GB2195366A

GB2195366A - Porous fabrics

Info

Publication number: GB2195366A
Application number: GB08721293A
Authority: GB
Inventors: Stanley Littman; Ronald Greenberg; Roland Bernier
Original assignee: Kendall Co
Current assignee: Kendall Co
Priority date: 1986-09-11
Filing date: 1987-09-10
Publication date: 1988-04-07
Anticipated expiration: 2007-09-10
Also published as: DE3729567A1; GB8721293D0; IT8748375A0; IT1211767B; JPS63114055A; GB2195366B; FR2604030A1

Description

1 GB2195366A 1

SPECIFICATION

Porous fabrics The present invention relates to porous fabrics that may be used as filters, membrane supports, 5 or more specifically, as battery separator fabrics, and especially to porous fabrics utilizable as separators in Lithium Batteries.

One of the most difficult problems that has been encountered today in the battery separator art, is to economically provide a porous fabric for use as a battery separator. Although the prior art has developed a microporous film that is used as a battery separator, it has not produced a 10 porous fabric or a film that is economical to use. The present invention has succeeded in providing a porous fabric that is inexpensive to produce where the prior art has not.

Typical prior art porous fabric is such as that made by the Celanese Corporation, located in

Charlotte, North Carolina, under the trade name "Celgard". This prior art film is an extruded plastic film that is drawn out to make it porous. Drawing out is defined as a stretching process. 15 Drawing of the film is done so as to make fractures occur in the film. The resulting fractures are what become micropores in the film. There are disadvantages with this prior art. One such disadvantage is that it is very expensive to produce the porous fabric due to the number of process steps. Because the porous fabric is expensive to produce, the selling costs are high.

Another disadvantage of this prior art is that the porous film is over engineered for use as a 20 battery separator. Micropores formed in the fabric are too small in size for the film to be economically used as a battery separator. The pores in the prior art range between 0.02 and

0.04 microns. This is a range that is far below what is necessary in a battery separator. The allowable range of pore sizes in battery separator fabrics is preferably between 1 to 12 microns, with an average of 6 microns.

The present invention provides sufficient flow and sufficient barrier properties to serve as well or better than the prior art and is less expensive to produce.

In U.S. patent No. 4,472,328, there is described a porous film or sheet. The prior art porous film is made by melt-molding a resin selected from a group consisting of a linear low density polyethylene, a high density polyethylene and a polypropylene into a film or sheet. The film or 30 sheet is then drawn out or stretched. During the stretching of the film, the film fractures, thus producing pores within the film or sheet.

This prior art has the same disadvantages as the previously mentioned prior art.

The present invention is superior to the prior art by reason of its economy, and its more realistically fitting the requirements of a battery separator material.

The present invention aims to provide a porous fabric that is more economical to produce than prior art products.

The present invention also aims to provide a porous fabric that may be used as a battery separator.

According to the present invention a porous fabric usable as a battery separator fabric comprises an unbonded fibrous web of thermoplastic polypropylene fibres, having top and bottom surface fibres and inner fibres, the surfaces of the top and bottom surface fibres of the fibrous web being seared and fused together by simultaneously applying heat and pressure. The heat and pressure may be applied to the top and bottom surfaces of the fibrous web by a pair of steel rolls. The heat is controlled so that it sears only the tops of the surface fibres while substantially leaving the top and bottom and inner fibres of the web unaffected. As the surface fibres are being seared, pressure is applied to insure that the seared fibres make contact with each other, thus securing themselves together. As the seared fibres make contact with each other they fuse together forming porous filrii-like top and bottom surfaces.

These pores are formed because a majority of spaces remain between the substantially so unaffected surface fibres. The unaffected surface fibres and the inner fibres along with the porous film-like surfaces, permits the fibrous web to substantially retain its original porosity. This results in a film-like fabric having excellent porosity.

The fabric is made by producing a fibrous web of thermoplastic fibres formed by carded, air 55. laid, wet laid, meltblown or spunbond process means, and passing this web between a pair of 55 steel rolls that are heated and that have pressure applied to them. The web may have a weight of between 20-40 grams per square yard (gsy) ( - grams per square metre (gsm)) while the preferred value is about 36 gsy ( gsm). The heat from the rolls sears the top and bottom surfaces of the surface fibres of the fibrous web.

Searing is defined for the purposes of this application as the minimum flowing of a thermplas- 60 tic fibre so as to substantially create a change in its surface structure. The temperature and pressure of the rolls is so controlled so that the fusion gradient will create stick points between the surface fibres, while permitting the top and bottom and inner fibres to remain substantially as fibres. The fusion gradient is defined for the purposes of this application as the temperature gradient across the web mass, while the web is in the nip of a pair of rolls, so as to only sear - 65 1 2 GB2195366A 2 the surfaces of the surface fibres to create stick points. Stick points are defined for the purpose of this application as the lowest level of initial adhesion between adjacent fibres. The rolls, when pressure is applied to them cause the seared fibres to spread out and contact other surface fibres to form top and bottom film-like surfaces while at the same time securing themselves together. It should be noted that a majority of the top and bottom surfaces fibres and the inner fibres of the web are substantially unaffected by the heat and the pressure of the rolls and remain essentially in their original state as whole fibres. Because a substantial amount of the seared thermoplastic fibres remain as whole fibres, spaces or pores remain between the fibres in the fibrous web, permitting the web to remain porous.

