OPTIMIZATION OF SINGLE-ZONE DRYING OF POLYMER SOLUTION COATINGS TO AVOID BLISTER DEFECTS

The optimal conditions for drying polymer-solvent coatings result from a trade-off between minimizing the residual solvent level and creating defects. Blistering defects can be caused by boiling the solvent within the coating. In this paper, we use a detailed drying model with automated constrained optimization to find optimal drying conditions for prototypical coatings that minimize the residual solvent without blistering the coating. The drying oven is assumed to have a single zone with fixed residence time. The optimal drying conditions include the oven air temperature and substrate-side and coating-side heat transfer coefficients The latter are constrained to physically reasonable values. According to our results, the optimal coating-side heat transfer coefficient is always equal to or greater than the optimal substrate-side heat transfer coefficient.     

Optimization of single‐zone drying of polymer solution coatings using mathematical modeling

Optimal conditions for drying polymer–solvent coatings result from a trade‐off between minimizing the residual solvent level and creating defects. This article describes an application of automated constrained optimization with a detailed mathematical drying model to find the optimal drying conditions for a prototypical coating in a single‐zone oven. The optimization process seeks oven conditions that minimize the residual solvent level for a fixed oven residence time without boiling the solvent within the coating. The optimal oven conditions include the air temperature and coating‐side and substrate‐side heat‐transfer coefficients. The conditions are constrained to physically reasonable values. According to our results, the optimal coating‐side heat‐transfer coefficient is always equal to or greater than the optimal substrate‐side heat‐transfer coefficient. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 149–165, 2000     

Optimization of operating conditions in a single‐zone drier for two‐layer polymer coatings

In industrial dryers, hot air is blown over wet coatings either from top or from both top and bottom sides to remove the solvent. Drying of multilayer coatings is a complicated process that involves simultaneous heat and mass transfer, transport, and thermodynamic behavior of polymer solutions. Airflow and its temperature are important operating conditions in a dryer. We report optimization of the operating conditions to minimize residual solvent without inducing blisters for two‐layer coatings using mathematical modeling. Our results reveal that optimal air flow on the bottom side of the coating is always greater than or equal to that on its top side—an agreement with the common notion of “backside drying.” Furthermore, the effect of coating thickness, residence time, individual layer thickness, initial solvent concentration, and reversal of the two layers on the optimal conditions is discussed. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

DRYING KINETICS OF A MULTICOMPONENT MIXTURE OF ORGANIC SOLVENTS

ABSTRACT

Drying kinetics of volatile organic solvents have been examined during the drying process of a pharmaceutical coating containing a multicomponent mixture of ethyl acetate, n-heptane, propanol-2, and toluene. A complete set of experiments was performed in two drying apparatuses, a laboratory air-dryer and an oven dryer, for a wide range of drying temperature, air velocity, initial coating thickness and drying time. An empirical kinetic model is used to predict the concentration of each organic solvent in the mixture during the drying process. The results show that both drying conditions and sample characteristics affect significantly the drying rate of solvents.     

DRYING OF SOLVENT COATED POLYMER FILMS

ABSTRACT

The ability to predict the drying behavior of coating is of great importance to the coating industry. For this purpose, the model developed by Vrentas and Vrentas has been used to predict the concentration profile as well as the thickness and the temperature change of the polymer film as a function of time. The model incorporates the coupling between heat and mass transfer and diffusion induced convection which occurs when a single layer of a binary polymer solution is dried in a convection heated oven.

The model was solved numerically by using the finite difference approximation and it was applied to the well characterized polyvinylacetate(PVAC)-toluene system. The effects of operating conditions on     

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.     

A HIGH AIRFLOW DRYING EXPERIMENTAL SET-UP TO STUDY DRYING BEHAVIOR OF POLYMER SOLVENT COATINGS

A High Airflow Drying Experimental Set-up (HADES) has been constructed to simulate conditions encountered in industrial dryers. In this experiment the drying gas flows parallel to the coating in a narrow gap (6 × 10^?3 cm). Measured heat transfer coefficients (HTCs) on HADES range from 7 W/m² K at 0.072 m/s to 26 W/m² K at 0.72 m/s. The range of HTCs could easily be extended by using higher gas flow rates or narrower gap. The drying results obtained in HADES compare well with model predictions for poly (vinyl acetate)/toluene and show expected trends for changes in drying temperature and flow rate. Experimental drying results for poly (methyl methacrylate)/acetone show an anomalous drying behavior where residual solvent content is minimized at intermediate drying gas flow rate. This phenomenon is likely due to formation of a glassy skin at the surface of the drying coating.     

