The production of nonwoven fabrics has long used melt-blown, coform and other techniques to produce webs for use in forming a wide variety of products. As used herein the term "meltblown fibers" means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into converging high velocity, usually heated, gas (e.g. air) streams which attenuate the filaments of molten thermoplastic material to reduce their diameter, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly disbursed meltblown fibers. Such a process is disclosed, for example, in U.S. Pat. No. 3,849,241 to Butin. Meltblown fibers are microfibers which may be continuous or discontinuous, are generally smaller than 20 and preferably less than 10 microns in average diameter, and are generally selfbonded when deposited onto a collecting surface. FIGS. 1a through 1c illustrate prior art machines which manufacture non-woven webs from melt-blown techniques. Additionally, prior art coform techniques are discussed in greater detail hereinafter.
FIGS. 1a-1c illustrate a typical approach for producing melt-blown fibers and nonwovens. Referring to FIG. 1a, a hopper 10 contains pellets of resin. Extruder 12 melts the resin pellets by a conventional heating arrangement to form a molten extrudable composition which is extruded through a melt-blowing die 14 by the action of a turning extruder screw (not shown) located within the extruder 12. As shown in FIG. 1c, the extrudable composition is fed to the orifice 18 through extrusion slot 28. The die 14 and the gas supply fed therethrough are heated by a conventional arrangement (not shown).
FIG. 1b illustrates the die 14 in greater detail. The tip 16 of die 14 contains a plurality of melt-blowing die orifices 18 which are arranged in a linear array across the face 16. Referring now to FIG. 1c, inlets 20 and 21 feed heated gas to the plenum chambers 22 and 23. The gas then exits respectively through the passages 24 and 25 to converge and form a gas stream which captures and attenuates the polymer or resin threads extruded from orifice 18 to form a gas borne stream of fibers 26 as is seen in FIG. 1a.
The melt-blowing die 14 includes a die member 36 having a base portion 38 and a protruding central portion 39 within which an extrusion slot 28 extends in fluid communication with the plurality of orifices 18, the outer ends of which terminate at the die tip. The gas borne stream of fibers 26 is projected onto a collecting device which in the embodiment illustrated in FIG. 1a includes a foraminous endless belt 30 carried on rollers 31 and which may be fitted with one or more stationary vacuum chambers (not shown) located beneath the collecting surface on which a non-woven web 34 of fibers is formed. The collected entangled fibers form a coherent web 34. The web 34 may be removed from the belt 30 by a pair of pinch rollers 33 (shown in FIG. 1a) which press the entangled fibers together. The prior art melt-blowing apparatus of FIGS. 1a-1c may optionally include pattern-embossing means as by patterned calender nip or ultrasonic embossing equipment (not shown) and web 34 may thereafter be taken up on a storage roll or passed to subsequent manufacturing steps. Other embossing means may be utilized such as the pressure nip between a calender and an anvil roll, or the embossing step may be omitted altogether.
It is well known in the art to vary a number of processing parameters in melt-blown fiber forming processes to obtain fibers of desired properties in order to form fabrics with desired characteristics. However, the majority of prior art techniques for varying fiber characteristics require more time consuming changes in machinery or process, such as changing dies or changing the resins. Therefore, those techniques may require that the production line be halted while the necessary changes are made, which results in inefficiency when a new material is to be run.
The prior art has previously taught that various effects can be obtained by the manipulation of air flow near the fiber exit in melt-blown fiber producing equipment. For example, Shambaugh, U.S. Pat. No. 5,405,559, teaches that the air flow provided in the melt-blown process can be alternately turned on and off on both sides of the die, thus reducing the energy required to produce melt-blown fiber. However, this teaching of Shambaugh has several drawbacks. Under some conditions, the complete shutting off of the air on either side will tend to blow the liquefied resin onto the air plates on the other side of the die, thereby clogging the machinery for typical production airflow rates (especially with high MFR polymers or other polymers normally used in non-woven web production). Further, such techniques would likely result in the deposition of resin globs or "shot" on the production web since the resin would be affected only minimally during the transition from airflow on one side of the die to the other. Finally, while the Shambaugh reference teaches switching air on and off for the purposes of reducing fiber size for a given flow, its main emphasis is that such switching saves energy by reducing the overall airflow requirements in the melt-blown process. Moreover, the low frequencies taught by Shambaugh would result in poor formation on a high speed machine. Fibers produced as given in the examples are coarser, e.g. larger diameters than typically found in non-woven commercial production.
U.S. Pat. No. 5,075,068, teaches the use of a steady state shearing air stream near the exit of the die in the melt-blown process for the purpose of increased drag on fibers exiting the die. The steady state air stream therefore draws the fibers further and enhances the quenching of the fibers. However, this patent teaches steady state airflow characteristics for varying fiber parameters in a spunbond fiber for producing a better fiber, but does not teach that airflow characteristics may be selectively altered to vary the characteristics of fibers in a desired manner.
Finally, U.S. Pat. No. 5,312,500, teaches alternating airflows at the exit of a spunbond fiber draw unit for laying a continuous fiber down in an elliptical fashion to form a non-woven web. This patent teaches that, among other techniques, varying airflows may direct fibers onto a foraminous forming surface to form a non-woven web. By varying the manner in which the fibers are deposited using airflow variation, this reference states that the characteristics of the web may be enhanced. However, this reference does not teach that the airflows may be used to enhance or vary the characteristics of the fibers themselves.
Therefore, it is an object of the present invention to provide highly sorbent meltblown and coform non-woven webs having desired characteristics through the production of fibers using perturbed airflows during fiber formation.
It is yet another object of the present invention to provide a process and apparatus for the formation of fibers and nonwovens having specific, desired characteristics by the simple, selective variation of the frequency and/or amplitude of perturbation of air flow during the production of the fibers.
It is yet a further object of the present invention to provide processes and apparati, using selective variation of the frequency and/or amplitude of a perturbing airflow in the formation of fibers, which allow for the production of non-woven webs and fabrics having desired characteristics.