Note: Descriptions are shown in the official language in which they were submitted.
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PAPER AND METHOD OF MAKING IT
FIELD OF THE INVENTION
This invention relates to a novel structure of paper
and to a method of making paper. Within the termi "paper"
we include all hydraulically deposited webs of fibres of
all kinds including for example fibres made from
cellulose, glass, asbestos, carbon fibre and mineral wool
or other synthetic materials~
BACKGROUND OF THE INVENTION
The conventional wisdom in the paper-making art is
that it is possible to form a properly coherent paper from
a slurry of cellulosic fibres such as wood, cotton or flax
fibres which have been subjected to an appropriate
treatment, for example by beating or refining. Binders or
other materials may be added. The inter-action between
the fibres is partly due to such frictioni as is caused by
mechanical intermeshing but is primarily by hydrogen
bonding between the hydroxyl groups existing on the fibres
and on fibrils formed by the treatme~nt to whiah tha ~ibres
have been subjected. Binders if present will act to
adhere the surfaces of the fibres together, or to form a
self bonding matrix in and around the ~ibrous web.
There are many fibres, particularly synthetic polymex
fibres and inorganic fibres, for which inherent
interengagement is weak and there are some applications
for which such adherence without binders is inadequate.
It is impossible, for example, to achieve high strength
without binders when the ~ibres in question are ~lass,
mineral wool fibres, quartz, alumina and so on, or when
non-fibrous materials, such as ion exchange resins or
silica gels or activated carbon, are incorporated.
Furthermore, although cellulosic fibres on the one hand
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can be interengaged with adequate strPngth and, on the
other hand, inorganic fibres such as glass fibres can, by
special treatments, form a paper of some strength, it has
been impossible in the past to make a material where
layers made from certain sorts of fibres form a strong
interface except by using large amounts of binder or, of
course, by adhesive lamination of pre-prepared webs.
It appears that the interengagement between fibres of
one type in one layer is taking place by one mechanism and
of another type in a separate layer by a different one,
even when the fibres in each layer are different types
only to the extent that they are fibres of the same
material but have been differently treated or are of
different length or of different thickness. One layer is
usually stronger than the other. The problem of forming
an interface is particularly difficult when the fibres are
different in each layer. This is especially so when the
fibres in each layer are of different materials.
Where great thickness was not required, papers have
generally been made of a single slurry and any special
surfaces were achieved by surface treatment of the web as
for example in the application of coatings or sizes.
For a lot of applications however it is the
properties of the surface of the paper which matter, the
~5 remainder of its bulk giving only mechanical strength or
the like. Alternatively, one property may be desired on
one surface and a different one within the thickness of
the paper or at the other surface.
In these cases therefore it would be economic to make
such papers in a single process by selecting the material
of a surface layer of the paper itself with a view to its
having a desired property and making the body of the paper
or its other surface of some other material.
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For example, in some chemical treatment/filtration
applications we wish to have a paper having inert surface
characteristics but a reactive material such as an ion
exchange compound incorporated in the body of the paper
between the two surface layers. Alternatively, for filter
papers a layer of glass fibres having desirable filtration
properties may be supported by a layer of cellulosic or
different glass fibres giving mechanical strength. We
would also wish to have the facility for building up a
comparatively thick paper from thin layers of fibres which
of their nature could not practicably be applied in the
full thickness - for example because of slow water removal
due to their extreme fineness.
It has previously been proposed to build up a
lS relatively thick paper board having the same type of fibre
across it by a continuous "wet-on wet" process in which a
first slurry of the fibres is deposited onto the wire of a
Fourdrinier papermaking machine and, while the first
slurry is still wet, a second slurry of the fibres is
deposited on the first so as to allow interlaminar mixing
of the fibres. In such processes i~ is known to introduce
the second slurry to the first at a sp~ed slower than that
at which the first slurry is moving.
A theoretical discussion of the "wet~on wet" process
is given by B. Radvan and A.J. Willis, in "Paper Industry
Conference Papers", available at Royal Hall, Exhibition
Centre, Harrogate 26-28th October 1982. Though not
specifically stated, it appears that the article relates
to conventional cellulosic paper board the fibres of which
are of the same physical and chemical characteristics
throughout the thickness of the paper. Radvan and Willis
address themselves to the problem of forming the second
layer on the first without disturbing the first so that
the number of fibres at the interface which lie across the
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board is reduced so that the board appears homogeneous~
To this end they employ the "Coanda effect" to introduce
the respective stocks at zero angle and zero speed
relative to a moving wire in which the stocks are
deposited. The success of such a process relies upon the
hydrogen bonding between the cellulosic fibres and the
presence in the slurries of a binder which is
conventionally present. ~he possibility of radically
affecting the fibrous structure of the web, e.g. through
repeated application of "micro-turbulence" throughout the
process of formation, is also suggested by Radvan and
Willis as something which may be discerned in the future,
though this suggestion is clearly within the same general
principle of kePping disturbance of the respective layers
to a minimum.
U.S. 2098733 discloses a practical method of forming
a thick paper board by depositing a second slurry on a
first slurry whils the first is still wet so as to allow
interlaminar mixing of the fibres. The fibres in the
2G first may be longer than those in the second slurry.
