ELVAX 9756 Copolímero de acetato de vinilo de etileno

ELVAX 9756 ethylene vinyl acetate copolymer is supplied as a pelletized random copolymer in which vinyl acetate comonomer units interrupt the polyethylene crystallite structure. The resulting resin has lower crystalline melting point, lower stiffness, and higher polarity than LDPE homopolymer. Its lot-to-lot consistency is controlled against the nominal values listed in the table below.

Incoming-resin qualification should include melt mass-flow rate, vinyl acetate content, density, and visual contamination. Sampling from 3 bags per lot with a clean stainless steel thief is typical before release to production. Melt-flow variation outside ±10% of the supplier’s nominal value has been associated with visible viscosity drift on slot-die coating and wax-coating lines.

The grade occupies a middle position within the ELVAX line. Relative to ELVAX 40W with nominal 40% vinyl acetate and 52 g/10 min, ELVAX 9756 has lower VA-derived polarity and a lower melt flow rate, so it produces higher melt viscosity at constant addition and less aggressive plasticization of nonpolar wax matrices. Relative to ELVAX 210 with nominal 28% vinyl acetate and 400 g/10 min, ELVAX 9756 disperses more slowly in low-shear blending but contributes less surface tack and less low-temperature adhesion to polar substrates. These differences define the grade’s practical use in compounds where wax compatibility, cohesive strength, and controlled open time take precedence over maximum adhesion to aluminium or polyester.

What Limits Screw Speed When ELVAX 9756 Is Run on a Twin-Screw Compounder?

On a 40 mm co-rotating twin-screw extruder with 44 L/D and modular screw geometry, the practical screw-speed ceiling for ELVAX 9756 is set by viscous heating rather than motor torque. Barrel zones from feed to die are typically set at 160 °C, 170 °C, 180 °C, 190 °C, 190 °C, 190 °C, and 180 °C. At screw speeds above 450 min⁻¹, melt temperature can rise to 220–230 °C because energy dissipation in kneading blocks is not removed quickly enough by barrel cooling. Degradation of the vinyl acetate segment begins to release acetic acid and shifts the melt from translucent to yellow; the odor threshold for acetic acid on the shop floor becomes detectable before the melt index rises significantly.

Melt pressure at the die plate is normally 2–4 MPa at 300 min⁻¹. Die-face pelletizing remains stable when the melt is filtered through a 200 µm breaker plate and the water-ring cutter is set to maintain pellet temperature below 60 °C at the outlet. The same compounding line can process ELVAX 210 at higher throughput with less restrictive screw geometry because its lower melt viscosity reduces shear heating; this difference in viscous heat accumulation is the main reason the two grades are not interchangeable in a fixed screw configuration.

Thermal stability of ELVAX 9756 in compounding is assessed by melt-flow shift and color development. At melt temperature 210 °C and residence time above 20 min, the melt flow rate can increase by 10–30% relative to virgin pellets because chain scission of the vinyl acetate fraction releases acetic acid. Hindered phenolic antioxidant at 0.1–0.3 phr and phosphite stabilizer at 0.1–0.2 phr are added when regrind ratios exceed 20% by mass. The stabilizer package is selected for low migration in food-contact applications and for compatibility with hydrocarbon tackifiers in hot-melt adhesives.

Addition of 1–5 wt% ELVAX 9756 to a 58–60 °C paraffin wax is performed in a jacketed vessel equipped with a Cowles disperser. The polymer is added slowly to wax at 120–150 °C under medium shear until a clear solution is obtained, typically 30–60 min. The resulting wax blend has smaller crystal domains and higher viscosity. At 5 wt% loading, congealing point measured by ASTM D938 typically increases by 2–5 °C, and kinematic viscosity at 100 °C by ASTM D445 may rise from 4–6 mm²/s to 12–18 mm²/s depending on the wax distillation range. Published data for this specific configuration is limited to wax-dependent ranges because feedstock composition and oil content control the final rheology more than the polymer grade alone.

Compared with higher-VA grades such as ELVAX 210, ELVAX 9756 gives better solubility in nonpolar paraffin and less phase separation on cooling, which is important for candle and coating systems that require low surface tack. Compared with lower-VA grades such as ELVAX 770 with nominal 9% vinyl acetate and 0.8 g/10 min, ELVAX 9756 disperses more easily and gives lower final blend viscosity at equivalent addition levels, while providing less hardness reduction.

Adhesion Build Rate in Slot-Die-Coated Hot-Melt Formulations

In hot-melt adhesive compounding with hydrocarbon tackifiers and paraffin wax, ELVAX 9756 is often formulated at 30–35 wt% polymer, 40–50 wt% tackifier, 15–25 wt% wax, and 0.5 wt% hindered phenolic antioxidant. The adhesive is applied with a slot-die coater at 175–185 °C and a coat weight of 25–50 g/m². On corona-treated polyethylene terephthalate, T-peel adhesion after 24 h by ASTM D1876 is governed by cohesive failure rather than interfacial debonding when the formulation is within this envelope. The open time on a 23 °C steel panel is typically 5–15 s; adding more high-softening-point tackifier extends open time, while the EVA grade controls the green-strength plateau.

