Effect of sisal fiber filler on thermal properties of bio-based polyurethane composites
Effect of sisal fiber filler on thermal properties of bio-based polyurethane composites
Ewa Głowin´ ska
This work is mainly focused on study of thermal and thermomechanical properties of obtained bio-based polyurethane (coded as bio-PU) composites via using different types of bio-components (bio-glycol, modified soybean oil and sisal fiber) in the procedure. The chemical structure, morphology and mechanical properties were also investigated and described in this manuscript in order to know more perfect characterization of produced composites. The bio-based polyurethane matrix of composites was synthesized via prepolymer method. Bio-PU composites were produced by dispersing 5 and 15 mass% of sisal fibers into the polyurethane matrix during their synthesis. To investigate the thermal stability of sisal fibers and bio-PU composites, the thermogravimetric method (TG) was used. Thermomechanical tests were performed by means of dynamic mechanical analysis (DMA). Based on the results of thermomechanical analysis, it was found that the sisal fibers amount has the impact on storage and loss modulus. Chemical structure was confirmed by FTIR spectra. Mechanical results and scanning microscopy images of the composites showed good interfacial adhesion between sisal fibers and the bio-based PU matrix.
Sisal fiber; Silanization; DMTA; Bio-polyurethane composites; Thermal stability
Bio-based composites are constantly under the unremitting
research attention. These materials are almost used in every
sector of industry. The vegetable oil-based polyurethanes
belong to these material groups, too. Vegetable oils (e.g.,
soybean or rapeseed oil) have become an attractive
resource that can be converted to bio-based polyols to
produce PU . The properties of the bio-based composites
including vegetable oil-based polyurethanes despite the
bio-based matrix are influenced by a number of variables as
form and size of fillers or processing conditions and
methods. Natural fiber fillers, which consist of cellulose,
hemicelluloses and lignin, are commonly used to produce
bio-based composites. The most important advantages of
these types of fibers are availability,
environmentalfriendly character, low price, good mechanical properties,
biodegradability. Nevertheless, there is some weakness
associated with poor compatibility and interfacial adhesion
between natural fibers and polymers. The low resistance to
moisture absorption made the use of natural
fiber-reinforced composites less attractive. In order to eliminate
these disadvantages, fibers are treating with
suitable chemicals, e.g., silane, alkaline or benzyl compounds
[2–4]. The knowledge about the thermal decomposition
process of natural fibers could help to predict fiber-based
composites process and estimate the influence on
composite properties by thermal decomposition of natural
fibers. The thermal stability of the fibers can be studied
using thermogravimetric analysis. The kinetic modeling of
the decomposition and also the knowledge about activation
energy are meaningful for a proper prediction of the
behavior of the materials under different working
conditions and the critical energy needed to start a
decomposition reaction, respectively . The thermal decomposition
process of common natural fibers was described in the
Well-known, naturally occurring fiber which can be
successfully used in composites production is sisal,
obtained from the Agave sisalana leaves. This fiber is
produced in the tropical regions such as Mexico, Brazil,
Tanzania, Kenya, Madagascar and China. Depending on
the size and form, natural filler has crucial factors on the
mechanical and thermal behavior of bio-composites [7, 8].
Sisal fiber represents the lignocellulosic fibers group,
except such as jute, coir, pineapple, bamboo, hemp, flax,
cotton, banana and straw fiber which can be used as
reinforcements in different polymers. This natural fiber
hydrophilic in nature and polymers which are hydrophobic
are characterized by poor interfacial compatibility [9, 10].
The modification of the natural fibers surface usually leads
to improving the adhesion between fiber and matrix and
enhances the physical and mechanical properties of
obtained composites. Treatment of natural fibers is
beneficial in order to brighten the water resistance of fibers or
enhance the wettability of the natural fiber surface by
polymers. The investigation of sisal fibers treatment
methods and sisal-based composites was subject of many
research papers. Many chemical methods are used in
surface modification, such as mercerization, silanization,
benzylation [2, 3, 11, 12]. The physical modification
methods, e.g., cold plasma treatment or corona treatment,
were used as well .
Silanization is one of the fiber surface treatment methods,
and it is conducted by using silane compounds [14, 15]. Prior
used, the silanes react with water or alcohol, in order to
obtain hydroxyl-silane groups (Si–OH), as coupling agent.
The presence of these groups leads to direct condensation
with hydroxyl groups in the natural fiber and reaction with
other functional groups derived from polymer matrix .
