Use of Alkyl Ketene Dimer (AKD) for Surface Modification of Particleboard Chips
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Eur. J. Wood Prod. (2009) 67: 37–45 DOI 10.1007/s00107-008-0275-z ORIGINALARBEITEN · ORIGINALS Use of alkyl ketene dimer (AKD) for surface modification of particleboard chips Ulrich Hundhausen · Holger Militz · Carsten Mai Published online: 28 August 2008 © The Author(s) 2008. This article is published with open access at Springerlink.com Abstract The study objective was to impart urea-formalde- ventionell verwendeten Paraffinprodukten moglich¨ ist. Al- hyde (UF) bonded particleboards higher and longer-lasting kylketendimer (AKD) wird in der Papierindustrie als Mas- hydrophobicity than that resulting from conventionally used seleimungsmittel eingesetzt und kann theoretisch mit Zell- paraffin. Alkyl ketene dimer (AKD) is a paper sizing agent wandbestandteilen Esterbindungen eingehen. Holzspane¨ that can theoretically esterify wood compounds and result wurden 1) mit einer wassrigen¨ AKD Losung¨ impragniert¨ in a surface modification. Particles were 1) impregnated und bei 130 ◦C getrocknet oder 2) mit einer AKD/UF- with an aqueous AKD-solution and cured at 130 ◦Cpriorto Harz Losung¨ bespruht.¨ Platten mit und ohne Paraffinbe- gluing, or 2) sprayed with a mixture of AKD-solution and handlung dienten als Referenzen. Im Vergleich zu Platten UF-resin in a single step. Boards with and without paraf- ohne Hydrophobierungsmittel wiesen Platten aus AKD- fin wax were used as controls. Thickness swelling after 2, impragnierten¨ Spanen¨ nach 2-, 24- und 48-stundiger¨ Was- 24, and 48 h immersion was decreased by 90, 62, and 59% serlagerung eine um 90, 62 und 59% verminderte Dicken- when the chips were impregnated with AKD in comparison quellung auf. Die Wasseraufnahme wurde um 91, 75 und to untreated control boards. Water uptake after 2, 24, and 60% reduziert. Die AKD Impragnierung¨ fuhrte¨ zu einer 48 h was reduced by 91, 75, and 60%. AKD-impregnation Verringerung der Querzugfestigkeit um 53%. Dies deutet with subsequent curing decreased the internal bond strength darauf hin, dass AKD die Verklebbarkeit negativ beein- by 53%, indicating that AKD impedes the adhesion. The flusst. Die Anwendung von AKD im Untermischverfahren mixture of AKD and UF-glue did not result in considerable fuhrte¨ nur zu geringer Hydrophobierung. FTIR Messungen hydrophobicity. Increased methyl/methylene and carbonyl an Furnierstreifen vor und nach einer Toluolextraktion wie- bands in FTIR-spectra after toluene-extraction suggest that sen darauf hin, dass ein Teil des AKD uber¨ Esterbindungen AKD partially formed ester bonds at the wood surface. auf der Holzoberflache¨ fixiert ist. Die Verwendung von Alkylketendimer (AKD) 1 Introduction zur Oberflachenmodifizierung¨ von Spanen¨ fur¨ die Herstellung von Spanplatten The crucial application restrictions of wood-based panels are ascribed to their poor dimensional stability in chang- Zusammenfassung Das Ziel der Untersuchung bestand ing climates and in contact with liquid water. While the darin, Harnstoff-Formaldehyd (UF) gebundene Spanplatten swelling and shrinking behavior of boards parallel to the starker¨ und dauerhafter zu hydrophobieren als es mit kon- plane complies with that of solid wood parallel to the fiber, the values perpendicular to the plane are significantly U. Hundhausen (u) · H. Militz · C. Mai greater (Amthor 1972). Swelling and shrinkage of wood Wood Biology and Wood Products, Burckhardt Institute, particles and hydrolysis of the adhesives limit the applica- Georg-August University Gottingen,¨ tion to low humidity conditions. The dimensional stability Busgenweg¨ 4, 37077 Gottingen,¨ Germany is influenced by board density, wood species, resin type, e-mail: [email protected] resin level, pressing conditions, particle geometry, and hy- 13 38 Eur. J. Wood Prod. (2009) 67: 37–45 drophobic agents (Haaligan 1970). To date, paraffin wax alteration of the wood cell wall structure can strongly emulsions are usually added to improve moisture resis- increase the water resistance of particleboards. Kajita tance. The typical amount of wax ranges from 0.3–1% and Imamura (1991) studied the board properties made based on the oven-dry weight of the particles (Youngquist from chips that were treated with a low-molecular-weight 1999). Higher additions show only marginal hydrophobic phenol-formaldehyde resin. Particleboards at 20% WPG effect and cause reductions in bonding strength (Muller¨ swelled less than 8%, while control boards swelled nearly 1962, Amthor and Bottcher¨ 1984); however, the major 30% during two-hour boiling. Subiyanto et al. (1989)re- problem of paraffin is that it decelerates the uptake of li- ported that boards made from acetylated chips bonded quid water and the swelling but cannot impart durable with isocyanate and phenol-formaldehyde swelled only hydrophobicity (Youngquist 1999). 