Cleaning Silicone Contaminated Parts

A. I hope to be corrected, Domenick, because I don't think it's possible to reliably remove silicone contamination from aluminum parts. In fact, I'm not so sure you can reliably do it even on steel parts where much more aggressive cleaning is possible. The rule is that parts simply must not ever get contaminated with silicone. Good luck.

A. The best way to eliminate silicone contamination is to bar the use of silicone oils in any processes prior to the painting operation. Once silicone gets on the surface of the parts, it is nearly impossible to completely remove it.

The following are some potential sources of silicone: mold release agents, defoamers used in rolling or forming oils, coolants, cleaners, water displacing lubricants like WD-40 ⇦this on eBay or Amazon [affil links] , hard surface cleaners that claim to leave surfaces "shiny", sealants and caulking materials, personal care products like skin creams, hair conditions, and lotions and many other common products.

When attempting to clean silicone oil off the surface of parts, it is very common for it to redeposit back onto the parts that were originally affected or onto other parts that entered the process free from silicone.

Silicone will cause paint defects at the parts per million level making it nearly impossible to detect at low levels.

A. No recommendation but found this:

GenSolveTM High-Performance Stripping Agents remove tenacious polymers from metal [...] substrates. They are effective at ambient temperatures with [...] coatings made of silicone [...]

Good luck,

A. I'm trying to clean some silicone myself and found out that sodium hydroxide or another strong base can de-polymerize the silicone back to cyclics and would probably work okay on aluminum (not sure about plastics yet). Also I hear Hexane will do the trick (probably better for metals) if you rinse it a few times.

A. Hi Yohands,

Silicones can certainly be detected. If the concentration is high enough then something like FT-IR will be a good tool, especially for any of the cutting fluids or coolants, otherwise it may need to be more specialised such as SEM with SIMS attachment.

What makes you suspect silicones, other than water break failure?

Cleaning of silicones is particularly difficult, but degreasing with standard halogenated degreasers followed by a good alkaline clean is usually sufficient. If you are finding that the film is still tenacious then it will come down to manual cleaning, this may involve some form of abrasive cleaning. If this is the case and the process is carried out prior to penetrant inspection you will need to carry out a pre-penetrant etch to remove smeared material.

A. Silicones are so difficult to remove that it is absolutely imperative that they are never allowed near any plating, painting or soldering process.
I would suggest that you should have a company policy banning them and make sure that your buying department understand the importance of this. Next you need to do a thorough inventory of the maintenance department and remove all those spray on wonder 'fix everything' lubricants they love. Hand creams are another common source. The ladies love them, particularly the ones they get as christmas presents! Health and safety departments are very good at handing out free samples of "skin care products" - they must get the message too.
There are no reliable cleaners, whatever the salesman tells you.
Just make sure that everyone understands the threat to the business (and their jobs!)

PS Oddly enough, I don't think that WD-40 has silicones but you should check

A. Hi Yohands,

Geoff's advice is sound, but knowing the sort of business Aerospace is it isn't always possible to avoid silicones or silicone based lubricants.

SEM analysis would be done on the surface of the contaminated part, not any of the solutions. It should be reasonably easy to pick up the Silicon peak with this analysis technique if it is present.

I get the feeling you are going to end up having to clean these parts very manually, that is swab degreasing with solvent and possibly scrub cleaning with a non-silicated cleaner.

As for trying to find the root cause of your problem you are going to have to look at the types of parts that are being processed, are they sheet, wrought or cast? What machining operations do they go through? What coolants/lubricants are used in the manufacturing process? Are these materials potential sources of contamination? Is someone using one of the "magic spray" cans people seem to have hidden away all over the place that are perceived to help with manufacturing?

At the end of the day you can't beat "walking the process", that way you can follow the parts from receipt to packing, will be able to see what operations are involved and what chemicals are utilised.

A. Silicon "on the surface" of the existing paint could be just a polish which was applied. I'd try scrubbing with something like Ajax ⇦this on eBay or Amazon [affil links] and retesting a sample area to see if that will remove the silicon. Steam cleaning with a detergent might work too.

A. I think worrying about the silicon residue from a waxing is unwarranted, however it's their hanger, so their rules.

Steam cleaning with detergent shouldn't be very labor intensive.

A. Hi Steve
The reason we don't want silicone is that it will not wet. Therefore the easiest test must be a simple water break check.
No swabbing a small area and hoping the rest is OK. Just throw a bucket of water over the aircraft or park it in the rain. You will soon see any residual silicone - and where it is so that you don't have to redo the whole thing if you find any.

Hi, Geoff. What was the specification number again that details throwing a bucket of water over an airplane?  :-)

Regards,

A. Lab swab testing is by hexane swabs.

A. I have been using two products by RPM Technology to completely remove silicone and silicone residue from metals and fiberglass. I use the Digisil to remove the larger bits left behind from mechanical removal. I put it on thick, cover it in plastic, and let it work over night. after a good cleaning and wash, I spray and scrub with the Di-Aqua, this removes the layer of slimy residue left behind. I am not affiliated with this company and I do not receive any compensation of any kind, it's just a great product that works for me, and I thought I'd pass on my $.02.

Cheers,

A. Hexane is the only cleaning agent that will come close to displacing silicone. The trick is to heat RO water to 120 degrees, mix in the dry Hexane and use it with clean wipers. Dispose of the wiper after one use and repeat.

P.S. Evaporative solids testing will not detect silicone. The presence of silicon can be determined relatively rapidly by elemental analysis, e.g., atomic adsorption or inductively coupled plasma (ICP). To localize the contaminant, a two dimensional technique such as scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDX) might be used. The SEM provides a characteristic visual observation of a very small area (e.g. a coating defect or a particle). EDX elemental analysis of that area identifying carbon, oxygen, and silicon can be used as evidence that some sort of silicone is the culprit.

A. I sometimes work at the BMW plant in Greenville/Spartanburg South Carolina. ANYTHING silicone or containing silicone is contraband and possession of it is immediate firing.
I also have noticed that even silicone rubber objects that touch glass always leave a footprint. When you wash the glass the mark stands out such as the suction cups that are used to install the glass. I have tried Windex, Goof Off, Xylene, and some kind of epoxy reducer made by Sherwin Williams. All no good.

A. Hi James,

You may try citric acid ⇦this on eBay or Amazon [affil links] which can remove silicone residue from plastic surface to certain extent.

Regards,
David

A. Hi Logan,

You don't say how high "High Temperature" is, but not all high temperature paints contain silicones, there are ones based on epoxies, polyamides, polyurethanes, epoxy/phenolics and others.

I would suggest you reach out to some of the paint manufacturers to better consider your options, companies like PPG, Akzo-Nobel, Randolph, Indestructible Paints and others have numerous paint systems that might better suit you without the introduction of silicones.

If you do have to use silicones, because of customer requirements then using normal good shop practices to prevent cross-over contamination should suffice to ensure you do not get adhesion issues.

A. I once heard of this silicone problem just occurring out of the blue, and they got a team in to find the culprit and found they had changed the compressor belts as well hey were old.
The new ones contained silicone and with the friction fine dust was actually going through the air supplying the paint gun and any other compressed air proses!