Influence of heat-treatment temperature on the structure and properties of polypropylene microporous membrane

Abstract

The polypropylene (PP) microporous membrane has been used in the Lithium battery as a separator. During the charge and discharge process, the temperature in the battery may rise. To clarify the microstructure change mechanism of membrane during the temperature increase, the PP microporous membranes were heat treated at different temperatures and the microstructure and properties were characterized using differential scanning calorimetry, scanning electron microscopy and atomic force microscopy. When the membranes were treated at temperatures lower than 130°C, no apparent dimension and microstructure changes were observed. The Gurley value (characterizing the air permeability) was kept around 240 s. When the membranes were treated at temperatures higher than 140°C, apparent dimensional shrinkage and the decrease or even disappearance of connecting bridges appeared. The corresponding Gurley value was increased to 1320 s, indicating worse air permeability property. The final heat-setting stage during the preparation of microporous membrane based on melt-stretching mechanism could stabilize the stretched connecting bridges and when the heat-treatment temperature was higher than heat-setting temperature, the crystals formed during heat setting could not support the stretched connecting bridges and resulted in the retraction or even disappearance of connecting bridges. These retracted connecting bridges were converted to new crystalline nodes sitting on both sides of separated lamellae.

Polypropylene-Based Porous Membranes: Influence of Polymer Composition, Extrusion Draw Ratio and Uniaxial Strain

“…According to the theory proposed by other authors [ 7 , 8 , 9 , 10 , 11 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 ], pores that were created during the cold extension were enlarged during the hot stretching step as the stress increased linearly. This is due to the larger flexibility of the tie chains that formed the interconnected bridges between the lamellae structures.…”

“…To avoid the use of solvents, due to their cost, environmental footprint and contamination risks, a dry process based on the stretching of extruded film with a specific structure called MEAUS (melt extrusion-annealing-uniaxial stretching) was developed in recent decades. This process is applicable to semi-crystalline polymers, and three consecutive stages are carried out: (a) fabrication of a precursor film with a row-nucleated lamellar structure; (b) annealing the film to increase the lamellar thickness and removes defects in the crystalline phase; (c) stretching the annealed film at low and high temperatures to create and enlarge the pores [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. After the stretching steps, the film should be heat-set in order to maintain dimensional stability and avoid shrinkage [ 6 , 24 ].…”

Cavitation‐Induced Stress Whitening in Semi‐Crystalline Polymers

“…These cavities can accelerate the breakage of materials which in turn shorten useful life of polymeric products. However, producing cavities is demanded in some special fields, for instance, manufacturing microporous membranes for separation process and polymer‐electret materials . Understanding the nucleation mechanism of cavities is the basis to control the appearance of cavities in polymers, especially for creating commercial products.…”

Porous Membranes Based on Polypropylene-Ethylene Copolymers. Influence of Temperature on Extrusion, Annealing and Uniaxial Strain Stages

“…Polymeric microporous membranes are conventionally produced by two main techniques, one based on phase inversion process and the other is via melt processing extrusion, where a semi-crystalline polymer film with a specific structure is uniaxial stretched in order to create the porous morphology. This stretching method is commonly called MEAUS (melt extrusion-annealing-uniaxial stretching) and consists of three consecutive steps: (a) Melt extrusion of a precursor film with a row-nucleated lamellar structure; (b) annealing of the film to increase the lamellar thickness and remove irregularities; and (c) stretching of the annealed film at room and high temperature to create pores and enlarge their size [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]. Finally, the membranes should be heat-set to prevent shrinkage and to maintain the mean pore size constant [3,21].…”