FILTRATION MEDIA MADE FROM MELT-BLOWN FIBERS WITH IMPROVED FILTRATION PROPERTIES

Abstract

The present invention relates to filtration media made from melt-blown fibers having improved barrier properties. The melt-blown fibers in the filtration media of the invention are made of a visbroken metallocene-catalyzed propylene homopolymer composition with specified melting temperature Tm and molecular weight distribution (MWD).

Claims

1. A filtration medium made of melt-blown fibers comprising a polypropylene composition, wherein: (i) the polypropylene composition comprises a propylene homopolymer (HPP) polymerized in the presence of a metallocene catalyst, (ii) the polypropylene composition has been visbroken, (iii) the polypropylene composition has a melting temperature (T.sub.m) measured by differential scanning calorimetry (DSC) according to ISO 11357 in the range from 152 to 160° C., and (iv) the polypropylene composition has a molecular weight M.sub.w (measured with GPC on the filtration medium) of between 20000-200000 g/mol, and a molecular weight distribution (MWD) of between 1.5-5.0.

2. The filtration medium according to claim 1, wherein the propylene homopolymer (HPP) has an initial MFR.sub.2 (230° C., 2.16 kg) determined according to ISO 1133 in the range of 20.0-250.0 g/10 min.

3. The filtration medium according to claim 1, wherein the propylene homopolymer (HPP) has a xylene cold soluble fraction (XCS) determined at 23° C. according to ISO 16152 in the range from 0.1 wt % to 1.5 wt %.

4. The filtration medium according to claim 1, wherein the propylene homopolymer (H-PP) has a content of 2,1 erythro regiodefects as determined from .sup.13C-NMR spectroscopy in the range from 0.30 to 0.80 mol. %.

5. The filtration medium according to claim 1, wherein the final MFR.sub.2 (230° C., 2.16 kg) determined on the filtration medium according to ISO 1133 is in the range of 600.0-5000.0 g/10 min, and/or the visbreaking ratio [final MFR.sub.2 (230° C./2.16 kg)/initial MFR.sub.2 (230° C./2.16 kg)] is in the range of 5 to 250, wherein the final MFR.sub.2 (230° C./2.16 kg) is the MFR.sub.2 (230° C./2.16 kg) of the polypropylene composition in the filtration medium and the initial MFR.sub.2 (230° C./2.16 kg) is the MFR.sub.2 (230° C./2.16 kg) of the propylene homopolymer (HPP).

6. The filtration medium according to claim 1, wherein: (i) the molecular weight (M.sub.w) ratio of the M.sub.w of the filtration medium to the M.sub.w of the propylene homopolymer (HPP) [M.sub.w(final)/M.sub.w(PP)] is <1, and (ii) molecular weight distribution (MWD) ratio of MWD of the filtration medium to MWD of the propylene homopolymer (HPP) [MWD(final)/MWD(PP)] is <1.

7. The filtration medium according to claim 1, wherein the polypropylene composition in the filtration medium has a molecular weight distribution (MWD) of between 2.0 to 3.2.

8. The filtration medium according to claim 1, wherein the propylene homopolymer (HPP) is polymerised in the presence of a metallocene catalyst complex of formula (I): ##STR00016## wherein Mt is H.sub.f or Zr; each X is a sigma-ligand, each R.sup.1 independently is the same different and is a CH.sub.2-R.sup.7 group, with R.sup.7 being H or linear or branched C.sub.1-6-alkyl group, C.sub.3-8 cycloalkyl group, C.sub.6-10 aryl group, each R.sup.2 is independently a —CH═, —CY═, —CH.sub.2—, —CHY— or —CY.sub.2— group, wherein Y is a C.sub.1-10 hydrocarbyl group and where n is 2-6, each R.sup.3 and R.sup.4 are independently the same or different and are hydrogen, a linear or branched C.sub.1-C.sub.6-alkyl group, an OY group or a C.sub.7-20 arylalkyl, C.sub.7-20 alkylaryl group or C.sub.6-20 aryl group, whereby at least one R.sup.3 per phenyl group and at least one R.sup.4 is not hydrogen, and optionally two adjacent R.sup.3 or R.sup.4 groups can be part of a ring including the phenyl carbons to which they are bonded, R.sup.5 is a linear or branched C.sub.1-C.sub.6-alkyl group, C.sub.7-20 arylalkyl, C.sub.7-20 alkylaryl group or C.sub.6-C.sub.20-aryl group, R.sup.6 is a C(R.sup.8).sub.3 group, with R.sup.8 being a linear or branched C.sub.1-C.sub.6 alkyl group, each R is independently a C.sub.1-C.sub.20-hydrocarbyl, C.sub.6-C.sub.20-aryl, C.sub.7-C.sub.20-arylalkyl or C.sub.7-C.sub.20-alkylaryl.

