Note: Descriptions are shown in the official language in which they were submitted.
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FILTER MEDIA
The present invention relates to adsorbent media,
particularly of a type formed from a sheet-form carrier
substrate incorporating particulate adsorbents therein, and
uses thereof in filtration of fluids i.e. by gas/solid
liquid/solid interactions wherein the contaminant atom, ion,
or molecule of gas or liquid adheres or is otherwise retained
by the solid adsorbent particle thereby removing it as a
contaminant from the filtered fluid. The invention thus finds
application in the adsorption of toxic, odiferous, nuisance,
radioactive or irritant chemicals from a gaseous or vapour
atmosphere, or from a body of liquid, particularly, but not
exclusively, in removal of dissolved organic substances from
aqueous liquids for example for personal protection.
Known adsorbent materials include activated carbon,
zeolites, silica and metal oxides including alumina which are
often used in particulate form for the purposes of maximising
the adsorbent surface. EP-A-0 432 438 describes a deodorising
unit for air cleaners and air conditioners formed from a
moulded mixture of such adsorbent particles, fine plastics
particles and a reinforcing fibre.
A water permeable container filled with a granular
adsorbent is described in JP-A-54 105 851, for use in
treatment of sewage, industrial effluent etc., containing
harmful substances such as heavy metals, N compounds,
phosphates etc. The granular adsorbents mentioned for that
purpose include activated carbon, ion exchange resins, chelate
resins, calcium silicate, powdered shell, zeolites, coal,
activated alumina, magnesium aluminate, and porous synthetic
resins.
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An adsorption filter element comprising a granular
adsorbent, such as active carbon, zeolite, silica gel and
stone, bonded using a hydrophilic binder e.g. acrylate
synthetic resin, covered with a mesh spacer (opening less than
10 mm, thickness 0.2-5 mm) and laminated with filter sheet,
e.g. woven cloth and non-woven cloth is described in JP-A-82
94 609.
A protective mask is described in US-A-4 643 182 which is
formed from a porous non-woven sheet material which is made of
glass fibres and a suitable binding agent such as polyvinyl
alcohol, and which may optionally contain a gas adsorbent
substance. The mask is intended to protect a wearer from the
effects of toxic gases and the like. The glass fibres are
from about 0.5 microns to 4 microns in diameter and may be
from 0.1 inches to 0.25 inches in length. The gas adsorbent
particles described there include activated carbon particles,
crystalline molecular sieve particles or zeolite particles,
and the mask may also include anion exchange resin particles.
It is known to make an activated carbon filtration medium
by spraying a liquid or foam containing activated carbon
particles onto a substrate. An advantage of this process is
that the substrate dries quickly thus speeding up the
manufacturing process. In addition, this process results in a
large proportion of the free surface area of the activated
carbon particles being available for chemical adsorption.
However, a disadvantage of this process is that the activated
carbon particles are poorly held on or within the substrate
and as a result there is a danger of said particles being shed
during normal use.
It is also known to manufacture an activated carbon
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filtration medium by a liquid or foam immersion process
whereby a substrate is immersed in a liquid or foam containing
activated carbon particles. This process allows many
different types of substrate to be impregnated with activated
carbon particles, said substrates being chosen according to
the atmosphere to which the filtration medium is to be exposed
in use. It also allows for a more even dispersion of
activated carbon particles within the matrix of the substrate.
However, in order to ensure that the activated carbon
particles have a sufficient surface area available for
chemical adsorption of impurities, the activated carbon
particles are held generally by entrapment within the matrix
of the substrate. Thus, said activated carbon particles are
poorly held within the substrate matrix and are easily shed
during normal use.
Activated carbon filtration media can, for example, be
employed to adsorb toxic, odoriferous, nuisance, radioactive
or irritant chemicals from a gaseous atmosphere or to adsorb
dissolved organic compounds from a liquid atmosphere.
However, the problem of shedding of the activated carbon
particles from the substrate of a filtration medium can lead
to the failure of the medium and can in itself represent a
hazard particularly where the medium is incorporated in
devices or garments for the protection of personnel.
Shedding is undesirable with any adsorbent material but is
a problem which is particularly acute with regard to activated
carbon due to the blackening of surfaces contacted by the shed
particles. Thus handling of filtration media during assembly,
maintenance operations etc. is unpleasant and requires clean-
up operations and other precautions to avoid contamination
with the shed materials. Often such problems are addressed by
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use of binders which can reduce the effective surface
available for adsorbent activity.
