A heavy-duty sack line starts with a long-stalk bubble and a 2.8:1–3.2:1 blow-up ratio

On a 45 mm 30:1 L/D grooved-feed blown-film extruder fitted with a 200–250 mm spiral-mandrel die, Tricolene LLDPE LLB1918SB is processed as a 50–80 µm monolayer for heavy-duty industrial sacks. The resin is specified as a butene linear low-density polyethylene with a nominal density of 0.918 g/cm³ (ASTM D1505-18) and a melt-flow rate of 1.9 g/10 min (ASTM D1238-20, equivalent to ISO 1133-1:2022 at 190°C/2.16 kg). Barrel temperatures are set from 180°C to 210°C, with die temperature at 210–215°C and melt temperature maintained between 208°C and 220°C. The die gap is 2.0–2.3 mm, the blow-up ratio is 2.8:1–3.2:1, and the frost-line height is 700–900 mm above the die. This long-stalk condition is necessary because butene LLDPE has relatively low melt strength and is prone to draw resonance and bubble chatter when the frost line is less than 500 mm. Gauge control is maintained at ±5% by a capacitance thickness scanner; excursions above ±7% create thick-thin bands that weaken the sack at the gusset fold. The corona treater is set to 38–42 mN/m wetting tension, measured by ASTM D2578-17, before two-colour flexographic printing. The converted sack is a side-weld or bottom-seal bag for 25–50 kg of fertiliser, petrochemical resin, or mineral powder. Seal settings on the bottom-seal bar are 135–150°C, 0.3–0.4 MPa, and 0.5–0.8 s dwell; seal strength is inspected by ASTM F88-21, with a minimum of 2.5 N/15 mm for 50 µm film. The melt temperature must not exceed 230°C, because gel particles from additive decomposition appear and slip additive can plate out on the die lip. When start-up regrind is added at 10 wt%, dart impact by ASTM D1709-18 Method A typically remains above 90 g for 50 µm film, but the exact value must be confirmed on the production line because film property variation depends on screw speed, gear-pump pressure, and frost-line position.

On the converting floor, the heavy-duty sack film is slit to 600–800 mm widths and converted on a bottom-seal or side-weld bag machine. The corona-treated surface accepts flexographic ink with a dry adhesion of 80–90% tape-off, determined by the converter’s internal tape test. The main upstream failure mode is bubble instability caused by high-extractable slip additive collecting on the die lip; the die lip is cleaned every 8–12 h to prevent deposit streaks. For bags intended for UN-certified dangerous goods, the sack body must pass drop tests and stack-pressure tests specified in UN Model Regulations Chapter 6.1.5; this requires seal strength above 3.0 N/15 mm and a reliable gusset fold without scoring. When the line runs 80 µm black sacks for light-sensitive resins, 2.0–2.5 wt% of carbon-black masterbatch is added, which raises melt pressure by 5–10% and requires a reduction in screw speed to avoid over-torque.

Why a 1.9 g/10 min butene copolymer is used as a sealant web in duplex lamination

In a duplex or triplex lamination structure, the LLB1918SB layer is produced as a 20–30 µm blown film and is bonded to a stiff outer substrate such as BOPET or BOPP using a two-component solvent-based polyurethane adhesive. The adhesive coat weight is kept between 2.0 g/m² and 3.0 g/m², and the substrate corona level is held at 42–46 mN/m before coating to prevent adhesive de-wetting. The blown film for lamination is run at a melt temperature of 190–210°C, a blow-up ratio of 2.0:1–2.5:1, and a die gap of 1.8–2.0 mm to reduce gauge scatter; the lower BUR shifts orientation toward the machine direction and improves web handling in the laminator. The advantage of the 0.918 g/cm³ density is a low seal-initiation temperature of 100–110°C, which allows form-fill-seal lines to run at jaw temperatures of 125–135°C without burning through the outer printed substrate. Seal strength plateau is typically 2.5–3.5 N/15 mm when sealed at 130°C, 0.35 MPa, and 0.5 s dwell, measured by ASTM F88-21. Hot tack measured by ASTM F1921-21 remains above 1.0 N/15 mm over a 110–150°C jaw-temperature window, which is necessary for vertical packing of granular products where the product load strikes the seal immediately after sealing. Because the slip additive migrates to the film surface over time, coefficient of friction stabilises at 0.20–0.25 after 48–72 h at 23±2°C (ASTM D1894-19). Laminated pouches made with this sealant web are used for detergent sachets, frozen-food bags, and dry powder packaging. Food-contact compliance is governed by FDA 21 CFR 177.1520(c) and EU 10/2011; the overall migration limit under EU 10/2011 is 10 mg/dm² for the plastic layer and must be verified on the finished laminate with the correct food simulant. The converter must also ensure that the polyurethane adhesive is fully cured, because residual isocyanate can increase extractable levels.

The primary failure mode in lamination is not film strength but delamination caused by low wetting tension after storage. The converter measures surface energy at the unwind stand and, when the value falls below 36 mN/m, the web is re-treated in-line at 0.8–1.2 kW corona discharge. If the web is not re-treated, adhesive coating defects appear as fish-eye de-wetting and the bond strength measured by ASTM D1876-08 can fall below 0.8 N/15 mm. A secondary failure mode is curl after lamination because the LLDPE layer and the PET or BOPP outer layer have different shrinkage forces; the lamination machine is set with tension-controlled unwinds and the film is conditioned for 24 h at 23±2°C before slitting. The terminal pouch must survive a 1.2 m drop test filled with 250 g of granular product; this is generally achieved when the sealant layer is 25 µm or greater and the seal strength exceeds 3.0 N/15 mm.

