Plastic foam is much less acidic and warp-resistant than traditional boards made of wood pulp. Still, the polystyrene in the plastic will eventually decompose if not properly covered or prepared.
Exposure to polluted air and ultraviolet light causes foam board to break down and release acidic vapors, so manufacturers have developed special buffers to coat the foam board. This will slow down the degradation but not prevent it completely.
Wrapping or completely covering the foam core with paint also reduces exposure to the elements. This also protects the artwork from the foam core.
Acid-free foam board is popular for mounting prints, photos and posters, which are bonded to the board in one piece with a hot press. The process is usually not reversible and not used for conservation.
Acid-free foam boards are also popular for mounting items that are sensitive to heat and moisture. Positional mounting is a cold-press process that is not usually reversible and is not suitable for conservation.