Patent Description:
Melt-blown webs are widely used in hygiene and filtration industry. Important properties of melt-blown webs include hydrostatic head and air permeability. Melt-blown webs may be made from polypropylene. In known ways of making melt-blown webs, polypropylene having a high melt flow index is subjected to a melt-blowing process. It is known to obtain polypropylene having a high melt flow index by viscosity reduction of propylene having a lower melt flow index, typically using peroxides or a hydroxylamine ester. The viscosity reduction is often also described as "vis-breaking", "melt-shifting", "modifying rheology" or "controlling rheology".

<CIT> discloses a process for making propylene polymer pellets comprising mixing a neat propylene polymer and a hydroxylamine ester compound to form a blend, where the neat propylene polymer exhibits a MFR of from <NUM> dg/min to <NUM> dg/min and pelletizing the blend. The pellets are used for making a non-woven fabric. The mixing and pelletizing steps occur at a temperature below that which substantially thermally degrades the hydroxylamine ester compound, preferably a temperature not greater than <NUM>ºC. The blend exhibits MFR which is <NUM> to <NUM> times higher than the MFR of the neat propylene polymer. The blend pellets are heated to form a high MFR polymer, from which fibers are created. The high MFR polymer exhibits a MFR of about <NUM> to about <NUM> dg/min.

<CIT> describes melt-blown webs comprising melt-blown fibers made of a polypropylene composition comprising a polymeric nucleating agent and wherein the polypropylene composition has been vis-broken without the use of peroxide. The visbreaking is performed by a hydroxylamine ester. An example of a commercially available hydroxylamine ester is Irgatec® CR76, which is commercially available from BASF.

<CIT> describes a method of preparing a rheology-controlled polypropylene characterized by comprising a stage of mixing a propylene polymer with at least one low-reactivity organic peroxide, wherein after the stage of mixing said polypropylene it comprises at least one peroxide having at least <NUM>% of active oxygen. As an example of the low-reactivity organic peroxide, Trigonox <NUM> is mentioned. As examples of the organic peroxide which is not a low-reactivity organic peroxide, Trigonox <NUM>, Trigonox <NUM> and Luperox <NUM> are mentioned. It is mentioned that the obtained polypropylene may be used for producing nonwoven fabric such as spunbond and melt-blown.

<CIT> discloses a method of preparing a controlled rheology polypropylene characterized by comprising a stage of mixing a propylene polymer with at least one low-reactivity organic peroxide.

<CIT> discloses a process for making normally solid, gel-free, propylene polymer material with a branching index of less than <NUM> and with a strain hardening elongational viscosity from normally solid, amorphous to predominantly crystalline propylene polymer material without strain hardening elongational viscosity, which comprises:.

It is an objective of the present invention to provide a melt-blown web with desirable properties such as a high hydrostatic head and a low air permeability.

Accordingly, the present invention provides a melt-blown web comprising melt-blown fibers obtained by.

Not according to the presently claimed invention, a process is provided for making a melt-blown web comprising melt-blown fibers, comprising the steps of:.

A melt-blown web is produced from a polypropylene composition comprising two or more types of peroxides having different decomposition temperatures. Heating at a lower temperature as in step a) of the process of the invention activates primarily the peroxide having a lower decomposition temperature and thus a partly vis-broken polypropylene is obtained. This partly vis-broken polypropylene composition, which may optionally be formed into pellets, is converted into melt-blown fibers at a high temperature. The preliminary vis-breaking and the optional pelletization and the final vis-breaking by a melt-blown process may be performed by different entities at different locations.

Surprisingly, the melt-blown web according to the invention has desirable properties such as high hydrostatic head and low air permeability. It was surprisingly found that these properties of the melt-blown article according to the invention are better than those of a melt-blown article made by melt-blowing a polypropylene which already has a high melt flow index.