An object of the present invention is to obviate or
mitigate the aforesaid problems. In particular it is an
object of at least one aspect of this invention to address the
shedding problem. Another object of the invention is to
provide improved filtration media suitable for general use in
fluid purification by adsorption of impurities from gaseous
and/or vapour atmospheres, or from a body of liquid. A
further object is to provide flexible adsorbent materials
which may be readily adopted in protective apparel. A still
further object is to provide an effective manufacturing
process for producing such adsorbent materials and filtration
media (hereinafter, simply "filter media").
According to a first aspect of the invention filter media
comprises a fibrous material, preferably a dry laid fibrous
web or the like flexible substrate incorporating an adsorbent
material in particulate form wherein the adsorbent material is
retained within the filter media using a fixing agent whereby
in use shedding of the adsorbent material therefrom is
inhibited.
The fibrous material is preferably provided as a non-woven
fabric, preferably as a dry laid fibrous web or the like
flexible substrate. Alternatively, the substrate may comprise
a woven fabric, a scrim fabric or any other suitable substrate
having a structure capable of retaining adsorbent particles.
Additional fluid-permeable fabrics may be incorporated in
the filter media, and may be of a different structure to the
fibrous material incorporating the adsorbent material. The
additional fluid permeable fabrics may incorporate different
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adsorbent materials. The filter media may be formed as a flat
flexible sheet which may be adapted to conform to a surface,
e.g. for the purposes of inclusion in a garment, as a
laminate, a scroll made up of such a sheet or laminate, or a
5 wadding.
According to another aspect of the invention there is
provided a process for forming a filter media comprising
providing a fluid-permeable carrier substrate containing
fibres which are selected from fibres capable of withstanding
prolonged contact with a fluid to be filtered, treating the
carrier substrate to introduce fixing agents including a
binder and a film-former, and particulate adsorbent
materials) to the carrier substrate, wherein the amount of
fixing agents, is just sufficient to retain the adsorbent
materials) in the carrier substrate whereby shedding of the
adsorbent therefrom is substantially avoided.
Preferably the filter media is formed by selecting a
fluid-permeable carrier substrate consisting essentially of
fibres and impregnating the substrate with a liquid containing
an adsorbent material in particulate form and fixing agents
for promoting particle retention in the substrate without
occluding the permeability of the substrate, and separating
the impregnated substrate and the liquid after a predetermined
period sufficient to obtain retention of the particulate
adsorbent material in the substrate.
A preferred liquid for imparting the desired non-shedding
characteristics contains a latex binder, such as an acrylics
or styrene butadiene latex or natural rubber based binder,
especially containing a film former and preferably also an
anti-foaming agent. The amounts are variable depending upon
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fibres selected but a limited degree of simple experimentation
is sufficient to determine the minimum amount of film former
adequate to obtain a satisfactory non-shedding characteristic.
The term film-former means a material capable of forming a
film when dry at standard conditions of temperature and
pressure (STP). Suitable materials include polyvinyl alcohol,
or polyvinyl alcohol/vinyl acetate copolymers, quaternary
ammonium salts of polyvinyl pyrolidine/vinyl acetate copolymer
amongst other similar film formers, but polyvinyl alcohol is a
preferred material.
Adsorbent materials may be selected according to the
intended use from adsorbents which would for the purposes
herein involve preferential selection from activated carbon,
zeolites, inorganic compounds such as silica and metal oxides
e.g, of titanium (TiOz), zinc (Zn0), and aluminium in
crystalline or micro-crystalline or fine particulate form and
mixtures of such adsorbents.
The activated carbon particles may comprise any one of
charcoal, peat, coconut shell, lignite and wood, but
preferably comprises charcoal or is lignite based.
Particle sizes are variable dependent upon the materials
selected but a useful working range is from about 0.1 to about
300 microns, preferably up to about 75 microns, and preferably
also not less than about 1 micron, more preferably not less
than 25 microns.
The carrier substrate material may be a non-woven fibre
fabric or similar sheet or mat capable of being impregnated
with an adsorbent-containing liquid. The impregnation may be
accomplished by a number of methods which would seek to
achieve saturation of the fibres to obtain uniform
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distribution of the adsorbent and fixing agents (treatment
liquor) throughout the fibrous material. These methods would
include application to the fibrous material by at least one of
the following treatments; saturation by soaking in a
convenient manner e.g. simply delivery of the appropriate
chemical treatment liquor from a hose over the fibrous
material; impregnation by immersion of the fibrous material in
a bath of the treatment liquor; forced impregnation into the
fibrous material by application of the liquor under pressure;
pouring of the treatment liquor over the fibrous material by a
curtain-coating device situated over a progressively advancing
web of fibrous material to drench the fibrous material;
spraying the treatment liquor upon the fibrous material; or an
equivalent treatment of a web or mat of the fibrous substrate.
