Absorbent nonwoven fabrics

Abstract

A permanently absorbent nonwoven fabric comprises a web of fibers, a crosslinkable latex binder, and a surfactant consisting of at least one bis-alkyl sulfosuccinate having alkyl substituents containing 13-14 carbon atoms. The preferred surfactant is bis-tridecyl sodium sulfosuccinate, and the amount used is generally 0.2-10.0 weight percent of the total fibers and binder. The surfactant may be incorporated into the binder latex prior to, during, or after polymerization.

Description

BACKGROUND OF THE INVENTION

This invention relates to so-called "nonwoven" fabrics of the type composed of a loosely assembled web of fibers bound together by a latex binder. Such bonded nonwoven fabrics have been formed by impregnating, printing or otherwise depositing an adhesive bonding material on a base web predominately comprising relatively long fibers, including those of textile length from about 0.5 inch to about 2.5 inches or more. The base web of the nonwoven fibers to which the latex binder is applied can be produced inexpensively and with low capital investment by carding, garnetting, interlaying, paper-making procedures, or other known operations for which automation is possible. The operation of bonding the fibers in place is much less expensive than conventional spinning and weaving. In comparison with woven fabric, the bonded nonwoven fabrics can be made in a much greater range of thicknesses, with a more homogeneous structure and no unravelling tendency, and with greater water absorbency, porosity and resiliency, when required.

To improve the absorbency of the bonded nonwoven fabrics, various emulsifiers have been used during the emulsion polymerization to produce the binder, and various wetting agents have been added to the impregnated medium by which the binder is applied to the fiber web. For example, bonded nonwoven fabrics for use as household wipes -- wiping clothes to remove and pick up liquids, wash furniture and cars, clean kitchens, etc. -- are typically absorbent because of the presence of anionic surfactants such as dioctyl sodium sulfosuccinate or sodium dodecyl benzene sulfonate. Such bonded nonwoven fabrics have not proven to be entirely satisfactory in use because the emulsifiers and wetting agents are generally easily extractable by water rinsing, machine washing and/or steam sterilization, thus rendering the fabric nonabsorbent. Such fabrics are especially not satisfactory for the common industrial, hospital and consumer wages where high absorbency after repeated usage is required.

Accordingly, it is an object of the present invention to provide a bonded nonwoven fabric incorporating a surfactant which resists extraction by water rinsing and even steam sterilization.

It is another object to provide such a fabric which is formed by bonding fibers with a crosslinkable binder latex incorporating a specific surfactant which resists extraction.

It is a further object to provide such a fabric which comprises fibers set with a commercially available binder to which a specific surfactant has added to impart permanent absorbency characteristics to the fabric.

SUMMARY OF THE INVENTION

It has now been found that the above and related objects of the present invention are obtained in an absorbent nonwoven fabric comprising a web of fibers, a crosslinkable binder, and at least one surfactant consisting of at least one bis-alkyl sulfosuccinate having alkyl substituents containing 13-14 carbon atoms. The surfactant is preferably bis-tridecyl sodium sulfosuccinate having branched alkyl substituents, and the crosslinkable binder is preferably an emulsion copolymer of at least two .alpha.,.beta.-monoethylenically unsaturated monomers, one an effectively monofunctional monomer and the other an effectively polyfunctional and hence cross-linkable comonomer. Generally the fabric includes about 5 to about 100 weight percent of the binder (based on the fiber weight), and about 0.2 to about 10.0 weight percent of the surfactant (based on the weight of the fibers and binder).

A preferred fabric comprises a loosely assembled web of fibers, about 10 to about 70 weight percent (based on the fibers) of crosslinkable binder, and about 0.3 to about 2.0 weight percent (based on the weight of the fibers and binder) of bis-tridecyl sodium sulfosuccinate having at least one branched alkyl substituent. The binder is preferably comprised of at least one .alpha.,.beta.-monoethylenically unsaturated monomer selected from the group consisting of alkyl acrylate, vinyl acetate, ethylene, vinyl chloride, and mixtures thereof, and at least one .alpha.,.beta.-monoethylenically unsaturated polyfunctional crosslinkable comonomer selected from the group consisting of a crylamide, N-methylol acrylamide and isobutoxy methyl acrylamide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The absorbent nonwoven fabric of the present invention is comprised of a loosely assembled web of fibers, a crosslinkable polymeric latex binder formed of at least one .alpha.,.beta.-monoethylenically unsaturated monofunctional monomer and at least one .alpha.,.beta.-monoethylenically unsaturated polyfunctional crosslinking comonomer, and at least one surfactant of the present invention. Various catalysts, emulsifying and wetting agents, protective colloids, buffering agents, plasticizers, thermosetting resins and the like known to those skilled in the art may optionally be used during formation of the binder or fabric.

THE FIBERS

The starting layer or mass can be formed by any one of the conventional techniques for depositing or arranging fibers in a web or layer. These techniques include carding, garnetting, air-laying, and the like. Individual webs or thin layers formed by one or more of the techniques can also be laminated to provide a thicker layer for conversion into a fabric. In general, the fibers extend in a plurality of diverse directions in general alignment with the major plane of the fabric, overlapping, intersecting and supporting one another to form an open, porous structure. When reference is made to "cellulose" fibers, those fibers containing predominantly C.sub.6 H.sub.10 O.sub.5 groupings are meant. Thus, examples of the fibers to be used in the starting layer are the natural cellulose fibers, such as cotton, silk and hemp, and the synthetic cellulose fibers, such as rayon, and regenerated cellulose. Often the fibrous starting layer contains at least 50% cellulose fibers, whether they be natural or synthetic, or a combination thereof. Other fibers in the starting layer may comprise natural fibers such as wool or jute; artificial fibers such as cellulose acetate; synthetic fibers such as polyamides (i.e., nylon), polyesters (i.e., "Dacron") acrylics (i.e., "Dynel," "Acrilan," "Orlon") polyolefins (i.e., polyethylene, polyvinyl chloride, polyurethane, etc.), vinyl resin fibers (i.e., the copolymers of vinyl chloride and vinyl acetate), siliceous fibers (i.e., glass and mineral wools), alone or in combination with one another.

