Note: Descriptions are shown in the official language in which they were submitted.
CA 022~638 1998-11-19
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Device ~n~l Process for Produci~g Cellulosic Tubular F;lm~
The present invention relates to a device for the production of cellulosic tubular films by
extruding a solution of cellulose in an aqueous tertiary amine oxide into a precipitant present
below the device, which device comprises an extrusion die having a substantially annular
extrusion gap, wherein a supply duct for precipitant and a discharge duct for used precipitant
are provided in the interior of the ring formed by the extrusion gap. The present invention also
relates to a process for producing cellulosic tubular films.
From U.S. Patent No. 2,179,181 it is known that tertiary amine oxides are capable of
dissolving cellulose and that cellulosic molded bodies such as fibers may be obtained from
such solutions by precipitation. A process for producing solutions of this kind is known, for
instance, from EP-A - 0 356 419. According to that publication, a suspension of cellulose in
an aqueous tertiary amine oxide is initially prepared. The amine oxide contains up to 40 % by
mass of water. The aqueous cellulose suspension is heated and water is drawn off under a
pressure decrease until the cellulose is dissolved.
For the production of cellulose fibers it is known from DE-A - 28 44 163 to provide an air
path or air gap between spinneret and precipitation bath in order to obtain a spinneret draft.
That spinneret draft is necessary, since stretching of the threads is rendered very difficult after
a contact of the formed spinning solution with the aqueous precipitation bath. The fiber
structure adjusted in the air gap is fixed in the precipitation bath.
A process for producing cellulosic threads is, furthermore, known from DE-A - 28 30 685,
according to which a solution of cellulose in a tertiary amine oxide is molded to filaments in
the hot state, the filaments are cooled with air and subsequently introduced into a precipitation
bath in order to precipitate the dissolved cellulose. The surfaces of the as-spun threads, in
addition, are wetted with water in order to reduce their tendency to adhere to neighboring
threads.
A device for producing seamless tubular films is known from WO 93/13670. According to
that known process, the cellulose solution is molded to a tube by an extrusion die having an
annular extrusion gap, which tube is drawn over a cylindrical mandrel and introduced into a
precipitation bath. In order that the extruded tube will not adhere to the surface of the
mandrel, its surface is coated with a water film in a manner that the inner side of the tube
coagulates and slides over the cylindrical mandrel. This has, however, the disadvantage that
the water fed for wetting the surface of the mandrel can ascend as far as to the extrusion gap
thereby wetting the die lip and causing the extrusion die to be cooled in addition to inducing
undesired coagulations already during the extrusion procedure proper. This is undesired,
CA 022~638 1998-11-19
because the cooled die cools the solution to be extruded, whose viscosity will thereby increase
so as to render impossible a perfect extrusion to films having uniform thicknesses. Add to this
that the known device is only cumbersome to reconvert if, for instance, films having different
thickn~sses are to be produced.
From EP-A - 0 042 517 a process for producing a dialysis membrane of cellulose is known, in
which membranes are produced from flat films or tubular films or hollow threads by means of
applopliate dies.
From WO 95/35340 a blowing process for producing oriented cellulose films by spinning a
cellulose solution in a plecipil~lion bath is known, in which the solution is extruded through a
film blowing die and an external air gap downwardly into the precipitation bath. The degree
of orientation may be increased by longitudinal and transverse stretching.
From DE-A - 195 15 137 a process for producing tubular films is known, according to which
a cellulose solution at first is extruded to a tube, which tube on its way from the annular die
outlet to the entry into the precipitation medium is stretched in the direction of extrusion and
widened, i.e., extended at a ratio ranging between 1: 1 and 1: 10 by a gas pl es~ul e prevailing in
the tube interior. By that extension, the tube is thus stretched transverse to the direction of
extrusion.
A device of the initially defined kind, i.e., for producing cellulosic tubular films by extruding
a solution of cellulose in a tertiary amine oxide into a precipitant present below the device,
which device comprises an extrusion die having a subst~nt~ y annular extrusion gap, wherein
a feed duct for pl~cipil~ll and a discharge duct for used precipitant are provided in the interior
of the ring formed by the extrusion gap, may be taken from WO 95/07811. In that device, a
spacer disc may be provided below the discharge duct in order to prevent the extruded tubular
film from collapsing in the precipitation bath.
The prior art devices for the production of tubular films are complex in structure. It is,
therefore, the object of the present invention to provide a device for the production of tubular
films, which is of simpler construction.
The device according to the invention for producing cellulosic tubular films by extruding a
solution of cellulose in a tertiary amine oxide into a precipitant present below the device,
which device comprises an extrusion die having a substantially annular extrusion gap, wherein
a supply duct for precipitant and a discharge duct for used precipitant are provided in the
interior of the ring formed by the extrusion gap, is characterized in that the supply duct for the
precipitant tt?lmin~tes below the discharge duct for the used precipitant.
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It has been shown that in the device according to the invention the liquid level in the interior
of the tube may be adjusted and, above all, kept constant in a simple manner. Furthermore, it
has proved advantageous to the precipitating procedure? if the precipitant is allowed to stream
opposite to the direction of extrusion, i.e., transport direction into the precipitation bath, in the
interior of the tube. The present invention differs from the prior art also in this respect, since,
as will be described below, fresh precipitant always gets into contact with film regions in
which the cellulose is present in the precipitated state already to a high degree. On the other
hand, the precipitant enriched most with amine oxide gets into contact with film regions in
which the cellulose is present to a yet slightly plecipil~ted extent.
A preferred configuration of the device according to the invention is characterized in that a
spacer is provided below the extrusion gap, which spacer preferably is designed to have a
substantially circular periphery.
