高リグニン含有ヤシバイオマスからのリグノセルロ
ースバイオポリマー及びそのバイオマイクロコンポ
ジットの創製と特性評価
メタデータ

言語: English
出版者:
公開日: 2021-06-23
キーワード (Ja):
キーワード (En):作成者: モハメド, アルハビ
メールアドレス:所属:

URL

https://doi.org/10.15118/00010387

論文内容の要旨（英文）氏名：Mohammed Abdullah Hamad Alharbi
論文題目：Fabrication and characterization of lignocellulosic biopolymers andtheir hybrid biomicrocomposites from palm biomasses with highlignin content（高リグニン含有ヤシバイオマスからのリグノセルロースバイオポリマー及びそのバイオマイクロコンポジットの創製と特性評価）
Abstract
Data palms and coconut are important agricultural products grown in Saudi Arabia
and used across the globe. At present, most of the plant waste generated from thesepalms is either incinerated or dumped in landfills. This work presented a more effective
method of utilizing high-lignin biomass, which is found in abundance in plant fibers andpalm leaves. It aims to fabricate biodegradable biopolymers from plant wastes and toutilize them as alternatives for petroleum-derived resins, by utilizing thermosetting
behavior of native lignin. A simple process that combines milling and hot pressing wasdevised to produce lignocellulose biopolymers using thermosetting biomasses with highlignin content derived from various palm wastes. Because lignin serves as an excellent
binder for plant fibers, it was used to fabricate lignocellulose biomicrocomposites withexcellent mechanical and thermal properties.
Lignocellulose contained in date fiber (DF), coconut coir (CC), date leaves (DL), andcoconut leaves (CLs) was pulverized and then hot-pressed to prepare lignocellulosic
biopolymers. Biomicrocomposites were prepared by mixing and hot-pressing CC powder
with construction and demolition waste (CDW) wood and recycled silk (RS) powder.
Fiber-based biopolymers with high cellulose and lignin content derived from DF andCC exhibited mechanical properties and thermal stability that are comparable to those of
petroleum-derived phenolic resins. The mechanical properties of the biopolymers were
derived from the alkaline pretreatment and pulverization of the fiber-based biomass. The
bending strength of the biopolymer increased with decreasing particle size of thepulverized lignocellulose. Alkaline pretreatment also improved the mechanical properties
of DF and CC biopolymers. Because of the excellent binding behavior of CC for cellulose,CDW/CC biomicrocomposites showed good mechanical properties and thermal stability.
The effects of milling and alkaline pretreatment on the chemical composition andcrystallinity of lignocellulose were investigated using Fourier transform infrared
spectroscopy and X- ray diffraction. The results revealed that the increase in mechanicalstrength was due to the removal of hemicelluloses by alkaline pretreatment. The biomass
pulverization increases the proportion of lignin exposed on the surface of a lignocellulose
particle, leading to improved mechanical properties of lignocellulosic biopolymers andbiomicrocomposites.
In summary, this study was conducted to develop an environment-friendly processthat combined milling and hot pressing to fabricate lignocellulosic biopolymers andbiomicrocomposites from various palm wastes. This process provides biopolymers withmechanical properties and thermal stability comparable to those of conventional phenolic
resins. A biomicrocomposite can be derived from cellulose-based biomass using palm
waste powder as a binder.

2

Acknowledgments
First, I want to Thank God (Allah) for his blessings and guidance. Thank you, Dad, foryour spiritual, emotional, and financial support and for the scientific insights you haveshared throughout my journey. Thank you, Mom, for your daily prayers and patience.
Thank you Khalty, Ibrahim, and Norah, for the spiritual and emotional support you
extended during challenging times and for sending raw materials at your expense.
Thank you, Professor Hirai, for accepting me in your laboratory and sharing your expertiseand vision about research on environmentally friendly materials from natural sources andfor encouraging me to prepare lignocellulosic materials. None of this would have beenpossible without your support, and I consider myself exceptionally lucky to have you as amentor. Thank you, Kuzuya Sensei, for agreeing to take on my supervision at the end of
my journey; I’m extremely grateful for your support, guidance, and thorough feedback.
Thank you, Professor Kamegawa and Sato Sensei, for being part of my thesis committee
and providing constant assistance.
Thank you to all my seniors and dear friends, Dr. Tuan, and Bien for their encouragement
and support and for educating me. Thank you to all my Japanese lab members for being
excellent colleagues and for teaching and helping me translate equipment manuals.
Thank you, Akioka, for your great collaboration and support; it has been a pleasureworking with you. A special thanks to Kuroda, Nakaizumi, Nakayama, Gando, Shoji,
Ijima, Inoue, Tamiya, Maegawa, Ume, and Narumi; I appreciate your assistance.
Thank you, Dr. Riky, for being an awesome friend and for offering important informationand great food. I want to thank you and Dr. Atchariya for sending raw materials fromIndonesia and Thailand. Thank you to my dear friends Nao, Ayana, Sun, Ayano, Mari,
Kato, Jonathan, Mie, Nina, and others whom I had the pleasure of knowing during thisjourney. I also would like to thank the international office, particularly Noda-san, Ikedasan, Kozue- san and Miyakawa-san, for their support and guidance.

Professional Acknowledgments
I would like to acknowledge the financial support extended by the Saudi Arabian Culture
Mission, the Custodian of the Two Holy Mosques Overseas Scholarship Program (Jan2019–March 2021). I would also like to thank Tabata-san, Miyamoto-san, and Yamaura
for their warm welcome and support during my internship at Air Water ECOROCA. I would
also like to thank ECOROCA and Tsukishima Kikai Co. Ltd. for donating raw materials.
Additionally, thank you to Kimura and Siree for their help using SEM.

3

Table of Contents

Abstract ...................................................................................................................................................... 1
Acknowledgments ................................................................................................................................... 3
Professional Acknowledgments .......................................................................................................... 3
List of Tables ............................................................................................................................................. 6
List of Figures ........................................................................................................................................... 8
1. Introduction ......................................................................................................................................... 13
2. Materials and preparation methods .............................................................................................. 22
2.1 Materials used in biopolymer preparation .................................................................................. 22
2.1.1 Main materials......................................................................................................................... 22
2.1.2 Supplementary materials ...................................................................................................... 23
2.2 Determination of chemical composition and moisture content ............................................... 23
2.3 Alkaline pretreatment .................................................................................................................... 24
2.4 Powder preparation ....................................................................................................................... 25
2.5 Biopolymer preparation................................................................................................................. 26
2.6 Materials used in biomicrocomposites ....................................................................................... 27
2.6.1 Main materials......................................................................................................................... 27
2.6.2 Supplementary materials ...................................................................................................... 28
2.7 Biomicrocomposite preparation ................................................................................................... 28
3. Characterization ................................................................................................................................. 31
3.1. Determination of apparent density ............................................................................................. 31
3.2. Bending strength........................................................................................................................... 31
3.3. Water absorption and thickness swelling.................................................................................. 31
3.4 Statistical analysis ......................................................................................................................... 32
3.5. Scanning electron microscopy (SEM) ....................................................................................... 32
3.6. Thermogravimetric analysis (TGA) ............................................................................................ 32
3.7. Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) ........... 36
3.8. X-ray diffraction (XRD)................................................................................................................. 38
4. Influence of chemical composition, particle size, and alkaline pretreatment onmechanical, thermal, and structural properties of lignocellulosic biopolymers .................. 40
4.1 Chemical composition ................................................................................................................... 40
4.2. Mechanical and physical properties .......................................................................................... 41
4.3. Water resistance ........................................................................................................................... 48
4.4 SEM morphologies ........................................................................................................................ 50

4

4.5 Thermal behavior and stability..................................................................................................... 55
4.6 FTIR spectroscopy ........................................................................................................................ 61
4.7 X-Ray diffraction (XRD) ................................................................................................................ 68
4.8 Activating lignin binding and optimal processing temperature ............................................... 71
4.9 Lignocellulosic biopolymer colors................................................................................................ 73
5. Effect of coconut coir and regenerated silk microparticles as blends and naturalbinders for construction and demolition waste (CDW) wood on the mechanical, thermal,
and structural properties of biomicrocomposites ........................................................................ 75
5.1 Mechanical properties and physical properties ......................................................................... 75
5.2. SEM morphologies ....................................................................................................................... 83
5.3 Thermal behavior and stability..................................................................................................... 86
5.4 FTIR spectroscopy ........................................................................................................................ 90
6. Discussion ........................................................................................................................................... 97
6.1 Sampling issue ............................................................................................................................... 97
6.2 Limitation ......................................................................................................................................... 97
7. Conclusions ........................................................................................................................................ 98
Data Statement ....................................................................................................................................... 99
References ............................................................................................................................................... 99

5

List of Tables
Table 1. Mechanical properties of silk-based resins and thermoplastics
Table 2. Prepared samples for biopolymers, their labels, and processing conditions
Table 3. Prepared samples for biomicrocomposites, their labels, and compositions(wt%)
Table 4. Prepared samples and their labels used in supplementary materials(biomicrocomposites)
Table 5. Thermal analysis to determine moisture and ash content of powders used in
the preparation of biopolymers and biomicrocomposites.
Table 6. Peak finding ranges associated with functional groups of celluloses, lignin,
hemicellulose, and β-sheets in biocomposites, determined for the summation of
associated peak areas according to the method described in [35], with slight modification
Table 7. Chemical compositions and (C+L)/(H+E+A) ratio of date palm fiber, coconut coir,date leaves, and coconut leaves
Table 8. Mean of mechanical and physical properties (± SD) of raw and alkaline
pretreated palm-based lignocellulosic biopolymers prepared from microparticles
Table 9. Influence of particle size on bending properties (± SD) of raw and alkaline
pretreated palm-based lignocellulosic biopolymers
Table 10. Mechanical and physical properties (± SE) of hot-pressed biopolymers(supplementary materials)
Table 11. Thermal properties of raw and alkaline pretreated palm-based lignocellulosic
biopolymers
Table 12. Thermal properties of raw coconut coir- based lignocellulosic analyzed under
non-oxidative atmosphere (argon gas)
Table 13. FTIR band assignments (cm-¹) for raw and alkaline palm-fiber- and leaf-based
biopolymers

6Table 14. Specific absorption-band wavenumbers (cm-¹) related to raw and alkaline
pretreated palm-fiber- and leaf-based biopolymers
Table 15. Summation of major peak areas in the “fingerprint” region associated withcellulose, lignin, hemicellulose, and pectin in lignocellulosic biopolymers that helps
determine the difference in chemical composition among plant-based biopolymers
Table 16. Specific absorption-band wavenumbers (cm-¹) related to hot-pressed plantbased biopolymers according to FTIR band assignments (cm-¹).
Table 17. Assignment of X-ray diffraction peaks for palm-based raw fibers and leaves,
powders, biopolymers, hot-pressed coconut fibers, milled CDW wood, cotton, bamboo,
and bagasse-based biopolymers
Table 18. Means of the mechanical and physical properties (± SE) of the neat
biopolymers
Table 19. Means of the mechanical and physical properties (± SE) of the CDW and CC
biopolymers and their hybrid biomicrocomposites
Table 20. Means of the mechanical and physical properties (± SE) of the RS and CC
biopolymers and their hybrid biomicrocomposites
Table 21. Means of the mechanical and physical properties (± SE) of the CDW and RS
biopolymers and their hybrid biomicrocomposites
Table 22. Prepared samples and their labels
Table 23. Influence of particle size on bending properties of CDW/CC and RS/CC
biomicrocomposites prepared at a mass ratio of 50:50
Table 24. Influence of mechanical and physical properties of lignocellulosic
biomicrocomposites prepared from microparticles (≤53 µm)
Table 25. Thermal properties of biocomposites and their corresponding biopolymers
Table 26. Summation of the major peak areas in the “fingerprint” region associated withcellulose, lignin, and hemicellulose in CDW/CC biomicrocomposites
Table 27. Summation of major peak areas in the “fingerprint” region associated with βsheets, cellulose, and hemicellulose in biomicrocomposites containing regenerated silk

7

List of Figures
Figure 1. Chart showing hierarchical information about utilization or end-life of plant fibers
commonly found in a literature review
Figure 2. Scheme presentation of plant fiber surface modification pathways adapted from
[22]
Figure 3. Schematic of papermaking concept also used for producing cellulose-basedmaterials
Figure 4. Technical lignin’s functions in application adapted from [23]Figure 5. Effect of hot-pressing temperature on fiberboards made from unripe coconut
husk derived from [24], showing lignin’s activation evident at a higher temperature due tothermal hydrolysis of cellulose and hemicellulose at the surface
Figure 6. a) Conversion process of coconut husk to binderless boards and b) sample
coir-board derived from [9]Figure 7. Major factors that influence optimal lignin binding to prepare lignocellulosic
biopolymers
Figure 8. Photographs of (a) Phoenix dactylifera tree from which fibers and leaves wereextracted for raw materials, (b) date fibers (c) Cocos nucifera tree, and (d) coconut leaves
Figure 9. Photographs of washed and dried raw and alkaline pretreated date fibers,
coconut coir, and date leaves
Figure 10. Photographs of powders prepared from raw and alkaline pretreated date andcoconut fibers and leaves, in addition to milled powders of cotton, recycled waste wood,bamboo, and waste bagasse
Figure 11. Flow process diagram depicting the production of lignocellulosic biopolymers
from raw and alkaline pretreated dates, coconut fibers and leaves, and waste bagasse inaddition to hot-pressed milled cotton, bamboo, CDW wood, and coconut coir fibers
Figure 12. Flow process diagram of the preparation of biomicrocomposites
Figure 13. Appearance of neat biopolymers and their hybrid biomicrocomposites

8Figure 14. Mass-loss percentage curves for (a) lignocellulosic powders consisting of
smaller microparticles, (b) lignocellulosic powders (pretreated with 1 wt% NaOH), and (c)lignocellulosic powders (pretreated with 3 wt% NaOH); corresponding DTG of palmbased (d) lignocellulosic powders consisting of smaller microparticles, (e) lignocellulosic
powders (pretreated with 1 wt% NaOH), and (f) lignocellulosic powders (pretreated with
3 wt% NaOH)
Figure 15. Mass-loss percentage curves for (a) Supplementary materials’ powders, and(b) microparticles used in biomicrocomposites, and (c), (d) their corresponding DTG,
respectively
Figure 16. Example of a method used to determine lignin’s peak (A1591), based on peakarea in a) DF and b) CC
Figure 17. Example of a method used to determine β-sheet compositions, based onpeak area
Figure 18. Means of mechanical and physical properties of raw and alkaline pretreatedpalm-based lignocellulosic biopolymers prepared from microparticles (out of 10 samples)
Figure 19. Influence of particle size on bending properties of raw and alkaline pretreatedfiber-based lignocellulosic biopolymers (out of three samples)
Figure 20. Influence of particle size on bending properties of raw and alkaline pretreatedleaf-based lignocellulosic biopolymers (out of three samples)
Figure21.

Mechanicalandphysicalproperties

ofhot-pressedbiopolymers

supplementary materials (out of three samples)
Figure 22. SEM micrographs of untreated and alkaline pretreated lignocellulosic
biopolymer

morphology

capturedat

600×

magnification

showing

biopolymer

morphologies of lignin-based adhesion
Figure 23. SEM micrographs of hot-pressed coconut coir fibers (CCPB) morphology offracture captured at 600× magnification representing morphologies of semi lignin-based
adhesion9Figure 24. SEM micrographs of DL, AK 3%DL, AK 1%CL, and AK 3%CL morphologies
of fracture captured at 600× magnification representing morphologies of poor lignin-based
adhesion
Figure 25. SEM micrographs of hot-pressed cotton, CDW wood and bamboo
morphologies of fracture captured at 600× magnification representing morphologies of
non-lignin-based adhesion
Figure 26. SEM micrographs captured at 120× magnification showing the morphology of
raw and alkaline pretreated lignocellulosic fibers and leaves before pulverization
Figure 27. SEM micrographs of raw and alkaline pretreated lignocellulosic microparticles
captured at 1200× magnification showing particle distributions and orientation of eachpowderFigure 28. SEM micrographs of milled powders of cotton, bagasse, CDW wood, andbamboo captured at 1200× magnification showing particle distributions and orientation ofeach powder. CDW wood and bamboo show the presence of microfibers
Figure 29. Mass-loss percentage of (a) untreated biopolymers prepared from smaller
particles and (b) untreated biopolymers prepared from larger particles. Corresponding
DTG of palm-based biopolymers (c) prepared from smaller particles and (d) preparedfrom larger particles
Figure 30. Mass-loss percentage of (a) biopolymers prepared 1 wt% alkaline pretreatedfibers and leaves and (b) biopolymers prepared 3 wt% alkaline pretreated fibers and leaf
microparticles. Corresponding DTG of palm-based biopolymers (c) prepared from 1 wt%
alkaline pretreated fibers and leaf microparticles and (d) prepared from 3 wt% alkaline
pretreated fibers and leaf microparticles
Figure 31. Mass-loss percentage curves for (a) hot-pressed supplementary materials,
the obtained hot-pressed biopolymers, and (b) corresponding DTG. The indicator showsa peak of lignin-based adhesion at around 230 °C, also noticed in bagasse-based
biopolymers
Figure 32. Mass-loss percentage curves under argon gas for (a) coconut coir- based
lignocellulosic biopolymer and (b) corresponding DTG curve10Figure 33. FTIR spectra of raw and alkaline pretreated lignocellulosic biopolymers
Figure 34. FTIR spectra of hot-pressed plant-based biopolymers
Figure 35. XRD diffraction patterns for (a) raw palm-based fibers and leaves, (b)lignocellulosic milled powders, (c) lignocellulosic biopolymers, (d) 1 wt% alkaline
pretreated biopolymers, (e) 3 wt% alkaline pretreated biopolymers, and (f) hot-pressed
supplementary materials
Figure 36. Schematic representation of activation of lignin for binding
Figure 37. Appearance of lignocellulosic biopolymers and hot-pressed biopolymers
Figure 38. Process of preparation and characterization of hybrid biomicrocomposites
Figure 39. Means of mechanical and physical properties (□) of neat biopolymers andtheir hybrid biomicrocomposites. The whiskers represent the maximum and minimum
values (out of three samples or more)
Figure 40. SEM Micrographs of coconut coir-microparticles, CDW wood powder, andregenerated silk microparticles captured at 190× and 600× magnifications showing
particle size and distribution
Figure 41. SEM micrographs of neat biopolymers and their hybrid biomicrocomposites at
190× magnification
Figure 42. SEM micrographs showing the effect of adding 20, 40, and 60 wt%
regenerated silk to the CDW/RS biomicrocomposites
Figure 43. Mass-loss percentage of (a) CDW, and CDW/CC, (b) RS and RS/CC, and (c)CDW/RS biomicrocomposites. Corresponding DTG of (d) CDW, and CDW/CC, (e) RS
and RS/CC, and (f) CDW/RS biomicrocomposites
Figure 44. Full mass-loss percentage curves for (a) regenerated silk biopolymer,
RS80/CC20, and CDW20/RS80 and (b) their corresponding DTG, confirming thermal
instability of regenerated silk (RS), RS80/CC20, and CDW20/RS80 biomicrocomposites
containing 80 wt% regenerated silk

11

Figure 45. FTIR spectra of a) CDW/CC, b) RS/CC, and C) CDW/RS biomicrocomposites
and their corresponding biopolymers

121. Introduction
Date palm (Phoenix dactylifera) and coconut palm (Cocos nucifera) are two
subspecies of the palm tree family (Arecaceae) widely spread in tropical and subtropicalregions. Palms have been cultivated for over 5000 years owing to the importance of theirprimary products (e.g., fruits and oils, as a source of nutrition) [1] and their secondary
byproducts (including fibers and leaves, as raw materials for producing wooden crafts,
ropes, and baskets) [2,3]. Top date- and coconut-producing countries generate large
amounts of high-lignin fibrous waste as a byproduct of fruit cultivation and pruning [4,5].
A single date tree annually produces about 1.25 kg of fiber and 8 kg of leaves [6], while
the cultivation of coconuts generates coir and pith in a coconut husk, which accounts forabout 35% of the total weight of the fruit [7]. Accordingly, most date palm waste is dumped
in landfills or incinerated [8]. Similar fates are assumed for coconut-waste fiber, where
only 15–20% utilization has been estimated [9].

