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: carol_correa@hotmail.com

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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