Control of the morphology of waterborne nanocomposite films

Nanocomposite materials are prepared by drying a mixed colloidal suspension of film‐forming latex and silica. The dispersion state of the silica particles in the dry film controls the properties. We describe, with the help of some examples, how the colloidal stability of the particles in the mixed suspension dictates the final morphology of the dry film. Next, some recent work aimed at controlling the morphology by means of formulation additives is described. Such additives are either surfactants or polymer dispersants (block copolymers) which stabilize the particles (latex and/or silica) in the drying suspension. As a result of the mixing‐and‐drying preparation process, suitable additives are necessarily different from the conventional dispersants used in the formulation of composites prepared by means of melt‐processing. Copyright © 2004 Society of Chemical Industry     

HIGH-INTENSITY DRYING OF PAPER

High-intensity contact drying denotes drying under suf- ficiently intensive heating conditions that, following a brief warmup period, the mist paper web operates at internal tem- peratures in excess of the ambient boiling point. A simplified, two-zone analytical model is first presented. The paper is depicted as having a dry layer, of ever-increasing thickness, adjacent to the hot surface. Heat conduction through this layer (the rate-limiting step) causes evaporation at the interface with the “wet zone.” The vapor is then considered to flow through the wet zone into the ambient. Results of bench-scale experiments are discussed. Drying rates as much as twenty times conventional rates are indicated. The drying rate increases with hot surface/boiling point tem- perature difference and applied mechanical pressure. The instsn- taneous drying rate decreases continuously after a brief warmup period. The vapor pressure at the hot surfacelpaper interface rises quickly to a peak value, followed by a continuously- diminishing level. The heat flux shows a rapid rise to boiling- like conditions, followed by a drop to a range compatible with measured drying rates. The temperature of the open side of the sheet is constant during drying, after its initial rise to the boiling point. The experimental results are compatible with pre- dictions of the model.     

HIGH-INTENSITY DRYING OF PAPER

High-intensity contact drying denotes drying under suf- ficiently intensive heating conditions that, following a brief warmup period, the mist paper web operates at internal tem- peratures in excess of the ambient boiling point. A simplified, two-zone analytical model is first presented. The paper is depicted as having a dry layer, of ever-increasing thickness, adjacent to the hot surface. Heat conduction through this layer (the rate-limiting step) causes evaporation at the interface with the “wet zone.” The vapor is then considered to flow through the wet zone into the ambient. Results of bench-scale experiments are discussed. Drying rates as much as twenty times conventional rates are indicated. The drying rate increases with hot surface/boiling point tem- perature difference and applied mechanical pressure. The instsn- taneous drying rate decreases continuously after a brief warmup period. The vapor pressure at the hot surfacelpaper interface rises quickly to a peak value, followed by a continuously- diminishing level. The heat flux shows a rapid rise to boiling- like conditions, followed by a drop to a range compatible with measured drying rates. The temperature of the open side of the sheet is constant during drying, after its initial rise to the boiling point. The experimental results are compatible with pre- dictions of the model.     

Drying mechanisms and stress development in aqueous alumina tape casting

Mass loss, shrinkage, Young’s modulus evolution and stress development of aqueous (alumina + latex) tape cast suspensions were observed during drying. Mass loss showed a constant drying rate period, followed by a falling rate period. Concurrently a linear shrinkage rate has been observed in the thickness direction, up to a drying point after which shrinkage abruptly stops. End of constant drying rate and end of shrinkage were not necessarily concomitant and depend on the latex/alumina ratio in the tape. A Young’s modulus value appears in the tape cast suspensions at the transition from liquid to solid like state. Then an increase of Young’s modulus is observed corresponding to latex film formation. The stresses generated by drying in the tape exhibited a first period of increase due to capillary pressure in the pores, a small relaxation immediately followed by a second stress increase due to latex film formation, and a stress plateau at the end of latex coalescence. Alumina powder granulometry and surface tension of the liquid had a preponderant influence on the first stress maximum whereas the properties of the latex and the drying conditions dominated the second stress maximum. By increasing latex proportion up to 25 wt.% on alumina basis, it was possible to make the first and the second stress rise concomitant.     

