Polyethylene Terephthalate and Polyethylene Share a Prefix, Not a Chemistry

The phrase “Polyethylene Terephthalate and Polyethylene Share a Prefix, Not a Chemistry” distinguishes the polyester repeat unit from the polyolefin backbone. Polyethylene terephthalate is a step-growth condensation polymer produced from purified terephthalic acid or dimethyl terephthalate and monoethylene glycol. Polyethylene is an addition polymer of ethylene with a saturated carbon-carbon backbone. In PET, the shared “ethylene” sequence in the name arises from monoethylene glycol, not from ethylene monomer. The PET repeat unit contains an aromatic p-phenylene ring and polar ester carbonyl groups; polyethylene contains only non-polar methylene units. This structural boundary changes density, thermal transitions, moisture response, gas transport, and solvent resistance. Bottle-grade PET at intrinsic viscosity 0.80 dL/g has a density near 1.38 g/cm³ under ISO 1183-1:2019, while HDPE pipe and bottle grades typically fall between 0.945 g/cm³ and 0.965 g/cm³. PET is a polyester, not a polyolefin.

For converters and resin selectors, the nomenclature similarity is a recurring specification risk. A polyethylene processing profile cannot be transferred to PET without accounting for hydrolytic degradation, acetaldehyde generation, and intrinsic viscosity loss. The following sections examine the property and processing boundaries that separate PET from polyethylene.

What Makes the Ester Linkage the Critical Failure Point During Melt Processing?

At melt processing temperatures above 260 °C, the ester carbonyl in PET is vulnerable to nucleophilic attack by residual water. Hydrolysis cleaves the terephthalate-ethylene glycol ester bond, generating terminal carboxyl and hydroxyl groups. The carboxyl end groups can then autocatalyse further hydrolysis. Residual moisture above 50 ppm in bottle-grade PET at 280 °C produces measurable intrinsic viscosity loss during normal screw residence. Desiccant dryer targets are therefore 30 ppm or lower. Amorphous PET with initial moisture near 0.25 wt% is dried at 160–170 °C for 4–6 h in a desiccant-bed dryer with air dew point −40 °C and airflow of 0.06–0.08 m³/min per kg/h. Crystalline recycled flake may require 140–150 °C to avoid pellet sticking. Polyethylene does not share this failure mode because the all-carbon backbone is largely inert to hydrolysis; moisture in polyethylene melt is handled by venting rather than chain-scission control.

Under High Ambient Humidity, the Drying and Melt Temperature Window Compresses

The operational boundary is not linear. If ambient relative humidity increases from 20% to 70% and drying conditions are unchanged, PET pellets reach higher equilibrium moisture before drying, and dried resin can regain moisture when exposed to plant air. At relative humidity above 60%, dried PET held in an unsealed machine hopper can exceed 30 ppm moisture regain within 15–30 min depending on pellet surface area and air contact. Production-scale systems therefore use closed-loop conveying from dryer to injection machine throat with dried-air dew point below −20 °C. Melt temperature measured at the nozzle for bottle-grade PET at 0.80 dL/g is maintained at 270–285 °C. Excursions above 285 °C raise acetaldehyde formation above sensory significance even when moisture is controlled. Hot runner residence time above 5 min at 280 °C can produce yellowing and oligomer deposition.

On production-scale preform injection equipment with 48 to 96 cavities, published equipment datasheets specify clamp force of 450–600 metric tons for the higher cavity counts. Barrel zones are profiled from 265 °C rear to 280 °C front, with hot runner manifold temperature at 280–285 °C and mould cooling water at 8–12 °C. Injection pressures reach 120–160 MPa and holding pressures 40–60 MPa for 0.80 dL/g bottle resin. Barrier screw designs with L/D 24:1 and back pressure 3–5 MPa are used to limit shear heating. Worn check rings, dead zones in hot runner drops, or excessive screw-tip free volume generate black specks and acetaldehyde excursions above 10 ppm. Acetaldehyde above 5 ppm in a water-contact preform is sensorially significant, so hot runner residence time is kept below 3 min in high-output beverage packaging lines.

