Material substitution in food contact blisters from plasticized or rigid PVC to glycol-modified polyethylene terephthalate is being driven by chlorine-free waste incineration, the removal of monomeric plasticizer migration under EU Regulation 10/2011 and REACH Annex XVII entries 51 and 52, and the ability of amorphous copolyester sheet to reproduce deep-draw blister geometries without the use of metal soap heat stabilizers. Industrial PETG sheet extrusion is not a direct drop-in on a PVC line. The resin must be pre-dried in a desiccant-wheel dryer with a supply dew point of -40 °C or lower to a residual moisture below 200 ppm, dried at 65–70 °C for 4–6 h, and fed to a single-screw extruder with an L/D ratio of 30:1 to 36:1, a barrier screw with compression ratio 2.5:1 to 3.5:1, and a melt pump that holds pressure variation below ±0.5 %. Melt temperature at the die entrance is maintained at 230–260 °C, with melt volume-flow rate measured according to ISO 1133-1:2022 at 230 °C and 2.16 kg in extrusion grades typically within 2–8 cm³/10 min to preserve melt strength during vertical take-off and downstream sag-limited thermoforming. The density of PETG is approximately 1.27 g/cm³ compared with 1.32–1.40 g/cm³ for PVC, so at equal sheet thickness the same mass of resin yields approximately 4–10 % more sheet area, although this is partially offset by drying energy and higher resin cost. PVC-specific stabilizers used to retard dehydrochlorination are no longer required, and the sheet contains no chlorine, no ortho-phthalate plasticizer, and no epoxidized soybean oil secondary plasticizer. On production-scale sheet lines, the main failure modes observed before optimization are edge curl caused by chill roll temperature imbalance, die lip oligomer buildup that starts to distort gauge after 48–72 h of continuous running, and moisture-induced silver streaks when plant relative humidity exceeds 60 % without sealed hopper loading.

What Limits the PETG Thermoforming Window for Blister Cavities?

The thermoforming window for monolayer PETG sheet is bounded by the onset of stress whitening at low sheet temperature and by excessive sag at high sheet temperature. The sheet surface temperature immediately before forming is typically controlled between 95 °C and 120 °C, measured by infrared pyrometer with emissivity set to 0.94; a variation of ±5 °C across the width is sufficient to produce measurable nonuniform cavity wall thickness. At temperatures below 90 °C, the elastic modulus remains high enough that corner stretching increases local stress beyond the yield point and creates visible haze bands, while above 125 °C the sheet sags more than 25 mm across a 500 mm web span and may drag on the transfer rails. Plug-assisted pressure forming with a syntactic foam or POM-C plug at 80–110 °C is used for draw ratios greater than 1:1, with vacuum level -0.6 to -0.9 bar and forming air pressure 4–6 bar on pressure-forming stations. Aluminum mold temperature is held between 15 °C and 25 °C to prevent PETG from sticking and to quench cavity sidewalls before shrinkage can develop. Gauge variation in the incoming sheet is amplified nonlinearly: because infrared absorption per unit area is inversely related to local thickness, a ±5 % thickness variation shifts local surface temperature by up to 6 °C under the same heater dwell. This produces thin corners in the thick sheet zones and webbing in the thin zones. Field-based process capability studies on industrial contact-heating machines indicate that maintaining a cap of ±2 % sheet gauge variation reduces downstream seal-leak rejects to below 1.5 % at a seal tester vacuum of -0.3 bar, while wider gauge variation can drive the same reject rate above 5 %.

Residual moisture is the primary process variable separating stable PETG extrusion from catastrophic melt degradation. PETG pellets can contain 0.25–0.50 % moisture after storage in ambient warehouse conditions at 50–60 % RH, and hydrolysis occurs rapidly when that moisture enters a melt stream above 230 °C. The target residual moisture is below 200 ppm, preferably below 100 ppm for sheet thinner than 300 µm, because the high surface-to-volume ratio of thin sheet makes any bubble or streak visible. Desiccant-wheel dryers with molecular sieve regeneration are operated at 65–70 °C for 4–6 h, with return air dew point of -40 °C or lower, and hopper residence time is sized for at least 4 h at maximum throughput. Drying at temperatures above 70 °C for more than 8 h can cause pellet surface softening and bridging in the hopper throat. A vented barrel section or vacuum pump on the extruder is used on some lines to remove volatiles, with vacuum levels of 20–40 mbar in the devolatilization zone. If wet resin is processed, the intrinsic viscosity of PETG can fall from 0.72 dL/g to 0.60 dL/g or lower, and melt strength loss appears as significant web sag and poor plug draw. The hydrolysis reaction is autocatalytic and rate increases as carboxylic acid end groups accumulate. Plant data from sheet lines with moisture analyzers show that a moisture excursion of 50 ppm above the 200 ppm limit can increase visible silver streak defects by an order of magnitude within one residence time.

