Note: Descriptions are shown in the official language in which they were submitted.
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-1-
NON-WOVEN FABRIC LAMINATE
This invention relates to a laminate .formed from non-woven
fabrics and to a method of treating a non-woven fabric
laminate. The laminate can be used as a separator in an
electrochemical device.
Non-woven fabrics can be made by processes which include
(a) melt blowing, (b) spinning, and (c) wet or dry laying.
The fibres of fabrics made by spinning and wet or dry laying
require bonding to one another for the fabric to have
integrity, so that it has the mechanical properties required
for satisfactory performance. In the case of fabrics made by
spinning, the fibres are bonded to one another by the
application of heat and pressure. In the case of fabrics
made by wet or dry laying, polyethylene is incorporated into
the fabric, either as fibres consisting essentially of
polyethylene or as bicomponent fibres consisting of a
polypropylene core and a polyethylene sheath. The
polyethylene in the fabric can provide the necessary bonds as
a result of heating the fabric to a temperature that is
greater than the softening point of the polyethylene.
Non-woven fabrics can be used to form an electrode separator
in an electrochemical device. Examples of such devices
include nickel-cadmium and nickel-metal hydride cells. The
separator should be inert towards materials with which it
comes into contact in the cell including in particular the
alkaline electrolyte and the electrode materials. It should
also have physical characteristics which enable it to
withstand the treatment encountered during assembly of the
device and during use. For example, it should be able to
withstand the stresses encountered during spiral winding of
the cell components. ~t should also be capable of resisting
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-2-
the growth of dendrites between the electrodes during
recharging.
A fabric that is made from spun fibres which are then bonded
together (a "spun bonded" fabric) has the disadvantage that
the bonds reduce the effective surface area of the fabric
that is available to ion transfer by effectively blocking the
pores of the fabric. The uneven current distribution that
results from this uneven pore distribution can give rise to
dendrite formation during recharging of a secondary cell,
ultimately leading to a short circuit in the cell. There is
therefore a compromise to be reached with such fabrics
between mechanical properties that are enhanced by bonds
between the fibres and aspects of electrochemical performance
which can be diminished by the bonds.
Furthermore, manipulation of a fabric formed from spun fibres
(whether or not bonded to one another), in particular prior
to and during assembly of a device with the fabric as a
separator, can lead to deformation of the fabric involving
relative movement of the fibres of the fabric (in regions
between bonds when present). This movement can involve for
example untangling of the fibres. It results in opening of
the structure of the fabric. This has the significant
disadvantage of reducing the effectiveness of the fabric as a
barrier and increasing the risk of failure of a device in
which the fabric is incorporated as an electrode separator,
generally by shorting of the electrodes.
The structure of a melt-blown fabric is stable when placed
under stress. A melt-blown fabric has the further advantage
of small fibres size (which is generally less than about
um, and often as low as 1 um or less) so that the fabric
can provide a separator with good barrier properties.
However, the fine size of the fibres of the fabric means that
.........--~...._.._.~
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98101671
-3-
the fabric is only able to withstand the application of small
stresses and small degrees of strain. A large stress or high
strain can result in fracture of the fabric.
Non-woven fabrics formed by wet or dry laying of fibres can
have satisfactory mechanical properties. However, especially
when bicomponent fibres are used, the fibre size can tend to
be undesirably large, often greater than 15 um. A further
disadvantage which arises from the use of bicomponent fibres
is their high cost.
The present invention provides a laminate of melt-blown and
spun fibre fabrics, the fibres of which have been treated by
graft copolymerisation of a vinyl monomer such as acrylic
acid.
Accordingly, in one aspect, the invention provides a laminate
formed from first and second non-woven fabrics which each
comprise fibres of a hydrophobic polymeric material, the
first fabric being formed from spun fibres and the second
fabric being a melt-blown fabric, the fibres of the fabrics
having undergone a copolymerisation reaction with a vinyl
monomer which is capable of reacting with an acid or a base
to form a salt directly or indirectly, the reaction involving
exposure of the laminate to ultraviolet radiation while
impregnated with a solution of the vinyl monomer resulting in
grafting of. the vinyl monomer to the surfaces of the fibres.
