SE448747B - MICROPOROSA POLYPROPENE FIBERS, PROCEDURES FOR PREPARING THEREOF AND THEIR USE IN A HYDROGEN ACID DEVICE - Google Patents

Description

448 747 performance which precludes the uncritical application of film technology to hollow fibers.

For example, U.S. Pat. No. 3,801,404 discloses a cold stretching / hot stretching process for producing microporous polypropylene films which comprises the steps of extruding a precursor film by a blown film method at a temperature of about 180 DEG-270 DEG. ° C and recording of the film with recording speeds of approximately 9 to 213 meters / minute and with a pull-out ratio of 20: l to 200: l. The precursor film is then optionally annealed, cold-stretched at a temperature below about 120 ° C, for example 25 ° C, hot-stretched at a temperature above 1200 and below the polymer melting temperature, for example between 130 and 150 DEG C. and finally heat-cured at a temperature in the range of about 125 ° C up to lower than the melting temperature of the polymer, for example about 130 to 160 ° C.

If extrusion or melt spinning temperatures of less than 230 ° C are used in the manufacture of the hollow microporous polypropylene fibers of the present invention, the structural uniformity of the hollow fibers with respect to inner and outer diameters and wall thickness is significantly reduced. The melt spinning temperature of the hollow fibers thus represents a process parameter that is not easily translated from film to fiber technology due to the inherent differences between fiber and film configuration, i.e. the concept of uniformity for inner and outer diameter fiber dimensions does not exist in film technology. The melt index of the polypropylene used for the production of the hollow fibers in question must also be regulated to preserve the structural and dimensional uniformity of the hollow fiber.

While the production of a film by the blown film method is similar to the production of a hollow film by the air injection method, there is also a significant difference. The uptake rate in the production of blown precursor films is limited in that with increasing higher uptake rates (keeping other process variables constant) the uniformity of the microporous film structure which is subsequently imparted to the precursor film decreases. Consequently, the process variable uptake rate cannot be controlled. ie increased) in a manner sufficient to impart increasingly higher levels of orientation to the precursor film structure. Higher degrees of orientation up to a certain threshold have a beneficial effect on the final permeability imparted to the film, but due to problems with uniformity of pore structure, a limit is imposed on the degree of orientation that can be imparted via uptake rate control to the precursor film.

When hollow fibers are produced by the method of the present invention, the problems of microporous structural uniformity are substantially reduced and higher uptake rates can be used to impart a higher degree of orientation, determined by wide angle X-ray diffraction of a (110) plane, to the hollow precursor fibers. , whereby a higher permeability potential is conferred on them. However, this increase in permeability potential is not easily achieved in the absence of regulation of the inner diameter (I.D.) of the hollow precursor fiber which in turn regulates the inner diameter of the subsequently formed hollow microporous fiber. Adjusting the dimensions of the hollow microporous fiber is an additional process variable that cannot be transferred from the film technology.

U.S. Pat. No. 3,558,764 discloses a cold stretching process for producing microporous films which comprises the steps of extruding a polymer at specifically defined temperatures to produce a precursor film, cooling the precursor film, expiring the precursor film at specifically defined temperatures (ie 5 to 10 ° C below the melting point of the polymer which is approximately 168 ° C for polypropylene), cold drawing the resulting film at a specifically defined drawing ratio and temperature and heat curing of the cold drawn film at a temperature of about 100 to about 150 ° C under voltage. The primary difference in this process from the previous cold stretching / hot stretching process is the absence of a hot stretching step. The cold stretching / hot stretching process described above represents an improvement over the cold stretching process according to said patent specification with respect to nitrogen flow.

On the other hand, when hollow precursor fibers are matured, cold stretched and thermoset generally in accordance with the above cold stretching procedures, especially when the thermosetting temperature is at or below the initial expiration temperature, the resulting hollow microporous fibers have varying degrees of shrinkage. roll up which is disadvantageous depending on the use for which the hollow fiber is used.

U.S. Patent No. 4,055,696 discloses a similar cold stretching process used to make hollow microporous polypropylene fibers instead of films. This method requires that the size of the pores be kept within a specified range by limiting the degree and temperature of cold stretching to 30 to 200% of the original fiber length and less than 100 ° C, respectively. The resulting cold drawn fibers which have previously been annealed are heat cured at a temperature at or above the initial annealing temperature used before stretching as described above. A separate hot stretching step used in the present invention is not included in the manufacture of these hollow fibers. Expired, cold-stretched, thermoset, hollow fibers made in accordance with the said patent tend to exhibit varying degrees of shrinkage depending on the relationship between the previous annealing temperature and the duration of the thermosetting temperature and the duration. In addition, there is no regulation of the inner diameter of the hollow fibers in said patent specification for improving the oxygen permeability thereof.

Japanese Kokai Patent Publication Sho 53 [1978] - 38715, published April 10, 1978, relates to an improvement in the method of making porous hollow polypropylene fibers described in U.S. Patent No. 4,055,696. The improvement includes control of the annealing temperature so that it is below 155 ° C and control of the heat-curing temperature after cold stretching so that it is from 155 to 17500 for from 3 seconds to 30 minutes. This method also fails to utilize either a thermal stretching step in addition to the cold stretching step required by the present invention or adjusting the inner diameter of the hollow microporous fibers to improve oxygen permeability.

A particularly significant use of hollow microporous fibers is as a blood oxygenation device illustrated in U.S. Patent No. 4,020,230, which discloses hollow microporous fibers made of polyethylene. As is well known, the properties required in a blood oxygenation 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 water to serve as a sufficiently hydrophobic character. , ease of manufacture, non-toxicity, relative inertness to body fluids and mechanical strength and handling properties suitable for facilitating the assembly and use of blood oxygenation devices. Microporous polypropylene films have previously been used as blood oxygenation membranes and all such films have been found to meet the above claims. However, due to the relatively small area of such films, relatively large volumes of blood must be removed from the body to achieve the required oxygen and carbon dioxide gas transfer. Hollow microporous polypropylene fibers, on the other hand, offer the advantage of being able to achieve the same gas transfer using much lower volumes of blood.

There has therefore been a continuous search for hollow microporous polypropylene fibers and a process for their production which exhibit a high oxygen permeability.

The present invention is a result of this application.

An object of the present invention is therefore to provide microporous hollow polypropylene fibers having high oxygen permeability values.

A further object of the present invention is to provide a process for producing hollow microporous polypropylene fibers having high oxygen permeability values.

These and other objects and features of the invention will become apparent from the following description and claims taken in conjunction with the accompanying drawings.

