Linear low-density polyethylene (LLDPE) is a copolymer family in which ethylene and an alpha-olefin comonomer are polymerized at levels that reduce solid-state density to 0.900–0.940 g/cm³. The comonomer—1-butene, 1-hexene, or 1-octene—becomes a short-chain branch after insertion. 1-Butene contributes an ethyl branch; 1-hexene contributes an n-butyl branch; 1-octene contributes an n-hexyl branch. These branch lengths change tie-molecule probability, lamellar thickness distribution, melting point, seal initiation, dart impact, and tear resistance. Spotting LLDPE comonomer families hidden in grade codes is necessary because the alpha-olefin is often not printed as an explicit C4, C6, or C8 field.

In polyolefin commerce, a grade code may begin with a density field, a melt-flow field, and a suffix that appears to the eye as an additive or catalyst marker. Some licensors encode comonomer identity in a letter position, such as B for 1-butene, H for 1-hexene, and O for 1-octene, but this pattern is not standardized under ISO 1872-1:2018 or a single ASTM classification system. The same letter may refer to a film additive package, a clarity package, or a production train in a different manufacturer’s naming system. A grade code alone therefore identifies the comonomer only when the supplier’s technical-data-sheet footnote or lot certificate confirms the alpha-olefin. The most reliable source remains the certificate of analysis, which lists the actual comonomer or the carbon-13 NMR branch concentration measured by ASTM D5017-13.

Can a Grade Code’s Alpha Suffix Mark Butene, Hexene, and Octene Across Gas-Phase and Solution Platforms?

Not consistently. Gas-phase fluidized-bed reactor suites such as Unipol and Innovene typically produce butene-based and hexene-based LLDPE because the alpha-olefin vapor pressure must remain compatible with reactor heat removal and bed density control. A gas-phase grade with a B suffix is often, but not always, a 1-butene copolymer; a gas-phase grade with an H suffix is commonly a 1-hexene copolymer. Solution-process platforms such as Dowlex and Sclairtech can incorporate 1-octene without the same dew-point limitations, and many high-dart film grades from solution trains are octene copolymers even when no O appears in the grade code. In metallocene-catalyzed families, the alpha-olefin may be omitted from the grade code entirely because the catalyst capability and reactor platform are the primary product descriptors. A converter using a metallocene hexene grade may see only a density field and a melt-flow field, with comonomer verification placed in the product specification rather than the resin name.

Absent an explicit comonomer field, a converter can infer branch identity from a cluster of measured properties that respond independently to branch length. At constant density and melt index measured by ASTM D1238 at 190 °C and 2.16 kg, moving from C4 butene to C6 hexene to C8 octene lowers the peak melting temperature by roughly 2–4 °C per increment, raises dart impact and Elmendorf tear, and shifts seal initiation downward. Differential scanning calorimetry according to ASTM D3418 is a rapid screening tool: a 0.918 g/cm³, 1.0 g/10 min film grade commonly shows a peak melting endotherm near 121–124 °C for butene, 118–121 °C for hexene, and 116–119 °C for octene. However, melt point alone cannot distinguish comonomer lengths if density varies by more than ±0.002 g/cm³ or if nucleating packages are present. Definitive identification requires ASTM D5017-13 carbon-13 NMR, which quantifies short-chain branch frequency and branch length in ethylene copolymers.

When lot-to-lot comparisons are made on the same blown film line, a shift in dart impact from 110 g to 210 g at 25 µm film thickness without any change in density or melt index is a strong practical signal that the hidden comonomer has changed from butene to hexene or octene. The same inference applies when Elmendorf tear in the transverse direction rises above 450 g while haze remains below 10%. These numerical envelopes are influenced by die gap, frost line height, blow-up ratio, and slip/antiblock additives, so they should be applied only within a fixed film structure and fixed process window.

Blown Film Puncture and Tear Response Shifts With Branch Length

The mechanical separation between butene, hexene, and octene LLDPE film grades becomes measurable when a blown film line is operated at a 2.3 mm die gap, 2.5:1 blow-up ratio, and a frost line height of 250 mm. At 25 µm film thickness and 0.8–1.2 g/10 min melt index, the dart impact and tear values in the table below are typical product-datasheet envelopes for non-oriented film without high loading of slip or anti-block. Variation from these ranges can exceed 20% when additives such as erucamide or silica are changed. The table is not a specification; it is a comparative screening guide for lot-to-lot investigation.

