WO2012126973A1

WO2012126973A1 - Fine polypropylene fibers and a method for their production

Info

Publication number: WO2012126973A1
Application number: PCT/EP2012/055052
Authority: WO
Inventors: Gaëtan HENRY
Original assignee: Total Research & Technology Feluy
Priority date: 2011-03-24
Filing date: 2012-03-22
Publication date: 2012-09-27

Abstract

The present application relates to fine polypropylene fibers as well as to a method for the production of such fine polypropylene fibers. Further, the present application relates to nonwovens comprising such fine polypropylene fibers.

Description

FINE POLYPROPYLENE FIBERS AND A METHOD FOR THEIR PRODUCTION

Field of the invention The present application relates to fine polypropylene fibers as well as to a method for the production of such fine polypropylene fibers. Further, the present application relates to nonwovens comprising such fine polypropylene fibers.

The technical problem and the prior art

Its unique combination of mechanical and chemical properties together with good processability has allowed polypropylene to become the polymer of choice for fibers and nonwovens used in a wide range of applications, such as for example in the construction and agricultural industries, for sanitary and medical articles, carpets and textiles.

This success is largely owned to the discovery of polymerization catalysts, the so-called Ziegler-Natta catalysts, i.e. transition metal coordination catalysts, specifically titanium halide containing catalysts, which have allowed the production of solid polypropylene at industrially interesting cost. Polypropylenes produced with Ziegler-Natta catalysts give acceptable properties in fibers and nonwovens, though for some applications, such as spunbonding or meltblowing, they need to be further modified, e.g. by visbreaking to narrow the molecular weight distribution and increase the melt flow index.

Recently polypropylenes produced with a metallocene-based polymerization catalyst, often referred to as "metallocene polypropylenes", have become commercially available and are increasingly used in the production of fibers, particularly for fibers and nonwovens produced by the spunbonding process. In addition to the good combination of mechanical and physical properties, metallocene polypropylene is of interest as it allows the production of finer fibers, i.e. fibers having a lower titer, than polypropylene produced with Ziegler-Natta polymerization catalysts.

To fully take advantage of the possibility to produce finer fibers and nonwovens comprising metallocene polypropylene fibers, converters have had to invest in new and cost intensive equipment, specifically designed for metallocene polypropylenes. Such an approach is of interest only, if from the start such new equipment would be fully used to produce such finer fibers. It is clear that this renders it for example very difficult for converters to make trial runs and produce test quantities for example for exploring the market potential.

Hence, there is a need for a process for the production of fine polypropylene fibers that does not require big investments and allows for the production of such polypropylene fibers and nonwovens having comparable mechanical properties to conventional polypropylene fibers and nonwovens.

In consequence, it is an objective of the present application to provide a process for the production of fine polypropylene fibers, said process being applicable to existing equipment.

It is further an objective of the present application to provide a process for the production of fine polypropylene fibers, such fibers having a final titer of 1.2 denier or less.

It is also an objective of the present application to provide a process for the production of nonwovens comprising such fine polypropylene fibers. It is a further objective of the present application to provide a process for the production of nonwovens being characterized by improved softness.

Furthermore, it is an objective of the present application to provide a process for the production of nonwovens having improved mechanical properties.

Additionally, it is an objective of the present application to provide nonwovens having improved softness or improved mechanical properties or both. In addition, it is an objective of the present application to provide nonwovens having improved web coverage for a given nonwoven weight.

Brief description of the invention

We have now discovered that any of these objectives, either individually or in any combination, can be achieved by providing the following process for the production of fine polypropylene fibers. Hence, the present application provides for a process for producing fine polypropylene fibers and nonwovens comprising said fine polypropylene fibers, said fine polypropylene fibers characterized by a titer of at most 1.2 denier, said process comprising the steps of

(a) providing a polypropylene composition comprising at least 50 wt%, relative to the total weight of said polypropylene composition, of a metallocene polypropylene to an extruder;

(b) subsequently melt-extruding the polypropylene composition to obtain a molten polypropylene stream; (c) extruding the molten polypropylene stream of step (b) from a number of fine, usually circular, capillaries of a die, thus obtaining filaments of molten polymer; and

(d) subsequently cooling and reducing the titer of the filaments obtained in step (c) to a final fiber titer,

wherein in step (c) the temperature of the molten polypropylene stream is at least Tmeit + 110°C, with Tmeit being the melting temperature, determined according to ISO 3146, of the metallocene polypropylene.

Further, the present application provides for the use of the above process for the production of fine polypropylene fibers consisting of a polypropylene composition comprising at least 50 wt% of a metallocene polypropylene,

(i) characterized in that the final titer of the so-produced fibers is at most 95 % of the final titer of a fiber produced under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C, or

(ii) characterized in that in step (c) the die pressure is at most 80 % of the die pressure under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C, or

(iii) characterized in that the elongation of the nonwoven, determined according to ISO 9073-3:1989, is at least 105 % of the elongation of a nonwoven having substantially the same weight per surface, said nonwoven having been produced under the same conditions but at a temperature of the molten polymer stream of at most Tmei_t + 100°C, or

(iv) characterized in that the tensile strength of the nonwoven, determined according to ISO 9073-3:1989, is at least 104 %, preferably at least 106 % of the tensile strength of a nonwoven having substantially the same basis weight, said nonwoven having been produced under the same conditions but at a temperature of the molten polymer stream of at most T_meit + 100°C, or

(v) characterized in that the bonding temperature is at least 2 °C lower than the bonding temperature of polypropylene fibers obtained under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + locrc,

with Tmeit being the melting temperature, determined according to ISO 3146, of the metallocene polypropylene.

The present application also provides for polypropylene fibers consisting of a polypropylene composition, said polypropylene composition comprising at least 50 wt%, relative to the total weight of said polypropylene composition, of a metallocene propylene polymer, said polypropylene fibers being substantially round and having a titer of at most 1.2 denier.

Detailed description of the invention For the purposes of the present application, the terms "polypropylene" and "propylene polymer" may be used synonymously.

For the purposes of the present application, the terms "fiber" and "filament" may be used synonymously.

For the purposes of the present application, the term "metallocene polypropylene" is used to denote a polypropylene produced with a metallocene-based polymerization catalyst. For the purposes of the present application, "fine fibers" and "fine polypropylene fibers" are characterized by a titer of at most 1.2 denier. Preferably, said titer is at most 1.1 denier, even more preferably at most 1.05 denier, and most preferably at most 1.0 denier. For the purposes of the present application, the term "tetrahydroindenyl" signifies an indenyl group wherein the six-membered ring has been hydrogenated to 4,5,6,7- tetrahydroindenyl. Throughout the present application, the melt flow index (M FI) of a polypropylene or a polypropylene composition is determined according to ISO 1133, condition L, at 230°C and 2.16 kg.

