Page 343 - Handbook of Adhesives and Sealants

Adhesive Classifications 301

removed, the adhesive has aggressive and permanent tackiness. The
substrates can be mated with very little pressure.
Reactivatable adhesives—The adhesive is applied to the substrate
and the solvent is evaporated. At this stage the substrate may be
stored or transported with a dry adhesive coating. At the time of
joining, the coating is moistened with the solvent and the adhesive
becomes tacky with pressure sensitive characteristics.
Resinous solvent adhesives—The adhesive is a resinous material
dissolved in the solvent. It is applied to porous substrates that are
then joined. The solvent evaporates leaving the resin to mechani-
cally lock the substrates together.

8.2.3.2.1 Contact adhesives. Contact adhesives are applied to both sub-
strates by spray or roll coating. Usually the solvent is allowed to evap-
orate under ambient conditions, but sometimes heat is applied to ac-
celerate drying. After some portion of the solvent evaporates, the
adherends are joined, and the adhesive rapidly bonds or knits to itself
with the application of contact pressure. There is a window of time in
which there is just enough tack exhibited by the adhesive to bond.
Contact adhesives generally have relatively high shear and peel
strengths. Strength and durability approach that of structural ther-
mosetting adhesives. A typical contact adhesive application is the
bonding of decorative surface materials to wood for kitchen counter
tops. The contact adhesives generally have less ‘‘green strength’’ or
immediate tack than pressure sensitive adhesives and cannot be ap-
plied to only one substrate.

8.2.3.2.2 Pressure sensitive adhesives. Pressure sensitive adhesives are
applied much like contact adhesives and instantly provide a degree of
tackiness. Unlike contact adhesives, their tackiness is permanent and
there is no optimal time range when the substrates must be joined.
Pressure sensitive adhesives are usually based on elastomer or ther-
moplastic solvent solutions. They are coated on the substrate or on a
film backing that is used as a carrier. Pressure sensitive tape (e.g.,
office tape) is made in this manner. Once the solvent evaporates, the
tape is ready to be applied or it is packaged so that it can be dispensed
at the point of assembly.
Most pressure sensitive adhesives are made from formulations
based on elastomer (natural, butyl, nitrile and styrene butadiene ther-
moplastic), acrylate, or silicone resins. The advantages and limitations
of each of these pressure sensitive types are shown in Table 8.6. Pres-
sure sensitive adhesives are specifically formulated for good flexibility,
tack, and peel strength.
Pressure sensitive adhesives can be applied from solvent solutions,
water dispersions, or hot melts. Table 8.7 shows the properties and