COATED PAPER SEPARATORS FOR ELECTROLYTIC CELLS

Abstract: Separators for electrochemical cells having a cationic starch as a component of their coating allow the cells to perform better in storage and performance tests.

Description: COATED PAPER SEPARATORS FOR ELECTROLYTIC CELLS The present invention relates to electrochemical cells having coated paper separators, wherein the separator coating comprises a starch and an additive, as well as to such separators . In the drive to improve the safety and performance characteristics of dry cell batteries (also referred to herein as zinc carbon, or carbon zinc, batteries) , various ingredients of the mix of the cell have been investigated, such as manganese dioxide, and various additives have been incorporated, in order to compensate for reducing the amount of mercury used. What was not previously investigated in any detail was the separator, or the coating of the separator. This is primarily because the only previously perceived purpose that the separator serves is preventing direct electrical contact between the two electrodes while, at the same time allowing ionic contact. What we have established is that, surprisingly, the separator and the coating of the separator can very substantially affect the performance and safety features of a cell, to the extent that the cells are even improved in the fresh SCA (Short Circuit Amperage) test. In co-pending application no. PCT/GB96/01318, incorporated herein by reference, we describe the advantages of making the coating of the separator from highly cross-linked starch, such as Vulca 90 (Trade Mark of National) , along with gellants, such as CELACO B1209 (Trade Mark of Courtauld) , which do not decompose during storage in zinc chloride solution. In addition, in co-pending application no. PCT/GB96/01319, incorporated herein by reference, we have also established the desirability of incorporating a polyoxyalkylene nitrogen containing compound, or additive, such as Crodamet C20. Crodamet C20 is a monoamine having two polyoxyethylene side-chains, the number of oxyethylene units being, on average, 20 moles per mole of Crodamet C20. Separators having coatings comprising the above ingredients result in dry cell batteries which perform markedly better than those comprising standard ingredients, m many standard battery tests. This is particularly so in regard to abuse conditions. There are many abuse tests, but we have devised two tests to assay leakage under abuse conditions (the HDCT and LDCT tests, described below) . The HDCT test assays those conditions which might be encountered where a flashlight was left in the "on" condition over a period of time, even after the battery had, to the user, gone "flat". The LDCT test simulates the conditions experienced by a battery in, for example, a clock. The advantage of these tests is that they can be performed relatively quickly, and t is not necessary to wait for a year or so to establish whether the battery will perform m a clock, for example, without leaking. In further co-pending application no. PCT/GB96/02739, incorporated herein by reference, we have also found that the nature of the paper used for the separator has a significant effect, and that high density papers which take at least four minutes to absorb a 50μl drop of water again perform better than any papers currently used in standard batteries. Despite the success of the various modifications made to the separators of the so-called carbon zinc cells m substantially reducing abuse leakage, there remains a drawback in that, overall, the performance of cells containing such additives is slightly down. Thus, the modifications that we have made to the separators yield cells which are far less likely to leak under abuse conditions and this, in itself, is extremely desirable and a major aim in battery manufacture. However, the down side is that these cells also tend to have a marginally reduced life compared to the cells of the art, so that it is necessary to balance the desirability of minimal leakage against the likelihood of reduced service life. What we have now, surprisingly, found is that, if a cationic starch is used in the manufacture of the separator coating for cells, then the storage properties of cells containing such separators are very substantially increased, the cells often still performing well even after storage under the most stringent conditions. In addition, we have found that it is possible to prepare cells which also have equivalent, or even better, performance than the cells of the art, and this can be achieved by providing the cells with a coated paper separator comprising not only a cationic starch, but also an additive having a low hydro-lipophilic balance (HLB) . Thus, in a first aspect, the present invention provides a coated paper separator for an electrochemical cell, the coating comprising a starch, characterised in that cationic starch constitutes a majority of the starch component of the separator coating. In an alternative aspect, the present invention provides a coated paper separator for an electrochemical cell, the coating comprising a starch and an additive, characterised in that the starch is a cationic starch and the additive has a hydro- lipophilic balance of less than 17. Previously, we had established that a particularly advantageous type of additive is generally classifiable as a surfactant, and is particularly preferably a polyoxyalkylene, nitrogen containing compound. Also, previously, we had established that it was particularly desirable for the starch to be highly cross-linked and that the most advantageous combination of highly cross-linked starch and additive was a starch such as Vulca 90 and a polyoxyethylene amine such as Crodamet C20. Crodamet C20 has an average of 20 oxyethylene units per amine and has an alkyl group on the amine averaging about twelve carbon atoms. Crodamet C20 has an HLB of 17. With the discovery that cationic starches significantly improved storage properties of cells, there was still the problem that under the Light Industrial Flashlight (LIF) test, performance was still approximately ten percent less than cells of the art, under certain circumstances. Previously, experiments with additives having lower HLB values had somewhat increased the results obtained in the LIF test, but had also resulted in very considerably worsened leakage. Surprisingly, with cationic starches, we have found that not only is performance substantially improved with additives having an HLB of less than 17, but that problems with leakage do not become significant, even at an HLB of as low as 5. In the accompanying tests, we show that, under the circumstance of the test, cationic starch in combination with