ORIGINAL PAPER
Obtaining nanocomposites of polyamide 6 and cellulosewhiskers via extrusion and injection molding
Ana Carolina Correa • Eliangela de Morais Teixeira •
Vitor Brait Carmona • Kelcilene Bruna Ricardo Teodoro •
Caue Ribeiro • Luiz Henrique Capparelli Mattoso • Jose Manoel Marconcini
Received: 8 August 2013 / Accepted: 27 November 2013 / Published online: 13 December 2013
� Springer Science+Business Media Dordrecht 2013
Abstract Nanocomposites of polyamides with cellu-
lose whiskers are difficult to obtain by conventional
processing of extrusion and injection molding because
of the low thermal stability of the cellulosic nanostruc-
tures and the relatively high processing temperature of
polyamides, which is higher than the temperature of
thermal degradation of cellulose whiskers. Thus, in this
study cellulose whiskers were coated with polyamide 6
(PA6) in order to increase their thermal stability and
prevent the formation of agglomerates. This coating on
cellulose whiskers allows their application to obtain
nanocomposites with polyamides, whose processing
temperatures are relatively high, around 250 �C. Cellu-
lose whiskers (CWs) were obtained from cotton fibers
by acid hydrolysis. The freeze-dried CWs were coated
with PA6 by dispersing them in formic acid; PA6 was
solubilized in this suspension. The cellulose-coated
whiskers (CCWs) were characterized by X-ray diffrac-
tion, differential scanning calorimetry (DSC), thermo-
gravimetry (TG), scanning electron microscopy (SEM-
FEG) and infrared spectroscopy. SEM-FEG and TG
results showed that the PA6 coating on CWs prevented
high agglomeration of dried CWs and promoted an
increase in their thermal stability from 180 to 280 �C,
allowing the use of CCWs to obtain nanocomposites
with PA6 using conventional processing routes, such as
extrusion and injection molding, at appropriate process-
ing temperatures. In this way, 1 wt% CCWs was used to
prepare nanocomposites with PA6. The PA6 ? 1CW
nanocomposites were compared to neat PA6 without
CWs. The samples were characterized by tensile tests
and DSC, and the results showed that the PA6 coating on
CWs was effective in raising the thermal stability of
CWs, improving the dispersion of CWs in the matrix of
PA6, resulting in a 45 % increase in the elastic modulus
of the nanocomposite with only 1 wt% of coated
cellulose whiskers in comparison to neat PA6.
Keywords Cellulose � Nanofibers �Whiskers �Nanocomposites � Polyamide-6 � Nylon-6
Introduction
Cellulose is the most abundant organic component on
earth; it is produced by plants, trees, bacteria and some
A. C. Correa (&) � E. de Morais Teixeira �V. B. Carmona � K. B. R. Teodoro � C. Ribeiro �L. H. C. Mattoso � J. M. Marconcini
National Laboratory of Nanotechnology for Agribusiness
(LNNA), Embrapa Instrumentation, PO Box 741,
Sao Carlos, SP 13560-970, Brazil
e-mail: [email protected]
V. B. Carmona
Graduate Program in Materials Science and Engineering
(PPG-CEM), Federal University of Sao Carlos (UFSCar),
Rod. Washington Luiz, Km 235, Sao Carlos,
SP 13565-905, Brazil
K. B. R. Teodoro
Graduate Program in Chemistry (PPGQ), Federal
University of Sao Carlos (UFSCar), Rod. Washington
Luiz, Km 235, Sao Carlos, SP 13565-905, Brazil
123
Cellulose (2014) 21:311–322
DOI 10.1007/s10570-013-0132-z
animals (tunicates). Cellulose is formed of long chains
of anhydroglucose units, bonded by b-1,4-glucosidic
linkages (C–O–C), produced via the condensation
polymerization of glucose. It is produced in the cell
walls of plants and trees and as a product of the
photosynthesis process (Eichhorn 2011), and it pre-
sents a semicrystalline structure in which the chains
are aggregated by intra- and intermolecular hydrogen
bonds. Cellulose crystals are completely insoluble in
water and in most organic solvents because of these
hydrogen bonds (Silva et al. 2009; Spinace et al. 2009;
Paakko et al. 2007). Native cellulose (cellulose I) can
be compared to a composite material consisting of two
phases, one of crystalline cellulose and the other of
amorphous cellulose. There is no raw material entirely
composed of crystalline cellulose; all of them exhibit a
significant portion of amorphous cellulose, whose
content varies from species to species (Eichhorn
2011).