FIBERS

The polypropylene fibers of the present application comprise

(i) a polypropylene component consisting of a polypropylene composition as defined below, and

(ii) an optional thermoplastic polymer component consisting of a thermoplastic polymer composition as defined below.

Fibers consisting of a polypropylene component only may also be referred to as "monocomponent fibers". Fibers consisting of a polypropylene component and one or more thermoplastic polymer components may also be referred to as "multicomponent fibers". Fibers consisting of a polypropylene component and one thermoplastic polymer component only may also be referred to as "bicomponent fibers", i.e. the term "multicomponent fibers" is understood to include "bicomponent fibers".

Preferably, the polypropylene fibers of the present application are monocomponent fibers or multicomponent fibers. More preferably, the polypropylene fibers of the present application are monocomponent fibers or bicomponent fibers. Most preferably, the polypropylene fibers of the present application are monocomponent fibers, i.e. consist of a polypropylene component only. In the multicomponent fibers of the present application, the one or more thermoplastic polymer components preferably cover at least 70 %, more preferably at least 80 %, even more preferably at least 90 %, still even more preferably at least 99 % of the surface of said multicomponent fibers, and most preferably covers the entire surface of said multicomponent fibers.

In the multicomponent fibers of the present application, the one or more thermoplastic polymer components preferably comprise at most 40 wt%, more preferably at most 30 wt%, even more preferably at most 20 wt% and most preferably at most 10 wt% of the total weight of said multicomponent fibers. Preferably, said one or more thermoplastic polymer components comprise at least 5 wt% of the total weight of said multicomponent fibers.

POLYPROPYLEN E COMPOSITION

The polypropylene composition, of which the polypropylene component comprised in the polypropylene fibers of the present application consists, comprises at least 50 wt%, relative to the total weight of said polypropylene composition, of a metallocene polypropylene as further defined below.

Preferably, said polypropylene composition comprises at least 60 wt% or 70 wt%, more preferably at least 80 wt% or 90 wt%, even more preferably at least 95 wt% or 97 wt%, and still even more preferably at least 99 wt%, relative to the total weight of said polypropylene composition, of the metallocene polypropylene. Most preferably, said polypropylene composition consists of the metallocene polypropylene. The remainder of said polypropylene composition preferably is a thermoplastic polymer composition as defined below.

Preferably, said polypropylene composition comprises at least 96.0 wt% or 97.0 wt%, more preferably at least 98.0 wt% or 98.5 wt%, even more preferably at least 99.0 wt%, still even more preferably at least 99.5 wt% and most preferably consists of propylene monomeric units, with wt% relative to the total weight of said polypropylene composition. Preferably the metallocene polypropylene used herein has a melt flow index of at least 5.0 dg/min, more preferably of at least 10 dg/min, even more preferably of at least 15 dg/min, and most preferably of at least 20 dg/min. Preferably the metallocene polypropylene used herein has a melt flow index of at most 150 dg/min, more preferably of at most 130 dg/min or 110 dg/min, even more preferably of at most 90 dg/min or 70 dg/min, still even more preferably of at most 50 dg/min, and most preferably of at most 40 dg/min.

Preferably, the metallocene polypropylene used herein has a molecular weight distribution (MWD), defined as M_w/M_n, i.e. the ratio of weight average molecular weight M_w over number average molecular weight M_n, of at least 1.0, more preferably of at least 1.5 and most preferably of at least 2.0. Preferably, the metallocene polypropylene used herein has a molecular weight distribution, defined as M_w/M_n, of at most 4.0, more preferably of at most 3.5, even more preferably of at most 3.0, and most preferably of at most 2.8. Molecular weights can be determined by size exclusion chromatography (SEC) as described in the test methods.

Preferably, the metallocene polypropylene used herein is characterized by a high isotacticity, for which the content of mmmm pentads is a measure. Preferably, the content of mmmm pentads is at least 90 %, more preferably at least 95 %, and most preferably at least 97 %. The isotacticity may be determined by ¹³C-N MR analysis as described in the test methods.

Preferably, the metallocene polypropylene used herein is a copolymer of propylene and at least one comonomer, said at least one comonomer being different from propylene. With respect to the comonomer content of said copolymer of propylene and at least one comonomer, it is preferably at most 2.0 wt%, more preferably at most 1.5 wt%, even more preferably at most 1.0 wt%, and most preferably at most 0.5 wt%, relative to the total weight of said copolymer of propylene and at least one comonomer. It is expressly noted that the present definition is meant to include a comonomer content of 0 wt%. In other words, the metallocene polypropylene may preferably be a propylene homopolymer or a copolymer of propylene and at least one comonomer, said at least one comonomer being different from propylene, with a comonomer content as defined above. Preferred comonomers are selected from alpha-olefins having from one to 10 carbon atoms. Preferred alpha-olefins are selected from the group consisting of ethylene, butene-1, pentene-1, hexene-1, octene-1 and 3-methyl-pentene-l. More preferred alpha-olefins are ethylene, butene-1 and hexene-1. Even more preferred alpha-olefins are ethylene and butene-1. The most preferred alpha-olefin is ethylene.

Preferably, the metallocene polypropylene used herein is characterized by a melting temperature T_mei_t of at most 160°C. Melting temperatures may be determined as described in the test methods. Preferably, the metallocene polypropylene used herein is characterized by a percentage of 2,1-insertions, relative to the total number of propylene molecules in the polymer chain, of at least 0.1 %. Preferably, the percentage of 2,1-insertions is at most 1.5 %, more preferably at most 1.3 %, even more preferably at most 1.2 %, still even more preferably at most 1.1 %, and most preferably at most 1.0 %. The percentage of 2,1-insertions may be determined as indicated in the test methods.

The polypropylene used herein may also comprise further additives, such as by way of example, antioxidants, light stabilizers, acid scavengers, lubricants, antistatic additives, nucleating agents and colorants. An overview of such additives may be found in Plastics Additives Handbook, ed. H. Zweifel, 5^th edition, 2001, Hanser Publishers. The metallocene polypropylene used herein is obtained by polymerizing propylene, or copolymerizing propylene and at least one comonomer, with a metallocene-based polymerization catalyst. In addition to a metallocene component, the metallocene- based polymerization catalyst preferably comprises a support and an activating agent. Such metallocene-based polymerization catalysts are generally known in the art and need not be explained in detail.

