Polyethylene terephthalate (PET) foam board is specified in core-grade configurations for structural sandwich panels where the controlled variables are density, closed-cell content, crystallinity, and thickness tolerance. The specification “PET Foam Board: Core Grade, Density and Sandwich Panel Uses” describes a material class in which the base resin is a semicrystalline aromatic polyester with a melting peak between 245 °C and 260 °C, a glass transition temperature near 75 °C, and an intrinsic viscosity (IV) after extrusion maintained above 0.70 dL/g to preserve melt strength. Core-grade PET differs from packaging-grade PET because the extruder feed stream typically contains chain extenders such as epoxy-functional styrene-acrylic oligomers or pyromellitic dianhydride (PMDA), added at 0.1–0.5 wt% to increase molecular weight and broaden the foaming window. The resulting foam board is produced on high-torque co-rotating twin-screw extruders with L/D ratios from 36:1 to 44:1. Physical blowing agents, usually carbon dioxide or a mixture of CO₂ and nitrogen, are metered through side injection ports into the melt at rates commonly reported between 0.5 wt% and 3.0 wt%. The extrudate is cooled through a calibrator and pulled as a continuous board with controlled skin density and thickness tolerance of ±0.5 mm for panels up to 50 mm. In recycled feedstocks, IV loss during drying and extrusion is compensated by the chain extender, but moisture must be held below 0.01% by mass to prevent hydrolytic degradation that reduces melt strength and creates irregular cell walls.

Core-Grade PET Resin and Melt Strength Requirements

Core-grade PET foam extrusion requires a melt with extensional viscosity high enough to stabilize expanding cell walls without suppressing nucleation. Published rheological data for chain-extended PET show that melt strength measured by a Rheotens device at 280 °C typically increases from approximately 5–10 cN for unmodified bottle-grade resin to 15–30 cN after chain extension. The increase is accompanied by a rise in zero-shear viscosity from roughly 300–500 Pa·s to above 900 Pa·s, depending on chain extender chemistry and concentration. Melt filtration is required at 20–40 µm mesh size when post-consumer flake is used because residual aluminum caps, label fragments, and silica particles produce die streaks and local cell-wall defects. Equipment manufacturers report that melt pressure variation at the die should be maintained within ±0.2 MPa to avoid density drift in the transverse direction. Batch-to-batch variability in recycled flake IV, measured by dilute solution viscometry using phenol/1,2-dichlorobenzene at 25 °C according to ASTM D4603 or ISO 1628-1, can shift nominal density by several kilograms per cubic metre if feed blending is not corrected.

The foaming window is narrow. At melt temperatures below 270 °C, carbon dioxide solubility increases but melt viscosity rises sufficiently to raise screw torque and create pressure spikes at the gear pump. Above 295 °C, thermal degradation accelerates, releasing acetaldehyde and reducing IV, which leads to open-cell collapse and gross density non-uniformity. The die lip temperature is normally set 5–10 °C below the melt temperature to maintain skin integrity while permitting expansion. On production lines, failure modes include prefoaming at the die entrance, post-extrusion skin blistering when residual blowing agent diffuses too rapidly into the ambient air, and centreline void formation when the calibrator cooling rate exceeds the crystallization rate of the foam matrix.

Density grade selection for core-grade PET foam board follows the design load path in a sandwich panel. Nominal densities used in industrial practice typically range from 60 kg/m³ for lightly loaded interior panels to 300 kg/m³ for local reinforcement under point loads. Most structural cores are specified between 100 kg/m³ and 200 kg/m³. Apparent density is measured according to ISO 845 or ASTM D1622-20 on conditioned specimens. The table below presents representative property ranges from published core-grade PET datasheets; values vary with crystallinity, skin thickness, and closed-cell content.

Why Does Closed-Cell Content Dominate Shear Modulus in Core-Grade PET Foam?

