J Mater Sci: Mater Electron (2016) 27:4344–4350 DOI 10.1007/s10854-016-4302-2

Preparation and properties of Barium titanate (BaTiO3) reinforced high density polyethylene (HDPE) composites for electronic application Jun Su1,2 • Jun Zhang1

Received: 7 October 2015 / Accepted: 6 January 2016 / Published online: 11 January 2016 Ó Springer Science+Business Media New York 2016

Abstract In this study, 3-Aminopropyltriethoxysilane (KH550) with polar amino groups is applied to modify the surface of Barium titanate (BaTiO3) particles. When volume fraction of BaTiO3 further increases to 50 vol%, the dielectric constant and loss of HDPE control increase from 2.5 and 0.02 to 18.5 and 0.10 at 10 MHz, respectively, with the volume resistivity of HDPE control decreasing from 3.6 9 1013 X m to 3.5 9 1011 X m. The increased amount of modified BaTiO3 can increase the flexural strength and modulus HDPE composites from 8 and 90 MPa to 14 and 780 MPa, respectively. However, the increase of modified BaTiO3 can restrict mobility of HDPE chains and decrease the crystallinity of HDPE composites.

1 Introduction The dielectric polymer/ceramic composite is one kind of the fastest growing materials in electric domain [1–4]. Taking fluoride polymer/ceramic for example, because of its polar and rigid structure in polymer matrix, it is commonly used to fabricate embedded capacitors in electronic devices [5]. The fluoride groups not only endow fluoride polymer/ceramic with electrical conductivity, but also raise the viscosity during manufacturing [6].

& Jun Zhang [email protected] 1

Department of Polymer Science and Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, People’s Republic of China

2

College of Mechanics Engineering, Nanjing Institute of Industry Technology, Nanjing 210023, People’s Republic of China

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High density polyethylene (HDPE) is one of the most commonly used general plastics with high crystallinity structure, which can improve the overall tensile strength and dielectric properties of HDPE to some extent [7]. Moreover, the crystal lattices can melt at processing temperature to decrease the viscosity of HDPE composites, improving the processability [8]. In addition, the non-polar molecular chains in HDPE can endow it with good resistance to oxidation and solvents, low dielectric constant and loss [9]. This can broaden the application of HDPE as embedded capacitors and electronic packaging materials. Apart from carbon nanotubes, carbon fibers and carbon blacks [12], Barium titanate (BaTiO3) possesses perovskite structure and is one kind of ceramic materials with high dielectric constant [10]. It also has the disadvantage of high dielectric loss [11]. Another drawback of BaTiO3 is the brittleness, which restricts the applications [13]. It is interesting to find whether the incorporation of BaTiO3 into HDPE matrix can overcome individual drawbacks of BaTiO3 and HDPE, making a composites with high dielectric strength and flexural strength. The interface between BaTiO3 and HDPE is not only significant to the mechanical and electrical properties of HDPE composites, but also critical to the viscosity of composites during manufacturing process [14–16]. In this study, BaTiO3 particles are firstly modified by coupling agent KH550 to improve the adhesion between BaTiO3 and HDPE. Then the varied amount of modified BaTiO3 are added into HDPE matrix. The aim is to evaluate the threshold of rheological and electrical properties of HDPE/ BaTiO3 composites and to investigate how overall properties of composites are affected by the increase of BaTiO3 content. The characterizations included Fourier Transform Infrared (FT-IR), Differential Scanning Calorimetry (DSC), resistivity, rheological, mechanical and dielectric properties.

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2 Experimental

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2.3.3 Differential scanning calorimetry (DSC) measurement

2.1 Materials High density polyethylene (HDPE 4902T and HDPE 6218), used in this study, is purchased from Sinopec Yangzi Petrochemical Cooperation Limited, China. Barium titanate (BaTiO3) particles are supplied by Songbao Electrically Functional Cooperation Limited. in Foshan, Guangdong province. The chemical name of the coupling agent KH550, supplied by Nanjing Shuguang Chemical Group Co. Ltd, is 3-Aminopropyltriethoxysilane and the structure is given below: 2.2 Sample preparation 2.2.1 Surface modification of BaTiO3 In this work, KH550 was used for surface treatment of the BaTiO3 particles. The content of KH550 was 1 % by weight (wt%) of BaTiO3 amount. The procedure is according to literature [17]. Then the modified BaTiO3 are extracted by diethyl ether, using soxhlet extractor. 2.2.2 Compounding of HDPE/BaTiO3 HDPE 4902 and HDPE 6218 are firstly blended at the ratio of 3:1. The obtained HDPE blends and BaTiO3, modified by KH550, are mixed by a two roll mixing mill purchased from Shanghai Rubber Machinery Works, China), respectively. The basic formulations of HDPE composites are HDPE blends 100 phr. The volume fraction of BaTiO3 in HDPE/ BaTiO3 composites is about 10, 20, 30, 40 and 50 vol%, respectively. The gained HDPE composites were heatly pressed at 170 °C.