The laminate of the invention has the advantage that it is
able to tolerate stresses imposed when it is being manipul-
ated, for example prior to and during assembly of an electro-
chemical device in which the laminate is incorporated as an
electrode separator. In particular, the structure of the
laminate remains stable under moderate loads and does not
exhibit the tendency to open as in the case of fabrics formed
i
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
_q_
from spun fibres. Furthermore, the laminate structure has a
reduced tendency to fracture when placed under load compared
with melt-blown fabrics.
A further advantage of the laminate of the invention is that
it can exhibit the good barrier properties which can be
obtained from melt-blown non-woven fabrics when it is used as
an electrode separator in an electrochemical device. This
arises from the small effective pore size that is presented
by the laminate. The effective size of the pores defined by
the fibres of the fabrics can be measured using a Coulter
porometer. Preferably, the effective pore size of the
laminate is less than about 30 um, more preferably less than
about 20 um, for example less than about 15 um. A small pore
size has the advantage of enhancing the ability of the
laminate to resist penetration of electrode materials, for
example as dendrites. The laminate of the invention can be
subjected to a calendering step during its manufacture.
Amongst other advantages, this can have the result of
reducing the effective pore size of the laminate.
A small pore size also enhances the ability of the laminate
to absorb and retain electrolyte once the fibres have been
treated to render them hydrophilic. A high electrolyte
absorption has the advantage of reducing the internal
resistance of an electrochemical device in which the laminate
is incorporated as an electrode separator, and of extending
the cycle life of the device.
Yet another advantage of the laminate of the present
invention arises from the fine structure presented by the
melt-blown fabric when the laminate is used as an electrode
separator. The laminate is able to combine the good physical
properties discussed above with an ability to absorb
contaminants. These contaminants, including ammonia and
.._ a ~. .__ .. _ . _...~._ _.. .._ _ ..___._.....i.. .
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-5-
metal ions, can be found in electrochemical devices following
the production and use of certain electrode materials, for
example nickel hydroxide and metal hydride electrodes.
Absorption of contaminants in the device has the advantage of
inhibiting self-discharge reactions. The shelf-life of a
device with a laminate of the present invention as its
separator can therefore be enhanced compared with devices
with previously known separators.
In the case of ammonia which can be formed by the reduction
of nitrate ion contamination from a nickel hydroxide electr-
ode, it has been found that there is a residual ion exchange
capacity in addition to the ion exchange capacity that is
measured using standard techniques such as the titration of
the acid form of the membrane with potassium hydroxide. It
is believed to be this residual ion exchange capacity that
enables ammonia and other contaminants to be absorbed in an
electrochemical device. The residual ion exchange capacity
is expressed in terms of the milliequivalents per gram of the
laminate, and is measured as described below. Preferably, it
is at least about 0.15 meq.g-', preferably at least about
0.25 meq.g-1, more preferably at least about 0.3 meq.g-1, for
example at least about 0.35 meq.g-1.
The laminate of the present invention also has the advantage
of reduced cost compared with products based on bicomponent
fibres such-as bicomponent polyethylene and polypropylene
fibres.
The fibres of the first fabric can be bonded to one another
prior to formation of the laminate, for example by localised
welds between the fabrics. The first fabric might then be a
spun bonded fabric.
i
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-6-
The fibres of the first fabric can be substantially unbonded
to one another in which the fabric is formed without a step
of bond formation by the application of heat and pressure.
There might be weak forces between the fibres of such
fabrics. For example, weak forces can result from a step of
calendering a fabric under moderate heat and pressure, which
can lead to localised deformation of the fibre material,
especially where fibres come into contact with one another.
