The Review of Powder Coatings

With the environmental regulations becoming more stringent and awareness of consumers increasing to protect environment, an urgent problem is to reduce the use of volatile organic compounds (VOCs). Powder coating is a kind of solid powder coating without any solvent. Due to its excellent application performance and environment-friendly, it is widely used in the field of metal coating, especially appliances of offices and home. In recent years, the use of powder coatings has been developed very fast and the requirements of functional powder coatings are also gradually strengthened. According to resin of the film forming, powder coatings can be divided into two types: thermosetting powder coatings and thermoplastic powder coatings. Each kind of powder coatings has its own advantages and disadvantages, and they will be applied in different fields. In order to improve the properties of powder coatings, there are many reports and studies about them. The compositions of powder coatings were extruded, crushed and screened to gain powder of coating. The powder always was stored at room temperature. The powder coatings often operated by two kinds which were electrostatic spraying method and fluidized bed dipping method. After that, the powder was heated to melt and cure. Finally, a smooth bright permanent film on articles was formed to achieve the purpose of decoration and corrosion. The process flow preparation of powder coatings always separated into dry process production and wet process production. The development courses of powder coatings about resins, pigments and fillers are summarized. The future development trends of powder coatings are illustrated.

Developing polydopamine modified molybdenum disulfide/epoxy resin powder coatings with enhanced anticorrosion performance and wear resistance on magnesium lithium alloys

Epoxy resin powder coating has been successfully applied on the corrosion protection of magnesium lithium alloys.However,poor wear resistance and microcracks formed during the solidification have limited it extensive application.There are limited approaches to exploit such anti-corrosion and mechanical properties of magnesium lithium alloys.Herein,the epoxy resin powder coating with polydopamine modified molybdenum disulfide(MoS_(2)@PDA-EP powder coating with 0,0.1,0.2,0.5,1.0 wt.%loading)was well prepared by melt extrusion to investigate its anticorrosion performance and wear resistance.The results revealed that the addition of MoS_(2)@PDA enhanced the adhesion strength between coatings and alloys,wear resistance and corrosion protection of the powder coatings.Among them,the optimum was obtained by 0.2 wt.%MoS_(2)@PDA-EP powder coating which could be attributed to well dispersion and efficient adhesion with coating matrix.To conclude,MoS_(2)@PDA-EP powder coating is meaningfully beneficial for the anticorrosive and wear performance improvement of magnesium lithium alloys.

Curing Kinetics, Mechanical Properties and Thermal Stability of Epoxy/Graphene Nanoplatelets(GNPs) Powder Coatings

Epoxy/graphene nanoplatelets（GNPs） powder coatings were fabricated using ultrasonic predispersion of GNPs and melt-blend extrusion method. The isothermal curing kinetics of epoxy/GNPs powder coating were monitored by means of real-time Fourier transform infrared spectroscopy（FT-IR） with a heating cell. The mechanical properties of the epoxy/GNPs cured coatings had been investigated, by evaluating their fracture surfaces with field-emission scanning electron microscopy（FE-SEM） after three-point-bending tests. The thermal stability of the epoxy/GNPs cured coatings was studied by thermo-gravimetric analysis（TGA）. The isothermal curing kinetics result showed that the GNPs would not affect the autocatalytic reaction mechanism, but the loading of GNPs below 1.0 wt % additive played a prompting role in the curing of the epoxy/GNPs powder coatings. The fracture strain, fracture toughness and impact resistance of the epoxy/GNPs cured coatings increased dramatically at low levels of GNPs loading（1 wt %）, indicating that the GNPs could improve the toughness of the epoxy/GNPs powder coatings. Furthermore, from FE-SEM studies of the fracture surfaces, the possible toughening mechanisms of the epoxy/GNPs cured coatings were proposed. TGA result showed that the incorporation of GNPs improved the thermal stability of the cured coatings. Hence, the GNPs modified epoxy can be an efficient approach to toughen epoxy powder coating along with improving their thermal stability.

STUDY ON UV CURABLE POWDER COATINGS BASED ON HYPERBRANCHED POLYMERS

By introducing semicrystalline moieties into hyperbranched molecular smictures three kinds of prepolymers of potential use as UV powder coatings were prepared and characterized.

