Polyethylene Properties

Engineering variability

• Short-term tensile modulus is far higher than long-term effective modulus because creep is significant.
• Yield and tensile behavior depend on temperature, strain rate and grade.
• Thermal expansion is very high relative to metals and must be accommodated in restrained piping.
• The 132°C value is a crystalline melting transition reported for PE100; useful service temperatures are much lower. The selected Vinidex table does not publish UTS or specific heat, so those cells are intentionally left non-numeric.

Selection note

Excellent for buried piping, water/gas service and chemically aggressive environments. Creep, thermal movement, support spacing and joining method are central design considerations.

Do not combine strength from one grade/condition with modulus, thermal data or temperature limits from another without checking compatibility. Where the overview shows a range, it is intended to expose variability—not establish allowable bounds.

Where it is specified

HDPE for pressure pipe (gas and water distribution), chemical tanks, geomembranes, blow-moulded containers and cutting boards. LDPE and LLDPE for film, packaging, cable insulation and squeeze bottles. PEX (cross-linked) for underfloor heating and hot water plumbing. UHMWPE for wear strips, conveyor guides, bulletproof fibre and orthopaedic joint bearings.

The family divides by chain branching and molecular weight rather than by chemistry. Branching lowers crystallinity, which lowers density, stiffness and melting point but raises clarity and toughness — so density is effectively the specification variable.

Chosen over what

Against PVC for pipe: PE is more ductile, tolerates ground movement and freezing far better, and is joined by fusion into a fully welded system with no mechanical joints to leak. PVC is stiffer and cheaper in large diameters. Gas distribution has moved almost entirely to PE for exactly the ductility reason.

Against steel pipe: no corrosion, no cathodic protection, lighter, and can be coiled and ploughed in — but far lower pressure capacity and unusable at elevated temperature.

Against polypropylene: PE stays tough at low temperature where PP becomes brittle, which is why cold-climate and refrigerated applications favour PE.

How it fails

Slow crack growth is the governing long-term mechanism in pressure pipe. A small defect grows quasi-brittle over years at stresses far below yield, with no visible deformation. It is precisely why modern PE100 resins were developed and why pipe is qualified by long-term hydrostatic testing to ISO 9080 rather than by short-term tensile data.

Environmental stress cracking in contact with surfactants, detergents and some oils under stress — measured by ASTM D1693.

UV degradation unless carbon black is added; this is why almost all outdoor and buried PE pipe is black, and coloured stripes rather than coloured pipe are used for identification.

Creep. PE deforms continuously under load, so allowable design stress falls markedly with time and temperature — a pipe rated at 20 °C has substantially reduced capacity at 40 °C.

Fabrication notes

Butt fusion and electrofusion are the standard joining methods and produce joints as strong as the pipe. Procedure control is everything — face preparation, heater plate temperature, pressure and cooling time — and joint failures are almost always procedural rather than material.

Adhesive bonding and painting are effectively impossible without surface treatment, because polyethylene has very low surface energy. Flame, corona or plasma treatment is required, and this is a frequent surprise in design.

Machining is easy but the material is soft and thermally expansive — coefficient of thermal expansion is roughly ten times steel — so tight tolerances are hard to hold and fixturing must not clamp hard.

UHMWPE cannot be conventionally moulded at all; its melt does not flow, so it is compression moulded or ram extruded and then machined.