Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF MANUFACTURING A DRAWN MULTIFILAMENT YARN
TECHNICAL FIELD OF THE INVENTION
The invention relates to a method of manufacturing a multifilament
yarn, such as a high performance polyethylene (HPPE) multifilament yarn. More
particularly, the invention relates to a method of manufacturing and/or
cleaning a
multifilament yarn, where no organic spin finish is required.
BACKGROUND OF THE INVENTION
Multifilament polyolefin yarn is typically manufactured by spinning of a
fluid, such as molten polyethylene or a solution (sometimes also referred to
as a gel) of
polyethylene in a spin solvent, through a spinneret with a multiplicity of
spinholes to
form filaments. Thereafter, the filaments are cooled and drawn. If a spin
solvent is
used, then at least some of this solvent is removed by for example evaporation
or
extraction. Examples of manufacturing processes are meltspinning process (as
for
example disclosed in EP1445356), solid state process (as for example disclosed
in
EP1627719) or gelspinning (as for example disclosed in WO 2005/066401). A
general
description is also presented in "Advanced Fibre Spinning Technology", Ed. T.
Nakajima, Woodhead Publ. Ltd (1994), ISBN 185573 182 7, all incorporated
herein by
reference.
One problem when manufacturing multifilament yarn of polyethylene
is that polyethylene is an electrical isolator and hence tend to build up
electro static
charge during manufacturing and since the individual filaments of the
multifilament yarn
is charged similarly (plus or minus), then individual filaments will electro
statically repel
each other. This means that the multifilament yarn will tend to loosen up
instead of
forming a unity. Furthermore, in case of filament breakage, the filament ends
will tend
to move as far away as possible from the main body of the multifilament yarn
reducing
the quality of the manufactured multifilament yarn by fluff formation or even
risk of a
loose filament (end) being caught in the manufacturing equipment and hence
even
requiring production stop to releasing the broken filament.
Traditionally, this has been solved by adding an organic spin finish to
the multifilament yarn during the drawing process. However, organic spin
finish is a
surface active product and should be avoided for environmental reasons.
One route to reduce influence of the spin finish has been suggested
in W02004/053212 where use of a volatile spin finish is suggested.
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OBJECTS OF THE INVENTION
It is the object of the invention to provide an improved method of
manufacturing a multifilament yarn.
It is another object of the invention to provide an improved
multifilament yarn or yarn construction.
DISCLOSURE OF THE INVENTION
The improvement may for example be one or more of a reduction of
the environmental impact, better logistics, reduced handling or another
improvement
provided by the present invention as discussed below.
In a first aspect of the invention, the object of the invention is
achieved by a method of manufacturing a multifilament yarn comprising the
steps of
providing a partially drawn multifilament yarn comprising partially drawn
filaments,
twisting the partially drawn multifilament yarn to a partially-drawn-twist-
level of 4 to 600
turns per meter to form a twisted partially drawn multifilament yarn, and post-
twist-
drawing the twisted partially drawn multifilament yarn by a draw ratio DRpt of
at least
1.1.
The partially drawn multifilament yarn may be provided directly from
the spinning process in the case where the spin holes of the multifilament
spinneret
has a contraction zone shaped to provide partially drawing or aligning of the
filaments
already in the spin holes for example as disclosed in WO 2005/066401. The
partial
draw multifilament yarn may also be a result of drawing in an airgap after the
spinneret.
Preferably, the partially drawn multifilament yarn is a solid stage yarn for
example after
cooling in a quench bath after the spinning. If the partially drawn
multifilament yarn is a
gelspun multifilament yarn, it is preferred that at least a part of the
solvent has been
removed (as compared to the solvent content during spinning through the
spinneret)
prior to twisting of the partially drawn multifilament yarn.
Drawing of the multifilament yarn takes place in multiple steps. The
twisting of the partially drawn multifilament yarn may be realized inline with
a drawing
step, or offline not connected with a drawing step. By inline is herein meant
that the
partially drawn yarn runs directly between a station where twisting of the
partially drawn
multifilament yarn is conducted and a station where the multifilament yarn is
drawn.
