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Pineapple Leaf Fiber
Evidence-based research had shown that elevated alkali treatment of pineapple leaf fiber
(PALF) compromised the mechanical properties of the fiber. In this work, PALF was subjected to
differential alkali concentrations: 1, 3, 6, and 9% wt/wt to study the influence on the mechanical
and crystal properties of the fiber. The crystalline and mechanical properties of untreated and alkali-treated
PALF samples were investigated by X-ray diffractometry (XRD), Fourier transforms infrared
spectroscopy (FTIR), and tensile testing analysis. The XRD results indicated that crystal properties
of the fibers were modified with 6% wt/wt alkali-treated PALF recording the highest crystallinity
and crystallite size of 76% and 24 nm, respectively. The FTIR spectra suggested that all alkali-treated
PALF samples underwent lignin and hemicellulose removal to varying degrees. An increase in the
crystalline properties improved the mechanical properties of the PALF treated with alkali at 6%
wt/wt, which has the highest tensile strength (1620 MPa). Although the elevated alkali treatment
resulted in decreased mechanical properties of PALF, crystallinity generally increased. The findings
revealed that the mechanical properties of PALF not only improve with increasing crystallinity and
crystallite size but are also dependent on the intermediate bond between adjacent cellulose chains.
Sustainable green technology developments have been elevated in recent times, especially,
in the polymer composite industry . Thus, the focus currently is on the identification
and characterization of plant fibers for a variety of applications, including fabrication
of scaffolds for tissue regeneration, drug delivery, and prosthetic design [2–4]. Plant fibers
have been shown to be good potential alternatives to synthetic fibers from petroleum-based
non-renewable sources . The use of plant fibers is attractive since they are available in
abundance and their properties which include biodegradability and low density facilitate in
achieving high specific strength composite designs . Among the numerous plant fibers,
pineapple (Ananas comosus) leaf fibers (PALF) which are considered agricultural waste,
have shown promising properties for polymer reinforcement based on the literature [7,8].
PALF consists of about 80% cellulose, 6–12% hemicellulose, and 5–12% lignin [9,10].
Cellulose, the largest constituent of PALF, exists in two distinct forms: The crystalline and
amorphous phases. In the crystalline phase, there are bundles of microfibrils, made of
assemblies of (1–4) b-D-glucan chains that are strongly hydrogen-bonded . The amorphous
phase consists of randomly arranged cellulose and hemicellulose that contribute
insignificantly to the structural and mechanical stiffness of the fiber. Therefore, several
works have shown the removal of the amorphous phase through hydrolysis leading to a higher degree of ordered crystalline regions with the ultimate goal of enhancing the
stiffness of the fiber [6,12].
Research has shown that the chemical treatment of natural fibers can not only enhance
their surface morphology but improve the mechanical properties of the fiber . The
treatment of natural fibers is related to their hydrophilic nature, which discourages good
fiber-matrix bonding with the majority of hydrophobic matrixes . It is for this reason
that researchers in the bio-fiber space have been exploring various treatment techniques
not only to improve the fibers matrix bonding ability for better composite reinforcement
applications , but also for other reasons including the study of the effect of alkali on
PALF reinforced composite’s mechanical, degradation, and water uptake properties 
and the production of nanocellulose from PALF .
The fiber stiffness evolvement depends on the methods and chemical concentrations
in treating the fiber . There are few literature reports of the use of 6% NaOH for fiber
treatment and the limited mechanical properties experienced by PALF when subjected to
the elevated percent NaOH treatment. However, the crystallographical information upon
the percent NaOH treatment has not been well characterized. Herein, the authors seek
to provide insightful information on the mechanical and crystallographical properties of
PALF when subjected to differential alkaline treatments. Furthermore, information on how
elevated alkali treatment of PALF limits the mechanical properties of the fiber despite the
enhancement in crystallinity and crystallite size is provided in this work.
2. Materials and Method
Sodium hydroxide (NaOH) was purchased from Sigma-Aldrich (St. Louis, MO, USA).
