Advances in Structural Adhesive Bonding

David A. Dillard

• Publisher: Woodhead Publishing
• Year: 2010
• ISBN:
  9781845698058
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Adhesive bonding is often effective, efficient, and often necessary way to join mechanical structures. This important book reviews the most recent improvements in adhesive bonding and their wide-ranging potential in structural engineering.Part one reviews advances in the most commonly used groups of structural adhesives with chapters covering topics such as epoxy, polyurethane, silicone, cyanoacrylate, and acrylic adhesives. The second set of chapters covers the various types of adherends and pre-treatment methods for a range of structural materials such as metals, composites and plastics. Chapters in Part three analyse methods and techniques with topics on joint design, life prediction, fracture mechanics and testing. The final group of chapters gives useful and practical insights into the problems and solutions of adhesive bonding in a variety of hostile environments such as chemical, wet and extreme temperatures.With its distinguished editor and international team of contributors, Advances in structural adhesive bonding is a standard reference for structural and chemical engineers in industry and the academic sector.

• Reviews advances in the most commonly used groups of structural adhesives including epoxy, silicone and acrylic adhesives
• Examines key issues in adhesive selection featuring substrate compatibility and manufacturing demands
• Documents advances in bonding metals, plastics and composites recognising problems and limitations

Part I

Adhesive selection

1

Key issues in selecting the right adhesive

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:

• adhesive chemistry

• adhesive form/structure

• mode or type of adhesive cure

• substrate compatibility

• surface pretreatment

• joint function and operating environment

• joint design

• manufacturing demands

• quality control/quality assurance (QA/QC)

• testing and evaluation

• end of life requirements

• aesthetics.

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:

• epoxy (or epoxide)

• polyurethane

• reactive acrylic

• toughened acrylic

• anaerobic acrylic

• cyanoacrylate

• silicone

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:

1.2.1 Epoxy (epoxide)

This is probably still the main workhorse of structural adhesive formulations. They bond well to a wide range of materials, especially metals, ceramics and most polymers, including many thermoplastics. They exhibit good chemical resistance, produce few volatiles during curing and have low shrinkage values. Therefore they have the capacity to form extremely strong and durable bonds with most materials in well-designed joints.

Owing to the nature of the chemistry and the curing reaction, great versatility in formulation can be achieved since there are many resins and many different hardeners. They are available in a wide variety of forms, from low-viscosity liquids to solid pastes or films. Development of toughened formulations has dramatically increased the demanding uses of these adhesives in many industries.

Throughout all the variations, the mechanism of curing (termed addition) is always the same. This mechanism requires precise quantities of resin and hardener, hence the need for accurate mix ratios, and the thorough mixing of resin and hardener in two-part systems. without these, curing will be affected and inferior properties may result, typically lower strength and stiffness and reduced environmental resistance.

Two-part epoxy adhesives start to react under ambient conditions once the two components have been mixed together and as such are often termed room-temperature (RT) curing adhesives. The reaction is strongly influenced by temperature and as a rule of thumb the reaction rate approximately doubles for every 10°C rise in temperature, that is, an epoxy which takes 1 hour to cure at 20°C, will cure in ~15 minutes at 40°C. Conversely the cure time will double as the temperature drops by 10°C. Complete cure times at ambient temperatures for two-part systems range from a few minutes to several days. It should be noted that, with only a few exceptions, at temperatures below 10°C, the rate of cure decreases significantly and the level of cross-linking may be compromised, giving rise to limited levels of cure, that is, lower cohesive strength, reduced modulus and lower resistance to aggressive environments.

Single-part epoxy adhesives are available in liquid, paste or film form. These adhesives require heat to cure. The resin and hardener are premixed but curing does not occur because the catalyst is in an inactive form at room temperature. It only becomes reactive as the temperature is raised, usually in excess of 100°C. The higher the temperature, the faster the reaction becomes and, in some instances, times of less than 10 minutes can be obtained.

1.2.2 Polyurethane

These adhesives, often abbreviated to PU or PUR, will bond to most materials, including plastics, glass, stone and metals. PU adhesives are chemically reactive formulations that may be one-part or two-part systems. They provide strong impact-resistant joints and have better low-temperature (cryogenic) strength t...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Part I: Adhesive selection
• Part II: Adherends, surfaces and pre-treatments
• Part III: Joint design
• Part IV: Environmental effects and durability of adhesives
• Index