Abstract
The dimensional stability and resistance to degradation of wood can be improved using high temperature heat treatment under anaerobic conditions; however, mildew growth can have a deleterious impact on its appearance and commercial value. In this study, wood samples were impregnated in copper-containing solutions at high pressure before being recovered and cured at high temperatures to create treated wood samples with nano copper particles. This copper impregnated wood (up to 6.35% copper content) suppressed the growth of Botryodiplodia theobromae Pat. and Aspergillus niger van Tieghem with 100% efficiency, and Penicillium citrinum Thom with 75% efficiency. However, the growth of Trichoderma viride Pers. was not suppressed. These results demonstrate that copper curing can be used to extend the scope, performance, and lifetime of heat-treated wood, enabling it to be used for a new range of applications.
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Anti-Mildew Properties of Copper Cured Heat-Treated Wood
Guijun Xie,a,b Yongdong Zhou,a,* Yongjian Cao,b and Lamei Li b
The dimensional stability and resistance to degradation of wood can be improved using high temperature heat treatment under anaerobic conditions; however, mildew growth can have a deleterious impact on its appearance and commercial value. In this study, wood samples were impregnated in copper-containing solutions at high pressure before being recovered and cured at high temperatures to create treated wood samples with nano copper particles. This copper impregnated wood (up to 6.35% copper content) suppressed the growth of Botryodiplodia theobromae Pat. and Aspergillus niger van Tieghem with 100% efficiency, and Penicillium citrinum Thom with 75% efficiency. However, the growth of Trichoderma viride Pers was not suppressed. These results demonstrate that copper curing can be used to extend the scope, performance, and lifetime of heat-treated wood, enabling it to be used for a new range of applications.
Keywords: Copper-containing compounds; Heat treatment of wood; Anti-mildew; SEM; XRD; XPS; Nano copper particles
Contact information: a: Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing, 100091; b: Guangdong Provincial Key Laboratory of Silviculture, Protection and Utilization, Guangdong Academy of Forestry, Guangzhou, 510520; *Corresponding author: zhouyd@caf.ac.cn
INTRODUCTION
Heat-treated wood processed at 160 to 260 °C under a low oxygen atmosphere is a new environmentally friendly material that has been widely used for decorative wall paneling applications. This type of heat-treated wood can be divided into three categories, with wood being processed under an atmosphere of nitrogen, or via treatment with steam, or oil (Yan-Jun et al. 2002; Esteves et al. 2013). These treatments render the wood more visually appealing (Unsal et al.2003; Johansson and Morén 2006), affording improved dimensional stability and degradation resistance properties after heat treatment (Dubey et al. 2012; Priadi and Hiziroglu 2013). However, its commercial quality is dramatically undermined, as mildew growth on wood is more likely to occur after it has undergone thermal treatment (Kun et al. 2010). Therefore, the identification of anti-mildew agents that can be applied to heat-treated wood would greatly increase its commercial applications.
The anti-mildew properties of heat-treated wood have been studied by many researchers, with Gu et al. (2010) reporting that Botryodiplodia theobromae Pat. caused less damage to heat-treated wood (185 and 205 °C for 1.5 h) from Pinus sylvestris var. mongolicaand Quercus mongolica Fisch., even though mildew growth was not totally suppressed. Sivonen et al. (2003) analyzed damage to wood from Pinus spp. caused by Coriolus versicolor and concluded that good anti-mildew properties could only be achieved by heat-treating pine wood at temperatures above 220 °C. Theander et al. (1993) found that heat treatment resulted in accumulation of oligosaccharides at the surfaces of wood from Pinus sylvestris and Picea asperata, with the nitrogen content of these carbohydrates causing yellowing of the wood over time. The growth of Penicillium brevicompactum on wood is closely related to its nitrogen content and the amount of low molecular weight polysaccharides present, with the growth of mildew (like Aspergillus spp.) being suppressed by substances generated in reactions that occur during the heat treatment process. However, feasible solutions that can effectively suppress mildew growth on heat-treated wood still need to be developed to enable its commercial application to be broadened.
