Polyethylene Synthesis Methods and Varieties
(1) Low-Density Polyethylene (LDPE)
When trace amounts of oxygen or peroxides are added as initiators to pure ethylene, compressed to approximately 202.6 kPa, and heated to about 200°C, the ethylene polymerizes into white, waxy polyethylene. This method is commonly referred to as the high-pressure process due to the operating conditions. The resulting polyethylene has a density of 0.915–0.930 g/cm³ and a molecular weight ranging from 15,000 to 40,000. Its molecular structure is highly branched and loose, resembling a “tree-like” configuration, which accounts for its low density, hence the name low-density polyethylene.
(2) Medium-Density Polyethylene (MDPE)
The medium-pressure process involves polymerizing ethylene under 30–100 atmospheres using metal oxide catalysts. The resulting polyethylene has a density of 0.931–0.940 g/cm³. MDPE can also be produced by blending high-density polyethylene (HDPE) with LDPE or through copolymerization of ethylene with comonomers such as butene, vinyl acetate, or acrylates.
(3) High-Density Polyethylene (HDPE)
Under normal temperature and pressure conditions, ethylene is polymerized using highly efficient coordination catalysts (organometallic compounds composed of alkylaluminum and titanium tetrachloride). Due to the high catalytic activity, the polymerization reaction can be completed quickly at low pressures (0–10 atm) and low temperatures (60–75°C), hence the name low-pressure process. The resulting polyethylene has an unbranched, linear molecular structure, contributing to its high density (0.941–0.965 g/cm³). Compared to LDPE, HDPE exhibits superior heat resistance, mechanical properties, and environmental stress-cracking resistance.
Properties of Polyethylene
Polyethylene is a milky-white, wax-like, semi-transparent plastic, making it an ideal insulation and sheathing material for wires and cables. Its main advantages include:
(1) Excellent electrical properties: high insulation resistance and dielectric strength; low permittivity (ε) and dielectric loss tangent (tanδ) across a wide frequency range, with minimal frequency dependence, making it nearly an ideal dielectric for communication cables.
(2) Good mechanical properties: flexible yet tough, with good deformation resistance.
(3) Strong resistance to thermal aging, low-temperature brittleness, and chemical stability.
(4) Excellent water resistance with low moisture absorption; insulation resistance generally does not decrease when immersed in water.
(5) As a non-polar material, it exhibits high gas permeability, with LDPE having the highest gas permeability among plastics.
(6) Low specific gravity, all below 1. LDPE is particularly notable at approximately 0.92 g/cm³, while HDPE, despite its higher density, is only around 0.94 g/cm³.
(7) Good processing properties: easy to melt and plasticize without decomposition, cools readily into shape, and allows precise control over product geometry and dimensions.
(8) Cables made with polyethylene are lightweight, easy to install, and simple to terminate. However, polyethylene also has several drawbacks: low softening temperature; flammability, emitting a paraffin-like odor when burned; poor environmental stress-cracking resistance and creep resistance. Special attention is required when using polyethylene as insulation or sheathing for submarine cables or cables installed in steep vertical drops.
Polyethylene Plastics for Wires and Cables
(1) General-Purpose Insulation Polyethylene Plastic
Composed solely of polyethylene resin and antioxidants.
(2) Weather-Resistant Polyethylene Plastic
Primarily composed of polyethylene resin, antioxidants, and carbon black. Weather resistance depends on the particle size, content, and dispersion of the carbon black.
(3) Environmental Stress-Crack Resistant Polyethylene Plastic
Uses polyethylene with a melt flow index below 0.3 and a narrow molecular weight distribution. The polyethylene may also be crosslinked via irradiation or chemical methods.
(4) High-Voltage Insulation Polyethylene Plastic
High-voltage cable insulation requires ultra-pure polyethylene plastic, supplemented with voltage stabilizers and specialized extruders to prevent void formation, suppress resin discharge, and improve arc resistance, electrical erosion resistance, and corona resistance.
(5) Semiconductive Polyethylene Plastic
Produced by adding conductive carbon black to polyethylene, typically using fine-particle, high-structure carbon black.
(6) Thermoplastic Low-Smoke Zero-Halogen (LSZH) Polyolefin Cable Compound
This compound uses polyethylene resin as the base material, incorporating high-efficiency halogen-free flame retardants, smoke suppressants, thermal stabilizers, antifungal agents, and colorants, processed through mixing, plasticization, and pelletization.
Crosslinked Polyethylene (XLPE)
Under the action of high-energy radiation or crosslinking agents, the linear molecular structure of polyethylene transforms into a three-dimensional (network) structure, converting the thermoplastic material into a thermoset. When used as insulation, XLPE can withstand continuous operating temperatures up to 90°C and short-circuit temperatures of 170–250°C. Crosslinking methods include physical and chemical crosslinking. Irradiation crosslinking is a physical method, while the most common chemical crosslinking agent is DCP (dicumyl peroxide).
Post time: Apr-10-2025