Block polypropylene/styrene-ethylene-butylene-styrene tri-block copolymer blends for recyclable HVDC cable insulation

Polypropylene (PP) is regarded as the most promising candidate for eco-friendly high-voltage direct current (HVDC) cable insulation material due to its excellent thermal and electrical properties. In this paper, a block polypropylene (BPP) was selected as the matrix and blended with styrene-ethylene-butylene-styrene tri-block copolymer (SEBS) by melt blending. The thermal behavior, dynamic mechanical behavior, crystallization behavior, and mechanical and electrical properties of the BPP/SEBS blends were investigated. It was found that SEBS could efficiently improve the flexibility while maintaining the excellent heat resistance of PPB. Furthermore, adding an appropriate amount of SEBS had a positive effect on suppressing the space charge and increasing the breakdown field strength. For the BPP/SEBS blend containing 20 wt% SEBS, a desirable combination of thermal, mechanical, and electrical properties was achieved. This work may help pave the way for developing eco-friendly PP insulating material in HVDC cable applications.


Introduction
Cross-linked polyethylene (XLPE) is widely adopted as the insulation material for modern extruded highvoltage direct current (HVDC) cables due to its extraordinary properties, such as its thermo-mechanical stability, chemical resistance, and electrical properties. However, XLPE cannot easily be recycled at the end of its service life because of its thermosetting property. In addition, there is some concern about the environmental impact caused by the addition of a peroxide crosslinking agent during the manufacturing process of XLPE [1][2][3]. Hence, the exploration of novel eco-friendly insulating materials to replace XLPE has been a hot topic in recent years.
Polypropylene (PP), a typical thermoplastic polymer, has been regarded as the most promising candidate for next-generation extruded cables since it possesses high mechanical strength, excellent insulation properties, and superb heat and chemical resistance [4,5]. In recent years, research on isotactic polypropylene (iPP) and its blends have been extensively reported. Zhou et al investigated the thermal, mechanical, and electrical properties of PP/POE blends [6]. Zha et al demonstrated how the space charge accumulation could be effectively suppressed by grafting maleic anhydride onto the molecular chain of PP [7]. Zhang et al introduced a βnucleating agent into PP to obtain a comprehensive improvement of electrical properties [8]. Gao et al investigated the trap distribution and dielectric breakdown of PP/elastomer for DC cable insulation [9]. Zhou et al documented the effects of different nanoparticles (MgO, TiO 2 , Al 2 O 3 , and ZnO) on the electrical properties of PP [10]. Gao et al investigated the influence of compatibility between the PP and the elastomer on the charge accumulation behavior [11].
Compared with iPP, block polypropylene (BPP) has a better impact property and flexibility due to the introduction of ethylene groups [12][13][14]. Zhang  composites for HVDC cables [15]. However, few studies have reported the use of BPP as insulating materials in the field of HVDC cables, thus requiring further development and characterization.
In this study, styrene-ethylene-butylene-styrene tri-block copolymer (SEBS), an outstanding thermoplastic elastomer with excellent flexibility, heat resistance, solvent resistance, and aging resistance, was blended with BPP by mechanical blending. The melting and crystallization behavior, dynamic mechanical behavior, mechanical properties, space charge distribution, and DC breakdown strength were investigated. The present work offers ideas for the development of recyclable HVDC cable insulation material.

Materials
The block polypropylene (BPP, K8003) used in this study was supplied by Shaanxi Yanchang Coal Yulin Energy and Chemical Co., Ltd, China. Antioxidant 1010 was provided by Guangzhou Telei Chemical Co., Ltd, China. The styrene-ethylene-butylene-styrene tri-block copolymer (SEBS, 503T) was purchased from Sinopec Baling Petrochemical Co., China. Analytical grade xylene was obtained from Tianjin Fuyu Chemical Co., Ltd, China.

Preparation of the BPP/SEBS blends
The BPP/SEBS blends of various SEBS contents ranging from 0 wt% to 25 wt% (table 1) were prepared by melt blending using a HAPRO RM-200C torque rheometer. A processing temperature, screw speed, and time of 210°C, 50 rpm, and 10 min, respectively, were chosen. For ease of description, the details in table 1 represent the blends at different SEBS contents.
All samples of different thicknesses for later testing were prepared by hot pressing using a flat vulcanizing machine. The procedure was as follows. First, the blend was placed in a preheated stainless steel mold and heated at 200°C for at least 8 min. Compression molding was then conducted under a pressure of 15 MPa for 5 min. Finally, the sheet was cooled using circulating water under the same pressure for 3 min. To eliminate the thermal history generated during the sample preparation, normalization processing was executed on all the samples before each batch of measurements.