The characteristics of inorganic nano-materials (small size, large specific surface area, quantum size effects) means that they can potentially be used as environmentally friendly, antibacterial, and anti-mildew materials for coating heat treated wood (Kartal et al.2009). They are an effective method for suppressing mildew growth, with cheap nano-copper particles widely used as effective anti-mildew agents, due to their inherent antifungal properties and low toxicity (Zhang et al. 2013; Yu et al. 2015). Initial methods developed to prepare nano copper particles were low yielding, energy-inefficient, and used toxic chemicals under non-aqueous conditions that required the presence of an inert atmosphere (Wu et al. 2006; Sastry et al. 2013). Consequently, new low-cost (Liu et al. 2012) liquid-phase reduction (Jain et al. 2015), and template methods (Yu et al. 2004) have been introduced for the cost-effective production of copper nanoparticles. For example, Khanna et al. (2007) prepared pure nano-copper particles using liquid-phase reduction conditions that used sodium formaldehyde sulfoxylate and sodium citrate (or tetradecanoic acid) as reagents. Heat-treated wood has previously been cured by dipping in copper containing suspensions at high pressures and temperatures. This results in liquid-phase reduction conditions that generate nano copper particles in situ (Yang and Fan 2014), with the resultant copper cured wood samples exhibiting improved anti-mildew properties.
This study confirms that copper nanoparticles can be used as an effective anti-mildew treatment for protecting heat-treated wood against unwanted fungal growth.
EXPERIMENTAL
Materials
Wood was harvested from 25-year-old Pinus massoniana Lamb. that did not exhibit any signs of decay or mildew, with each test group consisting of six individual wood samples. Copper solutions were prepared by mixing copper hydroxide, diethylamine alcohol, polyethylene glycol 200, and water. Copper ammonia solutions were prepared by adding water to a stirred solution of copper hydroxide dissolved in diethanolamine. Other copper-containing solutions (designated as CuG) were prepared by adding polyethylene glycol 200 to copper ammonia solution.
Methods
Preparation of copper-containing wood samples
The dimensions of wood samples were 50 mm (longitudinal) × 20 mm × 5 mm. Samples were pre-heated in an oven at 60 °C until their weight remained constant. Samples were then submerged in a copper containing solution for 30 min at a vacuum pressure between 0.09 MPa and 1.5 MPa for 40 min. The wood samples were then recovered and dried at 60 °C until their weight remained constant.
Heat treatment of wood samples
Wood samples were heat-treated using a custom-designed machine according to the following procedure. Pinus massoniana Lamb. wood samples that had been dipped in copper-containing solutions (and untreated controls) were placed on the iron wire net of a heating tank. The door of the heating chamber was closed, and the air pressure to the steam generator and carbonization unit set at 200 and 100 kPa, respectively, with oxygen being removed continuously from the heating tank over a period of 20 min. The steam carbonization process was then switched to an electrothermal carbonization process that was carried out at 220 °C for 3 h. After the heating process, the steam generator was turned on and the chamber allowed to cool viasteam evacuation until the temperature decreased to 140 °C. The heat-treated samples were then retrieved from the heating tank.
Table 1. Curing Conditions Used for the Heat Treatment of Pinus massoniana Wood
Samples with dimensions of 50 mm × 20 mm × 5 mm were directly heat-treated to produce test samples for anti-mildew tests, with these treated wood samples then ground into a 40-60 mesh powder for subsequent SEM, EDS, XPS, and XRD analysis.
Anti-mildew tests and performance evaluation of wood samples
The dimensions of the wood samples used in the anti-mildew tests were 50 mm (along the texture length) by 20 mm by 5 mm. Each anti-mildew test was conducted on 6 identical wood samples, with individual samples being treated using the different conditions listed in Table 1. The anti-mildew properties of each sample were investigated by evaluating their ability to suppress the growth of four different mildews: Botryodiplodia theobromae Pat., Aspergillus niger van Tieghem, Penicillium citrinum Thom, and Trichoderma viride Pers. Antimildew tests were carried out according to the GB/T 18261 (2013) standard. A typical test involved the following steps: hot mildew medium containing 2% (w/w) maltose and 1.5% (w/w) agar was first poured into a sterilized glass culture dish. The mildew medium was cooled to room temperature and mildew spores inoculated into the medium. The glass medium dish was stored at 28 °C at a relative humidity of 85% for 1 week to allow the mildew to grow. Two sterilized glass rods were inserted into the mildew medium, and the sterilized wood samples placed on top of the glass rods. The samples were left for 1 month before their degree of infection was determined. The anti-mildew efficiency (E) of the wood samples was calculated according to Eq. 1,
(1)
where D1 and D0 are the average degrees of infection of copper impregnated wood samples and wood controls respectively.
