阅读说明：本技术 废气净化过滤器 (Exhaust gas purifying filter ) 是由 水谷圭祐 小池和彦 石原干男 于 2020-02-18 设计创作，主要内容包括：本发明提供一种废气净化过滤器(1),配置于汽油发动机的排气通路。废气净化过滤器(1)具有：分隔壁(11),具有多个气孔(110)；多个腔室(12),由分隔壁(11)划分；以及密封部(13),在过滤器两端部将多个腔室(12)交替地封闭。在废气净化过滤器(1)中,分隔壁(11)的平均气孔径超过16μm且小于21μm,分隔壁(11)表面上的气孔(110)的平均表面开口径相对于分隔壁(11)的平均气孔径之比为0.66以上且0.94以下。(The invention provides an exhaust gas purifying filter (1) disposed in an exhaust passage of a gasoline engine. An exhaust gas purification filter (1) is provided with: a partition wall (11) having a plurality of air holes (110); a plurality of chambers (12) divided by partition walls (11); and a sealing part (13) which alternately seals the plurality of chambers (12) at both ends of the filter. In the exhaust gas purification filter (1), the average pore diameter of the partition wall (11) exceeds 16 [ mu ] m and is less than 21 [ mu ] m, and the ratio of the average surface opening diameter of the pores (110) on the surface of the partition wall (11) to the average pore diameter of the partition wall (11) is 0.66 to 0.94.)

1. An exhaust gas purification filter (1) is disposed in an exhaust passage of a gasoline engine, and comprises:

a partition wall (11) having a plurality of air holes (110);

a plurality of chambers (12, 121, 122) partitioned by the partition walls; and

a sealing part (13) which alternately seals the plurality of chambers at the two ends of the filter,

the average pore diameter of the partition walls is more than 16 μm and less than 21 μm,

the ratio of the average surface opening diameter of the pores on the surface of the partition wall to the average pore diameter of the partition wall is 0.66 or more and 0.94 or less.

2. The exhaust gas purifying filter according to claim 1,

the ratio of the surface opening diameter of 25 μm or more on the surface of the partition wall is 20% or less.

3. The exhaust gas purifying filter according to claim 1 or 2,

the surface opening ratio of the pores on the surface of the partition wall is 30% to 40%.

4. The exhaust gas purifying filter according to any one of claims 1 to 3,

in the state of accumulating the ash (23) of more than 20g/L and less than 40g/L,

at a position (X) of 10mm on the filter end face (15) on the side where the off-gas flows in₁₀) The gas permeability coefficient of the partition wall is k₁₀A central position (X) between the filter end surface on the exhaust gas inflow side and the filter end surface (16) on the exhaust gas outflow side_C) The gas permeability coefficient of the partition wall is k_cWhen the temperature of the water is higher than the set temperature,

gas transmission coefficient ratio k_c/k₁₀The value of (A) is 1.5 or less.

Technical Field

The present invention relates to an exhaust gas purifying filter.

Background

An exhaust gas discharged from an internal combustion engine such as a gasoline engine or a diesel engine contains particulate matter (hereinafter, sometimes referred to as "PM" as appropriate) called particulate matter. An exhaust gas purification filter is disposed in an exhaust passage of the internal combustion engine to collect PM in the exhaust gas and purify the exhaust gas.

As such an exhaust gas purifying filter, for example, patent document 1 discloses a diesel particulate filter (hereinafter, sometimes referred to as "DPF") that collects PM discharged from a diesel engine. Specifically, the following DPF is described in this document: in order to suppress an increase in pressure loss associated with an increase in PM accumulation, partition walls forming a plurality of chambers are formed of a porous base material having a porosity of 45 to 70%, and when the average pore diameter of the base material measured by a mercury intrusion method is (A) μm and the average pore diameter measured by a bubble point method is (B) μm, the average pore diameter difference ratio represented by { (A-B)/B }. times.100 is 35% or less, the average pore diameter (B) is 15 to 30 μm, and the maximum pore diameter measured by the bubble point method is 150 μm or less.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent application No. 2006-95352

Disclosure of Invention

The amount of PM discharged from a gasoline engine is overwhelmingly small compared to the amount of PM discharged from a diesel engine. However, since the number of PMs is limited, a vehicle having a gasoline engine (hereinafter referred to as a "gasoline vehicle") is also required to be equipped with a gasoline particulate filter (hereinafter referred to as a "GPF" where appropriate) capable of trapping PMs discharged from the gasoline engine.

In GPF, a catalyst for purifying exhaust gas is sometimes coated. From the viewpoints of ensuring the exhaust gas purification performance, regeneration treatment of the accumulated PM, and the like, it is conceivable to provide the GPF coated with the catalyst directly below the gasoline engine or immediately downstream of the start (S/C) catalyst. In this case, since the exhaust layout is restricted, it is desirable to reduce the filter volume. However, reducing the filter volume leads to an increase in pressure loss (hereinafter, sometimes referred to as "pressure loss" as appropriate). Further, in a gasoline engine, the temperature of exhaust gas is high and the flow rate is also high, so that the pressure loss tends to increase compared to a diesel engine. Further, clogging of the pores of the partition walls by the catalyst coat layer also causes an increase in pressure loss. Thus, GPF is required to further reduce the initial pressure loss.

Further, as described above, the amount of PM discharged from the gasoline engine is very small. Therefore, in the GPF, it takes a very long time for the PM accumulation to be transferred from the accumulation inside the partition wall to the accumulation outside the partition wall (wall accumulation) as compared with the DPF. Depending on the case, the deposition may not reach the outside of the partition wall. Further, when the catalyst is coated, pores of the partition wall are blocked, and the PM trapping rate is deteriorated. From these, GPF is required to be able to secure the initial PM trapping rate.

In addition to the above, the PM contains Ash (Ash) derived from engine oil or the like in addition to solid carbon (soot). Ash is a component remaining after the PM regeneration process. In gasoline vehicles, it is important to suppress the increase in pressure loss due to residual ash accumulated over long-term use. In addition, although DPF has been used for years to accumulate residual ash and increase pressure loss, the exhaust gas temperature is low and there is a margin in the exhaust gas layout, so that there has been no problem in suppressing the pressure loss increase after ash accumulation.

An object of the present invention is to provide an exhaust gas purifying filter capable of achieving initial pressure loss reduction, initial PM trapping rate maintenance, and suppression of pressure loss increase after ash deposition.

One aspect of the present invention is an exhaust gas purifying filter disposed in an exhaust passage of a gasoline engine, comprising:

a partition wall having a plurality of air holes;

a plurality of chambers partitioned by the partition walls; and

a sealing part for alternately sealing the chambers at two ends of the filter,

the average pore diameter of the partition walls is more than 16 μm and less than 21 μm,

the ratio of the average surface opening diameter of the pores on the surface of the partition wall to the average pore diameter of the partition wall is 0.66 or more and 0.94 or less.

