Surface Preparation

Good surface preparation is crucial to ensuring sufficient bond strength and reliable performance of bonded joints, particularly under hostile service or manufacturing conditions. Unsatisfactory surface preparation will result in premature and unpredictable bond failure at the adhesive/adherend interface either in service or at some further stage of the bonding process. Surface preparation is recognized as a most critical step in the adhesive bonding process and considerable effort may be expended in optimizing the surface treatment.

The purposes of surface treatments are to:

1. Remove contaminants that may interfere with bond formation;
2. Remove weak surface layers;
3. Produce a surface morphology that enhances the surface area available
for bonding and/or allows mechanical keying;
4. Chemically modify the surface to increase surface energy and
chemical compatibility with the adhesive.

The selection of surface treatment is largely dependent on the substrate, the required strength and durability of the joint and economic considerations (such as costs and time involved in preparation). Surface treatment processes often consist of a series of different steps. Surface treatments can be classified as either passive or active.
Passive surface treatments (e.g. solvent washing and mechanical abrasion) clean the surface, remove weakly attached surface layers and alter the surface topography without altering the surface chemistry. Active surface treatments (e.g. corona discharge or plasma treatment) alter the surface chemistry (i.e. introduction of functional groups). After completion of the surface preparation process, the adherends should be handled and stored carefully in order to prevent surface contamination prior to bonding. It is normally advisable that bonding be performed immediately following surface treatment to maximize performance.

It is normally important that the process of surface preparation only affects the chemistry and morphology of a thin surface layer of the adherend(s) and does not alter the mechanical and physical properties of the underlying substrate. There are many procedures available for engineering surfaces [e.g. 16, 20–24] but comparatively few for materials used non-engineering applications. Advice is usually sought on surface preparation from the adhesive manufacturer. Surface preparation procedures may often require potentially hazardous or environmentally damaging chemicals. All preparation should be carried out to COSHH specifications.