Polypropylene microporous films

As the separator, microporous polypropylene film (PORP by NPO UFIM, Russia) with the thickness of 25 pm was used. 

The most common material used is cellophane, which is a cellulose film, which acts as a membrane and is capable of resisting zinc penetration. The cycle life of cells utilizing this material is severely limited due to the hydrolysis of the cellophane in alkaline solution. Various methods have been tried to stabilize cellulose materials, such as chemical treatment and radiation grafting to other polymers, but none have, as of now proved economically feasible. The most successful zinc migration barrier material yet developed for the nickel—zinc battery is Celgard microporous polypropylene film. It is inherently hydrophobic so it is typically treated with a wetting agent for aqueous applications. 

As new membranes are developed, methods for characterization of these new materials are needed. Sarada et al. (34) describe techniques for measuring the thickness of and characterizing the structure of thin microporous polypropylene films commonly used as liquid membrane supports. Methods for measuring pore size distribution, porosity, and pore shape were reviewed. The authors employed transmission and scanning electron microscopy to map the three-dimensional pore structure of polypropylene films produced by stretching extended polypropylene. Although Sarada et al. discuss only the application of these characterization techniques to polypropylene membranes, the methods could be extended to other microporous polymer films. Chaiko and Osseo-Asare (25) describe the measurement of pore size distributions for microporous polypropylene liquid membrane supports using mercury intrusion porosimetry. 

The separator in a Li-ion battery is typically a thin (15/xm) microporous polypropylene film. It prevents the electrodes from shorting directly or through Li microdendrite growth on overcharge, and it also serves as a thermal shut-down safety device. When heated above ISO C (for example due to an internal short in a cell) the separator melts and its pores close, thus preventing current flow and thermal runaway. It is common to investigate the shut-down behavior of separators by measurement of cell impedance at selected frequencies, such as 1 kHz, dependent on temperature (Uchida [2003]). 

Microporous polypropylene film Microporous polypropylene film Microporous filled UHMW polyethylene separator Sintered PVC separator... 

The basic cell structure of this system consists of a lithium anode, a microporous polypropylene film separator, and a cathode that is usually composed of 90% V2O5 and 10% graphite, on a weight basis. When it is used in a reserve battery, the prevalent electrolyte is 2M LiAsFg + 0.4M LiBp4 in methyl formate (MF) because of its excellent stability during long-term... 

The most successful zinc migration barrier material yet developed for the nickel-zinc battery is Celgard . This is a microporous polypropylene film which has a typical thickness of 0.025 mm Polypropylene is inherently hydrophobic so the material is typically treated with a wetting agent for aqueous applications. One disadvantage of microporous materials. 

Membranes can be classified as porous and nonporous based on the structure or as flat sheet and hollow fiber based on the geometry. Membranes used in pervaporation and gas permeation are typically hydrophobic, nonporous silicone (polydimethylsiloxane or PDMS) membranes. Organic compounds in water dissolve into the membrane and get extracted, while the aqueous matrix passes unextracted. The use of mircoporous membrane (made of polypropylene, cellulose, or Teflon) in pervaporation has also been reported, but this membrane allows the passage of large quantities of water. Usually, water has to be removed before it enters the analytical instrument, except when it is used as a chemical ionization reagent gas in MS [50], It has been reported that permeation is faster across a composite membrane, which has a thin (e.g., 1 pm) siloxane film deposited on a layer of microporous polypropylene [61],... 

Korikov, A. R, Kosaraju, P. B., and Sirkar, K. K. 2006. Interfacially polymerized hydrophilic microporous thin film composite membranes on porous polypropylene hollow fibers and flat films. Journal of Membrane Science 279 588-600. 

Recently, microporous polypropylene (PP) materials (sheets, films and hollow fibers) have gained acceptance in various fields of separation technology. Usually, these microporous materials are prepared by using uniaxial or biaxial stretching of adequate preforms. Some examples of processing routes are ... 

LIB separators must provide electrical insulation between the positive and negative electrodes while permitting imi transport between them. They are microporous polyolefin films 10-30 pm thick with pores of 0.01-0.1 pm diameter. Most LIB separators are made of high-density polyethylene, although polypropylene is also used to a certain extent... 

The work on auxetic polymers arising from microporous structure has continued. Aider-son et al. [120], in an attempt to produce auxetic material in a more easily usable form, have used melt-spinning to produce auxetic polypropylene fibres. This work was developed further, by way of a study of the processing parameters for melt-spinning of auxetic polypropylene, polyester and nylon fibres [121]. Ravirala et al. [122] have produced auxetic polypropylene film using melt extrusion. Less conventionally, Alderson et al. [123] have produced auxetic polyethylene by a combination of powder compaction and sintering, without an extrusion step. 

Very different microporous separators for alkaline batteries are included in Table 16. The very thin (-25 ftm) films of stretched polypropylene ( Celgard ) are generally employed in combination with... 

They are fabricated from a variety of inorganic, organic, and naturally occurring materials and generally contain pores that are greater than 50—100 A in diameter. Materials such as nonwoven fibers (e.g. nylon, cotton, polyesters, glass), polymer films (e.g. polyethylene (PE), polypropylene (PP), poly(tetrafluo-roethylene) (PTFE), poly (vinyl chloride) (PVC)), and naturally occurring substances (e.g. rubber, asbestos, wood) have been used for microporous separators in batteries that operate at ambient and low temperatures (<100 °C). The microporous polyolefins (PP, PE, or laminates of PP and PE) are widely used in lithium based nonaqueous batteries (section 6.1), and filled polyethylene separators in lead-acid batteries (section 7.3), respectively. 

The materials used in nonwoven fabrics include a single polyolefin, or a combination of polyolefins, such as polyethylene (PE), polypropylene (PP), polyamide (PA), poly(tetrafluoroethylene) (PTFE), polyvinylidine fluoride (PVdF), and poly(vinyl chloride) (PVC). Nonwoven fabrics have not, however, been able to compete with microporous films in lithium-ion cells. This is most probably because of the inadequate pore structure and difficulty in making thin (<25 /rm) nonwoven fabrics with acceptable physical properties. 

Polypropylene and polyethylene microporous films obtained by this method are available from Cel-gard48.5o,54,55 Ube. The dry process is technologically convenient because no solvents are required. However, only a uniaxial stretching method has been successful to date, and as a result, the pores are slitlike in shape and the mechanical properties of films are anisotropic. The tensile strength in the lateral direction is relatively low. 

This simple mass transfer model based on simplified film theory has been proposed to describe the process of facilitated transport of penicillin-G across a SLM system [53]. In the authors laboratory, CPC transport using Aliquat-336 as the carrier was studied [56] using microporous hydrophobic polypropylene membrane (Celgard 2400) support and the permeation rate was found to be controlled by diffusion across the membrane. 

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