Melt blown web with good water barrier properties

Abstract

Melt-blown fiber comprising two polypropylenes which differ in their molecular weight.

Claims

1. A melt blown fiber consisting of a polypropylene composition comprising at least 95 wt %, based on the total weight of the polypropylene composition, of a mixture consisting of: (a) a first polypropylene having a weight molecular weight Mw in the range of 14 to 22 kg/mol, wherein the first polypropylene is a random propylene ethylene copolymer; and (b) a second polypropylene having a weight molecular weight Mw in the range of 70 to 100 kg/mol and a molecular weight distribution (Mw/Mn) in the range of 3.5 to 7.0, wherein the second polypropylene is a propylene homopolymer, wherein the mixture consisting of the first polypropylene and the second polypropylene has a melt flow rate MFR.sub.2 (230° C.) measured according to ISO 1133 in the range of more than 400 to 900 g/10min, wherein the mixture consisting of the first polypropylene and the second polypropylene has an ethylene content in the range of 0.4 to 12.5 mol-%.

2. The melt blown fiber according to claim 1, wherein the mixture has a weight molecular weight Mw in the range of 50 to 110 kg/mol.

3. The melt blown fiber according to claim 1, wherein the polypropylene composition has: a weight molecular weight Mw in the range of 50 to 110 kg/mol.

4. The melt blown fiber according to claim 1, wherein the weight ratio between the first polypropylene and the second polypropylene is in the range of 0.02 to 0.45.

5. The melt blown fiber according to claim 1, wherein the ratio of the weight molecular weight Mw of the mixture to the weight molecular weight Mw of the first polypropylene is in the range of 2.0 to 10.0.

6. The melt blown fiber according to claim 1, wherein: the first polypropylene has a molecular weight distribution (Mw/Mn) in the range of 1.5 to 3.0.

7. The melt blown fiber according to claim 1, wherein the mixture consisting of the first polypropylene and the second polypropylene has: a melting temperature Tm of at least 145° C.

8. The melt blown fiber according to claim 1, wherein the polypropylene composition has a xylene cold soluble fraction in the range of 2.8 to 35.0 wt.-%.

9. The melt blown fiber according to claim 1, wherein the second polypropylene has a comonomer content of at most 5.5 mol-%.

10. The melt blown fiber according to claim 1, wherein the first polypropylene has: (a) a comonomer content of at most 33.5 mol-%; and/or (b) a xylene cold soluble fraction in the range of 50 to 95 wt.-%.

11. The melt blown fiber according to claim 1, wherein the fibers have an average diameter of 0.5 to 5.0 μm.

12. A melt-blown web comprising melt blow fibers each consisting of a polypropylene compositing comprising at least 95 wt %, based on the total weight of the polypropylene composition, of a mixture consisting of: (a) a first polypropylene having a weight molecular weight Mw in the range of 14 to 22 kg/mol, wherein the first polypropylene is a random propylene ethylene copolymer; and (b) a second polypropylene having a weight molecular weight Mw in the range of 70 to 100 kg/mol and having a molecular weight distribution (Mw/Mn) in the range of 3.5 to 7.0, wherein the second polypropylene is a propylene homopolymer; wherein the mixture consisting of the first polypropylene and the second polypropylene has a melt flow rate MFR.sub.2 (230° C.) measured according to ISO 1133 in the range of more than 400 to 900 g/10min, wherein the mixture consisting of the first polypropylene and the second polypropylene has an ethylene content in the range of 0.4 to 12.5 mol-%.

