Comparison of cure, mechanical, electric properties of EPDM filled with Sm2O3 treated by different coupling agents

Polymer Testing 28 (2009) 235–242

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Material Properties

Comparison of cure, mechanical, electric properties of EPDM filled with Sm2O3 treated by different coupling agents Jun Su, Shuangjun Chen, Jun Zhang*, Zhongzi Xu Department of Polymer Science and Engineering, College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, People’s Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 October 2008 Accepted 18 November 2008

Composites based on ethylene–propylene–diene rubber (EPDM) were prepared. EPDM was reinforced with 100 phr Sm2O3 treated with coupling agents: stearic acid (SA), isopropyl tri(dioctylphosphate) titanate (NDZ102), bis-[-3-(triethoxysilyl)propyl]tetrasulfide (KH845-4), and N-b-(aminoethyl)-g-aminopropylmethyldimethoxysilane (SG-Si602), respectively. Cure, mechanical and electrical properties of the composites were investigated. It was found that carboxyl in coupling agents could retard EPDM cure while amino groups, P]O bonds and S atoms could accelerate EPDM cure. Amino groups enhanced composite mechanical properties by forming additional rigid C–C linkages, whilst S atoms boosted composite mechanical properties by generating flexible S–C linkages. P]O bonds might be subject to cleavage during vulcanization and form flexible P–C linkage. Thus, composites with NDZ102 and KH845-4 treated filler exhibited better mechanical properties than that with SG-Si602 treated filler. In addition, treatment of filler could reduce composite electrical properties due to interfacial improvement. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Ethylene–propylene–diene rubber Coupling agents Cure properties Mechanical properties Electric properties

1. Introduction Ethylene–propylene–diene rubber (EPDM) consisting of ethylene, propylene and unsaturated diene, is one of the popular synthetic rubbers. Due to its saturated backbone, EPDM possesses excellent resistance to heat and oxidation [1], whilst the non-polar structure endows EPDM with excellent electrical resistivity and resistance to polar solvents [2]. Thereby, it has broad application to thermoplastic vulcanizates, electrical insulation, waterproof rolls and so on. In addition, EPDM can not only vulcanize in a peroxide-cured system but also in sulfur-cured systems due to unsaturated diene. Earlier reports showed that rubber vulcanized by sulfur could accommodate more stress and exhibit higher elongation compared with peroxide-cured rubber [3], and that tensile strength and elongation at break of rubber with a mixed-cured system * Corresponding author. E-mail address: [email protected] (J. Zhang). 0142-9418/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2008.11.015

were higher than those of sulfur and peroxide-cured systems [4]. Commonly used fillers for EPDM include carbon black, silica, clay and fiber [5–8]. The surface functional groups of a filler could influence the cure properties of EPDM and finally affect the overall properties. Earlier reports observed that channel blacks with lots of oxygen functional groups, which were reported to be acid, would retard cure, whilst furnace blacks featuring a slightly alkali characteristic because of low oxygen content would accelerate vulcanization [5,9]. Pongdhorn et al. concluded that sulfuric atoms on the filler surface introduced by surface treatment would affect cure properties in three sulfur-cured systems, conventional, semi-efficient and efficient system [6]. Sm2O3, a rare earth oxide, with special shell structure, prominent physical, chemical, electrical and magnetic properties is often utilized in surface engineering [10]. So far, there have been few reports about the direct addition of rare earth oxide into a rubber matrix to strengthen mechanical properties. Therefore, it is interesting to

