RESIN COMPOSITION FOR POROUS FILM AND FILM PREPARED THEREFROM

Abstract: A porous film having a good water vapor permeability is obtained with a high productivity by melting a resin composi­ tion comprising 100 parts by weight of a polyolefin resin and 50 to 500 parts by weight of barium sulfate having an electric conductivity of 250 µS cm or less in terms of the electric con­ ductivity of a supernatant formed when the barium sulfate is added to water, and a particle size of 0.1 to 7 µm, forming the melt into a film and stretching the film to 2.5 to 14 times the original area at least in the uniaxial direction.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a resin composition for forming a porous film having a good water vapor permeability and air permeability and a superior water resistance, and to a porous film prepared therefrom. 2. Description of the Related Art Porous films have so far been obtained by filling various non-compatible substances into polyolefin resins, making the resulting compositions into films and stretching the films (see Japanese patent publication Sho 53-12542/1978, U.S. Patent No. 4,347,84 and Japanese patent application laid-open No. Sho 57-59727/1982). However, such films obtained according to these processes have such drawbacks that they are weak in the mechanical strengths, no uniform pore distribution is formed and they are too rigid, resulting in less-flexible film. Thus, for example, as proposed in G.B.-A- 2,151,538, the above problems have been improved by using barium sulfate as the non-compatible substance, but such use of barium sulfate causes clogging of the screen of the extruder due to its flocking and also lowers the screen life and further causes stretching breakage due to its flocks; hence such a process has not yet been satisfactory in the aspect of stabilized productivity. The screen life referred to herein will be defined in Examples. SUMMARY OF THE INVENTION An object of the present invention is to provide a resin composition which, when it is made into a porous film, prolongs the screen life up to e.g. 200 hours or longer and scarcely causes stretching breakage to thereby stably produce a porous film having a good surface condition. Another object of the present invention is to pro-vide a porous film produced by using the above resin composition. The present invention in the first aspect resides in a resin composition comprising 100 parts by weight of a polyolefin resin and 50 to 500 parts by weight of barium sulfate having an electric conductivity of 250 µS/cm or less in terms of the electric conductivity of a supernatant formed when said barium sulfate is added to water, and a particle size of 0.1 to 7 µm. The present invention in the second aspect resides in a porous film obtained by melting a resin composition comprising 100 parts by weight of a polyolefin resin and 50 to 500 parts by weight of barium sulfate having an electric conductivity of 250 µS/cm or less in terms of the electric conductivity of a supernatant formed when said barium sulfate is added to water, and a particle diameter of 0.1 to 7 µm, forming the resulting melt into a film and at least uniaxially stretching the film to 2.5 to 14 times of the original area. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Examples of the polyolefin resins in the present invention are low density polyethylene, high density polyethylene (HDPE), linear low density polyethylene (LLDPE), homopolymers of propylene (PP), butylene, etc., ethylene-propylene copolymer, ethylene-butylene copolymer, ethylene-vinylacetate copolymer or blends thereof. Particularly, HDPE, LLDPE, PP and blends thereof are preferred. The barium sulfate in the present invention is that having an electric conductivity of 250 (µS/cm) or less, preferably 100 (µS/cm) or less, and the lower the value, the better its effectiveness. If the electric conductivity of the barium sulfate exceeds 250 (µS/cm), the screen life is short due to the flocking of barium sulfate, stretching breakage occurs frequently, and the production stability becomes far inferior. Further, its average particle size is suitably 0.1 to 7µm, preferably 0.5 to 5.0µm. If the average particle size is less than 0.1µm, no good porosity can be obtained, while if it exceeds 7.0 µm, the stretchability becomes inferior so that it is difficult to obtain a good porosity. In addition, barium sulfate having a too large electric conductivity may be further washed with water or partially neutralized with an acid or a base, followed by washing with water, to thereby reduce the electric conductivity to the values defined in the present invention, and then used. In the resin composition of the present invention, the quantity of the barium sulfate blended is in the range of 50 to 500 parts by weight, preferably 100 to 400 parts by weight, based on 100 parts by weight of the polyolefin resin. If the quantity thereof used is less than 50 parts by weight, no good porosity can be obtained, while if it exceeds 500 parts by weight, the rigidity of the film increases so that no sufficient stretching can be carried out and the porosity is reduced. The electric conductivity refers to a value obtained by introducing barium sulfate (10 g) into purified water (100 mℓ), followed by heating at 100°C for 10 minutes, thereafter cooling and measuring the resulting supernatant by means of an electric conductivity meter. The particle size of the barium sulfate referred to herein is measured by means of an instrument for measuring the powder surface area. In addition, it is effective for raising the dispersibility of barium sulfate in the resin and enhancing the stretchability to subject the barium sulfate to surface treatment with a silicon,silane or resinic acid The porous film of the present invention may be produced for example according to the following process: Other additives