Transparent Wood Materials: Properties, Applications, and Fire Behaviour 3031234049, 9783031234040

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Springer Series in Materials Science 330

Igor Wachter Peter Rantuch Tomáš Štefko

Transparent Wood Materials Properties, Applications, and Fire Behaviour

Springer Series in Materials Science Volume 330

Series Editors Robert Hull, Center for Materials, Devices, and Integrated Systems, Rensselaer Polytechnic Institute, Troy, NY, USA Chennupati Jagadish, Research School of Physics and Engineering, Australian National University, Canberra, ACT, Australia Yoshiyuki Kawazoe, Center for Computational Materials, Tohoku University, Sendai, Japan Jamie Kruzic, School of Mechanical and Manufacturing Engineering, UNSW Sydney, Sydney, NSW, Australia Richard Osgood jr., Columbia University, Wenham, MA, USA Jürgen Parisi, Universität Oldenburg, Oldenburg, Germany Udo W. Pohl, Institute of Solid State Physics, Technical University of Berlin, Berlin, Germany Tae-Yeon Seong, Department of Materials Science and Engineering, Korea University, Seoul, Korea (Republic of) Shin-ichi Uchida, Electronics and Manufacturing, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan Zhiming M. Wang, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu, China

The Springer Series in Materials Science covers the complete spectrum of materials research and technology, including fundamental principles, physical properties, materials theory and design. Recognizing the increasing importance of materials science in future device technologies, the book titles in this series reflect the state-ofthe-art in understanding and controlling the structure and properties of all important classes of materials.

Igor Wachter · Peter Rantuch · Tomáš Štefko

Transparent Wood Materials Properties, Applications, and Fire Behaviour

Igor Wachter Faculty of Materials Science and Technology Slovak University of Technology in Bratislava Trnava, Slovakia

Peter Rantuch Faculty of Materials Science and Technology Slovak University of Technology in Bratislava Trnava, Slovakia

Tomáš Štefko Faculty of Materials Science and Technology Slovak University of Technology in Bratislava Trnava, Slovakia

ISSN 0933-033X ISSN 2196-2812 (electronic) Springer Series in Materials Science ISBN 978-3-031-23404-0 ISBN 978-3-031-23405-7 (eBook) https://doi.org/10.1007/978-3-031-23405-7 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

The ongoing push for a more sustainable future has elevated common wood to the status of a desirable bio-based functional material. Due to its renewable and biodegradable qualities, transparent wood, a sustainable resource, has the potential to replace traditional petroleum-based polymers. The factors influencing the fire behavior of transparent wood and corresponding wood-based composites are of interest to numerous groups of professionals with significantly divergent backgrounds and specialties. The main inspiration for writing this monograph was the lack of information on this topic. There is currently no book that covers the entire field of transparent wood and its composites in a comparable manner. In total, more than 130 studies directly related to transparent wood have been used in this monograph with another hundreds of references to accurately identify the evolution of ideas in the last decade. The reference list at the conclusion of each chapter contains all the sources cited in the book. During the past approximately 7 years, the interest of researchers in this promising material skyrocketed and experienced exponential growth which is documented by the growing body of research published in scientific databases. The monograph could be divided into two main parts. The first part summarizes the recent progress achieved in the field of transparent wood production and its various applications. The second, experimental part focuses to describe the behavior of transparent wood under a thermal load using a cone calorimeter. At the same time, this method can be used to describe different materials and compare them in terms of their flammability. These qualities can be used in industrial processing or energetics, though they are most frequently used in the field of fire protection. The author/editor also wishes to acknowledge the co-authors who spent their time and effort creating this book for their contributions. Trnava, Slovakia

Igor Wachter

v

vi

Preface

Acknowledgements This work was supported by the Slovak Research and Development Agency under the contract No. APVV-16-0223. This work also supported by the VEGA agency under the contracts No. VEGA 1/0678/22 and by the KEGA agency under the contract No. 016STU-4/2021.

Contents

1 Properties of Transparent Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Methods for Evaluating Properties of Transparent Wood . . . . . . . . . 1.3 Recent Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 3 5 10 11

2 Thin and Flexible Transparent Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Recent Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15 15 17 19 21

3 Fully Bio-based Transparent Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Recent Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23 23 26 30 30

4 Multilayered and Densified Transparent Wood . . . . . . . . . . . . . . . . . . . . 4.1 Multilayered Transparent Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Recent Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Densification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Recent Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

35 35 37 39 41 43 43

5 Transparent Bamboo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Preparation and Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

47 47 49 50

vii

viii

Contents

5.4 Recent Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

51 53 55

6 Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Recent Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59 59 62 65 67

7 Smart Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Recent Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

71 71 74 81 81

8 Smart Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Recent Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

87 87 89 93 93

9 Fire Properties of Transparent Wood and Its Components . . . . . . . . . 9.1 Composition and Thermal Decomposition of Wood Pseudo-components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Structure and Thermic Decomposition of Synthetic Composites of Transparent Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1 Epoxy Resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2 Acrylic Resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.3 Poly(Methyl Methacrylate) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.4 Poly(Vinylpyrrolidone) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.5 Polystyrene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.6 Poly(Vinyl Alcohol) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.7 Melamine–Formaldehyde Resins . . . . . . . . . . . . . . . . . . . . . . . 9.2.8 Flame Retardants in Transparent Wood . . . . . . . . . . . . . . . . . . 9.3 Cone Calorimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 Transparent Wood Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5.1 Mass Loss Rate (MLR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5.2 Smoke Production During Measurement . . . . . . . . . . . . . . . . . 9.5.3 Prediction of Time to Flashover, Fire Growth Rate Index, and European Fire Classification . . . . . . . . . . . . . . . . . 9.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

97 97 105 105 107 109 110 111 112 113 115 117 119 121 126 128 129 132 133

Chapter 1

Properties of Transparent Wood

Abstract Due to the distinctive structures that result from its natural growth, wood is a commonly utilized structural material with exceptional mechanical qualities. Different woods exhibit an incredible range of mesostructures depending on their kinds and geographic variances. In recent years, interest in transparent wood (TW), a biocomposite material with optical transparency in the visible range, has increased due to its enormous potential for environmentally beneficial applications, such as in the building sector and functionalized organic materials. The aim of this chapter is to describe the latest progress in the field of material properties (mechanical, optical, electromagnetic) of transparent wood.

1.1 Introduction A breakthrough material called transparent wood is derived from natural wood and retains excellent mechanical and optical properties [1–4]. Since the earliest civilizations, humans have used wood as a sustainable resource for a wide range of uses in daily life and production. The biomass material with the greatest accumulation and carbon storage in terrestrial plants is wood, which is a non-polluting, carbon-fixing, and sustainable resource that offers a constant source of materials and energy for human production and living [5]. Wood is made up of high-crystalline cellulose, hemicellulose, and lignin and has a hierarchical and mesoporous structure [6–8]. Even while cellulose and hemicellulose are visually colorless, lignin has a very complicated structure and a dark color [9]. Additionally, the porous nature of wood results in significant visible light scattering. As a result, the optical transparency of wood is typically regarded as zero. In order to create light-transmitting, continuousfiber reinforced composites, delignified or lignin chromophores deactivation bulk wood is infiltrated with a polymer that has a refractive index equal or similar to cellulose (n ≈ 1.5) to create transparent wood composites [9–11]. Wood has a porous and anisotropic structure (Fig. 1.1) with microscale cell lumen porosity [12]. Although a very minor cell fraction is aligned perpendicular to the tree stem, the vast majority of tubular cells are aligned in the axial direction (wood rays, for example, vessels and fibers) [13]. The selected orientation of wood fibers for the preparation of transparent © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 I. Wachter et al., Transparent Wood Materials, Springer Series in Materials Science 330, https://doi.org/10.1007/978-3-031-23405-7_1

1

2

1 Properties of Transparent Wood

Fig. 1.1 Microstructure of hardwoods includes ray cells, fibers cells, and vessels. Hardwoods typically have fibers cells with a diameter of 20 µm and a fiber length of 1 mm [14]

wood directly impacts the optical (anisotropy, haze, transmittance) and mechanical (tensile strength, modulus of elasticity, elongation, hardness, fatigue) properties of the resulting composite. The purpose of the polymer impregnation is to reduce light scattering within the material, which results from interfaces between the holocellulose structure (the primary component of delignified wood) and air spaces that are always present in transparent wood [2]. The combination of high mechanical qualities and light transmission has attracted interest as a structural material. Improvements in transparent wood preparation methods and future applications have advanced quickly since its original realization [10]. Associated research endeavours include improved optical characteristics through surface modification of delignified wood templates [15], adjusting mechanical properties through lamination [16], innovative preparation methods with greater thickness [17, 18], incorporating transparent wood into construction materials or optoelectronic devices (smart windows [19] and solar cells [20]), endowing transparent wood with multiple functionalities (luminescent TW doped with quantum dots [16, 21], wood embedded with luminescent dye [22], magnetic wood [23], thermal energy storage TW [24], heat shielding TW [25], application in transparent decoration [26], and X-ray shielding [27]. For the continued development of this class of optically functional materials, a deeper knowledge of its functional characteristics is necessary.

1.2 Methods for Evaluating Properties of Transparent Wood

3

1.2 Methods for Evaluating Properties of Transparent Wood To be able to access the functionality of transparent wood properly and reliably, it is necessary to use matching tests. In the following section, the most important tests with the corresponding equations are summarized. Haze measurement of transparent wood is usually performed using a UV−visible spectrometer. The haze value is calculated according to Eq. 1.1: ( haze =

T3 T1 − T2 T4

) × 100[%]

(1.1)

where T 1 denotes the incoming light beam, T 2 denotes the transmitted light beam, T 3 denotes the system’s diffused light beam, and T 4 denotes the system’s and the sample’s combined diffused light beam. The ASTM D1003 standard [28] describes the most popular technique for estimating haze and transmittance, which involves the use of an integrating sphere and a spectrometer. This technique was created primarily for materials like plastics or frosted glasses with haze levels around 30%, though. Therefore, using this approach directly on transparent wood may have certain drawbacks. The transmittance value is calculated according to Eq. 1.2: transmittance =

T2 × 100[%] T1

(1.2)

where T 1 is the incident light and T 2 is the light conveyed by the specimen. The TAPPI T 222 om-02 Standard [29] is typically used to determine the lignin content (Klason lignin). One gram of dry wood is extracted for 8 h with an ethanol/benzene mixture and then treated for 2 h at 20 °C with 15 mL of 72% sulfuric acid. The mixture in a beaker is diluted to a sulfuric acid concentration of 3% with the addition of 560 mL of deionized water, and it is then heated for 4 h. The mixture is filtered and rinsed with deionized water after cooling down. According to TAPPI T 211 om-02, the insoluble materials are dried and weighed before being transferred to a muffle furnace at 525 °C to determine the ash weight. The following equation is used to determine the amount of lignin: lignin content =

m1 − m2 × 100[%] m0

(1.3)

where m1 is the weight of insoluble materials, m2 is the weight of ash, and m0 is the oven-dried weight of the specimen. Bending tests (three-point flexural testing) are carried out in accordance with ASTM D7264 [30]. The flexural modulus E and flexural strength σ are calculated using Eqs. 1.4 and 1.5:

4

1 Properties of Transparent Wood

E= σ =

L 3m [Pa] 4wh 3

(1.4)

3F L [MPa] 2wh 2

(1.5)

where L, w, h, and F, are the span, width, thickness, and maximum force, respectively. The linear load–displacement curve slope, m, is taken between 10 and 40% of the maximum force. Another important mechanical test is the compression recovery ratio which is calculated following Eq. 1.6: ( compression recovery ratio =

Td Tr − To Td

) × 100[%]

(1.6)

where T r is the recovered thickness of immersing and drying, T d is the thickness of the densified wood, and T o is the thickness of the original wood. The final compression ratio (C d ) is calculated as follows (1.7): ( Cd =

rb − rc rb

) × 100[%]

(1.7)

where r b is the dimension in the R direction of the water-swollen sample before the compression testing. Holocellulose, which includes cellulose and hemicelluloses, makes up the majority of delignified wood. Equations 1.8 and 1.9 can be used to calculate the holocellulose volume fraction. Holocellulose is referred to in the following paragraph as “cellulose” for simplicity’s sake. W f − ρc [ 3 ] m ρf ) ( ] Wf Wm [ kg m−3 ρc = 1/ + ρf ρm vf =

(1.8) (1.9)

where V f is the volume fraction of cellulose, ρ c is the density of composite, ρ f is the density of holocellulose (1500 kg m−3 ), ρ m is the density of polymer [kg m−3 ], W m is the weight fraction of the polymer, and W f is the weight fraction of cellulose. These equations can also be used to calculate the volume fraction of lignin-modified wood in transparent wood or the volume fraction of delignified bamboo in transparent bamboo. The colour change of non-treated and transparent wood can be determined by colourimetry. The colourimetry uses the CIE L*a*b* colour space coordinates. In this way, the colour of the measured surface is expressed using three coordinates: • L*—coordinate on the axis indicating lightness

1.3 Recent Progress

5

• a*—coordinate on the axis between red and green • b*—coordinate on the axis between yellow and blue. To describe the total shift in this colour space, the total colour difference is used, which can be expressed as follows (Eq. 1.10): / d Et =

(

L ∗t − L ∗0

)2

)2 ( )2 ( + at∗ − a0∗ bt∗ − b0∗

(1.10)

where dE t is the total colour difference at time t, L ∗t is the value of L* at time t, L ∗0 is the value of L* at time 0, at∗ is the value of a* at time t, a0∗ is the value a* at time 0, bt∗ is the value of b* at time t, and b0∗ is the value of b* at time 0. To measure the thermal performance, the heat conductivity is calculated using Eq. 1.11: k=

] D [ QU + Q L × W m−1 K−1 2 ΔT

(1.11)

where QU is the reading of the upper heat flux sensor, QL is the reading of the bottom heat flux sensor, D is the sample thickness, and ΔT is the difference in temperatures on both sample surfaces. Table 1.1 provides a concise review of the experimental procedures at the various hierarchical levels for the mechanical characteristics of wood, along with examples of each technique’s benefits and drawbacks [31].

1.3 Recent Progress During the last few years, there has been a growing number of researchers dealing with describing optical, mechanical, electromagnetic, and thermodynamic properties in order to evaluate the functional and structural performance of transparent wood. The following section summarizes some of the latest and most influential results in this field. According to recent study findings, transparent wood has enormous promise in transparent structures, solar cells, windows, energy-saving applications, and luminous magnetic switches. All of these applications rely on the optical properties (transmittance, haze, refractive index, surface colour) of transparent wood which means they need to be properly described and well understood. Light transmission and haze are significantly impacted by nano- and microscale inhomogeneities at interfaces and structures. Although transparent wood qualities can be customized through interface manipulation, an in-depth knowledge of the mechanics governing light/transparent wood interaction is necessary to produce optically clear wood. In their work, Chen et al. go into detail about the effects of sample thickness and volume fraction of cellulose, the linear density of interfaces, interface compatibility (wood-polymer interface debond gaps), lignin content, and the refractive index of the polymer matrix on light-transparent wood interaction. They also discuss that the

6

1 Properties of Transparent Wood

Table 1.1 Description of the methods used to characterize the mechanical characteristics of wood and cellulose scaffolds at various levels of hierarchy and to identify the benefits and drawbacks of each [31] Level

Method

Parameter

Advantages

Disadvantages

Tissue

Tension, compression, bending, shear, etc

Elastic modulus E, strength σ, shear module G, shear strength τ of bulk wood

International standards; high-throughput tests; scale near practical application

A multitude of superimposed influence; test size matters; wide range of protocols for non-standardized tests inhomogeneity of samples

Single fiber

Microtensile test

Elastic modulus E, strength σ of single fibers

Excluding the influence of middle-lamella; excluding locking effect of microfibrils of neighboring cells

Chemical isolation may introduce damage; mechanical isolation is difficult; buckling during extension

Cell wall

Micropillar compression

Yield stress of cell wall

Test of single cell wall layer in a uniaxial stress state

High energy input by FIBa milling may introduce damage; vacuum conditions: 0% moisture content; sinking-in of sample: determination of elastic modulus E is not reliable

Nanoindentation

Reduced modulus of cell wall

High spatial resolution at subcell wall level; tests in ambient, controlled conditions

Complex loading situations: compression in longitudinal, radiant, and tangential; elastic, viscoelastic, and plastic deformation; reduced modulus is difficult to relate to elastic modulus E of cell wall (continued)

1.3 Recent Progress

7

Table 1.1 (continued) Level

Cell wall polymers: cellulose, hemicellulose, and lignin

Method

Parameter

Advantages

Disadvantages

Atomic force microscopy

Modulus of cell wall

High spatial resolution within cell wall; tests in ambient, controlled conditions; tests in water

Artifact due to the surface topography; unknown influences of surface conditions; modulus not clearly defined

In situ X-ray diffraction

Elastic modulus E of crystalline cellulose of cell wall

Characterization in situ in close-to-natural condition

Access to synchrotron facility beneficial; complex setup and evaluation; no information about amorphous cell wall matrix polymers

Acquisition of otherwise not accessible parameters

Structural changes due to the extraction; the highly artificial state of samples; no possibility of proof reliability and precision of obtained property

Compression of Elastic modulus extracted polymer E of individual matrix polymers

a

FIB–focused ion beam

main influence on haze in transparent wood has a forward scattering, caused by the mismatch in refractive index between the cell wall and the polymer [32]. Chen et al. hypothesized that the polymer is disseminated at the nanoscale inside the cell wall despite its poor miscibility. To test this hypothesis, small-angle neutron scattering experiments were performed and indeed proved nanoscale polymer distribution in the cell wall [33]. For the manufacture of transparent wood composites, the determination of the refractive index for delignified wood templates is essential. Chen et al. were able to accurately quantify the transmittance of samples of delignified wood submerged in particular liquids (immersion liquid method). In order to determine the refractive indices of the delignified wood samples in two fibre directions, a transmission model based on the Fresnel reflection/refraction theory was created, and at a wavelength of 589 nm, they measured the refractive indices for the delignified balsa wood template of 1.536 ± 0.006 and 1.525 ± 0.008, respectively. For the delignified birch wood template, the values were 1.537 ± 0.005 and 1.529 ± 0.006 [14].

8

1 Properties of Transparent Wood

The structure of the cellulose-based composites, the (mis-)matching of the refractive indices across these composites, and the polymer matrix all affect the optical properties of transparent wood, such as transparency and haze. Although data on cellulose refractive indices for different cellulose forms (fibres, powder, hot-pressed films, etc.) are available, these values might not match the actual refractive index of the transparent wood substrate. Therefore, Pang et al. reported a modelling approach using transparent wood samples to investigate light transmission and calculate an appropriate refractive index for such materials. This allowed them to retrieve the efficient refractive index of the transparent wood material, which has shown to be 1.54 ± 0.005 for the 633 nm light source [34]. Contrary to non-scattering absorbing media, the thickness dependence of light transmittance for transparent wood is complicated because optical losses are also related to increased photon path length from multiple scattering. This assertion was proposed by Chen et al., who found that the angle-integrated total light transmittance of transparent wood has an exponentially decaying dependence on sample thickness. Their findings using a photon diffusion equation demonstrate that the diffusion coefficient (scattering), in addition to the absorption coefficient, is a key factor in the overall light transmittance. Their model allows for the cross-comparison of the transmittances of samples derived from various wood species as well as the prediction of the total transmittance over a larger range of sample thicknesses [35]. Wu et al. observed the optical characteristics of natural wood and each transparent wood product to better understand the impact of various degrees of delignification. These characteristics included surface colour, optical transmittance, and optical haze. According to experimental findings, delignification increased the lightness of transparent wood while decreasing the redness and yellowness. Their findings provide insight into the production of transparent wood that can meet specific degrees of optical qualities. As anticipated, a longer delignification time may result in greater delignification. The extended treatment periods lead to a lower lignin concentration and a higher percentage of lignin removal as given in Table 1.2 [36]. Foster et al. looked in detail into how chemical modification affected the mechanical, optical, and water transport properties of transparent wood composites. They used two delignifying pretreatments, lignin oxidation and lignin modification, and Table 1.2 Impact of delignification time on lignin removal [36] Sample name

Treatment time (min)

Lignin content (acid-insoluble lignin and acid-soluble lignin) (%)

Lignin removal percentage (%)

NW

0

24

0

DW-1

30

16

33

DW-2

60

15

38

DW-3

90

13

47

DW-4

120

12

51

DW-5

150

9

64

1.3 Recent Progress

9

three interfacial modifications, acetylation, methacrylation, and treatment with 2hydroxyethyl methacrylate, to create transparent wood composites (Fig. 1.2). Understanding the effects of chemical modification on their basic properties serves to better tailor these composites for longevity in various applications [37]. Transparent wood is a viable choice for low-cost, light-transmitting buildings, and transparent solar cell windows due to its excellent mechanical qualities and lightweight. Its structural performance which is defined mainly by mechanical properties such as tensile strength, elastic modulus, sheer strength, bending strength, and compression recovery has been evaluated in many studies [11, 36−39]. Results showed that due to the high cellulose content and advantageous stress transmission methods, this element provides surprisingly strong reinforcement despite the fragility of delignified veneers. Due to resource depletion, environmental degradation, and climate change, interest in novel eco-friendly building materials is growing in the construction industry. Studies on these materials’ electromagnetic properties have been done [40, 41]. The benefits of transparent wood are as follows: It is biodegradable, its tensile strength is 8.6 times higher than that of glass, it serves as a good thermal insulator, and it can provide a uniform light distribution [2, 24, 25, 42]. In order to investigate a type of transparent wood with electromagnetic shielding or blocking properties, Cho et al. applied a transparent electrode to the transparent wood surface in their study. They then determined the impact of the applied polymer on the transparent wood’s electromagnetic properties to design a space that can regulate the spectrum efficiency of buildings. The researchers discovered that the suggested transparent wood can be employed as an environmentally friendly electromagnetic shielding material by examining the electromagnetic shielding performance with an indium tin oxide coating [43]. According to Majova et al. deep eutectic solvent delignification has the potential to replace oxygen delignification after kraft pulping because it has been shown that pulp with a higher initial Kappa number or lignin content

Fig. 1.2 Images of transparent wood composites from wood templates created using the two delignifying pretreatment methods show the proposed covalent, hydrophilic, and hydrophobic interactions between unmodified and modified wood templates and resin (lignin oxidation and lignin modification) [37]

10

1 Properties of Transparent Wood

possessed a greater fraction of easily removed lignin fragments [44]. These results were employed in a different study, where the deep eutectic solvent (oxalic acid and choline chloride) was used as a green solvent for the delignification of wood to produce a porous cellulose-based frame, infiltrated by acrylic acid, and carbon dots, resulting in a multiple-colour-emission transparent wood with excellent optical and mechanical properties, which can be used as a substitute encasing material for white light-emitting diodes [45]. Smart windows that use functional polymers to provide thermochromic [46−47] or electrochromic [48] properties are appealing in the context of energy-efficient constructions. Li et al. showed that a transparent wood manufactured by a standard NaClO2 delignification and by interface manipulation through acetylation of a wood template with exceptional optical properties can be used as a substrate for a haze-tuneable smart window. The functional layer was laminated on top of transparent wood using a polymer-dispersed liquid crystal film with optically adjustable properties. Therefore, the transmittance was tuneable by switching on/off electrical power. Because of their water solubility, low toxicity, and good transparency, polymers like polyvinyl alcohol have frequently been employed as polymer matrices [49−50]. The micromorphologies, chemical compositions, thermal stabilities, chemical functional groups, optical transmittance, and mechanical properties of the raw rubber wood and transparent wood prepared with infiltrated polymers (polyvinyl-pyrrolidone, polyvinyl alcohol, and methyl methacrylate) were characterized in the study of Yue et al. The results showed that the optical transmittance of polyvinyl alcohol TW (76.6%, λ = 600 nm, d = 0.70 mm) was similar to that of polyvinyl-pyrrolidone TW (73.4%, λ = 600 nm, d= 0.74 mm) and higher than that of methyl methacrylate RW (64.6%, λ = 600 nm, d = 0.73 mm), but methyl methacrylate TW had a higher mechanical strength (fracture strength of 230.14 MPa) [51]. Therefore, the final composite can have fine-tuned properties for a specific applications.

