FILTER MEDIUM, FILTER MEMBER PROVIDED WITH FILTER MEDIUM, AND PRODUCTION METHOD FOR RESIN FILM USING FILTER MEDIUM

Abstract

An object of the present invention is to prevent precipitation of antimony metal comprised in a molten resin and to prevent product defects of a resin film made of the molten resin due to the contamination of the antimony metal. According to the present invention, a filter medium for filtration of a molten resin containing antimony is provided, and the filter medium is formed of a material not substantially comprising molybdenum.

Claims

1. A filter medium for filtration of a molten resin containing antimony, being formed of a material not substantially comprising molybdenum.

2. The filter medium according to claim 1, wherein as a measure of the amount of antimony precipitated on the filter medium taken out after 24-hour immersion in an ethylene glycol solution containing 2% by weight diantimony trioxide kept at 170 C., the number of counts of X-rays at a wavelength specific to antimony generated by electron irradiation to the filter medium in an Electron Probe Micro Analyzer (EPMA) method is 1000 counts or less.

3. The filter medium according to claim 1, wherein the amount of iron dissolved from the filter medium during 24-hour immersion in an ethylene glycol solution containing 2% by weight diantimony trioxide kept at 170 C. is 20 ppm or less.

4. The filter medium according to claim 1, wherein as a measure of the amount of antimony precipitated on the filter medium taken out after 24-hour immersion in an ethylene glycol solution containing 2% by weight diantimony trioxide kept at 170 C., the number of counts of X-rays at a wavelength specific to antimony generated by electron irradiation to the filter medium in an Electron Probe Micro Analyzer (EPMA) method is 1000 counts or less, and wherein the amount of iron dissolved from the filter medium during 24-hour immersion in an ethylene glycol solution containing 2% by weight diantimony trioxide kept at 170 C. is 20 ppm or less.

5. The filter medium according to claim 1, wherein the material contains stainless steel.

6. The filter medium according to claim 5, wherein the stainless steel is an austenitic stainless steel mainly comprising iron, chromium, and nickel as components.

7. The filter medium according to claim 1, wherein the material does not substantially contain manganese or sulfur.

8. The filter medium according to claim 7, wherein the material does not substantially contain aluminum, titanium, phosphorus, silicon, or carbon as a component.

9. The filter medium according to claim 1, wherein the material contains carbon in an amount of 0.08% or less.

10. The filter medium according to claim 1, wherein the material contains at least one or more elements selected from copper, niobium, bismuth, lead, and tellurium.

11. The filter medium according to claim 1, wherein the material is a single material or a composite material selected from SUS304, SUS304L, SUS304LN, SUS304Cu, SUS304N1, SUS304N2, SUS304J1, SUS304J2, SUS304BF, SUS304FL, SUS347, SUS321, SUS630J2, ASK3000T, and SUSXM15J1.

12. The filter medium according to claim 1, having been subjected to a surface treatment selected from chrome plating, nickel plating, copper plating, ceramic composite nickel plating, titanium nitride sputtering, and silicon carbide sputtering or a composite treatment thereof.

13. The filter medium according to claim 1, wherein the molten resin is a thermoplastic resin.

14. The filter medium according to claim 1, wherein the material is a sintered metal non-woven fabric formed by processing of a metal wire into a fiber and then sintering of the resulting fibrous metal wire.

15. A filter member provided with the filter medium according to claim 1.

16. The filter member according to claim 15, being a leaf disc filter, a candle filter, or a pack filter.

17. A resin film production method comprising a step of producing a molten resin containing antimony, a step of filtering the molten resin produced in the production step, and a step of forming a resin film from the molten resin filtered in the filtration step, the filtration step being a step of filtering the molten resin using a filter medium formed of a material not substantially comprising molybdenum.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0025] FIG. 1 shows schematic views of a filter member provided with a filter medium of an embodiment of the present invention. (a) is a front view, (b) is a cross-section view, and (c) is an enlarged view around a hub part.

