US3515634A - Method of producing a fibrous fleece base material from three types of fibers - Google Patents

Description

June 2, 1910 E. SOMMER ETAL' 3,515,534

METHOD OF rnonucme A FIBROUS FLEECE BASE MATERIAL FROM THREE TYPES OF FIBERS Filed Dec. 12, 1966 SHRINK TENSION OVER APERIOD OF TIME MEASURED IN WATER AT SO'C.

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O 5 l I TIME SECONDS FIBER A CO POLYAMIDE OF AH-SALT AND ALKYLENE-BIS- IAMINOPROPYL ETHER ADIPATE) FIBER B COPOLY AMIDE OF 20% AH-SALT AND CAPROLACTAM INVENTORS;

E ERWIN SOMMER KLAUS BOEHME KLAUS GERLACH ATT'YS United States Patent 3,515,634 METHOD OF PRODUCING A FIBROUS FLEECE BASE MATERIAL FROM THREE TYPES OF FIBERS Erwin Sommer, Obernburg, Klaus Boehme, Erlenbach, and Klaus Gerlach, Ohernau, Germany, assignors t0 Glanzstoif AG, Wuppertal, Germany Filed Dec. 12, 1966, Ser. No. 600,357 Claims priority, application germany, Dec. 14, 1965, 2.9, The portion of the term of the patent subsequent to July 23, 1985, has been disclaiined Int. Cl. D2155 11/00; D21h 5/12 U.S. Cl. 162-146 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of producing a fibrous fleece web suitable as a base material for artificial leather. The method involves the steps of using a paperrnaking machine to waterlay a mixture of three specific types of fibers, removing only a minor amount of water from the resulting web, then subjecting the wet web to a brief heat treatment in the absence of pressure and tension, and finally drying the web in the absence of pressure and tension.

It is known that various woven or knitted textile fabrics, fibrous fleeces, felts, bats or the like have been used as a base material which can be saturated, impregnated, coated or otherwise treated with suitable polymer, usually from solution, so as to obtain a thin, flexible structure having the surface appearance of leather. It is especially desirable to obtain a finished artificial leather product which is impervious to water and which exhibits a satisfactory water vapour permeability (resistance to penetration by steam). These properties are usually best achieved by treating the base material or fibrous substrate with a polymer solution such as polyurethane. It is also desirable to obtain a finished artificial leather product which exhibits sufficient tensile strength and specific elongation characteristics which can be influenced to a considerable extent by the type of substrate employed as well as the polyurethane or other polymer added to the substrate. In general, it has been recognized that woven and knitted fabrics are not as suitable for use as a base material or substrate for artificial leather as are fibrous fleeces. Thus, a close resemblance to leather is much easier to obtain by the use of a fibrous fleece web.

Many different methods have been suggested for the production of fibrous fleece webs for artificial leather, and it is believed that fleece webs which contain a mixture of shrinkable fibers usually provide the best characteristics. For example, a method has been described wherein the fiber mixture in the fleece contains at least 50% by Weight of fibers that are capable of shrinking. A fleece web of such fibers first receives a needle treatment to improve the strength of the web, after which it is subjected to a shrinking treatment. In order to properly carry out this method, it is necessary to begin with a carded fleece which consists of individual fibers that are at least 12 mm. long. Shorter fibers cannot be used because they cannot be easily needled in the fleece web. In carded fleeces, the fibers always have a preferred direction, ie they are primarily oriented in the longitudinal direction of the fleece web. Therefore, an additional processing step is required to reorient the fibers in the carded fleece to form the ideal criss-cross position of the fibers. It is only in this way that one can be guaranteed a constant strength and elongation of the web in all directions, these 3,515,634 Patented June 2, 1970 characteristics being especially desired in an artificial leather.

