Fumed Silica: Properties, Uses in Plastics and Regulatory Status

Fumed silica (pyrogenic silica, CAS 7631-86-9) is a synthetic amorphous silicon dioxide made in a flame, used in plastics as a thickener and thixotrope for unsaturated polyester and epoxy resins, a reinforcing filler for silicone elastomers and a free-flow agent for powders. Its primary particles measure 5 to 50 nm and its specific surface area runs from 50 to 600 m2/g, so the practical questions are how a powder this fine is identified, regulated and handled.

Fumed silica shares CAS 7631-86-9 and EC 231-545-4 with every synthetic amorphous silica (its former fumed-specific CAS was 112945-52-5), is registered under REACH with 371 active dossiers (ECHA CHEM, 22 September 2026), is authorised for EU food-contact plastics as FCM No 504, is absent from the REACH Candidate List, and, as amorphous silica, is not on the California Proposition 65 list. Among plastic additives, it is a filler bought for its surface rather than its volume. Fumed silica is one of 12 filler substance pages in our database of plastic additives, each with the same identity, function and regulatory fields.

This page holds the identity and the compliance record together: the CAS trap between the current and the former number, the 2 flame routes that make the powder, the 3-level particle structure that EU food-contact law writes into its entry, the measured properties, the 3 uses inside unsaturated polyester, epoxy and silicone plus the free-flow use for powders, the 5 characterisation measurements, the adsorption interactions with antioxidants, slip and antistatic agents, the dated regulatory matrix across REACH, EU 10/2011, CLP, 21 CFR and Proposition 65, the dust profile that separates it from crystalline silica, the comparison with precipitated silica, and the 8 producers our records name.

Table T1. Fumed silica identity card.

Footnote: identity data from PubChem CID 24261 and ECHA CHEM; particle data from the fumed silica record in our source library; regulatory entries from the EU 10/2011 consolidation, ECHA CHEM and the OEHHA list cited in each section. Status as of 24 October 2026.

What Is Fumed Silica?#

Fumed silica is a synthetic amorphous form of silicon dioxide (SiO2, 60.08 g/mol) whose non-porous primary particles of 5 to 50 nm fuse into branched aggregates, which gives a white powder with a bulk density of only 160 to 190 kg/m3. The word "amorphous" is the defining one: the silicon and oxygen atoms carry no long-range crystal order, which separates fumed silica from quartz and cristobalite in structure, in law and in hazard data. Why does a material with the formula of sand behave as a thickener? Its nanometre-scale primaries, fused into aggregates with a very large surface area per gram, act on the liquid around them in a way a coarse mineral grain cannot, which the section on how fumed silica works in plastics sets out below.

By family, fumed silica is a filler, yet it is used less to fill volume than to control rheology and to reinforce. That places it at the opposite end of the filler range from ground carbonates and talc, which are added to fill volume, cut cost and raise stiffness. Fumed silica belongs to the synthetic amorphous silica class of the 13 filler types compared on fillers for plastics.

Is fumed silica the same as pyrogenic silica?#

Yes: fumed silica and pyrogenic silica are two names for the same synthetic amorphous silica, and "pyrogenic" records that it is formed in a flame. Pyrogenic amorphous silica is a third name for the same material, and the brand names Aerosil, Cab-O-Sil and HDK are producer trade names for it, listed with their owners in the supplier section below.

What is the CAS number of fumed silica?#

Fumed silica is CAS 7631-86-9 and EC 231-545-4, the numbers shared by every synthetic amorphous silica, while 112945-52-5 is the former fumed-silica-specific CAS number. The current number therefore does not distinguish fumed from precipitated silica; it identifies silicon dioxide as a substance. Which number belongs to which form is shown below.

A buyer comparing two data sheets with different CAS numbers is often looking at the same class of material. Both numbers resolve to the same record in our plastic additive CAS number lookup.

Why do safety data sheets still print CAS 112945-52-5?#

Safety data sheets still print 112945-52-5 because it is the former fumed-silica-specific number, and older documents keep it; PubChem records that the number was formerly listed under 7631-86-9. A data sheet that gives either number describes synthetic amorphous silica, and the grade name and the specific surface area identify the actual product. What a supplier must print on safety data sheets and GHS labels is set out on our compliance page.