1.4 Conclusion By removing the heavily light-absorbing lignin component from a nanoporous wood template, bulk infiltration of refractive index-matched polymer is used to create optically transparent wood. Optical (e.g., transmittance, haze) and mechanical properties of such material are highly dependent on the final thickness. While haze increased for the identical changes in wood thickness and cellulose volume fraction, optical transmittance dropped. The synergies between delignified scaffold and infiltrated polymer were shown by the mechanical qualities of transparent wood, as both components individually displayed inferior mechanical performance. Despite the delignified veneer’s fragility, delignified wood veneer has unexpectedly potent reinforcing effects in transparent wood/polymer biocomposites. Larger transparent wood structures may perform worse due to structural flaws (such as fibre misalignment) and improper impregnation. Also, the development of a recycling technique and the usage of a bio-based polymer could both result in products with

References

11

improved eco-friendliness. Considering all the findings, the suggested techniques for producing high-performing (lightweight, light-transmitting, inexpensive) biobased wood composites from rapidly growing softwoods could be applied in the construction, aviation, and automotive industries.

References 1. C. Chen et al., Structure–property–function relationships of natural and engineered wood. Nat. Rev. Mater. 5(9), 642–666 (2020) 2. Y. Li, E. Vasileva, I. Sychugov, S. Popov, L. Berglund, Optically transparent wood: recent progress, opportunities, and challenges. Adv. Opt. Mater. 6(14), 1800059 (2018). https://doi. org/10.1002/adom.201800059 3. Y. Huang et al., Wood derived composites for high sensitivity and wide linear-range pressure sensing. Small 14(31), 1801520 (2018). https://doi.org/10.1002/smll.201801520 4. E. Vasileva et al., Light scattering by structurally anisotropic media: a benchmark with transparent wood. Adv. Opt. Mater. 6(23), 1800999 (2018). https://doi.org/10.1002/adom.201 800999 5. I. Burgert, E. Cabane, C. Zollfrank, L. Berglund, Bio-inspired functional wood-based materials—hybrids and replicates. Int. Mater. Rev. 60(8), 431–450 (2015). https://doi.org/10.1179/ 1743280415Y.0000000009 6. H. Zhu et al., Wood-derived materials for green electronics, biological devices, and energy applications. Chem. Rev. 116(16), 9305–9374 (2016). https://doi.org/10.1021/acs.chemrev. 6b00225 7. H. Zhu et al., Extreme light management in mesoporous wood cellulose paper for optoelectronics. ACS Nano 10(1), 1369–1377 (2016). https://doi.org/10.1021/acsnano.5b06781 8. C. Huang, J. He, Y. Wang, D. Min, Q. Yong, Associating cooking additives with sodium hydroxide to pretreat bamboo residues for improving the enzymatic saccharification and monosaccharides production. Bioresour. Technol. 193, 142–149 (2015). https://doi.org/10. 1016/j.biortech.2015.06.073 9. M. Zhu et al., Highly anisotropic, highly transparent wood composites. Adv. Mater. 28(26), 5181–5187 (2016). https://doi.org/10.1002/adma.201600427 10. S. Fink, Transparent wood—a new approach in the functional study of wood structure. Holzforschung—Int. J. Biol. Chem. Phys. Technol. Wood 46, 403 (1992). https://doi.org/10.1515/ hfsg.1992.46.5.403 11. Y. Li, Q. Fu, S. Yu, M. Yan, L. Berglund, Optically Transparent wood from a nanoporous cellulosic template: combining functional and structural performance. Biomacromol 17(4), 1358–1364 (2016). https://doi.org/10.1021/acs.biomac.6b00145 12. T. Keplinger, E. Cabane, J.K. Berg, J.S. Segmehl, P. Bock, I. Burgert, Smart hierarchical biobased materials by formation of stimuli-responsive hydrogels inside the microporous structure of wood. Adv. Mater. Interfaces 3(16), 1600233 (2016). https://doi.org/10.1002/admi.201 600233 13. T. Nilsson, R. Rowell, Historical wood—structure and properties. J. Cult. Herit. 13(3 Supplement), S5–S9 (2012). https://doi.org/10.1016/j.culher.2012.03.016 14. H. Chen et al., Refractive index of delignified wood for transparent biocomposites. RSC Adv. 10(67), 40719–40724 (2020). https://doi.org/10.1039/D0RA07409H 15. Y. Li, X. Yang, Q. Fu, R. Rojas, M. Yan, L. Berglund, Towards centimeter thick transparent wood through interface manipulation. J. Mater. Chem. A 6(3), 1094–1101 (2018). https://doi. org/10.1039/C7TA09973H 16. Q. Fu, M. Yan, E. Jungstedt, X. Yang, Y. Li, L.A. Berglund, Transparent plywood as a loadbearing and luminescent biocomposite. Compos. Sci. Technol. 164, 296–303 (2018). https:// doi.org/10.1016/j.compscitech.2018.06.001

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17. X. Wang et al., Large-size transparent wood for energy-saving building applications. Chemsuschem 11(23), 4086–4093 (2018). https://doi.org/10.1002/cssc.201801826 18. H. Li, X. Guo, Y. He, R. Zheng, A green steam-modified delignification method to prepare low-lignin delignified wood for thick, large highly transparent wood composites. J. Mater. Res. 34(6), 932–940 (2019). https://doi.org/10.1557/jmr.2018.466 19. A.W. Lang et al., Transparent wood smart windows: polymer electrochromic devices based on poly(3,4-Ethylenedioxythiophene):poly(Styrene Sulfonate) electrodes. Chemsuschem 11(5), 854–863 (2018). https://doi.org/10.1002/cssc.201702026 20. M. Zhu et al., Transparent and haze wood composites for highly efficient broadband light management in solar cells. Nano Energy 26, 332–339 (2016). https://doi.org/10.1016/j.nan oen.2016.05.020 21. Y. Li, S. Yu, J.G.C. Veinot, J. Linnros, L. Berglund, I. Sychugov, Luminescent transparent wood. Adv. Opt. Mater. 5(1), 1600834 (2017). https://doi.org/10.1002/adom.201600834 22. E. Vasileva, Y. Li, I. Sychugov, M. Mensi, L. Berglund, S. Popov, Lasing from organic dye molecules embedded in transparent wood. Adv. Opt. Mater. 5(10), 1700057 (2017). https:// doi.org/10.1002/adom.201700057 23. W. Gan, L. Gao, S. Xiao, W. Zhang, X. Zhan, J. Li, Transparent magnetic wood composites based on immobilizing Fe3 O4 nanoparticles into a delignified wood template. J. Mater. Sci. 52(6), 3321–3329 (2017). https://doi.org/10.1007/s10853-016-0619-8 24. C. Montanari, Y. Li, H. Chen, M. Yan, L.A. Berglund, Transparent wood for thermal energy storage and reversible optical transmittance. ACS Appl. Mater. Interfaces 11(22), 20465–20472 (2019). https://doi.org/10.1021/acsami.9b05525 25. Z. Yu et al., Transparent wood containing CsxWO3 nanoparticles for heat-shielding window applications. J. Mater. Chem. A 5(13), 6019–6024 (2017). https://doi.org/10.1039/C7TA00 261K 26. L. Ding, X. Han, L. Chen, S. Jiang, Preparation and properties of hydrophobic and transparent wood. J. Bioresour. Bioprod. (2022). https://doi.org/10.1016/j.jobab.2022.02.001 27. N.A. Muhammad, B. Armynah, D. Tahir, High transparent wood composite for effective X-ray shielding applications. Mater. Res. Bull. 154, 111930 (2022). https://doi.org/10.1016/j.materr esbull.2022.111930 28. ASTM, Standard test method for haze and luminous transmittance of transparent plastics ASTM D1003–00. (West Conshohocken, 2000), pp. 7. https://doi.org/10.1520/D1003-00 29. TAPPI, Acid-insoluble lignin in wood and pulp. Test Method T 222 om-2 (2002) 30. ASTM, standard test method for flexural properties of polymer matrix composite materials ASTM D7264 (2015). https://doi.org/10.1520/D7264_D7264M-15 31. T. Keplinger, F.K. Wittel, M. Rüggeberg, I. Burgert, Wood derived cellulose scaffolds— processing and mechanics. Adv. Mater. 33(28), 2001375 (2021). https://doi.org/10.1002/adma. 202001375 32. H. Chen, C. Montanari, R. Shanker, S. Marcinkevicius, L.A. Berglund, I. Sychugov, Photon walk in transparent wood: scattering and absorption in hierarchically structured materials. Adv. Opt. Mater. 10(8), 2102732 (2022). https://doi.org/10.1002/adom.202102732 33. P. Chen et al., Small angle neutron scattering shows nanoscale PMMA distribution in transparent wood biocomposites. Nano Lett. 21(7), 2883–2890 (2021). https://doi.org/10.1021/acs.nan olett.0c05038 34. J. Pang et al., Light propagation in transparent wood: efficient ray-tracing simulation and retrieving an effective refractive index of wood scaffold. Adv. Photonics Res. 2(11), 2100135 (2021). https://doi.org/10.1002/adpr.202100135 35. H. Chen et al., Thickness dependence of optical transmittance of transparent wood: chemical modification effects. ACS Appl. Mater. Interfaces 11(38), 35451–35457 (2019). https://doi. org/10.1021/acsami.9b11816 36. J. Wu, Y. Wu, F. Yang, C. Tang, Q. Huang, J. Zhang, Impact of delignification on morphological, optical and mechanical properties of transparent wood. Compos. Part A Appl. Sci. Manuf. 117, 324–331 (2019). https://doi.org/10.1016/j.compositesa.2018.12.004

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Chapter 2

Thin and Flexible Transparent Wood

Abstract Consumer devices of today are created from hazardous and non-renewable materials. Additionally, they are rigid, heavy, and produced in an energy-inefficient manner using CO2 -producing processes. Also, synthetic textile fibres have always had strong application possibilities in the textile industry due to favourable physical characteristics. Although petroleum-based polymers, including polyethylene terephthalate and polyethylene naphtholate, can address the stiffness issue, they have a large carbon footprint and generate toxic waste. Scalable, flexible, and environmentally friendly techniques in electronics production could provide solutions to the problems in the field. Electronics should ideally incorporate such substrates without degrading the device’s performance. Wood has been extensively employed in the domains of construction, flooring, and furniture as one of the most sustainable materials. The development of transparent wood broadens wood’s uses, including light, thermal, electromagnetic, and energy management fields. Nevertheless, the majority of the reported transparent woods are made by impregnating polymers like polymethyl methacrylate, epoxy resin, or polyvinylpyrrolidone. Despite major advancements in functional transparent wood, programmable, light, stretchable, and thin transparent wood with shape memory has only recently been created during the last few years. This chapter explains the most current developments in materials that are ideal for creating smart transparent wood and have outstanding stimuli-responsive, shape memory, reprocessing, and self-healing capabilities.

2.1 Introduction In recent years, we have seen a rapid advancement in technology that has changed our civilization into one that is dominated and dependent on electronics. The demand for flexible electronic device technology is rising along with consumer preference for portability, convenience, and attractive design [1]. The stability of mechanical properties during repeated folding or bending is one of the most crucial characteristics of flexible electronic devices [2]. Transparent wood is a sustainable material that can be used for solar cell substrates or as building materials to cut down electrical energy use. A crucial component of flexible electronic equipment is a flexible transparent © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 I. Wachter et al., Transparent Wood Materials, Springer Series in Materials Science 330, https://doi.org/10.1007/978-3-031-23405-7_2

15

16

2 Thin and Flexible Transparent Wood

substrate. Since transparent plastic has excellent mechanical qualities, is lightweight, and is inexpensive, it is frequently utilized as the substrate for flexible electronic devices [3]. However, in recent years, interest in bio-based products has increased due to the need to lower carbon footprint, growing energy and water safety concerns, and the need for sustainable industrial growth. Wood is the perfect material to address this problem because it is natural, biodegradable, recyclable, frequently used, and reasonably priced [4, 5]. The optical properties of the natural wood can be easily modified to produce transparent wood by removing the light-absorbing chromophore groups present in the wood or by almost completely removing the lignin using delignification processes including oxidation, chemical modification, and enzymatic hydrolysis and filling the polymer with refractive index matching the wood substrate to eliminate the light scattering phenomenon. Natural fibres have recently received a lot of interest as an alternative to synthetic fibres in composite materials. High specific mechanical strength, great thermal insulation, outstanding flexibility, and exceptional biodegradability and compostability are all characteristics of natural fibres [6]. The potential of natural fibres in the field of high-performance composites has been shown in numerous research [7, 8]. As the most prevalent biopolymer on the planet, cellulose is extensively present in trees, crop waste, and other biomass. Cellulose-containing fibres have outstanding functional qualities and a high potential for sustainability [9, 10]. Wood cannot be plastically moulded since it cannot be heated to the point of melting, which restricts its use in a variety of applications. The dense and aligned wood-textile fibres have potential applications in wearable and smart textiles, including bioactive, magnetic, cooling, and electroconductive hybrid fibres (Fig. 2.1). Combining wood with a thermoplastic resin is one method for plastic moulding wood. As a result, research into the creation of wood-plastic composites by mixing resin and powdered wood is ongoing [11–14].

Fig. 2.1 Wood-textile fibres offer a lot of potential in a variety of applications, including wearable smart textiles [10]

2.2 Recent Progress

17

2.2 Recent Progress Jia et al. showed for the first time a straightforward top-down procedure for producing macroscopic wood fibres for textile applications, which involved removing lignin from natural wood directly using sodium chlorite before twisting and organizing cellulose nanofibers [10]. Because of their unique and advantageous ability to modify their topology via thermally driven bond exchange events without changing their cross-linking density, vitrimers have recently attracted a lot of attention as dynamic covalently cross-linked polymers [15–17]. According to Majova et al., deep eutectic solvent delignification has the potential to replace oxygen delignification after kraft pulping because it has been shown that pulp with a higher initial Kappa number or lignin content possessed a greater fraction of easily removed lignin fragments [18]. Lu et al. were able to produce flexible transparent poplar veneer by lignin removal and a subsequent epoxy resin infiltration into the delignified wood scaffold. Their results showed excellent optical properties, thermal stability, and tensile strength of the prepared material with a great potential to be used as a substrate in photovoltaics, solar cells, smart windows, etc. [19]. Successful preparation of flexible transparent wood through delignified wood template and deep eutectic solvent polymerization composed of acrylic acid and choline chloride was achieved by Yang et al. They discovered that the electrical signal of transparent wood has excellent stability and repeatability in numerous heating–cooling cycles, indicating that it has significant promise as a temperature sensor [20]. For the first time, Wang et al. obtained a programmable shape memory property by incorporating transparent and refractive index-matching epoxy vitrimers into the delignified wood template (NaClO2 delignification). The dynamic covalent networks of vitrimers, which include transition temperature-induced phase shift and topology freezing-induced rearrangement into the transparent wood system, are attributed with giving this composite material its programmable shape memory property. This unique property is achieved by thermal manipulation (Fig. 2.2) [21]. In the work of Kadumudi et al., they showed that a novel wood-based material reinforced with nanosilicate can produce flexible electronics which can roll and bend without losing their electrical function. According to the authors, the integration of sensors, CPUs, memory devices, and resistors into small and sophisticated circuits will unavoidably result in numerous intriguing applications foreshadowing a new era for soft robotics, cyborganics, smart biorobots, and artificial cyborg-like creatures [22]. Yang et al. developed a technique for directly producing wood-textile fibres from natural wood after being motivated by studies on the creation of cellulose nanofibers from natural wood. First, a deep eutectic solvent was used to treat the natural wood, giving it a highly porous structure and good flexibility that made it simple to cut into cellulose fibre bundles and then twist into wood-textile fibres [23]. To accomplish various delignification degrees, several treatment conditions for delignification methods were chosen by Cai et al. Paulownia wood was subjected to an alkaline

18

2 Thin and Flexible Transparent Wood

Fig. 2.2 Transverse (T-PSMTW) and longitudinal (L-PSMTW) programmable shape memory transparent wood with a thickness of 2 mm [21]

pretreatment, bleaching, and then epoxy resin and ethylene glycol diglycidyl ether impregnation. According to the findings, greater ethylene glycol diglycidyl ether addition improved flexibility, whereas excessive amounts of it decreased the mechanical properties of transparent wood. A higher delignification degrees produced higher transmittance [24]. Jiang et al. were able to combine additive manufacturing processes and highly transparent natural fibre-reinforced composites based on raw flax fibres to prepare composites exhibiting excellent optical and mechanical properties. 3D printing manufacturing allows using a novel and sustainable method to engineer transparent composites for functional devices, such as wearable electronics and soft robotics in multiplex geometries (Fig. 2.3) [25]. The chemical alteration, in which the hydroxyl groups in wood are substituted with benzyl groups, is a promising approach suggested by Abe et al. for imparting thermo-plasticity to wood. In the procedure, the wood powder was first treated with a very concentrated aqueous NaOH solution before being heated for a very long time while being stirred to react with a benzylation reagent, producing a translucent film

2.3 Conclusion

19

Fig. 2.3 System for additive manufacturing based on extrusion (left). Composites made of TFF and elastomer with a printed “loop” design (right) [25]

[26]. Since transparent wood is more environmentally sustainable and is determined to be safer than polyethylene, it may eventually replace it [27]. A summarization of the processes with the main characteristics of thin and flexible transparent wood is given in Table 2.1.

2.3 Conclusion Due to their favourable physical characteristics, synthetic textile fibres have always had promising application possibilities in the textile industry, but due to a lack of petrochemical resources, their development has been constrained. The studies mentioned above offered simple processes that can be used to create transparent wood, which is anticipated to open up several new opportunities in the development of flexible electrical devices and sensors. The study of flexible transparent wood also broadens the field of transparent wood research and offers excellent opportunities for its use in flexible wearable technology and flexible materials. However, due to the anisotropic hierarchical wood’s inherent hygro-mechanical complexity, a significantly more thorough mechanical investigation under various climatic factors is required than is now possible. As a result, this analytical improvement requires complementary multiscale modelling and simulation techniques rather than being limited to experimental findings.

