Oil-based and water-based magnetic inks

October 2010

1- Introduction

Beginning March 2010, one of our beloved readers asked whether we had already made a paper about water-based magnetic inks. He wondered about environmental concerns when using oil-based magnetic inks and was anxious to get our opinion.

We understand that, due to the impressive number of papers already published on our website, it may be somewhat difficult to retrieve exactly what our readers are looking for.

Among the papers already published on this topic:
• In the ''INFORMATION'' column: ‘‘a survey of Standard and Codes for Magnetic testing.’’
• In the ''OLDIES BUT GOODIES'' column: ‘‘magnetic testing: a product modified by the manufacturer, for sure".

So we have immediately taken the opportunity to write a paper to be published soon on our site.

Our paper titled "A survey of Standards and Codes for Magnetic testing" gave some details about when the first water-based magnetic inks were marketed in France and why they were so successful.

2- Water-based magnetic inks and main users

Car manufacturers and their subcontractors

In the 70s, we were both active agents of the beginning of fluorescent water-based magnetic inks by car manufacturers in France.

Foam problems, talked about in the paper “A survey of Standards and Codes for Magnetic Inks”, went along other problems on high-speed magnetic benches, as those for inspection of connecting rods:

• Due to their sturdiness, their long technical life and their cost many of these machines had already worked for a long time. They had been designed for oil-based inks only. So it did not come as a surprise when reservoirs, spraying systems, all the pipes, which were made of carbon steel, began to rust. Some of the first water-based formulae were quite corrosive; some rust was even seen settled on parts under inspection. More often than occasionally head/tail stocks also rusted and even seized. Problem solved when using stainless steel in place of carbon steel.

• Another problem was due to non-compatibility of water-based and oil-based carriers. Switching from oil to water many of the first users of water-based products did not care to thoroughly clean their machine, including reservoir, but also ALL THE INTERNAL COMPONENTS: pump, pipes, etc. Then the water-based ink and petroleum distillates made an unstable emulsion which wetted surfaces to be inspected poorly or not at all.

• Another problem came later: the head/tail stocks clogged making it much more difficult to properly maintain them. This problem stuck the French leader of this market at the end of the ‘80s. He did not succeed in solving this problem for a while and lost a significant market share to the complete satisfaction of its main competitor who overcame the problem almost completely.

Since then, the end of the ‘80s, the automotive industry uses mainly and by far fluorescent water-based inks for most of the mechanical parts due to a similar performance as the oil-based inks.

Nevertheless some “safety parts” may still be checked using fluorescent oil-based inks, for steering racks for instance. This is generally due to previous processes which may let some pollutants on the parts such as cutting oils and greases. Then oil-based inks easily wet the surface while dissolving the pollutants without any measurable effect on the sensitivity or reliability of the inspection. Water-based inks would then give a kind of “mayonnaise” with the oils and greases left on the parts.

Another point of interest is the material shelf life. Storing oil-based inks may go up to 5 years, or even more, while water-based magnetic inks have a far shorter shelf life: from 6 months only up to 18 months. Sometimes more. Therefore in a magnetic bench rarely used, better to use an oil-based magnetic ink.

Railways industry

In France, it is still a custom that users manufacture their own magnetic ink adding the fluorescent magnetic powder to the petroleum distillate. Therefore, the yearly consumption of specific hydrocarbons could have exceeded 80,000 litres (20,725 US gallons!) when the magnetic ink was to be prepared every day: using the product remaining from the day before was prohibited. This went until when some people had a look to the increasing cost of the oil carrier. Water-based inks were than considered and some parts (including some parts of the TGV, the very high-speed train) are now checked using them. Users may choose between oil-based or water-based inks, but some stayed with the oil-based products. We may nevertheless think that, though unavoidable, the change to water-based products only will be very cautious in an industry so entrenched with the “Safety first” motto.

Aerospace industry

Oil-based inks are almost always mandatory, water-based inks generally strictly forbidden. There is yet no evidence of any change of mind by the primes. Further less and less MT is performed as there are less and less ferromagnetic parts (except for some blades, bearings, some landing gears, etc). This trend is for ever if we think for instance of some Airbus A-350 landing gear parts which will be made of titanium alloys!

Who will pay for testing the water-based magnetic inks in Aerospace industry?

