POROUS FILM

Abstract: A porous film comprises a polyolefin resin, a filler and a polyester obtained from a polybasic acid, a polyhydric alcohol and a monobasic acid having 14 to 22 carbon atoms and/or a monohydric alcohol having 12 to 22 carbon atoms or a polyester obtained from a polybasic acid and a monohydric alcohol, the film being obtained by melting and moulding and then stretching. It has a good permeability to moisture and impermeability to liquid, being useful for a sanitary nap­ kin.

Description: This invention relates to porous film, and the purpose of the invention is to provide porous film which is required to have flexible hand feeling, good moisture permeable anti-leakage, and strength together in the field of sanitary materials, medical materials, and clothing material. Especially the purpose of the invention is to provide porous film which is suitable as a component for the fastening function in cooperation with a fastening tape such as moisture permeable anti-leakable film for disposable diapers, and which has the above-mentioned properties together. Many trials for obtaining porous films by kneading polyolefin resin with filler, melt-forming into film, and stretching the film uniaxially or biaxially have been made heretofore. These conventional porous films, in the case of uniaxial stretching, are disadvantageously anisotropic in strength, that is, the tear strength in the stretching direction and tensile strength in the transverse direction are very low. In order to improve said anisotropy in strength, a method in which film is stretched in a ratio as low as possible has been proposed, but it is difficult to obtain uniform stretched film by such method. The problem of anisotropy is resolved by applying biaxial stretching, but this method causes the disadvantage of remarkably decreased stretchability. On the other hand, mixtures comprising only polyolefin resin and filler will not give porous films with flexible hand feeling by stretching. Recently as the means to resolve these problems compounds which contain liquid hydrocarbon or other additives as third component in addition to polyolefin resin and filler have been proposed. For instance, liquid polybutadiene, liquid polybutene, and liquid polybutadiene with hydroxyl group on the ends are disclosed as the third component for improvement of the above-mentioned problems in Japanese Patent Laid-Open JP-A-5815538 and liquid polyisoprene rubber in Japanese Patent Laid-Open JP-A-58149925 respectively. However, the above-mentioned techniques for improvement can not satisfy fully the requirement from the view point of the balance between moisture permeability and tear strength in the stretched direction (longitudinal tear strength). Especially when the film is used, for instance, as a moisture permeable anti-leakage component member of disposable diapers which is served for fastening function with a fastening tape together, and if the longitudinal tear strength is low, the stress is centered locally on the moisture permeable anti-leakage film around the peripheral portion of said fastening tape to cause the breakage around the fastening tape when said fastening tape is fastened and unfastened or during use, thus the usable field is limited extremely. The inventors of the present invention had made effort to provide porous film which resolves the above-mentioned problems, and found that the longitudinal tear strength of porous film can be improved by adding particular polyester as third component, with retaining flexible hand feeling and good moisture permeable anti-leakage to attain this invention. A porous film of the invention comprises a polyolefin resin, a filler and a polyester obtained from a polybasic acid, a polyhydric alcohol and a monobasic acid having 14 to 22 carbon and/or a monohydric alcohol having 12 to 22 carbon atoms, the film being obtained by melting and moulding and then stretching. The invention film comprises the above defined polyester, including two preferable embodiments, a polyester obtained from a polyhydric alcohol, a polybasic acid and a monohydric alcohol having 12 to 22 carbon atoms; and a polyester obtained from a polyhydric alcohol, a polybasic acid and a monobasic acid having 14 to 22 carbon atoms. It is preferable that a dibasic acid such as adipic acid, succinic acid, a derivative of succinic acid and a dimeric acid is used as the polybasic acid. The monohydric alcohol to form a polyester with a polybasic acid has 12 to 22 carbon atoms. A monomer mixture of the same kind may be used for the polyester. It has a good permeability to moisture and impermeability to liquid, being useful for a sanitary napkin. The present invention will be described in detail hereinafter. Polyolefin resins used in the present invention refer to polymers which contain principally mono-olefin homopolymers and copolymers of ethylene, propylene, and butene, examples include high density polyethylene, low density polyethylene, linear