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DPCNews 028 - Use-by dates of PT/MT materials

Written by Administrator
Wednesday, 01 September 2010 14:49

September 2010

1- Introduction

In the June 21-25, 2010 PRI-NADCAP meeting, held in Singapore, the topic of the use-by dates of non-aqueous wet developer (NAWD) spray cans used in PT was raised.

Some in the attendance said that the used-by dates mentioned on these spray cans are for commercial purpose only. In other words, these spray cans could be used beyond their used-by date; should the spraying fulfil the requirement of an even coating, the developer is considered as right.
We think it very useful to give you some points to think about.

2- Some definitions

A product shelf life is the period of time during which the material may be stored and used as far as it keeps all of its features.

The use-by (“best before”) date of a material is the end of the shelf life; shelf life begins on the material’s manufacturing date.
Hence, the use-by date of a 5 year- shelf life product manufactured on May 4, 2010 is May 4, 2015.
In fact, end of May 2015 is enough: in no way, some few days more would have any detrimental effect if a 5-year time is considered, as far as good storage conditions are met.

3- Back to history

In France, giving a use-by date to PT/MT materials dates back to 1974 when one of us brought forth the idea of shelf life for the water-based fluorescent magnetic ink concentrates.
Beginning with a 6-month shelf life, due to the first tests results, it was then gradually extended to 18 months for products kept in sealed containers.
Before 1974, only batch numbers of PT/MT materials were taken care of.

As a general matter, batch numbers give no clue as for the manufacturing date, or for the use-by date of any material, except for some examples we give underneath.

A right warehouse management shall follow the FIFO - first in, first out - rule: the materials which go out of the warehouse first are the ones which entered first. However, this relies on two important points:

• First, the warehouse has to be put in order. The just arrived materials shall be put behind the previous delivery. Not always done this way! So, when tidying up, carrying out a physical inventory or moving, a not so rare occurrence was to lay one’s hands on old, outdated materials (“junk materials”, as we may call them).

• Suppliers should deliver materials newer than those previously delivered. We still have in mind a manufacturer which sent on a regular basis materials whose use-by dates were older than that of the previous shipments ... even some materials were delivered when their use-by dates were ...far behind!!  And yet this manufacturer was ISO 9001:2000 certified!

Were these old materials still efficient, meeting all the Quality Assurance requirements? Without a use-by date, it is almost impossible to have some confidence in such materials.
Did products whose use-by date is behind lose some performance and sensitivity?

Year after year, the traceability of a use-by date has become mandatory for all the NDT consumables.

4- The shelf life of PT/MT materials

How does a manufacturer specify the shelf life of his PT/MT materials?

The main parameter by far is the product long-term stability.

In the ‘60s, one of us had a professor in a university who taught him the statistical thermodynamics. Artful in drawing up paradoxical assessments, this teacher once said: “there are metastabilities which are stable".
He was perfectly right and this can be said, in some way, of PT and MT materials, made of a combination of up to 14 different raw materials!

Each of these raw materials, very different from each other, is stable.

When blended, though there is no chemical reaction per se, they may react for instance through molecular interactions, with consequences which may be detected later. It is a burden to anticipate how long such a metastable product will be stable enough to be used without any trouble.

Another point to be taken into account: storage conditions. They are given in the Material Safety Data Sheets (MSDS) and sometimes on the packaging label. Storing non frost-resistant products in freezing conditions may be detrimental to their performance.

PT/MT materials shelf life is not a theoretical data, it shall be accurately determined.

That is why PT/MT materials manufacturers carry out extended tests of long-term storage stability, which allow them to determine the shelf life of their materials, with a reasonable safety margin.

Some materials, especially penetrants, may show in fact a long shelf life if stored in suitable storage conditions. We know of 10-year (or more) old penetrants whose performances are unchanged though their declared shelf life is 5 years!  As a surprising example: one of us had the ability in 2007 to carry out tests of a renowned colour contrast penetrant: a sample of a batch manufactured few weeks before and packed in spray can, compared with what remained in a spray can manufactured ... exactly 30 years earlier!! The double-blind test did not allow for detecting the tiniest difference in performance, appearance and washability.

We draw your attention to the fact that the shelf life shall not be mixed up with the contractual warranty period granted by the manufacturer/supplier.

Therefore, the shelf life is not a commercial warranty but a kind of technical guarantee. Should a technical problem occurs in spite of adequate storage conditions (tightly closed containers, in the dark if transparent; kept in reasonable temperatures range, as a minimum), before the end of the contractual warranty period, the supplier shall provide substitute, investigate to know the cause for the problem and keep the user advised of the investigation results. Beyond the contractual warranty period, it becomes a standard commercial relationship!