How is fumed silica made?#

Fumed silica is made by flame pyrolysis of silicon tetrachloride (SiCl4), or by vaporising quartz in an electric arc at 3,000 °C (about 5,430 °F), and both routes produce non-porous primary particles of 5 to 50 nm. The 2 production routes recorded in our sources are listed below.

• Flame pyrolysis of silicon tetrachloride: SiCl4 is converted to silicon dioxide in a flame, the route that gives the material its "pyrogenic" name.
• Quartz vaporised in an electric arc: quartz is vaporised at 3,000 °C (about 5,430 °F), and the product is amorphous silica.

Both routes yield the same amorphous product, so the feedstock, silicon tetrachloride or quartz, does not change the identity of the powder. Precipitated silica comes from a different, wet precipitation route, and that difference in route is the origin of the porosity gap described in the comparison section.

What is the difference between silica and fumed silica?#

Silica is the common name for silicon dioxide in all its forms, and fumed silica is one synthetic amorphous form of it, distinct from crystalline silica such as quartz and cristobalite and from biogenic amorphous silica such as diatomaceous earth. The three types of silica relevant to plastics are compared below.

The distinction is more than chemistry. The respirable crystalline forms carry occupational exposure limits and a California Proposition 65 listing that the amorphous forms do not, so "silica" on a label is read with its form attached. The three forms are compared property by property on silica in plastics.

How Does Fumed Silica Work in Plastics?#

Fumed silica works through surface area rather than volume: its 5 to 50 nm primary particles fuse into branched aggregates with 50 to 600 m2 of surface per gram, so a small mass of powder presents a very large interface to the resin or polymer around it. That interface is the working part of the additive. What does it do in each use? The 3 effects of fumed silica in plastics are listed below.

• Thickening and thixotropy of liquid resins: in unsaturated polyester, epoxy and uncured silicone, fumed silica raises viscosity and makes the liquid thixotropic before cure.
• Reinforcement of silicone elastomers: in cured silicone rubber, fumed silica is the reinforcing filler.
• Free flow of powders: added to a powder, fumed silica keeps it pouring and dosing evenly.

Specific surface area, measured by nitrogen adsorption, is the property that drives coupling-agent demand and viscosity for every filler. Fumed silica sits at the top of that scale, and the consequence is that it thickens a liquid at a fraction of the loading a ground mineral needs. A coarse carbonate grain of the same mass offers far less surface, which is why ground minerals extend a compound while fumed silica changes how it flows.

Our source library records the functions of fumed silica but not a molecular mechanism for them. The surface chemistry that explains the thickening effect at molecular level is not yet verified in our records, so this page states the function and the structural fact behind it and goes no further.

What Are the Physical and Chemical Properties of Fumed Silica?#

Fumed silica is a white, non-porous powder with primary particles of 5 to 50 nm, a specific surface area of 50 to 600 m2/g and a bulk density of only 160 to 190 kg/m3 (0.16 to 0.19 g/cm3). The recorded properties are listed in Table T2.

Table T2. Physical and chemical properties of fumed silica.

Footnote: class values apply to synthetic amorphous silica in general. Melting point, pH and moisture are grade-specific and are not stated here; use the producer's technical data sheet.

Two densities appear in the table, and they measure different things. The bulk density of 160 to 190 kg/m3 is the powder as poured, with air included, and it governs handling, bag size and dust. The skeletal density of 2.2 g/cm3 is the solid silicon dioxide itself, and it is the density the silica contributes once it is dispersed in a compound, so a fumed silica never lowers the density of a part the way its powder bulk density suggests.

Particle structure: primary particles, aggregates and agglomerates#

Fumed silica is built in 3 levels: primary particles of 5 to 50 nm, which fuse into aggregates of 0.1 to 1 µm, which in turn cluster into loose agglomerates from 0.3 µm up to millimetre size. The hierarchy is written into EU food-contact law, where FCM No 504 describes synthetic amorphous silica as primary particles of 1 to 100 nm aggregated to 0.1 to 1 µm, with agglomerates of 0.3 µm to millimetre size. The two primary-particle ranges do not conflict: 5 to 50 nm is the value for fumed silica, and 1 to 100 nm is the legal specification for every synthetic amorphous silica.

