EP1587772B1 - Method for producing porous sintered bodies - Google Patents

Abstract

The invention relates to a method for producing highly porous sintered parts, according to which free-flowing thermoplastic moulding materials are expanded at a temperature ranging between 80 and 130 °C. Said method is essentially characterised by the use of expandable polystyrene as the expanding agent, in addition to corresponding binding agents. During the expansion process, encapsulated cellular polystyrene foam particles are formed, allowing the production of mechanically solid sintered bodies with a percentage volume of pores of 85 % and a uniform pore diameter. The method is used to produce open- or closed-pore ceramic and/or metallic sintered bodies.

Description

The invention relates to a method for producing a cellular porous shaped sintered body with the manufacturing steps of preparing a thermoplastic flowable molding compound by mixing ceramic u / o metal powder with binder components and incorporation of organic blowing agents, converting the molding material into a molten state and introduction into a molding device, foaming the molding material by means of the blowing agent, solidification of the foamed molding composition, spreading of blowing agents and organic components and sintering of the thus treated molding.

It is known to produce metallic and / or ceramic shaped bodies by pressing and sintering suitable starting powders. In some cases, a ductile binder has to be added to the matrix powder, for example a ductile metal powder in hard metal production, in order to obtain products which can be pressed and sintered.
A relatively recent technology for producing ceramic metallic sintered shaped bodies is the MIM (metal injection molding) process, in which the ceramic metallic matrix powder particles are mixed with organic binder components, and the mixture is usually brought into the desired shape in the thermoplastic state which solidifies molded part and then freed by pyrolysis u / o by dissolving and extracting its organic and / or inorganic binder components and finally sintered to form an approximately pore-free dense molded body. The shaping takes place alternatively for injection molding, for example by means of extrusion.

While it is usually the goal to bring sintered moldings in a possible non-porous final state, so are applications of sintered bodies are known in which a certain pore structure is needed. Targeted pore structures in sintered bodies are created, for example, by mixing the matrix starting powders with a pulverulent placeholder, wherein the placeholder particles usually chemically before or during the sintering process from the molded composite material removed u / o removed by thermal decomposition and take their place free spaces, or pores. It is also known to produce pore structures in moldings by blowing in gases, for example argon or nitrogen gas, into a molten metal. Alternatively, sintered bodies having a pore structure are produced by introducing blowing agents as additives as homogeneously as possible into a matrix material mixed with thermoplastic binder and heating this composite or molding compound to the evaporation or foaming temperature of the blowing agent. In this case, bubble-shaped gas spaces are formed in the, or foam formations of the thermoplastic or molten molding material, which stabilize upon cooling and transfer of the molding compound into a solid state and then allow extraction of the gas inclusions or the remaining propellant leaving pores. In parallel, the binder additives are extracted. The ready mechanical stabilization of the shaped body takes place by means of an additional sintering step. The achievable quality of porous sintered shaped bodies produced in this way, their mechanical stability, mechanical workability, homogeneity of the pore structure, percentage of the achievable pore volume, depend greatly on the process control chosen, on the auxiliaries, blowing agent and binder, as well as on the preparation of all in one Form mass introduced substances.

The wide range of organic and inorganic binders available today for these purposes is strongly influenced by the advances in MIM technology.
Likewise, a variety of different intumescent materials are described as propellants for creating pore structures in moldings made from powders.
However, individual specific combinations of matrix powder, binder and blowing agent in connection with the respective process control have a frequently unpredictable, mutual influence on the result or on the quality of such porous shaped bodies.

This is how the patent describes US 5 213 612 a method for producing a porous metal body, according to which an aqueous suspension of metal powder and foaming blowing agent are mixed within predetermined volume ratios, foamed and brought to the solid molding by drying. During the subsequent heating of the shaped body (foaming agent with metal powder distributed therein) to a first temperature level of 600-1200 ° C., foaming agent decomposition takes place in a reducing atmosphere with simultaneous particle-spreading and metallic binding of the powder particles. Finally, the temperature is raised to a sintering temperature adapted to the respective metal, and the metal powder is sintered to form a porous body. A useful foaming agent is an isocyanate-capped polyoxyethylene polyol, which eliminates the need for an additional binder. According to one embodiment, under 50% volume expansion is foamed.
A disadvantage of this method is the use of water in conjunction with polyurethane or polyethylene binders, which allows the mass thus formed little thermoplastic properties and thus foaming in only a very limited volume. He comes to shrinkage after foaming. The practically controllable pore content in the sintered body is 10-20% by volume, which generally precludes the formation of cellular pore structures.