[0026] FIG. 2 is a cross-section view of a filter using the filter member of an embodiment.

[0027] FIG. 3 is a schematic view of an apparatus for producing a resin film using the filter of an embodiment.

DESCRIPTION OF EMBODIMENTS

[0028] The embodiments of the present invention will be described in details below, referring to the drawings. The present invention is not limited to the embodiments described below, and various variations and modifications can be made without departing from the technical scope of the present invention.

Filter Medium

[0029] A filter medium (20) in an embodiment is intended for filtration of a molten resin containing antimony. The filter medium (20) is a sintered stainless metal non-woven fabric formed by sintering of a wire rod of SUS304L. The wire rod of SUS304L is one obtained by cutting work.

[0030] The filter medium (20) has a filtering accuracy of 1 to 80 m, the filtering accuracy herein being such a particle dimeter that 98% of particles having the dimeter can be collected in a single pass test. The filter medium (20) is a depth-type filter medium, and the collection efficiency can be adjusted by changing the weight per unit area of the filter medium (20) and/or the structure of the filter medium (20).

[0031] Moreover, as described above, molybdenum comprised in stainless steel causes precipitation of antimony metal, and therefore a wire rod of SUS304L, a molybdenum-free stainless steel, is used for the filter medium (20) of the embodiment. Accordingly, a molten resin containing antimony can be filtered without precipitation of antimony metal on the filter medium surface.

[0032] Herein, the wire rod used for the filtering medium (20) is not limited to SUS304L. The wire rod used for the filtering medium (20) may be any stainless steel not comprising molybdenum, and is preferably, for example, SUS304, SUS304LN, SUS304Cu, SUS304N1, SUS304N2, SUS347, SUS304J1, SUS304J2, SUS304BF, SUS304FL, SUS321, ASK3000T, SUS630J2, or SUSXM15J1 in addition to SUS304L.

[0033] The raw material of the molten resin containing antimony is preferably a thermoplastic resin. The raw material of the molten resin is preferably polyester, polyphenylene sulfide, polyamide, polypropylene, ethylene vinyl acetate, alicyclic olefin, or acrylic.

[0034] The molten resin containing antimony is preferably a polyester resin having an ester bond, i.e. an ester group-containing polymer obtained by polycondensation of a dicarboxylic acid with a diol or a hydroxycarboxylic acid. Examples of the dicarboxylic acid component include terephthalic acid, isophthalic acid, adipic acid, azelaic acid, sebacic acid, 2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, and the like. Examples of the diol component include ethylene glycol, 1,4-butanediol, diethylene glycol, triethylene glycol, neopentyl glycol, 1,4-cyclohexanedimethanol, polyethyleneglycol, and the like. Major examples of the hydroxycarboxylic acid include p-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid, and the like. Major examples of the polyester resin include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycyclohexylenedimethylene terephthalate (PCT), polybutylene terephthalate (PBT), modified bodies thereof, and the like.

[0035] The polymerization catalyst used for polymerization to give PET is an antimony metal compound, a germanium compound, a titanium compound, an aluminum compound, or the like, and an antimony (Sb) catalyst is dominantly often used for preparing PET raw materials in the resin film field throughout the world. In some cases, to PET raw materials prepared without the use of an antimony compound as a polymerization catalyst, an antimony compound is added for impartment of flame resistance and colorability.

[0036] The filter medium (20) of the embodiment is not limited to a sintered metal non-woven fabric, and may be formed of, for example, a stainless metal sintered body obtained by powder processing of a stainless steel not comprising molybdenum and sintering of the resulting powder. The filter medium (20) may also be a laminated body obtained by lamination of sintered stainless metal wire gauzes which are formed in a net shape using a sintered wire rod of a stainless steel not comprising molybdenum.

Filter Member

[0037] Next, a filter member (10) provided with the above-mentioned filter medium (20) will be described.

