DE69209831T2 - Non-woven fabric - Google Patents

Description

Technical area

The present invention relates to meltblown nonwoven mats, i.e., webs produced by extruding molten fiber-forming material through orifices in a nozzle into a high-velocity gaseous stream, which stream impinges on the extrudate and attenuates it into fibers, often in the microfiber range, averaging 10 micrometers or less.

General state of the art

During the more than twenty years that melt-blown fibers have been in widespread use, for example, in filtering, battery electrode separation, and insulation, there has been a recognized need for extremely small diameter fibers and webs with good tensile strength. However, it has always been recognized that the tensile strength of melt-blown fibers has been low, e.g., less than the tensile strength of fibers produced by conventional melt spinning processes (see the article "Melt-Blowing -- A One-Step Web Process For New Nonwoven Products," by Robert R. Buntin and Dwight D. Lohkcamp, Volume 56, No. 4, April 1973, Tappi, page 75, paragraph in columns 2 and 3). At least until 1981, it was generally doubted in the art "that melt-blown fabrics per se will ever have the strengths exhibited by conventional nonwoven mats produced by melt spinning, where fiber attenuation occurs below the polymer melting point, resulting in crystalline orientation with resulting higher fiber strength" (see the paper "Technical Developments In The Melt-Blowing Process And Its Application In Absorbent Products," by Dr. W. John McCullock and Dr. Robert A. VanBrederode, presented at Insight '81, Publishing Rights Marketing/Technology Service, Inc., Kalamazoo, MI, page 18, under the heading "Strength").

The low strength of meltblown fibers limited the applicability of the fibers and consequently various attempts were made to overcome this low strength. U.S. Patent No. 3,704,198 to Prentice teaches one such attempt in which a meltblown fabric is "fusion bonded" such as by calendering or point bonding at least a portion of the fabric. While the strength of the fabric may be partially improved by calendering, the fiber strength remains unaffected and the overall strength remains below the desired level.

Others in the art have proposed blending high strength bicomponent fibers into meltblown fibers prior to collecting the web or laminating the meltblown web to a high strength substrate such as a spunbond mat (see U.S. Patents 4,041,203, 4,302,495 and 4,196,245). These steps add cost and weaken the microfiber feature of the web, and are unsuitable for many purposes.

With respect to fiber diameter, there is a recognized need for fibers with uniformly small diameters and with extremely high aspect ratios, as described, for example, in U.S. Patent US-A-4,118,531 to Hauser (column 5) and in U.S. Patent US-A-4,215,582 to Kubik et al. (columns 5 and 6). Despite the ability to produce meltblown fibers with very small average fiber diameters, the fiber size distribution is relatively wide, as recognized by Hauser, with fibers in the range of 6 to 8 micrometers being suitable for use with Fibers with an average fiber diameter of 1 to 2 micrometers (Examples 5-7) are present. Difficulties also exist in eliminating "shots" with larger diameters, as described, for example, in the above-mentioned article by Buntin et al. on page 74, first paragraph of column 2. A shot is formed when the fibers break in the vortex flow of impinging air during the meltblowing process. Buntin states that a shot is unavoidable and has a larger diameter than the fibers.

U.S. Patent No. 4,622,259 to McAmish et al. relates to meltblown nonwoven fabrics particularly useful as medical fabrics and which are said to have improved strength. These fabrics are produced by supplying secondary air at high velocity to a point near where the fiber-forming material is extruded from the meltblowing die. As best shown in Figure 2 of this patent, the secondary air is supplied from each side of the stream of meltblown fibers exiting the meltblowing die, with the secondary air being supplied generally perpendicular to the fiber stream. The secondary air mixes with the primary air which has impacted the fiber-forming material and formed the fibers, and the secondary air is directed to travel in a direction more parallel to the path of the fibers. The mixed primary and secondary air then carries the fibers to a collector. The patent mentions that the use of secondary air produces fibres that are longer than those produced by a conventional melt blowing process and that the fibres have a lower autogenous bond after fibre collection; with regard to the latter property, the patent mentions that it has been found that the strength of the individual fibres is higher. stated that strength is dependent upon the degree of molecular orientation, and the patent mentions (column 9, lines 21-27) that the high speed secondary air used in the present process is essential to increasing the time and distance over which the fibers are attenuated. The cooling effect of the secondary air increases the likelihood that the molecular orientation of the fibers will not relax excessively as the speed of the fibers decreases as they accumulate on the screen.

The fabrics are produced from the accumulated spunlace by embossing the spunlace or by adding a chemical binder to the spunlace and it is mentioned that the fabrics have higher strengths, e.g. a minimum Grab tensile strength to weight ratio of more than 0.8 N per gram per square meter and a minimum Elmendorf tensile strength to weight ratio of more than 0.04 N per gram per square meter. It is also reported that the fibers have a diameter of 7 micrometers or less. However, no information is given that the process produces fibers having a narrow fiber diameter distribution or fibers with average diameters of less than 2. micrometers, essentially continuous fibers or spunlace fibers which are essentially shotless.

