STUDIES ON APPARENT KINETICS AND RHEOLOGICAL BEHAVIOR OF EPOXY/ACRYLATE IPNS AS VACUUM PRESSURE IMPREGNATION RESINS *

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1 Chinese Journal of Polymer Science Vol. 27, No. 4, (2009), Chinese Journal of Polymer Science 2009 World Scientific STUDIES ON APPARENT KINETICS AND RHEOLOGICAL BEHAVIOR OF EPOXY/ACRYLATE IPNS AS VACUUM PRESSURE IMPREGNATION RESINS * Jing-kuan Duan a, Chonung Kim a, b and Ping-kai Jiang a** a Shanghai Key Laboratory of Electrical Insulation and Thermal Aging, Shanghai Jiaotong University, Shanghai , China b Department of Electrical Engineering, Kim Chaek University of Technology, Pyongyang, Korea Abstract The apparent kinetics and cure behavior of novel interpenetrating polymer networks (IPNs) based on cycloaliphatic epoxy resin (CER) and tri-functional acrylate have been investigated by means of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR). The results of DSC measurements show that the curing reaction of the TMPTMA component is much earlier than that of the CER component, which can lead to the formation of the IPNs. In contrast to neat anhydride-cer system, the anhydride-cer/tmptma systems exhibit relatively lower curing temperatures. The activation energy for initiating the reaction of the anhydride-cer system slightly increases, whereas the activation energy for propagating the reaction markedly reduces during the full IPNs formation. The FT-IR spectroscopic changes are interpreted in terms of curing mechanism of CER and TMPTMA components. The extent of reaction is calculated from FT-IR absorption bands, which depends on the reactive group concentration. The experimental results of FT-IR measurements are in good agreements with those of DSC measurements. The rheological behavior of anhydride-cer/tmptma systems during IPNs formation is studied in this paper. It is confirmed that the introduction of TMPTMA monomer into anhydride-epoxy resin has significant effects on the rheological behavior of the system. Keywords: Interpenetrating polymer networks; Cycloaliphatic epoxy resin; Kinetics; Rheological behavior. INTRODUCTION The impregnation has been adopted as an important process in the electrical insulating industry for over fifty years [1]. The complete and void-free impregnation is necessary in order to achieve the maximum life and efficiency of the electrical apparatuses. Generally, solventless varnishes and the vacuum pressure impregnation (V.P.I.) technique should be the optimal candidate to achieve void-free coils [2]. Historically, polyester-based varnishes and epoxy-based varnishes have been the most commonly utilized materials in the V.P.I. process [3, 4]. The former is cheaper and requires less stringent manufacturing controls than the latter does, but most manufacturers usually prefer epoxy. Requires to the properties and species of the V.P.I. resins have been becoming higher and higher with the development of the electrical industry, e.g. low viscosity, environmental concerns and faster gelling, etc [1, 4]. In order to prepare a new type of V.P.I. resin with low viscosity, environmentally friendly character and faster gelling as well as excellent electrical properties, cycloaliphatic epoxy resin (CER), liquid anhydride and trimethylol-1,1,1-propane trimethacrylate (TMPTMA) have been taken as the matrix resin, the hardener and the reactive diluents in this study, respectively. In contrast with the conventional V.P.I. resins, the anhydride- CER/TMPTMA system shows several prominent characteristics, e.g. the lower viscosity, higher ionic purities and non-toxicity properties. * The work was financially supported by Shanghai Committee of Science Technology for Major Research Project of Shanghai City (No. 05dz22303). ** Corresponding author: Ping-kai Jiang ( 江平开 ), pkjiang@sjtu.edu.cn Received April 28, 2008; Revised June 16, 2008; Accepted June 20, 2008

