Silica paint

Caution: this post should be taken as a basis for further inquiry, not as Gospel truth. I am basically a layman who has experimented with sodium silicate as a coating, adhesive and encapsulant.

"These paints are based on potassium silicate (waterglass?)"

Waterglass usually refers to sodium silicate, which is cheaper and more commonly used, but has very similar properties.

"My questions are do surfaces need to be etched and if so do I need to use a particular acid or will building acid (weak Hydrochloric) do? Will the styrene stabilizer affect the silification process adversely and what is wet scrub resistance?"

Acid etching of masonry and concrete is usually done with dilute HCl, and is mainly to rid the surface of loose matter and free alkali. Hydrochloric acid, sometimes called muriatic acid, should work fine. Another function of a preliminary acid treatment might be to speed setting and bonding to the surface.

Wet scrub resistance simply means resistance to removal by a combination of water and brushing, such as would occur during routine cleaning of surfaces that would need to be sealed (food processing spaces, for example). Silicates generally "set" by absorbing carbon dioxide from the atmosphere (though other acidic gases will have a similar effect); while this process is ongoing they remain somewhat soluble in water, hence vulnerable to wet scrubbing. The addition of styrene seems to be for the purpose of waterproofing the coating during the setting period, but I would think that the plastic would also retard access by atmospheric gases, including CO2, so this is a little confusing. Perhaps this new coating has internal setting agents? For example, when I use sodium silicate and clay as a binder and filler for wood attacked by termites, I dissolve some bicarbonate in the silicate solution, and then use heat to release CO2 inside the filler, quick-setting it.

Piolenc,

thanks for that. I was hoping a few more chemists would chip in but hay ho. The etch process is supposed to open the pores of the masonry surface (when and where it is sintered (??)) to allow for better silification. I was hoping that somebody could explain the chemical processes involved with a few formulas etc.

Anyway, thank you Piolenc.

Richard.

Yes, an explanation of the chemical process called petrification, or silification, would be helpful. I am a painter who uses pottassium silicate to harden clay and lime painted surfaces, and often wonder what the chemical process is that makes the clay and lime "petrify." And is it the same process on clay as on lime. Any chemists want lend some insight?

Hi,

I am not a chemist but have used different of these silicate solutions for many applications.

Water-glass is a polymeric silicate that exists only as the (sodium or potassium)-salts of a hypothetical silicic acid.

The sodium or potassium is split off the molecules by CO2 that is forming carbonate and leaving polymerised silicic acid that is containing a lot of water. The water can be driven off the glass-like material by evaporation, elevated temperature helps but will give early cracking. The soluble part of the reaction-products has to be washed off - so potassium is preferred as it is easier to wash away.

If other ions are in the mixture: any metal-oxide or ? hydroxide these will react to highly insoluble silicates - very weather resistant stones. Also most natural carbonates (limestone) and mixtures of carbonates with silicates (clay, shale) may be used as source of metal-oxides.

There are lots of "modifiers" I do know nothing about that will result in faster or slower setting, please ask the manufacturer and please tell us.

The stuff exists without the sodium or potassium - made by hydrolysis of silanes (TEOS for example). This is "pure" H2SiO3 + H2O. Stripping all the H2O will leave SiO2. These are made and marketed by Monsanto under the brand: Syton. This is used in heat resistant metal-casting-molds to bind the zirconium-silicate to a hard, porous and fire resistant stuff. And in polishing very hard crystals and silicon. And more.

RHABE

Four reaction types can occur.

1. If an acid is present or is formed by other portions of the mix, the silica deposits out as a fairly hard coating.

2. Various metal oxides will react to form complex silicates. For example alumina from clay will react to form a potassium (or sodium) alumino-silicate.

3. If fluorides are present as well as metal oxides a complex metal-fluoro-silicate forms. Apparently this has properties similar to tooth enamel.

