JPH0724230A - Filter medium for fine particles - Google Patents

Abstract

PURPOSE:To obtain a filter medium having high precision of filtration separation of fine particles and a long filtration life by using a fiber assembly of superposed superfine fibers having specified properties and specifying the pore diameter and filling rate of the assembly. CONSTITUTION:In order to capture fine particles whose diameter is smaller than the average fiber diameter of superfine fibers by filtration with high precision, superfine fibers having 0.5-4mum fiber diameter are used and 10-40 such superfine fibers are superposed in the thickness direction to form a fiber assembly having <=25mum p[fl max. pore diameter, 1.0-1.8 ratio of the max. pore diameter to average flow rate and 0.05-0.35 filling rate of fibers. The controlled fibers are dispersed so as to attain a very uniformly dispersed state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter material for removing fine particles of micron or submicron order contained in a gas or liquid, and a filter cartridge using the filter material. The present invention relates to a filtering material which has a high filtering accuracy smaller than the equivalent circle diameter of fibers constituting the filtering material, has a long filtering life, and exhibits excellent processing ability, and a cartridge filter using the filtering material.

[0002]

2. Description of the Related Art Conventionally, in the development of a filter medium, if the filter medium is set so that the filter precision is increased, the filter life is shortened, and conversely, if the filter life is lengthened, the filter precision is decreased, and an effort is made to solve the problem. It has been. As a device for solving such a problem and increasing the filtration accuracy of the filter and prolonging the filtration life, the fiber diameter of the filter medium is changed in the depth direction of the filter medium as described in JP-A-60-21618. Further, as shown in Japanese Utility Model Application Laid-Open No. 60-28009, the packing density of the fibers is made to have a gradient in the depth direction, and further, as described in JP-A-1-297113, the fiber diameter and the average pore diameter are simultaneously set in the depth direction. The method of changing has been taken.

However, in order to change the fiber diameter, it is necessary to have many kinds of non-woven fabrics made under different spinning conditions, and there is a problem that a large loss occurs when the setting of the manufacturing conditions of the filter medium is changed. there were. In addition, JP-A-60-2
16818, the method of winding the filter medium around the filter core (tubular core member) while changing the production conditions is as follows.
Although such a problem can be solved, fluctuations in the flow and non-uniformity occur in the polymer piping due to unstable spinning conditions, and variations in polymer residence time cause variations in polymer viscosity, resulting in orifice hole There are problems that clogging occurs, spinning becomes unstable, and fiber diameter changes and yarn breakage occurs. In addition, there is a problem in that a new process is required to adjust the filling rate, which increases the cost.

Further, as described in Japanese Patent Publication No. 3-73222, the non-woven fabric formed by the water flow intersection has holes at the portion hit by the water flow, and is not suitable for the high precision filtration of fine particles for the purpose of the present invention. . A method of increasing the filling rate to about 0.4 to 0.85 in order to improve the filtration accuracy is also known, but it is not preferable from the viewpoint of prolonging the life, and even if such a filter medium is laminated. It is known that the stacking effect is small. Despite these efforts, no method is known for efficiently filtering particles smaller than the fiber diameter.

Further, when these filter materials having a low filling rate are stored in a state of being sprinkled on rolls, the filling rate of the filter material changes between the surface layer and the filter core side. There was also the problem of variations.

[0006]

DISCLOSURE OF THE INVENTION The present invention has improved the point that conventional filter media are insufficient, has high filtration accuracy of fine particles, and enables filtration of fine particles having a long filtration life. It is an object to provide a material and a filter cartridge.

[0007]

The present invention adopts the following means in order to solve the above problems. That is, the present invention has a maximum pore diameter of 25 µm or less in a fiber assembly in which 10 to 40 ultrafine fibers having an average fiber diameter of 0.5 µm to 4 µm are stacked in the thickness direction, and the maximum pore diameter and the average flow rate. Pore size ratio is between 1.0 and 1.8, and fiber packing ratio is 0.0
A fine particle filtering material (claim 1) characterized by being in the range of 5 to 0.35, and further having an average fiber diameter of 0.
Made of ultrafine fibers of 5 μm or more and 4 μm or less, with a basis weight of 5
It is characterized by comprising a fiber aggregate in the range of 40 g / m ² , and the fiber aggregate is characterized in that the adhesive points of the fibers are partially present in the thickness direction so that there is no difference between the surface layer and the inner layer. The filter material for fine particles according to claim 1 (claim 2), and further, a tubular core member having a liquid passage hole and a filter material layer arranged around the tubular core member are fixed at their ends. A filter cartridge having the above-mentioned structure, wherein the filter material layer is formed by winding at least three layers of the filter material according to claim 1 or claim 2.

