Silica or silicon dioxide (SiO2)

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Silica or Silicon dioxide (SiO2) is a compound of silicon and oxygen and is a colorless or white crystal with no odor and taste. It is mostly found in soils and rocks and is the second element mostly found in the earth’s crust following oxygen. It is utilized in many areas, such as moisture absorbent, polishing agent, component of building materials and substance used to increase the strength of the products.

Physical and chemical properties
• Trade name : silicon dioxide
• Chemical name : silicon dioxide
• Common name : silica, quartz, free crystalline silica, silica flour, silica
• Molecular formula : SiO2
• Molecular weight : 60.1 g/mole
• Appearance : White
• Physical state : Solid
• Odor : Odorless
• Melting point : 1,713 °C
• Boiling point : > 2,200 °C
• Flammability : Non-flammable
• Flash point : Non-flammable
• Specific gravity : 2.2-2.6
• Solubility : soluble with an alkali and insoluble in all an acid, except hydrofluoric acid
• CAS No. : 7631-86-9

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Silica is a compound of silicon (Si) and oxygen (O) that are arranged in the form of endless three-dimensional mesh tetrahedral and the silicon acts as the central atom surrounded by four oxygen atoms becoming SiO4. Changes of silica will happen very slowly, but it will happen very fast when mixed with alkali, vanadate, chloride and borates. However, its physical properties will not change much.

Silica is a synthetic polymer with a long polymer chain. The number of links, the branches of the chain, and the types of compounds linking to the chain can be modified to make the product become liquid, gel or elastomer.

Crytal-based types
1. Nesosilicate (Si : O2 = 1 : 4)
2. Sorosilicate (Si : O2 = 2 : 7)
3. Cyclosilicate (Si : O2 = 1 : 3)
4. Inosilicate (Si : O2 = 4 : 11)
5. Phyllosilicate (Si : O2 = 2 : 5)
6. Tectosilicate (Si : O2 = 1 : 2)

Source-based types
1. Ground mineral silica is located in the mineral or sand levels and its size is less than 200 meshes. It is cheap filler used as a heat resistant compound that does not affect the stability or ripeness of rubbers. Silica in the earth’s crust is the soil composed of fine silica that is used as a heat resistant compound.

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2. Diatomaceous earth (Kieselguhr) is silica obtained from evaporation into silicon dioxide containing less than 2% of water. It can be prepared by burning the silicon compound to evaporate and condense at low temperatures into very small particles. Fume silica is an additive that helps in reinforcement that provides drawing, tearing and abrading resistances especially in silicone rubbers.
3. Fume silica (pyrogenic silica) is used as a retardant to make the longer stability time of rubbers, which needs to increase the amount of catalysts, such as glycols or amines. The forming process should use wax, polyethylene or other lubricants to avoid the binding with the rollers or the adhesion to the mold after the rubbers are ripe.
4. Hydrated precipitated silica contains 10-14% of water with a particle size of approximately 10-40 nm. It is used as an additive of reinforcement, which will provide higher drawing, tearing and abrading resistances and hardness. It is commonly used in the semi-transparent and colorless products. This type of silica will serve as a retardant for the stability of rubbers so a greater amount of catalysts is required.

Chemical properties
Chemical properties of silica are relatively stable at room temperature and it does not react with many chemicals, but its form can be changed. The amorphous silica is more reactive than crystalline silica because it has more surfaces.

Acidic solutions do not affect the silica, except hydrofluoric acid that can react with the silica to become H2SiF6. Silica in different forms has different reactions with hydrofluoric acid depending on its density. Namely, the high-density silica can have less reaction than those with low density. Silica additives used in many industries are synthesized from the components of silicon dioxide. Naturally, this silicon dioxide occurs with the crystal structure of geometry.

When evaporation is conducted by burning silicon dioxide to vaporize, there will be a condensation on the room temperature and the silica with very small particles will then be obtained, which is referred to “silica fume”. This silica fume is commonly used as a reinforcing agent in rubbers, which provides the rubbers with the drawing and tearing resistances. However, the disadvantage found when using silica fume in the mix of rubbers is the longer time of stability. Therefore, it is necessary to also add the accelerating agent. In addition, there is precipitated silica that contains higher water content as compared to the silica fume, which is commonly used as an additive in products that do not need colors or translucence.

