Introduction

Linear low density polyethylene (LLDPE) offers excellent dielectric properties, mechanical toughness for wire and cable application, good resistance to chemicals and ease of processing, and thus it has been used as an insulation material in several applications [1, 2]. On the other hand, the metallocene linear low density polyethylene (mLLDPE) is a configured improvement of polyethylene to create highly regular polyethylene molecules with certain goal properties. This technology supports the use of polyolefin in different processes and applications [1]. Wang et al. [3] reported that the amount of space charge accumulated in the developed polyethylene polymerized using the metallocene catalyst is smaller than that in conventional polyethylene. Taniguchi et al. [4], Araoka et al. [5] and Notingher et al. [6] found that the dielectric properties were improved using metallocene polyethylene.

The chemical structure of natural rubber contains 100 % cis-1,4-polyisoprene. For this reason, the microstructure of NR is uniform and NR can be crystallized. It was observed that the highest rate of crystallization occurs at −26 or −24 °C [7, 8]. It was observed that the strain-induced crystallization occurred at a tensile strain >200 % even at high temperatures and the highest value of crystalline region of NR was 33 %. Natural rubber (NR) has best dielectric properties among butyl, EPDM rubbers etc. In addition, NR can be improved to give very high dielectric properties by vulcanization process [8]. However, it was shown that adding NR to LLDPE causes improvement in dielectric properties such as leakage current, carbon track [9].

Epoxidized material retains an unexpectedly high ability to achieve strain-induced crystallization [10], resulting in some of the high tensile strength characteristics that are associated with vulcanizates of unmodified natural rubber. Moreover, epoxidized natural rubber (ENR-50) was used as a compatibilizer for different composites [11–13]. On the other hand, the HVA-2 was also shown to be an effective compatibilizer by reducing the interfacial tension and improving adhesion between immiscible polymers [14–16]. Therefore, a dual compatibilizer system comprised of epoxidized natural rubber and HVA-2 were utilized to compatibilize 10 % rubber/mLLDPE system and used as dual compatibilizers to improve the interfacial adhesion between rubber and mLLDPE.

Magnesium oxide (MgO) has a high electrical resistivity of 1015 Ω cm at 600 K [17]. In addition, it has a high physical strength and stability and a melting point near 2852 °C [18]. MgO is an effective insulation material for polymer materials at a sufficient level of addition; therefore, it can be used to increase insulation properties [19–21].

Zinc borates are vital engineering substances which have been utilized in a variety of different applications, such as an environmentally friendly flame retardant agent in polymers, preservative in wood products, corrosion inhibitor in coating and lubricating additive in industrial oils [22–24]. In application of flame retardant agent, Zinc borate has been shown to reduce smoke in specific formulations [25] and its action is that of first cooling through water release and then forming borate glass to protect the substrate [25].

In our previous work [26–28], we did a series of studies on improvements to the composites to be used in wire and cables applications where the effects of blending different amounts of (90/10) natural rubber/epoxidized natural rubber (NR/ENR-50) with mLLDPE and HVA-2 on the mechanical, thermal and dielectric properties were investigated [26]. The highest tensile strength and best dielectric properties were observed in the 90/(9/1) mLLDPE/NR/ENR-50 blend. Therefore, this blend was used for further studies to improve the dielectric properties of the blend using MgO. In our second work, we studied the effects of the different loading of MgO blended with 10 % rubbers (9/1 NR/ENR-50) and mLLDPE in the presence of HVA-2 compatibilizer [27] on the thermal and dielectric properties. The results revealed that the 20 phr [parts per hundred parts of total polymer (rubbers and mLLDPE)] MgO composite had the best thermal stability and dielectric properties as compared to pure mLLDPE and other composites. In our third work [28], we focused on improvement the thermal properties of the best composite of our second work using zinc borate and the results revealed that 6 phr ZB/20 phr MgO/10 % rubbers (9/1 NR/ENR-50)/mLLDPE composite was best in thermal properties.

