Absorbent article

0001The invention relates to an absorbent article such as a sanitary towel or napkin and a disposable nappy or diaper having an excellent absorption property. In particular, the invention relates to an improvement of a nonwoven fabric to use in an absorbent article.

0002The invention applies to an absorbent article comprising a nonwoven fabric and an absorbent material such as fluff pulp, also called cotton-like pulp, an absorbent paper and an absorbent polymer. It may further comprise a leak-proof layer on the back. The disposable diaper or nappy is also called a paper diaper.

0003Various performances are required for the nonwoven fabrics constituting the surface layer of such absorbing articles, the most important of which are restriction of flow of liquid along the surface following a discharge of urine or menstrual blood, which could lead to leakage, reduction in return of liquid held in the absorbing layer to the skin and comfort for the wearer.

0004In the non-woven fabrics utilized so far for conventional absorbing articles, restriction in surface liquid flow has generally been obtained, for example, by the use of regenerated hydrophilic fibers represented typically by rayons. However, regenerated hydrophilic fibers are mixed solely or together with hydrophilic fibers uniformly into non-woven fabrics in the conventional materials and, since the surface of the hydrophilic fibers is water repellent, the proportion of regenerated hydrophilic fibers employed has to be very high in order that the surface of the non-woven fabric is sufficiently hydrophilic. Thus, the water absorption rate at the surface of the nonwoven fabric can indeed be increased, thereby decreasing liquid flow at the surface, but, since the regenerated hydrophilic fibers themselves absorb water which penetrates inside them, they have an intense water retaining power and show a great decrease in bulk upon wetting, the surface in contact with the skin becomes very sticky and the liquid return from the absorbing layer to the skin is increased to result in problems such as discomfort and the occurrence of skin rashes.

0005Recently, use of dry non-woven fabrics mainly composed of synthetic fibers has become popular as the surface material for the absorbent articles and the foregoing problems of the stickiness and liquid return inherent to the non-woven fabrics mainly composed of regenerated hydrophilic fibers have gradually been overcome. However, since conventional dry non-woven fabrics made of hydrophilic fibers have a water repellent surface, they cause substantial surface flow and then tend to leak liquid. In addition, there has been an attempt to treat the hydrophilic fibers with a surface active agent thereby rendering the surface of the non-woven fabrics hydrophilic to restrict the liquid flow at the surface. However their durability to water is insufficient if they are merely treated with the surface active agent and, after menstrual blood or urine has once permeated through them, the hydrophilic property of that portion is greatly reduced, and if menstrual blood or urine is again discharged at the same portion of the non-woven fabric, the liquid is then liable to flow thereby making it difficult to maintain the initial performance during extended use.

0006Further, as shown in Japanese Patent Laid-Open No.60-198151, it has also been attempted to treat the hydrophilic fibers having absorbed sweat or water with a high speed water stream to form non-woven fabrics thereby providing the durability to the hydrophilic property of the non-woven fabrics. However, the constitution of the fibers has not been considered in this case and, since the upper layer is mainly composed of hydrophilic fibers, liquid tends to flow readily at the surface and, further, sweat absorbing and water absorbing aids at the surface of the fibers are washed out by the high speed water treatment, to greatly reduce the hydrophilic property of the non-woven fabrics in practical use, so that again sufficient water absorbency is not achieved.

0007In US-A- 4366 206 water-swellable fibers are disclosed having a hydrophilic outer layer and a non-hydrophilic inner layer.

0008Although improvements have also been reported in commercially available articles, such as the use of particular water-absorbing high molecular weight materials as the absorbing material, there has not yet been found a material capable of satisfying both the requirements of high rate of absorption (to reduce surface liquid flow) and the low liquid return.

0009According to this invention we provide an absorbent article having as surface material, a non-woven fabric comprising a surface layer containing from 60 to 100% by weight of first fibers having a hydrophilic surface and a hydrophobic core and from 40 to 0% by weight of second fibers which are hydrophilic at least at their surface, and a rear face formed of at least by one or more layers containing from 0 to 50% by weight of said first fibers and from 100 to 50% by weight of said second fibers, in which after wetting the second fibers are more hydrophilic than the first fibers at the surface.

0010It is preferred that the surface layer consists entirely of the first fibers and the back layer consists entirely of the second fibers. It is preferred that said first fibers have been formed by rendering the surface of inherently hydrophobic fibers hydrophilic and that the second fibers are hydrophilic fibers or have been formed by rendering the surfaces of inherently hydrophobic fibers hydrophilic.

