EVAtech 160I/21 Composto do copolímero EVA, grau de espuma reticulada

EVAtech 160I/21 EVA Copolymer Compound, Crosslinkable Foam Grade, is supplied as a pelletized ethylene-vinyl acetate copolymer formulation for chemically blown, peroxide-crosslinked closed-cell foam conversion. The grade designation corresponds to a nominal vinyl acetate content of 16 wt% and a melt flow rate of 2.1 g/10 min; the specification window is typically 15.0–17.0 wt% vinyl acetate by FTIR, ASTM D5594-18, and 1.8–2.4 g/10 min by ISO 1133-1:2022 under 190°C/2.16 kg. Base compound density is 0.935–0.945 g/cm³, ISO 1183-1:2019, with an unfoamed Shore D hardness of approximately 38, ISO 868:2003. The formulation uses a narrow molecular weight distribution EVA base resin and a stabilizer package that does not contain amine-based antioxidants, which prevents premature consumption of peroxide radicals. In downstream use, the compound is dry-blended with 2–6 phr azodicarbonamide and 0.5–1.2 phr dicumyl peroxide before extrusion or molding. Finished foam density ranges from 0.12 g/cm³ to 0.35 g/cm³, depending on blow ratio, die pressure, accelerator loading, and crosslink density.

Production-scale twin-screw compounding on corotating lines with 44:1 to 52:1 L/D ratios is normally carried out with barrel set temperatures of 90–110°C and screw speeds of 250–400 rpm. At a screw speed of 350 rpm and a set temperature of 110°C, melt temperature measured by an immersed thermocouple typically remains below 125°C, which is below the azodicarbonamide decomposition onset. Field data from continuous foam extrusion lines show that incoming-lot melt flow rate drift outside ±0.10 g/10 min produces measurable changes in cell size distribution. On a 40:1 L/D single-screw foam extrusion line operating at 6 MPa die pressure, the cell size distribution shifts from 0.25–0.40 mm to 0.55–0.70 mm when the melt flow rate increases by 0.15 g/10 min. Die temperature deviation greater than ±5°C from the compound-specific optimum produces surface roughness and elevates open-cell content above 15%. High-shear dispersion of azodicarbonamide agglomerates is required to reach particle sizes below 20 µm; rotor-type batch mixers with intermeshing blades are not recommended for this dispersion threshold.

What Distinguishes This Grade from General-Purpose Foamable EVA Compounds?

Compared with common 18 wt% vinyl acetate foamable EVA grades having a melt flow rate of 2.5 g/10 min, the 16 wt% vinyl acetate base compound exhibits lower equilibrium moisture uptake. After 24 h at 23°C and 85% RH, the 160I/21 grade typically absorbs 0.08–0.12 wt% water, whereas 18% vinyl acetate grades can reach 0.18–0.25 wt%, as determined by Karl Fischer coulometry. The lower polar monomer content reduces hydrogen bonding with ambient moisture. Pre-drying at 70–80°C for 2 h in desiccant dryers with a dew point of −40°C is required when ambient relative humidity exceeds 60% or when regrind content is above 20%.

Compared with LDPE-based foamable compounds, the 160I/21 grade provides lower compression set at equal crosslink density and accepts higher filler loadings without complete loss of foam elongation. The continuous service temperature of crosslinked EVA foam is lower than that of crosslinked LDPE foam; EVA-based products are generally limited to 70–80°C, while LDPE foams can operate at 90–100°C. The grade is not silane-grafted; it therefore requires free-radical initiator addition at the conversion stage. This differs from moisture-cure silane foam systems that do not require peroxide addition but demand hot-water or steam exposure for cure completion.

Regrind levels above 20% are permitted only after on-line melt filtration using a 100 µm screen pack and verification of gel fraction. Reusing edge trim from already crosslinked foam is not recommended because crosslinked particles act as nucleating sites and produce microgels of 10–50 µm, observable as surface pinholes after expansion.

When Peroxide Crosslinking Is Superimposed on Endothermic Foaming

In the 160I/21 grade, the recommended crosslinking initiator is dicumyl peroxide, typically applied at 0.5–1.2 phr through a low-temperature tumble blender or gravimetric metering unit. Dicumyl peroxide exhibits a half-life in polyolefin melts of approximately 1 h at 135°C and less than 1 min at 175°C. The practical consequence is that compounding and extrusion zones before the die must remain below 125°C, while the expansion oven or cure section must reach 155–175°C to achieve gel fractions of 62–72%. The initiator decomposes to free radicals that abstract hydrogen from ethylene sequences; the resulting macro-radicals combine to form carbon-carbon crosslinks. At least 60% gel fraction, determined by extraction in boiling xylene per ASTM D2765-16, is required before azodicarbonamide gas evolution reaches its maximum. Lower gel fractions allow cell coalescence and density instability.

