Generally, the a-form is the most stable crystal with excellent
tensile strength but inferior toughness [17–20] while the b-form
has greater toughness but lower thermal stability. Several advantages
of the b-NA has been reported including lower crystalline density,
lower melting point [15,21,22] and higher toughness [23–25].
It was believed that the b-crystal can be achieved under stipulated
conditions such as a temperature gradient, under shearing or by
using a specific nucleating agent [4,15,20].
Among these
approaches, the addition of b-nucleating agent (b-NA) to iPP is
the easiest method for the preparation of iPP rich in b-phase [4].
Many researchers have attempted to produce iPP with high
fraction of b-crystal form. Jiang et al. [12] found a dramatic increment
of the impact strength by 191% when rare earth organic coordination
compound (WBG) was added. They reported that the
spherulite size was significantly decreased compared with that observed
in the neat iPP when WBG was incorporated at only
0.135 wt% [4]. Xiao et al. [26] utilized a hetero-nuclear dimetal
complex of lanthanum and calcium as a b-nucleating agent for
iPP and concluded that the optimum concentration of b-nucleating
agent was only 0.08 wt%. Wang et al. [27] promoted the b-phase in
iPP by utilizing multi-walled carbon nanotube supported calcium
pimelate and found excellent impact toughness compared with
that of the neat iPP.