This study is a continuation of our previous studies on ways to assess the mechanical and dielectric properties of 20 phr MgO blended with zinc borate for use in wire and cable applications, where mLLDPE is filled with (9/1) NR/ENR blend. Natural rubber (NR) has been chosen as a blend constituent for its good electrical characteristics. The other blend constituent is epoxidized natural rubber (ENR) containing 50 mol% of epoxy groups, which has been chosen for its reasonably good impact strength, good solvent and oil resistance properties, and its proven ability to act as an effective compatibilizer. MgO was chosen for its excellent dielectric properties. In addition, 2 phr HVA-2 was used as a compatibilizer for all compounds. Therefore, the objective of this article is to investigate the mechanical and dielectric properties of mLLDPE/(NR/ENR-50) (90/9/1 wt%)/20 phr MgO blend at different contents of zinc borate (2–8 phr). It is worth mentioning here that previous studies have not explored these composites and the study of mechanical and dielectric properties of mLLDPE/(NR/ENR-50)/MgO composite at different contents of zinc borate is still needed. Therefore, this is the first attempt to assess the suitability of these composites for wire and cable applications.

Experimental

Materials

Polymer matrix, metallocene linear low-density polyethylene (mLLDPE), Exceed 1018CA with a density of 0.918 g/cm3, a melt flow index of 1.0 g/10 min and a DSC melting peak at 119 °C was obtained from ExxonMobil Chemical Singapore Private Ltd. [29]. The second component was natural rubber (SMR-L grade) obtained from the Rubber Research Institute of Malaysia. The third component used in this study, epoxidized natural rubber with 50 mol% epoxidation (grade Epoxyprene 50), specific gravity of 1.03 and Mooney viscosity, M L, of 140 at a temperature of 100 °C, was obtained from the Malaysian Rubber Board, Malaysia. The compatibilizer used for the composite was HVA-2, a free radical crosslinking agent from DuPont Dow Elastomers. The physical properties of HVA-2 include a melting point of 195 °C and a density of 1.44 g/Cm3.

Magnesium oxide grade EMAG® 1000, a calcined natural magnesium oxide manufactured by Queensland Magnesia’s Pty Ltd, Australia, is mined from a high quality cryptocrystalline magnesite deposit. EMAG® 1000 is a non-toxic, pale pink powder characterized by high magnesium content and neutralizing properties. The chemical analysis and physical properties are mentioned in Table 1 [30].

Table 1 Chemical analysis and physical properties of magnesium oxide grade EMAG® 1000a
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The supplier of zinc borate was Borax Polymer Additives Group. Firebrake ZB grade zinc borate is a multifunctional boron-based fire retardant with applications in a variety of plastic and rubber compounds. Firebrake ZB releases its hydration water at temperatures exceeding 290 °C (554 °F) [31]. The chemical and physical properties of Firebrake ZB are shown in Table 2 [31]:

Table 2 Chemical and physical properties of Firebrake ZB (Borax Polymer Additives Group)
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Preparation of composites

All raw materials were initially dried in an oven at 80 °C for 24 h prior to processing. Compositions consisting of HVA-2 were mixed separately with rubber to obtain a homogenous mix using milling machine (Double Elephants Brand SX-160, China). This uniformly mixed compound was shredded to pieces manually using a pair of scissors. The pellets were compounded with mLLDPE, zinc borate and magnesium oxide using a twin screw extruder (Brabender PL2000 with L/D = 30 and D = 2.5 cm) at barrel temperature of 140, 145 and 150 °C at feeding zone, metering zone and die zone, respectively, at a speed of 45 rpm. The prepared compound was mixed in milling machine at 110 °C to obtain homogenous mixture. Test samples were prepared by compression molding at 205 °C under pressure of 68.7 × 105 Pa for 15 min. The rubber content percentage of NR/ENR-50 was fixed at 90/10 wt%, respectively. The compound formulations are given in Table 3.

