MX2008009681A - Salt-sensitive binders for nonwoven webs and method of making same - Google Patents

Abstract

A solution with a salt-sensitive polymer binder for use in fibrous webs, where the binder contains a copolymer of carboxylic acid monomer units and ethylenically unsaturated monomer units. The binder solution is prepared by emulsion-polymerizing a copolymer and then neutralizing the copolymer with base to make it water soluble. The binders provide a higher wet strength in concentrated salt solutions than in deionized water, and are particularly suitable for strengthening nonwoven fibrous webs in disposable articles such;as wet-wipes, personal care products, diapers, and the like.

Description

SALT SENSITIVE AGGLUTINANTS FOR NON-WOVEN NETWORKS AND METHOD TO MAKE THEMSELVES TECHNICAL FIELD The present invention relates generally to salt-sensitive binders for non-woven webs, and more particularly to water-soluble binder compositions which are converted from polymers to emulsion. The terpolymers of methacrylic acid / alkyl acrylate / hydrophobic monomer are preferred.

BACKGROUND OF THE INVENTION Wet strength is a desirable attribute for many disposable paper products that require to maintain their wet integrity for an extended period of time before and during their intended use. Such products include toilet paper, diapers, personal care products and pre-moistened items such as baby wipes and household cleaning towels. Wet strength, however, is often an unnecessary and undesirable feature in paper products. Many paper products are discarded after brief periods of use in landfills, incinerators, etc., which is inconvenient and can pose a significant load on the solid waste stream. Therefore, in many cases, it is preferred to direct the paper products used to the municipal sewage treatment facilities or to private septic systems. Unfortunately, this procedure is often not available if the product is "not cleaned with a water discharge". Clogging of sewer and septic systems may result if the product permanently maintains the strength properties resistant to hydrolysis. To address this problem, efforts have been made to make binders that will provide paper products with sufficient "temporary" wet integrity in the presence of salt solutions, but minimal integrity when exposed to large amounts of wastewater, so that they pass through plumbing and disintegrate in sewer and / or septic systems. Specifically, it has been attempted to produce disposable fibrous products that maintain a relatively high wet strength in the presence of solutions with high ion concentrations, but become more dispersible when they come into contact with solutions having a lower ion concentration. These water-dispersible polymer formulations, sensitive to ions, are well known in the art. There is disclosed, for example in U.S. Patent No. 6,429,261, to Lang et al., A polymer formulation comprising an activatable copolymer of acrylic acid, NaAMPS, butyl acrylate and 2-ethylhexyl acrylate, as well as a emulsion polymer of non-crosslinkable co-binder comprising poly (ethylene-vinyl acetate), wherein the polymer formulation is insoluble in a neutral salt solution containing at least about 1 weight percent salt, and wherein the activatable copolymer is water soluble, which contains up to about 200 ppm of one or more multivalent ions. The polymers in Lang are typically prepared by solution polymerization. U.S. Patent No. 6,683,129 to Eknoian discloses salt-sensitive binders in aqueous emulsions comprising methacrylic acid and acrylate monomers, such as butyl acrylate and 2-ethylhexyl acrylate. Ion-sensitive binders are applied as emulsion compositions. U.S. Patent No. 6,291, 372, to Mumick et al., Discloses ion sensitive binders for a fibrous material. A binder of a water-soluble polymer is described, which includes about 25 to about 85 weight percent of an unsaturated carboxylic acid ester terpolymer; as well as from about 5 weight percent to about 35 weight percent of a divalent ion inhibitor, and from about 10 weight percent to about 60 weight percent of a crosslinkable hydrophilic emulsion polymer. The polymeric binder is useful for bonding absorbent nets of the kind used in personal care products, such as pre-moistened wipes.

U.S. Patent No. 5,631, 317, to Komatsu et al., Discloses a process for producing a self-dispersible and salt-sensitive polymer. The formulations include a) from about 35 to about 65 weight percent acrylic acid; b) from about 10 to about 45 weight percent of an acrylic ester, such as 2-ethylhexyl (meth) acrylate or lauryl (meth) acrylate; and c) from about 20 to about 45 weight percent of a second acrylic ester, such as ethyl (meth) acrylate, isopropyl (meth) acrylate or n-butyl (meth) acrylate. The polymers in Komatsu are polymerized in a mixture of water and an organic solvent, and the solvent is subsequently evaporated, so that the binder is provided in an aqueous dispersion. Still other references of interest with respect to salt sensitive binders include the following: U.S. Patent No. 5,312,883 and U.S. Patent No. 5,31 7,063, both to Komatsu et al., Which describe water-soluble salt-sensitive polymers; and U.S. Patent Nos. 6, 127,593 and 6,433,245, both from Bjorkquist et al., which describe fibrous structures that are cleaned with a water discharge. Many of the references discussed above, such as Lang and Komatsu produce ion-sensitive binders by solution polymerization. Others, such as Eknoian, employ emulsion polymerization and provide binders as emulsion compositions. These procedures are typical in applications of salt sensitive binders. In contrast to the above processes, the emulsion polymers converted to water-soluble polymers, whereby the emulsion polymer is brought into solution by increasing the pH, until now, have not been used as salt-sensitive binders. Polymers solubilized in an alkali, polymerized in emulsion, have been used mainly in applications such as thickeners. The thickeners are added to the aqueous systems to increase the viscosity of a desired level and are often added to the materials such as paints, polishing and cleaning compositions, pharmaceuticals, among others. U.S. Patent No. 5,073,591 to Tsaur discloses a method for producing a soluble emulsion polymer in an alkali to be used as a thickener. The method comprises emulsion polymerizing in an aqueous medium a polymer having an acidic portion and an amino moiety. The emulsion polymerization occurs in an acid medium and then the pH of the emulsion is raised to dissolve the polymer. Tsaur indicates that when the pH of the emulsion increases, the dispersed composition dissolves, and the viscosity increases rapidly. U.S. Patent No. 4,384,096 to Sonnabend discloses a liquid emulsion copolymer containing a carboxylic acid monomer, a monovinyl ester monomer and an ester monomer of a nonionic vinyl surfactant. The emulsions are stable as dispersions in solutions with a pH below 5.0, but dissolve as the pH increases. It is said that dissolved emulsions are useful as thickeners in applications such as latex paints. Other references of interest include the Patent of the States No. 6,063,857, Greenblatt et al., Which discloses a soluble emulsion polymer in a hydrophobically modified alkali that is neutralized by at least 60% of its acid groups. The polymer contains, as the hydrophobic monomer, an ester of a surfactant. The polymers in Greenblatt can be used as thickening agents in paints, adhesives, non-wovens, textiles, etc. Similarly, U.S. Patent Application Publication No. 2004/0151886, by Bobsein et al., Discloses a composition for paper coatings containing 1) a binder copolymer and 2) a soluble emulsion polymer. in a hydrophobically modified alkali. United States Patent No. 4, 801, 671, by Shay et al., Discloses an alkali-soluble copolymer containing a monomer with characteristics of a surfactant and a monomer with a carboxyl functionality. Here again, the copolymers are used as thickeners. Although the substantial increase in viscosity associated with water-soluble emulsion polymers is beneficial in thickener applications, it is an undesirable property in the field of non-woven binders because the viscosity must be kept within processable limits.

Emulsion polymerization offers several advantages in the production of non-woven binders. For example, emulsion polymerization is cost effective and environmentally safe, allows the production of a composition with high solids content without the need to eliminate unwanted solvent. However, the present Applicants have observed that the emulsion binders may not "activate" as well as the binders in solution. "Activation" is a critical mechanism in salt sensitive applications, so that the polymer becomes insoluble in concentrated solutions of salts, although it remains dispersible when it comes into contact with solutions containing a lower ion concentration. Also, the mechanism that forms a film in the emulsion polymers is different and less predictable than in the polymers in solution. On the other hand, solution polymerization is disadvantageous from a processing point of view because the solvent must be removed, and the resulting compositions typically do not reach as high a solids content as can be produced with emulsion polymerization. Thus, despite contributions to salt-sensitive binders and the products that incorporate them, there is still a need for high-quality, salt-sensitive binders that can be produced safely and efficiently.

BRIEF DESCRIPTION OF THE INVENTION It has been discovered in accordance with the present invention that higher binder solutions can be prepared having salt sensitive polymer resins by emulsion polymerization of the desired copolymer, and then neutralizing the emulsion composition to produce a water soluble polymer. Particular preference is given to methacrylic acid terpolymers (MAA), such as the polymers of methacrylic acid / butyl acrylate / isobornyl methacrylate described in the following examples. There is provided in one aspect of the present invention, a method for producing a solution having a salt sensitive binder for a non-woven article, including the steps of: i) preparing an emulsion composition by emulsion polymerization in an aqueous medium a copolymer resin having from about 5 to about 80 weight percent units of a carboxylic acid monomer, and from about 10 to about 95 weight percent units of an ethylenically unsaturated monomer; i) converting the emulsion composition into a solution by neutralizing the resin with the base until it is soluble in water; and iii) controlling the viscosity of the solution to be less than about 2,000 cps at 23 ° C.