Figure 1. Chart showing hierarchical information about utilization or end-life of plant fibers
commonly found in literature review
Nowadays, waste utilization getting increasingly popular as an effective way to
produce environmental products and reduce upstream processes. However, utilizing
agricultural fibrous waste is one sector that has not been utilized efficiently, particularly in13

developing countries where inefficient open burning is standard waste management,
which remains a major environmental issue due to its contribution to greenhouseemission [11]. Moreover, biomass power generation generates fly-ash waste and requires
intensive chemical pretreatment processes, leading to the deterioration of energyproduction efficiency [12].
Burning agricultural biomass is a common waste management practice, which might
have severe environmental consequences. A recent study reported that 90% of thenational biomass burning emissions were caused by firewood crop, residue burned as
fuel, and crop residue field burning [10]. The burning of the biomass causes emissions
equivalent to 10% of the Chinese national total greenhouse gases (GHGs) emissions
including those from fossil fuel combustion and cement production. Moreover, theChinese GHG emissions from biomass are equivalent to 8% of worldwide, 22% ofdeveloping countries, and 34% of Asian GHG emissions from burning of biomass.
Therefore, the GHGs emitted from biomass burning can be a problem that cannot beignored.
Natural fibers are suitable candidates for biodegradable and environmentallyfriendly products owing to their low cost, biodegradability, and availability (Fig. 1).
Although lignocellulosic biomass can be used in various applications [11], it is not yet
utilized efficiently on a global scale to produce high-value-added products owing tointense competition from petroleum-derived materials. The utilization of natural raw fibers
as reinforcement materials in composite has been widely reported in past studies [12–16]. Other studies have also introduced a surface modification to enhance the weakadhesion between naturals fibers and matrix (i.e., alkaline treatment) by removing lignin
and waxes that hinder adhesion in composites with/without coupling agents (Fig. 2) [17–22]. Because binders, coupling agents, chemical pretreatment, or other complicatedprocesses have to be used to improve the adhesion of natural fibers and to achieve otherdesirable properties, significant challenges remain in utilizing natural fibers as reliable
composite reinforcement materials. Therefore, the self-binding of powdered high-lignin
natural fibers and biomass is a preferable alternative.

14

Figure 2. Scheme presentation of plant fiber surface modification pathways adapted from
[22]

Figure 3. Schematic of papermaking concept also used for producing cellulose-basedmaterials
15

Recently, the utilization of biomasses has gained considerable attention. For example,
new ways have been devised for the fabrication of cellulose-based materials from sugar
palm fibers (Fig. 3) [23, 24] and for the production of bioethanol, biogas, and biohydrogen[25] from various biomasses. However, high-lignin fibers might not be useful for these
purposes because acid hydrolysis is required to remove large amounts of unnecessary
lignin. Nevertheless, valorization of lignocellulosic fibrous waste by producing
nanofibrillated cellulose for low-lignin content fibers might have a less adverse impact on
the environment because high-lignin fibers require complete removal of lignin(delignification) using harsh chemicals.
On the other hand, technical lignin, a byproduct of chemical extraction, is used as
filler in composites [26] and PLA-lignin biocomposites [26–28]. The introduction oftechnical lignin in small amounts in composites improved their mechanical properties,
ultraviolet (UV) light resistance, and thermal stability [23]. However, an increase in thecontent of technical lignin hampers its mechanical properties because it plays the role offiller. Hence, chemical modification to enhance reactivity using esterification or grafting isoften required [29]. Furthermore, the chemical structure of technical lignins is alteredsignificantly, leading to severe damage to their functionality and impurities that limit theiruse in value-added products [30].

16Figure 4. Technical lignin’s functions in application adapted from [24]Moreover, chemical modifications are necessary to enhance reactivity using esterification
or grafting [31]. Other studies reported cellulose/protein or lignin/protein hybrid
biocomposites [32–35]. However, solvents such as ionic liquids are required to dissolveand destroy hydrogen bonds, which destroy the chemical structure of lignin responsible
for thermosetting behavior, an essential feature of native lignin.
Native lignin condensation in natural fibers produces binderless particleboards
prepared by hot pressing, as reported in many studies [36–39]. Lignin activation is related
to the disruption of crystalline regions in cellulose and a change in the chemicalcomposition achieved by hydrolyzing hemicellulose and other nonbinding constituents,
as shown in (Fig. 5). However, thermal pretreatment above 120 °C weakens lignin
adhesion [4], which results in inferior mechanical properties.
Binderless high-density, high-performance particleboards prepared from coconut fiberand a pitch prepared by hot-pressing non-pretreated milled powders, as shown in Fig. 6,reportedly have a strength (50 MPa) and stiffness (5 GPa) superior to those prepared bysteam explosion and extrusion [4,9]. Superior physical properties were attributed to thepreservation of intrinsic thermosetting and lignin condensation owing to high lignin content.
17Figure 5. Effect of hot-pressing temperature on fiberboards made from unripe coconut
husk derived from [25], showing lignin’s activation evident at a higher temperature due tothermal hydrolysis of cellulose and hemicellulose at the surface
Furthermore, binderless wood compacts have been hot-pressed using milled Japanesecedar, cypress wood microparticles (≤355 µm), and 20 wt.% wood microparticles (≤53µm) [40, 41]. The resulting composite has a bending strength ranging from 70–80 MPa,
attributed to factors such as pretreatment conditions and particle size. However, chemicalcompositions such as lignin and cellulose contents of wood powders have not been
investigated. Such studies rarely involve a synchronized general method that can beapplied to other natural fibers because correlating chemical composition and particle
sizes of milled agriculture biomass impacts native lignin thermosetting efficiency.

Figure 6. a) Conversion process of coconut husk to binderless boards and b) sample
coir-board derived from [9]18As shown in Table 1, binderless protein-based biopolymers have been prepared bythermal processing of commercial silk microparticles with 20 wt% added water using
spark-plasma sintering [42], Bombyx mori, and Eri non-degummed silk-based resin
prepared from milled microparticles (~70 µm). The resultant composite has a higher
bending strength (~122 MPa) and thermal resistance [43]. Regenerated silk (sericin-free)based thermoplastic is prepared by hot pressing followed by hot rolling of boiling waterwith pretreated resins [44] and regenerated silk-based thermoplastics are prepared from
nanoparticles [45]. As with binderless particleboards, a correlation between particle size
and amino acid composition is not well understood yet. However, findings show that manyfactors influencing the binding behavior and mechanical properties of protein-based
biopolymers and thermoplastics are attributed to the self-assembly and rearrangement of
the molecules of silk in the presence of bound water. This behavior is governed by theamino acid composition (the ratio of hydrophilic to hydrophobic amino acids), size, andorientation of particles.

Table 1. Mechanical properties of silk-based resins and thermoplastics
Silk-based
Biopolymer(raw materials)

Bending
Strength(MPa)

Strain at
Break
(%)

Young’s
Modulus(GPa)

Density(g/cm3)

Ref.

Regenerated silk+ 20 wt% water(sericin-free microparticles)
Bombyx mori(undegummed microparticles)
Eri silk(undegummed microparticles)
Regenerated silk resin(sericin-free microparticles)
Silk-based thermoplastics(sericin-free nanoparticles)

77–135-

2.4–3.5

-

[26]

122

-

8.7

1.45

[27]

95

-

8.2

1.32

[27]

93 ± 141.8 ± 0.2

4.7 ± 0.5

-

[28]

150

2.5

8

1.35

[29]

To the best of our knowledge, no studies have reported the influence of chemicalcomposition, alkaline pretreatment, and milled powders, consisting of microparticles ofdifferent sizes, on the production of binderless biopolymers from high-lignin-based orprotein-based biopolymers. Furthermore, no studies have reported utilizing coconut coir19

or regenerated silk microparticles as green binders for cellulosic waste such as
construction and demolition waste (CDW) wood nor protein/lignocellulosic hybrid
biomicrocomposites at the macroscopic (micrometer) level. However, incorporating
regenerated silk microparticles reported in Poly(butylene adipate-Co-terephthalate) andPoly(lactic acid) (referred to as PBAT/PLA) composites [45] results in poor mechanicalproperties due to weak adhesion and poor dispersion. Similarly, cellulosic biomass suchas CDW wood was used as a filler to increase the biological content and improve thermal
stability in wood-plastics composites [46]. However, it showed a poor effect onmechanical properties due to low reactivity between plastics and wood.
We hypothesize that the microfibrillation of lignocellulosic biomass microparticles
releases and distributes the lignin trapped in inner cell-wall layers to the surface, where
they become available for activation. However, chemical composition, which defines theratio of cellulose and lignin to hemicellulose, extractives, and ash contents (C+L)/(H+E+A),
plays an important role in enabling efficient lignin thermosetting. Therefore, higher(C+L)/(H+E+A) ratios, and the removal of non-structural materials (i.e., hemicellulose,
extractives) by mild alkaline pretreatment, enables a stronger binding interaction between
lignin units and with other constituents, thus resulting in superior mechanical and thermal
properties. Furthermore, (C+L)/(H+E+A) and microfibrillation influence processing
conditions such as hot-pressing temperature and lignocellulosic biopolymers’ color (Fig.
7).

20

Figure 7. Major factors that influence optimal lignin binding to prepare lignocellulosic
biopolymers
Therefore, we hypothesize that utilizing the ability of native lignin to bind and selfassemble (thermal fusion phase) silk found in abundant sources, such as coconut coirand regenerated silk, can produce natural binders for use in hybrid biomicrocomposites,
resulting in superior mechanical and thermal properties. Furthermore, these naturalbinders are better than traditional solvent systems such as polypropylene (PP) and urea
phenol-formaldehyde, which are typically used in lignocellulosic/protein blended
composites, wood-plastic composites, and conventional particleboards, respectively. No
studies have reported utilizing coconut coir and regenerated silk microparticles as green
binders of cellulosic waste (e.g., CDW wood or protein/lignocellulosic hybrid
biomicrocomposites) at the macroscopic (micrometer) level.

212. Materials and preparation methods
2.1 Materials used in biopolymer preparation
2.1.1 Main materials
Raw materials used in this research were natural date–palm fibers and leaves harvested
directly from a Phoenix dactylifera tree (from a local farm in Madina, Saudi Arabia), maturecoconut coir used for bird nests that was purchased from a commercial supplier in
Malaysia, and coconut leaves gathered from a local farm in Medan, Indonesia (Fig. 8).
Chemicals used for alkaline pretreatment are sodium hydroxide (NaOH, Chameleon
Reagent, 98%) and acetic acid (CH3COOH, acquired from Kanto Chemical Co. Inc.,

99.7%).
Figure 8. Photographs of (a) Phoenix dactylifera tree from which fibers and leaves wereextracted for raw materials, (b) date fibers, (c) Cocos nucifera tree, and (d) coconut leaves

22All the fibers and leaflets were washed with tap water to remove dirt and dust and thenair-dried at room temperature at (20 – 25 °C) for one week. Date and coconut leaves
required additional drying in an oven at 60 °C for 5 days and were then sun-dried for an
additional 2 days because of the high moisture content and chlorophyll. Fibers weremanually cut into small sizes and stored in vacuum-type containers until further
processing.
2.1.2 Supplementary materials
Cotton and coconut coir fibers are purchased from a commercial supplier, and recycled
waste wood milled powder is donated by Air Water ECOROCA Inc., Japan and bagasse
waste fiber by Tsukishima Kikai Co., Ltd, Japan. Bamboo flour with an average particle
size of approximately 75 µm is purchased from Nakawood, Naga Wood Co., Ltd, Japan.
The bagasse waste fibers are washed with warm water for 10 min and then air-dried for
5 days, unlike plant fibers that are used as received.
2.2 Determination of chemical composition and moisture content
Chemical compositions of fibers and leaves and coconut–palm trees were determined by
hot-water and ethanol exhaustive extraction (for 10–24 h) using the Soxhlet Method
adapted from [30]. The method had to be modified slightly to determine the water- andalcohol-soluble materials. Holocellulose content was determined by the method of Wiseet al. [31], using sodium chlorite, whereas α-cellulose content was determined using 17.5
wt.% sodium hydroxide (NaOH). Acid-insoluble lignin (ASL) content was calculated
according to TAPPI T222, using the Klason Method [32] with sulfuric acid (72 wt.%).
Lignin content correction was adjusted by subtracting ash content obtained from fibers.
Moisture and ash contents were determined using thermogravimetric analysis (TGA) in
air, wherein a 10–12 mg sample of each lignocellulose was heated at 10 °C/min to 700 °C
until complete combustion. Moisture and ash content was determined by weight loss
percentage measured at 130 and 700 °C, respectively. All the tests were repeated at least
three times to ensure accuracy.

23

2.3 Alkaline pretreatment
Raw date and coconut fibers and leaves were dissolved in 1 wt% and 3 wt% NaOH
aqueous solutions, with a fiber to solution ratio of 1:30 (w/v). The solution is heated to
95 °C in 30 min, kept for 15 min, and allowed to gradually cool for 30 min at room
temperature. The treated fibers and leaves were washed under tap water for 10 min
before being immersed in 1 wt% acetic acid at room temperature for 10 min and thenwashed again with running water until neutral pH levels were achieved. Finally, the fibers
and leaves dried in an oven for 5 days at 60 °C. The appearance of alkaline pretreatedfibers and leaves can be observed in (Fig. 9).

Figure 9. Photographs of washed and dried raw and alkaline pretreated date, andcoconut fibers and leaves.

24

2.4 Powder preparation
All the raw and alkaline pretreated fibers and leaves were pulverized into powder (Fig.
10), using a planetary alumina ball mill (Pulverisette 6, Fritsch, Germany) operating at
300 rpm for 45 repetitions of a 3-min milling cycle with a 2-min pause between cycles.
Extracted powders were then sieved to desired microparticle sizes, ≤53, 53–75, and ≤75–
106 µm, in a sieving shaker (Retsch AS 200, Japan). However, it was challenging toobtain milled powders of raw and alkaline pretreated date–palm leaflets, and alkaline
pretreated coconut–palm leaflets at 3 wt% in particles smaller than 53 µm by milling or
sieving owing to powders’ orientation.

Figure 10. Photographs of powders prepared from raw and alkaline pretreated date andcoconut fibers and leaves, in addition to milled powders of cotton, recycled waste wood,bamboo, and waste bagasse
25

Figure 11. Flow process diagram depicting the production of lignocellulosic biopolymers
from raw and alkaline pretreated dates, coconut fibers and leaves, and waste bagasse inaddition to hot-pressed milled cotton, bamboo, CDW wood, and coconut coir fibers
2.5 Biopolymer preparation
A schematic of the sample preparation is presented in Fig. 11. First, 0.9 g of each of thedifferent particle size powders was placed in stainless-steel Ø25 mm cylindrical molds,
which were placed in a hot press (AS-1 AH-2003, Japan) wherein the top and bottom
plates were set at 170 °C and wrapped by a heating belt at 170–180 °C. The processing
pressure was maintained at 20 MPa until the optimum temperature was achieved for eachmaterial. The prepared samples, their corresponding labels, and processing conditions
are presented in Table 2. Subsequently, the disc-shaped specimens were graduallycooled at room temperature and dried in a vacuum oven at 100 °C for 2 days before
testing. The supplementary materials were used to investigate the influence ofpulverization, and chemical composition on the different types of lignin, non-lignin, andsemi-lignin-based adhesion.

