WATER ABSORBING MATERIAL AND METHOD FOR ITS MANUFACTURE

Abstract: Use of a multi-layer film made of at least two kinds of thermoplastic resins having different melting points. The cross-sectional construction of a multi-layer film or a tape obtained from the multi-layer film or the like is applied to the cross-sectional construction of a fiber with the cross-­ sectional construction of the former left as it is, i.e., a very simple means for making fiber of a multi-layer film or a tape obtained therefrom is utilized, thus providing a conjugated fiber far superior to the conventional thermo-bondable con­ jugated fiber produced by a melt-spinning method and a water-absorbing material using such a superior conjugated fiber, and methods of manufacturing them.

Description: The present invention relates to a water absorbing material comprising a blend of a pulp having a water-absorbing property and multi-layer composite branched fibres, a method for the manufacture of said material and its use as an absorbent for paper diapers or toilet articles. Composite fibres are known as conjugate fibres formed by simultaneously extruding two kinds of spinning solutions from a single spinning nozzle having a partition provided therein to provide conjugate spinning, or by laminating together two kinds of fibres to form a single fibre. Known furtheron is melt composite spinning comprising using two kinds of thermoplastic resins different in melting point from each other, for example, crystalline polypropylene (m.p. 165°C) as a high melting point polymer and polyethylene (m.p. 125°C) as a low melting point polymer and simultaneously extruding both resins in their melted state from a single spinning nozzle to form a single fibre. In these conventional composite fibres, a spinning nozzle is used to effect spinning, and therefore, composite fibres obtained therefrom are limited to those of two component systems, and it has been extremely difficult to obtain composite fibres above three component systems. Moreover, in the aforementioned melt composite spinning, two kinds of polymers are made into a composite form in the state of a concentric circle or an eccentric circle, and in the case where they are made into a composite form with a low melting point polymer positioned externally and a high melting point polymer, the low melting point polymer is melted so that fibres may be thermally bonded with each other. For example, after a web has been formed, the low melting point polymer is melted but when the high melting point polymer is subjected to heat treatment at a temperature at which it is not melted, contact parts of the fibres become fused, resulting in non-woven fabrics or the like. This non-woven fabric eliminates the necessity of a binder such as a synthetic rubber emulsion, and therefore has an advantage that various disadvantages caused by the use of the binder may be avoided. However, in the above-described melt composite spinning, dies, nozzles and the like are complicated in construction, and for this reason, the composite fibres above three component systems may not only be obtained but thermoplastic resins used therefor are limited to polymers which can be thermally welded with each other. Furthermore, even in the composite fibres comprising two component systems, since it is difficult to change color of raw material, a colored non-woven fabric as a final product is hard to obtain. Despite the possession of characteristic of thermal adhesion, the range of utilization thereof has been greatly limited. US-A-4 134 951 discloses multi-layer composite split fibres which may comprise two or more stretched layers formed from thermoplastic resins differing in melting point from another. DE-A-2 319 735 discloses a filter material useful as a cigarette filter on the basis of a fibrilated material comprising longitudinal main fibres connected by connecting fibres and comprising a number of free ends obtained by splitting of the main fibres and/or the connecting fibres. GB-A- 1 522 952 describes a fibrous material made from films of a molecularly-orientable synthetic thermoplastic semi-crystalline organic polymer material, which, after drawing to a high degree of uniaxial orientation has the natural tendency to fibrillate to thereby obtain a helical crimp. DE-A-2 212 326 covers textile sheet materials comprising fibrillated fibres made of flat or blown films of polymers, copolymers or bi-component films. Laminates having a plurality of different thermoplastic resins laminated, for example such as multi-layer films, tapes, sheets, etc. are well known. Such laminates (hereinafter referred to as multi-layer films) have merits that they can be molded in two layers and in three layers or more by use of various molding means, such as by a calendering method, extrusion method, casting method and the like, and the materials can be colored. Furthermore, since lamination