EP4683952A1

EP4683952A1 - Polypropylene resin compositon

Info

Publication number: EP4683952A1
Application number: EP23782948.6A
Authority: EP
Inventors: Antonio RIEMMA; Andrea Felisati; Marco BOCCHINO; Claudio Cavalieri; Giampaolo Pellegatti
Original assignee: Basell Poliolefine Italia SRL
Current assignee: Basell Poliolefine Italia SRL
Priority date: 2023-03-21
Filing date: 2023-10-02
Publication date: 2026-01-28
Also published as: CN120787241A

Abstract

A polypropylene resin composition comprising: A) from 10 wt% to 70 wt% of an ultra-high-molecular-weight propylene homopolymer optionally containing up to 1.0 wt% of ethylene derived units; B) from 30 wt% to 90 wt% of an organic material selected from: organic acid esters; adipic acid esters, glyceric acid esters; phosphoric acid esters; paraffin; wax; and mineral oil.

Description

POLYPROPYLENE RESIN COMPOSITON

FIELD OF THE INVENTION

[0001] The present disclosure relates to polypropylene resin composition comprising an ultra- high-molecular-weight propylene homopolymer for use in the formation of a microporous membrane having good Gurley permeability, measured according to ISO 5636 and good mechanical properties.

BACKGROUND OF THE INVENTION

[0002] Microporous membranes made of polymeric materials are used in various applications, for example, filter membranes and separation membranes for medical and industrial use, and separators, such as battery separators and condenser separators. In particular, with the recent growing demand for secondary batteries as power supplies for mobile phones, mobile persona! computers, and automobiles, there is also a growing demand for battery separators.

[0003] EP 1 464669 relates to microporous membrane made substantially with high molecular weight polyethylene or a blend of polyethylene and polypropylene. However the air transmission rate (Gurley permeability) can be improved.

SUMMARY OF THE INVENTION

[0004] The present disclosure is directed to a polypropylene resin composition comprising:

A) from 10 wt% to 70 wt% of an ultra- high-molecular- weight propylene homopolymer optionally containing up to 1.0 wt% of ethylene derived units characterized in that:

- two melting points are present in the DSC thermogram, measured according to ISO 11357- 3, with heating and cooling rate of 20°C/min;

-the higher melting point, measured according to ISO 11357-3, with heating and cooling rate of 20°C/min, ranges from 155°C to 170°C;

- the fraction soluble in xylene at 25°C , measured according to ISO 16 152 - 2005 is comprised between 6.0 wt% and 2.0wt%;

-the isotactic pentads mmmm % measured with C¹³NMR measured with C¹³NMR as reported in the examples section ranges from 96.5 mol% to 90.0 mol%;

- the intrinsic viscosity (IV) measured in tetrahydronaphthalene at 135 °C ranges from 5.5 to 12.0 dl/g; - the melt strength measured according to ISO 16790-2005, at 250°C and applying an acceleration equal to 6 mm/s² is higher than 0.070 N;

B) from 30 wt% to 90 wt% of an organic material selected from: organic acid esters; adipic acid esters, glyceric acid esters; phosphoric acid esters; paraffin; wax; and mineral oil; wherein the sum of the amounts of component A) and component B) in wt% is equal to 100 wt% .

DETAILED DESCRIPTION OF THE INVENTION

[0005] The present disclosure is directed to a polypropylene resin composition comprising:

A) from 10 wt% to 70 wt% preferably from 20 wt% to 60 wt%; more preferably from 30 wt% to 57 wt% of an ultra-high-molecular- weight propylene homopolymer optionally containing up to 1.0 wt% of ethylene derived, preferably up to 0.6 wt% of ethylene derived units characterized in that:

- two melting points are present in the DSC thermogram measured according to ISO 11357-3, with heating and cooling rate of 20°C/min; preferably the lower melting point ranges from 135°C to 150°C;

- the higher melting point measured according to ISO 11357-3, with heating and cooling rate of 20°C/min„ ranges from 155°C to 170°C; preferably from 157°C to 168°C;