The DE 177 15 20 A1 describes a method for producing ceramic masses by casting, with honeycomb structure in the mass inside and with a smooth surface, are stirred in the plastics with pearl structure in the tempered casting slurry and the molded body solidifies under cooling. Preferred plastic is blowing agent-containing polystyrene which has been prefoamed depending on the desired bead size.
A disadvantage of this method is only unsatisfactory controllability of the bead distribution and arrangement in the casting slip, which is the use of the method with only moderate requirements for the mechanical minimum capacity of the cooled ceramic mass on the production of Shaped bodies with only low pore volume. The method does not provide for dispensing the polystyrene beads from the mass.

Another method of the type mentioned is in the EP 0 765 704 described. The essential features of the process lie in the separate preparation of two different components of a molding composition, on the one hand as an aqueous solution containing the foaming or blowing agent in a resinous binder and on the other hand, as a metal powder and a water-soluble, resinous binder solution , which are both brought together just before the planned foaming process. The foaming step takes place in an atmosphere with at least 65% humidity. The water-soluble resin binder stabilizes the pores formed in the bulk during foaming during the foaming and subsequent drying. The water-soluble resin binder with temperature-dependent viscosity allows a suitable adjustment of the viscosity of the molding compound in adaptation to the individual production steps. As examples of such a water-soluble resin binder, there are explicitly mentioned methyl cellulose, hydroxypropylmethyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, ammonium, ethyl celluslose and polyvinyl alcohol. Further, volatilizable hydrocarbons having 5 to 8 carbon atoms in the hydrocarbon radical are cited as means for forming gas bubbles or pores in the molding compound, specifically pentanes, hexanes, octanes, benzene and toluenes. The foamable suspension may additionally contain organic plasticizers. A variety of oils, esters, glycerines and other organics are listed explicitly. The possible addition of specific means for stabilizing the foam state and the shaped microcells is provided. Unlike the previous use of commercial polyurethane foam or blowing agent, a crack-free and therefore mechanically stable, porous sintered body should be finished by this method. The method steps detailed in the examples reveal the susceptibility of the method. In fact, according to this method, it is not possible to achieve sufficiently mechanically stable, porous sintered bodies with a high pore volume fraction for the majority of applications. The one used there The term sintered body with honeycomb structure has on this background at most limited information value.

The EP 0 460 392 A1 describes a method of producing foamable metal bodies by the steps of manufacturing, mixing metal powder and gas releasing propellant powder into a molding composition, heat compacting the molding composition under conditions permitting bonding and mechanical strengthening of the metal powders by diffusion, thereby gas tightly enclosing the propellant and simultaneously decomposing the propellant prevent. Furthermore, the compacted molding compound is brought to such a high temperature in an open container or in a mold that the matrix metal melts and the blowing agent decomposes to foam the melt. Depending on the heating and cooling rate, as well as the foaming time at maximum temperature, foam bodies of different pore size and structure are achieved. Titanium hydride, aluminum hydroxide and sodium bicarbonate are mentioned as blowing agents.
By this method, however, metal foams of high and homogeneous pore volume can only be produced unsatisfactorily. The low molding compound viscosity required for foaming causes heating to the usually high metal melting temperatures, which has many disadvantages. It comes during the foaming process for the unwanted combination of individual gas bubbles with the risk of collapse of the foaming molding composition and the formation of in their size distribution insufficient controllable pores.
The object of the present invention is thus to provide an improved process for producing a highly porous metallic U / o ceramic sintered body by means of foaming a molding composition with the aid of a blowing agent. The disadvantages of known processes, such as time-consuming and expensive process steps, high foaming temperatures, shrinkage of the molding after foaming and insufficient influenceability of the desired pore structure, even with only moderately high total pore volumes, should be avoided or brought to a significantly lower level.

This object is achieved for the method described above in an inventive manner by the method features mentioned in the claims.

