Patent Description:
During the last decades, many research projects focused on improving the creep properties of synthetic fibers, since such fibers are extremely suitable for a wide range of applications where lightweight and strength are driving factors. One example of synthetic fibers is UHMWPE fibers, which meet successfully the weight and strength requirements. The almost unmatched strength of UHMWPE fibers combined with ultraviolet resistance, chemical resistance, cut and abrasion resistance and other favorable properties are the reasons that these fibers found an almost immediate utilization in rope mooring, composite reinforcement, medical devices, cargo nets and the like.

UHMWPE fibers have however one drawback which acts as an impediment for their optimal utilization in long-term applications, this drawback being related to their creep behavior. It was observed that the ultimate failure mode of a system using UHMWPE fibers and in particular of those systems placed under a long-term load, is rupture or failure due to creep. More recently creep properties of UHMWPE fibers have been successfully improved making them suitable for above mentioned applications and resulting in first commercial creep optimized products such as the UHMWPE yarn sold by DSM Dyneema, the Netherlands under the trade name DM20.

Examples of UHMWPE fibers having a good creep behavior and a process for producing thereof are known from <CIT>; disclosing UHMWPE fibers having creep rates as low as <NUM> × <NUM>-<NUM> sec-<NUM> as measured at <NUM> under a load of <NUM> MPa and tensile strengths as high as <NUM> GPa.

<CIT> also disclose UHMWPE fibers having a good combination of creep rate and tensile strength, e.g. a creep rate of at most <NUM> × <NUM>-<NUM> sec-<NUM> as measured at <NUM> under a load of <NUM> MPa, and a tensile strength of at least <NUM> GPa.

<CIT> and <CIT> disclose UHMWPE fibers having substantially increased creep life time, e.g. reporting yarns with life time of greater than <NUM> hours as measured at <NUM> under a load of <NUM> MPa.

<CIT>, <CIT> and <CIT> also disclose processes for the preparation of gel spun UHMWPE fibers.

Since long it was recognized, amongst others by the inventors of <CIT>, that manufacturing fibers from branched UHMWPE polymers may produce fibers having good creep resistance. It was observed that good creep properties can be achieved by sufficient length and amounts of branches in the UHMWPE. Nevertheless increasing length and amount of branches may negatively affect the fiber spinning process. Furthermore, UHMWPE polymer with high levels of co-monomer are more difficult to produce and are economically less attractive, whereas length and number of branches may disrupt the fiber structure with negative effects on the fiber properties.

An aim of the present invention may therefore be to provide an improved UHMWPE fiber gel spinning process wherein above mentioned problems are absent or present to a lesser extent, while maintaining creep and strength properties at a commercially interesting level. Especially the gel spinning process disclosed herein may be more economical, and strike a balance between the amount of branches in the UHMWPE and the mechanical properties of the gel-spun fiber.

A process is disclosed herein for producing creep-optimized UHMWPE fiber comprising the steps of providing an UHMWPE composition having an intrinsic viscosity (IV) of at least <NUM> dl/g, a co-monomer content (CBR) of a least <NUM> short chains branches per <NUM> total carbon (SCB/1000TC), a mass averaged distribution of the co-monomer (CMAD) of at least <NUM>; dissolving the UHMWPE composition in a solvent to form a polymer solution having a UHMWPE concentration of between <NUM> and <NUM> wt%; spinning the polymer solution through a multi orifice die plate to form solution fibers; cooling the solution fiber to below <NUM> to form a gel fiber; drawing the fiber in at <NUM> least step to form a drawn fiber; removing at least a portion of the solvent before, during or after the drawing; whereby the ratio CMAD to CBR is greater than <NUM>.

It was observed that by providing an UHMWPE composition with a CMAD to CBR ratio greater than <NUM> creep optimized fibers could be produced with substantially better creep performance than the prior art products. Alternatively it was observed that yarns with strength and creep properties matching the ones of the prior art could be achieved by providing UHMWPE compositions with lower amounts of short chain branches (SCB). The inventors postulate that the balance of properties may have shifted by a more efficient use of a lower amount of short chain branches of the UHMWPE composition to achieve equal mechanical performance.