The fibrous starting layer or web suitably weighs from about 100 grains to about 2,000 grains per square yard and preferably weighs about 200 grains to about 800 grains per square yard. This fibrous starting layer, regardless of its method of preparation, is then subjected to at least one of the several types of bonding operations to anchor the individual fibers together to form a self-sustaining web. Some of the better-known methods of bonding are overall impregnation or printing the web with intermittent or continuous straight or wavy lines or areas of binder extending generally transversely or diagonally across the web and additionally, if desired, along the web.

THE MONOMERS

At least one .alpha.,.beta.-monoethylenically unsaturated monofunctional monomer forms the major part of the binder, generally from about 85 to about 99, preferably from about 95 to about 98, weight percent of such monomers based on the total binder monomers in the polymerization.

The monomers useful as this component include vinyl esters of the formula: ##EQU1## wherein R is a C.sub.1 -C.sub.12 alkyl group, and preferably a C.sub.1 -C.sub.3 alkyl group. Representative of such vinyl esters are the preferred vinyl acetate compound, as well as vinyl propionate, vinyl laurate, etc.

Other monomers useful as this component include the alkyl esters of acrylic and methacrylic acid having the formula: ##EQU2## wherein R is a C.sub.1 -C.sub.12 alkyl group, and R' is hydrogen or a methyl group. Representative of such alkyl esters are methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, etc.

Still other monomers useful as this component include the C.sub.1 -C.sub.12 dialkyl esters of maleic, fumaric and itaconic acids (e.g., dibutyl maleate, dioctyl fumarate, etc.), styrene, vinyl chloride, vinylidene chloride, ethylene and acrylonitrile.

The monomers useful as this component are effectively monofunctional at the processing parameters utilized (i.e., the polymerization temperature, time, etc.) -- i.e., they enter into chain growth but not crosslinking reactions.

The preferred monomers for this component are alkyl acrylates, vinyl acetate, ethylene, vinyl chloride and mixtures thereof, If desired, mixtures of the above monomers may be utilized as this component. The preferred mixtures of monomers on a weight basis include a 50-100/0-50 mixture of an alkyl acrylate (such as butyl acrylate) and vinyl acetate, a 60-100/0-40 mixture of vinyl acetate and ethylene, a 60-90/10-40 mixture of vinyl chloride and ethylene, and a 0-100/0-100 mixture of various alkyl acrylates such as ethyl acrylate and butyl acrylate.

THE CROSSLINKING COMONOMERS

At least one crosslinking .alpha.,.beta.-monoethylenically unsaturated polyfunctional comonomer forms a minor part of the binder to effect crosslinking of the binder, generally from about 1 to about 15, preferably from about 2 to about 5, weight percent of such comonomers based on the total binder monomers (including comonomers) used in the polymerization.

The comonomers useful as this component include

I. acrylamide and methacrylamide of the formula ##EQU3## wherein R is hydrogen or a methyl group; II. N-alkylol and alkyl ether derivatives of (I) having the formula ##EQU4## wherein R is hydrogen or a methyl group,

R' is hydrogen or a C.sub.1 -C.sub.6 alkyl group, and

R" is (CH.sub.2).sub.1-4, preferably a CH.sub.2 group;

Iii. n-alkyl substituted derivatives of (I) having the formula ##EQU5## wherein R is a hydrogen or a methyl group, and

R' is a C.sub.1 -C.sub.12 alkyl group;

Iv. hydroxy alkyl esters of acrylic or methacrylic acid having the formula ##EQU6## wherein R is hydrogen or a methyl group, and

R' is (CH.sub.2).sub.2-5, preferably a (CH.sub.2).sub.2 group;

V. glycidyl alkyl esters of acrylic and methacrylic acid (e.g., glycidyl acrylate) having the formula ##EQU7## wherein R is hydrogen or a methyl group, and

R' is (CH.sub.2).sub.1-5, preferably a (CH.sub.2).sub.2 group;

Vi. .alpha.,.beta.-ethylenically unsaturated carboxylic acids (for example, acrylic, methacrylic, itaconic, fumaric, and maleic acids) including the mono-alkyl esters of dioic acids (for example, the mono-alkyl maleate ester having a C.sub.1 -C.sub.12 alkyl group); and

Viii. diacetone acrylamide, and hydroxy methyl diacetone acrylamide.

The preferred comonomers are acrylamide, N-methylol acrylamide and isobutoxy methyl acrylamide.

THE SURFACTANT

The surfactant useful in the practice of the present invention is a bis-alkyl sulfosuccinate wherein the alkyl substituents each contain 13-14 carbon atoms, preferably 13 carbon atoms. The surfactant is preferably an alkali metal salt of the bis-alkyl sulfosuccinic acid, and at least one of the alkyl group substituents is preferably branched. The preferred surfactant is bis-tridecyl sodium sulfosuccinate, especially where at least one of the tridecyl groups is branched. Similar compounds having lower alkyl groups substituents (e.g., 12 carbon atoms and less), especially straight chain lower alkyl groups, are rapidly extracted during rinsing, washing or steam sterilization; similar compounds with higher alkyl group substituents (e.g., 15 carbon atoms and up) are so insoluble as to interfere with their incorporation into the binder latex, or, when incorporated, do not impart permanent absorbent properties to the fabric.

The amount of surfactant used is from about 0.2 to about 10.0, preferably about 0.3 to about 2.0 weight percent based on the combined weight of the fibers and binder. The surfactant may be used alone or in combination with various emulsifying and/or wetting agents. It may be added to the binder latex prior to polymerization, added intermittently or continuously during polymerization, or even added subsequent to polymerization.

