FR2550782A1 - PROCESS FOR PRODUCING POROUS CERAMICS - Google Patents

Abstract

PROCESS FOR PRODUCING POROUS CERAMICS IN WHICH A HIGHLY INFLATABLE PULVERULENT RESIN AND A SWELLING AGENT ARE MIXED IN PULVERULENT RAW MATERIALS FOR CERAMICS IN ORDER TO CREATE A SOIL; WE LEAVE THIS SOIL UNTIL IT BECOMES A GEL AND THE GEL IS DRYED AND BAKED TO FORM PORES IN THE CERAMIC BY EVAPORATING THE BLOWING AGENT FROM THE GELIFIED BODY. AS EXPANDABLE RESIN, A PULVERULENT RESIN CONSTITUTED MAINLY BY A HIGHLY WATER-ABSORBING RESIN IS USED AND, AS AN INFLATABLE AGENT, WATER OR AN AQUEOUS SOLUTION CONTAINING A SOLVENT MISCIBLE IN WATER, SUCH AS AN ALCOHOL OR A WATER-BASED SOLUTION, IS USED. A CETONE. THANKS TO THIS PROCESS, WE CAN EASILY OBTAIN POROUS CERAMICS WITH A HIGH PERCENTAGE OF VACUUMS, AT LOW COST, AND BY BEING ABLE TO ADJUST THE DIMENSIONS, SHAPES, ETC ... OF THE PORES AT WILL.

Description

Process for producing porous ceramics.

  The present invention relates to a method for producing

porous ceramics.

  Porous ceramics are lighter than general-purpose ceramic products and moreover their resistance to thermal and chemical influences is high. Consequently, they are in great demand to be used, for example, as furnace materials and insulating materials or to be used as absorbent materials, catalyst supports, etc. which uses pores To produce such porous ceramics, several processes have been proposed. Among these, we can cite: a process for forming a porous body by mixing inorganic materials beforehand, such as boron nitride in a lexagonal system which foams when evaporated at firing temperatures, in ceramic raw materials (Japanese patent 1983-15062); a method in which a combustible powder, such as sawdust, a polymer composite, is added to crude powder ceramics and voids are then formed by vaporizing the combustible powder during firing (US Patent 4,325,846) ; and a method in which a raw ceramic product, which has been transformed into mud using a dispersing liquid, is poured into the holes of a porous resin structure, such as a three-dimensional structure of polyvinyl acetal or polyurethane, the structure is dried and the previous structure is then baked in order to eliminate by combustion the synthetic resin present in the structure during the baking process (Japanese patent

1982-47756).

  These methods have advantages and disadvantages, respectively, and at this time there are still problems in producing porous ceramic products economically while maintaining a certain level of quality of the products obtained. In particular, as regards the method in which a foaming material is added to the firing temperatures, since it is necessary to adjust the firing temperature at which the raw ceramic product is melted to the temperature causing the decomposition and gasification of the foaming products by adjusting these temperatures at the same value, it is difficult to predict exactly in advance the appropriate temperature for cooking when the raw material used consists of natural products It is therefore difficult to adjust the degree of spumation, the dimensions and The form Regarding the process in which a combustible powder is added in advance, the following disadvantages ants appear when the mixture is treated by cooking and spraying When the additives are natural products, it is difficult to economically obtain a uniform powder as regards the shape, the particle size, etc. Furthermore, because the volume grains of the combustible powder fixes the void volume, in order to obtain a porous body with a high void percentage, a large volume of combustible powder must be mixed with the raw material for the ceramic product. it is usually difficult to produce porous ceramics with a void percentage greater than 60%. Furthermore, with regard to the process in which a crosslinked and steric porous body is formed, that is to say a sponge, with a polymer. synthetic and into which the ceramic raw material is then poured into the pores of the sponge, although this process is logically acceptable as an industrial process, it has the disadvantages following It requires many treatments to prepare the three-dimensional porous structure In addition, since it also requires that the quantity of polymeric products corresponds to the volume to be occupied by the empty voids to be formed, a large quantity of the polymer product for obtaining a ceramic with a high void percentage. It follows from the above that, for example when the raw material for the ceramic, dispersed in water, etc., is poured into the spongy body described above, the ceramic structure becomes weak and lacks solidity Thus, this process

is not realistic.

  The object of this invention is to provide a method for producing porous ceramics, in which ceramics are produced with a determined number and shape of pores, percentage of voids, etc., regardless of the type of component of the ceramic. . Another object of this invention is to provide a process for producing porous ceramics, which can be carried out at low cost and which is readily applicable to a casting process for producing ceramic structures with a specific shape using a

molding process.

