JPH03249252A - Device and method for melt blowing - Google Patents

Abstract

PURPOSE: To generate a corona zone at the discharging port of a die or thereabout by forming an electrostatic field between a pair of horizontal electrodes separated from the outlet of an orifice and a pair of emitter devices protruded outward from the die. CONSTITUTION: An electrode 42 is placed above a fiber-air flow 40 and an electrode 43 is placed below the flow and grounded. A pin 44 is attached to each of air knives 23 and 24 and the tip end 45 of the pin is inclined relative to the fiber-air flow 40. A high voltage is applied to the electrode 42 to generate an electrostatic field between the electrode and the pin playing the role of an emitter device.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention generally relates to a method and apparatus for producing fibrous, charged nonwoven fabrics. More specifically, the present invention relates to the production of meltblown charged IL fibers by applying a charge to the meltblown fibers during a meltblowing process.

There are multiple melt blow methods! It is a single step process in which a molten thermoplastic resin is extruded to form a mold. An astringent sheet of fast air blows the fibers onto a collector screen where they become entangled and aggregate to form a nonwoven fabric. These fabrics, referred to as meltblown nonwovens, have excellent properties for many applications, some of which are used to filter gases and liquids.

The microscopically small diameter of the entangled fibers of meltblown fabrics makes them ideal for filtering finely divided particles out of gaseous or liquid media. It is also known that the filtration efficiency of these nonwovens can be increased by applying an electric charge to the fibers. The charge on the cloth is often called an electret. U.S. Patent No. 4.215.68 discloses electrets of
No. 2, No. 4,375,718, No. 4,588,537
No. 4,592,815.

A method for applying a charge to molten, i.e., 14-temperature, 1M during the melt blowing process is disclosed in US Pat. No. 4,215,682. 5 at the high temperature of the polymer, i.e. in the molten state.
The static charge of the tui transfers charge into the polymer (because of its low electrical resistance) and remains trapped even after the polymer has cooled. This increases the charge lifetime of the electret.

The method disclosed in U.S. Pat. No. 4,215,682 is
Electrostatic charging is performed by passing the extruded fiber through an electrostatic field created by two charging sources above and below the extruded IN stream. Each charging source includes an electrode wire and a grounding sheath. Applying a high voltage charge to each electrode creates an electrostatic field between the wire and ground. A strong electrostatic field ionizes the air or other gas around the wire (using a grounded sheath around a portion of the wire can enhance this effect). The ions and charged particles thus generated are propelled into the fiber stream. The problem with this method is that fiber I
The goal is to propel charged particles into the I stream, particularly at or near the die exit where the fibers become hot again. A significant portion of the fibers does not pass through the corona zone. The above patent discloses that ions or charged particles are tlAH due to aerodynamic forces and electrostatic forces.
It states that it is propelled into a current and that fans can be used to aid this effect.

SUMMARY OF THE INVENTION In accordance with the present invention, a meltblowing apparatus includes an improved electrostatic charge device for generating a corona zone at or near the outlet of a meltblowing die. The melt blowing equipment may be of the conventional type, having a die with a row of orifices from which the molten thermoplastic resin is extruded to form a row 11. A converging hot air stream is blown onto the 1iAH to form a fiber-air stream.

The improved electrostatic charging device consists of a pair of grounded electron emitter devices mounted above the die (one on each side of the die outlet) and connected to a voltage source to charge the electrodes. a pair of current collecting electrodes (one on each side of the fiber-air flow);

The emitter device projects outwardly from the die generally in the direction of fiber-air flow flow and terminates near the die outlet. An electrostatic field between the electrode and the emitter device is established by a high voltage charge that creates a corona zone at or near the die outlet. The electrons and ions in the corona zone then contact the molten or hot fibers emitted from the die outlet. It is important that the corona zone be at the grounded emitter and not at the electrodes described in US Pat. No. 4,215,682. Therefore, a corona zone is established at or near the die outlet.

The preferred emitter takes the form of pins spaced along and on either side of the die outlet. The tip of the pin is shiny and ends near the die outlet.

