An investigation into the cohesive properties of toughened two-part structural epoxy adhesives

Abstract

The performance of a range of two-part toughened epoxy adhesives has been assessed in a number of modes. The structural behaviour of the adhesives in the bulk mode (cohesive) as a function of the level of cure applied has been evaluated through extensive mechanical experimentation. Thermal analysis has also been performed upon the adhesives in order to quantify the variation in both the degree of conversion and the glass transition temperature as a function of cure. Correlations between adhesive and bulk cohesive behaviour were made possible through the performance of fundamental joint strength tests. Fracture surfaces of flexural test specimens were analysed using SEM techniques allowing more detailed analysis to be made of the adhesive microstructure, specifically the morphology of the toughening phase. The mechanical performance of a range of adhesives in the presence of drawing oil was assessed as a function of cure level and substrate condition. The oil is used upon sheet steel by the automotive industry to aid forming processes and is frequently left upon substrates that are to be bonded. The effect of the presence of oil upon adhesive performance has been quantified through joint strength tests, absorption experiments and thermal analysis. The toughening components employed are of great importance to the fracture behaviour of the adhesive. When such elements are used correctly, significant gains in material performance can be made. Fracture mechanics tests were performed for various cure levels. Identical levels were also used in tests designed to monitor the phase out process of the toughening element in order to facilitate direct comparison. Using a bespoke light box and photocell, the turbidity of a model epoxy system was measured in both toughened and untoughened forms. The fracture behaviour is then directly correlated to the toughener phase out process. This method also allowed for deductions to be made on the morphology of the toughener. The work has also made many recommendations for improved processing controls as the performance of the adhesive can be tailored by alterations to not only cure temperature and durations but also substrate condition and joint design. These suggestions, when considered in combination, contribute significantly to the improvement of two part epoxy adhesive usage.

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