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Industrial Crops and Products 43 (2013) 529–537
Contents lists available at SciVerse ScienceDirect
Industrial Crops and Products
journal homepage: www.elsevier.com/locate/indcrop
Characterization and comparative evaluation of thermal, structural, chemical,
mechanical and morphological properties of six pineapple leaf fiber varieties
for use in composites
Alfredo R. Sena Neto a,∗ , Marco A.M. Araujo b , Fernanda V.D. Souza c , Luiz H.C. Mattoso b ,
Jose M. Marconcini b
a
PPGCEM/UFSCar, Rodovia Washington Luiz, km 235 – P.O. Box 676, Zip Code 13565-905, Sao Carlos, SP, Brazil
National Nanothecnology Laboratory for Agrobusiness (LNNA), Embrapa Instrumentation (CNPDIA), Rua XV de Novembro, 1452, Centro – Zip Code 13560-970, Sao Carlos, SP, Brazil
c
Embrapa Cassava and Tropical Fruits (CNPMF), Rua Embrapa, s/n, Embrapa – Zip Code 44380-000, Cruz das Almas, BA, Brazil
b
a r t i c l e
i n f o
Article history:
Received 18 May 2012
Received in revised form 27 July 2012
Accepted 4 August 2012
Keywords:
Lignocellulosic fibers
Renewable resources
Mechanical properties
Thermal analysis
Structure–property relations
Pineapple
a b s t r a c t
Natural fibers are candidates to replace conventional mechanical reinforcements in composites. Six cultivars of fibers of different pineapples varieties were characterized by tensile tests, thermogravimetry,
X-ray diffraction, scanning electron microscopy and infrared spectroscopy. The elastic modulus and tensile strength values were in the range of 15–53 GPa and from 210 to 695 MPa, respectively. The final
volatile loss temperatures for the six varieties were in the range between 175 and 195 ◦ C and the onset
temperatures in the range of 240–260 ◦ C. The high degree of cellulose crystallinity index influenced the
mechanical properties, hence suitable for composite reinforcement. This study aims to add information
value to the literature regarding pineapple leaf fibers and its characteristics for technical and engineering
applications. It was demonstrated that within the pineapple family, there are intrinsic variabilities for
natural materials, indicating different potential uses for each variety.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
The global population growth, corroborated with the average individual consumption increase has led to very high global
consumption levels of raw materials and finished products. Moreover, there is an increasing search for “green” or “environmentally
friendly” materials and methods.
Such trend and awareness to environmentally friendly behavior
and increased demand have progressively driven the primary sector to seek substitutes for materials with polluting attributes and/or
from non-renewable sources throughout the production process
(Leao et al., 2009).
Lignocellulosic fibers have emerged as a reinforcement alternative in polymer matrices to obtain composites for a wide variety of
applications (Sanadi et al., 1995; Leao et al., 2009; de Paoli et al.,
2009). There are some advantages over synthetic fibers, namely:
low densities and abrasiveness (relatively), high possible filling
levels resulting in high stiffness and high specific properties, recyclability, high bending resistance, biodegradability, wide variety of
fibers available worldwide, rural financial income generation and
∗ Corresponding author. Tel.: +55 16 2107 2884; fax: +55 16 2107 2902.
E-mail address: [email protected] (A.R. Sena Neto).
0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.indcrop.2012.08.001
low cost (Sanadi et al., 1994; Sanadi, 2004; Santos, 2006; Shanks
et al., 2006; Chattopadhyay et al., 2009). The main limitations are
high moisture absorption with decrease of mechanical properties,
occasional poor compatibility with hydrophobic resins, low processing maximum working temperature and seasonality (Santos,
2006).
Vegetable fibers have considerably complex structures, defined
by a wide variety of organic compounds such as lignin, hemicellulose, waxes, fatty acids, fats, pectins, among others (Rowell et al.,
2000; Martins et al., 2004). Lignocellulosic fibers are, in fact, bundles of smaller units: the fiber cells or ultimate fibers, which are
held together by binder agents (predominantly lignin and hemicellulose), also found on the outside of the fiber bundles and leaves.
