US3476638A - Impregnated nonwoven needled fabric consisting of a blend of synthetic fibers - Google Patents

Description

NOV. 4, 1969 K, w, PAULlG ET AL IMPREGNATED NONWOVEN NEEDLED FABRIC CONSISTING OF A BLEND OF SYNTHETIC FIBERS Filed NOV. l5. 1966 wwwa,

FIG l FIG 4 FIG 5 FIG 6` United States Patent O U.S. Cl. 161--148 6 Claims ABSTRACT OF THE DISCLOSURE An impregnated nonwoven fabric is disclosed, said fabric comprised of a needled batt which consists of a blend of stress-bearing synthetic fibers and nonstressbearing hydrophilic fibers, the binder being more heavily concentrated at discrete points corresponding to the ligations in the needled batt. The stress-bearing fibers consist of 25-70% crimped fibers and 454.0% uncrimped fibers.

This invention relates to synthetic leather. More particularly, it relates to an impregnated fibrous array suitable for use as a substrate to be finished into a leatherlike material for use in shoe uppers.

Leather substitutes generally are old in the art. Coated and impregnated papers have been used for many years, as have similarly treated woven fabrics. Although satisfactory for certain static uses suc-h as table-top ornamentation, luggage, notebook covers and the like, paper has insufficient, elongation to serve where the object must be molded or drawn to irregular contours, as in shoe formation. Woven fabrics impregnated with elastomeric polymers have found some limited utility in the shoe trade, particularly as quarter linings, but lack of extensibility and flex life has held such usage to low-priced shoes, where maximum service and comfort are not eX- pected. Prior art nonwoven fabrics, impregnated and coated, have also been the subject of intensive exploration in the shoe upper field. Although they do have greater elongation than paper or woven fabrics, in general conventional impregnated nonwoven fabrics have a relatively low tensile strength, tear strength, and flex life. By the use of special techniques and special binders, these deficiencies can be largely overcome, so that certain impregnated non-woven substrates can be produced which are acceptable so far as lasting processes are concerned, and which have good strength, scuf resistance, and moisture vapor permeability. In attaining such qualities, however, these recently developed substrates have not achieved the one property which distinguishes real leather from all nonleather prior art substrates of which we are aware: that is, the ability to take a limited permanent set.

The fact that a limited number of sizes of leather shoes will serve with relative comfort for a tremendous variety of shapes of individual feet is due in large part to the fact that leather, in addition to being extensible, will after a few wearings shape itself permanently to the highly individualized contour of the foot upon which it is worn. It is not by happenstance that the phrase corn- "ice fortable as an old shoe has been long current, and it is generally recognized that leather shoes, in the breakingin period, adjust themselves to the contour of the foot so that they feel comfortable, for broken in, even if worn only intermittently thereafter. This is because the interentangled fibers in real leather, under stress, can slip past each otherAand become rearranged along certain segments of their length, without becoming loosened from the main leather structure. It is characteristic of shoe upper leather that the fibrous structure will deform or rearrange readily under stress, but only up to a certain point, which is usually at 10% or less elongation. This degree of elongation is not spontaneously recoverable: that is, the leather fibers do not act like an elastic member under moderate repeated stress, but instead they take on a permanent set. If the leather is stressed beyond this point, inter-fiber slippage and fiber rupture occur.

Hitherto it has not been possible to our knowledge to reproduce the characteristic of permanent set in man made artificial leathers. Prior art non-'woven substrates for synthetic leather are in general of two types. Either they are relatively weak, and capable of excessive permanent deformation, or else they have been fortified to the point where they are extensible, but where there is no fiber flow under stress and no permanent deformation. Shoes made from the first class of material have so little recovery from stress that they rapidly become shapeless. Shoes made from the second class of material will conform during wearing to a foot, due to elasticity, but assume no permanent deformation, so that they feel like a new pair of shoes each time that they are worn. It is with improvements in the field of elongation versus permanent set that the present invention is concerned.

It is a primary object of this invention to provide an impregnated nonwoven substrate suitable for converting to a leather-like material.

It is a further object of this invention to provide such a substrate which matches Ythe elongation and the permanent set of real leather.

Other objects of the invention will appear more fully from the following description and the drawings, in which:

FIGURE 1 is a schematic plan view enlarged of one embodiment of the invention.

FIGURE 2 is a still further enlargement of a section of FIGURE 1.