It will be recognized that the fibrous fleece web prepared for use as a base material for artificial leather according to this known method cannot be initially produced on a paperrnaking machine because a fiber length of over 12 mm. cannot be handled on a paperrnaking machine. On the other hand, if one attempts to use the short fibers required for waterlaying on a paperrnaking machine, other serious problems develop, particularly if synthetic, non-absorbent or hydrophobic fibers are to be employed so as to increase the tensile strength of the final web product. Such problems are evident when attempting to pull the fibrous fleece base material or backing through the polymer-containing impregnating solution. At this point in the overall process of making an artificial leather product, the fibrous fleece web must have suflicient strength to prevent changes in its dimensions. The poor strength characteristics of the nonwoven waterlaid web are also quite evident in other operations requiring the web to be transported or pulled from one point to another after it leaves the screen of the paperrnaking machine, and especially after the web has been dried.

It has been suggested that the strength of the fibrous fleece web can be increased by using a certain proportion of short fibers which soften or melt at temperatures below the softening point of the remaining fibers in the mixture and then subjecting the non-woven web to a heat treatment, usually with steam or hot air. However, when using conventional extruded synthetic fibers, the desired strength can only be achieved if the heat treatment is carried out while simultaneously placing the web under pressure, for example by the use of suitable cal enders or press rolls. When pressed in this manner, the resulting web is changed into a paper-like product which is not at all suitable as a base material for artificial leather. Similar flat, paper-like structures are obtained when attempting to use various adhesives or bonding agents as additives to the fibrous dispersion prior to waterlaying on the paperrnaking machine.

Finally, various processes have been suggested which use specially treated or specially prepared polymer fibers which are mechanically processed to give them a special structure or shape adapted to improve the strength of the waterlaid web. For example, it is possible to use socalled fibrids, as disclosed in U.S. 2,999,788, or other types of fibers having an unusual structure. Aside from the fact that such fibers are more expensive to produce, they ordinarily result in a paper-like product rather than one which is suitable as a fleece backing or substrate for artificial leather products.

One object of the present invention is to provide a method for manufacturing a fibrous fleece web which is especially adapted for use as a base material or substrate in an artificial leather product. It is also an object of this invention to provide a method of producing such a fibrous fleece web which can be carried out on a paperrnaking machine by using a specific mixture of short fibers capable of being waterlaid from an aqueous dispersion and which can then be subjected to a heat treatment at relatively low temperatures to yield a strong non-woven web.

Yet another object of the invention is to provide a method for producing a fibrous fleece web suitable as a base material for artificial leather wherein it is possible to employ a high proportion of synthetic, non-absorbent or hydrophobic, solid fibers of relatively uniform size and/or structure, thereby avoiding any need for using fibers of unusual structure in terms of highly irregular shape, size or surface characteristics.

Still another object of the invention is to provide a waterlaid fibrous fleece web containing a minor proportion of shrinkable fibers which can be heat treated prior to drying the waterlaid Web at low temperatures and for short periods of time which not only avoid the use of complicated apparatus but also permits a rapid and economical production of the desired artificial leather substrate.

Yet another object of the invention is to provide a novel artificial leather base material or substrate as obtained by the method of this invention.

These and other objects and advantages of the invention will become more apparent upon consideration of the following detailed description of the invention. This description is intended to illustrate the method and product of the invention by means of specific materials, it being understood that the invention resides primarily in the processing steps and the particular product resulting therefrom so that it is not limited to the specific fibrous materials enumerated herein where equivalent materials may also be used within the scope of the appended claims.

