THIN SELF-SUPPORTING INORGANIC GREEN COMPACTS AND PROCESS FOR THE PREPARATION OF SUCH GREEN COMPACTS

Abstract: Thin self-supporting green compact, less than 25 mu thick, containing an inorganic substance and as a binder a polymer with a weight average molecular weight of more than 400,000, which polymer in a suitable solvent can form a thermoreversible gel above room temperature, the volume fraction of the inorganic substance being more than 45 % relative to the total amount of binder plus inorganic substance and a process to prepare such green compacts.

Description: THIN SELF-SUPPORTING INORGANIC GREEN COMPACTS AND PROCESS FOR THE PREPARATION OF SUCH GREEN COMPACTS The invention relates to a thin self-supporting inorganic green compact which contains an inorganic substance and a binder and is less than 25 μm thick. Such inorganic green compacts are described in EP-A-0210874. A drawback of the green compacts described in said patent, in this case films, is that cellulose is used as binder, which limits the stretchability of the green compacts. Because this means that the desired extremely thin films have to be produced in one go. The films are produced by first applying a thin layer of the solution containing binder and inorganic substance to a support, drying it and then separating the layer from the support. Such a process imposes very high intrinsic requirements on the equipment to be used to produce the films because irregularities in thickness or cracks easily occur. For the same reason such processes are extremely sensitive to the occurrence in the added inorganic substance of particles with dimensions similar to the layer thickness. In practice, it therefore appears to be hardly possible to produce very thin-walled objects without cracks in this manner, tests having shown that it is very difficult to remove the films from the support, in particular in the case of thin films (10-20 μ ) . Moreover, cellulose is sensitive to moisture, as a result of which the dimensional stability is adversely affected, which precludes specific applications. In the framework of the invention reference is made to a green compact as long as the organic binder is still present as such and has not yet been removed or converted. The aim of the invention is to provide thin green compacts of an inorganic substance bound with an organic binder which do not have the aforementioned drawbacks. The green compacts according to the invention are characterized in that the binder consists substantially of a polymer with a weight average molecular weight of more than 400,000, which polymer, incorporated in a suitable solvent, forms a thermoreversible gelling system with gel and dissolution points above room temperature, the volume fraction of the inorganic substance being more than 45% relative to the total amount of binder and inorganic substance. Such green compacts are easily obtainable without having to impose particularly high requirements on the equipment to be used. Moreover, it is possible to use said green compacts to apply inorganic layers, whether or not sintered, in objects. An example of such an object is a multilayer ceramic capacitor, in the case of which a low ceramic layer thickness is a great advantage with respect to the miniaturisation aimed at in this application. The invention therefore also relates to a process for the preparation of said green compacts and the use of the green compacts according to the invention in the application in objects of layers containing inorganic substance. DE-A-2334704 describes the preparation of fibres of polyethylene oxide in which substantial amounts of fillers can be incorporated. The polymer-solvent system described in said patent is a high-viscosity solution at room temperature, which causes problems with respect to the feeding of the extruder. Another drawback of the polyethylene oxide/water system used is that this system does not form a gelling system until it is cooled to below room temperature. At room temperature the product obtained therefore remains tacky as long as it contains solvent. Consequently, the solvent has to be removed to enable further processing. This is a particular drawback because the solvent has to be removed from the unstretched and thus thicker green compact, which is more difficult and takes longer. Thermoreversible gelling systems are in the scope of the invention understood to be systems characterized by the fact that the conversion into a gel of a polymer dissolved in a solvent is effected by means of a (rapid) drop in temperature. Examples of thermoreversible gelling systems with gel and dissolution points above room temperature are: polyalkenes with the usual solvents such as paraffins, toluene, xylene, tetralin or decalin; polyvinyl alcohol (PVAL) with ethylene glycol as solvent; polyacrylonitrile (PAN) with dimethylformamide as solvent, to which zinc chloride is added as complexing salt, in an amount of about 10 wt.