The fiber cells are structured in different layers, formed essentially by groups of nano-scale cellulose chains (fibrils) extending
helically along the axis of the fiber cells and interconnected by
amorphous regions composed of lignin and hemicellulose. The
helix angle between the fibrils in the secondary wall (S2) and the
axis of the fiber cell is known as microfibrillar angle (Rowell et al.,
1997; Martin, 2001; Silva et al., 2009).
The adequate mechanical properties of lignocellulosic fibers
applied as reinforcement in polymers are largely attributed to their
cellulosic fraction, given that this is responsible for the fiber’s crystalline organization. The pineapple fibers studied in this work have
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potential for the aforementioned applications due to their high
crystallinity (Reddy and Yang, 2005; Tomczak et al., 2007; Tomczak,
2010).
There are several factors that influence the properties of fibers
such as number of fiber cells, cell wall thickness, microfibrillar angle, cellulose content, molecular structure (Mukherjee and
Satyanarayana, 1986), cellulose crystallinity index (amount, orientation and the degree of polymerization) (Sanadi, 2004).
Cellulose is the main structural component of the lignocellulosic fibers, as it provides strength and stability to the cell walls
and the fiber as a whole (Paster et al., 2003). Therefore, the cellulose content in a fiber or bundle of fibers influences its properties
and consequently its applications. As an example of this cellulose
role, it is known that pineapple and banana fibers have a higher
cellulose content, which is probably related to the relatively higher
weight of the fruit they support and the fact that they are less perishable. Other fiber sources such as corn stover, bagasse, wheat, rice
and barley straw, and sorghum stalks all contain nearly the same
amount of cellulose. Fibers in these crops support relatively smaller
weights in comparison with bananas and pineapples (Reddy and
Yang, 2005).
Fibers with a higher cellulose fraction are more suitable for
fibrous applications, while the ones containing more hemicellulose
are preferable for producing ethanol and other fermentation products because hemicellulose is relatively easily hydrolysable into
fermentable sugars (Reddy and Yang, 2005). Mechanically, hemicellulose contributes little to the stiffness and strength of fibers or
individual cells (Thompson, 1983). Lignin is a highly crosslinked
molecular complex with amorphous structure and acts as binder
agent between individual fiber cells and between the fibrils forming the cell wall (Mohanty et al., 2000). Along with chemical
composition, as commented earlier, structural organization of the
fiber influences properties. Mechanically, the main structural component, called “ultimate” or individual fiber cells, are longer in
pineapple fiber than in most of other fibers, therefore these sources
can produce long fibers. That said, it is important to notice that a
lignocellulosic reinforcement’s suitability for a given application or
product is a result from different factors, such as chemical, structural and morphological aspects, as well as end use, and not the
cellulose content alone.
With the interdependence of the many given features of the
fiber system, it can be said that biofibers do not always show the
general relationship between crystallinity and strength observed
in pure cellulose fibers such as cotton and rayon, that is, the higher
the crystallinity, the higher the strength, that being the reason why
this relationship is relatively complex. The presence of substantial
amounts of noncellulosics, mainly lignin, which contributes to the
strength of fibers and the variations in the dimensions of unit cells,
is the major reason for the absence of a good relationship between
crystallinity and strength. However, biofibers with longer unit cells
as pineapple’s have higher strength (Reddy and Yang, 2005).
Despite the impossibility to establish a direct relation
between cellulose content and individual structural aspects, generally, higher cellulose percentages, lower microfibrillar angles
(Sukumaran et al., 2001), higher cellulose degrees of crystallinity
and aspect ratios lead to better mechanical properties and lower
extensibility. Thus, elongation of the fibers depends mainly on the
orientation and degree of crystallinity of the cellulose and the angle
of the microfibrils to the fiber axis.