When Sheet Gauge Variation Propagates to Heat Seal Leakage

In blister packaging, the heat seal flange is the structural boundary between cavity and lidding; if the flange gauge fluctuates, the seal platen applies uneven pressure and heat transfer. PETG sheet is extruded through a flex-lip die with automatic or manual die bolt positions, and the molten web enters a three-roll vertical or J-stack calender at roll temperatures of 40 °C, 55 °C, and 70 °C from casting roll to cooling roll depending on stack geometry. Roll gap is set 5–10 % below the final sheet thickness to maintain roll pressure and remove surface flow marks. A scanning beta or X-ray gauge positioned after the calender stack reports thickness across the web; for blister-grade PETG, the tolerance is typically ±2 % of nominal thickness, while commodity sheet may be produced at ±5 %. If the sheet is later indexed into a blister thermoformer, the flange area cut from thin regions can be 8–12 % below nominal thickness. During sealing with a lidding film coated with a heat-seal lacquer or extruded sealant, a seal temperature of 120–140 °C, dwell of 0.5–1.5 s, and jaw pressure of 60–80 psi are typical. Thin flange areas reach the sealant activation temperature faster; thick areas remain below the seal initiation temperature at the interface. The resulting heat seal strength measured according to ASTM F88 may vary from 8 N/15 mm on nominal gauge to less than 4 N/15 mm on thin flange segments. Dye penetration testing per ASTM F1929 or vacuum leak testing per ASTM D3078 reveals intermittent channel leaks. Production lines therefore reduce sheet gauge variation to ±1.5 % or better for pharmaceutical and high-acid food blister applications, and die bolt adjustments are tied to automatic gauge control loops with response intervals under 1 min.

Food contact blisters made from PETG meet the compositional requirements for ethylene glycol-1,4-cyclohexanedimethanol-terephthalate copolymers under FDA 21 CFR 177.1315 when the resin is produced from permitted monomers and complies with extractive limits specified in that section. Under EU Regulation 10/2011, the specific migration limit for terephthalic acid is 7.5 mg/kg food simulant and for ethylene glycol 30 mg/kg; 1,4-cyclohexanedimethanol is controlled through overall migration and residual monomer content rather than a separate high specific migration limit. Migration testing is performed according to EN 1186 series methods using food simulant A for aqueous foods, C for alcoholic foods above 10 %, and D2 for fatty foods, with contact time and temperature selected according to worst-case fill. Overall migration must remain below 10 mg/dm² for plastics. PETG used in food contact blisters is not formulated with ortho-phthalate plasticizers; therefore the phthalate restrictions under REACH Annex XVII entries 51 and 52 are not triggered by the polymer itself. The EU Packaging Directive 94/62/EC imposes sum of lead, cadmium, mercury, and hexavalent chromium below 100 mg/kg by weight for packaging; PETG sheet routinely complies with this threshold. In practice, the replacement driver is strongest for applications where PVC could face plasticizer migration concerns or where retailer packaging specifications require halogen-free materials, and weakest for high-barrier applications where PVC-PVdC laminates still provide lower oxygen transmission than monolayer PETG.

Regulation	Requirement	Test or limit	PETG implication
EU 10/2011	Overall migration	EN 1186, < 10 mg/dm²	Tested in simulants A, C, D2
FDA 21 CFR 177.1315	Copolyester composition	Extractive limits in section	Permitted for food contact
REACH Annex XVII	Phthalates in articles	Entries 51 and 52	Not triggered by PETG
94/62/EC	Heavy metal sum	< 100 mg/kg	Compliant sheet

Thermal Degradation Pathways in Stabilizer-Free PETG Extrusion

Because PETG contains no chlorine, the degradation chemistry is fundamentally different from PVC. PVC degrades by sequential dehydrochlorination at processing temperatures, producing conjugated polyenes and requiring thermal stabilizers such as calcium-zinc, organotin, or historically lead compounds. PETG degrades by hydrolysis of ester linkages, thermomechanical chain scission, and thermal oxidation. At melt temperatures above 260 °C, the ester bond scission rate accelerates, generating acetaldehyde, carbon dioxide, water, and low-molecular-weight oligomers. Acetaldehyde is the key organoleptic risk for food contact sheet; levels above 10 ppm in the sheet can be detected by trained sensory panels in packaged water or high-fat foods. The rate of degradation is a function of temperature, residence time, moisture, and acid end-group concentration. For this reason, melt residence time is typically limited to less than 5 min at 250 °C, and screw designs avoid stagnant zones with compression ratio above 3.5:1. Yellowing is measured by the b* value on a HunterLab or CIELAB colorimeter; production-grade PETG sheet for clear blisters has a b* value below 1.5, while thermally degraded sheet rises above 4. The processing window is narrower than rigid PVC in practical terms: melt temperature below 220 °C leads to incomplete melting and surface melt fracture, while above 270 °C leads to excessive oligomer generation and die lip deposits. The ±5 °C melt-temperature control band required for high-clarity PETG is tighter than the ±10 °C commonly accepted for rigid PVC.