Table 3 Compound formulation of mLLDPE/NR/ENR/MgO/HVA-2 composite samples
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Tensile strength test

The tensile strength was carried out according to ASTM 638-10 using a universal mechanical test (LLOYD, A trademark of AMETEK, Inc., UK) under ambient circumstances with crosshead speeds of 50 mm/min. The determination of result was carried out by the average test value of five samples for each formulation.

Short-time breakdown voltage test

In the present work, the breakdown voltage of mLLDPE and their composites were determined using a Digital Measuring Instrument, Haefely Trench (Switzerland). Steel Circle-Plane electrodes configuration was used to understand the breakdown characteristics of the material under AC voltages following ASTM-D149-09. The dimensions of the sample were diameter of 90 mm and thickness of 2.5 mm. A schematic diagram of the electrodes configuration used in the present work is shown in Fig. 1. The specimens were tested at a temperature of 25–26 °C and humidity of 56–74 %. AC voltage at frequency of 50 Hz was applied to the sample at the rate of 500 V/s till breakdown occurred. The samples were subjected to an average of five breakdowns to determine the breakdown voltage of the material.

Fig. 1
figure 1

Schematic diagram of the electrodes configuration (all dimensions in mm)

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Permittivity, dielectric loss and volume resistivity tests

The permittivity, dielectric loss and volume resistivity of mLLDPE and its composites were investigated by using a Measuring Bridge, Inst. PWR Supply, TETTEX Instrument (Switzerland). The same electrodes mentioned above were used to study these tests for mLLDPE and its composites. Dielectric constant ε r was determined according to ASTM D150-11.

Results and discussion

Mechanical properties

The effects of the loading of different weights of the ZB on the tensile strength, elongation-at-break and Young’s modulus of A1 and A2, B4, C1, C2, C3 and C4 composites were studied. The mechanical properties of A1, A2 and B4 composites were investigated in detail in our previous work [26, 27]. The present work was focused on the mechanical properties of C1, C2, C3 and C4 composites. The following deductions can be made about these properties.

Stress–strain curves

Stress–strain curves of A1 and A2, B4, C1, C2, C3 and C4 composites are given in Fig. 2. The increase in the loading of ZB content increased the plastic response, as indicated by the increased initial modulus and the decreased elongation-at-break more than virgin polymer. All samples showed neck formation and plastic flow. As can be seen in Fig. 2, for the all composites with ZB, a general increment in elastic modulus and yield stress is obtained with increasing the filler content as compared with A2 blend except C1 composite in which there was no change. The increment in elastic modulus and yield stress may be attributed to the two inflection points of mLLDPE that are smaller in size and the narrower distribution of the crystallites of mLLDPE and its polymerization method (metallocene) [32]. Interestingly, for C1, C2, C3 and C4 composites, their stress–strain curves resemble that of typical plastic, showing a strain-induced crystallization phenomenon at high strains (200 % elongation).

Fig. 2
figure 2

Stress–strain curve of A1 (pure mLLDPE) and A2, B4, C1, C2, C3 and C4 composite samples

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Tensile strength, elongation and modulus

Figure 3a–c shows the effect of the zinc borate (ZB) loading on the mechanical properties of A1, A2, B4, C1, C2, C3 and C4 composite samples, respectively. It is expected that the tensile strength and the elongation-at-break decrease with loading of ZB or MgO additives. When the MgO loading is 20 phr (parts per hundred resin), the tensile strength value decreases from 28.7 to 26.65 MPa, and the elongation-at-break value from 569.2 to 545.76 % as compared with A2 blend. At the same time, the loading of ZB at C1 composite, the tensile strength value decreases from 28.7 to 19 MPa, and the elongation-at-break value from 569.2 to 352.68 % as compared with A2 blend. On the other hand, the tensile strength and elongation-at-break of C1, C2, C3 and C4 composites slightly decreased with increased loading of ZB to reach lower values of 18 MPa and 337.2 % at C4 composite, respectively.

