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					    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 “amine-
formaldehyde” 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
   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 (cross-
linked) molecular
structure
               UF Condensation
Controlled by:
   Temperature (150 oC cure temperature)
   Time
   pH (>7, stable; <7, condensation)
      Reaction accelerated by “curing agent” or “hardener”
           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:
   Resole (cross-linking thermosetting resin)
   Novolac (resin with some thermoplastic
    properties)
Synthesis
Condensation reactions (resin cure)
       PF Synthesis: Resoles
Two stages:
   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.
                        Three-
                        dimensional
                        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                             Novolac
   A-stage is a low molecular        Fewer methylol groups,
    weight “impregnating” resin        thus much less cross-
    (use for making Impreg,            linking (linear structure)
    Compreg, and laminating           Formaldehyde must be
    materials)                         supplied to cure, along with
   B-stage is a bonding               heat
    (adhesive) resin                  Resin is soluble and
   Needs ONLY heat to                 thermoplastic
    polymerize                        Has an indefinite shelf life
   Continues to polymerize in        May be supplied in sheet or
    storage (limited shelf life)       flake form (dry)