Offline typically means that a winding step takes place between drawing and
twisting.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained more fully below with reference to
exemplary embodiments as well as the drawings, in which
Fig. 1 shows schematic flowcharts of methods according to the
invention, and
Fig. 2 shows schematic flowcharts of a method of manufacturing a
HPPE multifilament yarn according to the invention.
All figures show only steps which are necessary in order to elucidate
the invention, other parts being omitted or merely suggested.
DETAILED DESCRIPTION
The drawing of the multifilament yarn may be conducted in one or
more of the following variations (see Fig. 1): a) inline before twisting and
followed by
offline post-twist-drawing; b) inline before and inline after twisting of the
partially drawn
multifilament yarn; c) offline before twisting and inline after twisting; d)
offline before
and offline after twisting, so twisting of the partially drawn yarn takes
place in a step
separated (in time and/or space) from drawing of the partially drawn
multifilament yarn
before and after the twisting.
It is preferred that twisting the partially drawn multifilament yarn and
post-twist-drawing is conducted inline. The method according to the invention
may
incorporate more than one post-twist-drawing steps. If more than one post-
twist-
drawing steps are conducted in the method according to the invention, then it
is
preferred that twisting of the partially drawn multifilament yarn is conducted
inline with
at least one of the post-twist-drawing steps, and more preferably twisting of
the partially
drawn multifilament yarn is conducted inline with all post-twist-drawing
steps. The
method according to the invention may incorporate more than one twisting
steps. If
more than one twisting steps are conducted in the method according to the
invention,
then it is preferred that at least one of the twisting steps is conducted
inline with a post-
twist-drawing step.
Twisting of the partially drawn multifilament yarn may be conducted
on a dedicated (freestanding) equipment or may be combined with into one piece
of
equipment with one or more of a drawing oven, a quench bath, a winder, an
unwinder,
a spinning plate. In one embodiment, twisting the partially drawn yarn is
conducted by
an inline twister system for example as described in US 6,155,037. lnline
twister
systems described in US 6,155,037 is hereinafter referred to as an Uster
twister
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system. As this type of equipment allows for introducing a twist to a
partially drawn
multifilament yarn without having access to the ends of the yarn, this type of
equipment
is particularly advantageous for retrofitting of existing equipment, such as
an existing
drawing oven, or an existing winder or unwinder. One aspect of the invention
therefore
concerns a combination of an Uster twister system and a drawing oven, an Uster
twister system and a winder, or a Uster twister system and an unwinder.
In a particularly preferred embodiment, twisting the partially drawn
yarn is conducted by first winding an untwisted partially drawn multifilament
yarn
followed by twisting the partially drawn multifilament yarn by unwinding the
untwisted
partially drawn multifilament yarn using a twisting creel system. Preferably
the twisting
creel system is arranged to conduct the unwinding and twisting inline with the
post-
twist-drawing process wherein the unwinding speed and rotation speed of the
twisting
creel system is adjusted to the inline post-twist-drawing process.
The method according to the invention may further comprising the
step of tensioning the multifilament yarn in a tensioning station after
twisting the
partially drawn multifilament yarn and before post-twist-drawing of the
partially drawn
yarn. This allows to keep the tension in the partially drawn multifilament
yarn relatively
low during twisting while the tension during drawing may be kept sufficiently
high to
ensure the required drawing. Examples of suitable tensioning stations include
a
number of rolls, a break system and a friction system.
For all aspects of the invention, drawing of the multifilament yarn
typically takes place at an elevated temperature. It is preferred that at
least one
drawing step takes place at a temperature of above 100 C, and preferably at
least one
drawing step takes place at a temperature above 120 C. It is preferred that at
least one
drawing step takes place below the melting point of the multifilament yarn.