- the fraction soluble in xylene at 25°C measured according to ISO 16 152 - 2005 is comprised between 6.0 wt% and 2.0wt%; preferably comprised between 5.0 wt% and 2.5wt%; more preferably comprised between 4.0 wt% and 2.8wt%;

-the isotactic pentads, mmmm %, measured with C¹³NMR as reported in the examples section ranges from 96.5 mol% to 90.0 mol%, .preferably from 96.0 mol% to 93.0 mol%; more preferably from 95.5 mol% to 93.5 mol%;

- the intrinsic viscosity (IV) measured in tetrahydronaphthalene at 135 °C ranges from 5.5dl/g to 12.0 dl/g; preferably from 7.0 dl/g to 11.0 dl/g; more preferably from 8.0 dl/g to 10.0 dl/g;

- the melt strength, measured measured according to ISO 16790-2005 at 250°C and applying an acceleration equal to 6 mm/s² , is higher than 0.070 N; preferably higher than 0.080 N; more preferably higher than 0.090 N;

B) from 30 wt% to 90 wt%; preferably from 40 wt% to 80 wt%; more preferably from 43 wt% to 70 wt% of an organic material selected from: organic acid esters, as dioctyl phthalate, diheptyl phthalate and dibutyl phthalate, adipic acid esters, glyceric acid esters; phosphoric acid esters, such as trioctyl phosphate; paraffin, such as liquid paraffin and solid paraffin; wax; and mineral oil; wherein the sum of the amounts of component A) and component B) in wt% is equal to 100wt% . [0006] Preferably the organic material component B) is selected from liquid paraffin, solid paraffin; wax; and mineral oil; more preferably the organic material component B) is a mineral oil; even more preferably the organic material component B) is white mineral oil (CAS 8042- 47-5).

[0007] Preferably the propylene homopolymer of the present disclosure is not nucleated.

[0008] Preferably the homopolymer of the present disclosure shows a value of melt strength lower than 0.30 N.

[0009] Preferably the homopolymer of the present disclosure shows a polydispersity index, PI, measured according to ISO 6721-10, comprised between 4.5 and 7.5; more preferably between 5.0 and 7.2; more preferably from 5.5 to 6.5.

[0010] Preferably the homopolymer of the present disclosure shows a tensile modulus ranging from 2100 MPa to 1100 MPa; preferably from 1800MPa, to 1200 Mpa.

[0011] Preferably the homopolymer of the present disclosure shows a Charpy impact test at 23°C ranging from 4.0 kJ/m2 to 11.0 kJ/m2; preferably from 5.5 kJ/m2 to 9.0 kJ/m2.

[0012] The propylene homopolymers disclosed herein can be prepared by a process comprising polymerizing propylene optionally with ethylene, in the presence of Ziegler-Natta catalysts. An essential component of said catalysts is a solid catalyst component comprising a titanium compound having at least one titanium-halogen bond, and an electron-donor compound, both supported on a magnesium halide in active form. Another essential component (co-catalyst) is an organoaluminium compound, such as an aluminium alkyl compound. An external donor is optionally added.

[0013] Catalysts having the above mentioned characteristics are well known in the patent literature; particularly advantageous are the catalysts described in US patent 4,399,054 and European patent 45977. Other examples can be found in US patent 4,472,524.

[0014] The solid catalyst components used in said catalysts comprise, as electron-donors (internal donors), compounds selected from the group consisting of ethers, ketones, lactones, compounds containing N, P and/or S atoms, and esters of mono- and dicarboxylic acids. [0015] Particularly suitable electron-donor compounds are esters of succinic acid (succinates) Preferably, the succinate present in the solid catalyst component is selected from succinates of formula (I) below

[0016] in which the radicals Ri and R2, equal to, or different from, each other are a C1-C20 linear or branched alkyl, alkenyl, cycloalkyl, aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms; and the radicals R3 and R4 equal to, or different from, each other, are Ci- C20 alkyl, C3-C20 cycloalkyl, C5-C20 aryl, arylalkyl or alkylaryl group with the proviso that at least one of them is a branched alkyl; said compounds being, with respect to the two asymmetric carbon atoms identified in the structure of formula (I), stereoisomers of the type (S,R) or (R,S) [0017] Ri and R2 are preferably Ci-Cs alkyl, cycloalkyl, aryl, arylalkyl and alkylaryl groups. Particularly preferred are the compounds in which Ri and R2 are selected from primary alkyls and in particular branched primary alkyls. Examples of suitable Ri and R2 groups are methyl, ethyl, n- propyl, n-butyl, isobutyl, neopentyl, 2-ethylhexyl. Particularly preferred are ethyl, isobutyl, and neopentyl.

