SPECIAL ISSUE: NANOSTRUCTURED MATERIALS 2010

Processing and properties of carbon nanofibers reinforcedepoxy powder composites

C. Palencia • M. A. Mazo • A. Nistal • F. Rubio •

J. Rubio • J. L. Oteo

Received: 14 September 2010 / Accepted: 7 March 2011 / Published online: 18 March 2011

� Springer Science+Business Media B.V. 2011

Abstract Commercially available CNFs (diameter

30–300 nm) have been used to develop both bulk and

coating epoxy nanocomposites by using a solvent-free

epoxy matrix powder. Processing of both types of

materials has been carried out by a double-step process

consisting in an initial physical premix of all compo-

nents followed by three consecutive extrusions. The

extruded pellets were grinded into powder and sieved.

Carbon nanofibers powder coatings were obtained by

electrostatic painting of the extruded powder followed

by a curing process based in a thermal treatment at

200 �C for 25 min. On the other hand, for obtaining

bulk carbon nanofibers epoxy composites, a thermal

curing process involving several steps was needed.

Gloss and mechanical properties of both nanocom-

posite coatings and bulk nanocomposites were

improved as a result of the processing process.

FE-SEM fracture surface microphotographs corrobo-

rate these results. It has been assessed the key role

played by the dispersion of CNFs in the matrix, and the

highly important step that is the processing and curing

of the nanocomposites. A processing stage consisted in

three consecutive extrusions has reached to nanocom-

posites free of entanglements neither agglomerates.

This process leads to nanocomposite coatings of

enhanced properties, as it has been evidenced through

gloss and mechanical properties. A dispersion limit of

1% has been determined for the studied system in

which a given dispersion has been achieved, as the

bending mechanical properties have been increased

around 25% compared with the pristine epoxy resin. It

has been also demonstrated the importance of the

thickness in the nanocomposite, as it involves the

curing stage. The complex curing treatment carried out

in the case of bulk nanocomposites has reached to

reagglomeration of CNFs.

Keywords Carbon nanofibers � Epoxy powder

composites � Nanocomposite coatings

Introduction

The development of composite materials began in

the mid-twentieth century, as a consequence of the

C. Palencia (&) � M. A. Mazo � A. Nistal �F. Rubio � J. Rubio � J. L. Oteo

Departamento de Quımica Fısica de Superficies y

Procesos, CSIC, Instituto de Ceramica y Vidrio,

C/Kelsen 5, 28049 Madrid, Spain

e-mail: cristinapr@icv.csic.es

M. A. Mazo

e-mail: sandra@icv.csic.es

A. Nistal

e-mail: andresnistal@icv.csic.es

F. Rubio

e-mail: frubio@icv.csic.es

J. Rubio

e-mail: jrubio@icv.csic.es

J. L. Oteo

e-mail: meh@icv.csic.es

123

J Nanopart Res (2011) 13:6021–6034

DOI 10.1007/s11051-011-0331-1

necessity of little heavy materials combined with the

best mechanical properties and strength characteris-

tics of metals. Composite materials consist in the

combination of two or more components to obtain a

material of enhanced properties. Composites present

synergy properties respect to their components. Such

components in a composite must be physically

indistinguishable, chemically different, each inert

and insoluble, in such a way that an interface acts as

a boundary between the composite components.

Composite materials are formed by a major com-

ponent, named matrix, and one or more reinforcing

phases. Reinforcing phases are usually responsible

for the mechanical and functional properties in

composites, while the matrix acts as a continuously

material which generates cohesion to the composite,

as it provides the binding and proper distribution of

the reinforcing phases. Also, matrix phase prevents

cracks propagation.

The reinforcing phase choice is a very sensitive

matter, since composite properties would be deter-

mined by it. The growing interest shown for CNFs in

the last two decades has involved a great effort in

researches related to novel applications of such

materials, as it is well-known the high variety of

outstanding properties attributed to these materials.

Among others, most recent research papers have

reported the rule of CNTs and CNFs as reinforcing

phases in composites (Thostenson et al. 2001;

Hu et al. 2006; Gibson et al. 2007; Bokobza 2007).

Although in principle, CNFs show high quality

properties to act as reinforcing phase in nanocom-

posites formed with any kind of matrix, it seems that

scientific efforts have been directed to polymer

matrix above all, epoxy resins (Li et al. 2000; Ren

et al. 2003; Zhang et al. 2007; Thostenson and Chou

2006), since polymer matrixes are inexpensive and

lightweight materials, which made them appropriated

for a wide range of applications (Shen et al. 2007).

Obtaining CNFs-polymer composites of excellent

properties goes through the achievement of a well-

dispersed reinforced phase into the matrix. A proper

interfacial adhesion between composite phases is also

required, since it promotes the dispersion and facil-

itates the stress transmission from the matrix to the

reinforcing phase (Feng et al. 2010; Gojny et al.

