Hydrophobic silica

SiO2 surface modified by chemically bonded hydrophobic groups

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Hydrophobic silica is a form of silicon dioxide (commonly known as silica) that has hydrophobic groups chemically bonded to the surface. The hydrophobic groups are normally alkyl or polydimethylsiloxane chains. Hydrophobic silica can be processed in different ways; such as fumed silica, precipitated silica, and aerosol assisted self assembly, all existing in the form of nanoparticles.

Structure

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Hydrophobic silica has an orthorhombic crystal structure (its space group name is Pmna under the bipyramidal point group).[1] Orthorhombic structures are the product of stretching a cubic lattice along two of its orthogonal pairs, resulting in a rectangular prism shaped crystal structure.

Properties

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Hydrophobic silica displays water resistant properties because of its nanostructure and chemical properties. When applied to a surface of a material, the nanoparticles adhere to the host material and prevent liquids from permeating the rough texture. The water only comes into contact with the tips of the nanoparticles coating the outside of the material. Due to lack of attraction, the water is repelled from the hydrophobic silica.[3]

Processing

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Initially, silica is hydrophilic due to the presence of the silanol (Si-OH) groups on the surface of the particle. These silanol groups can chemically react with various reagents to render the silica hydrophobic. There are many different methods of processing silica to become hydrophobic, mainly by adding hydrocarbon groups.

Hydrophobic fumed silica

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Fumed silica can react with chlorosilanes in a fluidized bed reactor at 400 °C[4]

Hydrophobic precipitated silica

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Precipitated silica can be hydrophobized with e.g. alkylchlorosilanes or trimethylsilanol in the precipitated solution. The hydrophobised silica is filtered, washed, dried, and tempered to 300&#;400 °C to finish the reaction.[5]

Hydrophobic plasma polymer coated silica

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Silica particles can become hydrophobic through plasma polymerization. In this process, plasma polymerized 1,7-octadiene (ppOD) (related to the diene hydrocarbons) is used to deposit polymer films onto the silica particles. The ppOD films are deposited through the use of radio frequencies, along with a reactor containing a rotating chamber. Using low specific energy plasma conditions, the ppOD films chemically render the silica particles hydrophobic.[6]

By using the ppOD films, the hydrophilic polar Si-OH groups in the polymer itself are concealed by non-polar CxHy hydrocarbon groups, so when it's applied as a film to the silica particles, they become hydrophobic as well.[7]

Aerosol assisted self-assembly

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The goal of this process is to quickly and continuously create nanostructured particles deriving from a colloid precursor containing a solvent and silica particles. Aerosol assisted self assembly is a one step process with a high production rate. The process takes a few seconds in terms of reaction time, and there's no requirement for heating and chemically treating the particles after development.

The first part of the process is to create the colloid precursor which consists of the silica nanoparticles and the solvent. The initial silica nanoparticles are in an amorphous crystalline phase and the solvent is composed of trimethylsilyl chloride (TMCS) and ethyl alcohol. To synthesize hydrophobic nanostructured silica using this method, the colloid precursor containing the solvent and silica particles is sprayed by an aerosol generator. The droplets are then transported by a carrier gas to a furnace where they are heated. Upon entry into the furnace, the ethyl alcohol evaporates from the colloid precursor, allowing self-assembly to occur between the silica particles and the surface treating agent, TMCS.

The results of this process causes the silica particles to group together to combine into spherical nanostructured particles. By grouping these silica nanoparticles into a nanostructured particle, a certain percentage of porosity develops within the nanostructure related to the amount of TMCS concentration. Increasing the amount of TMCS concentration reduces the specific surface area of the silica nanostructured particles. The exhibited hydrophobicity is a result of the chemical reaction occurring between the silica particles and the TMCS. When the original SiO2-OH groups are replaced with hydrolytically stable Si(CH3) groups, this hydrophobicity occurs due to the prevention of the silica particles from interacting with water.[8]

Applications

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Hydrophobic silica is used to solve technical problems in a number of products including, but not limited to, paints, inks, adhesives, plastics, coatings, toners, defoamers, silicone rubber, sealants, cosmetics, food additives, polyester resins, cable gels, and greases. It's often manufactured as both single and multiphase composites in order to enhance properties such as dispersion, stability behavior, resistance to water, and functionality. Hydrophobic silica has been added to aerogels to increase their ability to adsorb organic pollutants without disrupting the network structure.[9]

Treated aggregated fumed silica

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Overview of DIN Standard Hydraulic Fittings

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Hydrophobic silica can be used to treat other surfaces to become hydrophobic, this is due to the morphology of the silica particles once they adhere to their host. The silica particles then alter the surface of its host material resulting in a hydrophobic surface.

