METALLIC SINTERED BODY AND PROCESS FOR PREPARING THE SAME

Abstract: A metallic sintered body having fine pores of uniform size, e.g. pore size of 10 µm. or less and moreover having a high porosity, e.g. 50 % or more. The sintered body is useful as a filter for precisely filtering off fine contaminants, and the filter has a high filtration capacity and a long filtration life. The sintered body is prepared by compacting fine metallic particles surface-treated or admixed with a thermosetting resin by means of hot pressing and sintering the resulting compact. Since the thermosetting resin hardens upon hot pressing to give a compact having a good strength, the workability is good and a multiplicity of the compacts can be sintered at one time.

Description: METALLIC SINTERED BODY AND PROCESS FOR PREPARING THE SAME The present invention relates to a metallic sintered body, and more particularly to a metallic sintered body having very fine pores of uniform size and moreover having a high porosity and a process for the preparation t!itreof. A filter medium for collecting dust or for purifying a gas or liquid is desired to have fine pores of uniform size and high porosities from the viewpoint of low pressure loss, precise collection of fine contaminants, high filtration capacity and long life. Usually, a sintered body of fine metal particles has been employed as a filter medium for collecting dust or for purifying air or a liquid. The sintered body is superior in heat resistance and mechanical strength to a paper filter and a filter medium made from a woven synthetic fiber, but it is in general difficult to form fine pores of uniform size. Te sintered body of fine metal particles has been prepared by sintering the particles in a mold at a high temperature under a high pressure. In the so prepared sintered body, relatively large pores and fine pores are present together and the pore size distribution is wide. Also, in case of preparing the sintered body having fine pores, it is hard to obtain the sintered body having a high porosity. In order to solve these problems, there are proposed various processes, e.g. a process in which a felt of a metal fiber having an irregular polygonal shape in section is impregnated with fine metal particles and then sintered. However, these known processes are still unsatisfactory for providing a sintered body having fine and uniform pores. It is an object of the present invention to provide a metallic sintered body having fine and uniform pores and moreover having a high porosity. A further object of the present invention is to provide a metallic sintered body useful as a filter medium which is low in pressure loss and is long in life and can precisely catch fine contaminants with high capacity. Another object of the present invention is to provide a process for preparing a metallic sintered body having characteristics as a filter medium and a molecular sieve. A still further object of the present invention is to provide Åa process for preparing a metallic sintered body, which is good in workability and can provide a metallic sintered body having uniform and fine pores and a high porosity. These and other objects of the present invention will become apparent from the description hereinafter. It has now been found that the above-mentioned objects can be attained when a metallic sintered body is prepared, by compacting a metal powder with a thermosetting resin by means of hot pressing and sintering the resulting compact. In accordance with the present inventiort, there is provided a metallic sintered body which is composed of fine metallic particles and has a porosity of at least 45 a a ratio of the intersecting point pressure to the initial bubble point pressure of not more than 1.35 and a filtration size of not more than about 15 Fm. The sintered body of the present invention has fine and uniform pores and a high porosity, and accordingly is very useful as a filter which has a high filtration capacity and can precisely collect fine contaminants. The present invention also provides a process for preparing a metallic sintered body. The process not only can provide a metallic sintered body having fine pores of uniform size and a high porosity, but also has the advantage that the workability is good. Fig. 1 is a schematic section view showing one instance of hot press machines as used in the present invention; Figs. 2A to 2E are microphotographs of the sintered bodies of the present invention; Figs. 3A and 3B are microphotographs of an irregular metal powder which can be used in preparing the sintered body of the present invention; Figs. 4A and 4B are graphs showing the relation ship between the pressure loss and the flow rate of