Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates to a non-woven material for the
support of a semipermeable membrane.
Semipermeable membranes are known to be technologically
weak and therefore require technological backing by a supporting element.
They are used, for example, in installations which operate on the principle
of reverse osmosis and ultrafiltration. They are supported, for example, by
perforated pipes or plates, i.e. porous and comparatively pressure-resistant
supporting materials, for example in the form of PVC or fine steel pipes, or
slabs of plastic. Since the perforation or channel interval of such support-
ing materials, on account of the pressures often made necessary by the processesinvolved, e.g. 1-140 bar~ must be kept fairly high for reasons of technological
strength, a mass that is non-compressible during operation is used as a drain-
age backing layer between the semipermeable membrane and the supporting element.
The most common semipermeable membranes are made of
cellulose acetate. The cellulose acetate is applied by extrusion in a fluid
state to a porous non-woven backing fabric in a layer thickness of about 100
to 200 ~m. During this coating procedure, the underside surface of the
cellulose acetate layer assumes all the irregularities of the surface of the
non-woven fabric which produces variations in the thickness of the cellulose
acetate layer. These variations are subsequently translated to the semi-
permeable membrane which is formed on the surface of the cellulose acetate
layer. The variations in the thickness of the semipermeable layer results in
differences in the filtering properties of the final product. Thus there is
a need to provide a non-woven backing material which has a very smooth surface
in the micro range.
; The cellulose acetate layer has a thickness in the range
of 0.1 to 0.2 mm. Therefore, cross drainage of fluids which have penetrated
into its surface cannot be expected. In order to obtain a low passing
resistance in spite of this fact, it is necessary that the non-woven fabric
employed as the carrier or backing material have a high porosity. After
subsequent coagulation of the cellulose acetate layer, the active membrane
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~roper is formed on this backing in a thlckness of 500 to 5,000 angstrom units.
The use of backings made from flat textile materials, for
example fabric, paper or non-woven material is already well known. So far,
the results obtained in this way have not been satisfactory. Great difficult-
ies arise owing to the unavoidable irregularities in the micro region in such
materials. In the case of fabrics, their structure was frequently reflected
on the surface of the highly sensitive membrane, and this could result in
hairline cracks and eventual destruction of the membrane.
Cellulose-based papers have poor technological strength
properties, and in the course of use are bacterially destroyed in a
comparatively short time. Papers of 100% synthetic composition with the
necessary high technological strength properties cannot be produced according
to methods known in the present state of the art.
For this reason the use of non-woven material has been
adopted. Non-woven materials of 100% synthetic, thermoplastic fibres, pre-
ferably polyester or polypropylene fibres produced by a dry method, e.g. by
teaseling or carding and strengthening in an essentially known manner by the
application of heat and pressure, have thus far proved the most suitable.
Such non-woven materials have extremely high wet strength properties.
A basic disadvantage of these dry-manufactured non-wovens,
however, is the lack of uniformity in their non-woven pattern, i.e. an inade-
quate distribution of individual fibres, together with a roughness of surface.
In transversely laid non-wovens, moreover, there is an uneven weight and
thickness distribution over the production width.
A certain improvement in the weight and thickness
distribution has been achieved by combining transversely and longitudinally
laid non-woven materials and calendering the materials. Even so, however,
a certain lack of uniformity remains over the production width, especially
higher weights and smaller thicknesses in the edge zones compared with the
centre zone of the non-woven material. This lack of uniformity of the non-
woven material results in a lack of uniformity of the membrane thickness
when the non-woven material is subsequently coated with the membrane
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solution so that irregular membrane properties are obtained. Since these
irregular membrane properties are unacceptable, comparatively wide edge ~ones
have hitherto had to be discarded before or after coating with the membrane.
This leads, of course, to a substantial increase in the total cost of membrane
production.
Aerodynamically laid non-woven or spun non-woven materials
have greater uniformity, to be sure, as far as their weight and thickness
distribution over the production width is concerned. However, their
comparatively irregular non-woven pattern owing to an irregular individual
fibre scattering or individual fibre distribution, again leads to comparatively
irregular membrane thicknesses when the non-woven material is subsequently
\ coated with the membrane solution, and hence to comparatively irregular
membrane properties.
Difficulties of the kind described have been difficult to
avoid owing to the specific characteristics of known textiles. Control
operations that were difficult to execute, as well as an extremely high rate -
of rejection, therefore have to be accepted and.result in an extreme increase
in the cost of these membranes.
The present invention seeks to develop a physiologically
unobjectionable backing, which is chemically, physically and thermally
resistant to a high degree and which avoids the aforementioned disadvantages
and would ensure the production of a semiper~eable membrane with excellent
uniformity by a comparatively simple manufacturing process.
Thus the present invention provides a non-woven material
for use as a backing for a semipermeable membrane which comprises bonding
fibres, the distribution of which corresponds to that of a wet-laid non-woven
material, the bonding fibres being fused and flattened, without restriction
of the porosity of the material in the surface region of the material.
The non-woven material of the present invention has fibres
which are oriented in the same way as those of a wet laid non-woven material
and are thermoplastically bonded and progressively compacted without restrict-
ing the porosity in the surface region.