2005; Gojny et al. 2004; Song and Youn 2005;

Fiedler et al. 2006). Obviously, chemical nature and

surface active sites of the phases play the main rule in

the interfacial adhesion between them, but also, in the

case of CNFs, it has to be taken into account the

entanglements and agglomerates formed. As it has

been reported in the bibliography, CNFs tends to

form entanglements because of both their surface

properties and their way of production (Feng et al.

2010; Gojny et al. 2005; Gojny et al. 2004; Fiedler

et al. 2006; Sandler et al. 2003; Ma et al. 2010).

In order to improve the interfacial adhesion between

CNFs and polymer matrixes surface modification of

one or the two phases can be carried out (Ma et al. 2007;

Li et al. 2005), but composite processing researches

can also improve interfacial adhesion, through remov-

ing entanglements and agglomerates of CNFs.

Indeed, composite processing research should leads

to an improvement in CNFs dispersion (Feng et al.

2010; Gojny et al. 2004; Sandler et al. 1999; Schulte

et al. 2005; Breton et al. 2004; Fidelus et al. 2005;

Song and Youn 2005; Miyagawa et al. 2006). Then,

the achievement of a proper composite processing,

seems to be the clue before going into the surface

modified CNFs composites properties in depth.

In the last few years there have been published a

variety of research works focused in CNFs polymer

composites. Although in some cases composites of

enhanced properties have been obtained, in most of

cases entanglements and agglomerates elimination is

still a problem to solve in composite processing to

obtain composites with the expected mechanical

properties. As far as the authors concerned, almost

all are referred to liquid polymer resins (Gojny et al.

2004; Fiedler et al. 2006; Sandler et al. 2003; Ma

et al. 2010; Song and Youn 2005). Powder polymers

possess interesting properties over liquid polymers,

due to the absence of solvents and the recyclability.

Such solvent-free polymers avoid pollution prob-

lems during the synthesis, manufacturing and appli-

cation of the product, but also decrease the

flammability danger and have no toxicity (Barletta

et al. 2007). Besides that, waste management is

simpler and products can be reused. Such properties

are the main reason for the use of powder resins as

coatings, which have been their main application

over the years. Nevertheless, as far as the knowl-

edge, the use of powder resins to develop bulk

composites has not been reported in the bibliogra-

phy, and the authors strongly consider the possibility

to obtain powder composites of excellent mechan-

ical properties combined with the advantage of the

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use of solvent-free resins. The authors think the

elimination of solvents in the composite processing

could improve the process, as liquid media favors

entanglements formation.

In this context, this study is focused in developing

a proper processing and the study of the resulting

properties of CNFs-epoxy powder nanocomposites.

Indeed, the objective is to develop both nanocom-

posite coatings as well as bulk nanocomposites.

Experimental

Materials

CNFs used in this study (Grupo Antolın Ingenierıa,

Spain) were those named as GANF. They were

synthesized according the floating catalyst method,

by pyrolysis of hydrocarbon gasses in the temperature

range between 950 and 1300 �C. The CNFs present a

wide range of properties, as it has been described

elsewhere (Martın-Gullon et al. 2006).

The epoxy powder matrix (GX) was provided by

Govesan S.A. (Madrid, Spain) and consisted in an

epoxy powder resin based on glycidyl ether of

bisphenol A, which presents a density between 1.2

and 1.6 g cm-3, and a grain size range between 10 and

100 lm. Different additives were used, as hardeners,

degasifiers, and dispersants. Both epoxy resin powder

and additives were solvent-free products, and were

used as received. The components of the epoxy powder

matrix employed are given in next Table 1.

Methods

The GANF-GX powder nanocomposites preparation

has been carried out into two stages: firstly a premix

of CNFs with GX through an extrusion process and,

secondly processing the composite by electrostatic

spraying or by mold compression. The next Scheme 1

shows these procedures.

The GANF-GX premixed nanocomposite powders

were processed by extrusion in a Werner Pleiderer

ZSK 25 P8 2 WLE type extruder with double

corotating screw, working in a temperature range

between 110 and 120 �C, remaining all the compo-

nents in a fluent state. The pellets obtained after

extrusions were grinded and sieved to obtain the

precured nanocomposite powders. These powders

were used for obtaining coatings or bulk nanocom-

posite materials.

Nanocomposite coatings of 110 lm thick were

obtained by electrostatic painting by using solid

precured powder nanocomposites. Steel plates were

used as substrates. Curing process was carried out in

an electric oven at 200 �C for 25 min.

Bulk nanocomposites were prepared in a brass

mold. Proper sized pieces according to ISO 178

three-point bending standard test were filled with the

precured nanocomposite powders and cured in an

oven up to 200 �C.

Characterization

Gloss properties measurements were made with a

Konica Minolta Multi Gloss 268 instrument, and

were carried out according to ISO 2813 standard test.

Three different angles (208, 608, and 858) were

registered, and each datum is an average of 20

measurements for each angle.

Differential scanning calorimetry (DSC) analysis

was conducted on a DSC 7 Perkin Elmer equipment.

