Greenflex ML 50 EVA Copolymer Resin,Foam & Footwear Grade

Greenflex ML 50 EVA Copolymer Resin, Foam & Footwear Grade is a free-flowing ethylene-vinyl acetate pellet supplied for chemically blown and crosslinked foam applications in footwear, sheet extrusion, and moulded cellular components. The resin is specified for footwear-grade processing by its melt index of 5.0 g/10 min determined at 190 °C under 2.16 kg load in accordance with ISO 1133-1:2022, and by a vinyl acetate content of 18 wt% determined by FTIR according to ASTM D5594-18a. Nominal density at 23 °C is 0.940 g/cm³ under ISO 1183-1:2019; tensile properties on compression-moulded sheet are 18 MPa tensile strength and 750% elongation at break under ISO 527-2:2012. Hardness of the uncompounded base resin is 90 Shore A per ISO 868:2003, and the melting peak by DSC is 82 °C under ISO 11357-3:2018. Vicat softening temperature is 68 °C by ISO 306:2022 method A50. The melt index and vinyl acetate content position the material between low-flow film grades and high-flow injection grades; the melt viscosity at processing temperatures is low enough for injection-expansion flow into multi-cavity moulds, while retaining sufficient melt elasticity to resist cell wall rupture during gas evolution.

In industrial formulation practice, the resin is not supplied as a ready-to-foam compound but is compounded with azodicarbonamide-based blowing agents, dicumyl peroxide crosslinking agent, zinc oxide or zinc stearate activators, stearic acid processing aid, and mineral fillers such as calcium carbonate. Compounding on a co-rotating twin-screw extruder with L/D 32:1 is typically performed with barrel temperatures from 90 °C to 120 °C and screw speed between 200 rpm and 300 rpm. The upper barrel temperature must remain below 130 °C because activated azodicarbonamide grades begin to release gas near 155 °C; incidental heat dissipation from the screw can create localized hot spots exceeding 140 °C if specific mechanical energy input rises above 0.15 kWh/kg. The material does not require pre-drying under normal indoor storage at 25 °C and less than 60% relative humidity. If storage has exceeded 60% relative humidity, pre-drying at 60 °C for 2 h in a desiccant hopper with a dew point below -20 °C is required to bring surface moisture below 0.1 wt%, as measured by ISO 15512:2019.

Filler loading is a dominant variable because calcium carbonate and talc raise compound viscosity and increase cell nucleation density. At 10 phr calcium carbonate, the melt flow index of the compound drops from 5.0 g/10 min to approximately 3.5 g/10 min, while the resulting foam hardness increases by 5 Asker C to 10 Asker C. At filler loadings above 40 phr, cell wall rupture becomes more frequent because filler agglomerates act as stress concentrators during gas expansion. Field data from production-scale internal mixers with 55 L chamber capacity indicate that filler dispersion is complete only after 8 min mixing at 80 °C to 100 °C and rotor speed 30 rpm to 40 rpm; shorter cycles leave visible filler agglomerates in the expanded midsole.

Which Processing Conditions Govern Blowing Agent Decomposition and Cure Timing?

Chemically blown crosslinked midsole expansion involves two coupled kinetic systems: dicumyl peroxide cure and azodicarbonamide decomposition. Dicumyl peroxide at 135 °C has a half-life of approximately 1 h; at 171 °C the half-life falls to roughly 1 min. The neat azodicarbonamide decomposition exotherm is centered between 200 °C and 220 °C, but activated grades containing zinc oxide or zinc stearate shift the gas-release onset to 155 °C–170 °C. In a typical injection-expansion cycle, the compound is melted at 90 °C to 110 °C, injected into a closed mould, and then heated under clamp load to 160 °C–180 °C to activate peroxide crosslinking before the blowing agent degrades. If platen temperature exceeds the upper set point by more than 5 °C, the activated blowing agent decomposes before sufficient crosslink density develops, causing cell coalescence and density inversion. If the platen temperature remains below 155 °C, the crosslink network is too weak to sustain gas pressure when the mould opens, and the foam collapses. The practical tolerance for platen temperature control is therefore ±5 °C across the cavity surface, with cavity-to-cavity variation not exceeding 4 °C on multi-station presses monitored by thermocouple grids. On production machines with clamp force between 150 t and 350 t, shot volume variation must be kept below 1.5% to hold midsole density within ±0.01 g/cm³.

The melt viscosity of Greenflex ML 50 measured by capillary rheometry according to ISO 11443:2021 at 190 °C and 100 s⁻¹ is typically in the range 900 Pa·s to 1,200 Pa·s, whereas a high-flow injection EVA with vinyl acetate content of 28 wt% may be below 500 Pa·s at the same shear rate. This difference is mechanistically significant: higher melt viscosity is required to prevent cell wall drainage and pin-hole collapse during expansion, but it narrows the flow-length-to-thickness ratio in thin midsoles. When foaming with microcellular injection methods, the screw recovery time and mold temperature must be adjusted so that no melt stagnates longer than 6 min in the barrel at 170 °C or higher; residence time beyond 8 min can initiate premature peroxide depletion and produce a hard, poorly expanded core.

