Chemistry: molecular biology and microbiology – Apparatus – Including condition or time responsive control means
Patent
1997-04-11
2000-06-13
Zirker, Daniel
Chemistry: molecular biology and microbiology
Apparatus
Including condition or time responsive control means
435 912, 4352886, 422101, B01L 1100, B01D 2702, C12M 302, D04H 104
Patent
active
060748699
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD OF THE INVENTION
The present invention pertains to fibrous webs, particularly melt-blown fibrous webs, and their preparation. Such fibrous webs are particularly suitable as filtration media, and for separating plasma or serum from blood. They are also useful wherever a substantially uniform porous medium is desirable.
BACKGROUND OF THE INVENTION
In one existing melt-blowing process, molten resin is extruded through a row of linearly disposed holes drilled in linear array on about 1 to 2 mm centers into a flat surface about 1 to 2 mm wide, with the surface being located as shown in FIG. 1 at the apex of a member having a triangular cross section, and with the angles at the apex being about 45.degree. to 60.degree. to the center line. Surrounding the apex 11 as shown in FIG. 1 are two slots 12, 13, one on each side, through which is delivered heated air, which attenuate the molten resin extruded through the holes, thereby forming a stream of fibers. The fibers are collected on one side of a moving screen which is separated from the nozzle tips by about 10 or more cm, with the other side of the screen being connected to a suction blower. In operation, most of the fibers are collected on the screen to form a low density web with a rough surface; however, a significant proportion of the fibers escape into the surroundings, and a suction hood is provided from which they are collected and sent to waste.
The collected web is quite weak, with tensile strengths well below about 1.5 kg/cm.sup.2. The fibers have a wide size distribution, with the largest fibers being ten or more times larger than the smallest, and the average fiber diameter is about five to seven or more times the smallest fiber diameter. Many of the fibers are twinned, with a twin being defined as two parallel fibers adhering to each other along a length of 20 or more times their average diameters, while others are roped, i.e., consist of two or more fibers twisted about each other in a form resembling a rope. Roped fibers behave in practice, for example in filtration, much like a single fiber of diameter about equal to that of the rope. Both twinning and roping cause the collected web to have a high pressure drop and low filtration efficiency. Fibers bonded in a twin are less efficient in a filter than two separate fibers. Shot--i.e., small pellets of unfiberized resin interspersed in the web--are also a problem. Typically, the collected web produced as described above reflects a compromise between making shot and rope. This web also tends to have a rough, rather fuzzy surface, that is undesirable for many applications, for example, for use in disposable clothing.
The system for producing webs described above is inefficient by virtue of its geometry. When the two air streams converge, a portion of the energy required to form fibers of the resin is dissipated in proportion to the component of their velocities perpendicular to the center line of the apparatus. A further inefficiency is the rectangular shape of the air stream which acts on each nozzle; if for example 0.5 mm diameter holes are located on 2 mm centers, a rectangular air stream 2 mm wide acts on each 0.5 mm diameter resin passageway. Because the liquid stream is circular, that portion of the air issuing from a corner of the rectangle farthest from the resin nozzle is relatively ineffective, generating a high degree of turbulence with a relatively small contribution to fiber formation. As a result of these inefficiencies, the cost of energy to compress and heat the air in this process is much larger than it would be if each resin nozzle were to receive its own supply of air through a circular annulus. Due to the high volume of air required to fiberize a given weight of resin, the distance from the resin nozzle outlet to the fiber collection surface in usual practice exceeds about 10 to 13 cm, and this relatively long passage through very turbulent air causes the undesirable roping and twinning in the fibers of the collected web. Attempting to operate at much less than 1
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Manteuffel Richard L.
Pall David B.
Pall Corporation
Zirker Daniel
LandOfFree
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