CZ310499A3 - Porous composite and use thereof - Google Patents

Abstract

The porous corpuscity with the input / output surface, which includes interspan-fiber-fiber fibers of astaple fibers. The composite has increased filtration efficiency where each other propletenámeltblown fibers define the density gradient from of coarse pores at the entry surface of the soft pores output surface. The method for producing a porous composite rests in that the melt of fibers is entrained in the hot air and thereafter staple fibers are injected. Porous Composite is suitable for fluid filtration.

Description

Porous composite and its use

Technical field

The invention relates to novel meltblown fibers (MB fibers) in which staple fibers such as polypropylene, polyethylene, polyesters, nylons, cotton, wool, glass fibers and / or particulate materials, in a manner controlled to achieve the desired density gradient across the obtained MB composite web. The invention also relates to said composites, which are preferably cold-electrostatically charged, to a method of such charging, to a method of using the composite, and to a method of making the composite.

BACKGROUND OF THE INVENTION

MB fiber nonwovens are used for air filters because their ultrafine fibers provide a large surface area. However, their high bulk density results in high air resistance and only surface filtration is available for most particle sizes. This considerably limits their use to disposable respirators, surgical masks and disposable surgical clothing.

The present invention contributes to solving the disadvantages of the prior art.

SUMMARY OF THE INVENTION

SUMMARY OF THE INVENTION The present invention provides highly porous meltblown (MB) fiber composites with controllable density across the filter thickness obtained by depositing staple fibers in a stream of molten fibers. These composites have a volume and a low pressure drop and therefore have an increased ability to maintain particles and a longer life without reducing the efficiency t

filtration. The efficiency of filtration can be further enhanced by the electrostatic charge supplied to the composite.

The invention relates to composite webs containing meltblown (MB fibers) intertwined with staple fibers which increase the volume and decrease the density.

to ·

The composite exhibits a density gradient between the two surfaces so that the density is highest near one side of the composite and lowest near the other side of the composite.

A key feature of the invention is that the concentration of injected fibers (mixing ratio of injected fibers to MB fibers) can be controlled to vary across the thickness of the composite web as needed. In general, it is more desirable if the filter has different degrees of filtration across the thickness of the filter or filter assembly. For example, the depth filter may be coarser on the open side where the aerosol (or filtered fluid) comes, so that larger particles get trapped in larger pores and openings of the web, with more space to catch them without significantly restricting air (or liquid) flow. filter, thus increasing the pressure drop. The aerosol containing the finer particles then travels through the denser side of the filter, where there are more very fine MB fibers to trap the particles on the larger fiber surface. Since most of the larger particles were trapped on the less dense side of the filter, the pores on the dense side would clog the particles over a longer period of time. Thus, a filter with a gradient running from coarser (less dense) to finer (denser) can filter an aerosol containing a mixture of coarse and fine particles while filtering very fine particles with a very high degree of efficiency, accompanied by low pressure drop and high filter life. The aerosol may be any type of gas and the liquid may be water or other liquid.

A density gradient filter running from coarser (less dense) to finer (denser) can further filter the aerosol from mostly very fine particles with excellent filtration efficiency and longer lifetime, as there is more space on the more porous side to trap particles that would otherwise tend to clog the side of the filter where the aerosol is delivered.

The MB process and the advantages of using MB fibers for filters are described by Hassenboehler (1994). The filtration efficiency (FE) of control and MB corona discharge electrostatically charged webs has been described by Tsai and Wadsworth (1994a). High FE MB fibers are caused by their ultrafine fibers. MB fibers in MB filter-grade nonwovens range from 0.5 to 10 µm, with an average fiber diameter value typically in the range of 2 to 4 µm. Although patents and publications on the meltblown process dating back more than 10 years indicate that MB fibers are cracking and only few cm in length, recent studies have shown that MB fibers are predominantly continuous (Milligan and Utsman, 1995x: Milligan, MW and Utsman, F., "An Investigation of the Meltblown Web Defect Known as Shot," International Nonwovens Research Journal 7, No. 2, 65-68, 1995; Milligan et al., 1992y: Milligan , MW, Lu, F., Buntin, RR, and Wadsworth, LC, “The Use of Crossflow to Improve Nonwoven Melt99 99 • 9 9 9

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Blown Fibers, Journal of Applied Polymer Science 44, 279-288 (1992). In any case, however, it is virtually impossible to detect the actual length (if the filament has broken) of the webs of MB fibers due to the intense entanglement of the fibers inherent in the MB process.

