CA1243074A - Battery separator composition of mixed ionomers - Google Patents
Battery separator composition of mixed ionomersInfo
- Publication number
- CA1243074A CA1243074A CA000541642A CA541642A CA1243074A CA 1243074 A CA1243074 A CA 1243074A CA 000541642 A CA000541642 A CA 000541642A CA 541642 A CA541642 A CA 541642A CA 1243074 A CA1243074 A CA 1243074A
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- Canada
- Prior art keywords
- copolymer
- acrylic acid
- melt index
- acid
- mole percent
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Abstract
IMPROVED BATTERY SEPARATOR
Abstract A composition suitable for use as a battery separator comprising a substantially homogeneous mixture of a copolymer of ethylene and acrylic acid having from 10 to 23 mole percent acrylic acid therein and a melt index of from about 0.1 to 5 in combination with a copolymer of ethylene and acrylic acid having at least 25 mole percent acrylic acid therein and having a melt index of at least about 10. The present invention is further directed to a process of forming a sheet product from the subject composition and the use of the formed product as a battery separator.
Abstract A composition suitable for use as a battery separator comprising a substantially homogeneous mixture of a copolymer of ethylene and acrylic acid having from 10 to 23 mole percent acrylic acid therein and a melt index of from about 0.1 to 5 in combination with a copolymer of ethylene and acrylic acid having at least 25 mole percent acrylic acid therein and having a melt index of at least about 10. The present invention is further directed to a process of forming a sheet product from the subject composition and the use of the formed product as a battery separator.
Description
3~
Background of the Invention $he instant invention is directed to a composition capable of forming a superior battery separator membrane and to a process of forming the membrane from the subject composition.
Alkaline battery systems, because of their high energy density, have great potential for replacing the more conventional lead-acid battery system in a number of terrestrial applications and~or where a light, portable energy source is required. Typical electrode combinations of such battery systems include silver-zinc, nickel-cadmium, and nickel-zinc. The potential of alkaline batteries utilizing nickel-zinc electrode combinations has not been fully realized due to the limitation of the repeated cycling without an irreversible loss of capacity upon repeated recharge. This limitation is due to the zinc electrode and the failure of the battery separator to inhibit zinc dendrite formation between the zinc and nickel electrodes which leads to 2n battery failure.
One of the recognized key components in extending the life and efficiency of the battery is its separator. The separator is a membrane located between the plates which freely permits electrolytic conduction. Contact between ~5 plates may be due to imperfections in the plate structure or due to warping or wrinkling of the plate during use.
Such macro deformations are readily inhibited by any type of sheet meterial which is coextensive with that of the plates. Contact may also occur due to the formation of 3Q dendrites or localized needle like growths on an electrode, such as zinc dendrites formed on a zinc electrode in an alkaline nickel-zinc battery system.
These dendrites bridge the gap between electrodes of opposite polarity either by puncturing the separator membrane located in the gap, or by passing through the pores of the separator. The high degree of solubility of ~2~3~
zinc oxide in alkaline electrolykes normally permits extensive loss of active material from the negative electrode through deposition of the zinc oxide in the separator pores and onto the positive electrode. These factors cause shorting out of the battery system end significantly reduce its effective life. The ability to produce a separator membrane which can effectively act as a dendristatic diaphragm is a required criteria for forming an effective battery 6ystem~
A battery separator which is capable of increasing the efficiency of a battery system and cause it to have a high energy density is highly desired, especially with respect to alkaline battery systems. It is generally agreed that such separators should be (a) resistant to degradation by the alkaline electrolyte and by oxidation due to nascent oxygen, (b) be very thin, a exhibit a high degree of inhibition to dendrite formation and growth, and (d) exhibit a high degree of electrolytic conductivity.
A considerable amount of effort has been directed to providing satisfactory separator materials for secondary alkaline battery systems, which illustrates he difficulty which has been encountered in providing the many diverse characteristics required for efficient functioning as a separator. Microporous separators, that is those that have discrete pores o from about 100 ko 5000 Angstroms;
usually in the form of a tortuous network, exhibit a high degree of electrolyte permeability and, therefore, a high degree of electrical conductivity. However, due to their porosity, such separator lack the ability to inhibit dendrite shorting. The zinc either deposits in the pores of the separator to eventually cause shorting between the positive and neqative electrode pair or replates onto the zinc electrode in the form of trees or needles (dendrites) which form bridge between electrodes of opposite 35 polarity.
~2~3C1 7g~
Separators which have been developed range from various organic microporous films or semi-permeable membranes to relatively rigid layers of inorganic, often ceramic, particles bonded together in some fashion. A
S further type of separator which has developed involYes inorganic particles contained in an organic matrixO
Despite the considerable effort in this field, the development of a viable separator material for secondary alkaline battery systems remains a primary obstacle to widespread utilization of such systemsO
- Microporous separators, such as those described in U.S. 4,287,276, are formed from an organic polymer matrix and contain inorganic particle utilize a wicking phenomenon due to the presence of inorganic material to carry the electrolyte through the separator. Some of the inorganic material or a secondary organic material in the matrix may be removed to provide a porous matrix to reduce the resistivity of such separators.
Separators have also been proposed which are in the for of a membrane that is of a sheet product having _- Yirtually no or very low porosity. The pore size of such membrane separators is normally lets than about 50 Angstroms and, therefore, readily inhibit dendrite penetration. However,-materials, such as polyethylene, used to form such separators exhibit. high resistivity (low conductivity, poor wetting and poor stability.
- Modification of polyethylene membranes has been attempted to overcsme the above discussed defects Copolymers Fred from polyethylene grafted by irradiation with a polar graft-polymerizable monomers, such as acrylic acid or methacrylic acid, have been suggested in U.S.
