5.2.4 Substrate Specification

The type and nature of the substrates to be bonded and the surface preparation that they may require are key factors in determining which adhesive to use. Some substrates, such as aluminium or wood, may be successfully bonded with a wide variety of adhesive types; other substrates, such as nylon or HDPE, may only be successfully bonded with a few and may require extra pre-treatment or surface preparation. In any case, not all substrates are the same, even within a substrate type, and specific attention should be given to the specification and manufacturer of a particular material, as mentioned in previous sections.

For example, 304 stainless steel is typically made by adding chromium and nickel. While the exact percentages added by weight at different manufacturers will differ, they will still be classified as 304 Stainless Steel. Adding to the complexity are other grades within the same ‘family’ of stainless steel (i.e. 316, 310). It is important to remember that while a substrate may belong to a ‘family’, they are similar but not identical.

Influences that are specific to the substrate type, such as condition, porosity, finish, acidity and alkalinity, will influence the adhesive’s performance and must therefore be considered during the adhesive selection process. The surface condition of the material, its processing history and the condition at the time of bonding may be more important than the bulk substrate material properties.¹⁷ This consideration is extremely important and often overlooked, and many issues have arisen from engineers assuming a material from one manufacturer the same as that from another manufacturer. In reality, the processes, storage conditions and surface conditions, among many other factors, may vary greatly between manufacturers.

Once a substrate has been specified in an application, great care should be taken if a change is required during production. The ‘new’ substrate should be validated in the process before the change is made to avoid any quality issues. If multiple suppliers of a material are to be used, each substrate should be validated independently to ensure process compatibility.

17) Edward M. Petrie, Handbook of Adhesives and Sealants, McGraw-Hill, 2007.