- from 50 to 98 wt % by weight of the polymer blend, of a first polymer which is a propylene polymer comprising propylene and at most 1%, relative to the total weight of the polymer, of one or more comonomers selected from the group consisting of ethylene and C₄-C₁₀alpha-olefins, and
- from 2 to 50 wt % by weight of the polymer blend, of a second polymer which is a heterophasic propylene copolymer comprising a propylene polymer and a rubber, wherein the rubber is present in an amount from 5 to 25 wt % relative to the total weight of the heterophasic propylene copolymer.

Preferably, the first and second polymers are prepared using a Ziegler-Natta catalyst.

With preference, in the second polymer, the rubber is ethylene propylene rubber (EPR). In an embodiment, the propylene polymer in the second polymer is a propylene homopolymer.

According to the invention the rubber, with preference ethylene-propylene rubber (EPR), is present in the polymer blend in an amount from 0.2 to 7 wt %, relative to the total weight of the polymer blend. With preference, the rubber content, relative to the total weight of the polymer blend is at least 0.3 wt %, preferably at least 0.4 wt %, more preferably at least 0.5 wt %. With preference, the rubber content, relative to the total weight of the polymer blend is at most 6 wt % or 4 wt %, preferably at most 2 wt %, more preferably at most 1.5 wt %. In a preferred embodiment, the polymer blend comprises the rubber in an amount of less than 2.0 wt % relative to the total weight of the polymer blend.

The rubber is present in an amount from 8 to 18 wt % relative to the total weight of the second polymer (i.e. the heterophasic propylene copolymer), preferably from 10 to 15 wt %.

The first polymer is selected from the group consisting of homopolymer polypropylene and random copolymers of propylene and one or more comonomers selected from the group consisting of ethylene and C₄-C₁₀alpha-olefin. Most preferably the propylene polymer is a propylene homopolymer, i.e. with a comonomer content of 0 wt %.

In an embodiment, the first polymer is a mini random propylene copolymer comprising from 0.05 wt % to 1 wt %, relative to the total weight of said copolymer, of one or more comonomers selected from the group consisting of ethylene and C₄-C₁₀alpha-olefins.

In another embodiment, the first polymer is a propylene hompolymer comprising at least two propylene homopolymer fractions of different melt flow index, wherein the ratio of the melt flow index of the fraction with the highest melt flow index and the melt flow index of the fraction with the lowest melt flow index is in the range from 3 to 400, preferably in the range of 5 to 200, more preferably in the range of 10 to 50 and most preferably in the range of 15 to 30.

With preference, the first polymer is a propylene homopolymer comprising at least two propylene homopolymer fractions of different melt flow index, wherein the propylene homopolymer fraction with the lowest melt flow index comprises from 55 wt % to 65 wt %, preferably from 55 wt % to 60 wt % of the propylene homopolymer.

The first polymer shows by one or more of the following properties:

- i. a xylene soluble content in the range from 1.5 wt % to 4.5 wt %; preferably in the range from 1.5 wt % to 3.5 wt %, most preferably in the range of 1.5 wt % to 2.5 wt % and/or
- ii. a melt flow index of at least 3.0 dg/min, more preferably of at least 4.0 dg/min, and most preferably of at least 5.0 dg/min; and/or
- iii. a polydispersity index in the range of 4.0 to 5.5.

The first polymer also shows very high isotacticity, for which the content of mmmm pentads is a measure. Preferably such tacticity is in the range from 97% to 99% of mmmm pentads, determined on the insoluble heptane fraction of the xylene insolubles fraction.

In a preferred embodiment, the fiber is drawn in the solid state at a draw ratio of at least 3 and shows a tensile strength of at least 45 cN/tex. In accordance with the invention, the fiber is drawn at a draw ratio of at least 3 and shows a tensile strength of at least 45 cN/tex and a toughness modulus of at least 100 MPa, preferably of at least 110 MPa.

Further, the invention discloses nonwovens comprising such fibers.

Additionally, the present invention provides a process for the production of high-tenacity fibers.

In a first embodiment, the fibers are produced according to a compact spinning process (short spin) wherein a polypropylene composition is subjected to a process comprising the step of melt-spinning and drawing at a draw ratio of at least 3 to obtain fibers having a tensile strength of at least 45 cN/tex.