Crodamet C20 has a result, under the LIF test, of only 88 percent of cells of the art. As described above, Crodamet C20 has an HLB of 17, which is above that required in the present invention. Without being bound by theory, we believe that the relatively hydrophilic cationic nature of the starch used in the present invention allows a relatively hydrophobic (or lipophilic) additive with a low HLB to be used. The relatively hydrophobic nature of non-cationic starches has meant, we have found, that the zinc / separator interface region tends to dry out during long duration performance tests like LIF, especially if the discharge time is small and the current drain high. LIF is a good example of a short discharge time coupled with a high current drain. This drying out on test leads to a raising of the internal resistance of the cell in the zinc / separator region and a consequent premature failing of the cell. This situation is worsened by gassing. This is why some testing is done with Araldite capped cells. Although many commercial cells are constructed to permit venting of waste gases at a predetermined pressure, some cells effectively prevent venting by having a tight seal, so that Araldite capping serves to emulate such cells. While Araldite capping prevents gas escaping during discharge, it also leads to lower discharge performance on LIF. This is very noticeable in the following Examples where LIF is given for "No Araldite" & "Araldite" capped cells . Thus, we have now found that an additive having an HLB of 14 performs very nearly as well as the prior art cell under the LIF test, with a value of 96 percent of the cell of the art, and an additive having an HLB of 11 performs effectively identically under the LIF test (99 percent) . In both cases, the likelihood of leakage is much reduced over the art. However, what is most surprising is that, once an HLB of 9 is reached, then the performance under the LIF test is more than 10 percent better than the cell of the art. Thus, in every way, a cell of the invention having additives which such low HLB's is superior to those of the art . From the foregoing, it will be appreciated that any additive having an HLB of less than 17 provides a cell which is superior:- a) to the cells of the art with regard to reduced leakage probability, and b) to cells having cationic starch in combination with Crodamet C20. Accordingly, any additive having an HLB of less than 17 is useful in the separators of the present invention, but it will be appreciated that additives having an HLB of 14 or less are preferred, as these allow the production of cells having substantially identical performance with cells of the art, but which are far less likely to leak under abuse conditions . Particularly preferred are additives with an HLB of less than 11, as these actually allow enhanced performance, even compared with the cells of the art. Our currently preferred HLB is 9, as this value allows a very similar LIF performance to additives having an HLB of as low as 5, but which is not associated with the marginally increased leakage of additives having such a low HLB. Nevertheless, the likelihood of leakage under abuse conditions of additives having an HLB of as low as 5 is still considerably less than the cells of the art. Thus, a preferred range of HLB is from 5 to 11, particularly from 7 to 10, with 8 or 9, especially 9, being most preferred. The HLB of an additive results from the balance between such hydrophobic constituents as alkyl chains, and such hydrophilic components as oxyethylene units. In Crodamet C20, the average alkyl chain length is 12 , whilst the average number of oxyethylene units is 20, effectively corresponding to two decameric polyoxyethylene units substituting an amine which is further substituted by a C 12 alkyl side chain. However, it will be appreciated that many compounds may make up the generic product known as Crodamet C20. To reduce the HLB, the number of oxyethylene units can be reduced, and the length of the alkyl side-chain can be increased. Crodamet C20 is largely based on coconut alkyl groups which have, as stated above, an average chain length of 12. Crodamet T5, which contains five oxyethylene groups, has an HLB of 12. Accordingly, it is generally desirable to lengthen the average alkyl chain, so that we have found that tallow provides a useful alkyl length, having an average of about 18 carbon atoms. Ethylan TT203 is an amine having two oxyethylene substituents and an alkyl substituent having an average length of 18 carbon atoms, and this has an HLB of 5. For an HLB of 9, Crodamet T5 (having five oxyethylene units) is preferred. However, it will be understood that the alkyl chain length and the number of oxyethylene units can be varied in any suitable manner in order to obtain an additive with a desired HLB. Further, for additives having a relatively low HLB, it is preferred that these additives should be non-ionic and, for the purposes of the present invention, non-ionic additives are preferred. As used herein, the term "cationic starch" relates to any starch which has, in its molecular structure, at least one cation, on average, per starch molecule in si tu . Cationic starch is well known in the art and is disclosed, for example, in "Starch, Chemistry and Technology (Academic Press, Inc., Eds. Whistler, R. , Bemiller, J. , and Paschalle, E., second edition, 1984) . In general, cationic starches can be prepared as disclosed in GB-A-2063282 and US-A-4613407, both of which are incorporated herein by reference. US-A-4613407 describes the use of a combination of cereal and tuber cationic starches as a wet-end cationic additive for the manufacture of paper. Many uses are generally suggested for cationic starch in both references, but no use had previously been suspected for electrochemical cells. As described in GB-A-2063282 , cationic starches may be prepared by solubilising normal starch in water and exposing said starch to a suitable cationising agent under alkaline conditions. The agents illustrated in GB-A-2063282 are lower alkyl halohydrins and lower alkyl haloepoxy compounds, and these are suitable to yield cationic starches with viscosities higher than 1000 and even 2000 Brabender units, measured at a concentration of 5% v/v in water. As described in the prior art, a generally preferable level of nitrogen content is between 0.2 and 2% w/w by dry weight of starch, although this can go up to as far as 2.8%. The only real ceiling is the practical limit imposed by chemical and cost considerations . In general, we prefer that the separator coating be made substantially as is known in the art, but incorporating cationic starch in place of some or all of the starch component of the