ArticlePDF Available

A New Strategy to Produce Hemp Fibers through a Waterglass-Based Ecofriendly Process

Authors:

Abstract and Figures

Natural fibers such as kenaf, hemp, flax, jute, and sisal have become the subject of much research as potential green or eco-friendly reinforcement composites, since they assure the reduction of weight, cost, and CO2 release with less reliance on oil sources. Herein, an inexpensive and eco-friendly waterglass treatment is proposed, allowing the production of silica-coated fibers that can be easily obtained in micro/nano fibrils through a low power mixer. The silica coating has been exploited to improve the chemical compatibility between fibers and the polymer matrix through the reaction of silanol groups with suitable coupling agents. In particular, silica-coated fibers easily functionalized with (3-Aminopropyl) triethoxysilane (APTS) were used as a filler in the manufacturing of epoxy-based composites. Morphological investigation of the composites through Scanning Electron Microscopy (SEM) demonstrated that the filler has a tendency to produce a web-like structure, formed by continuously interconnected fibrils and microfibrils, from which particularly effective mechanical properties may be obtained. Dynamic Mechanical Analysis (DMA) shows that the functionalized fibers, in a concentration of 5 wt%, strongly affect the glass transformation temperature (10 °C increase) and the storage modulus of the pristine resin. Taking into account the large number of organosilicon compounds (in particular the alkoxide ones) available on the market, the new process appears to pave the way for the cleaner and cheaper production of biocomposites with different polymeric matrices and well-tailored interfaces.
Content may be subject to copyright.
materials
Article
A New Strategy to Produce Hemp Fibers through a
Waterglass-Based Ecofriendly Process
Aurelio Bifulco , Brigida Silvestri * , Jessica Passaro , Luca Boccarusso , Valentina Roviello,
Francesco Branda and Massimo Durante
Department of Chemical Materials and Industrial Production Engineering (DICMaPI), University of Naples
Federico II, P.leTecchio 80, 80125 Naples, Italy; aurelio.bifulco@unina.it (A.B.); jessica.passaro@unina.it (J.P.);
luca.boccarusso@unina.it (L.B.); valentina.roviello@unina.it (V.R.); branda@unina.it (F.B.);
mdurante@unina.it (M.D.)
*Correspondence: brigida.silvestri@unina.it
Received: 4 March 2020; Accepted: 8 April 2020; Published: 14 April 2020


Abstract:
Natural fibers such as kenaf, hemp, flax, jute, and sisal have become the subject of much
research as potential green or eco-friendly reinforcement composites, since they assure the reduction
of weight, cost, and CO
2
release with less reliance on oil sources. Herein, an inexpensive and
eco-friendly waterglass treatment is proposed, allowing the production of silica-coated fibers that
can be easily obtained in micro/nano fibrils through a low power mixer. The silica coating has been
exploited to improve the chemical compatibility between fibers and the polymer matrix through
the reaction of silanol groups with suitable coupling agents. In particular, silica-coated fibers easily
functionalized with (3-Aminopropyl) triethoxysilane (APTS) were used as a filler in the manufacturing
of epoxy-based composites. Morphological investigation of the composites through Scanning Electron
Microscopy (SEM) demonstrated that the filler has a tendency to produce a web-like structure, formed
by continuously interconnected fibrils and microfibrils, from which particularly eective mechanical
properties may be obtained. Dynamic Mechanical Analysis (DMA) shows that the functionalized
fibers, in a concentration of 5 wt%, strongly aect the glass transformation temperature (10
C increase)
and the storage modulus of the pristine resin. Taking into account the large number of organosilicon
compounds (in particular the alkoxide ones) available on the market, the new process appears to pave
the way for the cleaner and cheaper production of biocomposites with dierent polymeric matrices
and well-tailored interfaces.
Keywords:
hemp fibers; waterglass; ecofriendly production; epoxy composites; silica coating;
web-like structure
1. Introduction
Recently, natural fibers such as kenaf, hemp, flax, jute, and sisal have become the subject of much
research as potential green or eco-friendly reinforcement composites, since they assure the reduction of
weight, cost, and CO
2
release with less reliance on oil sources [
1
3
]. The life cycle assessment (LCA)
method supports the environmental, social and economic advantages through the use of such fibers [
4
].
Microfibrillated celluloses (MFC) were obtained for the first time via wood pulp through repeated
passages in a high-pressure homogenizer [
3
,
5
9
]. Specifically, the fibers were disintegrated into fibrils
and microfibrils through mechanical action. MFC, also known as nanofibrillated cellulose (NFC) and
cellulose nanofibers or nanofibrils (CNF), were later produced in a diameter of 5 nm, from various
natural sources [
1
,
3
,
5
,
6
] through homogenization, microfluidization, microgrinding and cryocrushing.
There were, however, three severe drawbacks: mechanical energy consumption, fibers entanglement,
and clogging of the mechanical apparatus [
3
,
5
,
6
]. A large amount of the produced CNF with some
Materials 2020,13, 1844; doi:10.3390/ma13081844 www.mdpi.com/journal/materials
Materials 2020,13, 1844 2 of 11
non-fibrillated residual fibers was obtained, with the degree of fibrillation dependent on the mechanical
energy consumption [3,6].
Biological and chemical pre-treatments have made CNF production easier and more attractive
for commercial applications. Mild cellulose hydrolysis, catalyzed by some enzymes, promotes
fibrillation [
3
,
5
], whereas the introduction of negatively charged groups (through carboxylation via
TEMPO-mediated oxidation or via periodate–chlorite oxidation, sulfonation and carboxymethylation)
or positively charged groups (quaternization) on the cellulosic fibers [3,5] enhances delamination.
Alternative methods can be used to produce products of superior properties, which will hopefully
be discovered [3,5].
Another severe drawback is the hydrophilic character of cellulose and the formation of a strong
network held by hydrogen bonds [
3
,
5
,
10
12
]. For this reason, surface modification is necessary to
obtain good dispersion and compatibility in non-polar polymer matrices. These methods are well
described in the literature [3,5,1017].
In this paper, a new pretreatment that allows hemp fibers with diameters ranging from tens of
microns to tens of nanometers with the aid of a low power mixer is described. The method exploits
recent findings [
18
] that demonstrated the formation of a silica-based coating, following the use of
inexpensive and ecofriendly waterglass solutions. The silica-based coating demonstrated resistance
to washing and was also able to act as a protective and thermal shield. Fourier Transform Infrared
(FTIR) and solid-state Nuclear Magnetic Resonance (NMR) analysis strongly supported the formation
of –C–O–Si– covalent bonds between the coating and the cellulosic substrate. In this paper it is
shown that when eectively prolonging this eco-friendly process, the fabric becomes brittle and easily
produces silica coated hemp fibers with the aid of a low power mixer. The silica coated fibers could be
comfortably functionalized with (3-Aminopropyl) triethoxysilane (APTS) and then dispersed in epoxy
resin. The functionalized fibers were easily dispersed in epoxy resin and, in a concentration of 5 wt%,
strongly aected the glass transformation temperature (10
C increase) and the storage modulus of the
pristine resin.
Finally, it is worth remembering that nowadays, there is a large number of organosilicon
compounds available on the market, allowing the easy compatibilization of silica and polymer matrices
of very dierent chemical natures. Therefore, biocomposites with well-tailored interfaces and dierent
polymeric matrices can be obtained through similar cheap, clean production, as described in the current
paper, using an appropriate silane coupling agent (e.g., APTS).
2. Experimental
2.1. Materials
Sodium metasilicate (waterglass), 3-aminopropyltrimethoxysilane (APTS) hydrochloric acid and
Ninydrin (37% ACS) reagents were purchased from Sigma-Aldrich (St. Louis, MO, USA). Plain weave
hemp fabrics (grammage: 160 g m
2
, supplied by MAEKO S.r.l., Milan, Italy) and a two-component
epoxy resin system (SX10 by MATES S.r.l., Milan, Italy) were used for fabricating composite laminates.
