Stretch Wrap in Cold-Chain Logistics: Which LLDPE Film Holds Up

Machine-grade LLDPE stretch wrap for frozen distribution is specified as a cast coextruded structure with a skin layer containing polyisobutylene or ethylene-vinyl acetate tackifier, a core layer of 1-hexene or 1-octene copolymer with density 0.917–0.922 g/cm³, and a release layer containing a low-slip additive. On a five-layer cast line with a 2.4 m die, 18 °C chill roll, and 300–450 m/min line speed, film thickness uniformity of ±0.5 µm is the primary determinant of low-temperature puncture consistency. Failures observed in blast-freezer palletization are rarely pure tensile yield; they are initiated at frozen cornerboards, clipped banding, or ice edges where protrusion puncture dominates.

LLDPE differs from high-pressure LDPE by the controlled insertion of α-olefin comonomers along the ethylene backbone. 1-Butene, 1-hexene, and 1-octene produce different interlamellar tie-chain populations at equivalent density and melt index. The cold-chain issue is not oxidative embrittlement but low-temperature strain-rate sensitivity. At -25 °C and high-speed prestretch, films with insufficient tie-chain density fail by brittle crack propagation rather than ductile deformation. This distinction is evaluated by dart impact under ASTM D1709-16 and machine-direction or transverse-direction tear under ASTM D1922-15, not by melt temperature alone.

Does Comonomer Length Shift Low-Temperature Ductility in a Measurable Way?

At equivalent density and melt index, 1-octene mLLDPE introduces longer short-chain branches than 1-butene LLDPE, producing a more uniform sequence distribution and a greater number of load-bearing tie molecules after cast-film orientation. Laboratory comparisons under ASTM D1709-16 at -18 °C show that 1-octene resins increase dart impact values by roughly 25–40% relative to 1-butene grades at 20 µm gauge, while 1-hexene grades fall between them. Published data for this specific configuration is limited because most resin suppliers report ambient dart impact; however, the ranking direction is consistent across cast-film technical bulletins. Under ASTM D1922-15, transverse-direction tear often controls pallet overwrap failure. Transverse-direction tear strength of 1-octene films at -20 °C is typically retained to a greater extent than 1-butene films because crack-path energy dissipation remains plastic rather than brittle.

The structural reason is that the branched units are excluded from the polyethylene crystalline lamellae. Longer comonomer branches create more effective interlamellar tie molecules than shorter branches at the same branch frequency, and the tie molecules permit plastic deformation below -20 °C instead of brittle fracture. Comonomer distribution measured by Fourier transform infrared analysis under ASTM D5576-00(2021) should be requested when changing resin suppliers, because average density does not reveal whether the comonomer is randomly distributed or segregated in low-molecular-weight fractions.

Melt index alone does not discriminate low-temperature performance. A 2.0 g/10 min 1-octene resin processed at 190 °C/2.16 kg under ISO 1133-1:2022 may have the same ambient stretch force as a 2.0 g/10 min 1-butene resin, yet its low-temperature puncture performance differs because comonomer distribution changes the amorphous-phase mobility. Resin selection should therefore specify comonomer type and density in addition to melt index.

Cling at subzero temperatures is governed by the skin layer, not the core. Transfer of tackifier to the film surface is a diffusion-controlled process that slows as temperature decreases. Films passing ambient peel cling under ASTM D5458-95 may drop below industrial release-force targets at -20 °C. Cold-chain wraps therefore use higher-tack polyisobutylene grades or thicker cling skins, but excess low-temperature tack can create unwind difficulties on rotary tower wrapping arms operating at -5 °C to -10 °C in refrigerated docks. A prestretch unit set at 200–250% elongation at ambient conditions may produce higher neck-in and lower load-holding force at -20 °C if the core layer has a broader comonomer distribution.

Polyisobutylene cling resins used in cold-chain films have number-average molecular weight between 1,000 and 3,000 g/mol. Low molecular weight grades bloom to the surface rapidly at ambient warehouse temperatures, but they can be reabsorbed into the polyethylene surface when the pallet is moved into a freezer. High molecular weight tackifiers migrate more slowly but retain surface concentration better after freezing. Because peel cling under ASTM D5458-95 is typically measured at 23 °C, it does not predict the -20 °C release force. Operators report that film unwind force on a rotary arm at -10 °C can increase by 30–50% relative to ambient for the same film; the increase is due to tackifier stiffening and substrate modulus, not core-layer crystallization alone.

Puncture and Tear Propagation Under Pallet Vibration

Frozen pallet loads are subjected to vertical and rotational vibration in refrigerated trailers conforming to ASTM D4169-23 distribution-cycle practice. Stretch wrap must resist protrusion puncture from package corners, not only film-to-film cling. Protrusion puncture is measured under ASTM D5748-95; this standard is more predictive of corner cuts than dart impact. Films below 18 µm gauge frequently exhibit puncture energies that are adequate at room temperature but drop rapidly below -18 °C, particularly when the core layer contains a linear low-density grade with density above 0.925 g/cm³.

The failure sequence on a cold pallet is usually local puncture followed by tear propagation. Once a frozen carton corner or an irregular ice edge punctures the film, the stress concentration converts the puncture into a machine-direction or transverse-direction tear. A grade with adequate dart impact under ASTM D1709-16 can still fail if the tear propagation resistance under ASTM D1922-15 is low. Film evaluations for cold-chain use should therefore pair puncture, dart impact, and Elmendorf tear rather than relying on a single tensile property.

