TWI Industrial Member Report Summary 735/2002
E J C Kellar
For over fifty years the aerospace sector has successfully used adhesive bonding technology to great effect. The key issues of high strength and durability have been addressed through the implementation of highly controlled, rigorous surface pre-treatments prior to the bonding process. At present, the acid etch (chromic or phosphoric) followed by an anodising stage to produce a controlled oxide layer represents the surface pre-treatment favoured by the main fabricators. Irrespective of which acid is used, the presence of hexavalent chromium (Cr 6+) is required at some stage during the process.
Ever tightening world environmental legislation (especially in Europe and the USA) will make the use of toxic materials such as certain solvents and heavy metals (including chromium) more and more difficult in the future. There is a need therefore, to identify and develop 'green' solutions to these important processes.
Pre-treatments incorporating organo-silanes (commonly referred to as silanes) have been shown to offer considerable durability enhancement over simple mechanical treatments for Al alloys and between 50-70% to those prepared using conventional acid etch/anodise processes. However, the actual mode of action is not fully understood and design engineers are therefore reluctant to include silane use in performance specification, since variations in use are common. Acceptance can only be stimulated if there is sufficient supporting data and the effect of all major variables is evaluated thereby enabling full quality assurance to be achieved.
To maximise the benefits of TWI's Core Research Programme (CRP), TWI joined the venture and investigated the practicalities of silane solution application. In return for this contribution to the programme, it was agreed that results arising from ICOSAP would be made available to TWI Member companies.
The approach was twofold:
- Monitor and summarise the main findings arising from ICOSAP.
- Contribute experimentally to ICOSAP.
The latter task concentrated on how durability is affected by:
- Substrate position during silane application.
- Method of silane application.
- Effect of exposing substrate to boiling water prior to silane application.
- Define preparation and application parameters of a commercial silane coupling agent for maximising durability performance of a bonded joint.
- Investigate the chemical reactions associated with the silane and the alloy surface.
- Assess the effect of the adherend position during silane application on the durability of the joint.
- Assess the effect of hydrolysis (via exposure to boiling water) on the adherend surface prior to silanation on the durability of the joint.