Silanization does not provide to the destruction of the natural
fiber. In the result, the thermally stable material with good
adhesion to the polymer matrix can be obtained . Overall
silanes belong to the chemical compound which is efficient
coupling agents extensively used in composites and
adhesive formulations, e.g., in inorganic filler-reinforced
polymer composites such as glass fiber or mineral fillers. The
bifunctional structures of silanes have also been of interest in
applying them for natural fiber/polymer composites since
both glass fibers and natural fibers bear reactive hydroxyl
groups . Aminosilanes, especially
c-aminopropyltriethoxysilane (APS), are often used. Vinyl- or acryl-silane
modifies fibers by covalent bonds in the presence of
superoxide indicators .
Martin et al.  investigated the thermal degradation
stability of the sisal fiber (Agave sisalana) and its
constituents, cellulose, hemicellulose and lignin, exactly. The
researchers indicated that depending on the atmosphere
condition the exothermic peaks from DSC measurements
under the nitrogen atmosphere with the maximum at 297 C
were assigned to hemicellulose degradation. The
endothermal peaks at 365 C were assigned to degradation of
cellulose. Peaks indicated thermal degradation of the sisal fibers
and their constituents, at air atmosphere, exhibited of
twostep decomposition as visible two exothermic peaks. The
first step appeared at 376 C for raw sisal and at 351 C for
cellulose was the second step of the degradation appeared at
476 and 466 C, respectively. TG measurements carried out
under nitrogen atmosphere showed that the sisal raw sisal
fiber decomposed in two steps. The first step was assigned to
the hemicellulose degradation temperature, while the second
step was attributed to cellulose degradation. Similar results
were indicated also by Alvarez and Vazquez .
Kim and Netravali  have been investigated the
chemical modification of sisal fiber (Agave sisalana) and
their properties. Series of composites containing a different
type of sisal fibers were prepared. The effect of the tension
on the mechanical properties of the modified by
mercerization sisal fibers was tested as well. First, the sisal fibers
were modified by soaking in the 2 M sodium hydroxide
solution under the ambient condition with desired tensions
for 2 h. Moreover, the mercerized fibers without tension
were also prepared. The results indicated that tensile stress
and Young’s modulus of the fibers increase with increasing
tension weight up to 50 g per fiber. Tensile stress for
untreated fibers amounted to 283.5 MPa and the Young’s
modulus—5.2 GPa. For the sample with mercerized fibers
without tension, their parameters were: ca. 339 MPa and
6.1 GPa, respectively. Kim and Netravali  have used
modified sisal fiber to investigate the effect of the tension
on the mechanical properties of the sisal fibers-reinforced
composites prepared with the use of soy protein
concentration (SPC)-based resin. Composites with untreated
fibers, sisal fibers mercerized with tension (50 g of mass
per fiber) and slack-mercerized were prepared. Tensile
strength and Young’s modulus increased for mercerized
fibers, and the highest parameters revealed composite
containing sisal fibers mercerized with tension. The
researchers explained the reinforcing effect of the
mercerized sisal fiber as the impact of the solvent on the
fiber chemical content, which caused the reduction in the
hemicellulose and lignin. Cellulose is the polymer with the
highest crystallinity level which confirms the reinforcing
effect of the treated sisal fibers in the composites.
Sreekumar et al.  focused their work on the
improvement in sisal fibers/polyester interaction by using
various modifications methods including silanization. The
silane treatment of fibers caused increase in the tensile
strength and modulus of sisal-based composites. The SEM
of silane-treated fiber shows the presence of macropores,
rougher fiber surface and fibrillation.
Valadez-Gonzalez et al.  have been studying the
effect of fiber surface treatment on the fiber–matrix bond
strength of natural fiber-reinforced composites. Among
tested modification methods (alkaline and
preimpregnation), the silanization treatment of heneque´n (Agave
fourcroydes) fibers was carried out. The fiber surface
silanization results in a better interfacial load transfer
efficiency but did not improve the wetting of the fiber. The
silane treatment of the fiber surface increased the tensile
strength of the analyzed composite material from 21 to
27 MPa. Depositing a silane coupling agent to henequen
fibers has shown that adhesion between the natural hard
fibers and matrix plays an important role on the final
mechanical properties of the composite.
