Polyethylene (PE) foam is produced as either a non-cross-linked thermoplastic foam or a cross-linked thermoset foam. In the non-cross-linked route, low-density polyethylene or linear low-density polyethylene is plastified in a corotating twin-screw extruder with an L/D ratio of 30:1 to 48:1. A physical blowing agent such as isobutane or carbon dioxide is injected through a high-pressure metering system, or a chemical blowing agent such as azodicarbonamide is dispersed. Azodicarbonamide decomposition is exothermic and initiates above 200 °C; the resulting gas expands the melt at the die. Because the cell walls contain no covalent network, expansion depends on melt strength and die pressure. Cell coalescence, gas escape, and broad cell size distribution limit the minimum stable density. In the cross-linked route, an irradiation step using electron-beam equipment or a peroxide/silane grafting system creates a three-dimensional network before expansion. Gel content is measured by ASTM D2765 as a release criterion. Insufficient gel content produces asymmetric cell growth, pinhole defects, and slitting instability.

Because direct gas expansion depends on melt strength, the first measurable difference between the two foam types appears in cell morphology. In non-cross-linked PE foam, the expansion pressure must not exceed the elongational viscosity of the melt. If the melt pressure drops below the critical value at the die exit, cells open, gas escapes, and density reduction stops. In cross-linked PE foam, the network increases elongational viscosity at expansion temperature, allowing the foam to expand to lower density while retaining closed cells. Supplier micrographs indicate cell diameter for non-cross-linked PE foam between 0.3 mm and 1.5 mm, while cross-linked PE foam commonly exhibits cell diameter below 0.5 mm. This morphological difference changes surface roughness, edge quality, and thermal conductivity. Thermal conductivity of PE foam at 23 °C measured by ISO 8301 is typically between 0.035 W/m·K and 0.052 W/m·K depending on density and cell gas. Selection of Polyethylene (PE) foam for industrial conversion therefore reduces to a single comparison: Cross-Linked vs Non-Cross-Linked Polyethylene Foam: When the Difference Matters.

Why Does the Crosslink Network Control Compression Set and Cell Collapse?

The covalent network in cross-linked PE foam increases the elastic modulus of cell walls and restricts viscous flow under sustained load. Compression set testing under ASTM D3575, after 25% deflection at 23 °C for 24 h, typically shows non-cross-linked closed-cell grades between 25% and 40%, while cross-linked closed-cell sheets remain at or below 10%. At 80 °C, the difference widens because non-cross-linked cell walls begin to soften and creep; cross-linked foam maintains recovery as long as the service temperature stays below the oxidative degradation threshold of the crosslinked matrix, commonly listed at 100 °C in supplier data sheets. Compression set becomes the controlling specification in gaskets, flange seals, and HVAC sealing because a compressed foam that fails to recover generates a leak path at the interface. Open-cell content measured by ISO 4590 is also lower in irradiation-crosslinked PE foam, typically below 5%, which reduces capillary wicking and improves sealing in condensing environments.

In gasket and seal applications, compression set resistance combines with tensile and tear strength during installation and service. Published data for closed-cell PE foam sheets show tensile strength of 0.4–3.0 MPa for cross-linked grades and 0.1–0.5 MPa for non-cross-linked grades when tested under ISO 1798. Field observations on slitting lines indicate that non-cross-linked PE foam with density fluctuation above ±3% produces variable unwind tension, edge scalloping, and unstable tool registration during high-speed rotary die-cutting. Cross-linked grades are less prone to density drift but exhibit higher residual stress after slitting, which can produce curl if the foam is stored below 15 °C. Kiss-cutting of non-cross-linked PE foam at speeds above 60 m/min can generate adhesive build-up on the cutting tool because the torn cell walls release particulate debris; cross-linked foam produces a cleaner cut edge with less dust but requires sharper tool geometry due to higher tear resistance. These converting differences are observed on production-scale rotary die-cutting lines using high-speed steel or carbide stationary blades.

Short-cycle protective packaging rarely needs the recovery performance that separates cross-linked from non-cross-linked PE foam. The compressive load is transient, compression set is less important than energy absorption, and the thermoplastic nature of non-cross-linked foam allows edge trim and scrap to be routed into controlled closed-loop reclaim. Cross-linked PE foam is usually reserved for packaging applications requiring chemical resistance or repeated use, such as returnable dunnage and cases for sensitive instruments. Published data for cushioning curves of cross-linked versus non-cross-linked PE foam is limited; package designers typically generate dynamic cushioning data with shock test equipment per ASTM D1596.

