Gelation forming of ceramic compacts using agarose

Abstract

The development of a cast forming process for ceramics based on agarose gelation is described and the properties of green and sintered bodies are presented. For this process, a warm alumina slurry containing more than 50 vol.-% solids loading and ～1 wt-% agarose binder (Al₂O₃ basis) is cast into a relatively cold, non-porous mould, resulting in a tough green body formed by gelation (37°C) of the agarose molecules. The green compacts show uniform density distribution, with precise dimensions and very smooth surfaces. After drying, they can be sintered directly without special binder burn out procedures. Complex ceramic parts with thick and thin cross-sections can be formed. The process is illustrated for the preparation of a turbine rotor component.     

水性油墨用室温自交联纯丙乳液的合成及应用

以甲基丙烯酸甲酯(MMA)、丙烯酸异辛酯(2-EHA)、甲基丙烯酸(MAA)、双丙酮丙烯酰胺(DAAM)为聚合单体,己二酰肼(ADH)为交联剂,采用半连续种子乳液聚合法合成软核硬壳结构的水性纯丙乳液。利用ATR-FTIR、TEM、GPC、TG、DMA表征水性纯丙乳液的结构和性能。结果表明:乳液成膜过程中DAAM中的—CO—与ADH中的—NHNH_2发生反应,交联度增加,提高了乳液膜的耐水性、热稳定性、玻璃化转变温度和油墨在双向拉伸聚丙烯(BOPP)薄膜上的附着力。当DAAM用量(以MMA和2-EHA的总质量为基准,下同)从0增加到2%,m(ADH)∶m(DAAM)=0.7∶1.0时,乳液膜的吸水率从24.2%降到7.6%,油墨的附着力从87%升至100%;当m(ADH)∶m(DAAM)从0∶1增加到0.7∶1.0,DAAM用量为2%时,乳液膜的吸水率从28.1%降至7.6%,油墨的附着力从0增至100%。当DAAM用量为2%,m(ADH)∶m(DAAM)为0.7∶1.0时,水性纯丙乳液可用作环保型水性油墨的连接料。     

Gelation in the radical polymerization of dimethyl diallyl ammonium chloride

Within the framework of an irreversible aggregation model on a sample of polydimethyl diallyl ammonium chloride, we demonstrated that gelation occurred on a transient part from the autoacceleration stage until the dopolymerization stage. During part of the autoacceleration, the polymer solution was in a sol state on the segment of autoacceleration. Therefore, the use of the term gel effect in reference to the beginning of the reaction did not seem quite justified. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1394–1396, 2004     

室温自交联水性聚丙烯酸酯乳液的合成、结构与性能

以甲基丙烯酸甲酯(MMA)、丙烯酸丁酯(BA)、丙烯酸(AA)和双丙酮丙烯酰胺(DAAm)为共聚单体,采用半连续种子乳液聚合工艺,合成了具有核–壳结构的室温自交联水性聚丙烯酸酯乳液。以十二烷基硫酸钠(SDS)与辛基酚聚氧乙烯醚(OP-10)为复合乳化剂,研究了其用量对聚合反应稳定性的影响,讨论了不同DAAm用量下乳胶粒的粒径分布、漆膜耐水性和力学性能,确定了较佳的用量:复合乳化剂用量为单体质量的3.0%,w(DAAm)为4%。透射电镜(TEM)分析表明,乳胶粒具有明显的核–壳结构。热分析表明,以己二酰肼(ADH)为交联剂,交联后漆膜的玻璃化转变温度明显提高,红外光谱分析证明了酮肼交联反应的发生。     

Gelation of pectinic acids in the presence of calcium counterions

Pectins are polysaccharides which are able to gel in the presence of calcium ions if their degree of esterification is lower than 40%. From a purified pectin, a de-esterification was carried out by alkali and by enzymes in order to obtain samples with a different charge density and esterification pattern (random or blockwise distribution of carboxylic acid sites). The pectin-calcium interactions were studied in a dilute solution of calcium pectinates by conductimetry. If the concentration of highly charged polymer is increased, a multichain association can occur: the same process is observed with dilute solutions (c_p≤10-3 eq. 1^?1) but in the presence of an excess of calcium ions. This first step of the gelation was followed by light scattering and by viscometry. The gelling system can be described by a power law including a critical ratio (equivalent concentration of calcium ions/pectin carboxylic sites) and quasi-critical exponents. The values of these critical parameters have been discussed as a function of charge density of the pectins, their esterification pattern, their concentration, and the nature of the divalent counterion.     