Multiple types of lignocellulosic fibers are available according to
their species, places of origin, seasonality and mode of obtainment.
The following examples of lignocellulosic fibers are successfully
used in the reinforcement of composites: jute, flax, coconut, sisal,
cotton, bananas, curaua, acai, hemp, pineapple, soy, ramie sugar
cane, piassava, among others. The world annual production of natural fibers in 2010 was of 28.4 million tons, of which 7.5 million
tons were produced in India (FAOSTAT, 2010). In 2007, 12% of the
research groups in the Metallurgical and Materials Engineering
area were involved in research related to composites with lignocellulosic fibers (Satyanarayana et al., 2007). Notwithstanding this
substantial growth in research on lignocellulosic fibers, there is still
a considerable portion of untapped potential, such as the fibers
obtained from pineapple leaves (Souza et al., 2007; Amaral et al.,
2011).
There has been relatively little research carried out on fibers of
the pineapple plant leaf, and its introduction in industrial uses is
recent compared to other lignocellulosic fibers such as jute, flax and
sisal. For this reason, the pineapple plant is not profitably planted,
and most of the plant material, except for its fruit, is discarded due
to the lack of knowledge of its economic potential. Brazil is the third
largest producer with around 7% of the world production. Brazil’s
Northeast region has the highest production in the country. According to estimates made in 2004, the country’s production was about
1.4 million tons of fiber, with exports of only 1% of this total. Studies
indicated a yield of 1.22 tons per hectare, with the production of 40
leaves per pineapple, a mass of 0.065 kg per leaf and fiber income
of 2%. The values could lead to a production price per hectare of
US$ 434, considering the fiber price of US$ 0.36/kg (Satyanarayana
et al., 2007).
In the work reported here, five accessions and an hybrid of APGB
(Active Pineapple Germplasm Bank, located in Cruz das Almas, BA,
Brazil, at Embrapa Cassava and Tropical Fruits), with more than 600
cultivars of the genus Ananas and, Bromeliaceae families (Cabral
et al., 2004; Costa et al., 2011), were studied. They were characterized by morphology, cellulose crystallinity index and functional
groups; evaluating the thermal and mechanical properties for use
as reinforcement in composites with polymer matrices, emphasizing the different qualities and limitations of each variety.
2. Materials and methods
2.1. Materials
Lignocellulosic fibers were taken from six botanical varieties of
the genus Ananas (Bromeliaceae family) from APGB; and are listed
as follows:
-
A: Bromelia sp.
B: Ananas comosus var. comosus
C: Bilbergis sp.
D: Ananas comosus var. bracteatus
E: Ananas comosus var. erectifolius
F: Ananas macrodontes x Primavera
Initially, the leaves were immersed in water to be posteriorly calendered, so that the fibers could be separated from leaf
cover without being deformed or broken. The fibers were manually
extracted from hydrated and calendered leaves of each cultivar by
hand with the help of tweezers.
From this point, all six fiber varieties are cited as PALF (pineapple leaf fiber), or with its corresponding designated letter when
referring to a particular fiber.
2.2. Morphological analysis by scanning electron microscopy
(SEM)
The morphological characterization of the six PALF was performed by scanning electron microscopy (SEM). The aim was to
analyze the external morphology and structure of the fibers and
fiber cells, studying the effects of the extraction methods and
gathering useful information for the prediction of its behavior
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531
Fig. 1. Example of RX diffractogram with deconvoluted curves.
with matrixes in composites. The fibers were cut in segments of
1–2 mm length. The fiber samples were fixed with carbon conductive double-sided tape on aluminum stubs. The samples were
metalized with gold to present electrical conductive properties. The
equipment used in the analysis was a JEOL, model JSM 6510, with
a 2.5 kV accelerating voltage.