usually used, if necessary, are blended with the above-mentioned polyolefin resin and barium sulfate, followed by mixing the resulting blend by means of a Henschel mixer, super mixer or tumbling mixer, thereafter kneading the resulting mixture by means of a single- or twin-screw extruder, pelletizing the resulting material and forming the resulting pellets into a film by means of an inflation molding machine or a T-die molding machine. At that time, the above material may be directly formed into a film by means of an extruder without pelletizing it. The resulting film is then at least uniaxially stretched to 2.5 to 14 times, preferably 3 to 10 times of the original area according to a conventional process. The stretching may be carried out in a divided multistage manner or in the biaxial direction or more. Further, after the stretching, heat setting may be carried out in order to increase the form stability of pores. The porosity is determined depending on the quantity of the barium sulfate used, its particle size, the stretching ratio, and if the stretching ratio is less than 2.5 times, no sufficient porosity can be obtained, while if it exceeds 14 times, stretching breakage occurs, so that stabilized production is impossible. The porous film of the present invention can be used for various uses, but since the porosity may be varied in compliance with the use, it is preferred to adequately choose the stretching ratio. As the indication for the porosity of the porous film, water vapor permeability mentioned later is employed. The present invention will be described in more detail by way of Examples. Examples l to l3 and Comparative Examples l to 6 Various resins indicated in Table l and barium sulfate having various electric conductivity values and average particle diameters indicated in Table l were mixed in proportions indicated in Table l by means of a Henschel mixer, followed by uniformly kneading the mixtures by means of a twin-screw kneader and pelletizing the resulting materials. The resulting pellets were melted and formed to films by means of a T-die extruder and the resulting films were stretched to the stretching times indicated in Table l according to a rolling process. The physical properties of the films obtained are indicated in Table l. In addition, the evaluation of the physical properties of the films was carried out according to the following methods: (l) Water vapor permeability: According to the test procedure D in ASTM E96-66. (2) Screen life: When film-forming is carried out by means of an extruder of ll5 mm, provided with a screen of l50 meshes, with the quantity of the extruded resin composition being 200 kg/hr, the period of time until the resin pressure is elevated from 230 bar (kg/cm²) up to 300 bar (kg/cm²) is referred to as "screen life" herein. (3) Surface condition: According to visual judgement. (4) Electric conductivity of barium sulfate: A sample (l0 g) is weighed and placed in a 200 mℓ beaker, followed by adding distilled water (l00 mℓ), marking the position of the liquid surface, heating the mixture to l00°C on an electric beater for l0 minutes with stirring, adding distilled water up to the marked original position of the liquid surface, agitating the resultant, covering the beaker with a watch glass, allowing the liquid to cool down, filtering it with a filter paper, placing the filtrate in a 100 mℓ beaker and measuring its electric conductivity by means of an electric conductivity meter (CM-20E type (tradename of the meter manufactured by Toa Denpa Company)). (5) Particle size of barium sulfate: It is measured by means of an instrument for measuring the powder surface area (manufactured by Shimazu Seisakusyo), that is, by filling a sample (3 g) into a sample cylinder of 2cm² × 1 cm and measuring the time of air permeation (5 cm³) under 50 mm water pressure. According to the present invention, since the flocking of barium sulfate which is a non-compatible substance, is little and the screen life is long, the operability is notably improved, the operation period of the extruder is greatly prolonged and the stretchability is superior, so that it is possible to stably produce a porous film. Further, a film having a very good surface condition and a far superior appearance is obtained. Furthermore, since stretching up to 10 times which has so far been impossible can be carried out, it is possible to adequately vary the porosity by adjusting the stretching ratio in accordance with the intended use of the film to thereby choose the most useful film in accordance with its application; thus the resin composition and the process for producing the same, of the present invention are commercially very useful.

Claims: A resin composition comprising l00 parts by weight of a polyolefin resin and 50 to 500 parts by weight of barium sulfate having on electric conductivity of 250 µS/cm or less in terms of the electric conductivity of a supernatant formed when said barium sulfate is added to water, and a particle size of 0.l to 7 µm. A resin composition according to claim 1 wherein said electric conductivity is l00 µS/cm or less. A porous film obtainable by melting a resin composition comprising l00 parts by weight of a polyolefin resin and 50 to 500 parts by weight of barium sulfate having an electric conductivity of 250 µS/cm or less in terms of the electric conductivity of a supernatant formed when said barium sulfate is added to water, and a particle size of 0.l to 7 µm, forming the resulting melt into a film and stretching the film to 2.5 to l4 times the original area at least in the uniaxial direction.

Inventors: ITO, MICHIYASU, ITO, SYOICHI, MATSUMURA, SHUJI, SUZUKI, HISATOSI, TSUJI, SYOICHI
Applicants: MITSUI TOATSU CHEMICALS, INC.
Application year: 1986
Main classification: C08J5
IPC codes: C08J5, C08K3, C08L33, C08L23, C08J9, C08L7, C08L101, C08L21