Fig. 3
figure 3

Effect of a tensile strength, b strain-at-break and c Young’s modulus of pure mLLDPE (A1) and A2, B4, C1, C2, C3 and C4 composite samples

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The above results indicate that the mechanical properties of all composites are greatly deteriorated. The reductions in the properties are mainly due to the poor adhesion between the fillers and the polymer substrate and because of their inherent incompatibility as revealed by the results of scanning electron micrographs in our previous work [27, 28]. The decrease in elongation-at-break of the composites in the presence of ZB indicates hindrance by ZB to molecular mobility or deformability of composites which causes matrix to lose its elastic deformation ability. Hence, composites break at a lower elastic deformation. The next important reason for such a decrease is the existence of the cavities within the samples, formed via thermal decomposition of ZB filler and release of steam during the process and the voids or flaws in the mLLDPE matrix, which might grow into cracks [16]. Therefore, the composites with present ZB were more deteriorated than B4 composite.

An additional reason for the decreasing trends of the tensile strength and elongation-at-break for ZB-filled 10 % rubber/mLLDPE/MgO composite may be due to the small surface area of filler which gives rise to a weaker interaction between the filler and mLLDPE matrix thus acting as non-reinforcing filler. The field emission scanning electron microscope (FESEM) results in our previous works [27, 28] illustrated the poor dispersion and unwetted ZB and MgO fillers agglomerates in the composites, therefore the mechanical properties (tensile strength and elongation-at-break) of composites decreases with increasing filler loading.

Similar result was found by different works. Ramazani et al. [33] studied the effects of flame retardant fillers mixture, zinc borate and aluminum hydroxide on polypropylene including various ratios of their accumulation and they found that increasing filler quantity, in all samples, has caused a decrease in tensile strength and elongation-at-break. Hashim et al. [34], Yildiz et al. [24] and Sain et al. [35] observed a similar mechanical influence of zinc borate addition on polymer systems performance. On the other hand, Wang et al. [36], Niemelä et al. [37] and Liu et al. [38] reported that the magnesium hydroxide (MH) in polymer systems causes reductions in tensile strength and elongation-at-break properties.

The tensile modulus of the A1, A2, B4, C1, C2, C3 and C4 composites is shown in Fig. 3a. The results revealed that tensile modulus of the B4, C1, C2, C3 and C4 composites increases with increasing the ZB loading and in the presence of MgO. The increase in modulus with ZB and MgO loading in composites indicates the ability of ZB and MgO to impart greater stiffness to the polymer composites. Ramazani et al. [33] and Sain et al. [35] observed a similar mechanical influence of zinc borate addition on polymer systems performance as mention above. Also, Ismail et al. [39] studied white rice husk ash (WRHA) filled natural rubber (NR)/linear low density polyethylene (LLDPE) composites with and without compatibilizer, poly(propylene–ethylene–acrylic acid), (PPEAA). They found that increasing WRHA loading in NR/LLDPE composites resulted in reduction of tensile strength and elongation-at-break while tensile modulus increases.

It was also observed that the composite samples of C1, C2, C3 and C4 had higher tensile modulus than A1, A2 and B4 samples. These phenomena may be attributed to the greater increase in the amount of filler compared to other composites which imparts additional stiffness to the polymer composites. Another reason for the increase in the tensile modulus in C1, C2, C3 and C4 composites can be attributed to the crystallinity of these composites which were increased by 7.9, 9.7, 12.4 and 10.2 % as compared with pure mLLDPE as shown in our previous work [28]. It is well-known that crystallinity had a direct effect on tensile modulus where it increases with an increased crystallinity [40]. Mae et al. [41] reported that the relative elastic moduli of the polypropylene drastically increased with increased density.

