Comprehensive Guide to Types of Polyethylene

Polyethylene is the simplest polymer in terms of structure and is the most widely used polymer material. It consists of repeated –CH2– units. Polyethylene is produced through the addition polymerization of ethylene (CH2=CH2). The properties of polyethylene depend on its polymerization method. High-density polyethylene (HDPE) is formed through Ziegler-Natta polymerization under medium pressure (15-30 atm) with an organic compound catalyst. The polyethylene molecules produced under these conditions are linear with very long chains and molecular weights reaching several hundred thousand. In contrast, low-density polyethylene (LDPE) is produced through free-radical polymerization under high pressure (100-300 MPa) and high temperature (190–210°C) using peroxide catalysts, resulting in a branched structure.

Polyethylene is the most commonly used material for plastic packaging products. As it is produced from ethylene, a byproduct of petroleum processing, and since ethylene monomer is non-toxic, polyethylene materials, even with 200-300 ppm of ethylene monomer, remain non-toxic and suitable for direct contact with food and pharmaceuticals. The melting temperature of polyethylene significantly differs from its thermal decomposition temperature (above 315°C); it has good melt flow properties, allowing it to be easily processed and formed into various packaging products through extrusion, injection molding, compression molding, and blow molding. Polyethylene is a non-polar polymer, ensuring compatibility among different types, allowing them to be blended in any ratio to improve properties. Due to its low melting temperature and high thermal adhesion, polyethylene is frequently used as a heat-sealing material in soft plastic packaging.

Types of Polyethylene

Polyethylene can be classified based on the era of industrialization:

- **1939:** First generation - High-pressure polyethylene (LDPE)

- **1953:** Second generation - Low-pressure polyethylene (HDPE)

- **1977:** Third generation - Linear low-density polyethylene (LLDPE)

- **1984:** Fourth generation - Ultra-low density polyethylene (VLDPE)

- **1958:** Ultra-high molecular weight polyethylene (UHMWPE)

- **1990s:** Metallocene polyethylene (MPE)

These classifications involve various production techniques and often the addition of small amounts of α-olefins as comonomers, maintaining many characteristics of polyethylene despite minor variations in composition.

Characteristics of Polyethylene

Polyethylene is odorless, non-toxic, wax-like in touch, and exhibits excellent low-temperature resistance (operating temperature can reach -70 to -100°C). It has good chemical stability, resisting most acids and alkalis (except for oxidizing acids). It is insoluble in common solvents at room temperature, has low water absorption, and excellent electrical insulation. However, it is sensitive to environmental stress (chemical and mechanical) and has poor heat aging resistance. The properties of polyethylene vary depending on its type, primarily influenced by its molecular structure and density.

Types of Polyethylene

1. **LDPE (Low-Density Polyethylene):**

   - Characteristics: Density of 0.91-0.925 g/cm³, wax-like solid, tasteless, odorless, non-toxic, crystalline polymer with a crystallinity of 55-65%, melting point of 105-126°C, non-polar material prone to static electricity, low surface energy requiring corona treatment before printing or compounding, excellent transparency and heat sealing properties, good moisture resistance, poor gas barrier properties, moderate oil and organic solvent resistance.

   - Processing: Suitable for extrusion blow molding with a melt index (MI) of 2-6 g/10 min, extruded film MI of 8-15 g/10 min, high-temperature processing above 300°C for composite purposes but limited to short durations to prevent decomposition.

2. **MDPE (Medium-Density Polyethylene):**

   - Characteristics: Density of 0.926-0.94 g/cm³, properties similar to LDPE but with higher crystallinity (70-80%), increased melting temperature, hardness, and strength, positioned between LDPE and HDPE.

   - Processing: Suitable for calendering into sheets and films, particularly for vacuum forming applications.

3. **HDPE (High-Density Polyethylene):**

   - Characteristics: Density of 0.94-0.965 g/cm³, high rigidity, toughness, mechanical strength, solvent resistance, stress crack resistance, melting point of 126-136°C, crystallinity over 90%.

   - Processing: Used in garbage bags with MI < 1 g/10 min, rotational molding with MI of 3-20 g/10 min, injection molding with MI of 30-50 g/10 min, processing temperature of 180-250°C, high strength but poor transparency.

4. **LLDPE (Linear Low-Density Polyethylene):**

   - Characteristics: Excellent heat sealing properties, high thermal seal strength even with contaminated surfaces, high melt viscosity, sensitive to processing stress, requiring special screw design and high motor power.

   - Processing: Often blended with LDPE (50% LDPE + 50% LLDPE) to enhance properties and enable processing on LDPE equipment.

5. **mPE (Metallocene Polyethylene):**

   - Characteristics: Similar properties to LLDPE, produced using metallocene catalysts, high flowability under increased temperature or shear force, widely applied in plastic packaging.

6. **UHMWPE (Ultra-High Molecular Weight Polyethylene):**

   - Characteristics: Molecular weight over 3 million, almost zero melt flow, processed through thermal or cold pressing, superior to PTFE in comprehensive properties, extremely high strength, wear resistance, self-lubricating, used for mechanical wear parts, chain guides, bulletproof vests, and extensively in textiles, paper, packaging, transportation, machinery, chemicals, mining, oil, agriculture, construction, electrical, food, medical, sports, and refrigeration industries.

Polyethylene Formulation Considerations

1. Polyethylene and its copolymers (e.g., EVA, ethylene-vinyl alcohol copolymer) have good compatibility, allowing for performance improvement through blending.

2. Product properties can vary significantly with different production processes, even with the same material, highlighting the importance of production formula and process parameters.

3. When blending rubber with plastic, use powdered rubber or thoroughly mix in a two-roll mill before pelletizing. For functional additives like fillers, antioxidants, and antistatic agents, use masterbatches for uniform addition.

4. The key principle in formulation is to meet product requirements with the cheapest and simplest production method.

Main Methods:

Polyethylene is produced using liquid phase (solution and slurry) and gas phase methods, primarily employing Ziegler catalysts. For example, HDPE is obtained by polymerizing ethylene with a purity above 99% in the presence of titanium tetrachloride and diethylaluminum chloride catalyst at 0.1-0.5 MPa and 65-75°C in gasoline. The slurry is then treated to remove residual catalyst, neutralized, washed, and dried to produce the final product.