- Email: [email protected]
Optimization of delignification process for efficient preparation of transparent wood with high strength and high transmittance
Vacuum 158 (2018) 158–165
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Optimization of delignification process for efficient preparation of transparent wood with high strength and high transmittance
T
Jiankun Qina,b, Xiaowan Lia,b, Yali Shaoa,b, Kaixin Shia, Xin Zhaoa,b, Tianshi Fenga, Yingcheng Hua,b,∗ a b
College of Material Science and Engineering, Northeast Forestry University, Harbin, PR China Key Laboratory of Bio-based Materials Science and Technology, Ministry of Education, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Delignification Transparent wood Lamination Transmission Stretching strength
Transparent wood is an environmentally friendly material that could be used as building materials or substrates for solar cells to reduced electrical energy consumption. Many problems remain in the delignification process, such as energy consumption and gas pollution. In this paper, the optimum conditions for two stage delignification of low density balsa wood (Ochroma pyramidale) and high density of basswood (Tilia tuan) were studied. The optimized single layer transparent wood was laminated to produce transparent wood with high thickness. The results showed that the lamination method cut the delignification time in half and improved the dimensional stability of the transparent wood. Optical testing showed that the maximum transmittance of the same direction laminated transparent wood was similar to the single layer transparent wood with the same thickness. The mechanical experiments showed that the transparent basswood had a better tensile strength, which reached 75.12 MPa. Optimizing the process of the transparent wood could greatly reduce the preparation time and the energy consumption, so that mass production of enterprises could become a reality.
1. Introduction The rapid development of the world economy results in people having a higher demand for the living environment, where the population has begun to pay attention to sustainable design. Developed countries organized in the 1980's and began to explore and implement sustainable development of residential buildings/roads. The “green building challenge” [1] adopted novel technology, materials, and crafts in their designs, thus reducing the use of resource within the permissive range. The development of computer technology and the rapid aging of the population has created a demand for smart homes [2]. Intelligent housing is comprehensive system engineering, which spans architecture, structure, water supply and drainage, interior design, and materials. Wood is a material that has been used for thousands of years, where it is proven to reduce the diastolic blood pressure and increase the pulse rate when specific wood ratios are present within buildings (the ratio of the area covered with wooden material to the entire area of the ceiling, the walls, and the floors) [3]. This thought is in line with modern intelligent buildings. A novel type of biomass material, transparent wood, has attracted attention within intelligent building [4,5]. Transparent wood is the
∗
pouring of refractive index-matching polymer material into delignification wood or lignin retaining modified wood [6], which provides optical performance and improves its mechanical properties. Transparent wood has a high transmittance and a high haze for scattering light from the sun [7]. Transparent wood could be used in building materials [8], solar cell [9], magnetic materials [10,11], and luminescent materials [12]. Transparent wood has potential within intelligent building, due to its good optical transmittance (transmittance over 80%), high privacy haze (haze over 70%), and thermal insulation (thermal conductivity less than 0.23 W m-1 K−1) [13]. Transparent wood has unique light guiding effects, with a large forward to back scattering [14]. Transparent wood could be used in roofs or windows to provide buildings with good lighting, where it would obtain the best visual effect while enhancing the beauty and the comfort of the indoor environment. A natural porous wood appears opaque because visible light is absorbed by the lignin [15], making delignification an essential step in the preparation of transparent wood. Most studies use an excessive delignification time, which consumes a lot of energy and causes pollution. Yaddanapudi et al. [8] removed lignin from a 0.1 mm thick beech wood (Fagus sylvatica) with 5 wt % sodium chlorite (NaClO2) in an acetate
Corresponding author. No. 26 Hexing Road, Harbin, Heilongjiang Province, PR China. E-mail address: [email protected] (Y. Hu).
https://doi.org/10.1016/j.vacuum.2018.09.058 Received 7 April 2018; Received in revised form 29 September 2018; Accepted 29 September 2018 Available online 02 October 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved.
Vacuum 158 (2018) 158–165
J. Qin et al.
Fig. 1. (a) Cross direction layer, (b) same direction layer, (c) 5 layers cross direction multilayer transparent wood, and (d) 10 layers cross direction multilayer transparent wood.
that were cut into strips (70 mm × 10 mm, 3–5 specimens were tested for each type). The transparent epoxy resin was manufactured by the Wenzhou Tailiqi Company. The chemicals (sodium chlorite, sodium hydroxide, acetic acid, and hydrogen peroxide) were purchased from Shanghai Aladdin Biochemical Polytron Technologies, Inc. (China). The distilled water was produced in our own laboratory. The transmittance was textes by a dual beam ultraviolet visible spectrophotometer (TU1901). Their tensile behavior under the stretching process in the longitudinal and the transverse directions were recorded on a universal testing machine (CMT5504) with a speed of 2 mm/min. The temperature and the relative humidity of the room during the wood mechanical testing were 25 °C and 60%. A scanning electron microscope (SEM) JSM-7500F was used to observe and analyze the chord section and the cross section of the specimen.
buffer solution and allowed it to sit at 95 °C for 12 h. Li Yuanyuan et al. [16] obtained a centimeter thick transparent wood via interface acetylation manipulation. This method improves the transmittance of the transparent wood, but also increases the complexity of the preparation process. Fu et al. [17] used the traditional delignification process in transparent plywood (1 wt % NaClO2 in an acetate buffer solution for 6 h under 80 °C). NaClO2 produces many harmful gases during the delignification process, which is harmful to humans [18]. The process of removing lignin by NaClO2 should be shortened as much as possible to reduce the health risks and save energy. In this paper, we explored the changing rule of lignin during the delignification process. A novel lignin balance point was found. The two step delignification method of sodium chlorite and hydrogen peroxide (H2O2) was adopted. In order to simplify the delignification process of the transparent wood with a thickness greater than 2 mm, the veneers were laminated together at the cross direction (0°/90°) and the same direction (0°/180°) as seen in Fig. 1(a) and (b). The vacuum laminating method retained the porous structure and the thickness and filled additional cell lumens with epoxy resin. Fig. 1(c) and (d) show the 5 layers and the 10 layers cross direction multilayer transparent wood under the white-light point source. The direct light was dispersed into a fuzzy-looking halo, which dispersed sunshine. This showed that the transparent wood had a good light management effect [19], which made the lighting more evenly distributed in the room and saved energy. The main comparison specimens were original balsa wood (OBW), single layer transparent balsa wood (SLTBW), same direction multilayer transparent balsa wood (SMTBW), cross direction multilayer transparent balsa wood (CMTBW), original basswood (OB), single layer transparent basswood (SLTB), same direction multilayer transparent basswood (SMTB), and cross direction multilayer transparent basswood (CMTB).
