Polyolefin microporous films

In order to provide maximum dispersion and deagglomeration at these high loadings, extrusion compounding usually involves addition of the filler downstream after the polymer has fully melted. Appropriate selection of the screw configuration and the removal of trapped air are important, as also discussed in Chapter 3. 

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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. 

Monomers, such as n-vinyl pyrrolidinone and hydroxy ethyl methacrylate, have been used to enhance the biocompatibility of films and to control the air permeation and hydrophilicity of microporous films and of non-woven polyolefin materials [10], Significant opportunities exist to pursue other uses in the bio-materials area [11, 12], Attempts to produce grafted films to control gas permeation for use in food packaging applications have not met with success [13], Co-extruded films have proven to be more acceptable in this food packaging area. The modification of films and non-woven materials rely upon low-voltage, self-shielded electron beams. The development of lower cost, low-voltage EB equipment reduces the economic barriers to further development in this area (Fig. 4) [14, 15],... 

Li, J., Zhao, Q.L., Chen, J.Z., Li, L., Huang, J., Ma, Z., et al. Highly ordered microporous films containing a polyolefin segment fabricated by the breath-figure method using well-defined polymethylene-b-polystyrene copolymers. Polym. Chem 1, 164—167 (2010)... 

Separator structures fall into four main categories microporous films, nonwovens, gel polymers, and solid polymers. Microporous films contain small pores (5 to 10 nm in diameter) and are often used for low temperature applications. They are made from nonwoven fibers such as cotton, polyester, glass, polyolefins (PP and PE), PTFE, and PVC. Microporous separators are commonly used with organic electrolytes and in acidic systems. Nonwovens are manufactured as mats of fibers and bind through frictional forces. They exhibit consistent weight, thickness, and degradation resistance but they show inadequate pore order and are difficult to make thinner than 25 pm. Nonwovens are generally made from cellulose, PTFE, PVC, PVdF, or a combination of polyolefins and receive preference in alkaline systems [114]. 

Li-ion cells use thin (10 to 30 /u,m), microporous films to electrically isolate the positive and negative electrodes. To date, all commercially available liquid electrolyte cells use micro-porous polyolefin materials as they provide excellent mechanical properties, chemical stability and acceptable cost. Nonwoven materials have also been developed but have not been widely accepted, in part due to the difficulty in fabricating thin materials with uniform, high strength. ... 

Dry processes involve melting a polyolefin resin, extruding it into a film, thermal annealing, orientation at a low temperature to form micropore initiators, and then orientation at a high temperature to form micropores [9, 10]. The dry process involves no solvent handling, and therefore is inherently simpler than the wet process. The dry process involves only virgin polyolefin resins and so presents little possibility of battery contamination. 

Lower-density E-plastomers have found alternate use in cast film processes to make elastic film laminates with good breathability which contain laminates of liquid impermeable extensible polymeric films with extensible-thermoplastic-polymer-fiber nonwovens and nonwoven webs of polyethylene-elastomer fibers as the intermediate layers. The development relates to a breathable film including an E-plastomer and filler that contributes to pore formation after fabrication and distension of the film. The method and extent of distension is designed to produce a breathable film by stretching the film to form micropores by separation of the film of the E-plastomer from the particulate solids. This film is useful for manufacture of absorbent personal-care articles, such as disposable diapers and sanitary napkins and medical garments. In detail, these constructions comprise a liquid impermeable extensible film comprising polyolefins. The outer layer contains extensible-thermoplastic-polymer-fiber nonwovens, and an elastic intermediate layer contains nonwoven webs of fiber E-plastomers. The intermediate layer is bonded to the film layer and the outer... 

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. 

Asahi Chemical Industry carried out an exploratory investigation to determine the requirements for cellulose based separators for lithium-ion batteries. In an attempt to obtain an acceptable balance of lithium-ion conductivity, mechanical strength, and resistance to pinhole formation, they fabricated a composite separator (39—85 /cellulosic fibers (diameter 0.5—5.0 /pore diameter 10—200 nm) film. The fibers can reduce the possibility of separator meltdown under exposure to heat generated by overcharging or internal short-circuiting. The resistance of these films was equal to or lower than the conventional polyolefin-based microporous separators. The long-term cycling performance was also very comparable. 

Recently Serenyl used a flexible alkaline separator (FAS) in Silver—Zinc cells, which consists of a microporous polyolefin film, with inorganic filler. This can be folded around the silver and/or zinc electrodes to form conventional U wraps or heat sealed bags. They showed that the FAS was not attacked by the electrolyte and helps in inhibiting the shape change of zinc electrode. 

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... 

In contrast, hthium cells require thin micropo-rous separators based on the low conductivity of organic electrolytes. These microporous separators more closely resemble membranes with a thickness of 25 pm or less with pore diameters in the hundredths of microns. They most often are polyolefins consisting of polyethylene or polypropylene. Despite their thinness, up to three layers of separately extruded films may be used. 

Polyolefin materials used for the battery separator are based on a homopolymer or a blend of polyethylene (PE) and polypropylene (PP) in a number of combinations between high density polyethylene (HOPE) and ultrahigh molecular weight polyethylene (UHMWPE). The methods for manufacturing the microporous polyolefin membranes can be divided into the dry process and wet process. Both processes contain an extmsion step to produce a thin film, and employ one or more orientation steps to impart porosity and increase tensile strength. The membranes made by diy process show a distinct slit-pore microstmcmre, while those by wet process feamre interconnected spherical or elliptical pores. 

Funaoka H, Takita K, Kaimai N, Kobayashi S, Kono K (2003) Microporous polyolefin film and process for producing the same. US Patent 6,666,969... 

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