polymers
Article
The Role of Eucalyptus Species on the Structural and Thermal
Performance of Cellulose Nanocrystals (CNCs) Isolated by
Acid Hydrolysis
Oscar Gil-Castell 1,2 , Pablo Reyes-Contreras 3,4 , Pabla Andrea Barra 4 , Regis Teixeira Mendonça 5,6 ,
Isabel Carrillo-Varela 7 , José David Badia 8 , Angels Serra 2 and Amparo Ribes-Greus 1, *
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Citation: Gil-Castell, O.;
Reyes-Contreras, P.; Barra, P.A.;
Teixeira Mendonça, R.;
Carrillo-Varela, I.; Badia, J.D.; Serra,
A.; Ribes-Greus, A. The Role of
Eucalyptus Species on the Structural
and Thermal Performance of
Cellulose Nanocrystals (CNCs)
Isolated by Acid Hydrolysis.
Polymers 2022, 14, 423. https://
doi.org/10.3390/polym14030423
Academic Editor: Luminita Marin
Received: 29 November 2021
Accepted: 12 January 2022
Published: 21 January 2022
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
*
Instituto de Tecnología de Materiales (ITM), Universitat Politècnica de València (UPV), Camí de Vera s/n,
46022 Valencia, Spain; ogilcastell@doctor.upv.es
Department of Analytical and Organic Chemistry, Universitat Rovira i Virgili, C/Marcel·lí Domingo s/n,
43007 Tarragona, Spain; angels.serra@urv.cat
Centro Nacional de Excelencia para la Industria de la Madera (CENAMAD), Pontificia Universidad Católica
de Chile, Av. Vicuña Mackenna 4860, Macul, Santiago 8320000, Chile; preyes@leitat.cl
Centro de Excelencia en Nanotecnología (CEN), Román Díaz 532, Providencia, Santiago 7500000, Chile;
pbarra@leitat.cl
Facultad de Ciencias Forestales, Universidad de Concepción, Concepción 4030000, Chile; rteixeira@udec.cl
Centro de Biotecnología, Universidad de Concepción, Concepción 4030000, Chile
Centro de Investigación de Polímeros Avanzados, CIPA, Avenida Collao 1202, Edificio de Laboratorios,
Concepción 4030000, Chile; i.carrillo@cipachile.cl
Department of Chemical Engineering, School of Engineering, Universitat de València,
Avinguda de la Universitat s/n, 46100 Burjassot, Spain; jose.badia@uv.es
Correspondence: aribes@ter.upv.es
Abstract: Cellulose nanocrystals (CNCs) are attractive materials due to their renewable nature, high
surface-to-volume ratio, crystallinity, biodegradability, anisotropic performance, or available hydroxyl
groups. However, their source and obtaining pathway determine their subsequent performance. This
work evaluates cellulose nanocrystals (CNCs) obtained from four different eucalyptus species by
acid hydrolysis, i.e., E. benthamii, E. globulus, E. smithii, and the hybrid En × Eg. During preparation,
CNCs incorporated sulphate groups to their structures, which highlighted dissimilar reactivities, as
given by the calculated sulphate index (0.21, 0.97, 0.73 and 0.85, respectively). Although the impact
of the incorporation of sulphate groups on the crystalline structure was committed, changes in the
hydrophilicity and water retention ability or thermal stability were observed. These effects were also
corroborated by the apparent activation energy during thermal decomposition obtained through kinetic analysis. Low-sulphated CNCs (E. benthamii) involved hints of a more crystalline structure along
with less water retention ability, higher thermal stability, and greater average apparent activation
energy (233 kJ·mol−1 ) during decomposition. Conversely, the high-sulphated species (E. globulus)
involved higher reactivity during preparation that endorsed a little greater water retention ability and
lower thermal stability, with subsequently less average apparent activation energy (185 kJ·mol−1 ).
The E. smithii (212 kJ·mol−1 ) and En × Eg (196 kJ·mol−1 ) showed an intermediate behavior according
to their sulphate index.
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4.0/).
Keywords: cellulose nanocrystals (CNC); eucalyptus; crystallinity; thermal stability; kinetic analysis
1. Introduction
Eucalyptus’ wood is extensively used in the global papermaking industry in different
grades, such as printing, tissue or packaging [1,2]. Moreover, it is also used for chemical
conversion into cellulose derivatives such as viscose, cellulose-acetate, cellulose-xanthate,
carboxymethyl cellulose, and nanocellulose [3–5]. Diverse eucalyptus species have been
reported to show significant differences in morphological, anatomical, and chemical characteristics, which define their subsequent applicability. Depending on their adaptability to
Polymers 2022, 14, 423. https://doi.org/10.3390/polym14030423
https://www.mdpi.com/journal/polymers
Polymers 2022, 14, 423
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the environment in which they are grown, it is possible to select a certain species based on
factors such as weather resistance or growth speed. While E. globulus is widely applied for
manufacturing Kraft pulps due to its high wood density and fast-growing plantation, other
species such as E. nitens × E. globulus hybrids are used for papermaking in cold countries
due to their frost tolerance. As well as this, E. nitens has gained interest in recent years for
bleached hardwood Kraft pulp obtaining, boosted by its excellent growing rates [6]. In this
regard, variations in the structure and cellulosic components of milled wood, holocellulose and α-cellulose isolated from different eucalyptus species have been described in the
literature [7].
The wood of eucalyptus is mainly composed of polysaccharides (75%), followed
by lignin (18–22%), other water-soluble compounds (1–2%), and extractives (<1%) [8].
Polysaccharides involve both hemicellulose and cellulose. Among them, cellulose, the
most abundant renewable polymer on Earth, is composed of two β-D-glucopyranose
rings rotated concerning each other to form a unit of cellobiose. Each of its monomers
has three hydroxyl groups that interact with other cellulose molecules, forming intraand inter-molecular hydrogen bonds. According to the arrangement of these bonds, the
cellulose forms a rigid network, which creates the compact structure responsible for its
high viscosity in solution and its high tendency to crystallize. At this supramolecular level,
the cellulose structure is not uniform, and crystalline and amorphous regions can be found.
The proportion of these regions depends on the raw material and the treatments to which
the cellulose has been subjected to. Likewise, the molecular orientation and the H-bond
network within the crystalline regions can vary widely, giving rise to different polymorphs
(I, II, III, and IV), with cellulose I and II being the most studied forms [9–11].
The development and utilization of cellulose are of great significance for advanced
biomaterials manufacture. Among the cellulose-based products, cellulose nanocrystals
(CNCs) are emerging as an attractive material for multiple applications due to their renewable nature, high surface-to-volume ratio, high crystallinity, biodegradability, anisotropic
performance, available hydroxyl groups for functionalization, colloidal stability, excellent
mechanical properties, and good biocompatibility [12]. The CNCs are crystalline domains
extracted from wood through different methods such as acid hydrolysis, enzymatic hydrolysis, or natural chemicals called deep eutectic solvents, among other approaches [13,14].
During extraction by hydrolysis, the amorphous regions may be hydrolyzed, while the
crystalline regions must remain unaffected. CNCs are usually rigid, rod-like particles
with a width of several nanometers (2–20 nm) and lengths up to hundreds of nanometers
(100–1000 nm) [15]. Their characteristics depend on the source of the fibers and the hydrolysis conditions such as the acid concentration, temperature, and reaction time. From a
technological perspective, the CNCs have been used alone or in combination with other
materials. Such is the case of polymer biocomposites, in which CNCs act as fillers for
mechanical reinforcement purposes, along with permeability modulator of matrices, while
maintaining biodegradability [16–18]. Compared to other inorganic reinforcing fillers,
the CNCs are widely available, easily processable due to their non-abrasive nature, and
allow for high filling levels and significant cost-savings [19]. However, there are critical
aspects of CNCs that need to be understood before its utilization, which determine their
performance and assist to provide ubiquitous properties. For example, morphology, aspect
ratio, and functionality affect the rheological properties of CNCs. Moreover, crystallinity
is a critical feature due to its direct connection to strength/toughness modulus, which is
a desirable characteristic for composite applications. Although research has been carried
out regarding the modification of cellulose for advanced biomaterials manufacture [20–23],
there are few studies in the literature dealing with the structural variation of the CNCs
isolated from different species and their influence on the course of chemical reactions for
derivatives production.