The two components are reported by the supplier to be modified bisphenol A resin and modified
cycloaliphatic polyamines. The hemp fabric is reported by the producer to have been submitted to
several treatments: 1) 1.5/2 h in 0.2% soap solution; 1.5/2 h in 0.2% soda Solvay solution; 1.5/2 h in 0.1%
sodium hydroxide solution; 1.5/2 h in 1.5 gr/lt hydrogen peroxide solution.
The cellulose and lignin content were estimated to be equal to 70% and 5%–6% respectively.
2.2. Production of Functionalized Hemp Particles
Hemp fabric sheets were submitted to iterated soaking–drying cycles. In each cycle, the hemp
fabric sheets were soaked for 20 min in a diluted (0.01 M) solution of waterglass (Na
2
SiO
3
) acidified up
to Ph =2.5 with hydrochloric acid. The sheets were left to dry for 15 min at 80 C.
Materials 2020,13, 1844 3 of 11
After 30 cycles, the dry sheets could be easily torn and then reduced to powder with the aid of a
low power (350 W) mixer (IMETEC S.p.A., Azzano San Paolo, Bergamo, Italy). When the motion was
similar to wadding, water was added (i.e., 20 g of hemp in 100 ml of water) to successfully complete
the grinding. The obtained pulp was dispersed in a waterglass solution 0.1 M acidified up to pH =2.5
with hydrochloric acid and stirred for 24h (i.e., 20 g of hemp in 500 ml of solution). The suspension
was left to settle and the supernatant was substituted with distilled water in a stirred condition.
This operation was repeated three times. The hemp pulp was also washed three times in centrifuge
with water/ethanol mixtures (50% in volume). After drying at 80
C, the hemp was reduced to a powder
in the 350 W mixer. The powders were dispersed in water and ultrasonicated (20 kHz, 900–1000 W,
30 min). The final product was washed (three times) with a solution of EtOH/Water (50/50, vol/vol) in
centrifuge (10,000 rpm, 10 min, Thermo Fisher Scientific S.p.A., Waltham, MA, USA) and dried for
24 h at 40
C. The functionalization with surface amino groups has been performed by dispersing
the particles (indicated in the following as Hemp_SiO
2
) in a solution of EtOH/Water (80/20, vol/vol),
containing 3-aminopropyltrimethoxysilane (APTS) (10 vol.%) and subsequently acidified up to pH =5
with acetic acid. The functionalization occurs after only one soaking/drying cycle (soaking time/drying
time =20 min/10 min). The final product is then washed with a solution of EtOH/Water (50/50, vol/vol)
through three centrifugation cycles (10,000 rpm, 10 min). Finally, the APTS functionalized fibers
(indicated in the following as Hemp_SiO
2
_APTS) were dried for 24h at 80
C. Scheme 1shows a sketch
of the new fiber modification strategy.
Materials 2018, 11, x FOR PEER REVIEW 3 of 11
similar to wadding, water was added (i.e., 20 g of hemp in 100 ml of water) to successfully complete
the grinding. The obtained pulp was dispersed in a waterglass solution 0.1 M acidified up to pH =
2.5 with hydrochloric acid and stirred for 24h (i.e., 20 g of hemp in 500 ml of solution). The
suspension was left to settle and the supernatant was substituted with distilled water in a stirred
condition. This operation was repeated three times. The hemp pulp was also washed three times in
centrifuge with water/ethanol mixtures (50% in volume). After drying at 80 °C, the hemp was
reduced to a powder in the 350 W mixer. The powders were dispersed in water and ultrasonicated
(20 kHz, 900–1000 W, 30 min). The final product was washed (three times) with a solution of
EtOH/Water (50/50, vol/vol) in centrifuge (10,000 rpm, 10 min, Thermo Fisher Scientific S.p.A.,
Waltham, MA, USA) and dried for 24 h at 40 °C. The functionalization with surface amino groups
has been performed by dispersing the particles (indicated in the following as Hemp_SiO2) in a
solution of EtOH/Water (80/20, vol/vol), containing 3-aminopropyltrimethoxysilane (APTS) (10
vol.%) and subsequently acidified up to pH = 5 with acetic acid. The functionalization occurs after
only one soaking/drying cycle (soaking time/drying time = 20 min/10 min). The final product is then
washed with a solution of EtOH/Water (50/50, vol/vol) through three centrifugation cycles (10,000
rpm, 10 min). Finally, the APTS functionalized fibers (indicated in the following as
Hemp_SiO2_APTS) were dried for 24h at 80°C. Scheme 1 shows a sketch of the new fiber
Scheme 1. Overall scheme of the new fiber modification strategy to produce APTS functionalized
filler.
2.3. Synthesis of Composites
The synthesis of the composites epoxy/APTS functionalized hemp fibers was performed
involving the following two steps:
1. Mixtures of epoxy DGEBA (Bisphenol A diglycidyl ether) and amino functionalized fibers, with
weight percentages of hemp particles equal to 1, 2, 5 wt%, were stirred vigorously at 80°C for
2h in a closed system to allow the reaction between the primary amino groups and oxirane
rings of epoxy resin.
2. The amount of hardener needed for the curing (26 wt% of the epoxy resin) was added to the
mixture at room temperature and mixed for 5min, subsequently poured into a Teflon® mold.
The samples were cured for 24 h at 30 °C and post-cured for 4h at 80°C.
2.4. Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Analysis (EDX)
A Leica Stereoscan 440 Microscope (20 kV) (Leica Microsystems Cambridge Ltd., Cambridge,
UK), equipped with an energy Dispersive Analytical System (EDX) from Inca Energy 200, by using
AZtecEnergy EDS Software (v2.1, Oxford Instruments, Abingdon, UK, 2006) was used.
Scheme 1.
Overall scheme of the new fiber modification strategy to produce APTS functionalized filler.
2.3. Synthesis of Composites
The synthesis of the composites epoxy/APTS functionalized hemp fibers was performed involving
the following two steps:
1.
Mixtures of epoxy DGEBA (Bisphenol A diglycidyl ether) and amino functionalized fibers, with
weight percentages of hemp particles equal to 1, 2, 5 wt%, were stirred vigorously at 80
C for 2 h
in a closed system to allow the reaction between the primary amino groups and oxirane rings of
epoxy resin.
2.
The amount of hardener needed for the curing (26 wt% of the epoxy resin) was added to the
mixture at room temperature and mixed for 5min, subsequently poured into a Teflon
®
mold.
The samples were cured for 24 h at 30 C and post-cured for 4 h at 80 C.
Materials 2020,13, 1844 4 of 11
2.4. Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Analysis (EDX)
A Leica Stereoscan 440 Microscope (20 kV) (Leica Microsystems Cambridge Ltd., Cambridge, UK),
equipped with an energy Dispersive Analytical System (EDX) from Inca Energy 200, by using
AZtecEnergy EDS Software (v2.1, Oxford Instruments, Abingdon, UK, 2006) was used.
2.5. Fourier Transform Infrared Spectroscopy (FTIR)
The spectra were acquired with a Nikolet 5700 FTIR spectrometer (Thermo Fisher,
Waltham, MA, USA) (KBr pellets) with a resolution of 4 cm
1
and 32 scans and Thermo Scientific
OMNICSoftware Suite (v7.2, Thermo Fisher, Waltham, MA, USA, 2005).
2.6. Ninhydrin Test
A small quantity of the APTS functionalized fibers is soaked into a solution of ninhydrin
(2,2-dihydroxyindane-1,3-dione) EtOH/Water (80/20, vol/vol) solution. The presence of primary amino
groups can be easily highlighted. In fact, it reacts with ninhydrin forming a Schibase, with the
appearance of a characteristic blue-violet-color [19].
