Effect of Lignin Content on Properties of Flexible Transparent Poplar

Article Eﬀect of Lignin Content on Properties of Flexible Transparent Poplar Veneer Fabricated by Impregnation with Epoxy Resin

Mengting Lu 1 , Wen He 1,2,* , Ze Li 1 , Han Qiang 1 , Jizhou Cao 1 , Feiyu Guo 1 , Rui Wang 1 and Zhihao Guo 1 1 College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China; [email protected] (M.L.); [email protected] (Z.L.); [email protected] (H.Q.); [email protected] (J.C.); [email protected] (F.G.); [email protected] (R.W.); [email protected] (Z.G.) 2 Innovation Center of Eﬃcient Processing and Utilization of Forest Resources, Nanjing Foresry University, Nanjing 210037, China * Correspondence: [email protected]; Tel.: +86-138-0515-7249

Received: 12 October 2020; Accepted: 29 October 2020; Published: 5 November 2020

Abstract: In this work, poplar veneer (PV) rotary-cut from fast-growing polar was delignified to prepare flexible transparent poplar veneer (TPV). Lignin was gradually removed from the PV and then epoxy resin filled into the delignified PV. The study mainly concerns the effect of lignin content on microstructure, light transmittance, haze, tensile strength, and thermal stability of the PVs impregnated with epoxy resin. The results indicate that the lignin could be removed completely from the PV when the delignification time was around 8 h, which was proved by FTIR spectra and chemical component detection. Moreover, according to SEM observation and XRD testing, the porosity and crystallinity of the PVs were gradually increased with the removal of lignin. Also, the optical properties measurement indicated that the light transmittance and haze of the TPVs gradually increased, and the thermal stability also became more stable as shown by thermogravimetric analysis (TG). However, the tensile strength of the TPVs declined due to the removal of lignin. Among them, TPV8 exhibited excellent optical properties, thermal stability, and tensile strength. Consequently, it has great potential to be used as a substrate in photovoltaics, solar cells, smart windows, etc.

Keywords: fast-growing polar; ﬂexible transparent poplar veneer; optical properties; tensile strength; thermal stability

1. Introduction With portability, convenience and aesthetic design becoming increasingly important to consumers, the demand for flexible electronic device technology is also growing [1]. One of the most important properties of flexible electronic devices is their stable mechanical properties under repeated folding or bending [2]. Flexible transparent substrate is a vital part of flexible electronic equipment. Usually, transparent plastic, such as Polyethylene terephthalate (PET), Acrylonitrile Butadiene Styrene copolymer (ABS copolymer) and Polyvinyl chloride (PVC), etc., are used as substrate for flexible electronic devices because of their good mechanical properties, light weight, and low cost [3]. However, in recent years, the environmental problems caused by non-renewable and non-biodegradable resources are on the increase. As petroleum follow-on products, these transparent plastic products are neither biodegradable nor renewable, and their non-sustainability and non- environmentally friendly defects limit their application in the future [4]. Consequently, the demand for bio-based materials is urgent, and more attention is being paid to the sustainable production of biofuels, chemicals, and materials from lignocellulosic biomass [5–8].

Polymers 2020, 12, 2602; doi:10.3390/polym12112602 www.mdpi.com/journal/polymers Polymers 2020, 12, 2602 2 of 13

Wood is the most abundant bio-based resource on the earth. It has been widely used for construction structural materials and decorative materials due to its high ratio of strength to weight, aesthetics, low thermal conductivity, and biodegradability [9–11]. As a major component of wood, cellulose plays an important role on the contribution of the mechanical properties of wood, which is usually used as a reinforcement for bio-based composite materials [12,13]. In recent years, nanocellulose, which is isolated from cellulose by a series of physical and chemical treatments, has received extensive attention due to its extremely high aspect ratios, excellent mechanical behavior, biodegradability and so on [6,14–17]. Among the substrates for flexible electronic equipment, nanocellulose composites are expected to replace glass or plastics in displays, optoelectronics or automobiles because of their high transparency, excellent mechanical properties, safety, and sustainability [18]. However, during the fabrication of nanocellulose, the isolation process of cellulose always consumes a lot of energy, moreover, the intensive chemical treatment destroys the hierarchical structural of the cell wall, which leads to the oriented nanocellulose architecture no longer existing [10,19]. In order to reduce the expenditure of energy and retain the original oriented nanocellulose structure of wood, the bulk wood is directly subjected to delignification treatment to produce hierarchical micro and nano-scale cellulose fibers with a diameter less than the wavelength of visible light [20]. Subsequently, the delignified wood is impregnated by polymers whose refractive index match cellulose, such as epoxy resin and polymethyl methacrylate [3,4,21,22]. The fabricated transparent wood not only retains the natural wood structure, but also possesses the advantages of nanocellulose. Compared to natural wood, transparent wood has high transparency and toughness due to delignification and the penetration of transparent polymer [10,23]. At present, basswood [19], birch [22,24], and balsa [9] have already been successfully used to fabricate transparent wood. This transparent wood displays outstanding transparency, but the tensile strength is not satisfied because transversal wood is used as the substrate. Additionally, the dimension of these substrates is limited by the diameter of the trees, which is not suitable for a commercial process. Fast-growing poplar is a type of artificially planted wood, which grows fast and whose growth cycle is around 9–12 years [25]. As a fast-growing raw material, the density is low and its mechanical properties are not very good, so usually, it is used to produce ordinary plywood for decoration or low-grade furniture with low economic value added. Recently, the quarter-sawed lumber of fast- growing poplar has also been used to prepare transparent wood in a few research studies [7], similarly, this kind of substrate has a low utilization rate and it is difficult to process on a large scale. At present, the rotary cut veneer is the main processing unit material for fast-growing poplar, which has higher outturn percentage, and its dimensions and thickness are easily acquired according to need. Therefore, in this research, the rotary cut poplar veneer was used as the substrate to fabricate flexible transparent poplar veneer. Lignin was gradually removed from PV using acid sodium chlorite solution by controlling the delignification time, and epoxy resin was used as the filling in the delignified PV under vacuum impregnation, the preparation diagram is shown in Figure1. Although epoxy resin is an unsustainable material, here it was chosen to fill PV because its refractive index matches cellulose. The study mainly concerns the effect of lignin content on microstructure, light transmittance, haze, mechanical strength, and thermal stability of the poplar veneer impregnated with epoxy resin. Polymers 2020, 12, 2602 3 of 13 Polymers 2020, 12, x FOR PEER REVIEW 3 of 13

Figure 1. SchematicSchematic diagram diagram of of TPVs TPVs prepared prepared from PV.

