Patent Publication Number: US-2002009942-A1

Title: Absorbent material

Description:
TECHNICAL FIELD  
       [0001] The present invention relates to an absorbent material and, more particularly, to an absorbent material which is used as ink pads of ink-jet and bubble-jet printers, building materials such as wall and ceiling materials, materials for civil engineering work, medical materials such as dental materials, and other materials required to have excellent water absorption properties, water retention properties, form stability and easy processing.  
       BACKGROUND ART  
       [0002] Absorbent materials having various characteristics have hitherto been developed according to various purposes. To obtain absorbent materials having high water absorption percentage, a method of reducing a density of the absorbent material has been employed.  
       [0003] However, there is a problem that the volume decreases on water absorption when the density of the absorbent material is reduced, thereby causing a problem that water retention percentage decreases by reducing the density and water retention percentage decreases by vibration and so on.  
       [0004] Therefore, a water-absorptive polymer is mixed with the absorbent material to increase both the water absorption percentage and water retention percentage. However, there arises a problem that, when a water-absorptive polymer is mixed with the absorbent material, the water-absorptive polymer falls out on cutting or water absorption. A method of preventing this water-absorptive polymer from falling out includes, for example, a method of preparing a water-absorptive polymer in the form of large particles. However, the water-absorptive polymer in the form of large particles has a drawback that a change in volume of the absorbent material due to water absorption increases when the water-absorptive polymer in the form of large particles is mixed with the absorbent material because of large expansion on water absorption.  
       [0005] As a method of preventing the water-absorptive polymer from falling out when a water-absorptive polymer in the form of small particles is mixed with the absorbent material, a method of putting a mesh-shaped cover over the absorbent material has also been suggested. For example, Japanese Patent Laid-open Publication No. 91052/1981 discloses a method of sandwiching a layer containing a water-absorptive polymer between dry non-woven fabrics. However, this method has a problem that the position of the absorbent material is not fixed because the portion for seaming upper and lower covers is obstructive in the case of setting in a container box. In this method, there was also a problem that it is difficult to put a cover over an absorbent material having a complicated cut shape at low price.  
       [0006] Accordingly, an object of the present invention is to solve these problems of the absorbent material of the prior art and to provide an absorbent material which shows high water absorption percentage and high water retention percentage as well as small change in volume on water absorption and can be easily cut into a complicated form, and which can be obtained at low cost.  
       DISCLOSURE OF THE INVENTION  
       [0007] The present invention provides an absorbent material comprising a non-woven fabric containing water-absorptive fibers, wherein a water absorption percentage is not less than 400% by weight and a change in volume is less than 10% in a state where water is absorbed in an amount of not less than 400% by weight.  
       [0008] In this absorbent material, when highly water-absorptive fibers are contained in an amount of 2 to 20% by weight, more excellent water absorption properties can be obtained. Furthermore, it becomes possible to remarkably improve the water retention properties by using highly water-absorptive fibers of crosslinked sodium polyacrylate.  
       BEST MODE FOR CARRYING OUT THE INVENTION  
       [0009] The absorbent material of the present invention is an absorbent material comprising a non-woven fabric containing water-absorptive fibers, wherein a water absorption percentage is not less than 400% by weight and a change in volume is less than 10% in a state where water is absorbed in an amount of not less than 400% by weight.  
       [0010] As the water-absorptive fibers, highly water-absorptive fibers are preferably used. The term “highly water-absorptive fibers” used herein refer to fibers capable of absorbing deionized water in an amount twenty or more times as much as its own weight. By using the highly water-absorptive fibers, sufficient water absorption percentage and water retention percentage can be obtained.  
       [0011] As the highly water-absorptive fibers, a monomer having the composition resemblance to the water-absorptive polymer can be used. As carboxylic acid, for example, (meth)acrylic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, sorbic acid, cinnamic acid, crotonic acid, -acryloxypropionic acid, and alkali metal salts thereof can be used. Among them, (meth)acrylic acid is economically preferred. As sulfonic acid, for example, 2-(meth)acrylamide-2-methylpropanesulfonic acid, (meth)allylsulfonic acid, ethyl sulfonic acid (meth)acrylate, styrenesulfonic acid, vinylsulfonic acid, 2-(meth)acryloylpropanesulfonic acid, and alkali metal salts thereof can be partially used. The water absorption percentage can be improved by containing these sulfonic acids.  