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incorporate Sm2O3 into EPDM. Considering the poor adhesion between Sm2O3 and EPDM and strong agglomeration among Sm2O3 particles, coupling agents were often selected to modify the surface of particles, since chemical treatment of filler surface has become the most successful method to improve rubber–filler and reduce filler–filler interactions. Recently, much emphasis has been given to the application of coupling agents which serve, to some degree, to couple a filler to the rubber molecule on a ‘‘like-to-like’’ basis [6,11,12]. Generally, coupling agents possess two functionally active end groups, an alkoxy group which is capable of reacting with the hydroxyl group on the surface of common fillers, with the rest of the molecule having functional groups or special atoms, such as amino groups, double bonds and sulfuric atoms, which are compatible with rubber or can participate in vulcanization, leading to strong physical or chemical linkage between coupling agents and rubber molecules [13–15]. As a consequence, the coupling agent acts as a bridge between filler and rubber that enhances the rubber–filler interaction and the degree of reinforcement. In this work, Sm2O3 particles were treated with four types of coupling agents containing carboxyl, amino group, P and S atoms to compare the cure, mechanical and electric properties of EPDM containing the treated Sm2O3. The main aim is to investigate the correlation between the functional groups in the coupling agents and properties of composites. The properties of interest included cure characteristics, crosslink density, mechanical properties, morphology of fractured surface by scanning electron microscopy and electrical properties, including dielectric constant, loss and strength, surface and volume resistivity. 2. Experimental 2.1. Materials The rubber used in this study was ethylene–propylene– diene monomer (EPDM J-4045) containing 5-ethylidene-2norbornene (ENB) as diene, which was manufactured by Jilin Petrochem., SINOPEC. The EPDM consisted of 52.0 wt% ethylene, 40.3 wt% propylene and 7.7 wt% ENB. Compounding ingredients, such as dicumyl peroxide (DCP), zinc oxide, stearic acid, 2-mercapto benzimidazole (antioxidant MB) and polymerized 2, 2, 4-trimethyl-1, 2-dihydroquinoline

(antioxidant RD) were of reagent grade. The chemical/ commercial names and structures of the four types of coupling agent used are listed in Table 1. Stearic acid (SA) was supplied by Nanjing Chemical Plant; isopropyl tri(dioctylphosphate) titanate (NDZ102), bis-[-3-(triethoxysilyl) propyl]-tetrasulfide (KH845-4), and N-b-(aminoethyl)-gaminopropylmethyldimethoxysilane (SG-Si602) were obtained from Nanjing shuguang Chemical Group Co., Ltd. Sm2O3 particles were supplied by Liyan Fangzheng Rare Earth Technology Co., Ltd., Jiangsu Province. 2.2. Sample preparation 2.2.1. Surface modification of Sm2O3 In this work, SA, NDZ102, KH845-4 and SG-Si602 were applied for surface treatment of the Sm2O3 particles. The amount of each coupling agent was 1% by weight (wt%) of the Sm2O3 content. For instance, 1.0 g of SA was mixed with 100 ml xylene, and then stirred into a uniform solution. Sm2O3 (100 g) was then added to the solution with a further 30 min stirring to ensure a uniform distribution of the coupling agent on the Sm2O3 surface. The treated Sm2O3 particles were then dried at 60  C for 4 h in an oven until weight remained constant. Similarly, NDZ102, KH845-4 and SG-Si602 on the Sm2O3 surface were prepared with the same content for 100 g Sm2O3 using the same procedure as described earlier, but in different solutions, NDZ102 with isopropanol, KH845-4 and SG-Si602 with ethanol. 2.2.2. Compounding of EPDM vulcanizates with Sm2O3 EPDM and Sm2O3 treated with four coupling agents were mixed on a two roll mill (Shanghai Rubber Machinery Works, China) in accordance with ISO2393. The formulations of EPDM gum containing treated Sm2O3 are displayed in Table 2. Vulcanizates were cured in an electrically heated press at 170  C and 10.0 MPa for 15 min, and were conditioned for 24 h before testing. 2.3. Testing procedures 2.3.1. Fourier transform infrared spectroscopy Fourier transform infrared spectra (FT-IR) of untreated, SA treated, NDZ102 treated, KH845-4 treated and SG-Si602 treated Sm2O3 particles were obtained with a resolution of 4 cm1 in the range of 400–4000 cm1 in a Nicolet spectrometer (NEXUS 670 technique, America).