EP-A-0 322 16 describes a process for producing oriented, meltblown microfilters using a specific orientation chamber. This orientation process is described as enabling the production of fibers that generally have smaller diameters than fibers produced under conventional meltblowing conditions without an orientation chamber. It is also described that the fibers have a narrower diameter distribution than conventional, non-oriented microfibers.

EP-A-0 190 012 discloses a conventional meltblowing process whereby a nonwoven fabric is produced composed of fibers having an average single fiber denier of 0.005 to 2.0.

Disclosure of the invention

According to the present invention, there are provided novel meltblown fibers and nonwoven fabrics having greatly improved fiber diameter size distribution, average fiber diameter, fiber and web strength, and low wefts. The novel meltblown fibers exhibit significantly higher orientation and crystallinity as compared to prior oriented meltblown fibers as a result of being produced by a novel process which, in brief, comprises: extruding fiber-forming material at a low polymer feed rate of 0.01 to 3 g/hour/orifice onto a metering device and thereafter through the orifices of a 0.025 to 0.5 mm diameter die into a controlled high velocity gaseous stream at reduced pressure (about 70 kPa or less, with a 4 mm air gap) whereby the extruded material is rapidly distributed into the fibers; directing the dispersed or attenuated fibres and the gaseous stream into a first open end, i.e. the inlet end, of a tubular chamber located near the nozzle and extending in a direction parallel to the path of the dispersed or attenuated fibres as they leave the nozzle; supplying air having both radial and axial components into the tubular chamber such that the air blown along the axis of the chamber has a velocity sufficient to keep the fibres under tension as they pass through the chamber, and wherein the air is directed substantially along the entire length of the chamber perpendicular to the longitudinal axis of the chamber; optionally passing the refined fibres into a second tubular chamber in which cooled fibres are further drawn by air blown along the axis of the chamber; and collecting the fibres after they emerge from the opposite or outlet end of the last tubular chamber.

Short description of the drawings

Show it:

Figures 1 and 2 show a side view and perspective views, respectively, of various devices that can be used to carry out processes according to the invention for producing materials according to the invention;

Figure 3 is a graph showing the theoretical relationship of polymer flow rate to fiber diameter with respect to continuous submicron fibers; and

Figure 4 is a scanning electron microscope image of the submicron fibers from Example 5.

Precise description

A representative apparatus for producing blown fibers or a blown fiber web according to the invention is shown schematically in Figure 1. A portion of this apparatus which forms the blown fibers may be as described in the following documents: "Superfine Thermoplastic Fibers", Wente, Van A., Industrial Engineering Chemistry, Volume 48, page 1342 ff. (1956), or Naval Research Laboratories Report No. 4364, entitled "Manufacture of Superfine Organic Fibers", by Wente, VA, Boone, CD and Fluharty, EL, published May 25, 1954.

This portion of the device comprises a nozzle 10 having a series of aligned, side-by-side, parallel nozzle orifices 11, one of which is visible in the sectional view through the nozzle. The orifices open from the central nozzle cavity 12.

Fiber-forming material is introduced into the die cavity 12 from an extruder (not shown) through an orifice 13. On each side of the series of orifices 11 are air gaps 15 which transmit heated air at a very high velocity. This air, referred to as primary air, strikes the extruded fiber-forming material and is rapidly drawn, attenuating the extruded material into a fiber mass. The primary air is generally heated and supplied to both air gaps 15 at substantially equal pressure. The air is also preferably filtered to prevent dirt or dust from interfering with uniform fiber formation. The air temperature is generally maintained at a value higher than the temperature of the melt polymer in the die orifices. Preferably, the temperature of the air is at least 5°C above the temperature of the melt. Temperatures below this can cause excessive cooling as the polymer exits the die, making alignment in the chambers difficult. Too high a temperature can cause excessive polymer degradation or increase the tendency of the fiber to break.

From the meltblowing nozzle 10, the fibers extend to a primary tubular alignment chamber 17. In this specification, the term "tubular" refers to any axially elongated structure having open ends at each axially opposite end, with that axis surrounded by walls Generally, the chamber is a rather thin, wide, box-like chamber having a width slightly greater than the width of the nozzle 10 and a height (18 in Figure 1) sufficient to direct or orient the air so that it flows smoothly through the chamber without undue loss of velocity and so that pulp sprayed from the nozzle passes through the chamber without contacting the chamber walls. Too great a height would require excessively large volumes of air to provide a tension-generating air velocity. With a solid wall chamber 17, good results have been obtained with a height of 10 millimeters or more and it has been found that a height greater than about 25 millimeters is not necessary.