FIGURE 3 is a schematic cross-sectional view of the material of FIGURE 2 along the line A-A.

FIGURES 4, 5, and 6 are idealized sketches of the behavoir under stress of a pair of fibers in the product of this invention.

It has now been found that many of the properties of real leather, including permanent set, may be duplicated in a nonwoven fabric provided that fibers of a specific type, arrayed in a specific geometry, are bonded with a range of polymeric bonding agents which are distributed throughout the nonwoven fabric in a specific manner. The various individual elements of this invention, to the best of our knowledge, have never before been combined to bring out the unusual interplay of elements which effects the present unexpected results.

The process of the present invention comprises the following steps:

(1) Assembling a fibrous batt comprising a blend of crimped and uncrimped fibers, preferably in isotropic distribution.

(2) Needling the thus-formed batt to provide spacedapart areas of high fiber density interspersed among areas of lower fiber density.

(3) Saturating the needled batt with an elastomeric polymer or blend of such polymers.

(4) Hot-pressing the impregnated and dried fibrous batt so that the fibrous array is compacted and made more dense, approximating the apparent density of natural upper leather (around 0.8 gram per cubic centimeter).

FIGURE 1 represents a needled and saturated batt of textile-length fibers, magnified about 10 times. FIGURE 2 is a further 10fold magnification of a portion of FIG- URE 1, and FIGURE 3 is a cross-section along the line A-A of FIGURE 2. Referring particularly to FIG- URES 2 and 3, textile-length fibers, of a character specified below, are aggregated by a needling operation which forcibly reorients a portion of the fibers out of their normally horizontal planar orientation and carries them down and through the fibrous batt normal to the principal plane of the batt. Such localized rearrangements are sometimes called ligations, in the sense that they are the end result of a threadless stitching or sewing operation. In addition to the reorientation process, there is also a packing or aggregating effect, so that the fibers are, in localized areas, brought much closer together than they are throughout the main body of the fibrous batt. Local and spaced-apart areas f high capillarity are thereby established, so that after saturating such a batt with an aqueous polymeric binder dispersion and drying, the binder will be found to be concentrated to a considerable extent in irregularly-shaped core-like regions 4 of FIG- URES 2 and 3. In the areas between cores, the fibers 2 may carry some binder substance in the form of a coating or encrustation, and there may be scattered beads or nodes of binder substance 6 of FIGURES 2 and 3. But the fibers are for the most part free to move in the regions between the high concentrations 4 of binder substance, while within said high concentration areas they are essentially locked into place. Since the binder in these high concentration areas extends in depth down through the batt as well as laterally on the surface, as shown in FIGURE 3, these areas will be referred to herein as plugs.

We have found that in order to duplicate the elongation vs. permanent set characteristics of leather, it is desirable that a fibrous blend of both crimped and uncrimped synthetic fibers be used. By crimped fibers is meant synthetic fibers which have been artificially crimped, usually by a mechanical process, into a wavy configuration typically having between 8 and 20 peaks per inch, of such a nature that if the fiber is gently stretched to a straight configuration, it will be found to have been extended by between 10% and 33% of its original length.

In, addition, it has been found that tensile strength alone is not the primary index of fiber choice if a close simulation of real leather is to be realized. When batts of different fibers, but of the same weight, were uniformly saturated with a polyvinyl butyral dispersion to the same degree of add-on, the tensile strengths of one inch wide strips were: acrylic fibers 23, viscose rayon '28, polyester 35, polypropylene 50, nylon 52. Rather than use all nylon, however, we find that the moisture vapor transmission rate, strength, elongation, and permanent set of leather are best matched by the use of a fibrous blend of the following nature:

25%-70% crimped synthetic fibers 45 %-20% uncrimped synthetic fibers 30%-10% hydrophilic fibers such as viscose rayon.

The synthetic fibers in the above composition are the principal stress-bearing members, as explained more fully below. Nylon is the fiber of choice, but for economic or other considerations the crimped fiber fraction may be a blend of nylon with polypropylene fibers, which are also crimped. The viscose rayon ribers, present preferably to the extent of not more than 30% of the fibrous array, are valuable in increasing the moisture absorption, regain, and moisture vapor transmission of the product, and are not a substantial contributor to the strength of the product. For best results the fibers should be in random or isotropic distribution, so that the strength of the product is substantially equal in all directions. This may be accomplished by cross-lapping, air-lay techniques, or other means well known in the textile art.