a fluid; Fig. 5 is a graph showing the relationship between the volume of a contaminated water passing through a sintered body and the time; and Fig. 6 is a graph showing the relationship between the retention rate and the diameter of particles. The fine metal particles used in the present invention are a metal powder and a fibrous metal powder, such as stainless steel, steel, copper, zinc or their alloys with other metals. Any known metal powders such as pulverized powder and atomized powder are employed in the present invention. In the present invention, a stainless steel powder, e.g. stainless steel atomized powder, is suitably employed from the viewpoint of corrosion resistance, etc. The particle size of the metal powder is not more than about 100 meshes (Tyler mesh, hereinafter the same), preferably not more than about 200 meshes. Particularly, when a filter of which the filtration size is not more than about 15 zm., especially not more than about 10 pm., is desired, the metal powder haVing a particle size of not more than about 200 meshes (about 74 pom.) is preferably employed. The term "filtration size" as used herein means the particle diameter that 98 % of particles can be caught. The shape of the metal powder is not particularly limited, and an irregular powder is also usable in the present invention. In case of employing the irregular powder, there can be obtained a filter having an excellent property of collecting solid or gel contaminants, because of having an increased specific surface area and pores of complicated shape. For instance, the irregular powder is prepared by a particular atomization in which a molten metal, the temperature of which is made as low as possible, is atomized into-a liquid atomization medium having a large cooling effect so that atomized molten metal droplets are solidified within the period that no surface tension acts on the droplets. The thus obtained irregular atomized powder is novel, and the packing density thereof falls within the range of 1 to 4. The packing density of a known atomized powder is more than 4.5. Photographs of an irregular powder enlarged 42 times and 90 times by a scanning electron microscope are shown in Figs. 3A and 3B, respectively. The fibrous powder used in the present invention is short metal fibers. A stainless steel short fiber having a diameter of 2 to 20 ym. and an aspect ratio of the length to the diameter in the range of 3 to 50 is preferably employed. The stainless steel short fiber is prepared by subjecting a stainless steel long fiber to a heat treatment for crystal grain growth adjustment and then subjecting the heat-treated fiber to intergranular selective corrosion in an aqueous acidic solution; As a stainless steel long fiber, there are employed any known fibers of stainless steels such as austenitic stainless steels, martensitic stainless steels and ferritic stainless steels, preferably austenitic stainless steels such as the so-called 18-8 type stainless steel of which the basic composition is a low content of carbon, 17 to 19 % by weight of chromium and 8 to 10 % by weight of nickel. The length of the stainless steel long fiber to be treated is not particularly limited, but the fibers having a length of 2 to 20 cm., e.g. slivers, are convenient for the treating procedure. The use of a stainless steel long fiber having a diameter of less than 2 Fm. is not suitable, since upon intergranular selective corrosion in an acidic solution, the acid does not act on the long fiber merely to corrode the grain boundaries, but further corrode the crystal grains themselves. For this reason, when a stainless steel long fiber having a relatively small diameter, e.g. from.2 to 6 'am., is employed, it is desirable to pay attention to selection of the intergranular corrosion conditions such as the kind, concentration and temperature of the acidic solution and the immersion time so that only th,e grain boundaries are selectively corroded. On the other hand, when the diameter is more than 20 pm., it is difficult to shorten the stainless steel long fiber by the intergranular corrosion. A stainless steel long fiber having a diameter of 2 to 20 Fm. is commercially available. In general, the smaller the diameter of a stainless steel fiber, the longer the crystal unit and consequently the longer the short fiber produced tends to become. The heat treatment for crystal grain growth adjustment of a stainless steel long fiber is carried out in a non-oxidative atmosphere at a temperature of 9000 to 1,400 C., preferably 1,0000 two 1,2000C., for a prescribed period of time. By the heat treatment within the above temperature range, the stress of the distortion of crystals elongated at the time of