More
preferably, all drawing steps after the spinning takes place below the melting
point of
the multifilament yarn. Yet more preferably, all drawing steps after spinning
takes place
at a temperature of at least 5 C below the melting point of the multifilament
yarn, and
more preferably at a temperature of at least 10 C below the melting point of
the
multifilament yarn. Particularly, it is preferred that post-twist-drawing
takes place in a
range between the melting point of the multifilament yarn and 30 C below the
melting
point of the multifilament yarn, more preferably the post-twist drawing takes
place at a
temperature at least 5 C below the melting point of the multifilament yarn,
such as
10 C below the melting point of the multifilament yarn. For gelspun UHMWPE
multifilament yarn, it is preferred that drawing ¨ and particularly post-twist-
drawing ¨
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takes place at a temperature below 138 C, preferably at a temperature below
135 C
and more preferably at a temperature below 132 C. For gelspun UHMWPE
multifilament yarn, it is preferred that drawing ¨ and particularly post-twist-
drawing ¨
takes place at a temperature above 100 C, preferably at a temperature above
110 C,
more preferably at a temperature above 120 C, and more preferably at a
temperature
above 130 C.
Within the context of the present invention, monofilaments are
understood to mean elongated bodies of indefinite length and with length
dimension
much greater than their transversal dimensions, e.g. width, thickness or
radius, and
can have regular or irregular cross-section. The monofilaments may have
continuous
lengths, known in the art as filaments, or discontinuous lengths, known in the
art as
staple fibers. A multifilament yarn according to the present invention is an
elongated
body comprising a plurality of monofilaments.
By a gel-spun multifilament yarn is herein understood a multifilament
yarn manufactured by spinning a solution (sometimes also referred to as a gel)
comprising a polymer and a solvent for said polymer. The spinning of the
solution may
be conducted by converting a suspension of polymer and solvent into a solution
in an
extruder and extruding said solution through a spinneret containing more than
one
spinning apertures. Particularly preferred are gel-spun polyolefin
multifilament yarns,
which are spun from a solution containing the polyolefin and a solvent for
polyolefin,
which solvent can be removed from the multifilament yarn after spinning of the
yarn by
evaporation, like for example naphthenes such as decaline, tetralin or
methylcyclohexane.
Preferably, the gel-spun multifilament yarns obtained according to the
invention are high performance gel-spun multifilament yarns with a tenacity of
at least
1.5 N/tex, preferably at least 2.0 N/tex, more preferably at least 2.5 N/tex
or even at
least 3.0 N/tex. There is no reason for an upper limit of the tenacity of the
multifilament
yarns, but gel-spun multifilament yarns typically having a tenacity of at most
about 5 to
6 N/tex may be manufactured. Generally such high performance gel-spun
multifilament
yarns also have a high tensile modulus (also referred to as Young's modulus),
of e.g. at
least 50 N/tex, preferably at least 75 N/tex, more preferably 100 N/tex, most
preferably
at least 125 N/tex. There is no reason for an upper limit of the tensile
modulus, but gel-
spun multifilament yarns typically having a tenacity of at most about 180
N/tex.
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Tensile strength (also simply referred to as strength) tenacity and
modulus of multifilament yarns are determined by known methods, as based on
ASTM
D2256-97.
The multifilament yarns obtained according to the invention have a
titer of preferably at least 2 dtex, preferably at least 7 dtex and more
preferably at least
8 dtex. For practical reasons, the titer of the multifilament yarns of the
invention are at
most several thousand dtex, preferably at most 2500 dtex, more preferably at
most 500
dtex. The multifilament yarns comprising a plurality of filaments preferably
have a titer
per filament in the 0.2 ¨ 5 dtex range, preferably 0.5-2 dtex. Partially drawn
multifilament yarns comprising a plurality of filaments preferably have a
titer per
filament in the 0.5 ¨ 25 dtex range.
The multifilament yarn is preferably selected from the group
consisting of polyolefin (such as polyethylene and polypropylene), polyester
(such as
PET, vectran and PLA), polyamide (such as nylon and aramide) and more
preferably
the multifilament yarn is polyolefin. It is preferred that polyolefin used in
accordance
with the present invention is polypropylene or polyethylene, more preferably
ultrahigh
molecular weight polyethylene (UHMWPE).
By HPPE multifilament yarn is herein understood high performance
polyethylene, which is yarn comprising drawed polyethylene with a Young's
modulus of
at least 30 GPa. A particularly preferred type of HPPE is gelspun ultra high
molecular
weight polyethylene (UHMWPE), where the UHMWPE has an intrinsic viscosity (IV)
as
measured on a solution of UHMWPE in decalin at 135 C, of at least 5 dl/g,
preferably
at least 10 dl/g, more preferably at least 15 dl/g, most preferably at least
21 dl/g.
