polymers
Article
Plastic/Natural Fiber Composite Based on Recycled Expanded
Polystyrene Foam Waste
Wilasinee Sriprom * , Adilah Sirivallop, Aree Choodum
and Worawit Wongniramaikul
, Wadcharawadee Limsakul
Integrated Science and Technology Research Center, Faculty of Technology and Environment, Phuket Campus,
Prince of Songkla University, Kathu, Phuket 83120, Thailand; adey.siriwallop@gmail.com (A.S.);
aree.c@phuket.psu.ac.th (A.C.); wadcharawadee.n@phuket.psu.ac.th (W.L.);
worawit.won@phuket.psu.ac.th (W.W.)
* Correspondence: wilasinee.s@phuket.psu.ac.th; Tel.: +66-(0)-7627-6486
Abstract: A novel reinforced recycled expanded polystyrene (r-EPS) foam/natural fiber composite
was successfully developed. EPS was recycled by means of the dissolution method using an accessible
commercial mixed organic solvent, while natural fibers, i.e., coconut husk fiber (coir) and banana
stem fiber (BSF) were used as reinforcement materials. The treatment of natural fibers with 5% (w/v)
sodium hydroxide solution reduces the number of –OH groups and non-cellulose components in
the fibers, more so with longer treatments. The natural fibers treated for 6 h showed rough surfaces
that provided good adhesion and interlocking with the polymer matrix for mechanical reinforcement.
The tensile strength and impact strength of r-EPS foam composites with treated fibers were higher
than for non-filled r-EPS foam, whereas their flexural strengths were lower. Thus, this study has
demonstrated an alternative way to produce recycled polymer/natural fiber composites via the
dissolution method, with promising enhanced mechanical properties.
Citation: Sriprom, W.; Sirivallop, A.;
Choodum, A.; Limsakul, W.;
Wongniramaikul, W. Plastic/Natural
Keywords: natural fiber; recycled expanded polystyrene foam; natural fiber-recycled plastic composites; mechanical properties; dissolution
Fiber Composite Based on Recycled
Expanded Polystyrene Foam Waste.
Polymers 2022, 14, 2241. https://
doi.org/10.3390/polym14112241
Academic Editors: Alexey Iordanskii,
Valentina Siracusa, Michelina Soccio
and Nadia Lotti
Received: 21 April 2022
Accepted: 30 May 2022
Published: 31 May 2022
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Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
1. Introduction
Expanded polystyrene (EPS) foam is a thermoplastic that has been used to make a
wide variety of consumer products. Due to its low thermal conductivity, high compressive
strength, extremely light weight, versatility, durability, and moisture resistance, it is often
used as an insulator, in lightweight protective packaging, and in food packaging. With the
increased use of EPS foam, its waste has also increasingly accumulated around the world.
However, discarded foam waste is non-biodegradable and resistant to photolysis [1]. It can
break down when exposed to sunlight, rain, and ocean water (especially in tropical waters)
into its constituents, including styrene monomers [2] that are classified as possible human
carcinogens [3]. Styrene trimers may also increase thyroid hormone levels [4].
Fortunately, EPS foam waste has been reported as an excellent material for recycling [5].
To recycle polystyrene (PS) foams, they are typically cleaned first and then densified to
shippable logs using a thermal treatment [6]. The densified or compressed PS obtained
is then simply chopped up, heated, and recast into plastic pellets that can be used as
raw materials for plastic products [7]. Dissolution of PS foam with a suitable solvent has
become an alternative method for waste volume reduction or recycling, as it is one of the
least costly alternatives and uses less energy than melting or compressing the waste [8].
Various organic solvents, such as toluene, acetone, limonene, and other liquid hydrocarbons
have been reported for PS foam dissolution [5,8–11]. The dissolved EPS can be used for
packaging, similar to the brand new polymer [12], and various products can be produced,
e.g., nanofibers [13,14], or polymer–cement composites [5].
4.0/).
Polymers 2022, 14, 2241. https://doi.org/10.3390/polym14112241
https://www.mdpi.com/journal/polymers
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The loss of mechanical properties of the polymer during recycling has been reported,
and reinforcement with glass fibers or natural fibers is applied to conquer this problem
and extend the potential applications of recycled plastic materials [15–17]. There are recent
studies on the characterization of wood-recycled plastic composites, showing potential as
alternative materials [18,19]. Several kinds of natural fibers [20] have been reported as reinforcing fillers, e.g., banana/jute/flax fiber [21,22], kenaf/coir [23–25], and bagasse/Napier
grass fiber–polyester composites [26,27]. Nevertheless, most of the previous studies have
reinforce virgin plastic (polymer pellets) instead of recycled plastic, which may influence
mechanical properties of the filled composite.
The aim of this work was to investigate the feasibility to produce a novel composite
using EPS recycled via dissolution (r-EPS) and reinforced with natural fiber. A commercial
grade thinner containing mixed organic solvents was used, along with acetone, as a solvent
to recycle the EPS. Coconut husk fiber (coir) and banana stem fiber (BSF) were incorporated
into r-EPS to enhance the mechanical properties of the composite. The coir and BSF were
first treated with an alkaline solution and mixed with r-EPS to various filler loadings in the
composite sheets. The mechanical properties of both r-EPS and the composite sheets were
then evaluated.
