MXPA99005694A - Method and composition for increasing the strength of compositions containing high-bulk fibers - Google Patents

Abstract

Cross-linked cellulose fibers having free pendant carboxylic acid groups are disclosed. The fibers include a polycarboxylic acid covalently coupled to the fibers, and are cross-linked with a cross-linking agent having a cure temperature lower than the cure temperature of the polycarboxylic acid. Methods for producing the fibers and for producing a fibrous sheet incorporating the fibers are also disclosed.

Description

METHOD AND COMPOSITION TO INCREASE THE RESISTANCE OF COMPOSITIONS CONTAINING HIGHLY BULK FIBERS FIELD OF THE INVENTION The present invention is generally directed to a method and composition for increasing the strength of compositions containing highly bulked fibers. Specifically, the invention is directed to modified cellulose fibers to include free pendant carboxylic acid groups that impart increased resistance to products that have been prepared from these fibers.

BACKGROUND OF THE INVENTION Cellulose products such as absorbent sheets and other structures are composed of cellulose fibers, which, in turn, are composed of individual cellulose chains. It is common for the cellulose fibers to be entangled to impart beneficial properties such as the increased capacity of absorption, bulking and elasticity of the products containing said interlaced fibers. Highly bulky fibers are fibers that are generally highly interlaced and characterized by their high absorption and elasticity.

Intertwined cellulose fibers and methods for their preparation are widely known. Tersoro and Willard, Cellulose and Cellulose Derivatives, Bikales and Segal, eds, Part V, Wiley-lnterscience, New York, (1971), p. 835-875. The interlaced cellulose fibers are prepared by treating the fibers with an entanglement agent. Interlacing agents are generally compounds that perform two functions which, in the context of cellulose entanglement, covalently couple a hydroxy group from one cellulose chain to another hydroxy group in an immediate cellulose chain. In the entanglement processing, the hydroxy groups of the cellulose are consumed and replaced with entanglements (ie, in the covalent bonds that bind the crosslinker with the cellulose fiber). For example, the loss of hydroxy groups on the crosslinking of cellulose with a carboxylic acid crosslinking agent is accompanied by the formation of ester bonds. The sheet tension or strength of fibrous products that are derived from cellulose fibers are due in large part to the attractive interactions between one fiber and another. These interactions between fibers comprise the hydrogen bonding interactions between fibers that contain hydrogen bonding sites. For cellulose, the hydrogen bonding sites include the hydroxy groups of the individual cellulose chains. In general, the interlaced fibers have a greater capacity of absorption, bulging and elasticity than those non-interlaced or untreated fibers. On the contrary, due to the availability of its hydroxy groups as sites for hydrogen bonding, cellulose fibers that have not been treated have a higher binding capacity compared to other cellulose fibers. The result is that, although the fibrous products derived from the interlaced fibers possess beneficial absorbent properties, these products typically suffer from undesirable sheet resistance or low stress. The relatively low tensile strength is attributed mainly to the reduction of the hydrogen bond between fibers resulting from the depletion of hydrogen bonding sites of the fiber (eg, hydroxy cellulose groups) in the entanglement. As noted above, the crosslinking agents react at the hydrogen bonding sites of the fibers, converting these sites into entanglements that generally do not significantly participate in hydrogen bonding between the fibers. As a consequence, the absorbent properties that are associated with the interlaced fibers are accompanied by a corresponding reduction in the bonding capacity of the fibers with other fibers. Accordingly, there is a need in this art for highly bulked cellulose fibers having absorbent properties and, in addition, having the improved bonding capacity to increase the strength of the products including these fibers. The present invention satisfies these needs and also offers additional related advantages.

BRIEF DESCRIPTION OF THE INVENTION In one aspect, the present invention provides interlaced cellulose fibers with an interlacing agent, and a polycarboxylic acid that has been covalently coupled to the fibers through an ester linkage. Preferably, the crosslinking agent has a cure temperature lower than the cure temperature of the polycarboxylic acid. In a preferred embodiment, the polycarboxylic acid is a polyacrylic acid. Also disclosed are fiber sheets containing cellulose fibers containing free pendant carboxylic acid and absorbent products containing these fiber sheets. In another aspect of the invention, a method for producing cellulose fibers having improved bonding capacity is provided. The method produces cellulose fibers containing free pendant carboxylic acid groups. In the method, an interlacing agent and a polycarboxylic acid are applied to the fibers, and then cured at a temperature sufficient to cause an entanglement formation between the interlacing agent and the fibers, and furthermore, to cause an ester bond formation between polycarboxylic acid and fiber. Preferably, the formation of the ester linkage between the polycarboxylic acid and the fiber is the formation of a single ester linkage, and not the formation of extensive ester crosslinks.

Fiber sheets containing interlaced cellulose fibers having free, pendant carboxylic acid groups and absorbent products containing these fiber sheets are also disclosed. In a further embodiment to this aspect of the invention, a method is provided for producing a highly bulked cellulose fiber sheet having an increased tensile strength. In the method, the fibers that have not been treated are combined with those cellulose fibers that have free pendant carboxylic acid groups and that have been transformed into a fibrous sheet.

