Polyethylene Terephthalate (PET)

Polyethylene terephthalate (PET) is a linear condensation polyester produced from purified terephthalic acid or dimethyl terephthalate and ethylene glycol. The repeat unit contains aromatic terephthalate rings and aliphatic ethylene glycol segments, giving the resin a balance of stiffness, gas barrier, and melt processability. In unmodified form, PET crystallizes rapidly under orientation and heat, which is an advantage in biaxially oriented film and bottle preforms but a limitation in thick-section transparent parts. The materials covered by the designation PETG and PCTG: Glycol-Modified Copolyesters and Where They Belong are produced by replacing a fraction of ethylene glycol with 1,4-cyclohexanedimethanol, often abbreviated CHDM. This substitution interrupts chain packing and suppresses crystallization, placing PETG and PCTG between amorphous PET and polycarbonate in terms of clarity, toughness, and thermoformability.

Bottle-grade PET is often described by intrinsic viscosity values of 0.72–0.84 dL/g and a melt peak near 245–255°C when measured by differential scanning calorimetry. Glycol-modified copolyesters do not exhibit a comparable melting endotherm, because the cycloaliphatic diol prevents the formation of sufficiently ordered crystallites. The result is an amorphous resin that softens over a glass transition range rather than at a defined crystalline melting point. This distinction governs downstream conversion behavior, inspection criteria, and thermal performance limits.

What Converts Bottle-Grade PET into an Amorphous Copolyester?

The change is primarily compositional. Technical literature on melt-phase copolyesterification indicates that CHDM incorporation above approximately 20–25 mol% of total diol suppresses crystallization under standard differential scanning calorimetry cooling rates of 10°C/min. Commercial PETG therefore remains optically transparent after injection molding and sheet extrusion, while bottle-grade PET would develop haze. The molecular weight of PETG is still measured as intrinsic viscosity, commonly in the 0.70–0.80 dL/g range, but melt viscosity is lower at equivalent temperature because the irregular chain architecture reduces chain alignment.

PCTG is chemically similar but generally formulated with a higher CHDM ratio or an additional cycloaliphatic diol comonomer. This further reduces crystallinity and modulus while shifting impact behavior. The practical boundary is not fixed by a single classification code; it is defined by the comonomer feed ratio and the resulting thermal, rheological, and mechanical response. Because polar ester linkages remain, both PETG and PCTG are moisture sensitive and require controlled drying before melt processing.

Resin drying for PETG and PCTG is the first process step where hydrolysis-induced molecular weight reduction appears on production-scale equipment. Desiccant dryers with a closed-loop dew point of -30°C or lower are specified in supplier processing guides. Hopper residence of 4 h at 65–70°C reduces pellet moisture to below 0.08%. In plants where ambient relative humidity exceeds 60%, dried pellets exposed to plant air for more than 30–45 min can absorb enough moisture to generate splay at the die or silver streaks in molded parts. Hydrolysis of ester linkages produces chain scission, lowering notched impact strength before visible moisture defects appear. The failure is measurable as a shift in melt flow rate under ISO 1133-1:2022 and as a reduction in tensile elongation at break under ISO 527-2:2012.

Thermoforming Sheet and the Pre-Drying Threshold

Sheet extrusion lines processing PETG and PCTG use lower melt temperatures than bottle-grade PET. Typical supplier conditions for sheet of 0.7–1.2 mm gauge list melt temperatures of 230–250°C, die temperatures of 230–250°C, and polished roll-stack temperatures of 60–90°C to control surface quality without causing roll blocking. The screw configuration is usually a single-stage barrier design with L/D ratios between 28:1 and 32:1; high-compression screws designed for crystalline PET can generate excessive shear heating in amorphous copolyesters and reduce melt stability.

Thermoforming of PETG sheet is typically performed at sheet surface temperatures of 110–130°C. The window is narrower than polycarbonate because the copolyester does not have a sharp crystalline plateau. Below 105°C, deep-draw parts show stress whitening and corner cracking. Above 135°C, sheet sag becomes severe and wall-thickness variation increases. For medical trays with draw ratios above 2:1, the practical forming band can be no wider than ±5°C. Production lines use zoned ceramic infrared heaters and sheet-sag limit switches to hold the band. Slight over-temperature at the clamp frame produces localized thinning, while under-temperature at the plug assist produces dimensional recovery after trimming.

