Sodium silicate

Sodium silicate is a generic name for chemical compounds with the formula Na_2xSi_yO_2y+x or (Na₂O)_x·(SiO₂)_y, such as sodium metasilicate Na₂SiO₃, sodium orthosilicate Na₄SiO₄, and sodium pyrosilicate Na₆Si₂O₇. The anions are often polymeric. These compounds are generally colorless transparent solids or white powders, and soluble in water in various amounts.

Sodium silicate is also the technical and common name for a mixture of such compounds, chiefly the metasilicate, also called waterglass, water glass, or liquid glass. The product has a wide variety of uses, including the formulation of cements, coatings, passive fire protection, textile and lumber processing, manufacture of refractory ceramics, as adhesives, and in the production of silica gel. The commercial product, available in water solution or in solid form, is often greenish or blue owing to the presence of iron-containing impurities.

In industry, the various grades of sodium silicate are characterized by their SiO₂:Na₂O weight ratio (which can be converted to molar ratio by multiplication with 1.032). The ratio can vary between 1:2 and 3.75:1.^[1] Grades with ratio below 2.85:1 are termed alkaline. Those with a higher SiO₂:Na₂O ratio are described as neutral.

History

Soluble silicates of alkali metals (sodium or potassium) were observed by European alchemists already in the 1500s. Giambattista della Porta observed in 1567 that tartari salis (cream of tartar, potassium hydrogen tartrate) caused powdered crystallum (quartz) to melt at a lower temperature.^[2] Other possible early references to alkali silicates were made by Basil Valentine in 1520,^[3] and by Agricola in 1550. Around 1640, Jean Baptist van Helmont reported the formation of alkali silicates as a soluble substance made by melting sand with excess alkali, and observed that the silica could be precipitated quantitatively by adding acid to the solution.^[4]

In 1646, Glauber made potassium silicate, which he called liquor silicum, by melting potassium carbonate (obtained by calcinating cream of tartar) and sand in a crucible, and keeping it molten until it ceased to bubble (due to the release of carbon dioxide). The mixture was allowed to cool and then was ground to a fine powder.^[5] When the powder was exposed to moist air, it gradually formed a viscous liquid, which Glauber called "Oleum oder Liquor Silicum, Arenæ, vel Crystallorum" (i.e., oil or solution of silica, sand or quartz crystal).^[6]

However, it was later claimed that the substances prepared by those alchemists were not waterglass as it is understood today.^[7] That would have been prepared in 1818 by Johann Nepomuk von Fuchs, by treating silicic acid with an alkali; the result being soluble in water, "but not affected by atmospheric changes".^[8]

The terms "water glass" and "soluble glass" were used by Leopold Wolff in 1846,^[9] by Émile Kopp in 1857,^[10] and by Hermann Krätzer in 1887.^[11]

In 1892, Rudolf Von Wagner distinguished soda, potash, double (soda and potash), and fixing (i.e., stabilizing) as types of water glass. The fixing type was "a mixture of silica well saturated with potash water glass and a sodium silicate" used to stabilize inorganic water color pigments on cement work for outdoor signs and murals.^{[12][13][14][15]}

Properties

Sodium silicates are colorless glassy or crystalline solids, or white powders. Except for the most silicon-rich ones, they are readily soluble in water, producing alkaline solutions.^{[citation needed]} When dried up it still can be rehydrated back in water.^[16]

Sodium silicates are stable in neutral and alkaline solutions. In acidic solutions, the silicate ions react with hydrogen ions to form silicic acids, which tend to decompose into hydrated silicon dioxide gel.^{[17][failed verification]}

Heated to drive off the water, the result is a hard translucent substance called silica gel, widely used as a desiccant. It can withstand temperatures up to 1100 °C.^{[citation needed]}

Production

Solutions of sodium silicates can be produced by treating a mixture of silica (usually as quartz sand), caustic soda, and water, with hot steam in a reactor. The overall reaction is

