PET Metallised and Coated Film: Barrier Layers That Decide Recyclability

Polyethylene terephthalate (PET) film is converted into high-barrier webs by vacuum metallising or by depositing thin oxide and polymer coatings. These layers reduce oxygen and water vapour permeation for packaging, lidding, and labelling uses. In reclaimer decision trees, the applied barrier layer frequently overrides the base polyester substrate because it determines wash chemistry, flake density, melt filtration behaviour, and thermal degradation products. The recyclability of a PET metallised or coated film is therefore evaluated as a contamination vector in the PET reclaim stream rather than as a standalone film property.

Mechanical PET recycling lines operate on whole bottles, thermoforms, and selected flexible fractions. Post-consumer bales are sorted, ground to flake, hot washed at 80–85 °C in caustic solution, separated in float-sink tanks and hydrocyclones, dried, and extruded. Metallised and coated films enter this sequence as label facestocks, lidding films, or barrier inserts. Their compatibility depends on whether the barrier layer separates from the PET at the wash stage or remains as dispersed inorganic or metallic fines below the reclaimer’s filtration and ash limits.

Film-grade PET used for barrier converting typically has intrinsic viscosity of 0.60–0.65 dL/g per ISO 1628-5. The cast amorphous web is sequentially oriented at machine-direction draw ratios of 3.0:1–3.6:1 and transverse-direction draw ratios of 3.4:1–4.0:1. Heatsetting in a stenter at 200–230 °C develops crystallinity and dimensional stability. Before coating, the substrate is corona treated to a wetting tension of 42–52 dyn/cm under ASTM D2578. Metallisation-grade base film usually falls between 10 µm and 23 µm thickness, with haze below 2.5% under ASTM D1003. These production parameters determine the adhesion and cleanliness of the interface between PET and the subsequent barrier layer.

Why Does Vacuum-Deposited Aluminium Control Screen-Changer Pressure in Reclaim Lines?

Aluminium metallising is carried out on roll-to-roll vacuum chambers at working pressures below 10⁻³ Pa. Wire-fed resistance boats evaporate aluminium onto the cooled PET web to a metal thickness of 30–60 nm. Commercial packaging targets often require optical density between 1.8 and 2.6 and surface resistivity below 3 Ω/□. The metallised film then shows oxygen transmission of 0.3–1.0 cm³/(m²·day) at 23 °C, 0% RH under ASTM D3985, and water vapour transmission of 0.5–1.5 g/(m²·day) at 38 °C, 90% RH under ASTM F1249. These are established converting benchmarks, not formulations with universal sharp boundaries.

In flake form, aluminium metallised PET retains a bulk density near 1.30–1.40 g/cm³, close to clear PET, because the metal layer is only a nanometre-scale fraction of the total web mass. It may not separate cleanly in sink-float tanks or hydrocyclones. Ground fragments can carry metallic platelets with lateral dimensions of 0.5–5 mm, which creates a filtration problem. Reclaim extruders equipped with continuous screen changers and 40–80 µm screen packs experience pressure rise and throughput loss when aluminium-loaded flake blinds the screen surface. The mechanism is not simple retention; metal platelets accumulate as a low-permeability cake layer that reduces open filter area faster than uncoated PET flake.

Adhesion between aluminium and PET affects where the metal reports in the reclaim process. If the metallised layer remains bonded during wet grinding and caustic wash, it enters the extruder as metal-carrying flake. If it delaminates in wash, free aluminium fines may be removed by flotation or may contaminate the wash water and downstream filtration. Production-scale variance in oligomer bloom on heatset PET film is a known source of adhesion fluctuation. Low-molecular-weight cyclic oligomers at the film surface reduce aluminium nucleation density and create weak interfacial regions. This batch-to-batch variation means reclaimer behaviour cannot be predicted from a single adhesion value alone.

Transparent Oxide Coatings and the Inorganic Residue Trade-Off in Monomaterial Film

Silicon oxide and aluminium oxide barrier films are produced by electron beam evaporation or plasma-enhanced chemical vapour deposition. Coating thickness is typically 10–100 nm. These transparent deposits are promoted as recyclability-friendly because they are chlorine-free and non-conductive. Their oxygen transmission values generally range from 0.5 to 2.0 cm³/(m²·day) at 23 °C, 0% RH under ASTM D3985, with water vapour transmission from 0.5 to 2.0 g/(m²·day) at 38 °C, 90% RH under ASTM F1249. The residual mass added to the PET substrate is small, but the inorganic coating may be reduced to sub-1 µm fragments during grinding and extrusion shear.

