Enhanced Electrical Properties of Epoxy Resin with High Adhesion

In search of small dielectric constant (Dk) and low-dissipation (Df) energy substrates for high-frequency appliances, a benzoate-group-substituted bis...
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Enhanced Electrical Properties of Epoxy Resin with High Adhesion Eun Yong Lee,†,‡,§ Il Seok Chae,§,⊥ Jinkee Hong,*,∥ and Sang Wook Kang*,† †

Department of Chemistry, Sangmyung University, Seoul 110-743, Republic of Korea Shin-A T&C, #508 Byucksan-Kyungin Digital Valley II, 481-10 Gasan-Dong, Geucheon-gu, Seoul, Republic of Korea ⊥ WCU Program Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea ∥ School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 156-756, Republic of Korea ‡

S Supporting Information *

ABSTRACT: In search of small dielectric constant (Dk) and low-dissipation (Df) energy substrates for high-frequency appliances, a benzoate-group-substituted bisphenol A based resin was synthesized from bisphenol A dibenzolate and bisphenol A diglycidyl ether. Compared to the common bisphenol A diglycidyl ether epoxy resin, introduction of the benzoate group was considered to lead to increased hydrophobic character, which was supported by water absorption investigation as well as absorption peak investigation of the OH region via Fourier transform infrared spectra. The cured resin with few water molecules exhibited restricted motion of the segment, and, consequently, thermally stable properties (coefficient of thermal expansion and thermogravimetric analysis) were achieved. Ultimately, the developed epoxy resin showed dramatically reduced Dk (3.05 at 1 GHz) and Df (0.016 at 1 GHz) values as well as enhanced adhesive properties. The excellent overall properties lead to its promising use in various fields involving electrical devices.

1. INTRODUCTION Epoxy resin has been widely utilized as a representative thermosetting plastic for applications such as marine paints, precoated metals, composites, and adhesives.1−11 Recently, there has been increasing interest in high-performance epoxy resin because of its application in electronic products such as printed circuit boards, epoxy-molding compounds, and encapsulants. The growing demands for high-integrated and high-frequency electronics have accelerated efforts to develop epoxy resin for high electric insulation, which is typically evaluated in terms of the dielectric constant (Dk) and lowdissipation (Df) values.12,13 Also, the high adhesion properties of epoxy resin have played an important role for highperformance electronic devices utilizing multilayers. Thus, the development of epoxy resin has been continuously faced with not only high adhesion properties but also acceptable electrical properties. To enhance such physical properties, the introduction of specific chemical structures, such as dicyclopentadiene, biphenyl, naphthalene, and anthracene, into epoxy resin has been explored over the past decade.14−17 However, even though these chemical structures gave rise to low Dk, low Df, low coefficient of thermal expansion (CTE), high mechanical properties, and low water absorption, they have the disadvantage that the cured matrix can be relatively brittle.18 On the other hand, bisphenol A diglycidyl ether and rubber (CTBN, nitrile−butadiene rubber, and dimer) were polymerized to achieve high adhesion properties.19,20 However, even though the secondary alcohol that was formed enhanced the adhesion properties, the other properties such as electrical properties (Dk and Df), CTE, and mechanical properties were decreased by both the hydrophilic properties of the alcohol and hydrogen bonding. For these reasons, the design of a highperformance epoxy resin is one of urgent problems. © 2013 American Chemical Society

In this paper, we report on the benzoate-group-substituted bisphenol A based resin from the commercially available bisphenol A (Scheme 1). The introduction of the benzoate group to the main chain, instead of the hydroxyl group in the common bisphenol A diglycidyl ether epoxy resin, could lead to not only a physical cross-linking effect but also apparently increasing hydrophobic nature. Therefore, the developed epoxy resin showed an enhanced performance, i.e., low Dk, low Df, low CTE, low water absorption, high mechanical properties, and high adhesion properties.

