Colloidal silica
Colloidal silica
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Colloidal silicas are suspensions of fine amorphous, nonporous, and typically spherical silica particles in a liquid phase. It may be produced by Stöber process from Tetraethyl orthosilicate (TEOS).
Properties
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Usually they are suspended in an aqueous phase that is stabilized electrostatically. Colloidal silicas exhibit particle densities in the range of 2.1 to 2.3 g/cm3.
Most colloidal silicas are prepared as monodisperse suspensions with particle sizes ranging from approximately 30 to 100 nm in diameter. Polydisperse suspensions can also be synthesized and have roughly the same limits in particle size. Smaller particles are difficult to stabilize while particles much greater than 150 nanometers are subject to sedimentation.
Manufacture
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Colloidal silicas are most often prepared in a multi-step process where an alkali-silicate solution is partially neutralized, leading to the formation of silica nuclei. The subunits of colloidal silica particles are typically in the range of 1 to 5 nm. Whether or not these subunits are joined depends on the conditions of polymerization. Initial acidification of a water-glass (sodium silicate) solution yields Si(OH)4.
If the pH is reduced below 7 or if salt is added, then the units tend to fuse together in chains. These products are often called silica gels. If the pH is kept slightly on the alkaline side of neutral, then the subunits stay separated, and they gradually grow. These products are often called precipitated silica or silica sols. Hydrogen ions from the surface of colloidal silica tend to dissociate in aqueous solution, yielding a high negative charge. Substitution of some of the Si atoms by Al is known increase the negative colloidal charge, especially when it is evaluated at pH below the neutral point. Because of the very small size, the surface area of colloidal silica is very high.
The colloidal suspension is stabilized by pH adjustment and then concentrated, usually by evaporation. The maximum concentration obtainable depends on the on particle size. For example, 50 nm particles can be concentrated to greater than 50 wt% solids while 10 nm particles can only be concentrated to approximately 30 wt% solids before the suspension becomes too unstable.
Applications
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See also
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References
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Colloidal Silica |
Colloidal Silica
Colloidal Silica = Colloidal silicic acid
Cas No: -86-9
Silica sol, also known as colloidal silicic acid, is a stable suspension of spherical silicon dioxide (SiO2) nanoparticles in a liquid, that are hydroxylated on the surface.
Colloidal silicic acid is found in almost all industrial sectors.
The applications range from surface treatment in the paper industry, to use as a polishing agent in the electronics industry and use as an additive for varnishes, coatings and paints to improve weather and abrasion resistance.
It is also a common additive in cosmetics and in the food industry.
The mean particle size and distribution width define the field of application of the SiO2 particles.
Typical sizes range from 1 nm to 100 nm.
Colloidal silicas are suspensions of fine amorphous, nonporous, and typically spherical silica particles in a liquid phase.
Colloids are defined as a stable suspension of microscopic particles or molecules distributed throughout a second substance known as a dispersion medium.
They differ from other types of suspensions in that the colloid is evenly dispersed throughout the suspension, and does not separate or settle.
Colloids may be any combination of liquid, solid, and gaseous colloids and dispersion media.
Colloidal Silica Uses and Applications
Applications that use colloidal silica vary widely.
For more than one century, colloidal silica has been applied in many industries, eg as binder for inorganic paint, stiffener for hard coating reagent and especially as brasive particles for chemical mechanical polishing slurries.
It is also used in production of paper.
Colloidal Silica can be used to enhance or direct the movement of substances within various processes.
For example, Colloidal Silica is used in the paper manufacturing process to draw liquid from the finished paper quickly, thereby allowing the paper to dry faster while retaining its strengthening starch.
Similarly, colloidal silica can be used to absorb moisture in industrial settings where moisture levels are high.
Depending on the size of its constituent particles, colloidal silica may be used to enhance the movement of materials or to increase surface friction.
ATAMAN CHEMICALS Colloidal Silica
Colloidal silica is used in a broad range of industries and applications, including:
Densification of concrete, cement, and other materials
Fine retention in paper manufacturing
Enhanced bonding of waterborne adhesives
Improved surface friction and anti-slip properties
Wastewater filtration flocculant
Investment casting binding
Anti-soilant textile coatings
Anti-blocking aid for films
Scratch resistant surface coatings
Anionic coagulant
Ceramic fiber binder and rigidizing agent
Catalyst attrition resistance
Abrasive polishing agent
Strength-enhancing additive to plastics, mortar, and concrete
Colloidal silica is extensively used as a rheological additive in personal care products to control flowability.