Again the process is controlled so as to minimize the
number of fibres oriented generally~across the paper so
that the paper appears homogeneous. A size binder is
included in both slurries to achieve adequate bond
strength, and it appears that a silicata adhesive i~ also
employed.
A process in which a second slurry is deposited on a
partially dehydrated first slurry has also been employed
to produce papers which are at least primarily of asbestos
fibre. The fibres throughout the width of the paper are
of the same chemical and physical characteristics, but
those in one layer are more densely packed than those in
the other. In this process the degree of flocculation of
the fibres was controlled so as to provide some fibras
lying generally in the Z-direction in an attempt to
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improve bonding (U.S. 3353682~.
Paper made by the abovementioned known processes is
weakest at the interface between adjacent layers.
Even though the fibres throughout the width of the
paper made by such known processes were generally of the
same chemical and often physical nature, it was still
considered necessary to include a binder, often in large
amounts (see particularly U.S. 3353682) to achieve
adequate strength. The presence of such a binder i~,
however, highly undesirable in certain applicat:ions, such
as in scientific laboratory papers and battery separators
(see U.S. 4216280 which employs a single layer containing
~lass fibres, some coarse, some fine and no binder). In
other applications, such as filters for cleansing gases,
e.g. air filters, especially so~called "HEPA" (high
efficiency particulate air) filters, the pre~ence of more
than ~mall amounts of binder is highly undesirable though
such small amounts have somatimes, in the past, been
included to give the paper sufficient strength.
For a battery separator or HEPA~ filter, it is known
to be desirable to have one layer of relatively coarse
fibres and another of finer ones. To date, because it was
believed that too much binder would be required to form a
unitary structure, the two layers were separately
preformed and then laminated either with an adhesive or
mechanically (see, for example, U.S. 4262068) It would
be particularly advantageous to be able to provide a
battery separator or HEPA filter comprising a single
unitary sheet which contained no bind r, and which could
be made by a single process.
SUMMARY OF THE INVENTION
The present invention permits the manufacture in a
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single process of a paper having at least two material
layers which are inherently bonded together during the
papermaking operation from distinct slurries which will
usually be of different fibres ~i.e. different chemically,
physically or both), and which layers are joined at an
interface which comprises a region where the fibres of the
two distinct slurries are intermingled. If the first
slurry has fibres A and the second has fibres B then the
structure of the finished paper is lay~r A followed by
interface A+B followed by layer B and then optionally B+C,
C and so on. This is achieved in the present invention
without the necessary use of a binder.
The present invention provides a papermaking process
in which a plurality of layers of distinct slurries are
laid down one upon the other in the papermaking machine
such that a composite is built up in the wet state, a
second layer being applied to the first in a determinate
relationship of the composition of the two slurries at the
time of application and of the physical relationship
between the slurries at that time, whereby disturbance is
caused only in a surface region of the two layers to cause
penetration of the ~ibres of the second slurry among the
fibres of the first in that region but to lea~e
substantially undisturbed the fibres in the majority of
the thickness of each of the respective layers. It is
found that by such control we can have an adherence
strength at the interface which is equal to or,
freguently, greater than the strength of at least one and
possibly each of the respective layers, even in the case
where the fibres of the two layers are widely dissimilar,
and all this without necessarily using a binder material.
Thi~ is bacause the fibres are by the controlled
disturbance, intermingled at the interface betwPen
adjacent layers. This allows the fibres to become
interlink~d. Thus, on attempting to tear the adjacent
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layers apart from one another the fibres interengage so as
to resist the tearing. Hence the paper has a tendency to
tear along a plane in the weakest of the two adjacent
layers rather than at the interface. On attempting
delamination, a high percentage of the top ply fibres are
still physically attached to the bottom ply via the
interface.
By enabling adjacent layers to be bonded to one
another in this manner, a binderless unitary structure
having characteristics which vary through its width,
including variations in the chemical nature of the fibres,
variation in their physical characteristics (especially
fineness) and variations in the loading of other additives
which paper, for certain uses, desirably includes, e.g.
silica gel or particles of ion exchange resin in
laboratory filter paper, or perlite in battery separators,
may be obtained.
The physical relationships which are of primary
importance in a process of the invention are the relative
velocities of the two slurries at the time of application,
the height of the flow box nozzle of the second above the
first layer and the angle of that nozzle to the first
layer. By these variables we can control the degree and
extent of intermingling of fibres at the interface and the
degree and extent to which fibres are reorientated in the
interface region from the plane in which they are
predominantly deposited in either layer. The liquid
content of the respective layers may also play a part in
determining the characteristics of the final paper.
It is particularly preferred that the second slurry
is introduced to the first at a speed greater than that at
which the first slurry is moving.
