Wet non-woven fabric and method for producing the same

1 2

WET NON-WOVEN FABRIC AND METHOD making paper of the resulting mixture;

FOR PRODUCING THE SAME or pulverizing at least one of a highly water absorptive

polymer fiber and particle while it is swollen with DETAILED DESCRIPTION OF THE water togetner witn puip beating them; and

5 making paper of (he resulting mixture.

INVENTION 1. Field of the Invention

This invention relates to a wet non-woven fabric and a

BRIEF DESCRIPTION OF THE DRAWING

method for producing the same, and, more specifically, to a mG- 1 and nG- 2 m SEM Photos of a wet non-woven

wet non-woven fabric having an excellent water absorption fabric obtained in Example 2. FIG. 2 is an enlarged view of

property and a method for producing the same. 10 nG- 1 Wrinkled portions are pulp and non-wrinkled por

2 Prior Art t*ons wt a water absorptive polymer.

Non-woven fabrics having an excellent water absorption MOST PREFERRED EMBODIMENT OF THE

property have already been commercialized Most of mem INVENTION

are dry non-woven fabrics. To improve their water absorp- 15 xhe highly water absorptive polymer used in the present

tion percentage, absorptive polymer fibers or particles are invention is preferably a copolymer of a monomer having a

contained in the dry non-woven fabrics. In the paper making carboxylic acid group, a monomer having a hydroxyl group

case where the-absorptive polymer fibers or particles are that can form an ester bond upon reaction with a carboxylic

mixed with a wet non-woven fabric, as an absorptive poly- acid group, and a monomer having an alkali metal salt

mer fiber or particle generally used has a diameter as large 20 carboxylate group. The polymer has a crosslinking structure

as 0.5 to 2.0 mm. the water absorptive polymer fiber or and, e^Hts me water absorption aWUty to absorb 1200% by

... ... . . ,nrj. i. ./ .... weight or more and less than 3000% by weight of physi

particle swells to approximately 1000 times its original size , . . .. , J o- r J

,. .. r , .„. . . ., ological saline solution.

and has a diameter of several millimeters at mixing with ° . . .....

_ H Examples of the monomer having a carboxylic acid group

water. Consequently, the water absorptive polymer fiber or 25 ... acryJic ^ (abbreviated as ^ heremafter). meth.

particle has the disadvantage that it excludes other fibers at aaylic ad4 add and so forth ^ ^ ^ used

the time of paper-forming and falls off from the non-woven £ODe. or „, combination of two or more.

fabric when it is dried after paper-making. Examples of the monomer having a hydroxyl group

Furthermore, the water absorptive polymer fiber or par- 30 include hydroxyethylmefliacrylate (abbreviated as HEMA

tide has another disadvantage that an increase in its mixing hereinafter), hydroxy propylmeth aery late,

ratio with the fabric lowers the strength of the wet non- hydroxyethylacrylate, hydroxypropylacrylate, glyceryl

woven fabric. Therefore, to overcome mis disadvantage, me monomethacrylate, glyceryl monoacrylate. and so forth,

absorptive polymer fiber or particle is dispersed between ^ ■* used ^ 01 in combination of two or more,

layers of a laminate. However, this method requires addi- 35 Examples of the monomer having an alkali metal salt

tional cost for stacking layers, thus boosting costs. carboxylate group indude alkali metal salts of AA, meth

° acrylic acid, maleic acid and so forth. Examples of the alkali

Japanese Patent Publication No. 55202/1986 discloses a metal iadaat. sodium (abbreviated as Na hereinafter), potas

method which comprises the steps of impregnating a non- smm and so forth. They may also be used alone or in

woven fabric with a monomer before polymerization and w combination of two or more.

then polymerizing the monomer. Since the method requires The monomer having a hydroxyl group is preferably used

an intricate apparatus, a large amount of initial investment is in an amount not more than die equivalent mol of the

necessary, also inviting an increase in costs. monomer having a carboxylic acid group. It is advantageous

to use the monomer having a hydroxyl group in an amount

DESCRIPTION OF THE INVENTION 45 of at least 0.5% by weight based on the total weight of the

An object of the invention is to provide a wet non-woven monomer having a carboxylic acid group, the monomer

fabric having a uniform water absorption property. havin8 a hvdroxyl group and the monomer having an alkali

A „, .. ^ ... . • * J A. Ac metal salt carboxylate group. Below 0.5% by weight, the

Another object of the invention is to provide a method for .... , . Jr r ... , . .