2. Materials and Methods
2.1. Materials
The used and clean expanded polystyrene (EPS) foam boards were collected from
Prince of Songkla University, Phuket Campus. It was analyzed using gel permeation
chromatography (GPC) (Shodex Standard SM-105) equipped with Shodex GPC KF-806
M and KF-803 L (300 mm Length × 8.0 mm ID) using RI-Detector, obtaining the number average molecular weight (Mn ) (120,491 g/mol) and the polydispersity index (PDI)
(2.17). THF was used as the eluent at a flow rate of 1.0 mL/min at 40 ◦ C. The GPC system
was calibrated with polystyrene (PS) standards, with molecular weights ranging from
3790 to 3,053,000 g/mol. Acetone (99.98%) was purchased at the highest purity available from Fisher Chemical (Loughborough, England). Commercial thinner containing
mixed organic solvents, including toluene (70%), acetone (15.4%), ethyl acetate (4.9%),
2-butoxyethanol (3.9%), 2-propanol (2.9%), and 2-methyl-1-propanol (2.9%), was supplied
by TOA Paint (Thailand) Co., Ltd. (Samutprakan, Thailand) [28]. Coconut and banana
fibers were prepared from coconut husks and banana stems collected from a fruit garden in
Phuket, Thailand.
2.2. Dissolution of EPS Foam
An EPS foam board was broken into small pieces (30 g) before dissolving in 200 mL
of the mixed organic solvents (thinner: acetone in a 3:1 volume ratio). The mixture was
constantly stirred at 750 rpm for 4 h at room temperature to obtain a homogenous solution.
The dissolved EPS solution (r-EPS) was used for the preparation of natural fiber-reinforced
recycled EPS foam composites.
2.3. Preparation and Characterization of Natural Fiber
Coconut husk fiber (coir) and banana stem fiber (BSF) were cleaned by washing with
tap water and then dried in an oven at 100 ◦ C for 24 h. They were chopped into small
pieces and sieved to a length of 1 to 3 mm. An alkali treatment was applied on both coir
and BSF separately by immersing the chopped fibers in 5% aqueous sodium hydroxide
(NaOH) solution for 6, 12, or 24 h at room temperature. The treated fibers were vacuum
filtered and washed with distilled water until the water became neutral (pH = 7). Then, the
treated fibers were dried in an oven at 80 ◦ C for 24 h.
The chemical functionality of treated and untreated natural fibers was evaluated by
Fourier transform infrared spectroscopy (FTIR, Perkin-Elmer Frontier). Transmittance was
measured over a range from 4000 to 600 cm−1 . The surface topography and compositions of
Polymers 2022, 14, 2241
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treated and untreated natural fibers were observed by using a scanning electron microscope,
SEM Quanta 400, operated at 20 kV at 1000×.
2.4. Preparation of Natural Fiber-Reinforced Recycled EPS Foam Composites
Both untreated and treated coir and BSF were mixed with r-EPS at 2%, 5%, and 10%
by total weight to obtain natural fiber-reinforced recycled EPS foam composites. Next,
the composites were slowly poured to fill a 25 × 150 mm Petri dishes and then set aside
to dry at room temperature in the fume hood. The mass of samples was measured and
recorded until the constant mass was obtained within 72 h. With the ease of preparing the
composites using these conditions, the solvent blend could be recovered for further use as
a solvent for EPS recycling.
2.5. Mechanical Properties of Natural Fiber-Reinforced Recycled EPS Foam Composites
The tensile strength, flexural strength, and impact strength of recycled EPS foam and
recycled EPS foam natural fiber composites were investigated, as summarized in Table 1.
Each sample was cut into three specimens, and the same test was performed on these
replicates. The average test results with standard deviations (SD) are reported.
Table 1. Materials prepared from recycled EPS to evaluate the mechanical properties.
Material *
Tensile Strength
Flexural Strength
Impact Strength
r-EPS
r-EPS/u-coir (2, 5, and 10%)
r-EPS/u-BSF (2, 5, and 10%)
r-EPS/t-coir (2, 5, and 10%)
r-EPS/t-BSF (2, 5, and 10%)
X
X
X
X
X
X
X
X
X
X
X
* r-EPS = recycled expanded polystyrene foam; u-coir = untreated coir; t-coir = treated coir; u-BSF = untreated
banana stem fiber; t-BSF = treated banana stem fiber.
Tensile testing: Each sample was cut into three dumbbell-shaped specimens (gauge
length, L0 of 60 mm). The tensile test was conducted using the Instron (Model 5566)
universal testing machine with a 1kN load cell at 1 mm/min crosshead speed. The test was
continued until tensile failure occurred.
Flexural testing: Three-point bending flexural testing was carried out with an Instron
universal testing machine (Model 55R4502). The load cell and the crosshead speed were 1
kN and 1.13 mm/min, respectively, while the support span was 42 mm. The rectangular
specimens had 80 mm (L) × 12.7 mm (W) × 2.3 mm (T) dimensions.
Impact testing: Notched Izod impact testing was performed according to the ASTM
D256 standard method for determining the impact resistance of plastic samples. The testing
specimen was 65 mm (L) × 13 mm (W) × 2.5 mm (T). The depth under the notch of the
specimen was 10 mm. A pendulum energy of 1 joule was employed in the testing.