Thus, according to one aspect of the present
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invention there is provided a method of making paper which
involves applying a second slurry of fibres to a surface
of a travelling layer of a first slurry of fibres to form
a second layer thereon with intermingling and interlinking
of the first and second fibres, fibres of the respective
slurries being different from one another, the applying
being done at a time when the fibres of the first slurry
are mobile at the upper surface region of the first slurry
by virtue of its being in the wet state and when the
second slurry has a higher water content than that of the
first, characterised in that the fibres of at least one
said slurry are non-cellulosic and in that at the time of
application the velocity of the second slurry is
precontrolled in relation to the velocity of travel of tha
first slurry and the said applying is also done under
precontrolled conditions of angle (~) and height (h)
relative to the first slurry to control the thickness of
an interface layer formed by the application o~ the second
slurry between the first and second layers, the precontrol
being such as to cause a disturbance to an extent
sufficient to ensure bonding together of the first and
second layers irrespective of the nature of the first and
second fibres with a bond strength of the interface layer
at least as great as the bond strength in at least one of
the first and second layers.
The invention also discloses a method of making paper
which involves applying a second slurry of fibres to a
travelling layer of a first slurry of fibres, the said
fibres of the respective slurries being different from one
another, at a time when the first slurry has a consistency
o~ between 80% and 95% water and the second has a
consistency of between 98~ and 99.9% water, the second
slurry being applied to the first at a velocity greater
than the speed of travel of the first layer so as to form
a second layer and to provide disturbance only at an
interface between the surfaces of the said first and
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second layers, the said application of the second layer
being carried out under predetermined conditions of angle,
height and velocity relative to the first layer at the
time of application to control the thic~ness of an
interface layer between the first and second layers which
is formed by the intermingling and interlinking of the
different fibres to ensure the bonding together of the
layers irrespective of the nature of the fibre material of
the slurries while leaving fibres in the first and second
layers remote from the interface layer essentially
undisturbed.
Fu~thermore, the invention discloses a method of
making paper which involves applying a second slurry of
fibres to a travelling layer of a first slurry of fibres,
the said fibres of the respective slurries being different
from one another, at a time when the first slurry has a
consistency of between 80% and 95% water and the second
has a consistency of between 98% and 99.9~ water, the
second slurry being applied to the first at a controlled
velocity in relationship to the fixst layer so as to form
a second layer and to provide disturbance only at an
interface between the surfaces of the said first and
second layers, the said application of the second layer
being carried out under pre-determined conditions of
angle, height and velocity relative to the first layer at
the time of application to control the thickness of an
interface layer between the first and second layers which
is formed by the intermingling and interlinking of the
different fibres to ensure the bonding together of the
layers irrespective of the nature of the fibre material of
the slurries while leaving fibres in the first and second
layers remote from the interfa~e layer essentially
undisturbed~
The invention additionally provides a paper made by a
method according to any of the above aspects, and the
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resulting paper may have any one or more of the following
characteristics:-
(a) the interface layer formed has a bond strengthhigher than the bond strength of at least one of the first
and second layers,
(b) the interface layer formed is about 5 to 15% of
the total thickness of the resulting paper,
(c) one of the first and second layers contains
fibres which are finer than the fibres in the other layer,
the fibres in one, and preferably both, layers being
glass,
(d) in a paper having characteristic (c), the layer
containing the finer fibres has a thickness less, and
preferably considerably less, than the thicXness of the
layer containing the thicker fibres (the thickness of the
layer containing the finer fibres may represent 10-40%,
but is typically 25-30%, of the total thickness of the
paper) and
(e) the paper is free of binder.
A paper in accordance with one aspect of the
invention has a first layer comprising a first fibre and a
second layer comprising a second fibre different ~rom the
first, the fibres of at least one of the first and second
layers being non-cellulosic, the paper additionally having
an interface layer bonding the first and second layers
together and in which both the first and second Pibres are
intermingled and interlinked in orientations more
disordered than the fibres in the first and second layers,
the interface layer bonding together the first and second
layers with a bond strength at least as great as the bond
strength of at least one of the first and second layers.
Preferably one layer of such a paper comprises
cellulosic fibres and another layer comprises non-
cellulosic fibres, especially glass. Two adjacent layers
may each comprise non-cellulosic fibres, and the two
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adjacent layers may comprise respective fibres which are
the same as one another chemically, e.g. glass, but differ
in their respective thicknesses.
One layer of such a paper may contain fibres which
are finer than fibres in the other layer, so that th~
paper has a density change across its thickness.
Rreferably, the thickness of the layer containing the
finer fibres is less than that of the other layar.
A paper in accordance with another aspect of the
inventioh consists essentially of a first layer of a first
fibre and a second layer of a second fibre different from
the first, and wherein the fibres of the first and second
layers lie generally in parallel planes which planes are
parallel to the plane of extension of the paper, and an
interface layer between the first and second layers in
which the first and second fibres are intermingled and
interlinked to an extent sufficient to bond together the
said first and second layers, one of the said first and
second layers containing fibres which are finer than
fibres in the other of the said ~irst and second layers,
and wherein the paper has a density~change across the
thickness of the paper.
In such a paper, at least one of the fibres is
preferably non-cellulosic and may be inorganic or
synthetic organic, e.g. glass, polyester, polyamide or
polyolefin. Either or both of fibras respective layers
may be glass.
Even when the fibres are non-cellulosic, the ~ond
strength of the interface layer may be greater than the
bond strength of at least one of the first and second
layers.