The metallocene component can be described by the following general formula

wherein R^a, R^b, R^c, M, X¹ and X² are as defined below.

R^a is the bridge between R^b and R^c, i.e. R^a is chemically connected to R^b and R^c, and is selected from the group consisting of -(CR¹R²)_P- -(Si ^{1 2})_p- -(GeR¹R²)_p- -( ^p- - (PR^p- -(N⁺R¹R²)_P- and -(P R²^-, and p is 1 or 2, and wherein R¹ and R² are each independently selected from the group consisting of hydrogen, Ci-Cio alkyl, C₅-C₈ cycloalkyl, C₆-Ci₅ aryl, alkylaryl with Ci-Cio alkyl and C₆-Ci₅ aryl, or any two neighboring R (i.e. two neighboring R¹, two neighboring R², or R¹ with a neighboring R²) may form a cyclic saturated or non-saturated C₄-Ci₀ ring; each R¹ and R² may in turn be substituted in the same way. Preferably R^a is -(CR¹R²)_P- or -(SiR¹R²)_p- with R¹, R² and p as defined above. Most preferably R^a is— (SiR¹R²)_p— with R¹, R² and p as defined above. Specific examples of R^a include Me₂C, ethanediyl (-CH₂-CH₂-), Ph₂C and Me₂Si. M is a metal selected from Ti, Zr and Hf, preferably it is Zr.

X¹ and X² are independently selected from the group consisting of halogen, hydrogen, Ci-Cio alkyl, C6-C15 aryl, alkylaryl with C₁-C₁₀ alkyl and C6-C15 aryl. Preferably X¹ and X² are halogen or methyl. R^b and R^c are selected independently from one another and comprise a cyclopentadienyl ring.

Preferred examples of halogen are CI, Br, and I. Preferred examples of Ci-Cio alkyl are methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, and tert-butyl. Preferred examples of C5-C7 cycloalkyl are cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl. Preferred examples of C6-C15 aryl are phenyl and indenyl. Preferred examples of alkylaryl with Q-Cio alkyl and C₆-Ci₅ aryl are benzyl (-CH₂-Ph), and -(CH₂)2-Ph. Preferably, R^b and R^c may both be substituted cyclopentadienyl, or may be independently from one another unsubstituted or substituted indenyl or tetrahydroindenyl, or R^b may be a substituted cyclopentadienyl and R^c a substituted or unsubstituted fluorenyl. More preferably, R^b and R^c may both be the same and may be selected from the group consisting of substituted cyclopentadienyl, unsubstituted indenyl, substituted indenyl, unsubstituted tetrahydroindenyl and substituted tetrahydroindenyl. By "unsubstituted" is meant that all positions on R^b resp. R^c, except for the one to which the bridge is attached, are occupied by hydrogen. By "substituted" is meant that, in addition to the position at which the bridge is attached, at least one other position on R^b resp. R^c is occupied by a substituent other than hydrogen, wherein each of the substituents may independently be selected from the group consisting of C1-C10 alkyl, C5-C7 cycloalkyl, C6-C15 aryl, and alkylaryl with C1-C10 alkyl and C₆-Ci₅ aryl, or any two neighboring substituents may form a cyclic saturated or non-saturated C4-C10 ring. A substituted cyclopentadienyl may for example be represented by the general formula C₅R³R⁴R⁵R⁶. A substituted indenyl may for example be represented by the general formula C₉R⁷R⁸R⁹R¹⁰RⁿR¹²R¹³R¹⁴. A substituted tetrahydroindenyl may for example be represented by the general formula CgH4R¹⁵R¹⁶R¹⁷R¹⁸. A substituted fluorenyl may for example be represented by the general formula Ci₃R¹⁹R²⁰R²¹R²²R²³R²⁴R²⁵R²⁶. Each of the substituents R³ to R²⁶ may independently be selected from the group consisting of hydrogen, Ci-Cio alkyl, C₅-C₇ cycloalkyl, C₆-Ci₅ aryl, and alkylaryl with Ci-Cio alkyl and C₆-Ci₅ aryl, or any two neighboring R may form a cyclic saturated or non-saturated C₄-Ci₀ ring; provided, however, that not all substituents simultaneously are hydrogen.

Preferred metallocene components are those having C2-symmetry or those having Cr symmetry. Most preferred are those having C2-symmetry.

Particularly suitable metallocene components are those wherein R^b and R^c are the same and are substituted cyclopentadienyl, preferably wherein the cyclopentadienyl is substituted in the 2-position, the 3-position, or simultaneously the 2-position and the 3-position.

Particularly suitable metallocene components are also those wherein R^b and R^c are the same and are selected from the group consisting of unsubstituted indenyl, unsubstituted tetrahydroindenyl, substituted indenyl and substituted tetrahydroindenyl. Substituted indenyl is preferably substituted in the 2-position, the 3-position, the 4-position, the 5-position or any combination of these, more preferably in the 2-position, the 4-position or simultaneously in the 2-position and the 4-position. Substituted tetrahydroindenyl is preferably substituted in the 2-position, the 3- position, or simultaneously the 2-position and the 3-position.

Particularly suitable metallocene components may also be those wherein R^b is a substituted cyclopentadienyl and R^c is a substituted or unsubstituted fluorenyl. The substituted cyclopentadienyl is preferably substituted in the 2-position, the 3-position, the 5-position or simultaneously any combination of these, more preferably in the 3- position or the 5-position or both simultaneously, most preferably in the 3-position only, with a bulky substituent. Said bulky substituent may for exam pie be -CR²⁷R²⁸R²⁹ or -SiR²⁷R²⁸R²⁹ with R²⁷, R²⁸ and R²⁹ independently selected from group consisting of Ci-Cio alkyl, C₅-C₇ cycloalkyl, C₆-Ci₅ aryl, and alkylaryl with Ci-Cio alkyl and C₆-Ci₅ aryl, or any two neighboring R may form a cyclic saturated or non-saturated C₄-Ci₀ ring, it is preferred that R²⁷, R²⁸ and R²⁹ are methyl.