13. The melt-blown web according to claim 12 having a weight per unit area of at most 120 g/m.sup.2.

14. An article comprising a melt-blown web, the melt-blown web comprising melt blown fibers each consisting of a polypropylene composition comprising at least 95 wt. %, based on the total weight of the polypropylene composition, of a mixture consisting of: (a) a first polypropylene having a weight molecular weight Mw in the range of 14 to 22 kg/mol, wherein the first polypropylene is a random propylene ethylene copolymer; and (b) a second polypropylene having a weight molecular weight Mw in the range of 70 to 100 kg/mol and having a molecular weight distribution (Mw/Mn) in the range of 3.5 to 7.0, wherein the second polypropylene is a propylene homopolymer; wherein the mixture consisting of the first polypropylene and the second polypropylene has a melt flow rate MFR.sub.2 (230° C.) measured according to ISO 1133 in the range of more than 400 to 900 g/10min, wherein the mixture consisting of the first polypropylene and the second polypropylene has an ethylene content in the range of 0.4 to 12.5 mol-%.

15. The article of claim 14, wherein the article is selected from the group consisting of: filtration medium, diaper, sanitary napkin, panty liner, incontinence product for adults, protective clothing, surgical drape, surgical gown, and surgical wear.

16. The melt blown fiber according to claim 1, wherein the mixture consisting of the first polypropylene and the second polypropylene has a melt flow rate MFR.sub.2 (230° C.) measured according to ISO 1133 in the range of more than 440 to 900 g/10 min.

Description

EXAMPLES

(1) 1. Definitions/Measuring Methods

(2) The following definitions of terms and determination methods apply for the above general description of the invention as well as to the below examples unless otherwise defined.

(3) Quantification of Microstructure by NMR Spectroscopy

(4) Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used to quantify the comonomer content of the polymers. Quantitative .sup.13C{.sup.1H} NMR spectra were recorded in the solution-state using a Bruker Advance III 400 NMR spectrometer operating at 400.15 and 100.62 MHz for .sup.1H and .sup.13C respectively. All spectra were recorded using a .sup.13C optimised 10 mm extended temperature probehead at 125° C. using nitrogen gas for all pneumatics. Approximately 200 mg of material was dissolved in 3 ml of 1,2-tetrachloroethane-d.sub.2 (TCE-d.sub.2) along with chromium-(III)-acetylacetonate (Cr(acac).sub.3) resulting in a 65 mM solution of relaxation agent in solvent (Singh, G., Kothari, A., Gupta, V., Polymer Testing 28 5 (2009), 475). To ensure a homogenous solution, after initial sample preparation in a heat block, the NMR tube was further heated in a rotatary oven for at least 1 hour. Upon insertion into the magnet the tube was spun at 10 Hz. This setup was chosen primarily for the high resolution and quantitatively needed for accurate ethylene content quantification. Standard single-pulse excitation was employed without NOE, using an optimised tip angle, 1 s recycle delay and a bi-level WALTZ16 decoupling scheme (Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford, B., J. Mag. Reson. 187 (2007) 225; Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R., Severn, J., Talarico, G., Macromol. Rapid Commun. 2007, 28, 1128). A total of 6144 (6k) transients were acquired per spectra.

(5) Quantitative .sup.13C{.sup.1H} NMR spectra were processed, integrated and relevant quantitative properties determined from the integrals using proprietary computer programs. All chemical shifts were indirectly referenced to the central methylene group of the ethylene block (EEE) at 30.00 ppm using the chemical shift of the solvent. This approach allowed comparable referencing even when this structural unit was not present. Characteristic signals corresponding to the incorporation of ethylene were observed Cheng, H. N., Macromolecules 17 (1984), 1950).

(6) With characteristic signals corresponding to 2,1 erythro regio defects observed (as described in L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 2000, 100 (4), 1253, in Cheng, H. N., Macromolecules 1984, 17, 1950, and in W-J. Wang and S. Zhu, Macromolecules 2000, 33 1157) the correction for the influence of the regio defects on determined properties was required. Characteristic signals corresponding to other types of regio defects were not observed.

(7) The comonomer fraction was quantified using the method of Wang et. al. (Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157) through integration of multiple signals across the whole spectral region in the .sup.13C{.sup.1H} spectra. This method was chosen for its robust nature and ability to account for the presence of regio-defects when needed. Integral regions were slightly adjusted to increase applicability across the whole range of encountered comonomer contents.