In a second embodiment, the fibers are produced according to a traditional spinning process in at least two steps. The process comprises the steps of:

- a) providing a polypropylene composition to an extruder
- b) melt-spinning said polypropylene composition from a number of openings, to form molten filaments;
- c) cooling the molten filaments obtained by step (c) to obtain solidified fibers; and
- d) drawing said solidified fibers at a temperature of at least 70° C. and at most 150° C. and at a draw ratio of at least 3 to obtain fibers having a tensile strength of at least 45 cN/tex.

It is understood that in both embodiments of the process according to the invention the fibers are drawn in a solid state.

The polypropylene composition used in the process is the polypropylene according to the invention and described above which comprises a propylene polymer in a matrix phase and a rubber in a disperse phase; the propylene polymer comprising propylene and at most 1 wt %, relative to the total weight of said polypropylene composition, of one or more comonomers selected from the group consisting of ethylene and C₄-C₁₀alpha-olefins; wherein the polypropylene composition comprises a rubber in an amount from at least 0.2 wt % to at most 7 wt % relative to the total weight of the polypropylene composition.

Preferably, the polypropylene composition is produced by means of a polymer blend as described above.

DESCRIPTION OF THE FIGURE

FIG. 1 provides a graphical representation of a performance ranking of Busschaert-fibers, containing different levels of heterophasic propylene copolymer based on their toughness modulus, which is defined as the integration of the average stress-strain curve relative to the volume of the fiber. The average stress-strain curve is defined as the average stress strain curve of fibers drawn at D/R_max and D/R_max-10%.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the present application, the terms “polypropylene” and “propylene polymer” may be used synonymously.

The expression “% by weight” or “wt %” (weight percent), here and throughout the description unless otherwise defined, refers to the relative weight of the respective component based on the overall weight of the formulation.

Because of the low rubber content of the polypropylene composition, the drawn fiber of the invention is preferably produced by means of a polymer blend. Preferably, the polymer blend according to the invention comprises a propylene polymer and a heterophasic propylene copolymer. The preferred heterophasic propylene copolymer shows a rubber content of 5 to 25 wt % relative to the total weight of the heterophasic propylene copolymer. The preferred blend comprises:

- from 50 to 98 wt % by weight of the polymer blend, of a first polymer which is a propylene polymer comprising propylene and at most 1 wt %, relative to the total weight of the polymer, of one or more comonomers selected from the group consisting of ethylene and C₄-C₁₀alpha-olefins, and
- from 2 to 50 wt % by weight of the polymer blend, of a second polymer which is a heterophasic propylene copolymer comprising a propylene polymer and a rubber, with preference the rubber is ethylene propylene rubber (EPR).

The first polymer is a homopolymer polypropylene or a mini-random copolymer of propylene and ethylene or one or more C₄-C₁₀alpha-olefin. The preferred random copolymer is a copolymer of propylene and ethylene. The random copolymers of propylene of the invention comprise at least 0.05 wt % and at most 1 wt % of comonomer. The most preferred first polymer is a propylene homopolymer.

The second polymer is a heterophasic propylene copolymer. With preference the propylene polymer of the heterophasic propylene copolymer is a propylene homopolymer. The heterophasic propylene copolymer is chosen and provided in an amount sufficient to achieve a rubber content in the polypropylene composition ranging from at least 0.2 to at most 7% wt relative to the total weight of the polypropylene composition.

Preparation of the First Polymer

The polypropylenes used in the present invention are produced by polymerizing propylene and one or more optional comonomers in the presence of a Ziegler-Natta catalyst system or a metallocene catalyst system, which is well-known to the skilled person.

A Ziegler-Natta catalyst system comprises a titanium compound having at least one titanium-halogen bond and an internal electron donor, both on a suitable support, an organoaluminium compound, and an optional external donor. A suitable support is for example a magnesium halide in an active form. A suitable external donor (ED) is for example a phtalate or a succinate or a diether compound.

The organoaluminium compound used in the process of the present invention is triethyl aluminium (TEAL). Advantageously, the triethyl aluminium has a hydride content, expressed as AlH₃, of less than 1.0 wt % with respect to the triethyl aluminium. More preferably, the hydride content is less than 0.5 wt %, and most preferably the hydride content is less than 0.1 wt %. It would not depart from the scope of the invention if the organoaluminium compound contains minor amounts of other compounds of the trialkylaluminium family, such as triisobutyl aluminium, tri-n-butyl aluminium, and linear or cyclic alkyl aluminium compounds containing two or more Al atoms, provided they show polymerization behavior comparable to that of TEAL.

