Commercial aromatic thermoplastic polyesters are condensation polymers in which an aromatic diacid or diester—typically terephthalic acid or dimethyl terephthalate—reacts with a short-chain aliphatic diol. The resulting ester repeat units are classified under ISO 1043-1:2011 as poly(ethylene terephthalate), poly(butylene terephthalate), and poly(trimethylene terephthalate), abbreviated PET, PBT, and PTT respectively. The term polyester is therefore a family identifier, not a synonym for PET.

Textile fibre nomenclature compounds this confusion. ISO 2076:2013 defines a polyester fibre as a fibre formed from linear macromolecules containing at least 85 % by mass of an ester of a diol and terephthalic acid. PET meets this generic definition, but PBT and PTT fibres also satisfy the same structural criterion. Consequently, a garment labelled polyester is usually PET, but the label alone does not exclude PBT or PTT.

Within the same polyester family, copolyesters such as PETG and PCTG replace a portion of ethylene glycol with cyclohexanedimethanol; they are also polyesters but are not crystallisable in the same manner as PET. This further illustrates that polyester is a structural class, not a single polymer. The present comparison is confined to the semi-crystalline terephthalate homopolymers PET, PBT, and PTT.

Is PET the Same as Polyester?

No. PET is a specific aromatic polyester whose repeat unit contains two methylene units from ethylene glycol between terephthalate ester linkages. The compact diol segment restricts chain mobility, giving PET a higher glass transition temperature and slower crystallisation than PBT and PTT. Polyester, by contrast, is the broader class of polymers containing ester linkages in the main chain and includes aliphatic polyesters, unsaturated polyesters, and the aromatic thermoplastic polyesters discussed here.

The repeat unit of PET can be written as –[O–CH₂–CH₂–O–CO–C₆H₄–CO–]ₙ. PBT substitutes a four-methylene sequence, and PTT substitutes a three-methylene sequence in the diol residue. The methylene sequence is not trivial: it changes the number of flexible C–C single bonds between ester groups, which in turn alters glass transition, melting point, crystallisation rate, and hydrolytic stability. These differences are detected directly by differential scanning calorimetry under ASTM D3418-21.

Molecular weight of PET is expressed as intrinsic viscosity. ASTM D4603-18 determines IV in 60/40 phenol/1,1,2,2-tetrachloroethane at 30 °C; bottle-grade resins typically fall in the 0.80–0.84 dL/g range, while textile grades may be 0.55–0.70 dL/g. PBT and PTT molecular weights are also measured by solution viscosity, but the solvent systems and reference standards differ, so IV values are not directly transferable across the polyester family.

PET, PBT and PTT Inside the Polyester Family

The family relationship is controlled by the number of methylene groups in the diol segment. PET uses ethylene glycol, PBT uses 1,4-butanediol, and PTT uses 1,3-propanediol. The additional methylene units in PBT and PTT increase chain flexibility, reduce glass transition temperature, and modify crystalline phase formation. PET crystallises in a triclinic lattice and develops orientation slowly in cold stretch-blow molding; PBT crystallises rapidly from the melt and is therefore used in thin-wall electronic parts with short cycle times. PTT displays intermediate crystallisation and a pronounced strain recovery used in carpet and apparel.

The values in the following table are representative ranges drawn from published technical datasheets and are not engineering specifications.

The practical differentiation extends beyond transition temperatures. PBT crystallises fast enough to be injection molded with mold temperatures of 20–60 °C and still achieves dimensional stability; PET engineering compounds, unless oriented and quenched as in preform molding, generally require mold temperatures of 100–140 °C to develop crystallinity. PTT mold temperatures of 20–80 °C are reported in commercial processing literature. With 30 wt% glass-fibre reinforcement, PBT heat deflection temperature at 1.82 MPa rises to 190–210 °C under ASTM D648-16, while comparable PET compounds may exceed 200 °C depending on crystallinity.

PET food-contact compliance is supported by FDA 21 CFR 177.1630 for PET and by European Commission Regulation (EU) No 10/2011 with overall migration limits of 10 mg/dm². PBT and PTT food-contact grades are available but require separate compliance verification under the same framework; migration limits and conditions of use vary by resin grade and additive system.

On a production-scale PET preform injection molder with 96-cavity hot-runner tooling, the controlling variables are pellet moisture, melt temperature, residence time, and acetaldehyde generation. PET pellets are dried in closed-loop desiccant dryers at 160–170 °C for 4–6 h to reduce moisture below 50 ppm. The drying-air dew point is maintained at -40 °C to -50 °C. At ambient relative humidity above 60 %, hopper capacity must be derated or drying time extended because PET reabsorbs moisture rapidly.

