CA1180839A

CA1180839A - Composite binder composition for powder molding

Info

Publication number: CA1180839A
Application number: CA000382864A
Authority: CA
Inventors: Tadanori Hashimoto; Masaaki Hama; Osamu Kobayashi
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 1980-08-15
Filing date: 1981-07-30
Publication date: 1985-01-08
Also published as: GB2081733B; FR2488597B1; FR2488597A1; DE3132277A1; GB2081733A; NL8103692A; JPS5738896A

Abstract

ABSTRACT OF THE DISCLOSURE
When the composite binder composition of the invention comprising an incompatible mixture of a water-soluble polymer and a sparingly water-soluble organic substance dispersed in emulsion form is used in ceramic powder molding, it exhibits characteristics unexpectable from the behavior of each component and a high-quality powder-molded article is obtained. By utilizing this binder composition, the transmittance of a translucent arc tube for use in a high intensity discharge lamp can be greatly improved.

Description

3~

1 This invention relates to a binder beneficial to the production of an inorganic powder-molded article which is homogeneous in texture, high in density, excel-lent in strength and is easily releasable from the mold.
It has heretofore been known to use a suitable organic binder in producing an article by the press molding of an inorganic powder followed by repressing or sintering. In general practice, however, selection of the binder is made primarily on the basis of strength of the molded article in view of the ease in handling, whereas litt:Le attention has been paid to the homogeneity of the molded article which has a great deal of influence upon physica:L properties of the product. This originates from such condition imposed by the molding operation that in incorporating a binder, in order to ensure uniform mixing of the binder and the inorganic powdered material and to simpl:ify the handling of the mixture in the molding operation usîng molding equipments such as a press, the mixture is granulated by spray drying or other suitable means. When granulated, the powder particles become more closely packed in each granule and the granules become less collapsible in press molding so that the intergranular spaces remain as pores within the molded article, thus deteriorating the homogeneity of the latter.
A high density product of uniform texture is not obtained ~83~

1 from such a molded article, because when such a molded article is sintered, the densely pac~ed powder particles are first sintered within each ~r-anule and the inter-~ranular pores are left behind as large voids. Since the resulting sintered body has grains of non-uniform size and a high porosity, it is unsatisfactory in mechanical strengths, electric properties and optical properties; in addition, these characteristics vary from product ko product, resulting in a decline in commercial value of the product. The problem has become more serious with the recent increasing tendency of the powder molding to use a powdered raw material having finer particle size and more spherical particle shape in order to improve both the efficiency of sintering and the characteristics of the sintered articles. It would be no exaggeration to say that the above situation is the reason for a much lower reliability of the material, as represented by ceramics, produced by molding and subsequent sintering of inorganic powders compared with the reliability of metallic or plastic materials molded from the molten raw materials. In order to meet the situation, the following methods have been proposed as countermeasures, but each method is to impart homogeneity to the molded body at a considerable sacrifice of Gther characteristics.
In t;he first method, the geometrical form of the raw powder particles is controlled to decrease the bulk density so that the granules may become more easily 1 collapsible. This method utilizes the tendency of packing density to decrease with the increase in deviation of the particle form from a sphere. The molded body obtained by this method, however, has disadvantages in that although the molded body becomes apparently homo-geneous owing to complete collapse of the granules during molding operation, yet the low bulk density of the granules results in low densi~y of the molded body so that a high density product is not obtained upon sinter-ing; moreover, the shrinkage on sintering becomes large,resulting in inferior dimensional stability of the product.
The second method employs an elevated molding pressure to improve the homogeneity. This method is not generally applicable except for special cases because of a high molding cost resulting from required pressure increase of the commercial molding equipments. The third method is to improve hornogeneity of the molded body at a sacrifice of strength and releasability of the molded body by decreasing the amount of a binder or by using such a binder of low powder binding strength as is usually called lubricant. Since the molded body obtained by this method has a low strength, a special precaution is necessary for the molding and handling. Moreover, there is a certain limitation imposed on the shape of molded articles, because when a hollow article is to be manufactured by use of a mandrel, as is the case with the molding of a tubular body, this method cannot be employed because of the risk of failure in removing 3~1 1 the mandrel. The fourth method is the method generally called hot press molding which is carried out i.n two different modes. In one mode, a mold is used, while in the other an :isostatic pressure is applied by use of`
a pressure transmitting medium such as a gas or glass.
In another ca,e, the application range of the method is very limited owing to both the high cost due to elaborate equipments and the strict limitation placed on the shape of molded artLcles. In the fifth method, in order to eliminate ~he troubles caused by the molding of granules, the technique of injection or extrusion, both of which are common in the plastics molding, is used to obtain powder-molded articles~ Because of the exclusion of granules from the molding materials, the molded body has advantages of high homogeneity and high strength resulting from a high binder content. However, in order to impart the powder a necessary fluidity by adding an organic binder, solvent or the like, the additives content amounts to 10 times as much as is used in common granula~
tion. Consequently, the packing density of the powder is decreased and a long time is necessary to remove the binder by thermal decomposition prior to sintering, which is an economical disadvantage of the method. Another molding method requiring no granulation is the slip casting which is still in actual use. This method in-volves dispersing the powder material in a suitable medium such as water to make a slip which is poured into a gypsum mold. The gypsum absorbs the water leaving 0~3~