(8) For systems where only isolated ethylene in PPEPP sequences was observed the method of Wang et. al. was modified to reduce the influence of non-zero integrals of sites that are known to not be present. This approach reduced the overestimation of ethylene content for such systems and was achieved by reduction of the number of sites used to determine the absolute ethylene content to:
E=0.5(Sββ+Sβγ+Sβδ+0.5(Sαβ+Sαγ))

(9) Through the use of this set of sites the corresponding integral equation becomes:
E=0.5(I.sub.H+I.sub.G+0.5(I.sub.C+I.sub.D))
using the same notation used in the article of Wang et. al. (Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157). Equations used for absolute propylene content were not modified.

(10) The mole percent comonomer incorporation was calculated from the mole fraction:
E[mol %]=100*fE

(11) The weight percent comonomer incorporation was calculated from the mole fraction:
E[wt %]=100*(fE*28.06)/((fE*28.06)+((1−fE)*42.08))

(12) The comonomer sequence distribution at the triad level was determined using the analysis method of Kakugo et al. (Kakugo, M., Naito, Y., Mizunuma, K., Miyatake, T. Macromolecules 15 (1982) 1150). This method was chosen for its robust nature and integration regions slightly adjusted to increase applicability to a wider range of comonomer contents.

(13) MFR.sub.2 (230° C.) is measured according to ISO 1133 (230° C., 2.16 kg load).

(14) Number Average Molecular Weight (M.sub.n), Weight Average Molecular Weight (M.sub.w) and Molecular Weight Distribution (MWD)

(15) Molecular weight averages (Mw, Mn), and the molecular weight distribution (MWD), i.e. the Mw/Mn (wherein Mn is the number average molecular weight and Mw is the weight average molecular weight), were determined by Gel Permeation Chromatography (GPC) according to ISO 16014-4:2003 and ASTM D 6474-99. A PolymerChar GPC instrument, equipped with infrared (IR) detector was used with 3×Olexis and 1×Olexis Guard columns from Polymer Laboratories and 1,2,4-trichlorobenzene (TCB, stabilized with 250 mg/L 2,6-Di tert butyl-4-methyl-phenol) as solvent at 160° C. and at a constant flow rate of 1 mL/min 200 μL of sample solution were injected per analysis. The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with at least 15 narrow MWD polystyrene (PS) standards in the range of 0.5 kg/mol to 11 500 kg/mol. Mark Houwink constants for PS, PE and PP used are as described per ASTM D 6474-99. All samples were prepared by dissolving 5.0-9.0 mg of polymer in 8 mL (at 160° C.) of stabilized TCB (same as mobile phase) for 2.5 hours for PP or 3 hours for PE at max. 160° C. under continuous gentle shaking in the autosampler of the GPC instrument.

(16) Xylene cold soluble fraction (XCS wt.-%): Content of xylene cold solubles (XCS) is determined at 25° C. according ISO 16152; first edition; 2005-07-01.

(17) DSC analysis, melting temperature (T.sub.m) and heat of fusion (H.sub.f), crystallization temperature (T.sub.c) and heat of crystallization (H.sub.c): measured with a TA Instrument Q200 differential scanning calorimetry (DSC) on 5 to 7 mg samples. DSC is run according to ISO 11357/part 3/method C2 in a heat/cool/heat cycle with a scan rate of 10° C./min in the temperature range of −30 to +225° C. Crystallization temperature (T.sub.c) and crystallization enthalpy (H.sub.c) are determined from the cooling step, while melting temperature (T.sub.m) and melting enthalpy (H.sub.m) are determined from the second heating step. The crystallinity is calculated from the melting enthalpy by assuming an Hm-value of 209 J/g for a fully crystalline polypropylene (see Brandrup, J., Immergut, E. H., Eds. Polymer Handbook, 3rd ed. Wiley, New York, 1989; Chapter 3).

(18) The glass transition temperature Tg is determined by dynamic mechanical analysis according to ISO 6721-7. The measurements are done in torsion mode on compression moulded samples (40×10×1 mm.sup.3) between −100° C. and +150° C. with a heating rate of 2° C./min and a frequency of 1 Hz.