However, the very fine fibers also contribute to the high air resistance, which is recorded as a high pressure drop across the web if the bulk density of the web is not reduced in some way. The pressure drop in the MB web can be reduced by increasing the distance (DCD) between the nozzle and the collector or by increasing the fiber size by changing the process conditions. It has been demonstrated (Wadsworth, 1990) that MB media decreases FE as the DCD increases to produce a larger fleece. This results in a less uniform distribution of MB fibers on the collector and also allows multiple filaments to twist together. This bulkier, less regular fleece has larger pores and as a result a lower pressure drop but a lower FE.

Since increasing DCD or fiber size is not a good method of reducing air resistance within a web, the present invention allows reducing pressure drop and increasing the ability to trap particles without adversely affecting FE in MB nonwovens by mixing staple fibers such as polypropylene (PP) into the MB fiber stream. ), polyethylene (PE), polyesters, nylons, cotton, wool, glass fibers and / or particulate materials including activated carbon, superabsorbent powders and fibers and crushed textile materials. The diameter of the fibers whose cross-section varies from flat (cotton fiber is in the form of a flat twisted ribbon) to circular (in the case of synthetic fibers, almost any type of cross-sectional shape can be achieved by varying the spinner shape) generally ranges from 12 to 60. 12.7 to 76.2 mm (0.5 to 3.0 "; synthetic staple fibers are usually chopped from continuous filaments to a length of 25.4 to 50.8 mm, i.e. 1 to 2"). The wool fibers have very different lengths and are rather considerably longer, up to 50.8 to 152.4 mm (2 to 6 inches). For shredded textile, the thickness, length and width vary according to the shredding conditions, but the largest particle size of the shredded textile may be in the range of 1.59 to 12.7 mm (1/16 to 1/2 "). The activated carbon and superabsorbent particles may have a diameter of 1 to 300 µm.

MB fibers can be any meltblown processable fibers, usually synthetic. They may be mono-component or bi-component fibers of polyolefins, polypropylene (PP), polyethylene (PE), polyamides (nylon 6, nylon 6,6, etc.), polyesters, polyethylene terephthalate (PET), polycyclohexane terephthalate (PCT), polybutylene terephthalate (PBT) , polytrimethylene terephthalate (PTT) or any MB fibers or mixtures of MB fibers that can be processed by meltblown. For the composites according to the invention, which are intended to be: " ", ", " · 99 99 99 electrostatically charged to increase FE, shown by Wadsworth and Tsai (1997x: Wadsworth, LC. And Tsai, PP, 1997, "Recent advances and applications for Electrostatically Charged Filters", Proceedings of the Second International Conference on Nonwovens in Filtration) , Stuttgart, 80-85, 18-19 March) that PP, PE and PCT polyesters can be charged more easily for higher FE and much longer FE life after corona charging. However, any type of MB fiber that can be electrostatically charged is suitable. It has been shown that, if MB fibers can be electrostatically charged, injected fibers are not electrostatically charged to obtain good FE values (Wadsworth and Tsai, 1997x).

The nature of the MB fiber material is not essential to the extent that any material capable of forming fibers by the meltblown method is suitable for practicing the invention. Similarly, the nature of the staple fibers is not critical, and any material capable of thoroughly interweaving with the MB fiber defining a density gradient in the composite obtained is suitable.

In the following, the term "staple fibers" includes, unless specific staple fibers or textile materials such as PP staple fibers, all the aforementioned staple fibers and textile materials are mentioned. Since PP is the most widespread polymer for the meltblown method, the term MB here in the description of MB implies PP, although other types of MB fibers described herein may also be used.