Cellulose microfibrils present thin cross sections,
high strength and durability, biocompatibility, low
cost, low density and good mechanical properties
(theoretical elastic modulus of around 165 GPa), but
they also present some disadvantages, such as poor
solubility in organic solvents, low dimensional stabil-
ity, lack of antimicrobial properties and high moisture
absorption (not desirable for application in most
composites) (Eichhorn 2011; Hubbe et al. 2008; Roy
et al. 2009); furthermore, its thermal stability restricts
its application to polymers whose processing temper-
ature is relatively low or to water-soluble polymers. To
overcome these drawbacks, controlled chemical and/or
physical modifications of cellulose are required.
Cellulose nanostructures can be obtained by acid
hydrolysis, with strong acid solutions, such as hydro-
chloric and sulfuric acids, under controlled time and
temperature conditions. This process causes the removal
of amorphous regions while maintaining the crystalline
regions. After hydrolysis, particles with a needle-like
aspect, known as whiskers, are obtained. They present
lengths of 100 to 500 nm and diameters smaller than
20 nm. After acid hydrolysis, the obtained whiskers are
dispersed in aqueous medium. The acid applied in
hydrolysis can affect the dispersion characteristics of
whiskers in water. The use of only hydrochloric acid in
hydrolysis allows obtaining more thermally stable
whiskers. On the other hand, the whiskers are not
dispersed because chloride ions are easily removed
during dialysis, not remaining electrostatic forces
enough to cause the repulsion among particles, and
consequently aggregates of cellulose whiskers are
formed, causing precipitation (Roman and Winter
2004; Dufresne 2006; Wang et al. 2007; Correa et al
2010). The use of sulfuric acid results in more stable
aqueous suspensions because of the presence of sulfate
groups on the surface of whiskers, which promotes
electrostatic repulsion among particles and reduces the
formation of agglomeration. Although the presence of
sulfate groups reduces aggregation of cellulose crystals,
it also reduces the thermal stability of the crystals;
moreover, the aggregation of crystals cannot be com-
pletely avoided, even with the use of sulfuric acid in the
hydrolysis (Roman and Winter 2004).
The major problems in preparing nanocomposites
with CW are their thermal stability, difficulty of dispers-
ing CWs, and their low compatibility with nonpolar
solvents and matrices. This phenomenon occurs because
of the presence of a large number of hydroxyl groups,
which provides strong interactions between adjacent
whiskers, and these interactions are larger than the
interaction between them and the polymer, for example.
This effect generates a large agglomeration of whiskers,
especially after drying, reducing the efficiency on
mechanical reinforcement (Eichhorn 2011; Hubbe et al.
2008; Roy et al. 2009). Some strategies have been used to
overcome this undesirable effect, as reported by Hubbe
et al. (2008), Eichhorn (2011), Moon et al. (2011) and
Isogai et al. (2011), who tested the dispersion of cellulose
nanofibers in organic solvents, some chemical modifica-
tion or grafting on their surface. Cellulose whiskers can
also be coated with a hydrophobic polymer to avoid their
agglomeration after drying and improve their dispersion
in the polymer matrix (Azouz et al. 2012; Lin and
Dufresne 2013).
Polyamides are important as a material with high
performance and also excellent mechanical and ther-
mal properties (Leite et al. 2009). Furthermore,
polyamides have been used as matrices of nanocom-
posites filled with organoclays, layered silicates,
natural or synthetic nanofibers, etc., which have
shown interesting properties such as improving bar-
rier, thermal and mechanical properties, among others
(Leite et al. 2009; Qua and Hornsby 2011). Moreover,
a-cellulose can also be an effective reinforcement in
polyamides. Microcrystalline cellulose (MCC) has
been widely studied and used as a reinforcement in
composite materials with engineering thermoplastics
such as polyamide 6 and 66 because MCC presented
312 Cellulose (2014) 21:311–322
123
higher thermal stability than the cellulose whiskers,
allowing its application as a filler in composites to be
used in parts for the automotive industry (Kiziltas et al.
2013).
Nanocomposites of polyamides with cellulose
whiskers have not been widely investigated in the
literature because of the low thermal stability of the
cellulosic nanostructures and relatively high process-
ing temperature of polyamides, higher than the tem-
perature of thermal degradation of cellulose whiskers.
Thus, this study focused on coating cellulose
whiskers with polyamide 6 in order to avoid the
formation of agglomerates, promote an increase in
thermal stability of whiskers and enable their appli-
cation in polymers via conventional processing of
molten materials.
Materials and methods
Cellulose whiskers were obtained from commercial
cotton fibers. Analytical grades of sulfuric and formic
acids (Synth) were used. Cellulose membrane dialysis
tubing (Sigma) was used for dialysis. Nanocomposites
were prepared using polyamide 6 (Zytel 7301 NC010,
DuPont) with a melting temperature of 221 �C and
density of 1.13 g/cm3.