The layer containing the finer fibres preferably has
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a thickness less than the other layer.
In a paper in accordance with either of the above-
mentioned aspects, the thickness of the interface ragion
may be between 5 and 15% of the thickness of the paper,
especially when the paper has only two layers.
It is also preferred that a paper in accordance with
either of the above aspects is free of binder.
In a typical method embodying the invention, the
second slurry is applied to the first layer from flow box
nozzles set at a height of between 1 mm and 20 mm from the
surface of the first layer; the second slurry is applied
to the first layer from flow box nozzles set to issue at a
speed between 102% and 112% of the speed of the first
layer beneath the nozzles; and the second slurry is
applied to the first layer from flow box nozzles set at an
angle of between 2.5 and 12 to the plane of the first
layer.
In certain fields the use of a binder material is
highly undesirable. This is so in the field of scienti~ic
laboratory papers such as *ilter papers, etc. where the
object of using fibres such as glass fibres is to provide
a paper which is chemically highly inert and pure. The
presence of binders in such papers may be deleterious to
the results obtained, since they may introduce chemical
impurity and do reduce filtration efficiency. ~inders are
also highly undesirable in battery separators; the binder
would not be chemically compatible with the electrolytic
cell and would also restrict absorption of electrolyte
into the separator.
As explained above various aspects of the invention
provide a paper made integrally of different ~ibres in
respective layers with an interface between the layers
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comprising fibres of both layers and causing a cohesion
between the layers, and in particular such a paper when
free of binder. The fibres may be different either
chemically or physically or both. Furthermore, one or
other or each of the layers may have incorporated into it
in the slurry stage or deposited onto it non-fibrous
materials appropriate to the use of the paper and this in
the laboratory context may include particles of ion
exchange resin or in the ordinary context incorporating in
a surface layer (which may be of le~ser thickness than the
other layer or layers), furnishes for achieving a desired
surface characteristic. It is also possible to affect the
properties of the paper as a whole by controllin~ the
properties of the interface. For example, desirable
properties of, for example, a filter paper, battery
separator or gas-cleansing filter, such as an air filter,
especially a so-called "HEPA" filter, can be affec~ed or
even determined by control of the amount of disturbance
and hence of disorientation and intermingling which is
induced at the tim~ of application of the second or other
subsequent slurries.
The preferred parameters for t~e relative
consistencies and relationships of adjacent layers at the
time that a subsequent layer is applied to an earlier
layer are determined by the respective properties of the
two fibres involved.
It can be generally said that the greater the
dissimilarity between the fibres of the layers and
especially when one (or both) have a low bond strength
within the layer, the greater should be the angle of
incidence and the height from which the subsequent layer
is applied. In dependence upon the nature of the paper
required, each of the relative velocities, angle and
height may be chosen independently of one another.
Suitable angles of incidence may be between 1.5 and 20~
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preferably between 2.5 and 12. For many papers a
particularly suitable angle is about 4-6. The height
may lie between 1 and 50mm, preferably between 1 and 20mm,
more preferably between 1 and lOmm. The velocity
difference between the two slurries may be 2 to 15%,
preferably between 2 and 12%, and, for many papers, more
preferably about 5-7 (the second travelling faster than
the first).
For example to apply a glass layer (e.g. Johns
Manville 106 glass microfibres having an average diameter
of 0.49 - 0.58 micrometers) to a cellulosic layer (e.g.
cotton) the glass slurry is most e~fectively applied from
a height o~ 8mm at an angle of incidence of 7.0 to the
cellulosic layer: if the lower layer were a glass layer
(e.g. Johns Manville 104 glass microfibres having an
average diameter of 0.34 - 0.48 micrometers) it would
require 6.0mm and 4.0 respectively.
In all cases, control of the consistency of the
layers at the time of application is critical. A second
layer should be applied to a first when the first slurry
is still highly liquid and, in dependence upon the nature
of the paper to be mada, preferably contains between ~0-
95%, more preferably between 86.5 and 93.5%, more
particularly 87.5 - 92.5%, especially 89-91% by weight
water, and when the second contains between 98 and 9909%
water, more particularly 99.0 to 99.8%, especially 99.5-
99.7% by weight water (the rest in each case being solid
content). A third slurry if used may be applied at a
consistency of 85 to 95% water, more particularly 90%, at
which time th~ consistency of these first two layers,
taken toyether overall, may be between 89 and 91% o~
water. in this case, where the first two layers have
already consolidated to a certain extent, the formation of
an interface may be aided by mechanical disruption o the
face of the third layer by subjecting this layer to a
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change o~ direction by passing it over a roll immediately
prior to its being deposited upon the ~irst two (or more)
layers.
In each case an interface is formed of mixed fibres
which is about 5 to 15% of the total thickness of the two
layers, more usually about 10%. The extent of thickness
of the interface layer depends primarily, though not
solely, on the nature of the first layer rather than on
the consistencies and the variables mentioned above.
