US3222244A - Process for producing water-laid sheets from polyamide fibers pretreated with a tannin - Google Patents

Description

United States Patent 12 Claims. (a. 162-157) The present invention is related to a process for producing sheet or paper-like structures of synthetic polyamide. fibers, and more particularly, the invention is di- ,rected to an improvement in the formation of fibrous sheets or webs by steps which include laying or depositing fibers of a synthetic polyamide from an aqueous dispersion onto a supporting surface and subsequently removing water from the fibrous web.

Many different fibrous materials have been employed for the production of water-laid sheet-like or paper-like structures according to the generally well-known papermaking process. Depending upon the exact nature of the initial fibrous material and the conditions under which the fibers are treated during the paper-making process, it is possible to obtain sheet-like products with different properties and characteristics. Thus, paper is produced when using cellulosic fibers, and many attempts have been made to produce a similar sheet material from fibers or filaments derived wholly from synthetic polymers such as the known fiber-forming linear polyamides.

In general, the over-all process for making paper or similar water-laid sheet materials is substantially the same in each case regardless of the particular fiber, although there may be a number of minor variations such as pretreatment or aftertreatment of the fibers by beating or refining or the incorporation of additives such as binders, adhesives, thickening agents, fillers and the like.

In the usual paper-making process, the fibers are first dispersed in a liquid preferably water for obvious economical reasons, the dispersion of fibers being maintained by thorough mixing and/or the addition of a suitable dispersing agent. The resulting pulp or fiber dispersion is then transferred or flowed onto a moving screen or sieve for removal of at least part of the water and formation of a fibrous web or fleece. The moving screen employed for this purpose usually corresponds to one of two general types, the Fourdrinier machine and the cylinder machine. After the fibrous web has been formed, it is transferred from the moving screen onto other suitable apparatus for removal of any remaining water and formation of a solid sheet by the application of heat and/ or pressure and/or adhesives.

The paper-making process is particularly adapted to the manufacture of paper from cellulosic fibers, and it has been quite diflicult to treat fibers of synthetic polyamides or other polymers by this same process. A water-laid fleece or fibrous web of synthetic polyamide fibers cannot be transferred from the Fourdrinier or similar moving screen without being torn or pulled apart, because there is very little cohesion between the individual polymer fibers. Moreover, the strength of the fleece or fibrous web in the dry state is extremely low. In contrast to natural cellulosic fibers which can be beaten into very fine fibers or fibrils, it has been practically impossible to provide synthetic polyamide fibers with similar properties capable of forming a relatively strong and cohesive water-laid fibrous web. Thus, the lack of strength in ice the water-laid or subsequently dried web or fleece of synthetic polyamide fibers is partly due to the fact that it is impossible or at least highly expensive to form fibrils and partly due to the fact that synthetic polyamide fibers have very low swelling values and a high degree of water-repellency.

In spite of such obvious draw-backs, a number of at? tempts have been made to carry out processes for making sheet materials containing synthetic polyamide fibers in order to obtain the advantages of the specific physical and chemical properties inherent in the polymer fiber.

According to one known process, a sheet of synthetic polymer fibers is made by adding to the aqueous dispersion a relatively large proportion of cellulose fibers, for example, in amounts up to about 30% by weight. In the same manner, cellulosic fibers have been modified with minor amounts of synthetic polymer fibers in order to impart certain properties to the resulting paper product. It will be obvious that such procedures do not take full advantage of the preferred properties of a synthetic polymer such as the high molecular weight, fiber-forming, linear polyamides, and the admixture of very small amounts of a cellulose pulp to the synthetic fiber does not overcome the disadvantages in working With the waterlaid fibrous web.

Another process suggested by the prior art consists in the addition of certain adhesives or viscosity-increasing agents to the fiber dispersion or pulp in order to increase the strength of the fibrous web during transfer from the Fourdrinier. A typical example of this procedure is the addition of carboxymethyl cellulose to the aqueous dispersion of the fibers, usually in an amount of at least about 0.1 to 1% by weight with reference to the dispersing liquid such as water. It will be evident that the fiber dispersion or pulp then contains at least ten parts by weight or more of the additive for each part by weight of the fibers. Since the additive, such as carboxymethyl cellulose, is generally at least partly soluble in water, large amounts of the additive will be removed at the fibrous web is drained or dewatered, and it is therefore essential to recirculate the water in order to avoid sub-. stantial loss of the additive. Furthermore, the carboxymethyl cellulose or other additive remaining in the fibrous web must be Washed out during subsequent processing steps or at least removed from the finished product, since such additives generally impair the quality, color or other desirable properties of the final paper product.

