Analysis Method

Kinetic study of the epoxy curing at the silica particles/epoxy

interface using the fluorescence of pyrene label

D. Olmosa,b, A.J. Aznara,b, J. Gonzalez-Benitoa,b,*

aDepartamento de Ingenierıa de Materiales e Ingenierıa Quımica, Universidad Carlos III de Madrid, SpainbInstituto de Materiales y Quımica Alvaro Alonso Barba. Avda. Universidad 30, 28911 Leganes, Madrid, Spain

Received 1 October 2004; accepted 19 November 2004

www.elsevier.com/locate/polytest

Abstract

The cure process of an epoxy-amine system was studied in composite materials based on an epoxy polymer matrix filled with

silica particles. FT-NIR and Fluorimetry (based on the use of fluorescent labels) were used as analytical techniques. In order to

study the effect of the structure of the interphase, the silica particles were surface coated with two different silanes:

3-aminopropyltriethoxysilane and 3-aminopropylmethyldiethoxysilane. 1-pyrene-sulfonylchloride was chemically bonded to:

(i) the coated silica particles and (ii) to the epoxy polymer matrix, using its fluorescence response to independently monitor the

epoxy curing at the interface region and in the bulk respectively. The cure process was followed at different temperatures to

subsequently study its kinetics either by fluorescence or by FT-NIR. An extent of reaction in terms of integrated fluorescence

intensity was defined and the subsequent kinetic analysis allowed us to calculate apparent activation energies of the process.

Comparing the curing reaction in the polymer bulk and at the interface it could be concluded that the epoxy curing at the

interface proceeded faster but only during the first stages of the reaction. This result was attributed to a higher local

concentration of amino groups at the interface. Finally, a comparison between FTIR and fluorescence results showed that both

techniques provide complementary information about the curing reaction.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Fluorescence; Curing; Silane; Epoxy; Interface

1. Introduction

The use of composite materials, generally designed for

specific applications, has undergone an explosion during the

last century. Specifically, the study of the interfaces created

in composite materials is a key factor in order to properly

design high performance materials. Particularly, in the case

of polymer matrix composites, those based on an epoxy

polymer matrix reinforced with glass fibers or silica fillers

have received special attention from scientists [1–6] due to

their multiple and potential applications.

0142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymertesting.2004.11.008

* Corresponding author. Tel.:C34 91 6248870; fax: C34 91

6249430.

E-mail address: javid@ing.uc3m.es (J. Gonzalez-Benito).

It is well known that physical properties and mechanical

strength during the working life of composites are greatly

affected by the structure of the whole system. This depends

mainly on the curing conditions: temperature, curing time,

extent of the reaction, [7–9]. However, a deeper knowledge is

still needed and it is for this reason that large efforts have been

made to understand the relationship between the structure of

the network and the final properties of the material [10,12].

Several techniques have been used to monitor the curing

process in epoxy systems. Although FTIR [11–16] and DSC

[16–19] have been the most widely employed techniques,

they show some limitations, such as: low sensitivity at high

conversions [16] and non-selective information. The latter

means that they provide information about the whole system

and not only from specific sites such as the interfaces.

In particular, in the case of composites, there is still

Polymer Testing 24 (2005) 275–283

D. Olmos et al. / Polymer Testing 24 (2005) 275–283276

the necessity of a deeper knowledge of the curing process,

especially from an interfacial point of view.

There has been a large amount of literature concerning the

properties of the interfaces of composite materials. Most of

the researchers have focused their attention on the measure-

ment of mechanical properties by classical interfacial tests

such as: single fiber fragmentation tests, microcracking,

nanoindentation [20–22]. However, the main disadvantage

of these mechanical tests is their destructive character. On the

other hand, there are only few works related to the study of

curing processes at the interfaces of glass fiber reinforced

composites [23–27]. For instance, the curing at the interface

of composites has been studied using evanescent spec-

troscopy (either FTIR or fluorescence spectroscopy) [23,24].

