Types of Polyethylene: HDPE, LDPE, LLDPE, MDPE and UHMWPE Compared is a selection framework organized around density, branching topology, molecular weight distribution, and consequential processing behavior. Polyethylene is a semi-crystalline polyolefin in which crystallinity is governed by chain linearity and short-chain branch frequency. Density measured by ISO 1183 ranges from 0.910 g/cm³ for LDPE to 0.941–0.965 g/cm³ for HDPE, with MDPE and LLDPE occupying intermediate bands. Melt flow classification by ASTM D1238-20 at 190°C/2.16 kg gives widely different processability: LDPE flows at high melt index for coating and film, HDPE pipe grades are controlled at 0.2–0.7 g/10 min, and UHMWPE does not produce measurable melt flow and is characterized by intrinsic viscosity per ASTM D4020.

The resin families HDPE, LDPE, LLDPE, MDPE, and UHMWPE are differentiated primarily by density, short-chain branch frequency, long-chain branch content, and molecular weight. Table 1 summarizes typical commercial property ranges using standardized test methods.

What Differentiates HDPE Pipe Grades from LDPE Film Resins?

In ISO 4427 pressure pipe extrusion, HDPE grades such as PE100 are classified by minimum required strength of 10 MPa at 20°C for 50 years using ISO 9080 regression data. These resins have density 0.945–0.960 g/cm³, tensile yield strength 22–30 MPa per ISO 527-2 type 1B, and flexural modulus 900–1500 MPa per ISO 178. Pipe extrusion on a single-screw grooved-feed extruder with L/D 30:1 to 36:1 uses barrel temperatures from 200°C to 230°C and die temperatures from 190°C to 215°C. The principal long-term failure mode is slow crack growth, not short-term burst; pipe grades are therefore screened by environmental stress crack resistance per ASTM D1693 condition A and hydrostatic design basis per ASTM D2837.

LDPE film resin occupies a low-density, branch-rich region. Density is 0.910–0.930 g/cm³, tensile yield is 7–13 MPa, and flexural modulus is 150–400 MPa. Blown-film LDPE runs at lower melt temperatures of 160–200°C because the high-pressure autoclave or tubular reactor product contains long-chain branches that reduce melt viscosity and allow high draw-down. LDPE grades for extrusion coating typically have melt indices of 4–15 g/10 min to maintain melt curtain stability at line speeds above 200 m/min. The limitation of LDPE is inferior puncture and tear resistance compared with LLDPE; the limitation of HDPE is notch sensitivity and lower clarity in film.

Where gas distribution sockets and rotational-molded chemical tanks require slower crack propagation than ordinary HDPE but greater stiffness than LDPE, MDPE is selected. MDPE density is 0.926–0.940 g/cm³ and apparent melt index is 0.1–4.0 g/10 min; tensile yield is 14–20 MPa and flexural modulus is 400–800 MPa. Gas distribution piping under ISO 4437 and ASTM D2513 historically used MDPE because its moderate density reduces slow crack growth propagation in notched pipe sections. Rotational molding with MDPE powder, typically 35–70 mesh, uses oven temperatures of 280–320°C and biaxial rotation ratios near 4:1. The powder densifies into a bubble-free wall only if peak internal air temperature reaches 190–210°C; undercooked parts exhibit pinholes and low impact strength, while overcooked parts show oxidative yellowing and embrittlement. Because MDPE has fewer long-chain branches than LDPE, it has lower melt strength than LDPE but better environmental stress crack resistance than standard medium-melt HDPE. MDPE pipe-grade resins are often evaluated by notched pipe testing such as ISO 13479, with failure times above 1000 h under hydrostatic stress at 80°C.

