Page 350 - Handbook of Adhesives and Sealants

306 Chapter Eight

thickness may be only several mils thick. If there are gaps larger than
that between the substrates, the adhesive will not bond because there
is no applied pressure in the area of greatest gap. Pressure sensitive
tapes using foam (polyethylene or vinyl) carriers are used where gap
tolerances may be great. These adhesives are easily recognized in au-
tomobile protective side strips, mounting tape for wall dispensers, etc.
8.2.3.2.3 Reactivatable adhesives. Certain adhesives are known as sol-
vent activated adhesives. These systems can be applied to a substrate
via solvent coating and then dried to a non-tacky state. The coated
substrate is then usually stored or it proceeds to the next step in the
manufacturing operation. When it is time to bond the coated substrate
to another substrate, the coated piece is exposed to solvent either by
wiping it across a cloth saturated with solvent or by spraying a small
amount of solvent onto the coated surface. This partially liquefies the
adhesive, and it becomes tacky so that the substrates can be mated
under contact pressure.
Labels, nameplates, stamps, etc. are common applications for reac-
tivatable adhesives. Care must be exerted not to apply too much or
too little solvent to the dry adhesive film. The reactivating solvent is
generally applied by brush, spray, or roller coating. Too much solvent
could wash away most of the resinous component resulting in a
starved joint. Too little solvent could result in lack of sufficient tack
to bond to the other substrate. These adhesives are normally highly
formulated thermoplastic resins.

8.2.3.3 Resinous solvent adhesives. Many solvent adhesives are used
for their mechanical interlocking ability. They are formulated with a
relatively rigid, tough thermoplastic resin. The substrate is generally
porous. Once applied, the solvent evaporates into the air and through
the substrate so that the adhesive resin interlocks the pores of the
substrate together. Typical applications are the bonding of wood, card-
board, fabric, leather, and other porous substrates.
More dilute solvent based systems can be used to bond certain ther-
moplastics. In this application the solvent diffuses into the substrates
leaving the resinous portion of the adhesive in the joint. More infor-
mation is given on solvent cementing of thermoplastics in Chapter 15.

8.2.3.4 Hardening from the melt. Adhesives can also harden by cooling
from a melt condition. Hot melt adhesives are the most common ex-
ample of this type of adhesive. These are generally thermoplastic ad-
hesives that soften and melt when heated, and they harden on sub-
sequent cooling. The hot melt system must achieve a relatively low
viscosity when in the molten state to achieve wetting, and it must not
cool too rapidly or it will not have time to completely wet the rough-