Butene Comonomer Use in Film Grade LLDPE Short Chain Branching

Conventional film-grade linear low-density polyethylene is produced by low-pressure gas-phase, solution, or slurry copolymerization of ethylene and 1-butene using either a supported Ziegler-Natta catalyst or a single-site metallocene catalyst, and the resulting polymer is defined by ASTM D883-22 as a linear polyethylene of density 0.910–0.940 g/cm³. The comonomer introduces an ethyl branch of two carbon atoms along the polyethylene backbone, and this short-chain branch is the primary molecular feature that reduces the thickness of orthorhombic crystalline lamellae, lowers density, suppresses peak melting temperature, and shifts the heat-sealing initiation region downward compared with high-density polyethylene or low-branch-count LLDPE. In film extrusion grades, butene content is commonly in the range 2–6 mol%, corresponding to densities from approximately 0.925 g/cm³ down to 0.910 g/cm³, while melt index according to ASTM D1238-20 or ISO 1133-1:2022 is most frequently between 0.5 g/10 min and 2.0 g/10 min at 190 °C and 2.16 kg load. Density is measured routinely by the density-gradient column method of ASTM D1505-18 or ISO 1183-1:2019, but density alone cannot distinguish among different comonomer types because a hexene- or octene-based grade at the same density contains longer branches and can deliver different mechanical performance. Carbon-13 nuclear magnetic resonance spectroscopy under ASTM D5017-18 is the definitive laboratory method for quantifying butene-derived ethyl branch concentration and triad sequence distribution in the forms of EBE, EBB, and BEB structural sequences, and it is used during process capability studies and supplier qualification rather than as an online production tool. On a commercial blown-film line equipped with a 65 mm grooved-barrel extruder with 30:1 L/D ratio and a 200 mm spiral mandrel die set to a die gap of 1.2 mm, the observable consequences of varying butene content include changes in melt pressure, blow-up ratio tolerance, frost-line height, film blocking force, and tear anisotropy, all of which are traced to the molecular weight distribution and interchain short-chain branching distribution. Because Ziegler-Natta catalysts produce a broad distribution of comonomer incorporation across molar mass fractions, conventional butene-LLDPE often has a higher melt index ratio, I_21.6/I_2.16, than a comparable metallocene hexene-LLDPE, and this broad molecular weight distribution contributes to shear thinning behavior, lower melt fracture sensitivity, and reduced optical properties compared with narrow-composition-distribution resins.

What Limits Butene Incorporation in Gas-Phase Reactor Grades?

In condensed-mode gas-phase fluidized-bed reactors, the maximum butene incorporation is constrained by particle stickiness and agglomeration rather than by catalyst capability alone. The reaction temperature is normally kept below 88 °C for LLDPE production because the depressed crystallinity of the forming copolymer lowers the sintering temperature of the resin particle surface; if the sticking temperature is approached, fluidization collapses, distributor plate fouling occurs, and the cycle gas cooler can become blocked with fused polymer. Commercial suppliers therefore limit the molar ratio of 1-butene to ethylene in the cycle gas to producer-specific values often reported in the range 0.15–0.35, depending on the isopentane or n-hexane condensing-mode agent concentration, catalyst system, and target density. The copolymerization kinetics further restrict butene uptake because ethylene inserts far more rapidly than 1-butene with most supported Ti/MgCl₂ catalysts; published reactivity ratio data for some Ziegler-Natta systems place r₁ for ethylene in the range 40–80 and r₂ for 1-butene below 0.1, meaning that high ethylene-to-butene ratios are required in the reactor gas composition to achieve even 4–6 mol% butene in the copolymer. Published data for exact commercial reactor limits are limited because operating envelopes are proprietary, but production-scale experience on a 75 mm pilot gas-phase unit with a vertical product discharge and a cycle gas compressor confirms that butene-rich LLDPE grades must be produced with staged pressure reduction and extended degassing because residual butene is highly soluble in the amorphous phase. Batch-to-batch variation in butene incorporation of ±0.2 mol% is commonly observed when reactor conditions drift, and this level of variance is sufficient to shift density by approximately 0.001 g/cm³ and to move seal initiation temperature by 2–4 °C in downstream film converting. The polymer powder discharged from such a reactor must be pelletized under nitrogen or low-oxygen conditions, and residual butene must be stripped below lower flammability limit before pelletizing to prevent hydrolytic or oxidative gel formation during extrusion at 240–260 °C.

