## 3 Results
### Physical Properties  
In terms of physical properties, thickness swelling (TS) and water absorption (WA) both after 24 h water immersion were evaluated. Thickness swelling after 24 h water immersion for MUF bonded hazelnut husk panels (A) was determined with 17.5% while it was a little lower for walnut shells (B) with 13.2% (Table 5). Compared to the spruce reference board (C) with an average of 52%, the increase in thickness was much lower. The results for hazelnut (A) glued with MUF were a little better compared to those obtained by [31] and [35] (29.3% TS with a standard deviation of 3.5%), while the results for walnut (B) were higher compared to [34], who evaluated the thickness swelling with 10.2%. Thickness swelling after 24 h was significantly reduced when PUR was used to produce the boards. This means that it was only 9.7% for hazelnut (D) and 8.4% for walnut (E). The spruce boards showed an increase in thickness of nearly 52% for MUF and still 44.5% for PUR. The nutshells in combination with PUR performed better compared to the spruce particles. It was found that when MUF was used as the adhesive, there was a significant difference between the walnut and hazelnut shells, while the material did not have a big influence when bonded with PUR. The results for water absorption after 24 h water submersion were similar to those for thickness swelling after 24 h (Table 5). This means that the nutshell boards glued with MUF had a higher water uptake, namely 27% for hazelnut (A) and 25% for walnut (B), compared to the PUR bonded ones with 12.5% (D) and 11.5% (E), respectively. The values were much lower compared to the results of spruce particles with 64% for MUF (C) and almost 90% for PUR (F). Regarding water uptake over 24 h, there was no influence of the used nutshell type in combination with either MUF or PUR. The same trend of blocking and reduction of TS and WA was observed in the case of using hazelnut shells combined with wood particles [32], with a mean of 20% for TS and 70% for WA and from [34], in the case of walnut shells with the same value for TS as reported by [32] and lower WA of 37%. It was expected that thickness swelling and water absorption will be lower, compared to the spruce particleboards. The water uptake of lingo-cellulosic materials is strongly influenced by the number of free hydroxyl groups where water is bonded to. These hydroxyl groups are mainly present in the natural polymer cellulose. Hemicellulose is amorphous and has a hydrophilic character that is \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Properties** & **Board** & **Mean** & **Std. dev.** & **Min** & **Max** \\ \hline TS 24 h [\%] & A & 17.47 \({}^{\text{a,u}}\) & 2.88 & 13.56 & 21.91 \\  & B & 13.23 \({}^{\text{b,w}}\) & 1.35 & 10.98 & 15.21 \\  & C & 52.09 \({}^{\text{c,y}}\) & 7.84 & 42.69 & 64.84 \\  & D & 9.66 \({}^{\text{d,v}}\) & 1.48 & 8.33 & 12.66 \\  & E & 8.37 \({}^{\text{d,x}}\) & 1.26 & 6.48 & 10.29 \\  & F & 44.48 \({}^{\text{e,z}}\) & 11.01 & 30.92 & 64.39 \\ \hline WA 24 h [\%] & A & 26.96 \({}^{\text{a,u}}\) & 2.96 & 23.32 & 32.54 \\  & B & 24.54 \({}^{\text{a,w}}\) & 3.55 & 19.29 & 28.78 \\  & C & 64.00 \({}^{\text{b,y}}\) & 9.64 & 45.24 & 75.02 \\  & D & 12.44 \({}^{\text{c,v}}\) & 2.05 & 9.60 & 15.28 \\  & E & 11.49 \({}^{\text{c,x}}\) & 2.56 & 7.40 & 14.40 \\  & F & 89.99 \({}^{\text{d,z}}\) & 7.04 & 81.49 & 104.09 \\ \hline \hline \end{tabular} \({}^{\text{a,b,c,d,e}}\) values with the same letter were not significantly different (raw material), \({}^{\text{a,v,w,y,z}}\) values with the same letter were not significantly different (adhesive).  
\end{table} 
Table 5: Physical properties of the hazelnut, walnut, and spruce particle boards. Additionally increasing the water uptake. Lignin, however, is totally hydrophobic, which means that water cannot be absorbed within [52]. The high amount of lignin in the walnut shells (49.1%) compared to roughly 35% in softwood indicated a decreased amount of water absorption.