Barreto et al.  have described the properties of
cardanol-based bio-composites containing sisal fibers (from
Agave sisalana plant) treated by alkali solution. Based on the
results, it was found that the chemical treatment improved
the thermal stability of the mass loss process for sisal treated
with NaOH solution (5 or 10%) when compared to
nonmodified sisal fiber in its raw state. A variation at 15 C was
also observed when comparing the composite reinforced by
raw sisal with composite reinforced with sisal treated by
NaOH 10%. Bio-composites decomposed in three stages.
The first stage of thermal decomposition for bio-composites
reinforced with raw fiber occurred at 341 C, while for
treated fibers with NaOH occurred at 344 C. The second
stage for bio-composites with non-modified sisal was
recorded at 438 C, and modified sisal with NaOH 5% at the
range from 440 to 444 C. The last stage reported a
significant difference in temperature for non-modified sisal
(527 C) and sisal treated with NaOH solution (535 and
542 C) which indicates the better diffusion of the resin
inside the fibers treated with alkali solution.
In our previously work, effect of high hydroxylated
soybean oil (H3) content on the structure and mechanical
properties of polyurethane composites containing sisal fibers
was described . This work is mainly focused on thermal
and thermomechanical characterization of composites
containing non-modified and modified sisal fibers dispersed in
polyurethane matrix. The reason of sisal modification was to
enhance the thermal stability of fiber and, thereby, the
thermal stability of bio-polyurethane composites.
Materials and methods
The bio-based polyurethanes (bio-PU) obtained via the
prepolymer method were filled with the non-modified sisal
fibers (WN) and silanized (WS) sisal fibers (International
Fiber Corporation, Belgium). The sisal fibers length was
about 1500 lm and width 300 lm. The fibers were
degreased and dried prior to use. Prepolymer was
synthesized from diisocyanate MDI (BorsodChem, Hungary) and
a polyol mixture consisting of 75 mass% of commercial
polyether PTMG, Mn *2000 (Overlack, Poland) and
25 mass% of hydroxylated soybean oil H3, (Gdan´sk
University of Technology, Poland). The reaction was
conducted at 80 C for 1 h. During the second stage of the
process, the prepolymer chains were extended by using
bio-1,3-propanediol (DuPont, USA) with 0.3 mass% of
1,4-diazabicyclo[2.2.2]octane (DABCO, Merck, Germany)
as a catalyst. The molar ratio [NCO]prepolymer/
[OH]chain extender equaled 1:1. The bio-polyurethanes were
filled with 5 and 15 mass% of sisal fibers, and later samples
were molded using a hydraulic press. The procedure of
silanization modification was typical and was going
through treatment of the vinyltriethoxysilane solution on
sisal fibers. Procedure details were described in our
previous work .
TG Thermogravimetric analysis was performed in order
to characterize the thermal stability of the modified and
non-modified sisal fibers and prepared bio-polyurethane
composites. All specimens were tested with the use of
NETZSCH TG 209F3 analyzer. Samples about ca. 7 mg
were measured in the temperature range from 35 to 600 C
with the heating rate of 20 K min-1. Measurements were
taken under the nitrogen atmosphere. Thermogravimetric
(TG) and differential thermogravimetric (DTG) curves for
sisal fibers and bio-polyurethane composites were plotted
as the results of the TG analysis.
Dynamic mechanical analysis The dynamic mechanical
thermal analysis of bio-polyurethane composites containing
various amount of non-modified or silanized sisal fibers was
performed using DMA Q800 Analyzer (TA Instruments).
Rectangular samples with dimension ca. 30 9 10 9 2 mm
(length 9 width 9 thickness) were measured in the
temperature range from -100 to 150 C, where the heating rate
of 4 C min-1 was taken. As mechanical stress during
measurements, the single torsion mode was used. Tests were
executed under a nitrogen atmosphere at the operating
frequency of 10 Hz. In order to obtain the mechanical
characteristics of the prepared bio-polyurethanes, the variation
of storage modulus, E0 versus temperature, loss modulus, E00
versus temperature and Tan d versus temperature was
plotted. DMA tests were carried out for verifying the influence
of the sisal fiber modification on the composites
thermomechanical properties. Furthermore, this measurement
allowed to determine the glass transition temperature of the
soft segments of prepared bio-PU composites, which was
assumed as the transition in the Tan d curve. The storage
modulus E0 is provided to information regarding the material
stiffness and the stored energy. Loss modulus E00 is used to
measure the energy dissipated as heat, while loss factor (Tan
d) measures the degree of molecular motion .
