What Is LDPE 4 Plastic? The Resin Code Behind Bags and Films

Low-density polyethylene (LDPE) is a semicrystalline polyolefin produced through high-pressure free-radical polymerization of ethylene. Resin identification code 4 under ASTM D7611 identifies LDPE in the resin coding system used for sorting post-consumer plastic articles. The material is classified by ASTM D883 as a polyethylene with density in the range 0.910–0.925 g/cm³. The molecular architecture of LDPE differs from linear low-density polyethylene: LDPE contains short-chain branches and long-chain branches distributed along the ethylene backbone, with a broad molecular weight distribution. This architecture produces relatively low crystallinity, high melt strength, high extensional viscosity under strain, and low melt index sensitivity. The LDPE 4 plastic designation on a film or bag is a sorting and recovery identifier; it does not by itself constitute a food-contact declaration or a performance grade specification.

The high-pressure polymerization process is operated in either a continuously stirred autoclave reactor or a tubular reactor at pressures commonly reported between 100 MPa and 300 MPa and temperatures between 150 °C and 320 °C. Free-radical initiators, including organic peroxides or oxygen, generate ethylene radicals. Chain transfer and backbiting reactions create short-chain branches, while intermolecular chain transfer creates long-chain branches. Autoclave grades generally exhibit broader short-chain branching distribution and higher long-chain branching content than tubular grades, while tubular grades can be produced with narrower molecular weight distribution and, in some optical film formulations, lower haze. The choice of reactor platform affects gel content, melt strength, and film appearance. Commercial film producers qualify LDPE lots by melt index, density, haze, dart drop, and Elmendorf tear because batch-to-batch variation in high-pressure reactors is managed rather than eliminated.

How Does Resin Identification Code 4 Map to Density and Melt Rheology?

Resin identification code 4 under ASTM D7611 corresponds specifically to LDPE and is applied to bottles, film, and other articles where LDPE is the predominant resin by mass. Density is measured by ASTM D1505 or ISO 1183-1; commercial film grades typically fall between 0.917 g/cm³ and 0.924 g/cm³. Melt mass-flow rate is measured at 190 °C under 2.16 kg load using ASTM D1238 or ISO 1133-1. Film grades commonly range from 0.2 g/10 min to 2.0 g/10 min; extrusion coating grades range from 4 g/10 min to 12 g/10 min; injection molding grades may exceed 20 g/10 min. Melt flow rate alone is not sufficient to predict processing because LDPE exhibits broad molecular weight distribution and pseudoplastic behavior. Capillary rheometry at multiple shear rates is used to determine power-law index and entrance pressure drop. Melt strength, often measured by a Rheotens apparatus coupled to a capillary rheometer, is higher for LDPE than for linear low-density polyethylene of equivalent melt index, which supports bubble stability and draw resonance resistance.

At the molecular level, long-chain branches produce strain hardening in extensional flow. This is observed as an increase in extensional viscosity above the linear viscoelastic envelope at Hencky strain rates relevant to film blowing and extrusion coating. Long-chain branching frequency is influenced by reactor pressure, temperature profile, and chain transfer agent type. Published academic work using ¹³C nuclear magnetic resonance has reported long-chain branch concentrations on the order of 0.5–4 per 1000 carbon atoms for various high-pressure LDPE grades, but exact values are grade-specific and normally protected in supplier technical disclosures. The breadth of the molecular weight distribution also contributes to melt elasticity and die swell. For blown film dies, die swell influences draw-down distance and bubble diameter; operators typically set die gaps between 0.8 mm and 2.0 mm to balance extruder pressure and gauge variation.

Data compiled from published commercial technical bulletins and resin datasheets; ranges represent typical values, not specification limits.

Additive packages in LDPE films are formulated around migration and surface performance. Slip agents such as erucamide or oleamide migrate to the film surface over 24 h to 72 h and reduce coefficient of friction measured per ASTM D1894 from above 0.50 to approximately 0.10–0.25; migration rate depends on crystallinity, film thickness, and storage temperature. Antiblock agents, typically synthetic silica or diatomaceous earth at loadings of 1000–5000 ppm, reduce film-to-film blocking but increase haze as measured by ASTM D1003. Process stabilizer systems generally combine a hindered phenol primary antioxidant and a phosphite secondary antioxidant; long-term thermal stability is evaluated by oxidative induction time per ASTM D3895. UV-stabilized film grades for agricultural or construction film use hindered amine light stabilizers or carbon black, with performance verified through accelerated weathering under ASTM G154 or natural exposure. Additive dosing must be controlled during extrusion; overconcentration of slip can cause coefficient of friction below 0.05 and reel winding instability, while insufficient slip can generate blocking on finished rolls.

When Film Blowing Approaches the Upper Bubble Cooling Limit

For blown film production of LDPE, single-screw extruders with a 24:1 to 30:1 L/D ratio are commonly used with barrier screws and melt temperatures between 160 °C and 230 °C. The bubble is formed from an annular die, expanded to a blow-up ratio between 2.0:1 and 4.0:1, and cooled by a single or dual-lip air ring or internal bubble cooling system. Output is limited by bubble cooling capacity and frost-line height. If output exceeds the air ring heat removal capacity, the frost line rises beyond the stabilization zone, gauge variation increases, and bubble oscillation can initiate. LDPE high melt strength delays bubble instability compared with linear low-density polyethylene, but high stalk heights and internal bubble cooling are required for high-output lines producing films below 20 μm. Film properties are anisotropic because of molecular orientation in machine and transverse directions. Tensile properties are measured according to ASTM D882 or ISO 527-3; dart drop impact is measured according to ASTM D1709; Elmendorf tear strength is measured according to ASTM D1922; and puncture resistance is measured according to ASTM D5748.

