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Cure Kinetics Study of Two Epoxy Systems with FourierTranform Infrared Spectroscopy (FTIR) and DifferentialScanning Calorimetry (DSC)Dimitris S. Achilias a , Maria M. Karabela a , Eleni A. Varkopoulou a & Irini D. Sideridou aa Laboratory of Organic Chemical Technology, Department of Chemistry, Aristotle Universityof Thessaloniki, Thessaloniki, GreecePublished online: 13 Jul 2012.

To cite this article: Dimitris S. Achilias , Maria M. Karabela , Eleni A. Varkopoulou & Irini D. Sideridou (2012) Cure KineticsStudy of Two Epoxy Systems with Fourier Tranform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC),Journal of Macromolecular Science, Part A: Pure and Applied Chemistry, 49:8, 630-638, DOI: 10.1080/10601325.2012.696995

To link to this article: http://dx.doi.org/10.1080/10601325.2012.696995

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Journal of Macromolecular Science, Part A: Pure and Applied Chemistry (2012) 49, 630–638Copyright C© Taylor & Francis Group, LLCISSN: 1060-1325 print / 1520-5738 onlineDOI: 10.1080/10601325.2012.696995

Cure Kinetics Study of Two Epoxy Systems with FourierTranform Infrared Spectroscopy (FTIR) and DifferentialScanning Calorimetry (DSC)

DIMITRIS S. ACHILIAS, MARIA M. KARABELA, ELENI A. VARKOPOULOU, andIRINI D. SIDERIDOU∗

Laboratory of Organic Chemical Technology, Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki, Greece

Received February 2012, Accepted March 2012

This work was aimed at the study of cure kinetics of two commercial thermosetting epoxy systems, Epikote resin 816 LV/EpikureF205 and Epikote resin 240/Epikure F205, by Fourier Tranform Infrared Spectroscopy (FTIR) and Differential Scanning Calorime-try (DSC). The studied systems consist of a resin (A), based on a diglycidyl ether of bisphenol A and a hardener (B) basedon the Isophorodiamine (IPDA) a cycloaliphatic diamine. These systems are used for the building and civil engineering in-dustries, e.g. flooring compounds, adhesives, mortars and grouts. FTIR spectroscopy was employed to investigate the isother-mal curing kinetics at 30, 50 or 70◦C and DSC analysis to study the non-isothermal curing kinetics at different heating rates2.5, 5, 10 and 20◦C/min, from 20 to 300◦C. A kinetic model was employed to simulate the FTIR isothermal experimentaldata using two kinetic rate constants and incorporating also diffusion control at high degrees of conversion. Finally, the varia-tion of the effective activation energy with the extent of curing was estimated using isoconversional analysis of non-isothermalDSC data.

Keywords: Epoxy resin, epikote resin 816 LV, epikote resin 240, epikure F205, curing kinetics, FTIR, DSC, kinetic parameters

1 Introduction

Epoxy resins are an important class of thermosetting resinsthat widely used as adhesives, coatings, electronics, high-performance composite materials, aerospace industries be-cause of their excellent mechanical and chemical proper-ties, such as high tensile and compressive strength, goodchemical resistance, and high heat distortion temperature(1). Generally, the physical, electrical, transport and ulti-mate mechanical properties of epoxy resins depend stronglyupon the curing process (2). Therefore, the understandingof the curing mechanism and the ability to develop suitablekinetic models to simulate the curing reaction are essen-tial to predict and to control the final properties of thecrosslinked materials.

