Melt-blown fibrous electrets

© Start of patent duration: 1935 08 15 Longest possible duration:

© Issued: 1986 04 10 © inventor:

© dependence:

Ö 1 ÖÖU ± Ό © pamphlets considered to delineate the state of the art:

DE-OS 2232264 DE-OS 2345352 US-PS 3632443 US-PS 2740184

MAGAZINE TECHNICAL ASSOCIATION OF THE PULP AND

PAPER INDUSTRY (TAPPI) VOLUME 56 / N.4 / APRIL 1973 /

P.74-77 ROBERT R. 3URTIN-DWIGT D. LOHKAMP MELT

BLOWING-ONE-STEP WEB PROCESS FOR NEW NONWOVEN PRODUCTS

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The invention relates to an electret in the form of fibrous sheet material having a permanent electric charge-carrying fibers, which are produced in a simple manner and economically in a substantially one-step process and the fibers with microscopic

May have diameters; This uniquely enhances the properties of a

Electrets combined with those of microfibers.

In a known process for producing fibrous electrets (US Pat. No. 2,740,184), thermoplastic threads, fibers, fabrics or films are placed in an electrostatic field which is built up between parallel, closely adjacent electrodes. The fibrous material is heated to soften and then cooled in the presence of a field to form more or less permanent charges in the fibers.

U.S. Patent No. 3,571,679 describes the disadvantages of this method which is that it is difficult to form a sufficiently high, permanent charge in the treated fibrous web material because the application of high voltages to the charging electrodes causes an arc to pass through caused open pores of the fibrous web material. Therefore, in this US patent, the charging electrodes are covered with a poorly conductive foil to distribute the high applied voltage and to dampen a possible dielectric breakdown by the fibrous sheet material.

This covered electrode method is considered disadvantageous in U.S. Patent No. 3,998,916 because too long a period of time is required to charge the fibrous material to the desired state of charge. To avoid this disadvantage, US Pat. No. 3,998,916 uses a somewhat more elaborate or two-stage process. In this case, a film is first prepared and electrically charged, and then the film is fanned by transfer via needled rolls and arranged to form the fibrous web material in layers.

This use of films to make fibers is part of a historical development in the art which has gone from the production of fairly thick wax electrets to thinner films using polymers and techniques in which the process is controlled in several ways, for example, by controlling the temperature of the film during charging, by controlling the distance between the loader and the film and by controlling the loading period. According to US-PS No. 3,998,916 (see also the paper The Use of Polymers for Electrets J. Van Turnhout, Journal of Electrostatics, Vol. 1 (1975), pages 147-163), the film is charged by heating it to near its melting point, then stretching it over a curved plate and spraying positive or negative charges from a plurality of thin wires arranged above the curved plate , According to US Pat. No. 3,644,605, a thin polymer film is supported on a correspondingly extended dielectric plate and irradiated with electrons. Further, it is known (NASA Technical Report R-457 - December 1975) to conduct a sprayed or atomized liquid dielectric by corona discharge from a comb electrode or from a network of closely spaced wires and then collect on a dielectric film where the droplets in Cure the form of a film.

Although the production of fibrous web material by the interim forming of a film takes advantage of the known film loading techniques, this method of manufacture is time consuming and expensive. Furthermore, only limited fiber sizes can be obtained with this technique.

The disadvantages shown are eliminated by an electret of the type described above, which is inventively characterized in that the fibers are made by a melt-blown, have a mean diameter of less than 10 gm, of a hydrophobic material having a resistivity of 10¹ n.cm and carrying a charge with a half-life of at least one week at room temperature and 100% relative humidity of the environment.

In the meltblowing process, molten fiber-forming material is extruded through a plurality of nozzles into a high velocity gas stream where the extruded material is attenuated to form a fiber strand. The electret forming the electret according to the invention

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- 3 are irradiated on exiting the nozzles with electrically charged particles, such as electrons or ions, wherein they are at a remote location from the nozzle, where they are dimensionally stable solidified by cooling and where they carry a permanent electrical charge, are recorded. The recorded web or mat can be used directly or preferably to the desired

Size should be cut off.

