WO1991019034A1

WO1991019034A1 - Insulated collector for production of electrically charged meltblown webs

Info

Publication number: WO1991019034A1
Application number: PCT/US1991/003693
Authority: WO
Inventors: Peter Moosmayer; Erwin Zurcher; William Edward Deeds
Original assignee: Exxon Chemical Patents Inc.
Priority date: 1990-05-29
Filing date: 1991-05-28
Publication date: 1991-12-12

Abstract

The method and apparatus of the present invention employs an insulated, substantially nonpermeable collector drum for use in a meltblowing system equipped with means for electrically charging the meltblown fibers. The invention produces soft, electrically charged webs.

Description

INSULATED COLLECTOR FOR PRODUCTION OF ELECTRICALLY CHARGED MELTBLOWN WEBS

BACKGROUND OF THE INVENTION This invention relates generally to a process and apparatus for producing electrically charged nonwoven webs. In one aspect, the invention relates to the manufacture of electrically charged meltblown fibrous materials in which the charge is applied to the meltblown fibers during the meltblowing process. In another aspect, the Invention relates to electrically charging meltblown webs using an insulated collector drum.

Meltblowing is a one step process in which a molten thermopl stic is extruded to form a plurality of fibers. Converging sheets of high velocity air blows the fibers onto a collector surface where they are entangled and collected forming a nonwoven web. These webs, called meltblown nonwoven fabrics, have excellent properties for many uses, one of which is filtration of gases and liquids.

The microscopic diameters of the entangled fibers of the meltblown web are ideally suited for filtering finely divided particles out of a gaseous or liquid media. It is also known that the filtration efficiency of these nonwoven materials can be improved by applying a persistent electrostatic charge to the fibers. The charges on the web are frequently called electrets. U.S. Patents which disclose nonwoven fibrous electrets include

U.S. patents 4,215,682, 4,375,718, 4,588,537, and 4,592,815. A process for applying the electric charge to the molten hot fibers during the meltblowing process is disclosed in U.S. Patent

4,215,682. The electrostatic charging of the fibers in the hot or molten state of the polymer permits the charges to migrate into the polymer (since its electrical resistance is lower) and remain trapped upon cooling or cristal 1 ising of the polymer. This increases the charge life of the electret.

In the process disclosed in U.S. Patent 4,215,682, the charging is achieved by passing the extruded fibers through an electrostatic field and collected on a collector screen which, heretofore, have generally been made of metal.

Collection of the electrically charged fibers on a conductive and grounded screen can cause the fibers to lose their charge and can cause the fibers to pack more densely.

SUMMARY OF THE INVENTION The apparatus and method of the present invention employs an electrically insulated collector surface which produces improved electrets since the webs hold more charges. The use of insulation also appears to enhance web thickness and softness and filtration efficiency.

In accordance with the present invention an apparatus and method for producing electrically charged meltblown webs includes an electrically insulated collector drum for collecting the meltblown fibers in forming the web. The insulation may comprise a film of polyethylene, polypropylene or other substantially nonporous insulating material with suitable di¬ electric properties. Alternatively, the drum may itself be made of Insulated material such as molded polyethylene, polypropylene, PVC, etc. When using film, it may be placed over a conventional collector drum and secured in place.

* Filtration efficiency tests indicate that charged webs produced with an insulted collector drum have higher surface potentials, and lower pressure drop during filtration. Moreover, the webs collected thereon exhibit improved softness over similar webs produced with an uninsulated collector drum. In addition, filter aging tests indicate that insulated webs maintain high filtration efficiencies for long periods of time, which can be attributed to the high initial surface potential. Although the present invention is described in relation to filtration applications, it should be pointed out that electri¬ cally charged webs may have other applications. The filtration efficiency test described herein is an effective test for determining the charge of the webs, even if the webs are used for other applications. BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a schematic illustrating the main components of a meltblown line with electrostatic charging and the insulated collector drum of the present invention. Figure 2 is a cross-sectional view of a meltblowing die

Illustrating the flow of the fiber-air stream from the die discharge onto the insulated collector drum.