Patent 3,427,206; 3~615,865; 3,83 ~594; 3,928,497;
4,122,133; 4,230,549 and elsewhere as suitable materials ~3(~7~
to produce membrane separators. As the acrylic~-acid is grafted only in the amorphous regions of the polyethylene and not in the crystalline regions, the resulting separators exhibit irregular properties and instability as shown by high weight loss in accelerated oxidation tests.
Canadian Patent 1,124,436 describes battery separator formed from material prepared by copolymerizing ethylene with small amount of acrylic acid. Such materials have the defects of not being readily processable into sheet products, especially by commercially desired continuous methods, and of exhibiting high resistivity.
It is highly desired to form a membrane separator capable of use in an alkaline battery system. It is further desired to form a separator which is stable to oxidation and other conditions normally encountered in alkaline battery systems. It is still further desired to provide a separator which is capable of inhibiting dendristatic growth yet which has high conductivity. It is still further desired to produce a composition capable of readily forming into thin sheet form.
... . . ..
Summary of the Invention The present invention is directed to a composition, method of forming same and to separators suitable for alkaline battery systems; the composition comprising a combination of (a) a copolymer having a melt index of from 0 to 5 formed from ethylene and acrylir or methacrylic acid and having from about 10 to 23 mole percent ox said acid therein; and ~b) a copolymer having a welt index of about 10 or yreater formed from ethylene and acrylic or methacrylic acid havinq from about 25 to 35 mole percent acrylic acid heroin 35 The weight ratio of (a) to (b) is from 0.3:1 to 2:1.
~3~7~
The invention provides in a battery having an electrolyte, at least one pair of positive and a negative electrodes and a separator membrane between each adjacent positive and negative electrodes, the improvement comprising having the separator membrane formed from at least one sheet of material having a thickness of from 0.5 to 10 mils and formed of a composition comprising a mixture of (a) a copolymer of ethylene and acrylic acid or methacrylic acid, the acid substantially neutralized with an alkali or alkaline earth metal cation and the copolymer having from 10 to 23 mole percent of the acid therein and having a standard load melt index of from 0 to 5; and (b) a copolymer of e-thylene and acrylic acid or methacrylic acid, the acid substantially neutralized with an alkali or alkaline earth metal cation and the copolymer having at least about 25 mole percent of acrylic acid therein and having a standard load melt index of from about 10 to 100; the weight ratio of (a) to (b) being from 0.3 to 2.
- 5a -, 3~
Detailed Description of the Invent_ n It has been surprisingly found that a separator membrane having the desired properties of high inhibition to dendrite formation, high hydrolytic, oxidative and thermal stability under the conditions encountered in an alkaline battery, high conductivity and ease of processing into thin sheet material of good integrity can be formed by using the specific combination of copolymeric materials described in detail hereinbelow.
The composition found capable of achieving the combination of desired properties is formed from two distinct copolymers of ethylene and acrylic or methacrylic acid. The monomeric units of ethylene and acid of each of the copolymers is part of the polymer backbone. Stated another way, the subject copolymers are each a product of simultaneous polymerization of ethylene and acrylic or methacrylic acid or its precursor and not a graft copolymer formed from such monomers.
The first component of the subject composition a copolymer of ethylene and ac;ylic or methacrylic acid (the terms Wacid" or acrylic acida or Wmethacrylic acid" as used herein and in the appended claims shall interchangeably refer to both the acrylic acid and the methacrylic acid unless specifically indicated otherwise) in which the acrylic acid content is from 10 to 23, preferably from 15 to 23 mole percent of the copolymer.
The copolymer is required to have a standard melt index of from 0 to 5 and preferably from l to l This copolymer can be viewed as the low melt index (high molecular weight3, low acrylic acid content component of the present composition.
The second component of the subject composition is a copolymer of ethylene and acrylic acid. This second component is distinctly different from the first component 3~
as described above as it it required to have an acrylic acid content of at least 25 mole percent, preferably from 25 to 35 mole percent. This second copolymer is required to have a standard melt index of from at least about 10 to about 100, and preferably from about 10 to about 20. This copolymer can be viewed as the high melt lndex moderate molecular weight), high acrylic acid content material.
The copolymers of the present invention can be formed by various known methods For example, suitable copolymers can be formed directly from ethylene and - acrylic acid by copolymerizing the monomers using conventional free radical initiators, such as benzoyl peroxide; or can be formed from ethylene and a lower - Cl-C3 alkyl ester of the acrylic acid, preferably the methyl ester, and subsequently hydrolyzed with aqueous alkali metal hydroxide, preferably XOH. Conventional chain transfer agents can be used to obtain the proper degree of polymerization.
The subject composition and separators formed therefrom are a substantially homogeneous mixture of I- - ta) an ethylene/acrylic acid copolymer having from 10 to 23 mole percent acrylic acid therein and having a low melt index of from 0 to about 5, that is, a low melt index, low acrylic acid content copolymer; with (b) an ethylene/acrylic acid copolymer having from 25 to 35 mole percent acrylic acid therein and having a high welt index of prom 10 to 100, that is a high melk index, high acrylic acid content copolymer in which the ratio of pa) to (b) is within the ratio of from 0.3:1 to 2:1 produces a material providing a combination of low electrical resistance, high inhibition to dendr;te penetration and good stability under alkaline oxidizing conditions. Compositions formed from material or in amounts outside of the above required ranges do nut achieve the desired results.
