Method for preparing a uniform polyolefinic microporous hollow fiber

#The present invention relates to microporous polyolefinic hollow fibers having improved characteristics, and to a process and apparatus for preparing such fibers. It particularly relates to improvements in the spinning of hollow polyolefinic precursor fibers which yield microporous hollow fibers having improved structural and performance characteristics.

#Hollow porous fibers are well known in the art. See, for example, U.S. Pat. Nos. 4,020,230; 4,082,658, and 4,214,020, as well as European Published Patent Application No. 0 005 866, published Dec. 12, 1979. The advantages of permeable hollow fibers over permeable films are also well known. For example, porous hollow fibers possess a larger surface area per unit volume than a flat film of similar porous configuration. Consequently, there has been a growing tendancy to employ, if possible, hollow microporous fibers in those applications typically reserved for permeable films.

#One particular important use for hollow microporous fibers is as a blood oxygenator as illustrated by U.S. Pat. No. 4,020,230 which discloses hollow microporous fibers prepared from polyethylene. As is well known the properties required in a blood oxygenator membrane include good gas permeability with respect to gaseous oxygen and carbon dioxide, chemical stability, blood compatibility or substantially non-thrombogenic behavior in blood containing environments, sufficiently hydrophobic character to serve as a water vapor barrier, ease in manufacture, nontoxicity, relative inertness to body fluids, and mechanical strength and handling properties adequate for facilitating the assembly and use of blood oxygenation devices.

#FIG. 1 is a perspective view from above of an apparatus for simultaneously spinning six hollow fiber precursors in accordance with one aspect of the present invention.

#FIG. 2 is a schematic representation of a means for achieving hot stretching in multiple stages.

#FIG. 3 is a photomicrograph at a magnification of 46.5.times. of the wall of a microporous polypropylene hollow fiber produced in accordance with the present invention wherein the axis of the fiber runs from left to right.

#The essence of the instant invention comprises the instant inventors' discovery that a microporous polyolefinic hollow fiber having a novel combination of physical characteristics may be made by melt spinning a polyolefinic resin in a substantially vertically upward direction at a temperature of from about 10.degree. to about 90.degree. C. above the crystalline melting point of the polymer into a non-porous hollow precursor fiber while contacting the precursor with a substantially symmetric flow of a quenching medium such as air or other gas, and then converting the resulting non-porous hollow precursor fiber into a microporous hollow fiber by stretching the precursor fiber and then heat setting the stretched fiber. Preferably the precursor fiber is also annealed prior to stretching.

#Surprisingly, the instant inventors have found that polyolefinic microporous hollow fibers produced by this method exhibit a greater and more uniform molecular orientation and porosity, a more uniformly round shape and a more uniform wall thickness. As a result of this unique combination of characteristics, the microporous hollow fibers of this invention exhibit not only higher gas permeabilities than the typical microporous hollow fibers of the prior art but in addition are mechanically stronger and more durable than the prior art fibers. A further advantage of upward extrusion is the ability to spin larger hollow fibers. Using the down-spinning technique of the prior art, the internal diameter of the hollow-fiber is practically limited to a maximum of 300 microns with a 25 micron wall thickness using common fiber grade polyolefinic resins. Using upward extrusion, fibers with inner diameters as large as 1500 microns and 75 microns wall thickness may be spun with adequate orientation for conversion to the microporous state. In addition, a wider flexibility of resin choice is possible with the present process of producing polyolefinic microporous hollow fibers, both low melt index and high melt index resins being capable of use herein with equal advantage. This result is particularly surprising since with the downspinning technique of the prior art higher melt index resins are preferred due to their smaller die swell characteristics. The ability to utilize higher molecular weight (lower melt index) resins is an attractive advantage since they can be fabricated into a mechanically stronger fiber. The molecular weight of the resin can also be used to control the pore size of the fiber.

#While not wishing to be bound by any particular theory, it is believed that the foregoing advantages result from the faster drawdown and cooling rate provided by the upspinning/symmetrical quench system of this invention than that provided by the downspinning/cross flow quench system of the prior art. With the upspinning technique of this invention, the filaments are drawn down to their final diameter much more quickly than with the downspinning technique of the prior art. This faster drawdown allows greater stress to be applied to the molten hollow fiber at extrusion and hence a higher degree of molecular orientation to be imparted to the fiber. Moreover, the faster drawdown allows for more rapid cooling of the fiber due to the higher surface area and linear speed of the filament which further enhances the molecular orientation of the precursor fiber.