Table T3. Particle hierarchy of synthetic amorphous silica.

Which level does a compounder actually break down? The specification treats the aggregate as the unit of synthetic amorphous silica and the agglomerate as the loose cluster above it; our records do not state the level that dispersion reaches in a resin. The difference between an aggregate and an agglomerate is defined in our glossary.

Specific surface area: the number that separates fumed silica grades#

Specific surface area, measured by nitrogen adsorption under ISO 9277 (the BET method), is the property that separates one fumed silica grade from another, and it spans 50 to 600 m2/g for fumed silica against 5 to 100 m2/g for precipitated silica. A twelvefold spread inside one product class means that two fumed silica grades can differ more from each other than an average fumed grade differs from a coarse filler.

Surface area, not weight, sets how much coupling agent or surface treatment a filler needs and how strongly it thickens. A fumed silica at 50 to 600 m2/g needs far more treatment per kilogram than a coarse ground carbonate, and the same surface carries the adsorption effects described in the interactions section. How surface area, particle size and oil absorption are measured for every mineral is set out under filler properties.

What Is Fumed Silica Used For? 3 Uses in Plastics#

Fumed silica is used in plastics in 3 ways inside the polymer: as a thickener and thixotrope in unsaturated polyester resins, as a thickener and thixotrope in epoxy resins, and as a reinforcing filler in silicone elastomers. A fourth use sits outside the matrix: fumed silica keeps powders free-flowing. The 3 matrix uses are listed below in the order of the sections that follow.

1. Unsaturated polyester resins: thickener and thixotrope.
2. Epoxy resins: thickener and thixotrope.
3. Silicone elastomers: reinforcing filler, and thickener in the uncured silicone.

Table T4. Fumed silica uses by polymer or system.

Footnote: loadings for fumed silica are grade- and resin-specific and are not yet verified in our knowledge base; we do not print unsourced figures.

1. Thickener and thixotrope in unsaturated polyester resins#

In unsaturated polyester resins (UPR), fumed silica is the thickener and thixotrope: it raises the viscosity of the liquid resin before cure, which is the role our knowledge base records for it in UPR thermosets. The effect acts on the liquid resin only: once the thermoset cures, the silica stays in the solid part as a filler.

Why a nano-structured silica rather than a coarse mineral? The 50 to 600 m2/g of surface reaches the thickening effect at a low mass fraction, while a ground mineral would need a loading high enough to change the cured part. The loading itself is grade- and resin-specific and is shown as not recorded in Table T4. Curing agents, promoters and inhibitors for UPR sit outside this reference's scope, so the resin is named here and its cure chemistry is not.

2. Thickener and thixotrope in epoxy resins#

In epoxy resins, fumed silica plays the same role as in unsaturated polyester: it thickens the liquid resin and makes it thixotropic before cure. Epoxy here means epoxy used as a plastic, in thermoset parts and composites; epoxy adhesives and coatings sit outside this site's plastics-only scope, and so do the amine and anhydride hardeners. Fumed silica is one component of a wider epoxy package that also carries tougheners, other fillers, UV stabilizers and flame retardants. The full package of tougheners, fillers, UV stabilizers and flame retardants is covered under additives for epoxy resins.

3. Reinforcing filler in silicone elastomers#

In silicone elastomers, fumed silica is the reinforcing filler, and it is the use that turns a soft silicone polymer into a usable rubber part. The same powder also thickens the silicone before cure, which our records list alongside its UPR and epoxy roles. No tensile, tear or elongation value for a fumed-silica-filled silicone is recorded in our source library, so the reinforcing effect is stated here as a function and not as a percentage.

In the United States, 21 CFR 177.2600 lists silica among the fillers permitted in rubber articles intended for repeated use in contact with food; the section names silica, not a particular silicone grade. How fillers, antioxidants and peroxides combine in elastomers is covered under plastic additives in rubber and elastomers.

