HIFULL

Nano aluminium oxide is one of the key raw materials for electret masterbatches. So, how does nano alumina improve the filtration performance of mask materials and enhance mask quality?

Electret masterbatch is a special functional masterbatch used in conjunction with electret equipment. It is made with polypropylene as the base material and processed with specialized additives (such as tourmaline and alumina). The electret treatment of melt-blown fabric involves storing static electricity within the melt-blown layer of the mask, enabling it to adsorb harmful substances in the air, such as dust, bacteria, viruses, and aerosols. This thereby increases the mask's filtration efficiency to over 95%-99%. As nano alumina itself carries a positive charge, with a Zeta potential of around 70mV, its addition into the polypropylene electret masterbatch specifically used for masks can enhance the charge retention time and stability of the masterbatch, ensuring the quality of the melt-blown fabric and improving the filtration efficiency of the mask. Compared with traditional electret materials like tourmaline powder, it offers advantages such as being odorless and having slower charge decay. Meanwhile, compared with similar existing products, it features characteristics such as a high charge retention rate, low decay value, and a long charge preservation cycle.

Nano aluminium oxide has attracted considerable attention in the field of lithium‑ion batteries, where it contributes to performance improvements in cathode materials, separator membranes, and electrolytes – positively affecting battery capacity, cycle life, safety, and thermal stability.

For cathode materials, mechanical mixing allows the fine alumina particles to uniformly coat the cathode surface, forming a dense Al₂O₃ layer. This coating helps improve capacity retention and cycle life by suppressing side reactions and stabilising the electrode–electrolyte interface.

For separator membranes, the small particle size and good dispersibility of nano aluminium oxide enable the formation of a very thin (less than 4 μm) and uniform coating on the separator. This coating increases the porosity of the membrane, enhances electrolyte uptake, permeability, and ionic conductivity, improves wettability between the separator and the electrolyte, and reduces thermal shrinkage of the separator – thereby preserving its mechanical integrity and thermal stability under abuse conditions.

With regard to electrolytes, functional silane‑modified alumina coatings can improve the adhesion between the coating and the separator. Moreover, the functional groups can participate in free‑radical polymerisation of the electrolyte to form a gel‑type electrolyte, which contributes to better cycling stability and safety. In pouch‑cell tests for electrolyte leakage, studies have shown that within the same time period, cells without alumina addition exhibited a leakage rate of about 20%, whereas the addition of alumina reduced the leakage rate to between 5% and 10%.

Nano aluminium oxide is an ultrafine powder produced via a gas‑phase synthesis route, characterised by a high specific surface area. Aluminium oxide manufactured by this process typically consists predominantly of the γ‑crystalline phase, along with the α‑phase and minor amounts of transitional forms such as δ‑, η‑, θ‑, κ‑, and χ‑alumina. The primary particles form dendritic aggregates. This material generally exhibits small particle size, high purity, large specific surface area, low tap density, pronounced structural aggregation, good chemical stability, and a positive surface charge. Thin films of nano aluminium oxide show high transmittance in the visible range and effectively block ultraviolet radiation.

Typical properties include a primary particle size of 10–40 nm, an aggregate size (D₅₀) of approximately 100 nm, a purity of 98% or higher, a specific surface area in the range of 50–100 m²/g, and a tap density below 100 g/L. Its chain‑like aggregate structure is similar to that of fumed silica.

These features make nano aluminium oxide suitable for a variety of general industrial applications.

In powder coatings, it acts similarly to other nanoscale powders such as fumed silica. It helps prevent caking, improves flowability, enhances electrostatic spray performance, increases powder transfer efficiency, and improves both abrasion resistance and coating density.

In phosphor coatings for lighting devices, the material can serve as a selective UV reflector, a barrier to mercury diffusion, and an inorganic binder for the phosphor layer, forming a protective film that extends device lifetime. In 2000‑hour luminous flux and lumen depreciation tests, the incorporation of an alumina coating has been shown to moderately increase luminous flux and reduce lumen depreciation by about 3%, thereby improving the efficacy, lumen maintenance, and service life of fluorescent lamps.

In photographic ink‑jet paper coatings, the fine particle size and high specific surface area provide a highly porous structure that rapidly absorbs aqueous inks with high capacity, yielding prints with vivid colours and excellent gradation. Its positive surface charge not only aids dispersion in aqueous systems but also promotes strong interaction with the negatively charged dyes in the ink, enhancing dye fixation and thus ensuring image quality, high gloss, and superior print performance.

In thermal insulation materials, the same properties – small particle size, high porosity, and a high melting point – contribute to improved temperature resistance and mechanical strength of the composite.

In electret masterbatches for melt‑blown nonwovens, the intrinsic positive charge of nano aluminium oxide enhances the charging efficiency and charge retention of the electret, thereby maintaining the filtration efficiency of the melt‑blown fabric. Compared with conventional electret additives such as tourmaline powder, it offers advantages including odourlessness and slower charge decay.

As summer draws near, the demand for sun protection is on the rise, and a myriad of sunscreen products have flooded the market to meet this need. Thanks to its high energy, ultraviolet (UV) radiation finds widespread application in fields like photocatalysis and fluorescence. Under normal conditions, the collagen polymerization within our skin occurs at a relatively slow pace. However, when exposed to UV light, the skin ramps up the formation of free radicals. These oxygen free radicals, in turn, accelerate various oxidative reactions, leading to skin damage and premature aging. Therefore, the core of sun protection lies in shielding the skin from direct exposure to UV rays.