% relative to the amount of PAN. The green compact may have any simple shape obtainable via the usual continuous shaping methods, such as a fibre, film, or tube. The shape is not restricted to flat objects. Three-dimensional structures such as curved films or cylindrical shapes are also possible. If the green compact concerns fibres the thickness of the green compact is understood to be the diameter of the fibre. The binder can be removed according to commonly known methods, such as pyrolysis, extraction or firing. Good results, particularly as regards dimensional stability, were obtained by removing the binder by means of low-temperat e incineration with the aid of plasmas. If so desired, the object may then be subjected to sintering, in which case the object is brought to a certain temperature for a certain amount of time, in a known manner. The conditions to be applied are known in the literature and depend on the inorganic substance used. To obtain a continuous layer of inorganic material, the filler content must be as high as possible. In the case of the green compacts according to the invention the filler content is more than 45 vol.%, usually 50-99 vol.% inorganic substance relative to inorganic substance plus polymer binder. In some cases it may be useful to use an inorganic substance with a bimodal particle size distribution to obtain a high filler content. By sintering the green compact after the removal of the binder a self-supporting, whether or not continuous, object can be obtained. As inorganic sinterable substances use may be made of all substances or mixtures that are sinterable per εe or of inorganic substances that are not sinterable per se, to which suitable sintering additives are added, for example SiO~, Al-C , BaTiOn, Si^ . mixed with Y-O, or SiC mixed with boron compounds. The green compacts appear to be suitable for many applications. In some cases the green compact as such will meet the requirements, whereas in other cases the specific application will require a thin inorganic layer, which is obtained after the binder has been removed from the green compact, if so desired with subsequent sintering. The following applications are illustrative as examples, but the list is not to be regarded as limitative. If the inorganic substance has a high dielectric constant, thin dielectric layers can be obtained after firing and sintering, which are suitable for use in capacitors. In addition, thin conductive films can be produced, so that multilayer ceramic capacitors in particular present good possibilities for the use of the green compacts according to the invention. In addition, it appeared possible to produce very thin substrata for hybrid electronic structures. Good results have been obtained in this field using Al_0, as inorganic filler. An advantage of the green compact according to the invention in such applications is that the porosity of the green compact can be varied within wide limits to meet the demands involved in the various applications. The green films are suitable as thin flexible envelopes of objects for better protection from the influence of high-energy electromagnetic radiation. Several metallic fillers can be considered for this purpose. The green compacts are also very suitable for use as coatings. Abrasion-resistant coatings can be applied to objects using, for example, carbides or nitrides, in which case the * flexibility of the green compact appeared an advantage in particular in obtaining a good cover for complex shapes. It has been found that the green compact according to the invention has sufficiently high strength and modulus to be handled during the further processing. This is particularly the case if the modulus of elasticity of the green compact is at least 0.5 GPa. The invention also relates to a process for the preparation of thin-walled inorganic green compacts. A process for the production of filled fibres of organic material is known from US-A-4411854, in which process a high-molecular polyalkene with a weight average molecular weight of more than 4 x 10 dissolved in a suitable solvent with a binder content of less than 30 wt.