Obtaining cellulose whiskers: acid hydrolysis
The acid hydrolysis of cotton fibers was carried out
using 50 g of fibers to 1,000 ml of sulfuric acid solution
(60 % v/v) at 45 �C under mechanical stirring (Fisaton
713D) for 75 min. Then, the suspensions were diluted
into five parts of cold water to one part of acid
suspension. The diluted suspensions were washed by
centrifugations at 10,000 rpm for 15 min each. The
supernatant was removed from the sediment and
replaced by distilled water. The suspension was vigor-
ously stirred for 30 min and put in a piece of cellulose
dialysis membrane and closed with clamping closures.
Dialysis was carried out in tap water to remove free acid
from the suspension. This process was completed when
the suspension reached neutrality. The neutral suspen-
sion was vigorously stirred for 1 h, and after adding
several drops of chloroform, it was stored in a refrig-
erator for 24 h. After that, the suspension was frozen and
dried by a freeze-drying process. The obtained cellulose
whiskers were called CWs.
Coating cellulose whiskers with polyamide 6
Around 10 g of dried CWs was dispersed in 200 ml of
formic acid (85 % Synth) using a sonicator (Branson
450) for 3 min (50 % amplitude and around 80 W) in
order to obtain a homogeneous suspension of CWs in
formic acid. To this suspension 20 g of polyamide 6
(PA6) was added to be solubilized under continuous
magnetic agitation at 30 �C for approximately 30 min.
After all PA6 added had been solubilized, the suspen-
sion was poured slowly into water in order to
precipitate the mixture of PA6 and CWs, obtaining
coated cellulose whiskers (CCWs). The CCWs were
washed with tap water until neutrality, and the solid
was dried in an air-circulating oven at 50 �C for 24 h
and milled in a knife mill (Solab) in order to obtain
small particle sizes and improve the dispersion in the
nanocomposite. The control of this sample was PA6
solubilized in formic acid without CWs under the
same conditions as the coating to obtain the precipitate
PA6sol in order to compare the effectiveness of the
coating of CWs with PA6 (sample CCWs) with the
sample without CWs (PA6sol).
Nanocomposites
To prepare the nanocomposite with 1 %wt CWs, 3 %
of milled CCWs was incorporated into PA6 (as CCWs
have around 33 wt% CWs), obtaining the sample
PA6 ? 1CW. In order to compare to the performance
of the nanocomposite, a composition of 98 wt% PA6
(pellets) and 2 wt% PA6sol was prepared, called
PA6 ? PA6sol. The control was also PA6 processed
only from pellets (PA6p). All the samples were dried
in a vacuum oven at 60 �C for 24 h prior to extrusion.
The control (PA6p), PA6 ? PA6sol and nanocom-
posite (PA6 ? 1CW) were fed into an 18-mm co-
rotating twin-screw extruder with an L/D ratio of 40
(Laboratory Extruder ZSK 18 MEGAlab, Coperion,
Germany). The temperature profile was set as 180,
190, 200, 210, 220, 230 and 230 �C in the six heating
zones and dies, respectively. Remaining humidity was
removed through two separate vents and through a
third port attached to a vacuum pump. The screw
rotation speed was 600 rpm.
After extruding, the samples were dried for 24 h in a
vacuum oven at 60 �C and molded in ASTM D-638
tensile specimens in Arburg 270S 400-100 injection
molding equipment. An injection pressure of 2,500 bar,
Cellulose (2014) 21:311–322 313
123
mold temperature of 45 �C, and temperature profile of
175, 225, 240, 245 and 260 �C were applied in the five
heating zones.
Characterizations
Transmission electron microscopy (TEM)
The uncoated cellulose whiskers were examined by
TEM using TecnaiTM G2 F20 equipment in STEM
(scanning transmission electron microscopy) mode.
The images were acquired with a dark field (DF)
detector. A droplet of diluted suspension was depos-
ited on a carbon microgrid (400 mesh) and allowed to
dry. The grid was stained with a 1.5 % solution of
uranyl acetate and dried at room temperature.
Scanning electron microscopy (SEM-FEG)
The morphology of CCWs and PA6sol was examined
using a Jeol scanning electron microscope (JSM-
6701F) with a field emission gun. A small portion of
each sample was deposited on carbon tape, and a thin
layer (ca. 15 nm) of gold was sputtered on the surface.
Fourier transform infrared analysis (FTIR)
FTIR spectra of the samples were obtained in absor-
bance with a Perkin-Elmer Spectrum 1000 spectrom-
eter, using 32 scans, with 4 cm-1 resolution over a
wavelength range of 400 to 4,000 cm-1. The samples
were ground and pressed into KBr pellets (1 mg
sample/100 mg KBr).