However, the main drawback of these techniques is that the

information coming from the interface is also affected by the

response of the polymer matrix [24]. A better approach is to

use the fluorescence response of some molecules that can

be chemically bonded to the interfaces of the composites

[24–27]. Generally, the fluorescence from specific groups

(fluorophores) experiences changes when the medium where

they are immersed is chemically or physically modified

[24–27]. Thus, by following the fluorescence response of

these molecules (fluorophores) throughout the polymeriz-

ation, information about the cure process can be obtained.

In this work, the cure process of different composite

materials based on an epoxy polymer matrix filled with silica

microparticles has been studied using two different analytical

techniques. The first is the Fourier Transformed Infrared

Spectroscopy in the near range (FT-NIR), which is one of the

most commonly used techniques for the study of the curing in

polymeric systems. The second is based on the detection of

the fluorescence response coming from a fluorophore which

is chemically bonded to the system under study. In order to

study the curing process: (i) in the bulk and (ii) at the interface

of the composites, two different ways of inserting the

fluorescence groups were chosen: (a) labeling only the epoxy

matrix and (b) labeling only the interfaces. These exper-

iments will allow us: (i) to make a comparison between the

cure process in the bulk and at the interface (fluorescence

experiments) and (ii) to compare two analytical methods to

monitor the epoxy curing, FT-NIR and fluorescence.

2. Experimental

2.1. Materials

The silica particles used in this work were provided by

TOLSA (Madrid, Spain). The silanes, 3-aminopropyl-

methyldiethoxysilane (APDES) (I) and 3-aminopropyl-

triethoxysilane (APTES) (II), supplied by Fluka Chemika,

were selected as the coupling agents. 1-pyrenesulfonyl

chloride (PSC, Molecular Probes) (III) was used to label

independently either the epoxy bulk (polymer matrix of the

composite) or the interphases of the composite materials

(depending on the specific site to be analyzed). The matrix

of the composites was based on an epoxy system formed by

reaction of poly(bisphenyl A-co-epichlorohydrin) glycidyl

end-capped (DGEBA) (IV), MnZ348 g/mol (xZ0.03)

and ethylenediamine (EDA) (V) in stoichiometric amounts

(rZ1). The components of the polymeric system were

provided by Sigma-Aldrich. All the solvents used in this

work were HPLC quality. Toluene was always used either

from a freshly opened bottle or was previously dried under a

molecular sieve (4 A) 24 h before used.

D. Olmos et al. / Polymer Testing 24 (2005) 275–283 277

2.2. Sample preparation

The silica particles were immersed in 2% (v/v)

aqueous solutions of the two silanes selected as coupling

agents. After 15 min, the particles were filtered, exhaus-

tively washed with distilled water and subsequently

introduced into an oven at 110 8C for 1 h to polymerize

the silanes and to dry the samples. In order to remove

the possible remaining physisorbed monomers and

oligomers, the silanized silica particles were subjected

to a Sohxlet extraction with dried toluene for four hours.

Finally, the particles were vacuumed dried at 50 8C for at

least 8 h.

In order to follow the curing process independently

either in the bulk or at the interphases of the composite

materials it is necessary to label both regions. The reaction

between 1-pyrenesulfonyl chloride and primary amines is

reported to proceed practically 100% [29]. Therefore, it can

be assumed that there are no free chromophores or, in other

words, that every chromophore is chemically bonded either

to the bulk or to the surface of the particles, depending on

the specific study.

Tab

Sam

use

Sam

AP

AP

AP

AP

AP

The interphases were pyrene labelled by immersing 0.4 g

of the silanized silica particles in 50 ml of a 10K4 M

solution of PSC in acetonitrile for 15 min. Then the

particles were filtered and exhaustively washed with

acetonitrile.

The epoxy matrix was pyrene labelled by reaction of

PSC with ethylenediamine (EDA) in such amount that

the ratio of PSC in the polymeric system was

approximately 10K4 moles per kilogram of epoxy

mixture.

In Table 1 the sample codes referring to the fluorophore

location in the composites are grouped.

In every case, the samples were prepared by mixing the

stoichiometric amount of epoxy and amine (rZ1) with a

20% (w/w) of silanized silica particles. The silica filled

reactive mixture was placed in between two microscope

slides which are separated 0.6 mm by a Teflon spacer.

