Crosslinkable stabilizing composition for nonwoven substrates and method of making same

Abstract

A cross-linkable stabilizing composition for nonwoven substrates comprises an aqueous dispersion of a vinyl acetate-ethylene copolymer, a polyvinyl alcohol dispersion stabilizer, and a copolymer comprising maleic anhydride units or a copolymer comprising maleic acid units. The composition exhibits a free formaldehyde content of 5 ppm or less.

Description

Background Technology

[0001] This invention generally relates to compositions for use in nonwoven substrates. The invention also relates to methods for preparing them.

[0002] Polymer dispersions can be applied to nonwoven substrates to improve their dry and wet tensile strength. Such substrates can be used in products such as wet or dry wipes.

[0003] However, during the manufacturing of such products, formaldehyde may be generated as an undesirable byproduct due to the use of certain crosslinking agents. Furthermore, formaldehyde may be present in the polymer dispersion due to the use of certain free radical initiators in polymer formation. Formaldehyde may also be present due to the use of certain preservatives. The presence of formaldehyde in the dispersion and / or substrate is undesirable for both manufacturers of nonwoven products and end-use consumers. Efforts to use polymer dispersions with low levels of formaldehyde and free of formaldehyde-generating crosslinking agents often result in nonwoven products exhibiting insufficient wet tensile strength and dispersions lacking adequate stability.

[0004] Therefore, a composition that overcomes the above-mentioned defects is needed. A method for preparing such a composition is also desirable. Summary of the Invention

[0005] The following describes embodiments of a crosslinkable stabilizing composition for nonwoven substrates. In one embodiment, the composition comprises an aqueous dispersion of a vinyl acetate-ethylene copolymer, a polyvinyl alcohol dispersion stabilizer, and a copolymer containing maleic anhydride units or a copolymer containing maleic acid units. The composition exhibits a free formaldehyde content of 5 ppm or lower.

[0006] In some embodiments, the composition exhibits a viscosity of 4000 mPa⁻² or lower when measured at 23°C and using an LV-3 spindle at 60 rpm. In one such embodiment, the composition exhibits a viscosity of 200 to 4000 mPa⁻² when measured at 23°C and using an LV-3 spindle at 60 rpm.

[0007] In other embodiments, the viscosity of the composition does not change by more than 60% after 75 days at 20°C. In some of these embodiments, the viscosity of the composition does not change by more than 25% after 75 days at 20°C.

[0008] In some embodiments, the copolymer comprises ethylene units.

[0009] In one embodiment, the copolymer comprises an ethylene maleic anhydride polymer.

[0010] In some embodiments, the composition does not contain N-hydroxymethylacrylamide.

[0011] In other embodiments, the composition comprises a copolymer in an amount of 0.5% by weight or more based on the total weight of the monomers in the aqueous dispersion.

[0012] In some embodiments, the polyvinyl alcohol dispersion stabilizer comprises partially hydrolyzed polyvinyl acetate with an average degree of hydrolysis of 80 to 96 mol%.

[0013] The following describes embodiments of a method for preparing a crosslinkable stable composition for nonwoven substrates. In one embodiment, the method includes providing a copolymer comprising maleic anhydride units or a copolymer comprising maleic acid units. Vinyl acetate and ethylene are provided. Vinyl acetate and ethylene are copolymerized in the presence of a polyvinyl alcohol dispersion stabilizer to form a vinyl acetate-ethylene copolymer. A composition comprising the vinyl acetate-ethylene copolymer and having a formaldehyde level of 5 ppm or lower is formed.

[0014] In some embodiments, a vinyl acetate-ethylene copolymer is provided in an aqueous dispersion formed by emulsion polymerization. In one embodiment, the copolymer is mixed with the aqueous dispersion to form a composition.

[0015] In other embodiments, the copolymer is mixed with a polyvinyl alcohol dispersion stabilizer before the vinyl acetate-ethylene copolymer is formed.

[0016] In one embodiment, the copolymer is provided in an amount of 0.5% by weight or more, based on the total weight of the monomers in the aqueous dispersion. In another embodiment, the copolymer is provided in an amount of 0.5% to 5% by weight, based on the total weight of the monomers in the aqueous dispersion.

[0017] In some embodiments, the composition exhibits a viscosity of 50 to 4000 mPa-s when measured at 23°C and using an LV-3 spindle at 60 rpm.

[0018] In other embodiments, a nonwoven product is provided. In one embodiment, the nonwoven product comprises a nonwoven substrate treated with a crosslinkable stabilizing composition. In this embodiment, the nonwoven product exhibits a wet tensile strength of 25% or greater than its exhibited dry tensile strength, and a formaldehyde content of 1.5 ppm or less. In one embodiment, the copolymer and the vinyl acetate-ethylene copolymer are crosslinked. Detailed Implementation

[0019] It should be understood that, unless expressly stated otherwise, the invention may take various alternative orientations and sequences of steps. It should also be understood that the specific materials, apparatus, and methods described in the following specification are merely exemplary embodiments of the inventive concept. Therefore, unless expressly stated otherwise, specific properties, conditions, or other physical characteristics associated with the disclosed embodiments should not be considered limiting.

[0020] In one embodiment, a crosslinkable stable composition is provided. The composition has a low level of free formaldehyde. The composition can be applied to a nonwoven substrate. After application of the composition, the nonwoven substrate is typically allowed to dry and cure. Full curing is preferably achieved when the nonwoven substrate reaches the temperature leading to maximum crosslinking. Once dried and cured, the nonwoven substrate has high levels of wet and dry tensile strength and low levels of free formaldehyde. These properties make it suitable for products such as wet or dry wipes. However, the composition can also be applied to substrates used in other applications.

[0021] In one embodiment, the composition comprises an aqueous dispersion. The aqueous dispersion comprises a polymer dispersed in water, and will be discussed primarily in relation to copolymers. However, the aqueous dispersion may comprise homopolymers or mixtures of homopolymers and copolymers. In some embodiments, the polymer may be used as a binder in products comprising a nonwoven substrate.

[0022] Preferably, the aqueous dispersion comprises a vinyl acetate-ethylene (VAE) copolymer. In some embodiments, the VAE copolymer in the composition provides the desired dry tensile strength, tensile and / or elongation properties for the nonwoven substrate. The aqueous dispersion comprising the VAE copolymer can be formed by a polymerization method that copolymerizes vinyl acetate monomer, ethylene, and optionally additional monomers in water. In such embodiments, the vinyl acetate monomer content in the VAE copolymer is 66 to 100% by weight, preferably 68 to 95% by weight, more preferably 68 to 93% by weight, and most preferably 68 to 92% by weight, in each case based on the total weight of the copolymer monomers in the aqueous dispersion. The ethylene content in the VAE copolymer is 1% by weight or more. Preferably, the amount of ethylene in the VAE copolymer is 1 to 32% by weight. In some embodiments, the amount of ethylene in the VAE copolymer is 5 to 32%. In all cases, the % by weight of ethylene is based on the total weight of the copolymer monomers in the aqueous dispersion.