Table 1 Chemical/commercial name and structure of coupling agents used in this study. Chemical/commercial name

Molecular formation

Stearic acid (SA)

CH3(CH2)16COOH

CH3 Isopropyl tri(dioctylphosphate) titanate (NDZ102)

CH3

CH

O O

Ti {

O

P

O O

P

OH Bis-[-3-(triethoxysilyl)propyl]tetrasulfide (KH845-4) N-b-(Aminoethyl)-g-aminopropylmethyldimethoxysilane (SG-Si602)

(C2H5O)3Si(CH2)3–S–S–S–S–(CH2)3Si(OC2H5)3 NH2(CH2)2NH(CH2)3SiCH3(OCH3)2

( O

C8H17)2}3

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Table 2 EPDM/ Sm2O3 formulation (phr). EPDM

Untreated Sm2O3

100 100 100 100 100

100

SA treated Sm2O3

NDZ102 treated Sm2O3

KH845-4 treated Sm2O3

100 100 100 100

2.3.2. Curing characterizations Curing properties of EPDM gums were characterized at 170  C by an intelligent computer controlled moving die rheometer (MDR) 2000 (Wuxi Liyuan Electronic & Chemical Equipment Co., Ltd., China) with the arc of oscillation at 1. 2.3.3. Crosslink density Crosslink density of EPDM specimens was measured by the solvent swell method. EPDM samples were immersed in toluene for 72 h at room temperature. The crosslink density was determined by the Flory–Rehner equation [16]:

v ¼ 

SG-Si602 treated Sm2O3

" # 1 lnð1  VRÞ þ VR þ mVR2 V VR1=3  VR 2

ZnO

SA

DCP

RD

MB

5 5 5 5 5

1 1 1 1 1

4 4 4 4 4

0.5 0.5 0.5 0.5 0.5

0.5 0.5 0.5 0.5 0.5

2.3.6.3. Dielectric strength. The dielectric strength was determined following IEC 60243-1. The voltage source was a YOJ – 10kVA step-up transformer (Xuzhou Power Transformer Factory, China). The voltage on the circular sample with diameter of 100 mm was increased from zero until dielectric failure of the test specimen occurs. The power rating for this test was 1 kV/s for voltages under 20 kV, and 2 kV/s for voltages up to 20 kV. 3. Results and discussion 3.1. FT-IR analysis

(1)

where v is the crosslink density (mol cm3), VR is the volume fraction of EPDM rubber after immersion in toluene, V is the molar volume of toluene (cm3 mol1), m is the interaction parameter between rubber and toluene (0.49) [17]. 2.3.4. Scanning electron microscopy The dispersion of filler was carried out by scanning electron microscopy (SEM) (JEOL JSM-5900, Japan). The samples were fractured in liquid nitrogen, and then the fracture surface was sputtered with a thin layer of gold to avoid electrical charging during examination. 2.3.5. Mechanical properties Test specimens were cut from vulcanized sheets more than 24 h after vulcanization. The tensile and tear tests were carried out in accordance with ISO37 and ISO 34 using a CMT 5254 type electromechanical universal testing machine (Shengzhen SANS Testing Machine Co., Ltd. China) at a stable rate of 500 mm/min. The Shore A hardness of the specimens was measured using a LX-A rubber Shore A hardness degree tester (Jiangsu Mingzhu Testing Machinery Co., Ltd., China) in accordance with ISO 7619-1.