2.3. Tensile tests
The tensile properties of PALF were measured following an
adaptation of ASTM D3379 standard (ASTM, 1978) using an Emic
universal tensile testing machine, model DL 3000, with a crosshead
speed of 5 mm min−1 . The fibers were considered as bulky/solid
cylinders in the cross sectional area. The mean diameter of each
PALF was previously measured using a Mitutoyo IP65 digital
micrometer. For each sample, measurements were made at three
points in the effective length of the fiber used in the specimens
prepared for the tensile tests. The arithmetic mean of the values
measured at three points in the test area was obtained to calculate the mechanical properties of the fibers and for comparison
between the values. In the tensile tests the mean values of each
individual fiber were used, rather than the overall average value
of each cultivar. For each cultivar, a total of thirty samples were
tested.
2.4. Thermogravimetric analysis
Identifying the fiber types with higher degradation temperatures associated with improved mechanical properties is the main
goal of this study, since one of the limiting factors for using natural
fibers is their low oxidative degradation temperature (Kozlowski
and Przybylak, 2008). According to Bettini et al. (2010), polypropylene composites reinforced with coir fibers require caution when
choosing a temperature for processing and mixing of the materials, as the cellulose degrades at around 200 ◦ C. Other studies
(Hassan and Nada, 2003) assert that every polyethylene reinforced
with plant fibers is slightly less thermally stable than the neat PE.
This is a limiting factor for the processing of polymer composites with vegetable fibers, as they contain fairly large amounts of
cellulose. The melt temperature of the matrix must be lower than
the fiber degradation temperature. The thermogravimetric analysis
was conducted using a TGA Q500 thermal analyzer (TA Instruments). The analysis was performed within a temperature range
of 30–600 ◦ C, under synthetic air atmosphere (20%O2 and 80%N2 ),
using a heating rate of 10 ◦ C min−1 .
2.5. Fourier-Transform infrared spectroscopy (FTIR)
The transmittance spectroscopic analysis by FTIR was carried
out to detect characteristic chemical functional groups in the fibers.
A small mass of each PALF was milled with KBr, with 1 wt/wt%
of fibers and compressed into small discs. A spectrophotometer Spectrum 1000 (Perkin Elmer) was used. The spectra were
obtained using 32 scans per sample, with a 2 cm−1 resolution in
the 400–4000 cm−1 range.
2.6. X-ray diffraction (XRD)
As aforementioned, the cellulose contained in lignocellulosic
fibers has a portion of crystalline oriented zones in a nano or
micro-scale, which gives each fiber a certain degree of crystallinity.
This arrangement can be considered as an ordered phase permeating adjacent amorphous portions of the fiber structure. According
to some authors (Mukherjee and Satyanarayana, 1986; Sao et al.,
1994) the percentage of crystallinity is proportional to the mechanical properties of a material, including lignocellulosics. All six PALF
varieties were milled and deposited in aluminum holders. The
X-ray diffraction patterns were obtained using a Shimadzu diffractometer, model XRD 600 with Cu-K␣ radiation and 1.54 × 10−10 m
of wavelength value. The measure conditions were 30 kV, 30 mA
and scanning rate of 2◦ min−1 , from 5◦ to 40◦ . The resulting diffractograms were deconvoluted using Magic Plot Pro 1.5 software and
the cellulose crystallinity index Ic, was calculated by the following
equation:
Ic(%) =
1 − Aa At
∗ 100
(1)
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Fig. 2. Scanning electron micrographs of some analyzed PALF. (a) Fiber of cultivar B at a magnification of 500×; (b) defibrillation of cultivar E at a magnification of 2000×
and (c) defibrillation of cultivar C at a magnification of 1000×.
where Aa is the value of area under the curve corresponding to the
amorphous portion of a diffractogram and At is the total area of
the diffractogram, the sum of the areas of all its resulting deconvoluted peaks, including also the amorphous curve. An example of an
obtained diffractogram is showed in Fig. 1.