26

Table 2. Prepared samples for biopolymers, their labels, and processing conditions
Samplecode

Raw material

Alkaline
pretreatment

Size

Moisturecontent(%)

Total hotpressing
time (min)9.78 ± 0.5
9.85 ± 0.5
9.68 ± 0.5
9.18 ± 0.5
9.98 ± 0.5
9.51 ± 0.5
6 ± 0.5
9.02 ± 0.5
8.93 ± 0.5
7.7 ± 0.5
6.2 ± 0.5
8.14 ± 0.5
9.13
7.57
5.7

Optimum
hot-pressing
temperature
(C°)
180
170
170
180
180
180
150
170
170
160
140
140
180
180
140
140

DF
AK1%DF
AK3%DFCC
AK1%CC
AK3%CCDL
AK1%DL
AK3%DLCL
AK1%CL
AK3%CLCC (PB)
CottonBagasse
CDW
wood*
BambooDate fiber
Date fiber
Date fiber
Coconut coir
Coconut coir
Coconut coir
Date leaflets
Date leaflets
Date leaflets
Coconut leaves
Coconut leaves
Coconut leaves
Coconut coirCommercial cotton
Bagasse waste
Waste Wood

1 wt% NaOH
3 wt% NaOH
1 wt% NaOH
3 wt% NaOH
1 wt% NaOH
3 wt% NaOH
1 wt% NaOH
3 wt% NaOH-

≤53 – ≤ 106 µm≤53 – ≤ 106 µm≤53 – ≤ 106 µm≤53 – ≤ 106 µm≤53 – ≤ 106 µm≤53 – ≤ 106 µm
53 – ≤ 106 µm
53 – ≤ 106 µm
53 – ≤ 106 µm≤53 – ≤ 106 µm≤53 – ≤ 106 µm
53 – ≤ 106 µm(5 – 10 cm)(≤75–106 µm)(53 – 75 µm)(53 – 75 µm)

Bamboo waste-(≈75 µm)

5.7

140

4–6

7–9
8–11
8–11
7–9
7–10
7–10
3–4
5–7
5–8
4–6
4–6
4–6
7–9
7–9
3–4
5–7

2.6 Materials used in biomicrocomposites
2.6.1 Main materials
Raw coconut coir microparticles (particle size ≤53 µm, moisture content of 9.1%) wereobtained by ball milling and then sieving the mature coconut coir that was originallyharvested in Malaysia and purchased from a commercial supplier. CDW wood powder(mean particle size ≈114 µm, moisture content of 5.97%, ash content of 7.59%) was
donated by Air Water ECOROCA Inc. (Japan). Commercially available degummed B.
mori silk (sericin-free) microparticles (average particle size ≈7 µm, moisture content
8.18%, ash content 0.02%) were purchased from KB Seiren Ltd., Japan. The amino acidcomposition of regenerated silk microparticles (0.53% proline, 42.75% glycine, 10.51%
serine, 1.35% glutamine, 1.82% asparagine, 0.11% cysteine, 0.98% threonine, 0.28%
lysine, 0.19% histidine, 0.52% arginine, 30.61% alanine, 0.04% methionine, 2.35% valine,
0.76% phenylalanine, 0.68% isoleucine, 0.54% leucine, and 5.68% tyrosine) was
reported in [28].

272.6.2 Supplementary materials
CDW wood and coconut coir microparticles were sieved to different particle sizes ≤53 µm,
71–106 µm, and 75–160 µm, while raw and alkaline pretreated bagasse and coconut coir
were sieved to a microparticle size ≤53 µm.
2.7 Biomicrocomposite preparation

Figure 12. Flow process diagram of the preparation of biomicrocomposites
First, 0.9 g of blended microparticles was adjusted to obtain CDW wood/coconut coir,regenerated silk/coconut coir, and regenerated silk/CDW wood at mass ratios of 80:20,
60:40, 50:50, 40:60, and 20:80, respectively, and then mixed manually using an agate
mortar and pestle. A schematic of the sample preparation is presented in Fig. 12. The
samples were placed in stainless-steel Ø20 mm cylindrical molds and positioned in a hotpress (H300-05, AS One, Japan) wherein the top and bottom plates were set to 170 °C
28and wrapped by a heating belt at 180 °C to accelerate heat transfer to the mold. The
processing pressure was maintained at 20 MPa, and the samples were extracted when
the optimum temperature reached 170 °C for the CDW/CC and CDW/RS woodbiomicrocomposites and 180 °C for the RS/CC biomicrocomposites. Subsequently, thedisc-shaped biomicrocomposite specimens shown in Fig. 13 were gradually cooled atroom temperature and dried in a vacuum oven at 100 °C for 24–48 h to reverse theinfluence of moisture content before testing. The prepared samples, their correspondinglabels, and their compositions are presented in Tables 3–4.

Figure 13. Appearance of neat biopolymers and their hybrid biomicrocomposites

29

Table 3. Prepared samples for biomicrocomposites, their labels, and compositions(wt%)
Sample Code
CC
CDW
RS
CDW80/CC20
CDW60/CC40
CDW50/CC50
CDW40/CC60
CDW20/CC80
CDW80/RS20
CDW60/RS40
CDW50/RS50
CDW40/RS60
CDW20/RS80
RS80/CC20
RS60/CC40
RS50/CC50
RS40/CC60
RS20/CC80

Coconut coir
100
20
40
50
60
80
20
40
50
40
20

Composition (wt %)CDW wood
Regenerated silk
100
100
80
60
50
40
20
80
20
60
40
50
50
40
60
20
80
80
60
50
60
80

Table 4. Prepared samples and their labels used in supplementary materials(biomicrocomposites)
Sample code
BG
AK1%CC
AK1%BG

Microparticles
Waste bagasse
Alkaline pretreated coconut coir at 1 wt% NaOH
Alkaline pretreated waste bagasse at 1 wt% NaOH

30

3. Characterization
3.1. Determination of apparent density
Density was determined by an analytical balance (acquired from AUX120, SHIMADZU,
Co., Japan) based on five samples of each biopolymer using Archimedes’ principle.
3.2. Bending strength
Three-point bending strength, elastic modulus, and strain at break were evaluated using
a tensile tester with 1 kN maximum loading capacity (Autograph AGS-X, Japan) according
to the Japanese standard for testing flexural properties, JIS-K-7171 [33]. Small plateshaped samples were cut from each biopolymer and biomicrocomposite into 3-mm-wide
× 20-mm-long × 2-mm-thick test pieces. The crosshead speed and support span length
were set to 1 mm/min and 14 mm, respectively. Results were analyzed according tostatistical analysis of variants (ANOVA). A total of 10 samples were tested from eachbiopolymer prepared from powders consisting of ≤106 µm microparticles, including threesamples each of raw and alkaline pretreated date-fiber, coconut-coir, date-leaf, andcoconut-leaf biopolymer-prepared powders comprising ≤53-, ≤53–75, and ≤ 75–106 µm
microparticles, when possible. Furthermore, three or more samples were tested with
supplementary material-based biopolymers and hybrid biomicrocomposites.
3.3. Water absorption and thickness swelling
Water absorption (WA) and thickness swelling (TS) of the biopolymers andbiomicrocomposites were calculated according to Japanese industrial standard water
absorption and thickness swelling JIS-A-5905, sections 7.10 and 7.11, respectively [34].
Three or more samples of each biopolymer and hybrid biomicrocomposite werecompletely dried to a constant weight in a vacuum oven at 105 °C. The samples werethen cooled to a constant weight, and sample weight and thickness were measured
before the samples were immersed for 24 h in a flask containing 50 ml of distilled water.
The immersed samples were wiped with a dry cloth, and water absorption and thickness
swelling were measured using Eqs. 1 and 2, respectively, as follows:
𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 (%) =(𝑀𝑀₂− 𝑀𝑀₁)

𝑇𝑇ℎ𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 (%) =(𝑀𝑀₁)(𝑇𝑇₂−𝑇𝑇₁)
𝑇𝑇₁(1)× 100,(2)× 100,

31

where M₂ and T₂ are the weight and thickness of the sample immersed for 24 h and M₁
and T₁ are the initial sample mass and thickness, respectively.
3.4 Statistical analysis
Results were statistically analyzed using ANOVA carried out by data analysis software
OriginPro, Version 2020b at the OriginLab Co., USA. Mean comparison at 0.05 level ofsignificance (ρ ≤ 0.05) was performed according to Tukey’s method. Further, three ormore samples were used for evaluating the apparent densities, bending strengths, water
absorptions, and thickness swellings.
3.5. Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM) micrographs captured using JSM-6510 (JEOL, Ltd.,
Japan) were used to investigate fiber, powder, and biopolymer morphologies before andafter the alkaline treatment. First, the samples were fixed to an aluminum tray with
conductive tape and coated with a thin layer of platinum using a magnetron sputter MSP1S, SHINKKU VD, Tokyo, Japan. Then, they were placed in the SEM chamber andrecorded at 120×, 600×, and 1200× magnifications of the raw fibers, biopolymers, andmilled powders, respectively.
The SEM micrographs were used to investigate the material shape and size and tocompare the morphologies of neat biopolymers and their biomicrocomposites. First, thesamples were fixed to an aluminum tray with conductive tape and coated with a thin layerof carbon fiber using a carbon coating system (CC-40F; Meiwafosis, Osaka, Japan). Then,they were placed in an SEM chamber, and the micrographs were recorded at 190× and600× magnification, while neat biopolymers and biomicrocomposites at specified mass
ratios were captured at 190× magnification.
3.6. Thermogravimetric analysis (TGA)
Thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) were
performed using a thermogravimetric analyzer (EXSTAR TG/DTA 6300; Seiko
Instruments, Inc., Japan) to analyze the influence of the starting-material chemicalcomposition, alkaline pretreatment, and particle size on the thermal behavior of the palmbased lignocellulosic biopolymers. Sample weights (21–33 mg) of untreated biopolymers
were obtained from powders consisting of smaller and larger particles, alkaline pretreated32biopolymers, and hybrid biomicrocomposites heated at 10 °C/min in the range 30–700 °C
in the open. The samples were dried in an oven to reverse the influence of moisture before
testing, and the analysis was conducted under an oxidative atmosphere to ensurecomplete decomposition. The mass loss percentage was determined from the TGA
curves, where the onset temperature (Tonset) was defined as 1% mass loss from the TGA
curves and the end weight percentage was determined at 700 °C. Three majordegradation peaks were identified and selected from the DTG curves. The moisture andash content of the starting materials of the samples weighing 5–12 mg obtained by weightloss (%) measured at 130 and 700 °C are shown in Fig. 14, Fig. 15, and Table. 5.

Figure 14. Mass-loss percentage curves for (a) lignocellulosic powders consisting of
smaller microparticles, (b) lignocellulosic powders (pretreated with 1 wt% NaOH), and (c)lignocellulosic powders (pretreated with 3 wt% NaOH). Corresponding DTG of palmbased (d) lignocellulosic powders consisting of smaller microparticles, (e) lignocellulosic
powders (pretreated with 1 wt% NaOH), and (f) lignocellulosic powders (pretreated with
3 wt% NaOH)

33

Figure 15. Mass-loss percentage curves for (a) supplementary materials powders and(b) microparticles used in biomicrocomposites. (c) and (b) denote their corresponding
DTG, respectively

34

Table 5. Thermal analysis to determine moisture and ash content of powder used in the
preparation of biopolymers and biomicrocomposites
Starting Materials(powders)

Moisture Content at
130 °C

Date fiber (raw)Date fiber (1 wt% NaOH)
Date fiber (3 wt% NaOH)
Coconut coir (raw)Coco. Coir (1 wt% NaOH)
Coco. Coir (3 wt% NaOH)
Date leaf (raw)Date leaf (1 wt% NaOH)
Date leaf (3 wt% NaOH)
Coco. Leaf (raw)Coco. Leaf (1 wt% NaOH)
Coco. Leaf (3 wt% NaOH)
CDW wood
BagasseCottonBambooCoconut coir (raw)Regenerated silk (raw)
CDW wood (raw)

9.78
9.85
9.68
9.18
9.98
9.51
6
9.02
8.93
7.7
6.2
8.14
5.7
7.57
9.13
5.7
9.1
8.13
5.97

35

Degradation Maxima
Peaks (C)
i
ii
336
393
337
333
407
282
436
318
404
322
393
303
445
291
424
326
294
430
297
455
314
318
425
302
393
329
494
310
400
283
374
296
318
425

Remaining
Residues
(%) *
1.33
5.52
3.97
4.39
4.48
5.39
5.72
8.85
4.67
7.70
9.35
7.62
3.03
14.51
6.21
3.09
2.34
0.02
7.59

3.7. Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR)The Fourier-transform infrared spectroscopy (FTIR) spectra (JASCO FT/IR-6600; JASCO
Co., Japan) for the biopolymers were generated using the attenuated total reflectance(ATR) method for wavenumbers in the range 700–4000 cm-1 at 4 cm-1 resolution and 45
scans per sample to identify functional groups and note cellulose, lignin, hemicellulose,
and pectin contents and their peak area ratios in the obtained biopolymers. All spectra
were normalized using a method similar to the one described in [35]. The peaks attributed
to cellulose, hemicellulose, lignin, and pectin were identified by the second derivative
using graphing and data analysis software (OriginPro, Version 2019b; OriginLab Co.,
USA) according to a specified range and quantitative method [36,37]. All absorption
peaks attributed to cellulose, hemicellulose, lignin, and pectin in the “fingerprint” region
were calculated by the second derivative integration of the baseline curve for the specified
regions of interest using adjacent averaging as a smoothening method (Fig. 16). The
other peaks were identified by their maximum height. The summation of the major peakareas associated with cellulose (A895, A1160, A1317, A1370m, and A1420), lignin (A1505 andA1606), hemicellulose (A770, A1735, and A1745), pectin (A834 and A1246), and thecorresponding peak area ratios were used to depict the differences in chemicalcomposition among the obtained biopolymers.

Figure 16. Example of a method used to determine lignin’s peak (A1591) based on thepeak area in a) DF and b) CC

36

Figure 17. Example of a method used to determine β-sheet compositions, based onpeak area
The FTIR spectra (JASCO FT/IR-6600; JASCO Co., Japan) were generated by
performing 45 scans per sample for biomicrocomposites and their corresponding
biopolymers using the ATR method for wavenumbers in the range of 700–4000 cm-1 at 4
cm-1 resolution. The method helped identify the functional groups and their chemicalcompositions in the “fingerprint” region based on the method described in [40, 55]. It wasslightly modified to determine the cellulose, lignin, and hemicellulose contents in
CDW/CCs and the β-sheet, cellulose, and hemicellulose contents in RS/CC and CDW/RS
biomicrocomposites, as well as in their neat biopolymers. First, the spectra ofbiomicrocomposites were grouped with their corresponding biopolymers and normalized
to values of [0, 1].The peaks and peak areas associated with cellulose (A895, A1160,
A1312, A1370, and A1420), lignin (A1507 and A1595), hemicellulose (A1735 and A1745),
and β-sheets (A1620 and A1697) were then identified using second derivative integrationfrom a straight baseline. A 20-point adjacent averaging, used as a smoothing method,
and 10% threshold height peak filtering in the “fingerprint” region according to a specified
wavenumber range are shown in Table 6 and Fig. 17. The peak area summation
associated with cellulose, lignin, hemicellulose, and β-sheets was used to note thedifferences in the chemical compositions of the obtained biomicrocomposites. However,peak areas associated with the C-O-C antisymmetric stretching of cellulose (A1160) andaromatic vibration of phenol groups associated with lignin (A1507, and A1591) excluded
37

peak summations in RS/CC and CDW/RS biomicrocomposites due to either overlapping
or combined peaks found in the secondary structure of regenerated silk (amide III, amide
II, and amide I).
Table 6. Peak finding ranges associated with functional groups of celluloses, lignin,
hemicellulose, and β-sheets in biocomposites, determined for the summation of
associated peak areas according to the method described in [36], with slight modification
Assignment

Peak Area

Peak Finding
Ref
Range
β-Glycosidic linkage (amorphous cellulose)
A895
880–915[38,39]C-O-C antisymmetric stretching (cellulose)
A1160*
1140–1185[36]
CH2 symmetric bending (cellulose)
A1312
1300–1330[38,39]COH in plane bending (cellulose)A1370
1345–1385[38,39]CH2 symmetric bending (cellulose)
A1420
1400–1440[39,40]C=C aromatic skeletal vibration (lignin)
A1507*
1490–1530[36,41]
C=O aromatic skeletal vibration (lignin)
A1595*
1580–1710[36,41]
Carbonyl C=O stretching (hemicellulose)A1735, A1745
1710–1750[42]
C=O stretching in amide I (β-sheet)
A1620
1616–1622[43,44]
C=O stretching in amide I (β-sheet)
A1697
1697–1701[43,44]*Excluded from peak area summation in biomicrocomposites containing regenerated silk due
to peak overlapping among regenerated silk secondary structure and cellulose and lignin inplant fibers

The results were statistically analyzed using ANOVA to calculate the means andstatistical errors (± SE) from two samples for each biopolymer and biocomposite.
Graphing and data analysis software (obtained from OriginPro, Version 2020b; OriginLab
Co. USA) were used to perform the analysis.
3.8. X-ray diffraction (XRD)
X-ray diffraction (XRD) (Ultima IV Protectus; Rigaku Co., Japan) was conducted todetermine crystallinity indices (Xc) of raw and alkaline pretreated lignocellulosic
biopolymers and their corresponding raw fibers and powders. In addition to XRD
diffraction patterns of hot-pressed biopolymers, CuKα radiation was used, and XRD wasoperated at 40 kV and 30 mA. Samples were scanned at 0.2°/min in the 2θ range 5–50°.
Crystallinity (Xc) was analyzed according to an empirical method of calculating crystallinity
index (Xc) of lignocellulose fibers, as previously reported [45]. The method is represented
by Eq. 3 as follows:
𝑋𝑋𝑐𝑐(%) =𝐼𝐼(002)−𝐼𝐼(𝐴𝐴𝐴𝐴)
𝐼𝐼(002),(3)
38

where I (002) is the maximum intensity of the crystalline region and I(AM) is the minimum
intensity of the amorphous region between crystalline peaks at 16 and 22°.

39

4. Influence of chemical composition, particle size, and alkaline pretreatment onmechanical, thermal, and structural properties of lignocellulosic biopolymers
4.1 Chemical composition
Table 7. Chemical compositions and (C+L)/(H+E+A) ratio of date palm fiber, coconut coir,and date and coconut leaves
Plant fiber

αCellulose

Lignin(%)

Hemicellulose
(%)

Water
Extractive
(%)

Total
Extractiv
es (%)Ash(%)(C+L)/(H+E+A)*(%)
Date fiber

43± 0.5

20± 0.525 ± 0.5

8.1 ± 0.5

12 ± 23.2± 0.5

1.56

Coconut coir

46± 0.528 ± 0.5

16 ± 0.5

7.2 ± 0.5

10 ± 24.2± 0.5

2.43

Date leaves

26 ± 0.5

16 ± 0.5

18 ± 0.5

6.3 ± 0.5

27 ± 25.5± 0.5

0.83

Coconut
leaves33 ± 0.5

21 ± 0.5

19 ± 0.5

8.2 ± 0.5

16 ± 27.6± 0.5

1.27* The ratio of cellulose and lignin to hemicellulose, extractives, and ash content.