[0023] Optionally, in some embodiments, the range of usable polymers in the aqueous dispersion can be expanded by copolymerizing the additional monomer with vinyl acetate, or with vinyl acetate and ethylene. Typically, a suitable additional monomer (comonomer) is a monomer having a single polymerizable olefinic group. Examples of such comonomers are vinyl esters of carboxylic acids having 3 to 18 carbon atoms. Preferred vinyl esters are vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methyl vinyl acetate, vinyl neopentanoate, and vinyl esters of α-branched monocarboxylic acids having 9 to 11 carbon atoms, an example being VEOVA9. TM Or VEOVA10 TM Esters (available from Hexion, Columbus, OH). Other suitable comonomers include esters of acrylic acid or methacrylic acid with unbranched or branched alcohols having 1 to 15 carbon atoms. Exemplary methacrylates or acrylates include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, 2-ethylhexyl acrylate, and norbornyl acrylate. Other suitable comonomers include vinyl halides such as vinyl chloride, or olefins such as propylene. Typically, other comonomers are copolymerized in amounts of 0.5 to 30% by weight, preferably 0.5 to 20% by weight, based on the total amount of comonomers in the copolymer.

[0024] Optionally, based on the total amount of vinyl acetate and ethylene monomers in the aqueous dispersion, 0.05 to 10% by weight of additional monomers (auxiliary monomers) may be additionally copolymerized in the formation of the aqueous dispersion. The auxiliary monomers may include polymerizable olefinic groups and at least one additional functional group, which may be an additional polymerizable olefinic group that provides crosslinking. Other functional groups may include reactive groups, such as carboxylic acid groups or sulfonic acid groups.

[0025] Examples of auxiliary monomers are olefinically unsaturated monocarboxylic and dicarboxylic acids, typically acrylic acid, methacrylic acid, fumaric acid, and maleic acid; olefinically unsaturated formamides and formonitriles, typically acrylamide and acrylonitrile; and monoesters and diesters of fumaric and maleic acids, such as diethyl ester and diisopropyl ester, as well as maleic anhydride, olefinically unsaturated sulfonic acids, and their salts, typically vinyl sulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid. Other examples are pre-crosslinked comonomers, such as polyolefinically unsaturated comonomers. Examples are divinyl adipate, diallyl maleate, allyl methacrylate, or triallyl cyanurate. Also suitable are epoxy-functionalized comonomers such as glycidyl methacrylate and glycidyl acrylate. Other examples are silicon-functionalized comonomers, such as acryloyloxypropyltris(alkoxy)silane and methacryloxypropyltris(alkoxy)silane, vinyltrialkoxysilane and vinylmethyldialkoxysilane, where the alkoxy groups may be, for example, methoxy, ethoxy, and ethoxypropylene glycol ether groups. Additional monomers include hydroxyl or CO groups, examples of which are hydroxyalkyl methacrylates and hydroxyalkyl acrylates such as hydroxyethyl acrylate or hydroxyethyl methacrylate, hydroxypropyl acrylate or hydroxypropyl methacrylate, hydroxybutyl acrylate or hydroxybutyl methacrylate, and compounds such as diacetone acrylamide and acetoacetoxyethyl acrylate or acetoacetoxyethyl methacrylate.

[0026] The selection of monomers or monomer weight ratios is preferably carried out in such a manner that, typically, VAE copolymers exhibit a glass transition temperature T of -30 to +40°C. g The glass transition temperature (Tg) of VAE copolymers can be determined in a known manner according to ASTM D3418-82 by differential scanning calorimetry (DSC) at a heating rate of 20 °C / min as the starting temperature. Tg can also be approximated beforehand using the Fox equation. According to Fox TG, Bull. Am. Physics Soc. 1, 3, page 123 (1956), it is stated that: 1 / Tg = x1 / Tg1 + x2 / Tg2 + ... + xn / Tgn, where xn is the mass fraction of monomer n (wt% / 100) and Tgn is the glass transition temperature (in Kelvin) of the homopolymer of monomer n. The Tg values ​​of homopolymers are listed in Polymer Handbook, 2nd Edition, J. Wiley & Sons, New York (1975).

[0027] As described above, the aqueous dispersion containing the VAE copolymer is formed by a polymerization method. In some embodiments, polymerization can be carried out under conventional emulsion polymerization procedures. Examples of such procedures are described in the Encyclopedia of Polymer Science and Engineering, Vol. 8 (1987), John Wiley & Sons, pp. 659-677 and EP1916275 A1. In some embodiments, the polymerization process is carried out in a pressure reactor at a temperature of 50°C to 120°C. The pressure reactor can be used to form a mixture of water, vinyl acetate monomer, and ethylene. This mixture may also contain additional monomers and components that will be used to produce the aqueous dispersion of the VAE copolymer.

[0028] For example, in some embodiments, an aqueous dispersion of the VAE copolymer is formed by adding a certain amount of initiator to the mixture. The initiator can be a combination of redox initiators, such as those commonly used in emulsion polymerization. However, it is preferred that the VAE copolymer is formed by polymerization initiated with an initiator that does not produce a formaldehyde-generating component. Examples of suitable oxidative initiators are sodium, potassium, and ammonium salts of peroxydisulfuric acid, hydrogen peroxide, tert-butyl peroxide, tert-butyl hydrogen peroxide, potassium peroxydiphosphate, tert-butyl pervalerate, cumene hydroperoxide, and cumene monohydroperoxide. The initiator is typically used in amounts from 0.01 to 2.0% by weight, based on the total weight of the monomers.

[0029] Preferably, when using an initiator to form an aqueous dispersion, a reducing agent is also used to form the aqueous mixture and control the reaction rate. Suitable reducing agents are sulfites and bisulfites of alkali metals and ammonium, such as sodium sulfite, derivatives of hyposulfite such as zinc hyposulfite, or alkali metal formaldehyde hyposulfites, such as sodium hydroxymethanesulfite (Brüggolit), and (iso)ascorbic acid. In some embodiments, sodium isoascorbate, as a sodium salt of (iso)ascorbic acid, is preferably used as the reducing agent. The amount of reducing agent is preferably from 0.015 to 3% by weight based on the total weight of the monomers.

[0030] The oxidizing agent, more particularly the salt of peroxydisulfate, can also be used alone as a thermal initiator.

[0031] In some embodiments, the aqueous dispersion of the VAE copolymer is formed in the presence of one or more surfactants. However, the addition of surfactants is not necessary for the formation of the aforementioned aqueous dispersion. In fact, in some embodiments, adding surfactants to the polymerization reaction may not be preferred. However, in some embodiments, it may be advantageous to use a small amount (typically 1 to 4% by weight) of surfactant based on the total weight of the monomers used in the polymerization. Alternatively, one or more surfactants may be added to the aqueous dispersion after polymerization. When provided, one or more surfactants may be used to stabilize the aqueous dispersion of the VAE copolymer. In other embodiments, one or more surfactants may be added to improve the absorbency of the nonwoven substrate, or to form an aqueous dispersion of the VAE copolymer with a bimodal particle size distribution and to reduce the viscosity of the aqueous dispersion.

[0032] Preferably, any surfactant used does not contain an alkylphenol ethoxylate structure and is not an endocrine disruptor. Suitable surfactants can be anionic, cationic, or nonionic surfactants. Examples of suitable anionic emulsifiers are alkyl sulfates with a chain length of 8 to 18 carbon atoms, alkyl ether sulfates or alkyl aryl ether sulfates having 8 to 18 carbon atoms in the hydrophobic group and having up to 40 ethylene oxide units, alkyl sulfonates or alkyl aryl sulfonates having 8 to 18 carbon atoms, and esters and half-esters of sulfosuccinic acid with monohydric alcohols or alkylphenols.