3. Results and Discussion
3.1. Characterization of Natural Fibers
The treated natural fibers are shown in Figure 1, and their chemical functionalities are
shown in Figure 2. The FTIR of both untreated BSF and coir showed characteristic peaks of
cellulose, hemicellulose, and lignin. The absorption bands at 3300 and 2910 cm−1 indicate
hydroxyl groups (–OH) and –CH stretching vibrations, respectively, from the chemical
structures of cellulose and hemicellulose. The peak at 1733 cm−1 was attributed to C=O
stretching vibrations of the acetyl group in the hemicelluloses. Adsorption at 1507, 1436,
and 1250 cm−1 was attributed to C=C aromatic symmetrical stretching, HCH and OCH in
plane bending vibrations, and C-O stretching vibrations of the acetyl groups, respectively,
and these are typical absorption peaks of lignin [29]. After alkali treatment of BSF and
coir, all absorption peaks decreased with treatment time. The absorption peaks around
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Polymers 2022, 14, 2241
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respectively, and these are typical absorption peaks of lignin [29]. After alkali treatment
these are typical
absorptionwith
peaks
of lignintime.
[29].The
After
alkali treatment
ofrespectively,
BSF and coir,and
all absorption
peaks decreased
treatment
absorption
peaks
−1 of both fibers
of BSF1733–1250
and coir, all
absorption
peaks disappeared
decreased with
treatment
time. The absorption
around
cm
after
24 h of treatment,
indicating peaks
that
−1 of
1733–1250
cm−1 of cm
both
fibers
disappeared
after 24after
h some
of 24
treatment,
indicating
that lignin
around
bothbe
fibers
disappeared
h
of treatment,
indicating
that
lignin
and1733–1250
hemicellulose
might
removed,
matching
previous
reports
[20,30–32].
and
hemicellulose
might
be
removed,
matching
some
previous
reports
[20,30–32].
lignin and hemicellulose might be removed, matching some previous reports [20,30–32].
Figure 1. Treated fibers. (a) coir, and (b) BSF.
Figure1.1.Treated
Treatedfibers.
fibers.(a)
(a)coir,
coir,and
and(b)
(b)BSF.
BSF.
Figure
Figure 2. FTIR spectra of untreated and alkali-treated BSF and coir.
Figure 2. FTIR spectra of untreated and alkali-treated BSF and coir.
The
surface
topography
of BSF
coir, both
untreated
Figure
2. FTIR
spectra
of untreated
and and
alkali-treated
BSF
and coir. and treated with 5% NaOH
solution,
was investigated,
the SEM
micrographs
are shown
Figure
3. The
The surface
topographyand
of BSF
and coir,
both untreated
and in
treated
with
5% surfaces
NaOH
of
untreated
BSF
and
coir
were
not
smooth,
spread
with
nodes,
and
covered
with
irregular
Thewas
surface
topography
and
coir, both untreated
treated3.with
NaOH
solution,
investigated,
and of
theBSF
SEM
micrographs
are shownand
in Figure
The 5%
surfaces
(Figure
3a)
that
may
represent
lignin,
hemicellulose,
or and
impurities
[33].
After
alkali
solution,
was
investigated,
and
SEM
micrographs
are shown
in
Figure
3. The
surfaces
ofstrips
untreated
BSF
and
coir
were
notthe
smooth,
spread
with nodes,
covered
with
irregular
treatment
of 3a)
both
fibers
6 h,
the
layer hemicellulose,
of
substances
on
the
fiber
surface
seemed
of untreated
BSF
and
coirfor
were
not
smooth,
spread
with nodes,
and
covered
irregular
strips
(Figure
that
may
represent
lignin,
or impurities
[33].with
After
alkalito
be
removed
(Figure
3b),
thelignin,
FTIR
results
indicating
thatsurface
lignin[33].
and
hemicellustrips
(Figure
3a)fibers
that
may
hemicellulose,
orfiber
impurities
After
treatment
of both
formatching
6represent
h, the layer
of substances
on the
seemed
toalkali
be
lose
contents
were
decreased.
Some
holes
and
rough
surfaces
were
obviously
observed,
treatment
of
both
fibers
for
6
h,
the
layer
of
substances
on
the
fiber
surface
seemed
to
removed (Figure 3b), matching the FTIR results indicating that lignin and hemicellulosebe
especially
fordecreased.
coir 3b),
fiber,
that
thesurfaces
mechanical
interlocking
of hemicellulose
the fiber
and
removed
(Figure
matching
theimprove
FTIR
results
indicating
lignin and
contents
were
Somecould
holes
and rough
werethat
obviously
observed,
espepolymer
matrix
[33–35].
However,
after
alkali
treatments
for
12
or
24
h,
the
surfaces
of
contents
were
decreased.
Some
holes and
rough surfaces
were obviously
observed,
especially
for coir
fiber,
that could
improve
the mechanical
interlocking
of the fiber
and polythe
fibers
had
become
smoother
(Figure
3c,d),
so
that
potentially,
the
interfacial
bonding
cially
for coir
fiber,However,
that couldafter
improve
thetreatments
mechanical
thesurfaces
fiber and
mer
matrix
[33–35].
alkali
forinterlocking
12 or 24 h, of
the
of polythe
with
the
polymer
matrix
would
be3c,d),
weaker
thanpotentially,
with the
6the
hor
treatment
the fibers.