Blister sealing is the critical downstream operation where PVC and PETG diverge despite similar forming appearance. Heat seal initiation temperature for uncoated PETG to a compatible PETG-coated lidding film is typically 115–130 °C, compared with 105–125 °C for PVC to acrylic or vinyl acetate sealants. However, PETG has a higher modulus at typical seal jaw temperatures, so seal pressure must be increased or dwell extended to conform the flange surface. Heat seal strength according to ASTM F88 on 250 µm sheet with a solvent-based heat-seal lacquer or a coextruded PETG/PE lidding film reaches 10–15 N/15 mm for a dwell of 1 s at 130 °C, falling below 5 N/15 mm when seal temperature is below 110 °C or when the flange is contaminated by oligomer aerosol from die lip buildup. Hot tack measured immediately after seal opening on a J&B or Brugger system is typically 1.5–2.5 N/15 mm, which is adequate for high-speed blister lines running at 40–60 cycles/min. PETG lid films can be sealed using the same platen geometry as PVC, but the dielectric heating response is different; sealing stations using radio-frequency welding optimized for PVC do not heat PETG effectively because PETG has lower dielectric loss factor than polar plasticized PVC. For this reason ultrasonic or thermal conduction sealing is used for PETG blister trays. Peel failure modes also differ: PVC often exhibits cohesive failure in the lacquer, while PETG can exhibit interfacial adhesive failure if the sealant has not been designed for copolyester surface energy, typically 42–44 mN/m for corona-treated sheet.

Comparative Barrier Properties and Shelf-Life Determinants

Monolayer PETG is not a drop-in barrier replacement for PVC-PVdC laminates in oxygen-sensitive food blisters. The oxygen barrier of an amorphous copolyester sheet is moderate and must be measured according to ASTM D3985 on the exact sheet grade, thickness, and humidity condition; published data for specific PETG and PVC-PVdC blister structures is often limited because transmission rate is strongly dependent on plasticizer content, lamination layer, and relative humidity. Water vapor transmission rate is similarly measured according to ASTM F1249 at 38 °C and 90 % RH. The barrier gap is not the primary driver for PETG replacement; the driver is regulatory and waste-stream pressure. For dry foods, monolayer PETG is sufficient. For processed meat, cheese, and high-acid wet products requiring oxygen barrier, PETG must be coextruded with EVOH or coated with an oxygen barrier, which raises cost and complicates recycling. The table below summarizes comparative mechanical and thermal values used in material selection.

Property	Test method	PETG	Rigid PVC
Density	ISO 1183-1:2019	1.27 g/cm³	1.32–1.40 g/cm³
Tensile stress at yield	ISO 527-2	50 MPa	45–55 MPa
Elongation at break	ISO 527-2	>100 %	50–150 %
Flexural modulus	ISO 178	2100 MPa	2000–3000 MPa
Vicat softening temperature	ISO 306 A50	78–82 °C	70–85 °C
Water absorption, 24 h	ISO 62	0.2–0.3 %	0.04–0.4 %
Heat deflection temperature, 0.45 MPa	ISO 75-2/B	70–75 °C	65–75 °C
Seal initiation temperature	ASTM F88	115–130 °C	105–125 °C

Regrind management is a central cost variable in PETG sheet extrusion because edge trim and start-up scrap can represent 20–40 % of extruder throughput on narrow blister-grade lines. PETG regrind can be reintroduced at 30–50 % by weight if it is dried to below 200 ppm moisture and if the flake is free of paper labels, adhesive, and polyolefin lid film remnants. Each heat history reduces intrinsic viscosity by approximately 0.02–0.04 dL/g and raises yellowness index by 0.5–1.5 units; at high regrind ratios, this shifts sheet b* above 2.0 and lowers melt strength enough to affect plug-assisted draw uniformity. The scrap stream must not contain PVC trim or PVC dust because even 0.1 % PVC contamination can cause visible gassing, black specks, and localized degradation in PETG melts above 230 °C. Positive sorting and metal detection are therefore used before grinding. In plants where both PETG and PVC blister sheet are run on separate lines or at separate times, the grinders and hoppers must be purged with a purging compound or vacuum cleaned; cross-contamination is a major cause of field failure. Amorphous PETG is not compatible with reclaimed PET bottle flake; PETG contaminates PET recycling streams by acting as a crystallization disruptor and can raise haze in recycled PET products at levels as low as 1 %. On a continuous sheet line, PETG exhibits higher static charge accumulation than rigid PVC because the polymer is non-polar enough to hold charge and the sheet exits the calender at temperatures below the glass transition. Static dissipative systems with ionizing bars are required before the pull rolls and at the edge trim chopper; without them, edge trim can wrap around rolls and create web breaks. Die lip oligomer deposits are observed after 48–72 h of continuous extrusion and are managed by wiping the die lips with a copper-brass or bronze scraper during scheduled stops; the use of steel tools damages the chrome-plated lip surface. Some lines use automatic lip-cleaning devices but their effectiveness is limited for amorphous copolyesters. Plant humidity is normally maintained at 45–60 % RH because low humidity increases static charging and high humidity reintroduces moisture into dried resin through open hoppers. The combination of drying discipline, gauge control, thermal degradation avoidance, sealant compatibility, and scrap segregation defines the practical boundary for PETG as a PVC replacement in food contact blisters.