In the significant specification of thermoplastic, HFFR-insulated cables, such as IEC 60502 or BS 6724, HFFR compounds are used in jacket materials. Most jacket colors of volt power cables are black and contain CB. The prominent role of CB in polymer composites is to absorb UV and increase flame retardancy. CNTs may be utilized instead of CB in thermoplastic, clean, flame-retardant compositions to improve their mechanical performance and flame retardancy. CNTs are ideal for this application. Other CNT-compatible flame-retardant compositions include MH and HH. These components have a fine structure and can be used in flame-retardant formulations.
3.1. Effects of CB Contents on Flame-Retardant EVA/LLDPE Composites
The effects of CB on various types of flame-retardant materials were investigated. This study revealed that thermoplastic, clean, flame-retardant compositions, including a main flame-retardant MH (MAGNIFIN A-H10A) grade and various CB contents (
Table 1), showed a slight decrease in elongation at break and an increase in flame retardancy by 25.80% with an increase in the CB content. A similar tendency was observed when the primary flame retardant was changed from MH (MAGNIFIN A-H10A) to HH (Ultracarb LH15X), as shown in
Table 2. It was observed that the elongation at break decreased slightly, although the tensile strength and flame retardancy increased with the increased CB content. The presence of the filler results in a less complicated break formation process due to increased stress on the filler surface and, on the other hand, a decrease in chain mobility because of the polymer–filler interface, which lowers the combinations’ deformability. The elongation-at-break value is reduced as a result of these effects [
31,
32]. When MH (KISUMA 5B) grade was present, the tensile strength did not significantly alter when the CB content increased. Conversely, there was a noticeable effect of CB on the elongation at break. As the CB content increased, the elongation at break decreased (
Table 3). Generally speaking, a higher filler content can result in worse mechanical properties. Although a CB filler was used in the compounds, their tensile strength stayed nearly constant, and their elongation at break was significantly affected below 8 wt% of CB.
Furthermore, the LOI (%) increased as the CB content increased. Our investigation shows that, in black, thermoplastic, clean, flame-retardant compositions, CB increases flame retardancy and decreases elongation at break. Following the above discussion, the results show that, according to IEC 60502 standards [
33], no composites pass the V-0 of the UL-94 test with a minimal tensile strength of 9.5 MPa and a minimal elongation at break of 125%. A higher than 120 phr content of the primary flame retardant plus secondary (intumescent) flame retardants like RP, ZB, and BA must be compounded to pass the UL 94 test’s V-0.
Table 4 illustrates that even with increased flame retardancy, extra main and intumescent flame retardants may result in a decline in mechanical characteristics. Despite having high LOI values, formulations of C-17 to C-20 that contained CB exhibited a significant decrease in elongation at break. Many scientists are working to find a CB replacement with better flame retardancy in thermoplastic, clean, flame-retardant compositions without their sacrificing mechanical properties [
34,
35,
36]. For this specific goal, we introduced a novel material in our study that can replace CB in thermoplastic, clean, flame-retardant compositions.
3.2. Effects of CNT and CB Contents on Flame-Retardant EVA Composites
The relationships between each flame retardant and CNT/CB were investigated. The correlations between MH (Magnifin A-H10A) grade and CNT/CB were explored as a preliminary test, as shown in
Table 5. Because of the low-percolation-threshold requisite for high-aspect-ratio fillers, conducting polymer structures were created at low-CNT filler loading levels [
37]. The characteristic values were fewer than 6 phr CNT loadings compared to 2–6 phr CB components. Low particle components are essential for electronic clean-room applications (where particle contamination is a severe issue).
Figure 1,
Figure 2 and
Figure 3 compare CNT and CB in EVA/120 phr MH (Magnifin A-H10A) formulations.
Figure 1 illustrates the relationship between the elongation at break and CNT or CB content for EVA/120 phr MH (Magnifin A-H10A) formulations. Even though the content was small, different trends from other CNTs were seen. The elongation at break reduced with higher CNT contents at ranges of up to 4 phr. When compared to CB, CM-95 exhibited the lowest elongation at break. The size of the particle is thought to affect its mechanical properties. When elongation at break is examined between CNT50/CNT75 formulations and formulations including CB, it is clear that CNT50/CNT75 formulations exhibit greater values. CNT50/CNT75 may be used in HFFR formulations instead of CB to attain more excellent elongation at break values.
