ExxonMobil mLLDPE C4LL 2002.4XR is a metallocene-catalysed linear low-density polyethylene in which butene is the short-chain branching comonomer. The manufacturer-published nominal density is 0.920 g/cm³ when tested under ISO 1183-1:2019 or ASTM D1505. The melt mass-flow rate is 2.0 g/10 min at 190°C under a 2.16 kg load, determined according to ISO 1133-1:2022 or ASTM D1238. The grade is classified as a film resin, not as a high-viscosity blow-moulding grade, and is supplied as pellets with a formulated additive package identified by the 4XR suffix. Published data defining the exact slip and antiblock concentration for this additive package is limited; the supplier’s certificate of analysis is the controlling document for lot-specific additive levels.

The metallocene catalyst yields a narrow molecular weight distribution and a more uniform distribution of butene branches across both low- and high-molecular-weight fractions than conventional Ziegler-Natta C4-LLDPE. The practical consequence is a low-melting, low-crystallinity architecture that supports seal initiation and hot-tack in thin films. The melting peak for a butene mLLDPE with a nominal density of 0.920 g/cm³ is generally located between 120°C and 124°C under ISO 11357-3. The absence of long-chain branching differentiates this resin from high-pressure LDPE; melt strength and elongational viscosity are consequently lower, and bubble stability must be managed with air ring configuration and, where necessary, LDPE blending.

Rheological characterisation under ISO 11443 capillary rheometry at 190°C shows that the absence of long-chain branching reduces low-shear viscosity relative to high-pressure LDPE, but the effect narrows at shear rates typical of film dies because of the comparatively low melt flow rate. Extruder pressure is usually lower than an LDPE of similar melt flow rate but higher than a high-melt-flow LLDPE. The pressure drop across a 20/40/60 mesh screen pack should be recorded at a fixed screw speed to detect gel or melt-temperature drift; an increasing pressure trend without a corresponding screw speed change can indicate melt filter blinding or additive buildup on the screen pack.

Because the resin is hydrophobic, moisture absorption is not a relevant degradation mechanism. The only moisture-related handling risk is surface condensation on cold pellets. If pellets are stored below 10°C and transferred into a humid converting area at relative humidity above 60%, conditioning at 20–25°C for 4–8 h before extrusion prevents surface moisture defects. This is not a drying operation in the sense required for hygroscopic polymers such as polyamide or EVOH.

How does C4LL 2002.4XR compare with conventional butene Ziegler-Natta LLDPE?

Conventional butene Ziegler-Natta LLDPE grades typically display a broad molecular weight distribution with an Mw/Mn ratio of 3.5–5.0, while metallocene C4 grades are typically characterised in the range 2.0–2.5. The high-molecular-weight tail in Ziegler-Natta resins contributes shear thinning and bubble stability, but it also carries a higher extractables fraction and a less uniform comonomer distribution. In C4LL 2002.4XR, the butene branches are more homogeneously placed; this reduces the population of highly branched, low-melting, highly soluble chains and narrows the melting range. The grade therefore tends to provide a lower seal initiation temperature than a conventional C4 Ziegler-Natta resin of equivalent density and melt flow rate when measured under ASTM F88 using a seal strength threshold such as 4.5 N/15 mm, a dwell time of 0.5 s, and a jaw pressure of 0.35 MPa. Published data for this specific configuration is limited, and line trials are required to establish the seal curve on the intended packaging machine.

In dart impact testing under ISO 7765-1 or ASTM D1709, the metallocene C4 grade is generally expected to show improved impact resistance at equivalent thickness relative to a conventional C4 Ziegler-Natta LLDPE because the homogeneous branching distribution allows more effective tie-chain formation during film cooling. However, the measurement is profoundly affected by blow-up ratio, frost line height, film thickness, and quench rate. A change in blow-up ratio from 2.0:1 to 3.0:1 alters molecular orientation and can change dart impact by more than 20%. Therefore, comparative claims must be generated on the same blown film line with the same die gap and output rate. Elmendorf tear under ISO 6383-2 often remains anisotropic; machine-direction tear can be lower than transverse-direction tear in oriented film, and this should be considered for heavy-duty bag and shrink film designs.

Hexane extractables, measured under ASTM D5227, are normally lower in a metallocene C4 resin than in a conventional Ziegler-Natta C4 product of equal density, because the homogeneous short-chain branching distribution reduces the highly soluble low-molecular-weight fraction. This has direct relevance for food-contact compliance under 21 CFR 177.1520(c) and EU Regulation (EU) No 10/2011. The final migration value is structure-specific and must be confirmed on the finished packaging article.