~.Z~3~
The ~opolymer materials prior to formation into a sheet material, must be substantially neutralized with a strong base such as an alkali or alkaline earth metal hydroxide, preferably potassium or sodium hydroxide and most preferably p~tassi~m hydroxide. Thus, the process steps of forming the separator sheet product requires first forming the copolymers of ethylene and acrylic acid or their ester precursors, forming the alkali or alkaline earth metal salt o the acid of the copolymer, preferably 0 with KOH, forming a substantially uniform mixture of the copolymers and processing, such as by pressing, the mixture into a sheet product. When one uses a single ~opolymer of ethylene/acrylic acid, as suggested inCanadian Patent 1,~4,~K, and neutralizes it prior to formation into a sheet product, such material is difficult to process and/or the sheet tends to degrade. The present composition permits formation of the desîred salt product and processabili~y into a sheet product.
It has been presently found that by using the required combination of copolymers, as described above, one nexpectedly attains a composition which can be readily formed into a sheet product using conventional processing techniques. The present composition as its substantially neutralized salt has been unexpectedly found to be readily processable, capable of retaining its integrity during processing, substantially free of voids, pinholes and the like, and, as prepared, provides a sheet product having a high degree of conductivity, being substantially inert to degradation by alkaline electrolyte and having a high degree of inhibition to dendrite penetration The sheet product should not be greater than 10 mils thick, normally from 0.5 to 10 mils with the preferred thickness being 0.5-5 mils~ The sheet product can be used as a single sheet or can be laminated to itself to form a ~2~3~74
Background of the Invention $he instant invention is directed to a composition capable of forming a superior battery separator membrane and to a process of forming the membrane from the subject composition.
Alkaline battery systems, because of their high energy density, have great potential for replacing the more conventional lead-acid battery system in a number of terrestrial applications and~or where a light, portable energy source is required. Typical electrode combinations of such battery systems include silver-zinc, nickel-cadmium, and nickel-zinc. The potential of alkaline batteries utilizing nickel-zinc electrode combinations has not been fully realized due to the limitation of the repeated cycling without an irreversible loss of capacity upon repeated recharge. This limitation is due to the zinc electrode and the failure of the battery separator to inhibit zinc dendrite formation between the zinc and nickel electrodes which leads to 2n battery failure.
One of the recognized key components in extending the life and efficiency of the battery is its separator. The separator is a membrane located between the plates which freely permits electrolytic conduction. Contact between ~5 plates may be due to imperfections in the plate structure or due to warping or wrinkling of the plate during use.
Such macro deformations are readily inhibited by any type of sheet meterial which is coextensive with that of the plates. Contact may also occur due to the formation of 3Q dendrites or localized needle like growths on an electrode, such as zinc dendrites formed on a zinc electrode in an alkaline nickel-zinc battery system.
These dendrites bridge the gap between electrodes of opposite polarity either by puncturing the separator membrane located in the gap, or by passing through the pores of the separator. The high degree of solubility of ~2~3~
zinc oxide in alkaline electrolykes normally permits extensive loss of active material from the negative electrode through deposition of the zinc oxide in the separator pores and onto the positive electrode. These factors cause shorting out of the battery system end significantly reduce its effective life. The ability to produce a separator membrane which can effectively act as a dendristatic diaphragm is a required criteria for forming an effective battery 6ystem~
A battery separator which is capable of increasing the efficiency of a battery system and cause it to have a high energy density is highly desired, especially with respect to alkaline battery systems. It is generally agreed that such separators should be (a) resistant to degradation by the alkaline electrolyte and by oxidation due to nascent oxygen, (b) be very thin, a exhibit a high degree of inhibition to dendrite formation and growth, and (d) exhibit a high degree of electrolytic conductivity.
A considerable amount of effort has been directed to providing satisfactory separator materials for secondary alkaline battery systems, which illustrates he difficulty which has been encountered in providing the many diverse characteristics required for efficient functioning as a separator. Microporous separators, that is those that have discrete pores o from about 100 ko 5000 Angstroms;
usually in the form of a tortuous network, exhibit a high degree of electrolyte permeability and, therefore, a high degree of electrical conductivity. However, due to their porosity, such separator lack the ability to inhibit dendrite shorting. The zinc either deposits in the pores of the separator to eventually cause shorting between the positive and neqative electrode pair or replates onto the zinc electrode in the form of trees or needles (dendrites) which form bridge between electrodes of opposite 35 polarity.
~2~3C1 7g~
Separators which have been developed range from various organic microporous films or semi-permeable membranes to relatively rigid layers of inorganic, often ceramic, particles bonded together in some fashion. A
S further type of separator which has developed involYes inorganic particles contained in an organic matrixO
Despite the considerable effort in this field, the development of a viable separator material for secondary alkaline battery systems remains a primary obstacle to widespread utilization of such systemsO
- Microporous separators, such as those described in U.S. 4,287,276, are formed from an organic polymer matrix and contain inorganic particle utilize a wicking phenomenon due to the presence of inorganic material to carry the electrolyte through the separator. Some of the inorganic material or a secondary organic material in the matrix may be removed to provide a porous matrix to reduce the resistivity of such separators.
Separators have also been proposed which are in the for of a membrane that is of a sheet product having _- Yirtually no or very low porosity. The pore size of such membrane separators is normally lets than about 50 Angstroms and, therefore, readily inhibit dendrite penetration. However,-materials, such as polyethylene, used to form such separators exhibit. high resistivity (low conductivity, poor wetting and poor stability.
- Modification of polyethylene membranes has been attempted to overcsme the above discussed defects Copolymers Fred from polyethylene grafted by irradiation with a polar graft-polymerizable monomers, such as acrylic acid or methacrylic acid, have been suggested in U.S.
Patent 3,427,206; 3~615,865; 3,83 ~594; 3,928,497;
4,122,133; 4,230,549 and elsewhere as suitable materials ~3(~7~
to produce membrane separators. As the acrylic~-acid is grafted only in the amorphous regions of the polyethylene and not in the crystalline regions, the resulting separators exhibit irregular properties and instability as shown by high weight loss in accelerated oxidation tests.
