STUDY AND MODELLING OF COATED CAR PAINTING FILM BY INFRARED OR CONVECTIVE DRYING

In this study, we described and modelled some coupled material transformations with heat and mass transfer phenomena which occur during the convective or the infrared drying of coated films of model car paintings. For these very complex reactive systems -highly shrinking, hygroscopic, semi-transparent (infrared)- the vaporization process is coupled with the polymerization reaction itself which determines significantly the material properties of the dry paint coating. Three model systems (paint + support) were successively defined and investigated by association of five supports of different radiative or adhesive properties -polish aluminium, blackened aluminium, glass, galvanized iron and composite plastic (SMC)- with three model paintings : epoxy-amine system, polyurethane system and polyvinylalcohol (PVA). Two laboratory combined dryers (convective or infrared) were set up in order to control and regulate precisely the main process parameters: aerothermic conditions (temperature and velocity), spectrum (NIR or MIR) and flux density of infrared radiation.

Firstly, the kinetics of the polymerization were followed all along the process by three different techniques : DSC (Differential Scanning Calorimetry), SEC (Size Exclusion Chromatography), FT-ERS (FOURIER Transform Infrared Spectroscopy). The glass transition temperature of the material during the drying process was deduced from DSC data with the fractional conversion of the monomer. These two parameters were well correlated by the DI BENEDETTO'S relationship, thus defining some characteristic curve of the painting system. Drying curves -temperature profiles and drying rates- were determined for PVA systems for many operating conditions, principally infrared flux density, spectra type or air velocity. The two main coating properties necessary for the modeling were deduced from drying experiments 1: the mean radiative absorptivity as a function of the mean moisture content, 2: the water apparent mass diffusivity as a function of the temperature and of the local moisture content. A diffusive model - written with a mobile frame of coordinates (lagrangian coordinates)- associated with an overall heat balance of the support + coating system, has proved capable of predecting very well the drying curves and the temperatures profiles So, this whole set of results -associated with other material properties (rheological, adhesive, color, etc ⃛)- will be useful to rationally optimize the industrial dryers operating in the car manufactories.     

Experimental investigation of the drying process of water-based paints used in automotive industry

The objective of the government-funded research project “SiLat” (Simulation der instationären Lacktrocknung bei dreidimensionalen Objekten—simulation of paint drying processes on three-dimensionally shaped geometries) is the development of a CFD-software simulation add-on for the drying process of paints [11]. The software is intended to be used in the automotive industry to predict the intermediate drying process of water based paints on car bodies in continuous convective dryers. To validate the software, several laboratory scale experiments, using industrial water-based paint systems and a model base paint system applied on a sheet of metal, were carried out. Conditioned air was used for the drying process. The experimental setup was varied in three different ways: paint on a flat sheet was dried in impinging jet and parallel flow conditions. Furthermore, a Z-shaped sheet was dried during parallel flow conditions. Additional experiments were performed varying the flow parameters. During all experiments, the sheet temperature and weight of the painted sheet were monitored with a rate of 1 Hz. The evaporation of the organic solvents could be monitored using FTIR-spectroscopy. This contribution deals with the experimental investigations and simulation results, whereas the paint drying model is presented by Domnick et al. [5] in detail.     

THERMAL CONTACT CONDUCTANCE OF A COATED PAPER/METAL INTERFACE

A better understanding of the parameters associated with heating of the coated paper during drying process would permit a more accurate design and control of the process to achieve an improved coated paper quality and printability. Following the application of the coating, the coated paper passes through non-contact dryers (i.e. infrared dryers and impinging hot air dryers). When the coating has coalesced, further drying is achieved with steam-heated, contact dryers. One parameter affecting the heat transfer from a metallic dryer drum to the coated paper is the thermal contact conductance at the interface between the coated paper and the drum. In this paper, the thermal contact conductance of a coated paper/metal interface is determined and compared to that of the uncoated paper. Two types of base stock and one type of coating are considered. The thermal contact conductance values are given as a function of the applied interface pressure.     

