POROUS FILM, PROCESS FOR PRODUCING THE SAME AND ABSORBENT SANITARY ARTICLES

Abstract: A porous film comprises a polyolefin resin, a filler and an ester of dipentaerythritol having from 0 to 70% residual OH groups, based on the number of OH groups of unesterified dipentaerythritol. The film can be used as a liquid impermeable back sheet of absorbent sanitary articles, such as diapers, which comprise a liquid permeable surface sheet, a liquid absorber and a liquid impermeable back sheet; said liquid absorber being interposed between said liquid permeable surface sheet and said liquid impermeable back sheet.

Description: The present invention relates to a porous film, a process for the production thereof and absorbent sanitary articles. The present invention particularly relates to a porous film which has a good appearance and feel, excellent strength and proper moisture permeability, a process for producing the porous film with high efficiency, and absorbent sanitary articles such as a disposable diaper or a sanitary absorbent napkin containing the film. The film can also be used in, for example, clothing, packaging media, battery separators, filtering media and sanitary and medical materials. Many attempts to uniaxially or biaxially stretch a film obtained by mixing a polyolefin resin with a filler and then melt-molding the obtained mixture have previously been made to produce a porous film. However, in the uniaxially stretched film there are still problems with respect to the anisotropy in the physical properties thereof, particularly the tear strength in the longitudinal direction (stretching direction), and the surface strength. The biaxially stretched film has no problem with respect to the anisotropy in the physical properties thereof, but does have problems with respect to the stretching property and the surface strength. In addition, both films generally have a high stiffness, which is a disadvantage according their desired uses. The anisotropy in the physical properties of a film and the surface strength thereof can be improved by stretching with as low a ratio as possible to form a porous film. A low-melting point polymer, a rubber substance, a plasticizer or a surfactant can be added to give flexibility to the film. However, a porous film which satisfies the balance of physical properties of molding stability, porosity, stretching property, mechanical properties such as tear strength and tensile strength, and surface strength has not been found. To improve the aforementioned disadvantages of conventional porous films, a composition obtained by mixing polyolefin resin, a filler and liquid polybutadiene or liquid polybutene (JP-A-57-47334) and a composition obtained by mixing a linear low-density polyethylene, a filler and a polyhydroxy saturated hydrocarbon (JP-A-58-15538) have been proposed. In addition, a method using a polyester plasticizer such as an adipate or an epoxy plasticizer such as epoxidized soybean oil has also been proposed in JP-A-61-144331(1986), methods of using triglyceride have been proposed in JP-A-62-10141(1987) and JP-A-62-18435(1987), and a method of using a silicone oil has been proposed in JP-A-62-129321(1987). Methods of using ester compounds have also been proposed in JP-A-61-235439(1986), JP-A-62-18435(1987), JP-A-62-27438(1987), JP-A-62-280234(1987), JP-A-62-280235(1987), JP-A-62-288640(1987) and JP-A-63-22844(1988). However, none of these methods has achieved a porous film which satisfies all the properties such as strength, permeability, flexibility, appearance, feeling and printing properties. A disposal diaper and an absorbent (hereinafter referred to as "a diaper") comprise a liquid permeable surface sheet such as a gauze or a non-woven fabric which directly contacts the skin, a liquid absorbent such as pulp paper or an absorbent polymer sheet, and a liquid impermeable back sheet to prevent leakage of liquid. Recently, such a diaper has become remarkably popular owing to its convenience. Although a polyethylene film is generally used as the liquid impermeable back sheet, the polyethylene film has no permeability to air and water vapor, and easily becomes stuffy. In order to solve this problem, it has been proposed to use a porous film permeable to water vapor and impermeable to a liquid (JP-A-58-149303(1983), JP-A-60-185803(1985) and JP-A-59-69906(1984)). In each of these a porous film is obtained by kneading a polyolefin resin, an inorganic filler and a plasticizer, forming a film from the mixture and