# Enhanced Resistance to Fire of the Bark-Based Panels Bonded with Clay
## 3 Results and Discussion  
Thickness swelling of the bark composites bonded with clay was observable because the test specimen (n = 10) started to disperse after water immersion within one hour. For this reason, only the results for the UF-bonded bark panel are available: thickness swelling after 1 h: 2.5%, after 2 h: 4% and after 24 h: 8.68%; water absorption after 1 h: 6%, after 2 h: 7.7% and after 24 h: 41.74%. Therefore, larch bark composites bonded with clay are suitable to dry or slightly wet environments only, similar to standard wood cement boards. In the case of poplar bark panels bonded with clay and starch (F, G and H), the dimensional stability is different. A lower swelling in thickness was measured for the board glued with UF (3%), while the values for the formulations with clay and starch ranged from 28% (G) to 43% (F). A similar trend was followed for the water uptake. The lesser value was reported for sample G (43%), while 66% water uptake was calculated for sample F. In Table 3 are the values of thickness swelling and water absorption for bark boards, wood particle cement and wood wool cement boards. In the case of needing to use bark boards in a wet environment, into the bark board with UF can be added a small amount of nanoclay, which can improve water absorption and thickness swelling. Extensive research in adding nanoclay to particleboard has taken place in recent years. [PERSON] et al. [53] investigated the effect of adding nanoclay particles (3% and 6%) to resin on the properties of particleboard. Their research results showed that WA decreased by adding small amounts of nanoclay to UF-bonded boards, and TS was lower by 36%. In case of TS, these results were also confirmed in research by [PERSON] and [PERSON] [54]. The authors investigated the effects of different nanoclay loadings (2%, 4% and 6%) on the physical and mechanical properties of _Melia composita_ particleboard. The internal bond of the clay-bonded larch bark panels is moderate (Figure 4). Values in the range of 0.010 N/mm\({}^{2}\) to 0.035 N/mm\({}^{2}\) were measured in the case of the UF-bonded larch bark board (A). When poplar bark was bonded only with UF (E), the internal bond was 0.334 N/mm\({}^{2}\). The board with a 913 kg/m\({}^{3}\) density (G) had an internal bond of 0.3 N/mm\({}^{2}\), comparable with panel E. One of the reasons for this is that the clay gains its hardness and consistency when fired at temperatures over 800 \({}^{\circ}\)C. If it is only dried, as in the test, it is brittle. Trial samples made of bark with UF resin (A) could achieve values similar to those of industrially manufactured products (internal bond of 0.51 N/mm\({}^{2}\)). A raw particleboard with a similar thickness should have a minimum transverse tensile strength of 0.24 N/mm\({}^{2}\) (P1), according to EN 312:2010 [55] that states the requirements for particleboard. Similar to the cases of WA and TS, the internal bond can be improved by adding a small amount of nanoclay to UF-bonded bark boards. A statistically significant increase in IB occurred in the case of UF-bonded boards with the addition of nanoclay particles in the research by [PERSON] et al. [53]. Similar improvements can also be achieved in the cases of modulus of rupture (MOR) and modulus of elasticity (MOE). The authors also achieved a 39% increase in MOR by adding 6% nanoclay and a 73% increase in MOE with the same amount of nanoclay added. These results were confirmed by [PERSON] and [PERSON] [54]. In their case, MOR showed the most significant improvement (34%) with 6% nanoclay loadings and MOE 65%, also with 6% nanoclay loadings. [PERSON] et al. [56] investigated