The following should be taken into account:

The FE (η) of a fibrous web from a single fiber efficiency theory is expressed by the equation (Liu,

1986) η = le ^ (1) where S / is the filter area factor η ₅ is the single fiber efficiency a

S / = D / Z / (2) where Z / is the specific length or length of fiber per unit of filter area D / = fiber diameter.

The filter area factor is increased by

S * - Obh

9 ·· · 9 9 ······ · · · 9 9

999 999 999 999 99 99 where h = specific length of deposited fibers Dz, = size of deposited fibers.

The total filtration efficiency with the addition of staple fibers is then η - (4) where η o ~ the efficiency of a single fiber after mixing the fibers.

The overall efficiency of a single fiber (ηo) after admixing of fibers decreases due to a decrease in bulk density and the existence of coarse staple fibers. The total filtration efficiency may increase or decrease depending on the magnitude of the decrease in single fiber efficiency and the magnitude of the increase in the filter area factor.

According to Davies (1973), when the air flow through the web is a laminar, the pressure drop in the web is directly proportional to the bulk density function of the web and inversely proportional to the square of the average fiber diameter, i.e., the fiber diameter.

4? = tvuftti) d / (5) where Δρ = pressure drop across the web t = fleece thickness v = inlet velocity μ = air viscosity d / = average fiber diameter a = bulk density

For a bulk density in the range between 0.006 and 0.3, the bulk density function was experimentally determined as

Λα) = 64α ^{1 5 5} (1 + 56 ^{3 3} ) (6)

This means that the bulk density function decreases faster than the bulk density decreases. When mixing MB fibers with coarse staple fibers together with an increase in the average fiber diameter, the pressure drop decreases.

At the same other parameters, the pressure drop inside the web is directly proportional to the increase in web thickness. This thickness is then proportional to the filter area factor. Therefore, the filtration efficiency of the web can be expressed by the pressure drop across the web, ie.

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999 999 99 99 η = l-e ^ ”(7) where ψ-is referred to as filter quality and is a function of fleece and aerosol properties (Hinds, 1982).

The service life is defined as FE and the pressure drop should not exceed predetermined values. In some aerosols, such as sodium chloride particles, FE increases with particulate loading, so the filter lifetime is determined by the pressure drop. The pressure drop in the fleece increases due to particulate loading. The particulate capacity of the filter is the maximum amount of dust that the filter can retain before the pressure drop exceeds a predetermined value. It is important to relate lifetime to both pressure drop and load capacity. After obtaining the curve of pressure drop and particle load, the service life of the filter can be calculated, ie.

τ = M riCQ (8) where τ = lifetime (h)

M = load capacity (g / m ² ) η = filtration efficiency (% 100)

C = aerosol concentration (g / m)

Q = aerosol flow rate (m ³ / hm ² )

BRIEF DESCRIPTION OF THE DRAWINGS

Giant. IA is an electron microscope (SEM) MB image of the web at a magnification of 500x.

Giant. IB is an SEM image of the MB web of FIG. 1A in which polypropylene fibers were embedded.

Giant. 2 graphically depicts the pressure drop dependence on the particulate load of unloaded samples 1 and 2 of Table II.

Giant. 3 graphically illustrates the dependence of filtration efficiency on particulate loading of unloaded samples 1 and 2 of Table II.

Giant. 4 is a graphical representation of the pressure drop dependence on the particulate loading of charged samples 1 and 2 of Table II.

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Giant. 5 graphically depicts the dependence of filtration efficiency on particulate loading of charged Samples 1 and 2 of Table II.

Giant. 6 schematically illustrates a preferred embodiment of a device suitable for producing a novel composite according to the invention.

Giant. 7 schematically illustrates another preferred embodiment of a device suitable for manufacturing a novel composite according to the invention.

