Eco-friendly transparent poplar-based composites that are stable and flexible at high temperature

Farmed poplar could meet the human demand for transparent wood-based composites to replace glass, avoiding the consumption of natural forest resources. We removed the lignin of poplar using a potassium hydroxide (KOH) and deionized water solution, the waste black liquor could be converted into compound potassium fertilizer after being neutralized by phosphoric acid. Polyurethane (PU) was then added to the lignin-stripped poplar and hardened, the transparent poplar-based composite (TPC) has stable transparency at high temperatures, and flexibility – it elongates (about 15%) before breaking. These properties could provide more uses in hot environments requiring a flexible shape. The TPC PU provides transmittance of 85%, haze of 83%, and anisotropic light diffraction.


Introduction
To compete with glass, transparent wood-based composites (TWC) have been introduced as eco-friendly building materials, owing to their high optical transmittance and low thermal conductivity. When replacing glass as a building material, TWC can effectively save energy by guiding natural light into a building, reducing the use of articial light. TWC can also signicantly reduce greenhouse gas emission by insulating and reducing the use of air conditioning. 1-3 TWC is a kind of cellulose-based biocomposite, whose favourable properties and renewability are important drivers for applications ranging from smart furniture to solar cells. 4,5 However, if TWC is obtained using forest resources, its eco-friendly material properties will be diminished. Therefore, the raw material selection of TWC must conform to the principle of rational use of natural resources, in order to ensure its original intention to aid ecofriendly and sustainable development.
Poplar is the most widely distributed and adaptable tree species in the world, and it is mainly distributed in the temperate and cold temperate regions of the Northern Hemisphere, latitude 22-70 N, from low altitude to 4800 m. Poplar is mainly distributed in Russia, China, Canada, the United States, Italy, France and so on. [6][7][8] Poplar is an important agro-forestry tree in many nations due to its fast growth rate, short rotation period, multiple uses and high economical value. 9 Responsible use of farmed poplar can not only meet the human demand for TWC, but also avoid the consumption of natural forest resources. Therefore, poplar is used as the raw material of TWC, so that the transparent poplar-based composites (TPC) are always in line with the original intention of environmental protection and sustainable development. The lignin content of poplar is about 20-24% at a low level, and the physical and mechanical characteristics of poplar can be improved by adding polymers. 10 The production of TWC comprises the following steps: removing the lignin, bleaching its cellulose ber network, and adding a transparent polymer that inltrates the cellulose ber network. [11][12][13]15 The production of TPC mainly follows the above steps ( Fig. 1). However, potassium hydroxide (KOH) is used to replace sodium hydroxide (NaOH) for lignin removal from poplar, the black waste liquor could be converted into compound potassium fertilizer aer being neutralized by phosphoric acid (H 3 PO 4 ), 14 which reduces the environmental pollution caused by the production process. Polyurethane (PU) is used to replace epoxy resin as the inltrating polymer to make TPC PU. Compared with TWC, TPC PU has improved exibility and stability at high temperature. The transparency only had subtle changes aer it was treated by thermostatic hot air drying at 90 C for 100 h, and it has better elongation (about 15%) before breaking. TPC PE provides transmittance of 85%, haze of 83%, and anisotropic light diffraction.

Materials and chemicals
Longitudinal-cutting poplar veneer (PV, 80 mm Â 80 mm Â 3 mm) was selected and used in this study. As Table 1 shows, the sample of PV had lignin removed in a solution that included potassium hydroxide (KOH, >98%) and deionized water. Sodium hypochlorite (NaClO, >98%) and deionized water were then used to bleach the lignin-stripped PV. Ethyl alcohol absolute (C 2 H 6 O, >99.5%) was used to improve the limited swelling of the cellulose ber network. PU was then added as the inltrating polymer. KOH, NaClO, deionized water and C 2 H 6 O were purchased from Aladdin Biochemical Technology (Shanghai, China). PU and its hardener (polyisocyanate) were purchased from Wuhui Port Adhesive Co., Ltd. (Hangzhou, China).

Delignication
The sample of PV was immersed in solution that included KOH (2.7 mol L À1 in deionized water). Aer the solution had been boiled for 8 h at 120-130 C, the sample of PV was removed and the chemicals rinsed off in hot distilled water. Aer removing a large proportion of lignin, the sample of PV was immersed in NaClO solution (0.81 mol L À1 in deionized water) for about 24 h at 15-25 C until its color disappeared. Then, the PV was rinsed in hot distilled water again.

