Hydrophobic Silica

Hydrophobic silica is basically regular silica (silicon dioxide) that’s been given a special surface treatment so it hates water instead of loving it.

Normal silica is hydrophilic — its surface is covered in little -OH groups that attract water and soak it up. To flip that, manufacturers coat the surface with reactive silanes (or similar chemicals). At NLC, we currently only use polydimethylsiloxane for this treatment on both the fumed and precipitated silica. Those organic molecules latch onto the -OH groups and turn the surface water-repelling.

NLC has both hydrophobic fumed and precipitated versions, and a lot of the final properties still depend on what the starting hydrated silica was like.

A quick rundown on fumed vs. precipitated silica

Both are synthetic amorphous SiO₂, but they’re made differently, so they end up with different structures, properties, and best uses (coatings, rubber, silicone elastomers, cosmetics, food, etc.). The big differences mostly come down to how they’re produced.

Fumed silica tends to be finer and purer, so it’s the go-to when you need really good control over thickness/flow or strong reinforcement. Precipitated silica is coarser and more porous, which makes it cheaper and better for everyday bulk jobs like fillers or absorbents.

Some examples of applications:

Hydrophobic silica is basically silicon dioxide that’s been treated so it repels water instead of soaking it up. People love it because it helps products resist moisture, controls how thick or runny they are, soaks up stuff when needed, and keeps everything more stable. The surface gets coated with polydimethylsiloxane, which makes it great for situations where regular (water-loving) silica would just clump up or break down.

Here’s a quick look at some of the main ways it’s used:

Rheology control and thickening

It thickens things up, stops them from sagging or settling, and gives that nice thixotropic (thick when still, thinner when stirred) behavior so products don’t drip and stay mixed longer.

In adhesives, sealants, and greases → keeps the viscosity right and stops separation.
In paints, coatings, and inks → helps them stay put on the wall or surface and spread better.
In polar resins like epoxy, polyurethane, silicone, or vinyl ester → thickens without making the mix too stiff.

Anti-caking and free-flow agent

It keeps powders from sticking together by pushing moisture away, so they’re easier to pour, scoop, and dose.

In food additives, animal feed, vitamins, and dry mixes → keeps everything flowing freely.
In pharmaceutical powders (like for tablets) and toners → reduces clumping.
In fire extinguisher powders and plastic powders → makes them disperse better.

Foam control (defoaming)

It knocks down foam or stops it from forming in the first place.

Used in adhesives, paints, food processing, and powdered defoamers to keep unwanted bubbles under control.

Adsorption and carrier

It can soak up liquids, gases, or solids and turn sticky pastes or oils into free-flowing powders.

In aerogels → helps grab onto organic pollutants or oily stuff.
In composites and cable gels → boosts performance and stability.

Reinforcement and property enhancement

It strengthens materials and improves things like tensile strength, abrasion resistance, and weather toughness.

In rubber and silicone elastomers → makes them harder, more tear-resistant, and less likely to crack.
In plastics (PVC, polyester resins, etc.) → acts as a solid filler for better durability.
In coatings → improves covering power, adhesion, and even color.

Hydrophobicity and surface treatment

You can also use it to make surfaces themselves water-repellent, giving them a rough, non-wetting texture.

In construction materials or admixtures → better waterproofing.
In personal care and cosmetics → improves water resistance and how powders flow.

Pretty much all of these jobs can be done with either the fumed or precipitated versions of hydrophobic silica.

The Final Connection:

In the context of the silica we’ve been discussing, silane coupling agents (or related silicone treatments like polydimethylsiloxane) are exactly what manufacturers use to convert hydrophilic silica into hydrophobic silica. The silane reacts with the surface silanol groups, covering the water-loving –OH sites and leaving a water-repelling organic layer instead. This is why treated silica no longer absorbs moisture, clumps less, and performs better in non-polar systems.