As shown in Fig. 6, the invention combines the meltblowing method 1 by injecting 2 other fibers from the side into an air stream containing MB fibers 3. The MB fibers and other staple fibers are intensively mixed and intertwined in the stream and thrown at the edge of the formed web. The extent to which the injected staple fibers are concentrated on one side of the web depends on both the vertical and horizontal distances of the exit nozzle of the fiber delivery unit measured from the orifices of the MB spinner at the exit of the MB nozzle. Thus, it is possible to control the density gradient of the composite by shifting the position of the outlet nozzle of the staple fiber feed unit towards and away from the MB face of the nozzle and towards the center line and the center line of the spinneret orifices. Further, the density can be controlled by varying the amount of staple fibers injected relative to the MB fiber size. In general, the MB fibers closest to the nozzle outlet will have the highest proportion of staple fibers, while the farther fibers will contain fewer staple fibers. Similarly, the ratio of staple fibers in the composite is increased by increasing the speed of the staple fiber stream with or without increasing the amount of fibers supplied to the MB fiber stream.

The typical assembly shown in Figure 7 consists of a fiber delivery unit 1 with an outlet nozzle 2 at a vertical distance of 76.2 mm (3 ") downward from the exit edge of a row of apertures 5 of the spinneret MB (typically 20 to 35 apertures per 25, 4 mm (1 ”nozzles in one straight line of holes across the nozzle width) and at a horizontal distance of 203.2 mm (8”), damping extruded MB filaments, guided by slots 4 with loose knives located inside the nozzle on both sides of the nozzle, the fiber collector and the web conveyor 6 are shown for illustration. Either a rotary screen roller or a screen-coated conveyor belt is used (usually with suction of air below the surface of the roller or conveyor belt to assist in collecting MB fibers from the web and removing hot air). As can be seen from FIG. 6, the composite web 4 is carried by the conveyor belt and the composite web can be electrostatically charged in the space between the conveyor belt and the winder winding it. Although not shown in these schemes, the angle at which the fiber delivery unit transmits to the MB fiber stream is possible. »·· ···

8 staple fibers, by tilting the unit down from the nozzle to change from 90 ° as shown to the position where the fibers are delivered in more a tangential position, for example at 10 ° or less, if desired.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described in more detail by way of example, but it is intended only to illuminate it, not to limit it.

A vertically oriented MB 50 cm line from J&M Laboratories, Dawsonville, Georgia, USA, was used at The University of Tennessee to manufacture nonwoven and staple fibers. Fiber addition was performed using the injection system described. A thick (approximately 4 cm thick with a basis weight of 460 g / m ² ) a 50.8 cm (20 ") polypropylene (PP) staple fiber mat was introduced into an injection system that used a combination of rotating rollers with metal wire teeth and an air drive to receive individual fibers or small twisted strands of fibers into a stream of half-melted meltblown fibers and retardant air at a distance of about 4 cm from the final exit MB of the nozzle. Cotton fibers were also mixed with MB PP fibers to form low density composite webs. The mixing ratio and basis weight are shown in Table I. Corona charging of the nonwovens to improve their FE values was performed by Tantret ™ electrostatic charging technology, Technique II, developed by Tsai and Wadsworth (1995a), US Patent No. 5,401,446.

FE values were determined using TSI 8110 Automated Filter Tester and modified ASTM F1215 (Davis, 1993). Sodium chloride with an average particle size of 0.1 µm was used as the test particle for TSI 8110. The aerosol had a concentration of 15 mg / m ³ at a surface speed of 5.3 cm / s. Polystyrene microspheres of nominal sizes 0.6, 1.2 and 3 µm were used for the modified ASTM F1215 test. The actual bead size was 0.62, 1.07, 2.04 and 2.93 µm at a filtration rate of 30 cm / s.

The electron micrographs shown in Figures IA and 1B show that MB fibers are separated by coarse staple fibers to form a porous composite, while MB fibers without admixture are sealed together. The physical properties of the blended and blended MB fabrics are described in Table I. The carded PP, forming Sample 8 in Tables I and II, is composed of staple fibers used to be embedded in the MB fiber stream.