Pineapple leaves removed from the pineapple plant (Ananas comosus) were supplied by
Mawuli Farms, Nsawam, Ghana (5.8195 North, 0.3513 West).
2.2.1. Extraction and Alkali Treatment of PALF
Matured harvested leaves of the pineapple plant were soaked in water for 4 weeks
to soften the leaves for easy fiber isolation . This process encouraged microorganisms
including fungi and bacteria to digest the cement matrix of hemicellulose, freed the fiber
bundle, and enhanced the fiber isolation (Figure 1a). The isolated fibers were washed with
deionized water and air-dried (Figure 1b) for 15 days at 26 C . PALF samples were
treated with different concentrations by dissolving 0.15, 0.45, 0.90, and 1.35 g of NaOH
pellets in 15 g of deionized water to make: 1, 3, 6, and 9% wt/wt alkali solutions. PALF
samples were immersed in the alkali solutions for 1 h at 26 C. The fibers were picked out of
the solution and rinsed with deionized water repeatedly until pH 7 was obtained followed
by air-drying at 26 C for 24 h (Figure 1c). Three parallel experiments were conducted at
the four treatment conditions for characterization.
2.2.2. Fiber Tensile Test
Tensile tests were carried out on untreated, 1, 3, 6, and 9% wt/wt NaOH treated PALF
samples according to ASTM C1557-14  using Mark-10 ESM301 Force Test Stand in Basic
mode equipped with 1.5 kN load capacity. To avoid fiber slippage out of the holding jaws
(grip), the ends of the fiber specimens were mounted and aligned on cardboards using
cyanoacrylate (mounting tab). The tensile test was performed at a constant crosshead
displacement rate of 20 mm/min at a relative humidity of 76% and a temperature of 26 C.
Tensile tests were conducted on fibers using a gage length of 25 mm with at least three
replicates for each sample. The Force-displacement data were acquired directly using
the MESUR Lite software by Mark-10 for tensile properties determination. Generally, a
variation in fiber diameter for all the different NaOH treated PALF samples was observed.
The fiber diameter of the fractured end within the gage length was measured using a
micrometer screw gauge. The original cross-sectional area of the fractured fiber was
calculated using Equation (1):
A = πd²/4 (1)
where d is the diameter of the fiber and A is the cross-sectional area of the fiber. The
calculated original fiber cross-section of the fractured fiber surface was used to calculate
the tensile strength for each fiber tested. The average force at which each sample fractured
with the original cross-sectional area (determined using Equation (1)) of each fractured sample was used to calculate the tensile strength using Equation (2):
Tensile Strength (s) = F/A (2)
where F is the tensile force experienced by the fiber at fracture and A is the original cross-sectional area of the fiber at fracture.
2.3. Characterization of PALF
2.3.1. X-ray Diffraction (XRD) Analysis
The X-ray diffraction spectrum of each powdered sample was obtained using a Pan
Analytical diffractometer with monochromatic CuK radiation (λ= 1.54060), operated at 45 kV and 40 mA. The intensities of the scattered radiation were detected in the 2q range of
5–45°, with a scan step width of 0.05°.
The percentage crystallinity of untreated and differentially treated PALF samples was
determined using the peak height empirical method (Equation (3)) proposed by :
C = (I002 - Iam)/I002 *100% (3)
where I002 represents the intensity of the peak corresponding to the maximum intensity
of the peak; IAM gives the intensity of diffraction of the non-crystalline material (amorphous
band), which is taken as the valley between the crystalline peaks. Furthermore,
the crystallite sizes of the samples were estimated using the Debye-Scherrer relation in Equation (4):
D = Kλ/Bcosθ (4)
where D represents the crystallite size, λ represents the wavelength of the Cu-K radiation,
b represents FWHM fraction angles, K is the correction factor of 0.89, and q represents the
diffraction angle of the highest peak of the PALF samples.
