Linear low-density polyethylene (LLDPE) is a random copolymer of ethylene and an α-olefin comonomer, normally butene-1 (C4), hexene-1 (C6) or octene-1 (C8). The designation refers to the α-olefin carbon count, not the side-chain length; the short-chain branches generated are ethyl for C4, butyl for C6, and hexyl for C8. The change in branch length alters the frequency and distribution of defects within an otherwise linear backbone, governing density, toughness and optical haze. Density is conventionally determined by ASTM D792-20 or ISO 1183-1:2019 and melt mass-flow rate by ISO 1133-1:2022 at 190 °C with 2.16 kg. These standardized measurements alone do not reveal the comonomer topology responsible for film performance. C4, C6 and C8 LLDPE grades can be produced at laboratory density values from approximately 0.900 g/cm³ to 0.940 g/cm³, but the density at which a given toughness is obtained shifts strongly with comonomer type. In general, at equal density and melt index, C8 grades show the largest tie-chain population and the lowest crystalline order, C6 grades are intermediate, and C4 grades require higher density or lower melt index to approach comparable impact strength.

Selection among C4, C6 and C8 is not a simple quality ladder. A C4 grade at 0.930 g/cm³ may exhibit lower dart impact than a C8 grade at 0.912 g/cm³. This occurs because density is a bulk crystallinity indicator, whereas tie-chain population is controlled by branch architecture and intermolecular distribution. A density specification in a supplier datasheet therefore requires simultaneous reference to comonomer type and polymerization platform. At equal molar incorporation, C8 produces a larger density depression than C4 because the longer hexyl branch is less easily tolerated within the lamellar crystal.

How Does Short-Chain Branch Length Redistribute the Crystalline Network?

Copolymerization with an α-olefin inserts a side chain into the methylene sequence. A butene-derived ethyl branch creates a smaller free-volume perturbation than an octene-derived hexyl branch. During crystallization, ethyl branches may be partially accommodated at the lamellar fringe or deflected into the interlamellar amorphous layer, but the probability of chain expulsion from the crystal is lower per branch than for a hexyl branch. As a result, equal mole fractions of C4 and C8 do not produce the same density. Differential scanning calorimetry by ASTM D3418-21 shows a larger depression of peak melting temperature for C8 than for C6 or C4 at equivalent comonomer incorporation. The broader melting endotherm of C8 grades corresponds to a wider distribution of lamellar thicknesses and a greater concentration of tie chains crossing interlamellar amorphous zones. Tie chains are load-transferring segments that bridge adjacent crystallites; their density controls dart impact and puncture resistance more than crystallinity alone. In C4-rich LLDPE, a fraction of short-chain branches may concentrate in low-molecular-weight chains, especially with conventional Ziegler-Natta catalysts, reducing the tie-chain contribution of those chains and leaving the resin with a relatively coarse crystalline network. Single-site C8-LLDPE typically possesses a more uniform intermolecular and intramolecular comonomer distribution, which increases the number of stress-dissipating tie molecules at a given density.

In fluidized-bed gas-phase reactors, butene is favored because the comonomer is gaseous at reactor conditions and can be recycled without condensation. Solution-process lines in a continuous stirred-tank or loop configuration tolerate hexene and octene more readily because higher-boiling comonomers remain soluble in the hydrocarbon solvent. In solution processes, high reactor temperatures of 140–180 °C reduce viscosity but require a catalyst system with high thermal stability. Octene incorporation in a gas-phase reactor can cause sticky-particle agglomeration if the resin bed approaches its sintering point; this operational failure mode is controlled by maintaining the reactor dew point and bed temperature margin more than 5–10 °C below the resin melting point. On a 60 mm grooved-feed single-screw extruder with L/D 30 and a 2.0 mm die gap, a 1.0 g/10 min C8-LLDPE at 0.916 g/cm³ frequently exhibits 8–12% lower specific energy input than a C4 grade at the same melt index because the longer branch length reduces shear viscosity. However, the same C8 grade may show slower screw recovery in injection molding if the feed zone is not designed for low-bulk-density pellets; a feed-throat temperature below 40 °C and a rear barrel temperature above 160 °C prevent pellet deformation and bridging. Shared silos between C4 and C8 grades are not recommended; cross-contamination at levels above 5 wt% can shift dart impact and tear anisotropy in blown film without changing the nominal density by more than 0.001 g/cm³. Bulk pellet drying is unnecessary unless surface moisture exceeds 0.05 wt%, but storage at relative humidity above 60% can introduce bubble chatter when condensed moisture reaches the feed throat.

