Porous Polymer Fiber Filters

Novel microporous fibers and filaments are prepared from normally solid, thermoplastic, orientable, organic polymers which have reactive or reactable groups pendant from the polymer chain, employing a process which involves reactions of said pendant groups while the polymer is in an oriented condition.

It is well known that there are a large number of man-made thermoplastic polymers and resins which can be melt-extruded as monofilaments for the making of chopped fibers, or as continuous filaments. Many of these can be extruded as multifilaments or fine denier filaments. Fibers and filaments of large denier (generally greater than 15 denier) can usually be prepared from any thermoplastic polymer or resin which has a melt strength sufficient to withstand the tensile forces (gravitational or supergravitational) acting on the weakest portion of the extrudate.

The following references contain general information about the preparation and characteristics of high-denier monofilaments, low-denier multifilaments and staple fibers of olefin polymers: Encyclopedia of Polymer Science and Technology (1968), Vol. 9, pp 403-440, published by Interscience, New York; Kirk-Othmer Encyclopedia of Chemical Technology, (1981) third edition, Vol. 16, pp. 357-385, John Wiley & Sons, New York; Plastics World, June 1984, page 113; Man-Made Fiber and Textile Dictionary, published by Celanese Corporation; Man-Made Fibres by R. W. Moncrieff, John Wiley & Sons, New York; Fundamentals of Fibre Formation by Andrizej Ziabicki, John Wiley & Sons, New York. One specific technique for making microporous hollow fibers from various polyolefins is disclosed in U.S. Pat. No. 4,670,341. The hollow fibers of U.S. Pat. No. '341 are said to be formed by melt spinning a polyolefin/filler/plasticizer composition, followed by an extraction procedure, thus forming the microporous hollow fibers. The extraction procedure is chosen such that the polyolefin and the filler are insoluble.

Olefin/unsaturated carboxylic acid copolymers (especially ethylene/acrylic acid) in the form of pellets may be digested with an alcohol/caustic mixture causing the pellets to spontaneously convert into colloidal particles (U.S. Pat. No. 3,798,194) or the pellets may be digested in an amine and agitated to obtain small, non-colloidal, particles and fibers (U.S. Pat. No. 3,790,521). Such olefin/carboxylic acid copolymers are said to be hot-drawn as strands, the strands chopped into pellets, and the pellets digested with an alkali to obtain, upon shearing, small fibers (U.S. Pat. No. 3,801,551).

Ethylene polymerized with unsaturated organic carboxylic acids (ECA), e.g., acrylic acid (EAA), to form copolymers is taught, e.g., in U.S. Pat. No. 2,391,218; U.S. Pat. No. 3,520,861 and U.S. Pat. No. 4,351,931. Copolymers of ethylene and such acids can be made by grafting the acid onto polyethylene, by batch or continuous polymerization of mixtures of monomers of ethylene and the acid, by polymerization of mixtures of monomers of ethylene and the acid in a tubular reactor, and hydrolysis of copolymers of ethylene/alkyl acrylates which converts the ester groups to carboxylic acid groups. Also, block copolymers can be made whereby chain segments of polyacrylic acid and chain segments of polyethylene form long polymer chains. Any of these known ethylene/acid copolymers are suitable for use in the present invention, so long as they are of sufficient molecular weight to be formed into solid particles, fibers or filaments. Thus, the purview of the present disclosure includes, inter alia, ethylenic polymers containing a plurality of carboxylic acid groups in their molecular structure.

In addition to the ethylenic/acid (ECA) copolymers and terpolymers, other olefinic copolymers and terpolymers are within the purview of the present invention so long as the polymer is one which has reactive or reactable groups among the polymer chain as pendant side-groups which can be substantially reacted with a reagent, but where the polymer backbone substantially retains its molecular integrity.

It has now been discovered that there are unexpected characteristics resulting from a change in the known process of forming fibers of certain polymers, such as ethylene/unsaturated carboxylic acid (ECA) copolymers, said known process being that of digesting extruded strands of the polymer in a reagent which substantially reacts with side-groups pendant from the polymer molecule and then subjecting the so-treated polymer to shearing forces to cause fibrillation or particulation of the polymer strands. The said change in the process involves, as the principal distinguishing feature, the orientation (i.e. "stretching") of the polymer at a temperature below that at which stress-relaxation of the stretched polymer molecules is substantially encountered; this may be referred to as "cold-stretching" or "crystalline orientation".

