Standard Review

US MIL-I-17563C Test Ring Standard Review

MIL Standard Review

United States Military Specification Impregnation
MIL-I-17563C

The US Military Standard initially appeared in 1985. This was the first standard of its kind employing a sintered ring for the purposes of impregnation sealant examination and approval. From the impregnation industry point of view it represented a major step forward by giving some credence to this little-known process with a standard that was considered worthy of international cover. A number of companies supplying sealants to the impregnation industry have over the years signed up to this standard which is now increasingly used as a symbol of quality by suppliers and users alike.

Predominantly, the standard is written to include approval of both the impregnant and its application. The standard is well written and informative as well as it can be, having to cover an application that cannot be seen and measured. It follows therefore that there remains a void between approving the actual impregnant and the actual application of the process in the field. On the one hand the testing of the impregnant by the sealing of a test ring is simple to define, the ring either seals or it leaks. This is easily ascertained after impregnation by putting the ring on pressure test and observing the pressure tightness of the ring. However what it does not tell us is, why the ring has sealed or not sealed. We do not have any knowledge of the condition of the porosity, it could be that it would help or impede the sealing process. It does not tell us if the ring was processed correctly. We just cannot see what has happened.

In comparison, we can take a piece of metal and turn it down to a particular size and measure to ensure that this has been achieved. We can analyze the metal and ensure that it meets the intended specification. But with impregnation we can only assume. So what does the US MIL Standard do for us? Well it tells us that the sealant is fit for purpose and will effectively seal components that fall within the scope of the test ring and that objective tests can be carried out with these impregnated rings to ascertain immersion chemical and temperature resistance. So far so good! What the present approval compliance is not able to tell us is:

  1. that test rings were of a set standard in density and uniformity.
  2. whether the sealant submitted for the evaluation is identical to that which is made available to the market place.
  3. whether the sealant has any identity verification or traceability.
  4. whether the test rings were limited to two impregnation cycles.
  5. How well the sealant performed in quality and performance.

Test Rings

The standard states that it is reliant on the applicant sourcing and providing their own rings for the test approval. Invariably, these rings are made in small quantities, by different manufacturers and at different times which must lead to questionable standard of uniformity. It has been experienced that some rings were found to be so inconsistent and poorly made that they were potentially dangerous when it came to pressure testing. No doubt at the other end of the spectrum there are rings that were found to seal with ease i.e minimal porosity. Such rings, therefore provide an unstable and unsuitable platform for reliable evaluation.

Sealant

The applicant is conveniently allowed to seal the test ring himself. However, there is no suggested method to allow verification to show that this has in fact occurred. An applicant could easily seal the test rings with another supplier's sealant, known to comply with the specification without fear of detection. Furthermore, there is no means of confirming that the sealant being used in the field is one and the same as that approved to the standard.

Impregnation Limit

As the applicant is relied upon, at his own discretion to impregnate to a limit of two impregnations, again there is no way of verifying that this has been adhered to. Bearing in mind, having to deal with questionable standard of test rings that in themselves may not be taken for granted, the test assessment begins to look somewhat insecure. It must be assumed therefore that some licence in the sealing of these test rings would be conceivable although not in the spirit of the requirements of the specification.

With the absence of traceability and particularly having to cover a process that cannot be seen or measured, its application is clearly open to abuse, undermining the very foundation of credibility of the MIL-I-17563C Standard. The gulf between that which is approved and that which is being applied in the field, even if the original approval was legitimate, does not relate with the specific needs of the specification. For instance concern over reactive substances (MIL-I 3.3 Volatility) can be brought into question because we are now talking about a sealant that is in use and contaminated. This contamination can come from incoming components for processing of which origin is invariably not known. Therefore any contamination that it may harbour may be left in the sealant to contaminate subsequent components.

There can also be a vast difference between the scope of controls over the application of the process employed by the user as opposed to that exercised at the time of the preparation of the test rings for MIL approval. Many factors can influence the performance of the sealant. As has already been highlighted on this web site, the level of catalyst in the sealant can have a major influence on how the sealant behaves in the porosity. Because high levels of catalyst can breed fear of pot life instability, there is a tendency for process plants to operate at the lower end of reactivity which can lead to the sealant not curing or only partially curing within the porosity. If we add possible contamination such as water, oil, wash solutions etc., the sealant becomes even further removed from the laboratory prepared sample supplied for MIL approval.

It should be further understood that any liquid, vacuum impregnated into a porous component can exhibit a level of sealability to that component. Even water can provide a sufficient seal to get an otherwise leaky component through production pressure test. Providing of course that the duration of test is shorter that the time required to purge the water from the porosity.

As we can see it is becoming increasingly difficult to relate to that which was approved and that which is actually in use in the field, making the reality of the test procedure adrift from that in the real world. So what can be done about it?

Quality Performance Of The Sealant

Unbeknown to the market place, there can be considerable difference in quality and performance of impregnants. The present specification groups all approved sealants into one, fusing both high quality and poor quality together. Certain tests can be introduced so as to filter out the various layers of quality. This information is equally important to the end user of impregnation as it is to the intended applicator. The question of explosives is a point in question. Should a low quality sealant, able to pass the present approval tests be chosen for ordinance processing. First we need to understand the difference between a high quality sealant and a poor one.

