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Investigation on curing characterization of epoxy molding compounds with different latent catalysts by thermal, electrical and mechanical analysis
Thermochimica Acta 674 (2019) 68–75
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Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Investigation on curing characterization of epoxy molding compounds with different latent catalysts by thermal, electrical and mechanical analysis Da Eun Leea,b, Han-Jung Choa, Byung-Seon Konga, Hyung Ouk Choia, a b
T
⁎
KCC Central Research Institute, 17-3, Mabuk-ro 240beon-gil, Giheung-gu, Yongin-si, Gyeonggi-do, Republic of Korea School of Chemical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, Republic of Korea
ARTICLE INFO
ABSTRACT
Keywords: Epoxy molding compound Latent catalyst Cure kinetics Time-domain NMR Dielectric analysis
This research was conducted to investigate comprehensively curing characteristics of epoxy molding compound (EMC) with various latent catalysts by using their thermal, electrical and mechanical properties. The conversion prediction on isothermal condition for three types of organophosphine based catalysts was studied by model free method with differential scanning calorimetry (DSC). The T2 relaxation time was measured by time-domain nuclear magnetic resonance spectroscopy (TD-NMR) in order to compare the cross-linking density for each catalyst according to curing time. The results of DSC and TD-NMR showed that catalysts with lower latency reached curing in a shorter time. Furthermore, we observed the electrical-mechanical characteristics of EMC using additional thermal analysis. The ion viscosity was measured by dielectric analysis (DEA) according to the curing time, and the results suggested that the ion viscosity and the volume resistivity were lower for the catalyst with a slower curing rate. Dynamic mechanical analysis (DMA) was performed to compare activation energy of glass transition and glass modulus of final product. It was shown that the use of a catalyst with a low latency increases the modulus against external physical forces. In conclusion, our results demonstrated that the various analytical approaches are highly significant to understand the curing characteristics of EMC, which helps select appropriate catalysts in order to enhance the reliability of EMC.
1. Introduction Epoxy Molding Compound (EMC) is an electronic material used in the semiconductor industry, in order to protect electrical circuits [1]. The raw materials which compose EMC including epoxy resin, phenol or amine hardener, catalyst, filler, additives are evenly blended and moved to the mold. Most EMCs have short curing time at high temperatures, and this conversion of EMC is related with the reliability of semiconductor devices [2]. Quality issues such as cracks or delamination would occur, if the extent of curing was not enough or the rate of curing was too fast for materials to evenly mix. Therefore, to enhance the quality of product, EMC requires an appropriate curing under the temperature and time set during the process [3,4]. As catalyst plays the role of accelerator in the EMC curing, it is important to select a proper catalyst [5,6]. In order to have an effective curing reaction in practice, the catalyst should be stable before it is actually injected into the reaction. Recently, latent catalysts have been used to avoid accelerating reaction during storage in the manufacturing sites [7,8]. The latent catalysts delay reaction under storage temperature and enable to cure at curing temperature, but may reduce the total degree of conversion if
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latency of catalyst is too high. On the other hand, using catalyst with low latency cause problem during storage and reduce mechanical property of EMC due to a short curing time. This is why it is important to understand curing characteristics by selecting a catalyst with the appropriate latent and reactive activities. To observe thermal characteristics of epoxy resin, the DSC analysis has often been conducted. Previous studies have examined the differences in curing behavior according to the types of catalysts through the DSC analysis [9–11]. We predicted the conversion and reaction model of each curing system with DSC experiments, compared model fitting and model free method to verify the model free method to be more effective [12]. Recently, studies on predicting conversion as well as observing curing characteristics have been continued through various equipment such as delectric analysis (DEA), dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), and time-domain NMR (TD-NMR) [13–17]. TD-NMR is the equipment evaluating the crosslinking density of polymers using relaxation time of RF pulse. LaPlante et al. [13] used TD-NMR to evaluate cross-linking density of epoxy resin in a short time, with the epoxy resin/polyamidoamine curing system. DEA is the thermal scanning device to measure electric signals by
Corresponding author. E-mail address: [email protected] (H.O. Choi).
https://doi.org/10.1016/j.tca.2019.02.009 Received 28 November 2018; Received in revised form 15 February 2019; Accepted 18 February 2019 Available online 19 February 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. The molecular structure of EMC catalysts (a) TPP, (b) TPP-BQ, and (c) TPP-TPB.
