PET Sheet Stock: APET, CPET and PETG Sheet Compared

Polyethylene terephthalate (PET) sheet stock is manufactured in three principal grades: amorphous polyethylene terephthalate (APET), crystallizable polyethylene terephthalate (CPET), and glycol-modified polyethylene terephthalate (PETG). APET is a PET homopolymer quenched rapidly on chill rolls to retain a predominantly amorphous matrix. CPET generally uses higher intrinsic viscosity resin and, in commercial formulations, a mineral nucleating agent such as talc to promote controlled crystallization during heated-mold thermoforming. PETG incorporates cyclohexanedimethanol (CHDM) as a comonomer at approximately 15–35 mol%, which interrupts ethylene terephthalate sequencing and suppresses crystallinity even at slow cooling rates.

The selection of PET sheet stock: APET, CPET and PETG sheet compared in this article is evaluated against intrinsic viscosity retention, drying requirements, thermoforming window, thermal resistance, impact behaviour, and food-contact compliance. Differences are quantified using ASTM D4603-18 for intrinsic viscosity, ASTM D3418-21 for glass transition behaviour, ASTM D638-14 for tensile yield, ASTM D790-17 for flexural modulus, and ASTM D648-18 for deflection temperature under load.

APET remains amorphous during normal sheet extrusion and thermoforming, yielding high clarity and moderate thermal resistance. CPET is processed as a substantially amorphous sheet but is crystallized during thermoforming at mold temperatures above 160°C, producing a container with heat resistance suitable for dual-ovenable applications. PETG offers higher impact toughness and deeper draw capability but exhibits a lower continuous use temperature and is not considered ovenable.

Compositional Differences Across APET, CPET and PETG Sheet Grades

The molecular architecture of each sheet grade controls crystallization kinetics, mechanical response, and permeability. APET retains a glass transition temperature near 70–78°C and can crystallize only if exposed to temperatures above its cold crystallization onset, typically around 130–150°C, for sufficient time. In cast sheet production, chill roll temperatures below 30°C suppress crystallinity and preserve transparency. CPET sheet formulations compensate for slow inherent crystallization by adding talc or other inorganic nucleants at loadings between 0.5 wt% and 2.0 wt%, reducing crystallization half-time in the mold. PETG’s CHDM comonomer raises the glass transition to 75–85°C in some grades but removes the crystallization exotherm, so the material cannot be thermally crystallized.

Intrinsic viscosity directly influences melt strength, sag resistance, and impact toughness. APET sheet extrusion grades typically exhibit intrinsic viscosities of 0.76–0.84 dL/g. CPET grades typically require 0.95–1.05 dL/g to withstand severe frozen-food impact while retaining sufficient crystallinity after thermoforming. PETG sheet grades commonly run at 0.70–0.80 dL/g; higher melt strength is achieved through molecular weight or branching rather than crystallization.

Table 1. Comparative property ranges for APET, CPET and PETG sheet stock; crystallized CPET values depend on final crystallinity level and nucleant package.

On production cast sheet lines, APET and CPET are dried in desiccant-wheel dryers with a dew point of −40°C or lower. Resin bed residence of 4–6 h at 150–170°C reduces moisture to 30–50 ppm before extrusion. PETG is dried at 65–75°C for 4–6 h because the CHDM-modified backbone softens at lower temperature and may agglomerate in the dryer cone. Single-screw extruders with 30:1 L/D barrier screws and vacuum-vented barrels are specified for sheet widths up to 150 cm. Melt temperatures measured at the die lip generally range from 270°C to 290°C for APET, 280°C to 300°C for CPET, and 230°C to 250°C for PETG. Melt pumps upstream of the die reduce surging and improve thickness tolerance to below ±2% on cast lines.

Moisture control is more critical in PET sheet extrusion than in many polyolefin processes. At processing temperatures, residual moisture hydrolyzes ester linkages, lowering intrinsic viscosity. The practical target for APET and CPET is 30 ppm moisture; occasional excursions above 50 ppm result in measurable IV loss and increased gel formation in sheet. PETG can tolerate slightly higher moisture, but the lower processing temperature reduces headspace and venting efficiency, requiring careful hopper maintenance.

What Limits Thermoforming Latitude in Crystallizable CPET Sheet?

CPET sheet is supplied as a substantially amorphous product and must be thermally crystallized in the forming tool. The sheet surface temperature before forming is typically 110–130°C, but the mold face must be held at 160–210°C to initiate crystallization. Crystallinity levels of 30–45% are required for dimensional stability and resistance to bowing during oven heating. Cycle time is governed by crystallization half-time and part thickness; edges and corners can remain undercrystallized if mold temperature uniformity exceeds ±5°C.

APET thermoforming uses sheet surface temperatures of 110–140°C and chilled aluminum molds at 20–60°C. It remains amorphous because the mold cooling rate exceeds the critical cooling rate for crystallization. PETG forms at lower sheet temperatures, generally 82–121°C, and can be run on unheated or lightly heated tools, but deep draws may produce stress whitening if draw ratios exceed 4:1 or if sheet temperature falls below the low end of the range.