On a high-output blown film line with a 75 mm grooved-feed extruder, 30:1 L/D, and a chrome-plated spiral mandrel die, C4LL 2002.4XR processes with a die gap of 1.8–2.5 mm and a melt temperature of 190–230°C. Blow-up ratio is typically held between 2.0:1 and 2.8:1 because the low melt strength of the metallocene architecture makes ratios above 3.0:1 susceptible to bubble edge wander and gage variation unless internal bubble cooling and a dual-lip air ring are installed. Frost line height is a critical control variable; setting the frost line between 300 mm and 500 mm above the die balances machine-direction and transverse orientation. A frost line above 500 mm improves transverse orientation and can raise dart impact but increases the risk of bubble sag on older air rings, while a frost line below 300 mm raises machine-direction tear and can reduce overall film toughness.

Melt fracture is controlled by shear stress at the die lip rather than by melt temperature alone. Die gaps below 1.5 mm at high output can exceed the critical shear stress for sharkskin, producing a fine surface ridge pattern that is visible on the finished film. Fluoropolymer processing aids are commonly used at 200–800 ppm by weight to delay the onset of melt fracture; the additive must be dispersed during high-shear mixing to coat the die surface and is not a substitute for proper die temperature uniformity. If bubble stability is inadequate, converters commonly blend 10–30 wt% of a low-melt-index LDPE. This blend raises melt strength and permits higher blow-up ratios, but it also shifts seal initiation upward and may increase haze measured under ISO 14782.

In cast film, the grade is drawn onto chill rolls maintained at 15–30°C. The rapid quench suppresses spherulite growth and yields lower haze and higher dart impact than slower air-cooled blown film of the same gage. The narrow molecular weight distribution reduces draw resonance at high take-off speed, but web tension sensitivity is higher than LDPE because of lower melt elasticity. Edge pinning and air knife settings must be adjusted to control neck-in; excessive neck-in at the die reduces usable web width and increases edge trim.

When low seal initiation and high hot-tack define a coextruded sealant layer

When C4LL 2002.4XR is positioned as the sealant web of a coextruded flexible packaging structure on vertical or horizontal form-fill-seal equipment, the controlling quality parameters are hot-tack and cool-seal development rather than tensile yield. Hot-tack force under ASTM F1921 quantifies the still-molten seal’s resistance to pouch bursting before crystallisation. The narrow melting distribution of the metallocene C4 resin gives a rapid solid-to-melt transition and rapid hot-tack onset, which is advantageous on lines running seal dwell times below 1.0 s. Seal strength after cooling is tested under ASTM F88. The seal window must be established on the actual packaging machine because jaw temperature, dwell time, pressure, film thickness, and corona treatment level interact in a non-linear manner.

In coextruded barrier structures, this grade is often used as the food-contact sealant layer outside an EVOH or polyamide core. The low melt elasticity of the metallocene C4 resin can reduce interfacial viscosity mismatch with polyamide, but die temperature uniformity remains critical because the polyolefin layer is usually processed at 210–230°C, whereas polyamide and EVOH have their own tightly specified melt temperatures. A thermally separated multicomponent die is required when the viscosity ratio is unfavourable; otherwise interfacial layer distortion can appear as wavy line patterns in the film.

Comparing this butene-based grade with hexene-based metallocene grades at equal density and melt flow rate, the difference in comonomer length changes the entanglement network. Hexene and octene branches are longer and more effective in bridging crystalline lamellae; thus, a C6 mLLDPE generally shows higher puncture resistance under ASTM D5748 or ISO 7765-1 at the same gage. The C4 grade may show lower seal initiation due to its lower melting peak, but the toughness deficit remains the decisive selection criterion in abuse-prone applications. Therefore, C4LL 2002.4XR is selected when seal performance and moderate toughness are sufficient; C6 and C8 mLLDPE grades are preferred when heavy-duty pouches, agricultural stretch films, or extreme puncture resistance are required. The product can replace a conventional butene Ziegler-Natta LLDPE on blown lines that have adequate air ring control, but if the incumbent resin relies on a broad molecular weight distribution for bubble stability, a formulation adjustment or LDPE addition may be required.

Food-contact compliance must be verified on the final article. Polyethylene grades are generally covered by 21 CFR 177.1520(c) for olefin polymers, while European compliance is evaluated under EU Regulation (EU) No 10/2011 with an overall migration limit of 10 mg/dm² using test methods described in EN 1186. The exact additive package, coextruded layer composition, and food type influence the migration result. The product is not intended for retort or high-temperature sterilisation above its melting region; those applications require polypropylene or higher-density polyethylene structures.

In printed or laminated applications, the polyethylene surface requires corona treatment. A surface energy above 38 mN/m is necessary for solvent-based lamination and above 40 mN/m for water-based inks. Polyethylene films lose treatment over time due to additive migration and chain reorientation; storage above 30°C can accelerate decay. Converters should verify surface energy immediately before printing or laminating rather than relying on a value recorded during film winding.