CA1134572A

CA1134572A - Flexible microporous rubber base articles and process for producing these

Info

Publication number: CA1134572A
Application number: CA000329763A
Authority: CA
Inventors: Bruce S. Goldberg; Mahendra Shah
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-06-16
Filing date: 1979-06-14
Publication date: 1982-11-02
Also published as: PH15965A; GB2025425A; FR2443479B1; DE2924218A1; FR2443480A1; SE446518B; IT1117166B; IN157216B; AU526263B2; PL216321A1; DE2924218C2; GB2025425B; FR2443479A1; FR2443480B1; ATA425279A; PT69761A; AT381323B; LU81382A1; ES8201465A1; CH654583A5

Abstract

ABSTRACT OF THE DISCLOSURE

As an article of manufacture, a microporous flexible shape or sheet of a sulfur-free, cured polymeric material of a curable rubber, an ethylene-propylene copolymer or mixtures of the curable rubber and ethylene-propylene copolymer; the article possesses an average pore size of less than 2 microns, a predetermined flexibility, and improved toughness, when compared to prior art sulfur cured articles;
a continuous production of the cured rubber base material has been found to be especially advantageous.

Description

~L~L3~i7~

This invention pertains to microporous articles made of polymeric materials; more particularly, this invention pertains to cured polymeric compositions wherein the average pore size is less than 2 and, more commonly, less than 1 micron and wherein the article can be tailor-made to be of varying degrees of flexibility ranging from completely drapable material to a relatively stiff material, yet non brittle and tough haviny flexibility and toughness characteristics heretofore unknown in sulfur cured materials. A microporous sheet backed with a non-woven web is also disclosed having a number of properties heretofore unknown such as elongation greater than 25~, tensile strength up to 1000 psi and above, etc. This invention also pertains to a continuous process for producing cured polymeric compositions by electron beam irradiation at heretofore unknown, low irradiation levels; the cured polymeric material deposited on a backed material may be made of thickness heretofore not possible.

-BACKGROUND OF THE INVENTION

In commonly used electric storage batteries, such asthe well known 12-volt battery employed in cars, it has been a desiratum ~o have a battery separator between the battery plates as thin as it is possible to obtain so as to have the lowest possible electrical resistance. At the same time, it has been sought to obtain a battery separator which is reasonably flexible and yet does not develop failure in use such as hrittle failure.

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~9~3~

Generally, a battery separatox is needed as a spacer to prevent two plates from touching each other causing a short.
At the same time, a separator shall not impede the electrolyte flow. Also, a fine pore size is desirable ~o prevent dendrlte growth developing between adjacent plates. The result of dendrite growth is a battery "short". For one or more of the reasons given above, it has been necessary not only to increase the battery plate spacing, but also to use battery separators~
Varlous other problems have also resulted from spalling of the battery plates associated with the use of antlmony or calcium additives to the lead plates. Spalled deposits at the bottom of the battery have likewise caused shorts or premature failure of the battery. For this reason, it has been sought to have a battery which could be made in a manner whereby the battery separators could be fes~ooned around the plates or made in a serpentine fashion thereby isola~in~ one plate from the other.
~ owever, the prior art battery separators have been invariably rather stiff and inflexible; complex shapes could only be formed with great difficulty. In addltion to the above problems; overvoltage caused at the electrodes, particularly at an anode, has required the well known addition of battery water.
Only recently the overvoltage problem has been solved to a point such that maintenance-free batteries can be used with any degree of satisfaction. In no small part this has been a result of better plate or electrode materials or battery separators.
In the art of producing battery separators, commonly as a cheap and fairly short-lived separator, paper webs have been used. ~owever, these possess disadvantages and instead oE paper, ~ 36221/2 ~L~34S7æ

better quality batteries have, as separators, cured natural rubber compositions. A common disadvantage inherent in the use of rubber or natural rubber based battery separators is that a sulfur cure process not only is capital intensivel being a batch process, but it is also labor and energy intensive. Sulfur curing of natural rubber microporous articles results in stiff and brittle products. Moreover, in order to maintain the porosity provided by rehydrated silica, a battery separator must be sulfur cured in a water filled autoclave. Repeated raising and lowering of temperature of large amounts of water is very energy consuming.
Further, a sulfur cure process is capital intensive requiring compounding mixers, milling equipment, extruders, a battery of vulcanizers, etc.

In the curing of the rubber compositions, the cured articles are tested for cracking and brittleness. Unless very careful processing steps are followed in making sulfur cured separators, problems of brittle cracking often result. Dimensional tolerances are also difficult to maintain, for example, cured sheets from which battery separators are made require grinding.

BRIEF DESCRIPTION OF PRIOR ART

As a partial solution to the above problems associated with sulfur cured rubber products, phenolformaldehyde resin impregnated webs have been used as battery separators. The polyphenol resin is generally Gured to a B stage and produces a stiff battery separator.

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Processing of phenols and formaldehyde, disposal of residues thereof, and shortcomings of the end product, such as large pore si~e and poor oxidation resistancet has thus far limited the use of the process as well as the article.
As another approach to solviny the prior art problems, a polyvinyl chloride (PVC) impregnated web has been proposed as a battery separator. However, production of these impregnated webs requires using solvent systems and solvent removal which contribute to unwanted disposal and contamination problems.
A PVC web also must be heat-embossed or hot-embossed to produce the necessary ribbing for allowing electrolyte flow and necessary strength.
As another battery separator, a melt blown poly-propylene mat has been proposed. Moreover, the pore size of the mat has been excessive and unacceptable and electrical resistance characteristics have been hard to control7 These prior art efforts have required entirely new machinery and new processing techniques obsoleting existing facilities associated with sulfur curing of natural rubber.
A number of microporous articles and techniques for producing these permeable, microporous products have been disclosed, such as in UO S. Patents 2,274,260, 2,329,322,

2,336,75~, 2,686~142, 2,6~7,876, 3,298l869, 3,450,650l 3,773,540,

3,890,184, 3,900,341 and Canadian Patent 1,020,184 and references mentioned in these patent~ and further amplify the above description.

