Cure kinetics and properties of high performance cycloaliphatic epoxy

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Cure kinetics and properties of high performance cycloaliphatic epoxy resins cured with anhydride Maoping Lu, Yingchun Liu, Xiangxiang Du, Shiheng Zhang, Guokang Chen, Qian Zhang, Sa Yao, Liyan Liang, and Mangeng Lu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06442 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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Industrial & Engineering Chemistry Research

Cure kinetics and properties of high performance cycloaliphatic epoxy resins cured with anhydride

Maoping Lua, b, Yingchun Liua, b, Xiangxiang Dua, b, Shiheng Zhangc, Guokang Chenc, Qian Zhanga,b, Sa Yaoa,b, Liyan Lianga*, Mangeng Lua

a

Key Laboratory of Cellulose and Lignocellulosics Chemistry, Guangzhou Institute of Chemistry,

Chinese Academy of Sciences, Guangzhou 510650, PR China. b University

c

of Chinese Academy of Sciences, Beijing 10049, PR China.

Guangdong Provincial Engineering & Technology Research Center for Touch Significant Devices

Electronic Materials, Guangzhou 510650, PR China.

*Corresponding

author, E-mail address: [email protected]

Tel(Fax): + 86-20-85231033

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract: In this work, a cycloaliphatic epoxy resin hexahydrophthalic acid bis (3, 4epoxycyclohexylmethyl) ester (BE) was prepared and cured with anhydride. The curing kinetics and properties of the cycloaliphatic epoxy resin were characterized and compared with the (3’, 4’epoxycyclohexane)

methyl-3,

4-epoxycyclohexyl-carboxylate

(CE)

and

bis

(3,

4-

epoxycyclohexylmethyl) adipate (BA). The result shows that BE/MHHPA exhibits a higher reaction activation energy and a slower curing rate than CE/MHHPA and BA/MHHPA. The loss modulus of BE/MHHPA increased 53 % and 24 % comparing to CE/MHHPA and BA/MHHPA, respectively. Young’s modulus and initial degradation temperature of BE/MHHPA improved 31 % and 6.4 % than those of BA/MHHPA. Furthermore, superior moisture resistance and dielectric properties are expressed for BE/MHHPA among these epoxy resins. Those results demonstrated that the BE/MHHPA potential usefulness in the electronic packaging material field. Keywords: cycloaliphatic epoxy resin, curing kinetics, electronic packaging 1. Introduction In the past decades, thermosets have played a vital role in the polymeric materials owing to excellent thermal and dimensional stability, favorable chemical resistance and mechanical properties1. Epoxy resin as one of the most important thermosetting resins is widely employed in the fields of coatings, adhesives, paints, electronic component encapsulations and as matrix for the production of fiber-reinforced composites2-7. The attractive properties of cured polymers largely related to the intrinsic of chemical structure of the epoxy monomer or original epoxy oligomer8. In last years, many efforts and studies have been developed novel epoxy resins, which the heat and electric resistance were enhanced and used in


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electric encapsulations and microelectronic devices 9-13. The polymers used in the future microelectronic packaging must endow the following properties: low dielectric constant and loss; high thermal stability and moisture resistance for better solder resistance; high purity to present good stability14-16. Cycloaliphatic epoxies, cured polymer materials are widely employed for the electronic packing materials and optical devices, due to their characteristic properties including superior processability, high glass transition temperature, UV resistance, excellent mechanical and electrical properties6,

17-19.

Moreover, the synthesis of cycloaliphatic epoxides involves the peracid epoxidation of olefins rather than the condensation of epichlorohydrin with phenols, which results in that the cycloaliphatic epoxides are essentially free of chloride15,

20, 21.

Thus, due to the advantage of thermal stability and superior

processability of epoxy resins, a project aimed at the synthesis novel cycloaliphatic epoxy resins attracted much attention in recent years22-24. However, little research has been conducted compare novel cycloaliphatic epoxy resins to the commercial available bis (3, 4-epoxycyclohexylmethyl) adipate (BA). Herein, a cycloaliphatic epoxy, hexahydrophthalic acid bis(3, 4-epoxycyclohexylmethyl) ester (BE) was synthesized by the esterification and epoxidation reaction. The symmetric chemical structure and low polarity were introduced to the backbone of synthetic cycloaliphatic epoxy, which may improve the moisture resistance and dielectric properties. Meanwhile, the structure of six-membered ring in the middle of cycloaliphatic epoxy was designed to ensure the good thermal stability. The synthetic cycloaliphatic epoxy was cured with 4-methyl hexahydrophthalic anhydride (MHHPA). The thermal and mechanical properties, the mechanism of thermal decomposition, coefficient of thermal expansion, optical properties, the water absorption and its curing kinetics of the epoxy resin were investigated. In addition, the commercial available cycloaliphatic epoxy (3’, 4’-epoxycyclohexane) methyl-3,4epoxycyclohexyl-carboxylate (CE) and bis (3, 4-epoxycyclohexylmethyl) adipate (BA) were cured and



