Polyethylene Terephthalate (PET) Cups and Clamshells: Thermoformed Food-Service Formats

Polyethylene terephthalate (PET) is a semicrystalline polyester synthesized from ethylene glycol and terephthalic acid or dimethyl terephthalate. In amorphous sheet form, designated APET, it is extruded as roll-fed substrate for thermoformed cups and clamshells. The amorphous phase density is 1.33 g/cm³; the crystalline phase density is 1.45 g/cm³. Glass transition occurs at approximately 76 °C, and the melting point of strain-induced crystallites is observed at 245–255 °C. Thermoformed food-service articles are processed from sheet with intrinsic viscosity (IV) values typically between 0.72 dL/g and 0.84 dL/g measured by ASTM D4603 or ISO 1628-1. The IV value controls extensional flow during plug-assisted forming. Lower IV material reduces melt strength and increases incidence of sheet thinning at the sidewall–bottom transition of cups; higher IV material raises melt viscosity and can require elevated sheet temperatures that approach the onset of thermal crystallization.

How Does Intrinsic Viscosity Determine the Thermoforming Processing Window?

Resin drying is the first critical threshold. Undried PET absorbs ambient moisture at rates up to 0.25 wt% at 23 °C and 85% RH; hydrolytic degradation at melt temperatures above 270 °C reduces IV by chain scission. Desiccant dryers configured with a dew point of −40 °C and a residence time of 4–6 h at 160–180 °C are specified to reduce moisture below 0.005 wt% (50 ppm) before extrusion. A single-screw extruder with an L/D ratio of 30:1 to 36:1 and a barrier mixing section is used to deliver melt temperature of 265–280 °C to a flat die. Melt pressure stability within ±1.0 MPa is required to maintain sheet thickness tolerance of ±2%. Processors monitor post-extrusion IV drop; values exceeding 0.03 dL/g per pass indicate inadequate drying or excessive barrel residence time.

IV loss is not uniform across the sheet width. Edge bead regions can retain higher orientation because melt shear at the die lip generates localized viscous heating. In cup thermoforming, a distribution shift of 0.02 dL/g between center and edge sheet affects wall-thickness consistency. The practical response is to map IV across the sheet at startup using intrinsic viscosity measurement by ASTM D4603 and adjust die bolt temperatures to keep the melt curtain within a ±0.015 dL/g band. Without this control, deep-draw cups exhibit sidewall thin spots below 0.20 mm even when average sheet thickness is within specification.

Following extrusion, a three-roll vertical or inclined stack sets the cooling history. Chrome-polished rolls with surface roughness Ra 0.025 µm transfer gloss to the food-contact surface. Roll temperatures are set with the first roll at 60–80 °C, the middle roll at 70–90 °C, and the third roll at 50–70 °C to control cooling rate. If the sheet surface cools below Tg under constraint, frozen-in orientation develops. Subsequent reheating in the thermoformer then produces anisotropic shrinkage; cup rims can lose ovality when orientation is uneven. Sheet width variation across the die should not exceed ±1.5%. For clamshell stock, sheet thickness ranges from 0.35 mm to 0.75 mm; for deep-draw cups, thickness ranges from 0.45 mm to 1.20 mm. Thicker sheet requires multiple-zone infrared heaters with independent top and bottom control.

Barrier Gas Permeation and Flavor Scalping Benchmarks

APET food-service containers are not high-barrier formats. Published oxygen transmission rate values at 23 °C and 50% RH, normalized to thickness, commonly fall between 4 cm³·mm/(m²·day·atm) and 6 cm³·mm/(m²·day·atm) by ASTM D3985. Water vapour transmission rate by ASTM F1249 at 38 °C and 90% RH is reported between 1.5 g·mm/(m²·day) and 2.5 g·mm/(m²·day). These values are adequate for washed whole produce and bakery items but insufficient for oxygen-sensitive cut fruit or fatty fish without a barrier lid or coating. Flavor scalping by PET is governed by sorption of non-polar aroma compounds into the amorphous matrix. The partition coefficient increases as the polymer approaches Tg, so hot-fill cups above 70 °C surface temperature can exhibit higher aroma sorption than cold-fill cups.