2 570 J.K. Duan et al. TMPTMA, a tri-functional acrylate is often used as a cross-linking agent in rubber and other applications [5]. The application of TMPTMA as a kind of modifier for epoxy resins, particularly, CER has not been reported yet. Therefore, it is necessary to investigate the curing reaction mechanism and process of the anhydride- CER/TMPTMA system. Because two different reactions exist in the curing process of the anhydride- CER/TMPTMA curative systems, it is predicted that these systems would display the complicated rheological behaviors. In order to understand their progression in the viscosity during the cure process, and to optimize their processing cycle in the application of VPI resin, rheological analysis is used to investigate the advancement of viscosity as a function of curing time in the present study. EXPERIMENTAL Materials 3,4-Epoxycyclohexylmethyl 3,4 -epoxycyclohexane carboxylate (marketed under the trade designation UVR6105), is purchased from Dow Chemical Company, USA. The curing agent is methyl-tetrahydrophthalic anhydride (MeHHPA, marketed under the trade designation LHY-807), obtained from Shanghai Li Yi Science & Technology Development Co. Ltd., China. The latent accelerator is neodymium(iii) acetylacetonate ((Nd(III)AcAc), marketed under the trade designation FLQ-1) purchased from Qinyang Tianyi Chemical Co., Ltd., China. The modifier agent, trimethylol-1,1,1-propane trimethacrylatea (TMPTMA) is obtained from Jinshi Tech-development Co., Ltd., China. Initiator, dicumyl peroxide (DCP) is purchased from Aldrich Chemical Company. The chemical structures of the raw materials are shown in Fig. 1. Fig. 1 The chemical structures of UVR6105 (1), MeHHPA (2), TMPTMA (3) and DCP (4) Sample Preparation All the liquid raw materials are purified before using for sample preparation. Epoxy resin, curing agent and modifier are undergone an extended hold for degassing in a vacuum at 100 C for 30 min so as to remove voids. The latent accelerator and initiator are used without further purification. Epoxy resin and latent accelerator are weighed in suitable equivalences and mixed vigorously at 160 C until accelerator is dissolved completely in epoxy. Then, this mixture is rapidly cooled down to ambient temperature, MeHHPA and TMPTMA are added in suitable amounts according to the composition given in Table 1, mixed and degassed under a vacuum for about 30 min to give a homogeneous liquid mixture for use in DSC, FT-IR and rheological measurements.

3 Studies on Apparent Kinetics and Rheological Behavior of VPI Resins 571 Table 1. Composition of IPNs based on epoxy resin and TMPTMA Epoxy (pbw) a MeHHPA (pbw) Nd((III)AcAc (pbw) TMPTMA (pbw) DCP (pbw) a pbw denotes parts by weight. Measurements Differential scanning calorimetry (DSC) Chemical curing kinetics information is gathered with a heat flux differential scanning calorimeter (Perkin- Elmer Pyris 1 differential scanning calorimeter). Non-isothermal curing experiments are carried out at heating rates of 5, 10, 15 and 20 K/min, and isothermal curing experiments are conducted at 150 C, 180 C, and 210 C at a heating rate of 150 K/min. Fourier transform infrared spectroscopy (FT-IR) The curing process of the mixture of epoxy and TMPTMA is in situ monitored by FT-IR using a Paragon 1000 (Perkin Elmer, Inc., USA) with the resolution of 4 cm 1, and transmission measurements is employed. The sandwiched KBr pellets containing approximately 10 mg of the mixture are placed into a hot stage, and the reaction temperature is maintained at the desired temperature for a predetermined period. Rheological measurements The isothermal rheological measurements are conducted in real time by a Bohlin VOR Rheometer using parallel-disk configuration. The experimental conditions are the maximum strain of 0.18, a gap of about 1 mm, frequency and temperature are 0.2 Hz and 150 C, 160 C and 175 C, respectively. The sample chamber is heated to the desired temperature and stabilized at that temperature for one minute. The liquid sample is put into the chamber and measurement is started. RESULTS AND DISCUSSION Reaction Mechanisms There are two kinds of curing reactions proceeding sequentially during the formation of IPNs composing of TMPTMA and epoxy in this case: the free-radical polymerization of TMPTMA monomer and the polyaddition of epoxy. The curing reactions of these two components result in conversion of low molecular weight monomer or pre-polymers into two individual highly cross-linked, three-dimensional macromolecular structures, respectively. The polymerization of TMPTMA monomer can be divided into three stages: Initiation: Δ + RO ROOR RO RO + TMPTMA TMPTMA Propagation: (TMPTMA) n + TMPTMA (TMPTMA) n + 1 Termination: R + R R R Where RO is a radical of the initiator, dicumyl peroxide. TMPTMA is a monoradical of the TMPTMA monomer; ( TMPMTA) n is the propagating free radical with a degree of polymerization, n; and R is a macroradical. The free-radical reaction of TMPTMA monomer results in the formation of a three-dimensional network structure which is the first network of IPNs. The mechanism of polyaddition reaction of epoxy displays more complicated owing to the catalytic action of the metal acetylacetonate [6]. Many literatures have studied the catalytic behaviors of metal acetylacetionates