−

−

−

−

89 > 90

−

Lactic acid + choline chloride, H2 O2 treatment

Alkali pretreatment + NaClO2 delignification

NaOH + Na2 SO3 + H2 O2 delignification

NaClO2 delignification + ionic liquid treatment

Ashwood [23]

Paulownia wood [24]

Raw flax fibers (Linum usitatissimum) [25]

Birchwood plies [28]

97

−

−

Nacre-mimetic lapolose Hot pressing paper [22]

~ 80

62

Haze (%)

95

NaClO2 delignification

Balsa wood [21]

72

82

Transmittance (%)

60

NaClO2 delignification

NaClO2 delignification

Poplar wood [19]

Balsa wood [20]

Method

Wood type

∼ 370

4.2

3.74

102.3

Up to 84

2.59

1.14

67

Tensile strength (MPa)

Table 2.1 Recent progress achieved in the field of thin and flexible transparent wood fibres

−

1.5

2.0

13 cm long fibers

40 to 90 µm

2.0

1.5

1.0

Thickness (mm)

Moldability

Printable highly transparent natural fibre-reinforced composites

Bending angle 5:1

Wood-textile fibres with excellent mechanical, antibacterial properties, dyeable, hydrophobic

Flexible and foldable

Programmable shape memory

Excellent conductivity stability

2.5 mm bending radius

Special feature

20 2 Thin and Flexible Transparent Wood

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Chapter 3

Fully Bio-based Transparent Wood

Abstract Due to its high optical transparency, excellent thermal insulation, and great durability, transparent wood is a desirable structural material for energyefficient buildings, electronics, packaging, and nanotechnologies. The transparent wood enhances the aesthetic and practical qualities of wood. A lot of work has gone into making transparent wood with luminous, electrochromic, thermochromic, and photo-switchable functionalities by incorporating quantum dots, nanoparticles, or dyes. Because of their superior mechanical qualities and immense potential to function as renewable and CO2 -storing cellulose scaffolds for cutting-edge hybrid materials with embedded functionality, wood-derived cellulose materials obtained by structure-retaining delignification are gaining increasing attention. A wide range of characteristics is produced by applying various delignification protocols and numerous additional processes, such as polymer impregnation and densification. Due to the scarcity of bio-based monomers that combine advantageous processing with high performance, the sustainable development of biocomposites has been constrained. Nonetheless, because of its renewable and biodegradable qualities, transparent wood has the potential to replace traditional petroleum-based polymers because of the growing knowledge obtained during the last few years which is presented in the following chapter.

3.1 Introduction Sustainable development requires the creation of green materials that combine strength with useful characteristics from renewable resources. The most common renewable biomaterial produced by photosynthesis in plants is wood. The versatile properties of this material include low density, high modulus, high strength, high toughness, and low thermal conductivity. Its distinctive hierarchical microstructure and fibrous architecture with aligned fibres, composed of rigid cell walls made of high-strength cellulose fibrils embedded in a plant matrix of hemicelluloses and lignin, are the result [1, 2]. In contrast to conventional materials, wood has attracted a lot of attention recently for a variety of high-value-added products,

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 I. Wachter et al., Transparent Wood Materials, Springer Series in Materials Science 330, https://doi.org/10.1007/978-3-031-23405-7_3

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including flexible electronics [3], high-performance structural materials, energyefficient building materials, and other functional materials [4–7]. Wood naturally possesses well-oriented channels and a hierarchical microstructure [8]. Lignin– carbohydrate complex (LCC), which is mostly constituted of cellulose (35−50%), hemicellulose (20−35%), and lignin (5−30%), makes up the chemical composition of natural woods [9]. The intricate wood tissue, cell, and cell wall structure are typically altered somewhat unspecifically as a result of chemical delignification procedures. Lignin cannot be selectively removed from the cell wall by chemical treatments because of the closure assembly of cellulose, hemicelluloses, and lignin at the nanoscale [10, 11]. Wood’s porosity opens up possibilities for new functions. In comparison with the dense natural cell wall, the cellulose scaffolds produced by a structure-retaining delignification process have higher porosity and hydroxyl accessibility in the wet state, making it easier to incorporate polymers and inorganic materials into the hierarchical scaffolds to improve properties or implement novel functionality [12, 13]. To produce transparent wood and its composites with exceptional optical properties, it is necessary to incorporate interface manipulation of nanocellulose fibrils via acetylation to further improve the accessibility of polymer to the wood scaffold. A hydrophobic liquid has poor compatibility with hydrophilic wood, making it challenging for it to permeate the cell wall. It should be simpler to induce wood flow if the binder can penetrate the cell wall more easily. One chemical alteration of wood used to increase dimensional stability is acetylation [14]. According to Obataya and Shibutani [15], hydrophobic organic liquids can permeate the cell wall of acetylated wood, a sort of hydrophobized wood. The acetylated wood polymers allow more hydrophobic organic liquids to penetrate because they have fewer and weaker intermolecular hydrogen bonds. Acetylation, according to Shiraishi [16], is a useful technique for increasing the affinity of wood with methyl methacrylate (MMA), which can subsequently be polymerized into poly(methyl methacrylate) (PMMA) inside the cell wall. Seki et al. concentrated on the chemical altering of wood through acetylation to increase MMA penetration. They used a hydrophobic MMA monomer as a thermoplastic binder to impregnate the wood, which was then heated to polymerize the monomer into PMMA and investigated how wood acetylation affected the ability of solid wood impregnated with PMMA to be extruded [17]. Li et al. used interface manipulation through acetylation to produce a centimetre-thick transparent wood structure with outstanding optical properties of 60% transmittance and 76% haze. Without acetylation, the same optical parameters were achieved by 3-mm-thick samples. Figure 3.1 shows that more optical heterogeneity would be caused by cellulose aggregations and interface debonding, which would reduce optical transmittance and increase haze. Acetylated transparent wood had almost no interface debonding gap, resulting in increased compatibility between PMMA and the wood template [18]. Transparent wood’s hydrophilic and hydrophobic properties have a big impact on where it can be used. Wood’s many hydroxyl groups readily form hydrogen bonds with water, causing the material to absorb water and expand thus negatively affecting its use. In addition to esterification, acetylation, chemical or enzymatic

3.1 Introduction

25

Fig. 3.1 Low- and high-magnification images of the non-acetylated transparent wood, exhibiting the aggregation of nanocellulose fibrils and the interface debonding gaps (c–e), low-magnification and high-magnification images of the acetylated wood template (f–h) [18]

grafting with functional molecules, and modification with organosilicon compounds, the replacement of hydrophobic groups for hydroxyl groups has also been extensively studied [19–21]. Since transparent wood was initially studied [22], bio-based applications have been proposed. There are typically two crucial phases involved in the creation of transparent wood: (1) decolorizing wood by eliminating light-absorbing substances, primarily lignin (remove or modify) and (2) infiltrating a petroleum-based polymer with a refractive index compatible with wood substrates [23–27]. A chemical lignin removal also impacts hemicelluloses in close contact with lignin, which may also be determined from the mass loss after the chemical treatment [7]. Also, the inplane mechanical performance of transparent wood with thickness in the longitudinal direction is significantly worse, and the size of the structure is constrained by the cross-sectional area of the wood or tree. This limits the use of transparent wood when heavy, sturdy, and substantial building blocks are required. As a result, thick transparent wood that is thick in the longitudinal direction is more desirable but also more problematic [18]. PMMA or epoxy polymers encase the entire transparent wood with superior protection compared to natural wood, higher dimensional stability, and lower water sensitivity [4, 5]. As a result of their potential industrial and commercial uses transparent wood-based materials are highly sought after. Figure 3.2 depicts the material’s whole life cycle, starting with trees as renewable resources that are used to produce wood-based materials. They are subject to processing and modification to create biocomposite materials [28].

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Fig. 3.2 Wood-based biocomposites’ life cycle. To be considered sustainable, there must be a reduction in carbon dioxide emissions and total energy consumption [28]

3.2 Recent Progress The first ever transparent wood was created by Fink in 1992 [22]. It however was not a fully bio-based transparent biocomposite. Since then, there have been attempts to make a fully bio-based transparent wood primarily due to environmental reasons. Although many of the newly created materials were completely bio-based, they lacked in some areas and could be only used in a very specific application such as (1) mechanical energy conversion (decayed all-wood blocks without infiltrated polymer) [25], (2) photonic wood (white all-wood scaffold without infiltrated polymer) [29], (3) transparent nanopaper (all bio-based, however ultra-thin) [30–34], (4) radiative cooling material (natural delignified wood scaffold) [35], (5) high-performance structural materials (lack of transparency) [36–38], (6) hydrophobic wood films as optical lightning materials (bio-based, polymer matrix-free) [39], (7) high-performance densified cellulose materials (matrix-free wood-based materials) [40], and (8) natural fibre composites [41, 42]. Despite the above-mentioned materials and their applications, none of them can be considered a truly fully bio-based transparent wood. The definition of fully bio-based transparent wood for the purposes of this chapter is a biocomposite material consisting of delignified or lignin-modified wood scaffold with preserved microstructure and infiltrated with bio-based polymer resulting in a material with excellent optical and mechanical properties suitable for various applications such as in smart building, smart windows, solar panels, and electronics. In comparison with typical non-renewable polymers made from fossil resources, bio-based polymers may have advantages such as the utilization of renewable

3.2 Recent Progress

27

resources, carbon neutrality, and reduced global warming impact [43]. Fully biobased transparent wood can be prepared by using polymers which can be synthesized from raw renewable resources or organic wastes. Such polymers include polyvinyl alcohol [44, 45], furfuryl alcohol [46], gelatine [47], polycarbonates [48], polyacrylates [49], terpenes [50, 51], or polysaccharides [52]. Few studies proposed such procedures [53–57]. In load-bearing biocomposites, bio-based and biodegradable polymers have recently been studied as alternatives to fossil-based polymers. According to Lande et al. [58], furfuryl alcohol is also a bio-based polymer that can replace phenol– formaldehyde resins despite its hazardous properties (monomer and catalysts) and dark hue. Thermosets made from vegetable oils are another bio-based polymer that has been taken into consideration for plant fibre composites [56]. A comparison of the specific strength and specific stiffness of conventional thermosets and thermoplastics with poly(limonene acrylate) (PLIMA), which is considered a fully bio-based polymer, is shown in Fig. 3.3 [59]. The focus of Keplinger et al. was placed on structure-retaining delignification and following densification phases, as well as how experimental tests and modelling techniques affected mechanical characteristics [60]. Jiang et al. substantially improved the usability and scalability of transparent wood material for a variety of sustainable applications by offering insights into the production of advanced transparent wood material from the perspective of wood chemical composition [61]. The fact that cellulose and wood come from renewable sources does not, however, make wood composites eco-friendly materials (excessive energy requirements, greenhouse gas emissions, and water depletion). Green chemistry principles were explored by Montanari et al. Using a top-down strategy, a totally bio-based transparent wood

Fig. 3.3 PLIMA bio-based polymer’s mechanical characteristics in comparison with typical thermosets and thermoplastics [59]

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nanocomposite was produced. Natural wood underwent green delignification, biobased molecules stabilized the moisture, the bio-based monomer was infiltrated in a solvent-free process, and final curing resulted in a biocomposite with finely tuned properties [28]. To this date, only a handful of studies in which fully bio-based transparent wood was manufactured have been published. By creating a bag, medicine container, and drink straw, the potential for environmentally friendly packaging uses was shown by Hai et al. (Fig. 3.4) [62]. They employed chitosan, a chitin derivative with benefits including antioxidant, antibacterial, mechanical, thermal, and UVprotection qualities that may be obtained from a variety of sources including shrimp shells, lobster shells, mushrooms, and crabs [63−64], and it showed a promising result. An entirely bio-based transparent wood biocomposite was also created by a team under the direction of Montanari using a green synthesis process using a new limonene acrylate monomer from renewable resources. They emphasized the need for processing ideas that are appropriate for larger structures with controlled micro and nanostructure so that sustainable wood nanotechnologies transparent plastics and glass can compete with in applications like load-bearing panels, interior design, lighting, and various energy-saving initiatives [59]. Synthetic structural materials with great mechanical performance are often either heavy (such as steels and alloys) or produced using complicated techniques, which results in high costs or negative environmental effects. Therefore, Khakalo et al. created an all-wood composite material through a process consisting of delignification, ionic liquid-facilitated partial fibre

Fig. 3.4 Preparation process of transparent wood with all bio-based cellulose nanofiber and chitosan resin [62]

3.2 Recent Progress

29

surface dissolution, and heat-assisted densification which allowed for the encapsulation of the remaining fibres in a matrix of regenerated cellulose, creating compact composites with exceptional mechanical performance [65]. By infiltrating gelatine into the porous wood template, a bio-based hydrogel composite was created by Wang et al. Their work showed that wood structure is a great template for creating highperformance hydrogel composites, utilizing native porosity cell walls and aligned cellulose nanofibers for mechanical reinforcement and structural confinement [47]. Key research on transparent wood’s life cycle analysis and the environmental effects of its manufacturing and disposal was performed by Rai et al., who claimed that epoxy infiltration, sodium sulfite, hydrogen peroxide-based delignification (NaOH + Na2 SO3 + H2 O2 technique), and sodium hydroxide have the least negative effects on the environment. Compared to sodium chlorite delignification and PMMA, it showed about 24% less potential for global warming and 15% less for terrestrial acidification. In comparison with laboratory-scale production, the industrial-scale production model consumes 98.8% less electricity and has 28% less potential for global warming and almost 97% less toxicity for humans. Transparent wood’s end-of-life examination revealed that it had 107 times fewer ecological effects than polyethylene, indicating that it may be economically modified to replace traditional petroleum-based products [66]. Current development in the field of entirely bio-based transparent wood is summarized in Table 3.1. Table 3.1 Summarization of the recent progress achieved in the field of fully bio-based transparent wood Wood type Method

Transmittance Haze (%) (%)

Tensile Thickness Special strength (mm) feature (MPa)

Balsa veneers [59]

90

30

174

1.2

Young’s modulus 17 GPa

~ 85

~ 72

165.3

0.37

High hemicellulose content (~23 wt%)

0.1

Excellent UV shielding, thermal stability up to 315 °C

Peracetic acid delignification + Succinylation

Rotary-cut NaClO2 delignification basswood and pinewood [61] Fir wood veneer [62]

NaClO2 delignification 80

30–60 171.8

Birchwood NaClO2 delignification 40 plies [65]

∼ 430

Excellent mechanical properties

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3 Fully Bio-based Transparent Wood

3.3 Conclusion In the aforementioned studies, transparent wood composite materials made entirely of wood and from biomaterials were created. Due to their distinctive hierarchical structure and cellulose phase, which serves as an intrinsic reinforcement, woodbased multifunctional materials with good mechanical performance are being more and more evaluated for advanced sustainable applications. Based on these results, additional work might be done to produce different structures with highly aligned fibres, which would be valuable in the developing circular bioeconomy. Recent years have seen a lot of interest in the study of cellulose composites generated from wood, and, likely, these materials will soon be used in many different sectors. A thorough understanding of long-term performance is necessary for this, though, as well as a thorough understanding of mechanical behaviour under varied loading circumstances. Additionally, studies into the fundamental structure-property interactions and how they emerge from and are impacted by different processing parameters are lacking. The deployment of these useful wood- and cellulose-based materials in engineering applications will advance, but only if they can deliver dependable long-term performance, which necessitates a thorough understanding and control of all production parameters.

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Chapter 4

Multilayered and Densified Transparent Wood

Abstract The emergence of the concept of transparent wood has created a new frontier in wood modification, attracting academics to investigate it further and examine more functionality and production methods. In cutting-edge applications for architecture, new energy vehicles or spacecraft, it is very desirable to prepare a lightweight yet highly stable bio-based structural material that is sustainable and recyclable. In recent studies, transparent wood which has the potential to be widely used as new energy-saving material is mainly limited by its thickness which was either thin and highly anisotropic or thick and isotropic but weak. Therefore, new methods of its fabrication have been proposed to primarily increase the mechanical properties using multilayer lamination or densification.

4.1 Multilayered Transparent Wood Building materials made of wood have been utilized for ages and are increasingly popular in energy-harvesting systems [1, 2]. Residential and commercial building energy consumption accounts for approximately 40% of total energy consumption [3, 4]. Transparent wood is a biocomposite material that has high transmittance and adaptable optical properties [5, 6]. As a new biocomposite material with good transmission and mechanical qualities, transparent wood has been produced from various types of wood such as basswood [7–9], poplar wood [10, 11], balsa wood [12–15], pine wood [16], birch wood [17, 18], beech wood [19, 20], wood from Betula alnoides, Chinese fir (Cunninghamia), New Zealand pine (Radiata Pine), Oguman (Aucoumea klainea), black walnut [21], and paulownia [22]. It is made by removing or modifying the lignin groups found in natural wood to make it appear colourless and then infiltrating the material with a polymer with a similar refractive index as a wood cell wall to reduce light scattering. For the delignification of wood, a number of chemical-based treatments have been used, including sodium chlorite (NaClO2 ) [23], sodium hydroxide, sodium sulfite, and hydrogen peroxide (NaOH + Na2 SO3 + H2 O2 method) [24], and sodium hydroxide and sodium sulfide (NaOH + Na2 S) [25, 26]. Degradation, fragmentation, and hydrophilic lignin alteration occur as a result of alkaline treatment. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 I. Wachter et al., Transparent Wood Materials, Springer Series in Materials Science 330, https://doi.org/10.1007/978-3-031-23405-7_4

35

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4 Multilayered and Densified Transparent Wood

Because glass has high thermal conductivity, it is prone to thermal leakage. The use of glass materials is limited due to these features. As a result, alternative construction materials with desirable mechanical qualities, sunlight-controlling, and even lightemitting properties are being developed [27]. Engineering, building, and architectural research have traditionally focused on lightweight structural materials with high mechanical performance. Due to its low cost, simple processing, and abundance, natural wood is a particularly important type of structural material for buildings and furnishings [28–30]. However, natural wood cannot meet the need for modern engineering materials due to its poor physical and mechanical performance [31, 32]. Because of their exceptional mechanical qualities, partial or full delignification treatments while preserving the native wood’s hierarchical structure have received a lot of attention recently [33–37]. The development of transparent wood is still constrained by material and thickness. Currently, transparent wood can only be fabricated to a few centimetres in thickness, and the raw materials typically used are porous wood with a lighter density, which is constrained by mechanical strength during the actual application process. Researchers are therefore increasingly interested in the problem of thickening transparent wood. The intended experimental findings, however, cannot be attained since it is challenging for the chemicals to penetrate the thick wood. One approach for obtaining thick transparent wood is to laminate multiple thin transparent wood templates as shown in Fig. 4.1.

Fig. 4.1 Lamination of thin transparent wood templates [38]

4.2 Recent Progress

37

4.2 Recent Progress To enhance the mechanical and optical properties of transparent wood, Fu et al. prepared and tailored transparent plywood. The mechanical properties were enhanced in the transverse direction compared to single-ply transparent wood, and the structure–property relationships were investigated. Tangential veneers of balsa wood were delignified by sodium chlorite (the delignified wood samples were compressed by 75 kN for 25 min under 25 °C.), infiltrated by methyl methacrylate and five layers were laminated together resulting in 3.5 mm-thick samples. Lamination raised the maximum strength in the “transverse” direction from 15 to roughly 45 MPa [39]. The main purpose of Wang et al. study was to prepare engineering structural transparent wood materials with good mechanical properties and high light transmittance. Two kinds of tree species were delignified using sodium chlorite, infiltrated with epoxy resin and laminated in the same texture direction under a heavy object for 12 h. Three- and five-layer samples were prepared. It has been shown that the transmittance and mechanical properties of transparent laminated wood could be effectively controlled by selecting materials and assembly methods [40]. Wu et al. reported that there are still some problems associated with the traditional manufacturing of transparent wood. (1) The choice of raw materials also has a great influence on the performance of the prepared samples. (2) Removing lignin will damage and weaken the inherent structure of wood. Therefore, their experiment intends to investigate the performance of multilayered transparent wood under partial delignification, as well as the impact of layer lamination structure and the delignification procedure on the attributes. The delignified samples were infiltrated with methyl methacrylate, and then multiple pieces of wood templates with partial lignin removal were inserted into the impregnation solution in a staggered or sequential order. The results showed that by using partial delignification it is possible to efficiently limit the use of chemical reagents, save energy, and safeguard the environment to make transparent wood. In addition, the light transmittance of multilayer transparent wood may be enhanced to roughly 12.49% when compared to the original wood, and the mechanical characteristics can be raised by 24.21–35.79% [41]. Another problem arises from the delignification process itself such as energy consumption and gas pollution. The ideal conditions for the two-stage delignification of low-density balsa wood (Ochroma pyramidale) and high-density basswood (Tilia tuan) were investigated in the paper by Qin et al. [42]. Their results showed that: (1) Lamination and a two-stage delignification technique substantially speed up the preparation of transparent wood. (2) The H2 O2 treatment successfully stopped the yellowing of the transparent wood once the lignin concentration reached the equilibrium level. (3) As the wood thickness and tree density rose, the delignification process took longer. (4) As the number of strata and tree species density grew, the transmittance fell. (5) The transparent wood’s tensile strength improved as tree density increased.

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Delignification optimization of basswood was also the aim of Wachter et al. [43]. The study of Wu et al. mimics the manufacture of plywood by using an odd-layer laminated veneer with each fibre layer perpendicular to the other to create multilayer transparent wood. Poplar wood was utilized as the raw material for making a delignified wood template with sodium chlorite, and then transparent wood was made by impregnating it with epoxy resin, and then the physical and chemical properties were evaluated [38]. Figure 4.2 demonstrates how the layered transparent wood was created by staggered vertical lamination of thin wood. At the connecting line, the delamination interface is visible. The multilayer transparent wood has fewer internal fissures than transparent wood of the same thickness, and the epoxy resin adheres better to the cell wall. To our knowledge, Han et al. have prepared the highest-performing transparent wood composite using densified cellulose microfibers in combination with aramid nanofibers. The prepared material showed excellent tensile strength (341.7 MPa), toughness (4.4 MJ/m3 ) and Young’s modulus (24.7 GPa), due to low density (1.2 g/cm3 ), and a specific strength of 285 MPa cm3 g−1 , which is noticeably greater than certain common building materials, including concrete, alloys [44].