stretching the film formed. A diaper containing such a porous film has an advantage in that the stuffiness is improved. However, such a porous film has disadvantages in that, according to the plasticizer used, sufficiently uniform pores are not obtained and stretching spots are produced because of its poor stretching property. Furthermore the film has poor flexibility, feel and touch, lacks tear strength and tensile strength, and an eruption of the skin can occur due to bleeding of the plasticizer on the film surfaces over time. Therefore, a diaper without problems with respect to the film strength, permeability, appearance and feel has not been attained. Proposals have also been made in JP-A-57-203520(1982), JP-A-58-149925(1983), JP-A-59-62117(1984), JP-A-59-136334(1984), JP-A-59-140235(1984), JP-A-60-229731(1985) and JP-A-60-230825(1985), and Japanese Patent Application No. 62-85219. The present invention provides a porous film comprising, per 100 parts by weight of a polyolefin resin, from 25 to 400 parts by weight of a filler and from 1 to 30 parts by weight of an ester of dipentaerythritol with a carboxylic acid, said ester having from 0 to 70% residual OH groups based on the number with a carboxylic acid, said ester of OH groups of unesterified dipentaerythritol. The present invention also provides a process for preparing the porous film defined above, which process comprises melt-molding a composition containing the polyolefin resin, the filler and the ester of dipentaerythritol and stretching of the obtained sheet. The present invention further provides an absorbent sanitary article, for example a disposable diaper, comprising a liquid permeable surface sheet, a liquid absorber and a liquid impermeable back sheet composed of a porous film as defined above or which has been produced by a process as defined above, said liquid absorber being interposed between said liquid permeable surface sheet and said liquid impermeable back sheet. The film of the present invention has a good balance between the mechanical properties such as tear strength and tensile strength and permeability to water vapor, and good flexibility. A diaper using the film as a liquid impermeable back sheet has excellent strength, permeability, appearance and feel. The porous sheet has good extruding properties in melt-molding, particularly excellent stability in molding and produces a uniaxially stretched product having a good balance between mechanical properties such as tear strength and tensile strength, and permeability to water vapor, uniaxially and biaxially stretched products both having high surface strength and good stretching properties, particularly having few stretching spots which occur due to non-uniform stretching during stretching with a low ratio as well as exhibiting good flexibility. As a polyolefin resin, a high-density polyethylene, a medium-density polyethylene or a linear low-density polyethylene may, for example, be used singly or in a mixture of two or more. The melt index of the polyethylene is preferably from 0.01 to 10 g/10 minutes, more preferably from 0.03 to 5.0 g/10 minutes (measured with 2.16 Kg at 190°C in accordance with ASTM D-1238-70). Such a polyethylene may contain a branched low-density polyethylene obtained by a high-pressure method. Crystalline polypropylene can also, for example be used as the polyolefin resin. Propylene homopolymers or copolymers of propylene with other α-olefins maybe used singly or in a mixture of two or more. It is possible to mix such polyolefin resins with additives such as heat stabilizers, ultraviolet absorbers, antistatic agents, pigments and fluorescent agents. As a filler, an inorganic filler or an organic filler is used. Examples of inorganic fillers are calcium carbonate, talc, clay kaolin, silica, diatomaceous earth, magnesium carbonate, calcium sulfate, aluminium hydroxide, zinc oxide, magnesium hydroxide, calcium oxide, magnesium oxide, titanium oxide, alumina, mica, asbestos powder, glass powder, shirasu ballon, zeolite and silicate clay. Calcium carbonate, talc, clay, silica, diatomaceous earth and barium sulfate are preferred. Examples of organic fillers are cellulose powder such as wood powder and pulp. These fillers may be used singly or in a mixture. The average particle size of the filler is preferably not more than 30 µm, more preferably not more than 10 µm, most preferably not more than 5 µm. If the particle size is too large, the denseness of the pores in the stretched film deteriorates. Surface