After that, the samples were cured at different temperatures

(40, 50, 60 and 70 8C).

le 1

ple codes referring the fluorophore location and the techniques

d to monitor the epoxy cure

ple code Fluorophore

location

Techniques

TES-IR None FTIR

TES-B-Py Bulk FTIR, fluorescence

DES-B-Py Bulk Fluorescence

TES-I-Py Interphase FTIR, fluorescence

DES-I-Py Interphase Fluorescence

2.3. Measurements

The curing process of the composite materials was

followed by Fourier Transform Infrared spectroscopy in the

near range, FT-NIR, and fluorescence spectroscopy.

2.3.1. FT-NIR

The FT-NIR study was performed in a Perkin Elmer GX

FTIR Spectrometer. The study was carried out using a

SPECAC temperature controller to set the curing tempera-

ture in the oven where the samples were located. The FTIR

spectra were collected as a function of the curing time with a

homemade program. The curing was followed at 40, 50, 60

and 70 8C (G1 8C). All the spectra were obtained after 5

scans within the spectral range 7500–4000 cmK1 and with a

resolution of 4 cmK1.

2.3.2. Fluorescence

The fluorescence measurements were done in an

Edinburgh Instruments Co. fluorimeter, using an optical

fiber cable to both excite and collect the fluorescence of the

sample. The excitation and emission slits were set at 2.3 and

3.6 nm respectively. The fluorescence spectra were recorded

between 360–650 nm using an excitation wavelength of

340 nm, with a scanning rate of 120 nm/min. The cure

process was monitored by collecting the fluorescence

spectra at constant temperature using the same oven and

temperature controller as used for the FTIR measurements.

Four temperatures (40, 50, 60 and 70 8C) were also studied

in order to carry out the kinetic analysis.

3. Results and discussion

3.1. FTIR

In Fig. 1, as an example, some FT-NIR spectra of the

curing process for the APTES-IR sample at 40 8C, are shown.

Fig. 1. FT-NIR spectra for the APTES-IR sample during an

isothermal reaction at 40 8C.

D. Olmos et al. / Polymer Testing 24 (2005) 275–283278

In this figure, the typical bands of the epoxy group

(4530 cmK1) and amino group (4940 cmK1) can be

observed. The band at 4623 cmK1, which is generally

assigned to a combination band of the aromatic C–H bonds,

was taken as the reference band [14,15]. Since this band is not

involved in any chemical reaction, small changes in this band

could only be attributed to shrinkage of the system during the

curing reaction. On the other hand, as the epoxy curing

proceeds the absorbance of the epoxy (4530 cmK1) and the

primary amino groups (4940 cmK1) bands decrease, indi-

cating that the reaction is taking place.

The spectra were analyzed by integrating the epoxy band

(4530 cmK1) [14,15] and calculating the epoxy group

conversion using Eq. (1). In order to minimize the effect

of curing shrinkage and the changes in the refractive index

during the curing process, the epoxy band was normalized

using the absorption of the reference band centered at

4624 cmK1 [14]. Therefore, the conversion was calculated

using the following equation:

a Z 1 KAe;t=Ar;t

Ae;0=Ar;t

(1)

where Ae, Ar are the areas of the epoxy and the reference

bands respectively, and the subscripts t and 0 make

reference to any curing time, tZt, and the initial time,

tZ0, respectively.

In Fig. 2, the evolution of the epoxy conversion, aepoxy,

as a function of curing time is shown for the same sample

(APTES-IR) at four curing temperatures 40, 50, 60 and

70 8C.

Fig. 2 shows the typical behavior obtained with this kind

of system (diepoxy-diamine) [9,13–15]. There are two

regions that can be distinguished. In the first, during the first

stages of the reaction, the process is controlled by chemical

reaction and a very fast increase in conversion is observed.

The second, which corresponds to that region where the

curves level off, is usually attributed to the reaction

controlled by diffusion of the reactants. [7–9].

Fig. 2. Epoxy group conversion (aepoxy) as a function of curing time

at 40, 50, 60, and 70 8C (APTES-IR sample).

