Halloysite nanotubes as a novel β-nucleating agent for isotactic polypropylene

The nucleating ability of halloysite nanotubes (HNTs) towards isotactic polypropylene (iPP) was investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), polarized optical microscopy (POM) and scanning electron microscopy (SEM). HNTs are identified to have dual nucleating ability for α-iPP and β-iPP under appropriate kinetics conditions. The formation of β-iPP is dependent on the HNTs loading in the iPP/HNTs composites. The composite with 20 phr of HNTs is found to have the highest content of β-iPP. Under non-isothermal crystallization the content of β-iPP increases with decreasing of the cooling rate. The maximum β-crystal content is obtained at cooling rate of 2.5 °C/min. The supermolecular structure of the β-iPP is identified as β-hedrites with flower-cup-like and axialite-like arrangements of the lamellae. Under isothermal crystallization the β-crystal can be formed in the temperature range of 115-140 °C. Outside the temperature range, no β-iPP can be observed. The content of β-crystal reaches the maximum value at crystallization temperature of 135 °C. The formation of the β-iPP in the composites is correlated to the unique surface characteristics of the HNTs.