Carrillo et al. studied cellulosic components isolated from seven eucalyptus varieties
grown under the same conditions [7]. The results showed significant correlations for the
crystalline index (CrI), as obtained by X-ray diffraction (XRD), versus the peak of the
Polymers 2022, 14, 423
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derivative thermogravimetric curve (DTG) in wood samples and the DTG peak versus the
crystallite size in alpha-cellulose samples. These structural and thermal differences can
determine the production of cellulose derivatives from such species. During the production
of derivatives, cellulose is modified by substitutions at the hydroxyl groups present in
the C2, C3, and C6 positions of anhydroglucose units. This capacity to substitutions of
hydroxyl groups is known as cellulose reactivity. Indeed, in another study dealing with
cellulose from bleached kraft pulps of different eucalyptus species upgraded to dissolving
pulp, they demonstrated that E. benthamii and E. nitens were the less reactive pulps, whereas
other species such as E. globulus or the hybrid E. nitens × E. globulus (En × Eg) were more
reactive [24]. The accessibility is an intrinsic property of the pulp, which depends on
several chemical and structural features that could be roughly defined as the ease with
which the reaction sites (functional groups) can be reached by the chemical agents [25].
The accessibility of cellulose fibers is critical for the reactivity of cellulosic pulps, and it is
affected by many other inter-related and complex factors as fiber morphology, cell wall
architecture, pore system, surface area, supramolecular structure, chemical composition,
and molecular weight [26,27]. In this area, and considering the growing demand for
cellulose derivative materials such as CNCs for sustainable industrial applications, more
studies dealing with its quality and subsequent behavior are required.
This work aims to understand the structural and thermal performance of cellulose
nanocrystals (CNC) obtained from different eucalyptus species, all of them grown under
the same field and plantations conditions, i.e., E. benthamii, E. globulus, E. smithii, and the
hybrid En × Eg. The physicochemical properties of the CNCs were evaluated to determine
similarities and differences among the different species. In particular, the assessment of
composition using spectroscopy, the evaluation of surface morphology through microscopy,
the study of the crystalline structure employing diffraction and spectroscopic techniques,
and thermal properties using both differential scanning calorimetry and thermogravimetry
was carried out. In this regard, the assessment of the chemical composition, the crystalline
structure, and the detailed evaluation of the thermal stability of CNCs may offer knowledge on their compatibility, reinforcing ability and stability during application, and also
on the behavior in the course of preparation of thermoplastic-based biocomposites at high
temperatures [28]. Specifically, the crystalline arrangement and the chemical composition
are in turn the most influencing parameters during thermal decomposition and may be subsequently affected by the cellulose source and also by the extracting method [29]. Although
the pyrolytic decomposition of CNCs extracted from mango seeds, Kraft pulp, cellophane,
corn stalk or curaua fibers has been reported in the literature [30–33], the comparative
evaluation of the apparent activation energy (Ea) of CNCs from different eucalyptus species
during thermal decomposition has not been yet reported. Therefore, Kissinger, Flynn–Wall–
Ozawa (FWO), and Kissinger–Akahira–Sunose (KAS) methods were used to assess the
apparent activation energy (Ea) of such CNCs during thermal decomposition.
2. Materials and Methods
2.1. Materials
2.1.1. Eucalyptus Wood Samples
The eucalyptus species corresponding to E. benthamii, E. globulus, E. smithii, and a
hybrid of E. nitens × E. globulus (labelled as En × Eg), grew under the same field and
plantations conditions and were provided by Chilean forestry located at the Biobío region
(Southern Chile).
2.1.2. Chemicals and Reagents
Sodium hydroxide (NaOH, ≥98%, pellets (anhydrous)), sodium sulphide nonahydrate
(Na2 S ≥ 98%,), sulfuric acid (H2 SO4 , 95–98%), and glacial acetic acid (CH3 CO2 H, 100%),
sodium chlorite (NaClO2 , 80%) were purchased from Sigma Aldrich (St. Louis, MO, USA)
and Merck (Darmstadt, Germany). Deionized water was obtained from a Millipore Milli-Q
system (Milford, MA, USA) to prepare all the aqueous solutions.
Polymers 2022, 14, 423
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2.1.3. Kraft Pulping and Bleaching
Kraft pulping was performed in a rotatory digester equipped with four independent
Regmed reactors (Osasco, Brazil) with a volume of 1.5 L. For each reaction, wood chips
(100 g on a dry basis) with average size 30 × 20 × 0.4 mm3 and cooking liquor (400 mL)
with 18% active alkali (AA) and 30% sulphidity (both expressed on NaOH basis) were used.
Heating time to the maximum temperature (165 ◦ C) was 90 min and the H-factor was 800.
The resulting material from each cooking was disintegrated, and pulps were screened on a
0.2 mm slot screen. The pulp was then centrifuged to 35% consistency and weighted. The
exact moisture was determined, and the screened pulp yield was calculated.
Pulp bleaching was performed by chlorite delignification in an acidic media using a
NaClO2 charge of 10% oven dry-pulp (ODP) basics at 5% pulp consistency. Acetic acid
was added to acidify the mixture. The reaction was carried out in sealed poly(ethylene)
bags immersed in a thermostatic water bath at 90 ◦ C for 2 h. Finally, bleached pulps
were obtained after filtration, washed with abundant distilled water, and centrifuged to
35% consistency.
2.2. Characterization of Pulp
Eucalyptus pulps (300 mg) were hydrolyzed with 72% H2 SO4 (3 mL) at 30 ◦ C for 1 h.
Then, the acid was diluted to 4% and the mixture was heated at 121 ◦ C, 1.1 atm, for 1 h.
The resulting material was cooled and filtered through a porous glass filter. Solids were
then dried to a constant weight at 105 ◦ C and determined as insoluble lignin. The soluble
lignin concentration in the filtrate was determined through UV-spectroscopy by measuring
the absorbance at 205 nm, using the value of 110 L·g−1 ·cm−1 as the absorptivity. The
concentration of monomeric sugars and acetic acid in the soluble fraction was determined
via high-performance liquid chromatography (HPLC) in a setup from LaChrom-MerckHitachi (Tokyo, Japan) equipped with a refractive index detector and Bio-Rad Aminex
HTX-87H column (Hercules, CA, USA) at 45 ◦ C, a mobile phase of 5·10−3 mol L−1 of H2 SO4
and a flow rate of 0.6 mL min−1 . Glucose and xylose were used as external calibration
standards. The glucans content was calculated by multiplying the glucose content by 0.90;
the xylans content was obtained from the xylose content by multiplying by 0.88; and the
acetyl groups content was calculated by multiplying the acetic acid content by 0.70 [34].
2.3. Production of Cellulose Nanocrystals (CNCs)
CNCs from eucalyptus Kraft pulps were prepared by sulfuric acid hydrolysis using the
protocols published by Chen et al. [35], and Bondeson et al. [36], with some modifications.
