Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
2000-10-27
2002-05-07
Berman, Susan W. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C522S033000, C522S042000, C522S044000, C522S079000, C522S080000, C522S085000, C429S249000
Reexamination Certificate
active
06384100
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of graft polymerization and, more specifically, to graft polymerization of non-woven polyolefin fiber sheets, and to batteries in which graft polymerized non-woven polyolefin fiber sheets constitute separators which are positioned between the positive and negative plates.
2. Description of the Prior Art
Battery separators which are non-woven sheets of polyolefin fibers with an acrylic graft polymer on the fiber surfaces are known, being disclosed in a paper presented at the 1986 Power Sources Symposium held in Cherry Hill, N.J., J. Polym. Sci. 34, 671 (1959), and in WO 93/01622, published Jan. 21, 1993. The Symposium paper discloses the use of ionizing radiation to produce such separators, while WO 93/01622 discloses a method for producing such separators which involves immersion of non-woven sheets of polyolefin fibers in a solution of an acrylic monomer which also contains a photoinitiator, followed by ultraviolet irradiation of the sheets. Various publications, e.g., Yao and R{dot over (a)}nby,
Journal of Applied Polymer Science
, Vol 41, 1469-1478 (1990) disclose work carried out at the Royal Institute of Science, Stockholm, Sweden, which involved immersing polyolefin fibers and films in solutions of acrylic monomers and benzophenone, followed by ultraviolet irradiation of the fibers and films to produce an acrylic graft polymer on the surfaces of the fibers and films. Another publication,
Journal of Polymer Science, Polymer Letters Edition
, Vol. 19, pages 457-462 (1981) discloses the immersion of polyolefin films in an acetone solution of benzophenone and, after drying of the films, vapor phase and liquid phase copolymerization of an acrylic monomer with the polypropylene surfaces of the films to produce acrylic graft polymers.
A Journal article entitled “Chemical Absorptive Properties of Acrylic Acid Grafted Non-woven Battery Separators”, refers to work carried out by Leblanc et al. (citing P. Leblanc, Ph. Blanchard, S. Senyarich, Abstract No. 261 ESC/ISE meeting, Paris, (1997) and P. Leblanc, Ph. Blanchard, S. Senyarich,
Electrochem Soc
,. 145, 846, (1998)) as showing that ammonia in a NiMH cell dramatically reduces the self-discharge performance, and cites the latter reference for the statements:
“It was also demonstrated that this effect could be significantly reduced if the free ammonia in the cell could be removed. The tests showed that by using an acrylic acid grafted separator with an ammonia absorption of over 1.5×10
−4
mol/g then all the free ammonia could be eliminated, and the self-discharge performance improved to the levels normally associated with NiCd cells.”
The reference then refers to the process of WO 93/01622 as “capable of grafting non-wovens of all constructions” and states:
“A study was carried out to examine the effect of the non-woven type on the separator's ability to absorb ammonia using Kjeldhal's technique (see tablet). All the grafted materials were grafted to the same level using the same grafting conditions. The results show firstly that an acrylic acid graft is necessary for a non-woven to posses the ability to absorb ammonia. Furthermore, the amount of absorption is a function of the base non-woven material. The strongest correlation is with the fibre size of the non-woven, and therefore also surface area, with fine separators absorbing the most ammonia.
“Samples of the PP fine fibre spunbond were also prepared with two different graft levels, and their ammonia absorption measured (see table 2). These results show that the ammonia absorption is independent of the total amount of acrylic acid grafted onto the polymer backbone, and is further evidence that it is the base non woven which controls the degree of ammonia absorption.”
THE INSTANT INVENTION
The instant invention is based upon the discovery that a non-woven sheet of polyolefin fibers can be immersed in a solution of benzophenone, dried, immersed in a solution of acrylic acid, and subjected to ultraviolet irradiation, while the acrylic acid solution is still on its surfaces, to produce an acrylic graft copolymer on the surfaces of the fibers, and that the reaction proceeds more rapidly than when the non-woven sheet of polyolefin fibers is immersed in a solution of benzophenone and acrylic acid, and subjected to ultraviolet irradiation while the solution of acrylic acid and benzophenone is still on the surfaces of the fibers.
In a further aspect, the invention is based upon the discovery that a non-woven sheet of polyolefin fibers can be immersed in a solution of benzophenone, dried, immersed in a solution of acrylic acid, and subjected to ultraviolet irradiation, while the acrylic acid solution is still on its surfaces, and while the non-woven sheet with the acrylic acid solution on its surfaces is in contact with either air or an inert atmosphere, to produce an acrylic graft copolymer on the surfaces of the fibers.
In a still further aspect, the invention is based upon the discovery that a non-woven sheet of polyolefin fibers can be immersed in a solution of benzophenone, dried, immersed in a solution of acrylic acid, and subjected to ultraviolet irradiation, while the acrylic acid solution is still on its surfaces, and while the non-woven sheet with the acrylic acid solution on its surfaces is inside a polyethylene bag or tube, and is in contact either with air or with an inert atmosphere, to produce an acrylic graft copolymer on the surfaces of the fibers. In a preferred embodiment, after the volatile solvent in the photoinitiator solution vaporizes, a web of the polyolefin article is advanced through the solution of the acrylic monomer, and through a region where it is subjected to ultraviolet irradiation to cause the acrylic monomer to graft to the polyolefin surface, and sheets of a polyolefin such as polyethylene which are wider than the web are introduced above and below the web and the adjacent edges of the polyolefin sheets are sealed to one another to form a tube which surrounds the web, and it is the web surrounded by this tube which is advanced through the region where the web is subjected to ultraviolet irradiation.
In yet another aspect, the invention is based upon the discovery that the ammonia absorption capacity of a non-woven sheet composed of polyolefin fibers which have been graft polymerized with acrylic acid can be increased significantly by subjecting the sheet to corona discharge before the fibers are graft polymerized with acrylic acid.
In still another aspect, the invention is based upon the discovery of a method for producing a non-woven sheet of polyolefin fibers which is hydrophilic in at least one part and hydrophobic or hydrophilic to a different degree in at least one part. The sheet which is hydrophilic in at least one part and hydrophobic in at least one part can be produced by applying a solution of benzophenone or of another photoinitiator to a selected part or to selected parts of a non-woven sheet composed of polyolefin fibers, immersing the sheet, after evaporation of the solvent from the benzophenone or the like solution thereon, in an acrylic acid solution, removing the sheet from the acrylic acid solution, and irradiating the resulting sheet with ultra violet; the irradiation causes the acrylic acid to graft polymerize to the surfaces of the fibers where the benzophenone or the like solution was applied, making them hydrophilic, but does not cause graft polymerization to the surfaces of the fibers where benzophenone or the like was not applied, so that they remain hyrdrophobic. The sheet which is hydrophilic in at least one part and hydrophilic to a different degree in at least one part can be produced by applying a solution of benzophenone or of another photoinitiator to a selected part or to selected parts of a non-woven sheet composed of polyolefin fibers, applying a solution having a different concentration of benzophenone or of another photoinitiator to a different selected part or to different select
Berman Susan W.
Hollingsworth + Vose Company
Purdue David C.
Purdue John C.
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