Fig. 4.2 Micrographs of transparent plywood; a hierarchical structure of transparent plywood; b SEM image of single layer plywood; c SEM image of five-layer plywood (perpendicular); d SEM image of five-layer plywood (quasi-isotropic); e, f SEM images of interlaminar interfaces; and g highly aligned nanocellulose fibre [39]

4.3 Densification

39

Table 4.1 The procedures and main results of densified wood research Wood type

Transmittance (%) Tensile strength (MPa)

Thickness (mm)

New Zealand NaClO2 delignification pine (Pinus radiata) and Basswood (Tilia) [41]

12.49

~ 48

1.5 (3 layers)

scotch pine (Pinus sylvestris) and cherry wood (Prunus serotina) [40]

NaClO2 delignification

45–65 (3 layers) 35–55 (5 layers)

~ 25 (3 layers) ~ 45 (5 layers)

0.6 (3 layers) 1.0 (5 layers)

Poplar wood [38] NaClO2 delignification

~ 1.2 (3 layers) ~ 0.45 (3 layers)

10 (single 1.5 (3 layers) layer) 2.5 (5 layers) 25 (5 layers)

Balsa wood NaClO2 delignification (Ochroma pyramidale) [39]

83

50 (5 layers) 3.5 (5 layers)

Balsa wood, basswood [42]

Method

NaClO2 delignification + 59 basswood (2 H2 O2 treatment layers) 76.5 balsa (2 layers)

75.12 basswood 69.02 balsa

2.0 (2 layers)

A summarization of the procedures and main characteristics of densified transparent wood is given in Table 4.1.

4.3 Densification Due to its biodegradable, carbon–neutral, and essentially sustainable qualities, wood has drawn significant interest from researchers in response to environmental degradation and resource depletion. As transparent wood preparation technology has continued to advance, enhancing its performance has emerged as one of the main areas for future study. Further research is needed on the chemical and physical characteristics of delignified and densified wood since a recent research on increasing the mechanical strength of wood focuses on wood densification through the partial removal of lignin and hemicelluloses. Cellulose is the structural element of materials made of wood. The current consensus is that wood-based materials with a higher cellulose content produce stronger mechanical properties [33]. Thus, the creation of high-strength, densified wood was made possible by the partial removal of the matrix, particularly the lignin, from the wood [34, 45–49]. Depending on the lignin concentration, densification treatments, with or without increased temperature, typically

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4 Multilayered and Densified Transparent Wood

Fig. 4.3 Photographs of a natural wood; b delignified and densified wood; and c, d corresponding SEM images [50]

come next to improve the mechanical performance of these wood-derived composites [28, 34]. Li et al. concluded that there is a significantly positive correlation between the tensile strength of densified wood and the supramolecular structures of cellulose, such as crystallinity and the intermolecular hydrogen bond content. As depicted in Fig. 4.3), the densified wood underwent the following changes through alkali cooking and hot pressing when compared to natural wood. Densified wood showed a reduction in thickness of about four times when compared to natural wood. The cell walls and lumens of densified wood collapsed and formed a tighter bond inside, in contrast to natural wood, which had numerous lumens with diameters of about 10−20 µm [50]. The most crucial characteristic of transparent wood (depending on the application) is transparency, which is correlated with the degree of resin impregnation, the thickness of the wood template, etc. The mechanical performance of wood can be significantly improved with partial delignification and densification where transparency is not always required. Enhancing the properties of fast-growing woods through costeffective treatments is a value-added proposition. It is possible to modify fast-growing species’ inherent flaws, such as their thin cell walls, high porosity, short fibres, low density, and low mechanical resistances, by impregnating the wood with monomers [51], densification [52] and a combination of polymerization/densification [53]. Through heating/steaming and compression, wood densification is another crucial technique to enhance the qualities of low-density wood [54, 55]. The manufacturing of high-density wood without damaging the microcellular structure of the wood is possible due to the viscoelastic thermal compression method, which uses higher

4.4 Recent Progress

41

steam pressure to heat the wood over the lignin glass transition temperature (Tg) [56]. To preserve the integrity of the wood’s cellular structure, wood densification with pure thermal treatment, such as VTC, is frequently limited to a maximum of about 1 g/cm3 [57, 58]. Densification as a method for performance improvement is suggested by the significant association between wood density and mechanical performance. Wood becomes softer at higher temperatures and humidity levels [59, 60], which enables densification to approach the bulk wood cell wall density of 1.5 g/cm3 [61]. The ability to compress transverse to the fibre direction is also made possible by a partial elimination of cell wall polymers, especially stiff lignin, which also reduces cell wall fractures. Moreover, due to the distinctive hierarchical structure and innately reinforcing cellulose phase orientation, wood-based multifunctional materials with good mechanical performance are increasingly being taken into consideration for advanced sustainable applications even without the need for optical transparency [48]. Improved mechanical qualities come from compressing wood, which collapses the wood lumina and the porous wood cell walls. With delignification, hydrogen bonding makes it possible to densify wood at significantly lower pressures, which has the advantage of lowering processing costs and the number of defective products due to high-pressure-induced cracking [5, 62]. Therefore, it is necessary to investigate the microstructural, mechanical, chemical, and physical properties of delignified and densified wood for sustainable application.

4.4 Recent Progress High-performance wood-based materials with and without reinforcing polymers have been created by Frey et al. Strong fibre–fibre interfaces, which appear to facilitate stress transfer within the cellulose scaffold in the dry state, are the cause of the matrix-free densified cellulose materials’ high tensile properties, which reach stiffness and strength levels of up to 250 MPa and 40 GPa, respectively. Additionally, delignified wood-reinforced polymers exhibited excellent tensile properties with elastic moduli of up to 70 GPa and 600 MPa strength at a fibre volume content of 80%. This type of material is suitable and scalable for open-mould processing for manufacturing recyclable composite parts, eventually replacing materials like glass fibre composites [63]. For comparison, Li et al. prepared densified wood hot pressing after partial lignin, and hemicellulose was removed through alkaline solution cooking. The tensile strength and elastic modulus of the densified wood were improved up to 398.5 MPa and 22.5 GPa [50]. In the work of Wang et al., a new method was proposed to improve the transparency of transparent wood (Fig. 4.4). The densified wood template was creatively coupled with resin in the current preparation process, and the drying and oxidation phases were not included in the curing process, which significantly decreased the chemical cost and preparation difficulty. This method produced a natural optical material with enhanced structural qualities and good transparency that is a great replacement for energy-saving applications in industries like architecture and home furnishings [64].

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4 Multilayered and Densified Transparent Wood

Fig. 4.4 Preparation of densified transparent wood [64]

Yahyaee et al. studied the effect of organosolv delignification/polymerization on the set recovery of densified poplar wood. The innovative combination of delignification/polymerization increased the specific tensile strength of densified samples by 37% as compared to non-densified ones [65]. One of the objectives of Jakob et al. was to demonstrate the mechanical anisotropy of partially delignified and densified spruce wood. The samples were characterized at varying degrees of off-axis alignment. Off-axis tensile strength indicates a much sharper decline, and even a considerable loss below reference values is found at misalignment angles ≥ 60°, while the modulus of elasticity of treated samples shows a significant improvement across the complete range of misalignment degrees. It is determined that the observed decrease in off-axis performance is due to microcracks brought on by shear stress during densification [66]. Recent research on improving wood mechanical strength emphasizes wood densification through the partial removal of lignin and hemicelluloses. Wang et al., therefore, studied the microstructural, chemical, and physical properties of delignified and densified wood for sustainable application [67, 68]. All of the above-mentioned researchers used different wood species (such as balsa wood, poplar wood, and spruce wood) and different delignification methods (NaClO2 , Glycerol + NaOH, alkaline Na2 SO3 , maleic acid + NaOH delignification) resulting in a delignified and densified biocomposite material with favourable mechanical and optical properties suitable for various environmentally friendly uses. The limitations caused by scalable functionalization, effective processing, simple moulding, natural heterogeneity, and durability have thus far prevented wood products from being used more widely. A study by Khakalo et al. devised a method for fabricating multifunctional all-wood materials that use pressure-assisted consolidation, ionic liquid treatment, and delignification. In comparison with native wood, they were able to produce extremely compact, entirely additive-free, all-wood products with superior mechanical performance and gained multifunctionality [48].

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4.5 Conclusion Transparent wood is a type of composite transparent material that has been infiltrated with an appropriate polymer after the lignin has been removed or deactivated but has retained the anisotropic properties of the original wood. In the developing fields of wood technology and nanotechnology, transparent wood is a fascinating subject, both in an academic setting and in an industrial setting. Significant advancements have been made in the preparation of transparent wood with enhanced mechanical properties, the improvement of larger and thicker transparent wood structures, the realization of functionalized transparent wood, and the demonstration of applications in photovoltaic and smart building technologies. The aforementioned methods show that making the multifunctional material from wood is feasible, and they are therefore highly relevant in the developing field of sustainable high-performance materials.

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Chapter 5

Transparent Bamboo

Abstract The objective of this chapter is to acquaint the reader with information about recent progress made in the field of manufacturing biocomposite material— transparent bamboo. In the past few years, there has been a growing interest among researchers to develop a transparent material derived from fast-growing natural bamboo as a promising and sustainable alternative building material, smart house applications, electronic products, and aesthetically pleasing materials. Although bamboo has a significantly shorter growth cycle than wood, it is difficult to produce transparent items due to its high density and absence of lateral cell structures. Despite that, bamboo has many advantages over traditional wood but there are still many challenges which must be addressed before its practical applications.

5.1 Introduction The IPCC report 2022 stated that only the most drastic reductions in carbon emissions from now would help prevent an environmental catastrophe as the world is expected to reach the 1.5 °C level within the next two decades. Human activities are proven to be the main drivers behind issues such as more intense heat waves, glaciers melting, and our oceans getting warmer. Climate impacts are already more widespread and severe than expected. Climate change is already causing widespread disruption in every region in the world with just 1.1 °C of warming, and we are locked into even worse impacts from climate change in the near term. The report also finds that every tenth of a degree of additional warming will escalate threats to people, species, and ecosystems. Even limiting global warming to 1.5 °C which is a global target in the Paris Climate Agreement is not safe at all [1]. As a result, it is critical to reduce reliance on non-renewable resources such as petroleum while focusing on sustainable and green development. Biomass, particularly lignocellulosic resources, is abundantly available, renewable, and inexpensive to process. If biomass products are employed instead of conventional petrochemical- and mineral-based materials, this can be used to relieve severe

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 I. Wachter et al., Transparent Wood Materials, Springer Series in Materials Science 330, https://doi.org/10.1007/978-3-031-23405-7_5

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5 Transparent Bamboo

environmental concerns such as energy stress and pollution [2–5]. Thus, the development of transparent wood as an alternative transparent material for various applications is a promising strategy to solve the above issue. Typically, transparent wood is made of various kinds of tree species, such as balsa, basswood, paulownia, and poplar because of their high porosity and low density. However, bamboo provides several benefits compared to tree wood. The first ever published report on transparent wood was made by Fink in 1992 to observe wood structures within the wood in situ [6]. Zhu et al. used their ideas to create highly anisotropic and highly transparent wood composites with outstanding mechanical and optical properties [7]. Since then, a great interest in transparent wood has been observed. Many international research teams have published dozens of new articles on new methods of its fabrication and suggested numerous applications of transparent wood (TW), such as to produce construction materials [8-9], solar cells [10], magnetic materials [11], heat shielding [12], thermal energy storage [13], and electroluminescent devices [14] and combined with conducting polymers in electromechanical devices [15]. Nevertheless, the research on transparent bamboo has only begun recently with a handful of publications in the last few years [16–21]. In spite of the novelty of this research area, it has shown remarkable progress and potential for future applications. One of the most important non-wood forest products is bamboo. Bamboo is a perennial grass with lignified tissues and is widely distributed across temperate and tropical regions. It consists of more than 90% cellulose, hemicellulose, and lignin such as wood. Specifically, the percentages of each component are 40% cellulose, 31% hemicellulose, 21% lignin, 2% protein, 4% extractives, 1% starch, and 1% ash [22]. Lumen is structurally absent from bamboo. It features dense cell walls that are 1.7–2.5 times thicker than parenchyma cells and cellulose fibres that are oriented and have a circular cross-section. Bamboo is very strong because of its thick, multilayered walls [23]. However, the exact composition may vary due to variations within approximately 1500 species and different growth conditions as it is planted on 36 million hectares across Asia, America, and Africa [24]. The growth cycle of bamboo is only 3−5 years, while wood, as one of the traditional industrial materials, has a growth cycle of 20–60 years [25]. When comparing the amount of oxygen released into the atmosphere by the same volume of trees and bamboo, bamboo releases 35% more oxygen than trees [16]. Bamboo offers higher tensile strength, insect resistance, abrasion resistance, water resistance, and mould resistance than wood. Lignin is primarily responsible for bamboo’s stiffness. Bamboo is commonly utilized in furniture, construction, and bridges because its tensile strength is comparable to that of mild steel [26]. Designers are increasingly interested in using laminated bamboo materials in buildings, such as plywood, particle board, and other laminated bamboo structural elements [27]. Bamboo, on the other hand, has a lot in common with wood in terms of microstructure and chemical composition. As a result, bamboo has been hailed as one of the most potential replacements for wood in the next generation of transparent structural materials [18–19].

5.2 Preparation and Fabrication

49

5.2 Preparation and Fabrication Bamboo’s transparency is determined by its chemical makeup and structure. Bamboo and wood have the same chemical components as each other in terms of chemical composition. Bamboo’s main chemical components are cellulose, hemicellulose, and lignin, which account for more than 80% of its weight [28]. Colourless cellulose and hemicellulose, coloured lignin, and other compounds with differing refractive indices are among them (Fig. 5.1). Under visible light irradiation, these chemicals cause the material to undergo substantial light scattering and light absorption, resulting in an opaque colour [29]. Natural wood contains lignin and other light-absorbing components, and the porous nature of the wood scatters visible light, resulting in opacity, which means it cannot meet lighting demands and cannot be utilized as a building glass material. The opacity of wood is caused by two factors: (1) The wood contains a large amount of light-absorbing substance, lignin, which accounts for 20–30% of the total weight of the wood. (2) The porosity of the wood ranges from 30 to 80%, and a large number of pores have diameters larger than the wavelength of visible light (380–780 nm), resulting in severe light scattering. As a result, the chromogenic chemicals can be removed from the wood by removing the lignin [16]. The density of mature bamboo is usually around 0.65 g cm−3 [30, 31], which is much higher than that of wood used to prepare the TW. Using low-density wood as raw material for transparent composites is mainly due to its better permeability caused by its higher porosity [32] so the lignin removal or lignin modifying solution could easily access the lignin. Although attempts have

Fig. 5.1 Sample structure (a) a cross-section of the outer side (b) and inner side (c) observed by light microscopy combined with safranin and Astra blue staining [20]

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5 Transparent Bamboo

also been made to use high-density wood, such as birch wood, to prepare TW with higher strength, longer delignification time and more chemicals are required [33]. High-density bamboo would also need a longer time and more chemicals to remove its lignin due to its poor permeability. The current method of preparing TW is to remove the lignin from wood and then fill it with a polymer whose refractive index matches that of the delignified wood template [34–36]. Acid delignification, alkaline delignification, bio-enzymatic delignification, and lignin modification are techniques for removing the chromogenic groups from wood [37–39]. In addition, as an innovative and environmentally friendly solvent, the deep eutectic solvent may effectively encourage the fractionation of highpurity lignin from lignocellulosic biomass pretreatment. The findings of Li et al. emphasize the use of acidic deep eutectic solvent as a low-cost, high-performance material that is thermally stable and has a long delignification durability [40]. The most important aspect of resin impregnation is resin selection. Epoxy resins, poly(methyl methacrylate), polylactic acid, poly(vinyl) alcohol, and other polymers match the refractive index of the wood template, as well as the more environmentally friendly limonene acrylate made from orange peels, which was recently proposed [27].

5.3 Challenges Despite all the presented advantages of bamboo over traditional wood materials used for transparent wood manufacturing, there are still many challenges which have to be addressed. (1) After delignification with the sodium chlorite/glacial acetic acid solution, the structural integrity of the bamboo was destroyed; meanwhile, the transparent bamboo obtained exhibited yellow colour and a low transmittance of only 11% [18]. (2) Bamboo treated with H2 O2 (12 h) followed by NaClO solution (72 h) also loses its structural integrity, which is detrimental to its mechanical properties [41]. As a result, obtaining transparent bamboo composites with exceptional optical and mechanical qualities at the same time remains a difficulty. (3) Because of its poor permeability, removing the lignin from bamboo takes longer and requires more chemicals. The rate of resin filling is affected by permeability, which limits light transmission and the mechanical qualities of transparent bamboo. (4) The structure of bamboo differs greatly from that of wood from a structural standpoint [42]. As a result, when it is chemically treated, the treatment agent often has difficulty penetrating the material, and the desired effect of the modification is not obtained.

5.4 Recent Progress

51

(5) Recent research also found that when the TW fabrication technique was directly used to manufacture transparent bamboo, the transmittance of the obtained transparent bamboo was less than 10% [18]. (6) In order for transparent bamboo to become a suitable material for applications such as glass replacement in greenhouses, energy-saving buildings, skylights, smart windows and so forth in the future, it is necessary to achieve similar progress observed with traditional transparent wood where the dimensions of manufactured composites reached 185 cm in length [43] and 19 cm in thickness [44]. Large-scale transparent bamboo manufacturing is still extremely difficult due to the material’s intrinsic irregularities, hollow cylindrical shape with nodes, and large culm taper [21]. On the other hand, the dimensions of transparent bamboo only achieved dimensions of a few centimetres in length and a few millimetres in thickness. (7) One of the unique properties of transparent wood, which was described by Zhu et al., was its anisotropy [7]. However, due to the specific growth of bamboo, practically only longitudinal-orientated transparent bamboo can be manufactured in which the microstructure prevents the composite material from obtaining anisotropic properties. (8) Up to this date, no fully bio-based transparent bamboo (prepared from renewable bio-based polymers) was produced.

5.4 Recent Progress Many of these challenges have already been addressed in the latest research to this date. According to Wang et al., a strong and thermally insulating transparent bamboo composite was fabricated via an in situ lignin modification strategy combined with epoxy infiltration. As shown in Fig. 5.2, the first step was removing the lightabsorbing chromospheres (orthoquinone, conifer-aldehyde, and aromatic ketone) of lignin via the alkali H2 O2 treatment, while the aromatic skeletal lignin structure is preserved. Then, epoxy resin was infiltrated into the lignin-modified bamboo, and the transparent bamboo composite was then obtained after the solidification of the epoxy resin [17]. Moreover, sodium silicate (water glass) is a common commodity that is utilized to improve the peroxide bleaching of mechanical pulps since it is widely accessible, reasonably inexpensive, and relatively simple to use. Numerous hypotheses on the function of silicate in peroxide bleaching have been put forth [45– 47]. The precise stabilization mechanisms, however, are yet not fully understood. It was discovered that the results of the alkaline peroxide solution test did not always correspond to what was seen in a real pulp-bleaching environment. This suggests that transition metals behave somewhat differently in free solution than they do in a pulp suspension, where the activity of metals is dependent on location, activity state, counter ions, etc., [48]. Another research team led by Wu proposed a procedure to improve the light transmittance of transparent bamboo in the application of glass in the industrial

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5 Transparent Bamboo

Fig. 5.2 Fabrication of transparent bamboo and a scheme of the lignin modification mechanism according to [17]

field. They started by mixing 4 wt.% of sodium chlorite with water. Glacial acetic acid was added dropwise after fully stirring to generate a solution with a pH of 4.6. The dried bamboo samples were then placed in the solution, heated for 2–4 h at 80–90 °C, and subsequently microwaved (5–15 s). The samples were then heated for 2–3 h in a constant-temperature water tank before being stored in anhydrous ethanol for 24 h. The epoxy resin impregnation solution was made with a 2:1 ratio of epoxy resin to curing agent. The dried bamboo template samples were then immersed in the epoxy resin solution under a vacuum for 30 min. The impregnated samples were then placed in the silica gel sheet and cured at room temperature for more than 12 h to obtain transparent bamboo samples. The transmittance of transparent bamboo could reach up to about 15% [18]. Wang et al. reported that converting bamboo into a transparent material with great optical transmittance and good strength was achieved by pretreatment of the bamboo strips with 1% NaOH solution at 85 °C for 10 h and then using a 3% NaClO2 solution to delignify the samples at 85 °C for about 3 h until all bamboo strips became white. To evaluate the effect of the 1% NaOH precondition, delignified untreated bamboo was prepared at the same condition without the 1% NaOH precondition. Then a twopart epoxy resin (Clearcast 7000) was prepared according to the product instructions. The resulting transparent bamboo achieved a transmittance of 80% and a high haze of 81% [19].