Briefly, a H2 SO4 solution (55% wt.) was used with a reaction temperature of 60 ◦ C for 45 min
under constant magnetic agitation. All the acid hydrolysis experiments were conducted
using a 1:40 fiber to acid solution ratio. The reaction was stopped with cold distilled
water and the reaction products were washed three consecutive times by centrifugation
(10 min, 12,000 rpm). The resulting suspensions were extensively dialyzed for 5 days with
a membrane of 12 kDa against distilled water to a constant pH. Finally, the samples were
sonicated in an iced-water bath to avoid heating of the suspension at 750 W for 10 min using
pulses (10/15) and centrifuged for 10 min at 12,000 rpm. Thus, the resulting supernatant
containing the isolated CNCs was kept at 4 ◦ C. Afterwards, highly concentrated aqueous
solutions of such CNCs (1 g·L−1 ) were solvent-casted in circular molds and the water
allowed to evaporate until constant mass and consistent films with a diameter of 5 cm and
a thickness of 15 µm were obtained. They were stored in a desiccator at room temperature
and were the basis for further characterizations.
2.4. Characterization of Cellulose Nanocrystals
2.4.1. Chemical Oxygen Demand (COD)
Chemical oxygen demand (COD) measurements were used to determine CNCs yields
according to the procedure described by Wang et al. [37]. The colorimetric COD expresses
the amount of oxygen originating from potassium dichromate that reacts with the sample.
Polymers 2022, 14, 423
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The samples were oxidized with a hot sulfuric solution of potassium dichromate. Microcrystalline cellulose was used for calibration and further calculation. A proper aliquot
was taken and added in the COD reaction tubes from Merk Spectroquant COD cell test
(Darmstadt, Germany). The tubes were thoroughly mixed and incubated for 2 h at 148 ◦ C
using a Merck Spectroquant TR320 thermoreactor (Darmstadt, Germany). Once the samples
were cooled down to room temperature, the absorbance at 600 nm was measured in a
Merck Spectroquant Pharo 300 spectrophotometer (Darmstadt, Germany), and the COD
was determined by interpolation using calibration curves with Avicell PH-101 cellulose
from Sigma-Aldrich (St. Louis, MO, USA).
2.4.2. Field-Emission Scanning Electron Microscopy (FE-SEM)
The surface topology was analyzed by field-emission scanning electron microscopy
(FE-SEM) in a Zeiss Ultra 55 equipment (Oberkochen, Germany). They were prepared
in two ways: either small pieces of the films (1 cm diameter) were placed on the sample
holders with double-sided adhesive carbon tape; or aqueous low-concentrated dispersions
(2 mg·L−1 ) of the different samples were prepared, sonicated, and directly deposited onto
clean circular coverslips, and water evaporation was allowed overnight. Then, all the
samples were sputter-coated with a platinum layer for 15 s using a Leica EM MED020
sputter coater (Wetzlar, Germany). Finally, the micrographs were taken using a working
distance between 3 and 4 mm, a voltage of 2 kV, and magnification of 100,000×. The
size and distribution of the different samples were measured from the average of random
locations (n = 100) through the software Image J® 1.52p.
2.4.3. Fourier-Transform Infrared Spectroscopy (FTIR)
Fourier-transform infrared spectroscopy (FTIR) was used to study the different functional groups of eucalyptus nanocrystals and to evaluate structural changes. All the
experiments were conducted using an Agilent Cary 630 FTIR spectrometer (Santa Clara,
CA, USA) in the attenuated total reflectance mode (ATR). The solvent-casted film samples
were dried before measurement in an oven at 50 ◦ C for 24 h. The analyses were performed
in the wavelength range from 4000 to 650 cm−1 with a 4 cm−1 resolution and an average of
64 scans. From the intensities of bands at 1430 and 897 cm−1 , the infrared crystallinity index
(ICI) was calculated with Equation (1), as proposed by O’Connor et al. [38]. Then, Nelson
and O’Connor proposed a new infrared crystallinity ratio (ICR), considering the bands
at 1372 and 2900 cm−1 assigned to a C–H bending mode and C–H and CH2 stretching,
according to Equation (2) [39].
ICI (%) =
A1430
× 100
A897
(1)
ICR(%) =
A1372
× 100
A2900
(2)
2.4.4. X-ray Diffraction (XRD)
An X-ray diffraction instrument PANalytical Cubix with a PANalaytical X’Celerator
detector (Malvern, Worcestershire, UK) was used to determine the crystal size and the
crystallinity degree of the samples. Dried film specimens of 100 mm2 were considered
for the analysis. The CuKα radiation (λ = 1.5418 Å) was generated with a tension of
45 kV and a current of 35 mA and then monochromatized by using a Ni filter of 20 µm.
The experiment was measured and recorded in a reflection mode at the angular range of
2–90◦ (2θ) and a rate of scanning of 0.05◦ per 10 s. Results were analyzed using the X’Pert
Highscore software. An average spectrum of three different specimens was considered as
representative for each sample.
To determine the mean size of the crystalline domains, the Scherrer’s equation was
exploited according to Equation (3), where K is a dimensionless shape factor (0.96), λ is the
X-ray wavelength (0.154 nm), β is the line broadening at half the maximum intensity after
Polymers 2022, 14, 423
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subtracting the instrumental line broadening in radians and θ is the Bragg angle. The peak
of 2θ = 22.6◦ was observed to calculate the crystal size, which represented the diffraction of
the (002) plane.
Kλ
(3)
Lhkl =
β cos θ
The crystallinity index (CrI) was calculated employing the method proposed by
Segal et al. according to Equation (4), where I002 is the maximum density of the (002) reflection at 2θ = 22.6◦ and Iam is the intensity diffraction of the amorphous band at 2θ = 18.5◦ .
CrI (%) =
( I002 − Iam )
× 100
I002
(4)
2.4.5. Differential Scanning Calorimetry (DSC)
The calorimetric data were obtained using a Mettler–Toledo DSC 820e differential
scanning calorimeter (Columbus, OH, USA). The solvent-casted film samples, with a mass
of around 5 mg, were introduced into 40 µL aluminum crucibles perforated on top. The
method of analysis consisted of a heating scan between 25 ◦ C and 375 ◦ C with a rate of
10 ◦ C·min−1 . The test was carried out under an inert atmosphere of nitrogen with a flow
rate of 50 mL·min−1 . The samples were analyzed in triplicates with the aid of the STARe®
software, and the averages were taken as representative.
2.4.6. Thermogravimetric Analysis (TGA)
The thermal stability was assessed through a Mettler–Toledo TGA 851 thermogravimetric analyzer (Columbus, OH, USA). The solvent-casted film samples, with a mass
between 3 and 5 mg, were introduced into 70 µL alumina capsules. Next, they were
subjected to a dynamic assay, based on a heating segment from 25 ◦ C to 800 ◦ C with a
heating rate (β) of 5, 10, 20, and 30 ◦ C min−1 . The analyses were carried out under an inert
atmosphere with a feeding rate of argon at 50 mL per min−1 . The decomposition onset
and endset temperatures (To , Te ) were obtained by a tangential intercept method onto the
thermogravimetric (TG) curves. The peak temperatures were obtained from the maximum
of the derivative thermogravimetric curves (DTG), i.e., the maximum decomposition rate.
The samples were analyzed in triplicates with the aid of the STARe® software, and the
averages were taken as representative.
2.4.7. Kinetic Analysis for the Thermal Decomposition
The apparent activation energy (Ea) of the thermal decomposition process was calculated through kinetic analysis from the obtained thermogravimetric results at different
heating rates (β) [40–43]. Different approaches were applied, including Kissinger [44],
Flynn–Wall–Ozawa (FWO) [45,46], and Kissinger–Akahira–Sunose (KAS) [47] methods.
First, the Ea was estimated considering the method proposed by Kissinger, based on
the temperature at maximum decomposition rate (Tp ), the model for which is given by
Equation (5).
β
A· R ′
Ea
ln 2 = −
· f (α)
(5)
+ ln
R· Tp
Ea
Tp
where β is the heating rate (K·min−1 ), R is the gas constant (8.314 J·K−1 ·mol−1 ), Ea is the
apparent activation energy (J·mol−1 ), A is the pre-exponential factor (min−1 ) and Tp (K) is
the peak temperature of the main decomposition stage in the DTG curve. According to this
method, the activation energy can be calculated from the slope of the plots of ln (β/Tp 2 )
versus the reciprocal of temperature at the maximum reaction rate (1/Tp ), for the different
experiments at constant heating rate. Therefore, the activation energy can be estimated
regardless of the reaction mechanism and conversion.
