CONSOLIDATION OF CERAMICS THROUGH POLYSACCHARIDES GELATION

Abstract

Near-net shaping of ceramics is a key step in ceramic processing, because in this step a mass of powder is transformed into a piece with the desired geometry and size.  During consolidation many defects can be introduced in the piece, so that the control of any stage is necessary to reduce or avoid the presence of defects. The gelation of polysaccharides constitutes one the most interesting consolidation mechanisms  for  shaping  of  ceramics  due  to  the  relative  simplicity  of  the  method,  the  use  of  water  as  liquid  medium  and  the  low concentration  of  additives  necessary  to  obtain  the  bodies.  In  this  work  the  main  applications  of  polysaccharides  in  ceramic manufacturing  through  a  gelation  mechanism  are  reviewed.  The  chemical  structure  of  the  most  common  polysaccharides  and  their gelation mechanisms are described, including those yielding chemical gelation, as in the case of alginates in the presence of divalent ions, as well as polysaccharides that gel by a temperature change, either on heating as in cellulose derivatives, or on cooling, as in the case  of  agar  derivatives  and  carrageenans.  The  influence  of  other  polysaccharides  used  for  their  thickening  properties  but having  an important synergy with gelling polysaccharides is also analyzed. In addition to the structure and the basic gelation mechanisms, several examples showing the influence of polysaccharides on the rheology and shaping of ceramic slurries are presented.