These process conditions are in sharp contrast to the controlled conditions in known processes for making film electrets: the fibers move at extremely high speeds, are turbulently blown by a high velocity gas stream, and are crosslinked and dispersed in a large volume of diluting high velocity air. In this case, the electrically charged particles penetrate into the fiber strand and are sufficiently retained in the fibers produced by the melt-blowing process. Penetration of the particles into the fiber strand is necessarily done for a small fraction of a second (less than 1 ms) when the fibers are both near the source of the electrically charged particles and in a molten or nearly molten state. After this injection of the electrically charged particles, the fibers solidify extremely rapidly, and thereby the electrical charges in the fibers are frozen, so that the fibers received have a permanent electrical charge.

The permanent charge in the fibrous web material of the present invention differs from the temporary charge applied to other known fibrous products, often to aid manufacture, examples of which are the improvement of the coating of fibers with an oppositely charged liquid (US Pat. PS No. 2,491,889) or the improvement in the distribution and separation of the fibers and in the retraction of the fibers to a collector, to obtain a more uniform, fibrous mat (U.S. Patent Nos. 2,466,906, 2,810,426, 3,824,052, 3,003,304, and 3,490,115 and 3,356,156 for fibrillated filaments).

The charges applied in these manufacturing processes are only temporary. For example, it is possible that the fiber-forming material does not have sufficient resistivity or that too large an amount of conductive solvent is present in the formed fibers, so that a permanent charge can not be maintained. Furthermore, the charge can be applied only after the formation of the fibers, so that only a surface charge is present. In addition, the charging conditions, such as the applied voltage, may be insufficient for the formation of a permanent charge. It is also possible that the charge is neutralized after picking up the fibers. If, after the manufacture of the prior art fibrous mats, any residue remains on such a temporary load, it rapidly disappears during storage or use.

In contrast, the fibrous web material according to the invention has a permanent or permanent charge. When the fibrous sheet material of the present invention is stored under ordinary conditions, it can maintain an effective charge for many years. Under speed¹ For example, when stored at room temperature and at 100% relative humidity of the environment, the charge on the sheet material of the present invention generally has a half-life of at least one week and preferably six months or one year. With such a durability of the charge, the fibers according to the invention as well as the fibrous web material according to the invention can be appropriately called electrets; for the sake of further explanation, these are referred to herein as fiber electrets, electrets in the form of fibrous sheet material, or more generally as fibrous electrets.

For many electret in the form of fibrous web material according to the invention, the surface tension in the web material can be measured as a good measure of the size of the charge with an electrostatic voltmeter with the same probes. However, such a measurement is less accurate if the web material comprises a mixture of oppositely charged fibers.

However, such mixed charge web material is still advantageous, for example, to increase the filter properties, but the net charge measured on the web material is not representative of the total charge amount. In electrets made of fibrous web material according to the invention

- 4 No.380126 with permanent charges of only one polarity, the charge will generally be at least 10 "^8th C / g of the melt-blown fibers. For electrets in the form of fibrous web material with both positively and negatively charged fibers, the net charge will usually be at least 10 ".⁹ C / g of the melt-blown fibers. Further, the electric charge can be detected by other experiments, for example, by applying toner powder to the web material, but not necessarily numerically quantified measurement results are obtained.

As can be seen in the article Melt Blowing-A One-Step Web Process for New Nonwoven Products from Journal of the Technical Association of the Pulp and Paper Industry, Vol. 6, No. 4, April 1973, meltblowing is recognized special technique, whereby nonwovens produced by this technique are easily and safely distinguishable from other nonwovens. Meltblown fibers are long and tangled, forming a coherent mass from which it is impossible to separate out a whole single fiber.

According to a preferred embodiment of the invention, the fibers may contain polypropylene. Polypropylene-containing fibers are known per se in electrets, for example from DE-OS 2232264 and 2345352 and US Pat. No. 3,632,443, but not in connection with an electret of the genus of the invention.

According to a further feature of the invention crimped staple fibers can be provided, which are interspersed with the fibers produced by the melt-blowing process. These crimped staple fibers reduce the pressure drop across the fibrous electret.

The melt-blown charged fibers can be adjusted to a desired fiber diameter. For many purposes, microfiber sizes are required (ie, one that is best to be viewed under a microscope), and in some applications, the diameter should be particularly small. For example, the microfibers may not only have a mean diameter of less than 10 gm but even less than 1 gm.