~3~ 36221/2 BRIEF DESCRIPTION OF THE INVENTION

It h~s now been found that the microporous sheet material of predeterminable, tailor-made flexibility, improved toughness, and elongation can be produced with assured micro-porosity and other properties which are better than the best heretofore known sulfur-cured rubber microporous article. A
far-and~away greater flexibility of the new article has been accomplished without a sacrifice of the other properties or performance of the article in use, such as a battery separator.
The discovered material ranges from flexible drapable sheets to stiff, tough, yet non-brittle boardsO Hence, as one aspect of the invention, a battery separator has been disclosed which can now be readily shaped to any desired contour, can be made of various thicknesses, and can also be used in combination with a backing material. The newly discovered microporous ma~erial can be employed as microporous fibers, enzyme carriers, diffusers, fabric materials, and possesses numerous advantages which will be further explained herein.
As another aspect of the invention, a battery separator has been discIoed with a backing of heretofore unknown character possessing properties in combination such as low electrical resistance, reduced amount of microporous material (in combination with the bac~ing), and improved tensile, tear, toughness, elongation, and resistance to distortion. A superior combination has been discovered which is a synergistically coacting combination of the newly discovered microporous rubber base material and the flexible backing material.
Still further, a composition of matter suitable for ~3~7~ 36221/2 producing these microporous articles has been disclosed. It is a curable compositlon. This curable composition has been found to be especially suitable for electron beam curing. Moreover, the composition displays the superior properties when used with curatives heretofore unknown for that purpose, but curatives which are especially ef~icacious when subjected to electron beam irradiation. Synergistically coating polymeric compositions, each with the other, and with the curative(s) therefor, have reduced the irradiation levels to heretofore unknown levelsO Whereas typically for a cure, the prior art has suggested an irradiation dose for non-analogous products and heavily sensitized compositions o~ curable natural rubber from 20 to 40 megarads of styrene-butadiene or nitrile-butadiene rubber from 14 to 15 megarads and EPDM (ethylene-propylene-diene) copolymer from 12 to 14 mega~ads, the present microporous precursor composltion is effectively curable at an irradiation level less than 6 megarads, preferably at about 3 to ~ megarads. Although curing at higher levels is possible, e. g., at 6 megarads and up~ including up to 10, a number of properties suffer, such as flexibility; hence, for economic and best product performance, irradiation is to be carried out at a dose rate of less than 6.
A reduction in irradiation of such magnitude should be readily appreciated in an indus~rial environment.
In accordance with the present invention, the precursor, noncured compositions, as well as the cured compositions are believed to be novel compositions of matter.
Still further, in accordance with the invention, a novel process has now been discovered for producing microporous articles, such as shapes or sheets o~ manu~acture ~rom curable rubbers, e. g~, ~3~7~ 36221/2 natural rubber, copolymers of ethylene and propylene, and mixtures of curable rubber and ethylene propylene rubbers (copolymers).
The process has been found to be e5pecially useful as it confers a number of advantages heretofore not possible to obtain when using the conventional technology such as sulfur curing natural rubber to ohtain microporous articles. Thus, the present process provides a continuous operation with reduced number of process steps and allowing the employment of some of the existing compounding machinery and apparatus for producing the microporous sheets or shapes. Still further, in accordance with the present process, the steps which have been found necessary in the prior art processes and most objectionable from the standpoint of environ-mental problems, disposal of by-products, and energy requirements have now been eliminated.
As a further advantage of the present process, a very thin, flexible microporous article is produced which, in turn, permits a thin or thinner layer of the microporous nolymer material to be combined with an appropriate backing material.
When practicing the present process, an article can be produced without fear of distortion, handling problems, and material failuret such as brittle failure.
In accordance with the present invention, ~ailor-made articles o great flexibility can be produced resulting in the elimination of solvent systems and elimination of heating and cooling of large ~mounts of water as well as elimination of batch processing operations.
The present process advantages reside in the discovery of the steps leading to the flexible material which comprise the ~3~S~ 3G221/2 proper compounding of the coacting combination of curable rubber, e. g., natural rubber, ethylene propylene rubber (copoly~er), or mixtures of same with an especially suitable c:urative therefor properly proportioned (in combination with the polymeric material) and the above cured with rehydrated silica in the curing step.
While it can be appreciated that curing at higher levels is possible, such as up to 8 or even 10 megaradsv a number of disadvantages are evident, e. g., economic and safety factors, deteriorating properties, etc~, hence, the preferred range is

4 megarads and less, i. e., amount sufficient to cure the desired composition within a reasonable time.
In curing of the polymeric materials employed herein in admixture with the curative, the added rehydrated silica material does not apparently affect the ef~ectiveness of the irradiation, ~ut has indeed contributed to a product which, such as when irradiated at pre~erred levels of 3 to 4 megarads, confer properties on the end product such as on a battery separator heretofore not achievable.
The precise description of these compositions will be given below~ In general terms, the curable composition consists of a curable rubber, e. g., natural rubber, polyisopreme, and various variants thereof, styrene-butadiene rubber, nitrile-butadiene rubber, or mixtures thereof; these may be used by themselves~ but with considerably greater advantage when used in (rubber) combination with ethylene propylene monomer/(the last can also be used as the curable composition by itsel~) and as a curative for the above, a polyol diacrylate, a polyol tri-acrylate, a polyol tetraacrylate, a polyol~dimethacrylate, a polyol ~3~S~

