# Improved Durability of Wood Treated with Nano Metal Fluorides against Brown-Rot and White-Rot Fungi
## 1 Introduction  
Products made from wood such as those used in construction are known to be environmentally sustainable. In comparison to concrete and steel, wood-framed buildings have the lowest carbon footprint and can function as a carbon sink [1, 2]. The energy required to construct a house using timber is lower than steel or concrete [3]. It is estimated that use of timber instead of steel or concrete for new building projects can function in storing 20 gigatons of carbon [1]. Choosing timber over other construction materials can play an important tool in climate change mitigation strategies, although it is vulnerable to degradation by organisms such as fungi. Therefore, protection of wood products from decay aimed at extending their service life is highly important. Commercially, various preservative formulations and modification techniques are available to protect wood from wood-degrading organisms [4, 5]. In modification methods, the hydrophobicity of wood is increased to reduce its moisture content that increases its resistance to wood-degrading organisms and termites [4]. Recent chemical modification methods have shown that the combination of polyethylene glycol (PEG) with silica sol impregnation improved the hydrophobicity of wood, though results on durability against wood-degrading fungi have not been reported [6]. Another study reported that titanium alkoxide modified wood had mass losses up to 5% against _Coniophora puteana_, however the specimens were unleached [7]. Among preservative formulations, copper still remains the most extensively used active ingredient for wood protection, closely followed by boron [8; 9]. To provide sufficient protection the active ingredient in preservative formulations needs to be uniformly distributed inside wood. One method to improve the distribution of preservative formulations in wood has focused on using active ingredients in their nanoscopic form. Developments in this area has led to micronized copper (Cu) taking over 50% of the commercial Cu containing preservative market in North America [10]. In addition to copper, nanoparticles of active ingredients such as boron (B), silver (Ag), titanium dioxide (TiO\({}_{2}\)), and zinc (Zn) have been assessed for their biocidal efficacy against wood-degrading fungi [11]. Cu and Ag nanoparticles in particleboard resulted in mass losses greater than 10% [12]. Similarly, nanoparticles of copper oxide (CuO) and zinc oxide (ZnO) dispersed in propylene glycol tested in rubberwood against white-rot and brown-rot fungi led to mass losses greater than 10% [13]. Still, ZnO nanoparticles protected sapwood of Scots pine and beech wood specimens which had been artificially aged by leaching against white-rot fungi, _Tranetes versicolor_[14]. Yet the same treatment in pine wood resulted in high mass losses against brown-rot fungi _C. putteana_[14]. Combination of TiO\({}_{2}\) and ZnO were also evaluated in poplar wood which resulted in mass losses below 1% against _T. versicolor_[15]. Although these treated wood specimens were UV weathered, they did not undergo accelerated ageing through leaching. Another study reported that nano ZnO treatment alone did not protect against brown-rot fungus, _Rhodonia placenta_, but nano zinc borate reduced mass losses in wood to below 1% [16]. However, results on nano zinc borate treated specimens that were leached and exposed to _Rhodonia placenta_ were not reported. One nano-based treatment that showed resistance against _R. placenta_ and _C. putteana_ in spruce wood specimens that were leached was TiO\({}_{2}\)/Cerium (Ce) [17]. Still, it is not known if TiO\({}_{2}\)/Ce would protect wood from white-rot fungi, _T. versicolor_. Thus, so far there is no single nano-based treatment reported that protects wood against decay caused by _R. placenta_ and _T. versicolor_. Fluorides had been also widely used in wood preservative formulations. Fluoride-based preservatives were reported to have efficacy against basidiomycetes with average threshold values of 1 kg/m\({}^{3}\) in wood [18].