Adhesives have been used for about six millennia, but it was only from the first decade of the twentieth century that any significant development took place, with the introduction of synthetic materials to augment earlier natural materials.
The driving force for development has been the needs of particular industries rather than technological advances themselves. The introduction of synthetic resins began in about 1909, but although the growth in plywood manufacture was accelerated by World War I, little innovation was involved. Significant advances began with World War II and the development of epoxy and urea/ formaldehyde adhesives for the construction of wooden aircraft, followed by the phenol/formaldehyde/ polyvinyl formal adhesives for bonding aluminum, which cannot generally be welded. Later, adhesive bonding in conjunction with riveting was applied to automobile construction, initially to high-performance models but increasingly to massproduced vehicles. The fastening of composite materials is, with few exceptions, accomplished by use of adhesives. If the forces of adhesion are to be effective, intimate contact must be established between two components, one of them a liquid that will wet and flow across the other before solidifying so that the bond can resist and transmit any applied force. This change of phase from liquid to solid is achieved in a variety of ways.
Solution-Based Adhesives
The earliest adhesives were all natural products such as starch and animal protein solutions in water. These are still in use for applications where only low strength is required (e.g., woodworking or attaching paper and similar materials).In these cases, the cost has to be low because the uses are high volume. Until about 1930 these were the main adhesives used in all carpentry and furniture. Polyvinyl acetate adhesives are now probably the most important range of water-based adhesives. The base polymer is dispersed in water to give an emulsion that has to be stabilized, usually with approximately 5 percent polyvinyl alcohol. Solutions in organic solvents were first introduced in 1928, and they are now perhaps the most widely used adhesives both for manufacturing and for do-it-yourself purposes. Based on solutions of polychloroprene as base polymer dissolved in organic solvents, they provide a fairly strong ''quick-stick'' bond. Particular grades are extensively used in the footwear industry. Because of the toxic, environmentally unfavorable properties of the solvents, considerable efforts are being devoted to replacing these with water-based products, but these have not yet been entirely satisfactory.
Hot-Melt Adhesives
One of the oldest types of adhesive is sealing wax. Since about 1960, these hot-melt adhesives have been introduced initially for large-scale industrial use and more recently for small-scale and do-ityourself uses. Polyethylene is extensively used as the base for hot-melt adhesives since it is widely available in a range of grades and at low cost. Ethylene vinyl acetate is similarly a useful base, and the two are commonly used in combination to give effective adhesives with application temperatures in the range of 160-190_C. This means that the adhesives have an upper limit of service use of perhaps 140_C, and the materials being joined must be able to withstand the higher temperature. These adhesives are quite widely used in large-scale manufacturing. However there are a considerable number of applications where the temperature involved for normal hot-melt adhesives is excessive. Consequently, in the 1990s a group of special formulations evolved that have an application temperature in the range of 90 to 120_C without any loss of adhesive strength. The most recent developments are adhesives that are applied as hotmelts and are then ''cured'' by various means. They have all the advantages of ease of application and quick achievement of useful strength supplemented by a much higher service temperature. Curing may be achieved either by heating to a higher temperature than that of application or by irradiation with an electron beam.
High-Temperature Adhesives
All the adhesives considered so far can only provide useful bonds up to very limited temperatures, commonly 100_C or perhaps 150_C. There are demands, mainly military, for bonds that can withstand up to 300_C. To meet these needs, some adhesive base polymers have been developed that are based on carbon and nitrogen ring systems with a limited service life at these high temperatures.
Pressure-Sensitive Adhesives
Pressure-sensitive adhesives (e.g., Scotch Tape, first sold in 1940) are totally different from any others. These adhesives depend on an exceedingly highviscosity liquid that retains this state throughout its life and never cross-links or cures. The strength of the bond is dependent on the pressure applied to it as the bond is made. The useful life of pressuresensitive adhesives is generally limited to perhaps one or two years.
