As shown in Figure and Table , the addition of HAp to PLA results in a proportional decrease in the glass transition temperature, ranging from 59.8 °C for pure PLA to 54.7°C for PLA/40µHAp. As observed, the particle size of the filler has no impact on this issue. In most cases reported in the literature, the addition of HAp increases the Tg of the material as the presence of HAp particles reduces the mobility of the PLA chains . However, literature reports indicate that HAp may have no effect , or that the material containing it may exhibit a lower Tg than PLA -much depends on the properties of both the matrix and the filler. Much greater differences between the materials are observed in the position of the cold crystallization peak. As the filler content increases, the cold crystallization temperature shifts to lower values-even by about 10 °C in the case of the PLA/40µHAp composite. This shift is attributed to the nucleating potential of the HAp particles . Once again, no statistically significant differences are observed between composites with nHAp and µHAp. Moving on to the melting behaviour, it can be seen that the melting temperature of pure PLA is 152.5 °C. As the HAp content increases (regardless of type), a second melting peak begins to appear at lower temperatures, indicating the formation of a second type of crystallites in the system. The splitting of the melting peak for the PLA type used in this study has been widely reported in the literature . This paper does not focus on this phenomenon, as it is not relevant to the context of the current research. A similar trend in glass transition temperature is observed during the cooling scan. Furthermore, no exothermic effects are observed during cooling, indicating the ability of the material to crystallise from the melt. The exception is the PLA/20nHAp sample which shows a slight exothermic effect, possibly indicating the onset of polymer crystallisation. The second heating scan shows similar trends to the first scan (Figure , Table ).