THE CATALYST

Various free-radical forming catalysts can be used in carrying out the polymerization of the monomers, such as peroxide compounds. Combination type catalysts employing both reducing agents and oxidizing agents can also be used. The use of this type of combined catalyst is generally referred to in the art as "redox polymerization" or "redox system". The reducing agent is also often referred to as an activator and the oxidizing agent as an initiator. Suitable reducing agents or activators include bisulfites, sulfoxylates, or other compounds having reducing properties such as ferrous salts and tertiary aromatic amines (e.g., N, N-dimethyl aniline). The oxidizing agents or initiators include hydrogen peroxide, organic peroxides such as benzoyl peroxide, t-butyl hydroperoxide and the like, persulfates such as ammonium or potassium persulfate, perborates, and the like. Specific combination type catalysts or redox systems which can be used include hydrogen peroxide and zinc formaldehyde sulfoxylate; hydrogen peroxide, ammonium persulfate, or potassium persulfate, with sodium metabisulfite, sodium bisulfite, ferrous sulfate, dimethyl aniline, zinc formaldehyde sulfoxylate or sodium formaldehyde sulfoxylate.

In general, it is advantageous to utilize more water-soluble peroxides, such as hydrogen peroxide, rather than the more oil-soluble peroxides, such as t-butyl hydroperoxide, in the redox system catalyzing the monomer polymerization. Redox catalyst systems are described, for example, in "Fundamental Principles of Polymerization" by G. F. D'Alelio (John Wiley and Sons, Inc., New York, 1952) pp. 333 et seq. Other types of catalysts that are wellknown in the art can also be used to polymerize the monomers according to this invention, with or without the addition of reducing agents or other activating materials.

The activator is ordinarily added in aqueous solution and the amount of activator is generally 0.25 to 1 times the amount of catalyst.

EMULSIFYING AGENTS

The emulsifying agent can be any of the non-ionic or anionic oil-in-water surface active agents. In the following discussion of emulsifying agents, frequent reference will be made to a cloud point of a particular agent. The cloud points which are recited are based on 1 weight percent aqueous solutions of the agent. A relatively hydrophobic agent is one having a cloud point below 190.degree.F. and a relatively hydrophilic agent is one having a cloud point of 190.degree.F. or above.

A single emulsifying agent can be used or the emulsifying agents can be used in combination. When combinations of emulsifying agents are used, it is advantageous to use a relatively hydrophobic agent or a relatively hydrophobic agent in combination with a relatively hydrophilic agent. The amount of emulsifying agent used is generally from about 0.1 to about 10, preferably 0.5-7.0, weight percent of the monomers used in the polymerization.

Suitable nonionic emulsifying agents include polyoxyethylene condensates represented by the following general formula:

where R is the residue of a fatty alcohol, acid, amide, or amine having from 10 to 18 carbon atoms or an alkyl phenol having from 10 to 18 carbon atoms; and where n is an integer of 1 or above and preferably between 5 and 30. Some specific examples of polyoxyethylene condensates which can be used include polyoxyethylene aliphatic ethers such as polyoxyethylene lauryl ether, polyoxyethylene oleyl ether, polyoxyethylene hydroabietyl ether and the like; polyoxyethylene alkaryl ethers such as polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether and the like; polyoxyethylene esters of higher fatty acids such as polyoxyethylene laurate, polyoxyethylene oleate and the like as well as condensates of ethylene oxide with resin acids and tall oil acids; polyoxyethylene amide and amine condensates such as N-polyoxyethylene lauramide and N-lauryl-N-polyoxyethylene ethyl amine and the like; and polyoxyethylene thioethers such as polyoxyethylene n-dodecyl thioether.

Some examples of nonionic emulsifying agents which can be used include a polyoxyethylene nonylphenyl ether having a cloud point of between 126.degree. and 130.degree.F and marketed by GAF Corporation under the trademark "Igepal CO-630," and a polyoxyethylene nonylphenol ether having a cloud point of about 212.degree.F. and marketed under the trademark "Igepal CO-887." A similar polyoxyethylene nonylphenyl ether having a cloud point of about 86.degree.F. is marked under the trademark "Igepal CO-610" and is also a good emulsifying agent. Another agent is a polyoxyethylene octylphenyl ether having a cloud point of between 80.degree.F and 160.degree.F. and marketed by Rohm & Haas Company under the trademark "Triton X-100." Yet another agent is nonylphenoxy polyethoxyethanol marketed under the trademark "Triton X-305." Other emulsifying agents include a polyoxyethylene oleyl ether having a cloud point of between 80.degree.F. and 160.degree.F. and marketed under the trademark "Atlas G-3915," and a polyoxyethylene lauryl ether having a cloud point above 190.degree.F. and marketed under the trademark "Brij 35".

The nonionic emulsifying agents which can be used according to this invention also include a series of surface active agents known as "Pluronics." The "Pluronics" have the general formula:

where a, b and c are integers between 1 and 100. As the ratio of b to a and c increases, the compounds become less water soluble or more oil soluble and thus more hydrophobic, while as the ratio decreases the compounds become more water soluble and less oil soluble. An example of this class is "Pluronic L-64" which has a cloud point of about 140.degree.F. and a polyoxypropylene chain having a molecular weight of 1,500 to 1,800 and a polyoxyethylene content that is 40 to 50 percent of the total weight of the molecule. Another useful example is "Pluronic F-68", a polyoxyethylene-polyoxypropylene glycol having a cloud point of about 212.degree.F. and a polyoxyethylene content of about 80 to 90 percent of the total weight of the molecule.

A class of suitable emulsifying agents are a series of ethylene oxide adducts of acctylenic glycols sold commercially under the name "Surfynols." This class of compounds can be represented by the formula: ##EQU8## in which R.sub.1 and R.sub.2 are alkyl radicals containing from three to 10 carbon atoms, R.sub.2 and R.sub.3 are selected from the group consisting of methyl and ethyl, and x and y are integers having the sum in the range of 3 to 60, inclusive.