[0018] Particularly preferred are the compounds in which the R3 and/or R4 radicals are secondary alkyls like isopropyl, sec- butyl, 2-pentyl, 3 -pentyl or cycloakyls like cyclohexyl, cyclopentyl, cyclohexylmethyl.

[0019] Examples of the above-mentioned compounds are the (S,R) (S,R) forms pure or in mixture, optionally in racemic form, of diethyl 2,3-bis(trimethylsilyl)succinate, diethyl 2,3-bis(2- ethylbutyl)succinate, diethyl 2,3 -dibenzylsuccinate, diethyl 2,3-diisopropylsuccinate, diisobutyl

2.3-diisopropylsuccinate, diethyl 2,3-bis(cyclohexylmethyl)succinate, diethyl 2,3- diisobutylsuccinate, diethyl 2,3-dineopentylsuccinate, diethyl 2,3-dicyclopentylsuccinate, diethyl

2.3-dicyclohexylsuccinate. [0020] Particularly suitable electron- donor compounds are esters of phtalic acid and 1,3- diethers of formula:

[0021] wherein R¹ and Rⁿ are the same or different and are Ci-Cis alkyl, C3-C18 cycloalkyl or C7-C18 aryl radicals; R¹¹¹ and R^IV are the same or different and are C1-C4 alkyl radicals; or are the 1,3 -di ethers in which the carbon atom in position 2 belongs to a cyclic or polycyclic structure made up of 5, 6, or 7 carbon atoms, or of 5-n or 6-n' carbon atoms, and respectively n nitrogen atoms and n' heteroatoms selected from the group consisting of N, O, S and Si, where n is 1 or 2 and n' is 1, 2, or 3, said structure containing two or three unsaturations (cyclopolyenic structure), and optionally being condensed with other cyclic structures, or substituted with one or more substituents selected from the group consisting of linear or branched alkyl radicals; cycloalkyl, aryl, aralkyl, alkaryl radicals and halogens, or being condensed with other cyclic structures and substituted with one or more of the above mentioned substituents that can also be bonded to the condensed cyclic structures; one or more of the above mentioned alkyl, cycloalkyl, aryl, aralkyl, or alkaryl radicals and the condensed cyclic structures optionally containing one or more heteroatom(s) as substitutes for carbon or hydrogen atoms, or both.

[0022] Ethers of this type are described in published European patent applications 361493 and 728769.

[0023] Representative examples of said diethers are 2-methyl-2-isopropyl-l,3- dimethoxypropane, 2,2-diisobutyl-l,3-dimethoxypropane, 2-isopropyl-2-cyclopentyl-l,3- dimethoxypropane, 2-isopropyl-2-isoamyl-l,3-dimethoxypropane, 9,9-bis (methoxymethyl) fluorene.

[0024] Other suitable electron-donor compounds are phthalic acid esters, such as diisobutyl, dioctyl, diphenyl and benzylbutyl phthalate.

[0025] The preparation of the above mentioned catalyst component is carried out according to various methods.

[0026] For example, a MgC12»nROH adduct (in particular in the form of spheroidal particles) wherein n is generally from 1 to 3 and ROH is ethanol, butanol or isobutanol, is reacted with an excess of TiC14 containing the electron-donor compound. The reaction temperature is generally from 80 to 120° C. The solid is then isolated and reacted once more with TiC14, in the presence or absence of the electron-donor compound, after which it is separated and washed with aliquots of a hydrocarbon until all chlorine ions have disappeared.