The tensile strength of EVA/120 phr MH (Magnifin A-H10A) formulations is plotted against the CNT or CB content in
Figure 2. Even with a low content, different trends from other CNTs were seen. The tensile strength increased as the CNT content increased to a range of up to 6 phr. CM-95 exhibited the most substantial tensile strength compared to formulations containing CB. According to one theory, smaller particles may show greater tensile strength. As a result, CB had the lowest tensile strength, while CM-95 had the strongest. Tensile strength comparisons between the CNT50/CNT75 formulations and CB-containing formulations revealed that the CNT50/CNT75 formulations exhibited significantly higher values.
Figure 1 and
Figure 2 show that the CNT50/CNT75 formulations have higher values for both mechanical properties (elongation at break and tensile strength) than the CB-containing formulations. CNT50/CNT75 can be used in HFFR formulations instead of CB to provide more excellent mechanical properties (especially elongation at break). The stress intensity on the CB filler surface might cause small faults that lead to the breakdown of filled combinations with the polymer matrix. As a result, little fractures begin to form and grow until they reach the critical crack level. The formation and propagation of cracks in the same material without filler are accidental, and the collapse manifests at a reasonably high distortion. However, the presence of filler simplifies break formation due to increased stress and decreased chain mobility caused by the polymer–filler interface, reducing the combinations’ deformability and elongation-at-break values [
38,
39].
Figure 3 depicts the LOI (%) of EVA/120 phr MH (Magnifin A-H10A) formulations as a CNT or CB concentration function. The CNT50, CNT75, and CB formulations had identical flame retardancy trends as a content function. However, the CM-95 formulation had very low flame retardancy. Increasing concentrations improved the flame retardancy of the CNT50, CNT75, and CB formulations. Flame retardancy increases even with a 2 phr content of CNT50, CNT75, or CB. It is considered that these materials have a strong flame-retardant power with a combination of MH (Magnifin A-H10A), except CM-95. To slow down the polymer degradation rate, the polymer/CNT network structure layer served as a shield, reflecting incident radiation into the gas phase. Therefore, low loadings are required to achieve meaningful fire retardancy. Unfavorable alterations to the mechanical and physical properties of polymers can be prevented by using appropriate comonomer selection or other modifying groups. Reactive monomers and naturally flame-resistant polymers made of P, Si, N, Bi, and other random elements are examples of current advancements in HFFR polymers [
40].
The findings shown in
Figure 1,
Figure 2 and
Figure 3 clarify that the CNT50/CNT75 formulations outperformed the CB-containing formulations with regard to their mechanical properties without sacrificing flame retardancy. Increased mineral loading led to a decrease in the mechanical potential of the components [
41]. Kashiwagi et al.’s [
42] initial introduction of CNTs into PP was to reduce its flammability behavior. It was determined that the growth of the CNT structural network layer, which might protect the underlying polymer, was the most likely cause of the decrease in the heat release rate.
The relationships between each flame retardant and CNT/CB are continuous. The relationships between MH (KISUMA 5B) grade and CNT/CB are shown in
Table 6.
The elongation at break and tensile strength of EVA/120 phr MH (KISUMA 5B) grade formulations as a CNT or CB content function are shown in
Figure 4 and
Figure 5. Like MH (Magnifin A-H10A) grade formulations, different trends were found from other CNTs, although the content was relatively low. At a range of up to a 4 phr content, in the CNT50, CNT75, and CB formulations, the elongation at break did not change with an increase in the CNT content. On the contrary, in the CM-95 formulation, the elongation at break essentially decreased with the increase in content. Almost identical results were observed in the tensile strength tests. The effects of the mechanical properties of the MH Magnifin A-H10A formulations were different from those of the MH KISUMA 5B formulations. The results of the CM-95 formulation were different from those of the CNT50, CNT75, and CB formulations. However, MH KISUMA 5B is a synthetic MH product with a higher fatty acid content on the surface. The size distribution of the particles remained unchanged with the application of an organic agent. Nevertheless, it did cause a significant decrease in the specific surface area of 5–7 m
2/g without causing particle aggregates to form. The brucite surface’s clogged pores likely caused the surface area to decrease, which may have led to lower tensile strength values and increased elongation at break. According to Haveriku et al. [
43], fatty acid-based thermoplastic treatments often increase elongation and impact resistance while decreasing modulus, tensile strength, filler dispersion, and melt viscosity.
