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Thermochimica Acta 529 (2012) 29– 35

Contents lists available at SciVerse ScienceDirect

Thermochimica Acta

jo ur n al homepage: www.elsev ier .com/ locate / tca

tudy on the non-isothermal curing kinetics of a polyfurfuryl alcohol bioresin bySC using different amounts of catalyst

.C. Domíngueza,b,∗, J.C. Grivela, B. Madsena

Materials Research Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, Frederiksborgvej 399, DK-4000 Roskilde, DenmarkDepartamento de Ingeniería Química, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Avda. Complutense s/n. 28040 Madrid, Spain

r t i c l e i n f o

rticle history:eceived 27 July 2011eceived in revised form0 November 2011ccepted 12 November 2011vailable online 23 November 2011

eywords:olyfurfuryl alcohol

a b s t r a c t

The curing kinetics of a biomass-based polyfurfuryl alcohol resin with three different amounts of catalystwas studied by DSC non-isothermal measurements using seven heating rates. The change of the activationenergy of the curing process was obtained by the isoconversional methods of Kissinger–Akahira–Sunone,Flynn–Wall–Ozawa and Vyazovkin. The latter method provided maximum values of the activation energyof about 115, 95 and 80 kJ mol−1 before the gelation point for 2%, 4%, and 6% (w/w) amounts of catalyst,respectively. Based on a purely kinetic criterion, the most suitable amount of catalyst is assessed tobe 4% (w/w). The change of the activation energy during curing was found to consist of three stages:an initial stage, where the activation energy increases due to accumulation of reaction intermediates; a

ioresinifferentical scanning calorimetryuring kineticsodel-free-kinetics method

main stage, where the activation energy slowly decreases due to the increasing viscosity and gelling of theresin which leads to a constrained mobility of the polymer chains; and a final stage, where the activationenergy decreases more rapidly due to the formation of a rigid molecular network that restricts diffusionprocesses. Altogether, the obtained knowledge of the curing kinetics will form a valuable contribution tothe design of improved cure cycles for manufacturing of composite materials with a polyfurfuryl alcoholmatrix.

. Introduction

One of the present and future most important objectives in theevelopment of new composite materials is to obtain them fromenewable raw materials; thus, their production and cost are thenecoupled from the oil market and lower environmental impact cane achieved. To achieve this objective, the present study addresseshe kinetics of the curing process of a polyfurfuryl alcohol (FA)ioresin under development. The FA bioresin is based on furfuryllcohol, which can be obtained from pentosan-rich biomass, suchs bagasse (a wasteproduct from sugar cane production) [1,2]. Thisesults in a relatively cheap bioresin that can be used as matrixn the manufacturing of composite materials together with eitherraditional synthetic fibres, such as glass and carbon, or naturalbres, such as flax and jute. In the latter case, fully biomass-basedomposite materials can be obtained.

The curing process of the FA resin has been studied from a chem-cal point of view to determine the involved cross-linking reactions.

he curing mechanism for FA resins was originally proposed byunlop and Peters [3] and involves two reactions: firstly, con-ensation reactions of −OH group to obtain methylene linkages;

∗ Corresponding author. Tel.: +45 46775885; fax: +45 4677 5758.E-mail addresses: jucar@risoe.dtu.dk, jucdomin@quim.ucm.es (J.C. Domínguez).

040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.tca.2011.11.018

© 2011 Elsevier B.V. All rights reserved.

and secondly, condensation reactions of pairs of −OH groupsto obtain dimethylen ether linkages. Choura et al. [4] proposeda more complex mechanism which could explain the observedcolor change of the resin during curing, by the formation of lin-ear compounds with a high degree of conjugation. Also, severalpossible mechanisms were suggested for branching of the poly-mer chains such as Diels–Alder reactions between furan rings(dienes) in oligomeric molecules and conjugated dihydrofuranicsequences.

