# Improved Durability of Wood Treated with Nano Metal Fluorides against Brown-Rot and White-Rot Fungi
## 4 Discussion  
The biocidal efficacy of combination of MgF\({}_{2}\) and CaF\({}_{2}\) against _R. placenta_ and _T. versicolor_ was demonstrated with an _in-vitro_ agar test. In comparison Cu-amine was reported to have  
Figure 5: MAS \({}^{19}\)F (Larmor frequency: 376.4 MHz) NMR spectra of unleached treated (MgF\({}_{2}\) + CaF\({}_{2}\)) (black) and leached treated (MgF\({}_{2}\) + CaF\({}_{2}\)) wood (blue) specimens.  
Figure 6: Average mass loss caused by _R. placenta_ and _T. versicolor_. A—Ethylene glycol, B—NaF (1 M), C—MgF\({}_{2}\) (0.5 M), D—MgF\({}_{2}\) (0.8 M), E—MgF\({}_{2}\) + CaF\({}_{2}\) (0.5 M). The horizontal line is the mass loss cut-off at 3% for durability classification.  
limited protection against _T. versicolor_ in an _in-vitro_ agar test [25]. Thus, the combination of nano metal fluorides is more biocidal compared to Cu-amine against _T. versicolor_.  
Subsequently wood was treated with NMFs, leached, and characterized by MAS \({}^{19}\)F NMR and X-ray micro-CT. It was observed that NMFs were present in treated wood even after leaching. This is likely due to their lower solubility. Although the retention values for NMF treated wood are higher than those reported for micronized copper [8], they were lower compared to nano zinc-borate treated wood which had retention of 17.9 kg/m\({}^{3}\)[31]. Decay resistance tests demonstrated that wood treated with NMF combination of MgF\({}_{2}\) and CaF\({}_{2}\) and leached had mass losses below 3% against _R. placenta_ and _T. versicolor_. This result is in agreement with results from _in-vitro_ agar tests. Another significant finding of this study is that even after leaching, biocidal efficacy is provided by the combination of MgF\({}_{2}\) and CaF\({}_{2}\) against brown-rot and white-rot fungi.  
The synergistic biocidal efficacy of the NMF combination could be due to several factors. First, the differences in their solubility with MgF\({}_{2}\) having a higher solubility of 130 mg/L and CaF\({}_{2}\) of 16 mg/L may have resulted in an optimal release of fluoride that was biocidal to both fungi [30]. Secondly, a metastable phase of CaMgF\({}_{4}\) which has been reported before could have been formed that maintains sufficient fluoride aggregates within wood [32]. Thirdly, the NMF combination of low-water soluble fluorides in wood reduced the leaching of fluoride such that the amount remaining could still provide long-term protection against fungal decay. Further research needs to be done to determine the amount of fluoride present in wood at the end of the test and to investigate the tuning of the solubility of the metal fluorides in the combination that promotes its biocidal efficacy against fungi.  
Still, NMF treated wood provides a better alternative than highly soluble NaF and Na\({}_{2}\)SiF\({}_{6}\) formulations for wood preservation. Although decay resistance is reported for TiO\({}_{2}\)/Ce against _R. placenta_ and nano-ZnO against _T. versicolor_, there is no single nano-based wood treatment that had efficacy against both fungi after leaching [16; 17]. An advantage of the use of nano metal fluorides in treated wood that was shown in this study is that they are safe to handle and can provide durability without the need for additional fixatives in outdoor applications with no soil contact. Future characterization studies need to be done to determine the inclusion of nano metal fluorides in cell walls. In addition, the durability of NMF treated wood can be tested against termites (white ant), blue-stain and soft-rot fungi.