In the process of the present invention the molar ratio Al/Ti is not particularly specified. However, it is preferred that the molar ratio Al/Ti is at most 100.

If an external donor is present, it is preferred that the molar ratio Al/ED, with ED denoting external electron donor, is at most 120, more preferably it is in the range from 5 to 120, and most preferably in the range from 10 to 80.

Before being fed to the polymerization reactor the catalytic system preferably undergoes a premix and/or a pre-polymerization step. In the premix step, the triethyl aluminium (TEAL) and the external electron donor (ED)—if present—, which have been pre-contacted, are mixed with the Ziegler-Natta catalyst at a temperature in the range from 0° C. to 30° C., preferably in the range from 5° C. to 20° C., for up to 15 min. The mixture of TEAL, external electron donor (if present) and Ziegler-Natta catalyst is pre-polymerized with propylene at a temperature in the range from 10° C. to 100° C., preferably in the range from 10° C. to 30° C., for 1 to 30 min, preferably for 2 to 20 min.

The metallocene catalysts are compounds of Group IV transition metals of the Periodic Table such as titanium, zirconium, hafnium, etc., and have a coordinated structure with a metal compound and ligands composed of one or two groups of cyclopentadienyl, indenyl, fluorenyl or their derivatives. Use of metallocene catalysts in the polymerization of olefins has various advantages. Metallocene catalysts have high activities and are capable of preparing polymers with enhanced physical properties in comparison with the polymers prepared using Ziegler-Natta catalysts. The key to metallocenes is the structure of the complex. The structure and geometry of the metallocene can be varied to adapt to the specific need of the producer depending on the desired polymer. Metallocenes comprise a single metal site, which allows for more control of branching and molecular weight distribution of the polymer. Monomers are inserted between the metal and the growing chain of polymer.

The metallocene catalysts generally are provided on a solid support. The support should be an inert solid, which is chemically unreactive with any of the components of the conventional metallocene catalyst. The support is preferably a silica compound.

The polymerization propylene and one or more optional comonomers can for example be carried out in liquid propylene as reaction medium (bulk polymerization). It can also be carried out in diluents, such as hydrocarbon that is inert under polymerization condition (slurry polymerization). It can also be carried out in the gas phase. Those processes are well known to one skilled in the art.

Diluents which are suitable for being used in accordance with the present invention may comprise but are not limited to hydrocarbon diluents such as aliphatic, cycloaliphatic and aromatic hydrocarbon solvents, or halogenated versions of such solvents. Nonlimiting illustrative examples of solvents are butane, isobutane, pentane, hexane, heptane, cyclopentane, cyclohexane, cycloheptane, methyl cyclopentane, methyl cyclohexane, isooctane, benzene, toluene, xylene, chloroform, chlorobenzenes, tetrachloroethylene, dichloroethane and trichloroethane.

For the present invention the propylene polymers are preferably produced by polymerization in liquid propylene at temperatures in the range from 20° C. to 100° C. Preferably, temperatures are in the range from 60° C. to 80° C. The pressure can be atmospheric or higher. Preferably the pressure is between 25 and 50 bar.

Hydrogen is used to control the chain lengths of the propylene polymers. For the production of a propylene polymer with higher MFI, i.e. with lower average molecular weight and shorter polymer chains, the concentration of hydrogen in the polymerization medium needs to be increased. Inversely, the hydrogen concentration in the polymerization medium has to be reduced in order to produce a propylene polymer with lower MFI, i.e. with higher average molecular weight and longer polymer chains.

An example of suitable commercially available propylene homopolymer is marketed as Polypropylene PPH 5069 by Total. Such propylene homopolymer has a typical melt flow index of 6 g/10 min.

In an embodiment of the invention, the first polymer comprises at least two propylene homopolymer fractions of different melt flow index, wherein the ratio of the melt flow index of the fraction with the highest melt flow index and the melt flow index of the fraction with the lowest melt flow index is in the range from 3 to 400. Such bimodal propylene homopolymer is preferably produced in a polymerization unit having two loop reactors in series.

In such a sequential arrangement of polymerization reactors, the propylene homopolymer withdrawn from one reactor is transferred to the one following in the series, where the polymerization is continued. To produce propylene homopolymer fractions of different index, the polymerization conditions in the respective polymerization reactors need to be different, for example in that the hydrogen concentration in the polymerization reactors differs.

