Modeling Solvent Drying of Coated Webs Including the Initial Transient

ABSTRACT

A complete model for the drying of coated webs. as implemented in the spreadsheet DRYWEB for Lotus and for Excel, is described. The model predicts the temperatures and solvent levels throughout the length of the dryer. The initial transient as the coated web approaches the equilibrium temperature in the consiant rate period is calculated. The coating is divided into slices for a finite difference calculation of solvent diffusion to the surface, using a simple free volume model of diffusion. The semi-empirical constants needed can be found from one or two trial runs. The possible use of infra-red heating in addition to convection heating is included in both the constant and falling rates. The calculation of infra-red energy is discussed. The model assumes that the temperature is uniform from the top of the coating to the bottom of the base, and that assumption is justified.     

SiC/C纤维热力学生长机理研究

使用乙二醇作为稀释剂将硅溶胶稀释,得到不同浓度的硅溶胶溶液,用作涂覆商用碳纤维的原料。碳纤维表面改性后涂覆硅溶胶溶液,取出后在Ar气氛中于1 600 ℃保温30 min,硅溶胶涂层与碳纤维发生碳热还原反应形成SiC覆盖层。对制得的产物进行形貌和结构表征,并根据热力学数据对SiC包覆层的生长动力学和高温扩散机制进行研究。计算结果表明,反应过程为扩散控制。SiC/C纤维比碳纤维有更优异的耐高温氧化性能,为低成本制备新型陶瓷纤维提供可借鉴的思路和热力学依据。     

Behaviour of film-forming polymers in a microwave electric field

The application of a microwave electric field to the formation of polyurethane films supported by a glass substrate has been described previously. The present work concerns the investigation of the separated contributions of a polymer and its solvent during the film formation in microwaves. The measurement technique allows the kinetic formation to be observed by means of a specific infra-red pyrometer which indicates the temperature variation of the film surface when submitted to the electric field inside a waveguide associated to a microwave power generator at 2.45 GHz. Present data show that dielectric relaxation losses of structural elements of the polymers form the dominant heating process in microwave coating.     

Stabilization of ammonium azide particles through its microencapsulation with some organic coating agents

This work is devoted to reduce spontaneous sublimation of ammonium azide at ambient and elevated temperatures by means of two microencapsulation techniques involving solvent/non-solvent and solvent evaporation methods in which stearic acid, Viton and nitrocellulose (NC) have been tested as coating agents. Scanning electron microscopy (SEM) was employed to examine the coating morphology. The thermal behavior of pure and coated ammonium azide samples have been studied by using simultaneous thermogravimetery-differential thermal analysis (TG-DTA) and differential scanning calorimetry (DSC). The results showed that ammonium azide microparticle could be effectively coated with nitrocellulose through a solvent/non-solvent experiment in which the coating quality depends on some experimental factors such as coating agent to NH₄N₃ weight ratio and volume and addition time of non-solvent. The effect of these factors on coating quality and thermal properties of NH₄N₃ has been revealed by results of SEM and thermal experiments. The most stabilized coated ammonium azide was achieved by using 4.5% (w/w) of NC as stabilizer, and by addition of 30 ml n-hexane as non-solvent within 75 min, where the maximum sublimation temperature increases about 30 °C with respect to uncoated sample and reaches to 145.5 °C. The influence of the heating rate (5, 10, 15 and 20 °C/min) on the DSC behavior of the pure and coated ammonium azide particles at the optimum condition was verified, whereas sublimation temperature of the NH₄N₃ was increased as the heating rate was increased. Also, the kinetic parameters such as the activation energy and frequency factor of the sublimation processes for pure and coated ammonium azide were obtained from the DSC data by non-isothermal methods proposed by Kissinger and Ozawa. The results showed that, activation energy for sublimation of completely coated NH₄N₃ particles is considerably higher than (nearly 1.5 times) that of the pure one. Also, the first order rate constant of sublimation of completely coated sample is noticeably lower than that of pure sample (). The kinetic results led us to conclude that the applied microencapsulation technique caused efficient stabilization of volatile NH₄N_3.     

