Polyethylene terephthalate is a linear thermoplastic polyester formed by the polycondensation of purified terephthalic acid (PTA) and ethylene glycol (EG), although the older dimethyl terephthalate (DMT) route remains in use where PTA supply or catalyst selection favors transesterification. The repeat unit is C₁₀H₈O₄, with a repeat-unit molar mass of 192 g/mol and CAS registry number 25038-59-9. Across packaging, textile, and regulatory documents, the same polymer is identified by the labels PETE, PET and the Resin Code 1: One Polymer, Three Labels; these labels do not indicate different polymer grades or different melting points.

The resin is semi-crystalline but is commonly converted from the amorphous state to maintain clarity and orientation potential. Unmodified homopolymer PET exhibits a glass transition temperature of 70–80 °C, a cold-crystallization exotherm above the glass transition, and a melting peak near 245–255 °C depending on copolymer content and thermal history. Intrinsic viscosity rather than melt flow rate is the standard molecular weight proxy for PET, and the preferred method is ASTM D4603-18.

Why Does One Polymer Backbone Carry Three Different Designations?

The abbreviation PET is conventional in polymer science, engineering specifications, and international standards. PETE appears when a molded article is marked with the resin identification code because the four-letter acronym provides clearer legibility inside the triangular code symbol. ASTM D7611-20 assigns Code 1 to polyethylene terephthalate and supports either PET or PETE as the abbreviation. The code is a material identification system; it does not certify that an article will be accepted by a specific recycling facility, nor does it define food-contact compliance.

This distinction is operationally important. A resin supplier may certify PET against ASTM D4603-18 for intrinsic viscosity and ISO 1133-1:2022 for melt mass-flow rate, while the package marking drawing may specify PETE under Code 1. Because the chemical structure is identical, test data generated under one label can be transferred to the other, provided the same test method and sample conditioning are used. The Code 1 marking is also used for sorted post-consumer bales, but near-infrared sorting equipment identifies the polymer by its absorption spectrum rather than by the visual code.

In continuous melt-phase polymerization lines, PTA and EG are first converted to bis(2-hydroxyethyl) terephthalate and oligomers at 240–260 °C. Excess EG is removed, and the polycondensation stage is operated at 270–285 °C under vacuum below 1 mbar. The vacuum level is critical because the reverse hydrolysis and glycolysis reactions must be suppressed by removing the EG byproduct; high-surface-area, thin-film finishing reactors are used to overcome diffusion limitations in the viscous melt.

Melt-phase PET is pelletized as amorphous chips. Rapid cooling through the crystallization temperature range suppresses spherulitic growth and preserves the amorphous state required for later orientation. Melt-phase intrinsic viscosity is limited to 0.60–0.65 dL/g because higher molecular weight reduces reactor mass transfer and increases degradation. Solid-state polymerization raises IV by heating chips under nitrogen or vacuum at 200–220 °C; this step also reduces residual acetaldehyde. In production-scale SSP vessels, chip bed height, gas flow distribution, and temperature uniformity control batch-to-batch IV variation. Recycled flake can show IV variation greater than 0.03 dL/g, requiring blending or additional SSP to normalize the feed.

Table 1 summarizes typical grade windows. The density range reflects both amorphous and semi-crystalline states; orientation and annealing shift density toward the higher end of the range.

When Moisture Control Dictates Preform Molding Outcomes

PET hydrolyzes during melt processing if the pellet moisture content exceeds 0.005% by mass. Preform molders therefore use closed-loop desiccant dryers with a dew point below −40 °C, hopper temperatures of 160–170 °C, and residence times of 4–6 h for cold pellets. A rise in dryer dew point above −20 °C quickly produces hydrolytic cleavage; the resulting IV loss may not be uniform across the shot because pellets near the hopper walls can retain surface moisture while center pellets remain dry.

Injection molding barrel zones are set between 265–285 °C, and the nozzle temperature is held at or below 285 °C for bottle-grade resin. Residence time above 5 min at 285 °C accelerates thermal degradation and shifts part color. Multi-cavity preform tools require hot-runner temperature uniformity better than ±1 °C across cavities; otherwise shot-to-shot variation in IV and acetaldehyde becomes detectable in downstream blow molding.

