Use of alkyl ketene dimer (AKD) for surface modification of particleboard chips

Ulrich Hundhausen 0 1 Holger Militz 0 1 Carsten Mai 0 1 0 U. Hundhausen ( ) H. Militz C. Mai Wood Biology and Wood Products, Burckhardt Institute, Georg-August University Gottingen , Busgenweg 4, 37077 Gottingen, Germany 1 Die Verwendung von Alkylketendimer (AKD) zur Oberfla chenmodifizierung von Spa nen f ur die Herstellung von Spanplatten The study objective was to impart urea-formaldehyde (UF) bonded particleboards higher and longer-lasting hydrophobicity than that resulting from conventionally used paraffin. Alkyl ketene dimer (AKD) is a paper sizing agent that can theoretically esterify wood compounds and result in a surface modification. Particles were 1) impregnated with an aqueous AKD-solution and cured at 130 C prior to gluing, or 2) sprayed with a mixture of AKD-solution and UF-resin in a single step. Boards with and without paraffin wax were used as controls. Thickness swelling after 2, 24, and 48 h immersion was decreased by 90, 62, and 59% when the chips were impregnated with AKD in comparison to untreated control boards. Water uptake after 2, 24, and 48 h was reduced by 91, 75, and 60%. AKD-impregnation with subsequent curing decreased the internal bond strength by 53%, indicating that AKD impedes the adhesion. The mixture of AKD and UF-glue did not result in considerable hydrophobicity. Increased methyl/methylene and carbonyl bands in FTIR-spectra after toluene-extraction suggest that AKD partially formed ester bonds at the wood surface. Zusammenfassung Das Ziel der Untersuchung bestand darin, Harnstoff-Formaldehyd (UF) gebundene Spanplatten starker und dauerhafter zu hydrophobieren als es mit konventionell verwendeten Paraffinprodukten mo glich ist. Alkylketendimer (AKD) wird in der Papierindustrie als Masseleimungsmittel eingesetzt und kann theoretisch mit Zellwandbestandteilen Esterbindungen eingehen. Holzspane wurden 1) mit einer wassrigen AKD Lo sung impragniert und bei 130 C getrocknet oder 2) mit einer AKD/UFHarz Lo sung bespru ht. Platten mit und ohne Paraffinbehandlung dienten als Referenzen. Im Vergleich zu Platten ohne Hydrophobierungsmittel wiesen Platten aus AKDimpragnierten Spanen nach 2-, 24- und 48-stu ndiger Wasserlagerung eine um 90, 62 und 59% verminderte Dickenquellung auf. Die Wasseraufnahme wurde um 91, 75 und 60% reduziert. Die AKD Impragnierung fu hrte zu einer Verringerung der Querzugfestigkeit um 53%. Dies deutet darauf hin, dass AKD die Verklebbarkeit negativ beeinflusst. Die Anwendung von AKD im Untermischverfahren fu hrte nur zu geringer Hydrophobierung. FTIR Messungen an Furnierstreifen vor und nach einer Toluolextraktion wiesen darauf hin, dass ein Teil des AKD u ber Esterbindungen auf der Holzoberflache fixiert ist. 1 Introduction The crucial application restrictions of wood-based panels are ascribed to their poor dimensional stability in changing climates and in contact with liquid water. While the swelling and shrinking behavior of boards parallel to the plane complies with that of solid wood parallel to the fiber, the values perpendicular to the plane are significantly greater (Amthor 1972). Swelling and shrinkage of wood particles and hydrolysis of the adhesives limit the application to low humidity conditions. The dimensional stability is influenced by board density, wood species, resin type, resin level, pressing conditions, particle geometry, and hydrophobic agents (Haaligan 1970). To date, paraffin wax emulsions are usually added to improve moisture resistance. The typical amount of wax ranges from 0.31% based on the oven-dry weight of the particles (Youngquist 1999). Higher additions show only marginal hydrophobic effect and cause reductions in bonding strength (Mu ller 1962, Amthor and Bo ttcher 1984); however, the major problem of paraffin is that it decelerates the uptake of liquid water and the swelling but cannot impart durable hydrophobicity (Youngquist 1999). Besides the use of paraffin as a hydrophobic agent, a variety of treatments have proved as effective means to increase the water repellency of particleboards. Filcock and Vinden (2000) investigated the use of isocyanate resin to add water repellency to particleboards. Isocyanateimpregnated, not edge-sealed panels showed a thickness swelling of approx. 5% after 24 h as compared to control boards that revealed 17%. Several researchers reported that heat-treatment of particles leads to enhanced dimensional stability (Tomek 1966, Tomimura and Matsuda 1986, Boonstra et al. 2006). In addition, numerous studies on chemical modification have demonstrated that an alteration of the wood cell wall structure can strongly increase the water resistance of particleboards. Kajita and Imamura (1991) studied the board properties made from chips that were treated with a low-molecular-weight phenol-formaldehyde resin. Particleboards at 20% WPG swelled less than 8%, while control boards swelled nearly 30% during two-hour boiling. Subiyanto et al. (1989) reported that boards made from acetylated chips bonded with isocyanate and phenol-formaldehyde swelled only 4% during 30 days of water immersion, whereas the control boards swelled over 20 and 30%, respectively. Further studies corroborating the dimension stabilizing effect by acetylation were carried out by Okino et al. (2004), Youngquist and Rowell (1986), and Rowell et al. (1986). Papadopoulos and Gkaraveli (2003) examined the performance of one-layer particleboards made from chips modified with propionic anhydride. The thickness swelling after 24 h was reduced by 46% as compared to controls. Cell wall modification methods can strongly improve water-related properties but are expensive and difficult to implement in production processes of boards. The present study, therefore, investigates the surface modification by alkyl ketene dimer (AKD), a widely used paper sizing agent. It is assumed that a superficial change of particles, fibers, or flakes in reconstituted wood products might impart a high and particularly permanent hydrophobicity in combination with the binder. Internal sizing agents are used in the paper industry to accomplish resistance against fluids, improving paper properties like wet strength and printability. During the 1980s, the commonly-used rosin in combination with alum was increasingly substituted by cellulose-reactive systems, particularly based on AKD and alkenyl succinic anhydride (ASA) (Neimo 1999). Today, AKD is the most common internal sizing agent in Europes papermaking industry. Typical addition lies between 0.05 and 0.2%, based on dry mass of fiber (Davis et al. 1956, Neimo 1999). ASAs and AKDs hydrophobic effect is predominantly ascribed to an esterification with the wood fibers hydroxyl groups, as illustrated in Fig. 1 (Hubbe 2006, Seppanen 2007). The main difference between both sizing agents is the lower reactivity and hydrolysability of AKD. Thus, it is pre-emulsified by the AKD-manufacturer and shipped as a ready-to-use dispersion with maximum solid contents of 2025% to the pap (...truncated)