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TECHNICAL DATA
Oil Absorption Value in Silica Matting Agents: What OAV Means for Your Formulation
Oil absorption value is one of the most misunderstood parameters in silica selection. It directly controls how much binder a matting agent consumes — and getting it wrong leads to viscosity spikes, pot-life problems, or wasted material. This guide explains what OAV is, how to read it, and how to use it when building or adjusting a formulation.
What Is Oil Absorption Value?
Oil absorption value (OAV) measures how much linseed oil (in grams) a given silica grade can absorb per 100 g of powder before the mixture transitions from a crumbly paste to a coherent ball. It is expressed as g/100g and tested per ISO 787-5.
OAV is a proxy for the internal pore volume and surface area of the silica particle. High-OAV grades have large internal pore structures — they absorb more binder, which increases wet-film viscosity and reduces the effective binder-to-pigment ratio in the dry film.
How OAV Affects Your Formulation
- Viscosity Each gram of absorbed oil reduces the free binder available to lubricate the system. At equal loading, a grade with OAV 300 will increase viscosity significantly more than a grade with OAV 180. In high-solid systems (>70% NV), this can push viscosity beyond application window.
- Dosage correction When switching between grades with different OAVs, adjust loading to maintain equivalent free-binder levels. A rough correction: if OAV increases by 50 g/100g, reduce loading by ~0.5–1.0% to hold viscosity constant.
- Pot life In 2K polyurethane and epoxy systems, high-OAV silica sequesters NCO groups and reactive sites inside pores, shortening effective pot life. Prefer OAV <220 in 2K systems.
- Matting efficiency vs. OAV trade-off Higher pore volume generally improves matting efficiency (more light scattering sites per particle), but the viscosity penalty limits how much you can add. In UV and high-solid systems, low-OAV grades optimized for efficiency are preferred. See our guide on silica matting efficiency factors for detail.
- Sag resistance High-OAV silica absorbs binder and increases thixotropic index, which can improve sag resistance in vertical-application coatings — a useful side effect if rheology is otherwise controlled.
OAV Ranges by Application
| OAV Range (g/100g) | Typical Grade Type | Recommended Systems | Avoid In |
|---|---|---|---|
| 150–200 | Low-absorption, organic-treated | UV 100% solids, high-solid industrial | — |
| 200–260 | Standard wax-treated | Wood coatings, waterborne PUD | 2K systems above 60% NV |
| 260–320 | Medium-high, untreated or wax | Industrial coatings, coil coatings | UV and high-solid systems |
| 320–380 | High-absorption precipitated silica | Powder coatings, leather coatings | Any liquid system above 50% NV |
PRACTICAL RULE
Always measure viscosity after adding matting agent at your target loading before finalising the formulation. OAV from a TDS is measured in linseed oil — your resin system will behave differently. Run a Krebs Unit or cone-plate measurement at 25°C and compare to your application viscosity window before scaling up.
GMATT Series OAV Reference
| Product Series | OAV (g/100g) | Treatment | Primary Use |
|---|---|---|---|
| GMATT 100 | 180–220 | Untreated | Coil, leather (SB) |
| GMATT 200 | 200–260 | Wax-treated | Wood clearcoats, coil |
| GMATT 300 | 220–280 | Organic-treated | UV, thick-film wood |
| GMATT UV | 150–200 | UV-compatible | 100% solid UV |
| GMATT WB | 200–250 | Hydrophilic | Waterborne PUD/acrylic |
| GMATT 600 | 320–360 | Wax-treated | Industrial, leather, coil |
Frequently Asked Questions
Is higher OAV always better for matting efficiency?
Not necessarily. High OAV increases light-scattering capacity per particle, but limits the total amount of silica you can add before viscosity becomes unworkable. In practice, a mid-OAV grade at 5% loading often outperforms a high-OAV grade at 3% loading in the same system.
How do I compare OAV between two suppliers?
Confirm both use ISO 787-5 (linseed oil method). Some suppliers report dibutyl phthalate absorption (DBPA) instead — the values are not directly comparable. DBPA typically runs 10–15% lower than linseed OAV for the same silica. Always specify the test method when requesting TDS data.
Can OAV change during storage?
Yes. Silica that absorbs moisture during storage (especially untreated grades) can show apparent OAV changes because pre-adsorbed water partially fills pores before the oil titration starts. Store in sealed bags in dry conditions and re-test if the silica has been exposed to humidity for more than a few weeks.
What is the OAV impact in powder coatings?
In powder coatings, OAV does not affect liquid viscosity — it affects extrusion torque and melt flow during curing. High-OAV grades (320–360) improve flow levelling by absorbing resin during melt phase, creating a more uniform surface before gelation. See our powder coating application guide for more detail.
How does OAV relate to particle size?
OAV is primarily driven by internal pore structure, not external particle size. Two grades with the same D50 can have very different OAVs depending on how the silica was precipitated or fumed. For the relationship between particle size and matting performance, see our article on particle size and matting efficiency.
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