In accordance with the present invention, it has now been found that an excellent substrate or base material for artificial leather can be produced in the form of a non-woven fibrous fleece web by first waterlaying from an aqueous dispersion onto the screen of a papermaking machine a fibrous mixture consisting essentially of (I) 20 to 50% by weight of synthetic fibers which exhibit a shrink tension of 0.15 to 0.3 gram/denier when treated in water at 90-l00 C. for a period of 0.1 to 15 seconds, (II) 50 to 40% by weight of fibers selected from the class consisting of natural and synthetic fibers which are substantially resistant to shrinkage when treated in water at 90-100 C. for a period of 0.1 to 15 seconds, and (III) 30 to by weight of synthetic fibers capable of being at least softened so as to act as bonding fibers when heated at 90-100 C. in the presence of water for a period of 0.1 to seconds; then removing only sufficient water from the resulting waterlaid web to pro vide a residual water content of 50-70% by weight, with reference to the weight of the wet web; subjecting the still wet web with said residual water content to a short heat treatment of 0.1 to 15 seconds in the absence of pressure and tension on the web; and thereafter drying the web in the absence of pressure and tension thereon. It is particularly desirable to carry out the heat treatment step of the invention by subjecting the wet web to high frequency heating or infrared heating.

The papermaking machine used for waterlaying the fibrous mixture in the method of the invention is quite conventional and does not require a detailed description. In conjunction with this papermaking machine, such terms as waterlaying" and waterlaid are quite common in describing the manner in which a fibrous slurry or pulp is caused to flow upon the screen, sieve or other foraminous support of the papermaking machine. The term shrink tension is defined herein as the amount of tension in grams per denier which is developed when a length of fiber is held vertically between two clamps and is then dipped in water of 90-100 C. for a period of up to about 15 seconds. The term softened with reference to the type (III) fibers is employed herein because such fibers generally do not have a sharp melting point but begin to melt or soften at one temperature and finally become completely melted at a higher temperature. It is therefore common to refer to a softening range of such fibers rather than their melting point. Percentages referred to throughout the specification are by weight unless otherwise indicated.

In order to obtain a satisfactory product according to the method of this invention, it is extremely important not only to maintain the proportions of the different types of fibers in the fibrous mixture within the above-noted limits, but also to observe the particular properties in terms of shrinkage and softening point. The residual water content of the waterlaid web is also quite critical, and the heat treatment of the wet web must be carried out rapidly.

Suitable shrinking fibers (I) for the fibrous mixture can be readily selected by determining the necessary shrink tension as defined above. The ability of fibers to give the desired shrinkage depends upon the particular fiber-forming synthetic polymer and the manner in which it is produced, especially the amount of stretch imparted after spinning the fiber. Nevertheless, any synthetic fiber can be readily tested for its shrink tension, and one skilled in this art can classify all known fibers for this purpose as well as creating new or modified synthetic fibers with the desired shrinkage.

In this respect, however, a careful distinction must be made between the boiling shrinkage of the synthetic fiber (I) and its shrink tension as measured in water of 90 100 C. over a short period of time since only the shrink tension is important in the method of the present invention. The boiling shrinkage is measured as the percentage reduction in the length of the fiber when immersed in boiling water under conditions which permit the fiber to shrink freely. Fibers are known which exhibit a ver high boiling shrinkage but fail to provide the necessary shrink tension under the conditions of the method of this invention. For example, polypropylene filaments or fibers which have been spun and then stretched by a ratio of 3.7:1, i.e. 3.7 times their spun length, have a boiling shrinkage of 11% while their shrink tension at about 95 C. water temperature is only 0.13 gram/denier. Therefore, such fibers would not be suitable for the production of the fibrous fleece web according to this invention.

Synthetic fibers which are especially suitable as the shrinking fibers (I) are the fibrous copolyamides of hexamethylenediamine adipate (the so-called AH-salt) and caprolactam having a relatively low content of the AH-salt, e.g. approximately 20% AH-salt to caprolactam. Other suitable shrinking fibers are the fibrous copolyamides of the AH-salt and an alkylene-bis-(aminopropylether adipate) or polyester fibers such as polyethylene terephthalate which have been highly stretched.