  The economic advantage is even greater if mainly resins are used which are extremely expandable by application of water during the process for producing porous ceramics provided by this invention, since the use of these resins allows the use of as a blowing agent to form

the voids.

  By retaining the principles of this invention, its objects are achieved by a unique process for producing porous ceramics, in which the expandable powdery resin is first mixed with the powdery raw ceramic product, this mixture is then transformed into soil by the addition of blowing agent, the soil is poured into a mold so as to form a composite body containing swelling gels Then the composite body is heated and baked in order to gasify and evacuate the blowing agent so that the spaces

  filled with this blowing agent form voids in 20 l finished ceramic.

  The invention will be better understood on reading the

  detailed description, given below as an example

  only, from an embodiment of the invention, in conjunction with the attached drawing in which FIG. 1 is a diagram illustrating a production process according to

the present invention.

  Referring now in more detail to the present invention, the raw materials for ceramics to which the production process according to this invention can be applied comprises the following ores: siliceous ores, such as siliceous rocks, siliceous clay, diatomaceous earth ; alumina-based ores, such as diaspore, bauxite, molten alumina; aluminous silica ores, for example Gaerome clay and Ribushi clay, which are kaoliniths which are clay ores, or bentonite and agalmatolite which are ores based on montmorillonite or sillimanite magnesium ores such as magnesite and dolomite; lime-based ores such as limestones and wollastonite, chromium ores such as chromite and spinel; zirconium ores such as zyrcon and zyrcone; and other ores such as titanium ores (titanium oxide) and carbon ores

containing graphite.

  In addition to the natural minerals listed above, artificial minerals such as zirconia, silicon nitride, titanium oxide, fused alumina,

  synthetic magnesia, synthetic dolomite and synthetic mullite.

  These ceramic raw materials are used as single materials or mixtures of materials and in the form of a pulverized powder which is the same as in an ordinary method of manufacturing refractory products. The grain size can also be chosen according to the intended application and this makes it possible to obtain products which are

  cannot be obtained by the manufacturing process previously described using a polymer sponge.

  The swellable resins used in this invention are resins which can be transformed from soils to gels by absorbing a swelling agent composed mainly of water or an aqueous solution of a solvent

  miscible in water, such as an alcohol or a ketone.

  As the resins described above, the following types, listed below, can be used and all of those listed below are capable of increasing their

  volume by swelling from several tens of times to a thousand 5 times at most, by comparison with their dry form.

  Examples of these resins include: crosslinked products of polyvinyl alcohol or its derivatives; saponification products of copolymers of vinyl ester and unsaturated carboxylic acids (or of its esters); 10 saponificatoin products of copolymers of ethylene, vinyl ester and unsaturated carboxylic acids (or its derivatives); crosslinked products of alkaline salts of copolymers of alpha olefin and maleic anhydride; crosslinked polyethylene oxide products; polyvinyl pyrrolidone crosslinked products; hydrolysis products of starch and acrylonitrile or methacrylonitrile copolymers; saponification products of crosslinked polyacrylamides; crosslinked polyacrilic acid; saponification products of copolymers of hydroxyakyl acrylate and acrylamide; and crosslinked polyethylene sulfonates. Without exception, these resins are powders, but they have the following property When they come into contact with water serving as a blowing agent, each individual grain of powder of these resins forms a gel by absorbing water during a some time and, as the amount of water absorbed increases, the gel volume increases As can easily be expected from such a phenomenon, the shape of the gel is determined by the

  form of the expandable resin which is the solid body.

  This means that, starting from a spherical resin, a spherical gel is obtained. Furthermore, the size of the gel can be determined by the relative adjustment of the size of the solids of the expandable resin and of the

  volume of water used as a blowing agent.

  In other words, to obtain a gel having a large volume, it is arranged that the percentage of water absorption by the resin is high by increasing the amount of water to be absorbed by the resin; by staying within the water absorption capacity of this resin, it is possible to obtain a porous product using only a small amount of expandable resin. In addition, for a fixed void percentage, the number of pores by varying 10 the volume of each pore to enlarge or reduce it This means that the volume of each pore determines the number of por E for a fixed percentage of voids and this number of pores can in turn be determined by the number of gels Furthermore, the number of gels can easily be determined by choosing the combination of the grain size of the expandable resin and the percentage of water absorption (percentage expansion by volume per absorption of expandable resin The percentage of voids which is determined by the total volume of pores formed and by the size of the pores are the important factors which affect the physical and thermal characteristics of Pa ceramics For example, if the percentage of voids increases, the mechanical resistance and the filtration resistance decrease, but the gas permeability, the liquid permeability and the conductivity

are increased.

