Linear low-density polyethylene (LLDPE) film grades derive their puncture resistance, tear resistance, and seal integrity from the short-chain branching introduced by alpha-olefin comonomers. The comonomer type is not a minor additive adjustment. Replacing 1-hexene or 1-octene with 1-butene at constant melt index and density shifts the population of interlamellar tie molecules, the rate of secondary crystallization, and the distribution of branch lengths without necessarily changing the resin specification sheet. The result is a class of film failure that is difficult to detect during first-article inspection because the film may pass tensile and dart impact testing on the day of conversion. After 7 to 21 days of warehouse ageing, however, dart impact, Elmendorf tear, seal creep, and environmental stress cracking can deteriorate below specification. The failures are slow, morphology-driven, and frequently misclassified as contamination, film handling damage, or sealing machine drift.

Substituting LLDPE Comonomer: The Film Failures That Show Up Weeks Later

On a production blown film line built around a 75 mm grooved-feed extruder with 24:1 L/D barrier screw, 250 mm spiral mandrel die, 1.6 mm die gap, 2.5:1 blow-up ratio, and 600 mm frost-line height, replacement of a 1.0 g/10 min 0.918 g/cm³ 1-hexene LLDPE with a 1.0 g/10 min 0.918 g/cm³ 1-butene LLDPE produced a 25 µm film that initially met thickness and gauge-band requirements. Melt pressure at 120 kg/h decreased by approximately 9%, and the adapter melt temperature shifted from 218 °C to 214 °C. Bubble stability was retained only after the frost-line height was lowered to 500 mm and the blow-up ratio was reduced from 2.5:1 to 2.2:1. Same-day dart impact was within normal conversion limits, but after 14 days at 23 °C and 50% RH, the butene-substituted roll produced F50 values that were 36% lower than its day-one result, while the hexene control lost less than 10% over the same period.

The delayed loss is not detected by density or melt index verification. Density taken by ISO 1183-1:2019 and melt flow rate taken by ISO 1133-1:2022 remain unchanged because neither method measures tie-chain concentration or lamellar thickening. The property shift is better captured by destructive film tests repeated after standardized ageing. The table below shows representative shifts for a 25 µm blown film pair at 0.918 g/cm³ and 1.0 g/10 min melt index. The values are lot-specific and should not be treated as universal resin-grade data.

The observed gap narrows when the 1-butene grade has a bimodal molecular weight distribution or is blended with an octene-based LLDPE. Published data for direct C4-to-C6 substitution at identical screw speed, die gap, and blow-up ratio is limited because the response is highly dependent on catalyst technology, comonomer distribution, and additive package. The mechanism of delayed loss is nevertheless consistent: shorter 1-butene branches are less effective than 1-hexene or 1-octene branches at generating tie chains between adjacent lamellae when density is held constant. Over days of ambient storage, secondary crystallization consumes part of the remaining amorphous chain population, and the film loses ductility without a large change in crystalline density.

What Changes When 1-Hexene Is Replaced by 1-Butene on a 75 mm Grooved-Feed Blown Film Line?

The first shift observed downstream is usually not a packaging failure but a bubble-stability and gauge-control adjustment. 1-Butene LLDPE of equivalent density tends to exhibit lower melt tension and a narrower stable operating window at high frost-line heights. On the 75 mm line described, the bubble entered a vertical oscillation at 600 mm frost line within 20 min of the comonomer switch unless the blow-up ratio was reduced. Lowering the frost line to 500 mm restored gauge uniformity, but the change increased MD orientation. The resulting film showed improved haze but reduced MD tear resistance after ageing. The combination of lower melt temperature, increased shear heating in the barrier screw, and higher MD orientation produced a film that was visually acceptable on the winder but mechanically deficient after 14 days.

A second effect is the shift in seal initiation temperature. Because 1-butene LLDPE exhibits a broader melting endotherm and a higher average peak melting temperature when density is held constant, the seal jaw temperature required to achieve a 9.8 N/25 mm seal strength under 0.5 s dwell and 2.8 bar jaw pressure increases. The shift is often 4 °C to 8 °C. Packaging lines that were previously operating at the lower edge of the seal window continue to produce acceptable seal strength on the day of conversion, but the seals have a wider residual stress distribution at the seal shoulder. After chilled storage, slow peel and seal-edge cracking occur in the butene-rich film at a higher frequency than in the hexene control.

The primary equipment indication that the comonomer substitution has changed the rheological response is the melt pressure curve. At constant screw speed and 120 kg/h output, pressure before the screen changer typically falls from 310 bar to 282 bar when replacing 1-hexene with 1-butene LLDPE of the same melt index. This reduction is useful evidence that the resin has not been mistakenly dried or that the melt pump is not cavitating, but it does not predict the slow failure mode. A more useful process signal is the specific energy consumption, which decreases by roughly 0.02 kWh/kg to 0.03 kWh/kg. If the line control system uses specific energy as an indirect viscosity check, the window may accept the material while the downstream film properties degrade later.

Slow Crack Growth and Environmental Stress Cracking After 500 h in 10% Igepal CO-630

Compression-moulded plaques prepared from the butene-substituted film pellets and notched according to ASTM D1693 condition B, immersed in 10% Igepal CO-630 at 50 °C, show F50 failure between 180 h and 260 h. The 1-hexene reference typically exceeds 1,000 h. The failure distribution is also steeper for the butene-rich material, meaning that the lot-to-lot variance in slow crack resistance is more pronounced. This is the root cause behind field complaints where only certain batches of heavy-duty sacks fail after weeks of storage, while other batches made with the same nominal density and melt index remain intact.

