GB2098129A - Pliable microporous battery separator sheet - Google Patents

Abstract

Pliable, sintered, microporous separator sheets 2 for use in electric storage batteries are made by the incorporation of terpolymers consisting of linear ethylene vinyl acetate copolymer backbones with pendant grafted segments of poly (vinyl chloride). <IMAGE>

Description

SPECIFICATION Pliable microporous battery separator sheet This invention relates to tough, flexible, sintered battery separators based on terpolymers having linear ethylene vinyl acetate copolymer backbones and pendant grafted poly(vinyl chloride) segments.

Battery separators function to electrically isolate the positive and negative plates of electrolytic energy cells. Most separators for lead-acid, automotive, SIL (starting-ignition-lighting) batteries are formed from sheets of electrolyte-porous insulating materials. They prevent internal electrical shorts by physically spacing opposite polarity plates apart from one another and by suppressing dendritic crystal growth therebetween ("treeing").

Modern SIL Battery design seeks to reduce battery size and weight without sacrificing power or life.

Among the modifications that may be made to earlier designs is the elimination of mud ribs in the bottom of battery cases. This reduces overall case size and the amount of electrolyte needed to fill it.

Another is the provision of thinner battery plates so that a greater number of plates will fit into a smaller case and provide at least as much power as a battery having a larger case which accommodates fewer plates.

The role of the battery separator is critical to the successful implementation of such design modifications. To prevent "treeing", which is aggravated by the elimination of mud ribs, it is preferred to encase every other plate in a three-sided envelope folded around the bottom edge of the plate and sealed along its side edges. A suitable sheet-type separator material must be flexible enough to withstand such folding, porous enough to allow free electrolyte flow, and tough enough to undergo further processing steps in the battery fabrication process without damage.

High quality battery separators have been made by sintering micron sized poly(vinyl chloride) (PVC) particles in accordance with the method taught in U.S. patent 3,551,210, assigned to the assignee of this invention. The patent provides a method of laying down PVC powder and sintering it to form sheet stock at rates of 45.72 metres (150fit.) per minute or better. It is an economical process because PVC resin is relatively inexpensive and production rates are high. However, unmodified sintered PVC sheets tend to be too brittle for use as envelope-type separators in batteries without mud ribs. Thus, we sought a means of providing more flexible, tough battery separator sheet material that could be made by the sintering process.Many different approaches were taken to improve the flexibility and toughness of sintered PVC separators, but only those internally plasticized, vinyl chloridecontaining separators described and claimed herein proved successful.

It is therefore an object of this invention to provide an electrically insulating, microporous separator for use between the plates of an electrolytic storage battery comprised of sintered particles of a poly (vinyl chloride) based polymeric material which is both tough and flexible. A more particular object is to provide battery separators comprised of sintered particles containing at least a portion of a terpolymer consisting essentially of a flexible copolymer backbone of linear ethylene and vinyl acetate (EVA) that has grafted thereto a substantial weight portion of segments of poly(vinyl chloride).Another object of the invention is to provide a flexible battery separator made from such a grafted copolymer where the particles before sintering are in the form of substantially smooth hollow spheroids less than 60 microns in diameter so that the finished product has suitable microporosity for free electrolyte flow without inducing harmful dendritic growth.

A more specific object of the invention is to provide a battery separator made up of a portion of substantially pure poly(vinyl chloride) and a portion of a compatible ethylene vinyl acetate copolymer having grafted poly(vinyl chloride) segments.

Another object is to provide a mixture of particles of both resin types and sinter them by known means to create a battery separator which is sufficiently flexible and tough to withstand folding around a battery plate in an envelope configuration and subsequent battery fabrication without damage.

In accordance with a preferred embodiment of the invention, microporous battery separator sheets are formed from particles of a terpolymer having a backbone of copolymerized ethylene and vinyl acetate with grafted pendant segments of poly(vinyl chloride) (EVA-g-VC) or a mixture of the terpolymer and pure poly(vinyl chloride). The term terpolymer as used herein pertains to copolymers made up of three different kinds of mer units. Preferably, the polymer particles are formed by spray drying from a liquid polymerization suspension or emulsion so that they have the form of substantially smooth surfaced hollow spheroids less than 60 microns in diameter. To form separator sheet stock, a layer of such particles is deposited onto a smooth stainless steel belt and combed into a longitudinally ribbed particle bed of appropriate depth and width.The particles are sintered by known means for a time and at a temperature such that a cohesive yet microporous product results.