Controlling the morphology of ceramic and composite powders obtained via spray drying – A review

Spray drying is one of the most convenient methods for drying suspensions (slurries) and for granulation of materials. Spray dried powders have good flowability, narrow size distribution and controllable morphology. Morphology of powder particles (also called granules or microspheres) strongly affects the use and handling of powders. This review discusses the latest research on parameters that affect morphology and size of granules obtained by spray drying: atomization parameters, properties of sprayed slurry, mass transfer etc. The formation of hollow and dense granules is extensively reviewed. Granule size is affected by droplet size, slurry concentration and initial particle size. Morphology mostly depends on size distribution of initial ceramic particles, agglomeration tendency in the slurry and mechanical strength of the shell of a granule during the drying process compared to capillary force of the suspension liquid. Polymer additives (e.g. binders and lubricants) change the properties of granule shell and the evaporation of moisture; thus, polymer additives significantly affect morphology.     

Drying of Activated Sludge Under Partial Vacuum Conditions—An Experimental Study

A typical wastewater treatment system in a pulp and paper mill in Finland treats wastewater both mechanically and biologically. Sludges resulting from these processes have to be disposed of. One possible way of doing this is to incinerate them with solid fuel in the power plant of the mill. To minimize the amount of sludge and to make the use of the sludge energy efficient, it has to be dried before incineration. Mechanical drying of the sludge from biological wastewater treatment is difficult to carry out. Using secondary energies may provide a competitive way of arranging drying: a method of doing this by using partial vacuum evaporation to utilize the low temperature secondary heat in sludge drying is under development. A laboratory study to examine the behavior of activated sludge under partial vacuum evaporation conditions was carried out using a laboratory rotating evaporator to analyze the drying of activated sludge from three mills at 40-80°C boiling temperatures. This article presents the results from the tests. These are promising; it was possible to reach high dry solids content. Also, no boiling point temperature rise was detected, fouling of the evaporator seemed low and easily avoidable, condensate from the evaporator weas relatively clean and returnable to the processes of the mill.     

Aggregation Effects on the Compressive Flow Properties and Drying Behavior of Colloidal Silica Suspensions

The influence of aggregation phenomena on the compressive flow properties and drying behavior of nonaqueous and aqueous silica (SiO₂) suspensions of varying electrolyte (NH₄Cl) concentrations were studied. Compressive rheology measurements, including sedimentation and centrifugal consolidation, were first conducted to investigate consolidation behavior in the absence of solvent evaporation. The volume-fraction-dependent osmotic pressure and compressive yield stress were determined for dispersed and flocculated SiO₂ suspensions, respectively. Consolidation behavior then was studied in situ by simultaneously measuring stress evolution and solvent loss as a function of drying time. The observed drying stress histories of the films were complex, consisting of several characteristic regions. First, there was an initial period of stress rise to a maximum drying stress. These measured stress values exhibited good agreement with the osmotic pressure and compressive yield stress at equivalent SiO₂ volume fractions for the dispersed and flocculated systems, respectively. Beyond the maximum drying stress there was a subsequent region of stress decay, which coincided with the draining of liquid-filled pores. No residual drying stress was detected for films prepared from salt-free SiO₂ suspensions, whereas salt-containing films exhibited residual drying stresses likely due to salt-bridging effects. Microstructural characterization of dried films prepared from aqueous SiO₂ suspensions revealed nonuniformities in the spatial distribution of colloidal particles and precipitated salt, with the highest concentrations located at the outer edges of the films. Such features result from capillary-induced transport of these species during drying, and they have important implications on colloidal processing of ceramic thick films and bulk forms.     

a rview of: UNIFIED ANALYSIS AND SOLUTIONS OF HEAT AND MASS DIFFUSION

ABSTRACT

The objective of this study is the formulation of a finite element model that could be used to analyze the stress crack formation in a viscoelastic sphere resulting from temperature and moisture gradients during the drying process. Numerical solutions to the simultaneous moisture and heat diffusion equations describing moisture removal and heat intake process for the sphere are obtained. The distribution and gradients of temperature and moisture developed inside the sphere during drying are established. The calculated temperature and moisture gradients are used in a finite element analysis of the thermo-hydro viscoelastic boundary value problem to simulate the stresses in the body.