Hydrolysis is not avoided by average moisture alone. PET pellets exposed to ambient air at high relative humidity can pick up enough surface moisture during silo transfer to produce localized degradation even when the hopper dryer is functioning correctly. For this reason, conveying lines are closed and dry air is supplied to hopper throats in critical bottle operations.

At the reheating station of a linear stretch blow molder, injection-molded preforms are heated to 90–105 °C and simultaneously stretched in the axial and radial directions. The resulting biaxial orientation induces strain-hardening and converts the amorphous preform into a semi-crystalline bottle wall. Orientation increases tensile modulus and reduces oxygen and carbon dioxide permeability compared with unoriented sheet, but the molecular orientation must be locked by strain-induced crystallization before the part is ejected.

The reheat stretch blow window is narrow. If the preform surface temperature is too low, stress-whitening and uneven wall thickness result; if the surface exceeds the crystallization temperature too long, spherulitic crystallization produces haze and reduces clarity. Copolymerization with minor levels of isophthalic acid or diethylene glycol slows crystallization and widens the processing window. Bottlers use differential scanning calorimetry per ISO 11357-3 to verify that incoming resin or preform crystallinity remains within specification; published data for specific copolyester formulations is limited and is often controlled under proprietary brand specifications.

Acetaldehyde Generation Constrains Preform Processing for Water and Carbonated Soft Drink Applications

Acetaldehyde forms by thermal degradation of hydroxyethyl end groups, intramolecular elimination reactions, and thermo-oxidative cleavage of the polyester backbone. In water and carbonated soft drink containers, acetaldehyde is a sensory contaminant at low concentrations. Preform specifications commonly require residual acetaldehyde below 5 mg/kg, and still water applications may tighten the limit below 2 mg/kg depending on fill volume, headspace, and shelf-life testing. The standard static headspace method is ASTM F2013-10, though sensitive applications may add cryofocusing or mass-selective detection.

Reduction of acetaldehyde requires controlling melt temperature, screw speed, and barrel residence time simultaneously. Barrier screws with moderate compression ratios limit shear heating in preform machines. Acetaldehyde scavengers can be added, but their effect on clarity, regulatory status, and long-term taste must be evaluated for each food-contact application. Solid-state polymerization reduces residual acetaldehyde while increasing IV, which is why SSP resin is preferred for water and soft drink preforms over melt-phase resin.

In post-consumer bottle streams, Code 1 PET is sorted by near-infrared sensors that detect the overtone bands of the polyester backbone. The molded Code 1 symbol assists manual verification and consumer communication, but automated sortation is primarily spectroscopic. A single PVC bottle in a PET bale can cause severe degradation during extrusion because PVC releases hydrogen chloride at PET processing temperatures; polylactic acid, if not separated, can create haze and crystallinity disturbances. Material recovery facilities therefore rely on NIR separation and density flake sorting to reduce contamination.

Post-consumer PET flake is washed, dried, and often processed through solid-state polycondensation or chain extension to raise IV from 0.65–0.70 dL/g back into the bottle-grade range. Food-contact recycled PET is governed by the same polymer safety framework as virgin material: 21 CFR 177.1630 in the United States and Regulation (EU) No 10/2011 in the European Union, with recycling processes evaluated for decontamination efficiency before an opinion or letter of non-objection is issued.

Fiber, Film, and Sheet Grade Differentiation

Fiber-grade PET is produced at 0.55–0.65 dL/g because melt spinning requires low melt viscosity and stable flow through spinnerets. Bottle-grade resin at 0.80–0.85 dL/g would increase backpressure and degradation in high-speed spinning unless melt temperature were raised. Film-grade PET is typically cast at 0.55–0.65 dL/g and then biaxially stretched; sheet-grade PET used in thermoformed cups and trays is processed at 0.70–0.80 dL/g and may be formulated with chain extenders when high levels of plant scrap are reused.

All three conversion routes share the same polymer backbone and Code 1 resin identification, but the additive packages, molecular weight targets, and melt histories are route-specific. Misapplying a bottle grade to fiber spinning raises thermal degradation and pressure, while fiber grade in a carbonated soft drink preform reduces melt strength and may fail burst or top-load specifications. These boundaries are measurable differences in intrinsic viscosity, shear viscosity, and orientation behavior under the relevant conversion conditions.