MXPA99004111A - A method of producing an absorbent structure having improved strength properties - Google Patents

Abstract

A method of producing an absorbent structure that includes hydrophilic fibres and particles of superabsorbent material, comprising forming an air-laid structure from fibres and superabsorbent particles. At least a part of the air-laid structure is moistened to a moisture content in the moistened region of at least 15%calculated on the total weight of the structure within the moistened region, whereafter the structure is dried to a moisture content of at most 12%. The moisture treatment improves the tensile strength of the absorbent structure. Liquid dispersion in the structure is also improved.

Description

METHOD OF PRODUCTION OF AN ABSORBENT STRUCTURE WITH IMPROVED RESISTANCE PROPERTIES FIELD OF THE INVENTION The present invention relates to a method for producing an absorbent structure exposed to air that includes hydrophilic fibers and particles of superabsorbent material. Such absorbent structures are used in sanitary articles, such as diapers, incontinence guards, sanitary napkins and similar articles.

DESCRIPTION OF THE BACKGROUND OR? "THE INVENTION An absorbent structure in a sanitary article of the aforementioned type proposed for one-time use is commonly constituted by one or more layers of hydrophilic fibers, usually pulp soft cellulose.

The structure will also frequently include so-called superabsorbents, which are polymers that can often absorb their own weight in water or body fluid. Such an absorbent structure is flexible and comfortable to use and also has great absorption capacity. A drawback of these known absorbent structures is their relatively low strength properties, particularly their tensile strength, which sometimes can cause problems in some steps of the manufacture of the sanitary article in question, and also during the use of the aforementioned article. Various attempts have been made to improve the strength and structural coherence of such absorbent articles, inter alia, by mixing synthetic thermoplastic fibers with the subsequent heating of the absorbent structure; see for example, US 4,590,114. The thermoplastic fibers are mixed with this and contribute to a more coherent structure that has improved the strength properties. The drawback with this solution is the relatively high price of thermoplastic fibers and the negative influence that so-called thermal bonding has on the absorption properties. A method for producing an absorbent structure exposed to moisture, of relatively high tensile strength, is discussed in US Pat. No. 4,551,142, for example. This method includes providing an aqueous dispersion of the cellulose fibers and superabsorbent particles, the formation of this dispersion in sheets exposed to moisture which are then dehydrated, dried and compressed to a desired density. The procedure and equipment required in this process are completely different from those normally applied in the air exposure method, in which the pulp of Leaf or bale cellulose is crumbled dry to form the so-called soft pulp and exposed to the air to form a pulp mat together with the superabsorbent particles. US 516,569 discloses that an absorbent compound containing 40 to 85% superabsorbent particles is wetted with a moisture content of between 15 to 30% to bind the superabsorbent particles in the composite.

OBJECTIVES AND MOST IMPORTANT CHARACTERISTICS OF THE INVENTION The object of the present invention is to provide a method for producing an absorbent structure formed in accordance with a method of exposure to air and having considerably improved strength properties. This object is achieved, according to the invention, by moistening the structure exposed to air with a moisture content - at least 15%, preferably at least 20%, and more preferably at least 35%, calculated on the total weight of the structure, and then drying the structure at a moisture content of at plus 12%, preferably not more than 10%. When the absorbent structure is moistened or hydrated with a high moisture content, water can be compressed from the structure before drying it. The structure is preferably wetted with distilled water or deionized water. The inventive method is flexible, by virtue of the fact that it is only necessary to humidify or wet certain parts of the structure to obtain a better resistance. For example, the structure can be moistened with a suitable wetting pattern, for example, in a pattern of strips or in a network pattern. In the case of contoured structures where the different parts of the structure have mutually different unit area weights, it may be appropriate to wet only those parts that have the least weight per unit area and with this the least resistance.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an electron microscope photograph of an ana structure of soft pulp that includes superabsorbent particles. Figure 2 is a photograph corresponding to a corresponding treated structure according to the invention.