In the accompanying drawing, a diagram has been made to illustrate the shrink tension curves of both types of the copolyamide fibers plotted against time. The shrink tension was determined by clamping a fiber at both ends in a vertical position, the upper clamp being connected to the measuring head of an inductive dynamometer. The fiber between the clamps was then immersed in water of C. Under these conditions, the fiber does not longitudinally shrink but a force or tension in the fiber results which can be easily measured over a period of time. It will be seen that an initial shrinkage occurs quite rapidly and the fiber then tends to gradually recover over a period of time. For this reason, it is desirable to limit the shrinkage treatment according to the invention to a relatively short period of time such as up to 15 seconds in order to avoid an undesirable loss of shrinkage.

The content of fibers (I) in the fibrous mixture should be maintained within the prescribed limitsof 20-50% by weight. Within these limits, one can use a low amount of synthetic fibers having a relatively high shrink tension whereas fibers with the lowest shrink tension of about 0.15 gram/denier should be used in amounts of about 45-50%.

The shrink tension of any particular fiber-forming polymer can be varied simply by changing the degree of stretching of the individual filaments during the spinning process. This can be accomplished by a rapid drawing of the filament from the spinning nozzle and/ or by a higher stretching of the solidified or finished filament. The following summary provides an example of the manner in which the shrink tension, measured in a water bath at 90 C., depends upon the stretch imparted to a finished copolyamide filament of 20% by weight hexamethyleuediamine adipate (AH-salt) and 80% caprolactam:

The shrinkage of the fibers (I) takes place during the heat treating step of the invention at 90-,100 C. and must be accomplished without the use of pressure and/or tension on the wet waterlaid web, i.e. so that the web or randomly oriented layer of fibers can shrink freely. More than 50% of the shrinking fibers (I) must be avoided because the web or fibrous base material would then become too thick. Amounts of less than 20% of these fibers should also normally be avoided since fibers with a maximum shrink tension of about 0.3 gram/denier would still not be suflicient to yield the desired product, particularly because a corresponding higher content of fiber type (II) would provide a fleece web which is too loose and unsuitable for artificial leather products.

The natural or synthetic fibers of type (-II) according to the invention may be selected from many conventional fibers of the same kind of fibrous material or mixtures of different fibers. In order to achieve the highest possible strength and utility, it is advantageous to use at least about 20% by weight (with reference to all fibers in the fleece) of thermoplastic, non-absorbent, synthetic polymer fibers such as the conventional polyamide and polyester fibers, e.g. nylon and polyethylene terephthalate. It is also possible to use natural fibers or semi-synthetic fibers such as a regenerated cellulose fiber, e.g. in amounts of less than 30% by weight and preferably less than 20% by weight (with reference to all fibers in the fleece). In general, inorganic fibers should be avoided other than in very minor amounts of l or 2% by weight. Naturally, various additives such as pigments, dyes, dispersing agents, and the like can be incorporated in the fibrous composition in accordance with accepted practices.

Most importantly, the fibers of type (II) must be substantially free of shrinkage or exhibit only a slight or negligible degree of shrinkage under the processing conditions by comparison with the fibers (1). Such shrink resistant fibers represent the most conventional type of fibers used in textiles and are therefore readily available without requiring any special measures in their spinning, stretching or other preparation for use as staple fibers in the waterlaying of the fleece web.

According to the invention, the fibrous mixture being waterlaid should also contain 30-10% by weight of synthetic thermoplastic fibers which soften or melt at 90-100 C. in the presence of water, i.e. the fiber of type (III). The softening range of fibers can be readily determined by routine tests, and it will be recognized that this softening range is determined by a number of factors, including the particular fiber-forming polymer and the manner in which it is prepared, usually by modifying known polyamides and polyesters so as to substantially reduce their melting points or, more accurately, their softening ranges. Thus, many thermoplastic fiber-forming copolyrners are known which belong to a recognized class of so-called bonding fibers, i.e. which melt or at least soften at relatively low temperatures so as to adhere conventional fibers to each other. Such adherencenorrnally occurs at the points of intersection or contact of the individual fibers so that the fleecy or open structure of the fibrous web is not destroyed.

