Researche influence of holding time on the rheological properties of alumina slips containing with dispersing and hardening additives and the properties of corundum ceramics samples which made from them

The researches influence of holding time on the rheological properties of the slips and the properties of corundum ceramics samples based on alumina of the MARTOXID MR70 brand with dispersing and hardening additives (for casting thin-walled products) and only with the dispersing additive (for casting of thick-walled products) which made of them were carried out. The holding time has a significant effect on the rheological properties of the slips were established. The destruction of aggregates in initial alumina on particles were begining with increasing the holding time the slips to 24 hours as a result the dynamic viscosity of these slips decreases and reaches a minimum value. In the slips with combined additive (dispersing and hardening) with increasing the holding time more than 24 hours beginning the formation of a coagulation-condensation structure. In the slip with only a dispersing additive and in the slips without additive beginning the formation of a coagulation structure the dynamic viscosity of the slips increases and this process does not end at 672 hours. However, with a little mechanical mixing in all three slips their structure are fragile and easily almost completely destroyed and such slips are suitable for making quality products from them. The samples which made from the slips after a holding time of 24 hours and 720 hours have practically the same properties, so the increasing the holding time for more than 24 hours does not affect the properties of the samples. For making quality products the fresh slips are also suitable and it is necessary to choose the holding time of slips according to the size and configuration of products, their service conditions and production volumes. According to the electronicmicroscopic studies the formation a gel-like substance that envelops alumina particles and thereby prevents its settling in the slip which contains the hardening additive.

Analysis of hetero-coagulation structure of colloidal mullite precursor powder - experimental approach and computer simulation -

ABSTRACTTo control the hetero-coagulation structure of ultra-fine γ-A12O3 and SiO2 powders for mullite ceramics, hetero-coagulation structures in suspensions with different pH conditions were investigated by experimental approach and computer simulation. Electrophoretic mobility distribution, which was determined by a Laser Doppler method, was used for the estimation of hetero-coagulation structure. As results, a bimodal electrophoretic mobility distribution was observed when precursor powders were prepared by colloidal mixing processes at pH=9.5. Since each peak almost corresponded to the electrophoretic mobility of γ-A12O3 or SiO2 particles, hetero-coagulates were not formed in suspension. In the case of pH=6.0, heterocoagulates generated in suspension, however, uniform and fine hetero-coagulates could not be attained. On the other hand, narrow and single mode electrophoretic mobility distribution was observed after colloidal mixing at pH=4.4. The change of hetero-coagulation structure with different pH conditions were analyzed by using Brownian dynamic methods with considering hetero-coagulated system. The change of hetero-coagulation structure with different pH conditions were qualitatively simulated by this model.

Adsorption Modification of Fillers and Structure Formation in Solutions of Polymers

On addition of a solid disperse phase to a polymer one requisite for its effectiveness as an active filler is close similarity of the molecular properties of the surface of the particles and the polymer. This along with high “dispersity” (specific surface area) is mainly responsible for the high effectiveness of various types of carbon black—a hydrophobic filler of organic origin. For the same reason the majority of mineral fillers, which are usually of hydrophilic character, have relatively low capacity for the active filling of polymers, which is made evident in improvement in the mechanical and other properties of the filled material. As a consequence particular importance attaches to processes of adsorption modification, i.e., the alteration of the molecular properties of the surface of particles of hydrophilic fillers by orientated adsorption of surface-active substances (SAS). The hydrophobicization of the surface of the particles of filler which takes place here increases their “wettability” by the polymer phase and enhances the strength of its bond to the polymer, as has been investigated in detail in the work of P. A. Rebinder et al.. In this case modification leads to activation of the filler; in a number of cases however activation does not take place. As models for filled systems we may take concentrated suspensions of particles of the fillers in solutions of polymers containing additions of surface-active substances, which depending upon the concentration in the solution and proportionally to its increase, at first peptize the particles, lyophilizing them, and then stabilize them as a result of lyophobicization. This causes in sequence coagulation structure-formation in the system and then destruction of the resulting structures. A number of investigations have been devoted to the influence of fillers and pigments on the properties of polymers in the presence of surface active substances —in rubbers, and in lacquers and paints based on drying oils, but the laws of adsorption modification of mineral disperse phases and its influence on the filling effect in connection with the composition and molecular properties of the complex system: polymer-solvent/filler (or pigment)/surface-active substance have not been systematically studied. The present paper aims to investigate this question.