[0033] Suitable nonionic surfactants include, for example, alkyl polyethylene glycol ethers or alkyl aryl polyethylene glycol ethers having 8 to 40 ethylene oxide units. Preferred surfactants include nonionic, ethoxylated emulsifiers having branched or linear alkyl groups, or in the form of ethylene oxide-propylene oxide block copolymers.

[0034] Advantageously, the composition may comprise an aqueous dispersion of a VAE copolymer exhibiting the typical particle size and polydispersity of PVOH or surfactant-stabilized aqueous dispersions. For example, the aqueous dispersion may comprise a VAE copolymer exhibiting an average particle size of 0.3 to 5 micrometers on a volume basis. Preferably, the VAE copolymer exhibits an average particle size of 0.5 to 2 micrometers on a volume basis. Furthermore, the particle size distribution span may be 1.1 to 2.0, preferably 1.1 to 1.8. The span is defined as (D... v90 -D v10 ) / D v50 D v90 D v10 and D v50These are values ​​where 90%, 10%, and 50% of the distribution volume is below the particle diameter. Particle size can be measured by methods known in the art, such as laser scattering using a Horiba LA-920 particle size analyzer.

[0035] Aqueous dispersions of VAE copolymers can also exhibit high solids content. For example, based on the total weight of the aqueous dispersion, it may contain 50% by weight or more of solids. In some embodiments, the composition may have a solids content of 50 to 75% by weight based on the total weight of the aqueous dispersion. Preferably, the aqueous dispersion has a solids content of 50 to 60% by weight based on the total weight of the aqueous dispersion. The solids content of the dispersion can be calculated using known methods.

[0036] The composition also contains a polyvinyl alcohol (PVOH) dispersion stabilizer. The PVOH dispersion stabilizer is provided to stabilize the dispersion by helping to prevent the VAE copolymer from agglomerating. In one embodiment, the PVOH dispersion stabilizer is present in the composition at a level of 1 to 10 parts by weight per 100 parts by weight of polymer. Preferably, the PVOH dispersion stabilizer is present in the composition at a level of 2 to 8 parts, or more preferably 4 to 6 parts, per 100 parts by weight of polymer in each case.

[0037] PVOH dispersion stabilizers may contain polyvinyl alcohols known in the art, which are suitable for forming aqueous dispersions of VAE copolymers. Suitable polyvinyl alcohols include those that are partially or completely hydrolyzed polyvinyl acetate or mixtures thereof with an average degree of hydrolysis of 80 to 99.9 mol%. Suitable polyvinyl alcohols may also include ultra-low viscosity (3-4 cP for 4% aqueous solution), low viscosity (5-6 cP for 4% aqueous solution), medium viscosity (22-30 cP for 4% aqueous solution), and high viscosity (45-72 cP for 4% aqueous solution) varieties. Ultra-low viscosity polyvinyl alcohols have a mass average degree of polymerization of 150-300 and a weight average molecular weight of 13,000-23,000. Low viscosity polyvinyl alcohols have a mass average degree of polymerization of 350-650 and a weight average molecular weight of 31,000-50,000. Medium viscosity polyvinyl alcohols have a mass average degree of polymerization of 1000-1500 and a weight average molecular weight of 85,000-124,000. High-viscosity polyvinyl alcohol has a mass-average degree of polymerization of 1600-2200 and a weight-average molecular weight of 146,000-186,000. Preferably, the viscosity of the PVOH dispersion stabilizer is ultra-low, low, or medium.

[0038] The weight-average molecular weight and degree of polymerization of polyvinyl alcohol (PVA) can be determined using size exclusion chromatography / gel permeation chromatography. The viscosity of PVA can be measured using a Brookfield viscometer for a 4% aqueous solution of PVA. The viscosity of PVA can be described with reference to a 4% aqueous solution of PVA at 20°C.

[0039] In some embodiments, the PVOH dispersion stabilizer may comprise partially hydrolyzed polyvinyl acetate or mixtures thereof with an average degree of hydrolysis of 80 to 96 mol%. Partially hydrolyzed polyvinyl acetate with an average degree of hydrolysis of 86 to 90 mol% is particularly preferred, typically having a mass-average degree of polymerization of 150 to 2200 in each case. To adjust the viscosity of the resulting polymer dispersion, it may be advantageous to use a mixture of polyvinyl alcohols with different degrees of polymerization, in which case the degree of polymerization of the individual components may be less than or greater than the mass-average degree of polymerization of the mixture, for example, 150 to 2200.

[0040] In some embodiments, the PVOH dispersion stabilizer may comprise, alone or in mixture with partially hydrolyzed polyvinyl acetate, a fully hydrolyzed polyvinyl acetate having an average degree of hydrolysis of 96.1 to 99.9 mol%, typically 97.5 to 99.5 mol% in each case. The aforementioned fully hydrolyzed polyvinyl acetate may have a mass-average degree of polymerization of 150 to 2200.

[0041] Alternatively, or additionally, in some embodiments, the use of modified polyvinyl alcohol may be useful. For example, in some embodiments, the PVOH dispersion stabilizer may comprise polyvinyl alcohol containing one or more functional groups, such as one or more acetoacetyl groups. In other embodiments, the polyvinyl alcohol may comprise comonomer units. Examples of such polyvinyl alcohols include vinyl lauryl-modified polyvinyl alcohol and VERSATIC. TM Vinyl ester modified polyvinyl alcohol, which is available from Hexion Chemicals Company under the trade name VEOVA TM Purchased, for example, VEOVA TM 9 and VEOVA TM 10. Also suitable is ethylene-modified polyvinyl alcohol, for example, under the trade name EXCEVAL obtained from Kuraray America. TMThe polymer is known. The ethylene-modified polyvinyl alcohol described above can be used alone or in combination with standard unsubstituted polyvinyl alcohol. Preferred ethylene-modified polyvinyl alcohols have an ethylene fraction of up to 12 mol%, for example, 1 to 7 mol%, preferably 2 to 6 mol%, more preferably 2 to 4 mol%. Based on molecular weight data obtained via aqueous gel permeation chromatography, suitable ethylene-modified polyvinyl alcohols can in each case have a mass-average degree of polymerization of 500 to 5000, preferably 2000 to 4500, more preferably 3000 to 4000. Preferably, the ethylene-modified polyvinyl alcohol has an average degree of hydrolysis greater than 92 mol%, preferably 94.5 to 99.9 mol%, more preferably 98.1 to 99.5 mol%. In some embodiments, it may be advantageous to use a mixture of different ethylene-modified polyvinyl alcohols. Such mixtures can be used alone or as part of a mixture that also contains partially and / or fully hydrolyzed unmodified polyvinyl alcohol.

[0042] The composition comprises a copolymer used as a crosslinking agent. Specifically, and after curing, the copolymer is crosslinked with a PVOH dispersion stabilizer and a VAE copolymer. In some embodiments, the copolymer comprises maleic anhydride units. In these embodiments, the copolymer may contain 20 mol% or more of maleic anhydride units. In some embodiments, the copolymer contains 40 mol% or more of maleic anhydride units. In these embodiments, the copolymer may contain up to 70 mol% of maleic anhydride units, all based on the total monomer units of the copolymer.