The melt flow index (MFI₂) of the propylene polymer produced in the second reactor is calculated using:

Log(MFI_final)=w₁·Log(MFI₁)+w₂·Log(MFI₂)

wherein MFI_finalis the melt flow index of the total propylene polymer produced, MFI₁and MFI₂are the respective melt flow index of the propylene polymers fractions produced in the first and the second polymerization loop reactors, and w₁and w₂are the respective weight fractions of the propylene polymers produced in the first and in the second polymerization loop reactors as expressed in wt % of the total propylene polymer produced in the two polymerization loop reactors. These weight fractions are commonly also described as the contribution by the respective loop.

After the last polymerization reactor the first propylene polymer is recovered as a powder and can then be pelletized or granulated.

Preparation of the Second Polymer

The heterophasic propylene copolymers according to the invention comprise a matrix propylene polymer phase and a dispersed phase of a rubber, by preference ethylene propylene rubber (EPR).

The heterophasic propylene copolymer of the present invention as defined above is produced by sequential polymerization in a series of polymerization reactors in presence of a catalyst system, wherein in a first polymerization stage the propylene polymer is produced, and in a second polymerization stage the rubber is produced by copolymerizing ethylene and at least one further olefin that is different from ethylene. For example, the further olefin is polypropylene. Thus the rubber produced is EPR.

The matrix propylene polymer phase is a preferably propylene homopolymer. The polymerization is conducted to obtain a rubber content from 5 to 25 wt % relative to the total weight of the heterophasic propylene copolymer, preferably from 8 to 18 wt %, more preferably from 10 to 15%.

Examples of catalysts systems used for the production of such copolymers are Ziegler-Natta catalyst systems, as described above for the preparation of the first polymer. The catalyst system is added to the first polymerization stage.

The propylene copolymer can be prepared using a controlled morphology catalyst that produces rubber spherical domains dispersed in a polypropylene matrix. The amount and properties of the components are controlled by the process conditions.

The matrix propylene polymer, preferably propylene homopolymer, can be made for example in loop reactors or in a gas phase reactor. The propylene polymer produced in this way, in a first polymerization stage, is transferred to a second polymerization stage, into one or more secondary reactors where ethylene and propylene monomer is added to produce the EPR. Preferably this polymerization step is done in a gas phase reactor.

The average molecular weight of the rubber, for which the intrinsic viscosity η_Ris a measure, is controlled by addition of hydrogen to the polymerization reactors of the second polymerization stage. The amount of hydrogen added is such that the rubber as a intrinsic viscosity of at least 2.0 dl/g, measured in tetralin at 135° C. following ISO 1628.

The contribution of the second polymerization stage, i.e. the rubber content of the heterophasic propylene copolymer is from 5 to 25 wt % relative to the total weight of the heterophasic propylene copolymer.

An example of suitable commercially available heterophasic propylene copolymer is marketed as Polypropylene PPC 5660 by Total. Such heterophasic propylene copolymer has a typical melt flow index of 7 g/10 min.

After the last polymerization reactor the second polymer is recovered as a powder and can then be pelletized or granulated.

The first and/or second propylene polymers and/or the polypropylene composition according to the invention may contain additives such as, by way of example, antioxidants, light stabilizers, acid scavengers, flame retardants, lubricants, antistatic additives, nucleating/clarifying agents, colorants. An overview of such additives may be found in Plastics Additives Handbook, ed. H. Zweifel, 5^thedition, 2001, Hanser Publishers.

The antioxidants used in the propylene polymers of the present invention preferably have anti-gas fading properties. The preferred antioxidants are selected from the group consisting of phosphites, hindered phenols, hindered amine stabilizers and hydroxylamines. An example for a suitable antioxidant additivation is a blend of Irgafos 168 and Irganox 3114. Alternatively, phenol-free antioxidant additivations are suitable as well, such as for example those based on hindered amine stabilizers, phosphites, hydroxylamines or any combination of these. In general the antioxidants are added to the propylene homopolymer in an amount from 100 ppm to 2000 ppm with the exact amount depending upon the nature of the antioxidant, the processing conditions and other factors.

