The effect of specific nucleation on tensile mechanical behaviour of isotactic polypropylene

Commercial-grade isotactic polypropylene was modified with a specific P-nucleation agent NJ-Star (N,N'-dicyclohexylnaphthalene-2,6-dicarboxamide) in concentrations 0.03, 0.10 wt.% and with a specific alpha-nucleating agent Millad 3988 (1,2:3,4-bis-O-(3,4-dimethylbenzylidene)sorbitol) in a concentration of 1.0 wt.%. Specimens for mechanical studies were prepared by injection moulding. Two types of tensile mechanical testing were performed at room temperature: (1) stress-strain test encompassing the plastic behaviour well behind the yield point and (2) tensile creep in the region of non-linear viscoelasticity. The results derived from the stress-strain traces show a distinct decrease in Young's modulus and yield stress for samples containing the crystalline P-phase as compared with non-nucleated and alpha-nucleated samples. This decrease was more pronounced with samples containing the lower beta-nucleant concentrations (0.03 wt.%). Higher compliance of specimens containing the P-phase was also manifested in their creep behaviour. However, the creep rate of the specimen with the higher nucleant content (0.10 wt.%) did not rise with time so that its creep curve intersected the creep curves of non-nucleated and a-nucleated samples. Thus, at creep times longer than 1000 min, the sample with 0.1 wt.% of the beta-nucleant showed a lower compliance than non-nucleated polypropylene and at 10 000 min reached the compliance of the alpha-nucleated sample. The different softening effect of the beta-phase in the high-strain and low-strain regions has been ascribed to a specific structure of the amorphous interlayer induced by the presence of the beta-crystallites. (C) 2004 Elsevier Ltd. All rights reserved.