po

ao,Chine

Available online 22 April 2009

ining

diglycidyl ether of bisphenol A (DGEBA) were cured with 4,4-diaminodiphenyl methane

the

as biphenyl, naphthalene, uorene, heterocyclic ring, etc.[812].

The compound that contains uorene ring usually hasexcellent heat resistance, high refractive index, high trans-parency and low linear expansion coefcient [1315].

thermal properties of DGEBF cured with DDM [11]. In or-der to study the specic relationship between polymerstructures and properties of the epoxy resins based onbisphenol uorene, DGEBF was then cured with 4,4-(9-u-orenylidene)-dianiline (FDA), which also contains uorenestructure. Meanwhile, a commercially available epoxy re-sin, diglycidyl ether of bisphenol A (DGEBA) was also curedwith the same curing agents for comparison. The curing

0014-3057/$ - see front matter 2009 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel./fax: +86 10 82619667.E-mail address: jxu@iccas.ac.cn (J. Xu).

European Polymer Journal 45 (2009) 19411948

Contents lists available at ScienceDirect

European Poly

elsedoi:10.1016/j.eurpolymj.2009.04.012coatings, adhesives, insulating and substrate materialsdue to their superior electrical and mechanical properties,excellent moisture and chemical resistance, low shrinkageand good cohesiveness compared to many materials [13].With the development of advanced technology, many at-tempts have been made to prepare for high-performanceepoxy resins, especially to improve their thermal proper-ties [47]. An effective approach to enhance thermal prop-erties of epoxy resins is introducing various aromatic ringstructures into the skeleton of epoxy or curing agent, such

with a uorene or anthrone group between the two phenylgroups show a better thermal property. At present, the u-orene-containing epoxy resins are widely used for produc-ing molded product, forming interlayer insulation lm, andthe binder composition for soldering resist in printed wir-ing board manufacture and so on [1719]. However, thestudy on the basic theories of epoxy resins which containsuorene structure either in epoxy or in curing agent hasless been reported.

We have previously studied the curing kinetics andKeywords:Epoxy resinsFluoreneThermal propertiesKinetics

1. Introduction

Epoxy resins are widely used in(DDM) and 4,4-(9-uorenylidene)-dianiline (FDA). The curing kinetics, thermal propertiesand decomposition kinetics of these four systems (DGEBA/DDM, DGEBF/DDM, DGEBA/FDA,and DGEBF/FDA) were studied in detail. The curing reactivity of uorene epoxy resins waslower, but the thermal stability was higher than bisphenol A resins. The onset decomposi-tion temperature of cured epoxy resins was not signicantly affected by uorene structure,but the char yield and Tg value were increased with that of uorene content. Our resultsindicated that the addition of uorene structure to epoxy resin is an effective method toimprove the thermal properties of resins, but excess uorene ring in the chain backbonecan depress the curing efciency of the resin.

2009 Elsevier Ltd. All rights reserved.

industry eld as

Diglycidyl ether of 9,9-bis(4-hydroxyphenyl) uorene(DGEBF) as a resin containing uorene structure was rstprepared by Korshak et al. [16]. They found that polymersReceived 5 September 2008Received in revised form 2 April 2009Accepted 10 April 2009

(DGEBF) was synthesized by a two-step reaction procedure. In order to investigate the rela-tionship between uorene structure and material properties, DGEBF and a commonly usedKinetics and thermal properties of euorene structure

Zhen Dai, Yanfang Li, Shuguang Yang, Ning ZhBeijing National Laboratory for Molecular Sciences, Institute of Chemistry, TheBeijing 100190, PR China

a r t i c l e i n f o

Article history:

a b s t r a c t

The uorene-conta

journal homepage: www.xy resins based on bisphenol

Xiaoli Zhang, Jian Xu *

se Academy of Sciences, Box 50, Zhongguancun North First Street 2,

epoxy, diglycidyl ether of 9,9-bis(4-hydroxyphenyl) uorene

mer Journal

vier .com/locate /europol j

Eugene

kinetics, thermal properties and decomposition kinetics ofthese epoxy resins (DGEBA/DDM, DGEBF/DDM, DGEBA/FDA, and DGEBF/FDA) were studied in detail.

