Hydrophobic materials resist water. Hydrophilic materials have an affinity for water. Polystar Technologies’ sintered porous plastics are naturally hydrophobic but can be treated, at the customer's request, to be hydrophilic. For engineers and researchers who need to filter fluids, whether liquids or gases, but also want venting to take place, the degree of hydrophobicity exhibited by the venting or filtration media becomes very important. For this, Water Intrusion Pressure (WIP) and the Water Intrusion Test (WIT) become critical.
The amount of pressure that a material can withstand just prior to the intrusion of water is called the material's Water Intrusion Pressure (WIP), and is directly related to the pore size and hydrophobicity or hydrophilicity of the media being tested:
Hydrophilic porous plastic materials have a natural affinity for wetting fluids and have very low WIPs.
Hydrophobic porous plastic materials have a natural ability to withstand wetting fluids and tend to resist water intrusion. Therefore, their WIPs tend to be quite high.
By definition, hydrophobic materials will resist the flow of water through their pores up to their WIP. When water is introduced to one side of the media at pressures below the WIP, a small amount of water vapor may evaporate across the media due to differential pressure. The amount that evaporates will depend on the pore size of the media and the temperature of the water.
Water intrusion tests can be valuable for hydrophobic venting applications, as the largest through-hole may well determine the vent's ability to perform in various situations. This is important for barrier venting and filtration, as the WIP of the material under consideration demonstrates how water resistant or waterproof one can expect the material to be.