may be easily achieved through an adhesive layer, polymers which are hard to be fused together may be integrated. The object of the present invention is to provide a water-absorbing material having improved properties, specifically improved water-absorbing properties. The subject matter of the present invention therefore is a a water-absorbing material comprising a blend of a pulp having water-absorbing property and multi-layer composite branched fibres comprising at least two stretched layers formed from thermoplastic resins differing in melting point from one another, bonded by thermal fusion of said fibres. According to a preferred embodiment, the water-absorbing material comprises a pulp having a water-absorbing property which comprises a blend of a pulp and a high molecular absorbent. According to a further preferred embodiment, the branched fibres are short fibres having a length of 50 to 100 mm. The subject matter of the present invention furtheron is a method for the manufacture of the water-absorbing material according to claims 1 to 3, comprising blending short multi-layer composite branched fibres comprising at least two stretched layers formed from thermoplastic resins differing in melting point from one another with a pulp and a high molecular absorbent, heating said blend to a temperature in the vicinity of the melting point of the low melting point resin of said fibres and partially fusing the short fibres. A further object of the present invention is the use of this water-absorbing material as an absorbent for paper diapers or toilet articles. The multi-layer composite branched fibres comprise at least two stretched layers formed from thermoplastic resins differing in melting point from one another, bonded by thermal fusion. Said fibres are formed by splitting a base material comprising at least two multi-layer films formed of said thermoplastic resins having different melting points after said base material has been stretched, so that the fibres have the same section as that of the base material. The multi-layer films serving as the base material for said multi-layer composite branched fibres may be manufactured by various molding methods, such as a calendering method, an extrusion method and a casting method, wherein the extrusion method comprising an inflation step and a T-die method are preferred. The multi-layer films serving as the base material for said multi-layer composite branched fibres may be manufactured by various molding methods, such as by calendaring, extrusion or casting, wherein the extrusion method comprising an inflation step and the T-die method are preferred. The thermoplastic resin is preferably the resin from which a film may be molded, and the thickness thereof is not particularly limited but is preferably in the range of 30 to 100 µm. Specific examples of the thermoplastic resin are alpha-olefin polymers such as polyethylene, polypropylene, etc., polyamide, polyester, etc. Such resins can be copolymers with a monomer of a different kind, and can contain therein, as needed, an anti-oxidant, a lubricant, a ultraviolet absorbent, a delusterant, a stabilizer, a fire retardant, etc. Particularly, a color film is obtained by adding a colorant, and the composite fibre used accordingly to the present invention may be colored as will be described hereinafter, and a composite fibre rich in variety of color may be obtained. A plurality of resins used in the composite fibres used according to the present invention are different in melting point from one another because a heat fusion property must be imparted thereto. In case of three layers, it is necessary so that a resin having a low melting point is positioned externally. The difference in melting point between a thermoplastic resin having a low melting point and a thermoplastic resin having a high melting point is preferably that if the melting point of the former resin is Mp °C, then that of the latter resin is Mp + 10°C or over in consideration of the heat treating conditions, extrusion conditions of a multilayer film, stretching conditions in a post-process and the like, which will be described later. As for one example, polyethylene (m.p. 125°C) as a thermoplastic resin having a low melting point and crystalline polypropylene (m.p. 165 °C) as a thermoplastic resin having a high melting point are used. In the case where, as in the use of linear low-density polyethylene for a thermoplastic resin having a low melting point and the use of polyamide for a thermoplastic resin having a high melting point, both resins cannot be fused, an adhesive resin may be interposed between these resins for composition. As an example, this adhesive resin used comprises a resin composition in which polyethylene or polypropylene is denatured by acid anhydride such as anhydrous maleic acid and acids. Specically, anhydrous maleic acid denatured linear low-density polyethylene is