Characterization
A tensile test was carried out via an electronic universal testing machine (AG-J10KN, SHIMADZU) according to ASTMD638-2003. The dumbbell-shaped specimens with a gauge length of 25 mm and a cross section of 6 mm×1 mm were stretched at a crosshead speed of 50 mm min −1 at room temperature. A minimum of five specimens for each composite were repeatedly measured, the reported results of which were averages.
Differential scanning calorimetry (DSC) was performed using a power-compensation PerkinElmer DSC equipped with Pyris software to investigate the thermal properties and crystallization behaviors of the blends. Samples of approximately 5 mg to 8 mg were encapsulated in standard aluminum pans. The DSC measurements were run across a temperature range from 40°C to 200°C with a heating/cooling rate of 10°C min −1 under an N 2 atmosphere. The instrument was routinely calibrated by means of high-purity indium.
Dynamic mechanical thermal analysis (DMTA) measurement was carried out on TA Q800 equipment to evaluate the thermo-mechanical properties of the blends. The rectangle specimens with dimensions of 90×10×1 mm were examined from −80°C to 160°C in tension mode, with a fixed frequency of 1 Hz and a heating rate of 3°C min −1 .
X-ray diffraction (XRD) analysis was performed on an Empyrean intelligent X-ray diffractometer with Cu Kα radiation. The samples were measured from 5°to 60°.
The space charge measurement was conducted by using a pulsed electro-acoustic (PEA) system equipped with a LeCroy WaveRunner 610Zi (1 GHz, 10 GS s −1 digital oscilloscope) under a 40 kV mm −1 DC electric field for 1,800 s. The experiments employed a generator with an amplitude of 400 V, a pulse width of 8 ns, and a frequency of 2 kHz. A polyvinylidene fluoride (PVDF) sensor with a thickness of 9 μm was also used. The thickness of the film sample was approximately 260 μm. Prior to the test, the aluminum was evaporated on both sides of the sample, after which the sample was short-circuited for 24 h under vacuum at 80°C. The DC breakdown strength was evaluated by using a CS2674C dielectric strength tester. The sample was sandwiched between two opposing plate electrodes and immersed in silicone oil. The lower electrode was earthed, and an increasing DC voltage was applied to the upper electrode until breakdown. Breakdown voltage was recorded, and the sample thickness at the breakdown spot was measured. The corresponding field intensity could be obtained according to the formula: where E B represents the breakdown strength, U B is the breakdown voltage, and d is the sample thickness at the breakdown spot. Fifteen points were tested for each sample, and the two-parameter Weibull statistical distribution method was employed for data analysis.

Results and discussion
3.1. Thermal analysis Figure 1 presents the melting and the crystallization curves of the BPP/SEBS blends. The crystallinity degree (X c ) was calculated according to the DSC melting curves using the following equation: where DH m is the melting enthalpy of the blends, and D ¥ H m is the melting enthalpy of 100% crystalline PP homo-polymer. A value of D ¥ H m =209 J g −1 [16] was taken for calculating the crystallinity. According to figure 1(a), the BPP exhibited a high melting point of 166°C, which is beneficial in raising the operating temperature and increasing the transmission capacity of the cables. When adding different amounts of SEBS into the BPP, virtually no change in the melting point was observed. Therefore, BPP and its blends were all suitable for HVDC cable insulation applications from the point of view of their melting behavior. In addition, the BPP/SEBS blends displayed a lower X c than the virgin BPP. The SEBS content increased from 0 wt% to 25 wt%, whereas the X c was gradually reduced from 45% to 32% given that the distribution of SEBS in BPP hindered the orderly arrangement of BPP chains, thus restricting the BPP crystallization process [17]. The reduction of crystallinity is favorable to improving the mechanical properties of BPP [18].
It is evident from figure 1(b) that the crystallization temperature slightly decreased as the SEBS content increased. The crystallization temperature of the BPP/SEBS blend containing 25 wt% SEBS was lower than that of virgin BPP by 0.4°C; thus, no nucleation effect was observed for SEBS considering the decrease in the crystallization temperature.

