EARLY STUDIES ON THE USE OF STARCH IN CERAMICS...... 9. 4. STARCH IN ..... alumina suspensions and giving the bodies with high enough green strength was ...... urania and urania-ceria microspheres with improved sphericity were.
STARCH AND ITS DERIVATIVES IN CERAMIC PROCESSING Marek Sikora*, Piotr Izak** * University of Agriculture, Poland **University of Science and Technology, Poland
Cracow, 2006
2
1. INTRODUCTION........................................................................... 5 2. STARCH: ORIGINS AND PRODUCTION ..................................... 5 3. EARLY STUDIES ON THE USE OF STARCH IN CERAMICS...... 9 4. STARCH IN CERAMIC POWDER SYNTHESIS............................ 9 5. BINDER SYSTEMS..................................................................... 11 6. POROUS CERAMICS................................................................. 23 6.1. Porous structures .....................................................................................24 6.2. Diesels .....................................................................................................27 6.3. Filters .......................................................................................................28 6.4. Ceramic carriers .......................................................................................30 6.5. Silicon carbide (SiC).................................................................................31 6.6. Conductor films ........................................................................................31 6.7. Reduced thermal conductivity ..................................................................32 6.8. Sound insulators.......................................................................................32 6.9. Improved co-sensitivity.............................................................................32 6.10. Foamed ceramic panels .........................................................................33
7. CERAMICS MANUFACTURING ................................................. 33 7.1. Extrusion ..................................................................................................33 7.2. Ceramic glazers and pigments (colours), enamel inks .............................35 7.3. Bricks, tiles ...............................................................................................36 7.4. Glass fibers ..............................................................................................37 7.5. Glazing materials......................................................................................37
8. FOUNDRY MOLDS, REFRACTORIES ....................................... 38 9. ELECTRONIC CERAMICS ......................................................... 40 10. FUEL CELLS............................................................................. 43 11. ABRASIVES.............................................................................. 44 12. BIOCERAMICS ......................................................................... 44 13. RHEOLOGICAL AND SURFACE CHEMICAL STUDIES .......... 46 14. MISCELLANEOUS.................................................................... 49 15. USES OF THE OTHER SACCHARIDES IN CERAMICS .......... 56 16. LITERATURE............................................................................ 61
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Preface Development of science and rapid technological progress in many production branches influences also methods of ceramics production. Ceramic technology is not only interested in mechanisms of proceeding inside processes, but also in the method of their realization in macro as well as in micro scale. The word “process” means in this case all operations executed in order to obtain the final product, e.g. blending methods, forming, drying, and firing. These operations are often called elementary operations. Ancient processes at pottery production began by simple clay blending (ceramic powder) with water (medium). Now in many cases the synthetic materials are used, such as alumina, zirconia, and media, such as ethyl alcohol, as well as many different organics (additives), such as polyethylene glycol, stearic acid, polysaccharides, polyvinyl alcohol etc. Synthetic ceramic powders grant better mechanical endurance (after processing and firing), higher temperature resistance and/or better electric properties. An example of ceramic product obtained without clay processing is a spark plug for car engines. In fifties insulators were made of natural clays (porcelain), but now they are produced exclusively by the use of synthetic alumina, because of its lower tendency to cracking, and increased electrical resistance in applied conditions. However, synthetic ceramic powders have not usually enough plasticity for forming (they are also called “nonplastics”), so that they have to be admixed with some special chemicals, which allow the plastic flow, and grant advantageous features to the products. Added organics can have not only synthetic (polyvinyl butyral), but also natural origin (e.g. starch). In ceramic industry hundreds of organics are used. Many years ago these organics were applied by the use of try and error method. Nowadays, on the base of scientific research better and more specialized products are available. In many cases however, these organics are expensive, and which is more disadvantageous, they negatively influence the environment. Majority of organic chemicals used in ceramic technology have supremacy in comparison to inorganic ones. Namely, they can be fully removed in earlier phases of thermal processing. As a consequence, organic modifiers, considered as auxiliary substances should not worry by their remainders, but they should rather go in the direction of optimization of deformations (formability), and/or in the direction of giving technological properties (bonding) of ceramic mass by forming. Many details from these processes give metallurgy of powders, technologies of pigments and dyes, microprocessors technologies etc. Physicists, electricians, electronic engineers, and chemists of different interests work on winning, studying and application of ceramics. In this monograph, the efforts were concentrated on only one auxiliary substance, the starch, which has extremely significant meaning not only from standpoint of ceramic technology, but also from ecology and related sciences. Starch occurs numerously in the nature, it is cheap, and can be used at the production of different ceramic materials. During technological operations starch
4 behaves environmentally friendly, and does not threaten the working places. It should not be trivialized the fact, that starch belongs to so called “renewable raw materials”. The production of starch can be regulated in relatively short time, according to the needs of industry. These special features of starch were attempted to be shown in the form of a review, with simultaneous presentation of its wide application abilities in ceramic industry. This monograph is a “bunch of knowledge” useful for technologists, students, scientists and for those, who are more or less interested in technology of ceramics forming. Prof. dr hab. inż. Jerzy Lis
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1. INTRODUCTION The recent application of starch and its derivatives in ceramics splits into several areas, such as ceramic powder synthesis, ceramic powder binders, porous ceramic manufacture, production of structural ceramics, foundries, refractories, electronic ceramics, fuel cells application, abrasives, bioceramics and others. Instead of using nonrenewable, environmentally hazardous, and costly petrochemical-derived additives, which are difficult to treat from the standpoint of recycling, starch and its derivatives offer the ceramic industry: a set of applications, multifunctionality, differentiated properties, aqueous processing, and readily burn out. It is worth to point out, that starch belongs to the so-called renewable biological resources. Its production fluctuates under the influence of different factors (economy, climate), however can be raised considerably in relatively short period of time [Koch 1995]. Any binder should be burned out during sintering of ceramics. The products of starch (polysaccharides) pyrolysis are comparable to the those of polyvinyl derivatives commonly used in ceramic formulations [Sayaslan et al. 2000, Tomasik et al. 1989 a, Shanefield 1999, Pujari and Tracey 1993]. The main advantage of application of water-soluble saccharides in ceramics as compared to the synthetic organics is cleaner burnout of their solvents. The vapors of some organic solvents used in ceramics, e.g. benzene, trichloroethylene could even bring about leukemia [Shanefield 1999]. The aim of this work is to demonstrate the reader how broad range of applications in ceramics starch and its derivatives, as well as the other saccharides of natural origin could have. Reading the literature of the subject the authors were surprised of its immense quantity. For this reason some older patents, which already lost theirs actuality were omitted. In spite of this, initially relatively small work grew up to the actual shape.
2. STARCH: ORIGINS AND PRODUCTION Starch constitutes the major food reserve material of all the higher plants. The most abundant plants capable of synthesizing starch are cereals (corn, wheat, rice, barley, oat, rye, sorghum etc.), tubers (potato, tapioca, arrowroot, etc.), palm trees (e.g. sago), as well as leguminous plants (e.g. peas) [Whistler et al. 1984]. During plant growth, starch is deposited in the form of particles (starch granules) that are insoluble in cold water and are located inside small cells (plastids) within plant seeds and roots (Figure 1). The plant origin controls many important properties of starch, such as the granule size, shape, structure, water binding capacity, swelling and gelling ability etc. [Thomas, Atwell 1999].
6
a
b
c
Figure 1. Plant variety significantly influences the size and shape of starch granules. Scanning electron microscopy images of (a) tapioca, (b) potato, (c) corn starch granules (magnification 1000x). Starch granules are generally classified as consisting of two chemically distinguishable entities: amylose that is a mixture of essentially linear polymers, and amylopectin, which is a mixture of highly branched polymers [Banks and Greenwood 1975]. Both entities consist of glucan chains, which are polymers in which glucose mers bond to each other. Such bonding occurs through α (1-4) linkages (amylose) and/or branches of α (1-4) glucose chains that are linked to each other by α (1-6) linkages (amylopectin). In the Figure 2 the structural formulas of glucose (a), amylose (b), and amylopectin (c) are presented. 2OH H CH 6
4
H 6CH2OH H
5
4
O
HO HO
3 H H
2
H OH
1 OH
HO HO
H
5
O
3 H
α H
2
OH OH 1 β H
Figure 2 a. Pyranose structure of D-glucose. Numbers (1, 2, ….,6) denote the carbon atoms in the ring. The ring form is referred to as D-glucopyranose and can be in either the α (left) or β (right) configuration. Starch polymers contain only α linkages.
7 H CH 6 2OH 4
H O
5
O HO
H CH 6 2OH
3
4
H H 2 OH 1 O HO H
H O
5
H CH 6 2OH
3
H 4 H 2 OH 1 O HO H
H O
5 3
H CH 6 2OH 4
H OH 1 O HO
H 2 H
H O
5 3
H H 2 OH 1 H O
n
Figure 2 b. Amylose chain
H 6CH2OH 4
H O
5
O HO
3
OH H CH 6 2 4
H OH 1 O HO
H 2 H
O HO
5
H
3
H OH 1 O HO
5 3
H 2 H
4
H
H O
H 6CH2OH
H OH 1 O HO
H 2
OH H CH 6 2 4
H 2
H O
5
OH 1 O HO
H
H O
3
OH H CH 6 2 H 4
H 2
OH H CH 6 2 4
H O
5 3
3
H 2
OH H CH 6 2 H 4
OH 1 O HO
5
H
O
H
6
4
H 2
H OH 1 O HO
H
OH 1
H
H O
3
H O
5
5
CH2 H O
3
H 2 H
OH 1 O
H
n
Figure 2 c. Amylopectin Microstructure of starch granules consist of highly ordered and densely packed glucan chains. This packing is the reason why starch is found as insoluble granules in the plastids of plant cells. The structure of the starch granule according to Ball et al. [1996] is presented in the Figure 3. Industrial processes involving grinding, sieving, washing, and filtration are used to separate and remove starch granules from plant seeds and roots. Under chemical, physical and/or biological treatment, starch can be transformed into a broad range of different compounds with modified properties. The simplest physicochemical treatment of starch entails heating in the presence of acid (usually HCl), which results in white and yellow dextrins and/or so called British gums, which are water soluble to varying extents.
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Figure 3. Starch Granule Microstructure (Ball et al. [1996]). (A) A schematic view of a starch granule, which consists of a concentric set of amorphous (black) and crystalline (white) growth rings. (B) The structure of a crystalline growth ring of the granule is related to the molecular organization of amylopectin. Each black and white section represents amorphous and crystalline lamellae, respectively. Thus, the crystalline growth ring enlarged in this panel contains a regular succession of 11 amorphous and crystalline lamellae. This would amount to a 0.1 µm thick growth ring. (C) This panel enlarges a succession of 7 lamellae and relates them to the primary structure of a portion of an amylopectin molecule. Each line represents a -1,4 linked glucan chain. The chains are hooked together by -1,6 branches. The dotted line delimits the sections appearing in the crystalline (1) and amorphous (2) lamellae. Most -1,6 branches are included in the amorphous lamellae at the root of the chain clusters and that the glucan chains are pointing towards the granule's surface. (D) This panel relates a part of primary structure depicted in (C) to the secondary structure of a single cluster displaying the double helical structures. The 6 nm size of the crystalline portion corresponds to a length of 18 glucose residues. Enzymatic treatment of starch results in the formation of monosaccharides, oligosaccharides, and/or maltodextrins. Such enzymes as amylases (α-amylase, β-amylase, isoamylase, glucoamylase), pullulanase, and amylo glucosidase are most commonly used for this purpose. Readers interested in the properties of starch and its derivatives obtained upon chemical, physical °or biological treatment are recommended to read the works of Radley 1968, Petersen 1975, Radley 1976, Van Beynum and Roels 1985, Tomasik et al.1989 b, Kearsley and Dziedzic 1995, Tomasik et al. 1996, Whistler and BeMiller 1997, Jane et al. 1994, as well as Tomasik and Schilling 1998.
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3. EARLY STUDIES ON THE USE OF STARCH IN CERAMICS In the early studies on application of starch in ceramics the effects of polysaccharides in modifying the colloidal properties of clays were examined. In the practice the minor addition of starch or dextrins acted as binders. The effect of larger additions of starch products [Wilson and Bunnel 1947] (1- 3 %, based on weight of clay) was studied with a view to securing a plasticizing effect and to replace the ball clay. In that study, it was found that starch lowered the plasticity better than ball clay and increased the dry bond strength 4.5 times. At the same time, it produced a much whiter finished ceramic. Unmodified starches naturally absorb a large ratio of water on gelatinization. As a result of this large absorption of water, unmodified polysaccharides can increase the moisture content of a clay body, which, in turn increases shrinkage during drying and porosity during firing. [Kerr 1950]. The use of an oxidized starch or dextrins (British gums) for brick and tile manufacture was reported by Ellis-Anwyl [1946]. In that study, the addition of gelatinized starch improved workability as well as the binding and drying qualities of clays. Hoggat in 1945 [U.S. Patent 2 388 543] patented the use of starch to improve the physical and mechanical properties of plaster casts. In early studies described by Kerr [1950], unmodified starch, enzyme converted maltodextrins, dextrins, pregelatinized starch, thin boiling starch, and gelatinized starch were used in a variety of manufacturing applications including ceramics, coal and chalk briquettes, foundries, glass, and rockwool production. The recent application of starch and its derivatives in ceramics splits into several areas, such as ceramic powder synthesis, ceramic powder binders, porous ceramic manufacture, production of structural ceramics, foundries, refractories, electronic ceramics, fuel cells application, abrasives, bioceramics and others. All of them are mentioned in this review.