treatment of the filler is preferably carried out to improve the dispersion in the resin and the stretching properties of the film. A surface treatment with a fatty acid or a metal salt thereof is preferred. The amount of the fatty acid or the metal salt thereof used in the surface treatment is preferably not more than 10 parts by weight based on 100 parts by weight of the filler. A surface treatment with a large amount of such an agent is undesirable because white smoke generates or foaming takes place during kneading, pelletization or molding. The ester of dipentaerythritol is an ester of dipentaerythritol and a carboxylic acid or a partial ester of dipentaerythritol and a carboxylic acid. Examples of carboxylic acids are aliphatic mono- or dicarboxylic acids having from 1 to 30 carbon atoms and aromatic mono- or dicarboxylic acids having from 7 to 16 carbon atoms. Aliphatic monocarboxylic acids are particularly preferred from the viewpoint of their molding properties and stretching properties. More specific examples of carboxylic acids are aliphatic carboxylic acids such as acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, capronic acid, isocapronic acid, 2-ethylbutanoic acid, enanthic acid, caprilic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, stearic acid, eicosanoic acid, behenic acid, cerotic acid, melissic acid, succinic acid, glutaric acid, adipic acid, azelaic acid and sebacic acid; and aromatic carboxylic acids such as benzoic acid, phthalic acid, isophthalic acid and terephthalic acid. In the case of a monomolecular partial ester of dipentaerythritol, the above monocarboxylic acids can be used singly or in a mixture. When a dicarboxylic acid is used, the amount of the carboxylic acid used is preferably at most 0.5 moles per mole of dipentaerythritol, and the remaining hydroxyl groups are reacted with a monocarboxylic acid in such an amount to form the desired partial ester. The amount of residual OH groups in the esterified product of dipentaerythritol is from 0 to 70%, preferably from 3 to 70%, more preferably from 3 to 50%, based on the number of OH groups of one molecule of unesterified dipentaerythritol. If the amount is 3% or more, the improvement in the stretching properties and printing properties is better, while if the ratio is over 70%, the film forming properties of the film deteriorate. The ester of dipentaerythritol may be prepared by reacting dipentaerythritol with a slight excess of a carboxylic acid, stopping the reaction when the degree of esterification increases to a target value, and removing the excess carboxylic acid; or by introducing the starting materials in amounts necessary for the theoretical structre of the desired product, and carrying out a dehydration reaction. In the latter method, it is preferred to use an entrainer to increase the dehydration effect. In the film the amount of the filler used is from 25 to 400 parts by weight, preferably from 100 to 300 parts by weight, based on 100 parts by weight of the polyolefin resin, and the amount of the ester of dipentaerythritol used is from 1 to 30 parts by weight, preferably from 3 to 20 parts by weight, based on 100 parts by weight of the polyolefin resin. If the amount of the filler is too small, pores are not sufficiently formed in the stretched film resulting in a decrease in the porosity of the film. If the amount of the filler is too large, the kneading properties and dispersion are impaired resulting in deterioration of the moldability of the film and a decrease in the surface strength thereof. If the amount of the ester of dipentaerythritol is too large, satisfactory kneading properties cannot be obtained, and as a result sufficient moldability and stretchability cannot be secured. The mixing of the polyolefin resin, the filler and the ester of dipentaerythritol is performed using a ordinary blender or a mixer. A drum- or tumbler-type mixer, a ribbon blender, a Henschel mixer or a super mixer may be used as the mixer, but a high-speed stirring machine such as Henschel mixer is preferred. The kneading of the obtained mixture is appropriately performed using a known apparatus such as an ordinary screw extruder, a biaxial screw extruder, a mixing roll, a Banbury mixer or a biaxial kneader. The forming of the film may be performed using an ordinary film-forming apparatus in accordance with an ordinary method for forming a film. Blown film extrusion using a circular die or T-die extrusion molding may be suitably employed. The thus-formed