The method of the present invention includes: (a) extruding molten thermoplastic resin through a plurality of orifices to form a plurality of molten INs; (b) blowing hot converging air onto both sides of the fibers;
(c) forming a fiber-air flow by stretching l&N; (i) creating an electrostatic field between two electrodes on either side of the flow and 0) grounded electron emitters on either side of the die outlet; (d) collecting the charged fibers to form a fibrous charged cloth.

An important feature of the invention is the high rate of electron emission from the emitter, particularly the emitter pin. This creates a corona zone at or near the die outlet. U.S. Patent No. 4,215,
Unlike the device disclosed in '682, the present invention
Since the airflow flows substantially through the corona zone, it eliminates the problem of melting, or adhering, charged particles (substantially electrons and ions) to the hot fibers.

Tests have demonstrated that the charging of melt, or hot fibers according to the present invention results in filters of exceptional filtration efficiency. Although the invention has been described in connection with filter applications, it is noted that the charged fabric may have other uses as well. The filtration efficiency test is a valid test for determining the charge of a cloth, even if the cloth is used for other problems.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As noted above, the present invention relates to electrostatically charging a melt blown melt, i.e., hot lllllll, to produce a charged nonwoven fabric. A melt blowing process line equipped with an electrostatic charge device is illustrated in FIG.
2) and a rotating collector drum or screen (15) for separating ItIi and air. The cloth (16) is pulled out from the screen (15). A typical meltblowing process line also includes an air source connected to the die (11) via valved tubing (17) and a heating element (18).

As shown in FIG. 2, die (11) includes a body member (20.degree. 21), an elongated die nosepiece (22) coupled to body member (20), and air knives (23, 24). The nosepiece (22) has a triangular cross section and a tip (
30). The nosepiece (22) has a convergent portion (29).The nosepiece (22) is formed with a central long passageway (31), and a plurality of side-by-side orifices (32) are drilled at the tip 6a (30). Manufactured from high quality steel to provide durability.The molten polymer passes through the die passages (33,
34) (coat hanger form), and die passage (3
5, 31) from the extruder, and the orifice (
32) from ultra-thin horizontal I! Extruded as miscellaneous.

The air knife (23, 24) defines an air passageway (38, 37) with the body member (20, 21). Air knife (
2324) has an inwardly facing inclined surface, which converges with the inclined surface of the nosepiece (29).
9). As shown, each air passageway (3839
) can be adjusted freely. Air is passed from an air source through the air passages via tubing (17) and is discharged as thin sheets of hot air on either side of the molten gem.

The converging thin plates of hot air pull, or thin, IIN, m*-air! (40) to form the die outlet (
41).

According to the present invention, the melt blowing apparatus shown in FIG.
Apparatus is provided for applying an electrostatic charge to II as tan is ejected from the die outlet (41). An electrostatic field is established by two electrodes (42° 43) equidistant from the center line of the flow, one above the flow and the other below the flow. For reference, the center line of flow is in the direction of the orifice orientation.

The electrodes can be mounted on the same framework that supports the die to provide a common ground.

A protruding electron emitter, illustrated as a pin (44) in FIG. 2, is mounted on the air knife (23, 24).

The emitters protruding on both sides of the die outlet (41) can take various shapes, such as blades or wires;
The preferred emitter is a pin (44) as shown.

One row of pins (44) extends along the die above the die outlet (41) and a second row of pins extends along the die above the die outlet (41).
extends along the die below.

The pin (44) is angled toward the I-fiber-airflow and the tip (4
It ends with 5).

The tilt angle of the pin (44) is not critical, but helps position the pin tip (45) close to the air flow without interfering with the flow.

The distance between the pin (44) and the electrodes (42, 43) is as follows: -
The layers will be explained in detail.

A high voltage power supply is connected to the electrode (upper/lower), and the picture electrode is +
The polarity is controlled so that it can have /10 charges, +/- charges, -/10 charges or -/- charges. This is the electrode (
42, 43) and the grounded emitter (44). This, in turn, is the die outlet (41)
A corona band is generated around the emitter tip (45) near 4.