4. STARCH IN CERAMIC POWDER SYNTHESIS An American Patent [Nr. 5618767] described a method for producing components of SiC with the addition of carbon and/or carbon - containing binders. In this invention, the carbon was obtained at least in part by pyrolysis of the binders in the green component. The binder consisted of modified starch preferably with a sulfamate or a sulfonic ester, which was water-soluble. The process claimed by Henley et al. [U.S. Patent US 5370854 A 941206] comprised carbothermal reduction of oxide powder, e.g., SiO2, to its corresponding carbide or nitride by the addition of precursor pellets containing the oxide, a thermally decomposed binder material, and a carbon source directly into the heated reaction zone of a vertical, gravity-flow reactor. The binder was a mixture of wheat starch and corn starch, optionally in admixture with another binder, e.g., melamine. The binder material thermally decomposed to a
10 carbonaceous residue functioned both as additional carbon source and as a binder for the pellets. An International Patent [WO 9216457 A1 921001] described a similar process wherein calcined, porous precursor pellets flowed into a carbothermal reduction reactor. The pellets contained an oxide powder, a carbon source, and an organic binder. The reaction zone was kept at a temperature sufficient to devolatilize the pellets and carbothermally reduce the oxide(s) to corresponding carbide(s) or nitride(s). This process was capable of making high purity AlN by carbothermal reduction of alumina with nitrogen at 1700 ° C. In this case, pellets consisted of sub micron alumina powder, acetylene black, and wheat starch. Spray drying of ceramic suspensions was used in the manufacture of 20400 µm microgranules suitable for their subsequent transformation of solid solutions of Y2O3 in ZrO2 by plasma fusion treatment [Krasulin et al. 1980]. The geometry of the microgranules was largely affected by the content of the binder (e.g., poly (vinyl alcohol) or starch) in the suspension, initial moisture content, and spraying pressure. Rafaniello et al. [1981] obtained SiC-AlN powder by carbothermal reduction of silica and alumina, derived from an intimate mixture of silica, aluminum chloride and starch. The resulting single-phase SiC-AlN powder was then hot-pressed without additives to a high density, fine-grained, uniform microstructure. Tai et al. [1992] prepared sub micron powders of complex oxides via resin intermediates applying a starch based organic precursor. A commercially available water-soluble starch was used as the organic base for solution synthesis of ceramic powders. Calcination of amorphous resins at a temperature below 600 o C, yielded perovskite powders of Sr-doped LaMnO3 and Sr-doped La(Fe,Co)O3. Sr-doped LaCrO3 was calcined above 750 o C to ensure phase purity and to remove the organic residue. Tai and Anderson [1994] applied a commercially available water-soluble starch derivative as the sole organic precursor in the so-called Liquid-Mix synthesis of mixed-cation oxide powders and thin films. An acidified (HNO3) polymer was able to complex metal ions through carboxylate ligands. Loosely agglomerated fine powders, as well as dense thin films of complex oxides, were prepared using the same type of polymer. Oxide powders of Cr-doped lanthanum aluminate and yttrium aluminum garnet both crystallized in a single step, without forming any intermediate or second phases, when the amorphous resin intermediates were calcined at 650 o C and 750 o C for 2 h, respectively. It was shown that HNO3 could effectively reduce the viscosity of the polymer-nitrate solution to make it suitable for spin coating. Dense thin films of Y2O3 (8 mol %)doped ZrO2 were formed on silicon and Al2O3 substrates by spin coating the polymeric solution and heating at temperatures below 1000 o C. Raman et al. [1995] synthesized SiC from silicon alkoxides and various carbon sources. Tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and a mixture of TEOS and MTES were hydrolyzed in the presence of phenolic resin, ethylcellulose, polyacrylonitrile (PAN) and starch to incorporate the gel into the carbon source in the silica network. The gel thus obtained was carbonized at 800
11 o
C in an argon atmosphere to obtain the mixture of silica and carbon, which when heated to 1550 o C in argon yielded SiC. Characterization of the product by XRD, FTIR and SEM showed it to be beta SiC with different crystallite and grain sizes. The difference in sizes was attributed to the nature of the carbon source. The density of the SiC obtained by the sol-gel process was found to be lower than the values reported for SiC, and this was ascribed to the porous nature of the products generally obtained by this method.
5. BINDER SYSTEMS Starch and its derivatives can be widely used as the components of multiple binder systems in ceramic processing. The property requirements of temporary binders in the production of ceramics, especially tiles, were listed in the paper of Prampolini [1992]. The binders were mainly water-soluble linear polymers. Significant processes during their drying were chemical interaction with the particles resulting in strengthening and migration of the polymer to the surface, which reduced toughness. Organic polymers of longer chains, which were less soluble in water or insoluble in it, arranged themselves between the inorganic particles. They did not migrate to the surface on drying, but higher percentage of such binders was needed. The properties relevant to binding of starch and its derivatives, three cellulose derivatives: carboxymethylcellulose, methylcellulose and hydroxymethylcellulose, ligninsulphonates and several synthetic polymers were discussed. The effect of the above binders on the intermediate and final properties of high-alumina porcelain was compared. It was also concluded that binders were expensive, but their cost might be balanced by lower requirement for expensive foreign clays (Figs. 4, 5).
Figure 4. Modified starch during spray-drying migrates to the surface of granuledrops
T e m p e ratu re
D ep h t
F le x u ragl re en s tre n gth [M P a ]
12
D e p th H [m m ]
Figure 5. Change of mechanical properties of dryied surface by organic modificators Kucharski and Reyman [Polish Patent PL 77853 750530] patented the production of the sealing material for the preparation of ceramic-metal or glassmetal joints of sufficient strength at the temperature lower than 750 o C, which was made from a glass batch containing SiO2, Al2O3, Li2O, crystallization catalysts (Ag+, W 6+, and In3+), and catalyst accelerators (Sb2O3, starch, and CeO2). Thus, the batch containing pure glass sand - 788.0, Al(OH)3 - 62.5, Li2CO3 - 285.4, ZnO - 50.0, KNO3 - 45.0, AgNO3 - 0.6, WO3 - 0.1, In2O3 - 0.2, Sb2O3 - 0.2, starch - 0.6, CeO2 - 0.2, and NaCl - 2.0 g was melted at 1770 o C, fritted by quenching in cold water, ground with 7.5 wt. % clay, mixed with water, applied on the parts to be joined, dried at 100 - 120 o C, and heat-treated at 870950 o C and heated to crystallize at 660-730 o C. The glass-ceramic contained 6090 % crystal phase containing a solid solution mainly of Li metasilicate and had high electric resistance at elevated temperatures and high temperature of onset of softening. [UK Patent GB 2295351 A] described a composite article, which was manufactured by employing vacuum forming techniques with a dispersion including as the major solid constituent thereof an anhydrous crystalline mineral of acicular form, such as wollastonite. At least 60 wt. %, and preferably 85 wt. %, of the solid constituents of the dispersion comprised such a mineral. The mineral
13 had an aspect ratio of at least 10 to 1, and preferably 20 to 1. The dispersion included a binder system, which was starch, and/or colloidal silica and/or colloidal alumina, and/or latex. In the process described by German Patent [DE 4400131 A1 950706] a water-dispersible and/or water-soluble starch was added as the C-containing binder. A mixture consisting of SiC powder, carbon black 9.6 wt.%, and aqueous starch suspension (solids content 70 wt.%) 9.6 wt.% was dispersed in water together with a dispersant, e.g., lignin derivative, and the suspension spray dried to obtain granules. The granules were pressed to give rings that were heattreated at 600 o C, machined, and impregnated with Si and polished. Starch binders [European Patent EP 503833 A1 920916] especially useful for fuel briquettes to give products with good bonding strength and could carbonize with a clear flame, were obtained from gelatinized starch sulfamate and phosphate. Cooking an NH3-neutralized mixture of maize starch, sulfamic acid and H3PO4 at 140 ° C, for 3 min gave a gelatinized starch derivative, which was used to bind the coal dust in briquette manufacture. According to Japanese Patent [JP 59116171 A2 840704] ceramics were molded using a binder of starch derivatives and/or plant seed mucilage and/or its derivatives. The starch derivatives might be etherified, esterified, cross-linked, graft-copolymerized, dextrin-modified starch, or α-type starch. The plant seed mucilage might be guar gum, locust bean gum, or tamarind and it might be etherified, esterified, or graft-copolymerized. The use of the above binders gave ideal plasticity and flowability, and the molded products had smooth surface and high strength. Thus, Al2O3, binder (an aqueous mixture solution of etherified starch, crosslinked-etherified starch, and etherified guar gum), polyethylene glycol, di-Bu phthalate, glycerin, and surfactant were mixed to give a clay-type mixture which was molded to give a ceramic molded body having high bending strength. The application of different starch derivatives e.g. maltodextrins, dextrins as well as pullulan, dextran as ceramic binders lowering the viscosity of aqueous alumina suspensions and giving the bodies with high enough green strength was announced by Schilling et al. 1998. The paper of Sikora et al. [2002] described rheological behaviour of aqueous alumina suspensions with an admixture of differentiated quantities of maltodextrins and dextrins with various molecular weights in the range 900 – 63000 Da. In the Figure 6, a transition from pseudoplastic to nearly newtonian behaviour of 20 vol.% alumina slurries are shown. The other work of Sikora [2001] attempted to calculate statistically the optimal maltodextrins or dextrins addition dropping down the viscosity of aqueous alumina suspensions, in the temperature range 20 – 80 oC, as shown in the Figures 7 a-d and 8 a-d.
14
40
Shear Stress (Pa)
35 30 25
no additive 1%
20
3% 5%
15
10%
10 5 0 0
100
200
300
400
500
Strain Rate (1/s)
Figure 6. Aqueous suspensions of 20 vol.% alumina exhibit a transition from strongly-flocculated, pseudoplastic behavior to a Newtonian-like state upon the addition of various concentrations of 3,600 Dalton maltodextrin.
18
Mw
16
900 1800 3600 5650 5450 15000 63000
Yield stress [Pa]
14
12
10
8
6
4
2
0 0
1
2
3
4
5
6
7
8
9
10
Maltodextrin/dextrin concentration [% (m/m)]
Figure 7a. Yield stress changes of aqueous alumina slurries with an addition of maltodextrins/dextrins of different molecular weight (Mw) in the dependence on concentration, at 20oC
15
25
Mw
Yield stress [Pa]
20
900 1800 3600 5650 5450 15000 63000
15
10
5
0 0
1
2
3
4
5
6
7
8
9
10
Maltodextrin/dextrin concentration [% (m/m)]
Figure 7b. Yield stress changes of aqueous alumina slurries with an addition of maltodextrins/dextrins of different molecular weight (Mw) in the dependence on concentration, at 40oC 25
Yield stress [Pa]
20
M900 w 1800
15
3600 5650 5450
10
15000 63000
5
0 0
1
2
3
4
5
6
7
8
9
10
Maltodextrin/dextrin concentration [% (m/m)]
Figure 7c. Yield stress changes of aqueous alumina slurries with an addition of maltodextrins/dextrins of different molecular weight (Mw) in the dependence on concentration, at 60oC
16
40
35
Mw
30
Yield stress [Pa]
900 1800
25
3600 5650
20
5450 15
15000 63000
10
5
0 0
1
2
3
4
5
6
7
8
9
10
Maltodextrin/dextrin concentration [% (m/m)]
Figure 7d. Yield stress changes of aqueous alumina slurries with an addition of maltodextrins/dextrins of different molecular weight (Mw) in the dependence on concentration, at 80oC
0,08
Casson`s viscosity [Pa.s]
0,07
M900 w
0,06
1800 0,05
3600 5650
0,04
5450 0,03
15000 63000
0,02
0,01
0 0
1
2
3
4
5
6
7
8
9
10
Maltodextrin/dextrin concentration [% (m /m )]
Figure 8a. Casson`s viscosity changes of aqueous alumina slurries with an addition of maltodextrins/dextrins of different molecular weight (Mw) in the dependence on concentration, at 20oC
17
0,1
900 1800
0,09
Mw
Casson`s viscosity [Pa.s]
0,08
3600 5650 5450 15000
0,07 0,06 0,05
63000
0,04 0,03 0,02 0,01 0 0
1
2
3
4
5
6
7
Maltodextrin/dextrin concentration
8
9
10
Figure 8b. Casson`s viscosity changes of aqueous alumina slurries with an addition of maltodextrins/dextrins of different molecular weight (Mw) in the dependence on concentration, at 40oC
0,09
0,08
M900 w
0,07
Casson`s viscosity [Pa.s]
1800 3600
0,06
5650 0,05
5450 15000
0,04
63000 0,03
0,02
0,01
0 0
1
2
3
4
5
6
7
8
9
10
Maltodextrin/dextrin concentration [% (m/m)]
Figure 8c. Casson`s viscosity changes of aqueous alumina slurries with an addition of maltodextrins/dextrins of different molecular weight (Mw) in the dependence on concentration, at 60oC
18
0,09
0,08
Casson`s viscosity [Pa.s]
0,07
M900 w
0,06
1800 3600
0,05
5650 0,04
5450 15000
0,03
63000 0,02
0,01
0 0
1
2
3
4
5
6
7
8
9
10
Maltodextrin/dextrin concentration [% (m /m )]
Figure 8d. Casson`s viscosity changes of aqueous alumina slurries with an addition of maltodextrins/dextrins of different molecular weight (Mw) in the dependence on concentration, at 80oC The possible mechanisms of adsorbtion of added polysaccharides on the surface of alumina particles are presented in Figure 9. Adsorbed saccharides caused sorbate mediated steric hindrance, which counteracted the van der Waals forces, which in consequence changed the behaviour of aqueous alumina suspensions from pseudoplastic to nearly Newtonian. The work of Sikora et al. [2004] aimed to find and investigate some blends of polysaccharides, that could be useful as binders and plasticizers in the basic conditions of aqueous alumina suspensions. Aqueous suspensions containing 45 vol. % α - alumina, maltodextrin combined with small quantities of other polysaccharides (totally 5 g polysaccharides per 100 g of alumina) were thermally gelled, and their properties characterized by rheological techniques. Gelling formation was found in combined systems maltodextrin-agar, maltodextrin-agar-locust bean gum, maltodextrin-xanthan gum-locust bean gum, maltodextrin-xanthan gum and maltodextrin-carrageenan. In the Tables 1 and 2 some of rheological properties of such systems are done. The use of maltodextrin M040 was necessary for liquefying of dense aqueous alumina slurries. Saparov et al. [1984] described the production of alumina tubes and housings from a plastic alumina mixture in which the plasticizing binder was an aqueous suspension of paraffin, caustic soda, starch, polyvinyl acetate and water.