The curing kinetics of epoxy resins have been stud-ied experimentally with various techniques, which can begrouped into direct and indirect methods. The direct meth-ods allow the evaluation of the one or more reactive groups

∗Address correspondence to: Irini D. Sideridou, Laboratoryof Organic Chemical Technology, Department of Chemistry,Aristotle University of Thessaloniki, Thessaloniki GR-54124,Greece. Tel: +302310 997825; Fax: +302310 997769; Email:siderid@chem.auth.gr

as a function of time, such as gel permeation chromatogra-phy (GPC) (3, 4) nuclear magnetic resonance (NMR), elec-tron paramagnetic resonance, Fourier transform infraredspectroscopy (FTIR) and Raman spectroscopy (2, 5–12).Indirect analysis gives only an assessment of the degreeof cure by monitoring as a function of time, a physicalproperty directly correlated with the reactants’ conversion,such as thermal analysis by differential scanning calorime-try (DSC) (13–19) rheology, dielectric spectroscopy ultra-sonic measurements (20–21).

In the present work, FTIR spectroscopy was employedto investigate the isothermal curing kinetics of two com-mercial epoxy systems (30, 50 or 70◦C) and DSC analysisto study the non-isothermal curing kinetics of the two sys-tems at different heating rates 2.5, 5, 10 and 20◦C/min,from 20 to 300◦C. The studied epoxy systems were Epikoteresin 816 LV/Epikure F205 and Epikote resin 240/EpikureF205. The systems consist of a resin (A), based on a digly-cidyl ether of bisphenol A and a hardener (B) based on theisophorone diamine (IPDA) (Fig. 1).

Epikote 816 LV is of low volatility and mild odor whichgive the resin its agreeable handling characteristics. It pro-vides a good level of reactivity, mechanical performanceand chemical resistance. In addition, EPIKOTE 816 LV isconsidered to have a good resistance to crystallization. It is

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Cure Kinetics of Epoxy Systems FTIR and DSC 631

Fig. 1. Chemical structure of the used epoxy systems: Epikote resin 816LV/Epikure F205 and Epikote resin 240/Epikure F205. Datawere given by the manufacturer.

used for the building and civil engineering industries, e.g.flooring compounds, adhesives, mortars and grouts.

Epikote resin 240 has a low volatility and lack of odor,factors which are responsible for its agreeable handling

characteristics. EPIKOTE 240 does not crystallize on stor-age. It is used for the manufacture of heavily-filled com-positions for the building and civil engineering industriese.g. flooring compounds, mortars, grouts, adhesives, etc. It

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is used also for the formulation of solvent-free and high-solids coatings.

Epicure F205 is a low viscosity curing agent based onIsophorodiamine (IPDA). It is used in the formulation ofroom temperature curing compositions such as industrialfloorings, tank linings, solvent-free coatings, concrete ad-hesives, etc.

2 Experimental

2.1 Materials

Epikote resin 816LV / Epikure F205 and Epikote resin240/Epikure F205, consist of two components A and B andwas supplied by Resolution Performance Products (Hous-ton, Texas, USA).

Component A, which is a resin, for the first system(Epikote 816LV) is produced from bisphenol A andepichlorohydrin (DGEBA) (CAS No. 25068-38-6) with anumber average molecular weight, Mn < 700 (50–62% w/w)and containing an added proportion of Cardura E10P(glycidyl ester of neodecanoic acid) (CAS No. 26761-45-5) as reactive diluent. Component A for the second sys-tem (Epikote resin 240) is based on a blend of a bisphe-nol A resin (produced from bisphenol A and epichlorohy-drin (CAS No. 25068-38-6) and a bisphenol F resin (pro-duced from bisphenol F and epichlorohydrin (CAS No.2467-02-9) and containing an added proportion of a mono-epoxidised alcohol as a reactive diluent.

Component B, which is the hardener, for both systemsis the Epikure F205. It contains (a) Isophorodiamine (3-aminomethyl-3,5,5-trimethylcyclohexylamine) (CAS No.2855-13-2) 25–35% (b) Phenol, 4,4-(1-methylethylidene)bis-, polymer with 5-amino-1,3,3-trimethylcyclohexane-methanamine and (chloromethyl)oxirane or polymer ofBisphenol A diglycidyl ether with Isophorodiamine (CASNo. 38294-64-3) 25–35% (c) benzyl alcohol (CAS No.100-51-6) 35-50% and (d) salicylic acid (CAS No. 69-72-7)2–3%.