Examples of particularly suitable metallocenes are:

dimethylsilanediyl-bis(2-methyl-cyclopentadienyl)zirconium dichloride,

dimethylsilanediyl-bis(3-methyl-cyclopentadienyl)zirconium dichloride,

dimethylsilanediyl-bis(3-tert-butyl-cyclopentadienyl)zirconium dichloride,

dimethylsilanediyl-bis(3-tert-butyl-5-methyl-cyclopentadienyl)zirconium dichloride, dimethylsilanediyl-bis(2,4-dimethyl-cyclopentadienyl)zirconium dichloride, dimethylsilanediyl-bis(indenyl)zirconium dichloride,

dimethylsilanediyl-bis(2-methyl-indenyl)zirconium dichloride,

dimethylsilanediyl-bis(3-methyl-indenyl)zirconium dichloride,

dimethylsilanediyl-bis(3-tert-butyl-indenyl)zirconium dichloride,

dimethylsilanediyl-bis(4,7-dimethyl-indenyl)zirconium dichloride,

dimethylsilanediyl-bis(tetrahydroindenyl)zirconium dichloride,

dimethylsilanediyl-bis(benzindenyl)zirconium dichloride,

dimethylsilanediyl-bis(3,3'-2-methyl-benzindenyl)zirconium dichloride,

dimethylsilanediyl-bis(4-phenyl-indenyl)zirconium dichloride,

dimethylsilanediyl-bis(2-methyl-4-phenyl-indenyl)zirconium dichloride,

ethanediyl-bis(indenyl)zirconium dichloride,

ethanediyl -bis(tetrahydroindenyl)zirconium dichloride,

isopropylidene-(3-tert-butyl-cyclopentadienyl)(fluorenyl) zirconium dichloride isopropylidene-(3-tert-butyl-5-methyl-cyclopentadienyl)(fluorenyl) zirconium dichloride.

The metallocene may be supported according to any method known in the art. In the event it is supported, the support used in the present invention can be any organic or inorganic solid, particularly porous supports such as talc, inorganic oxides, and resinous support material such as polyolefin. Preferably, the support material is an inorganic oxide in its finely divided form. The polymerization of propylene and the one or more optional comonomers in presence of a metallocene-based polymerization catalyst to obtain a metallocene can be carried out according to known techniques in one or more polymerization reactors. The metallocene polypropylene used in the present invention is preferably produced by polymerization in liquid propylene at temperatures in the range from 20°C to 150°C. Preferably, temperatures are in the range from 60°C to 120°C. The pressure can be atmospheric or higher. It is preferably between 25 and 50 bar. The molecular weight of the polymer chains, and in consequence the melt flow of the metallocene polypropylene, is regulated by the addition of hydrogen to the polymerization medium.

Preferably, the metallocene polypropylene is recovered from the one or more polymerization reactors without post-polymerization treatment to reduce its molecular weight and/or narrow its molecular weight distribution, such as can be done by thermal or chemical degradation, and is often done for polypropylene produced with a Ziegler-Natta catalyst. An example for chemical degradation is visbreaking, wherein the polypropylene is reacted for example with an organic peroxide at elevated temperatures, for example in an extruder or pelletizing equipment. THERMOPLASTIC POLYMER COMPOSITION

The thermoplastic polymer composition used herein may further comprise one or more thermoplastic polymers different from the metallocene polypropylene as defined above.

Irrespectively of the number of components, which are comprised in the thermoplastic polymer composition, it is understood that their weight percentages, relative to the total weight of said thermoplastic polymer composition, add up to 100 wt%. Preferred suitable thermoplastic polymers may be selected from the group consisting of polyolefins, polyamides and polyesters, with the provision that the polyolefin is different from the metallocene polypropylene used herein. By "different from the metallocene polypropylene" is meant that the polyolefin differs in at least one characteristic from the above defined metallocene polypropylene. Said polyolefin may for example be different in composition, such as for example be based on an alpha- olefin different from propylene (e.g. ethylene, 1-butene, 1-pentene, 1-hexene or 1- octene), or be produced with a Ziegler-Natta catalyst instead of a metallocene-based polymerization catalyst, or have a different type of comonomer, or have a different content of comonomer, or have a different melt flow index.

Exemplary polyolefins for use herein are olefin homopolymers and copolymers of an olefin and one or more comonomers. The polyolefins may be atactic, syndiotactic or isotactic. The olefin can for example be ethylene, propylene, 1-butene, 1-pentene, 1- hexene, 4-methyl-l-pentene or 1-octene, but also cycloolefins such as for example cyclopentene, cyclohexene, cyclooctene or norbornene. The comonomer is different from the olefin and chosen such that it is suited for copolymerization with the olefin. The comonomer may also be an olefin as defined above. Further examples of suitable comonomers are vinyl acetate (H₃C-C(=0)0-CH=CH2) or vinyl alcohol ("HO-CH=CH₂", which as such is not stable and tends to polymerize). Examples of olefin copolymers suited for use in the present invention are random copolymers of propylene and ethylene, random copolymers of propylene and 1-butene, heterophasic copolymers of propylene and ethylene, ethylene-butene copolymers, ethylene-hexene copolymers, ethylene-octene copolymers, copolymers of ethylene and vinyl acetate (EVA), copolymers of ethylene and vinyl alcohol (EVOH).

Exemplary polyamides for use herein may be characterized in that the polymer chain comprises amide groups (-N H-C(=0)-). Polyamides useful in the present invention are preferably characterized by one of the following chemical structures

H-[-N H-(CH₂)_n-C(=0)-]_x-OH H-[-NH-(CH₂)_m-N H-C(=0)-(CH₂)_n-C(=0)-]_x-OH

wherein m and n may be independently chosen from one another and be an integer from 1 to 20.

Specific examples of suitable polyamides are polyamides 4, 6, 7, 8, 9, 10, 11, 12, 46, 66, 610, 612, or 613. Another example of a suitable polyamide is Nylon-MXD6, obtainable by polycondensation of meta-xylylene diamine with adipic acid and commercially available for example from Mitsubishi Gas Chemical Company.

Exemplary polyesters for use herein are preferably characterized by the following chemical structure

[-C(=0)-C₆H₄-C(=0)0-(CH₂-CH₂)_n-0-]_x

wherein n is an integer from 1 to 10₇ with preferred values being 1 or 2.

Specific examples of suitable polyesters are polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). Furthermore, preferred polyesters are poly(hydroxy carboxylic acid)s.

With respect to the melt flow index of the polypropylene composition as well as the thermoplastic polymer composition, it is preferred that they are within the same ranges and values as defined above for the metallocene polypropylene.