. The attainment of up to 10% permanent set in the products of this invention is not completely understood, but it appears to be a function of the interaction between the binder, the crimped fibers, and the uncrimped fibers. One possible basis for explanation is shown in FIGURES 4, 5 and 6, wherein 10, 10 represents a pair of localized high concentrations of polymeric binding material, 12 represents an uncrimped and 14 is a crimped textile length fiber, both 12 and 14 being bonded at the points 10, 10. These figures are admittedly hypothecated and idealized, since there are many fibers, both crimped and uncrimped, which are bonded at two separated points along their length by any adjacent pair of binder areas or plugs. Nevertheless, since there are substantial numbers of both crimped and uncrimped fibers in the product, there is a likelihood that for each uncrimped ber which follows a straight or cursive path between the points 10, 10, there is a crimped fiber following a similar path.

If stress is applied to the material in a lateral direction, the initial effect is shown in FIGURE 5, where the paths of the crimped and uncrimped fibers have to at least some extent become straighter. This is probably the situation in many of the fibers on the outer surface of a lasted shoe, with many of the uncrimped fibers in a generally straight configuration, and the crimped fibers in a similar configuration but capable of further extension due to residual crimp.

If the surface of the shoe is then called upon to meet local stress, as from enlarged joints or other deformities of the active or passive foot, the product of this invention is capable of elongating further due to the fact that the crimped fiber fraction between the binder areas 10, 10 is capable of further elongation. In the absence of such local stress, the fibrous configuration in the shoe may be assumed to approximate FIGURE 5, where the uncrimped fiber segments 12 lying between binder plugs 10, 10 stabilize the structure.

In the presence of local stress, the crimp is assumed to be locally removed from the crimped fiber segment 141ying between the binder plugs 10, 10, giving rise to the configurations shown in FIGURE 6. In this process of fiber extension in the crimped fiber segments, there is a certain degree of slippage of the straight fiber segments 12 through the binder plugs, or some plastic deformation of the binder plugs 10, 10, or both. For this reason, the binder material must have certain properties, as set forth below.

The tensioned segments of the uncrimped fibers remain bonded, and offer a degree of resistance to further deformation of the structure. As the crimped fiber segments 14 are stressed, removing the crimp, their resistance to further elongation, coupled with the residual resistance 0ffered by the uncrimped liber segments 12, is sufiicient to overcome or withstand the local stress. In order to match the unrecovered elongation or permanent set which is characteristic of natural leather, a level of 10% unrecovered elongation is preferred in the products of this invention. The test method used to determine permanent set is explained under Example I, below.

EXAMPLE 1 A blend of the following fibers was formed into an isotropic batt by cross-laying a garnett fleece of the following composition:

Percent 3 denier crimped polypropylene 40 3 denier nylon type 201, crimped 20 2.3 denier nylon type 420, uncrimped 20 3 denier viscose rayon, uncrimped 20 This batt was needled conventionally in a Hunter needle loom at a density of 350 needlings per square inch. The needled batt was then saturated with an aqueous emulsion containing the following ingredients:

75 dry parts of a soft, film-forming acrylic resin (Hycar 2600 x 83, from B. F. Goodrich) dry parts of nitrile rubber (Hycar 1572 x 42) 10 dry parts of semi-fluid acrylic resin (Daratack 74L,

Dewey and Almy) 5 dry parts of a zinc oxide dispersion, 60% solids 5 dry parts of a 60% clay dispersion 2 dry parts of a melamine resin 0.2 dry part of ammonium chloride catalyst.

The wet pickup Was adjusted so that about 150% solids by weight was added to the fibrous batt, so that after drying on dry cans the impregnated product consisted of about 40% fibers and 60% polymeric impregnant.

In this form, the density of the product is about 0.4 gram per cubic centimeter, and the material is too weak and soft to serve satisfactorily as a shoe upper material. The above product is therefore pressed in a platen press for 30 seconds at 150 p.s.i.g. with the top platen at a temperature of 295 F. and the bottom platen at room ternperature. This pressure should be sufficient to increase the density of the product to the preferred range of between 0.8 and 1.1 grams per cubic centimeter, matching the density of natural leather. The temperature differential helps to give a smoother and more compacted organization to the heated surface of the artificial leather, so that it can respond properly to the conventional coatings and finishes which are applied to the outer surface of shoe leather.

The properties of the above compressed artificial leather base were compared with the properties of a good grade of shoe upper leather.