The walls 26 along the width of the chamber 17 can be made of an air-permeable or porous material. Thereafter, a secondary cooling diffuse air stream can be introduced along the width of the chamber. This air stream serves the function of increasing the polymer solidification and/or crystallization rate in the cooling chamber 17. This secondary cooling air also helps to keep the fibers in the center of the chamber 17 and away from the walls 26. However, the air pressure of this cooling air stream must not be so high as to create turbulence in the chamber. Generally, a pressure between 2 and 15 psi has been found to be acceptable.

Orientation air is introduced into the alignment chamber 17 through the openings 19 located near the first open end of the chamber where the fibers contained in the primary air enter the chamber from the nozzle. The orientation air is preferably supplied from both sides of the chamber (ie from opposite sides of the fiber stream introduced into the chamber) around the curved surfaces 20, which surfaces may be referred to as Coanda surfaces. In general, Coanda surfaces with an infinite range of radiuses can be used. However, as the radius decreases to zero, the angle becomes too acute and the air tends to separate from the surface. Radii as small as 1/8 inch have been used, with radii generally in the range of 0.5 to 1.5 inches. With respect to the alignment chamber 17, a larger radius Coanda surface is preferred when the polymer used is less crystalline or has a slow crystallization rate. For polymers with low crystallinity, the air preferably exits an opening adjacent to the Coanda surface at an angle to a line perpendicular to the axial centerline of the chamber. At an angle of zero degrees, the air would exit the opening parallel to the axial centerline. In general, the exit angle of the orientation air has been between 0 and 90 degrees, although larger angles are possible. An air exit angle of 30 to 60 degrees is generally preferred. A smaller exit angle of the orientation air is acceptable when a cooling chamber is used before the orientation or alignment chamber, or when a highly crystalline polymer is melt blown.

The orientation air introduced into the chamber bends as it exits the opening and travels around the Coanda surface, providing a predominantly axial flow along the longitudinal axis of the chamber. The path of the air is relatively uniform and rapid, and it draws the fibers extruded from the meltblowing nozzle 10 evenly into the chamber. In contrast to the fibers exiting a meltblown nozzle, which normally vibrate relatively strongly shortly after leaving the nozzle, the fibers exiting the meltblowing nozzle in the inventive Process to run uniformly or evenly into the center of the chamber with a surprisingly planar distribution and to flow lengthwise through the chamber without major vibrations.

After the fibers exit the chamber 17, they normally exhibit a vibrational motion, represented by the vibration line 21 and the dashed lines 22, which represent the general outlines of the fiber stream. This vibration is due to the expansion or taper at the outlet of the chamber 17. However, this vibration does not result in significant fiber breakage behavior, as would be the case if the vibration occurred near the meltblowing die orifice. The alignment chamber provides significant reinforcement of the fiber so that vibrational behavior after the chamber, combined with a resulting increase in the maximum stress to which the fibers are subjected, can be better tolerated without fiber breakage.

As shown in Figure 1 with respect to the embodiment with an alignment chamber 17, the chamber 17 is preferably flared at its outlet end 23. It has been found that this taper provides that the fibers have a more random or isotropic arrangement in the fiber stream, but without fiber breakage. For example, a combined web of fibers according to the invention that has passed through a chamber without a flared outlet tends to have a fiber arrangement in the machine direction (ie more fibers tend to be aligned in a direction parallel to the direction of travel of the collector than to be aligned transversely to that direction). On the other hand, fiber webs collected from a chamber with a flared outlet have a better balance between machine and and transverse orientation. The taper may occur in height and width, that is, on the axis or in the plane of the drawing, as well as in a plane perpendicular to the side of the drawings. More usually, the taper occurs only on the axis in the plane of the drawing, that is, the large area sides or walls on opposite sides of the fiber stream passing through the chamber. It is believed that a taper at an angle (the angle 0) between the dashed line 25, which is parallel to the longitudinal axis of the chamber, and the flared side of the chamber of from 4 to 70 is ideal for smooth isotropic deposition of the fibers. The length 24 of the section of the chamber over which the taper occurs (which may be referred to as the randomization section of the chamber) depends on the velocity of the orienting air and on the diameter of the fibers produced. At lower velocities and for smaller fiber diameters, shorter lengths are used. Conically extended lengths in the range of 25 to 75 centimeters have proven to be suitable.

The orientation air enters the orientation chamber 17 at a high velocity, the velocity being sufficiently high to keep the fibers under tension as they pass lengthwise through the chamber. A planar, continuous path through the chamber indicates that the fibers are unbroken and under high tension. The air velocity required for orientation, which is determined by the pressure at which the air is introduced into the orientation chamber and the dimensions of the openings or gaps 19, depends on the fiber-forming material used and the diameter of the fibers. For most applications, optimum values for the corresponding tension have been found to be velocities corresponding to pressure conditions of about 70 psi (approximately 500 kPa) with an opening gap width of 19 (dimension 30 in Figure 1) of 0.005 inches (0.013 cm). However, in conjunction with some polymers, such as hexamethylenediamine adipic acid polymer, and the gap width mentioned, pressure conditions as low as 20 to 30 psi (140 to 200 kPa) have been used. When the chamber 17 is used primarily as a cooling chamber, pressure conditions as low as 5 psi can be used for the orientation air.