3. Results and discussion
The fiber sections showed with the SEM technique were randomly chosen among those that could be better focused and
viewed, from the basal to the apical portion of the leaf. All fibers
evaluated were manually collected and thus a large presence of
mucilage material was observed in all cultivars (pulp and outer layers of the leaves) and also cementitious material/other components
(Fig. 2). In PALF B and F, a separation formed by the parenchyma
cells was observed in the presence of external matter attributed to
the pulp of the leaf. The grooves found along the fiber showed a
lower presence of binder agents covering material on the sample
fibers and a higher relative amount of cellulose, which is the main
component of the fiber cells or ultimate cells. The large surface
area due to the fiber roughness plus the external area of the fiber
cells represents a large area available to interact with the composite
matrix, improving the reinforcement effect.
The appearance of the metalized fiber segments placed in the
sample holder showed defects and agglomerates, in addition to
diameter disparity along the fibers, which indicates a possible
diameter variation of the leaf section from which the fiber segment was removed. The presence of flaws and of disunited fiber
cells found throughout the region of the sample are the result of
handling and cutting of the fibers to prepare the sample.
PALF A showed the most cohesive and regular fiber bundles
among the six cultivars and minor defibrillation was observed,
since the fibers maintained higher integrity after defibration and
cutting.
As for the characteristics of the fiber cells of the six cultivars,
a regular and smooth surface morphology was perceived. PALF
A, B, D and E exhibited for the fiber cells a section ranging from
cylindrical to rectangular/flat. The PALF F and C showed a profile
with a rectangular/flat section and a morphology similar to two
conjoint microtubules. Since those PALFs come from natural
sources and did not undergo a more impacting pre or post processing, a fiber-to-fiber-variation is indeed expected.
The FTIR spectra of the six PALF showed similar shapes with
most of the peaks located on the same wave number range. The
shape of the spectra is similar to those seen for other types of
plant fibers and published in previous papers (Martins et al., 2004;
Correa, 2010; Tomczak, 2010). The 3450 cm−1 band observed in the
spectra corresponds to the hydroxyl groups. A peak wave number
of 2900 cm−1 was also identified, which is attributed to the asymmetric stretching of CH and CH2 (Sgriccia et al., 2008; Correa, 2010).
Both bands are characteristic of organic materials, exhibiting high
peak intensity, as seen in the spectra (Fig. 3).
The bands at 1730 and 1625 cm−1 correspond to the acetyl
groups and C O bonds, characteristic of the hemicellulose. These
bands are affected by the presence of pectin, whose peaks are
found at 1735, 1680–1600 cm−1 and also at the value of 1260 cm−1
(Garside and Wyeth, 2003; Stuart, 2004). A lignin peak is located
at the 1595 cm−1 band due to C C in-plane aromatic vibrations
(Garside and Wyeth, 2003). The cellulose is characterized by the
bands of 1170–1150 cm−1 , 1050 and 1030 cm−1 (Correa, 2010).
The resulting XR diffractograms (Fig. 4) were similar for the
six PALF. The peaks observed are attributed to the cellulose family crystal planes. The values found for the diffraction peaks at
2 and their corresponding crystallographic planes (Hermans and
Weidinger, 1948; Sao et al., 1994; Oh et al., 2005; Borysiak and
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Fig. 3. FTIR spectra of the six PALF studied, indicating the main peaks of hemicellulose, lignin and cellulose.
Fig. 4. XR diffractogram of the six PALF studied.
533
Doczekalska, 2008) were: 15◦ (0 0 1), 16◦ (1 0 1̄), 22◦ (0 0 2), 35◦
(0 4 0). These planes indicate the presence of cellulose I polymorphic form, known as native cellulose due to the fact it is the cellulose
form found in natural sources (Oı̌Sullivan, 1997).