Effects of direct stray current on the performance of cathodic disbonding epoxy powder coatings

The influence of the direction and magnitude of direct(DC)stray current on the disbonding performance of epoxy powder coating was studied by using electrochemical impedance spectroscopy and scanning electron microscopy technology.The results show that the application of DC stray current could accelerate the anodic dissolution.The peeling degree of the coating increases as the magnitude of positive DC stray current interference increases in the range of 2–8V.With 16V positive stray current disturbance,the degree of coating disbonding decreases.With the application of negative stray current,the coating peeling becomes more serious,and the degree of peeling increases as the DC stray current increases.

Fabrication and characterization of multi-scale coated boron powders with improved combustion performance:A brief review

Boron has high mass and volume calorific values,but it is difficult to ignite and has low combustion efficiency.This literature review summarizes the strategies that are used to solve the above-mentioned problems,which include coatings of boron by using fluoride compounds,energetic composites,metal fuels,and metal oxides.Coating techniques include recrystallization,dual-solvent,phase transfer,electrospinning,etc.As one of the effective coating agents,the fluorine compounds can react with the oxide shell of boron powder.In comparison,the energetic composites can effectively improve the flame temperature of boron powder and enhance the evaporation efficiency of oxide film as a condensed product.Metals and metal oxides would react with boron powder to form metal borides with a lower ignition point,which could reduce its ignition temperature.

Microstructure of Ni–Al powder and Ni–Al composite coatings prepared by twin-wire arc spraying

Ni–Al powder and Ni–Al composite coatings were fabricated by twin-wire arc spraying（TWAS）. The microstructures of Ni-5wt%Al powder and Ni-20wt%Al powder were characterized by scanning electronic microscopy（SEM） and energy dispersive spectroscopy（EDS）. The results showed that the obtained particle size ranged from 5 to 50 μm. The morphology of the Ni–Al powder showed that molten particles were composed of Ni solid solution, NiAl, Ni_3Al, Al_2O_3, and NiO. The Ni–Al phase and a small amount of Al_2O_3 particles changed the composition of the coating. The microstructures of the twin-wire-arc-sprayed Ni–Al composite coatings were characterized by SEM, EDS, X-ray diffraction（XRD）, and transmission electron microscopy（TEM）. The results showed that the main phase of the Ni-5wt%Al coating consisted of Ni solid solution and Ni Al in addition to a small amount of Al_2O_3. The main phase of the Ni-20wt%Al coating mainly consisted of Ni solid solution, Ni Al, and Ni_3Al in addition to a small amount of Al and Al_2O_3, and Ni Al and Ni_3Al intermetallic compounds effectively further improved the final wear property of the coatings. TEM analysis indicated that fine spherical NiAl_3 precipitates and a Ni–Al–O amorphous phase formed in the matrix of the Ni solid solution in the original state.

Preparation and Characterization of Carboxyl-terminated Poly(butadiene-co-acrylonitrile)-epoxy Resin Prepolymers for Fusion-bonded-epoxy Powder Coating

Liquid carboxyl-terminated poly（butadiene-co-acrylonitrile）（CTBN）-epoxy resin（EP） prepolymers were prepared with different contents of CTBN.The chemical reactions between EP and CTBN were characterized by Fourier ransform infrared（FTIR） spectroscopy and gel permeation chromatography（GPC）.The scanning electron micrograph（SEM） and dynamic mechanical analysis（DMA） of curing films showed phase separation,and the rubber particles were finely dispersed in the epoxy matrix.Mechanical properties analysis of curing films showed that impact strength and elongation at break increased significantly upon the addition of CTBN,indicating good toughness of the modified epoxy resins.Thermogravimetric analysis（TGA） showed that the incorporation of CTBN had little effect on the thermal stability of EP.Fusion-bonded-epoxy（FBE） powder coatings modified with CTBN-EP prepolymers were prepared.The experimental results demonstrate the ability of CTBN-EP prepolymers,toughening technology to dramatically enhance the flexibility and impact resistance of FBE coatings without compromising other key properties such as corrosion protection.

Preparation and Interfacial Stability of SiC_f/Ti-6Al-4V Composites Fabricated Using Powder Coated Fiber Method

SiCf/Ti-6Al-4V composites were fabricated by the powder-coated fiber method. The precursor fiber was prepared under the optimized parameter, and the composites were made using the vacuum hot pressure method. The influence of heat exposure time on products of thelinterfacial reaction was investigated using scanning electron microscope （SEM） and analytical transmission electron microscope （TEM） with energy dispersive spectrometer （EDS）. The main products are TiC and Ti5Si3 after vacuum exposing the samples at 700℃ for 50 h. The growth dynamics of interracial reaction products was analyzed quantitatively, which fitted the parabola rule. The activity energy of the reaction was 252 kJ·mol^-1.