FTIR spectroscopy Spectroscopic analysis of
bio-polyurethanes was carried out by means of a FTIR Nicolet 8700
spectrophotometer (Thermo Electron Corporation) and ATR
technique. The Specac Heated Golden Gate single-reflection
diamond ATR accessory was used. Spectra were registered
at room temperature for wavenumbers ranging from 500 to
4500 cm-1 at 4 cm-1 nominal resolution with 64 scans.
SEM analysis The microscopic analysis was conducted
on the cross section of samples using a Quanta FEG
scanning electron microscope under the following
conditions: magnification between 500 and 10,000, the
Everhart–Thornley detector (ETD) and a high vacuum at
30.00 kV at a working distance of 10 mm. Samples were
coated with gold prior to microscopic observation.
Tensile test The mechanical analysis in static condition
was conducted by the tensile test. Tensile strength (TSb)
and elongation at break (Eb) were recorded by using Zwick
Z020 tensile-testing machine according to EN ISO
standard 527-1:1996 and 527-2:1996. Dumbbell-shaped
samples were tested with normalized dimensions.
Measurements were taken at 50 mm min-1 speed rate.
Investigations were carried out at the room temperature.
Hardness Hardness (H) was measured according to the
PN-EN ISO 868:2005 standard. The circular samples
(U56 9 6 mm) were placed on a flat surface, and ten
measurements were taken per sample by applying a Shore
A durometer for 3 s.
Rebound resilience Rebound resilience was determined
by means of a rebound tester in accordance with the ISO
4662:2009 standard. Rebound resilience is measured by
dropping a free-falling pendulum hammer from a given
height, which impacts the sample. The amount of energy is
shown on a pendulum scale. Ten measurements were
recorded per sample.
Abrasion resistance Abrasion resistance (V) was tested
with an Abrasion Check instrument (Gibitre instruments) in
accordance with the ISO 4649:2010 standard. The value of
abrasion resistance was calculated according to the
where m1—the mass of the sample before the test (g); m2—
the mass of the sample after the test (g); 0.2—the required
mass loss of the standard sample (g); q—the density of the
analyzed material (g cm-3); and Dmw—the arithmetic
mean of the mass loss of three standard samples (g).
Density Density (d) was determined according to the
PNEN ISO 1183-1:2013-06 standard. The measurements were
performed in methanol at 23 ± 2 C.
Results and discussion
Thermal properties of sisal fibers (non-modified and
silanized), bio-PU composites containing 15 mass% of sisal
fibers (WN or WS) and reference material (matrix) were
determined by thermogravimetric analysis. Based on the
obtained data, the effect of sisal fiber modification and its
admixture on the thermal decomposition of the composite
materials was described below (Figs. 1–4).
Thermal decomposition of sisal fibers took place in two
steps (Fig. 4). Thermogravimetric analysis of the
nonmodified fibers (WN) and silanized fibers (WS) allowed
confirming similarity in the thermal degradation curve
curse. Figures 1 and 2 present the mass loss (%) and speed
of the mass loss (% min-1), respectively, of both of the
used fibers type. Differences visible in Fig. 1 are connected
with water content in the treated sisal fibers which led to
gently enhanced mass loss and little faster speed of the
mass loss. T5%, T10% and T50% for non-modified sisal fibers
were noted at 83, 255 and 360 C, while for silanized sisal
50 100 150 200 250 300 350 400 450 500 550 600
Fig. 1 Mass loss versus temperature plotted for the non-modified and
silanized sisal fibers
Fig. 2 DTG versus temperature plotted for the non-modified and
silanized sisal fibers
fibers at 100, 278 and 365 C, respectively. Figure 2 shows
distinct peaks, where the first peak appeared at 297 C and
the second peak appeared at 365 C. First step attributed to
the hemicellulose degradation which constitutes one of the
sisal fiber components. The second step corresponded to
the cellulose degradation which is the sisal fiber constituent
exhibited in the highest amount of the fiber. The same
results were obtained by Martin et al.  and also by
Alvarez and Vazquez . Figure 2 shows speed of mass
loss during thermogravimetric measurements. Silanization
of the sisal fibers allowed to improve their thermal
properties in bio-PU comparison with non-modified sisal fibers.
Silanized sisal fibers revealed lower speed of mass loss
than non-modified fibers.