X-ray diffraction (XRD)
The diffractograms were recorded on a Shimadzu
XRD 600 diffractometer operating at 30 kV, 30 mA
and CuKa radiation (k = 1,540 A). The samples were
scanned in 2h ranges varying from 5� to 40� (2�/min).
The diffractograms were fit by placing Pseudo-Voigt
shaped peaks after the deconvolution peaks using
Origin 7.5 software. The crystallinity index (Ci) was
estimated on the basis of areas under crystalline and
amorphous peaks after appropriate baseline correction
(Borysiak and Garbarczyk 2003).
The crystallinity index (Ci) was calculated using
Eq. 1:
Cið%Þ ¼ 1� Aa
At
� �� 100 ð1Þ
where: Aa is the area under the amorphous halo, and At is
the sum of areas under all peaks, including amorphous.
Thermogravimetric analysis (TGA)
Thermogravimetric analyses were carried out with TA
Instruments equipment (TGA Q500) from room temper-
ature to 600 �C (10 �C/min) under synthetic air atmo-
sphere (60 ml/min). The critical weight loss temperatures
were obtained from the onset points of the TG curves.
Differential scanning calorimetry (DSC)
These analyses were performed to evaluate the influ-
ence of cellulose whiskers on the thermal properties of
PA6. DSC measurements were carried out on DSC
Q-100 equipment (TA Instruments). The samples were
analyzed under nitrogen flow (60 ml/min); they were
heated from -20 to 250 �C at a heating rate of 10 �C/
min and cooled at the same rate from 250 to 0 �C. The
specimens’ Ci was calculated according to Eq. 2:
Cið%Þ ¼DHm � 100
DH�m � x
ð2Þ
where DHm is the heat of fusion of the nylon blend and
composites, DHm� is the heat of fusion for 100 %
crystalline nylon 6 (DHm� = 190 J/g)(Kiziltas et al.
2011), and x is the mass fraction for nylon 6 in
nanocomposites.
Mechanical properties
Tensile strength, modulus and elongation at break of the
injected specimens were determined according to
ASTM D638 using an Emic DL-3000 universal testing
machine fitted with a 3,000-Kgf load cell. The tests were
carried out at 23 �C using a crosshead speed of 50 mm/
min and distance between clamps of 100 mm. At least
seven samples were taken for each determination after
storage for 48 h at 50 ± 5 % relative humidity (23 �C).
Results and discussion
STEM observations (Fig. 1) showed individual cellu-
lose whiskers obtained after acid hydrolysis. As can be
314 Cellulose (2014) 21:311–322
123
seen, they present a needle-like aspect. The dimen-
sions were calculated with Image J software. The
length (L) and diameter (D) are approximately
205 ± 22 and 9 ± 4 nm. Compared to other nano-
sized structures, derived from other cellulose sources,
the measured diameters were similar to nanofibrils
from sugarcane bagasse (2–11 nm) (Teixeira et al.
2011), curaua (6–10 nm) (Correa et al 2010), banana
residues (5 nm) (Zuluaga et al. 2007) and eucalyptus
kraft pulp (7–21 nm) (Tonoli et al. 2012).
Cellulose whiskers were coated with solubilized
polyamide 6 (CCWs), and these modified whiskers
were compared to uncoated cellulose whiskers (CWs)
as well as the neat coating, PA6 solubilized in formic
acid and precipitated in water (PA6sol). The surfaces
of PA6sol and CCWs were observed by SEM, and the
micrographs are shown in Fig. 2a, b, respectively.
Figure 2a shows the smooth surface of the PA6sol
sample, as it is not a cryogenic fracture. The surface of
the CCWs (Fig. 2b) presents fibrillar structures with
diameters of around 25 nm, marked with arrows, and
spheres of approximately 30 nm diameter, marked
with circles, dispersed through the matrix, which may
be related to cellulose whiskers embedded in the
polymer. But the same CWs, prepared from the
aqueous suspension and observed by TEM (Fig. 1),
had an average diameter of 9 ± 4 nm. Thus, it can be
said that the whiskers were effectively coated by the
PA6 polymer. The amount of whiskers in the nano-
composite was 33 wt%, and this concentration has a
very high surface area. However, good dispersion of
whiskers in the matrix, showing no agglomerates
throughout the sample, was also observed as the
analyzed portion of the sample was the surface of the
solid supernatant, obtained after solubilization in formic
acid and precipitation in water; it can be considered a
representative portion of the sample as a whole.