The kinetic study was done considering a general kinetic

equation (Eq. (2)). Integrating it for a given time, yields

Eq. (3);

da

dtZ kf ðaÞ

ða

0

da

f ðaÞZ

ðt

0kdt (2)

t Z B=k (3)

where B is a constant given by BZÐ a

0 da=f ðaÞ and k is an

apparent rate constant with an Arrhenius like dependence.

Substituting kZAeKEa/RT, and taking natural logarithms in

Eq. (3) for each a value, expression (4) can be obtained.

Ln t Z C CEa=RT (4)

where C is a constant, R is the universal gas constant, T the

absolute temperature and Ea an apparent activation energy

of the process. Thus, plotting natural logarithm of time for a

constant conversion versus the inverse of absolute tempera-

ture should result in a straight line, from the slope of which

an apparent activation energy of the process can be

calculated. In Fig. 3, as an example, some of these Arrhenius

plots for the APTES-IR sample are shown. Similar plots

were obtained for the other samples under study. In every

case, the data can be fitted to a straight line with correlation

factors higher than 0.99.

In Table 2, the apparent activation energies calculated

using these plots are listed for the samples without any label

(APTES-IR; APDES-IR) and for samples labeled in the

polymer bulk (APTES-B-Py; APDES-B-Py)

Using DSC, Horie et al. determined an activation energy

of 53.9 kJ/mol for the curing reaction of the DGEBA-EDA

system [30]. Other researchers report apparent activation

energies for different diepoxy-diamine systems whose

values were between 46–58 kJ/mol [30–32]. In general

terms, the results from Table 2 are in good agreement with

the general trend observed for the different epoxy systems in

Fig. 3. Plot of the natural logarithm of curing time versus the inverse

of absolute temperature (KK1) for different curing conversions, aZ0.1–0.7, (APTES-IR sample).

Table 2

Apparent activation energies, Ea, (kJ/mol) obtained from the kinetic

analysis for each of the samples

a APTES-IR APDES-IR APTES-B-Py APTES-I-Py

0,1 42 – 53 37

0,2 45 60 46 46

0,3 45 58 46 52

0,4 49 56 47 53

0,5 47 54 51 54

0,6 47 53 – 55

0,7 49 56

Average 47G2 56G3 48G2 52G4

Fig. 4. Fluorescence emission spectra of pyrene moiety at different

curing times for the APTES-B-Py sample at 70 8C.

D. Olmos et al. / Polymer Testing 24 (2005) 275–283 279

previous works. These values seem to indicate that the

activation energy of the curing process is not greatly

affected by the incorporation of a filler in the system, at least

in the proportion used in this work. D. Olmos et al.[27]

found a slight decrease in the apparent activation energy of

the curing process when glass fibers are incorporated to the

epoxy system. Besides, this decrease of the activation

energy was higher when the reinforcement was coated with

APTES in comparison with that coated with APDES. This

fact was attributed to a catalytic effect of the hydroxyl

groups [27].

Comparing the labeled samples (APTES-B-Py, APTES-

I-Py) with the non-labeled one (APTES-IR), there are two

important facts that can be observed:

(a)

The bulk labelled sample (APTES-B-Py), shows no

differences with respect to the non-labeled sample

(Table 2). This indicates that the presence of the

fluorophore in the polymer bulk does not seem to affect

the general curing process;

(b)

Regarding the interface labelled sample (APTES-I-Py)

a slight increase in the apparent activation energy with

respect to the bulk labelled sample can be observed.

This result seems to indicate that the incorporation of

the fluorophore to the interface of the composite slightly

modifies the hydrophilic character of the siloxane

coating (APTES). Therefore, there is an increase in

the apparent activation energy.

3.2. Fluorescence

3.2.1. Study of the curing process in the bulk. Coating

type effect

In Fig. 4, the fluorescence spectra of the pyrene label

obtained at different curing times is shown. In particular, the

spectra in Fig. 4 correspond to the fluorescence emission of

the APTES-B-Py sample during curing at 70 8C. In spite of

very slight shifts, in every spectrum the emission spectra

show the typical bands observed in the pyrene derivatives.

