The distinction is critical for any application involving moisture or pressure.
- Closed-Cell Foam (e.g., PVC, PET, PU, XPS): The gas bubbles (cells) are fully encapsulated by the polymer wall. They do not connect. Result: It floats, it resists water absorption (typically <2% by volume), and it provides thermal insulation.
- Open-Cell Foam (e.g., Melamine Foam, Reticulated Polyurethane): The walls between cells are broken, creating a labyrinth of interconnected pathways. Result: It absorbs liquid and sound easily, but it acts like a sponge and will rot or freeze-thaw fracture if used outdoors.