trimethacrylate, a polyol tetramethacrylate, or mixtures there of. An ilLustrative, advantageous curative is trimethylol propane trimethacrylate. It is postulated that upon curing, the curative contributes signif.icantly to the end product perforll~ance.
In accordance with a broad aspect of the present inventio~, there is provided as an article of manu-facture, a microporous, flexible shape of a sulfur-free, cured polymeric material of a pore size less than 2 microns and of a pre-determined flexibility of curable rubber, ethylene-propylene polymer, or mixtures thereof, and a polyol acrylate, meth~
acrylate, or mixtures thereof as a precursor curative therefor~
In accordance with another broad aspect of the in-vention, there is provided a curable, rubber composition for microporous shapes comprising as a curable material, a curable rubber, ethylene-propylene rubber, or mixtures of same, and, as a curative therefor, a methacrylate or acrylate of a polyol,`
and rehydrated silica of 50 to 7Q~/O hydration as a micropore former therefor.
In accordance with yet another broad aspect of the invention, there is provided a process for producing micro-porous polymeric material comprising: compounding a sulfur-free curable composition- of a curable rubber, a copolymer of ethylene and propylene, or mixtures of same with a curative for curing the composition ~y electron beam irradiation and rehydrated silica, continuously forming a shape of said com-position, continuously curing said formed shape by irradiation at an irradiation leve~ of less than 8 megarads, and recovering said cured product.
The invention will now be described by reference to _ g _ ' 72.

the drawings wherein:
Fi~ure 1 i~ a schematic diagram illustrating the essential steps in a conventional. process for producing microporous articles by sulfur curing of a suitable rubber composition, and Figure 2 is a schematic illustrat:ion of the herein disclosed process.
By referring to the drawings herein, Figure 1 shows a conventional process wherein in a Banhury/mixer the com-pounding of natural rubber, the sulfur curative, rehydrated silica, and suitable processing additives, such as diphenyl ~- -guadadine mixing aid, oil, etc. are added. The sequence or order for the addition of these are varied, but generally, the curative and silica are added last. I'he mixture is mixed until a suitable drop (discharge) temperature has been reached. Thereafter, the discharged mixture is further processed such as on a two-roll drop mill until again the ;~
desired temperature for the mixing is achieved. From this .
mill, a suitable strlp is formed in a strip mill (often requiring milling on an additional strip mill for further processing thereof).
From the strip mill, the compounded, curable mixture goes to an extruder wherein a sheet such as of .300 inches thic~
is being extruded and is thereafter introduced into a water bath.
Subsequentl~ 3 a support web is added to the formed microporous ;.~
-9a-~3~S7~ 36221/2 article. A support web is needed so as not to distort the rubber upon vulcanization. As a support web, paper is conventional]y used. After forming a roll of the extruded sheet of appropriate size, e. g., in diameter, the roll is ready for curing.
Each wound up reel is t~en transferred to a vulcanizer wherein water at an appropriate temperature is raised to achieve the cure at about 350F.
The temperature is generally brought up at a steady rate of 40F/min. under air pressure so as not to distort the sheet.
As soon as the microporous article is vulcanized, it is then cooled and discharged. So as not to again introduce dis-tortion, cooling of the article is carefully conducted under pressure. Thereafter, drying of the cured article is carried out again in a batchwise manner in an appropriate dryerO In preconditioning, the support web is removed from the cured and dr-ed sheet.
Inasmuch as in curing there is some distortion observed and inasmuch as it requires processing so as to remove the unwanted distortion, each of the sulfur cured articles must be ground to obtain the desired contour. That is, proper dimensions and contours are obtained such as final thickness and ribbing for a battery separator. Thereafter, the article is slit to width and cut to length for packaging and sent to a manufacturer.
In referring to Figure 2, it should be noted khak the mixing of the componen~s while indicated to be simultaneous actually follows the procedure described below. For preparation of the master batch and compounding of the polymeric material, a ' more detailed description will be given. The present description --10-- .

~3~ 36221/2 will serve to illustrate the advantages of the present process and the steps in the process as shown in Figure 2.
As the process provides the most benefits when carried n continuously, the emphasis will be on the continuous aspects r~
of the operation. In a suitably sized Banbury mixer or suitably sized series of mixers, the compositions disclosed he~ein are mixed. A next batch can be milled in time sufficient so that a two-roll mill can be at all times kept operating to feed ultimately to the extruder the mixed and compounded composition so as to maintain a continuous operation. Thus, Banbury mixers, the two-roll mills, and the extruder~s) are operated such that at all times a continuous supply is provided to the extruder(s~.
The extruded sheet coming from an extruder is introduced in a water bath so as to maintain the rehydration level of silica and as shown in Figure 2. Again, if rehydration level can be appropriately controlled, the water bath may be optionald However, appropriate control of the amount of water in the mixture must be observed~
A suitable forming roll having the desired ribs or other configuration can be used to shape the article coming from the extruder and water baths. Advantageously, shaping of the sheet can be at an elevated temperature such as 110F to 140F.
After shaping, the continuously moving sheet is introduced into a water bath (optional) and therefrom into an electron beam unit which cures the composition at the indicated typical dose rate of 3 to 4 megarads.
From the electron beam unit, the cured sheet is then introduced into a dryer wherein the water of hydration is removed from silica and the microporosity thereby obtained. From the ~3~S~2 dryer, the sheet can then go to a finishing operation wherein the material is slit to width, cut to length, as well as packaged in the conventional manner.
Hence, as one aspect of the invention, a battery separator can now be readily produced according to the novel process and when so produced, the separator can be shaped to any desired contour and can be made of various thicknesses, including thicknesses heretofore unknown for rubber separators. These separators can also be made in combination with a backing material of thicknesses heretofore unknown for rubber separators. According to the present process, a very low resistance sheet is obtained of reduced thic]cness of the microporous material, improved tensile, tear, toughness, and resistance to distortion.
A superior product has also been produced by the present process as a coacting combination of the curable microporous rubber base material and a flexible backing material.

