Abstract
Surface characterisation and failure mechanisms of adhesively bonded glass butt joints were studied. Materials of relevance to the adhesive bonding of fibre optic components were employed: a fine annealed Grade A glass (Schott(TM) BK - 7), and a fast curing epoxy based optical adhesive. The joint behaviour and their durability under adverse environmental conditions were investigated, and the subsequent, failed joint fracture surfaces were examined using XPS, ToF-SIMS and SEM. Surface analysis techniques have been employed to characterise components of the adhesive systems and to interrogate the surface of failed joints with a view to establishing the locus of failure. Joints were immersed in pure water for period of 0 - 270 days at 35°C. Substantial reduction in the bond strength within a few days of water immersion was observed. Also for the joints left in for a prolonged exposure periods (> 100 days) the strength values reached a minimum value. The surface analysis of the joints has indicated a cochoidal brittle fracture through the adhesive in dry conditions, and an interfacial failure for wet joint fractures. Calculations of the polymer thickness on the interfacial surfaces indicated a decrease in overlayer thickness. This reduction reached a constant value for joints left in water for more than 100 days. During this investigation surfaces segregation of minor components of the adhesive such as amine and diluent was also identified by ToF-SIMS and XPS. In order to study this phenomenon further, reformulation of the adhesive has been carried out. Six adhesives were formulated, of which three resins were based on the change in diluent content of the system, three were based on the change in amine concentrations. This work has clearly identified amine segregation only at the interfacially failed joints. It is expected that when the joints cures very fast, even if the formulation contains very high proportion of amine, the segregation is minimal. Also, some evidence of diluent migration at the interfaces was observed during the reformulation studies.