These are the bands centered at 390, 398 and 420 nm, called

I1, I3 and I5, respectively [33,34]. Additionally, in this case,

it is also possible to observe a broad band without

vibrational structure between 450–650 nm. This band can

be assigned to the emission from an excited charge transfer

complex formed between the amino groups of the coating

and the pyrene moieties in the ground estate [31,32]. In the

case of the APDES-B-Py sample, similar spectra were also

obtained.

As the curing time increases, an increase in the

fluorescence intensity is also observed, which is in

accordance with that observed in other systems using the

fluorescence arising from other fluorofores [26,27,35]. This

increase in the fluorescence intensity is usually attributed to

an increase in the viscosity and rigidity of the medium. As

the curing reaction proceeds, the viscosity of the environ-

ment where the fluorophore is located increases. Therefore,

there is a decrease in the number of non-radiative processes

that may deactivate the excited estate of the fluorophore and,

consequently, an increase in its fluorescence quantum yield

which can be translated into an increase of its fluorescence

intensity [28].

In this case, a kinetic analysis of the curing process was

done using integrated fluorescence intensity ðhIiint ZÐ

IðnÞdnÞ

and defining a fluorescence conversion given by Eq. (5):

aI ZIt K I0

IN K I0

(5)

where It, Io and IN are the integrated intensity for a given

curing time, tZt, the initial time, tZ0, and the infinite curing

time, tZN. At this point, it is important to mention that this

curing conversion does not provide information on the

chemical changes occurring in the system, but of the physical

changes associated with variations in the viscosity of the

medium. For this reason, for tZN, aI(tZN) would be the

final conversion, although in terms of rigidity. Taking into

account this new definition of conversion, it is important to

mention that fluorescence conversions, aI, should not be

coincident with the real chemical conversions obtained by

FTIR, aepoxy.

In Fig. 5, the fluorescence conversion for the curing of

the APDES-B-Py sample at different temperatures is shown.

Fig. 5. Fluorescence conversion, aI, versus curing time at different

temperatures (40, 50, 60 and 70 8C) for the APDES-B-Py sample.

Table 3

Apparent activation energies (kJ/mol) obtained from the fluor-

escence data for the samples with the pyrene chemically bonded to

the epoxy matrix

a APTES-B-Py APDES-B-Py

0.2 – 46

0.3 32 44

0.4 34 46

0.5 35 47

0.6 36 47

0.7 37 48

Average 35 G 2 46 G 2

Fig. 6. Natural logarithm of curing time versus the inverse of

absolute temperature for the APDES-B-Py sample at different

fluorescence conversions.

D. Olmos et al. / Polymer Testing 24 (2005) 275–283280

It is observed how the profiles of fluorescence conversion

plots are similar to those obtained when the chemical

conversion, aepoxy, is plotted versus the curing time (Fig. 2).

There are two different regions that can be distinguished: the

first one at the beginning of the cure reaction, in which

the reaction rate is faster; and the second, in which the

fluorescence conversion levels off to a certain conversion

lower than 1, which is usually attributed to a reduction in

the curing rate when the reaction becomes diffusion

controlled [7–9].

As in the case of the kinetic analysis from the FT-NIR

data, it is possible to do a kinetic analysis from the

fluorescence data. Therefore, considering a general kinetic

equation, but now in terms of fluorescence conversion, Eq.

(6) could be used:

daI

dtZ kf ðaIÞ (6)

Operating in the same way as in the FTIR study, it is

possible to plot for a given conversion the natural logarithm

of curing time versus the inverse of the absolute tempera-

ture. Thus, an apparent activation energy of the curing

process can be calculated from the slopes of these plots. In

Fig. 6, as an example, the plots of the natural logarithm of

curing time versus the inverse of temperature at different

fluorescence conversions are shown for the APDES-B-Py

sample. In every case, similar plots were obtained that

can be fitted to straight lines with correlation factors higher

than 0.99.

In Table 3, the apparent activation energies (in kJ/mol)

of the systems for which the pyrene label is chemically

bonded to the epoxy bulk are listed.