coating. Preferred paper and other ingredients are generally as described herein. The cationic starch should form a majority of the starch component of the coating for the separator and, in general, we prefer that it should form all of the starch component, although it may occasionally be desirable to incorporate another component, such as Vulca 90. In general, there is no restriction on the nature of the cationic starch of the present invention. However, it may generally be desirable that the cationic starch should not be readily soluble in water, in which case we prefer that it should not be at all soluble in water (at least cold water) , and that it should not expand in the presence of water. However, the cationic starch should be at least partially cross-linked. Uncross-linked cationic starches do not perform satisfactorily, although there does not seem to be much difference in the performance of highly cross-linked starches and moderately cross-linked starches. The starch may be any suitable starch. Many suitable starches are known, and include potato, corn, wheat and tapioca starch. The nature of the cations is also not important, although it is generally preferred that the substance should not be toxic, for environmental reasons. Suitable cations are the sulphonium, phosphonium and ammonium ions, of which the latter are preferred, and these can be introduced into the starch using suitably alkylated molecules . Suitable alkyl amines will tend to result in the starch being substituted by trialkyl ammonium groups under the acidic conditions present in a zinc carbon cell. If the cationic starch used is in the form of, for example, dialkylamino substituted starch, then this precursor product may be converted into the cationic product by suitable treatment with acid, and this may be effected either in si tu , or prior to incorporation of the separator into the cell. A particularly preferred cationic starch of the present invention is a potato starch made by Roquette (Roquette Freres, 4 rue Patou, F-59022 Lille Cedex, France) under number LAB 2273, but other manufacturers also make appropriate grades of cationic starch. Separators generally require the use of gellants in their manufacture. We have found that the various soluble starch gellants and natural gums used to manufacture separators all appear to decompose during storage. However, etherified cellulose derivatives appear to be stable in aqueous solutions of zinc chloride, and these are particularly advantageous for use in the present invention. Suitable examples of gellants for use in the present invention include: Tylose MH200K (Trademark of Hoechst) , Tylose MH50, Culminal MHPC100 (Trademark of Aqualon) and Courtaulds DP 1209. Particularly preferred etherified cellulose derivatives should ideally swell and gel substantially immediately and remain stable in the presence of water over long periods, such as described in PCT/GB96/01318, and suitable etherified celluloses include methyl cellulose, ethyl cellulose, hydroxymethyl cellulose, carboxymethyl cellulose (including salts, such as the sodium salt) , hydroxyethyl cellulose, ethylhydroxyethyl cellulose, methylhydroxyethyl cellulose, 2- hydroxypropyl cellulose, methylhydroxypropyl cellulose and 2- hydroxypropylmethyl cellulose. We have also established that viscosity is an important factor in choosing a gellant. If the separator mix is outside certain viscosity limits, typically in the region of 3000 to 70000 cP (3 to 7.0 Pa.s), undesirable results and poor cells are usually obtained. Below 3000 cP (3 Pa.s), the mix is often so liquid that it soaks straight into the paper, which can lead to the tearing of the paper, for example. Above 70000 cP (70 Pa.s), the mix is generally too thick to spread onto the paper satisfactorily. Accordingly, it is desirable to provide a mix which falls within the limits defined above, and this is generally possible by using an etherified cellulose derivative having a viscosity of between about 20 cP (0.02 Pa.s) and about 300 cP (0.3 Pa.s) . As used herein (unless otherwise specified) the viscosity of a substance is defined in terms of a 2% w/v aqueous solution of that substance at 20°C at a neutral pH. Ideally the viscosity is between 50 and 100 cP (0.05 and 0.1 Pa.s). The additives useful in accordance with the present invention are suitably nitrogen containing compounds of any type that is suitable to be substituted by one or more polyoxyalkylene groups . Whilst amine and ammonium compounds are preferred, especially the amine compounds, other compounds which have substitutable nitrogen bonds are also suitable, such as carbamoyl, diazo and aci-nitro compounds. The individual alkylene moieties in the polyoxyalkylene substituents may be the same or different, but will generally be the same owing to the methods of manufacture employed for such compounds . Useful alkylene groups tend to be restricted to the ethylene and propylene groups, but the propylene groups are not as good as the ethylene groups at preventing gassing, so that polyoxyethylene nitrogen containing compounds are preferred, especially the polyoxyethylene amines. It will be appreciated that it is possible for any given polyoxyalkylene moiety to contain a mix of lower alkylene groups, such as methyl, ethyl and propyl. Where this is the case, then we prefer the average alkylene length to be two, or close to two, carbon atoms. Regarding the nitrogen atom, it is particularly preferred that this is substituted by at least one polyoxyalkylene group and one saturated or unsaturated alkyl group. This group is preferably an alkyl or alkenyl group. There is generally no advantage to alkynyl groups, although these are also envisaged. The group may be straight or branched and may be substituted by one or more substituents, such as hydroxy groups and halogen atoms, but it is generally preferred that the alkyl group is unsubstituted. The level of saturation is ideally complete or there are only one or two carbon-carbon double bonds. It is also preferred that the alkyl group should be straight chain and contain from 1 to 30 carbon atoms, provided that the compound has an HLB of less than 17. Compounds of the present invention may also contain more than one amine centre, in which case it is preferred that the individual amine groups are bridged by alkylene groups, preferably a short chain alkylene group, such as a trimethylene group . Where the nitrogen atom or atoms is substituted by an unsaturated group, particularly an alkenyl group, then the present invention provides separators comprising such compounds in accordance with the teaching of PCT\GB96\01319, which is incorporated herein by reference. In such separators, where an alkyl chain is disclosed in