The chemical structures of the samples were also
evaluated by FTIR, and the FTIR spectra of cellulose
whiskers, PA6sol and nanocomposites are shown in
Fig. 3.
Regarding the uncoated CWs, the band at
3,450 cm-1 was attributed to vibrations of OH groups
linked by hydrogen bonds. The band at 2,900 cm-1
was attributed to the asymmetrical stretching of -CH
and -CH2; the band at 1,636 cm-1 was related to the
deformation of OH bonds and at 1,370 cm-1 to the
symmetrical angular deformation of the CH bond
(Silverstein et al. 1979).
The band at 1,160 cm-1 was attributed to COC
binding, referring to the elongation of the glucose rings.
The band at 1,060 cm-1 was related to the stretching of
the CO groups (Kataoka and Kondo 1998). The bands at
894 and 672 cm-1 corresponded to the CH outside the
plane (Marcilla and Menargues 2005).
Concerning the polyamides, the bands at 3,300 and
3,100 cm-1 were related to the free NH groups.
However, in the region between 3,500 and
3,300 cm-1, free or associated OH and NH groups
may appear in superposition, which makes the alloca-
tion of the band difficult because of the displacement
of hydrogen bonds in the equilibrium, characteristic of
amides (Evora et al. 2002; Bittencourt 2008). How-
ever, the spectrum of the CCW sample showed that
peaks in this region were characteristic of both the OH
hydroxyl groups and NH amide, showing that there
was a polyamide coating of nanofibers.
The band at 1,650 cm-1 was attributed to the
symmetrical angular deformation of the NH group
and stretching of the C=O bonds, while the band at
1,450 cm-1 corresponded to asymmetric angular defor-
mations of the CH group. The band at 1,370 cm-1 was
due to stretching of the C–N bond with deformation of
NH, while the band at 1,270 cm-1 may have been due to
axial deformation of the CN bond.
It can be observed that the differences between the
FTIR spectra relating to the PA6sol and CCW matrix
Fig. 1 Scanning transmission electron micrograph of uncoated
cellulose whiskers from commercial cotton fibers (scale bar
500 nm)
Cellulose (2014) 21:311–322 315
123
were only due to the addition of PA6 over CWs,
showing no chemical interactions between the poly-
mer matrix and CWs because another peak did not
appear in addition to the existing PA6 or CWs or
significant alterations of them.
Thus, the chemical interaction of CWs and the PA6
matrix could not be confirmed. However, the possi-
bility was not excluded of secondary interactions such
as hydrogen bonding or dipole forces between the
amide groups of PA6 and the hydroxyl groups present
in cellulose whiskers.
The samples were characterized by X-ray diffrac-
tion to evaluate possible changes in the crystallinity of
PA6 due to the addition of CWs, which are more
crystalline than PA6. The XRD patterns are shown in
Fig. 4a. As the calculation of the crystallinity index
was made from the areas under the pseudo–Voigt
curves obtained by deconvolution of the X-ray
diffraction patterns, Fig. 4b–d shows these pseudo–
Voigt curves and the areas under each one, used to
calculate the crystallinity indexes of the samples.
Polyamide 6 can assume two crystallographic
forms, monoclinic a and monoclinic or pseudo-
hexagonal c (Khanna and Kuhn 1997). Phase a is
formed by the hydrogen-bonded sheets antiparallel to
each other, and in the phase c, the hydrogen-bonded
sheets are parallel, causing a torsion of chains in
zigzag planes. As a result of this phenomenon, the
crystalline density and heat of fusion of phase c(weaker interaction among chains) are smaller than in
phase a.
Phase a can be identified in the diffractogram of
PA6sol (Fig. 4c) as peaks a1 and a2, diffracting in 2h at
approximately 20� and 24�, respectively; a2 is related
to the distance between hydrogen bonds of polymeric
chains, and a1 is related to the distance between the
planes formed by these chains; the peak referred to
phase c appears as a shoulder of peak a1 in 2h between
21� and 22� (Bittencourt 2008).
Cellulose whiskers (CWs) present crystalline struc-
tures of cellulose I (Fig. 4b) with peaks at 15�, 17� and
22.7�. Cellulose I can present two polymorphs,
cellulose Ia, which has a triclinic cell containing one
chain, and cellulose Ib with a monoclinic unit cell
containing two parallel chains and Miller indices of
Fig. 2 MEV-FEG micrographs of a PA6sol and b CCWs (scales 100 nm) showing fibrillar (arrows) and spherical (circles)
nanostructures
Fig. 3 FTIR spectra of CWs, CCWs and PA6sol
316 Cellulose (2014) 21:311–322
123
(1�10) (110) and (200) (French 2013). Both cellulose Iaand Ib can present a peak at 20.5� for the (012) and
(102) reflections on the random pattern (French 2013),
appearing as a shoulder on the result of the diffrac-
togram of obtained cellulose whiskers (CWs).