%, relative to the binder plus solvent, is converted, at a temperate above the gelling temperature of the solution, into a fibrous, solvent-containing object, which object is rapidly cooled to below the gelling temperature and stretched. An additive was added to the fibres to improve the tensile strength and the modulus. It is advised to use less than 37.5% additive, relative to the amount of polymer plus filler. It has now appeared possible to use the process according to the invention to produce green compacts containing very large amounts of inorganic substance, which are di enεionally stable and εtretchable and contain little binder relative to the amount of inorganic subεtance. In thiε manner a thin, continuous inorganic object can be obtained after the removal of the polymer binder and, optionally, sintering. Preferably, the green compact is stretched biaxially because it has been found, surprisingly, that the maximum possible draw ratio does not decrease when the green compact is stretched biaxially. This is particularly surprising because the maximum draw ratio does decrease with an increasing filler content in the case of uniaxial stretching. Another advantage is that the thickness of the green compact decreases rapidly in biaxial stretching, which means that thin stretched green compacts can be eaεily obtained. Preferably, uεe is made of a polymer with the highest possible molecular weight. For it has been found that if the molecular weight is high, the concentration of polymer in εolution may be low and will nevertheleεs, after shaping, result in a coherent stretchable and dimensionally stable structure. It is important to use the lowest possible concentration of polymer binder in εolution becauεe then a higher inorganic εubstance/polymer binder volumetric ratio can be obtained after the removal of the solvent. This iε becauεe the allowable amount of inorganic εubεtance iε determined by the closeεt concentration of the particles in the εolution. For εpherical particles of uniform size the theoretical maximum amount of inorganic εubεtance iε thuε 60 vol.% relative to inorganic εubεtance plus polymer solution. In practice, use will generally be made of amounts of inorganic subεtance of between 20 and 40 vol.% in the εolution. The filler content w of the green compact iε defined aε: = V i /(V i + V ) (1) The maximum possible filler content of the green compact ( mϊ ) iε related to the filler content of the solution (a) according to the equation: v max (a) - a (a + M emelt/V (2 > These quantities are defined aε followε: a = V./(V. + V + V_), the filler content of the solution; V ,V. ,V = the volume of the polymer, inorganic substance and solvent, reεpectively; Memel,t. = the molecular weig J ht of the p c oly J mer between two cross-links in the equilibrium melt (g/mole) M = the number average molecular weight of the polymer (g/mole). Memel,t. can be determined according 3 to the method deεcribed in Advances in Polymer Science, 1974, part 16, Chapter 7. The Memel,t. to be determined for p coly _.ethy _.lene in thiε manner is 4000. M can be determined according to the usual methods, for example GPC. The polydisperεity defined aε the ratio of the weight average molecular weight and the number average molecular weight depends on the molecular weight distribution in the polymer and is between about 3 and 30 in the case of the aforementioned polymers. The maximum filler content of the green compact is determined by the maximum filler content of the solution, which, as already indicated above, is 0.6, according to equation (1) : v max (0 ' 6) = 0-6 ( 0.6 + M βnelt / ( 3 ) As an example, the values given in the w maχ (0-*5) and columns of Table 1 illustrate the ratios of the weight average molecular weight M , the number average molecular weight M and the maximum possible filler content of the green compactε in the caεe of polyethylene, on the baεiε of a polydispersity of 12, for a εolution filler content of 0.6 and 0.3, reεpectively. T A B L E M w M n w max (0 - 6) w max (0 - 3 [g/mole] [g/mole] [-] [-] 120,000 10,000 0.6 0.42 480,000 40,000 0.85 0.75 960,000 80,000 0.92 0.85 1,200,000 100,000 0.93 0.88 2,400,000 200,000 0.97 0.94 6,000,000 500,000 0.99 0.97 Preferably, use is made of the finest possible inorganic powder to obtain the smoothest possible surface. A good starting material can be obtained by εuεpending the inorganic powder in a suitable medium. The polymer solvent may be chosen as a suitable suspension medium. Surfactants may be added to stabilize the suεpension. If so deεired, the particle size can be reduced by subsequent grinding, for example in a pebble mill, or via an ultrasonic treatment. Preferably, a volatile solvent is used. The use of non-volatile εolvents iε deεcribed in US-A-3,926,851, where the solvent iε removed via extraction. Such a process is not only time-conεuming, but reεidual εolvent and/or extracting liquid are always retained in the end product, whereas only negligible residues of the solvent used are retained in the green compacts obtained with the proceεε according to the invention. Preferably, the green compact obtained iε εubjected to a relaxation step after cooling and stretching. After such treatment the green compact retains its εhape when heated. Such a relaxation εtep may consist in heating