Patent Application
20120087823
The invention relates to a process for producing porous sintered shaped metal bodies.
Metallic foams have some interesting properties: compared to the solid metal, their density is greatly reduced. However, they still have a high specific stiffness and strength. In the case of an impact, the cellular structure converts a great deal of kinetic energy into deformation energy and heat, so that metallic foams are well suited to incorporation in crash elements. Compared to polymer foams, metal foams have a significantly higher strength and heat resistance. Further potential applications include heat shields, filters, catalyst supports, sound-absorbing cladding or the production of very light, foam-filled rollers for the printing or paper industry.
Metal foams can be produced in various ways. In the “powder route”, a metal powder and a pulverulent blowing agent, e.g. titanium hydride powder (TiH3) are mixed, processed by uniaxial pressing or extrusion to form a foamable semifinished part and heated to the melting point of the metal. The blowing agent liberates gases and foams the molten metal. Depending on the time for which the temperature is held, the foam structure such as the relative density, pore size and the like changes. In the “melt route”, gas is blown into a metal melt and the foamed metal solidifies. To stabilize the bubbles in the melt, it is possible to add, for example, SiC particles to the melt.
Shaped metallic bodies can be produced by injection molding of thermoplastic compositions comprising metal powders together with a polymer as organic binder. These are highly filled organic polymer molding compositions. After injection molding, extrusion or pressing of the thermoplastic composition to form a green body, the organic binder is removed and the binder-free green body obtained is sintered. Porous shaped metallic bodies can also be obtained by concomitant use of blowing agents.
Thus, WO 2004/067476 discloses a process for producing a cellular sintered shaped body, in which metal powder is mixed with binder components and expandable polystyrene particles (EPS) are incorporated as blowing agent. This thermoplastically flowable molding composition is introduced into a housing mold for expansion of the molding composition, converted into a molten state and foamed. The foamed molding composition is solidified, the organic components are removed and the shaped body which has been treated in this way is sintered. The foaming step should occur with formation of individual expanded polystyrene foam particles which each take up a closed three-dimensional space in the molding composition and have a narrow diameter distribution. In this process, binder and foamable material are necessarily different from one another. Production of the molding composition is complicated and requires numerous successive steps.
According to WO 2004/067476, shaped bodies having simple geometries are obtained by pressing of a pulverulent EPS-comprising molding composition to form pressed bodies and subsequent foaming by means of steam in a perforated mold. Geometrically complex moldings are said to be obtainable by shaping and foaming of the molding composition by means of known injection molding processes. The process has the disadvantage that the pores produced by the EPS particles are very large, as a result of which only few stabilizing struts remain in the shaped body and these are also inhomogeneously distributed. Finally, materials whose mechanical properties are unsatisfactory for many applications are obtained.
DE 103 28 047 B3 describes a process for producing a component composed of metal foam, in which a plurality of metal foam building blocks which can be obtained by introduction of energy into and at least partial foaming of pellets comprising a metal powder and a blowing agent powder, e.g. a metal hydride, are arranged in three dimensions. The metal foam building blocks which have been arranged in this way are subjected to an after-treatment so that adjacent metal foam building blocks are joined to one another by positive locking, fusion and/or adhesively. A disadvantage of this process is that in the production of the metal foam building blocks it is possible for a partial collapse of the metal foam formed to occur, leading to uncontrollable formation of denser zones in the interior of a building block produced in this way and a low reproduction accuracy. Without adhesive bonding, the individual metal foam building blocks do not adhere to one another.
It is an object of the invention to provide a process for producing porous sintered shaped metal bodies, which is free of the above disadvantages.
The object is achieved by a process for producing porous sintered shaped metal bodies, wherein expandable polymer particles in which a sinterable metal powder is dispersed are foamed to form a shaped body and the green shaped body is subjected to a heat treatment in which the polymer is driven off and the sinterable metal powder sinters to give a porous sintered shaped metal body.
In an advantageous embodiment, the expandable polymer particles are introduced into a mold which is closed, preferably on all sides, after filling and the expandable polymer particles are foamed, for example by treatment with steam and/or hot air. The geometry (three-dimensional shape) of the mold usually corresponds to the desired geometry of the future molding.
