Polyethylene terephthalate (PET) is a semicrystalline thermoplastic polyester formed from ethylene glycol and purified terephthalic acid or dimethyl terephthalate. Its repeat unit is –OCH₂–CH₂–OCO–C₆H₄–CO– and its CAS registration number is 25038-59-9. ISO 1043-1 designates the material as PET. Commercial product lines are not a uniform material; they are segmented by intrinsic viscosity, comonomer content, crystallinity, carboxyl end-group concentration, residual acetaldehyde, color coordinates, and fines level. Product models are therefore specified by these analytical properties rather than by a single resin name.

Continuous melt-phase polymerization is followed by solid-state polymerization for high-viscosity grades. Melt-phase lines operate under high vacuum to remove ethylene glycol and water; antimony, germanium, or titanium catalysts are commonly used. Solid-state polymerization raises intrinsic viscosity by continuing esterification and transesterification in the solid state at 200–235 °C under vacuum or purified nitrogen for 12–24 h, depending on target molecular weight. This two-stage route is standard for bottle resin because it increases molecular weight while simultaneously reducing residual acetaldehyde and homogenizing crystallinity.

Intrinsic viscosity is not the sole specification axis

Intrinsic viscosity is measured according to ASTM D4603-18 or ISO 1628-5:2015 using a 60/40 phenol/1,1,2,2-tetrachloroethane solvent mixture at 30 °C. Typical unfilled product specifications are 0.58–0.68 dL/g for biaxially oriented film, 0.60–0.65 dL/g for staple fiber, 0.74–0.80 dL/g for water-bottle preforms, 0.78–0.84 dL/g for carbonated soft-drink preforms, and 0.90–1.05 dL/g for injection-molded engineering articles compounded with glass fiber or mineral fillers.

However, intrinsic viscosity alone does not define processability. Isophthalic acid and diethylene glycol comonomer concentrations, typically in the 1–3 mol% and 0.8–2.0 mol% ranges respectively for bottle grades, suppress melt crystallization and alter preform reheat performance. Carboxyl end-group concentration controls hydrolysis kinetics and is specified at ≤ 35 mmol/kg for many bottle resins. Solid-state polymerization does not simply increase molecular weight; it also reduces volatile organic by-products and narrows the carboxyl end-group distribution.

Biaxially oriented PET film is produced by flat-die extrusion onto a chilled roll at 20–30 °C, followed by sequential or simultaneous stretching. Oriented film reaches tensile modulus values of 4000–6000 MPa in the stretch direction as measured by ISO 527-3:2018. Clear film haze is commonly 1–3%. Film-grade PET is specified for capacitor films, flexible packaging, labels, and solar backsheets where dimensional stability and optical clarity are required.

Fiber-grade PET is melt-spun at 280–295 °C through spinnerets, then drawn to partially oriented yarn or fully drawn yarn. Drawn fiber tenacity is typically 4.0–8.0 cN/dtex with elongation at break of 15–35% under ISO 2062:2009. In staple fiber and continuous filament production, molecular weight uniformity is critical because periodic draw breaks on high-speed lines are associated with intrinsic viscosity fluctuation exceeding ± 0.02 dL/g.

Residual moisture in dried PET is the principal independent variable controlling melt-phase hydrolysis. Pellets equilibrated at 50% relative humidity may contain 0.2–0.4 wt% water; conversion without drying reduces molecular weight through ester bond scission and produces surface splay and preform haze. Desiccant wheel dryers with a dew point of ≤ −40 °C are specified. Drying at 160–180 °C for 4–6 h lowers moisture to ≤ 0.005 wt% (50 ppm) for general molding and ≤ 0.003 wt% (30 ppm) for bottle-grade preform molding. Hopper residence above 190 °C causes surface thermal oxidation and yellowing.

Melt processing is constrained by a narrow thermal window. Barrel temperatures of 270–290 °C are common for preform injection; prolonged residence above 5 min at 285 °C accelerates chain scission and acetaldehyde formation. Compounding lines using twin-screw extruders with 28:1–40:1 L/D ratios apply vacuum venting to strip residual volatiles. Mold surface temperatures for amorphous preforms are held at 10–15 °C to prevent premature crystallization, whereas heat-set bottle molds operate at 120–140 °C to promote stress-relaxed crystallinity.

What Limits Acetaldehyde Generation During Preform Injection Molding?

Acetaldehyde is a thermal degradation product generated by β-scission of vinyl ester end-groups and by melt-phase oxidation. Bottle-grade resin is routinely specified at ≤ 3 µg/g for carbonated soft drinks and ≤ 2 µg/g for water packaging, measured by headspace gas chromatography according to ASTM F2013-10. Preform limits are higher because molding adds acetaldehyde; many converters specify ≤ 8 µg/g in the finished preform. The compound partitions into package headspace and alters beverage flavor at concentrations that are not toxicologically significant.

The processing window is therefore governed by the balance between temperature, residence time, and intrinsic viscosity. Melt temperature should remain at 270–280 °C for bottles where flavor neutrality is critical. Hot-runner manifold temperatures are offset no more than 5–10 °C above the barrel set-point. Screw recovery time, cushion, and decompression are adjusted to keep melt residence time near 2–3 min. High-velocity valve-gated hot runners reduce shear heating and are preferred on 96–192 cavity preform lines. Acetaldehyde generation is also reduced by high-vacuum solid-state polymerization, which strips acetaldehyde to <1 µg/g before drying and melt conversion.

