PET vs PETG: Semi-Crystalline Precision vs Amorphous Clarity

PET (polyethylene terephthalate) and PETG (polyethylene terephthalate glycol-modified) share the same polyester molecular backbone — yet they behave as distinctly different engineering plastics. Adding glycol co-monomer during polymerization disrupts the crystalline ordering that forms in standard PET, producing an amorphous material with fundamentally different physical and processing characteristics. Engineering-grade PET (branded as Ertalyte by Ensinger and Mitsubishi) is semi-crystalline, dimensionally stable, and suited to precision machined parts. PETG is amorphous, optically clear, impact-resistant at low temperatures, and considerably easier to thermoform, bend, and fabricate — but it sacrifices chemical resistance and long-term dimensional stability. This is the highest-volume comparison query in the fedmat.com corpus, and for good reason: the two materials are routinely confused in purchasing and are rarely substitutable without engineering review.

TL;DR

• Crystallinity: PET is semi-crystalline; PETG is amorphous — this single difference drives most of their behavioral differences.
• Optical clarity: PETG is water-clear and essentially transparent in sheet form; PET (engineering grade) is typically translucent to opaque white.
• Chemical resistance: PET resists a wide range of solvents and acids; PETG is attacked by ketones, chlorinated solvents, and some alcohols.
• Strength: PET has higher tensile strength (~11,000 psi) and stiffness than PETG (~7,700 psi); PET holds properties better at elevated temperature.
• Thermoforming: PETG forms at lower temperatures and over a wider processing window — a major fabrication advantage for display and sign work.
• Food contact / FDA: Both are FDA 21 CFR compliant for food contact in standard grades.
• Cost: PETG sheet is typically less expensive than engineering PET rod/sheet; PET precision stock (Ertalyte) commands a premium.

Chemistry & Origin

Standard PET is a condensation polymer of terephthalic acid and ethylene glycol. When cooled slowly from the melt, PET chains align into a crystalline structure — this crystallinity is what gives engineering PET its superior stiffness, strength, and chemical resistance. Engineering-grade PET rod and sheet (Ertalyte) is manufactured specifically to develop controlled crystallinity throughout the cross-section, ensuring consistent mechanical properties for machined parts. This is distinct from the amorphous PET used in beverage bottles, which is quench-cooled to suppress crystallinity and maximize clarity.

PETG introduces a second glycol (cyclohexanedimethanol, CHDM) during polymerization. CHDM is a bulky molecule that physically prevents the polymer chains from packing into a crystalline lattice, locking the material in an amorphous state regardless of cooling rate. The result is a polymer that is permanently clear, substantially tougher at low temperature, and processable at much lower temperatures — but one that has lost the chemical resistance and dimensional stability that crystallinity confers.

Mechanical Properties

Engineering PET (Ertalyte) outperforms PETG in virtually every stiffness and strength metric at room temperature. Tensile strength is approximately 40% higher; flexural modulus roughly double. At elevated temperatures — above 70°C — PETG begins losing stiffness rapidly as it approaches its glass transition temperature; engineering PET retains usable mechanical properties well above 100°C.

PETG has the edge in low-temperature impact resistance. Its amorphous structure absorbs impact energy more effectively than the semi-crystalline PET, making it preferred for display cases, signage, and protective covers that may be handled roughly or installed in cold environments.

For precision machined parts requiring close dimensional tolerances, engineering PET is the correct choice. PETG's lower modulus and wider thermal expansion coefficient make it less suitable for tight-tolerance machined components.

Electrical Properties

Both PET and PETG are moderate electrical insulators but are not typically specified for primary electrical insulation. Volume resistivity for engineering PET exceeds 10¹⁵ Ω·cm; PETG is similar. Dielectric strength for both is in the range of 400–500 V/mil. Neither material approaches the dielectric performance of glass-reinforced thermosets (G10, FR4) or fluoropolymers (PTFE), and neither carries a UL94 V-0 flame rating in standard unfilled grades.

Thermal Properties

The temperature performance gap between PET and PETG is significant for engineering applications:

• PET (Ertalyte): Continuous service to approximately 110°C (230°F); intermittent to 120°C. Maintains dimensional stability to these temperatures due to crystalline structure supporting the amorphous phase.
• PETG: Continuous service to approximately 70°C (158°F). Above this temperature PETG softens noticeably and is unsuitable for structural loading. The lower service temperature also limits PETG's use in dishwasher-safe or sterilization-critical applications.

PETG cannot be steam-autoclave sterilized in standard grades. Engineering PET tolerates limited autoclave exposure but should be verified with the specific grade data.

Chemical Resistance

This is the most important differentiator for chemical process and industrial applications:

Engineering PET resists: dilute acids and bases, alcohols (methanol, ethanol, isopropanol), aliphatic hydrocarbons, oils and greases, and most aqueous solutions. It performs well in food and beverage contact and in pharmaceutical processing environments.

PETG is attacked by: ketones (acetone, MEK), chlorinated solvents (methylene chloride, trichloroethylene), concentrated acids, and some esters. PETG in solvent contact applications is frequently a design error. The amorphous structure, which permits crystal-lattice-free light transmission, is the same structural characteristic that makes it vulnerable to solvent diffusion.

Cost & Availability

PETG sheet is a commodity item widely stocked at competitive prices — it dominates the display, signage, and thermoforming markets. Engineering PET rod and sheet (Ertalyte) is a specialty precision plastic stocked by technical distributors; it commands a premium reflecting its controlled manufacturing and quality documentation.

PETG is commonly available in clear sheet from 0.030″ through 1.0″ and in limited rod sizes. Engineering PET is stocked in rod (0.25″–6″ diameter) and sheet (0.25″–4.0″) with closer dimensional tolerances.

When to Choose PET vs PETG

Choose engineering PET (Ertalyte) when:

• The application requires precision machined parts with tight dimensional tolerances.
• Operating temperature exceeds 70°C (158°F).
• Chemical exposure to solvents, acids, or cleaning agents is expected.
• Long-term dimensional stability under sustained load is required.
• Wear resistance in bearings, gears, or sliding components is a factor.

Choose PETG when:

• Optical clarity and transparency are required.
• The application is display cases, signage, light diffusers, or protective covers.
• Thermoforming or heat bending in a fabrication shop is required.
• Low-temperature impact resistance matters.
• Operating temperature stays below 60°C (140°F).
• Cost is a primary driver and structural demands are modest.

Common Alternatives

• Polycarbonate vs PETG — When clarity is required but higher temperature performance or impact strength is needed.
• Acrylic vs PETG — Both transparent; acrylic offers better UV resistance and optical quality.
• Delrin vs PET — When precision machined parts and chemical resistance intersect.

More related guides

Applications

• Display and Signage
• Bearings and Bushings
• Food Processing Components

Industries

• Medical
• Food & Beverage

Other comparisons

• Acrylic vs PETG
• Polycarbonate vs PETG
• Delrin vs PET

Spec sheets

• PET (Ertalyte) hub
• PETG hub