19 CH2OH
CH2OH
O
O OH
H O
Al2O3
O
OH
O H
O
Al2O3
CH2OH O
OH
OH
O
H O
b) O
OH
O H
CH2OH
CH2OH
O
O OH
H O H O
Al2O3
O
OH CH2OH
OH
a)
OH
OH
O
OH
H
c) O
OH
O H
Al2O3
O H H O CH2 O OH
O OH
CH2OH O d) OH
O
O
OH
Figure 9. Possible mechanisms of hydrogen bonds formation between hydroxyl groups on the surface of alumina and hydroxyl groups of polysaccharides a) at the first anomeric carbon atom, b) at the second carbon atom, c) with an interaction of water molecule, d) at the sixth carbon atom
20
Table 1. Rheological properties of aqueous alumina-polysaccharides systems before and after gelling as measured by oscillatory testing at the frequency of 1 Hz. τ max - maximum shear stress within the linear region of viscoelasticity (gel strength), G` - storage modulus, G``- loss modulus, η∗ - complex viscosity, tan δ [-] Sample
Alumina-M040 Alumina-M040agar Alumina-M040agar-LBG Alumina-M040xanthan Alumina-M040xanthan-LBG Alumina-M040carrageenan
τ max [Pa] Before After gelling gelling 0.29 ± 0.79 ± 0.01 0.03 0.37± 10.74± 0.02 0.13 0.48± 23.95± 0.02 0 7.28± 5.06± 0.25 0.20 2.33± 32.1± 0.09 0.11 2.13± 20.1± 0.08 0.66
G`[Pa] Before gelling 2.16 ± 0.09 4.05 ± 0.08 4.36 ± 0.20 126.6± 4.81 40.1 ± 1.70 51.0 ± 2.40
After gelling 12.88± 0.56 105.2± 0.49 276.2± 9.05 100.3± 3.39 341.3± 4.38 175.5± 7.13
η∗ [Pa.s] Before gelling 0.67 ± 0.02 1.04± 0.04 1.12± 0.03 20.60± 0.71 6.83± 0.28 8.78± 0.19
After gelling 2.47 ± 0.09 16.70± 0.57 43.95± 1.63 16.25± 0.64 54.55± 0.64 28.15± 1.20
tan δ [-] Before gelling 1.67 ± 0.03 1.28± 0.05 1.29± 0.03 0.22± 0.01 0.38± 0.01 0.26± 0.01
After gelling 0.68 ± 0.02 0.21± 0.01 0.07± 0 0.22± 0.01 0.09± 0.004 0.10± 0.004
Table 2. Rheological properties of aqueous alumina-polysaccharides systems before and after gelling as measured in combined CS/CR mode and pH values of alumina suspensions before gelling. τ 0s – static yield stress, τ 0d – dynamic yield stress, A – area of thixotropy Sample
Alumina-M040agar
τ 0d [Pa] Before gelling 0.22 ± 0.01 0.24 ± 0.01
After gelling 0.47 ± 0.02 2.4 ± 0.03
τ 0s [Pa] Before gelling 0.78 ± 0.03 1.13 ± 0.02
After gelling 1.9 ± 0.04 34.45 ± 1.53
A [MPa/s] Before gelling 7.55 ± 0.23 8.99 ± 0.16
After gelling 7.71 ± 0.21 15.25 ± 0.55
Alumina-M040agar-LBG
0.44 ± 0.01
4.1 ± 0.17
1.68 ± 0.02
57.04 ± 2.10
10.53 ± 0.39
17.37 ± 0.52
10.25 ± 0.18
Alumina-M040xanthan
4.76 ± 0.03
2.95 ± 0.14
20.03 ± 0.83
27.5 ± 0.40
15.54 ± 0.07
16.96 ± 0.50
10.35 ± 0.08
Alumina-M040xanthan-LBG
2.14 ± 0.10
6.5 ± 0.32
36.64 ± 0.99
43.2 ± 1.77
20.10 ± 0.20
25.24 ± 0.95
10.36 ± 0.11
Alumina-M040carrageenan
1.45 ± 0.06
1.88 ± 0.06
30.98 ± 1.04
54.45 ± 1.57
11.39 ± 0.33
12.52 ± 0
10.36 ± 0.03
Alumina-M040
pH [-] 10.41 ± 0.05 10.26 ± 0.24
21 Three mixtures with different content of starch were prepared (12-15 %). The best plasticizing effect was achieved at 15 % of starch concentration in the mixture. The same authors [Saparov et al. [1984] have published some indications on the production of the pipes and casings from Al2O3 (grade G-00), fired in tunnel kilns at 1560 o C, with paraffin, NaOH, starch, or polyvinyl alcohol as the binders). The pipes had diameter of up to 25 mm and 600 mm length. Both organic and inorganic binders were added to non-plastic materials such as zeolite and silica clay, and waste materials such as molten slags for extrusion molding. Bentonite and clay were used as inorganic binders; methylcellulose, acrylate, and starch were used as the organic binders [Fukazawa et al. 1997]. Terakura [Japanese Patent JP 06171995 A2 940621] described the production of heat insulating materials. In this process inorganic fiber molded articles having bulk density ≤100 kg/m3 and/or their scraps were crushed to average size 6 - 20 mm, mixed with aqueous binder and inorganic filler, and used as heat insulating materials especially for steam pipe, heating medium pipe, refrigeration pipes, etc. The aqueous binder was selected from inorganic watersoluble silicate, acidic phosphate, colloidal metal oxide, water-soluble polymer, and/or water-dispersing polymer or from organics such as: carboxymethyl cellulose, poly (vinyl alcohol), polyvinyl butyral, polyethylene glycol, ethyl cellulose, hydroxypropyl cellulose, and/or starch. Engelskirchen and co-workers [German Patent DE 4243703 A1 940630] described the production of dispersants and liquefying agents for ceramic suspensions. The nonaqueous components of the suspensions contained 0.025.0 wt.% salts of monovalent cations of the oxidation products of unsubstituted or glycosidically substituted homoglycans as dispersants and liquefying agents. The suspensions were manufactured by milling the nonaqueous components in the presence of water in a ball mill or rotary drum. A dispersion of potato starch in CCl4 was mixed with NO2, the mixture was heated at 50 ° C for 15 min and pressurized with oxygen to 2 bar, and further heated at 50 ° C for 3 h, at constant pressure, and an additional 3 h at decreasing pressure. The oxidized starch had on the average one carboxyl group per anhydroglucose unit. Al2O3 suspensions (water content 35 wt. %) containing 0.2 % of the NH4 salt of the liquefier had immediate and 2-day viscosity 130 and 340 mPa.s, respectively. According to the disclosure of Schulten and co-workers [German Patent DE 4203773 A1 930812], the metallic or ceramic parts were coated with a meltable material and a binder, heated first to the temperature above 500 ° C in vacuum or an inert atmosphere, and then at higher temperature to form a dense layer. The meltable material was selected from Si, Ti, Zr, Ca, Al, W/Si, Mo/Si, or SiO2 and the binder was selected from flour (?), starch, or a polymer such as polyethylene. Coating of graphite with SiC and MoSi2, aluminizing of steel, and Cu or Ag coating of natural objects such as walnuts and bamboo canes were some of the given examples. Two different copolymers of styrene: either (I) Me methacrylate with 2ethylhexyl acrylate, or (II) Bu acrylate with Et acrylate, or one of these
22 copolymers and starch thermally modified with Na2B4O7 or NH4Cl at 100-170 ° C, in weight ratio (0.5-2) :1, were added as aqueous dispersions stabilized with an anionic or nonionic surfactant to granulated ceramic mixtures to improve their compressibility and to give thin articles (plates, dishes) an improved, green mechanical strength [Czech Patent CS 252139 B1 880516]. The additive was miscible with water, did not cause coagulation of the ceramic suspensions, disappeared during firing, and was efficient in an amount less than 1 %. It increased the strength of granules and of pressed articles before and after drying by 120, 73, and 115 %, respectively. A binder composition for making green ceramic sheets described by Japanese Patent [JP 63025269 A2 880202] consisted of (A) a carbonyl groupcontaining water-soluble polymer (e.g. diacetone acrylamide copolymer or dialdehyde starch), (B) polyhydrazine derivatives containing more than 2 hydrazine groups, (e.g. adipic dihydrazide or sebacic dihydrazide), and (C) acrylic acid-Bu acrylate-styrene copolymer ammonium salt emulsion. Thus, the binder composition was mixed with Al2O3 powder, benzyl Bu phthalate, poly (oxyethylene octylphenyl ether, and water to form a slurry, which was formed into a green sheet on a polyester film, and sintered at 1600 ° C to give a circuit board with improved quality. In the preparation of a ceramic green body [Japanese Patent JP 2001145910 A2 20010529], an aqueous slurry containing a ceramic powder, a binder, and additives (i.e., a dispersing agent) was spray dried to give a powder which was then press molded to give a green body, wherein a binder containing starch or its derivatives was used. In the method, the moisture of the powder was controlled to be 1.0 - 5.0 %, or the powder was allowed to absorb moisture. In the method, the press-molded green body was dried. The starch derivative might be starch-derived dextrin, viscosity-decreased starch, starch alkenylsuccinate ester, and/or hydroxyalkylated starch. Also disclosed was the binder containing a starch derivative and poly (vinyl) alcohol. The obtained ceramic green bodies showed satisfactory strength without using a large amount of the binder. Binders played a vital role in formulating piezoelectric materials [Sangawar et al. 2001]. A number of binders such as poly (vinyl alcohol), methocel, starch, hydroxyprophylmethylcellulose, hydroxyethylcellulose, polyvinyl pyrrolidone, gum arabic, and polyethylene glycol were studied for solubility, viscosity, pH, thermal behavior and ash content incorporating them in a standard piezoelectric formulation. Further, the resulting piezoelectrics were studied for green density, sintered density, shrinkage characteristics, microstructure, compressive strength and electrical properties. The generated data indicated that poly (vinyl alcohol) was a potential candidate as a binder for this purpose. Loercks and Neisius [European Patent EP 503833 A1 920916] patented in 1992 the use of starch gelled phosphates and sulphonates as the binders of coal briquettes. Mundigler and Rettenbacher [European Patent EP 524920 A1 930127] patented also in 1992 the use of biodegradable starch containing products for construction, insulating, and packaging materials. Starch in such kind of
23 materials has been used in the form of a gel and was playing the role of a binder giving the stiffness after addition, mixing and drying. The production of ceramic materials from waste silica was described in Japanese Patent [JP 7762319 770523]. Waste SiO2 from clay processing was mixed with 0.2-10 % soluble starch derivative or its soluble hydrolyzate to obtain a raw material for ceramics. Thus, a mixture containing nonplastic waste SiO2, plastic SiO2, glass powder, dextrin, and water was extruded, dried, and fired at 1150 ° C to obtain a ceramic product having shrinkage 6.7 %, bending strength 343 kg/cm2, and water adsorption 7.2 % vs. 6.6 %, 333 kg/cm2, and 8.31 %, respectively, for a ceramic without the binder. Schilling et al. [2002] empirically correlated the rheology of aqueous suspensions of ultra fine (40 nm diameter) gamma-alumina powder with the concentration and structure of the following sugars and sugar alcohols: maltodextrin, sorbitol, maltitol, D-fructose, D-glucose and sucrose. It was shown that several monosaccharides, especially pentoses and sugar alcohols, significantly improved fluidity of aqueous suspensions and high-density pastes of alumina powder. In hexoses, the orientation of the 4-hydroxyl group played a key role in controlling alumina suspension rheology. The green strength and the sintering densification of slip cast alumina pellets were not affected by the addition of 5 wt.% of either arabinose, xylose, mannitol, or maltitol.