4 572 J.K. Duan et al. for the curing reactions of epoxy [7, 8]. Figure 2 depicts the possible schematic reaction between epoxy and anhydride, accelerated by neodymium acetylacetonate. These reactions play an important role in the chain extension, branching and cross-linking of the molecules formed in the reaction, which finally results in the network formation. This is the second network of IPNs. Fig. 2 The schematic reactions between epoxy and anhydride accelerated by aluminum acetylacetonate DSC Characterization As well known, it is crucial for the practice to understand and characterize the cure kinetics and mechanisms which can determine the chemical structure, cross-linking density and morphology as well as the ultimate performances of the cured products. Various available approaches have been taken for the determination of the kinetics and mechanisms of the polymerizations, in particular, the DSC measurement [9 11]. The curing reactions of the anhydride-cer/tmptma systems can be characterized by both non-isothermal and isothermal scanning DSC. The DSC trace of TMPTMA monomer and shifts of exothermic peaks during the formation of the IPNs with different TMPTMA concentrations are given in Fig. 3. It is observed from Fig. 3(a) that the non-isothermal cross-linking reaction of TMPTMA monomer shows a relatively very sharp exothermic peak between 188 C

5 Studies on Apparent Kinetics and Rheological Behavior of VPI Resins 573 and 198 C, indicating that TMPTMA monomers have undergone a high temperature curing reaction due to the function of the inhibitors in the polymerization; its reacting heat is concentrative and the reacting rate is rather fast. This result reflects a typical behavior generally observed for free-radical reactions, which is in good agreement with the above-described reaction mechanism of TMPTMA monomers. Fig. 3 Non-isothermal DSC traces at heating rate of 10 K/min a) TMPTMA monomer; b) Anhydride-CER/TMPTMA systems with different concentration of TMPTMA The non-isothermal curing of epoxy-anhydride system catalyzed by Nd(Ш)AcAc shows a broad exothermic peak with a small shoulder (around 187 C) between 179 C and 276 C in Fig. 3(b). The shoulder of the DSC exothermic peak has been observed in some uncatalyzed epoxy systems cured by diamine [12] and epoxy-anhydride catalyzed by dimethylbenzylamine (DMBA) [9]. In the present system, the shoulder could be a consequence of the superposition of two thermal events: the activation reaction between Nd(Ш)AcAc and anhydride, and the initiating reaction of epoxy resin. Here the shoulder could be suitably defined as the initiating reaction peak, and the second peak appeared in the zone of the main peak may be attributed to the propagating reaction of the epoxy curative system. Introduction of the different TMPTMA monomer concentrations into the epoxy-anhydride systems results in some distinct influences on the curing reaction of the epoxy curative systems. The shoulder temperature values tend to considerably decrease, while the values of propagating reacting peak temperatures also show a slightly decreasing tendency as the concentration of TMPTMA monomer increases. These effects reveal that the addition of TMPTMA into the epoxy curative systems makes the curing temperature values of epoxy curative systems shift towards lower temperatures. In Fig. 3(b), the first exothermic peaks of the anhydride- CER/TMPTMA systems may be attributed to the free-radical reaction of TMPTMA monomer. The reacting temperature value of TMPTMA monomer in epoxy curative systems sharply decreases in comparison with that of the neat TMPTMA monomer. The curing behaviors of these two components in the anhydride- CER/TMPTMA systems reveal that individual component in the IPNs can polymerize more rapidly than alone due to a solvent effect of the IPNs [13]. The curing enthalpies of epoxy curative systems take on a depressed tendency as the concentration of TMPTMA monomer increases. According to the basic assumption that the curing heat generated during the curing reaction is proportional to the rate of conversion [14], this effect predicts that addition of TMPTMA monomer can have some negative effects on the cross-linking reaction at the defined curing times. In the anhydride-cer/tmptma systems, the faster gelling of TMPTMA monomer causes the viscosity of the system to swiftly increase before the beginning of the curing reaction of epoxy resin, which blocks off the diffusion reaction of epoxy resin, resulting in the incomplete curing reaction of epoxy resin. The curing reaction abilities of thermosetting systems are determined by the activation energy of curing reaction. The non-isothermal DSC curves of neat epoxy and the CER/TMPMTA (100/20) system scanned at the different heating rates as depicted in Fig. 4 are useful to the theoretical calculation of ΔE by the Kissinger