2.2. Determination of lignin and extract The mass fractions of lignin was determined by the methods in Ref. [20]. The tests were repeated 3 times. 2.3. Delignification An 80 °C sodium chlorite (NaClO2, 1 wt%) in an acetate buffer solution (pH 4.6) was used during the delignification process, in accordance with previous reports [21]. A slice of the sample was removed every hour. After delignification, the woods were carefully washed with deionized water or hydrogen peroxide (H2O2, 5 mol/L) at 90 °C. The lignin content was measured to assess the effect that the lignin had on the transmittance. 2.4. Single layer transparent wood fabrication Before the impregnation solution was prepared, ethanol replaced the water in the lignin-removed woods (LRW). This could improve the permeability of the LRW. The mass ratio of epoxy resin to acetone to curing agent was 3: 3: 1. After the samples were thoroughly dissolved, the LRW (various NaClO2 treatment times, ranging from 1 h to 6 h) was placed at the bottom of a container, immersed in an epoxy resin solution, and incubated in a vacuum desiccator with a pressure of 1000 Pa for 2 h to ensure full impregnation. The infiltrated wood was rolled up with silicone paper and placed in a petri dish and then dried naturally at atmospheric pressure for 12 h.
2. Experimental 2.1. Materials The balsa wood (Ochroma pyramidale) samples were purchased from Yunnan Province, P.R China (with thicknesses of 1 mm, 1.5 mm, 2 mm, and 5 mm). The samples were 5-years-old and naturally dried with 10.7% moisture content. The density was 0.21 g/cm3. The basswood samples were purchased from Heilongjiang Province, P.R China (with thicknesses of 1 mm, 2 mm, and 5 mm). The samples were 10-years-old and naturally dried with 12.1% moisture content. The density was 0.49 g/cm3. The length and the width of the transmission samples were 20 mm each. The mechanical characterization used wood specimens
2.5. Multilayer transparent wood fabrication The impregnated multilayer delignified veneers were placed in 159
Vacuum 158 (2018) 158–165
J. Qin et al.
Fig. 2. (a) The fitting curves of the lignin residue content.
same direction and cross direction to prepare the multilayer transparent wood under vacuum. The multilayer transparent wood was sandwiched between two glass slides and dried naturally. The porous structure and the thickness of the wood were retained by vacuum impregnation.
Table 1 The fitting equation and the parameter. y (%) represents the instantaneous concentration of lignin, y0 (%) represents the equilibrium concentration, A1 represents the pre-exponential factor, x (h) represents the time, and t1 represents the time coefficient.
3. Results and discussion Fitting Equation
3.1. Lignin content and transmission of transparent balsa wood
y0 A1 t1 R2
Balsa is the lightest wood, weighing only 0.21 g/cm3 and a lignin content of 18.3%. It is a fast growing wood, with a high economic value. A 10-year-old balsa tree can reach 16 m high and be 50–60 cm in diameter [22]. There are plenty of holes in the balsa wood, making the lignin removal process simple. In order to compare the transmittance of the multilayer transparent wood and the same thickness of the primary transparent wood, the lignin content of the two groups were treated until they reached a balanced state. Balsa wood slices (1 mm) were immersed in a lignin removal solution (NaClO2, 2 wt%, pH 4.6) and incubated at 80 °C. The lignin content in each piece of wood was measured hourly. Fig. 2 shows 1–6 h of the lignin residue. As Fig. 2 (a) shows, about 60% of lignin was removed in the first hour. The lignin removal rate was significantly slower throughout the next 5 h. It took 3 h to balance the lignin at 20%. The majority of the chemicals were removed by rinsing the wood samples in distilled water or with an H2O2 treatment at 90 °C. The photograph shows the 1 mm transparent wood treat with NaClO2 (1 h-6h), then washed with the different solution, and the (b) distilled water or (c) H2O2. The first order exponential decline functions of the lignin residue over each hour were synthesized. The R2 was above 0.99, indicating that the lignin removal trend was in accordance with the law of function and the lignin residue approached y0 (Table 1). The two curves were compared to find that the main effect of the H2O2 was to remove the chromophores on the lignin, but less was removed in the lignin content [15]. Fig. 2 (b) and 2 (c) show that the transparent woods with various lignin contents that the 1 mm thick transparent wood sample that were placed on top of handwritten letters “1 h” to “6 h” were more translucent as time continued. The yellowish transparent wood was inhibited by the H2O2 treatment, which made the transparent wood more attractive. After heating for 1 h with H2O2, the yellow color of the “3 h” transparent wood nearly disappeared. This transparency was only
Distilled Water
1 h H2O2 Treatment
y = y0 + A1 e−x / t1 0.22242 ± 0.00809 0.77566 ± 0.01618 0.85854 ± 0.04771 0.9983
0.21311 ± 0.00735 0.78556 ± 0.01517 0.79182 ± 0.04177 0.9985
reached after at least 6 h without the H2O2. In order to ensure that the transparent wood had a better appearance, the following treatment with various thicknesses of LRWs (1 mm, 1.5 mm, 2 mm, and 5 mm) were used to improve the H2O2 treatment after 1 h. The treated LRWs were impregnated onto the ethanol solution for 8 h. Ethanol was used to the replace water in the LRW, which improved the permeability of the LRW. The transparent wood was obtained by impregnating the epoxy resin in a vacuum. As seen in Fig. 3(a) and (b), the transparencies at hour six were similar. The maximum transmission of the transparent wood reached 82%. The fitting curve of the lignin residue content demonstrated that the lignin reached equilibrium after 3 h. The transmittance of the transparent wood increased as the lignin content decreased and reached 80% after 3 h. A 1 mm veneer could be treated with NaClO2 for 3 h in order to obtain transparent wood with high transmittance. The transmittance of the same thicknesses of epoxy resin and glass were about 10% higher than the transparent wood. During the ultraviolet A (UVA) stage (320–400 nm), the transmittance of the transparent wood was significantly lower than the epoxy resin and the glass. During the UVA stage, the average transmittance of the transparent wood was lower than the epoxy resin, at about 45%, which was about 55% lower than the glass. UVA had a strong penetration, where it directly reached the dermis, destroyed elastic fibers and collagen fibers, and caused cataracts and skin cancer [23]. The 1.5 mm and 2 mm balsa woods were treated with the same methodology as the 1 mm wood. Fig. 4 shows the transparent wood transmission of the 1.5 mm and the 2 mm with various lignin contents. When the thickness increased, the transmittance of the transparent 160
Vacuum 158 (2018) 158–165
J. Qin et al.
Fig. 3. Transmission curves of the 1 mm transparent wood over various treatment times (1 h-6h) of NaClO2 (a) washing with distilled water and (b) washing with an additional H2O2 treatment.
Fig. 4. Transmission curves over various treatment times (1–6 h) of NaClO2 (a) 1.5 mm transparent woods washed with distilled water, (b) 1.5 mm transparent woods washed with an additional H2O2 treatment, (c) 2 mm transparent woods washed with distilled water, and (d) 2 mm transparent woods washed with an additional H2O2 treatment. 161
Vacuum 158 (2018) 158–165
J. Qin et al.
Fig. 5. Five-millimeter LRWs with various impregnation times under a vacuum.
Fig. 6. The framework was composed of delignification and was impregnated with the epoxy resin, where the optical properties were obtained. The lower right side shows a photo of the 2 mm thick transparent wood with a hand-painted picture “Main building.”