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The PP staple fiber webs had higher FE values than the unmixed webs for both the charged and uncharged webs as shown in Table II, except for the uncharged sample 2, which appears to have changed the regularity of the web. As can be seen from this table, the deposition of cotton fibers in MB fleeces showed the same trend. The pressure drop decreased from 1.9 mm H ₂ O for 34 g / m ² non-admixture (sample 1 in Table II) to 1.4 mm H ₂ O for PP staple fiber admixture (samples 1 and 2 in Table II) . The decrease in pressure drop by the addition of staple fibers was attributed to an increase in pore size or a decrease in bulk density. Reducing the seal decreases the FE value according to the single fiber efficiency theory (Davies, 1973). FE is the exponential function of the product of the filter area factor and the single fiber efficiency. A slight change in FE (samples 2 and 3 compared to sample 1 in Table II) after admixing means that the filter area factor increased by the same value as the decrease in single fiber efficiency. The pressure drop was not reduced by depositing 80% PP staple fibers in a 20 g / m ² fleece, as shown in Sample 5 in Table II, and by depositing 17 and 40% cotton fibers in 34 g / m ² nonwoven as samples 6 and 7 in the same. table. MB of a low basis weight web, for example 20 g / m ² , was not suitable for depositing a high proportion of PP staple fibers because the high pressure drop value is not compensated by an increase in FE. This was ascertained by comparing their filter quality values. Cotton had a higher ability to interweave with MB PP fibers because it has a finer fiber edge that is twisted due to the spiral structure.

Fleeces with the same FE value at lower pressure drop have better filter quality. For all MB staple fibers, the filtration quality improved as shown in Table II. However, the addition of cotton fibers showed less increase in filtration quality. Sample 8 in Tables I and II was a staple PP fiber mat used to blend into the MB fiber stream. It is given for comparison that its FE value is low compared to MB nonwovens.

Filter media having a small pore size as compared to a large particle size exhibits greater bumping. If the aerosol had a wider particle size distribution, the small particles would fill the cake pores and thus both the FE value and the pressure drop would increase. This is apparent from the FE values and the additive pressure drop (MB1) of Figures 2, 3, 4 and 5, where NaCl had an average particle size of 0.1 µm and a standard deviation of 1.9. The cubic shape of NaCl particles is also a clogging factor.

• V 99 999 999 • · · ·

The particles penetrate into the web if their size is small compared to the pore size of the web. The particles are deposited on the surface of the fibers and dendrites are formed by the accumulation of the particles. This build-up increases the FE value due to the increase of the surface area in the filter medium, as shown in Figures 3 and 5 for the admixture fleece. However, this type of load increased the pressure drop much more slowly than the non-adherent fleece blocking shown in Figures 2 and 4.

The service life of the non-woven fleece has increased due to the slower increase in pressure drop across the non-woven. The service life can be obtained from the pressure drop over time depicted in FIGS. 2 and 4. If a particular pressure drop was, for example, 3 mm H ₂ O, the service life by the curve approximation for unchanged non-webs was 73 min; fleece with admixture 211 min, for charged fleece without admixture 32 min and for charged fleece with admixture 72 min.

For the charged webs, the FE value was reduced in the initial filtration process, as shown in Fig. 5 for both non-doped and non-doped webs. The FE value decreased with particulate loading, because at the very beginning of filtration the charge was neutralized and filtration by electrostatic mechanisms was limited. After loading, the FE value increased because mechanical override electrostatic mechanisms. This is a typical load effect of an electrostatically charged filter, which is well documented (Hinds, 1982). The electrostatic filtration with NaCl load was reduced by the notional neutralization of the charge by NaCl particles rather than by the dissipation of the charge. It has been shown (Tsai, 1993) that PP material has low charge dissipation with storage time. Charged PP fabrics therefore have good durability.

It has been shown that the air flow through the MB fleece is within a wide range of basis weights of the fleece at filtration rates of laminaments (Tsai, 1993). When the web was treated with polystyrene beads with a surface velocity of 30 cm / s, equation (5) was fulfilled, as shown by the pressure drops in Tables III and IV. The samples in the two tables are those of Table II with the corresponding sample numbers. The pressure drop of each web was the same if the pressure drop at 30 cm / s was normalized to 5.3 cm / s.