  Figure 1 shows a diagram illustrating the process for producing baked goods, made from porous ceramics using the above raw materials. To disperse the gels in the raw material for the ceramic, either of the following methods can be used. One method consists in intimately mixing in advance the foamable resin powder and the powdery raw material for the ceramic, then pouring the blowing agent into the mixture to effect swelling and freezing. Another method consists in separately forming the gel particles. from the expandable resin by swelling it to a specified degree, then mixing these gelled particles with the raw material for the ceramic. The first process can be applied to a casting process when the desired shape of the ceramic product is obtained before drying. and baking It is a process which helps to facilitate operations and to save money, and the process provided by this invention makes such a process possible for the first time since there were difficulties when using a casting process in various types of conventional processes

  to produce porous ceramics.

  This process is applied as described below.

  When adding a blowing agent of specified percentage in the mixture of raw materials for ceramic and expandable resin, the gelation of

  the expandable resin takes place gradually over a period of time after addition of the blowing agent.

  Consequently, during the previous freezing, there is a period during which the expandable resin

  is partially transformed into gel and may still leak.

  At this stage of the gelation, the mixture can be processed by molding: three-dimensional or two-dimensional using either a metal mold or a template The time required for the gelation of the mixture to be complete (gelation of the expandable resin with the swelling agent) since its fluid state varies depending on the shape and size of the powder grains of the water-absorbent resin, but this time can be controlled by modifying the temperature of the blowing agent to be added The ready material to be fired, that is to say three-dimensional stabilized in the form of a composition of gels of expandable resin and raw materials for ceramics with a small amount of free blowing agent trapped in these raw materials for the ceramic, is heated to a drying temperature below 110 C for approximately one hour During this heating operation, the blowing agent in the gel is largely evaporated without causing displacement gelled bodies and raw material for ceramics, thus forming a practically full structural frame Then drying continues at a

  temperature below 170 ° C until the blowing agent in the gel is almost completely gone.

  Then, the porous ceramic is obtained by firing to melt the raw material of the ceramic. In order to increase the three-dimensional stability of the powder of ceramic raw material before drying and firing if necessary, various types of synthetic polymer resin can be added, tar, etc. which are commonly known as binding agents for inorganic powders. Thus, the raw material of the ceramic forms a three-dimensional structure in a stable state with the gel grains using a blowing agent or a binding agent. that we can obtain a ceramic fired with a stable shape without causing deformation by shrinkage, which could cause a variation in the percentage of voids, during drying and firing In the case where water is used as a blowing agent, Unlike free water, the water contained in gels after absorption of water has the property that it takes a relatively long time to develop. acer inside

  volume when heated.

  As a result, the evaporation rate of this water is slow As a result, there is no blistering phenomenon even if the drying temperature is too high In addition, the vapor does not collect inside the structure ceramic. So, for example, even if the drying temperature

  goes up to 130 to 150 C, there is no deformation or cracking in the ceramic.

  In addition, when the gasified vapor moves passing through the interstices between the raw material of the ceramic, the passage of this gasified vapor forms a vent In this way, the drying process of this material for the porous ceramic is completed when the vent is formed as described above and then the porous ceramic product is fired as it is In this way, a porous body is obtained

  sintered with a certain permeability to gas.

  As the raw materials for ceramics, the natural materials described above are used. However, it is possible to obtain, from zirconium, silicon nitride, etc., which have recently been used as artificial raw materials for ceramics, new porous ceramics having pro25 specific properties by treating these artificial raw materials in the same way as treating natural raw materials. Therefore, the production process provided by this invention is not necessarily limited to natural minerals. The advantageous characteristics of the production method according to this invention are as follows: 1 The combination of dimensions, shapes, etc. which are characteristic properties of the body and the combination of number, can be carried out in a simple and easy manner. pores in a given volume

  (apparent density) and the distribution of pores, which are the characteristic properties of the porous body.

  2 In the gelation process of the mixture of raw materials for ceramics and expandable resin, which are originally in powder form, during the initial stage of swelling the mixture must be maintained in the soil state, that is - i.e. in a sludge-like fluid state As a result, extremely porous products can be molded, with a void percentage greater than 70% and up to 90% and with a density less than 0.4 using a process As a result, it is not

  necessary to use press forming.

  3 By using expandable resins highly absorbing water, the volume of which can increase from several times to a thousand times their volume to dryness by absorption of water, concurrently using a blowing agent consisting mainly of water, including becout is weak, we get gelled bodies with

  a large number of pores Therefore, the cost of forming voids in a structure is low.