The study of the curing kinetics of resins is important for thedevelopment and optimization of the cure cycles used in the manu-facturing process of composite materials. Curing kinetic models canbe used to simulate the curing process, and in this way, the cycletime can be minimized. Kinetic models can also be used togetherwith models of chemorheology, heat transfer, and micromechanicsto calculate and minimize the generated internal stresses in com-posites and thereby improve the quality and performance of thematerials [5–12].

The curing kinetics of both the monomer FA, and the oligomerFA resins have previously been studied [13,14], as well as someblends of FA resins [2], using different thermal analysis techniques.

Moreover, different catalysts have been used for the curing process,such as maleic anhydride [2,14], ferric chloride hexahydrate [14],phosphoric acid [14], and p-toluenesulfonic acid [14–18]. In thepresent study, p-toluenesulfonic acid is used as catalyst, and since

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he studied FA bioresin is still under development, the amount ofatalyst was used as an experimental variable.

The main objectives of this work are: to obtain the curing kineticarameters for the FA resin, which will be needed for the opti-ization of the composite manufacturing process; to identify the

ependence of the curing activation energy with the curing degree;nd to determine the effect of using different amounts of catalyst.

. Theory

The curing process of a thermosetting polymer is commonlyescribed by a single step kinetic equation:

d˛

dt= k(T)f (˛) (1)

here represents the curing degree of the polymer; t is the time;(T) is the rate constant, which depends on temperature; and f (˛) ishe process mechanism function. k(T) is usually assumed to followhe Arrhenius equation (Eq. (2)):

(T) = k0 exp(

− Ea

RT

)(2)

here k0 is the pre-exponential factor, Ea is the curing activationnergy, R is the universal gas constant and T is the absolute temper-ture. Here follow descriptions of some of the widely model-freeinetic methods developed to estimate the parameters of the rateonstant function (Eq. (2)).

.1. Model-free kinetics methods

For curing processes of thermosetting polymers, under bothsothermal and non-isothermal conditions, several methods, suchs Borchardt and Daniels’ method [19], have been used to esti-ate the values of the pre-exponential factor and the activation

nergy. The main drawback of the method of Borchardt and Danielsnd similar methods is that when these methods are applied toata obtained under non-isothermal conditions where T and varyimultaneously, the method fitting procedure leads to the so-calledkinetic compensation effect”. This effect is avoided when isocon-ersional methods, also called model-free kinetics methods (MFK), aremployed. These methods are based on the isoconversional prin-iple according to which the curing rate at a given curing degreeepends only on temperature [20]. By applying this principle to Eq.1), the following Eq. (3) can be obtained [21]:

d ln (d˛/dt)˛

dT−1= − (Ea)˛

RT(3)

here (d˛/dt)˛ and (Ea)˛ are the curing rate and the activationnergy for a given curing degree. The isoconversional principle haseen succesfully applied to the curing process of a furfuryl alcohololymer by Guigo et al. [13], and therefore it is assumed to be appli-able in the present work. Moreover, MFK methods allow studyinghe change in the activation energy in response to temperature anduring degree during the curing process [13,22,23]. The profile ofhe activation energy in reponse to the curing degree can be used totudy the complexity of the curing process, and to identify whetherhe reactions are competitive, consecutive, reversible, or controlledy diffusion [20]. Therefore, MFK methods are a powerful tool toetermine if the curing kinetics are reaction rate controlled or dif-usion controlled, which is largely influenced by the viscosity of theolymer, and the proximity of the glass transition.

Several MFK methods have been developed in the literature.he methods have been classified either as differential meth-

ds such as the Friedman’s method [24], or as integral methodsuch as the methods of Kissinger–Akahira–Sunose (KAS) [25],lynn–Wall–Ozawa (FWO) [26], and Vyazovkin (VA) [21], depend-ng on if they use either the curing degree rate or the curing degree,

ica Acta 529 (2012) 29– 35

respectively, to calculate the curing activation energy. Friedman’smethod, a differential method, employs the instantaneous valuesof curing degree rates, and this method is therefore sensitive toexperimental noise. The method uses numerical differentiation ofthe experimental data to estimate the curing rates. This is the rea-son why the Friedman’s method is not used in the present study.Instead, the three mentioned integral methods (KAS, FWO, VA) areused. The methods of KAS and FWO are described by Eqs. (4) and(5), respectively:

ln

(ˇ

T2˛

)= A′ − Ea

RT˛(4)

log(ˇ) = A − 0.4567Ea

RT˛(5)

where

A′ = ln(

Rk0

Eag(˛)