PCT\GB96\01319, then this may be substituted with an unsaturated chain, provided that at least one alkyl is replaced by an unsaturated chain. The chain length of the polyoxyalkylene group is not particularly important to the present invention, provided that the HLB is less than 17, but we prefer that the chain length should be between 1 and 5, preferably with an average length of between 1 and 3 and especially around 2 or 3. Furthermore, compounds derived from tallow amines are preferred, and tallow alkyl groups contain between around 18 carbon atoms. Thus, the most preferred compounds of the present invention are mono- and di- amines wherein the free alkyl group has around 18 carbon atoms, the side chains are polyoxyethylene substituents having an average of 1 or 2 oxyethylene units each and, where the compound is a diamine, then the link between the two amine centres is trimethylene. The preferred compounds for use in the present invention are currently derived from tallow, and this has the following composition, by comparison with coconut, where chain length is the number of carbon atoms : Chain Unsaturation Acid Name Tallow Coconut Length Degree Oil Oil <8 0 1% 8 0 Ca rylie 5% 10 0 Capric 8% 12 0 Laurie 45% 14 0 Myristic 6% 18% 16 0 Palmitic 27% 11% 16 1 Palmitoleic 1% 18 0 Stearic 14% 2% 18 1 Oleic 50% 8% 18 2 Linoleic 3% 1% Any suitable paper may be used in accordance with the present invention, by which is meant any paper suitable for use as a separator. However, most papers used in conventional separators are from a single source of pulp and, while these papers are relatively cheap to manufacture, they tend to perform poorly in a number of tests. Nevertheless, we have established that it is possible to produce papers from a single source of pulp which perform well in tests, and such papers are characterised by their ability, at a temperature of about 20°C, to absorb a 50μl droplet of water in a period of between four and fifteen minutes. More preferably, this period is between five and fifteen minutes and is particularly preferably between five and ten minutes . If the paper absorbs the droplet of water in less than four minutes, then the density of the paper tends to be too low, and poor results may be obtained. If the paper absorbs the droplet in greater than fifteen minutes, then this causes practical problems during manufacture, as the individual cells need to be voltage tested soon after assembly, and the delay in absorbing the electrolyte from the mix would mean that there would be an unacceptable storage time before the cells could be tested. The characteristics of the papers which have the necessary absorption tend to be those of high beat and high density. Beating is performed on the pulp prior to formation of the paper, and the degree of beating can be measured by the use of the "Canadian standard freeness tester". The test is T 227m-58 of the Technical Association of the Paper and Pulp Industry and is described, for example, in "A Laboratory Handbook of Pulp and Paper Manufacture" (Auth. J. Grant, Pub. Edward Arnold, 2nd Ed. 1961, pp. 154 et seq. ) . Conventional papers, such as Enso 80, have a density typically in the region of 0.5g/cm 3 , and even PBDE100 only has a density of 0.62g/cm 3 . However, both papers may be used in accordance with the present invention, although higher density papers are preferred. The single pulp source papers which are preferred for the present invention have densities typically of 0.64g/cm 3 and above, with preferred densities being between about 0.65 and about lg/cm 3 , and more preferred densities being between about 0.65 and about 0.9g/cm 3 , although there tends to be very little to choose in this particular range of densities. For example, one particularly preferred paper of the present invention is made by Cordier (product code COK-70) and has a density of 0.64g/cm 3 , and another particularly preferred paper of the present invention is made by Munksjo (product code 114440) and has a density of 0.76g/cm 3 . A list of some of the preferred papers useful in the present invention is as follows: Cordier C0K~ 60 Cordier COK~ 70 Sibille Dalle 58060 (hereafter "SDMF") Munksjo 114440 Munksjo 114770 Tervakoski Oy Tertrans N75 0,75 Tervakoski Oy Terkab E70 10 The Cordier papers are available from Papierfabrik Cordier GmbH, Pfalz, Germany,* the Sibille Dalle papers are available from Sibille Dalle, Vitry sur Seine, France; the Munksjo papers are available from Munksjo Paper AB, Jonkpong, Sweden; and the Tervakosko papers are available from Oy, Tervakoski, Finland. Papers having a density of less than about 0.6g/cm 3 tend to yield poor results in the tests, whilst papers with densities in excess of about 1. Og/cm 3 tend to adsorb in greater than 15 minutes in the water droplet absorption test. In general, cells incorporating the separators with cationic starch still remain functional under adverse storage conditions. In one test, there was a 0% failure rate even after 26 weeks at 45°C. While not being bound by theory, it seems likely that this exceedingly good result is due to the interface between the separator and the can remaining moist . This is enhanced by the separators of the present invention. Other separators of the art tend to dry out, even though the overall moisture content of the battery remains substantially the same. This is a problem which had previously been recognised, but which had defied all attempts at resolution to date. Another advantage of using cationic starches is that the quality of manganese dioxide is not as important as in other cell constructs. This is a major advantage, as manganese dioxide is a major expense in the manufacture of dry cells, and great savings can be made if, for example, relatively cheap sources of electrolytic manganese dioxide (EMD) can be used. At present, such low grade material can only be used if it is combined with higher grade material in order to avoid leakage, but this is no longer necessary if cationic starches are used in accordance with the present invention. The present invention now makes it possible to use cheaper materials, such as are available from the People's Republic of China, for example. It will be appreciated that the present invention also provides electrochemical cells comprising separators of the present invention. There is also provided a coating mix suitable for the manufacture of coated separators of the present invention, said mix comprising cationic starch. Typical cells in which the separators of the present invention can be used include primary and secondary zinc carbon cells, including those cells known as Leclanche and zinc chloride cells, as well as alkaline cells. The electrolyte in zinc carbon cells is