Color change ratings were divided into 4 classes: 0 indicated no mildew growth was present; 1 indicated that less than 25% surface area of the wood sample was covered by mildew; 2 indicated that 25 to 50% of the wood sample was covered by mildew; 3 indicated that 50 to 75% of the wood sample was covered by mildew; and 4 indicated that greater than 75% of the wood sample was covered by mildew.
Scanning electronic microscopy (SEM) and energy dispersive spectroscopy (EDS)
Two copper containing wood samples were ground into powders that were subsequently analyzed using SEM (Zeiss SUPRA 40, Oberkochen, Germany) and EDS (ZEISS SUPRA 40).
X-ray photoelectron spectroscopy (XPS)
A total of 20 mg of copper containing wood samples were ground into powders and their elemental composition and valence states analyzed using a Thermo Fisher Scientific Escalab 250Xi XPS machine (Waltham, MA, USA).
X-ray diffraction (XRD) analysis
A total of 100 mg of copper containing wood samples were ground into powders and the crystal structures and sizes of their metal particles analyzed using a Bruker D8 XRD analyzer (Karlsruhe, Germany). The crystal sizes of the particles were calculated according to the Scherrer equation (Eq. 2),
(2)
where D is the crystal size, K is a constant; λ is the wavelength of the incident X-ray; β is the full width at half maximum (FWHM) of the diffraction peak; and θ is the recorded diffraction angle. The value of the constant K in the above equation is related to the definition of β, with K equal to 0.89 when β is referenced to the FWHM and equal to 1.0 when β is referenced to the integral breadth.
RESULTS AND DISCUSSION
Anti-mildew Properties of Copper Nanoparticle Treated Pinus massoniana Lamb Wood Samples
The ability of Botryodiplodia theobromae Pat., Aspergillus niger van Tieghem, Penicillium citrinum Thom, and Trichoderma viride Pers.,to grow on control wood samples, heat-treated wood samples, and copper impregnated wood samples was investigated. Four control samples were used to verify the reliability of the assay conditions in each case, with samples that were completely covered with mildew assigned an anti-mildew efficiency score of 0 (Fig. 1). The anti-mildew properties of the controls and heat-treated samples were all scored as 0, indicating that untreated and heat-treated wood samples exhibited very poor anti-mildew properties (Du et al. 2016).
However, heat-treated copper containing wood samples effectively suppressed the growth of Botryodiplodia theobromae and Aspergillus niger, with 100% inhibition of these fungi being observed when relatively high concentrations of copper were present in the dipping solution. It is likely that the micro/nano scale copper particles and oxides generated during the reduction of copper ions by polyethylene glycol 200 and the presence of reducing sugars and other reducing agents combine to suppress the growth of these mildew (Kamdem et al. 1998; Zhang et al. 2013; Yang and Fan 2014). There was 75% inhibition of the growth of Penicillium citrinum on impregnated wood samples treated with 6.35% copper-containing solutions, but no anti-mildew effect was observed for Trichoderma viride, reflecting the difficulty of preventing heat-treated wood being colonized by these aggressive fungi (Salem et al. 2016).
Fig. 1. Prevention of mildew growth on different types of Pinus massoniana wood samples
Exploration of the Anti-mildew Mechanism of Heat-treated Copper-containing Wood
Curing copper containing wood samples at 220 °C for 3 h effectively suppressed the growth of certain types of mildew; therefore, the structural morphology of these wood samples was investigated to elucidate how this treatment process might suppress fungal growth.
SEM and EDS Analysis of Heat-treated Copper-containing Wood
Heat-treated copper-containing wood samples that exhibited anti-mildew properties were analyzed using SEM and EDS. Figure 2 shows the SEM images and corresponding EDS results for copper-containing wood samples treated at 220 °C for 3 h. The SEM images show that particles adhered to the surface of the wood sample, with EDS analysis confirming the presence of copper. The EDS analysis in Fig. 2 also shows that the O/C ratio varied from 0.63 to 0.48 depending on the temperature and length of treatment time. In this respect, it is likely that dehydration byproducts are generated when wood is heat-treated, leading to a decrease in its oxygen content (Inariet al. 2006).
Fig. 2. SEM and EDS results for heat-treated wood samples dipped in suspensions containing different concentrations of copper
Figure 1 reveals that the amount of Penicillium citrinum that grew on the heat-treated copper-containing wood samples initially decreased with increasing concentrations of CuG in the dipping solution, but then it increased again at higher levels of CuG. After reaching a peak level, the conversion rate for reduction of Cu2+ to Cu0 and Cu+decreased as the concentration of Cu2+ increased. Further investigations will be required to gain a better understanding of the reaction pathways that are occurring.
