Pre-bonding

After a satisfactory verification film is produced, an assembly may be fabricated specifically for destructive inspection to validate that the verification film was accurate. This correlation allows the use of verification film rather than more expensive destructive inspection for future changes such as duplicate tool fabrication and tool or detail modification. Simple assemblies are usually not destructively inspected because of high confidence that the verification film is entirely representative of the expected bondlines. Complex or large parts may not be destructively inspected because of the cost of the details and assembly time. In these cases other means of validating the verification film are used. Meticulous pre-bond detail and post-bond assembly thickness measurements may be sufficient to prove bondline thickness control. Ultrasonic inspection and X-ray photography (discussed previously) may be sufficient to prove that details are in the correct places and bonds are good. 

During the basic step of peptide formation, two or more reacting components are pre-bonded by the enzyme molecule. A simple model of such reaction can he represented by the diagram in Figure 1.1. 

As described above, besides a certain participation of platelet physical adsorption, the interaction between platelet surface receptors and the pre-bond proteinic ligands on the material surface is mainly responsible for the ini-... 

Hybridization describes the pre-bonding promotion of one or more electrons from a lower energy subshell to a higher energy subshell followed by a combination of the orbitals into degenerate hybrid orbitals. 

Hybridization describes the pre-bonding promotion of one or more electrons... 

If the adhesive layer was always to represent the weakest link in the joint, the quality of the assembly would be limited to an assessment of the cohesive properties of that layer. Unfortunately it is the interface which tends to be of greater concern because it is here that joint failure is more commonly encountered, particularly after ageing. The most useful tests for indicating interfacial quality are in fact pre-bonding surface inspection procedures, which can nevertheless suggest potential joint performance. The tests used for assessing cohesive properties of the adhesive layer must necessarily be post-bonding activities. 

Pre-bonding. Fresh concrete is poured directly onto a layer of uncured adhesive spread over the prepared steel surface. This method, as used in the reinforcement of slab units by externally bonded plates, has been the subject of extensive research by the Wolfson Bridge Research Unit(lO). Open sandwich slabs, based on either a flat soffit plate or a plate curved slightly upwards to form a shallow arch have been recommended. The technique is described in some detail in Chapter 8. 

FIGURE 3.10 Orbital overlap schemes for dre-pre bonds, (a) pz-dxz in POCI3, (b) pz-dz in PF3, (c) pn-dz in P05, (d) p7t-dz -y in PO ", (e) dative bond transition metal-phosphine ti- and (f) dative n bond transition metal-carbon monoxide. 

Since the orbital overlap occurs in the region nearest to the electron donor (Figure 3.9b), dji-pre bonding is therefore polar, but it is weaker than pn-pn bonding. ... 

While it is possible to bond to a freshly abraded or cleaned metal surface, chemical treatments are preferred for rendering the metal surface inactive to corrosion over time. For low carbon steel, phosphatising is the recommended pre-bond surface preparation treatment. Stainless steel should be passivated or acid etched, while titanium is usually treated with a hydrofluoric acid pickle. Almninium or magnesium are best treated with a chromate conversion coating. Zinc and cadmium are generally prepared mechanically but a phosphate or chromic acid treatment may be used. Brass and copper may be treated with an ammonium persulphate etch or an acid-ferric chloride etch. 

All adherends require some form of pre-bonding preparation in order to obtain a good quality bond with optimum mechanical performance and durability. The objective of surface preparation is to ... 

Two examples of standard procedures used for pre-bonding preparation of metals quoted by many adhesive suppliers and widely used are those developed by the Boeing Airplane Company (BAC). 

Another critical issue with bonding of composite structures is pre-bond moisture, which can exist in a multiplicity of forms, each with its own distinct adverse consequences. Moisture absorbed in cured laminates and driven to the surface by the heat of the bond cycle prevents any adhesion from occurring without creating detectable symptoms. Such water lowers the energy level of the substrate, as described by Mahoney (1988) and can lead to the condition illustrated in O Fig. 44.8, in which a complete area failed to bond but was not detected. O Figure 44.8 shows a perfect replica of the original peel ply, known to be free of silicone, on both sides of the... 

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