preferred. It is of course that the composite fiber used accordingly to the present invention can be not only two and three-layer constructions but also four-layer or more, in which case, a multi-layer film is constituted in consideration of the heat treating conditions at a temperature at which the low melting point polymer is melted but the high melting point polymer is not melted. In forming a multi-layer film by way of a T-die method or the like, a die lip construction may be formed into a flat type but when a ribbed die is used, a film surface is formed into an uneven surface, which remains on the surface of each fibre even after being formed into a fibre, to bring forth a drape or volume in the composite fibre. The multi-layer film is stretched before being formed into a fibre. This stretching increases the strength of the composite fibre. Streching of the multi-layer film is carried out by any of stretching means such as a hot plate, a hot roll, an oven, etc. A stretching temperature and a stretching magnification are not particularly restricted but may be set to optimum conditions according to the composition and shape of the multi-layer film. However, the strength, drape and volume feeling of webs or non-woven fabrics obtained through posterior processes largely depend upon properties of the composite fibres. Preferable examples of the stretching temperature and stretching magnification are given: For example, in the case where a multi-layer film comprised of polyethylene and crystalline polypropylene is stretched by hot rolls after competion of slit process for longitudinally cutting it, the stretching temperature is preferably in the range of 100°C to 130°C and the stretching magnification preferably in the range of 4 to 10 times. A multi-layer film is formed into a fibre by splitting the film in a longitudinal direction. This split comprises discontinuously splitting surface of a film to form a fibre assuming a state or form (split fibre) which repeats a partial connection to provide a branch-like continuation. The coarseness and length of each fibre are determined according to the reed or pitch of a needle of a split roll, and their dimensions are made uneven to make the coarseness and length of the fibre uneven as the case may be. The composite fibre resulting from a fibrous form of a multi-layer film is used in the state of a continuous fibre or as a short fibre. The length of the short fibre is preferably in the range of 50 to 100 mm. This short fibre may be formed into a web via preliminary splitting or via direct carding process to form a non-woven fabric as well as gauze, diaper covers and carpets. The short fibre is blended with a pulp having a water-absorbing property, and when the blend is heated, a part of the short fibre is melted and as a result, a fusion partly occurs in the blend, whereby the pulp is solidified. Also, contraction of pulp caused by absorption of moisture may lso be controlled, and therefore, they can be used as an absorbing material for paper diapers and an absorbing material for toilet articles. The drawings show a composite fibre, a water-absorbing material using the composite fibre as a base material, and a method for producing them, according to the present invention. FIGS. 1 to 3 are sectional views, respectively, of a composite fibre. FIG. 4 is a flow chart for producing the composite fibre. FIG. 5 is a view showing a part of split yarns in an enlarged scale. FIG. 6 is a plan view showing a water-absorbing material partly enlarged. As shown in the section of FIG. 1, a composite fibre 1 consists of a resin layer having a high melting point 2 and resin layers having a low melting point 3, 4 forming external layers. A composite fibre 1 shown in FIG. 2 has an arrangement wherein surfaces of resin layers having a low melting point 4, are formed into uneven surfaces by use of a ribbed die. FIG. 3 shows a composite fibre 1 branched by partial connections. The aforesaid composite fibres 1 may be produced according to the process shown in FIG. 4. First, a multi-layer film 12 of a three-layer construction having approximately 3 µm of thickness is molded by an inflation film molding machine 11 provided with a cross head die for molding a three-layer. Subsequently, the multilayer film 12 is passed over heated stretching rolls 13, and thereafter are slit by means of a slitter 14 into a plurality of tapes 15 having the desired width. When this tape 15 is passed over a split roll 16 for splitting, a split yarn 17 comprised of a composite fibre having a three-layer construction is produced thereon. In this case, when each tape 15 is completely split, a single fibre as shown in FIG. 1 is produced. When a net-like fibre formed by being discontinuously split at regular intervals untied by carding process, a branched fibre as shown in FIG. 3 is produced. While in the aforementioned process, the multilayer film 12 