manufacturing the stainless steel fiber disappears, and soon crystal lattices arranged in parallel in the longitudinal direction of the fiber are formed, while weak grain boundaries of lattices are decreased with the lapse of time. As a result, the distance between the grain boundaries widens, thus growing into crystal grains continuing in the longitudinal direction. The heat treatment is carried out usually for 10 minutes to 5 hours. Since the crystal grains grow with the lapse of the heat treating time, the stainless steel short fiber having a desired length can be obtained by controlling the temperature and time of the heat treatment. In order to prevent lowering of physical properties due to oxidation, the heat treatment is conducted in an atmosphere in which no oxygen is present, e.g. in an atmosphere of an inert gas such as argon gas. The heat treatment for adjusting crystal grain growth comprehends a solution treatment conducted at a temperature of:9000 to 1,2000C. After the solution treatment, sensitization may be conducted at a temperature of 5000 to 8500C., if desired. Such a two stage heat treatment is effective in some cases, for instance, in case of using a stainless steel fiber having a relatively high carbon content of more than 0.08 % such as SUS 301 stainless steel. The heat treated stainless steel fiber is then subjected to selective corrosion of the grain boundaries by immersing in'an acidic solution, whereby bonding between the crystal grains is broken to produce short fibers. The so produced short fibers have no projections like fins, burrs or sags at both ends. As an acidic solution, there is employed an aqueous solution of an inorganic acid such as, for instance, nitric acid, hydrochloric acid, hydrofluoric acid or sulfuric acid. The acid may be employed in combination with a metal salt of an acid, e.g. a combination of sulfuric acid and cupric sulfate or a combination of sulfuric acid and ferric sulfate. A combination of hydrofluoric acid and nitric acid is perferable as an acid. The kind and concentration of the acid is selected according to the kind of the stainless steel so that only the crystal grain boundary is selectively corroded. Also, the concentration of the acid, temperature of the solution and immersion time are suitably controlled according to the diameter of the stainless steel fiber used and the desired length of the short fiber to be produced. In case of hydrofluoric acid, the concentration is usually selected from 2 to 5 % by weight. Also, nitric acid is usually employed in concentrations of at most 30 % by weight, and other acids than the above are usually employed in concentrations of at most 40 % by weight. The immersion is usually carried out at a temperature of 200 to 500C. Martensitic stainless steels and ferritic stainless steels are poorer in corrosion resistance and thus have a higher sensitivity to intergranular corrosion than austenitic stainless steels. Therefore, in case of martensitic or ferritic stainless steels, the intergranular corrosion treatment is conducted under a weaker condition as compared with the case of austenitic stainless steels. In order to avoid undesirable corrosion of metals, the acid solution may contain an inhibitor, e.g. an inorganic inhibitor such as a phosphate, a silicate or a chromate, and an organic inhibitor such as a high polymeric benzene derivative, quinoline, pyridine or polyethylene glycol. After the intergranular corrosion treatment, the produced stainless steel short fiber is washed with water or a hot water and then dried. Preferably, after washing the produced short fiber with a hot water at about 800C., passivation treatment is carried out to provide the short fiber with a corrosion resistance. The passivation treatment is carried out, for instance, by immersing the short fiber in a 30 % nitric acid solution at about 400C. for 1 to 3 hours. The passivation-treated fiber is washed with water at ordinary temperature and then dried. The thus, obtained short fiber, namely fibrous powder, is in the form of column having substantially the same diameter as that of the starting stainless steel long fiber, and is stable to corrosion. In the present invention, the metal powder and the metal short fiber may be employed alone or in admixture thereof. The mixture of the metal short fiber and the metal powder is preferably employed, since it is possible to form fine pores of uniform size with maintaining the porosity high and to obtain a filter having a high filtration capacity and a long life. Also, though the pore shape of sintered bodies made of an atomized powder alone is ununiform somewhat, the ununiformity in pore shape is improved by admixing the atomized powder