It is noted that <CIT> discloses a process for the manufacture of polypropylene pellets by adding to the polymer two free radical generators, G1 and G2, the half-life of G2 being at least <NUM> times longer than that of G1 at the pelletisation temperature. Non-sticky pellets with excellent reproducibility are obtained. <CIT> does not mention a melt-blown web. Examples of free radical generators G1 are given, each of which has a half-life time of <NUM> hour at a temperature between <NUM> and <NUM>ºC except for di-tert-butylperoxide which has a half-life time of <NUM> hour at a temperature of <NUM>ºC. Examples of free radical generators G2 includes diisopropylbenzene hydroperoxide which has a half-life time of <NUM> hour at a temperature of <NUM>ºC and a half-life time of <NUM> hour at a temperature of <NUM>ºC. In Example <NUM>, the pellets were converted into continuous filaments having a melt index of <NUM>/<NUM> measured at <NUM>ºC/<NUM> according to ASTM method D1238 condition E. This is lower than the melt index of filaments generally used for making a melt-blown web.

Further relevant information can be found in documents <CIT>, <CIT> and <CIT>.

Step a) involves melt-mixing under conditions at which mainly the first peroxide has the visbreaking effect. The composition obtained by this step may herein sometimes be referred as a partly-vis-broken composition.

The melt-mixing is performed at temperatures between <NUM>ºC and <NUM>ºC, preferably between <NUM>ºC and <NUM>ºC.

The duration of the melt-mixing may be suitably selected by the skilled person depending on the target viscosity properties, for example <NUM> to <NUM> hours.

This melt-mixing step may or may not be preceded by the step of obtaining a mixture (which may or may not be in the form of pellets) under conditions where substantially no visbreaking occurs. The mixture so obtained may herein sometimes be referred as a pre-visbreaking composition.

The pre-visbreaking composition may be obtained by mixing the propylene-based polymer, the first peroxide and the second peroxide at temperatures which do not exceed a temperature T1, wherein T<NUM>/<NUM><NUM> is at least <NUM>ºC higher than T1, and forming the mixture into pellets. The duration of the mixing may be suitably selected by the skilled person. Preferably, T1 is <NUM> to <NUM>ºC, for example <NUM> to <NUM>ºC.

Preferably, the propylene-based polymer has a melt flow index MFIA as measured according to ISO1133-<NUM>:<NUM> at <NUM> and <NUM> of e.g. <NUM> to <NUM> dg/min, for example <NUM> to <NUM> dg/min, <NUM> to <NUM> dg/min, <NUM> to <NUM> dg/min or <NUM> to <NUM> dg/min.

The composition obtained by step a) has a second melt flow index MFIB determined by ASTM D1238-<NUM> (<NUM> die) at <NUM> and <NUM>. Preferably, the ratio of MFIB to MFIA is <NUM> to <NUM>, for example <NUM> to <NUM>.

Preferably, MFIB is <NUM> to <NUM> dg/min, for example <NUM> to <NUM> dg/min.

The composition obtained by step a) may be formed into pellets before subjecting it to step b).

Step b) involves processing the composition obtained by step a), which may be in the form of pellets, by a melt-blown process to provide the melt-blown fibers. Step b) is performed at a temperature where both the first peroxide and the second peroxide have the visbreaking effect. The composition obtained by this step may herein sometimes be referred as a highly-vis-broken composition.

The melt-blown process to provide the melt-blown fibers is performed at temperatures between <NUM>ºC and <NUM>ºC, more preferably between <NUM>ºC and <NUM>ºC.

The melt-blown fibers have a third melt flow index MFIC determined by ASTM D1238-<NUM> Procedure C (<NUM> die) at <NUM> and <NUM>.

Preferably, the ratio of MFIC to MFIA is <NUM> to <NUM>, for example <NUM> to <NUM>.

MFIc is larger than MFIB. Preferably, the difference between MFIc and MFIB ,i.e. MFIC-MFIB, is at least <NUM> dg/min, more preferably at least <NUM> dg/min, more preferably at least <NUM> dg/min, more preferably at least <NUM> dg/min, more preferably at least <NUM> dg/min. Preferably, the difference between MFIC and MFIB ,i.e. MFIC-MFIB, is at most 100dg/min, for example at most 90dg/min.

Preferably, MFIC is <NUM> to <NUM> dg/min, for example <NUM> to <NUM> dg/min.

