Polyethylene (PE) is found to be one of the most commonly used plastics in the world, belonging to the group of polyolefins [
1]. Its wide applications, among others in the packaging, chemical, electrotechnical, and machine industries, result mainly from its easy processing, recyclability, and physical properties such as high wear resistance, high impact resistance, good chemical resistance, low density, physiological inactivity, and low price [
2]. Additionally it does not absorb water [
3]. Currently, in the cable industry, a broad range of polyolefins, alone or as blends, with various additives and conductive fillers are used as the materials to manufacture insulation and semi-conductive shields for the use in electric cables [
4]. The typical construction of medium and high voltage cables includes a conductor surrounded by an insulation and semi-conductive layers/screens. The inner and intermediate semi-conductive layers are most often a semiconducting cross-linked polymer layer applied by extrusion around the conductive element and over the insulation layer. The insulating layer is selected from crosslinked polyethylene (XLPE), ethylene-propylene rubbers, and ethylene propylene diene rubbers (EPDM rubbers). A bonded or strippable semi-conductive shield in these cables is based on low density polyethylene (LDPE), linear low density polyethylene (LLDPE) or medium density polyethylene (MDPE) compositions with ethylene-vinyl acetate, ethylene alkyl acrylate or methacrylate copolymers, and with an appropriate type and amount of carbon black (CB) [
4,
5,
6]. In order to achieve the electrical resistivity that will meet the requirements of cable standards (below 500 Ω·m), such composite has to be filled with large amount of CB (up to 40 wt. %) [
6]. The results present herein are the part of wider project on obtaining PE based nanocomposites containing carbon nanofillers, such as graphene nanoplatelets (GNPs), carbon nanotubes (CNTs), which can be used as semi-conductive screens in medium voltage (MV) cables. It can be expected that such nanocomposites can exhibit higher conductivity at a lower filler loading than conventional semiconductive composites with conductive CB.
The field of polymer nanocomposite research is currently one of the most rapidly developing domains of materials science and engineering. So far, nanocomposites containing CNTs have sparked greater interest in comparison to graphene-based nanocomposites, which resulted mainly from the poor repeatability of the graphene derivatives (GDs) preparation techniques [
7]. Graphene itself is a one atom thick, two-dimensional (2-D) sheet composed of sp
2 carbon atoms arranged in a honeycomb lattice [
8] with a carbon–carbon bond length equals to 0.142 nm [
9,
10]. Moreover, graphene exhibits various intriguing characteristics, such as high electron mobility at room temperature (250,000 cm
2/Vs) [
11,
12], exceptional thermal conductivity (5000 W/mK in plane) [
13], superior mechanical properties with Young’s modulus of ~1 TPa and ultimate strength of 130 GPa [
12], an extremely high surface area (theoretical limit of 2630 m
2/g), unique adsorption capability [
14,
15], and gas impermeability [
16], which makes it the perfect candidate for improving electrical and thermal conductivity, along with mechanical, thermal and gas barrier properties of polymers [
17,
18]. However, uniform distribution of both CNTs and GDs throughout a polymer matrix, that leads to the improvement of functional properties (most importantly mechanical properties and electrical conductivity) is a cause for some concern in thermoplastic polymer matrices [
19]. One of the latest solutions to solve the above mentioned problem is to utilize a mixture of CNTs and GDs in order to observe a so-called “synergistic effect” which in theory should result in a significant enhancement of properties of the obtained polymer nanocomposites. Several studies of hybrid system of CNTs and GDs that affect a variety of thermoplastic polymer matrices, such as PE [
20,
21,
22,
23], polypropylene (PP) [
24], PET [
25], poly(trimethylene terephthalate) (PTT) [
26,
27], poly(vinyl alcohol) (PVA) [
28], (thermoplastic) elastomers [
29,
30,
31,
32,
33], etc. have been already published. Significant enhancement of the carbon nanofiller dispersion was observed along with the improvement in electrical properties of the multi-phase composites [
34]. Generally, preventing nanofiller’s agglomeration leads to a “synergistic effect” on the properties of polymer composites, and the re-agglomeration during polymerization aids in achieving a lower percolation threshold in the composite [
30].
In our previous work [
35], we have presented how the addition of GNPs affect the selected properties of LDPE. While in this paper, LDPE-based nanocomposites with hybrid system of nanofillers comprising of CNTs and GNPs were prepared in order to enhance the dispersion quality of the melt blended materials, thus improving electrical and thermal conductivity, thermal stability and mechanical characteristics. The functional properties of the obtained hybrid nanocomposites were thoroughly studied and favorably compared with those of neat LDPE and those containing only CNTs, or previously published GNPs. Taking into consideration the possible potential application of semi-conductive LDPE based nanocomposites in the cable industry, for fabrication of nanocomposites, the multi-walled carbon nanotubes (MWCNTs) and GNPs with technical quality for industrial application were used. Herein, common industrial processing technique (melt processing) has been applied for preparation of LDPE based nanocomposites which is essential for mass production.