In most cases, preference is given to prefoaming the expandable polymer particles before introduction into the mold. During prefoaming, the expandable polymer particles are heated with mechanical agitation, e.g. by fluidization by means of a hot gas, in particular air and/or steam. Prefoamers which are suitable for this purpose are known to those skilled in the art from the production of EPS insulation materials. Temperatures of, for example, 60 to 120° C. are generally suitable. Under these conditions, the particles expand as a result of the vaporizing blowing agent and partially also as a result of the steam which has penetrated into them to form a closed-cell structure in the interior of the bead. During prefoaming, the polymer particles do not fuse with one another and remain as discreet particles.
The density of the future shaped bodies can be influenced via the degree of foaming which depends mainly on the duration of the heat treatment. The duration of the heat treatment in prefoaming is typically from 5 to 100 seconds.
Without the prefoaming step, nonuniform expansion and filling of the mold can occur during foaming of the expandable polymer particles in the mold, with the expandable polymer particles in the vicinity of the heated walls of the mold expanding to a greater extent than particles in the interior of the mold.
In general, the mold is only partly filled with the (optionally prefoamed) expandable polymer particles. During foaming, the polymer particles expand and positively fill the initially incompletely filled mold with foam. The polymer particles are fused to one another during this operation.
Particularly in the case of complicated geometries, it can also be advantageous to keep the empty volume in the mold low and optionally compact the bed of the (optionally prefoamed) expandable polymer particles introduced into the mold and in this way eliminate undesirable voids. Compaction can be achieved, for example, by shaking of the mold, tumbling motions or other suitable measures.
Foaming is usually effected by heating to, for example, from 60 to 120° C., preferably from 70 to 110° C., e.g. by heating the filled mold by means of steam, hot air, boiling water or another heat transfer medium. Foaming increases the volume of the polymer component of the particles, with the interstices in the bed being filled out by the expanding polymer particles and a shape-producing assemblage of the individual particles occurring as a result of force interactions between the particles whose volumes are increasing. The polymer particles melt on the mutual contact surfaces, so that the polymer particles fuse together to give a shaped body (green body). The mold proscribes the shape and volume of the green body. The shaped body which has a sufficient green strength can be taken from the mold.
The pressure during foaming is usually not critical and is generally from 0.05 to 2 bar. The duration of full foaming depends, inter alia, on the size and geometry and also the desired density of the molding and can vary within wide limits.
The process of the invention starts out from expandable polymer particles in which a sinterable metal powder is dispersed. The expandable polymer particles are preferably free-flowing or flow readily. The proportion by weight of the dispersed sinterable metal powder, based on the total weight of polymer and sinterable metal powder, is preferably from 60 to 95% by weight, in particular from 65 to 90% by weight. In the polymer particles, the polymer forms a continuous (coherent) phase in which the sinterable metal powder is dispersed.
The expandable polymer particles preferably comprise a physical blowing agent such as aliphatic hydrocarbons having from 2 to 7 carbon atoms, alcohols, ketones, ethers, halogenated hydrocarbons, carbon dioxide or water or mixtures thereof. Preference is given to isobutane, n-butane, isopentane or n-pentane or mixtures thereof. The expandable polymer particles generally comprise from 2 to 20% by weight, preferably from 3 to 15% by weight, of blowing agent, based on the polymer in the expandable polymer particles. The blowing agent is present in the expandable polymer particles as a molecular solution in the polymer and/or as included droplets.
The expandable polymer particles are preferably essentially spherical, but another shape such as rod-shaped or lens-shaped pellets is also possible. The expandable polymer particles generally have a diameter (or length in the direction of the largest dimension in the case of nonspherical particles) of from 0.5 to 30 mm, in particular from 0.7 to 10 mm.
The expandable polymer particles can be obtained in various ways.
The expandable polymer particles can be obtained, for example, by producing expandable thermoplastic polymer pellets by mixing a blowing agent and a sinterable metal powder into a polymer melt and pelletizing the melt. The expandable polymer particles are preferably produced by means of an extrusion process. Here, the blowing agent is mixed into a polymer melt via an extruder, the sinterable metal powder is mixed in and the polymer melt is pushed through a die plate and pelletized to give particles. The melt is usually cooled after introduction of the blowing agent. Each of these steps can be carried out by means of the apparatuses or apparatus combinations known in plastics processing. The polymer melt can be taken directly from a polymerization reactor or be produced in the mixing extruder or a separate melting extruder by melting of polymer pellets. Static or dynamic mixers are suitable for mixing in the blowing agent and the sinterable metal powder. Cooling of the melt can be carried out in the mixing apparatuses or in separate coolers. Possible pelletization methods are, for example, pressurized underwater pelletization, pelletization using rotating knives and cooling by spray misting of cooling liquids or pelletization by atomization.
