When a hot-melt adhesive comes into close contact with the surface to be bonded, a molecular layer of film at the surface of this substrate immediately attains a temperature approaching that of the hot melt. In addition a high degree of wetting, almost coalescence. of the hot melt and the material occurs. Immediately thereafter, the adhesive loses heat to the film over the entire area and temperature equilibrium is attained. Since the adhesive is in contact with a mass much larger than itself, the temperature of the entire system drops to the point at which the hot melt sets to a solid state with sufficient cohesive strength to bond the films together. Thus the uniqueness of hot-melt adhesives stems from the speed with which they produce a bond, which is almost instantaneously.
Although the use of hot-melt adhesives eliminates the cost of solvents required by some other adhesives, the principal cost reduction results from the time saved in their application. In addition, a lesser quantity of the hot melt can usually be utilized to produce an equivalent bond.
Hot-melt adhesives are used extensively in binding books made of loose sheets, especially those that are not rounded and backed, e.g., paperback books, telephone and other directories, etc. For books that are to be rounded and backed, however, the so-called DRYING MEMORY of the hot-melt adhesives causes problems. In addition, hot-melt adhesives alone do not lend themselves well to the binding of heavily loaded or coated paper, from which all particulate matter must be removed before application of the adhesive. See also: ADHESIVE BINDING ;ONE-SHOT METHOD (1) ;TWO-SHOT METHOD .
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