Paste Extrusion Research Articles

A Model for a Single Screw Paste Extruder with Non-Constant Channel Depth

A Model for a Single Screw Paste Extruder with Non-Constant Channel Depth

Preparation of extrudable alumina paste by coagulation of electrosterically stabilized aqueous slurries

Abstract The present work demonstrates the conversion of a low viscous concentrated (55 vol.%) alumina slurry into a thick extrudable paste by in situ coagulation. Highly concentrated (55 vol.%) electrosterically stabilized aqueous alumina slurry prepared using polyacrylate dispersant was coagulated by in situ generation of acetic acid. Acetic anhydride was used for acid generation and the miscibility of acetic anhydride in aqueous medium was increased by polyethylene glycol. Addition of acetic anhydride >0.16 mol/l converts the slurry in to a stiff paste, within 5 min. The paste exhibits extrudable consistency. Incorporation of 1% PVA in the slurry provides additional strength to the extruded tubes against deformation and development of drying cracks. The tubes sintered to 97% TD and the sintered samples showed a fine grained microstructure.

Preforming behavior of polytetrafluoroethylene paste

Due to its high melting point, polytetrafluoroethylene (PTFE) is processed by a number of unusual techniques including paste extrusion. The paste extrusion process is carried out at ambient temperature and is usually followed by sintering. It is necessary to preform the PTFE powder–lubricant mixture before extrusion to ensure paste densification. In this work, the preforming behavior of a series of homopolymer PTFE pastes was studied. It was found that preforming pressure is an important variable to be considered to ensure uniform densification of paste. Inadequate preforming pressure will cause variation in transient pressure during extrusion, which may potentially lead to a variation in the properties of the final product. It was also found that the required pressure level may quantitatively be correlated with the standard specific gravity or molecular weight of the resin. Furthermore, longer prefoming duration was found to increase the overall preform density, although this increases the extent of lubricant migration. Finally, it was found that an increase in the lubricant content increases the extent of the uniformity of preform density.

The effect of the binder on the manufacture of a 5A zeolite monolith

The effect of the binder on the manufacture, by a paste extrusion process, of a 2 mm square lattice channel zeolite monolith with a 0.98 mm wall thickness and an overall diameter of 20 mm has been studied using a variety of visual and analytical techniques. Crucial factors for manufacturing defect-free 5A zeolite monoliths have been found to be the use of a binder with good plasticity properties, such as Na–bentonite, extrusion conditions, and a well-controlled drying process. Pastes containing different amounts of water and binder were characterised from the relationship between pressure drop and extrudate velocity during flow from a circular barrel into a circular die-land. From the relationship between the extrusion pressure and the extrudate velocity, six extrusion parameters were derived for each paste. A higher extrusion pressure is required when there is either a decreased water content or an increased binder content.

Evaluation of slip in capillary extrusion of ceramic pastes

Abstract In this study, the capillary extrusion behaviour of an alumina paste material, made using submicron alumina powder, hydroxy ethyl cellulose (HEC), a polyelectrolyte dispersant, poly (methacrylate), and water is described. It confirms that the computed shear stress, obtained using estabilished analytical procedures, depends upon both the diameter and the length of the capillary tube. Additionally, for a given nominal shear rate, the shear stress so calculated is less for the smaller diameter capillaries than for the corresponding larger diameter capillaries. These trends are rationalised, along established lines, in terms of the preponderance of a material process slip at the wall of the capillary as opposed to cohesive or fully developed uniform flow. Moreover, for a given diameter of the capillary tubes the calculated shear stress is greater for the smaller length capillary tubes than the corresponding longer capillary tubes. It is concluded that these results are not amenable to meaningful analysis using classical capillary rheometery analytical procedures based upon the presumption of a fully developed flow. The extensive slip which inevitably occurs at the interface between the capillary and the extrudate negates the sensible application of these type of procedures. Instead, an attempt is made, using the same data, to interpret the slip characteristics of the paste-wall interface in terms of a quantity defined as the “reference slip thickness”, which is seen to depend upon the extrudate velocity (or the apparent shear rate) but is independent of the dimensions of the capillary tube.

