Water-disintegratable wiping sheet

Abstract: A water-disintegratable fibrous sheet having a layered structure comprises: (a) at least one first fiber web comprising pulp of conifer and pulp of hardwood; (b) at least one second fiber web comprising pulp of conifer, having a content of pulp of hardwood less than that in said first fiber web or containing no pulp of hardwood, said first and second fiber webs forming said layered structure; and (c) a water-insoluble or water-swellable binder being added to at least one of said first and second fiber webs.

Description: The present invention relates to a water-disintegratable wiping sheet that can be easily dispersed by a water stream. More particularly, the invention relates to a water-disintegrable wiping sheet comprising a plurality of water-disintegrable fibrous sheets laminated with each other, which water-disintegratable fibrous sheet have excellent in water-disintegratability and strength. Fibrous sheets are used for cleaning human skin, eg skin around anus, or cleaning a toilet room. The fibrous sheet is preferably water-disintegratable to be thrown away and drained in a toilet as it is. If it is not excellent in water-disintegratability, it requires a long time to be dispersed in a septic tank, and brings danger of clogging drainpipes of a toilet, when being thrown away and drained in a toilet. However, in general, a packed fibrous sheet impregnated with a cleansing liquid or the like has to be strong enough to endure conducting wiping operations while being impregnated with a cleansing liquid, and at the same time, has to keep water-disintegratability in the event of being thrown away and drained in a toilet. Therefore, a water-disintegratable fibrous sheet that had good water-disintegratability and strength sufficient to use is demanded. Unexamined Published Japanese Patent Application No H1-168999 discloses an easily water-dispersible cleaning product containing water-insoluble carboxymethylated pulp in a salt form of calcium. Unexamined Published Japanese Patent Applications No H9-132896 and No H9-132897 each discloses a water-disintegratable sheet, in which sodium carbonate is added to water-insoluble or water-swellable carboxymethyl cellulose. However, when a large amount of the binder, ie water-insoluble or water-swellable carboxymethylated pulp, is used to enhance sheet strength, water-disintegratability of the sheet is deteriorated. US 5,487,813 discloses a water disintegratable wiping sheet constructed in a manner to improve production capacity. The sheet comprises layers having different proportions of softwood and hardwood pulps, but does not disclose the particularly beneficial combination of the present invention. An object of the present invention is to provide a fibrous sheet and a wiping sheet that have good water-disintegratability and strength sufficient to practical use. The present invention provides a water disintegratable wiping sheet used in a wet state comprising two water-disintegratable fibrous sheets, wherein each of said water-disintegratable fibre sheets comprises: a first fibre web comprising pulp of conifer and pulp of hardwood without a binder; a second fibre web comprising pulp of conifer, said second fibre web containing no pulp of hardwood and said first fibre web and second fibre web forming a layered structure; and a water-insoluble or water-swellable binder added to only said second fibre web; said second fibre web has higher wet strength than the first fibre web, and said first fibre web is more water disintegratable than said second fibre web, and wherein said two water-disintegratable fibrous sheets have a wet strength of 75 g/25 mm or higher and are partially bonded one on the other with a water-soluble adhesive so that said second fibre web faces outside of said wiping sheet. An embodiment of the invention is described below with references to the accompanying drawings, in which: Figure 1 is a cross sectional view of a fibrous sheet having a two-layer structure; and Figure 2 is a cross sectional view of the wiping sheet of the invention comprising two fibrous sheets of Figure 1 laminated with each other. Pulp of conifer used in the invention is pulp produced by using conifer as a raw material. Representative examples of the pulp of conifer include bleached kraft pulp of conifer (NBKP) and kraft pulp of conifer is also included. Pulp of hardwood used in the invention is pulp produced by using hardwood as a raw material. Representative examples of the pulp of hardwood include bleached kraft pulp of hardwood (LBKP) and kraft pulp of hardwood is also included. What should be noted herein is that the average fibre length of the pulp of conifer and that of the pulp of hardwood are different from each other. The average fibre length (hereinafter referred to as "fibre length") of the pulp of conifer is longer than that of the pulp of hardwood. The fibre length of the pulp of hardwood is generally from 0.8 to 1.8 mm, whereas the fibre length of the pulp of conifer is generally from 2.0 to 4.5 mm. When fibre webs are produced by using