In blown film extrusion, the selection of butene, hexene or octene as the alpha-olefin comonomer in linear low-density polyethylene (LLDPE) alters short-chain branching length, molecular architecture, and the film’s mechanical, optical, and seal response. Resin density and melt index, measured under ASTM D1505 and ASTM D1238 respectively, do not capture these differences; two 0.918 g/cm³ LLDPE grades can diverge by more than 150 g in dart impact measured according to ASTM D1709 because of comonomer type and branch distribution. The selection therefore begins with molecular architecture, not with density or catalyst family alone.

Which LLDPE comonomer fits your film: butene, hexene or octene?

Butene is the lowest-cost comonomer and the most common in Ziegler-Natta LLDPE for commodity stretch wrap and textile bale wrap, but it produces two-carbon ethyl branches that are less effective at redistributing stress at the crystalline–amorphous interface. Hexene introduces four-carbon butyl branches and typically improves toughness at comparable density, making it a frequent choice for high-performance industrial liners and form-fill-seal packaging. Octene introduces six-carbon hexyl branches and is specified where downgauging, cold-temperature toughness, and seal performance dominate. Extrusion trial data indicate that moving from a butene-based 0.918 g/cm³, 1.0 g/10 min melt index resin to an octene-based resin of identical density and melt index can change dart impact by 40–70% when measured according to ASTM D1709, depending on film gauge, blow-up ratio, and extrusion temperature profile.

On a 60 mm single-screw blown film line with a 24:1 L/D barrier screw, a 200 mm spiral mandrel die, and a 1.5 mm die gap, the processing behavior of the three comonomer families differs in melt pressure, neck-height force, and bubble stability. Butene-rich LLDPE generally has the lowest shear viscosity for a given melt index, which can reduce screw amperage and increase output at a given melt temperature but narrows the bubble stability window at blow-up ratios above 3.0:1. Hexene and octene grades, particularly metallocene-catalysed grades with narrow molecular weight distribution, show more uniform neck-height force and less gauge variation but require careful control of internal bubble cooling and frost line height to prevent spiral die weld line roughening. A representative parameter set for an octene-based metallocene LLDPE at 0.918 g/cm³ and 1.0 g/10 min is melt temperature 204–218 °C, die pressure 280–360 bar, frost line height 2.5–4.0 die diameters, and blow-up ratio 2.0–2.8:1; excursions above 4:1 often induce bubble flutter unless 5–20 wt% of high-pressure LDPE is blended for melt strength.

Melt fracture and die build-up are limiting failure modes that differ by comonomer. Butene grades are prone to sharkskin melt fracture at die shear rates exceeding 350–450 s⁻¹ on narrow die gaps, which can force the use of fluoropolymer processing aids at 200–800 ppm. Hexene and octene grades frequently show a wider shear rate window before gross melt fracture but can form low molecular weight die deposits if the melt temperature is held above 220 °C for extended residence times. In long-run form-fill-seal lines, die lip cleaning intervals for octene metallocene LLDPE are commonly 6–12 hours longer than for butene Ziegler-Natta LLDPE when melt temperature is maintained below 215 °C.

Butene, hexene and octene produce distinct branch lengths that control tie-chain probability and fracture resistance

The differential performance is rooted in the physical length of the short-chain branch. Butene contributes a two-carbon ethyl branch after incorporation; hexene contributes a four-carbon butyl branch; octene contributes a six-carbon hexyl branch. In semi-crystalline polyethylene, longer comonomers are more efficient at interrupting the pendant crystallisable ethylene sequences, reducing lamellar thickness and increasing the population of load-bearing tie chains at equivalent branch content. This effect is quantified in tensile and impact testing according to ASTM D882 and ASTM D1709. A 25 µm blown film made from a 0.918 g/cm³ octene LLDPE can exhibit an ultimate tensile strength of 35–45 MPa in the machine direction and 28–38 MPa in the transverse direction, while a butene grade of the same density may display 5–10% higher tensile strength but 50–70% lower dart impact. The inverse relationship between tensile yield and dart toughness is characteristic of comonomer length effects, not of density change.

The following ranges are representative of blown film trial data for monolayer film at 25 µm, resin density 0.918–0.920 g/cm³, and melt index 1.0 g/10 min; specific commercial grades may fall outside these ranges due to molecular weight distribution and catalyst effects.

In blown film intended for high-clarity overwrap, the optical benefits of octene are measurable but not uniform. Haze measured per ASTM D1003 and gloss measured per ASTM D2457 improve with comonomer length because longer branches reduce the size and perfection of spherulitic scattering entities. However, the effect saturates when film is quenched rapidly; on high-throughput water-quench lines or in cast film processes, the optical difference between hexene and octene can be smaller than the variation introduced by die lip geometry and chill roll temperature. Film producers should therefore verify actual haze and gloss on their own equipment rather than extrapolate solely from resin type.

