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.     

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.     

铝粉粉末涂料邦定工艺的研究

本工艺通过设计聚酯/环氧型粉末涂料配方,采用邦定包膜技术将涂料粉末粒子与铝粉牢固黏结,静电喷涂试验表明该粉末涂料高度接近电镀铬效果,可作为电镀铬的替代产品.其中铝粉与粉末粒子良好粘结,铝粉使用量可降低30％;采用静电喷涂时,对不同金属元器件粘附良好,200℃固化10分钟,其涂膜拥有类似铬电镀的高光泽度金属效果,且色泽均一.相比溶剂型涂料60 ％～65％的成膜率,该粉末涂料成膜率接近100％,且未附着于被喷涂物件的粉末可以全部回收再利用.现有的邦定包膜技术具有高效环保、低成本等特点.     

Manufacture and characterization of free-standing epoxy-polyester films

The present investigation analyzes the deformation behaviour under static and dynamic loading conditions of electrostatically sprayed epoxy-polyester powder coatings by local and uniaxial tests, trying to account for the separate contribution of the raw polymeric material alone and of the adhesion to the underlying metal substrate. First, thermo-rheological properties of the basic material (i.e., the thermosetting powder paints) were characterized by differential scanning calorimetry (DSC) and rheometry. Secondly, free-standing films were manufactured by electrostatic spraying of the thermosetting powders onto stainless steel substrates pre-coated with an intermediate layer of silicon-based heat curable release coating. The resulting free standing-films were macroscopically characterized by combined dynamic-mechanical analyses (DMA) and tensile tests. Finally, local mechanical characterization of the coating performance was carried out by micro-scale depth sensing scratch and indentation on coatings ‘free-standing’ and ‘rigidly adhering’ onto metal substrate.     

纳米颗粒粒径对等离子喷涂法制备氧化锆纳米涂层的影响

利用扫描电镜(SEM)、场发射扫描电镜(FESEM)、透射电镜(TEM)、比表面积吸附法(BET)等分析测试技术,研究了3种不同纳米尺寸氧化锫粉粒的造粒性能、沉积效率以及对等离子喷涂涂层晶粒大小、涂层熔融性能、结合强度的影响.结果表明:纳米氧化锆粉体一次颗粒粒径大小显著影响纳米粉体的喷雾造粒性能、沉积效率、涂层表面粗糙度、涂层晶粒粒径和结合强度大小.本试验中,利用颗粒一次粒径范围为50～70 nm的纳米氧化锆粉体,等离子喷涂制备了晶粒粒径范围为80～120 nm,沉积效率为43%,涂层表面粗糙度为5.92 μm,结合强度为27 MPa的纳米结构氧化锆涂层.     

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.     

粉末涂料的静电涂装技术

粉末喷涂的喷涂效果在机械强度、附着力、耐腐蚀、耐老化等方面优于喷漆工艺,成本也在同效果的喷漆之下。针对粉末涂料的静电涂装及其前景和发展空间,分析了粉末涂料静电涂装的特点及技术要求,阐述了静电粉末喷涂的工作原理,介绍了粉末静电喷涂的基本原料及主要设备,提出了粉末静电喷涂工艺及施工要求,同时指出了粉末静电喷涂作业的常见问题及处理方法。     

An investigation on flowability and compressibility of AA 6061100 − x-x wt.% TiO2 micro and nanocomposite powder prepared by blending and mechanical alloying