typically as follows: Leclanche electrolyte - 5-20% zinc chloride, 30-40% ammonium chloride, remainder water; zinc chloride electrolyte - 15-35% zinc chloride, 0-10% ammonium chloride, the remainder water. Some other suitable cells for use in the present invention are described in Chapter 5 of the Handbook of Batteries and Fuel Cells (edited by David Linden, published by McGraw Hill) . Cells may have any suitable configuration, such as round, square or flat. Two useful tests used herein to establish leakage under abuse conditions are the High Drain Continuous Test (HDCT) and the Low Drain Continuous Test (LDCT) . The High Drain Continuous Test is intended to simulate abuse conditions, such as might be found in leaving a flashlight in the "on" condition over a period of time, even after the battery had, to the user, gone "flat". The Low Drain Continuous Test simulates the conditions experienced by a battery in, for example, a clock. HDCT results are measured in terms of the amount of leakage, whilst LDCT results are measured in terms of failure of the battery due to perforation or splitting of the can. These tests produce highly informative results in considerably less time than would otherwise be experienced in the conditions being simulated. Results are generally available in around 4 and 10 weeks respectively, although it will be appreciated that the amount of time required will depend on such factors as the cell which is to be tested and the extent to which it is desired to test the cell, for example. The Low Drain Continuous Test for an electrochemical cell is characterised in that the can is sealed but left uncovered, a high resistance is secured between the poles of the cell so as to complete a circuit, and the cell is monitored as to its condition. It will be understood that, in this test, monitoring the cell is intended to ascertain whether the cell fails during testing. The typical lifetime of a D-size zinc carbon cell is up to about 10 weeks when the resistance is about 300 Ω. Other resistances may be used as appropriate, although 300 Ω provides useful results. -An appropriate resistance for a C-size cell is about 500 Ω while, for an AA-size cell it is about 810 Ω. The omission of the bottom cover and the over tube is to expose the can to a surrounding atmosphere, thereby enhancing any failure that might occur, which is one reason why this test can be performed in 10 weeks, when it might take 2 years in a clock, for example. The High Drain Continuous Test for an electrochemical cell is characterised by the cell being preferably fitted with a bottom cover, a low resistance being secured between the top cover and a point on the can wall proximal to the top cover and, thereafter, sliding an over-tube onto the can so as to cover substantially as much of the can as possible without dislodging the resistance, weighing the resulting assembly, storing the cell at ambient temperature, preferably 20°C, weighing the cell at intervals during storage if desired, and determining the amount of electrolyte lost during storage by weighing to establish leakage. This last weighing may be effected by removing and weighing the over tube after storage or weighing the cell without the over tube but with the resistance, or both. Addition of the bottom cover during this test is particularly advantageous in preventing corrosion at the bottom of the can during the test . A suitable resistance for this test for a D-size cell is 3.9 Ω and about 5 Ω for an AA-size cell, and the test is typically carried out for 4 weeks, testing at weekly intervals. The normal discharge life for a D cell is about 6 hours in this test until the cell becomes useless. Testing for 4 weeks, for example, establishes how the cell stands up to abuse conditions. The present invention will now be illustrated with respect to the accompanying, non-limiting Examples wherein percentages are by weight, unless otherwise specified. The Test Examples are preceded by certain Test Protocols appropriate to the Test Examples. Unless otherwise stated, the zinc cans used in the present examples typically comprise 0.4% lead and 0.03% manganese and have a wall thickness of 0.46 + 0.03mm. The mix for the cathode typically comprises 52% manganese dioxide, 0.4% zinc oxide, 6% acetylene black and 41.6% zinc chloride solution (26.5% zinc chloride w/v) . Otherwise, cells are generally manufactured in accordance with EP-A-303737. Test Protocols Preparation of Separators The first step in the preparation of a separator is to prepare the paste to be used for the coating of the paper. The formulations used in the present Examples were as follows : Water 64.3% Additive 0.5% Gellant (as specified) 3.1% Starch (generally Vulca 90 or Roquette 2273, unless otherwise specified) 32.1% Suitable additives are commonly available as surfactants, typically as provided in "Industrial Surfactants Electronic Handbook" (published by Gower and edited by Michael and Irene Ash) . The following method was employed for making up the paste: mix the dry ingredients then add to the water and organic additive and place the resulting mixture in a paddle mixer, such as a Hobart mixer, and mix until a smooth paste is obtained. The separator paste is then coated onto the chosen paper. The technique used in the Examples is to run the coated paper between two rollers set apart by a predetermined distance in order to provide the desired coating weight when dry. The rollers are suitably set so that they run in opposite directions, with the forward roller running fastest. A suitable coating machine is made by Dixon's (Dixon's Pilot Coating Machine Model 160, UK). Suitable coating weights will be apparent to those skilled in the art. However, we prefer a dry coating weight of around 40gm " . The coated paper is then dried either by oven-drying at 100-140°C and/or by steam drum-drying at 100-150°C. Default cells in the Examples have the following constitutions, ES representing the cells of the art: ES Test Cells Starch 90.9% Roquette Vector L117 87.9% Roquette LAB2273 N* 0% 0.2% Cross-Linking Moderate Moderate Gellant 8.7% Tylose MH200K 8.4% Courtaulds 1209 Adhesive - 2.2% ISP PVP K120 Additive 0.4% Forofac 1110D 1.5% Low HLB Additive Paper Sibille Dalle WS64 Munksjo 114440 (M114440) gsm** 50 40 Gel Mixing Wet method Dry Powder Method Method Final Drying Oven Steam Drum (1st Oven) Method N* - nitrogen content of the cationic starch (% w/w) gsm** - grammes m " of dry mix on the paper Unless otherwise specified, all other cells are prepared with dry mixing of the gel components and with 40 gsm. Final drying is by steam drum drying. Essentially, we have found that the use of an adhesive, such as polyvinyl pyrrolidone (PVP) , together with steam drum drying greatly assists adhesion of the coating to the paper. We have also established that, when the starch is potato starch, the most advantageous gsm is about 40 which, we believe, effectively corresponds to a single grain layer on the paper. Wheat or corn starch is finer, and only needs to be coated at about 20 gsm, although the amount is less critical than for potato. HDCT High Drain Continuous Test) 1. Cell is manufactured as above. The bottom cover is added but no over-tube. 2. 