Processes embodying the invention may be carried out
so as to produce novel materials for two fields of use
which present particular di~ficulty, battery separators
and gas-cleansin~ filters such as air filters, aspecially
HEPA filters. Thus, a multilayer structure can be
produced for use as a gas-cleansing filter or battery
separator which structure will consist of two or more
layers of cellulose, synthetic organic or inorganic
fibres.
In particular, such processes allow the preparation
of a paper suitable for use, inter ~alia, as a battery
separator or gas-cleansing filter comprising a paper
having a density change across it, the fibres at the
interface between adjacent respective layers being
sufficiently intermingled and interlinked to provide
sufficient bond strength between the layers without the
necessity for binder to be present. Such binderless
graded density paper does not appear to have been
previously disclosed in tha literature.
A battery separator embodying the inventi~n is
particularly suitable for use in gas recombination
batteries which reguire separator integrity. The
separator, in a single unitary structure, provides
sufficient bulk to absorb and hold the electrolyte and
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efficiently prevents passage therethrough of bodies such
as small crystals harmful to the battery, while allowing
the gases to pass through it. The process of the invention
allows particularly efficient use of the fibres when
producing such battery separators.
In a proces~ in accordance with the invention, the
fibres in respective slurries may be dif~erent from one
another in either their physical or chemical
characteristics, or both. Furthermore each respective
slurry may contain in that slurry a mixture of fibres
different from each other in their physical and/or
chemical characteristics.
The fibre may be natural or synthetic, inorganic or
organic, for example, cellulosic (either natural or
regenerated) fibres such as wood pulp, cotton and
csllulose acetate, inorganic fibres such as glass,
asbestos and alumina, natural organic fibres such as
mineral wool and synthetic organic fibres such as
polyesters (e.g. polyethylene terephthalate), polyolefins
(e.g. polyethylene, polypropylene), acrylics (e.g.
polyacrylonitrile), carbon fibre and polyamides ~e.g.
nylon), especially aromatic polyamides (e.g. KevlarR -
Kevlar is commercially available from Du Pont). Kevlar is
particularly suitable for HEPA filters for use in the
nuclear industry because it is not attacked by the
hydrofluoric acid emitted by reactors.
Preferred papers made by processes embodying the
invention are those in which at least one of the fibres is
non-cellulosic and these may be selected from inorganic
and synthetic organic ~ibres, e.g. glass, polyester,
polyamide or polyolefin.
Other preferred papers hava one layer comprising
cellulosic fibres and another comprising non-cellulosic
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fibres, e.g. cellulose on glass, especially fine
cellulosic ~ibres on relatively coarse glass fibres.
The process is particularly applicable to forming
papers having two adjacent layers each of which comprises
non-cellulosic fibres. For example filters having
respective layers of polyester and glass are useful in gas
masks.
The fibres in the respective layers may be the same
as one another chemically but differ in their respective
thicknesses. Such papers, when made entirely out of
glass, provide especially suitable battery eeparators or
HEPA filters.
Where glass fibres are employed these may be of a
thickness within a range wider than for many other fibres.
Thusl the glass in the fine layer may be as fine as a
Johns Manville 100 microfibre (having an average diameter
of from 0.2 to 0.29 micrometers), while the glass in the
coarse layer may be as coarse as Johns Manville "Chop Pak"
fibres, which are either about 12.7 or 6.3mm in length and
15 micrometers in thickness.
Typically, a battery separator may have an average
weight/unit area of from 60-240g/m2 and comprise two
layers, viz a coarse layer of e.g. a mixture of John~
Manville 112 and 110 microfibres (average diameters 2.6
3.8 and 2.17-3.10 micrometers respectively) and a fine
layer of either Johns Manville 108 or 106 microfibres
~average diameter 0.59-0.88 or 0.49-0.58 respectively).
The glass may be a borosilicate glass with or without zinc
oxide ~e.g. Johns Manville type 475 or 753 respectively).
30A two-layer battery separator embodying the invention
may have:-
Layer (1) - a furnish of acid resistant glass
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micro-fihre having a nominal fibre diameter of 3.5 microns
and a grammage between ~0 and 250 g/m2.
Layer (2) - (and layer (3) if used) - a furnish of
acid resistant glass microfihre having a nominal value of
~.65 microns and a grammage between 5 and 100 g/m2.
In the case of a three ply structure layer (1) would
form the centre ply with layers (2) and ~3) forming the
outer surfaces of the structure.
The qualities which make such unitary glass
microfibre separators embodying the invention successful
are as ~ollows:
1. Their stability in sulphuric acid.
2. They are binder free multiple structures which
provide the ability to make more efficient use of the
different grades of glass fibres by
(a) u~ing a thin continuous layer of fine
fibres to give a fine pore structure and so reduce the
possibility of unwanted transfer between the electrodes,
and
~b) increasing the bulk of the material by use
of continuous layers of coarse fibres, capable of
absorbing comparatively large amounts of electrolyte pex
unit weight of separator.
3. Their improved strength as compared with the
thin, flimsy, single, mixed fibre, layer. In the unitary
structure embodying the invention, the thin layer of fine
fibres is strongly bonded without binder or mechanical
aids to the bulk layer of coarse fibres by which it is
supported.