Canadian Patent 1,124,436 describes battery separator formed from material prepared by copolymerizing ethylene with small amount of acrylic acid. Such materials have the defects of not being readily processable into sheet products, especially by commercially desired continuous methods, and of exhibiting high resistivity.
It is highly desired to form a membrane separator capable of use in an alkaline battery system. It is further desired to form a separator which is stable to oxidation and other conditions normally encountered in alkaline battery systems. It is still further desired to provide a separator which is capable of inhibiting dendristatic growth yet which has high conductivity. It is still further desired to produce a composition capable of readily forming into thin sheet form.
... . . ..
Summary of the Invention The present invention is directed to a composition, method of forming same and to separators suitable for alkaline battery systems; the composition comprising a combination of (a) a copolymer having a melt index of from 0 to 5 formed from ethylene and acrylir or methacrylic acid and having from about 10 to 23 mole percent ox said acid therein; and ~b) a copolymer having a welt index of about 10 or yreater formed from ethylene and acrylic or methacrylic acid havinq from about 25 to 35 mole percent acrylic acid heroin 35 The weight ratio of (a) to (b) is from 0.3:1 to 2:1.
~3~7~
The invention provides in a battery having an electrolyte, at least one pair of positive and a negative electrodes and a separator membrane between each adjacent positive and negative electrodes, the improvement comprising having the separator membrane formed from at least one sheet of material having a thickness of from 0.5 to 10 mils and formed of a composition comprising a mixture of (a) a copolymer of ethylene and acrylic acid or methacrylic acid, the acid substantially neutralized with an alkali or alkaline earth metal cation and the copolymer having from 10 to 23 mole percent of the acid therein and having a standard load melt index of from 0 to 5; and (b) a copolymer of e-thylene and acrylic acid or methacrylic acid, the acid substantially neutralized with an alkali or alkaline earth metal cation and the copolymer having at least about 25 mole percent of acrylic acid therein and having a standard load melt index of from about 10 to 100; the weight ratio of (a) to (b) being from 0.3 to 2.
- 5a -, 3~
Detailed Description of the Invent_ n It has been surprisingly found that a separator membrane having the desired properties of high inhibition to dendrite formation, high hydrolytic, oxidative and thermal stability under the conditions encountered in an alkaline battery, high conductivity and ease of processing into thin sheet material of good integrity can be formed by using the specific combination of copolymeric materials described in detail hereinbelow.
The composition found capable of achieving the combination of desired properties is formed from two distinct copolymers of ethylene and acrylic or methacrylic acid. The monomeric units of ethylene and acid of each of the copolymers is part of the polymer backbone. Stated another way, the subject copolymers are each a product of simultaneous polymerization of ethylene and acrylic or methacrylic acid or its precursor and not a graft copolymer formed from such monomers.
The first component of the subject composition a copolymer of ethylene and ac;ylic or methacrylic acid (the terms Wacid" or acrylic acida or Wmethacrylic acid" as used herein and in the appended claims shall interchangeably refer to both the acrylic acid and the methacrylic acid unless specifically indicated otherwise) in which the acrylic acid content is from 10 to 23, preferably from 15 to 23 mole percent of the copolymer.
The copolymer is required to have a standard melt index of from 0 to 5 and preferably from l to l This copolymer can be viewed as the low melt index (high molecular weight3, low acrylic acid content component of the present composition.
The second component of the subject composition is a copolymer of ethylene and acrylic acid. This second component is distinctly different from the first component 3~
as described above as it it required to have an acrylic acid content of at least 25 mole percent, preferably from 25 to 35 mole percent. This second copolymer is required to have a standard melt index of from at least about 10 to about 100, and preferably from about 10 to about 20. This copolymer can be viewed as the high melt lndex moderate molecular weight), high acrylic acid content material.
The copolymers of the present invention can be formed by various known methods For example, suitable copolymers can be formed directly from ethylene and - acrylic acid by copolymerizing the monomers using conventional free radical initiators, such as benzoyl peroxide; or can be formed from ethylene and a lower - Cl-C3 alkyl ester of the acrylic acid, preferably the methyl ester, and subsequently hydrolyzed with aqueous alkali metal hydroxide, preferably XOH. Conventional chain transfer agents can be used to obtain the proper degree of polymerization.
The subject composition and separators formed therefrom are a substantially homogeneous mixture of I- - ta) an ethylene/acrylic acid copolymer having from 10 to 23 mole percent acrylic acid therein and having a low melt index of from 0 to about 5, that is, a low melt index, low acrylic acid content copolymer; with (b) an ethylene/acrylic acid copolymer having from 25 to 35 mole percent acrylic acid therein and having a high welt index of prom 10 to 100, that is a high melk index, high acrylic acid content copolymer in which the ratio of pa) to (b) is within the ratio of from 0.3:1 to 2:1 produces a material providing a combination of low electrical resistance, high inhibition to dendr;te penetration and good stability under alkaline oxidizing conditions. Compositions formed from material or in amounts outside of the above required ranges do nut achieve the desired results.
~.Z~3~
The ~opolymer materials prior to formation into a sheet material, must be substantially neutralized with a strong base such as an alkali or alkaline earth metal hydroxide, preferably potassium or sodium hydroxide and most preferably p~tassi~m hydroxide. Thus, the process steps of forming the separator sheet product requires first forming the copolymers of ethylene and acrylic acid or their ester precursors, forming the alkali or alkaline earth metal salt o the acid of the copolymer, preferably 0 with KOH, forming a substantially uniform mixture of the copolymers and processing, such as by pressing, the mixture into a sheet product. When one uses a single ~opolymer of ethylene/acrylic acid, as suggested inCanadian Patent 1,~4,~K, and neutralizes it prior to formation into a sheet product, such material is difficult to process and/or the sheet tends to degrade. The present composition permits formation of the desîred salt product and processabili~y into a sheet product.
