Chapter 11 Binder and Bonds 1. Generals 1) Main roles of binders • improve the strength of the green tapes for easy handling and storage. • allow to form granules from hundreds of fine particles during the spray drying. 2) Additional roles • improve wetting. • delay sedimentation. • increase viscosity. 3) Requirements of binders for use in tape casting • compatibility with the system • ability to function as a stabilization aid • no interference either with solvent evaporation or with trapped air • easy burnout without leaving residues • effectiveness at low concentration • high molecular weight • low glass transition temperature (Tg) * Tg : the temperature at which the binder changes from a plastic, rubbery state to a brittle one.

2. Types of binders 1) colloidal clay minerals - fine kaolin, ball clay, bentonite - for the cases where alumina and silica are acceptable 2) molecular binders organic and inorganic binders

3. Clay binders - coagulated clay colloids are adsorbed and bridge the larger ceramic particles → a role of binder - clays containing the mineral montmorillonite are powerful binders → bentonite

4. Molecular binders Polymeric molecular that adsorb onto the particle surface forming organic bridges between them 1) Classification depends on the functionality of polymer molecules and most of the polymer binders used in ceramic processing are nonionic or slightly anioic.

2) Vinyl binders

Large one-dimensional molecules having the structure of H

H

C

C

H

R

  : mer n

n : deg ree of polymerization R : side group

i) polyvinyl alcohol (PVA, aqueous-based) - most common binder in ceramic processing - obtained by the hydrolysis of polyvinyl acetate (acetylene(ethylene) reacted with acetic acid) in the presence of a catalyst; acetate : CH3COO -

- water soluble due to hydrophilic –OH side group, but insoluble in many organic solvents - the ratio of vinyl alcohol to vinyl acetate is one of the major determinants of PVA properties. - the amount of vinyl alcohol functionality (expressed as mol% of vinyl alcohol units) is called the hydrolysis level. - partially hydrolyzed PVA: less than 20% acetate groups - fully hydrolyzed PVA: less than 2% acetate groups and dissolve in hot water

PVA Grades by Hydrolysis Grade

Hydrolysis (mol%)

Super

99.3+

Fully

98.0 – 98.9

Intermediate

95.0 – 97.0

Partially

87.0 – 89.0

Low

79.0 – 81.0

hydrophilic

Dissolution Procedures - Add the granules slowly using good agitation and cool water (85C for partially hydrolyzed grades >95C for fully and super hydrolyzed grades - The solution should be held at these temperatures for at least 30 min for stabilization.

ii) polyvinyl Butyral (PVB, non-aqueous-based) - most common binder in tape casting - dissolves in nonaqueous solvents - prepared by condensation reaction of an acid-catalyzed butyraldehyde (C4H8O) with PVA

- copolymer of PVB and PVA (~ 20%) due to uncompleted conversion of PVA and polyvinyl acetate (~ 2%) - thus, contains some residual acetate and alcohol groups These polar groups aid i) in the adsorption of the PVB to hydroxyl covered ceramic powders ii) adhesion of PVB iii) to lower viscosity even with a high molecular weight of PVB

3) Cellulose binders - soluble in water or nonpolar liquids - molecular weights : 50,000~500,000 - made up of a ring-type monomer unit having a modified α-glucose structure (C6H10O5)n

- the cellulose backbone is much less flexible than the vinyl backbone. - modification to the polymer occurs by changes in the side group-R. - the most often used in ceramic processing are methyl cellulose (MC), hydroxyethyl cellulose (HEC), and carboxymethyl cellulose (CMC). i) MC by treating with methyl chloride ii) HEC reacting with ethylene oxide

iii) CMC by reacting with chloroacetic acid (ClCH2-COOH) or sodium chloroacetate (ClCH2-COONa)

iv) Starch not soluble in cold water and break up with heat and agitation.

4) Polyethylene glycol (PEG) binders polymerized ethylene oxide

- molecular weights ranging 200~8,000 - much higher molecular weight of 20,000 g/mol is used as a binder in pressing and extrusion. - soluble in water and have limited solubility in a wide range of solvents. 5) Polyacrylic emulsion binders (Latexes) - aqueous dispersions of 0.05 – 0.5μm insoluble polymer particles. - typical composition is 100 parts polymer binders, 3 of emulsifier, 0.25 of electrolyte, and 100 of water.

5. Film-forming binders (waxes)

6. Dissolving and admixing binders i) binders with polar side groups are soluble in polor liquids. ex) PVA, Cellulose are powder-types ii) binders with nonpolor side groups are soluble in organic solvents. ex) PMMA, PVB, ethyl cellulose iii) PEGs are soluble in both water and organic solvents like alcohol and trichloroethylene.

7. Gelation Crosslinking of polymer molecules into a three-dimensional network with interspersed water as result of increasing the concentration of binder in solution

 polymer( solution)  [ polymer ]n( gel )  (   n) polymer( solution) Χ: conc. of dispersed polymer molecules before gelation

dehydrate and become hydrophobic

Gelation Gelatin in Solution

Cross-linking

Individual gelatin molecules are small

Gelatin strands bind to one another

Molecules diffusing in solution

Size increases Mobility decreases Viscosity increases

Gelation Cross-linking becomes extensive

Water in the interstices of gelatin matrix - infinite viscosity solid

1) Dehydration - opposite of Tg behavior - change in viscosity - reversible - methyl cellulose 2) Chemical reaction - temperature- sensitive - irreversible - examples i) coagulation of ionic binders at their PZC by adding a simple acid or base ii) cross-linking of anionic polymer into gel by highly charged cations iii) gelation of partially hydrolyzed PVA by soluble carbonates, sulfates, and borates iv) gelation of sodium alginate by alkaline earth ions such as Ca2+ : n   Nan a lg inate( sol )  Ca(2solution )  nNa( solution )  Can / 2 al g inate( gel ) 2 3) Changes in gelation temperatures - Electrolytes and glycerol, which have an affinity for water, reduce the gelation temperature. - Ethanol and propylene glycol raise the gelation temperature.

8. General effect of binders 1) Flocculation 2) Thickening-increasing viscosity 3) Decreasing liquid migration rate with binders of high MW

9. Polymer resins Moldable at an early stage and later capable of being hardened to form a structural bond in injection molding 1) Plastics polyethylene(PE), polypropylene(PP), polystyrene(PS)

2) Vinyl polymers polymethyl methacrylate

3) Thermoplastics - soften on heating and stiffen on cooling - reversible - linear polymers 4) Thermosettings - crosslinking between chains having unsaturated carbon bonds - not reversible - epoxy resins

Chapter 12 Plasticizers, Antifoaming Agents 1. Plasticizers 1) Characteristic - reduce the van der Waals forces binding polymer molecules together - cause polymers to pack less densely - lower Tg - increase flexibility of binder - decrease strength of binder - increase the chain mobility of binder molecules - low MW organic compounds with a high boiling point

2) Common plasticizers - Glycols and phthalates

- Water for water-soluble binders (ex. water in PVA)

2. Antifoaming agents (Defoamers) surfactants which migrate to the air/liquid interface and lower the surface tension - eliminate bubbles from a slurry. - aqueous defoaming surfactants : fluorocarbon, dimethylsilicones, higher MW alcohols and glycols, and calcium and aluminum stearate - using a low surface tension liquid like toluene as solvent

3. Lubricants an interfacial phase that reduces friction between the particles or between the particles and the die wall - stearic acid, stearates and various waxy substances are commonly used lubricants. HW) 11.11, 12.2