Free-flow agent for powders#

Fumed silica is also used as a free-flow agent, added to powders to keep them pouring and dosing evenly. The very low bulk density of 160 to 190 kg/m3 means a small mass spreads over a large powder volume. Our records name the function but not the powders it is used in. How powder, granule and dust-free delivery forms compare is set out under additive product forms.

Is fumed silica used in thermoplastics? Antiblock, matting and what the record shows#

Our knowledge base records no thermoplastic use for fumed silica: the antiblock and matting roles of synthetic silica in polyethylene and polypropylene film belong to precipitated silica, which works as an antiblock at below 0.5 wt% according to the 2023 cast-film review by Dziadowiec and colleagues. Absence from the record is not proof of absence, so this page says "no use recorded" rather than "not used". Silica is one of the 5 mineral classes compared on antiblock additives.

Porosity is the property the antiblock literature turns on. High pore-volume silica antiblocks are effective at 1,000 to 1,500 ppm, according to Van Essche, Kromminga and Schmidt in the Journal of Plastic Film and Sheeting in 2000, and our records attach that porous antiblock role to precipitated grades, not to non-porous fumed silica.

The same split holds for gloss reduction in thermoplastics. Why precipitated grades are the matting choice is explained under matting agents.

How Is Fumed Silica Characterised and Tested?#

Fumed silica is characterised by 4 measurements, specific surface area (ISO 9277), oil absorption (ISO 787-5 or ASTM D281), bulk density and primary particle size, and its content in a finished compound is measured as ash under ASTM D5630 or ISO 3451. The 5 methods and what each tells a formulator are compared in Table T5.

Table T5. Characterisation of fumed silica.

Footnote: methods for the viscosity and thixotropy of filled resins are not yet in our knowledge base.

Oil absorption, reported in grams of oil per 100 g of filler, is a proxy for binder demand and for viscosity in plastisols and thermosets, which makes it the second number after surface area for a thickener; no fumed-silica value is recorded in our source library. The ash methods are generic for filled plastics: ASTM D5630-22 and ISO 3451 burn off the polymer by muffle furnace (Method A, 5 to 50 g samples) or rapid ash (Method B, 2 to 10 g), and they exclude halogenated polymers. How a compound's silica content is measured by calcination is covered under ash content and filler content testing.

Surface area and oil absorption describe the powder; ash content describes the part. The full catalogue of methods is grouped under testing plastic additives.

How Does Fumed Silica Interact with Antioxidants, Slip Agents and Antistatic Agents?#

Synthetic silica interacts with other additives by adsorbing them on its surface: antioxidants and HALS lose activity at its acid sites, and slip and antistatic agents migrate more slowly to the surface, according to the additive-interaction records in our knowledge base. These records are written for silica and synthetic silica as a class, not for fumed silica by name. The 3 recorded interactions of synthetic silica are listed below.

• Antioxidants and HALS: adsorption at the acid sites of the silica surface causes activity loss, an effect shared with talc and kaolin.
• Slip agents, such as erucamide: the amide adsorbs on the silica; in polypropylene, erucamide and silica together lower the coefficient of friction more than either alone.
• Antistatic agents: adsorption on synthetic silica retards their migration to the surface, according to Ampacet.

A stabiliser package is therefore checked against the filler it shares a compound with. How phenolic and phosphite systems are dosed is covered under antioxidants for plastics.

With up to 600 m2/g of surface, fumed silica can adsorb more of a co-additive per gram than a coarse mineral. That comparative follows from the surface area and the class records; no fumed-silica-specific adsorption value is recorded.

Erucamide is the clearest case, because it relies on reaching the film surface to work. Erucamide and the other slip agents are compared on our slip page.

Surface treatment is the formulator's lever. Silanes are applied to silica as a surface treatment, and silanated synthetic amorphous silica carries its own EU food-contact entry, FCM No 87; our records do not state whether silanisation removes the adsorption effect. The silanes used to treat silica are profiled under silane coupling agents.

