Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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~TSODS OF MANZ1FACTQR.B OF HON9POV>3N FABRIC
This invention relates to a method of manufacture of a
aonwovea fabric made from cellulose and in particular from a
solution of cellulose,.
Cellulose fibres and filaments may be produced by
spinning a solution of cellulose in an amine oxide solvent
which is then leached into water or a dilute solution of
aqueous amine oxide, to produce cellulose filaments which can
then be cut into staple fibres. The process of extrusion and
coagulation is referred to as solvent spinning, and the fibres
of solvent-spun cellulose so produced are known under the
generic name of lyocel.l.
It is possible to produce small decitex fibres below 1.0
dtex by disintegrating staple fibres. However, this is costly,
and requires a high energy consumption.
The present applicant's international patent application
PCT/GB97/03391 discloses a method of manufacture of a nonwoven
cellulose fibre made from fibres formed by extruding a
solution of cellulose through at least one spinning jet and
subjecting the extrudate fibre to a high velocity gas flow,
the fibres being collected on a surface on which the fibre is
subsequently coagulated. The fibres tend to bond together
before coagulation forming a relatively dense nonwoven fabric
which is highly bonded.
In some applications a highly bonded, high density fabric
web is not desirable, for example for filtration materials,
or for materials which require a high water holding capacity.
The present invention provides a method of manufacture
of a nonwoven cellulose fabric having a high loft and a
relatively low density. -
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According to the present invention there is provided a
method of manufacture of a non-woven cellulose fabric in which
extrudate fibre produced from a spinning jet is subjected to
a high velocity gas flow, and the fibre is passed through a
vapour mist which at least partially coagulates the fibre
prior to its collection as a fibre web.
The extruded fibre is attenuated by the gas flow and may
be broken down into indefinite lengths.
The vapour mist may be formed from any suitable
coagulant. The mist is preferably an aqueous mist, although
lower alkyl alcohols, for example, may also be used.
Conveniently, atomised water may be sprayed from a nozzle
located below the spinning jet.
The cellulose solution is preferably a solution of
cellulose in an amine oxide solvent, typically a tertiary N-
amine oxide and in particular N-methylmorpholene-N-oxide
(NbIMO) . The cellulose solution may contain 4-22% by weight of
cellulose, preferably 10-15%, having a degree of
polymerisation of 200-5,000, and more typically 400-1,000.
In a preferred embodiment the cellulose solution
comprises 14% by weight of cellulose, 12% by weight water and
74 % by weight of NI~20, the cellulose having a degree of
polymerisation of about 600.
The attenuated fibre-forming microfibres or fibrils are
collected and are then further coagulated (alternatively
referred to as being "regenerated") by means of water, yr a
dilute aqueous solution of amine oxide containing up to 20%
amine oxide in water. The fibrils may be collected directly
into a coagulation bath, or may be collected onto a surface
and then further coagulated.
The gas, preferably air or dry steam, is blown onto tie
extruded fibres at a velocity of between 200 m. s'1 (metres per
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second) and 500 m.s'~ and has a temperature of between 100°C
and 155°C, preferably about 145°C for a cellulose
concentration of 14-I5%. The lower the cellulose content of
the dope, the lower the air temperature that can be used. The
gas velocity should be at least 50 times higher than the
velocity of the extrudate fibre emerging from the spinning
jet, and preferably between 1,000 and 20,000 times said
velocity. The air is directed onto the fibre extrudate at a
bias angle, preferably of between 15° and 45° relative to the
longitudinal axis of the extrudate, and more preferably about
30°. The air jets may also be biased at a second skew angle
relative to the spinning jet so that the air jet axes and
fibre axis do not intersect, the air jets being tangential to
the surface of the fibre extrudate.
The fibre collecting surface may be flexible. or
resilient. Preferably the fibre collection is made onto a bed
of foam Which is preferably soaked in water or aqueous ND~iO
solution.
The invention also provides a nonwoven lyocell fabric in
which the fibres are bonded or entangled together without the
use of a binder, the fabric having a density no greater than
175 g.dm'', and/or a tensile extension to break of at least
7.
°.
The invention will hereinafter be described in more
detail by way of e:~cample only, with reference to the
accompanying drawings in which:-
Figure 1 is a schematic drawing of an embodiment of
apparatus for the production of a nonwoven fabric according
to the present invention;
Figure 2 is a plain view of a spinning jet nozzle used in
the apparatus of Figure 1;
Figure 3 is a side elevation of the nozzle shown in
Figure 2, with internal passages ghosted: and
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Figure 4 is an axial cross-section through the nozzle
shown in Figure 2 and Figure 3.
With reference to Figure l, there is shown an extruder
having a nozzle 11 attached thereto. The extruder is fed
5 with a solution comprising 14% by weight cellulose, 12% by
weight of water and T4% by weight of N-methylmorpholene-N-
oxide (N~IO) . The cellulose has an average degree of
polymerisation of about 600.
The cellulose solution may be manufactured as is
10 described in WO 94/28217. The cellulose solution in the
extruder is held at a temperature of between 95 and 110°C,
preferably 105°C, and is forced through the nozzle to extrude
as a continuous filament of cellulose dope.
The nozzle 11 is shown in Figures 2 and 3 and may be
secured directly onto the extruder 10, or may be secured to
an adapter (not shows) which in turn is secured to the
extruder 10. The nozz7.e 11 has a hollow screw threaded stud
13 on its back face 12 and a central passageway 14 which
terminates in a jet aperture 15. The jet has a diameter of
between 0.2 and 0.3mm, and preferably about 0.27mm.
