Polyethylene terephthalate (PET) is a semicrystalline thermoplastic polyester obtained from the esterification of purified terephthalic acid and monoethylene glycol followed by high-vacuum polycondensation at 270–285 °C. The repeat unit has a molar mass of 192.17 g/mol. Intrinsic viscosity is measured according to ASTM D4603-18 in a 60:40 phenol/1,1,2,2-tetrachloroethane solvent at 30 °C. Commercial bottle resin exhibits an IV of 0.80–0.85 dL/g, textile filament grades 0.55–0.64 dL/g, and high-tenacity industrial yarn 0.85–1.00 dL/g. The glass transition occurs at 76–80 °C and the crystalline melt endotherm at 245–255 °C for homopolymer containing diethylene glycol below 2.0 mol%.

Because the letter string PET also appears in unrelated fields, procurement specifications and laboratory information systems require explicit material coding. The following table separates typical commercial PET grades by intrinsic viscosity and processing route.

Pet, PET Scan and St. Pete: Sorting the Meanings Behind the Letters

In documentation systems, the term PET identifies the polyester described by CAS Registry Number 25038-59-9. Positron emission tomography is a nuclear medicine imaging method based on coincidence detection of annihilation photons at 511 keV produced after positron emission by fluorine-18 fluorodeoxyglucose. The imaging contrast of a PET scan reflects tissue glucose uptake and is not governed by the intrinsic viscosity or crystallinity of poly(ethylene terephthalate). St. Pete is a toponym; when used on shipping documents it denotes a destination such as St. Petersburg, Florida, and has no entry in ISO 472:2013 as a material abbreviation. Lowercase pet in veterinary records indicates a domestic animal, not a resin. The only technically appropriate identifier for the packaging and fibre polymer is polyethylene terephthalate, abbreviated PET, with the CAS Registry Number 25038-59-9.

Predrying of virgin bottle resin before injection molding is conducted in desiccant dryers with a dew point below -40 °C and an air temperature of 160–170 °C. Residual moisture must fall below 50 ppm because hydrolysis at melt temperature reduces IV by 0.02–0.05 dL/g per processing pass. Injection molding of preforms uses barrel temperatures of 270–285 °C, hot-runner temperatures of 280–295 °C, and melt residence times below 5 min. Acetaldehyde generation is controlled by limiting the rear barrel temperature to 255–270 °C and filling the screw to 60–75% of shot capacity. Headspace gas chromatography according to ASTM F2013-10 quantifies acetaldehyde in preforms; water bottle specifications often require less than 5 ppm, carbonated soft drink grades less than 3 ppm, and sensitive water applications less than 2 ppm.

Injection molds for preforms are operated with chilled water at 8–12 °C to retain the amorphous state. Preform cycle times for 20–30 g preforms on 48–96-cavity tools are commonly 18–35 s, with clamp force specified at 500–650 metric tons for high-cavitation systems. Stretch blow molding reheats the preform to 90–105 °C; axial draw ratios of 2.0–2.5 and hoop draw ratios of 3.5–4.5 produce biaxially oriented containers with improved tensile modulus and barrier. Tensile yield strength of biaxially oriented sidewalls is measured according to ASTM D638-14 or ISO 527-2:2012, with values commonly 55–65 MPa for amorphous preforms and 150–200 MPa in the hoop direction for oriented containers.

What Limits Recycled PET Intrinsic Viscosity Retention During Twin-Screw Extrusion?

Post-consumer recycled PET flake entering extrusion frequently has an IV of 0.70–0.78 dL/g. Hydrolytic chain scission is the primary degradation mechanism because the ester linkage is sensitive to water; melt-phase recycling at 270–280 °C without vacuum devolatilization can reduce IV by 0.05–0.10 dL/g in a single pass. Twin-screw extruders with L/D 40:1 and two-stage vacuum venting at absolute pressures below 25 mbar limit IV loss, but residence time distribution and surface renewal remain critical. Solid-state polycondensation at 200–215 °C under nitrogen for 12–18 h is applied to raise recycled flake IV to 0.80–0.85 dL/g before melt processing.