[0027] In the solid catalyst component the titanium compound, expressed as Ti, is generally present in an amount from 0.5 to 10% by weight. The quantity of electron-donor compound which remains fixed on the solid catalyst component generally is 5 to 20% by moles with respect to the magnesium dihalide.

[0028] The titanium compounds, which can be used for the preparation of the solid catalyst component, are the halides and the halogen alcoholates of titanium. Titanium tetrachloride is the preferred compound.

[0029] The reactions described above result in the formation of a magnesium halide in active form. Other reactions are known in the literature, which cause the formation of magnesium halide in active form starting from magnesium compounds other than halides, such as magnesium carboxylates.

[0030] The Al-alkyl compounds used as co-catalysts comprise the Al-trialkyls, such as Al- triethyl, Al-triisobutyl, Al-tri-n-butyl, and linear or cyclic Al-alkyl compounds containing two or more Al atoms bonded to each other by way of O or N atoms, or SO4 or SO3 groups.

[0031] The Al-alkyl compound is generally used in such a quantity that the Al/Ti ratio be from 1 to 1000.

[0032] The electron-donor compounds that can be used as external donors include aromatic acid esters such as alkyl benzoates, and in particular silicon compounds containing at least one Si-OR bond, where R is a hydrocarbon radical.

[0033] Examples of silicon compounds are (tert-butyl)2Si(OCH3)2, (cyclohexyl)(methyl)Si (OCH3)2, (cyclopentyl)2Si(OCH3)2 and (phenyl)2Si(OCH3)2 and (1,1,2- tr imethy Ipropy 1) S i(OCH3 ) 3.

[0034] 1,3 -diethers having the formulae described above can also be used advantageously. If the internal donor is one of these diethers, the external donors can be omitted.

[0035] In particular, even if many other combinations of the previously said catalyst components may allow to obtain compositions according to the present invention, the component A) are preferably prepared by using catalysts containing a phthalate as internal donor and (cyclopentyl)2Si(OCH3)2 as outside donor, or the said 1,3 -di ethers as internal donors. [0036] The polymerization is generally carried out at temperatures of from 20 to 120°C, preferably of from 40 to 80°C. When the polymerization is carried out in gas-phase, the operating pressure is generally between 0.5 and 5 MPa, preferably between 1 and 4 MPa. In bulk polymerization, the operating pressure is generally between 1 and 8 MPa, preferably between 1.5 and 5 MPa. Hydrogen is typically used as a molecular weight regulator. The polymerization can be in gas phase or in slurry or in solution. In one or more reactors. Preferably the polymerizaiotn is carried put in two slurry reactors operating in series.

[0037] The composition of the present disclosure can be obtained with a process comprising the step of mixing component A) and component B) to perform a granulation in a mixer.

[0038] The composition of the present disclosure can be used in the formation of a microporous membrane having good Gurley permeability and good mechanical properties.

A method most suitably employed for obtaining the microporous membrane in accordance with the present disclosure comprises the steps of mixing organic material component B) with component A), heating and melting the mixture, extruding the melt into a sheet, orientating the sheet biaxially, either simultaneously or sequentially, and then extracting the liquid with a volatile solvent (dichloromethane for example).

Optionally the membrane can be subjected to a heat-setting process, this last step enhances the dimensional stability and prevents shrinkage or wrinkle formation upon heating, which is important for safety and control of the final performance.

[0039] The microporous membrane obtained with the composition of the present disclosure shows a good Gurley permeability, measured according to ISO 5636.

[0040] In addition the porosity measures according to the method described in the examples is particularly good, the porosity preferably ranges from 30.0% to 60.0%, more preferably from 33.0% to 51.0%; more preferably from 34.0% to 45.0%.

[0041] The puncture resistance of microporous membrane normalized for 20pm ranges from 700 g/20pm to 1200 g/20pm; preferably from 750 g/20pm to 1050 g/20pm; more preferably from 800 g/20pm to 998 g/20pm

[0042] A further object of the present disclosure is the use of ultra-high-molecular- weight propylene homopolymer containing up to 1.0 wt% of ethylene derived units component A) for producing a microporous membrane. [0043] A further object of the present disclosure is the microporous membrane preferably obtainable with the above process, having Gurley permeability normalized to 20pm thickness measured according to ISO 5636 lower than 1400 s/100ml720pm, preferably lower than 1300 s/100m/20pm.