0011It is preferred that the first fibers of the surface layer, when wetted, have a surface hydrophilic property of 0.5 ml or less and the second fibers of the back layer, when wetted, have such a property of 1 to 2 ml.

0012The absorbent article of the invention may comprise said nonwoven fabric, an absorbent and a leak-proof material and may be a sanitary napkin or towel or a disposable diaper or nappy.

0013The first fibers referred to herein may comprise a mixture of two or more different kinds of hydrophobic fibers having a similarly low hydrophilic property after wetting. In the same way, the second fibers referred to herein may comprise a mixture of two or more kinds of different hydrophobic fibers having, after wetting, a similar hydrophilic property which is higher than that of the first fibers.

0014In order that the non-woven fabrics can allow liquids such as body fluids to permeate rapidly to the absorbing layer, it is necessary that the surface of the fibers constituting the non-woven fabric is sufficiently hydrophilic at least in the situation where the absorbing article is constituted with the non-woven fabrics. However, since the liquid once absorbed in the absorbing layer is liable to return to the outside through the non-woven fabric if the hydrophilic property of the surfaces of the fibers is excessively high, it is necessary that at least the surface layer of the non-woven fabric to be in contact with the skin has a surface hydrophilic property that is somewhat reduced by the passage of water. In order to provide the surface layer of the non-woven fabric with such a property, it is necessary that the weight ratio of the first fibers, that is, the fibers having a hydrophilic surface and a hydrophobic core, and also low water retention in which the hydrophilic property after wetting is reduced as compared with that before wetting, comprises over 60% by weight and, most preferably, 100% of the surface layer.

0015The hydrophilic property of the first fibers after wetting.(Hw) is preferably less than 0.5 ml, more preferably less than 0.3 ml according to the test method for water absorption amount as described herein.

0016Then, it is necessary that the liquid absorbing rate at the surface of the non-woven fabric is not reduced even if the surface hydrophilic property is lowered due to the passage of water. For this purpose, it is necessary for the presence of a rearface layer comprising at least one layer having hydrophilic property at least at its surface which is not reduced so much after wetting, with the weight ratio of the second fibers having a surface hydrophilic property after wetting higher than that of the first fibers, being greater than that in the surface layer. In this case, for obtaining an effect that the liquid absorbing rate at the surface of the non-woven fabrics is not reduced even if the surface hydrophilic property is lowered due to the passage of water, it is necessary that the weight ratio of the second fibers constituting the rearface layer is at least more than 50 % and it may be 100 % so long as the fiber constituting the surface layer lies within the range described above.

0017Then, for attaining the purpose of maintaining the absorbing rate more effectively, it is desirable that the hydrophilic property of the second fibers after wetting (Hw) corresponds to more than 1 ml by the test method for the water absorption amount described later in the examples. Further, for reducing liquid return, it is better that the second fibers also have a hydrophilic surface and a hydrophobic inside and the water retainability of the fibers per se is lower and, most desirably, all of the second fibers are made of such fibers. Further, it is more preferable that the hydrophilic property of the second fibers after wetting corresponds to less than 2 ml according to the the test method for the water absorption amount as described later in the examples. For use in sanitary napkins, paper diapers and the like, it may be sufficient for the rear face layer to be a single layer but it may alternatively be a multi-layer structure in cases where a greater thickness is required.

0018The first fibers and the second fibers may be different from each other with respect to the surface layer and the rearface layer and it is desirable that the degree of the difference in the hydrophilic property between the first fibers and the second fibers is high because it reduces the amount of liquid returned.

0019The first fibers and the second fibers as described above can be prepared by applying a hydrophilic treatment to the surface of hydrophobic fibers and further controlling the durability of the hydrophilic treatment to the water content. Further, it is only necessary for the second fibers that they have hydrophilic property at least at the surface and those having hydrophilic property till the inside such as rayon fibers may also be used.

