DE69128834T2 - METHOD FOR PRODUCING SUBDENIER FIBERS, PULP-LIKE SHORT FIBERS, FIBRIDES, PRE-YARNS AND MATS FROM ISOTROPEN POLYMER SOLUTIONS - Google Patents

Abstract

A process for preparing subdenier fibers and structures thereof from isotropic polymer solutions is disclosed. The process comprises extruding a stream of the polymer solution into a chamber, introducing a pressurized gas into the chamber, directing the gas in the flow direction of and in surrounding contact with the stream within the chamber, passing both the gas and the stream into a zone of lower pressure at a velocity sufficient to attenuate the stream and fragment it into fibers, and contacting the fragmented stream in the zone with a coagulating fluid.

Description

This application is a continuation-in-part of U.S. patent application serial number 07/555194, filed July 20, 1990, which in turn is a continuation-in-part of U.S. patent application serial number 07/304461, filed February 1, 1989.

BACKGROUND OF THE INVENTION Field of the invention

The present invention relates to a process for producing subdenier fibers from isotropic polymer solutions which can be collected in the form of pulp-like short fibers, fibrids, rovings and nonwovens. The invention also relates to and includes products with new subdenier fiber structures produced according to the above-mentioned process.

Description of the state of the art

Various processes for producing sheet structures and nonwoven articles from discontinuous thermoplastic fibers are known in the art.

For example, Butin et al., US-A-3 849 241 and European Patent Publication 0166830 disclose directing gas streams onto a fiber-forming polymer in the molten state and then collecting the fibers on a screen.

Also known in the art is the flash extrusion of a continuous fibrillated polymer structure and its disintegration by directing a jet of fluid onto the structure at the moment of its formation. (See Raganato et al., US-A-4 189 455.)

In contrast to flash extrusion or the use of gas turbulence to produce fibers, others (JP-52-063424) have used liquid shear forces to produce film-like fibrids for the manufacture of nonwovens and papers. Similarly, US-A-2 988 782 discloses a process for producing a fibrid in which a solution of fully synthetic polymer (which may be, among other things, a poly(m-phenylene isophthalamide)) is added to a precipitant for the polymer under shear conditions.

US-A-4 187 143 discloses a process for the preparation of poly(amide-imide) fibrids in which solutions of poly(amide-imide) resins are introduced into a liquid precipitation medium while the latter is subjected to shear forces, thereby directly obtaining discrete fibrids which have a high degree of fibrillation and a strong tendency to form webs.

EP-A-0 381 206 (which has an earlier priority than the present application but was published after its priority date) discloses a process for producing subdenier fibers from a jet of optically anisotropic solution.

However, none of the above-mentioned documents discloses a process for producing subdenier fibers from isotropic solutions in which the beam is attenuated.

Morgan, US-A-2,999,788, describes the preparation of fibrids from various synthetic organic polymers and their use in the manufacture of synthetic sheet structures such as papers. Such papers, when made from poly(meta-phenylene isophthalamide) fibrids, are suitable for electrical applications, especially when combined with poly(meta-phenylene isophthalamide) short fibers (floc). As disclosed by Gross, US-A-3,756,908, the prior art poly(meta-phenylene isophthalamide) fibrids are film-like particles that act as a binder for the floc and provide good electrical properties. However, these fibrids have a disadvantage in that they over-seal the papers and thus have a porosity-reducing effect. Porosity is a valuable property because it facilitates the coating and saturation of the papers with varnishes and resins, which is a well-known process in the art for modifying and improving the properties of electrical papers.

It is an object of the present invention to produce novel pulp-like poly(meta-phenylene isophthalamide) fibrids. These pulp-like fibrids can be used to produce sheet structures, such as papers, that exhibit improved porosity and electrical properties. These sheet structures can be used to produce laminate and composite structures.

SUMMARY OF THE INVENTION

This invention provides a process for producing subdenier fiber from isotropic polymer solutions. The process comprises 1) extruding a jet of an isotropic solution of a polymer through a spinneret into a chamber, 2) introducing a pressurized gas into the chamber, 3) directing the gas in the direction of flow of and in ambient contact with the jet within the chamber, 4) directing both the gas and the jet through an orifice into a region of lower pressure at a velocity sufficient to attenuate the jet and fragment it into fibers, and 5) contacting the fragmented jet in the region with a coagulating fluid. A suitable gas for contact with the extruded jet in the chamber is air, and the region of lower pressure through which both the gas and the jet are passed may be air at atmospheric pressure. Preferably, the coagulating fluids are water, dimethyl sulfoxide or dimethylacetamide.

Preferred embodiments of the present invention include spinning isotropic polymer solutions of polyacrylonitrile, poly(m-phenylene isophthalamide), a copolymer of 3,4'-diaminodiphenyl ether and isophthaloylbis(caprolactam), and a blend of poly(m-phenylene isophthalamide) and a copolymer of 3,4'-diaminodiphenyl ether and isophthaloylbis(caprolactam).

The fragmented jet of subdenier fibers may be collected in the form of pulp-like short fibers, fibrids, rovings or nonwoven webs, and such products are intended to be part of the present invention.