2. EXPERIMENTAL SECTION 2.1. Materials. Bisphenol A diglycidyl ether (SE-187) was supplied from our commercial product line from Shin-A T&C Co. Bisphenol A, benzoyl chloride, and methyl ethyl ketone were obtained from Sigma-Aldrich. Phenol Novolac Hardener (softening point 110 °C, Kangnam Chemical) was used as a curing agent. 2-Ethyl-4-methylimidazole was utilized as a catalyst. All chemicals were used as received without further purification. 2.2. Preparation of Poly(bisphenol A-co-benzoatesubstituted epichlorohydrin), Glycidyl End-Capped (2). Benzoate-substituted bisphenol A based resin was prepared from bisphenol A diglycidyl ether and bisphenol A dibenzolate (1), as shown Scheme 1. Compound 1. Bisphenol A (500 mg, 2.19 mmol) was dissolved in methyl ethyl ketone (4 mL) as a solvent with 2ethyl-4-methylimidazole as a catalyst. Benzoyl chloride (508 μL, 616 mg, 4.38 mmol) was loaded into the reaction vessel, and Received: Revised: Accepted: Published: 15713

April 30, 2013 October 7, 2013 October 11, 2013 October 11, 2013 dx.doi.org/10.1021/ie401372q | Ind. Eng. Chem. Res. 2013, 52, 15713−15717

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Scheme 1. Preparation of 1 and Epoxy Resins 2 and 3

2.3. Measurements. 1H NMR spectra were recorded on a Bruker AVANCE 600 spectrometer with chemical shifts downfield from tetramethylsilane (TMS) as the internal standard. The epoxy equivalent weights were determined by the HBr−acetic acid method.22 IR measurements were performed on a 6030 Mattson Galaxy Series Fourier transform infrared (FT-IR) spectrometer; 64−200 scans were signalaveraged at a resolution of 2 cm−1. IR spectroscopic characterization was performed using a pressure cell equipped with quartz and CaF2 windows. CTE (based on ASTM E831) and thermogravimetric analysis (TGA) were performed with Mettler Toledo thermomechanical analysis (TMA) and TGA devices at a heating rate of 10 °C/min, respectively. The adhesion properties were measured by Automatic Mounting Press equipment (SimpliMet 1000, Buehler). Dk and Df were measured by an RF impedance material analyzer (Agilent Co., E4991A, based on JIS-C-6481).

the reaction was allowed to proceed for 10 h. After water was added to the reaction vessel, 1 precipitated. The resulting white solid was filtered and rinsed with cold methanol and dried in an oven. 1H NMR (CDCl3, 600 MHz, ppm, TMS): δ 1.73 (s, 3H of −CH3), 7.16 (d, 2H of benzolate), 7.33 (d, 2H of benzolate), 7.51 (t, 2H of benzene), 7.62 (t, 1H of benzene), 8.17 (d, 1H of benzolate). 13C NMR (CDCl3, 150 MHz, ppm, TMS): δ 165.2, 150.1, 148.8, 134.2, 130.8, 128.9, 122.0, 67.4, 25.3. Compound 2. 2 was prepared from 1 (7.4 g, 17 mmol) and bisphenol A diglycidyl ether (5 mL, 5.8 g, 17 mmol) via the transesterfication reaction in the presence of an imidazole catalyst, 2-ethyl-4-methylimidazole, and the reaction was allowed to proceed at 150 °C for 8 h. After reaction, the 1H signal of the carbinol methine proton was observed at about 5.6 ppm. This result was in agreement with the reaction between trichloroacetyl isocyanate and hydroxyl groups incorporated in the epoxy resin by Bogdal and Gorczyk.21 Also, the same diffusion coefficient was found out in the 1H peaks of two benzene groups (benzene, a; benzoate, b) in diffusion-ordered spectroscopy (DOSY), indicating that different benzene groups exist in the same polymer chain. Therefore, we could conclude that 2 were successfully synthesized. (see the Supporting Information for detailed NMR spectra). 1H NMR (CDCl3, 600 MHz, ppm, TMS): δ 1.60 (12H of −CH3), 2.78 (2H of −CH2 in oxirane), 2.86 (2H of −CH2 in oxirane), 3.37 (2H of −CH in methine), 3.94 (2H of −CH2O), 4.19 (2H of −CH2O), 4.36 (d, 2H of methylene), 5.63 (1H of methine), 6.81 (8H of benzene), 7.16 (8H of benzene), 7.21 (2H of benzoate), 7.57 (1H of benzoate), 8.02 (2H of benzoate). 13C NMR (CDCl3, 150 MHz, ppm, TMS): δ 166.0, 157.5, 134.0, 130.8, 130.4, 130.2, 129.3, 128.4, 114.7, 72.1, 69.9, 67.2, 50.5, 44.3, 42.2, 31.3. The equivalent weight values of 2 were 326.0−332.7 g/ equiv. Poly(bisphenol A-co-epichlorohydrin), glycidyl end-capped (3), was prepared from bisphenol A diglycidyl ether and bisphenol A and utilized as a comparison material. Curing 2 and 3 Resins. Phenol Novolac hardener was used as the curing agent with 2E4MZ.