Injection stretch blow molding converts the preform into a biaxially oriented hollow container by reheating to 95–110 °C and stretching with a mechanical stretch rod while high-pressure air forms the vessel. Axial stretch ratios of 2.5–3.0 and hoop stretch ratios of 2.0–2.5 are typical for 500 mL carbonated soft drink bottles. Sidewall crystallinity after orientation is 20–30% as measured by differential scanning calorimetry per ISO 11357-3:2018. Oriented crystallinity plus molecular alignment raises tensile modulus in the hoop direction above the isotropic value, which permits lightweighting while retaining top-load and burst strength.

Heat-set blow molding for hot-fill and pasteurization uses mold temperatures of 120–140 °C to raise effective crystallinity and reduce shrinkage. Standard cold-fill bottles are not specified for continuous service above 60 °C; heat-set PET can tolerate fill temperatures up to 85 °C without unacceptable panel deformation. Blow molding equipment with servo-controlled stretch rods and pressure profiling is required for repeatable material distribution in lightweight preforms.

Barrier and thermal property differentials versus polycarbonate and polyolefins

Comparative property measurements separate PET from polycarbonate, polybutylene terephthalate, and polyolefins in transparent packaging and engineering applications. Unfilled PET provides tensile yield strength of 55–75 MPa and tensile modulus of 2800–3100 MPa under ISO 527-2:2012. These values exceed unfilled polypropylene and are comparable to polycarbonate; however, PET is notched-impact limited at 2–4 kJ/m² and heat-deflection limited at 70–80 °C at 1.82 MPa per ISO 75-2/A:2020. Polycarbonate retains notched Izod values of 60–80 kJ/m² and is therefore selected where impact performance dominates.

In barrier performance, biaxially oriented PET film transmits far less oxygen than polyolefins at equivalent thickness. Oxygen transmission rates measured by ASTM D3985-17 at 23 °C and 50% relative humidity are commonly reported in the range 50–150 cm³/(m²·day·bar) for 25 µm oriented PET film; polyethylene and polypropylene films of equal thickness transmit oxygen at magnitudes of 1000–2000 cm³/(m²·day·bar). Polycarbonate is also a poor oxygen barrier under these conditions. This differential makes PET suitable for carbonated soft drink and oxygen-sensitive food packaging. Water-vapor barrier remains moderate, and aluminum oxide or silicon oxide coatings are applied when high moisture-barrier is required.

Polybutylene terephthalate crystallizes far more rapidly than PET; mold temperatures of 80–120 °C are used for PBT, while PET preform molds must remain cold to avoid haze. PBT offers better notch toughness and lower moisture absorption, but lower oxygen barrier and lower glass transition. Polyethylene naphthalate is a higher-barrier polyester with a glass transition near 120 °C and a melt temperature of 265–270 °C, used where oxygen barrier and thermal stability justify cost; it is outside standard PET mechanical recycling streams.

When glycol-modified PET replaces homopolymer PET in thermoformed trays

PETG is an amorphous copolyester in which 1,4-cyclohexanedimethanol replaces a portion of ethylene glycol. ISO 1043-1 classifies these copolyesters separately from homopolymer PET. PETG is selected for thick-wall extruded sheet and thermoformed trays because it does not crystallize during cooling and offers notched Izod impact of 9–12 kJ/m². Its tensile modulus is lower, 1900–2100 MPa compared with 2800–3100 MPa for PET, and its heat deflection temperature at 1.82 MPa is 63–70 °C. The material is therefore unsuitable for hot-fill applications above 65 °C, where heat-set PET or polycarbonate may be required.

Barrier data for PETG show higher oxygen and carbon dioxide transmission than oriented PET at equal thickness. This restricts PETG use in long-shelf-life oxygen-sensitive packaging. PETG extruded sheet is processed at lower melt temperatures than PET, but drying remains necessary; typical desiccant drying conditions are 65–70 °C for 4 h to <0.01 wt% moisture. Thermoforming of PETG sheet requires lower infrared heating intensity because amorphous sheet softens without crystallization, and edge-trim regrind can be incorporated at 20–50 wt% without loss of optical clarity.

Post-consumer recycling streams expose PET to polyvinyl chloride and other chlorinated polymers. PVC degrades at PET melt-processing temperatures above 260 °C, liberating hydrogen chloride that catalyzes PET hydrolysis and discoloration. Near-infrared sorting equipment separates PVC from PET bottles, and many bottle-to-bottle recycling operations reject flake with PVC contamination above 100 ppm. Metal contamination from aluminum caps must also be removed before melt filtration.

Unfilled PET is not recommended for continuous load-bearing service above 120 °C unless crystallized or heat-set. Amine-based additives, certain nucleating agents, and halogen-containing flame retardants can accelerate ester cleavage or corrode melt converting equipment; such additives require qualification before compounding. Food-contact PET is specified under FDA 21 CFR 177.1630 and European Commission Regulation 10/2011. Recycled PET intended for food contact in the European Union must meet the decontamination requirements of Regulation 2022/1616. Polyvinyl chloride contamination is considered a critical contaminant in mechanical recycling because of hydrogen chloride release during extrusion.