Morphology and surface chemistry of kissing bonds in adhesive joints produced by surface contamination

Kissing bonds (KBs) or zero volume debonds are interfacial defects in which the substrate and adhesive are in intimate contact or couple through a weak bond or thin layer of contaminant. These KBs can dramatically alter the failure strength of the joint causing premature failure. Adhesive bonds containing a layer of Frekote 700NC, artificial sweat and cutting oil lubricant were produced on chromic acid etched aluminium substrates. All the joints with contaminants were examined using analytical field emission scanning electron microscope (FESEM) fitted with energy dispersive X-ray spectrometer (EDS) and also wavelength dispersive spectrometer (WDS). Photoacoustic Fourier transform infrared spectroscopy (FTIR) and mechanical testing were also performed. These methods were used to investigate whether such joints qualified as KBs. FESEM revealed changes in morphology and surface/interface chemistry related to the corresponding contaminants. Elemental distribution from EDS showed migration of artificial sweat and absorption of oil contaminant into the adhesives particularly when curing at elevated temperature while mould release agent (Freekote 700NC) remained at the metal/adhesive interface or within the interphase. FTIR revealed the presence of contaminants including comprehensive information of chemical interaction of the adhesives and contaminants within the interphase. Further understanding of the effect of various contaminants on wetting of contaminated substrates was obtained by contact angle measurement. A direct relationship was discovered amongst the reduction in surface energy of the substrate contaminated with Frekote, morphological changes and a reduction in joint strength, showing that Frekote is a potential contaminant in the production of kissing bonds.