Still another technique which has been suggest-ed for the production of sheet structures of synthetic polymer fibers, including polyamide fibers, is the addition to the fiber dispersion of so-called fibrids. These fibrids may be characterized as ultra-fine fibers which can be obtained, for example, when a solution of a synthetic polymer is sprayed with high velocity onto the surface of a precipitation bath. The production of these fibrids and their use in the formation of water-laid fibrous webs from aqueous dispersions is described in detail in US. Patent No. 3,062,702. The use of such fibrids in the papermaking process does result in a stronger and more easily handled fibrous web, and it is possible in this manner to produce a sheet composed solely of synthetic polymer fibers. However, the methods required for the production of the fibrids are quite complicated and very expensive, particularly because large amounts of solvents and precipitating agents must be circulated and treated during production and recovery of the fibrids.

In addition to the production of fibrids according to the above mentioned US. patent, processes have also been developed for the so-called fibrillation of synthetic polymer fibers. In this case, an attempt is made to duplicate the effect of the beater in the usual paper-making process wherein cellulose fibers are beaten in order to fibrillate and hydrate the pulp material. However, synthetic polymer fibers generally cannot be hydrated, and any fibrillation is achieved only by using very expensive beating methods. Furthermore, as in the production of fibrids, it may be necessary to deviate substantially from the usual filament spinning processes in order to achieve satisfactory fibrillation.

Finally, as disclosed in the copending application by Sommer, Gerlach and Werner, Serial No. 240,342, filed November 27, 1962, it was found that the use of fibrids or fibrillation methods or the addition of additives such as carboxymethyl cellulose could be completely avoided by the use of synthetic polymer strands having smooth and flat surfaces for contact of the fibers with each other. It is essential in this process to withdraw water from the water-laid fibrous web on the supporting screen so as to obtain a residual moisture content of about 30 to 85% by weight with reference to the wet web. Although this process provides a satisfactory method of forming a strong web which can be readily withdrawn or transferred from the Fourdrinier or other supporting screen, it will be apparent that any further treatment of the fleece or web requires a supporting surface if the fleece is first dried to a moisture content of below 30% by weight with reference to the wet or moist fleece. Thus, as the waterlaid web is dried and the moisture content is diminished below the critical minimum value, the web will lose its required strength, thereby limiting the conditions under which the Water-laid web can be subsequently treated to form a final paper or sheet-like product.

Accordingly, the prior art has not found any completely satisfactory process for the production of waterlaid webs with subsequent conversion into a sheet-like product where the individual fibers consist substantially or at least predominantly of synthetic polymer fibers. Each individual process of the prior art has its own specific disadvantage with respect to a practical or commercial execution of the overall process. Therefore, it is an object of this invention to provide an improved as well as an alternative method of forming a sheet of synthetic polymer fibrous material, and as will'be more apparent hereinafter, the present invention is limited to sheet structures of synthetic polyamide fibers.

The primary object of the present invention is to provide a new process for the production of a sheet of synthetic polyamide fibrous material wherein the waterlaid fleece or web of fibers is sufliciently strong and coherent so that it can be transferred from the screen of the paper-making machine and otherwise handled during subsequent treatment, even after substantially complete drying, without being ruptured or pulled apart.

Another object of the invention is to provide a commercially practical and economical process for the felting or water-laying of synthetic polyamide fibers followed by formation of a sheet material, whereby the process can be carried out so as to obtain the advantages of conventional filament spinning methods and paper-making processes.

Yet another object of the invention is to provide a waterlaid or felted product consisting substantially or wholly of synthetic polyamide fibers in the form of a strong, continuous web or fleece, whether in the wet state or the dry state.

It is a further object of the invention to provide a means of pretreating polyamide fibers which can then be immediately dispersed in water as an aqueous pulp for waterlaying on a paper-making machine.

The following detailed description of the invention is especially directed to the pretreatment of the polyamide fibers since the remaining steps of the process can be carried out with conventional paper-making apparatus,

4 for example, as disclosed in the copending application Serial No. 240,342.

It has now been found in accordance with the present invention that synthetic polyamide fibers can be waterlaid upon a supporting surface without difficulty or damage to the web, even after being dried, if the polyamide fibers are first pretreated in an aqueous bath containing a natural or synthetic organic tanning agent, the excess bath liquor removed from the fibers and the pretreated fibers then dispersed in water to form an aqueous pulp suitable for water-laying on the supporting surface in order to form a strong, coherent fibrous web.