As presented in Table 7, raw palm fibers and leaves exhibited different cellulose, lignin,
hemicellulose, extractive, ash, and moisture contents. Although coconut coir and datefiber demonstrated comparable compositions and roughly similar cellulose, extractive,
and ash contents, coconut coir exhibited the highest lignin content (~28%), while datefiber showed the highest hemicellulose content (~25%). Furthermore, although dateleaves exhibited the lowest water extractive content (~6.3%), they showed the highest
total extractive content (~27%) of all the lignocellulose residues. Meanwhile, CLs showed
higher cellulose content (∼33%) and hemicellulose content (~19%) than date leaves, andthey exhibited the highest ash content (~7.6%) and roughly the same lignin content asdate fiber. Moisture content seemed to be influenced by the hemicellulose content, which
is attributed to moisture absorption, while lignin and cellulose crystalline regions are
considered hydrophobic [46,47]. Date fiber and coconut coir showed roughly similarmoisture contents of ~7 and ~6.7%, respectively, while coconut and date leaves exhibited
moisture contents of ~5.5 and ~3%, respectively.
Higher palm leaf extractive content similarly has been reported for oil palm leaves [48].
Hence, the date leaf defense mechanism against dehydration under extremely arid
conditions in North Africa and the Middle East [1] enables these leaves to produce waxy
40

layers. Furthermore, it is worth noting that the higher lignin content in CLs is perhaps
attributed to the broader leaf midrib, compared to the narrow midrib found in date leaves.
When preparing lignocellulosic binderless biopolymers, the chemical composition of
lignocellulose residue should be categorized into two groups: (i) structural materials (e.g.,cellulose and lignin) and (ii) non-structural materials (e.g., hemicellulose [46], extractives[48], and ash [49]), which degrade mechanical properties. Palm fibers showed higher
structural material content (63–74%) than palm leaves (42–54%), while date leaves
showed considerably higher total non-structural material content (~50.5%) than the otherthree materials (30.4–42.2%). These results show the similarities and differences withchemical compositions that are reported for sugar palm fibers [50], various parts of oilpalm trees [48], date leaflets [51], and different parts of date palm fibers (DFs) [52].
Chemical composition varies according to geographical location [12] or maturity of fibers,
such as coconut coir [4]. This indicates that the ratio of cellulose and lignin to
hemicellulose, extractives, and ash (C+L)/(H+E+A) is a good indicator of the efficient
application of agricultural biomass for producing binderless biopolymers. The ratio also
helps predict the mechanical and thermal properties of the resulting biopolymers.
4.2. Mechanical and physical properties
The mechanical properties of lignocellulosic biopolymers were strongly influenced by the(C+L)/(H+E+A) ratio, alkaline pretreatment concentration, and raw-material particle size.
Table 8 and Fig. 18 show that coconut coir (CC)-based biopolymers exhibited higher
bending strength (~109 ± 10 MPa) and strain (~1.75 ± 0.23%) compared to otheruntreated lignocellulosic biopolymers. DFs have a higher Young’s modulus (~6.02 ± 0.67GPa) than CC-based biopolymers and better mechanical properties than CLs. DF andCC exhibited higher densities (1.44–1.46 g/cm3) than CL (~1.42 g/cm3). Conversely, dateleaves (DLs) have the lowest bending strength (~33 ± 8 MPa), strain (~0.97 ± 0.17%),
Young’s modulus (~ 4.61 ± 0.37 GPa), and density (~1.32 ± 0.8 g/cm3).
Table 8 shows fiber- and leaf-based biopolymers demonstrating statistically significant
differences in bending strength, density, and Young’s modulus, suggesting a clear
association between (C+L)/(H+E+A) and superior mechanical properties of fiber-based
lignocellulosic biopolymers. However, according to the results obtained from Tukey’s
comparison test, DF and CC did not show any statistically significant difference in41

mechanical properties. Moreover, DL and CL did not show any statistically significant
difference in Young’s modulus, which may be because they have similar cellulose andlignin content.
Table 8. Mean of mechanical and physical properties (± SD) of raw and alkaline
pretreated palm-based lignocellulosic biopolymers prepared from microparticles
Lignocellulosic
Biopolymer

Bending
Strength(MPa)

Strain at
Break
(%)

Young’s
Modulus(GPa)

Density(g/cm3)

Water
Absorption(%)

Thickness
Swelling(%)
DF
101 ± 12a1.54 ± 0.13a,d 6.27 ± 0.29a 1.46 ± 0.01a,e
30 ± 4a20 ± 3a
AK1%DF116 ± 12e
1.71 ± 0.18e
6.75 ± 0.45a 1.47 ± 0.01e,f
43 ± 7e30 ± 5eeedfe
AK3%DF116 +6
1.53 ± 0.14
7.34 ± 0.56
1.48 ± 0.01
51 ± 731 ± 3eCC
109 ±10a1.75 ± 0.23a,f 6.02 ± 0.67a 1.44 ± 0.01a,c
12 ± 2b
11 ± 1bffagf
AK1%CC122 ±14
1.81 ± 0.20
6.40 ± 0.58
1.46 ± 0.01
20 ± 115 ± 2fafagf
AK3%CC
109 ± 81.62 ± 0.10
6.48 ± 0.39
1.46 ± 0.01
22 ± 419 ± 3f
DL
33 ± 8b0.97 ± 0.17b,h 4.61 ± 0.37b
1.37 ± 0.03b
46 ± 2c
41 ± 4c,gggehg
AK1%DL98 ± 111.49 ± 0.10
6.46 ± 0.32
1.47 ± 0.01
58 ± 237 ± 4g,hbhfig
AK3%DL32 ± 60.89 ± 0.12
4.00 ± 0.57
1.41 ± 0.01
62 ± 534 ± 2hCL
84 ± 9d1.45 ± 0.17d,i 5.39 ± 0.34b 1.42 ± 0.01c,k
40 ± 1d27 ± 2dhigjh
AK1%CL48 ± 81.29 ± 0.24
3.96 ± 0.99
1.39 ± 0.02
65 ± 338 ± 4iiihkh
AK3%CL33 ± 6
1.27 ± 0.19
2.96 ± 0.50
1.40 ± 0.01
62 ± 441 ± 5i
Different letters in the same column represent a significant difference based on Tukey’s testcomparison (p˂0.05) of mean properties among untreated biopolymers and untreated andalkaline pretreated biopolymers prepared from the same plant fiber.

Moreover, Table 8 and Fig. 18 show the positive influence of mild alkali pretreatment fordifferent types of plant fibers. Alkali pretreated fiber-based lignocellulosic biopolymers
show better mechanical properties compared to untreated fiber-based biopolymers.
Fiber-based biopolymers prepared from higher concentration alkali treatments, AK3%DF
and AK3%CC, resulted in a statistically significant difference in Young’s modulus andbending properties compared to AK1%DF and AK1%CC, respectively. AK1%DL showeda higher bending strength (∼98 ± 11), strain (∼1.49 ± 0.10%), Young’s modulus (∼6.46 ±0.32 GPa), and density (∼1.47 g/cm3) compared to untreated and alkali pretreated leafbased biopolymers. Conversely, AK3%DL, AK1%CL, and AK3%CL exhibited lower
bending strength, strain, Young’s modulus, and densities compared to untreated leafbased biopolymers.

42

Figure 18. Means of mechanical and physical properties of raw and alkaline pretreatedpalm-based lignocellulosic biopolymers prepared from microparticles (out of 10 samples)

43

Table 9. Influence of particle size on bending properties (± SD) of raw and alkaline
pretreated palm-based lignocellulosic biopolymers
Biopolymer
Bending
Strain atYoung’s Modulus(Processing powder’sStrength
Break
(GPa)
size)
(MPa)
(%)
DF (≤53 µm)
115 ± 4a1.69 ± 0.03a6.59 ± 0.01a,c
DF (53–75 µm)
97 ± 4b1.50 ± 0.03b5.99 ± 0.12bbc
DF (75–106 µm)
90 ± 3
1.39 ± 0.04
6.31 ± 0.26caa
AK1%DF (≤53 µm)
128 ± 21.90 ± 0.08
6.80 ± 0.35a
AK1%DF (53–75 µm)
106 ± 4b1.57 ± 0.11b
7.24 ± 0.02aaa
AK3%DF (≤53 µm)
124 ± 2
1.69 ± 0.15
7.63 ± 0.46abb
AK3%DF (53–75 µm)
112 ± 4
1.46 ± 0.08
7.15 ± 1.01a
CC (≤53 µm)
120 ± 3a1.85 ± 0.04a
6.18 ± 0.11aa,ba
CC (53–75 µm)
111 ± 6
1.79 ± 0.37
6.08 ± 1.58aba
CC (75–106 µm)
102 ± 8
1.54 ± 0.17
6.17 ± 0.41aaa
AK1%CC (≤53 µm)
140 ± 102.05 ± 0.17
5.70 ± 0.53a
AK1%CC (53–75 µm)
114 ± 1b1.68 ± 0.08b6.24 ± 0.61aaa
AK3%CC (≤53 µm)
118 ± 51.73 ± 0.12
6.67 ± 0.25aaa
AK3%CC (53–75 µm)
109 ± 3
1.58 ± 0.03
6.85 ± 0.12a
DL (53–75 µm)
34 ± 2a0.85 ± 0.07a
4.43 ± 0.29aba
DL (75–106 µm)
40 ± 41.05 ± 0.15
5.01 ± 0.18baa
AK1%DL (53–75 µm)
106 ± 6
1.57 ± 0.01
6.64 ± 0.31a
AK1%DL (75–106 µm)
99 ± 9a1.55 ± 0.03a
6.33 ± 0.31aaa
AK3%DL (53 – 75 µm)
30 ± 80.87 ± 0.11
3.62 ± 0.46aaa
AK3%DL (75–106 µm)
37 ± 20.90 ± 0.10
4.73 ± 0.04b
CL (≤53 µm)
95 ± 4a1.65 ± 0.07a
5.62 ± 0.44aa,ba,bCL (53–75 µm)
84 ± 7
1.54 ± 0.10
5.24 ± 0.05abc
CL (75 – 106 µm)
75 ± 1
1.39 ± 0.13
5.65 ± 0.10aaa
AK1%CL (≤53 µm)
55 ± 11
1.33 ± 0.08
4.56 ± 1.20a
AK1%CL (53–75 µm)
44 ± 6a1.36 ± 0.42a
3.20 ± 0.33aaa
AK1%CL (75–106 µm)
47 ± 4
1.52 ± 0.10
3.72 ± 0.71aaa
AK3%CL (53–75 µm)
33 ± 2
1.38 ± 0.10
2.73 ± 0.26a
AK3%CL (75–106 µm)
38 ± 3a1.37 ± 0.14a
3.49 ± 0.35b
Different letters in the same column represent the statistically significant
differences (p˂0.05) in the mean properties of each biopolymer prepared withdifferent-sized particles.

Table 9 and Figs. 19 and 20 demonstrate a positive influence of smaller milled-powder
particle size on bending properties and strains in untreated and alkaline pretreated fiberbased biopolymers, compared to milled-powders with larger particle size. Meanwhile,
untreated and alkaline treated leaf-based biopolymers showed either negative influenceor no statistically significant differences in mean bending strength, mean strain, and mean
Young’s modulus. Furthermore, untreated or alkali pretreated biopolymers prepared fromdifferent particle sizes, which did not show any statistically significant differences in44

bending properties, suggests that particle sizes being too similar to observe any
substantial difference, and/or smaller particles would not substantially affect more lignin
surface exposure.

45

Figure 19. Influence of particle size on bending properties of raw and alkaline
pretreated fiber-based lignocellulosic biopolymers (out of three samples)

Figure 20. Influence of particle size on bending properties of raw and alkaline pretreatedleaf-based lignocellulosic biopolymers (out of three samples)

46

These results establish the previously reported findings regarding the influence ofchemical composition on the bending strength [48,53] and particle size on the bending
properties [54] of binderless particleboards prepared from unstalked cotton. The results
indicate that processed fine particles exhibit a higher bending strength compared tocoarse particles. These findings are validated in Table 10 and Figs. 21a–d, which showthat hot-pressed coconut fibers report inferior bending properties compared to hotpressed milled coconut coir. Furthermore, cotton, CDW wood, and bamboo reported
much poorer mechanical properties compared to AK3%DL, AK1%CL, and AK3%CL due
to the absence or low concentration of lignin available for binding. Meanwhile, bagassebased biopolymers reported mechanical properties similar to CL, delivering similar valuesof (C+L)/(H+E+A).
Table 10. Mechanical and physical properties (± SE) of hot-pressed biopolymers(supplementary materials)
Lignocellulosic
Biopolymer
CCPB
CottonBagasse
CDW wood
BambooBending
strength(MPa)
71 ± 1124 ± 480 ± 437 ± 440 ± 9Strain atbreak (%)
2.35 ± 0.10
0.91 ± 0.20
1.22 ± 0.10
1.37 ± 0.55
1.20 ± 0.16

Young’smodules(GPa)
3.54 ± 0.25
3.13 ± 0.55
6.24 ± 0.35
3.00 ± 0.49
4.51 ± 1.05

Density(g/cm3)
1.33 ± 0.01
1.50 ± 0.02
1.51 ± 0.01
1.34 ± 0.01
1.20 ± 0.03

Water
absorption(%)
35 ± 332 ± 459 ± 1361 ± 20

Thicknessswelling(%)
26 ± 338 ± 646 ± 1133 ± 10

The results may provide evidence that adhesion in lignocellulosic biopolymers increased
when prepared from fibers and leaves treated after the removal of non-structural materials
by mild alkaline pretreatment and/or when prepared from powders consisting of smaller
particles, depending on chemical compositions of starting materials.

47

Figure21.

Mechanicalandphysicalproperties

ofhot-pressedbiopolymers

supplementary materials (out of three samples)
4.3. Water resistance
Table 8 and Fig. 18 also show a comparison of water absorption and thickness swellingof untreated lignocellulosic biopolymers and alkaline treated biopolymers. CC exhibited48

the lowest water absorption (~12%) and thickness swelling (~11%) among all the
biopolymers. DF showed almost double the water absorption (~30%) and thickness
swelling (~20%) of CC. Meanwhile, CL exhibited similar water absorptions, 46 and 40%,
respectively, but considerably different thickness swellings, ~41 and ~27%, respectively.
All the lignocellulosic biopolymers showed statistically significant differences in meanwater absorption and mean thickness swelling. Conversely, alkali pretreated biopolymers
showed an increase in water absorption and thickness swellings, compared to untreatedbiopolymers. AK1%CC exhibited the lowest water absorption (~20%) and thickness
swelling (~15%) of all the alkali-pretreated biopolymers. Although statistically significant
differences were observed in mean water absorption and mean thickness swelling
between untreated and alkali pretreated biopolymers, no significant statistical differenceswere found between 1 wt% and 3 wt% alkali pretreated biopolymers. The poor water
resistance in alkaline pretreated biopolymers is possibly due to the higher cellulose
content and, consequently, an increase in hydroxyl groups [47] or insufficiency of residual
sodium hydroxide in the biopolymers [55,56].
Untreated biopolymers showed better water resistance than previously reported
binderless particleboards fabricated using Paraserianthes falcataria wood [57] and waste
oil-palm trunks [58]. The water absorptions were found to be similar to those reported
previously for binderless particleboards fabricated from oil-palm cores [48], binderless allcellulose materials [59], and unstalked cotton [54]. The water resistance depends on thechemical composition of lignocellulosic binderless biopolymers and the hemicellulose
content in untreated biopolymers [25,53]. As shown in Table 10 and Figs. 21e and f, hotpressed CDW wood and bamboo show higher values of water absorption and thickness
swelling than alkaline pretreated leaf-based biopolymers. However, it is difficult tomeasure the water resistance for hot-pressed cotton because of the shattering of samples.
On the other hand, bagasse-based biopolymers reported similar water absorption andthickness swelling comparable to coconut leaf-based biopolymers. Hot-pressed coconut
coir (CCPB) reported higher water absorption (~35 ± 3) and thickness swelling (~26 ± 3),
compared to CC, which exhibited water absorption (~12 ± 2) and thickness swelling (~11
± 1).

49

4.4 SEM morphologies

Figure 22. SEM micrographs of untreated and alkaline pretreated lignocellulosic
biopolymer

morphology

capturedat

600×

magnification

showing

biopolymer

morphologies of lignin-based adhesion
The lignocellulosic-biopolymer microstructures were examined by SEM micrographs
captured at 600× magnification of lignin adhesion (Fig. 22), semi-lignin adhesion (Fig. 23),and poor or no lignin-based adhesion (Figs. 24–25). The micrographs show the bindingof lignin to cellulose and other constituents and display biopolymer cracking. The palm
fiber-based biopolymers, CLs, and AK1%DL have a denser microstructure, and smoothersurfaces than the other biopolymers. This is due to good wettability enabled by lignin
binding and microfibrillation. Furthermore, fiber-based biopolymers showed slightly bettermorphology and a less porous structure than CL and AK1%DL, possibly due to more50

efficient lignin condensation. Although the CL microstructure exhibited a proper
distribution of activated lignin, the cracks were visible due to a large quantity of unbinding
materials. As shown in Fig. 23, hot-pressed coconut fibers show semi-lignin morphology
similar to hot-pressed binderless particleboard, which could be because the partialhydrolysis of cellulose and hemicellulose enables lignin activation.

Figure 23. SEM micrographs of hot-pressed coconut coir fibers (CCPB) morphology offracture captured at 600× magnification representing morphologies of semi lignin-based
adhesion

51

Figure 24. SEM micrographs of DL, AK 3%DL, AK 1%CL, and AK 3%CL morphologies
of fracture captured at 600× magnification representing morphologies of poor lignin-based
adhesion
DL showed more debonding areas and lack of integration due to larger particles and low
lignin content relative to high unbinding-material content. AK3%DL, AK1%CL, and
AK3%CL exhibit microstructures similar to DL due to low lignin content, as shown in Fig.
24. Meanwhile, low lignin content or its absence in hot-pressed cotton, CDW wood, andbamboo results in debonding areas and voids, as shown in Fig. 25. This is due to the lackof adhesion and large particle size of starting materials.
Fiber-based biopolymers CL, AK1%DL, and bagasse microstructures indicated that thelignin adhesion was condensation based, whereas other biopolymers exhibited poor lignin
adhesion, semi- and non-lignin-based adhesion morphology. These observations
confirmed that efficient lignin wettability could be attributed to the higher (C+L)/(H+E+A)
ratio of lignocellulosic biopolymers and microfibrillation.

Figure 25. SEM micrographs of hot-pressed cotton, CDW wood, and bamboo
morphologies of fracture captured at 600× magnification representing morphologies of
non-lignin-based adhesion
SEM micrographs demonstrate the effect of alkaline pretreatments on palm fibers andleaves (Fig. 26). Raw coconut coir exhibits smoother surface morphologies than otherfibers and leaves, indicating lower extractives and hemicellulose content on the surface.
Figs. 27 and 28 depict the particle size and distribution of milled powders in which themicrofibers are noticeable in date palm leaves.

