Surface Preparation Techniques for Adhesive Bonding
By Raymond F. Wegman and James Van Twisk
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About this ebook
Surface Preparation Techniques for Adhesive Bonding is an essential guide for materials scientists, mechanical engineers, plastics engineers, scientists and researchers in manufacturing environments making use of adhesives technology. Wegman and van Twisk provide practical coverage of a topic that receives only cursory treatment in more general books on adhesives, making this book essential reading for adhesion specialists, plastics engineers, and a wide range of engineers and scientists working in sectors where adhesion is an important technology, e.g. automotive / aerospace, medical devices, electronics.
Wegman and van Twisk provide a wealth of practical information on the processing of substrate surfaces prior to adhesive bonding. The processing of aluminum and its alloys, titanium and its alloys, steels, copper and its alloys, and magnesium are treated in the form of detailed specifications with comparative data. Other metals not requiring extensive treatment are also covered in detail, as are metal matrix and organic matrix composites, thermosets and thermoplastics.
This new edition has been updated with coverage of the latest developments in the field including the sol-gel process for aluminum, titanium, and stainless steel, atmospheric plasma treatment for metals, plastics and rubbers and treatments for bronze and nickel alloys.
- Updated to include recent technological developments and chemicals currently prescribed for cleaning and surface preparation; a new generation of adhesives technologists can benefit from this classic guide
- Enables Materials and Process personnel to select the best process available for their particular application
- Practical coverage of a topic that receives only cursory coverage in more general books on adhesives: essential reading for adhesion specialists, plastics engineers, and a wide range of engineers and scientists working in sectors where adhesion is an important technology, e.g. automotive / aerospace, medical devices, electronics
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Surface Preparation Techniques for Adhesive Bonding - Raymond F. Wegman
considered.
1
Introduction
1.1 Adhesion
Adhesion is a surface phenomenon, i.e., adhesion is controlled by the condition of the surface of the adherend. The ASTM [1] defines adhesion as the state in which two surfaces are held together by interfacial forces which consist of valence forces or interlocking actions or both.
Adhesion between surfaces which are held together by valence forces is called specific adhesion; this is the same type of force which gives rise to cohesion. Cohesion is defined as the state in which particles of a single substance are held together by primary or secondary forces. As used in the field, cohesion is defined as the state in which the particles of the adhesive (or the adherend) are held together.
Adhesion between surfaces in which the adhesive holds the parts together by interlocking action is known as mechanical adhesion.
Both specific adhesion and mechanical adhesion are important to the understanding of how adhesion is affected by surface preparation. Allen [2] in his discussion of the fundamentals of adhesion concluded by stating … an adhesive bond achieves its strength from the combination of a variety of sources; (mechanisms)… For these mechanisms, the relative importance and the proper way which they should be combined will vary from one example to another, but none should be excluded without very careful consideration and exploration.
1.1.1 Specific Adhesion
According to the theory relating to specific adhesion, this type of adhesion involves the establishment of some kind of attraction between the atoms and the molecules which make up the adhesive and the adherends. These attractions may involve primary bonding forces, which tend to be quite strong; hydrogen bonding, which yields intermediate strength; and the weaker secondary (Van der Waals) forces.
Primary bonding may be covalent or ionic in nature. Covalent bonding involves the sharing of electron pairs between adjacent atoms. Ionic or electrostatic forces are the type of primary bonds that are found in ionic crystals. Another type of primary bond is the metallic bond which is similar to the covalent bond except that it involves the valence electrons in the metal. This type of bonding is discussed by Verink [3], Wegman and Levi [4] and Salomon [5].
Secondary bonding involves dipole-dipole interactions, induced dipole interactions and dispersion forces. Secondary bonding becomes important in adhesion when nonpolar or chemically inert surfaces are involved. Hydrogen bonding may be considered a special case of dipole interaction, since hydrogen bonds result from the sharing of a proton by two electron-negative atoms. However, hydrogen bond strengths are of the same order as a weak primary bond.
Another type of bond, discussed by DeLollis [6], is described as a chemisorbed bond. This type of bonding is proposed as the reason why adhesive promoters, such as primers and coupling agents, are successful in overcoming potentially weak boundary layers and result in good durable bonds.
In dealing with specific adhesion, good contact must be obtained between the adhesive and the surface of the adherend. To obtain this molecular contact with a solid adherend requires the wetting of the solid surface by the liquid adhesive. During the bonding process even the solid adhesive must go through a liquid phase. To understand the conditions of adequate wetting one must consider the role of surface energetics in adhesion. This, simply stated, requires that in order for a liquid to wet and spread on a solid surface the critical surface tension of the solid or solids must be greater than the surface tension of the liquid. In the case of polar solids such as metals and metal oxides this requirement is easily met, because the surface energies of the solid, provided that the surfaces are clean, are greater than 500 dyne/cm while the surface energies of the liquid adhesives are less than 500 dyne/cm [4]. However, even if good wetting and good contact between the adhesive and adherend are obtained, it may be difficult to obtain good durable bonds if there is a weak boundary layer on the adherend. Therefore, it is necessary to carefully select the proper surface preparation technique for the particular adherend, in order to make sure that such a weak boundary layer does not occur, which would make the bond practically useless. Surface energetics is further discussed by Kaelble [7].
1.1.2 Mechanical Adhesion
Mechanical adhesion results from an interlocking action between the surface structure of the adherend and the adhesive or primer.
Bickerman [8] proposed that adhesion was due to the inherent roughness of all surfaces. He accepted the fact that molecular forces of attraction caused an adhesive to wet and spread on the surface. Once this was achieved, however, Bickerman felt that mechanical coupling between the adhesive and the inherently rough adherend was more than enough to account for bond strength. Surface roughness can also account for a number of negative factors such as trapped gas bubbles, as described by DeBruyne [9], and imperfect molecular fit, as described by Eley [10]. Many arguments have been presented against Bickerman’s theory and inherent surface roughness should be considered to be a contributing factor only, rather than a basic factor in the theory of adhesion.
In 1977, Chen et al. [11] proposed that the surface of Forest Products Laboratory (FPL) etched aluminum consisted of fine finger-like structures of aluminum oxide which were determined to be about 50 Angstroms thick and approximately 400 Angstroms in height. Each of the fine fingers protruded from the tripoint of the oxide structure. This finding was made possible by advanced electron microscopy using such techniques as the scanning transmission microscope. Similar findings were made when the surface of phosphoric acid anodized aluminum was studied.
Further discussions and stereomicrographs of treated aluminum surfaces are presented in Chapter 2 of this book.
In 1980, Ditcheck et al. [12] described the morphology and composition of titanium adherends prepared for adhesive bonding. In this work they discussed macro-rough and micro-rough surfaces. From their determination of the morphology of various surfaces they predicted the order of bondability of the surfaces and the reliability of the resultant bonds. Their predictions were confirmed by Brown [13] using the wedge test and by Wagman and Levi [14] using stress durability testing. This work is described in more detail in Chapter 3. Some excellent micrographs of these surfaces were presented by Venables [15] using extended resolution scanning electron