2x NaOH + SiO₂ → (Na₂O)_x·SiO₂ + x H₂O

Sodium silicates can also be obtained by dissolving silica SiO₂ (whose melting point is 1713 °C) in molten sodium carbonate (that melts with decomposition at 851 °C):^[18]

x Na₂CO₃ + SiO₂ → (Na₂O)_x·SiO₂ + CO₂

The material can be obtained also from sodium sulfate (melting point 884 °C) with carbon as a reducing agent:

2x Na₂SO₄ + C + 2 SiO₂ → 2 (Na₂O)_x·SiO₂ + 2 SO₂ + CO₂

In 1990, 4 million tons of alkali metal silicates were produced.^[1]

Ferrosilicon

Sodium silicate may be produced as a part of hydrogen production by dissolving ferrosilicon in an aqueous sodium hydroxide (NaOH • H₂O) solution:^[19]

2NaOH + Si + H₂O → 2Na₂SiO₃ + 2H₂

Bayer process

Though unprofitable, Na₂SiO₃ is a byproduct of Bayer process which is often converted to calcium silicate (Ca₂SiO₄).

Uses

The main applications of sodium silicates are in detergents, paper industry (as deinking agent), water treatment, and construction materials.^[1]

Engineering

Adhesive & cement

Sodium Silicate adhesive properties are well known since at least first World War.^[20] The largest application of sodium silicate solutions is a cement for producing cardboard.^[1] When used as a paper cement, the tendency is for the sodium silicate joint eventually to crack within a few years, at which point it no longer holds the paper surfaces cemented together.

Sodium silicate solutions can also be used as a spin-on adhesive layer to bond glass to glass ^[21] or silicon oxide covered silicon wafers to one another.^[22] Sodium silicate glass-to-glass bonding has the advantage that it is a low temperature bonding technique, as opposed to fusion bonding.^[21] It is also less processing intensive than glass-to-glass anodic bonding,^[23] which requires an intermediate layer such as SiN to act as a diffusion barrier for sodium ions.^[23] Deposition of such a layer requires a low pressure chemical vapor deposition step.^[23] A disadvantage of sodium silicate bonding, however, is that it is very difficult to eliminate air bubbles.^[22] This is due in part because this bonding technique doesn't require bonding in vacuum and it also doesn't use field assistance like in anodic bonding.^[24] This lack of field assistance can sometimes be beneficial, because field assistance can provide such high attraction between wafers as to bend a thinner wafer and collapse^[24] onto nanofluidic cavity or MEMS elements.

Coatings

Sodium silicate may be used for various paints and coatings (e.g. for welding rods). They may be cured in different ways. In first approach, thin layer of sodium silicate may be slowly gelated by drying up and then turned into a hard film by heating. Before heating such layers aren't water resistant. In order to make them water-resistant high temperatures of 100 °C (212 °F; 373 K) is needed.^[16] It should be slowly raised to dehydrate film and avoid steaming (and blistering) and then it should be "fired" above 150 °C (302 °F; 423 K). The process must be relatively slow, infra-red lamps may be used at first.^[16]

The other approach, when high-temperature is not practical, the water resistance may be achieved by chemicals (or esters), such as boric acid (B(OH)₃), phosphoric acid (PO(OH)₃), sodium fluorosilicate, aluminium phosphate etc..^[16] Before application water solution of sodium silicate is mixed with curing agent.^[16]

Drilling fluids

Sodium silicate is frequently used in drilling fluids to stabilize borehole walls and to avoid the collapse of bore walls. It is particularly useful when drill holes pass through argillaceous formations containing swelling clay minerals such as smectite or montmorillonite.