The method chosen will be to some extent a matter of
convenience and many variants will be apparent to the skilled
plant designer. An immersion treatment such as dip-coating is
very flexible in that it allows simple equipment to be adopted
even for a continuous production, and permits a wide variety
of different substrates to be handled, with the additional
advantage of greater certainty of a substantially uniform
dispersion of the selected adsorbent throughout the carrier
substrate, provided that care is taken to ensure that the
period of contact with the chemical treatment liquor is
adequate to ensure that the fibrous material is heavily wetted
by that liquor. Thus, a batch dip bath or slow progressive
advancement of a web through a dip bath may be preferred for
thicker substrates. On the other hand a spray treatment
permits quick drying, rapid throughput with an overall fast
process rate, and also facilitates presentation of more of the
available free surface of the selected adsorbent. It will be
recognised that a spray coating plant can offer economies in
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plant volume by avoiding the need for the significant space
taken up by treatment baths and saturation holding tanks.
Thus a web of fibrous material of thin to medium thickness
suitably suspended between spray nozzles can be saturated
quickly by the pressure of the spray jets alone. Other ways
of saving space for such a treatment plant would include
passing a fibrous web under a header tank equipped with means
in contact with the web for applying treatment liquor under
pressure to forcibly impregnate the web to the desired degree
of saturation. The forced penetration may be achieved or
assisted by reducing ambient pressure on a surface of the web
opposite to the liquor receiving surface.
In selecting the treatment method those skilled in fibre
technology will naturally pay due attention to the physical
characteristics of the fibrous substrate to be treated. Thus
fibrous materials which may be disrupted by pressure
treatments would therefore be preferentially treated by low
pressure soaking treatment methods
The fibres to be used are preferably man-made durable
materials i.e. synthetic materials of good resilience and
impervious to, or at least highly resistant to degradation
under contact with the fluids) with which it is anticipated
normal usage as a filter media would expose the fibres to.
Thus one might readily consider polyesters, polyamides, and
the like. Preferably one would use polyester fibres in acidic
fluids, nylon or polyvinyl alcohol fibres in alkaline fluids,
and rayon viscose fibres for use in contact with vegetable or
mineral oil containing fluids. However these recommendations
are not exhaustive and are merely illustrative of the
materials contemplated for the purposes of this invention.
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The fibres may be between 0.5 dtex and 6 dtex but are
preferably in the range of 1 dtex to 3 dtex.
Preferably the fibres are synthetic materials formed into
a web, mat or similar flexible sheet-form substrate,
preferably as a non-woven fabric or as a laminar composite
material consisting of layers of woven and/or non-woven
fibres. Non-woven fabrics suitable for the purposes of this
invention are dry-laid fabrics which may be produced using a
carding process known in the art such as that process
particularly described in GB-A-2 151 667 to which reference is
directed. However non-woven fabrics are also obtainable by
entanglement processes, preferably by hydro-entanglement
processes known generally in the art of non-woven fabric
production. Such a fibre entanglement process subjects loose
fibres or filaments to a stream or streams of jetted fluid
e.g. using air streams (US-A-4 100 324) or using hydraulic
fluids (US-A-3 485 706) to intermingle the fibres.
The filter media contemplated for the purposes of this
invention preferably comprises a fibrous web which may contain
from 20% to 90°s(w/w), preferably from 30% to 60o by weight, of
the filter media sheet-form substrate.
The amount of fixing agents is suitably in the range of
from 5 0 (w/w) to 45 0 (w/w) , but preferably is from 15 0 (w/w) to
30o(w/w) in terms of the finished filter media product. In
relation to the treatment process, this is achievable using a
liquid containing the fixing agents, which may contain from
5o(w/w) to 95o(w/w) of a latex binder, preferably from
25%(w/w) to 65o(w/w) based on the solids (dry weight as
determined at STP) content of the treatment liquid. Likewise
for the film former material, the content thereof in a
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treatment liquor may be from 0.05o(w/w) and 5o(w/w), with a
preferred range of from 0.4%(w/w) to 1.2o(w/w). Where an
antifoaming agent is used, the amount thereof can be between
0.05o(w/w) and 5.Oo(w/w), the preferred range being from
5 1.5%(w/w) to 3.Oo(w/w).
