US2773286A - Process of forming non-woven porous fibrous synthetic leather sheet - Google Patents

Description

Uniad w a m o w PROCESS OF FORMING NON-WOVEN PQ QUS FIBROUS SYNTHETIC LEATHER j-Jolm Augustus Piccard, Swarthmore, Pa., and Bnynton G aham. larmp tl fi .a si no to du Pont 3 (le Nernours and Company, Wilmington, Del a corpor'ation of Delaware No Drawing. Application July 29, 1952, Serial No. 301,603

5 Claims. (Cl. 18-48.)

I This invention relates to synthetic leather and, more particularly, to the process of treating a non-wovenpolyvise a simple and rapid technique of producing. synthetic .leather cornpositions. In mostof the early art, pyroxylin was used to coat or impregnate various types of fibrous base materials to prepare leather substitutes. As time went on,pyroxyIin/oil/pigment compositions were. widely used as coating. or impregnating compositions for various woven or non-woven fibrous base materials. In the early stages of the synthetic leather industry, themain objective was to simulate the general appearance 10f leathetn, 1

InItodays markets, coated fabrics, particularly the vinyl-coated fabrics, are outstanding as leather substitutes in such applications as handbags, bookbindings, brief cases, card table covers, luggage, etc. In such applications, the coated. fabrics are satisfactorywbecause:the

'generaLappearance of leather is. simulated; and the coated 'jfabrics possesssome of the desirable propertiesofleather.

However, as compared to leather, the coated-fahricsrlack good tear strength, softness and the ability to breathe" or transpire water vapor and air; and although the coated fabrics are used in such applications as chair coverings, much is left to be desired, especially with respect to water vapor and air permeability. Up to the present time, 1

synthetic leather compositions have made little or no inroads into the boots, shoes, and gloves marketspm rinly because of their inability to breathe in addition to l'ack of good tear strength and softness. As used hereinafter, the term breathe means transpire water vapor andair.

In general, the use of a synthetic leather composition in boots, shoes, gloves, etc., is mainly dependent upon its ability to breathe, usually expressed in terms of water temperature of the binder material.

vapor permeability or leather permeability. Phyical tests on the water vapor permeability of leather indicate 7 that leather transpires water vapor about /3 as readily as free air. In general terms, shoe upper leather samples having a thickness of 0.016"-0.104" have a leather permeability within the range of LOGO-18,000 gms./ 100 sq.umeters/hr. when tested according toKanagy &Vickers Journal of American Leather Chemical Association 45,

211 -2421 (.April 1950), in an atmosphereof 23: C. and

890% relative humidity. Hereinaft'er,-the ability of syn- =thetic leather compositions 1 to transpire waterwaporwill =be expressed in terms of leather permeability in gins/100 sq; meters/hr. 1 Based upon comforttests, the minimum tolerableleather permeability for shoeupper leather is about 2,000 gms./ 1 00 sq. meters/hr. Preferably, for shee upper leather, the permeability value should be 4,000-20000 when tested at 23 C., and not greater than relative humidity.

fin- 'ohject of the, present invention is to provideasym ,1:

thetic leather having outstanding breathing qualities. A further object of the present invention is to provide a process of preparing a synthetic leather having the requisite properties for fabricating boots, shoes, gloves, garments, chair coverings, and other articles wherein a composition capable of breathing is required. A still further object is to provide a process of preparing a synthetic leather having a tenacity, flex life, elongation, tear strength, modulus and leather permeability equal or superior to the various types of genuine leather. Other objects will be apparent from the description given hereinafter. i

The above objects are accomplished according to the present invention by forming a compact, essentially impermeable, and continuous composite sheet by hot pressing, a composition comprising a structural fiber com ponent, an extractable pore-forming fiber component and a binder material, the proportion of pore-forming fiber, based upon the total volume of the sheet, being from 4 0570%, the weight ratio of structural .fiber to, binder material being from 1:2 to 2:1, and thereafter forming interconnecting poreshaving essentially the shape of the .porerform ing .fibers in thesheet by extracting the poreforming fiberQ t The preferred structural fiber is an, orientedsynthetic linear polyamide, for example, polyhexam'ethylene adiparnide, polyhexamethylene sebacamicle, polycaproamide, or an interpolyamide of the type disclosed in U,.S.'P. 2,285,009. IThePore-forming fiber is one. which ;is readily extractable with a solvent, e. g., waiter, acetone, etc.,. that is substantially inert, i. e., hasfno solvent action, toward ,the structural fiber and the binder material. The pre ferred p ore-forming fibers are cellulose acetate. and polyvinyl alcohol. The binder material may be selectedffrorn the large class of soft, elastic, at least initially thermoplastic, synthetic polymers, the preferred polymers being N-methoxymethyl polyhexamethylene adipamide, polyethylenetand its. derivatives, copolyesters made from ethylene glycol and 60 mol per cent of terephthalic acid and 40 mol per cent of sebacic acid, synthetic rubbers such as neoprene (polymeric 2-chloro-1,3-butadiene) and compositions containing vinyl chloride polymers or copolymers. i l i i The pore-forming fiber and the structural fiber componentsmust have softening temperatures above the flow Furthermore, the structural-fiber component and the binder material must be insolublein the solvent used'to extract the pore-form- "ing'fiber component.

The following-examples will serve to illustrate the principles and practice of the present invention and demonstrate the fcriticalness of. the specified relative proportions of ingredients employed. Parts are by weights unless otherwise indicated.

Example 1 A mixture of 1.688 parts of 0.01 average length, 3 denier/filament, polyhexamethylene adipamide staple and 4,125 parts, of 0.01" average length, idenier/filament, celluloseacetate staple was dispersed in, 600 parts of water by'stirring. To the dispersionwas added 12.05 parts of a 14% latex of N-methoxyrnethy1 polyhexamethyl ene adipamide having a DV (DV=dllution value, as defined in U. S. P. 2,430,923) of 55, together with 0.01 part of ootyl phenyl polyglycol ether (to maintain the fiber dispersion) and 0.02 part of acetic acid (to fleeculate the polyamide dispersion). After stirring briefly, 'a mat was formed by filtration through coarse filter paper, the area of the filter being such as to provide a retained weight of fibers plus'biuder of 3.0 oz./sq.,.ft. The. mat was dried and pressed for five: minutes at 14 0 C. under a pressure of 1930 lbs/sq. in, andthe cellulose 3 acetate component was then removed by extracting the mat for 18 hours with warm running acetone. The extra-cted product weighed 1.34 oz./sq. ft. and was leatherlike in appearance and feel. It had a tongue-tear strength (average resistance to propagation in the lnstron tester) of 1.6 pounds. (Tongue-tear strength is measured by cutting a 1" slit in the sheet to be tested and thereafter measuring the average force in pounds required to propagate the tear. The test is carried out in a manner similar to ASTM test method D39-39.) The product exhibited a leather permeability of 2369 gms./100 sq. meters/hr. when tested at 23 C. and 90% relative humidity. (In similar tests of water vapor permeability, various samples of representative shoe upper leathers exhibited the following leather permeabilities, all expressed in gms./100 sq. meters/hm Suede calf 13,932 Glazed kid 3,718 Scotch grain cow 6,161 Cretan butt 7,863 English calf 9,430

