The organic material of wood treated with inorganic nanoparticles has gained significant improvements in its stability against ultraviolet (UV) light and moisture degradation. Here, a new method was proposed to modify Chinese fir (Cunninghamia lancellata), which was treated by high-temperature steam and coated with titanium dioxide (TiO2) nanoparticles. Due to the synergistic effect, the treated Chinese fir can obtain the properties of surface hydrophobicity and photoaging resistance that can prevent the invasion of moisture and the damage of UV light from sunlight. Scanning electron microscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction were used to characterize the morphology, structure, chemical composition, hydrophobicity, and light-aging resistance of the treated Chinese fir. By means of comparative experiments, the Chinese fir was treated with 5 percent nano-TiO2 solution and annealed at 300°C. The surface water contact angle reached to about 130°, indicating the strong hydrophobicity. In addition, the total color difference (ΔE*) after accelerated light aging for 120 hours was only 45 percent of the control sample.
Wood, as a green environmental protection material, has many excellent characteristics including light weight (Corinaldesi et al. 2016), high ratio of strength to weight (Hoque et al. 2014), good elasticity, impact resistance, beautiful texture and tone, and easy processing (Falk 2010). Chinese fir (Cunninghamia lancellata), a commercial forest species south of the Yangtze River in China, has the characteristics of rapid growth, straight texture, uniform structure, and good quality. The wood of Chinese fir has been applied extensively in construction, furniture manufacturing, and interior decoration (Ridoutt et al. 2002, Bovea and Vidal 2004). However, wood has strong hygroscopicity due to its pore structure with abundant hydrophilic groups (Bledzki et al. 2005, Van Meel et al. 2011), which might cause some shortcomings, including warpage, deformation, and cracking. The reason is that moisture is easily absorbed by the wood's pores, which can increase the porosity between the wood's cells and open the material reaction channel so that the degradation products are washed away. Besides, wood material under a corrosive environment for a long time will degrade and change color (Muasher and Sain 2006), especially when exposed to ultraviolet (UV) light. When the wood is exposed to the natural environment, UV light of the sun's rays can lead to photochemical degradation on its surface. The UV light is thought to have the most damaging effect on wood through depolymerizing lignin in the cell wall (Hon and Chang 1984). Subsequently, the decomposed lignin in the surface of wood cells can deteriorate the physical, chemical, and biological properties, which further causes the surface's discoloration.
Therefore, it is necessary to develop a hydrophobic protection treatment for wood or wood products before use. Many traditional hydrophobic treatment methods, including chemical reagent impregnation and surface decoration, can improve the wood's hydrophobicity. However, the surface decoration cannot fundamentally inhibit the intrusion of moisture into the wood, making it difficult to protect the wood surface for a long time. Also, chemical reagent impregnation can affect the structure, composition, and mechanical strength.
The high-temperature steam treatment is an environmentally friendly treatment for wood materials. After the wood materials are treated with high-temperature steam, the number of hydrophilic hydroxyl groups on the cell wall decreases greatly and produces hydrophobic groups at the same time, which will change the chemical composition and structure of the cell wall. The mechanism is that the high temperature can increase the crystallinity of cellulose, preventing water molecules from quickly contacting hydroxyl groups, reducing the hygroscopicity of wood, and improving dimensional stability. Hemicellulose has weak heat resistance (Kamdem et al. 2002) and will degrade and generate free acetic acid at high temperatures. The formation of free acetic acid significantly reduces the favorite food of decay-causing bacteria (Clausen 1996, Johnston et al. 2016), thereby inhibiting the growth of decay-causing bacteria from the food chain and improving the dimensional stability, corrosion resistance, and durability of the wood.
Advances in nanotechnology provide new ways to enhance the hydrophobic properties of wood. For example, inorganic UV shielding agents (such as zinc oxide, silicon dioxide, and titanium dioxide [TiO2]; Salla et al. 2012, Devi and Maji 2013) have attracted increased attention due to their high chemical stability, thermal stability, nonmigration, and safety, and have been applied to increase light resistance in aging materials. As a common photocatalytic antibacterial agent, TiO2 has low toxicity, stable chemical properties, and high refractivity and photoactivity due to its small particle size and high activity (Sun et al. 2012). Based on previous research (Harandi et al. 2016), TiO2 nanoparticles deposited on the wood surface were recognized to improve the wood stability against UV light and moisture degradation, which takes advantage of the photocatalytic activity of TiO2 nanoparticles to protect the wood surface.