The process may also include the step of diluting the emulsion composition to a solids level of 10 to 35 percent before neutralization, and preferably at a level of 1 to 25 percent. Desirably, the viscosity of the solution should be controlled to be less than 1,000 cps, and more preferably less than 500 cps, or less than 200 cps. In another aspect of the present invention, there is provided a method for making a non-woven network with a polymeric binder, the method comprising the steps of i) preparing an emulsion composition by emulsion polymerizing in an aqueous medium a copolymer resin having about 5-80 weight percent carboxylic acid units and about 10-95 percent units of an ethylenically unsaturated comonomer; ii) converting the emulsion composition to a binder solution by neutralizing the resin with a base until it is soluble in water; iii) provide a fibrous network; and iv) applying the binder solution to the network, wherein the binder provides a characteristic wet strength index of less than 25 in deionized water, and an elevation of the wet strength index of at least 15 points in a solution of NaCI at 10 percent. Typically, the inventive method also includes the step of drying the fibrous web. In yet another aspect of the invention, there is provided a salt sensitive binder polymer comprising about 10-70 weight percent methacrylic acid units, about 0-90 percent of alkyl acrylate units having from 2 to 4 carbon atoms in the alkyl portion, and from about 2-55 weight percent of a hydrophobic monomer units selected from the group of alkyl (meth) acrylamides having 4 to 12 carbon atoms in the alkyl portion, straight or branched chain alkyl methacrylate units, having from 4 to 6 carbon atoms in the alkyl portion; bicycloalkyl (meth) acrylates with from 4 to 20 carbon atoms in the cycloalkyl portion; and combinations thereof, wherein the polymer of the binder is neutralized so that it is soluble in water. The alkyl acrylate units suitably comprise butyl acrylate. In some embodiments, the hydrophobic monomer units include a substituted or unsubstituted bicycloalkyl (meth) acrylate with 6-14 carbon atoms in the cycloalkyl moiety, for example, isobornyl methacrylate. Other suitable hydrophobic monomer units include n-butyl methacrylate and alkyl (meth) acrylamides with 6-10 carbon atoms in the alkyl portion, such as tertiary N-octyl acrylamide. Also, the polymer desirably contains less than 5 weight percent straight or branched chain alkyl acrylate units, ie, non-cyclic, with 8-12 carbon atoms in the alkyl portion; the presence of less than 5% by weight (or more preferably absence) of alkyl acrylates of (C8-Ci2) can be beneficial due to cost and processing considerations.

In yet another embodiment of the present invention, there is provided a salt-sensitive aqueous binder solution for a non-woven network comprising water, and a water-solubilized resin composition that is converted from a polymer to an emulsion, wherein the composition includes i) a copolymer with about 5-80 weight percent carboxylic acid units and 10-95 weight percent units of an ethylenically unsaturated comonomer, and ii) an amount of emulsifier effective to maintain a stable emulsion during the polymerization of the polymer. The binder provides a characteristic wet strength rating of less than 25 in deionized water and an elevation in the characteristic wet strength index of at least 15 points in 10% NaCl. The emulsifier is present in the binder solution in suitable amounts of 0.05 to 10 weight percent, or 0.2 to 5 weight percent of total solids. Preferably, the emulsifier includes one or more polymerizable surfactants that are polymerized in the main chain of the salt-sensitive resin. More preferably, the emulsifying agent consists of polymerizable surfactants. The copolymer resin may have from about 20 to about 65 weight percent carboxylic acid units. The carboxylic acid units are more preferably methacrylic acid; however, other carboxylic acid units may include beta-carboxyethyl acrylate or a monoalkyl ester of maleic acid such as monoethyl maleate. The copolymer desirably has from about 30 to about 70 weight percent units of the ethylene unsaturated comonomer. Exemplary comonomer units include alkyl acrylates with 1-4 carbon atoms in the alkyl portion, such as butyl acrylate. The units of the ethylenically unsaturated monomer may include units of a hydrophobic monomer that are selected from the group consisting of alkyl (meth) acrylamides having from 2 to 15 carbon atoms in the alkyl portion, straight chain alkyl methacrylate units or branched having from 4 to 12 carbon atoms in the alkyl portion; straight or branched chain alkyl acrylates having from 5 to 12 carbon atoms in the alkyl portion; Substituted or unsubstituted bicycloalkyl (meth) acrylates; and combinations thereof. The hydrophobic monomer units may be present in the copolymer in suitable amounts of 2 to 55 weight percent, and preferably 3 to 20 weight percent. The units of the ethylenically unsaturated monomer may also include units of a hardening monomer in preferred amounts of 2 to 55 weight percent, or 10 to 50 weight percent. The hardening monomer generally has a glass transition temperature in the range of 40 ° C to 140 ° C, and preferably 80 ° C to 120 ° C. The hardening monomer used in the invention is usually methyl methacrylate. The salt sensitive resin preferably has less than 0.25 percent crosslinkable monomers, the presence of which can adversely affect the dispersibility properties of the binder. The copolymer employed in the invention generally has a molecular weight of 40,000 to 400,000 g / mol, and more typically of 60,000 to 250,000 g / mol. The copolymer in the inventive binder solution is preferably solubilized at least to the point where it reaches its maximum optical transparency in water. Typically, the resin is solubilized by neutralizing it with a base that is stoichiometrically equivalent to 5-55 mole percent of the carboxylic acid units in the emulsion. Preferably, the cations in the base should be monovalent cations, such as sodium or potassium salts. The binder solution generally has a pH of from about 4 to about 9, with more preferred pH values being from about 6 to 8, or from about 6 to 7. The binder of the invention generally has salt sensitive properties, so which provides a characteristic wet strength index of less than 25 in deionized water and an elevation in the characteristic wet strength index of at least 35 points in a 10% NaCl solution. Preferably, the binder provides a wet strength index of less than 10 in deionized water with an index elevation of at least 50 points in a 10% NaCl solution. The binder has typical values of the characteristic wet strength index in a 10% NaCl solution of at least about 40, and a characteristic wet strength index value of less than about 10 in DI water. Preferably, the binder provides a characteristic wet strength rating of at least about 80 in 10% NaCl, and less than about 5 in DI water, and more preferably, provides an index of at least about 100 in NaCl. at 10%, and less than about 5 in DI water. The present invention also provides a disposable article having a non-woven web that is provided with a salt-sensitive polymeric binder comprising an emulsion polymerized water-solubilized resin composition having: (i) a copolymer with about to about 80 weight percent carboxylic acid units, and from about 10 to about 95 weight percent units of the ethylene unsaturated comonomer; and (ii) an amount of emulsifier effective to maintain a stable aqueous emulsion with the polymer as it polymerizes, wherein the binder provides a characteristic wet strength index of less than 25 in deionized water and an elevation in the characteristic wet strength index of at least 20 points in an aqueous solution of 0% NaCl. The disposable article is preferably in contact with a wetting composition that includes at least about 0.1 weight percent of an inorganic salt. The disposable article can be a wet wipe, a household wipe, a diaper, an incontinence undergarment or a feminine care product. The additional features and advantages of the present invention will become apparent from the discussion that follows.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is described in detail below with reference to the following drawings: Figure 1 is a graph comparing the salt-sensitive properties of a terpolymer of methacrylic acid with a corresponding acrylic acid terpolymer. Figure 2 is a graph of the wet strength ratings of Examples 1-9, plotted against the degree of neutralization. Figure 3 is a graph of the wet strength ratings of Examples 10-15, plotted against the amount of chain transfer agent used during the polymerization.

Figure 4 is a graph of the wet strength ratings of Examples 22-25, plotted against the amount of the acid monomer in the binder polymer.

DETAILED DESCRIPTION OF THE INVENTION The present invention is described in detail below with reference to the various examples. The modifications to the particular examples within the spirit and scope of the present invention, set forth in the appended claims, will be readily apparent to those skilled in the art. Unless stated otherwise, the terms should be considered in accordance with their ordinary meaning. Percent, for example, refers to percent by weight, unless the context indicates otherwise. Following are some exemplary definitions of the terms used in this specification and the appended claims. The phrase "an amount of emulsifier effective to maintain a stable aqueous emulsion" as the binder polymerizes, refers to an amount of emulsifying agent necessary to prevent the polymer and water from separating into non-emulsified phases for at least the time of polymerization. As a person skilled in the art will recognize, the effective amounts of emulsifier necessary to maintain a Stable aqueous emulsion will vary depending on the composition of the polymer and other factors. "Non-woven", "nonwoven web" and similar terminology, refer to materials formed of loosely assembled fibers, which are attached, in part, by a polymeric binder. The binder plays an important role in the properties of the material, such as the strength of the non-woven material. The phrase, "binder solution", and similar terminology, refers to an aqueous composition containing an emulsion polymerized binder, wherein the polymer is neutralized with a base at least to the point where the composition is no longer opaque . Aqueous polymer emulsions (also referred to herein as "latex"), are typically milky white opaque liquids. When the emulsion copolymer employed in the present invention is neutralized, it begins to solubilize and the liquid becomes more transparent. For the purposes of the present invention, the emulsion copolymer is preferably neutralized to an optimum point, whereby the aqueous composition becomes as transparent as possible. Similarly, the phrases "water solubilized" and "water soluble", when used with reference to an emulsion polymerized binder, mean that the polymer has been neutralized with a base at least to the point where it would be optically translucent as a 20 weight percent aqueous composition.