The vegetable fibers studied had values between 49% and 64% of
cellulose crystallinity index, within those expected, based in other
works: 44–60% (Reddy and Yang, 2005), 66.3% (Tomczak et al.,
2007) and 50.2% (Tomczak, 2010).
Fig. 5 and Table 1 show the results from thermogravimetry for
a 10 ◦ C min−1 heating rate under synthetic air atmosphere. The
volatile loss temperature is seen as a point on the thermogravimetric curve (Fig. 5b) where it can be seen a transition from a
linear mass decay (loss of volatiles) to a negative curvature decay
(degradation). The loss of volatiles is associated with loss of mass
corresponding to the volatile loss temperature point. This loss, in
all cases, was higher than 7% due to the tests carried out without
previously drying the fibers, while the values found in the literature
are in the 3–5% range (Ouajai and Shanks, 2005; Silva and Aquino,
2008; Rachini et al., 2009). The onset temperature, defined by the
“shoulder” area of the TG curves, ranged between 241 and 256 ◦ C.
This temperature can be used as a basis to set maximum working temperature limits for the fibers. The values found are slightly
higher than those found in other studies (200–240 ◦ C) (Hassan and
Nada, 2003; Kozlowski and Przybylak, 2008).
The DTG curves in Fig. 5a, show the main peaks of the two degradation stages observed for all studied varieties of pineapples. The
first stage, whose peak is between 250 and 320 ◦ C, shows contributions of the degradation of hemicellulose, pectin, lignin and the
onset of cellulose degradation. According to other thermal analysis
studies on lignocellulosic materials, it was shown that the hemicellulose degradation temperature ranges from 150 to 350 ◦ C, the
cellulose undergoes degradation from 275 to 350 ◦ C, and lignin
in the range of 250–500 ◦ C (Kim and Eom, 2001; Rachini et al.,
2009).
The second degradation stage of the DTG curve varies in the
range of 375–500 ◦ C, with the first temperature values including the
decomposition mechanisms of cellulose and lignin, which degrades
at temperatures of up to about 500 ◦ C (Rachini et al., 2009).
Evaluating the results, PALF E had higher thermal stability and
residual masses compared to the other cultivars. The accession E
is followed, in descending order, by the cultivars F, D, A, C and B.
As expected, there were no large variations between the aforementioned values or other thermal event values, whose deviations were
Table 1
Thermogravimetric data for the six PALF.
PALF
Volatile loss
mass (%)
Volatile loss
temperature (◦ C)
Onset temperature
(◦ C)
Residual
mass (%)
A
B
C
D
E
F
8.3
9.4
8.6
8.2
7.7
8.1
178
177
175
186
191
179
241
241
249
244
256
245
6.0
6.4
4.4
3.5
3.7
3.7
Table 2
Values of cellulose crystallinity index, Young’s modulus, tensile strength, stress–strain at break and cross-sectional area of fiber for the six PALF varieties.
PALF
Cellulose crystallinity
index (%)
Young’s modulus (GPa)
A
B
C
D
E
F
58.6
50
48.7
64.4
58.8
59.2
41.59 (±10.04a)
25.71 (±10.31b)
15.42 (±7.55b)
37.94 (±16.18abc)
37.39 (±23.75abc)
52.12 (±22.90a)
Tensile strength (MPa)
683 (±215a)
376 (±184b)
212 (±176b)
574 (±275a)
544 (±250abcd)
691 (±336acd)
Stress–strain at rupture
(mm/mm)
Cross-section area of fiber
(mm2 )
2.054 E−2 (±5.29 E−3a)
1.657 E−2 (±5.77 E−3a)
1.997 E−2 (±1.43 E−2a)
2.24 E−2 (±1.629 E−2a)
2.062 E−2 (±8.91 E−3a)
1.82 E−2 (±4.64 E−3a)
3.72 E−3 (±8.6E−4a)
3.86 E−3 (±2.94 E−3a)
8.0 E−3 (±4.44 E−3b)
2.56 E−3 (±1.37 E−3a)
4.43 E−3 (±3.04 E−3a)
3.45 E−3 (±1.81 E−3a)
Note: Significant differences at P < 0.05 between mechanical properties and cross-sectional area are followed by different letters.