A melt-blown web, which is a non-woven structure consisting of melt-blown fibers, is made by a melt-blown process. A melt-blown process is typically a one-step process in which high-velocity air blows a molten thermoplastic resin from an extruder die tip onto a conveyor or take-up screen to form fine fibered self-bonding web.

Preferably, the melt-blown fibers have an average filament fineness of at most <NUM>.

In some embodiments, step b) involves processing a mixture of the composition obtained by step a) and a further polymer by a melt-blown process to provide melt-blown fibers. The further polymer is preferably a propylene-based polymer. Preferably, the weight ratio between the pellets and the further polymer is <NUM>:<NUM> to <NUM>:<NUM>, for example <NUM>:<NUM> to <NUM>:<NUM> or <NUM>:<NUM> to <NUM>:<NUM>. However, more typically, step b) involves processing the composition obtained by step a) without a further polymer by a melt-blown process to provide melt-blown fibers.

The invention further provides an article comprising the melt-blown web according to the invention. Preferably, the article is selected from the group consisting of filter media (e.g. air filters such as clean room filters, ventilation filters, HVAC (heating, ventilation and air conditioning) filters, filters for face masks, filters for respirators, filters for gas masks, filters for vacuum cleaner and filters for room air cleaner; liquid filters such as water filters, filters for food and beverage and filters for chemicals and solvents), medical/surgical gowns, medical/surgical drapes, medical/surgical face masks, diapers, feminine hygiene products, sanitary napkins, adult incontinence products, absorbent mats, wipes (including household wipes and industrial clean up wipes and sanitary wipes), oil containment boom, food fat absorption wipes, protective apparel, masks (including industrial face masks), wet tissues articles used in electronics (e.g. battery separators and cable wraps) adhesives (e.g. hot melt adhesives), insulators (e.g. apparel thermal insulator and acoustics insulation article), composite non-wovens. The invention further provides a process for making the article comprising the process according to the invention for making the melt-blown web.

The invention further provides use of the melt-blown web according to the invention for making an article selected from the group consisting of filter media (e.g. air filters such as clean room filters, ventilation filters, HVAC (heating, ventilation and air conditioning) filters, filters for face masks, filters for respirators, filters for gas masks, filters for vacuum cleaner and filters for room air cleaner; liquid filters such as water filters, filters for food and beverage and filters for chemicals and solvents), medical/surgical gowns, medical/surgical drapes, medical/surgical face masks, diapers, feminine hygiene products, sanitary napkins, adult incontinence products, absorbent mats, wipes (including household wipes and industrial clean up wipes and sanitary wipes), oil containment boom, food fat absorption wipes, protective apparel, masks (including industrial face masks), wet tissues articles used in electronics (e.g. battery separators and cable wraps) adhesives (e.g. hot melt adhesives), insulators (e.g. apparel thermal insulator and acoustics insulation article), composite non-wovens.

Depending of the application and the properties sought such as permeability, hydrostatic head and mechanical properties, the weight per unit area of the melt-blown web is set. Generally it is preferred that the melt-blown web has a weight per unit area of at least <NUM>/m<NUM>, preferably in the range from <NUM> to <NUM>/m<NUM>.

In case the melt-blown web according to the instant invention is produced as a single layer web (e.g. for air filtration purposes) it preferably has a weight per unit area of at least <NUM>/m<NUM>, more preferably of at least <NUM>/m<NUM>, yet more preferably in the range of <NUM> to <NUM>/m<NUM>, still more preferably in the range of <NUM> to <NUM>/m<NUM>. It can also be produced as multilayer like SMS-web (spunbond, melt-blown, spunbond) or SSMMS (spunbond, spunbond, melt-blown, melt-blown, spunbond) e.g. for hygienic and/or medical applications. In this case the weight per unit area of the melt-blown web may typically be at least <NUM>/m<NUM>, more preferably of at least <NUM>/m<NUM>, yet more preferably in the range of <NUM> to <NUM>/m<NUM>, still more preferably in the range of <NUM> to <NUM>/m<NUM>.

The melt-blown web according to the invention being as a single layer web or a multilayer construction as described above containing the melt-blown web can be furthermore combined with other layers, i.e. polycarbonate layers or the like, depending on the desired end use of the produced article.

Preferably, the melt-blown web according to the present invention has a hydrostatic head (3rd drop, cm H<NUM>O resp. mbar), measured according to NWSP. <NUM> (R0) method of <NUM> as described in the experimental section of at least <NUM> mbar.

Preferably, the melt-blown web according to the present invention has an air permeability measured according to NSWP. R0 (pressure drop setting of 200Pa) of at most <NUM>/m<NUM>/sec.

The propylene-based polymer used according to the invention can be made by any known polymerization technique as well as with any known polymerization catalyst system. Regarding the techniques, reference can be given to slurry, solution or gas phase polymerizations; regarding the catalyst system reference can be given to Ziegler-Natta, metallocene or single-site catalyst systems. All are, in themselves, known in the art.

The propylene-based polymer may for example be a propylene homopolymer or a random propylene copolymer or a heterophasic propylene copolymer.

A propylene homopolymer can be obtained by polymerizing propylene under suitable polymerization conditions. A propylene copolymer can be obtained by copolymerizing propylene and one or more other α-olefins, preferably ethylene, under suitable polymerization conditions. The preparation of propylene homopolymers and copolymers is, for example, described in <NPL>.

The random propylene copolymer may at most <NUM> wt% of comonomer units. The comonomer units may be ethylene monomer units and/or an α-olefin monomer units having <NUM> to <NUM> carbon atoms, preferably ethylene, <NUM>-butene, <NUM>-hexene or any mixtures thereof.

The heterophasic propylene copolymer consists of.

Heterophasic propylene copolymers are generally prepared in one or more reactors, by polymerization of propylene in the presence of a catalyst and subsequent polymerization of an ethylene-α-olefin mixture. The resulting polymeric materials are heterophasic, but the specific morphology usually depends on the preparation method and monomer ratios used.

The heterophasic propylene copolymers employed in the process according to present invention can be produced using any conventional technique known to the skilled person, for example multistage process polymerization, such as bulk polymerization, gas phase polymerization, slurry polymerization, solution polymerization or any combinations thereof. Any conventional catalyst systems, for example, Ziegler-Natta or metallocene may be used. Such techniques and catalysts are described, for example, in <CIT>; <NPL>; <CIT>, <CIT> and <CIT>. Preferably, the heterophasic propylene copolymer is made using Ziegler-Natta catalyst.

The heterophasic propylene copolymer may be prepared by a process comprising.

The heterophasic propylene copolymer consists of a propylene-based matrix and a dispersed ethylene-α-olefin copolymer. The propylene-based matrix typically forms the continuous phase in the heterophasic propylene copolymer. The amounts of the propylene-based matrix and the dispersed ethylene-α-olefin copolymer may be determined by <NUM>C-NMR, as well known in the art.

The propylene-based matrix consists of a propylene homopolymer and/or a propylene copolymer consisting of at least <NUM> wt% of propylene monomer units and at most <NUM> wt% of comonomer units selected from ethylene monomer units and α-olefin monomer units having <NUM> to <NUM> carbon atoms, for example consisting of at least <NUM> wt% of propylene monomer units and at most <NUM> wt% of the comonomer units, at least <NUM> wt% of propylene monomer units and at most <NUM> wt% of the comonomer units or at least <NUM> wt% of propylene monomer units and at most <NUM> wt% of the comonomer units, based on the total weight of the propylene-based matrix.

Preferably, the comonomer in the propylene copolymer of the propylene-based matrix is selected from the group of ethylene, <NUM>-butene, <NUM>-pentene, <NUM>-methyl-<NUM>-pentene, <NUM>-hexen, <NUM>-heptene and <NUM>-octene, and is preferably ethylene.

Preferably, the propylene-based matrix consists of a propylene homopolymer.