1 behind a molded body. Although this method produces an apparently homogeneous molded body without requiring so large an amount of binder, yet it has a fundamental dlsadvantage of non-uniform particle distribution within the molded body owing to the non-uniform settling rate of the powder particles having varied density and particle size, under the influence of gravity during the course of casting. Moreover, since the gypsum mold is subject to wearing on repeated use, the dimensional precision of the molded body is unsatisfactory. Another problem is a prolonged time required for the drying of molded body subsequent to casting, because it is subject to fracture owing to uneven shrinkage caused by the difference in water content between inner and outer portions of the cast body under drying.
As outlined above, various methods have here-tofore been devised or worked out to produce from an inorganic powder a homogeneous molded body having both high bulk density and high strength, but none of them iS well established to meet all the requirements for the physical properties of the molded body. Consequently, the development of a method to meet the requirernents is eagerly awaited among the associated industrial circles.
Under the circumstances, the present inventors carried out extensive studies and, as a result, succeeded in producing a molded body having none of the above-mentioned disadvantages by keeping the condition of granules from excessively close pac]cing. The present inventors found a compo-site binder composition for powder molding, which is able to impart to the granulated particles a microstructure suitable for creating the above-mentioned condition within the granules. Based on this findirlg, the present invention has been accomplished.
An object of this invention is to provide a novel binder composition Eor use in powder molding.
Ano-ther object of -this invention is to provide a sintered product produced by sintering a molded body of inorganic powder using the above-mentioned binder composition.
According to the present invention, there is provided a binder compo-sition (hereinafter referred to as composite binder) for the molding of an inorganic powder, comprising at least ten percent by weight of at least one water-soluble polyrner selected from the group consisting of polyvinyl alcohol and isobutylene-maleic anhydride copolymer, and at least five percent by weight of at least one sparingly water-soluble organic substance selected from the group consisting oE waxes and fatty acids, wherein said waxes and fa-t-ty acids are in the form of an emulsion.
In another aspec-t, -the invention provides a method for manufacturing an alurnina sin-tered body of tubular form having good light transmittance proper--ties which comprises granulating by spray drying an alumina powder dispersed in water using a binder composition comprising polyvinyl alcohol and waxes to forrr, spherical granulates, and molding the resulting granulates by the isotactic press-molding technique into a tubular form.
In the accompanying drawings, Figure 1 is an electron photomicrograph (magnification: x 250) of granules formed by the granulation of an alumina powder using the present composite binder comprising polyvinyl alcohol and a wax emulsioni Figure 2 is an electron photomicrograph (x 250) of granules formedby the granulation of an alumina powder using polyvinyl alcoholi Figures 3 and 4 are enlarged electron photo~licrographs (x 2,500) of - 6a -t3~

