Page 323 - Handbook of Adhesives and Sealants

284 Chapter Eight

working life (sometimes defined as pot life or gel time) is limited. Sin-
gle part systems have all the reactive components in a single premixed
product. Generally, the single part adhesives require elevated temper-
ature cure. They have a limited shelf life, often requiring refrigeration.
Single part thermosetting systems are also available that cure at room
temperature by chemical reaction with the moisture in the air, by ex-
posure to radiation (visible, UV, electron beam, etc.), or by catalytic
reaction with a substrate surface.
Because molecules of thermosetting resins are densely crosslinked,
their resistance to heat and solvents is good, and they show little elas-
tic deformation under load at elevated temperatures. As a result, most
structural adhesives tend to be formulated with polymeric resins hav-
ing a thermosetting molecular structure.

8.2.2.2 Thermoplastic adhesives. Thermoplastic adhesives differ from
thermosets in that they do not cure or set under heat. Thermoplastics
are originally solid polymers merely soften or melt when heated. Their
molecular structure is either linear or branched as shown in Fig. 8.1.
Since thermoplastic molecules do not cure into a crosslinked struc-
ture, they can be melted with application of heat and then applied to
a substrate. Thermal aging, brought about by repeated exposure to
the high temperatures required for melting, causes eventual degra-
dation of the material through oxidation, and this limits the number
of reheat cycles. Once applied to the substrate, the parts are joined
and the adhesive hardens by cooling. Hot-melt adhesives, commonly
used in packaging, are examples of a solid thermoplastic material that
is applied in a molten state. Adhesion develops as the melt solidifies
during cooling.
Thermoplastics can also be dissolved in solvent to produce a flow-
able solution and then rehardened on evaporation of the solvent. Ther-
moplastic resins can also be dispersed in water as latex or emulsions.
These products harden on evaporation of the water. Wood glues, a
common household item, are thermoplastic resins that are dispersed
in water as either a latex or emulsion. They harden by evaporation of
the water and coalescence of the resin into a film form.
Thermoplastic adhesives are also preapplied to a substrate so that
they can be ‘‘activated’’ at an appropriate time. Some thermoplastic
adhesives make good use of this characteristic and are marketed as
heat, solvent, or moisture activated adhesives. The best known ex-
ample of a moisture activated adhesive is the mailing envelope ad-
hesive that is activated by moisture to become tacky and somewhat
flowable. When the moisture evaporates, the bond sets.
Thermoplastic adhesives have a more limited operating tempera-
ture range than thermosetting types. Although certain thermoplastics
may provide excellent tensile shear strength at relatively moderate