Comparison of Production Processes and Performance Between Polypropylene-Insulated and Crosslinked-Polyethylene-Insulated Low-Voltage Cables

Abstract

Traditional crosslinked-polyethylene (XLPE) insulation suffers from high recycling costs and low efficiency due to its thermosetting properties. In contrast, thermoplastic polypropylene (PP), with advantages of melt recyclability, low energy consumption, and excellent comprehensive performance, has emerged as an ideal alternative to XLPE. This study conducts a comparative analysis of low-voltage cables insulated with PP, silane-crosslinked XLPE (XLPE-S), and UV-crosslinked XLPE (XLPE-U), focusing on production processes, mechanical properties, thermal stability, and electrical performance. Tensile test results show that PP exhibits the highest elongation at break (>600%) before aging, and its tensile strength (>20 MPa) after aging outperforms that of XLPE, indicating superior flexibility and anti-aging capability. PP exhibits a lower thermal elongation (<50%) at 140 °C compared to XLPE, and its high-crystallinity molecular structure endows better heat-resistant deformation performance. The volume resistivity of PP reaches 9.2 × 10¹⁵ Ω·m, comparable to that of XLPE-U (3.9 × 10¹⁵ Ω·m) and significantly higher than XLPE-S (3.0 × 10¹⁴ Ω·m). All three materials pass the 4-h voltage withstand test, confirming their satisfied insulation reliability. PP-insulated low-voltage cables demonstrate balanced performance in production efficiency, energy consumption cost, mechanical toughness, and electrical insulation. Notably, their recyclability significantly surpasses traditional XLPE, showing potential to promote green upgrading of the cable industry and providing a sustainable insulation solution for low-voltage power distribution systems.

1. Introduction

Power cables are likened to the blood vessels of national economic development, undertaking the critical role of electric energy transmission. Traditional power cable insulation primarily uses crosslinked polyethylene (XLPE). However, as a thermosetting material, post-service recycling of XLPE can only be achieved through mechanical crushing, a method characterized by high cost and extremely low efficiency [1,2,3,4,5,6,7]. Thermoplastic polypropylene (PP) has demonstrated enormous potential to replace traditional XLPE insulation, poised to dominate the future cable industry. The standout advantage of PP lies in its ability to exhibit electrical and mechanical properties comparable to or even superior to XLPE without undergoing crosslinking, effectively avoiding environmental pollution from by-products, high production energy consumption, and long cycles associated with XLPE crosslinking. Additionally, its melt recyclability significantly enhances material circularity [8,9,10,11,12,13].

At present, international research on thermoplastic recyclable PP cable insulation has made breakthrough progress, with modified PP-insulated high- and medium-voltage power cables gradually entering application. Prysmian has produced nearly 50,000 km of PP-insulated medium-voltage cables, and 87/150 kV PP-insulated cables have been put into operation. China has also achieved grid operation of dozens of 10 kV, 35 kV, and 110 kV PP-insulated power cables. The widespread application of PP insulation is bound to drive technological upgrading and industrial innovation in the cable industry [14,15,16,17].

As a vital component of the power distribution system, low-voltage power cables are widely used in construction, industry, urban infrastructure, renewable energy, and other fields, playing an indispensable role in modern society to ensure stable operation and safe use of the power system [18,19]. This paper focuses on a comparative study of low-voltage cables insulated with PP and XLPE, conducting a comprehensive analysis from the perspectives of production processes and the key insulation performance indicators. It compares two typical crosslinking methods for traditional low-voltage XLPE insulation (silane crosslinking and UV crosslinking) with the direct extrusion method of PP insulation, exploring differences in production cycle, processing temperature, and overall energy consumption, as well as variations in mechanical, thermal stability, and electrical properties. The feasibility and superiority of polypropylene as a low-voltage cable insulation layer are comprehensively evaluated, aiming to provide references for the development of PP-insulated low-voltage cables against the backdrop of current research focused on PP-based cable insulation.