Colloidal silica forms strong ionic and hydrogen bonds with granular and fibrous material.
These bonds are thermally stable and chemically inert making them especially useful for:
Investment Casting Slurries
Vacuum Formed Refractories
Specialty Coatings
Catalysts
Insulation Board
Surface Modification Application
The silica particles in Colloidal silica adhere well to many surfaces via ionic and hydrogen bonding.
Once applied to a surface the silica particles provide an increased coefficient of friction that is useful for:
Paper & cardboard
Flooring
Polishing Slurries
Industrial and architectural coatings
Colloidal silica is available in grades with anionic sol charge as well as cationic sol charge.
When added to solutions Colloidal silica particles bond ionically with particles of the opposite charge and fall out of suspension.
This makes Colloidal silica an excellent flocculant for:
Paper retention and drainage
Beverage fining (beer and wine)
Insulation board
Properties
Colloidal silica consists of silica molecules suspended in liquid, thereby forming a liquid sol.
The process of creating colloidal silica is closely monitored to ensure that the silica molecules remain stable and separate within the liquid medium without collapsing into smaller component molecules or collecting into unstable silica gels.
The liquid dispersion medium exhibits greater density than water and must be electrostatically treated for enhanced ionic stabilization.
Colloidal silica is highly fluid with low viscosity.
Uses for colloidal silica vary depending on the size of the silica particles in the solution and the modifiable pH, ionization, and surface charge.
Usually they are suspended in an aqueous phase that is stabilized electrostatically.
Colloidal silicas exhibit particle densities in the range of 2.1 to 2.3 g/cm3.
Most colloidal silicas are prepared as monodisperse suspensions with particle sizes ranging from approximately 30 to 100 nm in diameter.
Polydisperse suspensions can also be synthesized and have roughly the same limits in particle size.
Smaller particles are difficult to stabilize while particles much greater than 150 nanometers are subject to sedimentation.
Selecting a Colloidal Silica
Selection of the appropriate grade of colloidal silica is heavily dependent on the end use application and the functionality you want the silica to achieve.
In general, colloidal silicas with a smaller particle size perform better as binders, while grades with higher particle size are more effective surface modifiers.
Other important factors to consider are the pH and ionic character of the end formulation as well the storage stability required
FUNCTIONS of Colloidal Silica:
Anti-Corrosion
Binder / Reinforcing Agent
Polishing Agent / Abrasive
Surface Modification
Adhesion Promotion
Flocculation / Retention Aid
Manufacture
Colloidal silicas are most often prepared in a multi-step process where an alkali-silicate solution is partially neutralized, leading to the formation of silica nuclei.
The subunits of colloidal silica particles are typically in the range of 1 to 5 nm.
Whether or not these subunits are joined together depends on the conditions of polymerization.
Initial acidification of a water-glass (sodium silicate) solution yields Si(OH)4.
If the pH is reduced below 7 or if salt is added, then the units tend to fuse together in chains.
These products are often called silica gels.
If the pH is kept slightly on the alkaline side of neutral, then the subunits stay separated, and they gradually grow.
These products are often called precipitated silica or silica sols.
Hydrogen ions from the surface of colloidal silica tend to dissociate in aqueous solution, yielding a high negative charge.
Substitution of some of the Si atoms by Al is known increase the negative colloidal charge, especially when it is evaluated at pH below the neutral point.
Because of the very small size, the surface area of colloidal silica is very high.
The colloidal suspension is stabilized by pH adjustment and then concentrated, usually by evaporation.
The maximum concentration obtainable depends on the on particle size.
For example, 50 nm particles can be concentrated to greater than 50 wt% solids while 10 nm particles can only be concentrated to approximately 30 wt% solids before the suspension becomes too unstable.
How does colloidal silica differ from fumed, fused, or precipitated silica?
Colloidal silica varies from other types of silica in several significant ways.
The most noticeable difference is that it's in liquid form, as opposed to powder.
In addition, it has the widest ranging surface area, and its aggregate size can be as small as the actual size of the primary particle.