Concrete and general masonry treatment

Concrete treated with a sodium silicate solution helps to reduce porosity in most masonry products such as concrete, stucco, and plasters. This effect aids in reducing water penetration, but has no known effect on reducing water vapor transmission and emission.^[25] A chemical reaction occurs with the excess Ca(OH)₂ (portlandite) present in the concrete that permanently binds the silicates with the surface, making them far more durable and water repellent. This treatment generally is applied only after the initial cure has taken place (7 days or so depending on conditions). These coatings are known as silicate mineral paint. An example of the reaction of sodium silicate with the calcium hydroxide found in concrete to form calcium silicate hydrate (or C-S-H) gel, the main product in hydrated Portland cement, follows. ^[26]

Na₂SiO₃ + y H₂O + x Ca(OH)₂ → x CaO.SiO₂.y H₂O + 2NaOH

Detergent auxiliaries

It is used in detergent auxiliaries such as complex sodium disilicate and modified sodium disilicate. The detergent granules gain their ruggedness from a coating of silicates.^[1]

Water treatment

Sodium silicate is used as an alum coagulant and an iron flocculant in wastewater treatment plants. Sodium silicate binds to colloidal molecules, creating larger aggregates that sink to the bottom of the water column. The microscopic negatively charged particles suspended in water interact with sodium silicate. Their electrical double layer collapses due to the increase of ionic strength caused by the addition of sodium silicate (doubly negatively charged anion accompanied by two sodium cations) and they subsequently aggregate. This process is called coagulation.^[1]

Refractory use

Water glass is a useful binder for solids, such as vermiculite and perlite. When blended with the latter lightweight fraction, water glass can be used to make hard, high-temperature insulation boards used for refractories, passive fire protection and high temperature insulations, such as moulded pipe insulation applications. When mixed with finely divided mineral powders, such as vermiculite dust (which is common scrap from the exfoliation process), one can produce high temperature adhesives. The intumescence disappears in the presence of finely divided mineral dust, whereby the waterglass becomes a mere matrix. Waterglass is inexpensive and abundantly available, which makes its use popular in many refractory applications.

Foundry

It is used as a binder of the sand when doing sand casting of all common metals. It allows for the rapid production of a strong mold or core by three main methods.^{[citation needed]}

• Method 1 requires passing carbon dioxide gas through the mixture of sand and sodium silicate in the sand molding box or core box. The carbon dioxide gas reacts with the sodium silicate to form solid sodium carbonate. This provides adequate strength to remove the now hardened sand shape from the forming tool. Additional strength occurs as any unreacted sodium silicate in the sand shape dehydrates.
• Method 2 requires adding an ester (reaction product of an acid and an alcohol) to the mixture of sand and sodium silicate before it is placed into the molding box or core box. As the ester hydrolyzes from the water in the liquid sodium silicate, an acid is released which causes the liquid sodium silicate to gel. Once the gel has formed, it will dehydrate to a glassy phase due to syneresis. Commonly used esters include acetate esters of glycerol and ethylene glycol as well as carbonate esters of propylene and ethylene glycol. The higher the water solubility of the ester, the faster the hardening of the sand.^{[citation needed]}
• Method 3 requires microwave energy to heat and dehydrate the mixture of sand and sodium silicate in the sand molding box or core box. The forming tools must be microwave transparent for this to work well. Because sodium silicate has a high dielectric constant, it absorbs microwave energy very rapidly. Fully dehydrated sand shapes can be produced in less than 1 minute of exposure to microwaves. This method produces the highest strength of sand shapes bonded with sodium silicate.

Since the sodium silicate does not burn during casting (it can actually melt at pouring temperatures above 1800 degrees F.) it is common to add organic materials to provide for enhanced sand breakdown after casting. The additives include sugar, starch, carbons, wood flour and phenolic resins.

Dye auxiliary

Sodium silicate solution is used as a fixative for hand dyeing with reactive dyes that require a high pH to react with the textile fiber. After the dye is applied to a cellulose-based fabric, such as cotton or rayon, or onto silk, it is allowed to dry, after which the sodium silicate is painted on to the dyed fabric, covered with plastic to retain moisture, and left to react for an hour at room temperature.^[27]

Metal repair

Sodium silicate is used, along with magnesium silicate, in muffler repair and fitting paste. When dissolved in water, both sodium silicate and magnesium silicate form a thick paste that is easy to apply. When the exhaust system of an internal combustion engine heats up to its operating temperature, the heat drives out all of the excess water from the paste. The silicate compounds that are left over have glass-like properties, making a temporary, brittle repair.