PRODUCTION OF FIBERS AN D NONWOVENS

The fibers of the present application are produced by commonly known production methods, such as for example described in Polypropylene Handbook, ed. Nello Pasquini, 2^nd edition, Hanser, 2005, pages 397-403 or in F. Fourne, Synthetische Fasern, Carl Hanser Verlag, 1995, chapter 5.2 or in B.C. Goswami et al., Textile Yarns, John Wiley & Sons, 1977, p. 371 - 376. Generally, fibers are produced by melting a polymer or a polymer composition in an extruder, optionally passing the molten polymer through a melt pump to ensure a constant feeding rate and then extruding the molten polymer or molten polymer composition through a number of fine capillaries of a spinneret to form fibers. These still molten fibers are simultaneously cooled by air and drawn to a final diameter and are finally collected. Optionally, the so- obtained fibers may be subjected to a further drawing step, though for the present application it is preferred they are as-spun, i.e. that no further drawing step is performed on the fibers.

Thus, the process for producing fine polypropylene fibers comprises the steps of

(a) providing a polypropylene composition as defined earlier in this application to an extruder;

(b) subsequently melt-extruding the polypropylene composition to obtain a molten polypropylene stream;

(c) extruding the molten polypropylene stream of step (b) from a number of fine, usually circular, capillaries of a die, thus obtaining extrudates of molten polypropylene; and

(d) subsequently cooling and reducing the titer of the filaments obtained in step (c) to a final fiber titer to obtain fine polypropylene fibers.

For the present application it is essential that in step (c) the temperature of the molten polypropylene stream, which may also be referred to as "melt temperature" (in contrast to "melting temperature") is at least T_mei_t + 110 °C, with T_mei_t being the melting temperature, determined according to ISO 3146 as further described in the test methods, of the metallocene polypropylene comprised in the polypropylene composition of step (a). Preferably, said temperature of the molten polypropylene stream is at least T_mei_t + 120 °C. Irrespective of the melting temperature of the metallocene polypropylene it is preferred that the temperature of the molten polypropylene stream is at least 260 °C and most preferred that it is at least 265 °C. In case the molten polypropylene stream comprises more than one polypropylene, the temperature of the molten polymer stream is determined in relation to the highest melting polypropylene comprised in the molten polypropylene stream.

In addition to the increased temperature of the molten polypropylene stream, it is preferred that the throughput per hole per min, i.e. the amount of molten polypropylene passing through a capillary of the die in a given time, is reduced to at most 95 %, more preferably to at most 90 %, and most preferably to at most 85 % of the throughput used in the production of polypropylene fibers not falling within the definition of "fine polypropylene fibers".

For the production of multicomponent fibers, the process comprises the steps of

(a') providing a thermoplastic polymer composition as defined earlier in this application to a further extruder;

(b') subsequently melt-extruding the thermoplastic polymer composition to obtain a molten thermoplastic polymer stream;

(c) extruding the molten polypropylene stream of step (b) from a number of fine, usually circular, capillaries of a die, thus obtaining extrudates of molten polypropylene;

(c') extruding the molten thermoplastic polymer stream of step (b') through a number of fine openings surrounding the capillaries of step (b), thus obtaining extrudates of molten thermoplastic polymer; and

(c") combining the extrudates obtained in steps (c) and (c') to form single filaments of an intermediate diameter, such that an extrudate of step (c') covers at least 70 % of the surface of the fine polypropylene fiber, and (d) subsequently cooling and reducing the titer of the filaments obtained in the previous step to a final fiber titer to obtain fine polypropylene fibers.

For the production of multicomponent fibers comprising more than two components, one or more further polymer compositions, such as for example a further polypropylene composition or a further thermoplastic polymer composition, may in turn be fed to separate extruders, subsequently melt extruded to form the respective extrudates, which are then combined with the extrudates of steps (c) and (c') to form single filaments.

It has come as a complete surprise to find that finer fibers can be produced when the fibers are produced at a higher temperature of the molten polypropylene stream than normally used in the production of fibers. In fact, before conducting the below described experiments, it had rather been expected that the well known lack of melt strength in polypropylenes having a very narrow molecular weight distribution would very much limit processability of the above described polypropylene composition. Quite unexpectedly, the present process also did not lead to processing problems caused by extruded filaments, which are still hot, sticking together as had been feared due to the delayed crystallization in consequence of the higher melt temperature.

Hence, the present application provides for the use of the above described process for producing the fine polypropylene fibers consisting of a polypropylene composition comprising at least 50 wt% of a metallocene propylene polymer, characterized in that the final titer is at most 95 %, preferably at most 90 % of the final titer of a fiber produced under the same conditions but at a temperature of the molten polypropylene stream of at most T_mei_t + 100°C, with T_mei_t being the melting temperature, determined according to ISO 3146, of the metallocene propylene polymer. Expressed differently, the above described process for producing the fine polypropylene fibers consisting of a polypropylene composition comprising at least 50 wt% of a metallocene propylene polymer is characterized in that the final titer is at most 95 %, preferably at most 90 % of the final titer of a fiber produced under the same conditions but at a temperature of the molten polypropylene stream of at most Tmeit + 100°C, with Tmeit being the melting temperature, determined according to ISO 3146, of the metallocene propylene polymer.

It has further been noted that producing the fine polypropylene fibers consisting of a polypropylene composition comprising at least 50 wt% of a metallocene propylene polymer, allows in step (c) to drastically reduce the die pressure.

Hence, the present application provides for the use of the above described process for producing the fine polypropylene fibers consisting of a polypropylene composition comprising at least 50 wt% of a metallocene propylene polymer, characterized in that in step (c) the die pressure is at most 80 % of the die pressure under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C, with Tmeit being the melting temperature, determined according to ISO 3146, of the metallocene propylene polymer.

Expressed differently, the above described process for producing the fine polypropylene fibers consisting of a polypropylene composition comprising at least 50 wt% of a metallocene propylene polymer, is characterized in that in step (c) the die pressure is at most 80 % of the die pressure under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C, with T_mei_t being the melting temperature, determined according to ISO 3146, of the metallocene propylene polymer.

The polypropylene nonwovens of the present application may be produced by any suitable methods. Such methods include thermal bonding of staple fibers, the spunlacing process, and the spunbonding process. The preferred method is the spunbonding process.

In addition to the above described process steps (a) to (d), and optionally (a') to (c"), the process for producing nonwovens comprising the polypropylene fibers of the present application further comprises the steps of

(e) collecting the fine polypropylene fibers obtained in step (d) on a support; and

(f) subsequently bonding the collected fine polypropylene fibers of step (e) to form a bonded nonwoven.

