Particulate anitblocking agents reduce blocking by creating a rough surface. The rough surface will then have less contact area with the neighboring surface, which is also rough. A softer, more compliant material will overcome this separation due to the deformation and flow of the polymer.
You could try a slip agent, such a migrating silicone or erucamide. These will diffuse to the surface and create a weak boundary layer. Do you need a silicone free surface?
If you are really hooked on the silica, consider either more silica or larger silica. Or maybe a combination of small and large. You might end up losing your Shore number however.
John Aspen Research, -
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