Effect of Natural Rubber/Epoxidized Natural Rubber (90/10) on Dielectric Properties and Crystallization of Metallocene Linear Low

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1 Effect of Natural Rubber/Epoxidized Natural Rubber (90/10) on Dielectric Properties and Crystallization of Metallocene Linear Low Effect of Natural Rubber/Epoxidized Natural Rubber (90/10) on Dielectric Properties and Crystallization of Metallocene Linear Low I.M. Alwaan a,b, A. Hassan *a and M.A.M. Piah c a Faculty of Chemical Engineering, Universiti Teknologi Malaysia b Faculty of Engineering, University of Kufa, Najaf, Iraq c Faculty of Electrical Engineering, Universiti Teknologi Malaysia Received: 23 April 2014, Accepted: 29 September 2014 SUMMARY The effects of different amounts of natural rubber/epoxidized natural rubber (90/10 NR/ENR-50) blended with metallocene linear low density polyethylene (mlldpe) with HVA-2 compatibilizer on the permittivity, dielectric loss, breakdown and volume resistivity at frequency of 50 Hz were investigated. Increasing the voltage with a range 1-5 kv increased the permittivity of both virgin mlldpe and its blends. The permittivity, dielectric loss and breakdown of mlldpe continuously decreased with increasing rubber content, but the AC volume resistivity of mlldpe continuously increased with the rubber content. The degree of crystallinity of the mlldpe composites was determined by differential scanning calorimetry. The lowest crystallinity and the maximum density of physical cross-links were observed in 10% rubber/mlldpe blend. Increasing the proportion of rubber decreased the melting points of the blends. The FTIR showed that the epoxy groups and double bonds were absent from all the blends while the carbonyl group appeared in all cases. Keywords: Metallocene linear low density polyethylene, Natural rubber, Epoxidized natural rubber, Dielectric; Crystallization 1. Introduction Investigation of the dielectric properties of insulating materials has led to better understanding and new applications of these materials. Polyethylene is used as an insulation material for wires and cables because of its excellent dielectric properties 1. Metallocene linear low density polyethylene (mlldpe) from single site, constrained geometry catalysts optimizes the polymer chain architecture to generate highly uniform polymer molecules with specific target properties. This technology of molecular synthesis and molecular control for polyolefin provides new options for replacing or modifying materials across a broad spectrum of processes and applications 2. * b Corresponding author: azmanh@cheme.utm.my, Tel: , a ism10alw@yahoo.com, c fendi@fke.utm.my Smithers Information Ltd., 2015 The effects of metallocene polyethylene properties such as density and crystallinity on dielectric behaviour have been previously discussed by researchers 2-4. They found that the dielectric constant and dissipation factor for metallocene polyethylene were about the same over various frequency ranges. For the dissipation factor at a power frequency of 60 Hz, mlldpe is as good as that of LDPE which is used as a control material. The AC breakdown strength of metallocene linear low density polyethylene increases as the crystallinity of the metallocene linear low density polyethylene increases. The accumulation of space charge decreases with increases in the crystallinity ratio. Wang et al. 5 reported that the amount of space charge accumulated in the developed polyethylene polymerized using the metallocene catalyst is smaller than that in conventional polyethylene. Taniguch et al. 6 studied the space charge behaviour in lowdensity polyethylene polymerized by using metallocene catalyst (m-ldpe). Four kinds of m-ldpe films were polymerized: mb was polymerized with butene, while and mh-1, mh-2 and mh-3 were polymerized with hexene at different densities. The experimental results revealed that the space charge amounts decreased when film density increased, but the types of comonomer (butene or hexene) had little effect on space charge properties. In the present investigation, mlldpe was filled with (9/1) natural rubber (NR)/ epoxidized natural rubber (ENR) blend. NR has been chosen as one of the blend constituents for its good electrical and high mechanical Polymers & Polymer Composites, Vol. 23, No. 7,