Delayed film failures in folded and boxed packaging are driven by the same slow crack growth mechanism. When film is folded into gussets, the outer crease develops a local strain concentration. The strain is initially accommodated by ductile yielding. Over 10 to 14 days, however, secondary lamellar thickening reduces the number of available tie chains, and the same crease becomes a site for microcrack initiation. A heavy-duty shipping sack line running 55 µm film on a 90 mm grooved-feed extruder at 180 kg/h recorded an increase in side-gusset cracking after storage from 0.3% of sacks to 2.8% when 25 weight percent of the 1-hexene LLDPE was replaced by 1-butene LLDPE. The first-article drop test and same-day tear testing did not predict the complaints.

The appropriate laboratory analogue for this field failure is not the Elmendorf tear test alone, because the Elmendorf test introduces a rapid cut and measures energy to propagate an existing tear. The fold-crack failure is better simulated by Gelbo flex testing according to ASTM F392, with pinhole formation measured after repeated flexing. In the butene-rich film, pinhole onset was detected at 120 to 180 cycles, while the hexene-rich control reached pinhole onset at 300 to 500 cycles. The result is consistent with a reduction in slow crack growth resistance rather than a loss in stiffness or yield strength.

Heat-seal performance is the failure mode most frequently misdiagnosed immediately after conversion because seal strength measured within minutes of sealing can pass the packaging specification while seal creep and cold-seal failures appear only after the packaged product has been chilled for two to three weeks. Substituting 1-butene for 1-hexene typically broadens the differential scanning calorimetry melting endotherm and raises the seal initiation temperature by 4 °C to 8 °C at a 25 µm film seal thickness. Under ASTM F88, with 0.5 s dwell, 2.8 bar jaw pressure, and 110 °C jaw temperature, seal strength can remain at 9–11 N/25 mm. However, when the package is filled with 1 L of a surfactant-containing liquid and stored at 4 °C, seal creep failure at the lap seal occurs after 14 days in butene-rich formulations, while the hexene-based control retains seal integrity beyond 30 days. The mode is a slow peel driven by residual stress at the seal edge and lower tie-chain density in the seal shoulder. Hot tack measured by ASTM F1921 also shifts; the plateau moves upward by approximately 6 °C and the peak force decreases by 12% to 18%.

A second delayed seal defect appears when the seal area is contaminated with light dust or release agent at 0.1 g/m² to 0.3 g/m². The butene-substituted film produces a caulk-like seal failure at the outer seal edge at lower jaw temperatures than the hexene control. The package may pass a vacuum-leak test immediately after filling and then develop leaks after distribution because the seal edge has a low-ductility interfacial layer. The failure is not a hot-tack problem but a slow interfacial debonding phenomenon. It is reproducible only when the sealed film is aged for at least 48 h before the peel test.

When Only 30 Weight Percent Butene LLDPE Is Processed Through a 40:1 L/D Tandem Sheet Line

Blending 30 weight percent of a 1-butene LLDPE into a 1-octene LLDPE cast stretch film formulation on a 40:1 L/D tandem sheet line with a 150 mm flat die and 45 °C polished rolls initially improves optics and lowers melt pressure by approximately 6%. The retained MD tear after elongation to 300% in a pre-stretch unit decreases below the octene-only control after 10 days. The delayed loss is worsened by antioxidant consumption during extrusion. The butene-rich fraction has a higher melt viscosity mismatch with the octene matrix, longer residence time in the die, and greater shear heating at the screw tip. Multiple-pass extrusion testing shows that oxidative induction time measured by ISO 11357-6:2021 falls faster in the butene-containing blend than in the octene-only control when extruder barrel temperatures exceed 230 °C.

Gel formation in the butene-containing blend is also a delayed failure indicator. On the tandem line, gel counts measured on the cast film after 14 days of roll storage increased from 0.4 gel/m² to 1.8 gel/m² at a 250 µm detection limit. The gels are not necessarily present on the day of extrusion; they form as oxidized polymer fractions crosslink during the early stages of warehouse ageing. This is a known limitation of butene LLDPE in high-shear cast film and stretch film operations, particularly when the formulation contains recycled edge trim that has already experienced one or more heat histories.

Confirmation of the delayed failure mechanism requires gel permeation chromatography, differential scanning calorimetry, and temperature rising elution fractionation. The butene-substituted film generally shows a higher peak melting temperature, a broader high-temperature crystalline fraction, and fewer interlamellar tie molecules than the hexene or octene reference at the same density. The latent property shift is therefore not a resin cleanliness problem but a morphological relaxation and stress-cracking phenomenon. A single-point melt index check under ISO 1133-1:2022 will not expose the failure potential. The practical control strategy is to run dart impact, Elmendorf tear, seal creep, and ESCR testing after 168 h of standardized ageing at 23 °C and 50% RH, and to avoid substitution decisions based on same-day film properties. For formulations containing more than 20 weight percent 1-butene LLDPE, published data for specific converter configurations remains limited, and the response must be confirmed on the target line using the actual die gap, chill-roll temperature, and edge-trim recycle ratio.