The exhaust gas purifying filter has the above-described specific configuration, and particularly, the average pore diameter of the partition wall and the ratio of the average surface opening diameter of the pores on the surface of the partition wall to the average pore diameter of the partition wall are in specific ranges. Therefore, according to the exhaust gas purification filter, initial pressure loss reduction, initial PM collection rate maintenance, and suppression of pressure loss increase after ash accumulation can be achieved.

The parenthesized symbols in the request range indicate the correspondence with specific means described in the embodiment described later, and do not limit the technical scope of the present invention.

Drawings

The above objects, and other objects, features, and advantages of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings. These figures are:

fig. 1 is a perspective view of an exhaust gas purifying filter according to embodiment 1;

fig. 2 is a sectional view of the exhaust gas purifying filter of embodiment 1 along the filter axial direction;

fig. 3 is a diagram showing the flow of exhaust gas in the exhaust gas purification filter of embodiment 1;

fig. 4 is a view showing an example of a reflected electron image of the surface of the partition wall obtained by a scanning electron microscope when measuring the surface opening diameters of the pores on the surface of the partition wall in the exhaust gas purification filter according to embodiment 1;

fig. 5 is a diagram showing an example of a binarized image obtained by binarizing the reflected electron image of fig. 4;

FIG. 6 is a view schematically showing the microstructure of PM;

fig. 7 is a sectional view of a partition wall showing an enlarged view of the side of the partition wall surface into which exhaust gas flows;

FIG. 8 is a view showing PM segregation in the vicinity of a surface opening of the partition wall shown in FIG. 7;

fig. 9 is a view showing a state in which ash contained in the PM remains after the PM shown in fig. 8 is subjected to regeneration processing;

fig. 10 is a view showing a case where PM is collected again in a state where the residual ash shown in fig. 9 is present;

fig. 11 is a view showing a case where the PM deposition and the PM regeneration treatment are further repeated from the state of fig. 10, whereby ash is crosslinked at the surface openings of the pores on the surface of the partition wall;

fig. 12 is a view showing a state in which the ash crosslinked at the surface opening shown in fig. 11 is peeled off by the flow of the exhaust gas, and the peeled ash is conveyed toward the sealing portion on the gas outflow side;

fig. 13 is a diagram showing a method of measuring a gas permeability coefficient of an exhaust gas purification filter;

fig. 14 (a) is a view showing an example of a tape for forming a seal portion attached to an upstream end surface of a measurement sample collected from an exhaust gas purification filter, and fig. 14 (b) is a view showing an example of a tape for forming a seal portion attached to a downstream end surface of a measurement sample collected from an exhaust gas purification filter;

FIG. 15 is a diagram showing an example of a graph of the relationship between the gas flow rate (X-axis) and the pressure loss (Y-axis);

FIG. 16 shows the gas permeability coefficient ratio k obtained in the experimental example_c/k₁₀Graph of the relationship with the rate of rise of pressure loss.

Detailed Description

(embodiment mode 1)

An exhaust gas purifying filter according to embodiment 1 will be described with reference to fig. 1 to 15. As illustrated in fig. 1 to 3, the exhaust gas purifying filter 1 of the present embodiment is disposed in an exhaust passage (not shown) of a gasoline engine and used. That is, the exhaust gas purifying filter 1 is a Gasoline Particulate Filter (GPF) capable of trapping PM2 (see fig. 6 described later) discharged from the gasoline engine. The directions of the double-headed arrows shown in fig. 1 to 3 are referred to as filter axial directions X of the exhaust gas purifying filter 1.

The exhaust gas purification filter 1 includes a partition wall 11, a plurality of chambers 12, and a seal portion 13. As illustrated in fig. 1 and 2, the partition walls 11 may be provided inside a cylindrical housing 14 formed in a cylindrical shape or the like, for example, in a lattice shape or the like in a cross-sectional view perpendicular to the filter axial direction X. In the exhaust gas purification filter 1, the partition wall 11 and the housing portion 14 can be formed of, for example, cordierite. The sealing portion 13 may be made of ceramic such as cordierite, for example, but may be made of other materials.

The plurality of chambers 12 are formed by being divided by partition walls 11. The chamber 12 is surrounded by the partition wall 11 and forms a gas flow path. The direction of elongation of the chamber 12 generally coincides with the filter axis X. In a cross-sectional view perpendicular to the filter axial direction X, the chamber may have a quadrilateral shape, for example, as illustrated in fig. 1. The shape of the chamber is not limited to this, and may be, for example, a polygonal shape such as a triangular shape or a hexagonal shape, or a circular shape. In addition, the chamber shape may be formed by a combination of two or more different shapes.

As illustrated in fig. 2, the plurality of chambers 12 are alternately closed by the sealing portions 13 at both ends of the filter. Specifically, the plurality of chambers 12 can have: a1 st chamber 121 that is open at a filter end surface 15 (upstream end surface) on an exhaust gas inflow side and is closed by a seal portion 13 at a filter end surface 16 (downstream end surface) on an exhaust gas outflow side; and a2 nd chamber 122 which is open at the filter end face 16 on the exhaust gas outflow side and closed by the seal portion 13 at the filter end face 15 on the exhaust gas inflow side. Thus, as illustrated in fig. 3, the exhaust gas G flowing into the 1 st chamber 121 from the filter end face 15 on the exhaust gas inflow side flows through the 1 st chamber 121, flows through the porous partition wall 11, and reaches the 2 nd chamber 122. The exhaust gas G reaching the 2 nd chamber 122 flows in the 2 nd chamber 122 and is discharged from the filter end face 16 on the exhaust gas outflow side.

The 1 st chamber 121 and the 2 nd chamber 122 can be alternately arranged adjacent to each other, for example, in both the transverse direction orthogonal to the filter axial direction X and the longitudinal direction orthogonal to both the filter axial direction X and the transverse direction. In this case, when the filter end face 15 on the exhaust gas inflow side or the filter end face 16 on the exhaust gas outflow side is viewed from the filter axial direction X, the 1 st chamber 121 and the 2 nd chamber 122 are arranged in a lattice pattern, for example. The 1 st chamber 121 and the 2 nd chamber 122 adjacent to each other are partitioned with a partition wall 11 interposed therebetween.

As illustrated in fig. 7, the partition wall 11 has a plurality of air holes 110. Specifically, the air hole 110 in the partition wall 11 includes a communication hole 111 that communicates between the 1 st chamber 121 and the 2 nd chamber 122 adjacent to each other. The air hole 110 in the partition wall 11 may include a non-communication hole 112 that does not communicate between the 1 st chamber 121 and the 2 nd chamber 122 adjacent to each other, in addition to the communication hole 111.