2. Experimental

2.1. Materials

9,9-Bis(4-hydroxyphenyl) uorene was purchased fromSuqian Ever-Galaxy Pharmacy & Chem. Co., China. Epichlo-rohydrin (ECH), tetraethyl ammonium bromide (TEAB),1,4-dioxane, sodium hydroxide (NaOH), acetone and etha-nol were obtained from Beijing Chemical Reagents Co.,Chinaandwereusedwithout furtherpurication.Diglycidylether of bisphenol A (DGEBA, EEW = 196 g/mol) and 4,40-diaminodiphenylmethane (DDM) were received fromShanghai Chemical Co., China. 4,4-(9-Fluorenylidene)-dian-iline(FDA) was purchased from Aldrich.

2.2. Synthesis of diglycidyl ether of 9,9-bis(4-hydroxyphenyl)uorene

9,9-Bis(4-hydroxyphenyl) uorene (175.21 g, 0.5 mol),TEAB (21.02 g, 0.1 mol), ECH (470.25 ml, 6.0 mol), and1,4-dioxane (96.71 ml, 1.1 mol) were added to a three-necked ask. The reaction mixture was stirred at 65 C

ethanol (5:1, volume ratio) solution. Then the white crys-talline powers of DGEBF were obtained [11].

2.3. Preparation of the cured epoxy resins

The chemical structures of epoxies and curing agentsare shown in Table 1. The epoxies were initially heatedto be melting and then a stoichiometric ratio of curingagent according to epoxy equivalent weight was mixedhomogeneously, which form four reactions of DGEBA/DDM, DGEBF/DDM, DGEBA/FDA and DGEBF/FDA. Thenthe mixtures were cured in their respective optimal curingcondition, decided by dynamic DSC tracing of epoxy andcuring agent compositions. The curing cycles are listed inTable 2.

2.4. Characterization

Differential scanning calorimetry (DSC) was performedon a Mettler Toledo 822e with a constant nitrogen owof 20 mL/min. The dynamic scanning experiments wereranged from 25 to 300 C at heating rates of 5, 10, 15 and25 C/min, respectively. Thermogravimetric analysis(TGA) was performed using a PerkinElmer Pyris1 at theheating rates of 10, 20, 30, 40 C/min in a nitrogen atmo-sphere from 25 to 750 C and the thermal stable residue

1942 Z. Dai et al. / European Polymer Journal 45 (2009) 19411948for 1 h, and then 120 ml of 50 wt.% NaOH aqueous solutionwas added drop wise over a period of 1 h. The reactantswas stirred for another 2 h at 65 C, and then distilled to re-move excess ECH and other solvent. The crude productswere ltered to remove the residual sodium chloride, andthe organic phase was washed several times with deion-ized water. After precipitated from its cold water, the prod-uct was puried through recrystallization in an acetone/

Table 1Chemical structure of epoxies and curing agents.

Component

Epoxy

DGEBA

DGEBF

Curing agent

DDM

FDAwas formed. FT-IR spectra were performed on a BrukerEquinox55 spectrometer in the range from 4000 to400 cm1. The cured epoxy resin was ground with potas-sium bromide for FT-IR measurement. Dynamic mechani-cal analysis (DMA) was carried out on a RheometricScientic instrument in air at a heating rate of 5 C/min.The specimen of 8 5 1.5 mm was loaded in a singlecantilever mode with a frequency of 1 Hz.

