In the long term, wood, adhesives, and bonded products
deteriorate at a rate determined by the levels of temperature,
moisture, stress and, in some instances, by concentrations of
certain chemicals and presence of microorganisms. Long-term
performance is equated with the ability of a product to resist
loss of a measured mechanical property over the time of
exposure. A durable product is one that shows no greater
loss of properties during its life in service than wood of the
same species and quality.
Many adhesives in bonded products have decades of documented
performance in many environments. Thus, it is
possible to predict with a high degree of certainty the longterm
performance of similar products. Well-designed and
well-made joints with any of the commonly used woodworking
adhesives will retain their strength indefinitely if the
moisture content of the wood does not exceed approximately
15% and if the temperature remains within the range of human
comfort. However, some adhesives deteriorate when
exposed either intermittently or continuously to temperatures
greater than 38°C (100°F) for long periods. Low temperatures
seem to have no significant effect on strength of
bonded joints.
Products made with phenol-formaldehyde, resorcinolformaldehyde,
and phenol-resorcinol-formaldehyde adhesives
have proven to be more durable than wood when exposed to
warm and humid environments, water, alternate wetting and
drying, and even temperatures sufficiently high to char wood.
These adhesives are entirely adequate for use in products that
are exposed indefinitely to the weather (Fig. 9–12).
Products well-made with melamine-formaldehyde,
melamine-urea-formaldehyde, and urea-formaldehyde resin
adhesives have proven to be less durable than wood.
Melamine-formaldehyde is only slightly less durable than
phenol-formaldehyde or resorcinol-formaldehyde, but it is
still considered acceptable for structural products. Although
considered less durable, melamine-urea-formaldehyde is also
accepted in structural products at a melamine-to-urea ratio of
60:40. Urea-formaldehyde resin is susceptible to deterioration
by heat and moisture (Fig. 9–12).
Products bonded with polyvinyl acetate and protein-based
adhesives will not withstand prolonged exposure to water or
repeated high–low moisture content cycling in bonds of high
density woods. However, if they are properly formulated,
these adhesives are durable in a normal interior environment.
Some isocyanate, epoxy, polyurethane, and cross-linked
polyvinyl acetate adhesives are durable enough to use on
lower density species even under exterior conditions, but
exterior exposure must be limited for most of these. Some
elastomer-based adhesives may be durable enough for
limited exposure to moisture with lower density species in
nonstructural applications or in structural applications when
used in conjunction with approved nailing schedules. Polyurethane
adhesives that chemically cure and still remain flexible
are among the most durable construction adhesives.