In the exhaust gas purification filter 1, the average pore diameter of the partition walls 11 (i.e., the average pore diameter inside the partition walls 11) is in the range of more than 16 μm and less than 21 μm.

The average pore diameter of the partition wall 11 was measured by a mercury porosimeter using the principle of mercury intrusion. Specifically, a test piece was cut out of the exhaust gas purification filter 1. However, the portion where the seal portion 13 exists is removed. The test piece was a rectangular parallelepiped having a dimension of 15mm in the longitudinal direction X15 mm in the transverse direction orthogonal to the filter axial direction X and a length of 20mm in the filter axial direction X. Next, a test piece was stored in the measurement chamber of the mercury porosimeter, and the pressure in the measurement chamber was reduced. Thereafter, mercury is introduced into the measurement chamber and pressurized, and the pore diameter and pore volume are measured from the pressure at the time of pressurization and the volume of mercury introduced into the pores 110 of the partition wall 11 of the test piece. The pressure is measured in the range of 0.5 to 20000 psia. In addition, 0.5psia corresponds to 0.35X 10^-3kg/mm²20000psia corresponds to 14kg/mm². The pore diameter corresponding to the pressure range is in the range of 0.01 to 420 μm. As constants for calculating the pore diameter from the pressure, the contact angle was 140 ℃ and the surface tension was 480 dyn/cm. The average pore diameter is the pore diameter distribution of the partition wall 11The middle cumulative pore volume is a pore diameter of 50% (pore diameter at which the cumulative value of pore volumes is 50%).

In the exhaust gas purifying filter 1, the ratio of the average surface opening diameter of the air holes 110 on the surface of the partition wall 11 to the average air hole diameter of the partition wall 11 is set to 0.66 to 0.94. The average surface opening diameter of the pores 110 on the surface of the partition wall 11 is measured as follows.

Surface openings 113 formed by the air holes 110 are formed in the surface of the partition wall 11 on the side where the exhaust gas G flows in and the surface of the partition wall 11 on the side where the exhaust gas G flows out. Here, a Scanning Electron Microscope (SEM) is used to obtain a reflected electron image of the surface of the partition wall 11 on the side where the exhaust gas G flows (i.e., the surface of the partition wall 11 facing the 1 st chamber 121). However, the surface of the partition wall 11 is removed from the portion where the seal portion 13 exists. In this case, the acceleration voltage can be set to 10kV and the magnification can be set to 300 times. Fig. 4 shows an example of a reflected electron image of the surface of the partition wall 11. In the reflected electron image of fig. 4, the black area is the surface opening 113 of the surface of the partition wall 11, and the light gray area is the skeleton portion 114 of the surface of the partition wall 11. Next, the photographic image was subjected to binarization processing using image analysis software (WinROOF, manufactured by sango corporation). The binarization process is performed to distinguish the surface openings 113 on the surface of the partition 11 from the skeleton portions 114 on the surface of the partition 11. Since the surface openings 113 and the skeleton portions 114 have different luminances from each other, the binarization processing is performed after removing noise remaining in the captured image and setting an arbitrary threshold value. Since the threshold value differs depending on the photographed image, a threshold value capable of separating the surface opening 113 and the skeleton portion 114 is set for each photographed image while visually checking the photographed image. Fig. 5 shows an example of a binarized image. In the binarized image of fig. 5, light gray areas are surface openings 113 on the surface of the partition walls 11, and black areas are skeleton portions 114 on the surface of the partition walls 11. For the surface openings 113 in the obtained binarized image, the equivalent circle diameter, which is the diameter of a perfect circle having the same area as the area of the surface opening 113, is calculated for each surface opening 113, and all the calculated equivalent circle diameters are integrated and divided by the number of surface openings 113 to obtain the surface opening diameter. As described above, the average value of the open pore diameters of the respective surfaces obtained from the binarized images obtained for the arbitrary 5 different portions of the surface of the partition wall 11 is set as the average open pore diameter of the pores 110 on the surface of the partition wall 11. Further, by dividing the average surface opening diameter of the air holes 110 on the surface of the partition wall 11 by the average air hole diameter of the partition wall 11, the ratio of the average surface opening diameter of the air holes 110 on the surface of the partition wall 11 to the average air hole diameter of the partition wall 11 can be obtained. That is, the ratio of the average surface opening diameter of the pores 110 on the surface of the partition wall 11 to the average pore diameter of the partition wall 11 can be determined by the equation of (average surface opening diameter of the pores 110 on the surface of the partition wall 11)/(average pore diameter of the partition wall 11).

In the exhaust gas purifying filter 1, the average pore diameter of the partition wall 11 and the ratio of the average surface opening diameter of the pores 110 on the surface of the partition wall 11 to the average pore diameter of the partition wall 11, which are defined as described above, are within the above-specified ranges. This enables the exhaust gas purifying filter 1 to achieve initial pressure loss reduction, initial PM collection rate maintenance, and suppression of pressure loss increase after ash deposition. Hereinafter, a mechanism by which such an effect can be obtained will be described with reference to fig. 6 to 12.

As shown in fig. 6, the PM2 contains a soluble organic component (SOF)22 and Ash (Ash)23 derived from engine oil or the like in addition to solid carbon (soot) 21 as a main component. As shown in fig. 7, the PM2 is trapped while passing through the air holes 110 in the partition wall 11. In addition, the arrows in fig. 7 indicate the flow of the exhaust gas G flowing through the air holes 110. In the case where the PM2 containing the ash 23 is collected by the partition wall 11 in which the average pore diameter of the partition wall 11 and the ratio of the average surface opening diameter of the pores 110 on the surface of the partition wall 11 to the average pore diameter of the partition wall 11 are within the above-described specific range, as shown in fig. 8, the PM2 segregates in the vicinity of the surface opening 113 formed on the surface of the partition wall 11 on the side where the exhaust gas G flows in. Further, as shown in fig. 9, ash 23 contained in PM2 remains after PM2 is regenerated. As shown in fig. 10, when the partition wall 11 traps the PM2 containing the ash 23 again in the state where the remaining ash 23 is present, the PM2 segregates in the vicinity of the surface opening 113 of the surface of the partition wall 11 and also accumulates in the vicinity of the remaining ash 23. As shown in fig. 11, by repeating the deposition of PM2 containing ash 23 and the regeneration treatment of PM2, the surface openings 113 on the surface of the partition wall 11 are crosslinked by the ash 23. Thereafter, as illustrated in fig. 12, the ash 23 crosslinked at the surface opening 113 is peeled off by the flow of the exhaust gas G at the time of regeneration of the PM2, and the peeled ash 23 is conveyed toward the gas outflow-side seal portion 13. The transported ash 23 is accumulated in the downstream-most portion 10 of the filter (also referred to as a filter bottom portion, see fig. 3). The ash 23 covering the outer surface of the partition wall 11 increases the pressure loss, but the ash 23 deposited on the downstream-most portion 10 of the filter hardly increases the pressure loss. That is, by using the partition wall 11 in which the average pore diameter of the partition wall 11 and the ratio of the average surface opening diameter of the pores 110 on the surface of the partition wall 11 to the average pore diameter of the partition wall 11 are in the above-described specific range, the cross-linking of the ash 23 occurs at the surface opening 113 of the partition wall 11, and the separation of the ash 23 is easily caused. As a result, ash 23 deposited on downstream-most portion 10 of the filter can be increased as compared with ash 23 covering the surface of partition wall 11. If the ash 23 deposited on the downstream-most portion 10 of the filter increases, the gas permeability of the partition wall 11 after deposition of the ash 23 also increases, resulting in a low pressure loss. From the above mechanism, it is considered that not only the initial pressure loss reduction of the exhaust gas purification filter 1 and the initial PM collection rate can be achieved at the same time, but also the increase in pressure loss after the accumulation of the ash 23 can be suppressed.