XPS Analysis of Heat-treated Copper-containing Wood Samples
XPS analysis of the surface of particles present in the heat-treated copper-containing wood sample (220 °C for 3 h) revealed the presence of peaks for Cu+ at 934.9 eV and 954.4 eV, with Cu0 peaks being detected at 932.9 eV and 952.6 eV, respectively (Fig. 3). These XPS results confirmed the presence of nano-copper and nano-cuprous oxide particles in these copper-impregnated wood samples. Only one peak was observed in the O 1s region, which was assigned to the presence of C-O bonds (Inari et al. 2006; Nzokou and Kamdem 2010). Three peaks were present in the C 1s region, corresponding to the presence of C-C, C-O, and C=O bonds. The O/C ratio of pure cellulose is 0.83 and the O/C ratio of pure lignin is 0.33 (Inari et al.2006; Nzokou and Kamdem 2010). The O/C ratio for the heat-treated copper-containing wood samples was between 0.42 and 0.48, suggesting that hydrothermal carbonization occurs during the steam treatment process at 220 °C, resulting in decomposition of the more labile hemicellulose component (Huang et al. 2010, 2013). This results in a decrease in the amount of accessible sugar feedstocks that are available for mildew growth, which also leads to a relative increase in lignin content, which is known to increase the ability of wood to suppress mildew growth.
XRD Analysis of Dipped and Heat-Treated Wood Samples
Figure 4 shows the different diffraction peaks that were detected for copper-treated wood samples at 43.3°, 50.4°, and 74.1°. These peaks match known crystal data for (111), (200), and (220) planes of cubic crystalline structures (Usman et al. 2013) for elemental copper, with no evidence of any cuprous oxide being present in the treated wood samples. Ding et al. (2013) reported on the potential leaching of nano copper particles (10, 50 nm, and ACQ) from treated wood samples, describing that micro/nano-scale copper particles with sizes of 50 nm exhibited the best resistance to leaching. In accord with the Scherrer equation and the XRD results, the average size of the nano copper particles present in the treated wood samples in this study was 100 nm, which is likely to result in good anti-leaching properties. Therefore, the excellent anti-mildew properties of the heat-treated copper-containing wood samples are attributable to the presence of elemental copper in micro/nano scale form that are resistant to leaching (Xu 2013).
The in situ process used to generate the micro/nano scale copper particles represents an eco-friendly coating method (Sedighi et al. 2014; Dobrovolný et al. 2015) that minimizes exposure of the user to potentially toxic nanoparticles (Raffi et al. 2010). Micro/nano scale copper particles in the heat treated wood adhere to and disrupt the cytoderm of mildew through a series of electrostatic interactions with the cell wall that serve to inhibit fungal growth (Mondal and Mani 2012; Civardi et al. 2015). Future work in this area will be directed towards determining the mechanism of action of the anti-mildew properties of these copper micro/nano scale particles, with the aim of identifying new treatments that can effectively suppress the growth of recalcitrant fungi (such as Trichoderma viride) on heat-treated wood.
Fig. 3. XPS analysis of heat-treated wood samples dipped in solutions containing different concentrations of copper
Fig. 4. XRD analysis of heat-treated wood samples dipped in suspensions containing different concentrations of copper
CONCLUSIONS
- Wood samples were dipped in copper-containing solutions and then heat-treated at 220 °C for 3 h to create copper-impregnated heat-treated wood samples. XPS and XRD analysis revealed that the surfaces of these treated wood samples contained significant amounts of nano copper particles.
- Heat-treated wood samples containing a copper content of 6.35% suppressed the growth of Botryodiplodia theobromaePat. (100% suppression), Aspergillus niger van Tieghem (100% suppression), and Penicillium citrinum Thom (75% suppression) on heat-treated wood.
ACKNOWLEDGMENTS
This research was jointly sponsored by the National Key Research Project “Chemical Basis Research of Reformation of Wood from Artificial Forest (2017YFD0600203)” and the Provincial Science and Technology Research Project of Guangdong “Research on the Mechanism of Mildew Growth on Heat Treated Wood and Its Preventive Treating Method (2014A040401043)”.
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Article submitted: February 27, 2018; Peer review completed: April 9, 2018; Revised version received and accepted: May 24, 2018; Published: June 6, 2018.
DOI: 10.15376/biores.13.3.5643-5655