GB1603635A - Battery separator - Google Patents

Description

(54) BATTERY SEPARATOR (71) We, TULLIS RUSSELL & COMPANY LIMITED, a British company, of Markinch, Glenrothes, Fife, KY7 6PB, Scotland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: Conventional battery separators, used for example in secondary lead-acid SL1 vehicle batteries, are formed by thin sheets of electrically insulating porous material which are immersed in the electrolyte between adjacent plates. Each separator has, on its face adjacent to the positive plate, a series of horizontally spaced vertical ribs which enable upward escape of the battery gases, such as nascent oxygen, which are evolved.

One form of existing separator is made of an absorbent paper impregnated with about 30% of a phenolic resin to give the paper wet strength and rigidity and to protect the cellulose from undue oxidation. The projecting ribs are formed either by deposit of strips of a hot melt adhesive, or by embossing the ribs and protecting them by the application of an adhesive.

An alternative existing separator is formed by extruding a thin sheet of sintered PVC, the ribs being formed integrally during the extrusion.

The phenolic resin impregnated paper separators tend to be brittle and the impregnation step together with the application of the hot melt adhesive to produce or protect the ribs involves significant expense. The extruded separators are also significantly more expensive.

The pores in the separators should be as small as possible to inhibit the passage through the separator of contaminants and prevent dendritic growth and it is a further criticism of existing commercial impregnated separators that the mean pore size is not as small as would be desirable. By pore size is meant the diameter of a pore of circular cross-section having the same cross-sectional area as the irregular pore shape existing in practice.

In accordance with the invention a method of making battery separators comprises the steps of preparing a furnish containing a synthetic pulp (as hereinbefore defined), forming a paper web from the furnish, subjecting the paper web to heat and pressure to consolidate the paper web over substantially its whole area and to emboss the paper web and thereby form projections on a face of the web, and cutting the paper web into battery separator sections.

In this context the term "synthetic pulp" means a pulp of polyolefin fibres having a fibrillated structure and made for example, by a flash spinning process. The invidual fibrils may be present in various sizes and various specific surfaces, the shape and size distribution not being unlike that of refined wood pulp. Each fibre usually has a central backbone with a number of laterally projecting branch fibrils.

Suitable examples of synthetic pulp would be derived from high density polyethylene, or polypropylene. The synthetic pulp may be filled or unfilled and, for example, a high density polyethylene synthetic pulp filled with between 25% and 35% by weight of china clay is suitable. The synthetic pulp may be modified, for example, with its surface oxidised to improve retention properties.

The fibres constituting the homogeneously dispersed synthetic component of the paper web are thermoplastic and this enables the web to be provided with additional strength and rigidity without brittleness, by the thermal consolidation substantially throughout its area.

This can provide a cost saving compared to the conventional phenolic impregnation. The thermoplastic nature of the paper web also enables the formation of projections integrally with, i.e. embossing, the paper web by a heating and pressing step. During this step the thermoplastic fibres will be softened and at least partially fused together before setting in the new configuration. Particularly when the paper is embossed with parallel ribs, as is preferred, these embossed ribs will also provide additional strength and rigidity. The ribs may be embossed in the cross web direction of the paper web but there may be advantages for the subsequent manufacture of the separators if the ribs are embossed in the longitudinal direction of the paper web.

Both embossing of the paper web and the thermal consolidation are preferably simultaneously carried out for example by passing the paper web through heated embossing calender rolls, or by preheating the paper web and passing it through embossing calender rolls.

Individual battery separator sections may readily be made by cutting the paper web into sections.

An important advantage of making the separator material from a furnish incorporating a significant quantity of synthetic pulp is that there is a lower average pore size, and narrower distribution of pore size, and a high porosity, that is pore volume, than is present in paper sheets of equivalent weight made from wood pulp. The weight loss due to oxidation of a separator incorporating synthetic pulp is similar to that of a separator of phenolicimpregnated cellulose paper without the need for the phenolic impregnation. This is a result of the acid resistance of the synthetic pulp fibres. For the application to battery separators, the maximum pore size should be less than 50 microns with a maximum mean value of about 20 microns. However, it is possible with a paper separator made in accordance with the invention to achieve a maximum pore size of about 25 microns with a mean value of about 10 microns.