Closed-cell content controls the shear stiffness of PET foam board because the cell walls behave as a load-transfer network between the face sheets. When closed-cell content falls below approximately 90%, the shear modulus drops more rapidly than the density would predict. In practice, core-grade PET foam is supplied with closed-cell content above 92%, measured by gas pycnometry or water displacement methods. Cell size also influences shear behaviour. Core materials with mean cell diameters between 100 µm and 250 µm show more uniform shear stress distribution than high-density boards with cell diameters below 60 µm, where localised cell-wall fracture can initiate edge-effect failures in short-beam shear tests.

Shear testing for these cores is conducted according to ASTM C273 or ISO 1922. The shear stress-strain curve of PET foam is nonlinear above roughly 35–45% of ultimate shear strain, and the reported shear modulus is usually the secant modulus at 0.1% shear strain. The measured shear modulus also depends on specimen thickness and skin removal. Removing the higher-density skin by machining reduces apparent shear modulus by 10–20%, because the skin region of extruded PET foam board often carries a higher degree of strain-hardening and crystallinity than the core centre. Designers therefore specify shear properties based on the machined condition when the foam surface is sanded before lamination. Published fatigue data indicate that the shear modulus degrades less than 5% over 10⁶ cycles at a load level corresponding to 30% of ultimate shear stress, while load levels above 50% produce a more rapid stiffness loss due to cell-wall yielding.

When Thermoforming Induces Density Gradients in Thick Core Panels

Thick PET foam boards above 25 mm can develop density gradients when thermoformed too rapidly. The boards are heated in multi-zone infrared ovens at surface temperatures between 140 °C and 180 °C before being shaped on a vacuum table. If the core centre remains below 110 °C while the surface reaches 170 °C, the softened outer layers deform independently of the semi-crystalline centre, producing shear localization along the thickness axis. In thick panels, the heating cycle is therefore staged, with a soak period of 2–5 minutes depending on board thickness and a final surface heat flux of 10–15 kW/m² to reduce the through-thickness temperature difference to below 20 °C before forming.

Density gradients are measured by cutting the thermoformed core into horizontal slices and testing each slice according to ISO 845. A density variation greater than ±10% across the thickness generally indicates insufficient thermal equilibration or excessive vacuum draw. Moulded PET foam parts used in wind turbine leading-edge protection and transport interior panels often retain a higher skin density at the outer radius, which increases local compressive strength but may reduce shear elongation in the core centre. If the tool temperature is below 60 °C, quench-induced crystallization is limited and the formed part may show post-mould shrinkage of up to 1.5% over 24 hours. For dimensional stability, tools are maintained between 70 °C and 100 °C during forming and cooling.

Fire performance requirements determine whether a PET foam core is acceptable for interior sandwich panel uses. PET foam does not emit styrene during combustion, which differentiates it from unsaturated polyester-based foam cores. The core is usually evaluated under FAR 25.853 for aircraft interiors when the panel is part of an occupied compartment, and under ASTM E84-21c for building interior finish applications. Cone calorimeter testing under ISO 5660-1 provides heat release data for marine and rail specifications. Reported total heat release and peak heat release depend heavily on face sheet material and adhesive, but the PET foam itself is classified as a thermoplastic melting foam with limited char formation. If the panel must meet a low heat-release specification, the core is generally used with fire-retardant adhesive films and non-flammable skins rather than relying on the foam alone.

Selecting Adhesives and Preparing Foam Surfaces for Sandwich Lamination

Sandwich panel lamination with PET foam cores is typically performed with epoxy, polyurethane, or structural methacrylate adhesives. Epoxy adhesives are applied at film weights between 200 g/m² and 400 g/m² for continuous lamination. Polyurethane adhesives are preferred when the face sheet is an aluminum skin and the cure temperature is limited to 80 °C because they tolerate a wider moisture window. Methacrylate adhesives are used for local repair because they develop handling strength rapidly, but their exothermic peak can exceed 130 °C in thick bond lines. If the bond line exceeds 5 mm, exothermic temperature rise can soften the adjacent PET foam and create low-density zones under the skin.

