Film Bubble Oscillation in High-Stalk Heavy-Duty Sack Lines

In heavy-duty industrial sack production, SABIC LLDPE 220HT is processed as a monolayer or as the outer skin of a coextruded structure on blown film lines equipped with internal bubble cooling and capacitance thickness gauging. The resin is characterized by a density of 0.922 g/cm³ and a melt flow rate of 2.0 g/10 min under ISO 1183 and ISO 1133-1; these values define the melt viscosity range that requires die temperatures between 205°C and 220°C. A typical 65 mm grooved-feed single-screw extruder with 30:1 L/D and barrier mixing geometry operates with a feed zone at 170–180°C, compression zone at 190–200°C, adapter at 205°C, and die zones at 210–220°C. The die gap is held at 2.0–2.2 mm, and the blow-up ratio is set at 3.0:1 to produce 50–80 µm final film for heavy-duty sacks. A high-stalk bubble configuration is used because 220HT, as a butene LLDPE with limited strain hardening, exhibits bubble hunting and MD gauge bands when the frost line is positioned below 7 die diameters from the die face. Moving the frost line from 5 to 9 die diameters is an effective corrective action for gauge variation on these lines; this change lowers the standard deviation of on-line capacitance gauging and improves dart impact reproducibility under ASTM D1709 Method A. The addition of 30 wt% LDPE increases long-chain branching in the melt, raises extensional viscosity, and stabilizes the bubble during transverse stretching. Tear resistance is measured under ASTM D1922, tensile properties under EN ISO 527-3, and the compound is qualified for shipping-sack use under warehouse handling protocols aligned with ASTM D3951. Slip and antiblock concentrates are limited to 0.5–1.5 wt% because higher silica loadings reduce cold-temperature dart impact and increase pinhole frequency at gusset folds. At shutdown, the 220HT melt should be purged with a lower-density LDPE grade above 200°C to prevent gel accumulation at the die lips; melt residence time above 220°C should not exceed 15 min.

Vertical form-fill-seal equipment running frozen vegetables, seafood, or prepared ice-glazed foods presents a sealing window in which 220HT film functions as the sealant web. The film is blown at 30–50 µm thickness with a 2.5:1 blow-up ratio and a 2.0 mm die gap to balance MD tear and TD elongation during bag-top folding. Food-contact compliance is supported by EU Regulation No 10/2011 and, for U.S. shipments, FDA 21 CFR 177.1520 when the grade is covered by the manufacturer’s food contact statement. Seal strength is tested under ASTM F88 on a 25.4 mm specimen at 300 mm/min; hot tack is measured on a J&B hot tack tester at 0.5 N/mm² seal pressure, 0.5 s dwell, and 200 mm/s peel speed. On high-speed vertical form-fill-seal lines cycling at 8–12 bags/min, seal jaws set below 115°C may produce ultimate seal strength below 4 N/15 mm on 40 µm film, causing burst pouches during drop loading; raising jaw temperature to 125–135°C restores a plateau above 6 N/15 mm. A melt blend of 10–20 wt% LDPE with 220HT widens the seal initiation window without eliminating the low-temperature dart impact required at -20°C; the trade-off is a small reduction in tensile modulus that is acceptable for gusseted pouches. The terminal product is a printed frozen food pouch with side gusset and back fin seal, typically 30–50 µm thick, qualified for impact testing under ISO 7765-1 at -20°C.

Does Silage Film Puncture Resistance Depend More on Film Processing Than on Resin Density?

For silage clamp covers and greenhouse tunnels, the puncture and tear performance of 220HT-based three-layer film is controlled as much by the frost line and blow-up ratio as by the resin density of 0.922 g/cm³. The outer skins are typically 60–80 wt% 220HT with 20–40 wt% LDPE or metallocene LLDPE, while the core carries a white-reflective or oxygen-barrier masterbatch. Ultraviolet stabilization is not provided by the neat 220HT resin; a HALS-based UV masterbatch is added at 2–4 wt%, and outdoor durability is assessed under ISO 4892-2 artificial weathering rather than by melt flow retention alone. Film thickness ranges from 150 µm for silage underlay to 200 µm for greenhouse covering. A 250 mm blown film die with 2.2 mm die gap, 2.8:1 BUR, and frost line fixed at 8–10 die diameters produces the expected isotropic tear balance. High frost line placement increases transverse orientation and improves Elmendorf tear under ASTM D1922 in the TD direction, but raising BUR beyond 3.2:1 can reduce machine-direction tear below the minimum required for silage bag insertion. The terminal product is a three-layer silage cover in which 220HT contributes adequate bubble stability at moderate BUR but does not match LDPE strain hardening. Converter trials on 65–90 mm three-layer lines remain the primary validation route. Published film property data for this exact UV-stabilized configuration is limited, particularly for tear after 12-month outdoor exposure.