Ideally the porosity of the paper separator should not be less than 60%. This determines to some extent the electrical resistance, wetting out properties and oxidation resistance of the separator. However, if the electrical resistance does not exceed a value of about 200 m.ohms cm2 after soaking for 20 minutes in a 1.28 specific gravity sulphuric acid, lower porosities would be acceptable. In practice it is found that this can be achieved in a paper separator made in accordance with the invention when the porosity is between 45 and 55%.

It is also possible to obtain a desirable characteristic of between 25 and 35% oxygen weight loss with the new separators.

It would be anticipated that the thermal consolidation previously referred to may to some extent reduce the porosity and increase the electrical resistance of the paper web, but we have surprisingly found that if the press pressure on the paper machine and the press pressure during thermal consolation, and also the moisture content of the paper web prior to thermal consolidation are carefully controlled, a low average pore size of about 10 microns, with a distribution of + 4 microns, and a porosity of 60% or above can be achieved. This may involve zero or minimum press pressures necessary for runnability on the paper machine, medium to high press pressures i.e. not more than 5 psi on the thermal consolidation rolls, and a moisture content in the paper web immediately prior to the thermal consolidation between 5 and 10% by weight. The moisture content is significant in that the water evaporates and provides a blowing effect during the thermal consolidation.

The furnish will usually contain at least 25%, and preferably at least 40% synthetic pulp by weight with an upper limit of 95%, or even 100%, any balance being a compatible pulp such as wood pulp. Although a paper could be made from a furnish containing a very high proportion of synthetic pulp using, for example, an Inclined Wire machine or a Sandy Hill Rotiformer (RTM) machine, it is difficult to manufacture paper webs from a furnish containing more than about 80% by weight of currently available synthetic pulp on a Fourdrinier machine. As paper making on a Fourdrinier machine is a cheaper process than on such other machines, and as a paper web made from a furnish containing between 60% and 80% of synthetic pulp is acceptable in the present context of battery separators, the paper web will preferably be made using a Fourdrinier machine.

We have found that the properties of the separators are significantly improved if the furnish incorporates polyester staple fibres, preferably at a level between 5 and 20% by dry weight of the furnish, that is pulp and polyester fibres. This is particularly valuable when the pulp contains a mixture of both synthetic and wood pulp in which case the polyester fibres will effectively replace a corresponding proportion of the synthetic pulp component.

Suitable polyester staple fibres will have a denier of between 1.5 and 10 and preferably, a length of between 3 and 6 mm. The inclusion of such polyester fibres has enabled us to achieve electrical resistance results in the range of from 50 and 100 m. ohms cm2.

The separator may be made from one, or two or more similar or different paper plies. For example the paper web may be made from at least two separate plies of which one incoporates a greater proportion of synthetic pulp than the other. A separator made from such a web would then be used with that ply containing the greater synthetic component towards the positive battery plate.

A suitable paper web may have a weight of between 75 and 300, but preferably between 85 and 200, and most preferably between 140 and 180 g/m2. The paper may be filled with specific absorbants for contaminants likely to be present or released within the battery with which the separator is to be used.

It is, of course, important for the battery separator to have a rapid wetting-out characteristic with substantially complete wetting when immersed in battery liquid in less than say 3 minutes. The significance of this is that batteries are frequently stored dry and are filled with the appropriate dilute acid immediately before use and the purchaser is seriously inconvenienced if he has to wait a long time for the battery to obtain its full potential. This wetting-out is potentially a particular problem when the battery separator is made from a paper web incorporating a significant quantity of synthetic pulp, owing to the hydrophobic nature of the polymer from which the paper is made. Conventional synthetic pulp normally incorporates a wetting agent, i.e. a surfactant, in the form of polyvinyl alcohol which facilitates dispersion of the synthetic pulp in water during the preparation of the furnish. However, it is undesirable to have significant quantities of polyvinyl alcohol in a battery and in any case this wetting agent does not produce adequate wetting-out characteristics when a battery incorporating the separator is filled. We have found that it is desirable to add a suitable chosen wetting agent at the paper making stage primarily in order to improve the wetting-out performance when the separator is used in a battery.

