The present invention generally relates to the formation of fibers and nonwoven webs by meltblowing processes. More particularly, the present invention relates to an improved die assembly of a meltblowing apparatus.
The formation of fibers and nonwoven webs by meltblowing is well known in the art. See, by way of example, U.S. Pat. No. 3,016,599 to R. W. Perry, Jr.; U.S. Pat. No. 3,704,198 to J. S. Prentice; U.S. Pat. No. 3,755,527 to J. P. Keller et al.; U.S. Pat. Nos. 3,849,241, 3,978,185 to R. R. Butin et al.; U.S. Pat. No. 4,100,324 to R. A. Anderson et al.; U.S. Pat. No. 4,118,531 to E. R. Hauser; and U.S. Pat. No. 4,663,220 to T. J. Wisneski et al.
Briefly, meltblowing is a process type developed for the formation of fibers and nonwoven webs; the fibers are formed by extruding a molten thermoplastic polymeric material, or polymer, through a plurality of small holes. The resulting molten threads or filaments pass into converging high velocity gas streams which attenuate or draw the filaments of molten polymer to reduce their diameters. Thereafter, the meltblown fibers are carried by the high velocity gas stream and deposited on a collecting surface, or forming wire, to form a nonwoven web of randomly disbursed meltblown fibers.
Generally, meltblowing utilizes a specialized apparatus to form the meltblown webs from a polymer. Often, the polymer flows from a die through narrow cylindrical outlets and forms meltblown fibers. The narrow cylindrical outlets may be arrayed in a substantially straight line and lie in a plane which is the bisector of a V-shaped die tip. Typically the angle formed by the exterior walls or faces of the V-shaped die tip is 60 degrees and is positioned proximate to a pair of air plates, thereby forming two slotted channels therebetween along each face of the die tip. Thus, air may flow through these channels to impinge on the fibers exiting from the die tip, thereby attenuating them. As a result of various fluid dynamic actions, the air flow is capable of attenuating the fibers to diameters of from about 0.1 to 10 micrometers; such fibers generally are referred to as microfibers. Larger diameter fibers, of course, also are possible, with the diameters ranging from around 10 micrometers to about 100 micrometers.
In these processes, the polymer is heated to a temperature that will allow extrusion through the die outlets, which typically are about 0.1 inch (0.25 centimeter) long. The portion of the die tip in which the outlets are located is referred to herein as the die tip apex. The attenuating air is typically heated to maintain the temperature of the die tip and the exiting polymer to allow extrusion to proceed without plugging the outlets. The meltblown equipment generally utilizes air that is about the same temperature as the expelled polymer. Because the polymer and air velocities are the highest in the vicinity of the die tip apex, the transfer of heat from the die tip and the molten polymer exiting from the outlets is the greatest in that vicinity as well. Maintaining the air temperature as just described aids in keeping the polymer in the outlets hot and the viscosity of the exiting polymer low.
However, it has been recognized that there are many advantages to using as a primary drawing medium attenuating air that is much cooler than the temperature of the polymer within the die tip and exiting from the outlets. One advantage is that the fibers quench more rapidly and efficiently, resulting in a softer web and less likelihood of xe2x80x9cshotxe2x80x9d, which, in one form, consists of fibers melted on the forming wire which form a stiff polymeric mass. Another advantage is that faster quenching may reduce the required forming distance between the die tip and the forming wire, thereby permitting the formation of webs with better properties, such as appearance, coverage, opacity, and strength.
With current die designs, the utilization of attenuating air at temperatures lower than those of the die tip and the exiting polymer would result in heat being transferred from polymer still present in the die tip. This loss of heat would increase the viscosity of the polymer and raise the pressure within the die tip to unacceptable levels. Furthermore, the increase in viscosity may be so extreme as a result of the temperature drop within the die tip to cause the polymer to practically solidify and plug the die tip.
Accordingly, there is a need for a meltblowing die that concentrates or focuses heat at the die tip, thereby permitting the use of attenuating air having temperatures significantly below the temperatures of the die tip and the polymer exiting therefrom.
The present invention addresses some of the difficulties and problems discussed above by providing a die that focuses heat at the die tip, and in particular at the die tip apex, by means other than heated attenuating air. Advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.
One embodiment of the present invention is an apparatus for forming meltblown material. The apparatus may include a die having a die tip and a heating element positioned proximate to the die tip. Furthermore, the die may include a body and a die tip apex. The body and die tip may form a passageway for expelling polymer, and still further, the die may include at least one air plate. The air plate and die tip may form channels for the passage of air. The heating element may radiate heat to the die tip. Also, the heating element may transfer heat to the die tip apex, and furthermore, may directly radiate heat to the die tip apex. Moreover, the heating element may be an infrared lamp having a periphery coated with a reflective material around a portion of the periphery. Additionally, the polymer may be about 150xc2x0 C. hotter than the air passing through the channels.
Another embodiment of the present invention is an apparatus for forming meltblown material that may include a die having a tip wherein at least one heating element may be embedded in the tip. Moreover, the heating element may be an electrical heating cartridge.
Still another apparatus for forming meltblown material may include a die having a die tip terminating in a die tip apex. The die tip may form at least one internal fluid passageway proximate to the die tip apex. The fluid passageway may be a conduit for a heated fluid for heating the die tip apex. Moreover, the die tip may form at least four internal fluid passageways for heating the die tip apex. Additionally, the internal fluid passageways may transport a fluid selected from the group comprising steam, oil, air, water, liquid metals, wax, and polymers. Furthermore, the fluid passageways may extend across the length of the die.
A further apparatus for forming a meltblown material may include a die. The die may further include a die tip terminating in a die tip apex and electrodes coupled to the die tip. A current may flow between electrodes heating the tip. Additionally, the current may flow the length of the die or alternatively, over the die tip apex. Furthermore, the die tip may form a passageway for expelling materials for forming a meltblown web and at least one electrode is positioned on either side of the passageway. Moreover, the apparatus may further include an electrical insulating layer.