Figures 3 and 4 show the TG and DTG, respectively, of
the prepared bio-PU composites and reference specimen.
Based on the results, it was found that composites
containing chemical-modified sisal fibers revealed similarity in
the thermal stability compared to non-modified sisal
fiberbased composite (Fig. 3). In the case of the thermal
Fig. 3 TG curves of bio-PU composites containing 15 mass% of
non-modified (WN) and silanized sisal fiber (WS), and reference
Fig. 4 DTG curves of bio-PU composites containing 15 mass% of
non-modified (WN) and silanized sisal fiber (WS), and reference
decomposition of the bio-PU composites containing sisal
fibers, the processes took place in three steps. In
comparison with the matrix, the difference is connected with
residues after the test, which appeared with higher the
value for composites containing sisal fibers. The residues at
ca. 600 C derived from ash, which constituents are
organic and inorganic impurities. Figure 4 presents the
DTG curves of the prepared composites. The first step
appeared at ca. 325 C is assigned to the decomposition of
urethane bonds in hard segments (isocyanate component
and a low molecular weight extender) . The second
step appeared at 353 C for bio-polyurethane composites
prepared with the use of untreated sisal fibers and at 379 C
for silanized sisal fiber-based composites. The little peaks
appeared in the temperature range from 340 to 380 C is
associated with decomposition of hemicellulose, lignin and
cellulose derived from the sisal fibers. In the case of the
reference sample, the board peak is caused by hydroxylated
soybean oil residue derived from polyol mixture. The third
step appeared at ca. 430 C for each polyurethane materials
is associated with the elastic segment degradation derived
from polyols polyurethanes constituents. Similar results at
the peaks appearance were obtained also by Pinto et al.
. Although the decrease in the sisal fiber speed of mass
loss by silanization, there is no visible improvement in
loss . The admixture of fibers resulted in a slightly
increasing speed of mass loss in comparison with the
Dynamic mechanical analysis
The dynamic mechanical behavior of the bio-PU
composites containing different amount and type of sisal fibers was
investigated with the use of dynamic mechanical analysis.
The variation of the storage modulus, E0 versus
temperature, variation of the loss modulus, E00 versus temperature,
Fig. 5 Storage modulus as a function of temperature for bio-PU
composites filled with non-modified (WN) and silanized sisal fiber
(WS), and reference material (REF)
and variation of tangent delta, Tan d versus temperature
were recorded (Figs. 5–7). In Table 1 the determined
parameters are correlated. It was observed that the sisal
fibers strongly influence on thermomechanical properties of
the prepared bio-PU composites. All curves indicated
distinct double-peak curves. DMTA results showed that
composites containing non-modified sisal fiber revealed the
decrease in the storage modulus (E0), loss modulus (E00)
and tangent delta (Tan d) with increasing fiber amount. The
most similar curves to the reference sample exhibited
composite with 5 mass% non-modified sisal fiber content,
which indicates a quite good interfacial bond strength and
adhesion between the matrix and the fiber (Fig. 5). In the
case of silanized sisal fiber-based composites regardless of
modified sisal fiber content, the composites are
characterized by similar thermomechanical behavior.
All curves indicated the linear behavior in the
temperature range from -100 to -50 C. The underlying cause of
this behavior is the presence of polyurethane matrix, which
occurs in the glassy state at the temperature range
mentioned above. At the temperature up to -50 C decreasing
trend is observed. This tendency correlates with the alpha
transition temperature . The midpoint (Fig. 5) of the
decreasing curve curses at the temperature range from -60
to ca. –20 C (ca. -40 C) is associated with the glass
Fig. 6 Loss modulus as a function of temperature for bio-PU
composites filled with non-modified (WN) and silanized sisal fiber
(WS), and reference material (REF)
transition temperature of the soft segments derived from
polyols mixture contribution while the temperature peak
assigned as the midpoint of the temperature range from
-20 to ca. 20 C (ca. 0 C) indicated on the soft segments
melting temperature, TmSS . These points are strongly
connected with peaks visible in the variation of Tan d
versus temperature graph (Fig. 6).
The storage modulus curves at the second temperature
range indicated the lower values for bio-based composites
correspond to reference sample. These results confirmed
that there is no reinforcing effect of the sisal fibers on the
The loss modulus (E00) strongly depended on amount and
type of sisal fibers. The E00 values decreased with increasing
sisal content (Fig. 6). The composite materials are
characterized by the loss modulus in the range from 78 to 169 MPa.