12 h (5 mm). When making the multilayer transparent wood, both the single layer and the multilayer transparent wood at the lowest lignin content were used for comparisons.
woods decreased. The maximum transmittance of the 1.5 mm transparent wood decreased to 80%, and the 2 mm transparent wood decreased to 77%. The transmittance of the transparent balsa wood increased as the lignin decreased, and decreased as the thickness increased. This also occurred with the thicker wood (5 mm). The shortest NaClO2 treatment time of balsa woods were 3 h (1 mm), 4 h (1.5 mm), 5 h (2 mm), and
3.2. The flow of resin in a vacuum The modified epoxy resin impregnation solution was impregnated in 162
Vacuum 158 (2018) 158–165
J. Qin et al.
Fig. 7. Optical transmittance curves of the transparent wood of the 2 mm SMTBW, the 2 mm CMTBW, the 2 mm SLTBW, the 5 mm SLTBW, the 5 mm CMTBW, and the 10 mm CMTBW.
Fig. 9. The transmittance curves of the 1 mm SLTB, the 2 mm SLTB, the 2 mm SMTB, the 2 mm CMTB, the 5 mm SLTB, and the 5 mm CMTB. Table 2 Tension strength and transmittance of the balsa wood and the basswood.
a 1000Pa environment via the vacuum impregnation method. The vacuum impregnation method eliminated the interlayer bubbles, which could enhance the bonding properties of the transparent wood and increase the transmittance. In order to study the flow of the impregnated liquid in wood, a 5 mm thickness transparent balsa wood with various impregnation times were compared. As Fig. 5 shows, the transmittance of the balsa wood increased as the vacuum impregnation times increased. The main flow of the epoxy resin is depicted by red arrows. Most of the epoxy resin was impregnated from bottom to top, and the small parts flowed from both sides to the middle. The gravity makes the flow of epoxy resin from bottom to top more difficult. This indicated that the thickness of the LRW was the main factor that affected the impregnation capacity and the transmittance. Multilayer transparent wood could address this problem. The thick woods were divided into thinner veneers, so that
Type
Transverse tension strength (MPa)
Longitudinal tension strength (MPa)
Transmittance
OBW 2 mm SLTBW 2-layers SMTBW 2-layers CMTBW OB 2-mm SLTB 2-layers SMTB 2-layers CMTB
1.02 ± 0.5 6.06 ± 1.5 9.06 ± 1.3
12.01 ± 4.1 69.13 ± 6.0 65.53 ± 3.5
3% 77% 76%
54.59 ± 2.5
54.59 ± 2.5
70%
4.12 ± 0.3 8.17 ± 0.3 6.13 ± 0.4 63.05 ± 2.7
45.12 75.12 70.23 63.05
1% 64% 59% 55%
± ± ± ±
2.6 2.4 3.0 2.7
Fig. 8. Stretching results of the 1 mm OBW, the 1 mm SLTBW, the 2 mm SLTBW, the 2 mm SMTBW, and the 2 mm CMTBW in the (a) parallel direction and the (b) upright direction. 163
Vacuum 158 (2018) 158–165
J. Qin et al.
basswood. The transmittance of the double-layer CMTB was 55%, which was slightly lower than the SLTB. This trend was similar to the transparent balsa wood. The difference between the multilayer transparent balsa wood and the basswood is shown in Table 2. Transparent basswood had a lower transmittance than the transparent balsa wood, but its mechanical properties were better. The laminating caused the transverse tension strength of the OB to increase from 4.12 MPa to 63.05 MPa. The increased longitudinal tensile strength was primarily caused by the enhanced epoxy resin. The maximum longitudinal tensile strength reached 75.12 MPa.
impregnated and laminated could improve the impregnation efficiency. 3.3. SEM and practicality picture Balsa wood is primarily composed of cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are polysaccharides, while lignin is a three-dimensional amorphous polyphenolic macromolecule that consists of three types of phenyl propane units that form a complex, highly branched, and amorphous structure. The local repartition of the compounds was not homogeneous. The lignin was primarily located at the surface of the fiber, while the backbone was primarily composed of cellulose. The intercellular layer accounted for 3/4 of the total lignin content and the secondary wall accounted for the remaining 1/4 [24]. The changes in the intercellular layer are shown in the red arrows in Fig. 6. The chlorine dioxide (ClO2) produced by the NaClO2 solution was the main component of the removal of lignin, which attacked the chromophore on the lignin and transformed the lignin into a free radical. The intercellular layer was cracked. The epoxy filled the cracks under vacuum and replaced the lignin to strengthen the cytoskeleton.
4. Conclusion The NaClO2 was used in the preparation of transparent wood is greatly shortened by a two-stage delignification process and lamination method. When the lignin content reached the equilibrium point, the yellowing of the transparent wood was effectively inhibited by the H2O2 treatment. The time of the delignification increased as the wood thickness and tree density increased. The shortest time required for the 1 mm balsa wood was a 3 h NaClO2 treatment and an 1 h H2O2 treatment. The flow direction of the epoxy resin in the LRW indicated that the thickness of the wood was the main factor that affected the permeation time. The SEM measurement showed that the LRW that was dehydrated with the anhydrous ethanol was well filled with epoxy resin in the vacuum. The ultraviolet spectrophotometer showed that the transmittance decreased as the tree species density and the number of layers increased. A maximum transmittance of the transparent balsa wood was 82%, higher than that of transparent basswood by 7% at a 300 nm wavelength. The transparent wood had a good barrier to UV radiation (less than 400 nm), which could be used in intelligent buildings to regulate light and reduce the UV ray damage. Tensile tests showed that the tensile strength of the transparent wood increased as the tree density increased. The transparent basswood had better tensile properties, where the tensile strength reached 75 MPa. Laminating allowed the delignification time of the thick wood to be reduced to that of a thin wood. The cross direction laminating reduced the risk of transparent wood being damaged from the weak side. The production of transparent wood with high strength and high transmittance in a short time reduces the cost of preparation and enables the industrialization of transparent wood.
3.4. Transmission and stretching strength of multilayer transparent balsa wood Fig. 7 shows the transmission of the 2 mm SLTBW, which was 76.5% and was similar to the 2 mm SMTBW. This was slightly higher than the CMTBW 6%. When more than two layers of transparent wood were made, they were stacked in a cross direction. The transmission of the 5 mm CMTBW was 54.4%, which was 10% lower than the 5 mm SLTBW. The transmission of the 10 mm CMTBW was 31.7%. When additional layers were present, the transmittance slowed. This could be related to the interlayer gluing layer. Compared to the thicker single layer transparent wood previously made [14,16], the multilayer transparent wood did not fully use the transmission rate. The multilayer transparent wood reduced the time required to remove the lignin. The multilayer transparent wood was composed of multilayer 1 mm veneer. The removal time for the lignin was 3 h. The multilayer transparent wood had a unique advantage within eliminating anisotropy. With the wood was impregnated with epoxy resin, the ultimate tensile strength parallel to the grain increased from 12.01 MPa to 69.02 MPa. The ultimate tensile strength of the 2 mm SMTBW was similar to the 2 mm SLTBW, but was 20% higher than the CMTBW. The CMTBW obtained the best upright direction mechanical property. The 2 mm CMTBW upright direction mechanical property was 5 times higher than the 2 mm SMTBW, 7 times higher than the 2 mm SLTBW, and 100 times higher than the 1 mm natural balsa wood (Fig. 8). The results showed that the transmission rate of the SMTBW was similar to the single layer transparent wood SLTBW with the same thickness. The cross direction multilayer transparent wood CMTBW enhanced the weakest direction.
