Polyethylene Properties at a Glance: Density, Crystallinity, Stiffness and Barrier

Polyethylene (PE) resin specifications are not established from a single physical constant. Four measurements—density, percent crystallinity, stiffness expressed as flexural modulus, and permeability to oxygen and water vapour—form the primary comparative set for resin selection and conversion control. Density is determined on compression-moulded specimens per ASTM D1505 or ISO 1183-1 at 23 °C after conditioning under standard laboratory atmosphere. Crystallinity is derived from the heat of fusion by differential scanning calorimetry per ASTM D3418 or ISO 11357-3 using 293 J/g as the reference enthalpy for fully crystalline polyethylene. Stiffness is normally reported from tensile modulus per ISO 527-2 or flexural modulus per ISO 178, and barrier is quoted as oxygen transmission rate per ASTM D3985 or water vapour transmission rate per ASTM F1249. Because the amorphous phase is continuous only at certain lamellar arrangements, these four indicators do not scale linearly across density classes. A resin with higher crystallinity may exhibit lower gas permeability but also reduced impact strength, and identical density values do not guarantee identical stiffness when short-chain branching distribution or thermal history differs.

Density measurement itself requires definition of specimen preparation. Compression-moulded plaques prepared per ASTM D4703 are free of extrusion orientation, but the cooling rate from the melt must be controlled because rapid crystallisation lowers density by approximately 0.002–0.005 g/cm³ relative to slow-cooled specimens of the same resin. Density gradient column methods are particularly sensitive to trapped air and surface wetting; specimens with internal voids cannot produce valid density values. For industrial arbitration, density is therefore reported alongside melt flow rate per ASTM D1238 or ISO 1133-1 because molecular weight and density together define the practical processing window.

What Actually Sets the Lower-Bound of Barrier Performance in Semi-Crystalline Polyethylene?

Barrier performance in polyethylene is governed predominantly by the amorphous free volume, not by the crystalline fraction alone. Semicrystalline PE consists of lamellar or spherulitic crystallites surrounded by amorphous interlayers; oxygen, carbon dioxide, and water vapour permeation occurs preferentially through these disordered regions. Increasing density from 0.918 g/cm³ to 0.955 g/cm³ reduces oxygen permeability because the amorphous phase becomes more tortuous and constrained by adjacent crystallites, increasing the effective diffusion path length. Published oxygen permeability coefficients at 23 °C and 0 % RH vary from approximately 150–300 cm³·mm·m⁻²·day⁻¹·bar⁻¹ for low-density polyethylene to 30–90 cm³·mm·m⁻²·day⁻¹·bar⁻¹ for high-density polyethylene. The same density progression lowers water vapour transport, although absolute water vapour values are affected by film surface condensation, gauge variation, and thickness normalisation errors.

Positron annihilation lifetime spectroscopy shows a reduction in the mean free-volume hole size from approximately 0.45 nm in branched low-density polyethylene to 0.30 nm in linear high-density polyethylene; this reduction is one of the mechanistic bases for the barrier improvement. However, the lower bound of barrier performance is not set by density alone. High melt-index grades with low chain entanglement characteristically exhibit poorer orientation stability and can develop microvoids at spherulite boundaries under high draw ratios, increasing permeability despite acceptable density values. The same effect occurs when processing produces unstable melt pressure and thickness variation; barrier layers with ±10 % gauge variation fail at the thinnest points, not at the average thickness.

Water vapour barrier follows a similar but not identical dependence. Polyethylene is non-polar, and the solubility of water in the amorphous phase is low; diffusion is controlled more by free-volume connectivity than by polar interactions. For film specimens normalised to 1 mm thickness, industrial datasheets typically report water vapour permeability coefficients of 0.3–0.6 g·mm·m⁻²·day⁻¹ for low-density polyethylene and 0.1–0.3 g·mm·m⁻²·day⁻¹ for high-density polyethylene at 38 °C and 90 % RH per ASTM F1249. Because water vapour permeability is highly sensitive to thickness and boundary-layer resistance, comparisons made without stating the method, relative humidity gradient, and thickness normalisation are technically invalid. Oxygen transmission rate also depends on relative humidity for certain grades; conditioned measurements should state not only temperature but also the moisture history of the specimen.

Density, Crystallinity, and Flexural Modulus Data for Extrusion-Grade Feedstocks

The following table consolidates representative ranges for four polyethylene classes. Values are from publicly available industrial datasheets and standards-based test data; they are not specification limits for any single grade. The crystalline fraction is determined by differential scanning calorimetry on samples cooled at 10 K/min and heated at 10 K/min, and the reference enthalpy of fusion is 293 J/g. Flexural modulus values are reported from three-point bending per ISO 178 at 2 mm/min.

Flexural modulus rises with density because the crystalline lamellae act as rigid reinforcing units. However, the correlation is not smooth in the 0.930–0.940 g/cm³ band, where short-chain branching distribution and molecular weight can shift stiffness by more than 100 MPa at a fixed density. This is a critical specification risk for medium-density PE used in pipe and rotomoulding: two resins with identical density and melt index can exhibit different flexural modulus because comonomer placement and lamellar thickness distributions differ. A one-point density measurement alone cannot distinguish a homogeneous copolymer from a blend or from a resin with broad chemical composition distribution unless paired with crystallinity data or an elution fractionation method such as temperature-rising elution fractionation.

