Induction-cured adhesives

Induction Curing Adhesives. Typically one-component epoxy adhesive systems are used for induction bonding of structural parts. However, two-component adhesives have also been used. The properly designed induction cured adhesive should provide the following application and performance properties ... 

Speed is the most important feature of induction heating. Workpiece heating rates greater than 40°C/s are possible. One supplier of induction curing epoxy adhesives claims production speeds of 600 parts per/hour are possible.21... 

When induction curing a one-part thermosetting adhesive, one has the choice of taking the adhesive to full strength or only to partial strength in the induction field (complete cure... 

Overheating of the adhesive is always a critical concern with induction curing. This is especially true when the adhesive cures with a high exotherm, such as epoxy-dicyandiamide systems. Several proprietary epoxy curing agents have been developed that provide lower exotherm yet faster cure rate than typical dicyandiamide reactions.27,28 Tertiary amines and modified polyamines29 are often used to accelerate the cure of dicyandiamide-epoxy... 

One of the advantages of using a two-component room temperature curing adhesive system is that induction curing can be used to spot-cure the adhesive in a joint (similar to a spot-welding process). The adhesive that is not spot-cured by induction will then fully cure at room temperature as the part moves down the assembly line. 

Turi, D., Supplier/User Cooperation Yields Beneficial Induction Cure Epoxy, Adhesives Age, June 1993, p. 22. 

Figure 7. Bonding metal to glass by means of electrical induction curing of a film adhesive.

New technologies which will impact the growth of adhesives in assembly line applications are robotics and induction curing. 

The process of building door assemblies requires the application of a structural adhesive to the outer door panel. The next step in the assembly of doors is the joining of the inner and outer panels. Finally, a hemming die is used to turn a small section of the outer panel over the inner panel to form a hem. Accurate bead placement and uniform material deposition are critical to achieve a structural bond. These requirements, combined with high production speeds, make hem flange bonding well suited for robotic automation. Fig. 11 depicts the entire door assembly line process complete with hemming die and induction cure fixture. The induction cure fixture applies localized... 

A unique feature of these imidazole catalysts is that they do not have the high exotherm that dicyandiamide produces when cured in epoxy resins. Thus, they do not char or burn when exposed to high cure temperatures for fast cure. This is an important factor for adhesives that are cured via induction or dielectric heating. These adhesive systems are also much safer to ship via ah freight than conventional dicyandiamide catalyzed epoxy formulations due to their low exotherm. 

Epoxy adhesives that cure via indirect thermal means (induction, dielectric, microwave, and several others)... 

The heat induced into the workpiece will conduct instantly into the adhesive, providing the catalyst for cure. If one of the two substrates is not electrically conductive, then an adhesive can be used that includes a small percentage of metal oxide. The metal oxide particles within the adhesive become heated in the induction field and provide the source of heat to cure the matrix material in the adhesive. (See Fig. 14.5.)... 

Ultrasonic welding is a frictional process that has been well established for heat welding thermoplastic parts. Like induction welding, it also has been adapted for curing structural adhesives such as epoxy. 

Both induction and dielectric heating involve relatively expensive capital equipment outlays, and the bond area is limited. Their most important advantages are assembly speed and the fact that an entire assembly does not have to be heated to cure only a few grams of adhesive. 

Another method to cure epoxies or preapplied (nylon) adhesive patches is induction heating. 

Chem. Descrip. Mercaptan polymer precatalyzed, accelerated Uses Epoxy hardener, accelerator in concrete adhesives, grouts, mortars, and coatings co-curative improves bond to glass and metal Features Fast cure eliminates induction times of polyamides and amido-amines... 

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