Tanks of penetrants are a good example. When the tank is large regular add-ons (which may be automatic) make up for the loss due to draining and unavoidable evaporation of some solvents. This may be done for years. Quality Assurance rules make it mandatory to perform some tests on the In-Service materials; but the concept of batch number and use-by date is not relevant, due to the blending of different batch numbers. The important information is that the material’s performances are kept in line with the requirements, whatever the proportion of different batch numbers and even if a very old product may be still in!

As for small tanks, it may be much more efficient to use brand-new materials every year, rather than to ask for analyses, at a cost.
When tanks are 10,000 litres (ca 2,600 US gal), 20,000 litres, 64,000 litres large, or even 240,000 litres (ca 62,000 US gal) (yes, such a big tank is in service ... in the USA), no one wants to replenish the tank every year in full - to the suppliers’ despair, as you bet!!

On the other hand, we have known a European plant in which the large fluorescent penetrant tanks were emptied and refilled with brand-new penetrants every six months, no question asked - no check of the in-use penetrants. These tanks were far larger than 100 litres (ca 26 US gal).
A kind of ‘‘tacit agreement’’ between users and the supplier? Anyway, the question arises!

The manufacturer is the “who of choice” who may decide for the shelf life of his materials, as he is supposed to know his products the best.

Is it in his best interest to give long or short shelf life?

Should the shelf life be too short, the user could try and find out on the market materials with a longer shelf life. On the other hand, if the shelf life is too long, the manufacturer may get complaints many years after delivery if there is some drop of performance and sensitivity.

An interesting point: a major prime requires a 6-month residual shelf life for the materials delivered any time.

As a general matter of fact, though it is not a fixed rule, manufacturers allow a 5-year shelf life to their bulk PT/MT materials, be they liquid or solid, packed in their sealed original containers, stored under shelter in a well ventilated cool place, out of direct sunlight and not subjected to extreme temperatures. Some other manufacturers state a 12 to 18 month only shelf life.

Spray cans are a different point. Leakages may occur at the valve, or on a weld or at the cup and concave bottom seaming, this latter point on tin plate cans.

Depending on the area, either some propellent, or part of the “active product” (the “useful” product), or both, if a liquefied gas is the propellent, escape from the pressurised can.

Should the leak be at the valve, the valve or the dip tube may be clogged if the active product contains solids: non-aqueous wet developer (NAWD), magnetic ink, white contrast aid paint. Solids may come together at the valve level and clog the spray can, which can no longer be used.

A manufacturer of spray cans of white contrast aid paint, who had problems of clogging in the dip tube, put the identification label upside down on cardboard boxes so that inside cardboard boxes, the spray cans were stored upside down, this avoiding any clogging risk. Later on, after improving his formula, the manufacturer faced very few clogging, and came back to a “standard” positioning of labels.

However, every one of us, one day or the other, had in hands a spray can either clogged or partially or even completely empty.

Some users notice that after a 2-year storage, spraying performances may be impaired. This makes it understandable why, depending on the formula, the technical quality of the products, of the spray cans, of the propellent, the spray cans shelf life may vary from 1 to 5 years.

5- When does the PT/MT shelf life begins?

Obviously, on the day when the product is manufactured. For spray cans, the day of filling is the starting date ... even if the material has been manufactured some weeks, sometimes some months before.

6- The use-by date: what is it?

The use-by date is determined by the manufacturer. From a purely technical point of view, a use-by date is not similar to a calibration date: a meter shall be calibrated not after the due date, save for a day’s difference: a due date of April 28, 2011, for a meter, means that the recalibration shall be done not later than the 28 of April. For chemicals, April 2015 means the end of April 2015, whatever the day of April they have been manufactured several years before. PT/MT materials are industrial products, stable for far longer periods than claimed (suppliers take some margin), not dairy products, nor meat or fruit.

Since many years, manufacturers have come to print on containers/cans a clear and understandable mark for the use-by date, instead of a shelf life after manufacturing.

Use-by dates may be quite close to the manufacturing date: this may be seen especially with many materials containing solids, such as white contrast aid paints for MT or some PT developers. Fluorescent magnetic ink concentrates and ready-to use fluorescent oil-based magnetic inks have quite short shelf lives. Fluorescent dyes are unglued from the magnetic particles by the liquid carrier, which becomes more and more fluorescent, while the active product is less and less. On the other hand, the MT black detection media may have longer shelf lives without any risk ... at least from this point of view!