The invention may be put into practice in various ways and one specific embodiment will be described by way of example to illustrate the invention with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic cross-sectional view of a fabric in accordance with the present invention to show the fibrous web; Figure 2 shows the fabric passing between the heated and pressure applying rolls; Figure 3 is a cross-sectional view of the fabric of Figure 2 after the fabric has passed between the heated and pressure applying rolls; and Figure 4 is a cross-sectional view of the fabric of Figure 3 to illustrate the porous nature of the fabric.

Figure 1 illustrates the initial stage in the preparation of a product in accordance with the present invention, which is a nonwoven fibrous web 10 of thermoplastic polypropylene fibres 12. As shown in Figure 2, the fibrous web 10 is passed between an arrangement of steel rolls 14, only two being shown, that are heated and that have pressure applied to them. Although it is preferred that the rolls be made from steel, other conventional materials may be used. These rolls may be heated to a temperature between 285'F (140"C) and 315OF (1570C), with the preferred temperature being 295'F (146'C). The pressure "F" applied to the rolls also subjects the fibrous web to a pressure of between 500-800 pounds per linear inch (PLI), (227-364 Kg per linear inch) ( Kg/linear cm) with the preferred pressure being 700 PLI (318 Kg per linear inch). The rolls 14 are enpowered to rotate about their axes, as indicated by the arrows "R" in Figure 2.

As the fibrous web 10 is passed between the nip of the rolls, the thermoplastic polypropylene fibres 12 on the top and bottom surfaces of the web that come in contact with the heated nip rolls, are seared. As these fibres are seared the pressure applied on the rolls causes the seared material to flow thus contacting other surface fibres and becoming secured thereto to form film like surfaces with pores. The preferred fibres are made by Hercules Inc. of Norcross, Georgia and sold as T-181 polypropylene fibres. This particular polypropylene fibre has a wider melting range than other fibres, thus permitting better control over the initial fusion of the fibres.

Although, polypropylene fibres are the preferred fibres, other fibres such as polyamide, poly ester, polysulphone, polyphenylene sulphide, cellulosic or blends thereof may be used with slightly different results.

In addition, the combination of heat and pressure produces a fabric that has an arrangement of film-like top 22 and bottom 24 surfaces and a plurality of pores 28 therein, as shown in Figures 3 and 4.

As the fibres on the top 22 and bottom 24 surfaces, as shown in Figures 2 and 3, are seared, a substantial amount of open area or spaces remain on these surfaces, thus leaving the 45 pores 28 therewithin, permitting the fabric to remain porous.

To further illustrate the unique structure of the present invention, Figure 4, the cross-sectional view of Figure 3, shows the top surface 22, the bottom surface 24, the unaffected inner fibres 12 and the pores 28 that remain after the seared material is cooled. Figure 4 also shows the porosity of the fabric, by illustrating the passage of a fluid indicated by arrows "F", disposed 50 therethrough. As illustrated in Figure 4 the pores 28 in the top 22 and bottom 24 surfaces along with the unaffected inner fibres 12 permit substantial porosity 30 throughout the fabric.

In testing the fabric it was unexpectedly found that the fabric retained substantial porosity throughout.

55- Porous construction in fabrics is necessary if the fabrics are to be used in batteries. Porosity 55 is essential because the pores in a fabric permit ionic contact to take place by allowing electrolytes to pass through them, thus making a connection between the electrodes. The porous fabric thus performs a needed function. A non-porous or semi- porous film does not permit sufficient passage of electrolyte to take place between the electrodes in a battery, thus 60- restricting ionic contact.

By using polypropylene fibres in the present invention it has price and performance advantages over the prior art. Another advantage the present invention has over the prior art is that it can deliver the same performance or better performance as the prior art at a substantially lower price.

The prior art is over-engineered for use as a battery separator because its pore size is finer v 3 GB2195366A 3 than required in many battery separator applications.

Still another advantage that the present invention has over prior art is that it can provide a cell life and cell voltage performance equivalent or better than the prior art, even though the present invention has larger pores and much wider pore distribution.

The following Table 1 is a compilation of test results between a sample of the present invention and a sample of the prior art. Both samples were compared to the allowable range of properties that are necessary in a fabric, if it is to be used as a battery separator.