Preferably, the IV is at most 40 dl/g, more preferably at most 30 dl/g, even
more
preferably at most 25 dl/g. Gelspun UHMWPE typically has a Young's modulus of
at
least 50 GPa. Preferably the HPPE multifilament yarn has a tenacity of at
least 1GPa.
Preferably the HPPE multifilament yarn comprises at least 90 wt-% UHMWPE
filaments and most preferably the HPPE multifilament yarn consists of UHMWPE
filaments.
By yarn construction is herein meant a collection of at least two
multifilament yarns connected for example by braiding, weaving, knitting,
twisting, heat
treatment involving partial melting, air entanglement and gluing.
By post-twist-drawing is herein meant the process of drawing a yarn
by a factor of at least 1.1 after twisting of the yarn by at least 4 turns per
meter and
before optionally converting the yarn into a yarn construction. It should be
observed
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that drawing of a yarn construction (such as drawing of a rope construction as
described in EP 0 398 434) is a fundamentally different process than post-
twist-drawing
since drawing of a yarn construction realize the fundamentally different
effect of
realigning filaments of the construction by creep and does not improve the
coherence
between the filaments of an individual yarn between the individual filaments
in one
multifilament yarn of the yarn construction. Furthermore, in addition to a
more coherent
filament bundle twisting of the partially drawn multifilament yarn before post-
twist-
drawing appeared to providing for a better heat and stress distribution in the
multifilament yarn during post-twist-drawing which again allowed for a more
uniform
drawing on filament level. A more uniform drawing leads to improved
processability of
the obtained multifilament yarn as well as improved yield of the manufacturing
process
since more uniform drawing appeared to lead to reduced filament breakage. Post-
twist-
drawing also results in a higher reduction of the titer as compared to drawing
a yarn
construction.
In a preferred embodiment, the partially drawn multifilament yarn is
twisted to a partially-drawn-twist-level of 10 to 500, and more preferably the
partially
drawn yarn is twisted to a partially-drawn-twist-level of 12 to 200 turns per
meter. The
optimum partially-drawn-twist-level depends on the titer of the multifilament
yarn, the
post-twist-drawing ratio, the desired twist level of the multifilament yarn
and the
optional twist level to be applied to the multifilament yarn after post-twist-
drawing.
In another preferred embodiment, the twisted partially drawn
multifilament yarn is post-twist-drawing by DRpt of at least 1.5, and more
preferably
post-twist-drawing by DRpt of at least 2, and most preferably post-twist-
drawing by DRpt
of at least 3. The upper limit of the post-twist-drawing ratio DRpt, depends
on the
condition of the yarn prior to the twisting of the partially drawn yarn, such
as for
example, the type of polymer, and the draw ratio prior to twisting of the
partially drawn
yarn, the temperature of the post-twist-drawing. Typically, the post-twist-
drawing ratio is
less than 1000; preferably DRpt is at most 100; more preferably DRpt is less
than 50;
more preferably DRpt is at most 25; more preferably DRpt is at most 10 or less
than 10.
The partially-drawn-twist-level is the number of turns per meter that
the partially drawn multifilament yarn has directly after the twisting of the
partially drawn
multifilament yarn. In a preferred embodiment of the invention, the partially
drawn
multifilament yarn is twisted to a level where the factor between the
partially-drawn-
twist-level and the draw ratio, DRpt, of the post-twist-drawing is between 2 ¨
250 turns
per meter, preferably the partially-drawn-twist-level / DRpt is between 2 ¨
120 turns per
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meter, more preferably the partially-drawn-twist-level / DRpt is between 2 ¨
20 turns per
meter, such as 5 to 20 turns per meter. In one embodiment, the twist level of
the
partially drawn multifilament yarn is sufficiently high that no further
twisting of the
multifilament yarn is required after post-twist-drawing.