5.5 Conclusion

53

Fig. 5.3 Method used to create flexible, large-diameter transparent bamboo [21]

Wu et al. used lamination to overcome the disadvantages of the current transparent bamboo veneer, which has a small thickness and limited light transmission. In their method, a 3.5 wt.% sodium chlorite aqueous solution was made, agitated thoroughly, and then glacial acetic acid was added dropwise to obtain a pH 4.6 solution. The bamboo samples were immersed in the solution for 23 h and then heated. The delignified bamboo templates were rinsed with distilled water and kept in anhydrous ethanol. The delignified bamboo templates with thicknesses of 0.3, 0.9, 1.5 and 2.1 mm were produced in this stage. In summary, the multilayer transparent bamboo exhibited high transmittance and superior mechanical properties [16]. Kurei et al. on the other hand found out that the best method for cellulose materials to maintain the original three-dimensional structure of bamboo is by optimizing the chemical process of alkaline treatment with alcoholysis followed by the Wise method [20]. The challenge of creating a large-scale transparent bamboo was recently addressed by Wang et al., who significantly forwarded the progress of transparent bamboo (up to 1.5 mm thickness) (Fig. 5.3). They used alkali pretreatment-cross-linkingdelignification and fabricated large-size and flexible transparent bamboo with excellent optical, mechanical (tensile strength was 78.5 MPa), and thermal properties (thermal conductivity of 0.35 W m−1 k−1 ) [21]. So far, there have been only several studies dealing with the fabrication of transparent bamboo. All of them used a different approach and obtained various results which are summarized in Table 5.1.

5.5 Conclusion In conclusion, raw bamboo provides several benefits compared with tree wood (e.g., the fast growth cycle of bamboo, bamboo releases 35% more oxygen than trees, higher tensile strength, insect resistance, abrasion resistance, water resistance, and

Lignin modification

NaClO2 + microwave processing

1% NaOH precondition + NaClO2 treatment

Alkali pretreatment-cross-linking-delignification

50 × 20 × 1.5

3 × 7.8 × 1.8

165 × 13 × 1

135 × 135 × 1.5

Length × width × t hickness

NaClO2 treatment

40 × 20 x max 2.1

a

Method

Dimensions (mm) (l × w × t)a

Epoxy resin

Epoxy resin (Clearcast 7000)

Epoxy resin (type E51)

Epoxy resin (type E51)

Epoxy resin (type E51)

Resin type

76.9

80%

15%

87%

0.3 mm thick—92.4% 1.2 mm thick—78.6%

Transmittance

Table 5.1 Preparation techniques of transparent bamboo from Moso bamboo (Phyllostachys heterocycla)

79.7

81%

–

90%

0.3 mm thick—43.5% 1.2 mm thick—70%

Haze

[21]

[14]

[13]

[12]

[11]

References

54 5 Transparent Bamboo

References

55

mould resistance). On top of that, these properties could be improved by manufacturing composite bamboo using different methods and various favourable fillers. The finished materials then achieve the desired mechanical properties, optical properties, low thermal conductivity, low coefficient of thermal expansion, and good water resistance which can be used as a new type of energy-efficient, renewable, safe and environmentally friendly material in plentiful applications. In addition, the utilization of thermosets is also suggested for future work since they facilitate composite recycling. Future research should now focus on environmentally friendly tailoring, with a life-cycle assessment that includes estimations of the total energy consumption and CO2 emissions, as well as a methodical application of green chemistry approaches.

References 1. H.-O. Pörtner et al. (eds.), Climate Change 2022: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. (Cambridge, UK, New York, NY, USA, Cambridge University Press, 2022) 2. T. Gurunathan, S. Mohanty, S.K. Nayak, A review of the recent developments in biocomposites based on natural fibres and their application perspectives. Compos. Part A Appl. Sci. Manuf. 77, 1–25 (2015). https://doi.org/10.1016/j.compositesa.2015.06.007 3. T. Li et al., Anisotropic, lightweight, strong, and super thermally insulating nanowood with naturally aligned nanocellulose. Sci. Adv. 4(3), eaar3724 (2022). https://doi.org/10.1126/sci adv.aar3724 4. W. Xu, X. Fang, J. Han, Z. Wu, J. Zhang, Effect of coating thickness on sound absorption property of four wood species commonly used for piano soundboards. Wood Fiber Sci. 52, 28–43 (2020). https://doi.org/10.22382/wfs-2020-004 5. H. Liu, J. Zhang, W. Jiang, Y. Cai, Characteristics of commercial-scale radio-frequency/vacuum (RF/V) drying for hardwood lumber. BioResources 14(3), 6923–6935 (2019) 6. S. Fink, Transparent wood—a new approach in the functional study of wood structure. Holzforschung—Int. J. Biol. Chem. Phys. Technol. Wood 46, 403 (1992). https://doi.org/10.1515/ hfsg.1992.46.5.403 7. M. Zhu et al., Highly anisotropic, highly transparent wood composites. Adv. Mater. 28(26), 5181–5187 (2016). https://doi.org/10.1002/adma.201600427 8. T. Li et al., Wood composite as an energy efficient building material: guided sunlight transmittance and effective thermal insulation. Adv. Energy Mater. 6(22), 1601122 (2016). https://doi. org/10.1002/aenm.201601122 9. H.S. Yaddanapudi, N. Hickerson, S. Saini, A. Tiwari, Fabrication and characterization of transparent wood for next generation smart building applications. Vacuum 146, 649–654 (2017). https://doi.org/10.1016/j.vacuum.2017.01.016 10. C. Jia et al., Scalable, anisotropic transparent paper directly from wood for light management in solar cells. Nano Energy 36, 366–373 (2017). https://doi.org/10.1016/j.nanoen.2017.04.059 11. W. Gan, L. Gao, S. Xiao, W. Zhang, X. Zhan, J. Li, Transparent magnetic wood composites based on immobilizing Fe3 O4 nanoparticles into a delignified wood template. J. Mater. Sci. 52(6), 3321–3329 (2017). https://doi.org/10.1007/s10853-016-0619-8 12. Z. Yu et al., Transparent wood containing CsxWO3 nanoparticles for heat-shielding window applications. J. Mater. Chem. A 5(13), 6019–6024 (2017). https://doi.org/10.1039/C7TA00 261K 13. C. Montanari, Y. Li, H. Chen, M. Yan, L.A. Berglund, Transparent wood for thermal energy storage and reversible optical transmittance. ACS Appl. Mater. Interfaces 11(22), 20465–20472 (2019). https://doi.org/10.1021/acsami.9b05525

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14. T. Zhang et al., Constructing a novel electroluminescent device with high-temperature and highhumidity resistance based on a flexible transparent wood film. ACS Appl. Mater. Interfaces 11(39), 36010–36019 (2019). https://doi.org/10.1021/acsami.9b09331 15. M. Wang et al., highly stretchable, transparent, and conductive wood fabricated by in situ photopolymerization with polymerizable deep eutectic solvents. ACS Appl. Mater. Interfaces 11(15), 14313–14321 (2019). https://doi.org/10.1021/acsami.9b00728 16. Y. Wu, J. Wang, Y. Wang, J. Zhou, properties of multilayer transparent bamboo materials. ACS Omega 6(49), 33747–33756 (2021). https://doi.org/10.1021/acsomega.1c05014 17. Y.-Y. Wang et al., High overall performance transparent bamboo composite via a ligninmodification strategy. Compos. Part B Eng. 235, 109798 (2022). https://doi.org/10.1016/j. compositesb.2022.109798 18. Y. Wu, Y. Wang, F. Yang, J. Wang, X. Wang, Study on the properties of transparent bamboo prepared by epoxy resin impregnation. Polymers (Basel) 12(4) (2020). https://doi.org/10.3390/ polym12040863 19. X. Wang, S. Shan, S.Q. Shi, Y. Zhang, L. Cai, L.M. Smith, Optically transparent bamboo with high strength and low thermal conductivity. ACS Appl. Mater. Interfaces 13(1), 1662–1669 (2021). https://doi.org/10.1021/acsami.0c21245 20. T. Kurei, R. Tsushima, Y. Okahisa, S. Nakaba, R. Funada, Y. Horikawa, Creation and structural evaluation of the three-dimensional cellulosic material. ‘White-Colored Bamboo’ 75(2), 180– 186 (2021). https://doi.org/10.1515/hf-2020-0030 21. K. Wang et al., Scalable, large-size, and flexible transparent bamboo. Chem. Eng. J. 451, 138349 (2023). https://doi.org/10.1016/j.cej.2022.138349 22. H. Rabemanolontsoa, S. Saka, Comparative study on chemical composition of various biomass species. RSC Adv. 3(12), 3946–3956 (2013) 23. L. Zou, H. Jin, W.-Y. Lu, X. Li, Nanoscale structural and mechanical characterization of the cell wall of bamboo fibers. Mater. Sci. Eng. C 29(4), 1375–1379 (2009) 24. L. Nayak, S.P. Mishra, Prospect of bamboo as a renewable textile fiber, historical overview, labeling, controversies and regulation. Fash. Text. 3(1), 1–23 (2016) 25. W. Lin, D. Chen, Q. Yong, C. Huang, S. Huang, Improving enzymatic hydrolysis of acidpretreated bamboo residues using amphiphilic surfactant derived from dehydroabietic acid. Bioresour. Technol. 293, 122055 (2019) 26. Z. Li et al., A strong, tough, and scalable structural material from fast-growing bamboo. Adv. Mater. 32(10), 1906308 (2020). https://doi.org/10.1002/adma.201906308 27. C. Montanari, Y. Ogawa, P. Olsén, L.A. Berglund, High performance, fully bio-based, and optically transparent wood biocomposites. Adv. Sci. 8(12), 2100559 (2021). https://doi.org/ 10.1002/advs.202100559 28. Y.Q. Xu, H. Chen, F. Yang, Effect of low-temperature plasma treatment on surface properties of bamboo silk decorative materials. Furniture 40, 21–27 (2019) 29. R. Vanholme, K. Morreel, J. Ralph, W. Boerjan, Lignin engineering. Curr. Opin. Plant Biol. 11(3), 278–285 (2008). https://doi.org/10.1016/j.pbi.2008.03.005 30. P.G. Dixon, L.J. Gibson, The structure and mechanics of Moso bamboo material. J. R. Soc. Interface 11(99), 20140321 (2014). https://doi.org/10.1098/rsif.2014.0321 31. M.S. Sulaiman et al., The classical mechanics engineered of Bambusa vulgaris and Schizostachyum brachycladum. J. Trop. Resour. Sustain. Sci 6 (2018). https://doi.org/10. 47253/jtrss.v6i2.561 32. A.J. Stamm, Density of wood substance, adsorption by wood, and permeability of wood. J. Phys. Chem. 33(3), 398–414 (1929). https://doi.org/10.1021/j150297a008 33. E. Jungstedt, C. Montanari, S. Östlund, L. Berglund, Mechanical properties of transparent high strength biocomposites from delignified wood veneer. Compos. Part A Appl. Sci. Manuf. 133, 105853 (2020). https://doi.org/10.1016/j.compositesa.2020.105853 34. W. Gan, S. Xiao, L. Gao, R. Gao, J. Li, X. Zhan, Luminescent and transparent wood composites fabricated by poly(methyl methacrylate) and γ-Fe2O3@YVO4:Eu3+ nanoparticle impregnation. ACS Sustain. Chem. Eng. 5(5), 3855–3862 (2017). https://doi.org/10.1021/acssusche meng.6b02985

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35. Y. Li, Q. Fu, R. Rojas, M. Yan, M. Lawoko, L. Berglund, A new perspective on transparent wood: Lignin-retaining transparent wood. ChemSusChem. 10 (2017). https://doi.org/10.1002/ cssc.201701089 36. A. Samanta, H. Chen, P. Samanta, S. Popov, I. Sychugov, L.A. Berglund, Reversible dualstimuli-responsive chromic transparent wood biocomposites for smart window applications. ACS Appl. Mater. Interfaces 13(2), 3270–3277 (2021). https://doi.org/10.1021/acsami.0c2 1369 37. M. Zhu et al., Isotropic paper directly from anisotropic wood: top-down green transparent substrate toward biodegradable electronics. ACS Appl. Mater. Interfaces 10(34), 28566–28571 (2018). https://doi.org/10.1021/acsami.8b08055 38. Z. Bi, T. Li, H. Su, Y. Ni, L. Yan, Transparent wood film incorporating carbon dots as encapsulating material for white light-emitting diodes. ACS Sustain. Chem. Eng. 6(7), 9314–9323 (2018). https://doi.org/10.1021/acssuschemeng.8b01618 39. C. Huang, W. Lin, C. Lai, X. Li, Y. Jin, Q. Yong, Coupling the post-extraction process to remove residual lignin and alter the recalcitrant structures for improving the enzymatic digestibility of acid-pretreated bamboo residues. Bioresour. Technol. 285, 121355 (2019). https://doi.org/10. 1016/j.biortech.2019.121355 40. W.-X. Li et al., Insights into bamboo delignification with acidic deep eutectic solvents pretreatment for enhanced lignin fractionation and valorization. Ind. Crops Prod. 170, 113692 (2021). https://doi.org/10.1016/j.indcrop.2021.113692 41. C. Zhou, I. Julianri, S. Wang, S.H. Chan, M. Li, Y. Long, Transparent bamboo with high radiative cooling targeting energy savings. ACS Mater. Lett. 3(6), 883–888 (2021). https://doi. org/10.1021/acsmaterialslett.1c00272 42. C. Huang et al., A sustainable process for procuring biologically active fractions of high-purity xylooligosaccharides and water-soluble lignin from Moso bamboo prehydrolyzate. Biotechnol. Biofuels 12(1), 189 (2019). https://doi.org/10.1186/s13068-019-1527-3 43. Q. Xia et al., In situ lignin modification toward photonic wood. Adv. Mater. 33(8), 2001588 (2021). https://doi.org/10.1002/adma.202001588 44. H. Li, X. Guo, Y. He, R. Zheng, A green steam-modified delignification method to prepare low-lignin delignified wood for thick, large highly transparent wood composites. J. Mater. Res. 34(6), 932–940 (2019). https://doi.org/10.1557/jmr.2018.466 45. J.L. Colodette, S. Rothenberg, C.W. Dence, Factors affecting hydrogen peroxide stability in the brightening of mechanical and chemimechanical pulps. I: Hydrogen peroxide stability in the absence of stabilizing systems. J. pulp Pap. Sci. 14(6), J126–J132 (1988) 46. L. Backman, G. Gellerstedt, Reactions of kraft pulp with alkaline hydrogen peroxide, in Proceedings 7th International Symposium Wood and Pulping Chemistry 1993 Conference, (1993) pp. 240–248 47. M. Fairbank, J.L. Colodette, T. Ali, E.L. Mclellan, P.W. Whitting, The role of silicate in peroxide brightening of mechanical pulp. IV: the role of silicate as a buffer during peroxide brightening (1989) 48. H. Hämäläinen, R. Aksela, J.M. Rautiainen, M. Sankari, I. Renvall, R. Paquet, Silicate-free peroxide bleaching of mechanical pulps : efficiency of polymeric stabilizers (2007)

Chapter 6

Solar Cells

Abstract Traditional energy production methods, such as burning fossil fuels, are harmful to the earth’s environment, resulting in major, planet-scale issues, and climate change. As a result, renewable and green energy technologies have become increasingly popular in recent years. Improvements in solar panel efficiency, in particular, have piqued researchers’ interest, as the sun is a year-round available source of energy that may be used efficiently for energy generation. However, due to reflection at the air/glass interface, a considerable portion of the incident solar energy is lost. The increased need for flexible electronics and solar energy conversion devices has fuelled a pursuit of high-quality paper-based materials with good mechanical flexibility and optical qualities including high transparency and haze. High optical transmittance and haze, superior mechanical characteristics, a smooth surface, and low thermal conductivity characterize transparent paper and wood. This qualifies it as a solar cell assembly substrate with potential in energy-efficient construction applications. High optical transparency is required for solar cell substrates, but the high optical haze is preferred to maximize light dispersion and, as a result, absorption in the active components.

6.1 Introduction The current global energy demand is 16 TW; by 2050, it is predicted to exceed 30 TW. The global energy situation is extremely concerning and is always under discussion. The sun provides approximately 174 × 103 TW of energy to the earth’s surface each year, of which almost 30% is reflected in space. This amount is 10,000 times greater than the world’s real annual energy use. The newest assessment also shows that a minimum of 50% of the total global rooftop area is required to meet the yearly global aggregated electricity demand [1]. Due to the year-round quantity of sunlight and technological breakthroughs in capturing light energy, solar energy has recently emerged as the most important and accessible source of renewable energy among the numerous sustainable energy alternatives. Solar photovoltaic power cells have become the primary means of harnessing solar energy over time since they are not only renewable but also safe and pollution free [2]. Photovoltaic arrays on © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 I. Wachter et al., Transparent Wood Materials, Springer Series in Materials Science 330, https://doi.org/10.1007/978-3-031-23405-7_6

59

60

6 Solar Cells

Fig. 6.1 Light propagation through (a) a single-layer coating and (b) multilayer coating on a substrate (ns > nn, ns and nn are the refractive indices of substrate and coatings, respectively [4]

their own are a relatively inexpensive and efficient way to generate electricity. As the demand for improved functionalized materials grows, surface coating processes and materials tend to evolve. Self-cleaning, photoactivity, and high transparency are some of the most desired characteristics of solar panels [3]. Light will not be reflected off the surface if destructive interference exists between the light reflected from the coating–substrate interface and the light reflected from the air–coating interface, as shown in Fig. 6.1 [4]. Plastic has been employed as a transparent flexible substrate in electronic and optoelectronic devices due to its favourable properties [5–8]. On the other hand, it is neither biodegradable nor renewable. For upcoming optical engineering applications, transparent lignocellulose nanopaper is a promising candidate light-management material. Lignin plays a crucial functional role in lignocellulose nanopaper matrices, much like it does in plant cell walls. However, lignocellulose nanopaper is unsuitable for a number of optical management systems due to its inherent light absorption feature. Because of its excellent biodegradability and renewability, cellulose-based transparent paper has recently attracted more interest as a developing flexible substrate [9–11]. Disintegrating the cellulose fibres via mechanical [12, 13], chemical [14– 16], or biological [17, 18] means, dispersing into solution, and finally reconstructing into transparent paper are all processes in the cellulose-based transparent paper manufacture process. As the foundation for optoelectronic devices, substrates are crucial. These substrates’ mechanical, optical transparency, and maximum processing temperature are all important features that determine their suitability for diverse applications. For flexible electronics, the optoelectronic device sector primarily uses glass and plastic substrates; however, recent research shows that transparent nanopaper based on renewable cellulose nanofibers could replace plastic substrates in many electronic and optoelectronic devices [19–22]. Because it is made of natural materials, nanopaper is an environmentally friendly substrate compared with plastic. The percentage of transmitted light that diffusely scatters is measured by optical haze,

6.1 Introduction

61

which is preferable in solar cell applications [23, 24]. Since transparent wood is more environmentally sustainable and is determined to be safer than polyethylene, it may eventually replace it [25]. Although optical haze is ideally maximized on transparent substrates integrated into solar devices, other optoelectronic devices, such as displays and touch screens, require various amounts of light scattering; for example, displays and touch screens demand excellent clarity and low optical haze [26, 27]. Lightmanagement capabilities, in addition to mechanical flexibility and biodegradability, are critical for flexible electronics and solar energy conversion devices, particularly thin film solar cells [28, 29]. High optical transparency is necessary for solar cell substrates. High optical haze is preferred to maximize light scattering and, as a result, the absorption in the active components (Fig. 6.2). The anisotropic transparent paper exhibits a very high light transmittance and haze. The light transmittance improves slightly with wavelength, reaching up to 90% when the wavelength is 800 nm. In the wavelength range of 400–800 nm, the anisotropic transparent paper also exhibits a very high haze. At a wavelength of 400 nm, the haze can reach 93% and gradually declines as the wavelength increases [30]. In addition to using the modified wood pulp to produce transparent paper for light management in solar panels, there have been studies using transparent wood directly as the substrate for solar cells. Wood is by far the most important structural material derived from renewable resources. The development of transparent wood technology has been extensively reviewed in recent studies [31–33]. High optical transmittance and haze are characteristics of transparent wood (over 70%) which makes it a great material to incorporate into solar panels.