Polymers 2022, 14, 423
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Then, both isoconversional FWO and KAS models were applied, which involve the
evaluation of the thermal decomposition as a function of the conversion degree (α). Neither
of these methods require knowledge of the particular thermal degradation mechanism.
log β = log
Ea
A· Ea
− 2.315 − 0.4567
R· g( a)
R· Tα
(6)
Finally, the Kissinger–Akahira–Sunose (KAS) method for a multi-heating rate application is based on Equation (7). Through this method, the apparent activation energy can be
estimated from the slope of the plot of ln (β/T2 ) versus 1/T at a given conversion degree.
β
Ea
A· R
−
ln 2 = ln
(7)
Ea· g(α)
R· Tα
Tα
3. Results and Discussion
3.1. From Eucalyptus Pulp to Cellulose Nanocrystals (CNCs)
Eucalyptus Kraft pulps were prepared and their yield and chemical composition were
determined. As gathered in Table 1, the unbleached Kraft pulps yields ranged between
49% and 57%, with the species E. benthamii being the one with the lowest yield. After
the bleaching stage, the yields of solids generally decreased, reaching values of 45% to
53%, since not only residual lignin is removed but also some low molar mass xylans and
glucans from polysaccharides fraction are detached. The obtained yield and chemical
composition found in this study are in agreement with the values reported in the literature
for eucalyptus wood with similar pulping conditions [48].
Table 1. Yield and composition of unbleached and bleached Kraft pulps along with CNCs obtaining
yield from the different eucalyptus species.
Unbleached Kraft Pulps
E. benthamii
E. globulus
E. smithii
En × Eg
Bleached Kraft Pulps
CNC
Yield
Glucans
Xylans
Lignin
Yield
Glucans
Xylans
Lignin
Yield
(%)
(% wt)
(% wt)
(% wt)
(%)
(% wt)
(% wt)
(% wt)
(% wt)
49 ± 2
57 ± 1
56 ± 2
57 ± 2
73 ± 1
71 ± 1
73 ± 2
75 ± 2
15 ± 1
18 ± 1
18 ± 1
19 ± 1
4.3 ± 0.9
3.8 ± 0.6
3.7 ± 1.1
4.6 ± 1.3
45 ± 2
53 ± 1
53 ± 2
52 ± 2
73 ± 1
65 ± 1
74 ± 1
71 ± 2
17 ± 1
19 ± 1
22 ± 1
22 ± 1
1.3 ± 0.6
1.3 ± 0.4
<1.0
1.7 ± 0.7
21.7 ± 1.0
14.6 ± 0.4
22.4 ± 1.5
24.9 ± 1.2
Cellulose nanocrystals (CNCs) were subsequently isolated, and the obtaining yields
were calculated. Table 1 gathers significant differences in the yield for the different eucalyptus species. While the E. globulus showed the lowest CNC yield, around 15%, the yield for
the hybrid En × Eg reached almost 25%, based on COD measurements. Although a wide
variety of CNC yield can be found in the literature according to specific conditions and
CNC source [37,49,50], these values generally agreed with other studies using a similar
CNC production strategy from eucalyptus, based on sulfuric acid hydrolysis with 55% wt.
H2 SO4 , 60 ◦ C and 45 min, such as reported by Chen et al. with yields near to 18% [35].
The variances found in the CNC yield can be ascribed to the differences in the properties
of cellulose from the diverse eucalyptus type, as reported by Carrillo et al. in wood and
holocellulose isolated from the same species [7].
3.2. Cellulose Nanocrystals (CNCs)
3.2.1. Morphology
It is well known that the origin of cellulose microfibrils and the hydrolytic conditions
strongly affect the geometric dimensions and morphology of CNCs. For this reason, the
surface morphologies of the CNCs obtained from the different eucalyptus species were
evaluated, and the size distributions in terms of diameter were calculated. In particular,
Polymers 2022, 14, 423
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the morphology of the low-concentrated dispersions was studied together with the surface
of the solvent-casted films. The obtained surface micrographs along with the diameter
histograms are shown in Figure 1.
Figure 1. Surface electron micrographs and diameter distribution of the CNCs obtained from the
different eucalyptus species.
On the one hand, the deposition of low-concentrated dispersions allowed the cellulose
nanocrystals to be clearly observed, with an appearance of individual separated CNCs
with average diameters of 9.5, 6.6, 7.2 and 8.5 nm for the E. benthamii, E. globulus, E. smithii
and En × Eg, respectively. For these CNCs, the aspect ratio was between 5 and 10, in line
with the standardized definitions for such nanoparticles [51]. On the other hand, after
the preparation of the solvent-casted films from highly concentrated solutions, a random
distribution of individual and aggregated stacked shapes with respective average diameters
of 14.8, 12.6, 16.7 and 15.5 nm were found in all cases [33]. The anionic stabilization via the
repulsion forces of electrical double layers caused by the surface functionalization during
hydrolysis may have prevented the aggregation of cellulose nanocrystals and explained
the formation of the self-assembled structures [52]. Although in these pictures a fibrous
appearance could be identified in the first plane, no signs of network-like structures or
inter-particle entanglements (or both) typical of cellulose nanofibers and nanofibrils (CNF)
could be detected [51]. Moreover, fracture signs transversal to these fibrous units could
be perceived, which have been reported as signs of an intermediate semi-fibrous stage
during the generation of cellulose nanocrystals by acid hydrolysis [53]. At this point, the
authors hypothesize that during the hydrolytic process for the preparation of CNCs, such
Polymers 2022, 14, 423
9 of 24
mentioned intermediate species remained in the dissolution, and given their lower density
and higher size, they were deposited on top, while the denser CNCs with lower size were
deposited in the inner part of the film overlapping the visualization of the CNCs after the
preparation of the solvent-casted films. According to the dimensions and morphology the
approximated aspect ratio in the range from 15 to 60 for all the species lies in the range of
long rod-shaped CNCs, with great potential for being used as good reinforcing agents in
the preparation of bionanocomposites with improved mechanical properties [51,54].
Overall, it can be remarked that the lower diameter for the CNCs from E. globulus
species were both in the highly dispersed CNCs and in the solvent-casted films. The
progression of the hydrolytic reaction in the E. globulus species may have reached a greater
extent and, consequently, resulted in lower diameters of the CNCs. In comparison to results
found in the literature, the dimensions of the obtained CNCs were comparable to those
found by other authors [19,55]. For instance, Wang et al. found diameters between 10 and
17 nm for CNCs from eucalyptus solid residues [37], Aguayo et al. reported diameters of
around 40 nm for CNCs obtained using severe hydrolysis conditions from rejected fibers
originated in the Kraft pulping process [56] and Duarte Urueña et al. reported diameters
varying from 4 to 15 nm in eucalyptus-based residues [57]. Then, Fortunati et al. found
diameters in the range of 5 to 10 nm in CNCs isolated from commercial microcrystalline
cellulose [58], Wulandari et al. achieved CNCs from sugarcane bagasse with an average
diameter of around 100 nm [59], and Kusmono et al. produced CNCs from ramie fibers
with diameters from 4 to 7 nm [60].
3.2.2. Chemical Structure
To evaluate possible functionalization caused by the acid hydrolysis process, the
− analyzed by Fourier-transform
chemical structure of the cellulose nanocrystals (CNCs) was
infrared spectroscopy (FT-IR). The acquired FTIR spectra are shown in Figure 2.
Figure 2. FTIR spectra of the CNCs obtained from the different eucalyptus species.