The model is used to solve a sample problem of drying a soybean kernel. The simulated drying curve for the soybean model is obtained and compared favorably with the experimental results reported in the literature. Tangential stress, as a criteria for failure, is shown to change from compressive to tensile stress as it approaches the surface. It reaches its peak value at the surface in one hour and then decays slowly. The effect of different drying conditions is studied and the results are discussed.     

Rheological Property and Stress Development during Drying of Tape-Cast Ceramic Layers

Rheological property and stress development of tape-cast ceramic layers derived from nonaqueous alumina (A1₂O₃)-poly(vinyl butyral) (PVB) suspensions were observed during drying. Casting suspensions exhibited strong shear-thinning behavior, with a low shear Newtonian plateau apparent viscosity >10² Pa^.s. The apparent suspension viscosity displayed a power-law dependence on the A1₂O₃ volume fraction during the initial stage of drying (<30% solvent loss). Stress development, measured by a cantilever deflection method, and parallel weight loss measurements were performed during the drying of tape-cast layers and pure binder coatings. Maximum drying stresses (σ_max) of 1.37-0.77 MPa were observed for plasticized tapes cast at gap heights of 150-400 μm. In contrast, nonplasticized tapes of similar thickness displayed a more gradual stress increase, with σ_max values approximately an order of magnitude higher than their plasticized counterparts. The stress histories of the corresponding binder coatings were quite similar to the tape-cast layers, albeit slightly lower σ _max values were observed. Stresses decayed beyond σ_max with a logarithmic time dependence to an almost constant value of 0.2-0.4 MPa for the plasticized tapes. Based on these observations, process methodologies have been offered to minimize stress development and retention in tape-cast ceramic layers     

Drying Characteristics In Superheated Steam Drying at Reduced Pressure

The superheated steam drying at reduced pressure is performed, and the effects of operational conditions such as drying pressure and temperature on the drying characteristics are examined. In order to obtain the basic guideline for the design of the superheated steam dryer at reduced pressure, the heat flux to sample was calculated and the optimal conditions were estimated.

After the sample temperature reached at the boiling point, the temperature was maintained at the boiling point and the drying rate became almost constant. Once the sample was dried out, the temperature suddenly increased up to the drying gas temperature. From the calculation of combined heat flux, the followings were found. The contribution of radiative heat transfer to the combined heat flux became larger as the drying pressure was lower. The combined heat flux had a maximum value against the drying pressure. The optimum drying pressure, which gave the maximum heat flux, became lower as the drying gas temperature decreased. It was found that reduction in the drying pressure is effective for the enhancement in drying performance.     

In Situ Determination of the Physical State of Biological Samples during Freeze Drying

The proceeding change of water content in a freeze-drying process, as well as the physical state of cell–sugar suspensions with Lactobacillus paracasei ssp. paracasei, was monitored by means of an in situ weighing system. Due to the pressure and temperature variations in the drying chamber, the weighing system was found to be influenced. This deviation of the weighing system was quantified by means of drying simulations with drying-inert Teflon dummies. This quantified deviation could be used for correction of the following freeze-drying processes. The applicability of this correction method was verified by drying model sugar solutions with known dry matter. Taking the quantified drift of the balance into account, the measured water contents for the model solution were proved to deviate less than 5% from the actual value. Finally, this verified correction method was applied to freeze drying of cell–sugar suspensions. As a result, the drying course of the cell–sugar samples could be depicted in the appropriate state diagram. Thus, the weighing system could be used to monitor the state transitions, which the bacterial suspension experienced during freeze drying, and qualified for specific investigations on the glassy state during freeze drying.     

Dimensionless Formulation of Convective Heat Transfer in Fry‐Drying of Sewage Sludge

Fry‐drying is an alternative for heat and mass transfer intensification. The process reuses waste oil as a heating medium for drying by contact with the wet sludge. At the end of the process, a stable derived fuel is obtained, a granular solid composed of the dried indigenous sewage solid and the impregnated oil. The fry‐dried sludge is storable and transportable without any pathogen elements. Knowledge about heat and mass transfer rates during the frying process is essential in order to assess the quality of the final product such as calorific value, oil uptake, porosity changes, etc. The heat transfer properties including transfer by free convection between the solid and the frying oil are fundamental for the process design and manufacturing of the fry‐dried product. The convective heat coefficient by temperature measurement and overall energy balance calculation is determined. The heat flux is calculated from the fry‐drying kinetics including moisture loss and oil intake kinetics. Various hydrodynamic regimes for convective heat transfer during the frying process are discussed (non‐boiling, boiling, and low‐boiling regime). A dimensionless formulation for estimating the convective transfer is proposed.     