In this study, Chinese fir decorated with TiO2 nanoparticles was treated with high-temperature steam, a new method developed to improve water resistance and decay resistance. It aims to simultaneously inhibit the effects of light and moisture on the physical properties of Chinese fir and impart good hydrophobicity to the modified materials. TiO2 nanoparticles were achieved by a sol-gel deposition method using an ethanol/titanium tetrachloride (EtOH/TiCl4) solution. A dip-coating process and subsequent annealing were used to realize nanoparticles on the wood surface. This method of deposition was utilized for the conformal covering of the wood structure as demonstrated by morphological investigation of the prepared samples by field emission scanning electron microscopy (SEM). Color change measurements and Fourier-transform infrared spectroscopy (FTIR) were used to analyze the lignin and carbonyl degradation of uncoated and coated wood during the UV exposure. Moisture protection of the coated wood was investigated by measuring the contact angle of the distilled water and wood surface. The measurements showed the UV and moisture protection of the TiO2 layer.
Materials and Methods
The high-temperature steam treatment
During the high-temperature steam treatment, water vapor produced by the steam generator, which was used to protect the treated Chinese fir, filled the whole tank. At the initial stage, the temperature was raised from room temperature to 100°C, with a heating rate of 20°C/h. Next, the rate of temperature from 100°C to 130°C was 10°C/h, and the temperature was incubated at 130°C for 30 minutes. Then, the temperature was increased to 180°C and kept for 1 hour. After the end of the heat treatment, the heat treatment box was naturally cooled to room temperature, and the treated test piece was taken out and equilibrated in a constant temperature and humidity chamber (Baranski 2018).
The preparation of TiO2 coating of Chinese fir
The EtOH/TiCl4 solution was arranged by the following steps (Rassam et al. 2012): (1) Solution A was prepared by mixing 10.5 mL of butyl titanate with 20 mL of absolute ethanol at room temperature. (2) Solution B was a mixed solution that contained 2 mL of deionized water, 5 mL of glacial acetic acid, and 20 mL of absolute ethanol. (3) Solution A was slowly added into Solution B with superspeed stirring at room temperature for 24 hours to obtain a transparent yellowish TiO2 solution. According to the pretest results, the Chinese fir treated by high temperature was immersed in TiO2 solution for 30 minutes, taken out, and dried for 1 hour. Finally, the TiO2-treated Chinese fir was dried in an oven at 105°C for 6 hours.
Before the FTIR measurement, the treated Chinese fir was cut into a thin piece, in which the three dimensions were 10 (longitudinal) by 10 (chordwise) by 2 mm (radial). The Fourier infrared spectrum was collected by FTIR (Vertex 70V, Bruker, Germany). The wavelength, which was scanned 32 times, ranged from 4,500 to 600 cm−1 with the resolution of 4 cm−1.
X-ray diffraction analysis
Each sample was cut into a thin piece with the following dimensions: 18 (longitudinal) by 18 (chordwise) by 5 mm (radial). An X-ray diffractometer (D8 ADVANC, Bruker) was employed to test the prepared samples. The parameters of the X-ray diffractometer were a radiant tube voltage of 40 kV, a radiant tube current of 40 mA, a copper target radiation λ of 0.154 nm, 2θ ranging from 10° to 70°, a step width of 0.22°, and a scanning speed of 0.5 seconds.
Based on the Segal method, the degree of crystallinity of cellulose (Crl) was calculated by the following equation:
where I002 is the diffraction intensity at the 002 plane diffraction peak, and Iam is the scattering intensity at the amorphous region.
Each sample was cut into a thin piece with the following dimensions: 5 (longitudinal) by 5 (chord) by 2 mm (radial). After the sample was baked to dry, it was fixed on the stage and sprayed with thin gold on the surface. The images of SEM were collected through a field emission scanning electron microscope (SU8010, Hitachi, Japan) with the test voltage 3 kV.
The surface color
The samples of Chinese fir were exposed to an artificially accelerating photoaging instrument for 0, 180, 540, and 900 hours. A color difference meter (NH310, Shenzhen Sankenchi Technology Co., Ltd.) was used to measure the chroma of each sample in the CIE (1976) L*a*b* color system with degree parameters L* (lightness), a* (red–green index), and b* (yellow–blue index). Five points of each sample were measured and the average value was computed. In the UV aging process, the lightness difference (ΔL*), red–green product index difference (Δa*), yellow–blue product index difference (Δb*), and color difference ΔE* of the wood surface were calculated with the following formulas (Eqs. 2 to 5):
In the equations, L0, a0, and b0 represent the brightness, red–green product index, and yellow–blue product index of the wood surface before exposed to UV light; Li, ai, and bi represent the brightness, red–green product index, and yellow–blue product index of the wood surface after the UV aging for i hours.
