The curing kinetics and thermal properties of epoxy resins cured by aromatic diamine with hetero-cyclic side chain structure

Thermochimica Acta 595 (2014) 22–27

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The curing kinetics and thermal properties of epoxy resins cured by aromatic diamine with hetero-cyclic side chain structure Xuhai Xiong a , Rong Ren a , Siyang Liu a , Shaowei Lu a , Ping Chen a,b, * a b

Liaoning Key Laboratory of Advanced Polymer Matrix Composites Manufacturing Technology, Shenyang Aerospace University, Shenyang 110136, China State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 18 May 2014 Received in revised form 28 July 2014 Accepted 29 July 2014 Available online 1 August 2014

The use of novel aromatic diamine containing phthalide structure (BAPP) as curing agent of diglycidylether of bisphenol A (DGEBA) epoxy resin has been studied. BAPP can react with epoxy monomers by an epoxy-amine condensation mechanism, which renders phthalide cardo incorporate into the network structure of the thermoset. The curing behavior and kinetics have been investigated by differential scanning calorimetry (DSC) and the kinetic analysis of non-isothermal cure shows that autocatalytic model is suitable to describe the cure mechanism. The thermal–mechanical properties have been determined with dynamic mechanical analysis (DMA) and the activation energies for relaxation were calculated according to the Arrhenius law. Thermogravimetric analysis exhibits a lower initial decomposition temperature, decreased rate of decomposition and higher char yield compared with DGEBA cured with commercially available 4,40 -diaminodiphenylsulfone (DDS). ã 2014 Published by Elsevier B.V.

Keywords: Aromatic amine Phthalide structure Epoxy resin Curing kinetics Thermal–mechanical properties

1. Introduction Curing agents play an important role in determining the practical use of epoxy resin [1–3]. The excellent properties, such as mechanical strength, heat resistance, durability and adhesiveness, are obtained by reacting the linear epoxy resin with suitable curatives to form three-dimensional crosslinked thermoset networks [4,5]. Therefore, design and development of novel curing agent is one of effective methods to improve properties of epoxy resin [2,5–8]. Despite the fact that epoxy resins could be cured by many types of hardeners, aromatic amines retain a prominent position in high-tech fields [9,10]. Epoxy resins cured with aromatic amine generally provide enhanced environmental (hydrolytic) stability, outstanding heat-resistant and mechanical properties [5,9]. 4,40 -diaminodiphenyl methane (DDM), m-phenylene diamine (m-PDA) and 4,40 -diaminodiphenylsulfone (DDS) are principal commercial aromatic-amine curing agents. These traditional aromatic amines with low molecular weight, however, bring on highly crosslinked materials, which are seized of greater brittleness. Generally,

* Corresponding author at: Liaoning Key Laboratory of Advanced Polymer Matrix Composites Manufacturing Technology, Shenyang Aerospace University, No. 37, Daoyi South Avenue, Daoyi Development District, Shenyang 110136, China. Tel.: +86 24 89723970. E-mail addresses: [email protected] (X. Xiong), [email protected] (P. Chen). http://dx.doi.org/10.1016/j.tca.2014.07.027 0040-6031/ ã 2014 Published by Elsevier B.V.

increasing molecular chain of curing agent will reduce crosslinked density and further improve the toughness of network, but heat resistance is decreased [6]. Therefore, the chain-extended aromatic amines with multiaryl skeleton may provide a good balance between mechanical and thermal properties. Phthalide moieties are the ever-popular hetero-cyclic side cardo groups and incorporated into polymer main chains to improve solubility, heat resistance and thermal oxidative stability [11–13]. Phenolphthalein and its derivations have been used largely as aromatic bisphenol monomers for preparing highperformance thermoplastics and thermosetting resins. Accordingly, phthalide-containing epoxy compounds were designed and synthesized, and various epoxy resin systems modified by polymers with phthalide cardo structure were developed [14–16]. Although phthalide-modified epoxy systems show a lot of advantages, a more cost-effective approach to introduce phthalide moiety into epoxy networks using phthalide-contained aromatic amine curing agents, which are of large molecular weight and have the potential to overcome the defects of traditional aromatic amines, has been not paid more attention. The present work aims to investigate the curing kinetics and mechanism of diglycidylether of bisphenol A (DGEBA) epoxy resin cured with aromatic diamine containing phthalide structure (BAPP) by using the non-isothermal DSC. The thermal properties including thermal mechanical and thermal stability of cured material will be also studied by means of DMA and TGA.