From Fig. 1, it is observed that the four coupling agents all absorbed bands at 2917 cm1, 2850 cm1 and 1464 cm1 assigned to asymmetric stretching vibration of methylene, symmetric stretching vibration of methylene, and methylene scissoring vibration, respectively [18]. In addition, the characteristic peaks of SA at 1701 cm1 and 935 cm1 are assigned to C]O stretching vibration and O– H transforming vibration of –COOH, respectively [18]. As for NDZ102, characteristic absorption peak appears at 1050 cm1 assigned to P–O stretching vibration [19]. KH845-4 absorbs characteristically bands at 1243 cm1, 1081 cm1, 961 cm1, and 489 cm1 assigned to Si–CH3 symmetric transforming vibration, Si–O–C rocking vibration, Si–H transforming vibration, and S–S stretching vibration, respectively [15]. Apart from peaks at 1259 cm1 and 1084 cm1, SG-Si602 absorbs at 1577 cm1 for –NH transforming vibration [20].

2.3.6. Electrical properties 2.3.6.1. Volume and surface resistivity. The volume and surface resistivity of composites were measured at room temperature by a high resistance meter (Shanghai Precision & Scientific Instrument Co., Ltd., China) following IEC 60093. 2.3.6.2. Dielectric constant and dielectric loss. The dielectric constant and dielectric loss were measured in the range of 5k–10 MHz (Agilent 4294A precision impedance analyzer, America) following IEC 60250.

Fig. 1. FT-IR spectra of four coupling agents.

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In Fig. 2, although the Sm2O3 spectrum overlapped some of the characteristic peaks mentioned above, there are still evident peaks, 1701 cm1, 1050 cm1, 1081 cm1, and 1259 cm1, to prove the adherence of coupling agents to the surface of Sm2O3 filler. Also, the sharp peak appearing at 3525 cm1 is assigned to the stretching vibration of the OH group [21]. According to earlier reports [13,22], alkoxy can alcoholze with hydroxyl on the filler surface and form strong interaction between filler and coupling agent, which provides evidence to demonstrate the adherence of coupling agents (NDZ102, KH845-4 and SG-Si602) to the filler surface. As for SA, the dehydration of hydroxyl and carboxyl could establish interaction between coupling agents and filler surface. 3.2. Effect of coupling agents on cure properties of rubber vulcanizates The rheographs and cure characteristics of EPDM with Sm2O3 treated by various coupling agents are shown in Fig. 3 and Table 3, respectively. The initial decline of torque is ascribed to the softening of the rubber caused by heating. Then, torque increases because of the formation of crosslinking bonds [3]. Clearly, ultimate cure state of vulcanizates indicated by maximum torque (MH) follows the order: vulcanizate with SG-Si602 treated Sm2O3 > vulcanizate with KH845-4 treated Sm2O3 > vulcanizate with NDZ102 treated Sm2O3 > vulcanizate with untreated Sm2O3zvulcanizate with SA treated Sm2O3, while cure rates obey the sequence: vulcanizate with SG-Si602 treated Sm2O3 > vulcanizate with KH845-4 treated Sm2O3 > vulcanizate with NDZ102 treated Sm2O3 > vulcanizate with untreated Sm2O3 > vulcanizate with SA treated Sm2O3. Modification of Sm2O3 can bring certain functional groups of coupling agents to the surface of filler [22]. SA introduces acidic carboxyl groups which would make DCP decompose into ions instead of radicals [23]. Only radicals, not ions, can initiate and participate in vulcanization, so the presence of carboxyl groups could reduce the radical number and finally retard cure rate. Although, vulcanizates filled with untreated Sm2O3 and SA treated Sm2O3

Fig. 3. Cure curves of EPDM with treated Sm2O3 by different coupling agents.