[0044] A further object of the present disclosure is the microporous membrane obtainable with the above described process.

[0045] The following examples are given in order to illustrate, but not limit the present disclosure.

EXAMPLES

CHARACTERIZATIONS

Xylene-insoluble and soluble fraction at 25°C

[0046] Xylene Solubles fraction has been measured according to ISO 16 152 - 2005; with solution volume of 250 ml, precipitation at 25°C for 20 minutes, 10 of which with the solution in agitation (magnetic stirrer), and drying at 70°.

Melt Flow Rate (MFR)

[0047] Measured according to ISO 1133 at 230°C with a load of 21.6 kg, unless otherwise specified.

Polydispersity (PI)

[0048] Some grams of molten homopolymer are submitted to a dynamic test in rate sweep with a parallel plate rheometer, at temperature of 200°C, according to the ISO 6721-10. G’ (storage modulus) and G” (loss modulus) are measured as function of frequency. From the rate sweep data, PI is defined by PI=105/Gc, wherein Gc is the crossover modulus as value of modulus at G’ = G

Melting temperature via Differential Scanning Calorimetry (DSC)

[0049] The melting points of the polymers (Tm) were measured by differential scanning calorimetry (DSC) on a Perkin Elmer DSC-1 calorimeter, previously calibrated against indium melting points, and according to ISO 11357-1, 2009 and 11357-3, 2011, at 20°C/min. The weight of the samples in every DSC crucible was kept at 6.0 ± 0.5 mg.

[0050] In order to obtain the melting point, the weighted sample was sealed into aluminium pans and heated to 200°C at 20°C/minute. The sample was kept at 200°C for 2 minutes to allow a complete melting of all the crystallites, then cooled to 5 °C at 20°C/minute. After standing 2 minutes at 5°C, the sample was heated for the second run time to 200°C at 20°C/min. In this second heating run, the peak temperature (Tp,m) was taken as the melting temperature.

¹³C NMR of homopolymer and propylene/ethylene copolymers

[0051] ¹³ C NMR spectra were acquired on a Bruker AV-600 spectrometer equipped with cryoprobe, operating at 160.91 MHz in the Fourier transform mode at 120°C.

[0052] The peak of the Spp carbon (nomenclature according to “Monomer Sequence Distribution in Ethylene-Propylene Rubber Measured by 13C NMR. 3. Use of Reaction Probability Mode ” C. J. Carman, R. A. Harrington and C. E. Wilkes, Macromolecules, 1977, 10, 536) was used as internal reference at 29.9 ppm. The samples were dissolved in 1,1,2,2-tetrachloroethane- d2 at 120°C with a 8 % wt/v concentration. Each spectrum was acquired with a 90° pulse, 15 seconds of delay between pulses and CPD to remove 1H-13C coupling. 512 transients were stored in 32K data points using a spectral window of 9000 Hz.

[0053] The assignments of the spectra, the evaluation of triad distribution and the composition were made according to Kakugo (“Carbon- 13 NMR determination of monomer sequence distribution in ethylene-propylene copolymers prepared with 8-titanium trichloridediethylaluminum chloride” M. Kakugo, Y. Naito, K. Mizunuma and T. Miyatake, Macromolecules, 1982, 15, 1150) using the following equations:

PPP = 100 Tpp/S PPE = 100 Tps/S EPE = 100 Tss/S

PEP = 100 Spp/S PEE= 100 Sps/S EEE = 100 (0.25 Syg+0.5 S₅₅)/S

S = Tpp + Tps + Tss + Spp + Sps + 0.25 Syg + 0.5 Sss

[0054] The molar percentage of ethylene content was evaluated using the following equation: [0055] E% mol = 100 * [PEP+PEE+EEE]The weight percentage of ethylene content was evaluated using the following equation:

100 * E% mol * MWE

E% wt. = >

E% mol * MWE + P% mol * MWp where P% mol is the molar percentage of propylene content, while MWE and MWp are the molecular weights of ethylene and propylene, respectively.