4% during 30 days of water immersion, whereas the Besides the use of paraffin as a hydrophobic agent, control boards swelled over 20 and 30%, respectively. a variety of treatments have proved as effective means Further studies corroborating the dimension stabilizing to increase the water repellency of particleboards. Fil- effect by acetylation were carried out by Okino et al. cock and Vinden (2000) investigated the use of isocyanate (2004), Youngquist and Rowell (1986), and Rowell et resin to add water repellency to particleboards. Isocyanate- al. (1986). Papadopoulos and Gkaraveli (2003) examined impregnated, not edge-sealed panels showed a thickness the performance of one-layer particleboards made from swelling of approx. 5% after 24 h as compared to con- chips modified with propionic anhydride. The thickness trol boards that revealed 17%. Several researchers reported swelling after 24 h was reduced by 46% as compared to that heat-treatment of particles leads to enhanced dimen- controls. sional stability (Tomek 1966, Tomimura and Matsuda Cell wall modification methods can strongly improve 1986, Boonstra et al. 2006). In addition, numerous stud- water-related properties but are expensive and difficult to ies on chemical modification have demonstrated that an implement in production processes of boards. The present Fig. 1 Reaction mechanisms of AKD with wood hydroxyl groups a and water b. R1 and R2 = C16–22 Abb. 1 Reaktionsschema von AKD mit Hydroxylgruppen des Holzes a und mit Wasser b. R1 und R2 = C16–22 13 Eur. J. Wood Prod. (2009) 67: 37–45 39 study, therefore, investigates the surface modification by 2 Experimental alkyl ketene dimer (AKD), a widely used paper sizing agent. It is assumed that a superficial change of particles, fibers, or 2.1 Wood and chemicals flakes in reconstituted wood products might impart a high and particularly permanent hydrophobicity in combination The raw material for the particleboards came from Pflei- with the binder. derer Holzwerkstoffe (Gutersloh,¨ Germany). Kaurit 350 Internal sizing agents are used in the paper industry to (BASF, Ludwigshafen, Germany) was used as UF-resin accomplish resistance against fluids, improving paper prop- (solid content of 66.5%). The AKD-dispersion Hydrores erties like wet strength and printability. During the 1980s, 157YC (Kemira, Leverkusen, Germany) is a commodity the commonly-used rosin in combination with alum was in- paper sizing product with a solid content of 16%, contain- creasingly substituted by cellulose-reactive systems, partic- ing 10% active AKD, 2% cationic starch, and 4% cationic ularly based on AKD and alkenyl succinic anhydride (ASA) polymers as stabilizers. HydroWax 138 (Sasol, Hamburg, (Neimo 1999). Today, AKD is the most common internal Germany) served as a paraffin reference (solid content of sizing agent in Europe’s papermaking industry. Typical add- 50%). Knife-cut spruce veneer were obtained from Danzer ition lies between 0.05 and 0.2%, based on dry mass of Furnierwerke (Kehl, Germany). fiber (Davis et al. 1956,Neimo1999). ASA’s and AKD’s hydrophobic effect is predominantly ascribed to an esterifi- 2.2 Board production cation with the wood fiber’s hydroxyl groups, as illustrated in Fig. 1 (Hubbe 2006, Seppanen¨ 2007). The main differ- Two different AKD-application processes were compared: ence between both sizing agents is the lower reactivity and In process 1, the AKD-dispersion was applied under vac- hydrolysability of AKD. Thus, it is pre-emulsified by the uum/pressure and subsequently cured. In process 2, the AKD-manufacturer and shipped as a ready-to-use disper- AKD-dispersion was mixed with the glue and sprayed on sion with maximum solid contents of 20–25% to the paper- the particles prior to pressing. maker whereas ASA must be emulsified on-site at the paper In process 1, the particles were filled into pillow cases mill. Both materials need to be stabilized to prevent hydro- to facilitate a better handling. Afterwards, they were sub- lysis and coalescing of the particles. Cationic starch is most mersed in a 1% (wt/wt) AKD-solution and subjected to commonly used as emulsifier. It is assumed that its quater- vacuum (60 min/80 mbar) and pressure (120 min/12 bar) in nary nitrogen facilitates the attachment of the droplets to order to accomplish a maximum penetration into the cell the negatively charged fiber surfaces. The ratio of AKD to lumina (AKD im). After impregnation, the cases were cen- starch is usually in the range of 1.5 : 1to4: 1 whereas the trifuged in a spin dryer to separate the bulk of non-absorbed ratio of ASA to starch is 1 : 2to1: 4 (Neimo 1999). The solution. The particles were then put into a drying cabinet lower starch content in AKD-dispersions can be presumably at 130 ◦C (24 h) to induce the reaction between AKD and considered as advantageous for the use in particleboards wood. After curing, the weight percent gain (WPG) was de- since the hydrophilic nature of an emulsifier works counter- termined as follows: active to the aspired hydrophobicity (Amthor 1972). m2 − m1 While AKD has been extensively investigated in pa- WPG[%]= × 100 (1) m per sciences (Lindstrom¨ and Soderberg¨ 1986, Quillin et al. 1 1992,Neimo1999, Karademir 2002, Wei et al. 2002, Hubbe where m2 [g] is the particle mass after curing and m1 [g] is 2006, Seppanen¨ 2007), there has been only one study found the dry particle mass before impregnation.