6 574 J.K. Duan et al. method [15], based on the equation derived from the condition for the maximum rate on a DSC curve: 2 d ln β / TP d(l / Tp ) ΔE = R (1) where β(dt/dt) is the heating rate in K/min; T p, the peak temperature in the DSC curve at the different heating rates in K; R, the gas constant; ΔE, activation energy in kj/mol. Fig. 4 Non-isothermal DSC traces at different heating rates a) neat CER; b) Anhydride-CER/TMPTMA (100/20) Equation (1) yields the linear plots of ln(β/t p 2 ) against 1/T p (depicted in Fig. 5), and the activation energy can be obtained from the slope of the corresponding straight line. The orders of the curing reaction of epoxy and their systems can be attained from Crane method [16] : where n is the order of the curing reaction. d ln β ΔE = ( + 2TP ) d(1/ TP ) nr (2) Fig. 5 Plot of ln(β/t P 2 ) versus 1/T P 1000 for neat anhydride-cer and the anhydride-cer/tmptma (100/20) system

7 Studies on Apparent Kinetics and Rheological Behavior of VPI Resins 575 In Eq. (2), if ΔE/nR >> 2T p, 2T p can be neglected, and then, the above expression can be modified as follows: d ln β ΔE = (3) d(1/ TP ) nr Equation (3) also yields the linear plots of lnβ against 1/T p (shown in Fig. 6), and the slope of the corresponding straight line, k can be obtained. The order of curing reaction, n can be calculated using the following equation: ΔE n = (4) kr Fig. 6 Plot of lnβ versus 1/T P 1000 for neat anhydride-cer and the anhydride- CER/TMPTMA (100/20) system In this study, the activation energy and order values of the initiating reaction (i.e., the first peak in Fig. 4a, and the second peak in Fig. 4b) and the propagating reaction (the second peak in Fig. 4a), and the third peak in Fig. 4b) of epoxy curative system have been calculated using Eqs. (1) and (4), respectively. The calculation results are listed in Table 2. Table 2. Non-isothermal DSC results for the CER and anhydride-cer/tmptma (100/20) system Sample Neat anhydride-cer Anhydride-CER/TMPTMA/DCP (100/20/0.02) β (K/min) lnβ T p1 (K) T p2 (K) T p3 (K) ΔE P1 (kj/mol) 62.3 ΔE P2 (kj/mol) ΔE P3 (kj/mol) 80.2 n n n As shown in Table 2, in contrast to neat epoxy, there is a slight increase in the activation energy value for the initiating reaction of epoxy/tmptma system from 62.3 kj/mol up to 64.4 kj/mol, the activation energy

8 576 J.K. Duan et al. value corresponding to the propagating reaction, however, decreases obviously from 93.6 kj/mol down to 80.2 kj/mol. It predicts that the addition of TMPTMA monomer could reduce the initiating reaction ability of epoxy curative systems, whereas it could improve the propagating reaction activation. Moreover, the slight decreases of two reacting orders have been observed. According to the characteristics of the system, the temperatures of 150 C, 180 C and 210 C were chosen in isothermal scanning DSC cure analysis. As can be seen from Fig. 7 which depicts the representative isothermal curing profiles of neat epoxy and the anhydride-cer/tmptma (100/20) system at the chosen temperatures, the curing reaction of neat epoxy does not take place at 150 C in the curing time range under consideration. When curing temperatures are higher than 180 C, the curing reactions of the epoxy resin are apparently observed. The curing reaction rates of the anhydride-cer/tmptma system become faster at the same curing temperatures as used for the curing of epoxy. The curves of TMPTMA monomer concentration versus the time t peak, where the peaks of the isothermal DSC curves occur at the different curing temperatures are given in Fig. 8. It is evident that the curing reaction rates (the time corresponding to the maximal exothermal peaks) of the systems are gradually retarded at 150 C whereas their curing rates are shortened markedly at 180 and 210 C with the increase of TMPTMA monomer concentration in the systems, which indicates that the anhydride- CER/TMPTMA systems have higher activation energies at lower temperatures. However, those systems exhibit lower activation energies at the higher curing temperatures in contrast to neat epoxy. Fig. 7 Isothermal DSC curing curves at different temperatures of neat anhydride-cer (a) and the anhydride-cer/tmptma (100/20) system (b) Fig. 8 Curves of TMPTMA monomers concentration versus the time of the peak formation at different curing temperatures for the anhydride-cer/tmptma systems

