What Limits the Seal Initiation Window of Coextruded BOPP Skin Layers on High-Speed Wrappers?

On tenter-frame biaxial orientation lines producing 15–25 μm transparent overwrap for confectionery and snack packaging, YUNGSOX PP Terpolymer 5050M is fed to the skin-layer extruder of a three-layer coextrusion die. The core layer is typically a PP homopolymer with a melt flow rate measured at 230 °C/2.16 kg according to ISO 1133-1:2022. The terpolymer skin is metered to a final oriented thickness of 1.5–4.5 μm; below 1.5 μm, seal strength after surface treatment can drop below 2.0 N/15 mm when tested at 140 °C, 0.3 MPa, and 0.5 s dwell per ASTM F88/F88M. Above 4.5 μm, the low-melting terpolymer layer tends to transfer to transverse-direction clips during the 155–165 °C tenter preheat zones, producing edge serration and periodic web breaks. The practical thickness window is therefore narrow and line-specific.

Barrel temperature profile for the skin extruder is set between 230 °C and 250 °C on a 65 mm single-screw with 30:1 L/D. The cast sheet is quenched at 20–25 °C, then oriented 4.5–5.5:1 in machine direction and 8–10:1 in transverse direction. Because the terpolymer skin has a lower melting peak than the PP homopolymer core, the preheat rolls are held at the lower end of the 120–135 °C range. If the rolls exceed 138 °C, the skin begins to stick and can cause film chatter at the subsequent stretching nip. The result is a hazy band with tensile imbalance that is measurable under ASTM D882 as a machine-direction yield stress differential greater than 5 MPa between centre and edge web positions.

For direct food contact in the European Union, the finished overwrap must comply with Regulation (EU) No 10/2011 Annex I and III, with overall migration below 10 mg/dm² in food simulant A, B, or D1 according to the intended food type. For the United States, the olefin polymer falls under 21 CFR 177.1520(c), provided that extractive limits for the specific use condition are met. Converters typically specify that the terpolymer layer contributes no heavy metals above 100 mg/kg total for Pb, Cd, Hg, and Cr(VI) under EU Directive 94/62/EC Article 11. These compliance boundaries do not constrain heat-seal performance directly, but they restrict the choice of slip and antiblock additives; only food-contact-grade silica, erucamide, and synthetic waxes with appropriate migration limits are used.

The finished BOPP overwrap is used for twist-wrapped candies, snack flow-pack, and biscuit sleeves where high-speed horizontal form-fill-seal machines run at 300–600 packages/min. At these speeds, the seal jaw contact time is often shorter than 25 ms, so the terpolymer must reach seal strength quickly. Hot-tack testing according to ASTM F1921 at a 0.5 N/25 mm threshold commonly requires 115–130 °C for a 2.5 μm terpolymer skin. If the converter replaces the skin with a random copolymer, the hot-tack threshold shifts upward by 10–18 °C, which is not acceptable on high-speed wrappers without longer dwell or heated fin wheels.

Across coextruded cast polypropylene lines producing cold-seal base webs for bakery flow-wrap and chocolate bar packaging, the terpolymer grade is extruded as the sealant skin on a 20–30 μm cast film. The core is typically a PP homopolymer or random copolymer with a higher melt temperature, while the terpolymer skin is held between 3.0 μm and 8.0 μm. Because cold-seal adhesives must wet the film surface consistently, the web is corona-treated inline to a minimum surface energy of 38 mN/m and a maximum of 42 mN/m according to ASTM D2578. Values below 38 mN/m cause adhesive dewetting at pattern-coating stations; values above 44 mN/m increase blocking risk during roll storage. The terpolymer’s lower crystalline fraction relative to homopolymer shifts the heat-seal initiation downward by 10–18 °C, allowing fin-seal jaw setpoints of 110–130 °C on machines running 30–45 m/min.