Surprisingly, many fibers can flow through the chamber for a long distance without touching the upper or lower surfaces of the chamber. However, a secondary cooling air stream is preferably introduced into the first chamber (17 or 37) in a diffuse manner perpendicular to the fibers through the side walls of the chamber. The secondary cooling air stream is preferred for polymers with a low crystallization rate, since these have an increased stickiness and consequently a tendency for isolated fibers to stick to the side walls of the chamber. Through its cooling or hardening effect, the cooling air stream also increases fiber strength, thereby reducing the likelihood of fiber breakage before, in or after the first chamber (17 or 37).

Generally, the chambers are at least about 40 centimeters long (although shorter chambers may be used for low production or when the first chamber serves primarily as an alignment chamber), with the chambers preferably being at least 100 centimeters long to achieve desirable alignment and mechanical properties in the fibers. With shorter chamber lengths, faster air velocities can be used so that fiber alignment can still be achieved. The inlet end of the first chamber is generally within 3-10 centimeters of the nozzle, and despite the turbulence typically present near the outlet of a meltblowing nozzle, already mentioned above, the fibers are aligned or drawn into the chamber in an organized array.

After exiting the alignment chamber or the last chamber (17 or 38), the solidified fibers slow down and, during this slowing down, they are combined at the collector 26 as a web 27, namely as a possibly misaligned mass of entangled fibers. The collector or the collecting device can be in the form of a cylindrical, fine-pored screen or drum, a rotating mandrel, or a belt conveyor. Behind the collecting device there can be a gas extraction device which assists in the gas extraction.

The accumulated fibrous web may be removed from the accumulator and wound onto a supply roll, with adjacent turns of the roll preferably separated by a spacer. At the time of fiber accumulation and web formation, the fibers are fully consolidated and aligned. These two characteristics tend to give the fibers a high modulus, and it is difficult to slow down and entangle high modulus fibers sufficiently to form a treatable, coherent web. Webs comprising only aligned meltblown fibers do not have the cohesion that an accumulated web of conventional meltblown fibers has. For this reason, the cohesive fibrous web is often fed directly to an apparatus for forming an integral, treatable web, for example, by bonding the fibers by uniformly calendering the web in regions or in places (generally in an area or range of 5 to 40 percent), by densifying the web into a continuous structure such as by hydraulic entanglement, by bonding the web together ultrasonically, by adding a binder to the fibers in dissolved or molten form and solidifying the binder, by adding a solvent to the web to solvent-bond the fibers together, or by making bicomponent fibers wherein the web is subjected to conditions such that one component melts, thereby fusing adjacent or intersecting fibers together. Further, the combined web may be applied to another web, such as a web passing over the collector, and a second web may be applied to the uncovered surface of the combined web. The accumulated or combined textile material may either not be attached to the carrier or cover sheet or intermediate layer, or it may be attached to the sheet or intermediate layer by bonding with heat, solvent or added binder.

The fiber diameter is on average less than 5 micrometers, preferably less than about 2 micrometers, with the diameter of the largest fibers preferably deviating from the average by no more than about 1.0 micrometer, and generally 90 percent or more of the fibers are within a range of less than 3.0 micrometers. Preferably they are within a range of about 2.0 micrometers or less, and most preferably within a range of 1.0 micrometer or less.

Figure 2A shows an embodiment that is suitable for the production of fibers with extremely low diameters, the diameters generally averaging about 2 micrometers or less, and a very narrow range of fiber diameters exists (e.g., 90 percent within a fiber diameter of 1.0 micrometer or less). The fiber-forming material from the extruder 30 is fed to a metering device which includes at least one precision metering pump 31 or discharge device or the like. The metering pump 31 tends to equalize the flow from the extruder 30. It has been found that for uniform, substantially continuous fibers of excessively small diameter, the polymer flow rate through each orifice in the die must generally be relatively low. For average fiber diameters of less than 1 or 2 micrometers, polymer flow rates for most polymers are in the range of 0.01 to 3 g/hour/orifice, with values between 0.02 to 1.5 g/hour/orifice being preferred. In order to achieve these low flow rates, conventional extruders are operated at low screw rotation speeds, even with a high density of orifices in the die. This results in a polymer flow rate with slight variations. It has been found that these slight variations in the flow state have a large adverse effect on the size distribution and continuity of the resulting meltblown fibers with extremely small diameters.