... „ . ^. , ^ ... crosslinking density of the resulting copolymer may be too

producing the wet non-woven fabric of the invention with smajj_ J

ease and at a low cost. *

, . . ... * • .„ The weight ratio of the monomer having a carboxylic acid

Other objects and advantages of the invention will tQ £ hayi ^ ^ ^ ^ ^

become apparent in the following description. ^ ^ ig prefaBbly m ^ range from b mto 1/10 ^

Firstly, according to the present invention, the above- total of me monomer having a carboxylic group and the

mentioned objects and advantages of the invention are J5 monomer navillg m alkali metal salt carboxylate group is

attained by a wet non-woven fabric which comprises (A) a preferably in the range from 70 to 99.5% by weight, more

highly water absorptive polymer in an amount of 5 to 60% preferably from 80 to 95% by weight based on the total

by weight and (B) pulp in an amount of 40 to 95% by weight of these monomers and the monomer having a

weight, based on the total weight of the components (A) and hydroxyl group. Below 70% by weight, the saline solution

W- 60 absorption percentage of the resulting copolymer may be

Secondly, according to the present invention, mere is insufficient, which is not desirable,

provided a method for producing a wet non-woven fabric addition to the above-mentioned monomers, other

which comprises the steps of: vinyl monomers such as vinyl acetate (abbreviated as VA

wet pulverizing at least one of a highly water absorptive hereinafter) and acrylonitrile may be used to provide the

polymer fiber and particle while it is swollen with 65 copolymer with plasticity. The amount of monomer for

water; plasticizing the copolymer is preferably not more than 30 by

mixing it with pulp; and weight

3 4

The polymerization method of the highly water absorptive tive fine polymer in the form of an amoeba preferably falls

polymer used in the present invention is not limited specifi- below 50 urn, more preferably below 20 urn, when it is dry.

cally. Water-base polymerization may be used if the mono- Above 50 um, the quality of the produced wet non-woven

mer composition is water soluble. Generally used sodium fabric deteriorates. The smaller the average diameter of the

persulfate and other suitable substances may be used as a 5 polymer, the better texture it achieves. In other words, the

polymerization initiator. uniformity, strength and other factors of the produced wet

The spinning method for obtaining the highly water non-woven fabric deteriorate. For underwater pulverization,

absorptive polymer fiber used in the present invention is a mixer with a blade can be used. When the concentration of

preferably generally used dry spinning. In the case of wet P"lp and the polymer of fiber at the time of pulverization is

spinning, an organic solvent must be used because water to higher than that at the time of forming paper, improved

cannot be used as a coagulant. After dry spinning, the highly pulverization efficiency can be achieved. After the highly

water absorptive polymer is drawn to at least 1.3 times by water absorptive polymer swells, its compatibility with pulp

dry heating while it contains not less than 10% by weight of is improved. The above-mentioned highly water absorptive

water, and then subjected to crosslinking treatment by dry fiber which exhibits adhesion may be used in conjunction

heating. Thereafter, the resulting fiber is suitably crimped 15 with the highly water absorptive fine polymer particle which

and cut. When the fiber is not drawn to 1.3 times or more exhibits adhesion. When the average diameters of the highly

while it contains not less than 10% by weight of water, the water absorptive fiber and the highly water absorptive

strength of the resulting fiber would be insufficient. polymer particle are reduced, their water absorption per

Although a fiber having a large molecular weight absorbs a centages for highly viscous substances such as blood and

large quantity of water, it is hard to be drawn because of its 20 substances containing a solid matter, for example, are

large molecular weight, and the strength of the fiber tends to improved.

decrease. In the method of the present invention, the wet pulverized

The highly water absorptive fiber produced by me above- matters of (he above-mentioned highly water absorptive

mentioned method and used in the present invention pref- polymer are then mixed with pulp, and paper is formed of

erably absorbs 1200% by weight or more and less than 25 the resulting mixture.