2. Materials Materials and Methods 2.1. Materials and Chemicals 2.1. Materials and Chemicals Rotary cut poplar veneer was obtained from Siyang County, Jiangsu Province (density: 0.429 g/cm3, Rotary cut poplar veneer was obtained from Siyang County, Jiangsu Province (density: 0.429 thickness: 1.0 mm), sodium chlorite (NaClO2, 80 % solid, Shanghai McLean Biochemical Technology g/cm3, thickness: 1.0 mm), sodium chlorite (NaClO2, 80 % solid, Shanghai McLean Biochemical Co., Ltd., Shanghai, China), Acetic acid (CH3COOH, AR, Nanjing Chemical Reagent Co., Ltd., Nanjing, Technology Co., Ltd., Shanghai, China), Acetic acid (CH3COOH, AR, Nanjing Chemical Reagent Co., China), ethanol (C2H6O, AR, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), epoxy resin Ltd., Nanjing, China), ethanol (C2H6O, AR, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), (AB style, JH-5511 Hangzhou Wuhuigang Adhesive Co., Ltd. Hangzhou, China). Deionized water epoxy resin (AB style, JH-5511 Hangzhou Wuhuigang Adhesive Co., Ltd. Hangzhou, China). was prepared using the laboratory ultrapure water machine (VE-A-10 L/h, Hongseng Co., Ltd., Deionized water was prepared using the laboratory ultrapure water machine (VE-A-10 L/h, Nanjing, China). Hongseng Co., Ltd., Nanjing, China). 2.2. Fabrication of Flexible Transparent Poplar Veneer 2.2. Fabrication of Flexible Transparent Poplar Veneer 2.2.1. Delignification Process 2.2.1. Delignification Process Rotary cut poplar veneer (PV) with a size of 50 mm (Length) 50 mm (Width) 1.0 mm (Thickness) Rotary cut poplar veneer (PV) with a size of 50 mm (Length)× × 50 mm× (Width) × 1.0 mm was dried at 60 ◦C for 24 h using a vacuum drying oven. The NaClO2 solution (5 wt%) with a pH of 4.6 (Thickness) was dried at 60 °C for 24 h using a vacuum drying oven. The NaClO2 solution (5 wt%) was prepared to remove the lignin from the PV. Then, the PV was wholly immersed into the NaClO2 with a pH of 4.6 was prepared to remove the lignin from the PV. Then, the PV was wholly immersed solution using a Teflon standoff as a shield, in which the mass ratio of the PV to the NaClO2 solution into the NaClO2 solution using a Teflon standoff as a shield, in which the mass ratio of the PV to the was 1:60. Then the delignification reaction was continued with an agitator at 75 ◦C. The PV was taken NaClOout after2 solution 1 h and was washed 1:60. withThen deionizedthe delignification water three reac times.tion was Then continued the washed with PVan agitator was placed at 75 into °C. The PV was taken out after 1 h and washed with deionized water three times. Then the washed PV fresh NaClO2 solution to repeat the above reaction process. In order to obtain different lignin contents, was placed into fresh NaClO2 solution to repeat the above reaction process. In order to obtain the PV was treated for 2 h, 4 h, 6 h, 8 h, and 10 h, respectively, separately named as PV2, PV4, PV6, PV8, different lignin contents, the PV was treated for 2 h, 4 h, 6 h, 8 h, and 10 h, respectively, separately and PV10. Subsequently, all delignified PVs were purged with a mixed solution of ethanol:deionized waternamed to as remove PV2, PV the4, PV remaining6, PV8, and chemical PV10. Subsequently, substances. Finally, all delignified the cleaned PVsPVs were were purged freeze-dried with a mixed to be solutionfurther used. of ethanol:deionized water to remove the remaining chemical substances. Finally, the cleaned PVs were freeze-dried to be further used. 2.2.2. Fabrication of Flexible Transparent PV 2.2.2. Fabrication of Flexible Transparent PV The curing agent was added to the epoxy resin at a mass ratio of 1:3, and after stirring evenly, the delignifiedThe curing PVagent was was immersed added to inthe the epoxy epoxy resin resin at a undermass ratio vacuum of 1:3, pressure and after for stirring 5 min, evenly, then, the delignified delignified PV was was immersed impregnated in the for epoxy 10 min re undersin under atmospheric vacuum pressure.pressure for In 5 order min, tothen, ensure the delignifiedsufficient penetration, PV was impregnated the above for impregnation 10 min under processatmospheric was repeatedpressure. threeIn order times. to ensure Subsequently, sufficient penetration,the excess epoxy the above resin was impregnation removed from process the surfacewas repeated of the PV three using times. a razor Subsequently, blade, and then the theexcess PV epoxy resin was removed from the surface of the PV using a razor blade, and then the PV was polymerized at 60 °C for 2 h to obtain different PV specimens, named as TPV2, TPV4, TPV6, TPV8, and TPV10, respectively.

Polymers 2020, 12, 2602 4 of 13

was polymerized at 60 ◦C for 2 h to obtain diﬀerent PV specimens, named as TPV2, TPV4, TPV6, TPV8, and TPV10, respectively.

2.3. Characterization The microstructure of the cross-section of all delignified PVs and TPVs was observed using an environmental scanning electron microscope (ESEM) (Quanta 200, FEI, Hillsborough, Oregon, USA). Before the testing, all specimens were dried at 60 ◦C for 12 h, and then the observation surfaces of all specimens were sprayed with gold using a particle sputtering instrument. The testing voltage was 15 kV and the analysis mode resolution was 3.0 nm FTIR spectroscopy was performed on a spectrometer (FTIR VERTEX 80V, Bruker, Karlsruhe, Germany) at room temperature, all delignified PV specimens were dried at 80 ◦C in a vacuum-dryer for 12 h before detection. Then a small quantity of each specimen was blended with KBr powder and 1 1 compressed to form a disk. The investigated range was 400–4000 cm− at a resolution of 4 cm− . The Van Soest method (GB/T 20806-2006 Determination of Neutral Detergent Fiber (NDF) in Feed) was used to determine the relative content of cellulose, hemicellulose, and lignin of all the delignified PVs. The X-ray diffraction (XRD) analysis of all delignified PVs was performed by X-ray diffractometer (Ultima IV, Rigaku, Tokyo, Japan) using Ni-filtered Cu Ka radiation (λ = 1.5406 Å) at 40 kV and 30 mA. Scattered radiation was recorded in the range of 5–50◦ at a scan rate of 5◦/min. According to ASTM D1003 “Standard Method for Haze and Light Transmittance of Transparent Plastics”, the light transmittance and haze of all the TPVs were detected using a UV–Vis spectrophotometer (Lambda 950, PE, Walsham, MA, USA) in the spectrum range of 200–800 nm. The tensile strength of all TPVs with a dimension of 50 mm (L) 5 mm (W) 1 mm (H) was × × measured using a microcomputer-controlled electronic universal testing machine (SANS 4304, MTS, San Diego, CA, USA). The tensile rate was 5 mm/min and each group had at least six specimens of which the averages were obtained. The thermal stability of all TPVs was investigated with a thermogravimetric analysis instrument (STA8000, Perkin Elemer, Waltham, MA, USA). Approximately 8 mg of specimen was used for each test and every specimen was repeated at least three times. The testing temperature range was 30–700 ◦C at a heating rate of 10◦/min under a nitrogen atmosphere.

3. Results and Discussion

3.1. The EFFECT of Deligniﬁcation Time on the Microstructure and Chemical Composition of PV

3.1.1. The Effect of Delignification Time on the Microstructure of PVs Natural wood is composed mainly of cellulose, hemicellulose, and lignin, of which cellulose and hemicellulose are colorless in the visible light range, while lignin plays an important role for the color of wood due to the strong absorption of visible light. Generally, the brown and opaque wood is mainly attributed to the strong absorption of visible light of the lignin and light scattering from its uneven structure [26]. In order to prepare transparent wood, it is necessary to remove lignin to reduce the light attenuation caused by light absorption [9,27]. In the study, the effect of delignification could be first judged by the color changes of the PV [28]. As shown in Figure2, it can be clearly seen that as the delignification time increased, the color of the PVs gradually changed from chalky yellow to white. After 8~10 h of delignification, The PV almost became plain white, and it was speculated that the lignin had been removed completely. Polymers 2020, 12, 2602 5 of 13 Polymers 2020, 12, x FOR PEER REVIEW 5 of 13

Figure 2. The physical objects and the microstructuremicrostructure ofof thethe cross-sectioncross-section ofof PV,PV,PV PV22,, PVPV44,, PVPV66, PVPV8,, and PV10 underunder 300 300 and and 5000 5000 magnifications, magniﬁcations, respectively. respectively.