A high quality sealant must:

  1. Exhibit good long term Pot life Stability
  2. Be stable with fast gel time - right amount of catalyst.
  3. Be able to cure correctly in the presence of reasonable levels of contamination.
  4. Have low viscosity and low surface tension for good penetration.
  5. Be easily removed from the component surfaces prior to curing avoiding the need for high shear washing action.
  6. Maintain test ring pressure tightness after elevated dry air temperature exposure.
  7. Have good all round chemical resistance.

Recommended Addition/Change To US MIL-I-Standard

  1. Test Ring - (Aluminium)

    Establish a central supplier of the test ring. Consider a test ring that is manufactured from commercially available powder. A ring that has a high degree of uniformity and repeatability.

  2. Gas Chromatography Verification

    Gas chromatography
    Typical GCE sealant trace

    For the applicant to provide a representative sample of the sealant to be tested by GC/FD. This provides a footprint of the product that can be held on record on an approved website. The applicant's confidentiality is not breached as such information could easily be available in the public domain. See trace of original Ultraseal PC 504.

  3. Test Ring Preparation

    For the central laboratory to acquire its own test rings from the approved central source and carry out the impregnation with the applicant's sealant according to applicants publicly declared information. Test rings to meet the existing requirements of MIL standard – max 2 impregnations. If the rings fail the sealant is rejected.

  4. Sealant Peroxide Test

    Before commencement of the MIL Standard application for approval, sealant containing AZDN as the catalyst should be checked for the addition of peroxide catalyst. This is known to enhance poor sealant performance. However such mixing of catalysts is also known to cause pot life instability if used in a production environment. A simple test is available for detecting the presence of peroxide.

  5. Gas Chromatography Chemical Testing

    MIL 4.5.5.6 requires that sealed and pressure tight test rings are subject to Table 111 and shall remain pressure tight when tested to 50 psi. Such a test does not examine the possibility that there could be some migration of the sealant into the test media. This could take place even if the test ring remains sealed for the duration of the test. To be certain that such a condition does not exist, GC/FD examination of the test media for elements of the sealant should be carried out. To overlook such a potential situation could lead to problems in the field.

  6. Sealant Contamination

    Sealants can be in use for many years and expect to become contaminated. Well designed sealants are able to cope with reasonable levels of contamination and still perform satisfactorily. It is therefore recommended that the MIL Standard test procedure incorporates a contamination test containing a mixture of a regular aqueous wash solution at 5% by volume and mineral oil at 2%. Antioxidant in these products can have a significant effect on curability of sealants that have low catalyst content.

  7. Raw Material Contamination

    Sealants based on methacrylate monomers can sometimes contain traces of polymer as a result of an over reaction manufacturing process. All incoming raw materials for sealant formulation need to first be checked for polymer contamination. The presence of polymer can effect proper penetration of the porosity. This is a test that can be carried out in conclusion of – MIL Ref: 4.3.1.1 Pot Life Test.

  8. Quality Grading Of Sealants

    It should be possible to award points to the testing of a sealant. These points would be collated to provide a star rating **** for high quality and * for low quality. These gradings could be on view on the information web page for impregnants. The design authority would have access to this information and thus be able to make a distilled choice based on particular needs.

    As an example:

    1. Record whether a sealant requires one or two impregnations to seal the test ring.
    2. Record final sealant weight pick-up and sealability of the test ring.
    3. Record weight loss of sealant after 24 hrs at 150ºC
    4. Register rates of ring leakage, using the ring chart as noted throughout the test evaluation.
    5. Grade test performance of contaminated sealants.

    Again the availability of such information to the market place will encourage effective quality assurance at all levels. It will allow the original MIL test approval to relate to the actual application of the process.

Test Laboratory Responsibility (Recommended)

Conclusion

If an impregnation standard is to have any meaning it must relate to the actual application of the process in industry. The key to this must be the test ring. It is the one thing that draws the whole process together.

It is appreciated that there may be good reason why the existing US MIL Standard Ring is required to be manufactured from pure aluminium. However, from a commercial point of view it is more important for the ring to have regular conformity in its density and available porosity.

The use of a GC/FD trace allows traceability between the original MIL Approval and the product supplied to the process plant. It also allows the user of the sealant to ensure that the sealant is not compromised. If there is any doubt, the user should have access to the original sealant trace against which he can check the sealant's identity. Such a check could be carried out at the appointed test laboratory. A regular trace of the sealant might also provide an early indication of contamination.

Such available information can also be useful in the event of a dispute between supplier and user of sealant. It also provides a useful base on which to apply an international standard, one that can be adopted by most impregnation users for internal quality audit. Often sealants are purchased primarily on price, because it is the only visible means by which it can be commercially judged. At this point in time the US MIL-I-17563C is the only means of judging sealant quality, which if it remains ineffective in its present form could provide a poor image of the impregnation industry. We have a collective responsibility to approach the US Military and make our recommendations known and request that this standard be updated to a point that it becomes an accredited document with audit traceability that both supplier and user can refer to with confidence.

It is important to bear in mind that once the sealant has been decanted from its drum into the process chamber it is in a totally different world from that of a laboratory environment. The mere presence of metal has a considerable influence as to how the sealant behaves. Unlike paint that is mixed and used once, sealant may be in use indefinitely.