Polyethylene terephthalate (PET) is a semicrystalline thermoplastic polyester produced commercially by direct esterification of terephthalic acid with ethylene glycol or by transesterification of dimethyl terephthalate with ethylene glycol under antimony-, titanium-, or germanium-based catalysis. The linear repeat unit has molar mass 192.17 g/mol, and the homopolymer typically exhibits solid density 1.38–1.40 g/cm³, glass transition 70–80 °C, and a melting endotherm between 245 °C and 255 °C when measured by differential scanning calorimetry according to ISO 11357-1:2016. The polymer is supplied as amorphous pellets, solid-state-polymerized pellets, or regrind, and its processing window is defined by moisture, residual acetaldehyde, intrinsic viscosity, and comonomer content.

The orthographic coverage of the material in trade documentation is broader than the standard name. Search records and safety-data-sheet indexes contain the string “Polyetylentereftalat, Terephtalat and 12 More Spellings of Polyethylene Terephthalate,” reflecting variants such as Polyethylene Terephtalate, Polyethylene Terepthalate, Polyethylenterephthalat, Polyetylentereftalat, Terephtalat, Polietilentereftalato, Polietilentereftalato, polyéthylène téréphtalate, polytéréphtalate d’éthylène, poly(ethylene terephthalate), poly(oxyethyleneoxyterephthaloyl), PET, and PETE. These designations refer to the same polymer with CAS registry number 25038-59-9, but raw-material specifications often list only one synonym, making normalization necessary for technical comparison.

When Polyetylentereftalat Enters the Resin Dryer

Hydrolysis is the dominant degradation mode during melt processing. At the injection-molding hopper, PET pellets can contain 0.2–0.3% moisture if exposed to ambient air, and moisture above 30 ppm at the barrel causes chain scission, carboxylic acid end-group buildup, and loss of intrinsic viscosity. Desiccant dryers with dew points at or below −40 °C are specified; drying at 160–180 °C for 4–6 h lowers moisture below 30 ppm, with verification by ISO 15512:2019. Pellet thickness above 3.5 mm, hopper hot spots, or insufficient regeneration of the desiccant bed can shift the required residence time to the upper bound or produce batch-to-batch moisture variance. Direct barrel venting is not a substitute for drying because hydrolytic damage occurs before volatile removal.

Industrial PET grades are distinguished primarily by intrinsic viscosity. Fiber and film grades are polymerized to lower molecular weight, while bottle grade requires solid-state polycondensation to reach intrinsic viscosity values above 0.78 dL/g. The table below summarizes representative commercial ranges.

What Limits Acetaldehyde Formation in Injection-Molded Preforms?

Acetaldehyde is a thermal degradation byproduct with organoleptic impact in water and beverage containers. Bottle-grade preforms commonly require acetaldehyde below 3 ppm, with sensitive beverages often specifying 2 ppm; analysis is performed by headspace gas chromatography according to ASTM F2013-10(2016). Acetaldehyde forms by β-scission of glycol end groups and by thermal decomposition of vinyl ester chain ends. Melt residence time above 5 min, barrel temperature above 285 °C, or hot-runner dead spots increase acetaldehyde concentration. Acetaldehyde scavengers can reduce headspace values, but amine-based additives may produce color bodies and are generally avoided in high-clarity preform formulations. Injection molders therefore balance cycle time, cavity fill, and degradation through melt temperature, screw recovery time, and hot-runner balance.

A comonomer such as isophthalic acid is commonly incorporated at 2–5 wt% to disrupt crystallization and improve preform clarity. Diethylene glycol formed during esterification acts as a similar defect; typical levels in bottle-grade PET are 1.0–1.5 mol%. These defects lower melting point by 5–15 °C and slow spherulitic crystallization, but they also reduce thermal stability and barrier performance. Copolymerized PET with high isophthalate content requires lower drying temperatures near 160 °C because pellet sticking occurs at lower temperature than in homopolymer. The balance between clarity, barrier, and heat resistance is controlled through final melting point, intrinsic viscosity, and comonomer content rather than through a single additive package.

Solid-State Polymerization and the Intrinsic Viscosity Response Surface

Solid-state polycondensation increases molecular weight without passing through a melt. Amorphous pellets are crystallized first at 150–170 °C to prevent agglomeration, then heated under vacuum or nitrogen at 200–220 °C. Ethylene glycol generated by transesterification is removed by purge gas or vacuum, shifting equilibrium toward higher molecular weight. Intrinsic viscosity rises from 0.60 dL/g to approximately 0.78 dL/g within 8–12 h, and reaches 0.85 dL/g only after extended residence of 18–24 h. Reaction rate depends on pellet crystallinity, pellet size, gas dew point, and residual catalyst. If the bed temperature exceeds 225 °C, partial fusion creates agglomerates and nonuniform residence-time distribution. Lower temperatures reduce sticking but require longer cycles and may not reach bottle-grade intrinsic viscosity. The process is routinely monitored by solution viscosity and carboxylic end group titration because melt flow rate alone does not capture end-group quality.