"Wet tensile strength" is the tensile strength of a net when wet. The wet tensile strength as used herein is measured according to the TAPPI UM 656 procedure and reported in gf / inch, consistent with the tests in the Examples illustrated below. "Wet strength index" as used herein, is defined as the transverse wet tensile strength standardized in gf / inch (adjusted for the basis in weight, see the following Examples) of a divided network between the amount of binder complement. "Characteristic wet strength index" refers to the Wet Strength Index that a binder or binder solution provides to a standard network when subjected to standard conditions, and soaked in a specific solution. For the purposes of this invention, the characteristic wet strength index is a property of the binder which is measured as set forth in the following Examples. Consequently, the characteristic wet strength index of a binder is determined by finding the average transverse tensile strength to the average machine of the samples that have been cut from a Whatman No. 4 filter paper, provided with an amount of binder solution equivalent to a complement of 10-16 percent, dried, and then soaked for 60 minutes in the prescribed solution. The resulting tensile strength is normalized to a basis weight of 1 12.5 gsm, which is a representative average basis weight for the test purposes. The characteristic wet strength index is then calculated by dividing the normalized wet tensile strength (in units of gf / inch) by the percent of the binder complement. "Salt sensitive", when used with reference to a binder, refers to the characteristic of a binder to provide a higher wet strength index in concentrated salt solutions compared to its wet strength index in deionized water . The compositions of the present invention typically exhibit a wet strength index in deionized water of less than 25 and a characteristic wet strength index in an aqueous solution of 10% NaCl that is at least 15 points higher than that exhibited in deionized water, that is, the binder exhibits at least an elevation of the index of 15 points in solutions with 10% salt. In preferred embodiments, the binder exhibits an elevation of the wet strength index in an aqueous solution of 10% NaCl of at least 20 points, 35 points or even more preferably at least 50 points. Thus, the binders used in the present invention are dispersible in deionized water and non-dispersible in solutions containing high concentrations of ions. Although 10% NaCl solutions are used as a reference for concentrated salt solutions, it is to be understood that the binders of the present invention will typically be non-dispersible in aqueous solutions having a salt content of less about 0.5 percent by weight or, perhaps even less. Fibrous networks exhibit a similar salt-responsive dispersibility behavior when provided with the binder. The dispersibility of a network is inversely proportional to the wet tensile strength, that is, high wet strengths correspond to a low dispersibility. The salt-sensitive binder compositions provided in the present invention comprise water and a water-solubilized, emulsion-polymerized binder composition, including a copolymer comprising from about 5 to about 80 weight percent of a monomer units of carboxylic acid and from about 10 to about 95 weight percent units of an ethylenically unsaturated comonomer. The carboxylic acid monomers used in the inventive binder compositions typically comprise metylic acid. Although acrylic acid can be used in certain embodiments, it has been surprisingly discovered that the polymers of the invention which include metylic acid generally have superior salt-sensitive properties as compared to those made with an acrylic acid. Without wishing to be bound by theory, it is believed that metylic acid exhibits substantially improved results compared to acrylic acid, because the emulsion / neutralization polymerization process of the invention is less compatible than with more hydrophilic monomers, such as Acrylic acid It is observed, by example, in Figure 1, that the acrylic acid terpolymer of Example 18 does not exhibit sufficient strength in saline solutions as compared to the corresponding metylic acid terpolymer of Example 17. Additional carboxylic acid monomers that may be suitable include one or more of the following C3-C8 ethylenically unsaturated carboxylic acid monomers: maleic acid, crotonic acid, itaconic acid, fumaric acid, aconitic acid, vinylsulfonic acids and vinylphosphonic acids, acryloxypropionic acid, metyloxypropionic acid, beta-carboxyethyl acrylate, maleate of monomethyl, monoethyl maleate, monomethyl fumarate, monomethyl itaconate and the like, fatty acids such as lauroleic acid, myristoleic acid, palmitoleic acid, oleic acid, ricinoleic acid, linoleic acid, linolenic acid, eleostearic acid, laconic acid, gadoleic acid, arachidonic acid, erucic acid, clupanodonic acid, nisinic acid, and combinations of the same. The carboxylic acid may be present in the copolymer in an amount of 5 to 80 weight percent, or of about 20 to about 55 weight percent. The units of the ethylenically unsaturated monomers are well known in the art. These monomers may comprise (meth) acrylates, maleates, (meth) acrylamides, itaconates, vinyl esters, styrenics, acrylonitrile, functional nitrogen monomers, functional monomers of alcohol and unsaturated hydrocarbons. In some embodiments, the units of an ethylenically unsaturated monomer they also comprise hydrophobic monomers and / or hardening monomers, as discussed in detail below. Preferably, the units of the ethylenically unsaturated comonomer comprise alkyl acrylate monomers. The alkyl acrylate monomers used in the polymer composition may include alkyl and hydroxyalkyl esters of acrylic acid, wherein the alkyl portion has from 1 to 4 carbon atoms. The alkyl acrylates suitably have from 2 to 4 carbon atoms in the alkyl portion. Exemplary alkyl acrylate monomers include methyl acrylate, ethyl acrylate, butyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxybutyl acrylate, and the like. Butyl acrylate monomer is especially preferred. The copolymer generally comprises from about 30 to 70 weight percent of alkyl acrylate monomers. In some embodiments, the copolymer consists essentially of alkyl acrylate monomers and carboxylic acid monomers, so that other components are not present in amounts that affect the basic and novel characteristics of the inventive binders, namely, their sensitivity to salt . Also, although it is recognized that butyl acrylate and similar lower alkyl acrylates may be somewhat hydrophobic, they are not considered a "hydrophobic monomer" for the purposes of this invention. As indicated above, the ethylenically unsaturated monomers may comprise hydrophobic monomers in amounts such that the salt-sensitive copolymer includes from 2 to 55 weight percent of units of the hydrophobic monomer. The presence of a hydrophobic monomer can improve the binding properties of the binder, particularly in applications where hard water dispersibility is desired. The hydrophobic monomer is selected from alkyl (meth) acrylamides having from 2 to 15 carbon atoms in the alkyl portion, straight or branched chain alkyl methacrylates having from 4 to 12 carbon atoms in the alkyl portion, acrylates of straight or branched chain alkyl having from 5 to 12 carbon atoms in the alkyl portion, substituted or unsubstituted cycloalkyl (meth) acrylate and combinations thereof. The alkyl (meth) acrylamide component includes alkyl carbonylalkyl (meth) acrylamides and having from 2 to 15 carbon atoms in the alkyl portion, and preferably 4-12 or 6-10 carbon atoms in the alkyl portion. Exemplary monomers include those such as tertiary butyl acrylamide, tertiary octyl acrylamide, isopropyl acrylamide, and N- (1,1-dimethyl-3-oxobutyl) acrylamide. A preferred alkyl acrylamide is the N-tertiary octyl acrylamide (8 carbon atoms in the alkyl portion), which may have the following structures.

N-tertiary octyl acrylamide The units of a hydrophobic monomer may further comprise an alkyl methacrylate which generally should have from 4 to 12 carbon atoms in the alkyl portion, preferably from 4 to 6 carbon atoms in the alkyl portion. Suitable alkyl methacrylate monomers include n-butyl methacrylate. The hydrophobic monomer units may also include straight or branched chain alkyl acrylates having from 5 to 12 carbon atoms in the alkyl portion. Alkyl acrylates of examples of this type include hexyl acrylate and 2-ethylhexyl acrylate. However, although the alkyl methacrylate or hydrophobic alkyl acrylate monomers may be suitable in many embodiments, the salt sensitive copolymer used in the invention, preferably has less than 5 weight percent of (meth) acrylates of straight or branched chain alkyl, having from 8 to 12 carbon atoms in the alkyl portion, for example, 2-ethylhexyl acrylate. In reality, typically the copolymer is substantially free of hydrophobic monomers such as 2-ethylhexyl acrylate. the hydrophobic monomer units comprise a Cycloalkyl (meth) acrylate, (including bicycloalkyl (meth) acrylates), should contain from 4 to 20 carbon atoms in the cycloalkyl portion. Suitable cycloalkyl (meth) acrylate monomers include isobornyl acrylate, isobornyl methacrylate, cyclohexyl (meth) acrylate, 3,5,5-trimethylcyclohexyl (meth) acrylate, 4-tert-butylcyclohexyl (meth) acrylate and mixtures thereof. Preferably, the monomer is a bicycloalkyl (meth) acrylate having from 6 to 12 carbon atoms in the cycloalkyl portion; isobornyl methacrylate having the following structure, it is especially preferred: Isobornyl methacrylate The ethylenically unsaturated monomers used in the salt sensitive copolymer may also include hardening monomers in amounts such that the copolymer includes from 2 to 55 weight percent of the monomer hardener units. More preferably, the hardener monomer units are present in the ranges of 10 to 50 weight percent, or 1 to 45 weight percent.