The metering device preferably uses a system of three precision pumps as shown in Figure 2A. The pumps 32 and 33 share the flow from the metering pump 31. The pumps 32 and 31 can be operated by a single drive, with the pumps being operated in fixed ratio to each other. In this arrangement, the speed of the pump 33 is continuously controlled so that pump 32 provides a polymer feed at a constant pressure as measured by a pressure transducer. Pump 33 generally serves as a drain to remove excess polymer feed from the extruder and pump 31 while pump 32 provides a smooth flow of polymer to die 35. More than one pump 32 may be used to supply polymer to a series of dies (not shown). Preferably, a filter 34 is provided between pump 32 and die 35 to remove any impurities. Preferably, the mesh size of the filter is in the range of 100 to 250 holes/square inch or more. While this system is preferred, other arrangements may be used which supply polymer to the orifices at the appropriately low rate and with substantially no fluctuations.

The polymer is fed to the die at a flow rate per orifice suitable for producing the desired fiber diameter, as shown, for example, in the hypothetical model of Figure 3, in which the Y axis represents the logarithm of the resin flow rate (in grams/minute/orifice) while the X axis represents the corresponding 0.9 density isotactic polypropylene fiber diameter in microns at two fiber speeds (upper line equal to 400 m/second and lower line equal to 200 m/second). This confirms the need to reduce the flow rate to produce uniform diameter microfibers. It will be seen that a very low polymer flow rate is required to produce continuous microfibers with very small mean diameters using the process of the invention. The total theoretical polymer feed rate to the die is dependent on the number of orifices. This suitable polymer feed rate is then provided, for example, by the measuring device. However, the Inventive methods for realizing uniform, continuous, high strength, small diameter fibers at such low polymer flow rates are not known or foreseeable from conventional melt blowing techniques.

Suitable orifice diameters for producing uniform fibers with average diameters of less than 2 micrometers are in the range of 0.025 to 0.50 mm, with a range of 0.025 to 0.05 being preferred (available, for example, from Ceccato Spinnerets, Milan, Italy, or from Kasen Nozzle Manufacturing Corporation, Ltd., Osaka, Japan). Suitable aspect ratios of these orifices are in the range of 200 to 20, with a range of 100 to 20 being preferred. For the preferred orifices, high orifice densities are preferred to increase polymer throughput. Generally, orifice densities of 30/cm are preferred, with densities of 40/cm or more being preferred.

In producing uniform fibers with average fiber diameters of less than 2 microns, the primary air pressure is reduced, thereby reducing the tendency of the fibers to break while continuing to attenuate and draw the polymer melt streams extruded from the die orifices. Generally, air pressure values of less than 10 pounds per square inch psi (70 kPa) are preferred, with about 5 pounds per square inch (35 kPa) above that being preferred, with an air gap width of about 0.4 mm. The low air pressure reduces turbulence, allowing a continuous fiber to be blown into chamber 17 or 37 prior to fiber breakage by the turbulence created during melt blowing. The continuous fiber fed to chamber 17 or 37 is then drawn through the orientation air (into chamber 17 or 37 and/or 38). The temperature of the primary air preferably corresponds almost to the temperature of the polymer melt (e.g. about 10ºC above the temperature of the polymer melt)

The fibers must be pulled out of the meltblown region at the outlet of the die face through the first and/or second chambers to maintain the appropriate tension of the stress line. The chambers (17 in Figure 1 and 37 and/or 38 in Figure 2A) ensure that the fibers do not exhibit the oscillation effect that normally occurs with meltblown fibers at the outlet of a meltblown die. When the fibers are subjected to these oscillation forces for randomization purposes, the fibers are strong enough to withstand the forces without breaking. The resulting aligned fibers are virtually continuous and no fiber ends were observed when the resulting microfiber webs were viewed under a scanning electron microscope.

From the nozzle orifices, the fiber-forming material is entrained in the primary air and then in the orientation air and secondary cooling air as described above for chamber 17 and chamber 37 (which may be used with or without chamber 38), respectively. In a preferred arrangement, the material exits chamber 37 and is further attenuated in chamber 38. Tubular chamber 38 functions in a similar manner to chamber 37. When secondary chamber 38 is employed, it serves primarily for orientation, in which case the air pressure is generally at least 50 psi (344 kPa) and preferably at least 70 psi (483 kPa) with an air orifice gap (not shown) of 0.005 inches (0.13 cm). When this secondary chamber 38 is used, the corresponding pressure values in the first chamber 37 with an identical gap width are generally between 5 psi and 15 psi (35 to 103 kPa). In this In this case, the first chamber 37 serves primarily as a cooling chamber, with a slight degree of alignment occurring.

The secondary chamber 38 is generally located 2 to 5 cm from the outlet of the first chamber, the first chamber having a taper as described above. The dimensions of the secondary chamber correspond substantially to the dimensions of the first chamber 37. When the secondary chamber 38 is present, its outlet end 40 preferably has a taper as described above with respect to the embodiment of Figure 1.

The randomization of the fibers is increased by the use of an air stream immediately before the fibers reach the flared outlet 40. This can be accomplished by an entangling air stream provided by the chamber walls. This entangling air stream can be provided through openings in the side walls (preferably along the width) and preferably near the outlet end 40 of the chamber 38. Such an air stream can also be used in an arrangement as described with respect to Figure 1.