Aggregated fumed silica can be applied to large surfaces to render them hydrophobic. Micro and nanoscale structures, resembling ball and block like forms, are attributed to the hydrophobic characteristics. Due to the change in the original surface's texture, the roughness of the surface causes its hydrophobicity to increase. This is because when water comes into contact with the rough surface, it only touches the tips of the rough texture and doesn't permeate any deeper through the rest of the air occupied structure. The water can't spread through the surface, thus yielding hydrophobic properties.[3]

Additional applications

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• Consumer goods
• Rheology control
• Suspension and stability behavior
• Mechanical/optical properties modification

References

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Fumed silica is available in both hydrophilic and hydrophobic forms. However, its hydrophilic nature makes it unsuitable for certain applications. Untreated fumed silica has many silanol groups on its surface, which absorb moisture and create a strong hydrogen bond network when added to a system. This bond network results in high viscosity and affects the stability of thickening and thixotropic properties. Polar solvents can easily break this bond network, causing instability in the system.

To overcome these issues, engineers treat fumed silica to convert its surface from hydrophilic to hydrophobic. Treating agents react with the surface silanol groups and form a layer of organic molecules, resulting in hydrophobic fumed silica. Hydrophobic fumed silica is beneficial in different product systems due to its good dispersing properties and ability to react with the product system through its silanol groups.

The Importance of Hydrophobicity in Fumed Silica

Hydrophobicity is a desirable property in fumed silica due to its many advantages in different applications. When treated to be hydrophobic, Its excellent dispersing properties, improved rheological properties, and moisture resistance make it a valuable material in various industrial and commercial applications.

Hydrophobic fumed silica has excellent dispersing properties, which enable it to form stable suspensions in water-based systems. It repels water, which reduces the tendency of the silica particles to clump together, resulting in a more uniform dispersion. This makes hydrophobic fumed silica useful in applications where uniformity of the suspension is important, such as in coatings and inks.

Hydrophobic fumed silica has improved rheological properties compared to hydrophilic fumed silica. The water-repellent nature of hydrophobic fumed silica makes it useful as a thickening and gelling agent in water-based systems, resulting in stable and consistent viscosity. This makes hydrophobic fumed silica ideal for use in adhesives, sealants, and other applications where controlled viscosity is critical.

Hydrophobic fumed silica has improved moisture resistance compared to hydrophilic fumed silica. The water-repellent nature of hydrophobic fumed silica makes it ideal for use in applications where moisture resistance is essential, such as in electronic coatings and sealants.

SystemFunctionRecommend DosageHydrophobic Fumed Silica SuggestHydrophilic Fumed Silica SuggestPolyurethane Sealantssag-resistance, thixotropic, extrusion efficiency, stability( moisture control)3%-7%HB-139
HB-151HL-200Acrylic Sealantssag-resistance, settling-resistance, rheology control, thixotropic, extrusion efficiency3%-7%HB-151HL-200Organic siliconesag-resistance, reinforcement, rheology control, thixotropic, extrusion efficiency, stability( moisture control)5%-15%HB-132
HB-630
HB-620
HB-612
HB-615HL-150
HL-200MS/SMP/SPU sealantsreinforce,extrusion efficiency,stability( moisture control)5%-10%HB-139
HB-151/butyl rubber-based sealantssag-resistance, reinforce3%-7%HB-151HL-200Polysulfide sealantReinforce, rheology control, thixotropic,3%-7%HB-151
HB-139HL-200Application In Sealants(HIFULL® Product Advisor)

Improved Flow Properties: Hydrophobic fumed silica has excellent flow properties, making it useful in the manufacture of high-quality coatings, such as automotive paints.

Cost-Effective: Hydrophobic fumed silica is cost-effective and can be used in small quantities to achieve desired properties. This makes it a popular choice in various industries, such as coatings, adhesives, and sealants.

Applications of Hydrophobic Fumed Silica

Hydrophobic fumed silica is a versatile material that finds its application in various industries due to its unique properties. It is a water-repellent material that has excellent dispersing properties, improved rheological properties, and moisture resistance.