When C8 Concentration Exceeds 7.5 mol% in Blown Film, Stiffness Collapses but Optics Improve

In blown film, increasing C8 content lowers density more efficiently than C4 on a molar basis. A C8 grade can reach 0.902 g/cm³ while retaining measurable crystallinity; a comparable C4 grade often plateaus above 0.912 g/cm³ unless additional α-olefin is used. The stiffness penalty arises from lower crystalline fraction and from thinner lamellae. Film stiffness measured as 1% secant modulus by ASTM D882-18 drops by approximately 40–60% when density is reduced from 0.918 g/cm³ to 0.902 g/cm³; this is tolerable in stretch hoods and retail packaging but inadequate for a stand-up pouch side wall. At the same time, total haze measured by ASTM D1003-21 falls below 5% for a 25 µm C8 film when surface haze is controlled; C4 films of identical density and gauge commonly show 9–14% total haze. The optical improvement is not merely a density effect. The longer hexyl branch disrupts lamellar organization more effectively per mole, producing smaller spherulitic domains and a narrower refractive-index contrast between crystalline and amorphous regions. Processing determines whether this intrinsic optical advantage is realized. A die gap of 1.8–2.4 mm, melt temperature in the range 205–225 °C, and a frost-line height of 2–4 die diameters are necessary to suppress surface haze and melt fracture for C8-rich films. Bubble stability at 2.5:1 blow-up ratio is usually better than C4 because the C8 resin has a broader stretching plateau; however, if the air ring is set at aggressive exhaust velocities, the bubble can enter a metastable breathing mode. This is a processing constraint rather than a material deficiency.

Dart Impact, Elmendorf Tear, and the Tie-Molecule Argument

Comparative film data are usually generated at equal gauge and equal density to remove the crystallinity variable. A 25 µm blown film produced from 1.0 g/10 min LLDPE at 0.918 g/cm³ can serve as a reference. Dart drop impact by ASTM D1709-15A Condition B typically places C8 at the upper end, C6 in the middle, and C4 at the lower end. The failure mode in C4 is often a straight-line tear along the machine direction, indicating fewer tie molecules and anisotropic stress dissipation. Elmendorf tear resistance by ASTM D1922-15 is also sensitive to branch length, but the tear anisotropy is not identical: C4 often displays higher machine-direction tear than transverse-direction tear, while C8 can be balanced in both directions when the film is fabricated at a low stalk height. Puncture energy by ASTM D5748-19 follows a trend similar to dart impact. The underlying structural explanation is that longer short-chain branches increase the number of elastically effective tie chains at constant density. The C6 grade is not simply a midpoint; hexene compresses the property difference between C4 and C8 at a lower cost position while still showing lower seal initiation than C4. For applications where dart impact must exceed 200 g at 25 µm without blending, C6 or C8 grades are selected over C4 unless coextrusion with higher-modulus layers is available.

Optical quality is assessed as total haze and clarity separately. Haze measures wide-angle scattering; clarity measures narrow-angle scattering. A C8 film can have low haze but still poor clarity if surface defects from a damaged die lip are present. In laboratory comparisons, internal haze is isolated by coating the film with a non-scattering wetting agent to remove surface roughness. The difference between internal haze and total haze becomes relevant when a C4 grade with high crystalline scattering is run at high melt temperature; the internal haze remains high even if the surface is smooth. Conversely, a C8 grade run through a dirty die or with inadequate purge can exhibit high total haze despite low intrinsic internal haze. Gloss, measured at 45° by ASTM D2457-13, correlates inversely with surface micro-roughness and is not a direct measure of crystalline clarity. Therefore, the optical comparison between C4, C6 and C8 should always specify whether the haze value is total or internal, and whether the film was made under controlled die and nip conditions.

What Limits Heat Seal Initiation in C4-Rich LLDPE?

Seal initiation temperature is connected to comonomer type through lowering of the crystalline melting distribution. C8-LLDPE seals at lower temperatures and maintains a broader hot-tack plateau than C4-LLDPE at comparable density. Seal initiation by ASTM F1921-12 or hot-tack by ASTM F2029-16 is often 10–30 °C lower for C8 relative to C4 at equal density. This difference is a direct consequence of smaller and more defective crystallites, which begin to melt and allow interdiffusion at lower temperatures. In high-speed form-fill-seal lines, hot-tack strength must exceed 100 g/15 mm within 150–250 ms of sealing to survive product drop; C8 grades can meet this criterion over a wider temperature window than C4 or C6. However, the lower seal initiation of C8 is accompanied by a reduction in grease resistance because the amorphous fraction is higher. A multilayer film may pair a C4 or HDPE layer for stiffness with a C8 sealing layer to balance seal performance and mechanical load-bearing. This application-specific split is common in retortable lidding and snack packaging; the C4 layer functions as the structural web, while the C8 layer provides the low-temperature seal. Selection should be validated on the actual packaging line using a sealant bar at 120–140 °C and 0.2–0.4 MPa jaw pressure for a dwell of 0.3–0.5 s, because laboratory seal curves do not reproduce the thermal inertia of commercial jaw assemblies.