Automotive repair

Sodium silicate is also used currently as an exhaust system joint and crack sealer for repairing mufflers, resonators, tailpipes, and other exhaust components, with and without fiberglass reinforcing tapes. In this application, the sodium silicate (60–70%) is typically mixed with kaolin (40-30%), an aluminium silicate mineral, to make the sodium silicate "glued" joint opaque. The sodium silicate, however, is the high-temperature adhesive; the kaolin serves simply as a compatible high-temperature coloring agent. Some of these repair compounds also contain glass fibres to enhance their gap-filling abilities and reduce brittleness.

Sodium silicate can be used to fill gaps within the head gasket. Commonly used on aluminum alloy cylinder heads, which are sensitive to thermally induced surface deflection. This can be caused by many things including head-bolt stretching, deficient coolant delivery, high cylinder head pressure, overheating, etc.

"Liquid glass" (sodium silicate) is added to the system through the radiator, and allowed to circulate. Sodium silicate is suspended in the coolant until it reaches the cylinder head. At 100–105 °C (212-221 °F), sodium silicate loses water molecules to form a glass seal with a remelt temperature above 810 °C (1,490 °F).

A sodium silicate repair can last two years or longer. The repair occurs rapidly, and symptoms disappear instantly. This repair works only when the sodium silicate reaches its "conversion" temperature at 100–105 °C. Contamination of engine oil is a serious possibility in situations in which a coolant-to-oil leak is present. Sodium silicate (glass particulate) contamination of lubricants is detrimental to their function.

Sodium silicate solution is used to inexpensively, quickly, and permanently disable automobile engines. Running an engine with about 2 liters of a sodium silicate solution instead of motor oil causes the solution to precipitate, catastrophically damaging the engine's bearings and pistons within a few minutes.^[28] In the United States, this procedure was used to comply with requirements of the Car Allowance Rebate System (CARS) program.^[28][29]

Safe construction

A mixture of sodium silicate and sawdust has been used in between the double skin of certain safes. This not only makes them more fire resistant, but also makes cutting them open with an oxyacetylene torch extremely difficult due to the smoke emitted.

Crystal gardens

When crystals of a number of metallic salts are dropped into a solution of water glass, simple or branching stalagmites of coloured metal silicates are formed. This phenomenon has been used by manufacturers of toys and chemistry sets to provide instructive enjoyment to many generations of children from the early 20th century until the present. An early mention of crystals of metallic salts forming a "chemical garden" in sodium silicate is found in the 1946 Modern Mechanix magazine.^[30] Metal salts used included the sulfates and/or chlorides of copper, cobalt, iron, nickel, and manganese.

Pottery

Sodium silicate is used as a deflocculant in casting slips helping reduce viscosity and the need for large amounts of water to liquidize the clay body. It is also used to create a crackle effect in pottery, usually wheel-thrown. A vase or bottle is thrown on the wheel, fairly narrow and with thick walls. Sodium silicate is brushed on a section of the piece. After 5 minutes, the wall of the piece is stretched outward with a rib or hand. The result is a wrinkled or cracked look.

It is also the main agent in "magic water", which is used when joining clay pieces, especially if the moisture level of the two differs.^[31]

Sealing of leaking water-containing structures

Sodium silicate with additives was injected into the ground to harden it and thereby to prevent further leakage of highly radioactive water from the Fukushima Daiichi nuclear power plant in Japan in April, 2011.^[32] The residual heat carried by the water used for cooling the damaged reactors accelerated the setting of the injected mixture.