Figure2 2shows shows the the microstructure microstructure of the of cross-sectionthe cross-section of PV of and PV all and delignified all delignified PVs with PVs di ff erentwith reactiondifferent times. reaction The times. cells ofThe PV cells were of arrangedPV were densely,arranged in densely, which the in cellwhich wall the was cell intact wall andwas theintact middle and lamellathe middle between lamella the between wood cellsthe wood was not cells destroyed. was not destroyed. However, However, after delignification after delignification for 2 h and for 42 h,h partand 4 of h, the part middle of the lamellamiddle waslamella destroyed, was destroyed, and obvious and obvious gaps appeared gaps appeared in the cross-section in the cross-section of the PV of duethe PV to thedue partial to the removalpartial removal of lignin. of lignin. Having been delignified delignified for 6 h, the middle lamell lamellaa was destroyed and the cell wall was slightly thinned, alsoalso manymany gapsgaps appeared appeared in in the the cross-section cross-section of of the the PV. PV. These These facts facts indicated indicated that that most most of the of ligninthe lignin had had been been removed. removed. The The middle middle lamella lamella had had almost almost completely completely disappeared disappeared in in PV PV88,, andand the transect of the cell wall showed a clear layered stru structure,cture, the intercellular space and cell cavity were obviously enlarged, and the porosity increased significantly.significantly. These phenomenaphenomena indicated that the lignin in in the the cell cell wall wall and and middle middle lamella lamella were were probably probably removed removed completely completely after after delignification delignification for for8 h. 8 h. When the delignificationdelignification time increased to 10 h, the intercellular space and cell cavity of the PV were furtherfurther enlarged, enlarged, however, however, the cellthe wall cell significantly wall significantly contracted contracted and collapsed. andThe collapsed. phenomenon The couldphenomenon be ascribed could to thebe damageascribed ofto the the cellulose damage crystal of the structure cellulose due crystal to a long structure delignification due to a time. long delignification time. 3.1.2. The Effect of Delignification Time on the Lignin Content of PV 3.1.2.Figure The Effect3 shows of Delignification the FTIR spectra Time of PVon the and Lignin all delignified Content of PVs. PV On the FTIR spectrum curve of 1 1, 1 PV, theFigure characteristic 3 shows the peaks FTIR located spectra at of 1505 PV cmand− all, 1425 delignified cm− and PVs. 1235 Oncm the− FTIRrepresent spectrum the aromaticcurve of skeletonPV, the characteristic vibrations and peaks the benzene located ring-hydrogenat 1505 cm−1, 1425 bond cm stretching−1, and 1235 vibration cm−1 represent of lignin, the respectively. aromatic 1 1 Theskeleton characteristic vibrations peaks and locatedthe benzene at 1462 ring-hydrogen cm− and 1367 bo cmnd− stretchingare ascribed vibration to the C–Hof lignin, bending respectively. vibration 1 absorptionThe characteristic of lignin. peaks The located peak located at 1462 at cm 1631−1 and cm− 1367is due cm− to1 are the ascribed conjugated to the carbonyl C–H bending (C=O) stretchingvibration vibrationabsorption of of lignin. lignin. The However, peak located with the at 1631 increase cm−1 ofis due the to delignification the conjugated time, carbonyl the intensity (C=O) stretching of these characteristicvibration of lignin. peaks wasHowever, gradually with weakened—of the increase note,of the these delignification absorption peaks time, could the intensity scarcely beof foundthese oncharacteristic the FTIR spectrum peaks was curves gradually of PV8 and weakened—of PV10. This fact note, indicated these thatabsorption the lignin peaks of the could PV was scarcely removed be completely after 8 h. found on the FTIR spectrum curves of PV8 and PV10. This fact indicated that the lignin of the PV was removed completely after 8 h.

PolymersPolymers2020 2020,,12 12,, 2602 x FOR PEER REVIEW 66 ofof 1313

Figure 3. FTIR spectra of PV and PVs with different delignification time. (a) FTIR spectrum of the 4000–500Figure 3. band,FTIR (spectrab) FTIR of spectrum PV and ofPVs the with 1600–1200 different band. delignification time. (a) FTIR spectrum of the 4000–500 band, (b) FTIR spectrum of the 1600–1200 band. 1 On the other hand, the intensity and position of the characteristic peaks located at 3430 cm− , 1 1 2913 cmOn− the, and other 895 hand, cm− werethe intensity nearly unchanged and position for allof FTIRthe characteristic spectra. These peaks peaks located are mainly at 3430 ascribed cm−1, to2913 the cm O–H−1, and and 895 C–H cm stretching−1 were nearly vibration unchanged of cellulose. for all The FTIR characteristic spectra. These peak peaks of hemicellulose are mainly ascribed located 1 atto1730 the O–H cm− and, which C–H is stretching caused by vibration the C=O stretchingof cellulose. vibration, The characteristic was not changed peak of except hemicellulose for the intensity located whichat 1730 was cm slightly−1, which decreased is caused after by delignificationthe C=O stretching for 8 h. vibration, These facts was revealed not changed that the except cellulose forand the hemicelluloseintensity which of thewas PV slightly were seldomdecreased affected after by delign the delignificationification for 8 process.h. These facts revealed that the celluloseThe relativeand hemicellulose content of cellulose, of the PV hemicellulose were seldom andaffected lignin by of the all delignification PVs was quantitatively process. measured by theThe Van relative Soest method content [29 of]. Thecellulose, results hemicellulose are shown in Tableand lignin1. The ligninof all decreasedPVs was sharplyquantitatively from 22.69measured to 8.31% by afterthe Van delignification Soest method for 2[29]. h, and The the resu ligninlts are content shown gradually in Table reduced 1. The withlignin the decreased increase ofsharply delignification from 22.69 time. to 8.31% The relativeafter delignification content of lignin for 2 wash, and about the 0.41%lignin aftercontent 8 h, gradually which indicated reduced thatwith the the lignin increase had of been delignification almost removed time. The completely. relative content When theof lignin delignification was about time 0.41% was after at 10 8 h,h, thewhich relative indicated content that of the lignin lignin decreased had been to 0.06%.almost Correspondingly, removed completely. the relative When contentthe delignification of cellulose time and hemicellulosewas at 10 h, the gradually relative increased content of with lignin the decreased increase of to delignification 0.06%. Correspondingl time. Finally,y, the relative contentcontent ofof cellulosecellulose andand hemicellulosehemicellulose increased gradually to increased 78.89% and with 20.66%, the increase respectively, of delignification when the delignification time. Finally, timethe relative was 8 h. content However, of cellulose the relative and contenthemicellulose of cellulose increased slightly to 78.89% decreased andafter 20.66%, 10 h respectively, of delignification. when the delignification time was 8 h. However, the relative content of cellulose slightly decreased after 10 Table 1. The relative content of cellulose, hemicellulose, and lignin of all PVs with different h of delignification. delignification time.

Table 1. TheSample relative content Hemicellulose of cellulose, (%)hemicellulose, Cellulose and (%) lignin of Ligninall PVs (%) with different delignification time. PV 15.30 61.80 22.69 PVSample2 Hemicellulose21.62 (%) Cellulose 70.06 (%) Lignin (%) 8.31 PV4 23.19 73.28 3.51 PV 15.30 61.80 22.69 PV6 21.58 75.66 2.71 PV8 PV2 21.6220.66 70.06 78.89 8.31 0.41 PV10 21.34 78.56 0.06 PV4 23.19 73.28 3.51 2.71 3.1.3. Effect of DelignificationPV6 Time on21.58 the Cellulose Crystal 75.66 Structure of PV PV8 20.66 78.89 0.41 As shown in Figure4, the di ffraction peaks appearing at 2θ = 15.8◦ and 22.6◦ are due to the PV10 21.34 78.56 0.06 I101 and I002 crystal planes of cellulose, and a small diffraction peak appearing at 2θ = 35◦ is the I040 crystal plane of cellulose [7]. These characteristic peaks represent a typical cellulose I crystal structure. 3.1.3. Effect of Delignification Time on the Cellulose Crystal Structure of PV The diffraction curves of all PVs displayed the same diffraction peaks, which indicated that the delignificationAs shown process in Figure did 4, not the change diffraction the cellulose peaks appearing crystal structure. at 2θ = On 15.8° the and other 22.6° hand, are the due intensity to the I of101 theandI 002I002di crystalffraction planes peak of gradually cellulose, increased and a small with diffraction the increase peak of appearing the delignification at 2θ = 35° time, is the nevertheless, I040 crystal itplane significantly of cellulose decreased [7]. These for PVcharacteristic10. According peaks to the represent Segal method, a typical the cellulose cellulose I crystallinitycrystal structure. of all PVsThe wasdiffraction calculated, curves as shown of all inPVs Table displayed2. The crystallinity the same of diffraction PV was 53.8%, peaks, which which increased indicated to 69.8% that forthe PVdelignification8, while the cellulose process did crystallinity not change decreased the cellulose to 62.7% crystal for PVstructure.10. These On facts the showedother hand, that the intensity removal of the I002 diffraction peak gradually increased with the increase of the delignification time,

