Pallet hood films are coextruded blown-film structures in which transverse shrinkage is generated by a deliberately unbalanced orientation state. The annular die diameter, die gap, blow-up ratio, frost line height, internal bubble cooling pressure, external air ring velocity, and collapsing frame geometry jointly determine how much lateral chain extension is frozen into the quenched web before gusseting and winding. A typical pallet hood film is produced at 80 µm to 180 µm total thickness from polyethylene skin layers and a recycled or lower-cost core, and is heated at 130 °C to 180 °C during application. At those temperatures molecular mobility increases and frozen transverse orientation relaxes, producing circumferential contraction around the pallet. The same orientation state changes slow tear resistance because a propagating tear under sustained load must either pull tie molecules out of oriented lamellae or rupture fibrils that bridge the crack faces. The direction dependence is not subtle: a tear path parallel to the transverse direction can follow boundaries between oriented fibrils, whereas a tear path along the machine direction must cut across the primary orientation axis. Higher transverse shrinkage therefore improves load retention and pallet stability but can reduce resistance to slow vertical tear propagation initiated by pallet corner contact, splinter penetration, or bottom-edge notching. This trade-off cannot be captured by a single tear value; it requires measurement of both free shrink and shrink force under controlled thermal conditions.

Process development on high-stalk blown film lines frequently encounters the conflict between transverse shrinkage and slow tear resistance when the blow-up ratio is increased to raise transverse orientation. The machine-direction drawdown also changes as haul-off speed and internal bubble cooling are adjusted, but the transverse direction is more sensitive because the bubble circumference is a direct multiplier of the die diameter. Fluctuations in ambient air temperature, external air ring velocity, or collapsing frame height can shift transverse free shrink by ±2 percentage points within a single production run. Such shifts are sufficient to change slow tear test results because the residual shrink stress superimposes a tensile load on the notch tip. Production lines operating with 2,000 mm dies and internal bubble cooling report that bubble circumference tension becomes the dominant instability mode when transverse free shrink is pushed above approximately 30 %. At that point the film may become more sensitive to edge tearing during trimming and to puncture during pallet wrapping even when laboratory Elmendorf tear values remain acceptable.

When transverse free shrink exceeds 25 %, slow tear propagation shifts from ductile pull-out to brittle craze rupture

At transverse free shrink values below approximately 15 %, slow tear propagation in polyethylene pallet hood film is generally accompanied by visible whitening and a ductile crack opening angle because the notch-tip plastic zone is large relative to the film thickness. As transverse orientation increases through higher blow-up ratio or faster quench, the average lamellar separation decreases and the plastic zone becomes constrained. The transition is gradual, but at transverse free shrink above 25 % measured by ASTM D2732-13, the tear path often becomes smoother and the crack opening angle decreases, indicating that craze fibrils are breaking before extensive chain disentanglement occurs. The standard trouser tear methods ASTM D1938-19 and ISO 6383-1:2015 impose a constant crosshead displacement rate of 250 mm/min, which is much faster than the quasi-static loading that occurs when a pallet hood film is under sustained shrink stress for minutes to hours. At the lower service strain rates, molecular relaxation allows even fewer tie chains to share the load, so the laboratory trouser tear value should be treated as a ranking indicator rather than an absolute field performance prediction. The shift from ductile pull-out to brittle craze rupture is accelerated by residual shrink stress because the stress intensity at the notch tip is the sum of the external tearing load and the frozen-in transverse stress. When the shrink force is high, tear can propagate without a large externally applied opening displacement, which is why film that appears undamaged after shrink application may still develop a slow tear once a pallet corner creates a small notch. Published data for pallet hood film slow tear at service strain rates is limited; the available comparative data confirm that increasing transverse orientation reduces machine-direction trouser tear resistance when the shrink stress is not relaxed by subsequent annealing.

On a three-layer pallet hood line with a 2,000 mm annular die, internal bubble cooling, and a blow-up ratio of 2.8:1, the selection of ethylene-octene LLDPE with a melt index between 0.2 dg/min and 1.0 dg/min and density between 0.918 g/cm³ and 0.924 g/cm³ is common because this resin maintains a larger tie-chain population than LDPE at equivalent orientation. The skin layers are often blended with 10–25 wt% LDPE to provide bubble stability at blow-up ratios above 3.0:1, but increasing LDPE content above approximately 30 wt% can reduce slow tear resistance because the long-chain branched architecture lacks the linear tie-molecule concentration needed to resist craze rupture. The core layer may contain up to 40 wt% recycled polyethylene if the contamination level of high-density polyethylene and polypropylene is kept below 5 wt%, since higher levels suppress transverse shrinkage and create brittle tear paths. Extruders of 70 mm to 90 mm diameter with 30:1 L/D barrier screws are used to keep melt temperature at 210 °C to 240 °C. The die gap is normally 1.8 mm to 2.4 mm, and output on a 2,000 mm die typically falls between 300 kg/h and 500 kg/h when internal bubble cooling is active. Incompatibility with moisture is generally not a concern for polyethylene, but recycled core material with more than 0.1 % moisture can generate bubble defects that act as local notch sites. Excessive slip additives above 1500 ppm are also avoided because surface migration can lubricate tear propagation paths and reduce slow tear resistance, especially in the machine direction.

What does slow tear testing measure that pendulum impact tear does not?