6. POROUS CERAMICS In the porous ceramic media production native starch in the form of granules has been used. According to Kerr et al. [1950] starch granules, depending on origin, have differentiated medium diameters ranging from 1-2 µm (amaranth starch) up to 100 µm (potato starch). The starch granules could be fired out during sintering, which results in the pores of relevant diameter. Depending on the needs one can find convenient starch granule size for the specific pore diameters in ceramic composition. In Table 3 the ranges of particle size distribution of more common starch varieties are presented. Table 3. Particle size distribution of common starch varieties Starch variety Corn Potato Wheat Tapioca Waxy corn starch Rice Sorghum Arrowroot
Particle size distribution [µm] Van Beynum &Roels 1985 Kerr 1950 3-26 5-26 5- 100 15-100 2-35 2-35 4-35 5-35 3-26 2-35 3-8 3-8 3-26 6-30 5-70 15-70
24
In order to achieve the granules with narrower particle size distribution one can use any of the separation methods, e.g. sieving, sedimentation. The native (untreated) starch granules are insoluble in water, so that an aqueous sedimentation method for starch granules separation could be applied.
6.1. Porous structures Bonekamp et al. [1989] investigated the pore properties of the macrovoid structure in a dense alumina matrix obtained by the addition of starch. These studies revealed that the homogeneity of the macropore structure depended on the degree of flocculation of the alumina suspension. The total porosity of the porous bodies increased linearly with the concentration of starch with respect to the total solids volume. A percolation threshold for a connective void-throat network was observed between 15 and 20 % (v/v) of starch depending on the suspension properties. Lyckfeldt and Ferreira [1998] formed porous ceramics using starch as both consolidator/binder and pore former. Simple and complex-shaped components of porous alumina were produced with ultimate porosities between 23 and 70 %. The large spherically shaped pores (10 - 80 µm) left by the starch particles dominated the overall pore structures. The average size of the small pores connecting the large pores controlled by the total solids loading and starch content in the originally prepared slips, varied between 0.5 and 9.5 µm. According to the authors the chemically modified starch gave better dimensional control of the connecting pores than native starch owing to more stable properties during water processing. Costa et al. [1996] compared several binders: PVA, paraffin and starch in the compaction of alumina-based ceramic tablets. These tablets were prepared by dry pressing and sintered at 1000 o C for 1 h. Samples were characterized by measuring water absorption, apparent porosity, total porosity and apparent density. The results showed that starch was the most effective of the binders, yielding tablets with high porosity and green strength, which remained adequate for handling after sintering. Guy and Evans [1997] incorporated an alumina powder into a starch-water mixture and compounded by twin-screw extrusion with superheating in the metering section. Decompression at the die produced foam, which could be pyrolysed and sintered. Average porosities of 69 % were obtained in the sintered foam, the highest individual value being 78 %. Microscopic examination revealed irregularity of structure. Thus, regions of high cell wall thickness and of apparent cell collapse accompanied regions of very low apparent density with a cell wall thickness less than 5 µm. Manufacture of ceramic bodies with open channels [U.S. Patent US 4814029 A 890321] comprised mixing of calcined Al2O3 particles, water, polypropylene, kaolin, corn starch, and cup grease, and organic mix comprising polypropylene powder (Hercoflat), water, methylcellulose, water-soluble polymer
25 (UCAR), and Calgon surfactant to an extrudable consistency in a high mixer. They were then extruded. The final multiple-core composite which contained approximately 676 cores was cut into 2.5 and 5 cm pieces which were fired at 1455 ° C with a 5 h soak. The resulting ceramic pieces had cross section approximately 1600 channels/in.2. European Patent [EP 48099 A1 820324] presented the gel materials, produced by precipitation sol-gel processes, and internal gelation, subsequent drying by evaporation into a gas mixture that contains an organic compound. The organic compound was one that was taken up by the gel during the drying when an open porous network had to be maintained. The spheroidal gel particles were made by feeding an aqueous solution of Al chloride hydroxide and a cationic starch derivative dropwise into 12 M NH4OH and the particles were soaked in the NH4OH solution, washed, heated in water, cooled, drained, and then dried in a saturated mixture of air and n-hexanol. Excess hexanol was removed from the particles by air and they were heated to 900 o C in air for debonding and sintered at 1400-1450 ° C in an Ar – 4 % H2 mixture. The sintered particles were substantially crack free. UK Patent Application [GB 2 192 869 A] described the method of preparing spherical microparticles of silica by adding an acid solution to a silicate solution before or after addition of an alkali metal alginate, ammonium alginate, starch, gelatin, pectin or mixtures thereof. The microparticles could be used to prepare macroporous materials having a controlled pore structure. Porous cordierite ceramics, formed from a mixture of ball clay, talc, alumina, and silica sand, were developed by direct consolidation method based on the properties of starch in water [Alves et al. 1998]. The gelling capability of starch granules in warm water allowed the transformation of a stable suspension into a rigid and stiff body that could be machined in the green state. Starch consolidation could be performed in nonporous molds, enabling materials with controlled porosity to be obtained by varying the starch content in the slip compound. The simplicity of the process as well as the low price of starches made this consolidation technique very promising from an industrial point of view. The results showed good agreement with the expected porosities of the cordierite-sintered bodies, thus enabling the design of porous textures and tailoring of the material to applications. The production of porous ceramic radiation plates described in US Patent [5219802 A 930615] comprised preparing a homogeneous mixture consisting of gairome, grog, low-expansion component selected from cordierite and petalite, sintering aid, calcined transition metal oxide, and organic components selected from carbon black and corn starch, and sintering the composition at 1000-1350 o C. Sintering time was decreased. A mixture consisting of gairome, grog, petalite, talc, carbon black, and corn starch, was molded, and the green plates sintered at 1150 o C for 2 h, to give a plate having porosity 60.64 %, pore size 0.2-30 µm, fracture strength 17.20 kg/cm2, and coefficient of thermal expansion (5.67 ± 0.2) × 10-6/degree.
26
Relative chan ge of dimen sions
Generally the organic binders behave as thermoplastics (they melt above 150oC), thus these features pass into the ceramic materials, and as a consequence during thermal treatment the ceramic details can deform.
Temperature [K]
Figure 9. Shape iteration (cylinder) of PAV samples during temperature treatment This problem can be eliminated usually by an addition of different organic polymers in order to widen their melting temperature interval in terms of pyrolysis, expansion and the other phenomena accompanying thermal treatment. Starch itself does not show thermoplastic effects, thus its use as modificator is fully justified.
Figure 10. Derivatogram of maltodextrin.
27
In the Figure 10 the analysis of maltodextrin by the use of derivatograph (Thermogravimetric Analizer TGA/SDTA 851e Mettler Toledo) is shown. The analysis was conducted in the temperature range 30 - 600 oC, at the heating speed 5 oC/min. As it is presented in the Figure 10 there are two main zones of polysaccharide decomposition in the atmosphere of air. At about 270 oC ether bonds break and maltodextrin supposedly decompose to pseudoglycol forms, which evaporate or subsequently transit to CO2 and H2O. At about 550 oC “caramelized” and “coked” parts decompose finally to CO2 and H2O. During the whole thermal treatment neither alcohols, nor oxides (NOx) form (Figure 11)
+
OH H2O NOx, (H2CO) CO/CO2 CO2 pseudoglicols
Figure 11. Thermal decomposition of maltodextrin (potato starch).
6.2. Diesels Wada et al. [Japanese Patent 07331213 A2 951219] patented the production of the ceramic filters for waste gases by joining ceramic sheets with adhesives prepared by mixing inorganic powders, starch, H2O, and polyacrylamides and/or poly (vinyl alcohol). Such filters were useful for filtering exhaust gases from diesel engines. Hiroshima sericite, corn starch, and H2O were mixed, heated, and mixed with polyacrylamide to give an adhesive, which
28 was applied to a ceramic sheet comprising sericite and aluminosilicate fibers and bonded to a ceramic sheet to give a joined ceramic showing good initial adhesion strength and no layer separation on sintering the ceramic. Nagai et al. [UK Patent GB 2302826 A] patented the production of an exhaust gas filter for passing exhaust gases of a diesel engine, thereby removing particulate matter from the exhaust gases. The filter was an extruded, honeycomb member made of aluminum titanate as the main component and of pore forming material. The pore forming material could be activated carbon, coke, synthetic resins, starch or graphite. The honeycomb had alternate cells closed at opposite ends. The closure material could be the same or a ceramic such as cordierite. SiO2, Fe2O3, Al2O3, TiO2, MgO or CaO could be added as strengthening agents. The manufacture of ceramics with continuous pores was described in Japanese Patent [JP 05097537 A2 930420]. The pores of molded products from spherical resin particles were filled with slurries comprising ceramic powder, thermosetting resins, and starch powder. The greens were prepared by heating at a temperature equal or higher than the gelation temperature of the starches to give the porous ceramics. The ceramics were useful for exhaust gas filters and / or catalyst supports. A corrugated honeycomb filter for treatment of diesel exhaust gases [Japanese Patent JP 63134020 A2 880606] consisted of heat-resistant inorganic fibers, ceramic material powder, and CeO2, and each cell of the honeycomb was 1-end closed and the closed end was arranged in an alternating manner with an open end of cell. The filter was effective for removing the particulates from the exhaust gases. Thus, aluminosilicate fibers (average length 10 µm, average diameter 3 µm) were dispersed in water. A solution containing sericite and petalite, CeO2, and water was added to the fiber suspension. An acrylic-vinyl copolymer emulsion, and then an AlCl3 solution were added. The mixture was neutralized with NH4OH to form an Al(OH)3 colloid. The colloidal solution was mixed with starch and cast the sheet, which was then corrugated and then 1 end was alternately plugged and calcined at 1250 ° C, to give a filter with 80 % porosity. The filter removed particulate at 1 g/min rate at 575 o C, vs. 670 o C for a conventional filter prepared without colloid formation.
6.3. Filters Kawakami et al. [Japanese Patent 03131580 A2 910605] claimed the manufacture of porous ceramics from the mixture of calcium phosphate or ZrO2 ceramic powder and water, foaming agent, and a thickener. The resulting slurry was molded, foamed, dried, and fired. The slurry contained 18 - 70 wt. % ceramic powder. As the thickener methylcellulose, hydroxypropylcellulose, or other watersoluble cellulose derivative, poly (vinyl alcohol), polyacrylic acid, polyacrylamide, poly (vinyl pyrrolidone), polyethylene glycol, pectin, and starch were used. The foaming agent was H2O2 or NaHCO3. The porous ceramics obtained were used for filters, prosthetic devices, liquid aggregates, etc.
29 Yamashita et al. [Japanese Patent 01056111 A2 890303] patented a method for preparing multilayer ceramic filters. The method comprised screening an organic fiber-containing ceramic slurry to form a porous ceramic raw support layer, spray-coating the surface of raw support layer with a second ceramic slurry, drying and calcining the coated support layer to burn the organic fiber. Thus, a ceramic slurry containing natural cellulose pulp, alumina-silica fibers, styrene-butadiene rubber binder, and anion-modified starch, was screened via an applicator to obtain a 1.1 mm porous support layer, which was then sprayed with an aqueous slurry containing calcined alumina, water, and styrene-butadiene rubber binder, dried, and calcined at 1500 o C under oxygen in an electric furnace to obtain a multilayer ceramic filter. The porous ceramic precursors useful for manufacture of filters and catalyst carriers [Japanese Patent JP 01160878 A2 890623] were manufactured by mixing a ceramic powder with at least two kinds of starches having different sizing temperatures and water, molding, and heat treating at a intermediate temperature between the sizing temperatures of starches. The authors of this method made full use of the differentiated properties, which starches of different origin have. One starch variety, with lower temperature of gelling was used as a binder (in the form of gelled starch) and another, with higher gelling temperature (not gelled, in the form of granules) was applied as a pore former in the same ceramic powder batch. In Table 4 one can find the gelling temperatures of various origin starches. Table 4. Gelling temperatures of various starches [Van Beynum and Roels 1985] Starch variety Corn Potato Wheat Tapioca Waxy maize Rice Sorghum
Range of gelling temperatures [o C] 75 – 80 60 – 65 80 – 85 65 - 70 65 – 70 70 – 75 75 – 80
Ceramic materials such as Al2O3, ZrO2, SiO2, mullite, and cordierite were mixed with 1-10 wt.% nonoxide whiskers, organic binder, and pore-forming materials and sintered in an oxidizing atmosphere to obtain porous ceramics [Japanese Patent JP 02199076 A2 900807]. Potato starch and rice husk were used in the preparation of SiC whisker-reinforced Al2O3-ZrO2 porous ceramics having high impact resistance. The porous ceramics were useful as filters, catalyst carriers, etc. Starch, water, ceramic powders (e.g., Cu oxide), and metal powders (e.g., Mn) were mixed, shaped, and heated for hardening and foaming to give plateshaped air filters for removal of malodorous substances [Japanese Patent JP 08126694 A2 960521].