When CWs were coated by PA6, crystals of both
cellulose and PA6 were expected to be observed by
X-ray diffraction. As can be seen in Fig. 4a, d, the
coating of CWs by PA6 (sample CCWs) was effective,
because the XRD pattern of this sample presented
peaks referring to both crystalline cellulose (CW) and
crystals of PA6 (PA6sol), but CWs did not induce the
formation of any other crystallographic form of PA6,
resulting in a physical mixture of CWs and PA6.
Thus, using the method of deconvolution of patterns
to calculate the crystallinity index (Fig. 4b–d), it was
possible to achieve a great increase in the crystallinity
of CCWs compared to PA6sol, as shown in Table 1.
Table 1 presents the data on the crystallinity index (Ci)
and temperature of initial thermal degradation (Tonset)
of CWs, CCWs and PA6sol. The crystallinity indexes
presented in Table 1 for CWs are greater than those
reported in the literature (91 %) (Teixeira et al. 2010),
but it is important to consider that another methodol-
ogy was used to calculate the crystallinity indexes than
deconvolution. Moreover, the calculation of CI was
made by deconvolution to allow using the same
method to calculate the Ci of PA6, CCWs and CWs.
Furthermore, as observed in Table 1, the crystallinity
of CCWs was ruled by the cellulose whiskers (CWs),
the more crystalline phase in the mixture, raising the
crystallinity of the PA6 (PA6sol) used as a coating.
Fig. 4 a XRD patterns of CWs, CCWs and PA6sol; deconvolution of the XRD pattern of b CWs, c PA6sol and d CCWs
Cellulose (2014) 21:311–322 317
123
The thermal stability was determined by thermo-
gravimetric analysis (TGA). Figure 5 presents the
thermal degradation profile of the samples. In Fig. 5a,
it can be noted that CWs presented lower thermal
stability (180 �C), which limits their use in polymers
whose processing temperatures are higher than
170 �C, because above this temperature not only
cellulose whiskers but also ash could be incorporated
into the nanocomposite, not providing the desired
reinforcement. Cellulose releases many combustible
volatiles during degradation, such as acetaldehyde,
propenal, methanol, butanedione and acetic acid,
which can increase the decomposition rate of the
polymer in composites (Qua and Hornsby 2011). This
effect can be observed when coated CWs (CCWs) are
compared to PA6sol.
However, when CCWs are compared to uncoated
CWs, the thermal stability of CCWs is increased by
100 �C, allowing their use as a master batch of CWs to
produce a nanocomposite with PA6.
As can be observed in the DTG graph (Fig. 5b), in
the temperature range of 180–350 �C, CWs present
multiple steps, suggesting different types of crystals.
Thus, multiple peaks on the DTG curves are possibly
cellulose crystals with different sulfonation degrees
(Correa et al. 2010), provided by large-scale produc-
tion of cellulose whiskers. More sulfonated crystals
start degrading at lower temperatures. However, the
sulfate groups present on the surface of crystals can
make them more polar, promoting better interaction
between these more sulfonated crystals and the
polyamide, leading to a substantial increase in the
thermal stability of CCWs.
DSC curves of PA6sol and CCWs are presented in
Fig. 6. The melting temperature on heating [Tm (�C)],
enthalpy of fusion [DHm (J/g)], crystallization tem-
perature on cooling [Tc (�C)], enthalpy of crystalliza-
tion [DHc (J/g)] and crystallinity indexes (%) obtained
from DSC analyses of PA6sol and CCWs are shown in
Table 2.
No significant alterations of melting and crystalli-
zation transition peaks were observed due to coating
CWs with PA6, keeping the profile of PA6sol. Kiziltas
et al. (2011) incorporated from 2.5 to 30 wt% of
microcrystalline cellulose (MCC) in PA6 and did not
find significant dislocation of melting and crystalliza-
tion peaks of PA6 due to the introduction of this filler
in the composite, concluding that the filler did not
influence the melting and crystallization temperatures,
Tm and Tc, respectively. Similar behavior was
observed for PA6 and CW nanocomposites.