DESCRIPTION OF PREFERRED EMBODIMENTS As mentioned previously, the present invention relates to an apparatus and method for collecting electrostatically charged meltblown molten or hot fibers for the production of electrically charged nonwoven webs wherein the fibers are collected on an insulated collector means (e.g. a rotating drum). The present invention is described below with reference to the apparatus and method disclosed in U.S. Patent 4,904,174. The present invention may also be used with other apparatus for producing electrically charged fibrous webs such as that disclosed in U.S. Patent 4,215,682. The disclosure in Patent 4,904,174 and U.S. Patent 4,215\682 are incorporated herein by reference. A meltblowing line with electrostatic charging equipment is illustrated in Figure 1 as comprising an extruder 10 for delivering molten resin to a meltblowing die 11 which extrudes fibers into converging hot air streams forming a fiber-air stream 12. The fiber-air stream 12 is sprayed onto a rotating collector drum 15 where the fibers are collected as a web. The collector drum 15 is provided with electrical Insulating film 16 on the collector surface. The Insulating layer 16 may be of polyethylene, polypropylene, or other nonporous or slightly porous material possessing insulating, dielectric properties (i.e. a nonconductor). The electrostatically charged fibrous web 17 is withdrawn from the insulating film 16. The typical meltblowing line will also Include means (heating elements 19) for heating the polymer to the meltblowing temperature and an air source connected to the die 11 through valved lines 18. As shown in Figure 2, the die 11 includes an elongate die noseplece 22 and air knives 23 and 24 secured to the die body, The nosepiece 22 has a converging section 29 of triangular cross section terminating at tip 30. A central elongate passage 31 is formed 1n the nosepiece 22 and a plurality of side-by-side orifices 32 are formed in the tip 30. Molten polymer is delivered to the extruder through die passage 31, and extruded as micros ized s1de-by-side fibers from the orifices 32.

The air knives 23 and 24 with nosepiece 22 define converging air passages 38 and 39 and an enlongate discharge opening 41. Air is delivered from an air source via lines 18 through the air passages 38 and 39 and is discharged onto opposite sides of the molten fibers as converging sheets of hot air. The converging sheets of hot air draw and attenuate the fibers forming a fiber and air stream 12 discharging from the die discharge 41. In accordance with the apparatus and method disclosed in

U.S. Patent 4,904,174, the electrically grounded meltblowing die 11 shown 1n Figure 2 hereof 1s provided with means for applying electrostatic charges to the fibers as they discharge from the die discharge opening 41. The electrostatic field is established by two electrodes 42 and 43, connected to a high voltage source, one placed_^above the stream 12 and one placed below the stream 12 but equidistant from the center line 12a of the stream 12. For re¬ ference purposes, the center line 12a of the stream is the direction of orientation of the orifices 41. The electrodes may be mounted on the same frame as that supporting the die 11 but have to be electrically Insulated against the die. Protruding electron emitter pins 44 are secured to the air knives 23 and 24. One row of pins 44 extends along the die above the die opening 41' and a second row of pins 44 extends along the die below the opening 41. The pins 44 are slanted toward the fiber-air stream and terminate in points 45. As described in U.S. Patent 4,904,174 the preferred dimensions a, b, c, d, and e shown in Figure 2 are as follows: DIMENSION PREFERRED RANGE (inches)

(a) emitter pin vertical spacing 0.3 to 0.9

(b) emitter pin tip spacing from die face 0.4 to 1.0 (c) electrode horizontal spacing 1.0 to 3.0

(d) electrode diameter 0.125 to 1.5

(e) electrode vertical spacing 1.5 to 4.0

A high voltage source is connected to electrodes 42 and 43 (top/bottom) and the polarity 1s controlled so that the electrodes may have a +/+ charge, - /+ or -/- charge. This establishes an electrostatic field between tne electrodes 42 and 43 and the ground emitters 44. This in turn creates a corona zone around the emitter tips 45 near the die discharge 41. The molten or hot fibers passing through the corona zone are charged by electrons or charged particles. The polarity of the electrodes is preferably +/+.