It is known (Hinds, 1982) that FE increases with increasing particle size from the mechanical filtration point where the particle size is larger than the largest penetration size, say 0.1 to 0.3 µm, depending on the fiber size and filtration rate etc. This effect is demonstrated in Table IV for uncharged webs. The same was true for the charged fleece as shown in Table III. However, an increase in the FE value for a charged web with a particle size was not as apparent as for the uncharged web. Electrostatic mechanisms do not have effective attraction to • 9 99 • 9 · 9

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large size particles moving at high speed (Brown, 1993). There is no theory to estimate the relationship between the FE value and the particle size of a charged web because it is difficult to characterize the charge in the web after electrostatic charging. Electrostatic strength has a large contribution to the efficiency of the small particle filter at low filtration rates. This is illustrated in Tables III and IV.

Finally, an admixture of 110% staple fiber PP into a 34 g / m ² MB fleece (sample 3 in Table II) did not achieve a better FE value and a lower pressure drop than an admixture of 50% staple fiber PP. Addition to MB fleece with a basis weight of 20 g / m ² (sample 4) did not reduce the pressure drop but increased the FE. The admixture of cotton fibers resulted in a higher FE value and pressure drop than the corresponding blend ratio with PP staple fibers. Although PP has a high dielectric barrier, this batch of PP staple fibers could not be corona loaded, possibly due to the use of an antistatic agent in fiber treatment (Tsai, 1996). In some cases, these researchers have found that heating the staple PP fibers and / or washing and drying them minimizes the ability of the antistatic treatment to interfere with efficient electrostatic charging of the fibers. Cotton has a high wettability that easily dissipates charge from the fibers so that it is neutralized by ions naturally present in the air. However, when both PP staple and cotton fibers were added to the PP MB nonwovens, the resulting composite fabrics could be easily electrostatically charged using the technology developed by Tsai and Wadsworth (1995a).

The composite fabrics obtained by injecting coarse staple fibers into the MB fiber stream reduced the pressure drop of the aerosol flowing through the fabric without decreasing the FE value of the fabric. The magnitude of the pressure drop reduction and the FE increase depends on the MB basis weight and the percentage of staple fibers deposited, as well as on the structural properties of the staple fibers. The admixed composites could be electrostatically charged to the optimum FE value by methods developed at The University of Tennessee.

Table I. Physical properties of fleece Sample Sample Substrate, Admixture Total C. flat mass mass wt. (g / m ² ) g / m ² ) % (g / m ² ) 1 MB1 34 0 0 34 2 MB1 + PP 34 50 17 51 3 MB1 + PP 34 110 37.4 71.4 4 MB2 20 May 0 0 20 May 5 MB2 + PP 20 May 80 16 36 6 MBl + cotton 34 17 5.8 39.8 7 MBl + cotton 34 40 13.6 47.6 8 carded mat PP 464 0 0 464

Table II. NaCl filtration efficiency and filter quality Sample Filtration efficiency, pressure drop and filter quality uncharged loaded FE Δρ Qf FE Δρ Qf (%) (mm H2O) (1 / mmH ₂ O) (%) (mm H2O) (1 / mm H2O) MB1 28.6 1.9 0.177 91.1 1.9 1,273 MB1 + PP 27 Mar: 1.4 0.225 94.5 1.5 1,934 MB1 + PP 30.4 1.6 0.228 93.2 1.5 1,792 MB2 15.5 1.1 0.153 72.7 1.1 1,180 MB2 + PP 21.8 1.1 0.224 84.1 1.1 1,672 MBl + cotton 37.9 2.3 0.207 97.7 2.5 1,510 MBl + cotton 36.2 2.3 0.195 98.0 2.3 1,701 carded mat PP 12.7 0.7 0.194 13.0 0.7 0.199

• · · · ···

Pressure drop (mm H ₂ O)

