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materials
Article
Study of Agave Fiber-Reinforced Biocomposite Films
Cindu Annandarajah 1, Peng Li 2, Mitchel Michel 1, Yuanfen Chen 3, Reihaneh Jamshidi 4,
Alper Kiziltas 5, Richard Hoch 6, David Grewell 7and Reza Montazami 2, *
1Agricultural and Biosystems Engineering, Iowa State University, Ames, IA 50011, USA;
cindu@iastate.edu (C.A.); mmmichel@iastate.edu (M.M.)
2
Department of Mechanical Engineering, Iowa State University, Ames, IA 50011, USA; pengnova@iastate.edu
3
College of Mechanical Engineering, Guangxi University, Nanning 530004, China; yuanfenchen@gxu.edu.cn
4Department of Mechanical Engineering, University of Hartford, West Hartford, CT 06117, USA;
jamshidi@hartford.edu
5Ford Motor Company, Dearborn, MI 48120, USA; akizilt1@ford.com
6Diageo, London NW10 7HQ, UK; Richard.Hoch@diageo.com
7Industrial and Manufacturing Engineering, North Dakota State University, Fargo, ND 58102, USA;
david.grewell@ndsu.edu
*Correspondence: reza@iastate.edu
Received: 20 November 2018; Accepted: 24 December 2018; Published: 29 December 2018
Abstract:
Thermoplastic resins (linear low-density polyethylene (LLDPE), high-density polyethylene
(HDPE), and polypropylene (PP)) reinforced by different content ratios of raw agave fibers were
prepared and characterized in terms of their mechanical, thermal, and chemical properties as well
as their morphology. The morphological properties of agave fibers and films were characterized
by scanning electron microscopy and the variations in chemical interactions between the filler and
matrix materials were studied using Fourier-transform infrared spectroscopy. No significant chemical
interaction between the filler and matrix was observed. Melting point and crystallinity of the
composites were evaluated for the effect of agave fiber on thermal properties of the composites, and
modulus and yield strength parameters were inspected for mechanical analysis. While addition of
natural fillers did not affect the overall thermal properties of the composite materials, elastic modulus
and yielding stress exhibited direct correlation to the filler content and increased as the fiber content
was increased. The highest elastic moduli were achieved with 20 wt % agave fiber for all the three
composites. The values were increased by 319.3%, 69.2%, and 57.2%, for LLDPE, HDPE, and PP,
respectively. The optimum yield stresses were achieved with 20 wt % fiber for LLDPE increasing by
84.2% and with 30 wt % for both HDPE and PP, increasing by 52% and 12.3% respectively.
Keywords:
agave fiber; polyethylene; polypropylene; thermoplastic; biocomposites; mechanical properties
1. Introduction
Over the past decades, conventional petroleum-based plastics have led to environmental
issues and sustainability concerns [
1
,
2
]. The abundance and sustainability of renewable fillers and
reinforcements make them attractive to the polymer industries, especially for automotive, packaging,
and construction applications [
1
,
3
–
5
]. Also, from the thermomechanical standpoint, bio-based
composites were either similar or superior to their petroleum-based equivalents.
Natural plant fibers have experienced rapid growth in the bio-renewable marketplace in recent
years. They have gained various advantages compared to traditional fibers, including lower density,
lower cost, prevalence, ease of processing, and low environmental impact [
6
,
7
]. Natural fiber
reinforcements, such as bast (jute, flax, and hemp); leaf (abaca, sisal); seed (coir); core (kenaf); grass;
reed; and wood (lignin and sludge), have been widely reported in the past decades [1,8–10]
Materials 2019,12, 99; doi:10.3390/ma12010099 www.mdpi.com/journal/materials
Materials 2019,12, 99 2 of 13
Recently, plant-based byproducts, such as agave fibers, have received considerable attention
from industry. As a leading automobile manufacturing enterprise, Ford has researched the use of
sustainable materials for their vehicles since 2000. Currently, Ford uses eight bio-based materials in
their vehicles, including soy foam, kenaf fiber, cellulose, wood, coconut fiber, rice husk, castor oil,
and wheat straw. Today, Ford is exploring agave-based parts for every single vehicle in their fleet.
Agave fiber comes from Agave Americana, belonging to the Agavaceae family, and the fibers used are
a co-product of agave tequila production [
11
]. Similar to other natural fibers, agave fibers are light,
low cost, and reproducible [
12
]. Despite many desirable properties of agave fiber-based composites,
there is only limited published information on agave fiber-based polymer composites. Singha and
Rana studied the effects of different fiber concentrations on mechanical properties of polystyrene
(PS)/agave-fiber composites and revealed that PS composites reinforced with 20% fiber loading by
weight exhibited the best mechanical properties [
13
]. In another study, Pérez-Fonseca investigated the
effect of hybridization of two natural fibers (pine/agave) on the mechanical properties of high-density
polyethylene-(pine/agave) based composites and observed that the addition of agave fibers improved
the mechanical properties of the composites [
14
]. Mendizábal’s group studied the effect of agave fiber
content in the thermal and mechanical properties of biocomposite [15].