Gas-phase titanium dioxide offers powerful absorption and shielding against both long-wave (320nm-400nm) and medium-wave (280nm-320nm) UV rays. Furthermore, titanium dioxide powder itself boasts a host of advantages: it is safe, non-toxic, physiologically inert, stable, resistant to degradation, and offers high tinting strength and excellent color compatibility. These qualities make it a cornerstone active ingredient in sunscreen skincare formulations. The particle size of nano titanium dioxide powder has a profound impact on its UV absorption capability, with the ideal range typically falling between 30nm and 50nm.

However, gas-phase titanium dioxide is inherently catalytic, meaning it also functions as a common catalyst. Consequently, when nano titanium dioxide is used, it must undergo surface treatment. This process deactivates the titanium dioxide's active sites while simultaneously enhancing the powder's dispersibility within cosmetics and skincare products.

The process by which semiconductors, under the irradiation of light, convert light energy into chemical energy, thereby facilitating the synthesis or decomposition of compounds, is termed semiconductor photocatalysis. Many semiconductor materials, such as TiO2, ZnO, and Fe2O3, possess suitable band structures and can serve as photocatalysts. Among them, TiO2 stands out as the most widely used semiconductor photocatalyst due to its chemical stability, strong redox capacity, non-toxicity, and low cost.

Titanium dioxide generally exists in three crystalline phases: anatase, rutile, and brookite. Among these, the brookite phase is unstable and, consequently, has been the subject of limited research. The band gap between the valence band and the conduction band in the anatase phase is 3.29 eV, which is slightly larger than that of the rutile phase, requiring higher light energy to induce electron transitions. However, the anatase phase boasts a longer-lived excited state and a surface that more effectively adsorbs oxygen in anionic forms. Therefore, the anatase phase exhibits superior photocatalytic activity. Gaseous titanium dioxide typically exists as a mixed phase of anatase and rutile, predominantly in the anatase form.

The photocatalytic applications of TiO2 primarily include the following:

(1) Photomineralization—Wastewater Treatment

When TiO2 is irradiated with ultraviolet light (< 385 nm), electrons in the valence band transition to the conduction band. The resulting photogenerated electrons and holes migrate to different sites on the surface to participate in redox reactions. For instance, water molecules are typically adsorbed onto the surface of titanium dioxide and are oxidized to generate hydroxyl radicals. Simultaneously, oxygen is reduced to form superoxide anions, which can be further reduced to peroxide anions. These anions possess moderate oxidizing capabilities. Collectively, these oxides are capable of oxidizing the vast majority of organic compounds and inorganic pollutants, mineralizing them into inorganic small molecules, carbon dioxide, and water. Consequently, titanium dioxide finds application in wastewater treatment, deodorization, and self-cleaning.

(2) Sterilization

The highly oxidizing hydroxyl radicals generated during the aforementioned process can effectively disrupt bacterial cell walls and viral proteins. As a result, gaseous titanium dioxide can effectively inhibit the proliferation of various microorganisms, including bacteria and viruses, on liquid surfaces and within liquids. It is also applicable to surface coatings to prevent discoloration caused by algae growth.

Nano titanium dioxide is an ultrafine powder with high specific surface area, produced via a gas-phase synthesis process. It features ultra-small particle size, high purity, and a unique mixed crystal structure of anatase and rutile, with anatase being the dominant phase. Rutile is the most thermodynamically stable crystalline form, while anatase typically converts to rutile at temperatures between 500 °C and 600 °C.

Industrially produced nano-TiO₂ exhibits small particle size, high purity, high specific surface area, excellent thermal and chemical stability, outstanding catalytic and photocatalytic efficiency, and UV photosensitivity. Its TiO₂ purity exceeds 99.5%, with a primary particle size of approximately 20 nm. The aggregates resemble those of fumed silica, forming three-dimensional, irregular chain-like structures ranging from 200 to 500 nm in size. Similar to fumed silica, the surface of nano-TiO₂ is rich in hydroxyl groups, giving the hydrophilic grade strong moisture absorption properties.

This material is suitable for catalytic and photocatalytic applications, as well as an effective UV shielding agent. It is widely used in self-cleaning building materials, solar cells, silicone rubber, ceramics and metal materials, lithium batteries, various bactericides, and cosmetics.

Application in Silicone Rubber

Nano titanium dioxide improves the thermal stability and flame retardancy of silicone rubber. Experiments show that increasing the amount of nano-TiO₂ in silicone rubber raises its decomposition temperature. In aging tests, silicone rubber containing nano-TiO₂ maintained better tensile strength and elongation at break after 5 days at 275 °C compared to unfilled rubber. In storage weight loss tests, adding fumed nano-TiO₂ to room-temperature-vulcanized (RTV) silicone rubber, high-temperature-vulcanized (HTV) silicone rubber, and liquid silicone rubber significantly reduced weight loss. This is because under high-temperature oxidative conditions, free radicals are generated that degrade organic substances into small volatile molecules. Titanium dioxide captures electrons and free radicals, thereby improving the thermal stability of the polymer.