DETAILED DESCRIPTION OF THE PREFERRED MODALITY The present invention is directed to cellulose fibers having a bonding capacity and improved methods in relation to said fibers. Specifically, the invention relates to those cellulose fibers having free pendant carboxylic acid groups, products containing these cellulose fibers, and methods related to the production and use of these fibers. The cellulose fibers of the invention show a high capacity for absorption, bulking and elasticity, and when said fibers are replaced by conventionally entangled fibers in a mixture between the interlaced fiber and the untreated fiber, the resulting sheet then has a tension or increased sheet resistance.

In one aspect, the present invention provides cellulose fibers having an improved binding capacity. These fibers include a polycarboxylic acid covalently coupled to the cellulose fibers. Because the polycarboxylic acid is covalently coupled to these fibers, the cellulose fibers of the invention have free pendant carboxylic acid groups. As used in the present invention, the term "free pendant carboxylic acid groups" refers to a carboxylic acid substituent per polycarboxylic acid, present after partially curing the polycarboxylic acid (e.g., after the formation of a bond of ester between a carboxylic acid group of the polycarboxylic acid and a hydroxy group of the cellulose fiber). Said carboxylic acid group hangs from the polycarboxylic acid and is released to form hydrogen bonds with other fibers, for example. The fibers of the present invention are produced by a "partial cure" of the polycarboxylic acid in the presence of the fibers. Although "cure" refers to the exhaustive reaction of an agent (eg, an interlacing agent) with the fibers, partial healing refers to less than an exhaustive reaction. For example, for many crosslinking agents, including the crosslinking agents of the polycarboxylic acid, it is desired and an exhaustive reaction is achieved between almost all the carboxylic acid groups of the agents and the fibers by means of, either reaction times and / or elevated curing temperatures. Partial healing refers to a non-exhaustive reaction, for example, coupling less than all the groups of a simple carboxylic acid group and preferably in a single carboxylic acid group of a polycarboxylic acid to a fiber. While the exhaustive reaction occurs at a compound curing temperature, less than the exhaustive reaction or only the partial cure occurs at less than the curing temperature of the compound. The degree of cure is also a function of the period when a curable agent is heated to a given temperature. Those skilled in the area of polycarboxylic acids will recognize that polycarboxylic acids are useful in this invention and can be present in the fibers in a variety of ways including, for example, the free acid form and the salts that are derived therefrom. Although the free acid form is preferred, it will be appreciated that all such forms are included within the scope of the invention. In the context of the present invention, suitable polycarboxylic acids include polycarboxylic acids having molecular weights of at least 500 g / mol, preferably within the molecular weight range of from about 500 to about 25,000 g / mol, most preferably 1 , 000 to about 10,000 g / moles and most preferably still from 1,500 to about 5,000 g / moles. The carboxylic acid can be a polymeric polycarboxylic acid. Suitable polymeric polycarboxylic acids include homopolymeric and copolymeric polycarboxylic acids. Representative homopolymeric polycarboxylic acids include, for example, polyacrylic acid polyaspartic acid, polyglutamic acid, poly (3-hydroxybutyric acid) and polymaieic acid. Examples of the copolymeric polycarboxylic acids are copolymers of polyacrylic acid such as polyacrylamide-co-acrylic acid, polyacrylic acid-co-maleic acid, polyethylene-co-acrylic acid, and poly (1-vinylpyrrolidone-acid-co-acid). acrylic, as well as other polycarboxylic acid copolymers including polyethylene-co-methacrylic acid, poly-methyl-acid-co-methacrylic polymethacrylate, polymethylvinyl ether-co-maleic acid, polystyrene-co-maleic acid, poly (3-hydroxybutyric acid) -co-3-hydroxyvaleric acid) and poly (vinyl) -acetate-co-maleic acid chloride In a preferred embodiment, the polymeric polycarboxylic acid is a polyacrylic acid In another preferred embodiment the polymeric polycarboxylic acid is a copolymer of the acid acrylic, and preferably a copolymer of acrylic acid and another acid, for example, maleic acid The representative polycarboxylic acids mentioned above ior are available in different molecular weights and molecular weight scales that are obtained from commercial sources. The polycarboxylic acids indicated above can be used alone or in combination with others to provide the cellulose fibers of the present invention having free pendant carboxylic acid groups. In order to appreciate more easily the chemical and structural properties of the polycarboxylic acid that has been useful in this invention, and particularly to be able to appreciate the relationship between the molecular weight, length and number of carboxylic acid groups of the polycarboxylic acid, will illustrate as an example the consideration of a representative polycarboxylic acid, polyacrylic acid. Accordingly, the polycarboxylic acid coupled to the fibers of the invention includes polyacrylic acids having molecular weights of at least about 500 g / mol, preferably within the molecular weight range of about 1,000 to about 15,000 g / moles, and most preferably from 1500 to about 5000 g / moles. Accordingly, the polycarboxylic acid includes polyacrylic acids having an amount of acrylic acid residues greater than about 7 (the acrylic acid repeats the units in the polymer), preferably from 10 to 200 residues of acrylic acid, and most preferably from 20 to 70 acrylic acid residues. As a consequence, the polycarboxylic acid includes polyacrylic acids having an amount greater than about 7 carboxylic acid groups, preferably from 10 to 200 carboxylic acid groups, and most preferably from 20 to 70 carboxylic acid groups. The polycarboxylic acid is