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.
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
One problem when manufacturing multifilament yarn of polyethylene is that polyethylene is an electrical isolator and hence tends to build up electro static charge during manufacturing and since the individual filaments of the multifilament yarn are 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 release 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
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.
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 according to claim 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 have a contraction zone shaped to provide partially drawing or aligning of the filaments already in the spin holes for example as disclosed in
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.
The invention will be explained more fully below with reference to exemplary embodiments as well as the drawings, in which
All figures show only steps which are necessary in order to elucidate the invention, other parts being omitted or merely suggested.
The drawing of the multifilament yarn may be conducted in one or more of the following variations (see
It is preferred that twisting the partially drawn multifilament yarn and post-twist-drawing are conducted inline. The method according to the invention may incorporate more than one post-twist-drawing step. If more 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 step. If more 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 dedicated (freestanding) equipment or may be combined 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
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 comprise 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 take place below the melting point of the multifilament yarn. Yet more preferably, all drawing steps after spinning take 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 - 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 aperture. 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 gelspun multifilament yarns typically having a tenacity of at most about 180 N/tex.
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 nylons and aramids) 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 drawn 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 1 GPa. 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 glueing.
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 that drawing of a yarn construction (such as drawing of a rope construction as described in
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-drawn by DRpt of at least 1.5, and more preferably post-twist-drawn by DRpt of at least 2, and most preferably post-twist-drawn 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 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 such 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 consists 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 than other 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 comprises 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-draw the multifilament 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-draw the multifilament 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 DRpt is 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 DRPT is less than 50; more preferably DRpt is at most 25; more preferably DRPT 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 from 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
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 allow 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. Examples 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 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.
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.
Bobbins of partially drawn UHMWPE yarn prepared in Example 5 were subjected to the inline drawing process as described in
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.
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.
Bobbins of partially drawn UHMWPE yarn prepared in Example 5 were subjected to the offline drawing process as described in
Two batches of bobbins (each about 400 g) were prepared.
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.
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.