High-tenacity PET filament is melt-spun through spinnerets with capillary diameters of 0.25–0.40 mm at 285–295 °C, quenched with air at 20–30 °C, and wound at speeds above 2500 m/min. Orientation-induced crystallization at high spinning speed creates a partially crystalline yarn with low elongation, which is subsequently hot-drawn at 160–220 °C at draw ratios between 3:1 and 6:1. The resulting tenacity for technical yarns is typically 7–9 cN/dtex with elongation below 15%, tested according to ISO 2062:2009. Intrinsic viscosity loss across spinning is typically 0.02–0.05 dL/g and is controlled by drying, barrel temperature, and residence time. Spin finish application and interlace level are adjusted for downstream weaving; no single spin finish is universal across warp knitting, tire cord, or geotextile conversion.

Why Is Intrinsic Viscosity Measured in Phenol/1,2-Dichlorobenzene at 25 °C?

Solution viscosity is the practical index of molecular weight because PET is not readily analyzed by gel permeation chromatography without specialized solvent systems. The standard dissolution medium is phenol/1,2-dichlorobenzene at 60/40 or 50/50 by mass, with measurement at 25 °C according to ISO 1628-5:2015. Reduced and inherent viscosities are extrapolated to zero concentration to yield intrinsic viscosity. The method is sensitive to moisture, undissolved gel, and oligomer content; unfiltered or poorly dissolved samples can bias the result by at least 0.02 dL/g. Mark–Houwink constants depend on solvent ratio and temperature, so intrinsic viscosity values from different laboratories cannot be compared unless the solvent system and calculation method are identical. This constraint is especially relevant when comparing virgin bottle-grade PET with post-consumer recycled PET, where oligomer and filler residues affect the extrapolation.

Oxygen and carbon dioxide barrier in PET packaging derive from the free-volume characteristics of the amorphous phase. Biaxially oriented PET at 35–45% crystallinity exhibits oxygen permeability in the range of 0.04–0.09 cm³·mm/(m²·day·atm) at 23 °C and 0% relative humidity, with permeability increasing at higher humidity. Carbonated soft-drink bottles rely on CO₂ retention rather than oxygen exclusion; strain-hardened sidewalls reduce creep but do not eliminate permeability. For beer and oxygen-sensitive beverages, monolayer PET is insufficient, and commercial structures incorporate oxygen scavengers, plasma-deposited silicon oxide coatings, or internal layers of MXD6 or EVOH. The exact scavenger concentration required for a 12-month shelf life depends on container surface area, headspace volume, and distribution temperature; published data for specific scavenger-polymer combinations in long-storage carbonated beverages is limited.

If Solid-State Polymerization Raises Intrinsic Viscosity Above 0.82 dL/g

High-intrinsic-viscosity PET increases melt pressure and shear heating during preform injection. At 285 °C and a shear rate of 100 s⁻¹, melt viscosity is sufficient to require injection pressures above 1400 bar on high-cavitation preform molds with 48–72 cavities. Hot-runner temperatures must remain within approximately 280–290 °C; lower temperatures produce flow marks and short shots, while higher temperatures accelerate acetaldehyde formation. Screw L/D ratios below 20:1 may deliver unmelted pellets to the metering zone, whereas compression ratios above 3.0:1 increase shear heating and viscosity nonuniformity. Back pressure and screw speed are reduced to limit frictional heat, but excessive reduction extends recovery time and residence time. Preform machines therefore require calibrated hot-runner balance, melt-pressure sensors, and cavity-to-cavity fill analysis when processing high-IV PET.

Post-consumer PET flake is sorted by near-infrared spectroscopy to reduce polyolefin, PVC, and other contaminants, then ground, washed in hot caustic solution at 80–90 °C, and dried before extrusion. Melt filtration and vacuum devolatilization remove particulates and volatile impurities, but solid-state polycondensation is often required to rebuild intrinsic viscosity from 0.70–0.75 dL/g to above 0.78 dL/g. Food-contact recycled PET is regulated by Regulation (EU) 2022/1616 in the European Union and by FDA no-objection letters in the United States, with challenge testing and migration limits derived from Regulation (EU) 10/2011 and 21 CFR 177.1630. Heavy metal restrictions under RoHS Directive 2011/65/EU apply to electronic articles, not packaging, but pigment residues in colored flake may affect color quality and are reduced by sorting and washing.