Coating powder beds with liquids and foams based on viscous formulations using a twin screw mixer:A continuous process study

Coating with viscous formulations has been essential in numerous industries as it can be a means for providing functionalization, additional properties, as well as other benefits. However, there have been scarce studies that have investigated and proposed methodologies in literature. Continuous coating of powders with viscous liquids poses as a promising technology, which has been mentioned in some studies, but has not yet been thoroughly investigated. This paper employs the use of image processing and analysis, in combination with statistical analysis of particles to evaluate the effectiveness of foams and liquids as a means of coating powder beds. Two different sizes of twin screw mixers that are working in continuous operation are employed, and a new continuous foaming device is fabricated and used for the experiments of coating. The effect of materials and process parameters (as for example rotational speed, and flowrate) on the quality of coating are investigated. Image analysis is used to assess the coating quality. The results clearly showcase the potential of using twin screw mixers for coating purposes and not only for mixing. The hypothesis that using large bubble foams to improve the coating of viscous liquids on particles is proven correct, as they provide higher quality coatings compared to their equivalent liquids, when used in the twin screw mixer. Surprisingly, using a larger scale twin screw mixer, does not show a substantial effect on the mixing, regarding quality, however there is still a requirement for mix optimization for achieving scale-up of this process. These results provide a new pathway for coating powders with viscous formulations in industrial applications, requiring less energy and effort in this process, and can pave the way towards introducing more sustainable industrial methodologies for coating.

Interfacial Shear Strength Measurements of SiC Fiber-Reinforced Titanium Composites

A continuous loading push-out test technique was used to measure the interfacial shear strength of SiC fiber reinforced Ti matrix composites. The interfacial shear strength of samples as-fabricated and after heat exposed at 700 and 800℃ for 50 h was successfully determined. It has been found that the interfacial shear strength of the specimen exposed at 700℃ was higher than that of as-prepared and exposed samples at 800℃. The shear strength of the as-prepared samples was about 112.45 MPa, and increased to about 153.77 MPa after heat-treating at 700℃ for 50 h, but decreased to 133.11 MPa after treating at 800℃ for 50 h. Scanning electron microscope （SEM） was used to investigate the interfacial morphology of the samples. The brittle phase was the main products of interface for samples exposed at 800℃, and the interface was easily peeled off.

Corrosion resistance properties of colored zirconium conversion coating and powder coating on cold-rolled steel

Jiuhe Corporation developed a new surface pretreatment for producing a phosphate-free and colored zirconium conversion coating on cold-rolled steel(CRS).Scanning electron microscopy(SEM)and energy-dispersive spectroscopy(EDS)were applied to study the surface morphology and elemental composition of the golden yellow-colored conversion coating.Elec trochemic al impedance spectroscopy(EIS)was used to investigate the electrochemical performance of the conversion coatings with different colors.The corrosion resistance properties of the powder coatings were characterized using the neutral salt spray test(NSS)and the tape adhesion test.The EIS and NSS results demonstrate that the colored zirconium conversion coatings exhibit excellent corrosion resistance properties compared with the bare CRS.The golden yellow-colored conversion coating has a maximum EIS value(748Ω·cm^(2))and achieves 0 mm degree of corrosion,as stipulated by EN ISO 4628-8.This paper proposes a hypothesis concerning the ZrO_(2)-(β-FeOOH)-polymer conversion reaction to explain the color changes.

Forced boiling of nonazeotropic immiscible mixture in a supercapillary microchannel array for ultra-high heat flux removal with chip junction temperature below 85℃