3.9Ω resistors are soldered between the cover and the top of the can adjacent the cover. Cells are weighed (w x ) . 3. Over-tubes are weighed (w 2 ) . 4. The over-tube is pushed on cell but not spun in. The cell is weighed (w 3 ) . 5. The HDCT cells are stored at 20°C for 4weeks . The normal discharge life for D cells on a 3.9Ω test is ~6h. This test for 4w represents an abuse test to simulate a consumer leaving equipment switched on. 6. At weekly intervals (lw, 2w, 3w & 4w) , 1/4 of the original cells are removed and measurements are taken. The complete discharged cell is weighed (w . 7. The over-tube is removed and weighed (w 5 ) . 8. The resulting cell with soldered resistor still intact is weighed (w 6 ) . 9. The HDCT leakage is W x - W 6 . LDCT (Low Drain Continuous Test) 1. Cell is manufactured as above. For LDCT no bottom cover is added and no over-tube. 2. 300Ω resistors are soldered between the cover and the top of the can adjacent the cover. 3. Cells are monitored at weekly intervals up to lOw. This would be the normal lifetime for a D cell on a 300Ω test. This test is a simulation of a cell being used on a long duration test such as a clock. 4. A failure is when perforation or splitting of the can is observed. This would allow 0 2 into the cell causing premature failure when on a long duration test . In the following Examples, various industry standard tests were employed. The cells tested were D-size, unless otherwise specified. Where not already defined, these tests are as follows : SCA - The cell is shorted and the current passed is measured on zero (very low) impedance meter. The resulting measurement is the SCA (Short Circuit Amperage) of the cell. LIF (Light Industrial Flashlight) - cells discharged across a resistance of 2.2Ω for 4 periods of 8 minutes in one hour, each period separated by about 2 minutes. This is repeated daily and the final result is given in terms of hours, being the cumulative total of eight minute discharge periods undergone until the failure voltage of 0.9V is reached. Motor (also referred to as DM herein) - cells are discharged across a resistance of 3.9Ω for 1 hour a day until the failure voltage of 0.9V is reached. The final result is the cumulative total of discharge time prior to failure of the cell. Toy (also referred to as DT herein) - similar to motor, except that cells are discharged across a resistance of 2.2Ω for l hour a day until the failure voltage of 0.8V is reached. The final result is the cumulative total of discharge time prior to failure of the cell. Continuous Toy (also referred to as DY herein) - cells are discharged continuously across a resistance of 2.2Ω until cell failure at 0.75V. The final result is the cumulative total of discharge time prior to failure of the cell. DP - cells discharged across a resistance of 2.2Ω for 8 periods of 4 minutes in one hour, each period separated by about 2 minutes. This is repeated daily and the final result is given in terms of hours, being the cumulative total of four minute discharge periods undergone until the failure voltage of 0.9V is reached. The mix used in the following Examples was 2.35 g H 2 0/Ah, 0.34% (w/w) ZnCl 2 /H 2 0, with 50% PRC Mn0 2 and 50% N65 Mn0 2 , unless otherwise specified. Also in the following Examples, the potato starches are: N-content Cross -Linking Roquette LAB2273 0.2% Moderate Roquette | 13-96 0.7% Moderate Roquette 114-96 0 2% Very High Performance of cells having different polyoxyethylene additives. Cells were made up with the following separators and tested in the SCA and HDCT tests v ES cells PI indicates the relative performa of the cells against ES cells The term EO indicates the oxyethylene content of the additive TABLE 1 Starch Vulca 90 Roquette LAB2273 Gellant Tylose MH200K ( ourtaulds 1209 Adhesive None ISP PVP K120 Gellant Mixing Wet Powder Paper Enso 80 Enso 80 Final Drying Oven Steam Drum ZnCI2/H2O 034 034 Alkyl EO Amine Alkyl EO HLB SC PI HDCT SC PI HDCT ES - - - - 68 100% 27 63 100% 30 Ethylan TT40 Tallow 40 18 68 98% 17 73 91% 06 Crodamet Coconut 20 17 65 97% 11 64 96% 05 # Tallow 15 14 64 97% 09 17 98% 08 Crodamet C5 Coconut 5 12 61 96% 12 - - - Crodamet T8 Tallow 8 11 - - - 66 99% 07 * Tallow 5 9 57 95% 19 69 103% 06 Ethylan Tallow 2 5 53 93% 20 67 103% 10 # Crodamet T15, Ethylan TT15 Crodamet T5, Ethylan TT05, Proxonic MT05 From the above, it can be seen that a combination of the cationic starch LAB2273 and tallow 5 gives a cell having lower likelihood of leakage (HDCT) and a greater fresh short circuit amperage Alternative potato cationic starches, obtained from Roquette Freres, yielded the following results, as shown in Table 2. Table 2 LIF NO Araldite Araldite cap % HDCT Starch N X-Link Gel/Adhesive Alkyl EO Paper No. Avg. No. Avg. of ES g Roquette LAB2273 0 2% Mod Courtaulds 1209/ Tallow 8 M114440 24 7 0 33 6 1 99% 07 ISP PVP K120 Roquette 1 13-96 0 7% Mod Courtaulds 1209/ Tallow 8 M114440 3 6 5 72 105% 01 ISP PVP K120 Roquette 1 14-96 0 2% V Courtaulds 1209/ Tallow 8 M114440 3 8 0 68 112% 01 High ISP PVP K120 TABLE 2 cont'd SCA No. DP DM DT DY PI 66 21 6 9 15 0 8 1 6 3 100% 100% 100% 100% 100% 69 3 6 5 15 7 8 6 6 3 101 % 94% 105% 106% 101 % Although this Example was performed using tallow 8 (HLB 11), rather than tallow 5 (HLB 9) , it can be seen that the nature of the cationic starch is not important, provided that it is at least moderately cross-linked. In Table 3, the effect that the nature of the additive has on performance, compared with ES cells, is established TABLE 3 LIF LIF NO Araldite Araldite cap % Starch N X- nk Gel/Adhesive Alkyl EO HLB Paper No Avg No Avg OfES ES 21 70 34 62 100% Vulca 90 Tylose MH200K Coconut 20 17 PBDE100 6 63 6 51 86% Roquette LAB2273 02% Mod Courtaulds 1209/ Coconut 20 17 Singer 70 15 59 33 48 81% ISP PVP K120 Roquette LAB2273 02% Mod Courtaulds 1209/ NONE - M114440 3 56 15 49 79% ISP PVP K120 Roquette LAB2273 02% Mod Courtaulds 1209/ Tallow 40 18 M114440 3 51 21 49 77% ISP PVP K120 Roquette LAB2273 02% Mod Courtaulds 1209/ Coconut 20 17 M114440 4 61 11 55 88% ISP PVP K120 Roquette LAB2273 02% Mod Courtaulds 1209/ Tallow 15 14 M114440 3 67 10 59 96% ISP PVP K120 Roquette LAB2273 02% Mod Courtaulds 1209/ Coconut 5 12 M114440 5 66 3 60 96% ISP PVP K120 Roquette LAB2273 