       [0012] When the highly water-absorptive fibers are crosslinked, it is possible to use 2-hydroxypropyl (meth)acrylate, hydroxyethyl (meth)acrylate, and their adducts with ethylene oxide and propylene oxide, and so on as a crosslinking agent. When the amount of these crosslinking agent is too large, the water absorption percentage is lowered. Therefore, it is necessary to appropriately set the amount taking the amount of the highly water-absorptive fibers and so on into consideration. Furthermore, when a water-absorptive polymer is prepared, a polyfunctional group is used. When the fibers are made, gelation hardly occurs by using those other than the polyfunctional group and, therefore, it is preferred in view of spinning. Crosslinking using the above crosslinking agent is preferably performed at the time when the fibers were completely formed after the completion of the spinning.  
       [0013] The highly water-absorptive fibers which are used particularly preferably in the absorbent material of the present invention include, for example, highly water-absorptive fibers of crosslinked sodium polyacrylate, highly water-absorptive fibers obtained by hydrolyzing the surface layer portion of acrylic fibers, highly water-absorptive fibers containing a maleic acid metal salt as a principal component, and so on.  
       [0014] Since these highly water-absorptive fibers have characteristics such as high swelling properties, the amount is preferably not more than 20% by weight taking deformation on water absorption into consideration. To obtain sufficient water absorption percentage and water retention percentage, the highly water-absorptive fibers are preferably mixed in the amount of not less than 2% by weight. That is, the amount of the highly water-absorptive fibers is within a range from 2 to 20% by weight, and preferably from 3 to 10% by weight, based on the total weight.  
       [0015] The highly water-absorptive fibers of a crosslinked sodium polyacrylate and the highly water-absorptive fibers containing a maleic acid metal salt as a principal component have flame retardance and good water retention properties, but have high swelling properties because of good water retention properties. Accordingly, there is a problem that, when these highly water-absorptive fibers are contained in a large amount, the degree of deformation on water absorption becomes large. Furthermore, since the fibers have high water absorption properties, the fibers themselves exhibit spot water absorption properties. Therefore, when the fibers are mixed in a very large amount, the water absorption rate of the absorbent material itself is decreased by blocking of water. Accordingly, in the case of mixing these highly water-absorptive fibers, the amount is preferably adjusted within a range from 2 to 15% by weight, and particularly preferably from 3 to 10% by weight.  
       [0016] Furthermore, the highly water-absorptive fibers of the crosslinked sodium polyacrylate are capable of performing high water absorption repeatedly because of very small reduction in water absorption percentage due to repeated water absorption, and which is superior in durability, light resistance and heat resistance on water absorption and cause small change with time. Therefore, the highly water-absorptive fibers are suited as highly water-absorptive fibers used in the absorbent material of the present invention.  
       [0017] Since the highly water-absorptive fibers obtained by hydrolyzing the surface layer portion of the acrylic fibers have lower swelling properties than those of the highly water-absorptive fibers of the crosslinked sodium polyacrylate and the highly water-absorptive fibers containing the maleic acid metal salt as the principal component, the highly water-absorptive fibers are preferably mixed in the amount within a range from 3 to 20% by weight, and more preferably from 5 to 15% by weight.  
       [0018] When the absorbent material of the present invention is particularly applied to electrical parts, it is preferred to mix flame-retardant fibers with them. Examples of the flame-retardant fibers include modacrylic, flame-retardant polyester, nylon, wool, flame-retardant rayon fibers, and so on, and these flame-retardant fibers preferably have Loi of not less than 26. Among them, particularly preferred are modacrylic fibers which easily impart hydrophilicity and are cheap. Such the flame-retardant modacrylic fibers include, for example, LUFNEN Manufactured by Kanebo Ltd., KANEKARON manufactured by Kaneka Corp ,and so on. That is, when using these flame-retardant fibers, there can be obtained those which pass UL Standards (UL&#39;94, MM, Vertical Burning Test, ASTM D3801) in the case of applying to electrical parts.  