Melting passing through the corona zone, i.e. high temperature t11. N is charged by electrons or charged particles.

As mentioned above, the equipment installed in the melt blowing process line includes electrodes, high voltage power supplies, and emitters. Specific examples of these elements are as follows.

! U: The electrodes (42, 43) can take the form of metal rods (conductive) as shown. The diameter of the rod is 2 inches (50
, 8mm+) to thin electric wire (e.g. 1/16 inch in diameter (
1,59jwe)).

As will be explained in detail later, the spacing between the electrodes is S width.
It should be as small as possible without obstructing the flow of airflow. Electrode diameter is 178” to 1” (3’, 2
~25.4jw) is desirable. Magnetically permeable, reticulated metal wires shaped like flat blades or ovals can also be used as electrodes. Openings between the metal wires forming a hedge or perforated material allow air to flow through the electrodes with minimal obstruction to 118-airflow. The shape of the electrode,
It is mounted on any type of insulating material, for example ceramic, and connected to a high voltage power supply.

J: Any high voltage DC power supply can be used. The power supply should have variable voltage settings (e.g., 10V to 25V) to allow for adjustment in creating the electrostatic field, and should preferably be capable of 10 and - polarity settings. Hatfield, Pennsylvania, USA
The high voltage power supply commercially available from 5IHCO, Inc., located in C. Field, has been found to be extremely suitable for the purposes of the present invention. The use of this power supply is as follows: Maximum voltage...25V DC output polarity...negative and positive maximum current...6.38A with electrodes connected Emitter: As mentioned above, the emitter , preferably in the form of sharp conductive pins spaced along and to the sides of the die outlet (41). The pin is slender, for example, the diameter is 1/16 ~
Ranges from 3/8 in (1.6 to 9.5 m+).

The pin (44) can be attached directly to the face of the air knife (23, 24) by welding or by inserting it into a properly drilled hole, as shown in FIGS. 2, 3 and 4. . Alternatively, the pin (44) is attached to a narrow plate (4647) (illustrated in Figure 5) and then this plate (4647) is attached to the air knife by means of a threaded bolt (50) passed through the slotted hole (49). Attach to (23, 24). This mechanism allows adjustment of the vertical spacing of the pin tips (45).

Each die outlet (4) is attached to both sides of the die outlet.
1) a pair of grounded blades (
(not shown) can also be used. A pair of wires attached to the sides of the die outlet (41) can also be used.

Another preferred emitter configuration is illustrated in FIG. In this embodiment, the outwardly projecting protrusions 23a, 24a) have sharp blades (51, 52) at their tips.
is provided on the air knife (23, 24).

The blades (51, 52) act as emitters in generating the corona zone. As shown in all figures, the emitter, regardless of its type, is grounded. Combinations of emitter configurations may also be used (eg, spaced apart pins on protruding air knives).

The rods (42, 43) and the emitter device are naturally connected to the die outlet (
41).

Electrostatic Element Spacing Electrode-to-emitter spacing can be important to the performance of an electrostatic device. The significance of the spacing will be explained with reference to FIGS. 3 and 4. The characters in the figure represent the following.

(a) Emitter pin vertical spacing (b) Emitter pin horizontal spacing (c) Emitter pin tip to die surface spacing (d) Electrode diameter (h) Electrode horizontal spacing (V) Electrode vertical spacing Pin tip (45) The spacing (a) is the fiber-air flow (40)
should be placed as close as possible without interfering with the flow of water. The distance (c) is large, preferably 0.2 to (h) in order to localize the corona zone near the discharge port (41).
It should be 0.6, most preferably 02-0.4.

The horizontal spacing (b) does not appear to be important as long as a sufficient corona zone is established along the length of the die outlet (41). The diameter (d) of the electrode should be large compared to the wire, but not so large as to impede I-fiber flow. The spacing (V) and (h) are small between the CliMH-airflow (40) and the die (
11) should be close to each other). Dimensions (d) and (V) should be determined so as not to impede the flow of fibers in stream (40). Dimension (h) should be greater than the distance between the tip (45) and its associated rod axis to avoid arcing between the rod and the air knife of the die.

