# Enhanced Resistance to Fire of the Bark-Based Panels Bonded with Clay
## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c} \hline
**Board** & **Thickness Swelling** & **Water Uptake** \\ \hline Larch bark board with UF (A) & 2.5\% (1 h), 4\% (2 h), 8.68\% (24 h) & 6\% (1 h), 7.7\% (2 h), 41.74\% (24 h) \\ Poplar bark board with UF (E) & 1\% (1 h), 2\% (2 h), 3\% (24 h) & 3\% (1 h), 6\% (2 h), 19\% (24 h) \\ Poplar bark board (F) & 7\% (1 h), 9\% (2 h), 43\% (24 h) & 43\% (1 h), 54\% (2 h), 66\% (24 h) \\ Poplar bark board (G) & 7\% (1 h), 9\% (2 h), 28\% (24 h) & 28\% (1 h), 34\% (2 h), 43\% (24 h) \\ Poplar bark board (H) & 7\% (1 h), 9\% (2 h), 36\% (24 h) & 36\% (1 h), 42\% (2 h), 48\% (24 h) \\ Wood wool cement board—Isolith & 1.8\% (24 h) & 36\% (24 h) \\ Wood particle cement board—Heraklith & 2\% (24 h) & 36\% (24h) \\ Wood wool cement bonded board—Knauf & N/A & N/A \\ \hline \end{tabular} Source: [42; 43; 44].  
\end{table} 
Table 3: Thickness swelling and water absorption for bark, wood particle cement and wood wool cement boards. adhesives based on UF/soy protein reinforced with clay for wood particleboard. Their results showed that a small amount of nanoclay (1%) improved the modulus of both rupture and elasticity. For comparison, a cement-bonded particle board has a transverse tensile strength of 0.5 N/mm\({}^{2}\), according to the manufacturer [57]. The bending strength of an Isolith wood wool cement-bonded board is 3.2 N/mm\({}^{2}\) (lengthwise) and 1.7 N/mm\({}^{2}\) (across), with a compression strength (at 10%) of 0.6 N/mm\({}^{2}\). The bending strength of a Heraklith wood particle cement-bonded board is 2 N/mm\({}^{2}\) (lengthwise), with a compression strength (at 10%) of 2.2 N/mm\({}^{2}\). The values of screw withdrawal resistance (Figure 5) were measured for both the surface and the edge, but only for the samples cut from the larch bark reference panel (16.3 N/mm for the surface and 12.6 N/mm for the edge). In the case of the other larch bark samples, the average value of screw withdrawal resistance at the surface was 1.4 N/mm, 87% lower than the value of the control sample. These values are within the required range for the boards' use in non-structural paneling applications. The measurements for the samples of the poplar bark boards bonded with UF (E) were similar for the edges (12.3 N/mm), but lower for the surface (10.76 N/mm). The board with a density of 700 kg/m\({}^{3}\) (F) and a thickness of 24.3 mm had the lowest values of screw withdrawal resistance from this series. Figure 4: Internal bond of the larch bark panels bonded with UF (A) and clay (B, C and D) and poplar bark panels bonded with UF (E) and clay and starch (F, G, H). Figure 5: Screw withdrawal resistance of the larch bark panel bonded with UF (A) and poplar bark panels bonded with UF (E) and clay and starch (F, G, H). All composite boards with larch and poplar bark and clay were exposed to fire, and they were compared with a reference panel manufactured with the same raw material and bonded with UF (Figure 6). These mineral-bonded larch bark boards revealed their properties against fire, both on the surface and edge, with a duration of flame propagation (flame height under 12 mm). The density of the panels and their thicknesses seemed to play non-significant roles when exposed to fire (from 10 to 12 mm) [38]. For the UF-bonded boards (A and E), the height of the flame was higher compared with the bark composites mixed with clay (B, C, D) and clay with starch (F, G, H). The clay acted as a fire retardant and improved the fire resistance of the boards by 12-15% for the surface and 27-39% for the edge [38]. The larch bark is lumpy [58] and the poplar bark more fibrous; for this reason, the flame height was lower for the former. No drop formation was detected, and the filter paper placed under the testing frame did not ignite. It is in harmony with [PERSON] et al. [25], who studied the effects of fiberboard coated with nano-kaolin-clay (NK). The results lead to the conclusion that finishing with NK, together with a phenolic resin, is an effective method for increasing fire safety and resistance.