The leveling of thermosetting waterborne coatings. Part IV: Effects of film thickness

The influence of film thickness on the leveling of a thermosetting waterborne coating (WBC) was examined. The theory of Patton has been revised to give the proper force balance and modified to include the changes in surface tension and viscosity during leveling by the evaporation of volatile components. However, the predicted leveling process by a numerical method did not coincide with the observed results. Thus, the driving force in the theory, the hydrostatic pressure gradient in the film, was considered to be insufficient to describe the leveling process of the WBC. The hydrostatic pressure gradient can be a component of the driving force to promote leveling, but the existence of an additional component is suggested.     

The leveling of thermosetting waterborne coatings. Part III: Leveling under controlled conditions

Experiments on coated films of water borne, coatings (WBC) under controlled conditions (i.e., 25°C, 65% RH) were carried out to study the mechanism of leveling. Data in a companion paper imply that the development of the surface tension gradients plays an important role in promoting leveling. The stress due to surface tension gradients is found to be much greater than the stress introduced by hydrostatic pressure gradients in the film caused by surface tension and the free boundary curvature of the film. This reconfirms that the use of a co-solvent, which evaporates faster than water, greatly accelerates the leveling of a WBC.     

POOL BOILING OF BINARY MIXTURES

Nucleate pool boiling heat transfer coefficients for pure water, pure n-propanol and eight of their mixtures including the azeotrope were measured by using a horizontal electrically heated stainless steel tube of 3.35 mm O.D. and 9.8 cm long as heating element. This binary system was chosen for the reason that it is suitable for simultaneous investigation of the mass diffusion effect (the F effect), the effect of slowing down in bubble growth rate caused by the exhaustion of the volatile component near the vapor-liquid interface, the Marangoni effect (the M effect), the effect of surface tension gradient caused by the evaporation of the component of lower surface tension, and the dynamic surface effect (the Y effect), the effect of surface tension gradient caused by the stretching of vapor-liquid interface, on the boiling heat transfer rate in mixture. It is concluded that the mass diffusion effect can qualitatively account for the experimental results of the system. The discrepancy between the experimental data and the theoretical prediction by considering the F effect is due to taking no account of the intercorrelated effects of M and Y.

The effect of interfacial properties such as surface tension and contact angle on boiling heat transfer rate and the incipience of boiling were also evaluated.     

APPLICATION OF GELATIN FOR ENCAPSULATING ASPIRIN INTO ETHYL CELLULOSE MICROCAPSULE IN AN O/W EMULSION

The application of gelatin protective colloid for microencapsulating aspirin in ethylcellulose was demonstrated using an oil-in-water emulsification/solvent evaporation technique. The gelatin concentration, alcohol co-solvent amount, and ethylcellulose viscosity were investigated by analyzing the recovered weight, particle size distribution, drug loading efficiency, aspirin release rate, surface characteristics, and release kinetics. Results showed that recovery increased with greater concentrations of gelatin (up to 1%). Adding co-solvent (ethanol) also changed the microcapsule particle size distribution. Higher recovery and release rates were obtained when the ethanol content in polymer solvent was at 25% and the viscosity of ethylcellulose was low (45cps). The release rate followed Higuchi matrix release kinetics, suggesting a monolithic system with aspirin uniformly distributed over the microcapsule.     

APPLICATION OF GELATIN FOR ENCAPSULATING ASPIRIN INTO ETHYL CELLULOSE MICROCAPSULE IN AN O/W EMULSION

The application of gelatin protective colloid for microencapsulating aspirin in ethylcellulose was demonstrated using an oil-in-water emulsification/solvent evaporation technique. The gelatin concentration, alcohol co-solvent amount, and ethylcellulose viscosity were investigated by analyzing the recovered weight, particle size distribution, drug loading efficiency, aspirin release rate, surface characteristics, and release kinetics. Results showed that recovery increased with greater concentrations of gelatin (up to 1%). Adding co-solvent (ethanol) also changed the microcapsule particle size distribution. Higher recovery and release rates were obtained when the ethanol content in polymer solvent was at 25% and the viscosity of ethylcellulose was low (45cps). The release rate followed Higuchi matrix release kinetics, suggesting a monolithic system with aspirin uniformly distributed over the microcapsule.     