The present invention will be described in detail below. In order to highly accurately filter and collect particles smaller than the average fiber diameter of the ultrafine fibers forming the filter medium in the filter medium used in the present invention, the filter medium is composed of ultrafine fibers having a fiber diameter of 0.5 μm to 4 μm. It is necessary to use a filter medium. Generally, the finer the fiber used for depth filtration is, the more the average value is 4
μm or less, more preferably 3 μm or less. If the average fiber diameter exceeds 4 μm, it is difficult to increase the filtration efficiency. It is preferable to make the fiber diameter small, but if the fiber diameter is smaller than 0.5 μm, there is a high risk that the filter medium will break when the filter medium is wound around the core member, and the manufacturing cost of the filter medium will be high. As a result of earnest research, the present inventor has found that the use of a fiber diameter of 3 times or less, preferably 2 times or less, of the particle diameter to be filtered is suitable for high-precision filtration of fine particles, which is the object of the present invention. Made it clear. Further, the value obtained by subtracting the fiber diameter from the value obtained by dividing the fiber diameter by the square root of the filling rate is approximately equivalent to the fiber gap distance when the fibers are uniformly dispersed in the filter medium, but the filling rate of the present invention Within the range, it is between 0.7 and 3.5 times the fiber diameter, and it is presumed that it is suitable for depth filtration. Further, the smaller the variation in the fiber diameter distribution is, the more easily the effect of the present invention can be enhanced, and preferably the CV% (the value obtained by dividing the standard deviation by the average value and expressed as a percentage) showing the fiber diameter distribution is 50%. It is preferably smaller. More preferably, the standard deviation of the fiber diameter is smaller than the average spherical equivalent diameter of the target particles.

When a thermoplastic polymer is used as a material for producing the ultrafine fibers constituting these filter materials, for example, a composite spinning method, a spunbond method, a melt blow method,
Electrostatic spinning method and the like can be mentioned. Among these methods, the spunbond method and the melt blow method are considered to be advantageous in terms of cost because the filter medium can be directly formed without a post-processing step. The form of the fiber aggregate may be a woven fabric or a non-woven fabric, but the non-woven fabric is more effective in improving filtration accuracy because the fibers are randomly dispersed. The cross-sectional shape of the fiber may be circular or any other shape, and in some cases, a modified cross-section is more preferable.

The basis weight of the fiber assembly is preferably in the range of 5 to 40 g / m ² , and more preferably 25 to 40 g.
/ M ² is preferable. When the basis weight is less than 5 g / m ² , problems such as an increase in the cost of winding the fiber assembly around the tubular core material and an increase in the risk of the filter material being cut for some reason during the processing step are likely to occur. On the other hand, if the fabric weight is more than 40 g / m ² , there are many problems in operability, such as wrinkles when wound around the filter medium.

In the present invention, it is important that 10 to 40 ultrafine fibers are stacked in the depth direction. The number of fibers in the depth direction is given by the following formula in which the basis weight is divided by the product of the specific gravity of the fiber constituent substance and the fiber diameter. Depth direction number of fibers = filtration weight (g / m ² ) / [fiber density (g / cm ³ ) × fiber diameter (μm)]

Conventionally, as disclosed in Japanese Patent Laid-Open No. 58-186412, it has been considered that the number of fibers in the depth direction should be large, but the present inventors have found that the number of fibers in the depth direction is 40 fibers. It has been clarified that it becomes difficult to uniformly disperse the fibers in the filter medium as the size becomes larger. Especially in the non-woven fabric made of ultrafine fibers produced by the melt-blowing method, suction is made to work from the lower surface of the collector so that the fibers collected in the collector are not blown off by the traction air jet flow, but the depth of the fiber layer is deep. Then, when the fiber is pulled due to the pressure loss of the fiber layer, the suction force acting to prevent the fiber from being blown off is weakened, and unevenness such as ropes is generated near the top of the sheet, so it is not possible to improve filtration accuracy by experiments. confirmed. Further, if the fiber layer is too deep, there is a problem that it becomes difficult to uniformly wind the fiber layer around the filter core. On the other hand, if the number of fibers in the depth direction is too small, it means that the basis weight is reduced, and problems such as strength tend to occur.