Polymers 2020, 12, x FOR PEER REVIEW 7 of 13 nevertheless, it significantly decreased for PV10. According to the Segal method, the cellulose Polymerscrystallinity2020, 12 of, 2602 all PVs was calculated, as shown in Table 2. The crystallinity of PV was 53.8%, which7 of 13 increased to 69.8% for PV8, while the cellulose crystallinity decreased to 62.7% for PV10. These facts ofshowed lignin increasedthat the removal the crystallinity of lignin increased of cellulose, the butcrystallinity excessive of deligniﬁcation cellulose, but treatmentexcessive delignification decreased the crystallinitytreatment decreased of cellulose. the crystallinity of cellulose.

(002)

(101)

(040) PV8

PV6 Intensity(a.u.)

PV4

PV2

PV10

10 15 20 25 30 35 40 2θ(deg)

Figure 4. XRD patterns of PVs with different delignification time. Figure 4. XRD patterns of PVs with different delignification time. Table 2. The relative crystallinity of cellulose for all PVs with different delignification time. Table 2. the relative crystallinity of cellulose for all PVs with different delignification time. Sample Relative Crystallinity (%) Sample Relative Crystallinity (%) PV 53.8 PV 53.8 PV2 61.4 PV4PV2 61.465.3 PV6PV4 65.367.2 PV8PV6 67.269.8 PV10 62.7 PV8 69.8 PV10 62.7 3.2. Microstructure and Properties of Flexible Transparent Poplar Veneer (TPV) 3.2. Microstructure and Properties of Flexible Transparent Poplar Veneer (TPV) 3.2.1. Microstructure of TPV 3.2.1.The Microstructure main reason of for TPV opaque wood is due to the existence of lignin and the porosity. In order to fabricateThe main transparent reason for wood, opaque therefore, wood is the due lignin to the needs existence to be of removed lignin and and the the porosity. interspaces In order in the to woodfabricate need transparent to be filled wood, with therefore, polymers, the whose lignin refractive needs to indexbe removed match and cellulose the interspaces (1.53). In in this the study, wood epoxyneed to resin, be filled whose with refractive polymers, index whose is about refractive 1.5, was index used match to fill cellulose the interspaces (1.53). In and this cell study, cavities epoxy of theresin, PVs whose with therefractive different index lignin is about contents 1.5, [was9]. Theused physical to fill the object interspaces of the di andfferent cell TPVscavities is of shown the PVs in Figurewith the5. Itdifferent is clearly lignin seen thatcontents the TPV [9]. The2 is yellowphysical and object semitransparent, of the different however, TPVs is the shown TPVs in gradually Figure 5. became colorless and transparent with the decrease of lignin content, finally, the TPV8 and TPV10 have It is clearly seen that the TPV2 is yellow and semitransparent, however, the TPVs gradually became almost complete achromaticity and transparency, which corresponds to the lignin content of the PVs colorless and transparent with the decrease of lignin content, finally, the TPV8 and TPV10 have almost listedcomplete in Table achromaticity1. and transparency, which corresponds to the lignin content of the PVs listed in Table 1.

Polymers 2020, 12, 2602 8 of 13 Polymers 2020, 12, x FOR PEER REVIEW 8 of 13

FigureFigure 5.5.The The physicalphysical objectobject and and the the microstructure microstructure of of TPV TPV2,2 TPV, TPV4,4, TPV TPV6,6, TPV TPV88and andTPV TPV1010 underunder aa magniﬁcationmagnification ofof 300300 andand 2500,2500, respectively.respectively.

TheThe microstructuremicrostructure of of the the cross-section cross-section of all of TPVs all TP is alsoVs is displayed also displayed in Figure in5 .Figure It clearly 5. indicatesIt clearly thatindicates the epoxy that resinthe epoxy was filled resin into was the filled vessels into and the cellvessels cavities and ofcell all cavities PVs, moreover, of all PVs, the moreover, impregnation the processimpregnation did not process destroy did the not framework destroy the of theframework cell wall of of the the cell PV wall [3]. Nevertheless,of the PV [3]. Nevertheless, PV2 and PV4 didPV2 notand provide PV4 did anot large provide amount a large of void amount space of duevoid to space thehigh due to lignin the high content, lignin and content, the epoxy and the resin epoxy did notresin form did a not complete form a filling complete in the filling wood in cells. the wood However, cells. the However, filling of the epoxy filling resin of inepoxy PV6 becameresin in fullPV6 andbecame dense, full as and just dense, a handful as just of a intercellularhandful of intercel spaceslular could spaces be found. could be When found. the When lignin the was lignin entirely was removedentirely removed from the cellfrom wall, the ascell shown wall, inas TPVshown8 and in TPVTPV108 ,and the epoxyTPV10, resin the epoxy was completely resin was filledcompletely in the cellfilled wall in the and cell cavity, wall and and there cavity, was and no there interface was no detachment interface detachment among the intercellularamong the intercellular space and epoxy space resin.and epoxy Notably, resin. the Notably, interface the could interface practically could not practi be observedcally not betweenbe observed the cellbetween wall andthe cell the polymerwall and forthe TPV polymer10. The for void TPV ratio10. The of void the PVs ratio gradually of the PVs increased gradually with increased the removal with ofthe lignin removal and of the lignin moving and channelthe moving of the channel epoxy resinof the increased. epoxy resin Subsequently, increased. Subsequently, the epoxy resin the enters epoxy the resin cell enters wall to the replace cell wall the locationto replace of thethe ligninlocation through of the microcapillary, lignin through which microcapillary, makes the microscopicwhich make voidss the decrease.microscopic Usually, voids thedecrease. refractive Usually, index the of refractive air is about index 1.0, of which air is is ab mismatchedout 1.0, which with is mismatched light. Therefore, with light. the decrease Therefore, of microporesthe decrease has of amicropores great influence has a on great the influence light transmittance on the light of transmittance the TPVs [30]. of the TPVs [30].

