1.1.13.1 Basic Classification

The broadest classification scheme categorises an adhesive as organic or synthetic. This manual focuses on synthetic adhesives, which are the most commonly used adhesives today by far. Modern adhesives systems that are used in demanding structural applications are made from synthetic polymers.

Adhesives may be classified by chemistry, common uses, means of turning from a liquid to a solid (which not all adhesives do), ease of use, method of application, speed of cure – in fact, there are so many characteristics that a diagram of these classifications would be a complex tangle of overlaps and contradictions. For the purposes of this text, we focus on function, chemical composition and type of reaction.

1.1.13.2 Function

One source suggests dividing adhesives into structural and non-structural classes, depending specifically on their ability to produce shear strength above 6.9 N/mm² (1,000 psi).¹ While the figure is somewhat arbitrary, the concept is sound: a structural joint (or bond) is expected to be load-bearing, strong, rigid, durable and able to last for the life of the assembly. Examples of non-structural applications include the attachment of lightweight, non-load-bearing components, coating, potting and gasketing.

While adhesive types like epoxies and acrylics are generally considered structural bonders, the conditions of the application – substrate, environment, geometry, etc. – will strongly influence the strength and durability of the joint. Likewise, in the right conditions, adhesives not normally considered ‘structural’ (silicones, threadlockers and even pressure-sensitive tape) may create a structural bond. The context and conditions of the application are key considerations.

1) Petrie, Digital Engineering Library; digitalengineeringlibrary.com, 280-281, McGraw-Hill, 2000.