A novel approach to the preparation of powder coating—Manufacture of polyacrylate powder coatings via one step minisuspension polymerization

A novel method for manufacturing powder coating through one step minisuspension polymerization is described. The conventional production of powder coating includes six steps—synthesizing resins, mixing the raw material, extrusion, cooling, pre-crushing and pulverization. Comparatively, the present method can simplify the complicated processes, reduce equipment and save energy. Before polymerization, the TiO₂ particles were treated with a reactive silane coupling agent 3-methacryloxypropyltrimethoxysilane (MPTMS) to obtain enough compatibility with the monomers. The powder coating was directly synthesized through employing one step minisuspension polymerization in the presence of titanium dioxide white particles. The powder coating was characterized using Fourier transform infrared spectra (FT-IR) and thermogravimetric analysis (TGA). The results show that TiO₂ particles and polymer are successfully linked up via MPTMS in the powder particles. The morphology of powder coatings produced via different methods was observed by scanning electron microscope (SEM). The powder coatings obtained from minisuspension polymerization consist of regular spherical morphology particles with narrow particle size distribution. The powder flowability and surface film smoothness were enhanced compared to the pulverization powder coating.     

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.     

Creep Behavior of Plasma-Sprayed Zirconia Thermal Barrier Coatings

Thermally sprayed ceramic coatings deposited from nanostructured feedstock powder have often demonstrated improved properties relative to coatings produced from conventional powders. This type of coating has been reported to exhibit better wear resistance and higher adhesion strength compared with conventional deposits. Powder consisting of hollow spherical particles has been reported to produce coating with lower unmelted particles and lower thermal conductivity. In this study, the thermo-mechanical properties of plasma-sprayed yttria-stabilized zirconia coatings deposited using each of these types of powder were investigated. Creep strain and creep rate were measured using free-standing thick coatings loaded in a four-point bend configuration at temperatures ranging from 800° to 1200°C in air under a range of loads. The creep exponent and activation energy were determined.     

细粉粉末涂料流动性能的表征研究

为了更好地解决细粉及超细粉末涂料颗粒在加工、气流输送、喷涂等工艺中遇到的问题,本文对影响细粉粉末涂料流动性能的各粒径参数进行了实验研究和回归分析。研究发现:工业上通常应用中粒径(D50)来表征细粉涂料流动性并不够准确。通过对不同特征粒径以及粒径分布跨度等因素的考察,本文提出了一种由代表粒径大小和粒径分布的D10-D50-D90多变量关联模型来表征细粉粉末涂料流动性的方法,使细粉涂料流动性的预测更为可靠和准确,为细粉涂料特别是超细粉末涂料的工业应用提供理论指导。     

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 the passivating coating type, particle size, and storage time on oxidation and nitridation of aluminum powders

Processes of nonisothermal oxidation, nitridation, and ageing of aluminum powders with different particle sizes (nano-sized powder, ASD-1 powder, and PAP-2 powder) are considered. Application of non-oxide coatings onto particles of aluminum nanopowders reduces their thermal stability. Owing to scale-shaped particles, the PAP-2 powder after long-time storage preserves high activity of oxidation and nitridation, which is commensurable with that of the aluminum nanopowder. The activity of the coarse ASD-1 powder consisting of spherical particles in terms of oxidation and nitridation is low and only slightly changes during ageing. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 2, pp. 61–69, March–April, 2006.     

Nylon 11/silica nanocomposite coatings applied by the HVOF process. II. Mechanical and barrier properties

Nylon 11 coatings filled with nominal 0–15 vol % of nanosized silica or carbon black were produced using the high velocity oxy‐fuel combustion spray process. The scratch and sliding wear resistance, mechanical, and barrier properties of nanocomposite coatings were measured. The effect of powder initial size, filler content, filler chemistry, coating microstructure, and morphology were evaluated. Improvements of up to 35% in scratch and 67% in wear resistance were obtained for coatings with nominal 15 vol % contents of hydrophobic silica or carbon black, respectively, relative to unfilled coatings. This increase appeared to be primarily attributable to filler addition and increased matrix crystallinity. Particle surface chemistry, distribution, and dispersion also contributed to the differences in coating scratch and wear performance. Reinforcement of the polymer matrix resulted in increases of up to 205% in the glass storage modulus of nanocomposite coatings. This increase was shown to be a function of both the surface chemistry and amount of reinforcement. The storage modulus of nanocomposite coatings at temperatures above the glass transition temperature was higher than that of unfilled coatings by up to 195%, depending primarily on the particle size of the starting polymer powder. Results also showed that the water vapor transmission rate through nanoreinforced coatings decreased by up to 50% compared with pure polymer coatings. The aqueous permeability of coatings produced from smaller particle size polymers (D‐30) was lower than the permeability of coatings produced from larger particles because of the lower porosities and higher densities achieved in D‐30 coatings. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2272–2289, 2000     