For the production of thermally bonded nonwovens the present fibers are cut into staple fibers having a length in the range from 5 to 30 mm. Said staple fibers are then carded, i.e. collected as a more or less continuous non-consolidated web on a support. In a final step the non-consolidated web is consolidated by thermal or chemical bonding, with thermal bonding being preferred.

In the spunlacing process continuous fibers or staple fibers are distributed randomly a support to form a non-consolidated web, which is then consolidated by means of fine high-pressure water jets and dried.

In the spunbonding process a thermoplastic polymer is melted in a first extruder, optionally passed through a melt pump to ensure a constant feeding rate and then extruded through a number of fine, usually circular capillaries of a spinneret.

For the production of multicomponent fibers, for example bicomponent fibers, further polymer blends are melted in further extruders, optionally passed through a melt pump, and then extruded through a number of fine openings surrounding the fine, usually circular capillaries of the spinneret. The various extrudates are then combined to form a single - essentially still molten - filament of an intermediate diameter. The filament formation can either be done by using one single spinneret with a large number of holes, generally several thousand, or by using several smaller spinnerets with a correspondingly lower number of holes per spinneret. After exiting from the spinneret, the still molten filaments are quenched by a current of air. The diameter of the filaments is then quickly reduced by a flow of high-pressure air. Air velocities in this drawdown step can range up to several thousand meters per minute. After drawdown the filaments are collected on a support, for example a forming wire or a porous forming belt, thus first forming an unbonded web, which is then passed through compaction rolls and finally through a bonding step. Bonding of the fabric may be accomplished by thermobonding, hydroentanglement, needle punching, or chemical bonding.

The nonwovens produced with the fine polypropylene fibers described above showed a surprising effect when used in the production of nonwovens. In comparison to polypropylene fibers produced at lower temperature of the molten polypropylene stream, fine polypropylene fibers produced in accordance with the process described in the present application allowed to produce nonwovens that were characterized by a higher elongation or a higher tensile strength or both. Hence, the present application provides for the use of the above described process for the production of nonwovens comprising polypropylene fibers consisting of a polypropylene composition comprising at least 50 wt% of a metallocene propylene polymer, characterized in that the elongation of the nonwoven, determined according to ISO 9073-3:1989, is at least 105 %, preferably at least 110 %, and most preferably at least 115 % of the elongation of a nonwoven having substantially the same weight per surface, said nonwoven having been produced under the same conditions but at a temperature of the molten polymer stream of at most T_meit + 100°C, with T_meit being the melting temperature, determined according to ISO 3146, of the metallocene propylene polymer. Expressed differently, the above described process for the production of nonwovens comprising polypropylene fibers consisting of a polypropylene composition comprising at least 50 wt% of a metallocene propylene polymer, is characterized in that the elongation of the nonwoven, determined according to ISO 9073-3:1989, is at least 105 %, preferably at least 110 %, and most preferably at least 115 % of the elongation of a nonwoven having substantially the same weight per surface, said nonwoven having been produced under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C, with T_mei_t being the melting temperature, determined according to ISO 3146, of the metallocene propylene polymer.

Additionally, the present application also provides for the use of the above described process for the production of nonwovens comprising polypropylene fibers consisting of a polypropylene composition comprising at least 50 wt% of a metallocene propylene polymer, characterized in that the tensile strength of the nonwoven, determined according to ISO 9073-3:1989, is at least 104 %, preferably at least 106 % of the tensile strength of a nonwoven having substantially the same basis weight, said nonwoven having been produced under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C, with T_mei_t being the melting temperature, determined according to ISO 3146, of the metallocene propylene polymer.

Expressed differently, the above described process for the production of nonwovens comprising polypropylene fibers consisting of a polypropylene composition comprising at least 50 wt% of a metallocene propylene polymer, is characterized in that the tensile strength of the nonwoven, determined according to ISO 9073-3:1989, is at least 104 %, preferably at least 106 % of the tensile strength of a nonwoven having substantially the same basis weight, said nonwoven having been produced under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C, with Tmei_t being the melting temperature, determined according to ISO 3146, of the metallocene propylene polymer. During the production of nonwovens it was surprisingly noted that the fine polypropylene fibers described herein allow a reduction in the bonding temperature. Such a reduction is of interest to nonwovens producers because it allows higher production speeds in the bonding of polypropylene fibers.

Therefore the present application additionally provides for the use of the above described process for the production of nonwovens comprising polypropylene fibers consisting of a polypropylene composition comprising at least 50 wt% of a metallocene propylene polymer, characterized in that the bonding temperature is at least 2 °C, preferably at least 4 °C and most preferably at least 6 °C lower than the bonding temperature of polypropylene fibers obtained under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C, with T_mei_t being the melting temperature, determined according to ISO 3146, of the metallocene propylene polymer.

Expressed differently, the above described process for the production of nonwovens comprising polypropylene fibers consisting of a polypropylene composition comprising at least 50 wt% of a metallocene propylene polymer, is characterized in that the bonding temperature is at least 2 °C, preferably at least 4 °C and most preferably at least 6 °C lower than the bonding temperature of polypropylene fibers obtained under the same conditions but at a temperature of the molten polymer stream of at most Tmeit + 100°C, with Tmeit being the melting temperature, determined according to ISO 3146, of the metallocene propylene polymer. Composites may be formed from two or more nonwovens, of which at least one comprises fine polypropylene fibers defined above. Said two or more nonwovens may either be bonded together, or they may be left "unbonded" to one another, i.e. just placed on top of each other. In particular, the composites comprise a spunlace or spunbond nonwoven layer (S) according to the present invention or a melt blown nonwoven layer (M) according to the present invention. Composites in accordance with the present invention can for example be SS, SSS, SMS, SM MSS or any other combination of spunlace or spunbond and melt blown nonwoven layers.

A first nonwoven or composite, said first nonwoven or composite comprising the fine polypropylene fibers defined above, and a film may be combined to form a laminate. The film preferably is a polyolefin film. The laminate is formed by bringing the first nonwoven or composite and the film together and laminating them to one another for example by passing them through a pair of lamination rolls. The laminates may further include a second nonwoven or composite, which can be but need not be according to the present invention, on the face of the film opposite to that of the first nonwoven or composite. In a preferred embodiment, the film of the laminate is a breathable polyolefin film, thus resulting in a laminate with breathable properties.

