T.O. 33B-1-1
2-69
2.6.6
Nonaqueous Solvent Suspended Developers.
2.6.6.1
Description.
Nonaqueous solvent suspended developers are supplied in the ready to use condition and contain particles of developer
suspended in a mixture of volatile solvents. The solvents are carefully selected for their compatibility with the
penetrants. Solvent developers also contain surfactants and dispersants whose functions are to coat the particles and
reduce their tendency to clump or collect together. Solvent developers are the most sensitive form of developers due to
the solvent action contributing to the adsorption and absorption mechanisms. In many cases where tight, small flaws
occur, the dry and aqueous developers do not contact the entrapped penetrant. This results in the failure of the
developer to create the necessary capillary and surface tension forces that serve to pull the penetrant from the flaw. The
nonaqueous developer solvents enter the flaw and dissolve into the penetrant. This action increases the volume and
reduces the viscosity of the penetrant. The manufacturer must carefully select and compound the solvent mixture.
Either excessive or inadequate volatility and solubility will adversely affect the performance of the developing action.
High volatility reduces the time for the developer to function before it evaporates, while low volatility increases the
drying time. Low solubility reduces the penetrant dissolving action, so the extraction of the penetrant from the flaw
will not be enhanced.
2.6.6.2
Application.
Nonaqueous wet developers are always applied by spraying. Proper spraying produces a thin, uniform layer that is very
sensitive in producing indications. Dipping, pouring or brushing are not suitable for applying solvent suspended
developer. Dipping and pouring increases the time the solvent is dissolving and diluting the entrapped penetrant so
that much of it ends up in the unevaporated liquid developer layer. During the drain, the penetrant will flow from the
flaw site, and any indications that do form will be weak and badly distorted. Application of solvent developer by
brushing will leave streaks and distort and smear flaw indications into unrecognizable forms. Nonaqueous wet
developer SHALL be applied only as a fine spray or mist.
2.6.6.2.1
NOTE
Excessive thickness of developer SHALL NOT be used. Parts that have received
excessive developer SHALL be completely reprocessed.
Spraying of nonaqueous developer is most often done with pressurized, aerosol containers. There are a few production
lines that use pressure pots and spray guns. Electrostatic spraying is possible but is seldom used due to the poor
throwing power of the spray. Like dry powder developers, solvent developers SHALL NOT be applied to a part until
the surface and any discontinuities are thoroughly free of moisture. The presence of any moisture will interfere with
the developer action and small flaws may be missed. Prior to spray application, the container SHALL be agitated.
Nonaqueous wet developer is usually a suspension and the particles settle out in a matter of minutes. The spray can or
gun must be held far enough from the surface to produce a light, moist film. Liquid flow on the part surface must be
avoided. The recommended technique is to apply a very thin, dry layer and build up the thickness with several passes
rather than applying a single, wet pass. The optimum coating thickness depends on the penetrant system type (i.e.
visible or fluorescent dye) and must be judged from its appearance, based upon prior experience. Type I penetrant
systems, the luster or surface texture of the part surface should not be completely hidden. If the metallic luster cannot
be seen, the developer layer is too thick, and small indications may be masked or too widely spread to be easily seen.
At the thin end of the optimum thickness range, there should be sufficient developer on the part surface to be clearly
visible. Coatings that are too thin may not extract a sufficient amount of entrapped penetrant to form an indication.
Also, too thin of a coat does not allow the penetrant to spread and magnify the indication. For Type II penetrant
systems, a thicker coating is required to provide a solid white background to contrast with the visible indication color.
2.6.6.3
Advantages.
a. They are packaged in portable aerosol containers.