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Th~s the present invention provides a permeable, bonded,
non-woven material for use as a carrier for a semi-permeable membrane,
comprising a calandered, wet-laid mixture of thermoplastic bonding fibres
of size 4 to 30 dtex and optionally non-bonding fibres of size 1 to 7
dtex, and the top surface of the material being comprised substantially of
bonding fibres which provide a smooth, highly porous upper surface layer.
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According to a special development, provision is made to
strengthen the backing by the use of a thermoplastic bonding agent in the
form of thermoplastic bonding fibres, which may or may not be blended with
nn-bonding fibres. The bonding fibres have a cut length between 6 mm and
36 mm, preferably 12 mm, and the non-bonding fibres a cut length between 3 mm
and 18 mm, preferably 6 mm. The thickness of the bonding fibres employed is
preferably between 4 - 30 dtex, preferably 6.8 dtex, that of any non-bonding
fibres used between 1 and 7 dtex, preferably between 1.3 and 3.3 dtex. The
term dtex is an international unit which measures the weight in grams of a
fibre which is 10,000 m. long. The term is a synonym for the thickness of
the fibre. The backing consists preferably of 10 to 100% by weight bonding
fibres and 90 to 0% by weight non-bonding fibres. As bonding fibres
unstretched polyester fibres and/or polypropylene fibres are especially suitable.
The desired use properties can be modified in a quite special way by the
specific properties of the non-bonding fibres added. Stretched polyester
fibres and/or polyamidimide fibres and/or fibres of aromatic polyamides have
proved particularly suitable.
The present invention also provides a method for making a
non-woven material for use as a backing for a semipermeable membrane which
comprises suspending in aqueous medium a mixture of non-bonding fibres and
bonding fibres, dewatering the suspension on a screen to obtain a non-woven
material in which the fibres are uniformly distrihuted with the bonding
fibres lying on the top surface of the material and calendering the non-woven
material at a temperature and a pressure whereby the bonding fibres are fused
and flattened.
As indicated previously, the bonding fibres comprise from
10 to 100 parts by weight and the non~bonding fibres comprise from 0 to 90
parts by weight of the non-woven material, the non-bonding fibres being
relatively thinner and shorter than the relatively coarse bonding fibres. This
difference in the fibre size results in the distribution of the fibres on the
screen during dewatering described previously.
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Parallel to the surface of the non-woven fabric, all fibres
have an almost uniform distribution. Directional effects as they are normally
found in dry laid non-woven fabrics are not found in the fabrics of this
invention.
After the dewatering step is completed, the resulting
non-woven fabric possesses an extremely smooth surface. This smoothness is
further increased by submitting the non-woven fabric to a heat calendering
process. As indicated above, this calendering process results in the binding
fibres at the surface of the fabric fusing. This fusing results in a rein-
forcement of the non-woven fabric. ~owever, the basic structure of the
individual fibres is not destroyed, and on microscopical examination, the
original distribution of the individual fibres is still visible. Only the
surface of the abric has been flattened.
The selection o the calendering conditions is readily
determined by a person skilled in the art. The desired result is readily ~-
apparent from an examination of the surface of the non-woven fabric.
The method of the present invention produces a non-woven
fabric which has in the micro range thickness variations of less than + 1%.
Preferably the non-woven bac~ing material is manufactured
by the method of this invention in a layer thickness of, for example, 150 x 10 6
m. The uniformity of w2ight and thickness distribution over the production
width of the non-woven fabric is substantially better than that of a non-
woven fabric produced by the previously described methods. The non-woven
patterns, i.e. individual fibre arrangement and individual fibre distribution,
are substantially better with the method of this invention than with a dry laid
method. The surface of the non-woven material obtained by this method is ~ ~-
substantially smoother than the surface of a dry laid non-woven material. .
The resulting non-woven material has a very high isotropic
strength which has been found to be of great importance for most applications
in the manufacture of semipermeable membranes.
The following example is used to illustrate the present
invention.
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Example 1
A mixture of 75% by weight of undrawn fibres having a titre thickness of
6.8 dtex and a length of 12 mm with 25% by weight of drawn polyester fibres
having a titre thickness of 1.8 dtex and a length of 6 mm was suspended in water.
The content of solid material was 0.8% by weight. The so formed suspension
was led to a screen device similar to a paper machine and once again diluted
in the head box with water so that the content of solid material was reduced
to 0.2% by weight. The water was drawn off by applying a vacuum to the under-
side of the screen and the speed was adJusted so that a non-woven fabric having
a weight per unit area of 175 g/m with a moisture content of 85% by weight
was deposited thereon. The excessive water was removed from the non-woven
fabric by means of a vacuum rotary drier. Air was passed through at a
temperature of 140 C until a moisture value of 40% was obtained. Immediately
thereafter, the non-woven fabric was submitted to a calender treatment with a
calender consisting of a steel and a cotton roller having a diameter of 350 mm
each, the surface of the non-woven fabric containing the binding fibres abutting
the surface of the steel roller. The calendering was carried out at a temperat-
ure of 200 C with a linear pressure of 130 kg/cm and a speed of 3.5 m/min. The
resulting non-woven fabric was found to have a surface which was extremely
~` 20 smooth and free of projecting fibre ends or slings.
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