Dimensions referred to herein as vertical or horizontal should be understood with respect to the orientation of the die. For vertically oriented dies, dimensions (V) and (h) would represent the horizontal and vertical dimensions, respectively. The present invention does not care about the orientation of the die.

The dimensions below represent broad and desirable ranges.

Dimensions Wide range Desired range Most desirable range Pin in in
+n(c) 0.25-1.5 0.4
~1.00.4~07(v) 1.0~5.0
1.5-4.8 2.0-3.0 Operation During operation, an electrostatic charging device is installed in the melt blowing process line. The line can be made of any thermoplastic that can be used in melt blowing processes. The preferred polymer is polypropylene, but other polymers may also be used.

For example, low and high density polyethylene, ethylene copolymers (including EVA copolymers), nylon, polyamide,
Polyester, polystyrene, poly-4-methylpentene-1, polymethyl methacrylate, polytrifluorochloroethylene, polyurethane, polycarbonate, silicone, and mixtures thereof.

Melt blowing process lines are less than 10μ, typically 1~
Produces 5μ fibers.

Once the line has been started and steady state operation has been achieved, the electrostatic charge device can be activated. Adjustments may be necessary to optimize the performance of the electrostatic device.

Experiment An experiment was conducted in which electrostatic charges were applied to nonwoven fibers during the production of nonwoven fabrics. Several variables were varied, including charging voltage, electrode polarity, and electrostatic charging device spacing and size. Test equipment and materials included: Melt-build o-die: 20 inches wide (>51 cm);
25 0.015in per 1in (2,541)
(0.38 liter) orifice; extrusion temperature below 450-550 (232-288°C): polymer flow rate 0.2-0.8 g/min per niorifice.

Electrode: Metal rod or wire hinge: 1/16 inch (1,59M) diameter (Steel) Resin: Polypropylene (PP commercially available from Exxon Chemical Company)
3145) Filtration efficiency measurement: The effect of electrostatic charge was determined by a wire test using the following equipment: Equipment: RefinedS
urgicos) FET system H (Poston City, 1985
June 4-6, 2018, International Nonwovens and Treated Materials Association (Inter
nationalionwovens anddis
Posable As5oc,) 13th Technical Symposium: L. C. Wadsworth (Wadsworth)
Automated test test equipment for rapid simulation of bacterial filtration efficiency (＾(JtOrRate(J ding(!)
St Appara-tus for Rapid S
imulation of Bacterial Fi
ltration efficiency) J)
Aerosol: 10% suspension of 0.8μ latex spheres in distilled water spray Counting: Optical particle counter Sample size: 5 x 5 in (12,7 x 12
．． 7CIR>Nonwoven fabric sample measuring instrument: Keighley electrometer
y Electrometer) Model 610C, 2
, 9 in (7,41) Metal Conical Probe Measurement Method Two Ta conical probes were mounted vertically with the large diameter end up. A plastic spacer was placed on top of the cone. The height of the spacer required to read the correct voltage on the metal plate connected to the power source was predetermined. Each city tested was placed on a plastic spacer. A grounded metal plate was then placed on top of the cloth and tested to measure the surface potential.

Test Procedure: Nonwovens were produced under different conditions (eg, varying voltage, polarity and spacing) and tested for filtration efficiency and surface potential. A large number of tests were conducted. The data shown in Table 1 are representative of the study.

Table 2 shows the results of a comparison of fabrics made without charge.

Experiments show that the electrostatic charging of IIn resulted in a charged molar mass with excellent filtration properties. Experiments also show that the following variables are important to achieve optimal results.

1 Electrode polarity (10/+) 2 Voltage (20 to 25 V) 3 Stiff electrode (at least 1/2 inch diameter (178
Tests using wire electrodes caused grounding problems).

4 Rigid electrodes with close vertical spacing (V) without disturbing the IIN of the flow (40) 5 Used as a rigid, sharp pin filter. Although the invention has been disclosed in the context of electrically charged, meltblown nonwoven fabrics, the invention can be applied to making electrically charged meltblown nonwoven fabrics useful in a variety of applications.