All the spectra were similar regardless of the eucalyptus species, showing the characteristic peaks of crystalline nanocellulose [56]. FTIR results corroborated that the lignin and
hemicellulose were completely removed during the isolation process of the CNCs, given
the absence of their characteristic peaks at 1250 cm−1 , 1510 cm−1 , and 1730 cm−1 related to
the stretching vibrations of C–O of the aryl group in lignin, C=C of the aromatic ring in
lignin, and C=O of carboxylic groups of hemicellulose and lignin, respectively [59,61,62].
The most dominant spectral bands in the
spectra of the CNCs were located at 3330
cm−1
−
−
−
1
and 1040 cm and are associated with the stretching vibration of –OH and C–O ether
𝑆𝑢𝑙𝑝ℎ𝑎𝑡𝑒 𝑖𝑛𝑑𝑒𝑥 (%) =
𝐴
− 𝐴
𝐴
× 100
Polymers 2022, 14, 423
10 of 24
groups, respectively. The peak around 1640 cm−1 is due to the O–H bending of adsorbed
water molecules, which were difficult to remove even after drying given the strong cellulose–
water interaction. Peaks observed at around 2900 cm−1 are associated with the asymmetric
and symmetric vibration of –CH2 groups. Scissoring motion of –CH2 in cellulose could be
identified in the range from 1300 to 1400 cm−1 . The peak at ~890 cm−1 is due to β-glycosidic
linkages. Finally, the presence of sulphate groups was perceived at 1205 cm−1 given the
S=O vibration and the small shoulder at 814 cm−1 caused by C–O–S group vibration.
These are remnant cues that come from the hydrolytic agent (H2 SO4 ) used for the
isolation of CNCs that may have established sulphate esters with the hydroxyl groups
on their surface during hydrolysis [60]. In this regard, it is important to remark that the
small peak of the C–O–S group vibration was less relevant for the E. benthamii species,
which may be less reactive and more resistant to substitution reactions during hydrolysis.
For comparative purposes, a sulphate index was established according to Equation (8),
relating the absorbance of the signal at 814 cm−1 of sulphates with the band at 2900 cm−1
of the C–H and CH2 stretching, considered as a reference as it remains unaffected in the
cellulose backbone.
Sul phate index (%) =
A814 − A2900
× 100
A2900
(8)
The obtained values are gathered in Table 2. In particular, the E. benthamii was by far
the less sulphated species, followed by the E. smithii, En × Eg, and E. globulus in increasing
order. The degree of incorporation sulphate groups may be therefore ascribed to the
dissimilar reactivity of the CNCs, as found in bleached pulps after acid hydrolysis [24] and
in holocellulose and α-cellulose isolated from different eucalyptus species [7,63]. Moreover,
cellulose sulphation has been reported in the literature as indicative reactions of CNC
formation, so that the generation of cellulose nanocrystals can be corroborated [35,37,64].
Table 2. Sulphate index of the CNCs obtained from the different eucalyptus species.
Sulphate Index (%)
E. benthamii
E. globulus
E. smithii
En × Eg
0.21
0.97
0.73
0.85
3.2.3. Crystalline Structure
Regarding the crystallinity evaluation, X-ray diffraction (XRD) analyses were carried
out and the obtained spectra are plotted in Figure 3. Four main reflection peaks were
identified at 2θ = 15.0◦ , 16.5◦ , 22.6◦ and 34.5◦ due to the (101), (101), (002) and (040)
crystallographic planes of the monoclinic cellulose I-β lattice [65]. The highly crystalline
structure of the obtained CNCs was thus corroborated and results were consistent with
cellulose crystal structure I [31].
From these spectra, and considering the maximum density of the (002) reflection at
2θ = 22.6◦ characteristic of the cellulose I and the intensity diffraction of the amorphous
band at 2θ = 18.5◦ , the lateral crystallite size (L002 ) and the crystallinity index (CrI) were
calculated using Equations (3) and (4), respectively, and gathered in Table 3. The CrI of the
CNC was around 65% and the L002 values varied between 10 and 11 nm regardless of the
eucalyptus species. The results indicate that the crystalline structure does not present more
than slight differences among the CNCs from the different species. In general, both CrI and
L002 were higher than those of analogous wood eucalyptus counterparts due to the acid
hydrolysis extraction of amorphous regions during CNC obtaining [7].
θ
β
Polymers 2022, 14, 423
101
11 of 24
Figure 3. X-ray diffraction (XRD) spectra of the CNCs obtained from the different eucalyptus species.
Table 3. Obtaining yield along with apparent crystallinity degree (CrI), lateral crystallite size (L002 ),
infrared crystallinity index (ICI), and infrared crystallinity ratio (ICR) of CNC obtained from the
different
eucalyptus species.
θ
θ
E. benthamii
E. globulus
E. smithii
En × Eg
Crystallinity Evaluation
CrI (%)
L002 (nm)
ICI (A1430 /A897 )
ICR (A1372 /A2900 )
65.8 ± 1.3
65.0 ± 1.7
64.4 ± 1.6
65.2 ± 1.9
10.9 ± 1.3
10.1 ± 1.1
10.1 ± 0.8
10.0 ± 1.4
1.041
1.014
1.014
1.011
0.997
0.983
0.988
0.987
Results were comparable to those reported before for CNC produced by acid hydrolysis with sulphuric acid from eucalyptus [64,66]. Even though slightly higher values
between 65 and 75% have been reported for eucalyptus CNCs with other hydrolysis approaches, the presence of such intermediate-semi-fibrous units may have prevented the
obtained CNC to reach such higher crystallinity indexes [57]. Moreover, it must be considered that the hydrolysis reactions during CNC production mainly cause changes in the
amorphous domains. However, it may also promote damage in the crystalline regions
and, therefore, influence the overall crystallinity resulting in such moderate CrI. Similar
values were calculated in other studies with acid hydrolysis isolation processes from other
biomasses [29,33,51,60].
Despite the evaluation of the crystalline structure by XRD being the most common
approach, a complimentary assessment of crystallinity through FTIR is also considered
in the literature [38,39]. Therefore, the crystalline structure was studied from the FTIR
spectra through some infrared crystallinity indexes defined in the literature. According
to O’Connor et al., the infrared crystallinity index (ICI), can be obtained using Equation
(1) from the ratio between the bands at 1430, assigned to crystalline areas, and 897 cm−1 ,
assigned to amorphous regions [38]. Then, Nelson and O’Connor proposed a new infrared
crystallinity ratio (ICR), shown in Equation (2), considering the bands at 1372 and 2900 cm−1
assigned to a C–H bending mode and C–H and CH2 stretching [39]. ICI correlates with the
overall degree of order in cellulose and is defined as an empirical crystallinity index while
ICR has been reported to be proportional to the crystallinity degree of cellulose [67,68].
Both infrared crystallinity values are gathered in Table 3.
Similar crystallinity parameters were found regardless of the eucalyptus species as
indicated by the XRD, although these methods are dependent on the quantity and size
−
Polymers 2022, 14, 423
12 of 24
of crystallites, whereas the infrared ratios are dependent upon the environment of individual molecules [39]. Only a slight increase was observed in ICI and ICR indexes for
the E. benthamii variety, which may be correlated to the presence of sulphate groups in
the CNCs and the progression of the hydrolysis reaction in the crystalline phase. The
E. benthamii was the one with a lower sulphate index; therefore, it seemed that the higher
the reactivity involving the greater incorporation of sulphates during hydrolysis, the lesser
ordered structure was perceived [24].
3.2.4. Thermal Properties and Thermal Stability
The thermal transitions of CNCs were assessed through differential scanning calorimetry (DSC) and the variations in the thermal stability were evaluated through thermogravimetric analyses (TGA).