3. RESULTS AND DISCUSSION Figure 1 shows the FT-IR spectra for epoxy resins prepared as described above. The sp3 stretching bands of the C−H and

Figure 1. FT-IR spectra of epoxy resins (a) 2 and (b) 3. 15714

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the main chain for epoxy resin 3 caused the motion of the segments to be increased, while it was restricted for epoxy resin 2, resulting in a decrease of the CTE value. To compare the thermal stability for epoxy resins 2 and 3, TGA was performed at a heating rate of 10 °C/min. The weight change with increasing temperature was observed for epoxy resins 2 and 3 in Figure 3. About 5% weight loss, the temperature for epoxy resin 3 was 389 °C and that for epoxy resin 2 386 °C. These data indicate that the effect of the introduction of the benzoate group on the thermal stability is negligible. Even though the glass transition temperature was decreased for epoxy resin 2, its thermal stability was unchanged because of the increased cross-linking density between the benzoate group and other polymer chains. For the applications of electronics, it was well-known that the Dk and Df values are very important factors. Thus, the electrical properties for epoxy resins 2 and 3 were determined by measuring their dielectric constant and dissipation factor at a frequency of 1 GHz. As shown in Table 1, in the case of the cured sample of epoxy resin 3, its Dk and Df values were 3.35 and 0.03, respectively. On the other hand, in the case of the cured sample of epoxy resin 2, its Dk and Df values were 3.05 and 0.016, respectively. The decreased dielectric constant of epoxy resin 2 was attributed to its increased hydrophobic character caused by substitution of the benzoate group in its main chain because it is known that the Dk value of water is 78.23 Furthermore, the decreased Df value could be explained by the decreased dielectric constant for epoxy resin 2 because the Df value is strongly correlated with Dk. From these results, epoxy resin 2, which was prepared for the purpose of achieving enhanced electrical properties, would be expected to be applicable to various applications employing electrical devices. For measurement of the adhesion properties for epoxy resins 2 and 3, automatic mounting press equipment was utilized. The molding process was performed at 80 bar and 180 °C for 20 min, and then the sample was cured with a curing agent and a catalyst at 180 °C for 2 h followed by cooling for 10 min. As shown in Figure 4a, for the case of the sample cured with epoxy resin 3, the upper side was clearly chipped off without adhesion. On the other hand, Figure 4b shows that when the upper side of the cured sample of epoxy resin 2 was chipped off, it was broken into various pieces, meaning that the adhesion properties were enhanced.

CC groups for epoxy resins 2 and 3 were observed at around 2900 and 1610 cm−1, respectively. The characteristic absorption bands of the glycidyl group of epoxide were observed at 910 and 835 cm−1 in addition to the absorption peaks at 3048 and 1178 cm−1, indicating the presence of an aromatic ring. Distinguishingly, the carbonyl peak was observed at 1722 cm−1 for epoxy resin 2, in addition to various peaks indicating the presence of the glycidyl group of epoxide and aromatic ring. In particular, the absorption peak of OH appeared at around 3400 cm−1 for epoxy resin 3, remarkably decreased when the OH peak was replaced by the benzoate group in epoxy resin 2, indicating that the effect of OH was negligible for epoxy resin 2. To confirm the effect of the OH groups on the water absorption values for epoxy resins 2 and 3, the weight changes were measured before and after dipping them into boiling water. As described in Table 1, the water absorption (wt %) was Table 1. Physical Properties of Epoxy Resins 2 and 3 CTE (ppm) 5% weight loss temp (°C) water absorption (wt %) Dk [1 GHz] Df [1 GHz]

2

3

58.95 386 0.911 3.05 0.016

98.18 389 1.277 3.35 0.03

1.277 for epoxy resin 3 because of rich OH groups in its main chain. On the other hand, when 1 was measured, its water absorption (wt %) was 0.911, indicating that the effect of the OH groups is negligible because of substitution of the benzoate group. To investigate the dimensional changes for epoxy resins 2 and 3, the CTE was measured as a function of the temperature at a heating rate of 10 °C/min, as shown in Figure 2. The CTE α value was calculated as follows: α=

1 ΔL L0 ΔT

where L0 is the initial length, ΔL is the change in length, and ΔT is the temperature range. The calculated CTE values for epoxy resins 2 and 3 were 98.18 and 58.95 ppm, respectively. This decrease in the CTE value for epoxy resin 2 was attributable to the decreased motion of the segments of the macromolecules. The water absorption by the OH groups in

Figure 2. TMA of (a) epoxy resin 2 and (b) 3. 15715

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Figure 3. TGA of (a) epoxy resin 2 and (b) 3.