1 granules shown in Figs. 1 and 2~ respectively; Fig. 5 is an electron photomicrograph (x 250) of granules formed by the granulation of an alumina powder using a wax emulsion; Fig. 6 ls an electron photomicrograph of a molded body formed from the granules shown in Fig. l;
Fig. 7 is an electron photomicrograph (x 250) of a molded body formed from the granules shown in Fig. 2;
Fig. 8 is an electron photomicrograph of a sintered body obtained from the molded body shown in Fig. 6;
and Fig. 9 is an electron photomicrograph of a sintered body obtained from the molded body shown in Fig. 7.
When an inorganic powder is mixed with a dispersion of the present composite binder in a solvent containing water as major component and granulated by spray drying or other means, as the solvent evaporates, the packing of inorganic powder at the spot where the sparingly water~soluble organic compound has been finely dispersed becomes somewhat coarse and the water vapor from the inside of' granules escapes through the coarsely packed aggregate of powder particles, leaving behind hollow granules which are easily collapsed when press-molded in spite of' the fact that the inorganic powder is closely packed. To the contrary, when granulation is carried out by using a conventional water-soluble polymer alone as binder, the surface layer of the granule is closely packed so completely that on dryin~ the granules the water vapor inside each granule does not completely escape and on subsequent cooling the surface l layer caves in to form a solid granule which is diffi-cultly collapsible upon press-molding and causes dif-ficulties described before. On the other hand, when granulation is carried out by using a sparingly water-soluble organic compound alone, the granules are easilycollapsible under a molding pressure owing to lack of binding power of the organic compound, but the molded body is extremely low in strength and is releasable from the mold with great difficulty, causing frequent rupture of the object in removing from the mold. Thereofre, such a binder is of no use in practical operation. Thus, a desirable binder effeet as shown in the present invention is not expectable when a water-soluble polymer or a sparingly water-soluble organic compound is used alone.
Also, a binder composition comprising those components whieh are compatible with each other does not impart to the granules such mierostructure as is produced by the binder of this invention. Only the binder composition aeeording to this invention comprising a uniform dis~
persion of incompatible binder components is able to exhibit an unexpectable and surprising effect as previously deseribed.
It has now beeome possible by use of the present composite binder to utilize a fine powder of l ~ or less in particle size to the best advantage.
Although such a fine powder has been ~nown to be desirable for use in powder molding on account of its excellent sinter eharaeteristics, yet it was difficult to utilize 1)83~

l said characteristics, because such a fine powder with small and uniform particle size yields, in the presence of a conventional binder, hard granules which are too hard to be collapsible under the molding pressure.
When the granulation is carried out by use of the present composite binder, the resulting granules can be sintered at a temperature lower than used before, thereby yielding a sintered body with grains more dense and more uniform in particle size compared with conventionally produced one~ resulting in remarkable improvement in physical properties and reliability of the sintered body.
In order to present more concretely the ad-vantages of the present composite binder, the dif~erence in the binder effect upon the granulation of an alumina powder between polyvinyl alcohol and the present composite binder comprising polyvinyl alcohol and a wax emulsion is illustrated below with reference to the accompanying electron photomicrographs.
(l) Difference in granules The shape of the granule (Fig. 1) formed by use of the present composite binder approximates a perfect sphere, whereas that of the granule (Fig. 2) formed by use of a conventional binder (polyvinyl alcohol) shows a cave-in in which is formed a secondary granule.
Observation of the enlarged surface of granules reveals that the granule (Fig. 3) formed by use of the present composite binder has pores distributed uniformly over the surface and the powder particles are moderately ~0~3~

1 packed, while the granule (Fig. 4) formed by use of a conventional binder has closely packed hard solid structure. Granules (Fig~ 5) ~ormed by the granulation with a sparingly water-soluble organic binder (~ax emulsion) have a fluffy surface which is due to the weak bond between primary particles and which interferes considerably with the flow of granules, contrary to one of the principal objects of granulation which is to improve the flowability and make easy the handling of a powder.

(2) Difference in molded bodies It is seen that the difference observed in the granules between the present composite binder and a conventional binder is faithfully reflected in the molded body. The granules formed with the present composite binder are collapsed under the molding pressure so completely that no vestige remains at all, yielding a homogeneous molded body (Fig. 6), whereas the granules formed with a conventional binder are very firm and l 20 their contours remain distlnct, though deformed under the molding pressure (Fig. 7). In the latter case, intergranular spaces and the cave-in on the granules remain after molding as large voids which are larger in size than the powder particles used as raw material and cannot be eliminated by sintering, resulting in marked deterioration of the performance of sintered product.