Chemical structure

O CH2 CH CH2

O

OCH2H2C

O

CH C

CH3

CH3

O CH2 CH CH2

O

OCH2H2C

O

CH

H2NH2C NH2

H2N NH2

3. Results and discussion

3.1. Curing kinetics of epoxy resins

The curing behavior and kinetics of the epoxies andcuring agents were studied by dynamic DSC. The reactivityof epoxies and curing agents can be directly obtained fromthe DSC curves. Fig. 1 shows the DSC curves of these fourepoxy resins with a heat rate of 5 C/min. The exothermic

Table 2Optimal curing conditions of epoxy resins.

System Curingtemperature(C)

Curingtime(h)

Post-curingtemperature(C)

Post-curingtime (h)

DGEBA/DDM 140 4 180 4DGEBF/DDM 155 4 185 4DGEBA/FDA 160 4 190 3DGEBF/FDA 170 3 200 4

Z. Dai et al. / European Polymer Journal 45 (2009) 19411948 1943peak temperature (Tp) refers to the curing temperature,and the Tp of DGEBA/DDM, DGEBF/DDM, DGEBA/FDA andDGEBF/FDA were 148, 156, 165 and 175 C, respectively.The Tp shifts to a higher temperature with the increasingof uorene ring in the reaction systems, and the inuencewas quite obvious by using a uorene-containing curingagent. This is because the curing mechanism of epoxyand amine curing agent is the primary amine changed intosecondary amine, then into tertiary amine which hascatalysis function and the process is critical to curing reac-tion. When uorene structure was in curing agents, thesteric hindrance of this process was more obvious, whileepoxies with uorene group have no inuence on thiscourse.

The curing kinetics of these four resins was studied by anon-isothermal DSC method. The curing activation ener-gies E were simulated by the method of Kissinger and Oza-wa, and the order of reaction n and the reaction rateconstant k were deduced from Crane and Arrhenius equa-

Hea

t Flo

w

a b c d140 160 180 200

Temperature / oC

e x

o

Fig. 1. DSC curves of curing process of epoxy resins: (a) DGEBA/DDM; (b)DGEBF/DDM; (c) DGEBA/FDA and (d) DGEBF/FDA. (Heat rate: 5 C/min).tion [2022]. Fig. 2 shows the DSC curves of these fourepoxy resins at different heat rates.

Kissingers method is based on the fact that Tp varieswith the heating rates and that it assumes the maximumreaction rate (da/dt) occurs at the peak temperatures. Theequation can be expressed as Eq. (1). So the Ek values ofcuring can be determined from a plot of ln b=T2p vs. 1/Tp.For Ozawas method, its on the assumption that the degreeof conversion at peak temperatures for different heatingrates is constant, as shown in Eq. (2). The Eo values of cur-ing can be obtained by plotting ln b vs. 1/Tp.

Ek Rdln b=T2pd1=Tp 1

Eo R1:052d ln bd1=Tp 2

where Ek and Eo are the curing activation energy simu-lated by Kissinger and Ozawas method. b is the heatingrate and Tp is the maximum peak temperature, and R isthe ideal gas constant. All the obtained results are sum-marized in Table 3.

The E values calculated from the Ozawa method wereslightly higher than that of the Kissinger method, but thetrend of E values determined by both of methods was con-sistent with each other. The E values were increased withthe uorene content in epoxy resins, and also the inuencewas much obvious when uorene structures were in cur-ing agents. This can also attribute to the steric hindrancein curing process; which leading to a higher curing temper-ature was required. The reaction type of these systems wasall the same as the order of reaction n were very close toeach other. The reaction rate constant k at three differenttemperatures was also calculated. The reaction rate con-stant also decreased with the increase of uorene content,which was corresponding to the increase of E values of dif-ferent systems.