[0038] The filter member (10) is provided with the above-mentioned filter medium (20) made of sintered metal non-woven fabric, a filter retainer (30), and a hub part (40), as shown in FIG. 1 (a). The filter member (10) is formed in an annular shape having an outside diameter of 304 mm, an inside diameter of 63.5 mm, and a thickness of 7.4 mm.

[0039] The filter medium (20) made of sintered metal non-woven fabric is formed in an annular shape. The filter medium (20) made of sintered metal non-woven fabric is supported on each axial end surface of the filter retainer (30). The filter medium (20) made of sintered metal non-woven fabric and the filter retainer (30) are arranged in a concentric fashion. As shown in FIG. 1 (b), the outer peripheral edge of the filter medium (20) made of sintered metal non-woven fabric is joined to the outer peripheral edge of the filter retainer (30) by welding all around, and thus, the filter member (10) is closed at the outer peripheral end (11). Moreover, at the inner peripheral end (12) of the filter member (10), the inner peripheral edge of the filter medium (20) made of sintered metal non-woven fabric and the inner peripheral edge of the filter retainer (30) are joined to a cylindrical hub part (40) by welding.

[0040] The filter retainer (30) is a laminated body obtained by lamination of a plurality of annular porous plates (31) and a plurality of annular wire gauzes (32) in the thickness direction (see FIG. 1 (a)). Inside the laminated body, as shown in FIG. 1 (c), a fluid passage (33) is formed so that a pore of the porous plate (31) is in communication with an opening of the wire gauze (32). Moreover, the hub part (40) has hub holes (41) formed over the entire periphery, the hub holes penetrating through the hub part in the radial direction. The hub hole (41) is in communication with the fluid passage (33) of the laminated body of the filter retainer (30).

[0041] In the filter member (10), a molten resin is filtered through the filter medium (20) made of sintered metal non-woven fabric and then flows into the fluid passage (33) in the filter retainer (30). The molten resin flows through the fluid passage (33) in the filter retainer (30) inward from the outer peripheral side in the radical direction, and discharges outside the filter member (10) through the hub hole (41) of the hub part (40).

[0042] Herein, since SUS304L as the material of the filter medium (20) made of sintered metal non-woven fabric in the filter member (10) does not comprise molybdenum, an element causing precipitation of antimony metal, antimony metal will not precipitate or adhere to the filter medium (20) made of sintered metal non-woven fabric even if antimony is comprised in the molten resin.

[0043] Moreover, the use of a stainless steel not comprising molybdenum, for example, SUS304L for forming of the filter retainer (30) and the hub part (40) can surely prevent precipitation of antimony metal in the filter member (10).

Filter

[0044] Next, a filter (50) having the above-mentioned filter member (10) will be described. The filter (50) is provided with a casing (51), a filter case (52), and a filter assembly (53), as shown in FIG. 2. The filter case (52) and the filter assembly (53) are housed within the casing (51) in such a state that they are combined.

[0045] The casing (51) has a bottomed cylindrical body part (54) and a lid part (55). The lid part (55) is removably attached to an opening end of the body part (54). The body part (54) is connected to an inlet passage part (57), the inlet passage part passing through a bottom (56) of the body part. The lid part (55) is connected to an outlet passage part (58), the outlet passage part passing through the axial end of the lid part. Moreover, the lid part (55) and the outlet passage part (58) are provided with heaters (59) for heating a molten resin (70).

[0046] The filter case (52) is formed in a column shape and in the central area, a recessed part (60) for housing the filter assembly (53) is formed. In the recessed part (60), the bottom is formed in a funnel shape, and in the central part of the funnel-shaped inside, a penetrating part (61) is formed.

[0047] The filter assembly (53) is provided with a cylindrical base (62), a pillar (63), a protecting member (64), and a plurality of the filter members (10) laminated in the thickness direction. In the base (62), the pillar (63) is fixed to the central part. To the end of the pillar (63), the protecting member (64) is fixed. A plurality of the filter members (10) are located between the base (62) and the protecting member (64) in such a state that the pillar (63) is inserted through the filter members. These filter members (10) are arranged at regular intervals in the axial direction of the pillar (63). A spacer provided between the filter members (10) generates a gap between the filter members (10), which results in prevention of the contact between the filter media (20) made of sintered metal non-woven fabric in the adjacent filter members (10).