- 10 _ ~f~

(3) Difference in sintered body A molded body made completely homogeneous by use of the present conposite binder yields a sintered body of excellent performance which, being substantially free from voids, has a density approaching the theoretical one and also a narrow distribution of sintered particle size (Figure 8). To the contrary, when a conventional binder is used in granulation, the inter-granular voids formed on molding remain in a great number at the boundaries of sintered granules as well as trapped inside the granules (Figure 9); also, the grain size of the sintered body is not uniform because of the locally uneven rate of growth originated from the non-homogeneous molded body.
As is apparent from the foregoing description, by applying the present composite binder to an inorganic powder, it has now become possible to produce easily a homogeneous high-density sintered article at low cost by using common equipments. Since the manufacture of such a sintered article has heretofore been possible only by use of special equipments and in limited geometrical forms, the industrial merit of ~he present binder may be said to be immeasurable.
The invention is described below in detail.
The water-soluble polymer for use in the present composite binder can be used in the molding in powder metallurgy, which include synthetic water-soluble polymers such as polyvinyl alcohol (PVA), and isobu~ylene-maleic anhydride copolymer. Ihese water-soluble polymers are used each alone or in combinations.
The sparingly water-soluble organic substance for use in the present composite binder can be any of the binders or lubricants used in the molding l~ ~

in powder metallurgy and in ceramic molding. ~uch organic substances in-clude paraffinic compounds and derivatives thereof such as liquid paraffin, paraffin emulsion, n-paraffin wax, isoparaffin wax, oxidized wax, poly ethylene wax (low moLecular polyethylene), microcrystallirle wax, chlGrinated wax, and wax emulsion; fatty acids such as stearic acid, stearic acid emulsion, lauric acid, palmitic acid3 isostearic acid, 1,2-hydroxystearic acid, behenic acid, myristic acid, butyl stearate, oleic acid, and linolic acid; fatty acid amides such as oleic acid amide, stearic acid amide, lauric acid amide, ricinoleic acid amide, erucic acid amide, hydrogenated beef tallow fatty acid amide, coconut fatty acid amide, behenic acid amide, and erucic acid amide; bis-fatty acid amides such as methylenebisstearamide, ethylenebi55tearamide, methylenebisamide and ethylenebisamide; and ester waxes such as cetyl palmitate, myricyl palmitate, and myricyl cerotate.
These sparingly wate:r-soluble organic substances are used each alone or in combinations. Of these organic substances, in view of the collapsibility of granules in molding operation, waxes and fatty acids are preferred. It is rnost preferable to use the waxes and fatty acids in the form of emulsion in view of controlling the microstructure of the grallules.
rrhe effective amount of the mixture of a water-~A - 12 -A1~ r l soluble polymer and a sparingly water-soluble organic substance is in the range of from 0.2 to 20% by weight based on the weight of inorganic powder If the binder is used in an amount below the lower limit, the strength and releasability of the molded body are both insufficient, while if the binder is used in excess of the upper limit, the collapsibility of the granules becomes unsatisfactory.
In case the water-soluble polymer or the sparingly water-soluble organic substance is used in the form of aqueous solution or emulsion, the "percent by weight'l is expressed in terms of said polymer or said organic substance, excluding the solvent, surface active agent and other additives. Hereinafter the same applies to the amounts of other additives. When added in an amount in the said range, the present composite binder manifests an appreciably favorable effect on the powder molding, but a more favorable effect is obtained by the addition of an amount in the range of from 0.3 to 15% by weight.
In view of the homogeneity of the te~ture of molded body and the ease of handling in molding operation, it is most preferable to keep the amount of addition within the range of from 0.5 to 10% by weight based on the inorganic powder.
The ratio between the water-soluble polymer 25 and the sparingly water-soluble organic substance can be varied depending on the characteristics of the powder and the conditions for the granulation and the molding.
However, if the proportion of the sparingly water-soluble , _ .~ _ ;D !33~