A considerable series of actual wearing comfort tests has indicated that membranes exhibiting a permeability in this test of 2,000 to 10,000 will provide wearing comfort equivalent to that obtained with glazed leathers and heavy shoe upper leathers, while permeabilities of 10,000 to 22,000 were found to provide wearing comfort equivalent to that obtained with the lightest shoe upper leathers.) The leather permeability test devised by Kanagy & Vickers, as described in the aforesaid article, was found to correlate with wearing comfort tests. In this test, a 3-inch diameter crystallizing dish was filled with 12-mesh calcium chloride, covered with a membrane of the substance under test, suspended inverted in an atmosphere of high humidity, and weighed at intervals. All values of leather permeability were measured at 23 C. and 90% relative humidity.

Example 2 A non-woven, fibrous, composite sheet was prepared by pressing the following plies of carded fibers into a composite sheet:

soap and shoe polish, the material had a leather permeability of 7151 and a resistance to penetration by liquid Water of 36 flexes, when tested according to the procedure descnibed in Federal Leather Specification KK-L-3l1, Method #271.1 (3/28/45). Representative shoe upper leathers have given the following valuesin this test:

Example 3 A non-woven fibrous sheet was prepared in a manner similar to that of Example 2. The top ply was composed of 40 parts of the copolyester staple binder of Example- 2 and 30 parts of cellulose acetate fiber pore-former, 3 denier/ filament, 1.5 in length. The middle ply was composed of parts of 3 denier/filament, 2.5 polyhexamethylene adipamide structural fiber, 60 parts of the copolyester binder fiber, and 180 parts of cellulose acetate pore-former. The base layer was composed of 60 parts of the polyamide structural fiber, 60 parts of the 00- polyester binder fiber, and 240 parts of the cellulose acetate pore-former. The composite structure contained an overall ratio of structural fiber/binder/pore-former of l6.5/2l.9/61.6. The product was 0.034" thick and weighed 1.3 oz./sq. ft It had a leather permeability of 14,284, a tensile strength of 1,739 lbs/sq. in., an elongation of 105%, a modulus of 3,101 lbs/sq. in., a tonguetear strength of 10.6 pounds, and a Schiltknecht flex life of 826,0004,645,000 flexes. After application of saddle soap and shoe polish, the product had a leather permeability of 7,555 and a Schiltknecht flex life of 1,677,000- 28,000,000.

Example 4 This example illustrates the pertinent physical properties of the initial non-woven, fibrous, impermeable structure of the present invention prior to extraction of the pore-forming fiber.

Each ply was prepared by carding the fibers into a bat,

and the plies were pressed together in cross-grain fashion is described in Bulletin #105 of Alfred Suter, 200 Fifth Avenue, New York, New York), and a cold crack temperature of below -70 C. After application of saddle A product was prepared as described in Example l except that the step of extracting the pore-forming cellulose acetate staple was omitted. The product weighed 3.0 oz./sq. ft., and it was very stiff and not leather-like in appearance or handle. It had a leather permeability of only 84 gms./ sq. meters/hr., a tongue-tear strength of only 1.0 lb./oz./sq. ft., and a Schiltknecht flex lifeof only 11,875 flexes. As a matter of comparison, the product described in Example 1 had a tongue-tear strength of 1.20 lbs./oz./sq. ft.; and a product similar to that described in Example 1 had a Schiltknecht flex life of 25,000-66,000. Thus, the step of extracting the poreforming fibrous component improved not only permeability to water vapor, but also tear strength and flex life.

Example Amat similar-to thatd scribed in E a pletl was prepared except that the ,ratio, of components was varied to provide a polyhexamethylene a-dipamide fiber/N-methoxymethyl polyhexamethylene adipa-mide binder fiber/cellulose acetate pore-former fiber ratio of 15/30/55 (structural fiber/binder ratio of -1/2,). The product was {permeable and had a tongue-tear strength of 1.1 pounds.

Example 6 when the fiber/binder ratio was less than 1/2, or greate than 2/1. As will be illustrated hereinafter, the tear strength ofthe synthetic leather compositions of the pres,- ent invention will vary with the length of the structural fiber. In Examples 1, 5. and 6, the length of thestruetural fiber was 0.01", a minimum value insofar as fabricating structures of substantial tear strengthh is concerned. In general, experience has shown that little additional strength is obtained by employing a structural fiber longer than about 1.5". l-Iowevenit may be more convenient,

from the standpoint of the type of textile machinery to I be employed, to use structural fibers from 0.5" to 8" in length. Hence, employing a fiber/binder ratio of f rom l/2 to 2/1 produces a composition having an optimum tear strength. This is a surprising discovery on the basis of what was known heretofore about fiber-reinforced compositions.

The following examples (7-15 inclusive) illustrate the effect of varying the contentof pore-forming fiber in the initial composition upon theleather permeability and tear strength of the extracted product.

Example 7 A matwas prepared as described inExamplel except that the ratio of structural fiber/binder/pore-formerwas intheratio of 37.5/37.5/25. The product had a leather permeability of 496 and a tongue-tear strength of 33.6 pounds.

Example8 A mat similar to that of Example Twas prepared except that the structural fiber/binder/pore-for-mer was in the ratio of 30/ 30/40. The resulting product had a leather permeability of 588.

Example'9 A mat similar to that of Example 7 was prepared Except, that the ratio of structural 'fiber/binder/porefOrmer was, in the ratio of 27.5/27.5/45. The extracted product had a leather permeability of 3,680.

Example 10 A mat similar to that of Example 7 was prepared except that the ratiotof structural fiber/binder/poreformer was in the ratio of 25/25/50. The product had a leather permeability-of 8,,85I3randa tougue1tear-.-stre gth ofi'2l7poundsy g E ample 1-1 A mat similar to Example 7 was prepared except that the ratio, of structural fiber/binder/pore-former 15115170. The extracted product had a leather permeability ot 17,895 and a tongue-tear strength of 1.4 pounds.

Example 13 A mat similar tothat of Example 7, was preparedexcept that-the ratio of structural fiber/binder/pore-former was .10/10/80. The. extr ct p u h a le her permeability of*23,7;21 and a tongue-tear strength of 0. Pounds.