It is customary, in some production processes, for polymers leaving a polymerization vessel to be melt-extruded through a die as strands which are chopped into pellets and cooled in a water bath. U.S. Pat. No. 3,801,551 discloses that such pellets may then be digested in alkaline material and then fibrillated using shearing forces. Since the strands are cut into pellets before the strands have cooled to the point at which crystallization occurs, then stress relaxation of the polymer molecules permits intertwining of the molecules. When such pellets are treated in accordance with U.S. Pat. No. 3,801,551 to prepare fibers, the fibers are relatively short and have very little porosity, if any. Polymers produced as small particles may also be melted, extruded as strands, and chopped into pellets before the intended end-use.

It has been found that if the extruded strands of polymer are allowed to cool to an extent, and for a time, sufficient to allow an appreciable amount of crystallization and are then cold-stretched (oriented), the crystallized molecules become substantially untwined and become substantially aligned in parallel relationship with the direction of orientation. This also draws the strands to narrower dimensions. Since the polymer, when oriented, is cold enough for the molecules to be crystallized, and not warm enough to allow stress-relaxation of the molecules, then the molecules remain dimensionally stable after the orientation is completed. When these oriented strands are treated with a reagent which reacts with the side-groups, (e.g. 0.5N NaOH) and subjected to shearing or crushing, the strands undergo fibrillation into fibers which are extensively porous. Most of the pores are small enough to be considered micropores. These micropores permeate the length and breadth of the fibers. Even if the oriented strands are chopped into pellets before being treated with alkali and subjected to shearing or crushing, the so-formed fibers are extensively porous and are longer than fibers prepared from pellets of the same dimensions treated in accordance with U.S. Pat. No. 3,801,551.

This same phenomenon is observed when cold-oriented films or strips are treated with alkali and sheared or crushed into fibers, and to a greater degree than when using films or strips which are stretched at high temperatures where stress relaxation of the polymer molecules is possible.

Novel microporous compositions, e.g., fibers or particles, are prepared by a method which comprises

a.) cold-stretching or orienting a normally-solid, thermoplastic, orientable, substantially non-elastic olefin polymer extruded in elongate form to a dimensionally stable form, wherein said polymer has pendent, reactive or polar side-groups in the molecular structure of the polymer, said cold-stretching or orienting being done at a temperature below that at which a substantial amount of stress relaxation of the stretched molecules is possible, said cold-stretching causing said molecules to align in substantially parallel manner along the direction of orientation,

b.) treating the cold-stretched or oriented polymer with a chemical reagent which reacts with the reactive groups in the polymer molecular structure, causing swelling of the oriented polymer,

c.) subjecting the so-swelled polymer to shearing or crushing or other physical force which causes the polymer molecules to undergo sufficient separation so as to form fibers exhibiting microfibrillar texture, and

d.) optionally, but beneficially, treating the so-formed fibers with a counter-reagent which substantially returns the reactive groups to their original chemical form or to a chemical form which is sterically smaller than the reacted groups created by the reagent, thereby obtaining void spaces (reticules) in the fibers. Depending on the concentration of reactive side-groups, the extent of cold-drawing, the thoroughness of the treatment with the reagents, and the thoroughness of the fibrillation or particulation, tile microporous particles or fibers will exhibit a surface area/weight ratio of at least about 0.5 m.sup.2 /gm, usually at least about 2 m.sup.2 /gm, preferably at least about 4 m.sup.2 /gm, and can be as much as 15 m.sup.2 /gm or more.

Alternatively, steps c and d above may be reversed, with the counter-reagent treatment preceding the physical force which causes the fibrillation.

FIGS. 1-6 are photomicrographs of various magnifications to illustrate the porosity of various embodiments of the present invention.

FIG. 7 is a flow diagram which illustrates the sequence of steps of an embodiment of the present invention for use as an aid in describing the invention.