2 I.M. Alwaan, A. Hassan and M.A.M. Piah strength characteristics. The other blend constituent, chosen because of its reasonably good impact strength, good solvent and oil resistance properties and proven effectiveness as a compatibilizer is ENR containing 50 mol.% of epoxy groups. Therefore, the objective of this research was to investigate the dielectric properties and crystallization of mlldpe at different rubber contents (90/10 NR/ENR-50) in order to assess the suitability of these blends for dielectric and engineering applications. 2. Experimental 2.1 Materials Metallocene linear low-density polyethylene (mlldpe), Exceed 1018CA, was obtained from ExxonMobil Chemical Singapore Pte. Ltd. Its density is g/cm 3, melt flow index is 1.0 g/10 min and DSC melting temperature 7 of 119 o C. The second component was NR (SMR-L grade) obtained from the Rubber Research Institute of Malaysia. The third component used in this study, ENR with 50 mol.% epoxidation (grade EPOXYPRENE 50), specific gravity of 1.03 and Mooney viscosity, M L, of 140 at a temperature of 100 o C, was obtained from the Malaysian Rubber Board, Malaysia. The compatibilizer used for the blend was N,N-mphenylenebismaleimide (HVA-2), crosslinking agent from DuPont Dow Elastomers. The physical properties of HVA-2 are: melting point 195 o C and density 1.44 g/cm 3. (Brabender PL2000 with L/D = 30 and D= 2.5 cm) at barrel temperatures of 140, 145 and 150 C at feeding zone, metering zone and die zone, respectively. Speed was set at 45 rpm. The prepared compound was mixed in a milling machine at a temperature of 110 o C to obtain a homogenous mixture. Test samples were prepared by compression moulding at a temperature of 205 o C and 15 min under 70 kg/cm 2 pressure. The percentage of rubber content was fixed at 90/10 wt.% in the NR/ENR-50 formulation. The compound formulations are given in Table Short Time Breakdown Voltage Test In the present work, the breakdown voltage of mlldpe and their blends were determined using a Digital Measuring Instrument, Haefely Trench (Switzerland). The steel Circle- Plane electrodes configuration was used to understand the breakdown characteristics of the material under AC voltages following ASTM-D standard 8. 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 Figure 1. The specimens were tested at a temperature of o C and humidity of 56-74%. AC voltage was applied to the sample, increasing at the rate of 500 V/s until breakdown occurred. Samples were subjected to an average of 5 breakdowns to determine the breakdown voltage of the material. 2.4 Permittivity, Dielectric Loss and Volume Resistivity Tests The permittivity, dielectric loss and volume resistivity of mlldpe and its blends were investigated by using a Measuring Bridge, Inst. PWR Supply, TETTEX Instrument (Switzerland). The same electrodes mentioned above were used to carry out these tests for mlldpe and its blends. Dielectric constant Ɛ r was determined according to ASTM 9 D Differential Scanning Calorimetry (DSC) Thermal properties of mlldpe NR, ENR-50 and their blends were determined by using a DSC7 (Perkin- Elmer, U.S.A.) differential scanning calorimeter. The samples were analyzed by DSC over the temperature range from ambient to 200 o C. A heating rate of 10 o C/min and a nitrogen atmosphere flow rate of 20 ml/min around the sample were used. The temperature Figure 1. A schematic diagram of the electrodes configuration, all dimensions in mm 2.2 Preparation of Blends All materials were initially dried in an oven at a temperature of 80 o C for 24 h prior to processing. Compositions containing of HVA-2 were mixed separately with rubber to obtain a homogenous mix using a milling machine. This uniformly mixed compound was manually shredded to pieces using a pair of scissors. The pellets were compounded with mlldpe using a twin screw extruder Table 1. mlldpe/nr/enr/hva-2 compound formulations Materials mpe 10% Rubber 20% Rubber 30% Rubber mlldpe a Rubber (90%NR+10%ENR) a HVA-2 b a Weight Percent, b Parts per hundred parts (Phr) of total polymer (rubber and mlldpe) 496 Polymers & Polymer Composites, Vol. 23, No. 7, 2015