(19) Grammage of the Web

(20) The unit weight (grammage) of the webs in g/m.sup.2 was determined in accordance with EN 29073-1 (1992) “Test methods for nonwovens—Determination of mass per unit area”

(21) Average Fibre Diameter in the Web

(22) The number average fibre diameter was determined using scanning electron microscopy (SEM). A representative part of the web was selected and an SEM micrograph of suitable magnification was recorded, then the diameter of 20 fibres was measured and the number average calculated.

(23) Air Permeability

(24) The air permeability was determined in accordance with DIN ISO 9237.

(25) Hydrohead

(26) The hydrohead or water resistance as determined by a hydrostatic pressure test is determined according to the WSP (wordwide strategic partners) standard test WSP 80.6 (09) as published in December 2009. This industry standard is in turn based on ISO 811:1981 and uses specimens of 100 cm.sup.2 at 23° C. with purified water as test liquid and a rate of increase of the water pressure of 10 cm/min.

Examples

(27) The two components PP1 and PP2 were melt-mixed in weight ratios as indicated in table 1 below in a Thermo PRISM TSE 24 twin-screw extruder at 200-240° C., followed by solidification of the resulting melt strands in a water bath and pelletization. The resulting compositions IE1 and 1E3 as well as the pure PP1 as CE1 were subsequently converted into melt-blown webs on a Reicofil MB250 line using a spinneret having 470 holes of 0.4 mm exit diameter and 35 holes per inch. Webs were produced at constant DCD (die to collector distance) of 200 mm, different melt temperatures as indicated in table 1 with a throughput of 10 kg/h.m and an air volume adapted in order to get a constant area weight of the resulting web. The resulting properties of the melt-blown webs are also indicated in table 1.

(28) TABLE-US-00001 TABLE 1 Properties of PP1, PP2, polypropylene composition (PC), melt blown fibers (MBFa) and melt blown web (MBW) CE1 CE2 IE1 IE2 IE3 PP1 [wt.-%] 0 100 5 13  22   PP2 [wt.-%] 100 0 95 87  78   MFR [g/10 min] 450 — 433 575  717   XCS [wt.-%] 6.9 81 10.4 16.8  24.6  C2 [mol-%] 0 11 0.55 1.43   2.42 Mw [kg/mol] 92 17 90 86  82   MWD [—] 5.0 2.1 5.1 5.7   6.2  Tm [° C.] 161 81 164 163  163   Tg [° C.] −0.5 −27 −1.0 −1.2   −1.6    Xc [%] 49 12 46 43  40   at melt temperatur 270° C. Fiber diameter [μm] 1.3 — 0.9 n.d.   1.0  Web weight [g/m.sup.2] 9.4 — 9.5 n.d.   9.5  Air permeability [mm/s] 1364 — 1071 n.d. 1213   Hydrohead (1st drop) [cm H.sub.2O] 45.2 — 53.8 n.d.  54.8  at melt temperatur 290° C. Fiber diameter [μm] 1.2 — 0.8 n.d.   0.9  Web weight [g/m.sup.2] 9.5 — 9.5 n.d.   9.5  Air permeability [mm/s] 1065 — 829 n.d.  827   Hydrohead (1st drop) [cm H.sub.2O] 77.1 — 82.2 n.d.  81.8  at melt temperatur 300° C. Fiber diameter [μm] 1.1 — 0.8 n.d.   0.8* Web weight [g/m.sup.2] 9.5 — 9.5 n.d.   9.5* Air permeability [mm/s] 638 — 715 n.d.  383*   Hydrohead (1st drop) [cm H.sub.2O] 12.7 — 105.6 n.d.  134*   *at melt temperature 310° C. PP1 is the the commercial propylene ethylene copolymer “Licocene PP1302” of Clariant PP2 is the the commercial propylene homopolymer “HL504FB” of Borealis AG