9 Studies on Apparent Kinetics and Rheological Behavior of VPI Resins 577 From both dynamic and isothermal scanning DSC curves of the anhydride-cer/tmptma systems, it is obvious that the rate of free-radical polymerization of TMPTMA monomer is much faster than that of epoxy resin, i.e., the formation of the TMPTMA network occurs prior to that of epoxy network. Moreover, the rate of the IPNs formation is faster than that of the individual network formation. The IPNs are in situ yielded in the anhydride-cer/tmptma systems during the curing reaction. In addition, the different cure mechanisms between epoxy resin and TMPTMA monomer may lead to the significant effects on the individual curing behavior of two reactions existing in the systems. FT-IR Characterization DSC measurements have already given the available information about the curing reaction process of the anhydride-cer/tmptma system in details, but because of the condition limitation of the apparatus, it is difficult to reflect completely the whole curing reaction process of the anhydride-cer/tmptma system, for instance, this technique can not easily provide the information of the post-cured samples. FT-IR which is a powerful technique to monitor the reaction kinetics of IPNs formation [17 19] has been employed to monitor the curing reaction of the anhydride-cer/tmptma system in order to make up the limitations of the apparatus and to obtain more curing information. Figure 9 depicts the FT-IR traces for the on-line monitoring of the curing reaction process for the neat epoxy and anhydride-cer/tmptma (100/20) system, and contains a series of spectra taken at various times during reaction at 160 C, respectively. It is very obvious that the characteristic bands for the anhydride, epoxy and unsaturated groups exist at 1875 cm 1 and 1786 cm 1, 901 cm 1 and 1635 cm 1, respectively. The peak centered at wave number 1730 cm 1 is attributed to the ester group from anhydride-epoxy reaction. The changes of the major peaks of relevance in epoxy and TMPTMA monomer reactions are quite evident as the curing time increases: (i) the decrease in the epoxide group absorption; (ii) the decrease in the anhydride group absorption; (iii) the decrease in the unsaturated group absorption; (iv) the increase in the ester group absorption can be observed. Additionally, the more apparent consumption rate of the double bond than that of the epoxide suggests that the free-radical polymerization of TMPTMA monomer is faster than the curing reaction of epoxy. Fig. 9 FT-IR traces of the on-line monitor curing reaction process at 160 C: the neat anhydride- CER (a) and the anhydride-cer/tmptma (100/20) system (b) It is well known that the conversion behaviors of the cured epoxy resins with catalyst are complex due to the involvement of epoxy-catalyst, hydroxyl-epoxy, and side reactions [18]. FT-IR measurements, however, provide a highly precise analysis of curing kinetics that can interpret the changes of chemical structures during curing. In the conversion study, the degree of conversion of different reactive groups of epoxy resin can be evaluated quantitatively by the disappearance or increase of the height of their characteristic absorption peaks. For calculation of the peak heights of reactive groups in FT-IR spectra, a range of wave number has been