Forms
1.Crystalline silica
This form of silica is found in the form of ore and its structural atoms are systematically and continuously arranged. The shape of this silica is definitely crystal under normal pressure and there are three types as follows:
• Quartz is the most common form and its stability temperature is less than 870 °C.
• Tridymite has a stability temperature of 870-1,470 °C.
• Cristobalite has a stability temperature of 1,470 °C and a melting temperature of 1,713 °C.

These three forms of crystals can be interchangeably inverted by heating or reducing the temperature. There are two styles of this inversion: (1) atoms within the new structure will be rearranged and (2) it is a quick inversion with a less change in its bond, but it can be quickly transformed into its original form. The α-quartz at 573 °C will be quickly transformed into β-quartz while quartz with high purity at 870 ° C can be converted into cristobalite, but if contamination is high, it can be converted into α-tridymite at 870 °C and is gradually changed into α-cristobalite at 1,470 °C, and if getting a high temperature of about 1,713 ° C, it will melt. During In the process of cooling down, tridymite and cristobalite will not be changed as quartz under normal condition, but they will be quickly transformed into γ-tridymite and then β-cristobalite as shown in the equation.

• α Tridymite ⇌ 117-163 °C ⇌ β Tridymite
• α Cristobalite ⇌ 220-260 °C ⇌ β Cristobalite
• α Quartz ⇌ 573 °C ⇌ β Quartz
• Quartz high ⇌ 870 °C ⇌ Tridymite high ⇌ 1,470 °C ⇌ Cristobalite high

All three forms of crystals occur in nature. Cristobalite that can be synthesized includes, for example, keatite, coesite, stishovite and silica.

Type of Silica
Crystal Density (g/cm3)
β-quartz trigonal 2.65
α-quartz hexagonal 2.53
γ-tridymite orthorhombic 2.26
β-tridymite hexagonal
α-tridymite hexagonal 2.22
β-cristobalite tetragonal 2.32
α-cristobalite cubic 2.2

2.Amorphous silica
This form of silica is biogenic silica and can be synthesized. It is an amorphous and non-crystal solid. Atoms within its structure are messily arranged and may be in the form of hydrate or anhydrate with several bonds, such as siloxane (-Si-O-Si-), silonol (-Si-OH-), silane (-Si-H), and organic silicon (-Si-OR or -Si-CR).

The synthesis can be achieved by heating into steam conditions. Precipitation of solutions can be classified into the following three types based on the products prepared:

1. Vitreous silica or silica glass is a non-porous solid and can be produced by melting amorphous silica crystals and then letting them cool.
2. Silica gel is a highly porous solid with an open pore structure and high surface area. It is divided into four types:
• Aquagel is a liquid gel containing a lot of water and can be prepared from a solution.
• Alcogel is a gel and can be prepared from alkoxysilane.
• Xerogel is a dry gel prepared by removing the water by evaporation. The structure is unchangeable from its original.
• Aerogel is a dry gel. Water or solvent is removed by a super critical extraction method. It can be prepared by drying at temperatures above the critical point.

3. Powder silica can be prepared by vaporization and precipitation of solution with a high dispersion of particles. When precipitated, this silica obtained will be very small with a high surface area.
• Aaquagenic can be prepared by precipitation in a solution or aquagel.
• Pyrogenic can be prepared by evaporation of silica using flames or arcs in the absence of oxygen and physical integration or chemical integration of silicon compound vapors from hydrogen flames and oxygen.
• Biogenic is a special type of aquagenic found in plants and diatoms.

Benefits
1. Silica
• Raw material as a component of building materials

2. Amorphous silica
• Component of catalysts
• Substance used to increase the strength and density of rubber products, plastics and
• Substance used to increase the adhesion of adhesive products
• Substance used to reduce the adhesion force between solids suspended in the liquid
• Substance used to increase the viscosity in many products, such as greases, inks, paints, drugs and cosmetics
• Emulsifier to make insoluble substances dissolve well, such as oil and water
• Anti-foaming substance
• Substance used to condition the surface into hydrophobic properties
• Substance used to increase the gloss
• Power dry
• Additive

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An important property of silica regarding the rubber reinforcement is its particle size. The silica with a small particle size provides high drawing, tearing and abrading resistances. The actual particle size per reinforcement unit is not the size of each particle, but it is the size of the adhesion as a group. Smaller particles will adhere to each other as a small group, while the larger particles will adhere to each other as a big group. The moisture or hydroxy bond of water is the secondly important property of silica, which will be in the form of density among silinol groups or free space for moisture.