In accordance with the present invention, an Insulated, rotating collector drum 15 which includes an electrical Insulating film 16 is placed downstream of and aligned with the die discharge opening 41. The fiber-air stream 12 impinges on the insulating film 16 where the fibers entangle and deposit forming a nonwoven web 17a on the film 16. The insulating film 16 prevents the flow of electrical charges from the charged web 17a to the collector drum 15. It is preferred that the film be nonporous (i.e. non- permeable to air). This appears to produce a fluffy, soft web. Air flowing through the drum such as that provided by a screen tends to pack the fibers tighter. The film 16 thickness may vary within wide limits but should provide the necessary insulation and be strong enough to withstand the operations without failing. Thicknesses of 1 to 10 mils are preferred. The film can be applied by wrapping a film panel around the drum and securing overlapped ends as by taping. As mentioned earlier, other web collecting means which provide the electrical insulating collector surface are possible. For example, a layer of dielectric material may be mounted around the periphery or a drum. Also, the drum itself may be composed of molded dielectric material having an impermeable collector surface. Operation In operation, the electrostatic charge equipment will be mounted on a meltblowing line. The line may employ any of the thermoplastic resins capable of use in meltblowing. The preferred polymer 1s polypropylene, but other polymers may be used such as low and high density polyethylene, ethylene, copolymers (including EVA copolymer), nylon, polyamide, polyesters, polystyrene, poly-4-methylpentene, polymethylmethacrylate, polytrifluorochloro- ethylene-*, polyurethanes, polycarbonates, silicones, and blends of these and other materials.

The meltblowing line produces fibers less than 10 microns in diameter, typically 1 to 5 microns.

The line is started and once steady state operation Is achieved, the electrostatic charge system may be activated.

, A rotating collector drum 15 which includes an electrical insulating film 16 cover (preferably made of polyethylene) is positioned and aligned with the meltblown fiber- air stream. The rate of rotation is adjusted in relation to the fiber-air stream flow rate to achieve the desired web thickness.

As the web is formed on the film 16 on the rotating drum 15, it is withdrawn as web 17 by conventional web take-up means. EXPERIMENTS

Experiments were carried out on the production of electrostatically charged webs produced with an electrically insu¬ lated collector drum and with a noninsulated metal screen drum. Other adjustable test parameters for the process were held constant to isolate the effects of the electrical insulation on the properties of the nonwoven web. Several properties including surface potential, filtration efficiency, pressure drop during filtration, and sample weight and thickness were measured. The test equipment and materials included the following: Meltblowing Die: 20 inch width with twenty 0.015 diameter orifices per inch; extrusion temperature: 450^* - 550^*F; polymer flow rate: 0.2 to 0.8 grams per minute per orifice. (The line used in Experiments 1 and 2 was similar to that described in

U.S. Patent 4,904,174. The line used in experiments 3 and 4 did not employ emitter pins.)

Electrodes: metal bars or wires Pins: 1/16 inch in diameter (steel) Resins: polypropylene (PP 3145 marketed by Exxon Chemical Co., 300MFR) Filtration Efficiency Measurements: The effect of electrostatic charge was determined by filtration tests using the following apparatus.

Apparatus: Refined Surgikos FET apparatus (described in "Automated Test Apparatus for Rapid Simulation for Bacterial Filtration Efficiency"; L.C. Wadsworth; 13th

Technical Symposium, International Nonwovens and Disposable Assoc; June 4-6, 1985; Boston) Aerosol : 10% suspension of 0.8 or 0.5 micrometer latex spheres in a distilled water fog. Counting: optical particle counter

Filtration Efficiency (X): (retained particles) x 100

(total particles) Electret Measurement System:

Sample Size: 5 x 5 inches nonwoven web specimens Instrument: Kelthly Electrometer Model 610C with

2.9 inch metal cone probe.

Measurement Method: The cone probe was vertically mounted with the large diameter end upward. A plastic spacer was placed on top of the cone. The height of the spacer required to give an accurate voltage reading of a metal plate connected to a power supply was previously determined. Each web tested was placed on the plastic spacer. A grounded metal plate was then placed on the web and the test was carried out to determine surface potential. Test Conditions and Results: Shown in Table I are the test conditions and comparative data for webs ("insulated webs") produced with Insulated drums (Tests 2A and 2B) and webs produced with uninsulated drums ("uninsulated webs") (Tests 1A and IB) produced with. emitter pins 0.5 Inch 1n length. Table II presents the test conditions and comparative data for insulated webs (Tests 4A and 4B) and uninsulated webs (Tests 3A and 3B) made using Simco apparatus without emitter pins. The Data in Tables I and II re¬ present the mean of three measurements with 5 x 5 inch web samples. The test results indicate that the insulated webs have significantly larger thickness than the uninsulated webs. As a result,; the insulated webs are a fluffier fabric than the uninsulated webs. The insulated webs are also noticeably softer to the touch. The Insulated webs have lower pressure drop (PD) during filtration data than the uninsulated webs. This can be attributed to the added fluffiness noted above. These data suggest that for a given filtration pressure drop, the insulated webs will have a higher throughput. The surface potential data indicate that insulated webs have much higher surface potential than the webs made without insulation. This is believed to be due to the retention of charges which may otherwise be lost through an uninsulated collector. High initial surface potential is important for the production of filters which maintain high efficiencies over extended periods of ambient aging.