As stated above, the polymers for use in the present invention are normally solid olefin copolymers or terpolymers which are (1.) thermoplastic, (2.) extrudable in elongate form, such as sheets, tubes, strands, strips, ribbons or films, (3.) orientable (stretchable) to dimensionally stable form at temperatures below that at which stress relaxation of the oriented molecules can occur, (4.) contain reactive side-groups pendant from the polymer "backbone", (5.) contain little or no groups directly in the polymer backbone structure which are as reactive as the reactive pendant groups under the conditions of reaction.

The polymers do not need plasticizers, fillers or special additives to function effectively in the present invention, although slip and antiblock additives, such as those disclosed in U.S. Pat. No. 4,897,437 and U.S. Pat. No. 4,751,262, both incorporated herein by reference, antioxidants, and inert fillers can be present in the polymers without affecting the polymer's performance in forming the microporous composition.

By "normally-solid" it is meant that the polymers are of sufficient molecular weight to be solid at ambient temperatures. "Thermoplastic" means that the polymers can be heat-softened or melted at superambient temperatures. "Extruded in elongate form" means that the heat-softened or melted polymer can be forced through an orifice to form a continuous elongated extrudate; this includes simultaneous extrusion of a plurality of strands to form multi-filaments, fibers, film and the like.

"Orientable" or "stretchable" means that the cooled extrudate, when subjected to sufficient tensile stress, will yield to the stress and become elongated in the direction of tensile stress. A "dimensionally stable form at temperatures below that at which stress relaxation can occur" means that after being oriented (elongated) under tensile stress, the polymer undergoes little, if any, elastic recoil as long as it is at a temperature below that at which "heat-shrinking" is encountered.

It is within the purview of the present concept to employ normally-solid, thermoplastic, orientable organic polymers comprised of chain segments which conform essentially to at least one of the following graphic representations of formulae: ##STR1## where R represents repeating olefin polymer units forming the stable polymer chain by way of polymerized olefin monomers; n is a number, representing a plurality of the repeating chain segments, that is sufficient to provide normally-solid polymers which are extrudable; X represents substantially reactive or polar side-groups on the polymer; X' represents substantially reactive or polar side-groups that are different from X but may or may not be reactable with the same reagents that react with X; Y represents side-groups that are not reactable with the reagents which react with X.

The reactive side-groups may comprise at least one selected from the group consisting of carboxylic acid groups, amide groups, nitrile groups, ester groups, phosphonate groups, thioester groups, carboxyl groups, sodium carboxylate, potassium carboxylate, lithium carboxylate, and ammonium carboxylate.

Thus, it is within the purview of the present concept to employ olefin polymers which have long chain molecular structures formed by the polymerization of monomers comprising at least one olefin or combinations of olefins, said molecular structures having pendant side-groups which may be substantially all alike or may not be all alike but which are reactive or reactable with a reagent which swells the polymer, and there may also be some pendant side-groups which are not reactive or reactable with the same reagent that is reactable with the said reagent. These non-reactive side-groups may be non-olefin moieties of olefin monomers that are pendant from the polymer chain, such as the alkyl moiety of C.sub.3 -C.sub.12 olefins, e.g., propylene, butene, heptene, hexene, octene, dodecene, styrene, and the like, or may be branches of polymerized olefin units which are pendant from the main polymer chain. Furthermore, it is possible, within the purview of the present concept, to employ monomers which are polymerized into the polymer chain through their olefin or vinyl unsaturation whereby pendant moieties of said monomers comprise relatively non-reactive groups such as halogens, esters, nitriles, phenyls, aryls; alkyls, imides, or amides. By "relatively non-reactive" it is meant that the groups undergo little or no reaction with the reagent under conditions at which the reactive groups undergo substantial reaction with the reagent. Polymers which contain such relatively non-reactive side-groups, may exert an influence on the reaction by contributing to the swelling, such as by hydration due to hydrogen bonding between water and the group.

As an example, consider polymers which contain side-groups having reactive carboxylic acid moieties and which also contain the pendant alkyl ester groups of an alkyl acrylate; the carboxylic acid groups can be reacted with a reagent, such as aqueous NaOH, under conditions wherein the Na salt of the carboxylic acid is formed, but substantially leaving the alkyl ester groups intact. Likewise, a halogen group on a polymer chain can remain intact (unreacted) under conditions wherein the carboxylic acid group is reacted. However, if the reagent is strong enough, and the reaction conditions stringent enough, even many of the "relatively non-reactive" groups, such as alkyl ester and halogen groups, may be caused to react. The neutralization of --COOH groups by reaction with NaOH is performed more readily than the saponification of --COOR groups with NaOH.

