Transcript Forestry 485

Forestry 485
Lecture 3-1: Urea and Phenol
Formaldehyde Adhesive Resins
Thermosetting Resins for Wood
Composites
Panel Products - three primary resins in use
in North America:
Urea formaldehyde (water resistant bond), used
exclusively for interior applications
Phenol formaldehyde (fully exterior, waterproof
bond), used almost exclusively for exterior
application and/or structural products
Polymeric MDI (water resistant to water proof),
used mainly as a core resin in OSB
Urea Formaldehyde (UF)
Synthesis
Cure chemistry
UF Synthesis
UF is an “amino resin” or “amineformaldehyde” class of adhesive
UF is synthesized by the reaction of urea
and formaldehyde to obtain stable, but
reactive intermediates
Storage stability due to reduced reactivity
of intermediates in alkaline conditions
(>pH 7)
UF Synthesis
Mild
alkali
(pH>7)
pH is used to control reaction rate. Occurs
very rapidly under acidic conditions.
Production of the reactive intermediates is
monitored and controlled by viscosity.
Note methylolation of urea residue.
UF Synthesis, continued
Other intermediates, such are
dimethylol urea, are also formed
Again, viscosity is monitored to indicate
the stage of the synthesis. Increasing
viscosity is correlated to polymer
growth; this indicates how “advanced”
the resin is
pH is maintained above 7 to slow
polymerization and thereby increase
storage life
UF Cure Chemistry
Polymerization of the reaction
intermediates
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Results from chemical reaction to build
polymer chain length and molecular weight
Polymerization is characterized as a
“condensation reaction”
Resin changes from low to high viscosity
liquid and ultimately to solid
Resin hardening corresponds to
development of mechanical strength of
adhesive bond
UF Condensation Reaction: Favored by acidic
conditions
Condensation
may be linear
or crosslinked
Result: Cured
network (crosslinked) molecular
structure
UF Condensation
Controlled by:
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Temperature (150 oC cure temperature)
Time
pH (>7, stable; <7, condensation)
Reaction accelerated by “curing agent” or “hardener”
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Mineral acid (e.g., HCl) or salts (e.g., ammonium chloride) See
Reading 3-1c
Ratio of formaldehyde to urea
Molar ratio varies from just >1.0 to approx. 1.5
Lower molar ratios favored in contemporary formulations to
reduce formaldehyde emissions. Most are <1.1
Formaldehyde scavengers may be used (see Reading 3-1d)
Phenol formaldehyde (PF)
adhesive resins
Two major types:
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Resole (cross-linking thermosetting resin)
Novolac (resin with some thermoplastic
properties)
Synthesis
Condensation reactions (resin cure)
PF Synthesis: Resoles
Two stages:
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Methylolation of phenol to produce methylol phenols.
First stage, or “A” stage, is alkaline catalyzed for the
synthesis of resoles
Alkaline A stage, P/F ratio=1:1 to 1:3 (i.e., molar
EXCESS of formaldehyde)
B stage is the condensation of intermediates to
produce water insoluble, but fusible products.
Reaction is monitored by viscosity and terminated
prior to complete polymerization.
Resole “A”
Methylolation may occur at ortho, meta, or para position.
Resole “B”
Note that an excess of formaldehyde is
used. This promotes full methylolation
of the phenol moieties.
Resole “C” (curing)
Process cure
temperature typically
190-200 oC. ONLY
heat is needed to
cure.
Threedimensional
cross-linked
structure is
formed.
Water
insoluble.
Novolac Synthesis
P/F ratio is > 1.0 (note error in figure on p. 78 of
Marra; in fact, note somewhat unorthodox
means of representing P:F ratio on p. 77 & 78)
Acid A stage, P/F ratio=1:1 to 1:0.6
Few methylols are formed; results in linear,
rather than cross-linked structure.
Has indefinite shelf life.
Soluble, (partially) thermoplastic.
Hardener in the form of additional formaldehyde
is needed for cure.
Novolac synthesis
Formaldehyde donors for cure:
Formalin
Paraformaldehyde
Hexamethylenetetramine (“hexa” hardener)
Resole vs. Novolac
Resole
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A-stage is a low molecular
weight “impregnating” resin
(use for making Impreg,
Compreg, and laminating
materials)
B-stage is a bonding
(adhesive) resin
Needs ONLY heat to
polymerize
Continues to polymerize in
storage (limited shelf life)
Novolac
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Fewer methylol groups,
thus much less crosslinking (linear structure)
Formaldehyde must be
supplied to cure, along with
heat
Resin is soluble and
thermoplastic
Has an indefinite shelf life
May be supplied in sheet or
flake form (dry)