9. The filtration medium according to claim 8, wherein the propylene homopolymer (HPP) is polymerised in the presence of a metallocene catalyst complex, wherein Mt is Zr, each X is independently a hydrogen atom, a halogen atom, C.sub.1-6 alkoxy group or an R′ group, where R′ is a C.sub.1-6 alkyl, phenyl or benzyl group, each R is independently a C.sub.1-C.sub.20-hydrocarbyl, C.sub.6-C.sub.20-aryl, C.sub.7-C.sub.20-arylalkyl or C.sub.7-C.sub.20-alkylaryl, each R.sup.1 independently is the same or different and is a CH.sub.2-R.sup.7 group, with R.sup.7 being H or linear or branched C.sub.1-6-alkyl group, C.sub.6-10 aryl group, each R.sup.2 is independently a —CH═, —CY═, —CH.sub.2—, —CHY— or —CY.sub.2— group, wherein Y is a C.sub.1-4 hydrocarbyl group and where n is 3-4, each R.sup.3 and R.sup.4 are independently the same or different and are hydrogen, a linear or branched C.sub.1-C.sub.6-alkyl group or C.sub.6-20 aryl groups, whereby at least one R.sup.3 per phenyl group and at least one R.sup.4 is not hydrogen, R.sup.5 is a linear or branched C.sub.1-C.sub.6 alkyl group or C.sub.6-20 aryl group and R.sup.6 is a C(R.sup.8).sub.3 group, with R.sup.8 being a linear or branched C.sub.1-C.sub.4 alkyl group.

10. The filtration medium according to claim 9, wherein the propylene homopolymer (HPP) is polymerised in the presence of a metallocene catalyst complex, wherein: Mt is Zr, each X is independently a chlorine, benzyl or a methyl group, each R is independently a C.sub.1-C.sub.10-hydrocarbyl or C.sub.6-C.sub.10-aryl group, both R.sup.1 are the same and are a CH.sub.2-R.sup.7 group, with R.sup.7 being H or linear or branched C.sub.1-C.sub.3-alkyl group, each R.sup.2 is independently a —CH═, —CY═, —CH.sub.2—, —CHY— or —CY.sub.2— group, wherein Y is a C.sub.1-4 hydrocarbyl group and where n is 3-4, each R.sup.3 and R.sup.4 are independently the same or different and are hydrogen, a linear or branched C.sub.1-C.sub.6-alkyl group or C.sub.6-20 aryl groups, whereby at least one R.sup.3 per phenyl group and at least one R.sup.4 is not hydrogen, R.sup.5 is a linear or branched C.sub.1-C.sub.6 alkyl group or C.sub.6-20 aryl group, and R.sup.6 is a C(R.sup.8).sub.3 group, R.sup.8 being the same and being a C.sub.1-C.sub.2-alkyl group.

11. The filtration medium according to claim 1, wherein the propylene homopolymer (HPP) is polymerised in the presence of a metallocene catalyst system comprising (i) a metallocene catalyst complex of formula (I): ##STR00017## wherein Mt is H.sub.f or Zr; each X is a sigma-ligand, each R.sup.1 independently are the same or can be different and are a CH.sub.2-R.sup.7 group, with R.sup.7 being H or linear or branched C.sub.1-6-alkyl group, C.sub.3-8 cycloalkyl group, C.sub.6-10 aryl group, each R.sup.2 is independently a —CH═, —CY═, —CH.sub.2—, —CHY— or —CY.sub.2— group, wherein Y is a C.sub.1-10 hydrocarbyl group and where n is 2-6, each R.sup.3 and R.sup.4 are independently the same or different and are hydrogen, a linear or branched C.sub.1-C.sub.6-alkyl group, an OY group or a C.sub.7-20 arylalkyl, C.sub.7-20 alkylaryl group or C.sub.6-20 aryl group, whereby at least one R.sup.3 per phenyl group and at least one R.sup.4 is not hydrogen, and optionally two adjacent R.sup.3 or R.sup.4 groups can be part of a ring including the phenyl carbons to which they are bonded, R.sup.5 is a linear or branched C.sub.1-C.sub.6-alkyl group, C.sub.7-20 arylalkyl, C.sub.7-20 alkylaryl group or C.sub.6-C.sub.20-aryl group, R.sup.6 is a C(R.sup.8).sub.3 group, with R.sup.8 being a linear or branched C.sub.1-C.sub.6 alkyl group, each R is independently a C.sub.1-C.sub.20-hydrocarbyl, C.sub.6-C.sub.20-aryl, C.sub.7-C.sub.20-arylalkyl or C.sub.7-C.sub.20-alkylaryl; and (ii) a cocatalyst comprising a compound of a group 13 metal.

