Tricolene LLDPE LLB1918 is a butene-comonomer linear low-density polyethylene manufactured by Haldia Petrochemicals Limited and used predominantly in blown-film packaging. The grade is defined by two primary specification limits: a melt mass-flow rate of 1.9 g/10 min measured at 190 °C under 2.16 kg load in accordance with ISO 1133-1:2022 Procedure A, and a nominal density of 0.918 g/cm³ determined by ISO 1183-1:2019 Method D. This combination places LLB1918 in the medium-slip, medium-impact segment of butene LLDPE film grades, distinct from fractional-melt high-strength grades used for geomembranes and from high-flow injection-molding grades used for caps and closures. The numeric suffix is significant: 19 encodes the nominal melt flow rate, while 18 encodes the nominal density. Because the density is controlled below 0.920 g/cm³, the resin retains sufficient amorphous tie-chain concentration to resist dart impact in thin films, while the melt index is high enough to permit economic output on conventional single-screw extruders without exceeding the torque limit of 45 mm to 75 mm drive systems.

What Does the Grade Designation LLB1918 Convey?

The comonomer is butene, the shortest conventional alpha-olefin used in commercial LLDPE. Incorporation of butene creates ethyl short-chain branches along the otherwise linear polyethylene backbone. These branches disrupt crystallinity and reduce density, but their short length is less effective at increasing tie-chain density than the hexyl branches derived from octene. Consequently, LLB1918 occupies a position defined by adequate impact strength and easier drawdown, rather than maximum low-temperature puncture resistance. The melt flow rate of 1.9 g/10 min is selected for blown-film lines running at 40 kg/h to 120 kg/h on 45 mm to 75 mm extruders. At these throughputs, the die pressure remains below the design limit of most spiral mandrel dies while the melt viscosity is still high enough to form a stable bubble at blow-up ratios of 2.0:1 to 2.8:1. The density of 0.918 g/cm³ supports dart impact resistance in heavy-duty sacks, but the absence of long-chain branching means the bubble does not self-stabilize as effectively as high-pressure LDPE.

Rheologically, LLB1918 displays shear-thinning behavior characteristic of broad-molecular-weight-distribution Ziegler-Natta LLDPE. Capillary rheometry at 190 °C across apparent shear rates from 100 s⁻¹ to 3000 s⁻¹ shows a decreasing viscosity curve with a power-law index below unity. This shear sensitivity reduces apparent viscosity inside the die and motor load at high screw speed, but the extensional viscosity is not reinforced by long-chain branching. The practical consequence is a narrow operating envelope in high-stalk bubble configurations. On a 45 mm single-screw extruder with 30:1 L/D, a barrier screw, and a 150 mm spiral mandrel die, melt pressure at 60 kg/h typically falls between 250 bar and 320 bar when the die gap is 1.5 mm. If the same die is used with a 0.8 mm die gap, the higher shear heating can raise melt temperature by 3 °C to 7 °C; this reduces die pressure but may shift bubble stability. Published data for the exact die pressure curve of this specific grade is limited, so converters should record start-up values against their own screw and die combination.

Size-exclusion chromatography of Ziegler-Natta LLDPE of this type typically shows a polydispersity index of approximately 4 to 6. That molecular weight distribution contributes to shear thinning and die pressure reduction but also creates a low-molecular-mass fraction that must be considered in direct food contact applications. The broader distribution increases melt strength relative to metallocene grades of equivalent melt index, yet it can reduce film clarity if the high-molecular-mass tail is not adequately homogenized.