The measurement was performed at 7 �C/min and up

to 275 �C under nitrogen atmosphere, in order to

Table 1 Nature, function,

and concentration of the

components in epoxy-

powder matrix

Component Nature Function Concentration (%)

GX DGEBA type resin Epoxy polymer base 90

Dispersant Not provided Dispersant 10

Dyhard 100 H2N–C(NH2)=N–CN Curing agent

Benzoin Ph–CH(OH)–C(=O)–Ph Degasifier

Byk 360 Acrylic polymer adsorbed

on silicon dioxide

Leveling agent

J Nanopart Res (2011) 13:6021–6034 6023

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examine the curing kinetics of the GX powder matrix.

Glass transition temperature (Tg), the initial temper-

ature of the cross-linking reactions as well as the

enthalpy of the curing processes (DH) were also

determined.

Mechanical properties for coatings and bulk

nanocomposites were determined according to dif-

ferent methods. Impact and bending tests were used

in the case of nanocomposite coatings, while three-

point bending tests were carried out in the case of

bulk nanocomposites. Impact tests were carried out

according to UNE-EN-ISO 6272 standard test by

dropping a stainless steel ball (20 mm diameter)

from 70 cm over the nanocomposite coating. UNE-

EN-ISO 1529:2002 was the standard test used to

carry out the bending test. A mandrel tester with a bar

of 5 mm thick was employed. The stainless steel

plate painted with the nanocomposite coating was put

into the tester and bent over the bar in a 1808 angle.

Coatings with no cracking or breaks were considered

to pass the bending test, and when a number of

crackings were appreciated, there were used as a

damage indicator.

Three-point bending tests for bulk nanocomposites

were conducted at room temperature in an Adamel

Lhomargy universal tester, equipped with a cell load

of 10 kN. The crosshead speed was 0.02 mm s-1,

and the span was 64 mm. Bulk nanocomposite

specimens were mechanized according to ISO 178

standard test. From the stress–strain curves obtained,

the maximum bending stress and the slope in the

linear trend of the curves were determined, in order to

calculate the flexural strength and Young modulus

(E) data, through Eqs. 1 and 2.

Flexural strength ¼ 3=2 S L=a b2� �� �

ð1Þ

E ¼ s L2= 4 a b3 1000� �

ð2Þ

where S is the applied strength, L the span,

s the linear section of the curve slope, and a and

b are the width and height of the tested sample,

respectively.

Scanning electron microscopy was used to exam-

ine the fracture surface. Characterization was carried

out using a field emission electron microscope

(FE-SEM, Hitachi 4700) operating at 20 keV.

Results and discussion

Processing precured nanocomposite powders

Processing of nanocomposites implies to achieve a

suitable dispersion of the reinforcement phase into

the matrix, being one of the main clues to obtain

materials with excellent properties. The development

of nanocomposites based in solvent-free resins

assumes, in principle, to deal with a solid state

processing of each component. Nevertheless, the

curing stage of such solvent-free resins includes, at

least in this case, a fluent state of the epoxy powder

resin at temperatures higher than Tg. The analysis of

the corresponding DSC curve allows studying some

complex processes involved in powder nanocompos-

ite processing and curing. Figure 1 shows the DSC

curve of the GX powder matrix used in this study.

First of all, it has to be noted that the DSC diagram

given in Fig. 1 is the typical one for a thermoset-

ting material. Processes occurring between 50 and

150 �C are related with the precured nanocomposite

powder processing (extrusion), while the ones in the

150–275 �C range have to be with curing stage.

Analysing the first interval, it can be seen at low

temperatures a small increase of the heat flow, that is

related with Tg. In this case, the Tg is registered at

55 �C, which is in the usual range for thermosetting

coatings (Gherlone et al. 1998). At higher tempera-

tures than Tg the DSC diagram shows a decrease of

the heat flow denoted by a fluent state of the material,

which seems to be the proper time to mix the

reinforcing phase with the matrix, with higher

possibilities of a better dispersion of the nanocom-

posite components. Extrusion processing parameters

were selected carefully accordingly with DSC data.

The DSC curve given in Fig. 1 indicates that the

temperature range related to the fluent state is around

80 and 140 �C. From some initial processing expe-

riences carried out in the lab, the authors can assure

that the fluent state takes place approximately at

(110–120) �C. Therefore, the extrusion temperature

in the heating zone chosen was 120 �C. As can be

seen in Fig. 1, although the observed fluent state

occurs at 120 �C, the whole interval covers 60 �C,

allowing the epoxy matrix to house reinforcing

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phases of a variety of natures and contents before the

cross-linking reactions begin.

The processing stage has been optimized by

carring out three consecutive extrusions of each

sample. An increase of the number of extrusions as

well as the higher amount of CNFs content affect

the fluent state, as the mix gets several viscosity

levels. An increase of the viscosity in CNFs/polymer

composites has also been reported by Guo and

co-workers (2010a, b) and Sandler et al. (2003),

who described how the agglomerates and entangle-

ments formed by CNFs reinforcing phase lead to

higher viscosities. The curing stage interval in the

DSC diagram is dominated by an increase of the

heat flow from 150 to 180 �C approximately. From

such temperature until 230 �C the curve shows

a plateau and after, the heat flow begins to decrease

to a minimum and increase again. Such interval is

denoted by cross-linking reactions, and the peak area

allows obtaining the enthalpy of the curing reaction.