It
mer had
matrix
[33–35].
However,
after
alkali
for 12
24 h, theof
surfaces
of the
fibers
become
smoother
(Figure
so treatments
that
interfacial
bonding
with
can
be
expected
that
the
alkali
treatment
of
fibers
for
longer
than
6
h
may
cause
damage
fibers
had
become
smoother
(Figure
3c,d),
so
that
potentially,
the
interfacial
bonding
with
the polymer matrix would be weaker than with the 6 h treatment of the fibers. It can be
by
hemicelluloses,
and
bound
cellulose
from
the fibers,
which
theremoving
polymer
matrix
would
belignin,
weaker
than
the than
6 h treatment
of thedamage
fibers.weakens
Itby
can
expected
that the
alkali
treatment
of fibers
forwith
longer
6 h may
cause
re-be
the
fiber
strength
[32].
Therefore,
6
h
treated
fiber
was
considered
the
most
suitable
for
expected
that the alkalilignin,
treatment
of fiberscellulose
for longer
than
h may which
cause damage
by
removing
hemicelluloses,
and bound
from
the6 fibers,
weakens
the
producing
fiber-reinforced
polymer
composites,
possibly
having
a
good
interfacial
bonding
moving
hemicelluloses,
lignin,
bound
from thethe
fibers,
weakens
the
fiber
strength
[32]. Therefore,
6 h and
treated
fibercellulose
was considered
mostwhich
suitable
for prowith
polymer
matrix
and a desirable
amount
of cellulose
on the
fiberbonding
surfaces.
fiberthe
strength
[32].
Therefore,
6 composites,
h treated
fiber
was
considered
the most
suitable
for producing
fiber-reinforced
polymer
possibly
havingexposed
a good
interfacial
The
mechanical
and
physical
properties
of
coir
and
banana
fiber
have
been
reported
ducing
fiber-reinforced
polymer
composites,
possibly
having
a
good
interfacial
bonding
with the polymer matrix and a desirable amount of cellulose exposed on the fiber surfaces.
in
thethe
literature,
summarized
in Tableamount
2 [36–39].
However,exposed
Yue et al.
reported
that
with
polymerasmatrix
and a desirable
of cellulose
on[40]
the fiber
surfaces.
the mechanical properties of many natural plant fibers may vary, to a large extent due to
inappropriate measurement.
Polymers
Polymers2022,
2022,14,
14,2241
x FOR PEER REVIEW
55 of 11
11
Figure3.
3. SEM
SEM micrographs
micrographs(at
(at1000
1000×)
Figure
×) of BSF and coir: (a) untreated, (b) 6 h treated, (c) 12 h treated,
and(d)
(d)24
24hhtreated.
treated.
and
Table The
2. Mechanical
andand
physical
properties
of the of
fibers.
mechanical
physical
properties
coir and banana fiber have been reported
in the literature, as summarized in Table 2 [36–39]. However, Yue et al. [40] reported that
Properties
Coir
Banana
the mechanical properties of many natural plant fibers may vary, to a large extent due to
Diameter
(µm)
150–250
[36]
100–250
[36]
inappropriate measurement.
3
1.2 [36,37]
Density (g/cm )
Tensile Strength (MPa)
175 [36], 131–220 [37]
Table 2. Mechanical and physical properties of the fibers.
Young’s modulus (GPa)
4–6 [36,37]
Elongation
at break (%)
30 [36], 15–30
[37]
Properties
Coir
35.1 ± 1.3 [38]
Surface energy (mJ/m2 )
0.8 [36]
161.8 [36]
8.5 [36]
2.0Banana
[36]
39.49 [39]
Diameter (μm)
150–250 [36]
100–250 [36]
Density (g/cm3)
1.2 [36,37]
0.8 [36]
3.2. Characterization of Recycled EPS Foam/Natural Fiber Composites
Tensile Strength (MPa)
175 [36], 131–220 [37]
161.8 [36]
Composite
materials
of r-EPS foam with
and coir at 2%, 5%, and8.5
10%
by weight
Young’s
modulus
(GPa)
4–6BSF
[36,37]
[36]
were Elongation
prepared using
both
the
untreated
(u)
and
the
treated
(t)
fibers
(treatment
at break (%)
30 [36], 15–30 [37]
2.0 [36] with 5%
NaOH
aqueous
solution
for
with 3 mm thickness
2) 6 h). Composite
Surface
energy
(mJ/m
35.1 ±sheets
1.3 [38]
39.49and
[39]130 mm
diameter were obtained. Their weights were increased from the total mass of EPS and fiber
by ~6%, which might be attributed to organic solvent trapping during the curing process.
3.2. Characterization of Recycled EPS Foam/Natural Fiber Composites
The distribution of fibers in the EPS matrix and the mechanical properties tensile strength,
Composite
of r-EPS
foam
with
BSF and coir
at then
2%, 5%,
and 10% by weight
flexural
strength,materials
and impact
strength
of the
composites
were
investigated.
were prepared using both the untreated (u) and the treated (t) fibers (treatment with 5%
NaOH
aqueous
solution
for 6 h).
