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Composition, structure, and mechanical properties of hemp fiber reinforced composite with recycled high-density polyethylene matrix

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Composition, structure, and mechanical properties of hemp fiber reinforced composite with recycled high-density polyethylene matrix

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Hemp fiber composites with recycled high-density polyethylene matrix were prepared in various compositions ranging from 20 to 40% of fiber volume fraction. The fiber-matrix interface was improved using 5% by weight NaOH-treated hemp fiber in each composite system. The surface morphology and chemical compound of hemp fiber after chemical treatment were analyzed by Scanning Electron Microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). This study indicated that hemp fiber-recycled high-density polyethylene (rHDPE) composites could achieve maximum tensile strengths on the order of 60 MPa. Among the tested samples, the composites with 40% of fiber volume fraction demonstrated the best mechanical properties with regard to tensile strength, elastic modulus, and flexural strength and modulus.
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Journal of Composite Materials
http://jcm.sagepub.com/content/early/2011/11/11/0021998311427778
The online version of this article can be found at:
DOI: 10.1177/0021998311427778
published online 11 November 2011Journal of Composite Materials
Na Lu, Robert H. Swan, Jr and Ian Ferguson
high-density polyethylene matrix
Composition, structure and mechanical properties of hemp fiber reinforced composite with recycled
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JOURNAL OF
COMPOSITE
MATERIALS
Article
Composition, structure, and mechanical
properties of hemp fiber reinforced
composite with recycled high-density
polyethylene matrix
Na Lu
1
, Robert H. Swan Jr
2
and Ian Ferguson
3
Abstract
Hemp fiber composites with recycled high-density polyethylene matrix were prepared in various compositions ranging
from 20 to 40% of fiber volume fraction. The fiber–matrix interface was improved using 5% by weight NaOH-treated
hemp fiber in each composite system. The surface morphology and chemical compound of hemp fiber after chemical
treatment were analyzed by Scanning Electron Microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR).
This study indicated that hemp fiber-recycled high-density polyethylene (rHDPE) composites could achieve maximum
tensile strengths on the order of 60 MPa. Among the tested samples, the composites with 40% of fiber volume fraction
demonstrated the best mechanical properties with regard to tensile strength, elastic modulus, and flexural strength and
modulus.
Keywords
Natural fiber composite, hemp fiber composite, recycled thermoplastic
Introduction
Natural fiber composites (NFC), fundamentally cellu-
lose fiber, have renewed interests in engineering com-
munity due to their unique material properties
including fast growth, low density (1/2 of E-glass
fiber), high specific strength and stiffness, excellent
sound-absorbing efficiency, and high shatter resis-
tance.
1–8
Currently, in the automobile industry, hemp
fiber composites have begun to replace fiberglass com-
posite and steel alloy as interior and exterior systems
for lightweight and fuel-efficient vehicles.
9
In the civil
and building construction industries, NFCs have
recently been used as non-load bearing members, such
as decking, in order to help mitigate the environmental
and health issues caused by using heavy metal–treated
wood.
10
The chemical composition of natural fiber consists of
cellulose (microfiber of the cell wall), hemicelluloses,
and lignin (biopolymer components of the cell wall).
The outer surfaces of plant fiber contain waxes, fats,
and pectin. The cellulose group is a highly crystalline
structure with theoretical Young’s modulus of
130 GPa
11
; therefore, many natural fibers exhibit
good mechanical properties. In particular, hemp, flax,
and kenaf have remarkable mechanical properties, with
a comparable specific strength but higher specific mod-
ulus than E-glass fiber, as presented in Table 1.
Thermoplastic materials were chosen as the poly-
meric matrix in this study due to its recyclability.
Every year, a large amount of postconsumer thermo-
plastic materials are generated worldwide. In 2005
1
Deparment of Engineering Technology and Construction Management,
Sustainable Material and Renewable Technology (SMART) Laboratory,
University of North Carolina at Charlotte, Charlotte, NC 28223,
United States
2
Department of Engineering Technology and Construction Management,
University of North Carolina at Charlotte, Charlotte, NC 28223, United
States
3
Department of Electrical and Computing Engineering, University of
North Carolina at Charlotte, Charlotte, NC 28223, United States
Corresponding author:
Na Lu, Department of Engineering Technology and Construction
Management, Sustainable Material and Renewable Technology (SMART)
Laboratory, University of North Carolina at Charlotte, Charlotte, NC
28223, United States
Email: na.lu@uncc.edu
Journal of Composite Materials
0(0) 1–11
!The Author(s) 2011
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alone, around 30 million tons of waste plastics were
generated in the United States while only 5.7% of this
waste material was recycled.