compared with the BE systems. 2. Experimental 2.1 Materials 3-cyclohexene-1-methanol, (3’, 4’-epoxycyclohexane) methyl-3, 4-epoxycyclohexyl-carboxylate (CE), bis(3, 4-epoxycyclohexylmethyl) adipate (BA) were purchased from Jiangsu Tatrachem Chemical Co., Ltd. (Jiangsu, China). Cis-1, 2-cyclohexanedicarboxylic anhydride (HHPA) and 2-ethyl-4methylimidazole (EMI) were provided by Aladdin Industrial Co., Ltd. (Shanghai, China). Anhydrous sodium bisulfate was purchased from Shanghai Macklin Biochemical Co., Ltd. 3-chloroperoxybenzoic acid (m-CPBA) was purchased from purchased from Beshine Chemical Technology Co., Ltd. (Beijing, China). 4-methyl hexahydrophthalic anhydride (MHHPA) was purchased from Guangzhou Qihua Chemical Co., Ltd. (Guangdong, China). All the other solvents were obtained from various commercial sources and used without further purification. 2.2 Synthesis of cycloaliphatic diepoxide 2.2.1 Synthesis of hexahydrophthalic acid bis(3-cyclohexene methyl) ester (Olefin) A 1000 mL three-necked flask, equipped with a magnetic stirrer and water separator with refluxing condenser, was charged with 3-cyclohexene-1-methanol (28 g, 0.25 mol) and HHPA (15.4 g, 0.1 mol) dissolved in 250 mL of toluene25. 2.17 g of sodium bisulfate anhydrous as a solid catalyst was added into the above mixture. The mixture was stirred rigorously at 130 °C refluxed for 8 h. After cooling, the organic layer of olefin was separated, washing with 5 % sodium carbonate solution, and then washing with deionized water to neutralization. Finally, the liquid olefin was dried with anhydrous sodium sulfate for overnight. After filtration and concentration, the desired olefin was purified by column chromatography with a yield of 33.12 g (92 %). FTIR (cm-1): 3018 (C=C), 2936 (C-H), 1732 (C=O),


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1649 (C=C), 1297 (C=O). 1HNMR (CDCl3, ppm): δ 5.68 (s, 4H, =CH), 3.98 (d, 4H, CH2-O), 2.86 (m, 2H, ring HCCOO), 2.19-1.02 (m, 22H, ring CH, CH2). 2.2.2 Synthesis of hexahydrophthalic acid bis(3,4-epoxycyclohexylmethyl) ester (BE) The hexahydrophthalic acid bis(3-cyclohexene methyl) ester was synthesized according to the method reported previously26. 34.52 g (0.2 mol) of m-CPBA was dissolved in 150 mL of methylene chloride and added into a 500 mL, three-neck, round-bottomed flask equipped with a dropping funnel. The mixture was stirred and cooled in an ice bath for 10 minutes. A methylene chloride solution (50 mL) of olefin (18 g, 0.05 mol) was added dropwise over 1 hour. After further stirring for 10 hours at room temperature, the white precipitate was removed with a Buchner funnel. The filtrate was washed with 5 % Na2CO3 and deionized water to neutralization. Then the organic layer was dried and purified with column chromatography on silica gel to give a 39 % yield of the product as a colorless liquid. FTIR (cm-1): 2930 (C-H), 1727 (C=O), 1297 (C=O) 908, 794 (C-C-O). 1HNMR (CDCl3, ppm): δ 3.90 (d, 4H, CH2-O), 3.18 (d, 4H, epoxide CHO), 2.82 (m, 2H, ring HCCOO), 2.18-1.01 (m, 22H, ring CH, CH2). 2.3 Curing reactions BE was mixed with MHHPA as hardener in a 1:0.85 of molar stoichiometric, 0.5 wt.% EMI as curing accelerator. The commercialize epoxy of CE and BA were cured with the same operation in the same ratio for comparison, termed as CE/MHHPA and BA/MHHPA, respectively. The mixtures were degassed under vacuum at 50 °C for 30 minutes, pre-cured at 130 °C for 2 h, and cured at 160 °C for 2 h, post cured at 190 °C for 1 h. 2.4 Characterizations The 1H-NMR spectra were performed on a Bruker DPX-400 spectrometer (400 MHz). Deuterated chloroform (CDCl3) containing tetramethylsilane (TMS) was used to dissolve the samples. Fourier