[0043] When dissolved in water, maleic anhydride units are converted into maleic acid units. Accordingly, an aqueous solution of the maleic anhydride copolymer is equivalent to an aqueous solution of the maleic acid copolymer. Therefore, in some embodiments, the copolymer comprises either maleic anhydride units or maleic acid units. By using such a copolymer, crosslinking can be achieved that allows the nonwoven substrate to exhibit the desired tensile strength without generating formaldehyde.

[0044] Preferably, the copolymer does not contain any formaldehyde-generating components. In some embodiments, the copolymer may contain units of styrene, vinyl ether, vinyl acetate, (meth)acrylate, and / or other unsaturated hydrocarbons such as propylene or butene. However, in some embodiments, it is preferred that the copolymer contains ethylene units. In some embodiments, the copolymer contains ethylene-maleic anhydride. In some embodiments, ethylene-maleic anhydride copolymers may be preferred due to their water solubility and the advantageous crosslinking functional groups per weight of polymer. Ethylene-maleic anhydride copolymers may be particularly preferred when the product using the composition is intended for food contact applications. In embodiments using ethylene-maleic anhydride copolymers, it is preferred that the ratio of maleic anhydride units to ethylene units is about 1:1 in molar terms.

[0045] The composition exhibits a free formaldehyde content of 5 ppm or lower. To achieve such a low formaldehyde content, it is preferable that the composition does not contain N-hydroxymethylacrylamide. N-hydroxymethylacrylamide contains unreacted formaldehyde and contributes to increasing the formaldehyde level in the aqueous dispersion. Furthermore, according to test methods used to measure formaldehyde in nonwoven substrates, N-hydroxymethylacrylamide can contribute even higher levels of formaldehyde to the nonwoven substrate. Therefore, in some embodiments, the aqueous dispersion does not contain N-hydroxymethylacrylamide. However, in some embodiments, the aqueous dispersion can be prepared with low levels of formaldehyde if a low level of N-hydroxymethylacrylamide, particularly a mixture of commercially available low-formaldehyde N-hydroxypropylacrylamide and acrylamide, is used to prepare the dispersion. In such embodiments, it is preferable that the aqueous dispersion exhibits a formaldehyde content of less than 15 ppm, preferably less than 10 ppm, and more preferably less than 5 ppm. Alternatively, other formaldehyde-free N-hydroxymethyl functional monomers can be used in the aqueous dispersion. In embodiments using N-hydroxymethyl functional monomers, the level of these monomers in the aqueous dispersion should be less than 1%, preferably 0.5% or lower, of the total monomer content in the aqueous dispersion.

[0046] Suitable N-hydroxymethyl functional monomers for preparing VAE copolymers are, for example, N-hydroxymethylacrylamide (NMA), N-hydroxymethylmethacrylamide, allyl N-hydroxymethylcarbamate, alkyl ethers (such as isobutyl ethers) or esters of N-hydroxymethylacrylamide, N-hydroxymethylmethacrylamide, or allyl N-hydroxymethylcarbamate. If the addition of an N-hydroxymethyl functional monomer is desired, N-hydroxymethylacrylamide and N-hydroxymethylmethacrylamide are preferred.

[0047] In some embodiments, the N-hydroxymethyl functional monomer can be provided as a mixture comprising other monomers. For example, NMA can be provided as a 48% aqueous solution of NMA and acrylamide in a 1:1 molar ratio, which can be marketed under a trade name. NMA-LF monomers (Solvay, Bristol, PA) are commercially available. In some embodiments, the NMA-LF level is selected such that the formaldehyde level in the aqueous dispersion is below 10 ppm, preferably below 5 ppm. It should also be noted that NMA and acrylamide do not need to be provided as a mixture, but they can be added separately to the reactor feed during polymerization. Suitable amounts of NMA relative to the total amount of NMA plus acrylamide are in the range of 20 mol% to 80 mol%, or in the range of 30 mol% to 70 mol%, or 40 mol% to 60 mol%.

[0048] Preferably, when a copolymer containing maleic anhydride units or a copolymer containing maleic acid units is provided, vinyl acetate and ethylene are also provided. Vinyl acetate and ethylene are copolymerized in the presence of a PVOH dispersion stabilizer.

[0049] The desired monomers and ethylene are introduced into the reactor. All monomers may form the initial feed, or all monomers may form the initial feed, or some monomers may form the initial feed, with the remainder forming the feed after polymerization initiation. The feeds may be separate (spatially and temporally), or all or some components may be fed into the reactor together. In some embodiments, a first amount of vinyl acetate monomer is introduced into the reactor as the initial feed, and a second amount of vinyl acetate monomer is fed into the reactor as a second amount.

[0050] In addition, the desired copolymer, PVOH dispersion stabilizer, and (if used) surfactant are directed into the reactor. In some embodiments, the copolymer may be added to the reactor when the PVOH dispersion stabilizer or (if used) surfactant is added. All of the PVOH dispersion stabilizer may form the initial feed, or all of the dispersion stabilizer may form the feed, or a portion of the dispersion stabilizer may form the initial feed, and the remainder may form the feed after polymerization initiation. Similarly, when a surfactant is used, all of the surfactant may form the initial feed, or all of the surfactant may form the feed, or a portion of the surfactant may form the initial feed, and the remainder may form the feed after polymerization initiation.

[0051] Surprisingly, the timing of copolymer addition has been found to affect the wet tensile strength exhibited by the nonwoven product. Therefore, in some embodiments, the copolymer can be introduced into the reactor at a predetermined, desired time. For example, in one embodiment, the copolymer can be added to the reactor before polymerization initiation. In this embodiment, all the copolymer can be added to the reactor before polymerization initiation, or all the copolymer can form the feed, or a portion of the copolymer can form the initial feed and the remainder can form the feed after polymerization initiation. In these embodiments, the copolymer can be fed into the reactor before providing a first amount of vinyl acetate monomer as the initial feed. Therefore, in these embodiments, the copolymer is fed into the reactor before the formation of the VAE copolymer.

[0052] In some embodiments, the copolymer is added to the reactor after polymerization is initiated. After the VAE copolymer is formed, it can be added to the other components of the composition. In fact, in some embodiments, the composition is formed by adding the copolymer to the other components of the composition after polymerization. In one such embodiment, the copolymer can be mixed with an aqueous dispersion of the VAE copolymer in the reactor or in another container as part of a post-polymerization addition process.

[0053] The copolymer is added in a predetermined amount to form the composition. In some embodiments, the copolymer is added in an amount of 0.5% by weight or more. In other embodiments, the copolymer is added in an amount of 5% by weight or less. Preferably, the copolymer is added in an amount of 0.5% to 5% by weight. More preferably, the copolymer is added in an amount of 1% to 3% by weight. In each case, the weight percentage of the copolymer added is based on the total weight of the monomers in the aqueous dispersion.

[0054] In some embodiments, undesirable materials such as residual monomers can be removed by employing known post-polymerization methods. Residual monomers may include, for example, unreacted vinyl acetate monomers. Preferably, for example, the content of unreacted vinyl acetate monomers in the composition is less than 1000 ppm after post-polymerization. In some embodiments, unreacted vinyl acetate monomers can be removed before the copolymer is added to the reactor. Unreacted vinyl acetate monomers can be removed by methods known in the art, such as stripping. In other embodiments, post-polymerization can be initiated with a redox catalyst. Volatile residual monomers can also be removed by distillation, preferably distillation under reduced pressure, and optionally in which an inert entrainment gas such as air, nitrogen, or vapor is passed through or through the aqueous dispersion. After removal of residual monomers and other undesirable substances, the aqueous dispersion of the VAE copolymer can be prepared for use in the composition. Typically, biocides, defoamers, and potentially other additives can be added to the composition.