       [0019] The absorbent material of the present invention can be mixed with flammable fibers, in addition to the flame-retardant fibers. When using the flammable fibers, it is preferred to mix taking the flame retardant properties of the flame-retardant fibers and kind of the flammable fibers into consideration. For example, when the modacrylic fibers are used as the flame-retardant fibers, there can be obtained those, which pass UL Standards, by mixing the modacrylic fibers in the amount of not less than 70% by weight. When an oily agent is coated on the fibers to impart the hydrophilicity, it is preferred to confirm by actually making a trial considering that it is influences by the amount of the oily agent, weight and density of the absorbent material as a product, flame retardance of the other fibers to be mixed, and so on. When using highly flame-retardant modacrylic fibers, there can be obtained those, which pass UL Standards, by mixing in the amount of not less than 50% by weight.  
       [0020] It is also possible to use a pulp as a kind of the water-absorptive fibers in the absorbent material of the present invention. In this case, since the fiber length of the pulp is short, binder fibers are preferably mixed so that the pulp does not fall out. As the binder fibers, there can be used heat-fusing binder fibers which are core-sheath or side-by-side type conjugate fibers and so on. The binder fibers may be made of a homopolymer, but those made of a copolymer are suitable because of large adhesive strength. As the binder fibers, for example, commercially available binder fibers such as ES type fibers manufactured by Chisso Corp., BELLCOMBI manufactured by Kanebo Ltd., and the like can be used. In this case, the binder fibers are usually mixed in the amount of not less than 10% by weight, preferably not less than 15% by weight, and more preferably not less than 20% by weight. That is, the larger the amount of the binder fibers, the larger the adhesive strength, thereby making it possible to prevent the pulp from falling out. However, since the binder fibers themselves are flammable, it is necessary to appropriately limit the amount to be mixed considering the amount of the flammable pulp in the case of imparting the flame retardance to the absorbent material. When using a large amount of conjugate binder fibers, the conjugate binder fibers form a network and, therefore, it is preferred to retain the shape of the absorbent material. Particularly, when the proportion of the conjugate binder fibers becomes 20% by weight, it becomes easier to exert the effect of retaining the shape.  
       [0021] The fineness and fiber length of these fibers are largely influenced by a method of and an equipment for producing the non-woven fabric constituting the absorbent material of the present invention. As the non-woven fabric constituting the absorbent material of the present invention, it is possible to appropriately use those selected from air-laid, cross-laid, wet, felt, and the like type non-woven fabrics. For example, when the non-woven fabric is a felt, the fineness is preferably adjusted to not less than 3 denier in average and the fiber length is preferably adjusted to not less than 50 mm in average. When non-woven fabric is an air-laid type non-woven fabric, the fineness of the binder fibers is preferably not less than 3 denier so as to increase the number of constituent fibers, whereas, the fiber length is preferably from 3 to 20 mm, and more preferably from 5 to 10 mm. Furthermore, in the case of the non- woven fabric produced by conventional carding, cross-laid and needle punching, the fineness and fiber length can be appropriately set according to carding. In this case, since the number of needle-punching is smaller than that of the felt, the shape of the absorbent material is preferably retained using the binder fibers by mixing the heat- fusing fibers and the like similar to the case of the non-woven fabric according to the air-laid method. The amount of the binder fiber to be blended varies depending on the degree of needle punching, but can be made slightly smaller than that in the case using the air-laid method. The non-woven fabric constituting the absorbent material of the present invention may be produced by any method described above, but is preferably a felt having large degree of interlocking of the fibers without using the binder fibers so as to impart the flame retardance.  
       [0022] By using dyed fibers partially as the fibers used in these non-woven fabrics, it becomes possible to classify products by color identification. In this case, it is preferred to use modacrylic fibers which can be easily dyed. In the case of the non-woven fabric produced by the air-laid method, not only classification of products but also further detailed information can be applied by using those colored by printing as the non-woven fabric and paper to be provided on the upper and lower surfaces.  
       [0023] The thickness of the absorbent material of the present invention is preferably adjusted to not less than 1 mm. That is, when the thickness is less than 1 mm, the liquid retention percentage per unit area is liable to be insufficient.  
       [0024] The density of the absorbent material of the present invention is preferably within a range from 0.03 to 0.28 g/cm 3 . That is, when the density is less than 0.03 g/cm 3 , there is a fear that the degree of deformation upon water absorption increases and the liquid retention percentage decreases by 10% or more upon vibration. To the contrary, when the density exceeds 0.28 g/cm 3 , the water absorption percentage is liable to be insufficient. Accordingly, the density of the absorbent material of the present invention is preferably adjusted within a range from 0.03 to 0.28 g/cm 3 , and more preferably from 0.08 to 0.20 g/cm 3 .  
