Dielectric Properties of Natural Rubber Composites filled with Graphite

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ScienceDirect Materials Today: Proceedings 16 (2019) 1338–1343

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ICAMMAS17

Dielectric Properties of Natural Rubber Composites filled with Graphite a

K.Ravikumara, K.Palanivelub, K.Ravichandranc * Dept. of Physics, Sri Sairam Institute of Technology, Chennai 600 044, Tamilnadu, India. b

c

Central Institute of Plastics Engineering and Technology, Chennai 600032, Tamilnadu, India.

Dept. of Rubber and Plastics Technology, MIT Campus, Anna University Chennai 600 044,Tamilnadu, India.

Abstract Natural rubber (NR) composites filled with graphite (G) at various loading level was prepared by two roll mixing mill. Curing characteristics and dielectric properties were investigated and compared with NR/carbon black (CB) composites. The minimum and maximum torque of NR/CB composites increases upto 40phr loading. The same trend was found in NR /G composites upto 30 phr. Scorch time and optimum cure time of NR/ G are relatively higher than NR/CB composites. Dielectric parameters such as dielectric constant and loss factor increases on increase in CB. Graphite composites show maximum dielectric constant up to 20 phr. The frequency dependant dielectric loss factor of NR/CB is shows that, they are more conductive than NR/G composites. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences. Keywords: Natural rubber, Graphite ,Carbon black, dielectric properties, vulcanization characteristics;

1. Introduction Graphite is one of the important allotropes of carbon and abundantly available in nature. Graphite has a layered structure called Graphene, held together by a weak Vander Waals force. The presence of π orbital over the entire Graphene sheet makes it a thermally and electrically good conductor. The thermal and electrical conductivity of graphite is about 209.34 W·m−1·K−1 and 2 x104Ω-1cm-1 respectively [1,2]. Therefore graphite filler is used in the elastomer industry as a filler to enhance electrical and thermal conductivity. Several authors are extensively studied the curing, electrical and dielectric properties of graphite filled polymer composites [3,4,5]. The presence of weak van der Waals forces between the graphite layers is attributed to relatively poor reinforcing properties in polymer. Further to understand the reinforcing effect of filler and the interfacial interactions between rubber matrix and graphite filler, dielectric spectroscopy studies was carried out. In this present study, the effect of graphite on vulcanization and dielectric properties of natural rubber composites has been investigated and the results were compared with NR/CB composites.

* Corresponding author. Tel.: +91-944-401-2674 ; fax: +044-222-32403 . E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences.

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2. Materials and Methods 2.1 Materials Natural Rubber (NR) used for the study is solid block rubber ISNR-CV3 obtained from Rubber Research Institute of India, Kottayam, Kerala, having Mooney viscosity {ML (1+4)} at 100ºC = 60. Carbon black medium reinforcing grade N660 was supplied by Cabot Corporations. Graphite powder (~20µm) was obtained from Sigma-Aldrich Germany. Other rubber additives such as activators zinc oxide and stearic acid, accelerators mercaptobenzothiazole sulphanemide (MBTS), Tetramethyl thiuram disulphide (TMTD) and vulcanizing agent sulphur were used in this study was supplied from rubber industries, Chennai. 2.2 Preparation of composites Natural rubber composites were prepared using a laboratory two roll mixing mill. Natural rubber was masticated by mixing mill at room temperature with a friction ratio of 1:1.4.After forming a smooth band, additives such as activators zinc oxide and stearic acid, filler (CB / graphite) accelerator and curing agent sulphur were added to the rubber as per the formulations (Table 1). Appropriate nip gaps were maintained and 3/4th cuts were made during the mixing process in order to get a uniform dispersive and distributive rubber compound. Table 1. Recipe for the rubber compounds Ingredients (PHR)