4. If a metal is present an inorganic metal silicate can form. I assume the sodium or potassium leaches out, but I'm not sure. An example was the inorganic zinc silicate used to protect steel surfaces. This used potassium silicate and zinc powder. I believe CO2 from the air reacted with the mix to form the final compound. It gave good galvanic protection and apparently still left a silicate protective coating even when much of the zinc had reacted (as in salt water).

Magnesium Fluoride and sodium silicate are used to harden concrete by reaction #3. It reacts with the free lime formed during cement hydration to form a magnesium sodium calcium silico fluoride.

I used to know the hypothetical formulae for these compounds but that was many years ago.

Hi Sceptic,

thank you for explaining the principles, that helps.

"If an acid is present..." This may be one task of hydrochloric acid pretreatment?

"Various metal oxides..." Do you know which ones to prefer? And how to speed up or slow down the reaction? I once tried CaO but got an immediate reaction with no adhesion nor solidification - too small reaction particles, loosely dispensed in a slurry.in

"Fluorides.." I did not know about this but would be cautious as some Silico-Fluorides are pretty poisonous.

"Metal is present..." Which grain size to use? "Iron"-powder from grinding should be good for a trial.

RHABE

Hi RHABE:

"If an acid is present..." This may be one task of hydrochloric acid pretreatment?

I've often thought that but have no positive experience or knowledge here.

"Various metal oxides..." Do you know which ones to prefer? And how to speed up or slow down the reaction?

They generally need to be very fine. Most will work, but CaO (or Ca(OH)2) need to be present in the material (eg set concrete) as they tend to react rapidly and hence get poor adhesion.

Some metal phosphates also work. I believe aluminium phosphate has been used successfully.

Don't know how to speed up the reaction. Often the problem is to slow it down enough to get it absorbed into the surface before the reaction occurs.

"Fluorides.." I did not know about this but would be cautious as some Silico-Fluorides are pretty poisonous.

True. Makes me wonder about fluoride in drinking water. Also sodium monofluoro acetate is better known as the poison 1080. Apparently not hard to make either.

Complex fluoro-silicates do tend to form a good strong coating which is quite inert.

"Metal is present..." Which grain size to use? "Iron"-powder from grinding should be good for a trial.

Zinc is the common one used as it gives cathodic protection. The finer the better as far as I know.

Iron will rust, although the alkalinity of the sodium silicate does inhibit this. Could be interesting to see if you can get a magnetic or magnetizable coating that way. Not sure what use it would be but interesting anyway.

On my list of things to try (when Mrs Sceptic runs out of jobs for me to do!) is to experiment with a few recipes of these silicate paints. I've always thought the field has a lot of promise.

Incidentally, rapid heating of sodium silicate will form a foam which is quite a good thermal insulator. Good for about 400-500C (flows at 550C).

Loading sodium silicate with kaolin and slowly heat curing gives a thermal insulator good to about 1500+C. Loading with alumina and it is good for at least 1800C. This last can be used for molten iron, but all these need to be cured first.

Hi Richard, I am a semi retired paint chemist and have just completed a 12 month project reviewing "waterborne Silicate Paints" which is quite complex but send your e mail address to me at ultrasonic@telkomsa.net and I will send you a copy.

In short the 3rd type of silicate paint known as 1-K silicate paint is a water based single pack product; two types of cure are used; the first as you mentioned is via the inclusion of 5% acrylic or co-polymer that holds the silicate on the surface while the silification reaction takes place (this can be quite slow and may even take a week or more); the true silicate reaction is that of the potassium silicate with the di-valent metal ions in the cement based substrate (Normally Calcium ions)

Type 3 as indicated is covered by the German spec that specifies the maximum of 5% organic matter on oven dried solids so as to be classified as an "Inorganic Coating".

The option of an acid wash is quite OK and could exposed some of the required di-valent ions (Beryllium; Magnesium or Calcium) in older or contaminated plaster surfaces.