The other day a client complained that even though he bought a cheaper filler to save on his costs, he never expected the resultant 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? The first point of reference is the Mohs hardness scale. This 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. Sand (or silica) is very abrasive and sits at number 7.
Sometimes, however, a talc 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 they contain so much free silica.
A water-washed kaolin is at 2 on the Mohs hardness scale, just above pure talc. However a kaolin that has simply been mined and milled would have sand milled into it and be much more abrasive. The water-washing process separates the abrasive sand from the 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 results from various local and imported fillers:
A local talc: 339 g.m-2
An Indian water-washed kaolin: 35 g.m-2
A local milled kaolin: 220 g.m-2
A local calcium carbonate: 69 g.m-2
An Egyptian calcium carbonate: 38 g.m-2
A local bentonite: 187 g.m-2
Some other extremely hard and abrasive fillers that you may have come across include fly-ash and zeolite. These are so abrasive that they wear through parts of the test equipment!
Egyptian calcium carbonates come from a soft limestone deposit with very low silica impurity. That makes them extremely soft and non-abrasive.
I guess the moral of the story is to get to know a bit more about how your filler is produced, otherwise have its abrasiveness tested.