Where a reduction of short chain branches in the prior art processes would improve production robustness such change would also negatively affects creep performance of the obtained yarns. In the light of the current invention it was observed that the deterioration of creep properties resulting from the overall reduction of SCB can be compensated by providing a UHMWPE composition having a mass averaged distribution of the co-monomer greater than the co-monomer content expressed in SCB/1000TC. Said characteristic of the UHMWPE composition may in less scientific terms be expressed as an inhomogeneous distribution of the co-monomer across the molecular weight, whereby the relative concentration of the co-monomer increases with increasing molecular weight of the polyethylene chains. The characteristic of the UHMWPE composition is expressed in the mass averaged distribution of the co-monomer, CMAD, and is determined by Formula <NUM>. <MAT> wherein <MAT> is the molecular weight distribution of the UHMWPE; br(M) is the co-monomer distribution, expressed as the number of branches per <NUM> total carbon in the molecules of UHMWPE, having molar mass M. The parameters <MAT> and br(M) for a UHMWPE composition can conveniently be established as provided in the Methods.

The UHMWPE composition provided to the process disclosed herein has an intrinsic viscosity of at least <NUM> dl/g, a short chain branching concentration (CBR) of a least <NUM> SCB/1000TC, a mass averaged distribution of the co-monomer (CMAD) of at least <NUM> and a ratio CMAD to CBR greater than <NUM>. Preferably the ratio CMAD to CBR is greater than <NUM>, preferably greater than <NUM>. Such increase of the ratio to higher levels may allow a further reduction of the total branching content of the UHMWPE composition of the inventive fibers. Increasing the ratio can be achieved by adjusting both, the CMAD or the CBR or a combination thereof and will depend on the intended improvement to be achieved.

Whereas there are different means to provide UHMWPE compositions according to the process disclosed herein such as producing a corresponding UHMWPE composition via an ethylene polymerization process or combination of such processes, the inventors identified that a suitable mean to provide the UHMWPE is in that the UHMWPE composition comprises at least <NUM> different UHMWPE polymers, A and B. Such a way to provide the UHMWPE composition is preferred since it does not rely on availability of a suitable commercial UHMWPE polymer, which to inventors best knowledge is not readily available, but can be provided by judiciously selecting different UHMWPE polymers A and B. By different is understood that the <NUM> UHMWPE polymers differ from each other by at least one physical or chemical property, such as molecular weight, co-monomer concentration, molecular weight distribution. In a preferred aspect the UHMWPE composition comprises UHMWPE polymer A having an IV of <NUM>-<NUM> dl/g, and a CBR of less than <NUM> SCB/1000TC, preferably of less than <NUM> SCB/1000TC, and/or a UHMWPE polymer B having an IV of <NUM>-<NUM> dl/g, and a CBR from <NUM> to <NUM> SCB/1000TC, more preferably from <NUM> to <NUM> SCB/1000TC and even more preferably from <NUM> to <NUM> SCB/1000TC. Such preferred combination of polymer A and B provides a UHMWPE composition with CMAD and CBR respecting the inventive characteristics.

In an aspect of the process disclosed herein the ratio of the IV of polymer A to the IV of polymer B is less than <NUM>, preferably at most <NUM>, more preferably at most <NUM>, even more preferably at most <NUM> and most preferably at most <NUM>. By providing polymers A and B with said specific IV ratio the robustness of the production process of creep optimized fibers may be further improved. An alternative way to characterize the UHMWPE composition according to such aspect is that the molecular weight distribution of the UHMWPE composition may show bimodality expressed as a double peak or at least a deviation from the typical monomodal distribution as described by e.g. Gaussian, Log-Normal or Schulz-Flory molecular weight distribution.

The UHMWPE polymers A and B of the UHMWPE composition may be combined in any ratio one to another, preferably the weight ratio of polymer A to polymer B is between <NUM> and <NUM>, preferably between <NUM> and <NUM>, more preferably between <NUM> and <NUM> and most preferably between <NUM> and <NUM>. The inventors identified that for ratios of polymer A to polymer B deviating substantially from the preferred ranges only small improvements in the process may be observed or substantial differences between polymer A and B, for example short chain branching or IV, are required.

