Che5700 陶瓷粉末處理

1. 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

2. Surfactants Che5700 陶瓷粉末處理 • 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+, NH4+ ions; sulfonate, carboxylate, etc) Lyo- solvent; lyophobic, lyophilic;

3. Surfactants (II) • Sodium dodecyl sulfate (sodium laural sulfate) – ionic surfactant CH3-(CH2)11-O-SO3- Na+ (other examples: sulfonate, lignosulfonate, carboxylate, phosphates, etc.) • Cationic surfactant: dodecyltrimethylammonium chloride: [C12H25N(CH3)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

4. 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

5. HLB and CMC • HLB = Σhydrophilic group No. – Σliophilic group No. + 7 • assigning numbers to structural groups (i.e. chemical nature of surfactants) • e.g. –SO4Na (38.7); -COOK (21.1); -COONa (19.1); -COOH (2.1); -OH (1.9); -O- (1.3); • for liophilic groups: -CH-, or –CH2-, or –CH3, 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.

6. Taken from JS Reed, 1995; • Dispersed in water: HLB > 8; • in oil: HLB < 6 • Too much surfactant  form micelle (CMC)

7. 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

8. Types of Binders: inorganic and organic

9. Binder Materials Che5700 陶瓷粉末處理 • 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

10. 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)

11. Vinyl Binders Backbone: C-C

12. Cellulose Binder • Modified by R1, R2 and R3; • Methyl cellulose: R = CH3 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

13. Hydroxyethyl cellulose: HEC, ethylene oxide substitute some H, -CH2-CH2O-; Molar substitution MS = average number of substitution by ethylene oxide

14. 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

15. 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

17. 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

18. 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

19. Hydrolyzed PVA, its Tg vs plasticizer and adsorbed moisture Temp. effect on elastic modulus; fromJS Reed, 1995;

20. 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)

21. 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;

22. 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.