Incorporation of an Adhesion Promoter in a Structural Adhesive: Aspects of Durability and Interface Chemistry

ABSTRACT

The durability and interfacial chemistry of adhesively bonded joints fabricated from clad aluminium alloys (2024-T3) has been investigated using the Boeing wedge-test geometry and XPS analysis. A silane, γ-glycidoxypropyltrimethoxysilane (GPS), has been incorporated directly within the adhesive formulation, and the effect of four different silane concentrations, 0, 0.5, 1, and 2% w/w, have been studied. As expected, the durability of the aluminium joints depends on the silane concentration contained within the adhesive formulation. The mechanical tests carried out showed that a silane concentration lying between 0.5 and 1% w/w would be optimum for the aluminium joints investigated in this work. The failure mode was apparently interfacial but the XPS analysis showed that some fractures occurred partly within the adhesive. The calculations of the C/N ratios from small-area XPS analyses on the adhesive interfacial failure surfaces have provided an indication of the manner in which failure may occur. It has been found that a depletion of curing agent takes place at the region where failure subsequently occurs, leading to a reduction in the mechanical properties at this region within the bond line.

Keywords:

• Adhesive
• Durability
• Epoxy
• Silane
• XPS

Notes

British term referring to a device for holding parts of a frame in place during construction. It usually consists of a steel bar along which slide two brackets between which the work is fixed, with one of the brackets pegged into a hole in the bar while the other is adjusted by means of a screw, which applies the required load.

*Negative value indicates that the analysis was performed before the crack front limit, positive value indicates after.

*Negative value indicates that the analysis was performed before the crack front limit; a positive value indicates after.

*Negative value indicates that the analysis was performed before the crack front limit; positive value indicates after.

*Negative value indicates that the analysis was performed before the crack front limit; positive value indicates after.

*Values in brackets correspond to the position relative to the crack front in mm.

**Two average values are provided for the adhesive without silane because analyses were performed before and after the crack front. For all other concentrations, only one value is provided.

The Royal Society is acknowledged for the provision of a University Research Fellowship to M-LA. The authors thank Nigel Porrit (QinetiQ), Steve Greaves, and Andrew Brown for their help in setting up the mechanical testing and XPS and SEM experiments, respectively.

Presented in part at the 27th Annual Meeting of the Adhesion Society, Inc., held in Wilmington, NC, USA, 15–18 February 2004.

*Current address: ELTRO LIMITED, Armstrong Mall, Southwood Business Park, Farnborough, Hampshire, GU14 ONR UK.