Overview

High-Density Polyethylene (HDPE) is a thermoplastic polymer made from ethylene with a high strength-to-density ratio. Its linear molecular structure gives it higher density, tensile strength, and chemical resistance than low-density polyethylene (LDPE). HDPE is one of the most widely used plastics worldwide — chosen for piping, tanks, geomembranes, packaging bottles, and industrial components due to its light weight, chemical inertness, weldability, and low cost.

ρ = 940-970 kg/m³ for HDPE (slightly less than water — 1000 kg/m³); LDPE is less dense at 910-930 kg/m³

Mechanical Properties

Property	Value (HDPE, typical molding/extrusion grade)
Density	940-970 kg/m³ (0.94-0.97 g/cm³)
Tensile Strength (yield)	20-30 MPa (3,000-4,500 psi)
Tensile Strength (ultimate)	25-40 MPa
Young's Modulus	800-1200 MPa (0.8-1.2 GPa)
Flexural Modulus	1000-1600 MPa
Elongation at Break	500-1000% (extremely ductile)
Hardness (Shore D)	60-70
Impact Strength (notched Izod)	30-200 J/m (tough)
Coefficient of Friction	0.2-0.3 (self-lubricating)

HDPE Floats in Water

With density of 940-970 kg/m³, HDPE is less dense than water and floats — useful for floating docks, marine barriers, and water surface applications. It also means that in water/wastewater piping, HDPE is neutrally buoyant to slightly buoyant and flotation is not a major concern during installation.

Thermal Properties

Property	Value
Melting Point	130-137°C
Glass Transition (T_g)	-100 to -80°C (remains tough in cryogenic service)
Deflection Temperature (0.45 MPa)	65-80°C
Continuous Service Temperature	60-80°C (above this, strength drops significantly)
Thermal Conductivity	0.45-0.52 W/m·K (insulator)
Specific Heat	1800-2000 J/kg·K
Coefficient of Thermal Expansion	100-200 × 10⁻⁶ /°C (very high — ~10x steel)

Chemical Resistance

HDPE is exceptionally chemically resistant:

Excellent: water, salt solutions, acids (concentrated HCl, H₂SO₄ up to 60%, most organic acids), alcohols, alkalis (NaOH, KOH)
Good: oils, greases, gasoline (brief exposure)
Poor/Avoided: strong oxidizers (concentrated HNO₃, halogens), aromatic hydrocarbons (benzene, toluene), chlorinated solvents
Porous to non-polar organics over time (flavor/odor scalping)

Pipe Grades (PE100, PE80)

HDPE pipe uses pressure-rated PE grades:

Grade	MRS (MPa)	Typical Use
PE80	8.0 MPa	Lower-pressure water/gas distribution
PE100	10.0 MPa	Standard for modern water/gas piping, higher pressure
PE100-RC	10.0 MPa	Resistance to crack propagation; trenchless installation

Common Pipe Applications

Water distribution: PE100 PN10/PN16 replacing ductile iron and PVC in many municipal systems
Natural gas distribution: The standard material for low/medium pressure gas lines
Sewer/drainage: Corrugated HDPE for gravity sewers and storm drains
Industrial: Chemical process piping, acid drains, landfill leachate collection
Marine/offshore: Flexible risers, subsea flowlines
Mining: Slurry lines, tailings transport (abrasion resistant)

Key Advantages as Piping Material

Flexible: can be bent in field; resists freeze-thaw cycles; accommodates ground movement (earthquake-resistant)
Weldable: heat fusion joints create monolithic, leak-free system (no gaskets, no thread sealant)
Corrosion proof: no rust, no galvanic corrosion, no internal tuberculation; smooth interior (C-factor ~150)
Light weight: ~1/7 the weight of steel — easier handling, lower installation cost
Long service life: 50-100 year design life for buried water pipe
Trenchless installation: HDD, pipe bursting, slip-lining without open trench

Thermal Expansion is Large

HDPE's coefficient of thermal expansion is 100-200 × 10⁻⁶/°C — about 10 times steel and 5 times PVC. A 30-meter HDPE pipe can change length by 60mm over a 20°C temperature swing. Design requires expansion loops, offsets, or proper burial restraint. Pressure rating also decreases significantly above 25°C.

Other Applications

Geomembranes: landfill liners, pond liners, canal liners (1-3mm sheets)
Packaging: milk jugs, detergent bottles, caps, food containers
Tanks: rotationally molded chemical tanks, water tanks (up to 50 m³)
Lumber/plastic wood: recycled HDPE for outdoor decking and boardwalks
Toys/playground equipment: no splinters, no paint, weather-resistant
Cutting boards and food processing: FDA-compliant grades

∑

Calculate HDPE Pipe Weight

Open pipe-weight-calculator →

Summary

HDPE (density 940-970 kg/m³) is a versatile thermoplastic valued for chemical resistance, flexibility, weldability, and light weight. As PE80/PE100 pipe it has become a standard for water and gas distribution, competing with and replacing ductile iron and PVC. Its major design considerations are high thermal expansion (10x steel), reduced strength at temperatures above 60°C, and UV degradation in outdoor use (requires carbon black stabilization for exterior service).

HDPE (High-Density Polyethylene)