Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and...
Reexamination Certificate
2000-02-16
2003-03-25
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
C429S247000, C429S248000, C429S249000, C429S250000, C429S254000
Reexamination Certificate
active
06537695
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of treating a non-woven fabric and to a treated non-woven fabric. The fabric can be used as a separator in an electrochemical device.
BACKGROUND OF THE INVENTION
Non-woven fabrics are used for separation applications relying on the porosity that is available from the non-woven structure. A balance has to be established between appropriate porosity and the physical properties of the fabric, in particular in terms of tensile strength of the fabric. The material and structure of the fabric have to be selected according to the requirements for the fabric when in use. An example of an application for a non-woven fabric is as an electrode separator in an electrochemical device such as an electrochemical cell. Examples of such cells include nickel-cadmium and nickel-metal hydride cells. The separator should be inert towards materials with which it comes into contact in the cell including in particular the alkaline electrolyte and the electrode materials. It should also have physical characteristics which enable it to withstand the treatment encountered during assembly of the device and during use. For example, it should be able to withstand the stresses encountered during spiral winding of the cell components. It should also be capable of resisting the growth of dendrites between the electrodes during recharging. Fabrics formed from polypropylene fibres have appropriate properties for use as separators in many electrochemical devices.
Non-woven fabrics can be made by processes which include (a) melt blowing, (b) spinning, and (c) wet or dry laying. The fibres of fabrics made by spinning and wet or dry laying require bonding to one another for the fabric to have integrity, so that it has the mechanical properties required for satisfactory performance. In the case of fabrics made by spinning, the fibres are bonded to one another by the application of heat and pressure so that the structure of the fabric is stable. In the case of fabrics made by wet or dry laying from polypropylene fibres, polyethylene is incorporated into the fabric, either as fibres consisting just of polyethylene or as bicomponent fibres consisting of a polypropylene core and a polyethylene sheath. The polyethylene in the fabric can provide the necessary bonds as a result of heating the fabric to a temperature that is greater than the softening point of the polyethylene.
A fabric that is made from spun fibres which are then bonded together (a “spun bonded” fabric) can have the disadvantage that the bonds reduce the effective surface area of the fabric that is available to ion transfer by effectively blocking the pores of the fabric. The uneven current distribution that results from this uneven pore distribution can give rise to dendrite formation during recharging of a secondary cell, ultimately leading to a short circuit in the cell. There is therefore a compromise to be reached with such fabrics between mechanical properties that are enhanced by bonds between the fibres and electrochemical performance which is diminished by the bonds.
A fabric that is formed by wet or dry laying of fibres has satisfactory mechanical properties. However, especially when bicomponent fibres are used, the fibre size can tend to be undesirably large, often greater than 15 &mgr;m.
An example of a process for treating a polyolefin non-woven fabric to render it hydrophilic is disclosed in WO-A-93/01622. The process involves impregnating a non-woven fabric formed from polyolefin fibres with an aqueous solution of a vinyl monomer (such as acrylic acid), and exposing the impregnated fabric to ultraviolet radiation while restricting exposure of the fabric to oxygen. The process results in copolymerisation of the vinyl monomer and the polyolefin of the fibres. Surprisingly, it also results in crosslinking of the material of the fabric. This can therefore result in an improvement in the tensile properties of the fibres. The resulting treated fabric can also be found to exhibit good resistance to degradation on exposure to alkaline electrolyte materials.
The technique disclosed in WO-A-93/01622 has been found to have beneficial effects on the physical properties of the polymeric material of the fibres of a non-woven fabric. However, acceptable physical properties of the separator have hitherto been derived from bonds between the fibres of the separator, through the application of heat and pressure or by incorporation of additional bonding materials or both.
SUMMARY OF THE INVENTION
In accordance with the present invention, a non-woven fabric is provided comprising substantially unbonded fibers and a vinyl monomer which is capable of reacting with an acid or a base to form a salt directly or indirectly by a reaction which involves exposure to ultraviolet radiation while impregnated with a solution of the vinyl monomer copolymerized to the surface of the fibers. In a preferred embodiment, the fabric has a tensile strength measured in the machine.direction which is at least about 1.5 times the tensile strength of the fabric prior to the copolymerization reaction.
In accordance with one embodiment of the fabric of the present invention, the fabric is produced by a process such as wet laying, dry laying or fiber spinning.
In accordance with one embodiment of the fabric of the present invention, the mean thickness of the fabric is less than about 400 &mgr;m. In another embodiment, the mean thickness of the fabric is at least about 80 &mgr;m.
In accordance with another embodiment of the fabric of the present invention, the vinyl monomer comprises an ethylenically unsaturated carboxylic acid or an ester thereof.
In accordance with yet another embodiment of the fabric of the present invention, at least about 40% by weight of the fibers comprise polypropylene.
In accordance with another embodiment of the fabric of the present invention, the mean thickness of the fibers is less than about 30 &mgr;m.
In accordance with the present invention, a laminate of the above-described fabric is provided with at least one further non-woven fabric comprising fibers of a hydrophobic polymeric material copolymerized with a vinyl monomer which is capable of reacting with an acid or a base to form a salt directly or indirectly by a reaction.
In accordance with the present invention, an electrochemical device has also been provided comprising an anode, a cathode, a quantity of an electrolyte, and an electrode separator formed from a fabric as set forth above or from a laminate as set forth above.
In accordance with the present invention, a method has also been provided of treating a non-woven fabric comprising substantially unbonded fibers which comprises impregnating the non-woven fabric with a solution of a vinyl monomer capable of reacting with an acid or a base to form a salt directly or indirectly, a solvent comprising a solvent which does not evaporate significantly when exposed to ultraviolet radiation, and exposing the impregnated fabric to ultraviolet radiation while the exposure of the fabric to oxygen is restricted, thereby causing the monomer and the fibers to copolymerize.
The present invention provides a technique for treating a non-woven fabric involving copolymerisation of a vinyl monomer to the surface of the fibres by an ultraviolet radiation initiated reaction which, applied to a fabric formed from substantially unbonded fibres, has been found to improve the physical properties of the fabric as well as render it hydrophilic.
Accordingly, in one aspect, the invention provides a non-woven fabric which is formed from fibres which are substantially unbonded and which has copolymerised to the surface of the fibres a vinyl monomer which is capable of reacting with an acid or a base to form a salt directly or indirectly by a reaction which involves exposure of the fabric to ultraviolet radiation while impregnated with a solution of the vinyl monomer.
In another aspect, the invention provides a method of treating a non-woven fabric which comprises:
(a) impregnating the non-woven fabric with a solution of a
Cook John Anthony
Gentilcore Giovanni
McLoughlin Robert Hamilton
Chaney Carol
Lerner David Littenberg Krumholz & Mentlik LLP
Scimat Limited
Yuan Dah-Wei D.
LandOfFree
Non-woven fabric treatment does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Non-woven fabric treatment, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Non-woven fabric treatment will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3081958