The modified Chinese fir was treated with UV aging for 0, 180, 360, 540, and 720 hours. The contact angle between the deionized water and the Chinese fir's surface was measured by a contact angle tester (OCA20, Dataphysics, Germany). The volume of the droplet was 5 μL deionized water and the test time was 90 seconds. Five measurements were taken for each sample and the average was calculated.
Mechanical wear resistance
The abrasion resistance of the hydrophobic layer on the surface of the Chinese fir was evaluated by a sandpaper abrasion test. The superhydrophobic Chinese fir surface was brought into close contact with the 1,500 mesh sandpaper under a pressure of 5 kPa, and was dragged in a straight line at a constant external force and speed of 25 cm. This process was a cycle, and a total of 10 cycles of abrasion resistance test was performed. The contact angle and the roll angle of the sample before and after the friction were measured by a contact angle measuring instrument.
The bacteriostatic activity of the Chinese fir samples was characterized by the bacterial inhibition loop method (agar diffusion test/CEN/TC 248WG13), which was evaluated using Escherichia coli (ATCC 25923). The bacterial culture medium was prepared using potato and glucose as nutrients, and the experimental procedure was as follows: 2 g of washed potato were cut into small pieces and added to 1,000 mL of water; the mixture was boiled for 30 minutes, and the potato pieces were filtered out. After that, the filtrate was diluted to 1,000 mL using distilled water. The mixture of 500 mL of diluted solution, 20 g of glucose, and 15 g of agar was heated until the glucose and agar were dissolved. The mixture was sterilized using an electric furnace, which was used as an agar medium. Finally, the sterilized mixture was placed in an agar culture dish at a constant temperature and humidity chamber for 24 hours. The worktable with ventilation was sterilized by ultraviolet light for 20 minutes. The Chinese fir samples were sawed to 20 by 20 by 10 mm, and then put into a conical flask (five pieces per bottle) with bacterial culture medium, and cultured in the bacterial culture box for 4 weeks.
During the heating process, the thermal weight loss of the Chinese fir sample was measured by the TGA209 F3 thermal analyzer. The following experimental conditions were listed: (1) the heating rate was 10°C/min, (2) the flow rate of N2 was 30 mL/min, (3) the temperature ranged from 0°C to 600°C, (4) the injection weight was about 5 mg, and (5) each group contained three test pieces during the experiment.
Results and Discussion
FTIR was applied to investigate the characteristics of Chinese fir before and after treatment by high temperature or TiO2 nanoparticles, which are shown in Figure 1. Compared with the Chinese fir, the spectrum of TiO2/Chinese fir exhibited a Ti–O–C absorption peak at a wavelength of 621 cm−1, indicating that the TiO2 nanoparticles successfully entered into and chemically bonded to the Chinese fir. From the spectrum of Chinese fir, it can be seen that the peak located at 3,420 cm−1 was the O–H stretching vibration peak of the hydroxyl. However, in the Chinese fir treated by high-temperature steam, the OH-1 absorption peak at 3,430 cm−1 was decreased dramatically in comparison with the C=O absorption peak intensity at 1,738 cm−1. The reason was that the free hydroxyl groups between the molecular chains of wood cellulose underwent a “bridge” reaction and generated ether bonds under the high-temperature steam (Gao et al. 2016) so that the number of free hydroxyl groups on the surface was significantly reduced. In addition, the hemicellulose underwent a severe degradation reaction, and the acetyl group in the hemicellulose was detached and hydrolyzed to form acetic acid, resulting in a decrease in the amount of carbonyl C= O (Asada et al. 2015, Baranski 2018). The spectrum of TiO2/Chinese fir treated by high temperature presented a synergistic effect.
X-ray diffraction analysis
Figure 2 shows the X-ray diffraction of Chinese fir before and after treatment by high temperature or TiO2. The characteristic peaks were located at 17°, 22.5°, and 35° when the Chinese fir was scanned using angle 2, which corresponded to the (101), (002), and (040) crystal surfaces of cellulose (He et al. 2018), respectively. By comparison, the new characteristic peaks at 25° and 38° were consistent with the characteristic peaks of anatase TiO2 crystal. This indicated that TiO2 in Chinese fir was still in the crystal form, which was better to maintain its photocatalytic performance. On the other hand, it shows that TiO2 successfully entered the interior of Chinese fir and combined with the cell wall of Chinese fir to form a stable structure.
