Bonding advantages gained through joint design can be useless if the surfaces to be bonded are not prepared to a degree consistent with the requirements. The amount of surface preparation depends on the required bond strength, desired environmental ageing resistance and economic practicalities. For maximum strength structural bonds, paints, oxide films, oils, dust, mould release agents and all other surface contaminants must be completely removed. There are four principal ways of preparing surfaces:
• solvent degreasing;
• abrasion, including emery paper, sand, shot or grit blasting;
• chemical etching and anodising;
• surface priming.
Given the correct adhesive and the appropriate surface preparation, almost any substrate can be bonded, provided that the operating conditions are not too extreme. However, the creation of the optimum surface can be expensive and may not be practical in many manufacturing situations. Fortunately, most applications do not require preparation at this level, and usually an acceptable compromise can be found at some point in the sequential process of:
• no treatment at all;
• solvent wash;
• solvent wash - abrade - solvent wash;
• solvent wash - abrade - solvent wash chemically etch;
• any of the above plus an appropriate primer.
Selection of an adhesive appropriate for a job depends on many things, including:
• forces in the joint;
• environmental conditions;
• required durability;
• application and process factors.
With respect to the substrate being used, different adhesives are more appropriate for different substrates. The issue of application and curing of the adhesives depends on:
• the adhesive selected;
• flexibility of the manufacturing process;
• type of manufacturing process;
• the substrate.
The Adhesive Database
Recent development have brought about the introduction of several adhesive selection databases. Technology transfer is a vital function of these systems and they are useful as education and training programmes.
The PAL System developed by Permabond has a selection logic which functions on non-quantitative data which are input in response to a series of searching questions. The adhesive databank is basically the Permabond product range but also accommodates generic type information. The selection process can be used to define the reason for rejection of a particular adhesive or family of adhesives. The PAL system contains a stress analysis package of limited capacity.
The ADHESYS System has been developed and marketed by AEA Technology. The software includes a selection programme, surface preparation details, health and safety information and a joint design and analysis capability for coaxial joint configurations. The databank is based on generic types of adhesives.
The STICK System has been developed by Lucas Engineering and Systems for use by Lucas operating companies, and its database contains forty generic types of adhesive. The selection process is based on an optimised rule search strategy which can display adhesives still under consideration at any stage during the selection process. The programme contains relevant information from adhesive manufacturers, health and safety sheets and applications. A `what if?' section is provided at the end of the system to allow responses to be changed.