Representative of the "Surfynols" are "Surfynol 465" which is an ethylene oxide adduct of 2,4,7,9-tetramethyl decynediol containing an average of 10 moles of ethylene oxide per mole of the surface active agent. "Surfynol 485" corresponds to "Surfynol 465" but contains an average of 30 moles of ethylene oxide per mole of surface active agent. "Surfynol 485" has a cloud point about 212.degree.F.

Anionic emulsifying agents which can be employed herein include anionic compounds obtained by sulfonation of fatty derivatives such as sulfonated tallow, sulfonated vegetable oils and sulfonated marine animal oils. Commercially available emulsifiers of this group are "Tallosan RC," a sulfonated tallow marketed by General Dyestuff Corporation; "Acidolate," a sulfonated oil marketed by White Laboratories, Inc.; and "Chemoil 412," a sulfonated castor oil marketed by Standard Chemical Company.

Various sulfonated and sulfated fatty acid esters of mono- and polyvalent alcohols are also suitable such as "Nopco 2272R," a sulfated butyl ester of a fatty ester marketed by Nopco Chemical Company; "Nopco 1471," a sulfated vegetable oil marketed by Nopco Chemical Company; "Sandozol N," a sulfated fatty ester marketed by Sandoz, Inc.; and "Stantex 322," an ester sulfate marketed by Standard Chemical Products, Inc.

Sulfated and sulfonated fatty alcohols are also useful as an emulsifier and include anionic agents such as "Duponal ME," a sodium lauryl sulfate; "Duponal L142," a sodium cetyl sulfate; "Duponal LS," a sodium oleyl sulfate which is marketed by E. I. DuPont de Nemours and Company; and "Tergitol 4," a sodium sulfate derivative of 7-ethyl-2-methyl, 4-undecanol, "Tergitol 7," a sodium sulfate derivative of 3,9-diethyl tridecanol-6, and " Tergitol 08," a sodium sulfate derivative of 2-ethyl-1-hexanol, which are marketed by Union Carbide Corporation, Chemical Division.

A particular useful class of anionic agents which can be employed comprises the C.sub.1 to C.sub.12 alkyl and C.sub.5 to C.sub.8 cycloalkyl esters of alkali metal sulfoalkanedioic acids having from three to about six carbons. Examples of these include diethyl sodium sulfosuccinate, di-n-octyl potassium sulfosuccinate, dicyclohexyl lithium sulfoglutarate, di(methylcyclopentyl) sodium sulfoadipate, dicycloheptyl cesium sulfomalonate, diamyl sodium sulfoadipate, etc. A useful member of this class is di-octyl sodium sulfosuccinate marketed by American Cyanamid Co. under the trademark "Aerosol-OT."

The half esterified, half ethoxylated derivatives of the aforementioned alkali metal sulfoalkanedioic acids are preferred emulsifiers. These agents have one of the carboxylic acid sites esterified with a C.sub.1 to C.sub.12 alkanol or C.sub.5 to C.sub.8 cycloalkanol, and the remaining carboxylic acid site condensed with from two to about 20 (preferably from eight to 16, and most preferably from 10 to 12) ethylene oxide units per mole to add a polyethoxylol group. Examples of these are: hexyl polyethoxylol sodium sulfosuccinate, isopropyl polyethoxylol potassium sulfoglutarate, decyl polyethoxylol lithium sulfoadipate, cyclohexyl polyethoxylol cesium sulfomalonate, cycloheptyl polyethoxylol sodium sulfosuccinate, cyclooctyl polyethoxylol potassium sulfosuccinate, etc.

Another preferred anionic emulsifying agent is sodium dodecyl benzene sulfonate, commonly called "SDBS" and used in a 20 weight percent solution.

OTHER POLYMERIZATION REAGENTS

A protective colloid may optimally be used to increase the adhesiveness of film prepared from the latex. When the protective colloid is used, it is desirable to decrease the amount of emulsifying agent used by an amount equivalent to the weight of the protective colloid, since the latter also aids the stability of the latex. This agent can be any of a wide range of compounds that is available for use as protective colloids, including many natural substances such as casein, natural gums, gelatins, agar, dextrin and globulin; suitably chemically modified polysaccharides such as hydrolyzed starch, hydroxyethyl cellulose, methyl and carboxymethyl cellulose; and synthetic colloids such as polyvinyl alcohol, alkali metal or ammonium salts of sulfonated polystyrene, water soluble interpolymers of acrylic acid and 2-ethylhexyl acrylate, copolymers of acrylamide and acrylic acid, partially hydrolyzed polyacrylamide having from 10 to 70 percent of its amide groups as carboxylic acid or alkali metal carboxylate groups, etc. The polyvinyl alcohols, which are prepared by hydrolysis of polyvinyl esters, typically polyvinylacetate, are preferred protective colloids and can be used with from 80 to 100 percent, preferably from about 90 to 100 percent of the ester groups hydrolyzed to hydroxyl groups.

A buffering agent can be used, to maintain the pH at a value from about 2.0 to 7.0, preferably from about 2.5 to 5.0, by periodic addition of the buffering agent. Suitable agents comprise the alkali metal or ammonium salts of weak acids; e.g., sodium carbonate, sodium bicarbonate, sodium acetate, potassium bicarbonate, lithium carbonate, potassium acid phthalate, potassium citrate, sodium acetate, potassium acid phosphate, and others well-known in the polymerization art.

Various polymerization modifiers may optimally be used in small quantities to modify the molecular weight of the copolymers formed during polymerization. For example, various polyfunctional agents, such as divinyl benzene and allyl methacrylate, may be used to increase molecular weight; various chain transfer agents, such as the mercaptans and alcohols, may be used to decrease molecular weight.

THE POLYMERIZATION PROCESS

The polymerization of the aforementioned monomers is performed by emulsion polymerization, generally under batch conditions; however, continuous processing can be employed if desired. The reactor used for the polymerization can be a jacketed kettle having stirring means with provisions to circulate a cooling medium through the jacket of the kettle to maintain the desired temperature. The aqueous medium is stirred to maintain dispersion of the monomers and the copolymer in the aqueous medium.