)(6)

A = log(

Eak0

Rg(˛)

)− 2.315 (7)

g(˛) =∫ ˛

˛0

1f (˛)

d˛ (8)

where is the heating rate, T˛ is the temperature for certain curingdegree, and ˛0 is the initial curing degree.

Vyazovkin developed a new MFK method that improved themain drawback of the previous methods which is the simplifica-tion of the way the temperature integral is approximated [21].The method by Vyazovkin is based on the assumption that g(˛)is independent of the heating rate (Eq. (9)).

g(˛) = A

∫ t˛

t˛−�˛

exp( −Ea

RT(t)

)dt = AJ [Ea, T(t˛)] (9)

where g(˛) is the integral of the reaction model and T(t) is the heat-ing program. The J-integral is defined in this expression. Accordingto Vyazovkin, the J-integrals are equal for all ˛-values regardlessof the heating rate employed. Expressed in a different way, thisassumption entails that the activation energy of the system is givenby the value that minimizes the following function:

˚(Ea) =n∑

i=1

n∑j /= 1

J[Ea, Ti(t˛)]J[Ea, Tj(t˛)]

(10)

where the subscripts i and j represent the number of heating ratesused to study the curing process. The reaction time for a certaincuring degree (t˛) can be calculated for an isothermal stage at anarbitrary temperature according to Eq. (11) [27]:

t˛ =∫ T˛

0exp(−Ea(˛)/RT)dT

exp(−Ea(˛)/RT0)(11)

where T˛ is an experimental value of the temperature corre-sponding to a given conversion at the heating rate and T0 is thetemperature at the isothermal stage. Similary, temperature for acuring degree using an arbitrary heating rate can be calculatedusing Eq. (12):

1/ˇ

∫ T˛

exp(−Ea(˛)/RT)dT − 1/ˇ0

∫ T˛,0

exp(−Ea(˛)/RT)dT = 0

0 0

(12)

where and T˛ are experimental values and T˛,0 is the temperatureat which a given curing degree is reached using a ˇ0 heating rate.

ochimica Acta 529 (2012) 29– 35 31

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J.C. Domínguez et al. / Therm

. Experimental procedure

.1. Materials

A FA resin under development (Furolite 050915A), and an acidatalyst based on p-toluenesulfonic acid dissolved in water (45%queous solution) (“S type + D”), were supplied by Transfuranshemicals BVBA (Belgium). According to the supplier’s datasheet,he specifications of the FA resin are: water content 5.40%, viscosity480 cPs at 25 ◦C, and monofurfuryl alcohol content below 1%.

.2. Measurement of DSC thermograms

The calorimetric measurements were performed on a STA 4491 Jupiter TGA–DSC equipment using low pressure pans with twooles in the lid. DSC runs were carried out from 30 to 250 ◦C usingeven different heating rates: 4, 6, 8, 10, 12, 16 and 20 ◦C min−1.ample weight was between 30 and 35 mg. Three different amountsf catalyst were used to cure the resin: 2%, 4% and 6% (w/w).xperiments were conducted under Ar atmosphere. At least fiveepetitions were carried out for each sample.

.3. Analysis of DSC thermograms

The DSC thermograms were analysed to study the curing kinet-cs of the FA resin. The curing heat of the crosslinking process

as calculated as the area under the curing exothermic peak. Aackground curve was initially obtained in order to avoid interfer-nces and noises from the reference crucible employed during theests. The curing degree of the resin was obtained by the followingxpression:

= (�H)t

�H∞(13)

here is the curing degree, (�H)t is the curing heat released fromhe beginning of the curing process until a certain time t, and �H∞s the total curing heat released.