The UHMWPE composition and/or the UHMWPE polymers A and B may be obtained by polymerization processes known in the art. Preferably, the UHMWPEs used according to the disclosure herein are obtained by a slurry polymerization process in the presence of an olefin polymerization catalyst at a polymerization temperature, whereby the polymerization catalyst may be a Ziegler (Z) or molecular catalyst (MC) characterized by their single-site nature, to which the well-known metallocene catalysts belong. Preferably, the Ziegler polymerization catalyst used is a Titanium based catalyst for producing UHMWPE. Examples of suitable catalysts are described in <CIT> or <CIT> included herein by reference. Molecular catalysts and therewith produced UHMWPEs are for example described in <CIT> included herein by reference. Whereas each individual catalyst system may provide UHMWPE polymers with specific characteristics and advantages for the present invention, the inventors identified that advantageous processing conditions can be achieved by selecting a polymer A produced by a ZN catalyst and polymer B produced by a single-site catalyst. Accordingly a preferred aspect of the process disclosed herein is a process wherein polymer A is a Z polymer and polymer B is a MC polymer.

In an aspect of the process disclosed herein, the UHMWPE composition used in the process disclosed herein has a polydispersity index (PDI), also commonly called molecular weight distribution Mw/Mn, of at least <NUM>, preferably at least <NUM>, more preferably than <NUM> and most preferably at least <NUM>. Such preferred UHMWPE composition may demonstrate even further improved gel spinning processing. By PDI in the context of the present application is understood the ratio of Mw/Mn. Since there may be conflicting teachings in the literature about the way to measure Mw and/or Mn values for a UHMWPE, resulting in a discrepancy of the molecular weight distribution, the herein understood PDI is the one as measured by SEC technique as further described in the experimental section. The PDI of the UHMWPE composition used in the process disclosed herein may have an upper limit of at most <NUM>.

Where the UHMWPE composition is attained by combining at least <NUM> polymer A and B, the individual polymers A and B are not bound by above limitations. In a preferred aspect of the disclosure herein, polymer A has a PDI of at least <NUM>, preferably at least <NUM>, more preferably than <NUM> and most preferably at least <NUM>. In another preferred aspect of the disclosure polymer B has a PDI of at most <NUM>, preferably of at most <NUM>, more preferably of at most <NUM> and most preferably of at most <NUM>. In a yet preferred aspect of the disclosure the PDI of polymer A is greater than the PDI of polymer B.

The co-monomer present in the UHMWPE composition, the UHMWPE polymer A and/or the UHMWPE polymer B has at least <NUM> carbon atoms and will result in short chain branches (SCB) of the UHMWPE. The nature of the co-monomer is not specifically limited other than that it comprises at least one polymerizable C-C double bond. Preferably the co-monomer is one or more monomers selected from the group consisting of alpha-olefins with at least <NUM> carbon atoms, cyclic olefins having <NUM> to <NUM> carbon atoms and linear, branched or cyclic dienes having <NUM> to <NUM> carbon atoms, more preferably the co-monomer is one or more monomers selected from the group consisting of <NUM>-butene, <NUM>-pentene, <NUM>-hexene, <NUM>-octene. Good results were obtained with <NUM>-butene and <NUM>-hexene providing ethyl and butyl branches to the UHMWPE composition, more preferably to the UHMWPE polymer B.

The disclosure herein further concerns the gel spun UHMWPE fiber obtainable by the herein described inventive process. The UHMWPE of the inventive fibers will have properties substantially corresponding to the properties of the UHMWPE composition used in the innovative preparation process described above. Nevertheless some or all properties of the UHMWPE composition may not be present at an identical level in the UHMWPE of the fiber of the invention due to the chemical, thermal and/or mechanical process to which said UHMWPE composition was subjected. According to claim <NUM> the gel spun fiber according to the invention comprises a UHMWPE having an intrinsic viscosity (IV) of at least <NUM> dl/g, a co-monomer content (CBR) of a least <NUM> SCB/1000TC, a mass averaged distribution of the co-monomer (CMAD) of at least <NUM>, whereby the ratio CMAD to CBR is greater than <NUM>. In an embodiment of the gel spun fiber according to the invention the ratio CMAD to CBR is greater than <NUM>, preferably greater than <NUM>. In a further embodiments of the gel spun fiber according to the invention the ratio CMAD to CBR is greater than <NUM>.