Such behavior is related with the curing reactions

between the hardener and other additives with the

matrix, that are responsible for the nanocomposite

hardening and curing. These processes take place in a

Fig. 1 Differential

scanning calorimetry of

epoxy powder matrix GX

Scheme 1 Procedures

carried out for preparing

CNFs epoxy nanocomposite

coatings and bulk ones

J Nanopart Res (2011) 13:6021–6034 6025

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wide temperature interval, meaning that the chosen

epoxy resin seems to be proper as a matrix phase in

CNFs composites, as the curing interval is long

enough to hardening and curing to occur in different

CNFs contents. Moreover, the fact that the processing

and curing intervals could be distinguished in the

DSC diagram indicate that the matrix composition is

suitable for an extrusion process, as the temperature

interval in which cross-linking reactions occur is

lower and differentiated compared with the curing

stage interval. That allows the matrix to house

different CNFs contents without expecting dramatic

changes in the temperature intervals. Extrusion

process does not interfere in the curing, as the

corresponding temperatures are so different.

As it has been mentioned before, the curing interval

in the DSC diagram is related with the curing stage. In

order to study the CNFs content influence in the

curing process there have been carried out precured

nanocomposite powders with 0.5, 1, and 1.5% CNFs

content. Higher amounts of CNFs contents were not

possible to add to the epoxy matrix and extrude, as the

mix reached high viscosity and density, which hinders

the extrusion process. The enthalpy of the curing

reactions in each case can be calculated from the heat

flow curve in DSC diagrams. Next Table 2 presents

the enthalpy of the curing reaction as well as the

maximum catalysis temperature.

A decrease of the enthalpy of reaction with higher

amounts of CNFs content in the precured nanocom-

posite powder is observed from data in Table 2. The

highest enthalpy of curing reaction is the one in the

epoxy matrix, which means that the presence of

reinforcing phase hinders the curing reactions. This

was also concluded by Gerson et al. (2010), who

studied SWCNTs/epoxy composites and found an

increase in the Tg for the nanocomposites compared

with the pristine epoxy, indicating that the reinforc-

ing phase was affecting the cross-linking reactions.

Regarding this study, it seems that the high-specific

volume characteristic of CNFs acts as an impediment

in the curing reactions, as it acts as discontinuously

points in the matrix. The tendency of CNFs to form

entanglements may be a key factor in the hinder role

of CNFs. What is more, while the increase of CNFs

content is always 0.5%, the decrease of the enthalpy

of reaction is of around 10 J g-1 between the epoxy

matrix and the nanocomposite powder reinforced

with 0.5%, each increase of CNFs content in 0.5%

means a decrease of the enthalpy of reaction of

5 J g-1. This could indicate that, although the new

phase hinders the curing reactions, the presence of

entanglements and agglomerates seems to be the key

factor. According to that, new additions of CNFs

should not lead to higher decreases in the enthalpy of

reaction, as such new amounts of CNFs may be

attracted to CNFs entanglements, and the macro-

scopic effect should not changed.

The nanocomposite processing includes not only

the precured powders processing, but also the curing

stage, that provides the nanocomposite its own prop-

erties. As the curing process implies the irreversible

hardening of the material, this process must be carried

out over the powdered material in its final form. What

is more, curing process and shaping process have to be

done in one single step. The two following sections

lead with this final stage in the cases of nanocomposite

coatings and bulk nanocomposites.

Nanocomposite coatings

The nanocomposite coatings were manufactured via

electrostatic paintings from the obtained precured

nanocomposite powders. The electrostatic painting

was carried out over a stainless steel plate by

projecting the nanocomposite powders through an

electrostatic painting instrument. Once the powders

have been projected and spread over the metal plate,

it is necessary to cure it. Attending at the DSC

diagram given in Fig. 1, the epoxy matrix gets cured

in the temperature interval between 180 and 230 �C.

Having into account the curing time, it has been

established that a curing process at 200 �C for 25 min

provides the nanocomposite its curing level, and

acquire its best properties.

The thickness of the coatings manufactured by this

process has been measured, and it was found that the

average thickness was around 110 lm.

Table 2 Enthalpy of curing reactions and maximum catalysis

temperature for extruded nanocomposite powders with 0%,

0.5%, 1, and 1.5% GANF content

Sample DH (J g-1) T max. (�C)

Ep-0G 105.9 209.8

Ep-0.5G 95.7 209.8

Ep-1G 90.2 208.8

Ep-1.5G 85.4 210.0

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As processing nanocomposites difficulties have

been long demonstrated, it was suggested to verify if

such conditions were good enough as processing

conditions or, by contrast, the processing method

could be improved. So that, it was carried out new

nanocomposite powder samples by increasing the

number of consecutive extrusions up to three, in the

processing stage. This limiting extrusion number was

imposed by the increase of viscosity achieved in

the extruder in the third consecutively extrusion

carried out, that made impossible the rise in the

extrusions number. Nanocomposite coating proper-

ties such as gloss and impact test were examined

in order to evaluate the effectiveness of the consec-

utive extrusions in processing CNFs-epoxy powder

nanocomposites.