The chemical structure of the systems main componentsused appears in Figure 1. All the materials in this studywere used as received without further purification.

2.2 FTIR Analysis

The FT-IR analysis was conducted in a FT-IR spectrome-ter, Spectrum One, Perkin-Elmer. The two components, Aand B, of each system were completely mixed by stirring atroom temperature until they formed a homogeneous mix-ture.

They were mixed at stoichiometric ratio as the supplierrecommends. The epoxy molar mass (EMM) of an epoxycompound is defined as the grams of resin containing 1g-equivalent of epoxide and is expressed in g/eq. The epoxymolar mass of Epikote resin 816LV is reported by the man-

ufacturer to be 192–204 = 198 g/eq and the epoxy molarmass of Epikote resin 240 is 185–196 = 190.5 g/eq.

The amine hydrogen molar mass (AHMM) of EpikureF 205 is reported by the manufacturer to be 104 g/eq.

Since it is assumed that one “amine hydrogen” re-acts with one epoxy group, the stoichiometric ratio ofthe hardener to use with the epoxy resin is given bythe ratio: (AHMM/EMM). In the case of Epikote resin816LV/Epikure F 205 in which AHMM = 104 g/eq andEMM = 198 g/eq the ratio AHMM/EMM = 0.525. Ifthis value is multiplied by 100, then the parts by weight ofamine per 100 parts of resin (52.5 phr) can be obtained.

In the case of Epikote resin 240/Epikure F 205, the stoi-chiometric ratio of the hardener to use with the epoxy resinis AHMM/EMM = 104/190.5 = 0.55. If this value is mul-tiplied by 100, then the parts by weight of amine per 100parts of resin (55 phr) is obtained.

A small amount of each system was placed between twotranslucent polyethylene strips, which pressed to produce avery thin film. This film was placed in a thermostated oven(TERMAKS, B8000 series, model B 8054) at several curingtemperatures (30, 50 or 70◦C). At pre-specified time inter-vals, each film was removed, placed on a NaCl crystal sinceKBr may act as a catalyst in the reaction, and the FTIRspectra of the samples were recorded at room temperatureimmediately after the oven cure. Spectra were obtained over1600–600 cm−1 region and were acquired with a resolutionof 4 cm−1 and a total of 64 scans per spectrum. For eachspectrum it was determined the height of the oxirane ringpeak absorption at 915 cm−1 and the p-phenylene grouppeak absorption at 830 cm−1, utilizing a base line tech-nique which proved the best fit to the Beer–Lambert law.The p-phenylene group absorbance the intensity of whichdoes not vary during the curing process is used as inter-nal standard. The percent conversion, which expresses thepercent amount of the epoxy groups reacted, is determinedaccording to the equation:

Degree of conversion (%) = 100[

1 − (A915/A830)t

(A915/A830)0

](1)

Where A represents the absorbance of the group and thesubscripts 0 and t denote zero and reaction time, respec-tively.

All the results presented in the following sections weretaken from an average of at least four experiments.

2.2 DSC-Analysis

The DSC study of the epoxy systems was carried outusing the differential scanning calorimeter Diamond DSCfrom Perkin-Elmer. The resin and the curing agent weremixed at a stoichiometric ratio at room temperature. Themixtures were sealed in aluminum pans and cured byheating in the DSC. Temperature and heat calibration ofDSC were carried out using Zinc and Indium standards.Non-isothermal experiments were performed at four

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Cure Kinetics of Epoxy Systems FTIR and DSC 633

different heating rates: 2.5, 5, 10 and 20◦C/min, from 20to 300◦C under nitrogen flow.