30 The preparation of an alumina ultrafiltration membrane with a mean pore size of 6.5 nm was performed by slip casting or tape casting of the support, dip coating or tape casting of the intermediate layer, followed by dip coating of the top layer [Lindqvist K. and Liden E. 1997]. Controlled porosity and pore size in the support were obtained by the addition of starch particles, which were subsequently burned out. An alternative process was developed, based on tape casting of the support and the intermediate layer, followed by lamination and sintering in one step. Very good linkage between the support and the intermediate layer was obtained, as well as an excellent control of the intermediate layer thickness and surface finish.
6.4. Ceramic carriers Chinese Patent [CN 1064069 A] described the porous ceramic carrier for immobilization of microorganisms. It contained 80 - 92 % ceramic materials, 3.514.5 % combustible substances, 3.5 % volatile substances, and 1 - 2 % additives. The combustible substances were organic substances such as wooden fibers, starch, carbon powder, and rice husk. The volatile substances were inorganic substances such as carbonates and sulfates. The mixture could be prepared into desired shapes and calcined at 1100 - 1300 o C. The carrier had a wide variety of applications in fermentation, petrochemistry, chemistry, wastewater treatment, etc. Immobilization of beer yeasts on this carrier was also described. The high-performance composite honeycomb ceramics for catalyst carrier [Chinese Patent CN 1277173 A 20001220] was manufactured from cordierite, aluminum titanate, ZrSiO4 + ZrO2, and Y2O3 by extruding and sintering at 13001400 °C. The ceramic contained 0-4 wt. % Fe2O3; the cordierite clinker was made from organic silicon, Al(NO3)3, and Mg(NO3)2; and the aluminum titanate was synthesized from TiO2 and Al2O3 at 1350-1400 °C. The process might comprise calcining cordierite, aluminum titanate, ZrSiO4, ZrO2, and Y2O3, and Fe2O3 respectively at 1300 -1400 °C, mixing, milling to 350-400 mesh, holding at 20 - 25 °C for half month, adding plasticizer CMC, glycerin, binder (MC, poly (vinyl) alcohol, composite phosphate), and pore-forming agent (starch, dextrin, and flour) pelleting, holding, adding lubricants, forming, drying, and sintering at 1300-1400 °C for 2 h. The obtained ceramic had high strength and porosity, and the catalyst carrier made of the title ceramic was suitable for treating automobile exhaust. In the manufacturing of the ceramics from gels of aqueous inorganic colloids, a liquid organic lubricant and a pore-forming aid were added successively to the gel [Japanese Patent JP 02192479 A2 900730]. The gel might also contain additives for lowering its surface tension and for forming glass by reacting with the inorganic component(s) in the gel. The pore-forming aid was selected from coal, carbon black, poly (vinyl alcohol), starch, CMC, PEG, sucrose, sawdust, camphor, cellulose, proteins, polysaccharides, and glucose. Aqueous SiO2 colloid was mixed with NaOH and ethanol to form a gel, a liquid
31 paraffin was mixed with the gel, dried, mixed with brown coal, pressed in molds, and sintered at 800 ο C to obtain a ceramic piece having 81.5 % porosity and average pore diameter 188 Å. The porous ceramics were useful as combustion catalyst carriers, fuel-cell electrodes, filters, and sound absorbers.
6.5. Silicon carbide (SiC) Porous SiC products useful for filter, abrasives, and catalyst supports [Japanese Patent JP 63248781 A2 881017] were manufactured from liquid resin components containing silicon and/or SiO2 powder, thermosetting resin, and polyvinyl alcohol with pore-forming agent, crosslinking agent, and hardening catalyst by reacting, removing the pore-forming agent to form porous bodies, and firing at the temperature above 1400 ° C, in an inert gas atmosphere. Thus, aqueous mixture containing SiO2 powder in resol resin, poly (vinyl alcohol), potato starch, HCHO, and maleic acid was molded, reacted in warm water at 80 ° C, for 24 h, washed to obtain a porous body, and fired in the atmosphere of argon, at 1700 ° C, for 24 h, to give a porous SiC product having density 0.44 g/cm3, porosity 84 %, and bending strength 74 g/cm2. Porous SiC ceramics having uniform pore distribution and high strength, hardness, and wear resistance [Japanese Patent JP 63190779 A2 880808] were manufactured from mixtures containing silicon or SiO2 powder, thermosetting resin, and poly(vinyl) alcohol with a pore-forming material, crosslinking agent, and hardening catalyst by reacting, hardening, and sintering at the temperature above 1400 ° C in an inert gas atmosphere. Thus, a mixture, comprising potato starch, HCHO, maleic acid, SiO2 powder, resol resin, polyvinyl alcohol and the balance water, was reacted 24 h at 80 ° C in a mold, washed, dried, and sintered 24 h at 1700 ° C to give a porous SiC having porosity 84 vol. %, apparent density 0.44 g/cm3, and bending strength 74 kg/cm2. The granules for manufacturing porous ceramics for sliding parts [Japanese Patent JP 02069367 A2 900308] comprised an organic powder having average particle diameter 10 - 60 µm. Porous SiC ceramics prepared from granules containing starch particles having average diameter 40 µm, had a friction coefficient that was half of that of SiC ceramics prepared from the same granules but containing no organic powder.
6.6. Conductor films Itoh et al. [1997] prepared CaTiO3 porous substrates required for spin coating of mixed conductor Ca(Ti,Fe)O3 films by a foam burning method. Mixtures of calcined CaTiO3 powder and foaming agents such as corn starch, wheat starch, D (+)-glucose and carbon black were heated gradually up to 500 o C, then kept at 500 o C for 2 h and finally sintered at 1350 o C. Relatively large pores above 15 µm in diameter were formed when 15 - 35 mass % corn starch
32 was used as a foaming agent. Pore diameter distribution broadened with wheat starch. Sintered compacts with pore diameters as small as 5 - 10 µm were prepared using 15 - 25 mass % carbon black or D (+)-glucose. Gas permeability was found to increase in the order corn starch > wheat starch > D (+)-glucose ≥ carbon black for a fixed amount of additive. An accelerated increase in the gaseous permeability was achieved with increasing amounts of foaming agent. A CaTiO3 porous substrate prepared with 20 wt. % carbon black or 40 mass % wheat starch exhibited sufficiently high mechanical strength for the thermal cyclic process of spin coating and subsequent sintering.
6.7. Reduced thermal conductivity Lyckfeldt et al. [1994] manufactured slip-cast cordierite-based materials with reduced thermal conductivity with controlled introduction of porosity. The porosity was obtained by addition of different kinds of fillers (hollow Al-silicate spheres, paraffin, polystyrene, carbon black or starch granules). The results showed that additions of corn or potato starch were most promising, with respect to the processing and porosity control. A homogeneous distribution of spherical pores (5-25 or 15-40 µm) was obtained after sintering. Slip-cast cordierite with 37 % porosity had a thermal conductivity of 1.7 W/mK (compared with 3.7 W/mK for fully dense cordierite), and a bending strength above 50 MPa. The porosity effect correlated very well with theoretical models by Maxwell and, hence, the thermal conductivity of the porous ceramic material could be predicted.
6.8. Sound insulators The porous ceramics useful for sound insulators and water-permeating paving materials [Japanese Patent JP 02283677 A2 901121] were manufactured from crushed feldspathic minerals containing 2.5-14.5 wt. % K2O + Na2O + CaO by adjusting the grain size, molding, and sintering. Thus, crushed feldspathic pottery stone was mixed with 10 % aqueous 50 % corn starch solution, press molded, and fired at 1250 o C to obtain a porous ceramic body having bending strength 70 kg/cm2.
6.9. Improved co-sensitivity According to Yoon and Choi [1995] the addition of 5 wt.% corn starch to porous ZnO showed different conductivity curves during increasing and decreasing temperature. The electrical conductivity decreased rapidly by desorption of OH-groups between 200 o C and 350 o C when the temperature increased in dry air. The CO gas sensitivity of starch-added ZnO samples was higher than that of ZnO without starch addition. The sensitivity of porous, starchadded ZnO to 200 ppm CO gas was much less in humid atmosphere than in dry
33 atmosphere, since water vapour increased the conductivity of porous ZnO in air, but decreased the conductivity in CO. Maximum sensitivity to 200 ppm CO gas balanced by air was about 100 in dry atmosphere and about 15 in an atmosphere with 23 % relative humidity.
6.10. Foamed ceramic panels Nagai et al. [U.S.Patent 4756956 A] patented the foamed ceramic panels, which comprised a base layer of foamed, inorganic raw materials, and a fine glass layer comprising fused and solidified ceramic paper applied to more than one surface of the base layer. The heat-foamable inorganic raw materials and the ceramic paper coating were fired to be fused together. This lightweight, ornamental or decorative chemically resistant foamed ceramics were useful for tiles and roofing tiles, and might be manufactured in large sizes. Wet-grinding a mixture containing feldspar, soda ash, ZrSiO4, and NaNO3 prepared a slip. The slip was passed through a 250-µm-mesh sieve, and Al2O3 fibers, starch, acrylic emulsion, and flocculant were added. The final pulp contained 1% solids and was fed to a paper machine to obtain a 50 x 50 x 1 cm sheet. The sheets were rolled at 20 kg/cm2, dried by far-IR radiation, and then 6 different pigments (each mixed with 201 cerazol oil) were applied. Next, a mixture consisting of acid clay, soda ash, NaNO3, zircon flour, and dolomite was ground to 44 µm (90 %), and pelletized to 1-2 mm. The pellets were spread to form a 50 x 50 x 1 cm layer that was covered with the sheet, and the combination was heated at 870 o C for 20 min, quenched, reheated at 850 o C, and cooled to give a panel.
7. CERAMICS MANUFACTURING 7.1. Extrusion Chen et al. [1997] studied the influence of various types of carbohydrates, such as starch, dextrin, lactose, and glucose, on paste extrusion and related to them water retention capacities. The bulk yield stress and the surface shear stress both decreased as the moisture content increased, however, the way in which the water that was present interacted with the carbohydrates had an important influence. The behaviour of some carbohydrates could be substantially accounted for by a consideration of packing effects, however, dextrin behaved differently. The carbohydrates functioned not only as binders but also as a means of retaining the liquid phase. Greenwood et al. [2001] developed a method of producing alumina fibers. The process used a water-based alumina and rice starch paste. An initial feedrod was made from a central cylindrical rod of alumina paste surrounded by a square shaped section of starch paste. This feed rod was subsequently extruded so that it was reduced in size by a determined ratio. The extrudate was cut into
34 short lengths and re-assembled to the exact dimensions of the initial feed rod so that it contained many smaller alumina rods. This latter specimen was reextruded and the steps repeated as many times as required to obtain a large amount of small diameter fibers. By selecting the die diameter and the number of extrusions the diameter of the fibers was easily controlled. Since all the fibers were in one easily handleable bunch, the bottleneck associated with sintering could be avoided. Hence an economic method of fiber production could be envisaged. Despite of significant progress in the field of ceramic powders consolidation technology, the methods of plastic forming are still of great importance in industrial practice. The ability of ceramic mass to plastic deformation can be modified by addition of special organic substances, which influence both the quality of obtained products and consumption of energy in extruding process. From the mechanical point of view the ability to plastic deformation can be bound to the number of “slipping planes” which determine the area of mutual displacement of the rigid parts (coagulation structures, grain aggregates) of the body.
glycerin
3
olein acid polisaccharide (NH4+)
Energy (J/g)
2,5
KSM Tylose H-4000
2 1,5 1 0,5 0 0
0,5
1
1,5
2
2,5
3
3,5
Organic plasticizer (wt.%)
Figure 12. Influence of organic additives content on the forming energy of extrudates Introducing of organic additives to ceramic mass lowers the forming energy in the process of extrusion (Fig. 12). Organic substances of swelling type (Tylose H 4000, and polysaccharide containing ammonia) decrease forming energy initially rapidly and then slower, and subsequently they slightly increase this energy proportionally to the increase of organic additive content. From practical point of view the content of organic additive should not be too high, in order not to produce the swelling effect. Moreover, its elevated
35
Flex. green strenght (MPa)
content does not rise the mechanical properties of extrudates. The decrease of forming energy and increase of mechanical strength are clearly evident in the Figures 12 and13.