The significant differences were observed for the composite
sample, with 15 mass% of non-modified sisal fiber; this
value is the lowest one compared to rest materials. The
elevated values of loss modulus of the obtained composites
are advantageous because less energy is irreversibly lost by
the materials . These results confirm also the decreasing
tendency in the hardness and rebound resilience properties
of the prepared bio-PU composites.
Table 1 DMTA properties of bio-polyurethane composites containing non-modified (WN) and silanized sisal fiber (WS), and reference material
Fiber type and content/mass%
Fig. 7 The Tan d as a function of temperature for bio-PU composites
filled with non-modified (WN) and silanized sisal fiber (WS), and
reference material (REF)
Table 1 shows the summary of the DMTA results of the
prepared bio-PU composites. The lowest value of the glass
transition temperature revealed matrix and composite with
15 mass% content of the non-modified sisal fiber
(-42 C). The highest value exhibited composite with
5 mass% content of the non-modified sisal fiber (-36 C).
The value of Tan d is correlated with the cross-linking level
between macromolecular chains (Fig. 7). The decreasing
trend with growing sisal fiber content in the composites
suggested the decline in the cross-linking in the prepared
materials. The loss factor (Tan d) indicates the damping
ability of the material, which is the ratio of the mechanical
dissipation energy and the storage energy. For good
damping materials, the intensity of these peaks should be
high . The values of Tan d of all investigated samples
were between 0.1 B Tan d B 0.2, which indicates that
synthesized materials have got good enough ability to
damping capacity; nevertheless, with increasing sisal fiber
content this ability is slightly declining .
Table 1 shows the DMTA properties of
bio-polyurethane composites containing non-modified (WN) and
silanized sisal fiber (WS) and reference material (REF).
The chemical structure of bio-based polyurethane
composites was investigated by FTIR-ATR analysis and is presented
in Fig. 8. Registered spectra of bio-PU composites
containing 15 mass% of sisal fibers (WN or WS) and
biopolyurethane matrix (REF) are similar to each other.
Characteristic for polyurethanes–urethane group
vibrations registered at 3313 cm-1 corresponds to the stretching
vibration of NH bond and at 1599 cm-1 is attributable to
the out of plane bending of NH. The stretching vibration of
CN was registered at 1533 cm-1 . The valent vibration
of carbonyl group present in the structure of synthesized
Fig. 8 FTIR-ATR spectra of bio-based polyurethane composites
containing 15 mass% of non-modified (WN) and silanized sisal fiber
(WS), and reference material (REF)
bio-polyurethane matrix and composites both for free and
hydrogen-bonded carbonyl groups manifests itself as a
multiplet at a wavenumber range from 1703 to 1736 cm-1,
while the stretching vibration of C=O group occurs at
1221 cm-1. The stretching vibrations of C–O–C group
were registered at 1103 and 1065 cm-1; the vibrations
correspond to free C–O–C group and hydrogen-bonded C–
O–C groups, respectively. The wavenumbers at 2854 and
2918 cm-1 correspond to the stretching vibrations of CH2
groups, while the wavenumber value at 1414 cm-1
interprets as deformation vibrations of the same groups .
In order to characterize the adhesion between sisal fiber
and polyurethane matrix, the cross sections of the prepared
bio-polyurethane composite samples were assessed by the
scanning electron microscopy. It is clearly visible that the
orientation of the short sisal fibers in the bio-polyurethane
matrix was disordered organized. It seems that fibers were
caked in polyurethanes matrix. Figure 9 shows that the
non-modified sisal fibers revealed sharp edges at the
matrix. These SEM images indicated on the insufficient
adhesion between sisal fiber and bio-PU matrix. This
Fig. 9 Bio-PU composites containing non-modified sisal fiber (WN)
in amount of 5 mass%
Fig. 10 Bio-PU composites containing silanized sisal fiber(WS) in
amount of 5 and 15 mass%
conclusion is supported also by the free, small spaces
observed along the sisal fibers. Consequently, the presence
of these spaces decreases the possibility to bear the external
stress by the composite. In Fig. 10 the SEM images of
bioPU composites containing 5WS and 15WS silanized sisal
fiber are presented. There is clearly visible better caked
sisal fiber distribution in comparison with non-modified
fibers. We found that the fiber modification improved
adhesion between polymer matrix and fiber. Similar results
were obtained also by Manikandan Nair et al. , Kim
and Netravali  and Jacob et al. , who investigated
the modified sisal fiber adhesion with polymer matrix used
the sisal fibers after such modification as benzylation,
mercerization and alkali treatment, respectively. In Fig. 10
the SEM graphs exhibit bubbles which derived probably
from moisture as residue from silanization or dampness
from the atmosphere. The bubbles occurrence caused stress
concentration which results in faster material breaking.