is stretched, it is to be noted that the tape 15 may be stretched. Conditions of various parts in the aforementioned process will be given below: Composition of composite fibre Example 1 Example 2 Low melting point resin layer 3 high density polyethylene (MI=1.0) linear polyethylene (MI=1.8) High melting point resin layer 2 polypropylene (MI=1.5) adhesive polyolefin Low melting point resin layer 4 high density polyethylene (MI=1.0) nylon 6 Conditions of extruding machines are as follows: A. Diameter of screw: 40mm ⊘ (for the low melting point resin layer 3) Temperature of cylinder: C : 170°C C : 230°C C : 220°C B. Diameter of screw: 40 mm ⊘ (for the high melting point resin layer 2) Temperature of cylinder C : 180°C C : 220°C C : 230°C C. Diameter of screw: 32 mm ⊘ (for the low melting point resin layer 4) Temperature of cylinder: Same as A above Die lip : 1 m/m Take-off speed: 10.7 m/min. Temperature of stretching roll: 120°C Stretching magnification: 5 times Split: 10 d The composite fibre 1 produced under the above-described conditions takes the form of a split yarn 17 as shown in FIG. 5. This split yarn 17 is cut into short fibres in the range of 70 to 90 mm. In case of the net-like fibre, when it is subjected to carding process, a branched fibre results. Webs which are well thermally fused are obtained from these short fibres. If approximately 2% of pigment is added to the high melting point resin layer 2, a colored thermally fused web may be obtained, the web being rich in drape and volume feeling. To use the aforesaid composite fibre 1 as a base material for an water-absorbing material for diapers or the like, the short fibres are untied into fibres by carding process, a suitable amount of fibres 17a and fibres 18 of pulverized pulp are blended while adding thereto high-molecular water absorbing agents 19 (for example, a copolymer of acrylic acid and vinyl acetate) and applying an air blow thereto, and the resulting blend is heated to a level in the vicinity of a melting point of the low melting point resins 3 and 4. It is noted that during such heating, the blend may be pressed into a flat form. Thereby, parts of the low melting point resins 3 and 4 are fused together to provide an absorbing material 20 in which the aforesaid two fibres 17a and 18 are mixed. The rates of the composite fibre, pulverized pulp and high-molecular water absorbing material are preferably 30%, 50% and 20%, respectively, and the amount of the composite fibre may be increased as the case may be. As described above, the composite fibre used according to the present invention comprises a multi-layer film as a base material, the multi-layer film being split into fibres. Therefore, coloring and composition of three component systems or more which are difficult in melt composite spinning may be easily carried out, and in addition, various composite fibres different in sectional construction may be obtained by changing a shape of die and besides uses of thermally fusion composite fibres are also widely ranged. Moreover, the composite fibres used according to the present invention are greatly efficacious as a base material for a water-absorbing material, particularly as a base material for a water absorbing material for toilet articles, paper diapers and the like, and therefore are extremely effective in industrial fields and widely used.

Claims: A water-absorbing material comprising a blend of a pulp having a water-absorbing property and multi-layer composite branched fibres comprising at least two stretched layers formed from thermoplastic resins differing in melting point from one another, bonded by thermal fusion of said fibres. The water-absorbing material according to claim 1 characterized in that said pulp having a water-absorbing property comprises a blend of a pulp and a high molecular absorbent. The water-absorbing material according to claim 1 or 2, characterized in that said branched fibres are short fibres having a length of 50 to 100mm. A method for the manufacture of the water-absorbing material according to claims 1 to 3, comprising blending short multi-layer composite branched fibres comprising at least two stretched layers formed from thermoplastic resins differing in melting point from one another with a pulp and a high molecular absorbent, heating said blend to a temperature in the vicinity of the melting point of the low melting point resin of said fibres and partially fusing the short fibres. The use of the water-absorbing material according to claims 1 to 3 as an absorbent for paper diapers or toilet articles.

Inventors: HASHIMOTO, AKIHIRO KAWASAKI JUSHI KENKYUSHO, KAMEI, YOSHISUKE KAWASAKI JUSHI KENKYUSHO OF SHOWA, NAKAMURA, AKIRA KAWASAKI JUSHI KENKYUSHO OF SHOWA, TANAKA, TOYOAKI KAWASAKI JUSHI KENKYUSHO OF SHOWA
Applicants: SHOWA DENKO KABUSHIKI KAISHA
Application year: 1986
Main classification: D01D5
IPC codes: D01F8, A61L15, D01D5, D04H1