The other day I was sitting with a customer who said, “I like using kaolin, but it has a high Oil Absorption”. What is this Oil Absorption thing, and why is it important?

Simply put, the Oil Absorption tells you how much resin or polymer the mineral absorbs, also known as the resin demand. This is more important in some industries than in others. It will be important whenever the mineral is mixed with an organic polymer, such as in paint, adhesives and rubber applications. The ceramic chemist though, is not too fussed about Oil Absorption.

How do we measure the Oil Absorption? This is where it gets a bit tricky. Even though standard tests have been developed (e.g. ASTM D281 and ISO 787-5:1980 ), they are very subjective.

What you do is add linseed oil from a burette to 100g of pigment and mix with a spatula until a smooth paste is formed. The number of grams (or millilitres) of oil absorbed is then called the Oil Absorption. One lab technician may mix the paste until it went shiny; another technician may decide on a different end point. See what I mean by being subjective?

The way most mineral producers overcome this problem is to get the same technician to always do the test. A machine called an Absorptometer has now been developed to make this test more reliable, but is not yet widely used.

Surprisingly, the Oil Absorption of one grade of a mineral powder will be different from another grade of the same mineral. This is because it depends a lot on the particle size, and hence the surface area. Makes sense, doesn’t it? The higher the surface area, the more oil would be absorbed. This table compares the Oil Absorption of some typical minerals of different particle sizes:

SOME TYPICAL OIL ABSORPTION VALUES OF VARIOUS MINERALS
MineralAverage particle size (D50)Oil Absorption (g/100g)
KAOLIN157
KAOLIN245
CALCINED KAOLIN150
CALCIUM CARBONATE0.722
CALCIUM CARBONATE217
CALCIUM CARBONATE515
DOLOMITE520
DOLOMITE1515
TALC235
TALC722
PYROPHYLLITE340
TITANIUM DIOXIDE0.426
BARIUM SULPHATE1513

These values may differ from your own data sheet for the same mineral of the same particle size. This is because minerals from different deposits can differ in Oil Absorption results. Not only does Oil Absorption depend on particle size, it also depends on:

Particle shape
Particle packing
Impurities in the mineral

The paint chemist is usually looking for fillers with a low Oil Absorption. This is because he wants his resin to be available for the good things it is needed for, like binding everything together and forming a film. He does not want it all absorbed on the filler surface. However this cannot be avoided completely as all the white minerals and fillers he uses absorb resin to some degree.

Can a high Oil Absorption ever be a good thing? Well, they say that a higher resin demand can lead to increased opacity in high PVC paints. I am not sure if that is true, except to say that our 1-micron kaolin is excellent in high PVC paints. I always thought it was because its particle size is closer to that of titanium dioxide, hence it acts as a great spacer for TiO2. I dunno, what do you think?

The other time a mineral should have a high Oil Absorption is when it gets used in ladies make-up, like face powders. Fine talc is really great for taking the shine off your face!

Oily or not, let’s face it, we need minerals and fillers in many aspects of our lives.

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