They are an important class of
adhesive used in the manufacture of outdoor grade plywood. In most of these
applications the adhesive is 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 140°C for several
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 fungus.
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 temperatures. 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 addition of elastomeric resins to improve toughness and peel
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
Because of their good peel strength and elevated-temperature properties,
nitrile-phenolic adhesives are used commonly for bonding linings to brake
shoes. They are also used in the aircraft industry for bonding of aluminum
facings to honeycomb cores. Other applications include electronics, footwear,
and furniture assembly.
are based on a combination of phenolic resin with polyvinyl
formal or polyvinyl butyral resins. Because of their excellent shear and peel
strength, vinyl-phenolic adhesives are one of the most successful structural
adhesives for metal. Room-temperature shear strength as high as 5,000 psi is
available. Maximum operating temperature, however, is only 93°C because the
thermoplastic constituent softens at elevated temperatures. Chemical resistance
and impact strength are excellent.
Vinyl-phenolic adhesives are
supplied in solvent solutions and as supported and unsupported film. The
adhesive cures rapidly at elevated temperatures under pressure. They are
generally used to bond
metals, rubbers, and plastics to
themselves or each other. A major application of vinyl-phenolic adhesive is the
bonding of copper sheet to plastic laminate in printed-circuit-board
are used to bond a variety of substrates. Normal service
temperature is from 20 to +93°C. Because of high resistance to creep and most
service environments, neoprene-phenolic joints can withstand prolonged stress.
Fatigue and impact strengths are also excellent. Shear strength, however, is
lower than that of other modified phenolic adhesives.
Temperatures over 150°C and pressure greater than
50 psi are needed for cure. Neoprene-phenolic adhesives are available as solvent
solutions and film. During cure these adhesives are quite sensitive to surface
contamination from atmospheric moisture and other processing variables.