The cross sectional area of the spacer may be larger than the area formed by the ring of the
extrusion gap. This enables stretching of the tubularly extruded solution transverse to the
transport direction.
The cross sectional area of the spacer may, however, also be smaller than the area formed by
the ring of the extrusion gap.
The spacer preferably is designed such that the size of the cross sectional area may be
changed. This is feasible, for instance, with a disc, or a ring, including displaceable members
in a manner analogous to an apertured diaphragm, in which the displaceable parts may slide
on or within one another, thereby enabling the external diameter to be changed.
A change in the size of the cross sectional area may, furthermore, be reached in that the spacer
is comprised of an elastic ring, for instance of rubber, to which air, water or the like may be
fed. The diameter of the elastic ring is, thus, controllable via the medium contained in its
interior. A variant of this embodiment consists in that no complete ring is used, but the inner
side is comprised of a solid part of, e.g., steel or plastics and an elastic part is attached to the
same. Another embodiment consists in a semi-shell, optionally comprised of segments, whose
fixed point is displaceable, thereby ch~nging the external diameter (umbrella principle).
The invention also relates to a process for producing cellulosic tubular films, in which a
solution of cellulose in a tertiary amine oxide is extruded through an extrusion die having a
substantially annular extrusion gap, wherein the solution is molded in a tubular manner, which
solution is transported into a precipitation bath and contacted with precipitant both on its irmer
CA 022~638 1998-11-19
side and on its outer side, whereby the cellulose is precipitated and the tubular film is formed,
which is characterized in that the precipitallt with which the tubularly molded solution is
contacted on its inner side contains dissolved tertiary amine oxide whose concentration
decreases in the transport direction of the tubularly molded solution.
In the process according to the invention, the tubularly molded solution may be stretched in
the transport direction andlor transverse to the transport direction.
Stretching of the tubularly molded solution transverse to the transport direction may be
effected by hydrostatic p,es~u,e or by gas pres~ule or by means of a spacer.
The tubular film molded by pleci~ila~ g the cellulose after transportation through the
precipitation bath is dried, wherein the tubular film is preferably kept under tension in order to
prevent shrinking.
The process according to the invention is particularly suitable for processing aqueous
solutions of cellulose in aqueous N-methylmorpholine-N-oxide (NMMO).
A plef~lled configuration of the device according to the invention will be explained in more
detail by way of the annexed drawing.
Fig. 1 schematically shows a cross section through the lower part of an extrusion device
substantially comprising an annular die 1 having an annular extrusion gap 2, a supply duct 3
for fresh precipitant (water or an amine oxide/water mixture) and a discharge duct 4 for used
precipitant.
The cellulose solution is pressed through the extrusion gap 2 via the annular spinning mass
space 5, thereby extruding the cellulose solution in the form of a tubular film 6 into the air
space present between the precipitation bath surface 7 and the lower side 8 of the annular die
1.
The tubularly extruded solution 6 is drawn off into the precipitation bath 7, in which it gets
into contact with precipitant on its outer side, thereby causing the dissolved cellulose to
coagulate and the amine oxide to be released into the precipitation bath. Precipitant is also in
the interior of the tube 6 such that the cellulose coagulates also on its inner side. In doing so,
amine oxide is released again.
In the Figure, the level 8 of the precipitant present in the interior of the tube 6 is on equal
height with the precipitation bath 7. Precipitant may be introduced into the interior of the tube
CA 022~638 1998-11-19
6 via the supply duct 3 and drawn off via the discharge duct 4. In this manner, the level 8 of
the precipitant may be adjusted in the interior of the tube 6.
In the process according to the invention, the level 8 of the precipitant present in the interior
of the tube 6 also may be adjusted to be higher or lower than the level of the precipitation bath
7. The composition of the precipitant in the interior of the tube 6 may differ from that of the
precipitation bath.
The tubular film 6 is drawn off via deflection means 10 and thereby stretched in the transport
direction, i.e., in the direction toward the precipitation bath 7.
The tubular film 6 is drawn over a spacer 9. That spacer 9 has the form of a circular disc
firmly connected with the die 1 by means of rods 11. The spacer 9 comprises continuous bores
9a for mass transfer. The supply duct 3 passes through the spacer 9. The disc also may be
replaced with a spacer ring.
The tube 6 is extended by the spacer 9, this corresponding to stretching transverse to the
transport direction. It is evident that such stretching transverse to the transport direction
increases with the size of the circular spacer.
By feeding fresh plecipitant via the supply duct 3 and discharging used, i.e. amine-oxide-
cont~ining, precipitation bath via the discharge duct 4, which terrnin~tes above the supply
duct 3, the concentration of amine oxide increases in the direction toward the level 8 or, in
other words decreases in the transport direction of the tubular film 6.
On the end of the supply duct 3 there may also be provided baffle plates (not illustrated) in
order to divert the liquid stream from the supply duct 3 upwardly in the direction of the level
8.
Stretching transverse to the transport direction may also be obtained by means of hydrostatic
pres~u-e instead of the spacer. To this end, the level 8 of the precipitant within the tube 6
simply is adjusted to be higher than the level 7 of the precipitation bath. In doing so, extension
is caused by the pressure of that part of the precipitant present in the interior of the tube 6,
which is above the level 7.
Extension also may be obtained by means of gas pleS~ule, wherein the die 1 must, of course,
be provided with a gas feed (not shown), by means of which the tubular film 6 is infl~te~l in
the space provided between the lower side 8 of the die and the precipitation bath 7. The gas
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may also be replaced, which means that operation is effected at a gas excess. In that case, also
a gas discharge duct (not illustrated), of course, is to be provided on the die.
