EVAtech 140S/33C Compósito do copolímero EVA, grau de espuma cruzável

EVAtech 140S/33C EVA Copolymer Compound, Crosslinkable Foam Grade is supplied as a pelletized ethylene-vinyl acetate compound formulated for peroxide-initiated crosslinking and chemical blowing-agent expansion. The grade designation identifies a high-vinyl acetate class with a nominal vinyl acetate content of 33% by mass, though published data for this exact commercial designation is limited. The numerical ranges in this document are therefore drawn from industrial compounding literature for high-VA crosslinkable EVA foam systems and should be revalidated on the target production line. The compound differs from lower-VA EVA compounds and non-crosslinkable ethylene copolymer foams primarily through reduced crystallinity, lower Shore hardness, and the formation of an insoluble gel network after cure. Components produced from this compound are commonly tested under ASTM D395-18, ASTM D638-14, ASTM D624-00, and ASTM D2240-15.

What Distinguishes a 33% Vinyl Acetate Crosslinkable Foam Compound?

The vinyl acetate content in EVAtech 140S/33C places it between standard 28% VA foam grades and softer 40% VA polymer blends. At 33% VA, polyethylene crystallinity is suppressed enough to produce expanded Shore A values in the 35–55 range at foamed densities of 0.20–0.35 g/cm³. An 18% VA EVA foam at equivalent density commonly measures 60–75 Shore A. The higher polar monomer fraction improves wetting of calcium carbonate and silicate fillers, permits higher filler loadings before tensile strength loss becomes unacceptable, and increases adhesion to polar solvent-borne inks and adhesives. The main penalty is moisture uptake; pellets exposed to ambient relative humidity above 60% require drying before compounding. Compared with a non-crosslinkable EVA foam, the crosslinkable formulation develops 40–70% gel by solvent extraction under ASTM D2765-16 and reduces compression set after 22 h at 70°C under 25% deflection. This makes the 33C grade appropriate for seals, footwear components, and gaskets where recovery after compression is specified, not for disposable low-density packaging foam where lower-cost non-crosslinkable polyolefin would be sufficient.

Differential scanning calorimetry under ISO 11357-3:2018 on high-VA EVA typically shows a broad crystalline melting endotherm from 40°C to 80°C, with the main peak near 60–70°C. Lower-VA grades such as 18% VA retain a distinct polyethylene-like melting endotherm above 85°C. This difference lowers the preform softening temperature in the mold and means the 33C grade requires shorter air-venting time before full press pressure is applied. The glass transition of EVA remains below -20°C, so low-temperature flexibility is retained after crosslinking and expansion. The same polar structure that improves filler wetting also increases dispersion viscosity when high-surface-area carbon black is added; formulations with carbon black above 5 phr should be screened for peroxide adsorption before scale-up.

On a corotating twin-screw extruder with L/D 44–52, the compound is mixed in barrel zones at 90–120°C. Melt temperature is held below 130°C when dicumyl peroxide is present. Pre-drying at 60–70°C for 2–4 h is required when storage relative humidity exceeds 60%; residual moisture causes surface porosity and variable blowing-agent dispersion. Side stuffing of fillers is preferred over single-pass high-shear kneading because frictional heat can push the melt into the peroxide scorch zone before the azodicarbonamide masterbatch is distributed uniformly. Formulations containing fillers with pH below 4.5 should be avoided, because acidic surfaces consume peroxide radicals and depress final gel content. For color-critical footwear products, titanium dioxide and organic pigments are added through a separate masterbatch at the feed throat to minimize color streaks.

Rheological and Thermal Boundaries for Screw Compounding

Melt flow rate for high-VA crosslinkable EVA foam grades is usually 2–6 g/10 min at 190°C/2.16 kg when tested to ISO 1133-1:2022. A melt flow rate below 2 g/10 min increases torque in reverse kneading blocks and can require barrel settings at the upper end of the 90–120°C window; a melt flow rate above 6 g/10 min may produce pellet cutting instability and lower melt pressure control. Dicumyl peroxide half-life data from organic peroxide suppliers place the one-hour half-life near 132°C. This establishes a practical melt-temperature ceiling of 120–130°C for compounding operations; if the melt remains above that ceiling for more than 60 s, localized scorch forms microgel particles that later appear as fish-eye defects in the expanded sheet. Compounding therefore uses low-shear screw profiles with distributive mixing elements and downstream melt thermocouples positioned before the pelletizer die. The die head zone is set at 100–120°C to avoid strand breakage while keeping residence time below 45 s.

Extruder barrel liners should be checked for wear at the kneading-block transition zone because high-VA EVA compounds are less abrasive than glass-filled polyamide but more shear-sensitive than polyethylene. If the melt temperature exceeds 130°C during a production run, the batch should be downgraded for scrap or sample evaluation rather than fed to the pellet silo, because scorched pellets cannot be re-melted without retaining fish-eye defects. For twin-screw lines with a gear pump before the pelletizer, the pump inlet pressure should be maintained above 10 bar to prevent cavitation at the lower melt viscosity generated by the VA fraction.