What's the difference between sodium silicate (water glass) and colloidal silica?
Colloidal silica consists of dense, amorphous particles of SiO2.The building blocks of these particles are randomly-distributed [SiO4]-tetrahedra.
This random distribution is what makes amorphous silica different from crystalline silica - ordered on a molecular level.
Sodium silicates are alkaline solutions with pH ranges of 12-13, compared to 9-11 for colloidal silica.
Sodium silicates are also composed of silicate monomers, as opposed to colloidal silica composed of polymeric silicates.
The composition of sodium silicates have a SiO2/Na2O ratio of approximately 3.4, whereas colloidal silica generally has a SiO2/Na2O ratio greater than 50.
Finally, the viscosity of sodium silicates is much higher - closer to that of a syrup, while colloidal silicas have viscosities close to that of water.
Where can colloidal silica be used?
Collodial silica can be used in numerous applications and it enhances functionality in an ever-growing number of products.
To give a couple of examples our products enhances the performance of waterborne coatings by delivering anti-soling properties as well as provides increased durability and strength in cementing operations.
Choosing the right colloidal silica can be a challenge.
Subtle differences in particle morphology, particle size, and ionic species can make all the difference.
Colloidal silica dispersions are fluid, low viscosity dispersions.
There are many grades of colloidal silica, but all of them are composed of silica particles ranging in size from about 2 nm up to about 150 nm
The particles may be spherical or slightly irregular in shape, and may be present as discrete particles or slightly structured aggregates.
They may also be present in a narrow or wide particle size range, depending on the process in which they were created.
The maximum weight fraction of silica in the dispersion is limited based on the average particle size.
Dispersions with a smaller average diameters have larger overall specific surface areas and are limited to low concentration dispersions.
Conversely, dispersions with larger average diameters have lower overall specific surface areas and are available in more concentrated dispersions.
The appearance of colloidal silica dispersion depends greatly on the particle size.
Dispersions with small silica particles (< 10 nm) are normally quite clear.
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Midsize dispersions (10-20 nm) start to take on an opalescent appearance as more light is scattered.
Dispersions containing large colloidal silica particles (> 50 nm) are normally white.
Standard colloidal silica dispersions are stable against gelling and settling in pH range of 8 - 10.5.
These colloidal silicas are charge stabilized with an alkali (normally alkalis of sodium, potassium, or lithium) or stabilized with ammonia.
Under these conditions, the particles are negatively charged.
The dispersion can be destabilized through the addition of excessive electrolytic species (sodium, calcium, chloride, lithium, potassium).
These colloidal silica particles can achieve additional anionic charge stability when as aluminosilicate sites are formed by incorporation of aluminum into the surface layer of the silica particles.
Low pH versions of colloidal silica are also available by the adsorption of cationic aluminum oxide onto the surface of the particles.
This results in a cationic particle that is stabilized with anionic species - commonly this is chloride.
These dispersions are stable below a pH of 4.
Low pH grades can also be obtained by completely deionizing the dispersion.
These grades do not require the presence of stabilizing ions and are also stable below a pH of 3.
Dispersion stability can also be enhanced with surface modification to incorporate silanes.
The silanol groups can be isolated silanol groups or even geminal (silanediol groups) or vicinal types.
Not only do these silanes provide reactive sites for the grafting of other chemicals, but they provide enhanced stability by physically preventing the formation of siloxane bridges that can result in the formation of aggregates or gel structures.
Particle size and pH are what differ most between the grades of colloidal silica.
Particle size can also be expressed in terms of specific surface area, i.e. the higher the specific surface area, the smaller the average particle size.
The average particle size also affects the maximum possible SiO2 content (i.e. small particles are only only stable in dilute sols while larger particles are stable at higher concentrations).
The pure silica sols are anionic and are typically sodium- or ammonium-stabilized to a pH of 9-11.
Through modification using Sodium Aluminate, however, the sols are stable down to a pH of 3-4.
Cationic silica sols are stable at pH 4-5, and deionized sols are stable at a low pH, typically 2-3.
Applications of Colloidal silica
In papermaking colloidal silica is used as a drainage aid. Colloidal silica increases the amount of cationic starch that can be retained in the paper.
Cationic stach is added as sizing agent to increase the dry strength of the paper.
