DPCNews 012 - The peelable developer

May 2009
Completed in January 2012

I-Introduction

This developer, in fact, is very similar to a non-aqueous wet developer and may be used with any penetrant.

The difference is that, after the solvent evaporation, a coating is left on the part; this coating acts as a fixing agent, which will be peeled off with the indications of discontinuities.

This developer comprises capillary agents, organic resin monomers, which polymerise when in contact with air at room temperature, and a silicon-free de-bonding agent (so as to ease its peeling off from the surface), all in an organic solvent.
However, we have found a peelable developer, which contained silicone!

This developer was designed in the early '70s when photographing with silver films made it necessary to use top-level equipment to get good pictures when contrast between the background and the indications is low, especially for tiny discontinuities detected with a colour contrast penetrant.

A professional photographer could be of some help, provided that he, nevertheless, got some clues from the user, especially when fluorescent products were used under UV-A radiation!

Furthermore, keep in mind that the film processing itself could lead to "information discrepancies" between what was seen by the inspector's eyes and what was seen on the picture.

Adding this form of developer in its range was then a "plus" for any PT materials manufacturer.

II-Why to use it?

It can be used with any kind of penetrant. The peelable developer gives an opaque capillary film in contrast with which an image of the indication will be visible either in daylight or artificial light, or under (UV-A) ultraviolet radiation, depending on the type of penetrant.

The main reasons to use it come as follows:

• PT and MT processes performance check:
A reference test block with known defects allows for the comparison of detectable defects when checking an in-use product with defects detected using brand-new materials. Furthermore, replicas may be kept by the Quality Assurance Department as evidence.

• Comparison and recording of performance of different PT or MT products and/or processes:
Thus, it is easy to check and record:

> PT: the impact due to modifying parameters: temperatures, pressures, times, excess of penetrant removal processes, hydrophilic emulsifier concentrations, etc.
As an example, it is easy to check the fluorescent background, depending on the in-service penetrant, or to compare this background to the reference material. This check or comparison is performed as per the washability test written in paragraph 7.6 in the ISO 3452-2 standard using the Type 2 reference Test Block described in the ISO 3452-3 standard.

> MT: the impact of different concentrations of magnetic particles in the magnetic inks.

> The assessment of a known defect enlarging in a part, either on an in-service part or during fatigue tests.

> The transfer of in-situ indications (got on large constructions) to a place more suitable for inspection.

> Inspectors' training to teach them in interpreting indications found while PT or MT.

III-Technique of application

Note that this developer is supplied in spray cans only as the resin cures when in contact with air, thus excluding packaging in cans.
Using this type of developer requires some know-how to avoid ''bubbles''. Bubbles appear when the developer is applied too quickly or if sprayed too close to the part surface. Doing so, the resin begins to polymerize while the organic carrier has not yet evaporated enough. It is then trapped in the polymerized layer and generates bubbles.

The right way to apply this developer is as follows:

• After wiping off the defective area with a clean rag, apply again the penetrant and remove the excess of penetrant the same way as previously. Instead of the currently used developer, apply a thin layer of the peelable developer.

• First, shake vigorously the spray can for one to two minutes to homogenize the product.

• Spray the area to be coated, while holding the spray can at 20-30 cm (8-12 inches). The application technique is important as the film should not be too wet or too dry and should be thin and even.

• After one or two layers, let the film dry for a while in order to fix the indication before further application.

• Apply three more layers, letting some scores of seconds between each spray. This is usually necessary to build up a coating thick enough to make it easy to peel off.

• Let the film dry for two to five minutes, at room temperature. It may then be peeled off by applying a pressure-sensitive tape over it and taking off the tape and the film. Cutting the edges of the film with a knife or with nail will ease the peeling off. Take care not to tear or stretch the replica during this operation.

• If using an appropriate adhesive tape, the replica can be glued to a card and stored as a permanent record for quality assurance purposes.

Note that the peelable developer replica shall be viewed back to front. Indeed, due to the thickness of the opaque coating, ''through'' looking leads to an apparent huge sensitivity loss, while, on the contrary, looking at the other side gives a higher sensitivity, when compared to any other form of developer.

IV-Restrictions for use

Some parameters are seen as a limit for using this developer:

• Surface roughness: when sprayed on a rough surface, it cannot be peeled off or comes as strips.

• Part's shape: only on quite simple shapes and developable surfaces.

• Part's material: the solvent of the first peelable developers was 1,1,1-trichloroethane (T-1,1,1). It could not then be used on austenitic steels, nickel-based alloys or titanium alloys. Later, T-1,1,1 has been replaced by acetone, which attacks numerous paints, elastomers and so-called plastics; but it may be used on austenitic steels, nickel-based alloys and titanium alloys!