The partially drawn multifilament yarn may be a homo-yarn or a
hetero-yarn. By homo-yarn is meant that the multifilament yarn consist of
filaments
having the same composition. By hetero-yarn is meant that at least one of the
filaments
of the multifilament yarn has a different composition that other of the
filaments of the
multifilament yarn. In one embodiment, at least one of the filaments of the
partially
drawn multifilament yarn is selected from the group consisting of (coloured or
uncoloured) polyester filaments, HD polyethylene filaments, UHMWPE filaments,
polypropylene filaments. Preferably, the partially drawn multifilament yarn
comprises at
least 75 number-% UHMWPE filaments and more preferably the partially drawn
multifilament yarn comprises at least 90 number-% UHMWPE filaments. Most
preferably, the filaments of the partially drawn multifilament yarn consist of
UHMWPE
filaments, as this provides the strongest and most homogeneous performance of
the
multifilament yarn.
Another aspect of the invention concerns a method of removing a
spin solvent from a gelspun multifilament polyolefin yarn. The method
comprising the
steps of providing a multifilament yarn comprising polyolefin filaments and a
spin
solvent, twisting the yarn to a twist level of 10 to 600 turns per meter, and
removing at
least a part of the spin solvent from the multifilament yarn after twisting
the yarn. The
multifilament yarn provided in the first step may be an undrawn, a partially
drawn or
post-twist-drawn gelspun multifilament yarn, however, it is preferred that the
multifilament yarn is partially drawn or post-twist-drawn. The removing of
spin solvent is
preferably by evaporation or extraction. Evaporation is particularly relevant
when the
spin solvent is a naphthene such as decaline, tetralin and/or
methylcyclohexane.
Extraction is particularly relevant when the spin solvent comprises paraffin.
Twisting of
the multifilament yarn before removing at least a part of the solvent from the
multifilament yarn allows conducting the conversion into a yarn construction
without
addition of spin finish to the multifilament yarn. In other words, this aspect
of the
invention allows removal of the spin solvent from a multifilament yarn in a
more
environmentally friendly way without jeopardizing the handlabiilty of the
yarn, since a
spin finish is not required or a smaller amount of spin finish is required. In
this
embodiment of the invention, it is also preferred to post-twist-drawing the
multifilament
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yarn, however, this is not a requirement. Preferred twisting levels and
optional post-
twist-drawing levels correspond to the levels as described herein for other
aspects of
the invention.
In one embodiment of the invention, the spin solvent content of the
multifilament yarn is already very low before twisting of the partially drawn
multifilament
yarn. In this embodiment, it is preferred that the spin solvent of the
multifilament yarn
during twisting is 10 ppm to 1000 ppm based on the weight of polyethylene and
spin
solvent. Preferably, the spin solvent of the partially drawn multifilament
yarn during
twisting is 20 ppm to 500 ppm.
In another embodiment of the invention, the spin solvent content of
the multifilament yarn during twisting of the multifilament yarn is at least
0.1 wt-%
based on the weight of polyethylene and spin solvent, preferably the spin
solvent
content of the multifilament yarn is at least 0.5 wt-%, more preferably the
spin solvent
content of the multifilament yarn is at least 2 wt-%, more preferably the spin
solvent
content of the multifilament yarn is at least 5 wt-%, such as for example at
least 10 wt-
%. Typically, the spin solvent content of the multifilament yarn during
twisting of the
multifilament yarn is below 90 wt-%.
The amount of solvent removed after twisting of the multifilament yarn
depends to a large extent on the content before twisting. Furthermore, the
lower the
solvent content of the post-twist-drawn multifilament yarn, the higher the
effect on
coherency of the twisting before removing the solvent from the yarn. In one
embodiment, at least 10 wt-% of the spin solvent present during the twisting
is removed
after twisting the multifilament yarn and preferably at least 90 wt-% of the
spin solvent
present during the twisting is removed after twisting the multifilament yarn.
Most
preferably at least 99 wt-% of the spin solvent present during the twisting is
removed
after twisting the multifilament yarn. It should be observed that if more
twisting steps
are utilized, then the solvent content during twisting is the solvent content
during the
first twisting step. The fraction of solvent removed from the multifilament
yarn after
twisting depends to some extent on the amount present before twisting. It is
preferred
that 100% of the solvent is removed, but since traces of solvent may stay in
the
multifilament yarn, in one embodiment at most 99.999% of the solvent is
removed. The
removed spin solvent is preferably collected and recycled by known means.