Several fiber pretreatment methods, including chemical treatment (alkali, maleated polyethylene,
acid, methyl methacrylate), and plasma treatment, have been reported to improve the agave
fiber reinforcement effect [
6
,
12
,
16
–
18
]. Alternatively, whether the raw fiber could enhance the
mechanical properties of polymers as fillers effectively is also of interest, especially in the automobile
manufacturing enterprise. Applications of plant-based byproducts effectively without further chemical
treatment have extensive environmental and economic impacts. Moscoso and his colleagues reported
that the specific stiffness and strength of PS/agave-fiber composites and foams were enhanced with
increasing fiber content [
19
]. Zuccarello systematically studied the influence of the variety, age, position,
and length of the agave fiber, as well as the fiber extraction method on the reinforcement effect of the
agave fiber, aiming to develop high performance agave reinforced biocomposite [20,21].
This study evaluated the application of agave fiber without any surface treatment in order
to determine whether raw fiber could effectively enhance the mechanical properties of polymer
composites applied in the automotive industry. Linear low-density polyethylene (LLDPE), high-density
polyethylene (HDPE), and polypropylene (PP) are chosen as the polymer matrices because they are
currently widely applied in automotive vehicles. The properties of biocomposite films containing
different fiber loading ratios in LLDPE, HDPE, and PP matrices were investigated in this study.
Composite films with various agave fiber contents (up to 30 wt %) were prepared by injection and
extrusion molding, and their mechanical and thermal properties were studied. Agave fibers were
washed to remove residual sugars prior to testing. Preliminary tests from the washing showed that
about 0.8 (g/g) amount of residual sugar per unit of fiber can be removed from the washing step. It is
envisioned that these sugars could be used to produce additional co-products through fermentation.
2. Materials and Methods
2.1. Materials
All matrix materials were used in pellet form. The LLDPE had a melt flow index of 20 g/10 min
at 190
◦
C and a density of 0.925 g/cm
3
(ExxonMobil Chemical Corporation, Houston, TX, USA).
The HDPE had a melt flow index of 20 g/10 min at 190
◦
C and a density of 0.952 g/cm
3
(Chevron
Phillips Chemical Company LP, TX, USA). The PP had a melt flow index of 20 g/10 min at 230
◦
C and
a density of 0.868 g/cm
3
(Formosa Plastics Corporation, TX, USA). Agave fibers were obtained from
Byogy Renewables, Inc. The fibers ranged from 3 to 7 mm in length and were used as received.
Materials 2019,12, 99 3 of 13
2.2. Preparation of Agave Fibers
Agave fibers and water were placed in a 70-liter tank at a 2:3 (solid: liquid) volume ratio.
The solution was heated to 70
◦
C and stirred for 24 h, after which the fibers were removed with
a filter. This wash cycle was repeated 6 times. The agave fibers were then laid onto drying trays and
dried at 105 ◦C for 18–24 h. The thickness of the fiber layer was less than 1.5.
2.3. Preparation of Biocomposite Film
The various polymers and agave fibers were blended in a Leistritz 27 mm co-rotating twin-screw
extruder with a barrel – l/d ratio of 25:1. The extrusion temperatures for the compounding process are
detailed in Table 1. In general, polymer matrix/agave fiber compositions with 0, 5, 10, 20, and 30 wt %
fiber-content were compounded at a screw speed of 225 RPM. After compounding, the extrudes were
pelletized. Subsequently, the pellets were placed into a Brabender plasticorder 19 mm single screw
extruder with a barrel – l/d ratio of 25:1 to produce films at a screw speed of 75 RPM. Table 1details
the processing temperatures from the feeder to the die for creating films. Figure 1shows images of
film samples with various fiber content.
Table 1. The extrusion temperature (◦C) parameters of extrusion for compounding and films.
The extrusion temperature (◦C) parameters of extrusion for compounding (twin screw extruder)
Heating Zone 1 (feeder) 2 3 4 5 6 7 8 9 10
11(die)
Resin
LLDPE
70 160 165 170 175 180 170 175 170 165 165
HDPE
70 165 170 175 170 185 185 180 175 170 165
PP 70 170 195 200 210 215 215 210 205 200 190
The extrusion temperature (◦C) parameters of extrusion for films (single screw extruder)
Heating Zone 1 (feeder) 2 3 4 5 (die)
Resin
LLDPE
90 165 180 175 165
HDPE
90 175 185 180 170
PP 90 150 190 180 180
2.4. Characterization
2.4.1. Scanning Electron Microscopy (SEM)
The morphology of the prepared fibers and the cross-sections of the biocomposite films were
investigated using SEM (NeoScopo, JCM-6000 Benchtop SEM) (Peabody, MA, USA). Prior to
SEM measurements, the samples were frozen with liquid nitrogen, then fractured along the axis
perpendicular to the extrusion direction, to prepare cross-sections.
2.4.2. Mechanical Analysis
Tensile test was conducted with on dynamic mechanical analyzer (DMA) (DMA-1, Mettler Toledo).
All measurements were performed at room temperature (25
◦
C) at a force rate of 0.5 N/min. Fibers were
used as received for single fiber tensile tests and dimensions were measured and taken into account
for stress and strain calculations. All composites were 10 mm
×
2 mm (length
×
width). The thickness
of the film was measured at three positions for each sample to count for potential variations due to the
extrusion conditions. The thickness of the biocomposite films with LLDPE, HDPE and PP matrices
ranged from 0.41 to 0.63 mm, 0.34 to 0.54 mm, and 0.31 to 0.47 mm, respectively. ASTM D882-18
Standard Test Method for Tensile Properties of Thin Plastic Sheeting and ASTM D3822/D3822M-14
Standard Test Method for Tensile Properties of Single Textile Fibers were followed while performing
the tensile testing.