It has been presently found that by using the required combination of copolymers, as described above, one nexpectedly attains a composition which can be readily formed into a sheet product using conventional processing techniques. The present composition as its substantially neutralized salt has been unexpectedly found to be readily processable, capable of retaining its integrity during processing, substantially free of voids, pinholes and the like, and, as prepared, provides a sheet product having a high degree of conductivity, being substantially inert to degradation by alkaline electrolyte and having a high degree of inhibition to dendrite penetration The sheet product should not be greater than 10 mils thick, normally from 0.5 to 10 mils with the preferred thickness being 0.5-5 mils~ The sheet product can be used as a single sheet or can be laminated to itself to form a ~2~3~74
2 or 3 ply laminated sheet product. The subject sheet product can be laminated to other products useful as battery separators. Such materials are exemplified by polyolefin based products disclosed in U.S. Patent 4,287,276 which are formed from a homogeneous ad-5 mixture of from about 5 to 20 weight percent of a polyolefin, fromabout lO to 60 weight percent of a plasticizer for the polyolefin and from about 30 to 75 weight percent of a particulate filler.
The polyolefin normally has an average molecular weight of at least lO0,000 and i5 generally selected from polyolefins having an average molecular weight of fxom lO0,000 to about 2,000,000.
The polyolefin can be selected from homopolymers such as poly-ethylene or polypropylene or from copolymers formed from an admixture of hydrocarbon olefinic monomers such as etllylene, propylene, butene and the like. Further, the polyolefin can be comprised of a mixtuxe of sigh molecular weight and low molecular weight polyolefins. It i6 preferable that the low molecular weight polymer be the major component of the polyolefin component.
The plasticizers useful in forming polyolefin-based membrane sheet products, as described in U.S. Patent 4,287,276 can be sol-uble or insoluble in water. Representative of water insolubleplasticizers are organic esters, such as the sebacates, phthalates, stearates, adipates end citrates; epoxy compounds such as epoxidized vegetable oil; phosphate esters, such as tricresyl phos--- phate-; hydrocarbon material, such as petroleum oil including lubricating oil and fuel oils; hydrocarbon resin and asphalt; low molecular weight polymers, such as polyisobutylene, polybutadiene, polystyrene, atactic, polypropylene, terpene resins, and linseed oil. Illustrative of water voluble plasticizers are ethylene glycol, polyethylene glycol, polypropylene glycol, glycerol and ethers and ester thereofO
Fillers found useful for forming polyolefin basad separator membranes have an average particle size of from about 0.01 to about 10 mi~ron~, a surface area of from lO0 to 385 moo and a pore volume (BET) 0~ at least 0.075 cc/gm. The most desir-able ~epa-ators are formed from the above described polyolefin and plasticizer with titania, alumina, ~agnasium hydroxide or calcium ~Z~7~
hydroxide. Conductive carbon black, in from about 0.25 to 5 weight percent, can also be used as an additional filler.
Battery separator membranes are generally produced by~blending the composition, forming the composition into sheets and subsequently extracting from the sheet at least a portion of the plasticizeer by means of a suitable solvent. The com.~osition of ale resultant separator will depend upon the degree of extraction of the plasticizer and normally will have from 7 to 30 percent of polyolefin, about 50 to ~3 percent filler and from 0 to 15 percent plasticizer.
Separator membranes can be formed from a composite of the battery separator membrane of U.S. Patent 4,~87,276, as well as similar membranes which have laminated to at least one of its surfaces a sheet product of an ethvlene/acrylic acid material as described in the presen$ application. Such a composite laminate material provides a separator product of good conductiYity and exceptionally high dendristatic inhibition.
The subject sheet product has bPen also found to be capable of forming a coating directly on battery electrode plats, such as by dipping tlle electrode plates into a solution containing the mixture of copolymers and air drying.
The following examples are made for illustrative purposes only and are not meant to be a limitation on the invention, as defined by the claims appended hereto. All parts and percentages are by weight unless otherwise indicated.
The determination of electrolytic resistivity (ohm-cm) or conductivity of ale samples below were conducted by the procedure described in "Characteristics of Separators for Alkaline Silver Oxide Zinc Secondary Batteries- Screening methods", edi~d by J.E. Cooper and A, Fleischer, Chapter 6, Electrical Resistance Direct Current ~1ethod by 3, Lander and R. Weaver, Pg. 53.
The determination of inhibition to zinc dendrite penetration of the samples below were conducted by the procedure described in characteristics of Separators for Alkaline Silver Oxide Zinc Secondary Batteries: Screening methods" by J.E. Cooper and A.
Fleischer, Chapter 12, Zinc Penetration by G.A. Dalin and F.
Solomon, Pave 129.
The stability of the samples as shown by weight Cain (~) or 103s (-I was determined by first ~u~ecting each of -9a-~30~7~
the samples to a 45 weight percent KOH solution~at room temperature for 24 hours, blotting, weighing duplicate samples prior to testing, subjecting each of the samples for 96 hours to a 45 weight percent aqueous KOH solution with nascent oxygen maintained at 80 C., blotting and reweighing the sample to determine any weight changeO
Changes of 20 percent or greater ore undesirableO
EXAMPLE I
A ~opolymer of ethylene/acrylic acid was formed by copolymerizing ethylene and acrylic acid monomers together using standard free radical polymerization technîqueO The monomers were present in amounts to form a polymer having 32 weight percent acrylic acid units therein. The Standard Load melt index (ASTM D-1238) of the polymer was 16.1. This polymer is identified as Copolymer A.