In the cleaning process according to this aspect of the invention, it is
highly preferred that the process further comprises a post-twist-drawing step.
Particularly, it was found to be advantageous to post-twist-drawing the
multifilament
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yarn by a draw ratio DRpt of at least 1.1 to form a HPPE multifilament yarn.
More
preferably the post-twist drawing draw ratio is at least 1.5, and even more
preferably
DRpt is at least 2, and most preferably by a DRpt of at least 3. The upper
limit of the
post-twist-drawing ratio DRpT, depends on the condition of the yarn prior to
the twisting
of the partially drawn yarn, such as for example, the type of polymer, and the
draw ratio
prior to twisting of the partially drawn yarn, the temperature of the post-
twist-drawing.
Typically, the post-twist-drawing ratio, DRpT, is less than 1000; preferably
DRpt is at
most 100; more preferably DR-1- is less than 50; more preferably DRpt is at
most 25;
more preferably DR-1- is at most 10 or less than 10.
Manufacturing of Gelspun multifilament HPPE traditionally requires
addition of spin finish during removal of the spin solvent as the filaments
would
otherwise repulse each other due to electrostatic charging effects of the
electrically
isolating polymer. Unfortunately, being a surface active agent, spin finish is
not a
desired additive for environmental perspective and particularly for medical
applications,
spin finish should be avoided. One aspect of the invention therefore concerns
a method
of manufacturing a gelspun HPPE multifilament yarn without adding a spin
finish. The
flowchart in Fig. 2 indicates this method. The method comprising the steps of
providing
(2) a solution of 3-25 wt-% UHMWPE in a spin solvent (based on UHMWPE and spin
solvent in the solution). The solution may for example be provided by
converting a
suspension of UHMWPE and spin solvent in an extruder, spinning (4) the
solution
through a spinneret having at least two spin holes, quenching (6) the solution
in a
quench bath, and drawing (7) to form a solid multifilament yarn comprising
partially
drawn UHMWPE filaments. Thereafter, twisting (12), the partially drawn
multifilament
yarn to a twist level of 2 to 600 turns per meter to form a twisted partially
drawn
multifilament yarn, and post-twist-drawing (13) the twisted partially drawn
multifilament
yarn by a draw ratio DRpt of at least 1.1 to form a HPPE multifilament yarn,
where
twisting, removal of spin solvent and post-twist-drawing are conducted without
adding a
spin finish to the multifilament yarn. At least a part of the spin solvent is
removed during
the process by evaporation or extraction. Twisting of the partially drawn
multifilament
yarn may be conducted inline, offline or a combination of inline and offline
with drawing
conducted prior to twisting. Twisting of the partially drawn multifilament
yarn may be
conducted inline, offline or a combination of inline and offline with post-
twist-drawing. It
was found to be highly advantageous to conduct drawing prior to twisting
offline with
the twisting and to conduct twisting inline with at least a part of the post-
twist-drawing
as this allows for good traceability and compact process equipment.
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The methods according to the present invention are particularly
advantageous in relation to preparation of polyethylene yarn to be used in
medical
applications, medical repair products, implants and medical devices, as the
methods
allows for preparation of polyethylene yarn without the use of a spin finish
and
particularly an organic spin finish, which organic spin finish typically is
not desired
inside the body or in contact with the body. Example of medical applications
are
medical components, like for example vascular prostheses, joint arthroplasty,
orthopedic and spine implants, for example meniscus implants, surgical
sutures,
meshes for example hernia meshes, fabrics, woven or non-woven, textiles,
sheets,
tapes, ribbons, bands, artificial joints, cables such as trauma fixation
cables, sternum
closure cables, prophylactic or per prosthetic cables, long bone fracture
fixation cables,
small bone fracture fixation cables, tube-like products for example ligament
replacement, endless loop products, bag-like, balloon-like products), tube-
like products
for example ligament replacement, endless loop products, bag-like products,
balloon-
like products, grafts, stent grafts, artificial veins, Y-shaped hollow
structures, skirts for
valve structures, such as heart valves and periphery valves and other medical
components.