Extrusion characteristics of alumina–aluminium titanate composite using boehmite as a reactive binder

Alumina–aluminium titanate composite was fabricated through paste extrusion using boehmite gel as a binder. The rheological characteristics of alumina paste containing various amounts of boehmite binder were studied at different shear rates. The results of investigations on the development of torque, yield stress, viscosity and fluidity are presented. In general, Bingham type plastic behavior was observed. Alumina pastes containing 18.5 vol.% of boehmite exhibits adequate plasticity and fluidity for extrusion. The torque value for this composition is 68 kPa at shear rate 28.5 s −1. The viscosity curves of alumina pastes showed “shear thinning” flow behavior. The mixture of 54.15 vol.% alumina and 18.5 vol.% boehmite resulted in 61% of theoretical green density on extrusion. These conditions were extended to synthesis of alumina–aluminium titanate composites, and the extrusion was carried out using 18.5 vol.% boehmite. Aluminium titanate was synthesized through sol–gel method and dispersed uniformly in alumina matrix through colloidal process. The composites could be sintered at 1400°C to 98% of its theoretical value. The SEM fractograph of sintered composite showed reasonably dense, uniformly distributed grains in the order of 2–3 μm. The present study shows that boehmite binder phase yields good workability for extrusion and contributes high green and sintered densities to the composites.

Zirconia toughened aluminas by hydro-thermal processing

Zirconia toughened alumina materials have been produced by a hydro-thermal powder synthesis route. The resultant sols were concentrated using filtration to form extrudable pastes. The extruded products when dried and sintered had uniform microstructures with improved mechanical properties compared to pure mono-phase rods produced by a similar route. The zirconia particles produced by hydro-thermal synthesis were too small to give the optimum toughening effect, as the zirconia phase is undoped. Two alternative methods were investigated in order to increase the zirconia grain size in the finished body. The hydro-thermally synthesized zirconia was freeze-dried to increase the flock size before mixing with the alumina sol and a commercial monoclinic zirconia powder was added to the alumina sol prior to concentration. The improvements in mechanical performance correlated well with theoretical predictions.

Extrusion of Metal–Ceramic Composite Pipes

Metal–ceramic composite pipes were prepared through simultaneous extrusion of different pastes by a multi‐billet extrusion method. ZrO2 and stainless steel powders were chosen, and an aqueous solution of water‐soluble polymer, hydroxypropyl methylcellulose (HPMC), was used as binder. The maximum extrusion pressure and the minimum amount of binder required reached their lowest values when the mixing fraction of ZrO2 powder was 0.4. The minimum amount of binder for forming the outer layers was 4%–5% higher than that for inner layers, even for the same powder. It was possible to decrease the binder content and broaden the extrudable range of the binder content by means of mixing coarse and fine powders.

Manufacture and Characterisation of Silicalite Monoliths

Multichannel monoliths containing up to 90% silicalite by weight and with cell densities up to 28 cells/cm2, wall thicknesses down to 0.6 mm and an overall diameter of 40 mm have been prepared from silicalite powder and sodium bentonite (as a binder) by the unit operations of paste preparation, extrusion, drying and firing. The manufactured monoliths, which show good strength, retain the crystal structure and micropore size of the source silicalite powder, and adsorption measurements made by using a dynamic flow system confirm that the monoliths exhibit an equilibrium performance broadly similar to that of commercial silicalite pellets. In addition, the manufactured monoliths possess a higher macroporosity than the commercial pellets. Regeneration of the monoliths was found to be possible at mildly increased temperature. These features augur well for the recovery and/or separation of organic compounds in simple pressure swing and thermal swing processes.

Drainage phenomenon of pastes during extrusion

The effects of elemental powder characteristics, binder content and its composition, as well as some additives on pressure change and drainage phenomenon of pastes during extrusion have been mainly investigated. The pastes consisted of a powder, zirconia or stainless steel, and a water-based binder, an aqueous solution of water-soluble polymer (hydroxypropyl methylcellulose). The drainage phenomenon has been found in extrusion of the stainless steel pastes with lower binder contents, while the zirconia pastes show a small probability of drainage in the range of the binder contents used in this investigation. It is shown that broadening particle size distribution by mixing powders with different average particle sizes has a significant effect on decreasing extrusion pressure and restraining occurrence of the drainage phenomenon, thus improving the extrudability of pastes. It is effective to increase the binder content in pastes, raise the mixing fraction of HPMC in binder and add plasticizer like glycerol, in order to reduce occurrence of the drainage phenomenon during extrusion of the stainless steel pastes.