2.7. Dynamic Mechanical Analysis (DMA)
DMA tests were carried out according to ASTM D4065-01 using a TA (RSA III) instrument in
a three-point bending configuration, adopting an oscillation frequency of 1 Hz under controlled
sinusoidal strain. The samples (50 ×12.5 ×3 mm3) were heated to 200 C at 3 C/min.
3. Results and Discussion
3.1. Preparation and Characterization of Hemp Fibers
Hemp fabrics were iteratively soaked into acidified waterglass solutions following the procedure
described in Section 2.2 and sketched in Scheme 1. The treatment is similar to the one reported in a
previous paper [
18
] where it was shown that the obtained silica-based coating was washing resistant
and able to act as a protective and thermal shield. Fourier Transform Infrared (FTIR) and solid-state
Nuclear Magnetic Resonance (NMR) analysis well supported [
18
] the formation of –C–O–Si– covalent
bonds between the coating and the cellulosic substrate. The hemp fabric is also the same as the one
used in the previous work [
18
] where SEM micrograph and Thermogravimetric Analysis (TGA) are
reported. In the latest paper, the eect of reiterating the exposure is described and exploited to obtain
micro/nano fibers.
Figure 1shows how hemp fabric mass changes (as wt%) with the number of soaking–drying
cycles. The treatment makes the fabrics progressively lose softness in terms of mechanical behavior.
Materials 2018, 11, x FOR PEER REVIEW 4 of 11
2.5. Fourier Transform Infrared Spectroscopy (FTIR)
The spectra were acquired with a Nikolet 5700 FTIR spectrometer (Thermo Fisher, Waltham,
MA, USA) (KBr pellets) with a resolution of 4 cm1 and 32 scans and Thermo Scientific™ OMNIC™
Software Suite (v7.2, Thermo Fisher, Waltham, MA, USA, 2005).
2.6. Ninhydrin Test
A small quantity of the APTS functionalized fibers is soaked into a solution of ninhydrin
(2,2-dihydroxyindane-1,3-dione) EtOH/Water (80/20, vol/vol) solution. The presence of primary
amino groups can be easily highlighted. In fact, it reacts with ninhydrin forming a Schiff base, with
the appearance of a characteristic blue-violet-color [19].
2.7. Dynamic Mechanical Analysis (DMA)
DMA tests were carried out according to ASTM D4065-01 using a TA (RSA III) instrument in a
three-point bending configuration, adopting an oscillation frequency of 1 Hz under controlled
sinusoidal strain. The samples (50 × 12.5 × 3 mm3) were heated to 200 °C at 3 °C/min.
3. Results and Discussion
3.1. Preparation and Characterization of Hemp Fibers
Hemp fabrics were iteratively soaked into acidified waterglass solutions following the
procedure described in Section 2.2 and sketched in Scheme 1. The treatment is similar to the one
reported in a previous paper [18] where it was shown that the obtained silica-based coating was
washing resistant and able to act as a protective and thermal shield. Fourier Transform Infrared
(FTIR) and solid-state Nuclear Magnetic Resonance (NMR) analysis well supported [18] the
formation of –C–O–Si– covalent bonds between the coating and the cellulosic substrate. The hemp
fabric is also the same as the one used in the previous work [18] where SEM micrograph and
Thermogravimetric Analysis (TGA) are reported. In the latest paper, the effect of reiterating the
exposure is described and exploited to obtain micro/nano fibers.
Figure 1 shows how hemp fabric mass changes (as wt%) with the number of soaking–drying
cycles. The treatment makes the fabrics progressively lose softness in terms of mechanical behavior.
After 30 cycles, the hemp fabrics became brittle enough to be easily torn up. Therefore, they
may comfortably be reduced to powder with the aid of a low power (350 W) mixer, as previously
described in the experimental section.
Figure 1. Percentage change as a function of the number of soaking–drying cycles.
In Figure 2, the SEM micrograph (at different magnification) of the obtained powders is shown.
As can be seen, fibers of diameter ranging from tens of microns to tens of nanometers are obtained.
In order to explain the results, it is worth reminding that plant fibers do possess a hierarchical
structure [20]. They consist of elementary fibers corresponding to single cells of 1–50 mm in length
Figure 1. Percentage change as a function of the number of soaking–drying cycles.
Materials 2020,13, 1844 5 of 11
After 30 cycles, the hemp fabrics became brittle enough to be easily torn up. Therefore, they may
comfortably be reduced to powder with the aid of a low power (350 W) mixer, as previously described
in the experimental section.
In Figure 2, the SEM micrograph (at dierent magnification) of the obtained powders is shown.
As can be seen, fibers of diameter ranging from tens of microns to tens of nanometers are obtained.
In order to explain the results, it is worth reminding that plant fibers do possess a hierarchical
structure [
20
]. They consist of elementary fibers corresponding to single cells of 1–50 mm in length and
10–50
µ
m in diameter. The central lumen allowing water uptake is surrounded by several cell walls
consisting of cellulose microfibrils (10–30 nm in diameter) embedded in a hemicellulose-lignin matrix.
In particular, they dier due to the composition of the matrix and the orientation of the microfibrils.
Each microfibril consists of 30–100 cellulose molecules in extended chain conformation.
Materials 2018, 11, x FOR PEER REVIEW 5 of 11
and 10–50 µm in diameter. The central lumen allowing water uptake is surrounded by several cell
walls consisting of cellulose microfibrils (10–30 nm in diameter) embedded in a hemicellulose-lignin
matrix. In particular, they differ due to the composition of the matrix and the orientation of the
microfibrils. Each microfibril consists of 30–100 cellulose molecules in extended chain conformation.
(a) (b)
(c)
Figure 2. Micrographs of the fibers at different magnifications: (a) general overview at 300 µm (b)
and increasing details at 40 µm (c) and 3 µm.
Recently the authors [18] proved that the exposure of hemp fabrics to inexpensive and
ecofriendly 0.1 M waterglass solutions allowed the formation of a well anchored silica-based
coating, additionally resistant to washing. Fourier Transform Infrared (FTIR) Spectroscopy and solid
state Nuclear Magnetic Resonance (NMR) analysis [18] demonstrated the formation of –C–O–Si–
covalent bonds as a result of the condensation reaction on the silica coating/cellulose interface.
Therefore, the above-described results may be due to the use of a lower concentration (i.e, 0.01 M)
combined with an elongation of the exposure time (see Section 2.3) [18]. This methodology may
allow a deep penetration of the waterglass aqueous solution through the hierarchical structure of
hemp fibers. The formation of the silicate layer could be a result of the observed brittleness and
effortless size reduction, in a low power mixer, to fibers of diameter from microns to tens of
nanometers. The obtained silica coated hemp fibers were easily functionalized with APTS as
described in Section 2.2.
In Figure 3, the SEM micrograph of hemp fiber following APTS functionalization is shown. As
can be seen, the general aspect of the fibers is similar to the one observed in Figure 2. Figure 3 shows
the EDX spectrum confirming the presence of nitrogen on the surface of the fibers.
Figure 2.
Micrographs of the fibers at dierent magnifications: (
a
) general overview at 300
µ
m (
b
) and
increasing details at 40 µm (c) and 3 µm.
Recently the authors [
18
] proved that the exposure of hemp fabrics to inexpensive and ecofriendly
0.1 M waterglass solutions allowed the formation of a well anchored silica-based coating, additionally
resistant to washing. Fourier Transform Infrared (FTIR) Spectroscopy and solid state Nuclear Magnetic
Resonance (NMR) analysis [
18
] demonstrated the formation of –C–O–Si– covalent bonds as a result
of the condensation reaction on the silica coating/cellulose interface. Therefore, the above-described
results may be due to the use of a lower concentration (i.e, 0.01 M) combined with an elongation of the
Materials 2020,13, 1844 6 of 11
exposure time (see Section 2.3) [
18
]. This methodology may allow a deep penetration of the waterglass
aqueous solution through the hierarchical structure of hemp fibers. The formation of the silicate layer
could be a result of the observed brittleness and eortless size reduction, in a low power mixer, to
fibers of diameter from microns to tens of nanometers. The obtained silica coated hemp fibers were
easily functionalized with APTS as described in Section 2.2.
