In a material screening sequence for POE vs EVA vs PEBA for Footwear Midsole: Rebound Rate and Density Table, the first controlled response is apparent density because closed-cell gas volume fraction determines the magnitude of mechanical energy returned under heel-strike compression. Polyolefin elastomer (POE) is a metallocene-catalysed ethylene-α-olefin copolymer; flexible foam grades contain comonomer fractions from 20 wt% to 45 wt% and exhibit solid density from 0.870 g/cm³ to 0.885 g/cm³. Ethylene-vinyl acetate (EVA) midsole grades are supplied with vinyl acetate contents from 14 wt% to 28 wt% and solid density from 0.930 g/cm³ to 0.950 g/cm³. Polyether block amide (PEBA) midsole grades are segmented block copolymers with solid density near 1.010 g/cm³ and Shore D hardness from 40 to 70. These solid-state values do not transfer directly to foam rebound; melt strength, blowing agent solubility, and cell stabilisation behaviour determine the final density and energy-return structure.

Vertical rebound resilience is measured under ASTM D2632-15; the test uses a 12.5 mm specimen and reports the percentage of drop height recovered. Apparent density is determined under ISO 845:2006 after conditioning for 24 h at 23 ± 2 °C and 50 ± 5 % relative humidity. Compression set is measured under ASTM D395-18 Method B at 50 % deflection for 72 h. Because foam rebound is strongly influenced by density below 0.15 g/cm³, any POE vs EVA vs PEBA comparison must state both density and rebound result in a single table. Without density, a POE foam reported at 60 % rebound and an EVA foam reported at 55 % rebound cannot be ranked because the POE sample may be thinner, lower-density, or closed-cell-rich.

What Would Invalidate a Direct POE–EVA–PEBA Rebound Comparison?

Specimen thickness and the presence of an integral moulded skin are the two most common sources of non-comparability. A 10 mm POE sheet with closed-cell content below 85 % can read 5–8 percentage points lower than a 12.5 mm sheet from the same formulation under ASTM D2632-15, because gas outflow through open surface cells lowers stored pneumatic energy. PEBA supercritical foam midsoles are frequently tested as laser-cut plaques without moulded skin; a POE injection-compression moulded midsole with an intact skin retains gas during impact and can appear artificially high in rebound. The same issue occurs when density is reported on a cut specimen under ISO 845:2006 while rebound is measured on a moulded part; the two datasets are not aligned. Conditioning history also matters. POE foam containing external open cells can gain moisture at 50 % RH and shift rebound by 2–4 percentage points because water plasticises the cell-wall amorphous phase. PEBA is less hygroscopic and typically shifts less than 1 percentage point under identical conditioning. Published data for this specific configuration is limited, but the moisture sensitivity is confirmed by dynamic mechanical analysis of water-conditioned EVA and POE foams.

Density and Rebound Bands Compiled From Supplier Datasheets

Representative ranges for midsole foams are shown in Table 1. The ranges are consolidated from publicly available supplier technical datasheets for chemically foamed POE and EVA, and supercritical CO₂-foamed PEBA. The exact compression set value within each band is influenced by peroxide or silane crosslinking density.

At equivalent density of 0.20 g/cm³, rebound resilience generally follows PEBA > POE > EVA. Below 0.12 g/cm³, PEBA may lose rebound if cell struts buckle during impact; POE at 0.15 g/cm³ can approach PEBA rebound within 3 percentage points when closed-cell content exceeds 90 %. The table therefore records a broad material platform, not a processing guarantee.

Cell size distribution supplies a second control variable that is absent from routine rebound-density tables. PEBA supercritical foams with median cell diameter below 50 µm generally return more energy than coarse-cell PEBA at the same density because thin cell walls recover elastically rather than buckling. Chemically foamed POE and EVA typically show median cell diameters between 80 µm and 150 µm; the wider distribution lowers rebound and increases compression set. Cell diameter is measured by scanning electron microscopy or X-ray microtomography with voxel resolution below 5 µm. This measurement is not standardised in ISO 845:2006 and is normally absent from supplier datasheets, which explains part of the scatter in Table 1.

