Hydrophilic ~ water loving. Such compounds have an affinity to water and are usually charged or have polar side groups to their structure that will attract water.

A hydrophilic system acts like a sponge and will absorb as much water as is available in its surroundings during the foaming process. It reacts with the amount of water needed for reaction and subsequently contains the excess water within its structure, as would a sponge. In dry circumstances, some, of the excess entrapped water can evaporate and result in a shrinking of the foam. When water is again present, the foam can absorb the water and return to its original size, as would a sponge (at least theoretically).

Hydrophobic ~ water hating. These compounds are repelled by water and are usually neutral (no charge.)

A hydrophobic system will absorb and mix with only as much water as is needed to complete the foaming and curing of the two components. A typical hydrophobic system will expand up to 30 times its initial volume. Historically, most hydrophobic foams were rigid and could be brittle if not carefully formulated to be more stable. As a consequence, concrete crack repair contractors initially used hydrophilic systems to stop water.