Influence of additives on interfacial phenomena during film formation of powder coatings

Film formation of powder coatings is illuminated from a physico-chemical point of view. Significant parameters influencing the film formation of powder coatings are surface (wetting) tension and viscosity of the polymer melt formed during the film formation process. A newly developed measuring device for the investigation of wetting at elevated temperatures is presented. This device allows systematic investigations of the temperature dependence of the wetting tension of powder coating binder systems. By combining analytical and surface tension measurements, it is possible to gain new information about the mechanism of action of additives in these melts. Hohe Stra?e 6, D-01069 Dresden, Germany.     

Process intensification in particle technology: Characteristics of powder coatings produced by nonisothermal flow‐induced phase inversion

The characteristics of powder coatings manufactured through a novel processing technique based on nonisothermal Nlow‐induced phase inversion granulation enhanced by fluid injection to promote phase inversion and particle formation from melt state is summarized. Experiments were carried out in a purpose‐built granulator, which operates in a parallel disk rotor‐stator arrangement, so that the mechanism of granulation could be studied. The product of this intensive granulation was compared with that of the conventional powder coating manufacturing process. Understanding the mechanism of intensive granulation helped to redesign the equipment that resulted in smaller particles. Pigment dispersion characteristics were improved by intensive granulation. Also, the particle size span can be significantly reduced by dry granulation and gas‐phase granulation, and the flowability can be improved by wet granulation. Chemical analysis of particles by Fourier transform infrared spectroscopic analysis showed that the injection of coolant fluid had no effect on the chemical composition. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

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。     

Powder production via electrohydrodynamic-assisted molten metal jet impingement into a viscous medium

In this paper, a novel method for preparing metal powder with the aid of electrohydrodynamic (EHD) force is introduced. A pneumatic based atomization apparatus was constructed for this study. Solder melt jet is injected from a 250 μm ID nozzle onto a viscous medium (transformer oil). As a result of jet impingement and penetration into the oil, the melt jet disintegrates into micro-sized droplets and ligaments by a combination of the natural jet breakup phenomenon and EHD atomization. Due to the presence of electrostatic forces, the disintegrated droplets and ligaments repel each other and therefore particle coalescence and agglomeration is minimized in this method. The breakup was captured with a high speed camera. The morphology and structure of the obtained powders were investigated using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD), respectively. Morphology of the particles was controlled by varying the oil temperature and the EHD voltage. Structure investigations show that a decrease in particle size leads to an increase in interior microstrain. Also, interior microstrain increases by decreasing the oil temperature. The main advantage of the developed method is to produce fine powder with controlled characteristics such as size, size distribution, morphology and structure.     

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     

Wet granulation: the effect of shear on granule properties

This paper presents a study of the wet granulation of fine cosmetic particles using a high-shear mixer granulator on a given particle and binder system. The shear effect on granule properties is highlighted. The granules formed under different impeller speeds are divided into size classes and further examined in terms of porosity, friability and binder content.

The main result of this study is that, depending on operating conditions, the granulation of a fine powder with a given binding liquid can result in the formation of granules of very different characteristics in terms of size, porosity and friability. Mechanical energy brought to the granulation system is as important as the physicochemical characteristics of the powder–binder pair.     

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。     

Calculation of detonation gas spraying

A model of processes in the barrel of a detonation facility for application of powder coatings, which takes into account the variable composition of the working gases in the barrel, is presented. It is demonstrated that varying the volume of the gas mixer is an effective tool for stabilizing particle parameters and coating properties if powders with wide fractions are used. The code proposed is a convenient instrument for predicting coating properties. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 6, pp. 112–120, November–December, 2007.     

The formulation of powder coatings using solid state shear extrusion

The feasibility of producing homogeneous powder coating formulations without melting is investigated using Solid State Shear Extrusion (SSSE). A detailed comparison between conventional melt extrusion and SSSE processing conditions of identical formulations is accomplished by following the behavior and degree of polymerization during curing. Commercially available polyester and epoxide powder coating premix formulations were processed by SSSE using a Brabender® laboratory‐scale twin‐screw extruder and further pulverized in a ball mill. Calculations based on particle size before and after comminution in single batch ball mill studies indicate a reduction in energy requirements for SSSE during pulverization. Metal coupons were coated with dry pulverized formulations using an electrostatic fluidized bed coating system. The dynamics of curing were followed for a series of bake time exposures. The sol was separated from the gel using Soxhlet methods and characterized by gel permeation chromatography (GPC). A comparison of experimentally determined gel points and gelation dynamics showed general agreement between both processing methods. The critical yield stress, determined from preliminary mechanical testing of modified thick adherend specimens crack‐line‐loaded in compression, was also determined. Results showed the SSSE process to be equivalent to melt extrusion.     