Application in Photocatalysis

In the photocatalytic process, photogenerated electrons react with oxygen molecules adsorbed on the surface. Molecular oxygen not only participates in reduction reactions but is also a source of hydroxyl radicals. The resulting reactive species — hydroxyl radicals (·OH), superoxide ions (·O₂⁻), and hydroperoxyl radicals (·HO₂) — can directly oxidize organic compounds into small inorganic molecules such as CO₂ and H₂O, thereby reducing airborne pollutant concentrations and alleviating environmental discomfort. In addition to organics, many inorganic substances also exhibit photoactivity on TiO₂ surfaces. For example, Cr⁶⁺ in industrial wastewater, when in contact with the TiO₂ catalyst surface, can capture photogenerated electrons and undergo reduction to the less toxic or non‑toxic Cr³⁺, helping purify contaminated water. Nano-TiO₂ can also break down hydrocarbons under visible light irradiation. By coating glass, ceramics, or tiles with a thin nano-TiO₂ layer, organic pollutants adsorbed on the surface are decomposed into CO₂ and O₂ via photocatalytic reactions, and the remaining inorganic residues are washed away by rain, achieving a self‑cleaning effect.

Other Applications

Fumed silica, a nanomaterial produced through the high-temperature hydrolysis of chlorosilanes in a hydrogen-oxygen flame, is performing a stunning feat of "invisibility" across modern industrial sectors. Its primary particles are spherical, ranging from 7 to 40 nm; its aggregate particles typically measure 100–500 nm; and its agglomerates average 1–200 μm. Through surface hydroxylation, fumed silica acquires unique physicochemical properties, allowing it to play a critical role in numerous applications while ultimately "disappearing". This seemingly paradoxical characteristic perfectly illustrates the fascinating magic of fumed nanomaterial science. Behind this remarkable vanishing act lies a delicate balance of surface chemistry, rheology, and interfacial science.

I. The Elegant Disappearance: From Solid Particles to Structural Networks

In epoxy resin composites, fumed silica is surface-modified with silane coupling agents, allowing its hydroxyl groups to form chemical bonds with the resin matrix. When added in the optimal amount and fully dispersed using appropriate equipment, the nanoparticles form a three-dimensional network under shear forces. During curing, this network crosslinks with the resin, eventually integrating completely into the polymer backbone. This integration boosts the composite's tensile and tear strength by 40%, while the nanoparticles become one with the matrix – achieving true "molecular-level disappearance".

Similarly, fumed silica's thixotropic regulation in coatings is a refined art. It constructs a reversible network via hydrogen bonding: under shear stress (e.g., during application), the network breaks down, viscosity drops instantly; upon standing, the 3D network rebuilds and viscosity recovers. In new energy vehicle coatings, just 0.83% fumed silica delivers perfect sag resistance. As the coating cures – solvent evaporating and crosslinking proceeding – the nanoparticles are firmly anchored within the crosslinked network, ultimately becoming an integral part of the coating structure.

II. Dimensional Transformation: From 3D Particles to Homogeneous Interfaces

In end products, fumed silica is rarely directly visible. This is because, thanks to its nanoscale particle size and excellent dispersibility, it disperses uniformly within the material matrix, forming stable suspensions or solutions. Existing as nanoparticles physically or chemically bonded to the matrix, it substantially enhances material properties – enabling performance improvements from the macro to the micro level.

In silicone rubber products, fumed silica interacts with rubber molecular chains to form a nano-filler network that significantly boosts tensile strength, tear resistance, and abrasion resistance. In room-temperature vulcanizing (RTV) silicone rubber, adding 10% hydrophobic fumed silica yields films that, when measured with a haze meter, show virtually the same transparency as films without the additive – pristine and clear.

Fumed silica may seem to have "disappeared" in the final product, but in reality it persists as nanoparticles within the material matrix, continuously performing its vital functions.

III. Energy Conversion: From Physical Entity to Functional Carrier

To the naked eye, fumed silica appears as a fluffy white powder – yet it also serves remarkably as a functional carrier.

In pharmaceutical applications, colloidal silicon dioxide acts as a drug carrier. Its strong adsorptive capacity encapsulates drugs within its porous structure, enabling sustained, slow release to prolong efficacy. Beyond controlled release, it can achieve targeted drug delivery, reducing drug loss in the body, enhancing therapeutic outcomes, and minimizing side effects.

In polyolefin catalysts and metal-oxide powder catalysts, fumed silica – thanks to its high purity – offers high specific surface area and pore volume to load more active components, without compromising catalytic efficiency. It reduces side reactions and enhances catalytic activity.

To expand the application of fumed silica in highly polar systems, organic silicon-based modifiers are often used to impart hydrophobicity to its surface. Polydimethylsiloxane (PDMS), commonly known as silicone oil, is a linear organosilicon polymer characterized by excellent thermal stability, low surface energy, and good compatibility, making it an ideal hydrophobic modifier.

During the modification process, PDMS interacts with the silanol groups on the surface of hydrophilic fumed silica through physical coating or chemical reactions. The primary reaction pathways include:

Condensation reaction: The terminal silanol groups of PDMS undergo dehydration condensation with the hydroxyl groups on the SiO₂ surface, forming Si–O–Si bonds;

Physical adsorption and coating: The long-chain molecules of PDMS form a hydrophobic layer on the surface of the nanoparticles, shielding the original hydrophilic groups.

