ExxonMobil mLLDPE C4LL 50026XR is a metallocene-catalysed ethylene-butene linear low-density polyethylene supplied as free-flowing pellets for cast film, extrusion coating, and thin-gauge blown film conversion. The XR suffix is a supplier-defined variant identifier; the exact additive or rheology modification associated with the suffix is disclosed on the supplier technical data sheet and should not be inferred from the base resin family. The polymer architecture is defined by a narrow molecular-weight distribution and a uniform short-chain branching distribution, which reduces the high-molecular-weight tail responsible for melt fracture in conventional Ziegler-Natta C4-LLDPE. Typical lot-release values are measured according to ASTM D792 for density and ASTM D1238 for melt flow rate at 190°C/2.16 kg; the grade code indicates a nominal density around 0.926 g/cm³ and a nominal melt flow rate around 5.0 g/10 min. Converters should nevertheless use the certificate of analysis for lot-specific values because deviations can affect gauge uniformity and melt pressure. The product is designed to compete with C6 and C8 mLLDPE grades in applications where stiffness, optical clarity, and line speed are valued simultaneously, but the shorter 1-butene branch requires tighter control of melt temperature and output when compared with higher alpha-olefin copolymers.

What separates a butene metallocene grade from hexene and octene metallocene alternatives?

At equivalent density and melt flow rate, 1-butene comonomer introduces shorter branches than 1-hexene or 1-octene. Shorter branches result in a more regular lamellar structure, which typically increases secant modulus when measured by ISO 527-3 or ASTM D882 while reducing Elmendorf tear and dart impact resistance when compared with a C6 or C8 mLLDPE at the same film gauge. The difference is not linear; it becomes more pronounced in film below 25 µm. Seal initiation temperature is generally higher than a C6 or C8 mLLDPE at equivalent density because butene branches are less disruptive to the crystal lattice, shifting the sealing window upward by approximately 5–10°C. This shift can be beneficial for boil-in-bag structures but may limit applications requiring ultra-low-temperature seals. In cast stretch film, the lower melt elasticity of the C4LL grade can reduce neck-in at the die and improve air-knife-assisted edge stability. A production-scale cast line running 0.020 mm gauge at 400 m/min may exhibit lower web flutter than an equivalent C6 grade, but the magnitude is equipment-dependent; published data for this specific configuration is limited.

On a 90 mm single-screw extruder with 30:1 L/D and a 200 mm cast film die, the resin is normally run with a reverse-temperature barrel profile from 180°C at the feed throat to 235°C at the adapter. Specific energy consumption is typically lower than a Ziegler-Natta C4 grade of the same melt index because the narrow molecular-weight distribution reduces viscous dissipation. Die pressure and motor amperage should be monitored at the beginning of the run; a gradual amperage increase may indicate gel accumulation on the screen pack, requiring a change from 100 mesh to 80/100/80 mesh breaker-plate screens. The resin is not hygroscopic; pre-drying is usually unnecessary when pellets are stored below 60% RH. If condensation is visible, a desiccant hopper dryer set to 60–80°C for 2–4 hours prevents surface moisture from destabilizing gauge at high line speeds.

For blown film, a die gap of 2.0–2.5 mm with a blow-up ratio between 2.0:1 and 3.0:1 and a frost line height of 500–700 mm is a common starting point. The lower melt strength of a butene mLLDPE compared with a C6 or C8 grade can reduce bubble stability at high blow-up ratio, so equipment with internal bubble cooling or an IBC control system improves gauge uniformity. Melt temperature should not exceed 280°C because oxidative degradation of the stabiliser package can increase odour and reduce adhesion in extrusion coating primers. For extrusion coating onto paperboard or oriented polypropylene, adhesion strength is measured by ASTM D1876 T-peel or ISO 8510-2. Avoid prolonged hold-up times at melt temperature because single-site resins can undergo chain scission; purging after shutdown with a 0.5–1.0 MI HDPE or purge compound of similar density reduces carbonization in the barrel.

