Surface Treatment of Materials for Adhesive Bonding

Sina Ebnesajjad, Cyrus Ebnesajjad

• Publisher: William Andrew
• Year: 2013
• eBook ISBN: 9780323265041
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Physics
• Index: Physics

Aimed at engineers and materials scientists in a wide range of sectors, this book is a unique source of surface preparation principles and techniques for plastics, thermosets, elastomers, ceramics and metals bonding. With emphasis on the practical, it draws together the technical principles of surface science and surface treatments technologies to enable practitioners to improve existing surface preparation processes to improve adhesion and, as a result, enhance product life.This book describes and illustrates the surface preparations and operations that must be applied to a surface before acceptable adhesive bonding is achieved. It is meant to be an exhaustive overview, including more detailed explanation where necessary, in a continuous and logical progression.The book provides a necessary grounding in the science and practice of adhesion, without which adequate surface preparation is impossible. Surface characterization techniques are included, as is an up-to-date assessment of existing surface treatment technologies such as Atmospheric Plasma, Degreasing, Grit blasting, laser ablation and more. Fundamental material considerations are prioritised over specific applications, making this book relevant to all industries using adhesives, such as medical, automotive, aerospace, packaging and electronics.This second edition represents a full and detailed update, with all major developments in the field included and three chapters added to cover ceramic surface treatment, plasma treatment of non-metallic materials, and the effect of additives on surface properties of plastics.- A vital resource for improving existing surface treatment processes to increase product life by creating stronger, more durable adhesive bonds- Relevant across a variety of industries, including medical, automotive and packaging- Provides essential grounding in the science of surface adhesion, and details how this links with the practice of surface treatment

Part I

Background and Theory

Outline

Chapter 1 Introduction to Surface Preparation

Chapter 2 Surface Tension and Its Measurement

Chapter 3 Surface Energy of Solids and Its Measurement

Chapter 4 Surface and Material Characterization Techniques

Chapter 5 Theories of Adhesion

Chapter 1

Introduction to Surface Preparation

Surface preparation or treatment for adhesion is defined as one or a series of operations including cleaning, removal of loose material, and physical and/or chemical modification of a surface to which an adhesive is applied for the purpose of bonding. In plastics bonding, surface preparation is aimed at increasing the surface polarity, improving surface wettability, and creating sites for adhesive. In summary, the objective of any surface treatment method is to enhance the adhesive bond strength and durability. The basic requirements for a good adhesive bond are the following: proper choice of adhesive; good joint design; cleanliness of surfaces; wetting of adherends (surfaces that are to be bonded together) by the adhesive; and proper adhesive bonding process (solidification and cure).

Keywords

surface preparation; adhesion; adhesive; plastics bonding; adherends; wettability

1.1 Definition of Surface Preparation, Adhesives, and Adhesive Bonding

Surface preparation or treatment for adhesion is defined as one or a series of operations including cleaning, removal of loose material, and physical and/or chemical modification of a surface to which an adhesive is applied for the purpose of bonding. In plastics bonding, surface preparation is aimed at increasing the surface polarity, improving surface wettability, and creating sites for adhesive. Metal surfaces are typically covered with oxide layers, rendering the surface highly polar and fit for good adhesive bonding. Surface preparation is intended to enhance the bonding strength to metal surfaces and, significantly, to improve the durability of the bond, especially when exposed to humid environments.

An adhesive is a material applied to surfaces of articles to permanently join them by a bonding process. It is a substance capable of forming bonds to each of the two or more part interfaces comprising the final object [1]. A feature of adhesives is the relatively small quantities required relative to the weight of the final object.

An entirely satisfactory definition for adhesion has not been found. One of the better definitions 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. The 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.”

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 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 in contact with the fasteners [3]. It is not unusual to obtain adhesive bonds that are stronger than the strength of the adherends (surfaces that are to be bonded together).

1.2 Introduction to Surface Treatment

The main reasons for applying surface treatments prior to bonding are:

1. to remove or prevent the later formation of a weak layer on the surface of the substrate;

2. to maximize the degree of molecular interaction between the adhesive or primer and the substrate surface;

3. to optimize the adhesion forces which develop across the interfaces and therefore ensure sufficient joint strength, initially and during the service life of the bond;

4. to create specific surface microstructure on the substrate.

Normally, optimum surface energy and structure is achieved by chemical surface treatment of plastic, rubber, and metal surfaces. The chemical composition and the morphology of the surfaces are altered so that the surface energy of the substrate is elevated sufficiently for adhesion. Chemical treatments also increase the chances that hydrogen, dipole, van der Waals, ionic, and/or covalent bonding can form at the substrate/adhesive interface. A “quick and dirty” test for adequate chemical treatment is to place a drop (bead) of water on the surface of the part and note whether the water spreads. If so, the contact angle it forms with the surface is very small. There are special fluids called dyne liquids that can fairly precisely bracket the numeric value of the surface energy of a material surface, e.g., between 40 and 42 dynes/cm.

Alternative methods to chemical treatment (with similar surface changes) include plasma, corona, and flame treatment methods. These methods act in ways similar to chemical treatments, usually with less hazardous waste generation. Mechanical abrasion is another means of surface preparation. This method is superior to other methods due to the simplicity of the process and significantly less waste generation. Mechanical abrasion works by providing a clean surface and increasing the contact area between the substrate and adhesive. In ordinary gluing of objects, where performance requirements are not stringent, these methods are not needed since maximum adhesion bond strength is not critical. When a strong adhesion bond is required, the minimum surface preparation consists of cleaning and removal of dirt and grease from the contact surfaces.

To give the reader an idea of the basic methods of surface treatment for adhesive bonding, short descriptions of degreasing, abrasion, and chemical treatments are provided in this chapter. These and the more complex methods have been descri...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Preface to First Edition
• Acknowledgments
• Part I: Background and Theory
• Part II: Surface Treatment Methods and Techniques
• Index