It is known that microfibers can lead to several advantageous properties, including the improvement of certain filtration properties; Furthermore, the combination of microfibers with permanent electrical charge to the electret according to the invention in the form of fibrous web material, which has unique filtration properties. A significant application of electrets according to the invention in the form of fibrous sheet material relates to respiratory protective devices, in particular cup-shaped face masks according to FIG. The use of the inventive electrets in the form of fibrous web material instead of the known web material of microfibers made by a melt blown process may improve the filter efficiency by a factor of 2 or more. The masks according to the invention according to Figure 3 can be manufactured cheaply, and their low cost and high efficiency leads to a wide range of applications, the known face masks are not accessible.

The invention will be explained in more detail below with reference to the drawings. Show it: Fig.l is a schematic view of an apparatus for producing an electret according to the invention in the form of fibrous web material, Figure 2 is a plan view taken along lines 2-2 in Fig.l with a schematic circuit diagram for a source of electrically charged particles of the apparatus of FIG. 3 is a perspective view showing the use of a face mask with the electret according to the invention in the form of fibrous web material; 5 shows a schematic representation of a test arrangement for testing the filter properties of the electret according to the invention in the form of fibrous web material, and FIG. 6 shows a graph of the particle penetration (ordinate) as a function of the FIG of the particle size (abscissa) in electrets according to the invention in the form of fibrous web material and for comparison in uncharged web material.

In Fig.l and 2, an apparatus -10- for the production of inventive

Electrets in the form of fibrous web material shown. Part of this device may be a conventional, melt-blown device; see. for example Report No. 4364 of the US Naval Research Laboratories of May 25, 1954 (VA Wente, CD Boone and

EL Fluharty: Manufacture of Super Fine Organic Fibers). Such a fiber blowing device

No. 380126 has a nozzle -11- with a series of narrow, juxtaposed nozzle orifices -12- for extruding the molten material and, on each side of the nozzle orifice row, slots -13- through which a gas, preferably air, is blown at high speed. The gas stream fibrously extrudes the extruded material, cools the fibers for solidification, and transports the fibers to a receiver -14- in the form of a fiber strand. The receptacle -14- shown in Fig.l has a provided with fine holes, drum or cylinder-shaped screen, but the transducer may also be designed differently, for example in the form of a flat screen or mesh or in the form of an endless belt, the order Rolling is guided around. To assist the deposition of the fibers and the removal of the gas, a gas suction device can be arranged behind the screen. The strand of blown fibers is applied to the receiver 14 as an irregularly crosslinked, coherent mass in the form of a handleable mat which can be removed from the receiver and wound onto a supply roll.

In order to irradiate the fibers produced by the melt-blowing process with electrically charged particles, one or more sources of such particles are arranged in the vicinity of the nozzle openings -12-. In the apparatus according to the Fig.l and 2, two sources -18 and 19- are used, u.zw. one source on each side of the fiber strand -15-. Each source has an electrical conductor -20 or 21-, which is connected to a high voltage source -22- and disposed within a metal jacket -23 or 24-, which is earthed via a resistor -25-. According to Figure 2, the conductors in insulators -26 and 27- can be attached. When a sufficiently high voltage (usually 15 kV or more) is applied to conductor 20 or 21, a corona is formed around it and the air or other gas around the conductor is ionized. The electrically charged ions or particles are driven into the fiber strand -15- by a combination of aerodynamic and electrostatic forces acting on the charged particles. The movement of the charged particles may be assisted by a fan or by a tension on the sheaths -23 or 24- which repels the particles. Instead of a cylindrical shell or a tube -23 or 24- can be on each side of the conductor -20 or 21- plane metal plates or any other suitable arrangement that causes a desired voltage gradient between the electrode and the surrounding shield. As sources of the electrically charged particles, electron beams and radiation sources such as X-ray apparatuses are suitable.

The sources -18 and 19- for the electrically charged particles are placed near the mouth of the nozzle -11-, where the fibers are in the molten or nearly molten state. Under these conditions, the mobility of the free carriers within the fibers is high, thereby facilitating the introduction of the charge into the fibers. The closer the source of the electrically charged particles is to the nozzle orifice, the more the fibers melt and the easier it is to transfer the charge.

As the fibers solidify and cool, the charges in the fibers freeze and the fibers become permanently charged (heating the fibers results in loss of charge). According to the usual terminology of electrets, this is a so-called single charge, which has the same polarity as the voltage applied to the conductors. Either a positive or a negative voltage may be applied to the source of the electrically charged particles and the sources of oppositely charged particles may be used on opposite sides of the fiber strand simultaneously.

