New polymer concentrates on the basis of polymer additives, like e.g. fillers and flame-retardants, are provided which have in particular an increased bulk density compared to the polymer additives as such. This increased bulk density leads to a substantial improvement in the processability of such concentrates, their dispersibility during compounding and the properties of the resulting polymer compound. Processing improvements include less dust, faster processing and more homogeneous additive dispersion. The invention also provides a process for preparing such new polymer concentrates, a process for preparing polymer compounds containing the new polymer concentrates, the respective polymer compounds and a process for preparing formed parts thereof. Such formed parts have more uniform properties such as density, wall thickness, and in case of the flame-retardants more homogeneous and consistent flame retardancy.