[0055] Preferably, the composition has a high solids content. For example, the VAE copolymer, PVOH dispersion stabilizer, and copolymer together typically comprise 45 to 65% by weight of the composition. More typically, the composition has a solids content in the range of 50 to 60% by weight, or 50 to 55% by weight. The solids content of the composition can be calculated using known methods.

[0056] Preferably, the composition is formulated to exhibit a minimized free formaldehyde content. For example, the composition exhibits a free formaldehyde content of 5 ppm or less. The free formaldehyde content of the composition can be determined using liquid chromatography according to ASTM D5910-05.

[0057] Furthermore, it is preferred that the composition exhibits the desired viscosity. Preferably, when measured at 23°C and using an LV-3 spindle at 60 rpm, the composition exhibits a Brookfield viscosity of 4000 mPa⁻² or lower. In some embodiments, the composition may exhibit a Brookfield viscosity in the range of 50 to 4000 mPa⁻² when measured at 23°C and using an LV-3 spindle at 60 rpm. Preferably, when measured at 23°C and using an LV-3 spindle at 60 rpm, the composition exhibits a viscosity in the range of 50 to 2000 mPa⁻². More preferably, when measured at 23°C and using an LV-3 spindle at 60 rpm, the composition exhibits a viscosity in the range of 300 to 1000 mPa⁻². The viscosity of the composition can be determined using commercially available viscometers, such as those available from Brookfield.

[0058] Preferably, the composition is stable, as evidenced by the relatively small change in Brookfield viscosity exhibited by the composition over a long period. In some embodiments, the Brookfield viscosity exhibited by the composition does not undergo a change of more than 60% after 75 days at 20°C. Preferably, the Brookfield viscosity exhibited by the composition does not undergo a change of more than 25% after 75 days at 20°C. More preferably, any change in the Brookfield viscosity exhibited by the composition after 75 days at 20°C is 10% or less. The Brookfield viscosity exhibited by the composition after 75 days at 20°C can be determined at 23°C using an LV-3 spindle at 60 rpm and a commercially available viscometer as described above.

[0059] After the composition is formed, it can be applied to a nonwoven substrate by any of a variety of application methods (including but not limited to spraying, saturation, foaming and printing).

[0060] Preferably, the nonwoven substrate comprises a fibrous material. The fibrous material used in the nonwoven substrate can be a natural fiber, such as cellulose fiber, or a synthetic fiber including, but not limited to, one or more of polyester, polyethylene, polypropylene, and polyvinyl alcohol, or viscose fiber, or any combination of these fibers, or a mixture of natural and synthetic fibers. The fibrous nonwoven substrate can be produced by various methods, including but not limited to air-laid, wet-laid, carded, and hydraulically entangled processes.

[0061] After the composition is applied to the substrate, the substrate can be dried using commercially available drying equipment such as an oven or another method. Drying can be carried out at a predetermined temperature and for a predetermined time. Drying temperatures and times known in the art are suitable when using the composition. Preferred drying temperatures and times are those that achieve maximum crosslinking, and in some embodiments, this temperature and time can be 150°C for 2 minutes or longer.

[0062] Once dry, nonwoven products exhibit the desired wet tensile strength. However, the specific wet tensile strength exhibited by a nonwoven product will vary between embodiments depending on, for example, the composition of the substrate and the amount of composition applied to the substrate. For example, an air-laid substrate containing 88% cellulose fibers and 12% bicomponent fibers comprising a polyester with a polyethylene sheath applied at a 20% addition amount based on the dry composition on the dry substrate by weight. In fact, in such instances, nonwoven products can exhibit wet tensile strengths ranging from 600 to 2000 g / 5 cm. Methods and apparatus known in the art for determining wet tensile strength are suitable for determining the tensile strength of nonwoven products. For example, wet tensile strength can be determined by testing according to ASTM D5035-95 using a commercially available tensile testing machine (such as the Instron 1122 model).

[0063] Similarly, once dry, nonwoven products exhibit the desired dry tensile strength. However, the specific dry tensile strength exhibited by a nonwoven product will vary between embodiments depending on, for example, the composition of the substrate and the amount of composition applied to the substrate. For example, an air-laid substrate containing 88% cellulose fibers and 12% bicomponent fibers comprising a polyester with a polyethylene outer sheath applied at a 20% addition amount based on the dry composition on the dry substrate by weight can exhibit a dry tensile strength greater than 2500 g / 5 cm. In this example, the nonwoven product can exhibit a dry tensile strength of 2500 to 6000 g / 5 cm. Methods and apparatus known in the art for determining dry tensile strength are suitable for determining the tensile strength of nonwoven products. For example, dry tensile strength can be determined by testing according to ASTM D5035-95 using a commercially available tensile testing machine (such as the Instron 1122 model).

[0064] Preferably, once dry, the nonwoven product exhibits a wet tensile strength within the expected range of its dry tensile strength. For example, in one embodiment, and once dry, the nonwoven product exhibits a wet tensile strength of 25% or greater of its dry tensile strength. In some embodiments, the nonwoven product exhibits a wet tensile strength of 25% to 60% of its dry tensile strength. Preferably, the nonwoven product exhibits a wet tensile strength greater than 25% of its dry tensile strength. In some embodiments, it may be preferred that the nonwoven product exhibits a wet tensile strength of 30% or greater of its dry tensile strength. In these embodiments, once dry, the nonwoven product may exhibit a wet tensile strength of 30% to 60% of its dry tensile strength. In some embodiments, once dry, the nonwoven product may exhibit a wet tensile strength of 40% or greater. In these embodiments, once dry, the nonwoven product may exhibit a wet tensile strength of 40% to 60% of its dry tensile strength.

[0065] In addition, nonwoven products exhibit formaldehyde content of 1.5 ppm or lower. The formaldehyde content of nonwoven products can be measured using commercially available extraction methods, such as extraction with deionized water at various pH values ​​according to ASTM D5910-96.

[0066] Example

[0067] The following describes embodiments within the scope of this invention. Comparative examples outside the scope of this invention are also described below. These embodiments are provided merely for the purpose of further illustrating and disclosing embodiments of the compositions and methods of their preparation.

[0068] Preparation and evaluation of the composition

[0069] Formaldehyde content:

[0070] The free formaldehyde content was determined using ASTM D5910-96.

[0071] Viscosity:

[0072] The measurements were taken at 23°C using a Brookfield viscometer (model DV-1) with an LV-3 spindle at 60 rpm.

[0073] Solid content:

[0074] The determination was performed using a Cenco digital moisture balance (manufactured by CSC Scientific company) and a 125-watt infrared lamp. 2–5 grams of sample were placed on the pan of the Cenco moisture balance and run under an automated program.