However, the forces will be capable of being overcome when
the fabric is placed under tension. It will be possible to
discern a boundary between the fibres of the fabric. There
will not be any intimate mixing of the materials of the
fibres as results from the formation of a weld. Features of
treated fabrics formed from unbonded fibres are disclosed in
the patent application filed with the present application,
claiming priority from UK patent application no. 9712690.8
and entitled NON-WOVEN FABRIC TREATMENT (bearing the agents'
reference P10599). Subject matter disclosed in the
specification of that application is incorporated in the
present specification by this reference.
Preferably, there are bonds between the first and second
fabrics of the laminate. For example, the bonds might be
formed by localised application of heat and pressure. The
heat and pressure can be applied by passing the laminate
between heated rollers with appropriately profiled surfaces.
Such treatment can lead to the formation of localised welds
between the fabrics. They will also tend to form bonds
between the fibres of the first fabric (which might already
be bonded to one another prior to lamination). Preferably,
the proportion of the area of the laminate in which the bonds
are formed is less than about 200, more preferably less than
about 15%, especially less than about 100, for example about
8~
o.
. ......_ ~
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
Preferably, the mean thickness of the fibres of the first
fabric (which might be measured as a mean diameter,
especially when the fibres have a circular cross-section)
from which the non-woven fabric is formed is not more than
about 30 um, more preferably not more than about 20 um. The
thickness of the fibres of the first fabric will often be at
least about 5 um, for example at least about 10 um.
Preferably, the mean thickness of the fibres of the second
fabric (which might be measured as a mean diameter,
especially when the fibres have a circular cross-section)
from which the non-woven fabric is formed is not more than
about 8 um, more preferably not more than about 5 um. The
thickness of the fibres of the second fabric will generally
be at least about 0.5 um.
Preferably, the ratio of the weight of the fibres of the
second fabric to the weight of the fibres of the entire
laminate is at least about 0.1, more preferably at least
about 0.2, especially at least about 0.9, for example at
least about 0.5.
Use of fabrics with different constructions or different
fibres or both can lead to a laminate with asymmetric
properties. This can lead to differing extents of the
copolymerisation reaction from one side of the laminate to
the other. -The presence of a surface of the laminate which
is relatively less hydrophobic than another surface has
advantages when the laminate is for use as a separator in
certain types of electrochemical devices. The hydrophilic
surface is more easily wetted by aqueous electrolyte which
can be advantageous in the region of a positive electrode.
This can inhibit flow of hydrogen gas from the negative
electrode to the positive electrode when the battery is being
recharged. The less hydrophilic surface lead to the creation
i
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98101671
_g_
of a three phase boundary at the surface of the negative
electrode (between the surface of the electrode, the electro-
lyte and oxygen gas generated at the electrode), which can
help to control the internal pressure in the battery on
recharging due to the generation of oxygen.
The laminate of the invention can include one or more fabrics
in addition to the first and second fabrics. For example,
the laminate of the invention can include the first and
second fabrics as discussed above, together with a third
fabric and possibly a fourth fabric. The third fabric can be
formed from spun fibres. The third fabric can have the same
construction as the first fabric. Spun fibres of a third
fabric can be bonded to one another prior to formation of the
laminate, that is as a spun bonded fabric. Generally,
however, the fibres of a third fabric will be bonded to one
another by localised application of heat and pressure by
which bonds between the fabrics of the laminate are formed.
The first fabric and a third fabric formed from spun fibres
can be arranged on opposite faces of the second fabric.
The ion exchange capacity of the laminate is measured in
meq.g-' according to the test routine referred to below, to
provide a measure of the extent of the graft copolymerisation
reaction between of the material of the fibres and the vinyl
monomer. Preferably, the ion exchange capacity is at least
about 0.25,-more preferably at least about 0.4, especially at
least about 0.6. Preferably, the ion exchange capacity is
not more than about 2.0, more preferably not more than about
1.6, especially not more than about 1.4, for example not more
than about 1.2. It has been found that useful increases in
the physical properties of a non-woven fabric, in particular
when formed from polypropylene fibres, can be obtained at low
graft levels corresponging to these values of the ion
exchange capacity.