After being treated with PDMS, the hydrophilic fumed silica transforms into a hydrophobic product, accompanied by significant changes in its physical and chemical properties:

Specific surface area: Decreases noticeably after PDMS treatment, indicating surface coverage by an organic layer that reduces nitrogen adsorption capacity;

Surface properties: The water contact angle increases markedly, rendering the powder unwettable and exhibiting typical hydrophobic characteristics;

Effective particle size: Slightly increases due to the presence of the surface organic layer.

Performance Advantages

Hydrophobic fumed silica modified with PDMS offers several general performance advantages that make it attractive for various industrial applications:

Rheology control: It provides excellent thickening, thixotropic, and anti-sagging properties, making it a suitable additive for silicone rubbers, coatings, and inks. In these systems, it improves product rheology, prevents flow on vertical surfaces, and enhances application efficiency and final product quality.

High filler loading capacity: Unlike unmodified hydrophilic fumed silica, the PDMS-treated version does not exhibit a pronounced “structure effect” (i.e., performance degradation due to excessive particle aggregation) even at high loadings. This allows silicone rubber formulations to achieve good mechanical properties while maintaining processability and long-term stability.

Transparency in clear systems: In transparent matrices such as clear silicone rubbers and coatings, the addition of PDMS-treated fumed silica does not impair optical clarity. At the same time, it imparts high hydrophobicity, improving water resistance and anti-soiling performance.

Anti-caking and flow aid properties: In powder materials, including powder coatings, the hydrophobic silica acts as an effective anti-caking agent and flow aid, preventing clumping during storage and transport, thereby improving production efficiency and product quality.

Application Examples

Adhesives and sealants: In curable polyurethane or silicone sealants, PDMS-treated fumed silica effectively controls rheological behavior, prevents sagging during application, and enhances cohesive strength.

Lubricants and cable gels: As a thickening and thixotropic agent, it disperses stably in polar media, preventing oil separation or additive settling, and improving the reliability of lubrication systems.

Powder coatings: As an anti-caking and flow-promoting agent, it significantly improves storage stability and spray fluidity, especially in ultra-fine powder systems.

Vinyl ester resins and composites: In resin systems, it regulates viscosity, inhibits filler settling, and enhances the weather resistance and hydrophobicity of the final products.

Fumed silica, as an important functional filler, is produced by the hydrolysis of volatile chlorosilanes in a hydrogen-oxygen flame. It features nano-scale primary particles (typically 7-40nm), a high specific surface area (ranging from 30-450m²/g), and a highly developed agglomerated structure.

However, hydrophilic fumed silica is rich in surface silanol groups (-OH), possessing strong polarity and hygroscopicity. In highly polar organic systems or environments sensitive to moisture, it tends to absorb moisture and agglomerate, leading to decreased dispersibility and causing product stability issues. To address these limitations, hydrophobic fumed silica has emerged, with dimethyldichlorosilane (DDS) treatment technology becoming one of the important pathways to achieve surface hydrophobic modification.

HIFULL® hydrophobic fumed silica is prepared by surface modification of hydrophilic fumed silica. Common modifiers include dimethyldichlorosilane (DDS), hexamethyldisilazane (HMDS), and polydimethylsiloxane (PDMS), among others.

After surface modification, the hydroxyl groups on the silica surface are reacted or shielded, rendering the material hydrophobic.

The working principle of DDS modification technology is based on a silanization reaction, where the silanol groups (-OH) on the surface of hydrophilic fumed silica chemically react with dimethyldichlorosilane. The surface polarity is effectively shielded, thereby significantly reducing surface energy and endowing the material with hydrophobicity and organic compatibility.

This modification not only reduces hygroscopicity but also improves dispersibility and rheological control capabilities in organic media. It is worth noting that hydrophobic treatment is not merely “masking” polarity, but rather achieving controllable adjustment of surface energy through molecular design, thereby adapting to systems of different polarities.

Characteristics and Differences of DDS-Modified Products

Through the DDS treatment process, various specifications of hydrophobic fumed silica products can be obtained, among which HB-151 and HB-152 produced by Hubei Huifu Nanomaterial Co., Ltd. are typical representatives.

HIFULL® hydrophobic fumed silica HB-151 is obtained by treating hydrophilic fumed silica with a specific surface area of 150m²/g via DDS; whereas treating hydrophilic fumed silica with a specific surface area of 200m²/g via DDS yields hydrophobic fumed silica HB-152.

These two product models exhibit significant differences in key parameters: the BET specific surface area of HB-152 is between 170±30m²/g, with a carbon content of 0.8-1.6%; whereas the BET specific surface area of HB-151 is between 120±30m²/g, with a carbon content of 0.6-1.2%.

It is precisely these differences in structural parameters that make HB-152 possess higher hydrophobicity, thickening and thixotropic properties, transparency, and reinforcing properties compared to HB-151.

In terms of physicochemical properties, hydrophobic fumed silica treated with DDS exhibits excellent stability. Taking HB-152 as an example, its loss on drying (dried at 105℃ for 2h) is ≤0.7, loss on ignition is ≤2.5%, silica content is ≥99.8%, and the pH value of the suspension (4%) is ≥3.9.