applying a constant alternating voltage and frequency to the sample, which allows observing the ion viscosity from curing in real time [14,15]. Furthermore, DMA evaluates thermal-mechanical characteristics by measuring the strength which reacts to mechanical stress applied to the sample. Sadeghinia et al. [16] have used DMA to compare modulus between the EMC of filler type and the EMC of non-filler type. Along with others, devices such as rheometer or TMA were used [17–19]. Also, there are studies that analyzed the effect of cure temperature [20–24]. Previous studies conducted individual analysis which limited observing the curing characteristics. To accurately understand epoxy curing system, it is crucial to obtain various curing
characteristics by different analytic methods and investigate comprehensively their interrelationships. The purpose of this study is to observe complex characteristics of the curing of epoxy resin/phenol hardener system according to the different selection of catalysts, using DSC, TD-NMR, DEA, and DMA. Three catalysts with different latency were chosen for comparing curing characteristics of EMC according to the catalyst selection. DSC and TDNMR were used to observe differences in curing rate and conversion. DEA allowed us to measure electric characteristics by ion viscosity as per curing time. Lastly, DMA measurement was conducted to observe the mechanical characteristics as per degree of conversion. Along with
Fig. 2. (a) DSC thermodynamic scans with different heating rate of 20, 10, and 5 K/min of TPP-TPB. (b) KAS plot at degree of conversion (0.1 – 0.9) of TPP-TPB. (c) The activation energy calculated from the slop of KAS plot according to conversion. (d) Isoconversional prediction curves at 150°C. 69
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(synthesized by mixing TPP and benzoquinone in acetone at room temperature), and TPP-TPB (Hokko Chemical Industry Co., Ltd.). The epoxy resin, curing agent, and catalysts were evenly blended with a constant content ratio (100 : 90 : 5) by a mixer. After these blended powders were melted and dispersed with twin-screw kneader at 100 °C for 5 min, cooled and crushed into fine powders. These fine powders were presented as name as TPP, TPP-BQ, and TPP-TPB on graph using by initial materials for DSC and DEA measurement.
Table 1 The results of activation energies at degree of conversion (0.2, 0.5, and 0.8). E of kinetics (kJ/mol)
= 0.2 TPP-TPB TPP-BQ TPP
155 ± 13 105 ± 12 135 ± 10
= 0.5 131 ± 2 114 ± 15 90 ± 7
= 0.8 126 ± 1 118 ± 16 63 ± 10
2.2. Differential scanning calorimeter (DSC) experiments
measuring the reaction rate and degree of conversion, the correlation between electrical-mechanical characteristics, would help us understand comprehensive curing characteristics.
The differential scanning calorimeter (Mettler Toledo, DSC 2/ 400 W) was used to monitor the curing kinetics. Three dynamic data (5 K/min, 10 K/min, and 20 K/min) were obtained with prepared powders of 5–10 mg under 50 mL/min N2 flow. The model free method was applied to compare activation energy according to the degree of conversion and conversion prediction curves were obtained by isoconversional method.
2. Experimental 2.1. Materials and sample preparation The epoxy resin was used as mixture of tetramethylbiphenol type (Mitsubishi Chemical Co., YX-4000HX) and o-cresolnovolac type (Nippon Kayaku Co., Ltd., EOCN1020-70). The curing agent consisted of phenol type (Meiwa Plastic Industries, Ltd., MEH-7800SS). The three latent catalysts were TPP (Hokko Chemical Industry Co., Ltd.), TPP-BQ
2.3. Time-domain NMR (TD-NMR) experiments The spin-spin relaxation time was measured by TD-NMR (Oxford instruments, MQR, UK) under 20.82 MHz. Each sample in a 10 mm
Fig. 3. T2 relaxation decay curves for cured EMC with catalysts (a) TPP-TPB, (b) TPP-BQ, and (c) TPP according to curing time of 2, 5, 7, 10 min. (d) Comparative curves of three catalysts with curing time of 5 min. 70
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Fig. 4. T2 relaxation time separated by T2, Solid and T2, Liquid for (a) TPP-TPB, (b) TPP-BQ, and (c) TPP. The signal intensity of a solid and liquid phase expressed as a percentage of (c) TPP-TPB, (d) TPP-BQ, and (f) TPP. Each catalyst was measured at curing time of 2, 5, 7, 10 min, respectively.
glass measuring tube was cured at 150 °C for 2, 5, 7, and 10 min using a heating block (Grant instruments, QBH, UK), and immediately transferred to a TD-NMR probe. The spin-spin relaxation decay was measured using CPMG pulse. The data points of 4096 echoes were collected with a relaxation delay of 0.5 s with 8 scans. The spectra were manually analyzed using Application Developer software version 0.10.4.330.
2.4. Dielectric analysis (DEA) measurement Dielectric cure monitoring was performed with LT-451 (Lambient Technology, USA). The flexible interdigitated electrode (Mini-Varicon) was used instead of parallel plate electrodes. The sensor has 1.5ʺ long, 0.004ʺ thick, and 0.004ʺ electrode space. Mini-Varicon can measure the
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Table 2 T2 relaxation time and signal intensity of solid and liquid domains according to curing time of 2, 5, 7, and 10 min. Curing time
2 min 5 min 7 min 10 min
TPP-TPB
TPP-BQ
TPP
T2,Solid (ms)
T2,Liquid (ms)
Ms (%)
ML (%)
T2,Solid (ms)
T2,Liquid (ms)
Ms (%)
ML (%)
T2,Solid (ms)
T2,Liquid (ms)
Ms (%)
ML (%)
10.511 3.652 1.840 0.620
48.189 34.962 21.559 10.969
21 53 68 79
79 47 32 21
4.359 1.496 0.228 0.153
35.806 16.778 2.966 2.512
49 74 80 87
51 26 20 13
2.574 0.316 0.137 0.175
21.453 5.222 2.811 –
61 82 91 100
39 18 9 0
Fig. 5. (a) Variation of ion viscosity (log IV ) during 600 s at 150°C for TPP-TPB, TPP-BQ, and TPP. (b) Comparison between ion viscosity and volume resistivity at 1 Hz and 600 s.
dielectric properties of EMC within approximately 0.004ʺ of the electrode surface. A sinusoidal AC voltage of 1 V at frequency of 1 Hz was applied under isothermal temperature of 150 °C.