Modeling and design of an industrial dryer with convective and radiant heating

Industrial equipment for drying polymeric coatings normally consists of a series of zones, each with a controlled temperature and airflow. Drying of a polymer–solvent solution is strongly affected by the variation of diffusivity, solvent vapor pressure, and solvent activity with temperature and composition. The equations of mass transfer by diffusion and of heat transfer by conduction and radiation describe changes in composition and temperature within the shrinking coating. This system of equations is solved by Galerkin's method with finite element basis functions. The boundary conditions on dryer airflow and temperature change at the entrance to each zone. In a few test cases, the predictions show how evaporative cooling can slow drying in early zones where the coating temperature drops below the dryer temperature, whereas in later zones the coating temperature rapidly approaches the dryer temperature. Infrared heating can be used to reduce the extent of evaporative cooling. In the test cases and experiments, “blistering” occurs in later zones where high oven temperature causes the solvent partial pressure to rise; dryer parameters can be chosen to maintain solvent partial pressure just below ambient pressure in order to avoid “blistering” with least sacrifice of process speed. © 1995 John Wiley & Sons, Inc.     

锅炉受热面复合陶瓷涂层抗高温SO2腐蚀性能

针对燃煤锅炉受热面存在的高温腐蚀问题,采用料浆法在20G钢材表面制备复合陶瓷涂层,烧结后的复合陶瓷涂层表面较为致密,涂层与基材具有良好的结合状态。对喷涂及未喷涂陶瓷涂层钢片在SO₂腐蚀气氛环境下的抗高温腐蚀性能进行试验,并采用热分析动力学方法对试验数据进行处理,结果表明,在400~500℃温度范围内,两种试样的腐蚀过程均符合一维扩散反应动力学模式,计算求得喷涂涂层钢片腐蚀反应活化能低于未喷涂涂层钢片,涂层具有较好的抗高温腐蚀性能。对腐蚀试验后试样的形貌、成分和物相分析结果显示,涂层在SO₂气氛下腐蚀后,表面生成K₂SO₄晶粒,但由于涂层较为致密,阻止了S的扩散,内部基本没有检测到K₂SO₄的存在。     

THE DRYING OF COATINGS ON THICK WEBS

The spreadsheet DRYWEB to predict the drying of coated webs assumes that the temperature at any location is constant across the coating and the web. But in drying a coating on a thick web, such as on a foamed plastics, the temperature across the web varies even though it will be uniform across the thin coating itself. To model heat conduction in the web a finite difference formulation is now used. Otherwise the spreadsheet, called DRYWEBTW, is very similar to DRYWEB. Examples show that with a thick, insulating web very little heats flows into the web from the coating. At an intermediate thermal conductivity more heat flows into the web and the drying rate is reduced. With a higher conductivity and with two-sided heating, heat can move by conduction from the backside of the web into the coating and thus increase the drying rate.     

Testing of starch-based carbohydrate polymer coatings for enhanced urea performance

In this study, starch–urea–borate adhesives were developed for coating the slow release urea. The physical properties of the developed adhesives were studied as a function of temperature, heating time, stirring rate, and pH. It was found that for certain specific adhesive composition, pH and stirring rate, the complete gelatinization time and corresponding adhesive viscosity do not remain constant with temperature. The suspension heated at 75°C reached its maximum viscosity after 21 min of heating, thereafter, remained constant over time. In contrast, the suspension heated at 80°C reached its peak viscosity after 12 min of heating. Further heating after 12 min caused a steady decrease in viscosity from its peak value of 450–339 cP. Once the adhesive physical properties were completely understood, a dripping solution technique was used to coat the urea granules with coating thickness ranging from 0.15 to 0.7 mm. It was noticed that the overall nutrients release time of the coated urea was three times higher than the uncoated urea. It was also concluded that the mechanical strength of coated urea strongly depends on the adhesive composition and coating thickness.     

Liquid Carbon Dioxide as Anti‐Solvent Coating Aluminum Hydride

This paper presents a novel surface coating to resolve the serviceability, stability, compatibility, and dehydrogenation of aluminum hydride (alane) using liquid carbon dioxide as an anti‐solvent, dispersant, drying medium, and inert atmosphere. The coated alane sample was characterized by SEM, XPS, XRD, and FT‐IR spectroscopy. The coating agent was uniformly applied on the surface of alane and the crystalline phase remained unchanged. The alane molecules became more stable after coating. The thermal stability of alane before and after coating was analyzed by DSC. The enthalpy of the coated alane increased under different heating rates, indicating increased thermal stability. The lower electric spark sensitivity of the coated alane showed improved serviceability and stability. The method developed in this study may be suitable for coating other hydrogen storage and energetic materials.     

