LDPE 2426H vs 2426K vs 2420H: Film Grade Selection for Extrusion Operations

Blown film converters comparing LDPE 2426H, 2426K, and 2420H are evaluating three low-density polyethylene grades that are frequently grouped on supplier shortlists because of overlapping density and melt flow rate ranges. The ordering decision is not resolved by a single viscosity value. It depends on the interaction between die gap, blow-up ratio, frost line geometry, downstream printing or lamination speed, and the additive package documented in the certificate of analysis. All three grades are normally supplied as general-purpose film extrusion resins with nominal density near 0.923 g/cm³, but their conversion behaviour differs under high-shear and elongational flow.

Why Do 2426H, 2426K, and 2420H Overlap in Melt Flow Rate but Diverge in Conversion Behaviour?

Published supplier technical bulletins place 2426H in the lower melt flow range, commonly reported as 1.8–2.0 g/10 min under ASTM D1238 at 190 °C/2.16 kg. 2420H is reported at 1.9–2.2 g/10 min, while 2426K is frequently listed at 2.0–2.4 g/10 min. The density ranges for the three grades overlap at 0.922–0.925 g/cm³ when measured by ASTM D1505 or ISO 1183-1:2019. These values place all three in the general-purpose film extrusion window, but the melt flow rate range is too broad to separate one grade from another without examining gel level, ash content, slip additive type, antiblock concentration, and molecular weight distribution.

The above ranges are not specification limits. They are consolidated from supplier technical bulletins for 25–50 µm blown film and are influenced by extrusion temperature, die gap, blow-up ratio, and additive loadings. Lot-to-lot variation can exceed the range width; the certificate of analysis remains the binding document for a given shipment.

On a 55 mm single-screw extruder with an L/D 30:1 barrier screw and a 0.8 mm die gap, 2426H typically produces higher backpressure and more stable melt temperatures at a given screw speed because of its lower melt flow rate and higher extensional viscosity. Operators record that bubble stability improves with 2426H under blow-up ratios of 2.5:1 to 3.0:1 and frost line heights between 6 and 8 die diameters. The higher melt strength resists sag and draw resonance in tubular film extrusion, which is beneficial when the film is run at 50–100 µm gauge or when downstream haul-off tensions fluctuate during printing or form-fill-seal conversion.

2426K, in contrast, shows lower extrusion head pressure and lower screw torque at identical temperature profiles. When run on the same die gap, converters commonly reduce barrel temperature set points by 3–5 °C in the metering zones to prevent melt fracture and to stabilise internal bubble cooling. The reduced viscous dissipation can permit 5–8% higher screw speed before the melt temperature limit is reached, but the same reduction in melt strength may require a lower blow-up ratio or a slightly higher frost line to prevent bubble flutter in thin gauge film below 25 µm.

Processing Window Constraints at 55 mm Blown Film Lines

Melt temperature measured at the die lip should be held within 180–200 °C for all three grades. For 2426H, the practical lower limit is governed by the onset of sharkskin melt fracture at the die exit; at melt temperatures below 175 °C and shear rates above 150 s⁻¹, surface defects can appear as regularly spaced transverse ridges. The upper limit is not determined by thermal degradation alone. It is determined by slip additive volatilisation and plate-out on the die lip, which can occur above 205 °C in continuous runs exceeding 8–10 h. External pre-drying is not required for these LDPE film grades under normal warehouse conditions, but surface condensation on pellets stored at relative humidity above 60% should be removed by preheating in a hopper dryer at 60 °C for 2 h before extrusion. Excess moisture from regrind above 5 wt% may require a vented screw configuration to prevent micro-bubble formation in film.

Specific mechanical energy input for LDPE blown film on 55 mm extruders typically falls within 0.20–0.28 kWh/kg, but published data for these three exact grades is limited. In practice, 2426H consumes energy at the upper end of that band because of higher melt viscosity, while 2426K runs closer to the lower end. The difference becomes operationally visible as a 4–7% reduction in extruder amps when changing from 2426H to 2426K without altering temperature set points. Film thickness uniformity measured across the web by ASTM D374 or an online capacitance gauge is usually better for 2426H and 2420H at high frost line settings, while 2426K can produce thinner bubble walls with fewer puncture defects when the line is configured with internal bubble cooling and a 1.2 mm die gap.