~ETAILED DESCRIPTION OF THE INVENTION
AND EMBODIMENTS THEREOF

In the essential aspect, the process for producing the curable composition, the microporous material or any shape, e. g., a sheet, is best described by the following general 3 example.
General Example A. ehydration of Silica The moisture content of the silica is determined first and then a correction is allowed for i~ before rehydration.
Rehydration levels of 66.5% or 69~0% are typically employed ~3~S~ 36221/2 but can xange from 65 to 70%. One thousand grams (1000 g) of silica are introduced into a blender and the corrected amount of water is pumped in at a rate of 800 to 900 cc/min.
The pumping time of water should be fairly short of the order of few minutes as otherwise the blend gets too wet.
After finishing the rehydration cycle, the blend is discharged and its moisture preserved. The blend should be in a powdered, friable form.

B~ Masterbatch Procedure _ The masterbatch preparation is desirable for obtaining a uniform mix of the curable composition, in tnis example, ethylene-propylene copolymer (EPM) and/or natural rubber.
Accordingly, the masterbatch consists of natural rubber, EPM, W stabilizer, and carbon black. A required amount of EPM and natural rubber grind (about 1000 g as an illustration) are placed into a Ban~ury mixer and mixed for about 3-4 min. (at the second gear speed) untll the temperature rises to 250F. Then the W stabilizer and/or carbon black (acting also as a W stabilizer) are (is) added and the batch is dropped (discharged) at 275F~
Total time is about 5 min. During this operation, a small amount of warm water (at about 150F) is going through ~ m the rotors and body of the Banbllry mixer to provide for temperature control. The total time required to make the masterba~ch should be about 5 minutes. The masterbatch coming out of the Banbury mixer is placed on the two roll mill (cold) and is sheeted out.

~ 3~,~72 36221/2 C. Com~oundina Procedure .. , _ A required amount of masterbatch (250-300 g) is milled on a two-roll cold mill until it became smooth (5 mins.) and T~
then placed in the Banbury mixer with diphenyl guanidine (DPG)--as a mixlng aid. The Banbury body temperature is 140F with no heat or cooling water circulated ~o the ~ nl rotor. The Banbury mixing speed is at its "slow" speed and when the temperature reaches 150F, one~half of the required amount of rehydrated silica and a curative, e. gD, trimethylol propane trimethacrylate (TMPTM) are added.
The composition is mixed until it again reaches 150F and then the result of the rehydrated silica is added and is allowed to mix until it again reaches 150-160F~ The composition is then dropped. A very unlform mix is obtained and the total Banbury mixing time is about 8 minutes~
Thereafter on a two-roll, this mixture is milled for about 7 to 8 minutes. Both mill roll temperatures are 140F. The milled sheet is then cut into small pieces and soaked in hot water for 30 to 45 seconds at 50-85C and is then calandered for contours and/or optionally a backing added thereto such as paper or heat-bonded polyester mat.
(The last is vastly more preferred as will be explained herein.) The temperature of both calander rolls is 130F~
The calandered sheet is cut into appropriate pieces, such as 15" x 9" pieces, and is irradiated in an electron beam (EB) unit. After Es curing, these sheets are then dried at about 50 to 100C to achieve the desired porosity.

~.~34~

D. Continuous Process In a continuous process, instead as indicated in the foregoing part of the ~xample, after milling, a sheet is introduced into an extruder. ~ shape obtained from the extruder is immersed into a water bath at a temper~ture of 50-85C so as not to lose any of the water of hydr~tion associated with silica. Depending on the ability to control the amount of water in the shape, this water bath may or may not be needed. A water bath at thls juncture does provide a ready means for careful control of the composition. From this water bath, the extruded shape travels through a forming roll such as to produce a sheet of the desired surface characteristics, for example, with ribs or other protuberances. If desired, a backing may be added to the polymeric material. Typically, an extruded sheet of the polymer, i. e., rubber or rubber and/or ethylene~
propylene copolymer mixture is backed in the formlng step.
From the forming roll, the sheet is again introduced into a water bath which is at a temperature about 25-85F, and then into an electron beam irradiation unit wherein the sheet is irradiated at a dose rate desirably 4 megarads or less. Irradiation at higher energy levels than 6 megarads, and sometimes even at that level causes the composition to become unduly embrittled. From the irradiation unit, the continuously moving sheet travels to a dryer where the water of rehydration associated with silica is being removed so as to obtain the desired porosity and pore si~e. From there, the sheet travels to the finishing operations ~ ~ 36~21/2 where it is being slit, cut, and packaged in appropriate containers for shipping to manufacturers utilizing the microporous articles such as for a conventional car battery.

Although the general example illustrates a small scale process, a scale-up of the process has followed the same steps as in the described examples and the continuous process illustration.
The description of the various components for the precursor composition is given below to illustrate the scope of the invention as well as to provide further elaboration `
on the embodiment discussed above.
As starting material, natural rubber is l~o. 1 smoked sheet possessing Mooney viscosities of about 25 to 30 at 175F.
On basis of plasticity, the natural rubber should be between 14 to 18 trheometer-50 scan). In place of natural rubber, synthetic polyisoprene, the various stereo specific variants and polymers thereof are also within the contemplation of the present invention as are mixtures of same with natural rubber.
Another polymer useful in the present process for the disclosed purposes is styrene-butadiene rubber (SBR), nitrile-butadiene rubber ~NBR), or mixtures of the above. I* is, of course, to be understood that before curing~ SBR and ~BR are polymers which are not thermoset. All of the above polymers may be used in admixture with each other.