In next Fig. 2, gloss properties measured at three

different angles of nanocomposite coating sam-

ples extruded one, two or three consecutive times

are shown.

As can be seen in Fig. 2, an increase of the number

of extrusions results in a subsequent gloss rise of the

related coatings. What is more, this trend seems to be

more significant between the second and the third

extrusions instead of first and second. This result is

consistent in each measured angle. That means that,

at least, the reduction of number and size of the

agglomerates is not easy to obtain, and consecutive

extrusions are necessary. The reduction of the

agglomerates may be achieved through the reduction

of the entanglements, followed by the elimination of

such size reduced agglomerates. For that, a process-

ing including two consecutive extrusions results in

better gloss properties compared with a sample

extruded only once (Fig. 2) probably due to the

reduction of the size of the entanglements. The third

consecutive extrusion seems to eliminate some of the

reduced entanglements, reaching a better dispersed

composite with improved gloss properties. Taking

into account that gloss properties are considerable

characteristics in coatings, results in Fig. 2 may

evidence that precured nanocomposite powders

extruded three consecutive times result in an

improvement of the resulting nanocomposite coat-

ings dispersion. The high-gloss values measured in

the case of nanocomposite coatings involves a

high quality of such coatings, so that, it could be

assumed that the extrusion processing has eliminated

entanglements and CNFs agglomerates. Therefore,

mechanical properties should also have been improved

as a consequence of a three consecutive extrusion

processing so that, there were measured. In this case

impact tests were examined; as such properties are also

indicative of the coatings quality. In next Fig. 3

photographs of the coatings after the impact tests are

Fig. 2 Gloss measurements of nanocomposite coatings

extruded one, two or three consecutive times, measured at

three different angles

Fig. 3 Photographs of the impact tested resulting nanocom-

posite coatings developed from nanocomposite powders

extruded one (a), two (b), or three (c) consecutive times

J Nanopart Res (2011) 13:6021–6034 6027

123

shown. The marks in the coatings have a diameter of

20 mm.

As photographs in Fig. 3 evidence, the coating in

the impact zone appears broken in the cases in which

the precured nanocomposite powders were processed

with one or two consecutive extrusions (Fig. 3a, b),

while in the case of three consecutive extrusions, the

nanocomposite coating does not show any damage

(Fig. 3c). What is more, it has to be emphasized that

the coating related to one-extruded powders shows

two rips, while the increment of one more extrusion

to the processing, results in a reduction of the coating

damage, as evidenced by the presence of only one rip

in the impact zone for the twice extruded coating.

Taking as a premise that the nanocomposite

properties are directly related to the dispersion

degree, these last results indicate that, on the whole,

an increase in the number of consecutive extrusions

let to achieve well-dispersed precured nanocomposite

powders which lead to high quality coatings, as gloss

properties and impact tests has revealed.

The nanocomposite coatings quality has been

determined by mechanical properties, such as impact

and bending tests. Both tests provide only qualitative

information. Referring to the impact tests, the starting

epoxy matrix did not resist the impact dropped from

heights higher than 10 cm, while the epoxy matrix

reinforced with CNFs and processed as shown before

did not show any damage neither ripping in the

coating when the impact test were carried out

dropping the ball from a height of 90 cm. That

means the processing described to obtain epoxy

powder nanocomposites reinforced with CNFs leads

the related coatings to an improvement of an impact

resistance nine times higher than the starting epoxy

powder matrix, evidencing that the three consecutive

extrusion processing has achieved a proper dispersion

of CNFs in the matrix.

The bending tests carried out over both the starting

epoxy powder matrix and the obtained nanocompos-

ites were overcome in both cases. At least, it means

that the initial bending properties of the starting

epoxy matrix have not been affected by the nano-

composite coating processing.

Bulk nanocomposites

Undoubtedly, a proper study of bulk nanocomposites

quality has to be with their mechanical properties,

such as three-point bending tests. In order to com-

pare the measured mechanical properties with those

obtained by other authors, the three-point bending

test has been carried out according to ISO 178. So

that, bulk nanocomposite specimens was prepared

according to certain dimensions. Again, curing and

shaping stages have been carried out in one single

step. Likewise it was done in shaping and curing

nanocomposite coatings, in this case, the extruded

and processed nanocomposite pellets were grinded

and sieved.