A three-layer coextrusion line running 50 µm T-shirt carrier bag film at 150 kg/h uses 220HT in the skin and core layers to increase dart impact and puncture resistance after transverse stretching at the bubble. The layer split is 10/80/10, with skins containing 80 wt% 220HT and 20 wt% LDPE, and the core carrying reclaimed trim at 10–20 wt% limited by gel accumulation in the screen changer and pressure rise across the melt filter. The die gap is 2.0 mm, the blow-up ratio is 3.5:1, and the frost line is kept at 6 die diameters to increase TD tear. Film property validation follows ISO 7765-1 for puncture and ASTM D1922 for tear; the controlling target for carrier bag conversion is a Dart impact of at least 100 g on 50 µm film and a TD tensile elongation above 600% under ISO 527-3. Silica antiblock is introduced at 0.5–1.0 wt% into the skins, and the static coefficient of friction is held below 0.35 under ASTM D1894 to maintain bag opening on wicket dispensers. The terminal product is a high-cycle T-shirt bag that withstands repeated top-load handling and carries printed retail branding.

When 220HT Is Let Down to 65 wt% in Collation Shrink Bundling Film

In collation shrink film for bottle and can bundling, 220HT is dry-blended or melt-compounded with LDPE at a letdown of 65 wt% to improve puncture resistance without raising the low-temperature seal initiation beyond the shrink tunnel’s hot-air profile. The film is produced as a 35–60 µm blown web on a 55 mm extruder with a 2.0 mm die gap and a 3.0:1 BUR, followed by high-stalk bubble cooling to preserve machine-direction shrinkage. Because butene LLDPE exhibits lower ultimate shrinkage than high-pressure LDPE, the converter must limit 220HT content to below 70 wt%; above this level, the residual shrink force under ISO 14616 falls below the force necessary to hold 6-pack bottle arrays during tunnel transit. The shrink tunnel is set to 160–180°C air temperature, and the web residence time is 3–5 s. Seal strength before shrinking is checked under ASTM F88, while tear after shrink is checked under ASTM D1922; the latter is critical because post-shrink cooling increases film brittleness. Equipment audits show that only blends containing long-chain-branched LDPE at 30–35 wt% maintain stable bubble geometry at 3.0:1 BUR. The terminal product is printed collation shrink bundling film for 6-pack or 12-pack beverage and canned food multipacks.

Corona-treated 220HT blown film enters the lamination stage at a wetting tension of 38–42 mN/m under ASTM D2578 and is subsequently laminated to BOPP, PET, or metallized substrates using a solventless polyurethane adhesive. The application is primarily in dry-food and powder packaging, where the 220HT web functions as the inner sealing layer in a two-ply or three-ply laminate. The blown inner web is produced at 25–40 µm thickness with a 2.5:1 BUR and a 2.0 mm die gap; lower BUR is preferred because the laminate requires higher machine-direction stiffness to resist crease cracking on the form-fill-seal line. Peel strength is measured under ASTM D1876 after 24 h adhesive cure and after a 72 h aged condition at 40°C; the minimum acceptable T-peel is typically 2.5 N/15 mm in dry-food lamination. Coefficient of friction after corona treatment is maintained below 0.30 static under ASTM D1894, and the sealant layer is qualified for migration under EU Regulation No 10/2011 and FDA 21 CFR 177.1520 where the laminate is intended for food contact. The terminal product is a stand-up pouch or gusseted pouch for dehydrated soups, pet food, and seasoning mixes, with 220HT selected for seal integrity after repeated flexing rather than for optical properties.