When the average pore diameter of the partition wall 11 is 16 μm or less, the initial effect of reducing the pressure loss is small. On the other hand, when the average pore diameter of the partition wall 11 is 21 μm or more, the initial PM trapping performance is lowered, and the pressure loss after the deposition of the ash 23 is also likely to increase. Further, when the ratio of the average surface opening diameter of the air holes 110 on the surface of the partition wall 11 to the average air hole diameter of the partition wall 11 is less than 0.66, the initial pressure loss reduction effect becomes weak. On the other hand, when the ratio of the average surface opening diameter of the pores 110 on the surface of the partition wall 11 to the average pore diameter of the partition wall 11 exceeds 0.94, PM2 containing ash 23 is easily accumulated inside the partition wall 11 and ash 23 is difficult to be peeled off, and the pressure loss due to accumulation of residual ash 23 increases due to long-term use of the vehicle.

In the exhaust gas purifying filter 1, the proportion of the surface opening diameter of 25 μm or more on the surface of the partition wall 11 on the side into which the exhaust gas G flows can be set to 20% or less. According to this configuration, PM2 containing ash 23 is less likely to accumulate inside partition wall 11, and the increase in pressure loss due to accumulation of residual ash 23 over the years of use of the vehicle is more likely to be suppressed. From the viewpoint of initial pressure loss reduction or the like, for example, the proportion of the surface opening diameter of 25 μm or more can be set to 10% or more.

By calculating the frequency from the surface opening diameters obtained from all the binarized images, the existence ratio of the surface opening diameters of 25 μm or more on the surface of the partition wall 11 can be grasped. Specifically, the presence ratio Ab of the surface open pore diameter of 25 μm or more is calculated from the frequency of the surface open pore diameters of the pores 110 obtained from the binarized image of the surface of the partition wall 11, and the average value of the presence ratios Ab of the surface open pore diameters of 25 μm or more obtained from the binarized images obtained for arbitrary 5 different portions of the surface of the partition wall 11 is set as the presence ratio Ab of the surface open pore diameter of 25 μm or more on the surface of the partition wall 11.

In the exhaust gas purifying filter 1, the surface opening ratio of the pores 110 on the surface of the partition wall 11 on the side where the exhaust gas G flows in can be set to 30% to 40%. By setting the surface opening ratio of the pores 110 on the surface of the partition wall 11 to 30% or more, the initial pressure loss reduction effect is easily obtained. Further, by setting the surface opening ratio of the pores 110 on the surface of the partition wall 11 to 40% or less, the PM2 containing the ash 23 is less likely to accumulate inside the partition wall 11, and the increase in pressure loss after the accumulation of the ash 23 can be suppressed.

The surface aperture ratio of the pores 110 on the surface of the partition wall 11 can be calculated by an equation of 100 × (the total value of the areas of the surface openings 113 in all the binarized images)/(the total value of the areas of all the binarized images). The entire binarized image is the binarized image of the 5 regions.

In the exhaust gas purifying filter 1, the porosity of the partition wall 11 can be set to 60% to 70%. By setting the porosity of the partition wall 11 to 60% or more, the initial pressure loss is easily reduced and the PM trapping performance is easily improved. Further, by setting the porosity of the partition walls 11 to 70% or less, the strength of the exhaust gas purification filter 1 itself is easily ensured, and cracks due to stress at the time of packaging and heat generation at the time of regeneration treatment of the PM2 are easily suppressed. The porosity of the partition wall 11 is preferably 62% or more, and more preferably 63% or more, from the viewpoints of initial pressure loss reduction, improvement in PM trapping performance, and the like. In addition, from the viewpoint of improving the strength of the exhaust gas purification filter 1, the porosity of the partition walls 11 is preferably 68% or less, more preferably 67% or less, and still more preferably 66% or less. These upper and lower limits can be arbitrarily combined.

The porosity of the partition wall 11 was measured by a mercury porosimeter using the principle of the mercury intrusion method. Specifically, the porosity of the partition wall 11 can be calculated from the following relational expression.

The porosity (%) of the partition wall 11 is equal to the total volume of pores/(total volume of pores + 1/true specific gravity of the partition wall material) × 100

In the case where the material of the partition wall is cordierite, 2.52 can be used as the true specific gravity of cordierite.

The exhaust gas purification filter 1 can be configured so that the filter end face 15 on the side from which exhaust gas flows is at the position X of 10mm in a state where ash 23 of 20g/L to 40g/L is accumulated₁₀The gas permeability coefficient of the partition wall 11 at (see FIG. 2) is k₁₀A central position X between the filter end face 15 on the exhaust gas inflow side and the filter end face 16 on the exhaust gas outflow side_CThe gas permeability coefficient of the partition wall at (see FIG. 2) is k_cThe gas permeability coefficient ratio k_c/k₁₀The value of (A) is 1.5 or less. According to this configuration, the pressure loss increase after ash deposition can be reliably suppressed.