In Fig. 6a, an endothermic peak was observed at
around 100 �C. This peak was attributed to the
evaporation of residual water or formic acid, as the
boiling point of formic acid is 101 �C (MSDS of
formic acid), and both the samples were solubilized in
formic acid and precipitated in water. Even after
drying them in an oven for 24 h, there might have still
been some residual water or formic acid. However,
related to the melting peak (Fig. 6a), a modification
was also observed in the form of this PA6sol peak
compared to CCWs. The presence of two distinct
melting peaks in PA6sol (Fig. 6a) can be attributed to
melting of c and a phase crystals (Qua and Hornsby
2011), as the presence of this phase can also be
observed by XRD (Fig. 4). In phase c, the hydrogen
bonded sheets are parallel to each other and in phase a,
antiparallel. Hydrogen bonding between chains in
antiparallel sheets makes the crystalline form more
thermodynamically stable in the a phase, with melting
temperatures between 220 and 229 �C. The melting
temperature of the less stable c phase is between 195
and 219 �C, and it is generally formed under particular
circumstances, such as fast cooling melts (Qua and
Hornsby 2011). In this study, the formation of phase ccould be attributed to the rapid removal of formic acid
through the precipitation of the solubilized PA6 in
water, because rapid solvent loss prevents anti-parallel
packing of hydrogen-bonded sheets, leading to the
formation of this metastable crystalline morphology
(Qua and Hornsby 2011). Cellulose whiskers dis-
persed in PA6 solubilized in formic acid can make its
loss a little slower, decreasing the formation of this
metastable c phase, perhaps because of the affinity of
whiskers with formic acid, which allowed the anti-
parallel packing of hydrogen-bonding sheets.
On cooling (Fig. 6b), PA6sol presented a crystal-
lization temperature (Tc) of 188.4 �C, whereas CCWs
presented a Tc of 191.0 �C. This slight increase in
Table 1 Crystallinity index (Ci) and initial temperature of
thermal degradation (Tonset)
Sample Ci (%) Tonset (�C)
CWs 97.3 180
CCWs 96.4 282
PA6sol 76.6 370
318 Cellulose (2014) 21:311–322
123
crystallization temperature can be attributed to nucle-
ation of PA6 crystals induced by cellulose whiskers,
resulting in a slight increase of the crystallinity of the
PA6 coating of CWs (Table 1).
Thus, this was an effective method for coating
cellulose nanofibers, increasing their thermal stability
without affecting the physical and morphological
properties, and it can be used to obtain PA6 nano-
composites reinforced with cellulose nanofibers
coated with the same polymer.
In this way, the CCWs were mixed with PA6 pellets
in order to obtain a nanocomposite filled with 1 wt%
of CW (PA6 ? 1CW). To evaluate the effect of the
filler (CW) on the PA6 matrix, PA6 ? 1CW was
compared to PA6 ? PA6sol, a blend of PA6 and
PA6sol, the neat coating of the whiskers. It was not
possible to analyze a nanocomposite prepared with
uncoated CWs because the temperature used in the
processing of the samples was higher than the thermal
stability of the uncoated CWs allowed, resulting in a
sample with fully degraded filler and black color, not
Fig. 5 TG (a) and DTG (b) curves of CWs, CCWs and PA6sol. Air atmosphere
Fig. 6 DSC curves of PA6sol and CCW samples: a heating and
b cooling
Table 2 Thermal properties and crystallinity index by DSC of
CCWs, PA6sol, PA6p, PA6 ? PA6sol and PA6 ? 1CW
Sample Tm (�C) DHm
(J/g)
Tc (�C) DHc
(J/g)
Ci (%)
CCWs 220.3 78.7 191.0 60.0 54.9
PA6sol 220.8 70.2 188.4 66.6 41.2
PA6p 222.5 71.0 187.3 64.2 37.2
PA6 ? PA6sol 222.4 72.2 186.7 67.7 37.8
PA6 ? 1CW 222.9 68.1 192.0 63.1 36.1
Cellulose (2014) 21:311–322 319
123
analyzed in this study. In order to evaluate the effect of
the addition of solubilized and precipitated PA6, both
were compared to the neat PA6, processed as the
nanocomposite and blend, called PA6p.
The thermal and mechanical analyses of the
nanocomposite (PA6 ? 1CW), PA6 ? PA6sol and
PA6p are presented below.
Figure 7a, b shows the DSC curves of the samples
on heating and cooling, respectively, and Table 2
presents the values of Tm (�C), melting enthalpy (DHm
in J/g), Tc on cooling (�C), crystallization enthalpy
(DHc in J/g) and crystallinity indexes (%) obtained by
DSC analyses of PA6p, the blend PA6 ? PA6sol and
nanocomposite PA6 ? 1CW. As shown in Fig. 7a, the
nanocomposite PA6 ? 1CW did not present signifi-
cant dislocations at the melting peak because of the
presence of cellulose whiskers compared to PA6 and
PA6 ? PA6sol.
