Preparation of highly oriented β polypropylene and its pore formation mechanism during stretching

Abstract

In this article, oriented β isotactic polypropylene (β‐iPP) with different aggregation structures were prepared by shearing and adding different β nucleating agents. It is found that the formation mechanism of oriented β‐iPP is nucleating agent induced and their crystalline structures strongly depend on the assembly of β nucleating agents. In the case of adding TMB‐5, the β lamellae orient slightly to form elliptical spherulite based on the spherical growth mechanism and the content of β‐crystal reduces substantially with the increasing shear rate. On the other hand, the “bundle‐like” β‐crystal rotates along the extrusion direction to form highly oriented β‐iPP. Based on the preparation of oriented “bundle‐like” β‐crystal, the morphological evolution of β‐iPP during stretching was systematically investigated by two‐dimensional wide‐angle X‐ray diffraction, two‐dimensional small angle X‐ray scattering and scanning electron microscopy. We found that micropores are directly initiated from the weak interfaces between β‐lamellae instead of β‐α phase transformation. In addition, since the β‐lamellae in oriented β‐iPP have identical deformation mode, the membrane with more uniform pore size distribution could be produced compared with that made by the traditional dry process.

Zeolite Coated Separators for Improved Performance and Safety of Lithium Metal Batteries

“…1a). 62,63 Conventional PP and PE-based separators exhibit a broad Gaussian pore distribution due to such manufacturing processes, as shown in Fig. 1a.…”

Dependence of lithium‐ion battery separator porous structure and performance on synchronous bidirectional drawing process regulation of β‐crystal polypropylene

“…For the dry process with asynchronous bidirectional drawing, casting films with pure β ‐phase PP ( β ‐PP) are extruded by mixing β ‐nucleating agents and common α ‐phase PP ( α ‐PP) based on the extruder and then undergo a bidirectional drawing process that involves longitudinal drawing along the mechanical direction (MD, or extrusion direction) and the following transverse drawing perpendicular to MD (transverse direction, TD) 14–16 . This manufacturing technique eliminates annealing steps and does not involve organic extractants such as methylene chloride compared to the other two processes, which has the merits of environmental friendliness, continuous production, low equipment investment cost and simple operation, which attracts much attention and has enormous potential for fabricating LIB separators 17–22 …”

“…[14][15][16] This manufacturing technique eliminates annealing steps and does not involve organic extractants such as methylene chloride compared to the other two processes, which has the merits of environmental friendliness, continuous production, low equipment investment cost and simple operation, which attracts much attention and has enormous potential for fabricating LIB separators. [17][18][19][20][21][22] Since ⊎-PP lamellae arrange loosely to form a bundle-like supramolecular structure, ⊎-lamella boundaries can separate directly when subjected tensile stress and retain a porous construction. [23][24][25] Lamellae along various directions present different deformation modes as longitudinal drawing proceeds, such that ⊎-lamellae along TD evolve into abundant crazes and cracks, and ⊎-lamellae along MD slip closely to form coarse ⊍-phase fibrils owing to ⊎-⊍ phase transformation.…”

Fabrication of Nano-Al2O3 in-Situ Coating Lithium-Ion Battery Separator Based on Synchronous Biaxial Stretching Mechanism of β-Crystal Polypropylene

“…Three commercialized manufacturing techniques are prevalent presently, namely, the dry process under unidirectional tensile, dry process under bidirectional tensile, and wet process, which all contain the extrusion process to produce casting films and tensile process to endow a porous structure. − During the extrusion of the dry process with bidirectional tensile, the cast film constituted by β-crystal polypropylene (β-iPP) is produced via adding β-nucleating catalysts into PP melt, directly undergoing sequentially bidirectional drawing that includes longitudinal tensile along the mechanical direction (MD) and transverse tensile along the transverse direction (TD). , Therefore, the dry process with bidirectional drawing makes allowance for the merits of the capability of continuous fabrication, simple technics, low cost, and environmental friendliness, possessing a great capacity for preparing separators. Whereas, the wide pore diameter dispersion of the separator confines massive application in high technology fields.…”