Preparation of the Polypropylene Composition

The first polymer (i.e. the propylene homopolymer and/or the random propylene copolymer) and the second polymer (i.e. the heterophasic propylene polymer) are blended together in a molten state. The amount of heterophasic propylene polymer used with the homopolymer and/or the random propylene copolymer, is calculated in order to have the desired amount of rubber in the polypropylene composition of the fiber. The desired amount of rubber ranges from 0.2 wt % to 7 wt % relative to the total weight of the polypropylene composition of the fiber.

For example a suitable blend according to the invention is a blend comprising from 50 to 98 wt % of a propylene homopolymer and from 2 to 50 wt % of an heterophasic propylene copolymer, relative to the total weight of the polymer blend. Preferably the blend comprises from 2 to 20 wt %, more preferably from 3 to 15 wt % of the heterophasic propylene copolymer relative to the total weight of the polymer blend, even more preferably from 5 to 10 wt %.

The propylene homopolymer and the heterophasic propylene copolymer are mixed together in pelletized, fluff or powder form prior to being introduced into an extruder. In certain instances, the polymers may be dry blended together prior to being introduced into the extruder. Alternatively the polymers may be introduced separately into the extruder at a position to achieve thorough mixing of the polymers within the extruder, such as with a gravimetric or volumetric dosing unit, which are commonly known in the art.

With preference, the first polymer (i.e. the propylene homopolymer and/or the random propylene copolymer) and the second polymer (i.e. the heterophasic propylene polymer) are chosen so the resulting polypropylene composition has a melt flow index of at least 3.0 g/10 min, more preferably of at least 4.0 g/10 min, and most preferably of at least 5.0 g/10 min and/or a melt flow index of at most 15 g/10 min, preferably at most 12 g/10 min, more preferably a at most 15 g/10 min, the melt flow index being measured according to ISO 1133.

Additives may be combined with the polymers during the extrusion process, such as, by way of example, antioxidants, light stabilizers, acid scavengers, flame retardants, lubricants, antistatic additives, nucleating/clarifying agents, colorants. An overview of such additives may be found in Plastics Additives Handbook, ed. H. Zweifel, 5^thedition, 2001, Hanser Publishers.

Process for Production of High Tenacity Drawn Fibers

The polypropylene composition of the present invention is used to produce fibers and yarns. In particular, it is used to produce staple fibers and high-tenacity yarns. These may in turn be used in the manufacture of nonwovens. The methods used in the production of the fibers and yarns as well as the nonwovens are known to the person skilled in the art and are for example described in Polypropylene Handbook, ed. Nello Pasquini, 2nd edition, Hanser, 2005, pages 397-403.

The polypropylene composition resulting from the blend of the first and second polymer is then extruded through a number of fine capillaries of a spinneret, thus obtaining molten fibers. The still molten fibers (also called “filaments”) are simultaneously cooled by air and melt drawn to an intermediate diameter. Subsequently they are solidified. Preferably, the solidified fibers are re-heated to a temperature of 150° C. or less, drawn to a draw ratio of at least 3, and then annealed. In a preferred mode the annealed fibers are cut to a length in the range from 1.5 mm to 200 mm, preferably in the range from 10 mm to 100 mm. Such “cut fibers” are generally referred to as “staple fibers”.

Staple fibers in general are produced either by the traditional spinning process or the compact spinning process. In the traditional spinning process staple fibers are produced in two steps. The first step includes fiber production, application of a spin finish to give certain desired properties to the fibers, and winding the undrawn fiber bundle or tow. The second step includes drawing of the fibers, optional application of a second spin finish, optional crimping or texturizing and cutting into staple fibers. The compact spinning, also called short spin, process is a one-step process, wherein fiber extrusion, drawing, and optional crimping or texturizing are performed in a single step.

The staple fibers produced in accordance with the present invention may subsequently be used to produce nonwovens. Preferably the production of nonwovens comprises the steps of carding, thus forming a web, which is then passed through a bonding step. Bonding of the web can be accomplished by thermobonding, hydroentanglement, needle punching, or chemical bonding. For geotextiles and nonwovens for automotive applications needle punching is preferred.

The nonwovens of the present invention are preferably geotextiles and nonwovens for automotive applications.

The fibers and yarns produced in accordance with the present invention have a high tenacity and an improved toughness modulus with respect to the prior art fibers and yarns. Preferably, the fibers and yarns produced in accordance with the present invention have a high tenacity of at least 45 cN/tex and a toughness modulus of at least 100 MPa. They also show improved elongational properties. In particular, the fibers and yarns of the present invention show an improved tensile strength in combination with improved elongational properties. The fibers and yarns of the present invention are characterized by tensile strength of 45 cN/tex or higher, in order to be well suited for nonwovens.