transform infrared (FTIR) spectra using KBr crystal plate method were recorded on a Bruker Tensor 27 FTIR spectrophotometer in the wavenumber range 400–4000 cm-1. Epoxy equivalent weight (EEW) was calculated according to the standard test method of GB/T 1677-2008. Differential scanning calorimetry (DSC, TA, instruments Q200, USA) was used to investigate the curing behavior from 25 °C to 290 °C under purging nitrogen gas atmosphere. The data of Tg were collected using heating rate 10 K/min for all of the cured samples. Dynamic mechanical analysis (DMA) was performed in tensile mode using a dynamic mechanical analyser (DMA Q800, V21.1, Build 51), which was operated at the heating rate of 2 K/min and the frequency of 1 Hz under an nitrogen atmosphere. The temperature was ranging from 20 °C to 280 °C and the size of samples is 2.0 × 0.2 × 0.1 cm. Coefficient of thermal expansion was measured by static mechanical analyzer (Netzsch DIL 402C Germany). It was operated at the heating rate of 5 K/min under an argon atmosphere. The temperature was ranging from 25 °C to 220 °C with the dimension of 10 × 0.4 × 0.4 cm. Thermal stability of the cured samples was evaluated by Thermogravimetric analysis (TGA). TGA was performed in the temperature range of 40 °C to 800 °C at a heating rate of 10 K/min under a continuous nitrogen flow (20 mL/min) on Germany Netzsch TG 209 F3. Thermogravimetric analysis/infrared spectrometry (TG-IR) was carried out on a TG 209F1 Libra thermo-analyzer instrument combined with a Nicolet IS 50 spectrometer. 13 mg of a sample was put in an alumina crucible and the temperature was ranging from 25 °C to 600 °C at the heating rate of 10 K/min under an nitrogen atmosphere. The mechanical properties of the dumbbell-shaped specimens with the dimension 2.5 × 0.4 × 0.2 cm were measured on a materials testing machine RGM-3030 at room temperature, using a stretching speed of 5 mm/min. The izod impact strength was carried out according to GB/T 1843-2008 using an impactometer (Xjjd-5, China) at 25 °C. The data reported are the average of five samples. The morphology and crack surface of the resins were studied by a Philips XL30 scanning


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electron microscope (SEM) at the accelerating voltage of 15 kV. The UV-vis spectra were obtained by a Shimadzu UV 2550 spectrophotometer in transmittance mode. The water absorption was determined by measuring the increased weight gain due to moisture uptake. Prior to the measurements, the specimens with dimension of 3.0 × 3.0 × 0.1 cm were dried at 80 °C for 16 h to remove moisture from the exterior surface. The average measurements were reported from four specimens. Dielectric properties were measured with a broadband dielectric spectrometer (Concept 80, Novocontrol Technology Company, Germany) in the frequency range from 1 Hz to 106 Hz at room temperature. Rectangular samples were prepared with the dimension of 3.0 × 3.0 × 0.2 cm. 3. Results and Discussion

O O +

O OH Toluene

O

NaHSO4

O

O

O Olefin O

O O

m-CPBA

O

CH2Cl2 O

O

BE

Scheme 1. Synthetic route of olefin and epoxide BE.

3.1 Synthesis and characterization of cycloaliphatic epoxy resin BE was designed and synthesized through two steps outlined in Scheme 1. The chemical structures of Olefin and BE were characterized by FTIR and 1HNMR spectroscopy. Figure 1 shows the FTIR spectra of Olefin and BE. The absorptions at 3018, 1649 cm-1 related to the cycloaliphatic double bond in Olefin (Figure 1a). But in the FTIR spectrum of BE (Figure 1b) , the absorption of double bond in Olefin was obviously disappeared, and new characteristic peaks of the oxirane ring appeared at 794 and 908 cm-1.



These results indicated that the C=C bonds in Olefin were converted into epoxy groups in BE.

Figure 1. FTIR spectra of Olefin and Epoxide.

In the 1HNMR spectra of Olefin and BE, all the signals corresponding to the compounds were observed in deuteron chloroform. For Olefin (Figure 2a), the chemical shift of the double bonds (=CH) were observed at around 5.68 ppm and the signal of double bonds were disappeared after epoxidation. The new signals located at 3.13-3.22 ppm (Figure 2b) correspond to oxirane ring appeared, thereby indicating that there was a chemical change occurring of double bond transformed into epoxy ring completely. That result was consistent with the spectra of FTIR.

Figure 2. 1HNMR spectra of Olefin and Epoxide.


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The epoxy equivalent weight (EEW) of epoxy resin was calculated by HCl/Acetone titration. EEWs of CE and BA were determined through the same method. Their theoretical and measured EEW values were shown in Table 1. The measured values are normally in accordance with that theoretical values. Table 1. Chemical structures of materials and epoxy equivalent weights (EEWs) of epoxy resins.