THERMAL CONTACT CONDUCTANCE OF A COATED PAPER/METAL INTERFACE

A better understanding of the parameters associated with heating of the coated paper during drying process would permit a more accurate design and control of the process to achieve an improved coated paper quality and printability. Following the application of the coating, the coated paper passes through non-contact dryers (i.e. infrared dryers and impinging hot air dryers). When the coating has coalesced, further drying is achieved with steam-heated, contact dryers. One parameter affecting the heat transfer from a metallic dryer drum to the coated paper is the thermal contact conductance at the interface between the coated paper and the drum. In this paper, the thermal contact conductance of a coated paper/metal interface is determined and compared to that of the uncoated paper. Two types of base stock and one type of coating are considered. The thermal contact conductance values are given as a function of the applied interface pressure.     

Modeling of the Seedless Grape Drying Process using the Generalized Differential Quadrature Method

Mathematical modeling of the grape drying process is important in understanding the transport phenomena involved in the production and processing of dried grapes. Drying models proposed in the literature have simplifying assumptions, and thus ignore important phenomena such as shrinkage and changes in transport properties which occur during the drying process. Consequently, a mathematical model is developed for the seedless grape drying process, which considers the effects neglected in previous models. Since an analytic solution to this nonlinear model is impossible, the generalized differential quadrature method is used to solve the models' equations. The model is validated with experimental data obtained from a laboratory scale convective tray dryer operating at 50–70 °C and an air velocity of 1.5 m/s. Model predictions are in close agreement with experimental data due to the inclusion in the model of shrinkage and variation in moisture diffusivity. Model results can serve as a framework to improve the performance of existing and novel dryers, and also in the design of process simulators for dryers.     

Model-Based Analysis of Convective Grain Drying Processes

ABSTRACT

Simulation results for convective drying processes in cross-flow packed bed grain dryers are discussed in this article. A mathematical model developed in order to enable easier design of convective dryers and optimization of operating conditions for agricultural materials (wheat, corn, sunflower seed, etc.) is used in the study. On the basis of calculated values of the state variables of the gas phase and the solids, a clear image of the process itself can be obtained, as well as an overview of advantages and disadvantages of a specific design, supporting and facilitating decisions about the choice of dryer type and operating scheme. The case of double passing of drying agent, with and without additional heating, for a cross-flow continuous dryer, as well as the case of different materials processed in a series of cross-flow batch dryers, is analyzed.     

Air convective drying and curing of polyurethane-based paints on sheet molding compound surfaces

In this study the kinetics and phase changes which occurred during the convective drying of a model car paint based on the polyurethane chemistry is described. The competition between the kinetics of the solvent removal and the phase changes, such as the gelation and/or the vitrification of the polyurethane, needs to be taken into account to avoid the defects in the resulting dried coating. The kinetics of polycondensation of a two-component polyurethane based on polyester triol and triisocyanate in a mixture of various solvents was studied as a 100 μm thick layer on sheet molding compound (SMC) and zinc plated steel plates. Differential scanning calorimetry (DSC) and Fourier-Transform infrared spectrometry (FTIR) and thermogravimetric analysis (TGA) during isothermal curings allowed us to determine the rate constants and the activation energies by using second and third order autocatalytic models. UPRES-A CNRS Q-5007, CPE Lyon, Bat. G308, Campus de la Doua, 69622 Villeurbanne Cedex, France UMR CNRS 5627, Institut National des Sciences Appliquées de Lyon, Bat. 403, 69621 Villeurbanne Cedex, France     

A Theoretical Model for the Nongray Radiation Drying of Polyvinylalcohol/Water Solutions

Abstract

A theoretical analysis of heat transfer and moisture variation was performed while a PVA solution was exposed to high-intensity nongray irradiation and/or air flow convection. Effective absorption coefficients were incorporated in the radiative transfer analysis. The influence of various radiation and convection parameters on the transfer of heat and moisture variation in the coated layers on an optically thick substrate was investigated. The effects of radiation and convection parameters on the transfer process were presented in terms of the rate of water content removal, heat transfer, and moisture distribution. Results were compared to those of drying when using convective heat. It is evident that the use of thermal radiation combined with convective heat will help in improving the drying rate. Numerical results show that both the radiative energy absorbed by the solution and the substrate and the distribution of water mass fraction in the solution are closely related to the rate of water removal from the solution during the process.     