Example 14 T016300 parts of water containing 0.2 part of alkyl 'aryl polyglycol ether were added, with stirring, 4.00parts of 0.5"", 3 denier/filament polyhexamethylene adipamide structural staple and 5.34 parts of 0.5", 25 denier/ filament polyvinyl alcohol pore-former fiber. The fiber dispersion was formed into a mat by filtration through an 8-rnesh wire screen. The mat was impregnated with 40 parts of a solution of polyvinyl chloride/di- .(2-ethylhexyl)tphthalate /60 disperse-d at 10% concentration in tetrahydrofurane/dimethyl formarnide 9.75/25. After drying, the impregnatedv mat was pressed for 10 minutes at C. and 1,245 lbs./sq. in.; and the polyvinyl alcohol. pore-former was finally extracted with 90 Curunning water. Complete extraction required 96 hours. The product had a leather permeability of. 3,805. The ratio of structural fiber/binder/pore-former was 30/30/40.

Example 15 TABLE I Percent Percent Leather Permeability Pore- Poreand Length of Example Fotpruer Former .Pore-Forming-Eiher y y Volume Weight v 0.01" Flock 10.5 Staple .leatherpermeability.

Example 16 To 16,000 parts of water containing 0.2 part of octyl amide staple and 4.125 parts of 0.01", 3 denier/filament cellulose acetate staple. The fiber dispersion wasformed into a that by filtration through an S-rriesh wire screen of area such as to provide a fiber weight of 2.3 oz./ sq. ft. The mat was impregnated with 12.05 parts of a 14% latex of N-met-hoxymethyl polyhexamethylene adipamide (DV=55), dried, pressed for five minutes at 140 C. and 1,930 lbs/sq. in. (structural fiber/binder/poreformer ratio of 22.5/22.5/S); and the cellulose acetate was finally extracted with warm running acetone.

The product had a leather permeability of 5,530 and a tongue-tear strength of 2.3 pounds. By comparison with the product described in Example 1, it will be seen that this increase in length of the structural fiber resulted in a corresponding increase in the tear resistance of the Example 17 A product was prepared as in Example 16 except that 0.5" polyhexamethylene adipamide staple was replaced with a. corresponding amount of 0.01", 3 denier/filament polyhexamethylene adipamide staple; and the 0.01" cellulose acetate staple was replaced with a corresponding amount' of 0.25", 1 denier/filament cellulose acetate staple. The product exhibited a leather permeability of 13,452 and a resistance to water liquid penetration of 172 flexes.

Example 18 The product was prepared in a manner similar to that of Example 16. In this example, 1.5 parts of 0.5" 3 denier/filament polyhexamethylene adipamide staple; 4.5

parts of 0.5, 3 denier/filament cellulose acetate staple;

and 10.7 parts of a 14% latex of N-methoxymethyl polyhexamethylene adipamide were combined to give a structural fiber/binder/pore-former ratio of 20/20/60. The product had a leather permeability of 13,706 and a tongue tear strength of 3.5 pounds.

A comparison of the permeabil-ities exhibited by the products of Examples 16--18 inclusive indicates that the permeability increased as the pore-forming fiber length was increased up to about 0.25", beyond which no further increase in permeability was obtained by increasing the pore-forming fiber lehgth, even when the pore-forming content was raised from 55 to 60%.

. Although little additional strength can be gained by increasing the length of the structural fiber beyond 1.5" and little additional ability to breathe can be gained by "increasing the pore-former fiber length beyond 0.25, it

may be convenient to use longer fibers. Various standard types of textile machinery which may be used in thepro' duction' of these synthetic leather compositions are designed to handle staple fibers in limited length ranges which may lie anywhere in the neighborhood of 0.5" to as long as 8''.

for these compositions using even longer fibers.

The following examples-illustrate the effect of fiber denier, i; e., both structural and power-forming fiber,

It is also possible to make usable mats upon leather permeability.

Example 19 v I I Aproduct was prepared as in Example 16 except that p 0.375", 1 denier/filament polyhexamethylene adipamide staple and 0.01", 1 denier/filament cellulose acetate poreformer were used. After extraction of the cellulose acetate pore-former, the product had a leather permeability of 3,992 and a water liquid penetration resistance of 1,227 flexes.

Example 20 A product was prepared as in Example 19 except that 0.01", 1.5 denier/filament cellulose acetate staple was employed. After extraction of the pore-former, the product had a leather permeability of 3,940 and a water liquid penetration resistance of 171 flexes.

Example 21 A product was prepared as in Example 19 except that 0.01", 3 denier/filament cellulose acetate staple was employed as the pore-former. After extraction of the poreformer, the product had a leather permeability of 4,888 and a water liquid penetration resistance of 80 flexes.

Example 22 A product was prepared as in Example 19 except that 0.5, 3 denier/filament polyhexamethylene adipamide staple and 0.01", l denier/filament cellulose. acetate staple were employed as structural fiber and pore-former, respectively. After extraction of the pore-former, the product had a leather permeability of 4,126 and a water liquid penetration resistance of 238 'fiexes.

Example 23 A product was prepared as in Example 19 except that 0.5", 1 denier/filament polyhexamethylene adipamide staple and 0.01, 1 denier/filament cellulose acetate were employed as structural fiber and pore-former, respectively. The extracted product had a leather permeability of 3,836.

' Example 24 A product was prepared as in Example 19 except that 0.5", 3 denier/filament polyhexamethylene adipamide staple and 0.01", 1.5 denier/ filament cellulose acetate were employed as structural fiber and pore-former, respectively. The extracted product had a leather permeability of 4,649 and a water liquid penetration resistance of flexes.

The results of Examples 19-24, inclusive, indicate that variations in structural fiber and pore-forming fiber thick- .ness result in little variation in the leather permeability of the synthetic leather compositions. Generally, increasing the thickness of the pore-forming fiber decreases the .re-

sistance to liquid water penetration.

Fibers which are finer than 1 denier are difficult to process on standard textile machinery; and when used as structural fibers in the present compositions, the fibers are too rigidly immobilized by the binder to give optimum strength, e. g., tear and tensile. In other words, the fibers of greater denier tend to shift within the binder under flexing or tearing, this action making the product more durable. Finer fibers, however, are rigidly held by the binder. Furthermore, fibers having a denier less than /2 have so much exposed surface, i. e., ratio of surface area to core volume is high, and are so soft that they no longer behave as normal textle staples. On the other hand, fibers coarser than 16 denier are extremely harsh and more closely resemble bristles.

The following Examples 25-26 illustrate the effects of thickness of the extracted sheets upon the value of leather permeability.