As a component, to impart the toughness, flexibility, and other desirable characteristics to the base composition, i. e., natural rubber, synthetic rubber, or mixtures of same, ~ S ~Z 36221/2 ethylene propylene rubber (copolymers) have pxovided unexpected and desired properties in the combi~ation with rubber or even by itself. Although in the prior art, ethylene propylene copolymers are often either designated as ethylene propylene polymer or ethylene propylene monomer or ethylene propylene rubber, the more accurate description is a "copolymer consisting of ethylene and propylene" in various proportions typically ranging from 20 to 80~ ethylene, balance propylene. (However, for sake of emphasizing the 'Irubber" aspect, it will be called ethylene-propylene rubber.) A particularly desirable combination of the ethylene propylene rubber has been found to be one which has ethylene content of about 60% by weight in the copolymer, the polymer having a ~ooney viscosity of about 30. This product is commercially available and known as EPCAR 305 and avallable from B. F. Goodrich & Co. Al~hough other EPDrl terpolymers have been investigated, the far-and-away preferred polymer is the ethylene propylene copolymer.
A more flexible product is obtained when rubber such as natural rubber is being used as the predominant or major component of the polymer in the composition up to and including 100% of the curable material. ~owever, greater rigidity and stiffness is obtained when higher amounts of etllylene propylene copolymer is used, e. g., in amounts from 20 to 30% or even 35%--more flexibility is ob~ained when the ethylene-propylene rubber is used in amounts as low as 3 to 5%. Appropriately cured microporous articles have been obtained solely from na~ural rubber or solely from ethylene propylene rubber, mixtures of these, or mixtures of these with other curable rubbers previously mentioned cured with the following curative.

~3~S72 The curative for the above composition, either for the rubber, e. g., natural rubber or the ethylene polymer copolymer or mixtures thereof typically is an acrylate or a methacrylate of a polyol. The polyol may be a di, tri, or tetra functional polyol r the acrylate or methacrylate being formed with the hydroxyl groups of the polyol. The polyol may have from 3 up to ;!
10 carbon atoms. Illustrative polyols from which the acrylates and methacrylates are formed are trimethylol propane, pentaerythri~ol, triethylene glycol, 1,6-hexane diol, etc. Mixtures of the acrylates and/or methacrylates of the above polyols are also included as curatives. Trimethylol propane trimethacrylate (TMPTM) has been found to be the species most suitable for the present purposes. Of the methacrylate or acrylate species, the methacrylate i5 preferred because of the vastly lesser problems of toxicity vis-a-vis the acrylate.
In the combination, typically the curative used is from 0.5 to 3 parts per weight per 100 parts of the curable rubber, ethylene propylene copolymer, or the mixtures oE same.
It has been found, however, that for certain articles of manufacture, such as battery separators, the amount of ethylene propylene copolymer in the mixture is desirably in the range from about 35 to 15~, most desirably, at about 20~. However, ratios of natural rubber to ethylene propylene to polymer such as of 70/10, 80/20, 75/25, 70/30, 60/40, 50/50, and up to 100 parts of ethylene propylene rubber have been evaluated. Appropriate microporous articles have been obtained solely from the curable rubber, e. g., natural rubber or solely from ethylene propylene copolymer cured with the above curativeO The pre~erred combination ~457Z 3622l/2 of the two is given above. Various proportions of these components give ~arious properties and tllUS allow to obtain the tailor-made charackeristics.
In addition to the above, carbon black is being used as an additive which improves the stability (as a W stabili~.er) of the porous article by itself or in combination with an anti-oxidant (W stabilizer).
Typically, carbon black is used from 0.5 to 3 parts per hundred (pph) of the polymer; and the stabilizer from 0.5 to 2 ppho Of the various stabilizers, a butylated p-cresol dicyclopentadiene was found to be preferred. It is available " rn~
as Wingstay-L from The Goodyear Tire & Rubber Co. Other stabilizers are such as styrenated diphenylamine, Wingstay 29 also available from The Goodyear Tire & Rubber Co. and polymerized 1,2 dihydro-2,2,4 tri~ethyl quinoline (FlectoAge from Monsanto Co . ) -The silica powder is for introducing the porosity inthe polymer. It is readily obtainableO One type is Hi-Sil 233 available from Pittsburgh Plate Glass Co. Generally, the surface area for silica should be greater than 50 m2/gr (B.E.T. procedure) and minimum oil absorption should be about 100 cc of oil or more per 100 gr of silica (ASTM method D-281-31).
The amount of silica being used is in proportions of rehydrated silica to rubber of 3.0:1 to 8 0, by weight, preferably 3.5:1 to 5.5:1 by weight at a silica rehydration level of about 65 to 70~ and up to 75%. In general, the greater the rehydrated silica to rubber ratio, the lower is the electrical resistance of a battery separator. At the grea~er silica ratiosl ~he cured article is also less flexible.