A suitable emulsifying agent of the anionic or nonionic types, or combinations thereof, is used in the polymerization. The amount of emulsifying agent is generally from about 0.1 to about 10, preferably from about 1 to about 5, weight percent of the monomers used in the polymerization.

A water soluble, free radical catalyst such as a water soluble peracid or salt thereof is used as the initiating catalyst and this can be used alone or in combination with an active reducing agent in a redox couple. The catalyst is used in concentration from about 0.01 to about 2, preferably from about 0.1 to about 0.5, weight percent of the monomers used in the polymerization.

The surfactant of the present invention is generally added to the polymerization latex in an amount of from about 0.2 to about 10.0, preferably 0.3-2.0, weight percent based on the combined weight of binder monomers and fibers.

If desired, the polymerization medium can also contain a minor quantity of a protective colloid to improve the adhesiveness of the product, generally from about 0.1 to about 3.0, preferably 0.5-1.5, weight percent based on the monomers used in the polymerization.

If desired, the polymerization medium can also contain a minor quantity of a buffering agent, generally from about 0.1 to about 0.5, preferably 0.2-0.4, weight percent of the monomers used in the polymerization.

Polymerization is performed conventionally--typically at a temperature of about 30.degree.-90.degree.C, preferably 40.degree.-70.degree.C, for sufficient time to achieve a low free monomer content; e.g., from 1 to about 10 hours, preferably from 5 to about 8 hours, to produce a latex having less than 3, preferably less than 1.5, weight percent free monomer.

The preferred procedure is a modified batch processing wherein the major amounts of some or all the comonomers and emulsifier are charged to the reaction vessel after polymerization has been initiated. In this manner, control over the copolymerization of monomers having widely varied degrees of reactivity can be ahieved.

It is preferred to add the .alpha.,.beta.-monoethylenically unsaturated monomers (e.g., vinyl acetate) intermittently or continuously over the polymerization period. The .alpha.,.beta.-monoethylenically unsaturated polyfunctional crosslinking comonomers (e.g., N-alkyl methacrylamide or acrylamide) are preferably slowly added during polymerization to avoid an excessive increase in viscosity of the latex which otherwise occurs when the entire amount of these comonomers is added to the initial charge to the polymerization vessel. (The catalyst is also preferably continuously or intermittently added during the polymerization.) From 40 to 100 percent of the amount of these comonomers can be added in this fashion, the balance, if any, being introduced with the initial charge.

The emulsifier used in the polymerization can also be added, in its entirety, to the initial charge to the polymerization zone or a portion of the emulsifier (e.g., from 90 to 25 percent thereof) can be added continuously or intermittently during polymerization.

The surfactant is preferably continuously or intermittently added during the polymerization, although at least a portion thereof may be charged to the reactor at the start of polymerization.

The latex produced by this polymerization can contain from about 35 to 65 weight percent solids comprised chiefly of the interpolymers. The preferred contents of solids are from 40 to 60, and most preferably from 50 to 60, weight percent.

APPLICATION OF BINDER TO FIBERS

The binder described above is suitably used to prepare nonwoven fabrics by a variety of methods known to the art which, in general, involve the impregnation of a loosely assembled mass of fibers with the binder latex followed by moderate heating to dry the mass. In the case of the present invention this moderate heating also serves to cure the binder by forming a cross-linked interpolymer.

The amount of binder, calculated on a dry basis, applied to the fibrous starting web, suitably ranges from about 5 to about 100 weight percent of the starting web, preferably from about 10 to about 70 weight percent.

If the surfactant has not been added already as part of the binder polymerization process, it is incorporated physically into the binder latex prior to application of the binder latex to the fiber mass. Thus an alcohol solution of the surfactant (with or without a plasticizer) may even be stirred into an available polymerized commercial binder latex formulated from the aforementioned monomers, cross-linking comonomers, and possibly additional surfactants to increase the longevity of the absorbency of the fabric to be produced.

The binder dispersion may be applied to the dry fibers after the formation or deposition of the web or mat so as to penetrate partially into or completely through the interior of the fibrous products. Alternatively, the binder dispersion may be applied to the fibers as they fall through the settling chamber to their point of deposition. This is advantageously obtained by spraying the binder dispersion into the settling chamber at some intermediate point between the top and the bottom thereof. By so spraying the fibers as they descend to the point of collection, it is possible to effect a thorough distribution of the binder among the fibers before they are collected into the product. In the production of certain fibrous products wherein a hot molten mass of a polymer, such as nylon or a fused siliceous mass or glass, is disrupted by jets of heated air or steam, the binder dispersion may be sprayed directly on the fibers -- while still hot and very shortly before their deposition -- so that quickly after deposition the binder sets and so bonds the fibers in proper relationship. Preferably, however, application of the binder dispersion to the fibrous product is made at room temperature to facilitate cleaning of the apparatus associated with the application of the binder dispersion. The binder dispersion may be applied to one or both surfaces of the fibrous product, or it may be distributed through the interior as well.

The binder of the present invention may be applied in conjunction with other binders, such as glue. Similarly, the use of potentially adhesive fibers within the fibrous product may also be resorted to in conjunction with the use of a binder of the present invention.

The aqueous dispersion of the binder may optionally contain from about 1/2 to 3% by weight of a wetting agent to assist penetration of the fibrous web or mat to which it is applied, and also it may optionally contain either a foaming agent to provide the binder in a foamed condition in the final product, or a defoamer when the ingredients of the aqueous dispersion have a tendency to give rise to foaming and in a particular case such foaming is undesirable. The conventional wetting agents, including the alkali metal salts of di(C.sub.6 -C.sub.12) alkyl sulfosuccinic acid, such as the sodium salt of dioctylsulfosuccinic acid, may be used. The wetting agent may also serve as the emulsifier in preparing the polymer latex or it may be added after production of the latex. Conventional foaming and defoaming agents may be employed, such as sodium soaps including sodium oleate for foaming and octyl alcohol or certain silicones for defoaming.