low density polyethylene, polypropylene, ethylene-propylene copolymer, polybutene, ethylene-vinyl acetate copolymer, and mixture thereof. Among these materials linear low density polyethylene is especially suitable because of flexibility and toughness thereof. In the present invention, inorganic fillers such as calcium carbonate, gypsum, talk, clay, kaolin, silica, diatom earth, magnesium carbonate, barium carbonate, magnesium sulfate, barium sulfate, calcium phosphate, aluminium hydroxide, zinc oxide, titanium oxide, alumina, mica, zeolite, and carbon black, and organic fillers such as powders of wood and pulp are used as the filler. These fillers may be used separately or in admixture. Fillers with a mean particle diameter of 30µm or less are preferably used, and more preferably fillers of 10µm or less, and especially 0.5 to 5.0µm are used. The surface treatment of filler is important for homogeneous distribution of the filler in resins, surface treatment agents which make the surface hydrophobic such as fatty acids or metal salts thereof are preferably used. Polyesters used in the present invention refer to polyesters of polybasic acid, polyhydric alcohol, monobasic acid with 14 to 22 carbon atoms, and/or monohydric alcohol with 12 to 22 carbon atoms. The combination of these acids and alcohols are selected from the view point of the balance of affinity of the polyesters for polyolefin and filler considering number of ester group in an unit weight of polyester and degree of branching of hydrocarbon chain. Polybasic acid, polyhydric alcohol, monohydric alcohol, and monobasic acid which constitute polyesters involved in the invention will be described in the following. Monobasic acids used in the present invention are monocarboxylic acids with 14 to 22 carbon atoms, polybasic acids includes dicarboxylic acids, tricarboxylic acids, and tetracarboxylic acids, monohydric alcohols are monohydric alcohols with 12 to 22 carbon atoms, and polyhydric alcohol includes diols, trimethylol propane, pentaerithritol, dipentaerithritol, sorbitol, and sucrose. If the number of carbon atom of monobasic acid is 12 or less or the number of carbon atom of monohydric alcohol is 10 or less, probably because the balance in affinity of the polyester for filler and polyolefin deviates from the acceptable condition, various disadvantages are caused, that is, the third component is locally deposited during film formation, film is unevenly stretched, or the longitudinal tear strength is reduced. Polyesters involved in the present invention may be polyesters which are obtained by dehydration condensation of these polybasic acids and polyhydric alcohols. Such end group-esterified polyesters are preferable that, in the case of carboxylic end group, most of the end groups are esterified with monohydric alcohol having a long hydrocarbon chain such as stearyl alcohol, oleyl alcohol, and Guerbet alcohol, and in the case of alcoholic end group, most of the end groups are esterified with monobasic carboxylic acid having a long hydrocarbon chain such as stearic acid, hydroxy-stearic acid, oleic acid, and isostearic acid, but all end groups are not necessarily esterified. Acids and alcohols with branched chain are more preferably used. Concrete examples of preferable polyester include polyesters of diethyleneglycol and dimeric acid both carboxylic end groups or alcoholic end groups of which polyesters are esterified entirely or partially with stearyl alcohol or stearic acid, polyesters of 1,3- butanediol and adipic acid both end groups of which are esterified with hydroxy stearic acid, hexa-ester comprising trimethylol propane, adipic acid, and stearic acid, octa-ester comprising penta-erythritol, adipic acid, and stearic acid, dodeca-ester comprising dipentaerythritol, adipic acid, and stearic acid, polyesters utilizing dimeric acid or hydrogenated dimeric acid instead of adipic acid which is the constituent of the above-mentioned polyesters, and polyesters utilizing isostearic acid in stead of stearic acid. The preferable proportion of polyolefin resin, filler, and third component utilized in the present invention is in the range from 50 to 250 parts by weight of filler and 5 to 50 parts by weight of third component based on 100 parts by weight of polyolefin resin. Polyolefin resin, filler, and third component may be mixed according to conventional processes. For instance, materials are preliminarily mixed in a Henschel mixer or in a super mixer, and kneaded in a twin-screw extruder. Film may be formed using conventional processes much as tubular film process and T-die process, and so on. Though conventional orientation methods such as uniaxial rolling, successive and simultaneous biaxial stretching may be applied, uniaxial stretching with a ratio of 1.5 to 3.0 is especially preferable. The porous film of the invention is unexpectedly improved in respect to tear strength, in particular against a tearing force applied at the