3.2.2.3.2.2. OpticalOptical PropertiesProperties ofof TPVsTPVs TheThe opticaloptical propertiesproperties (mainly(mainly involvinginvolving transparencytransparency andand haze)haze) ofof thethe substratessubstrates playplay aa crucialcrucial rolerole inin thethe preparationpreparation ofof optoelectronicoptoelectronic devicesdevices [[3311].]. In currentcurrent reportsreports onon glass,glass, plasticplastic andand nano-nano- paperpaper substrates,substrates, thethe maximummaximum transparencytransparency isis aboutabout 90%,90%, butbut thethe opticaloptical hazehaze isis lessless than 20% [[32].32]. OpticalOptical hazehaze isis defineddefined asas thethe abilityability toto scatterscatter incidentincident light,light, appearingappearing asas aa translucenttranslucent oror opaqueopaque statestate [[31].31]. AA higherhigher opticaloptical hazehaze meansmeans aa betterbetter interactioninteraction betweenbetween thethe lightlight andand thethe activeactive medium,medium, whichwhich can can improve improve the the effi ciencyefficiency of the of opticalthe optical device device [9]. Transparent [9]. Transparent wood with wood high with light high scattering light hasscattering potential has applications potential applications in solar cells, in OLEDsolar cells, lighting OLED systems, lighting liquid systems, crystal liquid display crystal (LCD) display backlight (LCD) units,backlight signage, units, etc. signage, [33]. etc. [33]. TheThe transmittancetransmittance andand haze haze of of all all TPVs TPVs in in the the spectral spectral range range of of 200–800 200–800 nm nm is is shown shown in in Figure Figure6. As shown in Figure6a,b the light transmittance of TPV was only 45% due to the high lignin content 6. As shown in Figure 6a,b the light transmittance of TPV2 2 was only 45% due to the high lignin content andand thethe incomplete incomplete filling filling of epoxyof epoxy resin, resin, which which increased increased the scattered the scattered light. With light. the With further the removal further ofremoval lignin fromof lignin the cellfrom wall the andcell intercellularwall and intercellular layer, the layer, porosity the graduallyporosity gradually increased, increased, which allowed which the epoxy resin to be fully permeated into the PV. Therefore, the light transmittance of TPV and allowed the epoxy resin to be fully permeated into the PV. Therefore, the light transmittance of4 TPV4 TPV was greatly enhanced, whose values achieved 68% and 77%, respectively. When the lignin was and6 TPV6 was greatly enhanced, whose values achieved 68% and 77%, respectively. When the lignin was completely removed, TPV8 and TPV10 reached high light transmittance of 81% and 82% respectively, which are near to the light transmittance of pure epoxy resin of around 88%. Figure 6c

Polymers 2020, 12, 2602 9 of 13

completelyPolymers 2020 removed,, 12, x FOR PEER TPV REVIEW8 and TPV 10 reached high light transmittance of 81% and 82% respectively,9 of 13 which are near to the light transmittance of pure epoxy resin of around 88%. Figure6c shows the 8 hazeshows of the all TPVshaze of in all the TPVs range in of the 400 range to 800 of nm,400 to in 800 which nm, TPV in which8 displayed TPV displayed the highest the haze highest value haze of 10 62%value at of 400 62% nm, at while 400 nm, the while haze valuethe haze of TPVvalue10 ofwas TPV about was 61%, about which 61%, was which slightly was lowerslightly than lower that than of 8 4 TVPthat8 .of Additionally, TVP . Additionally, TPV4 shows TPV shows the lowest the lowest hazevalue haze value at 400 at nm. 400 Notably,nm. Notably, the haze the haze values values of all of TPVsall TPVs decreased decreased slightly slightly with with the the increase increase of theof the wavelength wavelength beyond beyond 473 473 nm, nm, and and the the haze haze values values decreaseddecreased to to 46–53% 46–53% at at 800 800 nm. nm. In In addition addition to to the the outstanding outstanding optical optical properties, properties, the the TVP TVP should should be be flexible enough to be used in electronic devices. In this study, the flexibility of TPV8 was evaluated flexible enough to be used in electronic devices. In this study, the flexibility of TPV8 was evaluated duedue to to its its relatively relatively high high light light transmittance transmittance and and haze haze values values of of all all the the TPVs, TPVs as, as shown shown in in Figure Figure6d–f. 6d– f. The findings displayed that the lowest bending radius of TPV8 could reach 2.5 mm and it could be The findings displayed that the lowest bending radius of TPV8 could reach 2.5 mm and it could be tightly enlaced into a roll without transverse cracks, also, the original shape of TPV8 could be rapidly tightly enlaced into a roll without transverse cracks, also, the original shape of TPV8 could be rapidly restoredrestored after after the the force force was was released. released. In In addition, addition, it it still still maintained maintained good good optical optical properties properties under under the the bending state. Therefore, the fabricated TPV8 in this study has enormous potentials in photovoltaic bending state. Therefore, the fabricated TPV8 in this study has enormous potentials in photovoltaic applications,applications, such such as as photovoltaics, photovoltaics, solar solar cells, cells, smart smart windows, windows, and and light light shaping shaping di diffusersffusers [7 [7,34].,34].

Figure 6. Optical properties and flexibility of TPVs. (a) Transmittance curves, (b) the actual light Figure 6. Optical properties and flexibility of TPVs. (a) Transmittance curves, (b) the actual light transmittance comparison chart at 800 nm, (c) haze curves, (d) optical performance of TPV8 under transmittance comparison chart at 800 nm, (c) haze curves, (d) optical performance of TPV8 under bending, (e) the bending radius of TPV8,(f) curled diagram of TPV8. bending, (e) the bending radius of TPV8, (f) curled diagram of TPV8. 3.2.3. Thermal Stability of the TVPs 3.2.3. Thermal Stability of the TVPs The thermogravimetric curves (TG) and the differential thermal curves (DTG) of all TPVs and pure epoxyThe thermogravimetric resin are shown in curves Figure (TG)7. The and thermal the differ degradationential thermal of all curves TPVs mainly(DTG) of included all TPVs four and stagespure epoxy [35]. The resin first are stage shown from in roomFigure temperature 7. The therma tol 120 degradation◦C, is the dehydrationof all TPVs mainly drying included of the TPVs, four whichstages is [35]. mainly The duefirst to stage the evaporationfrom room temperature of the free moisture to 120 °C, and is absorbedthe dehydration water of drying TPVs, of and the the TPVs, TG curveswhich in is thismainly range due were to the relatively evaporation flat due of tothe little free weight moisture loss. and The absorbed second stage water occurred of TPVs, from and 150the ◦TGC tocurves 230 ◦C, in whichthis range is mainly were due relatively to the pyrolysisflat due to of little hemicellulose weight loss. and The partially second the stage lignin occurred of the TPVs, from also150 including°C to 230 °C, a small which quantity is mainly of due epoxy to the resin pyrolysis [36]. All of TPVs hemicellulose exhibited and a similar partially DTG the curve lignin in of this the stageTPVs, and also the including weight loss a small rate quantity was about of 7%.epoxy The resi thirdn [36]. stage All is TPVs between exhibited 230 ◦C a andsimilar 350 DTG◦C, which curve isin mainlythis stage ascribed and the to weight the pyrolysis loss rate of thewas remaining about 7%. ligninThe third and stage partially is between pyrolysis 230 of °C epoxy and 350 resin °C, [37 which,38]. Becauseis mainly of theascribed high ligninto the content,pyrolysis the of pyrolysis the remaining rate of lignin TPV2 andand TPVpartially4 increased pyrolysis rapidly, of epoxy and theirresin 2 4 weight[37,38]. loss Because reached of the 24% high and lignin 18%, respectively.content, the pyrolysis However, rate the of TPV TPV8 displayed and TPV the increased lowest weightrapidly, loss and andtheir pyrolysis weight loss rate, reached which further24% and indicated 18%, respectively. that the lignin However, had been the TPV removed8 displayed completely. the lowest The fourthweight stageloss and is from pyrolysis 350 to rate, 450 ◦whichC, which further is ascribed indicated to thethat pyrolysis the lignin of had cellulose, been removed residual completely. lignin, and theThe remainingfourth stage epoxy is from resin 350 [39 to]. The450 °C, weight which loss is rate ascribed of TPV to8 thewas pyrolysis at 64% at of this cellulose, stage, the residual weight lignin, loss rates and the remaining epoxy resin [39]. The weight loss rate of TPV8 was at 64% at this stage, the weight loss rates of TPV2 and TPV4 were 48% and 54%, respectively. Additionally, the DTG curves showed significantly that the pyrolysis rate of TPV2 and TPV4 was higher than that of TPV8, the decomposition rate of epoxy resin was significantly higher than that of the TPVs. It is the carbonization stage of the