Composite sheets with 3 mm thickness and 130 mm
3.2.1.
The
Distribution
of Fibers
in r-EPS
diameter
were
obtained.
Their
weights
increased
total mass
of loadings
EPS and
Images of both treated and untreatedwere
BSF and
coir infrom
r-EPSthe
at different
fiber
fiber
by
∼6%,
which
might
be
attributed
to
organic
solvent
trapping
during
the
are shown in Figure 4. The untreated fibers were gathered mostly at the center curing
of the
process. The
distribution
fibers
in the
EPS were
matrix
and the mechanical
properties
tensile
composite
sheet,
whereasofthe
treated
fibers
distributed
more evenly
in the matrix.
strength,
flexuralofstrength,
and impact
strength ofmatrix
the composites
then At
investigated.
This
is because
the improved
fiber-polymer
adhesionwere
[33–35].
the same
mass of the fibers mixed in the composite, the BSF spreads more thoroughly within the
3.2.1. The Distribution
of Fibers
in r-EPS
composite
sheet than does
the coir,
due to it being a greater density fiber (BSF has the
smaller
volume)
[41].
Increasing
the
fiberBSF
loading
caused
the fibers
to be fiber
moreloadings
evenly
Images of both treated and untreated
and coir
in r-EPS
at different
distributed
allFigure
over the
composite
sheet,
andwere
the fibers
weremostly
alignedatin
the
polymer
matrix.
are shown in
4. The
untreated
fibers
gathered
the
center
of the
comHowever,
it was
foundthe
thattreated
the composite
of EPS
with 10%
w/wevenly
untreated
coir
had some
posite sheet,
whereas
fibers were
distributed
more
in the
matrix.
This
excess
fibersofappearing
on thefiber-polymer
polymer surfaces.
The
results confirmed
wettability
is because
the improved
matrix
adhesion
[33–35]. Atthat
thethe
same
mass of
of
polymer
solution
was the
enhanced
by themore
alkali
treatment,within
as thethe
–OH
groups
thefibers
fibersby
mixed
in the
composite,
BSF spreads
thoroughly
composite
− Na+ groups [32,42], resulting in the reduction in the polarity of
were
modified
to
–O
sheet than does the coir, due to it being a greater density fiber (BSF has the smaller volthe
fibers
Consequently,
the treated
fibers
formevenly
stronger
interfacial
ume)
[41].[43].
Increasing
the fiber loading
caused
themore
fibersreadily
to be more
distributed
all
Polymers 2022, 14, 2241
over the composite sheet, and the fibers were aligned in the polymer matrix. However, it
was found that the composite of EPS with 10% w/w untreated coir had some excess fibers
6 of by
11
appearing on the polymer surfaces. The results confirmed that the wettability of fibers
polymer solution was enhanced by the alkali treatment, as the –OH groups were modified
to –O−Na+ groups [32,42], resulting in the reduction in the polarity of the fibers [43]. Consequently,with
the the
treated
fibersmatrix
more readily
form
interfacial
adhesionthe
with
the poladhesion
polymer
[36,43,44],
so stronger
they dispersed
throughout
composite
ymer better
matrixthan
[36,43,44],
so they dispersed
throughout
composite
sheet better
than the
the
sheet
the untreated
fibers. Maximizing
thethe
interfacial
adhesion
between
untreated
fibers.
Maximizing
the interfacial
between the
fibers and
fibers
and the
polymer
matrix would
provideadhesion
the final composite
material
withthe
thepolymer
highest
matrix would
provide the final composite material with the highest strength [34,45–48].
strength
[34,45–48].
Figure4.4. Digital
Digital photographs
photographs showing
showing the
the distribution
distribution characteristics
characteristics of
of fibers
fibers in
in the
the recycled
recycled EPS
EPS
Figure
(r-EPS)foam/natural
foam/natural fiber composite sheets prepared
prepared at
at different
different fiber
fiber loadings.
loadings.
(r-EPS)
3.2.2.
3.2.2. Mechanical
Mechanical Properties
Properties
The
tensile
strength,
The tensile strength, flexural
flexural strength,
strength, and
and impact
impactstrength
strengthofofr-EPS/natural
r-EPS/natural fiber
fiber
composites
and
r-EPS
(without
fiber)
were
evaluated
and
compared.
The
composites and r-EPS (without fiber) were evaluated and compared. The composites
compositeswith
with
untreated
strength
than
thethe
r-EPS,
whereas
composites
untreatedcoir
coir(r-EPS/u-coir)
(r-EPS/u-coir)had
hadlower
lowertensile
tensile
strength
than
r-EPS,
whereas
compowith
coir (r-EPS/t-coir)
provided
similar
tensile
strength.
On the
hand,
the
sites treated
with treated
coir (r-EPS/t-coir)
provided
similar
tensile
strength.