The calorimetric thermograms of the CNCs obtained from the different eucalyptus
species are plotted in Figure 4, in which a wide endothermal transition from 25 ◦ C to
150 ◦ C, followed by a sharp peak in the vicinities of 275 ◦ C with a small shoulder above
300 ◦ C, more visible in the E. smithii and En × Eg species, could be observed. The peak
temperatures and enthalpies for both transitions are gathered in Table 4.
Figure 4. Stacked calorimetric thermograms of the CNCs obtained from the different eucalyptus species.
Table 4. Peak temperatures and enthalpies for the water release (TH2O and ∆hH2O ) and CNC thermal
decomposition (Tcel and ∆hcel ), as obtained from the calorimetric thermograms.
H2 O Release
E. benthamii
E. globulus
E. smithii
En × Eg
CNC Thermal Decomposition
Tfree
∆hH2O
Tcel
∆hcel
(◦ C)
(J·g−1 )
(◦ C)
(J·g−1 )
69.2 ± 0.7
82.5 ± 2.3
69.8 ± 0.7
78.2 ± 1.9
57.1 ± 4.3
84.5 ± 4.3
49.8 ± 2.2
76.1 ± 6.6
279.7 ± 0.2
269.7 ± 1.8
278.6 ± 0.2
273.6 ± 0.6
114.0 ± 0.8
109.2 ± 0.8
115.2 ± 0.6
120.8 ± 1.0
The first endothermic process can be ascribed to a dehydration stage [69]. The peak
for this process (Tfree ) was located between 70 and 80 ◦ C for the different eucalyptus species,
with enthalpies (∆hH2O ) ranging from 50 to 85 J·g− 1 . The results indicate a correlation
between sulphate index and the Tfree and ∆hH2O values. The CNC from E. globulus has the
highest sulphate index and highest Tfree and ∆hH2O . The one from E. benthamii revealed
Polymers 2022, 14, 423
13 of 24
the lowest Tfree and ∆hH2O , correlated to a lower sulphate index. Finally, those CNCs from
E. smithii and En × Eg showed intermediate values according to their sulphate indexes.
These results may suggest the contribution of sulphate groups attached to CNCs, which
may have promoted structures with more capability to incorporate water.
Then, an endothermal acute transition and subsequent shoulder were found for the
thermal decomposition of the crystalline nanocellulose. This peak increased while the
sulphate index decreased. It was situated around 270 ◦ C for the E. globulus, at 274 ◦ C for
the En × Eg, followed by the E. smithii at 279 ◦ C and the E. benthamii at 280 ◦ C. These
emerging differences in the thermal stability of CNCs were presumable due to the effect
of the sulphate groups and were examined in detail with thermogravimetric stability
analyses [29,70,71].
The thermogravimetric thermograms with their first derivative curves obtained under
inert atmosphere at 10 ◦ C·min− 1 are plotted in Figure 5. The observed stages are the
two identified by the calorimetric results, and the char formation, given as the remnant
residue at the end of the thermogravimetric analyses. The progressive decomposition
behavior in the two stages were corroborated in the derivative thermogravimetric (DTG)
curves, in which water release together with a prominent peak and a wider process at high
temperatures associated with the CNC decomposition were perceived. This behavior has
been previously reported for CNCs obtained from eucalyptus [57,72], but also from corn
stalk [33], kenaf [63], and coconut husk [54]. The obtained thermograms were thoroughly
assessed and the values of the mass loss (∆m) and peak temperatures (TH2O , Tp ) of both
stages, along with the onset and endset decomposition temperatures (To , Te ) and the final
residue (R) were calculated and gathered in Table 5.
In general, humidity below 5% was found regardless of the eucalyptus species, as
suggested by the horizontal step of the first mass loss from 25 ◦ C to 150 ◦ C. The main
release of humidity was found below 100 ◦ C. A minor shoulder in the DTG curve could be
intuited around 120 ◦ C, due to the release of presumable bound water [73].
The next decomposition peak is from 260 to 280 ◦ C with a shoulder from 300 to 400 ◦ C.
Although temperatures around 350 ◦ C have been reported for the thermal decomposition
of cellulose, the presence of sulphate groups in the surface of CNCs replacing the hydroxy
groups may have catalyzed the decomposition and, therefore, decreased the stability of
the CNCs [29]. Nevertheless, the low-temperature decomposition stage may be attributed
to the degradation of amorphous regions that are more sulphated and accessible, and
at higher temperatures the degradation of the non-sulphated CNCs may occur [31,74].
Similar results have been reported for CNCs isolated from corn stalk [33], kenaf [63],
potato [75], and coconut husk [54]. On the other hand, these results also show that the
typical decomposition stages of hemicelluloses, lignin, and other minor components below
220 ◦ C were absent, so it was corroborated that these low thermal stability compounds
were eliminated during the production of CNCs.
Although differences among the eucalyptus species were non-critical, some observations can be highlighted, especially correlated to the presence of sulphate groups. The
CNCs from E. benthamii and E. smithii showed a lower water retention ability (∆mH2O )
than the En × Eg and E. globulus ones. As well as this, higher peak temperatures (TH2O )
were found for the more sulphated and less crystalline CNCs [60]. Afterwards, the thermal
stability, given by To , Tp , and Te , generally followed the sequence E. benthamii, E. smithii,
En × Eg, and E. globulus, according to the mentioned sulphate index. Furthermore, the
shoulder above 300 ◦ C was less relevant as the sulphate index increased, correlated to the
prevalence of a less crystalline structure with lower crystallite size [76].
Polymers 2022, 14, 423
14 of 24
Figure 5. Stacked mass variation thermograms (up) and first derivative traces (DTG) (down) of the
−
CNCs obtained from the different eucalyptus species analyzed at 10 ◦ C·min−1 .
Table 5. Peak temperatures and mass loss for the release of water (Tfree , ∆mH2O ), thermal decomposition of CNC (To , Tp , Te , ∆mcel ), and final residue (R) obtained from the thermogravimetric analyses at
different heating rates (β).