E is the activation energy which is variable with the degree of conversion ( ), R is the gas constant, and T is the absolute temperature. The conversion prediction was performed by the isoconversional method using following Eq. (2) proposed Vyazovkin’s study [28].
2.5. Dynamic mechanical analysis (DMA) measurement
t =
The dynamic mechanical analysis (DMA1, Mettler Toledo, Switzerland) was used to characterize frequency dependence of relaxations. The DMA applies oscillating force to the sample and measures resultant displacement of the sample. In this experiment, the samples of each catalyst were cured at 150 °C for 15 min and flattened to about 20 mm × 10 mm × 1 mm. The cured samples were placed in a single cantilever mode, and the modulus was measured at a temperature range of 30 °C–120 °C, and a frequency of 0.5, 1, 3 and 5 Hz.
This study used three types of organophosphine catalysts, each with different latent effect, to assess curing characteristics of EMC in each case. Organophosphine catalysts open the ring of epoxy resin with a nucleophilic attack, which starts the curing [2,3]. This ability of nucleophilicity amplifies when the basicity is larger and structural steric hindrance is less. [25] The structures of three catalysts are drawn in Fig. 1. Nucleophilicity of TPP was expected to be the largest, which led to increase the reaction rate. Kinetic study applied a model free method that offers the benefit of eliminating the need to measure parameters of reaction constants and reaction order without selecting a specific model. Also, model free method allows observation of activation energy as per extent of reaction calculated from Kissinger-Akahira-Sunose (KAS) method based on Eq. (1) [26,27].
T2
= ln
AR g ( )E
E RT
exp ( E / RT ) (2)
exp ( E / RT0)
where t is the life time at , T the temperature at , T0 is the temperature of isothermal condition. To perform the kinetic analysis, DSC experiment was conducted for three EMC samples. The curing peaks at three different heating rate were observed, from which changes in activation energy and conversion prediction curves at 150 °C were deducted. Each result is drawn in Fig. 2. Fig. 2b shows the linear relationship between ln ( / T 2 ) and 1/ T . The slope of each line represents the activation energies at degree of conversion ( = 0.1, 0.2, , 0.9 ) and the values at 0.2, 0.5, and 0.8 were indicated in Table 1. The variation of the activation energy is showed in Fig. 2c. In Fig. 2d, the conversion prediction curves at 150 °C were presented. After 10 min of curing, TPP almost reach 100% conversion, while TPP-BQ and TPP-TPB stops at about 80% and 60%, respectively. Also, the reaction rate was the fastest in the order of TPP, TPP-BQ and TPP-TPB. Another analytic method used to confirm the degree of conversion was TD-NMR. The spin-spin relaxation time measured by TD-NMR allows to measure the cross-linking density of the sample. For epoxy resin curing, T2 relaxation time was obtained by fitting the bi-exponential decay equation as below :
3. Result and discussion
ln
T 0
S (t ) = Ms exp
t T2, Solid
+ ML exp
t T2, Liquid
(3)
where S is the signal detected, M is the signal amplitude backextrapolated to time 0, t is the time elapsed, and T2 is the spin-spin relaxation time constant. Indices “Solid” and “Liquid” refer to the short and long components of the signal, which imply high-cross-linking density domain and low-cross-linking density domain, respectively [13,29]. In the curing process, high-cross-linking domain enlarges as degree
(1)
where is the heating rate, g ( ) is the integral form of the function according to degree of conversion ( ), A is the pre-exponential factor, 72
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Fig. 6. Experimental data of DMA measurement. (a), (c), (e) E (storage modulus) and (b), (d), (f) E (loss modulus) in the heating range of 30 °C–120 °C according to various frequency.