15
Previous studies have
shown that the properties of recycled high density poly-
ethylene (rHDPE) were similar to those of virgin
HDPE (vHDPE) and the price was 31–34% cheaper.
16
In this study, an investigation was performed to
determine the chemical, physical, and mechanical prop-
erties of hemp fiber composites with rHDPE matrix. A
compression molding technique was used to synthesize
hemp fiber composites with fiber volume fractions of
20%, 30%, and 40%, respectively, for both treated and
untreated fibers. Scanning electron microscopy (SEM)
and Fourier transform infrared spectroscopy (FTIR)
were used to investigate the surface morphology of
the fiber and the resultant composite. The tensile
strength, secant modulus, flexural strength, and flexural
modulus of the composites having different volume
fraction compounds were analyzed.
Experimental
Materials
Industrial hemp fibers were obtained from Hempline
Inc (Delaware, Ontario, Canada). The average density
of hemp fiber was 0.86 g/cm
3
with a typical diameter of
22.5 mm and length of 25 mm. The moisture content of
the raw industrial hemp fiber was approximately 6%.
The rHDPE pellets used in this study were obtained
from Customer Polymer Inc (Charlotte, North
Carolina, United States), which were recovered from
detergent bottle applications, having an average bulk
specific density of 0.98 g/cm
3
, a melt index (MI) of
0.45 g/10 min at 190C, and a melting temperature
range from 130 to 190C.
Composite manufacturing
For this study, the NFCs were prepared using both
treated and untreated hemp fibers. The treated hemp
fibers were prepared using an Alkali solution, which
contained a 5% concentration of sodium hydroxide
(NaOH), prior to the fabrication of the polymeric com-
posites. The hemp fibers were immersed in the NaOH
solution for 24 h at 60C to allow complete saturation.
After immersion, the hemp fibers were washed with
running distilled (DI) water with 1% of acetic acid to
neutralize any remaining NaOH molecules. The hemp
fibers were then removed from the DI water when their
pH level ranged from 6.8 to 7.2 using an Orion 2 Star
PH meter. The hemp fibers were then placed in a drying
oven at 60C for 24 h. The oven-dried hemp fibers were
then stored in desiccators prior to being used to man-
ufacture the polymeric composites.
The polymeric composite materials were fabricated
using both a C.W. Brabender 19.05 mm single-screw
extruder and Carver hydraulic press. Initially, the pel-
lets of the rHDPE were ground using a laboratory
miller manufactured by Arthur Thomas Co,
Swedesboro, New Jersey. The grounded rHDPE
powder was then processed into rHDPE films using
the single-screw extruder. The extruder was operated
at a temperature of 180C with an extruder rotational
speed of 60 rpm. The films that were extruded had a
typical thickness of 0.3 mm and were then cut into a
254 mm 254 mm sheets for use in the composite
manufacturing process.
A compression molding technique using the Carver
hydraulic press was used to manufacture the hemp fiber
composites with the rHDPE films using a fabricated
mold having the dimensions of 254 mm 254 mm.
