The PET vs PETG temperature windows for hot-fill, sterilisation, and heat deflection are separated by morphological differences rather than by glass transition alone. Polyethylene terephthalate (PET) is a semicrystalline polyester synthesised from ethylene glycol and terephthalic acid or dimethyl terephthalate, with a glass transition near 78–80 °C, a cold-crystallisation exotherm between 130 °C and 150 °C, and a melting range of 245–265 °C. Glycol-modified PET (PETG) replaces a substantial fraction of the ethylene glycol with cyclohexanedimethanol, which disrupts chain regularity sufficiently to eliminate crystallisation under commercial forming conditions. PETG therefore exhibits a glass transition of 78–85 °C and no measurable melting endotherm by differential scanning calorimetry. On a production line this distinction appears immediately in processing equipment: amorphous PET preforms are injection moulded through barrels at 270–295 °C into chilled moulds at 10–15 °C, while PETG sheet is extruded on a single-screw extruder with an L/D ratio of 24:1 to 32:1 at melt temperatures of 220–260 °C. Both resins require desiccant drying before melt processing; PET is dried at 160–170 °C for 4–6 h to below 50 ppm moisture, whereas PETG is dried at 65–70 °C for approximately 4 h to below 400 ppm moisture. The retained moisture limit is not a formality because hydrolytic degradation during hot-fill or steam sterilisation reduces molecular weight and shifts the practical deformation temperature downward.

How Do Heat-Set Morphology and Glycol Comonomer Content Alter Hot-Fill Limits?

Hot-fill beverage lines typically fill containers at 85–90 °C, hold the filled container for 10–60 s, invert it to pasteurise the closure, and then cool under vacuum that creates sidewall deflection stress. PET can be heat-set during two-stage stretch blow moulding to survive this operation. Injection-moulded PET preforms are reheated to 100–120 °C in a reheat stretch blow moulder, stretched, and blown into moulds maintained at 130–150 °C. The orientation and thermal conditioning produce a crystalline fraction in the sidewall of approximately 30–40 %, measured by density using amorphous PET density of 1.333 g/cm³ and crystalline PET density of 1.455 g/cm³ per ASTM D1505-18. Production heat-set blow moulding machines alternate hot and cold mould stations to set the bottle and release it without uncontrolled shrinkage. Under these conditions, the sidewall can withstand filling at 85–90 °C while maintaining sidewall shrinkage at or below 2–3 %. Amorphous PET without heat-setting does not have this resistance; cold-fill PET bottles can show diameter shrinkage and volume loss when exposed to sustained temperatures above 60–65 °C, especially in the shoulder and lower panel regions.

PETG cannot access the same heat-set pathway because the cyclohexanedimethanol comonomer suppresses strain-induced crystallisation at commercial stretch ratios. PETG blow moulds are run cold at 10–30 °C to preserve transparency and dimensional stability. The practical hot-fill ceiling for PETG is therefore bounded by its glass transition and by creep under top-load and sidewall vacuum. Industrial packaging datasheets generally place the continuous hot-fill limit for unreinforced PETG between 55 °C and 65 °C. At sustained fill temperatures above 65 °C, PETG containers exhibit panel deformation, thread ovality, and dimensional recovery loss. In a hot-fill line running at 85 °C, PETG is excluded from unsupported container geometries unless the pack is intentionally cooled before the liquid contacts the sidewall or the container is overmoulded with a high-temperature support structure. Published data for PETG performance at true hot-fill temperatures above 70 °C under pack cooling vacuum is limited.

Table 1. Comparative thermal data for PET and PETG relevant to hot-fill, sterilisation, and heat deflection.

Autoclave sterilisation for medical and pharmaceutical packaging imposes a 121 °C saturated-steam exposure with a gauge pressure of approximately 103 kPa for 15–30 min according to ISO 17665-1:2006. At that temperature both PET and PETG are above their glass transition temperatures, so shape retention depends on crystallinity, orientation, wall thickness, and the external pressure differential. Heat-set PET with a crystalline fraction above 30 % may tolerate steam sterilisation in rigid containers because the crystalline domains maintain part geometry while the amorphous domains soften. The steam environment nevertheless hydrolyses the ester linkages, and validation must include post-sterilisation intrinsic viscosity measurement by ASTM D4603-18 to confirm that molecular weight has not fallen below the part specification. For amorphous PET and unmodified PETG, the same autoclave cycle produces visible deformation at gate, thread, and sealing surfaces. On a 1,300 kN injection moulding machine, a medical manifold moulded in PETG shows repeated autoclave failure as corner warpage and lid-channel spread exceeding 0.5 mm relative to the CAD datum after the first sterilisation cycle. By comparison, an amorphous PET thin-wall cup will whiten, soften, and collapse under stack load, while a PET part crystallised at 140 °C for 2 h before machining may exhibit less than 1.0 % volumetric shrinkage.

Because PETG cannot endure saturated-steam sterilisation, medical device trays and pharmaceutical packaging are sterilised by ethylene oxide at 55 °C per ISO 11135:2014, by hydrogen peroxide gas plasma at 45–55 °C, or by gamma irradiation at 25–40 kGy per ISO 11137-1:2006. Dry-heat sterilisation at 160 °C is outside the operating envelope for both PET and PETG because oxidative yellowing and dimensional collapse occur before the cycle completes. The contraindication is not symmetrical: heat-set PET may survive steam sterilisation if the part is rigid and the wall is thick enough to resist the softening pressure differential, whereas PETG fails because it has no crystalline network to retain shape above its glass transition. This distinction is not visible in a single heat deflection temperature comparison.

When Heat Deflection Temperature Misrepresents Short-Term Load Capacity at 0.455 MPa

ASTM D648-18 and ISO 75-2:2013 measure the temperature at which a rectangular specimen deflects by 0.25 mm under a flexural load of 0.455 MPa or 1.82 MPa. ISO 75-2:2013 method B uses a flexural stress of 0.45 MPa, method A uses 1.80 MPa, and method C uses 8.00 MPa; ASTM D648-18 commonly reports values at 0.455 MPa and 1.82 MPa. The test is a short-time creep measurement at a controlled heating rate, typically 2 °C/min, not a maximum service temperature. For PET and PETG, the 0.455 MPa heat deflection values are often close: 70–75 °C for amorphous PET and 67–73 °C for PETG. That apparent similarity disappears under constrained hot-fill because the hot-fill stress state is biaxial tensile creep in a thin sidewall, not edgewise flexural bending of a 4 mm or 12.7 mm specimen.

Injection moulding process conditions further separate the materials. PET can be moulded in a cold mould at 10–15 °C to produce an amorphous transparent part, or in a hot mould at 130–150 °C to produce a crystallised opaque part with higher short-term load resistance. PETG is moulded at 220–260 °C with a mould temperature of 10–30 °C and remains amorphous regardless of mould heating because the comonomer suppresses crystallisation. The heat deflection temperature of an unreinforced PET part crystallised in a hot mould at 1.82 MPa is often reported in the range 80–120 °C, but the exact value depends on part thickness, crystallinity profile, and fibre reinforcement. Published HDT data for unreinforced heat-set PET sheet at 1.82 MPa is configuration-specific, and a resin supplier value cannot be transferred to a formed part without testing specimens cut from the sidewall according to ISO 75-2:2013. For incoming resin quality control, however, heat deflection testing remains useful provided the exact stress, conditioning history, and specimen geometry are reported with the standard code.