Example 25 A product was prepared in a manner similar to that described in Example 1 except that the thickness of the extracted Product was 0.0197, and its weightwas 0.67 (gm/sq. ft. The product had a leather permeability of Example 26 A product was prepared in a manner similar to that of Example 25 except that the, thickness of the resulting extracted product was 0.071". The leather permeability was 4,281.

The above examples indicate thatpermeability via in! ternal surface diffusion is only halved by an approximate 4-fold increase in the thickness. This is in contrast to the fact that with homogeneous films, the relationship 'between thickness and permeability is essentially "linear for relatively hydrophobic compositions, but varies from linearity with hydrophilicfilms wherein an increase in film thickness does not give the calculated decrease in permeability.

The following examples illustrate the use of various types of synthetic polymers as binders and the incorporation of such binder polymers into the compositions in various forms, e. g., as homogeneous films, from solvent solutions, from aqueous dispersions, etc.

Example 27 A mat consisting of 1.688 :partsof 0.01, 3 denier/filament polyhexamethylene adipamide staple and 4125 parts of 0.01", 3 denier/filament cellulose acetate staple was prepared from dispersion in 600 parts of water by filtration through a. filter with an area such as to provide a fiber Weight of 2.3 oz./sq. ft. The mat was dried, preheated for 15 minutes at 180 C. in an atmosphereof nitrogen, and then impregnated with 1.688 parts of a film f N-methoxymethyl polyhexamethylene adipamide (DV=86) by pressing for 10 minutes. at 160 C. and .1450 lbs/sq. in. The product had a structural fiber/ binder/pore-former ratio of 22.5/22.5/5.5. After extractionof the cellulose acetate, the leather permeability was 7,294; the product resisted 48 flexes before permitting water liquid penetration; and it resisted 213,000 flexes in the Schiltknecht machine before surface cracking developed.

Example 28 A mat was prepared in a manner similar to Example 16 using 0.5" cellulose acetate staple and replacing the latex with 29.6 parts of a dispersion of ,N-methoxymethyl polyhexamethylene adipamide (DI/=86) at 5.7% concentration in ethanol/ water 80/20. After drying, the impregnatedmat was pressed for 15 .minutes at 165 C. and 1245 lbs./ sq. in; and the cellulose acetate pore-former was finally extracted with acetone. The tough, permeable, pliable, leather-like-product was 0.028 thick and. Weighed 1.48 oz./sq. ft. It had a leather permeability of 13,778, a tensile strength of 3490 lbs/sq. in., an elongation of 38%, a modulus of 13,652 lbs/sq. in., and a tongue-tear strength of 16.0 pounds.

Example 29 Applied non-woven fibrous mat was prepared in a manner similar to Example 3. This composition was composed of a top layer of 1 part of a copolyester of ethylene glycol, 60 mol percent of terephthalic acid, and 40 mol percent of sebacic acid as the binder; and 1 part of cellulose acetate fiber, 3 denier per filament, 1.5" in length. An intermediate layer was composed of 3 parts of the copolyester binder, 3 parts of polyethylene terephthalate structural fiber, and 9 parts of cellulose acetate poreeforming fiber. A base layer was composed of parts of the copolyester binder, 5 parts "of polyhexamethylene adipamide structural fiber. and 15 parts .of cellulose'acetate fiber'pore-former. The layers were composited under heat and pressure, and the cellulose acetate was extracted. The resulting product C10 weigh d 13 czJsq. .-,ft., ha a leather permeation: 13, and a .Schiltkuecht .fiex life of 553.000 flexes.

Example 30 A product was prepared as in Example 27 except that the polyamide film was repl ced with an eq al. amoun of polyethylene. film, and the pressing waseondueted for 5 minutes at 175 C. and. 3390 lbs/sq. in, after preheating the mat 15 minutes at 180 C. in a nitrogen atmosphere. After extraction of the cellulose acetate, the product had a leather permeability of 8,987, a water liquid penetration resistance of 11,000 flexes, and a Schiltknecht flex life of 6,000. It will be noted that this composition ;was superior to natural shoe leather such as Scotch Grain Cowhide and Cretan Butt in the incompatible properties of high leather permeability and low water liquid permeability.

Example 31 A mat comprising 1.688 parts of .001", 3 deni'er/ filament polyhexamethylene adipamide staple and 4125 parts of 0.01, 2.5 denier/filament polyvinyl alcohol staple was prepared by the procedure described in Example 27. The mat was preheated 15 minutes at 180 C. in an atmosphere of nitrogen and then impregnated with 1.688 grams of a film comprising a polyvinyl butyral. resin/dibutyl sebacate composition 70/30 by pressing for 5 minutes at 155 C. at 1,450 lbs/sq. in. The polyvinyl alcohol pore-former was then extracted with hot running water. The extracted product had a leather permeability of 3,719, a water liquid resistance of 748 flexes, and a Schiltknecht flex life of 47,500.

Example 32 A product was prepared as in Example 31 except. that the polyvinyl acetal film was replaced with an equal amount 'of a film comprising N-methoxymethyl polyhex'ame'thylene adipamide (DV=55)/met hyl IO-phenol stearate 50/50. This film was impregnated into the polyhexamethylene adipamide/polyviny'l alcohol fiber mat by preheating the mat for 15 minutes at 180 C. in an atmosphere of nitrogen, followed by pressing for 5 minutes at 160 C. and 1,450 lbs/sq. in. pressure. The .product, after extraction of the polyvinyl alcohol, had a leather permeability of 12,428 and exhibited sure face cracking after 78,000 Schiltknecht .flexes.

Example 33 A mat Was prepared at a Weight of 3.3 oz./sq. ft. as described ,in Example 16, using 4.68 parts of 0.5", 3 denier/filament polyhexamethylene adipamide structural fiber and 11.46 parts of 0.5", 2.5 denier/filament polyvinyl alcohol pore-formingfib'er dispersed in 16,000 parts of water contain ng 0.2 .part of c yl ph ny p y y ether. After drying, themat as imp g it 65 parts of a dispersion at 7.2% concentration in methyl ethyl ketone of vinyl chloride/vinyl acetate /5 copo yrner/di-(2- hyly phtha at /60. The freshly impregnated mat was submerged in water, dried, and pressed for 10 minutes at C. and 1245 lbs/sq. in.; and the polyvinyl alcohol pore-former was finally extracted with running water at 90 .C. The extracted mat was split .down the middle to givea very leather-like product with a fiber-rich flesh side and a binder-rich grain side. The split product was 0.027" thick and weighed 1.00 oz./sq. it. It .had a leather permeability of 12,058, a tensile strength of 2,025 lbs/sq. in., an elongation of 48%, a modulus of 14,983 lbs/sq. in., a tongue-tear strength of 6.3 lbs., and a Schiltknecht flex life .of 285,000.

