What's the deal with silicone?

[JNieuwsma]

I work for an industrial valve manufacturer. Our customers in the automotive industry require silicone-free valves for painting applications since (as I understand it) the presence of silicone has a detrimental effect on paint adhesion. But other customers request silicone paint for high temperature applications. How can there be a whole family of silicone paints, yet silicone is bad for adhesion?

Thanks in advance for your explanations and insight.

Jonathan Nieuwsma

 

[rhodie]

I'll take a stab at this, but I'm not much more than a "paint chemistry layperson" myself...

To say "Silicone" is like saying "Mammals with four legs". There is a huge family of different types of silicones out there. I'm sure the material used in valve seats is different in many ways from the stuff used in paint.

A factor I might be concerned with it that the chemistry of paint I am passing through the valve could embrittle or degrade the silicone parts of the valve. This could lead to particles ending up in the paint and at the very least cause poor topcoat finishes.

Also, the type of cure could be a factor. The Silicone paint I am familar with is moisture cured, while other paints could be thermally cured, or even UV cured.

Like I said, take it for what it's worth...
Rhodie

 

[JNieuwsma]

Rhodie,

Thanks for your stab!

I'm not actually concerned about silicone IN the valve (the seat are metal). In fact, my question really isn't about valves specifically. It's more curiousity as to how silicone as a surface contaminant is bad for painting, but yet there is this category of silicone paint. Could it be that silicone as surface contaminant is bad for other types of paint, but wouldn#t be detrimental to silicone paint?
Anyone?

Thanks for enlightening me.

Jonathan Nieuwsma

 

[magic612]

I'm probably not the right person to answer this, but I hope this may help explain it. There are many different "flavors" of silicone that can be produced: hard rubber, soft rubber, foam rubber, liquid adhesive, pressure sensitive adhesive, and yes, paint that includes silicone.

If I understand it correctly, silicone typically is bad for parts to be painted PROVIDED that the silicone is uncured. Cured silicone parts are used all the time by powder coaters, platers & painters to mask all sorts of auto and motorcycle parts with no detrimental effect, because the silicone is in a cured, hardened state. Cured, it leaves no residue.

The reason silicone is generally termed "bad" for painting is because nothing sticks to silicone, except another silicone based product (which is why silicone RTV adhesive will bond two pieces of silicone rubber). Problem is, most paints don't like silicone and won't adhere to it, so if any is left on the surface, the resulting finish will have imperfections (fish eyes, etc.).

On the other hand, because nothing sticks to silicone (except silicone based products), it would likely be advantageous to have a silicone based paint if, say, a tape with a silicone adhesive used for masking was leaving residue on a part. The paint would be more likely to adhere without requiring another wash cycle.

Alternatively, having a silicone based paint means parts painted with it would be easier to clean since things won't tend to stick to it. Many silicone products are used for medical applications because of its cleanliness, so a silicone based paint would likely exhibit many of those beneficial aspects.

Sorry this is so long. I'll let someone else dissect my theory if I got any parts of this wrong, but I hope that helps.

 

[mbrown]

A lot of paints have at least some silicone in them already. Certianly all automotive and marine grade paints do. The trick is that silione can be hard to control, it tends to want to come out of solution under the best of situations which is part of the point since it migrates upward in baked coatings and forms a low friction surface.

If a silicone(one of the thousands of different types) either is added in excess or is sitting as a smear on the part it forms a barrier to adhesion. This is what causes fish-eyes on parts. Carbon based grease will do the same thing but since hydro-carbon solvents tend to dissolve carbon greases better it takes a lot more.

In fact the hammer tone paint that is put on a lot of tool boxes has an excess of silicone added specifically to cause a massive amount of fish-eyes, that what creates the hammer tone look.

 

[mx2mx2]

Another reason that silicone can be a problem is that it can migrate through the paint and penetrate the substrate. When this happens the paint will lose adhesion and flake. Also, re-painting will not adhere because the substrate is contaminated with silicone.

 

[kchemcat]

The biggest problem is the lack of profile on a surface of this type. A harder surface (silicon containing) can be more difficult to obtain a surface profile resulting in poor adhesion.

You may want to check with SSPC for your specific application.

 

[TuDzi]

Silicones typically have very low surface energy. The presence of silicone may change the wettability of the valve by other materials such as non-silicone paints. Another characteristic of silicone is that they out-gas quite a bit; i.e., they contain a lot of volatile smaller molecular weight species. This is typically a problem when using silicones in vacuum environments, but probably won't be relevant to automotive applications.

 

[robsalv]

Silicone contamination on a metal surface will provide poor adhesion - akin to having grease contamination.

But there are a range of paints that use silicon chemistry for good reason. An extract from an AKZONOBEL paper demonstrates why: Organic coatings degrade as a result of thermal oxidation, photo-initiated oxidation or by chemical attack. Silicon based inorganic coatings are much more resistant to these degradation mechanisms.

Hence your customer specifying a silicon based paint for a high temperature application.

Hope this helps.

Cheers

Rob