Presentation on theme: "Che5700 陶瓷粉末處理 Processing Aids May include the followings: Liquid/solvent Surfactant (wetting agent) Deflocculant Coagulant Binder/flocculant."— Presentation transcript:
Che5700 陶瓷粉末處理 Processing Aids May include the followings: Liquid/solvent Surfactant (wetting agent) Deflocculant Coagulant Binder/flocculant Plasticizer Foaming agent Antifoam Lubricant Bactericide/fungicide
Surfactants Che5700 陶瓷粉末處理 Lyo- solvent; lyophobic, lyophilic; Mostly having both polar and non-polar end-groups (hydrophobic and hydrophilic groups) Classified as: non-ionic surfactants; cationic surfactants (mostly with Cl- ) & anionic surfactants (e.g. Na +, NH 4 + ions; sulfonate, carboxylate, etc)
Surfactants (II) Sodium dodecyl sulfate (sodium laural sulfate) – ionic surfactant CH 3 -(CH 2 ) 11 -O-SO 3 - Na + (other examples: sulfonate, lignosulfonate, carboxylate, phosphates, etc.) Cationic surfactant: dodecyltrimethylammonium chloride: [C 12 H 25 N(CH 3 ) 3 ] + Cl - (commonly toxic materials) Surfactants can reduce surface tension: improve wetting property of liquid on solid Polyelectrolytes: polymer molecules with repeating ionizable groups (usually through electrical attraction onto solid surface) – sometimes also classified as surfactants
HLB Values Che5700 陶瓷粉末處理 In oil-water system, the surfactant is called emulsifier; Whether oil-in-water or water-in-oil: which one is the continuous phase, which one the dispersed phase? (relative quantity & …) Bancroft rule: the continuous phase is the one in which the emulsifier is more stable; (both oil drops and water drops are formed and adsorb surfactants; if if oil-oil coalescence is faster, oil will be the continuous phase;) HLB: an index to rate the relative strengths of hydrophobic and hydrophilic groups in a surfactant; HLB = 20 ( 1 – S/A) [scale of 0 – strong hydrophobic group; to 20 – strong hydrophilic group] (empirical) S: saponification number; A = acid umber of separated acid
HLB and CMC HLB = Σhydrophilic group No. – Σliophilic group No. + 7 assigning numbers to structural groups (i.e. chemical nature of surfactants) e.g. –SO 4 Na (38.7); -COOK (21.1); -COONa (19.1); -COOH (2.1); -OH (1.9); -O- (1.3); for liophilic groups: -CH-, or –CH 2 -, or –CH 3, or =CH- all 0.475 properties related to HLB: heat of hydration, micellization, dielectric constant, etc. log (CMC) = a + b log (HLB) critical micelle concentration: minimum concentration of surfactant molecules to form micelles (form aggregates) taken from book by Vold and Vold, 1983.
Taken from JS Reed, 1995; Dispersed in water: HLB > 8; in oil: HLB < 6 Too much surfactant form micelle (CMC)
Uses of Binders Che5700 陶瓷粉末處理 Quite a few uses are often found for a binder: Wetting agent (may improve wetting of particle) Thickener (increase apparent viscosity) Suspension aid: reduce settling velocity Rheological aid:control flow of a slurry Body plasticizer: help with pressing, extrusion; Liquid retention agent: reduce liquid migration (evaporation) in matrix; Consistency aid: alter amount of liquid required to produce a particular type of flow; Binder: improve strength of green body
Binder Materials Organic binders: be aware of different molecular weight effect; MBI index: adsorption of methyl blue dye/particle surface area; often used for inorganic binder PVA: polyvinyl alcohol; common binder material; partially hydrolyzed or fully hydrolyzed Che5700 陶瓷粉末處理
Binder MW: significant effect on viscosity; some may use viscosity data to determine MW; Gelation: some binder -solvent system,, when change in temperature, become poor solvency and gel; mostly reversible process; may use chemical methods to gel (e.g. pH change to PZC, reversibility poor here)
Cellulose Binder Modified by R1, R2 and R3; Methyl cellulose: R = CH 3 for all Rs Degree of substitution DS = average number of OH on anhydroglucose unit that have been reacted (0- 3 range); molar substi- tution; DS = 1.6 – 2.0 provides water solubility of methyl cellulose binder
Hydroxyethyl cellulose: HEC, ethylene oxide substitute some H, -CH 2 -CH 2 O-; Molar substitution MS = average number of substitution by ethylene oxide
More Binders Che5700 陶瓷粉末處理 polysaccarides: refined starch, easily dissolved in water; Some cellulose contain soluble function groups ionic type binder; e.g. Na carboxymethyl cellulose (Na-CMC) PEG: polyethylene glycol, HO-(CH2-CH2-O-)n-H, large variation of MW, several hundred – 20,000 g/mole Wax: paraffin compounds; microcrystalline wax – are saturated hydrocarbons, less crystalline, stronger but tougher than paraffins; In general: binder need to dissolve in solvent, then add ceramic powder, continue processing...problem of solvent selection
General Effects of Binders Che5700 陶瓷粉末處理 In general: small amount of non-ionic binder stabilization; increase concentration bridging flocculation; further increase may stabilize system again; For ionic binders, pH becomes very important; influence charge on both particle and binder, and hence adsorption behavior; Cellulose binders are les flexible than vinyls, waxes, and glycols; elastic effect Binders adsorbed may reduce surface roughness and coefficient of friction very specific to binder molecules
Different behaviors of polymers: Linear polymers: its chain can twist and coil e.g. HDPE; branched polymers: not so free to twist and coil LDPE; Cross-linked polymers: usually rigid structure, having a yield strength
Plasticizers Che5700 陶瓷粉末處理 For a binder system: glassy state (brittle, movement of molecule limited;) rubbery state (segment of molecules is able to flow and realign); elastic behavior to viscoelastic behavior ( increase of temperature) Glass transition temperature Tg Polymer films exhibit changes in resistance to mechanical deformation, thermal expansion, and specific heat at Tg Plasticizer: small molecules to reduce van der Waals forces between polymers, to cause polymers to pack less densely, to increase flexibility; decrease of Tg Adsorbed water: can function as a plasticizer
Temp. effect on elastic modulus; fromJS Reed, 1995; Hydrolyzed PVA, its Tg vs plasticizer and adsorbed moisture
Common Plasticizers Che5700 陶瓷粉末處理 Ethylene glycol: cheap, often used; effect related to MW; Stearic and oleic acid are plasticizers for waxes; oils and wax are used for thermoplastic polymers (PE, PS)
Foaming and Anti-foaming Agents Che5700 陶瓷粉末處理 Foaming agent: reduce surface tension of the foaming solution; used in fabricate light weight concrete and in beneficiation of some minerals (flotation) Anti-foam agent: defoaming agent; examples include – fluorocarbon, dimethylsilicones, higher-molecular weight alcohols and glycols, Ca and Al stearate;
Lubricants Che5700 陶瓷粉末處理 Reduce resistance to relative movement (sliding); to get objects out of a mold Boundary lubricant: adsorbed film of high lubricity; high adhesion strength, low shear strength Solid lubricants: fine particles with a laminar structure and smooth surface; plate-like particles: graphite, talc, graphitic BN etc.
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