these fibres and laminated each other and the fibre webs are different from each other in water-disintegratability and strength, a fibrous sheet can be obtained that has a characteristic feature in that water-disintegratability is enhanced by one of the fibre webs, and strength is maintained by the other of the fibre webs. As a result, a fibrous sheet excellent in water-disintegratability and strength can be obtained. The function of improvement in water-disintegratability and strength of the fibrous sheet in the invention is described in detail below. The pulp of conifer has been subjected to beating treatment for improvement in paper strength. Therefore, in the pulp of conifer, fibrillated fibres are strongly bonded to each other by hydrogen bonds. By adding thereto the pulp of hardwood which has the shorter fibre length and thus causes weak bonding strength, there generates unevenness in bonding strength of fibres, ie cellulose molecules. As a result, a fibre web containing both the pulp of conifer and the pulp of hardwood can be easily dispersed by a water stream, in other words, it exhibits excellent water-disintegratability. This is the first fibre web of the invention, mainly comprising pulp of conifer and pulp of hardwood. The language "mainly comprising" used herein means that the total content of the fibres is 90% by weight or more based on the total weight of the fibre web. On the other hand, a fibre web containing a smaller amount of the pulp of hardwood than the first fibre web or containing no pulp of hardwood has higher strength than the first fibre web, because the bonding strength among the fibres of the pulp of conifer is kept higher. This is the second fibre web of the invention, comprising pulp of conifer, containing no pulp of hardwood. By laminating the first and second fibre webs to form a layered structure, a fibrous sheet having excellent water-disintegratability can be obtained. In this layered structure, since the fibre webs (layers) have different volumes of swelling and different swelling rates, it has a structure that can easily incorporate water upon contacting water. Therefore, the wiping sheet of the invention, comprising two such fibrous sheets shown in Figure 2, is excellent in water-disintegratability in comparison to a fibrous sheet composed only of pulp of conifer. Furthermore, the fibrous sheet of the invention is excellent in strength in comparison to a fibrous sheet having a single-layer structure composed of pulp of hardwood and pulp of conifer. In the present invention, a water-insoluble or water-swellable binder is added to at least one of the first and second fibre webs constructing the fibrous sheets, to further improve strength of the fibrous sheet. The water-insoluble or water-swellable binder is more preferable than a water-soluble binder in the present invention, because the fibrous sheet is excellent in water-disintegratability due to its layered structure, so that the water-solubility of the binder is not necessarily required and the water-insoluble or water-swellable binder can enhance wet strength of the fibrous sheet upon use in a wet state. The water-disintegratable wiping sheet of the invention comprises two water-disintegratable fibrous sheets, each comprising a first fibre web mainly containing pulp of hardwood (eg LBKP) and pulp of conifer (eg NBKP) and a second fibre web mainly containing pulp of conifer (eg NBKP) and containing no pulp of hardwood. The first fibre web does not contain a binder, and only the second fibre web contains a binder. Figure 1 shows a schematic cross sectional view of this embodiment. In the present invention, the content of the pulp of hardwood in the first fibre web is preferably 60% by weight or less to the total fibre content of the first fibre web. When the content of the pulp of hardwood becomes larger than the upper limit, strength of the fibrous sheet becomes insufficient and it cannot withstand practical use such as wiping. The content of the pulp of hardwood is more preferably from 10 to 50% by weight to the total fibre content of the first fibre web. In this range of the content, both strength and water-disintegratability are excellent. The fibres used in the present invention mainly comprise the pulp of conifer and the pulp of hardwood as described above, and may further contain natural fibres such as cotton, synthetic fibres such as rayon, polypropylene, polyvinyl alcohol, polyesters and polyacrylonitrile, synthetic pulp made of polyethylene and inorganic fibres such as glass wool. These fibres form fibre webs to be laminated into a fibrous sheet. The fibre web used herein means a sheet-formed lump of fibres where the directions of the fibres are arranged to some extent. The total basis weight of the fibres constituting the fibrous sheet of the invention is preferably from 10 to 100 g/m 2 . When the total basis weight is less than the lower limit, strength necessary for wiping operations cannot be obtained. When the total basis weight is more than the upper limit, flexibility desirable in the fibrous sheet is lost. When the