Regarding the content of adsorbents) this would be in the
range of from 5o(w/w) and 75o(w/w), preferably from 15o(w/w)
to 65o(w/w) in the finished filter media. With regard to the
content of the adsorbents) in the treatment liquor, again
10 based on the dry weight as determined under STP, this would be
from 9o(w/w) to 99o(w/w), preferably from 1.5o(w/w) to
3.Oo(w/w). Ideally the particle size thereof is in the range
of 1 micron to 300 microns, preferably from 25 microns to 75
microns.
The treatment may be conducted by immersion in a bath of a
liquid containing the selected fixing agents and selected
adsorbent(s). This immersion treatment results in
impregnation of the carrier substrate. Alternatively the
liquid containing the selected fixing agents and selected
adsorbents) may be applied to a web of the carrier substrate
by a spraying process or by a curtain coating (drench or pour-
on) type of application.
According to a further aspect of the invention there is
provided an article of apparel, such as a protective suit,
face mask, industrial glove, apron or article of footwear,
including a fabric comprising fibrous materials, preferably as
a non-woven (e. g. dry-laid) fabric incorporating an adsorbent
material in particulate form which is retained therein by
means of a latex binder and a film former present in
sufficient quantities to avoid shedding of the particulate
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adsorbents) therefrom but permitting the fabric to retain a
high level of fluid permeability. The advantage of such
protective apparel is that the wearer may be protected against
skin contact with toxic or noxious substances in the air for
example.
According to a further aspect of the invention there is
provided a filter cartridge for location in a filtration
device comprising a non-woven fabric (e. g. dry-laid)
incorporating an adsorbent material in particulate form which
is retained in the fabric by means of a latex binder and a
film former present in sufficient quantities to avoid shedding
of the particulate adsorbents) therefrom but permitting the
fabric to retain a high level of fluid permeability. The
cartridge may include conventional components in addition to
at least one web, or layer, or bundle, or scroll or wadding
composed of the said dry-laid non-woven fabric treated in
accordance with the invention as hereinbefore described.
The invention will now be described further by way of the
following illustrative examples.
EXAMPLE 1
A filter media of polyester fibres having a length in the
range of 30 to 50mm and of between 1 and 3 dtex are formed
into a sheet which comprises a fibrous web using the carding
process detailed in GB-A-2 151 667.
The sheet may then be impregnated with particles of
activated charcoal by a process which involves immersing said
sheet for a predetermined period of time in a bath of
treatment liquor in which the activated charcoal is suspended.
In this particular example the activated charcoal has a
particle size in the range of 25 to 75 microns.
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The treatment liquor based on the dry weight as
determined under conditions of standard temperature and
pressure comprises 250 (w/w) of an acrylic butadiene, 20 (w/w)
of a polyvinyl alcohol, and an anti-foaming agent in a
suitable organic carrier medium. The bulk of the liquor is
made up of the particulate activated charcoal. The anti-
foaming agent is a carbon based compound.
During the immersion of the sheet in the treatment liquor
the activated charcoal particles are taken up into the
structure of the fibrous web and held in place by the binder
and the other fixing agents used.
Once the sheet has been immersed for a sufficient period
of time it is removed from the bath and the excess liquor
allowed to run off and dry. The final product is the filter
media.
In the filter media formed the activated charcoal remains
porous to the gas or liquid passing through the filter media
and also is not occluded to any significant degree by the
binding agent polyvinyl alcohol/acrylic butadiene which is
binding it to the fibres of the filter media.
EXAMPLE 2
A filter media of rayon viscose fibres having a length in
the range of 30 to 50mm and of between 1 and 3 dtex are formed
into a sheet which comprises a fibrous web using the carding
process detailed in GB-A-2 151 667.
The sheet may then be impregnated with particles of
activated charcoal by a process which involves immersing said
sheet for a predetermined period of time in a bath of
treatment liquor in which the activated charcoal is suspended.
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In this particular example the activated charcoal has a
particle size in the range of 150 to 175 microns.
The treatment liquor based on the dry weight as
determined under conditions of standard temperature and
pressure comprises 290 (w/w) of a styrene butadiene, and to
(w/w) of a polyvinyl acetate/Vinyl acetate copolymer in a
suitable organic carrier medium, and in this case no anti-
foaming agent is present in this case. The bulk of the liquor
is made up of the particulate activated charcoal.