By fiber is herein understood an elongated body, e.g. a body having a length and transverse dimensions, wherein the length of the body is much greater than its transverse dimensions. The fiber may have regular or irregular cross-sections. The fiber may also have a continuous and/or a discontinuous length. Preferably, the fiber has a continuous length, such fiber being known in the art as a filament. The term fiber as used herein may also include various embodiments including filament, staple fiber, tape, strip and ribbon. Within the context of the invention, a yarn is understood to be an elongated body comprising a plurality of fibers.

Preferably, the UHMWPE fibers and in particular those spun from UHMWPE compositions having ethyl or butyl branches, have a tenacity of at least <NUM> cN/dtex, more preferably of at least <NUM> cN/dtex, most preferably of at least <NUM> cN/dtex. Preferably, the inventive UHMWPE fibers and in particular those spun from UHMWPE compositions having ethyl or butyl branches, have an elastic modulus of at least <NUM> cN/dtex, more preferably of at least <NUM> cN/dtex, most preferably of at least <NUM> cN/dtex. It was observed that in addition to the excellent creep properties, the inventive UHMWPE fibers have also good tensile properties.

According to the invention, the inventive UHMWPE fibers are obtained by a gel spinning process, in the art such fibers being also referred to as "gel-spun UHMWPE fibers". For the present invention, by gel-spinning process is meant a process comprising at least the steps of (a) dissolving the composition in a solvent to form a polymer solution having a UHMWPE concentration of between <NUM> and <NUM> wt%, (b) spinning the polymer solution through a multi orifice die plate to form solution fibers, (c) cooling the solution fiber to below <NUM> to form a gel fiber, (d) drawing the fiber in at least one step to form a drawn fiber and (e) removing at least a portion of the solvent before, during or after the drawing. The gel-spinning process may optionally contain more than one drawing step wherein the gel fiber and/or the solid fiber are drawn with a certain draw ratio. Gel spinning processes are known in the art and are disclosed for example in <CIT>; <CIT> and in "<NPL>, these publications and the references cited therein being included herein by reference.

According to the invention, a gel-spinning process is used to manufacture the inventive UHMWPE fibers, wherein as already mentioned hereinabove, the UHMWPE composite is used to produce an UHMWPE solution, which is subsequently spun through a spinneret and the obtained gel fiber is dried to form a solid fiber.

The UHMWPE solution is preferably prepared with a UHMWPE concentration of at least <NUM> wt%, more preferably of at least <NUM> wt%. Preferably the UHMWPE concentration in the solvent is between <NUM> and 25wt%, more preferably between <NUM> and <NUM> wt%. Preferably, the concentration is between <NUM> and <NUM> wt% for UHMWPE with an IV of the UHMWPE composition in the range <NUM>-<NUM> dl/g, preferably <NUM>-<NUM> dl/g.

To prepare the UHMWPE solution, any of the known solvents suitable for gel spinning the UHMWPE may be used. Such solvents are also referred to herein as "spinning solvents". Suitable examples of solvents include aliphatic and alicyclic hydrocarbons, e.g. octane, nonane, decane and paraffins, including isomers thereof; petroleum fractions; mineral oil; kerosene; aromatic hydrocarbons, e.g. toluene, xylene, and naphthalene, including hydrogenated derivatives thereof, e.g. decalin and tetralin; halogenated hydrocarbons, e.g. monochlorobenzene; and cycloalkanes or cycloalkenes, e.g. careen, fluorine, camphene, menthane, dipentene, naphthalene, acenaphtalene, methylcyclopentandien, tricyclodecane, <NUM>,<NUM>,<NUM>,<NUM>-tetramethyl-<NUM>,<NUM>-cyclohexadiene, fluorenone, naphtindane, tetramethyl-p-benzodiquinone, ethylfuorene, fluoranthene and naphthenone. Also combinations of the above-enumerated solvents may be used for gel spinning of UHMWPE, the combination of solvents being also referred to for simplicity as solvent. In a preferred aspect, the solvent of choice is not volatile at room temperature, e.g. paraffin oil. It was also found that the process disclosed herein is especially advantageous for relatively volatile solvents at room temperature, as for example decalin, tetralin and kerosene grades. In the most preferred aspect the solvent of choice is decalin.