DESCRIPTION OF THE INVENTION The absorbent structure is formed by a conventional air exposure method sec [sic], for example EP 0 122 042, according to which the cellulose pulp is shredded in dry for soft cellulose pulp and a mixture of this pulp and superabsorbent particles are exposed to air to form a network, or exposed to air in molds on the so-called mat forming wheel, to form the absorbent structure. The superabsorbent particles can alternatively be applied as a layer between layers of pulp, instead of being mixed with pulp fibers. It is also possible to form two or more layers one on top of the other. These layers may contain mutually different superabsorbency percentages, different types of pulp fibers and / or superabsorbents, so as to impart the desired properties to the absorbent structure, such as the properties of liquid capture, liquid dispersion and liquid storage. The construction of such absorbent structures is well known in the art and will not be described in detail herein. The cellulose fibers can be made up of chemical pulp, mechanical pulp, thermomechanical pulp or chemithermomechanical pulp (CTMP). Chemically crosslinked cellulose fibers can also be used. Other types of hydrophilic fibers are regenerated cellulose (viscose), polyester and hydrophobic fibers that have been treated with a hydrophilizing agent. Several different types of superabsorbents can be used. A common feature of these superabsorbents is that they are polymers which are able to absorb and bind water or body fluid in amounts that correspond to several times their own weight. Examples of polymers used for this purpose are polyacrylates, alginates, polysaccharides, such as cellulose derivatives, starch derivatives, etc., and copolymers and graft polymers thereof. The superabsorbent particles will preferably be presented in an amount corresponding to at least 3% calculated on the total weight of the structure before wetting the superabsorbent particles, preferably at least 5% and not more than 80%, preferably not more than 70% and more preferably not more than 35%. The absorbent structure exposed to the air is wetted with water in the wetting region with a moisture content of at least 15%, preferably at least 20% and more preferably at least 35%, calculated on the total weight of the structure . The term "wetting", as used herein, also includes wetting the structure with water to the saturated concentration of the wetted regions. The water used will preferably be distilled or deionized water, which has the least possible effect on the absorbency of the superabsorbents after wetting and drying of the structure. The entire structure can be moistened, or only parts of said structure. For example, the structure may be wetted by extending longitudinal or transverse strips, in a network pattern or in some other pattern judged to be appropriate in the context. In the case of absorbent structures that have a contoured appearance, where the different parts of the structure have different weights per unit area, it may be appropriate to wet only those parts that have the least weight per unit area and therefore are the weakest . The structure is then dried in some suitable form for a moisture content of not more than 12%, preferably 10%, calculated on the total weight of the structure. Part of the liquid can be mechanically compressed - from the structure before drying of said structure. This may be appropriate when an excessive amount of liquid has been used to wet the structure. The absorbent structure will preferably remain moistened for a period of time of at least one minute. This is to allow the superabsorbent particles to absorb the liquid and expand. Dilation of the superabsorbent particles result in some adhesion between the superabsorbent and the pulp fibers, and it can be said that the expansion of the superabsorbents function as a bond in the fiber structure that increases the strength of said structure. As will be seen in Figures 1 and 2, the superabsorbent grains do not retain their original particle shape after the treatment, but instead "slide" along and between the fibers. After being dried, the structure can be compressed to a desired density, preferably between 0.06 and 0.3 g / cm3. The tensile strength produced in this way in the absorbent structure will be higher than the tensile strength of a corresponding structure that has not been subjected to a wetting treatment.

The absorbent structure can be incorporated as an absorbent body into an absorbent article, such as a diaper, an incontinence guard, a towel -hygenic and similar items. The absorbent structure is commonly contained between a liquid-permeable top sheet, which consists, appropriately, of a non-woven material or a perforated plastic film, and a liquid-impermeable sheet, usually composed of plastic film, for example, a polyethylene film. In certain cases, a material for capturing Liquid in the form of porous wadding or non-woven material can be placed between the material of the upper sheet and the absorbent body. The absorbent article can also be provided with elastic devices that shape and adapt the article to the user's body. With the intention of discovering how such a wetting treatment affects the properties of the absorbent structure, some tests were made on the following pulp / superabsorbent mixtures: A) STORA CTMP + 7% superabsorbent 1M7100 from Hoechst. B) KorsnSs Vigorfluff A (chemical pulp) + 30% superabsorbent 1M 7100. C) Weyerhaeuser NB 416 (chemical pulp) + 30% superabsorbent 1M 7100. D) Chemical pulp (200 g / m2) + 60% superabsorbent 1M 7100.

