PET Film Types: bOPET, Cast, Metallised and Heat-Sealable

Polyethylene terephthalate resin for film conversion is selected by intrinsic viscosity, carboxyl end-group content, and residual metal catalyst. Melt-phase polymer produced from terephthalic acid and ethylene glycol is converted at 270–290 °C only after closed-loop desiccant drying to a moisture content below 50 ppm; when ambient relative humidity exceeds 60%, the dryer outlet dew point is maintained below −40 °C. Hydrolysis at residual moisture above 80 ppm can reduce intrinsic viscosity by 0.02–0.04 dL/g during extrusion residence times of 10–15 min. Intrinsic viscosity measured per ASTM D4603 is specified in the range 0.62–0.84 dL/g for film resins; lower values favour melt flow in thin-gauge cast film, while higher values improve biaxial stretching stability. A typical cast film line uses a single-screw extruder with a barrier screw, L/D 30:1–33:1, a melt pump, and screen filtration with 20–40 µm elements. The melt is delivered to a flexible-lip die with automatic thickness scanning; transverse gauge variation on a 50 µm cast web is held below ±1.5%. The quench step on a polished chrome roll at 20–40 °C sets the amorphous state that is later oriented or thermoformed.

When Does Biaxial Orientation Produce Barrier Symmetry and Tensile Stiffness?

Stretching a quenched PET sheet biaxially initiates molecular orientation along machine and transverse axes and creates the property set identified with bOPET. The process reheats an amorphous sheet to 80–100 °C before longitudinal drawing and then stretches transverse in a tenter at 100–120 °C; sequential draw ratios of 3.0–4.0 in MD and 3.5–4.5 in TD are common. Orientation raises tensile strength measured per ASTM D882 to 200–260 MPa and modulus to 3.8–4.5 GPa, while elongation at break narrows to 80–150%. Clear bOPET at 12 µm shows haze of 2.0–3.0% per ASTM D1003, dynamic coefficient of friction of 0.40–0.60 per ASTM D1894, and density of 1.39–1.41 g/cm³ per ASTM D1505. Water vapour transmission rate for 12 µm film is 30–35 g/(m²·day) at 38 °C/90% RH per ASTM F1249; oxygen transmission rate is 100–130 cm³/(m²·day·atm) at 23 °C/0% RH per ASTM D3985.

On sequential tentering lines, the heat-setting stage constitutes the processing window that separates dimensionally stable film from poorly recrystallised film. Heat-setting at 190–230 °C with 2–5% machine-direction and transverse relaxation increases crystallinity to 40–55% and reduces free shrinkage at 150 °C/30 min to 0.5–1.5% when tested per ASTM D1204. Below 190 °C, residual shrinkage can exceed 2.0%; above 230 °C, elongation at break declines and surface haze may increase due to spherulitic crystallisation. Production logs on tenter frames with rail width adjustment of ±0.5 mm show that thermal expansion of the chain track, rather than film thickness variation, is the dominant source of bowing at heat-setting temperatures above 225 °C. Narrowly controlling this stage is critical for registration in metallising and coating; film with variation in heat-set temperature across the web develops non-uniform metal adhesion and seal response.

Cast PET film is produced without biaxial stretching and retains a largely amorphous structure because the chill-roll quench suppresses crystallisation. At thicknesses from 20–250 µm for lidding and print webs, and 300–800 µm for thermoforming sheet, cast PET exhibits tensile strength of 50–70 MPa and elongation at break of 300–450% per ASTM D882. The amorphous density is 1.33–1.37 g/cm³, and the film is reheated to 90–120 °C for thermoforming. Chill-roll surface finish is tightly specified: a roll roughness Rz of 0.4–1.0 µm replicates onto the film and controls air entrainment and gauge uniformity. Cast film is less brittle and more ductile than oriented bOPET, but tensile modulus is lower at 2.0–2.5 GPa. Because the film has low crystallinity, storage above 35 °C can induce slow densification and shrinkage if the wound roll contains residual stress from gauge variation.

Vacuum Metallisation, Optical Density, and Nucleation Defects

Vacuum metallisation converts a transparent bOPET substrate into a high-barrier web by evaporating aluminium onto a corona-treated surface under reduced pressure. The roll is placed in a vacuum chamber at 1×10⁻⁴–1×10⁻² mbar; the deposition source is typically a resistively heated intermetallic boat. Substrate surface wetting tension is raised to ≥50 mN/m per ASTM D2578 before metallisation because low dyne level causes discontinuous metal growth. Optical density is the primary process control variable; barrier-grade metallised PET is usually produced at OD 1.5–3.0. On a 12 µm BOPET base, OD 2.0 corresponds approximately to an aluminium layer of 30–60 nm, although published data for exact thickness as a function of boat geometry is limited. Winding tension during metallisation is maintained below 50 N/m for 12 µm film; higher tension produces hard rolls and slit-edge metal pick-off.

The barrier improvement after metallisation is not linear with optical density; defect density controls residual permeation. A clear 12 µm BOPET film with WVTR 30–35 g/(m²·day) falls to 0.5–1.0 g/(m²·day) after metallisation to OD 2.5, and OTR falls to 0.1–1.0 cm³/(m²·day·atm) per the same standards. Pinhole density is influenced by substrate surface roughness, winding tension, and contaminant particles; published data for pinhole density as a function of boat placement in resistively heated evaporation is limited. Metal adhesion is checked on production reels by tape test per ASTM D3359, with acceptance commonly at 4B or better. Migration of low-molecular-weight PET oligomers to the film surface can reduce metal adhesion; corona treatment of aged film, rather than freshly quenched film, is therefore limited to rolls stored below 30 °C and 50% RH.

When a Sealant Layer Lowers Seal Initiation Without Sacrificing Core Barrier

A heat-sealable PET film places a lower-melting copolyester layer on a bOPET core. The core retains a melting point of 255–260 °C per ASTM D3418, while the sealing layer is designed to soften and bond at 110–160 °C. Seal initiation temperature is measured with a laboratory tray sealer at 2 bar, 0.5 s, and 25 mm wide strips; seal strength per ASTM F88/F88M at 140 °C is typically 300–800 g/25 mm. The sealant layer thickness is held between 2–8 µm; below 2 µm, seal strength becomes dependent on substrate gauge variation, while above 8 µm, haze increases to 4.0–8.0% per ASTM D1003. The sealing layer must be coextruded or coated with sufficient melt strength to avoid squeeze-out during jaw sealing; excessive sealing pressure above 3 bar at 160 °C can displace the copolyester layer and expose the core, creating leakers in automated tray lines.

Compliance for heat-sealable PET in food-contact applications is governed by FDA 21 CFR 177.1630 in the United States and EU 10/2011 in Europe. Specific migration limits for monomers are referenced to EN 13130 migration testing; ethylene glycol is limited to 30 mg/kg and terephthalic acid to 7.5 mg/kg in EU food simulants. The film should not be exposed to sealing jaw temperatures above 190 °C for longer than 1.0 s unless the line is fitted with controlled cool-down stations; otherwise core-layer heat shrinkage per ASTM D1204 exceeds 1.5% and the heat-seal layer can laminate to the jaw contamination. On high-speed form-fill-seal equipment with cycle times below 40 cycles/min, seal layer pressure must be profiled rather than fixed; a constant pressure of 2 bar reduces wrinkles but may not clear air from the seal area.