fibrous sheet is used for wiping skin eg skin around anus or cleaning a damageable object, the total basis weight of the fibres is more preferably from 20 to 60 g/m 2 from the viewpoints of strength and softness. In the present invention, water-insoluble or water-swellable carboxymethyl cellulose is preferred as the binder. Solubility of carboxymethyl cellulose differs in accordance with a degree of etherification (DS), pH and so on. The water-insoluble or water-swellable carboxymethyl cellulose used in the present invention has a degree of etherification of from 0.3 to 0.6 and pH of 5.0 or more. What is called carboxymethylated pulp is included in the carboxymethyl cellulose. The carboxymethylated pulp can be also used as the water-insoluble or water-swellable binder in the present invention. The carboxymethylated pulp is in various salt forms, such as an acid type, a sodium salt, a calcium salt, an aluminium salt, a barium salt, a zinc salt, a copper salt and a manganese salt and the carboxymethylated pulp can be used in any salt forms in the present invention. However, any binder can be used in the invention if it is water-insoluble or water-swellable and can be dispersed in water. For example, water-insoluble polyvinyl alcohol can also be used instead of the water-insoluble or water-swellable carboxymethyl cellulose. In the present invention, further more, sodium carbonate and/or sodium hydrogencarbonate is preferably added to the fibrous sheet, when the water-insoluble or water-swellable carboxymethyl cellulose is used as the binder. The sodium carbonate and/or sodium hydrogencarbonate can enhance water-dispersibility of the water-insoluble or water-swellable carboxymethyl cellulose to some extent. The sodium carbonate and/or sodium hydrogencarbonate may be added to all of the layers constituting the fibrous sheet of the invention, and may be added to a part of the layers. The sodium carbonate and/or sodium hydrogencarbonate can be added to a layer containing no pulp of hardwood and the sodium carbonate and/or sodium hydrogencarbonate is not added to a layer containing pulp of hardwood. The sheet of the present invention can be used both in a dry state and in a wet state for wiping operations. When the sheet is used in a wet state, the sheet has to be prevented from water-disintegrating by water contained therein during wiping operations and also has to keep wet strength therein sufficient to endure wiping operations. Therefore, in the case where the carboxymethyl cellulose is used as the binder and the sheet is used in a wet state, a metallic salt that crosslinks the carboxymethyl cellulose is preferably added to the fibrous sheets. Wet strength of the fibrous sheets can be increased by crosslinking with the metallic salt. The metallic salt may be contained in all of the layers or may be contained a part of the layers. It is preferred that the metallic salt is in contact with the carboxymethyl cellulose in the fibrous sheet. Examples of the metallic salt include salts of magnesium, calcium, barium, strontium, manganese, zinc, cobalt and nickel. Among these, at least one metallic salt selected from the group consisting of salts of calcium, zinc and magnesium is preferably used. By using these salts, wet strength of the fibrous sheets can be improved. In the case where a metallic salt selected from magnesium, calcium, barium, strontium, manganese, zinc, cobalt and nickel is added to the fibrous sheets containing the water-insoluble or water-swellable carboxymethyl cellulose in all the layers, the metallic salt is preferably contained in an amount of 0.5 g or more per 100 g of the fibres. When the amount of the metallic salt is less than the lower limit, wet strength is not sufficient. Also, in the case where the carboxymethyl cellulose is used as the binder and the sheet is used in a wet state, the fibrous sheets are preferably impregnated with an organic solvent. The organic solvent can increase wet strength of the fibrous sheets, as well. Examples of the organic solvent include a monohydric alcohol such as ethanol and isopropyl alcohol, and a polyhydric alcohol such as propylene glycol, polyethylene glycol and propylene glycol monomethyl ether. The organic solvent may be contained in all the layers, or may be contained in a part of layers. The organic solvent is preferably added to the fibrous sheets in an amount of from 5 to 95 g per 100 g of the fibres from the standpoint of wet strength. The organic solvent is more preferably added in an amount of from 5 to 60 g to prevent deterioration of use feeling and rough dry skin of a user. In the water-disintegrable sheet of the present invention, other materials may be added if they do not spoil the effects of the present invention. For example, a surfactant, a disinfectant, a preservative, a deodorizer, a moistening agent, an alcohol and the like can be added. These materials may be added to the above-mentioned organic solvent or water, so as to improve the sheet. The water-disintegrable sheet of the invention can be used in a wet state for wiping operation, such as