The present invention relates to SiOZ flakes, especially porous SiOz flakes, wherein 0.70 ≤ z ≤ 2.0, especially 0.95 ≤ z ≤ 2.0, comprising a polymer additive in the pores, which provides enhanced (long term) efficacy. Further aspects of the invention are a process for their preparation and a polymer composition containing such SiOz flakes.
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