52

Figure 26. SEM micrographs captured at 120× magnification showing the morphology of
raw and alkaline pretreated lignocellulosic fibers and leaves before pulverization

53

Figure 27. SEM micrographs of raw and alkaline pretreated lignocellulosic microparticles
captured at 1200× magnification showing particle distributions and orientation of eachpowder54

Figure 28. SEM micrographs of milled powders of cotton, bagasse, CDW wood, andbamboo captured at 1200× magnification showing particle distributions and orientation ofeach powder; CDW wood and bamboo show the presence of microfibers
4.5 Thermal behavior and stability
As presented in Table 11, the chemical composition and particle size of starting materials
determine the thermal stability of lignocellulosic biopolymers. Weight-loss curves
calculated based on TGA exhibited onset temperatures in the range 211–220 °C forcoconut coir-, date fiber-, and coconut leaf-based biopolymers (Fig. 29a), whereas onset
temperatures slightly decreased, in the range of 206–216 °C, for biopolymers preparedfrom larger particles (53–75 µm) (Fig. 29b). Conversely, the date leaflet-based biopolymer
exhibited the poorest thermal stability of all the biopolymers. However, it showed slightly
improved onset temperature (~148 °C) when prepared from smaller particles. Table 5
also shows the effect of alkali pretreatments on the thermal stability of alkali preparedbiopolymers. Weight-loss curves calculated based on TGA showed slightly higher onset
temperatures (221–232 °C) for alkali pretreated fiber-based biopolymers (Figs. 30a and30b) compared to the corresponding untreated biopolymers. AK1%DL exhibited asignificantly higher onset temperature (~207 °C), compared to DL. AK1%CL and
AK3%CL showed a slightly lower onset temperature (~198 °C) and (~202 °C) comparedto CL. No significant differences were observed among onset temperatures for55

biopolymers prepared from alkaline pretreatment fibers and leaves at 1 wt% and 3 wt%.
However, AK3%DF and AK3%CC showed higher endset weight percentage residues
compared to AK1%DF and AK1%CC, whereas AK3%DL and AK3%CL reported lower
endset weight percentages compared to AK1%DL and AK1%CL.
Table 11. Thermal properties of raw and alkaline pretreated palm-based lignocellulosic
biopolymers
Biopolymer(Processing material’s
size)

Onset
Temperature
at 1% (C)

Degradation
MaximaRemaining
Peaks (C)Residues
(%)
i
ii
ii
DF (≤53 µm)
220
260
323
498
2.82
DF (53–75 µm)
210
260
323
473
2.70
AK1%DF (≤53 µm)
221
333
488
2.15
AK3%DF (≤53 µm)
224
345
495
4.57
CC (≤53 µm)
221
246
302
534
3.94
CC (53–75 µm)
216
246
302
523
3.32
AK1%CC (≤53 µm)
228
329
488
1.35
AK3%CC (≤53 µm)
232
334
487
2.35
DL (53–75 µm)
148
260
312
464
7.23
DL (75–106 µm)
127
298
400
6.34
AK1%DL (53–75 µm)
207
255
306
570
10.88
AK3%DL (53–75 µm)
210
334
496
3.59
CL (≤53 µm)
211
250
305
550
7.59
CL (53–75 µm)
206
252
305
552
5.94
AK1%CL (≤53 µm)
198
300
465
7.08
AK3%CL (53–75 µm)
202
315
526
4.33
CCPB (5–10 cm)
206
266
300
421
1.6
Cotton (≤75 µm)
40*
330
494
6.22
Bagasse (53–75 µm)
202
261
323
484
12.59
CDW wood (53–75 µm)
182
312
385
2.82
Bamboo (≈75 µm)
180
295
361
1.17
*Hot-pressed milled cotton is sensitive to moisture and absorbs moisture immediately
after drying.

DTG curves in Figs. 29c–d show small sharp peaks for all fiber-based biopolymers, CL,
and AK1%DL in the range of 200–250 °C, possibly attributed to release of nonfreezingbound water, aliphatic OH groups, and decarboxylation of acids in the lignin [47,60,61].
These peaks are observed in strong lignin adhesion-based biopolymers as well as inbagasse-based biopolymers, as shown in (Fig. 31b).
Decomposition peaks in the ranges 246–260 °C and 300–345 °C are attributed tohemicellulose and cellulose, respectively [42]. The broader peaks in the range 280–
560 °C are attributed to lignin decomposition and char formation [62,63]. The DTG curves,
shown in (Fig. 30c) and (Fig. 30d), of alkaline-pretreated biopolymers exhibited an56absence or a negligible value of hemicellulose decomposition peaks owing to effective
removal of hemicellulose by alkaline pretreatment. This differs from the major degradation
maxima peaks around 300 °C found in DL(75–106 µm) (Fig. 28d) and AK1%CL (Fig. 29c),
which are the combined hemicellulose and cellulose decomposition peaks.

Figure 29. Mass-loss percentage of (a) untreated biopolymers prepared from smaller
particles and (b) untreated biopolymers prepared from larger particles. Corresponding
DTG of palm-based biopolymers (c) prepared from smaller particles and (d) preparedfrom larger particles

57

Figure 30. Mass-loss percentage of (a) biopolymers prepared 1 wt% alkaline pretreatedfibers and leaves and (b) biopolymers prepared 3 wt% alkaline pretreated fibers and leaf
microparticles. Corresponding DTG of palm-based biopolymers (c) prepared from 1 wt%
alkaline pretreated fibers and leaf microparticles and (d) prepared from 3 wt% alkaline
pretreated fibers and leaf microparticles
The TGA curves were generated to obtain moisture content at 130 °C and ash content atcomplete combustion (700 °C). However, powdered lignocellulose materials thermallydecomposed at lower temperatures because the crystalline structures were destroyed bymilling and due to the absence of lignin thermosetting. Unlike the DTG curves for otherbiopolymers, these curves only show one major decomposition peak, which is the
combined decomposition peak for cellulose and hemicellulose, and a combustion DTG
peak, suggesting that these samples showed poor thermal stability.

58

Figure 31. Mass-loss percentage curves for (a) hot-pressed supplementary materials,
the obtained hot-pressed biopolymers, and (b) corresponding DTG. The indicator shows
the peak of lignin-based adhesion at approximately 230 °C, also observed in bagassebased biopolymers
This is also observed in the DTG curves of milled powders (Figs. 14–15) and non-lignin
based-adhesion materials (Fig. 31), suggesting poor thermal stability in non-lignin-basedadhesion materials. The fluctuation of DTG peaks at approximately 600 °C can be
attributed to the combustion of biopolymers under an oxidative atmosphere [64]. Thesepeaks disappear in a non-oxidative atmosphere (i.e., argon gas) (Fig. 32 and Table 12).
The low thermal stability of natural fibers is generally determined by their chemicalcomposition, including low crystalline-cellulose, lignin, and extractive contents [40].
Higher thermal stability of the lignocellulose biopolymers is determined by their higher
lignin content, more efficient lignin condensation due to small particle size, and removalof non-structural materials by mild alkaline-pretreatment. The results suggest that hotpressing high-lignin smaller particles had enabled lignin to chemically modify into a more
complex form or a more activated (i.e., condensed) lignin [61]. A mild alkaline
59pretreatment enabled a more efficient lignin-based adhesion in alkaline pretreated fiberbased biopolymers and AK1%DL, compared to untreated date leaf-based biopolymer. On
the other hand, an overlap in the hemicellulose and cellulose decomposition peaks
observed in non-lignin adhesion-based biopolymers and their corresponding milled
powders suggests poor thermal stability, because condensed lignin is more thermallystable than non-condensed lignin [61,65].

Figure 32. Mass-loss percentage curves under argon gas for (a) coconut coir- based
lignocellulosic biopolymer and (b) corresponding DTG curve
Table 12. Thermal properties of raw coconut coir- based lignocellulosic analyzed under
non-oxidative atmosphere (argon gas)
Lignocellulosic
Biopolymer(processing
powder’s size)
CC

Onset
Temperatu
re at 1%(C)

220

DegradationMaximaPeaks (C)
i

ii

ii

24
6

300

-

60

Remaining
Residues(%)
36

4.6 FTIR spectroscopy
FTIR spectra generated for the lignocellulosic biopolymers are shown in Fig. 33, andcorresponding band assignments typical for lignocellulose fibers are listed in Table 13.
Specific absorption-band wavenumbers corresponding to functional groups in each
lignocellulosic biopolymer explore different characteristics of both untreated and alkaline
pretreated fiber- and leaf-based biopolymers, which can explain the variations inmechanical and thermal properties.
Table 13. FTIR band assignments (cm−1) for raw and alkaline palm-fiber- and leaf-based
biopolymers
Peaknumber
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27

Assignment

Ref.

Range

Glucomannan
Aromatic C–H out-of-plane deformation (G)Ring vibration of Pectin
Aromatic C–H out-of-plane deformation (G)β-Glycosidic linkage
Aromatic C-H deformation in Guaiacyl and C-O stretchinggroups in primary alcohol
C-O stretching groups in primary alcohol
Aromatic C-H in-plane deformation (typical for S units)
C-O-C antisymmetric stretching of cellulose
C-C, C-O, and C=O stretch G condensed ˃ etherified
C-O stretching of pectin
Guaiacyl ring C=O stretch
CH2 symmetric bending of cellulose
CH2 symmetric bending of cellulose
CH bending deformation of cellulose
COH in plane bending of cellulose
C–H deformation of lignin and polysaccharides
C=C Aromatic skeletal vibrations of phenol groups in lignin
G˃S
Amid II N-H deformation (protein)
Amid II N-H deformation (protein)
C=C Aromatic skeletal vibrations of phenol groups ofaromatic ring S˃G
Absorption of water O-H
Amide I (protein)
Carbonyl C=O stretching of hemicellulose
C-H2 symmetric stretch
C-H3 antisymmetric stretch
O-H stretching

[38]
[36]
[66]
[36]
[38,39][36,39]760–780
800–829
829–845845–875
880–9151012–1042

[36]
[36]
[38,39]
[36]
[66]
[36]
[38,39][38,39][38,39][38,39][36][36,60]1045–1060
1100–1133
1140–1185
1220–1249
1239–1290
1239–1290
1300–1330
1320–1345
1345–1385
1400–1440
1444–1480
1490–1535

[67]
[67]
[36]

1530–1550
1550–1565
1574–1608

[68]
[67]
[42,69,70]
[71]
[71]
[71]

1620–1640
1645–1665
1710–1765
2800–3000

61

3150–3500

Figure 33. FTIR spectra of raw and alkaline pretreated lignocellulosic biopolymers
As presented in Table 14, leaf-based biopolymers exhibited more peaks associated with
pectin at approximately 834 and 1246 cm−1 compared to fiber-based biopolymers. Fiberbased biopolymers showed similar peaks. However, CC and AK1%CC exhibited an
additional peak at 851 cm−1 owing to the aromatic C–H out-of-plane deformation typicalof guaiacyl. AK3%DF and AK3%CC exhibited a disappearance of peaks amid the II N-H
deformation of proteins at 1540 and 1560 cm−1, carbonyl C=O stretching of hemicelluloseat (~1747 cm−1), and C-H2 symmetric stretching at (~2854 cm−1). AK3%DL and AK3%CL
showed a disappearance of peaks due to the presence of protein at 1653 and 1540 cm−1,
respectively. These results demonstrate the typical patterns of the effect of alkaline
pretreatment on the complete removal of hemicellulose and extractives and the partial
removal of lignin in plant fibers [72].

62Table 15 presents the summation of major peak areas associated with cellulose, lignin,
hemicellulose, and pectin among raw and alkali pretreated biopolymers to provide anestimation of chemical composition. Fiber-based biopolymers showed higher absorption
associated with cellulose, lignin, and hemicellulose, compared to leaf-based biopolymers.
Furthermore, CC reported the highest absorption of total cellulose (~3.79) and lignin(~1.56), while DF reported the highest values associated with hemicellulose (~1.76). CL
showed higher absorptions attributed to lignin, cellulose, and hemicellulose, while DL
reported slighter higher peak areas attributed to pectin (~0.15). These results are
consistent with the data presented in Table 6, suggesting that fiber-based biopolymers
have higher cellulose and lignin and lower extractive contents than leaf-based
biopolymers.
In alkali-pretreated biopolymers, the summation of the peak areas associated withcellulose increased, while that of hemicellulose and pectin dramatically decreased,compared to untreated biopolymers. AK3%DF, AK3%DL, and AK3%CL reported thehighest sum of peak areas of cellulose, compared to all other biopolymers. AK3%CC
reported a slightly lower sum of peak areas of cellulose compared to AK1%CC. Moreover,
the total peak areas associated with lignin reported the highest in AK1%CC (~1.95)
AK1%DF (~1.47) among all lignocellulosic biopolymers. These findings have beenvalidated by other studies [73], which evaluated the effect of alkaline pretreatment on
decreasing hemicellulose and increasing cellulose and lignin contents. While alkaline
pretreated leaf-based biopolymers exhibited a decrease in peak associated with lignin, It
is unknown if the effect of alkaline pretreatment on lignin-associated peaks in leaves canbe attributed to their low lignin content or higher extraction of lignin compared to plantfibers.

63

Table 14. Specific absorption-band wavenumbers (cm-¹) related to raw and alkaline
pretreated palm-fiber- and leaf-based biopolymers
Peak
no.
1

DF

AK1%
DF
770

770

2

817

3
4
5
6
7
8
9
10
11
12

896
1022
1120
1160
1233
1267

812
823
896
1025
1051
1161
1235
1267

13
14

1315
1331

15
16
17

AK3%
DF
766
773
814

CC
770

AK1%
CC
771

AK3%
CC
769

AK1%
DL
771

AK3%
DL
-

769

814

813

897
1030
1053
1113
1162
1231
1266

851
896
1028
1053
1121
1161
1233
1267

851
897
1026
1052
1119
1161
1229
1265

810
816
897
1026
1052
1118
1160
1227
1265

805
823
835
896
1028
1051
1116
1161
1233
1263

809
817
837
895
1031
1052
1113
1161
1235
1244
-

1316
1334

806
815
839
897
1023
1056
1112
1160
1233
1263
1271
1318
1331

834
897
1031
1054
1110
1159
1234
1246
-

1316
1337

1315
1335

1314
1336

1315
1332

1315
1331

1315
1333

1317
1337

1373
1419
1464

1372
1420
1459

1370
1422
1463

1373
1420
1458

1369
1421
1463

1371
1421
1463

1369
1429
1463

1370
1419
1462

1370
1420
1458

1369
1419
1463

18
19
20
21
22
23

1507
1540
1557
1592
1634
1652

1508
1542
1557
1592
1654

1507
1592
1652

1507
1541
1558
1592
1653

1508
1539
1591
1654

1507
1591
1633
1652

1507
1541
1559
1591
1654

1507
1541
1558
1593
-

1743

-

-

1516
1540
1557
1595
1634
1653
1659
1736

24

1745

-

25
26
27

2852
2921
3336
3348
3357

1732
1747
2881
2917
3336
3345
3355

1733

2853
2921
3337

2920
3337
3353

2919
3336
3343

2850
2920
3334
3348

2854
2922
3335
3344
3356

1733
1749
2852
2922
3335
3344
3359

2902
3334

64

DL
768
806

CL

AK1%
CL
772

AK3%
CL
769

-

811
821
834
898
1029
1053
1113
1161
1237
1249
1264

898
1029
1052
1113
1161
1233
1249
1263

1315
1332

1516
1541
1558
1592
1634
1653

1317
1330
1338
1370
1418
1457
1464
1507
1541
1558
1592
1635
1652

1735

1734

1734

2851
2920
3344
3355

2854
2922
3329
3345
3357

2854
2921
3342

1369
1421
1464
1504
1560
1592
1652

Figure 34. FTIR spectra of hot-pressed plant-based biopolymers

65As shown in Table 15, lignin to hemicellulose and pectin peak ratios agrees with later
findings, confirming the effect of alkaline pretreatment on removing non-structural
materials. Moreover, the highest cellulose to lignin peak ratios of HZ3%DL (~6.2),
HZ1%CL (~4.8), and HZ3%CL (~5.9) explain that the poor lignin adhesion can be
attributed to low lignin content. These results are also consistent with the data for hotpressed milled cotton, hot-pressed coconut fibers, and bagasse-based biopolymers
presented in Fig. 34 and Tables 15 and 16. However, it contradicts the data of hot-pressed
milled waste wood and bamboo, indicating that the chemical structure of lignin has beenaltered in these materials.
Table 15. Summation of major peak areas in the “fingerprint” region associated withcellulose, lignin, hemicellulose, and pectin in lignocellulosic biopolymers that helps
determine the difference in chemical composition among plant-based biopolymers
Samples∑A(cellulose)a∑A(lignin)b∑A(hemicellulose)c

DF
3.54
1.09
1.76
AK1%DF
4.86
1.47
0.23
AK3%DF
4.97
1.28
0.02
CC
3.79
1.56
1.45
AK1%CC
4.73
1.95
0.18
AK3%CC
4.42
1.69
0.03
DL
2.51
1.33
0.68
AK1%DL
3.19
0.92
0.28
AK3%DL
5.04
0.81
0.10
CL
3.08
1.15
0.75
AK1%CL
3.76
0.79
0.56
AK3%CL
4.61
0.78
0.21
CCPB
3.71
1.84
1.89
Cotton5.25
0.3
Bagasse
3.81
1.47
0.81
Waste
3.17
1.65
0.40
wood
Bamboo
4.17
1.63
1.18
a: ∑A (cellulose) = (A895+ A1160+ A1317+ A1370+ A1420).
b: ∑A (lignin) = (A1507 + A1592).
c: ∑A (hemicellulose) = (A770 + A1735 + A1745).
d: ∑A (pectin) = (A834 + A1246).
e: The ratio of peak areas of cellulose to lignin.
f: The ratio of peak areas of lignin to hemicellulose and pectin.