- Example 34 A mat was prepared in a manner. similar to that 2.0:

Example .33 replacing theplfasticiged vinyl chloride/vinyl acetate copolymer with an equivalent amount of polyvinyl chloride/di-(2-ethyl hexyl) phthalate 100/60, applied from dispersion at 6.2% concentration in tetrahydrofurane/dimethyl formamide 98/2. The impregnated mat was submerged in water, dried, pressed for minutes at 160 C. and 1245 lbs./ sq. in. using 0.030 shims; and the pore-former was extracted with running water at 90 C. The leather-like product was 0.031" thick and weighed 1.51 oz./sq. ft. It had a leather permeability of 7,908 and a tongue-tear strength of 4.1-7.1 pounds.

Example 35 A product was prepared as described in Example 16 except that the binder was composed of chlorosulfonated polyethylene/wood rosin/tribasic lead maleate/titanium dioxide/aluminum stearate/Captex (Z-mercaptobenzothiazole)/diphenyl guanidine 100/ 10/20/ 20/ 2/ 3/0.5, dispersed at 7.6% concentration in toluene. The freshly impregnated mat was submerged in methanol, dried, pressed for 10 minutes at 170 C. and 1,245 lbs./sq. in. using 0.030" shims; and the cellulose acetate poreformer was finally extracted with acetone. The leatherlike product was 0.028" thick and weighed 1.43 oz./ sq. ft. It had a leather permeability of 16,551 and a tonguetear strength of 8.0 pounds.

Example 36 A product was prepared in a manner similar to Example 28 except that the binder was polyethylene, and it was applied from a latex at 4.5% concentration. After drying, the impregnated mat was pressed for 10 minutes at 175 C. and 1,245 lbs./sq. in. using 0.030" shims; and the cellulose acetate pore-former was finally extracted with acetone. The leather-like product was 0.029" thick and weighed 1.48 oz./sq. ft. It had a leather permeability of 14,216 and a tongue-tear strength of 5.2 pounds.

Example 37 A mat comprising 1.688 parts of 0.01", 3 denier/filament polyhexamethylene adipamide staple and 4.125 parts of 0.01, 3 denier/filament acetate staple was preheated minutes at 65 C. and impregnated with 16.3 parts of a hot 10.34% solution in toluene of chlorinated polyethylene of 27.5% chlorine content. The solvent was allowed to evaporate, and the impregnated mat was pressed 10 minutes at 190 C. and 1,450 lbs./sq. in. After extraction of the cellulose acetate pore-former, the product had a leather permeability of 3,531, a water liquid resistance of 21,584 flexes, and a Schiltknecht flex life of 8,000.

Example 38 A mat was prepared in a manner similar to that described in Example 28; and the ethylene/vinyl acetate 2.36/1 copolymer binder, dispersed at 7.2% concentration in toluene, was employed. After drying, the impregnated mat was pressed for 10 minutes at 180 C. and 1245 lbs./sq. in. using 0.030" shims. After extraction of the cellulose acetate pore-former, the product was 0.026" thick and weighed 0.9 oz./sq. ft. The product had a leather permeability of 19,430, a resistance to liquid water penetration of 18,00024,000 flexes, and a tongue-tear strength of 15 pounds.

Example 39 A mat comprising 1.688 parts of 0.01" 3 denier/filament polyhexamethylene adipamide staple and 4.125 parts of 0.01", 2.5 denier/filament polyvinyl alcohol staple was prepared by the procedure described in Example 27.

.This was impregnated with 26.65 parts of 6.33% latex of .a vinylidene chloride-acrylonitrile copolymer/dioctyl Example 40 A mat prepared as described in Example 28 was imprgenated with a dispersion of neoprene (poly-2-chloro- 1,3-butadiene) zinc oxide/ magnesium oxide/phenyl-betanaphthylamine /10/10/2 at 7.2% concentration in benzene to give a structural fiber/binder/pore-former ratio of 22.5/ 22.5/ 55. The freshly impregnated mat was submerged in methanol, dried, pressed for 40 minutes at C. and at 1,245 lbs./sq. in., using 0.030 shims; and finally the cellulose acetate pore-former was extracted with acetone. The weight of the extracted product was 1.41 oz./ft., and its thickness was 0.028". It had a leather permeability of 8,638, a water liquid penetration resistance of flexes and a tongue-tear strength of 10 pounds.

The following example illustrates use of the synthetic leather compositions of the present invention in the fabrication of shoe uppers.

Example 41 In order to fabricate shoes from the synthetic leather compositions of the present invention, composite sheets consisting of fibers of polyhexamethylene adipamide, cellulose acetate, and a copolyester made from ethylene glycol and 60 mol percent of terephthalic acid and 40 mol percent of sebacic acid were prepared. In general, the initial non-woven water vapor-impermeable fiber sheets were made by carding the mixed fibers together and then hot-pressing at 500 lbs/sq. in. at C. the carded fibrous mat to form a composite sheet. In all cases, the fibers were 2.5" in length and 3 denier/ filament.

To prepare the non-woven fibrous impermeable sheets, the three fibrous components were blended on a Garnet card and were collected in the form of a hat or web of the desired thickness by wrapping the primary thin web from a doffer comb around a collecting drum a suitable number ofrevolutions. As indicated in Table II, some of the mats were formed from two webs of the same composition; and some of the mats were composed of two different webs. In one of the mats, an additional quantity of binder polymer was introduced in the upper layer of the mat by assembling a homogeneous film on top of the two webs. It should be noted that this did not remain in the form of a film in the final product, but was pressed entirely into the fibers and existed merely as an additional binder in the composition. The complete assembly of webs with or without film was laminated between cellophane and subjected to heat and pressure to melt the binder and make it flow into the fibrous webs. All of the compositions were cooled under pressure and extracted at room temperature in acetone. In Table II, the samples coded Ae-Ee have the same composition as samples A-E except that the initial fibrous impermeable mats were embossed with heat and pressure at a slightly lower temperature than that at which they were made, the embossing being carried out before extraction of the cellulose acetate fibers. This procedure represents an outstanding advantage of the process of the present invention whereby a synthetic leather composition with an embossed surface may be prepared by carrying out the embossing step prior to extraction of the pore-forming fiber. Obviously, embossing after extraction compresses the capillary structure of the compositions and decreases the water vapor permeability.

It will be noted that leather permeability values in the following table were measured at 23 C. and 80% relative 'humidity (R. H.), and this only applies to the examples in this table. v

TABLE II Wright Ratio of Structural L. P. V. Fiber/Binder/Pore-Former Grams/100 Tongue- Thick- Elonga- Oode sq. meters Tear, ness Tenacity tlon Modulus hr. at 80% Pounds (inches) (I). s. i.) (percent) (p.s. i.) Nylon T/10 2 Cellulose R. H; and

Acetate 23 C.