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A typical HEPA filter may have an average weight/unit
area of from 60-llo g/mZ and comprise two layers, viz. a
coarse layer of e.~. Johns Manville 112 microfibres
(average diameter 2.6-3.8~m), and a fine layer of John~
Manville 100 microfibres (average diametar 0.2-0.29
micrometres). The glass may be a borosilicate glass
containing a small amount of zinc oxide (e.g. Johns
Manville type 475).
A two-layer HEPA ilter embodying the invention may
have:-
(a) Layer (1) - a furnish of between 50% ---> 100%
glass microfibre having a nominal fibre diameter of 0~3
microns, 0-50%, preferably 5% ---> 50~, synthetic organic
Eibre and 0 ---> 10% acrylic binder with a grammage
between 20 ---> 200 g/m2.
Layer (2) - a furnish of 90% ---> 100% glass
microfibre having a nominal fibre diameter of 0.65 microns
or less and 0 ---> 10% additive, with a weight of between
5 and 100 g/m2.
(b) Layer (1) - a furnish of 50% ---> 100%
cellulose, 50% ---> 100% synkhetic organic fibre with a
grammage between 10 ---> 100 g/m2.
Layer (2) - a furnish of 90% ---> 100% glass
microfibre having a nominal fibre diameter of 0.65 microns
or less and 0 ---> 10% additive with a grammage between 5
and 100 g/m2O
A multi-density HEPA filter embodying the invention
thus produced has the following advantages over a
conYentional filter, which is a homogeneous mixture of
glass fibres.
1. Substitution of relatively fine fibres by
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coarser fibres in part of the thickness gives a reduction
in pressure drop without loss of strength and may also
mean that lower-cost materials can be used.
2. The material has a gradation of pore sizes from
the relatively large pores in the continuous coarse fibre
structure of tha layer we call a "pre-filter" layer
through a range of medium sized pores in the central
(mixed fibre) area, to the relatively small pores of the
continuous fine fibre structure of the filter layer.
3. The structure allows for more efficient depth
filtration to take place. Particles are in effect more
evenly distributed throughout the structure.
4. Loading capacity can be greatly increased by the
careful design of a filter/pre-filter material for
specific applications.
5. For a given loading, i.e. similar weights of
particulate matter, the pressure drop would be lower for
the multi-density filter than for a single ply filtar.
6. The layPr of fine fibres would be protected by
the pre-filter layer and hence the effective life of the
filter could be increased.
7. The amount and diameter of the fine fibres used
would be determined by the required filtratiQn
performance. This would lead to a more efficient use of
the expensive fine fibres.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
Figure 1 is a highly diagrammatic view o~ a
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Fourdrinier type machine modified to operate the
invention;
Figure 2 is a detailed scheme of the region o the
flow-box nozzle applying a second slurry;
Figures 3 to 7 are photomicrographs of partial
sections through various papers according to the
invention, at various enlargements, showing the region of
the interface.
DESCRIPTION OF THE EMBODIMENTS
A Fourdrinier machine 1 has the conventional flow-box
nozzle 3 to deposit a slurry onto a moving web (or
"wires") 4 to form a layer 5 of wet fibre. Water drains
conventionally from this into a sump 6 for
recycling/treatment. At a selected position along the
wire is provided a second flow box 7, fed with a different
slurry from a second header. The second head box nozzle 8
issues a stream 9 of the second slurry directly onto the
upper sur~ace of the layer 5 which at that time is of a
known consistency dependent on the ~constitution of the
first slurry, the speed of the wires, the speed of
drainage and the distance of the second box 8 from the
first. The nozzle is set at a height h above the s~rface
of the layer 5 and has a flow angle a to that layer. The
velocity of the layer 5 is V1 and the velocity of the
stream 9 as it leaves the nozzle is V2. The effect of
this and of making the consistency of this layer 5 be
about 90% water while the slurry stream g is about 99.5~
water, is to cause a disturbance of the upper surface only
~f the layer 5 and an intermingling of the fibre~ of the
two layers in the interface between them, indicated at 10.
Fibres of the second slurry if finer than those of
the first may be drawn down between them by gravity,
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drainage or suction so as to enhance the effect of the
disturbance in the interface region; if the ibres of the
second layer are coarser than those of the first they may
be thought of as stakes penetrating into the first layer
and anchoring the layers together.
~ he composite layer then passes to a SUCtiOII belt 11
and to drying rollers 12 in the conventional way.
I~ desired a third layer may be applied from a third
flow box 13 via an auxiliary wire 14 to be pressed onto
the composite layer at a time when that, as a whole, has a
water content of 89 to 91% and when the third slurry has a
water content of approximately 90%.
Examples of the manufacture of various specific
papers, using in each case the apparatus of Figures 1 and
2, follow.
EXAMPI.E 1: WOOD ON COTTON
A first slurry was made of cotton fibres and a second
of wood pulp. The first slurry was~run onto the wire and
at a position where its water content was 90~ the second
slurry was projected onto it with the second nozzle being
at a height h 3mm from the surface of the layer formed by
the first slurry at an angle of about 3, and at a
velocity V2 5% or 6% greater than that, V1~ of the layer
formed by the first slurry. The consistency of the second
slurry was at the time of contact 99.5% water.
