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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.

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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
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;


Surfactants ii
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


Hlb values
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 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.



Uses of binders
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


Types of binders inorganic and organic
Types of Binders: inorganic and organic


Binder materials
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


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)


Vinyl binders
Vinyl Binders

Backbone: C-C


Cellulose binder
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


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


More binders
More Binders H, -CH

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
General Effects of Binders H, -CH

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: H, -CH

  • 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
Plasticizers H, -CH

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


Hydrolyzed PVA, its Tg vs plasticizer and adsorbed moisture H, -CH

Temp. effect on elastic modulus; fromJS Reed, 1995;


Common plasticizers
Common Plasticizers H, -CH

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
Foaming and Anti-foaming Agents H, -CH

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
Lubricants H, -CH

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.