As used herein, "hardening monomers" refer to monomers having a glass transition temperature (based on a homopolymer of that monomer) of at least -40 ° C. Preferably, the hardening monomer has a glass transition temperature of more than 0 ° C, and suitably in the ranges of 40 ° C-140 ° C or 80 ° C-1 20 ° C. It should be understood, of course, that many monomers used in the invention can be characterized as a hardening monomer and a hydrophobic monomer. Most preferably, the hardening monomer comprises methyl methacrylate having a glass transition temperature of about 05 ° C. It is believed that the addition of monomers that raise the glass transition temperature of the salt sensitive polymer may also favorably affect the binder activation properties. Monomers with substantial irreversible crosslinking characteristics generally should not be used with the polymers of the present invention, because a significant amount of crosslinking will adversely affect the dispersibility of the copolymer in water. The copolymer should generally contain less than about 1.0 percent by weight, suitably less than 0.25 percent by weight of the pre-crosslinkable monomers. Desirably, the copolymer should also contain less than 0.25% by weight of post-crosslinkable monomers, and preferably contain no post-crosslinkable monomer. The pre-crosslinkable monomers are crosslinked with themselves during the polymerization to build the molecular weight of the polymer, and should include monomers containing at least two vinyl end groups such as divinylbenzene, among others. The post-crosslinkable monomers are crosslinked with themselves after the polymer has been formed. The post-crosslinkable monomers generally require a catalytic or thermal induction to crosslink, and often can also be crosslinked with cellulosic substrates. Examples of post-crosslinkable monomers include methylol, which contains monomers such as methylol acrylamide. In most embodiments, the copolymer employed in the present invention contains less than 0.1%, and preferably no monomer exhibiting a significant crosslinking ability. The copolymers used in the present invention are produced by emulsion polymerization. A general method for emulsion polymerization is described in U.S. Patent Application Publication No. 2003/0164476, by Guo et al. The emulsion polymerization is typically carried out in an aqueous medium at a pH of less than about 5.0, preferably at about 2.0 and at temperatures of less than 100 ° C, and preferably in the range of 40 ° C to 80 ° C. ° C. Typically, a seeded or unseeded process is used to copolymerize the monomers in water with a surfactant. Polymerization occurs once the monomer and initiator are added to the charge. Polymerization can carried out in batches, in steps or continuously with the batchwise and / or continuous addition of the monomer in the conventional manner. Suitably, at least one emulsifier is present in the polymerization. The emulsifier is present in an amount that is effective to maintain a stable aqueous emulsion of the copolymer as it polymerizes. The emulsifying agents may include surfactants and / or protective colloids. Emulsifiers perform many functions in emulsion systems, including solubilizing hydrophobic monomers, determining the size of latex particles (typically, more emulsifier results in smaller latex particles), decreasing latex sensitivity to electrolytes and providing stability to the emulsion both during and after the polymerization. The amount of emulsifying agent is typically from about 0.05 weight percent to about 10 weight percent, preferably from 0.1 to 5 percent, based on the total weight of the total solids. For salt sensitive binders, the emulsifier should generally be present in sufficiently low amounts, so that the salt sensitive properties of the binder are not materially affected by the emulsifying agent; it is known, for example, that an excess of surfactant can impede the mechanism of activation of the resin. The emulsifiers used in the invention are preferably anionic, nonionic or cationic surfactants. Suitable anionic surfactants include fatty acid soaps, sulphonated fatty acids, alkyl carboxylates, alkyl sulphates, alkyl sulfonates, alkali metal alkyl aryl sulfonates, alkali metal alkyl sulfates, sulfonated alkyl esters, alkylaryl disulfonates, hydroxylalkane sulphates, alkane sulphites and phosphates and polyethoxylated alkylphenols, as well as sulfosuccinic acid esters; specific examples include sodium dodecylbenzene sulfonate, sodium disodium butylnaphthalene sulfonate, sodium lauryl sulfate, disodium dodecylphenyl ether disulfonate, disodium n-octadecylsulfosuccinate, disodium dioctyl sulfosuccinate, among others. Nonionic surfactants include the addition products of 5 to 50 moles of ethylene oxide in adduct with straight chain or branched chain alkanols with 6 to 22 carbon atoms, or alkyl phenols of higher fatty acids, or primary and secondary alkylamines superiors; as well as block copolymers of propylene oxide with ethylene oxide and mixtures thereof. Poly (vinyl alcohol) can also be used as a non-ionic stabilizer. The cationic surfactants may include alkyl quaternary ammonium salts and alkyl quaternary phosphonium salts, such as: alkyl trimethyl ammonium chloride, diethyldimethyl ammonium chloride, didocosyl dimethyl ammonium chloride, dioctadecyldimethyl ammonium chloride; dioctadecyldimethyl ammonium methosulfate, ditetradecyldimethyl ammonium chloride, and natural mixtures of the above fatty groups, for example, di (hydrogenated tallow) dimethyl ammonium chloride; di (hydrogenated tallow) dimethyl ammonium methosulfate, ditallow dimethyl ammonium chloride and dioleyldimethyl ammonium chloride. Polyvinyl alcohol Cationically modified and the cationically modified starch can also be used as an emulsifying agent. Preferably, the surfactants used in the present invention are primarily anionic sulfate ether surfactants. Preferred surfactants are Disponil FES 993 or Standapol ES-40, commercially available from Cognis Corporation. In preferred embodiments, the emulsifying agent is a monomer of a surfactant agent that is polymerized in the polymer backbone. The use of a polymerizable surface active agent is advantageous because the emulsifying agent is polymerized in the main chain, there is essentially no free surfactant in the aqueous phase of the emulsion; this is desirable because the presence of free surfactants can adversely affect the activation mechanism of the inventive binder. The monomers of the polymerizable surface active agent are typically compositions having both hydrophilic and hydrophobic functional groups, and a polymerizable group. The polymerizable groups include allyl, acrylic, methallyl or methacryl groups. Suitable polymerizable surfactant monomers may include anionic surfactants such as sulfates, phosphates, sulfosuccinate half esters and sulfosuccinate diesters which carry copolymerizable reactive groups and monomers of a nonionic surfactant, such as polyethoxylated nonylphenoxy propenyl alcohols. In addition, ammonium or metal salts of unsaturated organic acids of C6 to C30 may be suitable; these can also be used alone or in combination with the above surfactants. Suitable polymerizable surfactants are described in U.S. Patent No. 5,332,854, to Yokota et al., And European Patent No. 1479699, the totalities of which are incorporated herein by reference. Preferred polymerizable surfactants include Hitenol BC 1025 (Montello Incorporated), Trem LF-40 (a sodium dodecyl allyl sulfosuccinate surfactant available from Henkel Corporation), and the Adeka Reasoap Series Surface Active Agents such as SR-10 (Asahi Denka Co., Ltd.). It is believed that Hitenol BC and SR-10 have the following respective structures: Hitenol BC SR-10 -CHO (CH2CH20) nS03NH4 The general structures of exemplary polymerizable surfactants are illustrated in Table 1 below: TABLE 1 Polymerizable surfactants TABLE 1 (Continued) Polymerizable surfactants TABLE 1 (Continued) Polymerizable surfactants TABLE 1 (Continued) Polymerizable surfactants Surfactants have a tendency to decrease surface tension in water, which normally has a surface tension of approximately 73 dynes / cm. Typically, surfactants used in the present invention reduce the surface tension of water by at least 30 percent when measured as 10% solids in water. Preferably, the surfactants reduce the surface tension of the water by at least 40, or even 50 percent when measured in 10% aqueous solutions. The surface tensions of the water and the aqueous solutions of 10% sodium, AMPS, Hitenol BC 1025 and Hitenol BC 05, are listed in Table 2, below.

TABLE 2 Surface tension activity As can be seen from Table 2, NaAMPS reduced surface tension in water by only about 19%; thus, the AMPS compounds are preferably not used as the primary emulsifying agent. Protective colloids can also be used as a stabilizing agent. Examples of suitable protective colloids are polyvinyl alcohols, starch and cellulose derivatives and vinyl pyrrolidone copolymers. The protective colloids can also be used in conjunction with other emulsifying agents.

