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Adhesion and Adhesives P1: FYK Revised Pages Qu: 00, 00, 00, 00 Encyclopedia of Physical Science and Technology EN001D-12 May 25, 2001 21:4 Adhesion and Adhesives K. W. Allen Joining Technology Research Centre, Oxford Brookes University I. History of Adhesion and Adhesives II. Mechanisms of Adhesion III. Classification of Adhesives IV. Solution-Based Adhesives V. Hot Melt Adhesives VI. Reactive Adhesives VII. Pressure-Sensitive Adhesives VIII. Surface Preparation for Bonding IX. Coupling Agents and Primers X. Joint Design XI. Failure of Joints XII. Test Methods GLOSSARY Phase A distinct form of matter: solid (crystalline, amor- phous, etc.), liquid, or vapor. Adherend That to which an adhesive bond is being made; Pre-treatment The preparation of an adherend surface sometimes referred to as a substrate. (Note: Biologists to increase the strength and/or durability of adhesive use the latter term in a quite different sense.) bonding to it. Adhesion An attraction between two substances which, Solvent based Any solution could be described as “sol- after they have been brought together, requires work to vent based,” but in the present context the term is used be done to separate them. By convention, this excludes for solutions in an organic solvent and implicitly ex- magnetic attractions. cludes aqueous (water)-based solutions. Adhesive A substance which fills the gap between two Surface tension/energy Because of the imbalance of surfaces and forms a relatively permanent and coherent forces at the surface of a liquid, the surface behaves bond (i.e., causes “adhesion” between them). as if it were a stretched skin. The tension in this skin, Curing An adhesive at an early stage in its use has to be a measured in force per unit length, is the surface tension. liquid so that it can wet and spread on the substrate, but If the skin is stretched, then work has to be done and then it has to become solid. This change from liquid to energy expended against that tension. This energy, in solid, however it is accomplished, is known as “curing.” energy per unit area, is the surface energy. Provided that 237 P1: FYK Revised Pages Encyclopedia of Physical Science and Technology EN001D-12 May 7, 2001 12:26 238 Adhesion and Adhesives a coherent system of units is used, the surface tension changes over the recent past is the gradual swing of this in dynes/cm or N/m is numerically identical to surface balance toward a better understanding of why and how energy in erg/cm2 or J/m2. and away from tradition and rule of thumb. Surfaces of high/low energy Generally, the surfaces of metals (or their oxides) are of high energy (200–1500), while most polymers are of low energy (20–50) Water I. HISTORY OF ADHESION is unusual, with a value of 72. AND ADHESIVES Vulcanize Originally used in rubber science to describe the cross-linking and eventual hardening from reaction The use of adhesion for practical purposes, either involv- with sulfur. Now extended to include any elastomer and ing a separate adhesive or taking advantage of the phe- its cross-linking or cure. nomenon in other ways, goes back a very long way in Wetting The extent to which a liquid will come into in- human history (also, various examples may be found in timate contact with a surface to which it is applied. It both zoological and botanical structures). Probably the may range from total spreading across the surface to oldest reference to the use of an adhesive is in the Book of remaining in droplets and making little or no contact. Genesis in the Bible, where we are told that the builders Good wetting of an adherend by an adhesive is a neces- of the Tower of Babel used “slime” or “bitumen” as mor- sary, but not sufficient, condition for achieving a strong tar. The earliest papyrus was made by using adhesion to adhesive bond. bring the separate reed strands into a coherent film, and dates back to 3000 B.C. A little later, flour-and-water paste was used to join the separate sheets into a continuous IT IS QUITE important to recognize the different mean- roll. As the Egyptian civilization developed, from per- ings of the words in the title of this chapter before looking haps 1400 B.C., richly decorated artifacts were produced in detail at either. The topic of adhesion covers the sci- by using natural product adhesives to secure veneers and entific study of all the forces available to operate between gems in place. While these techniques were used through- two surfaces in intimate contact and which may prevent out the Imperial Roman Empire, they appear to have been their separation without the application of some force. It lost with its fall (about 400 A.D.), and a dark period pre- encompasses a great many more phenomena than simply vailed until the 16th century. At that time, adhesives be- those of providing a practically useful joint between two gan to be used for inlaying, veneering, and assembling components. The permanence of print on paper or plas- furniture. However, even