Limited swelling of cellulose ber network
Although its cellulose ber network had been partially swollen during the removal of its lignin and bleaching its cellulose ber network, the sample of PV was been immersed in C 2 H 6 O (>99.5%, 100 ml) for 24 h at 15-25 C in order to maintain the existing swelling effect and retain the cellulose ber network with limited swelling.

Polyurethane inltration
Aer PU had been heated at 45-50 C for 5 min, PU and its hardener were mixed at a ratio of 1 to 1 (PU 50 ml, its hardener 50 ml), and this liquid resin (100 ml) immersed the sample of delignied PV. The liquid resin lled the cellulose-swollen structure of PV and its lumen in an RV-620-2 vacuum reactor (YBIF, Shanghai, China) at 25-30 C. All the above processes should be completed within 30 min. This kind of poplar-polymer composite was solidied at 25-30 C for 24 h, and its weight reached $20.5 g from $8.5 g before PU inltration.

Improving eco-friendliness by utilising waste material
Poplar is a kind of eco-friendly raw material of TWC. Its fast growth rate, short rotation period, multiple uses and high economical value allow it to meet human demand, and can avoid the consumption of natural forests. Previous research focuses on TWC from radial-cutting veneer for its easier in delignication, however, in our work, the TPC from longitudinal-cutting PV could obtain far larger breadth from the poplar trunk and better mechanical properties.
KOH is a kind of eco-friendly delignied material used for lignin removal from poplar. As shown in Fig. 2, the black liquor of KOH can be neutralized by H 3 PO 4 , and a compound potassium fertilizer (KH 2 PO 4 ) can be prepared by ltering and concentrating with a ve-effect evaporator. 14 In our work, the black liquor of KOH (68 ml) whose pH was adjusted from $14 to $7 by adding H 3 PO 4 (7 ml, >98%), then, KH 2 PO 4 ($19 g) was prepared by ltration and concentration.

Modication of cell wall contents and microstructure
During delignication and bleaching, the sample of PV has reduced water-absorbent properties and the hydrophobicity is obviously enhanced. Therefore, C 2 H 6 O was used to improve the limited swelling of the cellulose ber network more than water. C 2 H 6 O can also dissolve residues other than cellulose. Fig. 1 Longitudinal-cutting poplar veneer (PV) was selected and used from fast-growing farmed poplar trunk. KOH and deionized water were used to remove the lignin from the poplar, the resulting black liquor could be recycled into fertilizer. Sodium hypochlorite (NaClO) solution was used to bleach the sample of lignin-stripped PV, and ethyl alcohol absolute (C 2 H 6 O) was used to improve the limited swelling of its cellulose fiber network. Final addition of PU and hardening provides TPC PU, which can be used for applications requiring hot environments and a flexible shape. Fourier transform infrared spectroscopy (FTIR) was used to investigate the changes of lignin from natural PV to delignied PV. FTIR spectra were performed by FTIR-850 (Gangdong, Tianjin, China). In the FTIR spectrum, the band at 1505 cm À1 is characteristic of aromatic compounds (phenolic hydroxy groups) and is attributed to aromatic skeleton vibrations from lignin. 16,17 The band at 1235 cm À1 can be associated with hemicelluloses, and the band at 1735 cm À1 represents C]O functional group. [18][19][20] To compare with natural PV, the peaks of delignied PV at 1505 cm À1 , 1235 cm À1 and 1735 cm À1 almost disappeared which prove that lignin, hemicellulose and C]O functional groups were largely removed from PV (Fig. 3). Fig. 3 indicates that the absolute-drying weight of PV (80 mm Â 80 mm Â 3 mm) has reduced from $4.5 g to $2.1 g aer delignication and absolute-drying treatment.
Transparent polymer inltration is an important step in the production of TWC. Our TPC PU composite was examined using Quanta 450 scanning electron microscopy (FEI, US). Fig. 4 (a  and d) show the scanning electron microscope (SEM) image of radial-cutting PV and longitudinal-cutting PV before PU inltration. Fig. 4(b, c, e and f) show the SEM images of radialcutting PV and longitudinal-cutting PV aer PU inltration. Comparing to the SEM images (a and d), SEM image (b, c, e and f) show that the microstructure is well-preserved aer being lled with PU.