What Is the Regulatory Status of Fumed Silica?#

Fumed silica is registered under REACH as silicon dioxide (EC 231-545-4, 371 active dossiers), authorised for EU food-contact plastics as FCM 504, absent from the REACH Candidate List, listed in 21 CFR 178.3297 as a colorant for polymers, and, as amorphous silica, not on the California Proposition 65 list. Every listing that matters for plastics applies to synthetic amorphous silica as a class, so fumed and precipitated silica share it, while the crystalline forms sit under different entries. How these instruments fit together is mapped on plastic additive regulations.

Table T6. Fumed silica regulatory matrix, status as of 24 October 2026.

Three rows of the matrix read "not recorded" because the Annex XIV, Annex XVII, TSCA and Stockholm entries for silicon dioxide have not yet been checked against the official lists with a recorded date. They are left open rather than filled with an assumed "not listed".

Is fumed silica registered under REACH?#

Yes: fumed silica is registered under REACH as silicon dioxide, EC 231-545-4, which carried 371 active registration dossiers in ECHA CHEM on 22 September 2026. The registration covers the substance, not a particular grade, so one EC number stands behind every untreated fumed and precipitated silica on the EU market.

Since the 2020 REACH annex amendments made by Regulation (EU) 2018/1881, registrants must describe the nanoforms they cover, and that duty concerns fumed silica directly. Whether each individual registration covers a nanoform is not verified in our records, so buyers ask the supplier which registration and which nanoform description cover the grade they order. Registration, nanoform duties and the Annex V mineral exemption are explained on REACH and plastic additives.

Is fumed silica allowed in food-contact plastics?#

Yes in the EU: fumed silica falls under FCM No 504 (Ref 86240), the Regulation (EU) No 10/2011 entry for silicon dioxide, whose specification describes synthetic amorphous silica as primary particles of 1 to 100 nm aggregated to 0.1 to 1 µm, and silanated grades have their own entry, FCM No 87 (Ref 86285). Our records hold no specific migration limit for either entry, and the overall migration limit of 10 mg/dm2 (60 mg/kg for articles for infants) applies to every finished plastic article that contains them.

In the United States no filler is "FDA approved" for plastics as such: 21 CFR 178.3297 lists silica as a colorant for polymers at the level needed for the colouring effect, 21 CFR 177.2600 lists silica among the fillers for rubber articles intended for repeated use, and 21 CFR 172.480, the direct food additive listing, is not a plastics clearance. Which 21 CFR sections cover each filler is tabulated under FDA food contact rules.

Does the nano size of fumed silica matter under EU 10/2011?#

Yes, and the entry answers the question: Article 9(2) of Regulation (EU) No 10/2011 allows a nanoform only if Annex I explicitly authorises and specifies it, and FCM 504 does exactly that for synthetic amorphous silica by writing the 1 to 100 nm primary particles and their 0.1 to 1 µm aggregates into the entry. That is the attribute that sets fumed silica apart from most fillers: its particle form is a legal specification, shown in Table T3, and not only a line on a data sheet.

The pattern repeats for 2 other particle-specified materials. Carbon black, FCM No 411, carries a similar specification of 10 to 300 nm primary particles aggregated to 100 to 1,200 nm, and the organoclays FCM No 1030 and 1075 allow nanoform platelets only when they lie parallel to the surface and are fully embedded. Carbon nanotubes and graphene have no such entry, so they cannot be used in EU food-contact plastics.

A nanoform without its own specification therefore has no route into an EU food-contact plastic. The Union list, Article 9(2) and the overall migration limit are explained on EU 10/2011.

Is fumed silica an SVHC?#

No: fumed silica is not a Substance of Very High Concern and does not appear on the REACH Candidate List, which held 253 entries at its last update on 4 February 2026. No SCIP notification duty arises from the silica itself, because that duty attaches to Candidate List substances above 0.1 % w/w in articles.

Every plastic additive that is listed, with its entry date and grounds, is tracked on the SVHC Candidate List.