Evaluation of the wettability of spherical cement particle surfaces using penetration rate method

The wettability of cement particles is related to the fluidity of cement paste. This paper describes the mechanism of the higher fluidity imparted by the spherical cement particles in light of their wettability. In addition, the effects of gypsum on the wettability were also studied. This study has shown the following: (1) The weight of water and water-reducing agent solution penetrating the spherical cement powder bed is 24-150% higher than that for the ordinary Portland cement powder bed. This results in the improvement of the wettability of the particle surfaces of spherical cement. The high wettability of spherical cement contributes to its high fluidity. (2) The presence of many fine gypsum particles on the spherical cement particle surface reduces the wettability. (3) To prepare spherical cement, the optimum amount of gypsum added is determined by the acceleration of the formation of spherical particles and the wettability of particle surfaces.     

Force model considerations for glued-sphere discrete element method simulations

Despite knowing that particle shape plays a significant role in the dynamics of powder flow, most discrete element method (DEM) simulations utilize spherical particles. The reasons for using spheres are that (a) the contact detection scheme for spherical particles is simple, and (b) the contact force models for contacting spheres are well known (e.g. a Hertzian contact).Several schemes for modeling non-spherical particles have been proposed including those that involve polyhedra, ellipsoids, sphero-cylinders, and superquadrics. Perhaps the most common approach for modeling non-spherical particles, however, is using “glued spheres,” in which irregular particle shapes are produced by rigidly connecting individual, and possibly overlapping, spheres. The advantage of the glued-spheres approach is that even for complex particle shapes the simple spherical contact detection algorithm may be retained.Recent publications have focused on how approximating a given particle shape using a glued-sphere geometry affects the rebound of colliding particles [e.g. Price, M., Murariu, V., Morrison, G., 2007. Sphere clump generation and trajectory comparison for real particles. In: Fourth International Conference on Discrete Element Methods (DEM), Brisbane, Australia; Kruggel-Emden, H., Rickelt, S., Wirtz, S., Scherer, V., 2008. A study on the validity of the multi-sphere discrete element method, Powder Technology 188 (2), 153-165]. These investigations have focused on the errors introduced by approximating the geometry of the true particle shape. What has not been investigated, however, is how the spherical particle derived force models used in glued-sphere particle geometries influence the response of particle collisions. This paper demonstrates that in instances where more than a single component sphere in a glued-sphere model is involved in a contact, a modified force model must be used to produce an accurate force-deflection response.     

粉末涂料粒径对涂装产品质量的影响

介绍了用激光粒度分布仪检测粉末涂料粒径的原理,分析了粉末涂料的粒径对涂料的流平性、上粉率、稳定性、回收率及涂装产品质量外观等的影响,结合实际生产经验,根据涂装产品质量的要求,提出了控制粉末涂料粒径的生产工艺参数。     

Preparation of Monodisperse and Spherical Zirconia Powders by Heating of Alcohol–Aqueous Salt Solutions

A novel method is demonstrated which yields a spherical ZrO₂ powder of narrow size distribution through heating of a zirconyl chloride solution with an alcohol–water mixture as the solvent. The kind and composition of the solvent mixture greatly influenced the behavior of the precipitation and the morphology of the resulting particles. When 1-propanol or 2-propanol was employed as the alcohol of the solvent mixture, the resulting particles had a spherical shape and a narrow size distribution. The particle size and the particle agglomeration level could be controlled by the amount of hydroxypropyl cellulose (HPC) in the solution. As-prepared amorphous powder was crystallized to a mixture of metastable tetragonal phase and monoclinic phase at about 460°C. The metastable tetragonal phase was converted to the monoclinic phase as the calcination temperature was increased. After calcination, the spherical shape of the zirconia powder was retained, while its particle size was decreased slightly.     