R&D Needs in the Drying of Coatings

ABSTRACT

The key technologies involved in the drying of coatings are first reviewed and then the factors that influence the drying process are identified, leading to a discussion uf future R&D needs. The drying mechanisms in the constant and falling rate periods are discussed. Present and nticipated estrictions on the use of solvents will result in aqueous systems replacing some solvent systems. and in wider use of high solids coatings and 100% solids coatings. Cross-linking reactions to solidify monomers and low-molecular polymers now become important. Dryers will become chemical reactors. Further development of models of drying, of diffusion, of surface solvent levels, and of solvent vapor pressures will be needed, along with those of dryer defects and drying stresses. To accurately simulate a wide range of coating systems more physical property data is needed. Nan-convection heating and environmental concerns complete the discussion.     

Design of Binary Polymeric Coatings for Minimizing the Residual Solvent,Part I: Experimentation

Several binary polymeric coatings of poly(styrene)–tetrahydrofuran have been dried under quiescent drying conditions. Coatings are made using four different solutions of polymer having 5.01, 9.75, 10.06, and 15.14 wt% poly(styrene) initially. Experiments were performed for several different thicknesses for each polymer solution. Residual solvent remaining was lowest in the thicker coating. Results show that the thicker coating should be applied once rather than layer by layer to minimize the remaining residual solvent. The next layer is applied after complete drying of the previous layer. This practice should continue until the desired thickness is obtained. For instance, coatings of initial thickness 1010 µ m and final thickness 61 µm have residual solvent which is lower than the residual solvent in dried coatings of nearly the same final thicknesses using the layer-by-layer technique.     

IMPINGING JET DRYER

ABSTRACT

In coating and gravure printing, an impinging jet nozzle with high thermal efficiency for drying of coated film was developed.

Trial production 0f 40 kinds of nozzle enables to develop a high-performance impinging jet nozzle with heat transfer coefficient 1.5 times larger than that of current slit nozzle, through measurement of heat transfer coefficient, visualizations of air flow and heat transfer, and measuremenu of jet velocity and turbulence distribution. The purpose of the trial production was to expand a range of high heat transfer and promote turbulence compared with the current nozzle.

Paying attention to mass transfer within gravure ink coated film, drying characteristic of the film was analyzed by numerical solution of a set of equations governing the drying process in which concentration dependencies 0f the diffusion coefficient and the equilibrium vapor pressure were considered.

Applying these analyses. an industrial scale dryer with excellent drying efficiency has finally been developed.     

EFFECT OF DRYING ON HEAT TRANSFER OF BREAD DURING BAKING IN OVEN

ABSTRACT

The effect of drying on heat transfer in a loaf of bread during baking processing from dough in an electric oven was experimentally investigated. This study was performed to obtain informations for the development of an analytical model on baking processing of bread.

When the preheating period completed, a rise in temperature of bread was suspended with keeping the uniform temperature distribution. The uniform and constant temperature was about 373 K independent of the size of dough and baking temperature. While the over-all drying rate achieved a maximum and decreased gradually. Then, crust began to be formed on the dough surface. No constant drying rate period appeared under any experimental conditions carried out. Heating     

PORE EVOLUTION AND SOLVENT TRANSPORT DURING DRYING OF GELLED SOL-GEL COATINGS: PREDICTING ‘SPR1NGBACK’*

ABSTRACT

This paper reports predictions of drying phenomena in deformable porous gel coatings (i.e. a porous solid elastic network filled with air or solvent). Initially, a gelled coating is saturated with solvent, but as it dries, liquid-vapor menisci begin to recede into larger pores and the gel becomes a partially-saturated porous medium. The tensile capillary pressure in the liquid causes a compressive deformation on the solid skeleton and a consequent reduction in thickness and pore-size of the coating. A theory coupling the large deformation of the solid skeleton to capillary pressure in the interstitial liquid is used to predict the course of drying of dip-coated porous gel coatings. The theory predicts a ‘springback’ effect in late stages of drying as the effects of capillary pressure diminish, which matches with experimental observations.     