涂覆聚氨酯的聚酯/氨纶复合织物的定量分析研究

研究了涂覆聚氨酯(PU)的聚酯/氨纶复合织物(简称PU涂层复合织物)的定量分析方法,探讨了以四氢呋喃和环己酮作为溶剂在不同温度条件下对去除PU涂层复合织物中的PU涂层的效果,对去除PU涂层后的PU涂层复合织物进行其中的聚酯纤维含量(G)的测定并分析了未涂覆PU的相应聚酯/氨纶复合织物中的聚酯纤维含量(G0).结果表明:...     

Evaluating water transport through high solid polyurethane coating using the EIS method

Buried pipelines are commonly coated with high solid polyurethane to prevent corrosion. The diffusion of water through the coating plays an essential role in the performance and endurance of the coating. This study focuses on the evaluation of water diffusion through polyurethane coatings. The effects of thickness and ambient temperature on water diffusion through polyurethane, which is coated on mild steel and immersed in 3.5% (weight fraction) NaCl solution, are evaluated using electrochemical impedance spectroscopy (EIS). Experimental observations clearly indicate that the water diffusion coefficient strongly depends on service temperature and the thickness of coating. Higher temperature (lower than T _g) increases water uptake of the polymer but no measurable changes were observed for the temperature change studied. EIS results show that the mechanism of corrosion does not change in the range of thicknesses and temperature investigated in this study.     

用石蜡-CO_2超临界流体快速膨胀在流化床中进行细颗粒包覆

对超临界流体快速膨胀技术在流化床中进行细颗粒的表面包覆进行了研究 ,以实现细颗粒中关键成分的有效控制释放 .实验研究了含有包覆剂———石蜡的超临界二氧化碳流体通过微细喷嘴快速膨胀到装有细颗粒的流化床中 ,膨胀射流中所产生的微核在细颗粒表面均匀沉积 ,形成细颗粒表面薄层包覆 .结果分析表明 ,超临界流体快速膨胀前的温度是包覆过程的关键参数 ,通过控制操作过程参数可以获得良好的包覆结果     

The effect of microencapsulation with nitrocellulose on thermal properties of sodium azide particles

Based on the coacervation principle a solvent/non-solvent method has been used for microencapsulation of sodium azide (NaN₃) with fibrous nitrocellulose (NC). Scanning electron microscopy (SEM) was employed to examine the coating morphology. The thermal behavior of solid samples has been studied by means of thermogravimetry (TG) and differential scanning calorimetry (DSC). The results of TG–DTA analysis revealed that the main thermal degradation for the pure NC and NaN₃ occurs in the temperature ranges of 192–220 and 415–420 °C, respectively. The effects of some parameters, such as NC to NaN₃ weight ratio and volume and addition time of non-solvent, on coating quality and thermal properties have been investigated by SEM and thermal methods. The results of these experiments showed that the decomposition temperature of most stabilized coated sodium azide is about 50 °C higher than that of the pure sample. The DSC experiments were conducted to study the influence of the heating rate (5, 10, 15 and 20 °C/min) on the thermal decomposition processes of the pure NC, coated and pure NaN₃ samples. The results revealed that, as the heating rate was increased, decomposition temperature of the compounds was increased. Also, the kinetic parameters such as activation energy and frequency factor of the decomposition processes were obtained from the DSC data by non-isothermal methods proposed by ASTM E696 and Ozawa. Our finding showed that coated NaN₃ has lower decomposition rate with respect to the pure one.     

包膜尿素控制释放过程传质模型

根据质量守衡定律和F ick扩散定律建立了包膜尿素控制释放过程的传质模型,并用3种新颖的石蜡-松香混合物包膜尿素在蒸馏水中的释放数据和扫描电镜照片对模型进行了验证。结果表明,该模型能有效地描述和预测由扩散机制控制的包膜尿素的养分释放行为,拟合的相关系数R2分别为0.997 6,0.997 3和0.998 4。模型显示,包膜尿素的养分释放速率与膜的有效扩散系数Deff成正比,与尿素粒径的平方及膜的厚度成反比。     

钼电极表面玻璃基防氧化涂层的研究

针对钼在高温下极易氧化的问题,开发了一种在烤窑时防止钼电极被氧化的玻璃基涂层.该涂层是以钡硅酸盐玻璃为主体,以Cr2O3为难熔填料,加入有机粘结剂,用松油醇做溶剂,配制成悬浮液,采用刷涂法涂覆于钼电极表面.研究了Cr2O3的添加量对涂层防氧化性能的影响.通过能谱(EDS)和恒温失重分析,比较了有涂层和无涂层保护时,钼基体表面氧含量的差异.结果表明,该涂层在较宽的温度范围内对钼电极有很好的保护作用.     