3.2. Thermal properties and decomposition kinetics of curedepoxy resins

The TG and DTG thermograms of the cured epoxy resinsat a heat rate of 10 C/min are shown in Fig. 3. The degrada-tion temperature at 5% weight loss and the residual charyield at 750 C for DGEBA/DDM and DGEBA/FDA epoxy res-inswereat 384 C, 375 Cand17.1%, 17.9%, respectively, andfor DGEBF/DDM and DGEBF/FDA epoxy resins that were376 C, 360 C and 24.3%, 27.2%, respectively. DGEBA basedepoxy resins showed slightly higher initial decompositiontemperature, which due to the higher crosslink density ofDGEBA matrix resins. The crosslink density is importantfor deciding the initial decomposition temperature, becausethe chain breakage mainly occurs between intermolecularin the beginning of decomposition. The curing reactionwas dependent not only on the reactivity of the functionalgroup but also on the molecular mobility. It is well-knownthat a molecular unit with a higher aromatic ring contenthas a higher rigidity and a higher steric hindrance tomolec-ular motion. Themelt viscosity activation energies for DGE-BA and DGEBF were 23.84 and 56.90 kJ/mol, respectively

eTT

E

KOn(k(

1944 Z. Dai et al. / European Polymer Journal 45 (2009) 194119481. 5 oC/min2. 10 oC/min3. 15 oC/min4. 25 oC/min

Hea

t Fl

ow

exo

1 23 4

(a)[23]. Because of the much higher viscosity of DGEBF com-pared with DGEBA resin, it shifts to diffusion control witha slower rate of reaction during curing process. Fig. 4 showsthe FT-IR spectrum of cured epoxy resins, DGEBF basedepoxy resins still have characteristic absorption of epoxygroup even at the end of the cure process. The results indi-cated that the curing reaction of uorene-containing epox-ies was incomplete, which leads to a lower crosslinkdensity of epoxy resins. Therefore, the onset decompositiontemperature was not improved by uorene. But at the laterstage of decomposition, molecular internal interaction iscritical in chain breaking reaction. So the char yield of cured

75 100 125 150 175 200 225 250 275 300

1. 5 oC/min2. 10 oC/min3. 15 oC/min4. 25 oC/min

Temperature ( oC )

100 125 150 175 200 225 250 275 300

Hea

t Fl

ow

Temperature ( oC )

exo

12

34

(c)

ex

Fig. 2. DSC curves of curing epoxy resins at four different heating rates: (a

able 3he curing kinetic parameters of curing resins.

poxy resins DGEBA/DDM

issingers method Ek (kJ/mol) 49.20zawas method Eo (kJ/mol) 53.81

0.87T1 = 373 K) 0.02752(T2 = 423 K) 0.1795T3 = 473 K) 0.78761. 5 oC/min2. 10 oC/min3. 15 oC/min4. 25 oC/min

Hea

t Fl

ow

xo

12

34

(b)bisphenol uorene epoxy resins were higher than that ofbisphenol A epoxy resins, which corresponds to molecularinternal aromatization and cyclization of uorene ring.

The thermal decomposition kinetics of cured resins wasstudied by dynamic method [2426]. Fig. 5 shows the TGAcurves of cured epoxy resins at four different heating rates.The values of decomposition activation energy Ed, accord-ing to the non-isothermal kinetic theory, can be writtenas follows:

ln b 1:105 EdRT

ln 0:0048AEf a

3

1. 5 oC/min2. 10 oC/min3. 15 oC/min4. 25 oC/min

150 175 200 225 250 275 300

Hea

t Fl

ow

Temperature ( oC )

o

12 3

4

(d)

125 150 175 200 225 250 275

Temperature ( oC )

) DGEBA/DDM; (b) DGEBF/DDM; (c) DGEBA/FDA and (d) DGEBF/FDA.