Fig. 6.2 Diagram showing the distribution of incident light on a solar cell

62

6 Solar Cells

6.2 Recent Progress One of the earliest studies which was able to manufacture a novel transparent paper made of wood fibres with enhanced optical transparency (∼96%) and haze (∼60%) which is the most suitable substrate for solar cells and at a much lower cost was made by Fang et. al. They verified this with an organic solar cell by simply laminating a piece of transparent paper and observing a 5.34−5.88% increase in power conversion efficiency. To incorporate carboxyl groups into the cellulose, the primary wood fibres were treated using a TEMPO/NaBr/NaClO oxidation system. The hydrogen bonds between cellulose fibrils were weakened, causing the wood fibres to inflate and collapse, resulting in a high packing density and outstanding optical characteristics [34]. Another team led by Jia created a simple, scalable, and efficient method for fabricating anisotropic flexible paper with high transparency (~90%) and haze (~90%) by directly shear-pressing delignified wood resulting in a structural anisotropy which enables the transparent paper to uniquely scatter and polarize incoming light. Due to effective light scattering and increased light absorption, using anisotropic transparent paper as a light-management coating layer for GaAs solar cells can result in a 14% increase in overall energy conversion efficiency and an 18% increase in short circuit density. To remove the lignin, the wood slices were immersed in a 5% NaClO solution, and the reaction was carried out at room temperature until the wood slices were fully white. Basswood and NaClO solution had a mass ratio of 1:60. After that, the delignified white wood slices were rinsed three times in ethanol-water solution (50 wt.%) to eliminate any leftover chemicals. To prepare the transparent paper, a PET film was used to cover the delignified radial wood slice, which was then covered with a microporous membrane filter. By rolling a glass rod against the wood slice with steady pressure, the wood fibres were pushed down in one direction. Because of the strong hydrogen bonding, all of the wood fibres in the wood slice were drawn together and produced an aligned wood slice. The aligned wood slice was then covered with filter sheets and pressed at room temperature for 5 h [30]. Currently, solar cells, which are renewable energy sources, are the most efficient technology to convert natural energy to usable energy [35]. Perovskites are promising solar materials due to their particular characteristics like high electron mobility (800 cm2 Vs−1 ), high carrier lifetime (exceeding 300 ns), high absorption coefficient (greater than 105 cm−1 ), the low exciton binding energy (less than 10 meV), high photo luminesces (PL) quantum efficiency (as high as 70%), high carrier diffusion length (exceeding 1 μm), optimum band gap, and excellent structural defect tolerance [36–38]. In the study of Li et al. for the first time, perovskite solar cells with a power conversion efficiency of up to 16.8% were successfully assembled on optically transparent wood substrates, using a low-temperature process below 150 °C. Long-term stability was also demonstrated by the devices (It was found that the devices could retain 77% of their initial performance after 720 h of ageing). Their findings show that transparent wood might be used to assemble sustainable solar cells instead of

6.2 Recent Progress

63

Fig. 6.3 Methods used to produce patterned lignocellulose nanopaper and lignin-modified lignocellulose nanopaper [41]

glass, lowering the device’s carbon footprint [39]. Transparent wood preparation was performed according to the previous work of Li et al. [40]. The whole assembly procedure is explained in the article supporting information [39]. Recently, Jiang et al. created a lignin-modified lignocellulose nanopaper (LNP) that was superior to cellulose nanopapers in terms of UV protection and water resistance while maintaining well-preserved lignin structures. It had a comparable optical transmittance (90%). This transparent LNP was also shown to be a practical lightmanagement material that can greatly boost a GaAs solar cell’s power conversion efficiency (Fig. 6.3) [41]. To create a stable wet gel cake, the degassed lignocellulose nanofibril-water suspension was vacuum-filtered over a glass filter with a hydrophilic polytetrafluoroethylene membrane. The as-prepared wet lignocellulose nanofibril cake was stacked between two sheets of blotting paper for six hours before being subjected to either a cold-pressing procedure at 30 °C under a load of 250 N for 48 h or a hot-pressing procedure at 90 °C under a load of 250 N for eight hours to produce cold-pressed and hot-pressed LNP samples, respectively [41]. Table 6.1 summarizes the recent progress of transparent wood/paper preparation, with the most important mechanical and optical properties. The production of solar panels is quite demanding. The conversion efficiency of commercial photovoltaic panels is around 20%, which means large, irradiated areas have to be covered by them to provide sufficient power output. Therefore, if transparent paper/wood should become a reliable and commercially used solar cell substrate which increases efficiency, it is necessary to develop a simple, rapid, large-scale, and controllable manufacturing process. A new strategy for modifying lignin using an in situ, rapid, and scalable process that involves photocatalytic oxidation of native lignin in wood by H2 O2, and UV light was reported by Xia et al. Wood’s 3D hierarchically aligned open structures allow quick H2 O2 entry and effective UV light penetration, making thick photonic wood samples possible. The lignin-modified photonic wood has greater advantages as a structural material than delignified wood, including stronger wet mechanical

5 wt.% NaClO solution

TEMPO-oxidized system

1 wt.% NaClO2 solution

Basswood [30]

Southern yellow pine pulp [34]

Balsa wood (Ochroma pyramidale) [39]

Sugarcane bagasse fiber H2 O2 + UV light [41]

Method

Wood type

~ 45 μ m 89

86

∼ 96

50 μm 1.0 mm

~ 90

Transmittance (%)

0.8 mm

Thickness

Table 6.1 Properties of the transparent wood/paper and procedure conditions

90

70

∼60

~90

Haze (%)

−

0.67 MPa m1/2

3.2 MPa m1/2 38 MPa

∼ 1.88 J.m−3

−

− ∼ 105 MPa

Toughness

Tensile strength

–

16.8%

Increased from 5.34 to 5.88%

14%

Power conversion efficiency

64 6 Solar Cells

6.3 Conclusion

65

strength, improved water stability, superior scalability, and patterning possibilities. This photocatalytic oxidation approach was used to fabricate a huge sheet of photonic wood with a length of up to 185 cm, demonstrating its considerable potential for industrial-scale manufacture [42]. Similarly, to increase the transmittance of transparent wood, Chen et al. describe a method for UV-assisted delignification of wood in an alkaline environment [43]. A similar approach was presented by researchers from the University of Maryland who demonstrate a rapid, cost-effective, and sustainable method to fabricate patternable transparent wood based on a scalable solar-assisted chemical brushing method. The light-absorbing chromophore groups of lignin were eliminated during this process, improving the optical qualities of the resulting transparent wood without completely losing the aromatic structure. The solar-assisted chemical brushing offers greater production efficiency, and lower cost, and is more sustainable and controllable than solution-based delignification procedures. This low-cost, high-efficiency transparent wood manufacturing technology can also be used with solar energy, broadening the technique’s application to large-scale industrial production [44]. Despite that sodium silicate (water glass) is a common commodity that is utilized to improve the peroxide bleaching of mechanical pulps since it is widely accessible, reasonably inexpensive, and relatively simple to use. Numerous hypotheses on the function of silicate in peroxide bleaching have been put forth [45−47]. The precise stabilization mechanisms, however, are yet not fully understood. It was discovered that the results of the alkaline peroxide solution test did not always correspond to what was seen in a real pulp-bleaching environment. This suggests that transition metals behave somewhat differently in free solution than they do in a pulp suspension, where the activity of metals is dependent on location, activity state, counter ions, etc., [48]. Methods of large-scaled transparent wood scaffolds and the corresponding mechanical and optical properties are given in Table 6.2.

6.3 Conclusion The significance of renewable energy sources, including solar energy, was emphasized, and the use of thin film coatings to increase solar panel efficiency was thoroughly reviewed. It is commonly known that coating solar panels with antireflective and self-cleaning layers simultaneously is necessary; otherwise, the practical relevance of coating solar modules will be diminished. The optical properties of transparent wood/paper are tunable (transmittance and haze), and they also have great mechanical qualities, a smooth surface, and low heat conductivity. This qualifies it as a substrate for solar cell construction with potential in applications for energy-efficient buildings. The optical properties of the transparent wood can be adjusted through molecular and nanoscale materials design, leading to greater solar cell efficiency. The low-cost, high-efficient, eco-friendly, and sustainable manufacturing technologies employing the photocatalytic oxidation process may create a new

2–8

83

H2 O2 :DI water:ammonia = 5:5:1 + UV

Balsa wood [43]

1 (UV Index: 7 to 8)

≈ 85

brushed H2 O2 30 wt.% + NaOH 10 wt.% + solar irradiation

Balsa wood [44]

1–6.5

≈ 80

Immersion (10% NaOH + 30% H2 O2 ) + 20 W UV light irradiation

Balsa wood [42]

Processing time [hrs]

Lignin retained (%)

Method

Wood type

Table 6.2 Methods and properties of large-scaled transparent wood

82%

> 90%

−

Transmittance

75%

60–80%

−

Haze

>88%

−

−

90–96%

Reflectivity

31.4–46.2

≈ 20 (wet)

Tensile strength (MPa)

≈ 1.5

3.3 mm

9 mm

Thickness

66 6 Solar Cells

References

67

option for large-scale industrial production. These findings demonstrate that such transparent materials with high levels of haze and light transmission may be produced quickly with the aid of sunlight’s potent UV rays. The method’s adaptability makes it appealing for creating environmentally friendly materials for prospective solar cell substrate applications. The high-quality, quick, and environmentally friendly preparation of various derived useful materials based on delignified wood is also made possible by this process.

References 1. S. Joshi, S. Mittal, P. Holloway, P.R. Shukla, B.Ó. Gallachóir, J. Glynn, High resolution global spatiotemporal assessment of rooftop solar photovoltaics potential for renewable electricity generation. Nat. Commun. 12(1), 5738 (2021). https://doi.org/10.1038/s41467-021-25720-2 2. A. Sokliˇc, M. Tasbihi, M. Kete, U.L. Štangar, Deposition and possible influence of a selfcleaning thin TiO2 /SiO2 film on a photovoltaic module efficiency. Catal. Today 252, 54–60 (2015). https://doi.org/10.1016/j.cattod.2014.10.021 3. M.S. Mozumder, A.-H.I. Mourad, H. Pervez, R. Surkatti, Recent developments in multifunctional coatings for solar panel applications: a review. Sol. Energy Mater. Sol. Cells 189, 75–102 (2019). https://doi.org/10.1016/j.solmat.2018.09.015 4. L. Yao, J. He, Recent progress in antireflection and self-cleaning technology—from surface engineering to functional surfaces. Prog. Mater. Sci. 61, 94–143 (2014). https://doi.org/10. 1016/j.pmatsci.2013.12.003 5. P. Heremans et al., Mechanical and electronic properties of thin-film transistors on plastic, and their integration in flexible electronic applications. Adv. Mater. 28(22), 4266–4282 (2016). https://doi.org/10.1002/adma.201504360 6. G. Schwartz et al., Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring. Nat. Commun. 4(1), 1859 (2013). https://doi.org/10. 1038/ncomms2832 7. P. Zhu et al., Direct conversion of perovskite thin films into nanowires with kinetic control for flexible optoelectronic devices. Nano Lett. 16(2), 871–876 (2016). https://doi.org/10.1021/acs. nanolett.5b03504 8. Y. Liang, Z. Chen, Y. Jing, Y. Rong, A. Facchetti, Y. Yao, Heavily n-Dopable π-conjugated redox polymers with ultrafast energy storage capability. J. Am. Chem. Soc. 137(15), 4956–4959 (2015). https://doi.org/10.1021/jacs.5b02290 9. Y.H. Jung et al., High-performance green flexible electronics based on biodegradable cellulose nanofibril paper. Nat. Commun. 6, 7170 (2015). https://doi.org/10.1038/ncomms8170 10. D. Gaspar et al., Nanocrystalline cellulose applied simultaneously as the gate dielectric and the substrate in flexible field effect transistors. Nanotechnology 25(9), 94008 (2014). https:// doi.org/10.1088/0957-4484/25/9/094008 11. Z. Fang et al., Highly transparent paper with tunable haze for green electronics. Energy Environ. Sci. 7(10), 3313–3319 (2014). https://doi.org/10.1039/C4EE02236J 12. K. Abe, F. Nakatsubo, H. Yano, High-strength nanocomposite based on fibrillated chemithermomechanical pulp. Compos. Sci. Technol. 69(14), 2434–2437 (2009). https://doi.org/10. 1016/j.compscitech.2009.06.015 13. M. Jonoobi et al., Different preparation methods and properties of nanostructured cellulose from various natural resources and residues: a review. Cellulose 22(2), 935–969 (2015). https://doi. org/10.1007/s10570-015-0551-0 14. K. Zhang, P. Sun, H. Liu, S. Shang, J. Song, D. Wang, Extraction and comparison of carboxylated cellulose nanocrystals from bleached sugarcane bagasse pulp using two different oxidation methods. Carbohydr. Polym. 138, 237–243 (2016)

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15. L. Chen, J.Y. Zhu, C. Baez, P. Kitin, T. Elder, Highly thermal-stable and functional cellulose nanocrystals and nanofibrils produced using fully recyclable organic acids. Green Chem. 18(13), 3835–3843 (2016). https://doi.org/10.1039/C6GC00687F 16. C. Jia, L. Chen, Z. Shao, U. Agarwal, L. Hu, J.Y. Zhu, Using a fully recyclable dicarboxylic acid for producing dispersible and thermally stable cellulose nanomaterials from different cellulosic sources. Cellulose 24 (2017). https://doi.org/10.1007/s10570-017-1277-y 17. W. Wang, R. Sabo, M. Mozuch, P. Kersten, J.Y. Zhu, Y. Jin, Physical and mechanical properties of cellulose nanofibril films from bleached eucalyptus pulp by endoglucanase treatment and microfluidization. J. Polym. Environ. 23 (2015). https://doi.org/10.1007/s10924-015-0726-7 18. I.A. Sacui et al., Comparison of the properties of cellulose nanocrystals and cellulose nanofibrils isolated from bacteria, tunicate, and wood processed using acid, enzymatic, mechanical, and oxidative methods. ACS Appl. Mater. Interfaces 6(9), 6127–6138 (2014). https://doi.org/10. 1021/am500359f 19. J. Huang et al., Highly transparent and flexible nanopaper transistors. ACS Nano 7(3), 2106– 2113 (2013). https://doi.org/10.1021/nn304407r 20. H. Zhu et al., Biodegradable transparent substrates for flexible organic-light-emitting diodes. Energy Environ. Sci. 6(7), 2105–2111 (2013). https://doi.org/10.1039/C3EE40492G 21. M. Nogi, N. Komoda, K. Otsuka, K. Suganuma, Foldable nanopaper antennas for origami electronics. Nanoscale 5(10), 4395–4399 (2013). https://doi.org/10.1039/C3NR00231D 22. G. Zheng, Y. Cui, E. Karabulut, L. Wågberg, H. Zhu, L. Hu, Nanostructured paper for flexible energy and electronic devices. MRS Bull. 38, 320–325 (2013). https://doi.org/10.1557/mrs. 2013.59 23. Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, L. Han, Dye-sensitized solar cells with conversion efficiency of 11.1%. Jpn. J. Appl. Phys. 45(25), L638–L640 (2006). https://doi.org/ 10.1143/jjap.45.l638 24. M. Berginski et al., The effect of front ZnO: Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells. J. Appl. Phys. 101(7), 74903 (2007) 25. R. Rai, R. Ranjan, P. Dhar, Life cycle assessment of transparent wood production using emerging technologies and strategic scale-up framework. Sci. Total Environ. 846, 157301 (2022). https://doi.org/10.1016/j.scitotenv.2022.157301 26. W.A. MacDonald et al., Latest advances in substrates for flexible electronics. J. Soc. Inf. Disp. 15(12), 1075–1083 (2007). https://doi.org/10.1889/1.2825093 27. D.S. Hecht et al., Carbon-nanotube film on plastic as transparent electrode for resistive touch screens. J. Soc. Inf. Disp. 17(11), 941–946 (2009) 28. Z. Yu, A. Raman, S. Fan, Thermodynamic upper bound on broadband light coupling with photonic structures. Phys. Rev. Lett. 109(17), 173901 (2012). https://doi.org/10.1103/PhysRe vLett.109.173901 29. Y. Jiang et al., Enhancement of photovoltaic performance by utilizing readily accessible hole transporting layer of vanadium(V) oxide hydrate in a polymer-fullerene blend solar cell. ACS Appl. Mater. Interfaces 8(18), 11658–11666 (2016). https://doi.org/10.1021/acsami.6b02824 30. C. Jia et al., Scalable, anisotropic transparent paper directly from wood for light management in solar cells. Nano Energy 36, 366–373 (2017). https://doi.org/10.1016/j.nanoen.2017.04.059 31. L.A. Berglund, I. Burgert, Bioinspired wood nanotechnology for functional materials. Adv. Mater. 30(19), 1704285 (2018). https://doi.org/10.1002/adma.201704285 32. F. Jiang et al., Wood-based nanotechnologies toward sustainability. Adv. Mater. 30(1), 1703453 (2018). https://doi.org/10.1002/adma.201703453 33. Y. Li, E. Vasileva, I. Sychugov, S. Popov, L. Berglund, Optically transparent wood: recent progress, opportunities, and challenges. Adv. Opt. Mater. 6(14), 1800059 (2018). https://doi. org/10.1002/adom.201800059 34. Z. Fang et al., Novel nanostructured paper with ultrahigh transparency and ultrahigh haze for solar cells. Nano Lett. 14(2), 765–773 (2014). https://doi.org/10.1021/nl404101p 35. P. Roy, N. Kumar Sinha, S. Tiwari, A. Khare, A review on perovskite solar cells: Evolution of architecture, fabrication techniques, commercialization issues and status. Sol. Energy 198, 665–688 (2020). https://doi.org/10.1016/j.solener.2020.01.080

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Chapter 7

Smart Windows

Abstract Wood is an available and sustainable substrate which has the potential for large-scale nanotechnology functionalization. Most materials used in optical lighting applications must create a homogeneous illumination and have high mechanical and hydrophobic requirements. But they are rarely environmentally beneficial. The large heat loss/gain through windows contributes to high energy consumption in buildings. Furthermore, the traditional glass fabrication method causes numerous environmental issues. Transparent wood-based composites are gaining importance in smart window applications. To save energy, a novel material called optically transparent wood is being developed for light-transmitting structures in buildings. This material combines optical and mechanical performance. Buildings that are eco-friendly and energy efficient are desirable from a sustainability standpoint, especially considering the current global energy and environmental crisis. Therefore, this chapter highlights the recent progress and applications of transparent wood in the field of smart windows.

7.1 Introduction The need for energy consumption is rising as the global economy expands. Most of this energy comes from fossil fuels like coal, oil, and natural gas, all of which produce greenhouse gases and contribute to global warming. Recent reports claim that the global temperature has been steadily rising for the past 100 years [1, 2]. Today, more emphasis is placed on various energy-saving options in connection with modern housing in smart buildings. Actual heat leakage is highly dependent on individual buildings, but experts agree that the most sensitive places of the perimeter structure are windows and doors. Approximately 50% of total energy consumption is lost through windows through heat exchange between the building and the external environment [3]. At present, the use of triple glazing is already standard, i.e., three glass panes connected by the so-called warm frame that eliminates thermal glazing bridges. When choosing a suitable type of glass, it is necessary to consider the planned method of shading. We should not forget the possibility of using glass to increase safety or reduce noise from the external environment. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 I. Wachter et al., Transparent Wood Materials, Springer Series in Materials Science 330, https://doi.org/10.1007/978-3-031-23405-7_7

71

72

7 Smart Windows

A large portion of the world’s energy is used in the building sector, primarily as a source of light. A new trend in modern housing is the so-called smart windows, which with their properties meet the criteria in terms of energy savings and modern technology. Current innovative glazing technologies include low heat loss control, highly insulated windows and intelligent windows controlled by solar heat and daylight. Smart windows provide at least two throughput states that make them more than sufficient to cope with the changing nature of the environment, and in this respect, they overcome static simple transparent glazing [4]. Typical types of electrically activated smart windows include polymer dispersed liquid crystal, electrochromic and suspended particle devices [4–6]. Utilizing natural light with the help of environmentally friendly resources such as transparent wood is one technique to reduce the consumption of electricity in buildings. The material has a lot of benefits, including abundant resources, low density, excellent mechanical properties, renewability, ease of manufacture, and low manufacturing costs. The most astounding material property, optical transparency, has numerous uses in a variety of industries, from the building sector to photonics technology. Transparent wood can be a fascinating substitute to create optical activity, first and foremost optical gain and perhaps lasing, due to its compatibility with polymer technology [7]. Active chemicals, like dye molecules, can be incorporated into hierarchical wood structures to produce an effective optical gain material. Buildings using energy-saving materials offer significant reductions in indoor energy usage as an efficient light and heat managing material and are essential for the development of a carbon–neutral society. In terms of lighting, such a structure may fully utilize natural light to provide adequate lighting and thermal comfort in the building, while reducing the growing burden of increased energy consumption and pollution [8–9]. In both residential and commercial buildings, glass has long been employed as a light-harvesting material (e.g., windows and rooftops). On the other hand, it has a poor light-management capability aside from transmission, resulting in non-uniform illumination and uncomfortable glare, not to mention their brittleness and safety concerns. Wood is an abundant and environmentally benign substrate that can be functionalized using large-scale nanotechnologies. It provides great mechanical qualities in relation to its cost because of its unique structure and natural growing process. Current methods for making transparent wood composites rely on a complete (or almost complete) delignification process, which involves eliminating the majority of light-absorbing compounds (lignin and extractives) [10–12] or chromophoric components but leaving around 80% of lignin [13]. A variety of high-tech methods using woody, grassland, and vine plants are used to process and create biomass materials, which are new materials with good performance and high added value [14, 15]. Materials made of biomass have cheap processing costs, are readily available, and are renewable. If renewable biomass materials are used, developing and applying new materials can effectively alleviate the significant issues of energy tension and environmental pollution in the current condition of depletion of non-renewable resources. Transparent wood can be produced by removing lignin or modifying lignin chromophores in wood and then impregnating it with ecologically friendly polymers

7.1 Introduction

73

whose refractive index matches that of the cell wall. Although transparent wood was originally developed for the study of wood anatomy [12], it is now regarded as a wood-based composite for advanced functional (and simultaneously load-bearing) applications such as white-light-emitting diodes [16, 17], solar cells [18, 19], luminescent [20, 21] and heat shielding [22, 23] applications in panels and windows, application in transparent decoration [24], X-ray shielding [25], and organic material for non-cavity laser [7]. The development of this optical material included the addition of novel capabilities like luminescence and photoluminescence as well as switchable electrochromic and photochromic characteristics [21, 26–28]. When cellulose composite film is attached to the glass window, it offers a soft, consistent, and wide-area illumination toward the comfortable, healthy, but energy-saving living environment as shown in Fig. 7.1 [29]. One of the most important parameters for the use of transparent wood as a substitute for traditional windows is the ratio of the transmitted light flux to incoming luminous flux known as the transmission ratio, also known as transparency, transmittance, or transmission coefficient. It is mainly dependent on the thickness of the used wood scaffold, method of delignification (or lignin modification), and used infiltrating polymer. According to Table 7.1, the minimum visible transmittance ratio is 89% when flat glass is 2 mm in nominal thickness. As nominal thickness rises, the minimum transmittance ratio gradually declines, and when flat glass is 25 mm in thickness, the minimum transmittance ratio should be 67% [30].