66∑A(pectin)d
0.15
0.06
0.08
0.11
0.06
0.07
-

C/Le

L/(H+P)f

3.2
3.3
3.9
2.4
2.4
2.6
1.9
3.5
6.2
2.7
4.8
5.9
2
3.1
1.9

0.6
6
65
1
11
59
1.6
2.8
6.6
1.4
1.2
0.9
1
1.4
4.2

0.08

2.6

1.3

Table 16. Specific absorption-band wavenumbers (cm-¹) related to hot-pressed plantbased biopolymers according to FTIR band assignments (cm-¹)
Peak
#
1
2

CCP
B
770
806
817
852

Cotton

895
1021
1053
1113

9
10
11
12
13
14
15
16
17

895
1030
1052
1112
1122
1163
1233
1267
1315
1337
1372
1421
1459

18
19
20
21
22

1507
1541
1557
1591
1626

23
24

1654
1736

1159
1232
1263
1315
1335
1370
1427
1456
1464
1541
1559
1628
1634
1653
1732

25
26

2851
2894
2923
3317
3342
3360

3
4
5
6
7
8

27

768
-

2895
2916
3310
3333

Bagass
e
778
-

Waste
wood
768
811

Bamb
oo
812

835
-

836
-

896
1025
1052
1118

857
868
899
1028
1054
1109

1161
1236
1261
1318
1331
1372
1419
1457
1464
1508
1542
1559
1593
1635

1161
1228
1265
1315
1336
1370
1422
1456
1464
1509
1560
1591
1634

1652
1716
1733
2894
2902
2920
3309
3334
3356

1652
1731

1656
1734

2886
-

2854
2884
2920
3312
3331
3359

67

3313
3337
3342

897
1029
1053
1112
1122
1161
1236
1251
1258
1317
1331
1372
1421
1463
1509
1542
1560
1592
1631

4.7 X-Ray diffraction (XRD)
X-ray diffractograms of raw date and coconut fibers and leaves are shown in Fig. 35a,
milled microparticles in Fig. 35b, untreated palm-based lignocellulosic biopolymers in Fig.
35c, alkaline pretreated palm-based lignocellulosic biopolymers in Fig. 35e, and hotpressed supplementary materials in Fig. 35f. Meanwhile, XRD of untreated milled
microparticles are shown in (Fig. 35b), where date fiber, coconut coir, and coconut leaves
exhibited 2θ reflections at maximum peaks (19.96–20.74 °C) as similarly reported for ballmilled cellulose [74], compared to milled date leaf that exhibited 2θ = 21.37°. As shownin Figs. 35c–e, untreated and alkaline-treated lignocellulosic biopolymers and theircorresponding raw fibers and leaves exhibited 2θ reflections at maximum peaks between(21.8–22.66°) assigned to the (I002) plane of crystalline regions, whereas reflections
between (18–18.63°) assigned to amorphous regions (Iam), which are typical of native
cellulose found in most natural fibers [40].

Figure 35. XRD diffraction patterns for (a) raw palm-based fibers and leaves, (b)lignocellulosic milled powders, (c) lignocellulosic biopolymers, (d) 1 wt% alkaline
pretreated biopolymers, (e) 3 wt% alkaline pretreated biopolymers, and (f) hot-pressed
supplementary materials
68As shown in Table 17, biopolymers exhibited lower crystallinity (33.1–49.4%) than theircorresponding raw fibers (52–58%), owing to their chemical composition, microfibrillation,and the presence of crystalline regions in cellulose. Moreover, date fiber and coconut
leaf-alkaline pretreated biopolymers exhibited higher crystallinity (44.4–51.9%),
compared to their corresponding untreated biopolymers. Meanwhile, no noticeablechange was observed in the crystallinity of raw and alkaline pretreated coconut coir-based
biopolymers. The higher crystallinity indices found in AK3%DF, AK3%DL, and AK3%CL
show typical patterns found in plant fibers after alkaline pretreatment where thecrystallinity increased possibly owing to the removal of amorphous materials [66]. On theother hand, date-leaf-based biopolymers showed the highest crystallinity index (~49%)
among all biopolymers. These results verified the supplementary materials shown in Fig.
35f. However, the sharp peaks observed in bagasse- and waste wood-based biopolymers
associated with a high content of inorganic materials are typically found in ash waste [100].

69

Table 17. Assignment of X-ray diffraction peaks for palm-based raw fibers and leaves,
powders, biopolymers, hot-pressed coconut fibers, milled CDW wood, cotton, bamboo,
and bagasse-based biopolymers
SampleDate fiber (raw)Date fiber (microparticles)
DF (biopolymer)
AK1%DF
AK3%DF
Coconut coir (raw)Coconutcoir(microparticles)
CC
AK1%CC
AK3%CC
Date leaflet (raw)DL (microparticles)
DL
AK1%DL
AK3%DL
Coconut leaf (raw)Coconut leaf
(microparticles)
CL
AK1%CL
AK3%CL
CCPB
CottonBagasseWaste Wood
Bamboo2θ Crystalline
(002)
(°)
22.66
19.96
21.8
22.56
22.4
22.57

Intensity2θ (amorphous)(°)

Intensity

Crystallinity index
(%)

897
579
1826
1507
1530
1157

18.38
18.2
18
18.15
18.18
18.06

387
463
1222
826
735
558

56.8
20.0
33.1
45.2
51.9
51.8

20.26

650

18.05

507

22.1

22.24
22.21
21.99
20.8
21.37
21.86
21.72
22.08
21.78

1810
1486
1477
473
969
1445
1214
1690
923

18.24
18.48
18
18.18
18.07
18.63
18.15
17.98
18.38

1060
868
867
207
523
730
722
949
388

41.5
41.6
41.3
56.3
46.0
49.4
40.5
43.8
58.0

20.74

695

18.08

549

21.1

21.89
22.22
22.36
22.56
22.42
21.43
22.55
22.51

880
1685
2045
1247
3412
1033
1922
1300

17.99
18.08
17.98
19.07
17.98
18.02
18.87
19.08

552
938
998
578
1063
660
809
572

37.3
44.4
51.2
54
69
36
58
56

The higher crystallinity in DL is slightly higher crystallinity than its milled date-leaf powders(~46%), does not necessarily always reflect higher mechanical or thermal properties.
However, this suggests a weaker bonding mechanism similar to that in particleboards andnon-lignin-based biopolymers, possibly through hydroxyl groups at surface amorphous
regions where cellulose microfibrils interlink [47]. It is also possible that a strong
intermolecular hydrogen-bond network in the crystalline regions of the fibrillar structure
limits the availability of cellulose to bind with lignin during hot pressing.

70

4.8 Activating lignin binding and optimal processing temperature

Figure 36. Schematic representation of activation of lignin for binding
In general, natural fibers are governed by hydrogen bonds formed between cellulose
microfibrils [40], lignin–carbohydrate complex (LCC) bonds [75], and lignin inter-unit
linkages achieved by ether carbon–oxygen (C–O) or carbon–carbon (C–C) bonds [62].
However, the linkage attributed to the thermosetting behavior of lignin is difficult todetermine due to its complex structure. C–C linkages, often called condensed linkages,
can be attributed to the thermosetting of lignin, because these linkages are destroyed byharsh alkaline or acid pretreatment or by extreme conditions that also degrade othercomponents [76].
Therefore, lignin condensation by hot-pressing lignocellulosic fibers and leaves without
applying a mechanical or chemical pretreatment is difficult to activate owing to thepresence of intramolecular hydrogen bonds between cellulose microfibrils, higher thermal
resistance of cellulose, presence of non-structural materials on the surface, and low lignin
surface distribution. The lignin surface distribution increases radially toward the middle
lamella and the secondary-wall (S2) middle layer, which is similar to the pattern observedin wood [77], oil-palm fiber, coir, banana stems, and pineapple leaves [78]. Numerous
binderless particleboard studies have reported improvements in mechanical properties
with the activation of lignin binding after the hydrolysis of cellulose, hemicellulose, andextractives enabled by a steam explosion [53] or thermal pretreatment [25,79] at higher
temperatures. Although thermal degradation of cellulose and other constituents is
probably required to expose and activate lignin, it will lead to irreversible changes in lignin
thermosetting [4].
71Moreover, microfibrillation enables not only lignin release and redistribution but also lignin
chemical structure preservation, as shown in Fig. 36. Furthermore, milled-powder
crystallinity indices obtained from XRD data, shown in Table 17, were dramaticallyreduced (20–22.1%) by microfibrillation in date fiber, coconut coir, and coconut leaf
powders. The reduction represents the breakage of intramolecular hydrogen bonds
between cellulose microfibrils. Meanwhile, milled date leaf powders showed crystallinity(~46%) wherein waxy substances possibly prevented hydrogen-bond breakage, thereby
restricting lignin surface exposure, because crystalline regions remained intact,
necessary for activating lignin binding. Furthermore, alkali pretreatment is highly effective
in removing most of the non-structural materials at mild conditions (e.g., hemicellulose
and extractives) [80,81]. The efficiency of lignin binding enhanced the bending properties
in alkaline pretreated fiber-based biopolymers and AK1%DL, which is attributed to thecoupling efficiency of lignin and cellulose. Therefore, during hot-pressing, lignin
undergoes a glass transition phase in the range of 142–179 °C [82]wherein its wettability
improves, enabling it to form covalent bonds by condensation with cellulose and otherconstituents [70] and enabling unit–unit and double bonds to form between lignin units[62].
Hot-pressing temperatures of milled lignocellulose powders and efficient lignin binding
highly depend on chemical composition and, particularly, on degradation temperatures ofhemicellulose and extractives [40,42]. As the glass transition temperature (Tg) of lignin isalso in the range of degradation temperatures of hemicellulose and extractives, theoptimal curing temperature enables sufficient lignin melting while preventing thermalhydrolysis of hemicellulose and extractives, resulting in bumps and cracks due tohemicellulose thermal degradation and extractive volatility. Optimal processing
temperatures are lower for leaf-based untreated biopolymers (150–160 °C) and bagasse
fiber-based biopolymer (140 °C), compared to fiber-based biopolymers and AK1%DL, aspresented in Table 2.
Moreover, AK1%CL and AK3%CL showed a decrease in processing temperature(~140 °C), compared to CL, indicating excessive delignification or lignin depolymerizationaccording to the type of fiber and leaf under mild conditions. It is also possible that lignin
72

depolymerization cleaves and disrupts major bonds, reported with harsh chemicaltreatments (e.g., acid or steam explosion) or under severe conditions (e.g., highconcentration, temperature, and time) [62].
4.9 Lignocellulosic biopolymer colors
Lignocellulosic biopolymer colors indicate efficient lignin-based adhesion, which highly
depends on chemical composition (C+L/H+E+A) and particle size of hot-pressed natural
fibers. As shown in Fig. 37, DF, CC, and CL prepared from powders with higher lignin
contents (≥20 wt.%) and consisting of microparticles ≤106 µm were black and virtuallyindistinguishable. Similar colors were observed in alkaline pretreated fibers and AK1%DL,
because of the removal of non-structural materials by alkaline pretreatment. DL, on theother hand, appeared dark green owing to either low lignin content and/or large particles.
Furthermore, AK3%DL, AK1%CL, and AK3%CL exhibited dark brown color and bumps
on the surface attributed to excessive removal of lignin content. These observations
indicate that lignocellulosic biopolymers showed color variation depending on the degreeof lignin activation. Tables 8, 10, and 11 and Fig. 37 depict the correlations of color
variations and mechanical, physical, and thermal properties of other lignocellulosic
biopolymers hot-pressed from biomass sources (e.g., milled powders of bagasse, cotton,
waste wood, and fibers such as coconut coir). Bagasse-based biopolymers (BG) also
appeared black, indicating efficient lignin activation because of microfibrillation andpossibly, because lignin content ≥20 wt.%, whereas cotton, bamboo, and waste woodbased biopolymers appeared bright yellow due to the absence of lignin or lignin’s binding.
Hot-pressed coconut coir (CC), on the other hand, appeared brown and yellow, indicating
limited lignin availability for activation, similar to previously reported binderlessparticleboards [9,25]. These observations support recent evidence suggesting that ahigher (C+L/H+E+A) ratio and microfibrillation enable changes in lignin chemical
reactions, thereby improving lignin adhesion.

73

Figure 37. Appearance of lignocellulosic biopolymers
and hot-pressed biopolymers
74

5. Effect of coconut coir and regenerated silk microparticles as blends and naturalbinders for construction and demolition waste (CDW) wood on the mechanical,
thermal, and structural properties of biomicrocomposites

Figure 38. Process of preparation and characterization of hybrid biomicrocomposites
5.1 Mechanical properties and physical properties
Table 18. Means of the mechanical and physical properties (± SE) of the neat
biopolymers
SampleBending
Strength(MPa)

Strain at
Break
(%)

Young’s
Modulus(GPa)

Density(g/cm3)

Water
Absorption(%)

Thickness
Swelling(%)

CC
114 ± 1.8a
1.70 ± 0.06a 6.42 ± 0.17a 1.45 ± 0.00a
12 ± 0.82a
9 ± 0.85a
CDW
34 ± 2.3b
2.73 ± 0.37b 2.01 ± 0.24b 1.35 ± 0.01b
67 ± 3.67b
47 ± 4.33b
RS
95 ± 5.2c
1.57 ± 0.14a 5.56 ± 0.44a 1.36 ± 0.01b
9 ± 0.43a
7 ± 0.54a*Values with different letters in the same column indicate that the difference of means is statisticallysignificant at the 0.05 level (p ≤ 0.05) among CC, CDW, and RS biopolymers

As presented in Table 18, CC exhibited higher bending strength, Young’s modulus, andapparent density than regenerated silk-(RS)-based biopolymers, whereas RS exhibitedthe lowest water absorption (~9 ± 0.43%) and thickness swelling (~7 ± 0.54%) among all
the other samples. Moreover, CDW reported a similar density as RS, but it exhibited thelowest bending strength and highest strain at break, Young’s modulus, water absorption(~67 ± 3.67%), and thickness swelling (~47 ± 4.33%) among the other samples.
Furthermore, CC and RS showed statistically significant differences in the means ofbending strength and apparent densities, while all the mechanical and physical properties
of CDW were significantly different from those of CC and RS, except the density of RS.
75

Furthermore, CC has better mechanical and physical properties than CDW, which is
attributed to the formers’ chemical composition, particularly higher ratio of cellulose(reinforcing material) and native lignin (binding agent) to hemicellulose, extractives, andash contents (non-structural materials). A smaller microparticle size allows for more lignin
trapped between microfibrils to be exposed to the surface, thereby activating efficientlignin binding, as mentioned in Chapter 5. Meanwhile, the superior mechanical propertiesof silk-based biopolymers were attributed to the structural transition from a highly random
coil/α-helix structure (amorphous structure) to a β-sheet (crystalline structure) by hotpressing. This structural transition to β-sheets is governed by particle size (microparticles
or nanoparticles) and the presence of bound water or water uptake due to the higher ratioof hydrophilic to hydrophobic amino acids [27,28,83]. Therefore, regenerated silk-based
biopolymers reported bending properties equivalent to those of regenerated silk resin in[28] and slightly lower bending properties than non-degummed B. mori silk-based resins(bending strength of 122 MPa) [27], but inferior to those of regenerated silk nanoparticlebased thermoplastics (flexural stress of 160 MPa, flexural strain of 2.5%), as reported in[83].
Table 19. Means of the mechanical and physical properties (± SE) of the CDW and CC
biopolymers and their hybrid biomicrocomposites
SampleBending
Strength(MPa)

Strain at
Break
(%)

Young’s
Modulus(GPa)

Density(g/cm3)

Water
Absorption(%)

Thickness
Swelling(%)

CDW
34 ± 2.3a
2.73 ± 0.37a 2.01 ± 0.24a 1.35 ± 0.01a
67 ± 3.67a
47 ± 4.33a
b
a
a
a
b
CDW80/CC20
45 ± 2.0
2.24 ± 0.25
2.95 ± 0.37
1.36 ± 0.01
51 ± 3.10
40 ± 1.57a
CDW60/CC40
56 ± 1.6b
1.79 ± 0.12b 4.13 ± 0.22b 1.36 ± 0.01a
41 ± 1.58b
31 ± 1.23b
CDW50/CC50
54 ± 2.6b
1.39 ± 0.09b 4.45 ± 0.37b 1.35 ± 0.01a
39 ± 3.33b
27 ± 1.11b
CDW40/CC60
65 ± 3.0b1.60 ± 0.18b 4.66 ± 0.47b 1.37 ± 0.01a
35 ± 0.93b
23 ± 1.00b
b
b
b
b
b
CDW20/CC80
85 ± 2.8
1.28 ± 0.04
6.61 ± 0.19
1.43 ± 0.01
30 ± 1.19
19 ± 0.87b
b
b
b
b
b
CC
114 ± 1.8
1.70 ± 0.06
6.42 ± 0.17
1.45 ± 0.00
12 ± 0.82
9 ± 0.85bb*Values with the letter ( ) in same column indicate that the difference of means is statistically significant at
the 0.05 level (p ≤ 0.05) from those of CDW.

As shown in Table 19, incorporating CC exhibited an overall gradual increase in themechanical and physical properties of CDW/CC biomicrocomposites, exhibiting the bestproperties at 80 wt% CC (CDW20/CC80). Furthermore, CDW/CC biomicrocomposites at≤ 40 wt% CC loading showed overall statistically significant differences in mechanical andphysical properties, except density. This is because native lignin, which is the binding76

agent, accounts for only 28 wt% of the chemical composition of coconut coir, as reported
in Table 7. Therefore, efficient lignin-based adhesion might be noticeable only in coconutrich biomicrocomposites due to an increase in the ratio of cellulose and lignin to
hemicellulose, extractives, and ash, as mentioned in Chapter 4. The gradual increase in
the water-resistance of CDW/CC biomicrocomposites is also correlated to efficient lignin
binding. As discussed previously, efficient lignin binding corresponds to increasing
coconut coir content and decreasing hydrophilic constituents. These constituents containhydroxyl group sites that react with water, such as cellulose [47]. It is possible that, during
hot-pressing, efficient wettability of higher lignin content in CDW/CC biomicrocomposites
enables it to form covalent bonds by condensation [70]. The C–O and C–C bonds form
due to the lignin’s inter-unit linkages [62] and intermolecular hydrogen bonds between thehydroxyl groups of amorphous regions in cellulose. The aromatic skeletal vibration occurs
due to the presence of phenol and hydroxyl groups in lignin [47].
Table 20. Means of the mechanical and physical properties (± SE) of the RS and CC
biopolymers and their hybrid biomicrocomposites
SampleBending
Strength(MPa)

Strain at
Break
(%)

Young’s
Modulus(GPa)

Density(g/cm3)

Water
Absorption(%)

Thickness
Swelling(%)

RS
95 ± 5.2a
1.57 ± 0.14a 5.56 ± 0.44a 1.36 ± 0.01a
9 ± 0.43a
7 ± 0.54aaaaaaRS80/CC20
95 ± 1.8
1.49 ± 0.03
5.98 ± 0.09
1.37 ± 0.01
13 ± 1.12
9 ± 0.44aaaaabRS60/CC40
104 ± 1.8
1.62 ± 0.04
5.91 ± 0.21
1.39 ± 0.01
15 ± 0.59
10 ± 0.98a
a
a
a
b
b
RS50/CC50
105 ± 2.3
1.54 ± 0.04
6.33 ± 0.14
1.39 ± 0.01
16 ± 0.95
11 ± 0.47b
a
a
a
b
b
RS40/CC60
104 ± 1.5
1.61 ± 0.02
5.95 ± 0.14
1.41 ± 0.01
16 ± 1.22
12 ± 0.78b
b
a
a
b
b
RS20/CC80
109 ± 3.1
1.79 ± 0.04
5.65 ± 0.13
1.42 ± 0.01
18 ± 0.79
12 ± 1.42b
CC
114 ± 1.8b
1.70 ± 0.06a 6.42 ± 0.17a 1.45 ± 0.00b
12 ± 0.82a
9 ± 0.85a* Values with the letter (b) in same column indicate that the difference of means is statistically significant at
the 0.05 level (p ≤ 0.05) from those of CDW.