In one embodiment of the invention, pulp-like poly(m-phenylene isophthalamide) fibrids are prepared by spinning a polymer solution containing about 12 to 19 weight percent poly(m-phenylene isophthalamide) polymer. Hot air at a pressure equal to or greater than about 6 kg/cm2 is introduced into the chamber. Suitable solvents for the poly(m-phenylene isophthalamide) polymer include dimethylacetamide and a mixture of dimethylacetamide and dimethyl sulfoxide.

The invention also includes pulp-like fibrids produced by such a process. These pulp-like fibrids have a diameter of about 0.1 to 50 micrometers, a length of about 0.2 to 2 millimeters and a Schopper-Riegler Freeness value according to ISO standard 5267/1-1979(E) of about 100 to 2000 millimeters, the fibrids being capable of forming porous 100% by weight poly(m-phenylene isophthalamide) sheets. These wet laid porous sheets preferably have a porosity, measured according to Technical Association of the Pulp and Paper Industry (TAPPI) test method T 460 om-88, of about 0.1 to 200 seconds, and preferably of about 0.1 to 2.0 seconds per 100 cubic centimeters. These sheets have a dielectric strength that is about 10170 volts per g per m² (300 volts per ounce per square yard) or greater, and typically is between about 10170 and 23730 volts per g per m² (300 to 700 volts).

Wet-laid fabrics containing about 5 to 95% by weight of the above pulp-like poly(m-phenylene isophthalamide) fibrids are also claimed. These wet-laid fabrics can have a composition of pulp-like fibrids, film-like poly(m-phenyleneisophthalamide) fibrids and poly(m-phenyleneisophthalamide) staple floc.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1-6 are schematic cross-sectional views of the apparatus, mainly of spinning cells, for carrying out the invention.

DETAILED DESCRIPTION OF THE INVENTION

Several isotropic polymer solutions well known in the art can be used in the present invention. These solutions include: nylon 66 in sulfuric or formic acid; polyacrylonitrile, for example, co- and terpolymers of acrylonitrile, methyl acrylate and DEAM (diethylaminoethyl methacrylate) in dimethyl sulfoxide, dimethylacetamide or dimethylformamide solvents; polyether-urea-urethane polymers, for example, a polymer made from the reactants polytetramethylene glycol, methylenebis(p-phenylene isocyanate), ethylenediamine and 1,3-cyclohexanediamine in dimethylacetamide solvent; polyimides, for example, a terpolymer of oxydianiline, hexafluoropropylidenebisphthalic anhydride and sulfondianiline in N-methylpyrrolidone solvent; melt-processable aramids, for example copolymers of 3,4'-diaminodiphenyl ether and isophthaloylbis(caprolactam) in sulfuric acid; poly(m-phenylene isophthalamide) in dimethylacetamide; and polyvinylidene fluoride in dimethylacetamide. Other suitable fiber-forming isotropic polymer solutions well known in the art may also be used. If desired, more than one polymer may be included in the same isotropic solution to form suitable polymer blends. The isotropic polymer solutions used in this invention may be prepared by methods known in the art.

The isotropic polymer solution is extruded through a spinneret orifice into a chamber near a generally convergent-walled opening through which it will exit the chamber. A pressurized gas, inert to the isotropic polymer solution, is also introduced into the chamber near the opening and in ambient contact with the solution jet. The gas, preferably air, has a pressure of between 1.7 kg/cm2 and 7.2 kg/cm2 and a temperature of 20ºC to 300ºC when it is fed into the chamber. The velocity of the gas is such that as it exits the chamber through the opening the jet is attenuated and fragmented.

The gas and jet enter a region of lower pressure upon leaving the chamber, which is preferably air at atmospheric pressure. This is the region where the jet is brought into contact with coagulating fluid either before or after collection. Examples of Suitable coagulating fluids include water, alcohol and mixed solvents. A variety of products can be obtained depending on the type of coagulating fluid used and the method of contacting the jet with the coagulating fluid.

To produce a web, the fragmented jet is contacted with a coagulating fluid jet, e.g., water, at some distance from the orifice, such as 15 to 30 cm. The water jet will coagulate and disperse the jet, which can then be collected as a web on a screen belt that is transverse to the dispersed jet. If the jet contains an acid solution of the polymer, contact with water will dilute the acid and cause the polymer to precipitate from solution. The collected material can be further washed while on the screen belt or neutralized with dilute base, as is known in the art. The resulting web is formed by the random deposition of oriented, discontinuous subdenier fibers of widely varying morphology spun from an attenuated jet. The fleece can be stapled at the fiber crossing points to form a dimensionally stable surface structure.

To produce a pulp-like product, the coagulating fluid is caused to contact the exiting solution jet at the orifice and the product is collected over a basin of coagulating fluid. The pulp-like product consists of short, oriented subdenier fibers of varying morphology and lengths of up to 15.0 mm.