Materials 2019,12, 99 4 of 13
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Figure 1. Images of agave fiber reinforced thermoplastic composite films. (a-e), LLDPE reinforced
with agave fiber at 0 wt. %, 5 wt. %, 10 wt. %, 20 wt. % and 30 wt. %; (f-j), HDPE reinforced with
agave fiber at 0 wt. %, 5 wt. %, 10 wt. %, 20 wt. % and 30 wt. %; (k-o), PP reinforced with agave fiber
at 0 wt. %, 5 wt. %, 10 wt. %, 20 wt. % and 30 wt.%.
2.4. Characterization
2.4.1. Scanning Electron Microscopy (SEM)
The morphology of the prepared fibers and the cross-sections of the biocomposite films were
investigated using SEM (NeoScopo, JCM-6000 Benchtop SEM) (Peabody, MA, USA). Prior to SEM
measurements, the samples were frozen with liquid nitrogen, then fractured along the axis
perpendicular to the extrusion direction, to prepare cross-sections.
2.4.2. Mechanical Analysis
Tensile test was conducted with on dynamic mechanical analyzer (DMA) (DMA-1, Mettler
Toledo). All measurements were performed at room temperature (25 °C) at a force rate of 0.5 N/min.
Fibers were used as received for single fiber tensile tests and dimensions were measured and taken
into account for stress and strain calculations. All composites were 10 mm × 2 mm (length × width).
The thickness of the film was measured at three positions for each sample to count for potential
variations due to the extrusion conditions. The thickness of the biocomposite films with LLDPE,
HDPE and PP matrices ranged from 0.41 to 0.63 mm, 0.34 to 0.54 mm, and 0.31 to 0.47 mm,
respectively. ASTM D882 – 18 Standard Test Method for Tensile Properties of Thin Plastic Sheeting
and ASTM D3822 / D3822M - 14 Standard Test Method for Tensile Properties of Single Textile Fibers
were followed while performing the tensile testing.
2.4.3. Differential Scanning Calorimetry (DSC)
DSC was conducted under ambient conditions (Polymer DSC, Mettler Toledo). The composite
samples of both LLDPE and HDPE with agave fiber were heated from 25 °C to 150 °C at a rate of 10
1 c
m
Figure 1.
Images of agave fiber reinforced thermoplastic composite films. (
a
–
e
), LLDPE reinforced
with agave fiber at 0 wt %, 5 wt %, 10 wt %, 20 wt % and 30 wt %; (
f
–
j
), HDPE reinforced with agave
fiber at 0 wt %, 5 wt %, 10 wt %, 20 wt % and 30 wt %; (
k
–
o
), PP reinforced with agave fiber at 0 wt %,
5 wt %, 10 wt %, 20 wt % and 30 wt %.
2.4.3. Differential Scanning Calorimetry (DSC)
DSC was conducted under ambient conditions (Polymer DSC, Mettler Toledo). The composite
samples of both LLDPE and HDPE with agave fiber were heated from 25
◦
C to 150
◦
C at a rate of
10
◦
C/min. The composite samples of PP/agave fiber were heated from 25
◦
C to 200
◦
C at a rate of
10 ◦C/min.
2.4.4. Fourier-Transform Infrared (FT-IR) Spectroscopy
FT-IR spectra of bio-filler reinforced thermoplastic films were recorded in the range of
4000–650 cm
−1
(Frontier Optica FT-IR spectrometer equipped with UATR accessory, Perkin-Elmer)
(Waltham, MA, USA).
3. Results and Discussion
3.1. Morphology
Figure 2shows the images of fiber before and after washing and also the SEM image of a single
agave fiber. Figure below shows the raw agave fibers before (a) and after (b) washing. The color of
the agave fibers is lighter after washing compared to the unwashed fibers, because the residual sugar
in the fiber and the dust on the surface was washed. The washing pretreatment could improve the
thermal stability of agave fiber during the processing of composites [
2
]. Agave fiber has a visibly rough
surface and its diameter varies. During the compounding process, the fibers undergo a certain level of
thermomechanical damage. In general, the shorter the fiber, the lower the mechanical resistance [22].
Materials 2019,12, 99 5 of 13
Materials 2018, 11, x FOR PEER REVIEW 6 of 14
Figure 2. Agave fibers: (a) before and (b) after washing treatment; SEM images of agave fibers: (c) and
(d): at low magnification showing individual fibers, (e) and (f) variations in fibers’ diameter, and: (g)
and (h): surface roughness of agave fiber.
Figure 3 shows SEM cross-sectional images of PP: agave fiber; the morphology of the cross-
section of LLDPE and HDPE with agave fiber were similar. Figure 3a shows the control film (PP),
while Figure 3b-f show the variations of fibers in the polymer matrix for 80:20 wt. % PP-agave
composite film. This specific loading level was selected for morphological tests as it has the best
mechanical properties overall. In Figure 3c, it can be seen that there are three voids at the bottom part
of the image. These voids are likely water vapor bubbles. One explanation for their presence is that
although those pellets went through the drying process for more than 24 h, there was still a small
amount of moisture in the pellets. The moisture boiled during the extrusion process and caused the
formation of voids. Another possibility is that the chemical and polarity differences between
polymers and fibers in composites as the polymer is hydrophobic while the fibers are hydrophilic.
