2. Once painted onto a wall they absorb carbon dioxide, and the lime-based binders carbonate and water silicate-based binders solidify. They form SiO2 x H2O and K2CO3. Mineral silicate paint coats are considered durable and weather resistant.
3. Mineral paint contains inorganic pigments, and potassium-based, alkali silicate (water glass), also known as potassium silicate or liquid potassium silicate. When used the pure mineral paints do not form a film paint layer but instead permanently bond to the substrate through silicification.
4. The result is a highly durable connection between the paint coat and background. The water glass binding agent is highly resistant to UV light.
5. While dispersions based on acrylate or silicone resin over the years tend to grow brittle, chalky, and crack under UV, the inorganic binder water glass remains stable.
6. The chemical fusion with the substrate and the UV stability of the binder are the fundamental reasons for the extraordinarily high lifetime of silicate paints.
7. Silicate paints require siliceous substrate for setting. For this reason they are highly suitable for mineral substrates such as mineral lime and cement based plasters and renders and concrete.
8. Silicate coatings are alkali metal silicates that are made from naturally occurring materials such as sand and alkali. Alkali metal silicates are derived from a combination of silica (SiO2) and a carbonate of lithium, sodium or potassium to produce a silicate (SiO2/Na2O).
MOISTURE, ALGAE, DIRT, CONTAMINATION RESISTANCE
1. They are only of limited use for application on wood and metal. The permeability to water vapour of silicate paints is equivalent to that of the substrate, so silicate paints do not inhibit the diffusion of water vapour.
2. Moisture contained in parts of a structure or in the plaster may diffuse outward without resistance: this keeps walls dry and prevents structural damage. This addition helps avoid condensation of water on the surface of building materials, reducing the risk of infestation by algae and fungi.
3. The high alkalinity of the water glass binding agent adds to the inhibitory effect against infestation by microorganisms and completely reducing the need for additional preservatives.
4. Mineral paint coats are not prone to static charging and thermo-plasticity (stickiness developing under heat), which is common for surfaces coated with dispersion or silicone resin, soiling happens less, so fewer dirt particles cling to the surface and are easier to wash off. Silicate paints are incombustible and free of organic additives or solvents. Silicate paints colours are highly stable. As they are solely coloured with mineral pigments that do not fade with exposure to UV radiation.
5. Silicate paints are based upon mineral raw materials. They are environmentally compatible in manufacture and effect. Their high durability helps to preserve resources and their contaminant-free composition preserves health and environment. For this reason, silicate paints have gained popularity, especially in sustainable construction as well as historic building restoration work.
TYPES OF SILICATE PAINTS
1. Commonly three types of silicate paints are distinguished:
- Pure silicate paint consisting of two components
- A colour powder in dry or water-paste form
- The liquid binder water glass.
2. The addition of up to 5 mass percent of organic additives (e.g. acrylate dispersion, hydrophobisers, thickeners or similar) makes ready-to-use paint in containers possible.
3. These are also called "dispersion silicate paints". This paint range is still an active mineral paint, actively binding to a mineral background through Silification.
4. Cheaper silicate paints have more acrylics added and become passive paints without the same durability.
5. A combination of silica sol and water glass. The organic fraction is limited to 5 mass percent similar to dispersion silicate paint allowing for chemical setting and retaining of the silicate specific advantages.
6. The sol silicate paint allows use on non-mineral plaster. For these the bonding occurs chemically and physically. These paints can be applied easily and safely to nearly all common substrates.
WATERBORNE SILICATE COATINGS FORMULATION
1. Types of silicate-based coatings include silicate, silicate-organic emulsion and lastly sol-silicate.
2. Soluble silicates include those of the Group 1A elements of the Periodic Table (Li, Na and K). As Silicates are based on alkali metal oxides and silica, their solutions are alkaline. As the molar weight ratio of the silicon:alkali metal increases, the pH decreases:
3. Accordingly, when blending alkali metal silicates with organic emulsions, it is important to use higher ratios of silicon to alkali metal to achieve the best stability and a workable pH of 8 – 10 for most organic-based emulsions.
4. Viscosity of sodium silicate solutions is a function of concentration, density and ratio of sodium: silicon. Higher or lower ratios increase viscosity with a minimum viscosity reached at a 2.0 weight ratio.
5. From a structural standpoint, waterborne silicates are glasses that have a wide variety of molecular structures in which the anions are monomers, dimers, trimers, branched chains, and ring structures, as well as other three dimensional networks. Cations of alkali metals (Li+, Na+ and K+) attach to the anions (Si – O – ) to create a complex alkali silicate.
6. The ratio of alkali metal oxide to silica has a significant effect on coating properties as illustrated in the table below:
The higher ratio (High SiO2 low NaCO3, e.g. 3.75 to 1) gives:
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The lower ratio (Low SiO2 High NaCO3, e.g. 2 to 1) gives:
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Lower viscosity
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Higher specific weight
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Faster drying speed
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Greater solubility
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Faster curing speed
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Higher pH value
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Increased susceptibility to low temperatures
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Greater susceptibility to water influence
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Higher chemical resistance of coatings
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Higher tack and binding power
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COMMERCIAL PRODUCTS
1. Commercially available silicates are normally produced in ratios of 1.5 or higher. Coatings based on sodium silicate can be used and require a catalyst for ambient cure, but are susceptible to efflorescence. Solutions of sodium silicate can react or cure with dissolved polyvalent ions including Ca++, Al+++ and Mg++ to form insoluble silicates.
2. Mineral calcium carbonates (e.g. calcite) exhibit low reactivity with soluble silicate, whereas precipitated calcium carbonate provides high reactivity. The viscosity of sodium silicates is very high, whereas colloidal silicas (stabilized silica particles less than < 100nm in size) have viscosities closer to that of water. pH has a major impact on the viscosity of colloidal silicas and form gels at a pH < 7 and a Sol when a pH is >7. Liquid sodium and potassium silicates also can be reacted with a variety of acidic or heavy metal compounds to produce solid, insoluble bonds or films.
3. Neutralizing an alkali silicate with acidic materials (e.g., aluminum sulfate) polymerizes the silica and forms a gel. This produces a bond or film on surfaces where gellation occurs. Chemical setting agents that can be used in this manner include: mineral and organic acids, carbon dioxide (CO2) gas, and acid salts such as sodium bicarbonate and monosodium phosphate (NaH2PO4).
4. Silicate-emulsion paints comprise a low level of a polymeric organic emulsion (~5%) with an alkali silicate. The emulsion helps to enhance water resistance until the silification reaction is complete, which can take weeks. Higher levels of organic emulsions are generally incompatible.
5. As silicate paints are not generally flexible, they can be flexibilized by the addition of 1 to 5% by weight of glycerine or other polyhydric alcohols. Up to 30% of sorbitol can be used, provided the silicate solution is diluted to avoid excessive thickening.
6. Rubber lattices can also be employed as plasticizers. Incorporation of finely ground clays and similar fillers will improve flexibility to some extent. Silicate emulsions paints can also be formulated for use on aluminum, galvanized steel, steel, stone, brick, concrete, and previously painted surfaces that used an emulsion paint.
7. Sol-silicate paint is a combination of silica-sol and potassium silicate. An organic binder is incorporated at a percentage of 10% or lower. As opposed to most other silicate paints, sol-silicate paints bond to non-mineral substrates through both physical and chemical bonds. Silica sols are dilute solutions of dissolved silica that are at an acidic pH.
(Source: Womersleys, https://knowledge.ulprospector.com/9072/pc-waterborne-silicate-coatings-ultimate-eco-friendly-coating/)
