Page 433 - Handbook of Adhesives and Sealants

Adhesives Families 375

applied as a solution in alcohol, acetone, or water. It is coated on the
substrate, dried, and then cured under heat and pressure. However,
several forms of phenolic adhesives are available including spray dried
powders that are dissolved in water for application and films. Curing
is accomplished at temperature of approximately 280 F for several
minutes.
Adhesives for plywood are essentially solutions of low molecular
weight condensation products of phenol and formaldehyde in aqueous
sodium hydroxide. They provide bond strengths that are generally
greater than the strength of the wood substrate. Durability is good,
and the bond is essentially unaffected by boiling water, mold, or fun-
gus. These adhesives are suitable for exterior use, and their properties
are not appreciably affected by low temperatures. In the presence of
alkaline catalysts, crosslinking is brought about at elevated temper-
atures. Acid catalysts give room temperature cures, but also cause
degradation of wood and paper. Phenol formaldehyde resin is available
in the form of glue film, carried on tissue paper. Although expensive,
this form of adhesive is useful for faying of very thin or highly porous
veneers.
Phenolic resins have also been used to bond metal to glass. The bond
is somewhat brittle, however, and tends to shatter under impact or
vibration. As a result, phenolic resins are often modified by the addi-
tion of elastomeric resins to improve toughness and peel strength.

10.3.6 Modified phenolics
By modifying phenolic resin with various synthetic rubbers and ther-
moplastic materials, flexibility is greatly improved. The modified ad-
hesive is well suited for structural bonding of many materials, notably
metals. The most common type of modified phenolic adhesives are
vinyl-phenolic, nitrile-phenolic, and neoprene-phenolic. Some commer-
cially available phenolic-alloy adhesives are described in Table 10.13.

10.3.6.1 Nitrile-phenolic. Certain blends of phenolic resins modified
with nitrile rubber produce adhesives useful up to 300 F continuously.
Typical formulations may contain up to 50% by weight nitrile rubber.
The major advantage of incorporating nitrile into phenolic resin is to
improve the peel strength of the phenolic without significant reduction
in high temperature strength.
On metals, nitrile-phenolics offer shear strength of up to 5,000 psi
along with excellent peel and fatigue properties. Good bond strengths
can also be achieved on rubber, plastics, and glass. These adhesives
have high impact strength and elevated temperature capability during
service. Because of their good peel strength and elevated-temperature