Appropriate wetting agents appear to be organic sulphonates, such as Lumo 1683 sold by Zschimmer & Schwarz; WT 27 sold by GAF (UK) Limited (an organic sulphonate with polyester groups); or Warcowet 060 by Warwick Chemicals (a sodium di-octyl sulphosuccinate); quaternary ammonium chlorides, such as Gloquat (RTM) 1032 sold by ABM Chemicals; ethoxylated amides such as Ethomid (RTM) HT 15 sold by AKZO Chemie UK; and ethoxylated amines such as Ethomeen (RTM) T12 sold by AKZO Chemie UK.

Ethomid (RTM) HT 15 has a general formula of

and Ethomeen (RTM) T12 has a general formula of

In each case we find that only between two and five groups of ethylene oxide is acceptable so that 2 c (x + y) '5.

A preferred level of addition of the wetting agent is no more than 6%, e.g. between 1% and 6% by weight of dry synthetic pulp.

The wetting agent may be added during paper making, that is either added to the furnish or sprayed onto the paper web whilst the web is still wet. Alternatively it may be possible for the wetting agent to be incorporated in the dry pulp during manufacture of the synthetic pulp.

One particularly useful way af adding the wetting agent in the furnish and simultaneously improving the electrical properties of the resulting separators, involves the incorporation in the furnish of a porous silicate, pretreated with the wetting agent and particularly with an aryl-alkyl-sulphonate or a sulpho-succinate, preferably at a level of between 1% and 8% of pretreated silicate by dry weight of pulp in the furnish. The porous silicate is absorb ann and acts as a carrier for the wetting agent. An appropriate silicate is Dicalite (RTM) which is a diatomaceous silica and is composed of hydrated amorphous silica or a fused sodium potassium aluminium silicate which has an approximate analysis of 70% SiO2, 18% Awl203, 7% K2O, and 4% Na2O. Such an inclusion has enabled us to achieve electrical resistance results of 100 m.ohms cm2.

Some examples of the production of battery separator material in accordance with the invention will now be described by reference to the accompanying drawings, in which: Figure 1 is a diagram of a paper machine Figure 2 is a diagram of an embossing unit; Figure 3 is a section of an embossing roller of the Figure 2 unit; Figure 4 is a perspective view of a finished battery separator.

Example 1.

To a suspension of synthetic pulp named Ferlosa (RTM) C2KM from Montedison, was added 5% by weight of dry synthetic pulp, of Lumo 1683 from Zschimmer & Schwarz. This mixture of synthetic pulp and surfactant was then blended with soft wood sulphate wood pulp in a ratio of 30 parts dry wood pulp to 70 parts dry synthetic pulp, and the mixture was beaten to 30 SR. After the blending 0.5% of aluminium sulphate per total dry weight of fibre was added for improved retention of the surfactant.

The furnish was then introduced into a flow box 5 of a Fourdrinier paper machine, from which the furnish was laid on a moving wire 6 which passed under a dandy roll 7 and around a couch roll 8 through which suction was applied. The resulting wet paper web 9 was passed through a press section 10 around drying cylinders 11, and reeled onto a reel 12, by-passing the conventional machine calender rolls.

The machine parameters were adjusted so that the wet press roll pressure at the section 10 was adjusted to a minimum operating pressure; the temperature of the machine drying cylinders 11 was maintained below 1200C; and the production rate was adjusted to achieve a moisture content at reel up of between 5 and 10%. Immediately after paper manufacture the paper rolls were enclosed in a polythene wrapper.