The filtration efficiency data for the aged insulated webs are, in general, higher than the data for the uninsulated webs. While the efficiencies tend to be higher for the insulated webs, the sample weights of the insulated webs are actually less than their uninsulated counterparts. Therefore, the improved efficiency appears to be related to the increase in surface potential. While the improvement in the efficiency appears to be small, it should be noted that an increase in efficiency from, for example, 97% to 98% effectively decreases the number of unfiltered particles by a factor of one-third.

2 - five days after manufacture of web

Additional tests were carried out using 300 MFR polypropylene on the same apparatus used in Test 2A and 2B to determine effects of aging on surface potential and filtration efficiency. The data are presented in Table III.

These tests indicate that substantially all of filtration efficiency is retained even after long term aging.

Although the present invention has been exemplified in connection with electrically charged nonwoven meltblown webs used as filters, the Invention may be used to produce electrically charged webs useful in a variety of applications. The present invention may be useful for applications 1n which better flufflness and softness of the nonwoven fabric having low air pressure drop are desirable properties.

Claims

CLAIMS:

1. A meltblowing apparatus comprising

(a) a die having a plurality of orifices;

(b) means for extruding molten thermoplastic resins through the orifices forming fibers;

(c) means for applying an electric charge to the fibers extruded from the orifices; and

(d) means for collecting the charged fibers on a moving collector means, said collector means having an electrical insulating surface on which the electrically charged fibers are collected.

2. The apparatus of claim 1 wherein the collector means is a rotating drum.

3. The apparatus of claim 1 wherein the rotating drum is made of metal and includes a nonpermeable film of plastic covering the periphery of the drum.

4. The apparatus of claim 2 wherein the film is a polyolefin film having a thickness of 1 to 10 mils.

5. The apparatus of claim 4 wherein the film is made of pol ethylene.

6. A meltblowing apparatus for electrically charging webs produced thereby which comprises

(a) a meltblowing die for discharging an air- polymer fiber stream;

(b) a collector drum for collecting the fibers, said drum having a nonpermeable or slightly permeable layer of nonconductive material mounted thereon on which the fibers are collected; and (c) electrodes positioned above and below the f1ber-air stream and between the die and collecting drum for applying a high voltage to the polymer fibers between the die and collector.

7. The apparatus of claim 6 wherein the electrodes are capable of applying at least 20 kV above and below the stream.

8. The apparatus of claim 7 wherein the electrodes each apply positive polarity.

9. A process for preparing electrically charged meltblown webs which comprises:

(a) extruding a thermoplastic resin through orifices of a meltblowing die Into high velocity gas to form attenuated fibers;

(b) electrically charging the fibers;

(c) collecting the electrically charged fibers on a rotat'ing electrically insulated drum.

10. A process for forming a meltblown fibrous electrically charged web which comprises;

(a) discharging a plurality of thermoplastic fibers from an outlet of a meltblowing die into converging hot air streams to form a fiber-air stream;

(b) establishing an electric field between electrodes above and below the fiber-air stream to create corona zones;

(c) passing the fiber-air stream through the corona zones; and

(d) collecting the charged fibers on a moving insulated collector to form an electrically charged web.

11. The process of claim 10 wherein the corona discharge zone is positioned within 1 inch from the die outlet. 15

12. The process of claim 10 wherein the electric field is established by a voltage of 15 to 25 kv applied to each electrode and each electrode having positive polarity.

13. The process of claim 10 wherein the collector comprises a rotating drum having a nonconductive film covering the periphery thereof.

14. The method of claim 10 wherein the thermoplastic fibers are composed of a polyolefin.