Polyethylene terephthalate (PET) is an aromatic polyester produced by condensation of terephthalic acid or dimethyl terephthalate with ethylene glycol. PETG is the glycol-modified copolyester in which a portion of the ethylene glycol is replaced by 15–35 mol% cyclohexanedimethanol (CHDM). The comparison PET vs PETG: Where the Glycol Unit Changes Clarity and Crystallinity therefore centers on monomer topology rather than molecular weight alone. The cycloaliphatic CHDM ring interrupts the linear sequence of the PET chain and prevents the chain folding required for crystalline order. Bottle-grade PET typically exhibits intrinsic viscosity of 0.80–0.84 dL/g under ASTM D4603, while PETG extrusion grades commonly fall between 0.70–0.76 dL/g. This difference in chain structure does not simply alter viscosity; it determines whether the polymer remains transparent after reheating, orientation, or thermoforming.

Thermal Transitions and Crystallization Rate Differences

Under ASTM D3418-21 differential scanning calorimetry at 10°C/min, PET homopolymer shows a glass transition near 78°C, a cold crystallization exotherm with onset between 130–150°C, and a melting endotherm from 245–255°C. Cooling scans typically record a crystallization exotherm in the 170–200°C range, depending on diethylene glycol content and comonomer level. By contrast, PETG with CHDM content above 15 mol% exhibits a single glass transition near 80°C and no measurable melting peak or crystallization exotherm. The CHDM ring disrupts crystallographic registry sufficiently that quiescent crystallization is not observed under standard DSC cooling. Density measurements under ISO 1183-1:2019 reflect the same divergence: amorphous PET at 1.34 g/cm³ becomes 1.38–1.40 g/cm³ after orientation, while PETG remains near 1.27 g/cm³. The practical consequence is that PET can be converted from a ductile amorphous solid into a stiff semicrystalline article, whereas PETG remains essentially amorphous across all conventional melt-processing histories.

In 3.2 mm injection-molded plaques conditioned at 23°C and 50% relative humidity, PETG records total luminous transmittance above 90% and haze below 1% under ASTM D1003-21. PET can match this clarity only in a quenched amorphous condition, typically with mold surface temperature held at 15–30°C and section thickness below approximately 1.5 mm. Once crystallinity exceeds 5% or spherulite diameters become comparable to visible wavelengths, haze rises above 30%. The glycol unit does not eliminate light scattering directly; it removes the refractive index heterogeneity caused by crystalline domains. Because CHDM incorporates both cis and trans isomers, the chain loses long-range sequence regularity. As a result, PETG sheet heated to 90–110°C during thermoforming does not develop the spherulitic haze observed in PET sheet exposed to the same thermal history.

Does the Cyclohexanedimethanol Ring Suppress Strain-Induced Crystallization in Stretch Blow Molding?

In reheat stretch blow molding, PET preforms are heated to 90–110°C and biaxially stretched to areal draw ratios between 6:1 and 12:1. The mechanical work orients the chains and produces strain-induced crystallinity, raising density from 1.34 g/cm³ to 1.38–1.40 g/cm³. Tensile modulus increases to 2.8–4.0 GPa under ASTM D638-14. PETG containing 15–35 mol% CHDM does not undergo the same transition. The chain remains amorphous even after areal draw ratios exceeding 8:1, with density remaining near 1.27 g/cm³ and tensile modulus staying at 1.5–2.0 GPa. The absence of strain-hardening crystallinity limits blow-molded PETG to shallow containers or thick-wall packaging in which impact toughness is prioritized over top-load strength. Published data for high-draw PETG bottle configurations is limited because the material lacks the orientation stability required for carbonated beverage pressure retention. PET remains the conventional choice where ASTM D642 top-load and burst-pressure compliance is specified.

Desiccant drying of PET homopolymer is mandatory before melt processing. Moisture above 50 ppm at barrel temperatures of 270–285°C drives hydrolytic chain scission, reducing intrinsic viscosity from 0.80 dL/g to below 0.65 dL/g and producing brittle parts. Dryers are typically set to 150–170°C for 4–6 h with dew point below −40°C. PETG is processed at lower melt temperatures of 220–250°C and is less hydrolytically aggressive, but drying at 65–70°C for 4–6 h to below 200 ppm moisture is still required to prevent splay and molecular weight loss. On a 32 mm single-screw extruder with 24:1 L/D and a barrier screw, PETG can be run at 20–40 rpm with barrel setpoints of 220–240°C. PET bottle scrap is run at 270–285°C with vacuum venting to strip acetaldehyde. These processing differences arise because PETG’s fully amorphous structure does not provide the crystalline melt stability of PET and therefore cannot tolerate the same melt residence time.

When PETG Replaces PET in Thermoformed Medical and Retail Packaging Trays

Thermoformed PETG trays retain clarity after forming at sheet surface temperatures of 110–130°C because the CHDM unit prevents crystallization during heating and cooling. PET sheet must be quenched after extrusion and then formed under tightly controlled heating rates or it develops haze and dimensional instability. PETG offers notched Izod impact of 80–100 J/m under ASTM D256-23 at 23°C, compared with 30–50 J/m for amorphous PET and 20–40 J/m for oriented PET. This impact advantage makes PETG suitable for thin-walled hinged medical device trays where cracking during ethylene oxide sterilization and cold-chain distribution is unacceptable. However, heat deflection temperature at 0.455 MPa for PETG is around 70°C, whereas heat-set semicrystalline PET can withstand 200°C or more. Consequently, PETG is excluded from steam autoclave cycles above 121°C and from high-temperature retort packaging. The material selection trade-off is therefore clarity and toughness against thermal stability.

Under ASTM D543 chemical resistance testing, amorphous PETG exhibits stress crazing and environmental stress cracking in contact with ketones, esters, and some aromatic hydrocarbons. Semicrystalline PET shows higher resistance because the crystalline domains reduce solvent penetration. PETG should not be assembled with aggressive solvent-based adhesives or cleaned with methyl ethyl ketone. Food-contact status for PET is established under FDA 21 CFR 177.1630, while CHDM-modified copolyesters are typically covered under FDA 21 CFR 177.1630 or a specific food-contact notification. Migration testing is evaluated under EU Regulation 10/2011, with overall migration below 10 mg/dm². Recyclability also differs: PETG can contaminate the clear PET bottle recycling stream because its lower melting point and amorphous character alter melt viscosity and solid-state polymerization response. The National Association for PET Container Resources has published compatibility data that exclude PETG from clear PET bottle bales.