The reeled base paper was then thermally consolidated and embossed by means of the unit shown in Figures 2 and 3. Thus the base paper web 13 was unreeled from a reel 14, still with a moisture content between 5 and 10%, and at a speed of between 5 and 15 metres per minute passed around a heated cylinder 15 which was at a temperature between 170 and 200"C, through the nip of a pair of embossing rolls 16 and 17, around a chilled cooling cylinder 18, and reeled onto a reel 19. The rolls 16 and 17 may either be a male steel embossed roll 16 and a rubber roll 17 of controlled hardness, or a male steel embossed roll 16 and a female steel embossed roll 17 where the gap between the two rolls is controlled to give a separation on the flat area of the embossed design between 200 and 800 microns. An acceptable profile for the roll 16 is shown in Figure 3, in which the dimension A is 15.5 mm., B is 500 microns, C is between 100 and 200 microns, and D is 1200 microns.

It will be appreciated that the thermoplastic synthetic pulp component of the paper web 13 will be softened during passage around the cylinder 15, and the thermoplastic fibres will be partially fused together during passage through the rolls 16 and 17. These rolls compress the paper web over its full area and simultaneously emboss the web with parallel ribs in the cross web direction at 15.5 mm. centres. As the web is subsequently cooled, particularly after passage around the cylinder 18, the thermoplastic component sets in the new configuration.

The resulting embossed paper web, which has a weight of 160 g/m2 is subsequently unreeled from the reel 19 and cut into individual square battery sections, of which one having ribs 20 and a square side of 15 cm. is shown in Figure 4. It will be appreciated that the ribs 20 will have substantially the same profile as the roll 16 as shown in Figure 3.

Example 2.

The process as described in Example 1 was modified by reducing the initial surfactant addition to the synthetic pulp from 5% to 0.3% by weight of dry synthetic pulp. However, in addition further of the surfactant was applied to a level of 1% by weight of dry synthetic pulp to the paper web by spraying or roller application at the dandy roll or at some position between the dandy roll and the drying cylinders 11. It is possible by this modification to achieve lower addition levels of surfactant to the paper than can be achieved by the more conventional wet end addition. This modification also improves the ease of papermaking by reducing the foaming tendencies of the surfactants.

Example 3 The process as described in Example 1 was modified by replacing some of the dry synthetic pulp with 1.5 denier 6mm. polyester staple fibres as sold by Dupont under the name Dacron (RTM). The furnish consisted of 30% wood pulp, 60% synthetic pulp, and 10% polyester fibres. This modification showed that lower electrical resistance can be obtained by the addition of polyester fibres as electrical resistance figures in the range of 75 mQcm2 to 125 mQcm2 were obtained.

Example 4.

The process as described in Example 1 was modified by eliminating the initial surfactant addition to the synthetic pulp. Instead a porous silicate sold by Berk under the name Dicalite (RTM) BP1 was pretreated with 120% of the surfactant mentioned in Example 1.

This pretreated porous silicate was added to the total fibre blend of synthetic and wood pulp at a level of 6% by weight of the dry synthetic and wood pulp. This modification has shown the following advantages: (a) An easier method by which the surfactant may be added to the paper.

(b) Lower addition level of surfactant required because of higher retention on the absorbent silicate e.g. 3% to 4% by weight of synthetic pulp.

(c) Lower electrical resistance of converted separator than by the conventional wet end method of addition. Electrical resistance figures in the range of between 100 milcm2to 150 mQcm2 have been obtained.

Example 5.

The process as described in Example 1 was modified by reducing the proportion of wood pulp in the mixture of synthetic and wood pulp from 30% to 20% and increasing the beating of the pulp from 30 SR to 55" SR. This modification allows a higher level of synthetic pulp to be used thereby giving improved oxidation resistance.

Examples 6 to 10.

