Injection-moulded α- and β-polypropylenes: II. Tensile properties vs. processing parameters

Abstract

Resuming our comprehensive study, present Part II directs attention towards the effects of mould temperature and holding pressure on the tensile properties of neat and β-nucleated isotactic polypropylenes. A commercial-grade of isotactic polypropylene was modified with a β-specific nucleator. From both the original material (α-iPP) and the β-nucleated material (β-iPP), tensile test specimens were injection-moulded. Stress–strain measurements performed at room temperature revealed a positive influence of higher mould temperatures but negligible effect of holding pressure on the elastic modulus of injection-moulded specimens. It was suggested that at optimum processing conditions the elastic modulus of both α- and β-iPP may be comparable. The softening effect of β-form manifested itself in systematically lower yield stress values of β-iPP as compared to α-iPP. The yield strain of both α- and β-iPP decreased with the increase of mould temperature and the decrease of holding pressure. Prolonged solidification times at higher mould temperatures negatively influenced the strain at break of the specimens; the values of β-iPP exceeded by several folds those of α-iPP. The stress at break of β-iPP decreased correspondingly to strain at break. On the contrary, the increase of strain at break in α-iPP was followed by a decrease of stress at break.