Filter medium for filter, method for producing the same, and filter

Abstract

Provided are a filter medium for a filter, which makes it possible to obtain a filter high in collection efficiency, low in pressure loss and long in filter lifetime, a method for producing the same, and a filter using the filter medium for a filter. A filter medium for a filter is used as a constituent member of a filter and composed of a wet type nonwoven fabric, wherein the filter medium for a filter has a multilayer structure of two or more layers, and there is no interface between the above-mentioned two layers.

Claims

1. An air filter for an internal combustion engine obtained using a filter medium for a filter, which is used as a constituent member of the filter and which comprises a wet type nonwoven fabric, wherein the filter medium for the filter has a multilayer structure of two or more layers and there is no interface between two adjacent layers of the multilayer structure, wherein short-cut nanofibers comprising a fiber-forming thermoplastic polymer, having a single fiber diameter (D) of 100 to 1,000 nm and obtained by performing cutting so that the ratio (L/D) of the length (L) to the single fiber diameter (D) is within the range of 100 to 2,500, are contained only in one layer of the multilayer structure of the filter medium for the filter, thereby providing a layer structure in which an amount of the short-cut nanofibers gradually decreases in a thickness direction, and wherein a low-density layer is disposed on a fluid inflow side, and wherein when the delamination strength between the two adjacent layers of the multilayer structure is measured 10 times, for the remaining 8 values except for the maximum value and the minimum value thereof, the ratio of the maximum value/the minimum value is 1.5 or more, and wherein core-sheath conjugate type binder fibers are contained in the filter medium for a filter in an amount of 20 to 40% by weight, and wherein the thickness thereof is 0.5 to 4.0 mm.

2. The air filter according to claim 1, wherein the short-cut nanofibers are ones obtained by dissolving and removing a sea component from a conjugate fiber comprising an island component composed of a fiber-forming thermoplastic polymer and having an island diameter (D) of 100 to 1,000 nm and a sea component composed of a polymer more easily soluble in an alkaline aqueous solution than the fiber-forming thermoplastic polymer.

3. The air filter according to claim 2, wherein the sea component in the conjugate fiber is polyethylene terephthalate copolymerized with 6 to 12% by mole of 5-sodium sulfoisophthalic acid and 3 to 10% by weight of polyethylene glycol having a molecular weight of 4,000 to 12,000.

4. The air filter according to claim 2, wherein the island component in the conjugate fiber is a polyester.

5. The air filter according to claim 2, wherein the number of islands in the conjugate fiber is 100 or more.

6. The air filter according to claim 1, wherein in both surfaces of the filter medium for the filter, when the number of fibers on a surface on which more fibers are exposed is taken as DL and the number of fibers on the other surface on which fewer fibers are exposed is taken as DU, the DU/DL ratio is 0.8 or less.

7. The air filter according to claim 1, wherein the basis weight thereof is within the range of 30 to 300 g/m.sup.2.

8. The air filter according to claim 1, wherein a high-density layer is disposed on a fluid outflow side.

Description

EXAMPLES

(1) Examples and comparative examples of the present invention will be described in detail below, but the present invention should not be construed as being limited thereby. Incidentally, respective measurement items in examples were measured by the following methods.

(2) Melt Viscosity Curve

(3) (1) Melt Viscosity

(4) A polymer after drying treatment was set to an orifice whose temperature had been set to the melting temperature of an extruder at the time of spinning, melted and held for 5 minutes, and then, extruded by applying several levels of load. The shear rate and the melt viscosity at that time were plotted. The plotted points were smoothly connected to prepare a shear rate-melt viscosity curve, and the melt viscosity at the time when the shear rate was 1,000 sec.sup.1 was measured.