During the immersion of the sheet in the treatment liquor
the activated charcoal particles are taken up into the
structure of the fibrous web and held in place by the binder
and the other fixing agents used.
Once the sheet has been immersed for a sufficient period
of time it is removed from the bath and the excess liquor
allowed to run off and dry. The final product is the filter
media.
In the filter media formed the activated charcoal remains
porous to the gas or liquid passing through the filter media
and also is not occluded to any significant degree by the
binding agent polyvinyl acetate/vinyl acetate
copolymer/styrene butadiene which is binding it to the fibres
of the filter media.
EXAMPLE 3
A filter media of nylon fibres having a length in the
range of 30 to 50mm and of between 1 arid 3 dtex are formed
into a sheet which comprises a fibrous web using the carding
process detailed in GB-A-2 151 667.
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The sheet may then be impregnated with particles of
activated charcoal by a process which involves immersing said
sheet for a predetermined period of time in a bath of
treatment liquor in which the activated charcoal is suspended.
In this particular example the activated charcoal has a
particle size in the range of 75 to 125 microns.
The treatment liquor based on the dry weight as
determined under conditions of standard temperature and
pressure comprises 350 (w/w) of a natural rubber material, 20
(w/w) of a polyvinyl alcohol, and an anti-foaming agent. The
bulk of the liquid carrier material is made up of the
particulate activated charcoal. The anti-foaming agent is
silicon based compound.
During the immersion of the sheet in the treatment liquor
the activated charcoal particles are taken up into the
structure of the fibrous web and held in place by the binder
and the other fixing agents used.
Once the sheet has been immersed for a sufficient period
of time it is removed from the bath and the excess liquor
allowed to run off and dry. The final product is the filter
media.
In the filter media formed the activated charcoal remains
porous to the gas or liquid passing through the filter media
and also is not occluded to any significant degree by the
binding agent polyvinyl alcohol/natural rubber material which
is binding it to the fibres of the filter media.
An activated carbon filtration medium not only performs
the function of a chemical adsorbate in a gaseous or liquid
atmosphere but also acts as a particulate filter by means of
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physical and/or electrostatic entrapment of particles
contained in a flow of gas or liquid passing through the
medium, thus providing a means of reducing the amount of
impurities in the gas or liquid. The activated carbon
5 filtration medium must remain porous to the gas or liquid to
be passed therethrough. It must allow easy passage of said
gas or liquid through the medium to facilitate contact of
activated carbon within the medium and any impurities
contained within the gas or liquid thus enabling the activated
10 carbon, through the process of adsorption and aided by
particle entrapment by the substrate structure, to reduce the
amount of impurities in the gas or liquid.
An activated carbon filtration medium should exhibit some
of the following features: a rapid adsorption of impurities
15 from the gas or liquid passing through the medium; a high free
surface area of activated carbon particles available for the
adsorption of impurities; good fixation of the activated
carbon particles in the filtration substrate in order to
prevent shedding of said carbon particles under normal
conditions of use; be porous to the gas or liquid to be passed
through the medium; and be capable of physical and/or
electrostatic entrapment of solid particles in the gas or
liquid. The present invention enables these functions to be
retained yet the shedding problem is solved by providing a
substrate which is impregnated with activated carbon
particles, wherein said particles are fixed within the
structure of the substrate such that the free surface area of
the carbon particles is not occluded by more than a minimum
amount required for the continued efficacy of the carbon
particles as an adsorbate for impurities in gaseous or liquid
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atmospheres and, in such a manner, that said filtering means
does not shed the carbon particles in use.
EXAMPLE 4
A filter media is formed and treated as detailed in
Example 1 except in this case the activated charcoal is
replaced with a particulate Zeolite.
EXAMPLE 5
A filter media is formed and treated as detailed in
Example 2 except in this case the activated charcoal is
replaced with particulate Titanic.
EXAMPLE 6
A filter media is formed and treated as detailed in
Example 1 except in this case the activated charcoal is
replaced with a particulate Silica.
EXAMPLE 7
A filter media is formed and treated as detailed in
Example 3 except in this case the activated charcoal is
replaced with a particulate Zinc Oxide.
EXAMPLE 8
A filter media is formed and treated as detailed in
Example 3 except in this case the activated.charcoal is
replaced with a particulate Aluminium Oxide.