The embodiment described above is primarily used to realize continuous fibers with extremely small diameters, such as fibers with an average diameter of less than 2 micrometers, with very narrow ranges of fiber diameters, and with very high fiber strength. This combination of properties or characteristics is unique for a microfiber fabric and is highly desirable for certain uses, such as filtering or insulation.

As already described, it is assumed that the inventive aligned, meltblown fibers are continuous, which is clearly different from fibers formed by conventional meltblowing processes in that these fibers are typically discontinuous. The fibers are fed unbroken to the orientation chamber(s) (or first to the cooling chamber and then to the orientation chamber) and the fibers thereafter pass generally uninterrupted through the orientation chamber. The chamber(s) apply a tension along a tension line which aligns the fiber to a considerable extent and prevents the fibers from vibrating more until they have been fully aligned. There is no evidence of fiber ends or wefts (solidified beads of fiber-forming material which occur when fibers break, and where release of the tension allows the material to contract again) in the combined web. These features are also present in the embodiment in which the mean diameter of the fibers is less than 2 microns, which is particularly notable given the low strength of the extremely small diameter polymer streams exiting the die orifices. Furthermore, the fibers exhibit very little, if any, thermal bonding between the fibers.

The fibers can be made from a variety of different fiber-forming materials. The typical polymers used to form meltblown fibers include polypropylene, polyethylene, polyethylene terephthalate and polyamide. Polycaprolactam and hexamethylenediamine adipic acid polymer are particularly suitable materials because they form fibers with very high strength.

The fibers and textiles according to the invention can be electrically charged in order to improve their filtering properties, whereby the charges can be introduced, for example, during the formation of the fibers, as described in U.S. Patent US-A-4,215,682. However, the textile can also be charged after formation, as described in U.S. Patent US-A-3,571,679; see also U.S. Patents US-A-4,375,718, US-A-4,588,537 and US-A-4,592,815. Electrically charged fibers according to the present invention preferably contain polyolefins and in particular polypropylene, since these materials maintain a charge state particularly well.

Fiber spunbonds of the invention may contain other ingredients in addition to microfibers. For example, adhesive coatings may be sprayed onto a spunbond to improve the feel of the spunbond. Additives, such as dyes, pigments, fillers, surfactants, abrasive particles, light stabilizers, flame retardants, absorbents, medicaments, etc., may also be added to the spunbonds of the invention by adding them to the fiber-forming liquid of the microfibers, or by spraying these additives onto the fibers as they are being formed or after the spunbond has been collected or combined.

A complete fabric according to the invention can have very different thicknesses. In most applications, the fabrics have thicknesses in the range of about 0.05 to 5.4 microns. In some applications, two or more individually designed fabrics can be assembled as a thicker sheet product.

The invention is further described below with reference to the following illustrative embodiments.

example 1

Using the apparatus of Figure 2 without secondary chamber 38, an ultrafine submicron fiber was blown from polypropylene resin (Himont PF 442) with the extruder temperature at 435°F (224°C) and the die temperature at 430°F (221°C). The extruder was operated at 5 rpm (3/4 inch diameter, Model No. D-31-T, C.W. Brabender Instruments, Hackensack, New Jersey) with a bleed block. Excess polymer was bleeded to achieve a polymer flow rate of approximately less than 1 g/orifice/hour. The die included 98 orifices, each orifice having an orifice size of about 0.005 inch (125 micrometers) and an orifice length of 0.227 inch (0.57 cm). The primary air pressure was 30 psi (206 kPa) with a gap width of 0.01 inch (0.025 cm). The primary air temperature was 200ºC. The polymer was blown into the alignment chamber. The secondary orientation air was 70 psi (483 kPa) with an air gap width of 0.03 inch and at ambient temperature. The radius of the Coanda surface was 1/8 inch (0.32 cm). The chamber included an internal height of 1.0 inch (2.54 cm), an internal width of 4 inches (10.16 cm), and an overall length of 20 inches (including a flared outlet section).

The designed fibers had a mean fiber diameter of 0.6 micrometers, with 52% of the fibers having a diameter in the range of 0.6 to 0.75 micrometers. Approximately 85% of the fibers had a diameter in the range of 0.45 to 0.75 micrometers. (Fiber sizes and distributions were determined by scanning electron micrographs of the fabric and analyzed by a Bausch & Lomb Omicon Image Analysis System.) Some stranding of the fibers (approximately 3%) was observed.