12. The filtration medium according to claim 11, wherein catalyst system comprises as cocatalyst (ii) alumoxane, combinations of alumoxane with Al-alkyls, boron or borate cocatalysts, and combination of alumoxanes with boron-based cocatalysts.

13. The filtration medium according to claim 1, wherein the polypropylene composition is visbroken with any one or more visbreaking agents selected from peroxide, hydroxylamine ester and sulphur compound, or by purely thermal degradation.

14. The filtration medium according to claim 13, wherein the polypropylene composition is visbroken with a hydroxylamine ester selected from the group consisting of sterically hindered amine derivatives of the formula: ##STR00018##

15. A process for preparing a filtration medium comprising the steps of: (i) polymerizing a propylene homopolymer (HPP) in the presence of a metallocene catalyst system, (ii) mixing 90.000 to 99.999 wt % of the propylene homopolymer (HPP) obtained in step (i) together with 0.001-10.000 wt % of a hydroxylamine ester compound for visbreaking, (iii) pelletizing the mixture obtained in step (ii) in a pelletizer, and (iv) melt-blowing the blend pellets obtained in step (iii) and forming into a filtration medium.

16. A method of increasing the quality factor of a filtration medium to at least 0.7 when the weight per unit area of the filtration medium is 9.5±1.0 g/m.sup.2, the method comprising using a polypropylene composition, wherein (i) the polypropylene composition comprises a propylene homopolymer (HPP) polymerized in the presence of a metallocene catalyst, (ii) the polypropylene composition has been visbroken, (iii) the polypropylene composition has a melting temperature (T.sub.m) measured by differential scanning calorimetry (DSC) according to ISO 11357 in the range from 152 to 160° C., and (iv) the polypropylene composition has a molecular weight M.sub.w (measured with GPC on the filtration medium) of between 20000-200000 g/mol, and a molecular weight distribution (MWD) of between 1.5-5.0.

Description

EXAMPLES

[0261] 1. Measuring Methods

[0262] 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.

[0263] MFR.sub.2 (230° C.) is measured according to ISO 1133 (230° C., 2.16 kg load).

[0264] Quantification of Microstructure by NMR Spectroscopy

[0265] 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 optimized 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-d2 (TCE-.sub.d2) 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 rotatory 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 optimized 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.

[0266] 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).

[0267] 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.

[0268] 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 regiodefects when needed. Integral regions were slightly adjusted to increase applicability across the whole range of encountered comonomer contents.

[0269] 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))

[0270] Through the use of this set of sites the corresponding integral equation becomes:

E=0.5(I.sub.H+0.5(I.sub.C+I.sub.D))

[0271] 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.

[0272] The mole percent comonomer incorporation was calculated from the mole fraction:

E[mol %]=100*fE

The weight percent comonomer incorporation was calculated from the mole fraction:

E[wt %]=100*(fE*28.06)/((fE*28.06)+((1−fE)*42.08))

[0273] 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.

