Heavy-Duty Industrial Sack Film: Gauge Reduction and Seal Integrity

Braskem LF1020/21AF is a blown film extrusion linear low density polyethylene resin with a nominal solid-state density of 0.920 g/cm³ measured under ISO 1183-1 and a nominal melt flow rate of 1.0 g/10 min measured under ISO 1133-1 at 190°C with 2.16 kg load. The formulated additive package in the 21AF designation is intended to modify film separation and surface slip behaviour, but the exact additive composition is producer-specified and must be verified before food-contact use. In monolayer heavy-duty sack film, this resin is typically processed on a grooved-feed extruder with an L/D ratio of 25:1 to 30:1, a die gap of 1.2 mm to 1.8 mm, a die diameter of 200 mm to 350 mm, and a blow-up ratio of 2.2:1 to 2.8:1. Melt temperature at the die is maintained between 190°C and 210°C. Head pressure above 350 bar or melt temperature above 220°C increases the probability of oxidised gel formation, which appears as weak spots that fail under vertical drop impact or sealing stress. Because LF1020/21AF does not possess the melt strain hardening of LDPE, the bubble is often run in a high-stalk configuration to stabilise the molten membrane before frost line quenching. On production lines without internal bubble cooling, frost line height is usually held between 6 and 10 die diameters above the die face.

The principal process conflict in heavy-duty sack film is the simultaneous demand for gauge reduction and impact resistance. At final thicknesses of 100 µm to 150 µm, the film is frequently specified to meet a dart impact value under ASTM D1709-16a Method A, but converters cannot increase gauge indefinitely because sack weight and resin consumption are critical cost variables. The blown film line controls energy absorption by orienting the polymer below the frost line. A higher blow-up ratio shifts molecular orientation toward the transverse direction and helps offset the machine-direction orientation created by the collapsing frame. If dart impact failure occurs at the bottom gusset, the initiating tear is often associated with low transverse-direction tear strength under ASTM D1922. Raising the blow-up ratio from 2.2:1 to 2.8:1 can improve transverse-direction Elmendorf tear and reduce machine-direction split propagation, but bubble stability becomes more sensitive to plant air turbulence. An unstable bubble generates gauge variation that thins the sack side wall and creates premature failure under stacking load. The frost line position therefore becomes a critical variable. A frost line held too low freezes the melt before adequate stress relaxation, producing film with high machine-direction shrinkage and reduced seal integrity. A frost line held too high increases crystallinity development, increases haze, and may raise blocking tendency in the finished sack.

When extrusion pressure is excessive for a given grooved-feed throat, converters commonly blend LF1020/21AF with a low-melt-index LDPE at 10 wt% to 25 wt%. The LDPE addition reduces melt pressure and improves bubble stability under high draw, but it changes the failure envelope. LDPE long-chain branching reduces extension thickening under certain bubble conditions and lowers dart impact. Above 25 wt% LDPE, the film begins to lose the characteristic LLDPE tear resistance that protects the sack against sharp aggregate or granular product edges. The blend must be tested under ISO 527-3 for tensile strength and elongation, ASTM D1709-16a Method A for dart impact, and ASTM D1922 for tear propagation in both machine and transverse directions. If the sack is used for dusty powders or mineral fillers, the seal area may become contaminated during filling. Heat seal strength measured under ASTM F88/F88M should be confirmed at a seal jaw temperature of 110°C to 130°C, because the sealant melt must displace particles without creating a path for sifting. LF1020/21AF exhibits a lower seal initiation temperature than higher-density LLDPE grades, which supports high jaw speed on form-fill-seal sack lines. Hot-tack strength under ASTM F1921 is the controlling parameter during the fill cycle because the molten seal must maintain load before crystallisation is complete.

Table 1 summarises representative blown film start-up conditions for LF1020/21AF in monolayer heavy-duty sack production.