Injection molding of PCTG for thin-wall medical housings requires control of gate freeze and pack pressure. Melt temperatures of 240–270°C for PCTG and 230–260°C for PETG are common for general-purpose grades. Mold temperatures are held at 15–40°C to reduce cycle time, but low mold temperatures increase molded-in orientation and post-mold shrinkage. Clamp force is commonly estimated at 3.5–4.5 kN/cm² of projected part area for multi-cavity tools. Hot-runner systems with valve gates are preferred because cold-runner scrap contains gate blush, and regrind levels above 20 wt% can measurably reduce transmittance. Pack pressure is maintained until gate freeze; screw decompression of 3–6 mm after plastication reduces drool. Annealing at 70–80°C for 2 h can relax orientation but can change critical tolerances by 0.2–0.5%.

When Gamma Sterilization Demands PCTG over PETG

Gamma irradiation at 25 kGy generates free radicals and can shift the color and impact response of glycol-modified copolyesters. PETG can exhibit yellowness and reduced elongation after exposure because oxidative chain scission competes with crosslinking. PCTG grades with higher cycloaliphatic comonomer content generally show less post-irradiation yellowing and better retention of notched impact. Published data for this specific configuration is limited, but supplier technical bulletins for medical-grade PCTG report higher retained notched Izod after 50 kGy than PETG controls. Material selection for terminally sterilized medical packaging is validated under ISO 11607-1:2019, with seal strength tested to ASTM F88/F88M-21, microbial barrier tested to ASTM F1608-21, and accelerated aging performed to ASTM F1980-21.

Ethylene oxide sterilization remains suitable for PETG because chamber temperatures are generally below 55°C and moisture preconditioning does not approach the melt-processing hydrolysis threshold. Hydrogen peroxide plasma and gamma exposure are more demanding because residual oxygen and free radicals can alter surface chemistry. PCTG is specified where repeated sterilization cycles are expected, but each sterilization mode requires dose mapping and post-exposure mechanical testing. A PETG tray that passes a single 25 kGy dose may fail after 50 kGy in corner impact, while a PCTG housing of the same part mass may remain within acceptable notched impact limits.

Filament made from PETG is processed on fused filament fabrication equipment at nozzle setpoints of 235–245°C and bed temperatures of 70–80°C. Unlike dried injection-molding compound, filament spools left at 60% relative humidity for 24 h can absorb enough moisture to produce hydrolysis pinholes and reduced interlayer adhesion. Drying spools at 65°C for 4–6 h before printing is recommended. Cooling fan duty above 40% can suppress interlayer fusion and lower transverse tensile strength. Printed PETG specimens made to ISO 527-2:2012 typically show lower tensile strength than injection-molded coupons because of voids and raster boundaries. PCTG filament is selected where lower shrinkage and higher notched impact are required, but hot-end pressure and build-plate adhesion differ from PETG.

Values are typical ranges from supplier technical data sheets for general-purpose grades and are not specification limits.

Regulatory Compliance Matrix for Food-Contact and Medical Use

Material compliance documentation for PETG and PCTG relies on resin clearances and finished-article migration testing. For food-contact use in the United States, applicable poly(ethylene phthalate) copolyester resins may be cleared under 21 CFR 177.1630, provided end-use conditions and adjuvants comply with the regulation. In the European Union, compliance is assessed under Commission Regulation (EU) No 10/2011, with overall migration measured according to EN 1186-1:2002 against a limit of 10 mg/dm².

In cosmetic bottle and jar applications, PETG is selected for thick-wall clarity and resistance to stress cracking from fragrance emulsions, but the continuous-use temperature is limited to approximately 60–65°C before softening and deformation occur. PCTG can tolerate slightly higher static heat exposure but should not be specified for hot-fill above 75°C unless part design and wall thickness are verified by finite element analysis and pilot filling trials. Exposure to dishwasher drying cycles at 65°C can produce stress whitening at molded-in thread inserts and snap features. For repeated hot water, steam, or aggressive chemical exposure, polycarbonate or polysulfone is generally substituted. Amine-based additives and certain epoxy chain extenders should not be compounded into PETG or PCTG without reactive extrusion validation because uncontrolled viscosity shifts and gel formation can occur.