Designing of biodegradable polycaprolactone: Binary and ternary coatings to minimize the defects and cost of solvent(s)

Coatings of polycaprolactone (PCL) in methylene chloride and toluene were prepared using solution casting method. Coatings of two different polymer mass percentages (5% and 10%), and thicknesses (800 and 1600 μm) were prepared. A drying study on PCL-methylene chloride-toluene coatings was carried out. The binary coatings of PCL-methylene chloride and PCL-toluene served as control cases. The results showed that the drying behavior was external mass transfer controlled for the binary coatings but diffusion controlled for the ternary coatings for most of the drying period. The drying rate enhanced but residual solvent increased with the progressive increase in initial concentration of methylene chloride and decrease in initial concentration of toluene. This shows that the drying rate can be enhanced by partially replacing the costly solvent with cheaper one, that is, toluene with methylene chloride. The coatings were characterized by scanning electron microscopy to understand the surface morphology. The scanning electron microscopy results further indicate that replacing toluene with methylene chloride caused disappearance of cracks, wrinkles, and phase-separation. The coatings with high initial content of methylene chloride were smooth and dense, and more residual solvent content does not manifest in coating defects. Hence, replacement of toluene with methylene chloride leads to savings on material and energy costs for drying along with better quality coatings.     

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     

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.     

聚合物涂膜干燥研究进展

评述了人们对于聚合物涂膜形成过程及干燥机理的认识 ,并以涂膜干燥过程的模拟、涂膜缺陷的形成、扩散系数的测定及估算等方面的研究进展为依据 ,讨论了聚合物涂膜干燥过程中需要进一步研究和开发的领域     

CROSS-EFFECT OF HEAT AND MASS TRANSFER OF LU1KOV EQUATION: MEASUREMENT AND ANALYSIS

Abstract

This paper mainly focuses on cross-effect of heat and mass transfer of capillary porous media which A.B.Luikov set up on irreversible thermodynamics principle. On the basis of perfecting the equations of heat and mass transfer, the heat and mass transfer parameters are determined during drying processes, and thermal gradient coefficient δ and moisture gradient coefficient ξ are obtained which show the cross-effect of heat and mass transfer. Thus the fundamentals are provided for quantitative analysis of cross-effect of heat and mass transfer. The convective drying mathematical model under the first unsteady boundary condition is therefore proposed. By the application of Henry transform, the theoretical solution of unsteady drying process is given and its validity is verified     

DRYING KINETICS OF A MULTICOMPONENT MIXTURE OF ORGANIC SOLVENTS

Drying kinetics of volatile organic solvents have been examined during the drying process of a pharmaceutical coating containing a multicomponent mixture of ethyl acetate, n-heptane, propanol-2, and toluene. A complete set of experiments was performed in two drying apparatuses, a laboratory air-dryer and an oven dryer, for a wide range of drying temperature, air velocity, initial coating thickness and drying time. An empirical kinetic model is used to predict the concentration of each organic solvent in the mixture during the drying process. The results show that both drying conditions and sample characteristics affect significantly the drying rate of solvents.     

VACUUM DRYING OF DISPERSED MATERIALS FROM MULTICOMPONENT LIQUID SYSTEMS: VERIFICATION OF MODEL

ABSTRACT

Experimental data on vacuum drying of chemical and microbial synthesis products under laboratory and industrial conditions are presented. Kinetic relations of vacuum drying of dispersed materials with the removal of a multicomponent liquid system are verified on both laboratory and industrial scale dryers under different operating conditions. The results of the research were generalised and a number of empirical relations needed for drying process calculation are given. The relation between heat transfer coefficient and heat flux and motive force for the vacuum drying process is established.     

A HIGH AIRFLOW DRYING EXPERIMENTAL SET-UP TO STUDY DRYING BEHAVIOR OF POLYMER SOLVENT COATINGS

A High Airflow Drying Experimental Set-up (HADES) has been constructed to simulate conditions encountered in industrial dryers. In this experiment the drying gas flows parallel to the coating in a narrow gap (6 × 10^-3 cm). Measured heat transfer coefficients (HTCs) on HADES range from 7 W/m² K at 0.072 m/s to 26 W/m² K at 0.72 m/s. The range of HTCs could easily be extended by using higher gas flow rates or narrower gap. The drying results obtained in HADES compare well with model predictions for poly (vinyl acetate)/toluene and show expected trends for changes in drying temperature and flow rate. Experimental drying results for poly (methyl methacrylate)/acetone show an anomalous drying behavior where residual solvent content is minimized at intermediate drying gas flow rate. This phenomenon is likely due to formation of a glassy skin at the surface of the drying coating.