Each composite sample was manufactured by sand-
wiching a layer of manually distributed treated or
untreated hemp fiber in between two layers of rHDPE
Table 1. Typical mechanical properties of cellulose fiber vs. E-glass fiber
Material
Density
(g/cc)
Tensile
strength (MPa)
Elastic
modulus (GPa)
Specific
strength
(s/g)
Specific
modulus
("/g)
Elongation
at failure (%)
Moisture
absorption (%)
Cost
($/lb)
E-glass
11
2.62 3400 73 1297 28 4.8 N/A 1.10
Hemp
12
1.4 550–900 70 393–643 50 1.6 6–12 0.30
Flax
13
1.4 800–1500 60–80 571–1071 43–57 2.7–3.2 8–12 0.33
Ramie
14
1.5 500 44 333 29 3.6–3.8 8–17 0.34
Kenaf
14
1.45 930 53 641 36 1.6 10–12 0.24
Coir
13
1.25 220 6 176 5 1.5–4 8 0.20
Sisal
13
1.33 600–700 38 451–526 29 3–7 10–22 0.36
Jute
13
1.46 400–800 10–30 281–548 7–21 1.5 12–14 0.20
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films at a temperature of 180C under a constant pres-
sure of 1.5 MPa for duration of 15 min. The fibers were
placed using a disoriented (random) distribution. These
various built up sandwiches were used to fabricate the
desired final composites. The weights of hemp fiber and
rHDPE layers were controlled to maintain a 20%,
30%, or 40% fiber volume fraction. A summary of
the composite materials that were manufactured is pre-
sented in Table 2. The fiber volume fraction V
f
was
determined using the following Equations (1) and (2):
Vf¼ðWf=rfÞ=ðWm=rmÞþðWf=rfÞð1Þ
Vm¼1Vfð2Þ
where V
f
denotes the volume fraction of hemp fiber, W
f
is the weight of hemp fiber sandwiched in the compos-
ite, and r
f
is the density of hemp fiber. V
m
,W
m
, and r
m
represent the volume fraction, weight, and the density
of rHDPE matrix, respectively. Before manufacturing
the composite, the weight of fiber and rHDPE for each
layer was measured using a Denver Instrument bench-
top scale. The density of composite of each composition
was measured by displacement methods conforming to
American Society for Testing and Materials (ASTM)
D 792-08.
17
The measured density of each fabricated
composite is presented in Table 3.
Composite characterization and testing
SEM analysis. Surface morphology of the treated and
untreated hemp fiber, fiber distribution, and the fiber–
matrix interface were analyzed using A JSM-6764
SEM. The SEM specimens were selected from bulk
samples of the treated and untreated fibers and then
coated with a thin layer of gold using a Denton Desk
IV sputtering instrument. The SEM instrument was
operated at room temperature with 10 kV. The surface
morphology of the treated and untreated hemp fiber
and the hemp fiber–matrix interface of the rHDPE
composites were observed.
FTIR measurement. Chemical compound of untreated
and 5% NaOH treated hemp fiber were analyzed using
a Perkin-Elmer 100 Spectrometer (Boston,
Massachusetts, United States). A total of eight scans
were taken for each sample between 650 cm
1
and
Table 2. Description of the various composite tensile test samples
Composite designation
Hemp fiber
fraction (%)
Polymer
matrix
Polymer
fraction (%)
rHDPE 0% of treated hemp rHDPE 100
20 uHemp/80 rHDPE 20% of untreated hemp rHDPE 80
20 Hemp/80 rHDPE 20% of treated hemp rHDPE 80
30 uHemp/70 rHDPE 30% of untreated hemp rHDPE 70
30 Hemp/70 rHDPE 30% of treated hemp rHDPE 70
40 uHemp/60 rHDPE 40% of untreated hemp rHDPE 60
40 Hemp/60 rHDPE 40% of treated hemp rHDPE 60
rHDPE: recycled high-density polyethylene.
Table 3. Summary of tensile test results for hemp fiber composites
Composite
Density
(g/cc)
Maximum
strength (MPa)
SD
(MPa)
Strain
(%)
Secant modulus
at 2% strain (MPa) SD (MPa)
rHDPE 0.98 19.1 0.6 17.9 441 18.5
20 uHemp/80 rHDPE 1.00 15.7 2.4 4.5 546 34.8
20 Hemp/80 rHDPE 0.95 18.6 2.1 7.0 556 56.3
30 uHemp/70 rHDPE 1.03 27.4 4.9 5.4 682 98.4
30 Hemp/70 rHDPE 0.93 45.7 5.7 3.7 1670 163.5
40 uHemp/60 rHDPE 1.06 26.0 5.2 3.3 986 157.9
40 Hemp/60 rHDPE 0.89 60.2 7.3 3.0 2574 257.2
rHDPE: recycled high-density polyethylene.
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4000 cm
1
, with a resolution of 8 cm
1
. Each sample
was prepared in filament form.