Polymers 2020, 12, 2602 10 of 13

of TPV2 and TPV4 were 48% and 54%, respectively. Additionally, the DTG curves showed significantly that the pyrolysis rate of TPV2 and TPV4 was higher than that of TPV8, the decomposition rate of Polymers 2020, 12, x FOR PEER REVIEW 10 of 13 epoxy resin was significantly higher than that of the TPVs. It is the carbonization stage of the TPVs above 450 C, where the thermal decomposition was basically finished and the TG and DTG curves TPVs above◦ 450 °C, where the thermal decomposition was basically finished and the TG and DTG were relatively gentle. Thermogravimetric analysis further proved the thermal stability of TPV was curves were relatively gentle. Thermogravimetric analysis further proved the thermal stability of TPV improved due to the removal of lignin, which increased the penetration of epoxy resin and delayed was improved due to the removal of lignin, which increased the penetration of epoxy resin and the pyrolysis of PV [40]. Notably, the thermal stability of TPV10 is lower than that of TPV8, which is delayed the pyrolysis of PV [40]. Notably, the thermal stability of TPV10 is lower than that of TPV8, possibly ascribed to the decrease of cellulose crystallinity. which is possibly ascribed to the decrease of cellulose crystallinity.

Figure 7. TG and DTG curves of TPVs. (a) TG curves, (b) DTG curves. Figure 7. TG and DTG curves of TPVs. (a) TG curves, (b) DTG curves. 3.2.4. Density and Tensile Strength of TPVs 3.2.4.The Density density and of allTensile PVs andStrength TPVs of is TPVs displayed in Figure8a. Obviously, the density of the PVs gradually decreased with the increase of delignification time. After 10 h of delignification, the density The density of all PVs and TPVs is displayed in Figure 8a. Obviously, the density of the PVs of PV decreased to 0.279 g/cm3 due to the complete removal of lignin. On the other hand, the density gradually decreased with the increase of delignification time. After 10 h of delignification, the density of TPVs was significantly enhanced because of the filling of epoxy resin. It reached a maximum of PV decreased to 0.279 g/cm3 due to the complete removal of lignin. On the other hand, the density density of 1.241 g/cm3 for TPV , and the filling amount of epoxy resin was 237% of the mass of of TPVs was significantly enhanced8 because of the filling of epoxy resin. It reached a maximum PV8. This is because the microchannels and voids of PV gradually increased with the removal of density of 1.241 g/cm3 for TPV8, and the filling amount of epoxy resin was 237% of the mass of PV8. lignin, which correspondingly augmented the filling of epoxy resin. The tensile strength of TPVs was This is because the microchannels and voids of PV gradually increased with the removal of lignin, measured along the longitudinal direction of the PV, as shown in Figure8b. The tensile strength of which correspondingly augmented the filling of epoxy resin. The tensile strength of TPVs was TPVs was greatly reduced with the reduction of lignin content. The tensile strength of PV was 114 MPa, measured along the longitudinal direction of the PV, as shown in Figure 8b. The tensile strength of and the tensile strength of TPV , TPV , TPV , TPV , TPV decreased to 97 MPa, 88 MPa, 81 MPa, TPVs was greatly reduced with2 the 4reduction6 of lign8 in content.10 The tensile strength of PV was 114 67 MPa, and 54 MPa, respectively. In wood, cellulose existing in the cell wall is the skeleton material MPa, and the tensile strength of TPV2, TPV4, TPV6, TPV8, TPV10 decreased to 97 MPa, 88 MPa, 81 MPa, of the cell wall, and lignin permeates into the cellulose skeleton as a binder to strengthen the cell 67 MPa, and 54 MPa, respectively. In wood, cellulose existing in the cell wall is the skeleton material wall [41,42]. With the removal of lignin, the location of lignin was occupied by epoxy resin, and the of the cell wall, and lignin permeates into the cellulose skeleton as a binder to strengthen the cell wall voids of the cell wall were also filled. However, the epoxy resin could not play a role as binder among [41,42]. With the removal of lignin, the location of lignin was occupied by epoxy resin, and the voids the microfibrils, and although the epoxy resin could react with the alcohol group of cellulose, the main of the cell wall were also filled. However, the epoxy resin could not play a role as binder among the interaction between cellulose and epoxy resin was physical contact. Moreover, the pure epoxy resin microfibrils, and although the epoxy resin could react with the alcohol group of cellulose, the main used in this study has a low tensile strength, which is about 1 MPa. Therefore, the values of tensile interaction between cellulose and epoxy resin was physical contact. Moreover, the pure epoxy resin strength of TPVs significantly reduced with the increase of impregnation of epoxy resin. All the same, used in this study has a low tensile strength, which is about 1 MPa. Therefore, the values of tensile the tensile strength of TPV still reached 67 MPa, which is significantly higher than the values of the strength of TPVs significantly8 reduced with the increase of impregnation of epoxy resin. All the same, transparent plastic substrates used in flexible electronic devices (such as polyethylene terephthalate, the tensile strength of TPV8 still reached 67 MPa, which is significantly higher than the values of the whose the tensile strength is around 50 MPa [43]). transparent plastic substrates used in flexible electronic devices (such as polyethylene terephthalate, whose the tensile strength is around 50 MPa [43]).

PolymersPolymers2020 2020, 12, ,12 2602, x FOR PEER REVIEW 1111 of of 13 13

Figure 8. Density and stress-strain curves of PV and all TPVs. (a) Density diagram, (b) Stress strainFigure curve. 8. Density and stress-strain curves of PV and all TPVs. (a) Density diagram, (b) Stress strain curve. 4. Conclusions 4. ConclusionsIn this study, in order to fabricate flexible transparent PV, the rotary cut veneer from fast- growing poplar was first delignified followed by filling with epoxy resin. The research indicated that the cell In this study, in order to fabricate flexible transparent PV, the rotary cut veneer from fast- wall became thin and the porosity of PV was significantly augmented with the removal of lignin, growing poplar was first delignified followed by filling with epoxy resin. The research indicated that while the lignin was almost completely removed from the cell wall after delignification for 8 h. the cell wall became thin and the porosity of PV was significantly augmented with the removal of Meanwhile, the cellulose crystallinity of PV was greatly increased with the decrease of lignin content. lignin, while the lignin was almost completely removed from the cell wall after delignification for 8 SEM observation indicated that the epoxy resin had been filled into the cell cavities and cell wall h. Meanwhile, the cellulose crystallinity of PV was greatly increased with the decrease of lignin of the TPVs, and the filling amount of epoxy resin gradually increased with the decrease of lignin content. SEM observation indicated that the epoxy resin had been filled into the cell cavities and cell content. Correspondingly, the density, light transmittance, and haze of all TPVs distinctly increased, wall of the TPVs, and the filling amount of epoxy resin gradually increased with the decrease of lignin and the thermal stability became more stable; however, the tensile strength of all TPVs significantly content. Correspondingly, the density, light transmittance, and haze of all TPVs distinctly increased, declined. Among them, TPV exhibited excellent optical properties and thermal stability, of which the and the thermal stability became8 more stable; however, the tensile strength of all TPVs significantly light transmittance and haze achieved 82% and 62%, respectively. Also, the tensile strength of TPV8 declined. Among them, TPV8 exhibited excellent optical properties and thermal stability, of which still reached 67 MPa, which is significantly higher than that of traditional plastic substrates used in the light transmittance and haze achieved 82% and 62%, respectively. Also, the tensile strength of flexible electronic devices. Although TPV10 has excellent optical properties, its tensile strength and TPV8 still reached 67 MPa, which is significantly higher than that of traditional plastic substrates used thermal stability were lower than those of TPV8. Moreover, TPV8 still displayed fine optical properties in flexible electronic devices. Although TPV10 has excellent optical properties, its tensile strength and under bending state, and the minimum bending radius achieved 2.5 mm, which displayed outstanding thermal stability were lower than those of TPV8. Moreover, TPV8 still displayed fine optical properties flexibility. Therefore, as a type of wood-based flexible transparent material, TPV displayed great under bending state, and the minimum bending radius achieved 2.5 mm,8 which displayed potential to substitute for plastics as a substrate to be used in solar cells, OLED lighting systems, outstanding flexibility. Therefore, as a type of wood-based flexible transparent material, TPV8 liquid crystal display (LCD) backlight units, signage, etc. displayed great potential to substitute for plastics as a substrate to be used in solar cells, OLED Authorlighting Contributions: systems, liquidConceptualization, crystal display W.H. (LCD) and M.L.;backlight author, units, W.H. signage, and M.L.; etc. methodology, M.L.; software, Z.L.; validation, H.Q., J.C., F.G., R.W. and Z.G.; formal analysis, M.L.; investigation, F.G.; resources, Z.G.; dataAuthor curation, Contributions: W.H.; writing— Conceptualization, original draft preparation,W.H. and M.L.; M.L.; author, writing—review W.H. and M.L.; and methodol editing, W.H.;ogy, M.L.; supervision, software, W.H.Z.L.; All validation, authors have H.Q., read J.C., and F.G., agreed R.W. to and the Z.G.; published formal version analysis, of theM.L.; manuscript. investigation, F.G.; resources, Z.G.; data Funding:curation,This W.H.; research writing— was fundedoriginal by dr Jiangsuaft preparation, Agriculture M.L.; Science writing—review and Technology and Innovation editing, W.H.; Fund((JASTIF), supervision, grantW.H. number All authors CX(20)3174 have read and and The agreed APC was to the funded published by Department version of of the Finance manuscript. of Jiangsu Province.