       [0025] The water absorption rate of the absorbent material of the present invention is preferably not more than 5 seconds. This water absorption rate can be improved by coating an appropriate amount of the oily agent on the fibers. As the oily agent, a general oily agent can be used. Since a cationic oily agent is often inferior in hydrophilicity, nonionic and anionic oily agents are preferably used. The oily agent can be coated on the fibers on production of the fibers, production of the non-woven fabric, dyeing, and so on, and the amount varies depending on the method of producing the non-woven fabric but is preferably adjusted within a range from 0.3 to 3.0% by weight. That is, when the amount is too small, the hydrophilicity is insufficient. To the contrary, when the amount is too large, the operatablity is lowered particularly in the carding on production of the non-woven fabric.  
       [0026] In the present invention, the water absorption percentage of the absorbent material is measured in the following manner. That is, a test material of the absorbent material is allowed to stand on a 40 mesh wire cloth after sufficiently dipped in a liquid such as deionized water. After the test material became free from liquid drop, the weight is measured and a percentage (%) is determined by dividing a difference between the weight and an absolute dry weight of the same test material before water absorption by the absolute dry weight, followed by multiplication by 100. When the test material is made of fibers, the measurement is performed by putting the test material in a tea bag. When using the tea bag, a blank test in a state of putting no test material is performed and a correction of the water absorption percentage is performed by subtracting the water absorption percentage of the tea bag on calculation.  
       [0027] The water retention percentage of the absorbent material of the present invention is determined in the following manner. That is, the water-absorbed test material is dropped, together with the metal cloth, from the height of 30 cm in a state where the test material is allowed to stand on the metal cloth. Then, the water absorption percentage is measured again and the water retention percentage is calculated from the water absorption percentages before and after dropping.  
       [0028] In the present invention, the water absorption rate of the absorbent material is measured in the following manner. That is, a test material cut into pieces having a width of 3 cm and a length of 10 cm is dipped in a liquid at the lower portion of 1-2 cm while supporting perpendicularly the test material, and then a time required for the liquid level to reach the height of 5 cm is taken as a water absorption rate. As a standard solution, deionized water is used. 
     
    
    
     EXAMPLE 1  
     [0029] 5% by weight of highly water-absorptive fibers (OASIS manufactured by Technical Absorbent Co.) having a fineness of 9 denier and a fiber length of 51 mm and 85% by weight of flame-retardant modacrylic fibers (LUFNEN BR manufactured by Kanebo Ltd.) having a fineness of 3 denier and a fiber length of 51 mm were mixed, and then carding, cross-laid and needle punching were performed in a conventional production process of felt to obtain a felt having a thickness of 15 mm, a weight of 2500 g/m 2  and a density of 0.17 g/cm 3  as an absorbent material of the present invention. In the preliminary stage of the production of the felt, 2% by weight of an anionic hydrophilic oily agent was added to the above flame-retardant modacrylic fibers and fiber blending was performed, thereby to improve the water absorption rate of the resulting absorbent material.  
     [0030] The resulting felt was cut into pieces having a width of 3 cm and a length of 10 cm to produce a test material. Using this test material, the water absorption percentage and water absorption rate were measured by the method described above. As a result, the water absorption percentage was 490% by weight and the water absorption rate was 3.9 seconds. Furthermore, the thickness of the test material was 16 mm in a state where water was absorbed in the amount of 490% by weight. After water absorption, only the thickness changed without changing the area, and the volume changed by +6.7%.  
     [0031] Then, the water retention percentage of the test material was measured by the method described above in a state where the test material absorbed water in the amount of 490% by weight. As a result, it was confirmed that the water retention percentage after dropping is 470% by weight and the water retention is 96%, that is, the water retention properties are excellent. Even after this dropping test, no fibers were removed from the cut surface of the felt.  
     [0032] Furthermore, this felt was put in a desiccator containing water at the bottom and allowed to stand in a temperature controlled bath at 30 at 100% RH. Then, the weight after 72 hours was measured. The moisture absorption percentage was calculated from a difference between the measured weight and absolute dry weight. As a result, high moisture absorption percentage such as 25% was shown.  