In general the present novel process comprises: cold-drawing (orienting) the polymer which has been extruded in elongate form at a temperature (usually ambient) below that at which stress relaxation of the stretched molecules is likely to be encountered; cutting, slicing, or chopping the oriented polymer to desired physical dimension, if needed; reacting the reactive side groups of the polymer with a reagent, thereby changing the side groups and swelling the polymer; subjecting the polymer to stress which causes fibrillation or strand separation of the polymer; using a counter-reagent to return the side groups to their original form or to some other suitable form, thereby creating voids in the polymer. Optionally, the reaction with the counter-reagent can be done before subjecting the reacted polymer to fibrillation or strand separation stresses. If the beginning polymer at hand is one which has already been cold-drawn (oriented) then that initial step as outlined above need not be repeated.

Within the purview of the present invention, the following reactions are among those which may be performed in swelling the cold-stretched polymers:

Another polymer of interest here is a terpolymer of ethylene/carbon monoxide/unsaturated carboxylic acid (e.g. acrylic or methacrylic acid) wherein the carbon monoxide provides carbonyl (ketone) groups along the chain and the acid provides reactive groups along the chain.

The figures (photomicrographs) provided with this disclosure are intended to be visual aids for use in describing and illustrating the porosity in various embodiments of the present invention. In the lower right hand corner of each figure, there is a black bar shown above a set of numbers. Only the left hand numbers (i.e. the 1, 10, or 100) are of relevance in studying the pore dimensions. The 1, 10, or 100, as the case may be with a given figure, is the length of the black bar, given in microns.

FIG. 1 is a 2,000X photograph of a porous EAA fiber which has been reacted with an aluminum compound by contacting the sodium derivative fiber with a solution containing 0.1 wt. % aluminum sulfate for one hour at 22.degree. C.

FIG. 2 is a 5,000X photograph of the same fiber as in FIG. 1. The prominent dark hole (pore) seen just left of, and near to, the central portion of FIG. 1 is shown as a larger dark hole in FIG. 2.

FIG. 3 is a 100X photograph of ah EAA copolymer filament wafer prepared by treating an oriented fiber of EAA copolymer with aq. NaOH at a temperature below the temperature at which stress relaxation of the stretched fiber is encountered, thereby swelling the fiber, then washing out the NaOH, leaving pores throughout the fiber, and cutting the fiber into small segments (called "wafers" here). The cutting of the fiber into wafers (or other segment lengths) may, alternatively, be performed before the washing of the NaOH.

FIG. 4 is a 500X photograph of an EAA copolymer filament wafer or section, prepared substantially in accordance with the same procedure as the wafer shown in FIG. 3.

FIG. 5 is a 500X photograph of an EAA copolymer fiber, swollen with aq. NaOH, and then stressed to cause fibrillation by squeezing the fiber between tight rollers. In the photograph the dominant feature is that of the bundle seen as an aggregation of parallel laminae, these laminae being easily separated. While in such bundle form there is porosity between the laminae, as well as micro-porosity permeating each lamina.

FIG. 6 is a 10,000X photograph of an EAA copolymer wafer cut from fine hollow fiber prepared by extruding a small hollow strand, cooling it to about room temperature, cold-stretching the strand to form a fine hollow fiber, then reacting the fiber with aq. NaOH in the manner of the present invention to form micro-porosity throughout the walls of the fine hollow fiber. In the photograph, the large dark area is the hollow portion of the fine hollow fiber and the small dark holes in the wall of the fiber are tiny micropores which are distributed throughout the fiber.

FIG. 7 demonstrates an embodiment wherein the elongated copolymer is ethylene/acrylic acid copolymer wherein the acrylic acid groups are the reactive side groups. The elongated copolymer is oriented at a temperature below that at which stress relaxation can occur (i.e., it is "cold stretched") and is then reacted with NaOH. The reaction with NaOH causes the polymer to swell and the pendent carboxylic acid groups, --COOH, are converted to --COONa groups. The so-treated copolymer is subjected to physical stress, such as mashing, rolling between squeeze rollers, or other means which causes it to fibrillate (disintegrate) into easily separable, porous fibers. Optionally, the --COONa groups can be hydrolyzed back to --COOH groups by using aqueous HCl; this optional step can be done before the fibrillation step, if desired.

