When to Use hydrophobic colloidal silica？

Hydrophobic silicas are a unique class of materials with a wide variety of functional attributes and beneficial properties making it ideal for numerous applications ranging from adhesives, greases, and sealants to personal care and processing aid products. At Applied Material Solutions, we offer high-quality fumed and precipitated hydrophobic silicas to provide superior performance for your products.

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Benefits of Hydrophobic Silica

 Hydrophobic silica has several different benefits that make it a crucial ingredient in many different compounds across diverse industries. These benefits include:

• Adsorbent. Hydrophobic silica adsorbs solid, liquid, and gaseous materials to easily convert liquids and pastes into powder. This allows for easier handling and dosing.
• Anti-Blocking. Its anti-blocking properties prevent film sheets from sticking together due to static or other interactions.
• Anti-Sagging. Hydrophobic silica increases sag resistance, making it especially useful for paints and coatings as it prevents them from drooping or running while wet.
• Anti-Setoff. Hydrophobic silica prevents setoff, or the accidental transfer of ink between printed pages. It can be used to create anti-setoff sprays and powders.
• Anti-Setting. It can be used in chemicals to slow pigment or paint dry times, especially on metallic surfaces. This is a key additive in paints and coatings, especially for use on larger or more complex projects that require more time for application.
• Free Flow of Powders. Hydrophobic silica acts as an anti-caking material to prevent powders from caking or hardening. It controls the moisture to keep each particle separate and defined for free-flowing distribution or use.
• Foam Control. Hydrophobic silica can be used as an active ingredient in defoamers, which reduce or minimize foam, making it ideal for adhesives, paints, food applications, and more.
• Hydrophobicity Control. Hydrophobic silica naturally repels water and forces it to form droplets. Adding this material to compounds allows manufacturers to enhance hydrophobicity control in a product.
• Mechanical and Optical Properties Improvements. Adding hydrophobic silica can improve the clarity, transparency, durability, and other attributes of materials.
• Reinforcement. Hydrophobic silica reinforces the elastomeric properties of certain materials. It can increase the material&#;s tensile strength, resistance to tearing, and stabilization in the face of temperature changes.
• Pigment Stabilization and Dispersion. Adding hydrophobic silica to pigments can make the liquid material more stable and easier to disperse evenly. This allows for a more reliable application of complex pigments.
• Print Definition. Hydrophobic silica can reduce runoff, smudging, and other errors that reduce print definition and clarity. Inks with hydrophobic silica additives are clearer, crisper, and more precise.
• Processability Improvements. Carefully adding quantities of hydrophobic silica to pigments, inks, sealants, and personal care products allows for more precise application and better processing.
• Rheology Control. Hydrophobic silica can control the suspension of particles in water-based materials to ensure product consistency throughout the batch or container.
• Thickening. It can thicken pigments, sealants, adhesives, and other products to prevent running and poor application control.
• Thixotropy. Hydrophobic silica makes thick compounds thinner when they&#;re shaken, stirred, or otherwise disturbed. This temporary fluidity allows for easier mixing and preparation.
• Suspension and Stability Behavior. Hydrophobic silica can stabilize complex fluid products and consistently suspend a mix of ingredients throughout the batch.

Choose Applied Material Solutions for Hydrophobic Silica

At Applied Material Solutions, our top-quality fumed and precipitated hydrophobic silicas offer reinforcement and enhanced performance for products across a diverse range of industries. Our state-of-the-art treated silica production facility allows us to produce large volumes and reduce delivery times for our customers. To learn more about the properties and applications of hydrophobic silica, check out our product page today.

Hydrophobic colloidal silicon dioxide is a synthetic amorphous silica derivative in which the surface of the fumed silica particle has been modified by the addition of dimethyl silyl groups. The surface modification is achieved via a controlled chemical process that involves the attachment of dimethyl silyl groups, rendering the silica less wettable. It is approved for use in pharmaceutical products as an excipient and is supplied as a light, fine, white or almost white amorphous fluffy powder.

Hydrophobic colloidal silica occurs as a light, fine, white or almost white amorphous powder, not wettable by water.

Silicon Dioxide: Definitions and Types

Silicon and oxygen are the two most abundant elements in the earth&#;s crust. In nature, silicon almost always exists in combination with oxygen, either as free silica (SiO2), in conjunction with other elements (for instance, in silicates, which are the main minerals in rocks and soil), or as combined silica (SiO3). The different silicon compounds have substantially different chemical properties, applications, and hazards.

Free silica (Silicon dioxide) is a hard, low-reactivity, colourless substance that occurs naturally in rocks and minerals or can also be industrially produced in the form of synthetic amorphous silica. All forms of Silica, whether natural, synthetic, crystalline, cryptocrystalline or amorphous, are assigned a single CAS Registry number [-86-9]. For convenience, the different forms of Silica can be divided into three main groups as shown in the chart below:

Chemical Structure of Silica

At the fundamental level of Silica&#;s chemical properties is the Silica tetrahedron (SiO4), which consists of a central Silicon cation covalently bonded to four oxygen atoms, arranged in the shape of a tetrahedron. Silica tetrahedra may be linked and arranged in a variety of ways, from simple to complex three-dimensional frameworks. Crystalline forms of Silica exhibit a highly ordered crystal lattice, determined by the ordered arrangement of the Silica tetrahedra. Amorphous forms, on the other hand, have random, disordered lattices. The orientation of the bonds is random, and there is no long-range periodicity.

