# Enhanced Resistance to Fire of the Bark-Based Panels Bonded with Clay
## 2 Materials and Methods  
The raw materials used for these trials were larch and poplar bark particles (mesh sizes shown in Table 1), a urea-formaldehyde (UF) adhesive (10F102 MetaDynea Austria GmbH, Krems, Austria) with 66% solid content, pH 8.3-9 and viscosity 60-90 mPa*s and clay (Sibelco, Germany) with 40% chamotte and 0-0.5 mm grit and de-ionized water (pH value 7). Commercial corn starch was added for the adhesive formulations with poplar bark. The larch bark originates from a sawmill in Untermberg, Salzburg, Austria specialized in larch processing. The poplar bark was provided by Holz Deisl, Adnet, Austria. The bark was dried by means of a vacuum kiln dryer (Brunner-Hildebrand High VAC-S, HV-S1, Hannover, Germany) from 100% to 9% moisture content. The drying temperature was 60 \({}^{\circ}\)C at a pressure of 200-250 mbar. The bark was subsequently crushed in a 4-spindle shredder (RS40) at the Untha Co. in Kuchl, Austria, and repeatedly screened to obtain 0.5-20 mm particles. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Board** & **Particle Size** & **Density** & **Thickness** & **Adhesive** & **Press Time** & **Moisture** \\  & **(mm)** & **(kg/m\({}^{3}\))** & **(mm)** & **Adhesive** & **(min)** & **Content (\%)** \\ \hline A & 4–11 & 700 & 20 & 10\% UF + 52\%water & 7 & 8.6 \\ B & \textless{}4 & 600 & 19 & 55.5\%clay + 11\%water & 7 & 8.2 \\ C & \textgreater{}11 & 900 & 24.5 & 51.3\%clay + 8.2\%water & 7 & 8.5 \\ D & 4–11 & 950 & 33 & 54.8\%clay + 8.7\%water & 11 & 8.6 \\ E & 4–11 & 518 & 19.4 & 10\% + 5.2\%water & 7 & 3 \\ F & 4–11 & 720 & 24.3 & 40.7\%clay + 13.68\%water + 4.91\%starch & 7 & 3.5 \\ G & 4–11 & 913 & 22.4 & 43.66\%clay + 7.2\%water + 5.38\%starch & 7 & 3.9 \\ H & 4–11 & 750 & 25.4 & 33.7\%clay + 16.8\%water + 15.73\%starch & 7 & 4.4 \\ \hline \hline \end{tabular} Source: Authors’ compilation.  
\end{table} 
Table 1: Overall experimental schedule for larch-based boards bonded with and without clay. The bark grains were mixed with UF (A, E) and with clay, water (B, C, D, F, G, H) and starch (for the poplar boards), respectively, in a ploughshare mixer ENT type WHB-75 for 10 min. In the case of clay-bonded larch boards, the mineral and the water were added gradually. 1% ammonium sulphate hardener was used for the UF adhesive. Consequently, the glued particles were discharged and spread in a 45 cm \(\times\) 45 cm mold and pressed at 170 \({}^{\circ}\)C in a Hofer HLOP 280 (Taiskirchen, Austria) computer-controlled press for 7 min. A total of 16 boards (450 \(\times\) 450 mm), two for each type, were manufactured (Table 1), with a thickness ranging from 19 to 33 mm [38]. The properties of all boards were compared to wood wool cement-bonded boards (Isolith and Knauf) and wood particle cement boards (Heraklith) (Table 2). Thickness swelling (TS) and water absorption (WA) were determined according to EN 317:2005 [48]. The 50 \(\times\) 50 mm test samples were weighed and their thicknesses were measured, with a level of accuracy of 0.01 g and 0.1 mm. The specimens were immersed in de-ionized water (pH = 7 \(\pm\) 1) at 20 \({}^{\circ}\)C for 1 and two hours. Afterwards, the test samples were taken out and rinsed to remove excess water. Each sample was reweighed; the sample's size was taken from the same location prior to immersion in water. Based on the measurements, thickness swelling and water absorption after 1 and 2 h were calculated. To determine the internal bond (IB, Figure 1) of the test samples, the transverse tensile strength was tested according to EN 319:1993 [49]. Five 50 \(\times\) 50 \(\times\) 7 mm samples were fixed on testing plates with hot melt glue. The experiment was performed with a Zwick/Roell Z250 (Ulm, Germany) universal testing machine. The screw withdrawal resistance was made according to EN 320:2011 [50]. For this test, a wood screw (4.2 \(\times\) 38 mm) was drilled into both the surface and the edge (Figure 2) and the pull-out force was measured. A hole of 3 mm in diameter was drilled in the center of the 75 \(\times\) 75 mm samples.