5 4,5 4 3,5 3 2,5 2 1,5 1 0,5 0
Tylose H-4000 polysaccharide (NH4+)
1
1,05
1,1
1,15
1,2
1,25
1,3
1,35
1,4
Sw elling ratio ë
Figure 13. Influence of swelling ratio on flexural green strength and relative density of extrudates
7.2. Ceramic glazers and pigments (colours), enamel inks Mel'nik and Khodskii [1978] studied the influence of suspended additives (polyacrylamide, starch, etc.) on the properties of clayless enamel slips. The authors concerned the possible replacement of clay in enamel slips by these additives, as well as showed the effect of suspended additives and electrolytes on rheological properties of enamel slips. The mechanism of the interaction between the additives and particles in clayless enamel slips was also suggested. The colorant-transfer sheet for printing of ceramic [Japanese Patent JP 03023983 A2 910131] comprised: (a) transfer support having an ink layer containing 1:9 - 3:7 a mixture of glass frit and a pigment and a glass frit ink layer between the printing layer and the support and/or on the printing layer or (b) transfer support having water-soluble starch coating, which was successively laminated with a printing ink layer containing 1:9 - 3:7 a mixture of glass frit and a pigment and a covering layer. The sheet gave a clear printed image on a ceramic after firing showing peeling resistance. Thus, a support was coated with dextrin, gravure-coated with a compound comprising an acrylic resin, glass frit, and a solvent, coated with a compound comprising an acrylic resin, glass frit, a pigment, and a solvent, and overcoated with an acrylic cover to give the coloranttransfer sheet.
36 Characters or other patterns of hydrophobic pigments were formed on unfired ceramics with good clarity, according to Japanese Patent [JP 63132081 A2 880604] using transfer sheets printed with inks comprising water-soluble pastes (e.g., starch, CMC) and pigment (e.g., metal oxide) particles coated with water repellents (e.g., oils, fats, waxes). Jadeite-green pigments, Na1-xLixCrSi2O6 (x < 1), isomorph of natural jadeite [Chinese Patent CN 85108554 A 870617], were prepared by firing a mixture of SiO2, precursors of chromium oxide, Li2O, Na2O, and a carboncontaining reducing agent. The reducing agent was active carbon, charcoal, sawdust, starch, or rosin, and was added at 30-350 % the theoretical amount required for the reduction of the mixture. The firing was carried out at 800-1350 ° C, for more than 2 h, and the fired product was quenched. Thus, a mixture of Na2Cr2O7, SiO2 powder, and active carbon (125 % of theoretical amount) was dried at 100 ° C, for 12 h, ball-milled, fired at 1000 ° C, for 4 h, quenched, and ground to 320 mesh, to obtain a pigment containing 0.17 % Cr6+. This pigment showed satisfactory bright-green coloring effect in enamels, ceramics, colored glass, paints, plastics, etc.
7.3. Bricks, tiles Gribovskii and Rumyantsev [1975] reported preparation of acoustic tiles and plates from mineral wool impregnated with starch (binder) with the addition of surface-active substances. When a strip of desired thickness and shape was formed, electric current (110-120 V) was passed through it for 20-30 min. During heating, the starch turned into a gluing solution that bound individual grains or fibers, and fast evaporation of water took place. Kienow [1991] has obtained some flexible ceramic materials by mixing them with starch or long-chain cellulose and subsequent shaping by pressing. During firing starch became transformed to dextrin. On heating and cooling the individual grains expanded and contracted but the dimensions of the article remained unchanged. Choice of aggregate for refractory application depended on whether low or high thermal conductivity was required. Refractory masonry could be built without mortar and movement joints. If the masonry was intended for house building, the water-soluble dextrin should be replaced by amylopectin, and spinel was the best aggregate due to its low thermal conductivity. Mullite was mentioned as aggregate in dense ceramics for use in vehicles and aircraft. Subramanian et al. [1996] discussed the preparation of insulation bricks using the foaming process, via both mechanical and chemical means. The authors described five methods of producing lightweight insulating materials. For mechanical foaming, they investigated the optimum water content and the effect of stabilizers on the foaming reaction and green strength of the specimens. They studied also chemical foaming as they used phosphoric acid and aluminum powder with starch as a binder.
37 UK Patent [GB 2300632 A] described the brick making process, in which the clay was crushed and ground with water, the mix then being extruded, and the bricks dried and fired. The liquid syrup carbohydrate was added to the grinder or to the extruder. The syrup consisted of the partially hydrolyzed starch to a DE (dextrose equivalent) value of less than 50. Its application reduced considerably surface scumming in fired heavy clay articles.
7.4. Glass fibers [U.S. Patent 5690715] described the process for producing insulating materials with environmentally safe binding components, a long-chain starch and silicone. The starch was heated up to 50-60 o C, held at this temperature and sprayed on the glass fibers separately from the silicone; 6-8 % binder, consisting of starch and silicone, were used, then a spun-glass mat or slab might be shaped and dried at about 180 o C. A spun-glass mat, insulating mat or slab or adsorber was thus obtained which was water-proofed and held together exclusively by starch, resin and silicone, which might be used without any problems and had a uniform bulk density throughout. An emulsion was formed from silicone resin, silicone oil, a dust binder and the starch, which were atomized at 180-200 o C and sprayed onto the passing stream of glass fibers. Pach et al. [1996] studied the precipitation of calcium carbonate and calcium phosphates from solutions with and without additives such as hydroxyethyl cellulose, colloidal silica and potato starch, by XRD and SEM. A constant rate of 10-6 mol/s of reactant was added into the mixed solutions with and without additives. Nucleation frequency and morphology were dramatically altered by the addition of colloidal potato starch. It was found to be effective for non-specific nucleation of calcite and hydroxylapatite and specific nucleation of monetite. The nucleation frequency of calcite was increased by adding starch by about a factor of 10000. Starch also altered the shape of hydroxylapatite to fibrelike. Commercially available starch was used to coat the glass fibers, which then underwent attack at 80 o C by a simulated concrete pore solution at 60 days (150 and 450 mmol/L NaOH and KOH, resp.). Analysis of the final solution indicated that different coating materials give rise to different corrosion mechanisms [Grassi et al. 1985].
7.5. Glazing materials Chiavacci [1996] applied carboxymethyl cellulose (CMC), carboxymethyl starch (CMS), and the polyacrylates in the field of ceramic glazing materials. According to the author, starting with some of the properties of the polyelectrolytes, the mechanisms involved could be explained using simplified models (theory DLVO). This gave the ceramics operative indications on how to control rheology, water retention, binding effect and stability during glazing. A
38 number of developed in this way products with original characteristics were described. The microspherical glazes were described in European patent [EP 492280 A1 920701]. They consisted of particles having particle size 30-100 µm (optionally after removal of particles 100 µm), and contained up to 10 wt. % inorganic and/or organic binder, e.g., K silicate, colloidal SiO2, colloidal Al2O3, cellulose, CM cellulose, and/or starch. The granules were manufactured by milling the glaze to particle size 0.1 - 50 µm, mixing the particles with ≤10 wt.% of ≥1 of the above binders under addition of water, spray drying the mixture, optionally after preheating the mixture to 90 o C, to obtain the spherical particles, optionally after removing the particles 100 µm. The glazes were used for the manufacture of dry-glazed tiles. Coating the green tiles with an aqueous binder, e.g., CM cellulose or colloidal SiO2, evenly applying the granular material, and firing the green tiles manufactured the dry-glazed tiles. The glazed tiles had a smooth surface. Kerstan [German Patent DE 1471329 711104] proposed the manufacturing process of glazed ceramic products as floor tiles. In this process in order to avoid crazing due to differences in the expansion coefficients of substrate and glaze, to hydration swelling of the former, or to chemically reaction between the two, a white to gray interlayer was applied, 550 o C up to the temperature of beginning sintering, and the binders used were poly (vinyl alcohol) and (or) methylcellulose or water-soluble starch which evaporated at 250-550 o C. The quantities of binders and lubricants being added to the material to be sintered were 0.5-5 wt. %. Yamamoto [Japanese Patent JP 06187820 A2 940708] claimed the manufacture of oxide conductor ceramic flakes. The production involved spincoating an aqueous resin solution on a glass substrate, depositing an oxide film on the resin coating layer, dissolving the resin layer by soaking the substrate in a warm water (approximately 50 o C), filtering out oxide flakes, sintering the flakes for 90 min at 550 o C in air to give low-resistance flakes (thickness 1-5 µm, average diameter more than 50 µm and specific resistance less than 9x10-2 Ω.cm). The resin might be poly (vinyl alcohol), nitrocellulose, starch, glycogen,
43 inulin, or phthalocyanine. The flakes may be useful as an additive for Pb batteries as an active material in increasing their lifetime and energy density, for conductive pastes as a non-migrative oxide conductor, and for electromagnetic shielding materials. A reaction-limiting cathode for a nonaqueous alkali metal-oxyhalide battery [U.S. Patent US 4822700 A 890418] comprised carbon black, PTFE binder, and approximately 5-40 % ceramic material, which was unreactive with other battery components below a predetermined temperature T but was capable of reacting with the anode metal at .gtorsim.T to form substantially inactive products. The ceramic material was obtained by baking at 400 o C a ceramic paper containing Al2O3, SiO2, starch, neoprene rubber, and acrylic polymer. The cathodes containing an inert Al2O3-SiO2-based ceramic material did not affect the performance of lab and large engineering Li-SOCl2 batteries. However, the peak short-circuit currents of these batteries were decreased by nearly 35 % when cathodes containing the ceramic material were used. Potassium titanate and aramid fiber paper [Japanese Patent JP 63175200 A2 880719], useful as battery membranes and heat radiation plates, mainly comprised potassium titanate fibers and aramid fibers. Thus, potassium hexatitanate fibers, Kevlar 49 (aramid fibers), water, nitrile-butadiene rubber, polyacrylic emulsion, and carboxymethyl starch were mixed to give a slurry, which was formed into a sheet by paper making machine, and heat pressed at 6 kg/cm2 and 130 ° C for 5 min to give a paper having tensile strength 7.3 kg/15 mm, water absorption 3 mm, volume specific resistivity 5.9 × 1014 Ω-cm, vs. 1.3 kg/15 mm, 69 mm, and 1.5 × 1012 Ω-cm, respectively, for a paper prepared from Kaowool (Al2O3-SiO2 ceramic fiber) instead of aramid fibers. Ceramic materials [Japanese Patent JP 57082170 A2 820522] were mixed with dextrin or soluble starch and methylcellulose, or hydroxyethyl cellulose, poured into silicone rubber molds, and solidified. Thus, a mixture containing BaCO3, TiO2, PbO2, SiO2, Nb2O5, and MnO2 for manufacture of thermistors was mixed with dextrin, methylcellulose, and water, poured into a rubber mold, dried, and fired. The use of dextrin as a binder decreased during shrinkage.
10. FUEL CELLS Japanese Patent [JP 07187848 A2 950725] claimed the manufacture of porous multiple oxide bodies for fuel cell cathodes. The bodies were prepared by wet mixing presintered powder of a mixture containing La2O3, SrCO3, and Mn2O3 or MnO2 with 2.5-33 wt. % insoluble starch, molding, removing binder by heating at the temperature less than 400 o C, and sintering.
44
11. ABRASIVES Self-destructible solid abrasives useful for polishing and (or) finishing metals in a vibrating machine consisting of a core covered with a uniform thickness of binder and abrasive were described in French Patent [FR 2165294 730907]. As the covering wore away, the core was disintegrated by water or a lixivium, which was added at the end of the polishing operation. The core might be raw clay, wheat flour, plaster, starch, sawdust, or an inorganic salt. The abrasive might be corundum, SiO2, Al2O3, SiC, or ceramic. The binder was a thermoplastic resin, cement, glue, or other commercial adhesives. The binderabrasive mixture was applied to the core by mixing in rotating drums, by spraying, by immersing the cores held by an external support, or by rolling the cores on a cloth impregnated with the binder and abrasive.
