EFFECT OF STEPWISE CHANGE OF DRYING RATE ON MICROSTRUCTURE EVOLUTION IN POLYMER FILMS

The effect of the drying rate of the solvent on microstructure evolution in thin films of immiscible polystyrene/polycarbonate blends was investigated by applying a stepwise change in air velocity. The polymer blend was dissolved in tetrahydrofuran and cast on a glass substrate to form a thin liquid film. The drying rate of the solvent was measured by a gravimetric technique, and the phase-separated structure in the film was observed by optical microscopy. The results indicated that cellular structures of droplet arrays were formed in the phases evolved primarily during the early stage of drying, and the droplets then coalesced in the intermediate stage to form bigger droplets. The size of the first evolved phase decreased with increasing time at a high drying rate, whereas larger droplets were obtained with longer high drying rate regimes. These results suggested that the droplets were induced by secondary phase separation which was strongly affected by the drying rate of the solvent.     

Observation of Phase Transition from Dissolved Organic Semiconducting Materials to Solid Film during Drying

We studied the formation dynamics of low-molecular-weight organic semiconducting molecules of N,N′-bis[4-[bis(3-methylphenyl)amino]phenyl]-N,N′-diphenylbenzidine (DNTPD) during solvent evaporation. We dropped the DNTPD–toluene solution on a glass substrate and performed in situ measurements of mass, temperature, scattered light intensity, and photoluminescence (PL). We found that both light scattering and PL measurements were able to detect the onset of solid film formation from dissolved organic semiconducting materials. When a solid thin film forms during drying, sudden changes in scattered light and PL are observed. Furthermore, we also found that a period of time after the onset was necessary for completion of thin solid film formation. Observations and in situ PL measurement during thin-film formation by spin coating revealed that this period of time affected the optical properties of the film. This result indicates that quantitative information on the film formation process is quite important to obtain thin films with desired properties by coating and drying. Our in situ measurements were simple and practical approaches to monitor the formation dynamics of organic thin films during drying.     

聚酰胺酸溶液成膜过程中的传质行为

为了研究聚酰胺酸溶液成膜过程中的传质行为,采用自制液膜干燥实验装置在线测定聚酰胺酸溶液质量的变化.假设气、液两相传质通量相等的条件下,计算了溶剂气相传质系数和液面蒸气压.在此基础上考察了干燥温度、液膜厚度、溶液相对分子质量对液膜表面蒸气压的影响.结果表明,成膜过程中存在溶剂蒸发与聚酰胺酸溶液亚胺化反应的相互竞争.干燥初期溶剂蒸气压迅速升高,液膜表面溶剂的扩散为控制步骤;而干燥后期溶剂蒸气压较小,溶剂在膜内部扩散成为控制步骤.同时随着液膜厚度的增加、干燥温度的升高及溶液相对分子质量的减少,液面蒸气压的最大值呈现增大趋势.     

基于硝酸铵气孔诱导剂的多孔TiO2薄膜的制备

采用在TiO2溶胶中加入硝酸铵的方法并通过浸渍-提拉技术制备了气孔修饰的TiO2薄膜。气孔的形成与溶胶液膜中硝酸铵诱导的分相现象相关,硝酸铵一方面抑制了干燥过程中凝结水分的挥发,另一方面增大了水相的表面张力,促使了水以球形水珠的形态分散在湿凝胶的表面,水珠挥发后形成气孔。所制备的气孔修饰的TiO2薄膜表现了超亲水性能。随着硝酸铵用量的增加,薄膜表面气孔的分布密度增大,可见光的透过率减小。增加涂膜次数可以获得多层次网络气孔结构的表面形态,多层次气孔结构的多次散射有利于提高可见光的透过性能。     