These differences result in low compatibility between the materials generating voids or gaps at the
interface [15,23,24]. Cross-section of an agave fiber at different magnifications are shown in Figure
3e,f.
5
m
m
5
m
m
Figure 2.
Agave fibers: (
a
) before and (
b
) after washing treatment; SEM images of agave fibers: (
c
) and
(
d
): at low magnification showing individual fibers, (
e
) and (
f
) variations in fibers’ diameter, and:
(g) and (h): surface roughness of agave fiber.
Figure 3shows SEM cross-sectional images of PP: agave fiber; the morphology of the cross-section
of LLDPE and HDPE with agave fiber were similar. Figure 3a shows the control film (PP), while
Figure 3b-f show the variations of fibers in the polymer matrix for 80:20 wt % PP-agave composite
film. This specific loading level was selected for morphological tests as it has the best mechanical
properties overall. In Figure 3c, it can be seen that there are three voids at the bottom part of the
image. These voids are likely water vapor bubbles. One explanation for their presence is that although
those pellets went through the drying process for more than 24 h, there was still a small amount of
moisture in the pellets. The moisture boiled during the extrusion process and caused the formation
of voids. Another possibility is that the chemical and polarity differences between polymers and
fibers in composites as the polymer is hydrophobic while the fibers are hydrophilic. These differences
result in low compatibility between the materials generating voids or gaps at the interface [
15
,
23
,
24
].
Cross-section of an agave fiber at different magnifications are shown in Figure 3e,f.
Materials 2019,12, 99 6 of 13
Materials 2018, 11, x FOR PEER REVIEW 7 of 14
Figure 3. Cross-sectional SEM micrographs of PP/agave fiber films: (a) PP control group, (b–f) PP:
agave fiber 80:20 wt.%.
3.2. FT-IR Analysis of Composite Films
FT-IR spectroscopy was used to examine the interactions between the thermoplastic matrix and
the agave fibers. Figure 4a shows the FT-IR spectra of pure agave fibers, LLDPE control group, and
80:20 wt. % LLDPE: agave fiber composite. FTIR spectrum of pure agave fiber showed a broad peak
at 3331 cm−1, which correlates to the stretching vibrations of hydroxyl groups from the cellulose in
the agave fiber. The peak at 2923 cm−1 was assigned to C–H group. The peak at 1732 cm−1 was
corresponding to the C=O group of hemicellulose, waxes, pectin, and lignin [25]. The peak at 1616
cm−1 was due to H–O–H group stretching of absorbed moisture and for lignin C-H deformation. The
peak at 1517 cm−1 was due to the lignin aromatic ring vibration and stretching. The milder peaks at
1375 cm−1 to 1427 cm−1 were attributed to –CH, –CH2, or –CH3 groups [6]. The peak at 1317 cm−1 was
due to the –CH group from the cellulose. The peak at 1238 cm−1 was assigned to C–O–C and C=O
groups of lignin [26]. The peak at 1028 cm−1 was attributed to C-O stretching vibrations of the
cellulose. The peak at 893 cm−1 was due to –β glycosidic linkage of the agave fiber [6].
Figure 3.
Cross-sectional SEM micrographs of PP/agave fiber films: (
a
) PP control group, (
b–f
) PP: agave
fiber 80:20 wt %.
3.2. FT-IR Analysis of Composite Films
FT-IR spectroscopy was used to examine the interactions between the thermoplastic matrix and
the agave fibers. Figure 4a shows the FT-IR spectra of pure agave fibers, LLDPE control group, and
80:20 wt % LLDPE: agave fiber composite. FTIR spectrum of pure agave fiber showed a broad peak
at 3331 cm
−1
, which correlates to the stretching vibrations of hydroxyl groups from the cellulose
in the agave fiber. The peak at 2923 cm
−1
was assigned to C–H group. The peak at 1732 cm
−1
was corresponding to the C=O group of hemicellulose, waxes, pectin, and lignin [
25
]. The peak at
1616 cm
−1
was due to H–O–H group stretching of absorbed moisture and for lignin C-H deformation.
The peak at 1517 cm
−1
was due to the lignin aromatic ring vibration and stretching. The milder peaks
at 1375 cm
−1
to 1427 cm
−1
were attributed to –CH, –CH
2
, or –CH
3
groups [
6
]. The peak at 1317 cm
−1
was due to the –CH group from the cellulose. The peak at 1238 cm
−1
was assigned to C–O–C and
C=O groups of lignin [
26
]. The peak at 1028 cm
−1
was attributed to C-O stretching vibrations of the
cellulose. The peak at 893 cm−1was due to –βglycosidic linkage of the agave fiber [6].
Materials 2019,12, 99 7 of 13
Materials 2018, 11, x FOR PEER REVIEW 8 of 14
Figure 4. FT-IR spectra of agave fiber reinforced thermoplastic-based composite films: (a) LLDPE with
agave fiber; (b) HDPE with agave fiber; (c) PP with agave fiber.
The LLDPE control group showed peaks at 2915–2849 cm−1, 1473–1463 cm−1, and 730–719 cm−1.
These variations where the peaks at 2915–2849 cm−1, 1473–1463 cm−1, and 730–719 cm−1, are assigned
to the symmetrical stretching vibration of the C–H bonds. However, the LLDPE composites
reinforced with agave fibers exhibited additional peaks: 1) a broad peak between 3600 cm−1 and 3200
cm−1 was due to the –OH group in the cellulose of agave fiber; 2) the peak at 1712 cm−1 was assigned
to C=O group of hemicellulose; 3) the peak at 1614 cm−1 was due to H–O–H stretching; 4) the peak at
1376 cm−1 was attributed to –CH2– group; 5) the peak at 1271 cm−1 was due to C–O–C stretching; 6)
Figure 4.