2.3.2. FTIR Analysis
Untreated and differently treated PALF samples were cut into small particle sizes of
about 1 mm and analyzed using Nicolet MAGNA-IR 750 Spectrometer (Nicolet Instrument
Co., Madison, WI, USA). FTIR spectra were recorded with ATR attachment to determine
the functional groups of the PALF before and after differential treatments. The spectra
were recorded from 4000 to 500 cm-1 wavenumber with 16 scans and spectral resolution
of 4 cm-1.
2.4. Statistical Data Analysis
The data acquired was organized and analyzed using IBM SPSS Statistical Software,
version 22, and the Origin 9 Data Analysis and Graphing Software. The distribution of the
data was evaluated for normality assumption using Q-Q plots, Kolmogorov-Smirnov (KS),
and Shapiro-Wilk tests at p > 0.05 before the parametric analysis was conducted. The mean
and standard deviations were used to describe the data spread. One-way independent
analysis of variance (ANOVA) with Tukey’s HSD post hoc analysis was conducted to
measure the effect of the NaOH concentration on the tensile properties of the fibers at a
0.05 significance level.
3.1. Fiber Tensile Test
Figure 2 shows distinctive stress-strain curves obtained from a single fiber tensile
testing performed on untreated and differentially treated PALF samples. Table 1 shows the
ultimate tensile strength (UTS), Young’s modulus, and strain at break of all PALF samples.
It is observed that the strains at break of the treated PALFs are lower (<7.9%) than the
untreated PALF. In addition, the treated PALFs returned higher UTS (>630 MPa) values
as compared to the untreated PALF with treated PALF (6% wt/wt NaOH) recording the
highest UTS of 1620 MPa. The changes in the fiber stress and strain values are indications
of the increment in stiffness and the decrement in strain at break, respectively. Treated
PALFs recorded higher Young’s modulus as compared to the untreated PALF with treated
PALFs (1, 3, and 6% wt/wt NaOH) recording 24 GPa. One-way ANOVA followed by
Tukey’s HSD test showed that the NaOH treatment had a statistically significant (p < 0.05)
effect on the UTS, Young’s modulus, and strain at break of the fiber.
3.2. XRD Analysis: Crystallinity and Crystallite Size
Figure 3 shows XRD patterns of untreated and treated PALFs. The result indicates a
similar pattern of cellulose I crystal where the most intense peak was at 22 2q position
and corresponds to the (002) plane reflection. In addition, a minor peak at 15.1 2q position
which corresponds to (101) plane reflection is also identified for the untreated and treated
PALFs . However, a peak split is observed at (101) plane reflection occurring at 15.0 and
16.6 2q positions for treated PALF of NaOH 9% wt/wt . Table 2 presents the averaged
crystallinity and crystallite size of the untreated and treated PALFs. The crystallinity
of the treated PALFs (1–6% wt/wt NaOH) increased but decreased for treated PALF
(9% wt/wt NaOH). The crystallite size shows a wavy (zigzag) pattern for the treated
PALF (1–9% wt/wt NaOH). It increased from untreated PALF to treated PALF (1% wt/wt),
decreased to PALF (3% wt/wt), increased for PALF (6% wt/wt), and decreased again for
PALF (9% wt/wt).
4.1. The Influence of Crystal Properties on the Mechanical Behaviour of PALF
The key functional attribute of alkali treatment is to remove lignin and hemicellulose
from the PALF . However, the mechanical properties of natural fibers are determined
by the cellulose content which is made of an ordered crystalline phase and disordered
amorphous phase . When the fiber is loaded, the ordered crystalline phase made of
microfibrils transmits the load along the length of the fiber. Thus, the high-level ordered
crystalline phase lends the advantage of superior mechanical properties to the fiber .
The highest UTS which was recorded for the alkali-treated PALF and agrees with the
literature report  corroborates the XRD results. There was a common trend between
changes in the crystallite size and UTS of treated PALFs. However, no common trend
was identified between the UTS enhancement among the treated fibers and changes in
crystallinity. It was realized that an increase in crystallinity and crystallite size of treated
PALF improved the mechanical properties of the untreated PALF significantly (p < 0.05).