Solid-State Polymerisation and Intrinsic Viscosity Specifications for Bottle-Grade Resins

For carbonated soft-drink preforms, melt-phase polymer alone is insufficient. The amorphous melt-phase precursor typically exits at intrinsic viscosity 0.60–0.65 dL/g, measured in 60/40 phenol/1,1,2,2-tetrachloroethane at 30 °C according to ASTM D4603-18. Solid-state polycondensation raises intrinsic viscosity by heating crystallised pellets to 210–220 °C under vacuum below 1 mbar or under dry nitrogen for 12–20 h. Pellet crystallinity must exceed 30–35% before the material reaches solid-state reaction temperature to prevent agglomeration. The reaction removes ethylene glycol and water by diffusion through the semicrystalline pellet and becomes diffusion-limited as intrinsic viscosity approaches 0.84 dL/g. Carbonated soft-drink grades are specified at 0.80–0.84 dL/g; still-water grades are commonly 0.76–0.78 dL/g. Melt viscosity at 280 °C and 100 s⁻¹ for a 0.80 dL/g grade is approximately 200–400 Pa·s.

Amorphous PET sheet is thermoformed at 90–110 °C surface temperature; crystallisation accelerates above 120 °C and produces haze and loss of draw uniformity. PET resists dilute acids, aliphatic hydrocarbons, and oils. Strong alkalis and chlorinated solvents attack the ester linkage, particularly at elevated temperature. Cleaning-in-place operations with hot caustic above 2 wt% at 80 °C can etch the surface layer and reduce molecular weight at the exposed boundary. This is an operational limitation not encountered with polyethylene in the same cleaning environment.

When Recycled Flake Replaces Virgin Resin Without Solid-State Chain Extension

Post-consumer PET flake entering a bottle-to-bottle process is washed, density-separated, and dried to 25–50 ppm residual moisture. Without chain extension, the flake has lower intrinsic viscosity and higher carboxylic end-group concentration than virgin resin. Blending washed flake into virgin PET above 25 wt% can shift the blend intrinsic viscosity below 0.72 dL/g if the flake is not upgraded by solid-state polycondensation. Residual polyvinyl chloride in flake decomposes at PET melt temperatures and releases hydrogen chloride, which accelerates hydrolysis. Residual polyamide or amine additives cause aminolysis of the ester bond and generate amide termination. Recycled PET intended for food contact must comply with EU 2022/1616 challenge-test requirements and, in the United States, 21 CFR 177.1630. These regulatory citations address migration and safety, not processing stability. Published data for a specific flake source and solid-state line configuration is limited, but industrial practice is to process recycled flake through vacuum solid-state polycondensation at 200–215 °C to restore intrinsic viscosity before preform injection.

Barrier and Tensile Data Diverging From Polyethylene Norms

After rapid quenching and reheating to orientation temperature, biaxially oriented PET film is produced with stretch ratios of 3×3 to 3.5×3.5, developing machine-direction tensile modulus of approximately 4200–5000 MPa under ISO 527-3:2018. Oxygen permeability of semicrystalline PET at 23 °C and 0% RH is typically 1.5–4.0 cm³·mm/m²·day·atm by ASTM D3985-17. HDPE ranges from 1000–2500 cm³·mm/m²·day·atm because the non-polar, flexible polyolefin backbone has much higher free-volume transport for small gases. The aromatic p-phenylene ring in PET restricts segmental mobility and reduces oxygen solubility. Moisture vapour transport is controlled by the polar ester group; PET is therefore less effective as a water-vapour barrier than HDPE. In high-humidity packaging, this limitation is managed with orientation, crystallinity, and vacuum-deposited or plasma-applied barrier coatings. The physical properties of polyester and polyethylene are not interchangeable; the shared prefix does not imply shared transport, thermal, or chemical behaviour.