the green compact, for example under preεεure, to above the melting temperature of the binder for between 5 and 20 minuteε and then cooling it, also under presεure. Such a treatment can be very well combined with the lamination εtep in the production of laminates. It may be advantageous to subject the green compact to irradiation, in particular electron irradiation, before, during or after stretching, which resultε in products with reduced creep. Moreover, it appears to be possible to greatly shorten or even entirely omit the aforementioned relaxation step after such irradiation. In the process according to the invention varying draw ratios may be used. Moreover, the green compact may be stretched both biaxially and uniaxially. In the caεe of uniaxial εtretching the draw ratio will uεually be more than 5x, whereas in the caεe of biaxial εtretching the green compact will be stretched at least 1.5x in longitudinal and transverse direction, preferably between 3 and 15x. It is possible to use the same draw ratio in longitudinal and transverse direction or to use a higher draw ratio in longitudinal direction than in transverse direction or the other way round. Longitudinal and transverse εtretching may be alternated, but are preferably done simultaneously, for example in a εimultaneouε biaxial εtretching unit. The advantage of εtretching in a simultaneous biaxial stretching unit is that the film being stretched is clamped along its edges, which means that the film surface is free from equipment surfaces, whereas εtretching done with rolls may easily cause tear formation or an uneven thickness due to irregularities in the surface of the rolls or due to the occurrence of relatively large inorganic dust particles. The present process iε based on the use of combinationε of a high-molecular polymer and a εolvent, which combinationε form thermoreverεible εyεtemε. Polyvinyl alcohol or high-molecular linear polyalkeneε, εuch aε polyethylene, polypropylene and/or mixtures or copolymerε hereof are εuitable for use as polymer. The polymers may contain the uεual additiveε εuch as stabilizers, colourants, pigments, etc. Extremely suitable are solutionε of linear polyethylene with a weight average molecular weight of at least 4 x 10 5 g/mole, preferably of at least 8 x 105 g/mole. High-molecular linear polyethylene is understood to be polyethylene that may contain minor amounts, preferably at most 5 mol.%, of one or more copolymerised other alkenes such as propylene, butylene, pentene, hexene, 4-methylpentene, octene, etc. The polyethylene may contain minor amounts, for example at most 25 wt.%, of one or more other polymers, in particular an alkene-1-polymer such as polypropylene, polybutylene or a copolymer of propylene and a minor amount of ethylene. Preferably, use is made of a polymer with a narrow molecular weight distribution, for example with a weight average molecular weight and number average molecular weight ratio of less than 10, preferably 5. The advantage that can thuε be obtained is that a relatively larger amount of solvent may be used at the same average molecular weight. The weight average molecular weight can be determined according to known methods by gel permeation chromatography and light scattering. The concentration of polymer in the solution may vary, dependent in part on the nature of the solvent and the molecular weight of the polymer. Solutions with a polymer concentration of more than 60 wt.%, relative to the amount of polymer plus solvent, are rather difficult to handle on account of their high viscosity, in particular when use is made of polymer with a very high molecular weight, for example of more than 1 x 10 g/mole. In general, use will therefore be made of a polyalkene solution with a concentration of between 0.5 and 40 wt.%, in particular of between 2 and 20 wt.%. The conversion of the solution into a green compact can be effected in various manners, for example by extrusion via a spinneret or a flat-sheet or plate die. Of course, it iε also possible to cast the solution onto, for example, a tape or roll, roll or calender it. In the preparation of films and the like the εolution iε preferably caεt onto a tape or the like if the polymer concentration iε fairly low, whereaε it iε preferably extruded directly in the caεe of a high concentration. In the caεe of εpinning for example, the temperature of the εolution is preferably at least 100°C and more in particular at least 120°C, and the boiling point of the solvent iε preferably at leaεt 100°C and in particular at leaεt the same as the conversion or spinning temperature. The boiling point of the solvent must not be so high that it becomes difficult to evaporate it from the intermediateε obtained becauεe in that caεe an extraction εtep will be required. The conversion