Edge and face gluing of green timber using a one-component polyurethane adhesive

Excerpt

Gluing of unseasoned, green wood at high moisture content (MC), here referred to as green gluing, has mainly been applied for finger jointing using phenol resorcinol formaldehyde (PRF) adhesives (Parker 1994, Kreibich et al. 1998). In contrast to PRF adhesives, one-component PUR adhesives cure in the presence of water and could therefore be an interesting alternative for the use in green gluing. Finger jointing of green wood with one-component PUR adhesives was reported by Verreault (1999) and Lange et al. (2000) with promising results that comply with the requirements for structural or at least stud grade timber. One obvious limitation of PUR adhesives is that the glue lines must be thin, preferably of a maximum thickness of 0.3 mm, to prevent the formation of cavities in the glue line with resulting reduction of joint strength. In addition, the long-term durability and creep resistance of PUR adhesives are still not well understood and definitely critical issues that have to be considered in e.g. structural applications. The existence of temperature-dependent deformation in joints bonded with PUR adhesives was reported by Richter and Schirle (2002) and George et al. (2003). However, Radovic and Rothkopf (2003) showed that two different one-component PUR adhesives exhibited different creep behaviour and that the best performing PUR showed similar creep resistance as a PRF adhesive. Vick and Okkonen (1998) and Rajakaruna (2001) carried out accelerated durability tests of PUR adhesives and observed severe delamination of the glue line, but high strength of the joint. A low wood failure percentage of mechanically tested joints with PUR adhesives caused by high wood density and high extractive content of softwoods have also been reported (Richter and Schirle 2002). Furthermore, it has also been observed that the higher ductility of polyurethane adhesives can enable increased joint strength due to its higher fracture energy (Serrano 2002). Moreover, most studies regarding green gluing of timber have been related to finger jointing, and few studies (e.g. Properzi et al. 2003) have yet been focused on edge and face gluing. …