Sealant Layer Architecture in Coextruded Packaging Webs

SABIC COHERE VLDPE 8185 is an ethylene-1-octene random copolymer with a nominal density of 0.885 g/cm³ determined by ISO 1183 and a melt flow rate of 0.50 dg/min at 190 °C/2.16 kg per ISO 1133-1. The low crystalline fraction associated with that density shifts heat-seal initiation toward the range normally required for high-speed vertical form-fill-seal and horizontal form-fill-seal equipment, without moving the sealing window into the low-viscosity, low-melt-strength region that destabilises extrudate. On a three-layer blown-film line equipped with a 250 mm annular die, a die gap of 1.8 mm, and a blow-up ratio of 2.2:1, a sealant layer thickness of 12–20 µm is maintained when total web thickness is 60–100 µm. The sealant layer is typically blended with a linear low-density polyethylene of 0.918 g/cm³ density or with an LDPE of 2.0 g/10 min melt flow rate at 190 °C/2.16 kg to increase web stiffness and reduce blocking during roll conversion.

Starting barrel temperatures for a 60 mm screw with 30:1 L/D are shown in the table below. The die set point should be held at the upper end of the range only when the blend contains more than 40 wt% LDPE; a higher die temperature on a pure or high-content COHERE 8185 sealant layer increases die lip oxidation and can produce gel banks at the lip exit after 8–12 h of continuous run time.

Seal strength is measured according to ASTM F88/F88M after jaw sealing at 0.275 N/mm² pressure and 0.5 s dwell. Hot tack is evaluated under ASTM F1921 at a peel speed of 200 mm/s. A blended sealant layer containing 70 wt% COHERE 8185 and 30 wt% LDPE of 0.918 g/cm³ density is generally run with jaw temperatures between 95 °C and 115 °C on laboratory gradient bar tests; production setting depends on jaw pressure, film thickness, substrate gauge, and web speed. For direct food-contact use, compliance is anchored to FDA 21 CFR 177.1520 for olefin polymers and to EU 10/2011 with overall migration not exceeding 10 mg/dm² when tested by EN 1186-1 and EN 1186-3. The terminal packaged formats are fresh-cut produce pouches, frozen food packs, and peelable lidding films where seal integrity at low jaw temperature is the primary fitness-for-use criterion.

Operational boundary conditions are significant. If the sealant layer is stored or conveyed at relative humidity above 60 % and then fed without drying, surface moisture can produce micro-bubbles at the die exit but has a lower impact than on polar materials. The more serious limitation is blend incompatibility with high-stiffness HDPE regrind or with excessive processing aid packages based on low-molecular-weight polyethylene wax. Addition of more than 15 wt% HDPE regrind into the sealant layer raises seal initiation by 5–10 °C and increases stiffness to a point where peelable lidding behaviour transitions to destructive seal failure.

In heterophasic polypropylene compounds intended for soft-touch interior trim and thermoplastic olefin skins, COHERE 8185 is incorporated at 10–30 wt% through a co-rotating twin-screw extruder with 40:1 L/D and a melt temperature of 200–230 °C. The low density and high comonomer content produce discrete elastomer domains during melt compounding; the target domain size is below 5 µm as verified by reflected-light microscopy of microtomed surfaces. The compounding line is typically operated with the first kneading block placed 8–10 D downstream from the main feed to avoid premature melting that reduces downstream shear efficiency. When the addition is increased from 10 wt% to 30 wt%, notched Izod impact at -30 °C under ASTM D256 rises, while flexural modulus under ISO 178 falls. The practical upper limit on many lines is set by screw torque and by the onset of strand pellet instability. At 30 wt% addition, extruder barrel cooling becomes necessary because viscous heat generation is not fully balanced by the lower melting point of the ethylene-octene phase.

Published data for this specific configuration is limited; the actual impact-modification response depends on the base PP melt flow rate, the ethylene content of the PP impact copolymer, and the presence of ethylene-propylene rubber in the matrix. The main process conflict is that high screw speeds favour finer elastomer dispersion but also lower the effective melt viscosity and can produce pellet surface roughness. Production-scale twin-screw operators therefore typically reduce screw speed to 250–350 min⁻¹ and use a mild screw configuration with two kneading zones rather than three. The resulting compounds are used in injection-moulded door panel substrates, instrument panel retainer components, and soft-touch cowl side trim.

When Hot-Melt Adhesive Formulators Replace EVA with VLDPE 8185

Hot-melt adhesives based on COHERE 8185 are compounded in a heated sigma-blade mixer at 150–170 °C under a nitrogen blanket. The base formulation consists of 30–40 phr ethylene-octene polymer, 40–55 phr hydrogenated hydrocarbon tackifier, 0–20 phr paraffin or Fischer-Tropsch wax, and 0.5–1.0 phr hindered phenolic antioxidant. Melt viscosity is controlled by ASTM D3236 at 180 °C, and ring-and-ball softening point is measured by ASTM E28. The narrow composition distribution of the ethylene-octene backbone provides lower low-temperature flex fatigue than EVA of equivalent melt index, but the low crystallinity also reduces green strength at room temperature. That trade-off is addressed by shifting the tackifier type from C5 to C9 hydrocarbon resins or by adding 5–10 phr of a high-melt-index LDPE.

