Forestry 485
Lecture 3-2: Resorcinol,
melamine, and isocyanate
adhesives
Important Wood Adhesives
We have seen that UF and PF are the primary
wood adhesives, with usage of >1.5 and 1.0 x
109 dry pounds annually in the U.S., respectively
Related adhesive systems occupy special
niches, e.g., resorcinol and melamine
A different (chemically speaking) type of
adhesive, isocyanate (or MDI) is rapidly growing
in importance, particularly for OSB.
Resorcinol Formaldehyde (RF)
Resorcinol is closely related to phenol in
terms of chemical structure
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Resorcinol has two aromatic OH groups,
versus one for phenol
This renders resorcinol much more reactive
than phenol
Resorcinol plus formaldehyde at room
temperature reacts quickly to form a gel…this
“one stage” system is unsuitable for wood
adhesives
RF Synthesis
Does this structure remind you of
anything you’ve seen before?
RF Cure
High reactivity (10-15x PF) precludes formation of
resoles
Will cure at room temperature-useful for large gluelam
beams
Two-part system used for finger jointing lumber
RF Characteristics
Room temperature cure
Short pot life, generally 2-4 hours
High bond strength (fully structural)
Works well for difficult-to-bond substrates,
such as fire retardant or preservativetreated wood (FPL has developed
activating systems for these applications)
Dark reddish-brown glueline
Phenol-resorcinol formaldehyde
(PRF)
Resorcinol is expensive compared to
phenol
Since the two are very similar chemically,
phenol and resorcinol may be combined to
form a hybrid PRF adhesive system
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Example: RF may be added as a “hardener”
to resole PF at the time of use.
Melamine Formaldehyde (MF)
Like UF, this is an “amine formaldehyde”
resin
Unlike UF, MF forms fully waterproof
bonds
MF and UF may be blended to form
“fortified UF” or “melamine-urea
formaldehyde (MUF)” resins to take
advantage of MF adhesive qualities and
low cost advantage of UF
Melamine formaldehyde synthesis
Note that methylolation and polymerization
occur in similar fashion to UF.
As for UF, an acidic hardener is added to initiate polymerization.
MF Characteristics
High water resistance, high strength, high cost,
colorless
Has short storage life in liquid form; usually
supplied as powder
Used widely for surface coatings and laminates
Increased use in recent years for composite
flooring. MF content upwards of 30% in these
applications
Europeans make wide use of MF for exterior
products (e.g., hardboard siding)
Isocyanate Adhesives
Isocyanates are a separate class of
thermosetting adhesives
Isocyanate
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R-N=C=O where R is aliphatic or aromatic
Aromatic isocyanates are more important
as wood adhesives
Polyurethanes are also based on the
chemistry of these materials
Isocyanates: Generated from
toluene diamine and phosgene
Toluene diisocyanate –
volatile; not used for wood
adhesives
Diphenylmethane
diisocyanate – has low
volatility, an important
wood adhesive
Isocyanate=MDI
Two isocyanates are important as wood
adhesives:
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4, 4’ diphenylmethane diisocyanate
4,4’,4’’ triphenylmethane triisocyanate
Wood adhesives are a mixture of such
polymers and are generally called
Polymeric Isocyanates or pMDI(s) or
MDI(s) or simply isocyanate(s).
Cure Reaction of Isocyanate
Adhesives
R-N=C=O + HO-R’
R-NHC=O
OR’
urethane
Isocyanate groups react with hydroxyls to
form a urethane bridge.
Such reactions theoretically possible with
wood hydroxyls; chemical evidence is
limited, however.
Advantages of MDI
Dry strength comparable to other resins
MDI water resistance good; >UF, but < PF
long-term
No water in resin formulation, thus it is
possible to use higher MC furnish
Low resin application rates (typically <4%)
Can bond difficult materials, such as ag
particles (e.g., straw particleboard)
Disadvantages of MDI
High cost
Bonds to metal; must use platen release agent
or use only as core resin
Has low vapor pressure (this is an advantage),
but vapors are irritating to skin and eyes
Airborne MDI aerosols/blended particulates can
cause respiratory problems
Sticks in resin blenders…many factories use
jackhammers to periodically clean out blenders