Example 2

In this example, the same apparatus and polymer as in Example 1 were used, but without the chamber 38. In this example, the chamber 37 was provided with porous glass side walls and the chamber had a chamber length of 15.5 inches without the flared outlet section. Furthermore, the air brushes on the chamber 37 were adjustable so that the air could be supplied to the Coanda surface at different angles. The Coanda surface included a radius of 1 inch (2.54 cm) and an air exit angle of 45 degrees. The temperature of the extruder ranged from 190 to 225°C from inlet to outlet and the extruder had a rotational speed of 4 revolutions per minute (a screw diameter of 0.75 inches, 1.7 cm). Again, a bleed block was used to keep the polymer flow rate low and prevent excessive polymer residence time in the die. The polymer flow rate was 260 g/hour (2.6 g/minute/orifice). The die temperature was 186°C and the orifices each had an orifice size of 0.005 inch (0.013 cm). The primary air pressure was 10 psi (70 kPa) with an air gap of 0.005 inch (0.013 cm). The secondary orientation air was 20 psi (140 kPa) with an air gap of 0.03 inch (0.0076 cm). Cooling air was introduced through the porous glass walls at 10 psi (70 kPa). The collector was located 22 inches (56 cm) from the nozzle. When examined microscopically, the fibers had an average diameter of 1 micrometer.

Examples 3 - 15

In the examples, the same equipment and polymer were used as in Example 1 above.

The process conditions are listed in Table 1 below. Table 1

T1; - Extruder outlet temperature (ºC)

T2; - Drain block temperature (ºC)

T3; - Nozzle temperature (ºC)

Ta1-Ta2 - Temperature of the air flows (ºC), the primary air or the first orientation air

Pa1-Pa2 - Pressure values of above air flows (psi)

F₁ - Polymer flow rate of Examples 31-33 was about 2.5 g/hour/orifice

R - rpm of the extruder

Tm - melt temperature (ºC)

The size distribution of the fibres (in micrometres) was then determined and the results are shown in Table II below. Table II Median 90%+ Range Number

In Table II, the 90% range is the size range in which 90% or more of the fibers are found. Count is the number of fibers measured and St.Dev is the standard deviation. In general, narrower size distributions were observed at lower polymer flow rates. Examples 4 and 5 have higher extruder speeds and a much wider range of fiber diameters compared to Examples 3 and 6.

The last three examples in Table II (13-15) have smaller mean diameters than the other examples. It is believed that this resulted from the combination of the relatively low primary pressure and the relatively high air pressure from the alignment chamber openings.

In Example 15, an extraordinarily small mean diameter of the fibers was found over a very narrow range of fiber diameters. The scanning electron micrograph of the fibers from Example 15, shown in Figure 4, demonstrates this uniformity of fiber diameters (the small line under "5.0 kx" represents 1 micrometer).

Example 16

In this example, the same arrangement and polymer were used as in Example 2, except that a secondary chamber 38 was employed. The extruder and a ratio of metering pumps were used to control the bleed block. The extruder exit temperature was 240°C, with the bleed block and die temperatures being 250°C. The extruder was operated at 2 rpm.

The operation of the drain block was controlled by three precision pumps (Pump 1, "Zenith" pump, Model No. HPB-4647-0.297, Pumps 2 and 3, "Zenith" pumps, Model No. HPB-4647-0.160, purchased from Powell Equipment Company, Minneapolis, Minnesota). Pumps 1 and 2 were driven by a precision variable speed motor (Model No. 5BP56KAA62, Boston Gear Company, Boston, Massachusetts). These pumps were driven by a full-time gear drive that drove Pump 1 at five times the speed of Pump 2. The pump was driven by another precision speed motor of the same type. These pumps divided the incoming resin stream into two streams. The larger polymer stream from Pump 3 was removed from the system ("drained"). The smaller current from pump 2 was obtained.

The smaller stream was filtered through a filter bed of small glass beads with a mesh number of 240 openings/inch² filtered, with the filter capable of removing all foreign matter larger than 1 micron (1 micrometer). The stream was then directed into the nozzle and extruded through the orifices (0.012 inch, 0.03 cm diameter).

Primary air ("Air 1") was supplied to the nozzle at controlled temperature (210ºC), pressure (5 psi at an air gap of 0.01 inch) and volume per unit time.

Before beginning the actual formation and assembly of the fibers of the invention, the polymer flow rate through the die was measured by sampling the exiting resin stream at a point just after the die, and positioning a piece of weighed mesh screen at that point. The screen was reweighed after five minutes, and the weight of accumulated resin and the extrusion rate in grams/orifice/minute were calculated.

After these measurements, the resin flow was passed through two separate chambers.

The first alignment air stream was used to further direct the flow of melted and cooling resin through the first chamber. The orientation air pressure was 10 psi (70 kPa) with an air gap of 0.03 inches (0.0076 cm). Air was also introduced through the porous side walls of the chamber at 5 psi (35 kPa).