The treatment of the synthetic polyamide fibers by contacting them in a bath with the tanning agent can be carried out immediately after the polyamide has been spun and stretched into a continuous filament according to conventional spinning processes. On the other hand, the stretched and oriented continuous filament can first be cut into staple fibers of any desired length and then contacted with the tanning agent in an aqueous bath. It is advisable that the fibers not be completely dried after treatment with the tanning agent, but merelyseparated from the aqueous pretreatment bath and excess bath liquor removed by squeezing or centrifuging. Otherwise, it becomes dilficult to obtain a good dispersion of the pretreated fibers in the mixing tank of the subsequent paper-making process.

After the pretreated moist fibers have been dispersed as an aqueous pulp and waterlaid onto a supporting surface, the pulp water can be readily removed by conventional means and the resulting fleece or web then formed into a paper-like product. Thus, after partial or complete removal of water from the waterlaid web, it is generally necessary to apply heat and/ or pressure and/ or adhesives or binding agents in order to provide a fiat, sheet-like structure. Throughout these subsequent steps, the waterlaid fleece or web containing the adherent watersoluble polymer is sufficiently strong to withstand various mechanical operations such as transporting, reeling or unreeling, calendering or the like. While the invention is particularly useful in preparing sheets consisting essentially of synthetic polyamide fibers, it will be obvious that sheets consisting predominantly of the polyamide fibers, or even a smaller proportion, will have greater strength when such fibers are treated in accordance with the invention.

The polyamide fibers employed in the process of this invention are generally characterized as high molecular weight, linear polyamides which contain the recurring carbonamide group --CONH and which have been spun and stretched into a highly oriented filament. The most common polyamides for this purpose are polycaprolactam and polyhexamethylene adipamide, better known as nylon. Other linear polyamides, e.g. as obtained from homologous monomers, are also well known in the art and a number of polyamide copolymers have been developed into filaments or fibers. It will be understood that the present invention is generally applicable to all such synthetic polyamide fibers, regardless of their original source or method of formation. Nevertheless, it is especially advantageous that the present invention permits the use of the most common polyamide fibers as formed by the simplest and most conventional spinning and stretching methods.

The preliminary treatment of the polyamide fibers and the production of a waterlaid fleece or web can be carried out with filaments or fibers of any desired cross section. However, if the waterlaid fleece as removed from the paper-making machine, i.e. the supporting screen, has a relatively low weight per unit surface area with reference to the dry state, it is advantageous to use at least a small amount of fibers in the form of flat, smoothsurfaced bands or tapes as described in copending application Serial No. 240,342. In general, such strands, bands or tapes should be used where the dry waterlaid .5 sheet or the corresponding paper-like final product has a dry weight of less than about 50 grams per square meter.

The physical properties of the polyamide fibers may also be varied, depending upon the particular properties or characteristics desired in the final product. For example, the paper-making process itself is most advan tageously carried out with staple fibers having a cut length of from about 0.5 to 15 mm., preferably about 2 to 8 mm., and a denier of about 0.5 to 3, preferably 0.8 to 2, with reference to an individual fiber or filament. In addition, it is possible to incorporate a minor proportion of polyamide binding fibers having a lower melting point than the bulk of the fibers, so that under a hot pressure treatment, the waterlaid fleece can be bonded into a paper-like product by using a temperature slightly above the melting point of the binding fibers.

The tanning agents employed in the process of this invention are well-known substances in the art for the purpose of tanning leather products, and both the natural or vegetable tannins and the synthetic tanning compounds are described in detail in the literature of this art.

The vegetable tannins are derived from various parts of plants and trees, e.g. oak bark, and are usually characterized by the presence of tannic acid, sometimes referred to as digallic acid, in free or combined form. For example, the term tannin is sometimes used to denote the specific compound pentadigalloylglucose, which contains five tannic or digallic acid groups esterified by the hydroxyl groups of the glucose molecule. In order to avoid confusion, the terms natural tannin and vegetable tannin are employed herein with reference to both individual compounds and mixtures of these compounds as derived from natural sources. As individual compounds, the vegetable tannins can be divided into two major groups: (1) the hydrolyzable tannins, such as gallotannins or galloylsugars; and (2) the condensed tannins such as catecholtannins. For a more complete listing of the plant source of native vegetable preparations containing tannic acid and also for a classification of the individual compounds, reference is made to Hackhs Chemical Dictionary, by Grant, pp. 830-831, McGraW-Hill Book Co., Inc., New York, 3rd ed. (1944).