Finally, to produce roving or sliver, a jet of coagulating fluid is directed against the fragmented jet at a distance from the orifice between about 1.0 to 10.0 cm and the coagulated product is collected on a relatively rapidly moving screen; in this case, however, the jet employed is such that it lacks sufficient force to disperse the coagulated product before it is collected. The structure of the coagulated product is a substantially uniformly directed deposition of oriented, discontinuous subdenier fibers of widely varying morphology with essentially no stitching or bonding between the fibers.

A more detailed description of suitable equipment and operating methods is shown below.

Fig. 1 shows a schematic cross-section of a spinning cell with a tubular 1-hole spinneret (4) with an outlet (3) that extends into the chamber (9) with a cylindrical distributor (6). The distributor has an inlet (8) and a nozzle (10) with an orifice (11) with convergent walls which serves as an outlet from the cell. In operation, an isotropic polymer solution is metered through spinneret (4) and into chamber (9) where it is brought into contact with a pressurized gas introduced through inlet (8). As the gas is passed out of the chamber through orifice (11) whose walls converge towards a narrower opening, the polymer solution is attenuated and breaks up into elongated fragments. As the jet of elongated fragments exits orifice (11), they are brought into contact with a coagulating fluid. Depending on the manner of contact and the type of coagulating fluid used, a variety of products can be obtained.

Figure 2 shows a process in which the elongated fragments or fibers emerging from the spin cell (6) are contacted at a distance below the opening (11) with a fluid (26) from the spray nozzles (20) which serves to coagulate and disperse the fragments of jet (30) which are then deposited as a nonwoven sheet on a conveyor screen (32). If desired, a series of such nozzles can be used. These fragments are sub-denier fibers of widely varying cross-sections and with lengths of up to 10 cm, diameters of up to 10 micrometers and length-to-diameter ratios of at least 1000. The fibers on the screen can be washed, dried and wound onto a bobbin (not shown) in a continuous process.

Fig. 3 shows an alternative method of contacting the jet leaving the orifice (11) with coagulating fluid to produce sliver or sliver. In this case, an atomized jet of coagulating fluid (28) from the spray nozzle(s) (24) impinges on the jet leaving the orifice (11) at a distance of up to 10 cm below the orifice. The fibers in the jet have a momentum greater than that of the atomized jet of coagulating fluid and, accordingly, deflection of the jet and dispersion of the fibers are low. Under these conditions, the subsequent fiber deposition on the conveyor screen (32) is substantially rectified and the product is suitable as a sliver or sliver. In an analogous method, the spreading of the jet exiting the orifice (11) can be prevented by surrounding the jet with a curtain of coagulating fluid flowing in the same direction. The curtain of coagulating fluid initiates fiber coagulation and prevents spreading.

In each case, the jet containing the coagulated fibers is interrupted by a moving screen conveyor belt, causing the fibers to be deposited in a substantially uniform orientation over the screen. The sliver or roving that forms can be wound onto a bobbin (not shown). The fibers are similar to those of the nonwoven web previously described.

Fig. 4 shows a process for producing pulp-like short fibers. Fig. 4 shows a spinning cell (40) similar to that in Fig. 1 except that it has a conical nozzle (30) and a nozzle (35) built into the spinning cell housing. Coagulating fluid from nozzle (35) strikes the outer surface of nozzle (30) and runs down the slope of nozzle (30) to orifice (12) and contacts the exiting jet. This process results in the formation of pulp-like short coagulated fragments which can be distributed over a conveyor screen or collected in a collecting vessel (not shown) located below the spinning cell.

Fig. 5 shows a spinning cell (50) with inlet (51) for admitting hot air to heat the spinneret and prevent clogging, while inlet (52) admits cold operating air which is introduced in the second step. A shutter (54) prevents the hot air from mixing with the cold air in the spinning cell. Spent hot air can be removed from the chamber through outlet (53). The polymer solution and the cold air are discharged through outlet opening (55).

Fig. 6 shows a spinning cell (150) with inlet (151) for admitting hot air which heats the spinneret (104) to improve the flow properties of the solutions. The hot air then flows through a narrow annular gap (154) before exerting hydrodynamic drag on the extruded solution at the outlet of the spinneret (103). The air weakens and breaks the filaments as it flows out of the chamber through the orifice (130). The orifice (130) is an orifice of constant diameter and finite length. As the fractured filaments exit the orifice (130), they are immediately brought into contact with the coagulating fluid which passes through the orifice (152) to the orifice area. To prepare bulk samples of pulp-like fibrids, the polymer solution, exiting air and coagulant are collected in a water basin.

It will be apparent to those skilled in the art that a variety of modifications can be made to the above apparatus. Thus, if desired, multiple spinning cells can be used side by side in a linear fashion to facilitate the deposition of uniform sheets of considerable width. Similarly, a diverging channel formed by the walls aligned parallel and located at the exit of the orifice (11) will spread the emerging jet into a wider jet as it leaves the spinning cell.