The filling factor of the fiber is 0.05 or more and 0.
It may be 35 or less. If the filling rate is less than 0.05, it becomes difficult to perform highly efficient filtration of particles smaller than the average fiber diameter, which is the object of the present invention. On the other hand, when the filling rate is higher than 0.35, the filtration accuracy is slightly improved, but even if the filter media are laminated, the life is hardly changed and it is difficult to achieve a long filtration life.

The most important requirement for highly accurately filtering particles having an average particle diameter smaller than the average fiber diameter and being relatively monodisperse is to disperse the fibers in the filter medium so that the dispersed state is extremely uniform. is important. This is because when the particle size is larger than the fiber diameter of the filter medium, there is no difference in structure between the surface layer and the inside of the filter medium (cross-sectional direction), which is a problem when the fibers are evenly dispersed in the filter medium. Since it is often filtered only by the surface of the
This is completely different from the phenomenon that the filtration resistance is remarkably increased because the cake layer is formed on the surface layer portion of the filter medium on the most upstream side, and the filtration life is shortened.

In the case of a polydisperse system having a wide distribution of particle diameters, a number of kinds of the filtering materials of the present invention are used in combination, the particle diameters to be collected by the respective filtering materials are preliminarily determined, and the corresponding fibers are used. Effective filtration can be realized by laminating the filter material having the diameter, the filling factor, and the maximum pore diameter on the upstream side of the filter material having a larger target particle diameter.

In the present invention, it is important to control the dispersed state of the fibers, and as one of the methods for controlling the dispersed state, for example, in the melt blow method, a die for discharging a polymer and a collector for a nonwoven fabric are used. This can be easily achieved by changing the distance between them. The appropriate distance between the die and the collector varies depending on the spinning conditions and the size of particles to be filtered, but is generally preferably 3 to 60 cm, and more preferably 10 to 30 cm.

The dispersibility of the fiber is evaluated by cutting the cross section while preventing the filter material from being deformed (for example, by cutting it in liquid nitrogen), and observing the cross section with a scanning electron micrograph to melt the fiber. Examples include a method of counting the number of places where a plurality of fibers are entangled in a rope shape and fused and the number of fused fibers, or measuring the size of pores formed by the fibers with a porometer or the like. The better the fiber dispersibility, the more uniform the pore size, but it can be easily confirmed by, for example, measuring and comparing the ratio of the maximum pore size (maximum pore size) to the average flow pore size (mean flow pore). When the fiber assembly is a non-woven fabric, since the morphology of the fibers is not so uniform, it is necessary to increase the number of measurement points and perform the measurement, and to evaluate with an average value of at least 3 points.

In the filter medium that collects particles smaller than the fiber diameter, the ratio of the maximum pore diameter to the average flow pore diameter is 1.0-1.
It must be between eight. When the value of this ratio approaches 1.0, it approaches a microfiltration membrane, but it is desirable that the maximum pore size is larger than the average particle size in order to enhance the effect of laminating the filter media. On the other hand, if the value of this ratio is larger than 1.8, the presence of large pores causes channeling in the flow, which makes it difficult to increase the precision when filtering fine particles. The value of the ratio of the maximum pore diameter to the average flow pore diameter is influenced not only by the fiber dispersion state but also by the average fiber diameter and the fiber diameter dispersion. The smaller the average fiber diameter and the smaller the fiber diameter standard deviation, the more preferable. In particular, the average fiber diameter is 4 μm or less and the fiber diameter standard deviation is 50% or less of the average fiber diameter, and more preferably 3
It is preferably 5%. From this, it became clear that it is easy to adjust the value of the ratio, which is a requirement of the present invention, to a value of 1.0 to 1.8. Further, within the requirements of the present invention, it became clear that the larger the ratio of the maximum pore diameter to the average flow pore diameter, the greater the effect of increasing the number of laminated layers.

For the filtration of particles smaller than the fiber diameter,
It is important that the maximum pore size is 25 μm, more preferably 20 μm or less, and further preferably 15 μm or less in order to improve filtration accuracy. If the maximum pore size is larger than 25 μm, the filtration accuracy will decrease, which is not preferable.