Surface Fracture in Axisymmetric Paste Extrusion: An Experimental Study

In the extrusion of pastes, fractures can be found on the surface of product. Such fractures compromise strength and are often unacceptable aesthetically. Here, experiments show that increasing the length of the die has a beneficial effect on surface quality in all pastes examined. The materials were α-alumina pastes allied to those used for making catalyst that could additionally contain Bentonite clay and carbohydrates. Defects can be completely eliminated for a-alumina pastes with starch additives. Increasing extrusion velocity increases the depth of fracture in pastes with glucose additives. Reduced frequency and depth of fracturing is found when using tapered dies. All the materials show a decrease in the depth of fracture with increasing extrusion ratio, i.e. the ratio of barrel to die diameter. For a sample paste, the product of the frequency of fracture and diameter of the extrusion die yields a dimensionless quantity independent of extrusion velocity and extrusion ratio, having a value of two.

Gel-Assisted Extrusion of Alumina

Fabrication of alumina shapes through paste extrusion using boehmite gel, a monohydroxy aluminium oxide (AIOOH), as a reactive binder was studied and reported. The rheological characteristics of alumina with and without boehmite binder were studied at different shear rates. The critical quantity of boehmite binder was optimized from the results on effective solid loading, influence of medium pH, development of torque and viscosity. The alumina paste containing 15% (wt) boehmite binder resulted in 53.4% (vol) of total solids at pH 6.7 with extrudable consistency. The system exhibits a low torque value of 51.85 Pa at shear rate 28.5 S−1 and showed a plastic behaviour. The apparent viscosity of the system showed a shear dependent flow behaviour. Alumina-15 wt% boehmite resulted in higher particle packing, and 60.4% theoretical green density was achieved. The extruded alumina has sintered density of >98% of theoretical density after firing at 1500°C. The microstructure of sintered alumina showed average grain size <2 μm. The boehmite binder has several advantages, e.g. matrix compatibility, good workability, imparting higher green strength and also as a ‘sintering aid’ and can be successfully used for alumina ceramics.

Effect of particle packing on extrusion behavior of pastes

Two powders with different average particle sizes and size distributions were blended in various proportions. The influence of powder mixing on extrusion behavior of pastes containing a water-based binder has been examined. The bimodal mixed stainless steel powder exhibits a low extrusion pressure, small amount of binder and high green density when the mixing fractions of large and small particles are approximately equal. With regard to the mixing of zirconia and stainless steel powders, a similar result has been found, but the mixing fraction of powder corresponding to the optimal packing is shifted to stainless steel-side. In order to improve the extrudability of pastes, it is effective to mix two powders with a large difference in particle size.

On‐Line Monitoring of Ceramic Paste Extrusion

Fluctuations in pressure signals that are measured at a die during the extrusion of α‐alumina‐based ceramic pastes have been shown to contain information about the homogeneity of the paste that is fed into the extruder and phenomena that occur during the extrusion process. Standard error and fractal‐analysis techniques give quantitative measures that have shown good correlation with agglomerate breakdown and variation in solid‐liquid mixing. Gross extrudate surface fracture could be detected via frequency analysis. The potential use of these techniques for the feedback control of extruders is discussed.

Monolith honeycomb mixed oxide catalysts for methane oxidation

Low cost and active monolith catalysts for methane oxidation were developed. The preparation procedure includes the mechanochemical activation of mixed La–Ce rare-earth oxides and transition metal oxides, followed by their annealing, kneading with the binder, extrusion of plastic pastes, drying and calcination. For disordered oxide composites with the perovskite- or fluorite-like structures intermixed with alumina, their specific catalytic activity in the methane oxidation weakly depends upon the nature of transition metal cations (Mn, Ni, Fe, Co). In the temperature range of the methane + air mixture ignition (∼500°C), the performance of optimized monolithic catalysts is mainly determined by their specific surface area controlled by the binder composition and preparation procedure.

Magnetic resonance imaging (MRI) as a method to investigate movement of water during the extrusion of pastes.

