Adhesives Technology Handbook

Sina Ebnesajjad, Arthur H. Landrock

• Publisher: William Andrew
• Year: 2008
• ISBN:
  9780815516019
• Edition: 2
• Topic: Technik & Maschinenbau
• Subtopic: Werkstoffwissenschaft

Following the successful first, the second edition is a complete guide to all that is required to successfully bond materials. It is both a reference and a source for learning the basics for those involved in the entire product value chains. Basic principles of adhesion such as surface characterization, types of adhesive bonds, and adhesion failure topics are covered in addition to a description of common adhesive materials and application techniques.

• Provides the end user practitioners of adhesion technology with a complete guide to bonding materials successfully
• Covers most substrates, including plastics, metals, elastomers and ceramics, explaining basic principles and describing common materials and application techniques
• Arranges information so that each chapter can be studied selectively or in conjunction with others

1 Introduction and Adhesion Theories

1.1 Definition of Adhesives and Adhesive Bonding

An adhesive is a material that is applied to the surfaces of articles to join them permanently by an adhesive bonding process. An adhesive is a substance capable of forming bonds to each of the two parts when the final object consists of two sections that are bonded together.^[1] A feature of adhesives is the relatively small quantities that are required compared to the weight of the final objects.

Adhesion is difficult to define, and an entirely satisfactory definition has not been found. The following definition has been proposed by Wu.^[2] “Adhesion refers to the state in which two dissimilar bodies are held together by intimate interfacial contact such that mechanical force or work can be transferred across the interface. The interfacial forces holding the two phases together may arise from van der Waals forces, chemical bonding, or electrostatic attraction. Mechanical strength of the system is determined not only by the interfacial forces, but also by the mechanical properties of the interfacial zone and the two bulk phases.”

There are two principal types of adhesive bonding: structural and nonstructural. Structural adhesive bonding is bonding for applications in which the adherends (the objects being bonded) may experience large stresses up to their yield point. Structural adhesive bonds must be capable of transmitting stress without loss of integrity within design limits.^[3] Bonds must also be durable throughout the useful service life of a part, which may be years. A structural bond has been defined as having a shear strength greater than 7 MPa in addition to significant resistance to aging. Nonstructural adhesives are not required to support substantial loads but merely hold lightweight materials in place. This type of adhesive is sometimes called a “holding adhesive.” Pressure-sensitive tapes and packaging adhesives are examples of nonstructural adhesives.

The distinction between structural and nonstructural bonds is not always clear. For example, is a hot melt adhesive used in retaining a fabric's plies structural or nonstructural? One could argue that such an adhesive may be placed in either classification. However, the superglues (cyanoacrylates) are classified as structural adhesives even though they have poor resistance to moisture and heat.

1.2 Functions of Adhesives

The primary function of adhesives is to join parts together. Adhesives accomplish this goal by transmitting stresses from one member to another in a manner that distributes the stresses much more uniformly than can be achieved with mechanical fasteners. Adhesive bonding often provides structures that are mechanically equivalent to or stronger than conventional assemblies at lower cost and weight. In mechanical fastening, the strength of the structure is limited to that of the areas of the members in contact with the fasteners.^[4] It is not unusual to obtain adhesive bonds that are stronger than those of the strength of adherends.

Smooth surfaces are an inherent advantage of adhesively joined structures and products. Exposed surfaces are not defaced and contours are not disturbed, as happens with mechanical fastening systems. This feature is important in function and appearance. Aerospace structures, including helicopter rotor blades, require smooth exteriors to minimize drag and to keep temperatures as low as possible. Lighter weight materials can often be used with adhesive bonding than with conventional fastening because the uniform stress distribution in the joint permits full utilization of the strength and rigidity of the adherends.^[4] Adhesive bonding provides much larger areas for stress transfer throughout the part, thus decreasing stress concentration in small areas.

Dissimilar materials, including plastics, are readily joined by many adhesives, provided that proper surface treatments are used. Adhesives can be used to join metals, plastics, ceramics, cork, rubber, and combinations of materials. Adhesives can also be formulated to be conductive. The focus of this book is on adhesives for bonding plastics, thermosets, elastomers, and metals.

Where temperature variations are encountered in the service of an item containing dissimilar materials, adhesives perform another useful function. Flexible adhesives are able to accommodate differences in the thermal expansion coefficients of the adherends and therefore prevent damage that might occur if stiff fastening systems were used.

Sealing is another important function of adhesive joining. The continuous bond seals out liquids or gases that do not attack the adhesive (or sealant). Adhesives/sealants are often used in place of solid or cellular gaskets. Mechanical damping can be imparted to a structure through the use of adhesives formulated for that purpose. A related characteristic, fatigue resistance, can be improved by the ability of such adhesives to withstand cyclic strains and shock loads without cracking. In a properly designed joint, the adherends generally fail in fatigue before the adhesive fails. Thin or fragile parts can also be adhesive bonded. Adhesive joints do not usually impose heavy loads on the adherends, as in riveting, or localized heating, as in welding. The adherends are also relatively free from heat-induced distortion.^[4]

1.3 Classification of Adhesives

Adhesives as materials can be classified in a number of ways such as chemical structure or functionality. In this book, adhesives have been classified into two main classes: natural and synthetic. The natural group includes animal glue, casein- and protein-based adhesives, and natural rubber adhesives. The synthetic group has been further divided into two main groups: industrial and special compounds. Industrial compounds include acrylics, epoxies, silicones, etc. An example of the specialty group is pressure-sensitive adhesives.

1.4 Advantages and Disadvantages of Joining Using Adhesives

The previous discussion highlighted a number of advantages of adhesive bonding. This section will cover both advantages and disadvantages, recognizing that some of the points have already been mentioned.

1.4.1 Advantages^{[5, 6]}

• Uniform distribution of stress and larger stress-bearing area
• Join thin or thick materials of any shape
• Join similar or dissimilar materials
• Minimize or prevent electrochemical (galvanic) corrosion between dissimilar materials
• Resist fatigue and cyclic loads
• Provide joints with smooth contours
• Seal joints against a variety of environments
• Insulate against heat transfer and e...