Formulation for the cold-seal base web typically includes 1,000–2,500 ppm synthetic silica antiblock and 300–600 ppm erucamide slip. The silica prevents film-to-film blocking after rewinding, while the slip migrates to the surface over 24–72 h at 23 °C. If the film is converted into pouches before slip migration is complete, the coefficient of friction measured against itself under ASTM D1894 can exceed 0.55, which may cause misfeeds in high-speed flow-wrap infeed sections. If slip masterbatch is increased above 1,200 ppm erucamide, cold-seal adhesive anchoring can decline because excess amide creates a weak boundary layer; adhesion peel values below 2.0 N/15 mm are then observed when bond strength is tested after 24 h room-temperature ageing. The compounding ratio is therefore balanced between slip and adhesive compatibility, not just film-to-film friction.

The cast line uses a 75 mm or 90 mm extruder with 30:1 L/D for the core and a 45–65 mm satellite extruder for the terpolymer skin. Melt temperature at the die is held at 230–250 °C. The sheet is pinned to a polished chill roll at 18–22 °C; increasing the chill roll above 28 °C slows skin solidification and promotes blocking at the winder. Gloss measured at 60° per ASTM D2457 is commonly above 85 GU for a 25 μm film, and haze measured per ASTM D1003 is below 2.0% when antiblock particle size is maintained below 8 μm. The finished film is printed or pattern-coated with cold-seal adhesive and supplied as the outer web for chocolate tablets, snack cakes, and biscuits; the terpolymer sealant layer forms the interior of the package after centre-seam folding.

Extrusion Coating of Paperboard with Low-Coating-Weight Terpolymer Layers

For paper-based food service wraps, ream wraps, and dry-food carton liners, the terpolymer is applied at coating weights between 10 μm and 25 μm onto 40–80 g/m² clay-coated or uncoated paper. The extrusion coating line uses a 90 mm single-screw extruder with 30:1 L/D and a T-slot die. Melt temperature at the die exit is normally 260–290 °C; lower temperatures produce insufficient adhesion to paper because the melt viscosity remains too high to penetrate fibre interstices. A higher temperature than 295 °C accelerates thermo-oxidative degradation of the terpolymer, producing carbonyl groups that can shift the odour profile of the finished liner. The hot web drops through an air gap of 150–200 mm onto a nip-cooled chill roll at 15–20 °C with 250–400 N/cm nip pressure.

Adhesion to clay-coated board is not achieved by melt oxidation alone, as is typical for LDPE; the terpolymer surface is polar enough only after inline corona treatment or after the substrate receives a water-based polyolefin primer at 0.3–0.8 g/m². Without primer or corona, peel adhesion measured by ASTM D1876 may remain below 1.0 N/15 mm. With a properly primed clay-coated board, adhesion commonly reaches 1.5–3.0 N/15 mm and failure shifts from adhesive peeling to fibre tear. The finished structure is tested for heat-seal strength per ASTM F88/F88M at 130–150 °C, 0.3 MPa, and 0.5 s dwell; the target seal strength is above 2.5 N/15 mm for a 15 μm coating.

The end use includes burger wrap, sandwich wrap, and bakery tissue where the coated side must comply with FDA 21 CFR 177.1520 and Regulation (EU) No 10/2011. Because the coated paper may be used in microwave reheating, the formulation avoids migratory plasticisers and relies on high-molecular-weight slips with an overall migration limit below 10 mg/dm² under EN 1186 test protocols. The converter also verifies that the coating contains no polychlorinated biphenyls and that heavy metal content meets 94/62/EC Article 11 at ≤ 100 mg/kg. The terpolymer’s density of 0.89–0.90 g/cm³ per ISO 1183-1:2019 provides approximately 10–12% more coated area per tonne than LDPE at equal thickness, but downgauging below 8 μm is limited by pinhole formation at fibre intersections.

Where polypropylene cups require a peelable lidding seal without tearing the container flange, the terpolymer grade is formulated as the sealant layer in die-cut lidstock. The structure commonly consists of a 12 μm metallised PET barrier, a laminating adhesive, and a 25–40 μm cast PP terpolymer sealant web. The sealant layer is blended with 20–35 wt% of a polybutene or LDPE concentrate to produce peel force in the range of 2.0–6.0 N/15 mm when sealed against PP cup flanges at 145–160 °C, 0.3–0.5 MPa, and 0.8–1.5 s dwell. Seal strength below 2.0 N/15 mm risks leakage in distribution; seal strength above 6.0 N/15 mm shifts failure mode from peelable opening to flange tear or film elongation, which consumers reject.