RS/CC biomicrocomposites show the most formidable mechanical properties due tostrong adhesion, good dispersion, and compatibility between regenerated silk andlignocellulose after hot pressing, as shown in Table 20. The biomicrocomposites inherit
the superior properties of their neat biopolymers and show slight differences inmechanical and physical properties according to their mass ratio. However, water
resistance is slightly lower in biomicrocomposites than those in neat RS and CC. The
mean bending strength, strain, and Young’s modulus of RS/CC biomicrocomposites show
no significant differences from those of RS. The only significant difference is observed in77except the bending strength of RS20/CC80. Nevertheless, there were significant
differences in the densities of the RS/CC biomicrocomposites with coconut coir loading
amounts ≤ 50 wt% from those of RS. Additionally, there were statistically significant
differences in water absorption and thickness swelling from those RS. However, water
absorption with a CC loading amount of 20 wt% and thickness swelling with CC loading
amounts of 20–40 wt% were not significantly different from those of RS.
Table 21. Means of the mechanical and physical properties (± SE) of the CDW and RS
biopolymers and their hybrid biomicrocomposites
Biopolymer

Bending
Strength(MPa)

Strain at
Break
(%)

Young’s
Modulus(GPa)

Density(g/cm3)

Water
Absorption(%)

Thickness
Swelling(%)

CDW
34 ± 2.3a
2.73 ± 0.37a 2.01 ± 0.24a 1.35 ± 0.01a
67 ± 3.67a
47 ± 4.33a
b
b
b
b
b
CDW80/RS20
93 ± 2.6
1.81 ± 0.08
5.34 ± 0.31
1.39 ± 0.00
17 ± 1.52
12 ± 1.16b
b
b
b
b
b
CDW60/RS40
90 ± 1.4
1.50 ± 0.07
6.55 ± 0.25
1.39 ± 0.00
16 ± 0.65
13 ± 1.00b
b
b
b
b
b
CDW50/RS50
83 ± 2.9
1.42 ± 0.05
6.02 ± 0.29
1.38 ± 0.00
14 ± 1.47
12 ± 0.54b
CDW40/RS60
69 ± 2.4b
1.18 ± 0.03b 6.19 ± 0.25b 1.38 ± 0.00b
24 ± 1.18b
13 ± 0.85b
CDW20/RS80
62 ± 3.6b
1.08 ± 0.04b 6.38 ± 0.13b 1.38 ± 0.00b
21 ± 1.06b
12 ± 0.55b
RS
95 ± 5.2b
1.57 ± 0.14b 5.56 ± 0.44b 1.36 ± 0.01a
9 ± 0.43b
7 ± 0.54bb* Values with the letter ( ) in same column indicate that the difference of means is statistically significant at
the 0.05 level (p ≤ 0.05) from those of CDW.

Table 21 and Fig. 39 show that the mechanical properties of the CDW/RS
biomicrocomposites decrease gradually in regenerated silk-rich samples, exhibiting
maximum bending strength, strain, and Young’s modulus when prepared with 20 wt%
regenerated silk (CDW80/RS20). This was also observed for blended films prepared fromsilk fibroin/cellulose [84] and wool/cellulose [85], where the highest mechanical, physical,
and thermal properties were achieved with protein blending (20–25 wt%). CDW/RS
biomicrocomposites show statistically significant differences in all mechanical andphysical properties from those of CDW. Although CDW/RS biomicrocomposites showsuperior water resistance to CDW/CCs, they exhibit adverse effects by showing a slightdecrease in the water resistance of the regenerated silk-rich samples possibly due to thepoor dispersion and cracks during the drying of the CDW wood.

78

Figure 39. Means of mechanical and physical properties (□) of neat biopolymers and theirhybrid biomicrocomposites. The whiskers represent the maximum and minimum values(out of three samples or more)79

As shown in Tables 18–21, adding coconut coir and regenerated silk had a positive effecton the mechanical and physical properties of the CDW/CC and CDW/RS
biomicrocomposites. Additionally, RS/CC blends reported higher mean bending strengths,
strains, and Young’s moduli than the CDW/CC and CDW/RS biomicrocomposites.
RS/CC biomicrocomposites showed a better water resistance than CDW/RS
biomicrocomposites. Generally, the water resistance of the biomicrocomposites obtained
is attributed to the type of adhesion and its efficiency. Therefore, both β-sheet-enrichedcrystalline-based and activated lignin-based adhesions were attributed to the better water
resistance in RS/CC biomicrocomposites than lignin-based adhesion in CDW/CC or silkbased adhesion in CDW/RS. The obtained CDW/RS and RS/CC biomicrocomposites
showed a water resistance comparable to that of the previously reported raw and alkalinepretreated coconut coir-based biopolymers. In contrast, all biomicrocomposites showed
better water resistance than raw and alkaline-pretreated date fiber- and leaf-based
biopolymers (as discussed in Chapters 4.2 and 4.3) and binderless particleboards
reported in the literature [48,54,57–59]. These findings suggest that incorporating coconut
coir- and regenerated silk-based materials as binders or blends positively influences
water resistance when used in biomicrocomposites.
Utilizing coconut coir as a natural binder is highly dependent on more native lignin(chemically unaltered) and its efficient binding, while regenerated silk is highly dependent
on its ability to form more crystalline enriched β-sheets from amorphous microparticles.
Regenerated silk-based microparticles showed significant binding for CDW wood
because they primarily function as a binding agent, unlike 28 wt% of coconut coir-based
microparticles. Therefore, it is possible that cross-linking occurs among CDW wood,coconut coir, and regenerated silk microparticles during hot pressing via hydroxyl groups,
lignin inter-unit linkages, lignin phenol groups, and the secondary structure of proteins(explained in detail in Chapter 5.4).
These results suggest that coconut coir- and regenerated silk-based microparticles are
suitable for use as natural binders for cellulosic waste, such as CDW wood orprotein/lignocellulose blends. Additionally, the influence of the particle size of CDW woodandcoconut

coir

on

the

mechanical80

properties

of

CDW/CC

andRS/CCbiomicrocomposites was validated in the supplementary data (Tables 22–23), where
biomicrocomposites prepared from smaller particles achieved better mechanicalproperties than those prepared from larger particles. Table 23 shows the positive effectof blending raw and mildly alkaline-pretreated bagasse- and coconut coir-based
microparticles on the mechanical properties of lignocellulosic biomicrocomposites,
regardless of the mass ratio. Further, better mechanical properties than blends of CDW
wood- or untreated lignocellulosic-based microparticles were achieved.
Table 22. Prepared samples and their labels
Sample code
CDW
CC
RS
BG
AK1%CC
AK1%BG

Microparticles
CDW wood
Coconut coir
Regenerated silk
Waste Bagasse
Alkaline pretreated coconut coir at 1 wt% NaOH
Alkaline pretreated waste bagasse at 1 wt% NaOH

Table 23. Influence of particle size on bending properties of CDW/CC and RS/CC
biomicrocomposites prepared at a mass ratio of 50:50
Biomicrocomposite(Particle size

Bending
Strength(MPa)

Strain at
Break
(%)

Young’s
Modulus(GPa)

Density(g/cm3)

CDW (≤53 µm)/CC (≤53 µm)
CDW (71– 106 µm)/CC (≤53 µm)
CDW (75–160µm)/CC (≤53 µm)
CDW (106–160 µm)/CC(≤53µm)
CDW (106–160 µm) /CC (53–71 µm)
CDW (106–160 µm)/CC (75–106 µm)
CDW (106–160 µm) /CC (75–160 µm)
RS (≈7 µm) /CC(≤53µm)
RS (≈7 µm)/CC (≤53– 75 µm)
RS (≈7 µm)/CC (≤75–106 µm)
RS (≈7 µm)/CC (≤75–160 µm)

66
54
58
42
54
43
47
106
98
85
83

1.17
1.86
1.68
2.27
1.60
1.61
1.40
1.70
1.52
1.39
1.33

5.7
3.0
4.1
2.8
3.5
3.3
4.5
5.7
5.9
5.8
6.0

1.34
1.27
1.27
1.20
1.37
1.33
1.37
1.39
1.39
1.40
1.39

81

Table 24. Influence of mechanical and physical properties of lignocellulosic
biomicrocomposites prepared from microparticles (≤53 µm)
Biomicrocomposite
BG70/CC30
BG50/CC50
BG30/CC70
BG50/AK1%CC50
BG30/AK1%CC70
AK1%BG50/AK1%CC50

Compositions(wt.%)
BagasseCoconut coir
70
50
30
50
30
50

Bending
Strength(MPa)

Strain at
Break
(%)

Young’s
Modulus(GPa)

Density(g/cm3)

88
82
97
100
101
94

1.36
1.39
1.56
1.53
1.62
1.53

6.2
6
5.8
6.5
6.3
6.3

1.48
1.45
1.47
1.48
1.49
1.48

30
50
70
50
70
50

These results suggest that raw high-lignin or mildly alkaline-pretreated plant fiber sources
such as coconut coir or bagasse are suitable candidates for fabricating high-performancebiomicrocomposites or serving as binders for CDW wood. These findings may beevidence that suggests a potential shift from synthetic binder-based adhesion in WPC
composites and particleboards to native lignin- or protein-based adhesion, which does
not lead to degradation in mechanical properties. For example, incorporating 10–30 wt%
silk fibroin microparticles in PBAT/PLA blends, as performed in [86], exhibited no change
or decrease in flexural strength (from 38 to 32 MPa) compared to that of neat PBAT/PLA
blends (39–42 MPa). Moreover, mixing microparticles using melt blending by hot-pressing
does not require the use of the chemicals used in extraction or solvent systems for“solution blending,” such as technical lignins [87,88].

82

5.2. SEM morphologies

Figure 40. SEM Micrographs of coconut coir-microparticles, CDW wood powder, andregenerated silk microparticles captured at 190× and 600× magnifications showing
particle size and distribution
SEM micrographs of the starting materials presented in Fig. 40 confirm the particle size,
shape, and orientation of the starting materials. This helps explain the adhesion behaviorof the obtained biopolymers and biomicrocomposites. Regenerated silk exhibits uniform
and small microparticles, while the coconut coir microparticles are irregular and slightly
larger and exhibit microfibers. On the other hand, the SEM micrographs of the CDW wood
exhibited larger fibers and signs of degradation caused during the processing of CDW
waste.
Biopolymers and their corresponding hybrid biomicrocomposite microstructures wereobserved at 190× magnification (Fig. 41). As shown in the SEM micrographs (Fig. 41),
CC and RS both showed denser microstructures and smoother surfaces than CDW,
which was attributed to the thermosetting behavior of lignin and the self-assembly ofregenerated silk, respectively. The morphologies of coconut coir-based biopolymers, hotpressed CDW wood, and regenerated silk-based biopolymers were examined in [39, 43].
These studies proposed that lignin condensation and β-sheet-enriched crystallinetransitional phases were because of effective binding. Although CC and RS morphologies
83differ due to their binding behavior and microparticle size, CC exhibits a slightly roughermorphology than RS, possibly due to non-structural materials and cellulose nanofibers,in addition to lignin and their random distribution. RS shows a homogenous microstructure,
which is attributed to its smaller particle size and structural transition from an amorphous
structure to a β-sheet-enriched crystalline structure. Meanwhile, the lack of a binding
agent and the presence of microfibrils in hot-pressed CDW wood resulted in porous
structures and cavities.

Figure 41. SEM micrographs of neat biopolymers and their hybrid biomicrocomposites at
190× magnification
Furthermore, SEM micrographs (Fig. 41) illustrated phase separation and compositiondifferences in the obtained biomicrocomposites. Moreover, the incorporation of 50 and 80
wt% coconut coir microparticles in CDW50/CC50 and CDW20/CC80 caused changes in
the characteristics and a better morphology than that of CDW. More debonding areas and84

fiber pulling were still visible in CDW50/CC50. However, CDW20/CC80 showed dense
lignin adhesion similar to that in CC. These micrographs confirm that coconut coir-based
microparticles functioned as a binder for CDW by increasing the native lignin content,
thereby inducing more lignin-based adhesion. On the other hand, regenerated
silk/coconut coir blends at mass ratios of 50:50 and 20:80 exhibited morphologies
comparable to those of RS and CC, respectively. RS50/CC50 revealed a microstructure
including both lignin- and silk-based binding, whereas RS20/CC80 displayed more ligninbased adhesion than CC. This confirms that protein- and lignocellulosic-based
microparticles efficiently blend as macromolecules in the RS/CC biomicrocomposites.
The thermosetting behavior of lignin and self-assembly of silk phases might haveformulated a natural solvent system during hot pressing. This enabled the distribution ofsilk and coconut coir microparticles and integration in RS/CC biomicrocomposites.
CDW50/RS50 and CDW20/RS80 highlighted the ability of regenerated silk to function asa binder for CDW wood at much lower contents than coconut coir-based microparticles.
The CDW/RS biomicrocomposites with 50 and 80 wt% regenerated silk microparticles
displayed a better morphology than CDW50/CC50 and CDW20/CC80, respectively, due
to silk-based adhesion. This is caused by the transitional phase-based adhesion ofregenerated silk, which is attributed to its smaller particle size and absence of impurities,
while coconut coir’s lignin-based adhesion (accounting for 28 wt%) is attributed to thelarger particle size and presence of cellulose and other constituents. As shown in Fig. 42,
the effect of adding 20, 40, and 60 wt% regenerated silk to the CDW/RS
biomicrocomposites suggests that the regenerated silk microparticles are more effective
at lower mass ratios than coconut coir because they act as the sole binders. However,
CDW20/RS80, with the higher content of regenerated silk microparticles, showed cracks
and nanosized pores caused by CDW wood agglomeration and their uneven distribution,
moisture escape, and thermal degradation during drying.

85

Figure 42. SEM micrographs showing the effect of adding 20, 40, and 60 wt%
regenerated silk to the CDW/RS biomicrocomposites
5.3 Thermal behavior and stability
Table 25. Thermal properties of biocomposites and their corresponding biopolymers
SampleOnset Temperature at 1%(°C)

Maximum Degradation Peaks
Endset Residues
(°C)
(%)
i
ii
ii
CC
224
248
302
542
5.40
CDW
190
312
385
2.82
RS*
226
285
CDW80/CC20
213
300
410
2.4
CDW60/CC40
200
303
416
1.84
CDW50/CC50
211
297
455
2.49
CDW40/CC60
215
266
293
450
3.17
CDW20/CC80
225
253
275
505
3.05
RS80/CC20*233
284
RS60/CC40
223
269
306
544
1.66
RS50/CC50
221
275
306
524
1.22
RS40/CC60
219
256
304
513
1.85
RS20/CC80
218
253
281
514
2.44
CDW80/RS20
183
340
563
0.68
CDW60/RS40
186
297
342
581
0.63
CDW50/RS50
193
298
344
584
CDW40/RS60
196
298
345
598
CDW20/RS80*
190*Thermal behavior cannot be determined due to regenerated silk’s thermal instability above
285 °CAs shown in Table 25 and Fig. 43, the thermal behavior of the obtained
biomicrocomposites was determined by the type of added/blended starting materials andtheir mass ratio according to weight-loss curves and decomposition peaks obtained by
TGA and DTG. Figs. 43a–c show that RS and CC exhibited similar onset temperatures(at 1% weight loss) in the range of 224–226 °C. The high thermal stability of CC is causedby the microfibrillation and chemical composition of high-lignin coconut coir, which allows
for a more complex and thermally stable form of lignin, resulting in efficient lignin binding86[61]. Meanwhile, the thermal stability of RS is attributed to the formation of more
crystalline β-sheets [29]. Hot-pressed CDW reported a lower onset temperature (~190 °C)
than the other samples. Therefore, CDW/CC biomicrocomposites show an increase in
the onset temperature (200–225 °C) compared to that of CDW wood. On the other hand,a slight increase in onset temperatures by incorporating only 50 and 60 wt% regenerated
silk microparticles was exhibited in CDW50/RS50 and CDW40/RS60 (193–196 °C)
compared to that of CDW wood. RS/CC biomicrocomposites except for RS80/CC20 show
similar or slightly lower onset temperatures (218–221 °C) than CC. This suggests that
increasing the content of coconut coir microparticles in CDW/CC and RS/CC enhanced
thermal stability and showed similar trends in their bending strength and water resistance.
Although RS, CDW20/RS80, and RS80/CC20 exhibited high onset temperatures (190–
233 °C), the remaining residues cannot be determined because extreme vibration caused
thermal instability at around ~285° in regenerated silk, as shown in (Fig. 44). The
CDW/CC biomicrocomposites showed higher endset weight percentage residues (~1.84–
3.17%) than the RS/CC biomicrocomposites (1.22–2.44%), while the lowest endset
weight percentages were only reported in CDW80/RS20 and CDW60/RS40 at 0.68 and
0.63%, respectively. In addition, the higher endset weights observed in coconut coir- and
CDW wood-rich samples were attributed to the ash content in their starting materials
being higher than that of regenerated silk (Fig. 15 and Table 5).
The DTG curves in Figs. 43d–f show thermal decomposition peaks, which explain thethermal stability of biomicrocomposites. The presence of small sharp peaks in the rangeof 200–250 °C was possibly attributed to the release of non-frozen bound water andaliphatic OH groups or the decarboxylation of acids in lignin [47,60,61]. Similarly, thesepeaks were also observed in strong lignin adhesion-based lignocellulose biopolymers ( asmentioned in chapter four) and regenerated silk thermoplastics [29]. The absence of
these peaks in CDW, CDW/CC biomicrocomposites, and CDW80/RS20 may indicate
lower contents of condensed lignin and silk. Fig. 43d shows the DTG curve of CC,
exhibiting two decomposition peaks at 260 and 320 °C, attributed to hemicellulose andcellulose, respectively [42], while condensed lignin decomposes over a broader peak at
280–560 °C [62,89]. CDW wood, on the other hand, shows overlapping hemicellulose
and cellulose decomposition peaks and exhibited maximum DTG decomposition at87

300 °C, suggesting poor thermal stability similar to that of milled plant fibers (Fig. 15 andTable 5) and non-lignin-based adhesion biopolymers (as mentioned in chapter 4).
Specifically, condensed lignin has higher thermal stability than less condensed lignin.
Therefore,

CDW/CC

biomicrocomposites

exhibiteddecreased

maximum

DTG

decomposition peaks caused by enhanced thermal stability achieved by incorporating
more coconut coir-based microparticles due to cellulose and activated lignin, improving
thermal stability. Further, CDW40/CC60 and CDW20/CC80 showed hemicellulose andcellulose decomposition peaks in the ranges of 253–266 °C and 300–345 °C, respectively.
Fig. 43e illustrates the superior thermal stability of RS/CC biomicrocomposites containing
CC (40–80 wt%) to CDW/RS containing RS (20–60 wt%). However, RS, RS80/CC20,
and CDW20/RS80 were extremely unstable above 285 °C, as shown in (Fig. 44). Thesefindings confirm that condensed lignin and cellulose provide good thermal stability ataround 285 °C in RS/CC and CDW/RS, while regenerated silk provides enhanced thermal
properties at approximately 300–350 °C. Moreover, DTG peaks are exhibited in the rangeof 400–600 °C, and their fluctuations are attributed to char formation [63] and the
combustion of biomicrocomposites under an oxidative atmosphere [64]. In summary,
CC/RS biomicrocomposites showed superior thermal properties to CDW/CC and
CDW/RS biomicrocomposites. Although CDW/RS shows better thermal resistance than
CDW/CC, note that a higher regenerated silk content (~80 wt%) causes thermal instability.
A comparable enhancement in the thermal resistance of CDW/CC biomicrocomposites
was only observed when the coconut coir content was ≥60 wt% because the native lignin
content is responsible for higher stability and accounts for 28 wt% in coconut coir.