Moreover, it may be the chemical cross-linking density in rubber phases of C1, C2, C3 and C4 composites was greater than A1, A2 and B4 composites because zinc oxide plays different roles such as vulcanizer, activator, acid acceptor, thickener and anti-caking agent. In the rubber industry zinc oxide is used as activator. Its role in the mechanism of accelerated sulfur vulcanization has been extensively studied [42, 43] and the modulus increases with an increase crosslinking density [44, 45]. An additional reason may be that the appearance of carbonyl groups in the structure of composites as revealed by FTIR results in our previous work [28], cause increased interaction in the composites, thus the modulus increased in all composites.

Moreover, it may be because the cavities within the sample C2 which were formed via release of steam during the process were lower than other composites as revealed by a tensile strength nearly higher than other composites. Kishore et al. [46] studied the effect of dispersing hollow glass microballoons in thermosetting epoxy resins on tensile strength and modulus. They found that both tensile strength and modulus increased from 23.8 to 41.9 MPa and from 2 to 2.47 GPa, respectively, as the volume % of microballoons in the sliced sections decreased from 43.9 to 25.9. Mae et al. [41] studied the effects of local strain rate and density on the mechanical properties of the polypropylene syntactic foams. They reported that the elastic modulus decreases drastically once the microballoon is blended.

Dielectric properties

The dielectric properties of A1 and A2, B4, C1, C2, C3 and C4 composites were characterized in terms of relative permittivity, dielectric loss, dielectric strength and volume resistivity at frequency 50 Hz to determine the insulation value of composites. The dielectric properties of A1, A2 and B4 composites were studied in detail in our previous work [27]. The present work was concerned with the dielectric properties of C1, C2, C3 and C4 composites. The following explains the effect of each electrical property:

Relative permittivity

The permittivity of A1 and A2, B4, C1, C2, C3 and C4 composites were studied and the results revealed in Fig. 4. The permittivity slightly increases as the voltage increases from 1 to 5 kV in all compounds, as shown in Fig. 4. This result can be explained by the increase in the mobility of polar groups with the increase in voltage values and, hence, an increase in the effect of the environment that facilitates the orientation of the mobile groups [1].

Fig. 4
figure 4

Permittivity of A1 (pure mLLDPE) and A2, B4, C1, C2, C3 and C4 composite samples at different voltage (kV) and at 50 Hz frequency

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It was observed that the permittivity of C1, C2, C3 and C4 composites were lower than pure mLLDPE, A2 and B4 composites. The function of polar groups in the fillers of MgO and ZB can change the structure of the host material. An addition, the surface polar particles can act as physical cross linking centers, changing the structure of the material [47]. The surface function polar groups lead to an alignment of polymer chains close to the surface due to intermolecular forces between MgO and ZB fillers and epoxy polar group in ENR or new function polar groups (–C=O and –OH) of composites as revealed by FTIR results in our previous work [28]. These forces might also affect the periphery, thus affecting the layer. The aligned layer with its rigid structure can lead to an immobilization of molecules around the particle. This would lead to a decrease of permittivity (ε′).

Another reason for the decrease in the permittivity of C1, C2, C3 and C4 composites may be due to an increase in crystallization of composites with an increase in the ZB content, as recorded in our previous work [28]. Therefore, the dielectric permittivity decreases because of the changing number of mobile dipoles and charge carriers contributing to the signal [48, 49].

As mentioned previously, the MgO and ZnO play different roles such as vulcanizer, activator, acid acceptor, thickener and anti-caking agent and in the rubber industry they are used as activator and its role in the mechanism of accelerated sulfur vulcanization has been extensively studied [42, 43, 47]. In our previous work [28], it was clear from FTIR that the C–O–C and C=C bands of rubbers at 1250.56 and 1660 cm−1, respectively, were found to be absent in all composites. Therefore, it can be assumed that the opening of the double bond in NR and ENR-50 formed an inter-chain cross linking in the system. Therefore, the permittivity property of C1, C2, C3 and C4 composites may be decreased by an increase in the cross linking density of composites. The increased crosslink density of mLLDPE composites would hinder the motion of dipoles, resulting in enhancing the dielectric properties [50–52]. Therefore, loading of ZB in the composite improved the permittivity of composites.

