Overview
Polyethylene Medium Density (PEMD), also known as MDPE, is a thermoplastic polymer with a density range between low-density (LDPE) and high-density polyethylene (HDPE). It is produced via copolymerization of ethylene with alpha-olefins, resulting in a material with a unique balance of flexibility, toughness, and chemical resistance. PEMD was developed in the 1950s to address specific industry needs where neither LDPE nor HDPE provided optimal performance. The material's molecular structure features a moderate degree of branching, which gives it distinct advantages in applications requiring both durability and processability. PEMD is widely recognized for its environmental stress-crack resistance (ESCR), making it particularly valuable for products subjected to long-term mechanical or chemical stresses.
Physical and Chemical Properties
PEMD exhibits a density range of 0.926–0.940 g/cm³, placing it between LDPE (0.910–0.925 g/cm³) and HDPE (0.941–0.965 g/cm³). This intermediate density provides a balance of properties: greater stiffness than LDPE while maintaining better impact resistance than HDPE at low temperatures. The material typically has a melting point range of 120–130°C, with thermal stability up to about 80°C for continuous use. Chemically, PEMD is resistant to most acids, alkalis, and organic solvents at room temperature, though it may swell in some hydrocarbons. Its permeability to gases and vapors is lower than LDPE but higher than HDPE, a factor considered in packaging applications. The material's mechanical properties include tensile strength of 15–25 MPa and elongation at break of 100–600%, varying with molecular weight distribution and processing conditions.
Main Applications
PEMD's primary application is in pressure-rated gas distribution pipes, where its combination of crack resistance and durability meets stringent safety requirements. The material accounts for approximately 30% of plastic pipe systems in Europe and North America for natural gas transmission. In packaging, PEMD is used for heavy-duty sacks, shrink films, and caps/closures that require higher stiffness than LDPE but greater drop resistance than HDPE. The industrial sector utilizes PEMD for chemical tanks, geomembranes, and cable jacketing where environmental stress resistance is critical. Consumer applications include household containers, toys, and crates. Recent developments have expanded PEMD's use in rotomolded water storage tanks and large industrial containers, capitalizing on its impact resistance and weatherability when properly stabilized with additives.
Safety and Storage
PEMD is generally regarded as non-toxic and is approved for food contact applications when meeting FDA or EU regulations. However, processing at high temperatures (above 200°C) may release volatile organic compounds, requiring adequate ventilation in manufacturing environments. The material is not biodegradable but can be recycled under resin identification code #4. For storage, PEMD pellets or products should be kept in dry conditions below 40°C, away from direct sunlight to prevent UV degradation. Bulk storage in silos requires proper grounding to prevent static accumulation. When welding PEMD pipes or sheets, proper fume extraction is recommended, though the decomposition products are less hazardous than those from many engineering plastics.
B2B Procurement Guide
When procuring PEMD, buyers should specify key parameters including melt flow index (typically 0.1–10 g/10 min for most applications), density (within the 0.926–0.940 g/cm³ range), and additive packages (UV stabilizers for outdoor use). For pipe applications, verify compliance with standards like ISO 4427 or ASTM D3350. Packaging grades may require FDA or EU 10/2011 compliance documentation. Supply chain considerations include regional availability, as PEMD production is concentrated in areas with ethylene feedstock access. Lead times for specialty grades can range from 4–8 weeks. Pricing typically follows ethylene monomer costs with a multiplier, and large-volume contracts (20+ metric tons) may secure 5–10% discounts. Quality verification should include certificate of analysis for density, melt index, and antioxidant content.
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