A second copolymer of ethylene/acrylic asid was formed by copolymerizing ethylene and an alkyl ester of acrylic acid together by standard techniques to form a high molecular weight copolymer. The material way hydrolyzed and acidified to form a copolymer hav;ng a Standard Load melt index ox 0.5 ~ASTM D-1238) and having 16 percent free acrylic acid units therein. This polymer is identified as Copolymer B, 51.2 part of copolymer A, 73.8 parts of copolymer B
(A to B ratio of 1.45) and 900 parts of water were placed in a high speed mixer. To this mixture was added 87.5 parts of a 34.1 percent potassium hydroxide solution. The mixer was sealed, heated to 110 Cr and run for 30 minutes ~6 p5i~ and then cooled to ambient temperature. The mixture was removed from the mixer and dried under vacuum to form a dry, uniform composition of the potassium metal neutralized salt of copolymers A and B.
A portion of the dried composition was placed between 2 Mylar~sheets and pressed at 149 C. into a sheet of ~i~2~3~
about 5 mils thickness The sheet did not exhibit the presence of pin holes.
Samples of the sheet were tested according to the methods described above for Electrolytic Resistivity PER) (67.9 ohm-cm after 1 day, 67.0 ohm cm after 1 week, 68.7 ohm-cm after 30 days); Dendrite Penetration Time (DP) (greater than 200 min.) and Stability (4.1 percent weight loss). The formed sheet product exhibited good combination of properties as a battery separator.
EXAMPLE II
A composition and sheet product was formed in the same manner as described in Example I above except that 60 parts of the 34.1 % potassium hydroxide solution was usedO The resultant product exhibited an ER of 65 ohm-cm;
DP of greater than 200 and stability of 1.4 weight percent weight loss. The formed sheet product exhibited a good combination of properties as a battery separator EXAMPLE III
A composition and sheet product was formed in the same manner as described in Example I above except that 71.7 parts of copolymer A, 53.3 parts copolymer B (A to B ratio o$ 0.74) and 64.7 parts 34.1 % KO~ solution were used.
The resultant product readily pressed into a thin pin hole free sheet of about 5 Gil thick. The sheet product exhibited an OR of 44.7 ohm-cm, a DP of greater than 200 and stability of 14.8 perceng (wt.) saint The product exhibited a good combination of properties as a battery separator.
EXAMPLE IV
A composition and sheet product was formed in the same manner as described in Example I above except what 92.2 ~2~3~7~
parts copolymer A, 32.8 parts of copolymer B (A to B ratio of 0.35) and 69.5 parts of 34.1 4 potassium hydroxide solution were used. The resultant product readily pressed into a thin pin hole free sheet of about 5 mils thick.
5 The sheet exhibited an ER of 30~2 ohm-cm~ a DP of greater than 200 and stability of 14.8 percent (wt.) gain. The product exhibited a good combination of properties as a battery separator.
hXAMPLE V
For comparative purposes; a composition and sheet product was formed from copolymers A and B in a ratio of 0.11. The formed sheet product had poor stability as shown by swelling Jo an undesirable extent and exhibiting a 44.2 percent gain in weight when exposed, all above to hot alkaline solution.
EX~MPLE_VI
For comparative purposes only, several samples were formed by the process described in Example I above except that copolymer B was substituted by a ethylene/acrylic acid copolymer formed in the manner described for copolymer A except having an acrylic acid content of 20 percent and a Melt Index of 15 (Copolymer C3. merefore, these samples represènt a mixture of (a high acrylic acid, high melt index and (b) low acrylic acid, high melt index.
Table I below summariæes the results exhibited by these materials.
TABLE I
A/C ER DP Stability 1.3 64 >20~ + 9.6 0u66 52 ~200 ~36.9 0.31 3g ~200 ~32.2 ~3~7~
The results recorded in Table I show, when compared to sample of Examples I to IV, that compositions having comparable ratios o copolymers, do not exhibit the desired combination of properties when both are formed with high melt index materials. ER values are unexpectedly higher (undesired) and the stability is poorer in each instance.
.... . ..
The polyolefin normally has an average molecular weight of at least lO0,000 and i5 generally selected from polyolefins having an average molecular weight of fxom lO0,000 to about 2,000,000.
The polyolefin can be selected from homopolymers such as poly-ethylene or polypropylene or from copolymers formed from an admixture of hydrocarbon olefinic monomers such as etllylene, propylene, butene and the like. Further, the polyolefin can be comprised of a mixtuxe of sigh molecular weight and low molecular weight polyolefins. It i6 preferable that the low molecular weight polymer be the major component of the polyolefin component.
The plasticizers useful in forming polyolefin-based membrane sheet products, as described in U.S. Patent 4,287,276 can be sol-uble or insoluble in water. Representative of water insolubleplasticizers are organic esters, such as the sebacates, phthalates, stearates, adipates end citrates; epoxy compounds such as epoxidized vegetable oil; phosphate esters, such as tricresyl phos--- phate-; hydrocarbon material, such as petroleum oil including lubricating oil and fuel oils; hydrocarbon resin and asphalt; low molecular weight polymers, such as polyisobutylene, polybutadiene, polystyrene, atactic, polypropylene, terpene resins, and linseed oil. Illustrative of water voluble plasticizers are ethylene glycol, polyethylene glycol, polypropylene glycol, glycerol and ethers and ester thereofO
Fillers found useful for forming polyolefin basad separator membranes have an average particle size of from about 0.01 to about 10 mi~ron~, a surface area of from lO0 to 385 moo and a pore volume (BET) 0~ at least 0.075 cc/gm. The most desir-able ~epa-ators are formed from the above described polyolefin and plasticizer with titania, alumina, ~agnasium hydroxide or calcium ~Z~7~
hydroxide. Conductive carbon black, in from about 0.25 to 5 weight percent, can also be used as an additional filler.
