Effect of Natural and Synthetic Organics on the Processing of Ceramics

Abstract

Dry pressing has been shown to be an efficient and cost effective method of manufacturing ceramic ware. Dry pressed parts are typically manufactured with a low moisture content which has the further advantage of eliminating the drying step that is necessary for plastic formed ware, i.e., jiggered or ram pressed. Problems associated with the use of dry pressing in an industrial setting involve the high loss rate during the bisque firing process and the poor surface finish of the green (unfired) ware. It was the goal of this research to improve the surface finish of dry pressed ware to a level that is satisfactory for decorating of the bisque fired ware. The current surface finish of the dry pressed ware resulted in a decoration that was not aesthetically pleasing. The adsorption of organic additives, specifically dispersants, on the surface of particles is an important aspect of ceramic processing. The interactions between organic additives, specifically sodium poly[acrylic acid] and poly[vinyl alcohol], have been demonstrated to result in phase separation into distinct domains during the spray-drying process. This phase separation leads to a poly[vinyl alcohol]-rich film on the surface of the granulate which will increase the P1 value, the pressure at the onset of granule deformation, of the granulate. This negative interaction between the organics increases the surface roughness of the dry pressed ware. The presence of naturally-occurring organics, specifically humic acid and fulvic acid, interfered with the adsorption of poly[acrylic acid] on the surface of clay particles. The competitive adsorption between the naturally-occurring organics in clay and poly[acrylic acid] lead to a tertiary blend of organics in solution which will further aggravate the phase separation of the additives. Furthermore the presence of these organics in solution interfered with the titration method used to determine the amount of polymer which adsorbed on the particle surface. The amount of polymer dispersant which adsorbed on the surface of a kaolinite particle was predicted by a model based upon the mineralogy of the kaolinite platelet. Preliminary adsorption studies with commercial clays found a significant deviation from the predicted levels. Work with source clay minerals confirmed the model for adsorption. Washing and beneficiation by sedimentation of commercial clays was determined to improve the adsorption levels of poly[acrylic acid] to the predicted levels. The presence of digested lignite in a commercial clay, added by the supplier to improve plasticity, was found to not significantly effect the plasticity as determine by the HPASC. Instead the presence of sodium chloride, a byproduct of the process used to fraction the organics, was found to change the shear rheology of the clay. The digested lignite was prepared by treating lignite in hot sodium hydroxide. The result was a low molecular weight species that was observed to improve the rheology of a 30 volume percent clay suspension, but there was no significant improvement in the measured cohesion of a commercial kaolin clay. The roughness of the industrially prepared ware was determined using an optical interferometer to set a baseline for improvements in the surface finish of the dry pressed ware. Two routes were investigated to reduce the measured surface roughness of the dry pressed ware. The first involved the blending of dried granulate with different P1 values. The second involved the study of alternative binder systems to replace plasticized poly[vinyl alcohol] (pPVA) which has been demonstrated to undergo phase separation in the presence of poly[acrylic acid] (PAA) and poly[methacrylic acid] (PMAA) which are commonly used dispersants in the ceramic industry. Blending of dried granulate was determined to significantly improve the surface finish of the ware. The role of moisture content, controlled by the relative humidity, and granule size were determined. Moisture content was observed to have a significant role in compaction and storage of dried granulate at 90% relative humidity was observed to restore the attributes (P1, compact density, and green strength) of the granulate to those of the as-received material. Granule size was observed to be insignificant provided that the extreme fines, sub-230 mesh, were not tested. A reduction in the granule size distribution was observed to significantly reduce the measured RMS roughness of dry pressed ware. Granulate prepared at Alfred University had a significantly smaller mean diameter relative to the industrially prepared granulate. The result was an overall reduction in the depth of the features between compacted granules at the sample surface. This lead to a significant reduction in the surface roughness of the dry pressed ware. Alternative binders to replace a plasticized poly[vinyl alcohol] were observed to show improvements in the surface finish of the ware dry pressed in a semi-isostatic die. Blends of granulate prepared with alternative binders, selected on a basis of a normalized P1 value, were not observed to have a significant reduce the measured P1 value. Instead the change in the P1 value of the blended granulate was observed to be strongly correlated to the moisture content of the granulate. The measured RMS roughness was observed to slightly decrease with an increase in the moisture content of the granulate. The combination of shear and uniaxial pressure were observed to significantly decrease the P1 value of the granulate. The role of moisture content during uniaxial compaction was again observed. Above 3 wt% moisture plateaus were observed in the uniaxial P1 value, the compact density, green strength, and the RMS roughness. The P1 value in shear was observed to be nearly independent of moisture content and more than an order of magnitude lower than the uniaxial P1 value. The effects of organic binders on the plasticity of a granulated porcelain body were determined using the HPASC as a function of moisture content. Relative to the granulate with no organic binder system the addition of a plasticized poly[vinyl alcohol] and poly[ethylene glycol] were observed to decrease the maximum cohesion of the body and increase the pressure dependence at the maximum cohesion. The addition of lignosulphonate was observed to significantly increase the maximum cohesion, as well as increase the area under the curve in the shear rheology map, of the body and decrease the pressure dependence of the system. Based upon the criteria set forth in this thesis the addition of plasticized poly[vinyl alcohol] or poly[ethylene glycol] were determined to be detrimental to the plasticity of the clay body while the addition of lignosulphonate was observed to be beneficial to the plasticity. These changes in plasticity were related to the adsorption of lignosulphonate and the change in the surface tension of the water in the samples due to the organic additives. The results from a dynamic mechanical analyzer to determine the glasstransition temperature, indicated that the plasticizer used in this study, supplied by Buffalo China from their process line, is not an effective plasticizer for poly[vinyl alcohol]. Concentrated solutions of the two organics were observed to phase separate into distinct domains. Furthermore two peaks were observed in the Tan δ, the ratio of the loss modulus to the storage modulus, curves from the dynamic mechanical analyzer. Each peak corresponded to a glass transition temperature of an organic component in the test film, prepared using a glass fiber cloth as a substrate. An effective plasticizer would result in a shift of the glass-transition temperature of the organic. No shift was observed in the glasstransition temperature of the poly[vinyl alcohol] indicating that there was no associated complex between the organics. Comparison with a poly[vinyl alcohol] and glycerin mixture showed that glycerin is an effective plasticizer for poly[vinyl alcohol] resulting in the glass-transition temperature shifting to lower temperatures. In summary the most important aspect to improving the surface finish of dry pressed ware, i.e. facilitating compaction, is the selection of the organic additives. Additives which are observed to have a negative interaction, i.e. to phase separate into distinct domains, will result in an organic rich film at the surface of the granule thus increasing the P1 value of the granulate. The presence of water in the granulate can counteract this effect by plasticizing the organic. A suitable moisture content of the granulate is crucial to compaction. With little moisture, the binder will be hard and difficult to deform. Excess moisture results in poor flowability and the pressed part sticking to the die. The use of a binder which created an organic gelatin was observed to significantly improve the compaction of blended granule systems since the water which was trapped in the gelatin structure was available to plasticize the second organic binder. The highest green strength observed in this study resulted from a 50-50 weight percent blend of an agar binder system and a lignosulphonate binder system. Excessive amount of an organic which creates a gelatin will result in a reduction in the flowability of the granulate and sticking of the ware to the membrane surface after compaction.