V. C. Pandolfelli

Alternative gelling agents for the gelcasting of ceramic foams

Abstract In recent years, a new class of ceramic foams with porosity levels up to 90% was developed as a result of the association of the gelcasting process to the aeration of a suspension containing foaming agents. The gelation of foamed suspensions results from the in situ polymerization of water-soluble monomers. Nearly spherical pores and highly dense struts characterize the structure of these foams, which results in unprecedented properties, such as high permeability, low thermal conductivity, high specific surface area and high mechanical strength. The main drawbacks of this process are the inhibition of the polymerization in the presence of oxygen and the toxicity of the monomers. This work investigates two harmless alternative gelling agents that do not require atmosphere control to set foamed suspensions. The first route consists in the crosslinking of a previously dissolved polyvinyl-alcohol with the addition of an organotitanate. The other approach assessed the use of gelatin as a setting agent. This polymer gels the suspension due to changes in the structure of polypeptidic chains induced by temperature reduction. Gelatin-based systems presented higher storage modulus (G′) than the systems with crosslinked polyvinyl-alcohol chains. This characteristic prevented the formation of cracks during drying of gelatin-based samples, which constituted a limitation of PVAl-based systems. The results point out gelatin as a promising gelling agent to produce ceramic foams without the disadvantages of monomeric systems, such as the toxicity and the necessity of atmosphere control.

Monomer systems for the gelcasting of foams

The association of the gelcasting process with the aeration of ceramic suspensions allowed the development of a novel category of porous ceramics with unprecedented mechanical properties. One of the critical points in the processing of porous ceramics by this route involves the setting of the foams, which is based on a gelling reaction by the in situ polymerization of organic monomers dissolved in the liquid phase. A variety of monomeric systems that are known to be suitable gelling agents for setting ceramic suspensions into dense forms were investigated in this work, with a view to the production of ceramic foams. These systems, namely ammonium acrylate, N-hydroxymethylacrylamide, methacrylic acid, methacrylamide and methylene-bysacrylamide, were studied considering the main requirements to produce ceramic foams, which include short setting time and high wet green strength. The effects of monomer type on powder dispersion, reaction kinetics, and green strength of wet and dried gelled bodies were investigated. The results revealed that the chemical characteristics of each system can affect the dispersion and rheological properties of suspensions. Polymerization kinetics and wet and dry mechanical strength varied markedly, depending on the type of monomer used.

Prediction of Ceramic Foams Permeability Using Ergun's Equation

SiC-Al2O3 ceramic foams with 30 to 75 pores per linear inch (ppi) were used to access the application of pore size obtained by image analysis to estimate the cellular materials permeability through Erguns equation. Samples were sliced and the pore size distribution, cross-sectional porosity and pore density were evaluated as a function of the slice thickness based on optical and X-ray computerized tomography acquired images. The average pore sizes were used to calculate permeability constants (k1 and k2) of Erguns equation, which were compared to those experimentally obtained under water flow. Results indicated that the optical diameter distribution was very sensitive to the number of pore layers. Deviation between experimental and calculated values depended on the slice thickness. Nevertheless, the introduction of pore size obtained by image analysis into Erguns equations seems to give fair results to assess the permeability of ceramic foams.

Drying behavior of hydratable alumina-bonded refractory castables

Abstract The drying behavior of a hydratable alumina-bonded (HAB) refractory castable was evaluated by thermogravimetric tests and compared with an ultra-low cement composition (CAC). The key properties of monolithic refractory dry-out performance, such as fluid permeability and mechanical strength, were also determined. The results showed significant differences among the drying profiles ascribed to the distinct hydrated binding phases, which affect the castables’ physical properties to a differing extent. The consequences of these features on the explosive spalling tendencies of the compositions are discussed.

Root canal filling with cements based on mineral aggregates: an in vitro analysis of bacterial microleakage.

The aim of this study was to assess bacterial microleakage through 2 different cements used as root canal filling materials: mineral-trioxide-aggregate (White MTA, Angelus, Londrina, PR, Brazil); and an experimental material containing calcium aluminate plus additives (EndoBinder, University of São Carlos [UFSCar-Brazil], patent number PI0704502-6). Forty incisors were divided into 4 groups: (1) white-MTA, (2) calcium aluminate cement, and (3) positive and (4) negative control group. Falcon tubes were prepared for this experiment, divided into 2 separated chambers; the lower part was filled with RTF-transport media so that only the root apex was in contact with the transport media, whereas the coronal part was immersed in BHI containing Enterococcus faecalis to investigate bacterial microleakage. The evaluated materials used as root canal filling materials showed an absence of microbial growth. Both materials when used as the root canal filling were efficient in sealing root canals and preventing E. faecalis microleakage for the 30-day evaluation period using an in vitro model.

