Cleaning Sintered Rings

Q. I would like information on methods, materials and procedures to clean sintered steel rings (porous rings) used in the textile industry, more specifically, in drawing machines. The oil used in these rings is Syntheso XOL 12 - Klüber Lubrications. We currently have three procedure steps:

Boiled perchloroethylene bath (120ºC) - 30 min,
Ultrasonic bath in perchloroethylene (40HKz) - 30 min,
Oakite plus ultrasonic bath - 15 min.

But we are unsuccessful. Please provide any information about other procedures to clean this material. F.B.

A. I was able to find some information on the lubricant you mentioned. It is described as a synthetic ester used to impregnate sintered rings of textile machines reducing yarn tension and wear. My first thoughts are why would you want to clean this and how are you determining that you have not done an adequate job cleaning the rings? Based on the description, once you lubricate the sintered rings, there does not seem to be a reason to remove that oil. Additionally, it is unclear to me how you are coming to the conclusion that your cleaning process is unsuccessful.

It is possible that the surface tension of the lubricant is equal to or lower than that of the perchloroethylene or aqueous based cleaning process that you are using. The fact that it impregnates the sintered rings to begin with would suggest that the lubricant has a very low surface tension. I would expect the aqueous process (Oakite?) not to have a low enough surface tension to sufficiently penetrate the very small spaces in the sintered rings, so it is not too surprising that process is ineffective at removing impregnated lubricants.

Additionally, the ester-based lubricant would be even more difficult to remove since it will have a tendency to adsorb to the metal surface at the more polar, ester end of the molecule. However, I am a bit surprised that the perchloroethylene does not penetrate and clean or remove that to any significant extent. It is possible that it also cannot penetrate and displace the very small cavities of the sintered ring space due to surface tension. If that is the case, possibly the only other alternative would be to drive the lubricant off by use of elevated temperatures (i.e., boiling it out of the rings). However, if the lubricant goes through incomplete combustion (as it may in atmosphere), it will break down and char the surface. It may be necessary to use an inert atmosphere, or possibly a vacuum thermal degreaser to effectively boil and remove the lubricant without degrading the lubricant into various hydrocarbon fractions.