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.     

聚氟乙烯薄膜后处理工艺研究

考察了聚氟乙烯（PVF）薄膜干燥过程中溶剂初始含量、干燥温度对干燥前膜中溶剂含量和PVF膜的力学性能的影响。结果表明,干燥温度升高和溶剂初始含量减少会导致薄膜力学性能下降。当干燥温度为100℃时,采用配方4成型的PVF薄膜力学性能最佳。     

PVB流延膜干燥工艺研究

采用溶液流延法制备了一系列聚乙烯醇缩丁醛(PVB)膜材料,以热板干燥方式进行干燥,考察了干燥温度、初始料层厚度、溶液浓度和填充粒子含量对PVB流延膜干燥速率的影响。结果表明,这4种因素对流延膜的干燥速率均有重要影响,其中,较高的干燥温度、较薄的初始料层厚度以及较低的溶液浓度均有利于干燥速率的提高。     

喷雾冲击条件下液膜流动与传热模型

针对现有喷雾冷却计算模型的不足,以质量、动量、能量守恒方程为基础,建立喷雾冷却非沸腾区的液膜流动与传热方程,并采用数量级分析的方法对方程简化,最后运用数值方法对模型进行求解。给定液滴速度及液体温度,由模型计算液膜厚度、平均热通量与液体流出温度,与实验测试结果对照。结果显示,液膜厚度的计算结果与实验结果相差6%以内;平均热通量和液体离开待冷却表面的最终温度计算结果与实验结果相差10%以内,且超过60%的计算结果偏差小于5%。计算结果与实验结果的高度匹配证明该模型可较好地反映喷雾冷却过程的流动与换热。由模型可以获取不同位置处液膜厚度与温度,从而加深对喷雾冷却传热机理的理解。     

喷雾冲击条件下液膜流动与传热模型

针对现有喷雾冷却计算模型的不足,以质量、动量、能量守恒方程为基础,建立喷雾冷却非沸腾区的液膜流动与传热方程,并采用数量级分析的方法对方程简化,最后运用数值方法对模型进行求解。给定液滴速度及液体温度,由模型计算液膜厚度、平均热通量与液体流出温度,与实验测试结果对照。结果显示,液膜厚度的计算结果与实验结果相差6%以内;平均热通量和液体离开待冷却表面的最终温度计算结果与实验结果相差10%以内,且超过60%的计算结果偏差小于5%。计算结果与实验结果的高度匹配证明该模型可较好地反映喷雾冷却过程的流动与换热。由模型可以获取不同位置处液膜厚度与温度,从而加深对喷雾冷却传热机理的理解。     

流延法制备聚酰亚胺薄膜工艺研究

以均苯四甲酸二酐、4，4＇-二胺基二苯醚为原料，通过缩聚反应制备了聚酰胺酸，采用凝胶渗透色谱（GPC）考察了反应时间对产物重均摩尔质量的影响；测定了干燥过程中聚酰胺酸凝胶膜溶剂含量的变化；并利用电子万能试验机研究了聚酰胺酸薄膜的拉伸工艺。结果表明：聚酰胺酸的适宜反应时间为6h，此时其重均摩尔质量趋于稳定。随着干燥温度的升高和时间的延长，凝胶膜的溶剂含量逐渐减少。分析得到较好的拉伸条件是干燥温度为130℃，溶剂质量分数为30％左右的凝胶膜。聚酰亚胺薄膜经拉伸后，其拉伸强度和模量均随拉伸比的增大而增加。     

微流体技术应用于渗透汽化分离正丁醇-水体系的研究

将膜分离技术与微流体技术的优势相结合,开发设计了新型膜微反应器,该设备比表面积大且扩散距离短,可以强化渗透汽化过程。以正丁醇-水体系的分离效果为实验模型,分析了膜微反应器的渗透汽化性能。在不同料液温度、料液水含量及料液流速条件下,对膜微反应器中正丁醇-水体系的分离性能进行了系统的研究。当料液温度升高时,渗透通量增大。当进口料液水含量升高时,渗透通量明显增大。当流速升高时,渗透通量增大且趋势逐渐变缓。在实验研究的基础上,建立了渗透汽化传质模型,模型计算值与实验值吻合良好。     