The polypropylene fibers and filaments described herein can be used in carpets, woven textiles, and nonwovens.

The polypropylene spunbond nonwovens of the present invention as well as composites or laminates comprising it can be used for hygiene and sanitary products, such as for example diapers, feminine hygiene products and incontinence products, products for construction and agricultural applications, medical drapes and gowns, protective wear, lab coats, wipes, for example in sanitary but also in industrial applications, etc.

Test methods

Melt flow index (MFI) of polypropylene and polypropylene compositions is determined according to ISO 1133, condition L, at 230°C and 2.16 kg. Molecular weights are determined by Size Exclusion Chromatography (SEC) at high temperature (145°C). A 10 mg polypropylene or polyethylene sample is dissolved at 160°C in 10 ml of trichlorobenzene (technical grade) for 1 hour. Analytical conditions for the GPCV 2000 from WATERS are :

- Injection volume: +/- 400μΙ

- Automatic sample preparation and injector temperature: 160°C

- Column temperature: 145°C

- Detector temperature: 160°C

- Column set : 2 Shodex AT-806MS and 1 Styragel HT6E

- Flow rate: 1 ml/min

- Detector: Infrared detector (2800-3000 cm )

- Calibration: Narrow standards of polystyrene (commercially available)

- Calculation for polypropylene: Based on Mark-Houwink relation (logio(Mpp) = logio(Mps) - 0.25323 ); cut off on the low molecular weight end at M_PP = 1000. The molecular weight distribution (MWD) is then calculated as M_w/M_n.

Xylene solubles (XS), i.e. the xylene soluble fraction, are determined as follows: Between 4.5 and 5.5 g of propylene polymer are weighed into a flask and 300 ml xylene are added. The xylene is heated under stirring to reflux for 45 minutes. Stirring is continued for 15 minutes without heating. The flask is then placed in a thermostat bath set to 25°C +/- 1°C for 1 hour. The solution is filtered through Whatman n° 4 filter paper and 100 ml of solvent are collected. The solvent is then evaporated and the residue dried and weighed. The percentage of xylene solubles ("XS"), i.e. the amount of the xylene soluble fraction, is then calculated according to

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

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

The ¹³C-N MR analysis is performed using a 400 MHz Bruker NMR spectrometer under conditions such that the signal intensity in the spectrum is directly proportional to the total number of contributing carbon atoms in the sample. Such conditions are well known to the skilled person and include for example sufficient relaxation time etc. In practice the intensity of a signal is obtained from its integral, i.e. the corresponding area. The data is acquired using proton decoupling, 4000 scans per spectrum, a pulse repetition delay of 20 seconds and a spectral width of 26000 Hz. The sample is prepared by dissolving a sufficient amount of polymer in 1,2,4-trichlorobenzene (TCB, 99%, spectroscopic grade) at 130°C and occasional agitation to homogenize the sample, followed by the addition of hexadeuterobenzene (CeD6, spectroscopic grade) and a minor amount of hexamethyldisiloxane (H MDS, 99.5+ %), with HMDS serving as internal standard. To give an example, about 200 mg of polymer are dissolved in 2.0 ml of TCB, followed by addition of 0.5 ml of C₆D₆ and 2 to 3 drops of H MDS.

Following data acquisition the chemical shifts are referenced to the signal of the internal standard HMDS, which is assigned a value of 2.03 ppm. The isotacticity is determined by ¹³C-N MR analysis on the total polymer. In the spectral region of the methyl groups the signals corresponding to the pentads mmmm, mmmr, mmrr and mrrm are assigned using published data, for example A. Razavi, Macromol. Symp., vol. 89, pages 345-367. Only the pentads mmmm, mmmr, mmrr and mrrm are taken into consideration due to the weak intensity of the signals corresponding to the remaining pentads. For the signal relating to the mmrr pentad a correction is performed for its overlap with a methyl signal related to 2,1-insertions. The percentage of mmmm pentads is then calculated according to

% mmmm = AREA_mmmm / (AREA_mmmm + AREA_mmmr + AREA_mmrr + AREA_mrrm) · 100 Determination of the percentage of 2,1-insertions for a metallocene propylene homopolymer: The signals corresponding to the 2,1-insertions are identified with the aid of published data, for example H.N. Cheng, J. Ewen, Makromol. Chem., vol. 190 (1989), pages 1931-1940. A first area, AREA1, is defined as the average area of the signals corresponding to 2,1-insertions. A second area, AREA2, is defined as the average area of the signals corresponding to 1,2-insertions. The assignment of the signals relating to the 1,2-insertions is well known to the skilled person and need not be explained further. The percentage of 2,1-insertions is calculated according to

2,1-insertions (in %) = AREA1 / (AREA1 + AREA2) · 100

with the percentage in 2,1-insertions being given as the molar percentage of 2,1- inserted propylene with respect to total propylene.

The determination of the percentage of 2,1-insertions for a metallocene random copolymer of propylene and ethylene is determined by two contributions:

(i) the percentage of 2,1-insertions as defined above for the propylene homopolymer, and

(ii) the percentage of 2,1-insertions, wherein the 2,1-inserted propylene neighbors an ethylene,

thus the total percentage of 2,1-insertions corresponds to the sum of these two contributions. The assignments of the signal for case (ii) can be done either by using reference spectra or by referring to the published literature.

Melting temperatures T_mei_t were determined according to ISO 3146 on a DSC Q2000 instrument by TA Instruments. To erase the thermal history the samples are first heated to 200 °C and kept at 200 °C for a period of 3 minutes. The reported melting temperatures T_mei_t are then determined with heating and cooling rates of 20°C/min.

Fiber tenacity and elongation were measured on a Lenzing Vibrodyn according to ISO 5079:1995 with a testing rate of 10 mm/min. Tensile strength and elongation of nonwovens were measured according to ISO 9073- 3:1989. Examples

To illustrate the advantages of the present invention, two polypropylene compositions, denoted "ppi" and "PP2", were used under different fiber spinning conditions.

Polypropylene composition PPI consisted of a propylene homopolymer produced under standard polymerization conditions in an industrial-size polymerization plant with a metallocene-based polymerization catalyst with a dimethylsilyl-bridged bis(indenyl)zirconium dichloride as metallocene component.

Polypropylene composition PP2 consisted of a propylene homopolymer produced under standard polymerization conditions in an industrial-size polymerization plant with a commercial Ziegler-Natta polymerization catalyst, followed by visbreaking to increase the melt flow index as well as narrow the molecular weight distribution. Properties of the respective polypropylenes, which had been additivated with a sufficient amount of antioxidants and acid scavengers to reduce their degradation during processing, used in PPI and PP2 are given in Table I.