of conversion increases, and graph takes a rapid decrease as T2 relaxation time of two domains decreases. Fig. 3 shows bi-exponential decay graph as per curing time. Fig. 3 shows a sudden decrease in graph as the curing time increases, in all three samples. This means as the curing time increases, solid domain enlarges leading to the increase of degree of conversion. However, TPP-TPB (Fig. 3a) shows a gradual decreases as reaction time increases, while in the cases of TPP (Fig. 3c),
the shapes of graphs change for 2, 5, 7 min but in the 7, 10 min the shapes are almost still. It can be assumed that TPP reached 100% curing after 7 min. Each bi-exponentially decreasing graph in Fig. 3 has been separated into solid domain and liquid domain, to draw changes of T2 and M as per curing time in Fig. 4. T2, Solid and T2, Liquid decreased as per curing time, which means shortened relaxation time from limited molecule mobility in the epoxy resin curing process. In Fig. 4e, the 73
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resistivity result showed a similar tendency. That is, mobile molecule or ions of unreacted materials can affect electric characteristics such as resistance of the final cured material. Observing physical characteristics according to catalyst types was conducted through DMA experiment. DMA recorded the storage modulus (E ) and loss modulus (E ) of cured materials at 150 °C for 15 min on four different frequency. Each experiment result is drawn in Fig. 6. Glass transition temperature (Tg) in DMA is defined as the onset point where modulus starts to decrease from E or peak point at E . All three samples according to catalyst types show the Tg value shift right as frequency increases. The shift of Tg from the frequency change follows the Arrhenius equation as below:
lnF = lnA
Table 3 The glass transition temperature at 1 Hz obtained from peak point of E , glass modulus from E , and the activation energy for glass transition.
TPP-TPB TPP-BQ TPP
Glass modulus (MPa)
E of glass transition (kJ/mol)
91.8 93.2 94.8
1221 1455 1560
98 ± 9 164 ± 4 233 ± 7
4. Conclusion The comparison on reaction rate and prediction of conversion according to latency of the three catalysts (TPP-TPB, TPP-BQ, and TPP) were proceeded with DSC experiment. Under model free method, the conversion prediction curve for TPP-BQ and TPP reached 100% curing at 150 °C with reaction time of 10 min, but TPP-TPB reached around 90% curing level. The difference in conversion was also observed in the TD-NMR experiment as well. Relaxation time of T2 of TD-NMR was separated to solid domain and liquid domain, and relaxation time shortened as per curing time. Unlike TPP-BQ and TPP, liquid domain remained for TPP-TPB after 10 min. DEA experiment proceeded to observe changes in electric characteristics as per different curing rate of each catalyst. Comparing the increase of ion viscosity by curing time, the result of TPP-TPB was around 85% of TPP-BQ and TPP, which implies the existence of unreacted material in the cured material. Lastly, physical properties by different curing level were measured with DMA experiment. The faster curing rate and higher degree of conversion of material led to larger increase in glass modulus and activation energy from glass transition. As a result, curing rate and conversion differ according to the latency of catalyst which changes the electricalmechanical characteristics of the final product. Various analysis measurements and the correlation between each results should be confirmed to enhance reliability of EMC.
exponential decay curve was no longer separated into two domains after 7 min. This shows that the solid domain was only measured. Fig. 4b,d,f shows the changes of composition ratio with the curing time of the solid domain and liquid domain of each sample. This result which is of similar tendency with the former conversion prediction curve by DSC shows validity of the prediction. The T2 and M values are shown in Table 2. As the degree of conversion for each catalyst was checked previously in DSC and TD-NMR, DEA experiment was conducted to observe electric characteristics as per different conversion by catalyst. DEA monitored the dielectric properties such as permittivity, loss factor, and ion viscosity as a function of time in real time while EMCs were cured. Among these, ion viscosity is defined as below :
IV =
1
=
1 0
(5)
where F is frequency (Hz), A is the pre-exponential factor, E is the activation energy (kJ/mol), R is the gas constant (kJ/mol·K), and T is the temperature of glass transition [30]. Arrhenius plots using Eq. (5) of each sample are drawn in Fig. 7 and activation energy of glass transition can be calculated with the slope of each line. Table 3 shows the results of Tg, glassy modulus and activation energy. Glassy modulus and activation energy increased in the order of TPP-TPB, TPP-BQ and TPP. The result confirmed that the larger the glassy modulus for cured material is, the activation energy necessary to transfer from glass to rubber increase. Therefore, DMA experiment provides an indicator of physical transition of EMC cured materials.
Fig. 7. Arrhenius plot of glass transition for calculating the activation energy from each slope.