[0048] In the filter assembly (53), a through passage (65) where the molten resin (70) flows is formed in the central area. The through passage (65) is closed at one end (in the right side in FIG. 2), and is in communication with the outlet passage part (58) at the other end (in the left side in FIG. 2).

[0049] The pillar (63) has a plurality of pillar holes (66) formed on the outer peripheral surface. Each pillar hole (66) is in communication with the through passage (65) inside the pillar (63). Moreover, between the pillar (63) and the filter members (10), communication members (67) allowing connection between the pillar holes (66) of the pillar (63) and the hub holes (41) of the filter members (10) are formed.

[0050] In the filter (50), the molten resin (70) flows from the inlet passage part (57) into the recessed part (60) of the filter case (52) through the penetrating part (61) inside the filter case (52). The molten resin (70) flowing into the recessed part (60) enters an axial gap between the filter members (10), and then enters the filter medium (20) which is made of sintered metal non-woven fabric and provided in the filter member (10) located on each side of the axial gap, resulting in the filtration of the molten resin.

[0051] The filtered molten resin (70) flows into the fluid passage (33) in each filter retainer (30), flows through the fluid passage (33) inward from the outer peripheral side in the radical direction, and flows from the hub hole (41) of the hub part (40) into the through passage (65) inside the pillar (63) via the communicating member (67) and the pillar hole (66) of the pillar (63). The molten resin (70) flowing into the through passage (65) inside the pillar (63) discharges from the filter (50) through the outlet passage part (58).

[0052] The use of a stainless steel not comprising molybdenum, for example, SUS304L for forming the casing (51), the filter case (52), and the base (62), the pillar (63), and the protecting member (64) of the filter assembly (53) in the filter (50) can surely prevent precipitation of antimony metal in the filter (50).

Production Apparatus for Resin Film

[0053] Next, a resin film production apparatus (80) for producing a resin film using the filter (50) will be described. As shown in FIG. 3, the resin film production apparatus (80) is provided with an extruder (81), the above-mentioned filter (50), a film forming machine (82), a cooling machine (83), a stretching machine (84), and a take-up machine (85).

[0054] In the extruder (81), heat and shearing force are applied to a solid resin, and the resulting molten resin is extruded. The molten resin comprises antimony. The molten resin extruded by the extruder (81) is filtered with the filter (50) for removal of impurities. Herein, the filter medium (20) used in the filter (50) is formed of SUS304L. SUS304L does not comprise molybdenum, an element causing precipitation of antimony metal. Accordingly, during the filtration with the filter (50), antimony metal will not precipitate or adhere to the filter medium (20). The material used for forming the filter medium (20) is not limited to SUS304L, and may be any stainless steel not comprising molybdenum.

[0055] The molten resin filtered with the filter (50) is extruded from a slit nozzle provided in the film forming machine (82) and formed into a film. Herein, the nozzle may be formed of a stainless steel not comprising molybdenum, for example, SUS304L, as is the case with the filter medium (20) used in the filter (50). The use of such a stainless steel can prevent precipitation of antimony metal in the nozzle part. Moreover, in the resin film production apparatus (80), a part which the molten resin is in contact with may be formed of a stainless steel not comprising molybdenum. The use of such a stainless steel can surely prevent precipitation of antimony metal in the resin film production apparatus (80).

[0056] The film-shaped molten resin formed by extrusion from the nozzle of the film forming machine (82) is cooled with the drum-like cooling machine (83), and then stretched in any direction at any stretching ratio using the stretching machine (84) having a plurality of rotating rolls, to give a desired film. The edges at both the ends of the obtained film are cut, and then the film is taken up with the take-up machine (85).

Film Production Method

[0057] Next, a method for producing a resin film using the above-mentioned filter medium will be described. The resin film production method comprises a step of producing a molten resin, a step of filtering the molten resin, and a step of forming a resin film from the filtered molten resin.