Extrusion Melt Temperature Profiles and Die Pressure Windows

The recommended set-point profile for a barrier screw is feed zone 140 °C, compression zone 170 °C, metering zone 190 °C, adapter 210 °C, and die zones 215 °C. The melt temperature measured at the adapter should be maintained between 195 °C and 215 °C. At melt temperatures below 185 °C, the high-molecular-mass fraction may not fully plasticize, resulting in screw bounce, pressure variation, and visible flow lines. At melt temperatures above 225 °C, thermo-oxidative degradation accelerates; gel counts measured per ISO 18553 can increase, and dart impact decreases because chain scission reduces the high-molecular-mass tail responsible for impact resistance. These limits are more severe when recycled edge trim is introduced. Regrind of LLB1918 can be added up to 20 wt% without significant loss of optical uniformity if the regrind is dry and free of polypropylene contamination. Above 30 wt% regrind, converters often observe greater melt pressure fluctuation and an increase in fish-eye defects.

Film made from LLB1918 is normally evaluated at a reference gauge of 40 µm or 50 µm because the mechanical values shift with orientation and cooling rate. Blown-film tension in the machine direction increases with haul-off speed and frost line height, while transverse orientation increases with blow-up ratio. The film is tested for dart impact using ISO 7765-1 Method A or ASTM D1709 Method A, for Elmendorf tear using ISO 6383-2, and for tensile properties using ISO 527-3. The exact values depend on die gap, blow-up ratio, and cooling air temperature. A narrow die gap and high blow-up ratio increase transverse orientation and may raise tear resistance in the transverse direction while lowering machine-direction tear. This anisotropy is not a defect unless the end product requires balanced tear properties. Converters producing heavy-duty sacks often report that a blow-up ratio of 2.2:1 to 2.6:1 gives the best trade-off between bubble stability and dart impact.

The frost line height should be maintained at 2D to 5D of the die diameter. At low frost line heights, film clarity increases because the crystalline morphology is quenched, but the film tends to block if the air ring temperature is below 10 °C. At high frost line heights, melt relaxation improves mechanical properties but bubble stability falls because the molten region is longer. For LLB1918, a moderate frost line height of 3D to 4D is normally selected.

For heavy-duty sack lines, LLB1918 is processed at gauges between 50 µm and 120 µm. The film is converted on lines with collapsing frames and oscillating haul-offs. Bubble stability becomes critical at bubble diameters above 500 mm; at these dimensions, even small changes in ambient air speed can create gauge bands. Converters often blend 15 wt% to 25 wt% high-pressure LDPE with a melt flow rate of 0.3 g/10 min to 0.8 g/10 min to raise melt tension. The blend reduces the elasticity of the linear fraction but increases the maximum stable bubble diameter. In lamination films, the surface is corona treated to a minimum wetting tension of 38 mN/m and then bonded with solvent-based or solventless adhesives. Adhesion is evaluated according to ASTM F904 or ISO 11339. The seal initiation temperature for butene LLDPE of this density class is normally seen between 90 °C and 100 °C at 0.5 s dwell and 2 bar seal bar pressure. Because seal geometry and pressure distribution vary by line, the value must be confirmed on production sealing equipment.

In liquid pouch overwrap, the film is tested for hot tack using ASTM F1921 and for seal strength using ASTM F2029. Because LLB1918 is a butene grade, its hot tack window may be narrower than metallocene grades; this is relevant in high-speed vertical form-fill-seal lines where sealing pressure dwell is below 0.2 s. Published data for this specific configuration is limited, and packers should qualify the grade on their own fill-seal equipment with the actual filling temperature and product head-space.

When Downgauging or High Blow-Up Ratio Shifts the Processing Envelope

At gauges below 25 µm, LLB1918 may not generate enough melt tension to resist low-frequency oscillation of the bubble neck. The failure appears as lay-flat width variation greater than ±5% and is often misdiagnosed as die lip contamination. In this regime, the grade should not be processed at blow-up ratios above 2.8:1 unless it is blended with high-pressure LDPE. The addition of 10–30 wt% LDPE with a melt index of 0.3 g/10 min to 0.8 g/10 min increases extensional viscosity and reduces neck oscillation. The resulting film is tested for dart impact using ISO 7765-1 and for tear using ISO 6383-2. If the converter attempts to correct instability solely by increasing melt temperature, the bubble becomes more fluid and the oscillation typically worsens. The more effective correction is to reduce the frost line height or to introduce a dual-lip air ring that stabilizes the bubble without increasing melt temperature.