In a reclaimer extruder with 40–80 µm filter screens, sub-1 µm oxide fragments can pass through the screen pack. Some fraction reports to the final pellet as inorganic residue rather than being removed. Ash content in clear PET reclaimate is therefore the relevant quality indicator. Processes that are governed by film thickness and coating adhesion in packaging may generate reclaimate that fails bottle preform clarity or fibre spinning limits if oxide carry-over is not controlled. Published reclaimer-specific data on oxide-coated PET content in commercial flake streams remain limited; a universal recyclability claim requires line-specific flake assays, not only laboratory oxygen barrier performance.

In flexible packaging, oxide-coated PET is often sealed or laminated to polyolefin layers. That construction introduces additional separation issues that dominate over the oxide layer itself. The PET layer may be unrealistically difficult to liberate from the sealant web in conventional bottle reclaimer equipment. Barrier layer recyclability testing for laminates must therefore include the full structure, because the PET-specific benefit of an oxide coating cannot compensate for incompatible polyolefin lamination in a PET reclaimer.

When PVdC-coated PET enters a continuous reclaimer extruder, the processing temperature is sufficient to dehydrochlorinate the halogenated copolymer. PVdC degradation begins below 200 °C, while PET reclaim extruders commonly operate at 270–290 °C. The released hydrogen chloride attacks PET ester linkages and accelerates hydrolysis. This effect is more severe when flake moisture exceeds 100 ppm at the feed throat. Pre-drying at 160–170 °C to below 50 ppm moisture is mandatory for any reclaim stream containing PVdC residue. Melt pressure may rise as degraded material and char build on the screen pack. The change in reclaimate quality is measured by intrinsic viscosity loss under ASTM D4603 and by volatile acetaldehyde under ASTM F2013.

The production-scale consequence of releasing HCl inside a reclaimer extruder is not limited to polymer degradation. Vent ports and downstream metal surfaces in a 28:1–36:1 L/D twin-screw reclaim line are exposed to acidic volatiles. Maintenance intervals may shorten if PVdC-bearing film repeatedly enters the feed. This mode is fundamentally different from the mechanical screen blinding caused by aluminium, because it alters bulk resin chemistry rather than only filtration hydraulics.

When a Halogenated Barrier Layer Reaches the Reclaimate Extruder

The operational limit for PVdC-coated PET in a reclaimer is reached as soon as chlorine-containing fragments survive sorting and wash. Halogenated barrier layers cannot be removed by melt filtration because their decomposition products are soluble or gaseous at extrusion temperatures. The correct control is upstream removal: label wash separation, density discrimination, or optical sorting must divert PVdC-coated film before the extruder. If the coating remains on PET flake, the reclaimer may observe acid-catalysed hydrolysis, intrinsic viscosity reduction, and elevated acetaldehyde in the output compared with control PET flake processed under identical conditions.

Compliance screening for PET barrier films intended for recyclability uses a battery of methods. Oxygen and moisture transmission are measured by ASTM D3985 and ASTM F1249. Melt mass-flow rate can be compared under ISO 1133-1:2022. Intrinsic viscosity retention is measured by ASTM D4603. Material recovery frameworks such as ISO 15270 and packaging recovery standards such as EN 13430 provide the assessment boundary. However, final acceptance remains plant-specific because reclaimer equipment, wash chemistry, and screen configurations differ.

Hot wash caustic concentration is normally 0.5–2.0 wt% NaOH at 80–85 °C. Under these conditions, aluminium metallised PET does not dissolve as a bulk metal; aluminium can react with caustic to form aluminate species, but the reaction rate depends on exposed metal area and wash residence time. Complete removal of the metal layer cannot be assumed in a continuous bottle wash line. Flexural cracking during packaging transport also changes the barrier layer geometry and may increase oxygen transmission after metallisation. The increase is evaluated under flex-cracking conditions such as ASTM F392. A coating that detaches during consumer handling may separate in the reclaimer, while a highly abrasion-resistant coating may remain with the PET flake and require melt filtration.

Published data for specific PVdC contamination loads in commercial PET reclaim lines remain limited, and acceptance thresholds for silicon oxide or aluminium oxide reinforced PET flake vary by reclaimer. The barrier layer must therefore be specified alongside wash-line configuration, extruder screen pack, and ash limit to determine recyclability in a defined recycling system.