1993-03-01

Induction Heating of Adhesives for Bonding of Thermoplastics and Composites 930332

Structural adhesives have utilized different methods for accelerating cure and development of handling strength and final properties. These different methods have included fast curing chemistries, heated presses, microwave, radio frequency, high velocity hot air, and induction heating. Each method has particular advantages which the processor can use to maximize productivity and quality. This paper examines and reports on induction heating and its application for bonding of thermoplastics and composites.
Induction heating has had good acceptance in bonding applications where metal substrates are involved. Automotive hem flange bonding has been a particularly successful application of induction heating. With metal bonding, the substrate is heated by the electromagnetic field generated by a current passing through an induction coil. Heating to cure temperatures occurs within fractions of a second and temperatures can easily surpass the limits of any organic adhesive. Other advantages of induction heating are low energy consumption, safety for operators, and no direct contact with the substrate surface.
With composites and thermoplastic substrates, materials which are not electrically conductive or magnetic, the adhesive must be modified to be affected by the induction field. This generally involves the incorporation of electrically conductive or magnetic fillers into the adhesive.
In this work, modified two-component adhesives were used to bond sheet molding compound (SMC). The adhesives were formulated such that handling strengths were easily achieved within 60-seconds and as short as 45-seconds. This compares to cure times of 120-seconds or more with conventionally heated presses, which rely on thermal conductivity of the substrate and intimate heater block contact to affect the cure. Thus a 63% reduction the cure cycle has been achieved. This accelerated cure allows reduction in the number of bonding stations required or increasing the productivity of a single line. Additionally, properties of the induction cured adhesives are excellent and compare to the adhesive when cured by oven or heated press.
Used in much of this work was a solid-state, self-tuning, induction heater. The solid state control allowed automatic tuning to the resonant frequency of the tank circuit, coil, and load. Additionally monitoring and controlling the surface temperature of the substrate was possible. This allowed accurate and consistent curing of the adhesive.

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