Method and apparatus for making submicron diameter fibers and webs there from

Abstract

A method and device for the production of polymer filaments with a diameter of less than one micron. A plurality of polymer components are extruded through a spin pack and then attenuated using gas flows which are accelerated to achieve high velocity by means of a converging, diverging nozzle. The plurality polymer components may be extruded in an islands in the sea or segmented pie configuration. As a result of the high velocity gas flow, the plural components are split apart into their individual components resulting in filaments and fibers having a diameter or minor dimension of less than one micron.

Description

FIELD OF THE INVENTION

This invention relates to extrusion of polymer fibers, and in particular fibers that have a diameter of less than one micron.

BACKGROUND OF THE INVENTION

Fibers and filaments have been produced for many years using methods well known as melt blowing and spun laid fiber spinning. Melt blowing is typically associated with fibers of finite length whereas the spun laid process is typically associated with continuous filaments. In both of these technologies there has been an extended effort from many individuals to reduce the diameter of the fibers produced. Typical minimum fiber and filament diameters for these technologies is now on the order of 3 to 5 microns. One method for producing finer fibers is known as bicomponent spinning. In this method, disclosed for example in U.S. Pat. No. 5,162,074 to Hills and incorporated herein by reference, two or more polymers are extruded through specially designed spin packs which configure the filaments into arrangements known as side by side, sheath/core, islands in the sea or segmented pies. Of these arrangements, those of the islands in the sea and segmented pies are such that although the filaments of the combined components exceed 1 micron in diameter, the individual components can be separated from each other by post processing to result in filaments with a diameter of less than 1 micron. This post processing typically includes mechanical action to fracture the components apart at the segmented pie interfaces or chemically dissolving the sea polymer to leave only the islands polymer.

These post processing steps can be both costly an inefficient. An article “Spunbonded nonwovens made from splittable bicomponent filaments” by Schilde, Erth, Heye and Blechschmidt, Chemical Fibers International Vol 57 No 1, March 2007 describes multiple methods and the difficulties encountered in mechanical splitting of these fibers. Islands in the sea filaments provide the smallest known fiber diameters from melt polymers, ref “Spinning of Submicron diameter Fibers” and “Production of Sub-Micron Fibers in Nonwoven Fabrics” by Hagewood on Hills Inc website hillsinc.net where as many as multiple thousand of island fibers can exist within a single bicomponent filament. Complete removal of the sea polymer, however, is a known issue with this technology as evidenced by art disclosed to facilitate this process. See for example U.S. Pat. No. 6,861,142 “Controlling the dissolution of dissolvable polymer components in plural component fibers”.

As a result of these difficulties, recent developments to reduce the fiber diameters have focused primarily on reduction of the size and spacing of the spinneret holes as disclosed for example by Allen, US Application US2005/0087900 for spunbonding and by Berger, U.S. Pat. No. 7,192,550 for melt blowing.

An alternative method for making fine fibers has recently been introduced in U.S. Pat. No. 6,800,226 to Gerking which does not reduce the size of the spinneret holes, but rather adds a high velocity gas nozzle below a fairly conventional melt blowing spin pack. Gerking discloses that the high velocity gas causes the single polymer filament to spontaneously burst into a plurality of smaller filaments. Gerking, however, can not consistently reach the small fiber sizes achieved by the bicomponent methods.

Each of these methods have shortcomings. The bicomponent spun laid method using islands in the sea or segmented pie fibers requires post processing. The reduced spin hole size methods suffer from productivity reductions. Gerking's fiber bursting method suffers from a broad fiber size distribution with a significant amount of larger fibers. In addition, both Gerking and the new small hole methods rely on reducing the melt viscosity of the polymers which results in some loss of fiber properties. Thus, there still exists a need to produce fine fibers from melt polymers in a way that has high productivity and a narrow fiber size distribution.

SUMMARY OF THE INVENTION

In a preferred embodiment of the process a plurality of polymers is spun through a spin pack designed to produce islands in the sea or segmented pie bicomponent filaments and then through a converging diverging gas nozzle. The spin pack is designed such that the bicomponent filaments are extruded through a single row of holes. The tip of the spin pack is tapered to direct the gas flow toward the extruded filaments. The gas nozzle is designed with a converging diverging cross section so that the gas velocity may reach sonic or even supersonic velocity.

DESCRIPTION OF THE FIGURES

FIG. 1 shows an example of an “island in the sea” configuration

FIG. 2 shows an example of a segmented pie configuration.

FIG. 3 shows an example of a side by side configuration.

FIG. 4 shows an example of a spinning apparatus of the invention.

FIG. 5 shows an example of a configuration of gas nozzles.

FIG. 6 shows a second example of a configuration of gas nozzles.

DETAILED DESCRIPTION OF THE INVENTION

By “plurality” is meant more than one.

By “plural component configuration” in the context of polymer coextrusion is meant that a plurality of polymers form distinct extrudate phases that are present along the cross section of the entire length of the fiber. Each phase shares a boundary with at least one other phase and the number of phases does not necessarily equal the number of polymers in the plurality. In other words, some of the phases may be multicomponent.

The process of the invention is directed to a method for producing submicron fibers by melt spinning a plurality of polymers through a spinneret die in a plural component configuration and splitting the plural components into their individual parts by a high velocity gas nozzle.

