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The anisotropy and microstructure heterogeneity induced by filler presence and processing has been studied on injection‐moulded discs of poly(propylene) homopolymer (PP) filled with a wide range of concentration (0–60 wt.‐%) of uncoated platy magnesium hydroxide. Differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD) and scanning electron microscopy (SEM) techniques were used to help state the microscopic structure and justify properties measured by dynamic mechanical thermal analysis (DMTA). On the one hand, the anisotropy has been analysed by measuring samples in the parallel and perpendicular directions to the discs surface. On the other hand, the heterogeneity has been characterised by testing samples cut from different zones of the discs. It has been found that the nucleation activity of magnesium hydroxide on the α‐phase of poly(propylene) increases with the filler concentration up to the maximum level studied (60 wt.‐%). The polymer crystalline β‐phase was only detected in unfilled PP and in samples containing less than 20 wt.‐% of magnesium hydroxide. Remarkable differences in the polymer (α‐phase) and particle orientation degrees have been observed in the composites depending on the filler concentration, the disc zone, and depending on whether the analysis was carried out over the disc skin or core. The anisotropy degree of poly(propylene) crystals in the composites was independent on that of the filler particles up to 10 wt.‐% of Mg(OH)2 concentration; however, from 20 wt.‐% of filler the orientation of magnesium hydroxide particles clearly influenced the orientation and anisotropy of the polymer. Moreover, the differences of orientation in the specimen skin were mainly responsible for the heterogeneity of the discs. Composites with lower magnesium hydroxide concentration exhibited the higher heterogeneity. These factors were found to be the main causes of the different dynamic mechanical properties obtained for these materials when the experiments were performed in different zones and/or through different directions in the injection‐moulded discs.

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