By comparison, the synthetic tanning agents do not contain tannic acid, but instead these compounds are usually complex aromatic sulfonates wherein a plurality of polyhydroxyl aromatic rings are linked together, preferable with a methylene bridge. For example, such compounds which are suitable for the present invention can be obtained by combining polyhydroxybenzophenones, polyhydroxyanthraquinones, polyhydroxynaphthoquinones and reduction products thereof with a condensing agent such as formaldehyde and a sulfonating agent. These compounds are referred to herein as containing a plurality of phenolic hydroxyl groups, and the quoted term is employed in the broad sense of a hydroxy group attached to any aromatic ring. Also, the terms synthetic tanning agents, synthetic tannins and syntans are employed herein so as to have the same meaning and so as to clearly distinguish the natural tannins. As a treating agent for synthetic polyamide fibers, it is preferable to employ synthetic and natural tannins which are relatively water-soluble under the treating bath conditions and which have a molecular weight of about 200 to 1000.

For additional disclosure and discussions of typical synthetic tanning agents from which suitable compounds can be readily selected for purposes of this invention, note the following references:

Cheshire: 1. Int. Soc. Leather Trades Chemists, 27, 123

Thuau: Cuir Tech. 30, 160 (1943).

Fritz Stather: Gerbereichemie u. Gerbereitechnologie, 2nd. ed., pp. 287-288; Academie-Verlag, Berlin, 1951.

In pretreating the polyamide fibers, either in the form of continuous filaments or staple fibers, they are simply immersed or passed through an aqueous bath in which the natural and/or synthetic tanning agent has been dissolved. The aqueous bath should be adjusted to a pH in a range of about 1 to 4.5 with a concentration of the tanning agent of about 0.1 to 1, preferably 0.3 to 0.5, percent by weight. In general, the bath should be maintained at a temperature of about to 80 C., and it is preferably heated to an elevated temperature of about 35 to 60 C. in order to shorten the treatment time. The bath ratio of the aqueous treating liquor to the fibers or filaments can be maintained at about 20:1 or higher,- preferably about :1 to 50:1, as measured in parts by weight, and additional amounts of the treating agent can be added during continuous operation.

Under these conditions, the treatment time is relatively short, ranging from a few seconds up to a few minutes, preferably about seconds up to about three minutes. The synthetic polyamide fibers pretreated with the tanning agent have considerably added strength on the paper machine and in subsequent operations. In general, the pretreated fibers should retain about 5 to 15, percent by weight of the tanning agent, with reference to the weight of the dry fiber.

The pretreated synthetic polyamide fibers are converted into a paperor sheet-like structure in a conventional manner substantially in the following manner. About 0.01 to 0.5 and preferably 0.05 to 0.1 part by Weight of the pretreated and still slightly wet or moist fibers are first dispersed in parts by weight of water in the usual mixing box, beater or pulp tank arranged at one end of the paper-making machine. The fibrous dispersion is maintained by intense mixing or rapid agitation, and if necessary, with the addition of any suitable dispersing or wetting agent. The fibrous pulp is then discharged from the mixing box through an elongated opening onto the so-called wire or endless screen belt of a conventional paper-making machine such as the Fourdrinier. The bulk of the water can be removed through the screen by means of squeeze rollers, suction boxes or the like, and the waterlaid web or fieece can then be transferred to a second continuous belt or other conveying means for additional processing.

As one example, the waterlaid web can first be transferred to a continuous belt for conveyance between felt bands which remove additional water from the web, and complete drying can then be accomplished by conducting the web through a series of heated rollers. If desired, the web may also be air-dried at elevated temperatures or it can be rolled and stored for final drying. In most cases, the final solidification or binding of the fibrous web is most conveniently carried out while the web is substantially dry, and for this reason, it is especially desirable to maintain the strength of the web in the dry state.