To assess the potential of Magnetic Resonance Imaging (MRI) as a method of detecting water movement during the extrusion of pastes. Plug samples were made from mixtures of model materials and microcrystalline cellulose with two water contents at two different ram speeds to simulate ram extrusion. The extrusion process was stopped at different stages and analyzed for water distribution using MRI to assess the influence of water content and the speed of ram on water movement as the extrusion process progresses. Two types of water movement were detected: vertical and radial. When extruding at the faster ram speed, water moved predominantly in the vertical direction, whereas when extruding at a slower ram speed it moved predominantly in the radial direction. At the beginning of the extrusion process a greater water movement in the wetter formulations was observed. MRI appears to be a useful approach to non-invasive water mapping, and is expected to contribute towards a greater understanding of the role of water in the extrusion of pastes.

Plasticizing Aqueous Suspensions of Concentrated Alumina with Maltodextrin Sugar

Aqueous suspensions of submicrometer, 20 vol% Al2O3 powder exhibited a transition from strongly flocculated, thixotropic behavior to a low‐viscosity, Newtonian‐like state upon adding small amounts of maltodextrin (0.03 g of maltodextrin/(g of Al2O3)). These suspensions could be filter pressed to highly dense (57%) and extrudable pastes only when prepared with maltodextrin. We analyzed the interaction of maltodextrin with Al2O3 powder surfaces and quantitatively measured the resulting claylike consolidation, rheological, and extrusion behaviors. Benbow extrusion parameters were comparable to, but higher than, those of kaolin at approximately the same packing density of 57 vol%. In contrast, Al2O3 filter cakes without maltodextrin at 57 vol% density were too stiff to be extruded. Measurements of rheological properties, acoustophoresis, electrophoresis, sorption isotherms, and diffuse reflectance Fourier infrared spectroscopy supported the hypothesis that sorbate‐mediated steric hindrance, rather than electrostatic, interparticle repulsion, is important to enhancing the consolidation and fluidity of maltodextrin–Al2O3 suspensions. Viscosity measurements on aqueous maltodextrin solutions indicated that free maltodextrin in solution does not improve suspension fluidity by decreasing the viscosity of the interparticle solution.

Zeolite Monoliths for Air Separation: Part 1: Manufacture and Characterization

A 2 mm square lattice channel monolith with a 0.98 mm wall thickness and an overall diameter of 20 mm has been prepared from 5A zeolite powder and Na-bentonite (as a binder) by the unit operations of paste preparation, extrusion, drying and firing. The manufactured monolith, which shows good strength, retains the crystal structure and micropore size of the source 5A zeolite powder, and adsorption measurements made by the constant volume method confirm that the monolith exhibits an equilibrium performance equal to that of commercial 5A zeolite pellets. In addition, the monolith possesses a higher macroporosity than the commercial pellets, a feature which is expected to be of benefit in pressure swing adsorption air separation processes.

Two-bed catalytic system for NO x/SO x removal

A combined monolithic catalytic system has been developed by which NO x and SO x are successively removed from flue gases produced by the combustion of coal and other fossil fuels. The honeycomb catalysts used in this study have been manufactured at industrial scale by extrusion of pastes which contain all the precursors or their components. Nitrogen oxides are removed in the first stage of this combined process by selective catalytic reduction with ammonia, using a DeNO x monolithic catalyst based on Ti–V–W oxides dispersed in the structure of a natural magnesium silicate. The outlet gases pass through a second monolithic bed which oxidizes SO 2 to SO 3; this DeSO x monolithic catalyst is based on vanadium oxides and alkali pyrosulfates supported on diatomaceous earth. Laboratory scale activity tests were carried out using small honeycomb sections cut from the industrial catalysts. At a temperature of 450°C, molar conversions of 90% and 80% for NO x and SO x were achieved respectively, without any ammonia slip. The monolithic structure, and mechanical properties of both catalysts allows their operation in high dust configurations.

Liquid Phase Migration in the Extrusion of Icing Sugar Pastes

Liquid phase migration was found to affect the extrusion behaviour of ‘soft solid’ icing sugar pastes. Significant liquid drainage and apparent work hardening was observed with a simple aqueous liquid phase; the onset of phase migration with solutions of egg white in water was found to depend on the liquid phase volume fraction and the extrusion velocity. Flow visualization studies showed that work hardening was linked to the growth of an internal static zone of drier material at the die entry. Consolidation experiments con rmed that liquid migration is likely to occur in these pastes. An initial hypothesis for the observed behaviour is proposed. The modified plasticity approach described by Benbow and Bridgwater1 for modelling these pastes’ extrusion behaviour was not successful as the characterizable pastes exhibited weak yield stress behaviour and strong velocity dependence.