FT-IR spectra of agave fiber reinforced thermoplastic-based composite films: (
a
) LLDPE with
agave fiber; (b) HDPE with agave fiber; (c) PP with agave fiber.
The LLDPE control group showed peaks at 2915–2849 cm
−1
, 1473–1463 cm
−1
, and 730–719 cm
−1
.
These variations where the peaks at 2915–2849 cm
−1
, 1473–1463 cm
−1
, and 730–719 cm
−1
, are assigned
to the symmetrical stretching vibration of the C–H bonds. However, the LLDPE composites reinforced
with agave fibers exhibited additional peaks: (1) a broad peak between 3600 cm
−1
and 3200 cm
−1
was due to the –OH group in the cellulose of agave fiber; (2) the peak at 1712 cm
−1
was assigned to
C=O group of hemicellulose; (3) the peak at 1614 cm
−1
was due to H–O–H stretching; (4) the peak at
1376 cm
−1
was attributed to –CH
2
– group; (5) the peak at 1271 cm
−1
was due to C–O–C stretching;
Materials 2019,12, 99 8 of 13
(6) the milder peaks at 1165 cm
−1
and 1125 cm
−1
were attributed to O–C–O asymmetric stretching of
the cellulose; (7) the peak at 998 cm−1was assigned to C–O stretching vibrations of the cellulose.
Figure 4b shows the spectra of pure agave fiber, an HDPE control group, and an 80:20 wt %
HDPE: agave fiber composite. The results for HDPE composites were similar to those for the LLDPE
composites. Compared to the neat HDPE, there were several additional peaks exhibited in the
composites. The explanations of those additional peaks were the same as the LLDPE composites.
Figure 4c shows the spectra of agave fiber, neat PP, and an 80:20 wt % PP: agave fiber composite.
Both neat PP and the PP composites showed four peaks in the wavenumber range between 3000 cm
−1
and 2800 cm
−1
. The peaks at 2950 cm
−1
and 2868 cm
−1
were assigned to –CH
3
asymmetric and
symmetric stretching vibrations, respectively; the peaks at 2918 cm−1and 2839 cm−1were caused by
–CH
2
asymmetric and symmetric stretching vibrations, respectively [
27
–
29
]. The peak at 1456 was
assigned to –CH
2
– or –CH
3
groups, while the peak at 1376 cm
−1
was caused by –CH
3
groups. Various
small peaks also appeared in the wavenumber range between 1200 cm
−1
and 700 cm
−1
: the peak at
1168 cm
−1
was attributed to C–C, –CH
3
and C–H groups. The peak at 998 cm
−1
was assigned to –CH
3
groups, while the peak at 973 cm
−1
was attributed to –CH
3
or C–C groups. The peak at 841 cm
−1
was
assigned to the –CH
2
– group [
29
]. However, the PP composites reinforced with agave fibers showed
additional peaks: (1) a broad peak between 3600 cm
−1
and 3200 cm
−1
was due to the –OH group in
the cellulose of agave fiber; (2) the peak at 1712 cm
−1
was assigned to C=O group of hemicellulose;
(3) the peak at 1610 cm
−1
was due to H–O–H stretching; (4) the peak at 1376 cm
−1
was attributed to
–CH2– group; (5) the peak at 1275 cm
−1
was due to C–O–C stretching; (6) the peak at 1166 cm
−1
was
attributed to O–C–O asymmetric stretching of the cellulose; (7) the peak at 998 cm−1was assigned to
C–O stretching vibrations of the cellulose; (8) the peak at 900 cm
−1
was due to –
β
glycosidic linkage of
the agave fiber.
In general, the FT-IR spectra did not identify the production of chemical bonds at the interface
between the polymer matrix and the fibers in either LLDPE, HDPE, or PP composites.
3.3. Mechanical Properties
A single fiber tensile test using the ASTM method was performed and the tensile modulus of
agave fiber was 6778
±
823 MPa, which is in agreement with the previously reported value for tensile
modulus in literature (15). A typical stress-strain curve for agave fiber is presented in Figure 5. It can
be observed that the fiber is still in the linear region at 60 MPa stress, which is far beyond the yield
stress of the control films.
Materials 2018, 11, x FOR PEER REVIEW 9 of 14
the milder peaks at 1165 cm
−1
and 1125 cm
−1
were attributed to O–C–O asymmetric stretching of the
cellulose; 7) the peak at 998 cm
−1
was assigned to C–O stretching vibrations of the cellulose.
Figure 4b shows the spectra of pure agave fiber, an HDPE control group, and an 80:20 wt. %
HDPE: agave fiber composite. The results for HDPE composites were similar to those for the LLDPE
composites. Compared to the neat HDPE, there were several additional peaks exhibited in the
composites. The explanations of those additional peaks were the same as the LLDPE composites.
Figure 4c shows the spectra of agave fiber, neat PP, and an 80:20 wt. % PP: agave fiber composite.