In Figure 3, the SEM micrograph of hemp fiber following APTS functionalization is shown. As can
be seen, the general aspect of the fibers is similar to the one observed in Figure 2. Figure 3shows the
EDX spectrum confirming the presence of nitrogen on the surface of the fibers.
Materials 2018, 11, x FOR PEER REVIEW 6 of 11
Figure 3. Micrograph of hemp fiber after APTS functionalization and EDX spectrum.
The presence of amino groups on the surface of the functionalized fibers is also well supported
by the ninhydrin test and FTIR spectra. In fact, when a small quantity of the treated microfibers was
added to the ninhydrin solution (see Section 2.6), the characteristic blue-violet color appeared.
Finally, the occurrence of the amino functionalization is further confirmed by FTIR spectra
reported in Figure 4, which reports the spectra of hemp fibers (Hemp_SiO2 sample) before a) and
after b) functionalization with APTS (Hemp_SiO2_APTS sample). In detail, spectrum a) proves that
the hemp fibers are produced with a silica coating. In fact, spectrum a) is similar to the one
previously reported [18], thus differing from the hemp example for the presence of three bands; two
of them are due to the well-known stretching vibration of silicate structure, the third one resulting
from the condensation reaction between OH groups of hemp surfaces and silanols from silica
resulting in the appearance of C-O-Si bonds [18,21,22]. As can be seen, the exposure to the APTS
solution leads to the presence of the NH2 stretching vibration band on the FTIR spectrum [23].
Figure 4. Spectra of a) hemp fabric after functionalization with waterglass (Hemp_SiO2 sample), b)
the spectrum recorded after the functionalization with APTS (Hemp_SiO2_ APTS sample).
3.2. Composite Epoxy/APTS functionalized Hemp Fibers
3.2.1. SEM Observations
Figure 5 shows SEM micrographs of the epoxy/APTS functionalized hemp fibers at different
magnifications. Figure 5a suggests the formation of a web-like structure formed by fibrils and
microfibrils. As better highlighted in Figure 5b, the epoxy phase appears to completely coat the
entire surface of the fibers, due to a better chemical compatibility between the polymer matrix and
the functionalized filler. The formation of this web-like structure could be ascribable to the good
Figure 3. Micrograph of hemp fiber after APTS functionalization and EDX spectrum.
The presence of amino groups on the surface of the functionalized fibers is also well supported
by the ninhydrin test and FTIR spectra. In fact, when a small quantity of the treated microfibers was
added to the ninhydrin solution (see Section 2.6), the characteristic blue-violet color appeared.
Finally, the occurrence of the amino functionalization is further confirmed by FTIR spectra
reported in Figure 4, which reports the spectra of hemp fibers (Hemp_SiO
2
sample) before a) and after
b) functionalization with APTS (Hemp_SiO
2
_APTS sample). In detail, spectrum a) proves that the
hemp fibers are produced with a silica coating. In fact, spectrum a) is similar to the one previously
reported [
18
], thus diering from the hemp example for the presence of three bands; two of them
are due to the well-known stretching vibration of silicate structure, the third one resulting from the
condensation reaction between OH groups of hemp surfaces and silanols from silica resulting in the
appearance of C-O-Si bonds [
18
,
21
,
22
]. As can be seen, the exposure to the APTS solution leads to the
presence of the NH2stretching vibration band on the FTIR spectrum [23].
Materials 2018, 11, x FOR PEER REVIEW 6 of 11
Figure 3. Micrograph of hemp fiber after APTS functionalization and EDX spectrum.
The presence of amino groups on the surface of the functionalized fibers is also well supported
by the ninhydrin test and FTIR spectra. In fact, when a small quantity of the treated microfibers was
added to the ninhydrin solution (see Section 2.6), the characteristic blue-violet color appeared.
Finally, the occurrence of the amino functionalization is further confirmed by FTIR spectra
reported in Figure 4, which reports the spectra of hemp fibers (Hemp_SiO2 sample) before a) and
after b) functionalization with APTS (Hemp_SiO2_APTS sample). In detail, spectrum a) proves that
the hemp fibers are produced with a silica coating. In fact, spectrum a) is similar to the one
previously reported [18], thus differing from the hemp example for the presence of three bands; two
of them are due to the well-known stretching vibration of silicate structure, the third one resulting
from the condensation reaction between OH groups of hemp surfaces and silanols from silica
resulting in the appearance of C-O-Si bonds [18,21,22]. As can be seen, the exposure to the APTS
solution leads to the presence of the NH2 stretching vibration band on the FTIR spectrum [23].
Figure 4. Spectra of a) hemp fabric after functionalization with waterglass (Hemp_SiO2 sample), b)
the spectrum recorded after the functionalization with APTS (Hemp_SiO2_ APTS sample).
3.2. Composite Epoxy/APTS functionalized Hemp Fibers
3.2.1. SEM Observations
Figure 5 shows SEM micrographs of the epoxy/APTS functionalized hemp fibers at different
magnifications. Figure 5a suggests the formation of a web-like structure formed by fibrils and
microfibrils. As better highlighted in Figure 5b, the epoxy phase appears to completely coat the
entire surface of the fibers, due to a better chemical compatibility between the polymer matrix and
the functionalized filler. The formation of this web-like structure could be ascribable to the good
Figure 4.
Spectra of (
a
) hemp fabric after functionalization with waterglass (Hemp_SiO
2
sample),
(b) the spectrum recorded after the functionalization with APTS (Hemp_SiO2_ APTS sample).
Materials 2020,13, 1844 7 of 11
3.2. Composite Epoxy/APTS functionalized Hemp Fibers
3.2.1. SEM Observations
Figure 5shows SEM micrographs of the epoxy/APTS functionalized hemp fibers at dierent
magnifications. Figure 5a suggests the formation of a web-like structure formed by fibrils and
microfibrils. As better highlighted in Figure 5b, the epoxy phase appears to completely coat the entire
surface of the fibers, due to a better chemical compatibility between the polymer matrix and the
functionalized filler. The formation of this web-like structure could be ascribable to the good adhesion
at the interphase performed by the chemical coupling between the amino groups of APTS and oxirane
rings of the epoxy resin [3,24].
Materials 2018, 11, x FOR PEER REVIEW 7 of 11
adhesion at the interphase performed by the chemical coupling between the amino groups of APTS
and oxirane rings of the epoxy resin [3,24].
(a) (b)
(c)
Figure 5. Micrographs of the composites epoxy/APTS functionalized hemp fibers: (a, c) 50 µm scale
and its detail, (b) region at 10 µm.
3.2.2. DMA Results
Figure 6 shows the plot of glass transformation temperature (Tg) as a function of filler
composition. The Tg values were taken from DMA results as the Tanδ peak temperature (Figure 7).
Thus, Tanδ values were calculated as the ratio between the loss (E’’) and storage (E’) modulus. It is
possible to note that the composites do possess higher Tg than the neat epoxy resin, increasing with
the hemp particle content. This is indicative of a good interaction at the fiber/epoxy interphase, in
agreement with SEM micrographs (Figure 5).
Figure 6. Glass transformation temperature of the composites epoxy/APTS functionalized hemp
fibers as a function of composition.
Figure 5.
Micrographs of the composites epoxy/APTS functionalized hemp fibers: (
a
,
c
) 50
µ
m scale
and its detail, (b) region at 10 µm.
