| Summary: | In North America, almost all exterior grade plywood, particleboard, or flakeboard
is made with phenol formaldehyde (PF) resins. Formulations are available which can
satisfy a wide range of working and performance property demands of the users. PF
resins suffer from a few disadvantages which include the distinct dark brown color in
the cured glueline and the relatively high temperature (120°C at glueline) required for
curing. Modification of PF resins with several different chemical groups has been tried
to reduce or remove the drawbacks associated with PF resin. One particular
modification technique involves the addition of melamine into the PF resin system.
Although modification using melamine has been carried out in Europe and in Japan,
where the melamine is cheaper than phenol, no such attempt has been made in North
America, where the price of melamine is higher than that of phenol. Even though
melamine is an expensive chemical compared to phenol, its advantages may lie in the
lower cure temperatures and shorter press cycles that are required during hot pressing
operations.
In this study, an array of phenol-melamine-formaldehyde (PMF) resins were
synthesized by varying the formaldehyde/phenol and formaldehyde/melamine ratios at
pH 7.5 and 9.0. Melamine formaldehyde (MF) and PF resins were also synthesized for
comparison. The structure of all these resins, as well as commercial MF and
commercial PF resins, was characterized using Fourier Transform Infrared (FTIR)
spectroscopy, Proton Nuclear Magnetic Resonance (¹H-NMR) spectroscopy, Gel
Permeation Chromatography (GPC) and Differential Scanning Calorimetry (DSC). The
bond performance of these resins was evaluated by producing 3-ply plywood panels
and testing these panels for shear strength and wood failure under dry, wet and boiled
conditions.
Better bond performance was observed for PMF resins that were synthesized at
pH 7.5 than the ones synthesized at pH 9.0. Very reasonable wood failure results were
obtained for these resins and also for MF resins (synthesized and commercial) at lower
press temperatures(120°C) and lower press times (3 min.) compared with the PF resin.
The existence of co-condensation in PMF resins was confirmed with the IR and NMR
analysis. The majority of the co-condensation in resins prepared at pH 7.5 was by way
of methylene bridges. The bond performance was attributed to the presence of
melamine in the system and not to the level of coplymerization that occured between
phenol and melamine. These resins and also the MF resins were of much lower
molecular weight (<1,500) in comparison with the PF (>20,000) resins. DSC data of
PMF resins cooked at pH 7.5 showed that these resins exhibit two exotherms, the first
exotherm (150°C) corresponding to the condensation reactions which take place during
curing while the second exotherm (220°C) possibly corresponding to the elimination of
formaldehyde from the dimethylene ether links to form methylene cross-links. === Forestry, Faculty of === Graduate
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