Onother
the other
hand,
tensile
strength
of
all
composites
with
treated
BSF
(r-EPS/t-BSF)
was
higher
than
that
of
the tensile strength of all composites with treated BSF (r-EPS/t-BSF) was higher than that
r-EPS,
whereas
the
composites
with
untreated
BSF
(r-EPS/u-BSF)
showed
reduced
tensile
of r-EPS, whereas the composites with untreated BSF (r-EPS/u-BSF) showed reduced tenstrength
(Figure
5). As
in the
section,
the alkali
treatment
of fiber
not
sile strength
(Figure
5).described
As described
inprevious
the previous
section,
the alkali
treatment
of fiber
only caused the fiber surfaces to be less polar, but also increased surface roughness, enabling
not only caused the fiber surfaces to be less polar, but also increased surface roughness,
a stronger interlocking of fibers with the polymer matrix. Therefore, the alkali treated
enabling a stronger interlocking of fibers with the polymer matrix. Therefore, the alkali
fibers should provide better interfacial adhesion with the polymer matrix than untreated
treated fibers should provide better interfacial adhesion with the polymer matrix than
fibers [27,34,36,44,48]. On the other hand, the addition of untreated fibers resulted in
untreated fibers [27,34,36,44,48]. On the other hand, the addition of untreated fibers relower tensile strength, due to the unevenness of fiber distribution in the matrix and the
sulted in lower tensile strength, due to the unevenness of fiber distribution in the matrix
weak adhesion of fibers. Furthermore, it was found that the higher content of treated BSF
and the weak adhesion of fibers. Furthermore, it was found that the higher content of
distributed in the polymer matrix decreased the tensile strength of the composites because
treated BSF distributed in the polymer matrix decreased the tensile strength of the comof fiber agglomeration by fiber–fiber interactions, more so at higher fiber loadings. Similar
posites because of fiber agglomeration by fiber–fiber interactions, more so at higher fiber
results were observed by Ibrahim et al. [45] and Ramesh et al. [49].
loadings. Similar results were observed by Ibrahim et al. [45] and Ramesh et al. [49].
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2022,14,
14,xxFOR
FOR PEER REVIEW
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Figure
5.5.Tensile
Tensile
of
r-EPS
composites
with
untreated
and
treated
BSF
or
coir
fibers
at
2%,
Figure5.
Tensilestrengths
strengthsof
ofr-EPS
r-EPScomposites
compositeswith
withuntreated
untreatedand
andtreated
treatedBSF
BSFor
orcoir
coirfibers
fibersat
at2%,
2%,
Figure
strengths
5%,
or
10%
wt.
loadings.
5%,
or
10%
wt.
loadings.
5%, or 10% wt. loadings.
In
addition,
ititcan
can
be
observed
that
the
tensile
strength
of
the
treated
Inaddition,
addition,it
canbe
beobserved
observedthat
thatthe
thetensile
tensilestrength
strengthof
ofthe
thetreated
treatedBSF
BSFcomposite
composite
In
BSF
composite
(r-EPS/t-BSF)
was
higher
than
that
of
the
coir
composite
(r-EPS/t-coir).
This
indicates
(r-EPS/t-BSF)was
washigher
higherthan
thanthat
thatof
ofthe
thecoir
coircomposite
composite(r-EPS/t-coir).
(r-EPS/t-coir).This
Thisindicates
indicates
that
(r-EPS/t-BSF)
that
that
the
BSF
provides
better
reinforcement
in
the
composite
than
coir,
potentially
because
theBSF
BSFprovides
providesbetter
betterreinforcement
reinforcementin
inthe
thecomposite
compositethan
thancoir,
coir,potentially
potentiallybecause
because
the
the
the
the
BSF
contains
more
crystalline
cellulose
[20].
The
BSF
fibers
are also
less
thick,
which
BSF
contains
more
crystalline
cellulose
[20].
The
BSF
fibers
are
also
less
thick,
which
proBSF contains more crystalline cellulose [20]. The BSF fibers are also less thick, which proprovides
better
wettability
(fewergaps
gapsatatthe
theinterface)
interface)between
between the
the fibers
fibers and
the
polymer
videsbetter
better
wettability
(fewer
andthe
thepolymer
polymer
vides
wettability
(fewer
gaps at
the interface)
between the
fibers and
matrix.
This
was
confirmed
bybythe
tear
surfaces
of of
dumbbell-shaped
specimens
after
tensile
matrix.
This
was
confirmed
the
tear
surfaces
dumbbell-shaped
specimens
after
tenmatrix. This was confirmed by the tear surfaces of dumbbell-shaped specimens after tentesting
of
both
r-EPS/5%
t-coir
and
r-EPS/5%
t-BSF
(Figure
6).
The
composite
of
treated
siletesting
testingof
ofboth
bothr-EPS/5%
r-EPS/5%t-coir
t-coirand
andr-EPS/5%
r-EPS/5%t-BSF
t-BSF(Figure
(Figure6).
6).The
Thecomposite
compositeof
oftreated
treated
sile
coir
showed
fibers
slipping
out
from
the
polymer
matrix
more
frequently
than
what
was
coirshowed
showedfibers
fibersslipping
slippingout
outfrom
fromthe
thepolymer
polymermatrix
matrixmore
morefrequently
frequentlythan
thanwhat
whatwas
was
coir
observed
in
the
composite
filled
with
treated
BSF.
observed
in
the
composite
filled
with
treated
BSF.
observed in the composite filled with treated BSF.