When Thermoforming Demands Uniform Cell Wall Rupture Resistance

Thermoforming cross-linked PE foam differs from vacuum forming non-cross-linked PE foam because the crosslinked network permits deeper draw without wall rupture. Closed-cell cross-linked sheets of 25–50 kg/m³ density are heated by quartz or ceramic radiant banks to surface temperatures between 120 °C and 160 °C, then formed over male or female tools. The cell walls of cross-linked foam resist gas expansion and do not collapse when the tool applies localized stretching. Non-cross-linked PE foam has lower melt strength; at surface temperatures above 70 °C, the cell walls tear, gas escapes, and the sheet can shrink before forming. The thermoforming window for non-cross-linked PE foam is therefore narrow, and published processing guides for deep-draw non-cross-linked PE foam are limited. Cross-linked PE foam is preferred when a part must hold a drawn three-dimensional shape without springback, such as formed padding, gaskets with complex flanges, or automotive interior liners. Wall thickness uniformity after forming is measured by taking cross-sectional measurements at multiple points; cross-linked grades typically maintain thickness variation below 15%, while non-cross-linked grades may exceed 25% at corners.

Closed-cell skin formation differs between the two foam types and directly affects acoustic insertion loss and water absorption. Cross-linked PE foam retains a continuous skin layer that limits water absorption to ≤1% by volume under ASTM D3575 after 24 h immersion. Non-cross-linked PE foam, especially when slit or die-cut, exposes torn cell walls and can absorb between 2% and 5% by volume. In vibration isolation pads and HVAC pipe insulation, moisture uptake reduces thermal resistivity and can promote microbial growth on the foam surface. Acoustic performance of closed-cell PE foam is more modest than open-cell polyurethane and should be verified by impedance tube testing under ISO 10534-2 rather than estimated from density alone. Dynamic stiffness under ISO 10846-2 is controlled by foam density and cell wall stiffness; cross-linked grades provide a more repeatable dynamic stiffness across a temperature range from -20 °C to 60 °C, while non-cross-linked grades show greater temperature sensitivity. Published data for the specific frequency-dependent loss factor of PE foam in building underlay applications is limited.

Where conductor operating temperatures exceed 70 °C, cross-linked PE foam is specified in wire and cable insulation because the thermoset network reduces shrinkage during soldering or hot-stamp assembly steps. Non-cross-linked PE foam is seldom specified for continuous conductor temperatures above this limit. The dielectric constant of PE foam is broadly reported between 1.1 and 2.1 depending on density and blowing gas, with lower density producing lower dielectric constant. Test methods include IEC 60250 and ASTM D150. This application is a clear case where the crosslink network matters because sustained clamp heat would collapse non-cross-linked cell walls.

Material Property Data for Specification Engineers

The table below aggregates typical published ranges for closed-cell PE foam sheets. Values are not guaranteed minimums; processor-run trials under ASTM D3575 and ISO 1798 are required for lot acceptance.

Compliance requirements also separate the two foam types. Cross-linked PE foam used in automotive interior applications is commonly tested under ISO 3795 or FMVSS 302 for horizontal burn rate, and the closed-cell structure can be formulated with halogen-free flame retardants. Non-cross-linked PE foam often requires a higher loading of flame retardant to achieve the same classification because the less uniform cell structure affects dripping behaviour in vertical burn testing. Food-contact applications for olefin polymers are evaluated under FDA 21 CFR 177.1520, and cross-linked PE foam may be acceptable only if the crosslinking co-agents, blowing agents, and antioxidants meet the same olefin polymer restrictions. REACH and RoHS restrictions on azodicarbonamide decomposition residues may force converters to specify gas-expanded non-cross-linked PE foam or irradiation-crosslinked PE foam. Cross-linked PE foam cannot be melt-reprocessed through conventional polyolefin reclaim because of its thermoset network, while non-cross-linked edge trim and scrap can be recycled in controlled closed-loop streams. Peroxide-crosslinked PE foam should not be blended with amine-based antioxidants, which scavenge free radicals and reduce gel content. Non-cross-linked PE foam should not be thermoformed above 70 °C because cell wall collapse and sheet shrinkage occur.