Both neat PP and the PP composites showed four peaks in the wavenumber range between 3000 cm
−1
and 2800 cm
−1
. The peaks at 2950 cm
−1
and 2868 cm
−1
were assigned to –CH
3
asymmetric and
symmetric stretching vibrations, respectively; the peaks at 2918 cm
−1
and 2839 cm
−1
were caused by –
CH
2
asymmetric and symmetric stretching vibrations, respectively [27-29]. The peak at 1456 was
assigned to –CH
2
– or –CH
3
groups, while the peak at 1376 cm
−1
was caused by –CH
3
groups. Various
small peaks also appeared in the wavenumber range between 1200 cm
−1
and 700 cm
−1
: the peak at
1168 cm
−1
was attributed to C–C, –CH
3
and C–H groups. The peak at 998 cm
−1
was assigned to –CH
3
groups, while the peak at 973 cm
−1
was attributed to –CH
3
or C–C groups. The peak at 841 cm
−1
was
assigned to the –CH
2
– group [29]. However, the PP composites reinforced with agave fibers showed
additional peaks: 1) a broad peak between 3600 cm
−1
and 3200 cm
−1
was due to the –OH group in the
cellulose of agave fiber; 2) the peak at 1712 cm
−1
was assigned to C=O group of hemicellulose; 3) the
peak at 1610 cm
−1
was due to H–O–H stretching; 4) the peak at 1376 cm
−1
was attributed to –CH2–
group; 5) the peak at 1275 cm
−1
was due to C–O–C stretching; 6) the peak at 1166 cm
−1
was attributed
to O–C–O asymmetric stretching of the cellulose; 7) the peak at 998 cm
−1
was assigned to C–O
stretching vibrations of the cellulose; 8) the peak at 900 cm
−1
was due to –β glycosidic linkage of the
agave fiber.
In general, the FT-IR spectra did not identify the production of chemical bonds at the interface
between the polymer matrix and the fibers in either LLDPE, HDPE, or PP composites.
3.3. Mechanical Properties
A single fiber tensile test using the ASTM method was performed and the tensile modulus of
agave fiber was 6778 ± 823 MPa, which is in agreement with the previously reported value for tensile
modulus in literature (15). A typical stress-strain curve for agave fiber is presented in Figure 5. It can
be observed that the fiber is still in the linear region at 60 MPa stress, which is far beyond the yield
stress of the control films.
Figure 5. Elastic modulus (E) region of biocomposite films from stress vs. strain graph.
Figure 6 shows the elastic modulus (E) and Figure 7 shows yield stress (σ) of the composite films
and thermoplastic control films. The physical and mechanical properties of agave biocomposite films
are also summarized in Table 2. The results from Figures 6 and 7 and Table 2 show that the elastic
modulus of all the three composites increased as the fiber loading ratio increased and achieved the
highest elastic modulus with 20 wt. % agave fiber loading. The best achieved elastic modulus was
increased by 319.3%, 69.2%, and 57.5%, for LLDPE, HDPE, and PP, respectively. The overall
Figure 5. Elastic modulus (E) region of biocomposite films from stress vs. strain graph.
Figure 6shows the elastic modulus (E) and Figure 7shows yield stress (
σ
) of the composite
films and thermoplastic control films. The physical and mechanical properties of agave biocomposite
Materials 2019,12, 99 9 of 13
films are also summarized in Table 2. The results from Figures 6and 7and Table 2show that the
elastic modulus of all the three composites increased as the fiber loading ratio increased and achieved
the highest elastic modulus with 20 wt % agave fiber loading. The best achieved elastic modulus
was increased by 319.3%, 69.2%, and 57.5%, for LLDPE, HDPE, and PP, respectively. The overall
improvement in elastic modulus (E), with increasing agave fiber content can be attributed to the
reinforcement effect of agave fiber. A study by Lei et al [
30
], who investigated recycled high-density
polyethylene (rHDPE)/bagasse natural fiber reported similar trend an increase in modulus by about
50 % by adding 30 wt % of bagasses. Hence, incorporation of agave fiber into PP helps in good stress
transfer from the matrix to filler and as a result, successfully enhanced the elastic modulus of the
composite. The elastic moduli with agave fiber loading at 30 wt % were lower than those at 20 wt %,
probably because as fiber content increases, fiber curvature caused by the mixing process decreases the
composite property, especially when the fiber length is higher than a critical value [11,31].
Materials 2018, 11, x FOR PEER REVIEW 10 of 14
improvement in elastic modulus (E), with increasing agave fiber content can be attributed to the
reinforcement effect of agave fiber. A study by Lei et al [30], who investigated recycled high-density
polyethylene (rHDPE)/bagasse natural fiber reported similar trend an increase in modulus by about
50 % by adding 30 wt % of bagasses. Hence, incorporation of agave fiber into PP helps in good stress
transfer from the matrix to filler and as a result, successfully enhanced the elastic modulus of the
composite. The elastic moduli with agave fiber loading at 30 wt. % were lower than those at 20 wt.
%, probably because as fiber content increases, fiber curvature caused by the mixing process
decreases the composite property, especially when the fiber length is higher than a critical value
[11,31].
Figure 6. Elastic modulus (E) of biocomposites consisting of different fiber content.
Figure 7. Yield stress (σ) of the composite films and thermoplastic control films
Table 2. Mechanical properties of biocomposite films.