The sinterable metal powder is appropriately mixed in via a side extruder. For example, a substream of the melt stream initially obtained can be branched off via a melt valve into a side stream before passage through the die plate. The metal powder is added to the side stream and mixed homogeneously into the melt stream. Finally, the main stream and the additive-comprising side stream are mixed and discharged via the die plate. To be able to meter the metal powder with sufficient accuracy into the melt stream, the powder can be pasted beforehand. This means that it is incorporated into a liquid which is compatible with the melt and the metal powder so as to form a paste having a preferably high viscosity.
A suitable process is described, for example, in DE 10 358 786 A1, which is hereby fully incorporated by reference.
If temperatures above the flash point of the metal powder are reached during production of the expandable polymer particles, a suitable protective gas such as nitrogen or argon is preferably passed through the apparatuses.
As an alternative, pellets can firstly be produced by mixing a sinterable metal powder into a polymer melt and pelletizing the melt. These pellets can then be reshaped into beads in aqueous suspension in heated and stirred pressure vessels at temperatures in the vicinity of the softening point and at the same time impregnated with blowing agent. This conversion into beads gives bead-shaped particles having a defined particle size. The conversion into beads is generally carried out at from 120 to 160° C., e.g. about 140° C., over a period of from 1 to 24 hours, e.g. from 12 to 16 hours. Suitable processes are described, for example, in DE-A 25 34 833, DE-A 26 21 448, EP-A 53 333 and EP-6 95 109, which are fully incorporated by reference.
As an alternative, the pellets can be impregnated with blowing agent under superatmospheric pressure at a temperature below the softening temperature of the polymer. A pressure of from 25 to 70 bar (absolute), e.g. about 50 bar, is suitable for this purpose. The temperature can be, for example, from 25 to 60° C., e.g. about 40° C. A time of from 0.5 to 20 hours, e.g. about 8 hours, is generally suitable. For this purpose, a pressure-rated apparatus, e.g. an autoclave, is charged with the pellets, the blowing agent is added in such an amount that it preferably completely covers the pellets and the apparatus is closed. Air is displaced by an inert gas such as nitrogen. The apparatus is then heated and the desired pressure is set. The pressure is established as autogenous pressure of the blowing agent at the treatment temperature or is set by injection of inert gas.
As sinterable metal powders, mention may be made of, for example, aluminum, iron, in particular iron carbonyl powder, cobalt, copper, nickel, silicon, titanium and tungsten, among which aluminum, iron, copper, nickel and titanium are preferred. As pulverulent metal alloys, mention may be made by way of example of high- or low-alloy steels and also metal alloys based on aluminum, iron, titanium, copper, nickel, cobalt or tungsten. Here, it is possible to use either powders of finished alloys or powder mixtures of the individual alloy constituents. The metal powders, metal alloy powders and metal carbonyl powders can also be used in admixture. When mixed metal powders are used, the melting points of the components of the mixture should not differ too much from one another, since otherwise the lower-melting component flows and the higher-melting component remains. The maximum melting point difference is preferably 800° C. or less, in particular 500° C. or less and most preferably 300° C. or less.
Suitable metal powders are, for example, atomized metal powders which have been obtained by spraying of liquid metal with compressed gases.
Carbonyl iron powder is preferred as metal powder. Carbon iron powder is an iron powder which is produced by thermal decomposition of iron carbonyl compounds. To maintain flowability and to prevent agglomeration, it can, for example, be coated with SiO2. Iron phosphide powder can preferably be concomitantly used as corrosion inhibitor. Carbonyl iron powder has a small and uniform particle size; the particles have an essentially spherical shape. The melt viscosity of the composites with polymers is therefore very low and the melting point is uniform. Suitable carbonyl iron powders are described, for example, in DE 10 2005 062 028.
Further preferred metal powders are powders composed of aluminum and copper.
The particle sizes of the powders are preferably from 0.1 to 80 μm, particularly preferably from 1.0 to 50 μm.
Suitable polymers are thermoplastic polymers having a good uptake capacity for blowing agent, for example styrene polymers, polyamides (PA), polyolefins such as polypropylene (PP), polyethylene (PE) or polyethylene-propylene copolymers, polyacrylates such as polymethyl methacrylate (PMMA), polycarbonate (PC), polyesters such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), polyether sulfones (PES), polyether ketones or polyether sulfides (PES) or mixtures thereof. Particular preference is given to using styrene polymers.