The calculations of the curing degree from the DSC thermo-rams, and the calculations of the kinetic parameters by the variousethods were done with Microcal Origin© 8.1. and with in-built

unctions of Matlab© software [28]. The tests and calculations werearried out following the indications of the ICTAC Kinetics Commit-ee [29].

. Results and discussion

.1. Calorymetric measurements of the curing process

The characterization of the curing process of the FA resin wasarried out by DSC analysis using three different amounts of catalyst2%, 4% and 6%, w/w). During the tests, the mass loss changes as themount of catalyst is increased, being usually below 2% and nevereaching a 5% therefore the mass loss did not affect the kineticsf the curing process. Fig. 1 shows an example of thermogramsbtained for the three catalyst amounts tested, and with the sameeating rate of 10 ◦C min−1.

In Fig. 1 it is shown that the temperature at the curing peakecreases, and that the maximum heat flow at the peak increases, ashe amount of catalyst is increased. In addition, a small endothermiceak can be seen for all the three curves at ca. 125 ◦C. This peak isssumed to be due to evaporation of the volatiles in the resin. For allhe measured DSC thermograms, the curing heat for each amount

f catalyst, and for the various heating rates are shown in Table 1.

For the 2% (w/w) catalyst, the three fastest heating rates (12, 16nd 20 ◦C min−1) show lower curing heats than for the four slowestamps employed. Thus, in the case of the low amount of catalyst

Fig. 1. DSC curves for the indicated catalyst amounts at a heating rate of 10 ◦C min−1

(2%, w/w), the system becomes less reactive at the higher heatingrates. Similar observations were reported for FA resins by Guigoet al. [13]. In studies of other resin systems, a similar reduction ofthe curing heats has been found when the system was made lessreactive. In a study of epoxy resin, a reduction in the amount ofcuring agent (imidazole type) resulted in lower curing heat [30]. Ina study of phenolic resin, substitution of phenol by lignosulphonatemade the resin less homogeneous and reactive than the commercialresin [31]. In a study of an epoxy-anhydride system, the diminutionof the amount of accelerator increased the reduction of the curingheat when the heating rate was increased [32].

The average curing heat for the central heating rates (8, 10 and12 ◦C min−1) is the same for the 4% and 6% (w/w) amount of catalyst,284 ± 25 and 271 ± 6 J g−1, respectively. However, the curing heatcalculated for a 2% (w/w) catalyst is lower than for the other testedamounts, 208 ± 21 J g−1. Thus, the decrease in the reactivity of thesystem also depends on the amount of catalyst used to carry out thecuring of the resin. This reduction is assumed to be due to that thislow amount of catalyst is not capable of reaching all the reactivesites in the resin network either due to the low amount of catalystemployed and/or to the lower curing times used for high heatingrates. This will result in a lower cross-linking density. Therefore,the used amount of catalyst, besides its influence on the curingrate of the FA resin, may have an important impact in the finalproperties of the resin after the curing process has ended, sincea lower crosslinking density implies lower degradation and glasstransition temperatures, as well as (presumably) lower mechanicalproperties. As can be seen in Table 1, when the amount of catalystis increased above 2% (w/w), only for the highest heating rate of20 ◦C min−1, a reduction of the curing heat is found for the 4% (w/w)catalyst, whereas no reduction is found for the 6% (w/w) catalyst,which supports the above-mentioned explanation.

The values of the curing heats calculated for the FA resin(Table 1) are in general comparable to other resins with similar cur-ing processes, such as melamine-formaldehyde resins (260 J g−1)[33], novolac phenolic resins (129–144 J g−1) [34], and resol phe-nolic resins (98–132 J g−1) [31]. However, the curing heats of theFA resin are significantly lower than those obtained by Guigo et al.[13] for monofurfuryl alcohol (593–709 J g−1). This difference canbe explained by the higher degree of polymerization of the FA resinwith respect to the monomer, which leads to a lower curing heatneeded to become fully cured.

In Table 2, the curing temperature ranges for the different

amounts of catalyst are shown for DSC thermograms measuredwith the lowest, medium and highest heating rates (4, 10,20 ◦C min−1).