Experimental modeling of solvent-casting thin polymer films

An apparatus was designed and assembled to study the solvent removal from solution-cast thin polymer films. The computer interfacing of a thermogravimetric analyzer, spectrophotometer, electronic flowmeters, and control valves for the apparatus enabled the preprogramming of the carrier gas velocity, carrier gas solvent content, and temperature profiles to simulate the environment experienced in large parallel flow industrial driers. The apparatus has also been designed and operated to enable the visual observation of the drying film with an optical microscope. Initial experimental studies conducted with the apparatus involved the effect of temperature on solvent removal. The results indicate that high dryer gas temperatures can apparently cause skinning of the film surface resulting in slower solvent removal rates. The skin formation can be suppressed by higher solvent concentration in the carrier gas. The visual observations revealed the formation of standing waves in the film surface during drying at high gas velocities (>2OO cm/min). The wave formation at least partially overcomes the effect of skinning by increasing the surface area of the film, and may be the manifestation of flow instabilities involving circulation within the film.     

TEE EFFECT OF EXTENDED THIN FILM EVAPORATION DURING TEE CONSTANT DRYING RATE PERIOD

The extended thin film evaporation is analyzed during the constant drying rate period. The extended thin film is defined as the liquid film which the disjoining pressure dominates the fluid flow field and works as the driving force of replenishing the evaporating liquid. The results of the analysis show that the extended thin film evaporation can compensate the reduction of evaporation rate due to the increase of dry spots and keep the evaporation rate nearly constant. Experimental data in the literature are in good agreement with the results of this work. This implies that the extended thin film evaporation may be an important part of the mechanism for drying.     

TEE EFFECT OF EXTENDED THIN FILM EVAPORATION DURING TEE CONSTANT DRYING RATE PERIOD

ABSTRACT

The extended thin film evaporation is analyzed during the constant drying rate period. The extended thin film is defined as the liquid film which the disjoining pressure dominates the fluid flow field and works as the driving force of replenishing the evaporating liquid. The results of the analysis show that the extended thin film evaporation can compensate the reduction of evaporation rate due to the increase of dry spots and keep the evaporation rate nearly constant. Experimental data in the literature are in good agreement with the results of this work. This implies that the extended thin film evaporation may be an important part of the mechanism for drying.     

Microwave-Vacuum Drying Kinetics of Pharmaceutical Powders

Abstract

The drying kinetics (maximum drying rate, drying constants) and center temperature of selected powder (aspirin, paracetamol, lactose, and maize starch)–solvent (water, ethanol, methanol, and acetone) systems were monitored during microwave-vacuum drying. An experimental microwave-vacuum system (650 W and 2.45 GHz) operated at 61–81 kPa was used. The drying rate profile did not vary with the powder–solvent system; an initial warming-up period was followed by a constant-rate stage and two falling-rate periods. However, the drying kinetics were found to be both powder and solvent dependent, with the drying times for acetone-, ethanol-, and methanol-wetted materials being considerably shorter (up to 89.8%) than those of samples containing water. Although the general form of the temperature profile (short warming-up period, constant-rate stage, and decreasing temperature phase) was similar for all systems, the maximum temperatures varied quite significantly with solvent type, ranging from highest to lowest in the order water-ethanol-methanol-acetone. For most powder–solvent systems, reduced operating pressure facilitated increased drying rates and thus shorter drying times.     

THE USE OF A GAS CHROMATOGRAPH TO OBTAIN DRYING RATES DATA THROUGH SIMPLE LABORATORY EXPERIMENTS

A method making use of a gas chromatograph was investigated to acquire bound solvent drying rates. The solid to be dried was used as stationary phase in a chromatographic column, and the TCD detector was used to follow the decrease of solvent content of the effluent as inert gas was flown through the column. The measurement of the break-through curve allows the evolution of drying rate with time or solvent content to be obtained. The comparison of the experimental results with numerical simulation leads to the determination, through parametric adjustment, of the internal mass-transfer coefficient as a function of the solvent content during the second stage of bound solvent drying period. This parameter can lead to the value of the diffusivity as a function of solvent content.     