2.3 Theoretical Background

The curing of epoxy resins passes from the liquid state intothe rubber state through its gel point and then it is convertedinto a glass through the vitrification point. During curing,the three main reactions may take place, either simultane-ously or sequentially, depending on the reactants; reactivityand on the process temperature. These are the reactions ofepoxide ring, at the end of epoxy resin chains, with the pri-mary and secondary amine groups and the etherificationof the oxirane ring with a pendant hydroxyl group (Fig. 2).The etherification reaction is known to be not important atlow temperatures or for amine excess (22, 23).

The linear epoxy resins are converted during the curinginto three dimensional crosslinked thermoset networks.

According to this kinetic scheme and with the simplifyingassumptions of equal reactivity of primary and secondaryamino-hydrogen with epoxy groups and a stoichiometricratio of epoxy to amino-hydrogen groups, the followingequation was obtained (13, 15):

dα

dt= (k1 + k2α) (1 − α)2 (2)

Where α denotes conversion of the epoxy groups and therate constants k1 and k2 correspond, respectively, to catal-ysis by groups initially present in the resin and catalysis byhydroxyl groups newly formed in the reaction. These rateconstants depend on temperature with an Arrhenius-type

expression:

ki = Ai exp (−Ei/RT) (3)

Where Ai denotes the frequence factor of the ki rate con-stant and Ei its corresponding activation energy.

Equation 2 was extensively used in the literature anddescribes the experimental data when epoxy conversionis completed (final degree of curing approaching 100%).However, when the epoxy resins polymerize at tempera-tures below the glass transition temperature of the poly-mer being formed, the reaction does not reach completeconversion. As the conversion increases and the Tg of themixture approaches the reaction temperature, the diffu-sivity of the reactive functional groups becomes seriouslyrestricted. Thus, the curing reaction decreases considerablyeven when there may be a significant level of amino andepoxide groups available for reaction. Hence, a limitingconversion is usually achieved which is increased with thepolymerization temperature. Several equations have beenproposed to describe the effect of diffusion phenomena onthe polymerization kinetics [24]. One of them is the follow-ing (14, 15):

1k

= 1kchem

+ 1kdiff

⇒ kkchem

= 11 + kchem/kdiff

(4)

In the above equation the overall kinetic rate k is ex-pressed in terms of a reaction-limited term kchem and adiffusion-limited one kdiff (14, 15)

Chern and Poehlein (25) proposed a very simple equationcoming from the free volume theory to estimate the ratio of

Fig. 2. . Reactions occurred during curing of an epoxide with a primary amine.

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Fig. 3. FT-IR spectra of the epoxy system Epikote resin 816LV/Epikure F205, at different times curing, at 70◦C. (Color figureavailable online).

the diffusion controlled to reaction limited rate constant:kdiff

kchem= exp (−C(α − αc)) (5)

Where C is a constant that depends on the structure, systemand curing temperature and αc is a critical conversion.

Thus, the final equation which describes the overall re-action kinetics is (12):

dα

dt= (k1 + k2α) (1 − α)2 1

1 + exp (C(α − αc))(6)

3 Results and Discussion

The FTIR spectrum of Epikote resin 816LV / Epikure F205 and Epikote resin 240/Epikure F 205 film before cur-ing just after mixing showed peaks assigned to the epoxyresin and to the hardener. The most important of thesepeaks is that of the epoxide group at 915 cm−1 and thatof p-substituted aromatic ring at 833 cm−1 (6). Absorp-tion at 3344, 3282 and 3174 cm−1 is due to the stretchingvibration of the primary amino group (-NH2) on the hard-ener structure. It is obvious that the epoxide group peakand the primary amine peak, both decreased in size in thecourse of cure. On the contrary, new absorptions appeararound 3400 cm−1 due to secondary amine (N-H) and hy-droxyl groups, formed during cure. For quantitative studiesof epoxy cure the characteristic peak at 915 cm−1 is com-monly being used. Infrared spectra of the epoxy systemEpikote resin 816LV / Epikure F 205 during curing at dif-ferent times, at 70◦C, is illustrated in Figure 3.

