## 2 Materials and Methods  
The raw materials used for the particleboard production consisted of walnut and hazelnut shells. The hazelnut (_Corylus avelana_ L.) shells were provided by the Faculty of Forestry at the University of Zagreb (Croatia). The walnut (_Juglans regia_ L.) shells were provided by a family-owned walnut cracking company in Carinthia ([PERSON], Austria). Melamine urea formaldehyde (MUF) resin (Prefere 10G268) was provided by metaDynea (Krems, Austria) and polyurethane (PUR 501.0) was provided by Kleiberit Klebchemie (Kleiberit Klebchemie M. G. Becker GmbH & Co. KG, Weingarten, Germany). A total of 40 kg walnut shells and 30 kg hazelnut shells were available for the project. The shells were at first manually cleaned from impurities and afterward shredded in an R40 industrial four shaft shredder at Untha Company (Kuchl, Austria) using an 8-mm screen. The shredded nutshells were dried in a Brunner-Hildebrand High VAC-S, HV-S1 (Hannover, Germany) kiln dryer for three days to reach a moisture content of about 5%. After drying, the particles were sorted into three main size classes (fine-grained, middle-grained and coarse-grained) using a sieve shaker Retsch AS 200 (Haan, Deutschland). Particles in the middle-sized (3-6 mm) and fine (\(<\)3 mm) fraction were used for the board production. The percentage of particles in each group and nutshell type is listed in Table 1.  
A total of twelve boards (two for each group) was produced, of which the compositions are listed in Table 2. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Raw Material** & **\% Fine-Grained** & **\% Middle-Sized Grained** & **\% Coarse-Grained** \\ \hline Hazelnut & 13.4 & 67.1 & 19.5 \\ Walnut & 12.4 & 73.4 & 14.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Weight distribution according to the fraction size of the raw material. The 320 mm \(\times\) 320 mm boards were produced with a thickness of 10 mm and a density of 1000 kg/m\({}^{3}\) for nutshells and 700 kg/m\({}^{3}\) for the spruce control boards. The density of 1000 kg/m\({}^{3}\) could not be reached with wood particles due to the technical limitations of the hydraulic press Hofer HLOP 280. The walnut and hazelnut boards were composed of 70% middle-sized and 30% coarse-grained particles. For all panels, the particles were mixed with resin manually and then formed into a mat and pre-compressed. This mat was pressed afterward in a hydraulic laboratory press (Hofer HLOP 280, Taiskirchen, Austria) with a pressure of 3 N/mm\({}^{2}\). For the UF and MUF bonded boards, 10% resin based on weight was used including 1% ammonium sulfate as a hardener. These boards (Figure 1) were pressed at 160 \({}^{\circ}\)C for 6 min. The PUR bonded boards were also produced with 10% adhesive, but these were pressed at 60 \({}^{\circ}\)C for 20 min. After pressing, the panels were conditioned at 20 \({}^{\circ}\)C and 65% relative air humidity for one week before cut to test specimen size, which was done according to [40]. In terms of mechanical properties, bending strength (MOR) and modulus of elasticity (MOE) according to [41] and Brinell Hardness [42] were tested. For the bending properties [41], the three-point bending test was employed. The physical properties such as thickness swelling and water absorption after 24 h water immersion [43] and density [44] were also measured. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Board** & **Adhesive** & **Raw Material** & **Density (kg/m\({}^{3}\))** & **Press Time (min)** & **Press Temp. (\({}^{\circ}\)C)** \\ \hline A & MUF & Hazelnut & 1000 & 6 & 160 \\ B & MUF & Walnut & 1000 & 6 & 160 \\ C & MUF & Spruce & 1000 & 6 & 160 \\ D & PUR & Hazelnut & 1000 & 20 & 60 \\ E & PUR & Walnut & 1000 & 20 & 60 \\ F & UF & Spruce & 700 & 6 & 180 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Manufacturing parameters of particleboards made from hazelnut, walnut, and spruce.