Cast film extrusion of butene-LLDPE at line speeds above 250 m/min depends on the same short-chain branching architecture that governs melt rheology, because the ethyl branches reduce the plateau modulus and alter the characteristic relaxation time of the melt under extensional deformation. On a cast film line using a 90 mm diameter extruder with a 33:1 L/D barrier screw, a 1,800 mm deckled flat die, an air gap of 8–12 mm, and a chill roll held at 20–25 °C, typical mass temperatures for butene-LLDPE range from 240 °C to 260 °C, and melt pressures at the die entrance are generally maintained between 80 bar and 150 bar depending on melt index and screw rotation. Low melt temperatures must be avoided with butene-rich grades because high melt elasticity and broad molecular weight distribution can produce haze and micro-gels, while excessive temperatures above 280 °C initiate thermo-oxidative gel formation even in the absence of oxygen if residence time in the feed section is prolonged. Neck-in and edge bead formation in cast film are more pronounced with butene-LLDPE than with LDPE, and fluoropolymer processing aids are commonly added at 200–800 mg/kg to delay melt fracture, reduce die lip build-up, and stabilize the film edge. Draw resonance and edge weaving are controlled by adjusting the air gap and draw ratio, and commercial grade changes from 4 mol% to 6 mol% butene require rebalancing of chill roll temperature and line tension because the lower crystallinity of the higher comonomer resin delays solidification and increases blocking tendency. Pre-drying is not required for dry resin, but when pellets have been stored in cold warehouses and transferred into a warm production hall, surface condensation can occur and a hopper dryer operating at 60 °C for 2 h is recommended to prevent surface moisture from creating pinholes at high line speeds.

Thermal Fractionation and Molecular-Weight-Dependent Branching in Butene-LLDPE

Because butene-LLDPE made with heterogeneous Ziegler-Natta catalysts is not homogeneous in comonomer distribution, the short-chain branching concentration must be considered as a distribution function across both molar mass and crystallizable sequence length. Successive self-nucleation and annealing performed with a differential scanning calorimeter calibrated under ISO 11357-3:2018 separates the polymer into thermal fractions by stepwise cooling after self-nucleation, producing a series of melting peaks that reveal the breadth of lamellar thickness populations created by ethyl branches. For a density of 0.918 g/cm³, the SSA profile of a Ziegler-Natta butene-LLDPE typically spans from a high-melting fraction near 124 °C corresponding to long ethylene sequences to a low-melting fraction below 95 °C corresponding to highly branched, poorly crystallizable chains, while a metallocene butene-LLDPE of the same density shows a narrower fractionation window because the intermolecular branch distribution is more uniform. Gel permeation chromatography coupled with infrared detection reveals that the butene concentration is not constant across molecular weight fractions; low-to-mid molar mass fractions often contain more ethyl branches, whereas the high-molar-mass tail is more linear and contributes disproportionately to the high-melting fraction. This molecular architecture has direct implications for film mechanical properties because the high-molar-mass linear chains form tie molecules between lamellae, while the branched low-molar-mass fraction lowers seal initiation and improves optical clarity. The following table presents representative property ranges for film-grade butene-LLDPE across three short-chain branching levels, with all values collected from published supplier datasheets and peer-reviewed thermal analysis studies, and should not be interpreted as universal specifications.

Blown Film Processing Parameters and Bubble Stability

Blown film extrusion of butene-LLDPE generally operates on grooved-barrel extruders between 55 mm and 90 mm diameter with 24:1 to 33:1 L/D ratios, using spiral mandrel or rotating die systems with die gaps of 1.2–2.3 mm and blow-up ratios from 2.0:1 to 3.0:1. The processing window for bubble stability narrows as butene content rises because the lower melt crystallization onset shifts the frost line upward unless the external air ring and internal bubble cooling are retuned; on a commercial line with a 300 mm die and a dual-lip air ring, increasing butene content from 3 mol% to 5 mol% often requires a reduction in internal bubble cooling volume of 10–15% and a frost-line height adjustment from 400 mm to 600 mm to maintain bubble symmetry. The melt strength of butene-LLDPE is lower than that of an equivalent melt index LDPE, and this tends to limit the upper blow-up ratio and film gauge to below 100 µm in high-bubble configurations; helical instability and breathing oscillation may appear if the frost-line height is too high or if the air ring velocity exceeds 25 m/s. For blown film with thickness between 15 µm and 50 µm, typical mass temperatures at the die are 190–220 °C, and melt pressure before the die is normally 250–450 bar for sparsely branched LLDPE depending on the screw design and throughput. The high shear sensitivity of the broad molecular weight distribution permits stable operation at melt temperatures lower than those used for cast film, which reduces oxidative degradation and protects seal performance. However, butene-LLDPE with melt index above 2.0 g/10 min may exhibit excessive bubble sag and melt curtain instability on conventional air-ring configurations, while melt index below 0.5 g/10 min can exceed extruder torque limits and generate melt fracture unless processing aids are added. Short-chain branching concentration also influences the frost line position because ethyl branches reduce the crystallization rate under the same cooling air conditions, and this crystallization rate reduction is measurable as a broadening of the crystallization exotherm peak recorded by DSC under ISO 11357-3:2018 at a cooling rate of 10 °C/min.