The process as described in Example 1 was modified by replacing the Lumo 1683 surfactant in turn by between 4 and 6% of 2272R surfactant from Diamond Shamrock; Mersolat (RTM) HCA 76 surfactant from Bayer (these two being of similar chemical type to Lumo 1683; WT27 surfactant from GAF (UK) Limited; Gloquat (RTM) 1032 surfactant from ABM Chemicals; and Warcowet 0.6% from Warwick Chemicals. These examples demonstrated that surfactants other than the one mentioned in Example 1 have been found to be acceptable. Similar electrical properties were obtained.

WHAT WE CLAIM IS: 1. A method of making battery separators, the method comprising the steps of preparing a furnish containing a synthetic pulp (as hereinbefore defined), forming a paper web from the furnish, subjecting the paper web to heat and pressure to consolidate the paper web over substantially its whole area and to emboss the paper web and thereby form projections on a face of the web, and cutting the paper web into battery separator sections.

2. A method according to claim 1, wherein the consolidating and embossing steps are simultaneously carried out by passing the paper web through heated embossing calender rolls.

3. A method according to claim 1, wherein the consolidating and embossing steps are simultaneously carried out by preheating the paper web and passing it through embossing calender rolls.

4. A method according to any one of the preceding claims, in which the paper web has a weight between 75 and 300 g/m2.

5. A method according to claim 4, in which the weight is between 85 and 200 g/m2.

6. A method according to any one of the preceding claims, in which the paper web is made from a furnish incorporating a mixture of synthetic pulp and wood pulp, the proportion of synthetic pulp being at least 25% by dry weight of the pulp mixture.

7. A method according to claim 6, wherein the proportion of synthetic pulp is at least 40% by weight of the pulp mixture.

8. A method according to claim 6 or claim 7, wherein the proportion of synthetic pulp lies between 25% and 95% by dry weight of pulp mixture.

9. A method according to claim 6 or claim 7, wherein the paper web is made on a Fourdrinier machine, and the proportion of synthetic pulp in the mixture lies between 60% and 80%.

10. A method according to any one of the preceding claims wherein the paper web has a weight between 140 and 180 g/m2.

11. A method according to any one of the preceding claims, in which there is incorporated in the paper web a wetting agent selected from organic sulphonates, quaternary ammonium chlorides, ethoxylated amicdes, and ethoxylated amines.

12. A method according to claim 11, in which the wetting agent is added at a level between 1% and 6% by weight of dry synthetic pulp.

13. A method according to claim 11 or claim 12, in which the wetting agent is added in the furnish.

14. A method according to claim 13, wherein the furnish incorporates a porous silicate pretreated with the wetting agent.

15. A method according to claim 14, wherein the pretreated silicate is incorporated at a level between 1% and 8% by dry weight of the pulp in the furnish.

16. A method according to claim 14 or claim 15, wherein the porous silicate is pretreated with an aryl-alkyl-sulphonate or a sulpho-succinate.

17. A method according to claim 11 or claim 12, in which the wetting agent is sprayed onto the paper web while the web is still wet.

16. A method according to claim 17, wherein the furnish incorporates polyester staple fibres.

17. A method according to claim 18, in which the staple fibres have a denier of between 1.5 and 10.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (1)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   absorbent silicate e.g. 3% to 4% by weight of synthetic pulp.

   (c) Lower electrical resistance of converted separator than by the conventional wet end method of addition. Electrical resistance figures in the range of between 100 milcm2to

   150 mQcm2 have been obtained.

   Example 5.

   The process as described in Example 1 was modified by reducing the proportion of wood pulp in the mixture of synthetic and wood pulp from 30% to 20% and increasing the beating of the pulp from 30 SR to 55" SR. This modification allows a higher level of synthetic pulp to be used thereby giving improved oxidation resistance.

   Examples 6 to 10.