temperature and the dissolution temperature must not be so high aε to cause substantial thermal decomposition of the polymer. For this reason these temperatureε will usually be chosen below 240°C. The product obtained iε cooled to below the gelling point of the εolution. Thiε can be effected in any suitable manner, for example by pasεing the product through a liquid bath or through a chimney. When cooled to below the gel point of the polymer εolution, the polymer formε a gel. An object conεiεting of thiε polymer gel haε sufficient mechanical strength to be processed further, for example via the commonly uεed conveyors, rolls, etc. The gel thus obtained is then stretched. This gel may still contain substantial amounts of solvent, up to amounts that are hardly lower than those contained in the original polymer solution. It is also possible to remove a portion of the solvent, or even εubstantially all of the solvent, from the gel before the stretching, for example by evaporation or by washing it out with an extracting liquid. The possibility of varying the amount of solvent present after gelling without affecting the esεence of the stretchability leads to great flexibility in the control of the characteriεticε of the end product. Preferably, uεe iε made of more than 10 wt.% solvent relative to the total weight of polymer and solvent. The green compact obtained in this manner is characterized by better mechanical properties, such as lower porosity, good workability and a modulus of elasticity of > 0.5 GPa. It iε alεo poεεible to use the amount of solvent still present during stretching to control the porosity of the end product, in particular if use is made of a solvent that is removed after the stretching step. By using a solvent that can be removed during εtretching different effects can be combined. Preferably, the gels are stretched at elevated temperature, in particular at a temperature of over 75°C. The gels can be brought to the stretching temperature by pasεing them through a zone with a gaεeouε or liquid medium that iε maintained at the required temperature. A chimney with air as gaseous medium is very εuitable but it is also possible to use a liquid bath or any other εuitable device. In εtretching, (any) reεidual solvent will be separated from the product. Preferably, thiε is stimulated with the aid of εuitable methodε, for example by removing the εolvent vapour by paεεing a hot gaε or air εtream over the product in the εtretching zone or by εtretching in a liquid bath containing a medium that extracts the solvent. The green compact ultimately obtained must be free of solvent and the conditions are with advantage choεen εo that thiε εtate is already reached, or iε at leaεt virtually reached, in the εtretching zone. The denεity of the green compact ( P ___,_) is determined by dividing the weight of a specimen by the total macroεcopic volume as obtained, for example, via length x width x thickness. The theoretical maximum density (P tj , eor ) is determined from the known densitieε of the individual conεtituent componentε. The total poroεity (%) of the green compact followε from the ratio of the theoretical and the experimental denεity: poroεity x 00%. Mechanical propertieε from εtress/strain curves obtained at room temperature with the aid of a tensile εtrength teεter. Specimenε meaεuring 15 x 170 mm are stretched at a speed of 100%/minute. The modulus (E) follows from the maximum of the first derivative of the stress/strain curve. Both the modulus and the tensile strength (T) are relative to the original crosε-εectional area of the samples; the material properties are calculated by multiplying by a factor of 100/(100-porosity) . The elongation at break iε defined as the elongated length at the point of rupture divided by the initial length. The invention is further illustrated with the aid of the following examples without, however, being limited thereto. The concentration of inorganic subεtance given in Tableε 2 through 5 iε the filler content w, calculated according to formula (1). The concentration 0 was calculated as the polymer/ polymer plus solvent volumetric ratio. Example 1 10 parts by volume of polyethylene (M ~ 2 x 10 g/mole) were added to 30 parts by volume of Decalin, after which the suspension obtained was degasεed in a vacuum oven. Then 30 partε by volume of Al-O, (and a small amount of surfactant) were added to 60 parts by volume of Decalin and the εuεpenεion obtained was ground in a pebble mill for 72 hours. After degassing and grinding the aforementioned suspensions were mixed in a 1 : 1 ratio so that finally a εuεpension was obtained with a polymer : solvent : Al 2 0 3 volumetric ratio of 10 : 90 : 30 (p : s : i ) . The εuspension obtained was fed to a twin screw extruder equipped with a spinning pump, a