Azodicarbonamide decomposition is activated by zinc oxide or zinc stearate. The addition of 0.6–1.0 phr zinc oxide lowers the exothermic decomposition onset from approximately 195°C to 160–170°C. The rate of gas evolution must match the rate of melt strength development. At 3.5 phr azodicarbonamide and 0.8 phr dicumyl peroxide, the gas yield is approximately 220 cm³/g of blowing agent at STP; actual retained gas volume is lower because of diffusion loss through the expanding melt membrane. Foam densities below 0.15 g/cm³ require die pressures above 7 MPa and gel fractions above 65%. Published data for this specific configuration is limited; production validation using in-line rheometry and post-foam density measurement is necessary.

Vinyl acetate content influences vulcanization kinetics because vinyl acetate side groups reduce crystallinity and increase segmental mobility, but also dilute the methylene sequence length available for macro-radical propagation. Compared with a 9 wt% vinyl acetate EVA grade, the 16 wt% vinyl acetate compound typically requires 0.1–0.2 phr higher peroxide addition to reach the same gel fraction at 160°C. This offset should be considered when replacing other EVA grades on existing production schedules.

For compression-molded foam boards, the compound is blended with curative and blowing agent on a two-roll mill at 90–100°C for 8–12 min. The milled preform is then placed into a hydraulic press with platens at 155–165°C and cured under 3–6 MPa for 8–12 min per 10 mm of preform thickness. Venting is initiated during the final 20–30% of cure time when the crosslink density reaches the gel point; premature venting causes blowholes, while late venting produces buckled skins. Mold release agents containing amine functional groups must be avoided because amine functionality can interfere with radical cure and cause surface tack.

Physical Property Benchmarks and Compliance-Linked Test Data

The property set below represents typical values obtained from crosslinked foam sheets at 0.20 g/cm³ density after 24 h conditioning at 23°C and 50% RH. Values are not batch release limits unless a specific lot certificate states otherwise.

Compliance-relevant statements are bounded by the base polymer classification of EVA as an olefinic copolymer under FDA 21 CFR 177.1520 for food-contact use only when the finished foam article is tested for end-use migration. The product is not automatically food-contact compliant in all formulations because azodicarbonamide decomposition residues and peroxide fragments must be assessed. Under EU REACH, the base polymer does not require registration as polymer, but monomer and additive components must be listed or authorized. RoHS 2011/65/EU restrictions apply to lead, cadmium, mercury, hexavalent chromium, PBB, and PBDE in the final converted article; the 160I/21 grade itself is supplied without regulated flame retardants.

Die Pressure and Gel Fraction Define the Workable Envelope

The process window is narrower than that of low-melt-index LDPE foam compounds and slightly wider than that of high-vinyl-acetate EVA copolymers above 25 wt% vinyl acetate. Table 2 compares operational parameters at a fixed loading of 3.5 phr azodicarbonamide and 0.8 phr dicumyl peroxide.

On the same production line, the 160I/21 grade exhibits less screw slippage than 25% vinyl acetate grades because its lower polarity reduces barrel-wall adsorption in water-cooled feed zones. The main processing penalty compared with LDPE foam is the lower upper temperature limit and the higher pre-drying demand. Compared with 18% vinyl acetate EVA, the grade offers better cell size retention when regrind levels fluctuate between 5% and 20%.

Operational boundaries are explicit: storage in unopened original packaging is recommended at 5–35°C and below 60% RH for 12 months; opened packaging should be consumed within 30 days or re-sealed with desiccant. The product must not be processed in direct contact with copper or copper alloys above 120°C because copper ions can accelerate oxidative decomposition of the peroxide and produce acetyl odor in the foam. Combinations with amine-based nucleating agents, amine-functional silanes, or amine-cured epoxy concentrates are incompatible before the expansion stage. The foam is not inherently UV-stable; outdoor applications require carbon black, titanium dioxide, or hindered amine-free UV stabilizer packages that do not interfere with peroxide cure. When flame retardant grades are required, ammonium polyphosphate or magnesium hydroxide may be incorporated, but the dosage must be compensated by increased peroxide demand. All batches should be verified by torque rheometry and gel fraction analysis before release to full production.