Freezer-grade blown film for frozen vegetables, meats, and seafood is frequently produced at 35 µm to 80 µm, where room-temperature toughness values do not predict performance at -20°C or below. The low-temperature energy absorption of LF1020/21AF must be assessed using a conditioned dart impact procedure based on ASTM D1709-16a Method A, with the specimen, clamp, and dart held in a cold chamber for a minimum of 4 h before testing. The brittle point of the converted film, not the resin melt flow value, is the practical variable. When the film is exposed to sharp frozen product edges, crack initiation begins at microscopic notches around the seal perimeter. Puncture resistance measured under EN 14477 or ASTM D5748 provides a better approximation of the stress concentration created by frozen bone fragments and ice crystals. The film must survive both the filling operation and distribution vibration without splitting. The seal area is the most frequent failure site because the sealing operation melts and recrystallises the polymer, changing the crystalline morphology from the oriented blown film state to a low-orientation weld. A heat seal made at 115°C with a dwell time of 1 s and pressure of 0.3 N/mm² may develop adequate room-temperature strength, yet the failure mode can switch from ductile peeling to brittle fracture after low-temperature conditioning. The seal should be tested under ASTM F88/F88M after conditioning at the intended freezer temperature, not only at ambient laboratory conditions.

The anti-block and slip function associated with the 21AF additive package is migration-dependent. At freezer temperatures, the diffusion rate of slip additive to the film surface is substantially slowed. If coefficient of friction is tested immediately after conversion, the film may pass an ISO 8295 value below 0.10, but after storage at chilled temperatures the dynamic coefficient can rise. This increase may cause poor bag opening activity on horizontal form-fill-seal machines. The converter must confirm coefficient of friction after at least 48 h of side-by-side film aging at the expected storage temperature, not immediately after slitting. In multilayer structures, LF1020/21AF is commonly used as the core or sealant layer adjacent to a metalised outer layer. The metallic layer reduces oxygen transmission under ASTM D3985 and moisture vapour transmission under ASTM F1249, while the LLDPE layer provides puncture resistance and a broad sealing window. The sealant layer thickness should not be reduced below 15 µm unless a formal seal-through-contamination test has been performed. Frozen food packaging that may be microwave reheated should be cleared under FDA 21 CFR 177.1520(c) and, for the European market, Commission Regulation (EU) No 10/2011. Compliance is not automatic from the resin certificate alone; the converted film must pass overall migration testing under EN 1186-1 with a limit of 10 mg/dm² for the intended food simulant.

Operational boundaries for LF1020/21AF in frozen food packaging include the lower service temperature. Published data for this specific configuration is limited, but butene LLDPE films of this density class generally retain ductile behaviour in non-impacted tensile testing to approximately -30°C. Converter trials should verify package abuse at -18°C rather than relying solely on resin melt flow data. The film should not be used in direct contact with hot fill above 95°C because localised melting and distortion at the seal interface can compromise package integrity. Methyl ethyl ketone and aggressive oxygenated solvents used in ink cleaning can swell the surface and change slip behaviour; press-side cleaning should be controlled by a defined standard operating procedure. Reclaimed post-industrial LLDPE above 15% often introduces gel contamination that lowers dart impact and increases the probability of leaker formation. Any commercial regrind stream must be tested for melt flow shift, gel count using a film surface defect analyser, and seal contamination before use.

Table 2 provides a compliance matrix for applications where LF1020/21AF may be converted into food-contact or agricultural film.

Why Does Greenhouse Film Fail First at the Vent Fold Line?

The service life of a greenhouse covering film is controlled less by average film properties than by the fold line where the film is clamped to the frame or ventilated opening. At this location the film undergoes repeated flapping, abrasive contact, and concentrated UV exposure at an angle of incidence that maximises surface heating. LF1020/21AF as a base resin provides high Elmendorf tear and dart impact values, but it is not a UV-stabilised grade. Converter formulation must introduce a UV stabiliser masterbatch at a let-down ratio typically between 5% and 12%, depending on the active ingredient concentration and target lifetime. Hindered amine light stabilisers and UV absorbers must be dispersed in the LLDPE carrier without causing melt viscosity mismatch. If the stabiliser masterbatch carrier has a melt index below 0.5 g/10 min, dispersion in LF1020/21AF at 1.0 g/10 min remains acceptable if the extruder L/D is at least 25:1 and moderate mixing elements are used. Poor dispersion creates unstabilised zones that degrade first at the fold line.