DGEBF/DDM DGEBA/FDA DGEBF/FDA

51.65 54.36 61.4456.26 58.95 65.990.87 0.88 0.880.01907 0.01306 0.007160.1366 0.1037 0.074450.6450 0.5314 0.4720

; (b) D

Z. Dai et al. / European Polymer Journal 45 (2009) 19411948 1945100 200 300 400 500 600 7000

20

40

60

80

100W

eigh

t (%

)

Temperature (oC)

a

b

c

d

Fig. 3. TG and DTG thermograms of cured epoxy resins: (a) DGEBA/DDMhere b is the heating rate, f(a) is the differential expressionof a kinetic model function, a is the conversion of thermaldecomposition, A and R are pre-exponential factor andideal gas constant [27].

A single step decomposition were observed in thesefour kinds of cured epoxy resins, implied that the thermaldecomposition was controlled by a single activation en-ergy. The Ed for these four cured epoxy resins were calcu-lated by ln b versus 1/T plots at several decompositionconversions, which are summarized in Fig. 6. We investi-gated the process when a were 040%, because the re-search object were not the original resin at the latterstage of decomposition. The Ed of DGEBF/FDA resin werehighest at any conversions due to the highest content ofuorene in these systems, and that of DGEBA/DDM resinwere lowest since no uorene group in resin. The averagedecomposition activation energy of DGEBA/DDM, DGEBF/DDM, DGEBA/FDA and DGEBF/FDA were 149.58, 176.40,

4000 3500 3000 2500 2000 1500 1000

(d)

(b)

(c)

Tran

smitt

ance

(%

)

Wavenumber ( cm-1 )

(a)

Fig. 4. FT-IR spectrum of cured epoxy resins: (a) DGEBA/DDM250 300 350 400 450 500 550

-15

-10

-5

0

b

d

c

Der

ivat

ive

Wei

ght

(% / m

in)

Temperature (oC)

a

GEBF/DDM; (c) DGEBA/FDA and (d) DGEBF/FDA. (Heat rate: 10 C/min).181.97 and 249.40 kJ/mol. The decomposition activationenergy of resins were increased as uorene increased, thisis also because of a higher rigidity and of a higher resis-tance to molecular motion of uorene. Therefore, the ther-mal stability of epoxy resins was elevated by introducinguorene to the backbone of the resins.

3.3. DMA analysis of cured epoxy resins

The DMA curves of these four cured resins are shown inFig. 7. Because of the higher crosslink density, the storagemodulus values (E0) of DGEBA based epoxy resins are high-er than that of DGEBF in the glassy region. But the DGEBFbased resins have better retention of E0 at elevated temper-ature due to their higher rigidity of uorene skeleton in thechain backbone. The Tg of DGEBA/DDM, DGEBF/DDM, DGE-BA/FDA and DGEBF/FDA were 168, 260, 224 and 300 C,respectively, which determined by the peak temperature

500 940 920 900 880 860

(d)

(b)

(c)

(a)

; (b) DGEBF/DDM; (c) DGEBA/FDA and (d) DGEBF/FDA.

1946 Z. Dai et al. / European Polymer Journal 45 (2009) 1941194880

100%

)(a)of the tan d curve. The Tg values of these resins were en-hanced with the increase of uorene, which is related tothe stiffness of molecular chain and a low degree of freeconformational rotation. Moreover, the enhancement wasmore obvious when uorene comes from the epoxy unit.This is because the stoichiometric ratio of bisphenol epoxy

100 200 300 400 500 600 700

100 200 300 400 500 600 700

0

20

40

60

Wei

ght

(

Temperature (oC)

10 oC/min 20 oC/min 30 oC/min 40 oC/min

0

20

40

60

80

100

Wei

ght

(%)

Temperature ( oC)

10 oC/min 20 oC/min 30 oC/min 40 oC/min

(c)

Fig. 5. TGA curves of cured epoxy resins at four different heating rates: (a)

10 15 20 25 30 35 40120

140

160

180

200

220

240

260

280

300

E d (k

J/mol)

(%)Fig. 6. Ed curves at different conversions of cured epoxy resins: (d)DGEBA/DDM; (j) DGEBF/DDM; (.) DGEBA/FDA; (N) DGEBF/FDA.100 200 300 400 500 600 7000

