Influence of processing strategies on porosity and permeability of β nucleated isotactic polypropylene stretched films

Morphology Development, Structure and Dielectric Properties of Biaxially Oriented Polypropylene

“…For each cast film, a major melting peak characteristic to -form iPP was observed in the ~163-165 °C region (Table 2), with the high-isotacticity PP2-based cast films showing Tm ( ) higher than that of PP1. Additionally, double peaks in the form of 1 and 2 peaks were observed for the cast films below the major -melting peak at ~142 °C and ~150 °C, respectively-this result is in accordance with Offord et al [6], [10] and is typical for -form iPP crystallized both isothermally and nonisothermally below 125 °C. The double melting behavior reflects melting of the original 1-phase and subsequent recrystallization into more stable 2-phase during the DSC heating scan.…”

“…SEM-cross sectional imaging of the BOPP films, as exemplified in Figure 2c-d, revealed varying amounts of porosity and voids parallel to film surface, with the amount of voids/porosity increasing in proportion with the k of the corresponding cast film (Table 2). Mechanisms behind initial cavitation and elongation of voids by biaxial stretching in -iPP films were recently discussed by Offord et al [6], [10]similar mechanisms are likely playing a role in the void formation for the mixed / -phase iPP films studied here.…”

Influence of annealing on stress–strain behaviors and performances of β nucleated polypropylene stretched membranes

“…the monoclinic α form, hexagonal β form and the triclinic γ form [13][14][15][16]. The microstructure and mechanical behaviors of β-iPP have been studied recently through annealing process [1,2,16,17]. Davies [16] found that the ductility of β-iPP decreases with increasing annealing temperatures(T a ), which is quite different from the α-iPP with enhanced fracture toughness [15,18,19].…”

“…On the other hand, the β-iPP has been widely used to fabricate porous membrane for lithium-ion battery separator through a successively biaxial stretching, for the less dense β-crystal (0.921 g/cm 3 ) is likely to transform into more dense α-crystal (0.936 g/cm 3 ) during stretching, leading to the volume contraction and consequent formation of micropores [17,[22][23][24][25][26][27][28]. However, up to now, the separator made from biaxial stretching of β-iPP is still much poorer than those produced through "dry uniaxial stretch" [29][30][31] and "wet biaxial stretch" [32], leading to the limit of its applications in high tech fields such as power battery.…”

“…However, up to now, the separator made from biaxial stretching of β-iPP is still much poorer than those produced through "dry uniaxial stretch" [29][30][31] and "wet biaxial stretch" [32], leading to the limit of its applications in high tech fields such as power battery. Grant [17,26] optimized fabrication conditions based on annealing, which improved the quantity of perfected β-crystalline in the precursor films, and consequently produced higher permeability microporous membranes. However, the traditional annealing method only increased the porosity of β-iPP membranes to a certain extent and the pores distribution was still poor.…”