The diffraction intensity of nano-TiO2/Chinese fir decreased at 17° and approximately 22° to 23°, which contributed to the reduction of the ratio of the cellulose. In addition, the crystallinity of wood can be improved by high-temperature, high-pressure steam (TPS). The hydroxyl groups of cellulose molecular chains in the quasicrystalline amorphous region of cellulose can bridge and form ether bonds. The reaction made the microfilaments in the amorphous region more orderly and closed to the crystalline region, which made the high crystallinity of wood fibers increase. Thus, the intensity of TPS/Chinese fir was higher, 21 percent, than the untreated Chinese fir at 22°. The crystallinity of nano-TiO2/TPS/Chinese fir was slightly lower than TPS/Chinese fir but still higher than the untreated Chinese fir.
Figure 3 shows the SEM images of untreated Chinese fir, nano-TiO2/Chinese fir, TPS/Chinese fir, and nano-TiO2/TPS/Chinese fir after the aging treatment. The structure of the untreated Chinese fir was broken, with small cracks, and partially peeled off, indicating that the UV aging caused the Chinese fir to degrade and be destroyed. Compared to the untreated Chinese fir, TiO2 on the surface of Chinese fir was discontinuous; the average diameter was about 30 nm, demonstrating that the nano-TiO2 was partially lost during the aging process, and distributed unevenly. The surface of the TPS/Chinese fir was intact and smooth, which can protect the Chinese fir's surface against UV radiation. After the aging of nano-TiO2/TPS/Chinese fir, the surface was covered uniformly with a layer of TiO2 nanoparticles. It is shown that the TiO2 solution had entered the cell wall and the cell cavity to form a stable structure, indicating that the nano-TiO2 modified material has better antiseptic and antibacterial effects (Lu and Hu 2016).
TiO2 nanoparticles have a large refractive index, which can absorb and scatter UV rays, thereby imparting excellent UV shielding performance to the treated Chinese fir, and the color of the surface of the fir is protected. After the aging treatment shown in Figure 4, it can be seen that the irradiation time decreased the brightness index (L*), and increased (with the color of the fir becoming dark) the red–green index (a*) and the yellow–blue index (b*). Nano-TiO2 had a significant effect on the photochromic properties of Chinese fir. With the increasing concentration of TiO2, the surface of the Chinese fir was loaded with more nano-TiO2 that enhanced the UV-shielding functions and color stability of Chinese fir. The antiphotochromic effect of Chinese fir treated with 5 percent mass fraction TiO2 sol was the best. After 120 hours of accelerated photoaging, the total color difference (ΔE*) was only 45 percent of the original sample. The scattering effect of nano-TiO2 was related to the aggregation state of nanoparticles. The agglomerated TiO2 particles had a weaker ability to scatter UV rays than well-dispersed particles, which might be weak in their ability to scatter ultraviolet rays. Therefore, it was essential to control the concentration of TiO2.
Sample surface wettability
The hydrophobic properties of the Chinese fir were evaluated using the water contact angle (WCA). Figure 5 shows the WCA with a deionized water droplet on the surface of the Chinese fir sample. As shown in Figure 5a, the WCA of Chinese fir was about 59.6, demonstrating that the surface of Chinese fir had a mass of hydrophilic groups such as hydroxyl groups. When the Chinese fir was treated by TPS, the WCA of TPS/Chinese fir improved to reach 119.1°. The hydrophobicity had a negative relation with the number of hydroxyl groups. The hemicellulose in the Chinese fir was significantly degraded and produced organic acid at the same time, which decreased the number of hydroxyl groups on the surface. The WCA of nano-TiO2/Chinese fir had risen to 88.3° after the Chinese fir was modified by TiO2, which was attributed to TiO2 nanoparticles. The TiO2 nanoparticles made the surface of the Chinese fir form a micronano secondary coarse structure, so its hydrophobicity was significantly improved compared with the unmodified Chinese fir. When the Chinese fir was treated with nano-TiO2/TPS, the nano-TiO2/TPS/Chinese fir had the largest WCA (151.4°).