There are several important process variables that are critical in the production of high quality pulp-like fibrids by the process of the present invention, particularly if these fibrids are to possess properties needed to produce improved porous papers. Preferably, a spinneret arrangement similar to that shown in Figure 6 is used. The important process variables include the solution viscosity, the solution extrusion rate, the pressure of the hot air entering the cell, the opening of the air orifice (130) and the length of the air gap (measured as the distance between the outlet from the spinneret (103) and the outlet from the orifice (130)).

Solution viscosity is controlled by solution temperature and polymer concentration in the solution. In the work described here, solution viscosity was controlled by adjusting polymer concentration. Solution extrusion rate was controlled by nitrogen back pressure, which was applied to create forward motion of the solution. Air pressure can be easily controlled by a regulator.

The polymer concentration in poly(m-phenylene isophthalamide) solutions (in dimethylacetamide/dimethyl sulfoxide solvent) was varied between about 12 wt.% and 19 wt.% to investigate the effect of solution viscosity on the quality of the pulp-like fibrids. The fibrid diameter decreases with decreasing solution viscosity, while the concentration of large particle defects increases dramatically at the lower polymer concentrations. To achieve the goal of obtaining fibrids with the finest diameter and a minimal amount of large particle defects, it was determined that a solution with 16 wt.% solids gave the best results. The optimal polymer concentration will vary with the particular polymer-solvent combination used. Other possible solvents for poly(m-phenylene isophthalamide) polymer are known in the art and include dimethylacetamide itself.

The solution extrusion rate was controlled by nitrogen back pressure. High nitrogen pressure leads to high extrusion rate, which is preferred from a productivity standpoint, but it is often accompanied by a high concentration of large particle defects. For 16 wt. % solutions of poly(m-phenylene isophthalamide) (MPD-I) prepared under Using a 0.004-inch (0.102-mm) spinneret, a nitrogen back pressure of not more than 35.2 kg/cm² gauge (500 psig), and preferably not more than 28.1 kg/cm² gauge (400 psig), was required to produce high quality pulp-like fibrids.

The air pressure determines the air velocity, and the velocity changes near the capillary and the orifice. In this work, it was found that the best fibrid quality was obtained when the air pressure was set at the highest possible setting, which for the apparatus shown in Fig. 6 with the dimensions described in Examples 6-16, was about 80 psig (6.65 kg/cm2).

The pulp-like MPD-I fibrids of the present invention have different features and properties from fibrids known in the art. For example, MPD-I fibrids described in the prior art are flat, sheet-like materials with typical dimensions of 0.1 microns thick, 100 microns wide, and are chopped to various lengths. The sheet-like nature of these fibrids results in the sealing of papers containing them, resulting in low porosity.

In contrast, the improved pulp-like fibrids of the present invention have a substantially round cross-section with irregular fibrillar morphology. Unlike the prior art sheet-like fibrids, the MPD-I pulp-like fibrids of the present invention have a refined appearance, openness and are suitable for papermaking without the need for refinement. The pulp-like fibrids of the present invention do not cause closure of papers containing them. Therefore, when the pulp-like fibrids containing aromatic polyamides such as MPD-I are used to make electrical papers, an improved combination of electrical properties and porosity is obtained compared to similar prior art papers containing sheet-like fibrids.

In electronic papers or other high quality paper end uses, preferred dimensions for the pulp-like fibrids are 0.1-50 microns in diameter and 0.2-2.0 mm in length. More preferably, the pulp-like fibrids have diameters of 0.2-5.0 microns and lengths of 0.2-1.3 mm. The pulp-like fibrids of the present invention also have high freeness values. Preferably, the freeness values measured on a Schopper-Riegler apparatus are 100-2000 ml. More preferably, the pulp-like fibrids have freeness values of 500-1000 ml.

The pulp-like MPD-I fibrids of the present invention can be used alone or as blends with sheet-like fibrids and staple floc to produce papers with good electrical properties. "Staple floc" or "floc" as used herein means fibers in the form of short fibers. Preferably, floc contains fibers having a length of less than 2.54 cm, with the optimum length being about 0.6 cm. Suitable yarns or tows of the polyamide are cut to the desired floc length in any suitable manner, e.g., by using a helical saw cutter. Suitable fibers are those having a denier of about 0.5 and up to 10 or more. Deniers of less than about 5 are preferred. Particularly preferred are fibers having a denier of between about 1 and about 3.

In the manufacture of electrical papers by using blends of the pulp-like poly(m-phenylene isophthalamide) fibrides of the present invention with prior art film-like poly(m-phenylene isophthalamide) fibrides and poly(m-phenylene) isophthalamide staple floc, the preferred blend compositions are: 5-100 wt.% pulp-like fibrides, 0-60 wt.% film-like fibrides and 0-90 wt.% staple floc. Blends of 10-60 wt.% pulp-like fibrides, 0-33 wt.% film-like fibrides and 10-50 wt.% staple floc are particularly preferred.

TEST PROCEDURE

Before determining the tensile properties, the denier value of the sample fibers must be calculated. Methods for measuring the denier value of such non-round and variable diameter fibers are known and include measurement of the specific surface area, scanning electron microscopy measurement and direct measurement of a fiber group sample under the light microscope.