TEE EFFECIS OF EXTENDED THIN FILM EVAPORATION AND EXPWNAL DIFFUSION RESISTANCE DURING THE CONSTANT DRYING RATE PERIOD

Extended thin film evaporation with external diffusion resistance is analyzed for the constant rate period of the drying process, in which a polar liquid evaporates from porous bodies made of glass. The extended thin film is defined as the Liquid film in which the disjoining pressure dominates the fluid flar field and works as the driving force replenishing the evaporating Liquid. The results of the analysis shows that due to the existence of the evaporating thin Liquid film, the evaporation fran the extended thin film can compensate the reduction of evaporation rate caused by the increase of the dry spots and keep the drying rate the same as or even greater than that of the completely wetted surface. The external diffusion resistance makes the vapor concentration near the porous solid surface remain constant and therefore keeps     

THE USE OF A GAS CHROMATOGRAPH TO OBTAIN DRYING RATES DATA THROUGH SIMPLE LABORATORY EXPERIMENTS

Abstract

A method making use of a gas chromatograph was investigated to acquire bound solvent drying rates. The solid to be dried was used as stationary phase in a chromatographic column, and the TCD detector was used to follow the decrease of solvent content of the effluent as inert gas was flown through the column. The measurement of the break-through curve allows the evolution of drying rate with time or solvent content to be obtained. The comparison of the experimental results with numerical simulation leads to the determination, through parametric adjustment, of the internal mass-transfer coefficient as a function of the solvent content during the second stage of bound solvent drying period. This parameter can lead to the value of the diffusivity as a function of solvent content.     

A model for gel drying

A theoretical model for drying of a thin gel film is presented. The model is based upon the premise that as solvent is removed from any portion of a gel structure which is permeable by the solvent, the structure shrinks locally to fill the voids left by the solvent. The diffusion coefficient of solvent through the gel film is assumed to be an exponential function of concentration and temperature. The governing equations for the model indicate that for nonisothermal drying, the results of drying and shrinkage rates are functions of 13 independent dimensionless system variables. These results are obtained with the help of a computer solution of the proposed model. The computer results indicate that, except under extreme temperature conditions, the drying and shrinkage rates are most influenced by dimensionless groups M, P, and P?, defined by eq. (9) of the paper. Furthermore, the drying and shrinkage rates are essentially independent of groups M and P for the values of M and P greater than approximately 100 and 10, respectively. The effect of variable solvent diffusivity on approximate time to achieve the steady-state drying and shrinkage rates is approximately handled by defining a dimensionless time variable τ in terms of average solvent diffusivity. Finally, some experimental data on drying and shrinkage rates of isothermal drying of lyphogel film under natural convection condition are obtained. These data are found to be in qualitative agreement with similar computer predictions by the proposed model.     

TEE EFFECIS OF EXTENDED THIN FILM EVAPORATION AND EXPWNAL DIFFUSION RESISTANCE DURING THE CONSTANT DRYING RATE PERIOD

ABSTRACT

Extended thin film evaporation with external diffusion resistance is analyzed for the constant rate period of the drying process, in which a polar liquid evaporates from porous bodies made of glass. The extended thin film is defined as the Liquid film in which the disjoining pressure dominates the fluid flar field and works as the driving force replenishing the evaporating Liquid. The results of the analysis shows that due to the existence of the evaporating thin Liquid film, the evaporation fran the extended thin film can compensate the reduction of evaporation rate caused by the increase of the dry spots and keep the drying rate the same as or even greater than that of the completely wetted surface. The external diffusion resistance makes the vapor concentration near the porous solid surface remain constant and therefore keeps     

Atomic force microscopy characterization of ultrathin polystyrene films formed by admicellar polymerization on silica disks