Optionally the flexibility and softness of the fabric can be increased by the addition of a hydrophobic external plasticizer to the binder composition without loss of desirable properties. Examples of external plasticizers which are suitably used include dibutoxyethylphthalate, dibutyl phthalate, tricresyl phosphate and low molecular weight polyesters. A typical plasticizer as isodecyl diphenyl phosphate marketed by Monsanto Chemical Company under the trademark "Santocizer 148." These external components may be added just before application if their stability in the dispersion or solution is low, or they may be formulated into the aqueous dispersion of the binder and stored if their stability in aqueous dispersion is high.

Various thermosetting resins may optionally be added to the binder latex to improve the strength of the resultant fabric. Typical of the useful thermosetting resins are the condensates such as the melamine-formaldehyde and ureaformaldehyde condensates, the methylated and methoxy alkyl cyclic ureas, and the methylolated and methoxyalkyl carbamates. Typically the thermosetting resins are used in quantities not exceeding 20 weight percent, based on the monomers used in the polymerization, the resins being added in solution to the binder latex with agitation before, during or after polymerization.

An acid catalyst may be optionally included in the aqueous dispersion at the time it is applied to the fibrous web or it may be applied to the fibrous web before or after the copolymer is applied. Examples of acidic catalysts that may be employed include oxalic acid, dichloroacetic acid, para-toluenesulfonic acid, and acidic salts such as ammonium sulfate or chloride and amine salts, such as the hydrochloride of 2-methyl-2-aminopropanol-1.

The impregnated web is then drried and cured. Thus, the fabrics are suitably dried by passing them through an air oven or the like and then through a curing oven. Ordinarily, drying is effected at 150.degree.-200.degree.F. for 4-6 min., followed by curing at 300.degree.-310.degree.F. for 3-5 min. or more. However, other time-temperature relationships can be employed as is well known in the art, shorter times at higher temperatures or longer times at lower temperatures being used. For example, the curing step can be carried out at 280.degree.F. for about 15 min. or more. However, economic considerations make the use of excessively long times undesirable, and the upper temperature limit is governed by the nature of the fibers. Temperatures which degrade the fibers are, of course, avoided. However, if the fibers are heat resistant, temperatures even as high as 350.degree.F. or higher can be used with times of 5-10 min. or more. In some cases, if desired, the drying and curing can be effected in a single exposure or step, e.g., at 300.degree.F. for 5-10 min.

A preferred binder composition (including the surfactant of the present invention) is set forth below, all parts being by weight: 60-80 parts butyl acrylate 40-20 parts vinyl acetate 1.0-3.0 parts acrylamide 1.0-4.0 parts N-methylol acrylamide 1.5-3.5 parts bis-tridecyl sodium sulfosuccinate

EXAMPLES

The following examples illustrate the efficacy of the present invention, and are intended to be illustrative only and not limitative of the invention. All parts are by weight unless otherwise indicated.

The procedures utilized in Examples 14-15 to determine fabric absorbency are based on the following test format. A sample (4 in. .times. 4 in.) of the fabric is folded twice to give a 2 in. .times. 2 in. square which is then passed between steel rollers at 60 p.s.i. A paper clip is attached to weight the sample, which is then placed on the top of a water bath in a 400 ml beaker at ambient temperature. The time required for the sample to become saturated is the "wet" time and the time required for the sample to sink completely below the surface of the water is the "sink" time. The recorded "wet" and "sink" times are the average of four tests.

The initial absorbency test is conducted on a sample which has been conditioned to ambient or room temperature. The absorbency test after two extractions is conducted on a sample which has undergone two of the following extraction cycles. The sample (approximately 3 grams) is soaked while tumbling for 30 minutes in a half gallon jar containing 1,500 ml of distilled water, after which the liquid is squeezed out and the web is dried at room temperature. The absorbency test after steam sterilization is conducted on a sample which has been steam sterilized for 30 minutes at 130.degree.C prior to testing.

EXAMPLES 1-12

The identity and quantity of each of the additions utilized in each "addition" of these examples are indicated in Table I. Addition "A" is added to a 2 liter flask equipped with agitator, condenser and nitrogen purge. 10% of Addition "B" is also added to the flask, and the contents are then purged with nitrogen while heating to 70.degree.C. The contents are maintained for 15 minutes at 75.degree.C to insure initiation of polymerization, after which Additions "C" and "D," and the remainder of Addition "B" are added uniformly over a 4 hour period, the temperature of the contents being maintained at 75.degree.C. After the slow additions are completed, the contents are maintained for an additional 90 minutes at 75.degree.C, after which the contents are cooled (30.degree.C) and discharged.

EXAMPLE 13

The identity and quantity of the ingredients utilized in each "addition" of this example are indicated in Table II. Addition "A" is added to a 1 liter Paar reactor capable of withstanding 1,000 psig, and equipped with two pumps, a nitrogen purge, and evacuating means. The contents are then subjected to two purging cycles, each purging cycle consisting of a nitrogen purge to 100 psig, followed by evacuation to 25 inches of mercury. After completion of the two purging cycles, and while the contents are still under vacuum, mild agitation is commmenced and Addition "B" is permitted to be sucked into the reactor. The contents are then heated to 70.degree.C while adding a sufficient quantity of ethylene (Addition "C") to maintain a pressure of 750 psig. When the contents reach 70.degree.C, 10% of Addition "E" is added; after a 20 minute hold, Addition "C" is added uniformly over 4 hours, and Addition "E" is added uniformly over 6 hours, the pressure being maintained at 750 psig throughout the slow additions by the addition of ethylene (Addition "C"). After the slow additions are completed, the contents are held at 75.degree.C for 1 hour, and finally cooled, vented and discharged.

