Basic know-how about silica
1. What exactly is silica?
Silica is another name for silicon oxides - the most prevalent type being SiO2. It can be found in nature in crystalline form (as quartz sand), and it is the most abundant component of the earth's crust. Amorphous silica, on the other hand, is industrially manufactured in a variety of forms - including silica gels, precipitated silica, fumed silica, and colloidal silica.
2. What exactly does "colloidal" mean, anyway?
A colloid is a stable dispersion of particles - particles that are small enough that gravity doesn't cause them to settle, but large enough not to pass through a membrane and allow other molecules and ions to pass freely. Particle sizes range from about 1 to 100 nm.
3. 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.
4. 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 viscocities close to that of water.
5. How do you produce silica sols?
We use several different methods to produce the aqueous colloidal silica that meets our consistency standards. The general principle is to remove sodium from sodium silicate via cation exchange. Without the sodium, polymerization takes place and particles begin to grow. After growth, the sol is stabilized and concentrated to the desired content. For large particle sols, we start by building on smaller particles like a seed.
6. Can you explain surface charge and surface modification?
The majority of colloidal silica grades are anionic colloidal silicas. Their surface is composed mostly of hydroxyl groups with the formula of Si-O-H. However, other groups have also been identified including: silandiol, -Si-(OH)2; silanetriol, -Si(OH)3; surface siloxanes, -Si-O-Si-O-; and surface-bound water. This yields an anionic surface charge at alkaline pH and the particles are stabilized by cationic species such as sodium or ammonium.
Anionic colloidal silicas can be further stabilized by the incorporation of aluminum into the surface of the particle leading to the formation of -Al-OH- -groups. This results in very highly negatively charged surfaces even at a pH of 3. This increases the stability of the dispersion greatly!
In the case of cationic sols, the surface has coatined with of Al2O3. This reverses the charge of the surface of the particle to be positive, and the counter-ion is normally chloride. These sols are stable only below a pH of 4.
Surface modification with silanes reduces the surface charge of the particles, but the steric stabilization phenomenon allows these sols to be stable from pH 2-11.
Properties of colloidal silica
7. What are the differences between the grades of colloidal silica
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.
8. What about temperature change? Does it affect colloidal silica?
Sols should be stored at 5-35 ºC (40-95 ºF). If the sol is subjected to freezing conditions, it can lose its stability and precipitate. Highly elevated temperatures may accelerate the growth of micro-organisms and/or decrease the long-term stability of the silica sol.
9. How stable are colloidal silica dispersions?
The stability of a colloidal silica has many factors: pH, particle size, silica concentration and storage temperature.
The pH ranges listed in the section describing surface charge are very important to the stability of the sol. If the pH is adjusted outside of its "stable" range, the dispersion charge can destabilize and allow the particles to crosslink and gel.
Particle size and the resultant surface area are very important factors in dispersion stability. For very small particle dispersions, there is a phenomena called "Ostwald ripening." The surface area will slowly decrease as the dispersion attempts to spontaneously lower its overall charge. The larger the particles, the smaller this affect - and hence more stable the product.
Silica concentration is also very important factor in stability. The more concentrated a sol, the more likely the particles will be forced together and allowed to aggregate.
10. Can colloidal silica be mixed with other chemicals?
Positive counter ions balancing the negative surface charge are diffusely oriented around the particle. The negative potential is therefore declining by distance from the particle, and the repelling forces between particles extend for some distance out from the particle surface. As salt is added, the counter ions move much closer to the particle surface, which reduces the distance through which the repelling forces act. This causes a large reduction in sol stability by increasing the probability of inter-particle collision. As a rule, polyvalent cations are more effective in shrinking the diffuse layer making them more effective gelling agents for colloidal silica.
Alcohol, acetone and other polarized solvents can be mixed with colloidal silica. However, there is a limit to how much solvent can be mixed into the silica sol - gelling may occur. Compatibility is enhanced by increasing the dielectric constant of the solvent, decreasing the pH of the colloidal silica, decreasing the silica concentration, and to some extent increasing the particle size.
Emulsion resin and water-soluble resin
When they have the same pH and surface charge as the resin, colloidal silicas are compatible with both emulsion resin and water-soluble resin. However, in the case of emulsion resin, the emulsifier should be carefully selected, as it may cause gelling or separation.
