Diatomaceous Earth in Silicone Adhesives and Sealants — The Science Behind the Seal

The Unexpected Marriage of a 50-Million-Year-Old Fossil and Modern Industrial Chemistry

When you apply a bead of silicone sealant around a bathroom vanity, seal a double-glazed window unit, bond a car windscreen, or secure a rail carriage panel — you are probably not thinking about fossilised algae. But increasingly, the scientists and engineers who formulate those silicone products are.

Diatomaceous earth — the mineral formed from the microscopic silica skeletons of ancient diatom algae accumulated over tens of millions of years — is emerging as one of the most versatile and technically valuable functional fillers in the silicone adhesive and sealant industry. And the reasons why are rooted in some genuinely fascinating materials science.

Why Silicone Adhesives and Sealants Need Fillers

Unfilled silicone polymer on its own — polydimethylsiloxane (PDMS) — is a remarkable material. It remains flexible from -60°C to above 200°C, resists UV degradation, is chemically inert against most substances, and maintains its properties for decades. But it has a problem: it is mechanically weak, extremely tacky, and prohibitively expensive to use in its pure form.

Fillers are added to silicone formulations to solve these problems — improving mechanical strength, controlling tack and viscosity, reducing cost, and tailoring the final cured properties for specific applications. The most common filler is fumed silica (a synthetic amorphous silicon dioxide produced at very high temperatures). It works well, but it comes with significant drawbacks — high energy consumption in production, high cost, and supply chains concentrated in Europe and China.

This is where diatomaceous earth enters the picture. Natural, abundant, Australian-produced, and — as the research increasingly shows — genuinely effective at improving silicone adhesive performance in ways that are difficult to replicate with synthetic alternatives.

The Key Distinction — Fumed Silica vs Diatomaceous Earth Amorphus Silica

Both are amorphous silicon dioxide (SiO₂), but they are made very differently:

Fumed silica is manufactured synthetically by burning silicon tetrachloride in a hydrogen/oxygen flame at around 1,800°C. Extremely fine particles (~15 nanometres). Very expensive. Energy-intensive production. Synthetic amorphous silica is used in synthetic resins, plastics, lacquers, vinyl coatings, adhesives, paints, printing inks, and silicone rubber.

Natural diatomaceous earth (BSiO₂ product) — formed over millions of years by fossilised diatom algae. Naturally porous, high surface area. Mined and processed without chemicals or heat. Far lower embodied energy. Currently no Australian manufacturer using it as a direct sealant ingredient — an open market.

What the Published Research Shows

The most significant recent study was published in Scientific Reports (Nature Publishing Group) in August 2023, examining in detail how diatomite and various surface modifications of diatomite affected the performance of silicone pressure-sensitive adhesives (PSA).

What makes these results particularly significant is the mechanism. The improvement is not merely from adding bulk filler — it comes from DE's unique physical characteristics: its porous, high-surface-area structure, its natural amorphous silica chemistry, and its particle morphology derived from the original diatom frustule geometry.

The Specific Ways DE Improves Silicone Sealants

1. Rheology Control — The Anti-Sag Problem Solved

Anyone who has applied silicone sealant to a vertical surface knows the problem — the material tends to sag and slump before it cures, ruining a neat bead and requiring frustrating rework. This is a rheology problem — the sealant needs to be fluid enough to apply but thick enough to stay put.

DE is a highly effective thixotropic agent — it thickens the sealant under low shear (at rest) but allows it to flow under high shear (when applied). The porous, interlocking particle structure of DE creates a physical network that resists flow at rest but collapses under applied force. This is exactly the behaviour a sealant formulator wants — workable during application, immovable immediately after.

Imerys (one of the world's largest mineral companies) specifically markets their Celite DE range for this property, describing it as improving "hardness, tack control, cure and rheology for high-performance adhesives and sealants."

2. Thermal Resistance — Silicone Gets Even Better

Silicone polymers are already thermally excellent — most survive continuous service at 200°C. But in demanding applications such as aerospace, rail, automotive engine bays, and industrial ovens, even higher thermal performance is required. Adding DE pushes this ceiling higher.