12. BIOCERAMICS European Patent [EP 61108 A1 820929] described the use of calcium phosphate for bone implants. The latter should be impregnated with a broadspectrum microbiocide (Ag salt and/or an antibiotic), and coated with a tissuecompatible slowly degrading material. The coating might be a fat, polyglycolic acid, polyglycolic acid-lactic acid copolymer, alkylated cellulose derivative or starch. The granules of calcium phosphate were heated at 1180 o C for 6 h, cooled over 24 h, sprayed with PVP-I in a fluidized bed, and dried with air at 50 o C . The antibiotic-impregnated granules were coated with poly (glycolic acid), dissolved in hexafluoroacetone or hexafluoroisopropyl alcohol by spraying with air at 80 - 90 o C. The granules might be used to treat osteoporosis and osteomyelitis, with the antibiotic controlling infection and the ceramic being absorbed. In the work of Reis et al. [1997] bioactive glasses and glass-ceramics in the SiO2-3CaO.P2O5.MgO system were incorporated, in weight fractions up to 30 %, into two matrixes: ultra-high molecular weight polyethylene (UHMWPE) and biodegradable starch/ethylene-vinyl alcohol blends (SEVA-C). The composites were processed by compression and injection molding, after a previous compounding operation. The reinforcements were characterized by DTA, XRD, laser granulometry, and SEM/EDS. Two granulometric classes, below 30 µm and between 30 to 50 µm, were used. The mechanical properties of the molded composites were evaluated in tensile tests. The analysis of the resp. surfaces (by SEM/EDS and thin-film XRD) after different immersion periods in SBF assessed their bioactivity. The evolution of the solution pH and Ca, P, Si and Mg concentration was followed vs. time by ICP. Due to its water uptake properties, which enhance the accessibility of the solution to the inner particles of the bioactive fillers, the SEVA-C based composites exhibited a higher tendency to form a Ca-P film on the surface. It was possible to develop composites that
45 exhibited a bioactive behavior associated to a mechanical performance adequate to bone replacement applications. Ribeiro et al. [1998] described two innovative processing routes for producing bioactive porous ceramics. One of them was based on the use of a typical polyurethane (PU) foaming reaction to build up the original porous structure. The PU was then burn out and the porous ceramic structure was obtained by sintering. The second route was based on a microwave baking process using a powder, containing corn starch, sodium carbonate and sodium pyrophosphate, that were added to the bioactive ceramic. The green bodies were then sintered. By using the developed routes it was possible to produce hydroxylapatite or β-tri-calcium phosphate porous materials, combining and adequate micro and macroporosity with a mechanical performance matching the compressive behavior of human cancellous bone. The developed porous bioactive ceramics presented a combination of mechanical properties and morphological features that might be very useful in bone replacement and drug delivery applications, or as tissue engineering scaffolds. The method of consolidation called "starch consolidation" based on the gelling ability of starch in water, belongs to the group of direct-consolidation methods, and allows good homogeneity of the consolidated material. In the work of Rodriguez-Lorenzo et al. [1998], porous hydroxylapatite materials were manufactured using starch, both as consolidator/binder and as pore-forming agent. Slurries having solids content 50 wt. % were prepared and molded using plastic molds. After drying, burning out the starch and sintering, materials having porosity 45-69 % were obtained with the overall pore structure dominated by large (approximately 80 µm) spherical shaped pores left by the starch particles and smaller pores due to the contact areas of larger pores. Flexural strength of the materials was measured in 4-point bending tests. The strength of the materials decreased linearly with increasing total porosity. Biodegradable thermoplastic blends of corn starch with ethylene vinyl alcohol copolymers (40/60 mol/mol) were reinforced with bone-like ceramics (hydroxylapatite) in amounts up to 30 % by weight [Reis et al. 1997]. The materials were compounded either in a rotating drum (RD), or in a co-rotating twin-screw extruder (TSE). Then tensile samples were processed by conventional injection molding or using a shear-controlled orientation in injection molding (Scorim) technique. By using the later technique it was possible to induce anisotropy (copying bone structure) into the moldings and, for hydroxylapatite amounts up to 20 %, to obtain simultaneously higher values of stiffness and ductility (results of tensile tests). The goal of matching the minimum stiffness of cortical bone (around 7 GPa) was accomplished for 30 % hydroxylapatite composites processed by TSE + Scorim. Furthermore, the composites degradation was studied in simulated physiological solutions. It was found that an increase in the hydroxylapatite concentration led to a faster degradation rate, indicating that both the matrix and the reinforcement are being degraded by body fluids. The biologically active glass ceramic artificial bone claimed in Chinese Patent [CN 1087279 A 940601] comprised glass micro powders and shape and
46 pore-forming agent e.g. gelatin, methylcellulose, gum arabic, polyethylene glycol, or PVA in aqueous solution (100:0.5-1 wt.) and was sintered at 0.5-1 L/min of oxygen flow, followed by adsorption on polyurethane or polyethylene foam frame. The glass micro powder could be also mixed with paraffin wax as the shape and pore-forming agent (100:10-20 wt.), and with 2-5 wt.% of blowing agent (starch, dextrin, urea, etc). The glass ceramic artificial bone contained apatite and wollastonite crystal phase with 20-70 vol. % porosity and pore diameter of 101000 µm and had good biocompatibility. Reis et al. [1997] used bioactive glass as precursor for calcium phosphate (Ca-P) film deposition onto several polymer-based materials. Both bioinert (high molecular weight polyethylene), and biodegradable (corn starch-based blends, SEVA-C) polymers, unreinforced or reinforced with hydroxylapatite, were coated by the very simple proposed route. Also polyurethane foams, with an open-cell structure, were mineralized by the proposed method. In fact, it was possible to induce the growth of the Ca-P films not only at the surface, but also in the bulk of the polyurethane foam. These cellular materials were intended for cancerous bone replacement applications. The morphology of the formed films was strongly dependent on the used substrate, its polar character, and on the presence of hydroxylapatite in its composition, as observed by SEM. Nevertheless, a welldefined needle-like structure was observed in all samples at high magnifications. The Ca : P ratios of the films were between 1.5 and 1.7, i.e. in the range of tricalcium phosphate-hydroxylapatite. Raman spectroscopy and thin-film XRD evidenced the formation of mostly amorphous calcium phosphate films. After scraping the coating from the polymer surface and heat-treating the resulting powder at 1000 o C for 1 h, hydroxylapatite and beta-tricalcium phosphate typical peaks were found on XRD patterns.
13. RHEOLOGICAL AND SURFACE CHEMICAL STUDIES The rheological behaviour of concentrated mineral suspensions can be controlled by understanding and regulating the net inter-particle force. Manipulation of the surface chemistry via adjustments of the solution pH and electrolyte concentration can be used to control the electrical double layer repulsion, and produce either dispersed conditions, where a negligible yield stress is measured, or strongly attractive particle–particle interactions. Under the latter conditions, the suspension can possess a substantial mechanical strength. Electrolyte additions can also lead to an ion-specific short-range structural ‘hydration’ repulsion due to the presence of surface-adsorbed ionic species. Alternatively, a steric repulsion can be introduced by utilising a high concentration of a surface-associated polymer or polyelectrolyte. For cases in which a suspension with a high yield stress is desired, additional inter-particle attractions can be introduced by adding appropriate surfactants hydrophobic forces, high molecular weight adsorbing polymers polyelectrolytes bridging forces or non-adsorbing polymers polyelectrolytes depletion forces to the suspension.
47 Through appropriate manipulation of electrical double layer, structural, hydrophobic, steric, bridging and depletion forces, therefore, the rheological characteristics of mineral suspensions can be tailored to suit specific applications, e.g., dispersed materials for efficient pumping compression or aggregated conditions for rapid sedimentation clarification [Johnson et al. 2000]. Relatively long-chain starches added to aqueous suspensions in mineral processing operations can evoke bridging forces, when a polymer adsorbed on one particle also becomes attached to another one or more surfaces. Bridging interactions are most likely to occur when there is a low degree of surface coverage by adsorbing additives, such that suitable adsorption sites are readily accessible on the interacting surfaces [Johnson et al. 2000]. Orumwense et al. [1990] applied oxidized cassava starch (either hypochlorite or peroxide oxidized, charged and uncharged) for flocculation of kaolinitic clay suspensions and obtained well-flocculated systems at pH above 7. The authors revealed also that charged starch showed a larger flocculability than uncharged starch. The best results were yielded at 10 mg/l and more of applied starch. Percent solids between 2 and 4 were ideally suited for the process. Orumwense et al. [1992] studied also the effect of metal cations on the flocculation behaviour of kaolin. The effects of pH, concentration of magnesium and aluminum cations (Mg2+ and Al3+), and their presence in causticized cassava starch on the settling rate and percent reduction in sediment density was studied as well. Results showed that the presence of high concentrations of divalent (Mg2+) and trivalent (Al3+) ions, and the presence of these cations in causticized cassava starch significantly contributed to the existence of low settling rates at high supernatant clarity often observed in some clay slurry treatment systems by flocculation and sedimentation method. It was also established that for efficient flocculation, the systems had to be within the alkaline pH range (pH greater than 10). The further study revealed that in some flocculation systems, fast settling corresponded to high sediment volume whereas in others, the opposite was applied. Frisch et al. [1981] studied the effects of intermediate layers on the compaction of fine ceramic batches. The effect of soluble additives on the pressed density, ejection pressure, green density and elastic rebound of a spraydried clay-feldspar-grog composition was investigated. The additives used were: tetraethylene pentamine, ethanolamine, di-isopropanolamine, sugar, starch, cellulose, dimethylsulphoxide and polyvinylalcohol. They were added either to the slip before feeding it into a laboratory spray drier, or they were mixed with the spray-dried granules prepared on the technical scale. Depending on the nature of the additive four different compaction mechanisms were identified. Ouchiyama et al. [1987] applied starch in the mixtures with water, bentonite clay and α-alumina for production and investigation of porous cylinders of 3 x 3 mm dimensions prepared by extrusion and sintering. The abrasion of these cylinders in a rotating cell was measured. The possible relationship of this abrasion to fracture toughness (KIC) was studied. For this purpose, larger cylindrical test-pieces of the same material were prepared and provided with a chevron notch. Such test-pieces were suitable for determining the KIC of the
48 present porous materials by three-point bending. However, better correlation was found between abrasion and tensile strength, as determined by the indirect (Brazilian) method, probably because flow size was an important variable in addition to KIC. Subramanian and Natarajan [1988] performed zeta potential studies on hematite, corundum, and quartz samples using starches in order to understand the adsorption behavior of polymeric starch flocculants at the oxide mineralsolution interface and to correlate this information with their flocculation characteristics. The authors investigated effects of pH and calcium chloride, an organic electrolyte, on the zeta potential of iron ore minerals. The zeta potential of quartz was virtually unaffected by oxidized starch while that of hematite and corundum could be decreased to less electro-negative values by the starch addition. Cationic starch brought about the reversal of the sign of the zeta potential in all the three minerals. The addition of the calcium chloride imparted bulk stress, compressing the double layer further. Bates and Bridgwater [1995] successfully described ceramic paste rheology using the Benbow-Bridgwater equation. This expressed paste flow through holes and ducts and predicted pressure drops using well-defined rheological and geometric parameters. The approach was used for more complex flow and was adopted to describe the components of the flow of paste in the various parts of different shaped moulds. This approach used a physically based model, which applied over the wide velocity range found when a mould was filled. The long term purpose of this work together with work on binder composition and powder choice was to eventually enable a physically based computerized mould filling program to be written similar to those already available for plastic injection molding. Blackburn and Bohm [1997] characterized three ceramic pastes based on alumina with different binder systems using physically based equations and the pressure drop measured as they passed through an experimental honeycomb die. Using an additive approach the pressure drop through the honeycomb die was predicted from the paste parameters derived by the characterization method and the results compared with the experimentally determined pressure losses. In simple pastes based on a clay-starch binder system, where their pressurevelocity relationships were near-linear and the die entry velocity dependence was small, the predicted and experimental values were in reasonable agreement (plus or minus 10 %) but in more complex systems using polymer solution binders the fit was less accurate (-10 to – 45 %) for the best of the four models evaluated. Environmental SEM (ESEM) and Magnetic Resonance Imaging (MRI) were shown by Spitteler et al. [1998] to be powerful techniques for the analysis of water movement in pastes. A ceramic paste was studied containing alumina, bentonite and starch, which provided a representative model system for pastes used in the manufacture of catalysts. ESEM was shown to be able to image the surface of the paste under conditions of saturated vapor pressure and during subsequent flooding and drying of the sample. The magnetic resonance studies showed that the water in the paste was found to exist in two distinctly different
49 chemical environments, characterized by spin-lattice relaxation times (T1) of 0.2 s and 0.012 s. It was suggested that the faster T1 characterized the interaction of water with alumina and/or bentonite, while the slower relaxation time was associated with water in a starch-rich environment. The water/starch interaction disappeared following firing of the paste. Kim et al. [2000] reported rheological behaviors of alumina slurries with polysaccharides of different concentrations and molecular weights. The polysaccharide molecule adsorbed to the surface of alumina powder in an aqueous slurry. Rheological measurements were used to compare the strength of alumina particulate networks. The network strength of alumina slurry became weaker with an addition of polysaccharides of low molecular weight and stronger with the addition of polysaccharides of higher molecular weight. Tomasik et al. [2003] investigated several groups of organic compounds as potential dispersants for aqueous suspensions of micrometric aluminum oxide powder. They were compounds dissociating into bulky cations and small anions (tetralkylammonium chlorides and hydroxides, Methylene Blue), into bulky anions and small cations (phenols, mono- and poly-carboxylic acids, and fluorescein sodium salt), organic acids, and bases (tetralkylammonium hydroxides), and bulky, non-dissociating, polar compounds (Malachite Green and maltodextrin). These compounds were selected in order to contribute to the knowledge of how electrostatic interactions, the size and structure of the additive, and surface sorption onto alumina influenced the rheological properties of colloidal alumina suspensions. The study was focused on low molecular weight additives, of different shape to influence interparticle electrostatic interactions as the most essential in reducing the shear stress at a given strain rate. The size and structure effects of these low molecular weight additives might be overshadowed by interparticle electrostatic interactions. In the case of non-dissociating macromolecules (e.g., maltodextrin), electrostatic interparticle interactions were minimal, and sorption was a principal factor influencing rheology. Kim et al [2000] studied the rheology of alumina slurries with maltodextrin using experimental design and statistical analysis. Different levels of ultrasonication were applied to the slurries before or after adding maltodextrin. Viscosities of the slurries were measured after equilibration of the samples in a shaker. The viscosities of alumina slurries decreased with an addition of maltodextrin and increased with ultrasonication intensity. There were minor differences in the viscosities of the slurries depending on whether maltodextrin was added before or after ultrasonication.