film is then stretched, for example at a temperature of from 40 to 170°C, preferably from 40 to 80°C. In the case of unaxial stretching, roll stretching is generally used. A method in which stretching in the uniaxial direction (machine direction) is relatively strengthened by tubular stretching may also be used. Stretching may be performed in one stage or two or more stages. In biaxial stretching, low-ratio stretching can be performed as either simultaneous or successive stretching in the same way as in monoaxial stretching. Uniform stretching and the formation of pores can generally be achieved with a stretch ratio of 1.1 in at least one direction. The stretch ratio with which the formation of pores and uniform stretching can be achieved is, for example, from 1.1 to 3.0 in at least one direction. After uniaxial stretching or biaxial stretching, the obtained film may, for example, be heat-set at a temperature of from 100 to 180°C, preferably from 100 to 110°C, to stabilize the dimensional accuracy of the film. Surface corona or flame treatment can also be performed. The thickness of the porous film is preferably 0.01 to 0.5 mm, more preferably from 0.02 to 0.3 mm. The porous film of the present invention preferably has tear strength (MD) of not less than 10 Kg cm/cm² and a permeability of not less than 1000 g/m²·24hrs. In the absorbent sanitary article a gauze or a non-woven fabric is, for example, used as the liquid permeable surface sheet which directly contacts the skin. It is preferable to use a non-woven fabric made of polyolefin fibers such as polypropylene fibers or polyester fibers. Water-absorbing paper, pulp paper or a highly water-absorbing polymer is, for example, used as the liquid absorber. It is preferable to place one liquid absorber sheet on another to improve permeability to air to increase the bulk density. The thickness of the absorber is generally from 5 to 20 mm. In addition, a pressure sensitive tape may be used for fitting and an expansion member such as rubber may be used in the portions around the crotch to prevent any leakage therefrom. The method for producing the porous film according to the present invention has the following advantages: (1) The formation of many pores can be achieved with a low stretch ratio. (2) Stretching stress is low and low-temperature stretching is possible. (3) Uniform stretching can be performed with a low stretch ratio. The porous film according to the present invention has the following advantages: (1) Substantially no stretching spots are found. (2) The permeability to water vapor and permeability to gases are excellent, and the water-resistance pressure is high. (3) The flexibility is excellent. (4) The anisotropy of the physical properties is small. (5) The tear strength and the tensile strength are high. (6) The adhesive strength of the ink printed on the film and the abrasion resistance thereof are high, i.e. the printing properties are excellent. (7) The heat sealing properties are excellent and shrink packaging is possible. (8) No bleeding of the ester of pentaerythritol on the film surfaces takes place over time, and thus no trouble occurs in printing and other secondary processing. (9) The film is easily incinerated and generates no harmful gas. The porous film can be used in a permeable leakproof cover sheet for a paper diaper, as well as in other applications such as clothing, packages, battery separators, filtering media and medical materials. The absorbent sanitary articles according to the present invention have the liquid impermeable back sheet exhibiting the following advantages: i) Substantially no stretching spots are found and there is uniformity in stretching, so that thus the sheet is visually attractive. ii) The permeability to water vapor and the practical strength are excellent. iii) The flexibility and feeling are excellent. iv) No bleeding of the ester of pentaerythritol occurs and no trouble occurs during use. v) The sheet is easily incinerated and generates no harmful gas. Therefore, the absorbent sanitary article, for example a disposable diaper, according to the present invention exhibits excellent appearance and practical performance, and has great value as a commercial product. The present invention is further described in the following Examples. The raw materials used in the Examples and the Comparative Examples are shown in Tables 1 to 4. Symbol Polyolefin resin Melt index (g/10 min.) Density (g/cm³) A-1 Linear low-density polyethylene 1.0 0.920 A-2 Linear