The LOI (%) of EVA/120 phr MH (KISUMA 5B) formulations as a function of the CNT or CB content is shown in
Figure 6. Like the MH (Magnifin A-H10A) grade formulations, the CNT50, CNT75, and CB formulations showed almost the same trends of flame retardancy as a content function, whereas the CM-95 formulation showed very low flame retardancy. Flame retardancy increased with the increase in the CNT50, CNT75, and CB formulations, while flame retardancy did not change with the rise in the CM-95 content. The lowest LOI was found for CM-95, which has a smaller particle size.
The relationships between each flame retardant and CNT/CB are continuous. The relationships between HH (Ultracarb LH15X) and CNT/CB are shown in
Table 7.
Figure 7 and
Figure 8 demonstrate the elongation at break and tensile strength of EVA/120 phr HH (Ultracarb LH15X) formulations as a CNT or CB content function. They were not the same as for the MH Magnifin A-H10A and MH KISUMA 5B formulations; distinct tendencies were found for other CNTs despite the comparatively low content. The elongation at break reduced somewhat with increasing CNT or CB content in the CNT50, CNT75, and CB formulations throughout a range of up to a 4 phr content. In the CM-95 formulation, the elongation at break decreased with the increase in content. However, the tensile strength did not change with the increase in the CNT or CB content, and almost the same trends were observed for the four formulations. The mechanical properties of the MH Magnifin A-H10A formulations differed significantly from those of the MH KISUMA 5B formulations. The effects of the CM-95 formulation were different from those of the CNT50, CNT75, and CB formulations.
Moreover, the obtained tensile strength values were shallow at under 10 MPa in all contents of CNT or CB. From all tests of the relationships between each flame retardant and CNT/CB formulation, it was found that MH (Magnifin A-H10A) only showed the strongest mechanical properties among the various flame retardants in the CNT/CB formulations. In addition, CNT50 offered the best mechanical properties for the multiple types of CNTs. The favorable polymer–filler interaction could explain the increase in tensile strength and elongation at break. The reduction in both mechanical properties can be primarily attributed to a further rise in other CNTs or the CB network density, or the aggregation of particles and stress concentration. Furthermore, if the composite has a high interfacial area and a strong interfacial interaction between the fillers and the polymer matrix, the third phase as an interphase is generated [
38,
39]. This interphase dramatically influences the mechanical properties of polymer composites. We did not use functionalized fillers in the examined EVA composites, which could account for the enhanced interaction between them and the matrix molecules. A lack of interaction and poor wettability of the employed CNT/75, CM-95, or CB systems in EVA may significantly minimize polymer wrapping around CNTs and CB, influencing their tensile capabilities.
The LOI (%) of the EVA/120 phr HH (Ultracarb LH15X) formulations as a CNT or CB content function is shown in
Figure 9. Different findings were obtained for the MH Magnifin A-H10A and MH KISUMA 5B formulations; the CNT50 and CB formulations demonstrated more flame retardancy than the CNT75 and CM-95 formulations. The flame retardancy increased with increasing CNT or CB contents up to 2 phr contents and then stayed constant at higher content levels. CNT50 and CB consistently showed increased flame retardancy with combinations of various flame retardants in all tested/evaluated interactions between each flame retardant and CNT/CB composition. We discovered that a filler mix system including MH (Magnifin A-H10A) as the primary flame retardant combined with CNT50 had the optimum synergistic effect.
3.3. Effects of CNT50 Contents on Flame-Retardant EVA/LLDPE Composites
From the study of the relationships between each flame retardant and CNT/CB, it was found that the CNT50 formulations had the best results in terms of their mechanical properties and flame retardancy. CNT50 was the ideal composition length and size, with clean flame retardants included. CNTs with an outer diameter of 40–60 nm and a length distribution of less than 20 m can be substituted for CB in thermoplastic, clean, flame-retardant compositions to improve their mechanical and flame-retardant properties. Consequently, CNT50 was chosen for a thorough analysis in comparison to CB (
Section 3.1). The effects of CNT50 on various types of flame-retardant materials were investigated.
Formulations of EVA/LLDPE/MH (Magnifin A-H10A) (120 phr)/CNT50 were tested.