Dispersion of concentrated aqueous yttria-stabilized zirconia with ammonium polyacrylate

The dispersion of concentrated 0.53 μm yttria stabilized zirconia (YSZ) aqueous slip to achieve sintered compacts with high and uniform microstructure was investigated. 78 wt.% (37.5 vol%) and 83 wt.% (45.3 vol%) slips were prepared by dispersing the powder in water with ammonium polyacrylate (NH₄PA) at pH values in a range of 8–9.5. The influences of the amount of NH₄PA and pH on the rheological properties of 78 and 83 wt.% slips were studied. The minimum viscosities of 78 and 83 wt.% slips were obtained with the addition of 0.05–0.1 wt.% NH₄PA at pH 9. Increasing pH from 8 to 9.5 had no effect on the YSZ dispersion with NH₄PA. These dispersions yielded green densities of 63% of theoretical density (TD) and sintered to 98% TD at 1400 °C.     

Distinct dispersion stability of various TiO2 nanopowders using ammonium polyacrylate as dispersant

The dispersion stabilities of three titania (TiO₂) nanopowders with different particle sizes and surface chemistries in aqueous suspensions containing a common water-based dispersant, ammonium polyacrylate (PAA-NH₄), have been investigated and compared. According to adsorption isotherm and Fourier transform infrared spectroscopy analyses, the adsorption conformations of PAA-NH₄ are distinct for the different TiO₂ nanopowders. In addition, PAA-NH₄ exhibited the greatest adsorption affinity to the larger, hydrophilic TiO₂ nanopowder and the least affinity to smaller, hydrophobic nanopowder. From sedimentation and rheological results, the dispersion stability of the larger, hydrophilic TiO₂ nanopowder was demonstrated to be the greatest. Based on thermodynamic and kinetic calculations for the stabilization energies, the larger, hydrophilic TiO₂ nanopowder was also shown to be the best-stabilized powder, although it settles faster than the smaller, hydrophilic TiO₂ nanopowder; this is due to the greater affection of sedimentation flux on the larger nanopowder. In contrast, the hydrophobic TiO₂ nanopowder formed a gel-like structure in the aqueous suspension when the solid content was greater than 10 wt%, which is attributed to polymer bridging between PAA-NH₄-adsorbed TiO₂ nanoparticles.     

Gelation mechanism of agarose and κ-carrageenan solutions estimated in terms of concentration fluctuation

The gelation mechanism of agarose and κ-carrageenan aqueous solutions was investigated by using polarized light scattering and X-ray diffraction techniques in terms of the liquid-liquid phase separation. When an incident beam of He-Ne gas laser was directed to the gel prepared by quenching the agarose solution, the logarithm of scattered intensity increased linearly in the initial stage and tended to deviate from this linear relationship in the latter stage. If the linear increase in the initial stage could be analyzed within the framework of the linear theory of spinodal decomposition proposed by Cahn, the phase diagram indicated that the gelation is attributed to the phase separation due to the concentration fluctuation of solution. Furthermore, in the later stage showing the deviation of the linear relationship, light scattering under Hv polarization condition showed a X-type pattern indicating the existence of optically anisotropic rods, the optical axes being parallel or perpendicular with respect to the rod axis. In spite of the existence of the rods, no crystallites were confirmed by the corresponding X-ray diffraction and DSC measurements. For κ-carrageenan solutions, the logarithm of scattered intensity against time showed a constant value. This indicated that the gelation of κ-carrageenan solutions is independent of liquid-liquid phase separation but is due to the rapid formation of cross-linking points. Accordingly it turns out that the small difference of chemical structure between agarose and κ-carrageenan causes quite different gelation mechanism.     

Direct ink writing of aluminium oxynitride (AlON) transparent ceramics from water-based slurries

In this study, transparent AlON ceramics were fabricated via the direct ink writing (DIW) method from the water-based ceramic slurry. The solids loading of the ceramic slurry was optimised by changing the dispersant content, and the printability and water content were then adjusted by adding hydroxyethyl cellulose (HEC). The structure of the green body was complete and no impurity phases were detected. The effects of sintering temperature and dwell time on the bulk density, phase evolution, microstructure, in-line transmittance, and mechanical properties of the ceramics were studied systematically. High optical and mechanical properties of 10 × 10 × 0.9 mm³ single-phase AlON ceramic tiles were obtained by sintering at 1960 °C for 10 h in a nitrogen atmosphere: in-line transmittance of 81.90% at a wavelength of 780 nm, fracture toughness of 1.74 MPa·m^1/2 (2.94 N), and Vickers hardness of 18.56 GPa (2.94 N). This study provides a novel method for synthesising AlON transparent ceramics from water-based ceramic slurries.     