~19-~3~7~ 36221/2 Xrradiation of the novel compositions is accomplished by an electron beam unit rated at 850 kw and 50 mA and t ~ available under the name of Dynamitron from Radlation Dynamics, Incorporated at Melville, New York. For purposes of the present invention, any electron beam unit capable of imparting a radiation level of 6 megarads i5 acceptable. Time of irradiation and power needed is a function of sheet or shape thickness. Hence, any reference herein to the irradiation level is to the same sheet or shape thickness.
In using a backing, it has been found that the open structure of a non-woven web is of an excessive "pore size" to be acceptable as a battery separator; however, the flexibility of a proper web to which a sheet of the microporous article can be securely attached could heretofore not have been utilized for want of a proper and flexible microporous sheet. Consequently; a flexible web and a fairly stiff brittle microporous sheet still had to be of considerable thickness and hence, are not usedO
With a flexible microporous article and a flexible backing, the combination of the two allow the use of a thinner sheet of the micro-porous article, which is very advantageous not only because it provides less resistance in a battery, but also the more flexible sheet is less apt to be punctured, will not fail in flexing, and the flexible web, for example~ adds virtually no resistance to the combination when used in an electrolyte~ At the same time, the backing can be safely irradiated, provides a sufficient l'body"
to the polymeric material and allows use of a polymeric material as thin as 5 to 8 mils. A thicker layer~ or example, up to 25 mil55 can still be used~ Consequently, each use will dictate -~he appropriate thickness or the microporous layer with the backing ma~erial.

~L~3~57~

As a backing material 9 a polyester non-woven, heat-bonded (in distinction from an adhesive-bonded~ web has been found ~o be especially desirable. An average fiber length in these webs is typically about .8 inches. These webs are available from duPont and Co. such as under the trademark Sontara 8000.
Properties of these webs are determined on basis of electrical resis~ance, tensile and tear strength. For battery separators, electrical resistance added as a result of the backing should be no greater than 1 m~ in2/mil of thickness. Tensile strength should be about 100 lb/in2, elongation about 406. Tear strength for a base web of 1.2 oz/yd, standard size should be measured by grab breaking strength (ASTM Method D-1682-65) and should be about 22 and up in machine direction and 13 and up in cross-direction. Generally, webs of a weight from .75 oz/yd (yard) to 2.2 oz/yd are available.
Typical ranges for properties of the above material are as follows:
Grab Breaking Strength (lbs.) MD 20-30 XD 12-18 Grab 3reaking Strength (%) MD 25-50 XD 50-120 Weight (oz,/yd~ ) 102-2.2 Mullen Burst (lbs.) 30-40 Tongue Tear (lbs./in.) about 2.1 MD and about 3.1 XD
In the description herein, parts or percent are by weight~ unless otherwise indicated.
In further illustrating the present invention, specific examples are furnished in the Tables below. These examples show the various properties and characteristics of the composition in the various forms thereof. The examples are not to ~3~S7~ 362~1/2 limit the invention. In the description herein, parts or percent are by weight unl~ss otherwise indicated.

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~ 2 36221/2 In following the general example above, a composition was prepared consisting essentially of natural ruhber - 254 grams, TMPTM ~ 7.~ grams, Hi-Sil - 472 grams, water - 875 grams. A
cured product obtained from the above composit:ion had a resistance of 1.3 mQ in2tmil and 33 m~ in2. The above illustrates the relative ratios of silica to rubber and the reduced resistance, but flexibility is also reduced.
Another composition was obtained by following the general example; the constituents of the same were as follows:
-r~ ~
88.2 lbs. of 80% natural rubber; 15% EPCAR 306, 5~ Pliolite S-6F
(an 82.5% styrene, balance butadiene rubber (SBR) available from ~,n .
The Goodyear Tire & Rubber Co.~; 139 lbs. Hi-Sil 233; 1.3 lbs.
TMPTM; 1.7 lbs. DPG and 239.6 lbs. water. An electron beam cured article prepared from the above composition is suitable for forming various shapes or configurations of the cured material because the cured compositlon lends itself to ultrasonic welding. Accordingly, battery separators can be made as an envelope for a battery plate. It is to be understood that prior to curing/ styrene butadiene rubber and nitrile-butadiene rubber (NBR) are actually non-crosslinked, i. e., not thermoset polymers.
In the above described examples, weight loss ln chromic acid is a typical gross test to establish unsaturation in the polymeric composition as well as useful llfe; and acceptable weight loss in less than 35%; it also typifies completion of curing and process efficiency with respect to crosslinking n ~28-.

~3~ 36221/2 Similarly, shelf-life or storage stabilizi~y of the cured microporous article is indicative of product life and is approximated by exposure to fluorescent light; typically, the composition should be good for at least 14 days before it develops cracks and loses flexibility.
The various measures of toughness of the unbacked, cured material are: tensile strength which should be in the range from 200 to 4Q0 psi, preferably 300 to 400 psi. (For backed material, elongation in percent may be 20 to 90~, preferably 40 to 60%, and tensile strength up to 1200 psi.) Hence, it is now possible to produce very flexible shapes, i. e., conformable shapes when using thin sheets capable of great elongation; thicker sheets give tough, yet stiff products.
Flexibility (non-brittleness) is easily measured by the 180 bend test and the present compositionseasily meet this objective.
Again, while these values are generally pertinent to establish chemically desirable compositions, these values likewise can be used to establish the process variables vis-a-vis a standard.
A convenient measure of acceptable porosity is alcohol porosity and should be from 45 to 75%. Other measures of porosity have been given in the examples above and correspondingly, comparably acceptable values can be obtained from the above, first given value.
The electrical resistance norms for the battery separ~tor are easily achieved; typically for the present microporous article, a resistance of 1.0 to 2.5 mQ in2/mil is acceptable.
Rs mentioned before, dimensional stabili~y of the shape during processing is outstanding and careful conduct of the -29~

~L~3~57~

process eliminates grinding of the end product. These advantages for the unbacked and backed material show the various advantages of the present invention.
In use in a battery, the battery separator is tested by conventional tests known in the art, e. g., a "cold cranking"
test and "J-240" test identified by SAE testing procedures.
When employing a web, the thickness of the battery separator may be as little as 5 to 8 mils although typically a thickness of the separator is about 12 to 20 mils (backed) and from 10 to 20 mils without backing. Again, in a battery separator, the effectiveness of the thinner polymeric material on the web in combination is measured by the above two tests which also characterize the results of the process.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. As an article of manufacture, a microporous, flex-ible shape of a sulfur-free, cured polymeric material of a pore size less than 2 microns and of a predetermined flexibil-ity selected from the group consisting of curable rubber, ethylene-propylene polymer, or mixtures thereof, and a polyol acrylate, polyol methacrylate, or mixtures thereof as a precursor curative therefor.