The method thus serves to produce highly porous sintered shaped bodies with a cellular pore structure, ie the shaped body has comparatively thin cell walls, measured on the volume of the pores formed by them. The finished sintered bodies have a solid sintered skeleton of the matrix materials metal u / o ceramic, free of additives, or only with insignificantly small residual amounts of such, the molding compound originally added additives. They have high mechanical strength. The sintered cell walls are largely free of microporosity, but can also be manufactured on request in microporous execution.
Depending on the requirements, the cell-like pores have a largely homogeneously uniform mean pore diameter of preferably 0.1 to 10 mm in the finished sintered body, in contrast to a microporosity that is regularly smaller by at least one order of magnitude, as known from sintering technology , The pore volume in the sintered body is preferably
60-85 vol.%. Such high pore volume fractions are achievable only with strictly geometrically similar, for example honeycomb-like arrangement of the pores in the sintered shaped body.

For the formation of large-pored cellular structures, commercially available EPS (expandable poly-styrene) is preferably used as the polystyrene blowing agent, ie non-foamed polystyrene beads having a particle diameter of preferably 0.1 to 5 mm and containing as the blowing agent the volatile hydrocarbons pentane or hexane in a proportion of 1 to 8 % By weight.
For a specific influence on the foaming characteristic, it is also possible to use copolymers of the monomeric styrene with fractions of acrylic esters or acrylonitrile instead of the pure EPS polystyrene beads.

Mainly of the MIM technology is a variety of thermoplastic binder materials and combinations of individual binder components known. By means of a component selection familiar to the person skilled in the art, a broad variety of binders which can be adapted to the respective requirement can be achieved. For the intended implementation of the present invention, however, it is precisely the guarantee of a suitably low melt viscosity of the entire molding composition at the foaming temperature of 80 to 130 ° C. which is predetermined by the gas release of the blowing agent of great importance.
Based on the language used in MIM technology is then called a molten molding composition for the mixture of preferably organic binder components and matrix powder, if this has a low-viscosity, pulpy state. -

The suitable combination of inventive blowing agent and matched thermoplastic binder components allows foaming of the molding composition up to comparatively very high pore volumes, measured on the known prior art. According to a preferred embodiment of the method, sintered shaped bodies with greater than 30% by volume up to more than 85% by volume of cell-forming pores are produced in the sintered shaped body.
The plasticity of the molding material, which is sufficient for foaming, is still present at significantly more than 50% by volume of metallic u / o ceramic matrix powder and correspondingly lower binder content in the prepared, unfoamed molding composition. High proportions of matrix powder favor the subsequent sintering to mechanically strong sintered molded body substantially or make this possible. Known methods aimed at achieving high pore volumes did not allow comparably favorable volume fractions in practice. On the contrary, known methods require great compromises with regard to the simultaneous sintering stability and high pore volume in the sintered shaped body.
Mechanically strong sintered moldings with stable sintered skeleton and high pore volume fractions can be achieved according to the invention via the use of EPS as blowing agent, because this, in contrast to blowing agents according to the known state of the art, is not solely responsible for the release of gases for the purpose of gas bubble and pore formation in the molding compound. rather rather leads to the formation of foamed, mechanically bearing, self-contained polystyrene foam beads. This is the only way to avoid the collapse of foamed melts, which was dreaded in previous processes, beyond a certain, comparatively small pore size. It comes in the present method neither to unite individual small to a large gas bubble, or pore, nor to collapse foamed molding compositions for lack of sufficient thermoplasticity when exceeding the surface tension boundary between the gas bubble and molding compound.