3 Effect of Natural Rubber/Epoxidized Natural Rubber (90/10) on Dielectric Properties and Crystallization of Metallocene Linear Low was kept at 50 and 200 C for 5 min for the thermal history studies. The percentage of crystallinity was calculated according to Eq. (1) as the enthalpy of fusion ( H f ) taken from the DSC test divided by the enthalpy of 100% crystalline PE ( H o ). The latter was taken 10,11 as 279 J/g. (1) where k is the weight fraction of mlldpe in the blends. 2.6 Fourier Transform Infrared Spectroscopy (FTIR) To identify the chemical structures of mlldpe, NR and ENR-50 and their composites, FTIR spectroscopic analysis was carried out. IR spectra of samples were recorded on a Perkin- Elmer FTIR spectrophotometer. The measurements were taken over a range of cm -1 at room temperature, and a uniform resolution of 16 cm -1 was maintained in all cases. 3. Results and Discussion 3.1 Fourier Transform Infrared Spectroscopy (FTIR) The FTIR spectra of ENR and NR are presented in Figure 2. The characteristic bands observed were: 2963 and 2961 cm -1 ( C H aliphatic stretching), 1450 and 1449 cm -1 ( CH 2 bend), 1378 and 1376 cm -1 ( CH 3 symmetric bending), 701 and 737 cm -1 ( (CH 2 ) n wagging) respectively. The band of stretching C=C bonds of ENR and NR was observed at 1663 cm -1, and the unsaturated group (>C CH ) appeared in NR spectrum at 836 cm -1. The epoxide group of ENR-50 (C O C) appeared at 1251 and 876 cm -1 in FTIR spectrum of ENR-50. This result was similar to the observation by Latif 12 et al. aliphatic stretching), 1465 cm -1 ( CH 2 bend), weak peak at 1372 cm -1 ( CH 3 symmetric bending) due to the presence of hexyl side chain and 720 cm -1 ( (CH 2 ) 2 wagging) due to the presence of polyethylene chain. These results are in agreement with the work of Saha et al. and Vladimir 13,14 et al. The FTIR spectrum of 90/10/2, 80/20/2 and 70/30/2 mlldpe/rubber/hva- 2 blends are presented in Figure 2: 2919 cm -1 ( C H aliphatic stretching), 1465 cm -1 ( CH 2 rocking), cm -1 ( CH 3 symmetric bending) and cm -1 ( (CH 2 ) n wagging). A band at cm -1 was due to the epoxide group of ENR-50 (C O C). On the other hand, the C O C and C = C band of ENR 50 at 1251 and 1663 cm 1 respectively were found to be absent in all blends. Therefore, it can be assumed that the opening of the double bond in NR and ENR-50 may be due to the formation of interchain crosslinks in the system. However, the C = O band appeared at 1718 cm 1 in all compounds. It was clear from FTIR that the relationship between the rubber loading and the C=O band intensity at 1718 cm -1 was an inverse one, meaning that an increase in the concentration of rubber in the blends will result in a decrease in the concentration of C=O group in the blends Crystallinity Study Differential Scanning Calorimetry (DSC) The thermal and crystallization behaviour of rubber/mlldpe blends have been studied using DSC. Crystallinity was reduced by 7%, i.e., from 15% (neat polymer) to 8 % for 10% rubber/mlldpe in the blend and also by 3% for 20% rubber/mlldpe blend as shown in Figure 3. In contrast, the crystallinity of the 30% rubber (NR/ENR-50) blend increased to 4% more than pure mlldpe. Thus, the maximum reduction was in the 10% rubber blend. Dahlan 16 et al. found that when the percentage crystallinity of virgin polymer decreased the physical crosslinking in the blend was increased. In our case the crystallinity of 10 and 20% rubber blends decreased compared with virgin polymer (mlldpe), therefore the physical crosslinks would increase. Because the crystallinity of Figure 2. FTIR for mlldpe, ENR-50, NR, 10% rubber, 20% rubber, 30% rubber Figure 2 presents the FTIR spectrum of mlldpe: 2919 cm -1 ( C H Polymers & Polymer Composites, Vol. 23, No. 7,