EXAMPLE 14

The identity and quantity of the ingredients utilized in each "addition" of this example are indicated in Table II. Addition "A" is added to the reactor of Example 13, and two purging cycles similar to those of Example 13 are performed. Thereafter Addition "B" is allowed to be sucked into the reactor. Next the contents are heated to 50.degree.C., with sufficient ethylene (Addition "C") being introduced to maintain a pressure of 500 psig. Immediately upon the contents reaching the temperature of 50.degree.C., Addition "D" is uniformly added over a period of 6.5 hours, and 30 minutes after that temperature is reached, Additions "E" and "F" are added uniformly over a period of 5.0 hours. Upon completion of the slow additions, the contents are held for 15 minutes, and then Addition "G" is added over a 50 minute period. The contents are then agitated at 50.degree.C for 30 minutes, and finally cooled to 45.degree. C before venting and discharge.

EXAMPLE 15

Each of the binder latices prepared according to Examples 1-14 is diluted with water to 13 weight percent solids. Carded rayon test webs weighing 200 grains per sq. yard are then saturated with the diluted binder latices to provide a 20 weight percent dry resin add-on (i.e., 20 parts dry binder per 100 parts of web). The impregnated webs are then dried at room temperature, and cured for 2 minutes at 130.degree.C in a forced hot air oven. The dried and cured test webs are then tested, the results being indicated in Table III.

Table III illustrates that while all fabrics (except those formed with the binders of Examples 8 and 12) displayed a good initial absorbency of less than 300 seconds, only the fabrics containing the surfactant of the present invention also displayed a good absorbency of less than 300 seconds after two extractions or steam sterilization. More particularly, Examples 8 and 9 illustrate that the use of normal and even excessive amounts of a conventional anionic emulsifying agent such as SDBS (sodium dodecyl benzene sulfonate) does not provide permanent absorbency. Examples 10-12 illustrate that the addition of a second conventional anionic emulsifying or wetting agent such as Aerosol OT (di-octyl sodium sulfosuccinate), di-decyl sodium sulfosuccinate, and di-dodecyl sodium sulfosuccinate, respectively, does not provide permanent absorbency.

EXAMPLE 16

This example illustrates the utility of bistridecyl sodium sulfosuccinate as an additive for commercially available latex binders to improve the longevity of their absorbency characteristics.

Four commercial binder latices of the National Starch and Chemical Corporation of Plainfield, N.J. are tested according to the procedures of Example 15 for their utility in forming permanent absorbing fabrics. Each of the samples tested had sink times in excess of 300 seconds after two extractions. Next the same commercial binder latices are modified by the addition of 1.53 weight percent of a surfactant (based on the resin solids), the surfactant being added as a 20% active mixture of surfactant, benzyl butyl phthalate, and isopropanol (20/60/20 on a weight basis). When the solution contained di-octyl sodium sulfosuccinate as surfactant, the samples produced had sink times in excess of 300 seconds after two extractions. When the solution contained bistridecyl sodium sulfosuccinate as surfactant, the samples had the low sink times indicated in Table IV.

This example also illustrates the efficacy of bisalkyl sodium sulfosuccinates having higher molecular weight alkyl group substituents relative to those having lower molecular weight alkyl group substituents. This example further illustrates that the surfactant of the present invention may be added to the binder latex after polymerization of the binder latex has been completed.

To summarize, a permanently absorbent nonwoven fabric has been described which is particularly well suited for the industrial, hospital and consumer usages where high absorbency after repeated usage is required. The permanent absorbency is achieved by utilization of a specific surfactant which may be incorporated into the fabric either as a component of the crosslinkable binder during polymerization or as a blend of a crosslinkable polymeric binder and the special surfactant.

Now that the preferred embodiments of the present invention have been described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the spirit and scope of the present invention is to be considered as defined not by the foregoing disclosure, but only by the appended claims.

TABLE I __________________________________________________________________________ EXAMPLES 1 2 3 4 5 6 7 8 9 10 11 12 __________________________________________________________________________ INGREDIENTS ADDITION "A" WATER 430 430 430 430 430 430 430 430 430 430 430 430 SDBS (20%) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 TRITON X-305 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 AMMONIUM PERSULFATE 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 ADDITION "B" VINYL ACETATE 400 -- 200 80 -- 200 200 200 200 200 200 200 BUTYL ACRYLATE -- -- 200 -- 240 200 200 200 200 200 200 200 ETHYL ACRYLATE -- 400 -- 320 120 -- -- -- -- -- -- -- ACRYLONITRILE -- -- -- -- 40 -- -- -- -- -- -- -- BIS-TRIDECYL SODIUM SULFOSUCCINATE (80%) 10.0 15.0 25.0 7.5 25.0 25.0 25.0 -- -- -- -- -- DI-DODECYL SODIUM SULFOSUCCINATE (100%) -- -- -- -- -- -- -- -- -- -- -- 6.0 DI-DECYL SODIUM SULFO- SUCCINATE (50%) -- -- -- -- -- -- -- -- -- -- 25.0 -- GLYCIDYL METHACRYLATE -- -- -- -- 10.0 -- -- -- -- -- -- -- ACRYLIC ACID -- -- -- -- -- 15.0 -- -- -- -- -- -- ADDITION "C" WATER 120 120 120 120 100 100 100 120 120 120 120 100 SDBS (20%) 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 60.0 15.0 15.0 15.0 TRITON X-305 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 ACRYLAMIDE -- -- -- -- -- -- 15.0 -- -- -- -- -- N-METHYOL ACRYLAMIDE (60%) 10.0 20.0 25.0 7.5 -- -- -- 20.0 20.0 20.0 20.0 25.0 AEROSOL-OT -- -- -- -- -- -- -- -- -- 5.0 -- -- ADDITION "D" WATER 50 50 50 50 50 50 50 50 50 50 50 50 AMMONIUM PERSULFATE 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 __________________________________________________________________________