The UHMWPE solution is then formed into gel filaments by spinning said solution through a multi orifice die plate, also called spinneret. By multi orifice die plate is herein understood a spinneret containing preferably at least <NUM>, yet even more preferably at least <NUM>, most preferably at least <NUM> spinholes. Preferably, the spinning temperature is between <NUM> and <NUM>, more preferably said temperature is chosen below the boiling point of the spinning solvent. If for example decaline is used as spinning solvent the spinning temperature is preferably at most <NUM>.

The gel filaments formed by spinning the UHMWPE solution through the spinneret may be extruded into an air gap, and then into a cooling zone where they are cooled to below <NUM> to form gel fibers or gel filaments from where they are picked-up on a first driven roller. Preferably, the gel filaments are stretched in the air gap. In the cooling zone, the gel filaments are cooled preferably in a gas flow and/or in a liquid bath.

Subsequently to forming the gel filaments, said gel filaments are subjected to a solvent extraction step wherein the spinning solvent used to manufacture the UHMWPE solution is at least partly removed from the gel filaments to form solid filaments. The solvent removal process may be performed by known methods, for example by evaporation when a relatively volatile spinning solvent, e.g. decaline, is used or by using an extraction liquid, e.g. when paraffin is used as spinning solvent, or by a combination of both methods. Preferably the gel filaments are drawn with a draw ratio of preferably at least <NUM>, more preferably at least <NUM>, most preferable at least <NUM>.

Preferably, the solid filaments are also drawn during and/or after said removal of the solvent. Preferably, the drawing of the solid filaments is performed in at least one drawing step with a draw ratio of preferably at least <NUM>, more preferably at least <NUM>, even more preferably at least <NUM>. More preferably, the drawing of solid filaments is performed in at least two steps, even more preferably in at least three steps.

The inventive gel spun fiber or yarns comprising the gel spun fibers are suitable fibers and yarns for typical fiber applications. Hence one embodiment of the present invention concerns a product comprising the inventive gel spun fiber, preferably the product is selected from the group consisting of yarns, ropes, cables, nets, fabrics, and protective appliances such as ballistic resistant articles.

The inventive UHMWPE fibers have properties which make them an interesting material for use in ropes, cordages and the like, preferably ropes designed for heavy-duty operations as for example marine, industrial and offshore operations. Rigging ropes and ropes used in sports applications such as yachting, climbing, kiteflying, parachuting and the like are also applications where the fibers of the invention may perform well. In particular it was observed that the inventive UHMWPE fibers are particularly useful for long-term and ultralong-term heavy-duty operations.

Heavy duty operations may further include, but not restricted to, crane ropes, ropes for deep-sea deployment or recovery of hardware, anchor handling, mooring of support platforms for offshore renewable energy generation, mooring of offshore oil drilling rigs and production platforms such as offshore production platforms and the like. It was surprisingly observed that for such operations, and in particular for offshore mooring, the installation of ropes designed therefor may be optimized, e.g. the ropes can be installed using less complex hardware or smaller and lighter installation equipment.

The inventive UHMWPE fibers are also very suitable for use as a reinforcing element, for example in a liner, for reinforced products such as hoses, pipes, pressurized vessels, electrical and optical cables, especially when said reinforced products are used in deepwater environments where reinforcement is required to support the load of the reinforced products when free hanging. The invention therefore also relates to a liner and a reinforced product containing reinforcing elements or containing said liner, wherein the reinforcing elements or the liner contain the inventive UHMWPE fibers.

Most preferably, the inventive UHMWPE fibers are used in applications where said fibers experience static tension or static loads and in particular long-term and ultralong-term static tension or static loads. By static tension is herein meant that the fiber in application always or most of the time is under tension irrespective if the tension is at constant level (for example a weight hanging freely in a rope comprising the fiber) or varying level (for example if exposed to thermal expansion or water wave motion). Examples of applications wherein static tensions are encountered are for example many medical applications (for example cables and sutures) but also mooring ropes, and tension reinforcement elements, as the improved creep lifetime of the present fibers leads to improved performances of these and similar applications. A particular application of the inventive UHMWPE fibers is in crane ropes where the rope can reach an elevated temperature as result of (<NUM>) ambient temperatures and/or (<NUM>) internal heat generation due to friction around crane sheaves.