In one aspect of the present invention there is provided a process for producing hollow open cell microporous polypropylene fibers having an oxygen flow of at least 35 cm 3 / cm 2 minute at 0.7 kp / cm 2 comprising: (A) melt spinning at a temperature of at least 230 ° C of isotactic polypropylene having a melt index of at least 1 in a manner sufficient to obtain hollow, non-porous polypropylene precursor fibers, uptake of said precursor fibers at an elongation ratio of at least about 40, said melt spinning being performed on a sufficient to impart to the precursor fibers after uptake an average inner diameter of at least about 140 microns, an average inner diameter to average wall thickness of from about 8: 1 to about 40 microns and a degree of orientation. ¿Determined from the half-width of the wide-angle (110) X-ray diffraction arc, of not more than 250, and an elastic return of 50% increase at 25 ° C, 65% relative humidity and return time zero, of at least 50%; (B) annealing the precursor fibers at a temperature between about 50 ° C and less than 165 ° C for a period of about 0.5 seconds to about 24 hours; (C) cold stretching the non-porous hollow precursor fibers in their longitudinal direction at a temperature higher than the glass transition temperature of the precursor fiber and not greater than about 10 ° C to impart porous surface regions to the walls of the fiber which are perpendicular to the cold stretching. the direction; (D) hot stretching the expired cold stretched hollow fibers from (C) in the same direction of cold stretching at a temperature above the cold stretching temperature and below the melting point of the polypropylene to impart an open cell microporous configuration to the hollow fiber walls, said cold stretching and hot stretching a method sufficient to control the average inner diameter of the resulting hot stretched, hollow, microporous fibers to be at least 100 microns and to achieve a total degree of combined stretching of from about 80 to about 200%, a elongation ratio of from about 1: 3 to about 1:20 and an elongation rate of from about 10 to about 200% / minute; (E) heating the resulting hot stretched fibers from (D) under tension to produce dimensionally stable, open cell, hollow, microporous fibers having an average inner diameter of at least 100 microns. 1! According to another aspect of the present invention, there are provided hollow, open cell, microporous polypropylene fibers having an oxygen flow of at least 35 cm 3 / cm minute at 0.7 kp / cm 2 prepared by the above process.

The attached figure is a schematic illustration of a means for achieving hot stretching in multiple steps.

The hollow microporous fibers of the present invention are made of isotactic polypropylene having a weight average molecular weight ranging from about 100,000 to about 750,000 and a melt index: not less than about 1 (e.g., not less than about 5), in typical cases from about 1 to about 30 or higher, preferably from about 3 to about 15 and preferably from about 5 to about 10.

The term melt index is defined as used herein as the value obtained by performing ASTM D-1238 under conditions relating to temperature, applied load, time intervals and other operating variables specified therein for the particular polymer being tested, i.e. polypropylene.

If a fiber is made of polypropylene which has a melt index increasing below about 1, for example 0.5, the fiber shows an increasing tendency to break or split and an increasing greater variation in the uniformity of the inner diameter and the cross section of the fiber.

The density of the polypropylene should be approximately 0.90 g / cm3. to The isotactic polypropylene is converted / Eï_ïhollow precursor fiber by melt spinning. The molten polymer is caused to flow through one or more orifices (i.e., nozzles) of a spinneret capable of imparting the desired continuous hollow configuration to the fiber. In the preferred embodiment, for example, the melt is caused to flow kx 448 747 through one or more annular nozzles having a needle extending into each central portion thereof. A gaseous stream is then passed through the needle as the melt is pumped through the annular nozzle thereby imparting a hollow configuration to the fiber. Alternatively, the hollow channel of the fiber can be formed by passing molten polymeric material through an annular opening or a solid core capable of forming the desired hollow structure.

The temperature at which the polypropylene is extruded, i.e. melt spinning (assuming that spin variables other than those described herein are used) should be at least 230 ° C, preferably from about 240 to about 280 ° C and preferably from about 240 to about 250 ° C. If an extrusion temperature is more and more below about 230 ° C used, the uniformity of the fiber with respect to inner and outer diameters deteriorates more and more. In contrast, precursor films used in the cold-stretching / hot-stretching process of U.S. Pat. No. 3,801,404 can be prepared at extrusion temperatures as low as 180 ° C.

At extrusion temperatures that are more and more higher than about 280 ° C, the spin stress applied to the extrusion polymer must be significantly increased and there is a risk of polymer degradation.

When a hollow fiber air injection spinneret is used, the jet diameter, air flow rate, take-up speed, extrusion speed and pull-out ratio are controlled in a manner sufficient to achieve a hollow precursor polypropylene fiber having an average mean wall diameter and average wall thickness. ions described herein and a degree of orientation not greater than about 250 determined by half the width of a (110) wide angle X-ray diffraction arc.

The degree of fiber molecule orientation is determined by superimposing the fibers 448 in axiality to a thickness of 50 mg / cm 2. The fibers I are then irradiated with X-rays in a direction perpendicular to the axis direction of the fibers and the half width of a (110) wide-angle diffraction arc is recorded on film.

The angular spread of this (110) diffraction arc is then determined and shall not exceed 25 °.

The dimensions (i.e. inner and outer diameters) and wall thickness of the hollow fibers produced can be regulated in many ways. Initially, the diameter of the nozzle and the inert gas pressure selected to control the internal and external dimensions of produced fibers, respectively, are modified by the degree of magnification of fiber dimensions by release from the measured extrusion die through the spinneret.

Diameter and wall thickness can also be varied by varying the extrusion pressure through the spinneret and the take-up speed at which the fibers are drawn away from the spinneret. Changes in one of these values can be offset by changes in the other to achieve the desired results.

While the above process parameters are regulated with respect to achieving an inner diameter within a limited range, they are also regulated to impart the proper morphology to the precursor fiber to ensure that subsequent processing provides a microporous structure having a suitable gas permeability.

Accordingly, the melt spinning or melt extrusion step of the process is performed at a relatively high "pull-out" or "spin-pull" ratio so that the hollow fibers are spin-oriented when formed. The pull-out ratio is defined as the ratio of the rate of initial uptake rate of the hollow fibers to the linear rate of extrusion of the polymer through the spinneret orifice. The pull-out ratio used in the process of the present invention is at least 30, preferably at least 40 (for example from about 40 to about 100) and may be as high as about 700. Pick-up rates used to perform the The required pull-out ratios are usually at least about 200 meters / minute, typically from about 200 to about 1000 meters / minute and preferably about 200 to 500 meters / minute. High shear forces typically develop in the polymeric material that is not relaxed prior to fiber solidification.

The air flow rate, i.e. the speed at which the air passes through the needle in the central part of the nozzle hole will vary depending on the number of nozzle holes in the spin nozzle and is typically regulated so that they are from about S to about 70 cm 3 / minute / nozzle hole and preferably from about 10 to about 50 cm3 / minute / nozzle hole.

The temperature of the air as it exits the air injection nozzle is typically the same temperature as the melt spinning temperature of the polymer.

The hollow spin-oriented precursor fibers can optionally be cooled by passing them through a stream of gas, such as ordinary air at room temperature, or through an inert liquid, such as water, so that rapid cooling of the just spun hollow fiber is obtained as a result. The temperature of the cooling medium can be as high as 80 ° C and as low as 0 ° C (eg 0 - 40 ° C) depending on other spinning parameters. However, the preferred cooling temperature is 25 ° C and passage of the just spun fibers through ambient air results in a suitable cooling where the take-up roller is located about 1.5 to about 3 meters or further from the spinneret.

The resulting hollow precursor polypropylene fiber is non-porous and has a crystallinity of at least 30%, preferably at least 40% and preferably at least 50% (for example about 50% to about 60% or higher). The percentage of crystallinity is determined from the relationship: æ Crystallinity = va - v x 100 Va - Üc where Üa is the specific volume of the 100% amorphous polymer. Üc is the specific volume of the 100% crystalline polymer and Ü is the specific volume of the sample in question.

The specific volume of a polymer is l / D where D is the density of the polymer. The density of the polymer is determined by means of a density gradient column as described in ASTM D-1505-68. The hollow precursor polypropylene fibers should also exhibit an elastic return at zero return time upon exposure to a standard elongation of 50% at 2500 and 65% relative humidity of at least about 50%, preferably at least about 60% and preferably at least about 65%.