Sulzer mixer and a flat-sheet die and was proceεεed at about 170°C. After extruεion the film obtained was quenched in water and partially dried. Then the film obtained was biaxially stretched at 125°C and a draw ratio of 10 x 10 and waε heated to 170°C for 7 minutes in a press, 2 aatt aa pprreesεεεuurree ooff <about 100 kg/cm . The film obtained waε about 15 μm. thick, Example 2 The process of example 1 was repeated, but the molecular weight, the concentration of the polymer in the solvent and the total amount of inorganic substance were varied according to the data shown in Table 2. In testε a and b polyethylene with a relatively low molecular weight was used. The films obtained could not be stretched homogeneously. The maximum biaxial draw ratio waε low. T A B L E 2 o Mw susp c enεion 0 w λbi.ax thickneεs (g/mole) p : ε : i [-] [-] [-] [/m] 1) 2) 3) 4) 5) 6) 7) 8) 9) a) b) Example 3 Suspenεionε were prepared in the εame manner aε in example 1. The suspenεion was heated with εtirring to 160°C in an erlenmeyer flaεk in a εilicone bath. The εtirring waε εtopped after the Weiεenberg effect. After having been maintained at 160°C for about 2 hourε, the polyethylene εolution containing the suspended inorganic filler was poured into an aluminium dish. After the complete or partial evaporation of the solvent a filled film was obtained, which was stretched and relaxed in the εame manner aε in example 1. The data relative to the filler content of the polymer, the concentration of polymer in the εolvent and the inorganic subεtance content were varied aε εhown in Table 3. Experiments a and b refer to polyethylene with a relatively low molecular weight. The films obtained were brittle. A B E 3 No Mw εuεpenεion 0 w λ b,i.ax thickneεε ( g/mole ) p : ε : i [ - ] [ inorg . ] [ - ] [ /m ] a) b) 1) 2) 3) Example 4 Films were prepared according to example 3. The films were stretched both biaxially and uniaxially. Uniaxial stretching waε done with the aid of a shoe, at 130°C. The draw ratio waε determined with the aid of marks. Biaxial εtretching waε done according to example 1. The concentration of polymer in εolvent, the filler content and the draw ratio (biaxal and uniaxial) were varied while the molecular weight of the polymer remained unchanged. Table 4 shows the maximum uniaxial draw ratio (^ un; i ax ) at different filler contents, λu,ni.a„x ap r p c eared to decrease considerably J at high filler contents. At all filler contents a minimum biaxial draw ratio of 10x10 appeared feasible. The maximum poεεible biaxial draw ratio could not be determined due to equipment limitations. T A B E 4 o Mw εuεpenεion (g/mole) p : s : i 1) 2 x 10' 2) 2 x 10 3) 2 x 10* 4) 2 x 10 5) 2 x 10* 6) 2 x 10* Example 5 Polypropylene was procesεed in the εame manner aε in example 1. The data are shown in Table 5. T A B L E 5 The process of example 1 was repeated, on the understanding that use waε made of barium titanate powder and that no relaxation step waε carried out. The amountε were adjuεted εo that finally a suspenεion waε obtained with a polymer : εolvent : barium titanate ratio of 10 : 90 : 11 (p : ε : i). Partε of the film obtained were partly dried at room temperature for varying lengths of time, after which they were biaxially stretched at a draw ratio of 4 x 4. The amount of solvent still present at the beginning of the stretching procedure was determined gravimetrically. The mechanical properties and porosities of the resulting green compacts were compared. The green compacts obtained in tests 5 and 6 appeared to be self-supporting, but difficult to handle, due largely to their great senεitivity to electrostatic charging. The results are shown in Table 6. These results show, among other things, that the porosity can be varied within wide limits. T A B L E Test No. [Dec]* elongation E Porosity at break % GPa GPa [Dec] is the weight percentage of decalin at the beginning of the stretching procedure, calculated on the basiε of the total amount of polymer plus solvent. Example 7 Polyethylene (M =2.0 x 10 g/mole), decalin and barium titanate (Type X7R302H of TAM R , Buffalo, NY, USA) were uεed to prepare suspensions according to the process of example 1, with p:s:i volumetric ratios of 10:90:15.3 and 10:90:23.5, which suspensionε were extruded to obtain filmε. The filmε were biaxially stretched into 20-μm thick films, at a draw ratio of 4.5 x 4.5. Ten layerε of the stretched film were placed on top of each other and compresεed for 30 minutes at 180°C and a pressure of 10 MPa. The compresεed laminates were subsequently fired at 550°C, in a nitrogen atmosphere, and sintered for 2 hours at 1300°C. No bending or noπ uniform shape changes where observed upon binder burn-out and firing. After each of the deεcribed εteps the density of the samples was determined aε a percentage of the density of the barium titanate. The resultε are given in Table 7. T A B L E denεity/5.89 x 100% εample p:ε:i vol, green after after after compact com- firing εin- preεεion tering The relative dielectric conεtant of εample 3, determined at 25°C and 1 kHz, waε 3300, which agrees with the value specified by the supplier for the starting material. Example 8 The proceεε of example 1 waε repeated, but inεtead of Al 2 0, uεe was made of barium titanate in a p:s:i volumetric ratio of 10:90:11. Sampleε of the film obtained were subjected to biaxial stretching either immediately, that is, without drying, or after drying, in which caseε the decalin content was 37 wt.