The initiators are typically added to the aqueous medium to stimulate polymerization. The initiators are commonly soluble in water and can decompose at high temperature or through redox reactions. The Guo reference indicated above describes suitable thermal initiators and suitable redox initiator systems. Exemplary initiators include peroxygen compounds such as ammonium persulfate, potassium persulfate, sodium persulfate; peroxides such as hydrogen peroxide; organic hydroperoxides, such as eumeno hydroperoxide, t-butyl hydroperoxide; organic peroxides, such as benzoyl peroxide, acetyl peroxide, lauroyl peroxide, peracetic acid and perbenzoic acid; and azo-type compounds such as azodiisobutyl nitrile, azobisdimethyl valeronitrile, azobisisobutyl nitrile, azodiisobutyl amide, azobis (alpha-ethylbutyl nitrile) and azobis (alpha, gamma-dimethylcarponitrile). A chain transfer agent can also be added to control the molecular weight of the polymer. The chain transfer agent may be present in amounts ranging from 0% to about 5% based on the total weight of the monomers, and is preferably from about 0.2% to about 1.2%. The amount of the chain transfer agent in the polymerization is inversely proportional to the molecular weight; Thus, if more chain transfer agent is added, the molecular weight will be lower. The chain transfer agent can include any compound that is capable of transforming the free radicals. The transfer agents of the suitable chain include carbon tetrachloride, bromoform, bromotrichloromethane, triphenyl methane, mercaptopropionic acid, alkyl mercaptans and thioesters such as n-dodecyl mercaptan, t-dodecyl mercaptan, octyl mercaptan, tetradecyl mercaptan, hexadecyl mercaptan, butyl thioglycolate, thioglycolate Isooctyl and dodecyl thioglycolate. Other ingredients can be used in emulsion polymerization which are well known in the art, including chelating agents, buffering agents, inorganic salts and pH-adjusting agents. Emulsions prepared in this manner will generally have a solids content of about 20 to about 60%, and preferably about 40%. Prior to neutralization, the emulsion may be diluted with water until it has a solids content ranging from about 10 to about 35%, preferably 1 5 and 25%. Alternatively, the composition can be neutralized in solution and then diluted to the desired solids content, or perhaps not diluted. Generally, salt sensitive copolymers should comprise from about 25 to 100 weight percent, preferably 50 to 99.9 weight percent of the total solids in the binder composition. Water must be the primary medium in the binder composition, although organic solvents may be present in minor amounts, that is, less than about 20%.

The molecular weight of the copolymers is not particularly limited, although it affects the salt sensitivity properties of the polymer. The range of the desired molecular weight will depend on the components of the specific monomer and the desired application of the binder. Typically, however, the molecular weight of the polymer should be from about 40,000 g / mol to about 500,000 g / mol, and preferably about 60,000 g / mol to about 250,000 g / mol. In general, copolymers made in accordance with the present invention exhibit a decrease in solubility as the molecular weight increases, as more fully illustrated in the examples that follow. The emulsion polymer typically has a particle size of 10 to 1000 nm, preferably about 50 to 300 nm. The morphology of the polymer can vary from spheres to core-shell structures, voids, half-moons and the like. When monomers of a drastically different solubility or hydrophobicity are polymerized in an emulsion process, a wide variety of morphologies can result. As indicated above, the copolymer must be neutralized in solution in order to prepare the latex binder solubilized with water. To neutralize the copolymer, the base is added to the emulsion until the composition becomes optically clear; the pH is recorded at this point. Preferably, sufficient base is added right up to the point where the emulsion becomes as translucent as possible. Typically, about 5 to about 55 mol% of the carboxylic acid units in the emulsion are neutralized. Suitable bases include NaOH, KOH, Na2CO3, NaHCO3 and the like. Preferably, the base should be non-volatile and contain only monovalent ions. The emulsions typically become solubilized in water when the solution reaches a pH of 4.0 to 9.0, preferably from about 6.0 to 8.0, and even more preferably from about 6.0 to 7.0. The control of the neutralization and the pH of the final solution is an important part of the procedure. Polymers that are overneutralized may experience problems with viscosity or may exhibit excess leaching of the base in the wetting solutions. The use of a relatively mild pH value in the inventive compositions is of great advantage in the consumer's disposable applications. Such pH's are desirable because they are compatible with the wearer's skin and probably will not cause irritation with contact. Nevertheless, it is possible to use solubilized salt-sensitive polymers with pH values outside the desired range. To make a fibrous network more compatible with the skin of users in such circumstances, the polymer can be applied to the fibrous network and the network can be treated with acidic agents, buffers or the like. It should be understood that the degree of neutralization required for the binder solutions to exhibit the desired properties ( example, solubility, viscosity, salt sensitivity), will vary depending on the composition and properties of the polymer. Such factors will include such things as the amount of the acidic monomer in the polymer chain and the molecular weight of the polymer. The viscosity of the binder solution will also depend on many factors, for example, the monomer content of the polymer. However, regardless of the constitution of the solution, the viscosity can generally be controlled by regulating one or more of the following aspects: 1) the amount of solids content in the emulsion; 2) the molecular weight of the polymer; 3) the degree to which the composition is diluted either before or after neutralization; and 4) the degree of neutralization. The viscosity of the composition must be controlled so that it can be applied to a fibrous network by normal means, and also so that the composition suitably impregnates the network, whereby the fibers are at least partially bound. Thus, the viscosity of the binder composition should be below about 2,000 cps at room temperature (23 ° C). Preferably, it should be below 1000 cps, 500 cps, 200 cps and even more preferably below 150 cps at room temperature. Other adjuvants can also be incorporated into the binder solution as dictated by the nature of the desired composition, as is well known to those of ordinary skill in the art. Examples of traditionally used additives include plasticizers, surfactants, adhesives, thickeners, fillers, humectants, fragrances, pigments, titanium dioxide or other opacifiers, dyes, defoamers, bactericides, bacteriostats and encapsulated components that can be used in conventional amounts. The fibrous webs employed in the present invention are non-woven webs. The nonwoven structures of the present invention comprise the polymeric binder in combination with the fibers. The non-woven web is formed by any method known in the art, such as, but not exclusively, air-laying, wet-laying, dry-laying or carded fibers. The fiber network typically has a basis weight of 20-200 grams per square meter (gsm). The fibers in the nonwoven material may be isotropically oriented, or may be aligned with respect to a processing direction. The thicker nonwoven webs may also have the fibers oriented in the z-direction of the article, that is, perpendicular to the plane of the fabric. In addition to a binder material, the fibers in the nonwoven materials can be mechanically entangled to provide strength and cohesion. In the manufacture of nonwoven webs, the fibers are typically dispersed in a medium (air for laying with air and liquid for wet laying), and deposited in the form of a sheet in a support base. In air-laying procedures, fibers and other optional materials are typically suspended in air within a formation system and deposited as a structure similar to a sheet in a mobile forming mesh or a rotating cylinder, before application of the binder. Wet laying procedures include providing an aqueous slurry and drying the slurry to form the network. Fibers from any source of any suitable length can be used in the present invention. The fibers include those known in the art, including cellulose fibers of woody plants, such as deciduous or coniferous trees; non-woody plants such as cotton, flax, esparto grass, cotton, straw, jute and bagasse; and synthetic fibers, such as those derived from polyester, polypropylene, polyethylene, polyamides, polyacrylics and rayon. Other fibrous materials used in the technique and mixtures of any fiber, can be used in the present invention. Preferred fibers are those typically used in non-woven materials, especially wood pulp fibers having a length of less than 0.5 cm, such as kraft fibers. For wet laid nets, the fibers should generally be less than a maximum of 5 cm long and more preferably less than 2 cm long. For networks placed with air, the fibers should be less than 8 mm long, preferably less than 6 mm long. Such fibers provide good biodegradable decomposition and dispersion characteristics. The fibers are present in the network from 50 to 98 weight percent, depending on the final use of the disposable article. For many uses, the fibers constitute up to about 70 to 85 weight percent of the net.

Generally, the fiber network is formed or formed at least partially before application of the binder. The solution of binder is combined with the fibers by contacting the fibers with the composition by means known in the art, such as printing, sprayed with or without air, saturation, creping and foam application. He polymer can be combined with the fibers at the wet end of the procedure for making paper (for example, by adding to the provision of paper) or, preferably, after the product of paper is substantially formed (for example, via the addition at the end dry). After application, the fibrous network is typically subjected to a drying step to remove water and / or other liquid. Drying can achieved by subjecting the paper product to elevated temperatures, example, in the range of about 85 ° C to about 125 ° C for a sufficient time to achieve the desired level of dryness, typically a constant weight.

The amount of the binder composition that remains in The fibers are referred to as "complement". The percent of the complement It can be calculated as follows: n,, ", Binder / fiber combined weight - fiber weight, nnn,% Complement = ° - x] 00% Combined weight of binder / fiber The weight of the fiber is the weight of the fibers before any binder composition is applied. The weight of the fiber / binder is the weight of the dry product (dry to the touch). The fibrous webs will generally have a complement value of 2 to 50 weight percent, preferably 10 to 30 weight percent. The binder has a temporary wet strength in the presence of salt. Depending on the application, it may be desirable to apply the salt to the fibrous network during production. In such cases, the salt may be applied to the article during manufacture by conventional means such as spraying, coating, immersion, etc. Generally, the binder composition solubilized in water and the salt should not be mixed before they are added to the fibers. The reason for this is that the salt has the tendency to precipitate the polymer if the two are combined before the addition of the fibers. Thus, the disposable article that is being treated with the polymeric binder is preferably subjected to the drying step to remove the water and / or any other liquid prior to the addition of the salt. After drying, the salt component can be added to the fiber / binder substrate to develop the strength in use. The salt is typically applied to the network in a solution containing at least about 0.5 weight percent salt to ensure that the network maintains its temporary wet strength. It is preferred to use salts with monovalent cations such as NaCl, as opposed to salts having multivalent cations which tend to affect the bivability characteristics of the binder.

