Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 01/21283 CA 02383975 2002-03-05
PCT/GB00/03538
1
FILTRATION MEDIA AND THE MANUFACTURE THEREOF
This invention relates to the manufacture of filtration media and in
particular to the
manufacture of electrostatic filtration media suitable inter alia for
respiratory
filtration applications, and to novel filtration media produced thereby.
Filtration media are widely used in many applications, for example for the
capture
of airborne particles (bacteria, dust etc). In such filters it is desirable
for the
resistance to airflow to be low, without sacrificing the filtration efficiency
(ie the
effectiveness with which the filter captures the airborne particles). A known
measure intended to achieve these objectives is the creation of electrostatic
charge on the filter material. Such a charge serves to attract the airborne
material.
One particular field of application of such electrostatically-charged filter
media is
respiratory filtration.
US 4,798,850 describes the formation of filter material with a felt structure
composed of a blend of clean polypropylene fibres and clean fibres of an
addition
polymer comprising one or more halogen-substituted hydrocarbons. The felt is
made by carding fibres into a web and needling them to form a coherent fabric
structure.
In the carding operation, fibres are worked by a series of toothed rollers,
which
serve to disentangle the fibre and provide some mixing to increase the
homogeneity of the blend. The product from the carding machine is a continuous
web, which is peeled from the last main roller on the machine (doffer). The
orientation of fibres in the web leaving the doffer is substantially dictated
by the
orientation of fibres leaving the doffer and is predominantly in the machine
direction. In carding, the assembly of the web takes place mainly on the
doffer
and fibres are controlled by fibre to metal friction in the machine. The web
is
subsequently layered to produce a so-called batt structure that is then
mechanically bonded.
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In general, it is desirable to be able to produce filtration media having
satisfactory
filtration efficiencies and low resistance to airflow, without having
excessively high
weight or thickness. It is also desirable to be able to achieve these
objectives
without having to resort to multi-layer structures in which the filtration
medium is
laminated with, or bonded to, other material.
There has now been devised an improved method of forming non-woven filter
materials which offers significant advantages over the prior art.
According to the invention, there is provided a process for the manufacture of
a
filtration medium, which process comprises air-laying fibres to form a non-
woven
web.
The process according to the invention is advantageous over the prior art in
several respects, including the following:
(i) The fibre orientation in the web is more random (owing to the dispersion
of
loose fibres in air immediately before web formation). Web properties are
consequently more isotropic.
(ii) No carding step is required (as compared to the prior art) and
consequently
the resulting structure does not consist of individual layers of web assembled
one
on top of the other. A single integrated structure is produced.
(iii) The air-laid web structure can be characterised by pronounced
orientation
in the z-direction (or perpendicular to the web surface). This gives the
structure
higher bulk (for a given area density) than a carded web.
(iv) Using the sifting air-lay approach, fibres of 2-12mm can be converted
into
uniform web structures (in contrast to the prior art, which permits only
lengths of
typically 30-200mm to be processed (due to restrictions imposed by carding).
(v) A shorter web formation process is achieved as compared to carding.
(vi) Providing it is clean, short, waste fibres (eg polypropylene) can be used
in
the process assuming the length is at least 2mm. Such short fibres are
incompatible with the carding process.
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In the air-laying process, the manner of web formation is substantially
different
from the prior art and marked differences in fabric properties are obtained.
In air-
laying, fibres are transferred to either
(a) a rapidly rotating cylinder or roller clothed with teeth and interacting
with
either other toothed rollers or fixed carding plates or
(b) a sifting screen or rotor device in which fibres are circulated over a
mesh
screen and then passed through an air-stream to form a web structure.
The former approach (roller-based air-laying) is presently preferred. In both
processes, the mechanical working treatment is much shorter than that used in
carding but is sufficient to electrostatically charge the fibre. In contrast
to carding,
the effect can be created solely at the site of interaction between the feed
rollers
and the opening roller. No further working points (e8 worker rollers) are
required.
Electrostatic charging of the fibres is believed to be achieved as the fibres
are
separated between a set of feed rollers and a single rapidly rotating roller,
or as
they are contacted by the rotors and mesh yarns of the grid. Multiple rollers
as
used in carding are not required. In further contrast to carding, the charged
fibres
are then dispersed freely in a moving air stream to form an air/fibre mixture.
The
air then transports fibres from the rotating cylinder (or sifting area) to a
suctioned
mesh conveyor belt, screen or drum where the fibres are landed to form the
web.
The belt/drum acts as an air/fibre separator. The process is continuous and
web
weight depends on the speed of the landing drum or conveyor.
After web formation, consolidation of the web structure may be achieved using
needle-punching.
The weight of the filtration media produced in accordance with the invention
may
be varied from approximately 2008/m2 up to 10008/m2. For respiratory filter
applications basis weights in the range 350-500g/m2 would normally be
selected.
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To improve or modify performance characteristics (e8 flow resistance,
filtration
efficiency, dimensional stability and fluid transmission) ready-made fabrics,
scrims
or films can be attached to fabrics produced in accordance with the invention.
As mentioned above, the properties of the web formed in the process according
to
the invention are more isotropic than in the prior art. This may manifest
itself in a
lower ratio of the tensile strengths of the web in the machine and cross
directions
(MD:CD), ie the longitudinal and transverse directions of the web as it is
manufactured. Thus, according to a second aspect of the invention there is
provided a filtration medium comprising a non-woven web of fibrous material,
said
web having an MD:CD ratio of less than 2:1. More preferably, the MD:CD ratio
is
less than 1.5:1.
Preferably, a blend of two or more types of fibre is used in the process of
the
invention. Most preferably, the blend comprises (a) a polyolefin and (b) an
addition polymer comprising one or more halogen-substituted hydrocarbons. The
former component of the blend is preferably polypropylene and the latter may
be,
for instance, polyvinylchloride or polyvinylidene chloride.