Composite mechanical strength. Tensile and flexural
testing were conducted using an Instron 5582 constant
rate of extension (CRT) universal testing
machine in accordance with ASTM D638
18
and
D790
19
, respectively, under the following test condi-
tions of (i) a cross-head speed of 1.3 mm/min, (ii) air
temperature 23C, and (iii) 65% relative humidity. For
the tensile tests on the various composites manufac-
tured using treated and untreated hemp fibers, the typ-
ical tensile stress–strain behavior including analyses of
the maximum tensile strength, strain at maximum
Fiber pull-out Fiber
(a) (b)
(c) (d)
Figure 1. SEM image of (a) untreated hemp, (b) 5% NaOH treated hemp, (c) untreated hemp fiber being completely pull out from
matrix, (d) 5% NaOH treated hemp fiber–matrix interface. SEM: Scanning electron microscopy.
Figure 2. FTIR spectra of untreated hemp and 24 hour 5% NaOH treated hemp fiber. FTIR: Fourier transform infrared
spectroscopy.
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tensile strength, and the secant modulus at 2% strain
are presented and reported. For the flexural tests on the
various composites manufactured using treated hemp
fibers, the typical flexural stress–strain behavior includ-
ing analyses of the maximum flexural strength, strain at
maximum strength, and the flexural modulus at 1%
and 3% strain of are presented and reported.
Results and discussion
Surface morphology results
Figure 1(a) and (b) shows the surface morphology of
untreated and 5% NaOH treated hemp fibers.
Significant differences of surface morphology of trea-
ted/untreated hemp fibers are clearly observed. As can
be seen in Figure 1(a), the as-received fiber exhibited
smooth noncellulose structure boundary layers with
wax/protein composition and surface impurities.
Figure 1(b) indicated that the alkylation process
removed the weak boundary layer of noncellulose
structure, therefore, the surface roughness and surface
area of the hemp fiber have been significantly increased,
likely resulting in improved interfacial adhesion
between fiber and rHDPE matrix. Figure 1(c) shows
the SEM image of the fracture surface of a composite
with 30% untreated hemp fiber volume fraction.
Complete fiber pullout was often observed in the resin
rich region. This could be attributed to the poor fiber–
matrix interface due to the weak surface boundary
observed in Figure 1(a), suggesting that the failure
mechanism in the untreated/rHDPE composite could
have resulted from debonding. On the other hand,
Figure 1(d) presents a typical SEM image of the frac-
ture surface of a composite with 30% volume fraction
of NaOH treated hemp/rHDPE matrix. Fiber breakage
without pullout from the matrix was often observed in
many areas within the test specimen as shown in
Figure 1(d). This may suggest that there is improved
fiber–matrix interface adhesive strength after alkali
treatment.
FTIR results of NaOH treated hemp fiber
Figure 2 presents the FTIR spectra for both untreated
and 5% NaOH treated hemp fiber. The spectra show
various transmission bands. After 24 h of NaOH treat-
ment, the peak at 1000 cm
1
(OH group) is signifi-
cantly increased with associated hydroxyl group
available for fiber–matrix interface bonding. The reac-
tion of hydroxyl bonds with the carboxyl group is
given in the range 3200–3600 cm
1
. The peak in this
range has increased after the 24-h treatment.
The similar increases in intensity for both 1000 and
3200–3600 cm
1
band in hemp fibers with NaOH treat-
ment have also been reported in previous literature
.20
Compared to untreated fiber, the peak at 1250 cm
1
of treated hemp fiber is clearly removed. This peak
belongs to the C–O stretching of acetyl groups of
lignin. It appears that the lignin is completely removed
from the hemp fiber surface after NaOH treatment.
Also, the hemicelluloses group is partially removed
from the fiber surface after the NaOH treatment
as is evident by the decreased carbonyl peak at
1600–1650 cm
1
in treated hemp fibers.
The peak at 1740–1750 cm
1
in untreated hemp has
also been removed after the NaOH treatment. This is
due to the removal of pectin and wax that is present on
the hemp fibers. The peaks observed at 1100 cm
1
and
2850 cm
1
in untreated fibers also disappeared after
treatment. The disappearance of 1100 cm
1
peak
could be explained by the reaction of NaOH with a
secondary alcoholic group, and the peak at 2850 cm
1
Figure 3. Typical tensile stress–strain behavior of treated hemp
fiber composites.
