Toughened Epoxy Filled with Ferromagnetic Particles as High ... - piers

PIERS ONLINE, VOL. 7, NO. 7, 2011

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Toughened Epoxy Filled with Ferromagnetic Particles as High Temperature Resistant Microwave Absorbing Coating Zhenjiang Song, Jianliang Xie, Jianing Peng, Peiheng Zhou, and Longjiang Deng State Key Laboratory of Electronic Thin Films and Integrated Devices University of Electronic Science and Technology of China, Jianshe Road, Chengdu 610054, China

Abstract— In this work, microwave absorbing coatings consisted of toughened epoxy and ferromagnetic particles were prepared. The epoxy groups and isocyanate groups were investigated by Fourier Transform Infrared (FTIR) spectroscopy. Mechanical properties of the coatings are effectively improved comparing with the one employing unmodified epoxy in room temperature. After heat treat at 150◦ C for approximate 100 hours, the coatings still have special impact strength between 45 and 50 Kg · cm according to Chinese National Standards GB 1732–1993 and flexibility about 2–3 mm according to Chinese National Standards GB 6742-2007. Complex permittivity ε(f ) and permeability µ(f ) of the ferromagnetic particles were measured using network analyzer in the frequency range from 2 to 18 GHz, to explain the reflection loss behavior of the coatings tested in microwave chamber. The morphology of ferromagnetic particles and their random dispersion in the coatings were observed by field emission scanning electron microscope (FESEM).

1. INTRODUCTION

Electromagnetic absorbing materials are used for a wide range of businesses [1], expecially the kind of absorbing coatings because of their simple technics and convenient operation. With good performace properties, such as size stability, creep resistance, and strength, epoxy resins is studied as an attractive material at coatings [2]. However, epoxy resins also have disadvantage at brittleness due to the reported three-dimension network [3]. Therefore, we toughened the epoxy resins with a prepolymer which were isocyanate-terminated in this work. After mixing with ferromagnetic particles, the microwave absorbing coatings are prepared. 2. EXPERIMENTAL 2.1. Materials and Fabrication of Polyurethane

Polyester with an average molecular weight of 1,000 g/mol was used to fabricate the polyurethane with 2,4-toluene diisocyanate (TDI) and pretreated by drying under a vacuum to remove moisture. Dibutyl tin dilaurate (DBTD) and other chemical reagents were used as-received. The isocyanateterminated prepolymers of polyurethane (PU) were prepared by reaction between the hydroxyl of polyester and the isocyanate (NCO) of TDI. Firstly, 100 g polyester was added into 200 g dimethylbenzene as sovent in a three-necked glass flask in nitrogen atmosphere. After that, the flask was preheated to about 60◦ C in an oil bath. Then the mixture of the calculated TDI, 0.5 g DBTD, and dimethylbenzene were poured slowly into the flask with vigorous agitation. The temperature of polymerized was controlled at about 85◦ C for two hours. The progress of the reaction was monitored by measuring the content of NCO groups in the isocyanate-terminated prepolymers according to Chinese Chemical Industry-standard HG 2409-1992. When the content of NCO was almost constant, the reaction was considered to be complete. The chemical structure of isocyanate-terminated prepolymer is shown in Chart 1. CH3

O HN

NCO

C

CH3

O O

R

O

C

NH

NCO

Chart 1 Schematic chemical structure of isocyanate-terminated prepolymer.


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2.2. Preparation of Microwave Absorbing Coatings

Diglycidyl ether of bisphenol A epoxy resin (EP) having an epoxy value of about 0.02–0.04eq./l00 g was a commercial product(supplied by Wuxi Resin Factory of Blue Star New Chemical Materials Co., Ltd.). Before used, the solid EP was dissolved in the mixture of butanone and cyclohexanone. Microwave absorbing coatings was prepared by incorporating 80 wt.% ferromagnetic particles into the compound of EP and PU. The mixtures were poured into a mold and cured at room temperature for seven days. The spcimen was cut to a thin sheet with thickness of 3.5 mm, and then processed into cylindrical toroidal shape with inner diameters 3.00 mm and outer diameters 7.00 mm, for relative complex permittivity (εr = ε0 − jε00 ) and permeability (µr = µ0 − jµ00 ) test using the transmission/reflection coaxial line method. For mechanical properties test, the mixtures were coated on a 5 mm × 5 mm metal plate for impact strength and adhesion test, and a 5 mm × 12 mm rectangular sectors for flexibility test through an air compressor by atomizing method. 2.3. Characterization

Fourier transform infrared (FTIR) spectra were obtained using Bruker Tensor 27 at transmission mode in the spectral range from 4000 to 400 cm−1 . Firstly, the powder KBr was compressed into a pellet, then the liquid polmer were directly daubed onto the KBr pellet. The ferromagnetic particles and their random dispersion in the coatings were observed by field emission scanning electron microscope (Model JSM-7600F, JEOL, Tokyo, Japan). The moisture attaching to the ferromagnetic particles was analysed by thermo-gravimetric analyses (TG, Netzsch STA 449) carrying out from 40◦ to 800◦ with nitrogen protection, flow rate 22 ml/min, at the heating rate of 10 K/min · PU, EP, and toughened EP were examined by differentical scanning calorimetry (DSC) also using STA 449 at the heating rate of 10 K/min under nitrogen atmosphere. Agilent 8720 ET network analyser was used to measure the spcimen of cured coating within the frequency range of 0.5–18 GHz. Impact strength was tested according to Chinese National Standards GB 1732-1993 determination of impact resistance of film. Flexibility was tested according to Chinese National Standards GB/T 6742-2007 paints and varnishes-bend test (cylindrical mandrel). Adhesion was tested according to Chinese National Standards GB/T 5210-2006 paints and varnishes-pull-off test for adhesion. 3. RESULT AND DISCUSSION 3.1. The Chemical Structure of PU and the Toughened EP with PU