Changes of solubility parameters during evaporation of solvents as a factor influencing the self-stratification of epoxy/acrylic systems

Behaviour of solutions of binary mixture of epoxy/acrylic resins during solvent evaporation was examined. Properties of various liquid compositions at different stages of solvent evaporation were characterised by their solubility parameters and surface tension. Both parameters were calculated from the weight fractions of particular components determined by gas chromatography (GC). Phase separation during solvent evaporation and self-stratification were observed and characterized using optical microscopy and FTIR methods. The results of investigations indicate the possibility of using these methods in order to foresee behaviour of binary compositions.     

铸膜液流体力学特性与膜形态结构

通过对铸膜液的流体力学稳定性分析和计算机模拟,研究了与铸膜液流体力学特性相关的膜形态结构,结果表明流体力学不稳定性会导致膜面周期性地出现凹凸不平、聚集体和孔洞等结构。还探讨了过量分子作用势梯度、重力、粘度和表面张力等因素对这些形态结构的影响及消除这些不良结构的原则,用纹影仪观察了铸膜液的蒸发过程,证实铸膜液蒸发过程中出现流体力学不稳定性所致的不均匀结构和对流结构。用光学显微镜和电子显微镜观测了成膜后的形态结构,验证了理论分析结果。     

ATR-FTIR and SEM Gradient Study of Poly(2-ethylhexyl acrylate)/Poly (vinylidene fluoride-co-hexafluoro acetone) Blends

The segregation structure and the gradient structure of poly(2-ethylhexylacrylate) (P2-EHA)/poly(vinylidene fluoride-co-hexafluoro acetone) [P(VDF-HFA)] blends were confirmed by ATR-FTIR and SEM. For the P2-EHA/P(VDF-HFA) (50/50) blend, the P(VDF-HFA) and P2-EHA layers were observed at the free surface side and bottom side, respectively. The gradient structure was obtained for the P2-EHA/P(VDF-HFA) (30/70) blend. The domain morphology of a cross section of the P2-EHA/P(VDF-HFA) (50/50) blend was dramatically changed by the thickness of the blend film. Finally, we propose that the segregation structure and gradient structure formed in P2-EHA/ P(VDF-HFA) blends were caused by surface tension difference, rate of phase separation, rate of solvent evaporation, and molecular weight difference.     

ATR-FTIR and SEM Gradient Study of Poly(2-ethylhexyl acrylate)/Poly (vinylidene fluoride-co-hexafluoro acetone) Blends

The segregation structure and the gradient structure of poly(2-ethylhexylacrylate) (P2-EHA)/poly(vinylidene fluoride-co-hexafluoro acetone) [P(VDF-HFA)] blends were confirmed by ATR-FTIR and SEM. For the P2-EHA/P(VDF-HFA) (50/50) blend, the P(VDF-HFA) and P2-EHA layers were observed at the free surface side and bottom side, respectively. The gradient structure was obtained for the P2-EHA/P(VDF-HFA) (30/70) blend. The domain morphology of a cross section of the P2-EHA/P(VDF-HFA) (50/50) blend was dramatically changed by the thickness of the blend film. Finally, we propose that the segregation structure and gradient structure formed in P2-EHA/ P(VDF-HFA) blends were caused by surface tension difference, rate of phase separation, rate of solvent evaporation, and molecular weight difference.     