throughout the dark period, the tic, the integrity of composites, the durability of paint, the technique of adhesion was preserved and used in vari- way in which mussels or barnacles are attached to rocks or ous applications; for example, tar was used to make ships ship hulls, and the behavior of cancerous cells within the watertight. human body all involve adhesion (with no adhesives in- In 1690, the first commercial plant to manufacture ad- volved). In the reverse, the development of nonstick cook- hesives was opened in Holland, and this was followed ware and frictionless bearings requires an understanding in 1754 by the first patent being granted for making fish of the basis of adhesion so that it may be avoided. glue. Through the first half of the 19th century, devel- This science is of relatively recent development, with opment was slow, but gradually an increasing range of virtually no coherent work before the mid-1920s, although adhesives became available, including those using ca- eventually some earlier concepts were found to be relevant sein and natural rubber as base polymers. The develop- in unexpected contexts. ment of synthetic resins really began with the produc- In contrast, the technology of adhesives and their use tion in 1909 of “Bakelite,” a phenol-formaldehyde resin, dates back several millenia. Practical ways of uniting arti- and later of urea-formaldehyde resin, both of which were cles to provide a joint with resistance to an applied stress used in the production of plywood. The importance of air- have been explored and developed to a useful level by craft, accelerated by World War I, led to a considerable entirely empirical methods. These are used in another increase in the use of adhesives but no great innovations. wide range of instances, of which manufacturing plywood, These did not occur until the build-up to World War II securing cartons and packages, constructing aircraft, af- and under the pressure of national crisis, when epoxy fixing postage stamps, and manufacturing carpets are all and urea/formaldehyde/formic acid (Aerolite, 1937) for examples. wooden structures and aircraft and then the phenol/ The technology of each of the applications is a mixture formaldehyde/polyvinyl formula (Redux, 1942) for alu- of (often very great) experience together with a modicum minium aircraft adhesives were developed. of understanding of some of the more elementary con- Since then, two successive forces have led to fur- cepts from the science. Perhaps one of the more significant ther developments. First has been the steadily growing P1: FYK Revised Pages Encyclopedia of Physical Science and Technology EN001D-12 May 7, 2001 12:26 Adhesion and Adhesives 239 recognition of the advantages of using adhesive bonding instances where this notion seems reasonable—for exam- in place of other methods of joining and fastening. These ple, classical dental fillings using amalgam and sticking advantages include the absence of stress concentration, of porous materials (e.g., textiles, paper, or cardboard). saving of weight, reduction of the need for surface refin- However, there are many instances where this does not ishing (as there is no deformation from spot welding or seem possible (e.g., bonding of smooth, hard surfaces of rivets), and very quick curing for noncritical assembly. glass or aluminium). Consequently, there has been a growing demand for ad- With the development of more sophisticated techniques hesives in new situations (e.g., automotive structures re- for studying surfaces in the 1980s (ultra high-resolution quiring enhanced toughness). To satisfy these demands, electron microscopy), it became clear that these appar- several new chemistries have been developed, such as ently smooth surfaces, particularly the aluminium used anaerobic materials (1953), cyanoacrylates (1957), reac- for aerospace structures, had both pores and protrud- tive acrylates (1976), and polyurethanes. These develop- ing whiskers in the oxide layer which constituted their ments have been followed by an increasing awareness of outermost surface. These structures were on a scale of the deleterious environmental effects of many of the ma- Angstroms (10−10 m) rather than microns (10−6 m). The terials used, particularly the organic solvents, which had adhesive both penetrated into the pores and enclosed the been important constituents of many formulations. This whiskers to give a bond of great strength. Thus, the me- has led to considerable efforts to produce improved, more chanical interlocking explanation received fresh support environmentally friendly formulations, particularly water- when a much smaller scale was considered. based and hot-melt products. B. Adsorption II. MECHANISMS OF ADHESION The secondary molecular forces responsible for the phe- nomenon of physical adsorption of gases on solids also Explanations and theories of adhesion only began, in any provide a source of adhesive strength.
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