Drying of solids; estimation of the mathematical model parameters

In order to determine physical meaning of a thin‐layer mathematical model parameter, the Page model was modified and tested on numerous experimental data. Applicability of the model was tested on the drying kinetics data, X(t), of 11 different types of the porous materials and dried in a convective, vacuum and/or microwave dryers under the predetermined external process conditions. During the entire drying time the drying kinetics of all the investigated materials and heating methods were successfully correlated with the modified Page model. The evaluated values of a new parameter, t_k, corresponded to the time at which diffusion, as a governing mechanism of moisture movement through the material, started. The results were confirmed by the pore size distribution of some materials.     

Model-Based Analysis of Convective Grain Drying Processes

Simulation results for convective drying processes in cross-flow packed bed grain dryers are discussed in this article. A mathematical model developed in order to enable easier design of convective dryers and optimization of operating conditions for agricultural materials (wheat, corn, sunflower seed, etc.) is used in the study. On the basis of calculated values of the state variables of the gas phase and the solids, a clear image of the process itself can be obtained, as well as an overview of advantages and disadvantages of a specific design, supporting and facilitating decisions about the choice of dryer type and operating scheme. The case of double passing of drying agent, with and without additional heating, for a cross-flow continuous dryer, as well as the case of different materials processed in a series of cross-flow batch dryers, is analyzed.     

DRYING OF OPTICALLY SEMITRANSPARENT MATERIALS BY COMBINED RADIATIVE-CONVECTIVE HEATING

The objective of this work has been to basically elucidate the drying characteristics of an optically semitransparent material by combined radiative and convective heating. The experiments were conducted for a graphite suspension, a slurry of surplus activated sludge and a wet silica sand. The time-change of the drying rate as well as of the surface temperature of the brimfully wet material layer were measured under the step heating conditions using an infrared lamp bundle and a blast of hot air heated by an electric heater.

The experimental data obtained show satisfactory agreement with the calculated results from unsteady heat and mass transfer equations derived on the basis of a semitransparent drying model during the preheating and the constant drying rate periods.     

A Theoretical Model for the Nongray Radiation Drying of Polyvinylalcohol/Water Solutions

A theoretical analysis of heat transfer and moisture variation was performed while a PVA solution was exposed to high-intensity nongray irradiation and/or air flow convection. Effective absorption coefficients were incorporated in the radiative transfer analysis. The influence of various radiation and convection parameters on the transfer of heat and moisture variation in the coated layers on an optically thick substrate was investigated. The effects of radiation and convection parameters on the transfer process were presented in terms of the rate of water content removal, heat transfer, and moisture distribution. Results were compared to those of drying when using convective heat. It is evident that the use of thermal radiation combined with convective heat will help in improving the drying rate. Numerical results show that both the radiative energy absorbed by the solution and the substrate and the distribution of water mass fraction in the solution are closely related to the rate of water removal from the solution during the process.     

MODELING, SIMULATION AND DESIGN OF CONVECTIVE INDUSTRIAL DRYERS

The use of computers to perform simulations of chemical engineering processes has lead to the development of software tools that perform most tedious computations in the field of process analysis, mathematical modeling and design. In the case of dryers, these mathematical programming aspects can be dealt in a straightforward way. The mathematical models of all popular convective dryers are presented and analysed. The transport and thermophysical properties of materials and air involved in the developed mathematical models are briefly discussed. The simulation of convective drying processes facilitated by modern computer technology is outlined and discussed. Design of convective industrial dryers is described and performed through the simulation tools developed. Short-cut design techniques are introduced allowing concentrated information on design results for various levels of process parameters and variables to be integrated in generalized design curves that produce values of optimal dryer structures and operating conditions related to cost     

汽车低温涂装的工艺要点

简介了汽车低温涂装的工艺流程,着重介绍了低温面漆前处理、调配、喷涂、烘干时应注意的事项,讨论了低温金属色漆品种的选择及应用。     

DETERMINATION OF MOIST AIR DRYING ABILITY USING CHEMICAL POTENTIAL OF WATER VAPOUR

ABSTRACT

A thermodynamically simple method for calculation of chemical potential of water vapour, contained in the unsaturated moist air used as a drying medium in convective dryers, is developed. The introduced concept enables determination of the moist air drying potential for different patterns of air state change in convective drying processes and tracing new lines in psychrometric and similar diagrams that enables prediction of the optimal drying process. The proposed method also facilitates the evaluation of air consumption and efficiency of the specific drying process.     