Shear adhesion failure temperature can be evaluated by ASTM D4498 with a 1.0 kg load and 25 °C/min ramp; values in the range of 60–80 °C are common for wax-containing formulations but are highly dependent on tackifier softening point. Compared with ELVAX 40W, ELVAX 9756 gives lower wetting on aluminium and corona-treated polyester; an additional 2–5 wt% polar tackifier or a lower wax content may be required to match low-temperature adhesion. The trade-off is a reduction in cold-flow tendency and an increase in die-cutting cleanliness for label and tape applications.

For injection molding of ELVAX 9756, melt temperature is held at 180–200 °C and mold temperature at 10–30 °C because the grade has low crystallinity and cools slowly relative to polypropylene. A clamp force of 3–5 kN/cm² of projected area is sufficient for conventional parts, but full shot weight and hot-runner pressure loss must be evaluated because the high thermal expansion of EVA can cause flash if fill speed is excessive at the end of the stroke. Holding pressure of 40–60 MPa and gate freeze time of 2–4 s reduce sink marks and warpage in sections thicker than 3 mm.

Mold shrinkage measured by ASTM D955 on a 3.2 mm plaque is typically 1.5–2.5%, which is higher than LDPE homopolymer and must be compensated in tool design. The shear viscosity of ELVAX 9756 at 190 °C and 100 s⁻¹ is typically 800–1200 Pa·s; at 1000 s⁻¹ it falls below 300 Pa·s. These values explain why thin-wall filling requires melt temperature at the upper end of the range and why hot-runner temperature should not exceed 210 °C. The grade is less tacky than high-VA EVA and therefore easier to feed by screw conveyor from a dryer hopper; this is a practical difference from ELVAX 40W on humid production days.

When Pellet Pre-Drying Is Omitted in Humid Converting Environments

Moisture-related defects appear first as splay or surface roughness on extruded melt, not as catastrophic hydrolysis. The bulk EVA pellet absorbs little moisture, but condensation on pellets stored in outdoor silos or fed from open hoppers above 60% RH is sufficient to produce gas inclusion in the melt. Pre-drying at 60 °C for 4 h in a desiccant bed dryer with a dew point of −40 °C is the standard corrective action. Karl Fischer titration of a sample drawn after drying should read below 0.05% moisture.

When pre-drying is omitted, melt-pressure oscillation at the extruder head typically exceeds 0.5 MPa at 300 min⁻¹ and becomes visible as periodic surge at the die. Amine-based additive packages are not recommended in formulations containing vinyl acetate because residual acetic acid from thermal exposure can react to form salts that plate out on calender rolls or chill-roll surfaces; this is not a crosslinking mechanism but a surface-defect source. The same is true for zinc stearate in excess of 0.2 phr, which can generate clogging in screen packs when recycled EVA is processed at 200 °C for extended residence time.

In blown film extrusion at 40–60 µm gauge, the bubble is more sensitive to draft changes than LDPE because the EVA melt has lower melt strength. A blow-up ratio of 2.0–2.5:1 and a frost line height of 3–6 die diameters are typical starting points at a melt temperature of 170–190 °C. Dart impact measured by ASTM D1709-16A depends on gauge and film age; practical values are 200–400 g for 50 µm film, while Elmendorf tear by ASTM D1922 is generally higher in machine direction than in transverse direction. Film testing is performed after 40 h conditioning at 23 ± 2 °C and 50 ± 5% RH in accordance with ASTM D618.

The difference from high-VA film grades is lower blocking and lower coefficient of friction after winding, which reduces the need for external slip additives but lowers puncture resistance at −20 °C. Low-temperature flexibility is characterized by dynamic mechanical analysis; EVA with moderate VA content exhibits a loss tangent peak near −20 °C, whereas LDPE homopolymer retains higher modulus at that temperature but has lower clarity. This shift is responsible for lower low-temperature impact toughness of EVA films compared with metallocene LLDPE.

In extrusion coating onto paper and aluminium foil, ELVAX 9756 is processed at 190–220 °C with coat weights of 10–25 g/m². Adhesion to aluminium foil is improved by corona or ozone treatment; peel strength measured by ASTM D1876 is lower than that of acid copolymer or anhydride-grafted tie resins. The grade is not recommended for aggressive high-temperature retort structures because EVA softens above 70 °C. Compared with ethylene methacrylic acid copolymers, ELVAX 9756 offers lower metal adhesion but better moisture vapor barrier retention and lower corrosive by-product generation.

Regulatory compliance for ELVAX 9756 follows the EVA copolymer framework. Under EU 1907/2006, ethylene and vinyl acetate monomers are registered as monomers; the polymer itself is exempted from registration but must be supported by a safety data sheet reflecting the supplied pellet form. Under FDA 21 CFR 177.1350, EVA copolymers may qualify for food-contact use when the vinyl acetate portion and total extractives meet the migration limits specified in the regulation; confirmation is required for the specific antioxidant and slip additive package. Under RoHS 2011/65/EU, typical ELVAX 9756 does not contain the six restricted substances above threshold values, but verification against the actual certificate of analysis is required for electrical and electronic applications.