88

Figure 43. Mass-loss percentage of (a) CDW, and CDW/CC, (b) RS and RS/CC, and (c)CDW/RS biomicrocomposites. Corresponding DTG of (d) CDW, and CDW/CC, (e) RS
and RS/CC, and (f) CDW/RS biomicrocomposites

89Figure 44. Full mass-loss percentage curves for (a) regenerated silk biopolymer,
RS80/CC20, and CDW20/RS80 and (b) their corresponding DTG, confirming thermal
instability of regenerated silk (RS), RS80/CC20, and CDW20/RS80 biomicrocomposites
containing 80 wt% regenerated silk
5.4 FTIR spectroscopy
FTIR spectra were generated for the “fingerprint” and “single bond” regions of hybrid
biomicrocomposites and their neat biopolymers, as shown in Fig. 45. Fig. 45a shows
small differences in the spectra of the CDW/CC biomicrocomposites, CC, and CDW wood.
However, higher mass ratios of CC in CDW/CC biomicrocomposites show a gradualincrease in the intensities of the peaks associated with the β-glycosidic linkage ofamorphous cellulose at 895 cm-¹ [38,39]. It is also associated with the C–O stretching of
the guaiacyl ring at 1239–1290 cm-¹ [36], the aromatic skeletal vibrations of the phenol
90

groups of the aromatic ring in lignin at approximately 1594 cm-¹ [36], and the carbonyl
C=O stretching of hemicellulose at 1730–1745 cm-¹ [42].
Fig. 45b, c show that the peaks attributed to the lignocellulose-, cellulose-, and proteinbased functional groups overlap in case of the RS/CC and CDW/RS hybrid
biomicrocomposites. These peaks are assigned as follows: C–O–C antisymmetricstretching of cellulose (1140–1185 cm-¹) [90]; C–C, C–O, and C=O stretching of the Gband due to condensation ˃ etherification and the C-N stretching of the amide III band
(random coil) (1220–1249 cm-¹) [36,44]; the C=O aromatic skeletal vibrations of phenolgroups in lignin (G˃S) and the amide II band (N–H) attributed to the bending of β-sheetsand random coils in the region (1490–1570 cm-¹) [36,41,91]; the C=O aromatic skeletal
vibrations of phenol groups in lignin (S˃G) and the amide I band (C=O) stretching of silk’s
β-sheet region (1580–1720 cm-¹) [36,43,44]; C-H2 symmetric stretching, C–H3
antisymmetric stretching of lignocellulose, and C-H2 asymmetric stretching of the amide
B band of silk (2700–3000 cm-¹) [41,67]; and O-H stretching of lignocellulose and O–H
and N–H stretching of the amide A band of silk (3100–3600 cm-¹) [41,67].
By comparing the spectra of RS/CC and CDW/RS biomicrocomposites (Figs. 45b and c),significant differences in peak absorption can be observed, which is directly related to thetype and composition of added/blended starting materials. Therefore, the composition ofregenerated silk and coconut coir in RS/CC and CDW wood and regenerated silk in
CDW/RS biomicrocomposites can be observed from FTIR spectra (Figs. 45b and c),which mimic the FTIR spectra of their corresponding neat biopolymers. For example, silkrich biomicrocomposites showed an increase in the heights of the peaks at 1230 cm-¹
attributed to C–N stretching of amide III (random coil), 1520 and 1620 cm-¹ attributed toN–H deformation of amide II and amide I (β-sheets), and in the range 3250–3400 cm-¹
attributed to the O–H and N–H stretching of amide A. On the other hand, sharp peak at
1023 cm-¹ attributed to aromatic C–H deformation in guaiacyl and C–O stretching inprimary alcohol, which is typically found in plant fibers [36,39].

91

Figure 45. FTIR spectra of a) CDW/CC, b) RS/CC, and C) CDW/RS biomicrocomposites
and their corresponding biopolymers

92

Table 26. Summation of the major peak areas in the “fingerprint” region associated withcellulose, lignin, and hemicellulose in CDW/CC biomicrocomposites
Samples∑A(cellulose)a∑A(lignin)b

CDW
3.37 ± 0.04
1.62 ± 0.07
CDW80/CC20
3.02 ± 0.04
2.24 ± 0.11
CDW60/CC40
3.34 ± 0.07
2.37 ± 0.03
CDW50/CC50
3.15 ± 0.05
2.45 ± 0.08
CDW40/CC60
3.53 ± 0.42
3.04 ± 0.75
CDW20/CC80
3.48 ± 0.11
2.72 ± 0.05
CC
3.80 ± 0.10
3.35 ± 0.32
a: ∑A (cellulose) = (A895+ A1159+A1317+A1370+ A1420).
b: ∑A (lignin) = (A1509 + A1592).
c: ∑A (hemicellulose) = (A1735+A1745).

∑A(hemicellulose)c
0.27 ± 0.05
0.30 ± 0.03
0.42 ± 0.07
0.57 ± 0.02
0.65 ± 0.07
0.54 ± 0.01
0.63 ± 0.13

The results in Table 26 show an increase in the peak areas associated with cellulose,lignin, and hemicellulose in CDW/CC biomicrocomposites compared to those observedin neat hot-pressed CDW wood. This was attributed to coconut coir loading, which causes
composition changes due to the presence of lignin and hemicellulose; CC reported higher
summations of peak areas associated with cellulose, lignin, and hemicellulose than hotpressed CDW wood. Therefore, CDW/CC coconut coir-rich samples reported a higher
summation of peak areas associated with lignin and hemicellulose than hot-pressed CDW
wood. The CDW40/CC60 and CDW20/CC80 biomicrocomposites only show a noticeable
increase in the summation of peak areas associated with cellulose content compared withthat of hot-pressed CDW wood. The higher increase in peak areas based on functionalgroups associated with lignin establishes our previous findings for palm fiber- and leafbased lignocellulosic biopolymers ( as mentioned in chapter 4). That is, higher native
lignin content influences binding and thus affects their mechanical and thermal properties.

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Table 27. Summation of major peak areas in the “fingerprint” region associated with βsheets, cellulose, and hemicellulose in biomicrocomposites containing regenerated silk
Sample∑Aa∑Ab(β-sheet)(cellulose)RS
30 ± 0.1
RS80/CC20
32 ± 1.0
0.41 ± 0.00
RS60/CC40
31 ± 1.2
0.72 ± 0.01
RS50/CC50
26 ± 0.4
0.94 ± 0.18
RS40/CC60
18 ± 2.9
0.96 ± 0.06
RS20/CC80
12 ± 2.3
1.23 ± 0.24
CDW80/RS20
14 ± 1.4
1.41 ± 0.01
CDW60/RS40
27 ± 0.6
1.08 ± 0.18
CDW50/RS50
33 ± 0.6
0.60 ± 0.10
CDW40/RS60
29 ± 1.2
0.42 ± 0.13
CDW20/RS80
33 ± 0.4
0.15 ± 0.01
CC
2.32 ± 0.01
CDW
2.19 ± 0.00
a: ∑A (β-sheet) = (A1620 + A1697)
b: ∑A (cellulose) = (A895+ A1317+A1370+ A1420).
c: ∑A (hemicellulose) = (A1735+A1745).

∑Ac(hemicellulose)
0.04 ± 0.02
0.15 ± 0.02
0.02 ± 0.02
0.62 ± 0.12
0.29 ± 0.04

Moreover, the results presented in Table 27 describe the peak area summation
associated with the functional groups of β-sheets, cellulose, and hemicellulose, which
confirm and provide an estimation of composition changes in RS/CC and CDW/RS
biomicrocomposites. Moreover, gradual loading of CC and CDW content (≥50 wt%)shows a decrease in the summation of the peak areas associated with β-sheets in RS/CC
and CDW/RS biomicrocomposites compared to that in RS. Similarly, a steady rise incellulose content was observed in the RS/CC and CDW/RS biomicrocomposites
containing CC (≥20 wt%) and CDW (≥20 wt%). Additionally, smaller traces ofhemicellulose were observed from the summation of the peak areas associated with its
functional groups in RS40/CC60, RS20/CC80, and CDW80/RS20.
Based on these results, the numerically estimated cellulose and hemicellulose contents
confirm the composition of CC and CDW in CC/RS and CDW/RS hybrid
biomicrocomposites, respectively. Meanwhile, determining the lignin content in RS/CC
biomicrocomposites was immensely challenging using this method. However, it can beassumed that the reduction in the β-sheet peak areas was caused by greater or more
lignin-based adhesion. Two suggestions can be made from this trend: either lignin peaks
overlapped in the amide I and amide II bands of the secondary protein structure or there
94

were strong interactions among the functional groups associated with native lignin, amide
I and II. This made lignin’s functional groups challenging to distinguish from others. The
latter is highly probable, as indicated by the significant decrease in the summation of thepeak areas associated with β-sheets, which was exhibited only in RS40/CC60,
RS20/CC80 (Fig. 44 b), and CDW80/RS20 (Fig. 7 c) when the CC and CDW wood loading
amounts were ≥60 wt% and ~80 wt%, respectively. This is considered high compared tothat in the technical lignins reported in [92], which showed conformational changes toward
a “disordered” phase in thermoplastic zein/lignin bio-nanocomposites as little as 5 wt%
loading of technical lignin. However, we only observed an increase in peak area
associated with α-helices in RS40/CC60 and RS20/CC80 and a decreased peak area
associated with β-sheets in CDW80/RS20. This arrangement only indicated favorablehydrogen bonding between the functional groups of lignin and α-helices [92], possibly
caused by more lignin-based adhesion, rather than a conformational change toward a“disordered” phase. The conformational changes in the secondary protein structure bythermo-molding reveal that the wavenumbers and peak areas shift according to theconformation of β-sheets, α-helices, and β-turns [92]. In particular, higher-intensity peaksof β-sheets and α-helices imply a more “ordered” phase in the secondary structure ofproteins, while higher-intensity peaks of β-turns indicate a “disordered” phase in thepresence of additives [93]. However, RS/CC and CDW/RS biomicrocomposites showedthat conformation toward the formation of β-sheets, attributed to the peak at
approximately 1620 cm-1, was unhindered. In addition, possible hydrogen bonding
interactions among hydroxyl groups of silk cellulose and lignin were attributed to peaks
at approximately 3,300 cm-¹, a similarly to what observed in cellulose and wool blended
films [85].
FTIR data suggest strong reactivity between CDW wood, coconut coir, and regenerated
silk based on interactions between the functional groups associated with cellulose andlignin and the secondary structure of regenerated silk. Further evidence indicates that thematerials resulting from blending and hot-pressing native lignin in coconut coir andregenerated silk microparticles act as macromonomers instead of additives inbiomicrocomposites.

95

96

6. Discussion
6.1 Sampling issue
The sampling strategy may have a minor effect on the outcome of this study. However, it
is still subjected to the chemical composition and particle size if lignin’s chemical structure
behavior is preserved. The sampling is random, but is performed across locations in thecase of date fiber, date leaves, and coconut leaves. The sampling for coconut coir is fromthree different suppliers assuming that each supplier obtains coconut coir from various
locations. However, we did not intend to include these replications in this study because
the sampling strategy tends to generate variations in chemical composition in similartypes of plant fibers, even if they are obtained from the same tree in different locations.
For example, the cultivation conditions, including geographical location and location of
the fiber in the plant (500 mm from the root to the top), affect the tensile strength of Kenaf
fibers [12]. Furthermore, chemical compositions are widely varied, but still subject to(C+L)/(H+E+A) ratio and the particle size, which are the two primary factors influencing
the outcome of this study, assuming that lignin’s thermosetting behavior is preserved (nopre-heating or chemical pretreatment were applied). We have also selected two different
sources of palm fibers and leaves, bagasse, cotton, bamboo, and CDW wood, further
emphasizing the outcome of this study.
6.2 Limitation
Generally, the limitation of bio-based materials is their poor water resistance, compared
to conventional materials (e.g., plastics) due to their polarity to water. For example,
plastics such polypropylene(PP) and polycarbonate (PC) show water absorption after 24
h immersion in water in the range of 0–1.40% [94]. The reported properties are recordedin dry conditions; therefore, further research is required to investigate the long-term
effects of different weather conditions on mechanical and thermal properties. The
environmental conditions for the starting materials (microparticles), particularly the
moisture content, influence the processing of biomicrocomposites. Moreover, the shelf
time of the microparticles should be considered to determine if the materials naturally
degrade over time, which might be likely for synthetic additives added to thebiomicrocomposites, depending on the application. Finally, all this process requires a97

thorough life cycle assessment to evaluate and understand the environmental impact of
preparing these biomicrocomposites. Finally, we should also consider other factors thatplay a secondary role, such as moisture content, the effect of age, location, type of palm
tree, and season of harvest. Future research should focus on determining the effect ofthese factors.
7. Conclusions
This study presented an effective method of utilizing high-lignin biomass to produce highstrength lignocellulosic biopolymers by hot-pressing microfibrillated raw and alkaline
pretreated palm biomass microparticles (≤53, ≤53–75, and ≤106 µm) in the range 150–
180 °C. A higher CL/HEA ratio and microfibrillated powders comprising microparticles≤106 µm enabled more lignin exposure and efficient lignin activation. Compared toconventional methods, microfibrillation-enabled lignin exposure and distribution while
preserving lignin thermosetting during hot pressing by breaking hydrogen bonds incrystalline cellulose regions represented by milled-powder crystallinity indices in therange of 20–21%. Black lignocellulosic biopolymers indicated efficient lignin binding
activation. Chemical composition, (C+L)/(H+E+A) ratio, removal of non-structural
materials by mild alkaline pretreatment at 1 wt%, and particle size were the primaryfactors resulting in fiber-based, and date leaf-based lignocellulosic biopolymers, showing
mechanical and thermal properties superior to those of leaf-based lignocellulosic
biopolymers. These results were supported by the FTIR data in that fiber-based
biopolymers exhibited more peaks area associated with aromatic lignin than leaf-based
biopolymers that showed pectin-associated peaks. Furthermore, the summation of peakareas associated with cellulose, lignin, and hemicellulose and their corresponding peakarea ratios confirms the effective enhancement of mechanical and thermal properties dueto the removal of non-structural materials in alkaline pretreated biopolymers. Henceforth,most high-lignin plant fibers available in nature can be replicated, providing abundantsources of versatile environmentally friendly products. Leaf-based biopolymers might not
be suitable for high-strength materials unless mild chemical pretreatment is introduced toallow for lignin-based adhesion.

98

Furthermore, coconut coir-based and regenerated silk-based microparticles act asmultifunctional natural binders for CDW wood or blends used to prepare hybrid
biomicrocomposites. CDW, CC, and RS microparticles were mixed at specified mass
ratios and then hot-pressed in the range of 170–180 °C. Coconut coir and regenerated
silk acted as binders for CDW wood, and regenerated silk/coconut coir blends acted as
macromolecules, rather than additives in their respective biomicrocomposites. The
binding effect of the incorporation of RS (20–40 wt%) enhanced the mechanicalproperties and water resistance of CDW/RS. Meanwhile, the incorporation of CC slightly
improved the mechanical properties and water resistance of CDW/CC and RS/CC, butoffered greater thermal stability, as shown by thermogravimetric analysis (TGA/DTG).
SEM micrographs show that good interfacial adhesion is achieved by blending CC and
RS or adding them to CDW wood. FTIR data confirm the compositions and suggest
stronger reactivity among cellulose, lignin, and the secondary structure of the protein.
The findings of this study suggest the use of natural fiber-based microparticles inbiocomposites to enhance the mechanical and thermal properties and biological content.
Further, they offer insights into utilizing lignin’s and protein’s transitional phases assolvent systems during hot-pressing. This can be an alternative to conventional blending
methods (e.g., using ionic liquids to dissolve protein and lignocellulose to prepare hybrid
biocomposites). Although native lignin cannot be chemically isolated without disrupting
its thermosetting behavior, it can have multifunctional properties, as reported for technicallignin UV resistance and thermal stability. Furthermore, regenerated silk can be isolated
chemically from various silk sources but remains thermoplastic, which could mean thatsilk/lignocellulose blends could be recycled.
Data Statement
The raw, processed, and filtered data required to reproduce these findings are available
todownloadat

http://dx.doi.org/10.17632/vp5xkr9d9z.2

andhttps://dx.doi.org/10.17632/dw9mrddpbf.2.
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