with the metal short fiber, whereby the choking of the pores upon using the sintered bodies as filters is decreased and the filtration life becomes long. The ratio of the fibrous powder to the metal powder is from 20 : 80 to 80 : 20 by weight, preferably 40 : 60 to 60 : 40 by weight. The metallic particles, i.e. the metal powder, the metal short fiber or the mixture thereof, are surface-treated with a thermosetting resin, and are compacted by means of a hot press. The thermosetting resin can also be employed in the form of powder. In that case, the metallic particles are admixed with a powder of a thermosetting resin, e.g. a resin powder having a particle size of about 1/4 time that of a metal powder, and the mixture is compacted. The thermosetting resin is employed in an amount of 0.5 to 6 % by weight, especially 1 to 3 % by weight, based on the metallic particles. Examples of the thermosetting resin used in the present invention are, for instance, polyimide resin, phenol resin, alkyd resin, unsaturated polyester resin polycarbonate resin, epoxy resin, and the like. The polyimide resin is a polymer containing imide bondings in the main chain, and for instance, polypyromellitic imide which is a polycondensation product of pyromellitic dicyanhydride and a diamine, or its modified resin are employed. The use of the polyimide resin has the advantage that labors such as release of a gas from a sintering furnace and annealing treatment can be saved, thus increasing the workability, since generation of a gas at the time of sintering is a little as compared with other thermosetting resins such as phenol resin, and corrosion due to a gas. of high temperature and formation of carbon can be decreased. The phenol resin used in the present invention includes the so-called resol which is a mixture of phenol alcohols produced by the reaction of a phenol compound and an: excess aldehyde in the presence of an alkali catalyst, and the so-called novolak which is a dihydroxy diphenyl methane type compound produced by the reaction of an excess phenol compound and an aldehyde. Examples of the phenol compound are a monovalent phenol compound such as phenol, cresol, xylenol, p-alkylphenol or p-phenylphenol, a bivalent phenol compound such as catechol, resorcin or hydroquinone, and a trivalent phenol compound such as pyrogallol or phloroglucin. Examples of the aldehyde are formaldehyde, paraformaldehyde, hexamethylenetetramine and furfural. Inorganic and organic acids or alkalis, or their salts are employed as reaction catalysts and hardening agents for phenol resin. For instance, hydrochloric acid, oxalic acid, acetic acid and formic acid are employed as an acid catalyst, and sodium hydroxide, potassium hydroxide and pyridine are employed as an alkali catalyst. The surface treatment of metallic particles with a thermosetting resin is carried out in a usual manner. Usually, the metallic particles are immersed in or admixed with a solution of a resin in an organic solvent, and the solvent is evaporated. For instance, in case of employing a phenol resin as a thermosetting resin, resol or novolak is dissolved in an organic solvent such as an alcohol or acetone, and metallic particles are immersed in the solution. The solvent is then evaporated, or after taking out the particles, they are dried to remove the solvent, so that the surfaces of the particles are coated with the resin. The surface-treated metallic particles or a mixture of the metallic particles and a thermosetting resin powder is molded into a desired shape by means of hot pressing. For instance, the hot pressing is carried out by employing a hot press machine 1 as schematically shown in Fig. 1. The metallic particles 4 are placed in a depressed portion formed by under mold 2 and thickness adjusting ring 3. Upper mold '5 is then moved downward to mate with the ring 3, and the particles are compressed at an elevated temperature. The hot pressing is usually Carried out at a temperature of 50 to 250 C. under a pressure of 50 to 200 kg./cm2 for about 5 to about 30 minutes. Under such a condition, the hardening reaction of a thermosetting resin takes place, thus functioning as a binder for combining the metallic particles with each other. It is preferable to carry out the hot pressing at temperature at which hardening of a thermosetting resin takes place, and accordingly the hot press temperature is selected from the above range in accordance with the kind of the thermosetting resin used. For instance, in case of phenol resin, the hot pressing is preferably carried out at a temperature of 1500 to 2100C. When the hot press temperature is lower than 1500C., the hardening of the phenol resin is difficult to occur, and