Rheological Control and Application Performance

Researchers at Hubei Huifu Nanomaterial Co., Ltd. have found that in terms of application performance, hydrophobic fumed silica treated with DDS mainly affects the rheological properties of coatings through the hydrogen bonding of surface silanol groups and the entanglement of treatment agent molecules.

Due to its hydrophobicity, it more easily interacts with non-polar organic molecules in coating systems, forming a loose network structure.

Taking UV coatings as an example, when the addition amount of hydrophobic fumed silica HB-151 increases from 0.5% to 2%, at low shear rates, the viscosity can increase from 400Pa·s to 2800Pa·s, a 600% increase; at high shear rates, the viscosity also increases from 420Pa·s to 1050Pa·s, a 150% increase—while the thixotropic value increases from 0.9 to 2.8, maintaining a growth of around 210%.

These unique rheological characteristics make hydrophobic fumed silica treated with DDS widely applicable in industries such as room temperature vulcanized (RTV) silicone rubber, electronic potting adhesives, adhesives and sealants, coatings, inks, and powder coatings.

Polarity Matching and Application Systems

According to the principle of polarity matching, materials with lower surface energy disperse more easily in low-polarity media, while medium-to-high polarity systems require fillers with moderate polarity to balance interaction forces.

Due to its extremely low surface energy, hydrophobic fumed silica treated with DDS is particularly suitable for low-polarity systems, such as MSPolymer (silane-modified polyether), STP-E, polyacrylates, and polysulfides. Their intermolecular forces are weak, necessitating the use of hydrophobic fumed silica with extremely low surface energy, such as HB-151 and HB-152. These products are typically subjected to deep alkylation modification and can bond well with the matrix through van der Waals forces, providing effective thickening, thixotropic, and reinforcing effects.

In contrast, medium-to-high polarity systems such as polyurethane, polyols, polyamide, epoxy resins, and vinyl resins require the selection of hydrophobic products that retain a small amount of polar sites on the surface. This avoids excessive agglomeration while allowing residual silanol groups or designed polar functional groups to participate in interfacial interactions and even promote crosslinking reactions.

Market Prospects and Future Development Directions

With the continuous increase in global demand for high-performance materials, the market prospects for hydrophobic fumed silica are extremely broad. Especially in fields with stringent requirements for adhesive performance, such as high-end electronic packaging, wind power blade bonding, and aerospace structural adhesives, the application of hydrophobic fumed silica treated with DDS will drive related fields towards more efficient and reliable development.

Fumed silica, also known as white carbon black, is a nano-scale amorphous silicon dioxide powder generated through the high-temperature hydrolysis of silicon halides (such as silicon tetrachloride) in a hydrogen-oxygen flame.

However, the surface of untreated fumed silica is rich in hydrophilic hydroxyl groups, resulting in poor dispersibility in non-polar or weakly polar systems, which limits its application in hydrophobic systems. Therefore, the industry often employs organosilane modifiers for surface hydrophobic treatment, among which hexamethyldisilazane (HMDS) is one of the most commonly used modifiers.

The molecular formula of hexamethyldisilazane (HMDS) is [(CH₃)₃Si]₂NH. The modification reaction on the surface of fumed silica is essentially a condensation reaction between silanol groups and HMDS. This reaction converts the hydrophilic hydroxyl groups (–OH) on the surface of fumed silica into hydrophobic trimethylsiloxy groups (–OSi(CH₃)₃), while releasing ammonia gas.

By controlling the amount of HMDS, reaction temperature, time, and the specific surface area of the raw material, different degrees of surface coverage can be achieved, thereby regulating key indicators such as the hydrophobicity, carbon content, and specific surface area of the final product.

In industry, hydrophobic products with varying performances are typically obtained by treating hydrophilic fumed silica with different specific surface areas using HMDS. Based on actual production data, the results can be summarized as follows:

HB-612 & HB-615

Hydrophilic fumed silica with a specific surface area of 150 m²/g: After HMDS treatment, HB-612 and HB-615 are obtained.

Their physical and chemical indicators are basically consistent, but HB-615 has a higher carbon content, indicating a higher degree of surface treatment, superior hydrophobicity, and better thixotropy.

HB-612 is suitable for industries such as room temperature vulcanized (RTV) silicone rubber, adhesives and sealants, coatings, inks, and powder coatings.

It can serve as a thickening thixotropic agent and reinforcing agent for RTV silicone rubber. Its hydrophobicity endows RTV silicone rubber with a good shelf life. In coatings and inks, it plays roles in anti-settling, anti-sagging, and thixotropic control, and improves the free-flowing property of powder coatings, preventing caking.

On the basis of HB-612’s applications, HB-615, with its more outstanding and stable hydrophobicity and thixotropy, plays a significant role in elastomers, toners, high-temperature vulcanized (HTV) silicone rubber, and liquid silicone rubber (LSR).

It is an excellent reinforcing agent and rheology control agent for adhesives and sealants, and a superior reinforcing filler for elastomers, capable of preventing the structuralization of HTV silicone rubber and improving the uniformity and stability of pigments.