Figure 4. Typical tensile stress–strain behavior of untreated
hemp fiber composites.
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disappeared after NaOH treatment probably due to the
removal of a methane group.
Tensile strength
The tensile strength of the hemp fiber composites with
rHDPE were determined from data obtained in accor-
dance with ASTM D638. The tensile tests were con-
ducted using the standard dog bone-shaped test
coupon having manufactured dimensions of 12.7 mm
in width, 63.5 mm in length, and a thickness of
2.5 mm. Five coupons were made from each test
sample composite. Table 2 presents a summary of the
composite materials that were evaluated, which
included composites manufactured with treated and
untreated fibers.
Typical strain–stress curves of hemp fiber compos-
ites with different fiber–matrix volume fraction are pre-
sented in Figure 3 for treated fiber composites and
Figure 4 for untreated fiber composites. It should be
noted that the axial strains were calculated based on the
displacement of the CRT’s cross-head movement and
the initial clamp spacing for each test specimen. A con-
tinual improvement in maximum tensile strength and a
reduction in strain at maximum strength were observed
with the increase in hemp fiber volume fraction for the
treated fiber composites. There is a significant improve-
ment in the tensile stress–strain behavior of the treated
fiber composites (Figure 3) compared to the untreated
fiber composites (Figure 4), which may support the
findings from the SEM and FTIR, which suggests
there is improved interfacial adhesion due to the fiber
treatment. Overall, the hemp/rHDPE composites were
well behaved with regard to their initial stiffness and
each had a distinct rupture failure ranging from 3% to
7% strain, as can be seen in Figure 5, the maximum
tensile strength for the treated hemp/rHDPE composite
with 40% of fiber volume demonstrated an approxi-
mate three time improvement from the treated hemp
composite with 20% of fiber volume fraction, yielding
an maximum strength of 60.2 MPa and a strain at max-
imum strength of 3.0. The tensile testing results of the
treated hemp fiber with recycled HDPE matrix
exceeded the previous reported data regarding hemp
fiber composites manufactured with virgin Polylactic
Acid (PLA) matrix.
21
Since these manufactured NFCs are being consid-
ered for their potential use in the civil and building
construction sector as possible structural elements,
there is an interest at understanding their low strain
behavior. Due to the nonlinear behavior of the NFCs,
a secant modulus at 2% strain was selected to evaluate
the low strain behavior and stiffness. Figure 6 presents
the secant modulus at 2% strain as a function of hemp
fiber volume fraction from 20% to 40% for both the
treated and the untreated composites. There is an
observed continuous improvement in the composite
stiffness with the increase in fiber volume fraction for
the treated fiber composites. The greatest increase in
composite stiffness was observed for the 30% hemp
Figure 5. Maximum tensile stress of hemp fiber composites.
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fiber volume fraction having an elastic modulus of
1670 MPa as compared to the secant modulus of
556 MPa for the 20% hemp fiber volume fraction.
The secant modulus at 2% strain for the treated
hemp fiber composite with rHDPE matrix with a
40% fiber volume fraction was 2574 MPa. A summary
of the tensile properties of maximum strength, strain at
maximum strength, and secant modulus at 2% strain,
which were measured during this study with their cor-
responding results for the hemp fiber composites are
presented in Table 3.
Flexural strength
Based on improved tensile strengths of the treated
hemp fiber NFCs, flexural strength testing was con-
ducted only on composites manufactured from treated
hemp fibers. The flexural strength, strain at maximum
strength, and flexural modulus at 1% and 3% strain for
these NFCs materials were tested on the CRT testing
machine in accordance with ASTM D790. Each three-
point flexural bending test was conducted using a rect-
angular test coupon having typical dimension of
25.4 mm in width, 6.35 mm in thickness, and 127 mm
in length. Five coupons were made from each test
sample composite. The same treated hemp fiber com-
posites that were manufactured for the tensile tests were
used for the flexural tests as described in Table 2.