After pelletization, the compound is converted into closed-cell foam sheet in a hydraulic compression press or a continuous oven system. In compression molding, a pre-weighed preform is heated at 150–170°C under mold pressure sufficient to suppress premature gas expansion. The platen temperature should be controlled to ±2°C across the working area. If cure temperature falls below 150°C, gel development remains under 40% and compression set rises above 40% under ASTM D395-18 Method B. If cure temperature exceeds 170°C or hold time exceeds 12 min for a 10 mm preform, gas migration and cell coalescence produce a coarse internal structure and possible surface collapse. The mold opening step must release pressure rapidly enough to nucleate cells uniformly but not so fast that the parting-line skin tears. Production-scale observation on hydraulic presses indicates that platen temperature spreads greater than ±2°C create measurable sheet density variation; presses with validated PID controllers and heated platens are recommended for the 33C grade.

For multilayered foam sheet, coextruded or laminated skins are applied before molding, and the crosslinkable core layer must be matched to the skin rheology to prevent interlayer delamination at the foam-skinned interface. The die temperature for skin-core coextrusion should not exceed the peroxide scorch ceiling of 130°C. In continuous oven curing, the residence time and zonal air temperature must be tuned so that the surface cures before the core expands; otherwise the sheet exhibits a collapsed center and densified surfaces.

When ADC Decomposition and Peroxide Crosslinking Must Overlap in a Narrow Temperature Window

Differential scanning calorimetry of azodicarbonamide-modified EVA systems typically shows a blowing-agent decomposition exotherm with an onset near 200–210°C, while dicumyl peroxide cure exotherm onset is significantly lower. In a mold set at 150–170°C, ADC decomposition is delayed by heat transfer from the platen surface; gas evolution therefore peaks later in the cycle. At mold temperatures above 175°C, gas pressure rises before the gel network is sufficiently developed, causing cell rupture and irregular cell size distributions. At mold temperatures below 155°C, ADC conversion remains incomplete within normal 8–12 min residence times; residual blowing agent can produce surface blisters during post-cure trimming or heat-setting. The practical operating window is therefore narrow. A variation of ±5°C around the optimized mold temperature is sufficient to shift foam density by more than 0.05 g/cm³ in some formulations, according to supplier technical data. Crosslinking coagents such as triallyl cyanurate or trimethylolpropane trimethacrylate are used in some formulations to widen this window by increasing crosslink density without raising cure temperature.

The concentration of dicumyl peroxide and azodicarbonamide must be matched to the VA content because the vinyl acetate comonomer affects both radical crosslinking and gas solubility. A formulation designed for a 28% VA EVA will not transfer directly to the 33C grade without adjusting the peroxide level, as the higher VA fraction can increase the tendency toward polar byproduct formation and may require a slightly higher co-agent loading to reach the same gel content. Pilot trials should use thermocouple-instrumented preforms to record the centerline temperature lag; when the centerline is more than 8°C below the platen temperature at 50% of cycle time, the mold heating system or preform thickness should be modified rather than extending cycle time alone.

Applications for this crosslinkable foam grade are those requiring a closed-cell, low-density elastic material with Shore A below 55 and limited compression set after thermal aging. Footwear midsole and sockliner processors use the 33C grade because it can be molded, skived, and die-cut without excessive edge tearing. Gasket and seal fabricators specify the compound when ASTM D395-18 Method B compression set must remain below 35% after 22 h at 70°C with 25% static deflection. Thermal insulation and packaging components use the compound where the continuous service temperature does not exceed 80°C, as high-VA EVA can embrittle after long-term oxidative aging. For flame-retardant orders, halogen-free packages using magnesium hydroxide or aluminum trihydrate are added, but these fillers raise density and require rebalancing of the peroxide and blowing-agent levels. Compared with metallocene polyethylene foam, EVAtech 140S/33C gives lower hardness at equal density and better adhesion to polar inks, but it has higher moisture absorption and a lower maximum service temperature. Compared with PVC/NBR foam, it avoids plasticizer migration but offers lower inherent flame resistance and may require additional fire-retardant additives to satisfy IEC 60695-2-12 glow-wire testing at 650°C.

Lot-to-lot control during incoming inspection should include verification of melt flow rate and vinyl acetate content against the certificate of analysis. A shift of more than ±1.5 percentage points in VA content alters hardness and ink adhesion, while a shift of more than ±1 g/10 min in MFR changes screw torque and cell size distribution in the foaming line. Blowing-agent masterbatches with a mean azodicarbonamide particle size of 3–6 µm are preferred for uniform cell nucleation; coarser particle size distributions can produce pinholes and non-uniform density profiles. The product should be stored in moisture-barrier packaging and resealed immediately after use.

For a production engineer evaluating a change from a 28% VA crosslinkable foam grade to EVAtech 140S/33C, the pilot trial should compare hardness at equal density and compression set after 70°C aging, not tensile strength alone. The 33C grade will typically mold at a slightly lower melt viscosity and may require a 2–3°C reduction in barrel temperatures to maintain the same melt pressure. Because the higher VA content lowers the crystalline melting point, the preform may also soften earlier in the mold, reducing the time available to vent trapped air before full pressure is applied. These adjustments are specific to the crosslinking and blowing-agent package in use and cannot be transferred directly from one production line to another without confirmation on the target press.