In cold-fill cup applications, mechanical anisotropy is evaluated by tensile testing and high-speed puncture. APET sheet tensile strength at yield is reported at 50–60 MPa according to ISO 527-2, with elongation at yield of 4–6%. Elongation at break is above 200% in fully amorphous sheet. Instrumented puncture resistance is measured by ASTM D3763; clamshell hinge flexural performance is configuration-dependent, and published data for specific hinge geometries is limited. In freezer storage below −20 °C, high-speed impact can produce brittle failure at the cup rim or clamshell corner unless the part design redistributes stress through radiused corners and reinforced ribs.

Tool temperature control in cup-forming cavities creates the largest hidden process conflict. The plug-assist surface temperature must remain high enough to avoid chilling the sheet, but low enough to prevent PET sticking. A plug made of syntactic foam or PPSU is held at 100–120 °C; below 95 °C, contact chilling generates oriented amorphous regions that appear as stress-whitening in the sidewall. Above 125 °C, the plug picks up oligomer deposits and increases cycle-time variability. Sheet surface temperature at the forming station is maintained between 120 °C and 140 °C. The lower limit is critical: at 110 °C or below, incipient strain-induced crystallization is insufficient to suppress crazing. The upper limit is 145 °C before bulk thermal crystallization raises haze above 5% measured by ASTM D1003. Thus the practical forming window is ±5 °C around the midpoint; this justifies closed-loop scanning pyrometer control and zoned ceramic heaters with a response time below 1 s.

When Scrap Fraction Exceeds 20% in Sheet Extrusion

Post-industrial thermoforming skeletons and edge trim are ground through a rotor mill with a 6 mm screen. Dried flake is metered back into the sheet extruder. Scrap reincorporation above 20 wt% shifts IV distribution because each heat history degrades the polyester chain. A monolayer cup made with 40 wt% in-house scrap can show a reduction in melt-phase IV of 0.04 dL/g to 0.07 dL/g relative to virgin sheet, depending on dryer residence time and melt temperature. Acetaldehyde concentration in the sheet increases as a result of intra-chain ester cleavage; headspace values above 10 µg/L in the formed cup are detectable by ASTM F2013. Vacuum venting at −0.08 MPa on the extruder barrel removes volatile degradation products but may not fully compensate for degraded flake. Filters with 40 µm mesh are replaced when pressure differential exceeds 8 MPa; gel particles from degraded PVC contamination cause black specks and pinholes. Published data for exact acetaldehyde generation as a function of scrap fraction is limited and varies with flake bulk density and residual label adhesive.

Food-Contact Compliance and Heavy-Metal Limits

Polyethylene terephthalate food-service formats in the United States are covered under 21 CFR 177.1630, which addresses the polymer in contact with food. In the European Union, compliance is assessed under Regulation (EU) No 10/2011 on plastic materials and articles intended to come into contact with food. The overall migration limit is 10 mg/dm² of food-contact surface area. Specific migration for terephthalic acid under the same regulation is 7.5 mg/kg, and specific migration for ethylene glycol is 30 mg/kg. REACH registration applies to monomer and polymer imports above 1 tonne/year. PET does not require ortho-phthalate plasticizers, and heavy-metal limits for packaging are assessed under Directive 94/62/EC, where the sum of lead, cadmium, mercury, and hexavalent chromium must not exceed 100 mg/kg.

Processors should not assume that barrier data obtained at 0% RH translates directly to high-humidity distribution conditions. Sheet moisture regain after extrusion, warehouse storage time, and post-formed residual stress interact with oxygen transmission and aroma sorption in ways that require lot-specific verification under ASTM D3985 or ASTM F1249 conditions relevant to the intended cold-chain or ambient shelf-life window.