In the above, when the accumulation amount of the ash 23 is less thanAt a position X of 10mm from the filter end face 15 on the exhaust gas inflow side at 20g/L₁₀And a central position X between the filter end face 15 on the exhaust gas inflow side and the filter end face 16 on the exhaust gas outflow side_CSince no ash 23 or a very small amount of ash 23 is deposited outside partition wall 11, it is difficult to know the effect of stripping ash 23. Therefore, in the above description, the deposition amount of the ash 23 is set to 20g/L or more. On the other hand, if the deposition amount of the ash 23 exceeds 40g/L, the ash 23 which is separated and deposited on the downstream-most portion 10 of the filter increases, and the influence thereof is considered to be exerted on the central position X between the filter end face 15 on the exhaust gas inflow side and the filter end face 16 on the exhaust gas outflow side_CIt is difficult to know the effect of the separation of ash 23. Therefore, in the above description, the deposition amount of ash 23 is set to 40g/L or less.

k_c/k₁₀Is a gas permeability coefficient k_cAnd gas permeability coefficient k₁₀An index of the magnitude relationship between them. The more the ash 23 is accumulated, the smaller the value of the gas permeability coefficient of the partition wall 11 becomes. Further, the flow velocity of the exhaust gas G introduced into the chamber 12 of the exhaust gas purification filter 1 is at a position X10 mm away from the filter end face 15 on the exhaust gas inflow side₁₀Is located at a central position X between the filter end face 15 on the exhaust gas inflow side and the filter end face 16 on the exhaust gas outflow side_CThe treatment is quick. Namely, the following relationship is obtained: position X of 10mm from the filter end face 15 on the exhaust gas inflow side₁₀The amount of ash 23 deposited is larger than the center position X between the filter end face 15 on the exhaust gas inflow side and the filter end face 16 on the exhaust gas outflow side_CThe accumulated amount of ash 23 is large. When the gas permeability coefficient is replaced with the gas permeability coefficient, the gas permeability coefficient k after deposition of the ash 23₁₀And gas permeability coefficient k_cThe relationship between the gas permeability coefficient k_cGreater than the gas transmission coefficient k₁₀. That is, in the conventional exhaust gas purifying filter not having the configuration of the present invention, k is usually used_c/k₁₀The value of (c) becomes large. Specifically, when k is_c/k₁₀When the value of (3) exceeds 1.5, the rate of increase in pressure loss after deposition of the ash 23 from the initial pressure loss increases, andthis is not preferred, and will be described later in the experimental examples. This is considered to be because the position X is 10mm from the filter end face 15 on the exhaust gas inflow side₁₀Ash 23 on the surface of the partition wall 11 is not peeled off and accumulated in the air hole 110. Therefore, in order to suppress the rate of increase in pressure loss, k_c/k₁₀The value of (d) is preferably 1.5 or less. This is considered to be because the position X is 10mm from the filter end face 15 on the exhaust gas inflow side₁₀The stripping of the ash 23 from the surface of the peripheral partition wall 11 is promoted. However, not all of the ash 23 is separated and transported to the vicinity of the seal portion 13 of the downstream-most portion 10 of the filter and accumulated. A part of ash 23 remains in partition wall 11 in the vicinity of the surface of partition wall 11. As a result of the above, at k_c/k₁₀When the value of (d) is within the range of 1.5 or less, the rate of increase of the pressure loss after deposition of the ash 23 with respect to the initial pressure loss can be suppressed to be low.

Further, each gas permeability coefficient k₁₀、k_cThe measurement was performed as follows. First, 20g/L to 40g/L of ash 23 is accumulated in the exhaust gas purifying filter 1. The accumulation of ash 23 can be performed by operating a gasoline engine using gasoline containing 2% of ash derived from engine oil and accumulating the ash on the exhaust gas purification filter 1 mounted in the exhaust passage. Specifically, (1) PM2 is deposited under the conditions of the center temperature of the exhaust gas purification filter 1 of 800 ℃ for 9 minutes in an ideal stoichiometric atmosphere, and (2) PM2 is regenerated under the conditions of the center temperature of the exhaust gas purification filter 1 of 800 ℃ to 900 ℃ for 1 minute in an atmospheric atmosphere. By repeating the deposition of PM2 in the above (1) and the regeneration process of PM2 in the above (2), ash 23 is deposited on the exhaust gas purifying filter 1. By taking out the exhaust gas purifying filter 1 appropriately and measuring the weight, the amount of ash 23 accumulated can be grasped.

Then, with respect to the exhaust gas purifying filter 1 on which the predetermined amount of ash 23 is accumulated, the position X of 10mm from the filter end face 15 on the exhaust gas inflow side is set₁₀And a central position X between the filter end face 15 on the exhaust gas inflow side and the filter end face 16 on the exhaust gas outflow side_CSeparately digging out a measurement sample not including the seal part 13And (5) sampling. At this time, the sampling position X is set such that the position 10mm away from the filter end face 15 on the exhaust gas inflow side becomes the upstream end face₁₀The measurement sample (2). On the other hand, with the central position X_CA central position X is collected to form an upstream side end face_CThe measurement sample (2). Each measurement sample had a cylindrical shape with a diameter of 30mm and a length of 25mm in the axial direction of the filter. The shell portion 14 of each of the dug measurement samples can be formed by, for example, bonding.

Next, as illustrated in fig. 13, polyester tapes 315a and 316a are respectively bonded to both end surfaces 315 and 316 of the measurement sample 3 in the filter axial direction X. Next, the polyester tapes 315a, 316a are locally disappeared by, for example, an iron or the like, so that the alternating seal portions 13 are formed by the polyester tapes 315a, 316 a. As illustrated in fig. 14 (a), in the upstream end surface 315 which is the filter end surface on the exhaust gas inflow side of the measurement sample 3, for example, 13 chambers 12 are opened, and the remaining chambers 12 are closed by the sealing portions 13 made of the polyester tape 315 a. On the other hand, as illustrated in fig. 14 (b), in the downstream end face 316, which is the filter end face on the exhaust gas outflow side of the measurement sample 3, for example, 24 chambers 12 are opened, and the remaining chambers 12 are closed by the sealing portions 13 made of the polyester tapes 316 a. That is, the sealing portion 13 made of the polyester tapes 315a and 316a is formed instead of the sealing portion 13 made of ceramic. Although the measurement sample 3 in which the sealing portion 13 is formed by the polyester tapes 315a and 316a is described here in the measurement of the gas permeability coefficient, the same result can be obtained by using the measurement sample 3 in which the ceramic sealing portion 13 is formed.

Next, as illustrated in fig. 13, the gas is flowed from the upstream end face 315 of the measurement sample 3 toward the downstream end face 316 of the measurement sample 3, and the relationship between the gas flow rate and the pressure loss is measured by the palm porosimeter 4. Specifically, the pressure loss was measured when the gas flow rate was changed. In addition, the arrows in fig. 13 indicate the flow of gas. Then, a graph of the relationship between the gas flow rate (X-axis) and the pressure loss (Y-axis) was obtained. Fig. 15 shows an example of a graph of the relationship between the gas flow rate (X-axis) and the pressure loss (Y-axis). This relational diagram shows an actual measurement value (plotted point) of the palm hole ratio meter 4 and a calculated value (broken line) obtained by the following expressions (i) to (viii). The following describes equations (i) to (viii).

Pressure loss Δ P (unit: Pa) of the exhaust gas purifying filter 1 and condensation pressure loss Δ P when gas flows into the chamber 12_inletWith expanding pressure loss deltap of gas as it exits from the chamber 12_exitSum of Δ P_inlet/exit(unit: Pa) and the pressure loss Δ P of the gas passing through the chamber 12_channel(unit: Pa) and the pressure loss Δ P of the partition wall 11 when the gas passes therethrough_wAll(unit: Pa), satisfies the following relation of formula (i).