H2 O Release
β
(◦ C·min−1 )
Tfree
(◦ C)
∆mH2O (%)
CNC Thermal Decomposition
To
(◦ C)
Tp (◦ C)
Te (◦ C)
∆mcel (%)
R (%)
5
E. benthamii
E. globulus
E. smithii
En × Eg
69.5 ± 1.6
69.8 ± 1.6
69.1 ± 1.7
70.6 ± 1.7
2.5 ± 0.4
4.4 ± 0.7
3.4 ± 0.5
4.1 ± 0.3
260.0 ± 0.5
253.6 ± 2.6
255.0 ± 0.4
253.8 ± 0.9
270.0 ± 1.4
264.5 ± 1.7
269.4 ± 1.3
265.5 ± 2.7
295.1 ± 2.9
287.1 ± 0.8
292.9 ± 2.4
284.9 ± 0.9
69.3 ± 0.1
74.3 ± 0.2
69.4 ± 2.4
71.5 ± 0.9
28.3 ± 0.8
22.3 ± 0.4
26.5 ± 0.9
24.1 ± 1.1
10
E. benthamii
E. globulus
E. smithii
En × Eg
61.8 ± 0.5
87.9 ± 0.2
61.2 ± 1.6
61.1 ± 1.7
2.2 ± 1.0
4.8 ± 1.0
2.5 ± 0.4
4.4 ± 0.9
261.2 ± 0.2
260.1 ± 1.9
260.5 ± 1.5
261.4 ± 0.4
274.2 ± 0.6
271.4 ± 1.4
273.4 ± 1.3
271.8 ± 0.2
301.2 ± 2.6
295.0 ± 2.2
297.9 ± 3.1
293.5 ± 1.1
75.3 ± 0.1
78.4 ± 1.7
72.5 ± 0.7
73.4 ± 1.4
22.5 ± 0.9
16.8 ± 1.2
23.1 ± 0.7
24.1 ± 0.4
20
E. benthamii
E. globulus
E. smithii
En × Eg
80.9 ± 0.8
83.7 ± 1.9
79.0 ± 1.5
89.6 ± 0.4
3.6 ± 0.6
4.8 ± 0.9
4.1 ± 1.6
4.5 ± 0.1
265.5 ± 0.2
273.2 ± 0.7
265.3 ± 2.5
267.1 ± 1.8
278.7 ± 0.9
277.9 ± 0.1
279.8 ± 0.2
277.2 ± 2.8
309.6 ± 1.5
304.3 ± 2.1
308.1 ± 2.9
298.5 ± 5.6
78.2 ± 0.5
74.8 ± 0.2
78.7 ± 2.3
80.0 ± 0.4
18.2 ± 0.8
19.4 ± 0.6
14.8 ± 1.2
15.9 ± 0.9
30
E. benthamii
E. globulus
E. smithii
En × Eg
83.7 ± 1.9
87.9 ± 0.1
81.5 ± 3.0
89.7 ± 1.7
3.4 ± 0.2
4.9 ± 0.8
3.3 ± 0.9
3.8 ± 0.2
267.3 ± 1.0
272.9 ± 1.7
267.7 ± 1.0
269.5 ± 1.9
282.3 ± 1.3
282.8 ± 1.5
283.9 ± 0.1
281.4 ± 2.5
316.9 ± 0.9
310.3 ± 3.8
310.4 ± 0.3
304.2 ± 3.7
76.8 ± 0.1
74.2 ± 0.9
81.4 ± 0.7
76.9 ± 0.1
18.8 ± 0.7
20.9 ± 0.4
14.8 ± 0.9
19.8 ± 1.1
Polymers 2022, 14, 423
15 of 24
The most sulphated CNCs showed lower thermal stability due to the catalytic contribution of the sulfuric acid on the thermal decomposition process [70,71]. The presence of
the sulphate groups enabled the formation of carbon residues at the end of the pyrolytic
decomposition [31]. However, these residues, (R), could not be correlated. During heating,
sulfuric acid molecules are released and facilitate the removal of the hydroxyl groups in the
form of water or reacting with them through esterification, which causes the subsequent
removal of sulfuric acid [77–79]. At this point, the thermal stability was evaluated in
detail in the next section through kinetic analysis to understand the reaction mechanisms
involved in the degradation process.
3.3. Kinetic Analysis and Apparent Activation Energy during Pyrolysis
The thermal decomposition of the CNCs was assessed from a kinetic perspective,
including measurements at different heating rates of 5, 10, 20, and 30 ◦ C·min−1 . From
these results, the zero-decomposition temperatures (ZDT) were gained, assessing the
thermal decomposition behavior (TDB) when the heating rate tends to zero (β→0). This
allows avoiding the influence of the dynamic heating program during thermogravimetric
analyses [41,80,81]. For this purpose, results were fitted to the exponential relationship
shown in Equation (9), where a, b and k are parameters of the fitting.
TDB( β) =
a
1 + b· e−k· β
(9)
Figure 6 shows the evolution of the onset (To ), peak (Tp ) and endset (Te ) temperatures
as a function of the heating rate (β). Table 6 gathers the obtained values along with the fitting
parameters and regression coefficients for the CNCs of all eucalyptus species. In general,
the fitting lines showed a high degree of correlation (R2 ). Although all the ZDT values may
give appreciated information, the technological utility of the beginning and culmination
of the decomposition given by the onset and endset decomposition temperatures (To , Te )
is of great importance. When β→0, the value for the To was in the range between 244
and 253 ◦ C and decomposition finished at Te in the span from 276 to 289 ◦ C. As observed
before, the E. benthamii and E. smithii revealed higher thermal stability than the En × Eg
and E. globulus, correlated to the presence of sulphate groups and the crystalline structure
found before.
Afterwards, the apparent activation energy (Ea) was approximated for the thermal
decomposition of the CNCs, which brings valuable information to better understand its
thermal behavior. For this purpose, one of the simplest approximations for calculating Ea
was firstly carried out, as proposed by the Kissinger method [44]. Figure 7 shows the plot
of ln β/Tp 2 vs. 1000/Tp , along with the linear fitting for the different heating rates (β). The
representation of straight lines allowed for a linear fitting, from which the apparent (Ea)
was calculated and gathered in Table 7.
The values for the Ea were 362, 237, 295, and 275 kJ·mol−1 for the E. benthamii,
E. globulus, E. smithii, and En × Eg, respectively, being correlated to the abovementioned
thermal stability and sulphate index. The apparent activation energy was lower as the
number of sulphate groups increased, corroborating that sulfuric acid catalyzed the thermal
decomposition of CNCs. Moreover, these acid molecules can also promote dehydration reactions, causing the release of more molecules of water, which also contribute to catalyzing
cellulose decomposition processes. Consequently, the elimination of such hydroxyl groups
may reduce the presence of inter- and intramolecular hydrogen bonds, leading to a less
cohesive structure and therefore to lower apparent activation energy [82].
Subsequently, a deeper evaluation of the apparent Ea was carried out, considering the
isoconversional methods proposed by Flynn–Wall–Ozawa (FWO) [45,46] and Kissinger–
Akahira–Sunose (KAS) [47]. These methods allow for a more precise evaluation of the
apparent activation energy as a function of conversion degree (α) and represent a more
realistic way for understanding the variation of the Ea along the complete decomposition
process. The obtained isoconversion plots for the CNCs obtained from the different euca-
β→
Polymers 2022, 14, 423
16 of 24
lyptus species are shown in Figure 8. The FWO method involved the representation of log
β vs. 1000/T while the KAS approximation plots the ln β/T2 vs. 1000/T.
Figure 6. Evolution of the onset (To ), peak (Tp ) and endset (Te ) temperatures as a function of the
heating rate (β)
β of the CNCs obtained from the different eucalyptus species. Standard deviations
below 2 K were omitted for the sake of clarity.
Table 6. Values of ZDT along with the fitting parameters and regression coefficient for the thermal
∆ , ∆m ) when
β→ β→0. Standard error was omitted for the sake
decomposition of CNC (To , Tp , T
e
cel
of clarity.
ZDT
a
(◦ C)
(◦ C)
b
k
R2
E. benthamii
To
Tp
Te
253.0
265.5
289.0
271.5
286.9
338.7
0.04
0.04
0.09
0.06
0.05
0.03
0.998
0.991
0.998
E. globulus
To
Tp
Te
246.7
256.5
277.3
280.8
286.8
316.9
0.07
0.06
0.07
0.05
0.07
0.06
0.990
0.987
0.998
E. smithii
To
Tp
Te
246.8
270.1
283.3
268.8
292.4
314.8
0.04
0.02
0.06
0.10
0.07
0.07
0.996
0.972
0.957
En × Eg
To
Tp
Te
243.9
258.1
275.6
270.4
284.3
306.4
0.05
0.05
0.06
0.11
0.07
0.08
0.998
0.981
0.928
β
Polymers 2022, 14, 423
β
17 of 24
Figure 7. Application of the Kissinger method for the thermal decomposition of CNCs obtained from
the different eucalyptus species.
−
Table 7. Apparent activation energy (Ea) for the thermal decomposition of CNCs obtained from the
different eucalyptus species.