Study on required energy to deagglomerate pigmentary titanium dioxide in water

The dispersion process is used in various industries, including coatings, paper manufacturing, and food processing. All of these industries base their developments and technology of particle dispersion on theories that consider spherical particles in simple systems. The objective of this study was to improve understanding of the impact of these theories when applied to nonspherical/nonideal particles. Effective dispersion allows for a reduction in the use of the powder to achieve the specified final properties. This paper reports experimental results on the required energy (RE) used to disperse five different titanium dioxide (TiO₂) samples into liquids with different values of surface tension [deionized (DI) water and DI water containing surfactants]. The selection of the TiO₂ samples was based on different particle sizes with the same surface properties, and similar sizes with different surface properties. The results showed that the particle shape is fundamental in determining the RE for dispersion, once it defines the number of interactions among the particles. The larger the number of interactions, the more energy is required to deagglomerate the particles. An empirical equation was developed to describe the energy required for pigment dispersion as a function of the ratio between the liquid and the particle surface tensions and the particle shape factor.     

Study of the dispersion behaviour of l-leucine containing microparticles synthesized with an aerosol flow reactor method

l-leucine containing particles having salbutamol sulphate or sodium chloride as a main component have been produced by an aerosol flow reactor method. In the method, aqueous solute droplets were transferred into a heated laminar flow reactor where droplet drying took place. The geometric number mean diameter (GNMD) of the produced particles varied between 0.50 and 1.01 μm. Amino acid l-leucine, due to its surface-active nature in water, formed the outer layer of the initial droplet and in the product composite salbutamol and NaCl powders. The morphology of the amorphous salbutamol particles changed from spherical to wrinkled and that of the crystalline NaCl particles from faceted to rounded but fractured due to incorporated l-leucine. These powders mixed with coarse lactose powder were tested in a novel deagglomeration apparatus where they experienced continuous turbulent flows with jet flow rates from 15 to 90 l/min intended to disperse powder agglomerates. In general, the incorporation of l-leucine improved dispersion efficiency as well as decreased dependence on dispersing flow rate of all the powders. The influence of l-leucine was observed particularly at low flow rates: The particle number concentration of the dispersed NaCl particles increased ∼ 19 times and that of the salbutamol particles ∼ 12 times with 20 wt.% of l-leucine at a flow rate of 15 l/min. Added l-leucine affected the dispersion of salbutamol particles more than that of NaCl particles due to different particle surface. Moreover, the salbutamol-l-leucine agglomerates were reduced to the primary particles at high flow rates. This was not observed for the NaCl-l-leucine agglomerates. Fine particle fractions (FPF, D ≤ 5 μm) of NaCl-l-leucine and salbutamol-l-leucine composite particles at a flow rate of 60 l/min increased, respectively, from 0.14 to 0.29 and 0.19 to 0.39 with increasing l-leucine content. Commercial micronized salbutamol powder gave an FPF of 0.15.     

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.     

Improving the mechanical properties of laser‐sintered polyamide 12 through incorporation of carbon nanotubes

Well‐dispersed nanocomposite powder with spherical morphology and suitable particle size for processing by laser sintering was successfully produced by coating polyamide 12 (PA12) powder particles with carbon nanotubes (CNTs). Flexural, impact, and tensile test specimens produced by laser sintering the PA12–CNT powder showed no distortion and good definition. The density of the PA12–CNT laser‐sintered parts was higher than that of neat PA12, which the authors propose is mainly due to higher laser absorption by the CNT particles. Compared to the laser‐sintered PA12 parts, PA12–CNT parts showed enhanced flexural, impact, and tensile properties without sacrificing elongation at break. This enhancement may be attributed to the good dispersion of the CNT in the PA12 matrix and denser laser‐sintered parts. Considering the low weight percentage of CNT used (0.1 wt%), it would seem that the method used in this work is a cost efficient and effective way to produce polymer nanocomposite powders for laser sintering, while maintaining the optimum powder morphology for the laser sintering process and enhancing the mechanical properties of the laser‐sintered part. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers     

Experimental study of the mixing kinetics of binary pharmaceutical powder mixtures in a laboratory hoop mixer