The increase of percent conversion with the reaction timefor various reaction temperatures is shown correspondinglyin Figure 4a (Epikote resin 816LV / Epikure F 205) andFigure 4b (Epikote resin 240 /Epikure F 205). It is obvi-ous that an increase of reaction temperature leads to theincrease of reaction rate and to completion of reaction atshorter time. Also, it is observed that in all cases the reac-tion does not proceed up to 100% conversion but reaches alimiting degree of conversion. This limiting degree of con-version and the time at which this is reached are shown inTable 1. It is observed that Epikote resin 816LV/Epikure F205 system showed slightly higher values of limiting degreeof conversion and lower curing time values than Epikoteresin 240/Epikure F 205 system.

Moreover, based on Equation 6, the best fitting valuesof the kinetic parameters of curing were calculated and areillustrated in Table 2. A non-linear data fitting procedure

Table 1. Limiting degree of conversion and the time reached forvarious curing temperatures

Resin

CuringTemperature

(◦C)

Limiting degreeof conversion

(%)a

CuringTime(h)a

Epikote resin 816LV 30 69 21.5Epikote resin 816LV 50 75 10.0Epikote resin 816LV 70 80 5.0Epikote resin 240 30 65 24.6Epikote resin 240 50 72 21.0Epikote resin 240 70 77 8.5

a) Obtained from kinetic model.

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Cure Kinetics of Epoxy Systems FTIR and DSC 635

Fig. 4. Effect of isothermal temperature on the extent of conver-sion as a function of time curing of the epoxy systems: (a) Epicoteresin 816LV/ Epikure F205 and (b) Epicote resin 240/EpikureF205, at 30, 50 and 70◦C. The symbols represents experimentaldata from FTIR while the continuous lines represent the kineticmodel predictions. (Color figure available online).

was employed in this work based on Levenberg-Marquardtmethod and integrated into the OriginPro 7.5 software. Us-ing these values, differential Equation 6 is integrated to

provide the theoretical simulation curves of α as a functionof time. The kinetic model results are included in Figure4(a and b) as continuous lines. As can be easily seen, thekinetic theoretical model simulates the experimental dataadequately at all different experimental conditions and dur-ing the whole evolution of conversion. From the values re-ported in Table 2, it can be seen that the estimated values ofC were unaffected by temperature for the Epicote 816LV,while slightly affected for the Epicote 240. Concerning thevariation of C with temperature, no discernible trend wasalso observed in literature (12, 14). The estimated valuesrange between 12 and 30 and are similar to correspondingliterature values (12). In contrast, the critical conversion,αc, was found to increase with temperature, again in ac-cordance to literature data (12, 14). Since αc correspondsto the vitrification conversion, i.e., conversion above whichthe effect of diffusion-controlled phenomena is important,it is expected that increased temperature increases the mo-bility of molecules and thus the onset of diffusion controlmay be shifted to higher values. Regarding the kinetic rateconstants k1 and k2 it was found that the best fitting valuefor k2 in all cases is that of zero. Therefore, according to themodel fitting results it seems that catalysis by groups ini-tially present in the resin dominates the particular curing,instead of catalysis by hydroxyl groups newly formed in thereaction. By plotting k1 as a function of temperature in anArrhenius type plot, the Activation energy, E, can be calcu-lated from the slope of the straight line. These values for thetwo systems investigated appear in Table 2 and are similar tocorresponding literature values for different epoxy-aminesystems (12). Actually, according to Vyazovkin and Sbir-razzuoli (26) activation energy values for epoxy-anhydridecuring reactions derived from isothermal experiments varyfrom 28 to 100 kJ/mol.