3- And for the future ?

What are then the parameters which could drive the ultimate switch to water-based inks?

Material cost

An oil-based magnetic ink is used pure, while the water-based material is used at some percents in water. The automotive industry very quickly saw the interest of using water-based formula. Changing the material every shift made it easy to withdraw some quality assurance tests on in-service products. The only check is putting so-called “ghost parts” in the batches of inspected parts; these ghost parts have peculiar discontinuities and the inspector shall find ALL the parts. If not, the inspection is deemed as non-reliable.

Hygiene and safety

Once diluted at their strength of use, water-based magnetic inks are not labelled as hazardous materials. It may be, though rare, that the concentrates shall be labelled, more for environment concerns for that matter than for hygiene and safety strictly speaking. Oil-based inks are always classified as harmful with the following risks phrases:
• R65 Harmful: may cause lung damage if swallowed.
• R66 Repeated exposure may cause skin dryness or cracking.

Environment

Implementation of the environmental standard ISO 14001 drives every company to identify products that it uses which may give birth to volatile organic compounds (VOCs), so as to reduce their emission volume.

Water-based magnetic inks do not contain any VOC.

Oil-based inks are a “scratch-your-head” problem:

This is due to two European Directives :

• The Directive 1999/13/EC (*) which states that a ‘‘volatile organic compound (VOC) shall mean any organic compound having at 293,15 K a vapour pressure of 0,01 kPa or more, or having a corresponding volatility under the particular conditions of use.’’

• The Directive 2004/42/CE (**) which states that a ‘‘volatile organic compound (VOC) means any organic compound having an initial boiling point less than or equal to 250 °C measured at a standard pressure of 101.3 kPa.’’

Note however that the Directive 2004/42/CE, which seems to be more stringent than the previous one, is applicable only ‘‘to the use of organic solvents in certain paints and varnishes and vehicle refinishing products’’.

Therefore, this Directive is not applicable to PT/MT materials.

However this Directive states that :

• ‘‘Member States should, nevertheless, be able to maintain or introduce national measures for the control of emissions from vehicle refinishing activities involving the coating of road vehicles’’.

• A ‘‘coating means any preparation, including all the organic solvents or preparations containing organic solvents necessary for its proper application, which is used to provide a film with decorative, protective or other functional effect on a surface.’’

Should we consider application of magnetic inks as a functional coating ? But this applies only to road vehicles!

Volatile organic compounds (VOC) have a detrimental effect on air quality, hence the reason for these two Directives.

We may wonder why the same chemical could be considered as VOC or not, depending on the applicable Directive: in one case it is detrimental to air quality, while when reading the other Directive, this same chemical is cleared!

A well-known petroleum distillate is very commonly used by MT materials suppliers as a carrier liquid for ready-to-use inks, or sold under their own trademark as a carrier liquid. This same distillate is more or less specifically described in some specifications.
Its initial boiling point is 235°C (455°F) and its vapour pressure is 0,002 kPa at 293.15 K.
Using this petroleum distillate, these suppliers claim that their magnetic inks and liquid carrier based on this distillate show a 0% VOC content. They are then right and comply with the Directive 1999/13/EC.

Another petroleum distillate would solve the problem. Its initial boiling point is 255°C (491°F) and its vapour pressure is 0 kPa at 293.15 K.

This “best-choice” product is more expensive, so:

• Which manufacturer would accept to pay more for a petroleum distillate which would make its oil-based magnetic inks more expensive, for no technical advantage against competitors’?

• Which user would agree to pay more just to be less aggressive vis-à-vis air quality?

The only way to have all competitors on a par would be to have one Directive only about VOC! Everyone then would have to comply with exactly the same requirements. 

Effluent treatment
MT materials effluent treatment of MT materials will be the topic of a paper we will publish on our Website.

As a preview we can say:

• Water-based magnetic inks should be sometimes treated before disposal, this treatment being quite easy as the inks are highly diluted in water, between 1 and 5%.

• Unused or out-of-date oil-based magnetic inks and their containers should be disposed of in a suitable licensed facility.
Used materials may be settled.
Hydrocarbon blends are no longer burnt as in the past but they are now recovered to be reclaimed.

Which parameters may be counterproductive when switching to water-based inks?