     EXAMPLE 2  
     [0033] Absorbent materials No. 1 to 6 were produced in the same manner as in Example 1 except that a mixing ratio of the highly water-absorptive fibers (A) to the flame-retardant modacrylic fibers (B) used in Example 1 is set as shown in Table 1.  
     [0034] With respect to the respective absorbent materials, the water absorption percentage, the change in volume (change in thickness, area does not change) before and after water absorption, the water retention percentage, and the water absorption rate were measured. The results are shown in Table 1.  
     [0035] All of the above absorbent materials No. 1 to 6 passed VO in accordance with UL Standards.  
                                       TABLE 1                       No.   A (%)   B (%)   Water   Change in   Water retention   Water                                                            1    2   98    410    9   91   6.1       2    4   96    480    4   95   3.3       3    9   91    620    7   98   4.2       4   14   86    850    9   96   6.5       5    1   99    230    3   73   7.1       6   21   79   1030   16   90   10.5                  
 
     [0036] As is apparent from these results, the absorbent materials No. 1 to 4 show high water absorption percentage and high water retention percentage, small change in volume (change in thickness) before and after water absorption, and small water absorption rate(second).  
     [0037] To the contrary, the absorbent material No. 5 was insufficient in water absorption percentage because the amount of the highly water-absorptive fibers used is small such as 1% by weight. Furthermore, since the absorbent material No. 6 contains a large amount of the highly water-absorptive fibers, the water absorption percentage was sufficient. However, the change in volume (change in thickness) was large and the water absorption rate(second) was large.  
     [0038] As described above, when the absorbent material of the present invention is composed of a felt comprising the highly water-absorptive fibers and flame-retardant modacrylic fibers of the above Examples, the content of the highly water-absorptive fibers is preferably adjusted within a range from 2 to 20% by weight, and more preferably from 3 to 10% by weight, based on the total weight.  
     [0039] The felt thus obtained described above was cut into various complicated shapes. As a result, it could be easily cut into any shape without causing a problem. That is, it was confirmed that this felt has excellent cutting processability.  
     [0040] Furthermore, a change in water absorption change on repeated water absorption was examined by using the above absorbent material No. 3. As a result, the water absorption percentage was 600% by weight in the fifth water absorption, 540% by weight in the tenth water absorption, and 530% by weight in the twentieth water absorption, respectively, and the water absorption was reduced to only 500% by weight even in the fiftieth water absorption. That is, it was confirmed that the water absorption percentage is not largely reduced even if water absorption is repeated.  
     EXAMPLE 3  
     [0041] An absorbent material was produced in the same manner as in Example 1 except that highly water-absorptive fibers (fineness: 5 denier, fiber length: 51 mm) obtained by hydrolyzing the surface layer portion of acrylic fibers were used in place of the highly water-absorptive fibers used in Example 1 and that polyester fibers (Regular/full dull, fineness: 3 denier, fiber length: 76 mm) were used in place of the flame-retardant modacrylic fibers. As this absorbent material, two kinds of an absorbent material wherein 20% by weight of the above highly water-absorptive fibers and 80% by weight of the above polyester fibers are mixed (No. 7) and an absorbent material wherein 30% by weight of the above highly water-absorptive fibers and 70% by weight of the above polyester fibers are mixed (No. 8) were produced.  
     [0042] With respect to two kinds of these absorbent materials, the water absorption percentage, the change in volume (change in thickness) before and after water absorption, the water retention percentage, and the water absorption rate were measured in the same manner as in the above Examples. The results are shown in Table 2.  
                               TABLE 2                       No.   Water   Change in   Water retention   Water                  7   720   9   68   6.1       8   880   4   75   4.8                  
 
     [0043] It was confirmed that the absorbent materials of this Example are inferior in water retention percentage to those of Examples 1 and 2, but show high water absorption percentage and small change in volume (change in thickness).  
     [0044] Industrial Applicability  
     [0045] As described above, the absorbent material of the present invention shows high water absorption percentage and high water retention percentage as well as small change in volume on water absorption, and is superior in cutting processability. Therefore, the absorbent material of the present invention can be widely as an absorbent material required to have excellent water absorption properties, water retention properties, form stability and easily processing in various fields.