l organic substance is below 5% by weight, the collapsibility of the granules becomes insufficient in some cases.
Accordingly, said proportion should be 5% or more, preferably 10% or more, most preferably 20% by weight or more. The proportion of the water-soluble polymer should be at least 10%, preferably 20% by weight or more.
The water-soluble polymer or the sparingly water-soluble organic substance can be used, if necessary, in the form of aqueous solution or aqueous emulsion and both components are mixed to form a uniform dispersion.
The granulation of an inorganic powder with the present composite binder is carried out by utilizing the techniques generally used in the granulation of common powder materials. The composite binder, water-soluble polymer, or sparingly water-soluble organic substance is blended with an inorganic powder and the blend is mixed with a solvent, e.g. water. Alternatively, the binder components are dissolved or dispersed in water each independently or as a mixture and the resulting aqueous solution or emulsion is mixed with an inorganic powder. A most desirable solvent is water to which may be added an organic solvent so long as the advanta~e of the present composite binder is not injured. It is also possible to add a surface active agent, pH regulator or th~ like. The mixing or dispersion of an inorganic powder with the present composite binder is effected by the means co~monly used in mixing or dispersion o~
powdered materials, such as mixing by stirring with 3~g 1 rotating blade<" mixing by ball-milling, ultrasonic mixing, and the like.
The granulation is accomplished by any of the methods including drying and subsequent crushing of a slurry comprising an inorganic powder, a composite binder, a solvent, and additives; granulation in a rotating pan, granulation by ~neading, fluidized granu-lation~ and spray dryingO Of these methods, the fluidi~-ed drying or spray drying is particularly effective~
The molding of granules is accomplished by use of molding equipments generally employed in the dry molding of powdered materials. Such equipments include mechanical and hydraulic presses with metallic mold and isostatic presses with rubber mold. With respect to homogeneity of texture of the molded body and releasability from the mold, the advantage of the present binder is fully manifested in molding a tubular object by using the isostatic press.
The inorganic powders, to which the present composite binder is applicable, include powders of single metallic or nonmetallic elements, alloys and single oxides or non-oxide compounds thereof. These powders may be used each alone or as mixtures. Both the cations and anions of the metal oxides or non-oxide compounds of metals may comprise single element or plural elements. The present binder can be used with those powder systems containing oxides or non-oxide compounds and additives to improve the characteristics ~ ,~ _ 3~

1 of oxides or non~o~ides.
Particular metals for suitable ~letallic powders are aluminum o~ Group III of the periodic tahle (long form; the same applies hereinafter), silicon of Group IV; scandium, yttrium, lantha~oids and actinoids of Group IIIa; titanium, zirconium, hafnium and t~orium of Group IVa; vanadium, niobium, tantalum, and pro-tactinium of Group Va; chromium, molybdenum, tungsten, and uranium of Group VIa; manganese, technetiurrl, and rhenium of Group VIIa; iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum of Group VIII; copper, silver, and gold o~ Group Ib; zinc and cadmium of Group IIb; thallium of Group IIIb;
germanium, tin, and lead of Group IVb; arsenic, antimony, and bismuth of Group Vb; tellurium and polonium of ~roup IVb.
Oxides suitable for powders are oxides of above-listed metals. Other metal oxides include beryl-lium oxide~ magnesium oxide, calcium oxide, strontium oxide, barium oxide, lanthanum oxide, gallium oxide, indium oxide, and selenium oxide. Other suitable oxides containing two species of metals, commonly called double oxides, as classified with respect to crystal structure, include perovsk:Lte-type oxides such as NaNbO3, SrZrO3, PbZrO3, SrTiO3, BaZrO3, PbTiO3, AK~aO3~ BaTiO3 and LaAlO3; spinel-type oxides such as MKA12OI~, ZnA12OLI, 2 4, NiA12O4, NiCr2OL~, FeCr2O4, MgFe2O4, Fe3O4, and ZnFe204; illmen:Lte type oxides such as MgTiO3, MnTiO3, ,, ~, /~