Acknowledgements This research was funded by the National Natural Science Foundation of China (Grant no. 31470581) and Fundamental Research Funds for the Central Universities (Grant no. 2572016EBJ1). References [1] N. Kohler, The relevance of Green Building Challenge: an observer's perspective, Build. Res. Inf. 27 (4–5) (1999) 309–320. [2] M. Chan, D. Estève, C. Escriba, E. Campo, A review of smart homes- present state and future challenges, Comput. Methods Progr. Biomed. 91 (1) (2008) 55–81. [3] Y. Tsunetsugu, Y. Miyazaki, H. Sato, Physiological effects in humans induced by the visual stimulation of room interiors with different wood quantities, J. Wood Sci. 53 (1) (2007) 11–16. [4] Y. Li, Q. Fu, S. Yu, M. Yan, L. Berglund, Optically transparent wood from a nanoporous cellulosic template: combining functional and structural performance, Biomacromolecules 17 (4) (2016) 1358. [5] Y. Li, Q. Fu, X. Yang, L. Berglund, Transparent wood for functional and structural applications, Philos Trans A Math Phys Eng Sci 376 (2112) (2018) 20170182. [6] S. Fink, Transparent wood – a new approach in the functional study of wood structure, Holzforschung 46 (5) (1992) 403–408. [7] M. Zhu, J. Song, T. Li, A. Gong, Y. Wang, J. Dai, et al., Highly anisotropic, highly transparent wood composites, Adv. Mater. 28 (26) (2016) 5181–5187. [8] H.S. Yaddanapudi, N. Hickerson, S. Saini, A. Tiwari, Fabrication and characterization of transparent wood for next generation smart building applications, Vacuum 146 (2017) 649–654. [9] M. Zhu, T. Li, C.S. Davis, Y. Yao, J. Dai, Y. Wang, et al., Transparent and haze wood composites for highly efficient broadband light management in solar cells, Nanomater. Energy 26 (2016) 332–339.
3.5. Transmission and stretching strength of multilayer transparent basswood The basswood had a lignin equilibrium concentration equal to the balsa wood, but it required more time. The lignin equilibrium concentration time of basswoods were 6 h (1 mm) and 12 h (2 mm). It was possible to use the shortest balanced lignin concentration time (6 h) to produce the shortest time-consuming multilayer transparent basswood. The original basswood had a higher density (0.49 g/cm3) and was more compact inside. Its lignin content was 20.9%. It was more difficult to remove the lignin and to impregnate the epoxy resin. Its fabrication time was about twice as long as the Balsa wood. Fig. 9 shows that the transmittance decreased as the basswood chips thickness increased. The transmittance of 1 mm, 2 mm, and 5 mm transparent basswood reached 75%, 64%, and 19%. The transmittance of the double-layer SMTB reached 59%, which was close to the 2 mm single-layer transparent 164
Vacuum 158 (2018) 158–165
J. Qin et al.
[17] Q. Fu, M. Yan, E. Jungstedt, X. Yang, Y. Li, L.A. Berglund, Transparent plywood as a load-bearing and luminescent biocomposite, Compos. Sci. Technol. (2018) 164. [18] M.J. Nieuwenhuijsen, M.B. Toledano, N.E. Eaton, J. Fawell, P. Elliott, Chlorination disinfection byproducts in water and their association with adverse reproductive outcomes, Occup. Environ. Med. 57 (2) (2000) 73–85. [19] M.L. Brongersma, Y. Cui, S. Fan, Light management for photovoltaics using highindex nanostructures, Nat. Mater. 13 (5) (2014) 451–460. [20] T. Yokoyama, JFK, H. Chang, Microanalytical method for the characterization of fiber components and morphology of woody plants, J. Agric. Food Chem. 50 (5) (2002) 1040–1044. [21] H. Yano, A. Hirose, P.J. Collins, Y. Yazaki, Effects of the removal of matrix substances as a pretreatment in the production of high strength resin impregnated wood based materials, J. Mater. Sci. Lett. 20 (12) (2001) 1125–1126. [22] M. Osei-Antwi, J.D. Castro, A.P. Vassilopoulos, T. Keller, Shear mechanical characterization of balsa wood as core material of composite sandwich panels, Construct. Build. Mater. 41 (41) (2013) 231–238. [23] O.Y. Hao, G. Stamatas, N. Kollias, Dermal contributions to UVA-induced oxidative stress in skin, Photodermatol. Photoimmunol. Photomed. 25 (2) (2009) 65–70. [24] R. Vanholme, B. Demedts, K. Morreel, J. Ralph, W. Boerjan, Lignin biosynthesis and structure, Plant Physiol. 153 (3) (2010) 895–905.
[10] W.T. Gan, L.K. Gao, S.L. Xiao, W.B. Zhang, X.X. Zhan, J. Li, Transparent magnetic wood composites based on immobilizing Fe3O4 nanoparticles into a delignified wood template, J. Mater. Sci. 52 (6) (2017) 3321–3329. [11] W.T. Gan, S.L. Xiao, L.K. Gao, R.N. Gao, J. Li, X.X. Zhan, Luminescent and transparent wood composites fabricated by poly(methyl methacrylate) and gammaFe2O3@YVO4:Eu3+ nanoparticle impregnation, Acs Sustain Chem Eng 5 (5) (2017) 3855–3862. [12] Y.Y. Li, S. Yu, J.G.C. Veinot, J. Linnros, L. Berglund, I. Sychugov, Luminescent transparent wood, Adv Opt Mater 5 (1) (2017) 6. [13] Z. Yu, Y. Yao, J. Yao, L. Zhang, C. Zhang, Y. Gao, et al., Transparent wood containing CsxWO3 nanoparticles for heat-shielding-window applications, J. Mater. Chem. 5 (13) (2017). [14] T. Li, M. Zhu, Z. Yang, J. Song, J. Dai, Y. Yao, et al., Wood composite as an energy efficient building material: guided sunlight transmittance and effective thermal insulation, Advanced Energy Materials 6 (2016) 1601122. [15] Y. Li, Q. Fu, R. Rojas, M. Yan, M. Lawoko, L. Berglund, Lignin-retaining transparent wood, ChemSusChem 10 (17) (2017) 3445–3451. [16] Y. Li, X. Yang, Q. Fu, R. Rojas, M. Yan, L. Berglund, Towards centimeter thick transparent wood through interface manipulation, J. Mater. Chem. 6 (3) (2018) 1094–1101.
