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Binders1. Compaction behaviour of organic binders in alumina ceramics(PVA & PEG) General facts Mean grain size of raw material alumina is 1.3m The mean grain size of the spray dried agglomerates are typically between 100-200m Stages of compaction involves rearrangement of granules, then elimination of inter-granular porosity and third stage is reduction of intra-granular porosity through rearrangement of elementary ceramic particles They exhibit plastic behaviour over a significant range of deformation Property Tg
PVA 79C 31000 Hoescht 4-88 D Straight chain -OH on chain
PEG -60C 20000 PEG20M(union carbide) Straight chain di-ol
Mol Wt. Manufacturer/Product ID Chain
The Experiment Aqueous suspensions of 60 wt% alumina with 3 wt% binder was taken Particle size 200 m approx Residual moisture after spray drying 0.6wt% 3 samples: PEG (powder 1), PVA (2) & dried PVA (3) compacted @4mm/min Green samples analysed with the help of Hg porosimetry
The moisture content of 0.6wt% corresponds to 20wt% moisture if only absorbed in the organic phase. Tg of water plasticized-PVA should thus be located close to room temperature.
ResultsPEG No intergranular porosity PVA Marginal porosity Dried PVA Higher porosity
Densification due to intergranular & intragranular compression analysed for powder 2 Penetration of Hg through pore sizes greater than 1.3m is intergranular pore else intragranular Densification is enhanced if Tg is below ambient temperatures Intragranular porosity drops more sharply for powder3 than powder2 upon applying pressures of 10-100MPa Intergranular porosity drops more sharply for powder2 when compared to powder3
Powder1 shows the best results
2. US patent Ethylene acrylic acid co-polymer General FactsUsed as binder for ceramics particularly alumina ceramics Organic binders tend to require extended times during the 'burnoff' stage Large % of organics tend to produce cracks or blemishes Higher boiling point organics require a longer sintering step & length of the polymer hinders escape which eventually results in cracks This composition exhibits excellent wettability characteristics Co polymer of ethylene and alpha-beta unsaturated carboxylic acid The high concentration of oxides of many polymers are polar and attracted to the carboxylic groups in the co-polymer, increasing wettability. DEAE (diethyl amine ethanol) acts as a surfactant and good dispesing agent for thw copolymer Polyethylene ionomer containing Mg maybe used as a binding agent in injection moulding machinery. It has a low melting point of 90C. Methyl cellulose is another good ceramic binder and is marketed as Methocel 20-231. Hydroxyethyl cellulose may be added about 6wt% organic binder along with the new invention. It has a trade name HEC15000H. A gelation agent(3-25%) may be used to improve preform retention. It may be PEG, methyl cellulose etc. An important aspect of the invention is that a major portion of the aqueous organic portion consists of water. When the binder is driven off from the preform , the water vaporizes first, increasing porosity thus helping the homogeneous burnoff of the binder.
Commonly used configurations of today:
3. Carboxymethyl Cellulose General Properties Tg = 135C (dry state) .This drops to -85C on adding 0.2 to 0.5(g/g) water. Cellulose ethers are binders designed to improve water retention. Mol. Wt. Is of the order of 17000 Different grades of CMC are available in the market with varying viscosity, degree of substitution, pharma/food grade and coarse/fine particle size Online detailed product specifications requires special request Viscosity of CMC is found to drop significantly upon addition of certain cations like Na+, Ca++ and H+ The viscosity is found to drop linearly with temperature(C) It is pseudoplastic whose viscosity drops with shear rate
Merits Ensures uniform, controlled drying Prevents thermogelling cellulose ethers from migrating to the surface during drying Prevents formation of cracks or blisters The gelation temperature, the temperature above which the viscosity starts increasing can be adjusted by adding suitable additives for best results in the case of methyl celluloses. No references for gelation occuring in CMCs.