EEW (g/eq) Name

Structure

O O O

O

196

210

183

210

126

134

‐‐

‐‐

‐‐

‐‐

O

O

O

BA

O

Measured

O

O BE

Theoretical

O

O

O CE

O

O

O O

H3C O

MHHPA

O

H3C N EMI

N H

C H2

CH3

3.2 Curing kinetics The curing kinetic of BE/MHHPA was studied and compared to BA/MHHPA and CE/MHHPA from nonisothermal DSC analysis. Figure 3 displays the nonisothermal DSC thermal analytical curves. The kinetic parameters of samples is presented at Table 2 and Table S1. The cured epoxy resins of



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nonisothermal DSC curves and the glass transition temperature (Tg-DSC) are shown in Figure 4. It’s obvious that the exothermic peak temperature (Tp) increased with an increase of the heating rate (See Table 2). The curves show a major thermal curing process with a strong exothermic at higher temperature. Note that the exothermic peak (Tp) of the BE/MHHPA is higher than that of CE/MHHPA and BA/MHHPA. The activation energy (Ea) can be calculated from the peak temperatures (Tp) at different heating rates (β) following the Kissinger equation27, 28: ln

( ) ( ) 𝛽

𝑇𝑝2

= ln

𝐸𝑎 𝐴𝑅 ― 𝐸𝑎 𝑅𝑇𝑝

where A is the frequency factor and R is the gas constant. The Ea (Table 2) was calculated from the slope of a linear fitting plot of ln (β/Tp2) vs. 1/Tp (Figure 3b). The reaction activation energy (Ea) of BE/MHHPA is 79.76 kJ/mol, much higher than that of CE/MHHPA and BA/MHHPA systems. The lowered reactivity of BE is due to its ortho-positions structure and higher space steric hindrance of hexatomic ring on the molecule. The ortho-substituted of BE shows a higher potential barrier of internal rotation and hence higher energy of rotational isomerization29, which reduces reactivity of BE/MHHPA. However, the differences in the curing process of all samples are slight with the same heating rate, which is due to the same epoxy groups connected with alicyclic structure30.

Figure 3. DSC thermograms (a) and Kissinger plots of ln

( ) against 𝛽

𝑇𝑝2


1 𝑇𝑝

(b) for BE/MHHPA,

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BA/MHHPA and CE/MHHPA.

Table 2. Curing characteristics and the overall activation energy of cured BE/MHHPA, BA/MHHPA and CE/MHHPA.

Sample

Heating rate (K/min)

Tp (K)

5

434

10

451

15

462

20

470

5

437

10

456

15

465

20

474

5

449

10

459

15

465

20

477

CE/MHHPA

Ea (kJ/mol)

57.96

BA/MHHPA

56.73

BE/MHHPA

79.76

Figure 4. Nonisothermal DSC curves and Tg of the cured epoxy resins.

3.3 Dynamic mechanical properties



The dynamic mechanical analysis (DMA) were frequently used to investigated the glass transition temperature, cross-link density, stiffing and damping characteristics of cured thermosetting polymers. The changes of storage modulus, loss modulus and damping factor (tan δ) versus temperature are shown in Figure 5 and Table 3. According to the classical rubber elasticity equation, the crosslink density (ve) can be calculated by the following equation31: 𝐸′ = 3𝑣𝑒𝑅𝑇 where 𝑅 is the gas constant, T refers to the absolute temperature, 𝐸′ is the storage modulus of polymers in the rubbery plateau region and was selected at Tg+25 °C. The measured crosslink densities (ve) were presented in Table 3. Tg values were observed in the peak temperature of the tan δ curves. As can be seen in Figure 5 and Table 3, the cured epoxide of BE/MHHPA had the lowest cross-linking density compared with CE/MHHPA and BA/MHHPA. While there are two epoxide groups for each epoxide resin in our work, lower EEWs mean higher concentration of epoxide groups32. Compared with CE/MHHPA, a larger molecular weight and higher EEW approves that BE/MHHPA endows a lower cross-linking density. The storage modulus begins to decrease with the temperature increasing, and then exhibit the rubbery plateau. Glassy modulus is typically a function of chemical structure and chain packing33. BE/MHHPA has a higher modulus below 75 °C than CE/MHHPA and BA/MHHPA, as shown in Figure 5a. High modulus of BE/MHHPA is attributed to the hexatomic ring is linked by ortho-positions ester bonds, then the motion of these ester bonds are restricted at low temperature33. Moreover, chain packing seems to play significant roles in keeping the rigidity of network, due to long-range chain-chain stacking of chain regularity34. The lowest storage moduli in the glass region of the cured BA/MHHPA mainly result from four methylene of backbone. In contrast, the storage modulus of CE/MHHPA is higher than BE/MHHPA


