Thermally modified wood is widely used in cladding, decking, and other construction projects that are meant for outdoor exposure. The purpose of this study was to investigate changes in the color, microstructure, and chemical composition of heat-treated, Larix spp. wood that was exposed to artificial weathering. In this study, accelerated weathering tests (UV and moisture) were conducted over a period of 3000 h. Photodegradation of both heat-treated and untreated wood was evaluated in terms of color, microstructure, and chemical changes that were characterized using scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. Ultra-violet radiation caused the degradation of lignin and extractives of wood, resulting in an immediate color change of the wood. The SEM observation of the heat-treated wood showed deformations and cracks in both treated and untreated samples. Irradiation resulted in a pronounced reduction in the absorption intensity and broadening of the FTIR spectra. It was found that the industrial heat-treatment of wood products resulted in more color stability than untreated wood during the early stages of weathering. Thermal modification was found, however, was ineffective in improving the UV resistance wood over long-term photodegradation conditions.
Effect of Artificial Weathering on the Properties of Industrial-Scale Thermally Modified Wood
Dong Xing,a,b Siqun Wang,b and Jian Li a,*
Thermally modified wood is widely used in cladding, decking, and other construction projects that are meant for outdoor exposure. The purpose of this study was to investigate changes in the color, microstructure, and chemical composition of heat-treated, Larixspp. wood that was exposed to artificial weathering. In this study, accelerated weathering tests (UV and moisture) were conducted over a period of 3000 h. Photodegradation of both heat-treated and untreated wood was evaluated in terms of color, microstructure, and chemical changes that were characterized using scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. Ultra-violet radiation caused the degradation of lignin and extractives of wood, resulting in an immediate color change of the wood. The SEM observation of the heat-treated wood showed deformations and cracks in both treated and untreated samples. Irradiation resulted in a pronounced reduction in the absorption intensity and broadening of the FTIR spectra. It was found that the industrial heat-treatment of wood products resulted in more color stability than untreated wood during the early stages of weathering. Thermal modification was found, however, was ineffective in improving the UV resistance wood over long-term photodegradation conditions.
Keywords: Heat-treated wood; Artificial weathering; Photodegradation; UV resistance; FTIR
Contact information: a: Key Laboratory of Bio-based Material Science and Technology, Northeast Forestry University, Harbin, Heilong Jiang province, China 150040; b: Center for Renewable Carbon, University of Tennessee, Knoxville, TN, USA 37996; *Corresponding author: firstname.lastname@example.org
The use of heat-treated wood as an industrial product used for both indoor and outdoor applications is on the upswing. This highly desirable modification to wood maximizes its dimensional stability, durability, and attractive dark color (Nuopponen et al. 2003; Icel et al. 2015). Heat treatment process elements, such as duration of treatment, temperature of treatment, type of heating atmosphere, pressure, and catalysts vary (Shi et al. 2007). With the development of the manufacturing industry and controlling systems, thermal modifications to wood have resulted in several processes at the industrial level, such as Finland’s Thermowood® (or Premium Wood), the Retification® process (Retiwood, New Option Wood, France), France’s Le-Bois Perdure®, the Plato® process (Netherlands), and Oil-heat treatment (OHT®, Germany). Applications for heat-treated wood include furniture, parquetry, cladding, joinery, decorations, shutters, decking, and many others. Increased awareness of the existence and usefulness of wood products that do not contain any toxic preservatives has boosted the popularity of heat-treated wood in recent years.
Surface photodegradation of wood products, which is caused by the combined effects of ultraviolet (UV) and shorter wavelengths of visible radiation, heat, moisture, atmospheric pollutants, and microorganisms, has been extensively studied (Chaouch et al. 2010; Tomak et al. 2014). Chemical analysis of wood surfaces indicates that irradiation causes the degradation of hemicellulose, lignin, and the depolymerisation of cellulose (Forsthuber et al. 2014). Photochemical reactions on wood surfaces include the complex degradation of extractives, the reduction of the methoxyl content of lignin, the dissociation of carbon-carbon bonds, and the generation of carbonyl-based chromophoric groups (Decker 2005). In most outdoor conditions, the day-night cycle of ultraviolet radiation and humidity poses many challenges, swiftly inducing photodegradation and depolymerisation of wood components (extractives, lignin, cellulose, and hemicellulose). Among the constituent wood polymers, lignin is the most sensitive to light and has a structure that is vulnerable to the absorption of ultraviolet/visible light because of the chromophoric groups. This leads to the breakage of weak chemical bonds and the subsequent fading, darkening, and cracking of wood products.