Since use of spin finish is also not desired from an environmental
point of view, the methods according to the present invention are also
advantageous in
relation to preparation of polyethylene yarn to be used in other end-use
products and
applications like ropes, cables and cords, mooring lines, fishing lines and
fishing nets,
sports equipment, impact and abrasion resistant products and ballistic-
resistant
products.
EXAMPLES
Example 1: Preparation of HPPE yarn with drawing inline before twisting.
Fig. 1 a) shows a flow diagram of a method of preparation of a
polyolefin multifilament yarn where drawing inline with twisting is conducted
before
twisting. After twisting, the partially drawn multifilament yarn is wound on a
bobbin
before (at a later time) conducting offline post-twist-drawing. The twisting
may be
conducted in conjunction with winding of the partially drawn multifilament
yarn in a
combined twisting and winding unit, such as for example available from the
companies
Alma-Sauer, Dietze & Schell, Galan, and Barmag. The twisting and winding may
also
be conducted in two separate steps in which case the twisting for example may
be
conducted in a process and equipment as disclosed in US 6,155,037 and the
winding
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for example may be conducted on a precision cross winder, for example from
SSM,
Dietze & Schell, Comoli, Leesona, Georg Sahm GmbH, Oerlikon, and Barmag. The
winded and twisted partially drawn yarn is later (i.e. offline) post-twist-
drawing. This
corresponds to the process indicated in Fig. 1 a, where in addition to the
drawing (8)
conducted inline before twisting (12), further offline drawing (10) may be
conducted
prior to the inline drawing (8).
Example 2: Preparation of HPPE yarn with drawing inline before and after
twisting.
Fig. 1 b) shows a flow diagram of a method of preparation of a
polyolefin multifilament yarn where drawing inline with twisting is conducted
before as
well as after twisting. The twisting may be conducted with a twisting unit of
the type for
example disclosed in US 6,155,037 where a running yarn is twisted inline
during
transfer through the twisting unit. In one embodiment of drawing variation b),
one large
twisting step is utilized to achieve the desired twist level. In another
embodiment of
drawing variation c), multiple smaller steps of twisting (such as 2, 3, 4, 5,
6, 7, 8, 9 or
10 twisting steps) are utilized to achieve the desired twist level. The
multiple smaller
drawing steps may be arranged in between multiple smaller drawing steps or in
groups
of drawing steps between drawing steps. As indicated in Fig. 1 b), in addition
to the
inline drawing before (8) and after (14) the twisting (12), one or more
offline drawing
steps may optionally be utilized before (10) or after (16) the inline drawing
and twisting
steps (8, 12, 14).
Example 3: Preparation of HPPE yarn with drawing inline after twisting.
Fig. 1 c) shows a flow diagram of a method of preparation of a
polyolefin multifilament yarn where drawing inline with twisting is conducted
after
twisting. In this case, a partially drawn multifilament yarn is prepared by
drawing (10)
for example in a first drawing oven the as-spun multifilament yarn and
thereafter
winding the partially drawn multifilament yarn on bobbin for example on a
Twinstar
precision cross winder from Georg Sahm GmbH. At a later time, the partially
drawn
multifilament yarn is twisted and post-twist-drawing. The twisting preferably
takes place
on a combined twist and unwinder apparatus such as for example a twisting
creel
available from for example Alma-Sauer, Dietze & Schell, Galan, and Barmag.
Alternatively, a separate unwinder may be combined with a twisting unit of the
type for
example disclosed in US 6,155,037. After the inline post-twist-drawing (14),
the yarn
may optionally be further drawn offline (16). The advantage of this embodiment
is, that
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the unwinding and twisting can be finely tuned with the drawing process and
allow for
rapid start and stop in relation to yarn breakage. Furthermore, traceability
is easier to
handle as compared to the drawing variation of Example 4 since less offline
processes
are involved.
Example 4: Preparation of HPPE yarn with drawing offline before and after
twisting.