02% Mod Courtaulds 1209/ Tallow 8 11 M114440 24 70 33 61 99% ISP PVP K120 Roquette LAB2273 02% Mod Courtaulds 1209/ Tallow 5 9 M114440 18 74 61 72 111% ISP PVP K120 Roquette LAB2273 02% Mod Courtaulds 1209/ Tallow 2 5 M114440 12 77 26 71 112% ISP PVP K120 Without Araldite. Gel/ Adhesive Alkyl EO HLB Paper No DP DM DT DY PI ES 15 67 151 86 63 100% 100% 100% 700% 700% Vulca 90 Tylose MH200K Coconut 20 17 PBDE100 3 64 148 84 65 98% 95% 98% 98% 103% Roquette LAB2273 02% Mod Courtaulds 1209/ Coconut 20 17 Singer 70 3 47 148 83 64 91%. ISP PVP K120 70% 98% 97% 101% Roquette LAB2273 02% Mod Courtaulds 1209/ Coconut 20 17 M114440 3 58 156 79 63 95% ISP PVP K120 86% 103% 92% 100% Roquette LAB2273 02% Mod Courtaulds 1209/ Tallow 40 18 M114440 3 51 149 85 58 91% ISP PVP K120 76% 98% 99% 92% Roquette LAB2273 02% Mod Courtaulds 1209/ Tallow 15 14 M114440 3 67 148 82 62 98% ISP PVP K120 99% 98% 96% 98% Roquette LAB2273 02% Mod Courtaulds 1209/ Tallow 8 11 M114440 21 69 150 81 63 99% ISP PVPK120 103% 99% 95% 99% Roquette LAB2273 02% Mod Courtaulds 1209/ Tallow 5 9 M114440- 18 74 ■ 154 83 66 103% ISP PVP K120 109% 102% 97% 104% Roquette LAB2273 02% Mod Courtaulds 1209/ Tallow 2 5 M114 44 0 12 77 163 82 61 103%, ISP PVP K120 114% 107% 96% 97% With Araldite. Starch N X-link Gel/Adhesive Alkyl EO HLB Paper No DP DM DT DY PI ES 6 61 141 81 63 100% 100% 100% 100% Roquette LAB2273 02% Mod Courtaulds 1209/ Tallow 11 M114440 59 158 83 61 102% ISP PVP K120 97% 113% 102% 97% Roquette LAB2273 02% Mol Courtaulds 1209/ Tallow M114440 65 162 82 64 107% ISP PVP K120 108% 115% 102% 102% Roquette LAB2273 02% Mod Courtaulds 1209/ Tallow M114440 69 156 81 61 105% ISP PVP K120 113% 111% 100% 96% 105% In Table 4, there is illustrated the effect of additive in the HDCT test The results are in g/cell average leakage TABLE 4 ES Starch N X-link Gel/Adhesive Alkyl EO HLB GSM P Paappeerr No. Avg 10 3 0 Roquette LAB2273 0 2% Mod Courtaulds 1209/ Tallow 40 18 40 M1 14440 10 0 6 ISP PVP K120 Roquette LAB2273 0 2% Mod Courtaulds 1209/ Coconut 20 17 40 M114440 10 0 5 ISP PVP K120 Roquette LAB2273 0 2% Mod Courtaulds 1209/ Tallow 15 14 40 M1 14440 10 0 8 ISP PVP K120 Roquette LAB2273 0 2% Mod Courtaulds 1209/ Tallow 8 11 40 M1 14440 10 0 7 ISP PVP K120 Roquette LAB2273 0 2% Mod Courtaulds 1209/ Tallow 40 M1 14440 30 0 6 ISP PVP K120 Roquette LΛB2273 0 2% Mod Courtaulds 1209/ Tallow 40 M114440 10 1 0 ISP PVP K120 It can clearly be seen that all of the additives tested yield superior results over the cells of the art, and that even additives having an HLB of around 5 is advantageous gsm of coating Potato starches are relatively coarse (average particle size 0 04-0 06mm), while wheat (average particle size 0 006-0 015mm) & corn (average particle size 0 006-0 017mm) are relatively fine This has the following effects on AA size cells Table 5 This test compared 20 and 40gsm potato starch (Roquette LAB2273), using Crodamet 020 and Sibille Dalle MF60 paper gsm Starch SCA AW Pulse AZ Cont. 20 Cationic Potato 2 3A 122 cycles 1 6h 40 Cationic Potato 3 0A 144 cycles 1 7h AW pulse - cell is discharged for 15 seconds every minute across 1 8Ω until failure at 0 9V AW - cell is continuously discharged across 3 9Ω until failure at 0 75V 40gsm Potato is clearly superior to 20gsm Potato The reason appears to be that potato starch is so coarse that it does not give complete coverage of the paper at 20gsm Table 6 This test compared 40 and 50gsm potato starch (Roquette LAB2273) but using Tallow 8 Amine (Crodamet T8) and Munksjo 114440 paper This shows that If more than 40gsm Potato starch is coated then HDCT starts to increase The optimum coating weight for potato starch is apparently 40gsm, serving to maximise performance & minimise HDCT leakage In the following Table 7, various different papers are tested in accordance with the invention. The tests are performed with tallow 8 but, even so, it is clear that all papers coated in accordance with the present invention confer superior characteristics on the cell containing them. The starch was Roquette LAB 2273, the gellant was Courtaulds 1209, the adhesive was ISP PVP K120 and the additive was Tallow 8 amine. TABLE 7 In the following further Examples, standard highly cross-linked corn starch was compared with various cationic starches. The types of starch are given in Table 8. TABLE 8 Avg. Particle Starches Nitrogen Size (cm) Vulca 90 National Highly X-Linked Corn starch 0.0% 0.001 LAB2273 Roquette Mod. X-Linked Cationic Potato Starch 0.2% 0.005 LAB2469 Roquette Mod. X-Linked Cationic Corn Starch η 0.2% 0.001 620641 Roquette Mod. X-Linked Cationic Wheat Starch 0.2% 0.001 X-linked - cross-linked Mod - moderately Tests as described in Table 9 were used. TABLE 9 wherein 3R9/lh/0V8 indicates discharge across 3.9Ω for one hour a day until failure at 0.8V. Likewise, 15s/m indicates 15 seconds in every minute. In the LIF test, cells were discharged for 4 periods of 8 minutes every hour. AA indicates AA size batteries, while D indicates tests for D size batteries In Table 10, there is shown the effect of varying starch and starch thickness on AA size cells. It can be seen that, for AA size cells, cross-linked, cationic potato starch must be used in greater thickness, but does not perform as well as does highly cross-linked corn starch. In the Table, as in other tables, coconut 20 amine is typically Crodamet C20. In subsequent tables, tallow 5 and tallow 8 amines are typically Crodamet T5 and T8, respectively PRC EMD is electrolytic manganese dioxide obtained from the People's Republic of China, while N65 NMD is natural manganese dioxide from Mexico. TABLE 10 Discharge Performance Starch Vulca 90 Roquette LAB2273 Roquette LAB2273 Gel Courtaulds 1209 Courtaulds 1209 Courtaulds 1209 PVP PVP K120 PVP K120 PVP K120 Additive Coconut 20 Amine Coconut 20 Amine Coconut 20 Amine Sibille Dalle Paper MF60 MF60 MF60 gsm 20 20 40 PRC EMD / N65 NMD 90 /10 90 /10 90 /10 SCA 3.6 2.3 3.0 AI 3R9/1 h/0V8 2.00 1.60 1.70 A 1R8/15s/m/0V9 150 122 144 AZ 3R9/Cont./0V75 1.80 1.60 1.70 In Table 1 1, there is demonstrated the effect of varying paper thicknesses in AA cells, and it can be seen that thinner paper is advantageous, provided that it has the necessary structural integrity TABLE 11 Starch Roquette LAB2273 Roquette LAB2273 Gel Courtaulds 1209 Courtaulds 1209 PVP PVP K120 PVP K120 Additive Tallow 8 Amine Tallow 8 Amine Munksjo Paper 100021 114440 gsm 40 40 Thickness .0125cm .0150cm HMRA-F EMD / NMD 100 / 0 100 / 0 SCA 6.9 6.0 AW 1 R8/15s/m/0V9 174 157 AZ 3R9/Cont./0V75 2.03 1.95 In Table 12, there is shown the effect on discharge performance of differing starch and manganese dioxide compositions in AA cells. It can be seen that all compositions performed satisfactorily with the