Fiber from any melt processible polymers can be produced by this method, such as polyesters, polyamides, polyolefins and many other polymers, but it is preferable to choose polymers that will facilitate the bursting of the filaments along the defined interfaces of the components. For example, polyethylene terephthalate (PET) and polyethylene (PE) may be spun with polyester as the island polymer and polyethylene as the sea polymer. The choice of a high melt flow polyethylene with a standard viscosity polyester will enhance the bursting of the filaments along the weak boundaries without loss of polyester fiber properties due to excessive heating or polymer degradation.

When an island in the sea configuration is used then preferably the island fibers comprise more than 50%, or more preferably, more than 75% of the bicomponent filament, then the amount of sea polymer in the final product is reduced. Once the filaments have been burst into their individual components by the high velocity gas flow, there is no difficulty in separating the sea polymer from the island polymer. The bursting causes the island polymer to retain their shape as well defined filaments while the sea polymer fragments into particles and fibers. If a sea polymer is chosen such that it is an easily dissolvable polymer, such as polyvinyl alcohol, the removal issues in conventional bicomponent fibers are no longer present. As an alternative, the sea polymer can be chosen to add functionality to the final product. For example, in the combination cited above of PET/PE, the PE can be used as a bonding agent. A post heat treatment or calendering operation will cause the PE fragments and fibers to bond the PET filaments to create a strong nonwoven sheet.

Whether islands in the sea or segmented pie configurations are used, the final fiber size distribution in the nonwoven product will be more precise than conventional melt blowing. The fibers produced by the process of the invention do not have to be circular in cross section. It should be noted that as the percentage of island polymer is increased above about 50%, the island filaments tend toward hexagonal type packing creating flat sided filaments as opposed to circular cross sections. In addition, with segmented pie or hollow segmented pie filaments, the individual components are wedge shaped. In any of these cases, the diameter or minor dimensions of the individual components are controlled such that they are less than 1 micron.

Another preferred embodiment comprises the above method plus electrostatic charging of the filaments. Numerous methods for charging of the filaments would be known to one skilled in the art, such as Kubik U.S. Pat. No. 4,215,682; Deeds U.S. Pat. No. 5.122.048; and Moosmayer U.S. Pat. No. 4,904,174 all incorporated herein by reference. Any of these methods can be adapted to the current invention so that the individual filaments, once burst, will remain independent and not re-coalesce. The preferred method for inducing charge in the fibers is a corona discharge. Without meaning to be limited by mechanism, the addition of electrostatic charge induces a repulsive force between the individual filaments thus improving the overall fiber letdown.

The design of the spinneret of the process can best be appreciated with reference to the figures. In FIGS. 1-3 are shown examples of fiber configurations for a two polymer system. FIG. 1 shows an “island in the sea” configuration. FIG. 2 shows a “segmented pie” configuration, and FIG. 3 shows a side by side configuration. All three of these configurations can be used in the process of the invention, but the invention is not limited to them, and any configuration in which a plurality of phases coexist in the cross section of the fiber and along the length of the fiber can be used.

In FIG. 4 a schematic diagram shows the various major components of the apparatus. In the example shown in FIG. 4, two polymers are fed to the apparatus via inlets 41 and 42. The invention is not limited to two polymers and multiple inlets can be used.

The polymers then flow through a set of distribution plates (43) that feed the polymers to a tapered die tip (44). The polymers entering the tip are essentially in the desired configuration required before melt splitting, for example in the configurations of FIGS. 1-3.

Gas is fed to the apparatus through an inlet (48) and into a nozzle (45). The nozzle has the effect of accelerating the gas to a velocity in the range of 0.7 to 1.4 times the speed of sound. Fiber and gas then exit the apparatus together, and optionally past needles (46) to which an electrostatic charge is applied. The needles (46) are mounted in an electrostatic insulator plate (47) to prevent arcing to the bottom of the spin pack.

The gas nozzles may be arranged in the bottom plate as a row of individual circular nozzles corresponding to the polymer die holes on a one to one basis as shown in FIG. 5. Alternatively, the gas nozzle may be configured as a slot jet as shown in FIG. 6.

Although the invention has been described herein in a particular configuration, it will be understood that one skilled in the art will be able to make changes to the process and apparatus described here that fall within the scope of the invention and the claims below.

Claims

1. A method for producing submicron fibers by melt spinning a plurality of polymers through a spinneret die in a plural component configuration to produce a multiconstituent fiber, and splitting the plural components into their individual constituents by a high velocity gas stream, said high velocity gas stream being applied to the fiber while it exits the die in a way that multiple fibers are formed from at least one of the individual constituents.

2. The method of claim 1 in which the gas stream has a velocity of between 0.7 and 1.4 times the speed of sound.

3. The method of claim 1 in which the plural component configuration is “islands in the sea” or “segmented pie”.

4. The method of claim 1 further comprising the step of applying an electrostatic charge to the filaments.

5. An apparatus comprising a spin pack with distribution channels and orifices arranged such that a plurality of polymers can be coextruded in a plural component configuration, and a converging diverging gas nozzle.

6. An apparatus of claim 5 further comprising electrostatic corona discharge needles or bars.

7. A method for separating the individual components of multiple component fibers comprising co-extruding the filaments through a spinneret die and splitting the plural components into their individual parts by a high velocity gas nozzle prior to complete solidification of the components.