In the present invention, it is preferable that the fiber diameter distribution is narrow, but as a method for reducing the fiber distribution in the melt blowing method, the discharge amount per single hole is set to 0.01 or more and 0.75 g / min or less, and traction is performed. It was found that this can be achieved by making the distribution of the air flow, which is a fluid, in the nozzle width direction as small as possible.

The filter material used in the present invention can change the fiber diameter in the depth direction, as has been used conventionally.
It is also possible to construct the filter cartridge by a method such as changing the filling rate in the depth direction.

As a proper condition of the filtration operation carried out in the present invention, it is preferable to carry out the filtration at a linear velocity of 0.5 to 20 cm / min. When the filtration rate is less than 0.5 cm / min, the effect of the present invention is not sufficiently shown in improving the life, probably because the collecting mechanism changes. In addition, a low filtration rate means a reduction in the amount of filtration treatment, which is not industrially preferable. On the other hand, when it is higher than 20 cm / min, the filtering accuracy of the filter material of the present invention cannot be sufficiently increased.

As the particles to which the filter material of the present invention is most effectively used, not only particles having a nearly spherical shape such as polystyrene latex but also non-spherical particles are useful. A large effect was also observed on irregular particles such as blood cells and bacteria (particles that are deformed by external stimuli).

Next, in order to eliminate the difference between the inside and the outside of the performance of the filter medium stored in a roll, it is desirable to calender the filter medium of the present invention. The filtration material of the present invention has a ventilation resistance of 0.5 to 8.0 at a wind speed of 5.3 cm / sec.
Although it is often mmAq, it is preferable to press at a temperature of 1/2 or less of the melting point of the constituent material of the filter medium at a linear pressure of 5 to 100 kgf / cm ^{2 with} a calender roller or the like. Thereby, the air permeability is 20 to 100.
%, And the ventilation resistance is preferably adjusted to be, for example, between 0.6 and 16 mmAq. The calendering process slightly increased the accuracy and tended to shorten the life, but there was no difference in the characteristics between the inside and outside of the roll, and therefore no difference was found in the filtration characteristics.
With a filter medium having a ventilation resistance of more than 8.0 mmAq, no significant difference between the inner and outer layers was observed without calendar adjustment. On the other hand, the desired ventilation resistance could not be realized in the case where the ventilation resistance was less than 0.5 mmAq. If the linear pressure when using a calender is less than 5 kgf / cm ² , its effect is small and control is difficult, while 100 kgf / cm ²
It became clear that the filter material was crushed too much when it was larger, the life was too short, and the stacking effect was small. If the temperature at the time of calendering is too close to the melting point, the surface of the sheet will be melted and the surface of the filter material will be clogged with trapped particles to shorten the filter life, which is not preferable.
According to the experience of the inventors, it is preferable to carry out the treatment at a temperature between half the melting point and room temperature. In the present invention, it is preferable to uniformly disperse, in the filter medium, the places that are weakly bonded due to the pressure deformation. At this time, Japanese Patent Laid-Open No. 3-696
It is necessary that the bonding points are not dispersed only on the surface of the sheet as in 54, but are weakly bonded in the entire depth direction of the filter medium. This is extremely difficult to achieve at the high calendering temperature conditions as described in that patent.

Although the airflow resistance is slightly increased by calendering and the life is slightly decreased as a result, when the cartridge is made, the filter material is deformed by the winding tension and the airflow resistance is increased. This issue is not very important. There is no problem in processing by laminating two or more filter media in order to improve the processing efficiency. The number of bonding points may be adjusted by a hydroentangling method or heat treatment. Such a filter medium calendered at a relatively low temperature may be wall-folded (pleated) to be used by laminating 2 to 10 layers. Even if the filter of the present invention is laminated with another material such as a porous film or a membrane, or subjected to post-processing such as hydroentangling treatment or electret processing, the effect is the same. Also,
It is also one of the preferable modes to carry out the adsorption treatment at the same time by providing a layer made of granular or fibrous activated carbon or other porous material in the middle or before and after. The filter material of the present invention is also useful as a heat retaining material or a bacterial barrier material because of its ultrafine fiber characteristics and appropriate fluid permeability.