165
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Optimization of delignification process for efficient preparation of transparent wood with high strength and high transmittance
T
Jiankun Qina,b, Xiaowan Lia,b, Yali Shaoa,b, Kaixin Shia, Xin Zhaoa,b, Tianshi Fenga, Yingcheng Hua,b,∗ a b
College of Material Science and Engineering, Northeast Forestry University, Harbin, PR China Key Laboratory of Bio-based Materials Science and Technology, Ministry of Education, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Delignification Transparent wood Lamination Transmission Stretching strength
Transparent wood is an environmentally friendly material that could be used as building materials or substrates for solar cells to reduced electrical energy consumption. Many problems remain in the delignification process, such as energy consumption and gas pollution. In this paper, the optimum conditions for two stage delignification of low density balsa wood (Ochroma pyramidale) and high density of basswood (Tilia tuan) were studied. The optimized single layer transparent wood was laminated to produce transparent wood with high thickness. The results showed that the lamination method cut the delignification time in half and improved the dimensional stability of the transparent wood. Optical testing showed that the maximum transmittance of the same direction laminated transparent wood was similar to the single layer transparent wood with the same thickness. The mechanical experiments showed that the transparent basswood had a better tensile strength, which reached 75.12 MPa. Optimizing the process of the transparent wood could greatly reduce the preparation time and the energy consumption, so that mass production of enterprises could become a reality.
1. Introduction The rapid development of the world economy results in people having a higher demand for the living environment, where the population has begun to pay attention to sustainable design. Developed countries organized in the 1980's and began to explore and implement sustainable development of residential buildings/roads. The “green building challenge” [1] adopted novel technology, materials, and crafts in their designs, thus reducing the use of resource within the permissive range. The development of computer technology and the rapid aging of the population has created a demand for smart homes [2]. Intelligent housing is comprehensive system engineering, which spans architecture, structure, water supply and drainage, interior design, and materials. Wood is a material that has been used for thousands of years, where it is proven to reduce the diastolic blood pressure and increase the pulse rate when specific wood ratios are present within buildings (the ratio of the area covered with wooden material to the entire area of the ceiling, the walls, and the floors) [3]. This thought is in line with modern intelligent buildings. A novel type of biomass material, transparent wood, has attracted attention within intelligent building [4,5]. Transparent wood is the
∗
pouring of refractive index-matching polymer material into delignification wood or lignin retaining modified wood [6], which provides optical performance and improves its mechanical properties. Transparent wood has a high transmittance and a high haze for scattering light from the sun [7]. Transparent wood could be used in building materials [8], solar cell [9], magnetic materials [10,11], and luminescent materials [12]. Transparent wood has potential within intelligent building, due to its good optical transmittance (transmittance over 80%), high privacy haze (haze over 70%), and thermal insulation (thermal conductivity less than 0.23 W m-1 K−1) [13]. Transparent wood has unique light guiding effects, with a large forward to back scattering [14]. Transparent wood could be used in roofs or windows to provide buildings with good lighting, where it would obtain the best visual effect while enhancing the beauty and the comfort of the indoor environment. A natural porous wood appears opaque because visible light is absorbed by the lignin [15], making delignification an essential step in the preparation of transparent wood. Most studies use an excessive delignification time, which consumes a lot of energy and causes pollution. Yaddanapudi et al. [8] removed lignin from a 0.1 mm thick beech wood (Fagus sylvatica) with 5 wt % sodium chlorite (NaClO2) in an acetate
Corresponding author. No. 26 Hexing Road, Harbin, Heilongjiang Province, PR China. E-mail address: [email protected] (Y. Hu).
https://doi.org/10.1016/j.vacuum.2018.09.058 Received 7 April 2018; Received in revised form 29 September 2018; Accepted 29 September 2018 Available online 02 October 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved.
Vacuum 158 (2018) 158–165
J. Qin et al.
Fig. 1. (a) Cross direction layer, (b) same direction layer, (c) 5 layers cross direction multilayer transparent wood, and (d) 10 layers cross direction multilayer transparent wood.
that were cut into strips (70 mm × 10 mm, 3–5 specimens were tested for each type). The transparent epoxy resin was manufactured by the Wenzhou Tailiqi Company. The chemicals (sodium chlorite, sodium hydroxide, acetic acid, and hydrogen peroxide) were purchased from Shanghai Aladdin Biochemical Polytron Technologies, Inc. (China). The distilled water was produced in our own laboratory. The transmittance was textes by a dual beam ultraviolet visible spectrophotometer (TU1901). Their tensile behavior under the stretching process in the longitudinal and the transverse directions were recorded on a universal testing machine (CMT5504) with a speed of 2 mm/min. The temperature and the relative humidity of the room during the wood mechanical testing were 25 °C and 60%. A scanning electron microscope (SEM) JSM-7500F was used to observe and analyze the chord section and the cross section of the specimen.
buffer solution and allowed it to sit at 95 °C for 12 h. Li Yuanyuan et al. [16] obtained a centimeter thick transparent wood via interface acetylation manipulation. This method improves the transmittance of the transparent wood, but also increases the complexity of the preparation process. Fu et al. [17] used the traditional delignification process in transparent plywood (1 wt % NaClO2 in an acetate buffer solution for 6 h under 80 °C). NaClO2 produces many harmful gases during the delignification process, which is harmful to humans [18]. The process of removing lignin by NaClO2 should be shortened as much as possible to reduce the health risks and save energy. In this paper, we explored the changing rule of lignin during the delignification process. A novel lignin balance point was found. The two step delignification method of sodium chlorite and hydrogen peroxide (H2O2) was adopted. In order to simplify the delignification process of the transparent wood with a thickness greater than 2 mm, the veneers were laminated together at the cross direction (0°/90°) and the same direction (0°/180°) as seen in Fig. 1(a) and (b). The vacuum laminating method retained the porous structure and the thickness and filled additional cell lumens with epoxy resin. Fig. 1(c) and (d) show the 5 layers and the 10 layers cross direction multilayer transparent wood under the white-light point source. The direct light was dispersed into a fuzzy-looking halo, which dispersed sunshine. This showed that the transparent wood had a good light management effect [19], which made the lighting more evenly distributed in the room and saved energy. The main comparison specimens were original balsa wood (OBW), single layer transparent balsa wood (SLTBW), same direction multilayer transparent balsa wood (SMTBW), cross direction multilayer transparent balsa wood (CMTBW), original basswood (OB), single layer transparent basswood (SLTB), same direction multilayer transparent basswood (SMTB), and cross direction multilayer transparent basswood (CMTB).