A web or fleece obtained in this manner is sufliciently strong to be subjected to either hot or cold calendering for the formation of a compact, thin paper-like product. Also, the dry web can be sprayed with various impregnating or binding agents, such as adhesives or resins well-known in the art, so as to permanently adhere the fibers to each other. Furthermore, the waterlaid dry web can even be drawn through solutions or emulsions of such impregnating or binding agents, or it can be drawn through baths for the purpose of dyeing or adding fillers or pigments to the Web. In addition, the web or fleece can be solidified by using a proportion of polyamide fibers with a low melting point as the sole binding agent, the web then being subjected to heat and pressure alone. These and similar methods of converting the waterlaid web, Whether moist or dry, will be readily apparent to one skilled in this art, and in all cases, the web prepared according to this invention is strong enough to be easily handled and formed into a final sheet product.

The process of the invention is further illustrated by, but should not be limited to, the following examples. Parts and percentages are by weight unless otherwise noted.

Example I Polycaprolactam is spun in the usual manner to form a plurality of filaments having an individual denier of 1.2. The continuous filamentsare plied into a yarn of about 89,000 denier, and the yarn is then stretched 350%. After stretching, the yarn is conducted for a treatment time of 1.5 min. through a vessel which contains an aqueous 0.4% tannin solution which is adjusted to a pH value of 3.0 and has a temperature of 35 C. The tannin in this case is pentadigalloylglucose.

The yarn is then squeezed to remove excess bath liquor and, While still moist, cut to a length of 6 mm. The resulting staple fibers are then dispersed with the addition of a wetting agent in water in an amount of 1 part fiber to 200 parts water. This dispersion is poured onto the sieve or endless screen of a paper machine and drained to a residual moisture content of 600% (with reference to the weight of the dry fleece). The moist fleece is then conducted through the usual drying device of the paper machine and air-dried at about 96 C. The fleece or web can then be wound up and stored or immediately converted into a paper-like product.

In order to achieve the required properties of a paper, the dry fleece or web is conducted through a bath which contains a 0.5% methanolic copolyamide solution as a binding agent. After squeezing out the excess amount of liquid, the web is air-dried and thereafter calendered at 120 C. The resulting sheet structure has a weight per square meter of about 100 g.

Example 2 Polycaprolactam is spun by conventional means into a plurality of continuous filaments having an individual denier of 1.0. After stretching or elongating the filaments to 400% of their original size, they are cut to a length of 6 mm. The staple fibers are then stirred into an aqueous 0.4% tannin solution as a treating bath, the tannin in this case being pentadigalloylglucose.

The bath is maintained at a temperature of 35 C. and adjusted to a pH value of 3.0. The bath ratio of fiber to bath liquor in parts by weight is 1:50. After a contact period of one minute, the fiber mass is freed of excess bath liquor by centrifuging. A 0.05% dispersion of these moist fibers in water is poured onto the draining sieve or screen of a paper machine and dewatered to a residual moisture content of 400% (with reference to the weight of the dry fiber). The moist fleece or web is transferred into the usual drying device of a paper machine and airdried at about 96 C. The fleece web can then immediately be rolled up and, as described in Example 1, subsequently subjected to a treatment with any suitable binder to form a sheet or paper-like product.

Example 3 Polyhexamethylene adipamide is spun in the usual manner to form a plurality of continuous filaments having an individual denier of 1.4. The filaments are plied into a yarn of about 89,000 denier, and the yarn is then stretched 400%. The resulting filaments are cut to a length of 6 mm. so as to provide staple fibers. 1 kg. of these staple fibers is treated with a bath consisting of a 2% solution of m-digallic acid in water. The pH value of the bath is adjusted to 2.5, the bath temperature is 60 C., the bath ratio of fiber to liquor is 1:50, and the contact time of the tanning agent with the fiber material is about one minute. The treated fibers are then centrifuged to a moisture content of about 30% in a centrifuge.

An aqueous dispersion of these fibers, which contains 0.2% of fibers, is worked into a fleece or Web on a paper machine as described in Example 1. This fleece or web also has sufficient strength to be rolled up and/or subjected to an after-treatment with liquid binding agents by immersion therein. The final paper-like product can be formed by heating and/ or calendering in the usual manner.

Example 4 Polycaprolactam is spun through a slit nozzle With a plurality of openings, each having a dimension of 60 x 800 so as to form a plurality of filaments with an individual denier of 1.2. The individual filaments are plied into a yarn of about 89,000 denier and stretched 350%.

This yarn is then cut to a staple length of 6 mm. The staple fibers are stirred into a bath consisting of a 0.5% solution of tannin in water, the tannin in this case being pentadigalloylglucose.

The treating bath is adjusted to a pH value of 3, the bath temperature is 35 C., the bath ratio of fiber to liquor is 1:50, and the contact time of the tanning agent with the fiber material is about one minute. The treated fibers are then centrifuged to a moisture content of about 30%.