This work investigates the relationships between the components of powders, namely, the powder surface morphology, the flow characteristics and the compressibility of low-energy (microcomposite) and high-energy (nanocomposite) ball milled powders of Al 6061 alloy reinforced with TiO₂ particles. The morphology of the above powder as the function of reinforcement and the milling time was studied by using the scanning electron microscope (SEM). The changes in powder characteristics such as the apparent density, tap density, true density and flow rate were examined by the percentage of reinforcement and milling time. The cohesive nature of the powder was also investigated in terms of Hausner ratio and Kawakita plot. Further, the particle/agglomerate size of low-energy and high-energy ball milled powders was explained by the laser particle size analyzer. X-ray peak broadening analysis was used to determine structural properties of mechanically alloyed powders. The compressibility behavior was examined by the compaction equation proposed by Panelli and Ambrosio Filho to investigate the deformation capacity of the powder. The compressibility behavior, namely, the densification parameter (A) of the microcomposite powder (irregular morphology) was decreased significantly with increasing TiO₂ content due to the disintegration of TiO₂ particles and the cluster formation followed by its agglomeration. The compressibility behavior, namely, the densification parameter (A) of the nanocomposite powder (equiaxed and almost spherical) was decreased slowly with increasing TiO₂ content due to work hardening on the matrix powder. With increased milling time, the compressibility behavior of AA 6061-10 wt.% TiO₂ composite powders increased up to 30 h of milling due to embedding of TiO₂ particles with matrix and changes in powder morphology and finally decreased after 40 h due to work hardening effect.     

基于粒径分布的静电粉末喷涂上粉率研究

为研究粉末粒径分布对静电粉末喷涂上粉率的影响,采用不同目数标准检验筛将商用粉末涂料进行人工筛选.通过测量铝合金试样单位面积增质量来表征静电粉末喷涂一次上粉率,研究粉末涂料粒径分布对静电粉末喷涂一次上粉率的影响.此外,通过设计矩形凹槽模拟喷涂死角区,采用槽内试样单位面积增质量与槽外单位面积增质量的比值表征静电粉末喷涂死角...     

Characterization of powder flow: Static and dynamic testing

Many characterization techniques are available to determine the flow properties of powders; however, it is debated which method(s) are the most appropriate. In this study, sample fine powders with a medium particle size between 22 and 31 µm were characterized using a variety of techniques that tested powders under different stress states, ranging from static to dynamic. It was found that characterization techniques that were more dynamic such as fluidized bed expansion were best suited for predicting the fluidization performance while characterization techniques that were more static such as cohesion were better for predicting agglomeration. It was also found that results from static and dynamic characterization do not necessarily agree, where fine powders that showed good fluidization performance also displayed increased agglomeration, and vice versa. This suggests that flow properties are dependent upon the stress state and that no single technique is suitable for the full characterization of a powder. In other words, both static and dynamic characterization techniques must be employed to completely understand the flow properties of a powder and predict how it will behave under different process conditions.     

Granular flow and dielectrophoresis: The effect of electrostatic forces on adhesion and flow of dielectric granular materials

Electrostatic forces can significantly alter flow properties of granular materials and can adversely affect many industrial particulate processes in unpredictable ways. We investigate here the effect of higher order dielectric electrostatic forces, which are created by non-uniform electric fields, on the agglomeration, adhesion and flow of several granular materials, including pharmaceutical powders. We find that materials can adhere consistently and reproducibly to a metallic rod in a sufficiently strong electric field, which can be produced by either a DC source or tribocharged surfaces. These results provide a simple way to characterize material susceptibility to electrostatic agglomeration. The effect of applied non-uniform fields on the flow of grains falling from a cylindrical hopper is studied and found to significantly reduce the particle flow rate. The effects of humidity, particle size, coatings, and the grounding of equipment are also tested. Finally, contrary to common intuition, we find that grounding a metallic surface can actually exacerbate particle adhesion and agglomeration.     