An additional reason may be that the ZB has ZnO in its structure, which may be because the ε′ of ZnO is lower than neat polymers at around 1.7 [53] and according to conventional mixture rules for composites the permittivity of the compound should be lower than the base material.

On the other hand, the AC permittivity of the composites ranks as C4 > C2 > C1 > C3 composites as shown in Fig. 4. These results may be because the decreasing amounts of ZB in the composites increase the physical cross linking centers where zinc oxide works as cross linking activator in rubber industries [42, 43]. Moreover, lower permittivity was observed in C3 composite and this result may be because the C3 composite possesses higher physical cross linking than other composites with ZB. An additional reason for this result may be because the C3 composite possessed highest crystallinity as shown in our previous work [28], and it is well-known that as crystallization increases, dielectric permittivity decreases because of the changing number of mobile dipoles and charge carriers contributing to the signal [48].

Immobilization seems to be the main factor determining the ε′ of composites with particle surfaces of function polar groups. The immobilization of the particle surfaces overshadows the influence of the higher permittivity of the particles of MgO and ZB, thus reducing the overall ε′ of the compound.

Dielectric loss

The AC dielectric loss (ε″) of A1 and A2, B4, C1, C2, C3 and C4 composites were investigated and the results revealed in Fig. 5. It was shown that the dielectric loss of C1, C2, C3 and C4 composites increased as compared to A2 and B4 composites.

Fig. 5
figure 5

Dielectric loss of A1 (pure mLLDPE) and A2, B4, C1, C2, C3 and C4 composite samples at 50 Hz frequency

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Generally, the dielectric relaxation effects are observed only when the metal ions present in the blend network are in the divalent state. Among the constituent elements in these composites, divalent zinc ions having higher electronegativity may form dipoles with a pair of cationic vacancies and such types of dipole are responsible for the dielectric relaxation effects [54], by the addition of filler increasing the dipole polarity of the composite in the interfacial region, because fillers generally possess higher polarity than polymer. Therefore, the increase of filler loading would make dipole polarity rise and boost dielectric loss [55]. Addition reason may be that the presence of carbonyl and hydroxyl groups in all composites, as revealed by FTIR results in our previous work [28], causes an increase in the dipoles polarity of composites in the interfacial region.

On the other hand, the dielectric loss of C1 composite was highest among all other composites. This may be because the value of crystallinity of C1 composite was lower than other ZB composites, and as crystallization decreased, dielectric loss increased because of the changing number of mobile dipoles and charge carriers contributing to the signal [48]. The lowest dielectric loss among ZB composites was in C3 composite, therefore dielectric loss was improved in C3 composite. This may be attributed to the C3 composite possessing higher crystallinity than other composites as shown in our previous work [28]. Moreover, may be the cross linking density of C3 composite was higher than other composites and it is well-known that the dielectric properties were improved by an increase in the cross linking density. Furthermore, the increased crosslink density of mLLDPE composites would hinder the motion of dipoles [50, 52].

Breakdown strength

Figure 6 shows the AC breakdown strength of A1, A2, B4, C1, C2, C3 and C4 composites. It was showed that the breakdown of C1, C2, C3 and C4 composites decreased by 8.3, 15.8, 1.1 and 17.9 %, respectively, as compared with A2 blend. The explanation is given by the divalent zinc ions have higher electronegativity than other elements in the blend, it may form dipoles with a pair of cationic vacancies and such types of dipole are responsible for the dielectric relaxation effects [54]. Therefore, the addition of filler increases the dipole polarity of the composite in the interfacial region, because fillers generally possess higher polarity than polymer. For this reason, the increase of filler loading would make dipole polarity rise and decrease dielectric strength [55].