Extrusion coating adhesion and barrier structure limitations

When the resin is used as a sealant layer in flexible packaging, peel strength to aluminium foil or metallized film should be tested after lamination according to ASTM D1876. Adhesion values below 2.0 N/15 mm may indicate insufficient surface oxidation or inadequate primer coverage on the foil, not a resin-related failure. The C4LL grade is not a high-barrier material by itself; oxygen transmission rate measured by ASTM D3985 at 23°C/0% RH will be several orders of magnitude higher than EVOH or PVdC, and converters should not rely on the polyethylene layer alone for aroma or oxygen barrier in modified-atmosphere packaging. The material can be coextruded with polyamide or EVOH tie layers, but contact with excessive moisture at processing temperatures above 250°C can hydrolyse EVOH in the adjacent layer and reduce interlayer adhesion; the line should be run with the EVOH layer protected by a continuous polyethylene skin.

Film made from C4LL 50026XR is typically evaluated for tensile properties under ASTM D882, tear resistance under ASTM D1922, dart drop impact under ASTM D1709 Method A, and percentage haze under ASTM D1003. In thin-gauge cast stretch film, the combination of low gel content and narrow composition distribution supports downgauging from 20 µm to 15 µm only where pallet-load retention force, measured by ASTM D4649 or a stretch-film test stand, remains above the minimum specified by the user. Published data for this specific configuration is limited; therefore final downgauging decisions require pilot-line verification on the intended wrapper. For hygiene backsheet applications, the material must be tested for flexural modulus under ISO 178 and puncture resistance under ISO 6603-2, because thin-gauge handling can shift failure mode from ductile to brittle at high extension rates. The coefficient of friction of the film should be checked under ASTM D1894 if the pellets have been stored for more than 30 days, because surface slip additives may migrate or bloom unevenly under fluctuating warehouse temperatures.

When to specify C4LL 50026XR instead of a C6 or C8 metallocene resin

Selection of this grade is justified when the converter values high line speed, low melt pressure, and adequate mechanical toughness in non-abusive applications. In contrast, where high dart impact and Elmendorf tear in thinner films are load-bearing requirements, a C6 or C8 mLLDPE is likely required because the longer branch length increases tie-chain probability. The decision should be based on normalized data at the same thickness, not on nominal density and melt flow rate alone. Because butene comonomer is less efficient at disrupting crystallinity, the C4LL grade often provides higher stiffness at comparable density. This stiffness is quantified as secant modulus under ASTM D882 and may be 10–20% higher than a C6 grade of similar melt flow rate, but the exact difference is lot-dependent; comparative values should be generated on the target line, and published data for this specific configuration is limited. The grade is also preferred where low gel count and optical haze are critical, because the single-site catalyst and C4 chemistry reduce high-molecular-weight fractions that can form fisheye gels. In extrusion coating, the lower neck-in and better draw-down stability can increase maximum line speed before edge-bead formation becomes unacceptable.

Food-contact suitability is generally assessed under FDA 21 CFR 177.1520 for olefin polymers, provided the final additive package and end-use conditions are covered by a specific letter of no objection or equivalency. For European Union applications, verification against Regulation (EU) No 10/2011, including overall migration limits of 10 mg/dm² under OM2 conditions for fatty foods, is required before use. RoHS Directive 2011/65/EU Annex II restrictions on lead, mercury, cadmium, hexavalent chromium, PBB, and PBDE must be confirmed through supplier documentation; no intentionally added heavy metals are declared for this grade.

Pellets should be stored in a closed, dry silo or hopper to prevent surface oxidation. High ambient temperatures above 50°C can lead to additive blooming; inventory rotation should follow first-in-first-out with maximum ambient storage below 30°C where possible. If pellet surface slip additive concentration changes due to long storage, coefficient of friction measured by ASTM D1894 may shift. The resin should not be blended with high-cobalt or manganese pro-oxidant masterbatches intended for oxo-degradable films unless the stabilizer package and final environmental claims are verified, because metal-catalysed oxidation can accelerate degradation and alter film mechanical properties under ASTM D882.