CPET molds are typically oil-heated or cartridge-heated aluminum; female cavities require independent zone control because corners and undercuts dissipate heat at different rates. In production-scale thermoforming, underheated zones below 160°C produce hazy, low-crystallinity patches that can soften during oven reheating. Overheated zones above 210°C may initiate oxidative yellowing and stick to the plug assist. Published data for optimized CPET tools indicate that zone-to-zone variation should be maintained within ±5°C to hold part crystallinity between 30% and 45% across the formed geometry.

Mechanical property comparisons derived from standardized specimens show PETG has notched Izod impact values of 80–100 J/m, roughly three times those of APET at 20–30 J/m. CPET impact is dominated by crystallinity and lamellar morphology; over-crystallization above 45% can reduce puncture resistance in frozen applications. Flexural modulus data reflect these differences: PETG is more compliant, APET is stiffer, and CPET stiffness increases with density but may plateau with nucleant loading. The heat deflection temperature of crystallized CPET at 0.455 MPa exceeds 200°C, whereas APET and PETG remain below 80°C under the same load. This single property explains the use of CPET in dual-ovenable trays and the restriction of APET and PETG to cold-fill or warm-fill applications.

Crystallization Kinetics and Barrier Response in Finished Sheet

Crystallization increases density and reduces free volume. As CPET shifts from amorphous sheet to 30–45% crystallinity, density rises from approximately 1.33 g/cm³ to 1.39–1.45 g/cm³. This transition is not instantaneous; isothermal crystallization half-time in nucleated CPET is shortest between 170°C and 190°C, with slower rate below 160°C and above 200°C. Differential scanning calorimetry under ASTM D3418-21 can track the residual amorphous Tg and confirm the absence of a cold crystallization exotherm in fully crystallized CPET.

Oxygen and water vapour transmission differ among the three grades. Published permeability data using ASTM D3985-17 for oxygen and ASTM F1249-20 for water vapour indicate that PETG generally exhibits higher oxygen permeability than APET because CHDM disrupts chain packing. Crystallized CPET reduces oxygen transmission by approximately 20–40% relative to amorphous APET at equivalent thickness, although actual values depend on sheet thickness, nucleant content, and crystallinity uniformity. This barrier improvement is accompanied by greater haze and reduced clarity, which is acceptable for dual-ovenable trays but unsuitable for transparent packaging.

Chemical exposure resistance is grade-dependent. APET and CPET are susceptible to hot alkaline cleaners and prolonged contact with strongly acidic solutions. PETG is more sensitive to ketones, esters, and aromatic solvents and can develop stress cracking under tensile load. In food packaging, hot-oil contact above 70°C is not recommended for APET or PETG; CPET tolerates short-term hot-oil exposure but may absorb odour compounds if the crystallinity is below 30%.

When Regrind Ratios Exceed 35% in Extrusion of PET Sheet Stock

Edge trim and skeletal scrap are reintroduced as ground flake or repelletized regrind in most PET sheet operations. Regrind ratios of 20–35% are common for unpigmented APET and PETG. At regrind levels above 35%, thermal and hydrolytic degradation accumulate more rapidly because the recycled fraction has already passed through one or more heat histories. The effect is measurable as an intrinsic viscosity decline of 0.02–0.05 dL/g per pass when drying is not optimized, increased gel counts in extruded sheet, and a narrowing of the thermoforming window.

CPET is more sensitive to regrind composition. Mineral nucleants remain in the scrap, and repeated extrusion can over-nucleate the material, shifting crystallization onset and reducing the optimum mold residence time. On production lines with 120-inch cast extrusion, accumulation of fines from ground CPET edge trim has been observed to cause localized black specks in finished trays when screen packs below 100 mesh are used. PETG regrind at over 35% can increase yellowing because CHDM comonomer is subject to oxidation at high melt temperatures. Vacuum venting at 10–20 kPa absolute and a dual-vent screw configuration help remove adsorbed water and low-molecular-weight volatiles, but vent port fouling remains a recurring bottleneck on high-regrind PETG lines.

Food-contact compliance is evaluated under FDA 21 CFR §177.1630 for PET homopolymer and glycol-modified grades; CPET with mineral nucleating agents may require additional clearance under 21 CFR §174.5 or a food contact notification. In the European Union, sheet grades are assessed under Regulation (EU) No 10/2011 with overall migration below 10 mg/dm² for plastic food-contact materials. Dual-ovenable CPET trays are typically specified for heating from −40°C to 220°C. APET is used for bakery clamshells, deli trays, and cold cups where continuous temperatures remain below 60°C. PETG is selected for medical packaging, printed displays, and deep-draw trays; its continuous use temperature is limited to 60–65°C under load and it is not suitable for hot-fill above 70°C.