32 J.C. Domínguez et al. / Thermochimica Acta 529 (2012) 29– 35

Table 1Average values of curing heat for the FA resin.

Heating rate (◦C min−1) �H∞ (2%, w/w) (J g−1) �H∞ (4%, w/w) (J g−1) �H∞ (6%, w/w) (J g−1)

4 212 ± 8 167 ± 8 177 ± 126 219 ± 4 215 ± 11 224 ± 108 215 ± 6 259 ± 12 270 ± 30

10 222 ± 16 292 ± 21 266 ± 3512 187 ± 12 301 ± 10 277 ± 2616 159 ± 37 267 ± 7 276 ± 1820 165 ± 16 223 ± 17 271 ± 14

Table 2Temperature range for the curing process of the FA resin.

Catalyst (%, w/w) 4 ◦C min−1 10 ◦C min−1 20 ◦C min−1

Tstart (◦C)a Tend (◦C)b Tstart (◦C) Tend (◦C) Tstart (◦C) Tend (◦C)

2 58.3 ± 0.9 106.5 ± 2.6 71.1 ± 1.7 111.0 ± 2.8 80.3 ± 1.3 126.0 ± 3.64 52.5 ± 0.4 82.6 ± 2.1 62.9 ± 0.9 99.2 ± 1.5 69.7 ± 1.0 116.3 ± 1.76 49.0 ± 1.5 77.6 ± 1.4 63.7 ± 1.5 93.5 ± 2.6 66.6 ± 0.5 112.1 ± 1.5

phitaewttfittriiadccac

4K

rcdi

dvDtiibtaibt

Fig. 2. MFK method: Kissinger–Akahira–Sunone. Comparison between the activa-tion energies found for the curing process of the FA resin using different amountsof catalyst.

a Temperature at which the curing degree is 1%.b Temperature at which the curing degree is 98%.

For each of the three amounts of catalyst, the starting tem-erature of the curing process is consistently increased when theeating rate is increased, which is a behaviour widely described

n the literature [35–39]. The same behaviour is also observed forhe ending curing temperatures. In addition, the increase of themount of catalyst involves a reduction of both the starting andnding temperatures. This reduction is however more significanthen the amount of catalyst is increased from 2% to 4% (w/w),

han when it is increased from 4% to 6% (w/w). For the latter case,he decrease in starting temperature is negligible when the con-dence intervals are taken into account. Thus, it can be expectedhat the rate of cure of the resin will increase significantly whenhe amount of catalyst is increased from 2% to 4% (w/w), but theate will remain almost the same when the amount of catalyst isncreased from 4% to 6% (w/w). These observations should be takennto account in the assessment of the estimated values of the curingctivation energy; the changes of these values should be in accor-ance with the measured changes of the curing temperatures. Theuring degree of the FA resin as a function of temperature was cal-ulated from the obtained DSC curves by using Eq. (13). Next, thectivation energy of the curing process will be calculated from theseurves by applying the kinetic methods described in Section 2.

.2. Calculation of changes in activation energy; methods ofissinger–Akahira–Sunone, Flynn–Wall–Ozawa, and Vyazovkin

The activation energy as function of the curing degree for the FAesin with different amounts of catalyst (2%, 4% and 6%, w/w) wasalculated by the KAS and FWO methods according to the procedureescribed in Section 2. The results for the two methods are shown

n Figs. 2 and 3.By both methods, the curing activation energy is shown to be

ependent on the curing degree. The profiles obtained for the acti-ation energy can be divided into three different curing stages.uring the initial and final stages, the results show a high scat-

er, which reveals the low accuracy of the KAS and FWO methodsn calculating the activation energy in these two stages of the cur-ng process. During the main stage, however, two clearly differentehaviours were found for the activation energy depending onhe used amount of catalyst. For the 2% (w/w) amount of catalyst,

n almost constant activation energy in the range 70–80 kJ mol−1

s obtained (Figs. 2 and 3), which indicates that the process cane approximated as a single reaction. Moreover, it also indicateshat the kinetics of the curing process is not markedly diffusion

Fig. 3. MFK method: Flynn–Wall–Ozawa. Comparison between the activation ener-gies found for the curing process of the FA resin using different amounts of catalyst.