Atomic force microscopy was used to study the characteristics of polymer films formed via admicellar polymerization (the polymerization of monomers solubilized in adsorbed surfactant aggregates). The investigated system included cetyltrimethylammonium bromide (C₁₆TAB) as a cationic surfactant, styrene, 2,2′‐azobisisobutyrilnitrile as an initiator, and polished silica disk substrates. Our goal was to examine changes in the properties and morphology of the formed polymer films due to changes in the surfactant and monomer feed levels. Normal tapping and phase‐contrast modes in air were used to image the nanoscopic and microscopic morphologies of the polystyrene‐modified silica. The root‐mean‐square roughness of the surface before and after modification was statistically analyzed and compared. The images were captured with loading‐force set‐point ratios of 0.2–0.9, and this allowed us to examine the stability of the polystyrene films. In the first series, for which the feed ratio of C₁₆TAB to styrene was kept constant and the total feed concentration was varied, a uniform layer of a polystyrene film was observed along with some nanometer‐size aggregates at high feed concentrations of both C₁₆TAB and styrene. These droplets eventually agglomerated with the film beneath and formed larger macrodroplets in a ring arrangement. At lower concentrations, droplets and holes were observed that eventually agglomerated to form a bicontinuous thin film. In the second experimental series, the concentration of C₁₆TAB was kept constant, and the feed ratio of C₁₆TAB to styrene was varied. A smooth thin film was observed at high concentrations of styrene. This film could be deformed and/or removed to expose the silica surface beneath. At lower styrene loadings, the polystyrene film became unstable and formed dropletlike aggregates, possibly because of either the uneven adsolubilization of the styrene monomer within the admicelle or the dewetting effect during washing and drying. The structure of the polystyrene film formed on a smooth silica disk was very dependent on the amount of the surfactant fed to the system; this contrasted with the results on precipitated silica. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 36–46, 2003     

热塑性聚酰亚胺微电子薄膜的制备

以微电子业所急需的聚酰亚胺薄膜为背景,采用一种热塑性聚酰亚胺树脂(TPI),实验测定了聚合物溶液特性、干燥工艺及热拉伸性能。在化学环化过程中聚合物溶液粘度随时间逐步增大;15 h后粘度和重均相对分子质量及分布趋于稳定。薄膜溶剂含量在干燥初期急剧下降,干燥速率随干燥温度升高而增大。TPI树脂表现出良好的热塑拉伸性能,当温度高于其玻璃化温度时,最大拉伸比随升温速率降低而增大,而随拉伸载荷增加呈现出先增后降。TPI薄膜经拉伸处理后其力学性能得到明显提高,综合性能与日本钟渊TP E薄膜相当。     

SIMULATION OF SPRAY DRYING OF A SOLUTION ATOMIZED IN A PULSATING FLOW

Spray drying of NaCl solution was carried out under an intense oscillating flow field generated by a pulse combustor. A pulse combustion spray drying system was constructed. An optical analyzer was used to measure the particle diameter distribution of droplets atomized by a pulsating flow. The momentum, heat and mass transfer in both gaseous and particulate phases during spray drying inside the drying chamber were simulated using the computational fluid dynamics method. The simulated profiles of flow field, temperature and humidity of the gaseous phase, as well as the particulate phase, in the drying chamber were presented. The simulation showed changes of the flow field and particle trajectories in the drying chamber during one pulsating period. A large-scale vortex was observed in the upper part of the drying chamber because of the unstable state of flow field and particle trajectories. Short drying time and large evaporation rate are characteristics of pulsating spray drying. The influence of gas stream pulsation frequency on the drying process is also analyzed.     

CONDUCTIVE DRYlNG CHARACTERISTICS OF GELATINIZED RICE STARCH

The main objective of the experiment in thispaper is to investigate the influence of the ratio of the thickness of the plate and that of the film, and initial condi- tions on the conductive drying characteristics of thin films of gelatinized rice starch on a hot plate under constant heat and mass transfer to the surroundings. The plate was. supported on one arm of a sensitive balance which measured and recorded the weight loss due to drying. Tbennocouples situated at the upper and lower surfaces of the plate recorded the respective temperatures. The gelatinized starch was prepared by cooking a slurry of rice starchin a heated and agitated vessel at slightly above the gelatinization temperatureof the starch 70C. The film was applied after the plate was heated to its initial temperature which ranged from 70C to 119C. The weight 10s and the temperature of the upper and lower surfaces of the plate were recorded. The m3isture content of the dried sample was determined by measuring the weight loss after heating it in an oven at 90C for 24 hours.The dimensionless average moisture content was fitted to the drying rate eauation of the form The dimensionless temperature of tbe filmplate interface data was fitted to the following equation It was found that the critlcal moisture content Xc increases when both the intial temperature level and the ratio of the thickness of the plate to that of the film are raised. The dimensionless avenge moisture content profiles against dimensionless time and the dimensionless drying rate ploti against the dimensionless moisture content are not affected at all by this ratio. There was an initial cooling period fol lowed by a continuous rue in temperature. The dimensionless temperature is independent of the ratio of the thickness of rbe plate to that of the film. The dimen     