1 FeTiO3, CoTiO3, NiTiO3, ZnTiO3, LiNbO3 and LiTaO3; and garnet-type oxicles such as rare earth-gallium garnet represented by Gd3Ga5O12 and rare earth-iron garnet represented by Y3Fe5O12.
The metal non-oxide compound powders are powders of carbides~ nitride3, borides, and sulfides of the above-listed metals. The present composite binder is effectlvely applicable to carbides such as SiC, TiC, WC, TaC, HfC, ZrC and B4C; nitrides such as Si3N4, AlN, BN and TiN; and borides such as TiB2, ZrB2, and l,aB6.
Although the present composite binder is more or less suitable for any of the above-mentioned powders irrespective of the size and shape of powder particles, it is advantageously used in granulating a powder of 100 ~ or below in average part;icle size. With the decrease in particle size, the granulation with the addition of a conventional binder presents increased difficulties, whereas the present binder manifests its effectiveness to a greater degree with a powder of 20 or below, particularly 5 ~ or below in average particle size. ~lthough effectively applicable t;o a superfine powder of 0.01 ~l or below, the present binder is more effective when used with a fine powder of 0.01 ~ or above in average particle size. The term "average particle size" refers to the average particle size of primary particles suspended in a slurry just before granulation, said slurry being prepared by milling in a ball mill.
~he particle diameter is measured under a microscope.

1 When the slurry contains secondary agglcmerates; the smallest diameter of the single particle in an agglomerate is used in calculating the average partlcle diameter.
The present binder is used most advantageously in the granulation of an oxide powder a~ong inorganic powders, particularly such metal oxide powders as are used in the manufacture of translucent material, insuLat-ing materials, semiconductor materials, piezoelectric materials, magnetic materials and opto-electronic materials. Further, the present composite binder is advantageously used in the manufacture of translucent materials from the powders of A12O3, r~go, Y2O3, or Pbl-xLaxzrl-yTiyo3 (x- 0 to 1.0, y- 0 to 1 0) It is particularly e~fective for the manufacture of translucent materials from A1~03.
The present invention is illustrated below in further detail with reference to Examples and Compara-tive Examples, but the invention is not limited thereto.
In Examples all percentages are by weight unless other-wise indicated.

Example 1 As the water soluble polymer, there were useda 10% aqueous solution of polyvinyl alcohol (Poval 120 of Kuraray Co.; polymerization degree, 2,000;
saponification degree, 99 - 100 mole-%), a 3% aqueous solution of methylcellulose (Nakarai Kagaku Yakuhin Co.;
reagent grade), a 5% aqueous solution of gelatin ''`',. /~
_ ~r _ 83~

l (Nakarai Kagaku Yakuhin Co.; reagent grade). As the sparingly water--soluble organic substance, there were used a wax emulsion (MAXELON A of Chukyo Yushi Co.;
- solids content, 40%), a stearic acid emulsion (SEROSOL
920 of Chukyo Yushi Co.; solids content, 18%), and liquid paraffin. The amounts used were as given below.
As the inorganic powder, there was used a high purity alumina (purity, 99.99%; average partic:Le diameter, 0.5 ~;
Sumitomo Chemical Co., Ltd.). Magnesium nitrate (Makarai Kagaku Yakuhin Co.; extra pure reagent grade) was added as the sintering aid for alumina in an amount of 0.1%
in terms of magnesium oxide. The alumina powder together with the sintering aid was mixed with water to an alumina concentration of 40% and milled in a ball mill for 10 hours. To the slurry was added a composite binder of the composition shown in Table l; the amounts added were 2% of the water-soluble polymer and 1% of the sparingly water-soluble organic substance (3% in total), each in terms of solids, except for liquid para~fin.
The resulting slurry was granulated by spray drying at 180C. All of the granulates were in the form of nearly spherical bead having good flow properties. The granulate was molded by means of an isostatic press into a tubular specimen of 10 mm inner diameter x 150 mm length x 2 mm wall thickness. The moldability of each granulate was very good and the molded body was easily released from the mold without any adhe.cion. The strength of the molded body was sufficient enough for machining.
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1 The molded body in tubular form was externally ground ~o a wall thickness o~ 1 mm and presintered in the air at l,000C. On subsequent sintering in vacuum at 1,750C, the specimen showed good translucency as shown in m' able 1. In Table 1 are shown properties o~ the molded specimens and sintered specimens o~ alumina obtained by use of various binders in Example 1 and Comparative Examples 1 and 2 (described later). As is apparent from Table 1, all of the alumina specimens prepared by use of the present cornposite binders showed superiority in moldability and in physical properties of the sintered product.