[0274] The xylene solubles (XCS, wt.-%): Content of xylene cold solubles (XCS) is determined at 25° C. according ISO 16152; first edition; 2005-07-01

[0275] Number average molecular weight (Me), weight average molecular weight (M.sub.w) and polydispersity (Mal %) are determined by Gel Permeation Chromatography (GPC) according to the following method:

[0276] The weight average molecular weight M.sub.w and the polydispersity (M.sub.w/M.sub.n), wherein M.sub.n is the number average molecular weight and M.sub.w is the weight average molecular weight) is measured by a method based on ISO 16014-1:2003 and ISO 16014-4:2003. A Waters Alliance GPCV 2000 instrument, equipped with refractive index detector and online viscosimeter was used with 3×TSK-gel columns (GMHXL-HT) from TosoHaas and 1,2,4-trichlorobenzene (TCB, stabilized with 200 mg/L 2,6-Di tert.-butyl-4-methyl-phenol) as solvent at 145° C. and at a constant flow rate of 1 mL/min. 216.5 μL of sample solution were injected per analysis. The column set was calibrated using relative calibration with 19 narrow MWD polystyrene (PS) standards in the range of 0.5 kg/mol to 11 500 kg/mol and a set of well characterized broad polypropylene standards. All samples were prepared by dissolving 5-10 mg of polymer in 10 mL (at 160° C.) of stabilized TCB (same as mobile phase) and keeping for 3 hours with continuous shaking prior sampling in into the GPC instrument.

[0277] 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 Q2000 differential scanning calorimetry (DSC) on 5 to 7 mg samples. DSC is run according to ISO 11357/part 3/method C.sub.2 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 and heat of crystallization (H.sub.e) are determined from the cooling step, while melting temperature and heat of fusion (H.sub.f) are determined from the second heating step.

[0278] The glass transition temperature T.sub.g is determined by dynamic mechanical analysis according to ISO 6721-7. The measurements are done in torsion mode on compression molded 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.

[0279] Grammage of the web: The unit weight (grammage) of the webs in g/m.sup.2 was determined in accordance with ISO 536:1995.

[0280] Filtration efficiency: Air filtration efficiency was determined based on EN 1822-3 for flat sheet filter media, using a test filter area of 400 cm.sup.2. The particle retention was tested with a usual aerosol of di-ethyl-hexyl-sebacate (DEHS), calculating efficiency for the fraction with 0.4 μm diameter from a class analysis with 0.1 μm scale. An airflow of 16 m.sup.3.Math.h.sup.−1 was used, corresponding to an airspeed of 0.11 m.Math.s.sup.−1.

[0281] Pressure drop (Δp): The pressure drop was measured according to DIN ISO 9237 at an air speed (permeability) of 500 mm/s.

[0282] Quality factor: The quality factor (QF) is calculated based on the formula:

[00001]$\mathrm{QF}=\frac{-\ln \left(1-\eta \right)}{\Delta p}\times 100$

[0283] in which η is the collection efficiency for the particle size of 0.4 μm and Δp is the measured pressure drop in Pa.

2. Examples

[0284] The catalyst used in the polymerization process for the polypropylene homopolymer (HPP) of the inventive examples (IE1-1 to IE1-5) was produced as follows:

[0285] Preparation of MAO-Silica Support

[0286] A steel reactor equipped with a mechanical stirrer and a filter net was flushed with nitrogen and the reactor temperature was set to 20° C. Next silica grade DM-L-303 from AGC Si-Tech Co, pre-calcined at 600° C. (5.0 kg) was added from a feeding drum followed by careful pressuring and depressurizing with nitrogen using manual valves. Then toluene (22 kg) was added. The mixture was stirred for 15 min. Next 30 wt % solution of MAO in toluene (9.0 kg) from Lanxess was added via feed line on the top of the reactor within 70 min. The reaction mixture was then heated up to 90° C. and stirred at 90° C. for additional two hours. The slurry was allowed to settle and the mother liquor was filtered off. The catalyst was washed twice with toluene (22 kg) at 90° C., following by settling and filtration. The reactor was cooled off to 60° C. and the solid was washed with heptane (22.2 kg). Finally MAO treated SiO.sub.2 was dried at 60° under nitrogen flow for 2 hours and then for 5 hours under vacuum (−0.5 barg) with stirring. MAO treated support was collected as a free-flowing white powder found to contain 12.2% Al by weight.

[0287] Catalyst Synthesis

[0288] 30 wt % MAO in toluene (0.7 kg) was added into a steel nitrogen blanked reactor via a burette at 20° C. Toluene (5.4 kg) was then added under stirring. Metallocene C.sub.2 (93 g) was added from a metal cylinder followed by flushing with 1 kg toluene. The mixture was stirred for 60 minutes at 20° C. Trityl tetrakis(pentafluorophenyl) borate (91 g) was then added from a metal cylinder followed by a flush with 1 kg of toluene. The mixture was stirred for 1 h at room temperature. The resulting solution was added to a stirred cake of MAO-silica support prepared as described above over 1 hour. The cake was allowed to stay for 12 hours, followed by drying under N.sub.2 flow at 60° C. for 2 h and additionally for 5 h under vacuum (−0.5 barg) under stirring.