Table III. Filtration efficiency of charged fleece on polystyrene beads

Sample

Filter efficiency on polystyrene ball (5)

0,6 pm 1,0 pm 2,0 pm 3,0 pm 30 cm / sec 5.3 cm / sec 1 74.5 77.4 91.7 95.4 9.65 1.71 2 80.6 81.8 92.5 96.1 9.65 1.71 3 88.0 88.9 96.3 99.3 8.39 1.49 5 67.2 68.1 81.3 91.9 6.10 1.08 6 88.3 90.3 98.6 99.7 12.19 2.15 7 84.7 87.2 97.6 99.4 11.18 1.97

Table IV. Filtration efficiency of unloaded fleeces on polystyrene beads Sample Filter Efficiency on Polystyrene Ball (5) Pressure drop (mm H ₂ O) 0,6 pm 1,0 pm 2,0 pm 3,0 pm 30 cm / sec 5.3 cm / sec 1 30,5 46.0 88.1 95.5 9.65 1.71 2 31.3 44.6 76.1 96.7 8.13 1.44 3 33,4 45.7 85.9 97.2 8.51 1.50 5 28.6 40.4 79.1 92.8 6.10 1.08 6 39.8 56.4 93.8 98.8 10.54 1.86 7 45,9 59.6 94.6 99.2 10.67 1.88 Table V. Physical MB properties roun na base PP with admixture of PP staple fibers

at Accurate Products Laboratory

Sample Sample Substrate, Admixture Total C. flat mass mass wt. (g / m ² ) (g / m ² ) % (g / m ² ) 9 MB3 18 0 0 18 10 MB3 + PP 18 210 38 56

»

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Table VI. NaCl filtration efficiency and filtration quality of PP-based MB nonwovens with the addition of PET staple fibers at Accurate Products Laboratory

Sample Filter efficiency, pressure drop and filter quality uncharged charged

FE (%) Δρ (mm H2O) Qf (1 / mm H2O) FE (%) Δρ (mm H2O) Qf (l / mm H2O) MB3 ON ON ON ON ON ON MB3 + PP 27.3 1,2 0.266 79.8 1,2 1,333

Literature:

U.S. Patent No. 4,100,324 to Anderson et al.

US Patent No. 4,118,531 (Hauser)

Assoc. of the Nonwoven Fabrics Industry, The Nonwovens Handbook, pp. 53-55.

Brown, R.C., (1970), Air Filtration, Pergamon Press.

Davies, C.N., (1973), Air Filtration, Academic Press.

Davis, W.T., (1993), " Air Filtration Efficiency Testing Using ASTM F1215 ", Proceedings, TAPPI Nonwovens Conference.

Hassenboehler, C.B. and Wadsworth, L.C., (1994), "Melt Blown Webs Products for Filtration," Fluid / Particle Separation Journal, 7 (1), 31M-32M.

Hinds, W.C., (1982), Aerosol Technology, John Wiley & Sons.

Tsai, P.P., and Wadsworth, L.C., (1993), "Measurement of Melt Blown Geometry Properties by Air Flow Techniques," Book of Papers, 3rd TANDEC Conference, The University of Tennessee.

Tsai, P.P., and Wadsworth, L.C., (1994a), "Air Filtration Improved by Electrostatically Charging Fibrous Materials," Particulate Science and Technology, An International Journal, 12 (4), 323-332.

Tsai, P.P., and Wadsworth, L.C., (1994b), "Effect of Aerosol Properties on the Filtration Efficiency of Melt Blown Webs and Their Electrets", Book of Papers, 4th TANDEC Conference, The University of Tennessee * ·

Tsai, P.P., and Wadsworth, L.C., (1995a), "Method and Apparatus for Electrostatic Charging of a Web or Film," U.S. Patent 5,401,446.

Tsai, P.P and Wadsworth, L.C., (1995b), "Effect of Polymers and Additives on Electrostatic Charging of Different Filter Structures," 5th TANDEC Conference.

Wadsworth, L.C., and Lee, L. (1990), "Relationship Among Melt Blown Web Structure, Air Permeability and Filtration Efficiency," INDA Journal of New Research Research, 2 (1), 4348.