Although the curing process carried out in the

case of coatings was simple, this turned out to be a

complex process in the case of curing nanocompos-

ites with appreciable thickness (bulk nanocompos-

ites). Moreover, specimens obtained in a silicon

mold and cured at 200 �C for 25 min showed a

considerable degree of porosity and an irregular

shape. That proved that the key point in the shaping

and curing processes in bulk nanocomposites is their

thickness, as it hinders the escape of the resulting

gases in the curing process, leading to porous

specimens. Consequently, the curing experiments

for bulk nanocomposites were designed to avoid

porosity and shapeless nanocomposites. A series of

different experiments were developed, all of them

based in three premises: using a brass mold instead

of a silicon one, decreasing the heating and cooling

rates and extending the dwell times in the temper-

atures at which the nanocomposite show a fluent

state. Figure 4, presents the different curing exper-

iments developed.

The achievement of the best conditions in the

curing process to obtain non porous neither shapeless

bulk nanocomposites went through the optimization

of different factors: From experiment 1–2 the heating

and cooling rates were modified, decreasing them, but

the specimens obtained kept showing appreciable

porosity. Further, in curing experiment 3, it was

established a dwell time of 25 min at 120 �C, as at

this temperature the nanocomposite presents a fluent

state (see Fig. 1). As a result, specimens obtained

showed less porosity degree, indicating that the

longer dwell times in the curing process were needed.

So that, the following curing experiments was

focused in the raise of dwell times in the fluent state.

Experiment 4 consisted in two consecutive dwell

times of 3 h at 110 and 120 �C and, in the case of

experiment 5; a dwell time of 12 h at 120 �C was

6028 J Nanopart Res (2011) 13:6021–6034

123

employed. The nonporous and required-form speci-

mens were obtained in curing experiment 4.

In conclusion, the best curing process parameters

to obtain bulk nanocomposites of high quality were:

heating rate of 0.5 �C/min and dwell times of 3 h at

110 and 120 �C. In such conditions gasses formed as

a consequence of the cross-linking, hardening and

curing reactions may escape from the material, as the

dwell time in the fluent state and the heating rate are

longer enough. The use of dwelling times as well as

slow heating rates has also been employed in the

optimization of the curing stage in VGCNFs with

epoxy matrix (Seyhan et al. 2009).

Effect of reinforcing phase content

in nanocomposite properties

Likewise precured nanocomposite powder process-

ing is analogous to develop nanocomposite coatings

as well as bulk nanocomposites, it has been proved

the importance the nanocomposite thickness has in

order to manufacture the final nanocomposite, as

the curing and shaping process in bulk nanocom-

posites manufacture are longer and more complex.

So that, the analysis of reinforcing content phase

has in the nanocomposite has been carried out in

bulk nanocomposites. For that, the authors have

analyzing the three-point bending tests of bulk

nanocomposites manufactured with 0.5, 1, and 1.5%

of CNFs. Figure 5 shows the flexural strength

(a) and Young modulus (b) measured from the

strain–stress curves.

As can be seen in Fig. 5, the addition of CNFs to

epoxy powder matrix results in an appreciable

improvement in both the flexural strength and Young

modulus. It has to be noted that both show a very

similar trend as a function of CNFs content, which

may mean that these properties are related mainly

with nanocomposite processing, that is to say with the

dispersion degree in the nanocomposite. Having that

into account, it can be derived that one of the main

problems in manufacturing nanocomposites, above

all in powder solvent-free ones, is to achieve a high-

dispersion degree. In other words, the improvement

in mechanical properties of epoxy powder matrix by

adding CNFs as reinforcing phase is an evidence of

an achievement of a proper CNFs dispersion into the

epoxy matrix following the extrusion processing

mentioned before. The three extrusions processing

that this study reports may act reducing the number

and size of the agglomerates. Recently, Gershon et al.

(2010) published a work where it was reported the

relation between the size of the agglomerates in

CNFs/epoxy composites and the subsequent disper-

sion of the CNFs into the matrix. In such study it was

established a clear dependency of the agglomerates

size and the length of sonication time used to disperse

the CNFs in a liquid epoxy matrix.

A Young modulus of around 230 GPa has been

attributed to CNFs used in this study but, in this

study the values measured for the Young modulus

of the nanocomposites developed are smaller. This

may be due to the tendency of CNFs to kink itself,

which cause a reduction in their effective stiffness

Fig. 4 Curing experiments to obtain bulk nanocomposite specimens

J Nanopart Res (2011) 13:6021–6034 6029

123

(Guo and co-workers 2010a, b; Chasiotis and

co-workers 2010). However, Gershon et al. (2010)

measured a Young modulus for CNFs in epoxy

matrix of around 30 GPa, while the values observed

here are one order of magnitude lower. This

difference can be attributed to the nanofilament

diameter, and diameter range. As mentioned before,

the higher diameter range of CNFs sample could

have a negative effect in the dispersion through

difficulties in the reduction of agglomerates, which

has been traduced in the mechanical response.

Finally, it has been proved that CNFs can act as an

adequate reinforcing phase in solvent-free matrix if a

proper dispersion process is carried out. So that, and

considering the three consecutive extrusion process-

ing carried out such a proper one to get highly

dispersed CNFs-epoxy powder nanocomposites, fur-

ther improvements in nanocomposite properties could

be expected as the reinforcing phase content raise.

