Wetting ability - Affects penetrability and bleed-back characteristics.
Specific gravity - Penetrant chemicals used in a tank system must have a specific gravity less than one, to assure that water will not float on top of the penetrant and prevent the penetrant from covering the test object.
Flash point - The flashpoint of a material is the temperature at which enough vapour is given off to form a combustible mixture and a minimum value of around 93°C is typical.
Volatility - Highly volatile penetrant chemicals would evaporate too quickly to be practical.
Chemically inert - Penetrant materials must be as inert and non-corrosive as possible. Maximum sulphur, sodium and halogen levels are often specified by the nuclear and aerospace industries, to avoid the possibility of embrittlement or cracking.
Viscosity - Viscosity relates to the thickness or body of a fluid and is a result of molecular or internal friction. Excessive viscosity results in long dwell times, low viscosity leads to reduced dwell times but makes the penetrant prone to over washing.
Solubility - A penetrant must hold sufficient dye at ambient or high temperature and the dye must not come out of solution if the temperature drops.
Solvent ability - Having applied the penetrant, it becomes necessary to remove the surplus from the test specimen to ensure a clean, clear background. Volatile solvents, some flammable, some not, are often used. These must not dissolve the penetrant in defects.
Health hazard - Chemists developing new penetrant materials must comply with or exceed the most stringent health and safety requirements.
Electrical conductivity - Electrostatic spraying of penetrant is becoming increasingly popular in large automatic processors and even where electrostatic hand-spray guns are used. To be adaptable to electrostatic spraying, penetrant must readily accept and hold the electrical charge placed on the liquid particles.
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