Liu, B.Y.H. and Rubow, K.L. (1986), "Air Filtration by Fibrous Media," ASTM STP,

Claims (21)

PATENT CLAIMS A porous composite having an inlet and outlet surface comprising intertwined meltblown fibers and staple fibers having enhanced filtration efficiency, characterized in that the intertwined meltblown and staple fibers define a density gradient from coarse pores on the inlet surface to fine pores on the outlet surface. The composite of claim 1, wherein the density gradient is defined by varying the ratio of blending of staple fibers with meltblown fibers across the composite from the inlet surface to the outlet surface. 3. The composite of claim 2 wherein the weight ratio of blending staple fibers to meltblown fibers is in the range of about 0.17 to about 1.10. The composite of claim 1, wherein the staple fibers have a diameter of about 12 to 60 µm and a length of about 12.7 to 152.4 mm. The composite of claim 1, wherein the staple fibers comprise particles of 1 to 300 µm in size from a material selected from the group consisting of activated carbon and a superabsorbent powder. The composite of claim 2, wherein the staple fibers comprise shredded fabric with a maximum dimension in the range of about 1.59 to 12.7 mm. 7. The composite of claim 2 wherein the meltblown fibers are meltblown processable fibers. Composite according to claim 7, characterized in that the meltblown processable fibers are polyolefin fibers. ·· ·· I · to · ► ··· ··· ··· The composite of claim 5, wherein the meltblown fibers comprise polyolefin and the staple fibers comprise cotton. The composite of claim 9, wherein the polyolefin is polypropylene. 11. The composite of claim 1 having an electrostatic charge. Composite according to claim 1, characterized in that the filtration efficiency is defined by formula 4 in the description, wherein the symbols have the meaning given in the description. A method of using a porous composite having an inlet and an outlet surface comprising intertwined meltblown fibers and staple fibers having enhanced filtration efficiency, wherein the intertwined meltblown and staple fibers define a density gradient from coarse pores on the inlet surface to fine pores on the outlet surface, characterized in that a liquid comprising a mixture of undesirable particles having a particle size distribution from small particles to large particles is supplied to the composite and passed through the composite to obtain a filtered liquid having a lower undesirable particle content than the supplied liquid. The method of claim 13, wherein the fluid is supplied to the inlet surface and passes through the composite to the outlet surface, wherein larger particles are trapped in larger pores and smaller particles are trapped in smaller pores. The method of claim 14, wherein the particle size distribution comprises particles in the range of about 0.1 to 3 µm. A method of making a porous composite comprising intertwined meltblown fibers and staple fibers, wherein the intertwined meltblown and staple fibers define a density gradient from coarse pores on one surface to finer pores on the other surface of the composite, characterized in that the melt of fibers is entrained into hot 9 «99 • 9 · 9 9 9 999 999 9 · The air is then injected and staple fibers are injected therein to obtain a web of meltblown fibers intimately intertwined with the staple fibers. The method of claim 16, wherein the porosity of the composite from one surface to the other is controlled by controlling the exit nozzle distance of the fiber delivery unit measured from the meltblown spinneret orifices of the meltblown nozzle exit. Method according to claim 17, characterized in that the porosity of the composite is also controlled by controlling the speed and / or the ratio of the staple fibers injected into the meltblown fiber stream. The method according to claim 18, characterized in that the nonwoven obtained is further electrostatically charged. 20. A porous "used" composite having an inlet and an outlet surface, including intertwined meltblown fibers and staple fibers having enhanced filtration efficiency, wherein the intertwined meltblown and staple fibers define a density gradient from coarse pores on the inlet surface to fine pores on the outlet surface characterized in that the composite comprises undesirable particles trapped on the fibers, wherein the larger particles are located in the composite region having larger pores and the smaller particles are located in the composite region with smaller pores and thus form a gradient of entrapped particles from one surface of the composite to the other. 21. The composite of claim 20, wherein the meltblown fibers are electrostatically charged.