.. ...,.......... . ,_ .... . ..... .. _.,......,...?...
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
_g_
The gel fraction of the material of the laminate is measured
according to ASTM D2765-84, providing a measure of the extent
of crosslinking of the material of the fibres. Preferably,
the gel fraction is at least about 100, more preferably at
least about 200, especially at least about 300.
Preferably, the thickness of the laminate, measured using
test method DIN 53105 which involves lowering a 2.0 kg weight
onto a sample of the laminate of area 2.0 cm' at a speed of
2.0 mm.s-', is greater than about 80 um, more preferably
greater than about 100 um; preferably, the thickness is less
than about 400 um, more preferably less than about 250 um.
The method by which the laminate is made may include a
calendering step to reduce its thickness to a value within
the range referred to above, the reduction being by at least
about So, preferably at least about 150, more preferably at
least about 250, and less than about 600, preferably less
than about 450, more preferably less than about 400. Calend-
ering can have the advantage of reducing the effective size
of the pores in the fabric, giving rise to the advantages
discussed above. The calendering step may take place before
or after the material of the laminate is reacted with the
graft-polymerisation solution. Calendering the laminate
before the graft-polymerisation reaction has been found to
give rise to increased rates of the reaction. Calendering
the laminate after the graft-polymerisation reaction has been
found to give rise to enhanced electrolyte absorption. A
laminate that has been calendered after the graft reaction
can have an improved ability to absorb impurities, especially
ammonia, which might be present in the electrolyte system.
Moreover, fibres of the laminate are less likely to be
damaged physically as a result of the calendering step when
it is carried out after the graft-polymerisation reaction.
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-10-
The vinyl monomer which is graft-polymerised with the
material of the fibre surface can be capable of reacting with
an acid or a base directly to form a salt, or indirectly to
form a salt after appropriate work up, perhaps involving for
example hydrolysis or sulphonation. Preferred vinyl monomers
include ethylenically unsaturated carboxylic acids and esters
thereof such as acrylic acid, methacrylic acid, methyl
acrylate, and methylmethacrylate. Other vinyl monomers which
might be used include acrylamide, vinylpyridine, vinyl-
pyrrolidone and styrene-sulphonic acid.
Preferably, the material of the surface of at least some of
the fibres, for example at least about 40o by weight, pref-
erably at least about 600, more preferably at least about
800, comprises polypropylene. Preferably, at least 40o by
weight of the material of the fibres of the first fabric or
the second fabric or both is polypropylene, more preferably
at least about 600, especially at least about 800.
Preferably, the material of at least some of the fibres from
which the first or second fabric (or each of the fabrics) is
formed, for example at least about 40~ by weight, preferably
at least about 600, more preferably at least about 800, is
substantially homogeneous throughout the thickness of the
fibres. It can be preferred for many applications for the
material of substantially all of the fibres to be substan-
tially homogeneous throughout their thickness, so that those
fibres are formed only from polypropylene or another suitable
material (with appropriate additives where necessary).
In another aspect, the invention provides a method of
treating a laminate formed from first and second non-woven
fabrics which each comprise polymeric fibres, in which the
first fabric is formed from spun fibres and the second fabric
is a melt-blown fabric, the method comprising:
_ .t
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-11-
(a) impregnating the laminate with a solution of a
vinyl monomer which is capable of reacting with an acid
or a base to form a salt directly or indirectly, the
solvent being one which does not evaporate signif-
icantly in the subsequent step of exposing the fabric
to radiation, and
(b) exposing the impregnated laminate to ultraviolet
radiation while the exposure of the laminate to oxygen
is restricted, to cause the monomer and the material of
the fibres to co-polymerise.