On a 50 mm single-screw extruder with 36:1 L/D, POE chemical foam is run with a reverse barrel profile from 150 °C at the feed throat to 185 °C at the die. Die pressure is maintained above 70 bar to prevent premature azodicarbonamide decomposition. Melt flow index under ASTM D1238 at 190 °C/2.16 kg is typically 0.5–5 g/10 min for flexible POE foam grades; EVA with 18 wt% vinyl acetate runs in the 1.5–25 g/10 min range at the same condition. When the same line is switched from EVA to POE, barrel temperatures are reduced by 10–18 °C and die pressure drops by 10–15 bar at constant screw speed of 25 rpm. If screw speed is increased without raising feed throat cooling, POE pellets may bridge because the low melting onset permits early softening. Batch-to-batch density variance on 2.0 mm POE sheet is reported at ±0.012 g/cm³ when closed-loop melt pump pressure is used.

On a 350 t injection moulding machine with core-back expansion, POE chemical foam midsoles are produced by short-shot filling to 80–85 % of cavity volume, followed by core-back of 1.5–2.5 mm at 10–30 mm/s. If core-back speed exceeds 30 mm/s, cell rupture occurs and surface sink marks appear. EVA with equivalent expansion requires higher melt temperature and higher holding pressure. PEBA supercritical microcellular moulding is run with gas counterpressure and a clamp force reserve of at least 15 % to avoid mould flash at the split line.

When Supercritical CO₂ Foaming Replaces Chemical Blowing Agents in POE

Supercritical CO₂ is dosed at 0.4–0.8 wt% and maintained above 74 bar to create a single-phase melt solution. Cell nucleation occurs after the pressure drop at the die or gate; pressure drop rate and elongational viscosity of the polymer decide cell number density. POE has a narrow molecular weight distribution and lower melt strength than EVA or PEBA. At 190 °C, CO₂ plasticisation reduces POE viscosity enough that cell walls can coalesce before crystallisation stabilises the structure. The processing window for POE is therefore narrow: below 165 °C gas solubility is insufficient, and above 210 °C the cell walls collapse. PEBA under the same gas dosage maintains higher elongational viscosity and permits stable expansion down to 0.07 g/cm³; POE is generally limited to density above 0.12 g/cm³ without reactive modification. Cell nucleation density for POE under CO₂ is reported in the range 10^6–10^9 cells/cm³, while PEBA formulations with optimised pressure drop can reach 10^9–10^12 cells/cm³. Published data for this specific configuration is limited to laboratory-scale microcellular injection moulding.

Compression Set Does Not Scale Linearly With Rebound Rate

POE and EVA foams can have identical rebound resilience and still differ in compression set by 10 percentage points because rebound is dominated by gas-phase recovery, while compression set is dominated by plastic deformation of the solid cell-wall polymer. Under ASTM D395-18 Method B at 50 % deflection for 72 h at 23 °C, POE midsoles typically retain 12–20 % compression set. EVA midsoles with vinyl acetate content above 20 wt% retain 20–30 %; PEBA midsoles at 0.10 g/cm³ density retain 15–25 %. The POE advantage is tied to lower crystallinity and lower internal friction in the amorphous phase. Hardness measured by ASTM D2240-15 on a 6 mm stack of foam does not track rebound; the hardness range for POE in Table 1 may be shifted upward or downward by skin thickness without changing the rebound reading by more than 2 percentage points.

At -30 °C, EVA foam becomes stiffer because the amorphous vinyl acetate-rich phase approaches its glass transition; POE retains flexibility down to approximately -40 °C because the ethylene-α-olefin amorphous phase exhibits a lower tan δ peak. PEBA low-temperature response depends on the polyether soft segment and hard segment concentration; midsole grades with high poly(tetramethylene oxide) content maintain rebound better than EVA but can stiffen below -40 °C. The low-temperature modulus shift is not visible in a room-temperature rebound-density table. For cold-climate trail footwear, the density-rebound table should be supplemented with dynamic mechanical analysis storage modulus data over the range -40 °C to 23 °C.

Residual azodicarbonamide decomposition products in EVA and POE chemical foam are screened under REACH Regulation 1907/2006 Article 33 when a candidate-list substance is present above 0.1 wt%. PEBA supercritical foam avoids azodicarbonamide residuals because CO₂ or N₂ is used as the blowing agent; however, PEBA must be pre-dried to below 0.05 % moisture when relative humidity exceeds 60 % to prevent hydrolytic viscosity loss. POE pellets are less hygroscopic and may not require pre-drying below 0.05 % moisture. In peroxide-cured POE or EVA systems, amine-based antioxidant packages are avoided because amine additives scavenge free radicals and retard the cure response, thereby widening the density tolerance of the foam.