Polyethylene (PE) is a semicrystalline thermoplastic polymer of ethylene with repeat unit –(CH₂–CH₂)–n. For importers and customs brokers, the product definition is not settled by chemical formula but by the Harmonized System criteria for primary-form polymers. HS Classification for Polyethylene: Getting the Code Right the First Time requires simultaneous confirmation of polymer type, monomer weight distribution, physical form, and specific gravity. The relevant international scope is heading 3901, which covers polymers of ethylene in primary forms; subheadings 3901.10 and 3901.20 split polyethylene by the 0.94 specific-gravity boundary. Errors at the subheading level can trigger duty reassessments and binding tariff information reviews. The technical evidence chain—sampling, density measurement, comonomer identification—is examined below.

Why Is Specific Gravity, Not Bulk Density, the Operative Threshold?

Subheadings 3901.10 and 3901.20 use specific gravity rather than pour density or silo bulk density. The measurement should be reported dimensionless at 23 °C using ISO 1183-1:2019 immersion or density-gradient procedures, or ASTM D1505 density-gradient column methods. Bulk density per ISO 60:1977 indicates hopper flow and conveying behavior and has no tariff function. Specific gravity is especially classification-sensitive for PE because crystallization changes the amorphous/crystalline ratio; a lot near 0.940 may shift across the boundary if the specimen is cooled rapidly rather than annealed. Conditioning and test specimen preparation should follow ASTM D618, with a standard atmosphere of 23 ± 2 °C and 50 ± 10 % RH, before density testing.

Table 1 presents the six-digit subheading structure for ethylene polymers commonly encountered in PE trade. The first two rows are the primary commercial PE categories.

On a blown-film line, an LDPE grade with density 0.921 g/cm³ and MFR 1.0 g/10 min at 190 °C/2.16 kg may be processed through a 120 mm annular die at a die gap of 1.8 mm and a blow-up ratio of 2.2:1. The resulting film may have Dart impact strength measured by ISO 7765-1 or ASTM D1709, but the HS code for the pellet remains 3901.10 because its density is below 0.94. If the film is exported as garment packaging, heading 3920 for plates, sheets, film, foil and strip of non-cellular plastics may be applicable instead.

Melt flow rate is recorded during lot release but does not determine subheading. ISO 1133-1:2022 and ASTM D1238 define extrusion plastometer procedures for PE at 190 °C and 2.16 kg. A blow-molding HDPE grade may show an MFR below 0.5 g/10 min, while an injection-molding LDPE grade may exceed 20 g/10 min; both values are commercial specifications, not legal criteria. Primary form remains the controlling earlier step: Chapter 39 Note 6 limits heading 3901 to granules, powders, flakes, lumps, liquids, pastes, and similar bulk forms. If a shipment consists of mixed post-consumer PE flakes contaminated with polypropylene, the analysis may shift to waste-plastic heading 3915 before density is even considered.

When Ethylene-Alpha-Olefin Copolymers Pull the Goods into 3901.40

Chapter 39 Note 4 defines copolymers for headings 39.01 to 39.14 as polymers in which no single monomer contributes 95 % or more by weight of the total polymer content. If ethylene constitutes at least 95 % by weight, the resin is treated as an ethylene homopolymer, and the 3901.10 or 3901.20 density split applies. Commercial LLDPE often contains 5 % to 10 % by weight of butene-1, hexene-1, or octene-1 and therefore meets the copolymer threshold. When such a copolymer has specific gravity below 0.94, subheading 3901.40 may be more specific than 3901.10; however, national tariff rulings and WCO Explanatory Notes should be checked for the specific product. FTIR evaluation per ASTM D5576 can identify the comonomer type, and 13C NMR provides branch-level quantification for the technical file. For ethylene-alpha-olefin copolymers with specific gravity at or above 0.94, subheading 3901.40 is not available; national decisions may place the product in 3901.20 if it is regarded as polyethylene, or 3901.90 if not. The exact comonomer and specific-gravity records therefore determine the residual subheading.

On a gas-phase fluidized-bed reactor line producing LLDPE, density is controlled by comonomer content and chain crystallinity. A batch sampled from the first reactor turn of a grade change may have a density spread of 0.002 g/cm³ to 0.004 g/cm³ while the purge resin transitions from HDPE to LLDPE. Depending on reactor size, the off-spec transition may persist for 15 min to 30 min; published data for a specific line is limited. Off-spec pellets with density crossing 0.940 can invert the subheading if the shipment is not blended to a stable specification. Packaging records, silo discharge logs, and the ISO 1183-1 density certificate must agree; customs documents derived from a nominal product sheet rather than the shipped lot are a common source of misclassification.

ASTM D4976 and ISO 17855 Grade Naming Does Not Replace the Customs Inquiry

Specification ASTM D4976 defines PE materials for molding and extrusion by cell classification covering density, flow rate, and additives; ISO 17855-1:2022 provides the corresponding designation system for PE moulding and extrusion materials. A grade name such as “PE-HD” or “PE-RT” communicates performance class, not the six-digit tariff code. Mechanical property values generated under ISO 527-2 or ASTM D638 may support technical data sheets, but the customs inquiry requires the density or specific-gravity result from the shipped lot. A rotational-molding HDPE grade may show tensile yield stress of 23 MPa to 30 MPa and elongation at break above 500 %, yet the correct subheading is 3901.20 because the density exceeds 0.94.

Pressure pipe grades designated PE 100 exhibit a minimum required strength of 10 MPa at 50 years and 20 °C under ISO 12162, and the pipe product itself is governed by ISO 4427. Pipe-grade HDPE pellets typically have density from 0.945 g/cm³ to 0.960 g/cm³, confirming subheading 3901.20 before processing. On a grooved-feed single-screw extruder with 38:1 L/D, die melt temperature is generally set between 190 °C and 225 °C; higher melt temperature may promote thermo-oxidative degradation if residence time is prolonged. From a tariff perspective, the important record is not the extruder setting but the pellet density certificate. If the pipe compound contains carbon black above 2 % by weight, the density and filler content should be measured, but the polymer remains dominant and the heading remains 3901.

Vicat Softening, Environmental Stress Crack Resistance, and the Rigid Packaging Boundary

For rigid HDPE packaging, ISO 306/A50 Vicat softening temperatures are commonly reported from 118 °C to 130 °C, while LDPE film grades are lower at 85 °C to 100 °C. Environmental stress crack resistance under ASTM D1693 may vary from less than 10 h for low molecular-weight homopolymer to more than 1000 h for high molecular-weight HDPE with controlled comonomer. These properties influence application selection but do not alter the tariff code; HDPE packaging-grade pellets with density above 0.94 remain in 3901.20. If a blow-molded bottle is imported as a finished article, the analysis moves to Chapter 39 articles such as 3923, not to primary-form PE granules.

Recycled PE and off-spec lots introduce additional documentation risk. A washed LDPE film reprocessed into translucent pellets with density 0.925 g/cm³ may qualify under 3901.10 if it is a genuine primary-form commodity and not mixed waste. If the recycled stream contains polypropylene, the apparent density may remain within the PE range while the melt flow index and DSC thermogram reveal PP contamination; ISO 11357-3 can distinguish the melting peaks near 110 °C to 135 °C for PE and near 160 °C for PP. Ash content per ASTM D5630 can verify inorganic residue, but filler does not remove the product from heading 3901 if the polymer remains the dominant material. For border-line LLDPE, the file should include density, FTIR comonomer type, and ethylene weight percentage so that the code is substantiated before the goods arrive.