3.2.2. DMA Results
Figure 6shows the plot of glass transformation temperature (Tg) as a function of filler composition.
The Tg values were taken from DMA results as the Tan
δ
peak temperature (Figure 7). Thus, Tan
δ
values were calculated as the ratio between the loss (E”) and storage (E’) modulus. It is possible to
note that the composites do possess higher Tg than the neat epoxy resin, increasing with the hemp
particle content. This is indicative of a good interaction at the fiber/epoxy interphase, in agreement
with SEM micrographs (Figure 5).
Materials 2020,13, 1844 8 of 11
Materials 2018, 11, x FOR PEER REVIEW 7 of 11
adhesion at the interphase performed by the chemical coupling between the amino groups of APTS
and oxirane rings of the epoxy resin [3,24].
(a) (b)
(c)
Figure 5. Micrographs of the composites epoxy/APTS functionalized hemp fibers: (a, c) 50 µm scale
and its detail, (b) region at 10 µm.
3.2.2. DMA Results
Figure 6 shows the plot of glass transformation temperature (Tg) as a function of filler
composition. The Tg values were taken from DMA results as the Tanδ peak temperature (Figure 7).
Thus, Tanδ values were calculated as the ratio between the loss (E’’) and storage (E’) modulus. It is
possible to note that the composites do possess higher Tg than the neat epoxy resin, increasing with
the hemp particle content. This is indicative of a good interaction at the fiber/epoxy interphase, in
agreement with SEM micrographs (Figure 5).
Figure 6. Glass transformation temperature of the composites epoxy/APTS functionalized hemp
fibers as a function of composition.
Figure 6.
Glass transformation temperature of the composites epoxy/APTS functionalized hemp fibers
as a function of composition.
Materials 2018, 11, x FOR PEER REVIEW 8 of 11
Figure 7. Comparison between the experimental values of Tanδ (calculated as storage and loss
modulus ratio) for different volumetric percentage of filler content in the epoxy resin.
In Figure 8, the curves of the storage modulus (E’) in the function of temperature, obtained by
DMA tests, are reported. It is possible to observe that the values of modulus begin to decrease after
50 °C.
Furthermore, in the tempurature range of 75–90 °C, the E’ curves result in an intense drop,
which indicates a glass/rubbery transition.
Figure 8. Storage moduli of the different specimens measured by DMA.
Additionally, the trend of the average values of the storage moduli, reported in Figure 9, as a
function of hemp content, clearly depicts that the incorporation of microfibers for the resin generates
an increase in the modulus.
The experimental values were compared with the ones calculated through the “rule of
mixtures” (valid for long unidirectional fibers) [25–27]:
𝑬𝑽
𝒇𝑬𝒇𝑽
𝒎𝑬𝒎 (1)
and the “inverse rule of mixtures” (valid in case of short fibers or fillers) [25–27]:
𝟏
𝑬𝐕𝐟
𝐄𝐟
𝟏
𝐕
𝐦
𝐄
𝐦
(2)
where:
Figure 7.
Comparison between the experimental values of Tan
δ
(calculated as storage and loss modulus
ratio) for dierent volumetric percentage of filler content in the epoxy resin.
In Figure 8, the curves of the storage modulus (E’) in the function of temperature, obtained by
DMA tests, are reported. It is possible to observe that the values of modulus begin to decrease after
50 C.
Materials 2018, 11, x FOR PEER REVIEW 8 of 11
Figure 7. Comparison between the experimental values of Tanδ (calculated as storage and loss
modulus ratio) for different volumetric percentage of filler content in the epoxy resin.
In Figure 8, the curves of the storage modulus (E’) in the function of temperature, obtained by
DMA tests, are reported. It is possible to observe that the values of modulus begin to decrease after
50 °C.
Furthermore, in the tempurature range of 75–90 °C, the E’ curves result in an intense drop,
which indicates a glass/rubbery transition.
Figure 8. Storage moduli of the different specimens measured by DMA.
Additionally, the trend of the average values of the storage moduli, reported in Figure 9, as a
function of hemp content, clearly depicts that the incorporation of microfibers for the resin generates
an increase in the modulus.
The experimental values were compared with the ones calculated through the “rule of
mixtures” (valid for long unidirectional fibers) [25–27]:
𝑬𝑽
𝒇𝑬𝒇𝑽
𝒎𝑬𝒎 (1)
and the “inverse rule of mixtures” (valid in case of short fibers or fillers) [25–27]:
𝟏
𝑬𝐕𝐟
𝐄𝐟
𝟏
𝐕
𝐦
𝐄
𝐦
(2)
where:
Figure 8. Storage moduli of the dierent specimens measured by DMA.
Materials 2020,13, 1844 9 of 11
Furthermore, in the tempurature range of 75–90
C, the E’ curves result in an intense drop, which
indicates a glass/rubbery transition.
Additionally, the trend of the average values of the storage moduli, reported in Figure 9, as a
function of hemp content, clearly depicts that the incorporation of microfibers for the resin generates
an increase in the modulus.
Materials 2018, 11, x FOR PEER REVIEW 9 of 11
E is composite modulus
• Ef is fiber or filler modulus
• Em is the matrix modulus
• Vfand Vm are the volumetric fraction content of fibers and matrix respectively
In Figure 9, the average values of storage modulus at an environmental temperature are
reported for the different values of volumetric content of fibers. In this diagram, a comparison with
the results of the analytical formulae in the rule of mixture and inverse rule of mixture is reported.
The modulus increase expected by using Equation (2), i.e., “inverse rule of mixture”, would be
negligible for the very low values of loading (1 and 2 vol%) considered in the present work.
Figure 9. The difference between the experimental and theoretical values of moduli for different
volumetric percentages of filler content in the epoxy resin.
From 1 wt% of microfibers loading, the modulus increases up to 10% compared to pristine
polymer (see Figure 9). This may be ascribed to the establishment of a bond between
macromolecules and microfibers, influencing the rigidity of the resin. Moreover, it is worth
remembering that the “rule of mixture” is valid in the case of long unidirectional fiber-reinforced
composites, while the “inverse rule of mixture” is valid for short fiber-reinforced composites. In
particular, Figure 9 demonstrates that the two rules may may both be inadequate to fit the
experimental storage modulus of composites containing a filler in a web-like structure, as already
reported in literature [3].
4. Conclusions
When properly treated with a cheap and ecofriendly waterglass solution, hemp fabric
effectively produces silica coated fibers of diameters ranging from tens of microns to tens of
nanometers, with the aid of a low power mixer. The silica coated fibers can be functionalized with
(3-Aminopropyl) triethoxysilane (APTS) and then dispersed in epoxy resin. SEM micrographs of the
composites show a tendency to produce web-like structures formed by fibrils and microfibrils,
which are continuously interconnected, from which particularly useful mechanical properties may
be expected to result.
DMA analysis shows that the functionalized fibers, up to a concentration of 5 wt%, strongly
affect the glass transformation temperature (10 °C increase) and the storage modulus of the pristine
resin.
The reported results may be considered a case study. In fact, taking into account that many
organosilicon compounds are available on the market, the silica coating of the hemp fibers is
expected to be easily functionalized for the effective dispersion and tailoring of the interface in
different polymer matrices. Thus, the proposed new process may be a cleaner alternative to the
production of biocomposites with potentially improved mechanical performances.
Figure 9.
The dierence between the experimental and theoretical values of moduli for dierent
volumetric percentages of filler content in the epoxy resin.