3000% by weight of a physiological saline solution, more That is. me wet non-woven fabric of the present invention

preferably 1800% by weight or more and less than 3000% can be obtained by dispersing highly water absorptive fibers

by weight Above 3000% by weight, the fiber strength is apt and/or highly water absorptive fine polymer particles into a

to deteriorate. pulp slurry which has been suitably beaten to control its

Since the above-mentioned highly water absorptive fiber 30 freeness, forming paper with a paper-forming machine of a

exhibits excellent resistance to flame, it can provide a wet short net type or a long net type and drying paper. For the

non-woven fabric with resistance to flame even if it is mixed formation of paper, it is preferred that vibrations are given

with pulp in an appropriate amount to the paper forming net. The highly water absorptive fiber

The average diameter of the highly water absorptive fiber and/or water absorptive fine polymer particle used in the present invention when it is dry is preferably not swells TM me paper-forming step, but changes into an more than 50 um, more preferably not more than 20 um. amoeba-like form when it is dried in the paper. However, Above 50 um, the quality of the resulting wet non-woven bom of mem stm_ exhibit the same water absorption property fabric formed into paper deteriorates. That is. the wet as mat of me originalnon-woven fabric to be produced deteriorates in uniformity The pulp used in the present invention is not limited to and strength. any particular type. NBKR LBKP and other pulps that are

Furthermore, the highly water absorptive fiber used in the generally used may be used, but the present invention is not

present invention is preferably not more than 20 mm in limited to these. The wet non-woven fabric of the present

length invention contains at least 40% by weight of pulp. Below

Examples of the highly water absorptive polymer particle 45 40* by weight, paper-forming is difficult and it is not

used in the present invention include commercially available preferable economically.

sodium polyacrylate-base, graft polymerized starch-base According to another method of the present invention, at

and polyethylene oxide-base particles, but the highly water least one of the highly water absorptive fiber and particle

absorptive particle used in the present invention is not while it is swollen with water may be pulverized together

limited to these. so PulP and beaten, and then paper is made.

There are great differences in size among these commer- To produce the wet non-woven fabric of the present

dally available highly water absorptive polymer particles: a invention, generally used additives such as a paper strength

large one is almost 1 mm in diameter. When these particles enhancing agent, sizing agent, pigment, flame retardant and

absorb pure water and swell, they become approximately 10 antibiotic agent and a binder fiber may be used in such

times larger in diameter and, hence, they are not suitable for 55 amounts mat do not significantly impair the water absorption

the formation of paper. Even if the highly water absorptive percentage.

polymer is pulverized directly into powders to reduce its Thus, according to the method of the present invention,

size, the powders aggregate and become a big mass when the above-mentioned wet non-woven fabric of the present

they are dispersed into water. When it is pulverized into invention can be produced.

powders while it is dry, the powders are apt to easily absorb 60 The water absorptive wet non-woven fabric of the present

moisture because of their large surface areas. Therefore, they invention contains 5 to 60% by weight of a highly water

aggregate in storage, thus making it difficult to handle them absorptive polymer. Although different by each application

According to the method of the present invention, the purpose, when the content is below 5% by weight, the water

highly water absorptive polymer fiber or particle is wet absorption ability of the fabric is insuflicient for practical

pulverized while it is swollen with water. Thereby, the fiber 65 use. On the other hand, when it exceeds above 60% by

or particle does not aggregate but is uniformly dispersed weight, paper formation is difficult From a viewpoint of

The average diameter of the pulverized highly water absorp- water absorption and operation efficiency, the highly water