Seal initiation measured per ASTM F88 is commonly 5–12 °C lower for octene grades than for butene grades at equivalent density. In vertical form-fill-seal equipment running above 40 cycles/min, the lower initiation temperature can widen the dwell window, but hot tack measured per ASTM F1921 must be confirmed; a resin may seal at low temperature yet fail to hold a product load if hot tack strength is insufficient.

When downgauging pressures require a move from C4 to C6/C8

Downgauging from 40 µm to 25 µm in heavy-duty sacks or agricultural stretch film increases the mechanical burden on the film. Butene grades may satisfy the original gauge but fail dart and tear requirements after thickness reduction; hexene or octene grades maintain equivalent or better energy absorption with less material. In a production-scale comparison on a three-layer blown film line with a 75 mm grooved-feed extruder and 250 mm die, a 25 µm film with an octene skin and a butene core gave dart impact values comparable to a 40 µm all-butene film at 18–22% lower basis weight when tested under ASTM D1709. The result depends on layer ratio, die stagnation time, and internal bubble cooling; published data for specific configurations is limited, but the trend is consistent with the tie-chain mechanism.

In agricultural silage film, the transverse tear requirement often becomes the controlling property because the film must survive stretching over uneven baler surfaces. Octene-rich layers can raise transverse tear values by 30–60% relative to butene-rich layers at equal gauge, but this advantage is fully realised only when the frost line is held below 3.0 die diameters; excessively high frost line height preferentially orients the film in the machine direction and erodes the transverse toughness benefit.

Seal integrity and organoleptic limits in food contact structures

Resin compliance is governed by the finished food-contact article, not the pellet alone. In the United States, LLDPE copolymers are generally covered under 21 CFR 177.1520(c) as olefin polymers intended for food contact when the food-type and use-temperature conditions in the regulation are satisfied. In the European Union, the overall migration limit in EU Regulation (EU) No 10/2011 is 10 mg/dm² for plastic food contact materials, and the specific migration of the comonomer must comply with its assigned SML. Butene, hexene and octene have differing migration characteristics; longer-chain comonomers are generally less volatile and less prone to evaporative loss during extrusion, but residual comonomer content must be verified by headspace gas chromatography using a method such as EN 13628-1 or an internal validated procedure.

A compliance checklist for a monolayer LLDPE film used in food contact can be structured as follows.

Single-site catalysts alter the distribution of comonomer across chain lengths and therefore change the value of moving from butene to hexene or octene. Ziegler-Natta catalysts incorporate comonomer preferentially in low molecular weight chains, leaving high molecular weight fractions with lower branching; metallocene and other single-site catalysts incorporate comonomer more uniformly across the molecular weight distribution, which maximizes the effectiveness of octene and hexene in generating tie chains. Consequently, a Ziegler-Natta hexene grade may be outperformed by a metallocene octene grade at the same density and melt index, while the benefit of octene over hexene in a metallocene family may be smaller than the benefit of changing catalyst technology. This interaction is typically resolved with pilot-line data because published data for specific combinations is limited.

What does the distribution of comonomer across chain lengths reveal about film performance?

The molecular weight distribution and comonomer distribution act together. A narrow molecular weight distribution improves optics and reduces extractable species but may reduce melt strength and raise die pressure. A broad distribution aids bubble stability and tear at the expense of clarity and seal initiation. In a 25 µm film, the comonomer distribution can be inferred indirectly through hexane extractables performed according to 21 CFR 177.1520 or through crystallinity measurements by differential scanning calorimetry at 10 °C/min following ASTM D3418. A metallocene octene grade typically shows a lower and narrower melting endotherm than a Ziegler-Natta butene grade at equal density, which correlates with a 5–12 °C lower heat seal initiation but requires a more tightly controlled seal bar temperature window.

For heavy-duty shipping sacks and dunnage bags, octene-based LLDPE is typically specified in the core layer to resist puncture at -20 °C, while butene or hexene skins are used to lower raw material cost and maintain heat-seal performance. On a three-layer line producing 80–120 µm sacks, this asymmetric structure permits a 15–20% reduction in total film weight compared with a monolayer butene sack while maintaining Mullen burst strength and drop-test retention according to ASTM D3786 and ISTA 1A. The practical limit of this approach is reached when the skin layers exceed 30% of total thickness; at that point the loss in core layer energy absorption cannot be offset by layer ratio adjustment, and film failure shifts to the skin under sharp-edge puncture.