NR/G

NR/CB

100 5

100 5

2

2

Filler

0,10,20,30,40

0,10,20,30,40

MBTS

1

1

TMTD

0.5

0.5

Sulphur

1.5

1.5

Natural rubber ZnO Stearic Acid

2.3 Characterization and testing Curing or Vulcanization characteristics of unfilled and CB and graphite filled rubber composites were measured using an Oscillating Disc Rheometer (ODR) with amplitude of 3° arc at temperature 150°C.Curing parameters such as minimum (ML) and maximum torque (MH), scorch time (tS2) and optimum cure time (t90) were measured from the rheograph (torque vs time). Dielectric properties such as dielectric constant(ε´), dielectric loss(ε´´) , dissipation factor (tan δ) and volume resistivity (ρ) of rubber composites were measured at various frequencies from 50Hz to 5MHz by using Hioki High Tester LCR meter, model No IM3532 and text fixture 9262 as per ASTM D 150 at 30̊ C. 3. Result and discussion 3.1 Curing characteristics Curing or vulcanization characteristics are expressed in terms of the minimum torque (ML), maximum torque (MH), scorch time (ts2) and optimum cure time (t90). Curing parameters for NR/CB and NR/G composites are given in Table 2 and 3 respectively. Table 2 shows the vulcanization parameters of NR/CB composites. During curing reaction, minimum and maximum torque of rubber gradually increases on increase in CB, which reflects the increases in modulus value of composites. Increasing the CB content, increases the number of active sites present on the surface of carbon black particles, which forms a three dimensional network with NR matrix [6] and restricts the changes in rubber molecules configuration and improve the modulus values [7]. The scorch time and optimum cure time of NR /CB decreases on increase in CB. It is due to high surface area of CB which, as a consequence, generates more heat during curing process. Due to this excess heat the vulcanization reaction starts more readily [8].

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Table 2: Cure characteristics of NR filled Carbon black NR/CB

MH

ML

MH-ML

ts2

t90

(phr)

(dNm)

(dNm)

(dNm)

(min)

(min)

100/0

75

15

60

3.4

4.8

100/10

89

18

71

2.8

4.2

100/20

99

20

79

2.1

3.4

100/30

102

22

80

2.0

3.1

100/40

103

10

93

1.7

2.8

Table 3 shows the vulcanization parameters of NR/G composites. The minimum torque (ML) and maximum torque (MH) were found increasing up to 30 phr loading level. At 40 phr, MH was found to decrease. This may be attributed the soft and slippery nature of graphite fillers. Curing time and scorch time are found to decrease on increase in the graphite. Scorch time and optimum cure time of NR/G are found higher compared to that of NR/CB composites and it shows that carbon black is found to be better filler on processing safety aspect. Table 3: Cure characteristics of NR filled Graphite NR/G

MH

ML

MH-ML

ts2

t90

(phr)

(dNm)

(dNm)

(dNm)

(min)

(min)

100/0

75

15

60

3.4

4.8

100/10

87

16

71

3.3

4.0

100/20

103

18

85

2.6

4.3

100/30

104

20

84

2.2

4.1

100/40

98

13

85

2.3

4.4

3.2 Dielectric spectroscopy The term dielectric permittivity or dielectric constant (ε´) is used for describing the ability of polymers to store charges. The polymer, type of filler, loading level have significant affects the dielectric constant of elastomers. The frequency dependence dielectric constants of NR and NR/CB composites are shown in Fig.1a. Two types of variation were found in frequency dependence dielectric constant in NR/CB composites. In the low frequency region (up to log 3.5 Hz), shows gradual decrease of dielectric constant and at high frequency region (log 3.5-7.5 Hz), a flat line is observed. The higher value of dielectric constant in lower frequency range can be explained by the interfacial polarization (IP) or Maxwell-Wagner (M-W) polarization, which occurs due the presence of electrically heterogeneous materials, namely, polymer composites and blends. CB has very high electrical conductivity [9] compound to NR and forms some permanent dipoles at the polymer-filler interfaces, which enhance the polarization ability and hence improve the dielectric constant. As the percentage of CB increases, more active sites are formed and hence resulting in increase of dielectric constant [10]. These dipoles are relatively bigger in size compares to other dipoles, namely, orientation, ionic and electronic. On increase in frequency, the induced dipoles do not have sufficient time to align in the direction of applied electric field and dielectric constant gets reduced. The second part, the straight line represents the contribution from electronic polarization. Electronic polarization is the reason for optical properties of materials such as refractive index and optical attenuation [11].At 40phr loading, the dielectric constant was not as good as compared to that of 30 phr loading value and this may be due to agglomeration of CB.