Processing of the lidstock sealant web takes place on a cast film line with a 45–65 mm satellite extruder for the terpolymer blend and a 75–90 mm core extruder. The die gap is set at 0.5–0.7 mm, and the melt temperature is held at 230–250 °C. The film is cast onto a polished chill roll at 18–22 °C, then slit and aged for 24 h at 23 °C before lamination to allow slip migration to stabilise the coefficient of friction. The lidstock is corona-treated to 38–42 mN/m per ASTM D2578 on the outer print side, while the sealant side is kept below 36 mN/m to prevent blocking. If the sealant side is mistakenly treated above 38 mN/m, roll blocking can occur at stacked warehouse temperatures above 30 °C.

Compliance for dairy and snack packaging includes Regulation (EU) No 10/2011 overall migration below 10 mg/dm² and FDA 21 CFR 177.1520(c) for olefin polymers. The lidstock must also meet EU Directive 94/62/EC heavy metal limits of ≤ 100 mg/kg. For microwave reheating of dairy cups, the sealant layer must resist stress cracking when in contact with butterfat or yoghurt acids; the blend ratio of polybutene is limited to 35 wt% because higher levels can reduce the continuous-use temperature of the seal. The finished lidstock is used for PP cups containing dairy desserts, snack dips, and fruit preparations.

When Terpolymer Sealant Webs Must Match Hot-Tack Requirements of Vertical Form-Fill-Seal Lines

In vertical form-fill-seal packaging for frozen vegetables, dried pulses, and confectionery, the sealant web is typically a 25–50 μm cast or blown terpolymer film laminated to a printed BOPP or PET face. The VFFS sealing jaws close for 20–40 ms at 120–135 °C and 0.3–0.5 MPa pressure. The low melt peak of the terpolymer allows seal strength to develop within this short dwell, while the hot tack measured at 70 °C according to ASTM F1921 must remain above 0.5 N/25 mm at the moment the product weight falls into the still-warm seal. If the sealant is replaced by a standard PP random copolymer, the jaw temperature must be raised by 8–15 °C, which can wrinkle the outer printed layer and increase the incidence of seal-through-contamination failure.

When the product generates dust, powder, or oil droplets, the seal area may be contaminated, and the terpolymer’s caulking flow is critical. A formulation of 100% terpolymer provides deeper penetration around contaminant particles but can produce seal thinning at the side edges when the jaw gap is misaligned by more than 50 μm. Converters frequently blend 10–25 wt% of a higher-melting PP random copolymer to raise hot tack at 80 °C without increasing seal initiation beyond 125 °C. The processing boundary is narrow: adding more than 25 wt% random copolymer shifts the seal temperature window upward by 6–10 °C and causes the VFFS line to run on the edge of its thermal capacity.

The cast film line for the sealant web is operated with a 45–65 mm terpolymer extruder at 220–240 °C melt temperature and 18–24 °C chill roll. The film must be conditioned for 48 h at 23 °C before surface energy is raised to 38–42 mN/m for lamination. Low-molecular-weight fractions in the terpolymer can bloom to the surface during this conditioning, so the first 100 m of each roll is sampled for coefficient of friction according to ASTM D1894 and for haze according to ASTM D1003. If static COF exceeds 0.45 after 48 h, the roll is not used for high-speed VFFS because film tracking through forming collars becomes unstable.

Food-contact compliance is verified with Regulation (EU) No 10/2011 and FDA 21 CFR 177.1520, with specific attention to total migration under frozen food simulant conditions. The finished pouches are used for frozen vegetables, frozen fries, dried beans, and sugar confectionery. For frozen storage, the laminated film must also pass a drop test at -20 °C after 72 h conditioning to ensure that seal peel strength measured per ASTM F88/F88M does not fall below 2.0 N/15 mm. This requirement is not solely resin-dependent; it depends on jaw temperature, dwell, and product contamination, which is why line trials on the actual VFFS format are mandatory before grade substitution.