Figure 7 presents the flexural stress of the treated
hemp/rHDPE composites as a function of the flexural
strain. The flexural strains were calculated based on the
procedure outlined in ASTM D790 using the displace-
ment of the CRT’s cross-head movement. It is interest-
ing to observe that as the fiber fraction increased, there
was a proportional increase in bending strength and
stiffness. The most significant improvement bending
strength and stiffness was observed in the higher 40%
fiber fraction composite. It can be clearly seen that with
an increase in fiber volume fraction, there is an increase
in the maximum flexural strength as is presented in
Figure 8.
Figure 6. Secant modulus at 2% strain of hemp fiber composites.
Figure 7. Typical flexural stress–strain behavior of treated
hemp fiber composites.
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Based on the potential use of these manufactured
NFCs in the civil and building construction sector as
possible structural elements, there is a need in under-
standing their low flexural strain behavior. Due to the
more uniform behavior of the NFCs, the secant mod-
ulus at 1% and 3% strain were selected to evaluate the
low flexural strain behavior and stiffness. The flexural
moduli of the composites with different fiber volume
fraction are presented in Figure 9. The results indicate
that there is an increase in composite flexural moduli
with the increase in fiber volume fraction associated
with a corresponding reduction in strain. However,
the observed moduli reduce in stiffness as the flexural
strain increases. This behavior may be beneficial with
Figure 8. Maximum flexural strength of treated hemp fiber composites.
Figure 9. Flexural modulus at 1% and 3% flexural strain of treated hemp fiber composites.
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regard to absorbing impact loadings that a building
structure may incur. A summary of the flexural strength
properties including the maximum flexural strength,
strain at maximum strength, and flexural modulus at
1% and 3% flexural strain, which were measured
during this study with their corresponding results are
presented in Table 4.
Conclusion
Hemp fiber composites with rHDPE were manufac-
tured using extrusion and compression molding process
techniques. Prior to composite fabrication, the natural
hemp fibers were treated with a NaOH solution. The
effect of alkali treatment was investigated by FTIR and
SEM. FTIR results indicated that there is an increase in
the percentage of OH groups, which may provide
more reaction sites for fiber–matrix adhesion.
Therefore, the interfacial adhesion between the fiber
and matrix may possibly be increased. Pectin, wax,
and lignin were completely removed from hemp fiber
surface, which resulted in large surface area and
improved surface roughness. The FTIR also indicated
that the hemicelluloses group was partially removed.
SEM images of treated hemp fiber support the conclu-
sions from FTIR results. SEM images of fracture sur-
face of hemp fiber composites showed clearly improved
interfacial adhesion between the hemp fiber and the
polymer matrix. The resultant composites have demon-
strated promising mechanical properties with regard to
their tensile strength, tensile modulus and strain at
maximum strength, flexural strength, and modulus for
each hemp fiber composite that has been studied. Based
on these reported experimental results, the hemp fiber–
rHDPE composites with 40% fiber volume fraction
yielded very promising results of tensile strength and
modulus and flexural strength and modulus of
60.2 MPa, 2575 MPa, 44.6 MPa, and 2429 MPa (at
1% strain), respectively. The resultant composite have
a good potential for light load applications in civil
infrastructure industry, for instance, short-span bridges
and hurricane proof panels.
Funding
This research received no specific grant from any funding
agency in the public, commercial, or not-for-profit sectors.
Acknowledgments
The authors would like to express their gratitude to National
Science Foundation ADVANCE program and UNC
Charlotte Energy Production Infrastructure Center (EPIC)
for funding support, and Shubhashini Mysore Bhogaiah for
FTIR measurements.
Conflict of interest
None declared.
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Table 4. Summary of flexural test results for hemp fiber composites
Composite
designation
Maximum
flexural
strength
(MPa)
SD
(MPa)
Strain at
maximum
strength (%)
Flexural
modulus at
1% strain (MPa)
SD
(MPa)
Flexural
modulus
at 3% strain
(MPa) SD (MPa)
rHDPE 17.8 0.8 3.1 628 27.6 474 23.7
20 Hemp/80 rHDPE 32.5 3.6 3.6 1598 177 960 105.6
30 Hemp/70 rHDPE 37.1 5.4 5.8 2015 282.1 1217 170.4
40 Hemp/60 rHDPE 44.6 8.0 6.0 2429 437.2 1485 265.8
rHDPE: fiber-recycled high-density polyethylene.
Lu et al. 9
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