METHOD AND APPARATUS FOR
ULTRASONICALLY ASSISTED MELT
EXTRUSION OF FIBERS
This application is a divisional of application Ser. No. 08/992,862 now U.S. Pat. No. 6,036,467 entitled Apparatus for Ultrasonically Assisted Melt Extrusion of Fibers filed in the U.S. Patent and Trademark Office on Nov. 25, 1997, which is a Rule 62 file-wrapper-continuation of Ser. No. 08/721,773 entitled Apparatus for Ultrasonically Assisted Melt Extrusion of Fibers and filed in the U.S. Patent and Trademark Office on Sep. 25, 1996, now abandoned, which in turn is a continuation of application Ser. No. 08/264,548 entitled Method and Apparatus for Ultrasonically Assisted Melt Extrusion of Fibers and filed in the U.S. Patent and Trademark Office on Jun. 23, 1994, now abandoned. The entirety of application Ser. No. 08/264,548 is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The present invention relates to the melt extrusion of a thermoplastic polymer.
The melt extrusion of a thermoplastic polymer to form fibers and nonwoven webs generally involves forcing a molten polymer through a plurality of orifices to form a plurality of molten threadlines, contacting the molten threadlines with a fluid, usually air, directed so as to form filaments or fibers and attenuate them. The attenuated filaments or fibers then are randomly deposited on a surface to form a nonwoven web.
The more common and well known processes utilized for the preparation of nonwoven webs are melfblowing, coforming, and spunbonding.
Meltblowing references include, by way of example, U.S. Pat. No. 3,016,599 to Perry, Jr., U.S. Pat. No. 3,704,198 to Prentice, U.S. Pat. No. 3,755,527 to Keller et al, U.S. Pat. No. 3,849,241 to Butin et al., U.S. Pat. No. 3,978,185 to Butin et al, and U.S. Pat. No. 4,663,220 to Wisneski et al., See, also, V. A. Wente, "Superfine Thermoplastic Fibers", Industrial and Engineering Chemistry, Vol. 48, No. 8, pp. 1342-1346 (1956); V. A. Wente et al, "Manufacture of Superfine Organic Fibers", Navy Research Laboratory, Washington, D.C., NRL Report 4364 (111437), dated May 25,1954, United States Department of Commerce, Office of Technical Services; and Robert R. Butin and Dwight T. Lohkamp, "Melt Blowing—A One-Step Web Process for New Nonwoven Products", Journal of the Technical Association of the Pulp and Paper Industry, Vol. 56, No.4, pp. 74-77 (1973).
Coforming references (i.e., references disclosing a meltblowing process in which fibers or particles are commingled with the meltblown fibers as they are formed) include U.S. Pat. No. 4,100,324 to Anderson et al. and U.S. Pat. No. 4,118,531 to Hauser.
Finally, spunbonding references include, among others, U.S. Pat. No. 3,341,394 to Kinney, U.S. Pat. No. 3,655,862 to Dorschner et al., U.S. Pat. No. 3,692,618 to Dorschner et al, U.S. Pat. No. 3,705,068 to Dobo et al, U.S. Pat. No. 3,802,817 to Matsuki et al., U.S. Pat. No. 3,853,651 to Porte, U.S. Pat. No. 4,064,605 to Akiyama et al., U.S. Pat. No. 4,091,140 to Harmon, U.S. Pat. No. 4,100,319 to Schwartz, U.S. Pat. No. 4,340,563 to Appel and Morman, U.S. Pat. No. 4,405,297 to Appel and Morman, U.S. Pat. No. 4,434,204 to Hartman et al., U.S. Pat. No. 4,627,811 to Greiser and Wagner, and U.S. Pat. No. 4,644,045 to Fowells.
Some of the difficulties or problems routinely encountered with melt extrusion processes are, by way of illustration
only, thermal degradation of the polymer, plugging of extrusion dies, and limitations on fiber diameters, throughput, and production rates or line speeds. Fiber diameters generally are a function of the diameter of the orifices through which the
5 polymer is extruded, although the temperature and velocity of the attenuating fluid can have a significant effect. For some applications, fiber diameters of less than about 10 micrometers are desired. Throughput primarily is a function of the melt flow rate of the polymer, while production rates
10 depend in large measure upon throughput. In other words, throughput and production rates generally are dependent upon the viscosity of the molten polymer being extruded. The difficulties and problems just described result largely from efforts to manipulate melt viscosity to achieve desired
15 throughput and/or production rates. Accordingly, there are opportunities for improvements in melt extrusion processes based on improved melt viscosity control.
SUMMARY OF THE INVENTION
20 The present invention addresses some of the difficulties and problems discussed above by providing an apparatus and a method for the melt extrusion of a thermoplastic polymer, e.g., as fibers and nonwoven webs, which apparatus and method utilize ultrasonic energy to assist in the
25 melt-extrusion process. The apparatus includes a die housing and a means for applying ultrasonic energy to a portion of the molten thermoplastic polymer. The die housing defines a chamber adapted to receive the molten thermoplastic polymer an inlet orifice adapted to supply the cham
30 ber with the molten thermoplastic polymer, and an extrusion orifice adapted to receive the molten thermoplastic polymer from the chamber and extrude the polymer. The means for applying ultrasonic energy is located within the chamber.
35 In one aspect of the present invention, the die housing has a first end and a second end and the extrusion orifice is adapted to receive the molten thermoplastic polymer from the chamber and extrude the polymer along a first axis. The means for applying ultrasonic energy to a portion of the
4Q molten thermoplastic polymer is an ultrasonic horn having a
first end and a second end. The horn is adapted, upon
excitation by ultrasonic energy, to have a node and a
longitudinal mechanical excitation axis. The horn is located
in the second end of the die housing in a manner such that
, the first end of the horn is located outside of the die housing 45 & and the second end is located inside the die housing, within
the chamber, and is in close proximity to the extrusion
The molten thermoplastic polymer may be extruded as, by
50 way of example, a fiber. In such case, the longitudinal excitation axis of the ultrasonic horn desirably will be substantially parallel with the first axis. Furthermore, the second end of the horn desirably will have a cross-sectional area approximately the same as or less than a minimum area
55 which encompasses all extrusion orifices in the die housing. The present invention contemplates the use of an ultrasonic horn having a vibrator means coupled to the first end of the horn. Typically, the vibrator means will be a piezoelectric transducer. The transducer may be coupled directly
go to the horn or by means of an elongated waveguide. The elongated waveguide may have any desired input:output mechanical excitation ratio, although ratios of 1:1 and 1.5:1 are typical for many applications. The ultrasonic energy typically will have a frequency of from about 18 kHz to
65 about 60 kHz.
The present invention also contemplates a method of forming a fiber. The method involves supplying a molten