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over 80 °C, which is ascribed to difficulty in chain motions (relatively short and more rigid of backbone chains). Nevertheless, the ester bonds and hexatomic ring in the network obtain enough kinetic energy at a higher temperature. The storage modulus reduced dramatically due to the chemical bonds removed the energetic barrier for their motions34. The storage moduli of BE/MHHPA decreased to 7.7 MPa (from Table 3) at the rubber region, which could be attributed to mobility of hexatomic ring chains increased and low cross-linking density. Crosslinking density and the chemical structure of the chain segment for cured epoxides have an impacts on the values of Tg 35. In our case, the CE/MHHPA has the highest Tg (224 °C), which is attributed to higher cross-linking density and relatively shortly rigid segment between two alicyclic ring in comparison with the other two systems36. Though BE/MHHPA and BA/MHHPA have fairly close crosslinking densities, Tg is 154 °C and 124 °C respectively, suggesting the flexible methylene structure of BA/MHHPA plays a much more important role than the cross-linking densities in determining the Tg. Hence, the cured BA/MHHPA has higher chain mobility, which has lowest Tg values37. In spite of the Tg-DMA value differs from of Tg-DSC, its exhibits similar trend (Table 3). Loss modulus represents the energy dissipation on account of the internal friction related to the motion of the polymer chain. Figure 5b shows a main peak corresponding to α relaxation. BE/MHHPA takes 153 % and 124 % higher loss modulus value than CE/MHHPA and BA/MHHPA during the glass transition process. The greater loss modulus represents the higher damping energy dissipation and higher free molecular mobility in the chains of the BE/MHHPA, which will profit the transformation of the damageable vibration energy to heat30. The higher modulus and damping have a potential application for the thermosetting plastic34. The tan δ is the ratio of viscous component to elastic component or the ratio of loss modulus to storage



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modulus. Tan δ is quite sensitive to the structural variation of polymer materials38. The height of relaxation peak expresses a tendency to cross-linking densities and effect of network microstructure39. From Figure 5c, BE/MHHPA shows the highest tan δ (1.20) and means notably more viscous segments. One possibility for this result may be related to the steric hindrance with the present ortho-substituent from hexatomic ring26. The backbone of network rotation is restrained in glass region. With the temperature increasing, the hexatomic ring segmental mobility in polymer increased and the bonds attached to cycloaliphatic skeleton were rotation. Another results may be associated with the chain packing in the network and crosslinking density40, 41, the network of BE/MHHPA is looser than the others. By comparing the result of height of tan δ, CE/MHHPA exhibits the lowest of tan δ peak and highest Tg values, which is in accordance with the analysis of crosslinking density and rigidity structure in the molecular segments. Thus, the cured CE/MHHPA restricts the molecular motions and reduces plastic deformation ability of the network in the glassy relaxation process. Table 3. DSC and DMA date of the cured epoxides.

Storage modulus (GPa) Tg-DSC

Tg-DMA

Sample

ve

Height of

(10-3mol/cm3)

tan δ

Rubber (°C)

(°C)

Glass region a region b

CE/MHHPA

197

224

2.58

0.018

1.45

0.81

BA/MHHPA

109

124

2.34

0.0084

0.799

0.88

BE/MHHPA

139

154

2.62

0.0077

0.685

1.20

a

Storage modulus at 25 °C

b

Storage modulus at Tg-DMA + 25 °C


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Figure 5. Storage modulus (a), loss modulus (b) and tan δ (c) versus temperature for the

cured epoxy resins: CE/MHHPA, BA/MHHPA and BE/MHHPA.

3.4 Coefficient of thermal expansion Samples dimensional stability, which is a crucial property to evaluate the thermal resistance of epoxy resins using in packaging materials17. The values of average linear CTE were calculated from the slope of the length change versus temperature line at the range of 25 - 85 °C (Table 4). The results indicated that BE/MHHPA (7.6 × 10-5 K-1) and CE/MHHPA (7.4 × 10-5 K-1) presented lower CTE than BA/MHHPA (9.1 × 10-5 K-1). The low CTE values were attributed to rigid structure which limited the movement of molecular chain and leading to slightly smaller dimension change. In addition, high crosslinking density could be restricted segmental motion42. CE/MHHPA exhibits higher cross-linking density and rigid structure comparing with BE/MHHPA, thus the result presents lower CTE value for



