Greenflex ML 60 EVA Copolymer Resin,High Elasticity Foam Grade

When Foam Expansion Outpaces Crosslink Formation in Midsole Compounds

Compounding of Greenflex ML 60 for high-rebound midsoles begins with a batch cycle that keeps melt temperature below the onset of azodicarbonamide decomposition yet above the melting point of the resin and the peroxide dissociation threshold. In an internal mixer fitted with Banbury rotors and a discharge thermocouple, the resin is fluxed with zinc oxide, zinc stearate, and mineral filler before peroxide is added in a second-stage dump at 105–120 °C. A two-roll mill with a friction ratio of 1:1.2 to 1:1.4 then sheets the batch; stock temperature is maintained below 115 °C to avoid scorching during calendering. The subsequent compression-molding step relies on a thermal window of 155–170 °C, where dicumyl peroxide half-life is short enough to form a molecular network while the activated azodicarbonamide gas fraction remains below full decomposition. The central process conflict is kinetic: if the gel fraction exceeds roughly 25% before the blowing reaction completes, the elongational viscosity of the cell walls rises and the sheet cannot expand to the target density range of 0.18–0.25 g/cm³; if gas evolution dominates before sufficient tie points exist, cell coalescence creates internal splits that are rejected after skiving. A moving-die rheometer trace at 165 °C is used to define t_s2 values; production batches with a scorch time below 50–60 s tend to show pre-gelled particles in the molded bun. Expansion pressure inside a 300 mm × 300 mm mold cavity is governed by the loaded volume, blowing agent content at 2.5–5.0 phr, and the mold gap. When the sheet is loaded at 75–85% of cavity volume, internal cavity pressure stays low enough to prevent blow-by but high enough to force cell wall orientation. The molded bun is then cooled under pressure, stripped, and annealed in a forced-air oven at 40–60 °C for 24–48 h to release residual blowing gas and stabilize shrinkage. Acceptance testing follows ISO 845 for density, ISO 1798 for tensile elongation, and ISO 8307 for rebound; high-elasticity midsole foams are typically specified with ball rebound above 40% and compression set below 25% after 50% deflection per ISO 1856 method A. Insole and midsole designers further specify split tear resistance because perimeter die-cutting and stitching holes create stress concentrators; a tear strength below 2.5 N/mm on 20 mm-thick skived sheet is often rejected on production lines, especially when the foam is used with slender arch supports.

Where continuous lamination rather than block molding is required, Greenflex ML 60 is processed into roll stock through a flat-platen press or a continuous double-belt foaming line. The resin is first compounded into a homogeneous matrix with stearates and an endothermic–exothermic blowing agent blend to control cell size distribution across a 1.2 mm to 8.0 mm finished thickness. A calendar train with heated rolls at 80–95 °C produces a pre-sheet free of entrapped air, because any trapped moisture from filler surfaces creates pinhole defects during expansion. The pre-sheet is then foamed between heated platens at 150–165 °C; platen pressure is stepped from an initial 6–10 MPa to a lower holding pressure as blowing agent decomposition begins, allowing gas nucleation without losing the surface glaze. The result is a closed-cell sheet that can be skived to 0.8–1.5 mm layers for laminated orthotic covers or textile-laminated packaging liners. Surface roughening before lamination is controlled by the cooled release film; a matte side provides mechanical adhesion to polyurethane and polyamide films. The roll stock is tested for percent elongation before lamination because the printed film must stretch with the foam during die cutting; tensile elongation values below 200% per ISO 1798 tend to crack at the die edge when the composite is pulled over a radius.

Why Do 40 mm Mat Cores Sacrifice Rebound for Tear Propagation Resistance?

Sports mat and protective flooring cores require a different crosslink density than footwear components because high thickness increases the deformation path and heat build-up under repeated impact. A 40–60 mm mat core made from Greenflex ML 60 is typically foamed to a slightly higher density of 0.10–0.15 g/cm³ to balance cushioning and dimensional stability. Compression molding uses a blowing agent cup-blowing technology where the partially crosslinked sheet is placed in a hot mold with venting channels; the mold is opened in a controlled sequence to release gas without delaminating the center. Processors report that the center of thick mats remains hotter than the surface during expansion; if the mold opens too early, the still-expanding core separates at the mid-plane. This is a known failure mode on double-deck platen presses where heat transfer from both surfaces creates a core temperature lag of 8–15 °C over a 50 mm bun. Blowing agent level is typically reduced to 2.0–3.5 phr and peroxide is adjusted to a slightly higher network density to resist tensile tearing at cell walls. Resulting mats are cut by water-jet or oscillating-knife equipment into interlocking tiles or roll-up mats; edge sealing with a hot air lance prevents moisture ingress. Tear strength per ISO 8067 and indentation resistance after dynamic loading are more critical than rebound; competitive mats are often specified with residual indentation after 24 h recovery below 10% of thickness, while ISO 2439 is used to report indentation load deflection. Products intended for children’s play mats may also require migration testing under EN 71-3, which restricts the release of zinc, aluminium, and other elements from the colored foam surface.