The Product Is Not a Direct Substitute for Octene LLDPE in High-Stretch Applications

Compared with an octene-based LLDPE of the same melt index and density, LLB1918 is expected to show lower dart impact and lower puncture resistance at equivalent gauge because the short ethyl branches from butene are less effective than hexyl branches from octene in forming tie-chain density. The difference is most relevant in frozen-food packaging, stretch hood film, and agricultural film where impact loads occur at low temperature or high strain rate. However, butene copolymers of this density class often provide easier extrusion and lower haze than octene grades, because the shorter branch length produces smaller crystalline domains and a less diffuse interlamellar amorphous layer. Against high-pressure LDPE of similar melt index, LLB1918 has higher tensile strength and higher seal strength but lower melt tension and narrower bubble stability. This means LLB1918 is not a drop-in replacement for LDPE in coating, shrink, or protective film lines where bubble chatter is the limiting factor. The choice between LLB1918 and an octene grade should be made using ISO 7765-1, ISO 527-3, and ASTM D1003 on the same film line, not by comonomer ranking alone. Published data for this specific configuration is limited; the rankings described here reflect general butene-versus-octene differences reported in the published literature and require lot-specific confirmation.

Against metallocene LLDPE of the same melt index, LLB1918 is more shear-thinning because of its broader molecular weight distribution. This reduces die pressure at high screw speed and permits less aggressive barrel-temperature profiles, but it also produces a higher level of low-molecular-mass species that can migrate into fatty food simulants if the film is used in direct food contact. The metallocene alternative typically provides better hot tack and lower extractables at equivalent density, but may require higher die pressure and more precise temperature control. Against HDPE film grades, LLB1918 has much lower modulus and higher dart impact, but the seal initiation temperature is lower and the film is easier to weld on common impulse sealers. The choice of LLB1918 over HDPE is therefore made when flexibility and seal strength dominate, not when stiffness or moisture barrier is the primary requirement.

LLB1918 does not require routine pre-drying at ambient relative humidity below 60%. However, storage in unheated silos or jumbo bags during monsoon conditions can cause surface condensation. In those cases, the pellets should be dried at 70 °C for 2 h with a desiccant dryer set to −40 °C dew point. The grade is not supplied with an ultraviolet stabilizer package. Outdoor service requires addition of a hindered amine light stabilizer masterbatch at 2–3 wt%, with weathering performance validated under ISO 4892-2 or ASTM D4329. Cross-contamination with polypropylene must be controlled below 0.5 wt% because polypropylene domains do not disperse in the LLDPE melt and create fish-eye defects and seal failures. Food-contact compliance is not a property of the base resin alone; the final packaging article must be tested for specific migration under EU Regulation 10/2011 with the actual pigment masterbatch, adhesive, and ink system. The following table summarizes the compliance and test-method matrix applied to this grade in blown-film conversion.

On production-scale blown-film lines, the most frequently recorded defect with LLB1918 is bubble instability at blow-up ratios above 2.8:1, not melt fracture. The defect is mechanical in origin: the low extensional viscosity of the linear architecture cannot damp air-ring turbulence. Routine use of a dual-lip air ring or a ceramic-coated die lip reduces the frequency of these events. LLB1918 is therefore assigned to a narrow process envelope: sufficient melt flow for high output on standard extruders, sufficient density for impact strength in heavy-duty sacks, and predictable behavior when blended with high-pressure LDPE. Converters must request the current product data sheet, certificate of analysis, and any updated additive declaration before setting lot-specific limits, since published data for this specific configuration is limited to the primary melt-flow and density anchor points.