Polyethylene terephthalate (PET) is a semicrystalline thermoplastic polyester with the repeat unit −OCH₂CH₂O−COC₆H₄CO−. Importers of bottle-grade resin, preforms, washed flake, and regrind encounter a customs boundary defined by a viscosity number of 78 ml/g; material at or above this value is classified under HS 3907.61, while lower-viscosity PET enters HS 3907.69. Where recycled flake has been washed, granulated, and dried but is declared as plastic waste under HS 3915.90, tariff treatment can diverge sharply from primary-form PET. The divergence is not purely administrative: a shipment of washed food-contact flake with an intrinsic viscosity of 0.76–0.80 dL/g can be reclassified on audit, generating a duty gap between the lower waste-level duty and the primary-form duty plus any applicable anti-dumping or countervailing cash deposit. The technical assessment of PET imports therefore begins with molecular weight, carboxyl end group concentration, residual moisture, and contaminant load, not with the invoiced trade term.

Virgin and Recycled PET Imports: Duty, Documentation and the Duty Gap

Under the Harmonized System, the principal tariff split for PET in primary form occurs at the viscosity number threshold of 78 ml/g. HS 3907.61 covers polyethylene terephthalate having a viscosity number of 78 ml/g or higher; HS 3907.69 captures other PET including lower-viscosity material, off-spec resin, and some pelletised recycled grades that have not been solid-state polymerised. Waste, parings, and scrap of plastics are classified under HS 3915, with PET waste generally falling to HS 3915.90. The WCO Explanatory Note to heading 39.15 limits waste to plastics that are no longer fit for their original purpose; washed single-polymer flake that is directly usable in extrusion or injection moulding without recompounding falls outside that legal text in many enforcing jurisdictions. The resulting duty gap is the unpaid difference between the lower waste-level rate and the primary-form rate on the declared value, compounded by any anti-dumping or countervailing cash deposit that applies to the exporter and producer named on the entry.

Documentation defects in recycled PET supply chains frequently amplify the duty gap. A certificate of origin that lists the country of bottle collection rather than the country where washing, melt filtration, and pelletising occur can invalidate a preferential claim because the tariff shift from 3915.90 to 3907.61 reflects substantial transformation. The country of pelletisation is generally the origin for primary-form PET. When the origin certificate does not match the processing site, the entry is liquidated at the general rate and the preferential margin is lost. Similarly, a certificate of analysis issued only on the bale feedstock cannot be substituted for the pellet COA because the solid-state polymerisation step changes intrinsic viscosity, acetaldehyde, and carboxyl end groups. Importers are required to retain batch-level reconciliation between incoming bale lots and output pellet lots; the absence of that reconciliation is a documentation gap that customs auditors treat as a risk indicator.

Customs classification does not rest on the buyer’s declared end use. For PET, the viscosity number is the objective tariff indicator under HS 3907.61. A COA that reports intrinsic viscosity 0.82 dL/g under ASTM D4603-18 is inconsistent with a waste heading declaration because the material has been processed to a primary-form specification. The duty gap is quantified as the difference between the applied tariff under HS 3915.90 and the primary-form tariff under HS 3907.61 or 3907.69, multiplied by the entered value over the audit period. Where anti-dumping or countervailing orders are in force for the country of export, the gap also includes the AD/CVD cash deposit rate; published producer-specific rates are updated through administrative reviews and are not fixed in this text because they are country- and supplier-specific.

How Does the Viscosity Number of Washed Flake Change Tariff Heading 3907.61?

Viscosity number is determined by dilute solution viscometry in 60/40 wt% phenol/1,1,2,2-tetrachloroethane at 30 °C under ASTM D4603-18; the result is expressed in ml/g and is numerically related to intrinsic viscosity in dL/g. Resin producers specify bottle-grade virgin PET at 0.78–0.84 dL/g, while recycled pellet from post-consumer bottles typically ranges from 0.70–0.80 dL/g after solid-state polymerisation. Fiber-grade recycled material may be lower, in the 0.55–0.65 dL/g range, and therefore falls outside HS 3907.61. The legal threshold is not a quality statement; it is a tariff-setting physical property. A shipment of washed flake with intrinsic viscosity 0.76 dL/g and pelletised form can still be classified as primary resin even if the supplier calls it waste, because the material has undergone processing beyond collection and sorting.