Effect of Hollow Spherical Powder Size Distribution on Porosity and Segmentation Cracks in Thermal Barrier Coatings

The effect of characteristics of hollow spherical (HOSP) powders on porosity and development of segmentation cracks in plasma-sprayed thick thermal barrier coatings (TBCs) was investigated. Three powders with particle size ranges of 20–45, 53–75, and 90–120 μm were selected from a commercial HOSP powder feedstock for spraying the TBCs. The 20–45 μm powder has a higher deposition efficiency and a greater capability of producing segmented coatings than the other larger powders. Diagnostics of in-flight particles show that the average surface temperature and velocity of the particles sprayed from the fine powder is higher by 250°C and 50 m/s compared with those sprayed from the 90 to 120 μm powder, respectively, due to its greater ratio of surface area to mass. The lower porosity of the coating sprayed from the fine powder is mainly attributed to the decreased volume of intersplat gaps and voids.     

Effect of process conditions on spray dried calcium carbonate powders for thermal spraying

Powder preparation is an important stage in the production of thermal spray coatings with the desired characteristics. An important powder feature is flowability, which can be adjusted through particle morphology, particle size and size distribution. Combined, these features dictate the quality of the coating produced. To increase a powder's flowability, spherical particles within a particular size range are ideal. One way to achieve this is through spray drying. The aim of the present study was to investigate the effect of spray drying process parameters on the physical properties of calcium carbonate powder, with the goal of producing large, spherical particles ranging between 50 and 100 μm in preparation for thermal spray experiments. A key aspect was the use of ethanol to aide in the production of large spheres. A 2³ factorial design of experiments (DoE) was utilised to study the following process parameters: gas flow rate, feed flow rate and solids loading. The resulting powders were characterised in terms of particle size, morphology and production yield. Porous, hollow, spherical particles were produced in a suitable size range for thermal spraying, which was attributed to the rapid evaporation of ethanol. Statistical analysis was utilised to interpret trends between the spray drying parameters and powder characteristics quantitatively.     

Dominant effect of particle size on the CeO2 preferential evaporation during plasma spraying of La2Ce2O7

The inhomogeneous composition distribution of atmospheric plasma sprayed La₂Ce₂O₇ limits its operation in thermal barrier coatings. Two types of La₂Ce₂O₇ powders were used to investigate the effect of the powder size and structure on the preferential evaporation of Ce during plasma spraying. The results indicate that powder particle size is dominant for the Ce evaporation loss. Such effect is significantly intensified with the decrease of particle size. When the particle size is larger than 30 μm, the Ce loss is less or can be neglected. Two mechanisms including the element diffusion mechanism and convection mass transfer mechanism are proposed to well explain the evaporation rule. Moreover, the loss of Ce is higher using the agglomerate-sintered powder due to its higher thermal conductivity than the as-agglomerated powder. Correspondingly, the CeO₂ loss can be neglected in the coatings as the agglomerated powders with a size <30 μm were used.     

Applying dry powder coatings to pharmaceutical powders using a comil for improving powder flow and bulk density

A method for applying nano-sized silicon dioxide guest particles onto host pharmaceutical particles (a.k.a. “dry-coating” or “nanocoating”) has been developed using conventional pharmaceutical processing equipment. It has been demonstrated that under selected conditions, a comil can be used to induce sufficient shear to disperse silicon dioxide particles onto the surfaces of host particles such as active pharmaceutical ingredients (API) without significant host particle attrition. In accordance with previous studies on dry coating, the dispersed silicon dioxide adheres to the host particle surface through van der Waals attractions, and reduces bulk powder cohesion. In this work, laboratory and pilot scale comils were used to dry coat pharmaceutical API and excipient powders with 1% w/w silicon dioxide by passing them through the mill with an appropriate combination of screen and impeller. In general, the uncoated powders exhibited poor flow and/or low bulk density. After dry coating with a comil, the powders exhibited a considerable and in some cases outstanding improvement in flow performance and bulk density. This coating process was successful at both the laboratory and pilot scale with similar improvements in flow. The superior performance of the coated powders translated to subsequent formulated blends, demonstrating the benefit of using nanocoated powders over uncoated powders. This particle engineering work describes the first successful demonstration of using a traditional pharmaceutical unit operation that can be run continuously to produce uniform nanocoating and highlights the substantial improvements to powder flow properties when this approach is used.     