The cellulose dope is forced into the passageway 14 under
pressure, and is extruded through the jet 15. The nozzle 11
also has a plurality oi: gas outlet passageways 16, preferably
three, spaced around the central passageway 14. Each gas
passageway 16 is incla.ned with respect to the jet axis, and
they are circumferentially equally spaced around the jet 15
so that each gas stream exiting its respective passageway 16
has the same effect upon the extrudate filament.
The gas passageways 16 make a bias or convergence angle
with the longitudinal axis of the jet of between 15° and 45°,
and more preferably 30°. The passageways 16 are also skewed
so that the axes of the passageways 16 do not themselves
converge. The gas passageways are about 2.0 mm in diameter.
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The back face 12 of: the nozzle has an annular groove 17
therein which interconnects the ends of the three passageways
16.
When the nozzle is secured to the extruder, the central
passageway 14 is connected to the cellulose dope feed and the
annular passageway 17 is connected to a gas supply, preferably
compressed air.
With reference to Figure 1, compressed air is fed from
a source (not shown) through a flow regulator valve 21, a flow
meter 22, a heater 23 and a temperature sensor 24, to the air
passageway 17 in the nozzle. The sensor 24 may be connected
to the air heater 23 for control of the air temperature.
The extrudate filaments emerging from the nozzle 11 are
subjected to attenuation by high velocity air streams 25
emerging from the outlet passageways 16, and the filament is
drawn and may be fractured and either blown directly into a
coagulation bath 27 or, preferably, blown onto a support
surface 26 situated about 30 cm from the nozzle 11. The
cellulose dope flow rate is preferably about 0.2 g.min'1. In
the illustrated embodiment the support surface 26 is formed
by the outer peripheral surface of a rotatable drum 28, which
turns at about 10 revolutions per minute (rpm) to build up a
layer of nonwaven fabric on the drum.
The extrudate filaments pass through a fine mist 30 of
water vapour, generated by water sprayed from an air atomising
nozzle 32 located about 10 cm below nozzle 11.
The surface of the drum may be covered in compliant
material such as a layer of foam, preferably a 2mm thick layer
of polyurethane foam having an average cell size of 50~,m. The
foam may be soaked in water or an aqueous solution of DTI~IO.
Subsequent to the formation of the nonwoven fabric layer
on the drum 28, the drum 28 is immersed in a coagulant bath
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27 containing a suitable coagulant such as water or a dilute
solution of amine oxide in water, to coagulate the nonwoven
cellulose fabric on the drum. The fabric layer is then dried
on the drum after washing to remove solvent.
Table 1 below summarises the various conditions used in
the formation of extruded filament.
Table 1
Sample Method of Air Air flowrate Mean fibre
referencelaydown temperature (l.s'') diameter
(~Cm)
(C)
1 Collected dry 145 2.5 7
2 Collected on 148 2.3 5
wet surface
3 + water 147 2.3 5
aerosol
4 ~~ + compliant 145 2.3 7
surface
An air flow rate of 2.4 l.s'1 (litres/second) corresponds
approximately to air velocity of 250 m.s'1.
Samples 3 and 4 were made in accordance with present
inventions.
Table 2 below summarises web loftiness by way of
thickness measurements, carried out using a Mitutoyo Dial
Thickness Gauge, fitted with film/web measurement plates.
Because basis weights are different, the thickness is
normalised to a basis weight of 25 g.m'2.
Table 2
Sample Basis Weight Web ThicknessNormalised density
25reference(g.m'Z) (gym) Thickness (g.dm-')
(um/25g.m'Z)
1 25.3 113 112 223
2 28.3 145 128 195
3 28.0 168 150 166
4 22.5 163 181 138
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It can be seen that the samples according to the
invention produce less dense fabrics having a density no
greater than 17 0 g . dmu' .
With reference to Sample 4, it is believed that the foam
serves two purposes, namely (a) nascent fibres striking a hard
collector surface tend to fuse on impact - a compliant surface
removes some of the impact energy and reduces bonding and (b)
on compression of the foam at impact, coagulant contained in
its pores is expressed to the surface and aids rapid
regeneration.
To assess mechanical properties, strips were cut from the
webs, 5 mm in width, and tested on a Testometric PCX
materials tensile tester at a gauge length of 20 mm and at
cross head speed of 20 mm/min. Along with the absolute tensile
strengths, the tensile strengths are also shown normalised to
a basis weight of the web of 25 g.m'Z, which better reflects
the comparative mechanical properties, as basis weight
variations are eliminated.
Table 3 summarises the properties of the fabric webs
formed on the drum 26.
Table 3
Sample reference Tensile strength Elongation O Comments
(N) normalised to break (mm)
:?5 g.m'z
1 3.7 0.7 Clean breaks
2 0.2 0.85 Fairly clean
breaks
3 0.41 1.54 Sample maintained
integrity after
"breaking"
4 0.09 2.5 Sample maintained
integrity after
"breaking"
For samples 2, 3 and 4, where the surface of the drum is
partially immersed in the coagulation bath so that the drum
surface is wet, further coagulation takes place on contact
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with the wet drum or previously laid fibres.
For sample 1, the surface of the drum is dry and the
fabric is regenerated. after build up on the drum.
Samples 3 and 4, collected on the drum after passing
through the vapour mist, clearly had lower tensile strength
but greater extension to break whilst still maintaining some
integrity after the break point. These high loft material are
suitable for use as filters and high absorbency materials.
Whilst the invention has been described for the
attenuation of cellulose dope containing 14% cellulose, it can
be applied to other dopes as is described in PCT/GB97/03391,
the contents of which are hereby incorporated into the present
application by way of reference.
