Experimental study of particle trajectory in electrostatics powder coating process

Experimental studies of particle trajectory were conducted in an isolated Plexiglass coating booth. Polyester particles were injected with a Norsdon® electrostatics spray gun with a fixed distance from the gun to a grounded plate. Using a Dantec® Particle Dynamic Analyzer, the particle velocity and size distribution were measured simultaneously. The experimental results showed that the effect of electrostatics charging on the particle trajectory is strong in the close vicinity of the target, but can be neglected at locations away from the target. The influence on particle size and velocity profile due to the electric field between the charged particles and grounded target was weakened as more particles deposited on the target. When no charge is introduced on the coating powder, particle segregation is observed for particles larger than 100 μm. Particle gravitational settlement is noticed even near the gun tip. However, particle charging largely eliminates the segregation at a gun-to-target distance of 25 cm and helps break agglomerates formed in the spray system. The gun-to-target velocities of larger particles exhibited noticeable deviation from those of the flow field as the grounded target was approached. The study revealed that the onset of electrostatic coating is an important period that can affect the transfer efficiency and film thickness.     

Effect of process conditions on particle growth for spouted bed coating of urea

The aim of this work was to analyze the influences of operational variables on particle growth for urea coating in a conventional spouted bed (CSB). An aqueous polymeric suspension was the coating liquid sprayed on the spouted particles. The effects of inlet air temperature, coating suspension flow rate and atomizing air pressure on particle growth were analyzed by a central composite rotatable design (CCRD) of experiments. The results showed particle growth in the range of 1.1–2.6%, therefore, some results below the expected for a film coating (2–8%). A second-order polynomial model was obtained for estimating particle growth as a function of the statistically significant variables: air temperature, suspension flow rate and atomizing air pressure, with percentage of explained variation R² = 90.72%. The urea growth kinetics during coating was analyzed for the optimal operating condition and a linear growth coefficient of 1.13 × 10⁻³ min⁻¹ was obtained. The volatilization analyses showed that the polymer film coating provided a decrease of the nitrogen loss in the range of 3–57%. And, SEM analyses demonstrated a total, uniform and homogeneous covering of the urea particles surface.     

Influence of thermo-analytical and rheological properties of an epoxy powder coating resin on the quality of coatings on medium density fibreboards (MDF) using in-mould technology

Powder coating of engineered wood panels such as medium density fibreboards (MDF) is gaining industrial interest due to ecological and economic advantages of powder coating technology. For transferring powder coating technology to temperature-sensitive substrates like MDF, a thorough understanding of the melting, flowing and curing behaviour of the used low-bake resins is required. In the present study, thermo-analysis in combination with iso-conversional kinetic data analysis as well as rheometry is applied to characterise the properties of an epoxy-based powder coating. Neat resin and cured powder coating films are examined in order to define an ideal production window within which the resin is preferably applied and processed to yield satisfactory surface performance on the one hand and without exposing the carrier MDF too high a temperature load on the other hand to prevent the panel from deteriorating in mechanical strength. In order to produce powder coated films of high surface gloss – a feature that has not yet successfully been realized on MDF with powder coatings – a new curing technology, in-mould surface finishing, has been applied.     

粉末涂料制造工艺及其技术研究进展

本文从粉末涂料的制造、涂装的原理分析了其在涂料领域中的优势，探讨了粉末涂料的发展前景及技术发展趋势。     

细长钢管内壁涂塑工艺

采用流化床和热喷涂方法喷涂直径小于２０ｍｍ的长钢管内壁。叙述了工艺过程和涂层检测方法。     

Characterization of particle size evolution of the deposited layer during electrostatic powder coating processes

This experimental study investigated particle size evolution in deposited layers during typical electrostatic powder coating processes, using two powder (coarse and ultrafine) systems. Results disclosed that powder coating is a size-selective process in which the motions of in-flight particles are size-dependent. As a result, particles deposited on different regions of the substrate present some size discrepancy which accounts for a size-decreasing tendency along radial direction for both powders. In comparison with coarse powder, however, ultrafine powder can greatly alleviate the size discrepancy. Furthermore, it was also revealed that, due to the size-selective effect, locally deposited particles are with narrower size distributions than the original particles. The study still demonstrated that small particles are more prone to adhere on the substrate in coarse powder coating processes while large particles take the priority to deposit in ultrafine powder coating processes, due to different dominant factors. However, because of the intensifying back corona, the deposited particles show a size-decreasing tendency with extended spraying duration, which is commonly exhibited in both coarse and ultrafine powder coating processes. Nevertheless, in this study it was also found that charging voltage plays a limited influence on the size evolution.     