10 578 J.K. Duan et al. employed according to the wave numbers of different reactive groups as follows: the calculated ranges of anhydride group at 1875 cm 1, anhydride group at 1786 cm 1, unsaturated group at 1635 cm 1 and epoxide group at 901 cm 1 are from 1903 cm 1 to 1830 cm 1, from 1828 cm 1 to 1754 cm 1, from 1654 cm 1 to 1585 cm 1 and from 919 cm 1 to 865 cm 1, respectively. The degree of conversion of the functional group (α) at any time (t) can be expressed as: ( I ) α = 1 x t= t (5) ( Ix) t= 0 where I x is the peak height corresponding to the wave number x. The relationships between the conversions of the functional groups and the cure time obtained from the peak heights for the relevant functional groups are shown in Fig. 10. As shown in Fig. 10(a), it is clear that the reaction of anhydride group whose bands are at 1875 and 1786 cm 1 are prior to that of epoxy group, and the conversion rate of anhydride group is higher than that of epoxy group. These results show good agreement with the above-described reaction mechanism. The good agreement between the results observed from DSC and FT- IR techniques shows that the addition of TMPTMA monomer into epoxy can make the reactive rate of epoxy apparently faster than that of neat epoxy as depicted in Fig. 10(b). Moreover, it is found that the addition of TMPTMA monomer has some significant effects on the reaction of epoxy resin. In comparison with neat epoxy, the decay rate of anhydride group at 1786 cm 1 is apparently improved, the conversation of anhydride group at 1875 cm 1 is distinctly reduced, and the conversion of epoxy group is enhanced. In a word, the results of FT-IR measurements provide comprehensive, kinetic analyses of anhydride-epoxy resin constrained to intermolecular reactions, and it can be seen that the autocatalytic natures of DSC and FT-IR techniques are apparent. Fig. 10 Conversion of the functional groups as a function of the reaction time at 160 C: the neat anhydride-cer (a) and the anhydride-cer/tmptma (100/20) system (b) Rheological Measurements Unlike the thermoplastics, in which the isochronal complex viscosity is an extremely degenerated parameter and is adequate only for a linear viscosity body, thermosets and their blends with a modifier have shown an interesting rheological behavior during isothermal study [20]. In general, the development of the rheological properties of the thermosetting systems is complicated because of the dependence on the initial state and the kinetics rate of conversion from a liquid to a solid material. Like other polymers, the epoxy mixture is a viscoelastic material. During a cure process under continuous sinusoidal stresses or strains, its viscosity characteristics change, which is reflected in the variations of the rheological properties, such as the viscosity and modulus. The profiles of changes in complex viscosity (η * ) of neat anhydride-cer system, TMPTMA monomer and the anhydride-cer/tmptma systems subjected to different isothermal curing temperatures are shown in Fig. 11. It is found that the initially-low values of complex viscosity for TMPTMA monomer and neat

11 Studies on Apparent Kinetics and Rheological Behavior of VPI Resins 579 anhydride-epoxy resin increase abruptly in the specific time range as the curing reaction proceeds at the isothermal cure temperature, which is the typical viscosity profiles during the curing of thermoset systems. In addition, the value of t η, the time at which the gel formation begins, decreases as the isothermal cure temperature increases. This can be interpreted as that the rate of the curing reaction increases as the temperature increases. Fig. 11 Plots of η * at 150 C (a), 160 C (b) and 180 C (c) versus time for the blends with different concentration of TMPTMA For the anhydride-cer/tmptma systems, there exist two increasing regions of complex viscosity during curing process, in particular, the curing temperature is below 433 K, indicating that there are two reactions during the curing process of anhydride-cer/tmptma systems as described in the above experimental results. According to the changes of complex viscosity of the neat anhydride-cer system and TMPTMA monomer, the first increase stage of complex viscosity of anhydride-cer/tmptma system may be attributed to the freeradical polymerization of TMPTMA monomer, and the second increase stage may be ascribed to the crosslinking reaction of anhydride-epoxy resin curative system. Additionally, the t η TMPTMA value of TMPTMA monomer and the t η epoxy value of epoxy resin curative system have a lower value than those in their pure system, respectively, demonstrating that the addition of TMPTMA monomer into epoxy curative systems makes both the free-radical polymerization of TMPTMA monomer and the cross-linking reaction of epoxy resin curative system faster than those in their pure systems, respectively. This may be due to the dilutedness of epoxy curative systems to inhibitor in TMPTMA monomer, leading to the faster free-radical polymerization of TMPTMA monomer than that in neat TMPTMA monomer in which the inhibitor has relatively higher concentration. The smart exothermic reaction of TMPTMA monomer can make epoxy-resin-based curative complex systems own the much higher cure reactivity than pure epoxy resin curative system. Moreover, it is also found from Fig. 11 that the changes in viscosity of the anhydride-cer/tmptma systems strongly depend on