[0026]

【Example】

Examples 1 to 3 and Comparative Examples 1 to 3 The present invention will be specifically described with reference to Examples. The characteristic values described in the examples were determined by the following measuring methods. Fiber diameter Scanning electron micrograph of the fiber at a magnification of 1000 to 5000
The width of the side surface of 200 fibers arbitrarily taken from the photograph was measured and determined by arithmetic mean. The basis weight of the fiber packing ratio fibers weighed sample of 20cm square, it is converted to the basis weight per 1 m ^2. Further, the thickness of 5 arbitrary points of the sample was measured according to JIS L1096 under a load of 7 g / cm ² . The value obtained by dividing the basis weight by the specific gravity of the polymer was further divided by the thickness to obtain a dimensionless volume filling rate. Fiber dispersion state Using a Coulter Porometer II, ASTM F
Measurement was carried out according to 316-86, and the ratio of the maximum pore diameter to the average flow pore diameter was obtained. Particle Filtration Accuracy and Filtration Life JIS11 seed particles and 0.6 μm monodispersed alumina particles mixed at a mass ratio of 8: 2 are dispersed at 0.025 g / 1, and an aqueous solution is supplied at a linear velocity of 5 cm / min. After 3 minutes, the turbidity of the liquid before and after the filter medium was measured, and the collection efficiency was calculated by the following formula. Collection efficiency (%) = (1-outlet concentration / inlet concentration) × 100 Further, as for the filtration life, the differential pressure before and after the filter medium is measured with a digital manometer, and the time until the filtration pressure becomes 5 kgf / cm ² is measured. I asked.

A polypropylene resin having a melt flow rate of 300 to 1000 is used, and a basis weight of ultrafine fibers having a fiber diameter of 0.8 to 6.0 μm is 30 by a melt blow method.
A nonwoven fabric of g / m ² was prepared and a filtration test was performed. The results are shown in Table 1. In Table 1, the pore diameter ratio refers to the ratio of the maximum pore diameter to the average flow pore diameter.
Effect of collecting IS11 seed particles and 0.6 μm alumina particle mixture.

[0028]

[Table 1]

Example 4 The non-woven fabric used in Example 2 was prepared with a roll of 1000 m, left for 3 months and then rewound to sample 10 m each of the surface layer and the core, and a linear pressure of 20 kgf with a calendar roller. / Cm at 20 ° C. room temperature. Air velocity 5.3 cm / sec Ventilation resistance is
The surface layer portion had 3.6 mmAq and the core portion had 3.9 mmAq, but both were 4.8 mmAq by the calendar treatment. Although there is a slight increase in ventilation resistance, this does not cause any problem in actual use. 5 for each sample
When the filtration test was conducted twice, the collection efficiency was 92% and the life was 37 minutes for both, and no significant difference was observed between the two effects.

Comparative Example 4 The non-woven fabric used in Example 2 was prepared with a roll of 1000 m, left for 3 months, the surface layer and core of the roll were sampled, and a filtration test was carried out 5 times for each. The surface layer had no difference in performance from Example 1, but
The sample on the core side had a significant difference in the collection efficiency of 89% and the life changed to 38 minutes, and a significant difference was observed, and the difference in filtration performance was a big problem.

[0031]

INDUSTRIAL APPLICABILITY The filter material of the present invention has a high filtering accuracy for the target fine particles when used as a filter due to the controlled dispersibility of the fibers, and also enables filtration with a long filtration life, and The cost is greatly improved and the effect is extremely large.

Claims (3)

[Claims] 1. A fiber assembly in which 10 to 40 ultrafine fibers having an average fiber diameter of 0.5 μm or more and 4 μm or less are stacked in the thickness direction, and the maximum pore diameter of the fiber assembly is 25 μm or less, and The ratio of the maximum pore diameter to the average flow pore diameter is 1.0 to
It is in the range of 1.8 and the filling factor of the fiber is 0.05 to 0.3.
A filter material for fine particles, which is in the range of 5. 2. An ultrafine fiber having an average fiber diameter of 0.5 μm or more and 4 μm or less, and a fiber assembly having a basis weight in the range of 5 to 40 g / m ² , and the fiber assembly is partially formed in the thickness direction. The filter material for fine particles according to claim 1, wherein the adhesion points of the fibers are present so that there is no difference between the surface layer and the inner layer. 3. A tubular core member having a liquid passage hole,
A filter cartridge in which a filter material layer disposed around the filter material is fixed at an end thereof, and the filter material layer is formed by winding at least three layers of the filter material according to claim 1 or 2. A filter cartridge characterized in that