A static charge on the surface of the fibers, which may be opposite to the polarity of the radiation, may also form during the manufacture of the sheet material of the present invention. Such a charge is degraded quickly, u.zw. in a manner similar to the removal of a static charge applied to a finished, fibrous sheet material.

The temperature of the gas in the region of the fibers tends to decrease rapidly with increasing distance from the nozzle opening. For example, under the conditions of Example 1, the temperature of the air at the nozzle orifice is about 290 ° C and about 190, 150, 120 ° C and

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- 6 about 95 ° C in 1.25, 2.5, 3.75 and 5 cm distance from the nozzle. Therefore, the charges radiated into the molten or nearly molten fibers in the vicinity of the nozzle orifice are rapidly frozen in the fibers.

Several polymers whose dielectric properties allow the electrically charged particles to remain in the fiber without the charge flowing off can be used to make the melt blown fibers for web material of the present invention. Preferred is polypropylene which has a resistivity of about 10¹⁶ ii.cm. Other polymers, such as polycarbonates and polyhalocarbonates, which may be melt-blown and have suitable resistivities at the expected environmental conditions may also be used. In general, advantageous polymers have a resistivity of at least 10 -9.9 cm and prevent absorption of moisture levels which can prevent the desired charge half-life. Dyes, fillers and other additives can be mixed with the polymer if it does not lose the required properties, such as resistivity.

The diameter of the fibers produced varies with parameters such as the size of the nozzle orifice, the viscosity of the polymer and the velocity of the air stream. Microfibers prepared by the meltblown process are generally considered to be discontinuous, although their length to diameter ratios should approach infinity to produce suitable web material. It is assumed that the fiber lengths are up to 10 cm or more.

The process of making the fibers may be modified to incorporate other fibers or particles into the web material. For example, US Pat. No. 3,971,373 describes an apparatus and method for introducing particulates into web material from melt blown fibers. In particular, for filtering and cleaning various types of particles are suitable, such as activated carbon, alumina, sodium bicarbonate and silver, which are used to remove a component of a substance by adsorption, chemical reaction or amalgamation, as well as certain catalysts, such as hopkalite, which facilitate the conversion of catalyze dangerous gas into a harmless form. The particle size can vary, u.zw. at least 5 pm to 5 mm mean diameter. For respirators, the mean diameter of the particles is generally less than 1 mm.

Further, preformed fibers may be added during formation of the web made from the melt-blown webs; see. U.S. Patent Nos. 3,016,599 and 4,118,531. For example, staple fibers, including crimped staple fibers, can be added to a strand of melt blown fibers (using crimped staple fibers by receiving the crimped fibers from the web material by means of a rough roll) to produce looser or more porous web material with reduced pressure drop but good filter properties.

Within the scope of the invention, further measures or changes in the basic meltblowing process are possible. For example, the fibers made by the meltblown process may be collected in a pattern of densified areas and low density areas (U.S. Patent No. 4,042,740). Further, the gathered web material may be further processed from melt-blown fibers, for example, by chopping to form suitable fibers for use in other products, by densifying in a pattern (U.S. Patent No. 2,464,301), by spraying or by others suitable addition of additives to the web material, by laminating the web material to other webs or films or by deforming or cutting the web material.

FIGS. 3 and 4 show a preferred embodiment of the face mask according to the invention, in which electrets according to the invention can be used in the form of fibrous web material. The mask -28- has a substantially cup-shaped portion -29- to be placed over the mouth and nose of the user, and a band -30- for holding the mask. The edge of the mask should be adapted as closely as possible to the face shape and thus defines the air inlet for the user of the mask; ie most of the air inhaled by the mask user will flow through the mask. The cup-shaped part can be an inner fiber fleece

No.380126 airy fibers -31-, two layers -32 and 33- of an electret according to the invention in the form of fibrous web material and an outer nonwoven fabric -34- arranged from airy

Have fibers.

The invention is explained in more detail below by the examples. In order to study the filtration efficiency of the prepared sheet materials, two different tests are used in the examples; one test uses dioctyl phthalate droplets (DOP test) and the other test uses silica dust (National Institute for Occupational Safety and Health (NIOSH) test; these tests are described in detail in the Federal Register of the USA under the title 30 in Part 11.