蒸汽在过冷液体喷雾上直接接触相变换热过程实验研究

以水为工质,通过实验研究了饱和水蒸汽与过冷水喷雾逆流直接接触冷凝换热过程,考察了不同入口液相温度下液膜厚度及破碎长度变化、液膜轴向及径向的温度分布;基于实验数据计算出了液膜局部传热系数及总传热系数。实验研究的结果表明,直接接触冷凝换热过程中,低入口液相温度时的液膜厚度和破碎长度更大;液膜在径向方向上存在温度梯度变化,液膜表面的温度较高,中心存在1个最低温度;随着液膜运动轴向距离的增大,液膜温度逐渐升高,喷嘴出口处液膜的温升最快,在整个喷雾的冷凝换热过程中,液膜温升占喷雾换热总温升的80%～85%,因此相比液滴,液膜起主要换热作用;喷嘴出口处的局部传热系数最大,并随着轴向距离增大逐渐减小。实验得到总传热系数的值远大于传统的膜状冷凝传热系数,体现了蒸汽-过冷液体喷雾这类直接接触换热方式的优势。     

高性能聚砜中空纤维纳滤膜基膜的研制

采用干-湿法工艺制备聚砜基膜,研究了聚砜铸膜液中聚砜含量、添加剂种类、溶剂组成及含量和凝固浴温度对膜性能的影响。研究结果表明,铸膜液中聚砜含量和添加剂含量对膜性能影响较大;使用混合溶剂可有效改善膜的孔结构和提高聚砜基膜的性能;控制凝胶浴温度可以得到较高性能的基膜。通过选择最佳铸膜液组成及工艺条件,可以制备较高性能的基膜。     

膜蒸馏处理苯酚废水过程中苯酚传质模型的建立

采用质量守恒法建立了苯酚的传质模型,对膜蒸馏处理苯酚废水过程中苯酚含量进行了模拟。通过膜蒸馏实验,研究了不同温度和进料含量条件下进料侧和渗透侧苯酚含量变化规律,并进行了模型验证。结果表明,该模型可以很好的描述苯酚的传质过程:随温度升高,分子运动加快,苯酚质量损失系数增大,跨膜传质系数增大,加速了传质驱动力,苯酚在渗透侧含量升高。随着苯酚初始含量的增加,膜两侧含量差增大,加快了苯酚的传质过程。     

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     

The origin of residual internal stress in solvent-cast thermoplastic coatings

Experiments on polystyrene and poly(isobutyl methacrylate) coatings cast from toluene have shown that residual internal stress is independent of dried coating thickness and initial solution concentration. A theory is presented that predicts the magnitude of the stress and shows that it has no dependence on thickness or initial solution concentration. Internal strain is calculated from the volume of solvent lost after the coating has solidified. This solidification point is identified with the solvent concentration that is sufficient to depress the glass transition of the polymer to the prevailing experimental temperature. A one-to-one correlation is confirmed between the volume of solvent lost from solution during drying and the volume change of the polymer film.     

Operating window for the peeling of polyimide film

The peeling behavior of polyimide film coated on steel substrates was experimentally investigated and compared with existing models. An operating window for peeling, which is defined as a closed domain for steady and defect‐free peeling, is presented in terms of peeling force versus residual solvent content. The window is bounded by two major defects: the film becomes too brittle for peeling at high peeling force, and stick‐slip striation defect appears at low peeling force. There exists a critical residual solvent content below which the adhesion between the polyimide film and the substrate is too strong and then peeling is impossible. Existing models for predicting steady peeling and the onset of peeling defects have been modified and applied to setup the boundaries of the operating window. There also exists another operating window for drying of polyimide and is presented in the form of drying temperature versus film thickness. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Guidelines for dryer design based on results from non‐Fickian model