possessed almost the same MH, the former cured faster than the latter, indicating acidic functional groups in SA could retard cure rather than change the ultimate cure state. This trend partially conformed with reports [9] in which both cure rate and MH reduced in acidic condition. The possible reason was that the hydroxyl was not acidic enough to decrease both factors. KH845-4 contained sulfuric linkages which could cleave and generate active radicals at 170  C to participate in vulcanization [3]. According to an earlier report [12], the addition of S atoms into peroxide-cured system could enhance the mechanical properties remarkably. Therefore, MH and cure rate were both boosted. It is possible that the P]O bonds in NDZ102 may be subject to cleavage at high temperature and form radicals to react with unsaturated bonds. Thus, MH, cure rate and crosslink density increased. Contrary to the negative effect of acidic groups imposed on the cure process, it is reported that filler treated with NaOH exhibited a catalytic effect on vulcanization [24]. Thus, the amino of SG-Si602 would make the cure system alkaline, in which DCP decomposed into radicals rather than ions to initiate crosslinks [25]. As

Table 3 Cure characteristics of EPDM with treated Sm2O3 by different coupling agents. Sample

ML

tML

MH

tMH

N m min N m min EPDM with untreated Sm2O3 EPDM with SA treated Sm2O3 EPDM with NDZ102 treated Sm2O3 EPDM with KH845-4 treated Sm2O3 EPDM with SG-Si602 treated Sm2O3

Fig. 2. FT-IR spectra of treated Sm2O3 by different coupling agents.

ts2

t90

Cure rate

min min N m min1

0.12 0.35 1.38 22.98 1.42 8.82 0.056 0.12 0.33 1.37 29.08 1.45 8.72 0.043 0.13 0.33 1.44 25.73 1.47 9.08 0.059 0.13 0.38 1.55 22.00 1.65 9.03 0.066 0.13 0.37 1.78 20.07 1.22 9.68 0.084

ML: minimum torque; tML: time to minimum torque; MH: maximum torque; tMH: time to maximum torque; ts2: scorch time; t90: optimum cure time; Cure rate: (MHML)/(tMHtML).

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a result, the more radicals there were, the higher the MH and cure rate observed. Moreover, the contribution of amino on cure exceeding that of sulfuric atoms could clarify why SG-Si602 possessed the maximum MH and cure rate. Scorch time is the time taken for the minimum torque value to increase by two units. It is an indicator of premature vulcanization of the matrix. It is apparent from the Table 3 that the treatment of SG-Si602 can shorten scorch time while treatment of KH845-4 can extend scorch time. In other words, the scorch safety was the highest for vulcanizate with KH845-4 treated filler and lowest for vulcanizate with SG-Si602 treated filler. This trend was analogous to reports [3,4] in which addition of sulfur could increase the scorch safety. Due to highest cure rate, rubber with SG-Si602 treated filler might shorten the scorch time. 3.3. Effect of coupling agents on crosslink density of rubber vulcanizates The crosslink densities of vulcanizates with various coupling agents are listed in Table 4 and follow the order: rubber with SG-Si602 treated Sm2O3 > rubber with KH845-4 treated Sm2O3 > rubber with NDZ102 treated Sm2O3 > rubber with untreated Sm2O3zrubber with SA treated Sm2O3. This confirmed the trend of MH, indicating that degree of crosslinking would vary with functional groups adhered to the filler surface.

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between EPDM with SA treated Sm2O3 and EPDM with untreated Sm2O3, indicating that treatment of SA on filler surface had little effect. This could be explained by both crosslink density and filler dispersion of these two samples being almost the same. Nevertheless, treatment with the other three coupling agents brings apparent changes to mechanical properties of EPDM. The rise of hardness, 100% modulus, tensile strength and tear strength could be explained by increased crosslink density of vulcanizates and interaction between filler and matrix, which has been discussed above. In addition, rubber with KH845-4 treated filler exhibited higher tensile and tear strength than rubber filled with SG-Si602 treated Sm2O3. This is because the sulfuric atoms could participate in the peroxide-cured system. According to reports [3,4], the sulfur could react with the double bonds in ENB and form C–S linkages in a peroxide-cured system. As to SGSi602, the amino groups could makes DCP decompose with additional radicals and generate the most C–C linkages of all the samples. In addition, C–S linkages are more flexible than C–C linkages, so the former can withstand more stress than the latter. EPDM with SG-Si602 treated filler exhibited the lowest elongation at break, which was again attributed to the increased crosslink density. However, EPDM with KH845-4 and NDZ102 treated Sm2O3 possessed higher elongation at break than EPDM with untreated Sm2O3. This trend was also owing to flexible C–S [3,4] and possibly due to C–P linkages. 3.6. Effect of coupling agents on electrical properties