Cell morphology in crosslinked EVA foam is influenced by the release rate of nitrogen, carbon monoxide, and carbon dioxide during azodicarbonamide decomposition. The total gas yield of neat azodicarbonamide is approximately 220 mL/g, but gas yield alone is a poor predictor of cell size because cell nucleation is controlled by the number of dispersed filler particles and the solubility of gas in the polymer melt. In a well-dispersed compound based on Greenflex ML 50, the cell size distribution typically has a mean diameter from 60 µm to 120 µm at foam density 0.15 g/cm³, measured by optical microscopy following cross-sectioning. The skin layer is usually 200 µm to 500 µm thick and forms because the mould surface cools the melt rapidly before blowing agent decomposition completes; thicker skins are obtained when mould release agents are over-applied or when platen heat transfer is impaired.

For crosslinked midsole foam, the cured material is evaluated after mould cooling and post-vulcanization stabilization. Tensile strength of the foam at density 0.15 g/cm³ typically falls between 2.0 MPa and 2.8 MPa when tested according to ISO 1798:2017; elongation at break is generally 250%–350%. Compression set after 24 h at 50 °C and 50% compression is typically 10%–20% under ISO 1856:2018. Hardness of the expanded foam normally falls in the range 55 Asker C–65 Asker C, measured with an Asker C durometer using the footwear-trade procedure derived from JIS K 7312:1996. Rebound resilience at 0.15 g/cm³ is often measured with ISO 8307:2018; values from 35% to 45% are typical. These values are not intrinsic to the base resin alone; they are strongly dependent on filler loading, peroxide concentration, foam density, and post-mould ageing.

When the Footwear Moulder Compares Greenflex ML 50 with Film and Injection Grades

Direct comparative data for finished-part performance across all available EVA suppliers is limited; the differentiation below is drawn from the certified property profile of Greenflex ML 50 and common EVA foam formulation behavior. The primary difference from film extrusion grades is the higher melt index, which reduces injection pressure and improves cavity filling but also requires tighter cure timing. The difference from high-vinyl-acetate injection moulding grades is lower softness and lower rubber-like elasticity in the uncompounded state, which is corrected in footwear formulations by adding fillers, oils, or higher VA copolymers.

In addition to the property contrast, the grade differs from low-flow film EVA in molecular structure and additive requirements. Film grades with lower melt index require higher extrusion head pressures and are not practical for injection-expansion foam moulding because they do not fill the thin forefoot regions of a midsole before melt crosslinking begins. Injection grades with higher vinyl acetate content provide softer moquettes and interior components, but their lower melt viscosity at the same temperature permits more gas diffusion before cell walls set, which can collapse large cells and increase compression set by 10% to 15%. The 18 wt% vinyl acetate content of Greenflex ML 50 is therefore a mid-range point that preserves foaming melt strength while retaining enough polarity to accept high filler loadings and low-density expansion.

Compliance documentation for Greenflex ML 50 covers the base resin as delivered and is not automatically transferable to formulated foam compounds. The certificate of analysis reports melt index, vinyl acetate content, density, tensile properties, hardness, and moisture content against the methods already cited. Batch-to-batch variation in melt index is normally controlled within ±0.5 g/10 min and vinyl acetate variation within ±1 wt%; such variation can shift the optimum peroxide loading by 0.2 phr to 0.5 phr, which is why footwear compounders re-qualify cure when a new batch of resin is introduced.

For slabstock and sheet foam production, Greenflex ML 50 is calendered or extruded into preforms before hot-air or compression moulding. In calendering lines with roll temperatures between 70 °C and 90 °C, the resin exhibits sufficient hot-melt tack to maintain a uniform preform thickness of 2 mm to 10 mm. Sheet foam is then expanded in a continuous oven at 160 °C to 180 °C; residence time is usually 5 min to 10 min depending on sheet thickness and blowing agent activation. Thicker sheets above 10 mm require two-stage expansion to prevent a dense core caused by heat-transfer lag.

Dimensional tolerance after demoulding is another process boundary. Crosslinked EVA foam midsoles typically shrink 1.5% to 2.5% in length and width after 24 h ageing at 23 °C; uncontrolled post-cure shrinkage can reach 4% if demoulding occurs before the part reaches 50 °C. The standard practice on production lines is to anneal moulded midsoles in a forced-air tunnel at 60 °C for 2 h to stabilize dimensions; this step reduces subsequent shrinkage to less than 1% after 48 h at ambient conditions.

Foam processors should observe processing boundaries that are specific to this grade. Rework of partially cured EVA foam scrap is limited to 10 wt% addition into fresh compound because higher loadings reduce melt strength and increase hardness variance by ±3 Asker C on moulded midsoles. Avoid combining the resin with amine-based antidegradants or strong nucleophilic additives; these species scavenge peroxide-derived free radicals during cure and lower crosslink density at fixed peroxide loading. Aromatic solvents, ketones, and chlorinated hydrocarbons should not be used for cleaning equipment that will process the resin, because residues accelerate mould-fouling and can act as local cell nucleants. The recommended mould-release system is a water-based release agent applied at a dry-film thickness below 5 µm; thicker films reduce heat transfer and produce surface porosity. When these boundaries are not observed, the dominant failure modes are non-uniform cell size, density variation exceeding ±0.015 g/cm³, and surface blistering on expansion-moulded midsoles.