Chemical, physical and mechanical properties of a novel calcium aluminate endodontic cement

AIM To evaluate the influence of additives on several physical and chemical properties of a novel endodontic cement based on calcium aluminate in comparison with mineral trioxide aggregate (MTA). METHODOLOGY Manipulation tests, measurements of temperature, pH, ionic conductivity, compressive strength, apparent porosity, and pore size distribution were carried out on Gray-MTA (Angelus) and calcium aluminate cement (Secar-71, Kerneos) with and without various additives: a polymeric dispersant, CaCl(2) as plasticizer, and ZnO as radiopacifier. RESULTS The calcium aluminate cement without additives had a setting time of approximately 60min, and when combined with Li(2) CO(3) it decreased to 10min. The material also released Ca(2+) ions and alkalinized the medium. Moreover, the addition of additives (dispersant, plasticizer, and radiopacifier) improved its properties resulting in a material with a viscosity of 57mPas, enhanced handling properties, a mechanical strength of 81MPa, a porosity of 4% and pores with small diameter (0.25μm). MTA had no temperature increase (that indicates setting) up to 400min, a mechanical strength of 34MPa, and porosity of 28% with pores 2.5μm in diameter. CONCLUSION The novel cement set more rapidly, had better fluidity, improved handling properties, higher mechanical strength, and reduced porosity with lower pore size compared to Gray-MTA Angelus.

Effects of TiO2 addition on the sintering of ZrO2·TiO2 compositions and on the retention of the tetragonal phase of zirconia at room temperature

The production of tetragonal zirconia polycrystalline (TZP) ceramics and the identification of factors controlling retention of the tetragonal phase in the ZrO2·TiO2 system have been investigated. In this binary system, it was not possible to retain tetragonal zirconia polycrystals at room temperature for a range of compositions sintered above 1200 °C. A decrease in the martensitic transformation temperature of zirconia with titania addition was observed, but the effect was insufficient to retain the tetragonal phase at room temperature. In solid solution, the TiO2 additions act to suppress ZrO2 densification, this leading to grain growth when attempts are made to attain higher densities. The use of fine powders, fast firing or sintering in reducing conditions altered densification but was not able to generate a final grain size sufficiently small to avoid spontaneous tetragonal→monoclinic transformation on cooling. Based on the results obtained for ZrO2·MOx systems, the main factors involved in the retention of tetragonal zirconia at room temperature are discussed in an attempt to incorporate thermodynamical and the stress field effects.

Selection of dispersants for high-alumina zero-cement refractory castables

This article reports on the evaluation of novel short-chain molecules to improve the dispersion of zero-cement high-alumina refractory castables. Two approaches were investigated to enhance the dispersing efficiency of short-chain compounds: (a) the increase of the density of dissociable sites per molecule and (b) the improvement of the adsorption ability of the molecule at the usual pH range of high-alumina castables. Gallic acid and 2-phosphonobutane-1,2,4-tricarboxylic acid were selected based on these requisites and evaluated as potential dispersants with the help of zeta potential analysis, rheological measurements and free-flow tests. Results indicate that such compounds are able to efficiently disperse zero-cement high-alumina castables, enabling a reduction of their water consumption for a given fluidity level.

Processo de hidratação e os mecanismos de atuação dos aditivos aceleradores e retardadores de pega do cimento de aluminato de cálcio

One of the main aspects for the development of refractories castables is to master the knowledge regarding calcium aluminate cement (CAC), as this binder is the most applied in these products. The objective of this work was to analyze the available information in the literature in order to explain the understanding regarding the actions of retarder and accelerator additives in the setting mechanisms of CACs. The analysis of the compiled information pointed out that the retarder additives, in a general way, act in two ways: 1) inhibiting the process of cement dissolution, by the build up of insoluble barriers around the cement particles, and 2) enhancing the formation of more soluble hydrates, which increase the time necessary for the hardening beginning. On the other hand, the accelerators can induce the formation of less soluble hydrates, diminishing the time necessary for the setting or speeding up the beginning of the hydrate nucleation. The overall analysis pointed out that the action of some retarder and accelerators additives takes place in different periods of the setting time of the CACs. This aspect allows the combination of additives confering a good workability associated to a short demolding time.

Bioactivity of Calcium Aluminate Endodontic Cement

INTRODUCTION Calcium aluminate endodontic cement (CAEC) developed for use in root canal therapy has been produced with additives that improve handling properties and provide higher mechanical strength than mineral trioxide aggregate (MTA) according to prior studies. The aim of this study was to evaluate the bioactivity of CAEC containing accelerating additives (A-CAEC) in comparison with MTA, both in contact with simulated body fluid (SBF) solutions. METHODS pH measurements were taken for set cement samples immersed in water or SBF solutions prepared according to the Kokubo and Rigo techniques. The surface of these materials kept in contact with SBF solutions were also evaluated by means of scanning electron microscopy, energy-dispersive X-ray analysis, and X-ray diffraction. RESULTS Because of the calcium hydroxide dissociation, MTA cement is able to release more Ca(2+) ions and results in a higher pH increase compared with A-CAEC. This behavior enhances the supersaturation of Kokubo solution, resulting in the precipitation of calcium phosphate phases on the MTA surface. On the other hand, for MTA in Rigo SBF solution, the pH value attained was higher than for the Kokubo SBF solution as a result of the Mg(2+) ion precipitation, which inhibited the calcium phosphate phase formation. For A-CAEC, the optimal precipitation conditions of calcium phosphate phases are achieved in Rigo SBF solution. CONCLUSIONS MTA and A-CAEC present bioactivity in contact with SBF solution although the composition of this solution defines the type of phase precipitated.