FINITE ELEMENT SIMULATION OF THE THERMO-HYDRO STRESSES IN A VISCOELASTIC SPHERE DURING DRYING

The objective of this study is the formulation of a finite element model that could be used to analyze the stress crack formation in a viscoelastic sphere resulting from temperature and moisture gradients during the drying process. Numerical solutions to the simultaneous moisture and heat diffusion equations describing moisture removal and heat intake process for the sphere are obtained. The distribution and gradients of temperature and moisture developed inside the sphere during drying are established. The calculated temperature and moisture gradients are used in a finite element analysis of the thermo-hydro viscoelastic boundary value problem to simulate the stresses in the body.

The model is used to solve a sample problem of drying a soybean kernel. The simulated drying curve for the soybean model is obtained and compared favorably with the experimental results reported in the literature. Tangential stress, as a criteria for failure, is shown to change from compressive to tensile stress as it approaches the surface. It reaches its peak value at the surface in one hour and then decays slowly. The effect of different drying conditions is studied and the results are discussed.     

FINITE ELEMENT SIMULATION OF THE THERMO-HYDRO STRESSES IN A VISCOELASTIC SPHERE DURING DRYING

The objective of this study is the formulation of a finite element model that could be used to analyze the stress crack formation in a viscoelastic sphere resulting from temperature and moisture gradients during the drying process. Numerical solutions to the simultaneous moisture and heat diffusion equations describing moisture removal and heat intake process for the sphere are obtained. The distribution and gradients of temperature and moisture developed inside the sphere during drying are established. The calculated temperature and moisture gradients are used in a finite element analysis of the thermo-hydro viscoelastic boundary value problem to simulate the stresses in the body.

The model is used to solve a sample problem of drying a soybean kernel. The simulated drying curve for the soybean model is obtained and compared favorably with the experimental results reported in the literature. Tangential stress, as a criteria for failure, is shown to change from compressive to tensile stress as it approaches the surface. It reaches its peak value at the surface in one hour and then decays slowly. The effect of different drying conditions is studied and the results are discussed.     

Supercritical Fluid Drying of Aqueous 2-Propanol Suspension

Abstract

To prepare fine powders of oxides, supercritical fluid drying was applied. As the case of point, Y–Ba–Cu complex oxide was taken up. A suspension of Y–Ba–Cu complex oxide was synthesized by simultaneous hydrolyzation of 2-propanol solution of yttrium triisopropoxide–barium diisopropoxide–copper diisopropoxide. The supercritical fluid drying by carbon dioxide was confirmed to dry the suspensions, expecting no aggregation of particles throughout the drying procedure. First of all, solubilities of 2-propanol, water, and aqueous 2-propanol solution in high pressure carbon dioxide were measured. Judging from the observed solubilities, temperatures, and pressures in drying conditions were determined. The drying rates were measured and the dried powders were characterized by DTA, TGA, and SEM. The results of these experiments suggested that the supercritical fluid drying process is useful for drying of the suspensions of superconducting compound prepared by hydrolyzation of alkoxides.     

The role of boron-based additives in exterior wood coatings

Conclusion  Borates are clearly multi-functional coatings additives in the truest sense. Wood coatings can benefit from the biostatic, tannin stain resistance and flame retardant properties of borates. In addition, protection in storage is carried over into service and consequently borates can offer extremely cost effective solutions for the production of high performance coatings. Just one example, if zinc borate were being used for tannin stain resistance, it is unnecessary to use a separate dry film fungicide, in-can preservative or in-can corrosion inhibitor. Coupled with the fact that borates have diverse physical characteristics, in particular distinct aqueous solubilities and pH characteristics (anywhere between pH 4 and 11), it is possible to match the formulation environment and avoid incompatibility and stability problems. Other useful characteristics include their low effect on the optical properties of the dry paint film and, unlike certain organic additives, zinc borate will not alter the transmission properties of transparent coatings; so important at a time when semi-transparent woodstains are gaining popularity. Increasingly the coatings industry is being limited in the additives it can use; either because of specific product restrictions, a general trend to water-borne technology or because of rising raw material costs. It can be concluded from data reviewed here that the borates offer an environmentally non-harmful, safe and cost-effective alternative to the coatings formulator.     