An Instron 1122 was used to determine tensile strength and modulus according to ASTM D2101 Section 10.6 (elongation (10%)). For 1.0 inch (2.54 cm) specimen lengths, the clamps (clamps with 6/16 inch (0.95 cm) x 6/16 inch (0.95 cm) neoprene fronts) were set between 1.25 inch (3.2 cm) and 1.5 inch (3.81 cm) apart and operated at a crosshead speed of 0.1 inch (0.254 cm/min), while for 0.25 inch (0.64 cm) specimen lengths, the clamps were set with 0.75 inch (1.91 cm) between fronts and operated at a crosshead speed of 0.025 inch (0.064 cm/min).

Each filament sample end was taped to opposite ends of a rectangular strip with a rectangular cutout (opening) of the specified length of 2.54 cm or 0.64 cm (1 inch or 0.25 inch). The tape attachment was located some distance from the opening and a slight amount of slack in the fibers was allowed. A drop of adhesive was placed near the edges of the strip opening to secure the specified length of filament to match the length of the strip opening. The strip was attached to the top clamp of the Instron and one side of the strip was cut off. The opposite end of the strip was then attached to the bottom clamp and the other side of the strip was cut off, with the filament extending across the gap between the clamps. The Instron was turned on and the stress-strain relationship of the filament was fed directly into the computer which calculated the tensile properties.

The dielectric strength was measured according to ASTM D-149.

Porosity was measured according to TAPPI test method T460 om-88 "Air Resistance of Paper". Test results are given in seconds, which represent the number of seconds it takes a mass of 567 grams to force 100 cc of air through 6.4 square centimeters (1 square inch) of the paper tested. The higher the test result number in seconds, the lower the porosity of the paper.

The average fiber length for pulp-like materials was determined on a Kajaani Model FS-100 instrument according to the manufacturer's test procedure in "Kajaani FS100 Standard Procedure for Analysis", Document T3501.0-e, Copyright September 2, 1985, Kajaani Electonic Ltd., Kajaani, Finland.

The freeness value of the fibrid samples was tested according to the International Organization for Standardization (ISO) standard ISO 5267/1-1979(E), "Pulps - Determination of Drainability Part I - Schopper-Riegler Freeness Tester", using a block weight of 2.0 grams and a temperature in the range of approximately 20ºC to 25ºC. METRIC CONVERSION TABLE

The following examples are given to illustrate the present invention and are not intended to be limiting. Unless otherwise indicated, parts and percentages in the following examples are by weight.

EXAMPLE 1

A 25% solution of an acrylonitrile terpolymer having a composition of 91% acrylonitrile, 6% methyl acrylate and 3% DEAM (diethylaminoethyl methacrylate) with an inherent viscosity of 1.4 in dimethyl sulfoxide was prepared. The solution was prepared by placing the polymer powder and solvent in a resin kettle and then dissolving the polymer in the solvent by stirring. The solution was then hydraulically pushed into a spin cell similar to that shown in Figure 4 and spun through a single hole spinneret according to the conditions shown in Table 1. The spinneret had a diameter of 0.004 inches (0.1016 mm) and a length to diameter (L/D) ratio of 3.0. Referring to Figure 4, the spin cell had an air gap of 0.176 inches (4.47 mm) measured from the spinneret outlet (3) to the narrowest diameter (or throat) of the orifice (12) of the spin cell nozzle (30). The narrowest diameter of the orifice (12) was 0.062 inches (1.57 mm). The convergent wall of the orifice (12) was at an angle of 40 degrees to the spinneret axis, giving a conical angle of 80 degrees. Air heated to 80°C at a pressure of 80 psig (6.7 kg/cm2) was supplied to the spin cell to weaken and fragment the freshly extruded polymer. The discontinuous fibers exiting the spin cell were contacted with a jet of tap water over a moving screen conveyor belt at a distance of 17.357 inches (44.1 cm) from the tip of the orifice (12) to produce fibers up to 8 cm in length.

The fibers were laid over a moving screen conveyor belt and formed a random fiber web, which was transported together with the conveyor belt from the spinning chamber to a washing chamber. The fiber web was washed in this chamber to remove any remaining solvent residues, and then transferred to a drying chamber, where the washed fiber web was dewatered, partially dried and then wound up over a bobbin (or roller).

The fibers on the bobbin (or roller) looked like a carding ribbon and could potentially be used directly to make spun yarns. The fibers were tested for their physical properties and the results are given in Table I. TABLE I - POLYACRYLNITRILE FIBERS -

Alternatively, the discontinuous fibers exiting the spin cell were contacted with a jet of tap water at the tip of the orifice (12) to produce fibers less than 15 mm in length. These medium length fibers were collected over a basin of water, the water later being separated from the fibers by a standard filtration process. Finally, the fibers were washed to remove any residual solvent. These fibers can be wet laid down to form a paper by conventional methods known in the art.