TABLE 1

Physical Allowable Present Prior 10 Propertv Range Invention Art Fabric weight (gsy) 20-40 36.5 10-12 Thickness (mil) 1-4 2.5 1.0 15 Void Volume (%) min 30 56 38-46 Tensile, Dry (lblin) 7-1012-4 9.8/1.8 20/2 Air Permeability 20 Frazier (CFM/F2) (cubic feet/minute/ 25 squaie foot) 0-6 1.88 0 Pore size, max (microns) max 90 80 mean (microns) 1-12 8 0.02-0.04 30 Internal Resistance 10-50 29 2.5-10 KOH (35%) (milliohm/cm2 35 Cell Voltage, min. 2.0 2.7 2.5 initial (Volts) i As indicated in the table, the present invention permits a cell voltage that is higher than the prior art, despite the higher average pore size and higher internal resistance. A high cell voltage is a key requirement in a fabric for suitable performance as a battery separator.

prior art, although suited for use as a battery separator, has pores that are too fine for the particle size migrations it must resist.

The present invention battery separator has distinct advantages over the prior art, because it provides adequate filtration and improved higher void volume, while doing both at a substantially lower cost than the prior art.

An example of a fabric in accordance with the present invention is now given. This example is not intended to limit the present invention in any way.

EXAMPLE

A web of partially randomized, polypropylene 1.5 denier fibres, produced on web forming equipment such as cards, air formers, melt blowing, continuous filament or even wet-lay equip ment and weighing about 36 grams per square yard is passed between the nip of two steel rolls that are heated and that have pressure applied to them. The rolls are heated to a temperature of about 300OF (a temperature range of 285 to 320F (160Q is permissible) and are held in contact under about a 700 PLI pressure load. The web travels through the nip at a 60 speed of about 20 fpm. The web has a thickness of between about 2-3 mils after passing through the nips.

As the web contacts the heat, the surface of the polypropylene fibres on the top and bottom surfaces of the web are seared. As the fibres are being seared, the pressure exerted on the rolls, causes the seared fibres to make contact with each other securing themselves together. 65 4 GB2195366A This results in film-like surfaces being formed on the top and bottom of the fabric while leaving the surface fibres substantially as fibres. A majority of the polypropylene fibres within the web also remain substantially unaffected by the heat and pressure, thus retaining their original state as fibres.

Because the top and bottom fibres remain as whole fibres the initial porosity of the fibrous web is retained, thus permitting the web structure to retain porosity throughout. As shown in Table 1, the present invention has a higher air permeability than the prior art, which is a direct result of the pore size in the present invention. The product of the present invention is thus better suited for use as a Battery Separator.

Claims

CLAIMS -

1. A porous fabric comprising:

an unbonded fibrous web of thermoplastic fibres, having top and bottom surface fibres and inner fibres, the said top and bottom surface fibres being seared by heat and pressure so as to form film-like surfaces; and a plurality of pores disposed through the said top and bottom surfaces, the original state of the said fibres being otherwise substantially unaffected by said heat or pressure.

2. A porous fabric as claimed in Claim 1 in which the thermoplastic fibres are selected from the group consisting of: polypropylene, polyamide, polyester, polysulphone, polyphenylene sul- phide, or cellulosic fibres or blends thereof.

3. A porous fabric as claimed in Claim 1 or Claim 2 in which the temperature applied to said unbonded fibrous web is between about 285F (1400C) and about 320"IF (1600C).

4. A porous fabric as claimed in Claim 1, 2 or 3 in which the pressure applied to the said unbonded fibrous web ranges between about 500 and about 800 I'Ll.

5. A porous fabric as claimed in any one of Claims 1 to 4 in which the unbonded fibrous web 25 of thermoplastic fibres has a thickness of about 1 to about 20 mils to said searing thereof.

mms) prior to the

6. A porous fabric as claimed in any one of Claims 1 to 5 in which the said unbonded fibrous web of thermoplastic fibres has a weight of about 20 to about 40 grams/sq. yd.

7. A porous fabric as claimed in any one of Claims 1 to 6 in the form of a battery separator. 30

8. A porous fabric as claimed in Claim 1 in the form of a battery separator substantially as specifically described herein with reference to the example.

9. A method of making a battery separator fabric, comprising the steps of:

providing an arrangement of rolls to create a nip therebetween; heating the said arrangement of rolls; delivering an unbonded fibrous web of thermoplastic fibres to the said nip of the said rolls; rotating the said rolls sufficient to drive the said fabric therebetween and sear the upper and lower surfaces thereof while maintaining porosity therethrough.

10. A method of making a battery separator fabric as claimed in Claim 9, including:

heating the said rolls to a temperature range of about 285F to 320'F (140 to 1600C).

11. A method of making a battery separator fabric as claimed in Claim 9 or Claim 10, including: pressurizing the said fabric in the said nip to a range of about 500 to 800 PLI Kg per linear inch).

12. A method of making a battery separator fabric as claimed in Claim 9, 10 or 11 in which the said fabric is selected from the group comprising: polypropylene, polyamide, polyester, polysulphone, polyphenylene sulphide, and cellulosic fibres or blends thereof.

13, A method as claimed in Claim 9 substantially as specifically described herein with reference to the accompanying drawings.

Published 1988 at The Patent Office, State House, 66/71 High Holborn, London WC111 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.