By means of coordination of the chemical / physical properties of the binder components with the blowing agent according to the invention, which is familiar to the person skilled in the art, a further advantage of the inventive method is achieved in the foamed molding composition which has hitherto not been achieved. Usually, in a step following the foaming, both the binder components and the inflated polystyrene beads are predominantly discharged from the molding composition via a solution process in organic solvents, such as acetone or ethyl acetate. The mechanical dimensional stability is lost. The process according to the invention uses, as a preponderantly predominant binder component, already known high-polymer plastics, such as, for example, polyamides, which are insoluble in the abovementioned solvents customary for extraction.
Other binder components used are plasticizers, surfactants and release agents that are as soluble in acetone and ethyl acetate at temperatures above 30 ° C as the polystyrene. These solvent-soluble additional components can lead to microporosity of the (still unsintered) cell walls and facilitate the application of solvent and solutes therein. It is now the high-polymer plastic which can not be dissolved out of the foamed molding material during the extraction process which gives the metallic metallic powder particles sufficient mechanical strength even with a macroporous fraction of 85% by volume in the molding compound, on the one hand for the extraction step without volume shrinkage, as well as for manipulation of the extracted, unsintered shaped body, and finally for, with respect to shape preservation critical initial phase of the sintering process of the metallic u / o ceramic powder particles until the time of residue-free pyrolysis of the binder at 500 ° C.

The proportion of binder in the molding compound must be matched to the materials used in the molding compound and the process parameters for their processing. If this proportion is too high, it impairs the sintering together of the matrix powder during the subsequent sintering process. If the proportion is too low, the foamed molding composition falls below a minimum mechanical strength, which is indispensable for manipulability and further processing.

For the foaming process, the prepared molding material in a suitable shaping device is to be brought to a temperature suitable for volatilizing the blowing agents in the blowing agent, at the same time as the melting point of the molding compound. Foaming is all the more controlled and uniform the more evenly the polystyrene particles or EPS beads are distributed in the molding compound and the more homogeneous the temperature distribution in the molding compound.
When using a form provided with fine slots as a shaping device in a pressure-controlled autoclave, particularly good results can be achieved with regard to cell homogeneity, cell structure and volume fraction of the pores in the molding compound.

The process steps forming the molding compound and foaming can be carried out according to a number of different, previously practiced process.

For the production of geometrically complex moldings, the shaping and foaming of the molding composition has proven particularly useful by known injection molding.

Simply dimensioned shaped bodies, such as plates, rounds or spheres, can be obtained by pressing a pulverulent EPS-containing molding compound Producing compacts and subsequent foaming with steam in a form perforated by slots economically. According to a variant of the method, the compacts can optionally be laminated with a non-foamable surface layer in a subsequent powder pressing process. This will give you plates or discs with pore-free outer layer.

According to another economic step sequence according to the invention, the EPS is incorporated homogeneously into the molding compound melt at temperatures below 80 ° C. on a granulating extruder and the mass strands emerging from the perforated plate of the extruder are knocked off by means of so-called underwater granulation. In order not to have to accept premature gas losses from the EPS beads, it is expedient to carry out the underwater granulation under increased media pressure. Such EPS-containing molding compositions can be easily processed with the usual in plastics processing units to foamed molding compositions on.

After a similar process variant, EPS-containing granules are introduced directly into a vapor-permeable mold and foamed at the same time, as happens to a large extent with prefoamed EPS balls in the packaging industry. By means of this preferred method, the production of large-scale and large-volume moldings is feasible.

When incorporating the extrusion in the inventive method, the molding material is brought in a screw or piston press on melting and foaming simultaneously and pressed under high pressure of, for example 10 ⁶ to 10 ⁸ Pascal by a shaping tool. The melt emerging from the mold increases its volume under foaming and is brought to a so-called calibration with simultaneous cooling in its enlarged shape to solidification and thus deducted steadily.

According to a variant of the extrusion step sequence, the molding composition is cooled to prevent foaming after exiting the extrusion die under high pressure. In a subsequent In sequence, the shaped mass is reheated, foamed in a volume increase adapted shape, cooled and treated according to the features of the invention. This process variant is used primarily for the production of highly porous, large-area sintered moldings with either open or closed cell structure.

The process essential to the invention, in contrast to the preferred production of sintered compacts with closed pores or cells, always open cell structures, either either the ductility of the molten mass melt is too small for the speed and extent of foaming - and these can be controlled specifically or if the foaming process is influenced, for example, by increasing the proportion of EPS in the molding compound such that the molding composition to be provided locally to form and maintain closed cells is insufficient, so that the further expanding EPS beads receive direct surface contact with their adjacent neighbors ,

With regard to the selection of suitable for the process according to the invention metallic and ceramic matrix materials is only in so far as a limitation, as they must be in the form of sinterable powder, a requirement whose implementation belongs to the knowledge of Pulvermetallurgen. Preferred ceramic matrix materials are the oxides of aluminum, silicon and zirconium, as well as silicon nitride and mixtures thereof.
As metallic matrix materials, metals and alloys from the group Fe, Co, Ni, Cu, Ti, Ta, Mo, W and the precious metals, as well as metallic oxides, hydrides and hard metals have proven particularly useful.