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Fig. 5. (a) DTG and (b) TG curves of PALF under synthetic air atmosphere and heating rate of 10 ◦ C min−1 .
not higher than 10%. The PALF B and C underwent thermal event
occurrences at temperatures lower than the other cultivars. However, it should be noted that small differences do not necessarily
restrict possible applications of the fibers of the six PALF. Consequently, it is economically important to use fibers such as those of
variety B (A. comosus var. comosus) in view of its high annual production volume estimated at 300,000 tons of fibers in Brazil alone
(based on the production of pineapples in the country and the fiber
yield reported) (Aquino, 2006; Crestani et al., 2010; IBGE, 2011;
FAOSTAT, 2010).
Table 2 shows the values found for cellulose crystallinity index,
Young’s modulus, tensile strength, stress–strain at break and crosssectional area of the fibers. For the mechanical properties and the
cross-sectional area, statistical analyses were performed using Levene and Tukey tests.
The methodology used showed that the average cross-section
values are of the fibers of cultivars are similar, except PALF C that
showed higher size values. A and D had little deviation when compared to those associated with the other varieties.
Since the PALFs were manually extracted, binding and external leaf materials were present in the fibers during the diameter
measurements. However, the range of medium values obtained:
Fig. 6. Example of stress versus strain curve of a pineapple fiber (accession A).
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535
Fig. 7. Measured mechanical properties for PALF versus cross-sectional area. (a) Young’s modulus and (b) tensile strength.
50–91 ␮m, is in reasonable accordance with the typical values for
pineapple leaf extracted using distinct methods: 20–80 ␮m (Reddy
and Yang, 2005), and the curaua fiber: 25 a 64 ␮m (Tomczak, 2010).
The elastic modulus (E) values obtained in the tensile tests are
located in the 15–53 GPa range and the s values calculated are
in the 210–695 MPa range, indicating the fibers characteristic to
withstand high stresses and, consequently, enabling them to be
used as reinforcement in lower-mechanical property matrices, such
as polymeric ones. This complies with an important requirement
for its use as reinforcement in polymer composites (Mano and
Mendes, 2001; Callister, 2006). The means showed similar estimated values estimated, as cited in other works: Young’s modulus
25–36 GPa and tensile strength 362–748 MPa range (Mukherjee
and Satyanarayana, 1986); Young’s modulus 26.6–96.1 GPa and
tensile strength 87–310 MPa range (Tomczak et al., 2007). The values were higher than those found in the literature for other fibers,
such as sisal, jute and ramie (Rowell et al., 1997).
Statistically, it is possible to consider the moduli and tensile
strength of the fibers as significantly different, depending on the
cultivars compared. Cultivar C showed the lowest values, but as
previously discussed in the section on thermogravimetric analysis,
the lowest mechanical properties of the PALF C still enables it to be
used as reinforcement in certain polymer composites.
Among the characteristics analyzed in the tensile tests, the
stress–strain property (εr ) of the specimens showed the lowest
proportional variation between the PALFs with the mean values.
After carrying out Levene and Tukey tests, it was shown that
this property is not significantly different between any of the
Fig. 8. Different properties measured for PALF versus crystallinity. (a) Cross-sectional area; (b) Young’s modulus; (c) tensile strength and (d) stress–strain at break. The dotted
lines illustrate the magnitude of deviation for the group of fiber samples studied and its tendency with the variation of the properties measured.
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cultivars. The small strains at failure are explained by the fact that
the fibers have an elastic behavior with large Young’s modulus
values (Fig. 6).
