LLDPE Comonomers: C4 vs C6 vs C8 and What They Do to Film

Linear low-density polyethylene is not one material — it is a family defined by what you copolymerise the ethylene with. The backbone is the same; the comonomer is an alpha-olefin that grafts short side branches onto that backbone, and the identity of that comonomer — 1-butene (C4), 1-hexene (C6), or 1-octene (C8) — does more to determine how a film performs than either of the two numbers buyers usually fixate on.

Those two numbers are melt flow index and density. Both matter, but neither captures the comonomer. A buyer who specifies only "1.0 MFI, 0.918 density" can receive a C4 grade or a C8 grade and watch the same bag spec pass on one and split on the other. Understanding why starts with what the comonomer actually builds into the chain. For the broader PE landscape, see our guide to HDPE, LDPE and LLDPE grades.

Short-chain branching: the real variable#

When ethylene polymerises alone you get linear HDPE — long, regular chains that pack into dense crystals. Introduce an alpha-olefin comonomer and each insertion leaves a short branch dangling off the backbone. Butene leaves a two-carbon ethyl branch, hexene a four-carbon butyl branch, octene a six-carbon hexyl branch. This is short-chain branching (SCB), and it is the mechanism behind everything LLDPE does.

SCB disrupts crystallisation. The branches cannot fit into the crystal lattice, so they sit in the amorphous regions between lamellae, lowering density into the linear-low range (commonly around 0.915–0.925 g/cm3) and — critically — forming tie molecules. A tie molecule is a single chain that threads through one crystal, crosses an amorphous zone, and re-enters a second crystal, physically stitching the structure together. Tie molecules carry impact and tear loads. The more effective they are, the tougher the film.

Branch length is what makes a tie molecule effective. A longer side branch — hexyl from C8, butyl from C6 — anchors the amorphous entanglements more securely than a short ethyl branch from C4, so a given chain is more likely to act as a load-bearing tie rather than slipping under stress. That single structural fact drives the whole performance ladder below.

C4 vs C6 vs C8 — the performance ladder#

At matched MFI and density, toughness rises with comonomer carbon number. The gains are real and large — dart impact between a C4 and a C8 grade of the same headline spec routinely differs by a wide margin. Tear strength, puncture resistance, and low-temperature toughness follow the same order. The table sets out the practical picture; treat the property directions as robust and the magnitudes as indicative.

This is also why catalyst matters alongside comonomer. Metallocene LLDPE (mLLDPE) is not a comonomer — most mLLDPE is still C6 or C8 — but the metallocene catalyst distributes the short-chain branches far more uniformly across the molecular-weight distribution than conventional Ziegler-Natta chemistry, where the heaviest chains tend to carry the fewest branches. The result is higher toughness, better clarity, and tighter seal-initiation temperatures from the same comonomer. Specify the catalyst class and the comonomer together — they are independent levers, not substitutes.

Why MFI and density don't tell you enough#

Melt flow index (ASTM D1238 / ISO 1133, the 190 °C / 2.16 kg condition for PE) is an inverse proxy for average molecular weight — it tells you how the melt will process, not how the solid film will perform. Density (ISO 1183) tracks average crystallinity. Neither value encodes which comonomer was used or how the branches are spread along and across the chains. Two grades can land on the same MFI and the same density by entirely different structural routes. For the processing side of that number, see our explainer on melt flow index.

MFI tells you how it runs on the line; density tells you how stiff it is. The comonomer tells you whether the bag survives the drop test.

The practical consequence: a specification that omits comonomer type is incomplete. When the application is toughness-critical — heavy-duty shipping sacks, pallet stretch film, frozen-food packaging, pond and pit liners — the comonomer line on the datasheet is the line that decides pass or fail. A clear request-for-quote should name the comonomer (or at minimum a target dart impact) rather than leaning on MFI and density alone.

Cost vs performance — buying the right grade#

Comonomer pricing tracks the alpha-olefin feedstock. 1-butene is the cheapest and most widely available; 1-hexene and especially 1-octene carry premiums that flow through to resin price, so C8 LLDPE typically sits at the top of the LLDPE price stack and C4 at the bottom. The premium is only worth paying where toughness is the binding constraint.

• —Downgauging: if C6/C8 toughness lets you cut film thickness while holding performance, the resin premium can pay for itself in net cost per usable square metre.
• —Failure-critical applications: heavy-duty sacks, FFS film, and high-cling stretch wrap often simply will not hold spec on C4 — here C6/C8 is a requirement, not an upgrade.
• —General-purpose work: liners, lamination plies, and light-duty bags rarely justify a C8 premium; C4 or a C4/C6 blend is the correct commercial answer.
• —Recyclate streams: post-consumer LLDPE rarely declares its comonomer, so blend toughness is variable — see our recycled polymers buyer's guide before substituting recyclate into a toughness-critical film.

The discipline is matching comonomer to mechanical demand, not defaulting to the cheapest grade or reflexively buying the toughest. Specify comonomer type, MFI, density, and a target dart impact together — then a quote means something. The OmniaStrata desk can map a target film spec onto the right C4/C6/C8 grade and origin; start with our polyethylene sourcing service or talk to the desk with your dart, tear, and gauge targets in hand.