3.4. Effect of coupling agents on filler dispersion in rubber vulcanizates Scanning electron micrographs (Fig. 4) shows the effect of coupling agents on filler dispersion. Obviously, no matter which kind of coupling agent was used, fillers still aggregated just as the untreated ones did, and dispersion of fillers was not improved. This unexpected result was not similar to earlier reports [11,26,27] in which coupling agents could improve the filler dispersion and agglomeration. The possible reason might be high filler loading (100 phr) as well as the strong hydrogen bonding formed by hydroxyl on the filler surface. 3.5. Effect of coupling agents on mechanical properties of rubber vulcanizates Mechanical property data for EPDM with various coupling agent treated Sm2O3 are listed in Table 5. There was not significant change in mechanical properties

Table 4 Crosslink density of rubber with various coupling agents. Sample EPDM EPDM EPDM EPDM EPDM

with with with with with

Crosslink density (mol cm3) untreated Sm2O3 SA treated Sm2O3 NDZ102 treated Sm2O3 KH845-4 treated Sm2O3 SG-Si602 treated Sm2O3

0.001033 0.001017 0.001277 0.001404 0.001695

3.6.1. Effect of coupling agents on dielectric properties Like most polymers, raw EPDM is regarded as an insulator [28]. Several studies have shown that addition of inorganic fillers can increase the conductivity of the polymer [29]. The dielectric constant and dielectric loss, which are used to characterize molecular relaxations, were measured. The dielectric constant is a measure of the energy stored in a sample during a cyclic electric excitation. The dielectric loss is a measure of the energy lost into a system during cyclic electric excitation [30]. Fig. 5 shows curves of dielectric constants of EPDM filled various coupling agents treated filler in the following order: EPDM with untreated Sm2O3zEPDM with NDZ102 treated Sm2O3 > EPDM with SG-Si602-treated Sm2O3zEPDM with SA treated Sm2O3 > EPDM with KH845-4 treated Sm2O3. In addition, the dielectric constants of EPDM with various coupling agents remained almost unchanged in the frequency range of 5kw10 MHz. Composites with fillers, being heterogeneous, are also subjected to interfacial polarization, which occurs at the interface of dissimilar materials [31]: the hydrogen bonding with the filler surface could reduce slightly the dipole polarizability of the interfacial region. In addition, the effect of hydrogen bonding could be lowered by surface treatment. Thus, the decrease of dipole polarizability in the interfacial region would reduce molecular mobility to align to an cyclic electric excitation, and then reduce the dielectric constant [32]. For NDZ102, the Ti atom is the most polar among atoms of other coupling agents, which can explain the highest dielectric constant. The symmetric structure of

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Fig. 4. SEM micrographs of rubber vulcanizates with various coupling agents, A: EPDM with untreated Sm2O3; B: EPDM with SA treated Sm2O3; C: EPDM with NDZ102 treated Sm2O3; D: EPDM with KH845-4 treated Sm2O3; E: EPDM with SG-Si602-treated Sm2O3.