[0058] The production step is a step of producing a molten resin containing antimony. In the production step, a molten resin is produced by application of heat and shearing force to a solid resin. Moreover, to the molten resin, antimony is added as a polycondensation catalyst. Herein, the raw material of the molten resin is preferably polyester, polyphenylene sulfide, polyamide, polypropylene, ethylene vinyl acetate, alicyclic olefin, or acrylic.

[0059] The filtering step is a step of separating impurities by filtration from the molten resin produced in the production step. In the filtering step, the molten resin is filtered using the filter medium formed of a material comprising a stainless steel which does not comprise molybdenum. Molybdenum causes precipitation of antimony metal, and therefore, the use of, as the material of the filter medium, the stainless steel not comprising molybdenum can prevent precipitation of antimony metal in the filtering step.

[0060] The resin film forming step is a step of forming a resin film from the molten resin from which impurities have been removed in the filtering step. The resin film forming step comprises an extrusion sub-step, a cooling sub-step, a stretching sub-step, and a taking-up sub-step.

[0061] The extrusion sub-step is a sub-step of extruding the molten resin filtered in the filtering step from a slit nozzle into a film. The cooling sub-step is a sub-step of cooling the film-shaped molten resin formed by extrusion in the extrusion sub-step. The stretching sub-step is a sub-step of stretching the film-shaped molten resin cooled in the cooling sub-step in any direction at any stretching ratio to give a molten resin film having a desired form. The taking-up sub-step is a sub-step of taking up the resin film stretched in the stretching sub-step in rolls.

EXAMPLES

Measurement Method

(1) Amount of Dissolved Iron

[0062] Diantimony trioxide is dissolved in ethylene glycol heated to 110 C. to prepare an ethylene glycol solution containing 2% by weight diantimony trioxide. Into a glass container, 1 L of the ethylene glycol solution containing antimony was placed. In the solution in the container, a test material having a specific surface area of 150 cm.sup.2 (any of the materials described below, including SUS304L, 304LN, and SUS316L) is immersed. To the container, a reflux condenser is attached, and the test material immersed in the ethylene glycol solution kept at 170 C. in the container is left to stand for 24 hours. After that, the test material is taken from the ethylene glycol solution. In this test, due to the ion-exchange reaction between iron and an antimony compound, antimony metal is precipitated on the test material and an iron ion of the test material is dissolved in the post-reaction solution. The concentration of iron in the solution is determined by the method described below and the obtained value is defined as the amount of the dissolved iron.

1. Operation

[0063] The post-reaction solution in an amount of 1 g is precisely weighed out and placed into a 100-mL beaker. To this, 5 mL of sulfuric acid is added, and the mixture is heated at about 300 C. using a heater placed under the beaker for carbonation of a carbon compound in the post-reaction solution. To the post-reaction solution, nitric acid is gradually added, and the mixture is maintained at 300 C. for decomposition. After the post-reaction solution turns clear and colorless, the post-reaction solution is heated and concentrated to almost dryness. The almost dried substance is left to stand to be cooled to room temperature, 10 mL of hydrochloric acid is added thereto, and the mixture is heated to about 200 C. for dissolution of the dried substance. The post-reaction solution is cooled to room temperature, placed into a 25-mL measuring flask, and diluted by adding ion-exchanged distilled water to the marked line. In this measurement, a blank test is performed in an operation similar to the above, except that no test material is used, to obtain a blank value.

[0064] The solution obtained with the above operation is sprayed into argon plasma, and the amount of iron contained in the solution is measured at a wavelength of 259.94 nm with a high-frequency inductively coupled plasma emission spectrometer to obtain the iron concentration from a calibration curve previously prepared.