As it is well-known, curing of epoxy resins is a highlyexothermic process. Hence, reaction kinetics could be mon-itored by recording the amount of heat released with timeusing differential scanning calorimetry. Therefore, curing ofthe epoxy materials studied here was subsequently investi-gated by DSC under non-isothermal conditions at variousheating rates. Characteristic curves illustrating the changeof heat flow with temperature for the Epicote 240 resincured at different heating rates are presented in Figure 5.As it can be seen the reaction starts at approximately 20◦Cand the maximum rate appears in the interval from 80 to

Table 2. Optimum values of kinetic parameters at various temperatures

Resin Temperature (◦C) k1 × 103 (min−1) k2 × 103 (min−1) C αc E (kJ/mol)

Epicote 816 LV 30 8 — 30 0.60 34.550 14 — 30 0.6770 40 — 30 0.74

Epicote 240 30 7 — 15 0.50 31.450 14 — 15 0.5570 30 — 12 0.60

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Fig. 5. Normalized heat flow versus temperature obtained fromDSC for the epoxy system Epikote resin 240/Epikure F205, curedat different heating rates. (Color figure available online).

110◦C depending on the heating rate. Reaction stops near270◦C.

Kinetic analysis of curing using DSC data is usuallybased on the following single step kinetic equation (27,28):

dα

dt= β

dα

dT= kf (α) = k0e−E/RT f (α) (7)

Where t is the time, T is the temperature, k(T) is the rateconstant, k0 is the pre-exponential factor, E is the activationenergy and f (α) is the reaction model.

To evaluate the Arrhenius parameters k0 and E by Equa-tion 7, one has to separate the temperature, k(T) and con-version dependence, f (a) of the reaction rate. As it was re-ported previously, the currently dominating methodology,is force-fitting of experimental data to reaction models andestimate the optimum parameter values. When applied tononisothermal data, the model fitting methods tend to pro-duce highly unreliable values of Arrhenius parameters (27,28). This problem can be resolved when using model-freemethods that allow the activation energy to be evaluatedwithout assuming any reaction model (26–28). To use thesemethods, one has to perform a series of experiments atdifferent heating rates, β = dT/dt.

One of the most frequently used model-free methods todescribe the cure kinetics is that of Kissinger. This methodderives from the correlation between the peak temperature(Tp) and β and is based on a linear relationship between thelogarithm of β/Tp

2 with the inverse of the peak temperatureof the exothermic curing reaction through the followingexpression:

ln

(β

T2p

)= ln

AREa

− ER

1Tp

(8)

Fig. 6. Kissinger-type plots to estimate the effective activationenergy of the two epoxy resin systems from non-isothermal DSCmeasurements. (Color figure available online).

Accordingly, the effective activation energies of both sys-tems investigated here can be efficiently calculated. Thelogarithm plots of heating rate versus the reciprocal of theabsolute peak temperature are given in Figure 6. The effec-tive activation energies, E calculated from the slope of thestraight lines obtained were 59.0 and 63.5 kJ/mol for theresins Epicote 240 and Epicote 816LV, respectively. Thesevalues are in good agreement to other corresponding litera-ture values (see for example Zhang et al. (29) 54.6 kJ/mol).However, these values are higher than the correspondingones estimated previously from FT-IR measurements. Ofcourse this discrepancy is due to a number of reasons in-cluding the different experimental technique employed (inDSC the heat released is monitored, while in FT-IR theabsorbance of chemical bonds), the type of curing (i.e.,isothermal or non-isothermal), the temperature intervalemployed, as well as the theoretical model used in the cal-culations.

Although the Kissinger method produces unambiguousArrhenius parameters, it yields a single pair of Arrheniusparameters for the whole process in a manner similar to themodel-fitting methods. The resulting average values do notreflect changes in the reaction mechanism and kinetics withthe temperature and the extent of conversion. However,epoxy cures are known to involve multiple steps that arelikely to have different activation energies. Then, the con-tributions of these steps into the overall cure rate measuredby DSC should vary with both temperature and extent ofcure. This means that the effective activation energy deter-mined from the overall rate measurements should generallybe a function of these two variables (26–28).