Much of the following descriptions, examples, and explanations are directed to olefin polymers having carboxylic acid side-groups, such as ethylene/acrylic acid, "EAA", copolymers, which are among the preferred polymers in the present invention, but the invention is not limited to such polymers. It will be apparent to readers of this disclosure, who are skilled in these relevant arts, that other solid orientable polymers having reactive side-groups may be employed in substantially the same manner as described with the descriptions and explanations provided for said EAA polymers.

The highly porous polymers of the present invention are found to be quite useful in many applications where highly efficient separation of compounds or materials from fluid streams is desired. For instance, hardness values are efficiently removed from aqueous carriers, such as mineral water, brine, process streams, and the like. The fibers, especially those wherein the side-groups are --COOH, have been found to be effective in removing metal cations from solution in aqueous medium and from solution in organic or hydrocarbon media where the polymer is not dissolved or otherwise deleteriously affected by the organic or hydrocarbon; this includes monovalent and polyvalent metal ions, including heavy metals, transition metals, noble metals, rare earth metals, actinide series metals, light metals, alkali metals, and alkaline earth metals.

The porous fibers of the present invention are readily adaptable into the fiber and filament art for the making of monofilaments, multi-filaments, fine denier fibers, staple fibers, corded fibers, and the like. Filaments of greater than 15 denier and fine denier fibers are prepared in accordance with the present invention; "fine denier" generally means less than 15 denier and is so used here, though it should be realized that the extensive porosity of the instant fibers will have a noticeable effect on the relationship between fiber diameter and fiber weight expressed as "denier", especially with the hollow fibers. "Denier" is the weight in grams of a filament 9000 meters in length.

The lower the concentration of side-groups on a given polymer chain, the lower will be the amount of porosity one can obtain with that polymer. In the case of ethylene polymers, as the concentration of side-chain (such as --COOH groups) approaches zero, the lower will be the porosity and also the ability to draw fine denier fibers or multi-filaments is decreased, even though large denier monofilaments of polyethylene can usually be drawn (i.e., filaments of greater than 15 denier, especially greater than 30).

The porous fibers of the present invention can be formed into non-woven articles, yarns, woven articles, knitted articles, ropes, braided articles, pile loops, and the like. Masses of the polymer fibers can be joined at point-to-point contact by the application of enough pressure to cause sintering or radio-frequency bonding, so long as the "heat-shrink" temperature is substantially avoided. A modicum of heat-shrinking may be beneficial, however, in obtaining a "tight squeeze" of fibers wrapped around a core or mandrel, while sacrificing only a small amount of the porosity by way of the heat-shrinking.

Filter material comprising microporous fibers or filaments of the present invention can also be formed. The filter material can be made, for example, by blending various fibers with the fibers of the present invention, e.g., other olefinic fibers such as polypropylene or polyethylene, and subsequently thermally forming or otherwise integrating them into a filter structure. Other filter forming techniques, e.g., filter cartridge wrapping, are also applicable using the fibers of the present invention.

Filaments of ECA polymer granules which are cold-drawn (oriented) can be processed into a mass of microporous fibers. Such orientation results in ribbon-like structures with pendent fibrils and slot-like porosity and extensive microporosity. Moreover cold-drawing results in fibers of sufficient length to form felts or non-woven mats. The fibrils give the non-wovens enhanced strength. Filaments which are not cold-drawn give a ground-wood like material (short fibers) which can be cast into fragile sheets (or mats). The sheets of fibrous particles formed without cold-draw disintegrate or weaken when wet, or otherwise contacted, with water, whereas sheets or non-wovens derived from a cold-drawn filament are much stronger, even when wet, and can be treated much like a refined paper product made from wood fibers. A novel feature of this non-woven "paper" is that it exhibits ion exchange properties. Cold-drawing allows tiny derivative microporous "ribbons" of several feet length to be mechanically separated from a filament which would otherwise merely disintegrate into a fibrous, particulate mass resembling ground wood. The derivative microporous ribbons can be twisted into a threadlike bundle and even woven into an ion exchange "cloth" completely distinct from a nonporous, particulate or fibrous mass. The advantages of an ion exchange "paper" or "cloth" over a loose particulate mass of polymer are obvious, especially where new uses of the non-woven "paper" or cloth derive from microporosity, usually absent in the non-oriented particulate mass. Microporous fibers from the cold-drawn, oriented polymer can be sheared or ground into short lengths, if desired for use in an ion exchange bed. In such a case the microporous fibers exhibit good ion exchange properties, whereas the non-porous particulate polymer form exhibits very poor ion exchange chemistry. This is because the barrier properties of polyolefins conceal the chemistry, except where microporosity exposes the functional groups directly to the environment.

