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+, NH4

+ ions; sulfonate, carboxylate, etc)

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

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. –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.

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

Types of Binders: inorganic and organic

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)

Vinyl Binders

Backbone: C-C

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 BindersChe5700 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

PlasticizersChe5700 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.