(5) (2) Measurement of Island Diameter

(6) A fiber cross-sectional photograph was taken at 30,000 magnification under a transmission type electron microscope TEM, and measurement was performed. The measurement was performed utilizing the length measurement capabilities possessed by the TEM. Further, in the absence of the TEM, the photograph taken may be enlarged with a copier and measured with a ruler in view of a reduction scale. However, an average value (n=20) of major axes and minor axes in fiber cross-sections was used as the fiber diameter.

(7) (3) Fiber Length

(8) In a state where an ultrafine short fiber before dissolution and removal of a sea component was laid on a base plate, the fiber length thereof was measured at 20 to 500 magnification under a scanning electro microscope (SEM). The measurement was performed utilizing the length measurement function of the SEM.

(9) (4) Basis Weight

(10) Measurement was performed on the basis of JIS P8124 (Measuring Method of Basis Weight in GSM of Paper).

(11) (5) Thickness

(12) Measurement was performed on the basis of JIS P8118 (Testing Method of Thickness and Density of Paper and Paper Board).

(13) (6) Density

(14) Measurement was performed on the basis of JIS P8118 (Testing Method of Thickness and Density of Paper and Paper Board).

(15) (7) DL and DU

(16) Each of the both surfaces of the filter medium for a filter was photographed at 100 magnification using a scanning electron microscope, and then, a straight line was drawn. The number of fibers (all visually observable fibers) intersecting the line was counted. The number of fibers on a surface on which more fibers were exposed was taken as DL and the number of fibers on the other surface on which fewer fibers were exposed was taken as DU.

(17) (8) Ratio of Maximum Value/Minimum Value of Delamination Strength Between Two Layers

(18) When the delamination strength between two layers was measured with n=10 using a Tensilon universal tester manufactured by A & D Co., Ltd., for the remaining 8 values except for the maximum value and the minimum value thereof, the ratio of the maximum value/the minimum value was calculated.

(19) (9) Collection Efficiency

(20) When the flow rate at the time of sample passing was 16.7 cm/sec and the dust concentration was 1 g/m.sup.3, using ISO FINE dust, the transmittance of the dust weight before and after the sample was taken as the collection efficiency.

(21) (10) Pressure Loss

(22) The pressure loss was determined at the time of performing the measurement of the above-mentioned collection efficiency (flow rate: 16.7 cm/sec).

(23) (11) Filter Lifetime (DHC)

(24) The above-mentioned collection efficiency test was performed, and the dust retaining amount (weight increase) at the time when an increase in pressure loss reached 2 kPa was taken as the DHC.

(25) Incidentally, for the filter performances in Table 1, the collection efficiency, the pressure loss and the DHC were measured, constituting a filter by disposing a lower layer of a filter medium for a filter on a fluid inflow side and an upper layer of the filter medium for a filter on a fluid outflow side

Example 1

(26) Using polyethylene terephthalate having a melt viscosity of 120 Pa.Math.sec at 285 C. as an island component and modified polyethylene terephthalate having a melt viscosity of 135 Pa.Math.sec at 285 C., which was obtained by copolymerizing 4% by weight of polyethylene glycol having an average molecular weight of 4,000 and 9% by mole of 5-sodium sulfoisophthalic acid, as a sea component, spinning was performed at a weight ratio of sea:island=10:90 using a spinneret having an island number of 400, and taken up at a spinning speed of 1,500 m/min. The difference in alkali reduction rate was 1,000 times. This was drawn to 3.9 times, and cut to 1,000 m with a guillotine cutter to obtain an ultrafine short fiber precursor. This was subjected to alkali reduction with a 4% NaOH aqueous solution at 75 C. to reduce the weight by 10%. As a result, it was confirmed that ultrafine short fibers having a relatively uniform fiber diameter and fiber length were formed. The resulting fibers were used as short-cut nanofibers (fiber diameter: 750 nm, fiber length: 0.8 mm, L/D=1,067).