Elastic return, as used herein, is a measure of the ability of structured or shaped articles, such as hollow fibers, to return to their original size after being stretched.

The elastic return value is determined with an Instron Tensile Tester device that operates at an elongation rate of 10% / minute. After the fiber is stretched to the desired elongation value, the clamps of the apparatus return at the same speed until the distance between them is the same as at the start of the test, i.e. the original measuring length.

The clamps are again reversed in the direction of movement and stopped as soon as the stress begins to increase from zero. The elastic return is then calculated as follows: .t 31 448 747 13 Tbtallength Final.distance Elastic atelgmg = in stretch: ständ nstand - nellan clamp no .: steam cooked in stretched condition Measurements with Instron Tensile Tester devices are performed at room temperature, example 2S ° C, in air at 65% relative humidity. Although a standard elongation of 50% was used to identify the elastic properties of the precursor fibers, such elongation is only an example. In general, such precursor fibers will have elastic return values that are higher at elongations less than 50% and slightly lower at elongations substantially higher than 50% compared to their elastic return at 50% elongation.

The above process conditions are controlled for providing hollow precursor polypropylene fibers having an average inner diameter (I.D.) of at least 140 microns, preferably from about 140 to about 400 microns or higher, preferably from about 200 to about 300 microns. The above average internal diameter values have been found to be necessary to impart a high gas permeability potential to the hollow precursor fibers. When high gas permeability does not constitute a regulating factor in the desired end use of hollow fibers, the diameter values can be reduced to below 140 microns.

The dimensions of the hollow fibers are expressed as an average value because such dimensions will vary to some extent depending on where, along the fiber length, the dimensions are determined. Accordingly, the average inner and outer diameters are determined by cutting the cross-section of the fiber at 15.2 cm intervals during a total S interval along the fiber length and determining the fiber dimensions at each of these intervals. The fiber sections are then immersed in standard optical immersion oil and the dimension at each interval is determined using an optical microscope and optical measurement. The results were then used to calculate the mean value to determine the average inner and outer diameters.

The minimum wall thickness of the hollow precursor fibers shall be sufficient to prevent them from easily rupturing or otherwise undergoing physical deterioration at a rate which would render their use unattractive after being microporous by the methods described herein.

The maximum wall thickness of the hollow fibers is limited by the degree of permeability that is sought to be imparted to the final product.

The measurement of the average wall thickness is performed by determining the average outer diameter and the average inner diameter of the fiber, the wall thickness being half the difference in these average diameter values.

In addition, the average wall thickness can be expressed as a function of the average inner diameter of the hollow fiber. The ratio of the average inner diameter of the hollow precursor fiber to its average wall thickness may vary from about 8: 1 to about 40: 1, preferably from about 10: 1 to about 30: 1, and preferably from about 10: 1. l to about 20: l. More specifically, an average precursor fiber wall thickness of at least 10 microns is preferred and typically from about 10 to about 25 microns. g While it is the inner diameter, and associated wall thickness, of the final microporous hollow fiber product that is believed to be the primary regulatory factors for gas permeability, it is the inner diameter and wall thickness of the hollow precursor fiber that predetermines the maximum obtainable inner diameter. and the wall thickness of the final product. This is obtained as a result of the fact that the inner diameter of the precursor fiber shrinks approximately 25% when subjected to the two-step stretching process described herein. The average wall thickness of the hollow microporous fibers remains substantially unchanged by machining compared to the precursor fiber, although it can be reduced to a small extent.

The hollow precursor fibers are then subjected to a heat treatment or annealing step, whereby the amount of crystallinity and / or their crystal structure is improved. More specifically, this step in the process increases crystallite size and eliminates molecular misalignments. The annealing is carried out under an equilibrium of time and temperature to achieve the desired improvements as described above and which is still sufficient to avoid destruction or adverse effect on the precursor polymer structure (eg orientation and / or crystallinity).

The preferred annealing temperatures can range from about 130 to about 145 ° C for a period of about 30 minutes.

Consequently, when the annealing temperature is increased above about 145 ° C, the time during which the precursor fiber is matured is reduced.

Conversely, when the temperature drops below 130 ° C, longer and longer run times are used.

If the annealing temperature increasingly exceeds l45 ° C at an elimination time of 30 minutes, the precursor polymer fiber structure will be adversely affected and the gas permeability potential of the precursor fiber will be more and more reduced. If the annealing temperature increasingly becomes less than 130 ° C for 30 minutes, the gas permeability potential of the precursor fiber will also be more and more reduced.

In view of the above, the annealing is performed for time periods of about 0.5 seconds to about 24 hours at a temperature of from about 50 ° C to below the melting point of the polypropylene (i.e., 16,500 based on 448,747 differential scanning calorimetry).

The expiration step can be performed in a state under tension or tension-free state by depositing the precursor fiber in a static state in a heating zone which is maintained at the required elevated temperature, or by continuously conducting the precursor fiber through the heating zone. The elevated temperature can be achieved, for example, by using a conventional circulating air oven, IR heating, dielectric heating or by direct contact of the running fiber with a heated surface which is preferably curved to promote good contact.

The precursor fiber can be continuously passed through a sheathed tube or casing that radiates heat at the desired temperature. Alternatively, the precursor fiber may be wound under substantially no stress on a spool while undergoing expiration or simply placed in the heating zone in a loose state, such as a tangle of continuous fibers. To obtain the best results, it is recommended that the hollow fiber be kept at a constant length during the annealing step, i.e. under a stress sufficient to prevent a length elongation or shrinkage of more than about 5%. This can be achieved by passing the fibers in their longitudinal direction over and around a first stress isolating device through a heating zone maintained at the appropriate temperature and then over and around a second stress isolating device. Each stress isolating device may suitably take the form of a pair of oblique rollers.

Regulation of the ratio of the surface velocities of the two series of rollers allows isolation and regulation of the stress of the fibers between the rollers when undergoing annealing.

The resulting non-porous hollow precursor fiber is then subjected to a two-step stretching process and then subjected to heating.

At the first stretching step referred to herein as “cold- ~. 448 747 17 stretch ", the hollow precursor fibers are stretched at a temperature above the glass transition temperature (Tg) of the precursor fiber and which is not higher than about 100 ° C. Typical cold stretching temperatures can range from about 0 to about 100 ° C. , preferably from about 15 to about 70 ° C, and preferably at room temperature, for example 25 ° C. The temperature of the fiber itself is referred to as the stretching temperature.

Those skilled in the art of polymer technology know that the glass transition temperature (Tg) is the temperature at which the structure of a wholly or partially amorphous polymeric material changes from a glassy state to a viscoelastic state. The glass transition temperature of polypropylene is measured by depositing its specific heat against the temperature and noting the temperature at which there is a change in the slope of the curve. This measurement is commonly referred to as thermomechanical analysis and can be performed with commercially available instruments, such as a Thermomechanical Analyzer Model No. 990 manufactured by Du Pont. The glass transition temperature is also referred to as the second order transition temperature.

Cold stretching imparts porous surface regions or areas to the fiber wall which are extended perpendicular to the stretching direction.

The second stretching step, referred to herein as hot stretching, is performed at a temperature above the cold stretching temperature but lower than the melting point of the precursor fiber before or after cold stretching, i.e. the first-order transition temperature, determined by differential scanning calorimeter analysis.

Typical hot stretching temperatures will be greater than about 100 ° C and may range from about 105 to about 145%, preferably from about 130 to about 14 ° C and preferably from about 135 to about 145 ° C. Again, the temperature of the fiber itself, which is stretched, is referred to as the hot stretching temperature.

