Ester acceleration mechanisms in phenol-formaldehyde resin adhesives

An analysis of phenol-formaldehyde (PF) resins obtained by the addition of 0.5-5% glycerol triacetate (triacetin) as an accelerator during resin preparation showed the presence of intermediates involved in the acceleration mechanism. C-13-NMR spectroscopy, matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopy (MS), and gel permeation chromatography allowed us to identify some of the intermediates left over in the PF resin itself. The permanence in the resin of these labile intermediates, not easily observed otherwise, appeared to be due to the reaching of the diffusion-control led phase of the reaction. The mechanism involved appeared considerably more complex and different from any of the mechanisms presented previously. As a consequence of the evident complexity of the mechanism, it was not really possible to advance a complete mechanism of the reaction nor determine the real cause of the increase in the strength of the final network. The mechanism involved the phenate ion of the resin to apparently give a carbonyl or carboxyl group attached to the aromatic ring. Either directly or by subsequent rapid rearrangement after the initial attack, these C=O groups were found on sites different from the ortho position. The appearance gathered from NMR shift calculation indicated preferential positioning or repositioning to the para site and, surprisingly, to the meta sites of the phenolic ring. The shifts of these C=O groups correspond to those of an anhydride and to no other intermediate structures previously thought of. Anhydride-like bridges were clearly shown by MALDI-TOF MS to contribute to oligomer structures in which linkages between Phenol rings were mixed methylene bridges and anhydride bridges. These Structures appeared to be temporary, possibly due to the instability of the anhydride bridges; hence, they were in small proportions at any given moment of the reaction. MALDI-TOF analysis clearly indicated that these structures were at some moment an integral part of the structure of the liquid resin and that they existed parallel to the methylene bridges pertaining to a normal PF resin structure. Previous spectra showed that similar but not identical intermediates were present also in organic and inorganic catalyzed PF resins. (c) 2006 Wiley Periodicals, Inc.