The fibers were then captured by a second alignment chamber 38 when substantially cooled, which alignment chamber had an alignment air stream at a pressure of 60 psi (412 kPa) with an air gap of 0.03 inches (0.0076 cm) and an entrapment air stream adjacent the chamber outlet, this air stream was introduced through orifices at 5 psi (35 kPa). Pump 1 (31 in Figure 2A) was operated at 1730 rpm, with pump 2 (32 in Figure 2A) operating at one-fifth that speed. Pump 3 (33 in Figure 2A) was operated at approximately 900 rpm at steady state. The polymer feed rate was 1 g/hour/orifice. The designed fibers had a mean diameter of 1.1 micrometers, with all fibers (6 fibers counted) ranging from 0.07 to 1.52 micrometers.

For comparison, the same polymer was blown without these chambers (37 or 38 of Figure 2A). The melt blowing conditions were identical except that the primary air pressure was increased to 10 psi (70 kPa). The collected fibers had an average fiber size of 1.41 microns with a standard deviation of 0.37 microns. All fibers ranged from 0.5 to 2.1 microns.

Example 17

In carrying out this example, the method and apparatus of Example 16 were used. The polymer was polyethylene (Dow Aspun 6806, available from Dow Chemical Co., Midland, MI). The extruder was operated at 3 rpm and at an exit temperature of about 200°C. The die block and bleed block temperatures were also about 200°C. Gear pump 1 was operated at 1616 rpm while gear pump 3 was operated at 1017 rpm. The primary air temperature and melt temperature were both 162°C. The primary air pressure was 6 psi (32 kPa). The orientation air in chamber 37 was 50 psi (345 kPa) with an air gap of 0.01 inch (0.025 cm), with the pressure of the Cooling air pressure was 10 psi (70 kPa). Orientation air pressure in the second chamber was 50 psi (345 kPa), with entangling air stream pressure being 10 psi (70 kPa). The mean fiber diameter was 1.31 microns with a standard deviation of 0.49 microns (12 samples). All fibers were within the size range of 0.76 to 2.94 microns, with 94 percent of the fibers in the 0.76 to 2.0 micron range. The nozzle included 56 orifices, each 0.012 inch (0.03 cm) in size.

Example 18

The polymer of Example 17 was fed as in Example 16 above at a polymer feed rate of 0.992 g/hour/orifice (gear pump 31, gear pump 33 and extruder rotation speeds of 1670, 922 and 3 rpm, respectively). The primary air pressure (170°C) was 10 psi (70 kPa) with an air gap width of 0.01 inch (0.025 cm). The melt temperature was 140° with extrusion from a 200°C die (extruder exit temperature and block temperature were both about 170°C). The designed non-oriented fibers have a mean fiber diameter of 4.5 microns and a standard deviation of 1.8 microns. 93 percent of the fibers were in the range of 2 to 8 micrometers (47 fiber samples).

Examples 19 and 20

These examples were carried out in the same way as the previous example. The polymer used was nylon (BASF KR-4405) and a die insert with orifice diameters of 0.005 inches (0.013 cm) and 0.012 inches (0.03 cm) for the aligned and non-aligned Examples were used. The extruder was operated at 2 and 20 rpm with exit temperatures of 310 and 300ºC, respectively. Die and feedblock temperatures were 280 and 270ºC, and 275 and 270ºC, respectively. Gear pumps 31 and 33 were operated at 1300 and 1330 rpm, respectively. Melt temperatures were 231 and 234ºC, and primary air temperatures were 242 and 249ºC, respectively. Example 19 was a non-aligned example using only primary air at 7 ft³/min (0.2 m³/min) with an air gap of 0.01 inch (0.025 cm). The resulting fibers had a mean diameter of 1.4 micrometers with a standard deviation of 1.0. Ninety-five percent of the fibers (a count of 62) were in the range of 0.0 to 3.0 micrometers.

Example 20 was an aligned example using primary air at 3.5 ft³/min (10 psi or 70 kPa) with an air gap of 0.01 inch (0.025 cm). The first chamber 37 had orientation air at 20 psi (140 kPa) and sidewall air at 5 psi (35 kPa). The second alignment chamber had air at 40 psi (277 kPa) and entrapment air at 5 psi (35 kPa). The resulting fibers had a mean diameter of 1.9 micrometers with a standard deviation of 0.66 micrometers. 91.6 percent of the fibers (a count of 24) had diameters in the range of 1.0 to 3.0 micrometers.

The above examples are for illustrative purposes only. Various possible modifications and variations will be apparent to those skilled in the art and can be made without departing from the scope of the present invention, and the present invention should not be limited to the embodiments illustrated herein.

Claims (3)

1. A nonwoven, substantially shotless fabric comprising oriented, substantially continuous, meltblown fibers characterized in that the average diameter of the fibers is less than about 5 micrometers and at least 90 percent of the fiber diameters are within 2 micrometers of the average fiber diameter. 2. The nonwoven fabric of claim 1, wherein the average fiber diameter is less than 2 micrometers. 3. The nonwoven fabric of claim 1 or 2, wherein at least 90 percent of the fiber diameters are in a range of about 1 micrometer or less.