Stiffness reporting also varies with test geometry and strain rate. The tangent tensile modulus measured per ISO 527-2 at 1 mm/min can differ from the flexural modulus per ISO 178 because flexural testing introduces a stress gradient through the specimen thickness. For semicrystalline PE, the difference between tensile and flexural modulus is typically small at room temperature but increases at temperatures above 60 °C, where creep and viscoelastic relaxation become significant. Therefore, a stiffness specification must state the loading mode, specimen thickness, preconditioning, and strain rate. A value quoted as “modulus” without these conditions cannot be compared across material classes.

When Biaxial Orientation Raises the Barrier Ceiling

Orientation of polyethylene film and sheet modifies crystallinity, crystal orientation, and amorphous-phase relaxation behaviour. Blown film is biaxially oriented by the bubble’s transverse draw and the haul-off’s machine-direction draw; high-density polyethylene high-stalk film can develop a balanced orientation that raises tensile modulus and lowers oxygen permeability relative to cast sheet of the same gauge. In a typical high-stalk HDPE film, machine-direction orientation of 4:1 and transverse orientation of 2:1 can produce tensile modulus values exceeding 700 MPa in both principal directions. The same film measured as an unoriented compression-moulded sheet may show a flexural modulus near 900–1100 MPa but isotropic; the film’s modulus in the machine and transverse directions should be compared separately per ASTM D882, not ASTM D790. Orientation also tightens the amorphous tie chains between lamellae and reduces the fractional free volume available for diffusion; oxygen permeability decreases by approximately 15–35 % relative to an unoriented sheet of the same crystallinity. Published data for specific biaxially oriented HDPE film configurations are limited because commercial stretching temperatures and blow-up ratios vary; the stated reduction is therefore a processing-dependent range, not a material constant.

The barrier gain from orientation is not unlimited. At high transverse draw ratios above approximately 6:1, some linear grades develop microtears or interlamellar cavitation that can increase water vapour transmission while improving tensile strength. This is particularly observed in tenter-frame biaxially oriented films stretched near the lower end of the elongation-temperature window. The practical upper draw ratio is therefore set by the onset of cavitation, which is monitored by film density loss, haze rise, and disagreement between oxygen and water vapour permeability. When haze increases above 10 % per ASTM D1003, the barrier uniformity of the film may be compromised even if the average transmission value remains acceptable.

On a 55 mm single-screw blown-film extruder with a 25:1 L/D barrier screw and a 0.8 mm die gap, high-stalk high-density polyethylene is maintained at melt temperatures between 190 °C and 230 °C. The stalk height is held at 6–8 die diameters to allow lamellar orientation before the frost line. Blow-up ratio is limited to 1.5–2.0 for high-density grades; higher ratios increase transverse orientation but destabilise stalk geometry and produce gauge bands of ±12 %. Melt fracture on the same line appears at screw speeds above 80 min⁻¹ when the melt pressure upstream of the screen pack exceeds 35 MPa for a 0.45 dg/min high-density polyethylene. Replacing the 0.45 dg/min resin with a 0.95 dg/min grade lowers head pressure by 8–12 MPa but reduces die-lip draw stability and decreases dart impact strength per ASTM D1709 from approximately 300 g to 200 g. Batch-to-batch variation in melt flow rate from 0.45 dg/min to 0.55 dg/min shifts head pressure by 5–8 % on the same screw profile; torque variation is recorded at the gear reducer and used to define a limited processing window.

Injection moulding of high-density polyethylene with a 35 mm screw and 20:1 L/D at 220–260 °C requires clamp pressure sufficient for the projected area; parts with wall thickness 1.5–3.0 mm are typically moulded on 120–180 t machines. Shrinkage is anisotropic and directly coupled to crystallinity: mould shrinkage measured per ISO 294-4 is approximately 1.2–2.0 % for semicrystalline high-density polyethylene, with the larger value occurring in the flow direction. Mould cooling rate is the main control for crystallinity and hence stiffness in thick sections. At cooling rates below 10 K/min, the molten core of a 3 mm section can retain sufficient time for crystallite thickening, increasing flexural modulus but lowering impact resistance. Processing below the recommended melt temperature or with excessive residence time can produce gel particles from crosslinked fractions; these gels appear as discontinuities in film orientation and are detected by optical gel counting per ASTM D7310.

Barrier Classification Cannot Be Separated From the Amorphous Phase

Permeability measurements on polyethylene are often reported as a single coefficient, but the actual transport path is through the amorphous tie-chain network. Spherulite size, lamellar thickness distribution, and tie-chain density all influence the diffusion coefficient without necessarily changing the bulk density. For gas barrier applications, a higher-density high-density polyethylene does not automatically provide the lowest oxygen permeability if the film is quenched rapidly and contains fine spherulitic morphology with numerous low-crystallinity grain boundaries. Conversely, slow cooling at ≤5 K/min increases lamellar thickness and reduces amorphous free volume, lowering permeability but also reducing dart impact. This inverse relationship between barrier and impact is a known operational boundary for high-density polyethylene blow-moulded containers.

The table below lists the primary standard methods and the conditions that must be reported for valid comparisons. The test condition is part of the result; omitting it introduces classification errors when converting between film, sheet, and moulded parts.

A specification for barrier performance should include the sensor type and the humidity history of the specimen. Water vapour transmission rates measured per ASTM E96 gravimetric method can differ from ASTM F1249 modulated infrared values by 10–25 % for the same film, particularly at low transmission rates where boundary-layer resistance is significant. This is not an instrument defect but a consequence of method-specific driving force and detection sensitivity. Reports that do not identify the method, specimen thickness, and humidity gradient therefore fail as technical specifications.