2. As an article of manufacture, a microporous, flex-ible shape of a sulfur-free, cured polymeric material of a pore size less than 2 microns and of a predetermined flexibility selected from the group consisting of a curable rubber, ethyl-ene-propylene polymer, or rnixtures thereof, and a polyol acrylate, methacrylate, or mixtures thereof as precursor curative, wherein said shape has a backing material of an inert nonwoven heat-bonded fibrous polymer, said backing mat-erial having a shelf-life resistance to an electrolyte for electrical storage batteries of at least equivalent to said microporous article.

3. The article as defined in claim 1, wherein the poly-meric microporous article is a sulfur-free, cross-linked com-position of rubber.

4. The article as defined in claim 3, wherein the polymeric microporous article is a sulfur-free, cross-linked composition of natural rubber.

5. The article as defined in claim 1, wherein the curable rubber is natural rubber, styrene-butadiene rubber, nitrile-butadiene rubber, a polyisoprene or mixtures of same.

6. The article as defined in claim 2, wherein the curable rubber is natural rubber, styrene-butadiene rubber, nitrile-butadiene rubber, a polyisoprene or mixtures of same.

7. The article of manufacture as defined in claim 1, wherein the microporous polymeric article is a sulfur-free, cross-linked polymeric material selected from the group con-sisting of natural rubber, ethylene-propylene rubber, and mixtures thereof.

8. The article of manufacture as defined in claim 1, wherein the microporous polymeric article is a sulfur-free, cross-linked polymeric material of natural rubber and ethylene-propylene rubber in percent by weight of natural rubber to ethylene-propylene rubber from 95 to 75% natural rubber to 5 to 25% ethylene-propylene rubber.

9. The article of manufacture as defined in claim 2, wherein the microporous polymeric article is a sulfur-free, cross-linked polymeric material selected from the group con-sisting of natural rubber, ethylene-propylene rubber and mixtures of same.

10. The article of manufacture as defined in claim 2, wherein the microporous polymeric article is a sulfur-free, cross-linked polymeric material of natural rubber and ethylene-propylene rubber in percent by weight of natural rubber to ethylene-propylene rubber from 95 to 75% natural rubber to 5 to 25% ethylene propylene rubber.

11. The article of manufacture as defined in claim 1, wherein a curative for said polymeric material is a polyol di-acrylate, a polyol triacrylate, a polyol tetraacrylate, a polyol dimethacrylate, a polyol trimethacrylate, a polyol tetrameth-acrylate, or mixtures of same.

12. The article of manufacture as defined in claim 11, wherein the polyol is a trimethylol propane, pentaerythritol, triethylene glycol or 1,6-hexane diol.

13. The article of manufacture as defined in claim 11, wherein the polyol is a trimethylol propane.

14. The article of manufacture as defined in claim 2, wherein a curative for said polymeric material is selected from the group consisting of a polyol diacrylate, a polyol triacrylate, a polyol tetraacrylate, a polyol dimethacrylate, a polyol trimethacrylate, a polyol tetramethacrylate, or mixtures of same.

15. The article of manufacture as defined in claim 14, wherein the polyol is a trimethylol propane, pentaerythritol, triethylene glycol, or 1,6-hexane diol.

16. The article of manufacture as defined in claim 14, wherein the polyol is trimethylol propane.

17. As an article of manufacture, a battery separator for an electrical storage battery of a microporous, cured, flexible, sulfur-free polymeric material selected from the group consisting of a rubber, an ethylene propylene polymer or mixtures thereof, and a polyol acrylate, polyol methacrylate or mixtures thereof as a precursor curative therefor, said battery separator having a 180° bend flexibility, and a pore size of less than 1 micron.

18. The battery separator as defined in claim 17, wherein the same is of a cured polymeric material of natural rubber and ethylene-propylene rubber.

19. The battery separator as defined in claim 17, wherein the polymeric material is of a cured natural rubber and an ethylene-propylene rubber in weight percent ranging from 75 to 95% natural rubber.

20. The battery separator as defined in claim 17, wherein the polymeric material is 80% by weight natural rubber, 20% by weight ethylene-propylene rubber of about 50% ethylene monomer in said copolymer by weight.

21. The battery separator as defined in claim 17, wherein the microporous polymer material further contains carbon black.

22. As an article of manufacture, a battery separator for an electrical storage battery of a microporous, cured, flexible, sulfur-free polymeric material selected from the group consisting of a rubber, an ethylene-propylene polymer or mixtures thereof, and a polyol acrylate, polyol methacryl-ate or mixtures thereof as a precursor curative therefor, said battery separator having a 180° bend flexibility, and a pore size of less than 1 micron and an electrical resistance in an electrolyte solution for an electric storage battery of less than 3.5 m .OMEGA. in.2 /mil.

23. The article of manufacture as defined in claim 17, wherein the same has a backing of a polymer fibrous material of heat-bondable fibers having at least an equivalent resis-tance to an electrolyte for an electric storage battery to said polymer material.