Design and optimization of the coating thickness on chopped carbon fibers and sintering temperature for ZrB2-SiC-Cf composites prepared by hot pressing

Interface designation and low-temperature densifying were expected to inhibit the performance attenuation of fiber in the preparation of ceramic matrix composites. Nevertheless, the optimization of interface thickness and sintering temperature were still rarely investigated systematically. Herein, the performance of carbon fiber varied with coating thickness and the heat-treat temperatures were investigated by quantitatively. The tensile strength of carbon fibers degraded dramatically (≥30%) when coating thickness above a critical value. A larger dimension and ordered graphite crystallite structure resulted in the severe performance deterioration of carbon fibers as the sintering temperature exceeding 1500 ℃. The PyC coatings with an appropriate thickness acted as a critical role of changing the fracture mode of the ZrB₂-SiC-C_f and noticeably increased the fracture strain of the composite. The regular trend provided a general design strategy in construction of ZrB₂-SiC-C_f composite with optimal properties and could be extended to other ceramic matrix composites.     

Creation of superhydrophilic surfaces of paper and board

Corona, flame, atmospheric plasma, and liquid flame spray (LFS) techniques were used to create highly hydrophilic surfaces for pigment-coated paper and board and machine-glossed paper. All the surface modification techniques were performed continuously in ambient atmosphere. The physical changes on the surfaces were characterized by field emission gun-scanning electron microscopy (FEG-SEM), atomic force microscopy and Parker Print-Surf surface roughness. The chemical changes were analysed by X-ray photoelectron spectroscopy. The superhydrophilic surfaces, i.e. contact angle of water (CAW) <10°, were created mainly by modifying the surface chemistry of the paper and board by argon plasma or SiO₂ coating. The nano- and microscale roughness existing on paper and board surfaces enabled the creation of the superhydrophilic surfaces. Furthermore, the benefits and limitations of the surface modification techniques are discussed and compared. For example, the SiO₂ coating maintained its extreme hydrophilicity for at least six months, whereas the CAW of argon plasma-treated surface increased to about 20° already in one day.     

Effects of particle size and color on photocuring performance of Si3N4 ceramic slurry by stereolithography

Due to high absorbance of UV light and low solid loading, the stereolithography-based additive manufacturing of gray-colored and dense Si₃N₄ ceramic is of significant difficulty and challenge. The effects of geometric properties of ceramic powders on the curing performance were investigated by studying the absorption difference of the Si₃N₄ ceramic particles with different colors and particle sizes and ultraviolet light. The results show that the transmission of ultraviolet light and curing performance of the darker Si₃N₄ ceramic slurry are much poor. Under the same particle size, the Si₃N₄ ceramic slurry using lighter particles presents the smaller scattering coefficient. The scattering coefficient (～202) of the gray powder with ～0.8 μm average particle size is the smallest. Under the same color, the larger the particle size, the smaller the scattering coefficient. The smallest scattering coefficient of the white powder with ～2.0 μm average particle size is ～110.     

Modelling film formation and degradation of semi-transparent exterior wood coatings

Deposition and aggregation of small solid particles are encountered in many natural and industrial environments. These processes are of substantial significance for the development of a coating film on a wooden substrate. Formulating new coatings with improved performance and lower cost for exterior wooden joinery, mainly a trial-and-error approach, has a large influence on the initial film forming stage in a coating's life. Therefore modelling can supply insight in the particle–particle–substrate interaction. Two approaches are proposed. The first one uses a random point process to position the cluster centres and particles in a dry film. The second strategy starts with a random scattering of the particles in a wet film followed by Monte Carlo sampling and subsequently minimization of the total energy of the particle system. Surface roughness and gloss are calculated from the simulated surface structure. Next to these simulations, surface reconstruction of coated wood with confocal scanning laser microscopy (CSLM) is used to obtain surface roughness values and deduction of gloss applying the bidirectional reflection distribution function (BRDF) theory. As semi-transparent systems are the subject of gloss calculation on surfaces measured by CSLM and computer simulation, refractive index is estimated using an analytical solution of the reflectance and transmittance problem. Withal, coating and subsequent degradation simulation can become a valuable tool for screening purposes.     