Glass transition temperature (°C)
E RT
(4)
where is the electric conductivity, is the dielectric loss factor, is the angular frequency and 0 is the permittivity of free space. The loss factor is related to the energy loss caused by ion mobile and dipole rotation [15]. The change of ion viscosity (IV ) during 10 min of curing at 150 °C, is drawn in Fig. 5a. IV of all three samples increases as curing proceeds because dielectric loss decreases from the limited mobility of molecules. The values of each IV at 10 min are shown in Fig. 5b, which are 6.9 Ωcm (TPP-TPB), 8.0 Ωcm (TPP-BQ) and 7.9 Ωcm (TPP). TPP and TPP-BQ show a similar level of IV, whereas TPP-TPB shows about 85% of the former two. This shows that molecule mobility for TPP-TPB is still sufficient after 10 min of curing, which comes from unreacted materials. Fig. 5b not only shows the result of, IV but also the result of volume resistivity of curing from each cured material at 150 °C for 10 min. Volume resistivity is the result when applied 1 V of voltage to sample in room temperature, using DEA. Similar results of volume resistivity were recorded at 14.5 Ωcm and 14.2 Ωcm for TPP and TPP-BQ each, and TPP-TPB recorded 12.9 Ωcm around 90% of the former two. The IV result using in-situ experiment within DEA and the volume
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Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Investigation on curing characterization of epoxy molding compounds with different latent catalysts by thermal, electrical and mechanical analysis Da Eun Leea,b, Han-Jung Choa, Byung-Seon Konga, Hyung Ouk Choia, a b
T
⁎
KCC Central Research Institute, 17-3, Mabuk-ro 240beon-gil, Giheung-gu, Yongin-si, Gyeonggi-do, Republic of Korea School of Chemical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, Republic of Korea
ARTICLE INFO
ABSTRACT
Keywords: Epoxy molding compound Latent catalyst Cure kinetics Time-domain NMR Dielectric analysis
This research was conducted to investigate comprehensively curing characteristics of epoxy molding compound (EMC) with various latent catalysts by using their thermal, electrical and mechanical properties. The conversion prediction on isothermal condition for three types of organophosphine based catalysts was studied by model free method with differential scanning calorimetry (DSC). The T2 relaxation time was measured by time-domain nuclear magnetic resonance spectroscopy (TD-NMR) in order to compare the cross-linking density for each catalyst according to curing time. The results of DSC and TD-NMR showed that catalysts with lower latency reached curing in a shorter time. Furthermore, we observed the electrical-mechanical characteristics of EMC using additional thermal analysis. The ion viscosity was measured by dielectric analysis (DEA) according to the curing time, and the results suggested that the ion viscosity and the volume resistivity were lower for the catalyst with a slower curing rate. Dynamic mechanical analysis (DMA) was performed to compare activation energy of glass transition and glass modulus of final product. It was shown that the use of a catalyst with a low latency increases the modulus against external physical forces. In conclusion, our results demonstrated that the various analytical approaches are highly significant to understand the curing characteristics of EMC, which helps select appropriate catalysts in order to enhance the reliability of EMC.
1. Introduction Epoxy Molding Compound (EMC) is an electronic material used in the semiconductor industry, in order to protect electrical circuits [1]. The raw materials which compose EMC including epoxy resin, phenol or amine hardener, catalyst, filler, additives are evenly blended and moved to the mold. Most EMCs have short curing time at high temperatures, and this conversion of EMC is related with the reliability of semiconductor devices [2]. Quality issues such as cracks or delamination would occur, if the extent of curing was not enough or the rate of curing was too fast for materials to evenly mix. Therefore, to enhance the quality of product, EMC requires an appropriate curing under the temperature and time set during the process [3,4]. As catalyst plays the role of accelerator in the EMC curing, it is important to select a proper catalyst [5,6]. In order to have an effective curing reaction in practice, the catalyst should be stable before it is actually injected into the reaction. Recently, latent catalysts have been used to avoid accelerating reaction during storage in the manufacturing sites [7,8]. The latent catalysts delay reaction under storage temperature and enable to cure at curing temperature, but may reduce the total degree of conversion if
⁎
latency of catalyst is too high. On the other hand, using catalyst with low latency cause problem during storage and reduce mechanical property of EMC due to a short curing time. This is why it is important to understand curing characteristics by selecting a catalyst with the appropriate latent and reactive activities. To observe thermal characteristics of epoxy resin, the DSC analysis has often been conducted. Previous studies have examined the differences in curing behavior according to the types of catalysts through the DSC analysis [9–11]. We predicted the conversion and reaction model of each curing system with DSC experiments, compared model fitting and model free method to verify the model free method to be more effective [12]. Recently, studies on predicting conversion as well as observing curing characteristics have been continued through various equipment such as delectric analysis (DEA), dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), and time-domain NMR (TD-NMR) [13–17]. TD-NMR is the equipment evaluating the crosslinking density of polymers using relaxation time of RF pulse. LaPlante et al. [13] used TD-NMR to evaluate cross-linking density of epoxy resin in a short time, with the epoxy resin/polyamidoamine curing system. DEA is the thermal scanning device to measure electric signals by
Corresponding author. E-mail address: [email protected] (H.O. Choi).
https://doi.org/10.1016/j.tca.2019.02.009 Received 28 November 2018; Received in revised form 15 February 2019; Accepted 18 February 2019 Available online 19 February 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. The molecular structure of EMC catalysts (a) TPP, (b) TPP-BQ, and (c) TPP-TPB.