Is fumed silica listed under California Proposition 65?#

No: amorphous silica, which includes fumed silica, is not on the California Proposition 65 list (OEHHA list edition 31 July 2026, checked 25 September 2026); the listed entry is crystalline silica in airborne particles of respirable size, added for cancer on 1 October 1988. The listing is tied to form and to respirable particle size, which is why the amorphous forms sit outside it.

A Prop 65 warning for "crystalline silica" on a resin or filler data sheet therefore points to a crystalline component, such as quartz or the cristobalite in calcined diatomaceous earth, and not to the fumed silica. Listing dates for every additive are on California Proposition 65.

Is Fumed Silica Safe? Hazard Classification, Dust and Handling#

Fumed silica is not listed as a carcinogen by OSHA, IARC or NTP and amorphous silica is not on the California Proposition 65 list, but the fine powder is an inhalation irritant, so the hazard is the dust in handling, not the silica bound in a finished part. Under the CLP Regulation, Regulation (EC) No 1272/2008, silicon dioxide carries no harmonised Annex VI entry in application as of 22 September 2026; a classification process reached a Risk Assessment Committee opinion on 7 March 2025 whose content and silica form we have not yet verified. Harmonised entries and pending opinions are tracked under CLP classification of plastic additives.

The answer to "is fumed silica harmful?" depends on keeping amorphous and crystalline silica apart. The 3 points that separate fumed silica from crystalline silica are listed below.

• Classification: silicon dioxide has no harmonised CLP classification in application as of 22 September 2026.
• Listings: amorphous silica is not on the Proposition 65 list, while crystalline silica in respirable particles has been listed for cancer since 1 October 1988.
• Occupational limits: the EU binding limit of 0.1 mg/m3 (Directive (EU) 2017/2398) and the OSHA permissible exposure limit of 50 µg/m3 as an 8-hour time-weighted average, with an action level of 25 µg/m3 (29 CFR 1910.1053), are set for respirable crystalline silica and set no value for amorphous silica.

Dust is the exposure route that matters for fumed silica. A bulk density of only 160 to 190 kg/m3 means a small mass of powder becomes a large volume of airborne dust when a bag is opened or a hopper is filled. Buyers ask the producer for the grade's safety data sheet, and the dust-free delivery forms are compared on the product-forms page named in the uses section. No occupational limit specific to amorphous silica and no IARC group number for it are recorded in our source library, so none is given here.

What Are the Alternatives to Fumed Silica?#

The one alternative our knowledge base records for fumed silica is precipitated silica, the other synthetic amorphous silica, and it substitutes only where the job is reinforcement or surface effect rather than thickening a liquid resin. Other thixotropes for unsaturated polyester and epoxy are not recorded as fumed-silica alternatives in our source library, so they are not compared here.

Fumed silica vs precipitated silica#

Fumed and precipitated silica share one CAS number (7631-86-9), one EC number (231-545-4) and one EU food-contact entry (FCM 504), but fumed silica is non-porous with 50 to 600 m2/g of surface, while precipitated silica is porous with 5 to 100 m2/g and agglomerates of 1 to 40 µm. The function split follows the particle: fumed silica thickens resins and reinforces silicone, while precipitated silica reinforces rubber and thermoplastic elastomers, in tyres and shoe soles, and serves as antiblock and matting agent. The porous grade is profiled in full on precipitated silica.

Table T7. Fumed silica vs precipitated silica.

Footnote: the fumed value is powder bulk density; the precipitated value is recorded in our source without that label, so the two density cells are not a like-for-like comparison.

The choice between them is a choice of job, not of chemistry. A formulator who needs a liquid resin to hold its shape reaches for the high-surface pyrogenic grade, and one who needs rubber reinforcement or a rough film surface reaches for the porous precipitated grade.

Who Makes Fumed Silica? Trade Names, Grades and Suppliers#

Fumed silica is made by 8 producers recorded in our knowledge base, Evonik, Cabot, Wacker, Tokuyama, OCI, Heraeus, Orisil and Xunyuchem, and 6 of them sell it under their own trade names: Aerosil, Cab-O-Sil, HDK, Reolosil, Konasil and Zandosil. Two of the producers, Evonik and Tokuyama, also make precipitated silica. Each brand resolves to its CAS number in the plastic additive trade name lookup.