Thereafter, a 0.05% aqueous fiber dispersion is prepared to which there is added a copolyamide staple fiber with an individual denier of 1.2, a cut length of 6 mm. and a melting point of 170 C., in an amount of 10% with reference to the total amount of fibers. This fiber dispersion is worked up as described in Example 1 into a fleece or web, the operating speed of the paper machine being adjusted so that a fleece or web is obtained with a weight of 40 grams per square meter. The web can be rolled up after being air-dried. For the final solidification or bonding of the fibers, the web is subjected to a calendering treatment at 170 C. A paper-like product is obtained of high tensile and tearing strength.

The invention is hereby claimed as follows:

1. In a process for the production of a fibrous sheet material by water-laying synthetic polyamide fibers onto a supporting surface and withdrawing water from the resulting fibrous web, the improvement which comprises: pretreating said fibers in an aqueous bath containing an organic tannin having a molecular weight of about 200 to 1000, said aqueous bath being maintained at a pH of about 1 to 4.5; removing excess bath liquor from said fibers; and then dispersing the pretreated fibers into water to form an aqueous pulp slurry suitable for said waterlaying of a fibrous web.

2. A process as claimed in claim 1 wherein the organic tannin is a gallotannin.

3. A process as claimed in claim 1 wherein the organic tannin is a galloylsugar.

4. A process as claimed in claim 1 wherein the organic tannin is m-digallic acid.

5. A process as claimed in claim 1 wherein the aqueous pretreatment bath contains about 0.1 to 1% by weight of said organic tannin, and the bath ratio of the aqueous treating liquor to the fibers is about 20:1 to 50:1.

6. A process as claimed in claim 5 wherein the temperature of the aqueous pretreatment bath is about 20 C. to C.

7. A process as claimed in claim 6 wherein the pretreated fibers, after removing excess bath liquor therefrom, retain about 5 to 15% by weight of the organic tannin, with reference to the dry fiber.

8. In a process for the production of a fibrous sheet material by water-laying synthetic polyamide fibers onto a supporting surface and withdrawing water from the resulting fibrous web, the improvement which comprises: pretreating said fibers at a temperature of about 20 C. to 80 C. in an aqueous bath maintained at a pH of about 1 to 4.5 and containing about 0.1 to 1% by weight of a hydrolyzable vegetable tannin selected from the class consisting of gallotannins and galloylsugars, the ratio of the bath liquor to the fibers being about 20:1 to 50: 1; removing excess bath liquor from said fibers; and then dispersing the pretreated fibers into water to form an aqueous pulp slurry suitable for said water-laying of a fibrous web.

9 1O 2 9. A process as claimed in claim 8 wherein the tannin References Cited by the Examiner is a gallotannin. UNIT D E P A NT 10. A process as claimed in claim 8 wherein the tannin E S TE S is a galloylsugan 2,317,728 4/1943 Bristol.

11 A e 1 1 8 h th t 2,496,665 2/1950 Hermanson 162-157 c m c mm W mm e 5 2,869,973 1/1959 Hubbard et a1 162157 12. A process as claimed in claim 8 wherein the pretreated fibers, after removing excess bath liquor there- DONALL SYLVESTERPrZmary Exammer' from, retain about 5 to 15% by weight of the vegetable MORRIS O. WOLK, Examiner. tannin with reference to the dry fiber. 10

Claims (1)

1. IN A PROCESS FOR THE PRODUCTION OF A FIBROUS SHEET MATERIAL BY WATER-LAYING SYNTHETIC POLYAMIDE FIBERS ONTO A SUPPORTING SURFACE AND WITHDRAWING WATER FROM THE RESULTING FIBROUS WEB, THE IMPROVEMENT WHICH COMPRISES: PRETREATING SAID FIBERS IN AN AQUEOUS BATH CONTAINING AN ORGANIC TANNIN HAVING A MOLECULAR WEIGHT OF ABOUT 200 TO 1000, SAID AQUEOUS BATH BEING MAINTAINED AT A PH OF ABOUT 1 TO 4.5; REMOVING EXCESS BATH LIQUOR FROM SAID FIBERS; AND THEN DISPERSING THE PRETREATED FIBERS INTO WATER TO FORM AN AQUEOUS PULP SLURRY SUITABLE FOR SAID WATERLAYING OF A FIBROUS WEB.