wet-type tissue paper and a duster for cleaning a toilet room and its circumference. When the water-disintegrable sheet of the invention is packed as a product previously wetted, it is sold by sealed up to prevent the sheet from drying. Alternatively, the water-disintegrable sheet of the invention can be used by impregnating with water or a liquid drag upon use. A specific example of the wiping sheet of the present invention preferably has a structure in which the water-disintegrable fibrous sheets are laminated in such a manner that the second fibre webs face outside of the wiping sheet, as shown in Figure 2. This is because the outsides of the wiping sheet, ie sheet faces of the wiping sheet directly in contact with an article or human skin to be wiped upon use, are preferably excellent in strength. Furthermore, because the first fibre webs excellent in water-disintegratability face inside of the wiping sheet, the wiping sheet is dispersed from inside when it is disposed in a toilet and is in contact with a large amount of water, ie it is easily disintegrated by water in a short period. A plurality, greater than two in number, of the water-disintegrable fibrous sheets may be laminated with each other to form the wiping sheet in such a manner that a layer excellent in strength alternates with a layer excellent in water-disintegratability, ie in such an order from the first fibre web, the second fibre web, the first fibre web to the second fibre web. So as to produce the wiping sheet, the water-disintegrable fibrous sheets are partially bound with each other while being laminated. By binding them, strength thereof upon use can be further improved. The fibrous sheets are partially bound with a water-soluble adhesive, so that strength of the wiping sheet can be further improved without deterioration of water-disintegratability thereof. Examples of the water-soluble adhesive include water-soluble carboxymethyl cellulose, methylcellulose and water-soluble polyvinyl alcohol. The wiping sheet comprising the water-disintegratable fibrous sheets can be used in the form in that the wiping sheet is folded. The wiping sheet can be used in a wet state or in a dry state. The invention is further described in more detail by referring to the following example, but the invention is not construed as being limited to them. Example Bleached kraft pulp of conifer (ie NBKP) with a CSF (Canadian Standard Freeness) of 570 ml and bleached kraft pulp of hardwood (ie LBKP) with a CSF (Canadian Standard Freeness) of 720 ml were used as raw material fibres. As a binder, water-insoluble carboxymethyl cellulose (carboxymethyl cellulose chicorate produced by Nichirin Chemical Industries Ltd, DS: 0.42, pH 5.8) was used. The water-insoluble carboxymethyl cellulose was added in a solution containing the bleached kraft pulp of conifer (NBKP) dispersed therein in such a manner that the resulting solution having total weight of 100 g of the pulp and the binder contains 8 g of water insoluble carboxymethyl cellulose. Thereafter, in the solution containing the raw material fibres and carboxymethyl cellulose dispersed therein, 50 g of sodium carbonate was added per 100 g of the carboxymethyl cellulose, and the resulting solution was used as a raw material of a second fibre web. On the other hand, the bleached kraft pulp of conifer (NBKP) and the bleached kraft pulp of hardwood (LBKP) were mixed with water and the resulting solution was used as a raw material of a first fibre web. The resulting raw materials of the first fibre web and the second fibre web were subjected to two-layer paper manufacturing by using a paper machine (short mold - cylinder mold - Yankee machine). The raw material of the first fibre web was first manufactured by the short mold part of the paper machine to form into the first fibre web having a sheet shape with a basis weight of 11.25 g/m 2 . The raw material of the second fibre web was then manufactured by the cylinder mold of the paper machine to form into the second fibre web having a sheet shape with a basis weight of 11.25 g/m 2 as integrated with the first fibre web, to produce a fibrous sheet having a two-layer structure. At this time, an amount of the bleached kraft pulp of hardwood to that of the bleached kraft pulp of conifer in the first fibre web was varied to prepare Examples. The content of the bleached kraft pulp of conifer and the content of the bleached kraft pulp of hardwood based on the total weight of fibres in the first fibre web, in each of Examples 1-1, 1-2, 1-3, 1-4 and 1-5, are shown in Table 1 (% by weight). The crape ratio of each fibrous sheet was 10%. The thus-produced fibrous sheet was impregnated with a solution obtained by mixing propylene glycol monomethyl ether, calcium chloride and water in a ratio of 20/0.5/79.5 in an amount of 200 g per 100 g of the fibrous sheet, to obtain a water-disintegratable fibrous sheet. The water-disintegratable fibrous sheets thus impregnated were then allowed to stand at 20°C for 24 hours and then measured for water-disintegratability and wet strength. Water-disintegratability