When the hot stretching temperature used is increasingly less than 130 ° C, increasing degrees of shrinkage are observed in the final fiber product.

Hot stretching opens the porous surface areas imparted by the cold stretching to form an open-cell microporous structure.

The stretch in the two stretching steps must be consecutive, in the same direction and in that order, i.e. cold stretching and then hot stretching but can be done in a continuous, semi-continuous or batch process as long as the cold stretched fiber is not allowed to shrink to any significant degree (for example not more than about 10% based on the original precursor fiber length).

The sum of the total degree of stretching in the above cold and hot stretching steps may vary from about 80 to about 200% (for example about 80 to about 155%) and preferably from about 85 to about 120% (for example about 90%) based on the original length of the precursor fibers. When the total degree of stretching becomes less than about 80%, the resulting oxygen permeability at 0.7 kp / cm 2 becomes less than about 35 cm 3 / cm 2 per minute.

The ratio between the degree of stretching in the first (cold) and the second (hot) stretching steps is referred to here as the elongation ratio. The elongation ratio can vary from about 1: 3 to about 1:20, preferably about 1: 3 to about 1: 10 (for example 1: 3 to about 1: 5).

It is to be understood that the particular overall degree of elongation and elongation ratio is selected from the above range in a manner sufficient to control the final average inner diameter of the hot stretched 19 microporous fibers within the limits described herein.

The elongation rate, i.e. the degree of stretching per unit time by which the precursor fibers are stretched during both stretching steps is preferably the same for each step and may range from about 10 to about 200% / minute, suitably from about 10 to about 100% / minute, and preferably from about 15 to about 30% / minute (for example about 20% / minute).

At the preferred total elongation rate of about 80 to about 120%, the preferred elongation ratio is 1: 3 to about 1: 5, for example, 20% cold stretch and from about 60 to about 100% hot stretch.

The cold and hot stretching of the precursor fibers can be performed in any convenient manner using known methods. The hollow fibers can, for example, be stretched on a conventional draw frame placed in a heating zone which regulates the temperature of the fibers during stretching. Alternatively, the fibers can be cold and hot stretched in a continuous manner by means of two series of stress isolation devices (one series for each step) similar to those described in connection with the annealing step.

Accordingly, precursor fibers can be wound several times around a first pair of oblique rollers, passed through a heating zone, in which case they are, for example, brought into contact with a suitable heating device or medium and kept at a suitable cold stretching temperature and wound several times around a second pair. of oblique rollers. This arrangement allows the isolation and control of the longitudinal stress of the fibers between the two pairs of rollers during cold stretching. The fibers are then passed through a similar series of pairs of oblique rollers while heating to the appropriate hot stretching temperature.

The differential ratio between the surface velocity of each of the other pairs of rollers to the surface velocity of each surface pair of the first pair of rollers determines the stretching ratio and the elongation velocity which are set accordingly.

It is to be understood that in a continuous process, the cold stretched fibers may undergo shrinkage as they pass from the cold stretching step to the hot stretching step. This can occur as a result of heating the cold-stretched fibers when they enter the hot-stretching zone, such as a furnace with forced hot air, but before they are actually hot-stretched.

Accordingly, it is preferable to insert a tensioning device between the cold and hot stretching steps to prevent shrinkage greater than about 5% based on the cold stretched fiber length. Such a clamping device may suitably be in the form of a single pair of oblique rollers.

The heating zones which heat the precursor fibers to the appropriate cold stretching or hot stretching temperature are the same as described for pre-cold stretching and may suitably be in the form of a gas such as air, heated plate, heated liquid and the like. The preferred heating device is an oven with forced hot air which houses the stretching agent.

Following the cold and hot stretching processes described above, the stretched fibers are heat treated or drained while in the stretched state at a temperature of from about 125 ° C up to below the melting temperature of the polymer. As is known to those skilled in the art, the melting temperature can be determined by means of standard differential scanning calorimeters or with another known device which can detect thermal transitions for a polymer. The preferred heat treatment temperature can range from about 1300 to about 145 ° C. The most preferred heat treatment temperature is the same as the temperature used during hot stretching. Heat treatment does not change the fiber dimensions as they are after hot stretching. 3 now '448 747 21 The heat treatment step can be carried out in a batch process such as in an oven or autoclave or in a continuous manner. For example, the hollow microporous fibers can be rewound on a spool after hot stretching and subjected to the annealing process in this form. Alternatively, the hollow fibers can be stretched and heat treated in a continuous process by means of two pairs of driven rollers downstream of the stretching rollers moving at the same speeds, the material between the rollers continuously passing at a constant length after heat stretching through the heating zone. Accordingly, the stretching and heat treatment steps of the process can be performed in succession or they can be combined in a single in-line process.

The heat setting treatment is to be carried out while keeping the fibers under tension, i.e. so that the fibers are not free to shrink or can shrink only to a controlled extent not greater than about 15% of their elongated length but not under such a great tension that the fibers are stretched more than a further 15%. The tension is preferably such that substantially no shrinkage or elongation occurs, for example less than 5% change in elongated length.

The time for the heat setting treatment which is preferably carried out in conjunction with and after the heat stretching process should not be longer than 0.1 seconds at the higher heat setting temperatures and may generally be in the range of about 5 seconds to 1 hour. and preferably about 1 to 30 minutes.

Since the most preferred heat setting temperature is the same as the heat stretching temperature, it is preferable to perform both hot stretching and heating setting in the same heating means, such as a convection oven, in which case the total residence time in the oven for the hot stretching and heating setting steps may vary from about 10 to about 45 minutes and preferably from about 25 to about 35 m 448 747 22 minutes (e.g. 35 minutes) for hot stretching temperatures of about 130 to about 14 ° C.

The effect of the heat setting step is to improve the thermal stability of the microporous structure and reduce the shrinkage of the fibers.

In an alternative embodiment, the hot stretching and heat setting steps can be combined in a single step.

In this embodiment, hot stretching is achieved using a plurality of discrete hot stretching processes in succession at the appropriate hot stretching temperature. After the fibers have been cold stretched, for example, they are passed into an agent capable of stretching the fibers in a growth manner while being maintained at the appropriate hot stretching temperature so that the total degree of stretching of each increment amounts to the desired degree of total warm stretching.

The multiple stage hot stretching means may suitably be in the form of a plurality of rollers placed in an oven. The rollers are preferably located in a garland configuration similar to that described in U.S. Pat. No. 3,843.76l, the contents of which are hereby incorporated by reference. The use of a garland arrangement is preferred in that it provides extended exposure time in the furnace containing the multi-stage hot stretching agent, thereby eliminating the need for each heating step after the hot stretching is completed.

To illustrate a preferred method of achieving multiple stage hot stretching and combined heating, reference is made to the figure. Non-porous precursor fibers 5 that have expired are unwound from a supply roll 4, over guide rollers 6 and 7 into a cold stretching zone generally indicated at 2.