Buyers should compare grades by specific surface area and surface treatment, and ask for the technical data sheet, the safety data sheet and the REACH registration that covers the grade's nanoform. The producers and their recorded brand lines are listed in Table T8.

Table T8. Fumed silica producers and trade names.

Footnote: we list a company only where our knowledge base records it, and a trade name only where it is recorded for that company. Grade names and capacities are not shown.

The same producers sit inside a wider supply base for minerals. Producers of the other fillers are listed in the directory of mineral filler suppliers.

No fumed silica price is recorded in our knowledge base. Price follows the grade, its surface area and its surface treatment, and what drives the silica price by grade is tracked on our price guide.

Untreated and silanated (hydrophobic) fumed silica grades#

Fumed silica is sold untreated and surface-treated, and EU food-contact law separates the two: untreated synthetic amorphous silica falls under FCM No 504, while silanated silicon dioxide has its own entry, FCM No 87 (Ref 86285). The split is the clearest grade division our records hold, and it maps directly onto the two families of grades a buyer meets.

Surface-treated grades are what buyers search for as "hydrophobic fumed silica". Our knowledge base records the silane treatment class and the separate food-contact entry, not the individual treatment agents or water-repellency data, so a grade's treatment is read from its technical data sheet. Silanes, titanates and stearic acid are compared under filler surface treatment.

How Does Fumed Silica Relate to the Other Silica-Based Additives?#

Fumed silica is the higher-surface-area of the two industrial synthetic silicas, in a synthetic amorphous group that also includes precipitated silica, silica gel and colloidal silica, and it sits next to diatomaceous earth, the natural amorphous silica mined from fossil diatoms. The 4 synthetic amorphous silicas and their natural neighbour are listed below.

• Fumed silica: non-porous, 50 to 600 m2/g, thickener and silicone reinforcement.
• Precipitated silica: porous, 5 to 100 m2/g, rubber reinforcement, antiblock and matting.
• Silica gel: a physisorbing desiccant in plastics.
• Colloidal silica: a synthetic amorphous silica named in our class records, with no plastics data recorded.
• Diatomaceous earth: natural, CAS 61790-53-2, FCM 707, 80 to 90 % SiO2; calcined grades contain cristobalite.

Diatomaceous earth is the natural member used in plastics, as a film antiblock. The natural silica used as an antiblock is profiled on diatomaceous earth.

Fumed silica outside plastics#

Most of what search results say about fumed silica concerns coatings, adhesives, cosmetics and pharmaceuticals, which sit outside this site's plastics-only scope, and its US direct-food-additive listing, 21 CFR 172.480, is likewise not a plastics clearance. Those uses run under their own rules and are not covered here, which is why the uses section above names only unsaturated polyester, epoxy, silicone elastomers and powders.

Fumed silica and silica fume are different materials#

Silica fume, also called microsilica, is a concrete material and not the same product as fumed silica, despite the similar name. Concrete admixtures sit outside this reference's scope, so silica fume is not described further here.

Is fumed silica the same as silica gel, the desiccant?#

No: silica gel is a different synthetic amorphous silica, used in plastics as a physisorbing desiccant, while fumed silica is a non-porous thickener and reinforcing filler. Physisorption is reversible, unlike the chemisorption of calcium oxide, and silica gel dispersed in LDPE retains up to two-thirds less water-vapour capacity than pure silica gel, according to Sängerlaub and colleagues in Materials in 2019. How silica gel compares with calcium oxide in film is covered under desiccant masterbatch.

Is fumed silica a nanomaterial?#

Its primary particles are: at 5 to 50 nm they fall below the 100 nm line that defines a nanofiller, and both EU food-contact law (FCM 504) and REACH (since the Regulation (EU) 2018/1881 annex amendments) treat its particle form explicitly. The aggregates of 0.1 to 1 µm and the agglomerates above them are larger, which is why the EU entry specifies all three levels rather than the primary particle alone. The other nano-specified filler class in EU food contact, the organoclays, is covered under nanoclay (organoclay) in plastics.