0020The method of applying the surface hydrophilic treatment to the hydrophobic fibers can include, for example, a method of rendering hydrophilic property to the surface of hydrophobic synthetic fibers such as polyolefinic fibers, for example, of polyethylene and polypropylene, polyester fibers, polyamide fibers and polyacrylonitrile fibers using a surface active agent. Alternatively, the surface may be rendered hydrophilic by applying a surface chemical treatment such as chemically bonding those chemical materials having hydrophilic groups such as monomers having hydrophilic groups or polymers having hydrophilic groups, or a physical surface modification such as plasma fabrication and incorporation of chemical material having hydrophilic groups by kneading. For the chemical surface modification, those chemical materials having hydrophilic groups may be chemically bonded with the fiber surface, or chemical materials having hydrophilic groups may be bonded and crosslinked to each other to cover the surface of the fibers. Further, it is also possible to improve the water absorbing property by making the shape of the hydrophobic fibers into a profile configuration, or applying a hydrophilic treatment to the surface of them.

0021Since the method of surface hydrophilic treatment can control the durability to the water content by adequately selecting the processing conditions with the agent for giving surface hydrophilic property or fabrication conditions for the fibers, they may be applied to the first fibers and the second fibers. Among them, for the first fibers, hydrophilic treatment using a surface active agent can provide easy control for the durability against water content and an economical advantage in view of the cost as well. For the hydrophilic treatment to the surface of the hydorphobic fibers, a method of giving hydrophilic property in the production step of the fibers is customary but, as another example, non-woven fabrics may be prepared by using the hydrophobic synthetic fibers described above, and the hydrophilic property may be given to the surface of the hydrophobic fibers by applying chemical and physical surface modifications or the treatment with a solution of a surface active agent as the after fabrication.

0022For the hydrophobic fibers, polyesters which can be applied with various kinds of surface treatment and are less expensive in view of the cost are suitable to both the first fibers and the second fibers. Furthermore, for the first fibers, the polyolefinic fibers which can be easily applied with the hydrophilic treatment with low durability by a surface active agent and which are effective also as the thermo-fusible binder are also suitable.

0023In accordance with the fiber composition and the layer constitution as described above, the non-woven fabrics can provide those performances of rapidly absorbing the liquid into the absorbing layer and restricting the return of liquid from the absorbing layer that conflict to each other can be obtained simultaneously.

0024Then, descriptions will be made specifically below for the conditions to attain the foregoing properties of the non-woven fabrics most effectively.

0025The thickness of non-woven fabrics constitutes one of the major factors for the return of liquid from the absorbing layer to the outside of the non-woven fabrics. That is, if the thickness of the non-woven fabrics is decreased excessively, distance between the skin and the absorbing layer is extremely reduced when the absorbing article is put on under wet state to increase the liquids return and significantly worsen the feeling upon use. On the other hand, if the thickness of the non-woven fabrics is increased, the liquid return is surely decreased. However, if the thickness is excessively large, the entire absorbing article becomes thick, to bring about abnormal feeling to users and also result in a problems of increased manufacturing cost. Further, if the thickness of the surface layer is excessively large, the thickness is increased at a portion where the hydrophobic property is increased than that under dry state once after the permeation of the water content through the non-woven fabrics, and it is difficult for the water content dropped in the next time to intrude to the inside of the non-woven fabrics.

0026In this way, it is necessary that the thickness of the non-woven fabrics lies within a most adequate range in view of the liquid-returning property, feeling upon use and production cost. The range of the adequate thickness varies depending on the application uses and the entire thickness is desirablly from 0.3 mm to 0.8 mm under the load of 2.5 g/cm² when using the non-woven fabrics for the application uses of sanitary napkin and from 0.6 to 2.5 mm in the case of using them for the paper diapers.

0027In order that the non-woven fabrics have the thickness as described above, it is necessary to adequately select the fibers for constituting the non-woven fabrics and provide the non-woven fabrics with an appropriate weight per unit area. At first, it is necessary to use hydrophobic synthetic fibers having a surface rendered hydrophilic which show no remarkable reduction in the bulkiness upon wetting as has been described above. The size of the fibers may be within a range from 1.5 to 6 denier, because it is difficult to increase the bulkiness of the entire non-woven fabrics if the size is less than 1.5 denier, whereas the entire non-woven fabrics become rigid to worsen the feelings if the size exceeds 6 denier. Further, although higher weight per unit area is generally preferable for providing the non-woven fabrics with the bulkiness, it is desired on the other hand to suppress the weight per area as low as possible when the production cost is taken into consideration. Accordingly, it is preferable that the fibers has elasticity as high as possible, which can be attained, for example, by a method of avoiding the use of extremely fine fibers as described above or selecting hollow fibers or those fibers applied with sterical crimping.