In this study,a structure-optimized two-phase microchannel heat sink with sintered submicron nucleation sites was developed and tested.The copper-based microchannels had a rectangular cross-section with an equivalent hydraulic diameter of 222μm.The subcooled flow boiling characteristics were comprehensively compared between pure HFE-7100 and a non-azeotropic,immiscible binary mixture of HFE-7100 and water,considering heating areas of 1 and 5 cm^(2).The total heating power input to the test section were 100–1500 and 250–3000 W for a 1 and 5 cm^(2)heat source,respectively,with a flow rate ranging from 50 to150 L/h.Compared to pure HFE-7100,the non-azeotropic immiscible binary mixture of HFE-7100/water in the sintered porous microchannels exhibited a much higher overall heat transfer coefficient and lower power consumption.To maintain the junction temperature of a high power electronic chip below 85℃,the proposed supercapillary microchannel heat sink could effectively dissipate the heat flux of 1275 W/cm^(2)over 1 cm^(2)heat source and 500 W/cm^(2)over 5 cm^(2)heat source.In addition,the volume ratio of the binary mixture strongly influence the two-phase flow heat transfer characteristics.An optimal volume ratio exist in terms of the thermal resistance–pumping power minimization(HFE-7100:water=2:8 is recommended in this study).The findings of this investigation on the flow boiling properties of non-azeotropic immiscible mixtures help fill a gap in the related field.

Preparation of baicalin colon-targeted granules and its intervention effect on ulcerative colitis in rats

Baicalin(BA)is a flavonoid extracted from the dried root of Scutellaria baicalensis Georgi with excellent antioxidant and anti-inflammatory biological activities.In this study,Eudragit S100 was used as the colonic target material to prepare BA colonic targeting granules(EBCGs)based on plasticizer dry powder coating technology to improve the targeting transportation performance of BA.In vitro studies showed that EBCGs with pH-sensitive properties were successfully prepared by plasticizer dry powder coating,and in vivo animal imaging studies showed that EBCGs could deliver BA to the colon and inhibit the release of BA in the upper gastrointestinal tract(GIT).In vivo studies showed that EBCGs had good therapeutic effects in colitis,which reduced expression levels of tumor necrosis factor alpha(TNF-α)and interleukin-1β(IL-1β)and increased superoxide dismutase(SOD)activities in the colonic tissues of rats with colitis.In conclusion,Eudragit S100 could be used for the preparation of multi-unit oral colon-targeted formulations by plasticizer dry powder coating technology,and our prepared EBCGs had good colon-targeting properties,which could improve the therapeutic effect and provide a potential application for ulcerative colitis(UC).

An update on electrostatic powder coating for pharmaceuticals

Derived from dry powder coating of metals, electrostatic powder coating for pharmaceuticals is a technology for coating drug solid dosage forms. In this technology, coating powders, containing coating polymers, pigments, and other excipients, are directly sprayed onto the surface of the solid dosage forms through an electrostatic gun without using any organic solvent or water. The deposited coating powders are further cured to form a coating film. Electrostatic powder coating technology has many advantages compared to other pharmaceutical coating methods. It can eliminate the limitations caused by the organic solvent in solvent coating such as environmental issues and health problems. And electrostatic powder coating technology also surpasses aqueous coating due to its shorter processing time and less energy consumption, leading to a lower overall cost. Furthermore, the utilization of electrical attraction can promote the movement of coating powders towards the substrate, leading to an enhanced coating powder adhesion and coating efficiency, which make it more promising compared to other dry coating technologies. The objective of this review is to summarize the coating principles, apparatus, and formulations of different electrostatic powder coating technologies, giving their advantages and limitations and also analyzing the future application in the industry for each technology

Flow properties of fine powders in powder coating

Three types of nanoparticles and their combinations were blended into a fine powder, which has been used in the powder coating industry. To study their effects on flow properties, the modified powder samples were characterized using a variety of techniques that tested the powder under different powder states ranging from dynamic to static. It was found that all three nanoparticles improved the flow properties of the powder to some degree, though the amounts of the nanoparticles needed were different depending on their physical properties. Secondly, inconsistency among these powder characterization techniques was also found. This is attributed to the different states of the powder samples during a measurement including dynamic, dynamic-static and static states. It was confirmed that characterization techniques which test the flow properties of a powder under all three states are needed to fully describe the flow properties of the powder. Finally, the effects of combinations of nanoparticles were explored, and it was found that combinations of nanoparticles can intensify, weaken or combine the effects of their component nanoparticles. The effects of nanoparticle combinations are not a simple summation of the effects of their comnonent nanoparticles.