Of all exterior environmental factors, light contributes most to the weathering of wood above ground (Pandey and Pitman 2003). The precise pathways and mechanisms involved in complex photochemical reactions of wood are not yet clear (Abu-Sharkh and Hamid 2004). Heat-treated wood shows better color stability in the first stage of exposure to UV radiation and moisture than does untreated wood, which is probably attributed to the thermal modification of lignin-cellulose formation and the modified chromophoric lignin structure (Ayadi et al. 2003; Colom et al. 2003; Peng et al. 2015). A number of studies have been carried out on the wettability (Huang et al. 2012) and mechanical (Yildiz et al.2013), physical (Podgorski et al. 1996), and chemical changes (Pizzo et al. 2015) to which heat-treated wood is susceptible, as a result of artificial or natural weathering (Rosu et al. 2010; Saha et al. 2013). Despite the tremendous utility of heat-treated wood in many circumstances, research into the weathering performance of industry-scale heat-treated wood products, with biomass gas as a shield atmosphere, has been scarce.
The aim of this study was to investigate the changes in the microstructure, chemical composition, and surface aesthetics of industrial-scale heat-treated wood after artificial weathering exposure over the following time periods: 24, 72, 120, 360, 720, 960, 1440, and 3000 h. The color stability, surface microstructure, and chemical composition of the wood were evaluated. Results of this research on a softwood variety (Larix) could provide a reliable performance indicator for heat-treated wood in outdoor conditions.
Larix gmelinii (Rupr.) Kuzen is a fast-growing softwood species that is found across northern and eastern China. In this study, a 15-year-old Larix gmelinii tree was selected from a location in the Xiangfang District, Harbin, Heilongjiang Province of China. Larixspp. is one of the most commonly used species for furniture, decoration, and outdoor facilities in China because of its low price and extractives composition that make it resistant to exterior conditions; however, modification is often needed to compensate for its poor dimensional stability, weak durability, and aesthetic appearance.
All samples were taken from one log and cut into dimensions of 2000 × 100 × 20 mm in the longitudinal, tangential, and radial directions, respectively. The 12 specimens were initially conditioned at 103 °C for 48 h before heat treatment, and were then sealed in 12 plastic bags.
In this study, the wood samples were heat-treated using the industrial method with biomass gas as both a shield environment and a heat-transfer medium. After the wood sawdust was ignited in the bottom of a chamber, biomass gas formed from the combustion and was then injected into the upper chamber for the heat-treatment process. The exhaust biomass gases from the heat treatment were passed through a non-thermal plasma air purifier.
Table 1. Heat Treatment Process
The lumber pieces that were prepared for the thermal treatment were enclosed in the sealed chamber and treated from 170 to 210 °C for 6 h, respectively. Each heat treatment was repeated three times. The thermal treatment process is shown in Table 1. The treatment temperature was controlled to within ±1.0 °C with 10 detective thermocouples distributed around the chamber. Then, all of the specimens were conditioned at an ambient temperature and relative humidity of 65 ±1.0% until the artificial weathering test began.
Artificial weathering tests were conducted at the Key Laboratory of Bio-Based Material Science and Technology, Northeast Forestry University, Harbin, Heilong Jiang, China. The heat-treated and the untreated samples were exposed to cycles of ultraviolet light and moisture under controlled conditions in an Accelerated Weathering Tester (Model QUV/spray with Solar Eye Irradiance Control, Q-lab Corporation, Cleveland, OH, USA), which simulates dew and rain by condensing humidity. The tester can reproduce the damage that may occur to wood over months or years of outdoor conditions. The artificial cycle included 8 h of irradiation using the lamp UVA-340 light 0.77 W/m2/nm at 50 ± 3.0 °C black-panel temperatures, followed by 4 h of condensation (dew) at 40 ± 3.0 °C. Nine heat-treated and 3 untreated specimens were fixed on the specimen holder and the irradiation was interrupted after 20, 72, 120, 192, 360, 720, 1188, 1440, 2200, and 3000 h of exposure, respectively. Then, the 12 specimens were removed for the evaluation of changes in chemical and physical structure before and after artificial weathering.