polyfunctional and has the ability to provide a relatively greater number of useful carboxylic acid groups in the hydrogen bond between the fibers and in the increase in the strength of the fibrous sheets, meshes and mats that incorporate said fibers. The cellulose fibers containing pendant carboxylic acid groups and which have been formed in accordance with the present invention, include a polycarboxylic acid preferably having a molecular weight of at least 500 g / mol covalently to a cellulose fiber through a cellulose fiber. ester link. Although the polycarboxylic acid useful in the present invention is not an entanglement agent, it will be appreciated that multiple ester linkage formation may occur between a polycarboxylic acid and one or more cellulose chains or fibers and, therefore, such a link between Polycarboxylic acid and fibers are within the scope of this invention. For example, the polycarboxylic acid can form a single ester bond to a cellulose chain, two or more ester bonds to a chain, or two or more ester bonds between two or more chains or fibers. In any case, after covalent coupling to the fiber, the polycarboxylic acid has at least five free pendant carboxylic acid groups. In this paragraph, the crosslinking agents of the polymeric polyacrylic acid for the cellulose fibers have been described. See, for example, the patent of E.U.A. do not. 5,549,791, issued to Herrón et al. It was found that the crosslinking agents of the polyacrylic acid are particularly suitable to be able to form the entanglement bonds of esters with the cellulosic fibers. Unlike conventional interlacing agents that are very sensitive to temperature, polyacrylic acid is stable at high temperatures and, therefore, these crosslinking agents may be subject to high cure temperatures to efficiently and effectively provide highly interlaced fibers. Generally, these crosslinking agents of the polyacrylic acid penetrate into the individual fibers and are cured by subjecting the fibers treated with the interlacing agent to elevated temperatures (for example, example, an acrylic / maleic copolymer cures at about 187.7 ° C for 8 minutes, and a polyacrylic acid polymer cures at about 190.5 ° C for 30 minutes). The result is the formation of interlaced links between the fibers. Therefore, as indicated in Herron's patent, the fibers that are entangled provide an increased elasticity and an absorbent capacity to absorbent structures containing these fibers. Unlike the polyacrylic acid crosslinking agent treatment described in Herron's patent, in the present invention, polycarboxylic acids are not subject to high cure temperatures to cause the exhaustive polycarboxylic acid to be entangled with the fibers. Instead, in this invention, the polycarboxylic acid is cured at a significantly lower temperature in order to achieve the opposite effect, i.e., to be able to drive the covalent coupling of the carboxylic acid to the fibers and, at the same time, maintain sufficient groups of free carboxylic acids (e.g., non-interlaced) in order to impart the beneficial properties of binding to the fibers and the resistance to fibrous compositions incorporating these fibers. In the context of the present invention, the polycarboxylic acid is covalently and optimally coupled to the fiber through a single carboxylic acid group, forming a single ester linkage between the fiber and the polycarboxylic acid. The reaction through a simple carboxylic acid allows the remaining carboxylic acid groups of the polycarboxylic acid to participate in interactions between the fibers (e.g. hydrogen bonding) in fibrous compositions; thus allowing the resistance of those compositions. Accordingly, although the invention described in Herron is generally incorporated into polycarboxylic acid in the cellulosic fibers, due to the various treatments and goals, the resulting products are different. Herrón uses polyacrylic acid, as an interlacing agent. The present invention uses a polycarboxylic acid as a reinforcing agent to improve the binding capacity of the fibers. In example 2, we describe the effect of the curing temperature on the strength of the sheets of the fiber that incorporates the fibers of the present invention. Cellulose fibers having free pendant carboxylic acid groups have an effective amount of a polycarboxylic acid covalently coupled to the fibers through an ester linkage. This is, sufficient polycarboxylic acid to provide an improvement in strength (e.g., stress, sheet) in the compositions (e.g., fibrous sheets, webs, meshes, mats) containing the cellulose fibers to which the polycarboxylic acid has been covalently coupled and relative to conventional fibers lacking such free pendant carboxylic acid groups. As described in example number 1, fiber sheets that have been prepared from the combination of untreated fibers and fibers having free pendant carboxylic acid groups (for example DMDHEU polyacrylic acid / interlacing) have increased the strength of tension compared to those sheets of fiber prepared from fibers not treated and interlaced fibers that do not have any carboxylic acid pendant group (e.g., the DMDHEU that has been interlaced) only. Generally, the cellulose fibers are treated with a sufficient amount of such a polycarboxylic acid that an effective amount of polycarboxylic acid has been covalently coupled to the fibers. The polycarboxylic acid is preferably present in the fibers, in an amount from 0.1 to about 10% by weight of the total weight of the fibers. Most preferably, the polycarboxylic acid is present in an amount of from about 1 to 6% by weight of the total weight of the fibers, and in a particular preferred embodiment, from 2 to 4 percent by weight of the total weight of the fibers . At less than about 0.1 weight percent of the polycarboxylic acid, no significant improvement in binding capacity is observed and at an amount greater than about 10% by weight, the fibers begin to be disadvantageously brittle. For polycarboxylic acids with molecular weights between 1000 and 15,000 g / moles, the preferred range of polycarboxylic acid in the fibers (eg, from 0.1 to 10% by weight of