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2. Experimental 2.1. Materials Commercially available diglycidylether of bisphenol A (DGEBA)-based epoxy resin, with an epoxy equivalent of about 185–210 and average equivalent weight of 196, was supplied by Wuxi Resin Works, China, and dried at 100  C in vacuum for 1 h before use. The hardener, 3,3-bis[4-(4-aminophenoxy)phenyl] phthalide (BAPP), was synthesized according to the reported method [17]. The molecular structures of chemical structures DGEBA and BAPP are illustrated in Scheme 1.

2.2. Preparation of DGEBA/BAPP blend The prepolymer DGEBA/BAPP was prepared by mixing stoichiometric DGEBA and BAPP at 90  C under vigorous mechanical stirring for about 5 min, and then a homogeneous, visually transparent mixture was obtained. The mixture was poured directly into preheated teflon mold and then the mold was transferred into an oven at 100  C under vacuum for 5 min to drive off entrapped bubbles. The following temperature programs were used to finish the cure process in an air convection oven at 120  C for 1 h, 160  C for 2 h plus a post-cure period at 180  C for 4 h. Finally, the casting was removed from the mold and characterized.

2.3. Characterization Differential scanning calorimetry (DSC) measurements were conducted with a PerkinElmer Diamond DSC instrument. The DGEBA/BAPP mixture (about 7–9 mg mass) was loaded into sealed aluminum pan. All DSC experiments were performed under N2 protection and run twice, the first scan was conducted from 25 to 300  C at different heating rates of 5, 7.5, 10 or 15  C/min and the second scan was conducted from 25 to 200  C at a heating rates of 20  C/min. Dynamic mechanical analysis (DMA) was done on a TA Instruments Q800 DMA with an amplitude of 20 mm and a temperature ramp rate of 3  C/min. The scanning temperature was from 25 to 200  C and the driving frequencies were 1.0, 2.0, 5.0, 10 and 20 Hz. The cured DGEBA/BAPP specimens were cut to dimensions of 30 mm  6 mm 1 mm for the tension mode. Thermogravimetric analysis (TGA) was performed on a PerkinElmer TGA-7 thermal analyzer and the cured DGEBA/BAPP sample (around 5 mg) was heated from 25 to 700  C at a heating rate of 20  C/min under the purified nitrogen flow rate of 60 mL/min.

Fig. 1. Non-isothermal DSC thermographs of DGEBA/BAPP reactions.

3. Results and discussion 3.1. DSC analysis 3.1.1. Reactivity of DGEBA/BAPP The non-isothermal curing reactions of DGEBA/BAPP were studied using DSC at a heating rate of 5, 7.5, 10 and 15  C/min and the dynamic DSC thermograms and characteristic parameters are shown in Fig. 1 and Table 1, respectively. It can be seen that there is a single exothermic peak in each DSC curves and heating rate has a great influence on the shape of the exothermic curves. The exothermic peak is shifted to higher temperature and peak width is extended with increasing heating rate. In addition, the total heat release of cure reaction is independent of heating rate. In order to determine the glass transition temperature (Tg) of DGEBA/BAPP, the cured samples were carried out the second DSC test at a heating rate of 20  C/min. As seen from Fig. 2, Tg value shows a strong dependence on the heating rate of the first DSC test and decreases by about 10  C with shifting of heating rate from 5 to 15  C/min (see Table 1). The believable cause is that the increasing heating rates reduce the network chains packing density, resulting in a decrease of steric hindrance to molecular motion. 3.1.2. Activation energy of the non-isothermal cure The curing of epoxy resins is a multi-step chemical process that is complicated by the physical processes of gelation and vitrification [9,10]. Fortunately, the isoconversional analysis is a powerful

Scheme 1. Chemical structure of DGEBA and BAPP.

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Table 1 DSC analysis of DGEBA/BAPP resin at different heating rates.

b (K/min)

Tonseta ( C)

Tpeakb ( C)

Tendc ( C)

4Hd (J/g)

Tge ( C)

5 7.5 10 15

131.7 133.8 136.1 142.2

170.5 178.6 185.8 196.1

221.3 230.2 238.9 260.4

212.6 221.4 217.5 228.4

157.1 153.4 151.2 147.8

a b c d e

The onset cure temperatures. The peak exothermic temperature. The end cure temperatures. Total heat of cure reaction. Glass transition temperature measured by DSC.