[0075] Example 1

[0076] First, 300.0 grams of deionized water and 486.9 grams of 20% [amount of water] were added to the reactor. 205 aqueous solution (which is polyvinyl alcohol with an average hydrolysis level of 88%) and 4% Brookfield aqueous solution (which has a viscosity of 5.2-6.2 cP and is available from Sekisui), 162.5 g of 10% An aqueous solution of E60 (an alternating copolymer of ethylene and maleic anhydride with a number average molecular weight of 30,000, available from Vertellus) and 2.1 g of a 5% aqueous solution of ferrous ammonium sulfate were mixed in a 1-gallon pressure reactor. The pH of the mixture was 3.0. The reactor was purged with nitrogen, and then 1100.0 g of vinyl acetate was added to the reactor with stirring (200 rpm). The reactor was purged with ethylene, the stirring was increased to 1000 rpm, and 245 g of ethylene was added to the reactor. The temperature was raised to 32°C, and 7.3 g of a 4.75% aqueous solution of sodium isoascorbate was added to the reactor. The reactor contents were equilibrated. Both a 1.50% aqueous solution of hydrogen peroxide and a 4.75% aqueous solution of sodium isoascorbate were continuously fed into the reactor at a rate of 0.20 g / min. After polymerization initiation (indicated by a 1°C temperature rise), an additional 278.0 g of vinyl acetate was continuously metered into the reactor over 90 minutes at a rate of 3.09 g / min, and the reactor temperature was raised to 85°C over 60 minutes. During polymerization, the flow rates of hydrogen peroxide and sodium isoascorbate were maintained at approximately a 1:1 ratio, and the flow rates were adjusted to maintain a reaction temperature of 85°C. Polymerization proceeded for approximately 3.5 hours until no more heat of polymerization was released and the vinyl acetate level was below 1.5%. After 3.5 hours, the hydrogen peroxide and sodium isoascorbate feeds were stopped, the reactor was cooled to 50°C, and the reaction mixture was transferred to a degasser to remove unreacted ethylene. 1.5 g of... A mixture of VF defoamer (available from BASF) and 10.0 g of water was used to suppress foam formation during transfer. To reduce unreacted vinyl acetate monomer to below 0.1%, 25.0 g of an 8.0% aqueous solution of sodium isoascorbate and 20.0 g of an 8.0% aqueous solution of tert-butyl hydrogen peroxide were continuously added to the contents of the degasser over 15 minutes. The composition of Example 1 had a solids content of 53.7%, a pH of 3.9, and a Brookfield viscosity of 654 mPa-s. The residual vinyl acetate level in the composition was <1000 ppm, and the composition had a measured particle size level of 25 ppm after passing through a 100-mesh (149 μm) sieve. The composition of Example 1 was formed such that the copolymer of ethylene and maleic anhydride was present in an amount of 1% by weight, where the weight percentage of the copolymer was based on the total weight of the monomers in the composition.

[0077] Example 2

[0078] First, 200.0 grams of deionized water and 486.9 grams of 20% [amount of water] were added to the reactor. 205 aqueous solution and 324.6 grams of 10% An E60 aqueous solution was used to form a mixture in a 1-gallon pressure reactor. The pH of the mixture was adjusted to 3.4 with 24.5 g of a 10% sodium hydroxide aqueous solution, and 2.1 g of a 5% ferrous ammonium sulfate aqueous solution was added. The reactor was purged with nitrogen, and then 1100.0 g of vinyl acetate was added to the reactor with stirring (200 rpm). The reactor was purged with ethylene, the stirring was increased to 1000 rpm, and 245 g of ethylene was added to the reactor. The temperature was raised to 32°C, and 7.3 g of a 4.75% sodium isoascorbate aqueous solution was added to the reactor. The reactor contents were equilibrated. Both a 1.50% hydrogen peroxide aqueous solution and a 4.75% sodium isoascorbate aqueous solution were continuously fed into the reactor at a rate of 0.20 g / min. After polymerization initiation (indicated by a 1°C temperature rise), an additional 278.0 g of vinyl acetate was continuously metered into the reactor over 90 minutes at a rate of 3.09 g / min, and the reactor temperature was raised to 85°C over 60 minutes. During polymerization, the flow rates of hydrogen peroxide and sodium isoascorbate were maintained at approximately a 1:1 ratio, and the flow rates were adjusted to maintain a reaction temperature of 85°C. Polymerization proceeded for approximately 3.5 hours until no more heat of polymerization was released and the vinyl acetate level was below 1.5%. After 3.5 hours, the hydrogen peroxide and sodium isoascorbate feeds were stopped, the reactor was cooled to 50°C, and the reaction mixture was transferred to a degasser to remove unreacted ethylene. 1.5 g of... A mixture of VF defoamer and 10.0 g of water was used to suppress foam formation during transfer. To reduce unreacted vinyl acetate monomer to below 0.1%, 25.0 g of an 8.0% aqueous solution of sodium isoascorbate and 20.0 g of an 8.0% aqueous solution of tert-butyl hydrogen peroxide were added sequentially over 15 minutes. The composition of Example 2 had a solids content of 51.2%, a pH of 3.8, and a Brookfield viscosity of 1308 mPa-s. The residual vinyl acetate level of the composition was <1000 ppm, and the composition had a measured particle size level of 26 ppm after passing through a 100-mesh (149 μm) sieve. The composition of Example 2 was formed such that the copolymer of ethylene and maleic anhydride was present in an amount of 2% by weight, where the weight percentage of the copolymer was based on the total weight of the monomers in the composition.

[0079] Preparation of Examples 3, 4 and 5

[0080] First, 430.0 grams of deionized water and 487.5 grams of 20% [amount of water] were added to the reactor. A mixture was formed in a 1-gallon pressure reactor using a 205 aqueous solution. The pH of the mixture was adjusted to 3.2 with 4.4 g of a 50% phosphoric acid aqueous solution, and 2.1 g of a 5% ferrous ammonium sulfate aqueous solution was added. The reactor was purged with nitrogen, and then 1144.0 g of vinyl acetate was added to the reactor with stirring (200 rpm). The reactor was purged with ethylene, the stirring was increased to 1000 rpm, and 195 g of ethylene was added to the reactor. The temperature was raised to 32°C, and 7.3 g of a 4.75% sodium isoascorbate aqueous solution was added to the reactor. The reactor contents were equilibrated. Both a 1.50% hydrogen peroxide aqueous solution and a 4.75% sodium isoascorbate aqueous solution were continuously fed into the reactor at a rate of 0.20 g / min. After polymerization initiation (indicated by a 1°C temperature rise), an additional 286.0 g of vinyl acetate was continuously metered into the reactor over 90 minutes at a rate of 3.18 g / min, allowing the reactor temperature to rise to 85°C over 50 minutes. During polymerization, the flow rates of hydrogen peroxide and sodium isoascorbate were maintained at approximately a 1:1 ratio, and the flow rates were adjusted to maintain a reactor temperature of 85°C. Polymerization proceeded for approximately 3.5 hours until no more heat of polymerization was released and the vinyl acetate level was below 1.5%. After 3.5 hours, the hydrogen peroxide and sodium isoascorbate feeds were stopped, the reactor was cooled to 50°C, and the reaction mixture was transferred to a degasser to remove unreacted ethylene. 1.5 g of... A mixture of VF defoamer and 10.0 g of water was used to suppress foam formation during transfer. To reduce unreacted vinyl acetate monomer to below 0.1%, 25.0 g of an 8.0% sodium isoascorbate aqueous solution and 20.0 g of an 8.0% tert-butyl hydrogen peroxide aqueous solution were added sequentially over 15 minutes. The resulting aqueous dispersion of the VAE copolymer had a solids content of 52.0%, a pH of 5.0, and a Brookfield viscosity (60 rpm) of 248 mPa-s. The residual vinyl acetate content was <1000 ppm. The particle size level measured after passing through a 100-mesh (149 μm) sieve was 37 ppm.