Test Methods

The melt flow index was measured according to ISO 1133, condition L, using a weight of 2.16 kg and a temperature of 230° C.

Xylene solubles (XS) were determined as follows: Between 4.5 and 5.5 g of propylene polymer were weighed into a flask and 300 ml xylene were added. The xylene was heated under stirring to reflux for 45 minutes. Stirring was continued for 15 minutes exactly without heating. The flask was then placed in a thermostated bath set to 25° C.+/−1° C. for 1 hour. The solution was filtered through Whatman n° 4 filter paper and exactly 100 ml of solvent were collected. The solvent was then evaporated and the residue dried and weighed. The percentage of xylene solubles (“XS”) was then calculated according to

XS(in wt %)=(Weight of the residue/Initial total weight ofPP)*300

with all weights being in the same units, such as for example in grams.

Acetone insolubles are determined as follow: 100 ml of the filtrate of the solution in xylene (see above) and 700 ml of acetone are agitated overnight at room temperature in a hermetically sealed flask, during which time a precipitate is formed. The precipitate is collected on a metal mesh filter with a mesh width of 0.056 mm, dried and weighed. The percentage of acetone insolubles (“Aclns”) is then calculated according to:

Aclns (in wt %)=(Weight of the residue/initial weight ofPP)*300

with all weights being in the same units, such as for example in grams.

The amount of rubber in heterophasic propylene copolymer or in the polymer resulting from the polymer blend, is determined as the acetone insoluble fraction of the xylene soluble fraction.

The intrinsic viscosity of the rubber is determined using the acetone insoluble fraction of the xylene soluble fraction of the heterophasic propylene copolymer. The intrinsic viscosity is determined in a capillary viscometer in tetralin at 135° C.

The polydispersity index (PDI) is determined using the ratio of the weight average molecular weight of the polymer to the number average molecular weight of the polymer (Mw/Mn). Mw is determined by gel permeation chromatography (GPC) at 145° C. in 1,2,4-trichlorobenzene. The resulting solution is injected into a gel permeation chromatograph and analyzed under conditions well-known in the polymer industry.

Heptane insolubles were isolated as follows: The xylene insoluble fraction (see above) was dried in air for a minimum of 3 days and manually ground into small pieces, of which ca. 2 g are weighed into the extraction thimble of a Soxleth extractor and extracted with heptane under reflux for 15 hours. The heptane insoluble fraction is recovered from the thimble, and dried in air for a minimum of 4 days.

The polymer index (PI) is given as PI=10⁵Pa·G_c⁻¹. G_cis the cross-over modulus in Pascal determined at 230° C. using a dynamic rheometer in frequency sweep with a strain of 20% on an ARES from Tainstrument, branch of WATERS.

The isotacticity (mmmm %) is determined by NMR analysis according to the method described by G. J. Ray et al. in Macromolecules, vol. 10, n° 4, 1977, p. 773-778. It is performed on the dried product resulting of the extraction by boiling heptane of the xylene insoluble PP fraction.

The recovery compliance is determined at a temperature of 210° C. using a parallel-plate rotational stress rheometer. The sample is contained between two coaxial parallel discs in an oven filled with nitrogen. The test consists of monitoring the strain response when the stress has been deleted after a creep test. For the creep test a stress of 600 Pa is applied. Then the recovery compliance is the recoverable strain divided by the stress applied during the creep.

Filaments titers were measured on a Zweigle vibroscope S151/2 in accordance with norm ISO 1973:1995.

For Busschaert fibres tensile strength and elongation were measured on a Textechno Statimat ME according to norm ISO 2062-B (250 mm length with a testing speed of 250 mm/min).

The toughness modulus is defined as the integration of the average stress-strain curve of a set of tested fibers till break.

Examples

The advantages of the fibers of the invention over those of the prior art are shown in the following examples

Polypropylene Composition

The polypropylene composition of the fiber is obtained in blending two polymers. Their properties are given in table 1.