The ultraviolet radiation initiated polymerisation reaction
can be completed surprisingly quickly, for example by
exposing the impregnated laminate to radiation for as little
as 15 seconds, even as little as 5 or 10 seconds, and it has
been found that the fabrics of the laminate after reaction
contain a significant amount of grafted monomer, which can be
sufficient for the fabrics to be rendered wettable by aqueous
solutions such as might be found in certain electrochemical
devices. This is to be contrasted with techniques in which
graft-copolymerisation reactions are initiated using, for
example, electron bombardment (either of impregnated fabric
or of fabric prior to exposure to monomer solution), where
reaction times of many minutes can be required in order to
obtain a significant degree of grafting, and even after
reaction times of this order, the degree of grafting reaction
can be too low for many applications. Such techniques can
lead to homopolymerisation of the vinyl monomer and degrad-
ation of the material of the fabric or of the vinyl monomer
or both. They do not therefore lend themselves to continuous
processing in the manner of the present invention.
Details of the process~for rendering the polymeric fibres of
a non-woven fabric hydrophilic, involving impregnation with a
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-12-
solution of the vinyl monomer followed by radiation, are
disclosed in WO-A-93/01622. Subject matter disclosed in that
document is incorporated in the specification of this applic-
ation by this reference.
The exposure of the impregnated laminate to oxygen is
restricted during the irradiation, for example, by carrying
out the ultraviolet irradiation step in an inert atmosphere
such as an atmosphere of argon or nitrogen, or by sealing the
impregnated laminate between sheets of material which are
impervious to oxygen, but are transparent to ultraviolet
radiation of appropriate wavelength for initiating the co-
polymerisation reaction.
Preferably, the impregnation solution includes an initiator
for the polymerisation reaction. Preferably, the initiator
initiates the reaction by abstracting an atomic species from
one of the reacting materials, for example by abstracting a
hydrogen atom from polypropylene of the fabric fibres to
create a polymeric radical. Following such abstraction, the
polymeric radical, in contact with the monomer in solution,
can initiate the formation of a grafted branch. When an atom
is abstracted from the polymer of the fabric fibres, the
activated polymer can react either with another polymer
molecule so that the material of the fabric becomes cross-
linked, or with the vinyl monomer in a co-polymerisation
reaction. An example of a suitable initiator is benzo-
phenone. The mole ratio of the vinyl monomer to the
initiator is preferably at least about 50, more preferably at
least about 100, especially at least about 175; the ratio is
preferably less than about 1500, more, preferably less than
about 1000, especially less than about 500, more especially
less than about 350; for example the ratio may be about 200.
r T . ._..._.._. ..._...___.~.~".._~__...._...
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98101671
-13-
The impregnation solution may include a component by which
homopolymerisation of the vinyl monomer is inhibited.
Examples of suitable inhibitors include iron (II) and copper
(II) salts which are soluble in the reaction medium, a
preferred material for aqueous media being iron (II)
sulphate. It has been found, however, that the need for an
inhibitor can be avoided by selection of an appropriate
solvent for the graft polymerisation reaction which can
restrict the speed and degree of the homopolymerisation
reaction, for example as a result of its ability to act as a
heat sink. This can be an advantage when it is desired to
minimise the amount of contaminants in the laminate.
The impregnation solution may include additional components
to optimise reaction conditions such as surfactants to ensure
that the solution fully impregnates the laminate, an approp-
riate mixture of solvents to ensure homogeneity of the
solution, and so on.
A benefit of the present invention is that physical proper-
ties of the treated laminate (in particular, its tensile
strength or its ability to be wetted by aqueous solutions or
both) can be stable on prolonged exposure to an alkaline
solution. A laminate with stable physical properties is
particularly appropriate for use as a separator in electro-
chemical devices in which the electrolyte comprises an
alkaline solution. A test to determine stability on exposure
to alkaline solution involves storing a sample of a laminate
to a solution containing 30o by weight of potassium hydroxide
at 71°C for 21 days, and then comparing the selected property
of the exposed laminate to that of a fabric that has not been
exposed to the alkaline solution.
