Exploring the Mechanical and Dielectric Properties of Polypropylene and Bimodal Polyethylene Blends for Cable Insulation

By JianYunCable -

 


Introduction

In the ever-evolving field of cable insulation materials, traditional cross-linked polyethylene (XLPE) has long served as the industry standard for medium and low-voltage applications. However, XLPE's shortcomings—including high energy consumption during production and limited recyclability—have encouraged researchers to seek out more sustainable alternatives.

Polypropylene (PP) and bimodal polyethylene (BPE) are two such candidates. Individually, each offers unique advantages: PP boasts excellent dielectric strength and thermal stability, while BPE provides superior mechanical flexibility and processability. This study investigates the performance of various PP/BPE blend ratios, aiming to identify the optimal composition for eco-friendly cable insulation.

1. Objective and Methodology

The objective of the study is to evaluate the mechanical and dielectric performance of non-crosslinked PP/BPE blends with varying mass ratios. The experimental process includes:

  • Preparing blend samples using melt mixing

  • Observing the microstructure with polarized optical microscopy

  • Conducting impact and tensile tests

  • Measuring dielectric properties including volume resistivity, relative permittivity, and dielectric loss factor

Blend Ratios Examined

Seven different mass ratios of PP to BPE were evaluated:

  • 10% PP / 90% BPE

  • 20% PP / 80% BPE

  • 30% PP / 70% BPE

  • 40% PP / 60% BPE

  • 50% PP / 50% BPE

  • 60% PP / 40% BPE

  • 70% PP / 30% BPE

All samples were stabilized using 1% antioxidant to ensure thermal and oxidative resistance during processing.

2. Microstructural Observation

Polarized light microscopy revealed the spherulite formation and crystalline structure of both pure PP and the 30% PP / 70% BPE blend.

  • Pure PP formed large, well-defined spherulites during isothermal crystallization at 140°C.

  • The 30% PP / 70% BPE blend showed a denser crystalline structure with uniform dispersion, suggesting good compatibility between the two polymers.

This dense morphology contributes to improved mechanical performance and stable dielectric behavior in the blends.

3. Mechanical Properties

3.1 Impact Resistance

The impact strength of the PP/BPE blends was evaluated using notched samples and a pendulum impact tester. The results demonstrated a clear brittle-to-tough transition at the 30% PP / 70% BPE composition.

Blend RatioComplete FracturePartial FractureNo Fracture
10% PP / 90% BPE006
30% PP / 70% BPE033
50% PP / 50% BPE600

As BPE content decreases, the material becomes more brittle, indicating that BPE is the primary contributor to impact toughness.

Conclusion: The 30% PP / 70% BPE blend marks the transition point, balancing toughness and stiffness.

3.2 Tensile Properties

Tensile tests were conducted at a constant rate of 10 mm/min to analyze stress-strain behavior.

PropertyTrend with PP Increase
Elastic ModulusIncreases
Tensile StrengthIncreases
Elongation at BreakIncreases
Fracture EnergyIncreases

While higher PP content improves strength and stiffness, it also reduces material flexibility. Excessive PP (above 40%) makes the composite difficult to bend, limiting its suitability for cable applications that require coiling or vertical installation.

Conclusion: The 30% PP / 70% BPE blend offers mechanical properties comparable to or better than XLPE, while maintaining adequate flexibility.

4. Dielectric Properties

4.1 Volume Resistivity

All blends exhibited excellent insulation performance, with volume resistivity remaining around 10¹⁴ Îİ·m—similar to or better than XLPE.

Blend RatioVolume Resistivity (Îİ·m)
BPE (pure)4.0 × 10¹⁴
30% PP / 70% BPE2.4 × 10¹⁴
50% PP / 50% BPE7.3 × 10¹⁴
PP (pure)4.4 × 10¹⁴
XLPE (standard)≥ 1.0 × 10¹⁴

The variation in resistivity is largely attributed to differences in crystallinity and potential impurities introduced during processing.

4.2 Relative Permittivity and Dielectric Loss

Both properties were tested under 50 Hz AC conditions.

Blend RatioRelative PermittivityDielectric Loss (×10⁻⁴)
BPE2.070.53
30% PP / 70% BPE2.122.79
50% PP / 50% BPE2.163.43
PP2.122.03
XLPE≤2.35≤5

Conclusion: All blends meet the dielectric requirements for medium and low-voltage insulation. Minor variations are attributed to impurities or crystallinity, but remain within acceptable ranges.

5. Discussion

The data reveal a clear trend:

  • Higher PP content increases tensile strength and resistivity but decreases impact resistance and flexibility.

  • Higher BPE content enhances toughness and processability but may lower the upper service temperature.

The 30% PP / 70% BPE blend emerges as the optimal formulation, striking a balance between mechanical performance, electrical reliability, and process efficiency. It demonstrates superior compatibility, making it a strong alternative to XLPE for future non-crosslinked cable insulation systems.

Conclusion

This research confirms that PP/BPE blends offer a promising path forward for developing recyclable, environmentally friendly cable insulation. The key findings include:

  1. A 30% PP / 70% BPE ratio delivers the best balance between mechanical strength and electrical performance.

  2. The blend maintains comparable dielectric properties to XLPE while surpassing it in certain mechanical metrics.

  3. The microstructure analysis confirms good dispersion and crystallinity, which directly support the material's durability and insulation performance.

This blend formulation holds potential for commercial use in medium and low-voltage power cables, aligning with global trends toward sustainable materials and green manufacturing.

Keywords

  • Polypropylene (PP)

  • Bimodal Polyethylene (BPE)

  • Melt Blending

  • Cable Insulation Materials

  • Dielectric Properties

  • Mechanical Performance

  • Eco-friendly Polymers

References

  1. Cao Weikang, Xu Xin (2024). Mechanical and Dielectric Properties with Different Mass Ratio of PP and BPE. Wire & Cable, No. 4, 2024. DOI: 10.16105/j.dxdl.1672-6901.202404002

  2. GB/T 13022—1991: Plastic Film – Tensile Properties Testing Method

  3. IEEE Transactions on Dielectrics and Electrical Insulation.Source: JianYunCable

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