low-density polyethylene 2.0 0.921 A-3 High-density polyethylene 0.2 0.946 (Note) MI (melt index); measured at 190°C in accordance with ASTM D 1238-70. Density; measured at 20°C by a density gradient tube in accordance with ASTM D 1505. Symbol Filler B-1 Calcium carbonate subjected to surface treatment with a saturated fatty acid B-2 Barium sulfate B-3 Talc Number Third component(amount of residual OH groups:0%) C-10 Dipentaerythritol hexaoctanoate C-11 Dipentaerythritol hexastearate C-12 Dipentaerythritol trioctanoate/tribenzoate C-13 Dipentaerythritol hexabenzoate C-14 Pentaerythritol tetraoctanoate C-15 Dioctylphthlate (DOP) C-16 Dioctyladipate (ODA) C-17 Epoxidized soybean oil C-18 Polyether polyol C-19 Isocyanate polyester polyol C-20 Paraffinic process oil Examples 1 to 13 and Comparative Examples 1 to 14 Each of the polyolefin resins shown in Table 1 and each of the fillers shown in Table 2 were mixed with agitation in a Henschel mixer, and each of the third components shown in Table 4 was added to the obtained mixture and mixed therewith with agitation to obtain a mixture. The compositions of the thus-obtained mixtures are shown in Tables 5 and 6. Each of the obtained mixtures was kneaded by a biaxial kneader (PCM-30, produced by Ikegai Steel Co., Ltd.), pelletized and then subjected to blown film molding using an extruder of 40 mm  under the conditions described below to obtain a raw film having a thickness of 70 µm. Cylinder tempertaure: 110 - 190 - 190°C Head and die temperature: 200°C Die bore diameter: 100 mm Take-off speed: 8 m/min Blow ratio: 2.0 The thus-obtained film was uniaxially stretched (machine direction) with a roll stretching machine. The moldabilities during pelletization and formation of the raw film, the stretching temperature, the minimum stretching ratio (λ MIN ) with which uniform stretching with formation of many pores could be obtained, and the physical properties of the film at λ MIN are shown in Tables 7 and 8. As can be seen from Tables 7 and 8, there are differences in the moldabilities and the stretching properties between the cases using an ester of dipentaerythritol as a third component and the cases using plasticizers. When plasticizers are used, although molding is possible, the λ MIN values are increased and the degrees of longitudinal orientation (machine direction) of the stretched films are consequently increased, resulting in decreases in the longitudinal tear strength and transverse tear strength. Therefore, the physical properties such as the permeability and the strength are out of balance, leading to disadvantages according to the application. Example Polyolefin resin Filler Third Component Type Kg Type Kg Type Kg 1 - 3 A-1 3.5 B-1 6.0 C-10 0.7 4 " " " " C-11 " 5, 6 " " " " C-12 " 7 " " " " C-13 " 8, 9 " 3.9 " 5.6 C-10 " 10 A-2 4.5 " 5.0 " " 11, 12 A-3 3.5 " 6.0 " 1.0 13 A-1 3.7 B-2 5.8 " 0.7 Comparative Example Polyolefin resin Filler Third Component Type Kg Type Kg Type Kg 1 A-1 3.5 B-1 6.0 C-14 0.7 2 " " " " C-15 " 3 " " " " C-16 " 4 " " " " C-17 " 5 " " " " " 0.2 6, 8 " " " " C-15 1.0 7 " " " " " 1.5 9 " " " " C-18 0.7 10 " " " " C-19 " 11 " " " C-20 " 12 " 1.5 " 7.5 C-10 " 13 " 2.5 " 4.5 " 3.0 14 " 3.5 " 6.0 - - Examples 14 to 29 and Comparative Examples 15 to 18 Each of the polyolefin resins shown in Table 1 and each of the fillers shown in Table 2 were mixed with agitation in a Henschel mixer, and each of the third components shown in Table 3 was added to the obtained mixture and mixed therewith with agitation to obtain a mixture. The compositions of the thus-obtained mixtures are shown in Tables 9 and 10. Each of the obtained mixtures was kneaded by a biaxial kneader (PCM-45, produced by Ikegai Steel Co., Ltd.), pelletized and then subjected to blown film extruding using an extruder of 40 mm  under the conditions described below to obtain a raw film having a thickness of 70 µm. Cylinder tempertaure: 170 - 190 - 190°C Head and die temperature: 200°C Die bore diameter: 100 mm Take-off speed: 8 m/min Blow ratio: 2.0 The thus-obtained film was uniaxially stretched (machine direction)at 60°C with a roll stretching machine. The moldabilities during pelletization and formation of the raw film, the stretching temperature, the minimum stretching ratio ( λ MIN ) with which uniform stretching with formation of many pores could be obtained, and the physical properties of the film at λ MIN are shown in Tables 11 and 12. As can be seen from Tables 11 and 12. there are differences in the moldabilities