Through adhesive surface treatment for bonded joints
Through adhesive surface treatments have been defined as pretreatments for adhesive bonding where the adherend surface is physically attacked under a covering layer of uncured adhesive. In addition to the usual benefits associated with physical pretreatments, there is potential for novel chemical reactions between adhesives and adherends. It may be proposed that, in the absence of a coherent oxide layer, an adhesive will form primary chemical bonds to a reactive metal adherend. Primary bonds may endow an adhesive to adherend interface with greater strength and aqueous stability than secondary bonding alone can provide. Three techniques of through adhesive surface treatment have been investigated. The first uses an abrasive surface, flooded with adhesive, onto which the adherend is scrubbed. The second relies on abrasive filler in the adhesive. Agitation of uncured joints’ adherends causes the filler to abrade the bonding surfaces. Finally, the third technique employs the adhesive as an ultrasonic cleaning medium. Ultrasound in the uncured adhesive causes cavitation, the shock waves of which erode the adherend surface. After initial experiments to establish practical methods, all three techniques have been evaluated for a number of adhesive to metal combinations. The majority of tests show small but significant increases in initial strengths, coupled with possible increases in durability. Limited surface analysis studies have not discerned a reason for the changes, since primary bonding may not be their only cause. It is quite possible that changes in wetting can account for the small improvements observed in performance.
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