Why do some XPS sandwich panels fail to fully recover their thickness after prolonged loading and unloading?

This is attributed to the elastic-plastic behavior of XPS foam. When compressive stress exceeds its proportional limit (about 0.2–0.3 MPa), cell walls buckle beyond elastic buckling, forming permanent plastic hinges. Even after unloading, only part of the elastic deformation recovers, leaving a permanent compressive strain (ε_perm) of 1.0–2.5%. According to ISO 7616, panels with thickness change >5% after 24 hours of unloading are considered unacceptable. High-density XPS (>40 kg/m³) has better recovery. In engineering, to reduce permanent compression, pre-compression treatment can be applied: pre-compress XPS to 90% of its target thickness for 24 hours during production to “stabilize” the microstructure.

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Why do some XPS sandwich panels fail to fully recover their thickness after prolonged loading and unloading?

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