applying a constant alternating voltage and frequency to the sample, which allows observing the ion viscosity from curing in real time [14,15]. Furthermore, DMA evaluates thermal-mechanical characteristics by measuring the strength which reacts to mechanical stress applied to the sample. Sadeghinia et al. [16] have used DMA to compare modulus between the EMC of filler type and the EMC of non-filler type. Along with others, devices such as rheometer or TMA were used [17–19]. Also, there are studies that analyzed the effect of cure temperature [20–24]. Previous studies conducted individual analysis which limited observing the curing characteristics. To accurately understand epoxy curing system, it is crucial to obtain various curing
characteristics by different analytic methods and investigate comprehensively their interrelationships. The purpose of this study is to observe complex characteristics of the curing of epoxy resin/phenol hardener system according to the different selection of catalysts, using DSC, TD-NMR, DEA, and DMA. Three catalysts with different latency were chosen for comparing curing characteristics of EMC according to the catalyst selection. DSC and TDNMR were used to observe differences in curing rate and conversion. DEA allowed us to measure electric characteristics by ion viscosity as per curing time. Lastly, DMA measurement was conducted to observe the mechanical characteristics as per degree of conversion. Along with
Fig. 2. (a) DSC thermodynamic scans with different heating rate of 20, 10, and 5 K/min of TPP-TPB. (b) KAS plot at degree of conversion (0.1 – 0.9) of TPP-TPB. (c) The activation energy calculated from the slop of KAS plot according to conversion. (d) Isoconversional prediction curves at 150°C. 69
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(synthesized by mixing TPP and benzoquinone in acetone at room temperature), and TPP-TPB (Hokko Chemical Industry Co., Ltd.). The epoxy resin, curing agent, and catalysts were evenly blended with a constant content ratio (100 : 90 : 5) by a mixer. After these blended powders were melted and dispersed with twin-screw kneader at 100 °C for 5 min, cooled and crushed into fine powders. These fine powders were presented as name as TPP, TPP-BQ, and TPP-TPB on graph using by initial materials for DSC and DEA measurement.
Table 1 The results of activation energies at degree of conversion (0.2, 0.5, and 0.8). E of kinetics (kJ/mol)
= 0.2 TPP-TPB TPP-BQ TPP
155 ± 13 105 ± 12 135 ± 10
= 0.5 131 ± 2 114 ± 15 90 ± 7
= 0.8 126 ± 1 118 ± 16 63 ± 10
2.2. Differential scanning calorimeter (DSC) experiments
measuring the reaction rate and degree of conversion, the correlation between electrical-mechanical characteristics, would help us understand comprehensive curing characteristics.
The differential scanning calorimeter (Mettler Toledo, DSC 2/ 400 W) was used to monitor the curing kinetics. Three dynamic data (5 K/min, 10 K/min, and 20 K/min) were obtained with prepared powders of 5–10 mg under 50 mL/min N2 flow. The model free method was applied to compare activation energy according to the degree of conversion and conversion prediction curves were obtained by isoconversional method.
2. Experimental 2.1. Materials and sample preparation The epoxy resin was used as mixture of tetramethylbiphenol type (Mitsubishi Chemical Co., YX-4000HX) and o-cresolnovolac type (Nippon Kayaku Co., Ltd., EOCN1020-70). The curing agent consisted of phenol type (Meiwa Plastic Industries, Ltd., MEH-7800SS). The three latent catalysts were TPP (Hokko Chemical Industry Co., Ltd.), TPP-BQ
2.3. Time-domain NMR (TD-NMR) experiments The spin-spin relaxation time was measured by TD-NMR (Oxford instruments, MQR, UK) under 20.82 MHz. Each sample in a 10 mm
Fig. 3. T2 relaxation decay curves for cured EMC with catalysts (a) TPP-TPB, (b) TPP-BQ, and (c) TPP according to curing time of 2, 5, 7, 10 min. (d) Comparative curves of three catalysts with curing time of 5 min. 70
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Fig. 4. T2 relaxation time separated by T2, Solid and T2, Liquid for (a) TPP-TPB, (b) TPP-BQ, and (c) TPP. The signal intensity of a solid and liquid phase expressed as a percentage of (c) TPP-TPB, (d) TPP-BQ, and (f) TPP. Each catalyst was measured at curing time of 2, 5, 7, 10 min, respectively.
glass measuring tube was cured at 150 °C for 2, 5, 7, and 10 min using a heating block (Grant instruments, QBH, UK), and immediately transferred to a TD-NMR probe. The spin-spin relaxation decay was measured using CPMG pulse. The data points of 4096 echoes were collected with a relaxation delay of 0.5 s with 8 scans. The spectra were manually analyzed using Application Developer software version 0.10.4.330.
2.4. Dielectric analysis (DEA) measurement Dielectric cure monitoring was performed with LT-451 (Lambient Technology, USA). The flexible interdigitated electrode (Mini-Varicon) was used instead of parallel plate electrodes. The sensor has 1.5ʺ long, 0.004ʺ thick, and 0.004ʺ electrode space. Mini-Varicon can measure the
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Table 2 T2 relaxation time and signal intensity of solid and liquid domains according to curing time of 2, 5, 7, and 10 min. Curing time
2 min 5 min 7 min 10 min
TPP-TPB
TPP-BQ
TPP
T2,Solid (ms)
T2,Liquid (ms)
Ms (%)
ML (%)
T2,Solid (ms)
T2,Liquid (ms)
Ms (%)
ML (%)
T2,Solid (ms)
T2,Liquid (ms)
Ms (%)
ML (%)
10.511 3.652 1.840 0.620
48.189 34.962 21.559 10.969
21 53 68 79
79 47 32 21
4.359 1.496 0.228 0.153
35.806 16.778 2.966 2.512
49 74 80 87
51 26 20 13
2.574 0.316 0.137 0.175
21.453 5.222 2.811 –
61 82 91 100
39 18 9 0
Fig. 5. (a) Variation of ion viscosity (log IV ) during 600 s at 150°C for TPP-TPB, TPP-BQ, and TPP. (b) Comparison between ion viscosity and volume resistivity at 1 Hz and 600 s.
dielectric properties of EMC within approximately 0.004ʺ of the electrode surface. A sinusoidal AC voltage of 1 V at frequency of 1 Hz was applied under isothermal temperature of 150 °C.
