The material selection decision summarized as LDPE vs HDPE vs LLDPE Melting Points: A Converter's Comparison is not an academic tabulation of peak endotherm values; it is an operational boundary condition for screw speed, die temperature, seal jaw setting, and cooling time. Low-density polyethylene (LDPE) produced by high-pressure free-radical polymerization carries long-chain branches that broaden its melting endotherm and lower its peak temperature when measured by ASTM D3418-21 or ISO 11357-3:2018. Typical LDPE peak melting values fall between 105 °C and 115 °C, while high-density polyethylene (HDPE) homopolymer commonly peaks between 130 °C and 137 °C. Linear low-density polyethylene (LLDPE) produced with 1-butene, 1-hexene, or 1-octene comonomer generally exhibits a peak between 119 °C and 124 °C. Density ranges reported under ASTM D1505 or ISO 1183-1:2019 typically separate the three families as 0.910–0.940 g/cm³ for LDPE, 0.941–0.965 g/cm³ for HDPE, and 0.915–0.940 g/cm³ for LLDPE. These boundaries overlap for LDPE and LLDPE at equivalent density, which is why melting point must be interpreted with molecular architecture data.

LDPE vs HDPE vs LLDPE Melting Points: A Converter's Comparison

The melting point hierarchy observed in a second-heating DSC scan follows the crystalline lamellar thickness distribution rather than simply short-chain branch content. LDPE exits the high-pressure reactor with long-chain branches distributed randomly along the ethylene backbone; these branches create defective lamellae and a broad distribution of crystallite thicknesses. HDPE made with chromium, Ziegler-Natta, or metallocene catalysts has a predominantly linear backbone with minimal short-chain branches, permitting thicker and more uniform lamellae. LLDPE made by low-pressure copolymerization with 1-butene, 1-hexene, or 1-octene contains controlled short-chain branches that depress density and reduce average lamellar thickness, but the branch distribution is not identical to LDPE. The melting point depression in all three families is consistent with the Gibbs-Thomson relationship between lamellar thickness and observed melting temperature. The full enthalpy reference for crystalline polyethylene is approximately 293 J/g; typical LDPE crystallinity is 35–50%, HDPE 60–80%, and LLDPE 40–55%. Because LDPE and LLDPE overlap in density between 0.915 g/cm³ and 0.930 g/cm³, converter substitution at equal density cannot be based on melt point or density alone; rheological and seal testing are required.

The long-chain branching architecture of LDPE also affects melt rheology in a way that is not captured by a single melt point measurement. Capillary rheometry under ISO 11443:2021 at 190 °C and apparent shear rates of 100–1000 s⁻¹ shows greater shear thinning for LDPE than LLDPE of similar melt index. This difference appears on the production floor as lower motor amperage and lower melt pressure at equivalent screw speed. HDPE at the same melt index may display higher viscosity at low shear rates but faster solidification as the melt cools toward crystallization onset.

A second-heating scan at 10 K/min under nitrogen purge at 50 mL/min does not return a single thermodynamic melting point for polyethylenes. The instrument records an endothermic distribution whose onset, peak, and endset are governed by lamellar thickness distribution, molecular weight, branch length, and prior cooling rate. For LDPE, a broad endotherm typically begins between 85 °C and 95 °C and completes near 115 °C. For hexene- and octene-based LLDPE, the endotherm is narrower, with onset near 105 °C and endset below 130 °C. For HDPE, the onset often appears above 120 °C, with the peak at 130–137 °C and the endset above 140 °C. Thermal history is erased by holding the specimen at 180 °C for 3 min and cooling at a controlled rate, commonly 10 K/min, before the reported second heating trace. Sample mass is maintained at 5–10 mg in crimped aluminium pans to avoid thermal lag. Published data for specific catalyst and comonomer combinations is limited, particularly for bimodal HDPE reactor grades and C6–C8 LLDPE fractions, so converter comparisons should request the full DSC thermogram rather than relying on datasheet peak values.

The heating rate dependence means that a converter comparing legacy DSC data generated at 20 K/min against current 10 K/min data will observe a shift in apparent peak temperature of several degrees. This is not a resin property change; it is an instrumental effect. Extrapolating peak temperature to zero heating rate is rarely performed outside academic laboratories and is not required for routine converting decisions. Crystallization exotherms measured during cooling at 10 K/min separate HDPE from LDPE more clearly than heating traces. HDPE crystallization onset may exceed 118 °C, while LDPE crystallization extends to 85 °C or lower. This difference controls demoulding time in injection moulding and frost line height in blown film.

What Distinguishes LDPE Branching from Linear and Bimodal Grades at the Melting Point?

Long-chain branching in high-pressure LDPE introduces a broad distribution of inter-crystalline tie chains and defective lamellar folds. In a second-heating trace, this architecture produces a shallow melting onset and a tail extending over 20–30 °C. HDPE homopolymer with low branch content produces a sharper endotherm and a higher peak enthalpy. LLDPE exhibits an intermediate melting range; short-chain branches from hexene or octene create thin lamellae but do not produce the same long-chain branching rheology. The result is that LDPE retains higher melt strength and lower neck-in than LLDPE at equivalent melt flow rate, while HDPE solidifies at higher temperatures and permits faster demoulding. Density overlap between LDPE and LLDPE complicates melting point interpretation. A 0.918 g/cm³ LLDPE and a 0.918 g/cm³ LDPE have similar amorphous fractions but different melting endotherms because the branch distribution differs. The LLDPE material may show a sharper peak and lower seal initiation, while the LDPE material shows a broad low-temperature shoulder and higher melt strength. Converters therefore cannot substitute between LDPE and LLDPE at equal density without re-qualifying the extrusion profile and seal settings.