非对称膜的凝胶速度及膜结构的影响因素

本文研究了凝胶介质为醇／水溶液的情况下，在相转换法制备非对称尼龙６，６膜过程中，若干因素对凝胶速度及膜结构的影响。结果表明，醇中主链上碳原子数、醇的羟基数、醇的浓度、共溶剂的组成、聚合物的浓度、铸膜液及凝胶介质的粘度、以及凝胶介质的表面张力等因素对凝胶速度及膜结构都有显著影响。     

Evaporation of aqueous solutions of organic acids through spread films of sodium polystyrene sulfonate/dodecyl trimethyl ammonium bromide complex

The Wilhelmy plate technique was used to determine surface tension isotherms of aqueous solutions of formic and acetic acids with concentration up to 30 vol% and with a film of solution of sodium polystyrene sulfonate/dodecyl trimethyl ammonium bromide complex spread to the surface. The formation conditions of films with the extreme concentration of the complex were determined. Such films reduce the evaporation rate of binary liquids by 3-6% and demonstrate selective properties increasing the water content in the vapor by 2-5 abs%.     

Evaporation and pervaporation of a binary mixture from an inert carrier liquid

Bench-Scale experiments were carried out with the ternary mixture 2-propanol/water/glycerol. In a cylindrical vessel, the stirred and thermostated liquid was exposed to a preheated and either dry or humidified air stream. The mixture was evaporated either from a free surface or from the surface of a porous plate. On evaporating the mixture from a porous plate into a dry air stream, the less volatile component water escapes preferentially; in the case of evaporation from a free surface, water is retained in the mixture. In the former case, selectivity is liquid diffusion controlled, in the latter, the gas-side mass transfer and the thermodynamic equilibrium are the controlling mechanisms. On reducing the gas flow rate, the more volatile alcohol evaporates preferentially in both cases; the selectivity is controlled only by thermodynamic equilibrium. Humidification of the air stream with water vapour causes preferential evaporation of the alcohol at an increased rate. Furthermore, instabilities in the liquid boundary layer due to density gradients and surface tension effects may affect the selectivity of evaporation. Experimental results show that the selectivity of evaporation can be manipulated by choosing appropriate evaporation conditions. The theory, which applies the generalized Stefan-Maxwell equations to diffusion in the liquid, can describe these effects.     

Study of factors affecting the formation of a gradient structure in acrylate copolymer/fluoro-copolymer blends

The effects of the solution concentration and the rate of solvent evaporation on the formation of gradient structures were examined for poly(2-ethylhexyl acrylate-co-acrylic acid-co-vinyl acetate) [P(2EHA-AA-VAc)]/poly(vinylidene fluoride-co-hexafluoro-acetone) [P(VDF-HFA)] blends. When the 30/70 P(2EHA-AA-VAc)/P(VDF-HFA) blend was prepared from 10 wt% THF solution, a gradient structure was not formed. When the 70/30, 50/50, and 30/70 P(2EHA-AA-VAc)/P(VDF-HFA) blends were prepared by casting from 5 wt% tetrahydrofuran (THF) solution onto poly(tetrafluoroethylene) (PTFE), the chemical compositions of the surface and bottom sides of the blends were different depending on the blend ratio and rate of solvent evaporation. Therefore, it was concluded that the formation of a gradient structure is strongly influenced by the solution concentration and the rate of solvent evaporation.     

Influence of concrete properties on bleeding and evaporation

The bleeding of concrete and the evaporation of water from the surface of concrete, which affect plastic shrinkage cracking, were investigated. The quantities of water evaporated from concrete specimens exposed to different temperatures, relative humidities and windy conditions were evaluated. The evaporation of water from the surface of C25 and C35 concretes, which were made using PKC/B 32.5R and PC 42.5 cements, was determined. It was observed that the cement type affects the bleeding of concrete. Also concrete mixes containing high cement contents yielded minimum quantities of bleeding. It was observed that concrete mixes containing more mixing water yielded the maximum amount of water evaporation.     