In a further aspect, tie invention provides an electro-
chemical device, comprising an anode, a cathode, a quantity
i ~
CA 02294599 1999-12-15
WO 98158111 PCT/GB98/01671
-14-
of an electrolyte, and an electrode separator of the type
discussed above. Preferably, the cathode in the device
comprises nickel (II) hydroxide. An example of material
which can form the anode in such a device includes cadmium.
Alternatively, the anode may be a metal hydride electrode.
Other types of electrochemical device in which the separator
of the invention finds application include secondary cells
such as lead-acid cells.
Measurement of ion exchange capacity
A sample of a non-woven fabric weighing about 0.5 g is
converted into the acid (H') form by immersion in 1.0 M
hydrochloric acid at 60°C for 2 hours. The sample is washed
in distilled water until the washing water shows a pH in the
range of about 6 to 7. The sample is then dried to constant
weight at 70°C.
The dried sample is placed in a 100 ml polyethylene bottle to
which is added accurately 10 ml of approximately 0.1 M
potassium hydroxide. Additional distilled water can be added
to immerse the sample fully. A further 10 ml of potassium
hydroxide is added to a second polyethylene bottle, together
with the same amount of distilled water as that added to the
bottle containing the sample. Both bottles are stored at
60°C for at least two hours.
After being allowed to cool, the contents of each bottle are
transferred to glass conical flasks, and the amount of
potassium hydroxide in each is determined by titration with
standardised 0.1 M hydrochloric acid, using a phenolphthalein
indicator.
T...t ._.._..____~ ... .............v...~.... ....
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-15-
The ion exchange capacity, measured in milliequivalents per
gram, of the membrane in the dry acid (H~) form is calculated
according to the equation:
tz - ti
IEC =
lOW
where tl is the titration value of HC1 from bottle with the
sample, t~ is the titration value of HC1 from bottle without
the sample, and W is the weight of the dried membrane in acid
(H') form.
Measurement of residual absorption capacity
A weighed sample of a non-woven fabric laminate is converted
into the potassium salt form by immersion in 0.1 M KOH for
about 1 hour at 70°C. The sample is washed in distilled
water to remove excess KOH. Excess water is removed using a
paper towel.
An ammonia solution is prepared by mixing 120 ml of distilled
water and 5 ml 0.3 M NH3. The sample is immersed in the
solution and placed in an oven at 40°C for 2 hours. The
sample is then allowed to cool.
A 100 ml sample of the solution in the flask is then titrated
with 0.1 M HC1 to neutrality using methyl red as an
indicator.-
A control reading is obtained from the solution of ammonia in
distilled water, without the laminate sample.
Examples of the manufacture of an electrode separator from a
non-woven polypropylene fabric is set out below.
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98I01671
-16-
COMPARATIVE EXAMPLE 1
A spun bonded non-woven polypropylene fabric with a thickness
of 316 um, a fibre size of about 15 to 20 um, and a basis
weight of 60 g.m-=, was densified by passage through a set of
smooth rollers which were heated to a temperature of 125°C.
Its thickness following densification was 170 um.
The fabric was impregnated with a solution of acrylic acid by
passing the fabric around rollers located in a chamber with
an atmosphere of nitrogen so that the fabric passed through
the solution. The solution was formulated as follows
(percentages by weight):
Component wt.o
Acrylic acid 30.0
Benzophenone 0.25
Surfactant (Lutensol ON70''") 0.5
Water 69.25
The impregnated fabric was maintained in an atmosphere of
nitrogen and passed through an irradiation chamber defined by
quartz glass walls. Medium pressure mercury vapour lamps
were positioned parallel to one another on opposite sides of
the chamber outside the quartz glass walls. The lamps had a
power output of 120 W.cm-1 and were located 16 cm from the
fabric. Each lamp provided a parallel ultraviolet light beam
with a width of 10 cm. The total exposure time of the fabric
to the radiation was about 6 seconds.