The pre-moistened products produced in accordance with the present invention, such as wet wipes, may contain a wetting composition. The wetting composition should desirably contain at least 0.5 weight percent of an inorganic salt. The wetting composition may contain one or more additives, including, but not limited to, a solution of sodium chloride or sodium sulfate, preservatives, boric acid, bicarbonates, moisturizers, emollients, surfactants, humectants, alcohols, water and fragrances The wetting composition may be present up to 500 weight percent based on the weight of the nonwoven material, and preferably up to 350 percent. The wetting composition is generally added as a secondary treatment of the non-woven network. This treatment of the wetting solution may occur just before packing or after the non-woven net has been placed in the final container. Prepared in this way, wet-use products will have a stable wet strength of measurable tensile integrity and value, even if dispersed when placed in tap water, allowing the product to be cleaned with a discharge of water into sewer systems septic without blockade. Fiber typically begins to disperse immediately in water. The dispersion rate can be designed for different applications, varying factors such as the composition of the polymer, the molecular weight, the degree of neutralization, the pH of the solution or the type of the fibrous network.

Inventive binders are particularly suitable for applications involving pre-moistened articles, because the articles can be stored in a solution containing approximately 0.5 percent by weight or more of a salt, so that the articles maintain a high wet strength until they are discarded. Pre-moistened articles include those such as wet wipes, household wipes, pre-moistened baby wipes, pre-moistened toilet paper and pre-moistened household wipes. Inventive binder solutions may also be suitable for other disposable applications that employ salt sensitive binders, such as diapers, incontinent underwear, feminine care products and the like. The fibrous webs of the present invention must be non-dispersible in solutions containing more than 0.5% salt, although easily dispersible in typical wastewater. The fibrous webs used in the disposable articles of the present invention have binders that typically provide a characteristic wet strength index of at least about 40 in an aqueous solution of 10% NaCl, and preferably at least about 80, and more preferably at least about 100. Some binder compositions can provide wet strength ratings of more than 120 in 10 percent by weight NaCl solutions. In addition, binders generally provide a resistance index characteristic wet in deionized water of less than 25, and preferably less than 10, and still more preferably less than 5. The following examples are presented to further illustrate and explain the present invention and should not be taken as limiting in any way. appearance.

EXAMPLES Exemplary binder solutions were prepared by emulsion polymerizing the monomers to obtain a latex composition having approximately 30-55% solids. The emulsion polymerization process that was used to polymerize Examples 1-4 of the present invention is summarized below for illustrative purposes. An amount of 560 grams of deionized water was added to a 4 L glass reactor equipped with a condenser, thermometer and a stainless steel stirrer. Next, a solution of 121 grams of water and 27 grams of Disponil FES 993, an emulsifier, were mixed. The following components were then added to the water / surfactant mixture: 200 grams of methacrylic acid, 300 grams of butyl acrylate and 2.5 grams of dodecyl mercaptan. The mixture was stirred to provide a preemulsion of the stable monomer. 33 grams of the monomer pre-emulsion were added to the reactor charge. Then, 0.3 grams of ammonium persulfate in 5 grams was added of water to the reactor load. The reaction mixture was heated to 80 ° C. After 10 minutes, the pre-emulsion of the remaining monomer was added over a period of 4 hours to the reactor. Also, 2.4 grams of ammonium persulfate in 45 grams of water was added to the reactor for about 4.5 hours. After completion of the slow addition of monomer and initiator, the reaction mixture was cooled to 60 ° C. The dosage of the reducing component, which is composed of 10 grams of water and 0.7 grams of erythorbic acid, was started at an internal reactor temperature of 60 ° C. The reaction mixture was maintained between 55-60 ° C for 10 minutes. After finishing the addition of the reductant, the reaction mixture was cooled to 30 ° C. The product had 40 weight percent solids, a pH of 2.3, a viscosity of 18 centipoise, and a mean particle size of 131 nm. After the polymerization, the examples were diluted to the desired solids content, which unless otherwise specified, is about 20%. The binder solution was then prepared at room temperature by neutralizing the polymer according to the following procedure: the emulsion composition was stirred and an aqueous solution of sodium hydroxide was added at 15% solids until the desired degree of neutralization was achieved . Unless the context dictates otherwise, the compositions of the following examples are neutralized at least to the point where they are as translucent as possible. The resulting compositions have a wide variety of pH values, as can be seen from Table 7 below. As described in detail below, the binder solutions were applied to pulp substrates and tested for tensile strength after being moistened with three aqueous solutions having varying ion concentrations. The process for preparing the saturated pulp substrate is as follows: a pulp substrate is saturated with immersion with the binder solution and then thermally dried and post-treated to simulate the temperature conditions in commercial pulp manufacture. Sheets of Paper Filter Whatman # 4, commercially available from Whatman, Inc., are the pulp-based reserve and are stored under temperature (23 ° C) and humidity (50%) conditions controlled before use. The pulp is cut to approximately 1.5 x 57 cm strips and weighed to 0.01 grams. The Whatman pulp is saturated by immersion, passing the pulp through a binder bath and then passing the saturated sheets through pressurized clamping rollers of a double roller saturator (Werner Mathis VFM or a similar saturator) to squeeze the excess solution of the polymer. The saturated sheet is then placed in a heated drum dryer (Adirondack or similar dryer), set at 100 ° C until it is dry to the touch (usually about 2-3 passes). After initial drying, the pulp is placed in an oven set at 130 ° C for two minutes. The saturated dried leaf is reconditioned to controlled temperature and humidity conditions for a minimum of one hour. The sheet is reweighed at 0.01 grams and then calculated for the% of the complement. The following examples have complement values ranging from about 10% to about 16%. The procedure for the preparation of the strips for the traction and the soaking solutions is as follows: The saturated pulp is cut into tensile strips in the direction transverse to the machine of 2.54 x 10.16 centimeters (1 x 4 inches), using a Precision paper cutter (Test Machines, Inc. or a similar cutter). The tensile strips are weighed and the weight is used to calculate the basis weight in grams / square meter. The test solutions are prepared as follows: a) 10% by weight sodium chloride (NaCl) solutions in deionized water. b) A hard water solution of 200 ppm using 134 ppm Ca ++ of calcium chloride (CaCl2) and 66 ppm Mg ++ of magnesium chloride (MgCl2) in deionized water. c) Standard deionized water. The test of the traction and the normalization is carried out as follows: The test strips are soaked in the various solutions, and then the wet strength is measured. The solutions represent the type of media in which the pulp can be exposed, for example, conditions of concentrated salt for storage in use (10%) and various water conditions for disposal (200 ppm for disposal in hard water and deionized water for disposal in softer water). To determine the wet tensile strength, a tensile tester (Instron 5542 or a similar tester) conforms to the following parameters: a) pneumatic fasteners using 2.54 x 2.54 centimeters (x 1 inch) covers; b) a hole or 5.08 centimeters (2 inches) between the top and bottom covers; c) a crosshead speed of 2.54 centimeters / minute (1 inch / minute); and d) a load cell capable of measuring up to 20,000 gm / 2.54 centimeters (20,000 gm / inch). The traction strips were soaked (4-6 strips per soaking solution) for 60 minutes in the specified soaking solution, with emphasis on ensuring that the total wetting of the strips occurs. The level of the soaking solution used is as follows: a) 125 grams of a 10% NaCl salt solution per pull group (4-6 strips); and b) 45 grams of hard water or deionized water per traction strip soaked. After removal of the soaking solution, the pull strip is placed on an absorbent paper towel to remove the excess solution, and then immediately tested for tensile strength. The average tensile strength (gf / 2.54 centimeters) ((gf / inch)) is then normalized to a basis weight of the network of 1 12.5 gsm (representative basis weight), using the following formula: . . . ,,. , Average tensile strength (gf / inch) x 1 12.5 gsm Resistance to normalized traction = Actual basis weight (gsm) The untreated tensile strength is normalized to a basis weight of the standard fabric (1 12.5 gsm), in order to compensate for the slight effects of variations in the weight of the fabric. The wet strength index is then calculated for each example in order to minimize differences in wet strength due to varying levels of binder complement. It is calculated according to the following formula: , ..,,. f Normalized wet tensile strength Wet strength index = The wet strength index of each example was calculated in deionized water and in a 10% NaCl solution (hard water index was also calculated for examples 28-31). The following abbreviations of the compositions are used in the examples: AA Acrylic acid monomer BA Butyl acrylate monomer BC 025 Hitenol BC 1025, polymerizable polyoxyethylene alkylphenyl ether ammonium sulfate surfactant bCEA Beta-carboxy ethyl acrylate monomer ES-40 Standopol Surfactant ES-40 (sodium salt of alkyl alcohol sulphate ethoxylate) ibMA Isobornyl methacrylate monomer MAA Methacrylic acid monomer MEM Monomer of monoethyl maleate MMA Methyl methacrylate nBMA N-butyl methacrylate monomer tOA Tertiary octyl acrylamide monomer Trem LF-40 Polymerizable sodium dodecyl allyl dodecyl sulfosuccinate surfactant. The binders in Examples 1-4 contain butyl acrylate and methacrylic acid and the binders in Examples 5-9 also contain tOA. These examples are neutralized to varying degrees and the tensile results illustrate the effect that the amount of neutralization has on the salt sensitive properties of the binder. In these examples, the percent of sodium hydroxide used to neutralize the polymer is based on the weight percent of the dry polymer, ie (solids / solids). Examples 1 and 5 are not neutralized, and therefore are not covered by the present invention. They are included for comparative purposes.