CONDUCTIVE DRYlNG CHARACTERISTICS OF GELATINIZED RICE STARCH

The main objective of the experiment in thispaper is to investigate the influence of the ratio of the thickness of the plate and that of the film, and initial condi- tions on the conductive drying characteristics of thin films of gelatinized rice starch on a hot plate under constant heat and mass transfer to the surroundings. The plate was. supported on one arm of a sensitive balance which measured and recorded the weight loss due to drying. Tbennocouples situated at the upper and lower surfaces of the plate recorded the respective temperatures. The gelatinized starch was prepared by cooking a slurry of rice starchin a heated and agitated vessel at slightly above the gelatinization temperatureof the starch 70C. The film was applied after the plate was heated to its initial temperature which ranged from 70C to 119C. The weight 10s and the temperature of the upper and lower surfaces of the plate were recorded. The m3isture content of the dried sample was determined by measuring the weight loss after heating it in an oven at 90C for 24 hours.The dimensionless average moisture content was fitted to the drying rate eauation of the form The dimensionless temperature of tbe filmplate interface data was fitted to the following equation It was found that the critlcal moisture content Xc increases when both the intial temperature level and the ratio of the thickness of the plate to that of the film are raised. The dimensionless avenge moisture content profiles against dimensionless time and the dimensionless drying rate ploti against the dimensionless moisture content are not affected at all by this ratio. There was an initial cooling period fol lowed by a continuous rue in temperature. The dimensionless temperature is independent of the ratio of the thickness of rbe plate to that of the film. The dimen     

Experimental investigation of the morphology of salt deposits from drying sessile droplets by white‐light interferometry

This study presents a new approach to investigate the drying behavior and the structure of deposit resulting from drying of solid containing micro droplets. It is shown that deposit structure (porosity and “footprint”) depends on drying conditions. This dependency may contribute to better understanding of particle‐forming processes, such as fluidized bed coating. In the framework of this study, sessile droplets containing sodium benzoate dissolved in water were dried on thin glass plates in a small drying chamber. The drying conditions (temperature, moisture content and flow rate of drying gas) and material parameters (solid content of solution) were systematically varied. The drying rate of droplets was determined from the moisture balance of the drying gas. The final three‐dimensional shape of dried sessile droplets was measured using white‐light interferometry and transformed into a two‐dimensional profile using a Monte Carlo method. Moreover the mean porosity of dried droplets was calculated. By comparison of structural information and process conditions it is shown that the drying process may have a large influence on deposit structure. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2002–2016, 2018     

Film formation of monodispersed polystyrene latex at high temperature

This report discusses the drying behavior of monodispersed polystyrene latex at elevating temperature with particular attention to the relationship between water evaporation rate and morphological evolution during the film formation process. At the first stage, water evaporation rate was less influenced by the skin film formed at the latex/air interface, which was consistent with Croll's model. During this stage, a drying front advanced from the top film toward the bulk dispersion. At the final stage of film formation, the water evaporation rate was less than that of the initial stage, and another drying front developed from the interior region outside the system. Two distinct boundaries corresponding to the opposite directions of the second drying front between completely dried region and wet region were found if the film was peeled off the container surface. Besides, some particular morphologies were found in the completely dried region, which was likely related to preferable coalescence among the particles induced by capillary force because of water evaporation. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1835–1840, 2001