Simulation of non‐isothermal melt densification of polyethylene in rotational molding

Rotational molding involves powder mixing, heating and melting of powder particles to form a homogeneous polymer melt, as well as cooling and solidification. The densification of a loose powder compact into a homogeneous melt occurs over a wide range of conditions as the material passes from a solid state into a melt state. The numerical simulation of the non‐isothermal melt densification in the rotational molding process is presented in this work. The simulation combines heat transfer, polymer sintering and bubble dissolution models, and is based on an idealized packing arrangement of powder particles. The predictions are in general agreement with experimental observations presented in the literature for the rotational molding of polyethylenes. The simulation allows for systematic and quantitative studies on the effect of molding conditions and material properties on the molding cycle and molded part density. Results indicate that the densification process is primarily affected by the powder characteristics, which are accounted for in terms of the particle size and the particle packing arrangement. The material rheological properties become increasingly important as the powder characteristics lessen in quality. The simulation demonstrated that while certain combinations of processing conditions help reduce the molding cycle, they have a detrimental effect on the densification process.     

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.     

Pan‐milling to prepare ultrafine high‐density polyethylene powder with sodium chloride serving as grinding aid

In this article, our self‐designed pan mill equipment was used to pulverize high‐density polyethylene (HDPE) to prepare its powder product. This pan mill based on three‐dimensional shear forces shows much better pulverization effects on ductile polymer materials when compared with conventional pulverizing equipment based on impact force, and it can mill original HDPE pellets (particle size: 3–4 mm) into fine powder (particle size: 75 μm) at ambient temperature. To further improve the pulverization efficiency to obtain ultrafine HDPE powder, sodium chloride (NaCl), serving as a grinding aid, was comilled with HDPE. Taking the advantages of the cutting and isolating effects of NaCl crystals, HDPE can be pulverized into ultrafine powder with the particle size below 10 μm. Another advantage of NaCl as a grinding aid lies in an easy removal process through water wash, thus conveniently separating from hydrophobic HDPE powder. This technology provides a novel and efficient method to prepare the ultrafine powder of those polymers with high ductility and low melt point, and shows a promise in future commercial application. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Composition limits in granulation with active component in the binder

The process of reactive granulation is considered. Sodium carbonate primary particles react with dodecyl‐benzenesulfonic acid droplets to form granules where the active component is an anionic surfactant formed by the reaction. The effect of primary particle size on the maximum binder/solids ratio was systematically investigated and found to be directly proportional to the specific surface area of the primary particles regardless of how this surface area was achieved—whether by monodisperse powders or bimodal powder mixtures. The effect of binder viscosity on the maximum binder capacity has shown a nontrivial behavior: while the maximum binder content increased with increasing binder viscosity for fine primary particles, the opposite trend was observed in the case of coarse primary particles. This behavior was explained by detailed studies of primary particle wetting and binder penetration into particle beds, as well as by microtomography analysis of the internal granule structure. © 2014 American Institute of Chemical Engineers AIChE J, 61: 395–406, 2015     

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.     

Influence of liquid binder dispersion on agglomeration in an intensive mixer

In industrial scale mixer granulation, liquid binder is usually sprayed onto the agitated powder bed by means of a nozzle in order to enhance the agglomeration process. The early stage of this process, where granule nuclei are formed and grow, is not well understood. As it is desirable to model the agglomeration state right from the beginning of the process for the purposes of control and modeling, this nucleation step is therefore an important field of interest.To investigate the influence of binder droplet size on the nucleation stage of the agglomeration process, experiments were carried out with lactose and water in an intensive mixer. Water was sprayed in to the mixer with different nozzles to vary the size of the produced droplets. As a comparison, water was also directly poured into the turning mixer. Samples of the produced granules were taken at specific time intervals and analysed for size and water content. As the experiments were focused on examining short granulation times, the first samples were taken after only half of the water was added.Particle size distribution and liquid distribution in the wet granule samples were analyzed. It was found, that the droplet size of the binder liquid has great influence on agglomerate size and binder distribution at short mixing times, with increasing time, the mechanical stresses acting in the mixer becomes more and more dominating in the process. Preliminary comparisons are also carried out with single drop penetration tests in an attempt to correlate drop size to penetration time and also to produced granule size.In conclusion this paper studies the effect of different drop size conditions and subsequent spray flux on the behaviour of the nucleation and the early stages of the agglomeration process. The context of these findings for agglomeration in an intensive mixer is examined.     

TEM characterization and reaction mechanism of composite coating fabricated by plasma spraying Nb–SiC composite powder

Niobium carbide composite coatings with Nb₂C, NbC, Nb₃Si as the main phases were prepared in situ on the surface of TC4 titanium alloy by plasma spraying Nb–SiC composite powder. The microstructure of the coating was characterized in detail by TEM, and the reaction mechanism of Nb–SiC was revealed. Sub-micron and nano-scale NbC grains dispersed in Nb₃Si region, nano-Nb/Nb₃Si cellular eutectic region, and equiaxed Nb₂C nanograins region were formed in the coating. The research results show that Nb and SiC reacted firstly to form cubic NbC and Nb₃Si phases during the plasma spraying process. Then, NbC with a higher melting point took the lead in crystallization during the cooling process of the coating, forming sub-micron and nano-scale NbC granular fine grains. Nb₃Si with a lower melting point crystallized around the sub-micron and nano-scale NbC granular fine grains in the subsequent cooling process. In the plasma spraying process, the molten droplets formed Nb/Nb₃Si cellular eutectic structure under large temperature gradient and extremely fast cooling rate. The remaining Nb in the raw material powder formed a diffusion couple with NbC to generate fine and dispersed nano-equiaxed Nb₂C with cubic structure. The present investigation provides a reference for the reaction synthesis of advanced nanocomposites using Nb–SiC system.     