Blow-up ratio controls the balance of machine direction and transverse direction tear properties. At low blow-up ratios near 2.0:1, machine direction orientation dominates and transverse direction tear can drop; at higher blow-up ratios near 3.5:1, orientation becomes more balanced but bubble cooling and stability become more demanding. Gauge uniformity is monitored with an online capacitive or beta gauge; LDPE film lines typically target thickness variation below ±5 %. Frost-line height also affects haze and gloss: rapid quenching reduces spherulite size and improves clarity, while slow cooling increases crystallinity and haze. Selection of air ring geometry, die gap, and melt temperature is a constrained optimization rather than a single recipe; a change in one variable shifts the bubble temperature profile and the resulting orientation state.

In extrusion coating and laminating, LDPE grades with higher melt flow rate are used to deliver lower melt viscosity, lower neck-in, and higher drawdown. Melt temperatures are generally raised to 280–325 °C to promote adhesion to paper, paperboard, or aluminum foil through surface oxidation of the melt. Neck-in, the reduction in web width from die exit to chill roll, is a critical operational parameter; LDPE resins with long-chain branching and broad molecular weight distribution exhibit lower neck-in than linear low-density polyethylene at equivalent melt flow. Drawdown is the maximum line speed achievable before web breakup; it is evaluated on pilot coating lines because laboratory melt strength tests do not fully capture transient extensional behavior at commercial speeds. Published pilot-line data for specific neck-in values across all die configurations and air gaps is limited because neck-in depends on die-to-chill-roll distance, melt temperature, and resin lot. In liquid packaging board, LDPE extrusion coatings function as moisture barriers and sealing layers, while oxygen barrier is contributed by aluminum foil or other substrates. For applications requiring lower oxygen transmission, LDPE alone is not adequate; its gas permeability is high relative to EVOH, nylon, and metallized films.

Why Seal Initiation Temperature and Hot Tack Limit High-Speed Packaging

LDPE is used in horizontal form-fill-seal, vertical form-fill-seal, and bag-making lines because its melting range enables low-temperature sealing. Seal initiation temperature is the minimum jaw temperature at which a specified seal strength is achieved, often reported at 26 N/m or 0.5 lb/in, measured by heat seal methods such as ASTM F88. Film-grade LDPE seal initiation temperature is commonly reported in technical datasheets between 90 °C and 115 °C, depending on density, additive loading, and thickness. Hot tack strength is measured by ASTM F1921 and describes seal strength while the seal is still molten; it controls package integrity when product loading occurs before the seal solidifies. LDPE hot tack is acceptable for many dry-food and light packaging applications but may be insufficient for heavy product drop loading at high line speeds.

Seal strength of LDPE reaches a plateau above 120–150 °C; further temperature increases do not improve seal strength and can cause film distortion, shrink, or burn-through at the jaw. Seal integrity is also influenced by contamination: dust, powders, or oil in the seal area reduce peel strength and require higher seal temperatures or longer dwell times. In lamination structures, LDPE is often the sealant layer behind oriented PET or BOPP. Heat seal curves for laminated structures shift because the substrate restrains shrinkage and changes heat transfer. Seal-through-contamination performance can be improved by blending LDPE with ionomer or metallocene linear low-density polyethylene, but such modifications alter seal initiation and hot tack and must be revalidated with ASTM F88 and ASTM F1921. High-speed packaging lines typically specify a sealing window rather than a single temperature; for LDPE, a common upper limit is 130–150 °C depending on dwell time. Below seal initiation temperature, incomplete melting at the interface produces weak seals; above the upper limit, severe film distortion and edge burn-through occur. The operational window is therefore narrow relative to some sealant resins, and this is a primary technical limitation of LDPE in high-speed packaging.

In mechanical recycling, post-consumer LDPE film is recovered through municipal and retail collection streams, sorted by resin identification code 4, and processed in mechanical recycling lines. Incoming bales have low bulk density, often below 300 kg/m³ for baled film, and require size reduction, washing, drying, and melt filtration. Filtration screen packs with apertures from 100 μm to 250 μm are common to remove paper, dirt, and aluminum fragments; fine filtration below 80 μm increases melt pressure and can reduce throughput. Reprocessing is performed on single-screw or twin-screw extruders with vacuum degassing to remove moisture and volatile contaminants. The quality of post-consumer LDPE resin is lot-dependent: melt flow rate can shift upward due to chain scission or downward due to crosslinking and gel formation. Recyclers blend lots to target a specified melt flow rate and use screen pack pressure rise as a gel indicator. EN 15343 provides a traceability framework for recycled plastics, but it does not define food-contact suitability. Food-contact recycled LDPE is subject to region-specific authorizations and requires decontamination validation.

Regulatory Compliance Matrix for LDPE Film and Extrusion Coatings

Table 2 lists the primary standards and regulations used in technical datasheets, safety documentation, and quality assurance for LDPE articles. The list is not exhaustive; end-use applications may impose additional requirements, such as specific migration testing for food-contact additives or ignition resistance for construction film.

Under continuous thermal load, LDPE is generally limited to service temperatures below 80 °C; it is not suitable for retort, hot-fill above 90 °C, or medical steam autoclave cycles. The material has poor resistance to aliphatic and aromatic hydrocarbons, chlorinated solvents, and strong oxidizing acids; swelling and environmental stress cracking can occur under load and exposure. Environmental stress cracking resistance is measured by ASTM D1693 or ISO 22088, but this test is less commonly specified for LDPE film than for high-density polyethylene containers. Oxygen transmission is high, and water vapor transmission is moderate; barrier improvement requires lamination, coating, or coextrusion with barrier layers. The resin code 4 marking identifies the polymer family but not the additive package, recycled content, or food-contact status; converters must maintain separate documentation for each specific lot.