Generally speaking, the anionic colloidal silicas are compatible with anionic and non-ionic surfactants, whereas the cationic colloidal silicas are compatible with cationic and non-ionic surfactants. In both cases, the colloidal may become incompatible with the surface-active agent - depending on the surfactant's composition and impurities. This needs to be taken into account when selecting surface-active agents.
How to measure the product properties
11. How do I measure specific surface area?
Particle surface area can be determined through titration.
12. How do I know that I'm using the particle size I ordered?
All of our products are shipped with a certificate of analysis if requested detailing the specific surface area. Particle size can be estimated through back-calculation from surface area.
13. Is there an easy way to check whether the sol is still OK?
Colloidal silica does not have an infinite shelf life. While some of our particles have stated shelf lives of over a year, only your application will determine if the particles are still functional. Only experience and testing can determine if the silica sol in question will function properly in your application.
Practical advice about handling silica
14. Does shelf life influence the properties of colloidal silica?
Stability is what generally determines the shelf life of a sol. Checking the stability simply involves measuring the viscosity and specific surface area.
15. What kind of packaging is available?
Our colloidal silica is not only available in bulk deliveries, but also in plastic drums and IBCs. Please contact your local sales office office for packaging options for your region.
16. How does colloidal silica react with steel, plastic and other materials?
Colloidal silica does not react with stainless steel or plastic materials. The use of mild steel or iron is discouraged because the iron will discolor the product and destabilize the dispersion.
17. What types of pumps can I use with colloidal silica?
You can choose from a variety of pumps:
Piston membrane pumps, preferably with a pressure equalizer
Excenter screw pumps
It is recommended that all pumps are cleaned with water, and dried before and after use.
18. What about cleaning colloidal silica?
The simplest way to clean pipes, valves, pumps or spills is to rinse thoroughly with water right after use before the silica is allowed to dry. To clean tanks and containers that have accumulated solid deposits, you should first empty the tank and flush it with water. Next, inspect the walls and bottom of the tank and remove any solids with a high-pressure hose.
A second alternative is using caustic soda. If caustic is compatible with the tank and its contents, mix a solution of 4-5% caustic soda (NaOH) with an agitator and/or circulate the tank. This type of cleaning usually takes between 2-5 hours. For better results, heat the caustic soda to 50-60 ºC. If it isn't possible to use caustic soda due to restrictions and/or danger, follow the manual method to clean the tank/container. CAUTION: Caustic soda is extremely powerful; always protect yourself by wearing a full mask, helmet, rubber boots and goggles.
Safety and the environment
19. Is colloidal silica dangerous to the environment?
Since colloidal silica products consist of amorphous silica and water, they rank as one of the most environmentally-friendly, industrial chemical products. Colloidal silica does not usually present a problem, however, check with state and other relevant authorities.
20. Does it pose any particular health hazards?
All our silica sols are composed of amorphous silica which, unlike crystalline silica, has generated no confirmed cases of silicosis to date. As with all materials, you should review the corresponding Material Safety Data Sheet for health, safety and environmental information for each product. Because some products are slightly alkaline, you should always wear safety goggles and appropriate PPE when handling them. If you get colloidal silica in your eyes, rinse immediately with water. If problems persist, seek medical attention.
21. What exactly is colloidal silica used for?
Our customers use colloidal silica for a multitude of different purposes. You'll find a number of examples on this web page.
22. Can colloidal silica be modified?
Of course! Colloidal silicas can be modified to several configurations including but not limited to: adjustments to pH, stabilization ions, surface charge and surface modification. Contact your local sales office for more information.
23. Do you offer data or information on colloidal silica in my application area?
Technical information sheets have been developed for a variety of different applications. Contact your local sales office for more information.
24. Why should I use your colloidal silica?
To begin with, we are the worlds largest producer of colloidal silica. We have the know-how and capacity to provide you with world class colloidal silica products around the world! With our international productions locations, we can ensure a reliable supply of colloidal silica at competitive competitive prices - locally. Most importantly, no matter which of our facilities you use, we offer standard products of consistent quality. You can focus on your results, leave the silica supply to us.