Observing the melting peaks of PA6p and
PA6 ? 1CW, it can be noted that the introduction of
CCWs in PA6 promotes slight changes in Tm
compared to the control PA6p or PA6 ? PA6sol.
Regarding the Tc of PA6p, the semicrystalline phase of
the nanocomposite, Table 2 shows that the addition of
CCWs caused a slight increase in Tc compared to
PA6p, i.e., the cooling process began to form crystals
at higher temperatures than in pure PA6p. This fact
can be attributed to the performance of CWs as a
nucleating agent in the nanocomposite, inducing
crystal formation at higher temperatures, but since
an increase in the overall crystallinity of nanocom-
posites was not observed (Table 2), this effect could
be due to the interaction of nanofibers with polyamide
to restrict the mobility of the polymer chains (Oliveira
et al. 2011). The crystallinity indexes (%) obtained by
DSC analyses, shown in Table 2, follow the same
tendency as that obtained by XRD, i.e., CCWs present
the higher Ci (*55 %) because of their large number
of cellulose whiskers, followed by PA6sol with a
crystallinity index of *41 %. The PA6sol sample was
solubilized in formic acid and re-precipitated in water,
so this rapid removal of formic acid could prevent anti-
parallel packing of hydrogen-bonded sheets, leading
to the formation of the c phase, a metastable crystalline
morphology (Qua and Hornsby 2011), as discussed
above. Concerning the PA6p, PA6 ? PA6sol and
PA6 ? CW samples, the crystallinity was ruled by the
neat PA6; as the introduction of PA6sol and CWs in
PA6 was 2 and 1 %, respectively, it did not interfere
with the crystallinity of the samples.
Typical stress–strain curves are shown in Fig. 8,
and the values for the tensile strength (reported as
maximum tensile strength), elastic modulus and
elongation at break are presented in Table 3. Figure 8
shows the typical behavior of a ductile semicrystalline
material; the coating of PA6sol did not affect the
mechanical properties of the PA6 ? PA6sol sample
compared to the neat PA6p. The nanocomposite
PA6 ? 1CW presented an increase in the elastic
modulus of 30 % compared to PA6 ? PA6sol, with-
out loss of maximum tensile strength. However, if
PA6 ? 1CW is compared to PA6p, the increase in
elastic modulus is around 45 %, a significant increase
as well, considering that there was only 1 wt% of CWs
(or 3 % of CCWs) incorporated in the PA6 ? 1CW
nanocomposite, and there was no loss of maximum
tensile strength supported by the sample, but there was
Fig. 7 DSC curves of PA6p, PA6 ? PA6sol and PA6 ? 1CW
samples on a heating and b cooling
320 Cellulose (2014) 21:311–322
123
a considerable loss of elongation to break. This
increase in the elastic modulus reinforces the state-
ment that the CWs were homogeneously dispersed in
the polymer and suggests that there was an efficient
transfer of stress between the matrix and CWs whose
hydroxyl groups could form hydrogen bonds with the
amines, resulting in good interfacial adhesion (Qua
and Hornsby 2011).
Conclusion
It was possible to obtain a good dispersion of cellulose
whiskers obtained from commercial cotton fibers in
polyamide 6 via solubilization of this polymer in a
suspension of cellulose whiskers in formic acid
followed by its precipitation in water to obtain
cellulose whiskers coated by polyamide 6, with a high
concentration of whiskers. By FTIR and XRD mea-
surements, it was possible to observe peaks of both
phases, cellulose whiskers and PA6, suggesting that the
cellulose whiskers were not removed by washing,
supposedly being coated by polyamide 6. The SEM
micrographs also showed cellulose whiskers well
dispersed through the PA6 matrix. Thermal analysis
showed that the thermal stability of uncoated cellulose
whiskers was relatively low (180 �C), limiting their
use to polymers whose processing temperatures are
higher than 170 �C, but CCWs presented an increase of
approximately 100 �C in thermal stability, which
allowed its incorporation in polyamide 6 by conven-
tional processing of extruding and injection molding.
In this way, the CCWs were applied to PA6 to
obtain a nanocomposite with a concentration of 1 wt%
of whiskers (PA6 ? 1CW). The extruded and injec-
tion molded PA6 ? 1CW nanocomposite was evalu-
ated for mechanical strength and presented an increase
in elastic modulus of 45 % compared to the neat PA6p,
without loss of maximum tensile strength, and a
significant increase as well, since only 1 % of coated
whiskers was added to the nanocomposite, but the
nanocomposite presented a considerable loss in elon-
gation at break, which would be expected for a more
rigid sample.
Acknowledgments The authors gratefully acknowledge the
financial support provided by FAPESP (process no. 08/03606-
9), CAPES, FINEP and EMBRAPA.
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