when the temperature is higher than 2100C., thermal degradation of the phenol resin is easy to occur. Also, in case of polyimide resin, the hot press temperature is preferably selected from 220 to 2300C. The thus obtained compact of fine metallic particles has a relatively high strength and accordingly is easy to convey. Further, the sintering of the compact need not be conducted under pressure, and it is easy to prepare a sintered body having a desired porosity. Also, a multiplicity of the compacts can be sintered at one time without using molds, and accordingly the workability at the time of sintering is good. Moreover, since the thermosetting resin is gasified at the time of sintering, intercommunicating pores are easy to be formed. The compacts are placed in a sintering furnace, and sintered in an atmosphere containing substantially no oxygen usually at a temperature of 8000 to 1,2000C. for 30 minutes to 4 hours. The thermosetting resin thermally decomposes and gasifies by temperature rise, and at that time, carbon may deposit on the metallic particles and corrode their surfaces. In case that a polyimide resin has been employed as a thermosetting resin, such a phenomenon is little and the decomposition gas is substantially harmless. Preferably, in order to avoid the possible deposition of carbon, the gas removal is conducted, while the rate of generation of the decomposition gas is controlled by adjusting the rate of temperature elevation. The generated gas is removed by evacuation of the sintering furnace with or without introduction of hydrogen, argon or helium gas so that the inside of the sintering furnace is in the vacuum state or under an atmosphere of hydrogen, argon or helium. The sintered body prepared according to the present invention has fine, uniform pores and a high porosity. The pore size of the sintered body of the invention is not more than about 15 m., especially not more than about 10 pm., and accordingly the sintered body is particularly useful as a filter for precise collection of fine contaminants. The sintered body for use as a filter has a filtration size of not more than about 15 'am., especially not more than about 10 pm. Further, since the porosity is high and the pore size is uniform, the sintered body for use as a filter has the advantages that the pressure loss is small, the filtration capacity is high and the filtration life is long. The porosity of the sintered body of the invention is not less than 5 %, especially not less than 50 g. It is possible to prepare a sintered body within 1 % error of the porosity. Also, the ratio of the intersecting point pressure (P1) to the initial bubble point pressure (PO) of the present sintered body is not more than 1.35. The P1/PO value is a measure for the pore size distribution, namely the uniformity of pore size, and the nearer the P1/Po value comes to 1, the more uniform the pore size. The term "porosity" as used herein means the percentage of the pore volume in a sintered body. The term "initial bubble point pressure'' PO and the term "intersecting point pressure" P1 as used herein are those provided in the filtration size test of Japanese Industrial Standard (JIS) B 8356. The initial bubble point pressure is a pressure when a bubble generates first in the filtration size test, and is a measure for the maximum pore in a sintered body. The intersecting point pressure is a pressure at the intersecting point of the lines extrapolated from the large variation portion and the small variation portion on a curve of variation of the air pressure against the amount of flowing air in the filtration size test. In addition to the use as a filter, the sintered body of the present invention is also suitable for use as oil impregnated metals. The present invention is more specifically described and explained by means of the following Examples. Example 1 A SUS 30 stainless steel atomized powder (under 200 meshes and a fibrous powder (commercially available under the registered trademark "NASLON" made by NIPPON SEISEN CO., LTD:) which was stainless steel short fibers having a diameter of 12 Fm. and a length of 40 Pm. were admixed in a ratio of 1 : 1 by weight. The mixed powder was admixed with 1 % by weight of a polyimide powder (commercial name "Kerimid 601", product of Rhone-Poulene sold by Yakushi Kasei Kabushiki Kaisha) based on the weight of the mixed powder. The mixture was compressed at 2300C. and 100 kg./cm2. for 10 minutes by employing a hot press machine shown in Fig. 1 to form a compact of circular plate having a diameter of 60 mm. The compact was placed in a sintering furnace, and the temperature was gradually elevated under vacuum while removing the decomposition gas of