20

40

60

80

100

Wei

ght

(%)

Temperature ( oC)

10 oC/min 20 oC/min 30 oC/min 40 oC/min

(b)

60

80

100

eigh

t (%

)

10 oC/min

(d)and diamine curing agent was 2:1, the DGEBF/DDM systemhave more uorene content in the main chain. In addition,it is worthwhile to mention that the cured DGEBF/FDA re-sin shows a very broad glass transition, and a secondarytransition appeared in the range of 180200 C. The reasonis that the low crosslink density caused by the stiff of u-orene skeleton, which would restrain the internal rotationand motion of molecular segments in curing reaction. Fur-thermore, the molecular chains of resin begin to move andthe resin continues to post-curing with the temperatureincrease, and this enhanced interaction lead to a verybroad glass transition of resin. When its used in real appli-cations, this phenomenon deserves attention greatly inthat it would seriously affect the performance of resins.Therefore, the Tg of cured resin can be elevated drasticallyby introducing some rigid groups into main molecularchain, but the excessive aromatic ring may have negativeeffect on the performance of resins.

4. Conclusions

A uorene epoxy compound DGEBF was successfullysynthesized, and then DGEBF and a commonly used epoxyDGEBA were cured with DDM and uorene contained dia-mine FDA. The DSC analyses indicated uorene contained

100 200 300 400 500 600 7000

20

40W

Temperature ( oC)

20 oC/min 30 oC/min 40 oC/min

DGEBA/DDM; (b) DGEBF/DDM; (c) DGEBA/FDA and (d) DGEBF/FDA.

E(P

0.0

Z. Dai et al. / European Polymer Journal 45 (2009) 19411948 194775 100 125 150 175 200 225

106

107

108

109

Temperature ( oC)

E'

E''

(a)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

tan

(Pa)

106

107

108

109

E''

E'(c)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

tan

(Pa)epoxy resin needs higher curing temperature; especiallywhen a uorene group comes from curing agents. Thecuring kinetics was studied, and the results indicated thatthe reactivity of epoxy resin was depressed with the u-orene structure. The onset decomposition temperatureof cured epoxy resins were decreased with the increaseof uorene content, which is affected by a lower crosslinkdensity of epoxy resins. The char yield and the decompo-sition activation energy Ed of resins were increased due toaromatization and cyclization of uorene ring. The Tg ofthese cured epoxy resins were improved by introducinguorene, and the elevation was quite obvious when uo-rene structure were in epoxies. In DGEBF/FDA systems,which have too much uorene ring in main chain, will af-fect the properties of resins. Therefore, the presence ofuorene in resin skeleton is an effective way to enhancethe thermal properties of materials and to base on com-prehensive consideration of polymer properties, the addi-tion of the uorene group into one component of epoxyresin is a better way to improve the performance ofresins.

Acknowledgements

The authors thank the National Science Foundation ofChina (Nos. 50425312, 50373049, 50521302) and ChineseAcademy of Sciences Innovation Project for nancialsupport.

50 100 150 200 250

Temperature ( oC)

0.0

Fig. 7. DMA curves of the cured epoxy resins: (a) DGEBA/DDM100 150 200 250 300

107

108

109E'

''

(b)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8tan

a)

107

108

109

E''

E'

(Pa)

(d)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

tanReferences

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Eugene

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1948 Z. Dai et al. / European Polymer Journal 45 (2009) 19411948

Kinetics and thermal properties of epoxy resins based on bisphenol fluorene structureIntroductionExperimentalMaterialsSynthesis of diglycidyl ether of 9,9-bis(4-hydroxyphenyl) fluorenePreparation of the cured epoxy resinsCharacterization

Results and discussionCuring kinetics of epoxy resinsThermal properties and decomposition kinetics of cured epoxy resinsDMA analysis of cured epoxy resins

ConclusionsAcknowledgementsReferences