Page 432 - Handbook of Adhesives and Sealants

374 Chapter Ten

yurethane adhesives. These primers are also often used to improve
the adhesion of nylon tire cord to rubber.
Resorcinol and phenol-resorcinol adhesives cure on the addition of
formaldehyde. Commercially they are available as two part systems.
The resorcinol part is in liquid form (generally resin in a mixture of
alcohol and water), and it is mixed with a powder hardener (parafor-
maldehyde with fillers) before application. Once mixed, the pot life is
limited, and the mixed resin generates considerable heat of exotherm.
Generally, these adhesives are cold-setting, but they can also be ap-
plied to a substrate, dried, and then hot-pressed. Curing at room tem-
perature normally takes 8–12 hrs. For maximum durability, a cure at
moderately elevated temperature is recommended.

10.3.4 Melamine formaldehyde and
urea formaldehyde
Melamine-formaldehyde resins are used as colorless adhesives for
wood. These thermosetting resins are condensation products of un-
substituted melamine and formaldehyde. Because of their high cost,
they are sometimes blended with urea formaldehyde. Melamine for-
maldehyde is usually supplied in powder form and reconstituted with
water; a hardener is added at the time of use. Temperature of about
200 F is necessary for cure. Adhesive strength is greater than the
strength of wood.
Urea-formaldehyde adhesives are not as strong or as moisture-
resistant as the resorcinols. However, they are inexpensive, and both
hot-and-cold setting types are available. Maximum service tempera-
ture of a urea formaldehyde adhesive is approximately 140 F. Cold-
water resistance is good, but boiling water resistance may be improved
by the addition of melamine-formaldehyde or phenol-resorcinol resins.
Urea-based adhesives are used mainly in plywood manufacture.

10.3.5 Phenolics
Phenolic resins are the condensation product of phenol and formal-
dehyde. Phenolic or phenol formaldehyde is primarily used as an ad-
hesive for bonding wood. Because of their good heat resistance and
dimensional stability, they have also been used in brake linings, ab-
rasive wheels, sandpaper, and foundry molds. Solvent solutions of phe-
nolic resins filled with clay have been used as glass to metal adhesive
for attaching metal bases to light bulbs. Phenolics are relatively in-
expensive.
They are an important class of adhesive used in the manufacture of
outdoor grade plywood. In most of these applications the adhesive is