12 580 J.K. Duan et al. the curing temperature. This is likely due to that the curing temperature has important influences on the reaction progress of the systems. The dependence of the viscosity of these systems on the curing time corresponds to the cure kinetics network formation during conversion, which is in good agreement with the experimental results of DSC and FT- IR measurements. Figure 12 illustrates the evolution during IPNs formation of real part (G') of the shear modulus at 0.2 Hz at different curing temperatures. The developing tendency of G' is similar to that of complex viscosity as shown in Fig. 11. Fig. 12 Profiles of elastic modulus versus different curing temperatures: 150 C (a), 160 C (b) and 180 C (c) CONCLUSIONS The apparent kinetics and the cure behavior of the new IPNs based on CER and TMPTMA monomer have been characterized and monitored by DSC and FT-IR measurements. The experimental results of DSC show that during the formation process of the IPNs, they are subjected to two reactions: the free-radical polymerization of TMPTMA monomer and the curing reaction of epoxy resin. The anhydride-cer/tmptma systems display the more interesting results during curing: in contrast to neat epoxy, they show the lower reaction activities at lower curing temperatures whereas they have the higher reaction activities at higher curing temperatures, which may be vital to the V.P.I. process. And also, the anhydride-cer/tmptma systems are desired to improve the shelf time at room temperature and the drainage resistance properties at elevated temperatures of the V.P.I. resins. The experimental results of FT-IR measurements are in good agreement with those of DSC measurements.

13 Studies on Apparent Kinetics and Rheological Behavior of VPI Resins 581 The experimental results also outline the important variation of the dynamic rheological behavior of IPNs in the course of their formation. Conclusions drawn from the results are as follows: (i) with the curing temperature increases, the gel times of all systems tend to gradually shorten, which is in agreement with the general law of thermosetting system curing; (ii) when the curing temperatures are below 180, the overall viscosity of the mixture apparently displays the increasing trend of two stages, which implies that there are two reactions in the reactive mixture system, and these two reactions undergo a different reaction process; (iii) when the curing temperature is 180 C, the overall viscosity of the mixture shows a gradual increasing trend due to the superposition of the curing reaction of epoxy and the free-radical polymerization of TMPTMA; (iv) the addition of TMPTMA into epoxy reactive system has significant effects on the reacting rates of the two reactions. REFERENCES 1 Edward, A.B. and Gerg, C.S., IEEE Electrical Insulation Magazine, 2004, 20: 25 2 Thomas, W., Basic Impregnation Techniques, Proceedings of the Electrical/Electronics Insulation Conference, ed. by Electrical/Electronics Insulation Conference, Institute of Electrical and Electronics Engineers, New York, 1997, p Jerson, D.D. and James, F., 1985, U.S. Pat., 4,554,470 4 Chris, F., Improved Solventless Insulating Varnish, Proceedings of the Electrical/Electronics Insulation Conference, ed. by Electrical/Electronics Insulation Conference, Institute of Electrical and Electronics Engineers, New York, 1997, p Djomo, H., Morin, A., Damyanidu, M. and Meyer, G., Polymer, 1983, 24: 65 6 Lachenal, G., Pierre, A. and Poisson, N., Macromolecules, 1996, 27: Zhang, Z.Q. and Wong, C.P., J. Appl. Polym. Sci., 2002, 86: Smith, J.D.B., J. Appl. Polym. Sci., 1981, 26: Montserrat, S., Flaqué, C., Calafell, M., Andreu, G. and Málek, J., Thermochimica Acta., 1995, 269/270: Joseph, R., Nowers, J.R. and Balaji, N., Polymer, 2006, 47: Dean, K., Cook, W.D., Burchill, P. and Zipper, M., Polymer, 2001, 42: Jocelyne, G., Abed, S. and Jean, P.P., Polym. Eng. Sci., 1986, 26: Suthar, B., Xiao, H.X., Klempener, D. and Frisch, K.C., Polym. Adv. Technol., 1996, 7: Montserrat, S., Andreu, G., Cortes P., Calventus, Y., Colomer, P., Huntchinson, J.M. and Malek, J., J. Appl. Polym. Sci., 1996, 61: Kissinger, H.E., J. Res., 1956, 57: Crane, L.W., J. Polym. Sci., 1973, 11: Hsieh, K.H., Han, J.L., Yu, C.T. and Fu, S.C., Polymer, 2001, 42: Park, S.J., Kwak, G.H., Sumita, M. and Lee, J.R., Polym. Eng. Sci., 2000, 40: Kwak, G.H., Park, S.J. and Lee, J.R., J. Appl. Polym. Sci., 2000, 78: Bonnet, A., Pascault, J.P., Sautereau, H., Taha, M. and Camnerlin, Y., Macromolecules, 1999, 32: 8517