When LLDPE Replaces LDPE in Blown Film: Melt Strength, Motor Load, and Dart Impact

On blown-film lines, replacing LDPE with LLDPE at equivalent 190°C/2.16 kg melt index increases extrusion head pressure and motor load because LLDPE lacks long-chain branching and has a narrower molecular weight distribution. Screw designs for LLDPE generally require a longer feed section and high-shear mixing barrier; extruders with L/D 30:1 to 36:1 and grooved-feed or cooled-feed throats are specified. Die gaps are widened from 0.8–1.2 mm for LDPE to 1.5–2.5 mm for LLDPE to prevent melt fracture. The benefit appears in film mechanical data. Dart impact by ASTM D1709 for a 25 µm LLDPE film commonly falls between 150 g and 400 g, whereas LDPE film of the same gauge often falls below 200 g. Elmendorf tear by ASTM D1922 and tensile properties by ASTM D882 are also higher for LLDPE at equivalent film thickness, though LLDPE tear anisotropy is stronger.

Bubble stability is the primary processing conflict because LLDPE has lower melt strength than LDPE. When resin is run on a conventional single-lip air ring, frost line height is reduced and the film may flutter at draw-down ratios above 5:1. Processors adjust by blending 20–30% LDPE into LLDPE, adding dual-lip air rings, or selecting octene-based LLDPE for improved dart impact at higher pellet cost. Comonomer type changes short-chain branch length: butene-based LLDPE typically has dart impact 10–20% lower than hexene- or octene-based LLDPE at equivalent density and melt index.

Ultra-high-molecular-weight polyethylene does not undergo measurable melt flow under ASTM D1238-20 at 190°C/2.16 kg; intrinsic viscosity by ASTM D4020 is usually 20–40 dL/g, corresponding to molecular weight above 3.1 million g/mol. Conventional single-screw plastication is replaced by ram extrusion, compression molding, or direct compression molding of preforms. Compression molding of UHMWPE orthopedic preforms is conducted at 190–210°C under 3.5–10 MPa for 30–60 min; oxygen ingress during heating is limited by vacuum or inert-gas press operation because oxidative chain scission increases wear rate.

Surgical implant grades are specified under ASTM F648 and ISO 5834-2; tensile yield is 20–24 MPa, elongation at break is above 300%, and notched Izod impact frequently exceeds 100 kJ/m². Crosslinked UHMWPE bearing liners are irradiated at 50–100 kGy, and subsequent thermal or non-thermal treatment reduces residual free radicals. Published hip simulator comparisons show crosslinked UHMWPE volumetric wear is typically lower by 50% or more relative to conventional UHMWPE, depending on irradiation dose and counterface roughness. Gel-spun UHMWPE fiber processes use high intrinsic-viscosity resin in decalin or mineral oil; after extraction and hot drawing, fiber tenacity reaches 20–40 cN/dtex with modulus above 800 cN/dtex. The main boundary is thermal-oxidative degradation above 100°C in long-duration service; UHMWPE is not processed by injection molding or standard blown film.

Specification, Compliance, and Failure-Prevention Data Across the Five Polyethylene Families

Standards conformance is process- and end-use dependent. The five polyethylene families are not interchangeable under a single test specification. Food-contact use is governed by FDA 21 CFR 177.1520, which covers resins with different density and extraction limits; potable-water pipe adds NSF/ANSI 61 and EN 12201 for HDPE. Medical UHMWPE requires ASTM F648 and ISO 5834-2. Table 2 lists the primary material standards and typical conversion routes.

Failure prevention is tied to branch structure and molecular weight. In HDPE pipes, slow crack growth resistance is ranked by ISO 13479 notched pipe testing; in MDPE, the same mechanism is controlled by density reduction and tie-molecule formation. LDPE film lacks the dart impact and tear resistance required for heavy sacks; LLDPE compensation for low melt strength requires wider dies and internal cooling stabilization. UHMWPE oxidative stability is controlled not by melt processing but by gamma or electron-beam crosslinking and terminal sterilization; resin must be stored below 40°C in the absence of contact with copper or cobalt salts, which accelerate oxidation.