Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Extrusion molding
Reexamination Certificate
2001-05-16
2004-03-30
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Extrusion molding
C264S176100, C425S135000, C425S145000, C425S166000, C425S17480E, C425S224000, C425S464000
Reexamination Certificate
active
06713011
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates to an apparatus and methods for electrospinning polymer fibers and membranes.
Electrospinning is an atomization process of a conducting fluid which exploits the interactions between an electrostatic field and the conducting fluid. When an external electrostatic field is applied to a conducting fluid (e.g., a semi-dilute polymer solution or a polymer melt), a suspended conical droplet is formed, whereby the surface tension of the droplet is in equilibrium with the electric field. Electrostatic atomization occurs when the electrostatic field is strong enough to overcome the surface tension of the liquid. The liquid droplet then becomes unstable and a tiny jet is ejected from the surface of the droplet. As it reaches a grounded target, the material can be collected as an interconnected web containing relatively fine, i.e. small diameter, fibers. The resulting films (or membranes) from these small diameter fibers have very large surface area to volume ratios and small pore sizes. However, no practical industrial process has been implemented for electrospinning membranes containing a high percentage of small, e.g., nanosize, fibers. This is because with the production of small fibers, such as nanosize fibers, the total yield of the process is very low and a scale-up process, which maintains the performance characteristics of the films (or membranes), cannot be easily achieved.
U.S. Pat. No. 4,323,525 is directed to a process for the production of tubular products by electrostatically spinning a liquid containing a fiber-forming material. The process involves introducing the liquid into an electric field through a nozzle, under conditions to produce fibers of the fiber-forming material, which tend to be drawn to a charged collector, and collecting the fibers on a charged tubular collector which rotates about its longitudinal axis, to form the fibrous tubular product. It is also disclosed that several nozzles can be used to increase the rate of fiber production. However, there is no suggestion or teaching of how to control the physical characteristics of the tubular product, other than by controlling the charge and rotation speed of the tubular collector. For example, there is no teaching or suggestion of controlling jet formation, jet acceleration or fiber collection for individual jets. It is further noted that the spinning process of the '525 patent is used to fabricate tubular products having a homogenous fiber matrix across the wall thickness.
U.S. Pat. No. 4,689,186 is directed to a process for the production of polyurethane tubular products by electrostatically spinning a fiber-forming liquid containing the polyurethane. It is disclosed that auxiliary electrodes can be placed around the collector to help facilitate collection of the fibers. It is disclosed that the auxiliary electrodes can be arranged to facilitate separation or to prevent adhesion of the formed fibers. There is no teaching or suggestion of independently controlling jet formation, jet acceleration and fiber collection. It is also noted that the spinning process of the '186 patent is used to fabricate tubular products having a homogenous fiber matrix across the wall thickness.
The above mentioned references do not address the problems associated with producing membranes or other articles on an industrial scale, without adversely affecting the performance characteristics of the resulting products.
Thus, there is a need for improved electrospinning methods for producing fibers and membranes on an industrial scale which do not have the above-mentioned disadvantages.
SUMMARY OF INVENTION
According to the present invention, it has now been found that polymeric fibers can be produced by an elecrospinning process having improved control over fiber formation and transportation. In addition, membranes can be produced by electrospinning with the apparatus and according to the methods of the present invention on an industrial scale without the above-mentioned disadvantages.
In one aspect, the invention relates to a method for electrospinning a polymer fiber from a conducting fluid containing a polymer in the presence of a first electric field established between a conducting fluid introduction device and a ground source, which includes modifying the first electric field with a second electric field to form a jet stream of the conducting fluid. The conducting fluid introduction device is preferably a spinneret.
The second electric field can be established by imposing at least one field modifying electrode on the first electrostatic field. The field modifying electrode can be a plate electrode positioned between the conducting fluid introduction device and the ground source.
Preferably, the method includes feeding the conducting fluid to the conducting fluid introduction device at a controlled rate. The rate can be controlled by maintaining the conducting fluid at a constant pressure or constant flow rate.
In one embodiment, the method also involves controlling the electrical field strength at the spinneret tip by adjusting the electric charge on the field modifying electrode to provide a controlled diameter fiber.
In another embodiment, the method includes imposing a plurality of electrical field modifying electrodes to provide a controlled distribution of electrostatic potential between the spinneret and the ground source.
In another aspect, the invention relates to a method for electrospinning a polymer fiber from a conducting fluid containing a polymer in the presence of an electric field established between a spinneret and a ground source, which includes:
a) forming an electrospinning jet stream of the conducting fluid; and
b) electrically controlling the flow characteristics of the jet stream.
The flow characteristics of the jet stream can be electrically controlled by at least one electrode. The flow characteristics of the jet stream can also be electrically controlled by at least one pair of electrostatic quadrupole lenses. Preferably, the flow characteristics of the jet stream are electrically controlled by a plurality of pairs of electrostatic quadropole lenses and, more preferably, by also using an alternating gradient technique.
In one embodiment, the method involves electrically controlling the flow characteristics of the jet stream to provide a controlled pattern over a desired target area. The controlled pattern can be provided by applying a waveform to the potential on at least one pair of electrostatic quadropole lenses.
In yet another aspect, the invention relates to a method for forming a controlled-dimension and controlled-morphology membrane by electrospinning a plurality of polymer fibers from conducting fluid containing a polymer in the presence of an electric field established between a solution introduction device and a ground source, in which the method includes:
a) forming a plurality of electrospinning jet streams of the conducting fluid; and
b) independently controlling the flow characteristics of at least one of the jet streams.
Preferably, the flow characteristics of at least one of the jet streams are electrically controlled by at least one scanning electrode, more preferably, by at least one pair of scanning electrodes.
In one embodiment, the solution introduction device consists of a plurality of electrospinning spinnerets. Preferably, each spinneret produces an individual jet stream of the conducting fluid and, more preferably, the flow characteristics of each individual jet stream can be independently controlled.
Preferably, each spinneret has at least one scanning electrode for electrically controlling the flow characteristics of the individual jet stream. More preferably, each spinneret has two pairs of scanning electrodes for electrically controlling the flow characteristics of the individual jet stream.
It is contemplated that at least two spinnerets can deliver different solutions, wherein different solutions refers to different concentrations of polymer, different polymers, different polymer blends, different additives
Chu Benjamin
Fang Dufei
Hsiao Benjamin S.
Hoffmann & Baron , LLP
Tentoni Leo B.
The Research Foundation at State University of New York
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