Fiber Ratio
(wt. %)
Elastic
Modulus
(MPa)
Standard
Deviation
Yield Stress
(MPa)
Standard
Deviation
Specific Yield
Strength
(kN∙m/kg)
LLDPE:
agave fiber
0 88.4 9.29 1.9 0.51 1.83
5 241.5 27.16 2.0 0.15 2.13
Figure 6. Elastic modulus (E) of biocomposites consisting of different fiber content.
Materials 2018, 11, x FOR PEER REVIEW 10 of 14
improvement in elastic modulus (E), with increasing agave fiber content can be attributed to the
reinforcement effect of agave fiber. A study by Lei et al [30], who investigated recycled high-density
polyethylene (rHDPE)/bagasse natural fiber reported similar trend an increase in modulus by about
50 % by adding 30 wt % of bagasses. Hence, incorporation of agave fiber into PP helps in good stress
transfer from the matrix to filler and as a result, successfully enhanced the elastic modulus of the
composite. The elastic moduli with agave fiber loading at 30 wt. % were lower than those at 20 wt.
%, probably because as fiber content increases, fiber curvature caused by the mixing process
decreases the composite property, especially when the fiber length is higher than a critical value
[11,31].
Figure 6. Elastic modulus (E) of biocomposites consisting of different fiber content.
Figure 7. Yield stress (σ) of the composite films and thermoplastic control films
Table 2. Mechanical properties of biocomposite films.
Fiber Ratio
(wt. %)
Elastic
Modulus
(MPa)
Standard
Deviation
Yield Stress
(MPa)
Standard
Deviation
Specific Yield
Strength
(kN∙m/kg)
LLDPE:
agave fiber
0 88.4 9.29 1.9 0.51 1.83
5 241.5 27.16 2.0 0.15 2.13
Figure 7. Yield stress (σ) of the composite films and thermoplastic control films.
Materials 2019,12, 99 10 of 13
Table 2. Mechanical properties of biocomposite films.
Fiber Ratio
(wt %)
Elastic Modulus
(MPa)
Standard
Deviation
Yield Stress
(MPa)
Standard
Deviation
Specific Yield Strength
(kN·m/kg)
LLDPE:
agave fiber
0 88.4 9.29 1.9 0.51 1.83
5 241.5 27.16 2.0 0.15 2.13
10 266.7 41.65 2.1 0.06 2.09
20 370.7 46.55 3.5 0.35 3.55
30 324.2 43.64 3.3 0.29 3.23
HDPE:
agave fiber
0 435.1 50.83 3.8 0.06 3.99
5 617.6 42.58 5.1 0.1 5.28
10 657.6 68.23 4.8 0.47 4.89
20 736.0 82.06 5.4 0.64 5.34
30 640.3 56.38 5.8 0.12 5.68
PP: agave
fiber
0 560.4 40.04 8.1 0.51 9.33
5 766.7 118.18 6.1 0.23 6.9
10 829.7 40.73 7.0 0.65 7.78
20 882.7 48.31 9.0 0.76 9.63
30 738.8 111.43 9.1 1.17 9.36
The results from Figure 7and Table 2suggest that the yield strength of agave fiber reinforced
LLDPE films increased as fiber loading increased, and the highest yield strength was achieved at 20 wt
%, increasing by 84.2%. For the HDPE reinforced with agave fiber, the yield strength was improved
compared to that of the neat polymer. The highest yield strength was achieved at 30 wt %, increasing
by 52%. For agave fiber reinforced PP composites, the highest yield strength was achieved when the
fiber loading was at 30 wt %, increasing by 12.3%. However, the yield strength decreased when the
fiber loadings were at 5 wt % and 10 wt %, compared to neat polymer. This might be because when the
fiber concentration was lower than a critical value, the amount of fiber is not enough to restrain the
matrix, resulting in the development of large, uniformly distributed stresses at low strains [32].
In addition, the specific yield strength was also investigated, the results are shown in Table 2.
Compared to neat LLDPE film, the specific yield strength of LLDPE composites were increased as the
fiber loading increased. LLDPE composites reinforced with 20 wt % agave fibers loading achieved the
highest specific yield strength of 3.55 kN
·
m/kg, increasing by 94.0%. For HDPE composite, specific
yield strengths of all fiber contents were improved compared to the neat polymer. The highest specific
yield strength achieved with 30 wt % agave fibers loading, increased by 52.6%. Moreover, compared to
the neat PP film, the PP composites reinforced with 20 wt % agave fibers exhibited the highest specific
yield strength.
3.4. Thermal Analysis
Neat thermoplastics and composites reinforced with agave fibers underwent DSC analysis to
determine the changes in the samples’ melting points (T
m
). The effect of agave fiber ratio on melting
points of LLDPE, HDPE and PP are shown in Figure 8. There were no significant differences in T
m
between control groups and composites. Crystallinity of the composites were deduced from the DSC
glass transition temperature (T
g
) curves, Figure 9. The overall crystallinity of the composites is very
similar to the neat polymer matrices; however, both increases and decreases in the crystallinity values
were observed as the filler ratio increased. This behavior which was observed in all three composites,
could result from the fiber acting as either a nucleator or restrictor of the polymer chains, corresponding
to an increase or decrease of crystallinity [15,33].
Materials 2019,12, 99 11 of 13
Materials 2018, 11, x FOR PEER REVIEW 12 of 14
Figure 8. Effect of agave fiber content on melting points of LLDPE, HDPE and PP; no significant
variation in T
m
was observed.
Figure 9. Glass transition temperatures of various composites with different agave fiber contents.
