Aqueous processing of SiC green sheets II: Binder and plasticizer
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Well-dispersed SiC slurries in the presence of binder and plasticizer were prepared and investigated in this series of work. In this part, poly(vinyl alcohol) (PVA) 1788 was investigated as a potential binder for aqueous tape casting process. The minimum amount of binder was determined through a primary calculation. Effects of the binder on SiC slurries properties were analyzed in term of zeta potential measurement and rheological test. Coupled with PEI as dispersant and glycerol as plasticizer, they lead to homogeneous systems which seem compatible. The suspensions (formulated with 47.9 wt% SiC powder, 1 wt% dispersant, 3.5 wt% binder, and 3.5 wt% plasticizer) exhibited a shear-thinning behavior with a very limited time-dependent character. After tape casting and drying process, the properties of green tapes were evaluated in term of solid content and gap height. The optimal solid content of SiC was found to be near 22.25 vol%, and the gap height, near 200 m. The microstructure of green tape was characterized by TEM. Results showed that preparation of homogeneous green tapes with relative densities at about 51 vol% was feasible.
I. INTRODUCTION
Binders are added to provide strength for green tapes by forming interparticle bridges. After the evaporation of solvent, the tapes can then be easily manipulated and retained in desired shapes before sintering.1 In the selection of a certain binder for use in tape casting slips, the following requirements must be considered: compatibility with the system, no interference with solvent evaporation or with trapped air, a low glass transition temperature; effective at low concentration, etc.2 There is a range of binders that can be used in aqueous tape casting, of which the functional group can be nonionic, anionic, or cationic. Three groups of substances have been mainly used as binders for aqueous tape casting of ceramics: cellulose ethers, vinyl- or acrylic-type polymers, 1 , 2 and latex. 3 , 4 , 5 , 6 Cellulose is a natural hydrocarbon characterized by a ring-type monomer, the ring being a modified ␣-glucose structure. The most often used cellulose binders in aqueous tape casting are methylcellulose (MC),7 hydroxyethyl cellulose (HEC),8 and hydroxypropyl methyl cellulose.9 The vinyl- or acrylic-type polymers are characterized by a linear backbone consisting of carbon–carbon linkages, with side groups attached to every other carbon atom. Both of them have flexible backbones because of the rotatable
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Address all correspondence to this author. J. Mater. Res., Vol. 17, No. 8, Aug 2002
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carbon–carbon bonds, which can lead to polymers with coiled and curving configurations.4 The most commonly used vinyl- and acrylic-type polymers include poly(vinyl alcohols) (PVA),8 PAA,10,11 etc. True latex is prepared by polymerization of monomer or monomer blend, normally emulsified in an aqueous medium with the aid of anionic or nonionic surfactants. The range of typical properties of commercial latexes is given as follows: solid
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