Measurements of the surface color of the specimens were performed with a CM-2300 d spectrophotometer (D5003908, Konica Minolta Sensing, Inc., Japan) for different periods of artificial weathering exposure; the spectrophotometer was equipped with an integrating sphere according to the color system, CIE L*a*b*system (CIELAB) (International Commission on Illumination). Measurements were made over an 8 mm diameter spot with 10 different tests on the tangential surfaces of the 12 specimens before and after weathering. The equipment was calibrated before each color measurement was recorded.
The three coordinates of CIELAB represent the lightness of the color, its position between red/magenta and green, and its position between yellow and blue, respectively. The color difference (ΔE) related a measurement to a known L*, a*, or b* value that was calculated as a function of the artificial weathering time according to the following equations,
where, represent the test values of specimens before artificial irradiation and denote the test values of specimens exposed to t hours of weathering.
Scanning electron microscopy (SEM) analysis
Scanning electron microscopy analysis was used to examine surface details before and after 3000 h of artificial weathering. The surface of specimens were split for the dimensions of 8 × 8 × 2 mm and all blocks were sputter-coated (BAL-TEC SCD 005, Germany) with a gold layer for 140 sec and were mounted on standard aluminum stubs with electrically conductive paste. The specimens were scanned using an FEI Quanta 200 scanning electron microscope (Hillsboro, OR) at a 10-kV accelerating voltage. The temperature conditions were approximately 20 °C and the column vacuum stood at 0.83 torr.
Fourier transform infrared (FTIR) spectroscopy
The surfaces of the specimens were cut and ground. After grinding, specimens were dried at 105 °C for 24 h before pellet preparation. Approximately 2.3 ± 0.1 mg of the specimen was mixed with 250 ± 2 mg of the mixture, and this was used for preparing KBr pellets. The specimens were analyzed using FTIR (USA Nicolet Company Magna-IR 560 E.S.P) spectroscopy at a resolution of 4 cm-1 and 40 scans between wavenumbers of 4000 and 400 cm-1. Measurements were performed on four replicates per specimen.
RESULTS AND DISCUSSION
The color and visual changes observed during different exposure periods for both the control and heat-treated specimens are shown in Fig. 1. It was observed that the surfaces of all of the samples faded and became lighter immediately after the short-term artificial weathering. The light-induced photodegradation markedly reduced the integrity of both the heat-treated and untreated wood surfaces.
The color changes in the untreated specimens were much greater than those of the wood heat-treated for 360 h, showing that they resisted UV radiation better than the heat-treated samples. However, the surface color differences between the heat-treated and untreated samples became insignificant with long-term weathering exposure, which accords with previous studies (Huang et al. 2013). With regards to the moisture spray section of artificial cycles, water stains were seen on the samples after 72 h of artificial weathering. The water stains on all of the samples became larger with increased artificial time because of the uptake and release of water. However, it was difficult to analyze these phenomena by visual observation alone. Further investigations were performed and are presented below.
Fig. 1. The visual observation of heat-treated and untreated Larixspp. surfaces during different artificial weathering periods
Microstructure Analysis using SEM
Microstructural changes in the wood surface properties were characterized using SEM. Figure 2 shows the SEM microstructure on the tangential surfaces of both the controls and the heat-treated wood samples before and after artificial weathering for 3000 h. In Fig. 2a, the untreated sample has intact cell walls and some easy-to-recognize damage caused by splitting; however, the heat treatment had only a slight effect on the cell wall structure (Figs. 2b-d); a previous heat treatment study yielded similar results (Yildiz et al. 2013), with cracks between the S1 and S2 layers, visible changes in the pits, and abrupt transitions between earlywood and latewood.