4.Polyurethane binderIn 1995, a polyacrylic binder, Duramax B-1031, (Rohm & Haas) was developed. The binder provided sufficient green strength to allow shaping of unfired ceramic parts from alumina & SiC by drilling, lathing and grinding. Today, Dow chemicals has Duramax B-1022 with the same applications. Polyurethane emulsions are often obtained without low-molecular emulsifiers and therefore the burnout residue is low Polyurethane chains could adsorb easily on the surface of ceramic powders due to the presence of hydrophobic groups, those could be coordination centres, creating strong interparticle interactions. Hydrogen bonding bridges between the polymer molecules and the ceramic particles further increases green strength
Experimentation Alumina powder of size 0.7m Aqueous polyurethene dispersion 35wt% solid particles [polycarbonatediol (cyclohexane dimethanediol dimethyl propane derivative & cycloaliphatoc isocyanates)] Quantity of binder added was 1-5wt% in steps of 1wt% Mean size of spray dried granules was 200m Pressed at 150MPa Moisture in binder granules was 2.7 -2.9 wt% (sample1) & this was dried for 4hrs @ 80C to get 0.5-0.7 wt% moisture (sample2) Properties measured: 1. Tensile strength (using diametral compression method called 'Brazilian test')
2. Flexural strength (using 3 point bending method)
3. Glass transition temperature (differential scanning calorimetry) 4. Polymer Ceramic interactions are studied using IR spectroscopy 5. The thermal decomposition of polyurethane binder is studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA)
Results & Discussions: Tg of binder was 28C, several degrees higher than the pressing temperature it required plasticization for obtaining a good compaction. The moisture content of alumina provided this reduction in Tg Depending on binder concentration (15 wt.%), the green density varied between 59 and 57%of the theoretical density of corundum(3.98 g/cm3) The green density decreases with increasing of a polyurethane binder concentration & moisture helped increase green density
Variation of green strength with binder wt%
Tensile strength for the dried powder is greater and increases with binder wt% concentration
Because Tg of polyurethane binder is 28 C, it ought to be plasticized by water to achieve high green density, but then dried to yield high strength.
Binder Burnout Characteristics:Thermal decomposition takes place by two mechanisms depending upon the medium used is air(oxidation reaction) or nitrogen(depolymerization reaction). In air, the poly-urethane binder starts losing its weight at approximately 120 C. Upon reaching 200 C, the polymer loses about 10% of its weight. This pattern continues to approximately 470C where the weight loss reaches 90%. At 600C it reaches a plateau,leaving 0.4% residue. In nitrogen, the poly-urethane binder starts losing weight at approximately180C. Upon reaching 300C, the polymer has lost approximately 20wt%. At 475 C the weight loss reaches a plateau, leaving 0.1% residue.
Conclusions:1. The polyurethane binder provided high green strength of dry-pressed alumina compacts. Depending on binder concentration (15 wt.%)the tensile and exural strengths varied between1.67.0 and 2.012.7 MPa, respectively. 2. IR study revealed that the high green strength was controlled by hydrogen bonding between oxygen atoms of alumina oxide and urethane groups of binder phase. 3. The glass transition temperature of polymer influenced the binder behaviour in the ceramic processing. For decreasing Tg the binder had to be plasticized by water to achieve high green density, but then dried to achieve high green strength. 4. The polyurethane binder had a gradual burnout with a low char residue in both air and nitrogen.
5. US patent - Polyalkylene Carbonate
If the binder is in the form of an emulsion either with water or an organic like alcohol, then the ceramic binder must be dried to produce a free flowing powder prior to shaping the mixture into a green body The process is particularly suited for ceramic powders which should not be exposed to an oxidising atmosphere, because the binder composition of the present invention can be burned out in a non-oxidising atmosphere Polypropylene carbonate and polyethylene carbonate are preferred They decompose cleanly to produce cyclic carbonates, hence they dont require oxygen for removal. Ash content after burn out is 0.058wt% in air & 0.13wt% in nitrogen
Green density increase is shown below:
Density of the sintered product expressed as % of theoretical alumina density:
This binder decomposes completely even in inert atmospheres. Hence, the volume of gas produced during sintering is reduced thus reducing the possibility of cracks to form.