Polyurethane foams used in concrete repair to prevent water penetration through concrete cracks are said to be either hydrophobic or hydrophilic in behavior. Both types can be used to stop water in concrete crack repair, but hydrophobic systems are best suited for permanent repair of most leaking basement cracks unless there are structural issues (where epoxies coupled with carbon fiber staples are the best choice).
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. Today, there are hydrophobic formulations available which are flexible and stable, and are suitable for concrete crack repair. They bond well to concrete, do not shrink even in the absence of water, and can expand enough to fill voids economically.
Two component hydrophobic systems can be formulated where one side contains the catalyst and the other side contains the reacting ingredients. These systems require very little water to initiate the foaming reaction. There is typically enough water present in a crack to initiate the reaction. Thus unlike hydrophilic systems, where water is typically first mixed with the hydrophilic, these hydrophobic systems do not react until already present in the crack and have time to fully permeate all parts of the crack before beginning to foam. A hydrophilic system will usually be foaming before entering the crack, and can clog up the mixing unit before entering the crack. A hydrophobic system has time to not only permeate the crack but to fill any void behind the crack. A void is a separation of the foundation wall and the back-fill which can allow collection of water behind the crack and cause it to leak. A hydrophobic will react with the water, filling the void as it forms a rubber membrane behind the foundation which blocks water from reaching the crack to cause leaking. These systems work very well in residential basement concrete crack repair, especially when packaged in dual cartridges.
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).
A typical hydrophilic system is very flexible and has good adhesion to concrete, and thus bonds well to the sides of a concrete crack. This excellent adhesion, plus flexibility, can minimize the shrinkage in a crack. In the foaming process, a typically hydrophilic will expand two to four times its starting volume. It is an excellent product where there is limited movement around a crack which is greater than 3/8 of an inch (where its flexibility can be realized). It is also effective in situations where water is always present. It cannot be helpful in dealing with a void behind a crack, where the hydrophobic foams really shine.
In summary, hydrophobic systems, which have been modified for flexibility and good adhesion, are excellent choices for most basement concrete crack repair. Hydrophilics are good where there is a movement and/or water is always present.