2.2. Determination of lignin and extract The mass fractions of lignin was determined by the methods in Ref. [20]. The tests were repeated 3 times. 2.3. Delignification An 80 °C sodium chlorite (NaClO2, 1 wt%) in an acetate buffer solution (pH 4.6) was used during the delignification process, in accordance with previous reports [21]. A slice of the sample was removed every hour. After delignification, the woods were carefully washed with deionized water or hydrogen peroxide (H2O2, 5 mol/L) at 90 °C. The lignin content was measured to assess the effect that the lignin had on the transmittance. 2.4. Single layer transparent wood fabrication Before the impregnation solution was prepared, ethanol replaced the water in the lignin-removed woods (LRW). This could improve the permeability of the LRW. The mass ratio of epoxy resin to acetone to curing agent was 3: 3: 1. After the samples were thoroughly dissolved, the LRW (various NaClO2 treatment times, ranging from 1 h to 6 h) was placed at the bottom of a container, immersed in an epoxy resin solution, and incubated in a vacuum desiccator with a pressure of 1000 Pa for 2 h to ensure full impregnation. The infiltrated wood was rolled up with silicone paper and placed in a petri dish and then dried naturally at atmospheric pressure for 12 h.
2. Experimental 2.1. Materials The balsa wood (Ochroma pyramidale) samples were purchased from Yunnan Province, P.R China (with thicknesses of 1 mm, 1.5 mm, 2 mm, and 5 mm). The samples were 5-years-old and naturally dried with 10.7% moisture content. The density was 0.21 g/cm3. The basswood samples were purchased from Heilongjiang Province, P.R China (with thicknesses of 1 mm, 2 mm, and 5 mm). The samples were 10-years-old and naturally dried with 12.1% moisture content. The density was 0.49 g/cm3. The length and the width of the transmission samples were 20 mm each. The mechanical characterization used wood specimens
2.5. Multilayer transparent wood fabrication The impregnated multilayer delignified veneers were placed in 159
Vacuum 158 (2018) 158–165
J. Qin et al.
Fig. 2. (a) The fitting curves of the lignin residue content.
same direction and cross direction to prepare the multilayer transparent wood under vacuum. The multilayer transparent wood was sandwiched between two glass slides and dried naturally. The porous structure and the thickness of the wood were retained by vacuum impregnation.
Table 1 The fitting equation and the parameter. y (%) represents the instantaneous concentration of lignin, y0 (%) represents the equilibrium concentration, A1 represents the pre-exponential factor, x (h) represents the time, and t1 represents the time coefficient.
3. Results and discussion Fitting Equation
3.1. Lignin content and transmission of transparent balsa wood
y0 A1 t1 R2
Balsa is the lightest wood, weighing only 0.21 g/cm3 and a lignin content of 18.3%. It is a fast growing wood, with a high economic value. A 10-year-old balsa tree can reach 16 m high and be 50–60 cm in diameter [22]. There are plenty of holes in the balsa wood, making the lignin removal process simple. In order to compare the transmittance of the multilayer transparent wood and the same thickness of the primary transparent wood, the lignin content of the two groups were treated until they reached a balanced state. Balsa wood slices (1 mm) were immersed in a lignin removal solution (NaClO2, 2 wt%, pH 4.6) and incubated at 80 °C. The lignin content in each piece of wood was measured hourly. Fig. 2 shows 1–6 h of the lignin residue. As Fig. 2 (a) shows, about 60% of lignin was removed in the first hour. The lignin removal rate was significantly slower throughout the next 5 h. It took 3 h to balance the lignin at 20%. The majority of the chemicals were removed by rinsing the wood samples in distilled water or with an H2O2 treatment at 90 °C. The photograph shows the 1 mm transparent wood treat with NaClO2 (1 h-6h), then washed with the different solution, and the (b) distilled water or (c) H2O2. The first order exponential decline functions of the lignin residue over each hour were synthesized. The R2 was above 0.99, indicating that the lignin removal trend was in accordance with the law of function and the lignin residue approached y0 (Table 1). The two curves were compared to find that the main effect of the H2O2 was to remove the chromophores on the lignin, but less was removed in the lignin content [15]. Fig. 2 (b) and 2 (c) show that the transparent woods with various lignin contents that the 1 mm thick transparent wood sample that were placed on top of handwritten letters “1 h” to “6 h” were more translucent as time continued. The yellowish transparent wood was inhibited by the H2O2 treatment, which made the transparent wood more attractive. After heating for 1 h with H2O2, the yellow color of the “3 h” transparent wood nearly disappeared. This transparency was only
Distilled Water
1 h H2O2 Treatment
y = y0 + A1 e−x / t1 0.22242 ± 0.00809 0.77566 ± 0.01618 0.85854 ± 0.04771 0.9983
0.21311 ± 0.00735 0.78556 ± 0.01517 0.79182 ± 0.04177 0.9985
reached after at least 6 h without the H2O2. In order to ensure that the transparent wood had a better appearance, the following treatment with various thicknesses of LRWs (1 mm, 1.5 mm, 2 mm, and 5 mm) were used to improve the H2O2 treatment after 1 h. The treated LRWs were impregnated onto the ethanol solution for 8 h. Ethanol was used to the replace water in the LRW, which improved the permeability of the LRW. The transparent wood was obtained by impregnating the epoxy resin in a vacuum. As seen in Fig. 3(a) and (b), the transparencies at hour six were similar. The maximum transmission of the transparent wood reached 82%. The fitting curve of the lignin residue content demonstrated that the lignin reached equilibrium after 3 h. The transmittance of the transparent wood increased as the lignin content decreased and reached 80% after 3 h. A 1 mm veneer could be treated with NaClO2 for 3 h in order to obtain transparent wood with high transmittance. The transmittance of the same thicknesses of epoxy resin and glass were about 10% higher than the transparent wood. During the ultraviolet A (UVA) stage (320–400 nm), the transmittance of the transparent wood was significantly lower than the epoxy resin and the glass. During the UVA stage, the average transmittance of the transparent wood was lower than the epoxy resin, at about 45%, which was about 55% lower than the glass. UVA had a strong penetration, where it directly reached the dermis, destroyed elastic fibers and collagen fibers, and caused cataracts and skin cancer [23]. The 1.5 mm and 2 mm balsa woods were treated with the same methodology as the 1 mm wood. Fig. 4 shows the transparent wood transmission of the 1.5 mm and the 2 mm with various lignin contents. When the thickness increased, the transmittance of the transparent 160
Vacuum 158 (2018) 158–165
J. Qin et al.
Fig. 3. Transmission curves of the 1 mm transparent wood over various treatment times (1 h-6h) of NaClO2 (a) washing with distilled water and (b) washing with an additional H2O2 treatment.
Fig. 4. Transmission curves over various treatment times (1–6 h) of NaClO2 (a) 1.5 mm transparent woods washed with distilled water, (b) 1.5 mm transparent woods washed with an additional H2O2 treatment, (c) 2 mm transparent woods washed with distilled water, and (d) 2 mm transparent woods washed with an additional H2O2 treatment. 161
Vacuum 158 (2018) 158–165
J. Qin et al.
Fig. 5. Five-millimeter LRWs with various impregnation times under a vacuum.
Fig. 6. The framework was composed of delignification and was impregnated with the epoxy resin, where the optical properties were obtained. The lower right side shows a photo of the 2 mm thick transparent wood with a hand-painted picture “Main building.”