Figure6.6.Specimens
Specimensafter
aftertensile
tensiletesting:
testing:(a)
(a)r-EPS/5%
r-EPS/5%t-coir,
t-coir,and
and(b)
(b)r-EPS/5%
r-EPS/5%t-BSF.
t-BSF.
Figure
r-EPS/5%
t-coir,
and
(b)
r-EPS/5%
t-BSF.
Theflexural
flexuralstrength
strength
of
treated
BSF
and
coir
reinforced
r-EPS
composites
withdifdifofof
treated
BSF
and
coircoir
reinforced
r-EPS
composites
withwith
different
The
flexural
strength
treated
BSF
and
reinforced
r-EPS
composites
ferent
fiber
loadings
was
evaluated
by
three-point
bending
flexural
testing.
The
flexural
fiber
loadings
was
evaluated
by
three-point
bending
flexural
testing.
The
flexural
strength
ferent fiber loadings was evaluated by three-point bending flexural testing. The flexural
strength
ofboth
boththe
ther-EPS/t-coir
r-EPS/t-coir
andr-EPS/t-BSF
r-EPS/t-BSF
composites
wassmaller
smaller
than
that
ofthe
the
of
both the
r-EPS/t-coir
and r-EPS/t-BSF
composites
was smaller
than that
ofthat
the of
r-EPS
strength
of
and
composites
was
than
(Figure
7),
indicating
that
the
natural
fibers
diminished
bending
resistance.
This
is
possibly
r-EPS
(Figure
7),
indicating
that
the
natural
fibers
diminished
bending
resistance.
This
r-EPS (Figure 7), indicating that the natural fibers diminished bending resistance. This isis
because
orientation
of fibers may
at right
the angles
direction
thedirection
force acting
on
possiblythe
because
the orientation
orientation
of be
fibers
mayangles
be at
atto
right
angles to
toof
the
direction
of the
the
possibly
because
the
of
fibers
may
be
right
the
of
them
[49,50].
When
compared
with the
tensile test
discussed
previously,
the treated
fibers
forceacting
actingon
on
them
[49,50].When
When
compared
with
thetensile
tensile
testdiscussed
discussed
previously,
force
them
[49,50].
compared
with
the
test
previously,
added
in
the
r-EPS
enhanced
the
tensile
strength
of
the
material,
as
the
pulling
force
at
the
treated
fibers
added
in
the
r-EPS
enhanced
the
tensile
strength
of
the
material,
asisthe
the
the treated fibers added in the r-EPS enhanced the tensile strength of the material, as
right
angles
to
the
bending
force,
confirming
the
parallel
arrangement
of
the
fibers
to
the
pulling
force
is
at
right
angles
to
the
bending
force,
confirming
the
parallel
arrangement
pulling force is at right angles to the bending force, confirming the parallel arrangement
pulling
direction,
orpulling
the
perpendicular
arrangement
to the bending
direction.
In addition,
of the
the fibers
fibers
to the
the
pulling
direction, or
or the
the perpendicular
perpendicular
arrangement
to the
the
bending
of
to
direction,
arrangement
to
bending
the
flexural
strength
of
r-EPS/t-coir
was
found
to
be
greater
than
that
of
r-EPS/t-BSF
at
direction.In
Inaddition,
addition,the
theflexural
flexuralstrength
strengthof
ofr-EPS/t-coir
r-EPS/t-coirwas
wasfound
foundto
tobe
begreater
greaterthan
than
direction.
the
same
fiber
loadings.
This
may
be
attributed
to
the
fact
that
most
of
the
treated
coir
in
thatof
ofr-EPS/t-BSF
r-EPS/t-BSFat
atthe
thesame
samefiber
fiberloadings.
loadings.This
Thismay
maybe
beattributed
attributedto
tothe
thefact
factthat
thatmost
most
that
the
composite
was
aligned
in
the
direction
of
the
bending
force
[47,51],
more
so
than
in
of
the
treated
coir
in
the
composite
was
aligned
in
the
direction
of
the
bending
force
of the treated coir in the composite was aligned in the direction of the bending force
the
treated
BSF.soThe
flexural
strength
of the
composites
also increased
as
the fiber content
[47,51],
more
than
inthe
thetreated
treatedBSF.
BSF.
The
flexuralstrength
strength
ofthe
thecomposites
composites
alsoinin[47,51],
more
so than
in
The
flexural
of
also
increased
from
2%
tocontent
5%, butincreased
it decreased
to the
lowest
levelitfor
10% of either
treated
fiber.
creased
as
the
fiber
from
2%
to
5%,
but
decreased
to
the
lowest
level
creased as the fiber content increased from 2% to 5%, but it decreased to the lowest level
for10%
10%of
ofeither
eithertreated
treatedfiber.
fiber.This
Thisisislikely
likelydue
dueto
tothe
theless
lessuniform
uniformfiber
fiberdistribution
distributionwith
with
for
Polymers 2022, 14, x FOR PEER REVIEW
Polymers 2022, 14, x FOR PEER REVIEW
Polymers 2022, 14, 2241
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8 of 11
8 of 11
greater fiber loading in the polymer matrix. Similar results were obtained for the tensile
greater fiber loading in the polymer matrix. Similar results were obtained for the tensile
test,
as
This
is described
likely duepreviously.
to the less uniform fiber distribution with greater fiber loading in the
test, as
described
previously.
polymer matrix. Similar results were obtained for the tensile test, as described previously.