Cold-drawn, oriented filaments which are swollen, for instance EAA swollen in hot caustic, can be chopped into porous wafers. Cold-drawing not only produces porosity, but results in a capillary structure resembling the end grain of wood. Excellent ion exchange chemistry results from the induced microporosity. Non-porous granules of sodium salts of ethylene-methacrylic acid or ethylene-acrylic acid do not exhibit useful ion exchange chemistry.

While oriented, melt spun filaments of ECA polymer can be swollen to a microporous derivative form exhibiting ion exchange chemistry and even chopped into long fiber, which can be formed into a "pulp" or slurry. The non-wovens formed from melt spun fibers are weak, since no fibrils exist (in melt spun fibers) to entangle (fibril entanglement is responsible for good strength). Also the "fibers" are cylindrical in shape and exhibit a roughly circular cross-section compared to the ribbon-like structure which is favored by better strengths and also by better kinetics due to slot-like porosity of microporous ribbons.

Extruded monofilaments may be treated in a continuous manner in accordance with the present novel process by cold-stretching (orienting) the monofilament (preferably to near its tensile-break strength), treating with caustic to swell the polymer and react the reagent with the reactive groups on the polymer, and passing the so-treated monofilament through crushing rollers, which causes fibrillation of the monofilament into a multifilament bundle of microfibrillar texture. Such long multifilaments may be used, as is, in preparing, e.g., woven materials or wrapping on a filter mandrel; or, beneficially, may be twisted into a thread and used in preparing, e.g., woven materials or wrapped on a filter mandre. Such multifilaments provide benefits not found in ordinary melt-spun blown fiber, or spinnerette multifilaments because the instant fibers have fibrils along the surfaces of the fibers as well as having extensive microporosity.

The present particles or fibers, having extensive microporosity, provide greater access of reagents to the reactive groups on the polymer molecules, permitting as much as 90% or more of the reactive groups to be reacted with the reagent. The porosity is lost if the polymers become dissolved in a solvent or melted.

The following examples illustrate certain embodiments, but the invention is not limited to the particular embodiments illustrated.

In accordance with prior art, an ethylene/acrylic acid copolymer containing 20 wt. % acrylic acid and 80% ethylene, having a melt index of 300 grams/10 minutes, was melt extruded into a continuous filament of 1/8" diameter, and quenched in water to room temperature. One-inch long segments were cut from the strand and swelled in 0.5N NaOH for 2 hours at 50.degree.-55.degree. C. The segments were readily fibrillated by physical stress into fibers about 1/32 to 1/16 inch long, having a length-to-diameter ratio in the range of 10-15. The fibers were essentially non-porous.

In accordance with the present invention another strand as above was prepared and cooled, but was cold-stretched (oriented) about 100% (one foot stretched to about 2 feet). From this oriented strand were cut one-inch long segments which were swelled for 2 hours at 50.degree.-55.degree. C. in 0.5N NaOH. The segments were readily fibrillated by physical stress into fibers having a length-to-diameter ratio of 300-500 and the fibers were found to be extensively microporous. Fiber lengths of about 1/8" were found to be easily fabricable into mats and sheets using paper-making techniques.