The experimental values were compared with the ones calculated through the “rule of mixtures”
(valid for long unidirectional fibers) [2527]:
E=VfEf+VmEm(1)
and the “inverse rule of mixtures” (valid in case of short fibers or fillers) [2527]:
1
E=Vf
Ef
+1
Vm
Em
(2)
where:
E is composite modulus
Efis fiber or filler modulus
Emis the matrix modulus
Vfand Vmare the volumetric fraction content of fibers and matrix respectively
In Figure 9, the average values of storage modulus at an environmental temperature are reported
for the dierent values of volumetric content of fibers. In this diagram, a comparison with the results
of the analytical formulae in the rule of mixture and inverse rule of mixture is reported.
The modulus increase expected by using Equation (2), i.e., “inverse rule of mixture”, would be
negligible for the very low values of loading (1 and 2 vol%) considered in the present work.
From 1 wt% of microfibers loading, the modulus increases up to 10% compared to pristine polymer
(see Figure 9). This may be ascribed to the establishment of a bond between macromolecules and
microfibers, influencing the rigidity of the resin. Moreover, it is worth remembering that the “rule
of mixture” is valid in the case of long unidirectional fiber-reinforced composites, while the “inverse
rule of mixture” is valid for short fiber-reinforced composites. In particular, Figure 9demonstrates
that the two rules may may both be inadequate to fit the experimental storage modulus of composites
containing a filler in a web-like structure, as already reported in literature [3].
Materials 2020,13, 1844 10 of 11
4. Conclusions
When properly treated with a cheap and ecofriendly waterglass solution, hemp fabric eectively
produces silica coated fibers of diameters ranging from tens of microns to tens of nanometers, with
the aid of a low power mixer. The silica coated fibers can be functionalized with (3-Aminopropyl)
triethoxysilane (APTS) and then dispersed in epoxy resin. SEM micrographs of the composites show a
tendency to produce web-like structures formed by fibrils and microfibrils, which are continuously
interconnected, from which particularly useful mechanical properties may be expected to result.
DMA analysis shows that the functionalized fibers, up to a concentration of 5 wt%, strongly aect
the glass transformation temperature (10 C increase) and the storage modulus of the pristine resin.
The reported results may be considered a case study. In fact, taking into account that many
organosilicon compounds are available on the market, the silica coating of the hemp fibers is expected
to be easily functionalized for the eective dispersion and tailoring of the interface in dierent
polymer matrices. Thus, the proposed new process may be a cleaner alternative to the production of
biocomposites with potentially improved mechanical performances.
Author Contributions:
A.B., F.B., J.P., B.S. conceived and designed the experiments, A.B., J.P., L.B., V.R., performed
the experiments; all the authors contributed to analyzing the data and to writing the paper. All authors have read
and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Acknowledgments:
The valuable experimental support of Maria Cristina Del Barone, in charge of
LaMest - Laboratorio di MicroscopiaElettronica a Scansione e Trasmissione (SEM and TEM laboratory) at
Polymers Composites and Biomaterials Institute of National Research Council (IPCB-CNR) of Naples, is
highly acknowledged.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Eichhorn, S.J.; Dufresne, A.; Aranguren, M.; Marcovich, N.E.; Capadona, J.R.; Rowan, S.J.; Weder, C.;
Thielemans, W.; Roman, M.; Renneckar, S.; et al. Review: Current international research into cellulose
nanofibers and nanocomposites. J. Mater. Sci. 2010,45, 1–33. [CrossRef]
2.
Holbery, J.; Houston, D. Natural-fiber-reinforced polymer composites in automotive applications. Jom
2006,58, 80–86. [CrossRef]
3.
Siqueira, G.; Bras, J.; Dufresne, A. Cellulosic biocomposites: A review of preparation; properties and
applications. Polymers 2010,2, 728–765. [CrossRef]
4.
Alves, C.; Silva, A.J.; Reis, L.G.; Freitas, M.; Rodrigues, L.B.; Alves, D.E. Ecodesign of automotive components
making use of natural jute fiber composites. J. Clean. Prod. 2010,18, 313–327. [CrossRef]
5.
Nechyporchuk, O.; Belgacem, M.N.; Bras, J. Production of cellulose nanofibrils: A review of recent advances.
Ind. Crop. Prod. 2016,93, 2–25. [CrossRef]
6.
Spence, K.L.; Venditti, R.A.; Rojas, O.J.; Habibi, Y.; Pawlak, J.J. A comparative study of energy consumption
and physical properties of microfibrillated cellulose produced by dierent processing methods. Cellulose
2011,18, 1097–1111. [CrossRef]
7.
Turbak, A.F.; Snyder, F.W.; Sandberg, K.R. Microfibrillated Cellulose, a New Cellulose Product: Properties, Uses
and Commercial Potential; ITT Rayonier Inc.: Shelton, WA, USA, 1983.
8.
Turbak, A.F.; Snyder, F.W.; Sandberg, K.R. Microfibrillated Cellulose. U.S. Patent 4374702, 22 February 1983.
9.
Herrick, F.W.; Casebier, R.L.; Hamilton, J.K.; Sandberg, K.R. Microfibrillated cellulose: Morphology
and accessibility. In Proceedings of the Ninth Cellulose Conference; Applied Polymer Symposia; Wiley:
New York, NY, USA, 1983; Volume 37, pp. 797–813.
10.
Hubbe, M.A.; Rojas, O.J.; Lucia, L.A.; Sain, M. Cellulosic nanocomposites: A review. BioResources
2008,3, 929–980.
11.
Missoum, K.; Belgacem, M.N.; Bras, J. Nanofibrillated cellulose surface modification: A review. Materials
2013,6, 1745–1766. [CrossRef]
12.
Kalia, S.; Boufi, S.; Celli, A.; Kango, S. Nanofibrillated cellulose: Surface modification and potential
applications. Colloid Polym. Sci. 2014,292, 5–31. [CrossRef]
Materials 2020,13, 1844 11 of 11
13.
Mann, G.S.; Singh, L.P.; Kumar, P.; Singh, S. Green composites: A review of processing technologies and
recent applications. J. Thermoplast. Compos. Mater. 2018. [CrossRef]
14.
Gu, H.; Ma, C.; Gu, J.; Guo, J.; Yan, X.; Huang, J.; Zhang, Q.; Guo, Z. An overview of multifunctional epoxy
nanocomposites. J. Mater. Chem. C 2016,4, 5890–5906. [CrossRef]
15.
Dharmalingam, S.; Meenakshisundaram, O.; Elumalai, V.; Boopathy, R.S. An Investigation on the Interfacial
Adhesion between Amine Functionalized Lua Fiber and Epoxy Resin and Its Eect on Thermal and
Mechanical Properties of Their Composites. J. Nat. Fibers 2020. [CrossRef]
16.
Joseph, B.G.; Rajan, J.A.; Jeevahan, J.; Mageshwaran, G. Influence of alkaline treatment on improving
mechanical properties of jute fiber-reinforced epoxy (LY556) composites. FME Trans.
2019
,47, 83–88.
[CrossRef]
17.
Sepe, R.; Bollino, F.; Boccarusso, L.; Caputo, F. Influence of chemical treatments on mechanical properties of
hemp fiber reinforced composites. Compos. B Eng. 2018,133, 210–217. [CrossRef]
18.
Branda, F.; Malucelli, G.; Durante, M.; Piccolo, A.; Mazzei, P.; Costantini, A.; Silvestri, B.; Pennetta, M.;
Bifulco, A. Silica treatments: A fireretardant strategy for hempfabric/epoxycomposites. Polymers
2016
,8, 313.
[CrossRef]
19. West, R. Siegfried Ruhemann and the discovery of ninhydrin. J. Chem. Edu. 1965,42, 386. [CrossRef]
20.
Thomas, S.; Paul, S.A.; Pothan, L.A.; Deepa, B. Natural fibres: Structure, properties and applications.
In Cellulose Fibers: Bio- and Nano-Polymer Composites; Springer: Berlin/Heidelberg, Germany, 2011; pp. 3–42.
[CrossRef]
21.
Silvestri, B.; Pezzella, A.; Luciani, G.; Costantini, A.; Tescione, F.; Branda, F.
