Polyethylene Classification and Production Processes – Comprehensive Guide to PE Types and Manufacturing Methods

Polyethylene (PE) is one of the most widely produced synthetic resins globally. Based on differences in molecular structure and density, it forms a diverse product system. The three primary types are high-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE). Additionally, there are functional or specialty types such as ultra-high molecular weight polyethylene (UHMWPE) and metallocene polyethylene (mPE), which differ fundamentally in chain arrangement and branching structure.

1. High-Density Polyethylene (HDPE)
   HDPE, also known as low-pressure polyethylene, features a linear molecular chain with minimal branching and a high crystallinity of 80%–95%. Its density ranges from 0.941 to 0.960 g/cm³. This structure provides high strength, stiffness, excellent chemical resistance, and superior barrier properties against moisture and gases compared to other PE types.

2. Low-Density Polyethylene (LDPE)
   LDPE, also called high-pressure polyethylene, has a molecular chain with numerous long branches, resulting in a loose and disordered arrangement. It exhibits a crystallinity of only 55%–65% and a density of 0.910–0.925 g/cm³. The long-chain branching gives LDPE outstanding flexibility, transparency, and processability, though its mechanical strength and heat resistance are relatively low.

3. Linear Low-Density Polyethylene (LLDPE)
   LLDPE, known as the third-generation polyethylene, has a linear main chain with uniformly distributed short branches introduced via copolymerization of ethylene with α-olefins such as butene or hexene. Its density ranges from 0.915 to 0.925 g/cm³, with a crystallinity of 60%–70%. The short-branch structure provides a balance of HDPE's strength and LDPE's flexibility, particularly excelling in puncture resistance, tear resistance, and environmental stress crack resistance.

4. Ultra-High Molecular Weight Polyethylene (UHMWPE)
   UHMWPE has a molecular weight typically exceeding 1.5 million, far higher than conventional HDPE (200,000–500,000). Its extremely long molecular chains result in a crystallinity of 80%–85% and a density of 0.92–0.94 g/cm³. The ultra-long chains endow UHMWPE with exceptional wear resistance and impact strength, surpassing ordinary polyethylene, though it presents significant processing challenges.

5. Metallocene Polyethylene (mPE)
   mPE is synthesized using metallocene catalysts (e.g., zirconium or titanium complexes) and includes types such as metallocene LLDPE (mLLDPE) and metallocene HDPE (mHDPE). It features more uniform branching distribution, narrow molecular weight distribution, and comonomer content up to three times higher than conventional LLDPE, resulting in superior performance.

6. Medium-Density Polyethylene (MDPE)
   MDPE has a density between LDPE and HDPE, approximately 0.926–0.940 g/cm³, also known as medium-pressure polyethylene or Phillips process polyethylene. Its properties are close to HDPE, with improved environmental stress crack resistance.

7. Cross-Linked Polyethylene (PEX or XLPE)
   PEX is produced by chemically or radiatively cross-linking polyethylene molecular chains to form a three-dimensional network structure, altering the linear configuration. This significantly enhances heat resistance and mechanical performance.

8. Ionically Cross-Linked Polyethylene (Ionomer)
   Ionomer is an ethylene copolymer in which a portion of carboxyl groups are neutralized by metal ions (e.g., zinc, sodium). A typical example is DuPont's Surlyn resin, which exhibits unique "self-healing" characteristics.