不同管径毛细管的毛细蒸发特性实验

以无水乙醇为工质,采用3种不同管径的圆形毛细管进行毛细蒸发实验。研究结果表明：毛细管毛细蒸发过程中,蒸发界面移动后会延展形成一层液膜,这层液膜可以大幅提高液体蒸发速率。这是因为液膜蒸发引起的温度变化会通过Marangoni效应转化为表面张力差,使液膜在蒸发的同时得到不断补充,形成高效的蒸发-补充机制。随管径增大,液膜厚度增加,蒸发稳定性变好,速率变化幅度减小。工质整体蒸发速率随管径变大呈递减趋势。     

Influence of shrinkage-reducing admixtures on the reduction of plastic shrinkage cracking in concrete

Fresh concrete exposed to high evaporation rates is prone to plastic shrinkage cracking, especially in structures with large surface area/volume ratios. The present work shows that the reduction of the surface tension of the mixing water is an effective way for decreasing such cracking. In this study, conventional and high strength concretes with superplasticizers and shrinkage reducing admixtures (SRAs) were exposed to drying in the plastic state. Continuous monitoring of the surface displacement facilitated the identification of the different stages of plastic shrinkage cracking. Measurements of capillary pressure, settlement, internal temperature and evaporation rate were also made. The results show the effectiveness of SRAs in reducing plastic shrinkage cracking, even in high strength concrete. This is attributed to the reduction in the evaporation rate, delay of the peak capillary pressure due to the development of menisci in the pores and lower settlement.     

The Effect of the Disjoining Pressure on the Surface Evaporation of a Porous Medium

ABSTRACT

The behavior of surface evaporation of a porous medium is investigated experimentally. A theoretical analysis is used to describe the experimental results in this work. Quartz sand(10–20 mesh) is packed as the porous medium. Pure water with electric resistance greater than 10 MΩ and aqueous solution with various concentration of sodium sulfate are used as working liquid in the experiments. Electric light radiation is used as the heat source. Free convection is used in the experiments. The results of the experiment show that the concentration of the salt has great influence on the rate of surface evaporation. A model of microlayer evaporation is developed to predict the drop of evaporation rate. The agreement of experimental results and theoretical results show that the evaporation of the microlayer, driven by the gradient of the disjoining pressure, as well as the interface of the liquid surface curvature due to meniscus effects is important.     

醋酸纤维素纤维干法纺丝成形模拟

建立了基于Maxwell粘弹性本构方程的干法纺丝动力学模型,对醋酸纤维素纤维干法纺丝过程进行了模拟,预测了纤维成形过程中丝条的溶剂浓度、温度、速度、张力等沿纺程的变化关系。结果表明:纤维固化成形发生在闪蒸结束后离喷丝口不远处,此后丝条溶剂含量变化很小;丝条温度在闪蒸时略低于热吹风温度,闪蒸结束后迅速上升到热吹风温度;丝条张力主要来源于空气的摩擦阻力和卷绕张力,在纺程下半段以摩擦阻力为主;提高泵供量后溶剂蒸发速率变慢,丝条温度达到热吹风温度所需时间变长,丝条固化点离喷丝头越远;闪蒸速率随纺丝液溶剂含量增高而减小,同时丝条温度升到甬道温度的时间也越长。     

Temperature dependence of surface tension of poly(vinyl chloride)

The thermal dependence of the surface tension of poly(vinyl chloride) (PVC) containing typical additives was determined below the glass transition temperature. From the results, the surface tension of PVC melts was evaluated and possible differences between the real value and estimate are discussed. It is suggested that the surface tension does not affect the wall-slipping properties of PVC melts.     

Chitosan acetic-acid forming solutions and control of the properties determining their capability for electroforming

The effect of co-solvent additives of dimethylsulfoxide and ethanol on the electrical conductivity, surface tension, and structural and rheological properties of forming solutions of chitosan and their behavior under electroforming conditions were studied. It was shown that EtOH was the most effective cosolvent, exhibited greater surface activity than DMSO, and led to spraying of the forming solution to form spherical or ellipsoidal particles of diameter 100-800 nm.