4 I.M. Alwaan, A. Hassan and M.A.M. Piah Figure 3. The effects of rubber composition on the crystallinity of rubber/ mlldpe blends Table 2. DSC results of melting temperature ( o C) for rubber/ mlldpe blends compound formulations Materials Melting Temperature ( o C) T m 0% rubber % rubber % rubber % rubber materials and the following explains the effect of each electrical property: Figure 4. The effects of rubber composition on the permittivity of rubber/ mlldpe blends at different voltage (kv) and at 50 Hz frequency Relative Permittivity Relative permittivity is the ratio of the equivalent parallel capacitance, Cp, with a dielectric material to the vacuum (or air) capacitance, Cv, of the same configuration of electrodes. As shown in Figure 4, the permittivity of all the blends increased slightly as the applied voltage increased from 1 to 5 kv. This result can be explained by the increase in mobility of the polar groups with increase in voltage values, hence increasing the effect of the environment that facilitates the orientation of the mobile groups 20. the 10% rubber blend had lowest value, it had a maximum density of physical crosslinks. The 30% rubber blend had the highest crystallinity, which means a decrease of interfacial adhesion and decreased interaction between the stress concentration zones in the mlldpe matrix 16,17. Moreover, the crystallinity of the blends (rubber/mlldpe) increased with a decrease in chemical crosslinking caused by increased rubber loading. A similar observation was also reported by Moly 18 et al. They studied crystallization of LLDPE/EVA blends with and without dicumyl peroxide (DCP) as crosslinking agent. They found that increasing crosslinking caused a decreased crystallinity. Inoue and Suzuki 19 investigated the effects of crosslinking of ethylene-propylenediene terpolymer EPDM particles in PP matrix on the crystallinity behaviour, and they found that increasing crosslinking of EPDM caused the crystallinity of PP to decrease. The decrease in crystallinity on cross-linking is due to the fact that cross-linking provides some hindrance to the ordered arrangement of the polymer chains 18. Table 2 shows that the melting temperatures of mlldpe decreased slightly with increased loadings of rubber. 3.3 Dielectric Properties The dielectric properties of mlldpe and their blends were characterized in terms of relative permittivity, dielectric loss, dielectric strength and volume resistivity at a frequency of 50 Hz to determine the insulation value of the Figure 4 shows that the AC permittivity of mlldpe at a power frequency of 50 Hz decreased with increased loading of rubber. The literature reports that the permittivity of natural rubber is lower than that of LLDPE 21,22. According to conventional rules of mixture for composites, the permittivity of the compound should be lower than the base material. Therefore, all composites of mlldpe improved the neat mlldpe. It is possible that the decreasing AC permittivity of the compounds with increased rubber loading may be attributed to the crystallization of blends increasing with increased rubber content, as observed in Figure 3. It is well known that as crystallization increases, the dielectric permittivity decreases because of the changing number of mobile dipoles and charge carriers contributing to the signal Polymers & Polymer Composites, Vol. 23, No. 7, 2015

5 Effect of Natural Rubber/Epoxidized Natural Rubber (90/10) on Dielectric Properties and Crystallization of Metallocene Linear Low Reduced permittivity in the blends may also be attributed to crosslinking of rubber in the blend as revealed by the opening of the double bond in NR and ENR-50 in FTIR results. Furthermore, the increased crosslink density of mlldpe composites would hinder the motion of dipoles, resulting in lower permittivity 24,25. An additional reason may be that the concentration of carbonyl groups decreases with increasing rubber content. revealed by a decreasing peak intensity of the carbonyl groups 15 with increasing rubber content as shown in Figure 2. Thus the dipolar polarity in the blends would decrease with greater rubber content, causing a decrease in the permittivity of blends with loading of rubber 26. Permittivity decreased in all blends compared to neat mlldpe, although carbonyl polar groups also appeared in these