1 l 3 13, 088 24. 3 031 1, 839 93. 3 11.377 1 1 2 4, 818 18. 4 031 3, 205 102 22, 153 1 1 4, 13, 522 16. 8 045 1, 508 109. 5, 771 Three-Layer Composition 1 Mil Continuous T/lO 45/55 4 1 1 3 14, 348 18.0 032 1, 804 100 8, 412 (Embossed Surface) 1 1| 2 8, 839 21.0 026 1, 776 86. l 12, 786 (Embossed Surface) Three-Layer Composition Top. 1 Mil of Continuous T/10 45/55 Ee Mid l 1 3 11, 650 029 1, 490 100 7,1453

Bot. 1 l

(Embossed SuIrface) 1 Polyhexamethylene adipamide.

Z Copolyester of ethylene glycol and 60 mol percent terephthalic acid and 40 mol' percent scbacic acid. 3 Copolyester of terephthalic and sebacic acids in the stated ratioywith ethylene glycol.

structural fibers of I greater length, e. g., 2-2.5", and show the superior tear strength of the resulting synthetic leather compositions as compared to various types of genuine leather.

Examples 4249;are presented in tabularform in Table III, the preparation of these synthetic leather compositions being carried out in accordance with the general procedure: described in Example 41. It will be noted that The following examples mainly illustrate the use of 35 tear strengths of the compositions in the following table TABLE III Weight Ratio Structural Fiber Binder Example Denier Length Structural Fiber/Binder/Pore-Former Nature Fila; (Inches) Nature 1 Used As men Control Brown topgrain (upholstery) cowhlde... Do Red Morrocco finish topgrain upholstery cowhide. Do Yellow topgrain heifer hide (shoe upper)., Do High quality cotton duck (army tents,

tarpaulins) 18.5/ Polyethylene film 12.5/12 /75 T-.10**- Staple fiber (1.5, 7 den./ fila.) 3 22.5 T* J10 15 2-2.5 'I10** -d0 1. 5 1. 5 T-10" -do /20/60 top film hlor fonated poly- 3 2. 5 T-10 film w h top film ethylene as additional binder. of chlorosultonated polyethylene (0002) 27/18/55 Polyethylene Ter- 3-4 2 Polyethylene film ephthalate. /24/51 .do 3-4 2 do .do

Pore-Forming Fiber Thickness Trape- Percent Wt. of of Tenacity, zoidal Tongue- Elonga- Example Extracted Extracted #linJoz. Tear Tear LPV, g./100 tlon Denier Length Product Product yd. 2 Strength, Strength, sq. meters/hr. At Nature File; 1 (Inches) oz./yd. (Inches) #/oz./yd. #/0Z./yd- 2 Break men nylon=polyhexamcthylene adiparnidc. I v l Tl0=copolyestcr of ethylene glycol and 60 mol percent terephthallc acid and mol percent sebacic acid.

are expressed in terms of trapezoidal tear strength in lbs./oz./sq. yd. This test is described in Federal Specification CCC-T-191a dated October 5, 1945. For the sake of comparison, the trapezoidal and tongue-tear strengths (both expressed in lbs./oz./sq. yd.) are given for three different types of genuine leather. For these samples, the ratio of trapezoidal to tongue-tear strength ranges from 3 to 7. Hence, there appears to be no constant conversion factor.

It is to be understood that the foregoing examples are merely illustrative and that the present invention broadly comprises forming a compact, essentially impermeable and continuous, composite sheet by hot-pressing a composition comprising a structural fiber component, an extractable pore-forming fiber component, and a binder material, the proportion of pore-forming fiber, based upon the total volume of the sheet, being from 40-70%, the weight ratio of structural fiber to binder material being from 1/2 to 2/ 1, and thereafter forming interconnecting pores having essentially the shape of the poreforming fibers in the sheet by extracting the pore-forming fiber.

Fibers of nylon, i. e., synthetic linear polyamides such as polyhexamethylene adipamide, polyhexamethylene sebacamide, polycaproamide and interpolyamides, etc., are outstanding for use as the structural fibers in the synthetic leather compositions of the present invention. The use of nylon structural fibers produces an extracted composition having high tear strength, tensile strength, softness and fiex life. Polyethylene terephthalate homopolymer and copolymer fibers are also considered to be good structural fibers; and other natural and synthetic fibers which may be employed include polyacrylonitrile, acrylonitrile copolymers, cellulose acetate, viscose, polyvinyl acetals, cotton, wool and glass fibers. The length of the structural fiber may be varied depending upon the general strength properties required. As illustrated in the foregoing examples, structural fibers as short as 0.01 may be employed; but structural fibers having a length of about 1.5" give a product of substantially optimum strength properties. Using structural fibers longer than 1.5" imparts little additional strength to the present synthetic leather compositions; but, as mentioned hereinbefore, it may be convenient to use longer fibers; and the use of longer structural fibers, e. g., up to 8", is within the intended scope of the present invention. On the other and it should be substantiallyinert, i. e., have no solvent action, toward the structural fiber and the binder material. The duration of the extraction step depends upon the weight of the pore-forming fiber in the initial sheet, the thickness of the sheet, the solubility of the pore-forming fiber in the solvent, the temperature of the extracting liquid, and the degree of agitation.

As illustrated in the foregoing examples, products of satisfactory leather permeability may be formed using hand, structural fibers less than 0.01" in length add little to the strength of the sheet over that of a sheet composed wholly of the binder polymer.

It should be emphasized that the structural fiber component should not be excessively softened at the flow temperature of the binder material and should be insoluble in the solvent used to extract the pore-forming fiber.

The pore-forming fiber must be a workable staple fiber; that is, it should be adaptable to making bats. In this form, the pore-forming fiber is randomly disposed; and

extraction thereof results in the formation of a network of interconnecting capillaries or pores. Furthermore, the pore-forming fiber must not be excessively softened at the flow temperature of the binder and must be readily pore-forming fibers as short as 0.01, e. g., 0.01" cellulose acetate flock. However, a considerable increase in leather permeability is realized when longer pore-forming fibers are employed. For example, increasing the length of the pore-forming fiber from 0.01" (Example 16) up to a length of 0.25" (Example 17) produces a substantial increase in the leather permeability of the resulting product. However, as illustrated by comparing Examples 17 and 18, an increase in pore-forming fiber length from 0.25" to 0.5" resulted in substantially no increase in the leather permeability of the resulting sheet.

