POE Grade Selection Table: Density, Melt Index and Octene Content Compared

Polyolefin elastomer (POE) grades are predominantly metallocene-catalysed ethylene-1-octene copolymers with a narrow molecular weight distribution and a controlled intermolecular comonomer distribution. Selection among commercial grades is made on three primary release properties: density, melt index, and octene content. Density is measured per ISO 1183-1:2019 or ASTM D792. Melt index is determined at 190 °C under a 2.16 kg mass per ISO 1133-1:2022 or ASTM D1238-20. Octene content is typically measured by Fourier-transform infrared spectroscopy per ASTM D5576 or by ¹³C NMR. These values are coupled: higher octene incorporation reduces crystallinity and density, lowers the crystalline melting point, and increases elastomeric character. Melt index, controlled by molecular weight and long-chain branching, determines processability but does not by itself define the performance window. A grade selection table must therefore be read as a density-melt index-octene matrix. The comparison is limited to ethylene-1-octene grades because ethylene-butene POE variants follow a different density-comonomer relationship.

Commercial ethylene-1-octene POE grades typically span a density range from 0.857 g/cm³ to 0.908 g/cm³, melt indices from 0.5 g/10 min to 30 g/10 min, and octene contents from approximately 5 wt% to 38 wt%. Within this range, three selection clusters emerge. Low-density grades below 0.870 g/cm³ are soft, low-crystallinity, high-clarity materials for sealants, soft-touch compounds, and foaming. Mid-density grades at 0.870–0.890 g/cm³ balance flexibility with heat resistance for impact modification and wire-and-cable sheathing. High-density grades above 0.890 g/cm³ retain sufficient crystallinity for high-temperature TPO skins and thin-wall injection-molded components. The apparent simplicity of this density ladder is complicated by melt index variation, which can change tear strength, melt strength, dispersion quality, and injection pressure independently of octene content.

How Does Octene Content Rewrite the Crystallinity-Density Relationship in Metallocene POE?

In a homogeneous ethylene-1-octene microstructure, the octene branches interrupt methylene sequence length and reduce orthorhombic crystallite thickness. At octene contents below 10 wt%, density remains above 0.890 g/cm³ and the resin retains a measurable crystalline plateau. Above 25–30 wt% octene, density falls below 0.870 g/cm³ and the material transitions to a truly elastomeric solid with a broad melting endotherm rather than a sharp crystalline peak. Differential scanning calorimetry on commercial grades shows peak melting temperatures from 38 °C for a 0.857 g/cm³ grade to 104 °C for a 0.908 g/cm³ grade. The low-temperature melting fraction also shifts; a low-density POE may exhibit a secondary endotherm near 25 °C, which reduces blocking resistance but improves seal initiation. The octene distribution is therefore more informative than average octene content when comparing resins from different catalyst platforms. Two resins with the same density and melt index can differ in seal initiation temperature by 5–10 °C if one has a broader intramolecular comonomer distribution.

The following matrix compares six representative ethylene-1-octene POE types arranged by density and melt index. Octene content is reported as a band because it is rarely a specification limit on commercial technical data sheets and varies with production train and analytical calibration.

Octene content bands are typical values derived from melt-state FTIR correlation, not specification limits. DSC values are second-heat peak temperatures at 10 °C/min.

The table illustrates the inverse nonlinear trade-off between flow and flexibility. If a high-flow grade is selected without adjusting density, the resulting part may retain stiffness but lose impact strength. If a low-density grade is substituted at equal melt index, the melt strength in blown film may drop and the resin may fail to retain a bubble. The octene content band is useful when comparing resins across producers because density is affected by processing history and long-chain branching to a smaller but measurable extent.

When Low-Melt-Index POE Is Extruded Through a 40:1 L/D Twin-Screw Line

Extrusion of low-MI POE on a 40:1 L/D counter-rotating twin-screw line can generate melt temperatures 20–30 °C above the barrel set point due to viscous heating. If the POE is compounded with a peroxide masterbatch for silane grafting, this temperature overshoot can cause premature crosslinking and gel formation. Production-scale trials show that reducing the screw speed from 300 rpm to 150 rpm or increasing the feed-zone temperature by 15 °C reduces melt temperature by 8–12 °C. The processing window is narrowest for low-density POE grades because their low crystallinity removes the latent heat sink associated with melting; the entire energy input is converted into melt temperature rise. In a grade substitution, shear-thinning behaviour must be compared on a capillary rheometer at 1000 s⁻¹ before changing the melt index specification.

In PP impact modification, the POE grade is selected by matching the melt viscosity of the PP matrix. A PP homopolymer with an MFR of 35 g/10 min is typically compounded with 20–30 wt% POE and 10–20 wt% talc. If the POE viscosity is too high, the dispersed elastomer domains remain coarse, and Izod impact strength at -30 °C per ASTM D256 may fall below 6 kJ/m². A viscosity-optimised POE with a density of 0.885 g/cm³ and MI of 1.0 g/10 min can produce domain sizes below 1 µm and a ductile-brittle transition below -50 °C. In injection-molded TPO parts, the same grade may require a clamp force increase of 10–15% compared with a high-MI grade because the compound’s effective viscosity at the filling shear rate is higher.

