Low absorptivity, high temp paint

[vof1966]

I would appreciate any suggestion about a commercial paint having low absorptivity (less than 0.3), high emmisivity (if possible) and being able to be used for an application at about 700ºC.

Many thanks in advance. Best regards.

Vicente Olmos

 

[btrueblood]

try

http://www.cotronics.com

What is the substrate material?

What wavelengths are you trying not to absorb (infrared at 700 C, or sunlight?). Most metallic coatings are going to oxidize if exposed to air at that temperature, other coatings will diffuse into the substrate.

Silicon carbide has interesting alpha/epsilon properties at high temps...

 

[vof1966]

Substrate materials are steels A-283-B and A-387-91.

We are trying to reduce concentrated sunlight absorption on non-cooled surface edges. Air will be only at 60ºC, it is the steel that can reach 600-700ºC due to radiation. Tubes are going to be painted black with Pyromark, but there are some metallic surfaces whose absorption must be reduced, otherwise we would have about 1000ºC.

Many thanks for your answer. Best regards.

Vicente Olmos
Spain

 

[btrueblood]

Oh, from your address, is this for use in a heliostat/solar concentrator?

To reduce solar input, the white metals are usually the best. Try looking at rhodium plating. Also platinum and palladium.

There is a NASA handbook (can't remember title, sorry) that gives normal emissivity and absorbtivity for a wide variety of metallic and organic compounds.

 

[vof1966]

Well guessed, yes it is for a solar receiver.

In this case, added to the problem of low absortion required on some small non-cooled surfaces we probably cannot rely on any paint, as these surfaces may be scratched by other parts' expansions. If we could state a reliable value for A-387-91 reflectivity, we would probably let these parts unpainted, but if its absorptivity approaches 0.8 they will reach about 900ºC and will produce excessive stresses on the nearest tube. Then we would need a suitable paint.

Regards,

Vicente Olmos
Spain

 

[btrueblood]

Vof,

If the metal is in air, you can't count on the absorbtivity remaining stable over time. The oxide layer on the surface will build over time until the surface is very nearly "black" to thermal radiation. A rhodium or similar "noble metal" plating is your best bet; the noble metal will maintain a nearly oxide-free surface for a long time. Rhodium plating is what we used for rocket engine nozzle heat shielding, because its melting temperature is very high, limiting how much the metal will diffuse into the substrate.

You are quite correct about not trusting any paints at those temperatures, anything you can find will be pretty poorly adhered to metal surfaces, and prone to cracking/spalling due to repeated thermal cycling (differential expansion between metallic and ceramic components). There are porcelain enamels which hold up better than monolithic ceramic coatings, but I'm not sure the absorbtivity of the porcelains is low enough at those temperatures to be of much use. They also tend to degrade over time, due to thermal expansion differences.

Good luck!

 

[vof1966]

Frankly, I'm afraid our budget could not afford a rhodium plating. These zones are 5 m x 0.017 m.

Presently we are thinking on using stainless steel instead of A-387-91 for all the side membranes. Starting from an absorptivity coefficient of 0.4 (machine rolled), the first problem is to state how much this coefficient can raise when aging due to thermal effects. If it exceeds 0.5 problems could arise in some of this side membranes, mainly the central ones, as they could send too much heat to the attached tube.

For these ones we could consider a palladium plating, but we had to deal with some issues such as fabrication, thermal expansion, etc.

Best regards,

Vicente Olmos
Spain

 

[btrueblood]

Vicente,

My old Heat Transfer textbook (Holman, _Heat Transfer_, McGraw-Hill, 1986) gives a value of 0.54-0.63 for type 301, B stainless steel (presumably they mean a "Grade B" finish), and a value of 0.074 for polished stainless. In general, stainless steel above 300 C is going to slowly oxidize, and its emissivity and absorbtivity will both climb. This phenomena is pretty much the same for all "common" metals, and the emissivity becomes higher as the temperature gets hotter (thicker oxide layers build up, and become rougher and absorb light better). Another approach you might consider is replaceable foil insulations (aluminum may work).

 

[btrueblood]

Okay, Vicente, an old colleague finally came through for me. Try this link, and its associated contact links. The silicone-based low-alpha coatings may do the trick for you. We used a similar product on sunshields on Landsat thrusters, but had trouble getting the silicone paint to stay "stuck" on aluminum substrate through acceptance test firings (aluminum temperature ranged up to 400 deg. F). We wored with the paint manufacturer and finally found a primer that kept it adhered. I have no data for survivability of the coatings in earth/weather/rain environments. Good luck, and let me know what you learn. I'll keep my ears open, too, and will let you know if any other news comes up.

 

[vof1966]

Considering all aspects, we are working on three lines at the same time.

First, some modifications on the mechanical design. The final shape is going to be rather odd, but the width of the end plates will be reduced so that they will be better cooled. Considering stainless steel with an absorptivity of 0.65 we will model all this with NASTRAN to see the resulting stresses and will study the heat flux to analyze the boiling mechanism.

And in order to work more on the safe side, we will paint the plates with a suitable painting. About this, I could not find in your previous post any link to the sylicone-based coatings you mentioned, and it seems it would be very interesting for us.

Regards,

Vicente Olmos

 

[btrueblood]

Vincente,

If you have a way to maintain a "shiny" finish on the stainless steel, you may be okay. The 0.63 and higher absorbtivity numbers I think are for heavily oxidized, heavily scaled (rough) stainless steel. If you can inspect and maintain the surfaces on a regular basis to keep them fairly "bright" the alpha should stay low and you won't get the "runaway" degradation I described earlier.

Argh! I forgot to post the link! Here it is:

http://www.aztechnology.com/PDFs/Coatings Catalog.pdf

There may be other silicone-based paint manufacturers out there who can help you. Generally, you want to get a stable oxide (titania, zirconia, zinc oxide, lead oxide) in a heat-stable binder; it's that last part that's tricky.