0028Thus, if the easy control of the durability to water content of the surface hydrophilic treatment, reduction in the weight, easy of attaining the high elasticity and easy fabrication to non-woven fabrics are taken into consideration, polyester and polyolefin type fibers are most suitable as the fibers for non-woven fabrics.

0029Referring to the weight per unit of the non-woven fabrics, since the thickness is reduced as the weight per unit area is lower, whereas the thickness is increased as the weight per unit goes higher as a general trend, there is also an adequate range depending on the application uses. In the case where the fibers as described above used, the weight is desirably within the range from 10 to 30 g/m² as a whole and from 5 to 10 g/m² for the surface layer in the case of using them as sanitary napkins, whereas from 20 to 50 g/m² as a whole and from 7 to 15 g/m² for the surface layer in the case of using them as paper diapers.

0030Referring finally to the method of stabilized webs, any of the methods can optionally be selected provided that they have such a nature that the surface hydrophilic property of the constituent fibers is greater in the second fibers than in the first fibers in the situation where the nonwoven fabrics are finally formed. However, in the case where the hydrophilic treatment to the fiber surface undergoes damages in the course of forming the non-woven fabrics as in the entanglement by the high speed water flow, the hydrophilic property has to be provided by the after-treatment as described above, which requires much labors and brings about an extreme difficulty. Accordingly, for forming the non-woven fabrics without degrading the hydrophilic property of the starting fiber material, it is desirable to set the fibers to each other by the thermal fusion of the fibers. Further, the method of forming the non-woven fabrics stabilized by the fusion of the fibers is excellent as compared with other methods also in that the fabrics have sterically stabilized skeltons and can attain the physical property (thickness, feelings) of the non-woven fabrics as described above efficiency (with no remarkable increase in the weight per unit area).

Brief Description of the Drawings

0031Figure 1 is a cross sectional views of a device used for the measurement of water absorption amount. <tables id="tabl0001" num="0001"><img file="EP0245074B1_D0001.tif" /></tables>

0032This invention will now be described in more details by way of the examples.

Example 1 - 17 and Comparative Example 1 - 7

(Fibers and non-woven fibers)

0033Using fibers shown in Table 1 as the first fibers and the second fibers, various kinds of non-woven fabrics shown in Tables 2 and 3 were manufactured.

0034The method of processing the fibers with the agent providing hydrophilic property (hereinafter simply as a hydrophilic agent) shown in Table 1 are as below : <ul id="ul0001" list-style="none"><li>(A) Fibers were immersed in an aqueous solution of a surface active agent and then dewatered and dried.</li><li>(B) The hydrophilic agent was dispersed in a solvent, into which the fibers were immersed, then dewatered and dried followed by heat treatment (140°C for 30 minutes).</li><li>(C) The hydrophilic agent was dispersed in a solvent, into which the fibers immersed, then dewatered and dried followed by heat treatment (170°C for 10 minutes).</li><li>(D) The procedures were according to Example 1 of Japanese Patent Laid-Open No. Sho 55-122074.</li><li>(E) After placing fibers in a bell jar made of glass and evacuating the inside of the bell jar to lower than 10 Torr, an exhaust valve was closed and air was supplied up to 0.1 mmHg. Then, vapors of the hydrophilic agent were supplied up to 0.5 % based on the weight of the fibers and high frequency voltage (13.56 MHz) was applied to electrodes for about 3 minutes.</li><li>(F) Commercially available fibers were used as they are.</li><li>(G) Commercially available fibers were used as they are.</li><li>(H) Commercially available fibers were used as they are.</li><li>(I) Commercially available polyethylene - polypropylene composite fibers were washed with warmed water, dewatered and then dried.</li></ul>

0035In Examples 1 - 16 and Comparative Examples 1 - 6, non-woven fabrics were manufactured by way of heat bonding method (passing a hot blow at 140°C to a card web and setting ES fibers to other fibers through fusing by using polyethylene - polypropylene composite fibers)(ES fibers) or low melting polyester - polyester composite fibers (Melty) as binder fibers. In Example 17, fibers of the web were entangled under a high pressure water flow (spray pressure at 55 kg/cm², average amount of water stream supplied in the lateral direction : 250 cc/cm.min) to form non-woven fabrics. Then, after applying the same hydrophilic processing as in (B) above, the hydrophilic agent described in (A) above was sprayed from the side of the first layer and then dried. In Comparative Example 7, the fiber webs were subjected to the high pressure water flow processing under the same conditions as described above into non-woven fabrics, which were not applied with the hydrophilic treatment by the after treatment.