[0289] Dried catalyst was sampled in the form of pink free flowing powder containing 13.9% Al and 0.11% Zr.

[0290] Polymerization of HPP and Blending with Hydroxylamine Ester

[0291] The polymerization conditions of HPP used as inventive examples in IE1.1 to IE1.5 are indicated in Table 1. The polymerization were carried on a Borstar pilot plant, with prepolymerizer, loop and first gas phase reactor connected sequentially, in the presence of the catalyst described above. The result polymer powder was compounded and pelletized with 1.1 wt % of Irgatec CR76 (BASF), 0.1 wt % of Irganox 10101 and 0.05 wt % of calcium stearate on the ZSK 57 twin screw extruder, with melt temperature of 190° C.

[0292] The properties measured on pellets are shown in Table 1, too.

TABLE-US-00001 TABLE 1 Preparation of the Examples HPP Prepoly reactor Temperature [° C.] 28 Pressure [kPa] 55000 Loop reactor Temperature [° C.] 85 Pressure [kPa] 55000 MFR.sub.2 [g/10 min] 89 XCS [wt.-%] 0.8 Feed H.sub.2/C.sub.3 ratio [mol/kmol] 0.45 Amount [wt.-%] 60 GPR Temperature [° C.] 90 Pressure [kPa] 2500 MFR.sub.2 [g/10 min] 115 H.sub.2/C.sub.3 ratio [mol/kmol] 5.8 Amount [wt.-%] 40 Pellets MFR.sub.2 [g/10 min] 129 XCS [wt.-%] 0.9 T.sub.m [° C.] 155 T.sub.c [° C.] 117 M.sub.w [kg/mol] 121 M.sub.w/M.sub.n [—] 4.0 2.1 [mol %] 0.6 Tg below −20° C. [° C.] n.d. Tg above −20° C. [° C.] −0.1

[0293] The polymer used in comparison examples CE1.1-1.3 is a ZN based polypropylene homopolymer, which has been disclosed in EP3034522 as main polymer in IE1.1 and IE1.2. Said propylene homopolymer has been visbroken by using a co-rotating twin-screw extruder at 200-230° C. and using 1.1 wt % of Irgatec® CR76.

[0294] The polypropylene compositions of IE1 and CE1 have been converted into filtration media (filter) 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 different melt-blow temperatures and air volumes, with the same DCD (die to collector distance) of 200 mm and the same throughput of 10 kg/h/m.

TABLE-US-00002 TABLE 2 Processing conditions for the production of filters and properties of the obtained filters Melt-blow Air Web Fractional Pressure MFR Melt temperature temperature volume weight efficiency drop Quality (filter) M.sub.W MWD T.sub.m (filter) [° C.] [m.sup.3/h] [g/m.sup.2] [%] [Pa] factor [g/10 min] (filter) (filter) [° C.] CE1-1 270 360 9.4 25.08 57.4 0.504 860 62,300 3.94 162 CE1-2 290 210 9.4 48.67 122.1 0.547 1806 50,400 3.5 162 CE1-3 290 260 4.9 21.67 47.7 0.516 n.m. n.m. n.m. 162 IE1-1 270 230 9.9 20.75 26.3 0.886 1429 57,400 2.6 154 IE1-2 280 190 9.5 46.7 46.7 1.356 2222 50,700 2.5 154 IE1-3 290 90 9.2 55.78 52.8 1.547 2888 46,050 2.4 154 IE1-4 290 150 4.6 42.98 29.8 1.89 2934 46,500 2.4 154 IE1-5 298 70 8.9 64.82 60 1.744 3280 44,200 2.3 154

[0295] As can be seen from Table 2, the use of the polymer composition of the inventive examples yields filters with improved fractional efficiencies compared to the comparative examples. The benefit is shown more pronounced in the webs with lower thickness (i.e. lower web weights). With a similar web weight, 1E1-4 gives about 100% higher fractional efficiency than CE1-3.