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

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


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 = 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.