Analysing the particle size distributions in the composites with the addition of µHAp (Figures and), a reduction in the number of larger grains can be observed. This is due to the previously mentioned fact that, during the composite preparation process, the material is subjected to significant thermal and mechanical stresses, which cause fibre breakage. As a result, the aspect ratio decreases from nearly 4.0 to less than 3.0, and the d90 decreases from 3.65 mm to 2.83 mm when comparing the grains in the powder to those included in the composite. The observed reduction in particle size occurs to a similar extent for the coPBS_10%µHAp and coPBS_20%µHAp composites. The phenomenon of whisker length reduction (which, due to their shape, have the potential to reinforce the material ), together with the appearance of smaller particle size fractions, may negatively affect the reinforcing effect of HAp grains on the material. For the 'nano' type this phenomenon is not observed, and both AR and d90 remain at the similar levels. The above analyses assume that the apatite particles remain thermally stable at the static calcination temperature and that their geometry does not change significantly.