Chain extension with pyromellitic dianhydride at 0.10–0.30 wt% can restore IV, but addition above 0.50 wt% produces gel particles and melt-pressure instability. Melt flow rate is measured at 285 °C with 2.16 kg according to ISO 1133-1:2022; specifications are grade-specific. Recycled feedstock containing polyvinyl chloride, polypropylene, or paper labels must be separated before extrusion. Polyvinyl chloride contamination above 50 ppm liberates hydrogen chloride and generates black specks, while polyamide contamination causes yellowing and localized gel formation. Melt filtration at 40–80 µm is applied to remove particulate contamination.

Melt spinning of textile-grade PET is performed at extruder temperatures of 285–295 °C through spinnerets with capillary diameters of 0.25–0.40 mm. Take-up speeds for partially oriented yarn range from 1,500 to 3,500 m/min; drawn filament processes apply draw ratios of 3:1 to 6:1. Fully drawn and textured yarns typically display tenacity of 4.0–7.0 gf/den and elongation at break of 15–35% when measured by ASTM D2256-21. Spin finish is applied at 0.20–0.40 wt% to control static and friction; titanium dioxide delustrant at 0.30–0.50 wt% is added for matte fibers. Oligomer deposits on spinneret faces increase pack pressure; filtration through 20–40 µm media is standard.

Biaxially oriented PET film is produced by sequential drawing of amorphous cast sheet. Machine-direction draw ratios of 2.5–4.0 at 85–100 °C and transverse-direction draw ratios of 3.0–4.5 at 95–110 °C are used. Heat setting at 200–230 °C under restraint yields shrink values below 1.5% at 150 °C. Tensile strength of 12 µm film is 200–250 MPa in the machine direction when measured by ASTM D882-18. Oxygen transmission rate at 23 °C and 0% RH is typically 60–90 cm³/(m²·day·atm) when tested by ASTM D3985-17. Dielectric strength at 25 µm is 200–300 kV/mm by ASTM D149-20.

When Crystallization Half-Time Dictates Thermoforming Scrap Regrind Ratios

Thermoforming-grade sheet is produced from PET modified with 1,4-cyclohexanedimethanol or isophthalic acid to slow crystallization. The crystallization half-time for unmodified PET at 160–170 °C is on the order of 1–3 min; copolyester grades can extend this beyond 5 min, permitting wider processing windows. Sheet extrusion at 270–285 °C is quenched on chilled rolls at 20–40 °C to produce amorphous sheet with low haze. Thermoforming preheats the sheet to 110–140 °C; mold temperatures of 25–40 °C are used for clear amorphous containers, while crystallizing molds at 120–140 °C produce opaque, heat-stable parts. Regrind ratios of 30–50 wt% are common with virgin resin; higher regrind raises haze and lowers IV because of repeated thermal history. Moisture absorption at relative humidity above 60% before thermoforming can generate splay and must be addressed by drying at 65–80 °C for 2–4 h for amorphous sheet or 160–170 °C for crystallized regrind.

Compliance with food-contact regulations invokes FDA 21 CFR §177.1630 for polyethylene terephthalate in the United States; the regulation identifies acceptable polymers and extraction constraints. In the European Union, plastic food-contact materials are evaluated under EU Regulation No 10/2011, which sets the overall migration limit at 10 mg/dm² or 60 mg/kg for food simulants depending on article type. Specific migration of monoethylene glycol and terephthalic acid is limited under the same regulation; migration testing follows methods in the EN 1186 series. Recycled PET for food contact must comply with a positive approval process such as EU Regulation No 2022/1616 and operational phase criteria. Medical or pharmaceutical packaging made from PET must comply with USP <661.1> for plastic materials of construction and may require extractables profiling.