The fabric was then washed in de-ionised water to remove
unreacted components and then dried in an air oven at
approximately 70°C.
~. .
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-17-
The properties of the treated fabric are set out below, and
compared with the corresponding properties of the
polypropylene fabric starting material:
Unctrafted Grafted
Ion exchange capacity (meq.g-') 0 0.72
Gel content
(ASTM D2765-84) 0 53.3
Machine direction tensile
strength (N.m-') (ASTM D882) 2800 3100
Machine direction elongation (o)
(ASTM D882) >70 45
Electrolyte wicking rate (time) 60s 600s 60s 600s
(30o w/w KOH) (DIN 53924-78) (mm) 0 0 45 80
Electrolyte absorption (o
(AD 447301 US Air Force Manual) Non-wetting 290
Residual ion exchange capacity
(meq . g-1 ) 0 0 . 2 8
COMPARATIVE EXAMPLE 2
A melt blown non-woven polypropylene fabric with a thickness
of 200 um, a fibre size of about 3 to 5 um, and a basis
weight of 46 g.m-2 was impregnated with an acrylic acid
solution and irradiated as described above in Comparative
Example 1.
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-18-
The properties of the treated fabric are set out below, and
compared with the corresponding properties of the
polypropylene fabric starting material:
Unqrafted Grafted
Ion exchange capacity {meq.g-') 0 0.75
Gel content ( a )
(ASTM D2765-84) 0 43
Machine direction tensile
strength (N.m-') (ASTM D882) 0.8 1.077
Machine direction elongation
(o)
(ASTM D882) 11 8
Electrolyte wicking rate (time) 60s 600s 60s 600s
{30o w/w KOH)(DIN 53924-78)(mm) 0 0 39 132
Electrolyte absorption (o)
(AD 447301 US Air Force Manual) Non-wettinga 550
Residual ion exchange capacity
(meq.g 1) 0 0.48
EXAMPLE 1
A laminate was formed from a melt blown polypropylene fabric
and a fabric formed from spun fibres. The melt blown fabric
had a basis weight of 14 g.m-z and a fibre size of about 3 to
um. The spun fibre fabric had a basis weight of 36 g.m-
and a fibre size of about 15 to 20 Vim. The laminate was
created by the localised application of heat and pressure to
form localised welds between the fibres of the fabrics. The
~ T. _ __. _ ~ ._,....
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-19-
laminate had a thickness of 294 um. It was densified by
passage through a set of smooth rollers which were heated to
a temperature of 125°C. Its thickness following
densification was 170 um.
The laminate was impregnated with an acrylic acid solution
and irradiated as described above in Example 1.
The properties of the treated laminate are set out below, and
compared with the corresponding properties of the
polypropylene laminate starting material:
Unarafted Grafted
Ion exchange capacity (meq.g-') 0 0.4
Machine direction tensile
strength (N.m-') (ASTM D882) 1770 2400
Machine direction elongation (%)
(ASTM D882) 41 19
Electrolyte wicking rate (time) 60s 600s 60s 600s
(30o w/w KOH)(DIN 53924-78)(mm) 0 0 50 110
Electrolyte absorption (o)
(AD 447301 ~JS Air Force Manual) Non-wetting 240
Residual ion exchange capacity
( meq . g-1 ) 0 0 . 4
EXAMPLE 2
A laminate was formed bfrom a melt blown polypropylene fabric,
and two fabrics formed from spun fibres arranged on opposite
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-20-
surfaces of the melt blown fabric. The melt blown fabric had
a basis weight of 12 g.m-z and a fibre size of about 3 to 5
um. The spun fibre fabric had a basis weight~of 19 g.m-' and
a fibre size of about 15 to 20 um. The laminate was created
by the localised application of heat and pressure to form
localised welds between the fibres of the fabrics. The
laminate had a thickness of 229 um.