EXAMPLE 2

A 20% poly(m-phenylene isophthalamide) solution in dimethylacetamide solvent was hydraulically pushed into a spin cell similar to that shown in Figure 4 and spun through a single-hole spinneret under the conditions shown in Table II. The single-hole spinneret had a diameter of 0.004 inches (0.1016 mm) and an L/D ratio of 3.0. Alternatively, the single-hole spinneret had a diameter of 0.010 inches (0.254 mm) and an L/D ratio of 3.0. The solution was spun from both types of spinnerets.

Referring to Fig. 4, the spin cell had an air gap of 0.176 inches (4.47 mm) measured from the outlet (3) of the spinneret to the narrowest diameter (or throat) of the orifice (12) of the spin cell nozzle (30). The narrowest diameter of the orifice (12) was 0.062 inches (1.57 mm). The convergent wall of the orifice was at an angle of 40 degrees to the axis of the spinneret, giving a conical angle of 80 degrees.

The discontinuous fibers exiting the spin cell were contacted with a jet of tap water about 11 inches (28 cm) from the tip of the orifice (12) and collected over a moving stainless steel screen. A fibrous web of subdenier fibers formed on the screen. Individual fibers were tested for physical properties and the results are given in Table II. X-ray examination of the fibers showed an amorphous structure. The fibrous web, washed and dried, can be used as an inner layer for making laminates with similar poly(p-phenylene terephthalamide) layers and for high temperature insulation. TABLE II Poly(m-phenylene isophthalamide) fibers

EXAMPLE 3

In this example, 160 grams of a 20% poly(m-phenylene isophthalamide) solution in dimethylacetamide was diluted with 40 grams of dimethyl sulfoxide solvent. The mixture was hydraulically pushed into a spin cell similar to that shown in Figure 4 and spun through a single hole spinneret under the conditions in Table II. The single hole spinneret had a diameter of 0.004 inches (0.1016 mm) and an L/D ratio of 3.0. The spin cell had an air gap of 0.176 inches (4.47 mm) measured from the spinneret outlet (3) to the narrowest diameter (or throat) of the orifice (12) of the spin cell nozzle (30). The narrowest diameter of the orifice (12) was 0.062 inches (1.57 mm). The convergent wall of the orifice had an angle of 40 degrees to the axis of the spinneret, resulting in a conical angle of 80 degrees.

The discontinuous fibers leaving the spin cell were contacted with a jet of tap water at the top of the orifice (12) and collected over a water basin (not shown). The fibers were filtered, washed and slurried in water using a "Waring" mixer to further reduce the fiber length. The product was a subdenier pulp with a fiber length of up to 5 mm. These subdenier pulps are useful for making high quality paper, as a binder for poly(p-phenylene terephthalamide) papers and as a thickener.

EXAMPLE 4

A 30% solution of a copolymer of 3,4'-diaminodiphenyl ether and isophthaloyl bis(caprolactam) was prepared by dissolving the copolymer in dimethylacetamide. The solution was then hydraulically pushed into a spinning cell similar to that shown in Figure 4 and spun through a single-hole spinneret. The single-hole spinneret had a diameter of 0.004 inches (0.1016 mm) and an L/D ratio of 3.0. The air gap was 0.176 inches (4.47 mm) measured from the outlet (3) of the spinneret to the narrowest diameter (or throat) of the orifice (12) of the spinneret nozzle (30). The narrowest diameter of the orifice (12) was 0.062 inches (1.57 mm). The convergent wall of the orifice was at an angle of 40 degrees to the axis of the spinneret, giving a conical angle of 80 degrees. Air heated to 80ºC at a pressure of 83 psig (6.9 kg/cm²) was introduced into the spin cell as an attenuation fluid. The discontinuous fibers leaving the spin cell were contacted with a jet of tap water at a distance of about 11 inches (28 cm) from the tip of the orifice (12) and collected over a conveyor screen. A fibrous web of subdenier fibers was formed on the screen.

Alternatively, the discontinuous fibers leaving the spinning cell were contacted with water at the top of the orifice (12) and collected over a water basin as explained in EXAMPLE 3. The product in this case was subdenier pulp, which can be used, for example, for papermaking, as an asbestos substitute, or as a binder between poly(p-phenylene terephthalamide) layers for high temperature applications.

EXAMPLE 5

A 20% solution of a polymer blend of 70% poly(m-phenylene isophthalamide) and 30% of a copolymer of 3,4'-diaminodiphenyl ether and isophthaloylbis(caprolactam) was prepared in dimethylacetamide. The solution was then spun using a spin cell similar to that shown in Figure 4 with a single hole spinneret of 0.004 inch (0.1016 mm) diameter. The same solution was also spun using the same spin cell but with a spinneret of 0.010 inch (0.254 mm) diameter. Both spinnerets had an L/D ratio of 3.0. The spin cell had an air gap of 0.125 inches (3.175 mm) measured from the spinneret outlet (3) to the narrowest diameter (or throat) of the orifice (12) of the spin cell nozzle (30). The narrowest diameter of the orifice (12) was 0.062 inches (1.57 mm). The convergent wall of the orifice was at an angle of 40 degrees to the axis of the spinneret, giving a conical angle of 80 degrees. Air heated to 90ºC at a pressure of 60 psig (5.3 kg/cm²) was introduced into the spin cell as an attenuation fluid.

