# Lightweight Solid Wood Panels Made of Paulownia Plantation Wood
## 2 Materials and Methods  
The Paulownia European plantation timber (_Paulownia tometosa \(\times\) elongata_) was provided by Glendor Holding GmbH (Kilb, Austria) and sourced from 5- to 7-year-old trees. The plantation wood from Petrinja, Croatia, was delivered by Moserholz GmbH (Pettenbach, Austria) dried (m.c. = 12%) as rough-sawn lumber of approximately 100 cm length and 20-30 cm width and with a thickness of 20-25 mm. Prior to testing, the raw material was conditioned to constant weight at 20 \({}^{\circ}\)C and 65% relative air humidity for at least 14 days, until constant weight was achieved. For the manufacture of single- and three-layered SWPs with a thickness of 19 mm, two types of lamellae with the following dimensions were cut with a circular saw and planed: 112 \(\times\) 3.2 \(\times\) 2.2 cm for the single-layer SWPs and 98 \(\times\) 6.6 \(\times\) 1 cm for the 3-layer SWPs. A lamella width of 32 mm (single-layered board) was opted for according to the dimension of the core layer of a standard industrial blockboard from Moralt AG (Hausham, Germany). A lamella width of 66 mm (3-layered board) was chosen after optimising the available raw material in order to obtain the maximum yield. Both the one- and three-layered SWPs were glued using polyvinylacetate (PVAc) type Pattex(r) PV/H, category D3 (Pattex, Dusseldorf, Germany); melamine urea formaldehyde (MUF), type Dynea Prefere 4564 with hardener Prefere 5013, at a ratio of 100:8 (Dynea Austria GmbH, Krems, Austria) and polyurethane Kleiberit(r) PUR 501.0 (Kleiberit SE & Co. KG, Weingarten, Germany). The adhesive amount was as follows: for the PVAc, 330 g/m\({}^{2}\), for the MUF, 340 g/m\({}^{2}\) and 210 g/m\({}^{2}\) for the PUR. The single-layered SWPs were manufactured with Paulownia lamellae with 12% moisture content [22]. The lamellae were previous glued with PVAc, MUF and PUR and consequently cold-pressed at 0.68 MPa with screw clamps for two hours (Figure 1). The single-layer panels were calibrated using a wide belt sander, grit 60, to a thickness of 19 mm, a width of 446 mm and a length of 950 mm. Figure 1: Pressing a single-layered Paulownia solid wood panel with clamps, where the lamellae were longitudinally bonded (95 \(\times\) 44.6 \(\times\) 2.2 cm). The individual layers of the 3-layer SWP were formatted to 920 \(\times\) 920 mm and calibrated. The top layers (faces) had a thickness of 5.75 mm and the middle layer (core) 7.5 mm, considering the model of a standard spruce 3-layer panel from Binderholz GmbH (St. Georgen, Austria) of 19 mm thickness (2 \(\times\) 5.75 + 7.5 mm). The adhesive was applied to the individual layers of the 3-layer SWP with a notched trowel of tooth size B1 for all the adhesive types (Table 1). The 3-layer SWPs were pressed at 60 \({}^{\circ}\)C for 15 min in a veneer press OTT (Lambach, Austria) with a pressure of 0.6 N/m\({}^{2}\) for all panel types (bonded with MUF, PUR and PVAc) (Figure 2). For the manufacture of SWPs, lamellae with different thicknesses (32 and 66 mm) were used, as well as distinct amounts of adhesive (Table 1):  
The testing specimens were prepared from the 1-layer and 3-layer SWPs according to EN 326-1:2005 [23] and then stored in a standard climate (20 \({}^{\circ}\)C and 65% relative air humidity). For the density determination of the 1- and 3-layer SWPs (Figure 3), 6 test specimens were prepared for each type of board according to EN 323:2005 [24]. The mechanical properties of 3-point flexural strength (MOR) and modulus of elasticity (MOE) in bending were determined from 6 test specimens per board according to EN 310:2005 [25]. In the case of the 3-layer SWPs, 6 test specimens for each board were tested parallel and perpendicular to the grain. To test the quality of the bond for SWP 1 (24 h water storage), 10 test specimens per board were used to determine the compressive shear strength according to EN 13354:2009 [26], as depicted in Figure 4.