Low-density polyethylene (LDPE) and high-density polyethylene (HDPE) are both semicrystalline ethylene polymers, but short-chain branching, density, crystalline fraction, and melt rheology separate bag-grade film behaviour. In evaluating LDPE vs HDPE Bags: Strength, Cost and Recyclability Compared, the comparison cannot be reduced to resin price per kilogram. LDPE produced in high-pressure tubular or autoclave reactors contains long-chain and short-chain branches that depress crystallinity and produce a density of 0.910–0.940 g/cm³ under ASTM D1505. HDPE produced by gas-phase, slurry, or solution coordination catalysis is essentially linear and spans 0.941–0.970 g/cm³ under the same method. The resulting crystalline fraction commonly falls between 40–55% for LDPE and 60–80% for HDPE, and that difference controls tensile yield, tear anisotropy, seal initiation, and sorting behaviour.

Under ASTM D882 tensile testing of blown film, LDPE film tensile strength at break typically ranges from 8–20 MPa, while HDPE film commonly reaches 20–40 MPa in the machine direction. Secant modulus at 1% strain is also lower for LDPE, generally 100–250 MPa for blown film, compared with 400–800 MPa for HDPE. These values are method-sensitive and cannot be compared across different extrusion conditions, blow-up ratios, or test speeds. In puncture-dominated bag applications, LDPE often absorbs two to four times more energy than HDPE at equal gauge because lower crystallinity permits larger local deformation before failure. HDPE instead exhibits higher static load-bearing capacity and lower creep under continuous axial stress.

Why Does Dart Impact Reorder the Ranking of LDPE and HDPE Films?

Dart impact testing under ASTM D1709 exposes a failure-mode difference that tensile yield data do not predict. LDPE film at 25 µm typically shows higher F50 impact values than HDPE film of the same gauge because the lower crystalline fraction allows energy absorption over a larger deformation zone. HDPE instead fails through localised necking and split propagation along machine-direction orientation. Elmendorf tear testing under ASTM D1922 reveals the same anisotropy: HDPE blown film has lower machine-direction tear strength and is susceptible to zippering along molecular orientation lines, while LDPE tear resistance is more balanced between machine and transverse directions. The test method variant matters as well; ASTM D1709 Method A and Method B use different dart diameters and weights, and results are not interchangeable across methods.

These differences are most visible at gauges below 25 µm, where HDPE has insufficient material to resist crack propagation under high extension rates. Automatic bagging and steep-drop distribution cycles apply local stress over millisecond timescales, conditions closer to dart impact than to the quasistatic tensile test under ASTM D882. As a result, LDPE remains the resin of choice for industrial liners, high-puncture waste bags, and overwraps exposed to irregular loads, while HDPE is deployed for high-count thin sacks whose primary load is axial tensile force.

On a production-scale blown film line, the comparison changes with bubble geometry. LDPE processes at a blow-up ratio of 2.0:1–3.0:1 and a die gap of 0.8–1.5 mm on smooth-bore extruders with 24:1–30:1 L/D. HDPE high-stalk processing typically operates at 4:1–8:1 blow-up ratio and a die gap of 0.8–1.2 mm to develop the machine-direction orientation needed for down-gauging. The high-stalk HDPE bubble is less forgiving; gauge variation can exceed ±5% if melt temperature is not held within 10°C of the resin supplier target, and that variation directly affects dart impact, seal integrity, and print registration.

When Down-Gauging Is Permitted by Load Orientation, the Cost per Bag Declines

Polymer mass per unit area, not resin price alone, usually dominates bag economics. HDPE secant modulus is approximately 3–5 times that of LDPE, so a dry-goods bag carrying an axial load can often be produced at 25 µm in HDPE where a 50 µm LDPE film would be required for equal load retention. Using midpoint densities of 0.925 g/cm³ for LDPE and 0.950 g/cm³ for HDPE, the mass per square metre falls from approximately 46.3 g/m² to 23.8 g/m², a reduction of 48–49% before process scrap. Regional market assessments often place LDPE at parity to 15% above HDPE per kilogram, but integrated feedstock and comonomer positions can invert that differential. The mass saving overwhelms the density difference only when the application is not puncture-limited; if the thin HDPE design fails frequent ASTM D1709 drop tests, field returns eliminate the calculated material saving.

Converting economics add a second constraint. Film-grade HDPE has a melt flow rate of 0.04–0.7 dg/min at 190°C/2.16 kg under ASTM D1238, lower than most LDPE film grades at 0.2–2.0 dg/min. On a 55 mm 30:1 L/D grooved-feed extruder, high-molecular-weight HDPE raises backpressure at constant screw speed and can reduce specific throughput in kilograms per hour. Nevertheless, the thinner finished bag can increase bag count per hour by 20–40% if gauge variation is held within ±5%. LDPE extrudes at lower melt temperatures because its melting peak is 105–115°C, while HDPE requires 125–135°C and a narrower die-lip temperature window to avoid sharkskin. These differences mean the lowest resin price per kilogram does not automatically produce the lowest cost per bag.

Post-Consumer Separation and Melt Filtration Constraints

Resin identification code 4 identifies LDPE and code 2 identifies HDPE under ASTM D7611. Sink-float separation in water cannot isolate LDPE from HDPE because both densities are below 1.0 g/cm³; effective separation requires near-infrared sorting in the 1,100–1,700 nm spectral region, triboelectric separation, or froth flotation after surface treatment. Carbon black pigmentation absorbs across the same near-infrared region, so black film often cannot be positively sorted by automated optical units and must be removed before extrusion. In a mixed post-consumer bag stream, LDPE film is generally suitable for mechanical recycling into refuse sacks, agricultural film, and construction sheet when melt filtration removes hard contaminants. HDPE bag film is less tolerated in bottle recycling lines because thin film melts at a different rate and lowers bulk density. Under ISO 15270:2008, clean mono-polyolefin film recovery ranks higher than mixed polyolefin recovery, and food-contact recycled resin requires additional regulatory authorisation under Regulation (EU) 2022/1616 or FDA 21 CFR 177.1520, subject to feedstock traceability and contamination limits.

Mixed LDPE/HDPE regrind creates measurable optical and mechanical penalties. LDPE introduces long-chain branching and lowers the blend melting peak, while HDPE adds crystalline regions that raise haze and backpressure. On a recycling extruder with melt filtration through 80–120 mesh screens, processors often tolerate no more than 5–10% HDPE contamination in an LDPE film stream before dart impact and tear decline below lot-to-lot capability limits. Adding 2–5% of an LLDPE or EVA modifier can partially restore impact performance, but the modification must be validated against ASTM D1709 and ASTM D1922 on the final film, and the result is no longer a single-resin recyclate under ASTM D7611.

Seal integrity on form-fill-seal and bag-conversion equipment reveals a further boundary. LDPE seals at lower jaw temperatures and maintains seal strength over a broader window because lower crystallinity allows faster chain diffusion across the seal interface. HDPE monolayer bags can seal reliably only within a narrower temperature band. Operators of vertical form-fill-seal machines running 80–120 cycles/min often use coextruded structures with an LDPE or EVA seal layer to meet hot-tack requirements measured under ASTM F1921. That coextrusion improves seal performance but creates a recyclability penalty, because the multilayer structure cannot be sorted as resin code 4 or 2 without separating the layers.