The laminate was impregnated with an acrylic acid solution
and irradiated as described above in Example 1.
After the irradiation and washing steps, the laminate was
passed through smooth rollers heated to 97°C to reduce its
thickness further, to 151 um.
The properties of the treated laminate are set out below, and
compared with the corresponding properties of the
polypropylene laminate starting material:
_.
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-21-
Unarafted Grafted
Ion exchange capacity (meq.g-') 0 0.75
Gel content (o)
(ASTM D2765-84) 0 52.7
Machine direction tensile
strength (N.m-') (ASTM D882) 2070 2660
Machine direction elongation (%)
(ASTM D882) 74 55
Electrolyte wicking rate (time) 60s 600s 60s 600s
(30o w/w KOH) (DIN 53924-78) (mm) 0 0 22 70
Electrolyte absorption (%
(AD 447301 US Air Force Manual) Non-wetting 335
Residual ion exchange capacity
(meq. g-') 0 0.37
USE IN A BATTERY
An AA size alkaline spirally wound nickel-metal hydride
(Misch metal electrode) cell was dismantled and its
electrodes Teduced in area by approximately 300. Its
separator was replaced by one made in accordance with Example
2 above. The cell was reassembled with 30o KOH electrolyte.
The cell was repeatedly charged at 350 mA and discharged
through a 10 ohm passive load. The cell was found to be
capable of delivering 600 mA.h to a 1.0 V cut-off on
discharge.
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-22-
STRAIN RATE AND ELONGATION TO BREAK
Comparisons were made between the deformation characteristics
of the fabrics of Comparative Examples 1 and 2 and the
laminate of Example 2 above. The experiments were carried
out using a Lloyd Instruments tensile tester, model LRX
10030. The elongation (~) and force (N) to break were
recorded using different strain rates. Samples were taken
across the width of the fabrics and measured 50 mm wide and
200 mm long.
Strain COMP COMP EXAMPLE
EXAMPLE EXAMPLE 2
1 2
rate
(mm.min-') Extn Force Extn Force Extn Force
50 58 183 22 52 60 153
250 56 194a 18 54 53 162a
500 52 205 16 59 44 154
1000 46 ~ 186 ~ 10 j 68 ~ 36 ~ 140
(a - force to yield)
These results indicate that, at high strain rates such as
when assembling battery components by winding, a melt blown
fabric is unsuitable because of the poor ability to withstand
tensile forces. The spun fibre fabric and the laminate have
a greater ability to withstand tensile forces.
STRAIN RATE AND ELONGATION
The materials used in the experiment to assess the effect of
strain rate to a pre-determined extension on tensile
performance were analysed to determine the effect on the
maximum pore size of the fabrics. Pore sizes were measured
in um using a Coulter pyrometer.
_.__. . . ~ _ .. . .. i
CA 02294599 1999-12-15
WO 98/58111 PCT/GB98/01671
-23-
Strain Elongn (o) Maximum pore
size (pm)
rate
(mm.min-1) COMP EX 1 COMP EX 2 EX 2
0 0 76.72 20.04 14.43
1000 5 90.14 20.85 15.29
1000 7 90.14 2085 20.85
1000 10 ~ 109.2 ~ fail ~ 19.73
These results indicate that the melt blown fabric of
Comparative Example 2 can maintain its structure at high
strain rates but only to limited extension. It is therefore
poorly suited to incorporation in an electrochemical device
by winding.
The spun fibre fabric of Comparative Example 1 can withstand
large deformation at high strain rates. However, the pore
structure is disrupted as a result.
The pore structure of the laminated structure of Example 2 is
maintained at high deformation and at high strain rates.
