Low-Density Polyethylene (LDPE)

Low-density polyethylene (LDPE) is a branched polyolefin produced by high-pressure radical polymerisation. Commercial grades are specified by density per ASTM D1505-18 or ISO 1183-1:2019 and melt flow index (MFI) per ASTM D1238-23 at 190 °C with 2.16 kg. Density for LDPE typically falls between 0.917 and 0.930 g/cm³; film grades are usually supplied at 0.918–0.924 g/cm³ and 0.5–2.0 g/10 min. Linear low-density polyethylene (LLDPE) is produced by low-pressure polymerisation and contains short-chain branches but lacks the long-chain branching of LDPE. Substitution therefore requires matching MFI and density while accepting that molecular architecture, melt elasticity, and shear viscosity remain different.

LLDPE Grades You Can Substitute for LDPE: MFI and Density Matching

A substitution-grade LLDPE is selected by fixing the target LDPE density and MFI from the existing specification, then identifying an LLDPE grade within ±0.002 g/cm³ density and ±0.2 g/10 min MFI for blown film, or ±0.5 g/10 min for injection molding. LLDPE comonomer types—butene, hexene, and octene—change mechanical response even when density and MFI match. At the same nominal density, butene LLDPE has lower puncture resistance than hexene or octene LLDPE, while hexene and octene grades provide higher tie-molecule concentration and improved environmental stress crack resistance. The data in the following table represent typical industrial ranges, not single lot maxima or minima.

In blown film conversion, a 63.5 mm grooved-feed extruder with 30:1 L/D running a 0.920 g/cm³, 1.0 g/10 min hexene LLDPE in place of a 0.923 g/cm³, 0.8 g/10 min LDPE requires a die gap increase from 1.0 mm to 2.2–2.5 mm to suppress sharkskin. Melt temperature at the die lip shifts from 170–180 °C to 200–215 °C; extruder drive current increases by 10–25% at constant screw speed because linear LLDPE retains higher shear viscosity at film die shear rates. Bubble stability is maintained by raising blow-up ratio from 2.0:1 to 2.5–3.0:1 and increasing frost-line height to 25–40 cm.

What Limits Melt Extension When Linear LLDPE Replaces Branched LDPE on Blown Film Lines?

The absence of long-chain branching in conventional Ziegler-Natta LLDPE reduces strain hardening. In capillary and extensional rheometry, LDPE exhibits extensional viscosity growth, whereas LLDPE displays only weak strain hardening or none. For a 1.0 g/10 min LDPE, engineering stress at Hencky strain 2.5 can be 1.5–3.0 times higher than for an LLDPE at equivalent MFI. On film lines, this appears as early bubble instability when LLDPE is run under LDPE conditions. The corrective action is not to increase MFI excessively, because that lowers melt strength further; instead, bubble cooling is advanced and die gap is widened. Some bimodal or narrow molecular weight distribution LLDPE grades incorporate sparse long-chain branching or high-molecular-weight tails to recover strain hardening. Such grades can be specified by the same ASTM D1238 MFI and ASTM D1505 density but differ in melt flow ratio; a melt flow ratio I21.6/I2 of 20–25 is indicative of broadened molecular weight distribution, compared with 15–18 for standard narrow-MWD LLDPE.

For extrusion coating lines running 4.0–8.0 g/10 min LDPE at paper line speeds above 150 m/min, an LLDPE of equivalent MFI at 0.918–0.922 g/cm³ usually produces higher neck-in and lower draw-down. The replacement may require a fluoropolymer process aid at 300–600 ppm to reduce melt fracture, and the chill roll temperature is reduced from 15–20 °C to 8–12 °C to re-establish gloss. However, LLDPE coating contains amide-free stabiliser packages; migration tests under EU 10/2011 or FDA 21 CFR 177.1520 should be repeated even if the base resin is compliant.

Injection Molding and Rotational Molding Substitution Boundaries

Injection molding LDPE grades typically carry MFI 20–70 g/10 min and density 0.917–0.923 g/cm³; LLDPE equivalents with the same MFI and density are limited because most commercial LLDPE grades stop below 20 g/10 min except specialty grades. For thin-wall closures and caps, an LLDPE with MFI 25–35 g/10 min and density 0.924–0.928 g/cm³ may replace an LDPE of MFI 30 g/10 min only after rebalancing the mold gate size upward by 10–20%. Higher melt elasticity of LDPE promotes better shear thinning and less warpage; LLDPE's linear backbone can increase clamp tonnage demand by 5–15% at equal fill speed. Rotational molding presents a different density-matching problem. LDPE rotomolding grades at 0.925–0.930 g/cm³ and MFI 3.0–6.0 g/10 min have broader particle coalescence windows than LLDPE; an LLDPE replacement with equivalent density and MFI may require a 10–20 °C higher oven setpoint and an extended peak internal air temperature of 190–200 °C. Published data for specific rotomolding grade substitutions is limited.

Cast film and stretch wrap replacement follows a similar MFI and density logic but places more weight on weld clarity and tack. An LDPE cast film grade at 0.918–0.922 g/cm³ and 2.0–4.0 g/10 min can be substituted with an LLDPE of identical density and MFI, but the cast roll temperature must be raised from 10–15 °C to 18–25 °C to prevent crystallisation-induced haze. Static cling additives that performed adequately in LDPE may migrate differently through the more ordered amorphous phase of LLDPE; release values measured by ASTM D3354-15 can shift by 15–30% at identical addition levels.

When ESCR or Puncture Resistance Triggers an LLDPE Swap

Hexene-based LLDPE copolymers with 0.918–0.925 g/cm³ density replace LDPE in agricultural and industrial films when environmental stress crack resistance per ASTM D1693-15 Condition B and dart impact per ASTM D1709-20 are decisive. At equivalent density and MFI, hexene LLDPE can exhibit ESCR beyond 500 h, while LDPE often fails before 100 h. This substitution is not transparent to the conversion line; film clarity measured by haze per ASTM D1003-13 increases from 4–7% for LDPE to 8–15% for butene LLDPE, and gloss at 45° decreases. If optical performance is constrained, a metallocene LLDPE or blended LDPE/LLDPE system at 20–30 wt% LLDPE is used, accepting partial ESCR improvement. Tensile and puncture data should always be verified against the target resin certificate of analysis because comonomer type and additive package dominate the property scatter.

Masterbatch and compound substitution also depends on carrier resin MFI and density matching. A color concentrate based on LDPE with MFI 20 g/10 min will not disperse identically in an LLDPE carrier at equivalent MFI because LLDPE absorbs higher specific mechanical energy in a 25:1 twin-screw extruder, raising melt temperature and potentially reducing pigment dispersion. The carrier MFI should be raised by 1–2 g/10 min or screw speed lowered by 10% when switching to LLDPE. In blown film coextrusion, skin layers of LDPE cannot be replaced with LLDPE without adjusting die gap and bubble cooling; published data for moisture vapour transmission rate after LLDPE substitution are available from supplier permeability tables but remain product-specific.