As increasingly commented by the literature during the last 5 years, estimating the homogeneity of a powder mixture and following powder mixing processes is not a simple task. In this paper, we present the development and statistical validation of a sampling methodology for defining the number of samples required to provide a reasonable estimation of the homogeneity attained in a laboratory scale tumbler mixer. This method is then used to follow the mixing kinetics of a dilute binary powder mixture in a hoop mixer. Special attention is paid to the statistical meaning of the values obtained and the influence of the physical characteristics such as particle size and shape. The role of the particle shape of the majority powder is particularly emphasised and it is quantitatively demonstrated that spherical particles are harder to mix and more ready to segregate than particles with irregular shapes. The different mixing mechanisms at play are identified; the practical limits of use of such tumbler mixers with pharmaceutical powders are discussed.     

高频氢等离子体增强还原制备超细铜粉

利用高频感应热氢等离子体强化还原制备超细铜粉,考察了加料速率、还原氢气流量、氢气分布位置、反应区空间、冷却温度等因素对铜粉颗粒性能的影响,对制备的铜粉颗粒进行氧含量、XRD晶体结构、松装密度、粒度分布和比表面积的表征。结果表明,优化的工艺条件为反应区内径100 mm,加料速率4 g/min,淬火气氩气气量500 L/h,氢气气量500 L/h并通入少量载气,由氢等离子电离产生的氢自由基可强化反应实现瞬时还原,不仅可控制铜粉形貌,还能有效控制铜粉颗粒大小;利用该方法制备出粒径分布100?200 nm、分散性好的超细球形铜粉颗粒。该方法操作简便、产品纯度高、气氛可控、对环境污染小。     

Effect of powder,binder and process parameters on anisotropic shrinkage in tape cast ceramic products

The effect of powder, binder and process parameters on the properties of cast alumina tapes and their anisotropic shrinkage were investigated. Three alumina powders with different particle shapes (platelets, spherical, standard) and three PVB binders with different chain lengths were used. In addition, casting velocity and blade gap height were varied. The orientation of the particles in the tape was detected quantitatively by image analysis of micrographs. The shrinkage anisotropy is more than 12% for the platelet shaped powder and 8% for the standard powder, whereas the spherical particles lead to almost isotropic shrinkage. The influence of the organic binder chain length proved to be minor compared to the influence of the particle morphology. The variation of casting speed and blade gap height has no effect on anisotropic shrinkage in the investigated parameter range. This is explained by theoretical considerations of particle rotation in a sheared fluid.     

Dense nanostructured calcium phosphate coating on titanium by cold spray

This article deals with the understanding of building-up mechanisms of bioactive nanocrystalline hydroxyapatite coatings by Cold Spray, revealing very promising results in contrast to more conventional techniques such as Plasma Spray. A full characterization of feedstock and coatings is provided. The agglomerated structure of the powder proved to be suitable to obtain successfully thick hydroxyapatite coatings. A crystallite size below ∼20 nm in the powder and the as-sprayed coatings is calculated by the Rietveld X-ray refinement method and agreed by Transmission Electron Microscopy. Some wipe tests were carried out on Ti6Al4V substrates in order to study the deposition of single particles and the nanoscale features were evaluated. The resulting structure indicates that there is no delimitation of particle boundaries and the overall coating has been formed by effective compaction of the original nanocrystallites, leading to consistent and consolidated layers.     

Morphology and sintering behaviour of yttria stabilised zirconia (8-YSZ) powders synthesised by spray pyrolysis

This work is focused on the synthesis of nano-crystallised yttria stabilised zirconia (YSZ) powders by the spray pyrolysis method, the aim of the study being a better understanding of the influence of the spray pyrolysis parameters on the morphology of the produced powders. Spray pyrolysed powder consists of polycrystalline particles, which are spherical. Each particle consists of nanometric primary grains. The morphology of these polycrystalline particles was characterised by scanning electron microscopy (SEM), helium pycnometry, thermogravimetric analysis (TGA) and mass spectroscopy (MS), and the results are compared. Thus, particle size, particle size distribution and particle porosity were determined and correlated to the process parameters. Finally, by dilatometric measurements, sintering curves of pellets prepared from different sets of powders were analysed in regard of their morphologies. Two main conclusions could be deduced from these studies. Firstly, the process parameters influence both internal porosity and particle size distribution of the synthesised powders. Secondly, the morphologies of the spray pyrolysed nano-powders lead to particularly high sintering activities.