If changes in the mechanism are associated with changesin the activation energy, they can be detected by using themodel-free isoconversional methods (26–28). These meth-ods are based on the isoconversional principle that states

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Cure Kinetics of Epoxy Systems FTIR and DSC 637

that the reaction rate at constant extent of conversion isonly a function of the temperature:

ln(

dα

dt

)α

= ln [k0 f (α)] − Eα

RTα

(9)

In Equation 9, the subscript α indicates the values re-lated to a given extent of conversion. This statement holdsstrictly only for a single step reaction. For multi-step ki-netics, the global value of a does not uniquely determinethe composition of the sample. This ultimately causes Eato vary with a, that reflects the variation in relative contri-butions of single steps to the overall reaction rate. Reveal-ing the Eα-dependence assists in both detecting multi-stepprocesses and gaining insight into their mechanisms. Vya-zovkin and Sbirrazzuoli in a series of papers (26–28) usedthe isoconversional kinetic analysis to study the kinetics ofepoxy-amine systems. These authors, using an isoconver-sional approach estimated that the overall activation energyshould be a function of the extent of cure and take valuesin the range of 20 to 100 kJ/mol. According to isocon-versional methods the reaction rate at constant extent ofconversion is only a function of temperature.

In order to perform an isoconversional analysis initiallythe curing rate is estimated from data reported in Figure 5and are plotted as a function of conversion in Figure 7.From these data and using Equation 9 by plotting ln(dα/dt)as a function of 1/T at different extents of conversion theeffective activation energy at different α can be calculatedand is plotted in Figure 8.

As can be seen in Figure 8, the activation energy valuesstart from values similar to those found during isothermalFTIR experiments, i.e. in the range of 25 to 35 kJ/mol.Afterwards, the values continuously increase until almost60%, where an abrupt increase was observed in both sys-tems investigated. Therefore, it seems that the reaction ini-

Fig. 7. Curing rate vs. relative degree of curing for the epoxysystem Epikote resin 240/Epikure F205, cured at different heatingrates. (Color figure available online).

Fig. 8. Dependence of the effective activation energy on the extentof conversion for the epoxy system Epikote resin 240/EpikureF205 and Epikote resin 816LV/Epikure F205 cured in the DSCunder non-isothermal conditions. (Color figure available online).

tially starts with low activation energy, while afterwardswhen gelation and vitrification occurs the energy is signifi-cantly increased. The values estimated using the Kissingermethod were almost in the middle between the higher andthe lower value seen in Figure 8 for each resin studied.

4 Conclusions

In this study, the cure kinetic of two commercial thermoset-ting epoxy/amine systems used in the building and civil en-gineering industries was investigated. These were Epikoteresin 816 LV/Epikure F205 and Epikote resin 240/EpikureF205 and the evolution of epoxy conversion with time wasrecorded isothermally at 30, 50 and 70◦C using FTIR. Thelimiting degree of conversion varied between 70 and 80%depending on temperature and achieved after 22 to 5 h ofcuring, respectively. Epikote 816LV showed slightly highervalues of limiting degree of conversion and lower curingtime values compared to Epikote 240. A kinetic model wasemployed accounting of both kinetic and diffusional limita-tions at high conversions and the parameters were evaluatedby fitting to experimental data. It was found that catalysisby groups initially present in the resin dominates the partic-ular curing, instead of catalysis by hydroxyl groups newlyformed in the reaction. Furhtermore, DSC analysis wasused to study the non-isothermal curing kinetics at differ-ent heating rates and the variation of the effective activationenergy with the extent of curing was estimated using iso-conversional analysis of the non-isothermal DSC data. Theinitial activation energy values were similar to those foundduring isothermal FTIR experiments, i.e., in the range of 25to 35 kJ/mol, while afterwards they continuously increaseduntil almost 90 kJ/mol.

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