The cold stretching device comprises two pairs of inclined rollers 8 - 9 and 11 - 12 which are driven at peripheral speeds I ä! 441 747 23 S1, S2 and S3 and S4, respectively, by suitable propellants 10 and 13 to achieve the desired degree of cold stretching as described herein. For illustrative purposes, the cold drawing temperature is at room temperature and no heating or cooling agents are required for this step. However, if desired, suitable temperature control agents may be provided as described herein. The cold drawn fibers, now designated 15, are passed into the hot stretching means generally indicated by 3 over one or more guide rollers 14. Nip rollers are not used as they tend to crush the hollow fibers which is disadvantageous to the final product. The hot stretching means 3 comprises a single series of oblique rollers 16 and 17 and a plurality of additional multiple hot stretching rollers placed in an oven in a garland configuration. In order to reduce the unsupported fiber length between adjacent hot stretching rollers to a minimum which is relatively long in the preferred garland arrangement, at least one guide roller is provided between adjacent hot stretching rollers.

The single series of oblique rollers 16 and 17 helps to maintain the tension of the cold drawn fibers by regulating the peripheral speeds S5 and S6 thereof, respectively.

'Tension fi àhin has shrinkage, sagging and the like caused by any preheating of the fibers as they pass into the furnace but before they are hot stretched. Such tension helps to avoid any reduction in cold-drawn fiber properties caused by preheating. Although this tension step, in order to prevent fiber relaxation, may result in a small degree of stretching, the primary effect of this procedure is tension and the peripheral velocities S5 and S6 are accordingly regulated by propellant 18 to maintain a constant length between cold stretches. and hot stretch zones. This method thus constitutes only a preferred embodiment of a means for maintaining fiber tension before hot stretching. Other methods that prevent fiber relaxation during heating of the fibers before hot stretching can be used. The tensioned cold stretched fibers 15 are then transported downstream over guide rollers 19 and 20 on a first hot stretching roller 21. The fibers are hot stretched for the first time between roller 21 and the second tension roller 16. This occurs because the first downstream stretching roller 21 rotates at a peripheral speed S7 exceeding the peripheral speed S5 imparted to the fibers by the roller 16. It should be noted that a guide roller 19 is located between the rollers 16 and 21 to reduce the unsupported fiber length during the hot stretching step.

This process is continued for as many discrete steps as may be preferred. The fibers are stretched, for example, for a second time between the first hot stretching roller 21 and a second hot stretching roller 23. In this second hot stretching step, the peripheral speed of the second hot stretching roller 23 is S8. The peripheral speed S8 is larger than the peripheral speed S7 of the first hot-stretching roll 21. Thus, the fibers in the second hot-stretching step are blank-stretched with a hot-stretching ratio of S8 / S7. To again reduce the unsupported fiber length to a minimum, at least one guide roll 24 is located between the second and third hot stretch rollers 23 and 25. In a preferred embodiment, illustrated in the figure, the guide rollers are located approximately midway between adjacent hot stretch rolls.

In the embodiment illustrated in the figure, twenty stretching steps which occur in succession are provided. As illustrated in the figure, for the provision of twenty stretching steps, twenty-one hot stretching rollers are required. It should be noted that the second tension roller 16 is equivalent to the first hot stretching roller. In general, in the blanket hot stretching device in the preferred embodiment (n + 1) hot stretching rollers are required to provide n hot stretching steps in succession. Preferably, 2 to 40 stretching steps are preferred in the multi-step hot-stretching process. 448 747 Two preferred methods can be used to provide continuously increasing peripheral velocity with each additional downstream stretching roll. In a preferred embodiment, all the rollers are driven by a common drive mechanism. Each hot-stretching roll is thus driven at the same rotational speed.

However, each hot stretching roller has different diameters. More specifically, each additional downstream stretching roller has a larger diameter than the adjacent upstream roller. Thus, the roller 23 has a larger diameter than the roller 21 and the roller 57 the most downstream hot-stretching roller has a larger diameter than the diameter of the penultimate downstream roller 55.

As those skilled in the art are aware, the peripheral or surface velocity of a larger diameter roller rotating at its center at the same speed as a smaller diameter roller is greater than the smaller roller. The use of rollers of increasing diameter thus serves the purpose of providing differential peripheral speeds between adjacent hot stretching rollers.

A second preferred method of providing differential increasing peripheral speeds between adjacent hot stretch rollers is to provide separate propellants for each roll. In this preferred embodiment, each roller may have the same diameter. The increasing speed of adjacent downstream stretching rollers thereby becomes a function of the force imparted to each roller.

It is to be understood that the process variables described above in connection with the single increment hot-stretching process are applicable to the multi-stage hot-stretching process except that obvious modifications may be necessary when moving from the former to the latter process.

For example, as described above, the total degree of stretching in both stretching embodiments is the same except that in the multiple stage hot stretching, the total degree of stretching is achieved in several, preferably equal, increments. The elongation rate of each hot stretch increment is preferably also controlled to provide f a total residence time in the multiple hot stretch zone which is approximately equal to the total heat set residence time used in connection with single increment hot stretch and that obtained within elongation at break. the intervals described here for hot stretching in a single step.

The resulting hollow microporous fibers have an average inner diameter, as defined herein, of about 100 to about 300 microns or larger, preferably from about 200 to about 300 microns (e.g., 250 microns).

The average wall thickness of the hollow microporous fibers does not change significantly from that of the corresponding precursor fiber and the change in the ratio of average inner diameter to average wall thickness of the microporous fibers from the precursor fibers is due to the reduction in the average precursor thickness of the precursor fiber.

The ratio of average inner diameter to average wall thickness of the hollow microporous fibers will range from about 7: 1 to about 35: 1, preferably from about 10: 1 to about 30: 1. The particular wall thickness achieved is predetermined by the precursor fiber wall thickness which, as described above, will depend on the end use for which the fibers are to be used and the pressure to which they will be subjected. The particular wall thickness selected is preferably the minimum which will withstand normal operating conditions for a particular end use without undergoing physical deterioration at an unacceptable rate.

When the hollow microporous fibers are used for blood oxygenation, the wall thickness can range from about 10 to about 30 microns and the average inner diameter can range from about 200 to about 400 microns and still exhibit high gas permeability values and structural integrity.

When the average inner diameter of the hollow microporous fiber is reduced below 100 microns for a given wall thickness, the gas permeability at 0.7 kp / cm 2 decreases significantly.

When the average inner diameter of the microporous hollow fibers of the present invention is at least 100 microns and the ratio of inner diameter to wall thickness is not less than about 7: 1, such hollow fibers will have an oxygen flow at 0.7 kp / cm 2 of at least 35 cm 3. / cmz minute, typically from about 35 to about 85 cm 3 / cm 2 minute, and preferably from about 40 to about 60 cm 3 / cm 2 minute.

Oxygen flow Jg is determined by passing oxygen through a hollow fiber module which is discussed in more detail in the embodiments. The hollow fiber module allows the passage of gas under pressure (eg 0.7 kp / cm 2) through the interior of the hollow fibers, through the microporous hollow fiber wall and collection. The volume of gas collected over a period of time was then used to calculate the gas flow in cm 3 / cm 2 minute for the hollow fibers according to the equation: v J = -_í__. 9 (A) (T) wherein V is the collected volume of gas, A is the inner surface of the hollow fibers determined by the equation A = n¶'dl where n is the number of hollow fibers, d is the inner diameter of the hollow fibers in cm and l is the fiber length in cm, and T is the time in minutes it takes to collect the gas. 448 747 28 The pores of the hollow microporous fibers are substantially interconnected by tortuous paths which may extend from one outer surface or surface areas to another, i.e. open-celled. This term "open cell structure" means that the greater part of the hollow space or pore space within the geometric boundaries of the walls of the hollow fiber is accessible to the surfaces of the fiber walls.