2. Materials and Methods

2.1. Insulated Core Preparation

Traditional low-voltage cable insulations (XLPE-S and XLPE-U) were prepared via silane crosslinking and UV irradiation crosslinking, respectively, while thermoplastic polypropylene insulation (PP) was fabricated by melt extrusion. The UV irradiation crosslinkable insulation material, two-step silane crosslinkable insulation material, and PP insulation material were specifically sourced from Shenzhen Lantian Plastic Technology Innovation Co., Ltd., Shenzhen, China; Zhejiang Wanma Polymer Materials Group Co., Ltd., Hangzhou, China; and Nanjing Zhongchao New Materials Co., Ltd., Nanjing, China, respectively. The produced cable models were ZA-YJV22 0.6/1 kV 4 × 240 mm², ZA-YJV22 0.6/1 kV 4 × 240 mm², and WDZA-PPY23 0.6/1 kV 4 × 240 mm². During production, all insulation materials were melted on an SJ-90 extrusion platform to coat the conductor, with the designed insulation thickness set to 1.7 ± 0.1 mm. The extrusion temperatures for each cable are listed in

Table 1. Extruder temperature settings for three production methods.

Materials	Zone 1	Zone 2	Zone 3	Zone 4	Zone 5	Die Head
XLPE-S	160	180	205	205	205	205
XLPE-U	160	180	205	205	205	205
PP	160	180	190	200	200	200

. For UV-crosslinked cables, immediate irradiation treatment was conducted using a UVL-1 series UV irradiation device from Harbin Hapu Electric Co., Ltd., Harbin, China, after melt extrusion (local equipment photos are shown in Figure 1). Two-step silane-crosslinked cables required post-extrusion crosslinking: the whole reel of cores was placed in a saturated water vapor environment for 7 h of high-temperature steam curing to form the XLPE insulation layer (high-temperature steam equipment is shown in Figure 2). In contrast, PP-insulated cables were directly cooled after extrusion without additional treatment. All test specimens were obtained from finished cables using an XL-XP190 cable slicing machine by Dongguan Xilong Electrical Equipment Co., Ltd. (Dongguan, China).

2.2. Mechanical Property Tests of Insulation Layers Before and After Thermal Aging

The mechanical properties of cable insulation (i.e., tensile strength and elongation at break) are the most fundamental and critical indicators. Tensile strength measures the maximum tensile force a material can withstand before fracture, reflecting the stress calculated as the ratio of the maximum force at the instant of fracture (after initial yield deformation and significant cross-sectional area reduction) to the original cross-sectional area of the specimen. Elongation at break is expressed as the percentage ratio of the displacement value at fracture to the original length of the specimen.

Before testing, the insulated core was axially cut to remove the internal conductor, and dumbbell-shaped specimens with a thickness of 1 ± 0.05 mm were cut from the insulation layer. The tests were performed using a CMT6000 multi-functional electronic tensile testing machine (Mester Industrial Systems), following the stress–strain test procedures specified in ISO 527-2:1993. The specimens used were dumbbell-shaped, conforming to the 5A type specified with a thickness of 1 ± 0.05 mm. To ensure data accuracy, at least five tests were conducted for each insulation type, and the average value was taken as the final result [20].

Additionally, thermal-oxidative aging treatment was applied to each insulation type: the cut dumbbell specimens were placed in an oven at 135 °C for 168 h (with a spacing >10 cm between specimens), then removed, and their stress-strain properties were tested using the same method.

2.3. Thermal Extension and Crystallization Characteristic Testing

To evaluate the extension characteristics and permanent deformation of cable insulation under combined heat and load, a hot set test was conducted to assess the thermal resistance of the insulation materials. Due to the structural differences between thermosetting XLPE and thermoplastic PP, distinct test temperatures were applied. To achieve precise temperature measurement, all hot set tests were performed in a modified hot set oven, as shown in Figure 3.