TABLE 3 Composition of Examples 1-9 TABLE 4 Tensile Strength Values for Examples 1-9 The wet strength indices in Table 4 are plotted against the amount of base used to neutralize the emulsion and are shown in Figure 2. As can be seen from Table 4 and Figure 2, as the degree of neutralization increases, the binder in emulsion solubilized in water it becomes more dispersible in the solution of 10% NaCl and in deionized water. Examples 1 and 5 also illustrate that when the polymer is not neutralized, it has very limited salt sensitivity properties since the difference between the wet strength in 10% NaCl and the wet strength of DI is small. The compositions and tensile properties of the Examples 10-16 are shown below in Tables 5 and 6. These examples illustrate the effect of the chain transfer agent and the molecular weight on the wet strength index. The amount of the chain transfer agent present during the polymerization reaction is reported as the weight percent of the total content of the monomer. Molecular weight is measured by GPC analysis using polystyrene standards as the reference molecular weight.

TABLE 5 Composition of Examples 10-16 TABLE 6 Effect of molecular weight on wet strength The wet strength indices reported in Examples 10-15 for 10% NaCl and DI water are illustrated in Figure 3 and plotted against the amount of chain transfer agent. As shown in Figure 3, the use of higher amounts of chain transfer agents generally causes the polymer to become more dispersible in concentrated salt solutions and DI water. The polymers in Examples 10A-15B below, have the same compositions and corresponding molecular weights as those in Examples 10-15; that is, the polymer compositions of Examples 10A and 10B correlate with the composition in Example 10, and so on. The polymers in the following Examples are diluted to a target solids content of about 20% in Examples "A" and about 25 in Examples "B". The actual values of solids are indicated in Table 7 below. The emulsions were then neutralized with 7% NaOH on a solids / solids basis, and tested for the physical properties also reported in Table 7.

TABLE 7 Physical properties of Examples 10A-15B As shown in Table 7, the viscosity of the emulsion solubilized in water typically increases with the molecular weight of the polymer. It can also be seen that the viscosity increases rapidly as the solids content rises. The compositions for Examples 17-23 are shown below in Table 8. These examples were polymerized with various carboxylic acid monomers to determine the effects of each monomer and the salt sensitive properties of the binder. The wet tensile strengths and the wet strength indices provided by each binder composition are reported in Table 9.

TABLE 8 Composition of Examples 17-23 TABLE 9 Performance of carboxylic acid The results in Table 9 illustrate that for these examples, the choice of carboxylic acid monomer affects the salt sensitivity characteristics of the binder. For example, a comparison of Examples 17 and 18 shows that a solubilized BA / MMA / MAA emulsion binder provides a higher wet strength in concentrated saline solutions than a BA / MMA / AA monomer, while still having a good dispersibility in deionized water (see Figure 1). Similarly, it is observed in Examples 22 and 23 that a BA / MAA binder provides almost 8 times the wet strength of a BA / bCEA binder in 10% NaCl. It should be understood that although MAA appears to provide the best salt sensitivity performance in these examples, any carboxylic acid monomer can be used; in fact, acrylic acid or bCEA may be suitable in other polymer systems. The salt sensitivity properties are also affected by the amount of the carboxylic acid monomer, as can be seen from Examples 24-27 in Tables 10 and 1, below.

TABLE 10 Composition of Examples 24-27 TABLE 11 Comparison with variable levels of MAA The effect of the amount of methacrylic acid on the wet strength index is illustrated in Figure 4. Figure 4 shows that as the amount of methacrylic acid increases, the polymers generally provide a higher amount of wet tensile strength both in concentrated salt solutions and in DI water. As discussed above, in the preferred embodiments of the present invention, the binder compositions contain a hydrophobic monomer in addition to the carboxylic acid and the acrylate monomers. Examples 28-31 illustrate the effect of varying the amount and type of hydrophobic monomer in a binder having BA as the alkyl acrylate component and MAA as the carboxylic acid component.

TABLE 12 Composition of Examples 28-31 TABLE 13 Comparison of variable hydrophobic monomer The wet strength values in Table 13 illustrate that the presence of a hydrophobic monomer can improve the salt sensitivity properties of the polymer for certain applications. For example, it has been observed that all binders in Examples 28-31 provide acceptable wet strength values in 10% NaCl and DI water. Furthermore, it has been observed that in Examples 29-31, the networks have relatively low wet strengths in solutions having 200 ppm of divalent cations; the elevation of the index in the 200 ppm solution is no more than about 10.5 of that of DI water. Salt sensitivity properties of this type are particularly desirable for markets or applications where hard water dispersibility is required. The binders in Examples 32-36, shown below, are polymerized in a stable emulsion using a nonpolymerizable ether sulfate surfactant (ES-40) or a monomer of a polymerizable surfactant (BC 1025 or Trem LF-40). ). The amount of surfactant and chain transfer agent is reported as the percent by weight of the total monomer content that is not of the surfactant.

TABLE 14 Composition of Examples 32-36 TABLE 15 Strength values of Examples 32-36 As can be seen in Table 15, the use of polymerizable emulsifiers produces binders with salt sensitivity characteristics, which are equivalent to, if not higher than, non-polymerizable surfactants. The above examples are intended to more fully explain the invention as defined by the following claims. Many modifications and variations of this invention can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. For example, increasing or decreasing the various monomer ratios can influence the performance for traction of a network, as well as changing the level of neutralization. Other factors also affect the dispersibility of a network, such as the type of fibers, the structure of the substrate and the amount of binder used. Of course, it will also be understood that the specific wet strength properties will vary depending on the desired application. The specific embodiments described herein are offered by way of example only, and the invention is limited only by the terms of the appended claims, together with the full scope of equivalents to which such claims are entitled. In view of the above discussion, the relevant knowledge in the art and the references discussed above in relation to the Background and the Detailed Description, the descriptions of which are incorporated herein by reference, it is considered that an additional description is unnecessary.

Claims (2)