The melting behaviors of HDPE composites are characterized by DSC instrument (model: Q20, TA, USA). Samples are firstly heated from 30 to 180 °C at 40 °C/min, and then are held at 180 °C for 5 min to eliminate the effect of thermal history. In the next, the samples are cooled from 180 to 30 °C at 10 °C/min. Finally, the samples are heated again from 30 to 180 °C at 10 °C/min. The relative crystallinity (Xc) of the samples was calculated according to literature [7]. 2.3.4 Scanning electron microscopy (SEM) The dispersion of filler is observed with scanning electron microscope (model: JEOL JSM-5900). 2.3.5 Mechanical properties According to ISO 180, the impact strength is tested by Izod impact tester (model UJ-4), purchased from Chengde Machine Factory. According to ISO 178, flexural properties are tested by testing machine (model CMT 5254 type), purchased from Shengzhen SANS Testing Machine Co., Ltd. 2.3.6 Electric properties 2.3.6.1 Volume and surface resistivity According to IEC 60093, the volume and surface resistivity of HDPE composites are measured by using a high-insulation resistance meter, purchased from Shanghai Precision & Scientific Instrument Co., Ltd., China. 2.3.6.2 Dielectric constant and dielectric loss tangent The dielectric constant and dielectric loss tangent of HDPE composites are measured by Impedance Analyzer (model: Agilent 4294A), according to IEC 60250.

2.3 Testing procedures

3 Results and discussion 2.3.1 Fourier transform infrared spectroscopy (FT-IR) 3.1 FT-IR analysis Fourier transform infrared spectra of un-modified and KH550 modified BaTiO3 particles are obtained by Nicolet spectrometer (model NEXUS 670 technique). 2.3.2 Rheological characterization Rheological properties of HDPE composites are tested at 170 °C by Rheo-Stress instrument (model MCR302), obtained from Anton Paar, Austria.

FT-IR spectra are applied to detect functional groups on BaTiO3 particles. Figure 1 shows the spectra of un-modified BaTiO3, KH550 and modified BaTiO3 particles after extraction. In Fig. 1, it is observed that coupling agent KH550 absorbs at 2922 and 2855 cm-1, assigned to asymmetric stretching vibration of methylene and symmetric stretching vibration of methylene, respectively [8, 17]. After extraction by diethyl ether, modified BaTiO3 has

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Fig. 1 Spectra of BaTiO3 before modification and after extraction

absorption peak at 2922 and 2855 cm-1, confirming the chemical adherence of KH550 to the surface of BaTiO3 [14, 18, 19]. It is shown in Scheme 1 that KH550 has alkoxy groups, which can alcoholize with hydroxyl on surface of inorganic particles and form chemical bonds between surface of particles and KH550 [17]. 3.2 Effect of BaTiO3 loading on rheological properties of HDPE/BaTiO3 composites Figure 2 illustrates the complex viscosity, storage modulus (G0 ), G0 /loss modulus (G00 ) ratios and Van Gurp plots of HDPE/BaTiO3 composites. The Van Gurp plot is the function of the loss phase angle d dependence on the magnitude of the corresponding absolute value of the complex modulus |G*| [20]. In Fig. 2a, b, complex viscosity decreases with the increasing shear rate, while the G0 of HDPE/BaTiO3 samples ascend. The results of complex viscosity and G0 at the same shear rate follow the order: HDPE control \ HDPE with 10 vol% modified BaTiO3 \ HDPE with 20 vol% modified BaTiO3 \ HDPE with 30 vol% modified BaTiO3 \ HDPE with 40 vol% modified BaTiO3 \ HDPE with 50 vol% modified BaTiO3. Commonly, the random coils in HDPE can be unwrapped with the increasing shear rate [21]. It is shown in Fig. 2a that at higher shear rate (10–100 s-1), when volume fraction of BaTiO3 reaches 40 vol%, the complex viscosity of HDPE/