The activation energies obtained from the fluorescence

analysis are slightly lower than those obtained from the FT-

NIR data. Again, when the coating effect is considered, the

results show the same tendency previously observed in FT-

IR experiments. This result suggests that the information

obtained from fluorescence when the fluorophore is located

exclusively in the epoxy matrix is similar to that of the

whole system obtained using a conventional technique such

as FT-NIR [27].

3.2.2. Study of the curing process at the interface. Coating

type effect

In Fig. 7, the fluorescence spectra at different curing

times at 70 8C are shown for the coatings under study

(APTES, APDES).

The fluorescence spectra show the same general features

of the pyrene derivatives fluorescence (bands I1, I3, I5 and

‘excimer-like’ band). When these spectra are compared with

those shown in Fig. 4, it is found that the band assigned to

the formation of the complex is relatively more intense

when the fluorophore is at the interface. This result suggests

that the average of amino groups per pyrene molecule might

be higher when the chromophore is located at the interface.

This fact might be due to the presence of the amine groups

from the poliorganosiloxane coating. Additionally, when

both coatings are compared, the contribution of the broad

band to the whole spectrum is higher when the APTES

coating is concerned. This result may be explained

considering that the amount of silane present on the surface

Table 4

Apparent activation energies (kJ/mol) for the APTES-I-Py and

APDES-I-Py samples

a APTES-I-Py APDES-I-Py

0.2 43 40

0.3 42 41

0.4 39 42

0.5 40 42

0.6 40 41

0.7 39 41

Average 40G2 41G1

Fig. 7. Fluorescence spectra of the pyrene moiety for: (a) APTES-I-

Py and (b) APDES-I-Py samples as a function of curing time at

70 8C.

D. Olmos et al. / Polymer Testing 24 (2005) 275–283 281

of the silica particles is higher when the coating is APTES,

since there is a higher amount of amino groups.

Again, the integrated intensity was selected to calculate a

fluorescence conversion, aI (Eq. (5)). In Fig. 8, as an

example, the plots of the fluorescence conversion as a

function of the curing time for the APTES-I-Py sample, are

shown at different temperatures.

With the aim of comparing these results (at the interface)

with the previous ones (in the bulk), the kinetic study of

Fig. 8. Fluorescence conversion, aI, versus the curing time for the

APTES-I-Py sample, at four curing temperatures: 40, 50, 60 and

70 8C.

the epoxy curing at the interface of the composites was done

using the same kinetic model as previously explained.

Table 4, shows the results obtained for this kinetic analysis.

Again, the values of the apparent activation energies

obtained from the fluorescence data are slightly lower than

those obtained by FTIR (Table 2). However, when studying

the curing as a function of the coating, no significant

differences in the apparent activation energies are observed

between the two coatings, as happened when FT-NIR was

used. This result may indicate that the presence of pyrene in

the coupling region may produce a change in the hydrophilic

properties of the silica surfaces. The presence of small

amounts of pyrene (10K8 mol/g), induces an increase in the

hydrophobicity of the surfaces, which may be large enough

so as to observe no differences between the two coatings.

3.2.3. Comparison between the epoxy curing in the bulk

and at the silica/epoxy matrix interface

Fig. 9 shows the correlation, in terms of fluorescence

extent of reaction, aI, between the epoxy curing exactly at

the interface aII , and the epoxy curing in the bulk aB

I .

Independently of the surface coating there are two regions

that can be observed. At the first stage of the reaction, the

curing rate is higher at the interface. This behavior can be

attributed to a higher concentration of amino groups at the

interface. This result is in good agreement with those

obtained in a previous study when glass fibers are used as

reinforcement [27]. According to this work, when the epoxy

resin penetrates the interface, the increase in the viscosity is

faster due to a higher local concentration of the amino

groups (coming from the aminosilane coating).

Additionally, when both coatings are compared, the

region with the higher slope is more extended when the

fibers are coated with APDES (Fig. 9a) than when the fibers

are coated with APTES (Fig. 9b). This result can be

explained in terms of a more opened structure for the

diethoxysilane coating (APDES), which would also be in

accordance with the results obtained in previous works

[27,36]. H. Hamada et al. measured the effect of the

interfacial silane network structure on the interfacial

strength in glass fiber reinforced composites [36]. In this

work, the higher interfacial strengths were obtained for

composites in which the fibers were coated with APDES,

Fig. 10. aI versus aepoxy plots for the samples: (a) APDES-B-Py y

(b) APTES-B-Py.