[0056] The product of reactivity ratio nr? was calculated according to Carman (C.J. Carman,

R.A. Harrington and C.E. Wilkes, Macromolecules, 1977; 10, 536) as:

The tacticity of Propylene sequences was calculated as mm content from the ratio of the PPP mmTpp (28.90-29.65 ppm) and the whole Tpp (29.80-28.37 ppm).

Intrinsic viscosity

[0057] The sample is dissolved by tetrahydronaphthalene at 135 °C and then it is poured into the capillary viscometer. The viscometer tube (Ubbelohde type) is surrounded by a cylindrical glass jacket; this setup allows temperature control with a circulating thermostated liquid. The downward passage of the meniscus is timed by a photoelectric device. The passage of the meniscus in front of the upper lamp starts the counter which has a quartz crystal oscillator. The meniscus stops the counter as it passes the lower lamp and the efflux time is registered: this is converted into a value of intrinsic viscosity through Huggins' equation, provided that the flow time of the pure solvent is known at the same experimental conditions (same viscometer and same temperature). One single polymer solution is used to determine IV], Melt strength

[0058] The melt strength is measured according to ISO 16790-2005, by Haul-off Melt Strength Meter produced by Geottfert Werkstoff Pruefmaschinen, Germany. This system measures the extensional properties of polymer melts by drawing a vertical melt strand at a constant pull-off speed or with a linear or exponentially accelerating velocity. The HAUL-OFF system measures the force needed to elongate the strand, and calculates elongation stress, draw ratio and apparent elongation rate and viscosity. Polymer is melt and plasticized through a capillary rheometer, then is extruded from a hole die with a 1 mm of diameter, 30 mm of length and 180° inlet angle. The test is performed at 250°C. The distance from the capillary outlet to the center of the transducer pulley is 150 mm. The monofilament is stretched at each temperature test applying an acceleration equal to 6 mm/s² and, passing through an angular transducer, its tension is measured. The draw ratio (dimensionless value) and force (cN) values are recorded as the final result in addition to the entire curve. The value of the melt strength is the maximum force value of the curve. Charpy impact test

Charpy impact test is measured according to ISO 179-leA, e ISO 1873-2, on compression sample Tensile Modulus

Tensile Modulus is measured according to ISO 527-2, and ISO 1873-2 on compression sample

Porosity

Porosity is measured by measuring the mass and thickness of a film sample of 5 cm x 5 cm calculating the free volume in relation to the standard density given in equation below.

Porosity

In the equation, m is the mass of the sample, p is the density of the base material (0.915 g/cm³) , A is the area of the sample and d is the measured thickness.

Puncture resistance and deformation:

Determined according to the method ASTM D 4833 on microporous membrane. Punch diameter 8 mm, crosshead speed: 300 mm/min. The puncture resistance has been normalized for 20pm membrane.

Examples 1 - Preparation of homopolymer

Procedure for the preparation of the solid catalyst component

[0059] The solid catalyst used in the following examples was prepared according to the Example 10 of the International Patent Application WO 00/63261. Tri ethylaluminium (TEAL) was used as co-catalyst and dicyclopentyldimethoxysilane as external donor, with the weight ratios indicated in Table 1.

Polymerization

[0060] The polymerization run is carried out in continuous mode in a series of two reactors equipped with devices to transfer the product from one reactor to the one immediately next to it. The two reactors are liquid phase loop reactors. Propylene is the main solvent, hydrogen is used as molecular weight regulator. The gas phase is continuously analyzed via gaschromatography.

[0061] At the end of the run the powder is discharged and dried under a nitrogen flow. The main polymerization conditions and the properties of the polymer are reported in Tables 1 and 2. Table 1 - Polymerization conditions

The features of the polymers of example 1 are reported in table 2.

Table 2 The polymer of example 1 (component A) has been mixed with white mineral oil (CAS 8042-47- 5) sold by Kukdong component B).

