Page 358 - Handbook of Adhesives and Sealants

Adhesive Classifications 313

the articles are mated, or if one of the substrates is porous, the solvent
can evaporate through the pores.
Solvent-based adhesives may be chemically crosslinked, usually
through the application of a curing agent (either incorporated directly
into single part systems or added as a second part to the base resin)
and heat. The application of a solid thermosetting epoxy via solution
coating is an example. Such an adhesive may be used to coat and then
bond laminations of electrical steel for motor applications. The adhe-
sive is applied and then dried through solvent evaporation. Once the
parts are ready to be bonded, they are stacked under pressure. Heat
is then applied to achieve flow and final chemical crosslinking.
Solvent solution adhesives may also be uncrosslinked as in the case
of many pressure sensitive adhesives. In this case, a solid polymeric
resin, such as polyisobutylene, may be applied to a backing material
via solvent solution. Once the solvent is driven away or evaporated,
the resin increases in viscosity to a consistency in which it can operate
as a pressure sensitive adhesive.

8.2.4.3 Water-based adhesives. Water-based adhesives are usually
emulsions of thermoplastic resins. The properties of the emulsion are
derived from the polymer employed as well as the system used to
emulsify the polymer in water. There are several additives necessary
to stabilize and protect the emulsion. Other additives are used to ad-
just tack, drying time, viscosity, storage stability, etc.
Like the solvent-based adhesives, the water carrier is evaporated
through the air or diffused into the porous substrate. When dry the
resulting adhesive can be either a brittle, hard resin or a flexible,
tacky film depending on the adhesive formulation. Water-based ad-
hesives are like solvent-based adhesives in that they are formulated
as contact, pressure sensitive, reactivatable, and resinous adhesive
systems. They are used much the same way as their solvent based
counterparts.
Perhaps the most widely used emulsion-based adhesive is the poly-
vinyl acetate-polyvinyl alcohol copolymer that is known as ‘‘white’’
glue or wood glue. This adhesive hardens to a relatively rigid solid
when the water diffuses through the substrate.
Water-based adhesives are considered as replacements for solvent-
based adhesives for the purpose of reducing volatile organic emissions
in a manufacturing operation. Although there are several water-based
contact and pressure sensitive adhesives on the market, they are
slower drying than solvent-based adhesives, requiring about three
times more heat to dry. Forced drying of water-based adhesives costs
energy and causes corrosion problems in the ovens that are used. Wa-
ter-based contact adhesives also have a lack of immediate bonding