and the stretching properties between the cases using an ester of dipentaerythritol as a third component and the cases using plasticizers. When plasticizers are used, although molding is possible, the λ MIN values are increased and the degrees of longitudinal orientation (machine direction) of the stretched films are consequently increased, resulting in decreases in the longitudinal tear strength and transverse tear strength. Therefore, the physical properties such as the permeability and the strength are out of balance, leading to disadvantages according to the application. Example Polyolefin resin Filler Third component Type Kg Type Kg Type Kg 14 A-1 3.5 B-1 6.0 C-1 0.4 15 " " " " C-2 " 16 " " " " C-4 " 17 " " " " C-5 " 18 " " " " C-6 " 19 " " " " C-7 " 20 " " " " C 8 " 21 " " " " C-9 " 22 A-2 " " " C-2 " 23 A-3 " " " C-3 0.6 24 A-1 " B-2 " C-3 0.4 25 " " B-1 " D-1 " 26 " " " " D-4 " 27 " " " " D-5 " 28 " " " " D-7 " 29 " " " " D-8 " Comparative Example Polyolefin resin Filler Third component Type Kg Type Kg Type Kg 15 A-1 3.5 B-1 6.0 D-2 0.4 16 " " " " D-3 " 17 " " " " D-6 " 18 " " " " D-9 " Example Raw film-forming property Strechability Physical film property of Uniform fluidity Bubble stability λ MIN Tear strength (MD) (Kg·cm/cm²) Permeability (g/m²·24 hrs) Flexibility Ink adhesion 14 ○ O ○ 2.00 17.0 4560 O ○ ○ 15 ○ O ○ 1.90 20.0 4440 O ○ O ○ 16 ○ O ○ 1.83 22.0 4270 O ○ O ○ 17 ○ ○ 1.81 22.2 4200 O ○ O ○ 18 ○ O ○ 1.88 20 6 4390 O ○ O ○ 19 ○ O ○ 1.85 21.3 4320 O ○ O ○ 20 ○ O ○ 1.90 20.2 4370 O ○ ○ 21 ○ O ○ 2.00 17.8 4390 O ○ O ○ 22 ○ O ○ 1.96 16.1 5110 O ○ O ○ 23 ○ O ○ 1.95 13.8 4270 ○ O ○ 24 ○ O ○ 1.87 19.2 4490 O ○ O ○ 25 ○ O ○ 2.50 8.0 5160 O ○ XX 26 ○ O ○ 2.65 6.8 5280 O ○ XX 27 ○ O ○ 2.60 7.1 5230 O ○ X 28 ○ O ○ 2.60 7.2 5230 O ○ XX 29 ○ O ○ 2.85 5.1 5780 O ○ XX Comparative example Raw film-forming property Strechability Physical property of film Uniform fluidity Bubble stability λ MIN Tear strength (MD) (Kg·cm/cm²) Permeability (g/m²·24 hrs) Flexibility Ink adhesion 15 ○ X 16 X X 17 X X 18 X X The methods of measuring the moldability and the physical properties shown in Tables 7, 8, 11 and 12 are as follows: (Moldability) 1) Biaxial kneading property during pelletization ○: Stable pelletization is possible without producing venting up and surging. X: Stable pelletization is impossible because of the occurrence of venting up and surging. 2) Uniform fluidity during formation of a raw film ○: A molten resin is uniformly discharged from the whole periphery of a die clearance during blown film extruding X: Stable molding is impossible because a molten resin is not uniformly discharged from the whole periphery of a die clearance. 3) Bubble stability during formation of a raw film O ○: Bubbles do not sway. ○: Bubbles substantially do not sway. Δ: Bubbles sway, but formation of a film is possible. X: Bubbles vigorously sway, and formation of a film is difficult. (Physical properties) 1) Tensile strength In accordance with ASTM D 882 (Condition E), 10 mm width x 50 mm length, Tensile speed: 500 mm/min 2) Tear strength In accordance with JIS P-8116, the strength of a film in the machine direction thereof is measured. Sample size: 63 mm width x 76 mm length Notch length: 20 mm 3) Permeability In accordance with JIS Z 0208 At 30°C, 90% RH 4) Flexibility This is measured by feeling with the hands and decided on the basis of the following criteria: O ○: Very soft ○: Soft Δ: Slightly hard X: Hard 5) Printing property (ink adhesion) Evaluation is made in accordance with the following procedure: (i) Preparation of ink An ink used for an untreated polyolefin, Polymate G.T. (trade name), produced by Toyo Ink Manufacturing Co., Ltd. is diluted with a solvent GN502 so that the ratio of the ink to the solvent is 1 : 1 (by weight). (ii) Solid printing The ink is dropped on a film, and solid printing is performed using a #8 bar coater (the diameter of the core of the bar coater being 0.2 mm (8/1000 inch)). (iii) Drying Drying with air is performed at room temperature for 24 hours. (iv) Adhesion of cellophane tape A cellophane tape is adhered to the film on which the ink has been applied while being pressed by a rubber roller. (v) Judgment of ink adhesion The adhered cellophane tape is immediately separated from the film, and judgement is made on the basis of the area ratio of the ink remaining on the film. Mark Ratio of residual area O ○ 95% or more ○ 60% or more and less than 95% Δ 20% or more and less than 60% X 5% or more and less than 20% XX less than 5% Examples 