blends. One possible explanation may be that the mobility of the carbonyl group was obstructed by both the crosslinking of rubbers and increasing crystallinity in the blends Dielectric Loss The AC dielectric loss of mlldpe and its compounds continuously decreased with the rubber (NR/ENR-50) content. Pure mlldpe has a higher dielectric loss than other composites and 30% rubber (27/3 NR/ENR)/mLLDPE has a lower dielectric loss than the other composites, as shown in Figure 5. The dielectric loss improved in all blends and they gave better results compared to the virgin polymer (mlldpe). charges at interfaces. Therefore, the decrease of dielectric loss in mlldpe composites indicates that exist an interfacial polarization between rubber and mlldpe phases and increasing interfacial polarization with increases in rubber content were not sufficient to overcome the other factors that cause a decrease in dielectric losses. As mentioned in the discussion of FTIR results, a decrease in the peak intensities of the carbonyl groups with increasing rubber content, as shown in Figure 2, is attributed to a decrease in the concentration of carbonyl groups 15. Therefore, an additional reason may be that the dipolar polarity in the blends decreased as rubber content increased, causing a decrease in dielectric loss of blends with loading of rubber 26. As recorded in the literature 2-4,24,15, the dielectric loss and accumulation of space charge decrease with increases in the crystallinity ratio and crosslinking in the blends. Therefore, a possible reason for improvements in the dielectric loss with rubber loading can be attributed to a rise the crystallinity ratio, as shown in Figure 3 and the appearance of crosslinking in the blends, as revealed by FTIR results with rubber loading shown in Figure 3. Similar results of improved dielectric properties for LLDPE/NR blends were observed by different researchers. Abd- El-Messieh 28 et al. investigated the electrical insulating properties of waste polyethylene/ natural rubber blends at a frequency range of 1 khz up to 50 khz at room temperature. They found that the addition of such polyethylene to natural rubber decreased the dielectric loss (ε ). The leakage current and carbon track were developed by blends of linear low-density polyethylene with natural rubber (LLDPE/NR) either filled with or without alumina trihydrate 29,30. A series of linear low-density polyethylene (LLDPE)/natural rubber (NR) blends with 10 wt% of maleic anhydride grafted linear low-density polyethylene (LLDPE-g-MAH) as a compatibilizer containing different fillers were studied 31. The results revealed that the total partial discharge (PD) numbers decreased as the weight percentages of natural rubber in composite increased without any filler Breakdown Strength Figure 6 shows that the AC breakdown strength of mlldpe decreased continuously with increasing loading of rubber (NR/ENR-50) content. It is approximately 5.5 kv/mm in pure mlldpe, decreasing to a minimum of 4.57 kv/mm in 30% rubber/mlldpe as shown in Figure 6. An explanation for the decrease in the breakdown is given by the dilution effect as neat NR generally possesses lower breakdown Figure 5. The effects of rubber composition on the dielectric loss of rubber/ mlldpe blends at different voltage (1-5KV) and at 50 Hz frequency Dielectric losses data in the lowfrequency range depict a relaxation process that can only be attributed to an interfacial polarization known as Maxwell-Wagner-Sillars (MWS) effect 27. This phenomenon appears in heterogeneous media consisting of phases with different dielectric permittivities and conductivities, and is due to the accumulation of Polymers & Polymer Composites, Vol. 23, No. 7,