ΔP＝ΔP_inlet/exit+ΔP_channel+ΔP_wall……(i)

Further, Δ P_inlet/exitThe opening area A of the chamber 12_open(unit: m)²) The opening area A of the chamber 12 in the filter end face 15 on the exhaust gas inflow side_in(unit: m)²) The gas flow velocity V in the chamber 12_channel(unit: m/s) and air density ρ (unit: kg/m)³) And satisfies the following expression (ii).

[ numerical formula 1]

Further, Δ P_channel+ΔP_wallGas permeability coefficient k (unit: m)²) Length L of filter axial direction X of exhaust gas purifying filter 1 (unit: m), the hydraulic diameter a1 of the chamber 12 (unit: m), the thickness w of the partition wall 11 (unit: m), the coefficient of friction F (unit: dimensionless), reynolds number (unit: dimensionless), gas viscosity μ (unit: pa · s) and the gas flow velocity V in the chamber 12_channel(unit: m/s) satisfying the following formulae (iii) to (viii). In the formula (iii), e is an exponential function exp.

[ numerical formula 2]

[ numerical formula 3]

[ numerical formula 4]

[ numerical formula 5]

[ numerical formula 6]

[ number formula 7]

The pressure loss value is calculated based on the above equations (i) to (viii). The broken line based on the calculated values shown in the graph of the relationship between the gas flow rate (X axis) and the pressure loss (Y axis) illustrated in fig. 15 is a pressure loss value obtained by calculation. As understood from the expressions (i) to (viii), the filter length L and the opening area A of the chamber were measured in addition to the gas permeability coefficient k_openHydraulic diameter a₁And the thickness w of the partition wall 11, and the pressure loss values are calculated from these values, and these values are not changed even if the gas flow rate is changed. Therefore, by inputting an arbitrary value to the gas permeability coefficient, a calculated value in the relational graph between the gas flow rate (X axis) and the pressure loss (Y axis) can be derived.

For example, if the input gas permeability coefficient is large, the pressure loss value is lower than the measured value, and the calculated value is lower than the measured valueAnd (6) measuring. On the other hand, if the input gas permeability coefficient is small, the calculated value exceeds the actual measurement value. Therefore, in order to approximate the calculated value to the closest actual measurement value, the gas permeability coefficient k is calculated by the least square method so that the difference between the calculated value and the actual measurement value becomes the smallest. This calculated value becomes a gas permeability coefficient k. That is, the gas permeability coefficient k is a value obtained by back-calculating the gas permeability coefficient from the measured values of the pressure loss measured by the palm porosity meter by equations (i) to (viii). As described above, the gas permeability coefficient k at a predetermined position in a state where a predetermined amount of ash is deposited can be obtained₁₀、k_c。

The exhaust gas purification filter 1 described above can exhibit a sufficient effect when used in a state in which the partition walls 11 carry the catalyst, but can also be used in a state in which the partition walls 11 do not carry the catalyst. When the partition wall 11 carries a catalyst, the amount of the catalyst can be, for example, 30g/L to 150 g/L. In general, when a catalyst is carried, a part of the pores 110 in the partition wall 11 is blocked by the catalyst. Further, there are various methods for supporting the catalyst, for example, there are a case where the catalyst is uniformly supported in the partition walls 11, a case where a large amount of the catalyst is supported on the surface layers of the partition walls 11, and the like. In the present invention, it is important to have a surface opening diameter on the surface of the partition wall 11, and in view of this, a method of uniformly supporting the catalyst in the partition wall 11 is preferable. In the method of uniformly supporting the catalyst in the partition wall 11, the ratio of the average surface opening diameter of the pores 110 on the surface of the partition wall 11 to the average pore diameter of the partition wall 11 does not change with respect to the case where the catalyst is not supported. This tends to reduce the PM collection rate and increase the initial pressure loss, but the effect of the pressure loss after the deposition of the ash 23 is not changed.

< Experimental example >

The exhaust gas purifying filters of the examples and comparative examples will be described. In the present experimental example, each exhaust gas purifying filter will have a structure containing SiO₂: 45 to 55 mass% of Al₂O₃: 33 to 42 mass%, MgO: 12 to 18 mass% of cordierite having a chemical composition as a main component. The main component of cordierite means that 50 mass% or more of cordierite is cordierite. Therefore, in the production of each exhaust gas purifying filter of the experimental example, cordierite forming raw materials including an Si source, an Al source, and an Mg source were used so as to produce cordierite by firing.

Production of an exhaust gas purification filter

(example 1)

In the production of the exhaust gas purifying filter of example 1, a cordierite-forming raw material was prepared by blending porous silica (Si source), talc (Mg source) and aluminum hydroxide (Al source) so as to have a blending ratio (mass%) shown in table 1.

The porous silica used had a bulk density of 0.22g/cm³. For measuring the bulk density, Tap Densa manufactured by SEISHIN corporation, which is a Tap density method flow adhesion tester, was used. Specifically, after the cylinder of the measuring instrument is filled with silicon oxide, the silicon oxide is compressed by tapping, and the bulk density is calculated from the mass of the silicon oxide in a compressed state and the volume of the cylinder. Further, as the aluminum hydroxide, aluminum hydroxide having an average particle diameter of 3 μm and aluminum hydroxide having an average particle diameter of 8 μm were used in combination. The "average particle diameter" refers to a particle diameter at which a volume accumulation value in a particle size distribution obtained by a laser diffraction scattering method is 50%.

To cordierite-forming raw materials, water (solvent), methylcellulose (binder) and a dispersant were added in such a proportion (mass%) as shown in table 1, and the mixture was mixed with a mixer to produce raw material soils containing cordierite-forming raw materials. The dispersant is mainly used for suppressing aggregation of particles and improving dispersibility, and specifically, polyoxyethylene polyoxypropylene glyceryl ether having an average molecular weight of 4550 is used.

[ Table 1]

Here, in the raw material system using porous silica as in example 1, since there are many gaps between particles, a large amount of solvent (here, water) is required for the raw material to be converted into soil. In this way, in order to improve the dispersibility when the amount of the solvent is large, it is effective to enhance the stirring at the time of mixing the raw material soil. However, it is difficult to directly confirm whether or not the aggregation of the particles is suppressed by the stirring and whether or not the particles are dispersed in the raw material soil.