  4 In association with the swelling agent or the binding agent, the powdery raw material for the ceramic forms a resistant three-dimensional structure thanks to the initial drying process. The swollen gels are present in the structure and these gels form the pores.

  while maintaining their volume during the second drying.

  In this process, as the water contained passes through the spaces in the structure, the open pores communicating continuously between them provide vents

  that can be easily trained.

255078 Z

  We will now give a concrete description of a

  production method according to this invention by

referring to real examples.

  Example 1 As raw materials for ceramics, 100 grams of commercial diatomaceous earth containing 92% silica (i 02) as the main component and 4% alumina, 81% of the grains having a size less than 10 microns, are mixed with 10 grams of commercial bentonite 10 containing 68% silicic acid and 15% alumina,% of the total quantity having a size less than 45 microns As water-absorbent resin, 2 grams of powder are added to the mixture thus prepared Sumicagel S 50 commercial (saponification product of copolymer 15 res of vinybet acetate of methyl acrylate, manufactured by SUMITOMO CHEMICAL Co Ltd, JAPAN) Then, pour into the previous mixture 400 cc of water at room temperature and the mixture thus obtained is stirred. The viscosity of the mixture increases but it retains its fluidity. After three minutes, the mixture is poured into a molding container and allowed to stand. Five minutes later, this mixture is cured into a combination of gels and inorganic material and it becomes a molded product which retains its original shape. , even if it is removed from the molding container; therefore, it can be transported easily. Then, this sample is dried in an air bath at 80 ° C. for 12 hours. It is then dried at 150 ° C. for 8 hours. After drying, it is heated in an electric oven with a temperature gradient 30 of 10 o C per minute When the temperature reaches 1050 C, it is maintained at this temperature

  for 30 minutes, and then allowed to cool.

  Then, after two hours, the sample is taken out.

  The cooked product thus obtained is a porous ceramic having many pores with diameters ranging from

  0.3 mm minimum to 0.9 mm maximum, which are uniform

  evenly distributed in the three dimensions in the ceramic body The measurement of the physical properties shows a specific density of 0.15 and a percentage of voids

84%.

  EXAMPLE 2 Lightly cooked commercial magnesia containing magnesium oxide (90% in percentage) whose grain size is less than 90 microns is mixed with Kibushi clay and bento10 nite. case, the quantity used was 100 grams of lightly cooked commercial magnesia, 5 grams of commercial Kibushi clay and 5 grams of commercial bentonite. In the previous mixture, 2 grams of grain size polyviny alcohol are mixed. less than 100 microns We prepare separately, following

  the following procedure, an aggregate body of grains of gels.

  To three grams of KI Gel 201 (crosslinked product of an alkaline salt of copolymers of alpha olefin and of maleic anhydride, produced by KURARAY ISOPRENE 20 COMPANY LIMITED, JAPAN) is added a highly water-absorbent resin, the dimension of which is of grains is less than microns, 300 cc of water at room temperature and cc of methanol, and the mixture is stirred. It is then poured into the mixed powder of lightly cooked magnesia, Kibushi clay, bentonite and alcohol polyvinyl, the aggregate of gel grains; we mix it

  everything so that the mixture becomes homogeneous.

  This results in a viscous and hard mud When the mud is loaded into a cylindrical mold of 600 cc 30 and it is lightly manually pressed, a dense cylindrical molded product with a volume of 345 cc is obtained This molded sample is heated to 110 OC for 8 hours, and then at 1600 C During

7 hours.

  After heating, the sample is heated in an electric oven with a temperature gradient of 60 C per minute. When the temperature reaches 1400 C, it is maintained at this value for 30 minutes. Heating is then stopped and 6 hours later, the sample is removed from the electric oven. The product obtained is a porous ceramic having a large number of pores distributed in three dimensions in the ceramic body. The main dimension of these pores is approximately 0.3 mm The measurements indicate that the percentage of voids is 81% and that the specific gravity is 0.37.

t 5

Claims (2)

Claims   1 Process for producing porous ceramics, characterized in that: An expandable resin is mixed in a powdery raw material for ceramics; A blowing agent is mixed in the preceding mixture to bring it in the form of a gelled body; This gelled body is dried; and this dried gelled body is fired, whereby porous ceramics with numerous pores are obtained. 2 Process for producing porous ceramics, according to claim 1, characterized in that the expandable resin is a pulverulent resin containing an absorbent resin. strongly water, as the main component. 3 Process for producing porous ceramics according to   claim 1, characterized in that the blowing agent is water.   4 Process for producing porous ceramics according to claim 1, characterized in that the agent   swelling is an aqueous solution containing a solvent.