tool to explore the evolvement of cure mechanism by monitoring the variations in the effective activation energy with the extent of curing [18–21]. To perform isoconversional analysis, the original DSC data on exothermic peak were transformed into the fractional conversion (a) versus temperature curves at various heating rates, and the corresponding plots are shown in Fig. 3. The a values were obtained by the integration of the exothermic peak based on the following equation:

a ¼ Ha =Htotal

(1)

where Ha is the fractional enthalpy and Htotal is the total enthalpy of the cure reaction. It is clear from Fig. 3 that all a values increase very slowly at the beginning of curing and when the samples were heated to given temperatures, the a values show a sharply increase and then level off. Moreover, to obtain the same a value, the required temperature is increased with increasing heating rate. A modified integral method proposed by M.J. Starink based on Kissinger–Akahira–Sunose method, which is of more accurate, was used to estimates the activation energies (Ea) for different conversion levels (a). Starink equation is expressed as follows [22]: lnðb=T 1:92 a Þ ¼ const  1:0008  ðEa =RT a Þ

(2)

where b is the heating rate, R is the universal gas constant, Ta is the absolute temperature at a fixed value of the variable a. The activation energy can be calculated from the slope of the linear plot of ln(b/Ta1.92) against 1/Ta. The resulting Ea-dependencies are shown in Fig. 4. It can be seen that the Ea values are around 47.5–60.9 kJ/mol, the average value is 56.1 kJ/mol, which is quite typical for epoxy/aromatic amine copolymerizations [6,19]. In the interval 0.05  a  0.6, the Ea values are practically constant which

Fig. 2. Tgs of cured DGEBA/BAPP measured by DSC at a heating rate of 20 K/min.

Fig. 3. Conversion degree as a function of temperature.

suggests that the process follows a single step kinetic model. After a > 60%, the values of Ea decrease gradually to as low as 50 kJ as the degree of conversion increases. The same phenomenon was observed in the non-isothermal or isothermal cure of DGEBA/1,3phenylenediamine (m-PDA) using Vyazovkin method [19]. The decrease in Ea may indicate a change of the reaction mechanism. However, decreased amplitude of the Ea value is smaller than that reported in the other works [18,19]. The reason may be that when a > 60% epoxy curing system in the present consideration has passed the gel point and loses the ability to flow. In other hand, from Figs. 2 and 3 it is known the actual experimental temperature (a > 60%) is always above the glass transition temperature, hereat, the mixture of polymer network and monomer is not frozen and capable of engaging in further polymerization. Based on the aforementioned facts, it is reasonable to expect that the decrease in Ea results from increasing diffusion control. 3.1.3. Model-fitting kinetic analysis Reaction rate equation for thermosetting system is generally expressed da Ea ¼ Aexpð Þf ðaÞ dt RT

(3)

Fig. 4. Variation of effective activation energy calculated based on Starink method with conversion.

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Fig. 5. Plots of normalized y(a) and z(a) against a.

Fig. 6. Plots of ln[y(a)] versus ln[ap(1  a)].

where f(a) is the reaction model. Curing reaction of epoxy/amine system is assumed to follow autocatalytic reaction model (twoparameter SB model) [5,23,24]: da Ea ¼ Aexpð Þam ð1  aÞn dt RT

(4)

where f ðaÞ ¼ am ð1  aÞn , m and n are reaction orders. When m = 0, Eq. (4) is transformed into n-order model. Kinetic parameters (m, n) of the cure reaction were estimated using the Málek method [24–26]. In this method two parameters am and ap1, which are the maximum of y(a) and z(a), respectively, are first obtained based on Eqs. (5) and (6). yðaÞ ¼ ð

da ÞexpðxÞ dt

zðaÞ ¼ pðxÞð

(5)

da T Þ dt b

(6)

where x is the reduced activation energy, x ¼ Ea =RT; pðxÞ was calculated using the numerical approach of Senum–Yang Eq. (7):

pðxÞ ¼

x3 þ 18x2 þ 88x þ 96 x4 þ 20x3 þ 120x2 þ 240x þ 120

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(7)

examination shows that the values of am, ap1 and ap (the conversion degree on the maximum of curing exothermic peak) satisfy the conditions: 0 < am < ap < ap1 and ap1 6¼ 0.632, which strongly indicate that the reactions of DGEBA/BAPP follow autocatalytic reaction model. The kinetic exponents n and m and the pre-exponential factor A were obtained according to Eq. (8). The parameters, p, were first determined from the values of am and then a series of the plots of ln[y(a)] with respect to ln[ap(1  a)] at different heating rates were constructed and shown in Fig. 6. These data show good linearity. The values of n, m and ln A were calculated from the slope and intercept of fitting straight lines. The constant kinetic parameters thus were determined and listed in Table 2. Then, the averaged m, n and ln A along with previously calculated Ea are introduced into Eq. (4), and eventually the explicit reaction rate equation can be obtained as Eq. (9): da 56100 0:232 Þa ¼ 8:86  105 expð ð1  aÞ1:265 RT dt