As illustrated in Examples 19-24, variations in the denier of the structural fibers and pore-forming fibers appear to have little effect upon leather permeability. On the other hand, it has been found that decreasing the thickness of the pore-forming fiber results in the formation of a synthetic leather sheet having improved re sistance to penetration by liquid water. On the other hand, fibers which are finer than 1 denier are difiicult to process on standard textile machinery; and, when used as structural fibers in the present compositions, the fibers are too rigidly immobilized by the binder to give optimum strength, e. g., tear and tensile. In other words, the fibers of greater denier tend to shift within the binder under flexing or tearing, this action making the product more durable. Finer fibers, however, are rigidly held by the binder. Furthermore, fibers having a denier less than /2 have so much exposed surface, i. e., ratio of surface area to core volume is high, and are so soft that they no longer behave as normal textile staples. On the other hand, fibers coarser than 16 denier are extremely harsh and more closely resemble bristles.

It is in the binder material that the interconnecting capillaries or pores are formed upon extraction of the pore-forming fibers, the interconnecting capillaries or pores having the shape of staple fibers. Furthermore, the binder material holds together and is reinforced by the structural fibers. The binder material may be selected from a great variety of soft, elastic, initially thermoplastic, synthetic polymers which may be classified generally as elastomers, as set forth by H. L. Fisher (Industrial and Engineering Chem, August 1939, page extractable from the composite sheet with a solvent which is substantially inert, i. e., has no solvent action, to the structural fiber and the binder material. As illustrated in the foregoing examples, cellulose acetate is readily extractable using acetone and some grades of polyvinyl alcohol as readily extractable with water. Other suitable pore-forming fibers are those of sodium alginate, potassium metaphosphate polymer glass, and carboxymethyl cellulose.

The choice of a solvent for extracting the pore-forming fiber from the initial impermeable sheet depends upon the particular pore-forming fiber and the nature of the structural fiber and the binder material. The solvent should be one in which the pore-forming fiber is readily soluble;

942). The following polymers are preferred: N-methoxymethyl polyhexamethylene adipamide, copolyesters made from ethylene glycol, terephthalic acid and sebacic acid of the general types disclosed and claimed in copending applications U. S. Serial Nos. 150,811 and 150,812, filed March 20, 1950, in the name of M. D. Snyder, polyethylene and its derivatives, plasticized polyvinyl chloride, plasticized vinyl chloride/vinyl acetate copolymers, natural rubbers, synthetic rubbers such as neoprene (2- chloro-1,3-butadiene polymer) and various other compositions containing'vinyl chloride polymers or copolymers. Various other specific synthetic linear polymers which may be employed as a binder material with or without plasticizers include polyvinyl acetals, such as polyvinyl butyral or laural, chlorinated polyethylene, chlorosulfonated polyethylene, ethylene/vinyl acetate copolymers, vinylidene chloride/acrylonitrile copolymers, polyethylene terephthalate, etc., or any tough, pliable poly merie material which is at least initially thermoplastic and which melts or flows at a temperature below the deformation (softening) temperatures of the structural fiber and pore-forming fiber. By the term initially thermoplastic is meant that the binder material must melt and flow under the conditions of the hot-pressing 17 step. When the binder material is inthe form of individual fibers, the length of the fiber has no effect upon the properties of the extracted sheet, provided that the binder fibers have been uniformly dispersed throughout the composition before heat and pressure are applied.

In describing the binder material as a tough, pliable, at least initially thermoplastic, polymer, the following more specific requirements apply to those binder materials that are preferred:

1. The tensile strength should be at least 500 p. s. i.

2. The elongation must be at least 100%.

3. Materialsnot having a tensile strength and elongation greater than the above minimum specifications are satisfactory if the product of their tensile strength and elongation (where 100%:1) is at least 1,000.

4. The modulus must not be more than 25,000 p. s. i. and, preferably, not more than 5,000 p. s. i.

The binder material may be incorporated into the initial impermeable composite sheet in a variety of ways, several of which are illustrated in the foregoing examples. As illustrated in Example 2, the binder material may be in the form of fibers which may be carded along with the structural fiber and the pore-forming fiber to form a composite sheet by pressing the carded mixed fibers at elevated temperatures. Examples 1 and '7 illustrate mutual coagulation of a mixed dispersion of structural fibers, pore-forming fibers and binder polymers. Example 16 illustrates formation of a fibrous mat of a mixture of structural and pore-forming fibers followed by impregnation of the mat with a binder polymer in an aqueous dispersion. Other techniques of incorporating the binder material include impregnation of a fibrous mat with a binder polymer in solvent solution and impregnation of a fibrous mat with a binder polymer in the form of a powder or a homogeneous sheet. Other techniques of incorporating a binder material with the fibrous components (structural and pore-forming fibers) of the initial sheet include hot melt impregnation, impregnation by calendering or by spraying the binder material from aqueous dispersion or solvent solution onto one or both sides of a fibrous mat, followed by heat and pressure to impregnate the fibrous portion of the sheet with the binder material. Regardless of the technique employed to form a composite sheet, the sheet is consolidated prior to extraction by pressing at a temperature which is above the fiow temperature of the binder and below the softening temperature of the structural fiber and pore-forming fiber. The pressure used is sufiicient to cause the binder to flow and thoroughly impregnate the fibrous components of the sheet.

On the basis of numerous types of actual wearing comfort tests, it has been ascertained that membranes, e. g., leather and synthetic leather, exhibiting a leather permeability of 2,000 to 10,000 gms./ 100 sq. meters/hr. would provide wearing comfort equivalent to that obtained with glazed leathers and heavy shoe upper leathers. Furthermore, permeabilities of 10,000 to 22,000 were found to provide adequate wearing comfort equivalent to that obtained with the lightest shoe leathers.