E is the activation energy which is variable with the degree of conversion ( ), R is the gas constant, and T is the absolute temperature. The conversion prediction was performed by the isoconversional method using following Eq. (2) proposed Vyazovkin’s study [28].
2.5. Dynamic mechanical analysis (DMA) measurement
t =
The dynamic mechanical analysis (DMA1, Mettler Toledo, Switzerland) was used to characterize frequency dependence of relaxations. The DMA applies oscillating force to the sample and measures resultant displacement of the sample. In this experiment, the samples of each catalyst were cured at 150 °C for 15 min and flattened to about 20 mm × 10 mm × 1 mm. The cured samples were placed in a single cantilever mode, and the modulus was measured at a temperature range of 30 °C–120 °C, and a frequency of 0.5, 1, 3 and 5 Hz.
This study used three types of organophosphine catalysts, each with different latent effect, to assess curing characteristics of EMC in each case. Organophosphine catalysts open the ring of epoxy resin with a nucleophilic attack, which starts the curing [2,3]. This ability of nucleophilicity amplifies when the basicity is larger and structural steric hindrance is less. [25] The structures of three catalysts are drawn in Fig. 1. Nucleophilicity of TPP was expected to be the largest, which led to increase the reaction rate. Kinetic study applied a model free method that offers the benefit of eliminating the need to measure parameters of reaction constants and reaction order without selecting a specific model. Also, model free method allows observation of activation energy as per extent of reaction calculated from Kissinger-Akahira-Sunose (KAS) method based on Eq. (1) [26,27].
T2
= ln
AR g ( )E
E RT
exp ( E / RT ) (2)
exp ( E / RT0)
where t is the life time at , T the temperature at , T0 is the temperature of isothermal condition. To perform the kinetic analysis, DSC experiment was conducted for three EMC samples. The curing peaks at three different heating rate were observed, from which changes in activation energy and conversion prediction curves at 150 °C were deducted. Each result is drawn in Fig. 2. Fig. 2b shows the linear relationship between ln ( / T 2 ) and 1/ T . The slope of each line represents the activation energies at degree of conversion ( = 0.1, 0.2, , 0.9 ) and the values at 0.2, 0.5, and 0.8 were indicated in Table 1. The variation of the activation energy is showed in Fig. 2c. In Fig. 2d, the conversion prediction curves at 150 °C were presented. After 10 min of curing, TPP almost reach 100% conversion, while TPP-BQ and TPP-TPB stops at about 80% and 60%, respectively. Also, the reaction rate was the fastest in the order of TPP, TPP-BQ and TPP-TPB. Another analytic method used to confirm the degree of conversion was TD-NMR. The spin-spin relaxation time measured by TD-NMR allows to measure the cross-linking density of the sample. For epoxy resin curing, T2 relaxation time was obtained by fitting the bi-exponential decay equation as below :
3. Result and discussion
ln
T 0
S (t ) = Ms exp
t T2, Solid
+ ML exp
t T2, Liquid
(3)
where S is the signal detected, M is the signal amplitude backextrapolated to time 0, t is the time elapsed, and T2 is the spin-spin relaxation time constant. Indices “Solid” and “Liquid” refer to the short and long components of the signal, which imply high-cross-linking density domain and low-cross-linking density domain, respectively [13,29]. In the curing process, high-cross-linking domain enlarges as degree
(1)
where is the heating rate, g ( ) is the integral form of the function according to degree of conversion ( ), A is the pre-exponential factor, 72
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Fig. 6. Experimental data of DMA measurement. (a), (c), (e) E (storage modulus) and (b), (d), (f) E (loss modulus) in the heating range of 30 °C–120 °C according to various frequency.
of conversion increases, and graph takes a rapid decrease as T2 relaxation time of two domains decreases. Fig. 3 shows bi-exponential decay graph as per curing time. Fig. 3 shows a sudden decrease in graph as the curing time increases, in all three samples. This means as the curing time increases, solid domain enlarges leading to the increase of degree of conversion. However, TPP-TPB (Fig. 3a) shows a gradual decreases as reaction time increases, while in the cases of TPP (Fig. 3c),
the shapes of graphs change for 2, 5, 7 min but in the 7, 10 min the shapes are almost still. It can be assumed that TPP reached 100% curing after 7 min. Each bi-exponentially decreasing graph in Fig. 3 has been separated into solid domain and liquid domain, to draw changes of T2 and M as per curing time in Fig. 4. T2, Solid and T2, Liquid decreased as per curing time, which means shortened relaxation time from limited molecule mobility in the epoxy resin curing process. In Fig. 4e, the 73
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resistivity result showed a similar tendency. That is, mobile molecule or ions of unreacted materials can affect electric characteristics such as resistance of the final cured material. Observing physical characteristics according to catalyst types was conducted through DMA experiment. DMA recorded the storage modulus (E ) and loss modulus (E ) of cured materials at 150 °C for 15 min on four different frequency. Each experiment result is drawn in Fig. 6. Glass transition temperature (Tg) in DMA is defined as the onset point where modulus starts to decrease from E or peak point at E . All three samples according to catalyst types show the Tg value shift right as frequency increases. The shift of Tg from the frequency change follows the Arrhenius equation as below:
lnF = lnA
Table 3 The glass transition temperature at 1 Hz obtained from peak point of E , glass modulus from E , and the activation energy for glass transition.