Heparinconjugatedsilicananoparticlesynthesis. Mater. Sci. Eng. C. 2012,32, 2037–2041. [CrossRef]
22.
Passaro, J.; Russo, P.; Bifulco, A.; De Martino, M.T.; Granata, V.; Vitolo, B.; Iannace, G.; Vecchione, A.;
Marulo, F.; Branda, F. Water resistant self-extinguishing low frequency soundproofing polyvinylpyrrolidone
basedelectrospun blankets. Polymers 2019,11, 1205. [CrossRef] [PubMed]
23.
Califano, V.; Sannino, F.; Costantini, A.; Avossa, J.; Cimino, S.; Aronne, A. Wrinkledsilicananoparticles:
Ecientmatrix for β-glucosidaseimmobilization. J. Phys. Chem. C 2018,122, 8373–8379. [CrossRef]
24.
Nakagaito, A.N.; Yano, H. Novel high-strength biocomposites based on microfibrillated cellulose having
nano-order-unit web-like network structure. Appl. Phys. A-Mater. 2005,80, 155–159. [CrossRef]
25.
Mansor, M.R.; Sapuan, S.M.; Zainudin, E.S.; Nuraini, A.A.; Hambali, A. Stiness prediction of hybrid
kenaf/glass fiber reinforced polypropylene composites using rule of mixtures (ROM) and rule of hybrid
mixtures (RoHM). J. Polym. Mater. 2013,30, 321–334.
26.
Del Borrello, M.; Mele, M.; Campana, G.; Secchi, M. Manufacturing and characterization of hemp-reinforced
epoxy composites. Polym. Compos. 2020. [CrossRef]
27.
Haghi, A.K.; Zikov, G.E. Applications of Polymers in Construction Technology: Eects of jute/polypropylene
fiber on reinforcement soil. In Polymers for Advanced Technologies: Processing, Characterization and Applications,
1st ed.; Zikov, G.E., Bazylyak, L.I., Aneli, J.N., Eds.; Apple Academic Press: Palm Bay, FL, USA, 2013.
©
2020 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 (http://creativecommons.org/licenses/by/4.0/).
... Thermosetting polymers have been widely used in many industrial applications as adhesives, coatings and laminated materials because of their peculiar physical, chemical, electrical and adhesive properties [1,2]. At present, the reaction of bisphenol A (BPA) and epichlorohydrin, yielding diglycidyl ether of bisphenol A (DGEBA), is the main route for the world production of epoxy prepolymers. ...
... The charring ability of epoxy resins is well known; this is due to their degradation path that occurs mainly through a carbonization process (i.e., dehydration reactions) [32]. The production of acid species from the thermal decomposition of ester functional groups in the final products may promote the carbonization process, through dehydration reactions occurring between acid compounds and the epoxy resin [2,[33][34][35]. The use of an anhydride curing agent may strongly contribute to boost the char formation in the boundary layer (i.e., charring region), with respect to an aliphatic amino system [15]. ...
... As reported in the literature [2,15,37,38], hybrid systems are expected to anticipate the ignition (TTI) as compared to the neat cured resins. This anticipation does not occur in the case of DGEBA/MNA_2Si and is almost negligible in the case of BOMF/MNA_2Si. ...
Article
Full-text available
Thermosetting polymers have been widely used in many industrial applications as adhesives, coatings and laminated materials, among others. Recently, bisphenol A (BPA) has been banned as raw material for polymeric products, due to its harmful impact on human health. On the other hand, the use of aromatic amines as curing agents confers excellent thermal, mechanical and flame retardant properties to the final product, although they are toxic and subject to governmental restrictions. In this context, sugar-derived diepoxy monomers and anhydrides represent a sustainable greener alternative to BPA and aromatic amines. Herein, we report an “in-situ” sol–gel synthesis, using as precursors tetraethylorthosilicate (TEOS) and aminopropyl triethoxysilane (APTS) to obtain bio-based epoxy/silica composites; in a first step, the APTS was left to react with 2,5-bis[(oxyran-2-ylmethoxy)methyl]furan (BOMF) or diglycidyl ether of bisphenol A (DGEBA)monomers, and silica particles were generated in the epoxy in a second step; both systems were cured with methyl nadic anhydride (MNA). Morphological investigation of the composites through transmission electron microscopy (TEM) demonstrated that the hybrid strategy allows a very fine distribution of silica nanoparticles (at nanometric level) to be achieved within a hybrid network structure for both the diepoxy monomers. Concerning the fire behavior, as assessed in vertical flame spread tests, the use of anhydride curing agent prevented melt dripping phenomena and provided high char-forming character to the bio-based epoxy systems and their phenyl analog. In addition, forced combustion tests showed that the use of anhydride hardener instead of aliphatic polyamine results in a remarkable decrease of heat release rate. An overall decrease of the smoke parameters, which is highly desirable in a context of greater fire safety was observed in the case of BOMF/MNA system. The experimental results suggest that the effect of silica nanoparticles on fire behavior appears to be related to their dispersion degree.
... Cellulose is a widespread natural biopolymer of polysaccharide nature and with fibrous structure and unique properties due to which the scope of its use is increasing every year [27][28][29]. More often, cellulose-based materials are used as fillers for various types of composite materials of organic and inorganic nature [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. ...
Article
Full-text available
This paper shows that an eco-friendly electrospinning process allows us to produce water resistant sound absorbers with reduced thickness and excellent sound-absorption properties in the low and medium frequency range (250–1600 Hz) for which which human sensitivity is high and traditional materials struggle to match, that also pass the fire tests which are mandatory in many engineering areas. The structure and composition were studied through Scanning Electron Microscopy (SEM), Fourier Transform InfraRed (FTIR) Spectroscopy and ThermoGravimetric Analysis (TGA). The density, porosity and flow resistivity were measured. Preliminary investigation of the thermal conductivity through Differential Scanning Calorimetry (DSC) shows that they have perspectives also for thermal insulation. The experimental results indicate that the achievements are to be ascribed to the chemical nature of Polyvinylpyrrolidone (PVP). PVP is, in fact, a polymeric lactam with a side polar group that may be easily released by a thermooxidative process. The side polar groups allow for using ethanol for electrospinning than relying on a good dispersion of silica gel particles. The silica particles dimensionally stabilize the mats upon thermal treatments and confer water resistance while strongly contributing to the self-extinguishing property of the materials.
Article
Full-text available
β-Glucosidase (BG) was immobilized by adsorption on wrinkled silica nanoparticles (WSNs) giving an active and stable biocatalyst for the hydrolysis of cellobiose. WSNs exhibiting both a central-radial pore structure and a hierarchical trimodal micro/mesoporous pore size distribution were synthesized. They were used as matrix to immobilize BG, obtaining a biocatalyst (BG/WSNs) containing 150 mg of enzyme for gram of matrix. A complete textural and morphological characterization of BG/WSNs performed by Brunauer–Emmett–Teller (BET) method, Thermogravimetric (TG), Fourier Transform Infrared (FT-IR) and Transmission Electron Microscopy (TEM) analyses showed this matrix is able to generate a microenvironment particularly suitable for this enzyme. The immobilization procedure used allowed preserving for most part the secondary structure of the enzyme and, consequently, its catalytic activity. The kinetic parameters of the hydrolysis of cellobiose reaction performed with the biocatalyst were determined and compared with those of the free enzyme. It was found that the apparent KM value of the biocatalyst was slightly lower than that of the free enzyme, indicating that the enzyme-substrate affinity was increased. The complete hydrolysis of cellobiose was observed for four consecutive runs, showing a high operational stability of the biocatalyst.
