Apply hydrophilic emulsifiers as foam

August 2010

Introduction

Applying hydrophilic emulsifiers as foam is not a current technique.

Nevertheless some standards such as the ISO 3452-4:1998 one and some specifications such as the Rolls-Royce RPS 702 name it. But it is almost never talked about in training courses or in manufacturers/suppliers technical data sheets.

Many years ago some PT materials suppliers marketed foam generators; nowadays none does it.

In a PRI-NADCAP meeting, held in Rome (Italy) in February 2010 the topic of revising the ASTM E 1417 standard was talked about and emulsifying with foam technique should be considered.

Situation in the early ‘70s

Inspecting large structure parts or large mechanical components by immersion when using a post-emulsifiable penetrant system was not an easy task.

Lipophilic emulsifiers MUST be used only by immersion of parts, except for when they are in spray cans for inspection of small areas.

On the other hand, after the rinsing which mechanically wipes off a large part of the excess of penetrant from the surface, the hydrophilic emulsifier used to help for washing the surface may be sprayed on the part.

The hydrophilic emulsifier solution shall ‘‘work’’ as uniformly as possible on the entire surface during the entire contact time specified in the applicable document.

In fact what may occur?

• If the solution homogeneously wets the surface, then it will run down the surface as a continuous and homogeneous film. The bottom areas get more emulsifier than the areas at the top of the part; hence more background at the top, or an overemulsification of the bottom areas are likely.

• On the contrary, an imperfect wettability of the surface will make the water-based solution go down following preferential paths with a better wettability. Added to what is described previously one may see fluorescent drags.

How may one have at the same time a good wetting and the same emulsification quality on the entire vertical surface?

The first answer

The best answer was to duplicate what has been done for decades to clean the aircraft exterior surfaces: a detergent solution is sprayed as a creamy foam. The grip of the foam on the vertical surface is right when there is not too much foam: otherwise, too much foam would make “bundles” of foam detach from the surface due to gravity. This process requires using a foam generator. The solution is emulsified with air to give off foam.

Rolls-Royce, in the RPS 702 specification, states: ‘‘The emulsifier solution shall be applied to all areas of the component treated with penetrant. The application shall be total immersion or as foam. Foam shall not content emulsifier liquid flow or spray when applied’’.

In the ‘70s Willy BELLAERT designed an equipment suitable for applying water-based hydrophilic emulsifier solutions as foam.
Willy BELLAERT was the manager of NV BRENT SA, the Belgian subsidiary of BRENT CHEMICALS INTERNATIONAL plc, in Mariakerke near Gent.

The unit, called ARDROX® (*) BCP 51 WB (WB being for Willy BELLAERT), came as a 20 litre (ca 5.3 US gal) galvanized-steel pressurized can with a safety valve. It was filled up to a third of its volume with the diluted hydrophilic emulsifier.

A dip-tube with a hole just above the liquid level was the only specific device.

When tightly closed the can was plugged to the compressed air network through a regulator and an oil and water filter and pressurized up to 150-200 kPa (ca 21-29 psi). Then the unit was unplugged and brought where it was to be used.

When in use air pushed the liquid which mixed in the tube with the air under pressure and went to a spray lance through a reinforced flexible polyethylene pipe. This gave off a creamy foam of diluted hydrophilic emulsifier.

This simple process had several advantages:

• The hydrophilic emulsifier content is very low, in the 0.1 to 0.5% range. Hence, a lower cost both for the product and for waste water treatment.

• When collapsing, foam bubbles release energy, which "rubs" the surface and then constantly brings fresh emulsifier in contact with the penetrant on the part surface.

But there are some drawbacks:

• The foam from hydrophilic emulsifiers is stable enough to overcome the emulsification time, generally 2 minutes maximum. But this stability is a major problem in the waste water treatment installation. Have a look to the “Oldies, but Goodies” in this same monthly issue on our website.

• When compared to immersion foam application may be not as effective as foam does not enter blind holes or hollow parts.

Rolls-Royce patented the system in the ‘70s. As far as we know parts were put on a tray, at the bottom of a tank, and foam went through the tray.

For several years, application of foam has been the only way to spray hydrophilic emulsifier on large parts.

Next step

When the electrostatic spraying process was marketed very soon it was obvious that water-based products could not be sprayed with the available equipments: charging water droplets was impossible.

In the ‘80s a French company did find an answer using a Ransburg® (**) electrostatic spraying system. The idea was to not earth the can and the spray gun: results met expectations.

Our comments

We assume that this put an end to the foam technique. We may also think that the electrostatic spraying of the hydrophilic emulsifier may lead to streams with accompanying drawbacks as seen previously. In fact, we know of very few examples of hydrophilic emulsifier spraying in the industry on a large scale.

We would be happy to get comments from users, be there happy or not with the process: they could help others!

One of the main points against emulsifier spray, either electrostatic or not, is that it is almost impossible to be sure that the entire surface has been covered by the emulsifier. Immersion is closer to the 100% certainty … but only closer. Another point against spraying the emulsifier, whatever the way it is sprayed: on very large parts, or on complex-shaped parts it is difficult to make one sure that the contact time emulsifier/part is the same in any area of the part. Immersion is supposed to make the contact time almost equal everywhere.
This point may be countered: when parts are tall, for example, it is likely that they will enter the tank quite slowly, to prevent spatters or uncontrolled movements of the part which could then knock the walls of the tank. In the same way, taking the part out of the tank will be quite slow.
Easy to understand that the bottom of the part will have been in contact with the emulsifier much longer than the top. So much that washing the part should begin at the BOTTOM of the part! If a contact time of 3 minutes +/- 30 seconds is mandatory, 30 seconds is equal to 17% of 3 minutes. Not that negligible!

Two advantages of any spraying process (the emulsifier is not “recycled” as it is in a tank):

• The job is carried out with an emulsifier whose parameters are similar along the day, as only a brand new product is used every time.

• A VERY LOW CONCENTRATION is used: 0.1 to 0.5%, which may allow for a bit longer contact times.

Do not forget that a 3-minute time is VERY SHORT in industrial conditions. Further the contact time is not the time the part is in the tank only. We must add the time needed to move the part from the emulsifier application step to the washing step plus the time needed for water to begin washing the part!

Very often these additional times, not foreseen in specifications/procedures, come as … minutes … and the mandatory 3 minutes +/- 30 seconds … become 5 minutes +/- 30 seconds!
Even when using hydrophilic emulsifiers the contact time is not without influence on the reliability (this was a major reason why lipophilic emulsifiers have been deleted in favor of hydrophilic ones). When using a very low concentration of hydrophilic emulsifier, this allows for a greater ‘‘flexibility’’ of the contact times.

The end

This paper has one purpose: to open some ways of ‘‘freedom’’ to advisors, specs writers, auditors.

Notes

(*) ARDROX is now a trademark of CHEMETALL GmbH

(**) Ransburg is a trademark of ITW (Illinois Tool Works).

Reference

• ISO 3452-4:1998 Non-destructive testing - Penetrant testing - Part 4: Equipment, International Organization for Standardization, Geneva, Switzerland, 1998.

• Rolls-Royce, RPS 702, Non-destructive Testing, Fluorescent Penetrant Inspection, Derby, United Kingdom, Issue N° 7, December 2006.

• ASTM, E 1417 - 05e1: Standard Practice for Liquid Penetrant Testing, American Society for Testing and Materials, West Conshohocken, Pennsylvania, 2005.