was measured according to the test of water-disintegratability of toilet paper regulated under JIS (Japanese Industrial Standard) P4501. Specifically, a specimen obtained by cutting the water-disintegratable fibrous sheet into a square of 10 cm x 10 cm was put in a beaker containing 300 ml of ion-exchanged water, followed by stirring with a rotor. The rotational speed was 600 rpm. The state of dispersion of the fibrous sheet was continuously observed, and the time required for completion of dispersion was measured. (The results are shown in tables with a unit of second.) Wet strength was measured in such a manner that the above-obtained water-disintegratable fibrous sheet was cut to a test piece having a dimension of 25 mm width and 150 mm length, and wet strength of the test piece was measured with a Tensilon test machine at a chuck distance of 100 mm and a tensile speed of 100 mm/min. Wet strength was measured in both the machine direction (MD) and the cross direction (CD) of the fibrous sheet. Strength at breakage (gf) was taken as a test result of wet strength. (In the Table, the results are shown in terms of g/25 mm.) As Comparative Example, a raw material of a second fibre web was prepared in the same manner as in Examples. A raw material of a first fibre web was prepared by adding the water-insoluble carboxymethyl cellulose to fibres composed only of the bleached kraft pulp of conifer in the similar manner as in Examples, so to prepare a fibrous sheet from the first fibre web and the second fibre web. The resulting fibrous sheet was impregnated with the same solution as in Examples in an amount of 200 g per 100 g of the fibrous sheet, to obtain a water-disintegratable fibrous sheet. The resulting water-disintegratable fibrous sheet was measured for water-disintegratability and wet strength in the same manner as in Examples. The results obtained are shown in Table 1. Example 1-1 Example 1-2 Example 1-3 Example 1-4 Example 1-5 Comparative Example Amount of NBKP (% by weight) 90 70 60 50 40 100 Amount of LBKP (% by weight) 10 30 40 50 60 0 Water-disintegratability (second) 49 42 39 32 30 64 Wet Strength of MD (g/25 mm) 162 153 148 145 138 166 Wet Strength of CD (g/25 mm) 103 92 89 77 75 111 It is understood from the results shown in Table 1 that Examples are excellent in water-disintegratability while wet strength of Examples and Comparative Example are in the same level.

Claims: A water disintegratable wiping sheet used in a wet state comprising two water-disintegratable fibrous sheets, characterised in : each of said water-disintegratable fibre sheets comprises: a first fibre web comprising pulp of conifer and pulp of hardwood without a binder; a second fibre web comprising pulp of conifer, said second fibre web containing no pulp of hardwood and said first fibre web and second fibre web forming a layered structure; and a water-insoluble or water-swellable binder added to only said second fibre web; said second fibre web has higher wet strength than the first fibre web, and said first fibre web is more water disintegratable than said second fibre web, and wherein said two water-disintegratable fibrous sheets have a wet strength of 75 g/25 mm or higher and are partially bonded one on the other with a water-soluble adhesive so that said second fibre web faces outside of said wiping sheet. A water-disintegratable wiping sheet as claimed in claim 1, wherein a content of said hardwood pulp in said first fibrous web is 60% by weight or less to a total fibre content of first fibre web. The water-disintegratable wiping sheet as claimed in claim 2, wherein the content of said hardwood pulp in said first fibre web is 10 to 50 % by weight. The water-disintegratable wiping sheet as claimed in claim 3, wherein said binder is water-insoluble or water-swellable carboxymethyl cellulose. The water-disintegratable wiping sheet as claimed in claim 4, wherein said water-insoluble or water-swellable carboxylmethyl cellulose as the binder has a degree of etherification of 0.3 to 0.6 and a pH of 5.0 or more. The water-disintegratable wiping sheet as claimed in claim 5, wherein at least one of sodium carbonate and sodium hydrogen carbonate is added to said fibrous sheet. The water-disintegratable wiping sheet as claimed in claim 6, wherein at least one metallic salt selected from the group consisting of salts of calcium, zinc and magnesium is added to said fibrous sheet. The water-disintegratable wiping sheet as claimed in claim 7, wherein said fibrous sheet is impregnated with an organic solvent which is at least one compound selected from the group consisting of monohydric alcohol and polyhydric alcohol.

Inventors: KONISHI TAKAYOSHI, TAKEUCHI NACHITO, YAMADA DAISUKE, KONISHI, TAKAYOSHI, TAKEUCHI, NACHITO, YAMADA, DAISUKE, Konishi, Takayoshi, Research &amp; Development Div., Takeuchi, Nachito, Research &amp; Development Div., Yamada, Daisuke, Research &amp; Development Div.
Applicants: UNI CHARM CORP, UNI-CHARM CORPORATION
Application year: 1998
Main classification: B32B29
IPC codes: D21H27, A47L13, D21H17, D21H11, B32B5, A47K10, A47K7