Barrel and hot-runner setpoints are held between 270 °C and 295 °C for bottle-grade PET with intrinsic viscosity of 0.80–0.84 dL/g. Melt temperature above 310 °C accelerates acetaldehyde evolution and yellowing. Acetaldehyde is measured on the preform by static headspace gas chromatography under ASTM F2013-10; water-contact specifications commonly require values below 10 ppm. Screw design is normally 20:1–24:1 L/D with a compression ratio of 2.5:1–3.5:1, and melt residence time is held below 5 min to limit degradation.

Mold cooling water at 8–12 °C freezes the preform into an amorphous state; dwell times are short enough to avoid crystalline haze but long enough to prevent gate crystallisation. Contaminant control is critical. Polyvinyl chloride label residue in recycled PET can release hydrochloric acid during melting and accelerate hydrolysis; therefore, PVC is usually limited to below 50 mg/kg. Polyamide contamination above 30 mg/kg can cause aminolysis and intrinsic-viscosity loss, while silicone oil and certain ester lubricants may alter hot-runner flow. These are operational boundaries rather than universal resin specifications.

Preform IV is not merely a resin property; it is also a process control variable. Hot-runner gate design, barrel temperature profile, screw-speed profile, and back pressure determine the shear heating and molar mass retention. For carbonated-soft-drink preforms requiring burst strength, bottle-grade IV is often specified at 0.80–0.84 dL/g, while water preforms may be produced at 0.74–0.78 dL/g. Lowering screw back pressure below 5 bar may reduce shear heating but also causes unmelt and acetaldehyde heterogeneity. Published data for this specific configuration is limited, but the relationship between IV and bottle deformation is routinely monitored by ASTM D2463-15 for plastic bottle drop impact resistance and ASTM F2013-10 for acetaldehyde.

After preform injection, the amorphous preform is reheated in an oven to 105–120 °C and stretch-blow-molded at planar stretch ratios of 9:1–12:1. This orientation induces strain-hardening and crystallinity. Preheat temperature above 130 °C produces thermal crystallisation and haze; below 95 °C creates pearlescence and uneven wall thickness. These boundaries are standard in two-stage reheat stretch-blow molding.

Representative melt-conversion parameters for the three polyesters are summarised below. These ranges are compiled from equipment bulletins and resin supplier literature; actual settings depend on grade, crystallinity, and tool layout.

When Residual Moisture Remains Above 50 ppm During Polyester Melt Processing

At residual moisture above 50 ppm, PET undergoes hydrolytic chain scission in the melt. The reduction in molecular weight is measured as a loss of intrinsic viscosity by ASTM D4603-18 and appears on the molding floor as lower melt viscosity, preform haze, and reduced burst strength. For PET bottle preforms, moisture control is non-negotiable because the processing window is narrow: too little drying and hydrolysis occurs; excessive drying above 180 °C causes pellet sticking and acetaldehyde build-up. PBT and PTT also require drying, but their equilibrium moisture uptake is lower, and they are typically dried to 0.02 % or less at lower temperatures.

PBT is less sensitive to moisture than PET but not immune. Residual moisture above 0.02 % in PBT can produce silver streaks on part surfaces and lower tensile strength measured by ASTM D638-14. PTT occupies an intermediate position: it is dried at 130–150 °C, and extended residence above 270 °C for more than 5 min produces measurable discoloration and molecular-weight loss. The aromatic polyester family shares a common ester-linkage vulnerability to hot moisture, amines, and strong alkalis; processing and service environments must remain below the hydrolysis threshold.

PBT is specified for electrical connectors, relay bases, and sensor housings because its fast crystallisation permits short cycles and because glass-reinforced grades retain stiffness up to 190–210 °C heat deflection temperature under ASTM D648-16. Flame-retarded PBT grades are screened under UL 94 and glow-wire testing in accordance with IEC 60695-2-12. PBT has a limitation in continuous hot-water service above 60 °C; long-term hydrolysis reduces tensile strength measurable by ASTM D638-14.

PTT is used in fibre, carpet, and automotive interior applications where strain recovery and low-temperature flexibility are required. Comparative tensile recovery data for PET and PTT are limited when measured under the same standard method, so end-use fabric testing remains mandatory. PTT must avoid extended melt residence and strong alkaline detergent conditions in carpet cleaning above pH 10.