The preferred bonding fibers are the copolyamides of caprolactam with a higher content of the AH-salt, although there are any number of copolymers and modified homopolymers which can be tailor-made to provide the required softening or melting at temperatures below 100 C. in the presence of water. During the heat treating step, these bonding fibers do not hinder shrinkage, but serve to bind the fibrous web together.

A content of 10-30% of these bonding fibers im parts sulficient strength or coherency of the heat treated web or fleece, independent of the amount of the other two types of fibers. More than 30% of the bonding fibers is undesirable because one loses the required characteristics of the final artificial leather. Any higher strength with larger amounts of bonding fibers is achieved only at the expense of a lower softness and high resistance to bending. If the content of the shrinking fibers (I) is high, or preferably if the fibers (I) have a high shrink tension of close to 0.3 gram/denier, then the content of the bonding fibers (III) can be lowered to about 10% because the fibers (I) of high shrink tension also add to the strength and coherency of the fibrous fleece web.

In general, all of the individual fibers in the mixture must be of the usual staple length suitable for waterlaying on a papermaking machine, i.e. a length of about 3.0 to 6.0 mm. The individual denier of each type of fiber can vary over a relatively broad range, but it will be helpful to observe the following fiber characteristics with respect to length and denier:

The fiber mixture according to the invention is first dispersed in water to form an aqueous fibrous slurry which can then be waterlaid on a papermaking machine in the usual manner, i.e. so as to form a random, nonwoven web on the screen or sieve band. A suitable wetting agent or dispersing agent is preferably added to maintain a uniform distribution of the fibers in the head box of the papermaking machine. Depending upon the weight ratio of fibers to water in the slurry and the speed of the papermaking screen, waterlaid webs of varying thicknesses can be produced as desired. For use as an artificial leather substrate, the thickness of the waterlaid web before further processing should generally be about 0.6 to 0.8 mm.

At the so-called wet zone of the papermaking machine, the water content of the fibrous web is approximately by weight (with reference to the weight of the wet fibrous web). It is then necessary to reduce this water content to about 50-70% by pressing or squeezing the web, e.g. with suitable rollers exerting a light pressure thereon. No heat should be used for removing water at this point since this would result in a premature overheating of the fibrous mixture in the web. The still wet web with its residual water content of 50 70% by weight then has just the required amount of strength for further processing in accordance with the invention, i.e. the heat treatment step which follows immediately thereafter.

For this heat treatment, which should be carried out as rapidly as possible, it is especially advantageous to heat the web with infrared or high frequency irradiation. The still wet web is thus transferred or conveyed without pressure or tension between infrared rays or through a high frequency field. Such heating devices can be easily arranged immediately after the wet zone of the papermaking machine so that the fibrous web passes through the heating zone at the exit velocity of the papermaking screen. In this manner, the wet web can be subjected to the heat treatment for a period of not more than about 15 seconds. The composition of the fibrous mixture in the fleece web, the residual water content of the web and the sudden heating thereof all bear a functional relationship to each other. For obtaining the desired properties of the heat treated fleece substrate and the correspondingly essential properties of an artificial leather, it is of decisive importance to rapidly heat the wet fleece web so that its water content is suddenly heated to just below its boiling point, i.e. to 90-100 C. Only when this requirement is met can the maximum and essential shrinking tension be imparted to the fibers (I). At the same time, the bonding fibers (III) are at least softened or actually melted so as to strengthen the wet web.

The shrink tension of the fibers (I) causes the fleece web to shrink or contract in terms of its surface area while the thickness of the web tends to increase. This shrinkage effect is somewhat oifset or opposed by the binding forces which cause the cohesion of the wet fleece web (pressure applied during the previous step in which the web is squeezed to remove excess water above said residual content of 50-70%). Therefore, an incomplete shrinkage is caused by heating the wet web too slowly due to the slow development of the shrink tension, and the same undesirable result occurs if the maximum shrink tension of the fibers is too low, i.e. less than 0.15 gram/denier.