Iron concentration (g/g)=(SS0)V/W

[0065] Herein, S represents the iron concentration (g/mL) which is obtained from a calibration curve as a value corresponding to the luminescence intensity of the sample solution, S0 represents the iron concentration (g/mL) which is obtained from a calibration curve as a value corresponding to the luminescence intensity in the blank test and, V represents a fluid volume (mL) of an acid fluid in which a test material is dissolved, and W represents a fluid volume (g) of ethylene glycol.

2. Operation for Preparing Calibration Curve

[0066] An iron standard stock solution (1.0 mg Fe/mL) is diluted with hydrochloric acid to prepare iron standard solutions having a concentration in the range of 0 to 20 (g Fe/mL).

[0067] With the use of the iron standard solutions, a calibration curve representing the relation between the iron concentration and the luminescence intensity is prepared.

3. Measuring Apparatus

[0068] The high-frequency inductively coupled plasma emission spectrometer used in the measurement is a sequential ICP manufactured by Seiko Instruments & Electronics Ltd. (trade name, SPS1100).

(2) Amount of Precipitated Antimony

[0069] As described in above (1), a test material having a specific surface area of 150 cm.sup.2 is immersed in an ethylene glycol solution which contains 2% by weight diantimony trioxide and is maintained at 170 C., and left to stand for 24 hours. Then, the test material is taken from the ethylene glycol solution and the amount of antimony precipitated on the test material is measured with an EPMA method. In the EPMA method, electron beam irradiation to a sample induces the interactions between an irradiated electron and atoms comprised in the sample and thereby generates specific X-rays unique to each element. The number of counts of the specific X-rays unique to each element is measured to obtain the composition of the sample surface (at a depth of about 1 m).

(3) Cleaning Method for Filter Member

[0070] A used filter member to which a molten thermoplastic resin is adhered is pulled out from a casing and placed into a solvent cleaning tank or a heat treatment tank for removal of the thermoplastic resin. Then, the filter member is immersed in an acid or alkaline aqueous solution and rinsed with water. Ultrasound is applied to both surfaces of the filter member to remove foreign matter adhered to the filter member.

(4) Recoverability of Filter Member After Cleaning

[0071] The recoverability of the used filter member cleaned as described above is determined by measurement of the value of air flow resistance. Air is introduced into the inside of the filter member from the outer surface of the filter member. The flow resistance value at the time of the air introduction is measured in pascals with a mercury manometer, and the obtained value is used to evaluate the recoverability after cleaning. The recoverability of the filter member after cleaning is evaluated by the ratio (%) of the flow resistance value of the used filter to the flow resistance value measured in an unused filter.

(5) Detection of Foreign Matter Defects on Resin Film Surface

[0072] Detection of foreign matter defects with a size of 25 to 150 m on the resin film surface is performed with a surface defect inspection system equipped with a line sensor camera manufactured by NAGASE & CO., LTD. while a resin film is delivered at a speed of 1 to 15 m/min. Detection results are represented as the number of surface defects per unit area of the filter member (defects/m.sup.2).

(6) Detection of Corrosion in Filter Member

[0073] Detection of intergranular corrosion and pitting corrosion is performed by observation of the filter member surface with a scanning electron microscope (SEM).