HB-620 & HB-132

Raw material with a specific surface area of 200 m²/g: HB-620 is obtained after treatment; if deep modification is carried out (i.e., with a higher HMDS dosage or more thorough reaction), HB-132 is obtained.

It is noteworthy that the nitrogen adsorption specific surface area of HB-132 is lower than that of HB-620, but its carbon content is higher. This indicates that while deep modification partially blocks pores and reduces the specific surface area, it significantly enhances hydrophobicity.

HB-620 is suitable for HTV silicone rubber, LSR, coatings and inks, adhesives and sealants, and powder coatings. It has excellent hydrophobicity and stable thixotropy. It can serve as a reinforcing agent for HTV silicone rubber, preventing structuralization. In coatings and inks, it provides anti-settling, anti-sagging, and thixotropic effects. It is a good hydrophobic and rheology control raw material for adhesives and sealants, and can improve the free-flowing property of powder coatings, preventing caking.

HB-132 is widely used in silicone rubber, adhesives and sealants, defoamers, and toners. Due to its extremely low thickening effect and excellent operability, it allows for a high filler loading in polymers. HB-132 is very suitable for systems requiring low thickening and high thixotropy. It exhibits excellent reinforcing performance when used as a filler in silicone rubber, and plays a significant role in self-leveling one-component RTV (RTV-1), two-component RTV (RTV-2K) silicone rubber, and LSR. It can also improve the free-flowing property of powder coatings

HB-630

Raw material with a high specific surface area of 400 m²/g: HB-630 is obtained after HMDS treatment. This product has the highest specific surface area and carbon content among all HMDS-modified products, combining excellent dispersibility, transparency, and hydrophobicity.

HB-630 is suitable for elastomers, especially HTV silicone rubber and LSR, coating and ink systems, defoamers, and printer toner fields. With its high specific surface area, small particle size, and good dispersion uniformity, it possesses higher transparency, ultra-high hydrophobicity, and stable thixotropy.

It can be applied as a reinforcing agent for HTV silicone rubber and LSR. In coatings and inks, it provides anti-settling, anti-sagging, and thixotropic effects. It endows defoamers with good defoaming and anti-foaming capabilities, and improves the free-flowing property, uniformity, and stability of toner.

In modern materials science and industrial applications, fumed silica is a vital inorganic nanomaterial. Valued for its high specific surface area, excellent dispersibility, and superior surface activity, it is extensively utilized across sectors including coatings, rubber, plastics, cosmetics, and electronic packaging. With technological advancement, the demand for functionalized modification of fumed silica has grown significantly. In particular, the precise regulation of its hydrophobicity has become a critical factor in enhancing the overall performance of materials. Among these advancements, hydrophobic fumed silica obtained through treatment with different modifiers has garnered widespread attention due to its superior moisture resistance, weatherability, and stability.

Technicians at Hubei Huifu Nanomaterial Co., Ltd. conducted a systematic evaluation of the hydrophobic properties of three types of hydrophobic fumed silica, each processed via different modification techniques: HB-132 (treated with hexamethyldisilazane, HMDS), HB-139 (treated with polydimethylsiloxane, PDMS), and HB-151 (treated with dimethyldichlorosilane, DDS). The evaluation was carried out using the contact angle measurement method.

The technicians employed a tablet compression method to prepare samples, quantitatively pressing each type of fumed silica into uniform thin discs. Subsequently, a droplet was deposited onto the sample surface using a contact angle goniometer. The experimental results indicated the following order: HB-139 (126.5°) > HB-132 (122.3°) > HB-151 (120.7°).

A larger contact angle signifies stronger surface hydrophobicity. Therefore, among the three products, HB-139 exhibited the strongest hydrophobicity, with a contact angle exceeding 126°. This places it within the range of highly hydrophobic materials, approaching superhydrophobic levels (generally, a contact angle >150° is defined as superhydrophobic, while >120° is considered strongly hydrophobic). HB-132 followed, with a contact angle of approximately 122.3°, which still falls within the strongly hydrophobic category, though slightly lower than HB-139. HB-151 demonstrated relatively lower hydrophobicity, with a contact angle of about 120.7°. While it still possesses good hydrophobic properties, it is slightly weaker compared to the other two.

The core reason for this disparity in hydrophobicity lies in the fundamental differences in the modification effects of the various agents on the surface of the fumed silica. Firstly, the modifier used for HB-139 features a longer alkyl chain or higher reactivity, enabling it to more completely substitute the hydroxyl groups on the silica surface. This forms a dense hydrophobic layer that significantly lowers surface energy, making it difficult for liquid droplets to spread across the surface. Secondly, the modifier for HB-132 possesses slightly lower reactivity or a shorter molecular chain, resulting in a slightly lower substitution rate of surface hydroxyl groups compared to HB-139. This leads to insufficient compactness in the hydrophobic layer, causing a subsequent drop in the contact angle. Lastly, the modifier used for HB-151 may suffer from issues such as low reaction efficiency and weak hydrophobic activity in its functional groups. It can only partially cover the surface hydroxyl groups, and the residual hydroxyl groups retain a certain degree of hydrophilicity, ultimately resulting in the lowest contact angle.