Flexibility and color-stability at high temperature
Aer the gaps between the cellulose ber network had been inltrated by PU and these polymers hardened, the resulting TPC PU (80 mm Â 20 mm Â 3 mm) had improved exibility and color-stability against high temperatures compared to epoxy resin-inltrated TPC (TPC ER, 80 mm Â 20 mm Â 3 mm).
Flexible wood has increasingly attracted scientic interest due to its wide applications. 23 Aer removing lignin, bleaching the cellulose and inltrating with PU, TPC PU has better exibility including more elongation before breaking than TPC ER. Fig. 5(a) shows that the sample of PV breaks upon bending aer removing lignin and bleaching with cellulose. Fig. 5(b) shows that the sample of PV breaks upon bending aer removing lignin, bleaching cellulose and inltrating with epoxy resin. Fig. 5(c) indicates that the sample of PV becomes more exible upon bending aer removing lignin, bleaching with cellulose and inltrating with PU. Its bending radius is about 2.4 mm. The tensile strength of samples were tested by SmartTest (Joyrun, China). Fig. 5(d) shows the tensile strength test forcedisplacement curves. According to Fig. 5(d) and the equation for calculating elongation at break:   Elongation at break (%) ¼ L 0 À L/L Â 100% L of samples are 80 mm. Aer test of tensile strength, L 0 of TPC PU is about 92 mm and its elongation at break could be about 15%, L 0 of TPC ER is about 82 mm and its elongation at break is about 2.5%.
The transparency of TPC PU is more stable than TPC ER at high temperatures. We used Photoshop soware (Adobe, US) to collect RGB data and Lab data of the color at three positions (:, -, C) in photos of TPC PU and TPC ER before and aer thermostatic hot air drying at 90 C for 100 h. The hot air drying experiment involved a DGG-9203A electro-thermostatic blast oven (SLIC, Shanghai, China), and these photos of TPC PU and TPC ER were obtained by LiDE120 scanner (Canon, JP). As shown in Fig. 6, according to their respective change amplitude of RGB data and Lab data before and aer the hot air drying experiment, TPC ER became darker.

Optical properties of TPC
TWC shows high optical transmittance and haze, with minor reection on the outer surface. 1,24,25 Research interest has increased in modifying wood cell and cell wall assemblies as well as cell wall components. 26 Modied wood cells not only possess important engineering applications but also great potential in new technology elds. 27 Surface modication of the wood cell wall will help to tune light scattering properties. 16 The modied wood cell wall introduced strong scattering, resulting in diffused luminescence from embedded quantum dots. 28 In TPC, the light is diffracted in all directions by modifying its cell wall, and the optical haze is due to its structural anisotropy. The interface is critical for optical transmittance of TWC. 29,30 The interfacial space of TPC could be increased in a reasonable range by removing lignin, improving the limited swelling of cellulose ber network and inltrating with PU. Between the cellulose ber network of the poplar-polymer composite, the interfacial space linking lumen is also the light pathway for optical transmittance. Transmittance and haze were obtained by WGT-S transmittance and haze tester (SGIC, Shanghai, China). Fig. 7(a and b) show that the TPC with transmittance of 85%, haze of 83%. A photodiode power sensor S130C (Thorlabs, US) was used to record the scattered light intensity distribution in both the x and y directions. Fig. 7(c and  d) indicate that the TPC with anisotropic light diffraction, and lower refractive index uctuation in the x direction was obtained due to the aligned cellulose bres in this direction.  TPC is a kind of eco-friendly TWC due to poplar being the most widely distributed and adaptable tree species in the world. The lignin of poplar was removed by the solution that included KOH and deionized water, and the black liquor could be converted into compound potassium fertilizer aer being neutralized by H 3 PO 4 . Aer these gaps between its cellulose ber network had been inltrated, PU and these polymers have been hardened, this kind of TPC shows exibility, color-stability against high temperature, and optical properties.

Conclusions
The PU-inltrated TPC (TPC PU) was prepared by KOH delignication, NaClO bleach, C 2 H 6 O limited swelling, and PU inltration. This kind of TPC PU is from an farmed poplar and the black liquor side-product could be turned into a fertilizer. TPC PU is better than TPC ER in terms of color-stability at higher temperatures, and exibilityits elongation at break was about 15%. It also provided transmittance of 85%, haze of 83%, and anisotropic light diffraction. Furthermore, our future work will pay more attention to reduce the time cost in the preparation of TPC PU, to relieve the loss of poplar in the process of production, and to improve its volume and quality.

Conflicts of interest
There are no conicts to declare.