Comparing Blown and Cast Film Microstructure for Frozen Palletization

Blown LLDPE stretch wrap, processed on a 350 mm annular die with blow-up ratio 2.2:1 and frost line height 650–800 mm, produces a different balance of machine-direction and transverse-direction orientation than cast film. Blown films usually have higher transverse-direction tear resistance under ASTM D1922-15 but lower machine-direction stretch consistency. In cold-chain wrapping, high transverse-direction tear is advantageous when frozen pallet loads are stretch-wrapped over slip sheets, because transverse crack propagation from a puncture site is the main mode of film loss. However, blown LLDPE films often show higher frozen-film stiffness, increasing the wrapping-arm motor load at -25 °C. Processing conditions that raise the frost line too high can reduce toughness by forming larger spherulites; a lower frost line and higher melt temperature maintain a more amorphous tie-chain network.

Cast film generally offers superior thickness uniformity and higher prestretch capacity, but it may sacrifice transverse-direction tear resistance unless the core layer is formulated with 1-octene mLLDPE. On a cast line, the chill-roll temperature and vacuum-box settings determine the quench rate. A chill roll at 18 °C and a secondary cooling section of 4–6 m are typical for 20 µm cold-chain wrap. Higher quench rates suppress spherulite growth and improve dart impact under ASTM D1709-16.

Load retention at -20 °C is evaluated by machine-direction elastic recovery under ASTM D5459-95. Stress relaxation after stretching is slower in LLDPE than in low-density polyethylene; this is beneficial for frozen loads because the film continues to recover after pallet settling. However, if prestretch film is applied at high elongation above 250%, stress retention at -20 °C can decline faster than at ambient, especially when the core layer has a high polydispersity above 3.5. Published data for specific films at -25 °C and 24-hour relaxation is limited; plant trials using load-cell wrapping arms with ±1 N tension resolution provide more useful data than ambient laboratory tensile tests.

ASTM D5459-95 stress retention is normally reported as percentage of initial load after a fixed relaxation interval. At 23 °C, a typical cast LLDPE wrap may retain 80–90% after 24 h; at -20 °C, the retained value can be higher immediately after wrapping but may drop after the pallet is returned to ambient for cross-docking. Published data for repeated freeze-thaw cycles is limited, so load-retention tests should be run after 3 cycles from -20 °C to 10 °C to expose relaxation deficits.

Pre-stretched film contains oriented chains that have not fully relaxed. At freezer temperatures, these chains are immobilised in the amorphous phase, but when the outer wrap layer warms at dock doors, local stress relaxation can cause film slack. The result is a progressive loss of corner tension on partially wrapped pallets. Selecting a core resin with narrow molecular weight distribution below 3.0 polydispersity reduces long-chain relaxation and helps maintain corner force after temperature cycling.

When Blast-Freezer Airflow and Dry-Ice Proximity Depress Film Temperature

Blast freezers pull air across the pallet at 2–4 m/s with air temperature -30 °C to -35 °C. Film surfaces can reach -35 °C within minutes. In these conditions, LLDPE remains ductile because its glass transition is below -100 °C; the mechanical loss is driven by crystallinity and orientation. Films with density above 0.926 g/cm³ or high core-layer lot-to-lot variability can fail at the corners when the wrapper makes more than 200% prestretch at low temperature. Dry-ice shipments present film temperatures approaching -78 °C. At this temperature, published data for LLDPE stretch-wrap cling and puncture is limited; wraps should be reinforced with corner boards and strapping rather than relying on film integrity alone.

High-speed pre-stretching at 200 m/min applies strain rates that are closer to brittle conditions than quasistatic tests. An instrumented puncture test under ASTM D7192-19 at 3 m/s gives a better low-temperature ranking than ASTM D5748-95 alone. In machine trials, a rotating ring wrapper with a 0.75 kW prestretch motor may show overcurrent faults when a stiffer 23 µm blown film is used at -25 °C; the same motor handles a 20 µm cast 1-octene film without overload.

Batch-to-batch variance in comonomer distribution can present as a cliff-edge problem. Two lots with equivalent density and melt index can differ in low-temperature dart impact by more than 20% when the comonomer distribution broadens. Multilayer lines using in-line near-infrared gauges detect average thickness and density drift, but cannot detect tie-chain population. Periodic resin lot testing under ASTM D1709-16 after conditioning at -20 °C is therefore the only reliable release method.

Compatibility with frozen food and pharmaceutical loads requires that the film meet 21 CFR 177.1520 for olefin polymers and EU Regulation 10/2011 where applicable. Additive packages used for cold-chain cling must also be checked for migration at low temperature; although migration slows at -20 °C, reverse-driven bloom can occur during ambient warehouse staging before the pallet enters the freezer. Cold-chain failures are rarely due to melting or sealing. They arise from the intersection of three variables: film gauge below 18 µm, prestretch above 250%, and pallet protrusion sharpness. Once the film is cut or punctured, containment loss occurs within minutes under vibration. Increasing gauge by 2–3 µm often improves puncture survival more than changing resin grade, but it raises cost and may require wrapper recalibration.

Resin changeovers on a five-layer cast line should be validated by conditioning films at -20 °C for 48 h before conducting ASTM D1709-16 and ASTM D5748-95 tests, because ambient conditioning masks the crystalline changes that occur during the first freezer cycle.