% and < 4 wt.%, respectively. After stretching, the sampleε were compreεsed as in example 1. The results are summarized in Table 8. T A B E 8 b e f o r e c o m p r e s s i o n |after Icompressio draw elonga- tensile modu- dens, thick- poros. thick- por ratio tion strength lus of nesε ness at elaε- break ticity % GPa GPa g/ml μτa % μ % not dried:

Claims: C A I M S 1. Thin εelf-εupporting green compact that containε an inorganic εubεtance and a binder and iε leεs than 25 μ thick, characterized in that the binder consiεtε substantially of a polymer with a weight average molecular weight of more than 400,000, which polymer incorporated in a suitable εolvent formε a thermoreverεible gelling system with gelling and disεolution points above room temperature, the volume fraction of inorganic subεtance being more than 45% relative to the total amount of binder pluε inorganic εubεtance. 2. Green compact according to claim 1, characterized in that the volume fraction of inorganic εubεtance iε leεε than or eq ^uals 0.6/(0.6 + Me β mel- n t_/Mn) , Me„ m mel,t. being 3 the molecular weight of the polymer between two croεε-linkε in the equilibrium melt and M being the number average molecular weight of the polymer. 3. Green compact according to claim 1 or 2, characterized in that the polymer iε a polyolefin. 4. Green compact according to claim 1 or 2, characterized in that the polymer iε a polyvinyl alcohol. 5. Green compact according any one of clai ε 1-4, characterized in that the green compact iε obtained via biaxial εtretching. 6. Green compact according to any one of claimε 1-5, characterized in that the poroεity iε leεε than 80%. 7. Green compact according to any one of claimε 1-6, characterized in that the modulus exceeds 0.5 GPa. 8. Green compact according to any one of claimε 1-7, characterized in that the ratio of the weight average and the number average molecular weight iε leεs than 10 9. Green compact according to any one of claimε 1-8, characterized in that the inorganic substance iε εinterable. 10. Proceεs for the preparation of thin-walled inorganic green compacts containing sinterable inorganic substances and a high-molecular binder with a weight average molecular weight of more than 4 x 10 , in which the binder diεεolved in a suitable solvent, the concentration of the binder being less than 60 wt.% relative to the binder plus solvent, iε converted into a solvent-containing intermediate at a temperature above the gelling temperature of the εolution, εaid intermediate iε rapidly cooled to below the gelling temperature to form objects and iε stretched before, during or after the gelling, whether or not in the presence of the solvent, characterized in that the solvent-containing green compact is formed by adding an inorganic εubεtance, in an amount of more than 45 vol.% relative to the total volume of binder plus inorganic substance. 11. Proceεs according to claim 10, characterized in that a volatile solvent is used. 12. Procesε according to claim 10 or 11, characterized in that the green compact is stretched biaxially. 13. Procesε according to claim 12, characterized in that when the εtretching iε started the green compact still contains at least 10 wt.% solvent, relative to the total amount of solvent plus binder. 14. Proceεε according to any one of claimε 10-13, characterized in that the green compact obtained iε εubjected to a relaxation step. 15. Use of green compacts according to any one of claims 1-9 for the application in objects of layers containing inorganic substance. 16. Green compact aε deεcribed and illuεtrated with the aid of the exampleε. 17. Proceεs as described and illustrated with the aid of the exampleε. 18. Application aε deεcribed and illuεtrated with the aid of the examples.

Inventors: BASTIAANSEN CORNELIS WILHELMUS, HEFFELS FRANS, PIETERSE JAN KAREL, PONTENAGEL WILLIBRORDUS MARIE, BASTIAANSEN, CORNELIS, WILHELMUS, MARIA, HEFFELS, FRANS, PIETERSE, JAN, KAREL, PONTENAGEL, WILLIBRORDUS, MARIE, GERTRUD, FRANCISC
Applicants: STAMICARBON, STAMICARBON B.V.
Application year: 1990
Main classification: C04B38
IPC codes: C04B35, C08K3, C08L101, H01G4