polyimide. The temperature was elevated to 1, 150 C., and at that temperature, the compact was sintered for 3 hours to give a sintered body. The properties of the obtained sintered body are shown in Table 1. Examples 2 to 7 A SUS 304 stainless steel powder and a NASLON fibrous powder having a diameter of 12 am. and a length of 40 pm. were employed alone or in admixture thereof as a powder. The powder was surface-treated by admixing 500 g. of the powder with 50 ml. of a 30 % by weight tetrahydrofuran solution of a phenol resin (commercial name "SUMICON PM-40", product of Sumitomo Bakelite Kabushiki Kaisha) with agitation, during which tetrahydrofuran naturally evaporated. The powder was compressed at 2000C. and 100 kg./cm2. for 10 minutes by employing a hot press machine shown in Fig. 1 to form compacts of circular plate having a diameter of 60 mm. The compact were sintered in the same manner as in Example 1. The properties of the obtained sintered bodies are shown in Table 1. There are also shown for comparison in Table 1 the properties of Filter 05 (Comparative Example 1), Filter 07 (Comparative Example 2) and Filter 10 (Comparative Example 3) which are commercially available filters made of a stainless steel powder by UGINE CARBON CO., France. Figs. 2A to 2E show microphotographs of the sections of the sintered bodies obtained in Examples 2 (Fig. 2A), 3 (Fig. 2B), 4 (Fig. < 2C), 6 (Fig. 2D) and 7 (Fig. 2E). In Table 1, the initial bubble point pressure PO and the intersecting point pressure P1 were measured according to the filtration size test of JIS B 8356. DM shows the maximum pore diameter and Dm shows the intermediate pore diameter. Also, the flow performance was measured as follows: Water was passed through a sintered body by a pump, and when the pressure loss measured by a manometer 2 became 0l1 kg./cm., flow rate of water which passed through the sintered body was measured by a flow meter. The permeability was calculated from the flow rate of water and the pressure loss according to the Darcyls law. Further, the pressure loss against the flow rate of a liquid having a viscosity of 2,-000 poise was measured. The relationship between the pressure loss and the flow rate is shown in Fig. LIA, in which lines 1, 4, 5, Cl, C2 and C3 are for Examples 1, 4 and 5 and Comparative Examples 1, 2 and 3, respectively. Filtration efficiency and 98 % filtration size were measured as follows: Air cleaner fine test dust (ACFTD) having various particle sizes were employed as contaminants. ACFTD was dispersed in distilled water in a concentration of 0.5 g./liter, and the dispersion was filtered through a sintered body with suction of 30 mmHg. The filtration efficiency is shown by the percentage of ACFTD filtered off. In Table 1, the results of the measurement with respect to ACFTD having particle sizes of 3 Hm. and 5 pm. are shown. The 98 % filtration size showing a size of particles of which 98 % is filtered off was obtained from a graph of the filtration efficiency against the particle size of ACFTD obtained by the above measurement. Constant pressure filtration test was carried out as follows: ACFTD having an average particle size of 7 Fm. was dispersed as contaminants in distilled water in a concentration of 0.5 g./liter. The dispersion was filtered through a sintered body with suction of 30 mmHg, and the amount of the filtrate was measured with the lapse of time. The results are shown in Fig. 5. The amount of the filtrate obtained until the end velocity reaches 0.1 ml./sec. and the time required (filtration life) were also calculated according to the Ruth's equation of the constant pressure filtration. They are shown in Table 1. Table 1 --------------------------------------------------------------------------------------------------------------- Example No 1 2 3 4 5 --------------------------------------------------------------------------------------------------------------- Atomized powder/NASLON powder....... 1/1 1/0 2/1 1/1 1/1 thermosetting resin................. Polyimide Phenol Phenol Phenol Phenol resin resin resin resin Thickness of sintered body (mm)..... 1.00 1.76 1.59 1.65 1.02 --------------------------------------------------------------------------------------------------------------- Porosity (%)........................ 51.6 54.7 # 0.3 51.7 # 1 51.6 # 0.4 51.6 # 0.4 Filtration size test P0 (mmH2O)........................ 588 452 589 616 589 P1 (mmH2O)........................ 754 592 767 780 754 P1/P0............................. 1.28 1.31 1.30 1.27 1.28 PM ( m)........................... 6.1 8.1 6.2 5.9 6.2 Dm ( m)........................... 4.8 6.1 4.7 4.7 4.1 Flow performance Permeability (water)(x104 cm2).... 