the total fibers) corresponds to a range of about 0.001 to about 0.20 mole percent of the polycarboxylic acid (based on the molecular weight of 162 g / mole for an anhydroglucose unit). Accordingly, in the context of the present invention, the amount of polycarboxylic acid in the fibers is significantly less than the interlaced fibers of the polycarboxylic acid of low molecular weight, same as previously described; having an effective amount of an interlacing agent in the range of 0.5 to about 10 mole percent (see for example, U.S. Patent Nos. 5,137,537, 5,183,707 and 5,190,563). The polycarboxylic acid can be applied to the fibers by covalent coupling by any of the known methods in the production of treated fibers. For example, the polycarboxylic acid can make contact with the fibers as a fiber sheet that passes through a bath containing polycarboxylic acid. Alternatively, within the scope of the present invention, there are other methods for applying the polycarboxylic acid; examples of such methods are: aspersion of fibers, or spraying and pressing, or dipping and pressing with a solution of polycarboxylic acid. Preferably, the fibers of the present invention containing free pendant carboxylic acid groups, are cellulose fibers that have been entangled with an interlacing agent. The crosslinking agents preferably have a cure temperature lower than that of the polycarboxylic acid; for example, below 160 ° C. The use of crosslinking agents with cure temperatures below the cure temperature of the polycarboxylic acid, allows the total cure of the crosslinking agent, while only partially curing the polycarboxylic acid (as described above). Preferred crosslinking agents include urea derivatives, for example, methylolated urea, cyclic methylolated ureas, substituted cyclic ureas, methylolated with lower alkyl, cyclic dihydroxy ureas, dihydroxy cyclic ureas substituted with lower alkyl, cyclic dihydroxymethylolated ureas. Other preferred crosslinking agents include dimethyldihydroxyurea (DMDHU, 1,3-dimethyl-4,5-dihydroxy-2-imidazolidinone), dimethyloldihydroxyethyleneurea (DMDHEU, 1,3-Dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone). ), dimethylolurea (DMU, bis [N-hydroxymethyl] -urea), dihydroxyethylene urea, (DHEU 4,5-dihydroxy-2-imidazolidinone) dimethylethylene urea (DMEU, 1,3-dihydroxymethyl-2) -imidazolidinone) dimethylolethyleneurea (DDl, 4,5-dihydroxy-1,3-dimethylamino-2-imidazolidinone) and maleic anhydride. In a preferred embodiment, the crosslinking agent is a dimethyloldihydroxyethyleneurea (DMDHEU). The crosslinking catalysts can be used in combination with the crosslinking agent to promote the formation of entanglement. Usually, the interlaced cellulose fibers of the present invention, have free pendant carboxylic acid groups that can be prepared by applying a polycarboxylic acid as described above, and an interlacing agent having a cure temperature lower than the cure temperature of polycarboxylic acid to the cellulose fibers, and then curing the polycarboxylic acid and the crosslinking agent at a temperature sufficient to cause the formation of entanglement between the crosslinking agent and the fibers, and a bond formation of esters between the polycarboxylic acid and the fibers. In the context of the present invention, said formation of ester linkage between the polycarboxylic acid and the fibers does not represent an exhaustive formation of ester linkage, as it occurs in the entanglement of the fiber. The temperature sufficient to cause a bond formation of esters is lower than the cure temperature of the interlacing agent and may vary depending on the specific acid and the moisture content of the fibers among other factors. For the illustrative acid, the polyacrylic acid, the temperature sufficient to cause ester bond formation ranges from about 160 ° C to about 193.3 ° C. The use of a catalyst is optional, as described above, to promote an entanglement and a bond formation of esters between the polycarboxylic acid and the cellulose fiber in the method and can reduce the temperature that is required to cause the formation of bonds of esteres. While the catalysts can be used to effectively lower the cure temperature of both the crosslinking agent and the polycarboxylic acid according to the present invention, the use of catalysts will preferably not result in an exhaustive entanglement of the polycarboxylic acid at the fibers. The cellulose fibers of the invention can also be prepared with the aid of a catalyst. In said method, the catalyst is applied to the cellulose fibers analogously to the application of the carboxylic acid to the fibers as described above. The catalyst can be applied to the fibers before, after or at the same time when the polycarboxylic acid is applied to the fibers. Accordingly, the present invention provides a method of producing fibers by joining the free pendant carboxylic acid groups including the curing of the crosslinking agent and the polycarboxylic acid in the presence or absence of a catalyst. Generally, the catalyst promotes the formation of bonds between the crosslinking agent and / or the polycarboxylic acid and the cellulose fibers. The catalyst is very effective in increasing the formation of the ester bond (eg, the number of bonds formed) at a given cure temperature. Suitable catalysts include any catalyst that increases the rate of bond formation between the crosslinking agent and / or the polycarboxylic acid described above and the cellulose fibers. Preferred catalysts include alkali metal salts of phosphorus-containing acids, such as alkali metal hypophosphites, alkali metal phosphites, alkali metal polyphosphonates, alkali metal phosphates, and alkali metal sulfonates. Preferred catalysts include alkali metal polyphosphonates, such as sodium hexametaphosphate, and alkali metal hypophosphites such as sodium hypophosphite. When a catalyst is used to promote bond formation, the catalyst finds present in an amount in the variation or in scale between 5 to 20 weight% of the polycarboxylic acid. Preferably, the catalyst is present in an amount of about 10% by weight of the polycarboxylic acid.