(9)

In Fig. 7, a comparison between experimental and theoretical values is presented to check the predictability of Eq. (9). Obviously, the predicted rate is in fair accordance with the experimental rate. This demonstrates the autocatalytic reaction model is suitable to

By combining Eqs. (4) and (5) and yields Eq. (8) ln½yðaÞ ¼ lnA þ nln½ap ð1  aÞ

(8)

where p ¼ m=n ¼ am =ð1  am Þ The average Ea value calculated by the Starink method was introduced into Eqs. (5) and (6) and and function curves of y(a), which were normalized in order to simplify, and z(a) could be constructed, as shown in Fig. 5. This figure clearly indicates that conversion am associated with the peak values of y(a) curves are located in 0.10–0.18, and the z(a) curves show a “C” shaped contour experiencing a practically isoconversional peak value conversion, ap1, within 0.50–0.54. The values am and ap1 corresponding to different heating rates are shown in Table 2. A more detailed Table 2 Characteristic peak conversion values and calculated kinetic parameters for SB model at different heating rates.

b (K/min)

ap

am

ap1

p

m

n

ln A

5 7.5 10 15 Mean

0.508 0.479 0.484 0.471 0.486

0.159 0.123 0.155 0.136 0.143

0.535 0.516 0.516 0.508 0.519

0.189 0.140 0.183 0.157 0.168

0.260 0.216 0.181 0.273 0.232

1.202 1.271 1.162 1.424 1.265

13.64 13.76 13.57 13.81 13.70

Fig. 7. Comparison of predicted autocatalytic reaction model with experimental data.

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Fig. 10. TGA thermograms of cured DGEBA/BAPP systems.

Fig. 8. DMA thermograms of cured DGEBA/BAPP systems.

describe the progress of the non-isothermal curing reaction of DGEBA/BAPP system. 3.2. Dynamic mechanical thermal analysis The DMA curves conducted at a driving frequency of 1.0, 2.0, 5.0, 10 and 20 Hz for the DGEBA/BAPP networks are demonstrated in Fig. 8. Generally, peak temperature for tan d or loss modulus is identified as the glass transition temperature (Tg) and the area of peak is associated with the amount of energy loss caused by molecular motion. Seen from Fig. 8, tan d curves show single peak at a temperature range from 140 to 200  C, which is an indication that DGEBA/BAPP networks have homogeneous microstructure. Besides, it is obvious that storage modulus, loss modulus and relaxation peak express strong frequency dependency. As loading frequency increases, the network becomes somewhat stiffer. This is because the motion of network segments can follow the change of driving force when smaller frequency is employed. The increasing frequency also leads to a shift of loss modulus peak and tan d peak to a higher temperature and the shift of two peaks shows strong regularity. Herein, the Arrhenius law is adopted to correlate this interrelationship for calculating the activation energy of relaxation [26]. The activation energies of relaxation were determined based on Eq. (10).

dln f dð1=T g Þ

DE ¼ R

(10)

where 4E is the activation energy for specific relaxation, Tg denotes the corresponding relaxation temperature, R is the universal gas constant, and f is the loading vibration frequency. A series of f and Tg was obtained from the multi-frequency DMA measurements under fixed heating rate and the typical kinetic plots of ln f versus 1/Tg were drawn. According to the plots shown in Fig. 9, ln f against 1/Tg presents a good linear correlation and 4E was calculated from the value of the slope. As a result, the activation energy values calculated based on loss modulus peak temperature and tan d peak temperature are 638.8 and 587.9 kJ/mol, respectively. The results are higher than the 4E values (563 and 538 kJ/mol) for glass transition of cured DGEBA/ DDM system estimated by Raditoiu et al. [27]. This may be ascribed to more rigid molecular segment of BAPP than that of DDM. 3.3. Thermogravimetric analysis The thermal stability of the cured DGEBA/BAPP was assessed with TGA and compared with that of DGEBA cured with 4,40 diaminodiphenylsulfone (DDS), which was obtained from the literature [16]. Fig. 10 compares the thermal degradation behavior of DGEBA/BAPP and DGEBA/DDS in nitrogen atmosphere. DGEBA/ BAPP system exhibits a lower temperature of initial decomposition (Ti), slower rate of weight loss and higher char yield. The decreased Ti may be attributed to the lower crosslinking density and the relatively weaker tertiary amine crosslinks because of stronger basicity of BAPP than that of DDS [28]. On the other hand, the present of more aromatic structure may be the most significant factor contributing to the lower rate of weight loss and higher char yield. The latter indicates that the DGEBA/BAPP system is of higher limiting oxygen index (LOI) and better flame-retardant property according to the method of van Krevelen and Nijenhuis [29]. 4. Conclusions