Battery separator membranes are generally produced by~blending the composition, forming the composition into sheets and subsequently extracting from the sheet at least a portion of the plasticizeer by means of a suitable solvent. The com.~osition of ale resultant separator will depend upon the degree of extraction of the plasticizer and normally will have from 7 to 30 percent of polyolefin, about 50 to ~3 percent filler and from 0 to 15 percent plasticizer.
Separator membranes can be formed from a composite of the battery separator membrane of U.S. Patent 4,~87,276, as well as similar membranes which have laminated to at least one of its surfaces a sheet product of an ethvlene/acrylic acid material as described in the presen$ application. Such a composite laminate material provides a separator product of good conductiYity and exceptionally high dendristatic inhibition.
The subject sheet product has bPen also found to be capable of forming a coating directly on battery electrode plats, such as by dipping tlle electrode plates into a solution containing the mixture of copolymers and air drying.
The following examples are made for illustrative purposes only and are not meant to be a limitation on the invention, as defined by the claims appended hereto. All parts and percentages are by weight unless otherwise indicated.
The determination of electrolytic resistivity (ohm-cm) or conductivity of ale samples below were conducted by the procedure described in "Characteristics of Separators for Alkaline Silver Oxide Zinc Secondary Batteries- Screening methods", edi~d by J.E. Cooper and A, Fleischer, Chapter 6, Electrical Resistance Direct Current ~1ethod by 3, Lander and R. Weaver, Pg. 53.
The determination of inhibition to zinc dendrite penetration of the samples below were conducted by the procedure described in characteristics of Separators for Alkaline Silver Oxide Zinc Secondary Batteries: Screening methods" by J.E. Cooper and A.
Fleischer, Chapter 12, Zinc Penetration by G.A. Dalin and F.
Solomon, Pave 129.
The stability of the samples as shown by weight Cain (~) or 103s (-I was determined by first ~u~ecting each of -9a-~30~7~
the samples to a 45 weight percent KOH solution~at room temperature for 24 hours, blotting, weighing duplicate samples prior to testing, subjecting each of the samples for 96 hours to a 45 weight percent aqueous KOH solution with nascent oxygen maintained at 80 C., blotting and reweighing the sample to determine any weight changeO
Changes of 20 percent or greater ore undesirableO
EXAMPLE I
A ~opolymer of ethylene/acrylic acid was formed by copolymerizing ethylene and acrylic acid monomers together using standard free radical polymerization technîqueO The monomers were present in amounts to form a polymer having 32 weight percent acrylic acid units therein. The Standard Load melt index (ASTM D-1238) of the polymer was 16.1. This polymer is identified as Copolymer A.
A second copolymer of ethylene/acrylic asid was formed by copolymerizing ethylene and an alkyl ester of acrylic acid together by standard techniques to form a high molecular weight copolymer. The material way hydrolyzed and acidified to form a copolymer hav;ng a Standard Load melt index ox 0.5 ~ASTM D-1238) and having 16 percent free acrylic acid units therein. This polymer is identified as Copolymer B, 51.2 part of copolymer A, 73.8 parts of copolymer B
(A to B ratio of 1.45) and 900 parts of water were placed in a high speed mixer. To this mixture was added 87.5 parts of a 34.1 percent potassium hydroxide solution. The mixer was sealed, heated to 110 Cr and run for 30 minutes ~6 p5i~ and then cooled to ambient temperature. The mixture was removed from the mixer and dried under vacuum to form a dry, uniform composition of the potassium metal neutralized salt of copolymers A and B.
A portion of the dried composition was placed between 2 Mylar~sheets and pressed at 149 C. into a sheet of ~i~2~3~
about 5 mils thickness The sheet did not exhibit the presence of pin holes.
Samples of the sheet were tested according to the methods described above for Electrolytic Resistivity PER) (67.9 ohm-cm after 1 day, 67.0 ohm cm after 1 week, 68.7 ohm-cm after 30 days); Dendrite Penetration Time (DP) (greater than 200 min.) and Stability (4.1 percent weight loss). The formed sheet product exhibited good combination of properties as a battery separator.
EXAMPLE II
A composition and sheet product was formed in the same manner as described in Example I above except that 60 parts of the 34.1 % potassium hydroxide solution was usedO The resultant product exhibited an ER of 65 ohm-cm;
DP of greater than 200 and stability of 1.4 weight percent weight loss. The formed sheet product exhibited a good combination of properties as a battery separator EXAMPLE III
A composition and sheet product was formed in the same manner as described in Example I above except that 71.7 parts of copolymer A, 53.3 parts copolymer B (A to B ratio o$ 0.74) and 64.7 parts 34.1 % KO~ solution were used.
The resultant product readily pressed into a thin pin hole free sheet of about 5 Gil thick. The sheet product exhibited an OR of 44.7 ohm-cm, a DP of greater than 200 and stability of 14.8 perceng (wt.) saint The product exhibited a good combination of properties as a battery separator.
EXAMPLE IV
A composition and sheet product was formed in the same manner as described in Example I above except what 92.2 ~2~3~7~
parts copolymer A, 32.8 parts of copolymer B (A to B ratio of 0.35) and 69.5 parts of 34.1 4 potassium hydroxide solution were used. The resultant product readily pressed into a thin pin hole free sheet of about 5 mils thick.
5 The sheet exhibited an ER of 30~2 ohm-cm~ a DP of greater than 200 and stability of 14.8 percent (wt.) gain. The product exhibited a good combination of properties as a battery separator.
hXAMPLE V
For comparative purposes; a composition and sheet product was formed from copolymers A and B in a ratio of 0.11. The formed sheet product had poor stability as shown by swelling Jo an undesirable extent and exhibiting a 44.2 percent gain in weight when exposed, all above to hot alkaline solution.