Therefore, in the present experimental example, the raw material soil density deviation ratio was introduced as an index of new particle dispersibility in the raw material soil after mixing. Specifically, the raw soil before extrusion molding was performed by the die was taken out, and 8 raw soil sites were randomly collected. The collected raw material soil was put into a measuring device having a diameter of 25mm and a length of 20mm of a pressure measuring device "Autograph" manufactured by Shimadzu corporation, compressed under a pressure of 1kN, and the density of the raw material soil was calculated from the volume and weight of the taken-out raw material soil. The average value of the raw material soil densities calculated for the raw material soils of 8 sites was taken as the raw material soil density obtained by actual measurement. In contrast, the raw material soil density calculated in advance from the blending ratio of the raw materials was defined as the raw material soil density obtained by the calculation. The particle dispersibility can be determined by confirming the difference (deviation ratio) between the raw material soil density obtained by the actual measurement and the raw material soil density obtained by the calculation. The lower the raw material soil density obtained by actual measurement is, the less the wettability of the dispersant is, and the more air is present on the particle surface, so that the particle dispersibility is deteriorated. On the other hand, the closer the raw material soil density obtained by actual measurement is to the value of the raw material soil density obtained by calculation, the better the particle dispersibility becomes.

In example 1, a raw material soil was used in which the speed of the agitator and the number of times the raw material soil was repeatedly passed through the agitator were arbitrarily changed to adjust the following raw material soil density deviation rate to less than 10%. Further, when the speed of the mixer is increased, the raw soil density deviation rate tends to be decreased. Further, when the number of times the raw material soil repeatedly passes through the agitator increases, the raw material soil density deviation rate tends to decrease.

Raw material soil density deviation (%) of 100 × { (raw material soil density by calculation) - (raw material soil density by actual measurement) }/(raw material soil density by calculation)

The raw material soil adjusted as described above is formed into a honeycomb shape by extrusion molding. The molded article is dried and then cut into a predetermined length.

Then, the molded body was fired at 1430 ℃ to obtain a honeycomb-structured sintered body.

Next, the exhaust gas inflow end face and the exhaust gas outflow end face of the chamber were alternately filled with a slurry containing the same kind of ceramic material as the sintered body of the honeycomb structure by a dipping method and fired, thereby forming a sealing portion.

The exhaust gas purifying filter of example 1 was manufactured as described above.

(examples 2 to 8)

In example 1, the average particle diameter and the bulk density of the porous silica in the cordierite-forming raw material were changed. The larger the average particle diameter of the porous silica is, the larger the pore diameter of the formed partition wall is, and the smaller the bulk density of the porous silica is, the higher the porosity of the formed partition wall is. When the proportion of large particle diameter is increased in the formulation of aluminum hydroxide having different particle diameters, the surface opening ratio and the average surface opening diameter become large. Further, when the temperature rise rate at the time of firing is increased to 1200 ℃ to 1430 ℃, the pore diameter and the average surface opening diameter can be increased. These conditions were combined to produce the product. The total mixing ratio of aluminum hydroxide was the same as in example 1, and the mixing ratio of aluminum hydroxide having different particle diameters was changed, and the mixing ratio of the other raw material soil was the same as in example 1. Then, the exhaust gas purifying filters of examples 2 to 8 were produced by changing the temperature rising rate between 1200 ℃ and 1430 ℃ during firing using raw soil in which the raw soil density deviation rate was adjusted to less than 10% by optionally changing the speed of the stirrer and the number of times the raw soil was repeatedly passed through the stirrer, as in example 1.

(example 9)

In example 1, the exhaust gas purifying filter of example 9 was manufactured by using raw soil in which the speed of the agitator and the number of times the raw soil was repeatedly passed through the agitator were arbitrarily changed to adjust the raw soil density deviation rate to 10% or more.

(example 10, example 11)

In example 9, the average particle diameter and the bulk density of the porous silica in the cordierite-forming raw material were changed. Except for this, as in example 9, the raw material soil was adjusted so that the raw material soil density deviation rate became 10% or more, thereby producing the exhaust gas purifying filters of examples 10 and 11.

Comparative example 1

In the production of the exhaust gas purifying filter of comparative example 1, fused silica (Si source), talc (Mg source), and aluminum hydroxide (Al source) were blended so as to have the blending ratios (mass%) shown in table 2, thereby preparing cordierite-forming raw materials. Further, the fused silica used had a bulk density of 1.35g/cm³。

To cordierite-forming raw materials, water (solvent), methylcellulose (binder), lubricating oil, and graphite were added in the mixing ratios shown in table 2, and mixed by a mixer, thereby producing raw material soils containing cordierite-forming raw materials. The purpose of the lubricating oil is to accelerate the sliding between the raw soil and the molding machine and the metal part on the surface of the mold to accelerate the molding speed. As the lubricating oil, rapeseed oil, which is a vegetable oil, was used. Further, the raw material soil density deviation rate was adjusted to be less than 10%. Using the raw soil adjusted as described above, an exhaust gas purifying filter of comparative example 1 was produced in the same manner as in example 1.

[ Table 2]

Comparative examples 2 to 6

In comparative example 1, the average particle diameters of fused silica and talc in the cordierite-forming raw material and the blending ratio of graphite were changed. Except for this, in the same manner as in comparative example 1, the raw material soil was adjusted so that the raw material soil density deviation rate was less than 10%, and the exhaust gas purifying filters of comparative examples 2 to 6 were manufactured.

Determination of the characteristics of the partition wall

Partition wall characteristics were measured for the exhaust gas purifying filters of examples and comparative examples. Specifically, the porosity of the partition wall and the average pore diameter of the partition wall were measured by the above-described measurement methods. In this case, AutoPorIV9500 manufactured by shimadzu corporation was used as the mercury porosimeter. Further, the ratio of the average surface opening diameter of the pores on the surface of the partition wall to the average pore diameter of the partition wall, the existence ratio of the surface opening diameter of 25 μm or more on the surface of the partition wall, and the surface opening ratio of the pores on the surface of the partition wall were measured according to the above-mentioned measuring methods. In this case, Quanta250FEG manufactured by FEI corporation was used as the SEM. As image analysis software, WinROOF ver.7.4 manufactured by sango corporation was used. Further, according to the above measurement method, the gas permeability coefficient ratio k was measured_c/k₁₀The value of (c). In this case, CEP-1100AXSHJ manufactured by the company of Porous Materials was used as a palm porosimeter.

Evaluation-

For each exhaust gas purification filter, the initial PM trapping rate, the initial pressure loss, and the pressure loss after ash deposition were measured. In addition, regarding the initial PM collection rate, the initial pressure loss, and the pressure loss after ash deposition, the one having an outer shape of(filter diameter) × L120mm (filter length), thickness of partition wall of 8.5mil, and number of cells of 300 cpsi.