Melt Temperature Ranges Across Three Polyethylene Architectures

The table below consolidates the property windows most often encountered in converting plants. Values are not tight specifications; they span typical reactor grades and comonomer selections.

Screw torque and barrel residence time shift with crystalline melting range.

On a single-screw blown-film line with a 40 mm barrier screw and L/D 30:1, setpoints follow the melting hierarchy but are expanded by residence time and output rate. LDPE of melt flow rate 1.0 g/10 min at 190 °C/2.16 kg generally runs with barrel zones from 150 °C to 180 °C and die zone at 180–200 °C. The same die temperature with LLDPE of equivalent melt index may be 200–230 °C because the linear backbone and narrow molecular weight distribution increase screw torque and melt pressure. Without adding 10–20 wt% LDPE to the LLDPE feed, bubble instability appears as helical melt fracture and periodic gauge banding, particularly at blow-up ratios above 2.5:1. HDPE film grades with melt flow near 0.05–0.3 g/10 min require grooved feed sections and higher barrel torque; melt temperatures below 180 °C produce unmelts and melt fracture at the die lip, while temperatures above 220 °C degrade molecular weight and reduce bubble strength.

In cast film and sheet lines, the difference in melting range alters roll stack temperature requirements. LDPE calibrates at roll temperatures of 15–30 °C; HDPE and high-density LLDPE may require 40–60 °C to prevent curl and differential shrinkage. In coextruded barrier films, placing LDPE in the sealant layer and HDPE in the structural layer creates a thermal profile mismatch at the die. The HDPE layer solidifies first on the chilled air stream, while LDPE remains melt-softened; this can generate interfacial waviness unless the air ring cooling is biased and the die temperature is maintained at 200–230 °C.

When High-Density Homopolymer Replaces LDPE in Extrusion Coating

Extrusion coating lines running LDPE at 300–330 °C at the die use the high melt temperature not to overcome melting point but to oxidize the melt surface for adhesion to board, foil, or paper. HDPE substitution on the same line is constrained by its 130–137 °C melting peak and narrower processing window; die temperatures above 310 °C may initiate thermal degradation, while temperatures below 280 °C increase backpressure and produce poor draw-down. The lower melt strength of HDPE relative to LDPE increases neck-in at the die. Published data for this specific configuration is limited because die gap, air gap, and melt curtain profile dominate the result; a converter must measure neck-in on the actual line before substituting HDPE. LLDPE extrusion coating is similarly limited by neck-in and higher screw torque; blending with LDPE in 20–30 wt% ratios is a standard corrective measure.

When LDPE is selected as a sealant coating on aluminium foil for pharmaceutical blister lidding, the lower melt point and broad melting endotherm allow heat sealing against lacquered foil at 150–180 °C without distorting the PVC base sheet. HDPE would require higher seal temperatures and risks heat-seal blister formation. The standard peel test for such structures is ASTM F88/F88M-21; published data for HDPE blister lidding is limited compared with LDPE and LLDPE.

Heat seal initiation is not identical to peak melting temperature. In seal testing under ASTM F2029-18, LDPE webs begin to build seal strength when jaw temperatures reach 105–115 °C because surface crystallites melt and interdiffuse under 0.4–0.6 MPa seal pressure and 0.5–1.0 s dwell. LLDPE sealants initiate at 90–105 °C but lose hot tack at temperatures above 115 °C more quickly than LDPE, so converters may blend 20–30 wt% LDPE into LLDPE sealant layers to widen the operating window measured by ASTM F1921-18. HDPE sealant layers are limited to applications where seal temperatures above 130 °C are available; this is incompatible with high-speed vertical form-fill-seal films sealing against oriented polypropylene or polyethylene terephthalate without a separate sealant coextrusion layer.

In injection moulding of LDPE, barrel temperatures are typically set between 180 °C and 220 °C; the broad LDPE melting range and 35–50% crystallinity produce low warpage but extend cooling time because the recrystallization exotherm tails to 85 °C. HDPE moulded at 200–250 °C solidifies earlier because crystallization onset can exceed 118 °C, permitting shorter hold and cooling times at the same mould temperature. A mould coolant temperature of 20–40 °C is typical for LDPE, while HDPE parts may tolerate 50–70 °C coolant without excessive sticking. Shrinkage measured by ASTM D955-21 ranges from 1.5–3.0% for LDPE, 1.5–4.0% for HDPE, and 2.0–3.5% for LLDPE, with the largest variation occurring in the melt-flow direction.

Repeated extrusion of LDPE at temperatures above 220 °C in a twin-screw compounding step increases the risk of gel formation and molecular weight redistribution; thermal stability measurements by ISO 11358-1:2022 thermogravimetric analysis show onset of measurable mass loss above 300 °C in nitrogen but lower in air due to oxidative scission. Converters running edge trim back into HDPE film lines must avoid mixing LDPE fractions above 5 wt% where the melting point depression and film modulus change violate output specifications. This boundary is established by physical property testing under ISO 527-1:2019 and is not solely a melt point issue.