In general, the cellulose fibers of the present invention can be prepared by means of a system and apparatuses described in the patent E.U.A. No. 5,447,977 to Young, Sr. and others, which are incorporated herein by reference in their entirety. Concisely, the fibers are prepared by means of a system and apparatus consisting of a conveyor device for transporting the mat of the cellulosic fibers through a fiber treatment zone; an applicator for applying a treatment substance, such as an interlacing agent and a polycarboxylic acid from a source to the fibers to be subjected to the fiber treatment zone; a fiberizer to completely separate the individual cellulose fibers including the mat to form a fiber outlet consisting of substantially decomposed cellulose fibers; and a dryer coupled to the fiberizer for the residual moisture that has evaporated and to cure the interlacing agent and the polycarboxylic acid, and thus form dry and cured fibers. As used in the present invention, the term "mat" refers to any non-woven sheet structure consisting of cellulose fibers or other fibers that are not covalently bonded together. The fibers include fibers that have been obtained from wood pulp or other resources such as cotton cloth, hemp, grass, cane, husks, corn stalks, or other suitable resources of cellulose fibers that are can deposit inside a sheet. The mat of the cellulose fibers preferably in the form of an extended sheet may be one of several discrete sized bale plates or it can be a continuous roll. Each cellulose fiber mat is transported by means of a conveyor device, for example, a conveyor belt or series of drive rolls. The conveyor device carries the mats through the fiber treatment zone. In the fiber treatment zone, the interlacing agent and the polycarboxylic acid are applied to the cellulose fibers. The crosslinking agent and the polycarboxylic acid are preferably applied to one or both surfaces of the mat using any variety of methods known in the art, such as spraying, driving or immersion. Once the materials have been applied to the mat, the materials can be evenly distributed through the mat itself; for example, passing the mat through a pair of rollers. After the fibers have been treated with the interlacing agent and the polycarboxylic acid, the impregnated mat is fiberized by feeding it through a hammer mill. The hammer mill serves to separate the mat into its individual cellulose fibers, which are then blown in a dryer. The dryer develops two functions in sequence; first, by removing the residual moisture from the fibers, and second, by curing the crosslinking agent and the polycarboxylic acid according to the present invention. In one embodiment, the dryer consists of a first drying zone for receive the fibers and to remove the residual moisture from them through an instant drying method, and a second drying zone for curing. Alternatively, in another embodiment, the treated fibers are dried through an instant dryer to remove residual moisture and then transferred to an oven where the treated fibers are subsequently cured according to the present invention. Interlaced cellulose fibers having free pendant carboxylic acid groups provide beneficial absorbent properties that are characteristic of the entangled fibers including high capacity, bulk and relative elasticity to fibers that are not interlaced. Furthermore, because these fibers are interlaced with a low cure temperature interlacing agent, an interlacing is achieved at a lower temperature than the cure temperature of the polycarboxylic acid, therefore, minimizing any interlacing of polycarboxylic acid. Accordingly, although the hydrogen bonding sites have been consumed by the crosslinking agent, and due to the polycarboxylic acid component which has been cured at a partially high temperature, the hydrogen bonding sites are then added to the fiber in the process of interlacing. This hydrogen bonding site includes free pendant carboxylic acid groups of the partially cured polycarboxylic acid. The interlaced fibers of this embodiment, which have been cured at a temperature below the cure temperature of the carboxylic acid (for example, the temperature at which a exhaustive entanglement), have a considerable number of free pendant carboxylic acid groups compared with the fibers cured with the interlacing agent or at a higher cure temperature with the polycarboxylic acid alone. The fibers of the invention containing free pendant carboxylic acid groups can be formed into sheets or mats having a very high capacity for absorption, bulking, elasticity and increased tensile strength. For example, these fibers can be combined with other fibers, such as interlaced and non-interlaced fibers, including highly bulked fibers. Sheets and mats that consist of fibers that have free pendant carboxylic acid groups can be incorporated into a variety of absorbent products includingfor example, sheets of tissue, disposable diapers, products used for incontinence in adults, sanitary napkins and feminine hygiene products, such as tampons, bandages and other products, which are used for the absorption of waste in the packaging of edible meats. It has been observed that the interlaced cellulose fibers have free pendant carboxylic acid groups in the present invention, when they are used to conventionally replace entangled cellulose fibers in a laminate or mesh of interlaced fibers and non-interlaced fibers, they can increase the strength of sheet tension. As indicated above, the pendant carboxylic acid groups free of the fibers provide hydrogen bonding sites that increase the bonding capacity of the fibers to bond with other fibers. In another aspect, the present invention provides a method for producing a highly bulky fiber sheet having an increase in tensile strength. In the method, the untreated fibers are combined with the cellulose fibers in the present invention (for example the interlaced cellulose fibers containing free pendant carboxylic acid groups), giving rise to the formation of a sheet or mat. In a preferred embodiment, the cellulose fibers having free pendant carboxylic acid groups contain from 1 to 4% by weight of polycarboxylic acid in the fibers, with a polycarboxylic acid content of partially cured acid at a temperature between 148.8 ° C. 171.1 ° C. The cellulose fibers having free pendant carboxylic acid groups are present in an amount between 20 and 100, and preferably between 30 and 60% by weight of the total fibers that have been combined to form the sheet. The highly bulked sheet produced by the method has an increased tensile strength relatively compared to the sheet which was similarly prepared from highly bulked fibers lacking free pendant carboxylic acid groups. Examples 1 and 2 describe the preparation and properties of a fiber sheet that has formed from the interlaced fibers, containing free pendant carboxylic acid groups using a representative crosslinker such as (e.g. dimethyloldihydroxyethylene urea) and polycarboxylic acid (for example, polyacrylic acid). As shown in the examples, the incorporation of said entangled fiber to a sheet of the fiber increases the tension index of the sheet. In Example 1, the sheets were prepared from a mixture between the interlaced fiber and the untreated fibers (2: 1) (see, e.g., Example 1, Table 1). For these mixtures, adding from 0.5 to about 1.0% by weight of a representative polycarboxylic acid, a polyacrylic acid having a molecular weight of 10,000 grams / moles, to an interlaced cellulose fiber (4 weight percent of dimethyloldihydroxy ethylene urea) increases the tension index by 100%, relative to the sheets having the same mixture of fibers intertwined with the untreated fibers (for example, fibers intertwined with DMDHEU only, in the absence of a polycarboxylic acid). Example 2 describes the effect of the polycarboxylic acid content and the cure temperature for the fiber sheets incorporating the fibers of the present invention. Generally, by increasing the content of polycarboxylic acid in the fibers, the strength of the sheets incorporating the fibers is increased, and as the curing temperature of the fibers of the present invention increases, the strength of the sheets of the fiber decreases. they incorporate the fibers. The following examples illustrate the practice of the present invention and it is not intended to limit them therefrom.

