Abstract
The feasibility of using β-cyclodextrin (βCD) as an eco-friendly carrier of boric acid for the protection of strand-based wood composites against decay fungi was evaluated. The formation of a βCD–boric acid (βCD–B) complex was confirmed by the appearance of the boron–oxygen bond by using attenuated total reflection–Fourier transform infrared spectroscopy. Chemical shifts of around 6.25 and 1.41 ppm were also observed in 1H Nuclear Magnetic Resonance (NMR) and 11B NMR spectra, respectively. The βCD–B preservatives at two levels (5 and 10 wt.%) were uniformly blended with southern pine strands that were subsequently sprayed with polymeric methylene diphenyl diisocyanate (pMDI) resin. The blended strands were formed into a loose mat by hand and consolidated into 25 × 254 × 12 mm oriented strand boards (OSB) using a hot-press. The OSB panels were cut to end-matched internal bonding (IB) strength and fungal decay resistance test specimens. The vertical density profiles (VDPs) of the IB specimens were measured using an X-ray based density profiler and the specimens with statistically similar VDPs were selected for the IB and decay tests. The IB strength of the treated specimens was lower than the control specimens but they were above the required IB strength of heavy-duty load-bearing boards for use in humid conditions, specified in the BS EN 300:2006 standard. The reduced IB of preservative-treated OSB boards could be explained by the destabilized resin upon the addition of the βCD–B complex, as indicated by the differential scanning calorimetry (DSC) results. The resistance of the OSB panels against two brown-rot fungi (i.e., G. trabeum or P. placenta) was evaluated before and after accelerated leaching cycles. The treated OSBs exposed to the fungi showed an average mass loss of lower than 3% before leaching, while the untreated OSBs had 49 and 35% mass losses due to decay by G. trabeum or P. placenta, respectively. However, upon the leaching, the treatment provided protection only against G. trabeum to a certain degree (average mass loss of 15%). The experimental results suggest that protection efficacy against decay fungi after leaching, as well as the adhesion of the OSB strands, can be improved by increasing the amount of pMDI resin.
Highlights
The protection of wood composites by various biocides has been practiced since the end of the last century [1]
The C–H stretching (~2924 cm−1 ) in the βCD–boric acid (βCD–B) complex exhibited a lower intensity and shifted to a lower frequency, which possibly resulted from an altered environment around these bonds upon complexation
The formation of the βCD–B complex was indicated by the changes in OH groups and the formation of new peaks attributed to borate ester [25]
Summary
The protection of wood composites by various biocides has been practiced since the end of the last century [1]. Most of the conventionally used preservatives are either heavy metal based. Polymers 2020, 12, 274 compounds (e.g., chromated copper arsenate) or synthetic compounds (e.g., pentachlorophenol), which are considered toxic and harmful to human beings and our environment [2,3]. As one of the oldest preservatives, boron exhibits a low level of toxicity to mammals and the environment [4]. Borate-treated wood products are not corrosive to metal fasteners and are colorless and odorless after treatment [5]. Borates can penetrate into the wood at a higher moisture content, making them ideal for treating refractory wood species, such as Douglas fir [6]. Due to the mobility of borates in aqueous solutions, borates are vulnerable to leaching, which considerably reduces the life-span of borate-treated wood products in outdoor applications
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