vertical direction of the stretching, that is, strength to longitudinal tearing. A test about the longitudinal tear strength was carried out by applying to a test film a tearing force at the vertical direction of the stretching direction. Examples The invention will be described in detail referring to examples hereinafter, however the scope of the invention is by no means limited to the examples. The composition and property of polyesters used in the examples are listed in Table 1. These polyesters were obtained through usual dehydration esterification reaction. Example 1 100 parts by weight of linear polyethylene resin (Ultozex Trade Mark)) 3021F, Mitsui Petrochemical Industries, Ltd.), 150 parts by weight of surface treated calcium carbonate (mean particle diameter of 1µm), and 30 parts by weight of polyester E1 as the third component were preliminarily mixed in a super mixer of 20ℓ (Kawada Works Ltd.), and the obtained mixture was kneaded and pelletized using a twin-screw kneader PCM-45 (Ikegai Corporation). The pellets were fed to an extruder with a screw diameter of 50 mm and extruded through a T-die to form a film with a thickness of 70µm. Thus obtained film was extended uniaxially using a roll type uniaxially extending machine. Operation conditions are shown hereinunder. Width of film=400 mm, Preheating temperature=80°C, Extention temperature=50°C, Extention ratio=2.2, Winding speed=22 m/min. Properties of obtained porous film is shown in Table 2. Test procedures for each property are shown hereinunder. Moisture permeability:according to JIS 2-0208 Longitudinal tear strength:according to JIS P-8116 Examples 2 and 3 Porous films were prepared in the same manner as described in Example 1 excepting that the third component was changed as shown in Table 2. The evaluation result of properties of each film is shown in Table 2. Reference examples 1 to 4 Porous films were obtained in the same manner as described in Example 1 excepting that hydrocarbons shown in Table 2 were used as the third component instead of polyester. The evaluation result of properties of each film is shown in Table 2. Example 4, 5 and 6 Porous films were prepared in the same way as shown in Example 1, using polyesters listed in Table 3, and examined in respect to properties. Results are shown in Table 4. The moisture permeability of 1.0 g/100 cm²h or more determined according to JIS Z-0208 is demonstrated to be effective for prevention of stuffiness during use when the film is used as moisture permeable anti-leakage sheet for sanitary material of disposable diapers. As shown in Table 2 Reference examples 1 and 2 lie in the acceptable range of moisture permeability, but tear strength is extremely low, therefore the films are unusable in the field where film serves as a member for fastening function with fastening tape together. Reference examples 3 and 4 lie in the acceptable range of longitudinal tear strength, but moisture permeability is very low, therefore the effect in prevention of stuffiness can not be expected during use. On the other hand, in Examples 1 to 6 of the present invention both the moisture permeability and longitudinal tear strength are remarkably improved. That is, the longitudinal tear strength is improved to a higher level of approximately 4 times that of Reference examples 1 and 2 which have the moisture permeability of 1.0 g/100 cm² per hour or more, and in comparison with Reference examples 3 and 4 the longitudinal tear strength is increased by a level of 10 to 20% and the moisture permeability is increased to a higher level of 2.5 to 3.3 times.

Claims: 1. A porous film which comprises a polyolefin resin of linear low density polyethylene, a filler and a polyester, in which the polyester is derived from a polybasic acid, a polyhydric alcohol and a monobasic acid having 14 to 22 carbon atoms or a polybasic acid, a polyhydric alcohol, a monobasic acid having 14 to 22 carbon atoms and a monohydric alcohol having 12 to 22 carbon atoms, or a polybasic acid, a polyhydric alcohol, and a monohydric alcohol having 12 to 22 carbon atoms. 2. A porous film according to claim 1, which has been formed into a film by melting, moulding and then stretching. 3. A porous film according to claim 1 or 2, in which one monomer comprising the polyester has a branched-chain. 4. A porous film according to any preceding claim, in which the film has been uniaxially stretched to 1.5 to 3.0 times its original dimensions. 5. A porous film according to any preceding claim, which comprises 100 parts by weight of the polyolefin resin, 50 to 250 parts by weight of the filler and 5 to 50 parts by weight of the polyester. 6. A porous film according to any preceding claim, in which the polybasic acid is dibasic.

Inventors: KAWAGUCHI, HEIHACHIRO, KIMURA, AKIO, SHIRAI, HIDENORI
Applicants: KAO CORPORATION
Application year: 1987
Main classification: C08J5
IPC codes: B29K105, C08K5, C08L23, B29K23, C08J9, A61L15, B29C55, C08L67, C08J5, B29L7