Examples 1 to 8: Blown microfibers are made of polypropylene resin (Hercules Profax

6330) with a device according to Fig.l manufactured. In Examples 1, 2, 4 to 6 and 8, the following conditions are met: The width of the nozzle is 50 cm, and the temperatures of the melt in the nozzle, the nozzle itself and the air discharged from the nozzle are 346, 370 or 400 ° C. The air pressure at the nozzle is 0.43 bar and the extrusion speed of the polypropylene is 6.8 kg / h. The distance between the mouth of the nozzle and the receiver is 60 cm, the distance between the mouth and the ladders is 3 cm, and the distance between the center line -37- of the fiber strand and the conductors -20 and 21 - is 2.5 cm. 15 kV voltage is applied to each conductor -20 and 21 and 3 kV voltage to sheaths -23 and 24-. In Examples 3 and 7, substantially the same conditions are met, except that the melting temperature is 360 ° C, the air temperature 370 ° C and the air pressure 0.5 bar. Sheets of varying thickness and weight are made as shown in Table I. In most examples, a positively charged trajectory (denoted by a + in the table below and prepared by applying a positive voltage to the two electrodes -20 and 21- in Fig. 1), a negatively charged trajectory (-), and a non charged web (C) for comparison purposes. The pressure drop (ΔΡ) as well as the particle penetration (% P) measured by the DOP test are listed in Table I.

Table I

<td>example No.</td><td>basis weight g / m²</td><td>ΔΡ 10- "bar</td><td>% p</td>

<td>1 +</td><td>0.29</td><td>0.6</td><td>61</td>

<td>1 -</td><td>0.30</td><td>0.6</td><td>60</td>

<td>1 c</td><td>0.32</td><td>0.8</td><td>83</td>

<td>2 +</td><td>0.25</td><td>0.9</td><td>51</td>

<td>2 -</td><td>0.25</td><td>0.7</td><td>65</td>

<td>2C</td><td>0.25</td><td>0.7</td><td>80</td>

<td>3 +</td><td>0.26</td><td>0.9</td><td>54</td>

<td>3 -</td><td>0.26</td><td>0.9</td><td>58</td>

<td>3 C</td><td>0.28</td><td>1.0</td><td>78</td>

<td>4 +</td><td>0.33</td><td>1.1</td><td>44</td>

<td>4 -</td><td>0.33</td><td>1.2</td><td>53</td>

<td>4C</td><td>0.32</td><td>1.1</td><td>70</td>

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Table I

<td>example No.</td><td>basis weight g / m²</td><td>ΔP 10 "⁴ bar</td><td>% P</td>

<td>5 -</td><td>0.45</td><td>0.8</td><td>61</td>

<td>5 C</td><td>0.45</td><td>0.7</td><td>81</td>

<td>6 +</td><td>0.52</td><td>1.1</td><td>46</td>

<td>6 -</td><td>0.52</td><td>1.3</td><td>52</td>

<td>6 C</td><td>0.52</td><td>1.3</td><td>73</td>

<td>7 +</td><td>0.52</td><td>1.1</td><td>44</td>

<td>7 -</td><td>0.52</td><td>1.2</td><td>53</td>

<td>7 C</td><td>0.52</td><td>1.1</td><td>70</td>

<td>8th -</td><td>0.65</td><td>2.1</td><td>32</td>

<td>8 C</td><td>0.65</td><td>2.1</td><td>55</td>

Examples 9 to 12: Masks according to FIGS. 3 and 4 are produced from the web material of Examples 1, 1+, 2+ and 3+. The results of the NIOSH silica dust test are listed in Table II.

Table II

<td>example No.</td><td>initial inhalation IO^-4 bar</td><td>inhalation at the end IO "⁴ bar</td><td>evaporation at the end 10^-4 bar</td><td>particle -Penetration mg</td>

<td>9</td><td>7.9</td><td>13.9</td><td>9.3</td><td>1.39</td>

<td>10</td><td>8.1</td><td>14.7</td><td>10.0</td><td>, 66</td>

<td>11</td><td>11.6</td><td>16.6</td><td>16.4</td><td>19</td>

<td>12</td><td>12.0</td><td>17.8</td><td>13.8</td><td>, 23</td>

Charge decay tests

The drop in charge on the electret according to Example 6+ over a period of time is examined by storing samples of the web material in polyethylene containers under normal room conditions. Charge decay is determined by measuring the surface tension with a Monroe electrostatic voltmeter with the same probes and taking into account the relationship between the charge and the surface tension (Q = CV, where Q = charge, C = capacitance and V = surface tension) to calculate effective surface charge density. Table III shows the dependencies between the initial surface charge and the surface charge measured at various time intervals.

