If you formulate paints, coatings, or filled plastics, oil absorption number (OAN) is one of the most practically important properties of a mineral filler — yet it is frequently misunderstood or overlooked in purchase specifications. This guide explains exactly what OAN measures, how it is tested, and how it affects your formulation cost and product performance.
What Is Oil Absorption Number (OAN)?
Oil absorption number is the mass of raw linseed oil (in grams) that 100 g of a dry powder can absorb before forming a coherent, stiff paste. It is standardised under ASTM D281 (Standard Test Method for Oil Absorption of Pigments by Gardner-Coleman Method) and the equivalent ISO 787-5. The result is expressed as g oil / 100 g powder, sometimes written informally as ml/100g since linseed oil density is close to 0.93 g/ml.
OAN is a surface-area-dependent property: it reflects the total surface area of filler particles available to wet out with oil (or, by extension, with polymer binders and plasticisers). A higher OAN means the filler has more surface area per unit mass — generally because particles are smaller, more irregularly shaped, or more porous.
How OAN Is Measured — The Rubbing-Out Method
The ASTM D281 procedure is simple but requires care to reproduce:
- Weigh exactly 5 g of dry mineral powder onto a smooth glass plate.
- Add raw linseed oil dropwise from a burette, kneading the mixture thoroughly with a spatula after each addition.
- Continue adding oil and kneading until the paste just coheres — it should be stiff, not crumbly, and should not break or crumble when pressed. This is the endpoint.
- Record total oil added. Multiply by 20 to get OAN in g oil / 100 g powder.
Key sources of error: moisture in the powder (inflate OAN), subjective endpoint judgement, and inconsistent kneading force. Conditioning the powder at 105 °C for 2 hours before testing improves repeatability.
Typical OAN Values for Common Fillers
The table below shows representative OAN ranges. Actual values vary with particle size distribution, surface treatment, and ore source. Always request product-specific data sheets for procurement decisions.
| Mineral Filler | Typical OAN (g/100g) | Notes |
|---|---|---|
| Calcite (uncoated GCC) | 15–22 | Rajasthan high-purity grade; varies with fineness |
| GCC (uncoated, fine grade) | 15–25 | Finer grades approach upper end |
| Coated calcite (stearic acid) | 10–18 | Coating reduces OAN by 20–35% |
| Dolomite (uncoated) | 18–28 | Higher than calcite due to crystal habit |
| Talc | 30–50 | Platy shape creates high surface per mass |
| Titanium dioxide (TiO₂) | ~16–20 | Rutile grade; varies by surface treatment |
| Kaolin (hydrous) | 35–45 | Platelet shape; high surface area |
Why OAN Matters for Paint Formulation
In paint and coating formulation, OAN directly drives two critical design parameters: binder demand and Critical Pigment Volume Concentration (CPVC).
Binder Demand and Formulation Cost
Every unit of filler surface area must be wetted by the binder (resin + solvent or water). Fillers with high OAN demand more binder to fully coat their surfaces. If insufficient binder is present, the coating film will be porous, chalky, and mechanically weak. If excess binder is used to compensate, raw material cost increases substantially — binders are typically the most expensive component in a paint formulation.
As a practical rule of thumb: switching from an uncoated calcite (OAN ~20 g/100g) to a coated calcite (OAN ~14 g/100g) at 30% filler loading in an interior emulsion paint can reduce binder requirement by 8–12% without changing the Pigment Volume Concentration (PVC), translating to meaningful cost savings at volume.
Critical Pigment Volume Concentration (CPVC)
CPVC is the filler loading level at which just enough binder is present to fill all inter-particle voids and wet all surfaces. Below CPVC, the film is continuous and glossy. Above CPVC, porosity develops, causing scrub resistance to drop sharply. OAN is the primary input used to calculate CPVC for a mixed pigment/filler system. A higher-OAN filler lowers the CPVC threshold, meaning you hit the porosity cliff at a lower filler loading — directly limiting how much cost-effective filler you can include.
Why OAN Matters for Plastics Processing
In polymer compounding, OAN correlates with:
- Plasticiser absorption: Fillers with high OAN absorb more plasticiser from the polymer matrix (especially relevant in flexible PVC), reducing the effective plasticiser available and stiffening the compound more than expected.
- Rheology and melt viscosity: High-OAN fillers increase compound melt viscosity at equivalent loading, raising processing temperatures, torque demand, and energy consumption during extrusion or injection moulding.
- Dispersion quality: Fillers with lower OAN (particularly coated grades) have better surface compatibility with non-polar polymers (PP, HDPE, LDPE), leading to finer dispersion, fewer agglomerates, and better mechanical properties in the final compound.
For masterbatch manufacturers targeting 70–80% filler loading in PP or HDPE, the difference between an uncoated calcite (OAN ~20) and a coated calcite (OAN ~13) can be decisive for achieving pump-able melt flow at the extruder.
What Factors Affect OAN?
1. Particle Size
Finer particles have more surface area per unit mass. As particle size decreases from 10 µm to 2 µm, OAN typically increases by 30–60% for the same mineral chemistry. This is why ultra-fine calcite grades always carry a higher OAN than standard industrial grades.
2. Particle Shape
Irregular, angular, or platelet-shaped particles expose more surface than equidimensional (cuboid or spherical) particles of the same volume. Calcite, being roughly rhombohedral, has a relatively favourable OAN for its fineness. Talc and kaolin, with their extreme platelet aspect ratios, have much higher OAN despite similar mass-weighted particle sizes.
3. Surface Coating (Stearic Acid Treatment)
Coating calcite or GCC with stearic acid (typically 1.0–2.5% by weight) bonds a monomolecular layer of fatty acid to the mineral surface. This layer:
- Reduces the surface energy of the particle, making it more hydrophobic
- Physically blocks some of the surface sites that would otherwise absorb oil
- Reduces inter-particle friction and agglomeration
The result is a reduction in measured OAN of 20–40% compared to the uncoated base mineral. This is the primary technical reason why coated calcite powder commands a premium and is specified for demanding plastics and paints applications.
4. Moisture Content
Residual moisture occupies surface sites that would otherwise absorb oil during the ASTM D281 test. High moisture content artificially lowers measured OAN. Conversely, dried samples give higher (more accurate) OAN values. Always specify moisture content alongside OAN in purchase orders.
How to Specify OAN in a Purchase Order
When ordering mineral fillers, include the following OAN-related clauses in your technical specification:
- Test method: ASTM D281 or ISO 787-5 (specify which; results differ slightly)
- Maximum OAN limit: e.g., "OAN ≤ 18 g/100g" for coated calcite in PP masterbatch
- Sample conditioning: Specify drying at 105 °C for 2 hours before test if moisture sensitivity is a concern
- Certificate of Analysis: Request lot-specific OAN data with each delivery, not just catalogue values
For paint applications, typical maximum OAN limits are: interior emulsion extenders ≤ 22 g/100g; exterior coatings ≤ 18 g/100g; coated filler for premium paints ≤ 15 g/100g. For plastics masterbatch, ≤ 16 g/100g is a common threshold for high-loading coated calcite grades.
Frequently Asked Questions
Need Low-OAN Calcite or Coated GCC for Your Formulation?
Shikhar Microns supplies uncoated and stearic-acid-coated calcite powder from high-purity Rajasthan ore. OAN data and certificates of analysis available for every lot. Bulk supply across India.