Page 342 - Handbook of Adhesives and Sealants

300 Chapter Eight

For tape or film adhesives, this hardening process is usually accom-
panied by extruding, calendering, or casting the adhesive formulation
into thin films that are typically 5–10 mils thick. The product is B-
staged to a condition where it is either tack free or slightly tacky to
help application to vertical and contoured substrates. These films may
be in the form of either unsupported sheet or they may be reinforced
with glass fabric, paper, or another reinforcing media. These latter
systems are called ‘‘supported’’ tape and film adhesives. B-stage sup-
ported film is also called prepreg. Once the tape or film is in place
between substrates, the joint is heated under pressure so that the
B-stage adhesive becomes slightly fluid, wets the substrate, and flows
into the micro-roughness on the substrate. With additional heat and
time, the adhesive completely cures to a thermosetting structure.
B-staged solid adhesive formulations may also be ground to a pow-
der form and applied to the substrate by sifting onto the surface, or
the powder might be electrodeposited or coated on the surface in an-
other manner. The powder can also be formed into shapes or preforms
by the application of pressure and a die, much like how pharmaceu-
tical tablets are made. In this way shaped preforms can be made that
will conform to a specific joint geometry.

8.2.3.2 Solvent or water loss. Solvent solutions and water based la-
texes and dispersions harden by the evaporation of their carrier ma-
terial—either solvent or water. The carrier material’s function is sim-
ply to lower the viscosity of the adhesive so that it can be easily
applied to the substrate. Once applied, the water or solvent must be
removed either by evaporation into the air or by diffusion into a porous
substrate. Thus, solvent and water based systems often find use in
applications with porous substrates such as wood, paper, leather, and
fabric.
Once applied and dried, the adhesive can then provide a bond in a
number of ways. It could simply harden into a cohesive resin mass,
such as polyvinyl acetate or wood glues. It could form a film that is
then reactivated by solvent or water or by heat as in the case of a
laminating adhesive. It could also form a film that when dry has a
high degree of tack so as to be a pressure sensitive adhesive. There
are primarily four types of adhesives that harden by loss of solvent or
water:

Contact adhesives—The adhesive is applied to both substrates, sol-
vent is removed, and the substrates are mated under pressure so
that the adhesive coatings knit together.
Pressure sensitive adhesives—The adhesive is applied to one or
both substrates or to a carrier (film, cloth, etc.). Once the solvent is