6 I.M. Alwaan, A. Hassan and M.A.M. Piah value than neat mlldpe 32 and the blending of mlldpe with rubber will decrease the breakdown property of the mixture according to the rule of mixtures for plastic 33. The effects of metallocene polyethylene properties such as density and crystallinity on dielectric behaviour have been discussed by Han, S.J. and Gross 2, Pelissou et al. 34, Ishii 35 et al. and Ishimatsu 36. They found that the AC breakdown strength (ACBD) of metallocene linear low density polyethylene (mlldpe) increased as the crystallinity and density of the mlldpe increased. Moreover, by increasing the crystallinity the breakdown decreased, meaning that the effect of crystallinity on breakdown was less than the effect of the lower breakdown of neat NR. Another explanation may be appearance of a new polar group of C=O in blends as revealed by FTIR in Figure 2. This new polar group may cause the formation of dipoles inside the blends, resulting in the observation of effects of dielectric relaxation. The dipoles are responsible for the dielectric relaxation effects 37 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 Volume Resistivity Figure 7 shows that the AC resistivity of mlldpe increases continuously with increased rubber (NR/ENR-50) content. The AC volume resistivity of the pure mlldpe was lower than all the blends, while the highest volume resistivity was observed in the 30% rubber blend. The explanation is given by the volume resistivity of neat mlldpe, which is lower than natural rubber 32 ; therefore the blending of mlldpe with rubber will increase the resistivity property of the mixture according to the rule of mixture for plastic 33. As mentioned above, the dielectric properties were improved by increasing the crystallinity of polymer, and therefore, another reason for the increase in a volume resistivity is due to the increase in the crystallinity of composites that can be attributed to a change in the number of mobile dipoles and charges in the carriers contributing to the signal 23. Figure 6. The effects of rubber composition on the breakdown strength (KV/mm) of rubber/mlldpe blends at 50 Hz frequency Figure 7. The effects of rubber composition on the volume resistivity (Ω.mm) of rubber/mlldpe blends at 50 Hz frequency Other possible reason may be attributed to crosslinking of rubber in the blends as revealed by opening of the double bond in NR and ENR-50 in FTIR results. Furthermore, the increased crosslink density of mlldpe composites would hinder the motion of dipoles, resulting in a rise in volume resistivity 24,25,38. The volume resistivity of blends increased relative to pure mlldpe, and increased continuously with increasing rubber content. This phenomenon is attributed to three competing factors: polar carbonyl groups, crystallinity and crosslink density. Polar groups possess high polarity which makes the dipoles align as a result of cyclic electric excitation, which then decreases the volume resistivity. In the discussion of the FTIR results it was assumed that the opening of the double bond in NR and ENR-50 may be due to the formation of interchain crosslinks in the system. Furthermore, the crosslink density and crystallinity of mlldpe composites would hinder the motion of dipoles, resulting in elevated volume resistivity 24,25. Thereby, the interaction of polar groups, crystallinity and crosslink density contributes to the varied volume resistivity of mlldpe composites 24, Polymers & Polymer Composites, Vol. 23, No. 7, 2015

7 Effect of Natural Rubber/Epoxidized Natural Rubber (90/10) on Dielectric Properties and Crystallization of Metallocene Linear Low 4. Conclusions The effects of (90/10) NR/ENR-50 loadings on the dielectric properties, crystallinity and chemical structure of mlldpe/(nr/enr-50) blended with HVA-2 compatibilizer have been investigated. The FTIR spectrum revealed the absence of functional groups of epoxy and double bond between carbon atoms in all blends, and a carbonyl group vibration appeared in all compounds at 1718 cm 1. The maximum reduction of crystallinity was at 10% rubber blend. On the other hand, the crystallinity of 30% rubber (NR/ENR-50) blend was higher than that of virgin mlldpe. The relative permittivity of the mlldpe composite material at a frequency of 50 Hz showed an increase when the supply voltage was increased, but it decreased when the rubber content was increased. The mobility of polar groups for virgin mlldpe and its blends increased with increasing voltage. 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