The structure changes of polypropylene precursor film with different die draw ratio during annealing

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

“…Since β ‐PP lamellae arrange loosely to form a bundle‐like supramolecular structure, β ‐lamella boundaries can separate directly when subjected tensile stress and retain a porous construction 23–25 . Lamellae along various directions present different deformation modes as longitudinal drawing proceeds, such that β ‐lamellae along TD evolve into abundant crazes and cracks, and β ‐lamellae along MD slip closely to form coarse α ‐phase fibrils owing to β – α phase transformation 26–29 .…”

“…[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. [26][27][28][29] During the subsequent transverse drawing, crazes and cracks expand to a final porous construction, while ⊍-phase fibrils cannot transform into pores since the cross-hatched interlocking structure derived from the unique radial and tangential ⊍-lamellae solidifies ⊍-lamella boundaries, which is the fundamental reason for the wide pore distribution of a separator produced using the dry process with asynchronous bidirectional drawing.…”

Novel preparation of lithium‐ion battery wet‐processed separator based on the synergistic effect of porous skeleton nano‐Al2O3in situ blending and synchro‐draw

“…Both processes involve melt extrusion to produce cast films and pore formation under various drawings 12 . For the dry process, the polyolefin is added into the extruder to fabricate cast films including parallel stacked lamellae under a high‐level slit die drawing rate 13,14 . The cast film then undergoes annealing at suitable temperatures to perfect the supramolecular construction, followed by a uniaxial draw perpendicular to the stacked lamellae to generate a porous construction 15,16 .…”

“…12 For the dry process, the polyolefin is added into the extruder to fabricate cast films including parallel stacked lamellae under a high-level slit die drawing rate. 13,14 The cast film then undergoes annealing at suitable temperatures to perfect the supramolecular construction, followed by a uniaxial draw perpendicular to the stacked lamellae to generate a porous construction. 15,16 For the wet process, ultra-high molecular weight polyethylene (UHMWPE) and paraffin oil are mixed uniformly via the screw extruder and extruded to produce cast film based on the thermally induced phase separation (TIPS) mechanism.…”

Low-Cost and Large-Scale Fabricating Technology for High-Performance Lithium-Ion Battery Composite Separators with Connected Nano-Al2O3 Coating

“…For the dry process with uniaxial stretching, a high die draw ratio is imposed on the polypropylene melt to form the casting films with a representative shish-kebab structure. The casting film then experiences annealing and uniaxial stretching along the mechanical direction (MD) to produce a porous structure. , However, the casting film of the wet process is extruded based on the thermally induced phase separation of paraffin oil and ultra-high-molecular-weight PE, and it then undergoes biaxial stretchingz and solvent extraction process to form a porous structure . In the dry process with biaxial stretching, the β-nucleating agent is mixed with PP to fabricate β-crystal polypropylene (β-iPP) casting films, directly followed by the sequential biaxial stretching, which contains longitudinal stretching along the MD to create pores and transverse stretching along the transverse direction (TD) to enlarge pores .…”