The obtained composition has been melded to forma a sheet, the resulting sheet has been stretched in biaxial direction 7x7 times. Then the sheet has been treated with dichlorometane to remove component B). The features of the obtained microporous membranes are reported on table 3

Table 3

Gurley permeability has been measured according to ISO 5636, the value has been normalized for 20pm sheet.

Claims

What is claimed is: A polypropylene resin composition comprising:

A) from 10 wt% to 70 wt% of an ultra-high-molecular- weight propylene homopolymer optionally containing up to 1.0 wt% of ethylene derived units characterized in that:

- two melting points in the DSC thermogram measured according to ISO 11357-3, with heating and cooling rate of 20°C/min;

-the higher melting point, measured according to to ISO 11357-3, with heating and cooling rate of 20°C/min, ranges from 155°C to 170°C;

- the fraction soluble in xylene at 25°C measured according to ISO 16 152 - 2005, is comprised between 6.0 wt% and 2.0wt%;

-the isotactic pentads mmmm % measured with C¹³NMR as reported in examples section of the description, ranges from 96.5 mol% to 90.0 mol%;

- the intrinsic viscosity (IV) measured in tetrahydronaphthalene at 135 °C ranges from 5.5 to 12.0 dl/g;

- the melt strength measured according to ISO 16790-2005, at 250°C and applying an acceleration equal to 6 mm/s² is higher than 0.070 N;

B) from 30 wt% to 90 wt% of an organic material selected from: organic acid esters; adipic acid esters, glyceric acid esters; phosphoric acid esters; paraffin; wax; and mineral oil. The polypropylene resin composition according to claim 1 wherein in component A) two melting points are present in the DSC thermogram measured according to ISO 11357-3, with heating and cooling rate of 20°C/min; and the lower melting point ranges from 135°C to 150°C. The polypropylene resin composition according to claims 1 or 2 wherein in component A) the higher melting point, measured according to ISO 11357-3, with heating and cooling rate of 20°C/min, ranges from 157°C to 168°C. The polypropylene resin composition according to anyone of claims 1-3 wherein in component A) the fraction soluble in xylene at 25°C measured according to ISO 16 152 — 2005 is comprised between 5.0 wt% and 2.5 wt%. The polypropylene resin composition according to anyone of claims 1-4 wherein in component A) the fraction soluble in xylene at 25°C measured according to ISO 16 152 — 2005 is comprised between 4.0 wt% and 2.8 wt%. The polypropylene resin composition according to anyone of claims 1-5 wherein in component A) the isotactic pentads mmmm % measured with ¹³C-NMR ranges from 96.0 mol% to 93.0 mol%. The polypropylene resin composition according to anyone of claims 1-6 wherein component A) ranges from 20 wt% to 60 wt% and component B) ranges from 40 wt% to 80 wt%. The polypropylene resin composition according to anyone of claims 1-7 wherein component B) is selected from the group consisting of liquid paraffin, solid paraffin; wax; and mineral oil. The polypropylene resin composition according to anyone of claims 1-8 wherein component B) is mineral oil. The polypropylene resin composition according to anyone of claims 1-9 wherein in component A) the melt strength measured according to ISO 16790-2005 at 250°C and applying an acceleration equal to 6 mm/s², is higher than 0.080 N. The polypropylene resin composition according to anyone of claims 1-10 wherein in component A) the intrinsic viscosity (IV) measured in tetrahydronaphthalene at 135 °C ranges from 7.0 to 11.0 dl/g The polypropylene resin composition according to anyone of claims 1-11 wherein component A) ranges from 30 wt% to 57 wt% and component B) ranges from 43 wt% to 70 wt%. A process for obtaining the microporous membrane according to anyone of claims 1-12 comprising the steps of mixing organic material component B) with component A), heating and melting the mixture, extruding the melt into a sheet, orientating the sheet biaxially, either simultaneously or sequentially, and then extracting the liquid with a volatile solvent. The process according to claim 13 wherein the volatile solvent is di chloromethane. The use of the ultra-high- molecular- weight propylene homopolymer containing up to 1.0 wt% of ethylene according to claims 1-12 for producing a microporous membrane.