Paper making and fiber liberation – Processes and products – Non-uniform – irregular or configured web or sheet
Reexamination Certificate
2000-07-27
2001-07-10
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-uniform, irregular or configured web or sheet
C162S123000, C162S125000, C162S129000, C162S130000, C162S132000, C162S146000, C162S149000, C162S157600, C162S157700
Reexamination Certificate
active
06258210
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a water-decomposable fibrous sheet having high strength and capable of being readily decomposed and dispersed in water flow. More precisely, it relates to such a water-decomposable fibrous sheet having a multi-layered structure.
2. Description of the Related Art
To wipe the skin of human bodies including the private parts thereof, or to clean toilets and thereabouts, used are disposable cleaning sheets made of paper or non-woven fabric. For these cleaning sheets, water-decomposable cleaning sheets that could be directly disposed of in toilets after use have been developed, as being convenient for such purposes. The degree of their decomposability in water must be high in some degree. This is because, if poorly water-decomposable cleaning sheets are disposed of in toilets after use, they will take a lot of time until they are decomposed and dispersed in septic tanks, or will clog the drainpipes around toilets, etc.
For wiping off wet dirt and for easy and effective use, many cleaning sheets for wiper applications are packaged while being wetted with a liquid detergent chemical or the like, and are put on the market. Therefore, such water-decomposable cleaning sheets must have high strength in wet to such a degree that they are well fit for wiping with them wetted with such a liquid chemical or the like, but must well decompose in water after they are disposed of in toilets.
For example, Japanese Patent Publication No.24636/1995 discloses a water-decomposable cleaning article that comprises a water-soluble binder having a carboxyl group, a metal ion and an organic solvent. However, the metal ion and the organic solvent irritate the skin.
Japanese Patent Laid-Open No. 292924/1991 discloses a water-decomposable cleaning article of polyvinyl alcohol-containing fibers with an aqueous solution of boric acid infiltrated thereinto; and Japanese Patent Laid-Open No. 198778/1994 discloses a water-decomposable napkin of polyvinyl alcohol-containing non-woven fabric with a borate ion and a bicarbonate ion introduced thereinto. However, polyvinyl alcohol is not resistant to heat, and therefore the wet strength of the water-decomposable cleaning article and the water-decomposable napkin is lowered at 40° C. or higher.
Recently, various water-decomposable absorbent articles including sanitary napkins, panty liners, disposable diapers and others have been investigated in the art. In view of their safety, however, the water-decomposable fibrous sheets mentioned above could not be used as the top sheets for those absorbent articles that shall be kept in direct contact with the skin for a long period of time, as they contain a binder and an electrolyte.
On the other hand, Japanese Patent Laid-Open No. 228214/1997 discloses a water-degradable non-woven fabric having a wet strength of from 100 to 800 gf/25 mm (from 0.98 to 7.84 N/25 mm) as measured according to JIS P-8135, which is produced by mixing fibers having a length of from 4 to 20 mm with pulp followed by entangling them through treatment with high-pressure water jets. Since the constituent fibers are entangled therein, the non-woven fabric disclosed has a bulky feel. However, in producing the non-woven fabric, long fibers are entangled through high-pressure water jet treatment, whereby the non-woven fabric produced could have such a relatively high wet strength. Therefore, according to the technique disclosed, it is difficult to realize well-balanced bulkiness, strength and water-degradability for the non-woven fabric produced, and the non-woven fabric produced is unsuitable to disposal in flush toilets, etc.
SUMMARY OF THE INVENTION
An object of the invention is to provide a water-decomposable fibrous sheet which is well decomposed in water and has good strength enough for practical use even though no binder is added thereto.
According to one aspect of the invention, a multi-layered water-decomposable fibrous sheet with a plurality of layers each having a different fibrous constitution being integrated, may comprise:
a top layer on a top surface including fibrillated rayon of at least 3% by mass of all fibers constituting the top layer, the fibrillated rayon having a degree of beating of at most 700 cc and having primary fibers of a predetermined fiber length and microfibers extending from the primary fibers, the microfibers being entangled with and/or hydrogen-bonded to at least either of other microfibers and other fibers, and a back layer on a back surface including fibers containing no fibrillated rayon, the fibers entangled with and/or hydrogen-bonded to each other, or the back layer including the fibrillated rayon in an amount smaller than that in the top layer, the microfibers of the fibrillated rayon being entangled with and/or hydrogen-bonded to at least either of other microfibers and other fibers.
According to another aspect of the invention, a multi-layered water-decomposable fibrous sheet with a plurality of layers each having a different fibrous constitution being integrated, may comprise:
a top layer on a top surface and a back layer on a back surface each including fibrillated rayon of at least 3% by mass of all fibers constituting each layer, the fibrillated rayon having a degree of beating of at most 700 cc and having primary fibers of a predetermined fiber length and microfibers extending from the primary fibers, the microfibers being entangled with and/or hydrogen-bonded to at least either of other microfibers and other fibers, and
an interlayer disposed between the top layer and the back layer including fibers containing no fibrillated rayon, the fibers being entangled with and/or hydrogen-bonded to each other, or the interlayer including fibrillated rayon in an amount smaller than that in the top and back layers, the microfibers of the fibrillated rayon being entangled with and/or hydrogen-bonded to at least either of other microfibers and other fibers.
In this case, the fibrillated rayon content of the fibers constituting the top layer may be nearly the same as that of the fibers constituting the back layer.
Naturally in dry and even in wet with water, the water-decomposable fibrous sheet of the invention all the time keeps high strength. When it is immersed in a large amount of water after used and disposed of in toilets and others, it is readily decomposed. In the fibrous sheet of the invention, the microfibers of fibrillated rayon are entangled with and are further hydrogen-bonded to other fibers and other microfibers therein, thereby exhibiting their ability to bond fibers constituting the sheet and to enhance the strength of the sheet. When the fibrous sheet receives a large amount of water applied thereto, the entangled microfibers therein are loosened or the hydrogen bonds between the bonded microfibers therein are broken, whereby the fibrous sheet is readily decomposed in water.
The fibrous sheet of the invention has a multi-layered structure. In this, only the top layer may contain fibrillated rayon or the top layer may contain a larger amount of fibrillated rayon than the interlayer. The fibrous sheet of this embodiment is good, as having well-balanced decomposability in water and strength. In addition, fibrillated rayon is concentrated in the top surface of the fibrous sheet, and the microfibers extending from the fibrillated rayon therein are essentially contacted with the other layers of the sheet. As a whole thereof, therefore, the fibrous sheet can attenuate direct frictional force applied thereto. Accordingly, the surface of the water-decomposable fibrous sheet of the invention is highly resistant to friction. Fibrillated rayon is more expensive than ordinary rayon, and production of the fibrillated rayon takes a lot of time. Without requiring too much fibrillated rayon as a whole, the fibrous sheet of the invention can ensure good decomposability in water and high strength, so that production costs thereof can be low.
The water-decomposable fibrous sheet of the invention can be composed of materials not harmful t
Okada Kazuya
Takeuchi Naohito
Chin Peter
Darby & Darby
Uni-Charm Corporation
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