Sealing behavior and optical performance are the two downstream properties most directly governed by butene-derived short-chain branching, and film converters evaluate these properties before qualifying a resin for monolayer or coextruded structures. Heat-seal initiation temperature for butene-LLDPE is lower than for HDPE and for lower-branch-count LLDPE because ethyl branches reduce the crystalline fraction that must melt before chain interdiffusion can occur across the seal interface. According to ASTM F1921-18 hot-tack and heat-sealability testing, a Ziegler-Natta butene-LLDPE film of 25 µm thickness and 0.918 g/cm³ density typically develops an initial seal strength of 2–4 N/15 mm at seal bar temperatures between 95 °C and 105 °C, while final seal strength measured under ASTM F88/F88M-21 approaches 8–12 N/15 mm when the seal temperature reaches 115–125 °C. Hot tack windows are broader for butene-LLDPE than for metallocene hexene-LLDPE at the same density because the broad comonomer distribution melts over a wider temperature interval, but the presence of higher-melting linear fractions can limit ultimate cold-seal response. Film optical properties such as haze and clarity improve as butene content increases from 2 mol% to 6 mol% because the smaller lamellae scatter less visible light, and blown film haze measured under ASTM D1003-13 can fall from 12–15% at 2 mol% butene to below 8% at 5–6 mol% butene when other variables are held constant. Additive selection for butene-LLDPE film must account for the lower melting point and higher amorphous fraction; migratory slip additives such as erucamide and oleamide can bloom more rapidly and in higher surface concentration, and antiblock loadings above 5,000 mg/kg can reduce haze improvement and lower seal strength. The compliance matrix for food-contact use includes olefin polymer specifications set out in FDA 21 CFR 177.1520 and EU Regulation No 10/2011 with overall migration limits, and resin suppliers are expected to provide certificate-of-conformance data for butene comonomer residues and additive migration in the intended food category.

When Butene-LLDPE Is Substituted for Hexene-LLDPE in Stretch Film

Substituting Ziegler-Natta butene-LLDPE for hexene-LLDPE in machine-wrap stretch film is economically attractive but operationally constrained by short-chain branching length effects on extensional flow, puncture resistance, and cling retention. The butene-derived ethyl branch is shorter than the hexene-derived butyl branch, and at equivalent density and comonomer molar content it produces a less effective tie-chain network for distributing local stress; this is reflected in lower dart drop impact under ASTM D1709-15a and lower slow puncture resistance under ASTM D5748-20 at film gauges below 25 µm. Published comparative data from film converters indicate that a butene-LLDPE of 0.918 g/cm³ density and 1.0 g/10 min melt index can provide adequate elastic recovery and load retention in handwrap applications up to approximately 150–200% prestretch, but machine-wrap operations requiring prestretch above 250% generally favor hexene- or octene-LLDPE because the shorter branch length reduces strain-hardening and increases the risk of catastrophic film splitting at high-speed stretch rollers. On a 1,500 mm wide stretch-film line running at 400 m/min with in-line prestretch ratios of 2.0:1 to 3.0:1, butene-LLDPE formulations often require the addition of 10–20 wt% of a metallocene hexene-LLDPE or LDPE to restore bubble stability and split resistance; below this blend level, edge tear propagation and cling-induced web breaks become more frequent. The peel cling force measured under ASTM D5458-95 may be adjusted by erucamide addition in the range 1,000–3,000 mg/kg, but the higher amorphous content of butene-LLDPE accelerates additive migration and can produce excessive blocking if roll storage temperatures exceed 35 °C. Published data for the exact prestretch failure boundary of butene-LLDPE is limited because converter formulations are proprietary and equipment-dependent, yet the industrial consensus is that butene grades should be selected for lower-intensity stretch wrap and general-purpose film where seal and optical performance outweigh ultimate puncture and pre-stretch capability. The substitution should not be made without conducting a full film tear resistance and slow puncture comparison on the target line, using ASTM D1922-15 for longitudinal tear, ASTM D5748-20 for puncture, and ASTM D5458-95 for cling, because the molecular weight distribution and branching distribution of the alternative hexene-LLDPE can mask performance differences in short-duration laboratory tests.