EXAMPLES In general, the cellulose fibers of the present invention and the products containing these fibers can be prepared by means of a system and apparatus which are described in the patent E.U.A. No. 5,447,977 of Young, Sr. et al., Incorporated in its entirety in the present invention by simple reference.

EXAMPLE 1 The preparation and properties of fiber sheets formed from interlaced fibers containing free pendant carboxylic acid groups In this example both the preparation and the properties of the fiber sheets that have been formed from interlaced fibers containing free pendant carboxylic acid groups are described. This example demonstrates that a polycarboxylic acid can be added to another entanglement system of the fiber to achieve the bonding capacity of the fibers in sheets or mats. In the process, fiber sheets composed of individual cellulose fibers (Weyerhaeuser Co., New Bern, NC) were treated with a polyacrylic acid with a molecular weight of 10,000 grams / moles (HF-05, Rohm & Haas) and with dimethyloldihydroxyethylene urea (DMDHEU) at different proportions and according to the following procedure: Concisely, a fiber sheet of a roller was fed through a bath constantly saturated with an aqueous solution containing the polyacrylic acid and the DMDHEU adjusted to such concentrations to achieve a desired level of polyacrylic acid. (For example, from about 0.25 to about 1% by weight of the total composition) and DMDHEU (for example, from 2 to 4% by weight of the total composition) in addition to the fiber sheet. Then the treated fiber was moved through a roller grip assembly to remove enough solution to provide a fiber sheet with a moisture content of about 50%. After passing through the roll grip, the wet sheet of the fiber was fiberized when feeding the sheet through a hammer mill. The resulting fibers were blown through an instant dryer to a powder extractor where the treated cellulose fibers were collected. The treatment of the treated fibers was completed by placing the fibers in a laboratory oven and heating at approximately 165.5 ° C for a period of 5 minutes. Then, the interwoven fibers were added to the pulp fibers without treatment of southern pine kraft pulp (NB416, Weyerhaeuser Co., Federal Way, WA) in a fiber to fiber ratio of 2: 1 (treated: untreated) . The resulting blended fibers were then formed into test sheets using a standard TAPPI test sheet mold. It was determined the tension index of these test sheets using an Instron Stress Test Instrument. The results were summarized in table 1 TABLE 1 The tension index of fiber sheets interlaced with polyacrylic acid (PAA) and the combinations of dimethyloldihydroxyethyleneurea (DMDHEU) As indicated in Table 1 above, adding the polyacrylic acid to the cellulose fibers interlaced with a representative urea-based cross-linking agent, DMDHEU, increases the tensile strength of the sheets by incorporating said entangled fibers. At a constant entanglement of the DMDHEU (for example, 4% by weight), increasing the amount of polyacrylic acid (for example, from 0 to 1% by weight) increases the tensile strength of the sheets that have been prepared from the fibers. For example, the prepared sheets of the interlaced fibers having from 0.5 to 1% by weight of polyacrylic acid in the fiber have a tensile strength twice that of the sheets which were similarly prepared from the fibers intertwined with only the DMDHEU. In addition, the resistances of the sheets containing fibers treated with polyacrylic acid and with DMDHEU, prepared as described above, were determined in combination with the pulps of the untreated fiber (NB416 and NF405, Weyerhaeuser Co., Federal Way, WA). they used two interlacing systems of polyacrylic acid and dimethyloldihydroxyethylene urea to prepare the treated fibers: (1) PAA: DMDHEU (1: 1); and (2) PAADMDHEU (1: 3). The sheets were prepared by combining the interlaced and untreated fibers in the ratio of 2: 1 (interlaced: untreated) (designated as PAA: DMDHEU (1: 1 = and PAA: DMDHEU (1: 3) of Table 2 which is shown below) A control sheet composed of interwoven fibers with DMDHEU and untreated fibers (2: 1) was also prepared for comparison (designated as DMDHEU in table 2 below) .For these sheets, the breaking load, the tension index, and the percentage resistance increase in relation to the fibers intertwined only with DMDHEU, are summarized in the following Table 2 TABLE 2 Strength of fiber sheets interlaced with polyacrylic acid (PAA) and combinations of dimethyldihydroxyurea (DMDHEU) As shown in table 2, by adding the polyacrylic acid to the DMDHEU crosslinking agent we have as a result an increase in the strength of the sheets that were prepared from the fibers interlaced only with the DMDHEU. For the sheets that were prepared from the interlaced fibers where the ratio of polyacrylic acid to DMDHEU is 1: 1, the strength of the sheet is increased by 30% (eg, 36% increase for NB416, and 28% for NB405) compared to the sheets that were prepared from the interlaced fibers only with the DMDHEU. By decreasing the amount of polyacrylic acid in the interlaced fibers, compared to the interlacing agent DMDHEU, it appears that there is a decrease in the strength of the sheets containing these fibers (for example 17% increase for PAA: DMDHEU). (1: 3) compared to the 36% increase for PAA: DMDHEU 1: 1)).