J.C. Domínguez et al. / Thermochimica Acta 529 (2012) 29– 35 33

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of the FA resin during its curing process. It can be observed in Fig. 5

ig. 4. MFK method: Vyazovkin. Comparison between the activation energies foundor the curing process of the FA resin using different amounts of catalyst.

ontrolled [40], which then would be seen as a decrease in thectivation energy due to the constrained mobility of the polymerhains. For the 4% and 6% (w/w) amounts of catalyst, a reductionf the activation energy during the main curing stage is observed,hich indicates that the curing kinetics are diffusion controlled.

The change of the activation energy with the curing degree waslso calculated by the VA method. This method applies a more accu-ate approximation of the temperature integral, and therefore itrovides a more accurate approach of calculating the activationnergy, especially in the initial and final curing stages. Furthermore,he VA method has recently been found to be the most suitable

ethod for FA resin [41]. The results obtained in the present studyy this method are shown in Fig. 4.

The overall change of the curing activation energy of the FA resinalculated by the VA method is similar for all the amounts of cata-yst (on the contrary to the KAS and FWO methods). The activationnergy increases almost in parallel (particularly when the amountf catalyst is 4% or 6%, w/w) in the range of curing degree between

and 0.3. This initial increase in the activation energy is presum-bly related to the accumulation of reaction intermediates at thisarly stage of the curing process. When the curing process is fur-her progressing, these intermediates are consumed, and therefore,he accumulation of these intermediates stops [22,42]. This sameehaviour was also found by Guigo et al. [13] in a study of the FAonomer.The change of the activation energy calculated by the VA method

uring the initial curing stage (0 ≤ ≤ 0.3) is significantly more pro-ounced than in the case of the two previously used isoconversionalethods. In addition, the calculated values are significantly higher:

t = 0.3, the activation energies are about 115, 95 and 80 kJ mol−1

or the VA method versus about 76, 70 and 65 kJ mol−1, and 80, 72nd 66 kJ mol−1 for the KAS and FWO methods, for 2%, 4% and 6%w/w) catalyst, respectively.

During the main curing stage (approx. 0.3 ≤ ≤ 0.9), a decreas-ng behaviour is found for the activation energy for all the amountsf catalyst (Fig. 4). This decreasing behaviour is due to the gelationrocess of the FA resin that takes place at a curing degree of about.5 [13]. This same behaviour has been also reported for the FAonomer [13], in addition to a phenolic resin [23], which follows

similar curing mechanism as the FA resin, and for an epoxy resinn a recent study by Wan et al. [40]. The decrease of the activation

nergy continues after gelation due to that the reaction kineticsre increasingly affected by diffusion; the curing process mainlyelies on the mobility of molecular segments which have a short

Fig. 5. Experimental and predicted values of the curing degree of the FA resin withdifferent amounts of catalyst, and a heating rate of 10 ◦C min−1.

range motion. On the contrary to the KAS and FWO methods, theVA method reveals that for the 2% (w/w) catalyst, the curing kinet-ics are also influenced by a constrained mobility of the polymerchains. However, in agreement with the KAS and FWO methods,the reduction of the activation energy for the 4% and 6% (w/w) cat-alyst is much more marked. This reduction of the activation energyduring the main curing stage has also been found.

In Fig. 4, it can be seen that for all the three amounts of catalyst,the activation energy starts to decrease rapidly at ≥ 0.9. In thisfinal curing stage, the mobility of the polymer chains is severelycontrained so that the curing kinetics are no more reaction con-trolled, but only diffusion controlled.