As a result, it is widely used in applications such as inks, paints, adhesives, and sealants, cosmetics, pharmaceuticals, food and beverages, plastics and composites, energy, and rubber.

1. Inks: Hydrophobic fumed silica is used as a pigment stabilizer in ink formulations. It helps to prevent settling and improves the flow properties of the ink.
2. Paints and coatings: Hydrophobic fumed silica is used as a thickening and gelling agent in paint and coating formulations. It improves the viscosity, stability, and flow properties of the paint, resulting in high-quality coatings.
3. Adhesives and sealants: Hydrophobic fumed silica is used as a thickening agent in adhesive and sealant formulations. It provides high shear strength and excellent thixotropic properties, making it ideal for use in construction and automotive applications.
4. Cosmetics: Hydrophobic fumed silica is used in cosmetic formulations as a texture enhancer, a matting agent, and a viscosity control agent. It helps to improve the flow properties, stability, and texture of the cosmetic product.
5. Pharmaceuticals: Hydrophobic fumed silica is used as a flow aid and a disintegrant in pharmaceutical formulations. It improves the flow properties of the powder and helps to facilitate the dissolution of the active ingredient.
6. Food and Beverages: Hydrophobic fumed silica is used as an anti-caking agent and a carrier for flavors, vitamins, and other additives in food and beverage formulations.
7. Plastics and Composites: Hydrophobic fumed silica is used as a reinforcing filler in plastics and composites. It improves the mechanical properties, such as stiffness and toughness, of the material.
8. Energy: Hydrophobic fumed silica is used as a thickening agent and a rheology modifier in battery electrolytes and thermal insulation materials. It improves the performance and stability of these materials.
9. Personal Care: Hydrophobic fumed silica is used in personal care formulations as a matting agent, an anti-aging agent, and a sunscreen agent. It helps to improve the texture and appearance of the product.
10. Rubber: Hydrophobic fumed silica is used as a reinforcing filler in rubber formulations. It improves the mechanical properties, such as tear strength and abrasion resistance, of the rubber material.

The specific surface area of hydrophobic fumed silica can have an impact on its properties and performance in different applications. Generally, the larger the specific surface area of hydrophobic fumed silica, the better its ability to interact with the surrounding media and improve its properties.

Read More: How to Calculate Specific Surface Area of Fumed Silica

Here are some commonly used specifications and related parameters for hydrophobic fumed silica.

HIFULL Hydrophobic fumed silicaTreating AgentSpecific surface area(BET)pH value in 4% dispersionLoss on drying*2 hours at 105&#;Loss on ignition*2 hours at &#;Sio2 content* Based on ignited materialTamped density-based on dried materialCarbon content based on dried materialUnit m2/g %%%g/dm³%HB-612HMDS140±205.5&#;7.5&#;0.7&#;2.0&#;99.840&#;600.7&#;1.3HB-615HMDS160±305.5&#;7.5&#;0.5&#;4.0&#;99.840&#;601.0&#;1.7HB-620HMDS210±305.5&#;8.0&#;0.5&#;4.0&#;99.840&#;601.5&#;2.5HB-630 HMDS320±305.5&#;8.0&#;0.5&#;4.0&#;99.840&#;602.5&#;4.0HB-132HMDS120±407.0&#;8.5&#;0.5&#;4.0&#;99.840&#;602.5&#;4.0HB-135HMDS300±307.0&#;8.5&#;0.5&#;8.0&#;99.840&#;603.5&#;6.5HB-139PDMS100±205.5&#;7.5&#;1.5&#;10&#;99.840&#;604.5&#;6.5HB-151DDS130±203.7&#;4.5&#;0.7&#;2.5&#;99.840&#;600.6&#;1.2HB-152DDS160±203.7&#;4.5&#;0.7&#;2.5&#;99.840&#;600.8&#;1.6HB-151DDDS130±20&#;4.0&#;0.7&#;2.5&#;99.840&#;600.6-1.2HB-152DDDS170±20&#;4.0&#;0.7&#;2.5&#;99.840&#;600.8&#;1.6

HIFULL manufactures more than 10 grades of hydrophilic and hydrophobic fumed silica for your special choice. For product TDS, MSDS or sample-testing, price quotation, please check Datasheet or leave message to us.

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