Method for manufacturing an absorbent fibre layered product

Abstract: The invention relates to a method for manufacturing an absorbent fibre layer (15), and an absorbent fibre layer. In the method, at least one material layer (16) consisting of a mixture of natural fibres, such as wood fibres, and plastic fibres is formed by a dry-forming technique, and at least one other material layer (17) consisting of natural fibres, or a mixture of natural fibres and plastic fibres, and superabsorbent material (18) is formed on said material layer. The fibre layer (15) is bonded with heat. The invention is characterized in that the superabsorbent material (18) is added to the second material layer (17) to be formed in connection with its dry-forming stage, after which the upper surface of the fibre layer is compacted and its lower surface is moistened and compressed by hot calendering in order to compress the material layer (16) comprising no superabsorbent material into a liquid-spreading layer.

Description: The invention relates to a method for manufacturing an absorbent fibre layer, in which method at least one material layer consisting of a mixture of natural fibres, such as wood fibres, and plastic fibres is formed by a dry-forming technique, and at least one other material layer consisting of natural fibres, or a mixture of natural fibres and plastic fibres, and superabsorbent material is formed on said material layer, and which fibre layer is bonded with heat. When non-woven sanitary products are manufactured, an absorbent layer is formed by a dry-forming technique of natural fibre to which a bonding agent or bonding fibres are added to bond the formed material web into a porous fibre layer by the action of heat. A necessary number of such layers are formed on top of each other depending on the desired thickness of the product. Material layers formed in this manner are used in the manufacturing of different types of diapers, sanitary napkins and hospital wound dressings. The problem with products of this type is their limited absorbing capacity, which is also often incompatible with the efforts to diminish the size of the product. Such absorbent sanitary products are also known that comprise superabsorbent material which is added to improve the absorbing capacity. Although the absorbing capacity in such products has improved, the different layers of fibre, reinforcing material and superabsorbent material included in the same product make the products relatively expensive and complicate the manufacturing process. The purpose of this invention is to provide a method for manufacturing, in a simple manner, a fibre layer with a high absorbing capacity. EP-A-0 159 630 discloses a method for manufacturing an absorbent fibre layered product, comprising the steps of:- (a) depositing a first mixture of natural fibres, such as wood fibres, and of plastic fibres onto a moving wire, so as to form a first material layer by a dry-forming technique; (b) depositing on top of said first material layer, while said first material layer is travelling on the moving wire, either a second mixture of natural fibres, such as wood fibres, and superabsorbent material, or a second mixture of natural fibres, such as wood fibres, plastic fibres and superabsorbent material, so as to form a second material layer whereby there is formed from the two mixtures, by a dry-forming technique, a jointless material web; and thermobonding the fibres together of said material web by means of heat. In the description relating to Figs. 1-4 of EP-A-0 159 630, and in Figs. 1-4 of EP-A-0 159 630 themselves, there is no suggestion of any compaction or compression by hot-calendering. The use of heated patterned embossing rolls is mentioned in the description relating to Fig. 5 of EP-A-0 159 630, but only in relation to an embodiment which involves the placing of a pre-formed base sheet (on the belt) prior to the application of the first meltblown fibre stream. The present invention provides a method for manufacturing an absorbent fibre layered product, comprising the steps of:- (a) depositing a first mixture of natural fibres. such as wood fibres, and of plastic fibres onto a moving wire, so as to form a first material layer by a dry-forming technique; (b) depositing on top of said first material layer, while said first material layer is travelling on the moving wire. either a second mixture of natural fibres, such as wood fibres, and superabsorbent material, or a second mixture of natural fibres, such as wood fibres, plastic fibres and superabsorbent material, so as to form a second material layer; whereby there is formed from the two mixtures, by a dry-forming technique. a jointless material web; and thermobonding the fibres together of said material web by means of heat; characterised by the further steps of:- compacting a first surface of said material web, remote from said first material layer; moistening a second surface of said material web, remote from said second material layer; and thereafter compressing the moistened second surface of said material web by hot calendering so as to compress said first material layer into becoming a liquid-transport layer. The material web can be preferably hot-calendered by means of patterning facilitating the transportation of liquid to desired parts of the absorbent layer and providing rapid liquid transportation conduits on the compressed side of the fibre layer to different parts of the final product. A preferred product according to the invention is thus substantially a result of the interaction of two factors. Firstly, the fibre layer is made of substantially one layer, even though it is formed in several stages. Secondly, this one layer comprises both the required absorbent layer, formed by adding superabsorbent material to one side, and the liquid-spreading layer, formed by compressing the other side in a suitable manner. These two factors thus surprisingly result in a highly absorbent and at the same time easily manufacturable fibre layer, which comprises no concentrations of particles or other material weakening the mechanical resistance, nor joints formed by several superimposed webs. Other preferred embodiments of the method according to the invention are characterized by what is disclosed in the appended claims. The fibre layer according to the invention and its preferred embodiments are characterized by what is disclosed in the appended claims concerning it. In the following, the invention will be described by means of an example with reference to the accompanying drawings, in which Figure 1 shows the production line of a fibre layer according to the invention, Figure 2 shows an absorbent fibre layer according to the invention. Figure 1 shows a dry-forming line where a material web 2 is formed on a wire 1 in two stages by means of two formers 3 and 4. A mixture of air and fibre is blown into the formers, which extend transversely across the wire along its entire width, and the mixture is mixed and screened so as to form, according to a known technique, an even material layer on the wire 1 moving underneath. There can be as many formers as the desired layer thickness calls for, whereby one layer after another is formed on the same production line until the desired thickness is achieved. Furthermore, the ratio between wood fibres and polymer fibres in different layers may vary within the scope of the invention. The wood fibres are preferably mechanical pulp of relatively long fibres, and the plastic fibres may be of any suitable thermobonding quality, for example bicomponent fibres the core of which is polypropylene and the mantle polyethylene. Superabsorbent particles are added to the material web 2 either by means of a separate scattering device 7 or by adding the particles to the fibre material in the former 4. Suitable superabsorbent materials are for example activated carbon, activated clay, silica gels and cross-linked polyacrylates. The concentrations of different particles in layer 17 of the material web may be, for example: wood fibre (pulp fibre) 25-90%, plastic fibre 0-70% and superabsorbent particles 0-70%. The superabsorbent may also be in liquid form, whereupon spray nozzles are used instead of the scattering device to spray for example acrylic acid monomer in water dispersion on the material web. The monomers are cross-linked by means of heat and suitable radicals, whereby the superabsorbent contributes to bonding the fibres together. Thus the amount of plastic fibres in this layer can be diminished or the plastic fibres can be completely left out as unnecessary. After the forming stage, the material web is thermo-bonded and any possible liquid superabsorbent is cross-linked in a flow-through oven 8. After this, the upper surface of the bonded fibre layer 11 is compacted by a compactor 9,10 consisting of a heated calender roller 9 and its counter roller 10. The lower surface of the layer is moistened by a water nozzle 12 or a steam box and compressed by hot calendering by means of a calender roller 13 and its counter roller 14 to form a fibre layer 15 according to the invention. In the fibre layer of Figure 2, the material layer 16 comprising no superabsorbent material is compressed into a liquid-spreading layer as a result of hot calendering. Liquid has been found to spread faster, up to a certain limit, in a more densely packed fibre layer than in a more porous fibre layer. This is due to the higher liquid transportation capacity of more densely organized fibres as compared with less densely packed fibres. By exploiting this characteristic and by preferably forming patterns on the compressed side by a patterned calender roller, even faster liquid transportation conduits are provided to different parts of the product. The idea of the patterning is thus to form conduits to all parts of the product, the fibre structure in the conduits being further compressed to achieve a higher liquid transportation capacity, whereby the absorbing capacity of the product will be utilized to its optimum. The calendering may also be performed by means of a smooth roller, after which the final calendering of the fibre layer is performed by a patterned roller. The fibre layer 15 according to the invention, shown in Figure 2, thus comprises a lower material layer 16 consisting of a mixture of wood fibres and plastic fibres, and another material layer 17 formed on top of this and consisting of wood fibres, or a mixture of wood fibres and plastic fibres, and superabsorbent material 18. The superabsorbent material is shown in the figure in the form of particles. If fibrous or liquid superabsorbent material were used instead of the particles, such a substance could hardly be distinguished from the basic structure of the material, and therefore these alternatives are not shown separately. It is apparent from the cross-section of Figure 2 that the entire fibre layer 15 is substantially one and the same jointless layer, one side of which is the absorbent layer comprising superabsorbent material and the other side of which is the compressed liquid-spreading layer. The thickness of the layers can be varied and adjusted during the forming stage; the only essential thing is that the layers are formed on the same line and bonded together in one stage. The final product formed of the fibre layer is cut into a suitable size and its lower surface is possibly coated with a plastic layer impervious to liquid. Other finishing measures are determined by the use of the product. It is clear for one skilled in the art that the different embodiments of the invention are not limited to the examples described above, but they can vary within the scope of the appended claims.