EXAMPLE 9
A filter media is formed from vinyl acetate fibres as per
example 1 and treated as per said example.
EXAMPLE 10
A filter media of polyester fibres having a length in the
range of 30 to 50mm and of between 1 and 3 dtex are formed
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into a sheet which comprises a fibrous web using the carding
process detailed in GB-A-2 151 667.
The sheet may then be impregnated with particles of
activated charcoal by a process which involves spraying the
sheet a treatment liquor in which the activated charcoal is
suspended, so that it is saturated from all sides and in
effect immersing said sheet in the treatment liquor. In this
particular example the activated charcoal has a particle size
in the range of 25 to 75 microns.
The treatment liquor based on the dry weight as
determined under conditions of standard temperature and
pressure comprises 250 (w/w) of an acrylic butadiene, 20 (w/w)
of a polyvinyl alcohol, and an anti-foaming agent in a
suitable organic carrier medium. The bulk of the liquor is
made up of the particulate activated charcoal. The anti-
foaming agent is carbon based compound.
The effective immersion of the sheet in the treatment
liquor means that the activated charcoal particles are taken
up into the structure of the fibrous web and held in place by
the binder and any other fixing agents used. The final
product is the filter media.
In the filter media formed the activated charcoal remains
porous to the gas or liquid passing through the filter media
and also is not occluded to any significant degree by the
binding agent polyvinyl alcohol/acrylic butadiene which is
binding it to the fibres of the filter media.
EXAMPLE 11
A filter media of polyester fibres having a length in the
range of 30 to 50mm and of between 1 and 3 dtex are formed
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into a sheet which comprises a fibrous web using the carding
process detailed in GB-A-2 151 667.
The sheet may then be impregnated with particles of
activated charcoal by a process which involves continuously
dip coating the sheet in a trough of a treatment liquor in
which the activated charcoal is suspended, so that it is
saturated from all sides. In this particular example the
activated charcoal has a particle size in the range of 25 to
75 microns.
The treatment liquor based on the dry weight as
determined under conditions of standard temperature and
pressure comprises 25o (w/w) of an acrylic butadiene, 20 (w/w)
of a polyvinyl alcohol, and an anti-foaming agent in a
suitable organic carrier medium. The bulk of the liquor is
made up of the particulate activated charcoal. The anti-
foaming agent is a silicon based compound.
The effective immersion of the sheet in the treatment
liquor means that the activated charcoal particles are taken
up into the structure of the fibrous web and held in place by
the binder and any other fixing agents used.
The sheet is then allowed suspended so that the excess
liquor is allowed to run off and the sheet is then allowed to
dry. The final product is the filter media.
In the filter media formed the activated charcoal remains
porous to the gas or liquid passing through the filter media
and also is not occluded to any significant degree by the
binding agent polyvinyl alcohol/acrylic butadiene which is
binding it to the fibres of the filter media.
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EXAMPLE 12
A an article of protective apparel in the form of
surgical type face mask as shown in Figure 1 of the drawings
and comprising a central filtering portion 1 having a
surrounding edging strip 2, and securing straps 3 is formed
from the filter media as detailed in Example 1. This type of
mask is useful, for example to workers who are handling
radioactive and /or hazardous vapour emitting chemicals such
as mercury.
EXAMPLE 13
A filter media as per one aspect of the present invention
and as illustrated in Figure 2 of the drawings can be made
using the sheets as made and treated in accordance with the
present invention. The filtering media comprises a plastic
casing 10 which is designed to be push-fitted into separate
face covering breathing apparatus. The casing 10 comprises a
sidewall 11, a back-wall 12 having a number of perforations
12a formed therein, and a front-wall 13 formed in a lattice
type way and having a series of holes therein. Once fitted in
the breathing apparatus the passage of air to the user is via
the holes in the front-wall 13 through the filter media 14 and
out through the perforations 12a in the back-wall 12.
The filter media 14 is located and secured in the casing
10 so that any air being drawn through the filtering media has
to pass therethrough before it can be breathed in by the user.
The filter media 14 comprises a wad of material 15 which has a
number of individual layers at least one of which layers is a
sheet of material as made and treated in accordance with
Example 3.
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EXAMPIrE 14
A filter media as per Example 11 except the wad of
material 15 is with a number of layers of material made and
treated in accordance with the examples detailed above. The
5 wad can include several layers of a sheet made in accordance
with one example, for example Example l, or layers of sheets
made and treated in accordance with several of the examples.
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