[0081] Examples 3, 4 and 5

[0082] The compositions referred to below as Examples 3, 4, and 5 were formed by mixing samples of an aqueous dispersion of a VAE copolymer with different amounts of a 10% aqueous solution of poly(ethylene-co-maleic anhydride). The composition of Example 3 comprised an aqueous dispersion of a VAE copolymer and a certain amount of a 10% aqueous solution of poly(ethylene-co-maleic anhydride) such that the copolymer was present in an amount of 1% by weight. The composition of Example 4 comprised an aqueous dispersion of a VAE copolymer and a certain amount of a 10% aqueous solution of poly(ethylene-co-maleic anhydride) such that the copolymer was present in an amount of 2% by weight. The composition of Example 5 comprised an aqueous dispersion of a VAE copolymer and a certain amount of a 10% aqueous solution of poly(ethylene-co-maleic anhydride) such that the copolymer was present in an amount of 3% by weight. In each of Examples 3-5, the percentage by weight of poly(ethylene-co-maleic anhydride) is based on the total weight of the monomers in the composition.

[0083] The following reference will be made to a comparative example, namely Comparative Example 1, which is a sample of an aqueous dispersion of a VAE copolymer used to form the compositions of Examples 3-5, the compositions of which do not contain any copolymers containing maleic anhydride units or copolymers containing maleic acid units.

[0084] Table 1 below lists the free formaldehyde content of certain examples and Comparative Examples 1-2 (which are commercially available compositions comprising a vinyl acetate-ethylene-acrylamide-N-hydroxymethylacrylamide polymer with a glass transition temperature (starting) in the range of 7-13°C). The compositions are named as follows: 192 is available for sale and can be obtained from Wacker Chemical Corporation.

[0085] Table 1

[0086] Composition Formaldehyde (ppm) Example 2 3.5 Example 4 1.3 Comparative Example 1 3.98 Comparative Example 2 49.7

[0087] As shown in Table 1, the compositions of Examples 2 and 4 each exhibited free formaldehyde content of less than 5 ppm. In stark contrast, the composition of Comparative Example 2 exhibited free formaldehyde content of more than 45 ppm.

[0088] Furthermore, the stability of Examples 1-4 was measured by measuring the change in Brookfield viscosity of each composition after 75 days at 20°C using an LV-3 spindle at 60 rpm and a Brookfield viscometer (model DV-1). The changes in Brookfield viscosity of the compositions of Examples 1-4 are shown in Table 2 below.

[0089] Table 2

[0090]

[0091] As shown in Table 2, each of the compositions in Examples 1-4 did not experience a Brookfield viscosity change of more than 60% after 75 days at 20°C. In fact, the compositions in Examples 1-3 exhibited Brookfield viscosity changes of less than 25% after 75 days at 20°C. More preferably, the compositions in Examples 1-2 exhibited Brookfield viscosity changes of less than 10% after 75 days at 20°C.

[0092] Comparative Example 3

[0093] First, add 385.0 grams of deionized water and 9.0 grams of [unclear text - possibly a specific ingredient or ingredient]. MA80I is a surfactant stabilizer sold as an 80% aqueous solution of sodium di(1,3-dimethylbutyl)sulfosuccinate, available from Solvay USA, in 390.0 grams of 10% sodium di(1,3-dimethylbutyl)sulfosuccinate. An E60 aqueous solution was used to form a mixture in a 1-gallon pressure reactor. The pH of the mixture was adjusted to 3.2 with 23.6 g of a 10% sodium hydroxide aqueous solution, and 2.0 g of a 5% ferrous ammonium sulfate aqueous solution was added. The reactor was purged with nitrogen, and then 263.0 g of vinyl acetate was added to the reactor with stirring (200 rpm). The reactor was purged with ethylene, the stirring was increased to 900 rpm, and 195 g of ethylene was added to the reactor. The temperature was raised to 55°C. The reactor contents were allowed to equilibrate. An aqueous mixture of sodium persulfate (4.50%) and sodium bicarbonate (2.00%), as well as a 2.03% sodium isoascorbate aqueous solution, were continuously fed into the reactor at a rate of 0.40 g / min. Following polymerization initiation (indicated by a 1°C temperature rise), an additional 1492.0 g of vinyl acetate and 500.0 g of a solution, consisting of 383.1 g of deionized water and 19.5 g of..., were continuously metered into the reactor over 120 minutes at rates of 12.43 g / min and 4.17 g / min, respectively. 2403 (which is a 50% aqueous solution of sodium 2-acrylamide-2-methylpropanesulfonate and is available from Lubrizol Corporation) and 97.4 grams TLA3040 (which is a 40% aqueous solution of ethoxylated tridecyl alcohol having approximately 30 ethylene oxide units and is available from Solvay USA) was used. Furthermore, the reactor temperature was raised to 85°C within 30 minutes. During polymerization, the flow rates of sodium persulfate and sodium isoascorbate feeds were maintained at approximately a 1:1 ratio, and the flow rates were adjusted to maintain a reaction temperature of 85°C. Polymerization proceeded for approximately 3.25 hours until no more heat of polymerization was released and the vinyl acetate level was below 1.5%. After 3.25 hours, the sodium persulfate / sodium bicarbonate and sodium isoascorbate feeds were stopped, the reaction was cooled to 50°C, and the reaction mixture was transferred to a degasser to remove unreacted ethylene. 1.0 g of [unspecified ingredient] was added. A mixture of 670 defoamer (available from Solvay USA) and 10.0 g of water was used to suppress foam formation during transfer. To reduce unreacted vinyl acetate monomer to below 0.1%, 2.0 g of sodium isoascorbate dissolved in 20.0 g of deionized water and 2.5 g of tert-butyl hydroperoxide dissolved in 30.0 g of deionized water were added to the dispersion. The resulting aqueous dispersion of the VAE copolymer had a solids content of 54.8%, a pH of 3.4, and a Brookfield viscosity (60 rpm) of 390 mPa⁻². The residual vinyl acetate content was <1000 ppm. The particle size level, measured after passing through a 100-mesh (149 μm) sieve, was 48 ppm.

[0094] Comparative Example 4

[0095] Comparative Example 4 was formed in a manner similar to Comparative Example 3, except that 10% of the VAE copolymer was not added before forming the aqueous dispersion. An aqueous solution of E60 was added to the reactor. Conversely, for Comparative Example 4, a sample of the aqueous dispersion of the VAE copolymer was mixed with a certain amount of 10% poly(ethylene-co-maleic anhydride) aqueous solution, such that the copolymer was present in an amount of 2% by weight, which was based on the total weight of the monomers in the composition.

[0096] Preparation and evaluation of nonwoven products

[0097] Nonwoven products were then formed by treating the nonwoven substrate with one of the compositions of Examples 1-5 and Comparative Examples 1-4. The nonwoven products formed using the compositions of Examples 1-5 are hereinafter referred to as Examples 1A-5A, and the nonwoven products formed using Comparative Examples 1-4 are hereinafter referred to as Comparative Examples 1A-4A.