4. Conclusion
Biocomposite films made of thermoplastic (LLDPE, HDPE and PP) matrices and agave fiber
reinforcement were successfully prepared in order to investigate the effect of raw fiber addition on
chemical structure, mechanical and thermal properties. The level of fiber loading was varied between
0 and 30 wt. %. Compared to control films, the elastic moduli of composites reinforced with agave
fibers exhibited improvement as fiber content increased. The highest elastic modulus was observed
when the fiber concentration was at 20 wt. %. For LLDPE composites, the highest yield strength was
observed when fiber loading was at 20 wt. %; for HDPE and PP composites, the highest yield strength
was observed when fiber concentration was at 30 wt. %. The specific strength of agave fiber reinforced
biocomposite film was also investigated and it was observed that the specific yield strength of
LLDPE, HDPE, and PP reinforced with agave fiber was improved. The FT-IR spectra suggested that
the bonding between fibers and matrices was solely physical and no chemical interaction occurred.
Because there were no new chemical bonds created, the thermal properties of the composites were
dominated by the matrix resin. The melting temperature of biocomposites did not change
significantly by the addition of agave fibers compared to neat thermoplastic films. Although the
overall crystallinity of the composites did not change significantly, both an increase and decrease of
crystallinity was observed as the fiber ratio was increased. The results from mechanical and thermal
characterizations suggested that agave fiber fillers can be used to enhance desired properties in
biocomposites and the composites with 20 wt. % fiber loading was the optimum formulation.
Figure 8.
Effect of agave fiber content on melting points of LLDPE, HDPE and PP; no significant
variation in Tmwas observed.
Materials 2018, 11, x FOR PEER REVIEW 12 of 14
Figure 8. Effect of agave fiber content on melting points of LLDPE, HDPE and PP; no significant
variation in T
m
was observed.
Figure 9. Glass transition temperatures of various composites with different agave fiber contents.
4. Conclusion
Biocomposite films made of thermoplastic (LLDPE, HDPE and PP) matrices and agave fiber
reinforcement were successfully prepared in order to investigate the effect of raw fiber addition on
chemical structure, mechanical and thermal properties. The level of fiber loading was varied between
0 and 30 wt. %. Compared to control films, the elastic moduli of composites reinforced with agave
fibers exhibited improvement as fiber content increased. The highest elastic modulus was observed
when the fiber concentration was at 20 wt. %. For LLDPE composites, the highest yield strength was
observed when fiber loading was at 20 wt. %; for HDPE and PP composites, the highest yield strength
was observed when fiber concentration was at 30 wt. %. The specific strength of agave fiber reinforced
biocomposite film was also investigated and it was observed that the specific yield strength of
LLDPE, HDPE, and PP reinforced with agave fiber was improved. The FT-IR spectra suggested that
the bonding between fibers and matrices was solely physical and no chemical interaction occurred.
Because there were no new chemical bonds created, the thermal properties of the composites were
dominated by the matrix resin. The melting temperature of biocomposites did not change
significantly by the addition of agave fibers compared to neat thermoplastic films. Although the
overall crystallinity of the composites did not change significantly, both an increase and decrease of
crystallinity was observed as the fiber ratio was increased. The results from mechanical and thermal
characterizations suggested that agave fiber fillers can be used to enhance desired properties in
biocomposites and the composites with 20 wt. % fiber loading was the optimum formulation.
Figure 9. Glass transition temperatures of various composites with different agave fiber contents.
4. Conclusions
Biocomposite films made of thermoplastic (LLDPE, HDPE and PP) matrices and agave fiber
reinforcement were successfully prepared in order to investigate the effect of raw fiber addition on
chemical structure, mechanical and thermal properties. The level of fiber loading was varied between
0 and 30 wt %. Compared to control films, the elastic moduli of composites reinforced with agave
fibers exhibited improvement as fiber content increased. The highest elastic modulus was observed
when the fiber concentration was at 20 wt %. For LLDPE composites, the highest yield strength
was observed when fiber loading was at 20 wt %; for HDPE and PP composites, the highest yield
strength was observed when fiber concentration was at 30 wt %. The specific strength of agave fiber
reinforced biocomposite film was also investigated and it was observed that the specific yield strength
of LLDPE, HDPE, and PP reinforced with agave fiber was improved. The FT-IR spectra suggested that
the bonding between fibers and matrices was solely physical and no chemical interaction occurred.
Because there were no new chemical bonds created, the thermal properties of the composites were
dominated by the matrix resin. The melting temperature of biocomposites did not change significantly
by the addition of agave fibers compared to neat thermoplastic films. Although the overall crystallinity
of the composites did not change significantly, both an increase and decrease of crystallinity was
observed as the fiber ratio was increased. The results from mechanical and thermal characterizations
Materials 2019,12, 99 12 of 13
suggested that agave fiber fillers can be used to enhance desired properties in biocomposites and the
composites with 20 wt % fiber loading was the optimum formulation.
Author Contributions:
Conceptualization, D.G. and R.M. Data curation, P.L.; Formal analysis, P.L.; Methodology,
M.M.; Writing—review & editing, C.A., Y.C., R.J., A.K. and R.H.
Funding:
This research is supported in part by the Center for Bioplastic and Biocomposites (NSF Award No.
1439639). P.L. and R.M. would also thank Sarah Kreutner for technical review of the present work.
Conflicts of Interest: The authors declare no conflict of interest.
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