In marine buoyancy applications, closed-cell content is the primary safety parameter rather than mechanical recoverability. Greenflex ML 60 is formulated without hygroscopic fillers because any continuous water phase in the cell network increases the risk of buoyancy loss during long-term immersion. The foam is compressed into sheet or block form at 0.08–0.12 g/cm³ and the cell membranes must remain intact through post-molding machining. Closed-cell content is quantified by ISO 4590 or ASTM D6226, and values below 90% typically signal that the blowing reaction generated internal paths between adjacent cells. A marine collar or fender fabricated from the foam is then tested for water absorption after 7 days of immersion per ASTM D3575; accepted buoyancy-grade foam normally shows uptake below 3% by volume. The crosslinking system uses dicumyl peroxide at 0.8–1.2 phr with delayed-action blowing agent combinations to prevent gas release during injection of thick sections. In production, blocks are cut with a horizontal bandsaw, and the cut surface is heat-sealed by a hot wire at 200–220 °C to close open cells created by the blade. The heat-sealing step is not decorative; it restores the closed-cell barrier at the surface. Buoyancy aids manufactured from this material are generally covered with a textile shell, so UV testing per ISO 4892-2 is limited to the exposed shell, but uncovered foam should not be stored in direct sunlight beyond 500 h due to oxidative surface chalking. Pressure-sensitive adhesives used to laminate straps or attachment tabs must be selected from non-aromatic formulations; aromatic solvents can soften the foam surface and reduce local shear strength.

Orthotic Padding and the 40% Deflection Compression-Set Boundary

Orthotic and prosthetic interface padding is skived from crosslinked Greenflex ML 60 buns and must satisfy both mechanical endurance and dermal compatibility requirements. The base foam is prepared with a fairly low hardness grade, typically in the Asker C 30–45 range, and then heat-pressed into pads with a controlled skin layer. Heat-forming on aluminum tools at 110–130 °C allows the foam to take a three-dimensional heel cup or metatarsal pad shape; after cooling, the part is edge-sanded and solvent-wiped to remove loose cell debris. The compression set after 40% deflection for 24 h at 70 °C per ISO 1856 method B is the decisive mechanical limit because the pad is loaded cyclically under body weight. A foam that recovers poorly will bottom out against the shell and cause pressure sores; therefore a compression set above 15–20% is generally rejected for custom orthotics. If the foam is intended for direct skin contact in a medical device assembly, cytotoxicity testing according to ISO 10993-5 and sensitization per ISO 10993-10 may be required by the device manufacturer. Additive selection is restricted: only food-grade or medical-grade zinc stearate and peroxide decomposition residues that are volatile enough to be removed during annealing are used. Annealing after molding at 50–60 °C for 48 h reduces residual blowing gases such as carbon dioxide, ammonia, and nitrogen; failure to anneal creates an odor complaint in enclosed footwear and can lead to adhesive failure in laminated pads. The final pad is often bonded to polypropylene or carbon fiber shells with water-based polyurethane adhesive applied at 60–80 g/m²; bonding strength is limited by the foam’s surface energy, so corona treatment at 2.0–3.0 kW line speed may be used before lamination.

When automotive interior pads are converted from crosslinked EVA sheet, the processing route moves from compression molding to high-speed die-cutting, and the material must pass fogging and flammability requirements before it reaches an assembly line. Greenflex ML 60 is foamed into 5–15 mm sheet and then laminated with a nonwoven scrim or polyester facing; the adhesive layer is applied by roll-coating at 30–50 g/m². During die-cutting, the foam’s high elasticity causes dimensional recovery that can shift part geometry by 0.3–0.8 mm over a 400 mm length if the sheet is not aged for 72 h after molding. Automotive interior foams are typically tested for horizontal burning rate under FMVSS 302; EVA compounds can fail if the organic blowing agent residue or processing oil is too high, so formulations for this sector use mineral filler at low loading and avoid halogenated flame retardants unless a vehicle-specific specification requires them. Fogging is evaluated per DIN 75201 or SAE J1756; pass criteria vary by OEM, but gravimetric fogging values below 2 mg are a common acceptance band. The density window of 0.08–0.12 g/cm³ is selected to reduce head-impact accelerations while maintaining enough stiffness for die-cut edge retention. A production line that runs this sheet through a high-speed press at 40–60 strokes/min exposes the foam to rapid compressive cycles; if the sheet has not fully crosslinked, the die-cut edges smear and stick to the cutting rule. Therefore the gel fraction after foaming is checked by solvent extraction in xylene at 130 °C for 24 h, with a target of at least 50% for automotive die-cut parts. Published data for this specific resin configuration is limited because most OEM approvals are conducted under confidential part-level specifications; however, the generic behavior of crosslinked EVA foam is well documented in SAE and ISO test protocols.