Adhesives in Use

To achieve optimum bonding, performance and life in service from adhesives and sealants, it is absolutely crucial to follow carefully planned processes and procedures and to pay the utmost attention to quality at every stage. In fact, the major criticisms levelled against the use of adhesives are:

• Absolute cleanliness at all stages is essential. Surface preparation of the component is also crucial. To ensure consistent results on structural components, a purpose-built ‘clean room’ is required, in order to reduce contamination to a minimum.
• Pressure and heat may be required. Sophisticated equipment is required to produce pressure over the components in areas where adhesives are applied. This will often entail vacuum bags, purpose-built ovens or pressurised curing ovens (autoclaves).
• Inspection of the bonded joint is difficult. Special inspection techniques and test pieces are necessary to check the integrity of the bond. Prior to preparing the mating surfaces for ‘gluing’, it is necessary to carry out a ‘dry’ lay-up. i.e. a trial assembly of all related parts to check and adjust the fit if necessary. This procedure is essential to enable the final assembly ‘wet’ lay-up to proceed without delay, and without the risk of generating swarf or of contaminating specially prepared surfaces.

Surface Preparation

• Grease, oil or other contaminants must be removed by suitable solvents.
• An optimum surface roughness must be produced.
• Once pre-treated, a surface must be protected from harmful contamination until the bonding process is complete.
• Surfaces to be bonded are normally thoroughly cleaned/degreased in a suitable solvent. This may be followed by a chemical etch or light blasting treatment, followed by a water wash and subsequent drying.

Final Assembly

The adhesive is applied (usually within a specified time, otherwise re-processing may be necessary), and the assembly suitably clamped, or put in a nylon vacuum bag, and heated in an autoclave. The curing process then takes place under carefully controlled temperature and pressure conditions.

When cool, the component is inspected visually for positioning and for a satisfactory spew line. The glue-line thickness is also checked with a calibrated electronic probe, and specimen test pieces are tested for shear and peel properties. Following a satisfactory inspection, the component is finally given appropriate corrosion protection (usually over-painting).

Note: After commencing the final (wet) lay-up, curing of the adhesive must be carried out within a specified time (usually 12 hours). If this period is exceeded by a few hours, it is necessary to increase the temperature and pressure levels during curing (and to obtain an official ‘concession’ cover for this discrepancy).

If the permissible time between wet lay-up and curing is greatly exceeded (e.g. a full shift or day), it will be necessary to dismantle and not only re-commence the wet lay-up, but also to possibly repeat some of the preliminary surface preparation treatments (such as etching).

Typical (Abbreviated) Process

• Dry lay-up (i.e. ‘dummy run’).
• Prepare faces to be bonded (alumina blast, etch (pickle) anodise, etc.).
• Water wash and dry.
• Apply adhesive in clean room and clamp or apply vacuum bag.
• Cure in press/oven or autoclave (typically 120-170ºC).
• Release autoclave pressure when cool.
• Inspect Glue-line thickness (electronic probe), Specimen test-piece results (shear and peel), and positioning, uniform, continuous glue-line and so on.
• Carry out final post-cure surface treatments (e.g. over-painting of primer, sealant or top coat of solvent-resistant paint).