0.82 1.515 1.075 1.034 0.841 Flow rate at 0.1 kg/cm2 pressure loss (ml/cm2/min)...... 42 52 41 38 50 Filtration efficiency (%) Particles of 3 m diameter........ 94.2 88.5 95 95.5 94 Particles of 5 m diameter........ 98.3 95 98.6 99.2 98 98 % filtration size ( m)........... 4.8 10 4.5 4 5 Constant pressure filtration test Amount of filtrate (ml)........... 321 103.2 148.5 264.3 229.2 Filtration life (sec)............. 1100 456.2 668.3 1246 1070.3 ------------------------------------------------------------------------------------------------------------------- -continued Example No. 6 7 Com.Ex.1 Com.Ex.2 Com.Ex.3 Atomized powder/NASLON powder 1/2 0/1 - - Thermosetting resin................. Phenol Phenol resin resin Thickness of sintered body (mm)..... 1.43 1.59 2.0 2.0-2.04 1.93-1.96 Porosity (%)........................ 50.5#1 50.0#0.7 36 27.4-30 26.7-28.5 Filtration size test PO (mm H2O)..................... 698 807 527 417 325 P1 (mm H2O)..................... 932 992 789 786 392 P1/PO .......................... 1.34 1.23 1.50 1.89 1.21 DM ( m)......................... 5.2 4.5 6.32 8.7 11.2 Dm ( m)......................... 3.9 3.7 4.6 4.7 9.3 Flow performance Permeability (water) (x104 cmê)... 0.825 0.655 0.457 1.19 1.28 Flow rate at 0.1 kg/cmê pressure loss (ml/cmê/min).......... 35 25 14 38 40 Filtration efficiency (%) Particles of 3 m diameter.......... 99.5 99.8 93 85.5 45.5 Particles of 5 m diameter.......... 99.8 99.9 96.9 94.5 52.7 98 % filtration size ( m)........... 0.6 0.2 6.1 8 33 Constant pressure filtration test Amount of filtrate (ml)............. 118.9 105.8 28.4 92.5 74.7 Filtration life (sec) .............. 552.4 498.3 98.1 497.2 244.4 As is clear from Table 1, the porosity of the sintered bodies of the present invention is very high. Moreover, the pore size is very small. This would be understood from the values of the 98 % filtration size and the initial bubble point pressure PO. As stated before, the bubble point pressure is the force expressed in terms of the height of liquid column which is required to break a liquid membrane formed over the opening of a pore, and is a measure for. the maximum pore in a sinteredabody. ,The higher the initial bubble point pressure1 the smaller the pore size. Further, the P1/Po values of the sintered bodies of the present invention are smaller than those of filters put on the market and are near 1. This shows that the pore size of the sintered bodies of the invention is uniform, in other words, the pore size distribution is narrow. Like this, the sintered body of the present invention has fine pores of uniform size and moreover has a high porosity. Such a sintered body has not been known. For instance, a conventional sintered body of Comparative Example T has similar initial bubble point pressure and filtration size to those of the sintered bodies of the invention, but the porosity is small and also the pore size is not uniform. Fig. 4A shows the relationship between the pressure loss and the flow rate. From it, it would be understood that the sintered body of the present invention is low in pressure loss, despite that the pore size is very small. For instance, the pressure loss of the sintered bodies of Examples 1 and 5 is lower than that of Comparative Example 2, despite that the initial bubble point pressure is far higher than that of Comparative Example 2. Fig. 5 shows the relationship between the amount of the filtrate and the time. As is clear from Fig. 5, the sintered body of the present invention has a high capacity of collecting contaminants and a long life as a filter. This is also clear from Table 1. Like this, since the sintered body of the present invention has a high porosity and the pores are fine and uniform, the filtration capacity is high and the filtration life is long, and moreover precise collection of fine contaminants is possible. On the other hand, conventional sintered bodies as filters of Comparative Examples 1 to 3 cause choking of the pores in a short. period of time and the filtration life is short due to low porosity and ununiformity in pore size, as i-s clear from Table 1 and Fig. 5. Examples 8 and 9 The procedures of Example 1 were repeated except that a stainless steel irregular atomized powder having a particle size of 200 to 250 meshes (Example 8) or of under 250 meshes (Example 9) as shown in Figs 3A and 3B was employed alone as a powder. The-7properties of the obtained sintered bodies are shown in Table 2 together with those of Filter 15 made by UGINE CARBON CO., France (Comparative Example 4). In Table 2, the filtration size test, the measurement of 98 % filtration size and the constant pressure filtration test were carried out in the same manner as in Table 1. The flow performance in Table 2 shows data obtained by employing a liquid having a viscosity of 3,000 poise. The relationship between the pressure loss against the flow rate of a liquid having a viscosity of 3,000 poise is shown in Fig. LIB, in which line 8 is for the sintered body of Example 8 and line C4 is for the sintered body of Comparative Example 4. The results of the constant pressure filtration test are also shown in Fig. 5. Further, with respect to the sintered bodies of Example 8 and Comparative Example 11, results of the measurement of the filtration efficiency are shown in Fig. 6. Table 2 Example No. 8 9 Com.Ex.1 Particle size of powder (mesh)...... 