The porous hollow fibers of the present invention are further microscopic, i.e. the details of their pore configuration or arrangement are described only by microscopic examination. In fact, the open cells or pores in the fibers are smaller than those that can be measured using an ordinary light microscope because the wavelength of visible light that is approximately 5000 Å is longer than the longest flat or surface dimension of the open cell or pore. . However, the microporous hollow fibers of the present invention can be identified using electron microscopy techniques capable of resolving details of pore structure below 5000 Å.

The open cell microporous hollow fibers prepared in accordance with the present invention have an average pore size of 100 to 5000 Å and more commonly 150 to 3000 Å. These values are determined by mercury porosimetry as described in an article by RG Quynn on pages 21 - 34 in the Textile Research Journal, January 1963. Alternatively, an electron micrograph of the fibers can be taken and pore length and width determinations obtained by using an image analyzer or ruler to directly measure the length and width of the pores thereof, usually at 5,000 to 12,000 times magnification and scale conversion. to the appropriate size.

The pore length values that can be obtained by electron microscopy are usually approximately equal to the pore size values obtained by mercury porosimetry. The hollow microporous fibers of the present invention are also characterized by a reduced bulk density, sometimes hereinafter simply referred to as a "low" density.

The bulk density is also a measure of the increase in porosity of the fibers. This means that these microporous hollow fibers have a bulk density or total density which is lower than the bulk density of the corresponding hollow precursor fibers composed of identical polymeric material but which have no open-cell or other hollow structure. The term "bulk density" as used herein means the weight per gross or geometric volume of the fiber where gross volume is determined by immersing a known weight of the fiber in a vessel partially filled with mercury at 25 ° C and atmospheric pressure. at the level of mercury is a direct measure of gross volume.This method is known as the mercury-volumenometer method and is described in the Encyclopedia of Chemical Technology, volume 4, page 892 (Interscience 1949).

Thus, the hollow microporous fibers have a bulk density not greater than 95% and preferably about 40 to about 85% of the precursor fibers. In other words, the bulk density has been reduced by at least 5% and preferably about 15 to about 60%. The bulk density is. also a measure of porosity in that where the bulk density is about 40 to 85% of the precursor fiber, the porosity has increased by 60 to 15% depending on the pores or holes.

The final crystallinity of the microporous hollow fibers is preferably at least 35%, more preferably at least 45% and most preferably about 50 to 100% as determined by the density method just mentioned.

The hollow microporous fibers also have an elongation at break (ASTM D123-70) of not less than about 50%, and are preferably not less than about 100%. The surface area of the hollow microporous fibers described herein will have a surface area of at least 15 m 2 / g and preferably from about 20 to about 60 m 2 / g.

Surface area can be determined by nitrogen or krypton gas adsorption isotherms using a method and apparatus described in U.S. Pat. No. 3,262.319. The surface area obtained by this method is usually expressed as square meters per gram.

To facilitate comparison of different materials, this value can be multiplied by the bulk density of the material in grams per cubic centimeter resulting in a surface area quantity expressed as square meters per cubic centimeter.

The microporous hollow polypropylene fibers of the present invention exhibit not only good gas permeability but also good fluid flow and are suitable for a number of applications including blood oxygenation, ultrafiltration, dialysis, separation of gamma globulin from blood, for ascites treatment, and a variety of other uses that utilize hollow microporous fibers. For certain uses, it may be desirable to render the normally hydrophobic hollow microporous fibers of the present invention hydrophilic.

This can be achieved by any means known to those skilled in the art such as by impregnating the pores of the fibers with a suitable surfactant, such as high molecular weight nonionic surfactants available under the tradename Pluronics from Wyandotte Chemicals Corp. which are prepared by condensation of ethylene oxide with a hydrophobic base formed by condensation of propylene oxide with propylene glycol. Other surfactants include the series of nonionic surfactants available under the tradename Tween which are polyoxyalkylene derivatives of partial long chain hexitol anhydride fatty acid esters. Alternatively, the fibers may be treated with sulfuric acid, chlorosulfonic acid or other such agents to render the fibers hydrophilic.

To utilize the hollow fibers for blood oxygenation, bundles of hollow fibers containing the desired number of fibers can be made by applying an adhesive to each end of a group of pre-arranged parallel hollow fibers. The bundled fibers are then preferably introduced into an elongate fluid-tight tubular casing assembly made of a suitable material such as steel. Each end of the bundled fibers communicates with the outside of the casing while at either end of the casing a means for connecting each end of the fiber bundle to the ends of the casing is provided. Blood can thus be pumped through the hollow fibers. The tubular casing is further provided with valves which open into the interior of the casing and to the outer surface of each of the fibers in the bundles so that a means for circulating oxygen around the hollow fibers is provided. Even if the bundle of fibers were packed as tightly as possible, it should be sufficiently loosely packed to allow a gas to pass between the individual fibers and effectively surround each hollow fiber.

The oxygen gas can then be passed through the outer walls of the hollow fibers and oxygenate the blood passing inside the fiber while carbon dioxide is passed out of the blood through the hollow fiber.

Alternatively, oxygen can be conducted into the center of the hollow fibers and the blood circulated through the sheath whereby they come into contact with the outer surface of the hollow fibers.

Rather than utilizing a double-ended tubular sheath in which both ends are open to allow the passage of blood, it is possible to utilize a permeator in which hollow fiber bundles have been formed into a loop so that the ends of each of the fibers both exit through the same opening. in the tubular casing. 448 747 32 For further illustration of devices capable of utilizing hollow fibers for blood oxygenation, reference is made to U.S. Pat.

The invention is further illustrated by the following examples in which all parts and percentages are by weight unless otherwise indicated.

Example 1 Isotactic polypropylene having a melt index of 5, a weight average molecular weight of 380,000 and a density of 0.90 g / cm 3 is melt spun through a concentric five neck hole jet spinneret. Each jet hole of the spinneret is of the standard type pipe-in-opening with the pipe provided with a low-pressure supply, the pressure being regulated with an air flow measuring device set at 3.8, which indicates a flow rate of 120 cm 3 / minute. The outer diameter of each extrusion orifice (jet hole) of the spinneret is 1.391 mm and the inner diameter of each extrusion orifice is 0.772 mm. The diameter of the trachea inside each extrusion orifice is 0.332 mm. Tablets of polypropylene are placed in a 19.05 mm Barbender extruder and fed into the feed zone of the extruder by the action of gravity.

The extruder device is provided with a feed pump for regulating the melt pressure of the spinneret assembly to provide a passage through the spinneret of 23 g / minute. The temperatures of the feed zone, the feed and melt zones of the extruder are controlled by separate jacket sections. The temperature of the spinneret assembly is regulated with a separate electrically heated jacket and a constant extrusion, i.e. spin temperature of 24500 is maintained as indicated by a thermocouple in the spin nozzle assembly. An adjustable feed pick-up device collects the extruded fibers at a pick-up speed (TUS) of 500 meters / minute. The hollow precursor fibers are consequently drawn with a pull-out or spin ratio of 100: 1. The pick-up roller is located 3.05 meters from the spinneret and the extruded fibers are cooled by passage through air at room temperature, i.e. 25 ° C. The degree of orientation determined by X-ray diffraction analysis described herein is 160. The precursor fibers show an elastic return from 50% elongation at zero return time, 25 ° C and 65% relative humidity of 70%, an average inner diameter of 223 microns. , an average outer diameter of 257 microns and an average wall thickness of 17 microns. The resulting fibers are then blended at a constant length while still wrapped around the pick-up roll by placing the pick-up roll in an oven and heating it to 140 ° C for 30 minutes.