[0098] To form Examples 1A-5A and Comparative Examples 1A-4A, the compositions of the specific examples or comparative examples were sprayed onto an air-laid substrate containing 88% cellulose fibers and 12% bicomponent fibers, the bicomponent fibers comprising polyester with a polyethylene sheath. Each composition was applied to the air-laid substrate at an addition amount of 20%, based on the dry composition on the dry substrate by weight. After spraying, the nonwoven products were cured in an oven at 150°C for 2.5 minutes and conditioned at 72°F and 50% humidity for 24 hours.

[0099] The wet tensile strength of each example and comparative was tested by immersing the nonwoven product in water and a 1% surfactant mixture before testing in the transverse direction using Instron Model 1122.

[0100] In addition, the dry tensile strength in the transverse direction of each embodiment and comparative example was tested using an Instron model 1122. Table 3, provided below, lists the dry and wet tensile strengths of each embodiment and comparative example. Dry and wet tensile strengths are reported in g / 5cm in Table 3. The relationship between wet and dry tensile strengths is reported as a percentage in a separate column labeled "Wet / Dry". Free formaldehyde content for some embodiments and comparative examples is also provided in Table 3.

[0101] Table 3

[0102]

[0103] As shown in Table 3, the compositions of Examples 2A and 4A each exhibited free formaldehyde content of less than 1.5 ppm. In stark contrast, the composition of Comparative Example 2A exhibited free formaldehyde content of more than 10 ppm.

[0104] Furthermore, as shown in Table 3, treating the nonwoven substrate with the compositions of Examples 1-5 enabled the resulting nonwoven products to exhibit the desired dry and wet tensile strengths. Therefore, all nonwoven products of Examples 1A-5A possessed a wet tensile strength greater than 25% of the dry tensile strength exhibited therefrom. In fact, the composition of Example 2 enabled the nonwoven product of Example 2A to exhibit a wet tensile strength of 96% of the wet tensile strength exhibited by the nonwoven product of Comparative Example 2A.

[0105] Furthermore, Examples 2A and 4A, and Comparative Examples 3A-4A, illustrate the effect of using a polyvinyl alcohol (PVA) dispersion stabilizer in the composition forming for nonwoven products. For example, Examples 2A and 3A were each formed using a composition in which the copolymer was present in an amount of 2% by weight, based on the total weight of the monomers in the composition. However, the composition of Example 2 used a PVA dispersion stabilizer, while in the composition of Comparative Example 3, the aqueous dispersion of the VAE copolymer was stabilized by a surfactant. As shown in Table 3, the PVA dispersion stabilizer used in the nonwoven product of Example 2A exhibited a wet tensile strength that was more than 200% greater than that exhibited by the nonwoven product of Comparative Example 3A. Similar results were observed when Example 4A was compared with Comparative Example 4A, each of which included a composition in which an aqueous dispersion of the copolymer and the VAE copolymer was mixed to form the composition.

[0106] As will be apparent from the foregoing detailed description, various modifications, additions, and other alternative embodiments are possible without departing from their true scope and spirit. The embodiments discussed herein were chosen and described to provide the best illustration of the principles of the invention and its practical application, thereby enabling those skilled in the art to use the invention in various embodiments and to provide various modifications suitable for the particular intended use. It should be understood that all such modifications and variations are within the scope of the invention.

Claims

1. A crosslinkable stable composition for use in nonwoven substrates, comprising: Aqueous dispersions of vinyl acetate-ethylene copolymers, polyvinyl alcohol dispersion stabilizers, and copolymers containing maleic anhydride units or copolymers containing maleic acid units, wherein the compositions exhibit a free formaldehyde content of 5 ppm or lower. The condition is that the copolymer is dissolved in the aqueous dispersion and the copolymer contains 20 mol% or more maleic anhydride units or contains 20 mol% or more maleic acid units.

2. The crosslinkable stable composition according to claim 1, wherein the composition exhibits a viscosity of 4000 mPa-s or lower when measured at 23°C and using an LV-3 spindle at 60 rpm.

3. The crosslinkable stable composition according to claim 1, wherein the copolymer further comprises ethylene units.

4. The crosslinkable stable composition according to claim 1, wherein the composition does not contain N-hydroxymethylacrylamide.

5. The crosslinkable stable composition according to claim 1, wherein the composition comprises 0.5% by weight or more of the copolymer based on the total weight of the monomers in the aqueous dispersion.

6. The crosslinkable stabilizing composition according to claim 1, wherein the polyvinyl alcohol dispersion stabilizer comprises partially hydrolyzed polyvinyl acetate with an average degree of hydrolysis of 80 to 96 mol%.

7. The crosslinkable stable composition according to claim 2, wherein the composition exhibits a viscosity of 200 to 4000 mPa-s when measured at 23°C and using an LV-3 spindle at 60 rpm.

8. The crosslinkable stable composition according to claim 2, wherein the viscosity of the composition changes by no more than 60% after 75 days at 20°C.

9. The crosslinkable stable composition according to claim 3, wherein the copolymer comprises an ethylene-maleic anhydride polymer.

10. The crosslinkable stable composition according to claim 8, wherein the viscosity of the composition changes by no more than 25% after 75 days at 20°C.

11. A method for preparing a crosslinkable and stable composition for a nonwoven substrate, comprising: Provide copolymers containing maleic anhydride units or copolymers containing maleic acid units; Provides vinyl acetate and ethylene; In the presence of a polyvinyl alcohol dispersion stabilizer, vinyl acetate and ethylene are polymerized to form an aqueous dispersion of a vinyl acetate-ethylene copolymer; and A composition comprising the vinyl acetate-ethylene copolymer and having a formaldehyde level of 5 ppm or lower is formed. The condition is that the copolymer is dissolved in the aqueous dispersion and the copolymer contains 20 mol% or more maleic anhydride units or contains 20 mol% or more maleic acid units.

12. The method of claim 11, wherein the vinyl acetate-ethylene copolymer is provided in an aqueous dispersion formed by emulsion polymerization.

13. The method of claim 11, wherein the copolymer is mixed with the polyvinyl alcohol dispersion stabilizer prior to the formation of the vinyl acetate-ethylene copolymer.

14. The method of claim 11, wherein the copolymer is provided in an amount of 0.5% by weight or more, based on the total weight of the monomers in the aqueous dispersion.

15. The method of claim 11, wherein the composition exhibits a viscosity of 50 to 4000 mPa-s when measured at 23°C and using an LV-3 spindle at 60 rpm.

16. The method of claim 11, wherein the copolymer further comprises ethylene units.

17. The method of claim 12, further comprising mixing the copolymer with the aqueous dispersion to form the composition.

18. The method of claim 14, wherein the copolymer is provided in an amount of 0.5 to 5% by weight, based on the total weight of the monomers in the aqueous dispersion.

19. Nonwoven products, including: Nonwoven substrates treated with the composition of claim 1, wherein the nonwoven product exhibits a wet tensile strength that is 25% or greater than the dry tensile strength exhibited, and exhibits a formaldehyde content of 1.5 ppm or less.

20. The nonwoven product of claim 19, wherein the copolymer and the vinyl acetate-ethylene copolymer are crosslinked.

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