On Hot-melt, Heat-seal and Hot-set Adhesives

Abstract

To clear up the confusion I quote from the adhesives glossary published by the American Society for Testing Materials (ASTM, D 907-74) and from the Adhesives Handbook by J. Shields (CRC Press, 1970): Adhesive, hot-melt: An adhesive that is applied in a molten and forms a bond on cooling to the solid state (ASTM). Shields elaborates: An adhesive material applied at a temperature above its melting point. Usually thermoplastic, wax or 100% solids: adhesives applied between 150 and 2000C. In the technology of art conservation wax and wax-resin mixtures are adhesives of this type. They are applied in liquid form; they are absorbed by and penetrate through all the porosities of the materials to which they are applied and often stain them. Because of the depth of their penetration, total removal of such adhesives from the impregnated materials becomes impossible. Adhesive, heat-seal: use of heat reactivation to prepare a joint with a thermoplastic material present, as a thin layer, on the adherends, bringing adherend surfaces to their melting point and bonding under (Shields). The last word of the above definition-pressure-is of great importance. Indeed, heat-seal adhesives are usually described by the temperature, pressure and the length of time, that is, the dwell time, required to form a bond. See, for example, I. Skeist, Handbook of Adhesives (Reinhold: New York, 1962, p. 353, cited by Bernard when he describes his adhesive in A Poly (Vinyl Acetate) Heat Seal Adhesive for Lining, Conservation and Restoration of Pictorial Art, N. Bromelle and P. Smith, eds. (Butterworth: London, 1976): Skeist reports a minimum heat-seal temperature of 600C for AYAA, 1.5 sec. dwell-time at 60 lb. per sq. inch. The high viscosity of the heat-seal pressure-sensitive adhesive prevents it from penetrating into the voids (porosities) of the materials to be bonded even under the pressure required to form the bond. The result is a bond that forms only on those surfaces which are in direct contact with each other. This explains why heat-seal adhesives do not stain and, if prepared from stable theromoplastic materials, can be easily and completely removed from almost every material used as an art support, including absorbent and/or thin substances such as old silk or tissue paper. The differences between the above two types of adhesives are considerable. Yet in the paper Treatment of a Flood-Damaged Oil Painting on a Solid Support by David C. Goist, the author writes: When the wheat paste and paper were dry, each sheet was coated evenly with the Hot-Melt adhesive devised by Bernard Rabin (Journal of the AIC, XVI, 2, p. 24), although in his reference he quotes the title correctly: Rabin, B., A Poly (Vinyl Acetate) Heat Seal Adhesive for Lining, cited above.