(Specimens for Measurement)

0036Commercially available sanitary napkins (trade name; Lorie, manufactured by Kao Co) and disposable paper diapers (trade name Merries, manufactured by Kao Co.) were used while removing non-woven fabrics therefrom and, instead, placing non-woven fabrics shown in Tables 2 and 3 thereover respectively as the specimens to be measured as absorbing articles assuming the sanitary napkins and disposable paper diapers. Examples 1 - 7, Examples 9 - 17 and Comparative Examples 1 - 7 were used for the sanitary napkins, while Example 8 was used for the disposable paper diapers.

(Test Method)

(1) Surface liquid flow :

0037Test solution was dropped from above 1 cm of the surface of the specimen inclined by 45° and the flowing distance along the surface of the non-woven fabrics from the point of dropping to the point where the solution was absorbed to the inside of the specimen was measured, which was defined as the surface liquid flow for the first time. Then, one minute after the dropping of the test solution, the test solution was again dropped to a portion where the solution had been dropped previously and the flowing distance along the surface of the non-woven fabrics from the point of dropping to the point where the solution was absorbed to the inside of the specimen was measured, which was defined as the surface liquid flow for the second time. Those showing short surface liquid flow for the first time and the surface liquid flow for the second time showing no considerable change indicate that there was no remarkable lateral leak. The dropping condition was 0.1 g/sec for the specimen assuming the sanitary napkins and 0.5 g/cm for the specimen assuming the paper diapers.

(2) Returning amount :

0038The test solution was caused to be absorbed by 10 cc in the specimens assuming the sanitary napkin and by 150 cc in the specimens assuming the paper diaper, pressure was applied after a predetermined of time and the amount of the test solution returned from the inside through the non-woven fabrics was measured. As the returning amount is smaller, surface stickiness is reduced, the feeling upon use is better and wiping effect is more excellent.

(3) Water absorption amount :

0039Using the apparatus as shown in Figure 1, in which fiber webs 1 were packed at a density of 0.13 g/cm³ into a glass tube 2 of 14 mm inner diameter, ion exchanged water 5 was caused to be absorbed through a glass filter 4 into the fiber webs under a constant hydraulic pressure (atmospheric pressure) in a head portion 3 and the water absorption amount (ml) after one minutes was measured. The water absorption amount in the dry state is defined as Hd, while the water absorption amount after passing 600 cc of the ion exchanged water through the fiber webs and then dried was defined as Hw. Greater Hw means higher surface hydrophilic property after wetting. <tables id="tabl0002" num="0002"><img file="EP0245074B1_D0002.tif" /></tables><tables id="tabl0003" num="0003"><img file="EP0245074B1_D0003.tif" /></tables><tables id="tabl0004" num="0004"><img file="EP0245074B1_D0004.tif" /></tables>

(Effect of the Invention)

0040As can be seen from Examples, the surface liquid flow in the absorbing articles according to this invention is low both at the first and the second times and the liquid return from the inside to the surface is also low.

0041In Comparative Examples 1 - 7, absorbing articles were manufactured and the surface liquid flow and the liquid return thereof were measured by using the non-woven fabrics out of the range according to this invention and by the same procedures as in Examples. Since fibers having no substantial hydrophilic property were used for the surface layers in Comparative Example 1, 3 and 4, the surface liquid flow is high and leakage is liable to be caused. In Comparative Example 2 is constituted up to 100 % with fibers which are hydrophilic at the surface and the inside and the returning amount is remarkable and stickiness is liable to occur although the surface liquid flow is preferable.

0042In Examples 2 - 17 and Comparative Examples 5 and 6, the fabrics are constituted with fibers having a hydrophilic surface and a hydrophobic inside and show lower returning amount as compared with Example 1 comprising fibers having hydrophilic surface and inside. However, Comparative Example 5, among them, has a lower mixing ratio of the second fibers in the rearface layer than that defined in this invention and the surface liquid flow at the second time is increased. Further, in Comparative Example 6, the mixing ratio of the second fibers in the surface layer is greater than the range as defined in this invention and, accordingly, the returning amount is increased.

0043In Comparative Example 7, the surface hydrophilic treatment undergoes damage by the high speed water flow to remarkably reduce the hydrophilic property and, as a result, the surface liquid flow is increased.