Fig. 7.1 Broadband light management using the cellulose composite film connecting to the glass window [29]

74 Table 7.1 Minimum transparency of flat, colourless glass for visible light transmission [30]

7 Smart Windows Nominal thickness [mm]

The minimum value of visible light transmission ratio [%]

2

89

3

88

4

87

5

86

6

85

8

83

10

81

12

79

15

76

19

72

22

69

25

67

During the last few years, there have been numerous proposals on different fabrication methods and applications of transparent wood as light and heat-managing biocomposite which can be used in the building sector. It is important to note that substantial progress toward developing a commercially feasible transparent wood has been made during this period.

7.2 Recent Progress In 2017, Yaddanapudi et al. manufactured transparent wood with a maximum optical transmittance of 70% and a maximum haze of 49% for 0.1 and 0.7 mm thick wood samples using a delignifying solution containing 5 wt.% sodium chlorite in acetate buffer solution for 12 h at 95 °C [31]. Sun et al. created a broadband light-management cellulose composite film, composed of 75 wt.% cellulose bonded by phenol–formaldehyde resin, which shows high transparency (~86%) and haze (~90%) for effective light propagating and scattering, and a superior antiultraviolet capability (~83% absorptance and ~17% reflectance), thus enabling a soft, uniform, large areal, and safe illumination of the sunlight in buildings toward a comfortable, healthy yet energy-saving environment. They used a coniferous wood pulp to manufacture the cellulose film which was immersed in phenol-formaldehyde resin/ethanol solution for 20 min and hot pressed at the temperature of 80, 110, 140, and 160 °C with a pressure of ~20 MPa for 10 min [29]. The work of Höglund et al. investigated a thiolene thermoset as a new polymer category for eco-friendly biocomposites for applications where high optical transmittance and low haze are desirable. The lignin of balsa veneers (thickness of 0.9–0.2 mm) was modified according to previous work [10]. By submerging wood

7.2 Recent Progress

75

templates in ethanol and acetone under low pressure, the solvents were exchanged sequentially. The UV-initiator 1-hydroxycyclohexyl phenyl ketone was combined with stoichiometric combinations based on the thiol and allyl functionalities of PETMP (pentaerythritol tetrakis(3-mercaptopropionate) and TATATO (1,3,5-triallyl1,3,5-triazine-2,4,6(1H,3H,5H)-trione) (0.5wt.%). After an overnight infiltration at 55 °C, the templates were vacuum infiltrated for 5 h and sandwiched between glass slides and exposed to four 9 W 365 nm UV lamps. Due to the significant decrease in interfacial air gaps, the more environmentally friendly bleached wood template produces transparent wood with great optical transmission and significantly reduced transmission haze [32]. To enable efficient polymer infiltration, the extensive chemical treatment might severely destroy the original wood structure (e.g., the cell wall is partially degraded, and the growth ring patterns become less visible). Therefore, in the work of Mi et al., an aesthetic transparent wood was developed by spatial selectively removing the lignin of native wood material to make wood transparent and preserve its natural patterns simultaneously. Douglas fir wood was delignified (NaClO2 solution, 2 h, 4.6 pH), and infiltrated with a refractive-index-matched epoxy and it demonstrated excellent functions of optical transparency, UV-blocking, thermal insulation, mechanical strength, scalability, and aesthetics [33]. More designs can be made by layering appealing wood in a multilayered structure. For instance, different lattice shapes can be made by stacking two layers of aesthetically pleasing wood that are twisted at an angle concerning one another. Due to the high transmittance and inherent appeal of this feature, it may be possible to use it in the future on patterned ceilings (Fig. 7.2) [33]. The utilization of low-cost agriculture residues such as wheat straw and converting them into high-valued transparent composites with improved thermal insulation properties for light-transmitting building applications is critical for achieving long-term development. Tong et al. used a standard procedure to delignify wheat straw using NaOH (10 wt.%), Na2 SO3 (5 wt.%), and H2 O2 (30 wt.%) and impregnating wheat straw fibres with methyl methacrylate to manufacture transparent composites using a wheat straw with excellent properties (thickness of 3 mm, 30 wt.% of biocomponent achieved light transmittance of 74.63%, a haze of 54.87%). Transparent composite wheat straw (TCWS), as a result, has the potential to replace transparent wood in

Fig. 7.2. Possible arrangement of two layers of aesthetic wood templates [33]

76

7 Smart Windows

construction. Furthermore, the production of transparent composites maximizes the use of natural resources while adhering to the principle of sustainable development [34]. The potential application of transparent wood also includes indoor optical lighting management. These materials need to produce uniform illumination and require high mechanical and hydrophobic properties. They are, however, rarely environmentally friendly. Fu et al. created a bio-based, polymer matrix-free, luminescent, and hydrophobic film with excellent mechanical properties for use in optical lighting. To incorporate an organic CdSe/ZnS quantum dots solution, the microstructure of the delignified template was modified and tailored using a mild chemical treatment process. The impregnated quantum dots/treated wood was then compressed and dried before being coated with a thin hydrophobic hexadecyltrimethoxysilane layer via chemical vapour deposition, resulting in a material that could be used for a variety of optical applications such as lighting panels, indicators, photonics, and laser devices [35]. The luminescent and hydrophobic wood films from previous research could be enhanced and modified by a procedure developed by Höglund et al. who infiltrated gold and silver salt nanoparticles, which were reduced in situ to plasmonic nanoparticles via microwave-assisted synthesis into delignified wood to produce load-bearing materials with structural colour [36]. The harvesting of diffused visible light is made possible by using lignin-modified lignocellulose nanopaper prepared by Jiang et al., as a light-management layer atop the conventional glass. High transparency, haze, UV protection, an effective and affordable manufacturing method, and programmable optical management are just a few of this laminate’s primary benefits. It can even offer a pleasurable experience of visual beauty and comfort [37]. The need for UV shielding performance and its new application of transparent wood was studied by Liu et al., who manufactured a template made of delignified wood, a photochromic transparent wood by adding poly(methyl methacrylate) and UV/visible light switchable molecules [38]. Table 7.2 summarizes the properties of the transparent wood/paper and procedure conditions. Another branch of transparent wood research focuses on thermochromic and photochromic transparent wood applications. Many phase-change materials (PCMs) alter optical properties when they undergo a phase transition, indicating great potential in the preparation of smart optical materials. Smart products are typically defined as functional materials that can change one or more of their physical properties, such as colourimetric and emission spectra, in response to stimuli such as heat, pressure, mechanical effects, light, pH, solvent polarity, harmful chemical and biological entities, and electric and magnetic fields [39, 40]. Smart materials hold great promise for many intriguing applications because their features respond to external stimuli. PCMs, for example, are temperature-responsive materials that can reversibly store or release large amounts of thermal energy as their molecular arrangements change from one physical state to another [41]. Controlling optical properties in response to climatic conditions would be one way to advance existing research on transparent wood for window applications. The proposal was to create a temperatureand UV-responsive new type of window model out of transparent wood material

~80

81

~ 88

89

NaClO2 + acetate 2.1 buffer solution pH = 4.6

NaClO2 , 1 wt.% + 1.0 acetate buffer solution, pH = 4.6 + 1 M NaOH immersion

NaClO2 , 1 wt.% + 1.2 acetate buffer solution, pH = 4.6 ~0.045 0.2

NaOH (10 wt.%) and Na2 SO3 (5 wt.%) + H2 O2 (30 wt.%)

H2 O2 + UV light

NaClO2 delignification

NaClO2 delignification

Douglas fir, Balsa, Basswood [33]

Wheat straw [34]

Balsa wood [35]

Balsa wood [36]

Sugarcane bagasse fiber [37]

Balsa wood [24]

Poplar wood [38]

0.7

3 mm (30 wt.% of TCWS)

70 (0.1 mm thickness)

NaClO2 , 5 wt.% + 0.1–0.7 acetate buffer solution pH = 4.6

Beechwood [31]

70

69

~87

~86

0.13

Pulp was swelled, dispersed, infiltrated, hot-pressed

Cellulose fibers of Coniferous wood [29]

Transmittance (%)

Thickness (mm)

Method

Wood type

Table 7.2 Properties of the transparent wood/paper and procedure conditions

17.7 56.25

−

38

45.7

292

58.19

91.95

–

140

Tensile strength (MPa)

73

90

−

−

~55

~93

49 (0.7 mm thickness)

~90

Haze (%)

Photochromatic

Hydrophobic

UV shielding

Young’s modulus 3.80 GPa

Young’s modulus 24.4 GPa

Thermal conductivity 0.07 Wm− 1 k−1

Thermal conductivity 0.24 Wm−1 K−1

Elastic modulus is 2.5 GPa

Absorptance ~ 83

Additional properties

7.2 Recent Progress 77

78

7 Smart Windows

that can remain dark and opaque below a certain phase change temperature, such as representing night conditions, thereby reducing see-through properties to maintain privacy. This can be accomplished by incorporating thermochromic components. Simultaneously, such a window concept is capable of sensing daylight conditions, allowing the passage of sunlight inside the building by enhancing light transmission. In addition to the thermochromic effect, a smart transparent wood window would respond to hot weather or midday light by changing colour to a lighter hue (allowing partial light transmittance) rather than becoming completely dark. The colour/transmittance change is accomplished using photochromic components that are sensitive to the UV spectral portion of sunlight. Because of their ability to reversibly absorb and release thermal energy, organic phases change materials such as polyethylene glycol, fatty alcohols, paraffin, and fatty acids have been widely used for various energy-saving materials [42–45]. Therefore, the following research objectives were to prepare thermo-reversible, photochromic, transparent wood with excellent material properties for smart windows applications. In the work of Qiu et al., copolymer consisted of monomer styrene, butyl acrylate, and 1-octadecene was infiltrated into treated wood to obtain a flexible transparent wood with thermo-reversible optical properties. Their phase-change transparent wood could repeatedly turn from opaque (~23.7% of transmittance, ~ 98.3% of haze) to transparent (~74.9% of transmittance, ~36% of haze) with an increase in temperature, and vice versa during the cooling process. Standard delignification process with a 1.5% NaClO2 solution (pH = 4.6) at 80 ºC for 8 h was used, followed by 8% NaOH solution treatment at 80 °C for 8 h to remove most of the hemicellulose improving the flexibility of the delignified wood. Styrene, butyl acrylate, divinyl benzene as a cross-linker, and 2,2' -azobis-(2-methylpropionitrile) as an initiator were prepolymerized and infiltrated into the samples. The resulting material paves the way for wood-based materials to be endowed with actively tunable optical control properties [46]. A different approach was used by Samanta et al. who modified lignin chromophores by bleaching them using hydrogen peroxide in a strongly alkaline solution. Wood templates were then infiltrated with stoichiometric mixtures of thiol and allyl functionalized monomers of pentaerythritol tetrakis(3-mercaptopropionate) and 1,3,5-triallyl-1,3,5triazine2,4,6(1H,3H,5H)-trione containing a UV-initiator 1hydroxycyclohexyl phenyl ketone (0.5 wt.%) along with measured quantities of thermochromic or/and photochromic components. By changing the concentration of chromic components, the absolute optical transmission could be increased four times above the phase change temperature while the transmission at 550 nm could be reduced by 11–77% when exposed to UV. Furthermore, the chromic transparent wood composites demonstrated reversible energy absorption capabilities for heat storage applications, as well as a 64% increase in tensile modulus when compared to a native thiolene polymer [47]. The process of chromophores deactivation was also used by Al-Qahtani et al. who used the same type of parallel-cut wood (balsa); however, the thermochromic properties of transparent wood were achieved by using methyl methacrylate (MMA) as

7.2 Recent Progress

79

a polymer matrix with excellent mechanical and optical properties and a photoluminescent lanthanide-doped aluminium strontium oxide (SrAl2 O4 :Eu2+ , Dy3+ ) pigment characterized by good photo- and thermal stability. This transparent wooden substrate demonstrated a colour change from colourless in visible light to green under irradiation with UV as designated by CIE Lab colourimetric results [48]. Another material with exceptional mechanical and optical performance was reported by Wang et al. who investigated the possibilities and properties of polymer hydrogel combined with delignified balsa wood scaffold. Delignification was carried out at 80 °C using 1% sodium chlorite with 0.1 mol L−1 sodium acetate buffer at pH 4.6 for 12 h followed by TEMPO-mediated (2,2,6,6-tetramethyl-1-piperidinyloxy) oxidation and in situ polymerization of PNIPAM (poly(N-isopropylacrylamide)) resulting in a free-standing composite hydrogel with a high water content of 94.9 wt.% and high optical transmittance of 85.8% with anisotropic light scattering and thermochromic behaviour, reversibly changing between transparent and brightly white by a temperature change between 25 and 40 °C. This strong and thermochromic hydrogel could be used in a variety of applications such as smart windows, passive flexible displays, optical switches, and so on [49]. Similar results, using a slightly modified process of delignification and fabrication of thermochromic transparent hydrogel wood were achieved by Liu et al. [50]. The approaches using hydrogel could be considered the most environmentally friendly due to the high-water content of free-standing composite hydrogel of more than 90%. Another application of transparent wood was proposed by Muhammad et al., who successfully fabricated a form of poly(vinyl alcohol) /Gelatin/BaCO3 /transparent wood composites as a flexible shielding material for X-ray radiation application in the medical sector [25]. Table 7.3 summarizes the procedures and main results of the previously mentioned thermochromic transparent wood research. Transparent wood has been proposed for outdoor use. According to Li et al. [51], some issues should be addressed in future studies to allow for the industrialization of the technology, such as optical and mechanical stability and the desirability of increased cellulose content. When exposed to natural conditions, any polymeric material (organic or synthetic) is susceptible to environmental degradation. In the research of Wachter et al., lignin chromophores of radially cut basswood with dimensions of 100 × 50 × 1.2 mm were deactivated by using H2 O2 in an alkaline solution. Subsequently, the bleached wood templates were infiltrated by methyl methacrylate and 2-hydroxyethyl methacrylate polymers. The results show that UV-C radiation had a significant impact on the transmittance of the transparent wood samples. On the other hand, UV-C radiation had a much smaller effect on shore D hardness than it did on optical properties [52]. Therefore, it is essential to protect transparent wood from UV radiation to mitigate the negative effects, such as lowering the transmittance and mechanical performance resulting in its decreased efficiency and safety in various applications. This challenge was addressed by Bisht et al., who carried out a detailed study of the photostability of transparent wood under accelerated weathering conditions (up to 250 h of UV-A irradiation). Tangentially cut veneers of poplar wood were bleached using lignin modification and infiltrated with epoxy resin with the addition of 2-(2H-Benzotriazole-2-yl)-4, 6-di-test-pentylphenol (1% and 1.75%

65

Basswood wood [48]

Balsa wood Chromophores deactivation [47]

11–77 (dependent on UV exposure)

82.7 cold 39.8 hot

Balsa wood NaClO delignification [50]

Chromophores deactivation

~74.9

Balsa wood NaClO2 delignification [46]

Transmittance (%)

85.8

Method

Balsa wood TEMPO-mediated [49] oxidation of PNIPAM

Wood type

11.60 (hardness)

46.00

65 ± 2

11.60

2.19

0.32

Tensile strength (MPa)

88

90

~36

~80

Haze (%)

Pentaerythritol tetrakis(3-mercaptopropionate), 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione

Microparticles of lanthanide-doped aluminum strontium oxide phosphors

N-isopropylacrylamide

Copolymer of styrene, butyl acrylate, 1-octadecene

Poly(N-vinylpyrrolidone)

Thermochromic component

Table 7.3 Procedures and properties of the previously mentioned thermochromic transparent wood research

1.1

0.2

1.5

1.5

2.0

Thickness (mm)

80 7 Smart Windows

References

81

w/v) as a UV absorber. The results show that photo-degradation of transparent wood can be effectively controlled by incorporating a UV absorber into the resin polymer before infiltration into the wood substrate [53]. One of the least studied features of transparent wood is antimicrobial resistance. The results of Kunniger et al. showed that even in initial, un-weathered conditions, the evaluated coatings’ ability to defend against common microbes was insufficient. The coatings’ insufficient initial silver concentrations below 50 ppm and the resulting insufficient availability of free silver ions on the coating surface are the causes of the failure [54]. According to [55], there are some limitations of traditional architectural glass windows and doors. First, they can produce discomfort glare to threaten public safety. Second, the high brittleness of glass will make the cracks in glass very easy to expand. Third, there are drawbacks in the production process of the flat glass industry. On the other hand, transparent wood provides advantages over the traditional glass in the following areas: the optical properties of transparent wood meet the requirements of architectural glass [11, 56], transparent wood is safer and has excellent mechanical properties [57-59], higher aesthetic value [33, 60], good thermal insulation properties [30, 61]. The main challenges of transparent wood which need to be addressed are increasing its width, improving light transmission, improving flame retardancy, optimizing smart dimming, and adding functions such as self-cleaning properties and UV, and IR shielding [55].

7.3 Conclusion Wooden house components can bring people closer to nature and promote relaxation compared to the steel and concrete constructions frequently employed in modern buildings. Transparent wood is compatible with the trend toward sustainable development and satisfies the requirements for good lighting conditions, flame retardancy, heat insulation, and safety. It also has the advantage of a very diverse range of green and renewable material sources. Nevertheless, there is still some distance between transparent wood and actual large-scale application due to factors including method, time, cost, and safety. To reach large-scale and batch manufacturing and achieve sustainable development, architectural glass must receive more attention in the future. Also, only partially understood are the effects of treatments (such as bleaching and pulping delignification) on the spatial organization and chemical interaction of the remaining cell wall components, and little is known about the precise structural and chemical characteristics that govern the macroscopic mechanical properties.

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Chapter 8

Smart Buildings

Abstract To tackle energy and climate change challenges, renewable energy production and reduction of building energy consumption are required. To save energy, a novel material called optically transparent wood is being developed for light-transmitting structures in buildings. This material combines high optical transparency, outstanding mechanical characteristics, and superior thermal insulation. Due to its ability to regulate form, the transmitted light intensity distribution is adjustable. The resulting transparent wood also has excellent mechanical strength, good impact absorption, and excellent thermal insulation qualities. Transparent wood shows enormous promise as a cutting-edge practical and intelligent building material due to a combination of these properties. The following chapter summarizes the importance, recent progress, and possible applications of transparent wood in smart buildings.

8.1 Introduction Buildings consume around 40% of global energy and emit roughly a quarter of global greenhouse gas emissions [1–3]. Building energy efficiency is becoming increasingly important in meeting future energy and climate commitments [4, 5]. Current efforts are mostly focused on reducing building energy use by improving insulation, solar shading, and natural ventilation through the design of building envelopes [6–8]. Apart from lowering energy consumption, incorporating green renewable energy sources into building architecture is critical for making them energy-neutral or even energy-producing entities [9, 10]. Intelligent housing is a holistic system that integrates interior design, building materials, water supply and drainage, and architecture. Fabrication of green buildings has become a critical issue requiring rapid responses to preserve the world’s energy supplies and reduce the associated environmental load. The energy gain is highly dependent on weather conditions, and solar panel installation does not pay off for all kinds of buildings. Smart homes are in demand because of the advancement of computer technology and the rising ageing of the population [11]. Approximately, 1.5 trillion tonnes of cellulose are produced annually by the growth of biomass worldwide, making it a virtually limitless renewable resource [12]. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 I. Wachter et al., Transparent Wood Materials, Springer Series in Materials Science 330, https://doi.org/10.1007/978-3-031-23405-7_8

87

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8 Smart Buildings

Fig. 8.1 Annual municipal wood waste production and recycling in the USA between 1960 and 2018 [17]

In addition to being abundant, plant-derived cellulose is also inexpensive, biodegradable, non-toxic, and inert. Despite being used by humans for millennia, this material is still the subject of vigorous research [12–16]. The US Environmental Protection Agency reports that in 2018 only 17% of the nation’s total waste wood (3.1 million tons) recycled was municipal waste wood (Fig. 8.1) [17]. Wood recycling is a priority when considering improved resource efficiency and environmental effects e.g., carbon dioxide emissions from decomposing wood). Wood has good mechanical qualities because of its hierarchical structure and strong interactions between cellulose, hemicelluloses, and lignin. The cellulose nanoelementary fibrils of 3–5 nm in diameter, lignin, pectin, and other substances make up the cell walls. To supply the requirements for photosynthesis, ions, water, and other components are pumped into the wood trunk through vertically oriented channels [18–20]. While cellulose and hemicellulose are colourless, lignin serves as an important structural component in the support tissues of vascular plants and some algae and is mainly responsible for the colour and opacity of wood [21]. Additionally, the porous nature of wood results in significant visible light scattering [22]. As a result, the wood is always opaque and displays a particular colour. Kraft or sulfite pulping methods are frequently used to extract wood-derived cellulose as a fibrous pulp. In the study by Roberts et al., lignin and hemicellulose were removed from wood using a relatively mild sulfite pulping procedure, causing little disturbance to the wood’s natural porous structure. This produced aligned macroporous cellulosic monoliths known as cellulose scaffolds [23]. To manufacture transparent wood, it is necessary to remove or deactivate lignin from the wood to produce a cellulose scaffold which can be infiltrated with refractive

8.2 Recent Progress

89

index matching polymer. Due to its strong haze (above 70%), good optical transmittance (over 80%), and thermal insulation (thermal conductivity less than 0.23 W m−1 K−1 ), transparent wood offers the potential for use in intelligent buildings [24]. Unique light-guiding properties of transparent wood include significant forwardto-back scattering to produce optimum illumination, and therefore, it may find use in building roofs and windows [25]. Because of its economic and environmental benefits, wood has emerged as an important sustainable building material that might potentially replace steel and concrete [26]. Transparent wood is becoming a popular energy-saving building material because of its high optical transparency, outstanding mechanical characteristics, and superior thermal insulation.

