Procedural influence on the properties of particleboards made from AKD modified chips

Ulrich Hundhausen 0 Roman Stohldreier 0 Holger Militz 0 Carsten Mai 0 0 U. Hundhausen ( ) R. Stohldreier H. Militz C. Mai Wood Biology and Wood Products, Burckhardt Institute, Georg-August University Gottingen , Busgenweg 4, 37077 Gottingen, Germany Wood chips were treated with alkyl ketene dimer (AKD) using three different processes to impart water resistance to particleboards. In the first process, AKD was blended with UF resin. Thickness swelling and water uptake after a 24 h immersion period (20 and 69%) were lower than in the control boards (28 and 81%) but were higher than in the paraffin references (10 and 22%). In process 2, AKD and UF resin were sprayed separately on the chips resulting in a greater reduction of thickness swelling (15%) and water uptake (49%) than in process 1. Paraffin references revealed a thickness swelling and water uptake of 7 and 25%, respectively. An extension of the pressing time in processes 1 and 2 did not increase water repellence. In process 3, particleboards were made from AKD-treated chips that were cured at 130 C (24 h) prior to gluing. They showed a thickness swelling of 7% and a water uptake of 25%, whereas particleboards with paraffin exhibited levels of 8 and 29%. The thickness swelling and water uptake of boards with AKD increased when the curing time was reduced from 24 to 12 to 6 h (130 C). Changing the curing temperature from 130 to 100 C (12 h) had no effect on board properties. The IB of boards made from pre-cured chips with AKD (24 h/130 C) was 44% lower than in controls and 35% lower than in paraffin references. This indicates that AKD impedes the adhesion. - 44 und 35% reduziert. Dies weist daraufhin, dass AKD die Verklebung mit UF Harz beeintrachtigt. 1 Introduction Wood-based panels have become increasingly specialized and have been used in a wide range of applications in recent years. Their application in moist conditions has, however, been very limited due to their susceptibility to liquid and vaporous water. The moisture content essentially determines the different characteristics of the products. The uptake and release of liquid and vaporous water result in swelling and shrinking, which are most pronounced in the direction perpendicular to the plane. Thickness swelling is explained by a swelling of the wood itself and recovery from compression stress imparted during pressing. While the first is reversible, the latter is not. The uptake of moisture can, consequently, cause destruction of the adhesive-wood bonds and result in significant strength loss (Klauditz 1954, Neusser et al. 1965, Gatchell et al. 1966, Ernst 1967). The dimensional stability of panels is dependent on a number of process variables, e.g., resin level, board density, wood species, particle geometry, blending quality, and pressing conditions (Neusser et al. 1965, Haaligan 1970, Schneider et al. 1982). But even if these parameters are optimized, the use of a hydrophobic agent most commonly paraffin wax is still required. The disadvantage of paraffin, however, is that it only decelerates water uptake and does not offer permanent protection against water damage (Roffael and Schneider 1981). There has been extensive research done on wood modification to produce panels with high dimensional stability. These can be divided into active and passive methods depending on whether the cell wall polymers are changed or not. Furthermore, a distinction has been made between bulk and surface modification. The former implies that the modification reagent is distributed throughout the entire cell wall, the latter that the chemical alteration is confined to the wood surface (Hill 2006). Numerous studies have shown that thermally induced changes of the macromolecular components result in improved water repellency of wood-based panels (Tomek 1966, Burmester and Deppe 1973, Tomimura and Matsuda 1986, Boonstra et al. 2006, Paul et al. 2007). Chemical bulk modifications induced by various organic reagents have been successful in increasing the dimensional stability of boards (Youngquist and Rowell 1986, Fujimoto et al. 1988, 1991, Korai 2001, Papadopoulos and Gkaraveli 2003, Yildiz et al. 2005). In contrast to chemical and heat modification, the socalled impregnation modification method is considered to be one of the passive procedures. Such processes include an impregnation step using a solution of low molecular weight that diffuses into the cell wall and subsequently polymerizes. This leads to a condition where the reagent is locked without covalently reacting with cell wall compounds (Hill 2006). Phenol-formaldehyde resins were used for impregnating OSB strands (Wan and Kim 2006) and particles (Kajita and Imamura 1991); the boards displayed strongly reduced swelling following water submersion as compared to controls. Isocyanate-bonded waferboards which exhibited high dimensional stability were produced from DMDHEUtreated chips (Yusuf et al. 1995). Bulk modification methods are based on the premise that the reagents gain access to the interior of the wood cell wall. Solute exclusion measurements have shown that the maximum diameters of the cell wall micropores are within the range of 24 nm in fully swollen wood (Hill et al. 2004). To insure that penetration occurs throughout the cell wall in solid wood treatments, vacuum/pressure is usually applied prior to curing. This impregnation process is, however, timeconsuming and costly and is therefore not suited for manufacturing wood panels. It is necessary to consider whether bulk modification is adequate for products used in certain applications that do not exceed Use Class 3 (formerly Hazard Class) according to EN 335, e.g., particleboards as claddings or MDF boards in exterior doors. A surface modification of particles, fibers, or flakes in reconstituted wood products might be sufficient in combination with the adhesive to impart high and particularly durable water repellency. Alkyl ketene dimer (AKD) has been shown to improve dimensional stability of particleboards (Hundhausen et al. 2008). Contact angle and drop extension measurements on AKD-treated veneer strips have revealed strongly reduced surface wettability. Hydrophobicity has been primarily attributed to the formation of -keto-esters (Fig. 1), although extraction studies using FTIR-spectroscopy have indicated that only a small amount of the total AKD applied actually esterified wood hydroxyl groups. Besides esterification of wood hydroxyls, AKD may undergo other reactions, such as esterification with cationic starch (emulsifier) or hydrolysis with water. It is also noteworthy that the formed -keto-ester may be thermally cleaved during hot-pressing of the wood chips. Water resistance equal to that of paraffin treated references was attained when chips were impregnated and cured at 130 C for 24 h prior to board production (Hundhausen et al. 2008). In comparison, the admixture of AKD to UF glue did not have any great effect. This was most like (...truncated)