Fig. 6
figure 6

Breakdown strength (kV/mm) of A1 (pure mLLDPE) and A2, B4, C1, C2, C3 and C4 composite samples at 50 Hz frequency

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Another explanation may be appearance of a new polar groups of –C=O and –OH in composites as revealed by FTIR in our previous work [28]. This new polar group may cause the formation of dipoles inside the composites, resulting in the observation of effects of dielectric relaxation. The dipoles are responsible for the dielectric relaxation effects [56] by increasing the dipole polarity of the composite in the interfacial region. Therefore, the appearance of dipoles would make dipole polarity rise and dielectric strength decrease [55].

On the other hand, the breakdown of C3 was higher than all composites of ZB and this may be because the value of crystallinity of C3 composite was higher than other ZB composites as shown in our previous work [28]. Another reason may be that C3 composite possesses higher cross linking than other composites and as mention above the increase in cross linking will improve dielectric properties.

Volume resistivity

Figure 7 shows the AC resistivity of A1, A2, B4, C1, C2, C3 and C4 composites at 50 Hz frequency. The volume resistivity of C1, C2, C3 and C4 composites increased as compared to A1, A2 and B4 composites. It was observed that the volume resistivity of C1, C2, C3 and C4 composites was constant. These results agree with permittivity results that the ZB in the composites causes an increase in the physical cross linking centers and the mobility of particles would be decreased. Therefore, more obstruction would be generated finally leading to higher volume resistivity.

Fig. 7
figure 7

Volume resistivity (Ω mm) of A1 (pure mLLDPE) and A2, B4, C1, C2, C3 and C4 composite samples at 50 Hz frequency

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Another reason may be that magnesium oxide and zinc oxide are used as activator and its role in the mechanism of accelerated sulfur vulcanization has been extensively studied [42, 43, 47]. In our previous work [28], FTIR results revealed that the opening of the double bond in NR and ENR-50 formed an inter-chain cross linking in the composites. Therefore, the volume resistivity property may be improved by an increase in the cross linking density of C1, C2, C3 and C3 composites. The increased crosslink density of mLLDPE composites would hinder the motion of dipoles, resulting in enhancing the dielectric properties [50, 51, 56].

An additional reason for the increased volume resistivity of C1, C2, C3 and C4 composites is the increase in the loading of zinc borate. These results may be attributed to collision of electrons by ZB particles inside the matrix which would obstruct the electron avalanche with more filler concentrations in the composite, more obstruction would be generated finally leading to a higher volume resistivity. As mentioned above, the dielectric properties improved by increasing the crystallinity of polymer [48], therefore, the other reason for the increase in volume resistivity is due to the increase in the crystallinity of composites, as shown in our previous work [28].

Conclusion

The effects of ZB loadings on the mechanical and dielectric properties of 20 phr MgO/10 % rubber (9/1 NR/ENR-50)/mLLDPE composites with HVA-2 compatibilizer have been investigated. The tensile strength and the elongation-at-break decreased with the loading of ZB additive while the Young’s modulus increased. The strain-induced crystallization phenomenon greater than 200 % was observed in strain. The relative permittivity of the mLLDPE composite material showed an increase when the supply voltage was increased, but it decreased when mLLDPE/rubber/MgO composite was filled with ZB at a frequency of 50 Hz. Lower permittivity was observed in 6 phr ZB composite. On the other hand, the dielectric loss and volume resistivity of ZB composites increased as compared to pure mLLDPE or other composites, while the breakdown strength decreased in all ZB composites as compared to pure mLLDPE or other composites without ZB. The results of this research showed that all composites are suitable for wire and cable applications and 6 phr ZB composite is found the best composite because it has the lowest dielectric constant and loss and the highest break down and volume resistivity among the ZB composites.