In polymer solution coatings below the glass transition temperature of the pure polymer, the coating can go undergo a glass transition and develop stresses during drying. When stresses develop, a non‐Fickian model accurately describes solvent mass transport in drying polymer coatings. The non‐Fickian model includes the solvent transport due to both stress and concentration gradients. This article presents a non‐Fickian model, which predicts a lower residual solvent than does the corresponding Fickian model. We showed in an earlier article that the non‐Fickian model predicts trapping skinning (higher residual solvent under more intense operating conditions) at higher drying gas‐flow rates. In this article, the non‐Fickian model was used to investigate how the gas‐flow rate, dry film thickness, and substrate thickness affect the residual solvent for a single‐zone dryer. This work recommends guidelines for choosing gas‐flow rates, gas temperatures, and substrate thickness to minimize the residual solvent. The model predictions show that, at any gas temperature, the residual solvent is minimum at an intermediate gas‐flow rate. The trapping skinning effect is less evident in thicker coatings and substrates. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 477–486, 2003     

Self‐Expansion Behavior of Silk Fibroin Film

Summary: Silk fibroin cast film was prepared using a ternary solvent system of CaCl₂/CH₃CH₂OH/H₂O (1/2/8 in mole ratio). A drying temperature at casting influenced crystal structure of fibroin. When a drying temperature was set lower than 9 °C, the cast film became amorphous. When a drying temperature was set higher than 40 °C, a fibroin film of silk‐II structure was obtained. In order to produce a fibroin film of silk‐I structure, a preferable temperature range was from 20 to 26 °C. The crystal transformation from random coil structure into silk‐I could be made through exposure of an amorphous film to water vapor. As for the crystal transformation from silk‐I into silk‐II, the treatment with a glycerin solution was effective. In the course of the treatment a film showed self‐thinning and self‐expanding. The expansion ratio exceeded 40% at maximum. The film produced accompanying self‐expansion was ductile in nature.

The apparent self‐expansion percentage as a function of initial thickness of the film. The ductility of the film was classified into four stages from the observation of recovery behavior after folding: ?, very soft; ?, soft; ?, middle; ?, hard (see Figure 5 ).     

Effect of thermal history during drying and curing process on the chain orientation of rod-shaped polyimide

Chain orientation in polyimide (PI) film is influenced by the thermal history during drying and curing process. The amount of residual solvent and the degree of imidization, among other factors, play a major role in determining the chain orientation during the process. In the present study, poly(amic acid), the precursor of PI, coated on the glass substrate was imidized to PI through different drying and curing protocols. On the way of complete imidization, the residual solvent concentration and the degree of imidization were characterized using confocal Raman spectroscopy. The poly(amic acid) began to imidize quickly while retaining more solvent in the film as the initial drying temperature increased. The degree of in-plane chain orientation in fully imidized PI film made by different process protocols was compared using polarized Raman spectroscopy. The fully imidized PI showed the lowest degree of in-plane chain orientation when it was processed by the protocol with the highest drying temperature. The difference in the degree of in-plane chain orientation among different PI films significantly influenced the in-plane thermal expansion coefficient, while no significant change in crystallinity or glass transition temperature was observed.     

纳米氧化锌对天然胶乳膜干燥及硫化性能影响研究

采用失重法和溶胀法研究了普通氧化锌和纳米氧化锌活化的天然胶乳膜的干燥动力学,以及干燥过程中胶乳膜交联密度变化.结果表明:天然胶乳厚胶膜在成膜后仍含有大量水分并且去除困难.纳米氧化锌活化胶膜的干燥速率明显高于普通氧化锌胶膜.干燥温度对干燥过程影响显著,干燥时间随干燥温度升高明显降低.随着干燥的进行,2种氧化锌活化的胶乳膜的交联密度均迅速上升,在相同条件下,纳米氧化锌活化胶乳膜交联密度均比普通氧化锌的大.纳米氧化锌对天然胶乳膜的干燥及硫化过程均有促进作用.     

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.