Numerical investigation of coarse powder and air flow in an electrostatic powder coating process

The work presented here reports on the numerical simulation of an electrostatic powder coating process that uses a commercial computational fluid dynamic code, FLUENT v6.1. The purpose of this study was to understand the gas and particle flow fields inside a coating booth under given operating conditions and the effect of particle sizes on its trajectories and the final coating quality. The air and powder particle flows in a coating booth were modeled as a three-dimensional turbulent continuous gas flow with solid particles as a discrete phase. The continuous gas flow was calculated by solving Navier-Stokes equations including the standard k − ε turbulence model with non-equilibrium wall function and the discrete phase was modeled based on the Langrangian approach. Since the solid phase volumetric fraction was less than 0.1%, the effect of particle-particle interaction on particle trajectories was not taken into account. In addition to drag force and gravity, the electrostatic force including the effect of space charge due to the free ions was considered in the equation of motion and implemented using user defined scalars and functions. The governing equations were solved using the second order upwind scheme. Information was provided on the particle trajectories with respect to the particle diameters that could be used to develop suitable operating conditions for the use of fine powders in a powder coating process.     

Densification of monomodal quartz particle beds by tapping

A study has been undertaken to determine how the mean particle size of monomodal quartz particle beds and the bed preparation method affect initial and final bed apparent density and the kinetics of densification by tapping. The study was conducted on eight quartz particle size fractions obtained from a commercial quartz powder by sieving, each having a different mean particle size and a very narrow (monomodal) particle size distribution. The amplitude of the distribution and the particle shape were practically the same for all fractions. Densification experiments were conducted on the beds obtained with each fraction, using an assembly designed for this purpose. A kinetic model representing the densification process of monomodal powder beds by tapping is proposed, in which good relations are obtained between the parameters of the model and the foregoing variables.     

Novel method for preparing cellulose model surfaces by spin coating

A new, simplified method for preparing model surfaces of cellulose is introduced. Non-polar cellulose derivative trimethylsilyl cellulose (TMSC) was deposited onto untreated silicon substrate by spin coating, after which the coated TMSC was regenerated back to cellulose by vapour phase acid hydrolysis. By optimising the parameters of spin coating, a smooth cellulose film of ca 20 nm was obtained with roughness variation of max. 3 nm. With the well-defined morphology and chemical structure, combined with easy preparation, these model surfaces provide excellent means to explore the molecular level phenomena, taking place during various processes involving cellulose. Films were characterized using atomic force microscopy to illustrate the morphology and X-ray photoelectron spectroscopy to determine the chemical structure of the layers.     

The influence of particle size on the flow of initially fluidised powders

When particles are allowed to move over a horizontal surface, the effect of gas flow through them is to increase the distance over which they move, termed their mobility. This has already been shown for cases when gas is continuously passed through a current of particles, but this investigation shows that this is also true when the gas flow is only initially present. Experiments were conducted on a column of fluidised particles that were released into an enclosed channel by the removal of a wall, and the distance travelled by the particles was measured. The behaviour of fine particles (group A in the Geldart classification of fluidised particles) was distinct from that of larger particles.The mobility was modified when they were mixtures of different-sized particles. In particular, when there was no gas flow, the mobility was a maximum when the proportion of fine particles was 30% and the magnitude of this effect increased with the size of the coarser component of the mixture. All the different mixtures of particles acted in a similar manner with increasing mobility for a given gas flow rate with proportion of fine particles until roughly half the mixture was composed of fine particles, and there was then no further increase.     

Thermochemical and isoconversional kinetic analysis of a polyester–epoxy hybrid powder coating resin for wood based panel finishing