Fig. 1 d) shows a flow diagram of a method of preparation of a
polyolefin multifilament yarn where drawing is conducted offline before and
after
twisting. The partially drawn multifilament yarn is prepared as described in
Example 3
by drawing (10) for example in a first drawing oven the as-spun multifilament
yarn and
thereafter winding the partially drawn multifilament yarn on bobbin for
example on a
Twinstar precision cross winder from Georg Sahm GmbH. The partially drawn
multifilament yarn is thereafter at a later time twisted on a twist and rewind
equipment
for example available from SSM, Dietze & Schell , Comoli, Leesona, Georg Sahm
GmbH, Oerlikon, and Barmag. Finally, post-twist-drawing is conducted at a
later time
for example in a further drawing oven. The advantage of this embodiment is
that it
allows for realizing the improved coherence of the multifilament yarn in a
simple
constellation and does not require alignment of twisting equipment with one or
more
drawing stations. However, due to the process steps being conducted offline,
traceability needs to be handled carefully.
Example 5: Preparation of partially drawn HPPE yarn
Bobbins of partially drawn UHMWPE yarn were prepared by
preparing a solution of UHMWPE in decalin in an extruder, spinning the
solution
through a spinneret having 200 spin holes, quenching in a quench bath,
partially
drawing the as-spun yarn and removing more than 99% of the spin solvent from
the
yarn as compared to spin solvent content of the solution. No spin finish was
provided
during the process. Finally, the yarn was wound on bobbins each holding about
9000
m.
Example 6: Preparation of HPPE yarn utilizing drawing inline after twisting
Bobbins of partially drawn UHMWPE yarn prepared in Example 5
were subjected to the inline drawing process as described in Fig. 1 c) using
inline post-
twist-drawing ratio of 4 by feeding the drawing oven via a twisting creel
unwinder. Twist
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levels of 9 and 12 turns per meter were used. No offline post-twist-drawing
was used
and no spin finish was provided during the processes.
Twist level 9 turns 12 turns
per meter per meter
Total number of bobbins 75 17
Minimum number of defects per bobbin 0 0
Maximum number of defects per bobbin 16 10
Average number of defects per bobbin 3 1.5
It was therefore observed that the maximum number of defects per
bobbin as well as the average number of defects per bobbin decreased when the
twist
level was increased.
Comparative Example 1: Preparation of HPPE yarn utilizing no twisting
Experiment 6 was repeated using 0 turns per meter inline twist. The
resulting yarn showed very low coherency and no stable bobbins could be
prepared
after final drawing.
Example 7: Preparation of HPPE yarn utilizing drawing offline before and after
twisting
Bobbins of partially drawn UHMWPE yarn prepared in Example 5
were subjected to the offline drawing process as described in Fig. 1 d). A
twist level of
16 turns per meter was provided by unwinding using a ring-twister, parallel
winding and
a separate step of rewinding using cross winding. The bobbins of partially
drawn and
twisted UHMWPE yarn were hereafter drawn using an offline post-twist-drawing
ratio of
4. No spin finish was provided during the twisting or post-twist-drawing
processes.
Two batches of bobbins (each about 400g) were prepared.
= Batch 1: 50 bobbins of which 16 bobbins had one or more defects
= Batch 2: 75 bobbins of which 24 bobbins had one or more defects
It was therefore observed that a very high quality level and high consistency
was
realized utilizing a process involving twisting of the yarn and post-twist-
drawing despite
no spin finish was added during the process
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Comparative Example 2: Preparation of HPPE yarn utilizing no twisting
Experiment 7 was repeated using rewinding without twisting. The
resulting yarn showed very low coherency and no stable bobbins could be
prepared
after final drawing.
An individual feature or combination of features from an embodiment
of the invention described herein, as well as obvious variations thereof, are
combinable
with or exchangeable for features of the other embodiments described herein,
unless
the person skilled in the art would immediately realize that the resulting
embodiment is
not physically feasible.
Reference numbers in figures
2: Mixing
4: Spinning
6: Quenching and removing part of solvent
7: Drawing
8: lnline drawing
10: Offline drawing
12: Twisting
13: Post-twist-drawing
14: lnline post-twist-drawing
16: Offline post-twist-drawing
18: Un-drawn yarn
20: Multifilament yarn
