Abstract
This study involved the preparation and characterization of isotactic polypropylene (iPP),
sisal-fibre-reinforced polypropylene composite (SF-PC), treated sisal-fibre-reinforced
polypropylene composite (TSF-PC), silicon dioxide (SiO2) nanoparticles-reinforced
polypropylene composite (NS-PC), and dual filler-reinforced polypropylene composite (DFPC),
through the melt-processing technique.
Prior to the compounding process, sisal-fibres were modified by using sodium hydroxide
(NaOH) to improve the network microstructure, thermal stability, and water resistance.
Importantly, the alkali (NaOH) modification of the fibres led to the removal of the
amorphous component of the fibre. Hence, the untreated sisal fibre (USF) sample contained a
network of microstructure with micro-void in the fibre and the outer surface also not smooth,
while the alkali treated sisal fibre (TSF) sample exhibited surface roughness morphology, i.e.
irregular surface and not smooth. The C-O stretching vibration of the acetyl groups of lignin
in the USF vanished after the alkali treatment of SF. In this context, the PP-based USF
composites were not prepared, only TSF reinforcement materials were considered for PPbased
compounding. On the other hand, nanosilica (NS) particles were used as received, for
the compounding process.
In brief, the TSF were incorporated into PP at weight-to-weight ratios of 90/10, 80/20, and
70/30, and the resulting composites were characterized with various instruments. On the other
hand, the incorporation of NS particles in the same polymer at varied weight fractions of 1, 2,
and 3% was done separately. Moreover, the PP nanocomposites containing the hybrid fillers
of 3% NS and 10% TSF with their added functionalities of high degree of dispersion,
physical cross-linking, to simultaneously improve the PP-based material were prepared
through the melt-processing technique and were subsequently, characterized. In addition, the
PP nanocomposites containing the dual fillers, demonstrated the potentials for 3D-printing
filament construction and recyclability.
The morphology, together with the structural properties of the polymer nanocomposites were
investigated by using the: scanning electron microscopy (SEM), transmission electron
microscope (TEM), Fourier-transform infrared spectroscopy (FTIR) and X-ray Diffraction
(XRD) techniques; mechanical properties by conducting the: tensile test, Charpy impact test
and dynamic mechanical analysis (DMA); thermal properties by using the: thermogravimetric
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analysis (TGA), differential scanning calorimetry (DSC), and water absorption tests were
also conducted.
The addition of TSF, NS and the dual filler in PP resulted in moderated decreasing of tensile
strength together with impact toughness. TSF decreased the tensile strength from 42.3 MPa at
10 weight % TSF to 39.9 MPa at 30 weight % TSF. The NS nanocomposite containing 3%
reinforcement decreased the tensile strength from 45.5 MPa of neat PP to 42.6 MPa of the
reinforced PP. The dual reinforcement decreased the tensile strength from 45.5 MPa of neat
PP to 36.46 MPa at 3%-10% NS/TSF.
The nanocomposites containing the hybrid filler, 3%-10% NS/TSF in neat PP, showed that
the addition of the reinforcement, improved the elongation-at-break from 28% to a maximum
value of 639%. Agreeably, the tensile modulus for the SF-PC, presented an increase with the
addition of TSF at all weights %, because of the better interaction between the polymer
matrix and the reinforcement. The incorporation of NS in PP, showed increased ductility
characteristics as well and a well-proportioned tensile modulus of 1356 MPa at 3 weight %.
The 90PP-10TSF composite material exhibited the highest storage modulus (Eˈ) of about
4.5x1010 Pa because of the optimum distribution of TSF in the polymer matrix then start
decreasing at all test temperatures from -40 ℃ to 80 ℃. There was also a remarkable storage
modulus improvement amongst all the NS-incorporated PP nanocomposites. That is, with
increasing NS quantity, the storage modulus was most favorable or advantageous. At 3
weight % it was above 4.50x1010 Pa then start decreasing at all test temperatures from -30 ℃
to 75 ℃. Furthermore, the addition of TSF enhanced the tan delta for the TSF-PC sample in
comparison with the neat PP sample.
DSC secondary heating thermograph depicted that the addition of TSF had no much effect on
the melting temperature of the TSF-PC samples, while the cooling thermograph showed that
the addition of TSF to the polymer matrix, impacted slightly the crystallization temperature,
thereby suggesting a potential packing of the polymer chains. The 70PP-30TSF sample
showed the highest water absorption (more than 2%), followed by the 80PP-20TSF (less than
2%) and 90PP-10TSF (less than 1.5%) in 15 days, whereas the neat PP sample showed the
lowest absorption.
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The crystallinity level or percentage of the NS containing nanocomposites was reduced (from
neat PP to 2 weight % as 45.61%, 44.44%, 44.32% then increased for 3 weight % as
50.91%), based on the outcome of the nanoparticles inhibition on the packing structure or the
well-ordered molecules of the polymer. The relationships between the structure and the
property were interpreted, based on the mechanical data obtained. Based on the 30% and 50%
loss in weight of the reinforced composites, which were lower than the pristine PP matrix, the
results of their thermal stability decreased with increasing NS contents in the PP
nanocomposites. Nonetheless, it increased at a 30% loss in weight for the quantity that was
lower in silica in the PP nanocomposite (PP-1%NS). It was observed that the water
imbibition improved with increasing nanosilica. The increasing imbibition is associated with
the micro-voids produced in the process of ageing. Micro-voids behave as a mechanism for
absorption of water.
Most importantly, the results revealed that the incorporation of the hybrid filler to PP,
improved the composites thermomechanical properties, with a maximum elongation-at-break
of 639%. However, with a decreased tensile strength of 36.46 MPa at 3%-10% NS/TSF
compared to 45.5 MPa neat PP. Most importantly, the PP-based and the dual filler
nanocomposite materials were reprocessed to produce 3D-printing filaments. The 3D-printing
filaments confirmed the successful recyclability of PP and the dual filler nanocomposites,
paving the way for the translation of this nanocomposite into 3D printable specimen for
potential applications.