The invention further relates to composite articles containing the inventive UHMWPE fibers. In a preferred embodiment, the composite article contains at least one mono-layer comprising the UHMWPE fibers of the invention. The term mono-layer refers to a layer of fibers, i.e. fibers in one plane. In a further preferred embodiment, the mono-layer is a unidirectional mono-layer. The term unidirectional mono-layer refers to a layer of unidirectionally oriented fibers, i.e. fibers in one plane that are essentially oriented in parallel. In a yet further preferred embodiment, the composite article is multi-layered composite article, containing a plurality of unidirectional mono-layers the direction of the fibers in each mono-layer preferably being rotated with a certain angle with respect to the direction of the fibers in an adjacent mono-layer. Preferably, the angle is at least <NUM>°, more preferably at least <NUM>°, even more preferably at least <NUM>°, most preferably the angle is about <NUM>°. Multi - layered composite articles proved very useful in ballistic applications, e.g. body armor, helmets, hard and flexible shield panels, panels for vehicle armoring and the like. Therefore, the invention also relates to ballistic-resistant articles as the ones enumerated hereinabove containing the UHMWPE fibers of the invention.

The inventive UHMWPE fibers of the invention are also suitable for use in medical devices, e.g. sutures, medical cables, implants, surgical repair products and the like. The invention therefore further relates to a medical device, in particular to a surgical repair product and more in particular to a suture and to a medical cable comprising the UHMWPE fibers of the invention.

It was also observed that the inventive UHMWPE fibers are also suitable for use in other applications like for example, synthetic chains, conveyor belts, tensiarity structures, concrete reinforcements, fishing lines and fishing nets, ground nets, cargo nets and curtains, kite lines, dental floss, tennis racquet strings, canvas (e.g. tent canvas), nonwoven cloths and other types of fabrics, webbings, battery separators, capacitors, pressure vessels (e.g. pressure cylinders, inflatables), hoses, (offshore) umbilical cables, electrical, optical fiber, and signal cables, automotive equipment, power transmission belts, building construction materials, cut and stab resistant and incision resistant articles, protective gloves, composite sports equipment such as skis, helmets, kayaks, canoes, bicycles and boat hulls and spars, speaker cones, high performance electrical insulation, radomes, sails, geo-textiles such as mats, bags and nets, and the like. Therefore, the invention also relates to the applications enumerated above containing the UHMWPE fibers of the invention.

The invention also relates to an elongated object comprising a plurality of the UHMWPE fibers of the invention, wherein said fibers are at least partly fused to each other. In one embodiment said elongated object is a monofilament. In a different embodiment, said elongated object is a tape. By at least partly fused fibers is herein understood that individual fibers are fused at multiple locations along their length and disconnected between said locations. Preferably, said fibers are fully fused to each other, i.e. the individual fibers are fused to each other over essentially their whole length. Preferably, the fusing is carried out by at least compressing said plurality of UHMWPE fibers under a temperature lower than the melting temperature of the fibers. The melting temperature of the fibers can be determined by DSC using a methodology as described at pg. <NUM> of <CIT>. Processes of fusing UHMWPE fibers into monofilaments and tapes are known in the art and disclosed for example in <CIT>, <CIT> and <CIT>. It was observed that by using the fibers of the invention, monofilaments and tapes having optimized creep properties were achieved. Such products were suitable for utilization in applications such as fishing lines; liners; reinforcing elements; antiballistic articles such as armors; car parts; and architectural applications such as doors.

The invention will be further explained by the following examples and comparative experiment, however first the methods used in determining the various parameters used hereinabove are presented.

For practical purposes, the integration in Formula <NUM> can be substituted by a summation as shown in Formula <NUM> <MAT> where wi is the normalized weight fraction of the material fraction with molar mass Mi in the UHMWPE composition. The weight fraction wi can be determined, e.g., by SEC-IR.