14. MISCELLANEOUS Yoneda et al. [1980] studied the effect of binder materials (PVA and starch) on the formation of duplex structure in sintered Mn0.5Zn0.5Fe2O4. Duplex structure occurred when some grains in localized regions of the sintering material rapidly grew to become large. In this situation large grains coexisted with many
50 small grains. This grain structure adversely affected the magnetic properties of oxide magnetic materials. The formation of duplex structure was caused by various factors, such as the history of the raw materials, presintering temperature, sintering atmosphere etc. It was concluded that the excess amount of binder material (PVA or starch) caused the formation of duplex structure in Mn0.5Zn0.5Fe2O4. The UK Patent Application [GB 2281561 A] described an unsintered ceramic artifact, which comprised an alumina component, selected from partially hydrolyzed beta-alumina, partially hydrolyzed beta-alumina, and mixtures thereof, at least one water-soluble binder, and at least one water-soluble plasticizer. The artifact, e.g. a green ceramic tape, might be made from a ceramic slip comprising beta-alumina, beta-alumina, water-soluble binder, e.g. starch or cellulose ether, water-soluble plasticizer, e.g., glycerol or sorbitol, and water. Release agents for forming green ceramic sheets or tapes on a substrate comprised a first component, which was paraffin and/or a glycerol ester, and a second component, which was a fatty acid or a salt of a fatty acid. Siewert [1988] took an advantage of starch water-retaining properties. Admixing starch to hydrophobic plaster-of-Paris the author obtained the increase of compressive and flexural strength by 20 %. The workability period of such plaster mixture could be extended approximately to 140 %. Nasman and Wimmerstedt [1993] prepared an experimental setup to study the drying of gypsum plasterboards and measure sorption equilibrium. The quality of the cardboard and the effect of chlorides and starch were also studied. Cardboard quality was important; a more intensive drying required a less dense cardboard. The gypsum core had little effect on drying rate. Chloride content of the raw plaster of Paris had not to exceed 75 ppm. The starch migrated towards the surface of the gypsum core. It was important to use a starch sort, which could migrate into the core/cardboard interface to ensure a good bond. UK Patent Application [GB 2 198 125 A] described an investment for dental casting comprising a mixture of at least one refractory selected from alumina, zirconia, magnesia clinker, quartz, cristobalite and fused quartz with either a mixture of a soluble phosphate with magnesium oxide or hemi-hydrate gypsum, which acted as a binder. The investment further contained starch and at least one of selected from the group consisting of carbides, nitrides, borides, silicides and sulphides of transition metals of Groups IV, V and VI, which were added as expanding agents to the mixture. The investment might still further contain soluble starch. U.K. Patent [1 588 636] presented a refractory nozzle fitted to the outlet bore of a ladle for molten metals by a double layer of mortar and release agent, so that it may be readily removed and replaced when necessary. The release agent comprises powdered graphite, starch or other water-soluble polysaccharide and water. Rafaniello and Cutler [1981] described sinterable cubic aluminium oxynitride (AlON) preparation by carbon reduction of aluminium oxide in flowing nitrogen. Three different sources of alumina (alumina from clay, commercial alumina and alumina derived from AlCl3.6H2O) and two different sources of
51 carbon (carbon black and starch) were used. Pressed pellets of AlON powder were sintered in nitrogen atmosphere at 1950 o C to greater than 95 % of theoretical density. UK Patent [1559917] described the process for preparing shaped articles from rehydratable alumina, in which shaping means was water-immiscible phase (e.g. hot oil) into which the aqueous slurry of rehydratable alumina was dropped to form spheres, or heated tubes through which the alumina was extruded. The slurry contained combustible filler such as starch, cellulose, sawdust or carbon black. Covering films for decalcomania, which did not exhibit deformation during heating and slowly depolymerized on baking without damaging the surface of the ceramic article were described in Polish Patent [PL 125295 B2 830430]. The latter comprised ethers or esters of starch (particle size less than 50 µm) and methacrylate- or polystyrene-based coatings. Thus, corn starch in water was esterified with acetic anhydride at pH 10.5, and 45 ° C, in the presence of 5 % Na2SO4. The obtained starch acetate was mixed with methacrylate-based coating. A safe and nontoxic lubricant for forming ceramic products was presented in Polish Patent [PL 87008 761130]. The lubricant was prepared from mineral oil or its mixture with vegetable oil, polyoxyethylated surfactants, starch, carboxymethyl-cellulose, poly (vinyl) alcohol, and H2O. Thus, a lubricant contained mineral oil, unmodified starch, hypochlorite-modified starch, poly (vinyl) alcohol, polyethoxylated alkylphenol, polyethoxylated amide, NaNO2, and H2O. According to UK Patent [GB 1313750 730418] high yields of uncracked urania and urania-ceria microspheres with improved sphericity were manufactured by adding compounds containing urania or urania-ceria salts, a cellulose or starch gelling agent (Wisprofloc W - cationic starch derivative), and NH4NO3, NH4OAc, or HCO-NH2 modifying agents dropwise to an aqueous alcoholic solution, filtering off the gel spheres, and calcining them. Azuma et al. [Japanese Patent JP 63170264 A2 880714] presented SiC articles coated with alkali metal compounds and carbon, fired at 800-1300 o C in the presence of oxygen to form an alkali metal-containing glass layer, and the glass layer was removed to give oxidation-resistant SiC material having a dense crystal phase at the surface. The materials had high strength and excellent resistance to heat and thermal impact. Thus, a sintered SiC article was coated with a methylcellulose solution of Li2CO3 and starch, dried, and fired for 24 h at 1100 o C to give a Li-containing glass coating. The coated article was immersed in H3PO4, heated 15 min at 280 o C, washed, and dried to give an article having β-SiC single-phase surface. Czech Patent [CS 226644 B 860215] described improved bonding of rock wool and ceramic fillers in building boards with SiO2 by introducing Ca, which reacted with the colloidal binder particles to replace Na, increasing flocculation of the fine mixture components, and decreasing shrinkage of the boards on firing due to improved binder distribution. Thus, rock wool, halloysite, bentonite, Ca(OH)2, SiO2 hydrosol, 50 % asphalt emulsion, potato starch jelly, and
52 flocculant Herkofloc were consecutively worked up to a 2 % aqueous suspension which was dewatered, molded, and dried at 150 o C. The boards had density of 410 kg/m3, thermal conductivity 0.059 W/m.K, bending strength 1.5 MPa, and flammability in Group A of the Czech standard. Fundamental studies were performed by Goel et al. [1996] with aqueous suspensions of colloidal α-Al2O3 to evaluate the role of sucrose, maltodextrin, and oxalic acid on viscosity, sedimentation and filtration characteristics, plastic flow behavior of filter cakes, and sinterability. Maltodextrin and oxalic acid systems exhibited superior results, including filtration to high packing-densities and clay-like plasticity with minimal cracking. The German Patent [DE 3013261 A1 811008A] described dust free, granulated Pb additive for use in the glass, ceramic, and battery industries, which possessed no pollution problems, and was produced by adding an aqueous solution of macromolecular organic compounds, together with stabilizing chemicals and additives, to PbO-containing powders and granulating the mixture in a conventional manner. These materials included polysaccharides: alkyl celluloses, carboxycellulose, galactose, mannose, starch, poly (vinyl) alcohol, polyacrylates as 0.3-0.5 % aqueous solutions and acids, alkalis, and/or carbonates, silicates, metal oxides, and nitrates as stabilizers. Thus, PbO was placed in a pan granulator, and was mixed with 3 % solution of methylcellulose, of average viscosity. After mixing for 10 min, a dust-free, soft free-flowing, granulated material was obtained and remained unchanged upon storage. Coarse and fine granules were removed by screening. Dust contents were 2 mg/100 g material, as compared with the raw material, which contained 120 mg/100 g, respectively. Vandermeer described the process [International Patent WO 9915322 A1 990401] of forming an aqueous slurry containing ceramic fiber, cationic starch and silica sol, and passing the slurry through a porous screen under a vacuum pressure to deposit the solids content onto the screen to produce a shaped product. The slurry contained a solid content of 0.5-3, ceramic fiber 0.5-2.0, SiO2 0.01-0.7, cationic starch 0.005-0.2 wt.% (as to total weight of the slurry), and balance of water, where the SiO2 sol had 50 % SiO2 having size 7-200 nm, specific surface area 10-100 m2/g. Porous fibril-based preforms [European Patent EP 887325 A1 981230] suitable for injection with molten metal or alloy were manufactured from a pasty mixture with hydrosorbent binder by: (a) injection molding into a chilled die cavity at the pressure higher than 2000 psi, and freezing to obtain a preform; (b) ejecting the frozen preform from the die, and holding under vacuum for freeze drying the aqueous phase; and (c) heating the preform to remove the hydrosorbent phase. The fibrils were selected from metals, C, or ceramics, and were used at 5 - 45 % by volume. The initial binder was a shear-resistant gel from hydrosorbent and water with an optional surfactant, typically corn starch, gelatin, or cellulose was used as the hydrosorbent. The binder was optionally modified by addition of colloidal SiO2 and Al2O3 for increased heat resistance. The process was suitable for preparation of stable porous preforms to be infiltrated with molten metal or alloy in manufacturing of metal-matrix composites.
53 The typical molding mixture was prepared from Kaowool mineral fiber, colloidal SiO2, and the balance as mixture of starch and water with a minor surfactant, and was suitable for injection molding of preforms hardened by freezing at –15 ° C followed by freeze drying at 50 ° C, and firing at 1100 ° C. The gasket, with good elasticity and useful for sealing bonded steel or stainless steel pipes, was prepared from fibrous materials (e.g., ceramic fibers, inorganic fibers, polymeric fibers), fillers (e.g., clays, silica), and binders (e.g., starch, latex) by forming circular shape with protuberances at the inner wall [Japanese Patent JP 97-90877 970409]. Water-permeating ceramic blocks were manufactured from mixtures of aggregates containing more than 75 wt. % steel making slag (max. size less than 9.5 mm) and sintering binders by molding and sintering preferably at 800-1250 o C. The binders were selected from feldspar, clay, natural stone and glass powder. The molding pastes were selected from CMC, methylcellulose, starch, water glass and cement [Japanese Patent JP 09030873 A2 970204]. The fired ceramic cores [International Patent WO 9615866 A1 960530] used in investment casting mold assemblies were manufactured from the core mix containing: (a) 20-50 % alkali-earth borate as the acid-soluble binder; (b) refractory filler as powdered Si3N4 or AlN, as the balance; and (c) optional CaCO3 or starch powders as the leaching promoters. The core binder was preferably Ca borate or Mg borate, and could be leached with AcOH, sulfamic acid, formic acid, and/or HNO3. The AlN-based core specimens were leached in aqueous 35 % HNO3 at 80 o C with stirring, and showed the binder loss in 14 min. Kanno [1992] described the preparation of the β-SiC film on a heated graphite substrate by a chemical vapor deposition using CH3SiCl3 and hydrogen as source gases at deposition temperature of 1523 o K, total gas pressure 70 torr, and gas flow rate 180 mL/min. The fabricated β-SiC film oriented direction due to the small molar ratio of Si/C (.tplbond.1). Because of the temperature gradient on the graphite substrate, free C deposited together with β-SiC in the both edge region of the substrate. A-doped β-SiC powders (an average particle size 0.29 µm) and standard β-SiC powders (an average particle size 0.31 µm) were mixed with the addition of 1 % starch paste in the presence of hot water. The resultant green rod was normally sintered at 2323 o K under argon flow. The sintered rod had bulk density of 2.14 - 2.17 g/cm3. The electrical properties and the surface temperature of the SiC ceramic were investigated in relation to use as a heater. Granules prepared from argillaceous slate or shale [Spanish Patent ES 2011989 A6 900216], preferably wastes from manufacture of slate or shale products, consisted of these materials expanded by release of gases produced by heating and had a glassy or siliceous outer surface, particle size 1-25 mm, density 0.4 - 1, and Mohs hardness 3 - 6. They were obtained by grinding and exfoliation of wastes to the particle size less than 3-20 mm, classification and selection of desired particle size, heating in a furnace to 1100-1200 o C for >1 h with injection of approximately 25 % air and/or oxygen, rapidly discharging and cooling the granules to 300 o C, and cooling slowly to ambient temperature. When ground to the size less than 1 mm, the powder was granulated with a
54 binder, e.g., starch or cellulose esters, the granules were heated for 90 oC to dissolve, except carrageenan, where full dissolution was achieved at