On June 3, 1958, the USS Nautilus, the world's first nuclear submarine, visited Everett and Seattle. In Seattle, crewmen dressed in civilian clothing were sent in to secretly buy 140 quarts (160 liters) of an automotive product containing sodium silicate (originally identified as Stop Leak) to repair a leaking condenser system. The Nautilus was en route to the North Pole on a top secret mission to cross the North Pole submerged.^[33]

Firearm cartridges

A historical use of the adhesive properties of sodium silicates is the production of paper cartridges for black powder revolvers produced by Colt's Manufacturing Company during the period from 1851 until 1873, especially during the American Civil War. Sodium silicate was used to seal combustible nitrated paper together to form a conical paper cartridge to hold the black powder, as well as to cement the lead ball or conical bullet into the open end of the paper cartridge. Such sodium silicate cemented paper cartridges were inserted into the cylinders of revolvers, thereby speeding the reloading of cap-and-ball black powder revolvers. This use largely ended with the introduction of Colt revolvers employing brass-cased cartridges starting in 1873.^[34][35] Similarly, sodium silicate was also used to cement the top wad into brass shotgun shells, thereby eliminating any need for a crimp at the top of the brass shotgun shell to hold a shotgun shell together. Reloading brass shotgun shells was widely practiced by self-reliant American farmers during the 1870s, using the same waterglass material that was also used to preserve eggs. The cementing of the top wad on a shotgun shell consisted of applying from three to five drops of waterglass on the top wad to secure it to the brass hull. Brass hulls for shotgun shells were superseded by paper hulls starting around 1877. The newer paper-hulled shotgun shells used a roll crimp in place of a waterglass-cemented joint to hold the top wad in the shell. However, whereas brass shotshells with top wads cemented with waterglass could be reloaded nearly indefinitely (given powder, wad, and shot, of course), the paper hulls that replaced the brass hulls could be reloaded only a few times.

Food and medicine

Cosmetics

Sodium silicate and other silicates are the primary components in "instant" wrinkle remover creams, which temporarily tighten the skin to minimize the appearance of wrinkles & under-eye bags. These creams, when applied as a thin film and allowed to dry for a few minutes, can present dramatic results. This effect is not permanent, lasting from a few minutes up to a couple of hours. It works like water cement, once the muscle starts to move, it cracks and leaves white residues on the skin.

Food preservation

File:"Raise More Poultry...on Farms and Back Yards...More Eggs and Poultry Will save Beef and Pork." - NARA - 512571.tif Waterglass has been used as an egg preservative with large success, primarily when refrigeration is not available. Fresh-laid eggs are immersed in a solution of sodium silicate (waterglass). After being immersed in the solution they were removed and allowed to dry. A permanent air tight coating remains on the eggs. If they are then stored in appropriate environment, the majority of bacteria which would otherwise cause them to spoil are kept out and their moisture is kept in. According to the cited source, treated eggs can be kept fresh using this method for up to five months. When boiling eggs so preserved, the shell is no longer permeable to air, and the egg will tend to crack unless a hole in the shell is made (e.g. with a pin) in order to allow steam to escape.^[36]

Homebrewing

Sodium silicate flocculant properties are also used to clarify wine and beer by precipitating colloidal particles. As a clearing agent, though, sodium silicate is sometimes confused with isinglass which is prepared from collagen extracted from the dried swim bladders of sturgeon and other fishes. Eggs preserved in a bucket of waterglass gel, and their shells are sometimes also used (baked and crushed) to clear wine.^[37]

Aquaculture

Sodium silicate gel is also used as a substrate for algal growth in aquaculture hatcheries.^[38]

See also

• Precipitated silica
• Sodium carbonate
• Sodium stannate
• Sodium germanate

References

Further reading

• Ashford's Dictionary of Industrial Chemicals, third edition, 2011, page 8369.

External links

• Centre Européen d'Etudes des Silicates
• International Chemical Safety Card 1137
• ChemSub Online : Silicic acid, sodium salt
• ChemSub Online : Sodium metasilicate

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