8.2 Recent Progress Many research teams all around the world are proposing new methods of transparent wood fabrication and its carbon–neutral and energy-saving applications. Making sure a building reflects infrared light away is an excellent method to lower the amount of cooling it needs. Materials for passive radiative cooling are expertly designed to accomplish this (Fig. 8.2). Li et al. created a structural material with a mechanical strength of 404.3 MPa, which is more than eight times that of natural wood and a specific strength three times that of Fe–Mn–Al–C structural steel. The designed material’s cellulose nanofibers backscatter solar energy and emit significantly in the mid-infrared spectrum, resulting in constant sub-ambient cooling during the day and night. The cooling effect on energy savings can range from 20 to 60%, with the effect being greatest in hot and dry conditions. Fabrication of the cooling wood is straightforward, easy, and environmentally benign. The natural wood block was initially cut in the direction of

Fig. 8.2 Cooling wood serves as a demonstration of passive sun radiative cooling. When used as a building material, the cooling wood exhibits strong infrared emissivity and solar reflectance

90

8 Smart Buildings

growth, which is suitable for industry-cutting procedures for large-scale wood panels [8]. A different approach to manufacturing transparent cellulose-based passive radiative coolers was suggested by Gamage et al. by varying the material preparation method. They applied the principle of electrospinning porous fibre network films and casting homogeneous films to control the solar light interaction from highly reflective (>90%, porous structure) to highly transparent (≈90%, homogenous structure). The cellulose materials show strong thermal emissivity and minimal solar absorption, making them suitable for daytime radiative cooling [27]. Another application of modified wood was developed by Sun et al., who have demonstrated a simple, safe, low-cost, and environmentally acceptable method for functionalizing wood to produce electricity efficiently from mechanical energy input. The piezoelectric output was improved over 55 times by increasing the elastic compressibility of balsa wood using a fungal decay pretreatment. They envisioned the possibility to make large-scale wood floorings, allowing the production of electricity from human activities [28]. To use transparent wood as an energy-saving building material, it is necessary to not only retain its advantageous properties but also to ensure efficient large-scale production. For practical uses, thick or large-size transparent wood is particularly desirable. Wang et al., therefore, reported a transparent wood with dimensions of 300 × 300 × 10 mm, light transmittance of up to 68%, and a haze of up to 82%. Rather than using the entire wood template, the prepolymerized methyl methacrylate solution was infiltrated into delignified wood fibres to create the large-size transparent wood (Fig. 8.3). Large-scale translucent wood with any thickness and any size can be easily created using this process, and the preparation efficiency is roughly three times higher than the previous methods. Furthermore, the transparent wood produced by this approach does not require high-value raw materials and may be made directly from wood processing residues like sawdust and branches, resulting in increased resource use [29]. A similar approach was proposed by Dong et al., who used low-value wood (residual, damaged, decayed, disposed, or fractured) instead of small wood chips and turned it into a lightweight and strong structural material through delignification, combined with partial dissolution and regeneration of cell walls [30]. Li et al. adopted a different, green, and universal steam approach for the highly efficient removal of lignin from thick, large wood samples to produce transparent wood composites. They were able to delignify (0.84% lignin content) basswood and pine wood samples up to a thickness of 40 mm. The results showed a haze of 40% and a transmittance of 97% for 20-mm-thick samples. The procedure consisted of steaming the samples with a boiling H2 O2 aqueous solution (30 wt.%) and subsequent infiltration of epoxy resin solution into prepared wood templates [31]. Another application of transparent wood suitable for smart buildings was reported by Liu et al. who fabricated luminescent transparent wood by encapsulating multicolour carbon dots and poly(vinyl alcohol) into a delignified wood framework to detect formaldehyde gas (Fig. 8.4). The delignification of poplar wood (30 × 30 × 7 mm) was performed using NaClO2 (2 wt.%) in an acetate buffer solution (pH =

8.2 Recent Progress

91

Fig. 8.3 Synthesis process of conventional transparent wood and transparent fibre wood, respectively [29]

4.6) at 80 °C for 6 h and a subsequent vacuum-assisted polymer solution infiltration (poly(vinyl alcohol) (10 wt.%) and carbon dots (Abs = 3) in a volume ratio of 20:3). A stimuli-responsive building material with the real-time and visual self-detection ability of formaldehyde gas was prepared. It exhibited 85% optical transmittance, tuneable room-temperature phosphorescence, and ratio-metric fluorescence emission [32]. Li et al. demonstrated that delignification was shown to be unnecessary to produce transparent wood. Their high-lignin content (up to 80 wt.%) transparent wood showed improved mechanical and optical properties as well as a more efficient and environmentally friendly manufacturing process [33]. Transparent wood prepared by Wang et al. exhibits reconfigurable shape memory behaviour, excellent optical properties (transmittance of ∼90% and tuneable light-guiding effects), and low thermal conductivity (212.8 mW m−1 K−1 ). In addition, the tensile strength, modulus, and toughness of transparent wood were 60.14 MPa, 2.09 GPa, and 1.19 MJ m−3 , respectively [34]. Transparent wood is increasingly used in building construction to increase energy efficiency, resulting in cooler exterior surfaces, and increasing moisture condensation on the building’s exterior envelope. Therefore, the development of functional coatings is needed to prevent the growth of microorganisms. Reports on the effectiveness of silver against bacteria are available [35–37]. However, there were no records of what happened during natural weathering. Kunniger et al. showed that silver concentrations below 50 ppm in the coatings and the associated availability of free silver ions on the coating surface are insufficient [38]. Even though transparent wood is a relatively new material which currently is still not used in commercial, large-scale, and batch manufacturing, there have been a few studies focused on the applications of this material in architecture, the building industry, or civil engineering. The first study of its kind proposed a transparent wood

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8 Smart Buildings

Fig. 8.4 Schematic of luminescent transparent wood preparation (up); application of prepared transparent wood for ratio-metric fluorescence formaldehyde detection

façade, structural elements such as different types of beams and a concept design of a building. When the interior has to be illuminated by sunshine but privacy is also important, transparent wood should be chosen (Fig. 8.5) [39].

Fig. 8.5 Application of transparent wood as a material that disperses and transmits light while preserving the privacy

References

93

The following research tested whether transparent wood is a suitable material for load-bearing architectural constructions. Numerical analysis has shown that transparent wood is suitable for use in load-bearing structures [40].

8.3 Conclusion In conclusion, the aforementioned studies offer straightforward, environmentally friendly, quick, and highly effective methods to create thick, large delignified wood scaffolds with different applications in flexible electronics, energy-efficient building materials, and solar energy conversion devices. The addition of different capabilities to the transparent wood creates new possibilities for the design and creation of high-tech, intelligent, and practical building materials and accessories. However, for the developing field of functionalizing wood and cellulose materials, a better understanding of the primary materials selection, chemical treatment methods, and subsequent processes on the structure-property relationships is also crucial.

References 1. C. Ballif, L.-E. Perret-Aebi, S. Lufkin, E. Rey, Integrated thinking for photovoltaics in buildings. Nat. Energy 3 (2018). https://doi.org/10.1038/s41560-018-0176-2 2. B. Svetozarevic et al., Dynamic photovoltaic building envelopes for adaptive energy and comfort management. Nat. Energy 4(8), 671–682 (2019). https://doi.org/10.1038/s41560-0190424-0 3. P. Farese, How to build a low-energy future. Nature 488(7411), 275–277 (2012). https://doi. org/10.1038/488275a 4. D.M. Kammen, D.A. Sunter, City-integrated renewable energy for urban sustainability. Science (80-. ) 352(6288), 922–928 (2016). https://doi.org/10.1126/science.aad9302 5. J. Rogelj et al., Energy system transformations for limiting end-of-century warming to below 1.5 °C. Nat. Clim. Chang. 5(6), 519–527 (2015). https://doi.org/10.1038/nclimate2572 6. S.B. Sadineni, S. Madala, R.F. Boehm, Passive building energy savings: a review of building envelope components. Renew. Sustain. Energy Rev. 15(8), 3617–3631 (2011). https://doi.org/ 10.1016/j.rser.2011.07.014 7. L. Gustavsson, A. Joelsson, Life cycle primary energy analysis of residential buildings. Energy Build. 42(2), 210–220 (2010). https://doi.org/10.1016/j.enbuild.2009.08.017 8. T. Li et al., A radiative cooling structural material. Science (80-. ) 364(6442), 760 LP–763 (2019). https://doi.org/10.1126/science.aau9101 9. G.A. Barron-Gafford et al., Agrivoltaics provide mutual benefits across the food–energy–water nexus in drylands. Nat. Sustain. 2(9), 848–855 (2019). https://doi.org/10.1038/s41893-0190364-5 10. M.H. Rafiei, H. Adeli, Sustainability in highrise building design and construction. Struct. Des. Tall Spec. Build. 25(13), 643–658 (2016). https://doi.org/10.1002/tal.1276 11. M. Chan, D. Estève, C. Escriba, E. Campo, A review of smart homes—present state and future challenges. Comput. Methods Programs Biomed. 91(1), 55–81 (2008). https://doi.org/10.1016/ j.cmpb.2008.02.001

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12. D. Klemm, B. Heublein, H.-P. Fink, A. Bohn, Cellulose: fascinating biopolymer and sustainable raw material. Angew. Chemie Int. Ed. 44(22), 3358–3393 (2005). https://doi.org/10.1002/anie. 200460587 13. C. Moulherat, M. Tengberg, J.-F. Haquet, B. Mille, First evidence of cotton at Neolithic Mehrgarh, Pakistan: analysis of mineralized fibres from a copper bead. J. Archaeol. Sci. 29(12), 1393–1401 (2002). https://doi.org/10.1006/jasc.2001.0779 14. R.J. Moon, A. Martini, J. Nairn, J. Simonsen, J. Youngblood, Cellulose nanomaterials review: structure, properties and nanocomposites. Chem. Soc. Rev. 40(7), 3941–3994 (2011). https:// doi.org/10.1039/C0CS00108B 15. T. Huber, J. Müssig, O. Curnow, S. Pang, S. Bickerton, M.P. Staiger, A critical review of allcellulose composites. J. Mater. Sci. 47(3), 1171–1186 (2012). https://doi.org/10.1007/s10853011-5774-3 16. L. Jabbour, R. Bongiovanni, D. Chaussy, C. Gerbaldi, D. Beneventi, Cellulose-based Li-ion batteries: a review. Cellulose 20(4), 1523–1545 (2013). https://doi.org/10.1007/s10570-0139973-8 17. United States Environmental protection agency, National overview: facts and figures on materials, wastes and recycling (2018). [Online]. Available https://www.epa.gov/facts-and-figuresabout-materials-waste-and-recycling/national-overview-facts-and-figures-materials 18. M. Zhu et al., Transparent and haze wood composites for highly efficient broadband light management in solar cells. Nano Energy 26, 332–339 (2016). https://doi.org/10.1016/j.nan oen.2016.05.020 19. L. Donaldson, Cellulose microfibril aggregates and their size variation with cell wall type. Wood Sci. Technol. 41(5), 443–460 (2007). https://doi.org/10.1007/s00226-006-0121-6 20. L.J. Gibson, The hierarchical structure and mechanics of plant materials. J. R. Soc. Interface 9(76), 2749–2766 (2012). https://doi.org/10.1098/rsif.2012.0341 21. M. Zhu et al., Highly anisotropic, highly transparent wood composites. Adv. Mater. 28(26), 5181–5187 (2016). https://doi.org/10.1002/adma.201600427 22. J. Wu, Y. Wu, F. Yang, C. Tang, Q. Huang, J. Zhang, Impact of delignification on morphological, optical and mechanical properties of transparent wood. Compos. Part A Appl. Sci. Manuf. 117, 324–331 (2019). https://doi.org/10.1016/j.compositesa.2018.12.004 23. A.D. Roberts et al., Enzyme immobilisation on wood-derived cellulose scaffolds via carbohydrate-binding module fusion constructs. Green Chem. 23(13), 4716–4732 (2021). https://doi.org/10.1039/D1GC01008E 24. Z. Yu et al., Transparent wood containing CsxWO3 nanoparticles for heat-shielding window applications. J. Mater. Chem. A 5(13), 6019–6024 (2017). https://doi.org/10.1039/C7TA00 261K 25. T. Li et al., Wood composite as an energy efficient building material: guided sunlight transmittance and effective thermal insulation. Adv. Energy Mater. 6(22), 1601122 (2016). https://doi. org/10.1002/aenm.201601122 26. E. Tawfik, F. Soliman, Change of organic phase of wood to transparent wood. 7, 15–18 (2021). https://doi.org/10.9790/264X-0701011518 27. S. Gamage et al., Reflective and transparent cellulose-based passive radiative coolers. Cellulose 28(14), 9383–9393 (2021). https://doi.org/10.1007/s10570-021-04112-1 28. J. Sun et al., Enhanced mechanical energy conversion with selectively decayed wood. Sci. Adv. 7(11), eabd9138 (2022). https://doi.org/10.1126/sciadv.abd9138 29. X. Wang et al., Large-size transparent wood for energy-saving building applications. Chemsuschem 11(23), 4086–4093 (2018). https://doi.org/10.1002/cssc.201801826 30. X. Dong et al., Low-value wood for sustainable high-performance structural materials. Nat. Sustain. 5(7), 628–635 (2022). https://doi.org/10.1038/s41893-022-00887-8 31. H. Li, X. Guo, Y. He, R. Zheng, A green steam-modified delignification method to prepare low-lignin delignified wood for thick, large highly transparent wood composites. J. Mater. Res. 34(6), 932–940 (2019). https://doi.org/10.1557/jmr.2018.466 32. Y. Liu et al., Luminescent transparent wood based on lignin-derived carbon dots as a building material for dual-channel, real-time, and visual detection of formaldehyde gas. ACS Appl. Mater. Interfaces 12(32), 36628–36638 (2020). https://doi.org/10.1021/acsami.0c10240

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Chapter 9

Fire Properties of Transparent Wood and Its Components

Abstract Since transparent wood has the potential to become a suitable building material, its fire characteristics also become more important. During its preparation, several components are combined. One of them is wood modified by one of the many methods used, and others are synthetic (or natural) polymers. The scope devoted to this chapter is not able to cover all the specific materials due to the variety of combinations that can be prepared in this manner. The first part, therefore, contains a description of the individual components, their chemical composition, and their thermal decomposition. Frequently used components are processed as separate subsections. However, in some cases, groups of materials that include many substances are used to produce transparent wood. These are therefore combined into common units for the sake of greater clarity and fluidity of the text. The second part of this chapter is devoted to the fire properties of a specific type of transparent wood. It contains a description of its production as well as information regarding the cone calorimeter through which the samples were measured. It also contains the measurement results and, finally, the prediction of some other properties. The obtained data are evaluated in the form of graphs and compared with the results of other authors.

9.1 Composition and Thermal Decomposition of Wood Pseudo-components As already described in the previous chapters, transparent wood is a composite made of partly treated wood mass and partly synthetic (or natural) polymer. The wood itself is not a chemically pure substance and is made up of a wide range of components, also called pseudo-components. Carbohydrates contained in wood are collectively referred to as holocelluloses and can be divided into cellulose and hemicelluloses. Cellulose was first isolated in 1839 by the French chemist Anselme Payen [1] and for determining its structure, along with other carbohydrates, Herman Fisher was awarded the Nobel Prize in Chemistry in 1902 [2]. It is a polymer formed by β-dglucopyranose units, which are connected in positions 1 → 4 (Fig. 9.1). Cellulose macromolecules are mostly regularly arranged in parallel (crystalline) and partly free, disordered (amorphous) form [3]. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 I. Wachter et al., Transparent Wood Materials, Springer Series in Materials Science 330, https://doi.org/10.1007/978-3-031-23405-7_9

97

98

9 Fire Properties of Transparent Wood and Its Components

Fig. 9.1 Chemical formula of cellulose

The term hemicellulose was used for the first time by Ernst Schulze in 1891. He thus designated the components of the cell wall that are easily soluble in dilute alkalis [4]. Hemicelluloses include all other polysaccharides, except cellulose. Their chains are significantly shorter compared to cellulose. The main building blocks are xylans, glucans, mannans, and arabinans [5]. The aromatic component of wood is called lignin (Fig. 9.2). It was discovered in 1938, similarly to cellulose, by Anselme Payen [6]. It is a phenolic polymer formed by radical coupling polymerization of three monolignols (p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol). Its structure is amorphous [7]. In the case of softwood, more than 95% of the total number of structural units are coniferyl alcohol units. In contrast, lignin in hardwood contains, in addition to coniferyl alcohol units, a significant number of sinapylalcohol-derived units. Cellulose adds strength to wood fibres. Hemicelluloses act as a matrix for cellulose and a link between fibrous cellulose and amorphous lignin. Lignin is a cementing material for wood fibres [8]. The composition of wood varies significantly not only between tree species, but also between individual trees, or even between different parts of the same tree. Based

Fig. 9.2 Proposal of the chemical formula of lignin [9]

9.1 Composition and Thermal Decomposition of Wood Pseudo-components

99

Table 9.1 Content of pseudo-components in different types of wood Bamboo fibre

Cellulose

Hemicellulose

Lignin

References

54.6

11.4

21.7

[10]

Bamboo

45.78

23.94

14.46

[11]

Larch

37.4

28.7

43.3

[12]

Southern yellow pine

40.9

29.4

29.1

[12]

Sitka spruce

46.2

26.6

27.6

[12]

Picea orientalis

54.12

21.43

24.37

[13]

Pinus sylvestris (juvenile)

32.6

21.9

27.1

[14]

Pinus sylvestris (mature)

41.5

21.9

27.7

[14]

Balsa wood (1 year)

45.04

16.93

22.07

[15]

Balsa wood (3 years)

44.57

17.15

24.92

[15]

Balsa wood (5 years)

42.03

18.02

24.05

[15]

Acer rubrum

40.7

30.4

23.3

[16]

Fagus grandifolia

36.0

29.4

30.9

[16]

Fraxinus americana

39.5

29.1

24.8

[16]

Liriodendron tulipifera

39.1

28.0

30.3

[16]

Quercus alba

41.7

28.4

24.6

[16]

Quercus nigra

41.6

34.8

19.1

[16]

Pinus bungeana

45.8

30.8

22.6

[17]

Populus tomentosa

39.2

33.2

24.1

[17]

Pinus sylvestris

42.8

15.9

27.5

[18]

on the data from Table 9.1, it can be said that wood contains approximately 15–35% hemicelluloses, 35–55% cellulose, and 15–35% lignin. From the formula of cellulose, it can be deduced that it consists of approximately 44.5% carbon, 6.2% hydrogen, and 49.3% oxygen. However, the product obtained by its isolation may have a different quality and also contain a small number of other elements, such as nitrogen or sulfur (Table 9.2). The elemental composition of hemicelluloses generally differs only slightly from cellulose. The largest part is oxygen (about 50%), followed by carbon (about 40%) and hydrogen (about 6%). The most common structural unit of hemicelluloses is xylan [19–21]. Its chemical composition does not significantly deviate from the values common for hemicelluloses. The representation of chemical elements in lignin is significantly different from the composition of carbohydrates. Due to the aromatic rings in its molecule, the carbon content increases, the hydrogen content decreases slightly, and the amount of oxygen decreases significantly. Individual monolignols contain more than 60% carbon, approximately 25% oxygen and 6.7% hydrogen. Similar to cellulose, different lignin extraction methods produce different products. Their carbon content ranges from approximately 50−70%, hydrogen content between 4 and 6%, and oxygen content in the range of 20–40% of their weight.

100

9 Fire Properties of Transparent Wood and Its Components

Table 9.2 Ultimate analysis of individual pseudo-components of wood Cellulose

C

H

N

O

S

References

42.8

6.4

0

50.8

0

[22]

Xylan

40.4

6.1

0

53.04

0

[22]

Lignin

63.7

6.0

0.90

27.2

2.29

[22]

Cellulose

44.46

6.22

0

49.32

0

Calculation using chemical formula

Coniferyl alcohol

66.66

6.71

0

26.62

0

Calculation using chemical formula

P-coumaryl alcohol

71.99

6.71

0

21.30

0

Calculation using chemical formula

Sinapyl alcohol

62.86

6.71

0

30.43

0

Calculation using chemical formula

Cellulose

42.96

6.3

0

50.74

0

[23]

Xylan

43.25

6.2

0

49.9

0

[23]

Lignin

57.7

4.38

0.11

34.00

3.22

[23]

Hemicellulose

39.26

5.76

0.02

54.95

0.01

[24]

Cellulose

41.36

6.49

0.02

52.11

0.02

[24]

Lignin

56.82

5.21

0.11

33.62

1.32

[24]

Cellulose

41.46

5.97

0.07

52.48

0.02

[25]

Hemicellulose

40.69

5.75

0.04

53.49

0.03

[25]

Lignin

58.67

5.43

1.13

34.51

0.26

[25]

Hemicellulose

38.1

6.0