Table I

SPUN BON D NONWOVENS

Polypropylene compositions PPI and PP2 were used to produce spunbond nonwovens on a 1.1 m wide Reicofil 4 line with a single beam having about 6800 holes per meter length, the holes having a diameter of 0.6 mm. Targeted fabric weight was 12 g/m². The nonwovens were thermally bonded using an embossed roll. Further processing conditions are given in Table II. In Table II the temperature of the molten polypropylene stream is denoted as "Die - Melt temp.".

The calender temperature, denoted "calender temp.", in Table II is the bonding temperature at which the highest values for max. tensile strength were obtained. The calender temperature is measured on the embossed roll using a contact thermocouple.

Properties of the nonwovens obtained are shown in Table III, with MD denoting "machine direction" and CD denoting "cross direction".

Table II

*) Due to unstable spinning no nonwovens could be produced. Table III

*) Due to unstable spinning no nonwovens could be produced.

The results show that an increase in melt temperature, i.e. the temperature of the molten polypropylene stream, leads to a reduction in the final titer of the fibers of more than 10 % of the titer under "low temperature extrusion" conditions, as can be seen from the results of Example 1 and Example 2.

Interestingly, and also very surprisingly, the reduction in final fiber titer by increase of the melt temperature is only possible with polypropylene compositions comprising a metallocene polypropylene. A polypropylene composition run under "high temperature conditions" (see Ex. 3) could not be processed into a nonwoven due to unstable spinning. It has further surprisingly been found that the "high temperature extrusion conditions" allowed the production of nonwovens with improved tensile strength as well as increased elongation, and that in both, MD and CD direction.

Claims

Process for producing fine polypropylene fibers and nonwovens comprising said fine polypropylene fibers, said fine polypropylene fibers characterized by a titer of at most 1.2 denier, said process comprising the steps of

(a) providing a polypropylene composition comprising at least 50 wt%, relative to the total weight of said polypropylene composition, of a metallocene polypropylene to an extruder,

(c) extruding the molten polypropylene stream of step (b) from a number of fine, usually circular, capillaries of a die, thus obtaining filaments of molten polymer; and

wherein in step (c) the temperature of the molten polypropylene stream is at least Tmei_t + 110°C, with T_mei_t being the melting temperature, determined according to ISO 3146, of the metallocene polypropylene. 2. Process according to claim 1 further comprising the step of

(e) collecting the filaments obtained in step (d) on a support; and

(f) subsequently bonding the collected filaments of step (e) to form a bonded nonwoven.

Process according to any of the preceding claims, wherein in step (c) the temperature of the molten polymer stream is at least T_mei_t + 120°C, with T_mei_t being the melting temperature, determined according to ISO 3146, of the metallocene polypropylene obtained in step (a).

4. Process according to any of the preceding claims, wherein the metallocene polypropylene is a copolymer of propylene and at least one comonomer, said at least one comonomer being different from propylene, with a comonomer content of at most 2.0 wt%, relative to the total weight of said metallocene propylene polymer, of the at least one comonomer.

5. Process according to any of the preceding claims, wherein the metallocene polypropylene has a melt flow index MFI, determined according to ISO 1133, condition L at 230°C and 2.16 kg, of at least 10 dg/min and of at most 150 dg/min.

6. Process according to any of the preceding claims, wherein the metallocene polypropylene has a melting temperature of at most 160°C, determined according to ISO 3146.

7. Process according to any of the preceding claims, wherein the process is characterized in that the final titer of the so-produced fibers is at most 95 % of the final titer of a fiber produced under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C,

8. Process according to any of the preceding claims, wherein the process is characterized in that in step (c) the die pressure is at most 80 % of the pressure under the same conditions but at a temperature of the molten polymer stream of at most Tmeit + 100°C,

with Tmeit being the melting temperature, determined according to ISO 3146, of the metallocene polypropylene. Process according to any of the preceding claims, wherein the process is characterized in that the elongation of the nonwoven, determined according to ISO 9073-3:1989, is at least 105 % of the elongation of a nonwoven having substantially the same weight per surface, said nonwoven having been produced under the same conditions but at a temperature of the molten polymer stream of at most Tmeit + 100°C,

with Tmei_t being the melting temperature, determined according to ISO 3146, of the metallocene polypropylene. 10. Process according to any of the preceding claims, wherein the process is characterized in that the tensile strength of the nonwoven, determined according to ISO 9073-3:1989, is at least 104 %, preferably at least 106 % of the tensile strength of a nonwoven having substantially the same basis weight, said nonwoven having been produced under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C,

with Tmei_t being the melting temperature, determined according to ISO 3146, of the metallocene polypropylene.

Process according to any of the preceding claims, wherein the process is characterized in that the bonding temperature is at least 2 °C lower than the bonding temperature of polypropylene fibers obtained under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C,

12. Use of the process of any of claims 1 to 6 for the production of fine polypropylene fibers consisting of a polypropylene composition comprising at least 50 wt% of a metallocene polypropylene, characterized in that the final titer of the so-produced fibers is at most 95 % of the final titer of a fiber produced under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C, or characterized in that in step (c) the die pressure is at most 80 % of the die pressure under the same conditions but at a temperature of the molten polymer stream of at most T_meit + 100°C, or characterized in that the elongation of the nonwoven, determined according to ISO 9073-3:1989, is at least 105 % of the elongation of a nonwoven having substantially the same weight per surface, said nonwoven having been produced under the same conditions but at a temperature of the molten polymer stream of at most Tmeit + 100°C, or characterized in that the tensile strength of the nonwoven, determined according to ISO 9073-3:1989, is at least 104 %, preferably at least 106 % of the tensile strength of a nonwoven having substantially the same basis weight, said nonwoven having been produced under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C, or characterized in that the bonding temperature is at least 2 °C lower than the bonding temperature of polypropylene fibers obtained under the same conditions but at a temperature of the molten polymer stream of at most T_mei_t + 100°C, with Tmeit being the melting temperature, determined according to ISO 3146, of the metallocene polypropylene.

13. Polypropylene fibers consisting of a polypropylene composition, said polypropylene composition comprising at least 50 wt%, relative to the total weight of said polypropylene composition, of a metallocene propylene polymer, said polypropylene fibers being substantially round and having a titer of at most 1.2 denier.