Kellar Ewen J.C.,     TWI, UK

Abstract:

In a world where there are many hundreds of commercially available adhesives, spanning many chemical compositions, physical forms and curing requirements, it is very daunting for an engineer to make an appropriate selection based on an application he or she is working on. In addition to the nature of the adhesive, other important factors to consider include surface pretreatment, substrate compatibility, joint design, manufacturing demands, quality control and end of life requirements. This chapter seeks to introduce the reader to the key areas associated with adhesive selection and provide an overview of the selection approach. Additionally, this chapter provides some comment on what is available to assist engineers with selection in terms of software and web resources and concludes with some thoughts on future trends and drivers which are likely to affect the adhesives industry and end-users alike.

Key words

adhesive forms

adhesive selection

chemistry

end of life

internet resource

selection software

surface pretreatment

1.1 Introduction

It is difficult to estimate the total number of commercial structural adhesives that are available to the modern engineer but it most likely equates to several hundreds worldwide. Add to this the fact that some formulations span several brands differing perhaps only in product name or in some small change in filler or additive and the task of effective adhesive selection has the potential to become overwhelming. Thankfully, if a systematic approach is adopted, considering a number of key parameters, selection can be simplified. However, it should be stressed that all selection must be followed up with an appropriate test programme to ensure fitness for purpose. The major key parameters include:

Depending upon the application, different selection parameters will adopt different levels of priority and it is not uncommon for an iterative approach to be taken. Initial application requirements will be put forward by the engineer or designer, which at first pass appear to be of the utmost priority but upon subsequent review there may be other factors that have a greater impact upon selection. For example, a key requirement that is often stated is that the adhesive should be as strong as possible. This might result in the selection of a rigid heat cure epoxy, but the epoxy may exhibit a very low strain to failure of say, < 0.3%, which would be very strong but could result in catastrophic failure with little or no yield. A re-evaluation of the design might point to the fact that a better candidate may be a ‘weaker’ toughened epoxy or acrylic adhesive which fails at greater strain levels (e.g. 1–5%). The joint is more robust and a greater volume of adhesive carries the applied load. Another example might be for a glass bonding application where structural performance is superseded by aesthetic requirements and a water-clear ‘colour’ is preferred over other superior properties.

Although virtually all of the parameters listed are covered in greater detail in subsequent chapters of this book, it is useful to introduce them in a way that emphasises the complex interrelationships that they have with each other.

1.2 Adhesive chemistry

The majority of structural adhesives are based upon six main types of chemical composition:

Each chemical type offers a range of properties which often overlap each other but also possess some unique attributes. In recent years, innovative chemists have formulated many hybrid systems (e.g. epoxy/acrylic and epoxy/polyurethane) designed to bridge the remaining gaps and so provide the adhesives engineer with an almost continuous palette of joining materials from which to choose. However, the base resin chemistry is only one part of the final adhesive product. Adhesives contain other components in quantities ranging from > 10% to < 0.01%, each selected for specific property-modifying attributes including adhesion promotion, thermal expansion control, toughening, rheology control, bond-line control, thermal/electrical conduction, colouring, cure control, mix ratio control, and so on. In most cases the full properties of the adhesive are regarded as proprietary information and are rarely, if ever, disclosed. That said, general adhesive properties are still defined by the base chemistry: