WO2022122722A1

WO2022122722A1 - Melt-blown web made of polypropylene

Info

Publication number: WO2022122722A1
Application number: PCT/EP2021/084569
Authority: WO
Inventors: Abderrahman MEDDAD; Fahad AL-MUTAIRY; Henrica Norberta Alberta Maria Steenbakkers-Menting; Hendrik Kormelink; Tinashe Victor Mandishonha NDORO; Gert Boven; Ronald Julianus Peter SCHIPPER; Marco DOHMEN
Original assignee: Sabic Global Technologies B.V.
Priority date: 2020-12-08
Filing date: 2021-12-07
Publication date: 2022-06-16
Also published as: CN116724153A; US20240035203A1; EP4259864A1

Abstract

The invention relates to 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 180 ºC and 240 ºC, preferably between 200 ºC and 220 ºC, wherein the first peroxide has a half-life time of 1 hour at a first temperature T1/21 and the second peroxide has a half-life time of 1 hour at a second temperature T1/22, wherein T1/22 is higher than T1/21 and b) processing the composition obtained by step a) by a melt-blown process at temperatures between 240 ºC and 300 ºC, preferably between 245 ºC and 280 ºC, to provide the melt-blown fibers.

Description

MELT-BLOWN WEB MADE OF POLYPROPYLENE

The present invention relates to a melt-blown web made of melt-blown fibers made of a polypropylene composition, and its applications.

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”.

W02007126961 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 50 dg/min to 400 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 250 ^ºC. The blend exhibits MFR which is 1 to 4 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 400 to about 3500 dg/min.

EP3034522 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.

US20160311944 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 70% of active oxygen. As an example of the low-reactivity organic peroxide, Trigonox 31 1 is mentioned. As examples of the organic peroxide which is not a low- reactivity organic peroxide, T rigonox 101 , T rigonox 301 and Luperox 130 are mentioned. It is mentioned that the obtained polypropylene may be used for producing nonwoven fabric such as spunbond and melt-blown.

EP3081677A2 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.

EP384431 A2 discloses a process for making normally solid, gel-free, propylene polymer material with a branching index of less than 1 and with a strain hardening elongational viscosity from normally solid, amorphous to predominantly crystalline propylene polymer material without strain hardening elongational viscosity, which comprises:

(1 ) mixing a low decomposition temperature peroxide with a linear propylene polymer material, which material is at room temperature to 120°C, in a mixing vessel in the substantial absence of atmospheric oxygen or its equivalent,

(2) heating or maintaining the resulting mixture in the substantial absence of atmospheric oxygen or its equivaket at room temperature to 120°C for a period of time sufficient for decomposition of the peroxide, for a substantial amount of fragmentation of the linear propylene polymer material to occur, and for a significant amount of long chain branches to form, but insufficient to cause gelation of the propylene polymer material;

(3) treating the propylene polymer material in the substantial absence of atmospheric oxygen or its equivalent to deactivate substantially all the free radicals present in said propylene polymer material.

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 a) melt-mixing a propylene-based polymer, a first peroxide and a second peroxide at temperatures between 180 ^SC and 240 ^ºC, preferably between 200 ^ºC and 220 ^ºC, wherein the first peroxide has a half-life time of 1 hour at a first temperature T1/2I and the second peroxide has a half-life time of 1 hour at a second temperature T1/22, wherein T1/22 is higher than T1/2I and b) processing the composition obtained by step a) by a melt-blown process at temperatures between 240 ^ºC and 300 ^ºC, preferably between 245 ^ºC and 280 ^ºC, to provide the melt-blown fibers.

According to another aspect, the present invention provides a process for making a melt-blown web comprising melt-blown fibers, comprising the steps of: a) melt-mixing a propylene-based polymer, a first peroxide and a second peroxide at temperatures between 180 ^ºC and 240 ^ºC, preferably between 200 ^ºC and 220 ^ºC, wherein the first peroxide has a half-life time of 1 hour at a first temperature TI/₂1 and the second peroxide has a half-life time of 1 hour at a second temperature TI/₂2, wherein T1/22 is higher than T1/2I and b) processing the composition obtained by step a) by a melt-blown process at temperatures between 240 ^ºC and 300 ^ºC, preferably between 245 ^ºC and 280 ^ºC, to provide the melt-blown fibers.

According to the invention, 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 US4897452 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 20 times longer than that of G1 at the pelletisation temperature. Non- sticky pellets with excellent reproducibility are obtained. US4897452 does not mention a melt-blown web. Examples of free radical generators G1 are given, each of which has a half-life time of 1 hour at a temperature between 105 and 119 ^ºC except for di- tert-butylperoxide which has a half-life time of 1 hour at a temperature of 146 ^ºC. Examples of free radical generators G2 includes diisopropylbenzene hydroperoxide which has a half-life time of 1 hour at a temperature of 154 ^ºC and a half-life time of 0.1 hour at a temperature of 207 ^ºC. In Example 1 , the pellets were converted into continuous filaments having a melt index of 190 g/1 Omin measured at 190 ^ºC/2.16 kg according to ASTM method D1238 condition E. This is lower than the melt index of filaments generally used for making a melt-blown web.

Step a)

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 180 ^ºC and 240 ^ºC, preferably between 210 ^ºC and 230 ^ºC.

The duration of the melt-mixing may be suitably selected by the skilled person depending on the target viscosity properties, for example 0.01 to 0.03 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 T1/2I is at least 50 ^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 0 to 60 ^ºC, for example 10 to 30 ^ºC.

Preferably, the propylene-based polymer has a melt flow index MFIA as measured according to ISO1133-1 :2011 at 230 °C and 2.16 kg of e.g. 0.1 to 60 dg/min, for example 0.1 to 1.0 dg/min, 1.0 to 5.0 dg/min, 5.0 to 15 dg/min or 15 to 60 dg/min. The composition obtained by step a) has a second melt flow index MFIB determined by ASTM D1238-13 (2 mm die) at 190°C and 2.16 kg. Preferably, the ratio of MFIB to MFIA is 5 to 100, for example 8 to 80.

Preferably, MFI_B is 10 to 300 dg/min, for example 100 to 200 dg/min.

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

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 240 ^ºC and 300 ^ºC, more preferably between 245 ^ºC and 280 ^ºC.

The melt-blown fibers have a third melt flow index MFIc determined by ASTM D1238- 13 Procedure C (1 mm die) at 230°C and 2.16 kg.

Preferably, the ratio of MFIc to MFIA is 5 to 100, for example 8 to 80.

MFIc is larger than MFIB. Preferably, the difference between MFIc and MFIB ,i.e. MFIc- MFIB, is at least 30 dg/min, more preferably at least 40 dg/min, more preferably at least 50 dg/min, more preferably at least 60 dg/min, more preferably at least 70 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, MFI_C is 100 to 300 dg/min, for example 150 to 250 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 5 urn.

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 80:20 to 100:0, for example 90:10 to 100:0 or 95:5 to 100:0. 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 1 g/m², preferably in the range from 1 to 250 g/m².

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 1 g/m², more preferably of at least 4 g/m², yet more preferably in the range of 7 to 250 g/m², still more preferably in the range of 8 to 200 g/m². 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 0.8 g/m², more preferably of at least 1 g/m², yet more preferably in the range of 1 to 30 g/m², still more preferably in the range of 1 .3 to 20 g/m²

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 H2O resp. mbar), measured according to NWSP.080.6 (R0) method of 2015 as described in the experimental section of at least 65 mbar.

Preferably, the melt-blown web according to the present invention has an air permeability measured according to NSWP.070.1. R0 (pressure drop setting of 200Pa) of at most 440 l/m²/sec.

Propylene-based polymer

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 a-olefins, preferably ethylene, under suitable polymerization conditions. The preparation of propylene homopolymers and copolymers is, for example, described in Moore, E. P. (1996) Polypropylene Handbook. Polymerization, Characterization, Properties, Processing, Applications, Hanser Publishers: New York.

The random propylene copolymer may at most 10 wt% of comonomer units. The comonomer units may be ethylene monomer units and/or an a-olefin monomer units having 4 to 10 carbon atoms, preferably ethylene, 1 -butene, 1 -hexene or any mixtures thereof.

The heterophasic propylene copolymer consists of

(a) a propylene-based matrix, wherein the propylene-based matrix consists of a propylene homopolymer and/or a propylene copolymer consisting of at least 70 wt% of propylene monomer units and at most 30 wt% of comonomer units selected from ethylene monomer units and a-olefin monomer units having 4 to 10 carbon atoms, based on the total weight of the propylene-based matrix, wherein the propylene-based matrix is present in an amount of 60 to 95 wt% based on the total heterophasic propylene copolymer and

(b) a dispersed ethylene-a-olefin copolymer, wherein the dispersed ethylene-a-olefin copolymer is present in an amount of 40 to 5 wt% based on the total heterophasic propylene copolymer and wherein the sum of the total amount of propylene-based matrix and total amount of the dispersed ethylene-a-olefin copolymer in the heterophasic propylene copolymer is 100 wt%.

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-a-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 W006/010414; Polypropylene and other Polyolefins, by Ser van der Ven, Studies in Polymer Science 7, Elsevier 1990; W006/010414, US4399054 and US4472524.

Preferably, the heterophasic propylene copolymer is made using Ziegler-Natta catalyst.

The heterophasic propylene copolymer may be prepared by a process comprising

- polymerizing propylene and optionally ethylene and/or a-olefin in the presence of a catalyst system to obtain the propylene-based matrix and

- subsequently polymerizing ethylene and a-olefin in the propylene-based matrix in the presence of a catalyst system to obtain the dispersed ethylene-a olefin copolymer. These steps are preferably performed in different reactors. The catalyst systems for the first step and for the second step may be different or same.

The heterophasic propylene copolymer consists of a propylene-based matrix and a dispersed ethylene-a-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-a-olefin copolymer may be determined by ¹³C-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 70 wt% of propylene monomer units and at most 30 wt% of comonomer units selected from ethylene monomer units and a-olefin monomer units having 4 to 10 carbon atoms, for example consisting of at least 80 wt% of propylene monomer units and at most 20 wt% of the comonomer units, at least 90 wt% of propylene monomer units and at most 10 wt% of the comonomer units or at least 95 wt% of propylene monomer units and at most 5 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, 1 -butene, 1 -pentene, 4-methyl-1 -pentene, 1 - hexen, 1 -heptene and 1 -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 0.1 dg/min, at least 0.2 dg/min, at least 0.3 dg/min, at least 0.5 dg/min, at least 1 dg/min, at least 1 .5 dg/min, and/or for example at most 50 dg/min, at most 40 dg/min, at most 30 dg/min, at most 25 dg/min, at most 20 dg/min, measured according to ISO1133-1 :2011 (2.16 kg/230°C). The MFI_PP may be in the range of for example 0.1 to 50 dg/min, for example from 0.2 to 40 dg/min, for example 0.3 to 30 dg/min, for example 0.5 to 25 dg/min, for example from 1 to 20 dg/min, for example from 1.5 to 10 dg/min, measured according to ISO1133-1 :2011 (2.16 kg/230°C).

The propylene-based matrix is present in an amount of 60 to 95 wt%. Preferably, the propylene-based matrix is present in an amount of 60 to 80 wt%, for example at least 65 wt% or at least 70 wt% and/or at most 78 wt%, based on the total heterophasic propylene copolymer.

The propylene-based matrix is preferably semi-crystalline, that is it is not 100% amorphous, nor is it 100% crystalline. For example, the propylene-based matrix is at least 40% crystalline, for example at least 50%, for example at least 60% crystalline and/or for example at most 80% crystalline, for example at most 70% crystalline. For example, the propylene-based matrix has a crystallinity of 60 to 70%. For purpose of the invention, the degree of crystallinity of the propylene-based matrix is measured using differential scanning calorimetry (DSC) according to ISO1 1357-1 and ISO11357- 3 of 1997, using a scan rate of 10°C/min, a sample of 5 mg and the second heating curve using as a theoretical standard for a 100% crystalline material 207.1 J/g.

Besides the propylene-based matrix, the heterophasic propylene copolymer also comprises a dispersed ethylene-a-olefin copolymer. The dispersed ethylene-a-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 0.05 to 2.0 micrometers, as may be determined by transmission electron microscopy (TEM). The amount of the dispersed ethylene-a-olefin copolymer in the heterophasic propylene copolymer may herein be sometimes referred as RC.

The amount of ethylene monomer units in the ethylene-a-olefin copolymer may e.g. be 20 to 65 wt% with respect to the ethylene-a-olefin copolymer. The amount of ethylene monomer units in the dispersed ethylene-a-olefin copolymer in the heterophasic propylene copolymer may herein be sometimes referred as RCC2.

The a-olefin in the ethylene-a-olefin copolymer is preferably chosen from the group of a-olefins having 3 to 8 carbon atoms. Examples of suitable a-olefins having 3 to 8 carbon atoms include but are not limited to propylene, 1 -butene, 1 -pentene, 4-methyl- 1 -pentene, 1 -hexen, 1 -heptene and 1 -octene. More preferably, the a-olefin in the ethylene-a-olefin copolymer is chosen from the group of a-olefins having 3 to 4 carbon atoms and any mixture thereof, more preferably the a-olefin is propylene, in which case the ethylene-a-olefin copolymer is ethylene-propylene copolymer.

The MFI of the dispersed ethylene a-olefin copolymer (before the heterophasic propylene copolymer is mixed into the composition of the invention), MFIrubber, may be for example at least 0.001 dg/min, at least 0.01 dg/min, at least 0.1 dg/min, at least 0.3 dg/min, at least 0.7 dg/min, at least 1 dg/min, and/or for example at most 30 dg/min, at most 20 dg/min, at most 15 dg/min at most 10 dg/min, at most 5 dg/min or at most 3 dg/min. The MFIrubber may be in the range for example from 0.001 to 30 dg/min, for example from 0.01 to 20 dg/min, for example 0.1 to 15 dg/min, for example 0.3 to 10 dg/min, for example from 0.7 to 5 dg/min, for example from 1 to 3 dg/min. MFIrubber is calculated according to the following formula:

wherein

MFIheterophasic is the MFI (dg/min) of the heterophasic propylene copolymer measured according to ISO1133 (2.16kg/230°C),

MFImatrix is the MFI (dg/min) of the propylene-based matrix measured according to ISO1133 (2.16kg/230°C), matrix content is the fraction of the propylene-based matrix in the heterophasic propylene copolymer, rubber content is the fraction of the dispersed ethylene-a-olefin copolymer in the heterophasic propylene copolymer. The sum of the matrix content and the rubber content is 1 . For the avoidance of any doubt, Log in the formula means log™.

The dispersed ethylene-a-olefin copolymer is present in an amount of 40 to 5 wt%. Preferably, the dispersed ethylene-a-olefin copolymer is present in an amount of 40 to 20 wt%, for example in an amount of at least 22 wt% and/or for example in an amount of at most 35 wt% or at most 30 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-a-olefin copolymer is 100 wt% of the heterophasic propylene copolymer.

First peroxide and second peroxide.

The first peroxide has a half-life time of 1 hour at a first temperature TI_/21 and the second peroxide has a half-life time of 1 hour at a second temperature TI/₂2. T_V22 is higher than Tv₂1 , i.e. the first peroxide is reactive at a lower temperature than the second peroxide. For example, TI/₂2 - TI/₂1 is 5 to 20 ^ºC or 10 to 15 ^ºC.

Preferably, TI/₂1 is 120 to 145 ^ºC, preferably 125 to 140 ^SC, more preferably 128 to 137 ^ºC. Examples of the first peroxide include 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane (e.g. Trigonox™ 101 manufactured by AkzoNobel), which has TI/₂1 of 134 ^ºC.

Preferably, the first peroxide has a half-life time of 0.1 hour at a temperature of 140 to 180 ^ºC, more preferably 150 to 170 ^ºC ^ºC.

Preferably, TI/₂2 is higher than 145 ^ºC and at most 180 ^ºC, preferably at most 170 ^SC, for example higher than 145 ^ºC and at most 150 ^ºC or at least 155 ^ºC and at most 170 ^ºC. Further, the second peroxide preferably has a half-life time of 0.1 hour at a temperature of 165 to 188 ^ºC. Examples of the second peroxide include 3,6,9-Triethyl- 3,6,9-trimethyl-1 ,4,6 triperoxonane (e.g. Trigonox™ 301 manufactured by AkzoNobel), which has TV₂2 of 146 ^ºC and a half-life time of 0.1 hour at a temperature of 170 ^ºC and 3,3,5,7,7-pentamethyl-1 ,2,4-trioxepane (e.g. Trigonox™ 31 1 manufactured by AkzoNobel), which has TI/₂2 of 166 ^ºC and a half-life time of 0.1 hour at a temperature of 185 ^ºC. Preferably, the amount of the first peroxide with respect to the propylene-based polymer is 100 to 2000 ppm .

Preferably, the amount of the second peroxide with respect to the propylene-based polymer is 100 to 8000 ppm, preferably 1000 to 8000 ppm .

Other additives

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 noted that the invention relates to the subject-matter defined in the independent claims alone or in combination with any possible combinations of features described herein, preferred in particular are those combinations of features that are present in the claims. It will therefore be appreciated that all combinations of features relating to the composition according to the invention; all combinations of features relating to the process according to the invention and all combinations of features relating to the composition according to the invention and features relating to the process according to the invention are described herein.

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 prod uct/com position comprising certain components also discloses a prod uct/com position 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.

Following materials were used.

PP22: a propylene homopolymer having a MFI of 22 dg/min according to ISO1133- 1 :2011 (230°C/2.16 kg)

PP11 : a propylene homopolymer having a MFI of 1 1 dg/min according to ISO1133- 1 :2011 (230°C/2.16 kg)

PP3: a propylene homopolymer having a MFI of 6 dg/min according to ISO1 133-1 :2011 (230°C/2.16 kg)

Stabilizer package: Irganox 3114, Irgafos 168, Ca stearate in a weight ratio of 40:85:35 Trigonox 101 (2,5-Dimethyl-2,5-di(tert-butylperoxy) hexane) having T_V21 of 134 ^ºC Trigonox 301 (3,6,9-triethyl-3,6,9-trimethyl-1 ,4,7-triperoxonane) having Ti/₂2 of 146 ^ºC Comp PP: propylene homopolymer HL712FB available from Borealis, having an MFI of 1200 dg/min according to ISO1 133 according to the product data sheet. Analysis of HL712FB revealed that HL712FB contains a stabilizer package comparable to the stabilizer package used for Ex1 , Ex2 and Ex3 and analysis also revealed that it contains 2,5-dimethyl-2,5-di(tert-butylperoxy) hexane (referred to in Table 2 as ‘Irgatec’).

Components shown in Table 1 were melt-mixed at a temperature shown in table 1 for 0.025-0.030 hours and made into pellets.

MFI of the compositions of the pellets were measured by ASTM D1238-13 (2 mm die) at 190°C and 2.16 kg.

The pellets were subjected to a melt blow process in a Hills melt blown pilot line using a die with holes of 0.25 mm diameter and 35 holes per inch. The melt temperature was set at 250°C and the air temperature at 275°C. The processing parameters are summarized in Table 1 .

MFI of the fibers of the melt-blown webs were measured by ASTM D1238-13 Procedure C (1 mm die) at 230°C and 2.16 kg. Hydrostatic head of the melt-blown web was determined. Hydrostatic head as determined by a hydrostatic pressure test was determined according to NWSP.080.6 (RO) method revised in 2015. Test was done using Textest FX3000 hydrostatic head tester, 100 cm² 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 60 mbar/min on a fabric specimen of 100 cm² at 23°C 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 20cm² fabric sample taken from a fabric prepared as described herein using a Texas Instruments (Lab Air 3300) machine with a pressure drop setting of 200 Pa. Specimens are clamped into place and the flow rate of air through the sample is increased until the pressure drop reaches 200 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.070.1 .R0 (pressure drop setting of 200Pa).

Accordingly, polypropylene with various MFI were mixed with Trigonox 101 and Trigonox 301 and heated at temperatures of 214 to 226 °C, at which temperatures visbreaking occurs mainly due to Trigonox 101 . The MFI of the pellets were measured at 190 °C instead of 230 °C so as to limit the occurrence of visbreaking during the MFI measurement.

These pellets were made into melt-blown web at a temperature of 250 °C or 290°C as indicated in the above Table. The MFI of the melt-blown web were measured at 230 °C using the half die method (ASTM D1238-13 Procedure C (1 mm 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.

The compositions of the examples of the invention were better processable in a melt- blown process than the compositions of CEx5, CEx6 and CEx7. CEx5 and CEx7 needed a higher temperature (290°C) for conversion of the compositions into a melt blown web using a melt-blown process and due to its too low MFI, the composition of CEx6 could not be converted into a melt blown web.

Claims

CLAIMS 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 180 ^ºC and 240 ^ºC, preferably between 200 ^ºC and 220 ^ºC, wherein the first peroxide has a half-life time of 1 hour at a first temperature TI_/21 and the second peroxide has a half-life time of 1 hour at a second temperature T1/22, wherein T1/22 is higher than T1/2I and b) processing the composition obtained by step a) by a melt-blown process at temperatures between 240 ^ºC and 300 ^ºC, preferably between 245 ^ºC and 280 ^ºC, to provide the melt-blown fibers. The melt-blown web according to claim 1 , wherein T1/2I is 120 to 145 ^ºC, preferably 125 to 140 ^ºC, more preferably 128 to 137 ^ºC. The melt-blown web according to any one of the preceding claims, wherein TI ₂2 is higher than 145 ^ºC and at most 180 ^ºC, preferably at most at most 170 ^ºC, for example higher than 145 ^ºC and at most 150 ^ºC or at least 155 ^ºC and at most 170 ^ºC. The melt-blown web according to any one of the preceding claims, wherein the second peroxide has a half-life time of 0.1 hour at a temperature of 165 to 188 ^ºC. The melt-blown web according to any one of the preceding claims, wherein the first peroxide is 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane and/or the second peroxide is 3,6, 9-Triethyl-3,6,9-trimethyl- 1 ,4,6 triperoxonane and/or 3,3,5,7,7-pentamethyl-

1 ,2,4-trioxepane. The melt-blown web according to any one of the preceding claims, wherein the amount of the first peroxide with respect to the propylene-based polymer is 100 to 2000 ppm and the amount of the second peroxide with respect to the propylene- based polymer is 100 to 8000 ppm, preferably 1000 to 8000 ppm. The melt-blown web according to any one of the preceding claims, wherein the propylene-based polymer is a propylene homopolymer, a random propylene copolymer or a heterophasic propylene copolymer.

8. The melt-blown web according to any one of the preceding claims, wherein the propylene-based polymer has a first melt flow index MFI_A determined according to ISO1133-1 :2011 at 230 °C and 2.16 kg of 0.1 to 60 dg/min, for example 0.1 to 1 .0 dg/min, 1 .0 to 5.0 dg/min, 5.0 to 15 dg/min or 15 to 60 dg/min.

9. The melt-blown web according to any one of the preceding claim, wherein the composition obtained by step a) has a second melt flow index MFI_B determined according to ASTM D1238-13 (2 mm die) at 190°C and 2.16 kg of 10 to 300 dg/min, for example 100 to 200 dg/min.

10. The melt-blown web according to any one of the preceding claim, wherein the melt- blown fibers have a third melt flow index MFIc determined by ASTM D1238-13 Procedure C (1 mm die) at 230°C and 2.16 kg of 100 to 300 dg/min, for example 150 to 250 dg/min.

11 . The melt-blown web according to any one of the preceding claim, wherein MFIc- MFIB is at least 30 dg/min, more preferably at least 40 dg/min, more preferably at least 50 dg/min, more preferably at least 60 dg/min, more preferably at least 70 dg/min and/or wherein MFIc-MFIs is at most 100dg/min, for example at most 90dg/min, wherein MFIc stands for the melt flow index of the melt blown fibers determined by ASTM D1238-13 Procedure C (1 mm die) at 230°C and 2.16kg and wherein MFIB stands for the melt flow index of the composition obtained by step a) and determined according to ASTM D1238-13 (2 mm die) at 190°C and 2.16 kg.

12. The melt-blown web according to any one of the preceding claim, wherein the composition obtained by step a) is formed into pellets and the pellets are subjected to step b).

13. Use of pellets for making a melt-blown web, wherein the pellets are made by a) melt-mixing a propylene-based polymer, a first peroxide and a second peroxide at temperatures between 180 ^ºC and 240 ^ºC, preferably between 200 ^ºC and 220 ^ºC, wherein the first peroxide has a half-life time of 1 hour at a first temperature TI/₂1 and the second peroxide has a half-life time of 1 hour at a second temperature TI/₂2, wherein TI/₂2 is higher than T1/2I , and forming the composition obtained by step a) into pellets. An article comprising the melt-blown web according to any one of claims 1 -12, preferably wherein the article is selected from the group consisting filter media (e.g. air filters such as clean room filters, ventilation filters, filters, 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 articles, composite non-wovens. Use of the melt-blown web according to any one of claims 1 -12 for making an article selected from the group consisting of filter media, (e.g. air filters such as clean room filters, ventilation filters, filters, 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 articles, composite non-wovens.