Claims: A method for manufacturing an absorbent fibre layered product (15), comprising the steps of:- (a) depositing a first mixture of natural fibes, such as wood fibres, and of plastic fibres onto a moving wire (1), so as to form a first material layer (16) by a dry-forming technique; (b) depositing on top of said first material layer (16), while said first material layer (16) is travelling on the moving wire (1), either a second mixture of natural fibres, such as wood fibres, and superabsorbent material (18) or a second mixture of natural fibres, such as wood fibres, plastic fibres and superabsorbent material (18) so as to form a second material layer (17) so as to form from the two mixtures, by a dry-forming technique, a jointless material web (11); and thermobonding the fibres together of said material web (11) by means of heat; characterised by the further steps of:- compacting a first surface of said material web (1 1), remote from said first material layer (16); moistening a second surface of said material web (11), remote from said second material layer (17); and thereafter compressing the moistened second surface of said material web (11) by hot calendering so as to compress said first material layer (16) into becoming a liquid-transport layer. A method according to claim 1, characterised in that the hot calendering of the fibre layer is performed by a patterned roller (13) to provide patterning which facilitates the transportation of liquid in the plane of the fibre layer. A method according to claim 1, characterised in that the hot calendering of the fibre layer is performed by a smooth roller (13), after which the final calendering of the fibre layer is performed by a patterned roller to provide patterning which facilitates the transportation of liquid in the plane of the fibre layer. A method according to claim 1, 2 or 3, characterised in that the superabsorbent material (18) is added in the form of particles or fibres to the flow of fibres forming the second material layer (17) in the forming stage of the layer. A method according to claim 1, 2 or 3, characterised in that the superabsorbent material (18) is added in liquid form to the second material layer (17) immediately after the layer has been formed.

Inventors: FAVRE STEFAN, GUSTAFSSON HELMER, FAVRE, STEPHAN, GUSTAFSSON, HELMER
Applicants: YHTYNEET PAPERITEHTAAT OY
Application year: 1995
Main classification: D04H13
IPC codes: D04H3, A61F13, D04H13