Analysis of adhesion in an aggressive environment of a protective paint coating on an aluminium alloy surface treated by air cold plasma

2024 aluminium alloy is commonly used for the fuselage of aircraft or helicopters because of its high strength. The fuselage bears the weight of the movers of the aircraft and the oil or petrol tanks. It works where the climatic conditions are very harsh, i.e. very low temperature and salt fog, and in presence of aggressive chemical agents. The aluminium fuselage needs to be protected from water, heat and aggressive chemical agents, such as hydrocarbons, lubricating oils or salt fog, by a paint coating. It is important that the adhesion between the paint and the aluminium alloy remains strong for a long time. A complex and critical process is industrially used to enhance both wettability and adhesion of paint to the aluminium alloy. Cold plasma represents an efficient, non-polluting and economical alternative to clean and activate aluminium surfaces. The present work describes the possibility to use an aluminium alloy, pre-treated by cold plasma and then protected by a paint coating, in the presence of water or high temperature, as well as aggressive environmental conditions. Adhesion between the paint and the aluminium surface was characterized by a set of mechanical and physical-chemical tests. Many experimental tests were carried out in order to understand the interaction between plasma and workpiece material. The results obtained by the cold plasma treatment and the commonly used industrial process have been compared. Also, the nature of the relationship between wettability and adhesion was investigated. The results show that cold plasma allows to eliminate cleaning by a solvent and to reduce the quantity of Alodine 1200^® or XPD^® primer applied and, therefore, to reduce the use of polluting substances and the weight of the whole helicopter or aircraft.     

A Quasi-stationary Approach to Drying Kinetics of Fluidized Particulate Materials

A modified quasi-stationary method has been proposed to describe drying kinetics of particulate materials dried in convective dryers with active hydrodynamic regimes. Both our own results and literature data were used. These include sliced celery, cranberry, diced carrot, wheat and polystyrene granules dried in four types of dryers: pulsed fluid bed dryer, fluid bed dryer with a mixer, spouted bed dryer with a draft tube, and vortex dryer. The method was validated by comparing experimental data with results of modeling in terms of a reduced moisture content and material temperature. A new form of the generalized drying curve has been proposed with the reduced time and the index of hydrodynamic intensity as parameters. The equations developed can be used to calculate the total drying time and determine the temporal variation of the moisture content and material temperature.     

Infrared,Convective, and Sequential Infrared and Convective Drying of Wine Grape Pomace

In this paper, effects of infrared drying and/or convective drying on drying kinetics of wine grape pomace were examined, and drying characteristics, sterilizing efficacy, and effects on pomace's polyphenols and pro-anthocyanidins content were determined. Infrared drying had the highest drying rate, which reduced the drying time by more than 47.3% compared with other methods. Sequential infrared and convective drying had a faster drying rate than convective drying. Five empirical models were chosen to fit the drying curves and the Midilli et al. model had the highest R ² and lowest RMSE and χ ² . For sterilizing efficacy, infrared drying and convective drying (90°C) performed the best with minimum survival yeasts, molds and bacteria, while higher drying temperatures resulted in better pasteurization efficacy. Sequential infrared and convective drying did not yield a satisfactory sterilizing effect, with efficacy not being enhanced by prolonging the infrared drying. The wine grape pomace dried by infrared drying had the highest content of polyphenols and pro-anthocyanidins, showing that decreasing the drying temperature led to less damage to these two substances.     

DETERMINATION OF MOIST AIR DRYING ABILITY USING CHEMICAL POTENTIAL OF WATER VAPOUR

A thermodynamically simple method for calculation of chemical potential of water vapour, contained in the unsaturated moist air used as a drying medium in convective dryers, is developed. The introduced concept enables determination of the moist air drying potential for different patterns of air state change in convective drying processes and tracing new lines in psychrometric and similar diagrams that enables prediction of the optimal drying process. The proposed method also facilitates the evaluation of air consumption and efficiency of the specific drying process.