Based on previous studies on pineapple fiber (Mukherjee and
Satyanarayana, 1986) and curaua fiber (Tomczak et al., 2007), the
mechanical properties of the fibers decrease with increasing of
cross sectional area, probably due to an increase in the occurrence
of tensile concentrating defects. The PALFs studied in this work
showed a declining trend in Young’s moduli and s (Fig. 7) with
increasing cross-sectional area, which is in accordance with the
above. Stress–strain at rupture showed a softly declining trend with
increasing cross-sectional area, but not as noticeable as the other
two mechanical properties.
Fig. 8 shows the dependence of the different properties on the
cellulose crystallinity index of the fibers.
The structural characteristics of the high cellulose crystallinity
index of pineapple fibers reflect on interesting mechanical properties at a practical level (Mukherjee and Satyanarayana, 1986). By
plotting graphics of Ic versus Young’s modulus or tensile strength,
a increase tendency is observed, showing that if higher mechanical
properties are desired in natural fibers, those with higher crystallinity should probably be preferred. For the fracture strain there
is an increase tendency with increasing crystallinity, however less
pronounced than other properties and small compared to the deviations.
Since both the increasing of Ic and the reduction of the cross sectional area of the fibers lead to an increase of mechanical properties,
Fig. 8 illustrates the dependence of the Ic on the transverse area. It
is noted that the former has an inverse relationship with the area,
indicating the possible reduction in the internal binder, amorphous
or contaminant materials of the fibers and also the concentration
of defects and weak links. This leads to a behavior close to that
corresponding to fiber cells which consists primarily of cellulose of
higher crystallinity index, thus contributing to higher mechanical
properties.
4. Conclusions
Six PALFs of APGB cultivars were comparatively characterized
and evaluated for potential application as mechanical reinforcements in polymeric composites. The mechanical properties coupled
with thermogravimetric analysis indicated that the six PALF meet
the requirements to be used as fibrous reinforcement in a reasonable range of possible composite applications, especially when
comparing those properties with some of the fibers traditionally
used with this purpose.
Peaks relating to cellulose I and its crystalline planes were identified by XRD spectra, as they are characteristic of plant fibers.
The photomicrographs indicate the presence of envelope materials from the leaves and different quantities of impurities, remnants
of the manual fiber extraction process. The different types of fibers
show different susceptibilities to the separation between fiber cells.
Two types of fiber cells were identified from the cultivars: similar
to tubular and flat aspects. There may be influences of this morphology in the matrix–reinforcement interaction. The FTIR analysis
confirmed the presence of type I cellulose, lignin and hemicelluloses, characteristic of lignocellulosic fibers without chemical
modification. The six PALF had onset temperature at around 245 ◦ C,
reaching satisfactory values for use as reinforcement in polymer
matrices that can be processed at this temperature range. Cultivars E and F were more thermally stable. The cellulose crystallinity
index stood between 49 and 64%, values that are within the range
observed in other studies.
The Young’s modulus ranged from 15 to 53 GPa, and tensile
strength between 210 and 695 MPa, confirming to all six PALFs,
requirements to be used as reinforcement in polymeric composites.
Statistical analysis showed significant differences between the six
varieties, except for stress–strain. A great variability in the average
values of the fiber’s mechanical properties were observed, which
allows one to consider them as statistically similar, except for accession C, whose properties were lower than accession A.
The mechanical properties showed an indirect relation with
cross-sectional area and a direct relation with crystallinity. Analyzing the Ic versus the medium of the cross sectional area for each
of the six PALF it was observed that an inverse relationship exists
between the cross sectional area and the cellulose crystallinity
index, possibly pointing that with a reduction of the cross sectional area of the fiber, there is a lower presence of amorphous
materials, impurities, waxes, fats and defects while maintaining
crystalline phase. This justifies the search for natural fibrous materials of smaller dimensions and higher crystallinity index when
higher mechanical strength and modulus are desired.
Acknowledgements
The authors would like to thank CNPq/PIBIC, CAPES, FINEP and
EMBRAPA for the financial support.
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