The pendulum impact tear method ASTM D1922-15 and ISO 6383-2:1983 applies a high-rate tear impulse that often causes polyethylene films to yield and deform before the tear front advances. The test is useful for quality control because it is rapid and reproducible, but it does not reproduce the sustained loading that causes slow tear failure in a pallet hood film. The trouser tear method ASTM D1938-19 and ISO 6383-1:2015 measures the force required to propagate a pre-existing notch through a rectangular specimen at 250 mm/min, allowing craze formation, fibril bridging, and chain disentanglement to contribute to the measured force. In films with high transverse orientation, the difference between the two tests becomes large because the impact method is often dominated by notch-tip blunting while the trouser test is sensitive to the stability of the craze fibrils. Slow tear resistance is therefore better correlated with the ratio of shrink force to trouser tear force than with Elmendorf tear alone. A film with high Elmendorf tear but low trouser tear may still fail in slow tear if the shrink stress is high. This distinction is critical because pallet hood films are not simply stretched; they are heated, shrunk, and then held under residual stress for the duration of transport or storage. The standard test methods for shrink stress, ASTM D2838-18 and ISO 14616:1997, should be used alongside slow tear testing to determine whether the residual stress is sufficient to promote brittle craze rupture. If the constrained shrink stress is large relative to the machine-direction trouser tear force, the operating window is considered narrow because small notches can propagate without additional significant external load. Published data for this specific ratio in pallet hood film is limited.

Bubble geometry, shrink force, and hoop stress in pallet hood application

Shrink force measured by ASTM D2838-18 or ISO 14616:1997 is the constrained stress generated when the film is heated; free shrink measured by ASTM D2732-13 is the dimensional change when the film is unrestrained. A pallet hood film can have high free shrink but low shrink force if the oriented amorphous phase relaxes without generating high stress, or it can have moderate free shrink and high shrink force if the crystalline network dominates. During application the hood is constrained by the pallet load, so the hoop stress generated by transverse shrinkage is more relevant to slow tear initiation than free shrink alone. The hoop stress acts circumferentially around the pallet and creates a state of tension in the film wall. Any small cut or notch at a pallet edge concentrates this stress and may initiate a slow tear. The bubble geometry used to produce the film controls the distribution of that residual stress. Frost line height below approximately 8 die diameters quenches the bubble rapidly and retains orientation, while a frost line above 12 die diameters allows molecular relaxation and reduces transverse free shrink. Increasing internal bubble cooling pressure at constant blow-up ratio also increases quench rate and may raise transverse free shrink without changing the gross bubble dimensions. On a production line with an 1800 mm die and collapsing frame angle of approximately 25° per side, the transverse stress at the bubble collapse is already significant, and gusset folds become regions of higher local orientation. These gusset edges can act as preferential tear initiation sites if they are creased by the nip rolls or if thickness variation exceeds 10 % of nominal. The application temperature selected for the shrink frame must be high enough to release orientation but not so high that the film melts or the shrink stress exceeds the yield strength of the palletized product. Typical polyethylene pallet hood films are shrunk at 130 °C to 160 °C, with higher temperatures used for higher free shrink values and lower for delicate loads.

Laboratory evaluation of pallet hood film should separate transverse free shrink, shrink force, and directional slow tear resistance rather than combining them into a single specification. The table below lists the standard methods applicable to this evaluation and identifies the limitation of each method. When a film is compared across different suppliers, the same test conditions should be used because thickness, notching direction, and thermal history all change the measured values. Specimens for slow tear testing should be cut from the flat web before gusseting, and the notch should be oriented so that propagation direction is explicitly recorded as machine direction or transverse direction. Condensed-water or oil immersion methods used for shrink testing can alter surface slip and should be followed by the conditioning specified in the relevant standard.

After the test matrix is generated, the critical process window is determined by the combination of transverse free shrink, machine-direction trouser tear force, and shrink force. If the film passes the shrink requirement with a margin of only 2–3 %, but the machine-direction slow tear force is near the lower end of the supplier specification, the processing window is narrower than the nominal values suggest. The film should be sampled at the beginning, middle, and end of the master roll because transverse orientation can vary across the web width due to collapse-frame tension and gusset geometry. Production-scale failure modes include spontaneous web breaks at the gusset edge during winding, excessive stretch marks in the collapse frame, and tearing at the bottom edge of the hood during heat application. Each of these can be traced to the same imbalance between transverse shrink stress and slow tear resistance. The use of internal bubble cooling can reduce the severity of the problem by increasing output without requiring a higher blow-up ratio, but it also shifts the frost line and may increase orientation if the quench rate is not matched to the line speed.

Transverse free shrink is a direct proxy for residual TD orientation in PE hood films

In the absence of molecular architecture differences, transverse free shrink measured by ASTM D2732-13 correlates with the amount of residual transverse orientation in the quenched film. The measurement is not sufficient to predict slow tear resistance because it does not capture tie-chain continuity, shrink force, or local thickness variation, but it provides a fast indication of the orientation state. A film with 20 % transverse free shrink at 130 °C is expected to exhibit different slow tear behavior than a film with 30 % transverse free shrink at the same temperature. The direction of the effect is not always monotonic at low values because small additions of high-molecular-weight linear polyethylene can improve tie-chain density without increasing shrinkage. At high values the relationship becomes more predictable: additional orientation reduces the energy required for slow crack propagation because the available plastic zone is smaller and the notch-tip stress is amplified by residual shrink force. Films that require both high transverse shrink and high slow tear resistance are therefore formulated with high-tie-chain resins and processed at the lowest blow-up ratio that still meets the shrink specification. The operating window is bounded on one side by insufficient transverse shrinkage and on the other side by inadequate slow tear resistance. This boundary is best established by measuring transverse free shrink, shrink force, and machine-direction trouser tear force on the same master roll, rather than relying on supplier datasheet values that may have been generated on different film thicknesses and test conditions. Published data for this specific configuration is limited, but the underlying principles of orientation, tie-chain pull-out, and craze rupture are well documented in the blown-film polymer literature.