As styrene polymers, preference is given to using clear, colorless polystyrene (GPPS), high-impact polystyrene (HIPS), anionically polymerized polystyrene or high-impact polystyrene (A-IPS), styrene-α-methstyrene copolymers, acrylonitrile-butadiene-styrene polymers (ABS), styrene-acrylonitrile (SAN), acrylonitrile-styrene-acrylic ester (ASA), methyl acrylate-butadiene-styrene (MBS), methyl methacrylate-acrylonitrile-butadiene-styrene (MABS) polymers or mixtures thereof or with polyphenylene ether (PPE).
To achieve better dispersion of the particles in the polymer melt, dispersants can optionally be added. Examples are oligomeric polyethylene oxide having an average molecular weight from 200 to 600, stearic acid, stearamide, hydroxystearic acid, magnesium, calcium or zinc stearate, fatty alcohols, ethoxylated fatty alcohols, fatty alcohol sulfonates, ethoxylated glycerides and block copolymers of ethylene oxide and propylene oxide, and also polyisobutylene.
The polymer is driven off by means of a heat treatment. The sinterable metal powder is sintered to give a porous sintered shaped body. The term “drive off” comprises upstream decomposition and/or pyrolysis steps. The heat treatment can be carried out in a single-stage or multistage process. Preference is given to driving off the polymer (binder removal) at a first temperature in a first step and sintering the resulting binder-free shaped body at a second temperature. The second temperature is generally at least 100° C. higher than the first temperature. If the shaped body is exposed directly to the sintering temperature, severe soot formation on the shaped metal body is frequently observed, presumably due to excessively rapid pyrolysis.
Binder removal and the sintering process can be carried out in the same apparatus; however, different apparatuses can also be used. Suitable furnaces for carrying out binder removal and/or sintering are convection box furnaces, shaft retort furnaces, convection shuttle kilns, hood-type furnaces, elevator furnaces, muffle furnaces and tube furnaces. Belt furnaces, combi-chamber furnaces or shuttle kilns are suitable for carrying out the steps of binder removal and sintering in the same apparatus. The furnaces can be provided with facilities for setting a defined binder removal atmosphere and/or sintering atmosphere.
To carry out binder removal, the shaped body is preferably exposed suddenly to the binder removal temperature and not heated slowly to the binder removal temperature, since otherwise the polymer can run and the foam structure can be lost. It is thus generally not a good idea to leave the shaped body in the heating zone of the furnace during the heating phase. To carry out binder removal in the laboratory, it is possible to use, for example, a tube furnace having a long interior tube and to position the specimen in the tube but outside the heating zone during the heating phase. As soon as the target temperature has been reached, the specimen can be pushed into the heating zone. Binder removal can be carried out industrially using, for example, belt furnaces in particular.
Binder removal is preferably carried out in a defined atmosphere. In general, preference is given to an inert atmosphere or reducing atmosphere, with a reducing atmosphere being particularly preferred. In the case of metals such as aluminum, zinc or copper, it can be advantageous to carry out binder removal under slightly oxidizing conditions in order to increase the green strength. Better removal of residual carbon and a strength-increasing oxide skin on the surface of the metal powder particles are achieved in this way.
A temperature of from 150 to 800° C. is generally suitable for binder removal. In the case of iron, a temperature of about 700° C. has been found to be useful, and a temperature of from 400 to 600° C. has been found to be useful in the case of aluminum. The duration depends greatly on the size of the shaped bodies.
Binder removal is followed by a sintering process. This sintering process can be carried out at a temperature of from 250 to 1500° C. In the case of iron, a sintering temperature of from 900 to 1100° C. has been found to be useful, and a temperature of not more than 650° C. has been found to be useful in the case of aluminum. The sintering atmosphere can be matched to the metal used. In general, an inert atmosphere or reducing atmosphere is preferred, with a reducing atmosphere being particularly preferred.
As reducing atmosphere during binder removal and/or sintering, hydrogen or mixtures of hydrogen and inert gas, e.g. a hydrogen/nitrogen mixture, have been found to be useful. The mixture of hydrogen and inert gas preferably comprises at least 3% by volume of hydrogen.
The shaped body can sometimes undergo “after-foaming” during pyrolysis. It can be advantageous to carry out the pyrolysis in a mold having perforated walls, with greater filling of the mold and also compaction and conglutination taking place.
High-strength porous metallic light-weight bodies are achieved according to the invention.