Surprisingly, the best mechanical properties have

been obtained in the case of CNFs content of 1% and,

raising these CNFs contents implies a low decrease in

mechanical properties (see Fig. 5). These results are

in accordance again with the study by Gershon et al.

(2010), who demonstrate the appearance of a limit of

dispersion of the CNFs in the epoxy matrix, which

was evidenced by mechanical properties, similarly

than in this study.

Table 3 gives the percentage improvements

achieved in epoxy nanocomposites with increasing

CNFs contents.

An exhaustive analysis of data shown in Table 3

corroborates the hypothesis that mechanical proper-

ties can be used as a dispersion reference. As can be

seen, the percentage of improvement of either the

flexural strength or Young modulus, are quite similar

in each CNFs content; at least, in the case of 0.5 and

1%. This may indicate that, in such CNFs contents,

mechanical properties are directly related with nano-

composite dispersion, together with CNFs efficiency

as reinforcement. However, in the case of 1.5% CNFs

content the three-point bending test show two main

differences with the ones of lower CNFs contents. On

one hand, a decrease of flexural strength and Young

modulus, related to 1% content, but even higher

compared with the 0.5% content nanocomposite. On

the other hand, the increase of flexural strength and

Young modulus for 1.5% CNFs content, does not

present the same trend referring to 1% CNFs content.

As a consequence, it can be derived that reinforcing

epoxy matrix with 1.5% content means a decrease in

the dispersion degree, as the improvement in flexural

Fig. 5 Flexural strength (a) and Young modulus (b) in three-point bending test of epoxy powder matrix and bulk nanocomposites

reinforced with 0.5, 1, and 1.5% of CNFs

Table 3 Improvements achieved in epoxy powder nanocom-

posites with increasing CNFs contents

CNFs

content (%)

Flexural strength

improvement (%)

Young modulus

improvement (%)

0 – –

0.5 16 17

1 24 25

1.5 19 23

6030 J Nanopart Res (2011) 13:6021–6034

123

strength and Young modulus is different and lower

compared with 1% content. This is a fact that clearly

indicates that there is a dispersion limit of CNFs in

the epoxy matrix and, in this case this limit is referred

to a CNFs loading of 1%. This dispersion limit has

been also commented by Gershon et al. (2010) but in

his work he found that such limit was around 3% for

a solvent-free system and 3.5% in the case of a liquid

one. Taken into account that both CNFs (those used

by Gershon and the ones employed in this study)

present similar densities (1.87 and 1.97 g cm-3,

respectively), the lower dispersion limit found for

the CNFs may be due to the higher range of diameters

(between 30 and 300 nm) compared with the one in

Gershon0s work (between 100 and 200 nm). This

higher range of sample diameters may create a higher

increase of viscosity during the extrusion stage,

reaching a lower dispersion limit.

However, both mechanical properties are higher in

the 1.5% CNFs/epoxy matrix compared with the

pristine epoxy and it is an evidence itself of the

higher mechanical properties of CNFs referring to a

polymer matrix, either when they form entanglements

and agglomerates.

Bending mechanical properties in bulk nanocom-

posites were expected to be higher and, although the

high-gloss values observed for nanocomposite coat-

ings mean the absence of entanglements and agglom-

erates, it may be possible that during the curing

process reagglomeration could has been occurred

(Sandler et al. 2003). The short time needed (25 min)

for curing nanocomposite coatings has not been

enough to house the reagglomeration, while the long

curing time for obtaining bulk nanocomposites has

favored the reagglomeration. This reagglomeration

could also be responsible for the lower dispersion

limit found when compared with the ones reported by

Gershon et al. (2010).

As mentioned before, only a few papers have been

published in relation with solvent-free polymer

matrixes and CNFs and in such cases, the mechanical

response of nanocomposites has been evaluated

mainly through dynamic mechanical analyses from

values such as the storage modulus (G0), the loss

modulus (G00), and tand. Taken into account differ-

ences between both mechanical tests, Young modulus

can be compared with values of G0, as such parameter

is related to the composite elastic modulus. Guo and

co-workers (2010a, b) found a considerable increase

of the storage modulus of an epoxy resin when it was

reinforced by 0.1% of CNFs, and such increase was

assigned to a high dispersion of CNFs in the matrix.

Besides, it was reported that the increase of CNFs

content in the nanocomposite leads to a stabilization

of G0, that was ascribed to the presence of agglom-

erates. Therefore, the results are in agreement with

the ones obtained by Guo. In other recent publication,

Guo and co-workers (2010a, b) explained that the

increase of G0 and G00 in epoxy resins when

reinforced with CNFs is due to the interactions

between CNFs, which let the formation of a network

structure, responsible for the enhanced mechanical

properties. In this study, although it is believed that

the network structure is formed at a CNFs loading of

1% and it was reasonable to expect the maintenance

of such network with higher CNFs loadings, it has

been found a slight decrease in mechanical properties

with CNFs loadings. This decrease is higher than 1%

and it seems to be due to the reagglomeration

phenomenon occurred during the curing process as

it has also been reported by other authors (Ma et al.