The mixing torque in a sigma-blade unit remains lower than that of EVA-based formulations because the polymer phase does not contain vinyl acetate groups that increase hydrogen bonding and polar interaction with the mixer walls. The limiting factor is wax compatibility. If more than 20 phr of a high-melting paraffin wax is used, the wax phase separates above 120 °C and produces a mottled adhesive surface. End uses are low-temperature packaging adhesives, hygiene assembly adhesives, and bookbinding where cold flex resistance is tested at -10 °C or lower under mandrel bend conditions. Data for a specific formulation should be generated because the grade itself is not an adhesive; the final adhesion to corrugated board, polyester film, or aluminium foil is dominated by the tackifier and substrate surface treatment.

Masterbatch producers use COHERE 8185 as a carrier resin for carbon black and organic pigments because the low crystalline chain segment distribution accelerates wetting of high-surface-area filler without excessive torque rise on a co-rotating twin-screw extruder. The compounding equipment is a 32–44:1 L/D co-rotating twin-screw unit with a melt temperature of 170–200 °C. Carbon black loadings of 40–50 wt% are produced with the carrier resin making up the balance, while additive masterbatches are produced at 10–30 wt% active ingredient. The resulting pellets are quantified by density per ASTM D1505 and by melt flow rate per ISO 1133-1 at 190 °C/2.16 kg. The carrier selection is based on the fact that the high comonomer content lowers the final melt flow dilution penalty when the masterbatch is let down at 2–5 % into film or moulding compounds.

The operational boundary is low-temperature pellet handling. Carrier-rich carbon black masterbatch produced with COHERE 8185 may agglomerate in silo storage above 35 °C if the carbon black structure is high; external silo cooling or a maximum pellet surface temperature of 30 °C after strand pelletising is required. The dominant terminal use is film-grade black masterbatch for agricultural stretch film, garbage bags, and irrigation pipe, where the carrier resin must not increase the density of the let-down polyolefin by more than 0.002 g/cm³ and must not introduce gels above 300 µm in a 50 µm cast film.

Extrusion Lamination Melt Draw and Adhesion Control

In extrusion lamination and extrusion coating, COHERE 8185 is used as a heat-seal or low-integrity tie component in combination with LDPE. The blend is composed of 30–50 wt% COHERE 8185 and 50–70 wt% LDPE of 4.0–8.0 g/10 min melt flow rate to stabilise the melt curtain and reduce neck-in on a slot-die line. Equipment conditions on a 90 mm extruder with 30:1 L/D, a 1500 mm slot die, an air gap of 150–250 mm, and a line speed of 80–200 m/min are typical. The melt temperature at the die is held between 260 °C and 280 °C when the substrate is aluminium foil or oriented polyester because oxidation of the melt surface during the air gap contributes to adhesion. The same temperature range is not automatically suitable for paper substrates because excessive oxidation can produce off-taste and brown discoloration at the paper-polymer interface.

Adhesion to aluminium foil is routinely checked after lamination by peel testing under ASTM D1876. The extrusion lamination structure is then evaluated for heat seal strength under ASTM F88/F88M. The COHERE 8185-containing layer can seal directly against itself in sachet structures, but seal strength decreases if the coating layer thickness is below 15 µm because the substrate acts as a heat sink. The main processing conflict is melt draw instability. If the COHERE 8185 content exceeds 50 wt% in the blend, the melt curtain begins to oscillate at line speeds above 120 m/min, producing edge weave and uneven coating weight. In those cases, lowering the die gap to 0.6 mm and reducing the air gap to 150 mm partially recovers stability, but the safer corrective action is to keep COHERE 8185 at 30–40 wt% and use a higher neck-in allowance. End products are sachet webs, aluminium barrier laminates for dry food, and strip-package sealants in pharmaceutical packaging where the seal layer must not require a separate cold-seal latex.

For crosslinked foamed sheet and profile production, COHERE 8185 is compounded with azodicarbonamide at 2–5 wt%, zinc oxide at 1–3 wt%, and dicumyl peroxide at 0.5–1.0 wt% on a two-roll mill at 100–110 °C. The low crystalline backbone permits lower shear heating during crosslinking and delays the onset of plate-out on the roll surface. Published data for this specific configuration is limited; the formulation must be validated on the target line because the blowing agent decomposition profile interacts with the exact ethylenic branching distribution and with the level of residual transition metal residues in the grade. The terminal uses are low-density foam gaskets and cushioning layers where compression set under ASTM D395 is the controlling end-use property.