Samples of the expired precursor fiber are then subjected to varying degrees of cold stretching at ambient temperatures as shown in Table I, Experiments 1-6 and then to varying degrees of thermal elongation at 140 ° C which are also shown in Table I, Experiments 1-6. The elongation rate for both hot and cold stretching is also shown in Table I. Cold and hot stretching are achieved using a conventional Bruckner stretching frame and the elevated temperatures during hot stretching are achieved by placing the stretching frame in a forced heat oven. The hot stretched fibers are left in the oven for 30 minutes to achieve heating at the same temperature as used for hot stretching, i.e. 140 ° C. The fibers are kept at a constant length during heating through the tension frame.

In Experiments 7-10, the precursor fiber sample preparation is varied with respect to spin temperature, take-up speed, draw ratio, throughput speed and airflow meter setting as shown in Table I. The degree of orientation (determined by X-ray diffraction analysis described herein) for the precursor fibers prepared in accordance with s is 1st ° And for experiments 9 and 1o 22 °. The elastic 448 747 34 recoil (ER) from 50% elongation at zero recoil time for experiments 9 and 10 is 64%. The elastic return for experiments 7 and 8 was not determined. The degree of cold stretching and hot stretching as well as the elongation rate are also shown in Table I.

The resulting heat-treated microporous hollow fibers are then tested for surface area by nitrogen absorption as described herein and also for oxygen flow. The oxygen flow is determined as follows.

Twenty of the hollow microporous fibers from each experiment with a length of 40.6 cm are pre-arranged in a parallel fiber bundle configuration and then placed in loops so that the 40 open ends of the fibers lie next to each other and are flush in a single plane. The open ends of the fiber loop are then inserted into a short length (31.75 mm) of a hard plastic tube having an inner diameter of 3.18 mm. The fibers are then coated with epoxy resin 12.7 to 15.2 cm from the open looped fiber ends. The plastic tube is then slid down over the resin-coated section so that approximately 5.1 cm of the uncoated fiber bundle protrudes from the tube, with the open ends of the looped fiber bundle extending out of the tube. When the resin has hardened, the open ends of the looped fiber bundle are trimmed at the same level as the plastic tube. To preserve the open circularity of the open fiber ends, clean cutting is achieved by first immersing the fibers in liquid nitrogen, then dipping them in isopropyl alcohol to fill the inner cavities with liquid, re-immersing the fibers in liquid nitrogen for about 1 hour. 5 minutes for freezing the alcohol and then placing them across a small block of wood that is also immersed in the liquid nitrogen. The open ends of the fibers can then be easily cut with a razor blade against the wooden block without damage. The tubular fiber unit is then sealed in a Sweglok insert with epoxy resin that leaves a 19.05 mm extension of the tube exposed over the insert. The epoxy-baked fiber assembly is then inserted into a 17.8 cm length of 9.52 mm diameter copper pipe 448 747 and the Sweglok: C> insert is sealed with suitable devices.

For convenient access, a 3-way T-device is attached to the outer end of the copper tube (with respect to the Sweglok device) and one of the outlets of the T-device is sealed.

One end of a rubber hose is attached to the open opening of the T-device and the other end is inserted into an inverted graduated cylinder which is filled with water and immersed in a water bath. Oxygen is then passed through the open fiber ends through the fiber walls and collected in the measuring cylinder. The gas pressure is maintained first at 0.35 kp / cm 2 and then at 0.7 kp / cm 2 as shown in Table I. The gas flow (Jg) in cm 3 / cm 2 minute is determined from the equation described here.

As can be seen from the results in Table I, oxygen permeability values or flows greater than about 80 cm 3 / cm 2 per minute can be obtained from hollow microporous fibers made in accordance with the process of the present invention. Such permeability values, when normalized to flow per micron fiber wall thickness, represent a significant improvement over normalized gas permeability values for microporous films in flow per micron film thickness when such films are prepared in accordance with the cold stretch / hot stretch process of U.S. Pat. , 80l, 404.

The normalized flow of the hollow microporous fibers having an oxygen permeability at 0.7 kp / cm 2 of 82.9 cm 3 / cm 2 minute is obtained, for example, by dividing this permeability by the average wall thickness of 15 microns, which gives a normalized flow per micron. fiber wall thickness of 5.5.

Similarly, a microporous film made by the cold stretching / hot stretching process of U.S. Pat. No. 3,801,404 and having a film thickness of about 25.5 microns has an oxygen flow of about 44 cm 3 / cm 2 per minute.

When this gas flow is normalized for comparison with the normalized gas flow of the hollow microporous fibers of the present invention, a flow per micron film thickness of 1.73 is obtained. Similar comparisons can be made with experiments l - 9 in example l.

Thus, hollow microporous fibers can be prepared in accordance with the process of the present invention which have a normalized flow which is about 3 times the normalized flow of microporous films produced by the process of the above-described patent. 1) m.nm Üov Slwn m .. maN an am am SN van øßw cn av om men on ~ on SS SSS SSS SS SS å ä 2 S ~ SSS SS ... S 2 8 S3 SS S 7 ... I 0.: .. SS SS .SSS 3 S 2 2 SS S3 ... m 2 SS S3 SS S 4 ... S S2 SS I SSS .SS 3 3 2 than S3 ... S 2. SS SSS SS S 7.

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Lä: Lä ... wnnnmlSßwnmzmaw _: wo: mm .n 448 747 38 Comparative Example 1 Example 1 is repeated except that the inner diameter of the precursor fiber is reduced to below 140 microns as shown in Table II. The degree and elongation rate of the cold stretch and the hot stretch as well as the process variables are also compiled in Table II. Note that the microporous hollow fiber wall thickness is assumed to remain substantially unchanged and has not been determined empirically.

Experiments 1-10 illustrate the reduced oxygen permeability obtained when the average precursor fiber inner diameter is substantially below 140 microns, for example about 86 microns, compared to the gas permeability obtained from the experiments in Example I which used the precursor fiber inner diameter. l40 microns. The maximum oxygen flow obtained was only 1.0 cm3 / cm 2 per minute.

Experiments 7 - 10 illustrate a significant reduction in the gas permeability when the hot stretching step is eliminated or the elongation ratio is selected so that the degree of cold stretching is greater than the degree of warm stretching.

Experiments 11 - 14 illustrate unsuccessful attempts to improve the gas permeability of cold stretched fibers by allowing them to relax 10% (ie experiments 11 and 12) and by allowing the cold stretched fibers to relax 10% at a temperature of 130 ° C (ie runs 13 and 14). ).

Experiments 15-26 illustrate the gas permeability values obtained under varying process conditions using average internal precursor fiber diameters of 110 microns.

As can be seen, the gas permeability is significantly reduced compared to the gas flow for precursor fibers having average inner diameters used in the experiments in examples. 448-747 9 3 ß.m fi F! now .