12 h (5 mm). When making the multilayer transparent wood, both the single layer and the multilayer transparent wood at the lowest lignin content were used for comparisons.
woods decreased. The maximum transmittance of the 1.5 mm transparent wood decreased to 80%, and the 2 mm transparent wood decreased to 77%. The transmittance of the transparent balsa wood increased as the lignin decreased, and decreased as the thickness increased. This also occurred with the thicker wood (5 mm). The shortest NaClO2 treatment time of balsa woods were 3 h (1 mm), 4 h (1.5 mm), 5 h (2 mm), and
3.2. The flow of resin in a vacuum The modified epoxy resin impregnation solution was impregnated in 162
Vacuum 158 (2018) 158–165
J. Qin et al.
Fig. 7. Optical transmittance curves of the transparent wood of the 2 mm SMTBW, the 2 mm CMTBW, the 2 mm SLTBW, the 5 mm SLTBW, the 5 mm CMTBW, and the 10 mm CMTBW.
Fig. 9. The transmittance curves of the 1 mm SLTB, the 2 mm SLTB, the 2 mm SMTB, the 2 mm CMTB, the 5 mm SLTB, and the 5 mm CMTB. Table 2 Tension strength and transmittance of the balsa wood and the basswood.
a 1000Pa environment via the vacuum impregnation method. The vacuum impregnation method eliminated the interlayer bubbles, which could enhance the bonding properties of the transparent wood and increase the transmittance. In order to study the flow of the impregnated liquid in wood, a 5 mm thickness transparent balsa wood with various impregnation times were compared. As Fig. 5 shows, the transmittance of the balsa wood increased as the vacuum impregnation times increased. The main flow of the epoxy resin is depicted by red arrows. Most of the epoxy resin was impregnated from bottom to top, and the small parts flowed from both sides to the middle. The gravity makes the flow of epoxy resin from bottom to top more difficult. This indicated that the thickness of the LRW was the main factor that affected the impregnation capacity and the transmittance. Multilayer transparent wood could address this problem. The thick woods were divided into thinner veneers, so that
Type
Transverse tension strength (MPa)
Longitudinal tension strength (MPa)
Transmittance
OBW 2 mm SLTBW 2-layers SMTBW 2-layers CMTBW OB 2-mm SLTB 2-layers SMTB 2-layers CMTB
1.02 ± 0.5 6.06 ± 1.5 9.06 ± 1.3
12.01 ± 4.1 69.13 ± 6.0 65.53 ± 3.5
3% 77% 76%
54.59 ± 2.5
54.59 ± 2.5
70%
4.12 ± 0.3 8.17 ± 0.3 6.13 ± 0.4 63.05 ± 2.7
45.12 75.12 70.23 63.05
1% 64% 59% 55%
± ± ± ±
2.6 2.4 3.0 2.7
Fig. 8. Stretching results of the 1 mm OBW, the 1 mm SLTBW, the 2 mm SLTBW, the 2 mm SMTBW, and the 2 mm CMTBW in the (a) parallel direction and the (b) upright direction. 163
Vacuum 158 (2018) 158–165
J. Qin et al.
basswood. The transmittance of the double-layer CMTB was 55%, which was slightly lower than the SLTB. This trend was similar to the transparent balsa wood. The difference between the multilayer transparent balsa wood and the basswood is shown in Table 2. Transparent basswood had a lower transmittance than the transparent balsa wood, but its mechanical properties were better. The laminating caused the transverse tension strength of the OB to increase from 4.12 MPa to 63.05 MPa. The increased longitudinal tensile strength was primarily caused by the enhanced epoxy resin. The maximum longitudinal tensile strength reached 75.12 MPa.
impregnated and laminated could improve the impregnation efficiency. 3.3. SEM and practicality picture Balsa wood is primarily composed of cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are polysaccharides, while lignin is a three-dimensional amorphous polyphenolic macromolecule that consists of three types of phenyl propane units that form a complex, highly branched, and amorphous structure. The local repartition of the compounds was not homogeneous. The lignin was primarily located at the surface of the fiber, while the backbone was primarily composed of cellulose. The intercellular layer accounted for 3/4 of the total lignin content and the secondary wall accounted for the remaining 1/4 [24]. The changes in the intercellular layer are shown in the red arrows in Fig. 6. The chlorine dioxide (ClO2) produced by the NaClO2 solution was the main component of the removal of lignin, which attacked the chromophore on the lignin and transformed the lignin into a free radical. The intercellular layer was cracked. The epoxy filled the cracks under vacuum and replaced the lignin to strengthen the cytoskeleton.
4. Conclusion The NaClO2 was used in the preparation of transparent wood is greatly shortened by a two-stage delignification process and lamination method. When the lignin content reached the equilibrium point, the yellowing of the transparent wood was effectively inhibited by the H2O2 treatment. The time of the delignification increased as the wood thickness and tree density increased. The shortest time required for the 1 mm balsa wood was a 3 h NaClO2 treatment and an 1 h H2O2 treatment. The flow direction of the epoxy resin in the LRW indicated that the thickness of the wood was the main factor that affected the permeation time. The SEM measurement showed that the LRW that was dehydrated with the anhydrous ethanol was well filled with epoxy resin in the vacuum. The ultraviolet spectrophotometer showed that the transmittance decreased as the tree species density and the number of layers increased. A maximum transmittance of the transparent balsa wood was 82%, higher than that of transparent basswood by 7% at a 300 nm wavelength. The transparent wood had a good barrier to UV radiation (less than 400 nm), which could be used in intelligent buildings to regulate light and reduce the UV ray damage. Tensile tests showed that the tensile strength of the transparent wood increased as the tree density increased. The transparent basswood had better tensile properties, where the tensile strength reached 75 MPa. Laminating allowed the delignification time of the thick wood to be reduced to that of a thin wood. The cross direction laminating reduced the risk of transparent wood being damaged from the weak side. The production of transparent wood with high strength and high transmittance in a short time reduces the cost of preparation and enables the industrialization of transparent wood.
3.4. Transmission and stretching strength of multilayer transparent balsa wood Fig. 7 shows the transmission of the 2 mm SLTBW, which was 76.5% and was similar to the 2 mm SMTBW. This was slightly higher than the CMTBW 6%. When more than two layers of transparent wood were made, they were stacked in a cross direction. The transmission of the 5 mm CMTBW was 54.4%, which was 10% lower than the 5 mm SLTBW. The transmission of the 10 mm CMTBW was 31.7%. When additional layers were present, the transmittance slowed. This could be related to the interlayer gluing layer. Compared to the thicker single layer transparent wood previously made [14,16], the multilayer transparent wood did not fully use the transmission rate. The multilayer transparent wood reduced the time required to remove the lignin. The multilayer transparent wood was composed of multilayer 1 mm veneer. The removal time for the lignin was 3 h. The multilayer transparent wood had a unique advantage within eliminating anisotropy. With the wood was impregnated with epoxy resin, the ultimate tensile strength parallel to the grain increased from 12.01 MPa to 69.02 MPa. The ultimate tensile strength of the 2 mm SMTBW was similar to the 2 mm SLTBW, but was 20% higher than the CMTBW. The CMTBW obtained the best upright direction mechanical property. The 2 mm CMTBW upright direction mechanical property was 5 times higher than the 2 mm SMTBW, 7 times higher than the 2 mm SLTBW, and 100 times higher than the 1 mm natural balsa wood (Fig. 8). The results showed that the transmission rate of the SMTBW was similar to the single layer transparent wood SLTBW with the same thickness. The cross direction multilayer transparent wood CMTBW enhanced the weakest direction.
