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Adhesion durability assessment.

Duncan, B C; Lodeiro, M J (2004) Adhesion durability assessment. NPL Report. DEPC-MPR 004

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As part of a DTI funded Measurements for Materials Systems project, Interfacial Adhesion Strength, a number of test methods for adhesion strength have been studied as means for quantifying adhesion strength. The pull-off methods and three-point bend were evaluated, in experimental and Finite Element (FE) studies, for their ability to quantify the strength of adhesion between adhesive and adherend. In principle, these tests might be suitable for large scale screening of adhesives and surface treatments.
Durability trials have been performed, using three test systems – anodised aluminium, galvanised steel (oiled) and glass fibre reinforced polypropylene (corona discharge) – bonded with epoxy adhesives, to investigate the influence of surface treatment and measurable surface characteristics on adhesion strength performance under hot/wet conditioning (70 °C, 85% RH). Experimental matrices were chosen using design of experiments approaches and the results from the pull-off and 3-point bend measurements analysed statistically to determine any correlations with contact angle, surface roughness and colour measurements.
The study on the aluminium system indicated that there appeared to be minimal degradation of the bond strength on conditioning. The reductions in bond strength measured were most likely due to the decrease of the cohesive strength of the adhesive occurring due to moisture absorption by the adhesive. There were no correlations with surface characteristics observed.
The zinc layer on the galvanised steel corroded locally under the adhesive layer, where moisture condensed in pores in the adhesive in contact with the surface, but not where the surface was open to the environment. It is believed that the rate of corrosion depends on the rate at which moisture permeated through the adhesive – the 3-point bend sample has a much shorter diffusion path than the pull-off sample and had correspondingly higher levels of corrosion. Some correlations between bond strength and surface characteristics were observed but these must be treated with suspicion since the mode of surface degradation was different under the bond layer than it was for the exposed, travelling surfaces measured. The use of travelling specimens to observe changes in surfaces to predict bond performance is therefore questionable.
The polypropylene surfaces have very low bond strengths and failure in the bond tests was interfacial. The corona treatment reduces the contact angle (increasing surface energy for bonding) and there is a correlation between contact angle and bond strength.
Overall, the correlations between surface characteristics and interfacial adhesion strength were disappointingly poor, mainly as processes other than the surface changes are involved in the degradation of joint performance.

Item Type: Report/Guide (NPL Report)
NPL Report No.: DEPC-MPR 004
Subjects: Advanced Materials
Advanced Materials > Adhesives
Last Modified: 02 Feb 2018 13:16
URI: http://eprintspublications.npl.co.uk/id/eprint/3049

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