30 to 37 and Comparative Examples 19 to 25 When a liquid impermeable back sheet was formed, each of the polyolefin resins shown in Table 1 and each of the fillers shown in Table 2 were mixed with agitation in a Henschel mixer, and each of the third components shown in Table 4 was added to the obtained mixture and mixed with agitation to obtain a mixture. The thus-obtained mixture was kneaded with a biaxial kneader (PCM-30, produced by Ikegai Steel Co.,Ltd.), pelletized and then subjected to blown film extruding by an extruder of 50 mm  under the conditions described below to obtain a raw film having a thickness of 70 µm. Cylinder temperature: 170 - 190 - 190°C Head and die temperature: 200°C Die bore diameter: 150 mm Take-off speed: 10 m/min Blow ratio: 2.5 The thus-obtained film was uniaxially stretched (machine direction) with a roll stretching machine to obtain a sheet having many pores. A liquid absorber comprising a sheet of pulp paper and a water absorbing polymer, and a liquid permeable surface sheet composed of a non-woven polyester fabric were placed on the thus-formed liquid impermeable back sheet in order, and these superposed sheets were united. A pressure sensitive tape and a stretch rubber member were provided on the united sheets to form a disposable diaper. The moldability, stretch ratio, appearance and physical properties of each of the liquid impermeable back sheets formed, and the results of wearing the disposable diapers of the present invention are shown in Tables 13 and 14. The methods of measuring and the criteria for judging are described below: (Moldability) The moldability in compound molding (pelletization) and in film molding is collectively judged on the basis of the following criteria: ○: Stable molding is possible without producing any trouble in both compound molding and film molding. X: Stable pelletization is impossible because venting up or surging occur during compound molding, or stable pelletization is possible but stable film molding is impossible because of non-uniform extrusion of a molten resin from a die opening or poor stability of bubbles during the film molding. (Sheet appearance) 1) Uniformity in stretching (visual judgement) O ○: The whole film is completely and uniformly stretched and uniformly porous; uniformity is obserbed. ○: The film is substantially uniformly stretched and porous, and thus substantially no unevenness is observed. Δ: Non-uniformity is recognized to some extent but is not remarkable. X: Non-uniform stretching and non-uniform whitening are observed over the whole film, and the appearance is not good. 2) Flexibility (judged by touch with the hands) O ○: Extremely flexible and extremely good feel ○: Flexible Δ: Slightly hard X: Hard and poor feel (Physical properies) 1) Permeability In accordance with JIS Z 0208 (at 30°C, 90% RH) 2) Tear strength In accordance with JIS P-8116, the strength of a film in the machine direction is measured. Sample size: 63 mm width x 76 mm length Notch length: 20 mm Example 38 and Comparative Example 26 When a liquid impermeable back sheet was formed, after a compound had been pelletized by the aforementioned method, the formed pellets were then subjected to T-die molding by an extruder of 65 mm  under the conditions described below to form a raw film having a thickness of 90 µm. Cylinder temperature: 170 - 190 - 190°C Head and die temperature: 230°C T-die lip width: 450 mm Take-off speed: 16 m/min The thus-obtained film was stretched (machine direction) by a roll stretching machine, and then stretched (transversal direction) by a tenter to obtain a porous sheet. A disposable paper diaper was formed using the formed liquid impermeable back sheet in the same way as that described above. The moldability, appearance and physical properties of each of the liquid impermeable back sheets and the results of use of the disposable diapers formed using the sheets are shown in Tables 13 and 14. As can be seen from a comparison between Tables 13 and 14, there are differences in the moldability, appearance and physical properties of the sheets between the case using the ester of dipentaerythritol as a third component and the case using the plasticizer which is added to the composition for each of the liquid impermeable back sheets. Accordingly, there are differences in performance between the diapers formed by using these back sheets. The disposable diapers of the present invention are excellent commercial products because of their good quality and appearance.