Acknowledgments:Funding: This researchThe authors was funded are grateful by Jiangsu for theAgricultur fast-growinge Science poplar and veneerTechnology used forInnovation the experiments Fund((JASTIF), from the Tongyuan Wood Industry Co. Ltd. of Siyang county. grant number CX(20)3174 and The APC was funded by Department of Finance of Jiangsu Province. Conﬂicts of Interest: The authors declare no conﬂict of interest. Acknowledgments: The authors are grateful for the fast-growing poplar veneer used for the experiments from the Tongyuan Wood Industry Co. Ltd. of Siyang county. References Conflicts of Interest: The authors declare no conflict of interest. 1. Hyeokjo, G.; Jihyun, H.; Haegyeom, K.; Dong, H.S.; Seokwoo, J.; Kisuk, K. Recent Progress on Flexible ReferencesLithium Rechargeable Batteries. Energy. Environ. Sci. 2014, 2, 538–551. 2. Rim, Y.S.; Bae, S.; Chen, H.; De, M.N.; Yang, Y. Recent Progress in Materials and Devices toward Printable 1. andHyeokjo, Flexible G.; Sensors. Jihyun,Adv. H.; Mater.Haegyeom,2016, 28K.;, 4415–4440.Dong, H.S.; [CrossRef Seokwoo,][ PubMedJ.; Kisuk,] K. Recent Progress on Flexible Lithium Rechargeable Batteries. Energy. Environ. Sci. 2014, 2, 538–551. 2. Rim, Y.S.; Bae, S.; Chen, H.; De, M.N.; Yang, Y. Recent Progress in Materials and Devices toward Printable and Flexible Sensors. Adv. Mater. 2016, 28, 4415–4440.

Polymers 2020, 12, 2602 12 of 13

3. Tang, Q.; Fang, L.; Wang, Y.; Zou, M.; Guo, W. Anisotropic Flexible Transparent Films from Remaining Wood Microstructures for Screen Protection and AgNW Conductive Substrate. Nanoscale 2018, 10, 4344–4353. [CrossRef][PubMed] 4. Song, J.; Chen, C.; Zhu, S.; Zhu, M.; Dai, J.; Ray, U.; Li, Y.; Kuang, Y.; Li, Y.; Quispe, N.; et al. Processing Bulk Natural Wood into a High-Performance Structural Material. Nature 2018, 554, 224–228. [CrossRef][PubMed] 5. Yan, W.; Yajing, W.; Feng, Y.; Jing, W.; Xuehua, W. Study on the Properties of Transparent Bamboo Prepared by Epoxy Resin Impregnation. Polymers 2020, 12, 863. 6. Michelin, M.; Gomes, D.G.; Romani, A.; Polizeli, M.; Teixeira, J.A. Nanocellulose Production: Exploring the Enzymatic Route and Residues of Pulp and Paper Industry. Molecules 2020, 25, 3411. [CrossRef] 7. Rao, A.N.S.; Nagarajappa, G.B.; Nair, S.; Cathoth, A.M.; Pandey, K.K. Flexible Transparent Wood Prepared from Poplar Veneer and Polyvinyl Alcohol. Compos. Sci. Technol. 2019, 182.[CrossRef] 8. Weiss, M.; Haufe, J.; Carus, M.; Brandão, M.; Bringezu, S.; Hermann, B.; Patel, M.K. A Review of the Environmental Impacts of Biobased Materials. J. Ind. Ecol. 2012, 16, S169–S181. [CrossRef] 9. Li, Y.; Fu, Q.; Yu, S.; Yan, M.; Berglund, L. Optically Transparent Wood from a Nanoporous Cellulosic Template: Combining Functional and Structural Performance. Biomacromolecules 2016, 17, 1358–1364. [CrossRef] 10. Jiang, F.; Li, T.; Li, Y.; Zhang, Y.; Gong, A.; Dai, J.; Hitz, E.; Luo, W.; Hu, L. Wood-Based Nanotechnologies Toward Sustainability. Adv. Mater. 2018, 30.[CrossRef] 11. Yang, X.; Berglund, L.A. Structural and Ecofriendly Holocellulose Materials from Wood: Microscale Fibers and Nanoscale Fibrils. Adv. Mater. 2020.[CrossRef] 12. Li, Y.; Fu, Q.; Yang, X.; Berglund, L. Transparent Wood for Functional and Structural Applications. Philos. Trans. R. Soc. A 2018, 376.[CrossRef][PubMed] 13. Li, Y.; Vasileva, E.; Sychugov, I.; Popov, S.; Berglund, L. Optically Transparent Wood: Recent Progress, Opportunities, and Challenges. Adv. Opt. Mater. 2018, 6.[CrossRef] 14. Klemm, D.; Kramer, F.; Moritz, S.; Lindstrom, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: A New Family of Nature-Based Materials. Angew. Chem. Int. Ed. Engl. 2011, 50, 5438–5466. [CrossRef][PubMed] 15. Lv, T.; Huang, J.; Liu, W.; Zhang, R. From Sky Back to Sky: Embedded Transparent Cellulose Membrane to Improve the Thermal Performance of Solar Module by Radiative Cooling. Case Stud. Therm. Eng. 2020, 18. [CrossRef] 16. Chen, Z.; Xiao, P.; Zhang, J.; Tian, W.; Jia, R.; Nawaz, H.; Jin, K.; Zhang, J. A Facile Strategy to Fabricate Cellulose-Based, Flame-Retardant, Transparent and Anti-Dripping Protective Coatings. Chem. Eng. J. 2020, 379.[CrossRef] 17. He, W.; Wu, B.; Lu, M.; Li, Z.; Qiang, H. Fabrication and Performance of Self-Supported Flexible Cellulose Nanofibrils/Reduced Graphene Oxide Supercapacitor Electrode Materials. Molecules 2020, 25, 2793. [CrossRef] 18. Park, S.Y.; Yook, S.; Goo, S.; Im, W.; Youn, H.J. Preparation of Transparent and Thick CNF/Epoxy Composites by Controlling the Properties of Cellulose Nanofibrils. Nanomaterials 2020, 10, 625. [CrossRef][PubMed] 19. Zhu, M.; Song, J.; Li, T.; Gong, A.; Wang, Y.; Dai, J.; Yao, Y.; Luo, W.; Henderson, D.; Hu, L. Highly Anisotropic, Highly Transparent Wood Composites. Adv. Mater. 2016, 28, 5181–5187. [CrossRef] 20. Mi, R.; Li, T.; Dalgo, D.; Chen, C.; Kuang, Y.; He, S.; Zhao, X.; Xie, W.; Gan, W.; Zhu, J.; et al. A Clear, Strong, and Thermally Insulated Transparent Wood for Energy Efficient Windows. Adv. Funct. Mater. 2020, 30. [CrossRef] 21. Lang, A.W.; Li, Y.; De, K.M.; Shen, D.E.; Österholm, A.M.; Berglund, L.; Reynolds, J.R. Transparent Wood Smart Windows: Polymer Electrochromic Devices Based on Poly(3,4-Ethylenedioxythiophene): Poly (Styrene Sulfonate) Electrodes. ChemSusChem 2018, 11, 854–863. [CrossRef] 22. Jungstedt, E.; Montanari, C.; Östlund, S.; Berglund, L. Mechanical Properties of Transparent High Strength Biocomposites From Delignified Wood Veneer. Compos. Part A 2020, 133.[CrossRef] 23. Fu, Q.; Yan, M.; Jungstedt, E.; Yang, X.; Li, Y.; Berglund, L.A. Transparent Plywood as a Load-Bearing and Luminescent Biocomposite. Compos. Sci. Technol. 2018, 164, 296–303. [CrossRef] 24. Wu, Y.; Zhou, J.; Huang, Q.; Yang, F.; Wang, Y.; Liang, X.; Li, J. Study On the Colorimetry Properties of Transparent Wood Prepared From Six Wood Species. ACS Omega 2020, 5, 1782–1788. [CrossRef] 25. Zou, W.; Sun, D.; Wang, Z.; Li, R.; Yu, W.; Zhang, P. Eco-Friendly Transparent Poplar-Based Composites that are Stable and Flexible at High Temperature. RSC Adv. 2019, 9, 21566–21571. [CrossRef] 26. Jacucci, G.; Schertel, L.; Zhang, Y.; Yang, H.; Vignolini, S. Light Management with Natural Materials: From Whiteness to Transparency. Adv. Mater. 2020.[CrossRef] Polymers 2020, 12, 2602 13 of 13