Synthesis and properties of polydimethylsiloxane/graphene oxide modified,water-based polyurethane/polyacrylate

In this study, we prepared polydimethylsiloxane–graphene oxide (PG) modified waterborne polyurethane acrylate (WPUA) films and enhanced their hydrophobicity, thermal stability, and mechanical properties. The prepared films were characterized by Fourier transform infrared and energy spectroscopy analysis; this confirmed the successful incorporation of PG into WPUA. As shown by scanning electron microscopy and atomic force microscopy analysis, the surfaces of the WPUA films were confirmed to have turned smoother and more compact after PG addition. With an optimized amount of PG (0.1%) incorporated into the WPUA film, it exhibited a contact angle improvement of 21.9°, an enhanced decomposition temperature at 5% weight losses of 25 °C, and an elongation at break improvement of 482.92% over those of pure WPUA. The newly synthesized PG–WPUA showed considerable enhancements in its hydrophobicity and thermal and mechanical properties and could be deemed to have potential value as an alternative contender for practical applications in coatings painted on tunnels and highways. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47926.     

Dispersion behaviour of laser-synthesized nanometric SiC powders in aqueous medium with ammonium polyacrylate

The dispersion behaviour of laser-synthesized nanometric SiC powders in water using ammonium polyacrylate (molecular WEIGHT=10,000) as dispersant was investigated. The influence of oxidation, presoaking time, ammonium polyacrylate (NH₄PA) concentration, and pH on suspension stability and coagulation rate of aggregates was determined. The stabilization mechanism is discussed. Excellent dispersion stability was obtained for oxidized (500 °C) powders containing 2.45 wt.% NH₄PA at pH 9.25 after a lengthy aging treatment.     

Polymerization of methylmetacrylate in the presence of quaternary ammonium salts

Summary At 50°C, trioctyl methyl ammonium salts influence the rate of bulk polymerization of methylmetacrylate initiated by dibenzoyl peroxide. Acetate increases it, chloride, chlorate, and bromate increase it only on start, hydroxide, bromide, tetrachlorozincate, trifluoroacetate and trifluoromethanesulfonate decrease it, iodide and dichromate inhibit the polymerization, whereas fluoride, perchlorate, some (substituted) benzoates as well as sulfonates do not change the reaction rate. The influence on kinetic parameters of polymerization and the formation of intermediates and products suggest that the exchange of anions between the quaternary ammonium salt and dibenzoyl peroxide is the primary mechanism of action of these salts and other effects accompany it.     

Kinetics of copper electrodeposition in the presence of triethyl-benzyl ammonium chloride

Quasi-steady state hydrodynamic voltammetry at a rotating-disc electrode and electrochemical impedance spectroscopy were used to investigate the influence of triethyl-benzyl-ammonium (TEBA) chloride on the kinetics of copper electrodeposition from sulphate acidic electrolytes. SEM and X-ray diffraction analysis were used to examine the morphology and the structure of copper deposits. The kinetic parameters (i ₀, _c, k ₀), obtained by both Tafel and Koutecky–Levich interpretations lead to the conclusion that TEBA acts as an inhibitor of copper electrodeposition process, as a consequence of its adsorption on the electrode surface. The influence of TEBA on the kinetics of copper electrodeposition was explained in terms of a reaction model confirmed by the simulated impedance spectra. TEBA acts only as a blocking agent competing for adsorption active sites of the cathodic surface with cuprous ions without changing the reaction pathway corresponding to the absence of the additive.     

Graft copolymerization of acrylamide onto cellulose in presence of comonomer using ceric ammonium nitrate as initiator

The graft copolymerization of acrylamide (AAm) and ethylmethacrylate (EMA) monomers onto cellulose has been carried out using ceric ammonium nitrate (CAN) as initiator in presence of nitric acid at (25 ± 1)°C and varying feed molarity from 7.5 × 10^?2 mol dm^?3 to 60.0 × 10^?2 mol dm^?3 at fixed feed composition (f_AAm = 0.6). The graft yield (%G_Y) has shown a linear increasing trend upto a feed molarity of 37.5 × 10^?2 mol dm^?3. The composition of grafted copolymer chains was found to be constant (F_AAm = 0.56) during feed molarity variation but shown variations with feed composition (f_AAm) and reaction temperature. The grafting parameters have shown increasing trends up to 7.5 × 10^?3 mol dm^?3 concentration of ceric (IV) ions and decreased on further increasing the concentration of ceric (IV) ions beyond 7.5 × 10^?3 mol dm^?3. The IR and elemental analysis data were used to determine the composition of grafted chains (F_AAm) and reactivity ratio of acrylamide (r₁) and ethylmethacrylate (r₂) comonomers. The reactivity ratio for acrylamide (r₁) and ethylmethacrylate (r₂) has been found to be 0.7 and 1.0 respectively, which suggested for an alternate arrangement of average sequence length of acrylamide (mM?₁) and ethylmethacrylate (mM?₂) in grafted chains. The rate of graft copolymerization of comonomers onto cellulose was found to be proportional to square concentration of comonomers and square root to the concentration of ceric (IV) ions. The energy of activation (ΔEa) of graft copolymerization was found to be 9.57 kJ mol^?1 within the temperature range of 20–50°C. On the basis of experimental findings, suitable reaction steps have been proposed for graft copolymerization of selected comonomers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2546–2558, 2006     