7- Does the batch number give any clue for the use-by date?

Some manufacturers use a batch number numbering which allows for knowing the materials manufacturing date. Users must know the system, otherwise ...

First example
A PT/MT manufacturer uses a very simple numbering for bulk materials:


• YY = the two last figures of the year of manufacturing.
• MM = month of manufacturing.
• DD = day of manufacturing.
• XYZ = order number of the materials manufactured on the DD day.

Therefore, the batch number 100324-028 means the 28th product manufactured on March 24, 2010.
If its shelf life is 5 years, its use-by date is March 24, 2015.

Second example
A manufacturer uses the underneath code for bulk and spray cans:
•  The first two digits of the batch number come for the year of manufacturing.
•  The letter that follows comes for the month: A for January, B for February, C for March, D for April, E for May, F for June, G for July, H for August, J for September, K for October, L for November and M = December. The letter ‘I’’ is not used to avoid any confusion with the digit ‘‘1’’.

So, the batch number 09C8483 was manufactured on 2009, March.

Third example
A manufacturer uses another code for bulk and spray cans:
4 digits, a letter and two digits.
The first 4 digits are the batch number, the letter is for the month (January = A) and the last 2-digits number come for the year.

Hence, the batch number 6347B10 was manufactured in February 2010.

Nevertheless, many manufacturers use complex systems and do not give any clue to their customers as for how to know the use-by date by a quick look at the can. We still find suppliers who do not clearly display the use-by date.

8- What to do of out of date materials?

The question is for both users and manufacturers.

8.1- Users

Before asking an authorised subcontractor to dispose of an out-of-date material, one must be aware that this does not come for free and that it may be a waste of energy and time. Maybe “think first” is worthwhile!

Due to the Quality Assurance and materials traceability requirements, manufacturers keep a typical sample of every batch for long times. If any discrepancy appears on the sample, as it happens sometimes, the supplier sends a “warning” message to all the users of the relevant batch.

If a user thinks of pushing forward the use-by date of a product, as the manufacturer has a typical sample, the user may ask him, either directly or through a representative, for a lengthening of the use-by date. The manufacturer may, or may not, acquiesce in such a request.  We know of a manufacturer who considers such a request as far as the residual shelf life of the product is at least one month.

No standard/procedure prevents a user for asking for a lengthening of the use-by date.

This process is applicable only to never-used materials, kept in sealed containers and in suitable storage conditions.

Before going this way, better to be sure that the product seems to be in good condition. For instance, check a spray can for a continuous-without-surge spray, no clogging of the tube or of the valve. A liquid material shall be checked for an unusual smell, separation, turbidity, etc.

If any visible/smelling difference is detected, it is not worthwhile to ask the supplier for a test.

If everything seems OK, the user gives the manufacturer the relevant batch number. If the manufacturer has no sample left, he asks the user to send him a sample. The manufacturer may also ask for a sample, even if he has still enough in his laboratory ... just to “make up his mind” about the way the material is stored: a rusty spray can, a container whose label is mould...make the manufacturer have second thoughts about lengthening the use-by date of a product so badly stored.

Once tests are carried out, the manufacturer may opt for no lengthening, or, if test results are satisfactory (no loss of performance for any parameter), for a shelf-life lengthening; this is generally for a unique period of six months or one year, depending on the “normal” shelf-life of the product. On a case-to-case occurrence, a second lengthening may be accepted.

As for any service, there is a cost incurred. The manufacturer may charge these costs or not, in part or in full. When a batch is manufactured, these analyses costs are spread on the whole batch. For a specific analysis, these costs are borne by a smaller volume. If there are only 10 litres of penetrant, if the cost is close to the price of a brand-new 10-litre can...obviously, forget about retesting the product. Better to ask the manufacturer first!
This service shall not be mistakenly considered as the regular check of in-service bulk materials as required by relevant specifications.

8.2- By the manufacturer

To ease inventory burden of their customers, manufacturers/suppliers do their best to ship only one batch number of a given material. This is even a clause in some orders. Think of a user receiving 4 different batch numbers for a 200 litre order! Unbelievably, this happens!!
Nevertheless, a given product is manufactured in a finite quantity: for some materials, a batch may comprise 20,000 litres; for others, a batch may be 1,000 litres maximum. There are often technical reasons for these limits; it also could be that 1,000 litres are the average quantity sold in one year; it is then useless to manufacture much larger batches. We know of very specific PT materials whose a batch is 100 litres...and even some thixotropic or high temperature PT materials for which a batch is ... 10 litres!!