Fumed silica, commonly known as white carbon black, is an ultrafine nanopowder characterized by its small particle size and large specific surface area. As one of the most successfully industrialized nanomaterials, it boasts a wide range of applications. However, the abundant hydroxyl groups on its surface render it highly polar and prone to moisture adsorption, which impairs its dispersibility in polymer matrices. To address this, surface modification is essential to enhance its dispersibility and interfacial bonding in polymers, thereby broadening its application scope.

Surface-modified hydrophobic fumed silica retains the appearance of a fine white powder. The following methods can be employed to roughly distinguish hydrophobic products: the water solubility test, the methanol value method, carbon content determination, and infrared spectroscopy.

I. Water Solubility Method

This method involves placing a layer of fumed silica powder, approximately 1 cm thick, into a 250 mL stoppered conical flask, followed by the addition of about 100 mL of distilled water. After sealing with the stopper, the mixture is shaken vigorously. Untreated fumed silica will "dissolve" in the water, forming a transparent or semi-transparent viscous substance. In contrast, surface-treated hydrophobic fumed silica will either float entirely or partially on the water surface. The degree of surface treatment can be roughly assessed by the turbidity of the water phase; the clearer the water, the higher the degree of treatment and the superior the hydrophobicity.

II. Methanol Value

The methanol value is determined by measuring the volume of methanol required to completely dissolve 0.2 g of surface-treated fumed silica in 50 mL of pure water. Specifically, the surface-treated fumed silica is added to a beaker containing pure water and stirred using a magnetic stirrer. Initially, the powder floats on the water surface. Methanol is then added to the beaker via a burette, with the tip inserted below the water surface. As methanol is introduced, the floating powder gradually diminishes. The volume of methanol consumed, denoted as A (mL), is recorded when the fumed silica is fully dissolved. The methanol value is subsequently calculated using the appropriate formula.

Generally, the greater the methanol consumption during the test, the higher the degree of surface treatment and the better the hydrophobicity.

III. Carbon Content Determination

The sample, contained within a crucible and covered with a suitable catalyst, is placed in an oxygen stream within a high-frequency induction furnace for combustion. The carbon combustion products are converted into carbon dioxide via a platinum catalyst, and the concentration is quantified using the infrared detector of an infrared high-frequency sulfur-carbon analyzer.

IV. Infrared Spectroscopy Method

Infrared spectroscopy operates on the principle that energy absorption causes molecular vibrational energy level transitions; recording this process yields the infrared absorption spectrum of the molecule. Prior to surface treatment, fumed silica possesses a significant number of surface hydroxyl groups, exhibiting a strong absorption peak at 3450 cm⁻¹ in the infrared spectrum. Following surface treatment, the hydroxyl absorption peak weakens, while a C-H stretching vibration peak of methyl groups emerges near 2970 cm⁻¹.

Comparative Analysis of Methods

Among the methods described above, infrared spectroscopy serves as a qualitative measurement. It readily determines whether the surface modifier is chemically bonded to the fumed silica surface, but it cannot quantitatively indicate the degree of surface treatment or the level of hydrophobicity.

The water solubility method can only roughly classify fumed silica products as hydrophilic, hydrophobic, or semi-hydrophobic; it cannot serve as a precise criterion for assessing hydrophobicity.

The methanol value method yields relatively accurate hydrophobicity results. However, due to the susceptibility to human error in testing and judgment, it is generally not adopted as the standard for hydrophobicity evaluation.

The carbon content method delivers the most accurate results with minimal instrumental error and is frequently employed as the standard method for testing the hydrophobicity of surface-treated products.

Fumed silica, also known as white carbon black, is an extremely fine amorphous silica produced by the high-temperature hydrolysis of halosilanes in an oxyhydrogen flame. Its surface is rich in a large number of silanol groups, giving it strong polarity and high hydrophilicity. Untreated hydrophilic fumed silica has a strong tendency to agglomerate, which can lead to excessive stress concentration in organic phases, causing various defects and severely affecting the performance of polymers. To expand the application fields of fumed silica, it is necessary to reduce the number of silanol groups on its surface, transforming it from strongly hydrophilic to hydrophobic, thereby improving its compatibility and bonding strength with organic phases.

Characteristics of Hydrophobic Fumed Silica

Hydrophobically modified silica exhibits distinct characteristics: it can be well dispersed in organic phases through its hydrophobic groups, while also forming strong interactions with organic phases via its silanol groups. This establishes a stable connection between the otherwise incompatible inorganic silica and the organic phase, achieving purposes such as reinforcement, anti-settling, anti-sagging, and scratch resistance.

Commonly Used Hydrophobic Modifiers

Hydrophilic fumed silica is produced via the high-temperature hydrolysis reaction of silicon halides in a hydrogen-oxygen flame. Since hydroxyl groups remain on the surface of its particles, it exhibits hydrophilic properties. Hydrophilic silica can be wetted by water and disperses readily in water.

The primary particle size of hydrophilic fumed silica ranges from 7 to 40 nm, with an average aggregate particle size of 150 to 300 nm, and an average agglomerate particle size of 1 to 50 μm. Its specific surface area falls between 90 and 400 m²/g, and the pH of a 4% aqueous solution is between 3.7 and 4.5.