	TABLE 1

		Second polymer:
	First polymer:	heterophasic propylene
	propylene homopolymer	copolymer

MFI (g/10 min)	6	7
XS (wt %)	2.2	14
PI	4.1	4.8
PDI	6.5	7.1
mmmm	97.3	—
EPR (wt %)	—	13

Five polypropylene compositions have been prepared by blending and tested. The compositions differ from the resulting amount of rubber. Examples 2, 3 and 4 in accordance with the invention, show a rubber content in the range from 0.2 to 7 wt % by weight of the polypropylene composition. Examples 1 and 5 are comparative examples.

The particulars of the compositions are given in table 2.

TABLE 2

propylene	heterophasic
homopolymer	propylene copolymer	Resulting EPR
content (wt %)	content (wt %)	content (wt %)
in the blend	in the blend	in the blend

Ex 1	100	—	—
(comparative)
Ex 2	95	5	0.65
Ex 3	90	10	1.30
Ex 4	50	50	6.50
Ex 5	—	100	13
(comparative)

Fiber Spinning

The propylene polymers were spun into fibers on a Busschaert fiber spinning pilot line equipped with two dies of 112 circular holes each of a diameter of 0.3 mm and an L/D ratio of 3.2. The melt temperature was kept at 280° C. The filaments were drawn between two pairs of godets with the temperature of the first pair being at 80° C. and the second pair being at 90° C. Draw ratio was between 3 and 5. The targeted fiber titer was in the range from 5 to 7 dtex per filament. This was achieved by keeping the winder speed after the drawing step at constant speed of 1200 m/min and adapting the take-up speed, i.e. the speed at which the fibers are collected directly after melt spinning. It is noted that long spin fibres (multifilament), produced on the Busschaert fiber spinning pilot line, typically reach 40-60 cN/Tex at 20-60% elongation at break.

The respective draw ratio and the fiber properties are indicated in table 3.

	TABLE 3

	Product

Ex 1				Ex 5
(comp)	Ex 2	Ex 3	Ex 4	(comp)

EPR (wt %)	—	0.65	1.30	6.50	13
draw ratio	3.5	3.4	3.1	3.9	3.9
D/R
tensile	47	47	45	47	38
strength
(cN/tex)
elongation at	33	36	48	42	36
break (%)
toughness	97	118	145	113	76
modulus
(MPa)

The results clearly demonstrate the advantages of the present invention:

- The fibers show a tensile strength of at least 45 cN/tex. Their tensile strength are improved relative to the comparative example 5. Their elongational properties are improved relative to the comparative example 1.
- The toughness modulus of the fibers is improved for the fibers of the invention relative to the comparative examples.

The results further show that for a polypropylene composition comprising a rubber content in the range from at least 0.2 wt % to less than 6.0 wt % relative to the total weight of the polypropylene composition, preferably to less than 2.0 wt %, a tensile strength of at least 45 cN/tex can be obtained with a draw ratio lower than in the comparative examples. With such a content of rubber, the drawn fibers according to the invention show higher tensile strength and improved elongational properties at a lower draw ratio compared to drawn fibers prepared with higher content of rubber.

Without being bound by any theory it is believed that good results are obtained in a first hand by the combination of the stiffness-to-impact balance properties of an heterophasic propylene copolymer (in which the rubbery phase is more homogeneously dispersed and size controlled than in a mere blend of propylene polymer with an elastomeric polymer or a rubber), with a propylene polymer having a high isotacticity to produce a propylene composition. In the second hand, it is believed that the targeted tensile strength of at least 45 cN/tex in combination with a toughness modulus of at least 100 MPa obtained with the inventive fibers results from the properties of the polypropylene composition combined with the described production process of the fibers in which the fibers are drawn in a solid state.

FIG. 1 provides a graphical representation of a performance ranking of Busschaert-fibers, containing different levels of a heterophasic propylene copolymer (% of copo in blend), based on their toughness modulus (in Mpa), which is defined as the integration of the average stress-strain curve relative to the volume of the fiber. The average stress-strain curve is defined as the average stress-strain curve of fibers drawn at D/R_max and D/R_max-10%. D/R_max is determined as the D/R that still allows spinning a small bobbin before a fibre break occurs. The curve demonstrates that a significant improvement can be achieved by blending about 2 to 50 wt %, preferably 2 to 20 wt %, and most preferably about 3 to 15 wt % of the heterophasic propylene copolymer into a polypropylene homopolymer, with the EPR content in the heterophasic propylene copolymer being 13 wt % relative to the heterophasic propylene copolymer weight.

In conclusion it was found that the high-tenacity fibers of the present invention show the desired combination of tensile strength and elongational properties.