Holz als Roh- und Werkstoffs2 (1994) 218-2.22 9 Springer-Verlag 1994 Fast vs. slow-reacting non-modified tannin extracts for exterior particleboard adhesives A. Pizzi, A. Stephanou 218 Non-fortified, non-modified tannin extracts, rather than chemi- cally-modified tannin adhesives intermediates, can be used as effectively to produce excellent exterior grade particleboard at fast pressing times of industrial singnificance, with considerable advantages in both handling and cost. This can be achieved by simple pH-controlled reactivity adjustments of the tannin extract in the glue mix. This new concept is shown to be applicable to all types of polyflavanoid tannin extracts: Pine, mimosa, pecan, que- bracho, and gambler tannin extracts are compared as regards their performance and capabilities. The new concept applied to the faster-reacting tannins, such as pine tannin and pecan tannin extracts, is shown to produce fast hot-pressing rates and excel- lent high moisture content tolerances which are undreamed of for the slower-reacting tannins, modified or non-modified, which are traditionally associated with thermosetting wood adhesives. Industrially used techniques to control well viscosity and pot-life of the glue-mixes of accelerated tannin adhesives are also pre- sented. 5chnell- start langsam-reagierende, nicht-modifizierte Tanninextraktezum Verleimen yon 5panplatten for die Aut~enverwenduno Nicht-verst~irkte und unmodifizierte Tanninextrakte k6nnen anstelle von chemisch modifizierten Tanninen zur Herstellung hochwertiger Spanplatten far die Auflenverwendung eingesetzt werden. Sie erm6glichen kurze Pret3zeiten und bieten sowohl auf der Kostenseite als auch in der Handhabung bedeutende Vorteile. Dies kann durch einfache pH-Steuerung der Reaktivit~it des Tanninenextraktes im Klebstoffgemisch erreicht werden. Das neue Konzept konnte ftir alle Typen von Polyflavonid-Tanninen best~tigt werden. So wurden Tanninextrakte aus Kiefern, Mimosa, Hickory, Quebracho und Gambir (Katechu) aufihre Verwertbarkeit untersucht. Mit den schnell-reagierenden Tanninen aus Kiefern und Hickory konnten sehr kurze Preflzeiten und ausgezeichnete Feuchtetoleranzen gegentiber den Sp~nen erzielt werden, die bisher bei herk6mmlichen, langsam- reagierenden Tanninklebern unvorstellbar waren. Weiterhin werden industrielle Verfahren zur Kontrolle der Viskosit~it und der Topfzeit der Klebstoffgemische aus diesen beschleunigt reagierenden Harzen vorgestellt. 1 Introduction Tannin-based adhesives for exterior grade particleboard based on mimosa tannin extract and to a lesser extent on quebracho Tannin extract have been in commercial operation in several Southern Hemisphere countries since 1971(Pizzi 1991). The adhe- sives based on these two tannins are much slower reacting with formaldehyde than the newer generation of adhesives based on the faster reacting procyanidin/prodelphinidin-type tannins such as pine bark and pecan nut pith tannin extracts, introduced since University of the Witwatersrand, Johannesburg, Private Bag3 Wits 2050, South Africa Dedicated to Prof, Dr. D. Fengel on occasion of his retirement 199o (Pizzi 1991; Pizzi et al. 1993). The structural reasons for the differences in reactivity towards formaldehyde of the two slow reacting tannins, mimosa and quebracho, and two faster-react- ing tannins, pine and pecan, are well known. They are based on the structure of the A-rings of the majority of flavonoid units composing the two sets of tannins: resorcinol-like in mimosa and quebracho, phloroglucinol-like in pine and pecan nut tannins. (Pizzi 1983; Pizzi and Stephanou a993a). Their reactivity has always been thought to put the fast tannins at a disadvantage in relation to the slower-reacting tannins traditionally used for wood adhesives (Pizzi 1983). This because hardening reactions under industrial conditions were thought to be not easily con- trollable, leading to problems of precuring, and particularly because of the traditional pHs of operation of tannin adhesives, these because of the pot-life of the adhesive glue mix would be too short (Pizzi 1983). Furthermore, because the main tannin adhesive based on mimosa was based on structural modifica- tions of the tannin extract itself (Pizzi 1978), without any synthe- tic resins fortification the ineffectiveness of such modifications on the fast tannins appeared to deny their use for lower-cost nmffortified exterior grade adhesives. This paper describes instead how the high reactivity of these tannins permits application feats in wood adhesives which are impossible to achieve with the slower, traditional tannins and introduces the solutions now used industrially to overcome the problem of pot-life of the glue-mix which has been perceived as a blocking problem in the industrial exploitation of the fast tan- nins. It also addresses and corrects the misconception that any flavonoid tannin slower or faster-reacting, needs to be chemical- ly or structurally modified, or fortified by addition of synthetic resins, to perform as an excellent thermosetting wood adhesive. 2 Experimental 2.1 Gel time and reactivities Gel time of the reaction of tannin extracts with formaldehyde are an indication of the reactivity of the tannin sites and hence of some of the likely structural characteristics of the tannin repeat- ing units important for the preparation of adhesives and resins (Pizzi and Stephanou 1993; Pizzi et al. 1993e). The gel time dependance on pH at 94~ was determined over the pH range 4, 5 to 1o for five commercial tannin extracts, namely mimosa (Acacia Mearnsii, de Wildt) bark extract, quebracho (Schinopsis balansae) would extract, pine (Pinus radiata) bark extract, pecan (Carya illinoensis) nut pith extract and Gambier (Uncaria Gambit) shoots extract and for two modified, non-fortified, tan- nin adhesives (Pizzi and 1978; Pizzi and Stephanou 1993b) based respectively on mimosa and quebracho tannins. The results are shown in Figs. 1 and 2. Gelation is defined as the point at which the polycondensates formed by the reaction of tannin extract and formaldehyde ceases to be a viscous liquid and becomes a soft, elastic, rubbery solid, lO g of liquid tannin extract 40% (quebra-