With characteristics such as hydrophilicity and oleophobicity, hydrophilic fumed silica delivers remarkable effectiveness in rheology control during processing, reinforcement of silicone elastomers, thickening of non-polar liquids, and flow assistance for food and industrial powders. It is suitable for industries including coatings and paints, adhesives and sealants, unsaturated polyesters and films, inks, silicone rubber, cosmetics, pharmaceuticals, elastomers, food, and powdered materials.

Hydrophobic fumed silica is produced through a chemical reaction between hydrophilic fumed silica and modifying agents. Common modifying agents include hexamethyldisilazane (HMDS), dimethyldichlorosilane (DDS), polydimethylsiloxane (PMDS), and specialty silanes. After treatment, the hydroxyl groups on the silica surface are either reacted or shielded, giving the material hydrophobic properties.

Unlike hydrophilic silica, hydrophobic fumed silica cannot be wetted by water. Even though its density exceeds that of water, it can float on the water surface. Hydrophobic fumed silica has weak hygroscopicity and does not easily reabsorb moisture. Compared to the hydrophilic type, its particle size increases while its specific surface area decreases. The pH of its 4% isopropanol-water solution (in a 1:1 volume ratio) ranges from 3 to 10.

Hydrophobic fumed silica features properties like oleophilicity, non-polarity, and low moisture absorption. It plays a significant role in aiding dispersion, adjusting rheology, thickening polar liquids, reinforcement, anti-corrosion, water repellency, and scratch resistance. It is applicable to industries such as room temperature vulcanization (RTV) silicone rubber, adhesives and sealants, coatings, inks, powder coatings, liquid silicone rubber, silicone defoamers, printing toner, colorants, vinyl ester resins, lubricants, and cable gels.

With continuous exploration and innovation in the application functions and fields of fumed silica, Hubei Huifu Nanomaterial Co., Ltd.(HIFULL®) has achieved ongoing breakthroughs and results in the application of fumed silica nanopowder.

To date, HIFULL® offers 9 grades of hydrophilic fumed silica (HL-30/HL-50/HL-90/HL-150/HL-200/HL-260/HL-300/HL-380/HL-450) and 12 grades of hydrophobic fumed silica: 5 treated with hexamethyldisilazane(HMDS) (HB-132/HB-612/HB-615/HB-620/HB-630), 3 with polydimethylsiloxane(PDMS) (HB-139/HB-139B/HB-139C), 2 with dimethyldichlorosilane(DDS) (HB-151/HB-152), and 2 with special silanes (HB-701/HB-202N).

Among them, the low specific surface area hydrophilic fumed silica HL-30 and HL-50 are new grades launched by HIFULL®. Compared to previous fumed silica products, these additions fill the gaps in the high, medium, and low specific surface area product series, making product applications more precise and segmented.

As a hydrophilic fumed silica with a specific surface area of around 100m2/g, HL-90 features low specific surface area and easy dispersion. Its excellent dispersibility and stability make it widely used in fields such as chemical mechanical polishing and metal cutting fluids, serving as a superior polishing and grinding aid. Meanwhile, HL-260 and HL-450, which offer excellent reinforcement effects without affecting product transparency, are highly acclaimed in the liquid silicone rubber (LSR) field. Although HL-90, HL-260, and HL-450 demonstrate outstanding performance in these specific application areas, they can still function as raw material additives in other fields.

To further focus on the industry and professional quality, HIFULL® has coordinated its Application Technology R&D Center, Production Technology R&D Department, Quality Control Department, and Production Department. After years of effort and hundreds of breakthroughs, they have developed raw material additives specifically for applications in the food and pharmaceutical fields. Characterized by being colorless and odorless, highly safe, stable, high in purity, high in specific surface area, and highly active, this series of products is set to be released in 2024.

HIFULL® Food Additive-Silicon Dioxide is an excellent anti-caking agent, flow aid, and dispersion/grinding aid in powdered or granular foods. It effectively resolves issues of moisture absorption and caking in powdered or granular foods caused by environmental humidity, air, and compression during production, transportation, and storage, thereby improving the production efficiency of powdered foods and extending their shelf life.

Fumed silica, commonly known as "white carbon black," is a nano-scale amorphous silica powder obtained by high-temperature hydrolysis of silicon halides in an oxyhydrogen flame. Inside the reaction furnace, halosilanes hydrolyze into individual silica molecules and rapidly grow into spherical particles of 7-40nm. These particles are the primary particles of fumed silica. In a molten state, primary particles continue to move along with the flame in the reaction furnace and collide with each other, forming three-dimensional branched structures composed of multiple fused spherical particles. These are known as aggregates of fumed silica, typically ranging in size from 100-500nm. As aggregates continue to move and collide with the airflow in the pipeline, they connect through interactions to form a fluffy, flocculent powder, which we call agglomerates of fumed silica. The average size of agglomerates ranges from 1-200μm.

Due to this special preparation process, the structure and characteristics of fumed silica differ from traditional silica. It possesses numerous properties such as nano-scale particle size, high specific surface area, high surface activity, high insulation, high thermal stability, high chemical inertness, and high purity. The surface silanol groups (Si-OH) exhibit high chemical activity and can undergo chemical reactions with organic groups. Furthermore, silanol groups can form three-dimensional network structures through hydrogen bonding. The presence of silanol groups also makes it possible to modify the surface of fumed silica. By grafting functional chemical groups onto the surface, hydrophobic fumed silica with more diverse functions can be obtained.