A 10 gm filament of ethylene (80%)/acrylic acid (20%) copolymer of 1/8" diameter was cold-drawn (by hand) by a factor of 1.5-2.0 and cut into 1" segments and placed in a swelling bath containing 0.5N NaOH at 55.degree.-60.degree. C. The filament segments, swelled for 4 hours, turned white (they had been essentially clear) and rubbery during swelling. The filaments were drained, washed with de-ionized water, and placed in a bottle with de-ionized water (5% polymer in water) and allowed to shake vigorously on a laboratory mechanical shaker overnight (about 16 hours). An aqueous fiber slurry resulted with fibers ranging up to 1" in length. The slurry was cast onto a 100-mesh wire screen and the water was removed by suction (vacuum) applied to reverse side of the screen. The mat was allowed to air dry and was examined by microscopy. Fibers of 10-30 microns diameter were con, non with fibrils (branching) of 2-5 microns being very abundant. The fibers were found to be ribbon-like with fibrils composed of small appendage ribbons split from the parent ribbon, and were found to be extensively microporous. The so-formed mat was tough and very strong.

These ribbon-like fine fibers having fibril appendages are believed to be novel and are very useful by way of the tiny fibrils which entangle and give rise to mats, sheets, and articles of exceptional strength without the need to apply supplemental bonding. Furthermore, these novel fibers are quite useful as a blend component with other fibers, such as wood fibers, cotton fibers, cellulose fibers, nylon fibers, wool fibers, polyester fibers, other olefin polymer fibers (e.g. polypropylene), mineral fibers (e.g. asbestos), fiberglass, and other natural or synthetic fibers. The bonding tenacity of the fibril appendages provides considerable strength to mats drawn from a slurry of the blended fibers and also to hot castings of such blends.

Still further, the present fibers with the fibril appendages provide strongly bonded composites with non-fibrous pigments, fillers, polymers, resins, or other additives, such as clay, calcium carbonate, titanium dioxide, and other inorganics. The fibers are highly water-absorbent and readily dispersed into water to form a synthetic pulp resembling wood pulp; it also behaves much like quality wood pulp in formation of non-wovens. Non-woven products also resemble bleached refined wood pulp (paper) under microscopic examination.

An EAA copolymer (80 wt. % ethylene-20 wt. % acrylic acid and a 300 grams/10 minutes melt index) was extruded through a spinnerette die into a 500 micron diameter strand at 125.degree. C. and subsequently drawn to a 10 micron diameter filament. The 10 micron filament contained orientation resembling a cold-drawn filament. The melt spun fibers were cut from the core (of the haul off unit) in a sheet. The sheet was divided into strips parallel with the fiber length axis. The strips were then mechanically chopped into fibers of 7-10 mm length. The fibers were placed in a Waring blender with water and dispersed, then recovered and redispersed in an excess of a solution containing 0.5N NaOH. The mixture, containing about 15 wt. % chopped fiber by weight, was stirred and heated at 60.degree.-65.degree. C. for about 10-12 minutes. The swollen fibers were drained and thoroughly washed with deionized water and cast onto a 60 mesh stainless steel wire screen. Excess water was drawn off under vacuum and the mat was air dried. The white mat (or felt) exhibited very little strength, due to the absence of pendent fibrils, but did possess microporosity of a fibrillar or slot-like texture. The microporous chopped fibers were useful as intermediates for forming high melting (m.p. above 200.degree. C.) aluminum salt derivative fibers, formed by contacting the microporous chopped fibers with an aqueous solution containing 0.5 wt. % aluminum sulfate. The derivative fibers contained 1.3 wt. % aluminum by weight and were also microporous.

An ethylene/acrylic acid copolymer (20% AA by wt., 300 melt index) cold-drawn and caustic treated to cause swelling, is fibrilated and washed with water. The so-produced fibers, having extensive microporosity, are found to exchange cations in solutions containing metals, thereby creating ionomers (metal carboxylates) with the carboxylic functionalities on the polymer. The following metals are exchanged, though not all at the same rates:

These metal ionomers of these microporous polymers are novel and useful. They can be used for adding metal values to other materials or for subsequent exchanges of cations. They can also be molded into various shapes and configurations.

Surprisingly, the instant microporous polymers are capable of forming ionomers with polyvalent metals by cation exchange, whereas prior art involving EAA copolymers and divalent Mg and Ca have involved melt-compounding MgO and CaO into the polymer, thereby creating crosslinked polymer with the crosslink being essentially irreversible.