(27) On the other hand, as binder fibers, core-sheath conjugate type binder short fibers (fineness: 1.1 dtex, fiber length: 5 mm, no crimp, core/sheath=50/50, core: polyethylene terephthalate having a melting point of 256 C., sheath: copolymerized polyester having a softening point of 110 C., which was mainly composed of terephthalic acid, isophthalic acid, ethylene glycol and diethylene glycol) and, in addition thereto, polyethylene terephthalate short fibers (fineness: 2.2 dtex, fiber length: 5 mm, crimped, triangular in cross section) were mixed at a predetermined weight ratio (short-cut nanofibers/binder fibers/other fibers=5/30/65, basis weight: corresponding to 50 g/m.sup.2), followed by stirring. The resulting mixture was put as a first slurry in TAPPI (a square type sheet machine manufactured by Kumagai Riki Kogyo Co., Ltd., hereinafter the same), and about a half of water was withdrawn. A second slurry (binder fibers/other fibers=30/70, corresponding to 50 g/m.sup.2) was additionally put therein in a halfway stage of forming wet paper on an undersurface thereof, followed by weak stirring so as not to form an interface, and water was completely withdrawn to obtain wet paper of 100 g/m.sup.2. Thereafter, rotary dryer drying (120 C.2 minutes) was performed to obtain a sheet. The physical properties obtained are shown in Table 1.

Example 2

(28) A sheet was prepared by performing the treatment/processing under the same conditions as in Example 1 with the exception that the ratio of the fibers of the upper layer used in Example 1 was changed (nanofiber-mixed layer: short-cut nanofibers/binder fibers/other fibers=20/30/50, 50 g/m.sup.2). The physical properties obtained are shown in Table 1.

Example 3

(29) A sheet was prepared by performing the treatment/processing under the same conditions as in Example 1 with the exception that the ratio of the fibers of the upper layer used in Example 1 was changed (nanofiber-mixed layer: short-cut nanofibers/binder fibers/other fibers=1/30/69, 50 g/m.sup.2). The physical properties obtained are shown in Table 1.

Example 4

(30) A sheet was obtained under the same conditions as in Example 1 with the exception that in the same raw fiber constitution as in Example 1, the basis weight of the upper layer was changed to 75 g/m.sup.2, the basis weight of the lower layer to 75 g/m.sup.2 and the total basis weight to 150 g/m.sup.2. The physical properties obtained are shown in Table 1.

Comparative Example 1

(31) The slurries used in Example 1 were each separately subjected to papermaking using TAPPI to obtain wet paper layers, and thereafter, they were laminated on each other, followed by the same drying process as in Example 1 after the lamination to obtain a sheet (different type two-layer sheet making). The physical properties obtained are shown in Table 1.

Example 5

(32) A sheet was prepared by performing the treatment/processing under the same conditions as in Example 1 with the exception that the ratio of the fibers of the upper layer used in Example 1 was changed (nanofiber-mixed layer: short-cut nanofibers/binder fibers/other fibers=25/30/45, 50 g/m.sup.2). The physical properties obtained are shown in Table 1.

Example 6

(33) A sheet was prepared by performing the treatment/processing under the same conditions as in Example 1 with the exception that polyethylene terephthalate short fibers (fineness: 0.1 dtex (diameter: 3 m), fiber length: 3 mm, L/D=1,000) were used in place of the short-cut nanofibers of the upper layer used in Example 1. The physical properties obtained are shown in Table 1.

Comparative Example 2

(34) Using the short-cut nanofibers/binder fibers/other fibers=2.5/30/67.5, the same fibers as used in Example 1, a single-layer wet type nonwoven fabric was obtained by papermaking at a basis weight of 100/m.sup.2 using TAPPI, followed by heat treatment. The physical properties obtained are shown in Table 1.