The result is in good agreement with the principle that higher fiber crystallinity enhances
fiber stiffness . It is noticed in literature that the UTS of untreated PALF can range from
170–1627 MPa  and with 6–7% alkali treatment, the UTS was improved by 18–53% [12,28].
In our work, the UTS of the untreated PALF improved by 157% from 630 to 1620 MPa for
the 6% alkali treatment. The variation in mechanical properties of PALF could be due to the
inherent fiber properties influenced by climatic and growing conditions [6,9,27]. Whereas,
crystallinity enhanced the strength and stiffness of the fiber, it decreased the strain at break
which in effect was a measure of the fiber ductility . Although there were reports that
suggested the strength of the fiber correlates with increasing crystallinity to a limit [12,30],
there is no evidential explanation of how elevated alkali treatment affects crystal properties
leading to the drastic decrease in fiber strength. Therefore, in this study, it was deduced
that excessive breakdown of alkali sensitive hydroxyl groups that bond adjacent cellulose
chains were hydrolyzed during the alkali treatment resulting in very weak intermediate
bond formation between cellulose chains. This resulted in the decreased fiber strength
for the treated PALF (9% wt/wt NaOH). The decrease in fiber strength at elevated alkali
treatment was also reported by other researchers where modification in the cellulose chain
arrangement and the emergence of new defects at elevated alkali concentrations were linked to the decrease in fiber strength [31,32]. Future works, therefore, would be aimed
at strengthening the intermediate bonds between cellulose chains. It is interesting to note
that no common trend was identified between the UTS and alkali treatment probably due
to inherent structural variations among the treated fibers.
4.2. Effect of NaOH Treatment on the Crystalline Properties of PALF
The peak splitting emanated from the penetration of the crystalline regions by the
high alkali treatment which scissions the long cellulose chains into shorter chains . Peak
splitting was intense for the 9% treatment and resulted in decreasing the crystallinity from
6%. The peak split observed in the treated PALF in our work is reported in literature for the
untreated PALF . The peak split that occurs with the untreated PALF could be attributed
to the source of the PALF. The properties of PALF have been reported to be influenced
by plant variety, growing, treatment, and climatic conditions [6,9,26]. The crystallinity
of the PALF was improved as the concentration of NaOH treatment increased. It was
noted that the release of acetyl groups and uronic acid substituents through hemicellulose
and lignin removal further enhanced the degradation of cellulose and resulted in a lower
crystallinity at the harsh treatment condition of PALF (9% wt/wt NaOH) . The elevated
concentration of alkali dissociated to give an excessive number of Na+ ions which aided
in breaking down the crystalline regions [35,36]. Studies on the crystallite sizes from the
XRD data suggested that increasing NaOH concentration generally increased the crystallite
size relative to that of the untreated PALF. Line broadening at full-width-half-maximum
is inversely related to the crystallite size according to Debye-Scherrer relation . The
decrease in crystallite size for treated PALF (3 and 9% wt/wt NaOH) could be attributed
to the relatively high degree of line broadening induced by the alkali. Compared with
other PALF samples, the peak at 22 was the sharpest and highest for the treated PALF
(6% wt/wt NaOH) probably due to its ability to achieve the highest removal of hemicellulose
and lignin constituents as indicated by the FTIR pattern . The XRD result indicated
a common trend between crystallinity and NaOH treatment. It was observed that the
increasing concentration of NaOH achieved an increasing effect in crystallinity of PALF
with the highest results realized for PALF (6% wt/wt NaOH). However, no trend was
identified between the concentration of NaOH treatment and crystallite size. Structurally,
lignin as a phenolic unit serves as a protective barrier encasing cellulose and hemicellulose.