The melt flow index (MFI) of the propylene-based matrix (before the heterophasic propylene copolymer is mixed into the composition of the invention), MFIPP, may be for example at least <NUM> dg/min, at least <NUM> dg/min, at least <NUM> dg/min, at least <NUM> dg/min, at least <NUM> dg/min, at least <NUM> dg/min, and/or for example at most <NUM> dg/min, at most <NUM> dg/min, at most <NUM> dg/min, at most <NUM> dg/min, at most <NUM> dg/min, measured according to ISO1133-<NUM>:<NUM> (<NUM>/<NUM>). The MFIPP may be in the range of for example <NUM> to <NUM> dg/min, for example from <NUM> to <NUM> dg/min, for example <NUM> to <NUM> dg/min, for example <NUM> to <NUM> dg/min, for example from <NUM> to <NUM> dg/min, for example from <NUM> to <NUM> dg/min, measured according to ISO1133-<NUM>:<NUM> (<NUM>/<NUM>).

The propylene-based matrix is present in an amount of <NUM> to <NUM> wt%. Preferably, the propylene-based matrix is present in an amount of <NUM> to <NUM> wt%, for example at least <NUM> wt% or at least <NUM> wt% and/or at most <NUM> wt%, based on the total heterophasic propylene copolymer.

The propylene-based matrix is preferably semi-crystalline, that is it is not <NUM>% amorphous, nor is it <NUM>% crystalline. For example, the propylene-based matrix is at least <NUM>% crystalline, for example at least <NUM>%, for example at least <NUM>% crystalline and/or for example at most <NUM>% crystalline, for example at most <NUM>% crystalline. For example, the propylene-based matrix has a crystallinity of <NUM> to <NUM>%. For purpose of the invention, the degree of crystallinity of the propylene-based matrix is measured using differential scanning calorimetry (DSC) according to IS011357-<NUM> and IS011357-<NUM> of <NUM>, using a scan rate of <NUM>/min, a sample of <NUM> and the second heating curve using as a theoretical standard for a <NUM>% crystalline material <NUM> J/g.

Besides the propylene-based matrix, the heterophasic propylene copolymer also comprises a dispersed ethylene-α-olefin copolymer. The dispersed ethylene-α-olefin copolymer is also referred to herein as the 'dispersed phase'. The dispersed phase is embedded in the heterophasic propylene copolymer in a discontinuous form. The particle size of the dispersed phase is typically in the range of <NUM> to <NUM> micrometers, as may be determined by transmission electron microscopy (TEM). The amount of the dispersed ethylene-α-olefin copolymer in the heterophasic propylene copolymer may herein be sometimes referred as RC.

The amount of ethylene monomer units in the ethylene-α-olefin copolymer may e.g. be <NUM> to <NUM> wt% with respect to the ethylene-α-olefin copolymer. The amount of ethylene monomer units in the dispersed ethylene-α-olefin copolymer in the heterophasic propylene copolymer may herein be sometimes referred as RCC2.

The α-olefin in the ethylene-α-olefin copolymer is preferably chosen from the group of α-olefins having <NUM> to <NUM> carbon atoms. Examples of suitable α-olefins having <NUM> to <NUM> carbon atoms include but are not limited to propylene, <NUM>-butene, <NUM>-pentene, <NUM>-methyl-<NUM>-pentene, <NUM>-hexen, <NUM>-heptene and <NUM>-octene. More preferably, the α-olefin in the ethylene-α-olefin copolymer is chosen from the group of α-olefins having <NUM> to <NUM> carbon atoms and any mixture thereof, more preferably the α-olefin is propylene, in which case the ethylene-α-olefin copolymer is ethylene-propylene copolymer.

The MFI of the dispersed ethylene α-olefin copolymer (before the heterophasic propylene copolymer is mixed into the composition of the invention), MFlrubber, may be for example at least <NUM> dg/min, at least <NUM> dg/min, at least <NUM> dg/min, at least <NUM> dg/min, at least <NUM> dg/min, at least <NUM> dg/min, and/or for example at most <NUM> dg/min, at most <NUM> dg/min, at most <NUM> dg/min at most <NUM> dg/min, at most <NUM> dg/min or at most <NUM> dg/min. The MFIrubber may be in the range for example from <NUM> to <NUM> dg/min, for example from <NUM> to <NUM> dg/min, for example <NUM> to <NUM> dg/min, for example <NUM> to <NUM> dg/min, for example from <NUM> to <NUM> dg/min, for example from <NUM> to <NUM> dg/min. MFIrubber is calculated according to the following formula: <MAT> wherein.