Heat seal initiation temperature is influenced more by density, antioxidants, and slip/antiblock migration than by the small melt flow rate differences among these three grades. In laboratory seal tests using ASTM F2029 or ASTM F88, blown films from 2426H commonly exhibit a seal initiation temperature 5–8 °C higher than 2420H when the 2426H contains larger antiblock particles. The difference narrows when films are corona treated before printing, because surface oxidation increases polar energy and can lower the practical seal initiation temperature. Haze measurements according to ASTM D1003 show that 2426K is frequently the clearest of the three in thin-gauge films, but the ranking is not guaranteed because haze is coupled to die-line cleanliness, screw cooling, and frost line turbulence. Coefficient of friction values measured by ASTM D1894 are typically targeted in the 0.10–0.25 range for high-speed packaging, but exact values require the converter to state whether erucamide, oleamide, or a synthetic slip additive is used.

Hot tack strength measured with ASTM F1921 is strongly affected by molecular weight distribution and the low-molecular-weight tail. Grades with higher melt flow rate, such as 2426K, may seal at lower temperatures but can exhibit a narrower hot tack plateau at packaging speeds above 40 cycles/min. 2426H tends to retain hot tack over a wider temperature window, which is an advantage when sealing through contaminated surfaces or when film temperature varies on vertical form-fill-seal machines. However, published data for this specific configuration is limited because hot tack values depend on jaw dwell time, jaw pressure, and anvil temperature uniformity.

When 2420H Is Added to LLDPE-Rich Blends

2420H is frequently ordered not as a neat resin but as a coextrusion or blend partner with linear low-density polyethylene. In a 70:30 LLDPE/LDPE blend, the LDPE component lowers shear viscosity and stabilises the bubble. 2420H provides an intermediate melt flow rate that creates a viscosity ratio closer to unity with a typical 1.0 melt index LLDPE. The reduced viscosity mismatch lowers interfacial instability in 3-layer coextruded film die packages and permits higher drawdown. In blown film lines with an L/D 30:1 extruder, replacing 2426H with 2420H at the same weight fraction can lower extruder amps by 4–7% and shorten purging time when the LDPE skin layer is run against an LLDPE core. The penalty is often lower dart impact and slightly higher haze after film ageing, but the loss is acceptable in lamination or overwrap film where seal performance and bubble stability control the order.

For food contact applications, all three grades are generally supplied under a food-contact statement referencing FDA 21 CFR 177.1520(c) for olefin polymers, provided that the finished film meets extractives and density limits. Converters must verify the exact subparagraph and the supplier’s REACH and RoHS 2011/65/EU declarations, because additive packages such as slip, antiblock, and antioxidants can alter the overall migration profile under Regulation (EU) No 10/2011. Films made from 2426H with high antiblock loadings may require additional migration testing if the food contact surface exceeds 10 mg/dm² overall migration in simulant D1 for fatty food contact. Published data for this specific configuration is limited, and the finished article converter remains responsible for compliance under the applicable national food contact legislation.

Transition Purging Is Governed More by Additive Carryover Than by Viscosity

Grade changes among 2426H, 2426K, and 2420H require less purge than transitions from LDPE to HDPE or EVA, but residual additive packages can persist for 15–30 minutes on a 55 mm extruder. If 2426K with high slip is ordered after 2420H without a purge compound, the slip additive may mask the seal initiation temperature of the first 50–100 kg of production, producing intermittent coefficient of friction values outside the 0.10–0.25 target. Die-line residues from antiblock particles in 2426H may appear as gel-like specks in thin film even though the specks are not true gels. Purging with a cast acrylic or high-MI LDPE purge resin is recommended after 8–10 h of continuous operation with 2426H before switching to high-clarity 2426K. Melt memory effects in the blown film die can maintain the previous grade’s haze and dart impact signature for several roll changes. Since all three grades are low-density polyethylene with similar melt flow rates, purging time is governed less by viscosity than by additive carryover and the condition of the die lip.