Table 8 shows the formulations in detail. The mechanical properties of the CNT50 formulations were higher than those of the CB formulations, as seen in
Figure 10 and
Figure 11. When the CNT50 content increased, the elongation at break and CB content increased slightly. Furthermore, the tensile strength increased as the content of CNT50 or CB increased, but at a faster pace than the CB content. In particular, CNT50 outperformed CB in terms of both mechanical properties. These findings indicate that it may be an acceptable flame retardant if CNT50 can attain the same or better flame retardancy than thermoplastic clean flame-retardant formulations. Surprisingly, CNT50 formulations had stronger flame retardancy than CB formulations, as illustrated in
Figure 12. The flame retardancy increased as the CNT50 or CB content increased. The CNT50 formulations grew at a faster rate than the CB formulations. Furthermore, the volume resistivity of the CNT50 and CB formulations is more significant than 1 × 1015 Ωcm, making them suitable for usage in jacket materials and wire and cable insulation.
These findings are consistent with earlier research findings that the electrical properties of a polymer are proportional to its size [
44]. As a result, larger sizes result in worse electrical characteristics. The electrical characteristics deteriorated as the size/volume decreased. However, when the size was substantial, the electrical characteristics were strong. Small-sized polymers have low electrical properties, specifically electrical resistivity, whereas large-sized polymers have greater volume resistivity [
45]. Results show that the electric resistivity of a polymer varies according to the thickness of the layer [
46]. Because of its electric volume resistivity, the thickness and size of a polymer are significant when choosing one. Volume resistivity is a type of electrical insulation. According to the overall mechanical properties, flame retardancy, and electrical properties of CNT50/CB formulations, CNT50 is a suitable material for thermoplastic, clean, flame-retardant compositions instead of CB.
The experimental results confirmed the main points of this study. CNTs with an outer diameter of 40–60 nm and a length distribution of less than 20 nm can be used instead of CB in thermoplastic, clean, flame-retardant compositions to improve mechanical properties and flame retardancy. Another significant difference between clean, flame-retardant materials and routine thermoplastics is that the extruding temperature of clean, flame-retardant materials is 160–200 °C, whereas that of routine thermoplastics is 200–250 °C. Because clean, flame-retardant materials are primarily composed of low-softening-temperature-grade polymers such as EVA, ethylene alpha-olefin, or ethylene ethyl acrylate, the extruding temperature of clean flame-retardant materials is lower than that of routine thermoplastics such as polyethylene. Sheets of test specimens for mechanical properties and flame retardancy were prepared via hot pressing and compressing at 180 °C for 10 min with a thickness of 2 mm. The above materials are preferably extruded from 160 °C to 200 °C onto conductors to prepare the insulated cable and check the processability. This extruding method is identical to the standard thermoplastic method. During the cable extrusion of the above compositions, the non-CNT50 content composition (M-1) and the 2–4 phr CNT50 composition (M-2 and M-3) demonstrated the finished cables’ best processability and excellent surface smoothness.
Table 9 shows the results of the EVA/LLDPE/MH (KISUMA 5B)/CNT50 formulations. The non-CNT50 content composition (run number M-5) and the 2–4 phr CNT50 content composition (run numbers M-6 and M-7) demonstrated the best processability and final cable surface smoothness. As indicated in
Table 10, intumescent flame retardants such as RP, ZB, and BA were utilized in the EVA/LLDPE/MH (MAGNIFIN A-H10A) formulations to increase their flame retardancy and achieve V-0 of the UL94 test. The test specimen and cable extrusion operations were unchanged from previous methods.
These findings indicate that the compositions of 2–4 phr of CNT50 (run numbers M-10 and M-11) exhibit good mechanical and flame-retardant qualities. In particular, all compositions satisfy the UL94 test’s V-0 requirement, and their volume resistivity is sufficient for use as an insulating material for wire and cable. At 2–4 phr of CNT50 content, the elongation at break of run numbers M-10 and M-11 were somewhat diminished.
Table 4 illustrates how the elongation at break was significantly reduced when CB was utilized instead of CNT50 in run numbers M-10 and M-11. Furthermore, every compound passed the thermal aging test (100 °C × 136 h), demonstrating exceptional thermal characteristics. For completed cables, the non-CNT50 content composition (run number M-9) and the 2–4 phr CNT50 compositions (run numbers M-10 and M-11) exhibited the best processability and superior surface smoothness.