Effect of activators on the properties of nickel coated diamond composite powders

Nickel coated diamond composite powders were fabricated via a newly developed direct electrodeposition technique. The effects of activators on the coating of diamond were firstly investigated and diamond grinding wheels were then prepared from Ni-coated diamond composite powders with different activators. The microstructural characterizations of this composite powders were finally conducted by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction, and the mechanical and tribological properties of as-prepared diamond grinding wheels were also measured. There are changes in microstructures and properties of the composite powders with activators. The activator concentration also has an influence on the morphologies and phase structures of the Ni coating on diamond particles.The composite powders with more compact coating of nickel can be prepared by adding 1 g dm^（-3） or more AgNO_3 as an activator to electrodeposit nickel on diamond. The mechanical and tribological properties of diamond grinding wheels were significantly improved when the coating phase structure of Ni crystal grew with（111） plane orientation on the surface of diamond particles. The wheels made from nickel coated diamond composite powders possessed the advantages of easy preparation and outstanding tribological properties. Therefore, Ni coated diamond composite powders exhibit a great potential to be extensively applied in diamond cutting and grinding tools.

A core-shell composite pigment with rutile TiO_(2) intensification for UV inhibition

Organic pigments generlly have bright colors but poor ultraviolet(UV)resistance.To improve the UV for UVinhibition is proposed by afacile sol-gel method inthis work.Acore-shellstructure,witha homo resistance and extend the applications,a core-shell composite pigment with rutile Tio intensification geneous sol-gel TiO_(2) shell containing additional nanosized rutile TiO2 particles and with the pigment as the core,was established taking advantage of UV resistance of TiO2 and binding ability of sol-gel.While the sol-gel TiO2 shell alone has already shown obvious ultraviolet shielding effect,as tested over different sol-gel aging times and Tio2 loadings,the UV resistance of the fluorescent pigments was further enhanced by binding the nanosized rutile TiO2 in the sol-gel shell At a sol-gel TiO2 to rutile TiO2 ratio of 2:1,the UV exposure time is extended about eight times compared with that of the original pigment and twice as that of the modifed pigment with pure sol-gel TiO2 for the same color change.Therefore,the novel core-shell composite pigment intensified with nanosized rutile Tio2 particles is proved to be effcient in improving the uV resistance of organic pigments.

Coating particles using liquids and foams based on viscous formulations with industrial mixers: Batch operation

Particle or powder coating with viscous liquids has been essential in industry for surface modification purposes to induce and enhance specific functionalities.This paper evaluates the performance of using foams (of different bubble diameters) versus liquids as a means of coating powder beds based on viscous liquid formulations.Coating with viscous liquids present numerous industrial challenges and therefore preparing foam equivalents can render the liquid component weak enough (through pre shearing to form the foam),to allow it to break up and coat particles under the shear forces exerted in a mixing device.In this study,two shear mixers are used;the first type consists of paddles in different configurations attached to a single rotating shaft,whilst the second type is a commercial twin screw mixer (TSM).The quality of coating achieved on the bulk powder bed using liquids and foams (stained with a dye) is assessed by image analysis to determine the homogeneity of the color distribution.In addition,scanning electron microscopy provides a tool to further investigate the coating quality of individual particles from the bulk product.The results show that large bubble (centimeter size) foams are much more effective at distributing within the fluidized powder bed compared to the starting viscous liquid and small bubble foams (submillimeter size).Furthermore,there is a maximum ratio of foam to powder beyond which agglomeration occurs and is insufficient to fully coat the particles.Coating of individual particles is achieved in the case of the TSM,whereas SEM proves that the single shaft paddle mixer crushes the particles and subsequently granulates them together to form granules of a size comparable to the size of coated particles seen after coating with the TSM.

Effect of electrode voltage for NaCl coating on baked potato chips in continuous electrostatic coating system

Powder coating is an important process in the food industry,especially for snack foods such as potato chips,to create variety in food products.Electrostatic coating has been adopted in order to provide better transfer efficiency and lower the dust produced during coating.Studying the effect of electrode voltage on coating efficiency and evenness of baked potato chips using the developed electrostatic powder coating system was the main purpose of this research.Different types of NaCl(refined and table salts)were coated on baked potato chips at 0,30,40,50,60,and 70 kV.After coating with either refined or table salt,transfer efficiency,adhesion after coating,coating evenness and texture of samples were determined.Higher transfer efficiency,adhesion and coating evenness were observed when electrostatic coating was conducted at 30-50 kV.Most samples with higher transfer efficiency,adhesion and evenness were obtained after electrostatic coating.However,it did not significantly affect hardness of baked potato chips.