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ACKNOWLEDGMENTS This research was supported by a 2013 Research Grant from Sangmyung University.

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(1) Lin, C. H.; Chiang, J. C.; Wang, C. S. Low dielectric thermoset. I. Synthesis and properties of novel 2,6-dimethylphenol−dicyclopentadiene epoxy. J. Appl. Polym. Sci. 2003, 88, 2607. (2) Lin, C. H.; Wang, C. S. Novel phosphorus containing epoxy resins I. Synthesis and property. Polymer 2001, 42, 1869. (3) Jin, F. L.; Park, S. J. Thermal properties of epoxy resin/filler hybrid composites. Polym. Degrad. Stab. 2012, 97, 2148. (4) Li, C.; Zuo, C.; Fan, H.; Yu, M.; Li, B. Novel silicone aliphatic amine curing agent for epoxy resin: 1,3-Bis(2aminoethylaminomethyl)tetramethyldisiloxane. 1. Non-isothermal cure and thermal decomposition property. Thermochim. Acta 2012, 545, 75. (5) Chen, X.; Lu, Y.; Zhang, X.; Zhao, F. The thermal and mechanical properties of graphite foam/epoxy resin composites. Mater. Des. 2012, 40, 497. (6) Sinh, L. H.; Son, B. T.; Trung, N. N.; Lim, D.-G.; Shin, S.; Bae, J.Y. Improvements in thermal, mechanical, and dielectric properties of epoxy resin by chemical modification with a novel amino-terminated liquid-crystalline copoly(ester amide). React. Funct. Polym. 2012, 72, 542. (7) Chen, Y.-J.; Yang, C.-Z.; Guo, Z.-H.; Zhang, Y.; Cheng, J.; Fang, Z.-P. Irradiation treatment of multiwalled carbon nanotubes and its effect on the thermal properties of their composites with epoxy resin. Carbon 2012, 50, 3119. (8) Łukaszczyk, J.; Janicki, B.; Kaczmarek, M. Synthesis and properties of isosorbide based epoxy resin. Eur. Polym. J. 2011, 47, 1601. (9) Wan, J.; Bu, Z.-Y.; Xu, C.-J.; Li, B.-G.; Fan, H. Preparation, curing kinetics, and properties of a novel low-volatile starlike aliphaticpolyamine curing agent for epoxy resins. Chem. Eng. J. 2011, 171, 357. (10) Xu, Y.; Peng, H.; Wang, X.; Su, S. Comparative study of different polymerically-modified clays on curing reaction and thermal properties of epoxy resin. Thermochim. Acta 2011, 516, 13. (11) Xing, W.; Jie, G.; Song, L.; Wang, X.; Lv, X.; Hu, Y. Flame retardancy and thermal properties of epoxy acrylate resin/alphazirconium phosphate nanocomposites used for UV-curing flame retardant films. Mater. Chem. Phys. 2011, 125, 196. (12) Lubin, G., Ed. Handbook of composites; Van Nostrand Reinhold: New York, 1982. (13) Dusek, K. Epoxy Resins and Composites. I. Adv. Polym. Sci. 1985, 78, 15. (14) Hsiue, G.-H.; Wei, H.-F.; Shiao, S.-J.; Kuo, W.-J.; Sha, Y.-A. Chemical modification of dicyclopentadiene-based epoxy resins to

Figure 4. Adhesion properties of epoxy resins (a) 3 and (b) 2.

4. CONCLUSIONS The benzoate-substituted epoxy resin was successfully prepared from 1 and bisphenol A diglycidyl ether, resulting in low Dk, low Df, low CTE, low water absorption, high mechanical properties, and high adhesion properties. The synthesized epoxy resin showed relatively low water absorption (wt %) compared with the corresponding epoxy resin 3. The decreased water absorption restricted the motion of the segments of the polymer chain, resulting in a decreased CTE value. Furthermore, the low water absorption caused the Dk and Df values to be decreased because, when water was incorporated, the Dk and Df values increased. These electrical properties along with enhanced adhesion properties are expected to allow epoxy resin 2 to be used in various fields involving electrical devices.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR for 2 and 2D DOSY. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Tel: +82 2 2287 5362. Fax: +82 2 2287 5362. E-mail: [email protected]. Author Contributions §

E.Y.L. and I.S.C. contributed equally as the first authors.

Notes

The authors declare no competing financial interest. 15716

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