The blend may contain other fibres, either alternatively or in addition to
those
mentioned above. Examples of other fibre types which may be included are
polyethylene and "modacrylic", ie a copolymer comprising from 35 to 85 weight
percent acrylonitrile units and preferably having the balance made up
substantially
of other addition polymer-forming units, being halogenated hydrocarbon such as
vinyl chloride or vinylidene chloride.
The components of the blend may be present in any suitable proportions.
Preferably, the weight ratio of (a):(b) is in the range 70:30 to 30:70. Most
preferably, the two classes of fibre are present in approximately equal
proportions
ie in each case between 45% and 55% by weight.
Preferably, the linear density of the two classes of the fibres in the blend
is similar
and is in the range 0.1 - 10 dtex (dtex = weight in grams of 10,OOOm of
fibre).
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Most preferably, the fibres are of less than 3.3 dtex. In terms of fibre
diameter, the
diameter is most preferably 12Nm or less.
The fibres are preferably substantially free from any fibre finishes, oils or
other
5 extraneous matter prior to blending. Such chemicals are ideally removed from
the
fibres by an aqueous scouring process using a solution containing a synthetic
detergent, sodium carbonate or a potassium carbonate solution. Other scouring
regimes may also be suitable. The scouring process should be followed by
thorough rinsing and drying stages prior to further processing.
Likewise, all mechanical processing machinery must be thoroughly cleaned,
preferably by chemical means, to remove all fibre finish, waxes, grease, anti-
static
agents or other chemical residues.
Currently preferred embodiments of the invention will now be described in
greater
detail, by way of illustration only, with reference to the accompanying
drawings, in
which
Figure 1 is a schematic diagram of a roller-based air-laying process; and
Figure 2 is a schematic diagram of a sifting-based air-laying process.
Roller-Based Air-Laying
Roller-based systems can take many forms. A basic embodiment is illustrated in
Figure 1. In a roller-based air-laying process raw fibres are transferred
first from a
feed conveyor 11 to a clothed feed roller system 12 and then to a rapidly
rotating
cylinder 13 which is clothed with teeth and interacts with fixed carding
elements
14,15 or some other clothed surface (eg clothed rollers). Electrostatic
charging of
the fibres is achieved as the fibres are opened on the clothed cylinders
12,13. An
air knife 16 displaces fibres from the cylinder 13 on to a perforated conveyor
17 to
which suction is applied from below. A non-woven web of fibre is built up on
the
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6
perforated conveyor 17 from which the web is drawn off and consolidated by
needle-punching.
Siftin4-Based Air-Laying
An example of a sifting-based air-laying process is illustrated in Figure 2.
In such a
process, loose fibre is contained within a drum 21 having a grid 22 at its
base.
Rotors 23 within the drum 21 displace fibres in an air stream on to the top
surface
of a perforated conveyor 24, to which suction is applied from below. Again,
the
non-woven web is built up on the conveyor from which it is drawn off and
consolidated by needle-punching. Airflow in the system is constrained between
a
pair of rollers 25,26, the downstream one of which 26 also applies compression
to
the web. Other systems that use rotating rollers or brushes instead of a
static grid
and rotors may also be used.
Gihrc Rlcniie
Examples of fibre blends which may be used are:
a) Polyvinylchloride / Polypropylene
b) Polyvinylchloride / Modacrylic / Polypropylene
c) Polyvinylchloride / Polypropylene / Polyethylene
d) Polyvinylchloride / Modacrylic / Polyethylene
In each case, the proportion of PVC in the blend is approximately 50%. All the
fibres have diameters of 12Nm or less and lengths in the range 2 to 12mm.
Experimental results have indicated that the method of the invention provides
marked performance benefits in the filter media compared to the prior art:
RdamsonJon 0115 92471 e_4
I I
180/58lP/WO
CA 02383975 2002-03-05
(i) Up to a 20% reduction in the weight of the fabric can be achieved whilst
maintaining a bacterial filtration efficiency of at least 99.9997°!0.
(ii) Up to a 39% reduction in the resistance to flow can be achieved (compared
to the existing art) whilst maintaining a bacterial filtration efficiency of
at least
99.9997%.
(iii) Bacterial filtration efficiencies of at least 99.99997% can be achieved
with a
single layer air-laid structure. No laminated or incorporated layers (e8
meltblown
fabrics) are required.
Typical results (resistance to flow and filtration efficiency) for fabrics
produced
using the method of the invention (specifically, the roller-based air laying
approach) are given in Table 1. These samples were a 50:50 blend of
polyvinylchloride and polypropylene.
Table 1
Tvaical Test Results for Air-Laid Media
Sample Fabric weightResistance to Bacterial Filtration
ref flow
(glmz) Efficiency (%)
(a~fi0 Ilmin (cmH20)
2E 402 1.4 99.9997
6C 433 1.8 99.9994
4E 483 1.6 _ - . 99.9998
6B 491 2.1 89.999
4B 529 ~ .8 99.999
7A 597 2.1 >99.999991
AMENDED SHEET
FIADfa(IXS121i 3~VI(L~ IJ~4U
HdamsonJon ' 0115 92471
180158IPIW0
CA 02383975 2002-03-05
AN tests were carried out on a pad of the respective fabric measuring
7.5x5.3cm
and welded into a plastic housing with 22mm cylindrical inlet and outlet.
'Resistance to flow was measured in accordance with BS EN ISO 9360-1:2000.
For bacterial efficiency, no standard currently exists. However, all products
were
tested in accordance with the former draft standard prEN 13328-1 Part 1.
AMENDED SHEET
FIADfafIgS181T ;i.VRi. 10:4U