1. NOVELTY OF THE INVENTION CLAIMS 1 .- A method for producing a solution of polymer binder sensitive to salt, activatable by ions, for a nonwoven article, the method comprises the steps of: i) preparing an emulsion composition by emulsion polymerizing in an aqueous medium a resin of a copolymer containing from about 5 to about 80 weight percent units of a carboxylic acid monomer and from about 10 to about 95 weight percent units of an ethylenically unsaturated comonomer; ii) converting the emulsion composition into a solution by neutralizing the copolymer resin with a base until the resin is soluble in water; and ii) controlling the viscosity of the solution to be less than about 2,000 cps at 23 ° C, where the salt-sensitive binder, activated by the ions, provides a characteristic wet strength index of less than 25 in deionized water and an elevation of the characteristic wet strength index of at least 15 points in an aqueous solution of 10% NaCl.
2. 2. The method according to claim 1, further characterized in that it comprises the step of diluting the emulsion composition to a solids level of 10% to 35% before neutralizing the copolymer resin. 3. - The method according to claim 1, further characterized in that it comprises the step of diluting the emulsion composition to a solids level of 15% to 25% solids before neutralizing the copolymer resin. 4. The method according to claim 1, further characterized in that the viscosity of the solution is controlled to be less than about 1,000 cps at 23 ° C. 5. - The method according to claim 1, further characterized in that the viscosity of the solution is controlled to be less than about 500 cps at 23 ° C. 6. - The method according to claim 1, further characterized in that the viscosity of the solution is controlled to be less than about 200 cps at 23 ° C. 7. - A method for making a nonwoven web with a polymeric binder, the method comprising the steps of: i) preparing an emulsion composition by emulsion polymerizing in an aqueous medium a copolymer resin containing from about 5 to about 80 weight percent carboxylic acid units and from about 10 to about 95 weight percent units of an ethylenically unsaturated comonomer; ii) converting the emulsion composition to a binder solution by neutralizing the copolymer resin with a base, at least until it is soluble in water; iii) provide a fibrous network; and iv) applying the binder solution to the fibrous network, where the binder is activatable by the ions so as to provide a characteristic wet strength index of less than 25 in deionized water and an elevation in the characteristic wet strength index of at least 15 points in an aqueous 10% NaCl solution. 8. The method according to claim 7, further characterized in that it comprises the step of drying the network. 9. A polymer of a salt sensitive binder, activatable by the ions, comprising: i) from about 10 to about 70 weight percent of methacrylic acid units; ii) from about 10 to about 90 weight percent of alkyl acrylate units having from 2 to 4 carbon atoms in the alkyl portion; and iii) from about 2 to 55 weight percent of hydrophobic monomer units selected from the group of alkyl (meth) acrylamides having from 4 to 12 carbon atoms in the alkyl portion, straight chain alkyl methacrylate units or branched having 4 to 6 carbon atoms in the alkyl portion; Bicycloalkyl (meth) acrylates with 4 to 20 carbon atoms in the cycloalkyl portion; and combinations thereof, wherein the salt-activated binder polymer, activated by the ions, is prepared by emulsion polymerization in an aqueous medium and neutralized so that it is soluble in water, and wherein the binder provides a characteristic wet strength index of less than 25 in deionized water and an elevation in the characteristic wet strength index of at least 15 points in an aqueous solution of 10% NaCl. 10. - The salt sensitive binder polymer according to claim 9, further characterized in that the alkyl acrylate units comprise butyl acrylate. eleven . - The salt sensitive binder polymer according to claim 9, further characterized in that the hydrophobic monomer units comprise a substituted or unsubstituted bicycloalkyl (meth) acrylate having from 6 to 14 carbon atoms in the cycloalkyl moiety. 12. - The salt sensitive binder polymer according to claim 1, further characterized in that the hydrophobic monomer units comprise isobornyl methacrylate. 13. - The salt sensitive binder polymer according to claim 9, further characterized in that the hydrophobic monomer units comprise n-butyl methacrylate. 14. The salt sensitive binder polymer according to claim 9, further characterized in that the hydrophobic monomer units comprise an alkyl (meth) acrylamide having from 6 to 10 carbon atoms in the alkyl moiety. 15. - The salt sensitive binder polymer according to claim 14, further characterized in that the hydrophobic monomer units comprise tertiary N-octyl acrylamide. 16. - The salt sensitive binder polymer according to claim 9, further characterized in that the polymer comprises less than 5 weight percent straight or branched chain alkyl (meth) acrylates having from 8 to 12 carbon atoms in the alkyl portion. 7. The salt sensitive binder polymer according to claim 9, further characterized in that the binder provides a characteristic wet strength rating of less than 25 in deionized water and an elevation in the characteristic wet strength rating. of no more than 15 points in an aqueous solution containing 200 ppm of divalent cations. 18. A solution of a salt-sensitive, water-activatable binder, aqueous, for a non-woven network, comprising: a) water; and b) a water-solubilized resin composition converted from an emulsion copolymer, wherein the resin composition includes (i) a copolymer with from about 5 to about 80 weight percent carboxylic acid units, and from about 10 to about about 95 weight percent of units of an ethylenically unsaturated comonomer; and (ii) an amount of emulsifier effective to maintain a stable emulsion during the polymerization of the polymer, wherein the salt sensitive binder, activable by the ions, provides a characteristic wet strength index of less than 25 in deionized water and an elevation in the characteristic wet strength index of at least 15 points in an aqueous solution of 10% NaCl. 19. The composition according to claim 18, further characterized in that the emulsifier is present in an amount from about 0.05 to about 10 weight percent, based on the total weight of the solids. 20. The composition according to claim 18, further characterized in that the emulsifier is present in an amount of about 0.2 to about 5 weight percent, based on the total weight of the solids. 21 - The composition according to claim 18, further characterized in that the emulsifier includes one or more polymerizable surfactants that are polymerized in the main chain of the copolymer. 22. The composition according to claim 18, further characterized in that the emulsifier consists of one or more polymerizable surfactants that are polymerized in the main chain of the copolymer. 23. The binder solution according to claim 18, further characterized in that the copolymer comprises from about 20 to about 65 weight percent carboxylic acid units. 24. The binder solution according to claim 18, further characterized in that the carboxylic acid units comprise methacrylic acid. 25. - The binder solution according to claim 18, further characterized in that the carboxylic acid units comprise a monoalkyl ester of maleic acid. 26. - The binder solution according to claim 25, further characterized in that the monoalkyl ester of maleic acid is monoethyl maleate. 27. The binder solution according to claim 18, further characterized in that the units of the ethylenically unsaturated comonomer comprise an alkyl acrylate having from 1 to 4 carbon atoms in the alkyl portion. 28. The binder solution according to claim 27, further characterized in that the alkyl acrylate units comprise butyl acrylate. 29. - The binder solution according to claim 18, further characterized in that the units of the ethylenically unsaturated monomer comprise hydrophobic monomer units that are selected from the group consisting of alkyl (meth) acrylamides having from 2 to 15 carbon atoms. carbon in the alkyl portion; straight or branched chain alkyl methacrylates having from 4 to 12 carbon atoms in the alkyl portion; straight or branched chain alkyl acrylates having from 5 to 12 carbon atoms in the alkyl portion; a substituted or unsubstituted bicycloalkyl (meth) acrylate; and combinations thereof. 30. - The binder solution according to claim 29, further characterized in that the copolymer includes from 2 to about 55 weight percent of the hydrophobic monomer units. 31. The binder solution according to claim 29, further characterized in that the copolymer comprises from about 3 to about 20 weight percent of the hydrophobic monomer units. 32. - The binder solution according to claim 18, further characterized in that the units of the ethylenically unsaturated monomer comprise units of a hardening monomer. 33. - The binder solution according to claim 32, further characterized in that the copolymer includes from 2 to 55 weight percent of the monomer hardener units. 34. The binder solution according to claim 32, further characterized in that the copolymer includes from 10 to 50 weight percent of the monomer hardener units. 35. - The binder solution according to claim 32, further characterized in that the hardener monomer units have a vitreous transition temperature in the range of 40 ° C to 140 ° C. 36. - The binder solution according to claim 32, further characterized in that the units of the hardening monomer have a glass transition temperature in the range of 80 ° C to 120 ° C. 37. - The binder solution according to claim 32, further characterized in that the units of the hardening monomer comprise methyl methacrylate. 38. - The composition according to claim 18, further characterized in that the copolymer contains less than 0.25 weight percent crosslinkable monomers based on the total weight of the solids. 39. The composition according to claim 18, further characterized in that the copolymer has a molecular weight of from about 40,000 to about 500,000 g / mol. 40. The composition according to claim 18, further characterized in that the copolymer has a molecular weight of about 60,000 and 250,000 g / mol. 41 - The binder solution according to claim 18, further characterized in that the resin of the copolymer is solubilized at least to the point where the solution reaches its maximum optical transparency in water. 42. - The binder solution according to claim 18, further characterized in that the resin of the copolymer is it solubilizes in water neutralizing it with a stoichiometrically equivalent amount of 5% and 55% of the carboxylic acid units present in the emulsion. 43 - The binder solution according to claim 42, further characterized in that the base contains cations that are monovalent. 44 - The binder solution according to claim 18, further characterized in that the binder solution has a pH of about 4 to about 9. The binder solution according to claim 18, further characterized in that the binder solution has a pH of about 6 to about 8. 46. - The binder solution according to claim 18, further characterized in that the binder solution has a pH of about 6 to about 7. 47. - The binder solution in accordance with claim 18, further characterized in that the binder provides a characteristic wet strength rating of less than 25 in deionized water and an elevation in the characteristic wet strength rating of at least 35 points in an aqueous solution of 0% NaCl . 48. - The binder solution according to claim 18, further characterized in that the binder provides a characteristic wet strength rating of less than 10 in water deionized and an elevation in the characteristic wet strength index of at least 50 points in an aqueous solution of 10% NaCl. 49. - The binder solution according to claim 18, further characterized in that the binder provides a characteristic wet strength index of at least about 40 in an aqueous solution of 10% NaCl, and a characteristic wet strength index of less than about 10 in deionized water. 50. - The binder solution according to claim 18, further characterized in that the binder provides a characteristic wet strength index of at least about 80 in a 10% aqueous solution of NaCl, and a characteristic wet strength index of less than about 5 in deionized water. 51 - The binder solution according to claim 18, further characterized in that the binder provides a characteristic wet strength index of at least about 00 in an aqueous solution of 10% NaCl, and a characteristic wet strength index of less of about 5 in deionized water. 52. - A nonwoven web including a salt sensitive polymer binder, activatable by the ions, the improvement in which the binder comprises: a) an emulsion polymerized resin composition, solubilized in water, having: (i) ) a copolymer with from about 5 to about 80 weight percent carboxylic acid units, and from about 10 to about 95 weight percent units of an Edenically unsaturated comonomer; and (ii) an amount of emulsifier effective to maintain a stable aqueous emulsion with the polymer as it polymerizes, wherein the binder is activatable by the ions, so as to provide a characteristic wet strength index of less than 25 in water deionized and an elevation in the characteristic wet strength index of at least 20 points in an aqueous solution of 0% NaCl. 53. - The improvement according to claim 52, further characterized in that the non-woven network is in contact with a wetting composition containing at least about 0.1% by weight of an inorganic salt. 54. - The improvement according to claim 52, further characterized in that the article is selected from the group consisting of a wet wipe, a household wipe, a diaper, an undergarment for incontinence and a product for female care.