2010). Ma et al. (2010) published a work in which

reagglomeration of CNFs in an epoxy matrix was

observed during curing. He concluded that reagglom-

eration was present in the curing due to the van der

Waals forces and Coulomb attractions between CNFs

and that such reagglomeration was a time-dependant

process. It was also suggested that such process could

be more significant at higher CNFs contents, as the

distance between them are reduced. Assuming that

results, the authors can assume that reagglomeration

can be expected in this case, as the curing

stage comprises long dwelling times at 110 and

120 �C.

Surface fracture of the CNFs-epoxy bulk nano-

composites has been observed by FE-SEM. Micro-

photographs taken at low magnification are presented

in Fig. 6.

Microphotographs shown in Fig. 6 reveal the

presence of residual porosity in each of the samples.

However, three-point bending mechanical properties

have been improved with CNFs. That evidences the

reinforcement effect of CNFs on epoxy matrix

nanocomposites. As can be observed in Fig. 6,

fracture surface corresponding to the epoxy matrix

presents a considerable number of holes that must be

resin pull-out brands, while fracture surfaces corre-

sponding to epoxy matrix reinforced with CNFs

J Nanopart Res (2011) 13:6021–6034 6031

123

seems to have less pull-out brands. Resin pull-out

brands are due to the resin ripping during the fracture

progression. The presence of CNFs reinforcing phase

improves the cohesion in the material, so that it can

withstand greater mechanical requirements. Never-

theless, it has not been observed any difference in

pull-out brands in fracture surface microphotographs

of epoxy matrix reinforced with any of the CNFs

contents.

Fracture surface microphotographs obtained at

high magnifications have allowed the observation of

CNFs (see Fig. 7).

In spite of the dispersion achieved with the

extrusion processing developed, CNFs entanglements

Fig. 6 Low magnification

microphotographs of CNFs-

epoxy bulk nanocomposites

obtained by FE-SEM

Fig. 7 Fracture surfaces

at high magnifications

6032 J Nanopart Res (2011) 13:6021–6034

123

have been observed (see Fig. 7a, b). However, CNFs

seems to be well integrated into the matrix and, as

can be seen in Fig. 7c, CNFs reinforcement effect

may act through the CNFs fracture. The higher

flexural strength and Young modulus characteristic of

CNFs compared with the ones for polymer matrixes

allow the CNFs to act as mechanical reinforcement.

The progression of the crack through the resin occurs

faster and, in CNFs presence this progression become

to occur slower, due to the mechanical strength

opposed by CNFs. So that, fracture in bulk nano-

composites are denoted by the presence of CNFs as

junction points between the resin.

In Fig. 7b it can be seen that not all the nanofiber is

in contact with the matrix, which evidences that a

higher interfacial adhesion could be achieved. The

dispersion degree obtained with the proposed extru-

sion processing has decreased the number and size of

entanglements and has improved the interfacial adhe-

sion between the matrix and the reinforcement phase.

Nevertheless, surface modification of the reinforce-

ment phase may improve even more mechanical

properties in bulk nanocomposites as well as nano-

composite coatings. In this sense, Guo and co-workers

(2010a, b) modified CNFs by silanization and devel-

oped polymer composites in which a decrease of

agglomerates were found resulting in a better disper-

sion into the polymer matrix. Also, Guo reported a

rougher CNFs surface after treatment with HNO3,

which is expected to improve mechanical properties of

composites.

Conclusion

A double-step processing has been employed to

prepare CNFs-epoxy powder nanocomposites. The

suitable processing to obtain such nanocomposites

involves a proper choice of each of the additives as

well as the previous premix of the components before

the extrusion. Three consecutive extrusions have

been needed to obtain nanocomposites free of neither

entanglements nor agglomerates.

Nanocomposite coatings presented high gloss

properties as well as impact resistance, and the high

quality of such nanocomposite coatings evidence the

absence of agglomerates.

While the curing process for nanocomposite

coatings has proved to be easy and fast, the thickness

factor has turned to be of great importance for

obtaining bulk nanocomposites. So that, a complex

curing process involving dwell times at the fluent

state temperatures as well as slow heating rates was

needed. Mechanical properties measured for bulk

nanocomposites have revealed that the addition of

small contents of CNFs to the epoxy matrix results in

an improvement of the mechanical properties. Nev-

ertheless, the best flexural strength and Young

modulus were obtained with 1%. This value seems

to be the dispersion limit for the studied system for

the given process conditions.

Higher mechanical response of bulk nanocompos-

ites was expected and the possibility of reagglomer-

ation during the curing process could be related to

this point. Besides, once the processing has been

optimized, surface modification of the reinforcing

phase should be proposed, as it could improve, even

more, mechanical properties of both nanocomposite

coatings and bulk nanocomposites.

Acknowledgments The authors thank the financial support

from Ministerio de Ciencia e Innovacion and Centro para el

Desarrollo Tecnologico Industrial (CDTI) under Project

CENIT DOMINO no. CEN-2007-1001.

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