CE/MHHPA. The long and flexibility fatty chains for BA/MHHPA facilitate segment motions and free volume expansion, which presents high CTE. 3.5 Epoxy networks mechanical behaviors The tensile stress-strain curves of cured cycloaliphatic epoxies are shown in Figure 6. It is obvious that all samples reveal high tensile strengths. Remarkably, the cured BE/MHHPA exhibits a high Young’s modulus of 2.13 GPa. It is in consistence with the tendency of storage modulus at glassy region. Furthermore, BA/MHHPA breaks at larger elongation and therefore illustrates a higher tensile toughness compared to BE/MHHPA and CE/MHHPA. It is ascribed to BA expressed linear monomer structure of flexible methylene groups26 and the BA/MHHPA exhibited lower crosslinking density as the aforementioned. Thus, the flexible chains of the BA/MHHPA led to liable mobility of the chain segments and lower space steric hindrance of the network. Herein, new fracture surfaces rapidly generated and the necessary strain energy increased for the continuation of fracture due to lower space steric hindrance on the network of BA/MHHPA43. The results were consistent with the characterization of SEM. From Table 4, it is obvious that BA/MHHPA has a higher impact strength (1.86 KJ/m2), which is evaluated better toughness compared to those of BE/MHHPA and CE/MHHPA. The higher impact strength and elongation of BA/MHHPA is ascribed to its superior toughness.


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Figure 6. Tensile stress-strain curves of cured epoxy resins.

3.6 Morphology analysis The fracture surfaces of the samples after the impact tensile test were analyzed by SEM (Figure 7). The mode of crack growth as well as deformation mechanisms were displayed on the rupture surface provide information 44. The cracks initiate at a stress concentration point (see Figure 7a, b and c), and then propagate from a slow crack growth zone (near the crack initiation point) to a rapid crack growth zone (away from crack initiation point). The roughness of the fracture surfaces is often associated with the ability of a material to resist crack propagation45-47. CE/MHHPA and BE/MHHPA show a relatively smooth surface in the fracture (see Figure 7d and f), which indicates a brittle fracture due to the poor tensile toughness compared with BA/MHHPA. The fracture surface of the BA/MHHPA shows much rough with massive embossments and a lot of branched cracks. The crack propagation indicates of increased plastic deformation and energy absorption. Therefore, the brittleness of BA/MHHPA has been reduced, which is consistent with the result of the tensile stress-strain curves.



Figure 7. SEM micrographs of tensile test specimen fracture surfaces of specimens: (a), (d) CE/MHHPA, (b), (e) BA/MHHPA, (c),(f) BE/MHHPA.

3.7 Optical properties of cycloaliphatic resins Light transmittance is a vital optical properties of encapsulation materials. Transparency has a direct effect on encapsulation efficiency and reliability of products. The optical transmittances of the cured epoxies are shown in Figure 8. BA/MHHPA expresses a higher transmittance than those of BE/MHHPA and CE/MHHPA at the wavelength of 800 nm. With the UV wavelength decreasing, the trend of transmittance change is almost the same. As the all samples show a transmittance above 86 % in the visible region of 380nm-780nm (See Table 4), indicating that the polymers possess superior light output property48. The digital photographs of all specimens are shown correspondingly in Figure 8. These digital photographs can be clearly presented through CE/MHHPA, BA/MHHPA and BE/MHHPA, which demonstrates highly transparent of samples. It is suggested that the excellent optical properties of the asprepared epoxy resin possess a promising application in electronic packing.


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Figure 8. The Ultraviolet-visible (UV-Vis) spectra curves of the cured resins and the digital photograph (a), and the samples of CE/MHHPA (b), BA/MHHPA (c) and BE/MHHPA (d) on the digital photographs, respectively.

3.8 Moisture resistance of cycloaliphatic resins Water absorption during lifetime is considered as one of the chief reasons for the separated layer in encapsulated electronics17, 49. The moisture resistance properties of cured epoxy resins were conducted by gravimetric measurement. The moisture absorption content versus time of the specimens was measured by the below equation: 𝑀𝑡 =

𝑊𝑡 ― 𝑊𝑜 𝑊𝑜

× 100%

where Wo is the original weight for the dry sample, and Wt is the measured weight for the wet sample after being wiped at different times. From Table 4, the cured CE/MHHPA has the highest water absorption about 1.37 % among the three epoxides after immersed in deionized water for 48 h. The higher cross-linking densities were caused by the shorter segmental lengths, which are easier to be opened up by the bulky and rigid nodes42, 50. In addition, the structure of high polarity is beneficial for moisture diffusion and absorption onto high polar region, and then hydrophilic nodules attract water molecules and attach them by hydrogen bonds, leading to more equilibrium water uptake17. The CE is equipped



Page 20 of 34

with high polarity of chemical structure compared with the symmetric structure of BE and BA in the view of micro-molecules. As a result, CE/MHHPA shows the highest water absorption. For the same reasons, BE/MHHPA expresses best moisture resistance performance. Table 4. Performance of the cured BE/MHHPA, BA/MHHPA and CE/MHHPA