Sulfur vapour used for pest control is an operational incompatibility that is often underestimated. Sulfur fumigation generates acidic oxidation products that can deactivate hindered amine light stabilisers and accelerate surface chalking. Greenhouse films exposed to weekly sulfur burns should be washed to remove sulfur deposits, but mechanical cleaning is not always possible. The base polymer can survive, but the stabiliser package is consumed more rapidly, and the film loses tensile elongation. Tensile strength retention after weathering should be evaluated under ISO 527-3 after accelerated aging under ISO 4892-2 or ASTM G155, using a schedule that includes condensation cycles. If the film is classified under EN 13206 for greenhouse covering films, the manufacturer must verify the stated light transmission and thermic effect. LF1020/21AF does not contain an IR-blocking additive; if thermal retention is required, an EVA or mineral-filled masterbatch is added, but this may reduce dart impact and complicate the bubble. The film should be tested at 150 µm to 200 µm final thickness for tear, tensile, and flex cracking resistance. Gelbo flex testing under ASTM F392 is more relevant at the vent fold line than a simple dart impact test because the failure mode is cyclic bending, not single-point impact.

The blown film process for greenhouse covering requires a high blow-up ratio close to 2.8:1 to reduce machine-direction orientation and provide balanced tear properties. A die gap of 1.6 mm to 2.0 mm is preferable to maintain bubble stability at high gauge. The bubble must be protected from greenhouse film line drafts; an unstable bubble creates variation in film thickness that becomes an early failure point under wind load. The use of LF1020/21AF in a three-layer greenhouse film is typically as the core layer, where its toughness contributes without placing the additive package on the exposed surface. The skin layers carry the UV stabiliser and, in some structures, anti-drip additives. If the core layer is loaded with regrind, the maximum level should be limited to 15% unless the converter has performed tensile retention tests under ISO 4892-2 that demonstrate equivalent performance. Reclaimed material with degraded catalyst residues can initiate oxidation at the fold line. The lower density of LF1020/21AF gives high impact strength relative to 0.926 g/cm³ LLDPE, but the film modulus is lower. If the greenhouse structure requires high snow load resistance, the structure design must account for the modulus of the film rather than the film alone providing structural capacity.

When Collation Shrink Demands Higher MD Tear Than LDPE Alone Can Provide

Collation shrink film for bottles and cans is traditionally produced from LDPE-rich formulations because high ultimate shrink and controlled shrink force are required. The limitation of LDPE is puncture and tear when the shrink film wraps over metal or glass edges. LF1020/21AF is introduced into these formulations at 20 wt% to 50 wt% to improve dart impact and tear resistance while retaining a processable bubble. Above 50 wt% LLDPE, the ultimate shrinkage measured under ASTM D2732 and shrink force measured under ASTM D2838 decrease, and the film may not achieve the tight bottle-to-bottle registration required on high-speed collation lines. The exact ceiling depends on the LDPE melt index and the bubble cooling configuration. If the LDPE component has a melt index below 0.3 g/10 min, the blend can tolerate more LLDPE without losing bubble stability. If the LDPE component is 0.8 g/10 min or higher, the blend viscosity mismatch becomes visible as die-line streaks and uneven thickness.

The blown film line for collation shrink requires a low frost line and a low blow-up ratio, typically 2.0:1 to 2.5:1, to preserve biaxial orientation in both directions. A high BUR increases transverse shrink and reduces machine-direction shrink, producing a bag that tightens unevenly around the product group. The die gap is usually 1.2 mm to 1.5 mm. The melt temperature is kept at 180°C to 200°C to avoid excessive oxidation in the LDPE phase. The film is then slit and sealed on a bottom-seal or side-seal collation machine. The seal strength must be high enough to survive shrink tension after the film exits the shrink tunnel. If the film shrinks too aggressively before the seal has cooled, the seal peels open. LF1020/21AF contributes a broader sealing window than some metallocene LLDPE grades, which helps on machines where the seal dwell time is 0.3 s to 0.8 s. Hot tack under ASTM F1921 is the governing parameter. A packaging line with tunnel temperatures above 200°C risks overheating the film surface and producing localised melt holes. Shrink tunnel temperature and air velocity must be mapped across the tunnel width, and the film should be evaluated at the maximum tunnel temperature expected during summer operation.