Table 5 Mechanical properties of EPDM with treated Sm2O3 by different coupling agents. Sample

EPDM with untreated Sm2O3 EPDM with SA treated Sm2O3 EPDM with NDZ102 treated Sm2O3 EPDM with KH845-4 treated Sm2O3 EPDM with SG-Si602 treated Sm2O3

Hardness Modulus Tensile Elongation Tear at 100% strength at break strength kN m1

Shore A

MPa

MPa

%

54

1.32

2.20

222

7.17

54

1.39

2.26

230

7.63

55

1.44

2.74

264

8.25

57

1.63

3.03

275

11.22

60

1.92

2.81

194

8.56 Fig. 5. Dielectric constant of EPDM with treated Sm2O3 by different coupling agents.

J. Su et al. / Polymer Testing 28 (2009) 235–242

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filler and matrix. The increase of such pathways indicates easier current carrier motion which facilitates passage of current [34]. Table 6 also presents the dielectric strength data which follow the order: EPDM with KH845-4 treated Sm2O3 < EPDM with SG-Si602 treated Sm2O3zEPDM with NDZ102 treated Sm2O3 < EPDM with untreated Sm2O3 < EPDM with SA treated Sm2O3. The lowest dielectric strength of EPDM with KH845-4 treated filler could be explained by the lowest polarity of filler, which facilitates the electric penetration in the compositions [35]. As for the highest value of dielectric strength of EPDM with SA treated filler, it might be explained by the relatively large aggregates in the matrix, observed by SEM, that prevent current carrier motion to the greatest extent [34]. 4. Conclusions Fig. 6. Dielectric loss of EPDM with treated Sm2O3 by different coupling agents.

KH845-4 and six alkoxy groups in each end could boost treatment efficiency, resulting in the smallest dielectric constant. In terms of SA and SG-Si602, the similar molecular structure and approximate polarity of C and N atoms could lead to almost the same dielectric constant value. It is observed from Fig. 6 that values of the dielectric loss of EPDM treated with coupling agents are all lower than those of EPDM with untreated filler and dielectric loss declined in the range of 5k–10 MHz. Fig. 6 shows that the values of dielectric loss of EPDM with treated filler are all lower than those of EPDM with untreated filler. The dielectric loss is typically correlated with the dipole polarity of composites. The reduction in polarity makes it difficult for dipole alignment in cyclic electric excitation [33]. 3.6.2. Effect of coupling agents on volume and surface resistivity and dielectric strength Resistivity is a measure of the resistance the material exhibits to the passage of current. Table 6 shows that all of the composites with treated filler have lower volume and surface resistivity than EPDM with untreated filler. Lower polarity means more compatibility between filler and matrix which means more inter-connective pathways or conductive channels and current density formed between

Table 6 Dielectric, volume and surface resistivity of EPDM with treated Sm2O3 by different coupling agents. Sample

EPDM with EPDM with EPDM with untreated SA treated NDZ102 Sm2O3 treated Sm2O3 Sm2O3

EPDM with KH845-4 treated Sm2O3

EPDM with SG-Si602 treated Sm2O3

Volume 3.32  1014 2.48  1014 8.05  1013 6.94  1013 2.30  1012 resistivity (U m) Surface 4.16  1012 2.62  1012 9.24  1011 6.08  1011 3.31  1011 resistivity (U) Dielectric 31.3 33.7 29.7 28.3 29.8 strength 1 (MV m )

The surface treatment of Sm2O3 with coupling agents has significant influence on cure, mechanical and crosslinking measurement of EPDM composites with Sm2O3. The reason is that the treatment could result in some functional groups being attached to filler surface. It was found that acidic carboxyl groups could make DCP partly decompose into ions and then retard cure, whereas, the amino groups which are alkali could make DCP decompose more radicals and then enhance crosslink density. The presence of sulfuric linkages can participate in vulcanization to increase crosslink density and help composite possess optimum mechanical properties due to more flexible C–S linkages. In addition, the P atoms might accelerate vulcanization just as S atoms did. The variation of electric properties of EPDM loaded with various coupling agent treated filler was studied. The value of dielectric constant of composites with KH845-4 treated Sm2O3 is the lowest compared to others because of the minimum polarity of sulfuric atom. Furthermore, the number of conductive channels and the current density increased with the decrease of polarity, resulting in relatively low dielectric strength, volume and surface resistivity of composites.

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