[Brief explanation of drawings]

FIG. 1 is a schematic perspective view showing the main elements of a meltblowing process line equipped with the electrostatic device of the present invention; FIG. 3 is a schematic cross-sectional view of the preferred electrostatic charge device of the present invention; FIG. 4 is a cross-sectional view of the die of FIG. 3 showing the horizontal spacing of the emitter pins in relation to the die outlet; The front view, FIG. 5, is an enlarged perspective view showing the adjustable attachment of the emitter pin to the die, and FIG. 6 is a schematic perspective view showing another emitter that may be used with the present invention.

Claims (1)

[Scope of Claims] 1. A melt blowing apparatus for producing melt blown and electrostatically charged cloth, comprising: (a) a long die with a plurality of orifices formed at the tip of the die; (b) a plurality of horizontally arranged (c) a device for extruding a molten thermoplastic resin through an orifice in the die tip to form a fibril; (d) a pair of long, horizontal electrodes spaced from the outlet of the orifice of said die tip, one electrode above said fiber-air flow and the other electrode above said fiber-air flow; Fiber - an electrode below the air flow; (e) 1 coupled to and projecting outwardly from said die;
a pair of emitter devices, one emitter device disposed above the die tip and the other emitter device below the die tip, each emitter device having an outer end proximate the exit of the die tip orifice; (f) a device for applying a high voltage to the electrode to establish an electrostatic field between the electrode and the emitter device. 2 a first row of pins spaced along and above the die tip orifice and coupled to the die; and a second row spaced along and below the die tip orifice coupled to the die. 2. The apparatus of claim 1, wherein each emitter device includes a pin. 3. The outer ends of the pins are pointed, and the outer ends of the first row of pins are within 1/4 to 1 inch (6.35 cm) vertically from the outer ends of the second row of pins. ~25.4mm)
3. The apparatus of claim 2, wherein the apparatus is spaced apart. 4 The electrode has a diameter of 1/16 to 2 inches (1.6 to 5
0.8 mm) and 1.0 to 5.0 inches (2.5
2. The device of claim 1, wherein the rods are spaced apart from each other by 4 to 12.8 cm. 5. In a meltblowing apparatus in which a stream of fibers and air is discharged from a long die outlet, a device for charging the fibers in the stream: (a) fixed to the die above the die outlet; (b) a first row of conductive pins extending outwardly from the die and terminating above the direction of the die orifice; (b) fixed to the die below the die exit; (c) a second row of pins extending outwardly from and terminating below the orientation of the die orifice; (c) horizontally spaced from the die in the orientation of the die; (d) a first electrode disposed above the facing direction of the die; (d) horizontally spaced from the die in the facing direction of the die and disposed below the facing direction of the die so as not to impede the flow of the fibers; (e) generating an electric field between the pin and the electrode and a corona band at the tip of the pin; , a power supply sustained to each of the electrodes. 6 the electrodes are spaced horizontally equidistant from the die, each electrode having a diameter of 1/8 to 11/2 inches (3.2 to 3 inches);
8.2 mm), the voltage is 15-25 KV, and each electrode has positive polarity. 7. A method of forming a charged cloth of meltblown fibers comprising: (a) discharging a thermoplastic fiber-air stream from the exit of a meltblowing die; (b) electrodes above and below the air stream; An electrolytic force is established between grounded emitters arranged above and below the die exit, horizontally spaced apart from the die exit, and closer to the die exit than the electrodes, to create a corona zone at the emitter. (c) passing the fiber-air stream through the corona zone. 8 The emitter is 1 inch horizontally from the die exit.
(2.54cm). 14. The method according to claim 13. 9 the electrodes are equidistantly spaced both from the die and from the fiber-air stream, and the horizontal spacing of the electrodes from the die exit is greater than or equal to 1 inch and 3 inches;
(7.62 cm) or less, and the horizontal spacing of the emitter from the die exit is 0.2 to 0.2 of the horizontal spacing of the electrodes.
14. The method of claim 13, wherein the 10. The electric field is established by applying a voltage of 15 to 25 KV of polarity to each of the electrodes.
Method described.