Kissinger
Ea
(kJ·mol−1 )
E. benthamii
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
E. globulus
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
E. smithii
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
En × Eg
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
361.5
237.6
FWO
R2
274.9
(kJ·mol−1 )
KAS
R2
Ea
(kJ·mol−1 )
287.1
306.0
284.0
212.0
131.5
89.1
78.0
87.8
R2
1.00
281.3
299.4
278.6
210.2
133.9
93.8
83.6
93.3
0.98
0.97
0.97
0.93
0.89
0.92
0.95
0.97
0.95
233.1
243.6
248.7
187.8
112.0
81.2
81.1
88.1
1.00
0.99
0.97
0.97
0.99
0.98 β
0.98
0.98
α
236.3
247.3
252.7
188.4
108.5
75.7
75.3
236.3
1.00
254.9
257.6
237.8
188.0
157.2
102.9
81.4
74,0
0.99
1.00
0.97
0.98
0.99
0.99
0.99
0.99
259.2
261.9
241.0
188.5
156.0
98.5
75.6
67.5
0.99
1.00
0.97
0.97
0.99
0.99
0.99
0.99
1.00
216.6
219.3
209.8
182.0
122.0
93.9
94.0
92.4
0.98
0.97
0.99
1.00
1.00
0.99
1.00
0.99
219.0
221.8
211.7
182.4
119.0
89.1
88.9
86.7
0.98
0.97
0.99
1.00
0.99
0.99
1.00
0.99
β
295.2
Ea
0.98
0.96
0.97
0.93
0.88
0.91
0.94
0.97
1.00
0.99
0.97
0.97
0.99
0.97
0.97
0.97
Polymers 2022, 14, 423
18 of 24
Figure 8. Application of Flynn–Wall–Ozawa (FWO) (left) and Kissinger–Akahira–Sunose (KAS)
(right) methods for the CNCs obtained from the different eucalyptus species.
As gathered in Table 7, a high coefficient of determination (R2 ) was generally obtained
during the whole conversion for both FWO and KAS methods, implying a strong correlation
and validating the applied methodology for the evaluation of the apparent activation energy.
Given the similar results from the FWO and KAS approximations with analogous values
of activation energies with the same trend, the evolution of the Ea during the advance
of the decomposition process was calculated and plotted in Figure 9 as an average from
both methods.
Polymers 2022, 14, 423
19 of 24
Figure 9. Evolution of the apparent activation energy (Ea) as a function of conversion calculated as
an average for the FWO and KAS methods.
As expected, the apparent activation energy smoothly varied during the thermal
decomposition of CNCs, suggesting the occurrence of simultaneous degradation reactions
with dissimilar natures [83]. In general, values were in the same order of magnitude as
those reported in the literature for CNCs isolated from Kraft pulp, cellophane, mango
seeds, or corn stalk [32,33]. However, differences can be established for CNCs with crystal
structures I and II. Whereas CNCs with crystal structure II usually show an increasing trend
for the activation energy as a function of the conversion degree, the CNCs with crystal
structure I have been reported with a first increase in the activation energy for conversions
around 0.3 followed by a decrease in conversions close to 0.6 with an increase again for
higher conversions [31]. Such is the case of the CNCs assessed in this study, which all have
a cellulose crystal structure I. Analogous behavior was observed with crystal structure I
obtained from bleached Kraft cellulose through acid hydrolysis with sulfuric acid [31,32].
For a detailed discussion of the obtained results, the decomposition reactions at
specific conversion degrees must be considered. For the low conversion range, involving
the beginning of degradation up to α = 0.3, the volatilization of the remnant amorphous
regions mainly occurs. During this stage, the sulphate groups play a crucial role, as they
α
may have been attached prominently to amorphous domains, in which they contribute to
reducing the thermal stability and activation energy. If the different eucalyptus species are
compared, the E. benthamii showed the highest activation energy, which is again associated
with the lowest sulphate index, and higher crystallinity degree and crystallite size of the
CNCs [31,32]. Then, the E. smithii, E. globulus, and En × Eg showed lower values for the
activation energy at such conversion degree. Afterwards, for conversions above α = 0.4 and
up to α = 0.6 the apparent activation energy progressively decreased with the conversion
α it
degree for all the species, where the decomposition of the cellulose units occurs. Finally,
α
reaches a plateau up to α = 0.8, where the residue of the thermal decomposition is generated
with non-relevant differences among the dissimilar CNCs.
α considering the preparation of polymer bionanocomFrom a technological perspective
posites, the apparent activation energy for the thermal decomposition of CNCs is usually
higher than that of the most extended polymer matrices. This behavior may suggest that the
combination of these CNCs with polymer matrices will result in higher activation energy
for the overall system. However, the low percentage generally applied for reinforcing,
slightly affects the overall decomposition process of the material, as demonstrated by
Motta et al. with poly(styrene) (PS)/nanocellulose [84] or by Dhar et al. with poly(lactide)
(PLA)/nanocellulose [85], both with filling percentages up to 1% wt, or by Coelho de Car-
Polymers 2022, 14, 423
20 of 24
valho et al. with poly(hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)/nanocellulose with
filling percentages up to 3% wt [86]. For higher percentages, agglomeration usually occurs,
and thermal stability is generally prevented due to other phenomena such as nucleation,
porosity, or interphase issues, as demonstrated for some biopolymers, including PLA [87]
or PHBV [86].
4. Conclusions
CNCs from eucalyptus Kraft pulps were prepared by sulfuric acid hydrolysis and
solvent-casted films were prepared. Their evaluation exhibited some differences in terms of
physico-chemical properties, especially given the dissimilar progression of the hydrolytic
reaction and the incorporation of sulphate groups.
The CNCs obtained from the E. benthamii species were those less sulphated, with
higher average diameter, less water incorporation ability with weaker interactions and
higher thermal stability. Conversely, the E. globulus was the most sulphated of the analyzed
species, giving, as a result, small hints of a slightly less crystalline morphology with a
little more capability to incorporate water into their structure and also with lower thermal
stability. The E. smithii and En × Eg revealed an intermediate behavior according to their
sulphate indexes.
In the beginning of degradation up to α = 0.3 the volatilization of the remnant amorphous regions occurs and the sulphate groups contribute to reducing the thermal stability
and activation energy. For conversions above α = 0.4 and up to α = 0.6, the apparent
activation energy progressively decreased with the conversion degree for all the species.
Only for conversions above α = 0.8 was a plateau observed and non-relevant differences
among the dissimilar CNCs were found.
The evaluation of the apparent activation energy for the thermal decomposition
under pyrolysis conditions served to corroborate the abovementioned performance, highlighting the reactivity or stability of CNCs from the different eucalyptus species. Particularly, it was pointed out the crucial role of the sulphate groups, that catalyzed the
thermal decomposition.
Author Contributions: Conceptualization, P.R.-C., R.T.M., A.R.-G. and A.S.; methodology, O.G.-C.,
P.R.-C. and I.C.-V.; software, O.G.-C. and P.R.-C.; validation, O.G.-C., P.R.-C. and J.D.B.; formal
analysis, O.G.-C. and P.R.-C.; investigation, O.G.-C., P.R.-C., I.C.-V. and P.A.B.; resources, P.R.-C.,
R.T.M., A.R.-G. and A.S.; data curation, O.G.-C., P.R.-C., J.D.B., A.R.-G. and A.S.; writing—original
draft preparation, O.G.-C., P.R.-C., J.D.B., A.R.-G. and A.S.; writing—review and editing, O.G.-C.,
P.R.-C., R.T.M., J.D.B., A.R.-G. and A.S.; visualization, O.G.-C. and P.R.-C.; supervision, R.T.M.,
A.R.-G., J.D.B. and A.S.; project administration, P.R.-C. and A.R.-G.; funding acquisition, A.R.-G. and
P.R.-C. All authors have read and agreed to the published version of the manuscript.
Funding: Financial support was provided by the Innovation Found for Competitiveness of the
Chilean Economic Development Agency (CORFO) under Grant no. 13CEI2-21839. Generalitat
Valenciana is thanked for the post-doctoral contract for O. Gil-Castell (APOSTD/2020/155). I.
Carrillo-Varela thanks ANID/Fondecyt-Postdoctoral grant (No. 3200114). R. Teixeira Mendonça
thanks ANID/Fondecyt grant (No. 1200504).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data is contained within the article.
Conflicts of Interest: The authors declare no conflict of interest.
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