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- 9 Table III

<td rowspan="2">example No.</td><td colspan="3">Proportion of remaining surface tension after several days of storage</td>

<td>100 d</td><td>200 d</td><td>325 d</td>

<td>6 +</td><td>0.96</td><td>0.94</td><td>0.94</td>

In addition, measurements of the charge drop in samples of the web material according to Examples 6 + and 6 C are performed, u.zw. after storage in a desiccator at 20 ° C and 100% relative humidity. The samples are placed in the desiccator 120 days after their preparation. The proportion of surface charge remaining after various periods of time is listed in Table IV.

Table IV

<td rowspan="2">example No.</td><td colspan="5">Proportion of remaining surface tension after several days of storage</td>

<td>5 d</td><td>10 d</td><td>25 d</td><td>100 d</td><td>180 d</td>

<td>6 +</td><td></td><td></td><td>0.99</td><td>0.98</td><td>0.97</td>

<td>6 C</td><td>0.35</td><td>0.15</td><td>0.1</td><td></td><td></td>

In addition to the surface charge decay studies, the change in particle penetration through web material of Example 6 + is measured after various storage periods (in an environment of 100% relative humidity); the results are listed in Table V. The measurements are carried out by means of a device -39- according to FIG. The air entering a 7.6 cm diameter aerosol delivery tube is passed through an absolute filter 41 to ensure that the particle concentration residue is minimized. The marker aerosol is injected downstream of the absolute filter through inlet 42 and passed through section 43, where the aerosol may optionally be neutralized using a krypton 85 radiation source. As the marking aerosol, fuming silica dust according to the NIOSH silica dust test is used.

The delivery of the aerosol source is measured by means of an aerosol photometer -44- attached to the production tubing. The aerosol photometer uses a photodiode -45- to measure the weight of the particles scattered forward by the particles passing through the particle beam from a helium-neon laser. The scattered amount of light is a measure of the aerosol concentration when the size distribution of the aerosol population is constant over the measurement period. An aerosol sample is withdrawn from the main aerosol stream through line -47- and passed through the test filtration medium -48-. By suitable valve control, the size and concentration of the marking particles of 0.15 to 3 gm in the flow direction in front of and behind the filtration medium are measured, u.zw. using a particle measuring system (ASAS-200 aerosol spectrometer) connected to line -49-. There are continuous measurements of the pressure drop across the filter (by a pressure gauge -50-), the

W dew point temperature in line -51- and air temperature performed. The with this

Data obtained from the test device allow a description of the filter penetration as a function of

Particle size in place of a reference to the mass.

Typical results of the penetration in the web material according to Examples 3 + (Quadra10

No.380126 te), 6 + (circles) and 6 C (full points) with the device according to Figure 5 are shown in Fig.6. Maximum particle penetration occurs at particle sizes of 0.3 to 0.6 μα, where neither diffusion nor internal deposition is very efficient. However, as shown, the electrets of the invention in the form of fibrous web material lead to improvements in all

Particle sizes.

As stated above, Table V shows the penetration results with the apparatus of Fig. 5 after the test web material has each been exposed to 100% relative humidity for different periods of time. The results listed in Table V are cumulative particle penetrations, u.zw. measured for particle sizes smaller than a given diameter (0,3,

1 and 3 gm); ie the result given in the column for the particle size 3 gm is the percentage of the particles up to 3 gm which has passed through the investigated web material; the result given in the column for the particle size 1 gm is the percentage of the particles that have passed through, up to 1 gm particles; The same applies to the column with particle sizes up to 0.3 gm

Table V

<td rowspan="2">Storage time in Days at 100% relative humidity</td><td colspan="3">cumulative mass penetration in% at different particle sizes</td>

<td>0.3 gm</td><td>1 gm</td><td>3 gm</td>

<td>0</td><td>0,012</td><td>0.24</td><td>2.4</td>

<td>1</td><td>0.019</td><td>0.30</td><td>3.3</td>

<td>7</td><td>0,008</td><td>0.34</td><td>3.0</td>

<td>30</td><td>0.009</td><td>0.24</td><td>1.7</td>

<td>180</td><td>0,008</td><td>0.29</td><td>2.6</td>