Thermal mechanical analysis
For practical HVDC cable insulation, the material should have not only adequate flexibility at room temperature to meet the requirements of cable installation but also appropriate rigidity at a high temperature to meet the cable operation requirements [2,3,17]. Therefore, it is necessary to focus on the dynamic mechanical thermal analysis. The plots of the storage modulus (E′) and loss tangent (tanδ) as a function of temperature for the BPP and BPP/SEBS blends at various compositions are presented in figure 2. All samples exhibited a rather similar tendency wherein the storage modulus decreased as the temperature increased (figure 2), which was in accordance with the polymer relaxation properties. The virgin BPP exhibited a storage modulus of approximately 1,600 MPa at room temperature, which was far greater than that of XLPE [3]. Consequently, virgin BPP cannot be practically used in cable manufacturing due to its high rigidity. The introduction of SEBS into the BPP matrix significantly decreased the storage modulus of the blends given the decrease in crystallinity of the blends, as evidenced in the DSC study, and the elastic character of the SEBS. For BPP-25, the storage modulus decreased to approximately 600 MPa. This suggests a great decline in the rigidity and enhanced flexibility, which is desirable for HVDC cable installation. Furthermore, BPP-25 showed a remarkably improved modulus at temperatures above 110°C compared with XLPE [3], which ensured sufficient mechanical integrity of the cable without the need for crosslinking and is an advantage for cable operation under high temperature.
According to the tanδ curves, the BPP exhibited two obvious loss peaks at around 8°C and −40°C. PPB was synthesized via two-step polymerization. In the first stage, propylene homopolymerization was performed, and ethylene-propylene copolymerization was carried out in the second stage. The resulting PPB was a multicomponent system [19][20][21]. The peak at 8°C is associated with the glass-transition temperature of polypropylene in the amorphous regions (labeled as Tg 1 ), while another peak at −40°C is attributed to the glasstransition temperature of ethylene propylene rubber (EPR) (labeled as Tg 2 ) [19]. When SEBS was added into BPP, a new loss peak corresponding to the EB segment relaxation at approximately −50°C was not observed, even when the SEBS content reached 25 wt%. Moreover, the peak position of Tg 2 shifted toward the lower temperature, and the peak area of Tg 2 increased with increasing SEBS content. In general, if two polymers have excellent compatibility, their Tg shift toward each other, and ultimately only a single Tg exists if they are in complete miscibility [22][23][24]. Petermann and Gohil [25] reported good compatibility between PP and poly(1- butene) due to rather similar molecular configurations of both polymers. Thus, the above results indicate strong interaction and good compatibility between the EB segments of SEBS and the EP segments of EPR. It is believed that the lower glass-transition temperature and the increased peak area of Tg 2 play important roles in the enhanced toughness of the BPP/SEBS blends [13].

Mechanical properties
It is of paramount importance to take mechanical properties into account in the choice of cable insulating materials. The tensile stress-strain curves of BPP/SEBS blends at various compositions are presented in figure 3.
The corresponding values of the tensile strength, elastic modulus, and elongation at break obtained from the tensile test are listed in table 2. The stress and elongation curves with the fracture point are shown in figure 4. One can see that the incorporation of SEBS softened the BPP, as expected. Furthermore, a sharp increase in the elongation at the break of the BPP/SEBS blends with increasing SEBS content was detected, indicating that the toughness of the blends improved [13]. As discussed above, this is attributed to a decrease in the crystallinity of the blends and the elastic character of the SEBS. Interestingly, the addition of SEBS to the BPP matrix first resulted in a slow increase and then a rapid increase in the tensile strength. The tensile strength increased from 24.7 MPa of BPP to 37.2 MPa of BPP-25, suggesting that the capability of resistance to the external force damage of the BPP/SEBS blends substantially improved, which is of great significance for practical cable applications. The increase in the tensile strength of the BPP/SEBS blends can be explained as follows. First, an increase in the SEBS content resulted in a decrease in the crystallinity, which loosened the intermolecular arrangement and led to a decrease in stress [26,27]. Second, as the PS segment of the SEBS was in a glassy state at room temperature, it was difficult to move and thus acted as a physical crosslinking point, which hindered the slip between the molecular chains and increased the stress [28][29][30]. Third, a strong interaction was observed between PPB and SEBS due to the presence of EPR in the PPB, which strengthened the intermolecular mutual entanglement. As a result, more force was required to untangle the molecules during stretching, thus increasing the stress [31].  Clearly, the experimental results showed that the influence of the latter two factors was greater than that of the former, such that the tensile strength increased with an increase in the SEBS content.

XRD analysis
XRD measurements were performed to further investigate the effect of SEBS incorporation on the BPP crystal structure. Figure 5 presents the XRD diffraction patterns of the BPP blends with various SEBS contents. The virgin BPP sample exhibited characteristic diffraction peaks at 2θ of 14.1°, 16.9°, and 18.5°, which corresponded to the (110), (040), and (130) lattice planes of the α-monoclinic crystal of PP, respectively [32]. The same diffraction peaks were also observed in the diffraction patterns of all the blend compositions. Furthermore, no other additional peak related to other crystal phase was observed, implying that the addition of SEBS did not change the crystalline form of PP.

