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The hydrophilic emulsifier led to a dramatic change in FPI processing when it was first used in Europe.

Few among our readers know this time!

Till the end of the 60s, the only available emulsifier was the lipophilic one comprising mainly mineral oil and surfactants.

The main characteristics of the lipophilic emulsifier are shown underneath:

AND THE HYDROPHILIC EMULSIFIER

WAS THOUGHT OF!

LIPOPHILIC EMULSIFIER

Dilution

Use as supplied: ready-to-use.

Water tolerance

Emulsifier shall be thrown away if a 5% water content is measured.

Penetrant tolerance

The emulsifier shall withdraw a content of penetrant up to 20% without any performance impairing.

Action

Both chemical and mechanical action.

- Mechanical action starts to remove some of the excess penetrant as the mixture drains from the part.

- Then the emulsifier diffuses into the remaining penetrant.

- Mixture is then removed by air/water spray washing.

Emulsification time

Of utmost importance: may lead to fast and important loss of sensitivity.

Wastes

The very high content of mineral oil makes waste treatment costly.

Quality managers in Aerospace industries were worried that the emulsification time was by far the most important parameter to meet when using post-emulsifiable penetrants. An overtime as low as 15 to 30 seconds has a significant effect on the inspection sensitivity and reliability.

In this case, sensitivity rapidly lowers, the more with larger discontinuities sizes.

It was in a sense the Achilles' heel of the PE FPI.

As the lipophilic emulsifiers were the only alternative everyone had to do with this inconvenience.

Norman HYAM (†), a Brit, may be at the origin of the hydrophilic emulsifier idea.

He worked not only in the PT & MT methods but also for the surface treatments industry by designing trendsetter speciality chemicals intended:

Prior to inspection, to remove every pollutant without detrimental action on the parent metal or on the mechanical characteristics of the metallic alloys and their fatigue resistance limit.

After inspection to provide a short to long term corrosion inhibition for metal surfaces.

His work about detergents led him to the hydrophilic emulsifier design: hence the "detergent remover" wording sometimes used for this product.

Indeed hydrophilic emulsifiers are mainly a blend of surfactants.

The main characteristics of hydrophilic emulsifiers are displayed underneath:

HYDROPHILIC EMULSIFIER

Dilution

Used diluted in water, generally at:

- 0.5% (foam application)

- 2.5% (spray application)

- from 5% to 27% depending on applications and AMS-2644 approval (immersion application)

Water tolerance

Completely miscible in water.

Penetrant tolerance

- Varies with emulsifier concentration.

- About from 1% to 5% (by volume) for a solution with a 10% concentration of emulsifier. Depends on emulsifier.

Action

Both chemical and mechanical action.

- No diffusion takes place.

- The emulsifier breaks up the penetrant into micelles and prevents them from recombining or reattaching to the surface of the part.
- The mechanical action of the rinse water removes the displaced penetrant from the part and allows fresh remover to come in contact with it and to lift it from the surface.
- The mixture is removed by air/water spray washing.

Emulsification time

Not so critical as with the lipophilic emulsifier.
A difference of one minute may lead to a small difference for discontinuities detectability.

Wastes

The emulsifier being oil-free, water treatment is much easier to achieve.

Though hydrophilic emulsifiers, when compared to lipophilic ones, show a better sensitivity less critical to emulsification time, hydrophilic emulsifiers needed time to be accepted. Here some reasons:

First of all:

- Changing established processes gives rise to risks; someone has to take the responsibility for them!

- Some people are reluctant to any change as this modifies their working habits: it's called inertia.

Furthermore the competitors who did not have hydrophilic emulsifiers in their line of products panicked, or kind of, and try to counter by playing down this new product, hoping to nip it in the bud. They were as if on the verge to lose some market shares, at least in Europe.

One of their points: as hydrophilic emulsifiers are diluted in water, water would corrode parts.

This statement with no proof to sustain it led to nowhere.

The first French "indexing system" automatic process line commissioned in France in 1973 used a hydrophilic emulsifier for the first time in France: a double ''first''.

Then the entire French Aerospace industry went to use hydrophilic emulsifiers.

Many papers dealt with the topic detailing why these emulsifiers constantly gave better results than lipophilic ones. Among them:

- Norman H. HYAM, "Quantitative Evaluation of Factors Affecting the Sensitivity of Penetrant Systems", Materials Evaluation, February 1972, pages 31-38.

- Jean VAERMAN, "Fluorescent Penetrant Inspection Process. Automatic Method for Sensitivity Process", 11th World Conference on NDT Las Vegas, November 1985.

- Jean VAERMAN, "Fluorescent Penetrant Inspection, Quantified Evolution of the Sensitivity Versus Process Deviations", Proceedings of the 4th European Conference on Non- Destructive Testing, September 1987, Pergamon Press, Maxwell House, Fairview Park, Elmsford, New York, Volume 4, pp. 2814-2823.