TPP-TPB TPP-BQ TPP
Glass modulus (MPa)
E of glass transition (kJ/mol)
91.8 93.2 94.8
1221 1455 1560
98 ± 9 164 ± 4 233 ± 7
4. Conclusion The comparison on reaction rate and prediction of conversion according to latency of the three catalysts (TPP-TPB, TPP-BQ, and TPP) were proceeded with DSC experiment. Under model free method, the conversion prediction curve for TPP-BQ and TPP reached 100% curing at 150 °C with reaction time of 10 min, but TPP-TPB reached around 90% curing level. The difference in conversion was also observed in the TD-NMR experiment as well. Relaxation time of T2 of TD-NMR was separated to solid domain and liquid domain, and relaxation time shortened as per curing time. Unlike TPP-BQ and TPP, liquid domain remained for TPP-TPB after 10 min. DEA experiment proceeded to observe changes in electric characteristics as per different curing rate of each catalyst. Comparing the increase of ion viscosity by curing time, the result of TPP-TPB was around 85% of TPP-BQ and TPP, which implies the existence of unreacted material in the cured material. Lastly, physical properties by different curing level were measured with DMA experiment. The faster curing rate and higher degree of conversion of material led to larger increase in glass modulus and activation energy from glass transition. As a result, curing rate and conversion differ according to the latency of catalyst which changes the electricalmechanical characteristics of the final product. Various analysis measurements and the correlation between each results should be confirmed to enhance reliability of EMC.
exponential decay curve was no longer separated into two domains after 7 min. This shows that the solid domain was only measured. Fig. 4b,d,f shows the changes of composition ratio with the curing time of the solid domain and liquid domain of each sample. This result which is of similar tendency with the former conversion prediction curve by DSC shows validity of the prediction. The T2 and M values are shown in Table 2. As the degree of conversion for each catalyst was checked previously in DSC and TD-NMR, DEA experiment was conducted to observe electric characteristics as per different conversion by catalyst. DEA monitored the dielectric properties such as permittivity, loss factor, and ion viscosity as a function of time in real time while EMCs were cured. Among these, ion viscosity is defined as below :
IV =
1
=
1 0
(5)
where F is frequency (Hz), A is the pre-exponential factor, E is the activation energy (kJ/mol), R is the gas constant (kJ/mol·K), and T is the temperature of glass transition [30]. Arrhenius plots using Eq. (5) of each sample are drawn in Fig. 7 and activation energy of glass transition can be calculated with the slope of each line. Table 3 shows the results of Tg, glassy modulus and activation energy. Glassy modulus and activation energy increased in the order of TPP-TPB, TPP-BQ and TPP. The result confirmed that the larger the glassy modulus for cured material is, the activation energy necessary to transfer from glass to rubber increase. Therefore, DMA experiment provides an indicator of physical transition of EMC cured materials.
Fig. 7. Arrhenius plot of glass transition for calculating the activation energy from each slope.
Glass transition temperature (°C)
E RT
(4)
where is the electric conductivity, is the dielectric loss factor, is the angular frequency and 0 is the permittivity of free space. The loss factor is related to the energy loss caused by ion mobile and dipole rotation [15]. The change of ion viscosity (IV ) during 10 min of curing at 150 °C, is drawn in Fig. 5a. IV of all three samples increases as curing proceeds because dielectric loss decreases from the limited mobility of molecules. The values of each IV at 10 min are shown in Fig. 5b, which are 6.9 Ωcm (TPP-TPB), 8.0 Ωcm (TPP-BQ) and 7.9 Ωcm (TPP). TPP and TPP-BQ show a similar level of IV, whereas TPP-TPB shows about 85% of the former two. This shows that molecule mobility for TPP-TPB is still sufficient after 10 min of curing, which comes from unreacted materials. Fig. 5b not only shows the result of, IV but also the result of volume resistivity of curing from each cured material at 150 °C for 10 min. Volume resistivity is the result when applied 1 V of voltage to sample in room temperature, using DEA. Similar results of volume resistivity were recorded at 14.5 Ωcm and 14.2 Ωcm for TPP and TPP-BQ each, and TPP-TPB recorded 12.9 Ωcm around 90% of the former two. The IV result using in-situ experiment within DEA and the volume
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