Adhesive Choice

modified epoxy, and urethane adhesive are commonly used in weldbonding aluminum. Epoxy and polyimide adhesives are used for titanium. Polyimides are particularly suitable for titanium because of their very high temperature resistance (matching the resistance of titanium). Epoxy and modified-epoxy adhesives are available in one-or two-part liquid, paste, capillary, or unsupported-film form.  

The most suitable adhesive for general weldbond applications is modified-epoxy one-part paste type containing metal filler. Most adhesives used for weldbonding are arbitrarily selected from adhesives that were developed for other purposes, resulting in a compromise when used for weldbonding. Adhesives specifically developed for weldbonding should show improvements over general-purpose adhesives.  

Fillers for epoxy adhesives used in weldbonding include silica (Cab-O-Sil, 7% by wt) to prevent run-off, 3% strontium chromate to provide corrosion inhibition, and conductive metal powders. Viscosity of adhesives in paste form has an important effect on the weld and bonded joint. Viscosity must be low enough to allow the force of the electrodes to push the adhesive out of the interface contact area, yet sufficiently high or thixotropic so that it will not flow out of the joint during the cure cycle. 

Film adhesives have been found by some workers to be impractical because spot-welds could not be made consistently, due to the carrier.On the other hand, film adhesives have been found to provide substantial benefits because they improve workmanship in the production facility. The first requirement for any adhesive for the weld-through process of weldbonding is that the adhesives have the capability of being moved under pressure of the welding electrodes in order for metal-to-metal contact to occur at the joint interface. The second requirement is that the heat resulting from the spot weld cause only limited detrimental effect on the strength of the bond.  

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The two predominant mechanisms of failure in adhesively bonded joints are adhesive failure or cohesive failure. Adhesive failure is the interfacial failure between the adhesive and one of the adherends. It indicates a weak boundary layer, often caused by improper surface preparation or adhesive choice. Cohesive failure is the internal failure of either the adhesive or, rarely, one of the adherends. 

Today s new engineered composites provide designers with a wealth of options and challenges. Many structural adhesives are currently available, and the correct adhesive choice is crucial. Factors to consider include VOC content, surface preparation, working and fixture times, requirements of the application, and dynamic fatigue resistance. 

Table 5.1 Production factors affecting adhesive choice...

Adhesive choice. Factors most likely to influence adhesive selection are listed in Table 7.4. With regard to the controlling factors involved, the many adhesives available can usually be narrowed to a few candidates that are most likely to be successful. The general areas of concern to the design engineer when selecting adhesives should be the material to be bonded, service requirements, prodnction reqnirements, and overall cost. 

Application consideration. There are, of course, certain applications that require specific types of adhesive. Again, available equipment may limit adhesive choice, and special adhesives can be very expensive. Generally speaking, however, the performance requirements are much more rigid than application requirements. 

In addition to mechanically fastening, adhesives in electrical applications are required to conduct heat, conduct or isolate electricity, provide shock mounting, seal, and protect substrates. Properties required for various applications cover the range from life of a few seconds to many years. Operating temperatures are from -270°C to 500°C. Adhesives are used in quantities of less than a microgram to more than a ton. The adhesive choice is governed by these considerations plus strength. 

Adhesive choice can therefore be influenced by actual joint tolerance control and dimensions and where a cyanoacrylate or an anaerobic adhesive might be ideal (polymeric substrates or a threaded metallic joint, respectively), the measured bond gap may be too large and an alternative adhesive such as a polyurethane or an epoxy may be better suited. 

This adhesive choice is driven by a complex dynamic distribution of the thermal flux with regard to the time against the deviation of the thermal insulation properties. 

It may be desirable to use a primer or sealant. Adhesive choice is particularly important with cadmium coatings. 

Adhesive choice is clearly fundamental apart from selecting a type which will meet the basic requirements of any particular concept, the designer should take this process further. The most suitable adhesives will ... 

There are two common adhesive choices cyanoacrylic and methyl metacryhc. Cyanoacrylic acid butyl ester is usually preferred over the methyl and ethyl esters because of its slower cure (polymerization) rate. Consequently, the polymerization produces less heat it also causes less tissue irritation. Cyanoacrylate glue is commonly used for approximation of skin after various surgical procedures [44], and is occasionally used to embo-lize blood vessels [45,46] and to occlude fistulas [47]. 

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