Refuse sack downgauging and the puncture-tear threshold at 25 µm

Refuse sack conversion shifts the dominant failure mode from tensile yield to tear propagation and puncture when the web is reduced from 40 µm to 25 µm. LLB1918SB is processed on a 50 mm extruder with a 250 mm die at a melt temperature of 205–215°C and a blow-up ratio of 3.0:1–3.5:1 to shift more orientation into the transverse direction. At 25 µm, converter specifications commonly require Elmendorf tear values not lower than 50 g in the machine direction and 80 g in the transverse direction, tested by ASTM D1922-19. Puncture resistance is checked by ASTM D5748-19, with values above 2.5 kgf for 25 µm film being typical for this butene LLDPE class. The addition of 10–15 wt% post-industrial reclaim lowers puncture resistance by 10–20% and can create micro-gels that appear as fisheyes; reclaim use above 20 wt% is therefore avoided on thin-gauge bags. Carrier bag conversion from the same film uses a 135–150°C, 0.3 MPa, 0.6 s bottom-seal condition; insufficient seal temperature below 125°C produces weak seals because the seal bar fails to melt the 0.918 g/cm³ layer through the full gauge. The terminal products are 50 L household refuse sacks and high-density carrier bags with a maximum load of 6 kg. The base resin contains slip and anti-block so roll blocking is controlled; however, sacks with high print coverage on both surfaces show a higher blocking tendency because the ink layer acts as a barrier to slip-agent migration. In that condition, the converter reduces the rewind roll diameter from 600 mm to 400 mm or adds 0.5–1.0 wt% of silica-based anti-block masterbatch. Under EU 94/62/EC, the sum of lead, cadmium, mercury and chromium VI in packaging components is limited to 100 ppm; refuse sack producers using post-industrial reclaim must maintain heavy-metal conformity on each batch.

Carrier bags produced from the same film are often side-gusseted and die-cut, which creates stress concentrations at the gusset fold. The converter must ensure film tear resistance is not degraded by over-stretching during gusseting; a gusset depth of 100–150 mm is common for a 400–500 mm layflat bag. When LLB1918SB is blended with 5–10 wt% LDPE to improve bubble stability on older extruders, dart impact decreases slightly but the film becomes stiffer and easier to convert. This blend is not recommended for food-contact refuse sacks if the LDPE grade has no food-contact compliance. The terminal article is a 5–10 kg carrier bag or a 50–80 L refuse sack with a draw tape or fold-over top.

For collation shrink bundling of bottle packs, a 40–50 µm film is blown on a 45 mm extruder at 180–200°C melt temperature, using a 3.5:1–4.0:1 blow-up ratio and frost-line height 900–1,100 mm. The higher BUR increases transverse shrink tension, but the resin must be handled with a long-stalk bubble because a low frost line creates unstable necking. Shrink tension measured by ASTM D2838-18 is typically 0.30–0.60 MPa in the transverse direction for butene LLDPE of this density, which is lower than LDPE-rich formulations; the packaging design therefore compensates by using a tighter shrink tunnel at 160–180°C air temperature. In some collation shrink formulations, 10–15 wt% LDPE is added to increase transverse shrink tension and reduce bubble flutter, but this blend reduces dart impact by approximately 10–15%. Corona treatment is set to 38 mN/m for flexographic printing, and the slip agent must be allowed to migrate for 48 h at 23–25°C before slitting to prevent roll blocking. The terminal product is a shrink bundle wrap for 6–12 PET bottles; the film’s low density provides flexibility at low temperatures, but the resin system is not recommended for high-shrink applications where more than 40% machine-direction shrink is required because the molecular orientation of butene LLDPE cannot recover sufficiently. Heavy-metal content in the packaging is kept below 100 ppm as required by EU 94/62/EC. Published data for this exact collation-shrink configuration is limited, so line trials with the specific shrink tunnel and package-size specification are required before adoption.

Stretch hood line trials for LLB1918SB are normally run with a 10–20 wt% metallocene LLDPE blend to raise dart toughness and reduce pinholing on pallet corners. A 70 mm extruder with a 300 mm spiral die and 2.2 mm die gap is set at 210–220°C melt temperature and 3.8:1–4.2:1 blow-up ratio. The film, at 60–80 µm, is collapsed through a gusseting frame and slit to 1,200–1,600 mm layflat. Elastic recovery after 100% elongation is checked by ASTM D5459-22; for a butene LLDPE/metallocene blend, the permanent set is usually 30–40%, leaving a recovery of 60–70%. That recovery is sufficient for stretch hood application where the film is stretched over a pallet and then relaxes to hold the load, but it is lower than an all-metallocene stretch film. The terminal product is a stretch hood bag for building material, cement, and cold-store pallets; the film must withstand pallet corner puncture and maintain tension during transport. The processing boundary is set by melt pressure: above 350 bar the high-molecular-weight fraction begins to shear-heat, creating gels; screw speed is reduced or die temperature increased to stay below that value. Pellet moisture above 0.05% can occur after open storage at RH>65%; a dehumidified hopper at 50°C for 2 h is used before extrusion to avoid steam splay and surface pitting. For industrial pallet hoods, no direct food-contact requirement normally applies; REACH 1907/2006 Article 33 communication applies if candidate-list substances are present above 0.1 wt%.