Acknowledgements This research was funded by the National Natural Science Foundation of China (Grant no. 31470581) and Fundamental Research Funds for the Central Universities (Grant no. 2572016EBJ1). References [1] N. Kohler, The relevance of Green Building Challenge: an observer's perspective, Build. Res. Inf. 27 (4–5) (1999) 309–320. [2] M. Chan, D. Estève, C. Escriba, E. Campo, A review of smart homes- present state and future challenges, Comput. Methods Progr. Biomed. 91 (1) (2008) 55–81. [3] Y. Tsunetsugu, Y. Miyazaki, H. Sato, Physiological effects in humans induced by the visual stimulation of room interiors with different wood quantities, J. Wood Sci. 53 (1) (2007) 11–16. [4] Y. Li, Q. Fu, S. Yu, M. Yan, L. Berglund, Optically transparent wood from a nanoporous cellulosic template: combining functional and structural performance, Biomacromolecules 17 (4) (2016) 1358. [5] Y. Li, Q. Fu, X. Yang, L. Berglund, Transparent wood for functional and structural applications, Philos Trans A Math Phys Eng Sci 376 (2112) (2018) 20170182. [6] S. Fink, Transparent wood – a new approach in the functional study of wood structure, Holzforschung 46 (5) (1992) 403–408. [7] M. Zhu, J. Song, T. Li, A. Gong, Y. Wang, J. Dai, et al., Highly anisotropic, highly transparent wood composites, Adv. Mater. 28 (26) (2016) 5181–5187. [8] H.S. Yaddanapudi, N. Hickerson, S. Saini, A. Tiwari, Fabrication and characterization of transparent wood for next generation smart building applications, Vacuum 146 (2017) 649–654. [9] M. Zhu, T. Li, C.S. Davis, Y. Yao, J. Dai, Y. Wang, et al., Transparent and haze wood composites for highly efficient broadband light management in solar cells, Nanomater. Energy 26 (2016) 332–339.
3.5. Transmission and stretching strength of multilayer transparent basswood The basswood had a lignin equilibrium concentration equal to the balsa wood, but it required more time. The lignin equilibrium concentration time of basswoods were 6 h (1 mm) and 12 h (2 mm). It was possible to use the shortest balanced lignin concentration time (6 h) to produce the shortest time-consuming multilayer transparent basswood. The original basswood had a higher density (0.49 g/cm3) and was more compact inside. Its lignin content was 20.9%. It was more difficult to remove the lignin and to impregnate the epoxy resin. Its fabrication time was about twice as long as the Balsa wood. Fig. 9 shows that the transmittance decreased as the basswood chips thickness increased. The transmittance of 1 mm, 2 mm, and 5 mm transparent basswood reached 75%, 64%, and 19%. The transmittance of the double-layer SMTB reached 59%, which was close to the 2 mm single-layer transparent 164
Vacuum 158 (2018) 158–165
J. Qin et al.
[17] Q. Fu, M. Yan, E. Jungstedt, X. Yang, Y. Li, L.A. Berglund, Transparent plywood as a load-bearing and luminescent biocomposite, Compos. Sci. Technol. (2018) 164. [18] M.J. Nieuwenhuijsen, M.B. Toledano, N.E. Eaton, J. Fawell, P. Elliott, Chlorination disinfection byproducts in water and their association with adverse reproductive outcomes, Occup. Environ. Med. 57 (2) (2000) 73–85. [19] M.L. Brongersma, Y. Cui, S. Fan, Light management for photovoltaics using highindex nanostructures, Nat. Mater. 13 (5) (2014) 451–460. [20] T. Yokoyama, JFK, H. Chang, Microanalytical method for the characterization of fiber components and morphology of woody plants, J. Agric. Food Chem. 50 (5) (2002) 1040–1044. [21] H. Yano, A. Hirose, P.J. Collins, Y. Yazaki, Effects of the removal of matrix substances as a pretreatment in the production of high strength resin impregnated wood based materials, J. Mater. Sci. Lett. 20 (12) (2001) 1125–1126. [22] M. Osei-Antwi, J.D. Castro, A.P. Vassilopoulos, T. Keller, Shear mechanical characterization of balsa wood as core material of composite sandwich panels, Construct. Build. Mater. 41 (41) (2013) 231–238. [23] O.Y. Hao, G. Stamatas, N. Kollias, Dermal contributions to UVA-induced oxidative stress in skin, Photodermatol. Photoimmunol. Photomed. 25 (2) (2009) 65–70. [24] R. Vanholme, B. Demedts, K. Morreel, J. Ralph, W. Boerjan, Lignin biosynthesis and structure, Plant Physiol. 153 (3) (2010) 895–905.
[10] W.T. Gan, L.K. Gao, S.L. Xiao, W.B. Zhang, X.X. Zhan, J. Li, Transparent magnetic wood composites based on immobilizing Fe3O4 nanoparticles into a delignified wood template, J. Mater. Sci. 52 (6) (2017) 3321–3329. [11] W.T. Gan, S.L. Xiao, L.K. Gao, R.N. Gao, J. Li, X.X. Zhan, Luminescent and transparent wood composites fabricated by poly(methyl methacrylate) and gammaFe2O3@YVO4:Eu3+ nanoparticle impregnation, Acs Sustain Chem Eng 5 (5) (2017) 3855–3862. [12] Y.Y. Li, S. Yu, J.G.C. Veinot, J. Linnros, L. Berglund, I. Sychugov, Luminescent transparent wood, Adv Opt Mater 5 (1) (2017) 6. [13] Z. Yu, Y. Yao, J. Yao, L. Zhang, C. Zhang, Y. Gao, et al., Transparent wood containing CsxWO3 nanoparticles for heat-shielding-window applications, J. Mater. Chem. 5 (13) (2017). [14] T. Li, M. Zhu, Z. Yang, J. Song, J. Dai, Y. Yao, et al., Wood composite as an energy efficient building material: guided sunlight transmittance and effective thermal insulation, Advanced Energy Materials 6 (2016) 1601122. [15] Y. Li, Q. Fu, R. Rojas, M. Yan, M. Lawoko, L. Berglund, Lignin-retaining transparent wood, ChemSusChem 10 (17) (2017) 3445–3451. [16] Y. Li, X. Yang, Q. Fu, R. Rojas, M. Yan, L. Berglund, Towards centimeter thick transparent wood through interface manipulation, J. Mater. Chem. 6 (3) (2018) 1094–1101.
165