Before testing, the insulated core was axially cut to remove the internal conductor, and dumbbell-shaped specimens with a thickness of 0.8–2.0 mm were cut from the insulation layer [20]. The specimens were suspended in the oven, and a 0.2 MPa load was applied via a lower fixture. For XLPE insulation specimens, after the oven temperature reached 200 °C, the temperature was maintained for 15 min, and the distance between marked lines was measured to calculate the elongation rate (i.e., the ratio of length increment to original length, expressed as a percentage). For PP insulation, the test began at an initial temperature ≤ PP’s crystalline melting temperature, which was set to 130 °C in this study. The temperature was increased stepwise by 2 °C until the maximum elongation exceeded 40%. The corresponding temperature was defined as the highest thermal deformation temperature of PP insulation.

The crystallization and melting properties of materials before and after aging were tested using a differential scanning calorimeter (DSC). Approximately 8 ± 0.5 mg of sample was weighed with an electronic balance, placed into an aluminum crucible, sealed with a lid, and then placed in the sample cell of the DSC analyzer. High-purity nitrogen was used as the protective gas during the test. The temperature control protocol was set as follows: the temperature was raised from 25 °C to 150 °C, where the heat flow change during the first stage was recorded as the crystallization curve, and that during the second stage was recorded as the melting curve. Both the heating and cooling rates were set to 10 °C/min, and the flow rate of the protective nitrogen was 150 mL/min. The crystallinity was calculated using Equation (1) [21,22].

$$X_c={\displaystyle \frac{\mathsf{\Delta }{H}_m}{\mathsf{\Delta }{H}_{100}}}\times 100\%$$

(1)

where ΔH_m (J/g) represents the melting enthalpy of the sample, and ΔH₁₀₀ = 288 J/g and 209 J/g correspond to the melting enthalpies of fully crystallized XLPE and PP, respectively.

2.4. Electrical Performance Tests

Measuring the insulation resistance of low-voltage cables at their maximum operating temperature systematically evaluates the electrical insulation performance of the insulation layer, identifies manufacturing defects in a timely manner, prevents leakage and short-circuit faults during operation, avoids high maintenance costs and power outage losses caused by sudden failures, and enhances the operational stability and reliability of the power system. Before testing, the insulated core was immersed in water at the cable’s designed maximum operating temperature ±2 °C for at least 1 h to ensure full moisture saturation. The test temperature was set to 90 °C ± 2 °C in this study. The DC voltage range for the test was set between 80 V and 500 V, applied continuously until the reading stabilized (no less than 1 min and no more than 5 min) [23]. The test setup is shown in Figure 4, and the volume resistivity is calculated using the following formula:

$$\rho ={\displaystyle \frac{2\pi L_{i}R}{\ln \left(\frac{D_i}{d_i}\right)}}$$

(2)

where $\rho$ is the volume resistivity (Ω·cm); $L_i$ is the length of the insulated core sample (cm); $R$ is the measured insulation resistance (Ω); $D_i$ is the outer diameter of the insulated core (mm); and $d_i$ is the inner diameter of the insulated core (mm). To ensure accuracy, each insulated core was measured three times, and the average value was taken.

The voltage strength that the insulation layer can withstand and its continuous working duration directly affect the safety of cables and electrical equipment. The voltage withstand test evaluates insulation reliability: a qualified product should not break down under the rated voltage for a specified time. In this study, to systematically evaluate the characteristics of the three insulation materials, we not only conducted a 4-h 4U0 power-frequency voltage-withstand test on the insulated cores but also performed a room-temperature AC breakdown strength test on 0.1-mm-thick slices (axially rotated from the cores). This test investigates the critical electric field strength at which the insulation permanently loses its insulating properties, providing a comprehensive analysis of the stability and reliability of the three materials as low-voltage cable insulations [21].

