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    Topic review

    Pineapple Leaf Fiber

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    Submitted by: Eric Gaba


    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.

    1. Introduction
    Sustainable green technology developments have been elevated in recent times, especially,
    in the polymer composite industry [1]. 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 [5]. 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 [6]. 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 [11]. 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 [12]. The
    treatment of natural fibers is related to their hydrophilic nature, which discourages good
    fiber-matrix bonding with the majority of hydrophobic matrixes [13]. 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 [14], but also for other reasons including the study of the effect of alkali on
    PALF reinforced composite’s mechanical, degradation, and water uptake properties [15]
    and the production of nanocellulose from PALF [16].
    The fiber stiffness evolvement depends on the methods and chemical concentrations
    in treating the fiber [12]. 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
    2.1. Materials
    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. Methods
    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 [6]. 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 [17]. 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 [18] 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 [19]:
    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 [20]. 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 [20]. 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).

    4.0 Discussion
    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 [23]. 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 [24]. 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 [25].
    The highest UTS which was recorded for the alkali-treated PALF and agrees with the
    literature report [26] 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 [27]. It is noticed in literature that the UTS of untreated PALF can range from
    170–1627 MPa [9] 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 [29]. 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 [33]. 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 [28]. 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) [34]. 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 [37]. 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 [20]. 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 [38]. 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 [34].
    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 [17]. 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 [41]. 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 [34].
    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 [42].

    5. Conclusions
    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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