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BaTiO3 begins to be non-sensitive to the variation of shear rate and decreases slowly with the increasing shear rate. The reason is that the polar groups on BaTiO3 surface can interact with HDPE chains and delay the response of HDPE chains to the variation of shear rate, making themselves less sensitive to the variation of shear rate. In Fig. 2c, it is shown that when BaTiO3 loading is lower than 50 vol%, The G0 /G00 of samples are all lower than 1 at lower shear rate and exceed 1 at higher shear rate, showing the transition from liquid-like state to solid-like state [22]. What’s more, the rheological transition points shift to lower shear rate with the increasing of BaTiO3 content. Only G0 /G00 of HDPE with 50 vol% BaTiO3 is higher than 1 over the testing shear rate, showing solid-like behavior. These phenomena are explained by the fact that more modified BaTiO3 particles have stronger interaction with HDPE matrix, hindering HDPE chains in response to higher shear rate. Figure 2d shows the Van Gurp plot of HDPE/BaTiO3 composites [20, 22]. It is shown that with the increase of BaTiO3 fraction, Van Gurp curves can shift to higher value of |G*|. HDPE with higher volume fraction of BaTiO3 needs higher |G*| to obtain the phase angle, meaning more interacted structures formed with the increased amount of BaTiO3 particles. 3.3 Effect of BaTiO3 loading on DSC characteristics of HDPE/BaTiO3 composites Table 1 shows the effect of BaTiO3 loading on DSC characteristics of HDPE/BaTiO3 composites. It is found that the onset melting temperature (Tmonset ), the peak melting temperature (Tmpeak ) and final melting temperature (Tmfinal ) of HDPE/BaTiO3 composites are not influenced by the addition of BaTiO3. By comparison, the relative crystallinity (Xc) and enthalpy of fusion (DH) of HDPE/BaTiO3 composites decrease with the increased BaTiO3 loading. Generally, crystallinity is determined by the chain’s ability to pack into crystal lattices [8]. The polar amino groups on the surface of BaTiO3 particles can restrict the motion of HDPE chains and hinder the pack of HDPE chains into crystal lattices, leading to the descent of DH and Xc [7]. 3.4 Effect of BaTiO3 loading on electrical properties of HDPE composites 3.4.1 Effect of BaTiO3 loading on dielectric properties of HDPE composites

Scheme 1 Molecular structure of KH550

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The variation of dielectric constant and loss at different frequency are listed in Figs. 3 and 4. It is observed that

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Fig. 2 Rheological properties of HDPE/BaTiO3 composites: a complex viscosity; b storage modulus; c curves of G0 /G00 ratios; d Van Gurp plot

Table 1 DSC characteristics of HDPE composites

Sample (vol%)

Tmonset (°C)

Tmpeak (°C)

Tmfinal (°C)

DH (J/g)

Xc (%)

0

120.9

133.5

140.8

185.5

66.9

10

122.6

133.0

140.0

112.0

64.7

20

121.3

132.0

137.2

76.7

60.9

30

122.1

132.2

136.7

54.4

54.9

40

122.7

132.7

135.9

38.5

47.2

50

123.0

131.2

134.7

26.2

Tmonset

Tmpeak

onset melting temperature, peak position in melting temperature range, temperature, DH enthalpy of fusion, Xc relative crystallinity

with the increase of BaTiO3 content, HDPE with 30 vol% BaTiO3 has a remarkable increase of dielectric constant and loss, meaning the threshold of dielectric properties of HDPE/BaTiO3 is 30 vol%. The dielectric threshold is lower than rheological threshold. It is observed in Scheme 2a, the crystals melts at the testing temperature of rheological properties (170 °C). It is shown in Scheme 2b that there is extra interfacial polarity between crystal lattices and amorphous phase at 24 °C. So the overall polarity in HDPE composites is boosted, reducing the threshold of dielectric constant and loss of HDPE/BaTiO3 composites [23, 24]. Moreover, the further addition of 50 vol% BaTiO3 increases the dielectric constant and loss of HDPE control from 2.5 and 0.02 to 18.5 and 0.10 at 10 MHz. BaTiO3 possesses high dielectric constant and loss, while HDPE exhibits low dielectric loss [25–27]. It has been confirmed

37.8 Tmfinal

final melting

Fig. 3 Dielectric constant of HDPE composites at different frequency

that the addition of 50 vol% BaTiO3 can boost the overall dielectric constant of HDPE composites, without raising dielectric loss of HDPE composites greatly.

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Fig. 4 Dielectric loss of HDPE composites at different frequency Fig. 5 Cole–Cole cycles of HDPE composites Scheme 2 Models of 30 vol% BaTiO3 in HDPE matrix: a at testing temperature of rheological properties (170 °C); b at testing temperature of electric properties (24 °C)

