Are abrasive fillers wearing your equipment away?

The other day a client complained that even though he had shopped around for a cheaper filler to save on costs, he never expected high wear on his machinery to eat a hole in his profits.

 How can you tell if a filler is going to be abrasive or not? A good starting point is the Mohs hardness scale, which gives you an idea of the hardness of various minerals compared to each other. Diamond is the hardest mineral in the world, so of course it sits at number 10 on the hardness scale. Pure talc is the softest mineral at number 1. 

Mohs hardness of minerals





Zirconium silicate


Titanium dioxide, rutile






Barium sulphate




Calcium carbonate









Sand (or silica/quartz) is very abrasive with a Mohs hardness of 7. Sometimes a mineral is just dug from the ground and milled along with the sand in the deposit. This pushes its abrasiveness sky high. So, the thing to remember is not all talcs are soft! The cheapest ones unfortunately are usually also the most abrasive because the least careful selection has gone into their production.

 With kaolin, abrasiveness also depends how it has been processed. Water-washed kaolin sits at 2 on the Mohs hardness scale, just above pure talc. Water-washing removes most of the free silica (sand) that occurs as an impurity in kaolin deposits.

 However, a kaolin that has simply been mined and milled would have sand milled into it and be much more abrasive.

 An easy way to check whether a particular kaolin is milled or water-washed, is the check the silica content on the data sheet. If it is above 60%, then it is a milled kaolin. Water-washed kaolin has around 47% silica, close to the theoretical value for pure kaolin.

 Another way to find out a mineral’s abrasiveness is to test it in the laboratory on an instrument such as the Einlehner tester. Here are some abrasiveness results for various mineral fillers:


Water-washed kaolin: 35 g.m-2

Milled kaolin: 220 g.m-2

5μ calcium carbonate: 69 g.m-2

Bentonite: 187 g.m-2


Apart from silica, some other extremely abrasive fillers that you may come across include fly-ash and zeolite. These are so abrasive that they wear through parts of the abrasion tester!

With calcium carbonate, one can check whether it comes from a marble deposit or a limestone deposit. Marble is much harder than limestone, even though they are both the same mineral!

I guess the moral of the story is to get to know how your filler is produced, check its silica content, or have its abrasiveness tested.










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