Obviously water-based inks formulae are more complex than oil-based inks’: their physical chemistry is the result of compromises as we explain underneath.
The liquid carrier comprises water and an aqueous solution of biodegradable surface agents which shall completely wet the surfaces while producing no or very few foam. pH of diluted inks shall be high enough to avoid corrosion of the parts under inspection but at the same time shall not be too high to prevent any adverse effect on operator’s skin. An anti-foam agent is necessary; some manufacturers use also a bactericide.

Some industries are reluctant to water-based inks use because they think:

• On oily or greasy smooth surfaces the ink may poorly wet, leaving some areas without any MT materials - hence a low reliability of the inspection. This may occur on parts insufficiently degreased before MT.

• Corrosion may be seen on points of contact between parts after inspection (parts put in a basket for example) by galvanic effect, or in areas where the magnetic ink would stay for a long time (blind holes, for example, by oxygen concentration cell effect, the hole bottom becoming an anodic pole).

We nevertheless think that the aerospace industry could find it favourable to use water-based magnetic inks, if only for in-house inspections:

• Less control on the in-service material, if it is replaced often enough (thus lowering the Quality Assurance burden: less "paper" to be filled in, less "non productive" time, end of the "pseudo-control" made by measuring the settlement volume in the pear shaped centrifuge tube; hence a better consistency of the materials performances).

• Less costly waste, which can be disposed of in a sewer without preliminary treatment, whatever some documents can say about it (it is just enough to make a ratio between the mass of MT chemicals thrown away per day, and the mass of the "natural effluents "left by the tens, or hundreds, of people in the company, who go to the toilet at least once a day! What mass of nitrogenous products, among others, thrown away this way every day!)

• Lower risk of dermatitis, or to inhale hydrocarbons. Wearing gloves is however recommended.

• No storage of large quantities of hydrocarbons which, even not classified "flammable", are nevertheless combustible - and are a good “nutrient” of any fire.

• Fewer square meters (feet) for storage: a 200 ml dose is often enough to make ten litres of magnetic ink (1 gallon for 50 gallons).

We are anxious to get the reactions of our readers and from users.

Keep in mind that in the so-called “general industry” (vessel manufacturing for example) and in services companies, mainly for cost saving, black water-based magnetic inks are a hit: they even are used instead of black oil-based inks in spray cans.

4- Available materials

As you probably know now magnetic inks are made of magnetic powders mixed with either an oil-based or a water-based carrier.

Magnetic powders colours come as black, more rarely red, dual (red under white light and fluorescent orange under UV-A), fluorescent yellow/green under UV-A. The particles sizes depend according to the particles and their purposes: from < 0.5 µm to about 10 µm for the black particles, from about 2.5 µm to 10/15 µm for the particles which fluoresce under UV-A. Some other yellow/green fluorescent particles are in the 10 µm to about 30 µm range, larger so less mobile. They are also brighter; that’s why they are called ‘‘superbright’’.

Users may choose in a wide range of materials. They may buy ready-to-use oil-based magnetic inks. They also may buy the magnetic powder from an MT materials supplier and the carrier liquid either from the same supplier or from another source. They then mix the magnetic particles with the carrier at the recommended concentration. Some users and suppliers may add other additives to the petroleum distillate, such as mineral oil, when specified in the applicable documents.

Water-based magnetic inks are supplied either as liquid, or more rarely as powder concentrates. They are never supplied ready-to-use except for spray cans.

Users may buy the magnetic powder and the conditioning agent as liquid or powder. This conditioning agent comprises several materials such as surface active agents, corrosion inhibitors, rheological agent, foam abating agent, bactericide, etc.

Oil-based magnetic inks are also supplied in spray cans.

A French manufacturer/supplier recently marketed a completely new range of water-based magnetic inks packed in spray cans with carbon dioxide propellent, for better hygiene and safety conditions.

Reference

(*) Council Directive 1999/13/EC of 11 March 1999 on the limitation of emissions of volatile organic compounds due to the use of organic solvents in certain activities and installations.

(**) Directive 2004/42/CE of the European Parliament and of the Council of 21 April 2004 on the limitation of emissions of volatile organic compounds due to the use of organic solvents in certain paints and varnishes and vehicle refinishing products and amending Directive 1999/13/EC.