Characterization of polymers in particulate systems

Small amounts of polymers (ca. 1 wt.-%) are often used in the processing of ceramics to bond the particles. The purpose is to enhance mechanical prope rties, notably the strength, of intermediate or green products obtained aft er consolidation of the powder. In this paper, the influence of polymers on mechanical properties of green (Mn,Zn)ferrite products is investigated. Fr equently used polymers are methyl methacrylate - ethyl acrylate copolymer a nd poly(vinyl alcohol) (PVA). Because PVA generally produces low-strength c ompacts and is sensitive to moisture, a blend of the copolymer and PVA was used. The glass transition temperatures (T-g) of the polymers and green pro ducts were determined. The good correspondence between the T-g's indicates that the polymers dominate the mechanical behaviour when tensile stresses a re applied. In practice, compacts usually have an axisymmetric shape and ar e subject to diametral compression during handling. Therefore, ring-shaped compacts were crushed at different velocities. The crushing velocity has a strong influence on the compact strength when the copolymer is used and onl y a small effect when the compact contains the PVA blend. This indicates th at the copolymer can absorb a rapid buildup of stress through plastic defor mation. By contrast, the rate of plastic deformation in the PVA blend is re latively slow. Further insight is obtained by subjecting rectangular compac ts to three-point bending. By interpreting the experimental data with a mec hanical model, the deformation of compacts containing acrylic is found to b e elasto-plastic. With compacts containing the PVA blend, a yield point is observed. This indicates that elasto-plastic deformation is preceded by ela stic deformation.