Drying Characteristics In Superheated Steam Drying at Reduced Pressure

Abstract

The superheated steam drying at reduced pressure is performed, and the effects of operational conditions such as drying pressure and temperature on the drying characteristics are examined. In order to obtain the basic guideline for the design of the superheated steam dryer at reduced pressure, the heat flux to sample was calculated and the optimal conditions were estimated.

After the sample temperature reached at the boiling point, the temperature was maintained at the boiling point and the drying rate became almost constant. Once the sample was dried out, the temperature suddenly increased up to the drying gas temperature. From the calculation of combined heat flux, the followings were found. The contribution of radiative heat transfer to the combined heat flux became larger as the drying pressure was lower. The combined heat flux had a maximum value against the drying pressure. The optimum drying pressure, which gave the maximum heat flux, became lower as the drying gas temperature decreased. It was found that reduction in the drying pressure is effective for the enhancement in drying performance.     

Rheological Control of the Coffee Stain Effect for Inkjet Printing of Ceramics

The rheology of inkjet printing inks must be well controlled in order to be able to form small droplets. One solution is to use low volume fraction dispersed suspensions, but this leads to a common problem during drying called the coffee stain effect. It is caused by particle migration from the center to the edge of a drying drop and leads to nonuniform printed structures. This article describes an approach, to suppress the coffee stain effect by a sufficiently fast increase in viscosity after deposition. Due to the viscosity limitations during printing, inks with tailored rheology and drying behavior need to be developed. Ceramic inks were prepared and printed. First, a binder was added to study the influence of viscosity on printability and the coffee stain effect. Second, the use of a high vapor pressure solvent for faster drying was investigated. Eventually, an ink with the combination of binder and fast drying agent was prepared. This ink showed a considerable decrease in drying time as well as a rapid increase in viscosity after deposition and was suitable to completely suppress the coffee stain effect. Plateau‐like structures were achieved by adapting the drying temperature to permit particle movement to a certain degree.     

Drying of Activated Sludge Under Partial Vacuum Conditions—An Experimental Study

Abstract

A typical wastewater treatment system in a pulp and paper mill in Finland treats wastewater both mechanically and biologically. Sludges resulting from these processes have to be disposed of. One possible way of doing this is to incinerate them with solid fuel in the power plant of the mill. To minimize the amount of sludge and to make the use of the sludge energy efficient, it has to be dried before incineration. Mechanical drying of the sludge from biological wastewater treatment is difficult to carry out. Using secondary energies may provide a competitive way of arranging drying: a method of doing this by using partial vacuum evaporation to utilize the low temperature secondary heat in sludge drying is under development. A laboratory study to examine the behavior of activated sludge under partial vacuum evaporation conditions was carried out using a laboratory rotating evaporator to analyze the drying of activated sludge from three mills at 40–80°C boiling temperatures. This article presents the results from the tests. These are promising; it was possible to reach high dry solids content. Also, no boiling point temperature rise was detected, fouling of the evaporator seemed low and easily avoidable, condensate from the evaporator weas relatively clean and returnable to the processes of the mill.     

Problems Associated With Spray Drying Of Sugar-Rich Foods

ABSTRACT

Stickiness is a major reason that limits the spray drying of various sugar-rich food products.Higher hygrmopicity of amorphous powder, increase in solubility of sugars with temperature, and lower melting point and glass transition temperature, contribute to the aickiness problem. So far, the glass transition temperature has been widely accepted as a ben indicator for stickiness. There are various manawm that have been applied to spray dry such product. Some of them are the addition of drying aids, modilication of drier design and use of mild drying temperature conditions. This review paper highlights the major research works that deal with the stickiness property of sugar-rich foods.