High temperature binders
Investment casting - used in moulds
An abrasive - for polishing silicon wafers
Carbonless paper
Catalysts
Moisture Absorbent
It increase the bulk & taped density of powder & granules also
Colloidal silica is also be used in Lubrication of Tablet
Stabilizing and rigidizing refractory ceramic fiber blankets
Abrasion resistant coatings
Increasing friction - used to coat waxed floors, textile fibers and railway tracks to promote traction
Antisoiling &#; fills micropores to prevent take up of dirt and other particles into textiles
Surfactant &#; used for flocculating, coagulating, dispersing, stabilising etc.
Liquid silicon dioxide (colloidal silica) is used as a wine and juice fining agent.
Absorbent
Colloidal silica is used in concrete densifiers and polished concrete.
In manufacturing Quantum dots, small semi-conductors used in various scientific research settings.
Binders in ceramic compounds for high temperature applications
Clarification of wine, beer and fruit juice concentrate
Polishing agents in wafer and memory-chip production
A component of silicate-based paints and plasters
Retention aids in papermaking
Colloidal Silica IMPROVES PERFORMANCE OF TONER FOR LASER PRINTER
With our Colloidal Silica portfolio we can provide multiple solutions to enhance the performance of the formulations of Toner manufacturers.
Colloidal Silica in powder form with its uniform particle shape and very narrow particle size distribution can be used to improve the performance of toner.
This results in enhanced durability and high performance of the toner.
With the right surface modification, the Colloidal Silica particles can act as a spacer - we are able to customize our Colloidal Silica particles based on your formulation.
In addition, Colloidal Silica particles offer multiple possibilities to improve the performance of the Toner like improving the cohesion, charge stability and free flow of the toner particles.
OUR Colloidal Silica GRADES OFFER THE FOLLOWING ADVANTAGES:
Cohesion
Tribocharge/ Charge stability
Free-flow
Spacing
Colloidal Silica IMPROVES PERFORMANCE OF SPECIALTY COATINGS
Colloidal Silica can be used in a variety of coating applications to improve coating properties.
For example adding Colloidal Silica in certain coatings can improve properties such as binding, hardness, anti-blocking and scratch resistance.
Colloidal Silica nanoparticles are spherical, available in different particle sizes and their surface can be functionalized.
Moreover, low refractive index values can be achieved with Colloidal Silica used in antireflective coatings.
Antireflective coatings help to reduce reflections on optical lenses and other optical surfaces.
Through our diverse product portfolio, we can offer aqueous dispersions as well as dispersions in other solvents.
Colloidal Silica particle surface can be modified with hydrophobic and hydrophilic ligands to get the desired properties.
OUR Colloidal Silica GRADES OFFER THE FOLLOWING ADVANTAGES IN SPECIALTY COATINGS:
Very narrow particle size distribution, no fines or large particles
Particle hardness for better abrasion resistance
Customized surface modification possible to achieve best performance
Dispersions in water and a variety of solvents available
High temperature resistance and UV stability
MEMBRANES
Colloidal Silica can improve the performance of the membrane by enhancing properties such as thermal and mechanical stability as well as selectivity.
Membrane technology is accepted as an effective separation process for many industrial applications.
Membranes are available in a wide range of materials (Ceramic, Polyether Sulfone (PES), among others) and pore sizes (from micro to nano).
Depending on the application, different types of membranes offer advantages, but also have drawbacks, e.g. fouling.
OUR COLLOIDAL SILICA PARTICLES CAN HELP TO IMPROVE THE PERFORMANCE OF YOUR MEMBRANE IN SEVERAL WAYS:
Increased mechanical and thermal stability by incorporating Colloidal Silica to the membrane matrix
Decreased fouling and increased membrane wettability and durability by making the membrane surface more hydrophilic through modification with COLLOIDAL SILICA
Smoother surfaces and narrow pore size distribution
Selective metal ion absorption through assistance with Colloidal Silica
Catalyst support on membrane surfaces i.e. membrane reactor
Surface properties of our COLLOIDAL SILICA particles can be custom designed to ensure material compatibility of your specific application.
Colloidal Silica is the most popular binder used in the precision investment casting industry today. It offers the investment caster a safe, economical, easy to use slurry component that performs well as either primary or backup slurry.