TWO-SIDED DRYING OF COATINGS ON POROUS WEBS

A relatively simple spreadsheet-based model of the drying of coated webs has been extended to cover coatings on porous webs Two cases have been modeled. In one the coating is assumed to fully saturate the porous web, with the drying being completely symmetrical about the center plane except for differences in the air velocities and temperatures. The drying rate in the constant rate period is then increased, but because of the greater thickness that the coating now occupies, diffusion is much slower and the falling rate drying can be slower, even though it takes place on both sides. In the other case the coating is assumed to lie on top of the porous web, with solvent vapor diffusing through the web. Here the constant rate drying is only slightly faster than in the base case, but in the falling rate period the drying can be greatly accelerated Both models predict the temperatures and solvent levels throughout the length of the dryer.     

普通石墨材料用Al_2O_3涂层抗氧化性能研究

以普通石墨电极为例,设计并制备了氧化铝高温抗氧化涂层。采用氧化失重分析法对有涂层和无涂层石墨电极进行对比研究,同时采用扫描电镜对涂层的形貌进行观察。研究结果表明,700℃时,无涂层石墨电极的氧化失重率已经是有涂层石墨电极的10.7倍;涂层与石墨基体之间没有形成明显的扩散层。在实际使用过程中,涂层能够很好的阻止氧气向石墨基体表面扩散,大大降低了石墨材料的氧化损耗。     

Delamination behavior of lithium-ion battery anodes: Influence of drying temperature during electrode processing

One essential process step during electrode processing for lithium-ion batteries is the drying of the wet particulate electrode coating. The electrode film solidifies during evaporation of the solvent and a porous film is formed. In this study, we focus on the influence of drying temperature on the internal electrode structure of the dry film. Anode slurries that consist of graphite and an aqueous binder system were coated and subsequently dried. To assure defined and controllable drying conditions, a laboratory set-up with a temperature-controlled substrate carrier and an impingement dryer was used. To facilitate a scale-up to continuously passed dryers, the choice of experimental temperatures was based on a calculation of steady-state temperatures that result from gas temperatures that are commonly applied in industrial drying processes. The delamination behavior of the differently dried electrodes was investigated by means of a 90° peel test. The results show a strong dependency of electrode adhesion on drying temperature. A lower adhesion force at higher temperatures hints at a variation in binder content at the interface between the copper substrate and the coating layer. The formation of a consolidation layer at the air-film interface during drying is identified as a possible explanation and a criterion for consolidation layer formation is suggested.     

TWO-SIDED DRYING OF COATINGS ON POROUS WEBS

ABSTRACT

A relatively simple spreadsheet-based model of the drying of coated webs has been extended to cover coatings on porous webs Two cases have been modeled. In one the coating is assumed to fully saturate the porous web, with the drying being completely symmetrical about the center plane except for differences in the air velocities and temperatures. The drying rate in the constant rate period is then increased, but because of the greater thickness that the coating now occupies, diffusion is much slower and the falling rate drying can be slower, even though it takes place on both sides. In the other case the coating is assumed to lie on top of the porous web, with solvent vapor diffusing through the web. Here the constant rate drying is only slightly faster than in the base case, but in the falling rate period the drying can be greatly accelerated Both models predict the temperatures and solvent levels throughout the length of the dryer.     

Stress development in drying fibers and spheres

Stress development during drying is a critical factor that affects the final structure and properties of a coated fiber or spherical product. Stress development during drying of the coating is due to nonuniform shrinkage and physical constraints. In this study, a large deformation elasto‐viscoplastic model is developed to predict stress development in drying fibers and spheres after the coatings solidify. From the model, stress evolution in the drying fibers/spheres can be predicted by a partial differential equation of diffusion in one dimension, a first‐order partial differential equation of pressure distribution, and two ordinary differential equations on local evolution of the stress‐free state. The system of equations is solved by the Galerkin/finite element method in the one dimensional axial/spherical symmetric coatings. Solutions show changes in solvent concentration and viscous stress as the coating dries. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3934–3944, 2003