200-250 under 250 Thermosetting resin................. Polyimide Polyimide Thickness of sintered body (mm)..... 2.7 1.76 2.0 Porosity (%)........................ 61 55 31 Filtration size test PO (mm H2O)..................... 233 452 235 P1 (mm H2O)..................... 288 592 310 P1/PO .......................... 126 1.31 1.32 DM ( m)......................... 16 8.1 Dm ( m)......................... 12 6.1 Flow performance Permeability (water) (x104 cmê)... 7.9 8.8 2.73 Flow rate at 0.1 kg/cmê pressure loss (ml/cmê/min).......... 0.058 0.023 98 % filtration size ( m)........... 16 10 31 Constant pressure filtration test Amount of filtrate (ml)............. 636 103.2 31 Filtration life (sec) .............. 2989 456.2 66.3 Comparing the sintered body of Example 8 with a conventional sintered body of Comparative Example 4, both having a similar initial bubble point pressure to each other, the sintered body of the present invention has a far higher porosity than the conventional sintered body. Therefore, the pressure loss of. the former is far lower than.that of the latter, as is clear from Fig. 4B, and also the water permeability of the former is higher than that of the latter. Accordingly, the sintered body of the present invention has a high filtration capacity and a long life as compared with the conventional sintered body. This is also clear from the results of the constant pressure filtration test and Fig. 5. Also, as is clear from Fig. 6, the filtration efficiency of the sintered body of the present invention against contaminants having a small particle size is higher than that of the conventional sintered body, and it is understood that the sintered body of the present invention is made up of fine pores.

Claims: WHAT WE CLAIM IS: 1. A metallic sintered body which is composed of fine metallic particles and has a porosity of at least 45 %, a ratio of the intersecting point pressure to the. initial bubble point pressure of, not more than 1.35 and a filtration size of not more than about 15 pm. 2. The sintered body of Claim 1, wherein the porosity is at least 50 % and the filtration size is not more than abort 10 pm. 3. The sintered body of Claim 1, wherein the fine metallic particles are a powder selected from the group consisting of a metal powder and a fibrous powder. 4. The sintered body of Claim 1, wherein the fine metallic particles are a mixture of a metal powder and a stainless steel fibrous powder in a ratio of 20 : 80 to 80 : 2,0 by weight, said metal powder having a particle size of not more than about 200 meshes and said stainless steel fibrous powder having a diameter of 2 to 20 pm. and an aspect ratio of the length to the diameter in the range of 3 to 50. 5. The sintered body of Claim 4, wherein the ratio of the metal powder to the fibrous powder is from 40 : 60 to 60 : 40 by weight. 6. A process for preparing a metallic sintered body which comprises compacting fine metallic particles with a thermosetting resin at a temperature of 500 to 2500C. under a pressure of 50 to 200 kg./cm2. for about 5 to about 30 minutes, and sintering the resulting compact at a temperature of 8000 to 1,2000C. for 30 minutes to 4 hours. 7. The process of Claim 6, wherein the thermosetting resin is a member selected from the group consisting of a polyimide resin and a phenol resin. 8. The process of Claim 6, wherein the fine; metallic particles are a powder selected from the group consisting of a metal powder and a fibrous powder. 9. The process of Claim 6, wherein the fine metallic particles are a mixture of a metal powder and a stainless steel fibrous powder in a ratio of 20 : 80 to 80 : 20 by weight, said stainless steel fibrous powder having a diameter of 2 to 20 pm. and an aspect ratio of the length to the diameter in the range of 3 to 50. 10. The process of Claim 6, wherein the sintering is carried out in an atmosphere containing substantially no oxygen.

Inventors: ISHIBE, HIDEOMI, OKAZIMA, TADAYUKI
Applicants: NIPPON SEISEN CO., LTD.
Application year: 1982
Main classification: B22F3
IPC codes: C22C49, B01D39, B22F3