Gelcasting of alumina foams using agarose solutions

Aqueous gelcasting of dense or cellular ceramics by using biopolymers as gel-formers, instead of monomers, is a promising technology mainly in terms of environmental aspects. The main difficulty of using biopolymer solutions in processing of cellular ceramics by foaming method is their high viscosity, which prevents the foaming capacity of the ceramic suspension. In this work, the procedure for preparing concentrated agarose solutions (4 wt.%) by dissolving under overpressure conditions was evaluated for the gelcasting of alumina foams, and the rheological behaviour of alumina suspensions containing agarose was studied. The viscosity of the gelling solution obtained under overpressure conditions was lower than that prepared by simply heating at 90 °C, thus providing high foaming capacity of the alumina suspensions and consequently manufacturing of highly porous ceramics (86–90%). The microstructure of alumina foams was typically composed of approximately spherical cells interconnected by circular windows. The use of different agarose concentrations in alumina suspensions effected the rheological conditions, which resulted in changes in the pore and window sizes of the resulting ceramics. Depending on agarose concentration (0.50–1.0 wt.% on a dry solids basis) in the starting (35 vol.%) alumina slurry, the mean pore size ranged from 529 to 375 μm, while the mean window size varied from 113 to 77 μm.     

A comparative study on water-based coatings prepared in the presence of oligomeric and conventional protective colloids

Poly(vinyl acetate-co-butyl acrylate) latexes were prepared by using semi-continuous emulsion polymerization method in presence of two different protective colloids which were oligomeric N-methylol acrylamide and conventional poly(vinyl alcohol). The effects of these protective colloids on colloidal, surface and film properties of latexes were examined. Poly(vinyl acetate-co-butyl acrylate) synthesized with oligomeric N-methylol acrylamide, which was characterized by FT-IR, NMR and MALDI-TOF MS, was found to have lower viscosity, finer particle size, better latex stability, lower polydispersity, higher T_g and better film forming behavior compared to those synthesized from the poly(vinyl alcohol).     

Rheological properties of water-coal slurries based on brown coal in the presence of sodium lignosulfonates and alkali

The effect of the oxidized surface of brown coal on the structural and rheological properties of water-coal slurries was found. The kinetics of structure formation processes in water-coal slurries based on as-received and oxidized brown coal was studied. The effect of lignosulfonate and alkali additives on the samples of brown coal was considered.     

Dewatering of organic slurries using reinforced thermosensitive porous gels

Summary The use of a thermosensitive porous gel for dewatering organic slurries was investigated. The gel was synthesized from N-isopropylacrylamide (NIPAM), the monomer and N,N’-methylenebisacrylamide (MBAA), a crosslinker. A porous gel was produced by simple radical polymerization at higher temperature than the LCST (Lower Critical Solution Temperature) of the N-isopropylacrylamide polymer. At such a high temperature, phase separation occurs as polymerization proceeds, and a porous structure is formed. The thermosensitive porous gel, which was supported by stainless steel wire, was prepared and the effect of preparation conditions such as the concentration of crosslinker and monomer used on the dewatering performance of the gel was examined. The effects of these parameters on gel structure and the relationship between gel structure and absorption rates were also examined.     

Oxygen reduction on platinum in aqueous sulphuric acid in the presence of ammonium

The oxygen reduction reaction (ORR) was studied using a rotating disk electrode. The kinetic current density (cd) at 0.7 V_RHE was reduced by more than a factor of 10 on pre-reduced Pt and a factor of 2.5 on pre-oxidised Pt when comparing ORR rate for pure sulphuric acid to that in 0.18 M H₂SO₄ + 0.1 M NH₃ solution which was the highest ammonium concentration studied. The Tafel slope at low cd on pre-reduced Pt is about −90  mV/decade in the presence of ammonium compared to about −74  mV/decade in pure acidic solution. The Tafel slopes at high cd is not affected by ammonium. The Tafel slope of ORR on pre-oxidised Pt surface is not significantly affected by ammonium. The exchange cd is lower in the presence of ammonium both for pre-reduced and pre-oxidised Pt. The direct origin of the loss of ORR activity has not been firmly resolved. Adsorption and oxidation of ammonium including intermediate species as well as nitrogen-oxygen species formed at high potentials may be involved.