On production lines, the property differences also appear as process behavior. A 60 mm single-screw extruder with L/D 30:1 running at 90 rpm may require a die head temperature of 210–220 °C for butene LLDPE, while a hexene metallocene of equivalent MI can require 215–225 °C to control melt pressure because of its narrower molecular-weight distribution. If a processor sees an unexplained motor-load increase after a grade-code change that leaves density and melt flow unchanged, an undisclosed shift from C4 to C6 or C8 is one possible cause. Purging with an HDPE transition resin at 180–220 °C before shutdown reduces carryover of unmelts and additive residues when changing comonomer families.

Comonomer identity also interacts with molecular architecture. A Ziegler-Natta C4 grade can have a high-density crystalline fraction that raises modulus and heat-seal temperature, while a metallocene C6 grade at the same density has lower crystalline thickness and a more uniform junction distribution. High-temperature gel permeation chromatography by ASTM D6474-20 frequently shows polydispersity index near 3.0–4.5 for Ziegler-Natta LLDPE and below 2.5 for metallocene LLDPE. The comonomer marker in the grade code does not convey this architecture variable, and it is this variable that often determines film haze, hot tack, and bubble stability. A converter should therefore read the hidden comonomer signal in combination with the supplier’s rheology data, such as melt flow ratio I21/I2 under ASTM D1238 at 190 °C. Ziegler-Natta C4 film grades often exhibit I21/I2 of 24–30; metallocene C6 film grades can show 16–22 at similar MI. This difference is an indirect clue that the resin is not a conventional butene gas-phase grade.

When a Grade Code Omits the Comonomer Marker, Melt Flow Ratio and Carbon-13 NMR Close the Gap

Carbon-13 NMR by ASTM D5017-13 resolves the methine and branch resonances; butene branch ends produce resonances assigned to two-carbon branches, hexene branch ends produce four-carbon branches, and octene branch ends produce six-carbon branches. Whole-polymer integration is calibrated against model copolymers. Routine lot release may instead use FTIR according to ASTM D5576, which provides total methyl content but not branch length. Therefore a supplier’s stated butene or hexene may originate from NMR, while an FTIR-only result can be blind to a mixture of comonomers. When a converter encounters a grade code with no obvious comonomer marker and needs to resolve a film property drift, ASTM D5017-13 is the most direct structural method. Density, melt index, and melt flow ratio provide only indirect inference.

Extrusion Coating and Food-Contact Certification Boundaries for C4, C6, and C8 Resins

Extrusion coating and heat-seal applications illustrate why hidden comonomer families cannot be evaluated by density and melt index alone. In extrusion coating, a butene LLDPE with melt index 4–8 g/10 min and density 0.924–0.930 g/cm³ is often blended with LDPE at 20–30% to reduce neck-in and draw resonance on coating lines running above 80 m/min. An octene LLDPE at equivalent melt index and density may exhibit a lower peak melting point by 4–6 °C, which can reduce seal initiation but also narrow the high-speed coating window if die temperature is not corrected. If the grade code does not identify the comonomer, the converter should request the supplier’s seal initiation measurement made on the actual film structure, for example 0.5 MPa jaw pressure and 1 s dwell, or an equivalent seal-strength test under ASTM F2029. The density field alone will not reveal the comonomer because a butene grade at 0.926 g/cm³ and an octene grade at the same density can differ in seal initiation by 5–8 °C.

Food-contact converters often encounter LLDPE grade codes in which the comonomer is not visible. In the United States, olefin polymers are regulated under FDA 21 CFR 177.1520; the permitted uses and temperature ceilings are established by the polymer type and condition of use, not by grade-code suffix. Under 21 CFR 177.1520(c) 3.1b, a polyethylene may be used in contact with aqueous, acidic, and low-alcohol foods up to certain temperatures; fatty-food conditions require higher molecular weight or lower extractable limits. In the European Union, plastics in food contact are regulated under Regulation (EU) No 10/2011; LLDPE with authorized monomers and an overall migration limit of 10 mg/dm² may comply, but the declaration of compliance must list the actual grade and manufacturing specifications. The grade code is not a regulatory certificate. A polymer with a hidden comonomer marker must be confirmed by the supplier’s certification for each production lot.