Influence of nucleating agents on morphology and properties of injection-molded polypropylene

Isotactic polypropylene (i-PP) samples containing three nucleating agents: 1,3:2,4-bis(3,4-dimethylobenzylideno)sorbitol (DMDBS), bis(4-propylbenzylidene)propilsorbitol (NX8000), and lithium2,2-methylene-bis(4,6-di-tert-butylphenyl)phosphate (NA-71) were prepared via a twin-screw extruder. The role of these additives on i-PP properties was studied using polarized light microscopy (PLOM) with hot stage, atomic force microscopy (AFM), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and Haze-Gard Plus equipment. All the investigated nucleating agents were effective regarding the onset of crystallization, although the influence of the sorbitol derivatives depends on concentration. WAXD measurement indicated that these additives do not affect the typical pattern of α-crystalline i-PP. The nucleated PP samples have a fibrillar transcrystalline structure (compared to spherulites structure of the neat i-PP), which reduces the crystalline phase interaction with visible light and consequently improved its optical feature. The most refined fibrils were formed by i-PP containing lithium organophosphate additive. Compared to DMDBS, NA-71 and NX8000 were more effective at lower concentration (2000 ppm). For example, the addition of 2000 ppm NA-71 increases the crystallization temperature (T_c) of i-PP from 113 to 132°C and its crystallinity index (X_c) from 45% to 51%. Whereas the T_c and X_c of i-PP containing the same amount of DMDBS were 125°C and 49%, respectively. The highest crystallinity index (58%) was recorded when 4000 ppm NX8000 was added to the i-PP before processing. The developed samples can be used as film products or injection-molded parts in automotive interiors.