EXAMPLE 2 The effect of the polyacrylic acid content and the curing temperature on the fiber sheets formed from the interlaced fibers containing free pendant carboxylic acid groups This example illustrates the effect of the polyacrylic acid content and the cure temperature on the fibrous sheets taking into account the bulge, absorptive capacity, and tensile strength of said fibers that were formed from the interlaced fibers containing groups. of carboxylic acid free pendants. A test of the absorption capacity is developed on a test pad by recording the initial sample weight in dry (Wi) in grams. The test pad is then placed in a sieve with wire support and immersed in synthetic urine, a saline solution containing 135 meq / l sodium, 8.6 meq / l calcium, 7.7 meq / l magnesium, and 7.95% urea by weight (based on total weight), plus other ingredients, available in the "National Scientific" under the brand called RICCA in a horizontal position for 10 minutes. The pads are then removed from the synthetic urine solution and allowed to drain for 5 minutes. Therefore, the pads are placed under .0703 kg / cm2 for 5 minutes. The wet pad (W2) in grams is reweighed. The total capacity under load is reported as W2-W ?. Unit capacity under load is calculated by dividing the total capacity by dry weight, (W2-W1 / W1).

A dry pad tension integrity test is developed on a 25.8 cm2 by 25.8 cm2 test pad. When holding a dry test pad on two opposite sides. Approximately 7.6 cm in length of the pad is visible between the fasteners. The sample is pulled vertically on an Instron test machine and the tensile strength is measured to report it in N / m. The tensile strength is converted into the stress index, Nm / g, by dividing the tensile strength by the basis weight g / m2. In this example, the polyacrylic acid (PAA) was combined with the dimethyloldihydroxyethylene urea (DMDHEU) in different proportions and applied to a fibrous sheet as described above in example 1. In one of the groups of experiments, the treated fibers were cured resulting at 165.5 ° C, a temperature that totally cures the DMDHEU cross-linking agent, but partially cures the PAA (for example, PAA is covalently coupled to the fibers since the polycarboxylic acid retains the free pendant carboxylic acid groups), to provide the interlaced fibers of DMDHEU. The untreated fibers (NB416) were then added to the interlaced fibers at a fiber to fiber ratio of 2: 1 (interlaced: untreated) and formed into test sheets as described above in Example 1. Then the bulging, the absorption capacity and the tension index of said sheets were determined for the different combinations of DMDHEU: PAA. The results were summarized in the following table 3.

TABLE 3 The effect of the polyacrylic acid content on the fiber sheet strength The results show that the polyacrylic acid can be added to a pulp fiber interlacing system to improve the bonding capacity of the fibers in sheets or mats. For the intertwining system of the DMDHEU that was previously used, the greater increase in sheet strength for fibers containing a polyacrylic acid grade of 1% up to 2% by weight of the total fibers treated. In another group of experiments, PAA: DMDHEU treated fibers (for example 4% DMDHEU, 1% PAA) were cured and cured at different temperatures (171.1 ° C, 182.2 ° C, 193.3 ° C) and were combined with the fibers that had not been treated and which had been formed into sheets as described above.An control sheet composed of an interlaced DMDHEU fiber was also prepared with untreated fibers for comparison.For these sheets, both the bulking and the absorption capacity and the index were measured. The results are summarized in Table 4 below: TABLE 4 The effect of the cure temperature on the strength of the fiber sheet The results illustrate that increasing the cure temperature to the fibers treated with polyacrylic acid results in a more complete reaction between the polyacrylic acid and the cellulose fibers, resulting in the availability of fewer carboxyl groups to improve the bond on the sheet. The results generally indicate that a loss of sheet strength occurs with an increase in cure temperature, for these sheets containing polyacrylic acid. Although the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes may be made therein without departing from the spirit and scope of the present invention.

