The effect of molar ratio and pH during synthesis on the performance of phenol-melamine-formaldehyde adhesives

• Main Author: Sidhu, Avtar Singh
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/7938

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.