24. The article of manufacture as defined in claim 22, wherein the backing material is a non-woven, fibrous polyester.

25. A curable, rubber composition for microporous shapes comprising as a curable material, a curable rubber, ethylene-propylene rubber, or mixtures of same, and, as a curative therefor, a methacrylate or acrylate of a polyol, and rehy-drated silica of 50 to 70% hydration as a micropore former thereof.

26. The curable rubber composition as defined in claim 25, wherein the same comprises 80% by weight natural rubber, 20% by weight ethylene-propylene rubber having 60%
by weight of ethylene as copolymer thereof, as a curative therefor trimethylolpropane trimethacrylate and rehydrated silica of a 60 to 70% hydration, carbon black and a stabil-izer therefor, said curable composition being curable via electron beam irradiation at less than 6 megarads.

27. The curable composition as defined in claim 25, wherein the same comprises 80% by weight natural rubber, 15% by weight ethylene-propylene rubber, and 5% by weight styrene-butadiene rubber, as a curative therefor trimethyl-olpropane trimethacrylate and rehydrated silica of a 60 to 70% hydration, carbon black and a stabilizer therefor, said curable rubber composition being curable via electron beam irradiation at less than 6 megarads.

28. In a curable polymeric material of natural rubber, ethylene-propylene copolymer or mixtures of same, the im-proved combination for curing of same, which comprises a polyol methacrylate or a polyol acrylate as a curative for said polymeric material for electron beam irradiation.

29. In a process for producing microporous polymeric material, the improvement comprising:
compounding a sulfur free curable composition of a compound selected from the group comprising curable rubber, ethylene-propylene polymer, or a mixture of same with a curative for curing the composition by electron beam irradiation, said curative therefor being an ethylenically unsaturated curing agent comprising a polyol acrylate or polyol methacrylata and rehydrated silica;
continuously forming a shape or said composition;
and continuously curing said formed shape by irrad-iation at an irradiation level of less than 10 megarads.

30. The process as defined in claim 29, wherein curing is at an irradiation level of less -than 8 megarads.

31. In a process for producing microporous polymeric material, the improvement comprising:
compounding a sulfur free curable composition of a a curable rubber, a copolymer of ethylene and propylene, or mixtures of same with a curative for curing the composi-tion by electron beam irradiation and rehydrated silica;
continuously forming a shape of said composition, continuously curing said formed shape by irrad-iation at an irradiation level of less than 8 megarads, and recovering said cured product.

32. The process as defined in claim 29, wherein curing is at an irradiation level of less than 6 megarads.

33. The process as defined in claim 29, wherein the curable rubber composition is natural rubber, a polyisoprene, styrene-butadiene rubber, nitrile-butadiene rubber, or mix tures of same.

34. The process as defined in claim 29, wherein the curable rubber composition is natural rubber.

35. The process as defined in claim 29, wherein the curable rubber composition is natural rubber and styrene-butadiene rubber mixture.

36. The process as defined in claim 29l wherein the curable composition is an ethylene-propylene rubber.

37. The process as defined in claim ;29, wherein the curable composition is an a mixture of natural rubber and an ethylene-propylene rubber.

38. The process as defined in claîm 37, wherein an admixture of rubber and a copolymer of ethylene-propylene is from 65 to 95% by weight natural rubber and balance ethylene-propylene rubber.

39. The process as defined in claim 38, wherein in the admixture, rubber is 80% by weight.

40. The process as defined in claim 37, wherein the ethylene and propylene rubber has a Mooney viscosity of about 30.

41. The process as defined in claim 29, wherein during the said continuous forming of said shape the curable com-position is extruded.

42. The process as defined in claim 29, wherein during the said continuous forming of said shape the curable com-position is extruded and in shaping to final form, backed with a sheet of an inert polymer, non-wovenl heat-bonded web.

43. The process as defined in claim 29, wherein said curative is a polyol diacrylate, a polyol triacrylate, a polyol tetraacrylate, a polyol dimethacrylate, a polyol tri-methacrylate, a polyol tetramethacrylate or mixtures thereof.

44. The process as defined in claim 43, wherein the curative is a polyol acrylate or methacrylate and saicl polyol is trimethylol propane, pentaerythritol, triethylene glycol, 1,6-hexane diol or mixtures of these acrylates, methacrylates or both methacrylates with acrylates.

45. The process as defined in claim 29, wherein the formed shape is electron beam irradiated within an irradia-tion dose of 4 and less than 4 megarads.

46. The process as defined in claim 29, wherein the formed shape is a battery separator in a form used in an electric storage battery.

47. The process as defined in claim 46, wherein the formed shape of said battery separator is electron beam irradiated and the recovered product is of a predetermined flexibility.

48. The process as defined in claim 47, wherein said battery separator is backed with a non-woven, inert, heat-bonded polymeric material of a polyester.

49. The process as defined in claim 29, wherein the sulfur free curable composition comprises 80% by weight natural rubber, 20% by weight copolymer of ethylene and propylene rubber having 60% by weight of ethylene as co-polymer thereof, as a curative therefor trimethylolpropane trimethacrylate and rehydrated silica of 60 to 70% hydra-tion, carbon black and a stabilizer therefor, said curable composition being curable via electron beam irradiation of less than 6 megarads.

50. The process as defined in claim 21 t wherein the curable composition is cured of an irradiation level of 3 to 4 megarads.

51. In a process for producing microporous polymeric material, the improvement comprising:
compounding a sulfur free curable composition selected from the group consisting of a curable rubber, an ethylene-propylene rubber of mixtures of same with a) a curative for curing the same by electron beam irradiation, said curative being an acrylate or methacrylate of a di, tri, or tetra functional polyol or a mixture there-of, and b) rehydrated silica;
maintaining a predetermined moisture content in said curable composition during processing of same while forming a shape of said composition; and curing said formed shape by irradiation at an irradiation level of less than 8 megarads.