Powder coating is an established technology especially for the surface finishing of metallic substrates for example in the automotive industry. Moreover, powder technology holds also great promises for the coating of non-conventional substrates like plastics or wood due to the lack of solvents and good recoverability. Here, low-temperature curing resins are required and especially mild processing conditions are demanded by the substrates. Advanced characterization methods need to be established that allow the precise balancing of the processing conditions required for adequate melting, flowing and curing of the powder with the process conditions that can be tolerated by the temperature-sensitive substrates. In the present contribution it is shown that differential scanning calorimetry (DSC) in combination with isoconversional kinetic analysis (ICKA) provides great potential for this purpose. DSC is a standard thermo-chemical method that can be successfully used to study both the melting and curing processes of powder coatings and to determine, for example the glass transition temperature of the cured coating directly from the measured thermograms. However, still more information can be extracted from the enthalpy signals when more sophisticated methods of data post-treatment and analysis are employed. Isoconversional kinetic analysis techniques such as the Kissinger–Akahira–Sunose (KAS) or the advanced Vyazovkin (VA) approaches allow calculating the time-dependencies of physical and chemical processes at various temperatures based on the estimates of activation energies which are obtained from DSC raw data. These analyses allow for example to calculate the time required for a certain degree of cross-linking in the coating after processing the coating under specified curing conditions. In the present contribution the application of ICKA of DSC measurements for the analysis of the flowing and curing behaviour of a powder coating based on a polyester–epoxy hybrid resin is illustrated and the potential of this approach to predict optimal curing times for arbitrary curing temperatures is demonstrated. This is especially useful when temperature-sensitive substrates like wood-based panels are coated. Additionally, the potential to relate the thermo-chemical properties of the powder coating to the surface properties of the coated substrates is discussed.     

Numerical simulations of ultrafine powder coating systems

Numerical simulations for gas–solid two-phase flows were conducted for an experimental coating booth and an industrial coating booth to study the effect of the coating powder size on the performance of the coating process. To optimize coating parameters, simulations were conducted for different coating parameters, such as the size of the coating part, the distance between the coating part and the spray gun, the air flow rate and particle flow rate from the spray gun, the position of the pattern adjust sleeve of the spray gun, and the electrostatic field, in order to increase the coating process efficiency and coating quality.

In numerical simulations, the air flow field is obtained by solving three-dimensional Navier–Stokes equations with standard κ– turbulence model and non-equilibrium wall function. The second phase, the coating powder, consists of spherical particles and is dispersed in the continuous phase, the air. In addition to solving transport equations for the air, the trajectories of the particles are calculated by solving the particle motion equations using Lagrangian method. It is assumed that the particle–particle interaction can be neglected due to low particle volume fraction in coating systems. The electrostatic field is predicted by solving the Laplace equation.     

A mechanistic study of the effect of pigment loading on the appearance of powder coatings: The effect of surface topography on the optical properties of powder coatings: Modelling and experimental results

Gloss is a critical property for many powder coating applications and is related to the amount of light reflected by the coating layer on a substrate. Gloss of powder coatings can, depending on the composition, vary from excellent to matt. It is well known in the powder coatings industry that increasing loadings of pigment, especially TiO₂, causes a detrimental loss of gloss. In order to understand the cause of this phenomenon two questions have to be addressed: firstly, what is the relation between the optical properties and the surface topography of the powder coating and, secondly, how do the powder coating composition and curing conditions affect the surface topography? In order to answer the first question, the typical features of the surface of a cured powder coating have been studied in detail. Using a white light interferometer, it has been shown that the surface topography consists of both short wave and long wave patterns. Each of these patterns could be described by using two statistical parameters only, the root mean square height of the roughness and its correlation length. The effect of both the short and long wave roughness on the gloss has been simulated with a single wave pattern model, based on an approximation of the Kirchoff scattering theory [9]. These simulations illustrated that neither the short nor the long wave roughness on itself determines the optical properties. In order to quantify the combined effect of the long and short wave surface features a two-scale modelling approach was followed. The predictions of this model were in good agreement with experimental gloss data of coatings containing different amounts and types of pigments.     

Estimation of the effective particle sizes within a paper coating layer using a void network model

A range of calcium carbonate-based tablets and paper coating layers, with latex or starch as binder, were prepared. Their dry porous structures were analysed by mercury porosimetry. Some of these samples were also examined by electron microscopy. The porous space of these structures has been simulated using a network model named Pore-Cor, which creates network structures with percolation behaviour and porosity matching those of the experimental sample. Representative particles were grown between the cubic pores and cylindrical throats of the void network model until they touched up to four of the adjacent void features. The sizes of these representative particles, or skeletal elements, have been previously shown to be realistic for the case of unconsolidated sand and glass beads. The size distributions of these skeletal elements were compared with each other and with experiment using a Mann-Whitney test. The sizes of the skeletal elements were found to increase with the particle size of the calcium carbonate powder. The properties of the binders, used in the paper coating formulations, were found to have a major influence on the sizes of the skeletal elements, whose sizes also increased with coating thickness. These findings give insights into the wet structure and the drying process of paper coatings.