The mechanism is elegant. The inorganic silica mineral structure of DE — essentially SiO₂ in a porous amorphous matrix — is itself highly thermally stable. As the polymer matrix softens at high temperature, the DE particle network maintains the structural integrity of the adhesive, preventing flow and loss of bond. The 225°C thermal resistance documented in the published research represents a meaningful improvement over standard silicone PSA formulations.

In rail carriage applications — where silicone adhesives are used to bond window panels, floor panels, and interior trim — thermal performance is a safety-critical specification. The same applies in aerospace interior panels and industrial oven seals.

3. Tack and Adhesion — Getting the Balance Right

Pressure-sensitive adhesives must walk a fine line — tacky enough to bond on contact but cohesive enough not to transfer to the substrate when removed. DE helps formulations find this balance more precisely than with polymer adjustment alone.

The research found that small additions of DE — 0.1 to 0.5% by weight — consistently increased adhesion compared to unmodified adhesive. The mechanism relates to the cohesive-adhesive balance: the DE particles create a more compact internal structure that improves cohesion without sacrificing the surface tack needed for initial bonding. Beyond about 1–3% loading, the cohesive benefits begin to outweigh the adhesive surface, reducing tack — so formulation precision matters.

4. Cure Behaviour and Shrinkage Reduction

Silicone sealants cure by a condensation or addition reaction that inevitably involves some volumetric shrinkage. In critical sealing applications — double glazing units, structural glazing facades, precision electronics encapsulation — shrinkage creates stress at the bond line and can lead to adhesion failure or seal cracking over time.

DE's rigid mineral particles act as an inert reinforcement network during cure, physically resisting the polymer's tendency to contract. The 0.1% shrinkage result documented in the research is dramatically lower than typical unreinforced silicone sealants — representing a significant improvement in long-term joint integrity.

5. The Natural Ingredient Story

This is increasingly important commercially. The construction sector, automotive industry, and consumer products market are all under pressure to reduce synthetic chemical content and move toward more sustainable formulations. A silicone sealant that uses naturally occurring amorphous silica (DE) rather than energy-intensive synthetic fumed silica carries a genuinely better environmental profile — and an ingredient story that resonates with green building certification programs like Green Star, LEED, and BREEAM.

BSiO₂ product is Australian-sourced, naturally occurring, never chemically processed, and carries independent laboratory certification from three accredited Australian laboratories. For a sealant manufacturer seeking an Australian-supply-chain natural silica ingredient, there is no other domestic option.

Which Applications Use DE-Filled Silicone Adhesives?

The Particle Size Question — Why Micronising Matters

This is where the practical challenge lies for any natural DE producer wanting to supply the adhesives and sealants market. The research used DE at particle sizes up to 10 µm — and the general consensus in the formulation chemistry community is that sub-10 µm particles perform best as fillers in silicone systems, offering the optimal balance of surface area, dispersion, and rheological contribution without grit or settling.

BSiO₂'s current standard grades have a D50 of 156 µm — well above the target for adhesive filler applications. However, this is exactly why Stage 4 of BSiO₂'s development plan — the acquisition of micronising equipment (specifically a fluidised bed jet mill) — is commercially critical. Micronised DE to D90 <10 µm would open the adhesives and sealants market directly.

A Note on Surface Treatment

The published research also explored surface modification of DE particles using NaOH and KOH solutions to alter the silicon-to-other-elements ratio of the particle surface. Surface-treated DE consistently outperformed untreated DE in adhesion performance across the test matrix — a finding that points toward an interesting product development pathway.

Commercially, silane coupling agents are the most widely used surface treatment for mineral fillers in silicone systems — they create a chemical bridge between the inorganic mineral surface and the organic polymer matrix, dramatically improving stress transfer and interfacial adhesion. BSiO₂ is monitoring this area and welcomes discussions with adhesive formulators interested in developing surface-treated DE grades for specific applications.