Claims: A porous film comprising, per 100 parts by weight of a polyolefin resin, from 25 to 400 parts by weight of a filler and from 1 to 30 parts by weight of an ester of dipentaerythritol with a carboxylic acid, said ester having from 0 to 70% residual OH groups based on the number of OH groups of unesterified dipentaerythritol. A porous film according to Claim 1, wherein the proportion of residual OH groups in said ester of dipentaerythritol is from 3 to 70%. A porous film according to Claim 1 or 2, wherein said polyolefin resin is polyethylene or crystalline polypropylene. A porous film according to Claim 3, wherein said polyethylene has a melt index of 0.01 to 10 g/10 minutes. A porous film according to Claim 3 or 4, wherein said polyethylene is high-density polyethylene, medium-density polyethylene, low-density polyethylene or a mixture of two or more thereof. A porous film according to Claim 3, wherein said crystalline polypropylene is a propylene homopolymer, a copolymer of propylene and another α-olefin, or a mixture of two or more thereof. A porous film according to any one of the preceding Claims, wherein said filler is calcium carbonate, talc, clay, kaolin, silica, diatomaceous earth, magnesium carbonate, calcium sulfate, aluminium hydroxide, zinc oxide, magnesium hydroxide, calcium oxide, magnesium oxide, titanium oxide, alumina, mica, asbestos powder, glass powder, shirasu ballon, zeolite, silica clay, wood powder or pulp. A porous film according to any one of the preceding Claims, wherein said filler has an average particle size of 30 µm or less. A porous film according to any one of the preceding Claims, wherein said filler has been subjected to surface treatment with a fatty acid or a metal salt thereof. A porous film according to Claim 9, wherein the amount of said fatty acid or said metal salt thereof used for surface treatment of said filler is not more than 10 parts by weight per 100 parts by weight of said filler. A porous film according to any one of the preceding Claims, wherein said ester of dipentaerythritol is an ester of dipentaerythritol with an aliphatic mono- or dicarboxylic acid having from 1 to 30 carbon atoms or with an aromatic mono- or dicarboxylic acid having from 7 to 16 carbon atoms. A porous film according to Claim 11, wherein said mono- or dicarboxylic acid is acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, capronic acid, isocapronic acid, 2-ethylbutanoic acid, enanthic acid, caprilic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, stearic acid, eicosanoic acid, behenic acid, cerotic acid, melissic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, benzoic acid, phthalic acid, isophthalic acid or terephthalic acid. A porous film according to any one of the preceding Claims which contains from 100 to 300 parts by weight of said filler and from 3 to 20 parts by weight of said ester of dipentaerythritol per 100 parts by weight of said polyolefin resin. A process for producing a porous film according to any one of the preceding claims, which process comprises melt-molding a composition containing the polyolefin resin, the filler and the ester of dipentaerythritol and stretching of the obtained sheet. A process according to Claim 14, wherein said stretching is uniaxial stretching, biaxial stretching or multi-stage stretching. A process according to Claim 14 or 15, wherein said stretching is performed with a stretch ratio in at least one direction of from 1.1 to 3.0. A process according to any one of Claims 14 to 16, wherein said stretching is performed at from 40 to 80°C. A process according to any one of Claims 14 to 17, further comprising subjecting the stretched film to heat-treatment at from 100 to 110°C. A process according to any one of Claims 14 to 18, further comprising subjecting the stretched film to corona-treatment or flame-treatment. An absorbent sanitary article comprising a liquid permeable surface sheet, a liquid absorber and a liquid impermeable back sheet composed of a porous film as claimed in any one of claims 1 to 13 or which has been produced by a process as claimed in any one of claims 14 to 19, said liquid absorber being interposed between said liquid permeable surface sheet and said liquid impermeable back sheet. An article according to Claim 20, wherein said liquid permeable surface sheet is made of gauze or a non-woven fabric. An article according to Claim 21, wherein said non-woven fabric is composed of polyester fibers or polyolefin fibers. An article according to any one of Claims 20 to 22, wherein said liquid absorbent is a water absorbing paper, pulp paper or a highly water-absorbing polymer. An article according to any one of claims 20 to 23, which is a disposable diaper.

Inventors: BIZEN, KUNIO, HASEGAWA, RYUICHI, HAYASHI, KOJI, KASHINO, MINORU, SUZUKI, TASUKU
Applicants: MITSUBISHI KASEI CORPORATION, MITSUBISHI KASEI VINYL COMPANY
Application year: 1988
Main classification: C08J5
IPC codes: A61L15, C08K5, A61F13, C08J5