The water loss during the 'heat treatment step is very low, usually not more than about Immediately after the heat treatment, the fleece web can be readily transferred or conveyed through a suitable drying zone wherein any heating means can be employed to remove the re maining water provided that pressure and/or tension on the web is avoided.

In previous methods, the fleece substrate has been subjected to shrinkage by various means. In working with needled webs of long fibers obtained by the carding process, the best known method has been to immerse the needled web in hot water. However, this technique cannot be employed directly with waterlaid webs produced on a papermaking machine. Although one would achieve about the same degree of surface area shrinkage as in the method of the present invention, the high absorbency of the waterlaid web would cause the volume and weight of the Web to increase to such an extent that the web would be deformed or even destroyed at the exit from the bath. This disadvantage might be avoided by leading the web through two rotating screens, but then the web would be placed under pressure and/pr tension at the same time that the bonding fibers (III) are softened or melted and the shrinking fibers (I) are imparting their tension, thereby preventing the desired shrinkage effect on the web to a very considerable extent. Fleece webs produced in this manner simply would not be useful as a substrate for artificial leather.

It might also be possible to achieve a good surface area shrinkage by heating with saturated steam at temperatures above 100 C. In this case however, the heating zone must be maintained under an elevated pressure requiring expensive pressure resistant apparatus which is capable of continuously treating the Web contained therein. Fur- Fleece Composition, percent;

thermore, it is more difiicult if not almost impossible to achieve a rapid heating of the web under carefully controlled conditions, and the high temperatures can easily lead to damage of the fibers.

The following examples serve to illustrate the method of the invention and the fibrous products obtained thereby, without limiting the actual scope of the invention.

EXAMPLE 1 A fiber mixture having the following composition is first dispersed in water:

(I) 50% by weight of copolyamide fibers having a titer of 1.4 denier and a staple length of 6 mm. (The copolyamide consists of 20% by weight AH-salt and by weight caprolactam. The stretch ratio of the fibers is 3.1: 1.)

(II) 20% by weight of polyamide fibers (polycaprolactam) having a titer of 1.4 denier and a staple length of 6 mm.; 20% by weight of viscose-tubular-fibers having a titer of 2.5 denier and a staple length of 3 mm.; and

(III) 10% by weight of copolyamide fibers having a titer of 2.5 denier and a staple length of 3 mm. (The copolyamide consists of 40% by weight AH-salt and 60% by weight caprolactam.) The fiber mixture is further diluted with water which contains a wetting agent until it reaches a fiber concenetration of 0.05%. The wetting agent is an oxyethylated long chain fatty-alcohol which is a typical fiber dispersing agent. The resulting fibrous slurry is waterlaid on a. papermaking machine, and near the end of the screen, the water content of the waterlaid Web amounts to 81%. The wet web is then passed through a wet press where the water content is reduced to 59%, and immediately thereafter the web is passed at a velocity of about 5-6 meter/ second through a heating zone equipped with an infrared heater where the web is heated on both surfaces for a short period of time such that the water in the web reaches a temperature of -98" C. The retention time of the web in the heating zone is 12 seconds. After the web leaves the infrared zone or passage, it is dried in a conventional suspension dryer. No pressure or tension is applied to the web during these heating and drying steps.

The web exhibits the following characteristics (as determined on a test ribbon of 1.5 x 10 cm.)