(initial PM trapping Rate, initial pressure loss)

The initial PM collection rate is measured as follows. The manufactured exhaust gas purification filter was attached to the exhaust pipe of a gasoline direct injection engine, and exhaust gas containing PM flowed through the exhaust gas purification filter. At this timeThe number of PM in the exhaust gas before flowing into the exhaust gas purifying filter, that is, N is measured_inN, the number of PM in the exhaust gas flowing out of the exhaust gas purification filter_outThrough 100 × (N)_in-N_out)/N_inThe initial PM trapping rate is calculated by the equation (1). At this time, the measurement conditions were that the temperature was 450 ℃ and the flow rate of the exhaust gas was 2.8m³In terms of a/minute. For the measurement of the number of PM, a PM particle number counter "AVL-489" manufactured by AVL corporation was used. On the other hand, the initial pressure loss was measured as follows. The pressure before (upstream) the exhaust gas purification filter and the pressure after (downstream) the exhaust gas purification filter are measured by the pressure sensor while the initial PM trapping rate is measured, and the difference is defined as the initial pressure loss. At this time, the measurement conditions were that the temperature was 720 ℃ and the exhaust gas flow rate was 11.0m³In terms of a/minute. In each measurement, each exhaust gas purification filter in an initial state in which PM is not deposited and which is not coated with a catalyst is used.

In the present experimental example, in the case where the initial PM trapping rate is 70% or more, the initial PM trapping rate is assumed to be sufficiently secured and is "a". When the initial PM collection rate is 60% or more and less than 70%, the initial PM collection rate is assumed to be "B". When the initial PM collection rate is less than 60%, the initial PM collection rate is not secured and is set to "C". In the case where the initial pressure loss is 6kPa or less, the initial pressure loss reduction effect is sufficiently obtained and is referred to as "a". In the case where the initial pressure loss exceeds 6kPa and is not more than 7kPa, "B" is assumed to obtain the initial pressure loss reduction effect. When the initial pressure loss exceeds 7kPa, the initial pressure loss reduction effect is not obtained, and "C" is assumed.

(pressure loss after deposition of Ash)

In the initial state where no PM is deposited and in each exhaust gas purification filter to which no catalyst is applied, ash is deposited in an amount of 20g/L to 40 g/L. The accumulation of ash is performed by operating a gasoline engine using gasoline containing 2% of ash derived from engine oil and accumulating the ash on an exhaust gas purification filter mounted in an exhaust passage. Specifically, the exhaust gas is purified by repeating (1) the steps ofDeposition of PM such as PM at a center temperature of the filter of 800 ℃ for 9 minutes, and (2) PM regeneration treatment such as regeneration treatment of PM at a center temperature of the exhaust gas purification filter of 800 ℃ to 900 ℃ for 1 minute in an atmospheric atmosphere, thereby depositing ash in the exhaust gas purification filter. At this time, the amount of ash accumulated is grasped by appropriately taking out the exhaust gas purifying filter and performing weight measurement. Then, similarly to the initial pressure loss, the pressure before the exhaust gas purification filter and the pressure after the exhaust gas purification filter are measured by the pressure sensor, and the difference is defined as the pressure loss after ash deposition. In the present experimental example, in the case where the pressure loss after deposition of ash at the time of deposition of 30g/L of ash is 13kPa or less, the effect of suppressing the increase in pressure loss after deposition of ash is sufficiently obtained and is referred to as "a". Similarly, when the pressure loss after the deposition of ash exceeds 13kPa and 15kPa or less, the effect of suppressing the increase in pressure loss after the deposition of ash is assumed to be "B". In the case where the pressure loss after the deposition of ash exceeds 15kPa, the effect of suppressing the increase in pressure loss after the deposition of ash cannot be obtained, and "C" is assumed. When the initial pressure loss is P_freshAnd the pressure loss after ash deposition is P_ash-loadedWhen passing 100 × (P)_ash-loaded-P_fresh)/P_freshThe rate of increase of the pressure loss after ash deposition to the initial pressure loss was determined by the following equation. FIG. 16 shows the gas transmission coefficient ratio k_c/k₁₀And rate of rise in pressure loss. Fig. 16 shows the results of the exhaust gas purifying filter of example 1 as a representative example of each example.

The results of the above experiments are summarized in table 3.

As shown in table 3, it was confirmed that: the exhaust gas purifying filters of examples 1 to 11, in which the average pore diameter of the partition wall and the ratio of the average surface opening diameter of the pores on the surface of the partition wall to the average pore diameter of the partition wall are within the specific range specified in the present invention, can achieve the initial reduction in pressure loss, the maintenance of the initial PM collection rate, and the suppression of the increase in pressure loss after ash deposition.

On the other hand, the exhaust gas purifying filters of comparative examples 1 to 6, in which the ratio of the average pore diameter of the partition wall and the average surface opening diameter of the pores on the surface of the partition wall to the average pore diameter of the partition wall is outside the specific range specified in the present invention, could not achieve any of the initial pressure loss reduction, the initial PM collection rate retention, and the suppression of the pressure loss increase after the ash deposition.

When the exhaust gas purifying filters of examples 1 to 8 and the exhaust gas purifying filters of examples 9 to 10 are compared, it is understood that when the proportion of the surface opening diameter of 25 μm or more on the surface of the partition wall is 20% or less, PM containing ash is less likely to be deposited inside the partition wall, and the increase in pressure loss due to the deposition of residual ash is easily suppressed when the vehicle is used for a long period of time. Further, it is found that the initial pressure loss reduction effect is easily obtained by setting the surface opening ratio of the pores on the surface of the partition wall to 30% or more. It is found that by setting the surface opening ratio of the pores on the surface of the partition wall to 40% or less, PM containing ash is less likely to accumulate inside the partition wall, and the increase in pressure loss after ash accumulation can be suppressed. Further, when the exhaust gas purifying filters of examples 1 to 8 and the exhaust gas purifying filter of example 11 were compared, it was found that the initial pressure loss reduction effect was easily obtained by setting the surface opening ratio of the pores on the partition wall surface to 30% or more. Further, it is found that by setting the surface opening ratio of the pores on the surface of the partition wall to 40% or less, PM containing ash is less likely to be deposited inside the partition wall, and the increase in pressure loss after deposition of ash can be suppressed.

The exhaust gas purifying filters of examples 1 to 9 had a gas permeability coefficient ratio k of 20g/L to 40g/L in the state of ash being deposited_c/k₁₀The value of (A) is 1.5 or less. With this configuration, as shown in fig. 16, it can be said that the pressure loss increase after ash deposition can be reliably suppressed.

The present invention is not limited to the above embodiments and experimental examples, and various modifications can be made without departing from the scope of the invention. The configurations shown in the embodiments and examples can be arbitrarily combined. That is, although the present invention has been described with reference to the embodiments, it should be understood that the present invention is not limited to the embodiments, structures, and the like. The present invention also includes various modifications and modifications within an equivalent range. Moreover, various combinations and modes, even including only one of the elements, and other combinations and modes above or below the elements, also fall within the scope and idea of the present invention.