   The process as described in Example 1 was modified by replacing the Lumo 1683 surfactant in turn by between 4 and 6% of 2272R surfactant from Diamond Shamrock; Mersolat (RTM) HCA 76 surfactant from Bayer (these two being of similar chemical type to Lumo 1683; WT27 surfactant from GAF (UK) Limited; Gloquat (RTM) 1032 surfactant from ABM Chemicals; and Warcowet 0.6% from Warwick Chemicals. These examples demonstrated that surfactants other than the one mentioned in Example 1 have been found to be acceptable. Similar electrical properties were obtained.

   WHAT WE CLAIM IS: 1. A method of making battery separators, the method comprising the steps of preparing a furnish containing a synthetic pulp (as hereinbefore defined), forming a paper web from the furnish, subjecting the paper web to heat and pressure to consolidate the paper web over substantially its whole area and to emboss the paper web and thereby form projections on a face of the web, and cutting the paper web into battery separator sections.

   2. A method according to claim 1, wherein the consolidating and embossing steps are simultaneously carried out by passing the paper web through heated embossing calender rolls.

   3. A method according to claim 1, wherein the consolidating and embossing steps are simultaneously carried out by preheating the paper web and passing it through embossing calender rolls.

   4. A method according to any one of the preceding claims, in which the paper web has a weight between 75 and 300 g/m2.

   5. A method according to claim 4, in which the weight is between 85 and 200 g/m2.

   6. A method according to any one of the preceding claims, in which the paper web is made from a furnish incorporating a mixture of synthetic pulp and wood pulp, the proportion of synthetic pulp being at least 25% by dry weight of the pulp mixture.

   7. A method according to claim 6, wherein the proportion of synthetic pulp is at least 40% by weight of the pulp mixture.

   8. A method according to claim 6 or claim 7, wherein the proportion of synthetic pulp lies between 25% and 95% by dry weight of pulp mixture.

   9. A method according to claim 6 or claim 7, wherein the paper web is made on a Fourdrinier machine, and the proportion of synthetic pulp in the mixture lies between 60% and 80%.

   10. A method according to any one of the preceding claims wherein the paper web has a weight between 140 and 180 g/m2.

   11. A method according to any one of the preceding claims, in which there is incorporated in the paper web a wetting agent selected from organic sulphonates, quaternary ammonium chlorides, ethoxylated amicdes, and ethoxylated amines.

   12. A method according to claim 11, in which the wetting agent is added at a level between 1% and 6% by weight of dry synthetic pulp.

   13. A method according to claim 11 or claim 12, in which the wetting agent is added in the furnish.

   14. A method according to claim 13, wherein the furnish incorporates a porous silicate pretreated with the wetting agent.

   15. A method according to claim 14, wherein the pretreated silicate is incorporated at a level between 1% and 8% by dry weight of the pulp in the furnish.

   16. A method according to claim 14 or claim 15, wherein the porous silicate is pretreated with an aryl-alkyl-sulphonate or a sulpho-succinate.

   17. A method according to claim 11 or claim 12, in which the wetting agent is sprayed onto the paper web while the web is still wet.

   16. A method according to claim 17, wherein the furnish incorporates polyester staple fibres.

   17. A method according to claim 18, in which the staple fibres have a denier of between 1.5 and 10.

   20. A method according to claim 17 or claim 19, wherein the staple fibres are

   incorporated at a level between 5% and 20% by dry weight of the furnish.

   21. A method according to any one of claims 18 to 20, wherein the staple fibres have a length lying between substantially 3mm and 6mm.

   22. A method according to any one of the preceding claims, in which the paper web is made from at least two separate plies of which one incorporates a greater proportion of synthetic pulp than the other.

   23. A method according to any one of the preceding claims, in which the projections are spaced parallel ribs.

   24. A method according to any one of the preceding claims, in which the synthetic pulp is a filled pulp.

   25. A method according to claim 24, in which the pulp is filled with china clay.

   26. A method according to claim 1, substantially as described with reference to any one of the examples.

   27. A battery separator which has been made by a method according to any one of the preceding claims.