In addition to viscosity, carboxyl end group concentration influences melt processing and can indicate hydrolytic degradation. Virgin bottle-grade PET commonly has carboxyl end groups of 15–22 mmol/kg by ASTM D7409-15; recycled material may reach 20–35 mmol/kg. Elevated carboxyl end group concentration reduces melt viscosity and increases acetaldehyde regeneration during injection moulding. From a tariff perspective, a high-carboxyl recycled flake is not automatically waste; it may still exceed the 78 ml/g viscosity threshold and therefore remains classifiable under HS 3907.61. The technical defect is a processing risk, not a customs reclassification justification.

Thermal analysis of imported flake under ISO 11357-3 at 10 °C/min shows a glass transition at 76–80 °C, cold crystallisation at 130–150 °C, and a melting endotherm at 240–255 °C. A wide melting peak with a shoulder near 220 °C typically indicates melt-history differences or contamination by lower-melting polyolefin residues. Such variation does not usually change tariff classification, but it alters downstream melt robustness. In a twin-screw extruder with L/D 32:1, barrel zones are commonly held between 260 °C and 285 °C. If the feedstock is undried recycled flake with residual moisture above 50 ppm, hydrolytic chain scission reduces intrinsic viscosity by approximately 0.02–0.04 dL/g per pass and raises carboxyl end groups by 3–6 mmol/kg. Melt-pressure fluctuation at the screen changer, typically in the range of 60–120 bar, is the first field indicator that incoming flake has not been dried to 0.005% moisture. Injection preform moulding requires a melt temperature of 270–290 °C; when the melt temperature exceeds 295 °C, acetaldehyde evolution accelerates and yellowing becomes measurable. The operating window is therefore narrow: insufficient barrel heat produces crystalline haze in preforms, while excess heat pushes acetaldehyde above the 0.5 ppm limit commonly applied to bottled-water preforms under 21 CFR 177.1630 and EU Commission Regulation 2022/1616 for recycled plastic food-contact materials.

Batch-to-batch variance in recycled PET is dominated by the non-PET fraction. Near-infrared sorting on a commercial flake line reduces polyvinyl chloride and polyolefin contamination, but residual particles at 0.1 wt% can produce gel defects in sheet extrusion and black specks in thermoformed trays. A recycled food-contact process operating under EU Regulation 2022/1616 must demonstrate decontamination through a challenge test or an approved recycling process; the certificate of analysis alone is not sufficient for import release. Import documentation for food-grade recycled PET therefore includes the no-objection letter or EU recycling process authorisation, the recycled content certificate, and the batch COA showing contaminant and migration limits. Where such documentation is missing, customs may still release the shipment against a higher tariff heading or require a bond; the duty gap is not resolved by the absence of a food-contact claim because the physical form of the material already determines primary classification.

When Solid-State Polymerisation Crosses the Threshold Between Waste and Primary Resin

Solid-state polymerisation is the unit operation that most often moves recycled flake from the waste tariff category into primary resin. In a typical SSP vessel, granulated recycled PET is held at 190–220 °C under vacuum below 1 mbar for 8–14 h; the polycondensation reaction raises intrinsic viscosity from 0.65–0.70 dL/g to 0.80–0.84 dL/g while removing volatile contaminants. The operation is batch-dependent: if feedstock carboxyl end group concentration exceeds 35 mmol/kg, the required SSP residence time increases, and the final acetaldehyde level may fail the 0.5 ppm food-contact criterion. From a customs perspective, SSP-treated pellets with a viscosity number above 78 ml/g are primary-form PET under HS 3907.61; they are not waste or scrap. The classification does not depend on the ratio of post-consumer content. A pelletised recycled resin containing 100% post-consumer PET but meeting 0.80 dL/g intrinsic viscosity and <0.5 ppm acetaldehyde is dutiable as a primary polyester, regardless of the sustainability certificates attached.