TABLE II ______________________________________ EXAMPLE 13 14 ______________________________________ INGREDIENTS ADDITION A WATER 330 260 SODIUM ACETATE 0.85 0.5 IGEPAL CO-977 8.5 SDBS (20%) 3.2 2.2 SODIUM BISULFATE 2.40 FERROUS SULFATE 44 ppm SODIUM PERSULFATE 1.7 ADDITION B VINYL ACETATE 20 VINYL CHLORIDE 30 ADDITION C ETHYLENE (cp GRADE) 750 psig 500 psig ADDITION D VINYL ACETATE 280 BIS-TRIDECYL SODIUM SULFOSUCCINATE 15.0 N-METHYLOL ACRYLAMIDE (60%) 16.7 IGEPAL CO-977 4.0 SDBS (20%) 2.0 WATER 130 45 SODIUM ACETATE 1.0 SODIUM FORMALDEHYDE SULFOXYLATE 0.41 ADDITION E WATER 40 1.5 AMMONIUM FERSULFATE 2.40 VINYL CHLORIDE 30 ISO-BUTOXY METHYL ACRYLAMIDE 8.5 BIS-TRIDECYL SODIUM SULFOSUCCINATE 10 ADDITION F WATER 35 SDBS (20%) 35 ADDITION G BIS-TRIDECYL SODIUM SULFOSUCCINATE 10 SANTOCIZOR 140 15 ISOPROPYL ALCOHOL 5 ______________________________________

TABLE III __________________________________________________________________________ INITIAL ABSORBENCY ABSORBENCY AFTER 2 ABSORBENCY AFTER STEAM EXTRACTIONS STERILIZATION EXAMPLE NO. WET SINK WET SINK WET SINK __________________________________________________________________________ 1 <1 2.0 6.6 12.9 2 <1 1.9 5.9 107 3 <1 2.5 26.2 40.8 2.0 <10 4 <1 2.1 25.5 70.5 5 <1.0 1.5 <1 5.4 6 <1 1.7 2.7 58.1 7 <1 1.5 1.5 7.6 8 >300 >300 >300 >300 9 <1.0 2.8 >300 >300 10 2.0 5.0 >300 >300 >300 >300 11 <1.0 2.05 >300 >300 12 >300 >300 >300 >300 13 <1 3.0 4.1 7.4 14 <1 3.0 37.5 37.5 __________________________________________________________________________

TABLE IV ______________________________________ SINK TIME BINDER LATICES After 2 Extractions ______________________________________ 25-4280* 215 25-4445* 54 25-4260* 200 25-2833** 80 ______________________________________ *A crosslinkable binder latex containing an acrylate copolymer, and at least one conventional emulsifier. **A crosslinkable binder latex containing a vinyl acetate/acrylate copolymer, and at least one conventional emulsifier.

Claims

We claim:

1. An absorbent nonwoven fabric consisting essentially of

a. a web of fibers,

b. about 5.0 to about 100 weight percent, based on the fibers, of crosslinkable binder, and

c. about 0.2 to about 10.0 weight percent, based on said fibers and binder, of at least one surfactant consisting of at least one salt of a bis-alkyl sulfosuccinate having alkyl substituents each alkyl containing 13-14 carbon atoms.

2. The fabric of claim 1 wherein said surfactant consists of at least one alkali metal salt of bis-tridecyl sulfosuccinate.

3. The fabric of claim 1 wherein at least one of said alkyl constituents is branched.

4. The fabric of claim 1 wherein said surfactant consists of bis-tridecyl sodium sulfosuccinate hving at least one branched alkyl substituent.

5. The fabric of claim 3 wherein said binder consists essentially of a copolymer of at least two .alpha.,.beta.-monoethylenically unsaturated monomers, one of said monomers being effectively monofunctional and another of said monomers being effectively polyfunctional, said effectively monofunctional monomers comprising from about 85 to about 99 weight percent and said effectively polyfunctional monomers comprising from about 1 to about 15 weight percent of said binder.

6. The fabric of claim 5 wherein said copolymer is a crosslinkable emulsion copolymer.

7. The fabric of claim 5 wherein at least one of said .alpha.,.beta.-monoethylenically unsaturated monofunctional monomers is selected from the group consisting of alkyl acrylate, vinyl acetate, ethylene, vinyl chloride, and mixtures thereof, said alkyl acrylate having a C.sub.1 -C.sub.12 alkyl group.

8. The fabric of claim 5 wherein at least one of said .alpha.,.beta.-monoethylenically unsaturated polyfunctional monomers is selected from the group consisting of acrylamide, isobutoxy methyl acrylamide, and N-methyol acrylamide.

9. The fabric of claim 3 containing about 0.3 to about 2.0 weight percent, based on said fibers and binder, of said surfactant.

10. The fabric of claim 3 containing about 10 to about 70 weight percent, based on said fibers, of said binder.

11. The fabric of claim 1 comprising on a weight basis:

a. a loosely assembled web of said fibers;

b. about 10 to about 70 weight percent, based on said fibers, of said binder containing at least one .alpha.,.beta.-monoethylenically unsaturated monofunctional monomer selected from the group consisting of alkyl acrylate, vinyl acetate, ethylene, vinyl chloride, and mixtures thereof, said alkyl acrylate having a C.sub.1 -C.sub.12 alkyl group, and at least one .alpha.,.beta.-monoethylenically unsaturated polyfunctional monomer selected from the group consisting of acrylamide, N-methylol acrylamide and isobutoxy methyl acrylamide, said monofunctional monomers comprising from about 95 to about 98 weight percent and said polyfunctional monomers comprising about 2 to about 5 weight percent of said binder; and

c. about 0.3 to about 2.0 weight percent, based on said fibers and binder, of bis-tridecyl sodium sulfosuccinate having at least one branched alkyl substituent.

12. The fabric of claim 3 wherein said binder and said surfactant comprise on a weight basis about 60-80 parts butyl acrylate, 40-20 parts vinyl acetate, 1.0-3.0 parts acrylamide, 1.0-4.0 parts N-methylol acrylamide, and 1.5-3.5 parts bis-tridecyl sodium sulfosuccinate.