Article
Full-text available
In this paper, for the first time, inexpensive waterglass solutions are exploited as a new, simple and ecofriendly chemical approach for promoting the formation of a silica-based coating on hemp fabrics, able to act as a thermal shield and to protect the latter from heat sources. Fourier Transform Infrared (FTIR) and solid-state Nuclear Magnetic Resonance (NMR) analysis confirm the formation of –C–O–Si– covalent bonds between the coating and the cellulosic substrate. The proposed waterglass treatment, which is resistant to washing, seems to be very effective for improving the fire behavior of hemp fabric/epoxy composites, also in combination with ammonium polyphosphate. In particular, the exploitation of hemp surface treatment and Ammonium Polyphosphate (APP) addition to epoxy favors a remarkable decrease of the Heat Release Rate (HRR), Total Heat Release (THR), Total Smoke Release (TSR) and Specific Extinction Area (SEA) (respectively by 83%, 35%, 45% and 44%) as compared to untreated hemp/epoxy composites, favoring the formation of a very stable char, as also assessed by Thermogravimetric Analysis (TGA). Because of the low interfacial adhesion between the fabrics and the epoxy matrix, the obtained composites show low strength and stiffness; however, the energy absorbed by the material is higher when using treated hemp. The presence of APP in the epoxy matrix does not affect the mechanical behavior of the composites.
Article
The objective of this work is to develop a natural fiber-reinforced epoxy composite with enhanced compatibility between resin and the fiber, achieved by amino silane treatment of Luffa fiber. Amine modification on the surface of the Luffa fiber is confirmed by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Composites with different volume fractions (2%, 4%, 6%, and 8%) of amine functionalized/un-functionalized Luffa fiber and epoxy resin are fabricated. The functionalized/un-functionalized Luffa epoxy composites are subjected to various studies such as tensile, flexural, and impact in the area of the effect of amine functionalization of fiber/epoxy composites. Dynamic mechanical analysis and fatigue analysis are carried out to enable a study on the effect of amine functionalization. Variations in thermal stability of the composites are studied using TGA analysis. A maximum tensile strength value of 18.3 MPa is reached for the 6% amine functionalized composite compared to the plain epoxy of 9.4 MPa. The amine functionalized fiber-reinforced composites show improved thermal, mechanical, and morphological properties as a result of improved interaction between the fiber and matrix.
Article
The adoption of natural‐based fibers in place of inorganic reinforcements is an effective approach to reduce the environmental and economic impact of composite materials. In particular, hemp is an attractive solution due to its mechanical, physical, and growing properties. The present article deals with the manufacturing of thermoset hemp‐reinforced composite materials. In particular, the investigation moves into the production by resin transfer molding and by resin powder molding with the use of epoxy polymeric material. To describe the effects of the technological cycle onto the characteristic of realized products, different manufacturing parameters have been combined during the braiding of reinforcement and the polymerization of the final composite. Computed tomography, microscopical analysis, and tensile tests have been used to observe the main effects of the manufacturing process and mechanical properties of the materials. Furthermore, elastic moduli of the materials have been estimated by means of modified rule of mixture and Halpin‐Tsai models in order to verify their effectiveness in forecasting stiffness of the hemp‐reinforced composites in the early design phase. The article extends the existing knowledge base on hemp‐reinforced thermoset composites manufactured with different processes. Results also illustrate relations existing between error introduced by calculation models and the intrinsic variability in mechanical properties.
Article
Composites are currently in a wide range of applications such as aerospace, automotive, house hold items, marine industries etc. There is always a need for improvement in the synthesis of composites without compromising on the mechanical properties and physical properties. In this article, the jute fiber reinforced polyester composite is synthesized by alkaline treatments and the mechanical properties are evaluated. The objective of this article is to discover any significant changes in the mechanical properties before and after alkaline treatments of fiber reinforcement. The specimens are prepared with and without alkaline treatments. SEM images of specimens before and after alkaline treatments are discussed for microscopic analysis. The mechanical properties such as tensile strength, flexural strength and impact strength are determined for the specimens and the results are compared. From the results, it is found that alkaine treated jute fiber-reinforced epoxy composites exhibit better mechanical properties than untreated fiber.
Article
Biocomposites are considered as the next-generation materials as these can be made using natural/green ingredients to offer sustainability, eco-efficiency, and green chemistry. Nowadays, biocomposites are being utilized by numerous sectors, which include automobile, biomedical, energy, toys, sports, and so on. In this review article, an effort has been made to provide a comprehensive assessment of the available green composites and their commonly used processing technologies for the sake of materials’ capabilities to meet up with demands of the present and forthcoming future. Various types of natural fibers have been investigated with polymer matrixes for the production of composite materials that are at par with the synthetic fiber composite. This review article also highlights the requirements of the green composites in various applications with a view point of variability of fibers available and their processing techniques. This review is specially done to strengthen the knowledge bank of the young researchers working in this field.
Chapter
The world wide cement production in 2007 was 2.77 billion tons [1]. Asia is the first producer (70%), followed by European Union countries (9.5%). Indeed, cement industry can be considered strategic in fact, from one side it produces an essential product in building and civil engineering for the construction of safe, reliable, long lasting buildings, and infrastructures. On the other side it is very important from the economic point of view (for example Indian cement industry is playing a very import role in the economic development of the country). However, cement industry environment wise is also responsible for a large use of not renewable raw materials (clays and calcium carbonate) and fossil fuels (e.g. clinker, the main cement constituent, is obtained at T = 1,500°C) resulting in heavy emissions of carbon dioxide (CO2) in atmosphere. In fact, in 2006, the European cement industry used an energy equivalent of about 26 Mt of coal for the production of 266 Mt of cement [2-21] and it is estimated that 1 tone of CO2 is emitted for each tone of cement produced. This induces cement industry to consider the possibility to introduce waste of different nature and origin in cement productive process. Two routes are currently taken into consideration: one involves the use of waste as alternative fuel the other considers waste as a new cement constituent. In this chapter, we will show how to convert the recycled wastes into wealth with particular application in construction industries.
Article
Natural Fibers Reinforced Composites (NFRC) are finding much interest as a substitute for glass or carbon reinforced polymer composites, like for instance automobile interior linings (roof, rear wall, side panel lining), shipping pallets, construction products (i.e. composite roof tiles), furniture and household products (i.e. storage containers, window and picture frames as well as food service trays, toys and flower pots) as well as fan houses and blades. However, a notable disadvantage of lignocellulosic fibers as reinforcements is their polarity which makes it incompatible with hydrophobic thermoplastic matrix. This incompatibility results in poor interfacial bonding between the fibers and the matrix. This in turn leads to impaired mechanical properties of the composites. This defect can be remedied by chemical modification of fibers so as to make it less hydrophilic. In this paper experiments have been performed to further the development of natural fiber reinforced composites. Untreated and treated surfaces of hemp fibers were characterized using Fourier Transform Infrared (FTIR) spectroscopy and Scanning Electron Microscopy (SEM). Fiber-matrix adhesion was promoted by fiber surface modifications using an alkaline treatment and (3-Glycidyloxypropyl) trimethoxysilane coupling agent. The mechanical behaviour of epoxy matrix composite reinforced with woven hemp was studied and mechanical test results show that silane treatment of hemp fibers improves, both tensile and flexural properties of the composites, although no high values are obtained.
Article
Epoxy is a crucial engineered thermosetting polymer with wide industrial applications in adhesive, electronics, aerospace and marine systems. In this review, basic knowledge of epoxy resins and the challenge for the preparation of epoxy nanocomposites are summarized. The state-of-art multifunctional epoxy nanocomposites with magnetic, electrically conductive, thermally conductive, and flame retardant properties of the past few years are critically reviewed with detailed examples. The applications of epoxy nanocomposites in aerospace, automotives, anti-corrosive coatings, and high voltage fields are briefly summarized. This knowledge will have great impact on the field and will facilitate researchers in seeking new functions and applications of epoxy resins in the future.