EX~MPLE_VI
For comparative purposes only, several samples were formed by the process described in Example I above except that copolymer B was substituted by a ethylene/acrylic acid copolymer formed in the manner described for copolymer A except having an acrylic acid content of 20 percent and a Melt Index of 15 (Copolymer C3. merefore, these samples represènt a mixture of (a high acrylic acid, high melt index and (b) low acrylic acid, high melt index.
Table I below summariæes the results exhibited by these materials.
TABLE I
A/C ER DP Stability 1.3 64 >20~ + 9.6 0u66 52 ~200 ~36.9 0.31 3g ~200 ~32.2 ~3~7~
The results recorded in Table I show, when compared to sample of Examples I to IV, that compositions having comparable ratios o copolymers, do not exhibit the desired combination of properties when both are formed with high melt index materials. ER values are unexpectedly higher (undesired) and the stability is poorer in each instance.
.... . ..
Claims (6)
1. In a battery having an electrolyte, at least one pair of positive and a negative electrodes and a separator membrane between each adjacent positive and negative electrodes, the improvement comprising having said separator membrane formed from at least one sheet of material having a thickness of from 0.5 to 10 mils and formed of a composition comprising a mixture of (a) a copolymer of ethylene and acrylic acid or methacrylic acid, said acid substantially neutralized with an alkali or alkaline earth metal cation and said copolymer having from 10 to 23 mole percent of said acid therein and having a standard load melt index of from 0 to 5; and (b) a copolymer of ethylene and acrylic acid or methacrylic acid, said acid substantially neutralized with an alkali or alkaline earth metal cation and said copolymer having at least about 25 mole percent of acrylic acid therein and having a standard load melt index of from about 10 to 100; the weight ratio of (a) to (b) being from 0.3 to 2.
2. In a battery having an electrolyte, at least one pair of a positive and a negative electrodes and a separator membrane positioned between adjacent positive and negative electrodes, wherein the improvement comprises that each of the said separator membranes has a thickness of less than 10 mils, and is formed from (A) a membrane composed of an admixture of from 7 to 30 weight percent of at least one polyolefin selected from polyolefins having an average molecular weight of at least 100,000; from 0 to 15 weight percent plasticizer;
and from 50 to 93 percent of a filler having an average particle size of from about 0.01 to 10 micron, a surface area of from 100 to 385m2/cc and a pore volume of at least 0.075cc/gm; and (B) a sheet material comprising a mixture of (a) a copolymer of ethylene and acrylic acid or methacrylic acid, said acid substantially neutralized with an alkali or alkaline earth metal cation and said copolymer having from 10 to 23 mole percent of said acid therein and having a standard load melt index of from 0 to 5; and (b) a copolymer of ethylene and acrylic acid or methacrylic acid, said acid substantially neutralized with an alkali or alkaline earth metal cation and said copolymer having at least about 25 mole percent of acrylic acid therein and having a standard load melt index of from about 10 to 100; the weight ratio of (a) to (b) being from 0.3 to 2, said sheet material laminated to at least one major surface of said membrane (A).
and from 50 to 93 percent of a filler having an average particle size of from about 0.01 to 10 micron, a surface area of from 100 to 385m2/cc and a pore volume of at least 0.075cc/gm; and (B) a sheet material comprising a mixture of (a) a copolymer of ethylene and acrylic acid or methacrylic acid, said acid substantially neutralized with an alkali or alkaline earth metal cation and said copolymer having from 10 to 23 mole percent of said acid therein and having a standard load melt index of from 0 to 5; and (b) a copolymer of ethylene and acrylic acid or methacrylic acid, said acid substantially neutralized with an alkali or alkaline earth metal cation and said copolymer having at least about 25 mole percent of acrylic acid therein and having a standard load melt index of from about 10 to 100; the weight ratio of (a) to (b) being from 0.3 to 2, said sheet material laminated to at least one major surface of said membrane (A).
3. The battery of Claim 1 wherein said sheet material copolymer (a) contains from 15 to 23 mole percent of acrylic acid based on said copolymer and has a standard load melt index of from 0.1 to 1 and copolymer (b) contains from 25 to 35 mole percent of acrylic acid based on said copolymer and has a standard load melt index of from 10 to 20.
4. The battery of Claim 2 wherein said sheet material copolymer (a) contains from 15 to 23 mole percent of acrylic acid based on said copolymer and has a standard load melt index of from 0.1 to 1 and copolymer (b) contains from 25 to 35 mole percent of acrylic acid based on said copolymer and has a standard load melt index from 10 to 20.
5. The battery of Claims 2, 3 or 4 wherein filler of sheet (1) of the laminate is titania having a surface area of from 125 to 385m2/cc and a pore volume of from 0.08 to 0.8 cc/gm.
6. The battery of Claims 2, 3 or 4 wherein the sheet (1) of the laminate further contains up to 10 weight percent of a conductive carbon black having a surface area of at least 100m2/cc.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000541642A CA1243074A (en) | 1982-09-02 | 1987-07-08 | Battery separator composition of mixed ionomers |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/414,004 US4434215A (en) | 1982-09-02 | 1982-09-02 | Battery separator |
| US414,004 | 1982-09-02 | ||
| CA000428045A CA1228949A (en) | 1982-09-02 | 1983-05-12 | Battery separator composition of mixed ionomers |
| CA000541642A CA1243074A (en) | 1982-09-02 | 1987-07-08 | Battery separator composition of mixed ionomers |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000428045A Division CA1228949A (en) | 1982-09-02 | 1983-05-12 | Battery separator composition of mixed ionomers |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1243074A true CA1243074A (en) | 1988-10-11 |
Family
ID=25670032
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000541642A Expired CA1243074A (en) | 1982-09-02 | 1987-07-08 | Battery separator composition of mixed ionomers |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1243074A (en) |
-
1987
- 1987-07-08 CA CA000541642A patent/CA1243074A/en not_active Expired
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