[0074] The resin used was polyethylene terephthalate raw material (IV: 0.62, including 200 ppm antimony polymerization catalyst, 30 ppm trimethyl phosphoric acid (TMPA), 65 ppm magnesium acetate, and silica particles with a diameter of 80 nm). The polyethylene terephthalate raw material was dried under a reduced pressure of 2 mmHg at 170 C. for 2 hours to give a dried PET raw material with an adjusted water absorption rate of 15 ppm. The dried PET raw material was completely melted in the first extruder (length/diameter ratio, i.e. L/D: 25) of a single-screw tandem extruder, and the molten resin was delivered to the second extruder (L/D: 25) of the single-screw tandem extruder. The resin adjusted to a temperature of 285 C. was supplied at a discharge rate of 3 tons/hour from the extruder (81) shown in FIG. 3 to the filter (50) having the filter member (10) shown in FIG. 1. The filter member (10) provided in the filter (50) was one that has 200 filter media made of a sintered metal non-woven fabric having a diameter of 12 inches and a filtering accuracy of 5 m, and such a filter member was applied in the filter apparatus. The molten resin filtered with the filter (50) was supplied to the film forming machine (82) shown in FIG. 3 having a T die with a width of 2200 mm, and was extruded from a slit nozzle of the die into a film sheet. The film sheet was appressed on the surface of the drum-like cooling machine (83) shown in FIG. 3 having an outside diameter of 1800 mm, a surface temperature of 22 C., and an outer peripheral surface plated with chromium, while electrostatic charge is applied to the surface. Then, the film sheet was delivered through the stretching machine (84) and taken up with the take-up machine (85) to give a resin film having a thickness of 2500 m. Before the supply of the molten resin to the filter member, the filtration pressure was measured with filtering pressure gauges attached upstream and downstream of a pipe for supplying the molten resin extruded from the extruder (81) to the filter (50). In Comparative Example 1, the material of the filter medium in the filter member was SUS316L, which is conventionally used for a filter medium, and in Examples 1 and 2, the material was SUS304L and SUS304LN, respectively. The lifetime (days) of the filter member, the foreign matter defects on the resin film surface (defects/m.sup.2), the recoverability (%) of the filter member after cleaning, the lifetime (days) of the cleaned filter, the existence of corrosion in the filter member, the amount of the precipitated antimony (counts), and the amount of the dissolved iron (ppm) are shown in Table 1.

TABLE-US-00001 TABLE 1 Comparative Example 1 Example 2 Example 1 SUS304L SUS304LN SUS316L Lifetime of filter member (days) 45 48 15 Foreign matter defects on resin 0 0 5 film surface (defects/m.sup.2) Recoverability of filter member 95 98 74 after cleaning (%) Lifetime of cleaned filter (days) 40 41 13 Corrosion in filter member Not Not Not detected detected detected Amount of precipitated 0 0 8000 antimony (counts) Amount of dissolved iron (ppm) 5.2 5.2 40

[0075] Herein, the lifetime (days) of the filter member represents the number of days for which the molten resin has flown until the filtering pressure reaches 25 MPa. The lifetime (days) of the cleaned filter means the time period to the exchange of the filter medium.

[0076] Thus, it is understood that a long lifetime of the filter member can be obtained by using SUS304L or SUS304LN instead of SUS316L as the material of the filter medium. It is also understood that a lifetime of the filter cleaned for reuse as long as that of a new filter can be obtained by using SUS304L or SUS304LN. Moreover, no foreign matter defects are observed on the surface of the resin film obtained in the case of using SUS304L or SUS304LN, and therefore it is also clear that the use of such a material allows formation of an excellent resin film. Furthermore, while the amounts of the precipitated antimony in Examples 1 and 2 are both 0 count, the amount of the precipitated antimony in Comparative Example 1 is 8000 counts, which is significantly large. While the amounts of the dissolved iron in Examples 1 and 2 are both 10 ppm or less, the amount of the dissolved iron in Comparative Example 1 is more than 20 ppm.

Other Examples

[0077] The filter member in the above embodiment comprises a leaf disc filter, but the filter member is not limited to this type and for example, a candle filter, a pack filter, a wire gauze filter, or the like can be used. The filtering accuracy of these types of filters can be selected according to the customer request, and for example, a cut filter with a filtering accuracy of 0.1 to 500 m may be used. The same will apply to the filtering accuracy of a leaf disc filter.

INDUSTRIAL APPLICABILITY

[0078] As described above, the present invention is useful for a filter medium, a filter provided with the filter medium, and a production method for a resin film using the filter medium.

REFERENCE SIGNS LIST

[0079] 10 Filter member [0080] 20 Filter medium [0081] 30 Filter retainer [0082] 40 Hub part [0083] 50 Filter [0084] 80 Resin film production apparatus [0085] 81 Extruder [0086] 82 Film forming machine [0087] 83 Cooling machine [0088] 84 Stretching machine [0089] 85 Take-up machine