3. Results

3.1. Mechanical Property Test Results of Insulation Layers Before and After Thermal Aging

As shown in Figure 5 and Figure 6, PP exhibits the highest elongation at break before aging, mainly due to its flexible molecular chains. In contrast, XLPE forms a network structure through crosslinked PE chains, which makes PP possess better elasticity and flexibility than XLPE. After aging, PP shows the highest tensile strength. Unlike the two types of XLPE, whose tensile strength decreases due to molecular chain scission and damage to crosslinked structures, the tensile strength of PP increases instead. Owing to the coexistence of amorphous and crystalline phases in polymer materials, there are differences in the positions of their structural matrices. Among them, PP and XLPE-U exhibit good consistency, while XLPE-S shows poor dispersibility—and this poor dispersibility becomes particularly severe for XLPE-S after aging. In summary, PP consistently shows a significantly higher elongation at break than XLPE both before and after aging; its toughness and tensile properties are also superior to those of XLPE, thus meeting the requirements for cable insulation applications.

3.2. Hot Set Test Results of Insulation Layers

As shown in Figure 7, the hot set elongation of PP insulation at 140 °C is far lower than that of UV-irradiated and silane-crosslinked XLPE, which can be attributed to the excellent thermal properties of PP. PP features a linear molecular structure with high symmetry and regularity, enabling ordered molecular stacking to form a dense and highly ordered crystalline structure, thus exhibiting high crystallinity. In contrast, the crosslinked structure of XLPE restricts molecular chain mobility: although its crystallinity decreases, the heat resistance exceeds that of polyethylene (~110 °C). It is inferred that polymer crystallinity plays a more critical role in heat resistance temperature—higher crystallinity correlates with better heat resistance. Therefore, at 140 °C, PP insulation exhibits much smaller deformation than XLPE. Moreover, its crystalline structure can maintain the material’s strength, thereby ensuring stable thermal stability, making it more suitable for cable insulation applications [24].

The crystallization and melting curves of the three materials are shown in Figure 8, and their crystallization temperature, melting temperature, and crystallinity are listed in

Table 2. Crystallization, melting temperature, and crystallinity of materials before and after thermal oxidative aging.

Materials	Unaged			Aged
	Tm/°C	Tc/°C	Xc/%	Tm/°C	Tc/°C	Xc/%
XLPE-S	122.03	102.82	34.72	121.23	102.82	34.69
XLPE-U	123.65	105.73	32.89	122.36	105.24	32.81
PP	162.24	106.7	37.42	161.75	106.7	37.4

. It can be observed that after aging, both the melting temperature and crystallinity of the materials slightly decrease, with little difference in crystallinity. For commercial cable insulation materials, their antioxidant components can meet the aging requirements during the service life of cables; therefore, no significant changes occur in the crystallization and melting properties of the materials after aging [25].

3.3. Electrical Performance Test Results

As a non-polar polymer, PP exhibits excellent electrical insulation performance, typically with a higher resistivity than XLPE. However, after graft blending modification to adjust its mechanical properties, the volume resistivity of the blend may decrease while mechanical properties are significantly improved. The volume resistivities of the three insulation materials at the maximum allowable conductor operating temperature were tested. As shown in

Table 3. Volume resistivity and 4-h voltage-withstand test results.

Materials	Volume Resistivity (Ω·m)	4-h Voltage-Withstand Test
PP	9.2 × 1015	No breakdown
XLPE-U	3.9 × 1015	No breakdown
XLPE-S	3.0 × 1014	No breakdown

, PP has a volume resistivity of 9.2 × 10¹⁵ Ω·m, UV-crosslinked polyethylene 3.9 × 10¹⁵ Ω·m, and silane-crosslinked polyethylene 3.0 × 10¹⁴ Ω·m—the latter value is notably lower, nearly an order of magnitude below the former two [26].