Published data for this specific LF1020/21AF/LDPE collation shrink blend is limited; converter trials are required. However, general blend rules apply. The addition of LF1020/21AF above 30 wt% raises the oxygen transmission and lowers the shrink force, so the film may not be suitable for moisture-sensitive foods. The film should be evaluated under ASTM D2732 at 150°C in both machine and transverse directions. Shrink tension under ASTM D2838 should be measured on the final film because slip additives in the 21AF package may interact with the shrink tunnel and lower the film-to-product coefficient of friction. If the film is used in a corrugated tray overwrap, the bottom seal must be protected from corrugated edges; a puncture test under ASTM D5748 is useful. The operational boundary is the low-temperature behaviour of the shrink film after the product has been stored in unheated warehouses. Shrink film that has been under strain may lose elongation; a cold-conditioned tensile test under ISO 527-3 at 0°C provides additional assurance.

Stretch hooder lines on palletising conveyors require film with high elongation at break, high puncture resistance, and retained load tension after repeated corner flexing. LF1020/21AF is used in blown stretch hood film at 90 µm to 130 µm. The film is stretched over the top corners of the pallet load and released, and the elastic recovery of the LLDPE phase creates the containment force. A film with excessive LDPE content loses recovery and relaxes. The molecular weight distribution of LF1020/21AF permits high draw without immediate failure. Tensile properties are measured under ISO 527-3, with machine-direction elongation at break values typically reported in the region of 600% or higher for well-optimised blown film, although converter conditions affect the observed value. Puncture resistance is measured under ASTM D5748. Cling performance is modified through cling additives or cohesive layers; the base film without cling will slide, so the surface layer must be formulated with a cling masterbatch. The coefficient of friction on the outside surface should be controlled to allow the film to slip over the pallet wrapper without generating static charge. A high-slip surface may reduce the required film tension but can create unstable pallets during transport.

The blown film production of stretch hood film uses a blow-up ratio of 2.0:1 to 2.5:1, a die gap of 1.2 mm to 1.6 mm, and a melt temperature of 190°C to 210°C. The film is allowed to relax slowly after the nip; fast cooling before the frost line suppresses elastic recovery and creates a film with high permanent deformation. The pallet load retention force is measured by wrapping a standardised test pallet and recording film tension with a load cell, but there is no single ASTM method for this system-level property. The converter should correlate stretch hooder settings with film elongation, puncture, and cling values measured under the standards above. A common failure mode is corner puncture during stretching over sharp pallet edges. The film must have sufficient dart impact and slow puncture resistance to survive repeated rapid stretching. If corner puncture occurs, the operator often increases film gauge, but an alternative is to increase the blow-up ratio moderately to improve transverse-direction elongation. Above 2.8:1, bubble instability can increase gauge variation, which generates weak bands along the pallet circumference. The film should be slit to width with minimal edge damage, because nicked edges propagate tears during stretch. Edge quality can be monitored under magnification and checked for web breaks after 30 min of continuous hooder cycling.

Low-temperature stretching is an operational boundary. At warehouse temperatures below 5°C, the film becomes stiffer and corner puncture probability increases. Converter trials should include cold pallets and cold film, because both ductility and cling level shift with temperature. The 21AF additive package reduces blocking on the roll, but prolonged storage above 40°C can accelerate slip additive migration and reduce coefficient of friction. Rolls should be stored out of direct sunlight and away from ozone sources. Ozone from nearby corona treaters or electric motors degrades the film surface and reduces seal and cling performance. If corona treatment is used for printing, a minimum wetting tension of 38 mN/m to 40 mN/m measured by ISO 8296 is typical, but higher treatment increases surface oxidation and can interfere with cling. The film should not be exposed to amine-based compatibilisers or nitrile solvents in cleaning, because such compounds can stress-crack the stretched edge region. This is especially relevant when the stretch hood line is located in a facility that also handles plasticiser-containing PVC films, because plasticiser migration onto the LLDPE surface lowers the coefficient of friction and weakens load retention.

Blown film lines producing perforated produce bags and bread bags operate in the 18 µm to 30 µm gauge range, where the bubble stability of LF1020/21AF at high line speed is more important than extreme mechanical strength. The melt flow rate of 1.0 g/10 min permits sufficient output without excessive melt pressure, while the 0.920 g/cm³ density provides adequate clarity for product visibility after anti-block addition. For very high-speed bag conversion, the film should be evaluated for hot tack under ASTM F1921 and coefficient of friction under ISO 8295. If coefficient of friction is too high, bags will not open on wicket dispensers. If coefficient is too low, the film may slip on the bag machine drive belts and create registration faults. A moderate blow-up ratio and relatively high frost line give a clear film, but above 10 die diameters frost line height, the film may exhibit excessive blocking in warm warehouses.