Note: Descriptions are shown in the official language in which they were submitted.
36221/2
~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.
'~
36221/2
~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,
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~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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36221/2
Z
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.
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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
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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~,
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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
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~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
;.~
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~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
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~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
36221/2
~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
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~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.
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36221/2
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
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~ ~ 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,
-16-
~ 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.
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36221/2
~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
-18
~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.
-20-
36221/2
~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
-21-
~3~S7~ 362~1/2
limit the invention. In the description herein, parts or percent
are by weight unl~ss otherwise indicated.
-2~-
3~57~ 36221/2
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27--
~ 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~
36221/2
~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.
-30
