Page 442 - Handbook of Adhesives and Sealants

382 Chapter Ten

into two distinctive groups: saturated (thermoplastic) and unsaturated
(thermosetting).
Unsaturated polyester resins are commonly used for casting, glass
fiber laminates, and adhesive systems. Unsaturated polyesters are
fast-curing, two-part systems that harden by the addition of catalysts,
usually peroxides. Styrene monomer is generally used as a reactive
diluent for polyester resins. Cure can occur at room or elevated tem-
peratures depending on the type of catalyst. Accelerators, such as co-
balt naphthalene, are sometimes incorporated into the resin to speed
cure.
Unsaturated polyester adhesives exhibit greater shrinkage during
cure and poorer chemical resistance than epoxy adhesives. Certain
types of polyesters are inhibited from curing by the presence of air
leaving a tacky resin surface. However, they cure fully when protected
from air by covering or enclosing between two substrates. Depending
on the type of polyester resin used, the adhesives can be quite flexible
or very rigid. Tensile shear strengths from several hundred to
thousands of psi can be realized. Polyester adhesives bond to metals,
ceramics, and glass reinforced laminates. Applications include patch-
ing kits for the repair of automobile bodies and repair cement for con-
crete flooring. Polyester adhesives also have strong bond strength to
glass-reinforced polyester laminates.
Saturated polyester resins exhibit high peel strength and are used
to laminate plastic films such as polyethylene terephthalate (Mylar).
They also offer excellent clarity and color stability and have been used
for optical equipment. This type of polyester adhesive, available in
solution or solid film form, can be used as a high performance hot melt
system. They can also be chemically crosslinked with curing agents
such as the isocyanates for improved thermal and chemical stability.
Typical applications for saturated polyester adhesive films are the
lamination of flexible printed circuitry material where the metallic
conductor foil is bonded to the polymeric dielectric film in a continuous
lamination process.

10.3.9 Polyurethanes
Like epoxies, polyurethane reactive adhesives include systems that
are available as 100% solids or solvent based and as one or two part
formulations. These adhesives can cure at room or elevated tempera-
tures. Unlike epoxies, the polyurethane adhesives are flexible and
have both relatively high shear and peel strength.
Polyurethane adhesives are made with isocyanate resins as building
blocks. Most adhesives are made with either toluene diisocyanate