The dispersed ethylene-α-olefin copolymer is present in an amount of <NUM> to <NUM> wt%. Preferably, the dispersed ethylene-α-olefin copolymer is present in an amount of <NUM> to <NUM> wt%, for example in an amount of at least <NUM> wt% and/or for example in an amount of at most <NUM> wt% or at most <NUM> wt% based on the total heterophasic propylene copolymer.

In the heterophasic propylene copolymer in the composition of the invention, the sum of the total weight of the propylene-based matrix and the total weight of the dispersed ethylene-α-olefin copolymer is <NUM> wt% of the heterophasic propylene copolymer.

The first peroxide has a half-life time of <NUM> hour at a first temperature T<NUM>/<NUM><NUM> and the second peroxide has a half-life time of <NUM> hour at a second temperature T<NUM>/<NUM><NUM>. T<NUM>/<NUM><NUM> is higher than T<NUM>/<NUM><NUM>, i.e. the first peroxide is reactive at a lower temperature than the second peroxide. For example, T<NUM>/<NUM><NUM> - T<NUM>/<NUM><NUM> is <NUM> to <NUM>ºC or <NUM> to <NUM>ºC.

Preferably, T<NUM>/<NUM><NUM> is <NUM> to <NUM>ºC, preferably <NUM> to <NUM>ºC, more preferably <NUM> to <NUM>ºC. Examples of the first peroxide include <NUM>,<NUM>-Dimethyl-<NUM>,<NUM>-di(tert-butylperoxy)hexane (e.g. Trigonox™ <NUM> manufactured by AkzoNobel), which has T<NUM>/<NUM><NUM> of <NUM>ºC.

Preferably, the first peroxide has a half-life time of <NUM> hour at a temperature of <NUM> to <NUM>ºC, more preferably <NUM> to <NUM>ºC ºC.

Preferably, T<NUM>/<NUM><NUM> is higher than <NUM>ºC and at most <NUM>ºC, preferably at most <NUM>ºC, for example higher than <NUM>ºC and at most <NUM>ºC or at least <NUM>ºC and at most <NUM>ºC. Further, the second peroxide preferably has a half-life time of <NUM> hour at a temperature of <NUM> to <NUM>ºC. Examples of the second peroxide include <NUM>,<NUM>,<NUM>-Triethyl-<NUM>,<NUM>,<NUM>-trimethyl-<NUM>,<NUM>,<NUM> triperoxonane (e.g. Trigonox™ <NUM> manufactured by AkzoNobel), which has T<NUM>/<NUM><NUM> of <NUM>ºC and a half-life time of <NUM> hour at a temperature of <NUM>ºC and <NUM>,<NUM>,<NUM>,<NUM>,<NUM>-pentamethyl-<NUM>,<NUM>,<NUM>-trioxepane (e.g. Trigonox™ <NUM> manufactured by AkzoNobel), which has T<NUM>/<NUM><NUM> of <NUM>ºC and a half-life time of <NUM> hour at a temperature of <NUM>ºC.

Preferably, the amount of the first peroxide with respect to the propylene-based polymer is <NUM> to <NUM> ppm.

Preferably, the amount of the second peroxide with respect to the propylene-based polymer is <NUM> to <NUM> ppm, preferably <NUM> to <NUM> ppm.

The melt-mixing in step a) may involve mixing further additives such as nucleating agents, stabilizers, e.g. heat stabilizers, anti-oxidants, UV stabilizers; colorants, like pigments and dyes; clarifiers; surface tension modifiers; lubricants; flame-retardants; mould-release agents; flow improving agents; plasticizers; anti-static agents; external elastomeric impact modifiers; blowing agents; inorganic fillers such as talc and reinforcing agents; and/or components that enhance interfacial bonding between polymer and filler, such as a maleated polypropylene. The skilled person can readily select any suitable combination of additives and additive amounts without undue experimentation.