The hydrophilicity of the treated Chinese fir was evaluated by measuring the contact angle of distilled water. Supplemental Data Figure S1 shows the dynamic contact angle of the water drop versus time curve. As shown in Figure S1a, the contact angle of the drop on untreated Chinese fir, nano-TiO2/Chinese fir, TPS/Chinese fir, and nano-TiO2/TPS/Chinese fir (70°, 90°, 110°, and 120°, respectively) did not considerably change after 20 minutes. After UV exposure for 720 hours, the contact angle of distilled water on the surfaces of four samples was measured. The results in Figure S1b show that the contact angle of the untreated Chinese fir decreased to about 0° after 55 minutes, and the rest reached stability gradually. It indicated that both TiO2 and TPS could enhance the aging resistance of Chinese fir. TiO2 had better performance than TPS due to the photocatalytic behavior of the TiO2 nanoparticles
Figure S2 shows the antibacterial properties of Chinese fir before and after treatment by TiO2 or TPS, in which the four samples were incubated in E. coli culture dishes. As shown in Figures S2a and S2c, we found that untreated Chinese fir and TPS/Chinese fir had no antibacterial properties when cultured with E. coli. However, nano-TiO2/Chinese fir and nano-TiO2/TPS/Chinese fir can inhibit the growth of E. coli, and the bacteriostasis circle had a diameter of 2.5 mm, which indicated that TiO2 nanoparticles had high antimicrobial properties. Under visible or UV irradiation, TiO2 induced photochemical reactions to activate oxygen in water or air, which can produce reactive oxygen species, highly charged active electrons, and negative oxygen ions. These products can penetrate the bacterial membrane, and then destroy its membrane structure, the double helix structure of DNA molecule, preventing bacterial growth and metabolism.
Figure S3a shows the relationship between temperature and quality retention rate of Chinese fir before and after treatment by TiO2 or TPS. The weight loss of Chinese fir composited with nano-TiO2 was different from the untreated Chinese fir. The initial temperature of weight loss, from high to low, was nano-TiO2/TPS/Chinese fir, nano-TiO2/Chinese fir, TPS/Chinese fir, and untreated Chinese fir; the initial temperature of the treated Chinese fir was 30°C higher than the untreated Chinese fir. The quality retention rate of nano-TiO2/TPS/Chinese fir was 17°C higher than the untreated Chinese fir, which confirmed that TiO2 was not simply physically filling the inside of the Chinese fir but that it formed a chemical bond or a hydrogen bond with Chinese fir. Also, TPS treatment can oxidize free hydroxyl groups on the surface of Chinese fir to aldehyde groups, ketone groups, or carboxyl groups, which further enhanced the thermal stability of Chinese fir.
In this study, Chinese fir was treated with nano-TiO2/high-temperature steam to improve the aging resistance and hydrophobicity. When the treatment temperature was 300°C and the concentration of nano-TiO2 was 5 percent, the modified Chinese fir had the best physical and mechanical properties. The main conclusions were as follows:
The weight gain rate of nano-TiO2/Chinese fir was closely related to the initial water content of Chinese fir. When the initial water content of the Chinese fir was approximately 7 to 8 percent, the weight gain rate of nano-TiO2/Chinese fir composite was the largest. Moreover, the weight gain rate decreased with the increase in the water content of Chinese fir.
By comparing the SEM images of the nano-TiO2/Chinese fir prepared under different initial moisture contents, the nano-TiO2 was shown to be distributed in the cell cavity and on the cell wall surface, which showed a close relationship with the weight gain rate.
X-ray diffraction analysis showed that the characteristic peak of anatase TiO2 crystal appeared in nano-TiO2/Chinese fir composite at 25°C, indicating that the anatase crystalline TiO2 was chemically bonded with the Chinese fir. They are combined and distributed in the cell wall and cell cavity. Also, the overall diffraction peak intensity of nano-TiO2/Chinese fir composites decreased with the increase of weight gain rate, and some of the characteristic peaks of Chinese fir even disappeared.
The heat resistance of nano-TiO2/Chinese fir was significantly improved, and the initial temperature of rapid weight loss was about 30°C, which was higher than that of the untreated Chinese fir. The percentage of residual mass was also higher, about 13 percent, than untreated Chinese fir. The WCA could be increased to about 130°, showing strong hydrophobicity, as well as high antimicrobial activity and mechanical wear resistance.
This work was supported by the Social Science Project of Qiqihar City (No. QSX2018-05JL) and the National Natural Science Foundation of China (Grant No. 31570550).
The authors are, respectively, Associate Professor, School of Fine Arts and Art Design, Qiqihar Univ., Qiqihar City, China (firstname.lastname@example.org [corresponding author]); and Associate Professor, School of Industrial Design and Ceramic Art, Foshan Univ., Foshan City, China (email@example.com). This paper was received for publication in October 2019. Article no. 19-00050.