CTE a

Impact strength

(ppm/K)

(KJ/m2)

CE/MHHPA

74

1.02

BA/MHHP

91

1.86

Sample

Transmittance (%) Water absorption (%) 380 nm

800 nm

87.3

91.5 1.37

86.9

92.1

86.9

91.7

1.33

A 1.20 BE/MHHPA a

76

1.16

Average linear CTE at 25 - 85 °C

3.9 Thermogravimetric analysis The TGA and DTG curves of the polymers under N2 atmosphere are illustrated in Figure 9 and the data are summarized in Table 5. All the samples exhibit similar decomposition profiles and the progress of degradation takes place in one stage. CE/MHHPA has slightly higher Td5 (276 °C) than that of BA/MHHPA (264 °C) in TGA curves, while BE/MHHPA has the highest Td5 (281 °C). The statistic heat-resistant index (Ts) is a popular index to evaluate the thermal stability of the epoxy resins determined by the following equation51: 𝑇𝑠 = 0.49[𝑇𝑑5 + 0.6(𝑇𝑑30 ― 𝑇𝑑5)] The Ts value (see Table 5) of BE/MHHPA is slightly higher than BA/MHHPA and CE/MHHPA, which indicates that BE/MHHPA has relatively higher thermal stability compared with other epoxy resins.


Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 5. The thermal degradation behaviours of the cured BE/MHHPA, BA/MHHPA and CE/MHHPA.

Sample

Td5a (°C)

Td30b (°C)

Tpc (°C)

Tsd (°C)

CE/MHHPA

276

350

373

157.0

BA/MHHPA

264

360

379

157.6

BE/MHHPA

281

356

376

159.7

a

5 % weight loss of temperature.

b

30 % weight loss of temperature.

c

Maximum Peak temperature at the curves of weight loss rate.

d

heat-resistant index

Figure 9. TG and DTG thermographs of CE/MHHPA, BA/MHHPA and BE/MHHPA at heating rate of 10 K/min in N2 atmosphere.

3.10 TG-IR analysis To further analyze the thermal pyrolysis process of the samples, the volatile products were investigated by TG-FTIR technique and presented in Figure 10. From Figure 10 (a),(c) and (e), the degradation of



cured polymers mainly occurred between 30 and 40 minutes (320-430 °C). For the pyrolysis process of BE/MHHPA (see Figure 10 (c) and (f)), the absorption peak locating at around 3570 cm-1 was fit well to the gas product H2O, which may be come from the decarboxylation reaction52. The peak at 3044 cm-1 can be associated with C=C-H stretching vibration. The main range of 3000-2800 cm-1 was assigned to saturated C-H53. For the CO2 absorption peak was mainly appeared at 2350 and 2306 cm-1. The volatile product of CO2 was generated through the decarboxylation reaction and the breakage of carbonyl groups. The absorption peak locating at 1870 cm-1 and 1805 cm-1 correspond to the C=O of cyclic anhydride. The peak at 1740 cm-1 is attributed to the absorption of carbonyl compounds. It mainly ascribed to the decarboxylation reaction of dicarboxylic acid. The aromatic compounds locating at1602, 1504 and 1450 cm-1,54 which were transformed by six-membered ring. The bands at 1217 and 1168 cm-1 had been related to the C-O-C stretching vibration. While the size, morphology and weight are equal to each other in tests, the intensity of the characteristic peak in the three-dimensional spectra can expression the amounts of degradation products27. It also could be seen that BA/MHHPA releases much more pyrolysis products than those of CE/MHHPA and BE/MHHPA throughout the whole thermal degradation process. The absorption peaks of BA/MHHPA resin are much higher nearby 3570 and 2350 cm-1 than those of BE/MHHPA and CE/MHHPA (Figure 10 and Figure S1). The significant difference of BA/MHHPA are attributed to the structure of type Ⅱ (Scheme 2), which occurred decarboxylation and reaction decarboxylation reaction at high temperature. In addition, the pyrolysis process for CE/MHHPA and BE/MHHPA appear slightly similar behavior. It can be explained that the chains segments with common structure of type Ⅰ.


Page 22 of 34

Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O O

NHR2 O

O O

+

O

O O O

O

BA/MHHPA

O

type I

type II

NHR2+ O

O

O O O

O

O

CE/MHHPA

O O

O

O

O

O

O

O

O

O

NHR2+

O

O

O

O BE/MHHPA

O O O

O

O

type III

Scheme 2. Schematic diagram of the different features of samples for thermal pyrolysis process.

Figure 10. Three-dimensional FTIR spectra of the gaseous volatiles evolved during the heating processes of (a) CE/MHHPA, (c) BA/MHHPA, (e) BE/MHHPA and the absorbance of pyrolysis products with different temperature of (b) CE/MHHPA, (d) BA/MHHPA, (f) BE/MHHPA.