V-The replica storage

Peelable developer replicas may be used for a while, provided that they are kept away from natural or artificial light and from UV radiation, to prevent any fading of the colour or of the fluorescence of the indications.
Indeed, submitted to such exposure, the original yellow-green fluorescence turns to a brilliant blue, then to a bright white, before disappearing. This is a phenomenon of the colour or fluorescence fading. This is due to the photolysis of the dyes. As the concentration of the yellow-green dyes in fluorescent penetrants is generally lower than that of the optical brightener, the blue fluorescence of the latter remains after the yellow-green dye has faded. Then, the deterioration goes on, and the blue fluorescence turns to white before disappearing completely.
Similarly, non-fluorescent dyes may be destroyed if replicas are exposed for too long to natural or artificial daylight.

If properly protected from day light and UV light, these replicas may be used and stored for many years without any change.

Take care also when handling them.

VI-The peelable developer "passing away"

This developer is still mentioned in many training courses and in the ISO 12706 standard as well as in some specifications.

The peelable developer was mentioned in the 2000 issue of the ISO 3452-2 standard, while it is no longer specifically mentioned in the 2006 issue. The same way for the American SAE-AMS 2644 specification.

In fact, this developer form has not been on the market for circa eight years now (this paper was initially written in April 2009), and it is almost impossible to use it even for training as the spray cans are likely to be out-of-service!

Main reasons for this situation:
• Spray cans was the only way to get the right coating thickness/toughness/whiteness.
• Digital still cameras are generally now the means of choice to record indications in many lighting conditions (though sometimes specific precautions shall be considered).

This developer was used only on a small scale. It was then difficult for the manufacturers to have filling-in companies agreeing to fill in only some scores, or 300 max spray cans. This was always a manual process carried out in the filling company's laboratory. Whatever its retail price, this developer was helpful in specific cases.

Due to the dramatic development of the digital photographing, sales of these spray cans decreased, dramatically also! The manufacturers had unsalable stocks (the expiry date was a barrier!). They had to pay for destruction. Quite often they sold fewer cans than they destroyed! Furthermore, even before the expiry date, some of the spray cans were unusable.

Valves are not completely leak-proof: tiny quantities of developer found their way out, and the resin polymerized on the valve and the tip: the spray can was then clogged. That was why the manufacturers recommended that, after every use, the spray can be put upside-down and briefly activated to clear the nozzle and the valve.

Using still cameras has numerous advantages over the peelable developer:
• Surface roughness is no longer a concern.
• A simple and fast means to get a photograph.
• Picture retouching is possible and easy, even the colour reversal (which is useful for pictures of fluorescent penetrant testing on plastic materials).
• Easy recording and filing.
• Sending pictures all over the world to involved people is simple.
• Low cost.

VII-Current situation

Most of the in-use specifications require real reference photographs (scale 1/1 colour pictures) of reference test blocks such as the PSM-5® or the Type 1 and Type 2 from the ISO 3452-3 standard. The replicas made of peelable developers are no longer accepted.

References

• ISO 3452-2:2006 Non-destructive testing - Penetrant testing - Part 2: Testing of penetrant materials, International Organization for Standardization, Geneva, Switzerland, 2006.

• ISO 3452-3:1998 Non-destructive testing - Penetrant testing - Part 3: Reference test blocks, International Organization for Standardization, Geneva, Switzerland, 1998.

• ISO 12706:2009 Non-destructive testing - Penetrant testing - Vocabulary, International Organization for Standardization, Geneva, Switzerland, 2009.

• SAE-AMS 2644E: Inspection Material, Penetrant, Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvania 15096, USA, 2006.

We, Pierre CHEMIN and Patrick DUBOSC, welcome any comment, any idea. If you have some examples you would like to see discussed here, please give us all the useful indications. If you require confidentially, we would modify locations, names and some parameters to prevent any traceability.
Nevertheless, we are convinced that our site may be a kind of surge-valve: the topic is NOT to target this company, or that auditor; but it is always to make users think, to make them ask themselves, or others, the right questions.

We may also give advice, once again on a confidential basis if needed: please, feel free to ask questions, to document our data basis: about Material Safety Data Sheets (MSDS), about environment, a chemical name you don't understand, a Penetrant process you have heard about, etc.
We have plenty of examples, some being out of all the specifications/standards, which led to the discontinuities detection, when the "current, normal, processes" prevented discontinuity finding.