Colloidal Silica systems are very stable; able to form a long life ceramic slurry with a large range of refractory materials due to the binder&#;s chemical inertness. This versatility allows Colloidal Silica to form the basis of ceramic shells used for the casting of a large range of metal alloys.
Ceramic shells formed with colloidal silica binder&#;s offer several advantages for the investment process. The exceptionally strong bonds formed by the colloid enables ceramic shells to have a superior green and fired strength. Benefiting the investment caster with:
An increased maximum pour weight
Reduced material usage in the back up coat
Better handling
Furthermore, use of colloidal silica binders has allowed investment casters a greater freedom of design with an increased level of intricacy to castings.
Ceramic shells using colloidal silica binders also display excellent refractory properties, thermal resistant to temperatures as high as &#;&#; , with good resistance to thermal shock and crucially little shrinkage.
Since colloidal silica products consist of amorphous silica and water, they rank as one of the most environmentally-friendly, industrial chemical products.
Does it pose any particular health hazards?
Colloidal silica products are aqueous dispersions of amorphous silica.
Colloidal silica is not classified as harmful, but as mildly irritating.
Because the products can have a drying effect on the skin, protective gloves should always be used. In case of skin contact, wash the area of contact with plenty of water.
The use of safety glasses is always recommended.
In case of eye contact, rinse with large amounts of water and seek professional medical advice.
For further information, please reference the Safety Data Sheets for each product.
Mini-Encyclopedia of Papermaking Wet-End Chemistry
Additives and Ingredients, their Composition, Functions, Strategies for Use
COLLOIDAL SILICA
Composition: Despite the fact that colloidal silica has the same chemical formula as quartz sand, SiO2, the two materials could hardly be more different in their effect on paper machine operations.
The key difference is size.
The subunits of colloidal silica particles are typically in the range of 1 to 5 nm.
Whether or not these subunits are joined together depends on the conditions of polymerization.
Initial acidification of a water-glass (sodium silicate) solution yields Si(OH)4.
If the pH is reduced below 7 or if salt is added, then the units tend to fuse together in chains.
These products are often called "silica gels." If the pH is kept slightly on the alkaline side of neutral, then the subunits stay separated, and they gradually grow.
These products are often called silica sols.
Hydrogen ions from the surface of colloidal silica tend to dissociate in aqueous solution, yielding a high negative charge.
Substitution of some of the Si atoms by Al is known increase the negative colloidal charge, especially when it is evaluated at pH below the neutral point.
Because of the very small size, the surface area of colloidal silica is very high.
Functions: Key part of drainage-aid programs marketed by Eka Chemicals and Nalco; also capable of increasing the amount of cationic starch that can be retained as a dry-strength agent.
Strategies for Use: The conventional procedure is to add the colloidal silica very late in the approach flow to a paper machine, typically just after a set of pressure screens.
A drainage rate increase is expected only if the furnish already has been treated with a suitable high-mass cationic polymer such as cationic starch or cationic poly-acrylamide. The effect is most pronounced when the net amount of cationic additives is enough to render the system at least slightly cationic before the addition of the micro-particle.
For this reason it can be helpful to treat highly anionic furnish with a highly cationic material such as alum, poly-aluminum chloride (PAC), polyamine, or polyethyleneimine (PEI). In addition to scavenging excess anionic colloidal charge, such additives are expected also to make the molecules of the subsequently added cationic starch or cationic PAM adsorb with more loops and tails extending into solution.
The function of the colloidal silica appears to involve
(a) release of water from polyelectrolyte bridges, causing them to contract, and
(b) acting as a link in bridges that involve macromolecules adsorbed on different fibers or fine particles.
These effects create more streamlined paths for water to flow around the fibers.
The tendency of microparticles to boost first-pass retention also will tend to have a positive effect on initial dewatering rates.
It has been reported that paper produced by means of a microparticle retention and drainage program has a more open, porous structure, though the effect may become obscured by subsequent wet-pressing and calendering operations. Papermakers often are able to "trade away" chemical-induced drainage improvements in favor of improving formation uniformity.
This is possible by either
(a) decreasing the headbox solids by increasing the amount of white water recirculated,
(b) increasing the proportion of hardwood fiber relative to softwood, or (c) increased refining.
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