Influence of MA‐g‐HDPE compatibilizer content on quality and adhesion/bending strength of polyethylene/O‐MMT coating films

High density polyethylene/organo‐modified montmorillonite composites whit various concentrations of maleic anhydride grafted high density polyethylene (MA‐g‐HDPE) as compatibilizer (5–20 wt %) have been prepared by melt process. The extruded composite powders are applied on the treated steel surfaces using spray electrostatic powder technique, followed by oven curing at various temperatures (180°C–220°C) and times (15–45 min). The surface uniformity of produced coating films is studied by scanning electron microscopy. Comparison of micrographs of the coatings shows the composite coating films are measured using standard methods. The uniformity, adhesion, and bending strength of the coating films are compared to select high performance coatings. The results indicate that the presence of 15 wt % MA‐g‐HDPE in the coatings shows the highest properties (adhesion and bending strength) and more surface uniformity. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40926.     

Fluidized bed spray granulation: Investigation of the coating process on a single sphere

The coating and granulation of solid particles in a fluidized bed is a process which converts pumpable and atomizable liquids (solutions, slurries, melts) into granular solids in one step by means of drying. The solution to be processed is sprayed onto a fluidized bed. Particle growth can take place either via surface layering or agglomeration. In the case of surface layering the atomized droplets deposit a thin layer of liquid onto the seed particles. The solvent is then evaporated by the hot fluidizing, leaving behind the dissolved material on the surface. Although fluidized bed spray granulation and film coating have been applied in industry for several years, there is still a lack of understanding of the physical fundamentals and the mechanisms by which spherical granules are formed. Hence a new method was developed which allows the direct observation of the subsequent particle-forming mechanisms such as droplet deposition, spreading, wetting and drying. The authors present a laboratory scale apparatus in which a single freely suspended particle can be coated under well defined and constant coating and drying conditions. With this device, particle-growth-rate and the development of particle morphology were measured and investigated under various experimental conditions.     

Melt versus solvent coating: Structure and properties of block–copolymer‐based pressure‐sensitive adhesives

Pressure‐sensitive adhesives (PSAs) composed of a styrene–isoprene–styrene triblock copolymer and a midblock‐associating resin were prepared via solvent and hot‐melt coating. The formulations and thermal histories up to the point of coating were identical, yet significant differences in the properties were observed as a function of the coating method. The solvent‐coated PSA showed superior shear holding power, and the hot‐melt‐coated PSA performed better in tack and peel tests. Two factors resulting from the processing conditions were responsible for these property differences. The quick cooling process occurring after hot‐melt coating led to a poorly defined microstructure and, therefore, less physical crosslinking. Rheological data for melt‐pressed and solvent‐cast PSA films confirmed these microstructural differences. The increased solubility of the tackifier in the solvent additionally created a composition gradient in the solvent coating. Annealing improved the long‐range order of both hot‐melt and solvent coatings, producing a body‐centered cubic microstructure identified by small‐angle X‐ray scattering. This microstructure improved the shear strength of both types of adhesive coatings, whereas the peel and tack properties of the solvent coatings remained inferior to those of the hot‐melt coatings because of differences in the surface compositions. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3355–3367, 2002     

Statistical modelling of the spouted bed coating process using positron emission particle tracking (PEPT) data

Coating of particles larger than about 1 mm can be achieved in a spouted bed, a particle mobilization device in which a strong particle circulation occurs, rapidly upwards in a lean central “spout” region and downwards in a slowly moving annular settled bed. In a spouted bed coater, a spray nozzle is placed at the base of the spout, spraying upwards into a distinct coating zone. The coating formation in a spouted bed is inter alia a function of (i) the particle motion, that is, how often and where particles enter and traverse the coating zone and (ii) the extent of droplet collection by individual particles passing through the coating zone. The coating model proposed here is based on the statistical history of individual particles, whose projected area governs the collection of spray droplets in the coating zone. Positron emission particle tracking (PEPT) has been used to determine the particle trajectories, the distribution of cycle times and the size and voidage of the spout. Whilst the model is not capable of delivering absolute values of coating mass a priori, it can predict deviations from a mean, which can itself be determined from an overall mass balance. To validate the model, a spouted bed coating process was studied in which coarse PVC spheres were coated with the hot‐melt coating material polyethylene glycol (PEG) 1500. Coating mass distributions, derived from the weight data of individual particles before and after manual coating removal, compared (for the studied conditions) very well with the predictions of the model.