3.11 Dielectric properties The excellent dielectric properties are essential parameters to evaluate encapsulating material12, 55. The curves of dielectric constant and dielectric loss for cured epoxy resins at ambient temperature relating to



Page 24 of 34

the frequency are showed as Figure 11. In Figure 11a, all the samples present relative stable dielectric constant at high frequency (105 Hz), demonstrating the cured resins have fairly stability of dielectric properties. It was observed that BE/MHHPA had the lowest dielectric constant and dielectric loss of the epoxy resins through the wide frequency range from 1 to 106 Hz. Attractively, the value of dielectric loss is approximately 0.008 for BE/MHHPA, which reduced about 71 % and 33.3 % compared with CE/MHHPA and BA/MHHPA, respectively (Figure 11b). Dielectric property could be explained by the Debye equation: 𝑘 ― 1 4𝜋 𝜇2 = 𝑁 𝛼𝑒 + 𝛼𝑑 + 𝑘+2 3 3𝑘𝑏𝑇

(

)

where k is the dielectric constant, N is the number density of dipoles, αe is the electronic polarization, αd is the distortion polarization, μ is the orientation polarization related to the dipole moment, kb is the Boltzmann constant, and T is the temperature. Dielectric properties are determined via the relaxation and orientation of dipoles for polymers56. Symmetric chemical structure is dulled to polarize relaxation and greater space hinder is insensitive to orientation of dipoles. For BE/MHHPA, the structure of the highly symmetric and low polarizable hexatomic ring, which linked two epoxy moieties by ester bonds and thus diminished the N. The skeleton of BE on the network decreased the electric polarizability, which corresponding to reduced αe. The steric hindrance of ortho-substituent for chain segments was apathetic in orientation of dipoles, then the μ further decreased. Therefore, the lowest dielectric constant of BE/MHHPA is ascribed to the reduced N, αe and μ. The increase of the electric polarizability is due to asymmetric chemical structure. Thus, the dielectric constant of CE/MHHPA is high. BA/MHHPA possesses symmetrical structure and minor polarity

on

the

main

chains.

However,

the

flexible

chain

segments

of

BA/MHHPA

play a fundamental role in the rapidly orientation of dipoles57. Thus, BA/MHHPA showed a higher


Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dielectric constant compared with BE/MHHPA. In general, the factors that influence the dielectric constant also affect the dielectric loss58. The relaxation happened while the polarization does not follow the change of the externally electric field, the result presented as dielectric loss. As the above discussion, the great symmetry and steric hindrance of BE/MHHPA is restrained the relaxation of the dipoles, and result in extremely low dielectric loss . The reason for the high dielectric loss may be the rapid orientation between two alicyclic rings of CE/MHHPA at the relatively short segment and the change of relaxation polarization with frequency. From these results, BE/MHHPA possess excellent dielectric properties, which could satisfy the requirement to be severed as potential electronic materials.

Figure 11. Dielectric constant (a) and dielectric loss (b) of CE/MHHPA, BA/MHHPA and BE/MHHPA at heating rate of 10 K/min in N2 atmosphere.

4. Conclusions The BE/MHHPA was successively prepared using MHHPA as curing agent. The curing kinetics and properties of BE/MHHPA was systematically compared with commercial epoxy resins. The value of reaction activation energy (Ea) is 79.76 kJ/mol for BE/MHHPA, which is higher than CE/MHHPA (57.96 kJ/mol) and BA/MHHPA (56.73 kJ/mol), respectively. The high Ea is likely to that ortho-substituted



BE/MHHPA possess a higher potential barrier of internal rotation. Moreover, BE/MHHPA showed a higher glass transition temperature, low CTE coefficient, comparable moduli, light transmittance and crosslink density than those of BA/MHHPA, which can be ascribed to the rigid structural unit of BE. Further, BE/MHHPA exhibited excellent Young’s modulus, thermal stability, moisture resistance, lowest dielectric constant, lowest dielectric loss and highest damping among three epoxy resins. The thermal degradation process of BE/MHHPA is rapid and concentrated on the temperature of 320-430 °C. In summary, the synthesized cycloaliphatic epoxy resin has the potential as packing materials with desirable thermal and mechanical properties.

Supporting Information

Kinetic parameter, kinetic model and absorbance of pyrolysis products.

Acknowledgements

This work was supported by National Key R&D Program, China (No.2017YFD0601003), Guangzhou Science and Technology Plan Project - Pearl River Technology Stars Program (No.201710010084, 201806010023) and Guangzhou Science and Technology Plan Project (201707010274).

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74x47mm (600 x 600 DPI)


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Cure kinetics and properties of high performance cycloaliphatic epoxy

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