A coherent two-layer paper was formed after the
conventional drying and pressing stages the two layers of
which were separated only with difficulty and which showed
under the microscope an interface layer, extending to
about 10% of the thickness of the paper, where there was
great intermingling and di orientation of the wood and
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cotton fibres.
EXAMPLE 2: GL~SS ON COTTON
A first slurry was made of cotton fibres and a second
of ~ohns Manville 104 glass microfibres. The first slurry
was run onto the wire and at a position where its water
content was 90% the second slurry was projected onto it
with the second nozzle being at a height h, lOmm from the
surface of the layer formed by the first slurry at an
angle of about 6, and at a velocity V2 about 5% greatex
than that, V1, of the first slurry. The consistency of
the secon~ slurry was at the time of contact 99.6% water.
A coherent two-layer paper was formed after the
conventional drying and pressing stages the two layers of
which were not separable, in the sense that the bond
strength between the layers was greater than the fibre-
fibre bonding in the glass layer.
Examination of the structure of the paper showed an
intermediate layer 10 in which fibre from the two layers
5,9 were intermingled and disorientated. The thickness of
the layer 10 was about ~0% of that of the paper.
Photomicrographs of sections through this sample are seen
in Figures 3, 4 and 5 which are respectively at x500,
x1800 and x5500 magnifications. Figure 5 is also of
interest showing at 15 a glass fibre penetrating a cotton
fibre. This product provides a particularly efficient
strong, flexible liquid filtration medium.
EXAMPLE 3: GI~SS ON COTTON
Example 2 was repeated except that the glass fibres
were Johns Manville 106 microfihres~ the nozzle was spaced
at 13mm from the surface of the layer and at an angle. of
9~. A photomicrograph at x550 of th~ paper thus prepared
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- 24 -
is Figure 6.
EXAMPLE 4: GL~SS ON GLASS
A first sluxry was made of Johns Manville 108B glass
microfibre and a second of Johns Manville 104 glass
microfibre. 108B is coarser than 104~ The first slurry
was run onto the wire and at a position where its water
content was 91.5~. The second slurry was projected onto
it with the second nozzle being at a height h 4mm from the
surface of the layer formed by the first slurry at an
angle of about 3, and at a velocity V2 5 or 6% greater
than that Vl, of the layer formed by the first slurry.
The consistency of the second slurry was at the time of
contact 99.7% water.
A coherent two-layer paper was formed after the
conventional drying and pressing stages, the two layers of
which were not ~eparable, in the sense that the bond
strength between the layers was greater than the fibre-
fibre bonding in the glass layer.
Examination of the structure o~ the paper showed an
intermediate layer 10 on which fibre from the two layers
5,9 were intermingled and disorientated, a photomicrograph
at x550 of the paper produced being seen at Figure 7.
This structure provides a particularly efficient pre~
filter (or depth) filter, especially a HEPA filter, or
battery separator.
EXAMPLE 5: GLASS ON GLASS~POLYETHYLENE
Example 4 was repeated except that a first slurry was
made of 90% Johns Manville 108B glass microfibre and 10%
Solvay Pulpex polyethylene fibres, and a second of Johns
Manville 104 glass microfibre.
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A coherent two-layer paper was formed after the
conventional drying and pressing stages the two layers o~
which were not separable, in the sense that the bond
strength between the layers was greater than the fibre-
fibre bonding in the glass layer.
Examination of the structure of the paper showed an
intermediate layer 10 on which fibres from the two layers
5,9 were intermingled and disorientated.
Similarly, a quantity of granules of ion-exchange
lo material or other non-fibrous material may be incorporated
in one of the slurries.
A strong, flexible laboratory liquid filter with high
wet strength is obtained.
EXAMPLE 6: GLASS ON POLYESTER
In a manner similar to Examples 4 and 5 a slurry of
glass microfibres was laid down on a slurry of polyester
fibres, with the same satisfactory results.
This is useful as a highly efficient gas face mask
medium.
EXAMPLES 7 - ll
Papers particularly suitable for HEPA filters and
battery separators were made as follows.
Equipment
1. Black Clawson 8' HCVT Tile Hydrapulper, 24" Diameter
Vokes Rotor and Drive Assembly - 100 hp. 1800 r.p.m.
Westinghouse Motor.
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2. Semtile Chests, 3000-gal. and 7000-gal. capacities
with side-entering lightning mixers.
3. Black Clawson Secondary Flow Box - installed on a 36-
inch Fourdrinier.
4. Fourdrinier Paper Machine 36", described in detail as
follows:
Sandy Hill Corporation Fourdrinier Paper Machine with
a wire width of 36" and designed to operate at speeds
from 5 f.p.m. to 300 f.p.m. Headbox with multiple-
type operation - static, pressure or vacuum ~
equipped with manifold-type inlet and various
specially designed homogenizer and distributor rolls
and Neilson slice. Fourdrinier table adjustable for
inclined operation of 3" in 15' table length. The
press section consists of two main presses, the first
one being a straight through plain press and the
second press a plain reversing press. The rolls are
cast iron with various speci~l rubber and stonite
covers. One smoothing press has a straight through
run. The drier sections consist of seven and five
driers with integrally cast journals and two felt
driers on the bottom and top first section felts.
combination horizontal and vertical size press is
provided and equipped with various composition
coverings. It was not used in this series of runs.