Therefore, small quantities of several batches of the same product may be stored in the manufacturer’s warehouse. What can be done with them? Well, the manufacturer blends them in a manufacturing tank, adding other raw materials if needed, to make a new batch whose “birth date” is the day of this manufacturing.

There is nothing to prevent a manufacturer to “recycle” an out-of-date batch with brand new raw materials, and have a completely new product with a new batch number ... and a new “life expectancy”.

As far as the new batch meets all the requirements, this recycling procedure is not wrong per se. Once again, these products are not for food.

Of course, such a procedure cannot be done with spray cans.

9- Are use-by dates that important?

Except for a very strict management of the warehouse, if no use-by date is written, how can one know that a batch is “older” than another one? As seen before, every manufacturer has his own system to number batches.
The first idea which comes to mind is quite simple: ask the supplier to clearly print the use-by date on EVERY CONTAINER, INCLUDING EVERY SPRAY CAN.
If only all the users asked for that ...

Nowadays, thanks to a Quality Assurance system enforced in every company, plus the use-by date printed on every can by renowned suppliers, no out-of-date products should be put in service.

10- Where do we find the use-by dates?

They are seen on every pack and also on “over-packaging”, i.e. on the cardboard containing 4 x 5 litre cans of penetrant or developer, for instance.

They are also a mandatory information on certificates of conformity, laboratory test reports, analysis reports, etc.

Some time ago - that means more than a decade - use-by dates were printed on small stickers, glued on labels, and which unglued quite easily and quickly. This was an easy way for some “unscrupulous” suppliers to “rejuvenate” products whose use-by date was almost gone, or really behind!! Ink jet printing allows for similar “tricks”, as ink is easily removed with ... acetone.

Nowadays, in most cases, use-by dates are printed on the container labels, or silk-screen or offset printed on containers.

Thus, use-by dates marking is finalised and not prone to forgery. Anyway, other documents should be modified accordingly if a “dishonest” user tried to modify dates on packaging.

11- Other identifications systems

In the mid ‘80s, one of us was asked by the French automotive industry to use a bar-code identification system for the 10-litre containers of fluorescent water-based magnetic ink concentrate, delivered on pallets of 800-litre batches per month to several plants.
Some PT/MT materials manufacturers still use this system.

We may guess that the near future will see a large use of RFID tags. RFID comes for Radio Frequency IDentification. These RFID tags comprise an antenna connected to a chip, which may “answer” when “questioned” by an RFID transceiver. This system allows for retrieving an item, even if put in the wrong place in a warehouse. It will also be used in supermarkets to quickly issue an invoice for all the items that the customers have in their trolley.

AIRBUS announced its wish to implement the RFID technology in the A 350 XWD program. (*)

A similar requirement is made by BOEING for its B 787 Dreamliner.

The relevant specification and standard are:
• ATA Spec 2000 Chap 9-5.
• SAE- AS5678.

Two European standard drafts, respectively prEN4617 (UHF passive) and prEN4618 (HF passive), are circulating.

12- Conclusion

In our opinion, used-by dates are of the utmost importance for materials traceability and for the management of the Quality Assurance program.

This may be applicable only to materials, which are used in large tanks, in which add-ons are made using different batches along the time. When such tanks are in service, periodic tests are mandatory every month, every two months, three months or four months, depending on primes’ procedures and on products (penetrants, hydrophilic emulsifiers, water-based developers, etc.). MT detection media used in benches are also checked on a regular basis- or the material is replaced every shift, for instance.

Let us remind you that any PT/MT material, though contractually guaranteed by the manufacturer/supplier, shall be checked for performance and other parameters after the relevant procedures every day, every shift, or any other specified periodicity.


(*) Philippe CANTEAU, Opening meeting ‘‘Innovation & Normalisation’’ (Editor’s note: ‘‘Innovation & Standardization’’), AFNOR, Saint Denis, France, May19, 2010.

• ATA Spec 2000 Chapter 9, Automated Identification and Data capture (AIDC), ATA 1301 Pennsylvania Avenue, NW Suite 1100, Washington, DC 20004, 2009.

• SAE - AS5678, Passive RFID Tags Intended for Aircraft Use, Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvania 15096, 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.

Last Updated ( Saturday, 21 May 2011 15:17 )