Space charge characteristics
When a high direct current electric field was applied on the insulating materials, space charges were generated and accumulated, which became obstacles for developing HVDC cables [33]. Figure 6 presents the space charge distributions in the BPP/SEBS blends at different SEBS contents during polarization under a DC electric field of 40 kV mm −1 , which was applied for 1,800 s. A large amount of hetero-charge accumulation was observed in BPP, i.e., the negative charge near the anode and positive charge near the cathode. In general, the hetero-charge is generated from the ionization of impurities or additives, inevitably introducing them into polymers during  manufacturing [34][35][36]. With the addition of SEBS, a gradual reduction in hetero-charge was observed. In the case of BPP-20, almost no charge accumulation was observed. The present results suggest that SEBS had a positive effect on the space charge inhibition. Similar results were reported by Dong et al [37]. In general, aromatic rings can act as strong charge trapping sites [38][39][40][41][42][43]. Teyssedre et al computed the effect of the physical and chemical defects on the trap-level distribution in the polymer material using density functional theory (DFT) modeling, the results of which showed that the aromatic rings would introduce deep traps in the polymer material [40,41]. Li et al analyzed the space charge characteristics in XLPE that was modified by polycyclic aromatic compounds using the integration current method and quantum chemical calculations. Deep traps in aromatic rings are formed from the deviation of the electron cloud under electric fields, which forces the electron to occupy lower energy levels. The consequent decrement of the lowest unoccupied molecular orbital (LOMO) and increment of the highest occupied molecular orbital (HOMO) led to the appearance of deep trap sites in the aromatic rings. On the macro level, the apparent dipoles of the aromatic rings built pairs of positive and negative potential wells, wherein the electrons and holes served as capture carriers [43]. Thus, the SEBS containing aromatic rings introduced deep trapping sites into the composites when SEBS was added into PPB. These deep traps could capture the charge carriers that were liberated by the ionization of impurities and block the movement of the charge carriers, thereby decreasing the quantity of carriers transported to the extraction electrode and resulting in the suppression of hetero-charge formation [44][45][46][47]. On the other hand, an increase in the SEBS content increased the trapping sites and formed a dense trap layer near the cathode and anode, thus capturing the homo-charges injected from the electrodes following the application of the DC electric field. This increase in homo-charge resulted in an apparent decrease in the PPB hetero-charge [48,49]. Thus, the space charges were significantly suppressed due to the deep traps introduced by the SEBS aromatic rings. However, when the content of SEBS exceeded 20 wt% and continued to increase, superfluous trapping sites introduced additional homo-charges. Heavy homo-charge accumulation near the cathode was observed in the bulk of PPB-25 ( figure 5). Thus, PPB-20 possessed the optimal space charge suppression performance, which is highly favorable for HVDC cable insulation applications.

DC breakdown strength
The DC breakdown strength is one of the most important parameters used to evaluate the insulating property of the materials. Figure 7 shows the Weibull plots of the DC breakdown strength of the BPP/SEBS blends with various components at room temperature. One can see that the DC breakdown strength of BPP/SEBS blends increased with increasing SEBS content. When the SEBS content increased from 0 wt% to 20 wt%, the DC breakdown strength increased from 287.8 kV mm −1 to 327.5 kV mm −1 . The resonance structure brought by the SEBS chain played an important role in improving the breakdown strength. Thanks to the existence of delocalized π-electrons, the benzene rings could absorb and dissipate the energy of the high-energy electrons by producing relatively stable anions and cation radicals in the transport process, which weakened the attack of the electrons on the polymer molecular chains [50][51][52]. Hence, the breakdown strength was improved. Notably, the DC breakdown strength of BPP-25 was lower than that of BPP-20. This may be attributed to the space charge accumulation, which resulted in serious local electric field distortion and thus decreased the breakdown strength of the insulation materials [53].

Conclusions
BPP/SEBS blends were prepared by melt mixing, and the feasibility of the BPP/SEBS blends was evaluated for HVDC cable insulation based on the melting and crystallization behavior, dynamic mechanical behavior, mechanical properties, space charge distribution, and DC breakdown strength. All the samples exhibited a high melting temperature of about 166°C and sufficient high-temperature rigidity, ensuring the BPP/SEBS blends can operate under high temperatures without the need for crosslinking. The addition of SEBS effectively improved the flexibility of the BPP/SEBS blends. Interestingly, the tensile strength of the BPP/SEBS blends showed a remarkable increase with increasing SEBS content. Meanwhile, BPP-20 presented the optimal space charge suppression performance and the highest breakdown field strength. This work provides a reference for developing environmental protection HVDC insulation material.