The EVA copolymer backbone portion of the terpolymer constituent provides the sintered separator material with toughness and flexibility. The grafted vinyl chloride segments add strength and are acid resistant. Whether the terpolymer containing particles are sintered alone or in combination with substantially pure poly(vinyl chloride) resin powders, the resulting separators are substantially internally plasticized by the presence of only a small weight portion of EVA. They are flexible enough to be enveloped around preformed battery plates at room temperature in automatic folding and heat sealing equipment without developing cracks along the folded bottom edge or at the corners. They are tough enough to tolerate further battery assembly procedures without damage.

The invention and how it may be performed are hereinafter particularly described with reference to the accompanying drawings, in which: Figure 1 is a view of a ribbed sheet of sintered EVA-g-VC powder prior to folding around a battery plate.

Figure 2 shows an expanded lead battery plate encased by a three-sided EVA-g-VC envelope in accordance with the invention. The separator is folded along the bottom edge of the plate and heat sealed along the side edges.

Figure 3 is a cross sectional view along 3:3 of Figure 2 showing the battery plate encased in the ribbed separator envelope.

The present invention relates to the incorporation of particles containing EVA-g-VC terpolymer in a microporous sintered battery separator to provide the flexibility and toughness required of an envelope type separator in the manufacture of compact, lead-acid, automotive SIL batteries. The synthesis of such terpolymers for use in liquid impermeable coating and moulding compositions is well known.

See, e.g., Great Britain patent number 1,075,643A Process for the Production of Polymers of Vinyl Chloride filed August 18, 1964 as a patent of addition to Great Britain patent 1,021,324 dated September 4, 1963. However, the use of EVA-g-VC terpolymers as flexibilizing agents for microporous sintered battery separators was neither anticipated nor suggested prior to the present invention.

In a preferred practice, a battery separator is prepared by sintering particles comprised of EVA-g VC terpolymer alone or a combination of terpolymer and PVC particles. The sintering particle mix is prepared so that the finished separator contains from about 2 to 40 percent total resin weight, and preferably 5 to 20 percent, linear ethylene vinyl acetate copolymer. Poly(vinyl chloride) segments are grafted to the vinyl acetate copolymer backbone so that the total weight poly(vinyl chloride) is at least 60% of the terpolymer weight. The molecular weight of the terpolymer should be at least 70,000, the maximum molecular weight being determined by the desired physical properties for the separator material. The poly(vinyl chloride) portion of the terpolymer particularly provides battery separators with structure integrity, acid resistance, and strength.The ethylene vinyl acetate copolymer provides increased flexibility and toughness. A high molecular weight EVA-g-VC terpolymer containing a large proportion of grafted vinyl chloride will yield a relative stiff but strong material. A low molecular weight terpolymer containing a small proportion of grafted PVC will yield a relatively soft, flexible material. Thus, adjusting the molecular weight of a terpolymer and the proportion of graft PVC present can modify the physical properties of the polymer over a large range. The mere formation of a mixture of ethylene vinyl acetate and poly(vinyl chloride) by dissolution in a common solvent has been found to improve neither the strength nor flexibility of sintered separators.Thus, we have found it necessary to actually chemically link the polymeric ethylene vinyl acetate and PVC constituents in order to make strong, tough, sintered separator materials. We believe that a preferred ethylene to vinyl acetate ratio in the copolymer backbones of the subject terpolymers is about 1:1. Other ratios of ethylene to vinyl acetate should also work, although we found ethylene homopolymerwith grafted vinyl chloride segments to be an unsuitable separator material.

However, it is preferred to limit the amount of acetate in the terpolymer to less than about 20 weight percent. Leaching of acetate by battery acid may lead to the formation of acetic acid in the battery electrolyte. Acetic acid cannot be tolerared because it in turn may form lead acetate which uselessly ties up a battery's active lead materials.

