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

• Published in: European polymer journal 2004-07, Vol.40 (7), p.1317-1323
• Main Authors: Raab, Miroslav, Ščudla, Jaroslav, Kolařı́k, Jan
• Format: Article
• Language: English
• Subjects: Applied sciences; Creep; Exact sciences and technology; Mechanical properties; Nucleating agents; Organic polymers; Physicochemistry of polymers; Polypropylene; Properties and characterization; Stress–strain behaviour; β-Phase in polypropylene
• ISSN: 0014-3057; 1873-1945
• DOI: 10.1016/j.eurpolymj.2004.02.027

Commercial-grade isotactic polypropylene was modified with a specific β-nucleation agent NJ-Star ( N, N ′-dicyclohexylnaphthalene-2,6-dicarboxamide) in concentrations 0.03, 0.10 wt.% and with a specific α-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 β-phase as compared with non-nucleated and α-nucleated samples. This decrease was more pronounced with samples containing the lower β-nucleant concentrations (0.03 wt.%). Higher compliance of specimens containing the β-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 α-nucleated samples. Thus, at creep times longer than 1000 min, the sample with 0.1 wt.% of the β-nucleant showed a lower compliance than non-nucleated polypropylene and at 10 000 min reached the compliance of the α-nucleated sample. The different softening effect of the β-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 β-crystallites.