Formulas <NUM> and <NUM> are also applicable if a blend of at least two polymers A and B is used <MAT> where XA and XB are the mass fractions of the polymers A and B in the blend (XB = <NUM> - XA) and the subscripts A and B indicate that the corresponding sums must be calculated for the polymer A or polymer B, respectively.

If more than two polymers (A, B, C, etc.) are blended, the Formula <NUM> takes the form of Formula <NUM> <MAT> where k = A, B, C, etc., Xk is the mass fractions of the polymer k in the UHMWPE composition and whereby ΣkXk = <NUM>.

Both the continuous definition, Formula <NUM>, and its discrete version, Formula <NUM>, emphasize the asymmetry of the co-monomer incorporation into the low and high molar mass part of the molecular weight distribution.

Molecular catalyst polymerized UHMWPE: <NUM> UHMWPE polymers have been synthesized as ethylene homopolymers or copolymers of ethylene with <NUM>-butene or <NUM>-hexene. The polymerization procedure as described in <CIT> with the molecular catalyst (MC) of Example <NUM> described therein. Details of the produced polymers I, III, IV, V, VII and VIII are reported in table <NUM>.

Ziegler catalyst polymerized UHMWPE: <NUM> UHMWPE polymers have been synthesized according to the general preparation process described in <CIT> with a supported Ziegler catalyst (Z). Details of the produced polymers II and VI are reported in table <NUM>.

Prior to gel-spinning the fibers, the prepared UHMWPE polymers have been blended by tumbling and later dispersion in the spinning solvent to form UHMWPE compositions. In case of blends of polymers, CBR and CMAD have been established by considering the CBR and CMAD of the individual polymers and their weight ratio in the composition.

A process such as the one disclosed in <CIT> was used to produce UHMWPE fibers from the described UHMWPE polymers or compositions. In particular, the UHMWPE solution was extruded with at a temperature setting of <NUM> through a spinneret having a <NUM> spinholes into an air atmosphere containing also decalin and water vapors with a rate of about <NUM>/min per hole.

The spinholes had a circular cross-section and consisted of a gradual decrease in the initial diameter from <NUM> to <NUM> with a cone angle of <NUM>° followed by a section of constant diameter of <NUM> length, this specific geometry of the spinholes introducing a draw ratio in the spinneret of <NUM>.

From the spinneret the fluid fibers entered an air gap and then into a water bath, where the fluid fibers were taken up at a velocity <NUM> times higher than their velocity at the spinneret outlet, introducing a draw ratio in the air gap of <NUM>.

The fluid fibers were cooled in the water bath to form gel fibers, the water bath being kept at about <NUM> and wherein a water flow was being provided with a flow rate of about <NUM> liters/hour perpendicular to the fibers entering the bath. From the water bath, the gel fibers were taken-up into an oven at a temperature of <NUM> wherein partial solvent evaporation occurred to form solid fibers.

The solid fibers were drawn in a first step at around <NUM> and in a second step at around <NUM> by applying a total solid draw ratio during which process most of the solvent evaporated. The total solid draw ratio is the product of the solid draw ratios used in the first and second drawing step.

All reported samples were drawn to achieve a modulus of approximately 1200cN/dtex and a strength of approximately <NUM> cN/dtex.

The fibers' creep rates and the measurement conditions (temperature and load) for the Comparative Experiments A to D and of the Examples <NUM> to <NUM>, are reported in Table <NUM>. From said table it can be seen that for equal type of branching and comparable total short chain branching concentration the fibers of the invention have substantially increased creep rates. Alternatively it can be observed that similar creep rates can be attained by the inventive fibers at a substantially lower total amount of short chain branches CBR of the UHMWPE composition.

Claim 1:
A gel spun UHMWPE fiber wherein the UHMWPE of the fiber has an intrinsic viscosity (IV) of at least <NUM> dl/g determined as described in the METHODS OF MEASUREMENT,
a co-monomer content (CBR) of a least <NUM> SCB/1000TC determined as described in the METHODS OF MEASUREMENT,
a mass averaged distribution of the co-monomer (CMAD) of at least <NUM> determined as described in the METHODS OF MEASUREMENT,
whereby the ratio CMAD to CBR is greater than <NUM>.