The use of the three isoconversional methods has proved thatthe activation energy of the curing process of the FA resin is highlydependent on the amount of catalyst; it shows a clearly differentbehaviour when using 2% (w/w) catalyst compared to 4% or 6%(w/w). Therefore, based on a purely kinetic criterion, it is assessedthat the most appropriate amount of catalyst to be added to theresin is 4% (w/w) since it shows significantly higher curing ratesthan those obtained with 2% (w/w) catalyst, and it shows curingrates that are almost equal to those obtained with 6% (w/w) cata-lyst. The latter result is probably due to that saturation is reachedin the range of 4–6% (w/w) [33]. However, other concerns can beconsidered in the selection of the proper amount of catalyst: e.g., afast curing reaction is not always desirable, and moreover, a loweramount of solvent water coming from the catalyst could improvethe final properties of the materials by limiting voids growth. Forthese concerns, a 2% (w/w) catalyst could me more recommend-able.

4.3. Prediction of curing degree

In order to compare the predictions made through the isocon-versional MFK methods employeed in the present work, Eq. (12) hasbeen used to obtain the predicted curing degree of the FA resin asa function of temperature for the three amounts of catalyst. Thesepredicted values together with the experimental values are shownin Fig. 5.

In general, the three isoconversional methods are all well pre-dicting the experimentally measured change of the curing degree

that the values predicted by the KAS and FWO methods are similar.The predictions made by the VA method result in slightly highercuring degrees at a given temperature than those achieved by the

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AS and FWO methods. However, during the initial curing stage,nly the VA method is able to accurately predict the experimentalctivation energy profile for all three amounts of catalyst. Similarbservations have been made in the literature for other resin sys-ems [13]. Altogether, it can be said that the accurate predictions

ade by the three isoconversional MFK methods demonstrate thathese methods can be used in the design of improved cure cyclesor manufacturing of composite materials with a FA matrix.

. Conclusions

The curing process of a FA resin was studied by non-isothermalSC measurements using seven different heating rates. Threemounts of catalyst were used: 2%, 4% and 6% (w/w). The curingeat for each amount of catalyst were obtained, and it was found toe increased when the amount of catalyst was increased. Thus, theeactivity of the system was found to be a function of the amountf catalyst.

The evolution of the curing degree of the resin as function ofemperature was obtained. The change of the activation energy dur-ng curing was calculated by using the model-free kinetics methodsf Kissinger–Akahira–Sunone, Flynn–Wall–Ozawa, and Vyazovkin.he two first methods showed similar activation energy profiles,nd with values from 55 to 80 kJ mol−1 during the main curingtage, which is close to the values obtained by the single-valueethods of Ozawa and Kissinger. The Vyazovkin method was used

o have a more accurate approximation of the change in the activa-ion energy during the initial and final curing stages. By this method,he obtained profile of the activation energy is similar for all themounts of catalyst. The obtained profile is in agreement with pre-ious results in the literature for comparable resin systems. Thectivation energies calculated by the Vyazovkin method are rangingrom 80 to 115 kJ mol−1.

Altogether, based on a purely kinetic criterion, the results showhat the most appropriate amount of catalyst for the FA resin is 4%w/w).

It is demonstrated that the presented kinetic models and meth-ds, and the obtained values of the kinetic parameters of the FAesin, allow calculation of the time required to achieve a certainuring degree of the resin under certain operating conditions. Thisill form an important contribution to the design of optimized

ure cycles for the manufacturing of composite materials with aA matrix.

cknowledgements

The authors are grateful for the support from “Ministerio deducación” (Spanish Government) for financial support (Programaacional de Movilidad de Recursos Humanos del Plan Nacional de

-D + i 2008-2011). The research has been funded by the Europeanommunity’s Seventh Framework Programme (FP7/2007-2013)nder grant agreement no. 210037 (WOODY).

eferences

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[2] N. Guigo, A. Mija, L. Vincent, N. Sbirrazzuoli, Eco-friendly composite resinsbased on renewable biomass resources: polyfurfuryl alcohol/lignin thermosets,Eur. Polym. J. 46 (2010) 1016–1023.

[3] A.P. Dunlop, F.N. Peters, The Furans, Reinhold Publishing Co., 1953.[4] M. Choura, N.M. Belgacem, A. Gandini, Acid-catalyzed polycondensation of

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