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Claims (7)

42 44.3 _74? PATENT REQUIREMENTS 1. A process for producing hollow, open-cell microporous polypropylene fibers having an oxygen flow of at least 35 cm 3 / cm 2 minute at 0.7 kp / cm 2 characterized in that it comprises: (A) melt spinning at a temperature of at least 230 ° C of isotactic polypropylene having a melt index of at least 1 in a manner sufficient to obtain hollow, non-porous polypropylene precursor fibers, uptake of the precursor fibers with an elongation ratio of at least about 40, whereby the melt spinning is performed in a manner sufficient to impart to the precursor fibers after uptake an average inner diameter of at least about 140 microns, a ratio of average inner diameter to average wall thickness of from about 8: 1 to about 40: 1, a degree of orientation determined from half the width of a wide-angle (110) X-ray diffraction arc of not more than 25 °, and an elastic return from 50% elongation at 2S ° C, 65% relative humidity, and return time zero, of at least 50%; (B) annealing the precursor fibers at a temperature between about 50 ° C and less than 165 ° C for a period of about 0.5 seconds to about 24 hours; (C) cold stretching the non-porous hollow precursor fibers in the direction of their length at a temperature higher than the glass transition temperature of the precursor fiber and not higher than about 100 ° C to impart porous surface areas! to the walls of the fiber which are perpendicular to the cold stretching direction; ! (D) hot stretching the expired, cold stretched hollow fibers from (C) in the same direction for cold stretching at a temperature above the cold stretching temperature and below the melting point of the polypropylene to impart an open-cell, microporous configuration to the hollow fiber walls, wherein the degree of cold stretching and hot stretching is carried out in a manner sufficient to control the average inner diameter of the resulting hot-stretched, hollow microporous fibers to at least 100 microns and to achieve a total degree of combined stretching of from about 80 to about 200 microns. %, an elongation ratio of from about 1: 3 to about 1:20 and an elongation rate of from about 10 to about 200% / minute; (E) heat treating the resulting hot stretched fibers from (D) under tension to produce dimensionally stable, open-celled, hollow, microporous fibers having an average inner diameter of at least 100 microns. A method according to claim 1, characterized in that the spinning temperature is from about 240 to about 280 ° C, the pull-out ratio at which the hollow precursor fibers are taken up is at least about 40, the average inner diameter of the hollow precursor fibers after uptake is regulated so as to be from about 140 to about 400 microns, the cold stretching is carried out at a temperature of from about 15 to about 70 ° C, the hot stretching is carried out at a temperature of about 130 to about 145 ° C, the total degree of combined cold and hot stretches are from about 80 to about 155% and the elongation ratio is from about 1: 3 to about 1:10. 3. The method of claim 1, wherein the melt spinning temperature is from about 240 to about 250 ° C. 4. A method according to claim 1, characterized in that hot stretching and heat treatment are combined into a single step by successively hot-stretching the cold-stretched hollow fibers in a plurality of discrete stretching steps. f-L (ü 44 448 747 A method according to claim 1, characterized in that the melt spinning takes place at a temperature of about 240 to about 280 ° C, the isotactic polypropylene has a melt index of at least about 5, the uptake of the hollow precursor fibers takes place with a pulling ratio of from about 40 to about 100, the precursor fibers after taking up have an average inner diameter of from about 200 to about 300 microns, a ratio of average inner diameters to an average wall thickness of from about 1011 to about 30 microns; the annealing of the hollow precursor fibers takes place at a temperature of about 130 to about 145 ° C for about 30 minutes; the cold stretching of the non-porous precursor is performed at a temperature of from about 15 to about 70 ° C with a degree of stretching of about 20% based on the original precursor fiber length, and an elongation rate of from about 10 to about 100% / minute; The stretching of the cold stretched hollow fibers is carried out at a temperature of about 130 to about 145 ° C with a degree of stretching of about 60 to about 100%, and an elongation rate of about 10 to about 100% / minute and the heat treatment of the resulting the hollow fibers occur at a temperature of about 130 to about 160 ° C. 6. .6. Whole open-cell microporous polypropylene fibers having an oxygen flow of at least 35 cm3 / cm2 per minute at 0,7 kp / cm, characterized in that they are produced by a process comprising: (A) melt spinning at a temperature of at least 230 ° C of isotactic polypropylene having a melt index of at least 1 in a manner sufficient to obtain hollow non-porous polypropylene precursor fibers, uptake of the precursor fibers with an elongation ratio of at least about 40, whereby the melt spinning is performed in a manner sufficient to feed the precursor fibers after uptake impart an average inner diameter of at least about 140 microns, an average inner diameter to average wall thickness ratio of from about 8: 1 to about 40: 1, a 448,747 w orientation degree determined from half the width of an (110) X-ray diffraction arc of not more than 250, and an elastic return from so% clearance at 2s ° c, relative humidity, and return time zero, of at least 50%; (B) annealing the precursor fibers at a temperature between about 50 ° C and below 16,500 for a period of about 0.5 seconds to about 24 hours; (C) cold stretching the non-porous hollow precursor fibers in the direction of their length at a temperature higher than the glass transition temperature of the precursor fiber and not higher than about 100 ° C to impart porous surface areas to the walls of the fiber which are perpendicular to the cold stretching - the direction; (D) hot stretching the expired cold stretched hollow fibers from (C) in the same direction for cold stretching at a temperature above the cold stretching temperature and below the melting point of the polypropylene to impart an open-cell, microporous configuration to the hollow fiber walls, wherein the degree of cold stretching carried out in a manner sufficient to control the average inner diameter of the resulting wart-stretched hollow microporous fibers to at least 100 microns and to achieve a total degree of combined elongation of from about 80 to about 200%, an elongation ratio of from about about 1: 3 to about 1:20, and an elongation rate of from about 10 to about 200% / minute; (E) heat treating the resulting hot stretched fibers from (D) under tension to produce dimensionally stable, open-oily, hollow, microporous fibers having an average diameter of at least 100 microns. .gu fp 448 747 46 Use of hollow, open-celled, microporous polypropylene fibers having an oxygen flow of at least 35 cm / cm2 per minute at 0,7 kp / cm2 and produced by a process comprising: (A) melt spinning at a temperature of at least 230 ° C of isotactic polypropylene having a melt index of at least 1 in a manner sufficient to obtain hollow non-porous polypropylene precursor fibers, uptake of the precursor fibers having an elongation ratio of at least about 40, wherein the melt spinning is carried out in a manner sufficient to impart to the precursor fibers after uptake an average inner diameter of at least about 140 microns, a ratio of average inner diameter to average wall thickness of from about 8: 1 to about 40: 1, a degree of orientation determined from half the width of an (110) X-ray diffraction arc of not more than 250, and an elastic return from 50% elongation at 25 ° C, 65% relative humidity, and return time zero, of at least 50%; (B) annealing the precursor fibers at a temperature between about 50 ° C and less than 165 ° C for a period of about 0.5 seconds to about 24 hours; (C) cold stretching the non-porous hollow precursor fibers in the direction of their length at a temperature higher than the glass transition temperature of the precursor fiber and not higher than about 100 ° C to impart porous surface areas to the walls of the fiber which are perpendicular to the cold stretching - the direction; (D) hot stretching the expired cold stretched hollow fibers from (C) in the same direction for cold stretching at a temperature above the cold stretching temperature and below the melting point of the polypropylene to impart an open-cell, microporous configuration to the hollow fiber walls, wherein the degree of cold stretching is carried out in a manner which is sufficient to control the average inner diameter of the resulting stretched hollow microporous fibers to at least 100 microns and to achieve a total degree of combined stretching of from about 80 to about 200%. an elongation ratio of from about 1: 3 to about 1:20, and an elongation rate of from about 10 to about 200% / minute; (E) heat treating the resulting hot stretched fibers from (D) under tension to produce dimensionally stable, open-celled, hollow, microporous fibers having an average diameter of at least 100 microns in a blood oxygenator. 1, »