Claims (24)

NOVELTY OF THE INVENTION CLAIMS

1. - Interlaced cellulose fibers containing free pendant carboxylic acid groups, consisting of cellulose fibers interlaced with an interlacing agent and a polycarboxylic acid covalently coupled to the fibers, further characterized in that the crosslinking agent has a cure temperature below the temperature of curing of the polycarboxylic acid and further characterized in that the polycarboxylic acid provides the fibers with free pendant carboxylic acid groups.

2. The fibers according to claim 1, further characterized in that the polycarboxylic acid is covalently coupled to the fibers by means of an ester linkage.

3. - The fibers, according to claim 1, further characterized in that the polycarboxylic acid has a molecular weight in the range of about 500 to about 20,000 grams / moles.

4. The fibers according to claim 1, further characterized in that the polycarboxylic acid has a molecular weight in the scale of about 1,500 to about 5,000 grams / moles.

5. - The fibers according to claim 1, further characterized in that the polycarboxylic acid is a polyacrylic acid.

6. - The fibers according to claim 1, further characterized in that the polycarboxylic acid is present in the fibers in an amount of about 0.1 to about 10% by weight of the fibers.

7. The fibers according to claim 1, further characterized in that each polycarboxylic acid provides at least about five free pendant carboxylic acid groups to the fibers.

8. - The fibers according to claim 1, further characterized in that the interlacing agent is a maleic anhydride.

9. - The fibers according to claim 1, further characterized in that the interlacing agent is a urea based crosslinking agent.

10. The fibers according to claim 9, further characterized in that the urea based crosslinking agent was selected from the group consisting of dimethyloldihydroxyethyleneurea, dimethylolurea, dihydroxyethyleneurea, dimethyletylenediurea, dimethyldihydroxyethylene urea and mixtures thereof.

11. - The fibers according to claim 1, further characterized in that the interlacing agent is a mixture of maleic anhydride and a urea-based cross-linking agent.

12. - A fiber sheet constituted by cellulose fibers interlaced with an interlacing agent and a polycarboxylic acid covalently coupled to the fibers, further characterized in that the interlacing agent has a cure temperature below the cure temperature of the polycarboxylic acid, and characterized further because the polycarboxylic acid provides groups to the free pendant carboxylic acid fibers,

13. - The sheet of the fiber in accordance with the claim 12, further characterized in that the polycarboxylic acid is a polyacrylic acid.

14. - The fiber sheet according to claim 12, further chacterized because it is constituted by non-interlaced cellulose fibers.

15. - The fiber sheet according to claim 14, further characterized in that the non-interlaced cellulose fibers are present in an amount of about 10 to 80% of the total fiber weight.

16. - An absorbent product that is constituted by cellulose fibers interlaced with an interlacing agent and a polycarboxylic acid covalently coupled to the fibers characterized by the interlacing agent and because they have a cure temperature below the curing temperature of the polycarboxylic acid, and further characterized in that the polycarboxylic acid provides the fibers with free pendant carboxylic acid groups.

17. The absorbent product according to claim 16, further characterized in that the carboxylic acid is a polyacrylic acid.

18. The absorbent product according to claim 16, further characterized in that it is constituted by non-interlaced cellulose fibers.

19. - A method for producing interlaced cellulose fibers containing free pendant carboxylic acid groups and consisting of: the application of a polycarboxylic acid; the application of an interlacing agent at a cure temperature below the cure temperature of the polycarboxylic acid to the cellulose fibers and the cure of the polycarboxylic acid and the crosslinking agent at a temperature sufficient to cause the formation of entanglement between the interlacing agent and the fibers and, to cause the formation of an ester linkage between the polycarboxylic acid and the fibers and also to produce entangled cellulose fibers containing free pendant carboxylic acid groups.

20. - The method according to claim 19, further characterized in that the curing of the polycarboxylic acid and the cross-linking agent at a temperature sufficient to cause the formation of the entanglement between the interlacing agent and the fibers, in addition to the formation of the ester bond between the polycarboxylic acid and the fibers, consists in heating the cross-linking agent to the curing temperature.

21. - The method according to claim 19, further characterized in that it consists of adding an effective amount of a catalyst to the cellulose fibers (prior to curing).

22. - A method for producing a highly bulked cellulose fiber sheet having increased tensile strength, consisting of: the combination of untreated fibers and interlaced cellulose fibers containing free pendant carboxylic acid groups, interlaced cellulose fibers by means of an interlacing agent and a polycarboxylic acid covalently coupled to the fibers, further characterized in that the crosslinking agent has a cure temperature below the cure temperature of the polycarboxylic acid and characterized further because the polycarboxylic acid provides the fibers of groups free carboxylic acid pendants; forming the combined fibers into a sheet to produce a highly bulked cellulose fiber obtaining an increase in tensile strength.

23. - The method according to claim 22, further characterized in that the interlaced cellulose fibers having free pendant carboxylic acid groups are present in an amount of from about 20 to about 90% of the weight of the total fibers.

24. - The method according to claim 22, further characterized in that the untreated fibers consist of highly bulky fibers.