The water boiling process in silane crosslinking may cause slight damage to the substrate, and residual trace moisture adversely affects electrical properties. In irradiation-crosslinked XLPE-U, the crosslinking assistant Trimethylolpropane trimethacrylate (TMPTMA) needs to be added to enable the rapid completion of the crosslinking reaction. During crosslinking, TMPTMA is grafted onto molecular chains, introducing localized traps into the material and effectively increasing its volume resistivity. Due to its high chain regularity and relatively high crystallinity, polypropylene (PP) exhibits hindrance to carrier transport by spherulites/lamellae, which reduces carrier mobility. As a result, PP inherently possesses a relatively high volume resistivity [27]. This explains why UV-crosslinked XLPE and unmodified PP outperform silane-crosslinked materials in terms of electrical performance.

3.4. Production Process Parameters Comparison

The PP extrusion process features simple equipment and low investment, requiring only a common extruder. In contrast, silane crosslinking equipment includes a high-temperature steam chamber, entailing high energy consumption and substantial investment. UV crosslinking equipment adds UV irradiation devices, with energy consumption and investment slightly lower than silane crosslinking but still higher than PP. Thus, considering both equipment investment and production energy consumption, PP insulation extrusion is undoubtedly the most economical and cost-effective choice.

A horizontal comparison of

Table 4. Comparison of the main parameters of the three processes.

Materials	PP	XLPE-U	XLPE-S
Extrusion Temperature (°C)	200	180	180
Production Speed (m/min)	36	29	21
Production Cycle (h)	1	1	10
Energy Consumption & Cost	Low	Medium	High
Environmental Protection	Easy	Medium	Difficult

shows that silane crosslinking has the slowest production speed and longest cycle, while PP and UV crosslinking are similar, with PP having a slight advantage. The PP cable process is more convenient than silane crosslinking, eliminating the steps and time required for crosslinking in a steam chamber. Compared to UV irradiation, PP does not require control of irradiation intensity, enabling simpler quality control, faster production speed, and shorter cycles. The extrusion temperature of PP is slightly higher due to its higher melting point.

In terms of environmental protection, the traditional high-temperature curing process for silane-grafted polyethylene consumes significant energy and time, featuring complex technology, high cost, high energy consumption, and poor environmental protection. UV irradiation equipment consumes more energy than PP extrusion but less than silane crosslinking. Both silane and UV crosslinking require additional equipment, whereas PP insulation only needs a common extruder. Therefore, PP has the lowest energy consumption and cost. In summary, PP demonstrates the optimal comprehensive process, making it more suitable for low-voltage applications [28,29,30,31].

4. Conclusions

This paper investigates three different insulation production processes for 240 mm² cables with the same cross-section: PP insulation extrusion, UV irradiation crosslinking, and silane crosslinking. Parameters such as temperature settings, energy consumption, and production speed were recorded, with key comparisons focusing on production speed, production cycle, extrusion temperature, environmental friendliness, and cost. After production, samples of the three cables were taken to measure their mechanical properties, thermal stability, electrical insulation performance, etc. The comprehensive properties of the three materials were compared and evaluated.

(1)

Under the same extrusion process, PP low-voltage cable insulation exhibits the most excellent tensile properties. While XLPE-U and XLPE-S show stronger thermal load capacity due to their constructed physical crosslinking networks, PP insulation materials still meet the operational requirements of cables and maintain thermoplastic characteristics.

(2)

The PP insulation material maintains a high volume resistivity of 9.2 × 10¹⁵ Ω·m, ensuring no breakdown in the 4-h dielectric withstand test and sustaining excellent electrical insulation properties.

(3)

In full-process production and recycling, PP-insulated cables feature high production speed and short delivery cycle. Meanwhile, by eliminating the irradiation or high-temperature steaming process in insulation crosslinking, their production energy consumption and cost are significantly reduced. The thermoplastic properties of PP ensure green recycling after cable decommissioning.