Effects of different nano-agglomerated powders on the microstructures of PS-PVD YSZ coatings

Plasma spray physical vapor deposition (PS-PVD) is a technology that combines the advantages of traditional atmospheric plasma spraying (APS) and electron beam physical vapor deposition (EB-PVD). As the feedstock of the PS-PVD, nano-agglomerated powder is critical on determining the microstructure of the obtained coating. In this study, a method to characterize the cohesion of nano-agglomerated powders was investigated. The nano-agglomerated powders fractured into smaller particles under ultrasonic waves. Their particle size distributions were measured to quantitatively compare their cohesiveness. The change rate in the percentage of powders with particle size less than 5 μm was selected as the value for the cohesion comparison. A high change rate corresponded to a faster fracture and lower powder cohesion. Furthermore, the fracture behavior and heat and mass transfer process of nano-agglomerated powders in the plasma torch were studied by combining 3-D simulation and observation of the microstructures of PS-PVD coatings sprayed with different powders. To obtain a quasi-columnar coating, the nano-agglomerated powder required high cohesion. Finally, a suitable powder was selected and quasi-columnar structure coatings were obtained by optimizing the PS-PVD parameters.     

High heat flux burner-rig testing of 8YSZ thermal barrier coatings: Influence of the powder feedstock

Burner-rig thermal cyclic testing of Thermal Barrier Coating (TBC) samples fabricated using different 8YSZ powders was conducted to investigate the influence of the chemical and phase compositions of the powder feedstocks. Four different powder feedstocks were selected. The chemical and phase compositions among the 8YSZ powders were systematically varied while the powder particle size and other physical characteristics were kept nominally the same. The coating process was also selected to achieve similar microstructure among the samples. The testing revealed that (1) higher impurity content (esp. silica) is detrimental to the cyclic life of the TBC; (2) coating porosity has a significant influence on the cyclic life of the TBC, the higher the porosity, the higher the cyclic life, for the range of porosity of the tested samples; (3) a low monoclinic content in the feedstock powder has not been shown to have a positive effect on the cyclic life of the TBC.     

静电粉末喷涂中一次上粉率浅析

分析了静电粉末喷涂中影响一次上粉率的主要因素以及在应用中如何提高一次上粉率。     

Numerical Study of the Effect of Particle Size on the Coating Efficiency and Uniformity of an Electrostatic Powder Coating Process

This paper reports on a research project that studies the effect of particle size on the coating efficiency and coating uniformity in a powder coating system using the computational fluid dynamics as a modelling tool. The numerical simulations are conducted for different particle sizes with different distances between the spray gun and the coating part and different positions of the powder spray gun pattern adjuster sleeve (PAS). This study can provide detailed information on air flow pattern and particle trajectories inside the powder coating booth, and the coating film thickness on the coated part as well as the particle transfer efficiency (PTE). In numerical simulations, the air flow field is obtained by solving three‐dimensional Navier‐Stokes equations with standard κ‐ϵ turbulence model and non‐equilibrium wall function. The second phase, the coating powder, consists of spherical particles that are 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 the Lagrangian method. It is assumed that particle‐particle interaction can be neglected. The electrostatic field is modelled by solving the Laplace equation.     

溶剂与添加剂对溶剂沉淀法制备PA12粉体的影响

通过改变溶剂沉淀法中复合溶剂中乙醇的浓度或添加剂（SiO₂）的含量制备PA12粉体,并使用激光粒度分析仪、霍尔流速计等对PA12粉体的性能进行了测试与分析。研究发现,乙醇的浓度与添加剂的含量均对PA12粉体的性能存在密切的关联;随乙醇浓度的提高,PA12粉体平均粒径先减后增、粒径分布均匀性提高与流动速率提高、密度降低;随着SiO₂含量的增加,PA12粉体的流动速率与粒径分布均匀性提高,平均粒径与密度呈现先减后增的趋势。