Comparative Exarnple 1 The procedure of Example 1 was repeated, except that 3% (based on alumina powder) of a water-soluble polymer was used alone in place of the composite binder.
The water-solub:Le polymers used were the same polyvinyl alcohol, methylcellulose, and gelatin as used in Example 1. The results of evaluation for the moldability of granulates and physical properties of sintered specimens were as shown in Table 1.

Comparative Exarnple 2 The procedure of Example 1 was repeated, except that 3% (based on alumina powder) of sparingly water-soluble organic substance was used alone in place ofthe composite b:inder. The molding of granulates and ~0 ~ ,~_ 33~

1 the physical properties of sintered specimens were evaluated. The sparingly water-soluble organic sub-stances employed were the same wa~ emulsion, stearic acid emulsion, and liquid paraffin emulsion as used in Example 1. The granulates obtained were inferior in flow properties and were difficult to handle. Upon molding a tubular body, the mandrel, used as the core~
sticked so firmly to the wall of molded tube that the molded tube cannot be removed. By use of a release agent, the molded tubular body could be released from the mold.
However, owing to insufficient strength, the molded body was broken upon external machining and the intended specimen was not obtained. The results of evaluation were as shown in Table 1.

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1 Example 2 An inorganic powder was prepared by therrnall~
decomposing basic magnesium carbonate (Nakarai Kagaku Yakuhin Co.; extra pure reagent grade) ln the air at 900C
to obtain magnesium oxide and adding, as an additive, magnesium ~luoride (Nakarai Kagaku Yakuhin Co.; extra pure reagent grade) to the magnesium oxide in an amount of 0.2% based on the magnesium oxide. To the above in~
organic powder, were added the same polyvinyl alcohol and wax as used in Example 1 in amounts of 2% and 1%
(based on magnesium oxide), respectively. The granulation and molding were carried out as in Example 1 to obtain a tubular molded body which was presintered in the air at 400C for 2 hours and then sintered in vacuum at 1,400C
for 2 hours. The molded body was highly homogeneous and excellent in releasability and strength. The sintered body was translucent.

Comparative Example 3 The granulation~ molding and sintering were carried out in the same manner as in Example 29 except that 3% of the same polyvinyl alcohol as used in Example 2 was used as the sole binder. The molded body was not homogeneous and showed deformed contours o~ the spray-dried granules. The sintered body showed little trans-lucency.

3~

1 Comparative Example 4 The procedure, including granulation, moldingand sintering, of' Example 2 was repeated, except that 3% of' the same wax as used in Example 2 was used as the sole binder~ The molded body was much inferior in releasability and no tubular molded body was obtained.
The fragments of the molded body were of low strength and diff'icult to handle.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A binder composition for the molding of an inorganic powder, compris-ing at least ten percent by weight of at least one water-soluble polymer selected from the group consisting of polyvinyl alcohol and isobutylene-maleic anhydride copolymer, and at least five percent by weight of at least one sparingly water-soluble organic substance selected from the group consisting of waxes and fatty acids, wherein said waxes and fatty acids are in the form of an emulsion.

2. A method for manufacturing an alumina sintered body of tubular form having good light transmittance properties which comprises granulating by spray drying an alumina powder dispersed in water using a binder composition compris-ing polyvinyl alcohol and waxes to form spherical granulates, and molding the resulting granulates by the isotactic press-molding technique into a tubular form.

3. A method according to claim 2, wherein the binder composition con-tains at least 10% by weight of polyvinyl alcohol and at least 5% by weight of waxes.

4. A method according to claim 2, wherein the total amount of polyvinyl alcohol and waxes is 0.2 to 20% by weight based on the weight of alumina powder.

5. A method according to claim 2, wherein the waxes are used in the form of emulsion.

6. A method according to claim 2, wherein the alumina powder is a powder, 10µ or less in average particle size.

7. An alumina sintered body of tubular form, having good light transmit-tance properties, homogeneity, strength of molded body and releasibility from a mold, when prepared by the method of claim 2, 3 or 4.