The calender stack consists of eight rolls with the
intexmediate rolls bored for steam. Each roll is
constructed of chilled iron and is precision ground
and carried in anti-friction bearings. Also included
is a 36" diameter Pope type reel with a 36" face and
capable o~ winding rolls up to 40" in diameter.
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Stock Pre~aration
Each furnish was dispersed in the Hydrapulper at 3.0
pH for designated periods of time. The furnish was then
pumped to either the 7,000-gal. Secondary stock chest or
the two 3,500-gal. Primary Stock Chests and adjusted to
the required consistency and ph.
Papermakinq
Primary System
The Fourdrinier wet-end was used for the primary
layer. Each furnish was pumped from the machine chests
and metered with a Foxboro Flow Controller to the suction
of the fan pump where white water from the wixe was added
to give the required papermaking consistency. From the
fan pump, the diluted furnish was metered with a Foxboro
Flow Controller (total flow) through a five-pipe manifold
into the headbox.
Secondary System
The Black Clawson Secondary Flow Box was installed
over the fourth foil box and used to form the secondary
layer. The furnish was pumped from a 7,000-gal. stock
chest and metered with a Foxboxo Flow Controller into the
flow box.
The edges of the first press were taped to prevent
any pressure being applied to the sheet.
~5 All dryer cans were felted during the trial~
In all Examples, the secondary slurry was deposited
on the primary slurry at an angle of 4 15' from a height
of lOmm. The second slurry issued at a speed of around 8%
faster than the primary.
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Example 7 (HEPA filter)
Furnish - Primary slurry:-
90% Johns Manville glass 1212 microfibre
~average thickness 2.6-3.8 micrometres),
type 475 (borosilicate glass containing a
small quantity of zinc oxide).
10~ Johns Manville Chop Pak A20 BC ~" glass
fibre (average thickness 15 micrometres).
Water content when second slurry impinges -
93.32%
Weight basis of primary layer - 54g/m2.
Second slurry:
100~ Johns Manville glass 104 microfibre
(average thickness 0.34-0.48 micrometres)
type 475.
Water content - 99.765%.
Weight basis of secondary layer - 26g/m2.
Example 8 (HEPA filter) (Best Method)
Furnish - Primary slurry:-
90% Johns Manville glass 110 micro~ibre
(average thickness 2.17-3.10 micrometres)
type 475
~0% Johns Manville Chop Pak A20 BC ~ 99 glass
fibre.
Water content when second slurry impinges -
94.37%.
Weight basis of primary layer - 52g~m2.
Second slurry:-
100~ Johns Manville glass 100 microfibre
(average thickness 0.2-0029 micrometres~
type 47~.
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Water content - 99.765%.
Weight basis of secondary layer - 26g/m2.
This gave a particularly efficient HEPA filter, there
being a substantial density gradient from the coarse
primary to the fine secondary layer.
Example 9 (HEPA filter)
Furnish - Primary slurry:-
90% Johns Manville glass 110 microfibre
(average thickness 2.17-3.10 micrometres),
type 475.
10% Johns Nanville Chop Pak A20 BC ~" glass
fibre (average thickness 15 micrometres)~
Water content when second slurry impinges -
93.72%.
Weight basis of primary layer - 55g/m2.
Secondary slurry:-
100% Johns Manville glass 106 microfibre
(average thickness 0.49-0.58 micrometres).
Water content - 99.769%. ~
Weight basis of secondary layer - 29g/m20
Exam~le lO (Battery separator)
Furnish - Primary Slurry:-
90% Johns Manville glass llO microfibre, type
475
10% Johns Nanville Chop Pak A20 BC ~" glass
fibre.
Water content when second slurry impinges
94.41%.
Weight basis of primary layer - 75g/m2.
$
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- 30 -
Second Slurry:-
100% Johns Manville glass 108A microfibre
(average thickness 0.59-0088 micrometres)
type 753 (a zinc oxide free borosilicate
glass).
Water content - 99.755%.
Weight basis of secondary layer - 30 g/m2.
Example 11 (Battery separator~
Furnish - Primary slurry:-
90% Johns Manville glass 110 microfibres, type
475.
10% Johns Manville Chop Pak A20 BC ~" glass
fibre.
Water content when second slurry impinges -
91.60%.
Weight basis of primary layer - 155g/m2.
5econdary slurry:-
100% Johns Manville glass 108A microfibre, type
475.
Water content -`9~.752%.
Weight basis of secondary layer - 40g/m2.
Delamination tests were conducted on papers embodying
the invention by adhering double sided tape to both faces
of the paper and a pulling member to the other face of the
two sided tape. The pulling members were then pulled
apart from one another and the paper examined to determine
where tearing occurred.
With papers such as those of Examples 7-11, tearing
occurred in either the primary layer or the secondary
layer, the interface remaining in tact.
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