Fig. 9. Plots of ln f versus 1/Tg.

A novel epoxy system was developed using aromatic amine containing phthalide cardo structure as curing agent. The curing behavior and kinetics were studied in detail by non-isothermal DSC. The average values of the apparent activation energy obtained by Starink equation were 56.1 kJ/mol. Model-fitting kinetic analysis showed the curing mechanism of DGEBA/BAPP system followed autocatalytic model. The viscoelastic properties of the cured material were

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characterized by DMA and single peak appeared in tan d curve at a temperature range from 140 to 200  C. Activation energies for the glass transition determined from tan d and loss modulus peaks are 638.8 and 587.9 kJ/mol, respectively. The results obtained from TGA analysis indicated the cured material was provided with a decreased rate of thermal decomposition and a higher char yield. Acknowledgements The financial supportfrom NationalNatural Science Foundation of China (No. 51303107), National Defense 12th Five-Year Fundamental Research Program (No. A352*******) and program for Liaoning Excellent Talents in University (No. LR2013002) is gratefully acknowledged. References [1] A. Catalani, M.G. Bonicelli, Kinetics of the curing reaction of a diglycidyl ether of bisphenol A with a modified polyamine, Thermochim. Acta 438 (2005) 126–129. [2] T. Maity, B.C. Samanta, S. Dalai, A.K. Banthia, Curing study of epoxy resin by new aromatic amine functional curing agents along with mechanical and thermal evaluation, Mater. Sci. Eng. A 464 (2007) 38–46. [3] E.S. Balcerzaka, H. Janeczeka, B. Kaczmarczyka, Epoxy resin cured with diamine bearing azobenzene group, Polymer 45 (2004) 2483–2493. [4] D. Rosu, F. Mustata, C.N. Cascaval, Investigation of the curing reactions of some multifunctional epoxy resins using differential scanning calorimetry, Thermochim. Acta 370 (2001) 105–110. [5] M. Ghaemy, M. Barghamadi, H. Behmadi, Cure kinetics of epoxy resin and aromatic diamines, J. Appl. Polym. Sci. 94 (2004) 1049–1056. [6] M.F. Mustafa, W.D. Cook, T.L. Schiller, Curing behavior and thermal properties of TGDDM copolymerized with a new pyridine-containing diamine and with DDM or DDS, Thermochim. Acta 575 (2014) 21–28. [7] G.R. Saada, E.E.A. Elhamidb, S.A. Elmenyawyb, Dynamic cure kinetics and thermal degradation of brominated epoxy resin–organoclay based nanocomposites, Thermochim. Acta 524 (2011) 186–193. [8] Y. Zhang, S. Vyazovkin, Macromol. Chem. Phys. 206 (2005) 342–348. [9] T. Dyakonov, Y. Chen, K. Holland, Thermal analysis of some aromatic amine cured model epoxy resin systems—I: materials synthesis and characterization, cure and post-cure, Polym. Degrad. Stab. 53 (1996) 211–242. [10] B.A. Rozenberg, Kinetics, thermodynamics and mechanism of reactions of epoxy oligomers with amines, Adv. Polym. Sci. 75 (1985) 113–165. [11] C.P. Yang, Y.Y. Su, J.M. Wang, Synthesis and properties of organosoluble polynaphthalimides based on 1,4,5,8-naphthalene tetracarboxylic dianhydride, 3,3-bis[4-(4-aminophenoxy)phenyl] phthalide, and various aromatic diamines, J. Polym. Sci. Part A: Polym. Chem. 44 (2006) 940–948. [12] Z.G. Wang, T.L. Chen, J.P. Xu, Gas and water vapor transport through a series of novel poly(arylether sulfone) membrane, Macromolecules 34 (2001) 9015–9022.

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