Fig. 9. Fluorescence extent of reaction at the interface, aII , versus

fluorescence conversion in the bulk, aBI : (a) APDES and (b) APTES

coatings.

D. Olmos et al. / Polymer Testing 24 (2005) 275–283282

and this strength decreased as the content of APTES

increased. This result suggested that the increase in the

siloxane crosslinking of the silane decreases the interfacial

strength of the composite, since this crosslinked structure

depresses the penetration of the resin into the interphase,

thus hindering the reaction between the silane organofunc-

tional groups and the resin. J.G. Iglesias et al reached the

same conclusion when they evaluated the mechanical

properties of glass fiber reinforced composites in which

the fibers were also coated with these silanes [3]. All this

evidence would help to justify a higher interpenetration

between the epoxy and the polysiloxane coating for the

APDES coating (Fig. 9). [27].

3.2.4. Comparison FT-NIR-fluorescence

Finally, a correlation between FT-NIR and fluorescence

data is shown in Fig. 10 in which the fluorescence

conversion in compared with the epoxy group conversion.

Only at 40 8C is a nearly linear trend is observed. As

temperature increases, the plots deviate from linearity as can

be seen in the plot for 70 8C. With FT-NIR a chemical

reaction is followed, so higher changes in conversion are

observed from the beginning until the gel conversion is

reached. At this point, the viscosity of the system increases

very rapidly to effectively infinite and little changes in the

chemical reaction can take place until the vitrification is

reached where the reaction stops. On the other hand, with

fluorescence, physical changes of the system associated with

the chemical reaction are followed. Therefore, lower

changes in the fluorescence conversion are observed at the

beginning of the cure process since the increase of the

viscosity is lower. However, at a certain conversion, the gel

point, there is an abrupt increase of the viscosity and

consequently in the mobility of the system.

However, a key aspect is that there must be a simple

relationship between the aepoxy and, aI which means that a

change in fluorescence corresponds to a change in chemical

conversion. In Fig. 10, two different parts are observed: in

the first, at low conversions, the plots present a moderate

slope and in the second, at higher conversions, the plots have

greater slope.

These results suggest that FT-NIR and fluorescence

actually can be considered as complementary techniques to

study epoxy curing processes. FT-NIR, seems to be more

useful when the first stages of the curing reaction are

concerned because bigger changes are observed with curing

time, while the fluorescence should be recommended to

follow the curing processes at the last stages of the epoxy

D. Olmos et al. / Polymer Testing 24 (2005) 275–283 283

cure reactions due to its higher sensitivity. However, both of

them allow kinetic parameters to be obtained that could help

in designing curing conditions for preparing epoxy based

materials.

On the other hand, only the fluorescence technique can

be used to monitor the curing process at the interface in a

composite material, additionally allowing kinetic studies of

the cure reactions to be carried out.

4. Conclusions

Two different techniques have been used to monitor the

curing process in a composite material, FT-NIR and

fluorescence. FT-NIR and Fluorescence actually can be

considered as complementary techniques to study epoxy

curing processes. FT-NIR seems to be more useful when the

first stages of the curing reaction are concerned because

bigger changes are observed with curing time, while

fluorescence should be better to follow the curing processes

at the later stages of the reaction due to its higher sensitivity.

With reference to the kinetic analysis, the activation energy

of the epoxy curing reaction remains constant during the

whole process. This result suggests that there are no changes

in the mechanism as the reaction proceeds.

Finally, these experiments have allowed us to compare

the curing process in the bulk with that at the interface,

obtaining valuable information related to the curing rate of

the process in both regions of composites materials.

Acknowledgements

Authors gratefully acknowledge to the Contrato-Pro-

grama del III PRICIT de la CAM and EPOXIL (MAT2000-

0391-P4-02) and FIBRODONT (MAT2001-0677-P3)

projects for financial support.

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