27. Wu, Y.; Zhou, J.; Huang, Q.; Yang, F.; Wang, Y.; Wang, J. Study On the Properties of Partially Transparent Wood Under Different Delignification Processes. Polymers 2020, 12, 661. [CrossRef] 28. Zhang, T.; Yang, P.; Li, Y.; Cao, Y.; Zhou, Y.; Chen, M.; Zhu, Z.; Chen, W.; Zhou, X. Flexible Transparent Sliced Veneer for Alternating Current Electroluminescent Devices. ACS Sustain. Chem. Eng. 2019, 7, 11464–11473. [CrossRef] 29. Van Soest, P.J.; Wine, R.H. Use of Detergents in the Analysis of Fibrous Feeds. IV. Determination of Plant Cell-Wall Constituents. J. Assoc. Off. Anal. Chem. 1963.[CrossRef] 30. Cai, H.; Wang, Z.; Xie, D.; Zhao, P.; Sun, J.; Qin, D.; Cheng, F. Flexible Transparent Wood Enabled by Epoxy Resin and Ethylene Glycol Diglycidyl Ether. J. For. Res. 2020, 18.[CrossRef] 31. Yang, W.; Jiao, L.; Liu, W.; Dai, H. Manufacture of Highly Transparent and Hazy Cellulose Nanofibril Films via Coating TEMPO-Oxidized Wood Fibers. Nanomaterials 2019, 9, 107. [CrossRef] 32. Fang, Z.; Zhu, H.; Yuan, Y.; Ha, D.; Zhu, S.; Preston, C.; Chen, Q.; Li, Y.; Han, X.; Lee, S.; et al. Novel Nanostructured Paper with Ultrahigh Transparency and Ultrahigh Haze for Solar Cells. Nano Lett. 2014, 14, 765–773. [CrossRef][PubMed] 33. Fang, Z.Q.; Zhu, H.L.; Li, Y.Y.; Liu, Z.; Dai, J.Q.; Preston, C.; Garner, S.; Cimo, P.; Chai, X.S.; Chen, G.; et al. Light Management in Flexible Glass by Wood Cellulose Coating. Sci. Rep. 2014, 4.[CrossRef] 34. Hu, T.; Núria, B.; Qi, Z. A Transparent, Hazy, and Strong Macroscopic Ribbon of Oriented Cellulose Nanofibrils Bearing Poly (Ethylene Glycol). Adv. Mater. 2015, 27.[CrossRef] 35. Kim, S.; Kim, E.; Choi, K.; Cho, J.K.; Sun, H.; Yoo, J.W.; Park, I.; Lee, Y.; Choi, H.R.; Kim, T.; et al. Rheological and Mechanical Properties of Polypropylene Composites Containing Microfibrillated Cellulose (MFC) with Improved Compatibility through Surface Silylation. Cellulose 2019, 26, 1085–1097. [CrossRef] 36. Yang, R.; Liang, Y.; Hong, S.; Zuo, S.; Wu, Y.; Shi, J.; Cai, L.; Li, J.; Mao, H.; Ge, S.; et al. Novel Low-Temperature Chemical Vapor Deposition of Hydrothermal Delignified Wood for Hydrophobic Property. Polymers 2020, 12, 1757. [CrossRef] 37. Hao, J.; Yanyan, J.; Jiantuo, G.; Lei, W. Enhancement of Thermal and Mechanical Properties of Bismaleimide Using a Graphene Oxide Modified by Epoxy Silane. Materials 2020, 13, 3836. [CrossRef] 38. Hao, B.; Yang, R.; Zhang, K. A Naringenin-Based Benzoxazine with an Intramolecular Hydrogen Bond as Both a Thermal Latent Polymerization Additive and Property Modifier for Epoxy Resins. RSC Adv. 2020, 10. [CrossRef] 39. Shen, D.K.; Gu, S.; Bridgwater, A.V. Study On the Pyrolytic Behaviour of Xylan-Based Hemicellulose Using TG–FTIR and Py–GC–FTIR. J. Anal. Appl. Pyrol. 2010, 88, 213. [CrossRef] 40. Dong, Y.; Ma, E.; Li, J.; Zhang, S.; Hughes, M. Thermal Properties Enhancement of Poplar Wood by Substituting Poly (Furfuryl Alcohol) for the Matrix. Polym. Composite. 2019, 41, 1066–1073. [CrossRef] 41. Li, K.; Wang, S.; Chen, H.; Yang, X.; Berglund, L.A.; Zhou, Q. Self-Densification of Highly Mesoporous Wood Structure into a Strong and Transparent Film. Adv. Mater. 2020.[CrossRef] 42. Zhou, Y.; Sheng, X.; Garemark, J.; Josefsson, L.; Sun, L.; Li, Y.; Emmer, Å. Enrichment of Glycopeptides Using Environmentally Friendly Wood Materials. Green Chem. 2020, 22, 5666–5676. [CrossRef] 43. Helkouly, H.; Rushdi, M.; Abdel-Magied, R.K.F. Eco-Friendly Date-Seed Nanofillers for Polyethylene Terephthalate Composite Reinforcement. Mater. Res. Express 2020, 7.[CrossRef]

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aﬃliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).