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Fig. 1. Dielectric constant (a) NR/CB; (b) NR/G Fig 1(b) shows the frequency dependant dielectric constant of NR/G composites. The low frequency effect or IP polarization is found to be very low. It may be due to anisometry in electrical conductivity of graphite particles and its orientation in polymer matrix [2]. Dielectric constants of NR/CB composites are found to be superior to that of NR/G composites.

Fig. 2. Dielectric loss (a) NR/CB; (b) NR/G Fig 2(a) and 2(b) shows the frequency dependent dielectric loss (ε´´) spectrum of NR/CB and NR/G composites respectively. The dielectric loss factor is directly related to the relaxation phenomenon of elastomers and measures the exponential decay of polarization with respect to time [12].The value of dielectric loss of NR/CB and NR/G is higher than that of NR. This can be attributed to relaxation involving the polarization phenomenon, namely, orientation and interfacial [10]. The decrease in dielectric loss with increases in the frequency is due to various types of losses, namely, losses due to non-uniformity of dielectrics, losses due to electrical conductivity and CB- rubber structure formation [13].The higher value of dielectric loss in low frequency region is due to the free charge motion within the materials. In both the cases, dielectric loss increase through addition of CB and Graphite. This can be attributed to the moderate increase in conductivity of the filled system. Filled samples show some fluctuations referred to as relaxation peaks. These multiple relaxation peaks arise due to the flexible side group or sometimes in the motion of main chains clearly showing contribution of dielectric property analysis of filled polymers to a better understanding of structure property relationships [14].

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Fig. 3. Volume Resistivity (a) NR/CB; (b) NR/G Fig 3(a) and 3(b) show the variations in volume resistivity (ρ) vs. frequency of NR/CB and NR/ G composites respectively. It is found that, the addition of filler decreases resistivity of composites. This can be attributed to the higher conducting nature of fillers, which promotes conducting process in NR composites. The decrease in the resistivity with increasing frequency (f) can be attributed to increase in conductivity (σ), which can be shown by the equation (1). σ (f) ~ f s ( s arbitrary constant ~ 0 to 1)

(1)

Frequency dependence of ac conductivity can be explained by the Maxwell-Wagner (M-W) two layer models or the heterogeneous model [15].From the frequency dependant resistivity analysis of NR/CB and NR/ G composites, CB black is found to be better conducting filler compared to graphite. 4 Conclusions In the present study, NR/CB and NR/G composites were prepared at different phr level. It was found that minimum and maximum torque values of CB composites are higher than the graphite composites. The scorch time and optimum cure time of NR/CB decrease through increase in the CB. It is due to the excess heat generated in curing process and hence accelerates the curing reaction. Scorch and optimum cure time of graphite composites is higher than CB composites and it shows graphite is very poor in processing safety aspect. Frequency dependant dielectric constant value of NR/CB composites is more significant than the NR/G composites. It shows that interface formed by the CB particles induce more dipoles than graphite particles. Electrical conductivity properties such as dielectric loss and volume resistivity of CB filled composites are better than that of NR/G composites. This may be due to electrical anisometry behaviour of graphite particles. References [1] Junping Song, Lianxiang Ma, Yan He Haiquan Yan, ZanWu & Wei Li , Chinese Journal of Chemical Engineering, 23(2015), 853-859. [2] William Hes, Charles Head , ‘Microstructure , Morphology and general physical properties, Carbon Black: Science and Technology’ Second Edition, Jean-Baptiste Donnet,Roop Chand Bansal and Meng Jiao Wang, Marcel Dekker, Inc. New York.1993. [3] Ieva Kranauskaite, Jan Macutkevic, Polina Kuzhir,Nadeja Volynets, Alesia Paddubskaya, Dzmitry Bychanok, Sergey Maksimenko,Juras Banys,Remigijus Juskenas,Antonino Cataldo,Federico Micciulla,Stefano Bellucci,Vanessa Fierro, Alain Celzard, Phys. Status Solidi A, (2014),1-11. [4] K. Lalkishore, K. Ramkumar, M. Satyam, Journal of Applied Physics 61, (1987),397.

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