The air permeability and moisture vapor transmission of the artificial product were also equal or superior to the natural leather.

The permanent set of the product of Example I was compared with the permanent set of the natural leather by cycling one-inch wide strips of both materials to an elongation of for seven cycles on the Instron machine. After 24 hours standing after the last cycling, both the product of Example I and the natural leather showed an unrecovered deformation, or permanent set, of 5%. A Widely known and used poromeric synthetic shoe material, in a comparable test, recovered its original dimensions completely in less than 24 hours, indicating that it had taken no permanent set. As explained above, this property of conforming permanently to local stresses is most important in shoe leather, and is an outstanding advantage of the product of this invention.

The particular types of fibers selected for use in the practice of this invention may be varied, provided that the preferred ratio of crimped to uncrimped stress-bearing fibers set forth above is not widely departed from.

By stress-bearing fibers is meant those fibers, present usually to the extent of 70% or more of the total ber content, which largely determine the physical properties of the fibrous part of the product. In general, at least 70% of the weight of the fibrous element will consist of fibers which have a tenacity of at least 3 grams per denier under standard conditions, and a wet tenacity which is at least of the dry tenacity. Nylon is a particularly valuable fiber for use in this invention, `since it enhances the tear strength of the product, which is reflected in sewing or stitch-tear resistance needed to produce durable shoes. The nylon, preferably present to the extent of at least 20% of the fibrous element, may for economic or other reasons be blended with other suitable stress-bearing synthetic fibers, such as the polypropylene of Example I. It is desirable that the artificial leathers of this invention be not too soft and too readily deformable, nor should they be too rubbery, bouncy, or resilient. Most of the commercially available film-forming acrylic used as binders for nonwoven fabrics yield films which are too soft, and are marked by low tensile strength, high elongation, and high creep. Nitrile latices in general yield films which are too rubbery and elastic. Typical moduli of elongation were run on films cast from the acrylic latex, the nitrile latex, and the blend of the two lattices identified above in Example I.

Modulus in pounds per square inch at- We have found that a desirable range of modulus for the binder used in this invention is from. l0 to 25 pounds per square inch at elongation. One convenient, though not restrictive method of arriving at such a modulus is to use as binding agent a major portion (50% or more) of a soft, film-forming acrylic resin, a minor (10%-40%) of a rubbery elastomeric resin such as a nitrile rubber, and a still smaller portion (2%-20%) of a semi-fluid compatible polymeric plasticizer. Other binder combinations will suggest themselves to those skilled in the art, the important consideration being that the binder should adhere well to the fibers and should have a modulus of between l0 and 25 pounds per square inch at 100% elongation.

Having thus described our invention, we claim: 1. An impregnated nonwoven fabric suitable as a base for artificial leather which comprises a needled batt of textile-length fibers consisting of a blend of stress-bearing synthetic :fibers which have a dry tenacity of at least 3 grams per denier and a wet tenacity which is at least 75% of the dry tenacity,

said stress-bearing synthetic fibers comprising between 25% and 70% crimped synthetic fibers and between 45% and 20% uncrimped synthetic fibers,

the balance of said blend comprising between 30% and 10% of non-stress-bearing hydrophilic fibers,

said needled batt of textile-length fibers being bonded with a soft polymeric binding material,

said binding material being more heavily concentrated in a set of discrete and spaced-apart points corresponding to the ligations in said needled batt than it is in the areas between said points,

said impregnated nonwoven fabric having a permanent set of not greater than 10%.

2. The product according to claim 1 in which the orientation of the textile-length fibers is isotropic throughout the principal plane of the fabric.

3. The product according to claim 1 in which the uncrimped synthetic stress-bearing fibers are nylon and the crimped synthetic stress-bearing fibers are a blend of nylon and polypropylene.

4. The product according to claim 1 in which the References Cited textile-length fibers comprise 25% to 70% of a blend of crimped nylon and crimped polypropylene; 45% to 20% FOREIGN PATFNTS of uncrimped nylon; and 30% to 10% of hydrophilic 9191500 2/1963 Great Bumm' ber.

5. The product according to claim 4 in which the 5 ROBERT F' BURNETT Prlmary Exammer hydrophilic fiber is viscose rayon. L. M. CARLIN, Assistant Examiner 6. The product according to claim 1 in which the modulus of the binder is between 10 pounds and 25 U.S. Cl. X.R.

pounds per square inch at 100% elongation. 10 161-154, 170

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