It is further noted that the term 'comprising' does not exclude the presence of other elements. However, it is also to be understood that a description on a product/composition comprising certain components also discloses a product/composition consisting of these components. The product/composition consisting of these components may be advantageous in that it offers a simpler, more economical process for the preparation of the product/composition. Similarly, it is also to be understood that a description on a process comprising certain steps also discloses a process consisting of these steps. The process consisting of these steps may be advantageous in that it offers a simpler, more economical process.

When values are mentioned for a lower limit and an upper limit for a parameter, ranges made by the combinations of the values of the lower limit and the values of the upper limit are also understood to be disclosed.

The invention is now elucidated by way of the following examples, without however being limited thereto.

Components shown in Table <NUM> were melt-mixed at a temperature shown in table <NUM> for <NUM>-<NUM> hours and made into pellets.

MFI of the compositions of the pellets were measured by ASTM D1238-<NUM> (<NUM> die) at <NUM> and <NUM>.

The pellets were subjected to a melt blow process in a Hills melt blown pilot line using a die with holes of <NUM> diameter and <NUM> holes per inch. The melt temperature was set at <NUM> and the air temperature at <NUM>. The processing parameters are summarized in Table <NUM>.

MFI of the fibers of the melt-blown webs were measured by ASTM D1238-<NUM> Procedure C (<NUM> die) at <NUM> and <NUM>.

Hydrostatic head of the melt-blown web was determined. Hydrostatic head as determined by a hydrostatic pressure test was determined according to NWSP. <NUM> (R0) method revised in <NUM>. Test was done using Textest FX3000 hydrostatic head tester, <NUM><NUM> samples of the fabric prepared as described herein are clamped into place over a water filled test head. Water pressure underneath the sample is increased at <NUM> mbar/min on a fabric specimen of <NUM><NUM> at <NUM> with purified water as test liquid. The test is terminated when three drops of water penetrate the sample.

Air permeability is a measure in volume of air per unit time per unit area of fabric of the barrier properties of a fabric. Air Permeability was determined on a <NUM><NUM> fabric sample taken from a fabric prepared as described herein using a Texas Instruments (Lab Air <NUM>) machine with a pressure drop setting of <NUM> Pa. Specimens are clamped into place and the flow rate of air through the sample is increased until the pressure drop reaches <NUM> Pa. A measurement is made of the flow rate of air and volume of air per unit area per unit time. This procedure is according to NSWP. R0 (pressure drop setting of 200Pa).

Accordingly, polypropylene with various MFI were mixed with Trigonox <NUM> and Trigonox <NUM> and heated at temperatures of <NUM> to <NUM>, at which temperatures visbreaking occurs mainly due to Trigonox <NUM>. The MFI of the pellets were measured at <NUM> instead of <NUM> so as to limit the occurrence of visbreaking during the MFI measurement.

These pellets were made into melt-blown web at a temperature of <NUM> or <NUM> as indicated in the above Table. The MFI of the melt-blown web were measured at <NUM> using the half die method (ASTM D1238-<NUM> Procedure C (<NUM> die)) which allows MFI measurement of high flow polypropylene.

The melt-blown webs obtained according to the invention were found to have a high hydrostatic head and a low air permeability. Compared to the melt-blown web using comparative polypropylene CEx4 (with only one peroxide), the hydrostatic head was higher and the air permeability was lower. Compared to the melt blown web using comparative polypropylene CEx5 (with only a hydroxylamine and no peroxides), the hydrostatic head was higher.

Claim 1:
A melt-blown web comprising melt-blown fibers obtained by
a) melt-mixing a propylene-based polymer, a first peroxide and a second peroxide at temperatures between <NUM>ºC and <NUM>ºC, preferably between <NUM>ºC and <NUM>ºC, wherein the first peroxide has a half-life time of <NUM> hour at a first temperature T<NUM>/<NUM><NUM> and the second peroxide has a half-life time of <NUM> hour at a second temperature T<NUM>/<NUM><NUM>, wherein T<NUM>/<NUM><NUM> is higher than T<NUM>/<NUM><NUM> and
b) processing the composition obtained by step a) by a melt-blown process at temperatures between <NUM>ºC and <NUM>ºC, preferably between <NUM>ºC and <NUM>ºC, to provide the melt-blown fibers.