This conclusion proved also the results of the mechanical
In order to characterize the prepared composites, their
mechanical properties were investigated. The results of
tensile strength and elongation at break are shown in
Table 2. Moreover, hardness, rebound resilience, abrasion
resistance, density were also studied. Bio-PU composites
containing sisal fibers, non-modified (WN) and silanized
(WS) revealed similar results in terms of tensile tests. The
addition of the fibers generated the tensile strength and
elongation at break reduction in compassion to reference
sample (REF), bio-polyurethane matrix. Nevertheless, in
the case of the bio-PU composites containing silanized
sisal fiber, with increasing sisal fiber, the increasing tensile
strength was noted. Bio-polyurethane composite prepared
with the silanized sisal fibers usage revealed similarity in
the tensile strength (ca. 9.50 MPa) to each other, despite
the non-modified sisal fiber-based composites exhibited
considerable distinction (12.19 and 9.43 MPa for 5 and
15 mass% fiber content, respectively). The elongation at
break of the bio-polyurethanes composites was decreasing
with increasing sisal content which was excepted.
Bio-polyurethane composites containing dispersed sisal
fiber are characterized by lower hardness values than
reference sample—bio-polyurethane matrix. Comparing the
influence of sisal fiber type on the bio-polyurethane
hardness, it was observed that modification slightly changed this
parameter. The highest impact on the hardness reduction had
addition of the 5 mass% of the non-modified sisal fibers.
Overall, it was noted that with increasing fiber content
hardness also increases and the bio-polyurethane composites
have similar hardness to the reference sample (Table 2).
Rebound resilience decreased with growing sisal fiber
addition. Some higher reduction was visible for bio-PU
composites filled with silanized sisal fibers. The results are
connected with the decrease in the mobility of polymer
chain with the growing amount of sisal fiber. The growing
fiber content resulted in harder and stiffer composites,
which better assimilate the energy (Table 2).
Based on the abrasion resistance test results, it was found
that the measured parameters decreased with increasing sisal
fibers content, especially in the case of modified fibers. It
was noted that both types of the sisal fibers (non-modified
and silanized) addition caused decrease in the density,
especially in the case of bio-polyurethane composites filled
with silanized fibers (Table 2).
Table 2 Mechanical properties of the obtained bio-based polyurethane composites containing non-modified (WN) and silanized sisal fiber
(WS), and reference material (REF)
Fiber type and content/mass%
Series of bio-PU composites containing dispersed diverse
quantity of sisal fibers (non-modified and silanized) were
successfully obtained and analyzed. Based on
thermogravimetric, thermomechanical and mechanical results, it
was found that the sisal fibers can be added to the bio-based
polyurethane matrix in the maximum quantity of
15 mass% and obtained materials characterize of good
properties. Generally, thermal decomposition of bio-PU
composites took place in three steps. Moreover, the
chemical modification of sisal fibers slightly increases the
thermal stability of sisal fibers and bio-PU composites and
follows with lower rate. Overall, based on the analyzed
DMA data, it can be stated that the higher silanized sisal
fibers content had a quite positive effect in the whole range
of temperature. Regardless of modified sisal content, the
composites are characterized by similar thermomechanical
properties. On the whole, sisal fibers addition to the
biopolyurethane matrix provided to decrease in the composites
mechanical properties compared to reference sample. It
was found that with increasing sisal fiber in the bio-PU
composites, the tensile strength and elongation at break
decreased. In the case of bio-PU composites containing
silanized fibers, the tensile strength of these materials was
similar although fibers content. In the case of comparison
between both bio-PU composites types there were no
significant differences in the value of hardness, rebound
resilience and abrasion resistance.
Acknowledgements The authors wish to thank DuPont Company
(USA) and International Fiber Corporation (Belgium) for kindly
providing the bio-glycol and sisal fibers.
Open Access This article is distributed under the terms of the
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appropriate credit to the original author(s) and the source, provide a
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