocks must coalesce through the diffusion adhesion mechanism to provide an optimum sized critical mass of polymer for the particular adhesive requirement. Methods of bringing the CTUcial number of particles together at the proper point of intersection for optimum bonding represents much of the ingenuity and skill involved in the utilization of emulsion polymers in adhesive applications. 

Small Cu clusters, more polarizable than a single Cu atom, interact more strongly with the nondefective Si02 surface. Still, the role of defects for the diffusion, adhesion, and nucleation processes is crucial. All Cu clusters interact with the E ... 

Solvent and Diffusion Adhesives (Adhesive Varnishes, Contact Adhesives). 244... 

The dissolutirai and/or swelling of the molecular structure of the surface layers caused by contact and diffusion adhesives result in the following disadvantages ... 

With some diffusion adhesives, solvent residues can still be found in the adhesive joints after several months. 

Typical organic solvents for diffusion adhesives are methylene chloride (for PC, ABS, PMMA), xylene, cyclohexanone (ABS), tetrahydrofurane (THF adhesive for PVC-U, ABS, SAN, SB), ethyl and butyl acetate (ABS), alcoholic-aqueous solution of resorcin and phenol (for PA 6, PA 6.6 and PA 12), DMF, higher primary alcohols, A-methyl-pyrrolidone (for PA 11), methylethyl ketone (for CA, CAB, CP, CN, ABS, SAN, SB, PMMA), xylene, toluene for PS. It is remarkable that so much of these solvents is used despite the known high hazard potential. 

Physical adhesion This mechanism is controlled by diffusive bonding, where the diffusivity increases with increasing contact temperature according to Fields law. This can be maximised by substrate preheating. Because of the small diffusion depth (produced by the rapid solidification), the diffusive adhesion generally plays only a minor role as an adhesion mechanism. 

Diffusion Adhesion arises through the interdiffusion of molecules in the adhesive and adherend. Solvent or heat welding of thermoplastic substrates is considered to be due to diffusion of molecules. 

FIGURE 23.9 Schematic representation of (a) chemical bonding, (b) molecular entanglement, (c) mechanical interlocking, (d) diffusion adhesion, and (e) electrostatic adhesion between the matrix and the CNTs. 

Adhesives can be classified on the basis of chemical composition [29], setting mechanism [30], and adhesion mechanism as pressure-sensitive, adhesion, and diffusion adhesives [31]. Lucre [32] proposed a flexible classification in which each adhesive is characterized and classified according to various characteristic features, such as chemical basis, form of application, application temperature, thermal behavior, and uses. A classification such as this provides for very detailed coverage of the individual features of an adhesive and is a helpful guide for industrial users. However, it is extremely comprehensive and goes beyond the scope of this treatise. For this reason, a classification based on the setting mechanism is given here [30]. 

Over-diffusion (adhesion) When the extent of the interdiffusion of materials causes a weakening of the material in the diffusion zone. Examples Weakening by formation of Kirkendall porosity microfracturing due to stresses caused by phase changes in the diffusion zone. 

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