Figure 7. Flexural strength of r-EPS composites with treated BSF or coir at 2%, 5%, or 10% wt. fiber
Figure 7. Flexural strength of r-EPS composites with treated BSF or coir at 2%, 5%, or 10% wt. fiber
loadings.7. Flexural strength of r-EPS composites with treated BSF or coir at 2%, 5%, or 10% wt.
loadings.
fiber loadings.
The impact strength of the composites was found to be increased from that of r-EPS
The impact strength of the composites was found to be increased from that of r-EPS
(Figure 8), and it increased with the loading of treated fibers. Moreover, the r-EPS/t-coir
(Figure 8), and it increased
increased with
with the
the loading
loading of
of treated
treated fibers.
fibers. Moreover,
Moreover,the
ther-EPS/t-coir
r-EPS/t-coir
composite showed higher impact strength than the r-EPS/t-BSF composite at the same
composite at
at the same
composite showed higher impact strength than the
the r-EPS/t-BSF
r-EPS/t-BSF composite
loading. This indicates that the coir may absorb greater force during the impact test than
This indicates
indicates that
that the coir may absorb greater force during the impact test than
loading. This
the BSF. Since the coir is larger in size or lower in density than the BSF, the greater volume
density than
than the
the BSF,
BSF, the greater volume
the BSF. Since the coir is larger in size or lower in density
of the coir presents in the composite at the same mass added [41].
of the coir presents in the composite at the same mass added [41].
[41].
Figure8.8.Impact
Impact
strength
r-EPS
composites
with
treated
or at
coir
at 5%,
2%,or5%,
orwt.
10%
wt.
Figure
strength
of of
r-EPS
composites
with
treated
BSF BSF
or coir
2%,
10%
fiber
Figure 8. Impact strength of r-EPS composites with treated BSF or coir at 2%, 5%, or 10% wt. fiber
loadings.
fiber loadings.
loadings.
Conclusions
4.4.Conclusions
4. Conclusions
This study
study has
has demonstrated
demonstrated the
the production
production of
of r-EPS
r-EPS foam
foam composites
composites reinforced
reinforced
This
This
study
hasbydemonstrated
theprocess.
production
ofnatural
r-EPS foam
composites
reinforced
with
coir
and
BSF
the
dissolution
The
fibers
were
treated
with
5%
with coir and BSF by the dissolution process. The natural fibers were treated with 5%
(w/v)
with coir
and BSF
by12„
theor
dissolution
process.
The natural
fibers
were
treated
with
5% (w/v)
(w/v)
NaOH
for
6,
24
h.
The
FTIR
analyses
revealed
that
the
number
of
hydroxyl
NaOH for 6, 12,, or 24 h. The FTIR analyses revealed that the number of hydroxyl groups
NaOH
for
12,, or 24 h. The
FTIR analyses
revealed
that the number
of hydroxyl groups
groups
and6,non-cellulose
components
in the
fibers decreased
with treatment
and
and
non-cellulose
components
in the fibers
decreased
with treatment
time, and time,
SEM imand
non-cellulose
components
in
the
fibers
decreased
with
treatment
time,
and
SEM
imSEM imaging
thattreatment
alkali treatment
modified
thestructure.
fiber structure.
6 h treated
BSF
aging
showed showed
that alkali
modified
the fiber
The 6 The
h treated
BSF (2%
aging
showed
that alkali
treatment
modified
the fiber
The 6 hintreated
(2%
(2%
wt.)
reinforced
r-EPS foam
composite
provided
the structure.
greatest increase
tensile BSF
strength
wt.) reinforced r-EPS foam composite provided the greatest increase in tensile strength by
wt.)
reinforced
r-EPS foam
composite
providedreinforced
the greatest
increase
in tensile
strengthcoir
by
by about
70%, whereas
r-EPS
foam composite
with
10% wt.
of 6 h treated
Polymers 2022, 14, 2241
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showed the maximum increase in impact strength, by about 210% compared to r-EPS. This
was due to the good adhesion and interlocking of treated fibers with the polymer matrix.
This study presented an alternative method to produce recycled polymer/natural fiber
composites via the dissolution method, with promising enhanced mechanical properties.
Author Contributions: Conceptualization, W.S.; data curation, W.S., A.C., W.L. and W.W.; formal
analysis, W.S. and A.C.; funding acquisition, W.S. and W.L.; investigation, A.S.; methodology, W.S.;
project administration, W.S.; resources, W.S.; supervision, W.S.; validation, W.S. and W.L.; visualization, A.S.; writing—original draft, W.S.; writing—review and editing, W.S., A.C., W.L. and W.W. All
authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Faculty of Technology and Environment, Prince of Songkla
University, Phuket Campus, and the Graduate School, Prince of Songkla University.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: All data are available from the corresponding author on reasonable request.
Acknowledgments: The authors would like to thank the English correction service of the Research
and Development Office (RDO), Prince of Songkla University (Seppo Karrila).
Conflicts of Interest: The authors declare no conflict of interest.
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