Weight per surface area: 400 grams/meter Thickness: 2.5 mm. Surface area shrinkage: 50% Strength:

Longitudinal6.53 kg. Lateral-8.15 kg. Elongation at break:

Longitudinal-41.0% Lateral--78.0%

EXAMPLES 27 Different fiber mixtures were processed in the same manner as described in Example 1. The results can be seen in the following table:

TABLE Fleece Properties, 10 X 1.5 cm. strip Elongation,

Fiber F1ber Iii Fiber Strength, kg.- percent Thick- Area Area I, III, ness, shrinkage, weight,

Example A B C D E F Long. Lat. Long. Lat. mm. percent gJmJ Fiber IA=Copolyamide of 20% AH-Salt and 80% caprolaetam, titer of 1.4 denier and staple length of 6 mm; Fiber IIB =Polycaprolactam, titer of 1.4 denier and staple length of 6 mm.

Fiber Regenerated cellulose, strength of 4 g. ]den., titer of 1.3 denier, staple length of 3 mm.

Fiber IID =Regenerated cellulose, strength of 3.1 g.{den., titer of 1.2 denier, staple length of 3 mm.

Fiber IIE=Regenerated cellulose, tubular or hollow spun, titer of 2.5 denier, staple length of 3 mm.

Fiber IIlF=C0polvamide of 40% AH-Salt and 60% caprolaetam, titer of 2.5 denier, staple length of 3 mm;

9 EXAMPLE 8 A fiber mixture having the following composition is first dispersed in water:

(I) 50 parts by weight of copolyamide fibers having a titer of 1.4 denier and a staple length of 6 mm. (The copolyamide consists of 20% by weight AH-salt and 80% by weight caprolactam. The stretch ratio of the fiber is 2.5 :1). v

(H) 20 parts by weight of polyamide fibers (polycaprolactam) having a titer of 1.4 denier and a staple length of 6 mm.; and 20 parts by weight of viscose-tubular fibers having a titer of 2.5 denier and a staple length of 3 mm.

(III) 10 parts by weight of copolyamide fibers having a titer of 2.5 denier and a staple length of 3 mm. (The copolyamide consists of 40% by weight AH-salt, and 60% by weight caprolactam) The fiber mixture is processed on a papermaking machine, as described in Example 1. The fleece web has the following characteristics after drying:

Weight per surface area: 392 grams/meter Thickness; 2.40 mm. Surface area shrinkage: 44% Strength:

Longitudinal-5 .35 kg. Lateral-9.13 kg. Elongation at break:

Longitudinal-67.2% Lateral60.0%

EXAMPLE 9 An aqueous fiber dispersion of the following composition is first prepared:

(I) 50 parts by weight of copolyamide fibers having a titer of 1.4 denier and a staple length of 6 mm. (The polyamide consists of 20% by weight AH-salt and 80% by weight caprolactam. The stretch ratio of the fibers amounts to 3.1:1.)

(H) 30 parts by weight of viscose-fibers having a titer of 1.4 denier and a staple length of 3 mm.

(II) 20 parts by weight'of copolyamide fibers having a titer of 2.5 denier and a staple length of 3 mm. (The polyamide consists of 40% by weight AH-salt and 60% by weight caprolactam.) The fiber mixture is processed into a waterlaid fibrous fleece web as described in Example l. The web has the following characteristics after drying:

Weight per surface area: 430 grams/meter Thickness: 2.28 mm. Strength:

Longitudinal-42.66 kg. Lateral11.04 kg. Elongation at break:

Longitudinal-44.7% Lateral-79. 1

EXAMPLE 10 An aqueous fibrous dispersion of the following composition is prepared:

(I) 50 parts by weight of copolyamide fibers having a titer of 1.4 denier and a. staple length of 6 mm. (The copolyamide consists of 20% by weight AH-salt and 80% by weight caprolactam. The stretch ratio of the fibers is 2.5:1.)

(II) 40 parts by weight of polyamide fibers (polycaprolactam) having a titer of 1.4 denier and a staple length of 6 mm.

(III) 10 parts by weight of copolyamide fibers having a titer of 2.5 denier and a staple cut length of 3 mm. (The copolyamide consists of 40% by weight AH-salt and by weight caprolactam.)