Paint adhesion on POE-rich TPO skins is an additional constraint. High octene content increases non-polar surface character and reduces surface energy, which can lower crosshatch adhesion per ISO 2409 below the required Gt 0 rating unless flame, plasma, or chlorinated polyolefin adhesion promoters are used. A POE grade with density above 0.890 g/cm³ and octene content below 15 wt% gives a better paint-adhesion baseline. Conversely, low-temperature impact after aging per ISO 6603-2 improves as octene content increases; the selection therefore becomes an adhesion-impact trade-off rather than a single-property optimisation.

Processing-Limited Grade Substitution Across the Density-Melt Index Matrix

Melt index values below 1.0 g/10 min are conventionally used in profile extrusion, calendered sheet, and film where high melt strength and low crystallinity are required. Values from 1.0 to 5.0 g/10 min provide a balance between melt-strength retention and injection-molding fill pressure. Values above 10 g/10 min are restricted to thin-wall injection molding, high-speed compounding, and melt-blown nonwoven applications. However, melt index at 2.16 kg is a low-shear measurement and does not capture shear thinning. A long-chain-branched POE with a melt flow ratio I10/I2 of 10–18 may process at high shear like a linear grade with a higher nominal melt index while still retaining melt strength. Melt index alone therefore cannot guarantee equivalent mould filling, dispersion quality, or pressure-drop behaviour across supplier boundaries.

Wire-and-cable compounds use POE with a density of 0.870–0.902 g/cm³ and MI of 1.0–3.0 g/10 min. For flame-retardant sheathing, a mid-density grade is preferred because it resists deformation at 120 °C better than a 0.870 g/cm³ grade but retains sufficient flexibility. Oil resistance after 168 h at 100 °C in IRM 902 oil per ISO 1817 improves as density increases. Peroxide crosslinking in low-voltage insulation benefits from a low-crystallinity POE because the elastomer melts earlier and permits more uniform peroxide dissolution. The residual peroxide concentration after steam vulcanisation increases if the POE grade has a high octene content, but the resulting network may show lower hot-set rupture strength unless the coagent level is raised. Silane-grafted POE compounds show similar sensitivity: the silane grafting efficiency drops if the melt temperature exceeds 230 °C because the initiator half-life is consumed before uniform alkoxysilane grafting occurs.

Footwear foaming imposes a different rheological boundary. EVA/POE compounds expanded with azodicarbonamide at 170–190 °C require a minimum melt elasticity to prevent cell coalescence. A low-density POE with 0.857 g/cm³ and MI of 1.0 g/10 min produces a foam with lower compression set than an equivalent Shore A EVA grade, but only when the POE melt strength is boosted with 10–20 phr of EVA or a long-chain-branched POE. High-MI POE above 5 g/10 min may cause cell coalescence and shrinkage because the melt viscosity is too low to contain the gas pressure from the blowing agent. Crosslinkable foam formulations based on dicumyl peroxide require a POE with a low crystallinity to allow peroxide dispersion below the dosed rubber-plateau temperature; otherwise the resulting foam exhibits localised hard spots and uneven expansion.

Why Does Octene Distribution Control Optical Haze More than Average Comonomer Content?

In polyolefin packaging films, haze is governed by the number and size of crystalline aggregates that scatter visible light. Two POE grades with the same average octene content can differ in haze by 2–5% if one has a broader intramolecular comonomer distribution. The broader distribution creates longer methylene sequences, which crystallise into lamellae thick enough to scatter light. This is particularly relevant in blown and cast sealant films containing a low-density POE. A metallocene catalyst with a constrained geometry produces a narrower distribution than older multi-site catalysts, but differences remain among producers. Therefore haze measurement per ASTM D1003 should be paired with octene content data rather than used as a proxy for comonomer uniformity. Published data for this specific configuration is limited, but the optical difference is consistently observed in film trials.

Medical tubing applications impose a different constraint. The POE must meet extractable requirements under ISO 10993-1 and USP Class VI. Grades with a melt index above 5 g/10 min tend to have a larger low molecular weight tail, which can raise hexane extractables and increase the risk of migration. Therefore a lower-MI grade in the 0.5–2.0 g/10 min range is selected despite the increased extrusion pressure. Sterilisation resistance is generally acceptable up to 50 kGy gamma radiation, but surface tack may increase after sterilisation if the POE density is below 0.880 g/cm³. Pre-drying is not normally required because POE does not hydrolyse; however, surface moisture from condensation at RH > 60% can cause bubble formation in cast film, and a hopper dryer at 70–80 °C for 2–4 h is applied only when pellet surface moisture exceeds 0.05 wt%. In all cases, residence times above 15 min at 220 °C should be avoided because oxidative chain scission and discoloration may occur in the absence of sufficient antioxidant protection.