The invention is illustrated by the following examples.
a) Extrusion of polystyrene with carbonyl iron powder:
4.0 kg of polystyrene (obtainable under the designation 158K from BASF SE, Ludwigshafen, Germany) were compounded with 16 kg of carbonyl iron powder (carbonyl iron powder EQ, obtainable from BASF SE) in an extruder and the melt was pelletized by die-face pelletization to give pellets having an average particle size of about 3 mm.
b) Pressure impregnation with pentane:
The pellets were then immersed in pentane S (80% of n-pentane, 20% of isopentane) and maintained at a pressure of 50 bar and a temperature of 40° C. in a pressure autoclave for 4 hours. This gave polymer particles loaded with about 5% by weight of pentane.
c) Production of the green body:
The pellets were introduced into a closed cube-shaped steel mold having an edge length of 4 cm and the mold was heated to about 100° C. by means of steam for 10 minutes. The polymer particles expanded during this treatment and fused to give a green body which was taken from the mold.
d) Binder removal and sintering:
The green body was sawn into smaller cubes by means of a saw and subsequently placed in a porcelain boat in a fused silica tube. The fused silica tube was installed horizontally in a hinged high-temperature tube furnace (model LOBA 11-50 from HTM Reetz, Berlin, Germany). The fused silica tube projected out of the furnace at both ends. The porcelain boat was firstly placed in an outer end of the fused silica tube, i.e. outside the heating zone. Nitrogen was passed through the fused silica tube.
The furnace was set to 700° C. As soon as the furnace reached a temperature of 700° C., the nitrogen flow was reduced by 50% from 20 l/h to 10 l/h and supplemented by a hydrogen flow of 10 l/h. The porcelain boat was subsequently pushed inside the fused silica tube into the middle of the furnace. After the specimen had reached a temperature of 700° C., it was left in the heating zone for 10 minutes. It was then pulled from the heating zone again to the end of the fused silica tube.
The furnace was then set to 900° C. As soon as the set temperature had been reached, the porcelain boat was pushed back into the middle of the furnace. After a sintering temperature of 900° C. had been reached, the specimen was left in the heating zone for 15 minutes. The porcelain boat was then pulled out of the heating zone again and the furnace was switched off. After cooling, the specimen was taken out.
e) Examination of the mechanical properties:
The examination of the mechanical properties was carried out by a method based on the test standard DIN EN 826—compressive strength of insulation materials. Here, the compressive stress at 10-100% deformation and also the E modulus can be determined. Specimens having the same composition which had all been subjected to binder removal at 700° C. for 10 minutes but had been sintered at different temperatures (900° C. and 1000° C.) for different residence times were examined.
The following results were obtained:
Kneading of polystyrene with carbonyl iron powder:
70 g of polystyrene 158K (from BASF SE, Ludwigshafen, Germany) were melted in a kneader (model Messkneter H60 from IKA Staufen, Germany). 280 g of carbonyl iron powder EQ were subsequently added a little at a time. The mixture was subsequently kneaded for 30 minutes. After kneading, the product was discharged and roughly pelletized. The coarse pellets were subsequently milled to an average diameter of about 5 mm in a mill. The further steps were carried out in a manner analogous to example 1.
Kneading of polystyrene with aluminum:
200 g of polystyrene 158K (from BASF, Ludwigshafen, Germany) were melted in a kneader (from Linden, Marienheide, Germany). 622 g of coarse aluminum powder ASMEP123 CL (from ECKA, Fürth, Germany) were subsequently added a little at a time and the mixture was subsequently kneaded for 30 minutes. After kneading, the product was discharged and roughly pelletized. The coarse pellets were subsequently milled to an average diameter of about 5 mm in a mill. The further steps were carried out in a manner analogous to example 1, with binder removal and sintering being carried out in one step at 600° C. over a period of 5 minutes.
Kneading of polystyrene with copper
200 g of polystyrene 158K (from BASF, Ludwigshafen, Germany) were melted in a kneader (from Linden, Marienheide, Germany). 910 g of copper Rogal GK 0/50 (from ECKA, Fürth, Germany) were subsequently added a little at a time and the mixture was subsequently kneaded for 30 minutes. After kneading, the product was discharged and roughly pelletized. The coarse pellets were subsequently milled to an average diameter of about 5 mm in a mill. The further steps were carried out in a manner analogous to example 1, with binder removal being carried out at 700° C. over a period of 5 minutes and sintering being carried out at 850° C. over a period of 10 minutes.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09161664.9 | Jun 2009 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2010/057618 | 6/1/2010 | WO | 00 | 11/30/2011 |