Sintered bodies produced by the process according to the invention have a wide field of application. The focus is on the application in the field of lightweight components and parts with relatively low thermal conductivity, as well as in the case of open-pored sintered moldings in the field of mechanical filters and catalysts.

The invention is described in more detail by the following process examples.

Example 1 describes the preparation of a porous chromium nickel steel sintered body. Water-atomized chromium nickel powder grade 316 L (Pamco, Japan, 90% particle size less than 15 microns) is in a kneading aggregate with binder components, composed of polyamide, plasticizer, wetting and release agent (the binder), in a weight ratio, 93 , 5% by weight of 316 L powder, 6.5% by weight of binder are thoroughly mixed and kneaded at about 100 ° C. until a low-viscosity melt is present.
This mass is discharged from the kneading unit, solidified by cooling and ground to powder of a particle size smaller than 0.3 mm. 140 g of this powder are mixed with 13 g of EPS beads (Styrofoam P 656 from BASF, particle size 0.3 to 0.4 mm) in a laboratory mixer and at room temperature under a pressure of 200 bar to a powder compact of dimensions 60 x 90 x 7.2 mm ³ pressed.
This compact is placed in a 20 mm high Al frame of dimensions 70 x 100 mm ² , its top and bottom surfaces are covered with filter paper and fine mesh and then each with 6 mm thick Al plates, so that a closed, pressure-resistant and yet vapor permeable form arises. The vapor permeability is ensured by holes in the plates of 4 mm diameter and 3 mm spacing.
The mold filled with compact is exposed for 4 minutes in a steam autoclave with steam at 120 ° C. under steam pressure of less than 0.7 bar. After cooling the autoclave to less than 100 ° C, the mold is removed and cooled to about 30 ° C under cold water. The molded article of dimensions 70 × 100 × 20 mm ³ inflated compact is removed after removal from the mold from the filter paper and dried for 2 h at 60 ° C. He loses 2.5 wt.% Of moisture.
Thereafter, the molding is treated for 24 hours, resting on a perforated plate, in 50 ° C warm ethyl acetate as a solvent. Subsequently, the solvent-soluble and dissolved in it substances, already porous shaped body is removed from the bath and freed from the solution by means of vacuum distillation. The remaining, still unsintered Shaped body has a weight of 137 g with respect to the foamed molding unchanged outer dimensions. From a comparison with the weight of the molding compound
(140 g + 13 g = 153 g) results in a weight loss of 16 g, which, based on 17.2 g of theoretically extractable material, corresponding to a share of 93.0%. As the first stage of the final molding sintering, the not yet extracted portion of polystyrene and binder components, above all polyamide in volatile form, is removed from the molding by means of pyrolysis at 500 ° C. With the further sintering process for 60 min at 1320 ° C, a sintered compact of dimensions 61.5 x 88 x 17.3 mm ³ and 130 g of weight is produced.
This corresponds to a density of about 1.4 g / cm ³ or a pore volume of 82%.
The average diameter of the largely uniformly sized pores, or cells in the sintered shaped body is about 0.60 mm.

Example 2 describes the preparation of a porous Al ₂ O ₃ sintered body.
For this purpose, a sinterable Al ₂ O ₃ powder of 3 microns average particle size and 99.80% purity (grade CT 3000 SG, Fa. ALCOA) in a kneading unit with binder components (polyamide, plasticizer, wetting and release agent) at 100 ° C. mixed thoroughly and kneaded until a low-viscosity melt is present. The weight fractions are 86.0% by weight of CT 3000 SG and 14.0% by weight of binder components.
According to Example 1, the kneaded mass is discharged from the kneading unit, cooled and ground into powder of a particle size smaller than 0.3 mm.
Then 65 g of this powder mass is mixed with 25 g of EPS beads (Styrofoam P 656, BASF, particle size 0.3 to 0.4 mm) in a laboratory mixer and at room temperature under 200 bar pressing pressure to a compact of the dimension 60 x 90 x 12 mm ³ pressed.

