Holz als Roh- und Werkstoff 53 (1995) 2m-2o4 © Springer-Verlag 1995 Autocondensation-based, zero-emission,tannin adhesives for particleboard A. Pizzi, N. Meikleham, B. Dombo, W. Roll Interior grade particleboard prepared with tannin adhesives to which no aldehyde hardener was added were prepared. The hardening of the tannin adhesive was induced by the reaction of autocondensation tannins undergo when catalyzed by a lignocetlulosic suhstrate or by a weak Lewis acid as alkali dis- solved silica. The more alkaline the tannin extract solution the higher the ceiling internal bond strength that the particleboard bonded with it can reach. Differences between four commercial tannins are discussed. Pecan nut tannin and pine tannin need only lignocellulosic induced autocondensation to give excellent interior grade particleboard. Slower reacting tannins such as mimosa need instead both the catalytic effect of the substrate as well as that of a weak Lewis acid such as silica. Only pecan nut tannin appears at this stage to be able to give boards of acceptable dry internal bond strength at industrially significant pressing times. Tanninkleberfor Spanplatten, hergestelltdutch Autokondensation und ohne Emissionyon Schadstoffen Spanplatten for die Innenverwendung wurden mit Tanninldebern hergesteltt ohne Zugabe yon Aldehyden als H~irter. Die H~irtung der Tannine erfolgt durch Autokondensation. Diese wird katalysiert durch ein lignocellu- losisches Material oder eine schwache Lewis-S~ure wie in Alkali gel6ster Kiesels~iure. ]e alkalischer die Tanninl6sung ist, desto h6her ist die Querzugfestigkeit, die erreicht werden kann. Unterschiede zwischen vier kommerziellen Tanninen werden diskutiert. Tannine aus Hickorynu6 und Kiefern k6nnen schon mit Lignocellulosen zur Autokondensation gebracht werden und eignen sich ftir hochgradige Spannplatten zur Innenverwendung. Langsamer ragierende Tannine aus Mimosa-Arten ben6tigen aufler diesem Substrat noch eine schwache Lewis-S~ure wie z.B. Kiesels~ure. Zur Zeit scheint aber nur Hickory-Tannin geeignet zu sein, Spanplatten mit ausreichender Querzugfestigkeit innerhalb industriell vertretbarer Pregzeiten herzustellen. 1 Introduction Thermosetting polyflavonoid tannin-formaldehyde resins have performed well in industrial applications as wood adhesives for exterior partideboard for more than 20 years (Pizzi 1983). While the reactions of these natural polymeric materials with formaldehyde to give polycondensates have been used exten- sively, autocondensation reactions characteristic of, and A. Pizzi ENSTIB, Universit~ de Nancy 1, BP 1041, Epinal, France A. Pizzi; N. Meikleham Dept. of Chemistry, University of Witwatersrand, ]ohannesburg, South Africa B. Dombo; W. Roll Bakelite AG, Duisburg, Germany inherent to tannins have never been used to prepare adhesives in absence of aldehydes. Tannins autocondensations are known but their effect on polymeric systems has neither been studied nor even been considered for resin preparation and hardening. Recently a predominantly prodelphinidin tannin, pecan (Carya iIlinoensis) nut pith tannin extract has been found to undergo with ease fairly rapid autocondensation reactions (Pizzi and Stephanou 1993a, b, c; Pizzi 1994). Although this reaction leads to noticeable increases in viscosity (Pizzi and Stephanou 1993a) of concentrated (40%) tannin solutions, gelling does not occur: the reaction by itself then does not lead to cross-linked, hardened resins. Even more recently however, a series of catalysts to induce accelerated autocondensation of this and other tannins has been described and its mechanism studied in depth on the natural polymers (Meitdeham et al. 1994). The autocondensation to gelation of tannin extracts has been found to be induced by weak Lewis acids (Meikleham et al. 1994; Pizzi 1994), and in particular by dissolved silica (Meikleham et al. 1994; Pizzi 1994), boric acid (Meikleham et aL 1994; Pizzi 1994) and others, when used in very small proportions (3%) of tannin extracts solids. For example, addi- tion of an alkaline solution of silica to a 40% tannin extract solution at a pH of 10 to 12 gels and hardens the tannin solu- tion in just a few minutes even at ambient temperature (Meikleham et al. 1994; Pizzi 1994). Even the addition of finely powdered silica ("silica smoke", Aerosil) to a tannin solution at pH 12 causes a similar effect (Meikleham et al. 1994; Pizzi 1994). A dissolved silica catalyzed system is of particular inter- est because both the tannin resin and the catalyst are of natural origin and not toxic. Even of greater interest has been the find- ing that the presence of a lignoceUulosic substrate catalyses and accelerates not only the curing of synthetic formaldehyde- based resins (Pizzi et al. 1995; Pizzi 1994) but also the autocon- densation and hardening of tannin extracts alone. This is par- ticularly noticeable at high pH (Pizzi et al. 1995; Pizzi 1994). The preparation of particleboard edusively by application of a tannin extract at high pH, with no formaldehyde, or rather no aldehydes being used, or at most by the addition of a few percentages of dissolved silica, is then a real and exciting possi- bility. It would give a panel completely composed of natural, non-toxic materials and consequently very environmentally acceptable. This paper then describes the results obtained by applying the induced, catalysed autocondensation of tannins for interior and exterior grade particleboard. 2 Particleboard preparation and testing 2.1 Substrate catalysis - Tannins alone Duplicate single layer laboratory particleboard of 400 × 400 × 12 mm dimension were prepared using industrial core particles 201