In addition, it is illustrated that the dielectric constant remains little changed in the range from 10 kHz to 80 MHz. However, the dielectric loss of HDPE composites decreases sharply in the range from 10 kHz to 1 MHz, and then decreases slowly from 1 to 80 MHz [28, 29]. Generally, Cole–Cole model and Debye relaxation are commonly utilized to examine the dielectric properties of materials [30–34]. Cole–Cole semicircles are often plotted by the curves of dielectric constant versus dielectric loss [35]. Curves of dielectric loss versus dielectric constant of HDPE samples are shown in Fig. 5. It is shown that there are Cole–Cole semicircles for HDPE/BaTiO3 composites. Moreover, such semicircles can be ascribed to one Debye relaxation, due to the interfacial polarization. HDPE control possesses non-polar structure, so there is not obvious semicircle found in curves of HDPE control. With the increase of BaTiO3 particles, the Cole–Cole semicircles of HDPE/BaTiO3 composites become obvious, meaning that the interfacial polarization between BaTiO3 particles and HDPE matrix is enhanced. 3.4.2 Effect of BaTiO3 loading on surface and volume resistivity of HDPE composites Table 2 shows results of surface and volume resistivity of HDPE composites. Because of the non-polar structure of

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HDPE, the passage of current in HDPE matrix is very difficult [2, 3]. It is shown that when BaTiO3 content reaches 30 vol%, the volume resistivity of HDPE control decreases by two orders of magnitude. Moreover, with the increase of BaTiO3 amount, the volume resistivity of HDPE composites further decrease slightly. This is because that the increased amount of BaTiO3 particles greatly boost the chance of particles contacting each other, facilitating the passage of electric carriers in the HDPE/BaTiO3 composites [1, 4]. By comparison, the surface resistivity of HDPE composites are not affected by the incorporation of BaTiO3. Although the volume resistivity is reduced by the increase of BaTiO3, the volume resistivity of obtained composites are still higher than 1.0 9 1011. These HDPE composites can be regarded as non-conductive materials, meaning that the BaTiO3 can only greatly affect the dielectric properties rather than resistivity of HDPE composites [6, 11]. The threshold of volume resistivity of HDPE composites is 30 vol%, which is also lower than rheological properties of HDPE composites. According to Scheme 2b, the conductive crystal lattices can act as bridges between BaTiO3 particles to transfer the current carriers [17]. So even when BaTiO3 particles depart from each other at 30 vol%, the passage of current is still boosted by the presence of crystal lattices.

J Mater Sci: Mater Electron (2016) 27:4344–4350 Table 2 Volume and surface resistivity of HDPE composites

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Volume fraction (vol%)

Volume resistivity (X m)

Surface resistivity (X)

0

3.6 9 1013

6.1 9 1011

4.6 9 10

13

6.9 9 1011

1.5 9 10

12

7.3 9 1011

8.2 9 10

11

6.1 9 1011

40

5.4 9 10

11

4.6 9 1011

50

3.5 9 1011

1.5 9 1011

10 20 30

Fig. 6 Effect of volume fraction of BaTiO3 on flexural properties of HDPE/BaTiO3 composites

3.5 Effect of BaTiO3 loading on mechanical properties of HDPE composites Figure 6 shows the flexural strength and modulus of HDPE/BaTiO3 composites at different volume fraction of BaTiO3 particles. It is observed that the addition of 50 vol% BaTiO3 can boost the flexural modulus and strength of HDPE control from 90 and 8 MPa to 780 and 14 MPa, respectively. Commonly, inorganic fillers can reinforce the polymer matrix to some extent [14, 23]. It is also reported that the crystal lattices in polymer matrix can provide extra strength for materials to resist outside forces [7]. In this study, although crystallinity of HDPE composites drops with the increased content of BaTiO3 particles, the increased BaTiO3 can still increase the flexural modulus and strength of HDPE/BaTiO3 composites [8].

4 Conclusion In this work, Fourier transform infrared spectra prove the chemical adherence of KH550 to the surface of BaTiO3 particles. It is found that the threshold of dielectric properties and volume resistivity of HDPE/BaTiO3 composites is 30 vol%, which is lower than rheological threshold of HDPE/BaTiO3 composites (40 vol%), because of the presence of crystal lattices.

The incorporation of BaTiO3 into HDPE can fabricate composites with high dielectric constant and low dielectric loss. The addition of 50 vol% BaTiO3 increases dielectric constant and loss of HDPE control at 10 MHz from 2.5 and 0.02 to 18.5 and 0.10, respectively. Meanwhile, the addition of 50 vol% BaTiO3 can decrease the volume resistivity of HDPE control from 3.6 9 1013 X m to 3.5 9 1011 X m, meaning that HDPE with 50 vol% BaTiO3 is still non-conductive. In terms of mechanical properties, the increased amount of modified BaTiO3 can increase the flexural strength and modulus HDPE composites from 8 and 90 MPa to 14 and 780 MPa, respectively. However, with the increase of modified BaTiO3 can restrict mobility of HDPE chains and decrease the crystallinity of HDPE composites. Acknowledgments The authors gratefully acknowledge the financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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