In general, the wearing comfort of boots, shoes, gloves, etc., is determined or evaluated by measuring the leather permeability of the material from which the article is fabricated. With synthetic leather compositions made by the process of this invention, the leather permeability is substantially directly dependent upon the proportion of pore-forming fibers in the initial impermeable sheet. It is to be understood that the initial sheet, i. e., before extraction of the pore-forming fibers, is substantially continuous and vapor-impermeable. As mentioned hereinbefore, generally satisfactory comfort for boots and shoes is obtained when the shoe upper material has a leather permeability of at least 2,000 gms./100 sq. meters/hr. in an atmosphere of 23 C. and 90% R. H. Expressed in terms of one of the critical limitations of the present invention, it has been found that the total volume of voids, i. e., interconnecting pores produced by extraction of the-pore-forming fiber, should be no less than 40% of the total volume of the poroussheet to give at least satis factory wearing comfort. Hence, since the densities of the components, i. e., structural fiber, binder material and pore-forming fiber, of the composite are substantially the same, this means that the proportion of the pore-forming fiber should be at least 40% of the total volume of the initial composite sheet. On the other hand, the foregoing examples clearly illustrate that the strength of the synthetic leather compositions of this invention drops off appreciably when the volume of the pore-forming fiber in the initial unextracted tructure is substantially greater than 70% of the whole. This means that the strength of the synthetic leather compositions of the present invention is substantially below optimum when the total volume of voids in the final composition is greater than 70% of the whole. Resistance to penetration to liquid water is also unduly decreased at greater than 70% poreformer. The optimum volume of voids or pores in the final composition appears to be from 5060%. As illustrated in the foregoing examples, the value of leather permeability increases as the total volume of voids increases; but the amount of void space, is limited by the ultimate strength desired. For example, kid leather has a tonguetear strength of only 3.15 pounds; and coated fabrics, for example, a woven cotton fabric coated with N-methoxymethyl polyhexamethylene adipamide, which have been successfully fabricated into shoes, have a tongue-tear strength of 2.62 pounds. On this basis, the very minimum tongue-tear strength of synthetic leather compositions is in the neighborhood of one pound (tongue-tear) although at least a tongue-tear of 3 to 5 pounds is preferred.

As mentioned hereinbefore, the prevailing problem in the production of a synthetic leather having satisfactory wearing comfort is that of preparing a sheet material of satisfactory tear strength, tenacity, flex life, softness, coupled with the ability to transpire water vapor and air. In addition to the direct effect of pore-former content upon leather permeability, it has been found that the weight ratio of structural fibers to binder material is also a critical factor in the preparation of a comfortable material of satisfactory strength. As illustrated in Examples 1, 5 and 6, optimum tear strength is obtained when the weight ratio of structural fibers to hinder material is between l/ 2 and 2/1. Synthetic leather compositions of this invention in which the weight ratio of structural fibers to binder material is outside this range possess tear strengths substantially below the optimum. This is surprising and is not predictable on the basisof prior art on the preparation of impermeable non-woven sheets.

In addition to possessing optimum tear strength when the ratio of structural fibers to binder fibers is within the above specified limits, it is surprising that the present synthetic leather compositions are more durable in general service than conventional coated fabrics made from the same fiber and binder, i. e., film-former, material. For example, a woven nylon fabric coated with a typical binder polymer of this invention does not possess the unique combination of tear strength, tensile strength, softness, and flex life, which are characteristic of the synthetic leather compositions of this invention. Furthermore, the coated fabrics are substantially impermeable to air and water vapor. In addition, the formation of a synthetic leather composition by extraction of a poreformer fiber results in producing a composition which is generally superior to all types of known synthetic leathers, especially with respect to the combination of tear strength, flex life, tensile strength, pliability and water vapor and air permeability. In general, the present synthetic leather compositions surpass all previous leather substitutes and, frequently, even natural leather itself.

The process of the present invention is exceptionally versatile with respect to preparing a synthetic leather composition of the desired internal structure and surface texture. As mentioned hereinbefore, embossed sheets may be readily and efliciently produced by employing embossing rolls or pressure-applying surfaces during composition and fusing of the combination of structural fibers, binder and pore-forming fibers. Hence, embossing may be carried out in conjunction with a necessary step in the process; or it may be carried out immediately thereafter under conditions of lower temperature and pressure than employed in the fusing step. Embossing the extracted composition results in substantially decreasing the permeability of the structure and is generally to be avoided.

Another outstanding advantage of the present process is that the cross-sectional structure of the present synthetic compositions may be tailored for the desired end use by laminating or fusing different plies of one or more of three basic components together, the binder component usually being in fiber, powder or film form. On the other hand, initial fibrous mats composed of a mixture of the structural and pore-forming fibers may be carded and thereafter impregnated with the binder material by any of the methods mentioned hereinbefore. Examples 2, 3 and 41 illustrate laminating various plies containing one or more of the three basic components together in order to control the face-to-back concentration of structural fiber, binder material and pore-former. For example, the top ply of the initial unextracted composition may be composed entirely of the binder polymer in the form of a homogeneous film. The middle ply may be composed of various amounts of the structural, binder and pore-forming fiber; and the bottom ply may be composed essentially of the structural fiber. Such a structure after lamination and extraction of pore-former would form a synthetic leather composition having a relatively smooth or non-fiber-like upper surface (usually referred to as the skin side); and the bottom side would be substantially fibrous (usually referred to as the flesh side). It is obvious that the concentration of the structural fiber, binder material and pore-former may be varied from the face to the back of the sheet within reasonable limits by empolying the general practices of the present invention.

An additional advantage of the present invention is that it provides a synthetic leather having a unique combination of tear strength, tensile strength, softness, pliability, fiex-lifeand ability to transpire water vapor and air. A further advantage is that it provides a process of preparing such synthetic leather compositions especially useful in fabricating boots, shoes, gloves, etc., wherein the properties may be tailored to the desired end use. A still further advantage is that it provides an ecomonical process of preparing a synthetic leather which is equal or superior to the various types of genuine leather,

As many widely different embodiments'may be madewithout departing from the spirit and scope of this invention, it is to be understood that said invention is -in no way restricted except as set forth in the appended claims.

We claim:

1. The process of forming non-woven porous fibrous sheet which comprises forming a non-woven fibrous sheet comprising essentially structural fibers from the group consisting of staple fibers of synthetic linear polyamide, polyethylene terephthalate, polymers of acrylonitrile, polyvinyl acetals, regenerated cellulose, cotton and wool, from 40% to by volume based on the total volume of the sheet, of pore-forming fibers selected from the group consisting of polyvinyl-alcohol, cellulose acetate, sodium alginate and carboxymethylcellulose, and a soft elastomeric binder material having a flow temperature below the deformation temperature of said fibers, the Weight ratio of structural fibers to. binder material being within the range of from 1:2 to 2: 1, hot pressing said sheet at a temperature above the flow temperature of the binder and below the deformation temperature of said fibers, and thereafter extracting said pore-forming fibers from the sheet with a liquid which is a solvent for said pore-forming fibers and a non-solvent for said structural fibers and binder material whereby said structural fibers are substantially uniformly distributed throughout said porous sheet.

2. The process of claim 1 in which the structural fibers are a synthetic linear polyamide.

3. The process of claim 2 wherein the length of the synthetic linear polyamide fibers is within the range 0.01" to 8.0" and the length of the pore-forming fibers is within the range of 0.01" to 2.5".

4. The process-of claim 1 wherein the pore-forming fibers are cellulose acetate.'

5. The process of claim 1 wherein the pore-forming fibers are polyvinyl alcohol.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain Dec. 31, 1946