As described in Example 1, the fiber mixture is processed to a fleece web on a papermaking machine. The web has the following characteristics after drying:

Weight per surface area: 402 grams/meter Thickness: 2.93 mm. Strength:

Longitudinal3.27 kg.

Lateral5.25 kg. Elongation at break:

Longitudinal26.6%

Lateral-5 1.3

Similar results are achieved as in the foregoing examples if the fleece web is heated in a high frequency field in place of infrared heating, and in both cases a very rapid heating is achieved so as to impart the essential shrinkage and bonding of the fleece. In addition, this technique of rapid heating while the fleece is still wet has the advantage that it can be carried out at the wet end of the papermaking machine and before the fleece web is dried. Substantial savings in time and expense therefore result from this technique, as well as gaining very desirable properties for an artificial leather substrate. The wet strength of the fleece web containing 50-70% water is fully sufficient to transfer and convey the web through the heating zone without distortion or a tendency for the web to pull apart. The dry strength of the same web is too low to withstand drying before being subjected to shrinkage. The method of the invention is also advantageous because the heating temperature can be maintained slightly below the boiling point of water so as to avoid any possible damage to the fibers.

The dried fleece web produced by the method of the invention is an excellent base material or substrate for artificial leather products. The surface coating or impregnation of the substrate with a suitable plastic simulating a leather surface can be accomplished by conventional methods without distorting or damaging the fibrous fleece base material of the present invention.

The invention is hereby claimed as follows:

1. A method for producing a fibrous fleece web suitable as a base material for artificial leather, which method comprises:

waterlaying from an aqueous dispersion onto the screen of a papermaking machine a fibrous mixture consisting essentially of (I) 20 to 50% by weight of synthetic fibers which exhibit a shrink tension of 0.15 to 0.3 gram/ denier when treated in water at 100 C. for a period of 0.1 to 15 seconds,

(II) 50 to 40% by weight of fibers selected from the class consisting of natural and synthetic fibers which are substantially resistant to shrinkage when treated in water at 90-100 C. for a period of 0.1 to 15 seconds, and

(III) 30 to 10% by weight of synthetic fibers capable of being at least softened so as to act as bonding fibers when heated at 90-400 C. in the presence of water for a period of 0.1 to 15 seconds;

removing only suflicient water from the resulting waterlaid web to provide a residual water content of 50- 70% by weight, with reference to the weight of the wet web;

subjecting the still wet web with said residual water content to a short heat treatment of 0.1 to 15 seconds suflicient to heat said residual water to just below its boiling point in a range of 90-100 C. in the absence of pressure and tension on said web; and

thereafter drying said web in the absence of pressure and tension thereon.

2. A method as claimed in claim 1 wherein said wet web with a residual water content of 50 to 70% by weight is subjected to high frequency heating for said period of 0.1 to 15 seconds.

3. A method as claimed in claim 1 wherein said Wet web with a residual water content of 50 to 70% by weight is subjected to infrared heating for said period of 0.1 to 15 seconds.

4. A method as claimed in claim 1 wherein said fibrous mixture consists essentially of (I) 20 to 50% by weight of a copolymer fiber of about 20 parts by weight of hexamethylenediamine adipate to about 80 parts by weight of caprolactam, said copolymer fiber having been produced with a stretch ratio of at least 2.6: 1;

(II) 50 to 40% by weight of a fiber selected from the class consisting of polyamide and regenerated cellulose fibers; and

(III) 30 to 10% by weight of a copolymer fiber of about 40 parts by Weight of hexamethylenediamine adipate to about 60 parts by weight of caprolactam.

5. A method as claimed in claim 4 wherein the fibers of component (II) are a combination of polyamide fibers 5 and regenerated cellulose fibers such that said cellulose fibers constitute not more than 30% by weight, with reference to the total weight of all fibers in said fibrous mix- 3,394,047 7/1968 Sommer 162-146 HOWARD R. CAINE, Primary Examiner US. Cl. X.R. 162-457

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