The discontinuous fibers leaving the spinning cell were brought into contact with a jet of tap water at the top of the orifice (12) and collected over a water basin as explained in EXAMPLE 3. The fibers were then filtered, washed and dried. The product was pulp-like short fibers which can be used as an asbestos substitute or as a binder. Thin filter cakes of the pulp-like short fibers were hot pressed at about 260°C to form non-porous membranes.

EXAMPLE 6-16

The pulp-like fibrids used in these examples were prepared as follows. A 19% poly(m-phenylene isophthalamide) solution in dimethylacetamide was diluted to 16% solids with dimethyl sulfoxide. The solution was spun at 25°C through a 0.004-inch (0.102-mm) single-hole spinneret with an L/D ratio of 3. The spin cell was similar to that shown in Figure 6 and had an air gap of 0.155 inches. (3.94 mm) measured from the outlet (103) of the spinneret to the outlet of the orifice (130) which had a diameter of 0.062 inches (1.575 mm) and a length of 0.062 inches (1.575 mm). The spinning solution pressure was 28.1 kg/cm² (400 psig) and the attenuation air pressure was 5.2 kg/cm² (74 psig).

The discontinuous fibers leaving the spinning cell were brought into contact with a jet of tap water at the tip of the orifice (130) and collected over a water basin. The fibers were then washed several times with water in a household mixer to remove the solvent (the final dimethylacetamide content was 0.16%, with no dimethyl sulfoxide detectable). The fibers obtained were in the form of pulp-like fibrids.

Fibrid quality was examined by mixing at high speed at 0.04 wt% solids in distilled water in a domestic kitchen mixer for about one minute. The high quality fibrids separated easily in the mixer and remained evenly dispersed in water without clumping. The aqueous dispersions were poured into tissue paper thin hand swatches (3-4 g/m2), dewatered and dried. The sheet samples were examined for pulp clumps. The samples were found to be fine-grained and uniform and contained few or no clumps, indicating the high quality of the pulp-like fibrids. Clumps may be knotted filaments or solid polymer that has evaded fibrillation during spinning.

The fibrid diameters measured by scanning electron microscopy were 1-20 micrometers with very few particle defects. The average length of the pulp-like fibrids was determined to be 0.47 mm using the Kajaani method. The pulp-like fibrids had a freeness value of 773 ml measured using a Schopper-Riegler apparatus.

Prior to making sheets, the pulp-like fibrids were opened by placing the required total weight of wet lap block in a standard 1-quart household mixer approximately 3/4 full of water and mixing at medium speed for 1-2 minutes so that no lumps or strings were present. A total of 2.8 g of ingredients were used to make sheets of nominal 2.0 ounces/yard2 basis weight 8 by 8 inches (20.3 cm by 20.3 cm). Hand samples consisting of the pulp were poured in a standard deckle box. The pulp-like fibrids (which were supplied in the form of a dilute slurry) were gently mixed in the deckle box with 10 liters of water. A vacuum was applied which allowed the sheets to be formed on a removable wire screen. Further dewatering was achieved by gently pressing the sheet and wire screen between two layers of blotting paper using a Noble and Woods sheet press. The wire screen was removed and replaced with a fresh sheet of blotting paper, the paper sheet sandwich was pressed again, and the sheet was then removed and dried between fresh layers of blotting paper.

The pulp-like fibrids (P) were used alone or in combination with film-like poly(m-phenylene isophthalamide) fibrids (F) and/or poly(m-phenylene isophthalamide) staple floc (5). The film-like fibrids were prepared according to the process disclosed by Gross, US-A-3,756,908, which disclosure is hereby incorporated by reference, and had an average Kajaani length of 0.25 mm and a Schopper-Riegler Freeness value of 330 ml. The staple floc was prepared according to the process disclosed by Alexander, US-A-3,133,138, which disclosure is hereby incorporated by reference, and had a cut length of 6 mm and had a dtex value of 2.22 (2 denier) per filament.

The pulp-like fibrids, film-like fibrids and/or staple floc were mixed together in a deckle box prior to applying vacuum. Samples of the sheets were hot pressed for 1 minute at 70.4 kg/cm2 (1000 psi) on a Farrel-Watson-Stillman press, Model No. 9175-MR. The sheets were tested for basis weight, dielectric strength, porosity, elongation at break (Elong-b), modulus and density. The sheet properties are given in Table III below. TABLE III TABLE III - CONTINUED

* respected values

The benefit of adding pulp-like fibrids is clearly evident from these data. Example 13, containing 50 wt% sheet-like fibrids and 50 wt% staple floc, represents compositions of commercially available papers.