Sample bodies measuring 50 x 100 mm and containing a mixture of pulp fibers and superabsorbent particles were die cut. The samples were compressed to a bulk value of 6 cm3 / g (density 0.17 g / cm3) and then conditioned for 24. The tensile strength of the samples was measured in an Instron apparatus. The sample bodies were then moistened with deionized water, practically at saturation, and then dried at 50 ° C in 10-14 hours. The samples were conditioned and the bulk adjusted to 6 cm3 / g (density 0.17) g / cm3). The tensile strength of the samples was measured in the same way as described above. The results obtained are given in the following tables.

Table 1 Resistance to the tension of the samples before and after the treatment.

The values of the measurement constitute the average value of seven measurement processes.

It will be noted that the resistance to stress multiplied as a result of the treatment. When the chemical pulp has a high content of superabsorbent particles, as in Example D, the tensile strength is low. The resistance tension increases with the treatment. However, the resistance is still low compared to the resistance of the other treated chemical pulps containing only 30% superabsorbent.

In order to discover how the absorbent properties were affected by the treatment, measurements were made with respect to absorbance and dispersion distance. The measurements were carried out according to the following methods: The samples were placed in the test equipment with a horizontal part and a part inclined at 60 °, and allowing the absorption of liquid according to the principle of self- 'infinite' font. After a certain period of time (the absorption time), the measurement to which the liquid was dispersed was determined (the dispersion distance). The amount of liquid that was absorbed (the absorption) was also measured with the help of scales. The sample was placed in liquid and allowed to freely absorb, then the sample was placed on a grid and loaded with a weight of 7.0 kg for a period of 5 minutes. The sample is weighed to give free absorption under load. The values of the measurement obtained can be used to calculate a measure of the potential use of the sample (degree of use) according to: Abs. - Sample weight x 100 (%) Abs. Free - Sample weight The results are given in Table 2 below.

Table 2 Absorption properties before after treatment.

The values of the measurement constitute the average measurement of three measurement processes.

It will be observed from these measurements that the dispersion distance increases after the treatment, which also gives a higher degree of utilization. On the other hand, a slightly low free absorption was obtained after loading the sample, which can be explained because the capacity of the superabsorbent to absorb liquid has deteriorated after the treatment. However, this is compensated by the improved dispersion distance, and therefore the absorption was approximately the same before and after the treatment. When the pulp has a high content of superabsorbent, about 50%, the distance of dispersion will not be increased so much by the pulps having a lower content of superabsorbent. Neither the potential use will increase much. This is demonstrated by Example D where the dispersion distance and the utilization potential remain essentially unchanged by the treatment. Thus, a superabsorbent content below 60%, especially a content below 50%, and more preferably a content below 40 or 35% is preferred.

Claims (7)

1. A method for producing an absorbent structure including hydrophilic fibers and particles of superabsorbent material, said method consists of the formation of an air-exposed structure of fibers and superabsorbent particles, characterized by the wetting of at least a part of said structure exposed to the air at a moisture content in the wetting region of at least 15%, preferably at least 20% and more preferably at least 35% calculated on the total weight of the structure within the wetting region, and then the drying of the structure at a moisture content of not more than 12%, preferably not more than 10%.

The method, according to claim 1, characterized in that the absorbent structure contains 3-80%, preferably 5-70% and more preferably 5-35% superabsorbent particles, calculated on the total weight of the structure before of wetting.

3. The method according to claim 1 or 2, characterized by wetting the structure with distilled or deionized water.

4. The method, according to one or more of the preceding claims, characterized by compression 'of water of the wetted structure, before the drying of said structure.

The method, according to one or more of the preceding claims, characterized by the wetting of only certain parts of the structure.

6. The method, according to claim 5, characterized by the wetting of the structure in such a pattern or within such parts of the structure that the resistance to the total tension of said structure is improved.

7. An absorbent article, such as a diaper, an incontinence protector, a sanitary napkin or a similar article, characterized in that said article includes an absorbent structure, according to one or more of claims 1 to 6.