In Fig. 1, FTIR spectrum of NCO shows a broad and intense absorption at around 2270 cm−1 . Besides, the N-H bands are characterized at 3298 cm−1 and 1536 cm−1 . The band around 1109 cm−1 is attributed to the vibration of eater -C-O. The C = O from the chemical reaction of NCO and OH are observed at 1730 cm−1 [4, 5]. Consequently, the desired preplymer is obtained (Chart 1). From Fig. 2, we could speculate the main chemical strcture of the toughened EP. At the prime phase of mixing EP with PU, epoxy characteristic absoption at 910 cm−1 and absorption

Figure 1: FTIR spectrum of the polyurethane compose.

Figure 2: FTIR spectrum of the mixture of EP and PU.


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at 2270 cm−1 belonging to NCO group are distinct shown in Fig. 2. In the inset of Fig. 2, FTIR spectrum of the wavenumbers range of 850 to 1900 cm−1 are given.After toughen process finished, the FTIR spectrum exhibits an intensive absorption band of urethane group at 1730 cm−1 , with a drop of NCO group absorption at 2270 cm−1 , and the pendant secondary hydroxyl groups (-OH) absorption on the epoxy resin spectrum at 3470 cm−1 shifted to lower wavenumber, like the absorption band of N-H at 3298 cm−1 [6]. The epoxy characteristic absorption at 910 cm−1 also shifts to a lower wavenumber. The epoxy cycle does not react with NCO groups without a catalyst [6]. However, catalyst DBTD engaged in the prepared process of PU prepolymer may survive, so the reaction of oxazolidone formation may occure, and the main chemical groups reaction are shown in Chart 2. 3.2. The Microstructure of the Microwave Absorbing Coatings and Its Molecular Level Explain of High Temperature Resistance

Figure 3 shows the random dispersion of particles in as-prepared coatings. In Fig. 4, from DSC curve of the toughened EP, the exothermal peak in the range of 150–270◦ belongs to the reaction of oxazolidone formation, which is the chemical structure for high temperature resistance [7]. Different from conventional epoxy resins which is rather brittle resulted from the three-dimension network, our EP toughened with PU exhibites excellent flexibility due to its semi-IPN stucture [8]. We have tested the mechanical properties of the coatings, and listed in Table 1. Cured at room temperature for seven days, the coatings show good mechanical properties. Even high temperature treated, the coatings prossess promising flexibility based on its molecular level thermal stability.

O R NCO

HO

R'

R NH C O

R'

O O R CH CH2 OCN R'

C O

R' N

R CH CH2

Chart 2 Urethane formation and oxazolidone formation.

Figure 3: SEM of coating section.

Figure 4: DSC of EP,PU,and the mixture of EP and PU.

Table 1: Mechanical properties of the microwave absorbing coatings. Coatings Treated coatings

Impact strength /Kg · cm 50 50

Flexibility /mm 2 2–3

Adhesion /MPa 14.78 ——

Coatings: cured at room temperature for seven days Treated coatings: high temperature treated at 150◦ for 80 hours after cured completely.


(a)

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(b)

Figure 5: Electromagnetic parameters of the coatings: (a) complex permittivity ε; (b) complex permeability µ.

Figure 6: Reflection loss plot of coatings with thickness at 0.80 mm and 0.90 mm. 3.3. The Microwave Absorbing Property of the Microwave Absorbing Coatings

Based on the complex permittivity ε(f ) and permeability µ(f ) of the testing sample, we can obtain the reflection loss RL of this microwave absorbing coatings. The following equation is given for a single-layer absorbing layer [9]: RL (dB) = 20 log | (Zin − Z0 ) / (Zin + Z0 ) | p √ Zin = Z0 (µr /εr ) tanh {j (2πf t/c) µr εr }

(1) (2)

where Z0 is the impedance of free space, Zin is the input impedance; µr and εr are the the relative complex permeability and permittivity of the absorbing layer, respectively; f is the frequency of the electromagnetic wave; t is the coatings thickness; c is the velocity of light. The complex permittivity and permeability of the spcimen of the coatings are shown in Fig. 5. Using the Equation (1), (2) and the parameters tested aboved, the RL can be evaluated as shown in Fig. 6. Microwave absorbing coatings with 0.80 mm and 0.90 mm thickness are designed. The RL values exceeding −6 dB are achieved in the frequency range of 8 GHz to 18 GHz with the thickness 0.9 mm. So our prepared coatings may be applied as microwave absorbers for X- and Ku-band frequencies. 4. CONCLUSION

The toughened EP with PU are prepared and characterized by structural, compositional andmorphological tests. Coatings with good mechanical properties are obtained, expecial at high temperature resistance. Molecular level reasons for aboved merits of designed coatings are given. Electromagnetic parameters and RL of the microwave absorbing coatings indicatepotential in practical application. This kind of coatings has light weight as well as excellent impact strength, flexibility, and adhesion.


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