(35) TABLE-US-00001 TABLE 1 Com- Com- par- par- ative ative Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Fiber Diameter Fiber Length ple 1 ple 2 ple 3 ple 4 ple 1 ple 5 ple 6 ple 2 Raw Upper Short- 750 nm 0.8 mm (L/D = 1067) wt % 5 20 1 5 5 25 2.5 Fiber Layer Cut Constitu- (dense) Nano- tion fibers Core- 1.1 dtex 5 mm (no crimp) wt % 30 30 30 30 30 30 30 30 Sheath Conjugate Type Fibers Other 2.2 dtex 5 mm (crimped, wt % 65 50 69 65 65 45 65 67.5 Fibers triangular in cross section) 0.1 dtex 3 mm (no crimp, circular in cross section) Design Corresponding Basis Weight g/m.sup.2 50 50 50 75 50 50 50 100 Lower Short- 750 nm 0.8 mm (L/D = 1067) wt % Layer Cut (coarse) Nano- fibers Core- 1.1 dtex 5 mm (no crimp) wt % 30 30 30 30 30 30 30 Sheath Conjugate Type Fibers Other 2.2 dtex 5 mm (crimped, wt % 70 70 70 70 70 70 70 Fibers triangular in cross section) 0.1 dtex 3 mm (no crimp, circular in cross section) Design Corresponding Basis Weight g/m.sup.2 50 50 50 75 50 50 50 Average Short- 750 nm 0.8 mm (L/D = 1067) wt % 2.5 10 0.5 2.5 2.5 12.5 0 2.5 Cut Nano- fibers Core- 1.1 dtex 5 mm (no crimp) wt % 30 30 30 30 30 30 30 30 Sheath Conjugate Type Fibers Other 2.2 dtex 5 mm (crimped, wt % 67.5 60 69.5 67.5 67.5 57.5 67.5 67.5 Fibers triangular in cross section) 0.1 dtex 3 mm (no crimp, 0 0 0 0 0 0 2.5 0 circular in cross section) Structure Properties Basis Weight g/m.sup.2 101 104 101 151 101 99 102 101 Confirmation Thickness mm 0.82 0.48 0.85 1.1 0.69 0.35 0.85 0.59 Density g/cm.sup.3 0.12 0.22 0.12 0.14 0.15 0.28 0.12 0.17 Ratio of Numbers of Fibers on Both Layers 0.64 0.28 0.76 0.62 0.64 0.46 0.84 0.93 (Coarse Layer/Dense Layer) Delamination Strength Ratio between two 1.73 1.86 1.65 1.77 1.20 1.80 1.30 1.86 adjacent layers (Maximum/Minimum) Filter Performance Collection Efficiency % 99.83 99.99 99.67 99.99 99.99 99.99 95.87 99.74 Pressure Loss Pa 543 765 476 678 876 987 476 768 DHC g/m.sup.2 456 376 676 576 245 354 564 221 Remarks *1 *2 *3 *4 *5 *6 *7 *8 *1 The collection efficiency and the pressure loss are well balanced. (Standard) *2 The collection efficiency tended to be increased, because the nanofiber ratio was increased. *3 The pressure loss was decreased, resulting in an increase in DHC, because the nanofiber ratio was decreased. *4 The collection efficiency was increased, because the basis weight was increased. *5 The pressure loss is high, and the DHC is low, because the density is increased. *6 The pressure loss is high, because the nanofiber ratio is large. *7 The collection efficiency was decreased, because of no presence of nanofibers. *8 The thickness was decreased (the density was increased), resulting in high pressure loss and short lifetime, because of the single layer.

INDUSTRIAL APPLICABILITY

(36) According to the present invention, there are provided a filter medium for a filter, which makes it possible to obtain a filter that is high in collection efficiency, low in pressure loss and long in filter lifetime and has high collection efficiency, low pressure loss and a long filter lifetime, and a filter using the filter medium for a filter. The filter is also useful as a filter for an indoor air conditioner, a cooler, a heater (electric or oil-fired), an automotive air conditioner, an air cleaner, a clean room, an indoor humidifier or the like, a microfilter and a liquid filter, as well as an air filter for an internal combustion engine such as an intake air filter for an internal combustion engine. Thus, the industrial value thereof is extremely large.