Page 470 - Handbook of Adhesives and Sealants

406 Chapter Ten

10.4.2.3 Polyvinyl alcohol. Polyvinyl alcohol is a water soluble ther-
moplastic synthetic resin that is supplied generally as a formulated
water solution for the bonding of porous materials such as paper and
textiles. With wood and paper, polyvinyl alcohol adhesives lose water
rapidly and consequently set quickly, allowing pressure release in
minutes after assembly. Wet tack properties, combined with quick set,
make them quite useful in automatic labeling operations, packaging,
and continuous laminating of foil and paper. Vinyl alcohol adhesives
can be heat sealed.
Water resistance is only fair but sufficient for most interior appli-
cations. Resistance to grease and oil is excellent. The adhesives are
also odorless and tasteless. They consequently find application in the
packaging of food.
The addition of polyvinyl alcohol to polyvinyl acetate emulsions is
often used to slow down the speed of set and to improve flexibility and
tack. However, the heat resistance and resistance to long term creep
of the pure polyvinyl acetate resin are generally lowered.

10.4.2.4 Thermoplastic elastomers. Thermoplastic elastomers are
block copolymers. The best known materials are styrene-butadiene.
They do not have the good adhesion characteristics of nitrile, neo-
prene, or natural rubber. However, they are lower in cost and have
better heat aging resistance than natural rubber. Styrene-butadiene
rubber (SBR) is the largest volume synthetic rubber manufactured in
the U.S. These materials are not useful as adhesives in their natural
state and are often heavily compounded with plasticizers, fillers, and
other resins.
SBR adhesives have very good toughness and low water absorption.
Formulated compositions have very good tensile strength character-
istics. They are used commonly as pressure sensitive adhesives, hot
melt adhesives, sealants, and binders. They are used for the splicing
of tire threads and as adhesives to bond fabrics to themselves as well
as other materials.

10.4.2.5 Ethylene vinyl acetate. This random copolymer is often for-
mulated as a hot-melt adhesive containing up to 30% vinyl acetate in
polyethylene. As with all hot melt formulations, melt viscosity is very
dependent on molecular weight of the base resin. Tackifiers are added
to reduce viscosity and improve wetting and adhesion. Waxes are
added to lower cost and reduce viscosity. Inorganic fillers are used to
lower cost and increase viscosity. Antioxidants are required to protect
the adhesive during application and during its service life.
Use includes cardboard, book binding, and iron-on clothing articles.
The adhesives will creep at relatively low stress. This can be lessened