Plastic and nonmetallic article shaping or treating: processes – Formation of solid particulate material directly from molten... – With subsequent uniting of the particles
Patent
1992-07-22
1993-11-09
Theisen, Mary Lynn
Plastic and nonmetallic article shaping or treating: processes
Formation of solid particulate material directly from molten...
With subsequent uniting of the particles
156167, 264 12, 264518, 264115, 425 7, B29B 910
Patent
active
052600033
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention is based on a process for producing microfibres and non-woven microfibre webs from thermoplastic polymers by the melt-blowing technique in which a polymer melt flows through at least one orifice in a melt die and is separated into fibers by a gas which impinges on the melt from both sides immediately after its exit from the orifices. The invention also relates to a device for carrying out the process. The melt-blowing process has been disclosed in numerous publications (see e.g. U.S. Pat. Nos. 3,755,527, 3,978,185, 4,622,259 and 3,341,590), and German Patent No. 2,948,821. According to the melt-blowing technique a stream of polymer melt extrusion issuing from a melt orifice is separated into individual fibers and drawn out while attenuated by means of an inert gas, in most cases air, which has a temperature higher than or equal to the temperature of the melt and is blown against the melt in the direction of flow. One main object is to increase the economic efficiency of the process by appropriately regulating the melt viscosity. Thus the prior art discloses the use of polymers with an extremely low viscosity and correspondingly high extrusion flow rates, since this enables relatively fine fibers to be produced with a lower degree of energy consumption by reducing the temperature of the melt and the gas stream. The following parameters are known to have a crucial effect on the economic efficiency of the process: the melt per orifice, rate over the whole length of the die,
According to the prior art the gas temperature is adjusted to a value higher than or equal to the temperature of the melt. In all known processes the gas stream issues from the die in direct proximity to the melt orifices and on either side thereof via exit slots arranged in the longitudinal direction of the die. Complicated hydrodynamic brake means and air distributing systems have to be provided in the gas inlets to ensure a uniform rate of flow over the entire slot length. PCT application WO 87/04195 describes appropriate technical means for achieving optimum results.
The use of relatively large gas exit slots (1 mm to 3 mm) has also been disclosed. One disadvantage of this method is the high quantity of gas required since high rates of flow are necessary in particular for the production of very fine fibers of an average diameter of <3 .mu.m. The rate of flow at the slot exit is usually 0.5 to 0.7 times the sonic speed of the gas (0.5 V.sub.s to 0.7 V.sub.s ; V.sub.s =sonic speed).
SUMMARY OF THE INVENTION
A principle object of the invention is to provide an additional increase in the economic efficiency of the melt-blowing process. In particular the object is to provide higher economic efficiency in the production of fibers of mean fibre diameters of <10 .mu.m, preferably <5 .mu.m. An additional object is to considerably increase the melt throughput per orifice and thus the total spinning capacity of the installation when producing fibers of mean diameters of between 0.5 .mu.m and 3 .mu.m.
According to the invention the above objects are achieved by accelerating the rate of flow of the gas to supersonic speed in Laval nozzles arranged mirror-symmetrically in relation to the melt orifices and reducing the rate of flow of the gas in flow channels arranged downstream of the Laval nozzles, and having a constant cross-section or a cross-section tapering in the direction of flow to a rate just below sonic speed and by directing the polymer melt into the gas stream issuing from the flow channels. "Just below sonic speed" is understood to be a range between 0.8 V.sub.s, preferably 0.9 V.sub.s and 0.99 V.sub.s (0.8 V.sub.s <V<0.99 V.sub.s, preferably 0.9 V.sub.s <V<0.99 V.sub.s). Whereas in the known melt-blowing process the rate of flow of the gas stream at the exit to the slot nozzles is considerably lower than sonic speed, the solution provided by the invention is based on a gas exit speed just below sonic speed ("transonic speed") which is obtained in a particular manner. Th
REFERENCES:
patent: 3806289 (1974-04-01), Schwarz
patent: 4001357 (1977-01-01), Walz et al.
patent: 4919854 (1990-04-01), Walz
patent: 5080569 (1992-01-01), Gubernick et al.
Berkenhaus Dirk
Kreimer Armin
Nyssen Peter R.
Wagner Wolfram
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