Page 369 - Handbook of Adhesives and Sealants

Adhesive Composition and Formulation 325

Figure 9.2 Dependence of tack, peel, and shear on resin/tackifier ratio. 4

rials which when added to the adhesive increase the viscosity when it
is under rest. Therefore, thixotropic agents provide sag resistance and
the ability for an adhesive to remain in place on a vertical substrate.
However, when a slight force is applied, such as in the act of stirring
or extruding the adhesive, the system then acts as if it had a lower
viscosity and flows with relative ease. Thixotropy will be discussed
further in the next section. Scrims, carriers and woven reinforcements
are other methods commonly used to control bondline thickness.
The removal of solvent from a true solution of a polymer leaves a
film of the polymer. If the same polymer is available as an emulsion,
removal of the liquid by evaporation does not necessarily leave a co-
herent film. The individual globulars of polymer will only merge to
form a film if the polymer is well above its glass transition tempera-
ture. Drying of some emulsions will produce a powder unless its glass
transition has first been lowered by the addition of a plasticizer or
film former to the emulsion. Film formers are carefully chosen to lower
the glass transition temperature without appreciably lowering the
strength of the film.
Many polymers have a limited life and are subject to aging processes
even before they are used in production. To delay these aging pro-
cesses, antioxidants, antihydrolysis agents, and stabilizers are added
to adhesive formulations. Antifungal agents or biocides are used in
many water-based adhesive systems.
Aqueous suspensions comprise a wide range of adhesives. These
contain as additives the various soaps, surfactants, and wetting agents
necessary to stabilize the emulsion or latex. Additives are also incor-
porated into aqueous formulations to provide system stability under
repeated freeze-thaw cycles during storage.
This description of various additives used in the formulation of ad-
hesives is generic and brief. There are literally thousands of additives
that can be used in adhesive systems. The choice depends on the com-
position of the adhesive system, how it is to be used, system cost, and