It is attached to the hemicellulose unit through an ester bond which is alkali sensitive. In
the presence of a high concentration of NaOH, the ester bond becomes hydrolyzed, thus,
exposing the hemicellulose . The decrease in crystallinity and crystallite size observed
for PALF (9% wt/wt NaOH) relative to PALF (6% wt/wt NaOH) might have occurred
probably since at alkali concentrations above 6% wt/wt NaOH, regions of highly ordered
cellulose chains of microfibrils (major reinforcing element of the fiber) became disordered
due to the breakdown of alkali sensitive hydroxyl groups linking the cellulose chains in the
fiber leading to decreased crystallinity. For the observed fluctuation in crystallite size, we
suspect the presence of the varying number of Na+ ions in the solution may have strained
the crystalline region of the fiber at varying degrees. The strain induced possibly changed
the structure of the fiber due to the presence of NaOH .
4.3. Effect of NaOH Treatment on the Chemical Composition of PALF
The FTIR spectrum fingerprint of untreated PALF obtained in this work was consistent
with the established specific functional groups present in PALF, as reported by other
researchers [39,40]. After the treatment, the unique lignin peaks disappeared followed by
the appearance of additional peaks indicating the mercerization effect of the NaOH on
the lignocellulosic fiber . Additionally, the peak at ~1730 cm?1 which is characteristic
of the carbonyl (C=O) absorption of the hemicellulose and lignin component of the fiber
diminished as the alkali concentration increased. This is evident since the hemicellulose and
lignin constituent of the fiber are alkali sensitive . Moreover, it was observed that there
was a progressive decrease of the peak at 1730 cm?1 as a function of alkali treatment up to 6%. However, upon 9% alkali treatment of the PALF, the peak at ~1730 cm?1 reappeared
suggesting that a higher concentration of the base could be due to the ionic effect or
structural variation among the fibers. The double peaks at ~1234 and ~1202 cm?1 might
have originated from the influence of the alkali on the cellulose chains .
4.4. Potential Polymer Reinforcement Using PALF
The mechanical properties of PALF characterized in this work showed promising
results which makes it a potential reinforcement material for matrixes such as polyester,
polypropylene, and epoxy. The challenge often faced with natural fibers is the poor
fiber-matrix interaction between hydrophilic fiber and hydrophobic matrix. However, the
research attempts made towards fiber surface modifications, interfacial incompatibility
enhancement, and tensile properties enhancement by chemical treatments such as demonstrated
in this work, have shown exciting results [7,8]. The prosthetic field among the other
fields has seen a limited application of PALF as a reinforcement material for prosthetic
socket design. Especially in resource-limited regions of the world where accessibility
and cost of materials limit amputees who are financially constrained but need prosthetic
devices, the use of plant fibers such as characterized in this work can serve as an alternative
to synthetic fibers. Our future work is targeted at investigating how PALF can be used to
reinforce the type of prosthetic resin used for prosthetic socket design.
Additionally, the yield in PALF ranges from 1.6–2.5% per kilogram of pineapple leaf.
Considering that PALF is rich in cellulose makes it a good plant material for other industries
such as the paper industry. In this industry, plant sources with high cellulose content and
low lignin are most preferred due to the low specific energy consumption required for the
pulp refining process .
In this work, PALF was successfully extracted from pineapple leaves and subjected
to different concentrations of alkali treatment. The removal of lignin and hemicellulose
(amorphous phase) and the improvement in crystalline properties of PALF were indicated
and confirmed by FTIR and XRD, respectively. These spectroscopic results were further
corroborated by the high UTS recorded for the alkali-treated PALFs which ranged from
1090–1620 MPa. Although the high concentrations of NaOH improved the crystallinity
and size of the PALF crystallite, it enhanced the mechanical strength and stiffness of the
fiber just to a limit after which the fiber strength and stiffness drastically decreased. It was
revealed that fiber strength and stiffness are not only enhanced by increased crystallinity
and crystallite size, but also by the intermediate bond between adjacent crystalline chains.
This work helps understand the processing conditions that could potentially allow the use
of this material in a variety of matrixes, for applications ranging from aerospace, paper
industry, and biomedical applications.
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