mixture. TABLE 12 Starch Vulca 90 Roquette LAB2273 Roquette 620641 Roquette LAB2469 Gel Tylose MH200K Courtaulds 1209 Courtaulds 1209 Courtaulds 1209 PVP NONE PVP K120 PVPK120 PVPK120 Additive Coconut 20 Amine Tallow 5 Amine Tallow 5 Amine Tallow 5 Amine Munksjo Paper 100021 100021 300542 300542 gsm 20 40 20 20 PRC EMD /N65 NMD 100/0 100/0 100/0 100/0 SCA 6.6 6.3 64 67 Al 3R9/1h/0V8 2.38 320 2.36 244 AW1R8/15s/m/0V9 182 168 176 180 AZ 3R9/Cont /0V75 2.01 184 1.94 201 PRC EMD / N65 NMD 50/50 50/50 50/50 50/50 SCA 5.8 5.5 61 59 AI3R9/1h/0V8 161 1.58 1.62 165 AW1R8/15s/m/0V9 128 124 129 132 AZ 3R9/Cont /0V75 156 1.49 154 156 In Table 13, there is shown the effect of different starches and additives in two leakage tests on AA cells The JIS test for AA batteries involves continuous discharge across 5Ω for 48 hours and observing leakage For D cells, discharge is across 2Ω for 48 hours The DAT (discharge abuse test) test involves discharge across 15Ω for 4 weeks for AA cells and discharge across 5Ω for 4 weeks for D cells, before measuring leakage It can be seen that cross-linked, cationic corn starch gives the best results TABLE 13 Table 14 illustrates how the results of the JIS test were scored The total for each batch of 20 forms the leakage index TABLE 14 LOCATION OF LEAKAGE Score Externally visible 50 1 Visible after jacket removal 15 2 Gassing audible on jacket Release 7 3A Seπous Under top cover 5 3B Minimal Under top cover 3 4A Serious Under bottom cover 5 4B Minimal Under bottom cover 3 5A SΘΠOUS Between PVC & Can still wet 3 5B Minimal Between PVC & Can still wet 2 6A Serious Between PVC & Can dπed out 1 6B Minimal Between PVC & Can dned out 0 7A Seπous In air space still wet 3 7B Minimal In air space still wet 2 8A Seπous In air space leakage gelled 2 8B Minimal In air space leakage gelled 1 9A Seπous In air space dπed out 1 9B Minimal In air space dned out 0 10A Seπous Can wall perforated 1 10B Minimal Can perforated 0 11A Serious Bottom of can perforated 1 11B Minimal Bottom of can perforated 0 In Table 15, there is shown the results of the JIS and DAT tests on D cells. Once again, it can be seen that cross-linked, cationic corn starch gives the best results. Also demonstrated is the fact that can wall thickness is important TABLE 15 In Table 16, the effect of different starches of the Invention on D cells is shown, Once again, it can be seen that cross-linked, cationic corn starch gives the best results TABLE 16 Starch Roquette LAB2273 Roquette LAB2469 Gel Courtaulds 1209 Courtaulds 1209 PVP PVP K120 PVP K120 Additive Tallow 5 Amine Tallow 5 Amine Munksjo Paper 100021 300542 gsm 40 20 PRC EMD / N65 NMD 100 / 0 100 / 0 SCA 6.3 6.7 DP 2R2/4*8m/0V9 10.4 10.7 DY 2R2/Cont./0V75 7.6 8.1

Claims: Claims 1. A coated paper separator for an electrochemical cell, the coating comprising a starch and an additive, characterised in that cationic starch constitutes a majority of the starch component of the separator coating. 2. A separator according to claim 1, wherein the additive is a polyoxyalkylene, nitrogen containing compound. 3. A separator according to claim 1 or 2 , wherein the additive has a hydro-lipophilic balance of less than 17. 4. A separator according to any preceding claim, wherein the additive has a hydro-lipophilic balance of less than 14. 5. A separator according to any preceding claim, wherein the additive has a hydro-lipophilic balance of less than 11. 6. A separator according to any preceding claim, wherein the additive has a hydro-lipophilic balance of 9 or less. 7. A separator according to any preceding claim, wherein the additive has a hydro-lipophilic balance of greater than 5. 8. A separator according to any preceding claim, wherein the additive has a hydro-lipophilic balance of from 5 to 11. 9. A separator according to any preceding claim, wherein the additive has a hydro-lipophilic balance of from 7 to 10. 10. A separator according to any preceding claim, wherein the additive has a hydro-lipophilic balance of from 8 to 9. 11. A separator according to any preceding claim, wherein the additive has a hydro-lipophilic balance of about 9. 12. A separator according to any preceding claim, wherein the additive is non-ionic. 13. A separator according to any preceding claim, wherein the cationic element of the starch is nitrogen-based and the level of nitrogen content is between 0.2 and 2% w/w by dry weight of the starch. 14. A separator according to any preceding claim, wherein cationic starch forms substantially all of the starch component 15. A separator according to any preceding claim, wherein the starch is cross-linked. 16. A separator according to any preceding claim, wherein the starch is highly cross-linked. 17. A separator according to any preceding claim, wherein the starch is substantially insoluble in water at room temperature. 18. A separator according to any preceding claim, wherein the starch is a potato, corn or wheat starch. 19. A separator according to any preceding claim, wherein the starch is a wheat starch. 20. A separator according to any preceding claim, wherein the coating further comprises an etherified cellulose as a gellant. 21. A separator according to claim 20, wherein the gellant is able to swell and gel substantially immediately in water at room temperature and to remain stable therein over long periods . 22. A separator according to claims 20 or 21, wherein the gellant has a viscosity of between about 20 cP (0.02 Pa.s) and about 300 cP (0.3 Pa.s), preferably between 50 and 100 cP (0.05 and 0.1 Pa.s) . 23. A separator according to any preceding claim, wherein the additive is a mono- and di- amine polyoxyalkylene compound wherein the free alkyl group has around 18 carbon atoms, the side chains are polyoxyethylene substituents having an average of 1 or 2 oxyethylene units each and, where the compound is a diamine, then the link between the two amine centres is a trimethylene link. 24. A separator according to any preceding claim, wherein the paper, at a temperature of about 20°C, is able to absorb a 50μl droplet of water in a period of between four and fifteen minutes, preferably five and fifteen minutes and is particularly preferably between five and ten minutes. 25. An electrochemical cell comprising a separator according to any preceding claim. 26. A coating mix suitable for the manufacture of a separator according to any of claims 1 to 24, said mix comprising cationic starch.

Inventors: RANDELL CHRISTOPHER FRED, WHITE NEAL CHARLES, RANDELL, CHRISTOPHER, FRED, WHITE, NEAL, CHARLES
Applicants: EVER READY LTD, RANDELL CHRISTOPHER FRED, WHITE NEAL CHARLES, EVER READY LIMITED, RANDELL, CHRISTOPHER, FRED, WHITE, NEAL, CHARLES
Application year: 1997
Main classification: H01M6
IPC codes: H01M2, H01M6