When comparing Examples 6 and 7 with Example 13, note the dramatic increase in porosity in Examples 6 and 7, which is accompanied by good dielectric properties. In addition, it should be noted that the papers of Examples 6 and 7 have high elongation and low modulus compared to those of Example 13. The high elongation and low modulus, i.e. high flexibility, is advantageous for certain applications where rolling of the paper is necessary. However, since these papers are also highly porous, they can be saturated with resins or varnishes to make them more rigid. Therefore, these papers are more versatile. Saturation with resins or varnishes is also well known in the art as a method for improving mechanical and electrical properties. Example 9 illustrates a ternary blend of pulp-like fibrids, foil-like fibrids and floc with improved porosity and dielectric strength. These benefits can be achieved with a concentration of 33% pulp-like fibrids, 33% foil-like fibrids and 33% floc. However, the foil-like fibrids tend to counteract the porosity benefit introduced by the pulp-like fibrids (see Examples 10, 14 and 16).

The porosity of unpressed sheets is a suitable porosity indicator for pressed sheets, especially when the porosity of pressed sheets is very low (high porosity values, i.e. greater than 1800 seconds). It would be inappropriate or impractical to carry out a porosity test over such a long period of time. In addition, for sheets with high porosity (low porosity values, i.e. less than 0.1 seconds), the porosity determinations can be controlled by the practical ability to carry out time measurements at these points.

Comparison of Example 11 with Example 12 shows that the porosity benefits can be obtained by replacing the film-like fibrids in a 25% film-like fibrid and 75% floc sheet with 25% pulp-like fibrids. Dielectric strengths of greater than about 200 are commercially important and for high porosity papers these values can be increased by saturation with resins and varnishes.

Example 15 also shows the porosity benefits obtained when pulp-like fibrids are added.

Claims (18)

1. A process for producing subdenier fiber from isotropic polymer solutions comprising 1) extruding a jet of an isotropic solution of a polymer through a spinneret orifice into a chamber, 2) introducing a pressurized gas into the chamber, 3) directing the gas in the direction of flow of and in ambient contact with the jet within the chamber, 4) directing both the gas and the jet through an orifice into a region of lower pressure at a velocity sufficient to attenuate the jet and fragment it into fibers, and 5) contacting the fragmented jet with a coagulating fluid in said region. 2. A process according to claim 1, wherein the polymer in solution is polyacrylonitrile. 3. A process according to claim 1, wherein the polymer in solution is poly(m-phenylene isophthalamide). 4. A process according to claim 1, wherein the polymer in solution is a copolymer of 3,4'-diaminodiphenyl ether and isophthaloylbis(caprolactam). 5. A process according to claim 1, wherein the polymer in solution is a blend of poly(m-phenylene isophthalamide) and a copolymer of 3,4'- diaminodiphenyl ether and isophthaloylbis(caprolactam). 6. A method according to claim 1, wherein the region of lower pressure is air at atmospheric pressure. 7. A process according to claim 1, wherein the gas in contact with the extrudate in the chamber is air. 8. A process according to claim 1, wherein the subdenier fiber is collected in the form of fibers, rovings or nonwoven webs. 9. A method according to claim 1, wherein the coagulating fluid is selected from the group consisting of water, dimethyl sulfoxide and dimethylacetamide. 10. The process of claim 1, wherein the polymer solution contains about 12 to 19 weight percent poly(m-phenylene isophthalamide) and the gas is air at a pressure of about 6 kg/cm2 or greater. 11. The process of claim 10, wherein the polymer solution contains about 12 to 19 weight percent poly(m-phenylene isophthalamide) in dimethylacetamide solvent. 12. The process of claim 10, wherein the polymer solution contains about 12 to 19 weight percent poly(m-phenylene isophthalamide) in a mixed solvent of dimethylacetamide and dimethyl sulfoxide. 13. A pulp-like poly(m-phenylene isophthalamide) fibrid having a diameter of about 0.1 to 50 micrometers, a length of about 0.2 to 2 millimeters, and a Schgpper-Riegler Freeness value according to ISO Standard 5267/1-1979(E) of about 100 to 2000 milliliters, wherein the fibrid is capable of forming 100% by weight porous poly(m-phenylene isophthalamide) sheets, and wherein the sheets have a porosity, measured according to TAPPI Test Method T 460 om-88, of about 0.1 to 200 seconds per 100 cubic centimeters. 14. A wet-laid sheet consisting of the fibrids of claim 13 and characterized by a dielectric strength of about 10,170 volts per g per m² (300 volts per ounce per square yard) or greater. 15. The wet laid sheet of claim 14 wherein the porosity measured according to TAPPI test method T 460 om-88 is about 0.1 to 2.0 seconds per 100 cubic centimeters. 16. A wet-laid sheet containing about 5 to 95 weight percent of the fibrids of claim 13 and characterized by a dielectric strength of about 10,170 volts per g per m² (300 volts per ounce per square yard) or greater. 17. The wet laid sheet of claim 16 wherein the porosity measured according to TAPPI test method T 460 om-88 is about 0.1 to 2.0 seconds per 100 cubic centimeters. 18. A wet-laid sheet comprising a composition of the fibrids of claim 13, film-like poly(m-phenylene isophthalamide) fibrids and POLY(m-phenylene isophthalamide) staples and characterized by a dielectric strength of about 10,170 volts per g per m² (300 volts per ounce per square yard) or greater.