To differentiate between the different silica analogues, new CAS Registry numbers have been assigned. These are shown in the chart below:

Synthetic Amorphous Silicon Dioxide

Synthetic amorphous silicon dioxides are further divided into three main types, namely:

• Pyrogenic or fumed silicon dioxide
• Precipitated silicon dioxide, and
• Surface-modified silicon dioxide

Even though they share the same chemical structure and synthetic origin, Synthetic silicas exhibit different properties, as briefly reviewed below:

1). Pyrogenic (Fumed) Silicon Dioxide

Pyrogenic silicon dioxide is produced using a high-temperature process in which silicon tetrachloride is vaporised in an oxygen-hydrogen atmosphere according to the following chemical reaction:

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SiCl4 + 2H2 + O2 -> SiO2 + 4HCl

The raw materials used in the silica production process are all inorganic and very pure. As a result, the synthesis produces only hydrochloric acid (which is easily removed) and silica in very high states of purity (typically > 99.9%). Silicon dioxide produced pyrogenically exists in the form of chain-like, branched aggregates, giving rise to a fluffy, light powder. (The term &#;fume&#; alludes to the method of manufacture, which involves the use of a flame.) Varying processing conditions allows the production of silica products with different specific surface areas, typically between 50 and 400 m2/g.

The pyrogenic method for producing Silica was invented in by Harry Klopfer, a scientist at Degussa (now part of Evonik AG). This method is what is still used by Evonik (for the production of pyrogenic silica marketed under the AEROSIL® brand name) and Cabot Corporation (for the Cab-O-sil® fumed silica brand). Note that these silica grades can be used in their native (unprocessed) state. They can also be further processed (for example, spray drying, granulation, or surface chemical modification) to turn them into other technical silica grades.

2). Precipitated Silicon Dioxide

Precipitated silicon dioxide is silica produced in an aqueous solution at temperatures >60 °C. In this process, sodium silicate (waterglass) undergoes controlled neutralisation with either concentrated sulphuric acid or hydrochloric acid. The Silica precipitates out as a slurry of (hydrated) silica, which is washed and filtered to remove by-products. It is then dried in hot air and milled or passed through a classifier.

Precipitated silicon dioxide has been known since the mid 17th century. It was not until the s that its practical uses and industrial production were fully established. Currently, Precipitated silica is produced in volumes that are up to x10 greater than for Pyrogen silica. The method and conditions have been fine-tuned and now permit the production of many types of synthetic silica that are structurally and characteristically different, including exhibiting internal pore volume/specific surface area, larger particle sizes, and water content.

3). Surface-Modified Silicon Dioxide

The Silica grades described thus far are available for use in their native or unmodified state. These materials have freely accessible silanol groups (Si-OH) on the surfaces of Silica particles, rendering them hydrophilic. Frequently, it is desirable to have hydrophobic silica, i.e a product that repels water.  Hydrophobicity can be achieved through a post-synthesis step in which the silanol groups are reacted with organic groups. The added organic groups are tightly bound to the surface (via covalent bonds) and are only broken via thermal decomposition.

Pharmaceutical-approved hydrophobic silica is produced by reacting hydrophilic silicon dioxide with dimethylchlorosilane immediately after the production of Silica particles in the hydrogen flame chamber. This process is also conducted at high temperatures and allows dimethysilyl groups to be bound irreversibly onto the surface of the silica via siloxane bonds. This produces a material that, while appearing identical to the precursor Silica, is very hydrophobic, repels water and does not absorb moisture from the environment.

Colloidal Silica, Silica Colloids (Sol-Gels) and Fumed Colloidal Silica

&#;Colloidal&#; is used in reference to both pyrogenic and precipitated silica. It may be confused with Silica colloids, which are also obtained via the wet chemical route. Note that the International Union of Pure and Applied Chemistry (IUPAC) defines colloids as systems (dispersions) in which particles of colloidal size (1 nm&#; nm) of any nature (solid, liquid, or gas) are dispersed in a continuous phase of different composition or state.

Thus, in the strictest sense, the term &#;colloidal silica&#; applies to stable dispersions (or sols) consisting of discrete particles of amorphous silica having sizes of between 5 and 100 nm. These colloidal silicas are commercially available in the form of sols or dried powders (e.g., xerogels, dry precipitates, aerogels, or calcinated coarcervates). In a broad sense, however, many other forms of silica (other than wet or dry silica sols above) are colloidal on the grounds that they are composed of particles in a colloidal state of subdivision (1- nm). Moreover, the silica particles and aggregates are self-supporting and stabilised dispersions of silica particles in a continuous air phase and are unaffected by gravitational forces. Finally, fumed silica is commonly referred to as &#;colloidal&#; because the silica powders are made by condensing a silica precursor from a vapour phase. In this sense, fumed silica particles are dispersed in a gas during its production process.

Physical Form

The SiO2 molecules in synthetic silicon dioxide do not exist in isolation. While the primary structure is the tetrahedron, consisting of one silicon atom bonded to four oxygen atoms, tetrahedrons arrange into networks. During the synthesis process, minute droplets of SiO2 initially aggregate into so-called nuclides, which combine stochastically into even larger particles, facilitated by weak physical interactions such as van der Waal&#;s forces.

Another important property of synthetic silica is its specific surface area. Fumed silicon dioxide, in particular, has only one surface, which is external and little or non-existent internal pore volume. Precipitated silica, on the other hand, is mesoporous and exhibits an internal surface. Generally, the higher the specific surface area, the greater the degree of agglomeration. It is these aggregates that partly contribute to the unique functionalities of amorphous silicon dioxide.

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