Analogously to Example 1, the compact is processed to a foamed compact of dimensions 70 x 100 x 20 mm ³ and then stored for the extraction of soluble substances in ethyl acetate as a solvent.
The molded article present after vacuum distillation is 62 g and has the unaltered dimensions of 70 × 100 × 20 mm ^3. The weight loss compared to the weighing-in amounts at this point to 28 g, which corresponds to 89% of the theoretically extractable amount of substance of 31.5 g equivalent.
After pyrolysis of the remaining portions of the polystyrene and the binder components at 500 ° C in air and sintering at 1550 ° C for 60 minutes, the sintered compact has the dimensions 60 x 86 x17 mm ³ and a weight of 56 g.
This corresponds to a density of about 0.64 g / cm ³ , or a pore volume of 84%.
The mean diameter of the macropores is 0.60 mm.
The sintered body is mechanically stable or insensitive to breakage so that it can be manipulated and used without restrictive precautions with only a slight risk of damage.

Claims (15)

1. A process for producing a cellular shaped sintered body, which comprises the manufacturing steps of preparation of a thermoplastically flowable molding composition by mixing of ceramic and/or metal powder with binder components and incorporation of organic and/or inorganic blowing agents, conversion of the molding composition into a molten state and introduction into a shaping device, foaming of the molding composition by means of the blowing agent, solidification of the foamed molding composition, removal of blowing agents and organic components and sintering of the shaped body which has been treated in this way, characterized in that expandable polystyrene particles are used as blowing agent and the foaming step is carried out at temperatures of from 80° to 130°C in a mold which leaves room for expansion of the molding composition to form individual polystyrene foam particles which each take up a closed space in the molding composition and have a narrow diameter distribution.
2. The process as claimed in claim 1, characterized in that bead-shaped polystyrene particles having a mean diameter of from 0.1 to 5 mm and a small scatter of the diameter are used.
3. The process as claimed in claim 1 or 2, characterized in that a copolymer of monomeric styrene and acrylic esters or acrylonitrile is used as blowing agent.
4. The process as claimed in any of claims 1 to 3, characterized in that polystyrene comprising pentane or hexane as expandable agent is used.
5. The process as claimed in any of claims 1 to 4, characterized in that the blowing agent is incorporated as solid, not preexpanded pellets into the mixture for the molding composition.
6. The process as claimed in any of claims 1 to 5, characterized in that small proportions of other thermally unstable, gas-releasing substances are additionally mixed into the molding composition physically separately from the expandable polystyrene particles to form micropores in the shaped body.
7. The process as claimed in any of claims 1 to 5, characterized in that space occupier particles which are chemically soluble or can be volatilized by means of pyrolysis are added to the molding composition in addition to the polystyrene particles and physically separately therefrom to form micropores in the shaped body.
8. The process as claimed in any of claims 1 to 5, characterized in that a proportion by volume of cell-forming pores of greater than 30% and less than 85%, based on the volume of the sintered shaped body, is formed on foaming.
9. The process as claimed in any of claims 1 to 8, characterized in that cell-forming pores having a mean diameter of 0.1 - 10 mm and a proportion of 60 - 85% by volume, based on the state in the sintered shaped body, are produced.
10. The process as claimed in any of claims 1 to 9, characterized in that the removal of blowing agents and organic components is effected by dissolution of these in organic solvents.
11. The process as claimed in any of claims 1 to 10, characterized in that the removal of the blowing agent is effected pyrolytically.
12. The process as claimed in any of claims 1 to 11, characterized in that the shaping and foaming process occurs after an extrusion process.
13. The process as claimed in any of claims 1 to 12, characterized in that the metal powder selected from the group consisting of Fe, Co, Ni, Cu, Ti, Ta, Mo, W and noble metals is introduced as pure metal, as oxide, nitride and/or hydride into the mixture for the molding composition.
14. The process as claimed in any of claims 1 to 13, characterized in that the metal powder is introduced in the form of a type of cemented carbide into the mixture for the molding composition.
15. The process as claimed in any of claims 1 to 14, characterized in that a mixture of various binder components having a predominant proportion by weight of polyamide is used.