The grafted poly(vinyl chloride) segments on the electrolyte vinyl acetate copolymer backbone serve to make the terpolymer compatible with pure poly (vinyl chloride) resin where mixtures of the two are used to form sintered battery separators. In the compositions, the poly(vinyl chloride) segments provide the physical properties of prior sintered PVC separators, particularly, acid resistance, strength, and processability. The electrolyte vinyl acetate portions of the terpolymers provide additional toughness and flexibility.

While making the terpolymer is not part of this invention, certain steps should be followed to assure consistent high quality materials. The terpolymer can be formed by emulsion or solvent polymerization according to well known practices such as those set out in British patent number 1,075,643 cited above. Terpolymers may also be formed by dissolving ethylene vinyl acetate copolymer containing the desired ratio of ethylene to vinyl acetate in vinyl chloride monomer under heat and pressure in the presence of a graft initiator such as a peroxide. The initiator causes the growth of the vinyl chloride monomers onto the EVA copolymer backbone. The length of the grafted vinyl chloride segments is determined by the concentration of monomer in the polymerization solution as well as duration, temperature and pressure of the reaction.Another method of making the terpolymer is to dissolve ethylene vinyl acetate copolymer in a solvent and thereafter introduce vinyl chloride monomer under pressure until a desired degree of grafting is obtained. Vinyl chloride may also be grafted in this manner beginning with an EVA-water-solvent emulsion.

Past experience with the production of poly(vinyl chloride) battery separators has shown that optimum results are obtained by spray drying the polymer particles to be sintered from their solution or emulsion polymerization stocks. This creates hollow spheroidal particles less than 60 microns in diameter. When sintered, these particles yield battery separators with the desired degree of porosity and resistance to "treeing".

Figure 1 shows a ribbed sheet of battery separator material which could be made by sintering particles containing EVA-g-VC terpolymer for flexibility and toughness. The separator sheet 2 comprises a web portion 4 and rib portions 6. The thickness of the web portion 4 is generally less than about 0.635 mm (0.025 inch) and preferably about 0.483 mm (0.019 inch). The thickness at the rib portions 6 is generally less than about 1.27 mm (0.05 inch) and preferably 1.02 mm (0.04 inch). Ribs 6 act primarily as spacers between adjacent plates. Figure 2 shows the separator sheet 2 of Figure 1 folded in an envelope configuration over an expanded metal battery grid 8. It is bottom fold 10 that is most prone to crack in unplasticized sintered PVC separators. The separators of this invention, however, can be severely folded without tearing or cracking. The separator is heat sealed along side edges 12 of grid 8 so that it will remain folded along line 10 and not slip off the grid. Figure 3 shows a cross sectional view of the separator enveloped grid 8. The smooth separator surface 14 lies adjacent expanded portion 16 of grid 8. The grid retains a lead oxide battery paste 18.

The present invention provides for battery separators containing sintered terpolymer particles. The terpolymer has a backbone of ethylene and vinyl acetate copolymer to which poly(vinyl chloride) segments are grafted. The EVA toughens the separator and provides it with enough flexibility, e.g., to fold around the bottom of a battery plate at room temperature without cracking or breaking.

Claims (5)

1. A pliable microporous battery separator sheet, for use between the plates of an electrolyte storage battery, comprising sintered particles of a terpolymer consisting essentially of a linear ethylene vinyl acetate copolymer backbone and grafted thereto segments of poly(vinyl chloride) in an amount of at least 60% of the terpolymer weight, the ethylene vinyl acetate portion of the terpolymer providing the separator sheet with toughness and flexibility.

2. A pliable microporous battery separator sheet according to claim 1, in which said particles of terpolymer before sintering are in the form of substantially smooth surfaced hollow spheroids less than 60 microns in diameter.

3. A pliable microporous battery separator sheet according to claim 1 or 2, in which 2-40 percent by weight of said terpolymer consists of said ethylene vinyl acetate copolymer and the balance consists of said poly(vinyl chloride), said ethylene vinyl acetate providing said separator sheet with flexibility and toughness so that it can be folded around a battery plate at room temperature and heat sealed at the plate edges without damage.

4. A pliable microporous battery separator sheet according to any one of the preceding claims, in which the sheet includes sintered particles of poly (vinyl chloride).

5. A pliable microporous battery separator sheet substantially as hereinbefore particularly described and as shown in Figures 1 to 3 of the accompanying drawings.