Before lamination, the foam surface is usually abraded or corona-treated because extruded PET foam board has a higher-density skin with lower surface energy than the internal foam. Surface treatment increases polar surface energy from roughly 34–38 mN/m to above 44 mN/m, measured with a dyne test pen. Release agents from the extrusion calibrator and machining dust from cutting must be removed with a dry vacuum and an isopropanol wipe. If the core has been stored at relative humidity above 60%, moisture absorption in the cell walls is less of a concern than for aramid or wood cores, but drying at 40 °C for 12 hours is often performed to remove surface condensation before adhesive application. Vacuum bag processing should be limited to –0.8 bar relative pressure for low-density cores below 100 kg/m³, because higher vacuum can crush the foam and reduce panel thickness by more than 5%.

In marine and wind energy applications, core-grade PET foam is used where long-term moisture resistance and reduced resin uptake are required. The closed-cell structure limits water absorption in the core, but water can penetrate machined kerfs and scoring cuts if those channels are not filled with adhesive or sealant. Marine panels with fiberglass skins laminated by vacuum infusion require the core to be cut with kerf spacing between 20 mm and 50 mm to allow resin flow without creating resin-rich pathways that raise panel density. Infusion resin viscosity must be below 500 mPa·s to fill the kerfs and perforations without leaving dry zones at the foam-skin interface. Wind turbine blade shells and shear webs use PET foam in density classes from 100 kg/m³ to 200 kg/m³, with higher-density inserts at root connections and where edgewise loads concentrate.

Fatigue Life and Environmental Aging Data in Structural PET Foam Cores

Fatigue and environmental aging data for PET foam derive from sandwich beam tests rather than foam coupons alone, because face sheet stiffness influences the stress state inside the core. Four-point bending fatigue tests on sandwich beams, run according to ASTM C393 for static properties and ASTM C1360 for cyclic loading, show that PET foam cores retain a higher fraction of initial shear stiffness after cyclic loading than many rigid PVC foams at the same density. Published test programmes report that a 130 kg/m³ PET foam core cycled at 40% of ultimate shear stress for 10⁶ cycles experiences a stiffness loss of approximately 8–12%. At 60% load level, the stiffness loss increases and the hysteresis loop area grows, indicating progressive cell-wall yielding rather than a single crack front.

Water ageing is assessed by exposure to seawater or distilled water at 40 °C for periods up to 1000 h. Mass gain of a closed-cell PET foam board is usually below 2% by mass under these conditions, with most of the uptake occurring in the cut surface layer and machined kerfs. The effect of absorbed water on shear properties is small when specimens are tested after drying to constant mass, but testing in the wet condition can reduce shear modulus by 5–10% for low-density grades. In contrast, environmental ageing at elevated temperatures above the glass transition of the foam matrix produces larger property shifts. Continuous exposure above 80 °C can increase crystallinity and densify the cell structure, raising compressive strength while reducing elongation. The operational boundaries for structural PET foam are therefore defined by the combination of load, temperature, moisture, and cyclic duration rather than by a single density specification.

Building and transport panel producers specify PET foam board for interior partitions, trailer sidewalls, bus floors, and architectural cladding. In these sandwich panel uses, the core density is selected from the compressive load under floor traffic or wind suction, and the panel thickness is set by deflection limits under ISO 2248 or ASTM D7249 test conditions. A 100 kg/m³ PET foam core laminated with 0.5 mm aluminum skins is used in trailer sidewalls where panel mass must be reduced without sacrificing point-load resistance. For building cladding, the panel is usually qualified under ASTM E330 for wind load and ASTM E283 for air leakage. Core-cutting and thermoforming waste can be reground, dried, and recompounded into non-structural PET products, but the recycled content in a new core-grade foam board is limited by the loss of IV after repeated heat histories. Published data for closed-loop recycling of post-industrial PET foam scrap into structural foam core are limited; the main recycled stream remains bottle-grade flake blended with virgin resin and chain extenders. The final core density and cell geometry are therefore controlled by extrusion melt history, blowing agent concentration, and panel curing parameters, not by coupon-level density alone.