Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Producing multilayer work or article
Reexamination Certificate
2000-02-02
2002-01-15
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Producing multilayer work or article
C156S148000, C156S167000, C156S176000, C156S181000, C156S244240, C156S244270, C156S436000, C156S437000, C156S441000, C156S500000, C264S103000, C264S210800, C264S211120, C264S211140, C264S555000, C425S072200, C425S363000, C425S382200, C425S383000, C425S464000
Reexamination Certificate
active
06338814
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods and apparatus for forming spunbond webs and, more particularly, to formation of spunbond webs by deflection of extruded fibers onto a non-horizontal web-forming surface, such as a moving screen belt.
2. Description of the Related Art
Non-woven fabrics made from melt-spinnable polymers are commonly produced using spunbond processes. The term “spunbond” refers to a process of forming a non-woven fabric or web from an array of thin, melt-spun polymeric fibers or filaments produced by extruding molten polymer from orifices (the orifices can be, for example, those of a long, generally rectangular spinneret or of a plurality of spinnerets). Below the spinneret, the extruded fibers form a vertically oriented curtain of downwardly moving strands that are at least partially quenched before entering a long, slot-shaped air aspirator positioned below the spinneret. The aspirator introduces a rapid downwardly moving air stream produced by compressed air from one or more air aspirating jets. The air stream creates a drawing force on the fibers, causing them to be drawn between the spinneret and the air jet, thereby attenuating the fibers. Upon exiting at the bottom of the aspirator, the drawn fibers are randomly laid on a forming surface, such as a moving conveyor screen belt (e.g., a Fourdrinier wire), to form a continuous non-woven web of fibers. The web is subsequently bonded using one of several known techniques to form a stable, non-woven fabric. A common bonding method involves lifting the web from the moving screen belt and passing the web through two heated calender rolls. Often, one of the rolls is embossed, causing the web to be bonded in numerous spots. Carded or air-laid webs can also be formed from such polymeric fibers.
It has long been understood that the distances between the spinneret, aspirator and web forming surface are important parameters in the formation of non-woven webs, and the ability to adjust these distances is highly desirable, if not essential, in any apparatus designed to produce a variety of non-woven webs having different properties (e.g., denier, weight, texture, polymer composition, etc.). For example, in U.S. Pat. No. 3,802,817 to Matsuki et al., the disclosure of which is incorporated herein by reference in its entirety, it is recognized that good fiber formation and high production require optimization of the distance L from the spinneret to the aspirator (i.e., the “spinning” distance), and optimum web formation requires optimization of the distance M from the aspirator to the wire belt (i.e., the “laydown” distance). Unless the machine is dedicated to producing only one type of fabric, these distances must be varied in accordance with the desired fiber denier, the particular polymer being spun, and the fiber cross-sectional shape. In a large production spunbond machine, the spin beam containing the spin pack assembly (of which the spinneret is the bottom element) has a weight of several tons, since the spin beam is a pressure vessel using boiling Dowtherm or a similar liquid/vapor material to provide uniform heat. The beam also contains metering pumps to control the polymer rate through the spinneret. For an apparatus making a web of three meters or more in width, the forming table, which controls and drives the Fourdrinier wire, also weighs several tons. The aspirator is not quite as heavy and is typically more compact than the beam and forming table; nevertheless, the aspirator may weigh over one ton for a three-meter machine.
Conventionally, two of the three major machine elements must be moveable in the vertical direction to enable independent adjustment of the spinning distance L and the laydown distance M. Moving the spin beam is particularly problematic, since the spin beam is fed molten polymer through heated piping from a screw extruder that also weighs several tons. If the piping is fitted with rotary joints, it is possible to raise and lower the beam while the extruder is at a fixed height; however, these rotary joints are prone to leakage when operated at 280° C. and over 1000 psig polymer pressure, which are normal operating conditions. The extruder can be raised and lowered with the beam to allow shorter piping and no rotary joints, but then the additional several ton weight of the extruder must be lifted along with the beam.
If, instead, the spin beam remains at fixed height and the forming table and aspirator are adjusted vertically, considerable additional expense will be incurred in providing the capability to move these components, particularly the forming table. Also, if the forming table is moved vertically, independent adjustment of spinning distance L cannot be achieved on a machine with multiple beams and aspirators. Independent adjustment is desirable in any number of circumstances where plural beams and aspirators produce plural arrays of fibers. One example is in the production of a laminated fabric where, for example, there are three laydown zones and the middle zone produces fibers of much finer denier than the first and second zones, capturing a fine denier web between two coarser denier webs. Such independent adjustments cannot be achieved by raising or lowering a horizontal forming table and are very expensive if achieved by individually raising or lowering each spin beam with its related piping, pump drives and quench ducts.
Another problem encountered with spunbond processes occurs when the spinning distance L is kept particularly short in order to permit formation of fine fibers at high production speeds. U. S. Pat. No. 5,545,371 to Lu, the disclosure of which is incorporated herein by reference in its entirety, describes a process wherein the distance L is less than 50 cm, and this distance is adjusted to control the diametric size of the filaments (fiber denier). Adjusting the spinning distance L will also change laydown distance M, unless laydown distance M can be independently adjusted by raising or lowering the screen wire, as is shown in the Lu patent. When the spinning distance L is short (e.g., less than 50 cm), the process can be operated under certain conditions of polymer flow (normally expressed in grams/spinneret orifice/minute) and aspirator air velocity. However, if the polymer flow or air velocity is too high, the fibers will not cool sufficiently and will break between the spinneret and the aspirator. Each break is followed by a drip of polymer at the leading end of the new fiber that is forming from the spinneret orifice where the break occurred. If the process conditions are such that very few fiber breakages occur, economical web production can take place. On the other hand, if frequent fiber breakages occur, these drips can land on the Fourdrinier belt while still hot and become bonded to the belt as the belt passes through the compaction rolls. The belt must then be stopped and production lost in order to periodically clean the belt. Until the belt is cleaned, the drips stuck to the belt can snag fibers in the web and cause web damage.
Accordingly, there remains a need to improve upon conventional spunbond web formation techniques for adjusting the spinning and laydown distances and to mitigate the detrimental effects of polymer drips that occur during spunbond web formation.
SUMMARY OF THE INVENTION
Therefore, in light of the above, and for other reasons that become apparent when the invention is fully described, an object of the present invention is to provide a process and apparatus that significantly reduces the cost of spunbond machinery by eliminating the need to raise and lower the spin beam and the forming table while preserving the ability to independently adjust the spinning distance L and the laydown distance M.
Another object of the present invention is to separate a significant portion of polymer drips from the extruded fibers before the drips can land on the web-forming belt, thereby reducing belt damage and downtime resulting from polymer drips.
Yet another object of the present inven
Hills, Inc.
Tentoni Leo B.
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